Zeolites have been widely used as catalysts, ion exchangers,andadsorbents since their industrial breakthrough in the 1950s and continueto be state-of the-art adsorbents in many separation processes. Furthermore,their properties make them materials of choice for developing andemerging separation applications. The aim of this review is to putinto context the relevance of zeolites and their use and prospectsin adsorption technology. It has been divided into three differentsections, i.e., zeolites, adsorption on nanoporous materials, andchemical separations by zeolites. In the first section, zeolites areexplained in terms of their structure, composition, preparation, andproperties, and a brief review of their applications is given. Inthe second section, the fundamentals of adsorption science are presented,with special attention to its industrial application and our caseof interest, which is adsorption on zeolites. Finally, the state-of-the-artrelevant separations related to chemical and energy production, inwhich zeolites have a practical or potential applicability, are presented.The replacement of some of the current separation methods by optimizedadsorption processes using zeolites could mean an improvement in termsof sustainability and energy savings. Different separation mechanismsand the underlying adsorption properties that make zeolites interestingfor these applications are discussed. This is the case of petrochemicalproducts or alcohols produced by fermentation, both of which requirepurification steps after being produced.3 On the otherhand, the separation and purification of naturally occurring mixtures,such as air or natural gas, is of high environmental, economical,and/or practical interest.6Separation processes areof extremely high importance for our society,comprising 10–15% of the global energy use and 40–90%of the total process cost.1 As climate change and resourcescarcity threaten to become more severe, cost reduction graduallyaligns with sustainability objectives, and the focus of the industryis drawn from traditional separation procedures, with distillationbeing the main exponent, to alternative technologies or process designs.The development of improved methods for separations, such as hydrocarbonseparations, the separation of olefins from paraffins, or the removalof carbon dioxide from industrial exhaust gas or from the atmosphere,could have an enormous impact on our progress toward sustainability.1Until a not too distantpast, many separation processes did nottake the ecological axis into account, mainly due to the wide availabilityof fossil fuels as an energy source and the lack of environmentalawareness.7 At the same time, the versatility of the technique,i.e., the growing range of conditions and process designs available,and, most of all, the vast dimension of possible adsorbents make itan ever evolving technology that is gaining the interest and recognitionof the academic and industrial communities.Adsorption represents one of the possiblealternative separationtechnologies for many fluid mixtures. It is a mature technology thathas been used for separation applications, such as water treatment,drying, hydrogen purification, air separation, or hydrocarbon separation.10 Zeolites are microporous crystalline aluminosilicates with well-definedpore sizes, and for this reason, they are able to discriminate betweenmolecules of high practical interest, such as CO2, ethylene,isobutene, or xylenes, with a precision of tenths of Å (10–11 m).11 Additionally, theirstructural and compositional richness allows for fine-tuning of theiradsorption properties to address a targeted separation. They havebeen successfully applied to drying of gases and liquids,12 to the separation of nitrogen from oxygen,4 to the separation of linear from branched hydrocarbons,13 and to the separation of xylene isomers amongother separations.14 Nonetheless, theyare potentially applicable to many different separations which arecurrently performed by different technologies, such as CO2 removal, olefin–paraffin separation, and the separation ofhydrogen isotopes, or which still are in development, e.g., the separationof mono- from multibranched hydrocarbons.The industrial developmentof adsorption processes is tightly boundto the commercialization of the first zeolitic materials.In this review, wepresent the state-of-the-art zeolites as adsorbentsand their potential to address separations of current industrial interest. We aim this review at a broad audience interested in chemical separations.Readers with experience in zeolites and catalysis will be able toget into the field of zeolites as adsorbents and to find out aboutpromising applications which may be accessible to them. On the otherhand, readers with knowledge in adsorption may get an overview onthe variety of relevant separations currently being targeted and alsoon the vast possibilities that zeolitic adsorbents offer. Readerswith interest in pursuing a specific separation out of the ones includedhere will be provided with a solid starting point and a rich and updatedliterature background.218 have boosted their use in industrial and domesticapplications, which take advantage of their unique properties. Inthe following sections a short historic review of zeolites and theiruse  was in 1858,22 and the first report on zeolite synthesis wasin 1862, with the synthesis of levyne.23 The first industrial success of these materials was based on theirion-exchange properties, as water softeners for laundry compositions.26 This still remains one of their major applications.For the next years no noticeable discovery was made by chemistsin reference to zeolites, and it was not until 1840 that Damour demonstratedthe reversible hydration and dehydration of these materials.27 and further studied byGrandjean in 1909.28 Selective adsorptionand exclusion of molecules, i.e., the molecular sieve effect, wasfirst described in 1924 by Weigel and Steinhoff, who observed howwater, methanol, ethanol, and formic acid were adsorbed on chabazite,while acetone, diethyl ether, and benzene were excluded.29 This effect could not be explained until thestructural porosity of zeolites was described following the firststructural elucidations of these materials.32 In 1932, McBaincoined the term “molecular sieve”, referring to zeolitesand their very high selectivity when applied to adsorption processes.33 McBain’s work was a turning point inzeolite science, as it encouraged a young researcher, Richard M. Barrer,currently considered the father of zeolite science, to dive into thesematerials’ field of research.34The adsorptionof species other than water was first reported byFriedel in 189637 and others with no natural counterpart, which he named as zeolitesP and Q.40Barrer studied the separation of mixtures of many different moleculeson zeolites and realized the great potential of these materials asadsorbents for separation processes. Over the next 20 years he successfullyattempted the synthesis of zeolites by mimicking the crystallizationconditions of natural zeolites , thus obtaining some syntheticanalogues of natural zeolites, such as chabazite and mordenite,41 During histime at the Imperial College (1954–1976), Prof. R. M. Barrerachieved the first zeolite synthesis using tetraalkylammonium cations,44 which, in retrospect, has turned out to be the most fruitful strategyfor obtaining new zeolitic materials to the present time and stillremains state of the art , he varied the synthesis conditionsby means of lowering the temperature to 25–150 °C andusing more reactive silica sources and more alkaline media. This ledto the rapid obtention of zeolites A and X  or inorganic nature, depending on the synthesisconditions and on whether the material has been subjected to postsynthesistreatments . Natural zeolites and manysynthetic zeolites contain metallic cations, which are usually hydratedand account for Cronstedt’s discovery.The propertiesthat make zeolites especially useful as catalysts and adsorbents stemfrom their crystalline structure and also from their composition.Zeolites are crystalline microporous aluminosilicates, the frameworkof which consists of corner-sharing TOedra see 2, where 55 There is as well the special case of zeolites with no aluminumat all, known as zeosils or pure-silica zeolites. Logically, theycontain no extraframework cations. Pure-silica and high-silica zeolitesare under intense research, as they present a hydrophobic surfaceand generally also larger thermal and chemical stabilities than traditionalzeolites, which make them very appealing for adsorption applications.58According to their Si/Al ratio, aluminosilicatezeolites can beclassified as low-silica zeolites ,medium-silica zeolites , and high-silica zeolites (Si/Al > 5).59 is a straightforward way to obtain a quick overviewon the rich compositional variability of zeolites. Apart from Si andAl, which are not necessarily present in all zeolite-like materials(zeotypes), other atoms can be found in tetrahedral coordination inthe framework, such as B, Be, Co, Fe, Ga, Ge, Mg, P, Ti, and Zn. Itmust be noted that the presence of some of these “heteroatoms”can facilitate the crystallization of specific structures which areotherwise not achievable and also of materials with different chemicalproperties.On the other hand, the T atoms can be differentfrom Si and Al.There are many compositional variants of zeolites which present structuresanalogous to or different from aluminosilicate zeolites. An advanced“chemistry search” in the Database of Zeolite Structures61 Importantly, very often,B-silicate zeolites are produced with different structures than Al-substitutedzeolites.65 Also, the relatively high lability of B in framework positions allowsits isomorphic exchange by Al through secondary synthesis treatments,enabling Al incorporation in zeolites that can not be synthesizeddirectly as Al silicates.66For instance, isomorphic B incorporation in zeoliteshas been frequentlyachieved, resulting in acid microporous catalysts of milder acid strengththan Al-containing zeolites.71 The preferential location and strong directing effect toward D4r-containingzeolites have been attributed to the larger germanium atomic radius,74 which introduces framework flexibility and high tolerance in thecrystal structure to relatively acute T–O–T bond anglesresulting in zeolites having low framework densities as predictedby Brunner.75Anotherinteresting case of the structure-directing effect of frameworkheteroatoms is that of Ge, which can replace Si. It has been observedthat Ge incorporation very often directs to the formation of doublefour ring (D4r) containing zeolites, where Ge tends to be placed.4 and PO4 tetrahedra. They have proven interesting foradsorption and heat exchange applications, even though frequentlythey present more limited thermal and chemical stabilities if comparedto other zeolites.76 There is a series of AlPO-relatedmaterials, which are in concept heteroatom-substituted AlPOs.78 The possible “heteroatoms” include Si, Fe, Mg, Mn,Co, Zn, Ti, V, and/or Cr among others.79 In silicoaluminophosphate (SAPO) materials, part of the T positionsof the framework are occupied by Si atoms. Silicon substitution inSAPOs follows conceptually more complicated patterns than Al substitutionin aluminosilicate zeolites, as Si can “replace” a singleP atom (isolated Si), but also larger framework fragments, yieldingwhat is known as Si islands or Si-rich domains (see 83Aluminophosphate (AlPO)materials are isoelectronic with pure-silicazeolites and present a perfectly alternating sequence of AlOains see 3. SAPOs 84 The acidity/basicity of specific adsorption sites depends as wellon the composition of the framework. The ratio of tetravalent  to trivalent  atoms, usually the Si/Al ratio,largely defines the polarity of the material. For instance, zeoliteswith a higher Al content  adsorblarger amounts of polar compounds, such as water, than high- or pure-silicazeolites.55 Additionally, the charge-balancingextraframework cations can contribute with their specific chemicalproperties to the chemistry of the material.85Depending on the T atoms present in a framework,the chemical andphysical properties of the material will vary. The presence of atomswith redox properties, such as Ti, Co, Fe, or V, may have a greatinfluence on the redox chemistry of the material.45 thus resulting in a large number (millions) of different hypotheticalporous structures.86 More than 250 differentzeolitic structures are known to exist at the present time, of whichsome can be found in nature and others are synthetic. Each structureis given a three-letter code and registered in the Database of ZeoliteStructures,59 where a thorough structuraland crystallographic description is provided.However, the most important feature of zeolites and the onethathas made them interesting for any application, since they were firststudied in detail by Barrer, is their structure-derived porosity.The flexibility of the T–O–T angle allows for differentspatial dispositions of the tetrahedra,4 tetrahedra, i.e.,the primary building units of zeolites, can be linked following differentarrangements, which result in secondary building units (SBUs), compositebuilding units (CBUs), or the so-called “tiles”. SBUscontain a maximum number of 16 T atoms and were initially intendedto be the sole descriptor of zeolite structures; i.e., a single SBUtype  should suffice forthe description of each framework. At the same time, different SBUscould be used to describe a single framework, and different frameworkscould be described using the same SBU. However, in 2007 it was realizedthat the SBUs were insufficient for the universal description of zeolitestructures, and the listing of new SBUs ceased. Instead, the broaderconcepts of CBU and/or tiles were introduced and recommended. It mustbe noted that there is an overlap between these descriptors, and somearrangements of tetrahedra can belong to two or all three of thesekinds of descriptors. For instance, the double 4-ring belongs to allthree of them and is named differently in each case . Examples of typical building units are given in The structuraldescription of zeolites is usually performed interms of their building units. The TOm is the numberof n-rings defining the polyhedron and ∑mi the total number of faces. Thus, the d6r building unit could be expressed more generally as [4662] and the sod building unitas [4668].87 In somecases, instead of polyhedral building units, chain building unitsmay be useful for structural description.A more general notation of the CBUs, also applicablefor new structuresand building units, follows the scheme n-rings arecalled windows). Polyhedral units with windows smaller than or equalto 6R are named cages, and only a few very small molecules, e.g.,water, can penetrate these. The sod building unitshown in lta building unit shown in Another approach forthe structural description of zeolites isbased on the size, connectivity, topology, and geometry of their poresystems. The pores are the void spaces inside the framework that arenot occupied by framework atoms. These pores can be accessible orinaccessible to molecules of various sizes, depending on how theyare connected and the window size (the 884 tetrahedra (8–9R).Small pore zeolites have a minimum pore diameter between3 and 5 Å, which corresponds to rings consisting of 8–9TOMedium pore zeolites have a minimum pore diameter between5 and 6 Å, which corresponds to 10-rings (10R).Large pore zeolites have a minimum pore diameter between6 and 7.5 Å, which corresponds to 12R.Extra large pore zeolites have a minimum pore diameterabove 7.5 Å, which corresponds to rings of more than 12 tetrahedra.According to the minimumwindow size of the largest pores presentin their structure, zeolites can be classified as follows:Within these groups, there are many structures withdifferent poresizes and shapes. If a zeolite presents more than one kind of pore,it will be classified according to the largest pore present. For example,the STW framework presents intersecting channels with different minimumwindow sizes, i.e., 8R and 10R, and is considered a medium pore zeolite.The topology of the pore system can be of high importance as well,as it has a large impact on the interaction and diffusion of moleculesinside the pores.2.32.3.190 Through imitation of the natural process, some zeolites were obtained,mostly analogues of minerals existing in nature. However, it was bymodifying it that the structural and compositional richness of thesematerials started to become apparent.The synthesis of zeolites is usually performed following the hydrothermalmethod, which mimics the natural conditions that lead to the crystallizationof zeolites. This includes a source of the T atoms , a structure-directingagent ,a mineralizing agent , temperaturesbelow 600 °C, and autogenous pressures.49 started the search for new zeolites in 1949.Typical Al sources include different kinds of alumina, aluminum hydroxides,aluminum alkoxides, and aluminates. In the case of AlPOs and SAPOs,P is most frequently added as phosphoric acid.91The synthetic processesthat have led to the most discoveries of new zeolitic structures andcompositional variants follow these guidelines in general terms butpresent many singularities. The T atom source is usually an oxidizedform of the T atom. For instance, typical Si sources are amorphous,fumed, or colloidal silica, silicates, alkyl silicates, and otherzeolites/materials. These kinds of Si sources with enhanced surfacearea and solubility were a key for success when Milton and co-workers+, Ca2+, or K+. Nonetheless, the most remarkable type of SDAs and the onesthat meant a breakthrough in zeolite science are organic SDAs (OSDAs),which are in most cases amines and alkylammonium cations.48 These OSDAs were initially referred to as “templates”,93 a term which is still frequently (and inaccurately) used to addressOSDAs in general. It has its origin in the so-called “templateeffect” that some OSDAs possess, in which their presence inthe synthesis gel leads to the crystallization of a specific structurewith matching topological features.92 Othermolecule types, such as alkylphosphonium cations, alkylsulfonium cations,phosphazenes, crown macrocycles, metal complexes, and self-assembledmolecules, have been used as OSDAs but with a quantitatively moremodest degree of success than nitrogen OSDAs.94 The way in which these OSDAs favor the crystallization of a specificstructure is not yet fully understood, despite the large researcheffort put into it.96 However, the rational design of OSDAs inthe search for particular zeolites has given good results in somecases.100 In this sense, recent advancesin data mining and artificial intelligence have allowed us to computationallypredict to some extent which OSDAs may lead to the crystallizationof targeted zeolite structures.101 In general terms, linear OSDAs favor the crystallization of 1D structures;branched OSDAs favor the crystallization of interconnected 2D and3D structures; and bulky OSDAs favor the crystallization of structurespossessing cavities. The lower charge density of the OSDAs in comparisonwith the alkaline and alkaline-earth cations allows for less chargedframeworks, thus facilitating the obtention of final materials witha higher Si/Al ratio.55 More than one kind of inorganicor organic SDA may be present in the synthesis gel, and both may actas SDAs; however, they also may have been added to increase basicity,as explained below. Additionally, the T atoms present in the synthesisgel can have a structure-directing effect, too, as they may favorthe crystallization of structures bearing specific CBUs. This is thecase for Ge, or Be, which favors D4R and 3R, respectively.48The role of the SDAs is of large importance, as theynot only promotethe crystallization of specific structures but also may remain insidethe pores of the final material to some extent and act as charge-balancingions. The first SDAs that were used in zeolite syntheses were cationsof inorganic nature, such as Na102 By doing so, the environmental impact of zeoliteproduction as well as its cost would be lower, thus easing the requirementsfor scale-up.103 The OSDA-free synthesis of zeoliteswas reported for the first time in 1985 for high-silica ZSM-5,104 thus being a rare example of such synthesesuntil the publication of Xie et al. in 2008 reporting the crystallizationof zeolite Beta in the absence of OSDA.105 In this report, the use of seeds (see below) of zeolite Beta wascrucial for growing the desired zeolite. Since then, a large numberof researchers have devoted large efforts to expanding the range ofzeolites synthesized through OSDA-free synthesis routes108 and also to understanding the synthesis parameters that govern thegrowth of zeolites in such conditions.111On the other hand, there is an increasinginterest in reducingthe amount of OSDA needed for the synthesis of certain zeolites  or even to find ways to recycle it or dispense with it.112 Possible mineralizing agents are the hydroxide and fluoride anions.113 Hydroxide anions are the most widely used mineralizingagent, and they are frequently added along with the SDA. If an extraamount of hydroxide anions is needed, it is usual that inorganic  bases are used for low-silica zeolites andorganic  bases for high- and pure-silica zeolites.The source of fluoride anions can be hydrofluoric acid, which in turndecreases the pH of the gel (this may be desirable for preventingOSDA decomposition), or ammonium fluoride. Some zeolites have beensynthesized both from gels containing hydroxide and from fluoride,and there are interesting consequences to the use of one or the other.The fluoride anion has in some way a structure-directing effect, too,in which it favors the formation of certain CBUs and phases with lowerdensities.115 On the other hand, zeolitessynthesized from fluoride-containing gels tend to present an extremelylow amount of defects.116 The H2O/SiO2 ratio is important as well, especially in highsilica gels in fluoride media, as it affects which kind of frameworkswill be obtainable based on the density of the final material andthe size of the crystals.118The mineralizingagent intervenes directly in the breaking andformation of T–O–T bonds and helps to establish a dynamicequilibrium that ends in the formation of the zeolite.119 When preserved, the seeds promote the crystallizationof their same structure, as the nucleation step is skipped and thecrystals can start to grow immediately.96 When sacrificed, the seeds promote the crystallization of a phasethat may or may not share structural resemblance with the parent structure.Another way of influencing the synthesis outcome is to introducecrystal seeds of a certain zeolite “parent” structurein the synthesis gel. The seeds can be preserved or dissolved intoanionic species with one or more T atoms .112 Higher temperatures and longercrystallization times favor the obtention of more dense, usually morestable phases instead of more open phases. On the contrary, the pressurein the gas phase does not seem to have any effect on the synthesis.49 Other synthesis parameters that have a remarkableinfluence on the product obtained are aging of the gel at lower temperatureprior to the hydrothermal process and stirring/rotation speed duringthe crystallization.112Crystallization temperature and time have a decisive effect inthe synthesis of zeolites.As can be seen, thereare many different variables that affectthe results of the hydrothermal synthesis of zeolites. The complexityof these heterogeneous systems has not allowed for a full rationalizationof the crystallization mechanisms or of the specific conditions thatlead to the crystallization of a specific phase with a well-definedcrystal size and composition. However, general trends on how eachand every one of these parameters affect the synthesis outcome areunderstood and applied to new synthetic processes in order to reducethe range of possible results.2.3.2The advances in zeolite synthesis are a major drive for their applicationin new and established processes. Whereas the previous section dealswith the conventional synthesis methods, in this section, an overviewon more recent synthetic trends and methods will be given, includingnonconventional synthesis methods. The production of hierarchicalzeolites and nanocrystalline zeolites and the control of heteroatomdistribution will also be discussed here.120 Zeolite interconversion takes place in gels, where the silica sourceis replaced by a parent zeolite which shares common building unitswith the target zeolite, an example of which is the transformationof faujasite into chabazite.123 On the other hand, new zeolitescan be obtained by topotactic transformation in which some labileatoms are removed by mild secondary treatments followed by condensationor pillarization of the remaining zeolitic layers, providing new fullyordered microporous materials.126 Solid-phase transformation by applying highpressure on the zeolite has been also described to transform the structureof one parent zeolite into another zeolite of different topology.128 The presence of radicals in the synthesis media has been claimedto accelerate the crystallization of zeolites129 as has been shown for microwave heating,130 while using ionic liquids as the solvent instead of water, i.e.,ionothermal synthesis, decreases the operating pressure, allowingthe synthesis to be carried out at ambient pressure.133Modification of parameterswithin the context of hydrothermal synthesis has been a relativelysuccessful strategy to obtain new zeolitic materials throughout theyears. However, the rate of discovery of new zeolitic structures andmaterials can be accelerated through the implementation of nonconventionaltechniques, such as interzeolite and topotactic conversions, ionothermalsynthesis, and microwave- or radical-assisted synthesis.138 although other nonconventional methods have been applied as well.139 The aggregation of these nanosized crystalsgives rise to “hierarchical” zeolites with an effectivemeso- or even macroporosity.140 Moreover,changes in the effective pore size can directly be induced by ionexchange, functionalization, or steaming, all of which are explainedin 141 Accordingly, these methods are used to producehierarchical zeolites, which feature not only their original microporositybut also additional mesoporosity. The presence of a multimodal poresize distribution of micro-, meso-, and/or macropores leads to anincreased accessible surface area, shorter diffusion distances, andhigher mass transfer rate when used as adsorbents in separation/adsorptionprocesses. Therefore, hierarchical zeolites have been extensivelystudied in the past decade.145Diffusion of molecules inside the pores of a zeolite is ofparamountimportance in adsorption processes see 3.1.2 sin149 The charge density, size, and concentration of the SDA play an importantrole in this sense.150 The distribution of Al has been shown to have an effect on the adsorptionproperties of zeolites, as well. For instance, when the aluminum isevenly distributed throughout the framework, the adsorption capacityof aluminosilicate zeolites is enhanced and the adsorption heat remainsconstant.151 In the case of SAPOs, the Sidistribution can be controlled by modifying the synthesis conditionsand also has an important effect on the catalytic and adsorptive behaviorof the materials.152Heteroatom distribution in zeolitic materials has a largeinfluenceon their interaction with molecules. It has been found that selectivityin many acid-catalyzed reactions by Al-containing zeolites stronglydepends on the Al distribution in the zeolite framework, which canbe controlled by modifying the synthesis conditions.2.3.3154Even though many zeolites can be obtained by direct synthesis withtailored composition and structure, it is common that further processing,i.e., postsynthetic treatment, is needed to achieve the desired propertiesin the final material. Ion exchange, desilication, surface functionalization,calcination, and steaming are some of the most frequently used methods.Ion exchange of aluminosilicatematerials allows us to modify their acid–base, redox, and texturalproperties (pore sizes and interaction with adsorbates). It is usuallyperformed in an aqueous solution with a high concentration of thecationic species to be exchanged. After reaching equilibrium, thezeolite is filtered, washed, and dried and can be subject to furtherexchange or modifications. Exchange of small cations, such as metalsor ammonium, is the usual case. If the zeolite pores are too narrowfor the extraframework species to diffuse, ion exchange may not bepossible, which is usually the case for OSDAs.156 Supported metal zeolitic materials are most frequently producedby impregnation or ion exchange and, more recently, by chemical vapordeposition.154Functionalizationof zeolite surfaces allows us to introduce newchemical and/or physical properties to zeolitic materials. Functionalizationby supporting transition and noble metals on zeolitic materials hasbeen widely used for catalyst preparation see 2.4.2, as158 The amine groups help decreasethe hydrophilicity of the zeolite and keeps good affinity toward CO2, thus improving the recyclability of the material.Another type of functionalizationinvolves grafting of nonmetallicfunctional groups to the zeolite surface, examples of which includeinorganic acids, amines, and silanes. Amine grafing on zeolites hasgained attention in the last years for carbon capture applications,although in this case the protagonists are still amine-grafted mesoporoussilicas.159 It is usually performed by chemical vapor depositionof alkoxysilanes, such as tetramethylorthosilicate or tetraethylorthosilicate,followed by calcination.Silanization is a type of surface functionalization of zeolitesthat can be used to selectively deactivate external surface acidityand to tune their textural and catalytic properties, by decreasingthe pore size and the diffusivity.112 Calcination at high temperatures in oxidizing or inert  atmospheres is afrequent method to activate zeolites. If the zeolite has been synthesizedin the presence of an inorganic SDA or has been subject to ion exchange,these inorganic cations lose their hydration sphere upon calcination,thus allowing for their interaction with other species. In the caseof OSDA zeolites, calcination in air leads to the combustion of theseorganic species, thus freeing the pores. Calcination of ammonium-exchangedzeolites leads to the obtention of their acidic H-form with the correspondingrelease of ammonia.41 Additionally, calcinationcan result in annealing of silanol groups; i.e., connectivity defectsdisappear as the silanols react with each other.160 A specific method that allows for P-removal in zeoliteswhich have been synthesized using a P-containing OSDA is hydrogenationat high temperature followed by calcination in air.94Activation of zeolites, upon whichextraframework species are modifiedor removed, is crucial prior to their use as catalysts or adsorbents.161Steaming processes involve high temperatures andan atmosphererich in water. These promote the hydrolization of the T–O–Tbonds and can have diverse effects in the final material, dependingon the severity of the treatment. In the first place, the hydrolysisof the Si–O–Al (or other heteroatoms) happens, uponwhich the acidic properties of the material are modified. Furthersteaming leads to the  loss of the heteroatoms present inthe framework, e.g., dealumination, and the formation of silanol groups.After this, the Si–O–Si bonds start to be hydrolyzed(desilication), and the formation of mesoporosity occurs. Finally,partial or complete loss of crystallinity can happen.2.3.4163 This transformation of the powder into aggregates is known as shapingor structuring of the adsorbent or catalyst. Shaping or structuringis important to prevent or minimize pressure drop, material loss,and erosion of equipment and can greatly affect the adsorption capacityand kinetics  and spherical.Granulation leads to the gradual formation of beadsor pellets from the powder by controlled addition of water in theform of either droplets or a spray. The resulting beads are usuallysmall .Extrudates that are intended for use as a single piece are calledmonoliths. Monoliths usually are designed to have macroscopic holesin the direction of flow which allow for fast mass transfer and reducedpressure drop.167 It is still mostly used at a laboratory scale, but thereare some commercial providers.1683D printing is a relativelynew technique which cangive rise to pieces with virtually any shape.Zeolites are mostly synthesizedas a powder. Postsynthesis modification is carried out on the zeolitepowder. However, prior to industrial use, this powder needs to betransformed into larger aggregates in order to improve its mechanicaland physical properties with special focus on the final use.tics see 3.1.2.163162 Binder content in the finalmaterial usually ranges from 5 to 30%, and a compromise must be soughtbetween optimal mechanical properties and adsorption properties, aslarger amounts of binder tend to make the material more resistantto abrasion but also less porous and thus more prone to present diffusionalresistances. Binderless zeolitic adsorbents can also be produced.In this case, an “intermediate” binder is used to produceshaped adsorbent particles, which are then submitted to further treatment,e.g., hydrothermal or thermal, after which the resulting shaped materialconsists only of zeolite. These binderless particles tend to presentless problems related to reduced adsorption capacity and kinetics.After shaping, the material is usually dried and calcinedto remove water and labile components and to achieve the final mechanicalproperties.2.455 In general terms, traditionalaluminosilicate zeolites are thermally stable up to 700 °C, canbe dissolved in acids and strong bases, and partially retain theircrystallinity upon steaming at high temperature.153 Specifically, steaming at high temperatures has been usedas a postsynthetic treatment to remove aluminum from the framework,increase its stability, and modify defect distribution, the most knownand illustrative case of this being the development of a fluid catalyticcracking (FCC) catalyst Ultra Stabilized Y zeolite (USY).161 It is common that high- and pure-silica materialspresent even larger thermal  and chemical stabilities (only soluble in hydrofluoricacid and concentrated strong bases). AlPOs are somewhat less thermallystable than zeolites, retaining their crystallinity at temperaturesup to 1000 °C and up to 600 °C in a moist atmosphere.55 SAPOs tend to be moisture sensitive and slowlycollapse if exposed to ambient moisture after long periods of time.In the absence of water, however, their stabilities resemble thoseof AlPOs. It is frequent that AlPOs and SAPOs undergo changes of structureupon hydration.171 The effect of mechanical stress on zeolites,e.g., by excessive grinding, can lead to a partial or even total lossof crystallinity.173Zeolitesand related materials present high thermal and moderate to high chemicalstabilities.175 Furthermore, their chemical properties can be tailored by syntheticor postsynthetic procedures for specific applications. When extraframeworkspecies present in their pores possess acid–base or redox properties,these are transferred to the containing zeolite to a greater or lesserextent. Below, a brief review of interesting applications and theunderlying properties of zeolites is provided. Probably due to theirearly commercial availability, zeolites of type LTA  and FAU  are the most frequently addressed ones in all types of applications.The most important propertyof zeolites, and the one on which their applicability as catalysts,adsorbents, and ion exchangers depends, is their structural porosity.Closely related to this feature, their narrow pore size distribution,i.e., very regular pore sizes, makes them useful for applications,in which size or shape selectivity is involved.2.4.126 which still remainsone of their major uses. In the 1950s, zeolites A, X, chabazite, mordenite,and others were tested for their ion-exchange properties.178 Depending on their pore size, these materials can act as ion exchangersfor diverse cations. Logically, if the cation’s size  is larger than the pore opening, theexchange will not be possible to a great extent. This size exclusiontogether with the different affinities of ions when using zeolitesas exchangers can allow for ion separation and, more specifically,ion sieving.179 For instance, zeolite 4A (sodiumform of zeolite A) proved useful for the separation of Ni2+ and Co2+ cations from an aqueous solution, in which thecobalt is preferably exchanged.178As mentioned in 179 The general conclusions on ion-exchange affinitiesare as follows:High silica zeolites tend to prefer cations with lowcharge density , while low silica zeolites prefercations with high charge density .+ or Mg2+, tend to present slow exchangekinetics.Cations that have high heats of hydration,such as LiOther cations are usually preferredover transitionmetal cations .Since then, a great number of ion-exchange isotherms and selectivitiesof natural and synthetic zeolites with ANA, CHA, HEU, EDI, ERI, FAU,FER, GIS, KFI, LAU, LTA, MER, MFI, MOR, PHI, SCO, and STI structureshave been determined and were reviewed by Dyer in 2007.181 as follows fromIt must be noted that ion-exchange isotherm measurementsface animportant problem when dealing with dilute ion solutions and low silicazeolites. Introducing sodium-exchanged A, X, or Y zeolites into purewater will cause an almost immediate alkalinization of the aqueousphase due to the slow exchange of sodium cations with hydronium cations,184 Zeolite 4A has been used sincethe late 70s as a component in laundry detergents, replacing phosphatesin their function as water softeners and thus avoiding the environmentalhazard of these, i.e., eutrophication.185 A synthetic zeolite with GIS structure showing better performancethan zeolite 4A was commercialized in 1994 for the same application.187 Natural zeolites, more specifically clinoptilolite, have been widelyused for ammonium removal from water. Heavy metal cation removal fromwater and wastewater using zeolites has been reported as well, withclinoptilolite being again the most frequently addressed material.Furthermore, the use of zeolites in radioactive ion removal from wastestreams has been known since the 1960s, when zeolite 4A was demonstratedto be highly selective toward radioactive strontium exchange.188 Natural zeolites chabazite and clinoptilolite and synthethic zeoliteswith CHA, FAU, and LTA structures have been used for the mitigationof the effects of nuclear accidents or the presence of radioactivewaste and, more specifically, for removal of radioactive cesium.190The use of zeolites as ion exchangers for industrial applicationshas been reviewed by several authors.191 This allows for the removal of other contaminants different thancations, such as particulate matter, anions (F–),192 or organic contaminants.It must be noted thatthe use of zeolites for water treatment purposesmay involve processes other than ion exchange, such as filtration,surface precipitation, or adsorption.2.4.2+-exchanged X zeolite for the crackingof hydrocarbons and discovered it was much more active than the existingsilica–alumina catalysts.49 Thatsame year, they developed methods for metal dispersion in A, X, andY zeolites and performed catalytic tests on the resulting materials.197 Shortly thereafter, and persuaded by Milton and co-workers, researchersin other companies started studying zeolites for their potential useas catalysts. In 1959, zeolite Y was commercialized as an acid catalyst for isomerization and crackingprocesses by Union Carbide.200 In the coming years, other companies stepped in on this researchfield, such as Socony Mobil Oil Company, USA, and started producingtheir own zeolite-containing catalysts.202 Soon zeolite cracking catalysts were implemented instead of theold amorphous silica–alumina catalysts in every refinery.Industrial applicationof zeolites in catalysis was first envisaged by the Union Carbidezeolite research group in the 1950s. In 1954, Milton and Breck studiedthe use of partially H203 A description of some of the most importantexamples is provided below.208209 Zeolites with FAU structure, more specifically Y-type rare-earthexchanged (REY) and ultrastabilized Y zeolites (USY), have been usedin this application, and the latter remains the preferred catalystfor this process. The superior catalytic properties of USY as comparedto amorphous silica–alumina are partly due to the presenceof extraframework aluminum species, which favor the initial formationof carbenium ions, ultimately leading to an increase in the product’soctane number.210 Furthermore, zeolitesZSM-5 and Beta have been used as an additive in FCC catalyst compositions,as they increase the yield to light olefins and the octane numberof the gasoline, respectively.Fluid catalytic cracking (FCC) is a process used forthe production of gasoline from heavy oil fractions.211 Zeolite USY is usedas an acid catalyst in the hydrocracking unit, along with a hydrogenation–dehydrogenationcatalyst, which can be a noble metal, such as Pt or Pd, or a transitionmetal, such as W or Ni, depending on the sulfur content of the feed.Hydrocrackingis a process in which heavy unsaturatedand aromatic fractions are converted into lighter saturated compoundsin the gasoline, diesel, or kerosene fractions by hydrogenation, cracking,and isomerization.212 ZSM-5 (Mobil Oil Corp.),213 and other proprietary catalysts presumably containing SAPO-11(Chevron),216 Beta, ZSM-22, or ZSM-23 (Mobil Oil Corp.).218Dewaxing of lubricants and fuels is a processthat startedusing zeolites as its catalyst in the late 1960s. Acidity and shapeselectivity are crucial to this process in which long-chain linearalkanes undergo cracking and/or isomerization to form branched species.In order to selectively transform the linear alkanes, medium porezeolites have been preferably used. Industrial dewaxing processeshave used catalysts based on mordenite (British Petroleum Co.),6–C10 fraction) produces branchedalkanes and aromatics . Reformingitself happens in the presence of hydrogen and an alumina-supportedPt–Re or Pt–Re–Sn catalyst; however, postreformingshape-selective reactions are necessary to improve the quality ofthe product. The first zeolitic catalyst used for this process waserionite, which allowed for selective cracking of the remaining short-chain n-paraffins to produce liquefied petroleum gas .219 Later, ZSM-5 wasintroduced as the shape-selective catalyst,220 which also allows the entry of monobranched paraffins and benzeneand toluene. The monobranched paraffins undergo cracking in the poresof ZSM-5, and the resulting olefins alkylate the aromatic species.221Catalytic reforming of naphta (mainlylinear paraffinsin the C5–C6 fraction) produces branchedparaffins. Thecatalyst system needed for this reaction presents an acidic functionand a hydrogenation function. Apart from superacidic chlorinated aluminaand sulfated zirconia, noble-metal-supported zeolites have been usedfor this process, such as Pt-loaded modified mordenite207 and other Pt-promoted proprietary zeoliticcatalysts.204Isomerization of light straight runnaphta and isobutene (2-methylpropene). It can be carried out in the presenceof a zeolitic catalyst, more specifically modified and nonmodifiedferrierites.n-butene and isobutane, yielding iso-octanesis industriallycarried out using liquid sulfuric or hydrofluoric acids. Several processesfor alkylation wielding a zeolitic catalyst have been developed butare not operational at a large scale.226 Pt-supported Y zeolite227 and other FAU-structuredmaterials have been reported.Alkylation of olefins with paraffins,mainly 6+ iso-olefins. Phosphoricacid supported on silica was the first catalyst used for this purposeand remains the most widespread one.228 Some processes have been developed that use zeolitic adsorbents,such as Ni-mordenite229 and modified ZSM-5.232Olefinoligomerization needs propene and butenes asa starting material and yields COil refining.234 and ZSM-5238 catalysts have been commercialized for this application.Methanol to olefins (MTO) is a process that convertsmethanol into light olefins (ethene and propene). SAPO-342–C4 olefins upon cracking of heavier hydrocarbons.207Catalytic cracking for propene productionuses primarilyZSM-5 catalysts, which favor the formation of light C2–C8 range produces H2 andBTX and is carried out in the presence of a bifunctional  catalyst. Light paraffins in the C2–C4 range can be aromatized using a zeolitic catalyst,239 such as Ga/HZSM-5.240 Hydrocarbons in the C6–C8 range canbe converted into benzene, toluene, and H2 using an L-typezeolite.241Aromatization of light paraffins andolefins in theCOil refining and petrochemistryinterface.p-Xylene  from toluene . ZSM-5 and otherproprietary  catalysts are used.Toluene disproportionation–transalkylationprocessesare designed to produce benzene and xylenes can be producedby alkylation and/or transalkylation processes of benzene and/or toluenewith olefins using medium and large pore zeolitic catalysts, suchas ZSM-5, modified mordenites, MCM-22, Beta, and Y, depending on thedesired outcome.2O2 and NH3 in the presenceof titanium silicalite-1 (TS-1), and the Beckmann rearrangement happensin the vapor phase in the presence of silicalite-1 (S-1).247ε-Caprolactam is the precursorto Nylon-6 andmay be produced from cyclohexanone by ammoximation and Beckmann rearrangement.MFI-structured materials are employed as catalysts for these two stepssee 6; moresPetrochemistry.Since then, zeolites have been used as catalysts in a wide varietyof industrial processes, especially in oil refining and petrochemistryand processes at their interface. Zeolites with MFI, FAU, and MORstructures are the ones that have found more application niches.2.4.327 However, it was not until McBain identified the possibility of carryingout extremely selective adsorption processes using these materialsand coined the term “molecular sieves” that the wayto a practical application of these was cleared.33 As said in 248 Since then, various applicationsof zeolites as adsorbents for the separation of mixtures have beendeveloped and commercialized. Due to this review’s focus beingon the use of zeolites as adsorbents, a thorough review of the applicationof zeolites in industrial and potential adsorption and separationprocesses will be provided in The use of zeolitesas adsorbents stems ultimately from their microporosity and regularpore size. The studies of Damour in 1840 and Friedel in 1896 on thereversible adsorption of molecules by zeolites were the first to shedlight on the adsorption properties of these materials.249 The idealadsorbent:is selective toward one or more components of the mixturepresents a large (working) adsorption capacityis easily regenerableis durable and stable at relevant conditionsis cheapcanbe shaped to achieve optimal mechanical and dynamicalpropertiesFrom the point of view of the adsorption process, there are certainparameters to consider when deciding upon which adsorbent better suitsthe needs of the separation.a priori reasonable option for manyseparations. Furthermore, through postsynthetic modifications, tailoredzeolitic adsorbents can be obtained.It is difficult to find an adsorbent that fulfills allof the abovecharacteristics for a given separation, but in any case, the bestcompromise needs to be sought. The selectivity in adsorptive separationssee 3 canbe 18 Zeolite A is produced hydrothermally in its Na form and is commonlyreferred to as zeolite 4A, due to its pore size of 4 Å. Upon50–70% exchange of Na+ per Ca2+, zeolite5A (5 Å pore size) is obtained, and, analogously, K+-exchange of 4A yields 3A (3 Å pore size).250 These materials achieve to discriminate between moleculessmaller than their pore size and larger ones thanks to the molecularsieving mechanism. For instance, zeolite 3A selectively adsorbs waterfrom gas streams and liquid solvents,252 and zeolite 5A selectively adsorbs linear over branchedhydrocarbons in the gasoline range.254 These arebeautiful classic examples of how the postsynthesis modification ofthe pore size of a single zeolite via ion exchange allows for substantiallydifferent molecular sieve separations to be performed.It is not necessary todig too deep into zeolite science to findvery representative examples of tailoring of a zeolite’s adsorptionproperties. Zeolites 3A, 4A, and 5A, along with 13X, are the oneson which most adsorption studies have been carried out and the onesmost used in industrial separations, probably due to their early commercialavailability and relatively low production cost.256 These structure changes can affect the adsorption properties ofa material, in which case they are generally referred to as gatingeffects.257 In the case of zeolites, twotypes of gating effects have been reported to be especially relevant,i.e., trapdoor phenomena and guest-induced framework deformation.“Trapdoor” zeolites involve a specific case of cationrelocation in which the interaction of a specific molecule with aninitially pore-blocking cation leads to a displacement of the cationand subsequent penetration of the molecule inside the pores. Thesewere first described by Shang et al. in 2012, after observing remarkablyhigh CO2/CH4 selectivities on low silica exchangedchabazites.258 Since then, trapdoor phenomenain zeolite adsorption have received a great deal of attention andresearch effort, due to the large selectivities achievable.266 These phenomena are usually susceptible to external stimuli, suchas temperature,267 and are usually triggeredby molecules that interact strongly with the extra framework cationspresent in the zeolite, such as water and carbon dioxide.Withzeolites being outstandingly shape- and size-selective adsorbents,it is not surprising that even slight changes in cation distributionor framework structure can be of high importance for their applicabilityas adsorbents. For instance, cation relocation coupled with a slightframework symmetry change in zeolites upon adsorption of water isa largely known phenomenon.257 Zeolites were traditionally considered as relativelyrigid solids, but some adsorption and diffusion results could notbe understood until flexibility was taken into account.270 Zeolite framework flexibility is the underlying reason for close-fittingmolecules being able to diffuse inside the pore system.271 Furthermore, guest-induced flexibility in zeolites is a more recentdiscovery, beautifully exemplified by the adsorption of ethene onmolecular sieve ITQ-55.272 Another interestingcase, in which cation relocation and structure change take place simultaneously,is that of CO2 adsorption on RHO and related zeolites.276Onthe other hand, framework deformation can be as well triggeredby guest species or temperature and is related to framework flexibility.Overall, zeolitic adsorbents present very promising propertiesand potential applicability for separation processes under development.The very active search for new structures that can address currentproblems and the key findings of the last ten years support the notionthat these unique materials continue to be of high scientific andtechnical relevance.3277 In the case of a solid–gasor solid–liquid system, this interface is the surface of thesolid. Adsorbable molecules in the fluid phase are adsorptive or sorptive; adsorbed molecules are called the adsorbate or sorbate; and the solid materialreceives the name of adsorbent or sorbent . The opposite process, inwhich molecules leave that surface and go back to the fluid phase,is called desorption.Adsorption is defined as the enrichment in the concentration ofmolecules, atoms, or ions present in a fluid phase in the vicinityof an interface.277dp <20 Å.Microporous, with dp < 500 Å.Mesoporous, with 20 Å< dp.Macroporous, with 500 Å < Adsorbents need to possess a high specificsurface area, as themaximum adsorption capacity will depend on it. Porosity increasesthe surface area per volume of material, thus porous materials area common choice as adsorbents. Porous materials with pores with diametersbelow 100 nm are known as nanoporous materials and can be classifiedinto different groups according to their pore size:There are a large number of examples of nanoporous materials,suchas activated carbons, carbon molecular sieves, carbon nanomaterials,zeolites, metallosilicates, mesoporous silicas, metal–organicframeworks (MOFs), or covalent organic frameworks (COFs). Zeolitesand related materials belong to the microporous materials group.3.13.1.1P, equilibrium is reached, and the amount adsorbed Q may be calculated by the pressure drop (volumetric method)or the gain in mass (gravimetric method). Generally speaking the amountadsorbed will increase with pressure, although the shape of the isothermmay vary greatly depending on the adsorbate–adsorbent pairand the specific conditions of the experiment.Adsorption phenomena are most frequently studied by measuring adsorptionisotherms. In a typical experiment, the temperature is set constant,and a clean sample of adsorbent is exposed to certain values of pressure(or concentration) of the desired adsorptive/s. At each pressure 277Years of accumulatedadsorption isotherm data have allowed us to establish a classificationof typical isotherm shapes see 7, which Type I(a) and I(b) isotherms belong to microporoussolids, suchas zeolites. The steep low-pressure regime is due to the strong interactionsthat take place in the close-fitting pores of these materials. Thesteeper this region, the stronger the interactions. Above a certainpressure, saturation is reached, and the limited micropore space ofthe solid cannot take in more molecules, thus the horizontal asymptote.At pressures close to the condensation pressure, another steep increasemay be seen, which is due to capillary condensation of the adsorptiveoutside the micropores, possibly in the space between adsorbent particles.Type II isotherms are given by relatively weak adsorption on nonporousor macroporous adsorbents, where multilayer adsorption and capillarycondensation take place. Some microporous materials present mixedfeatures of type I and II isotherms to some extent, as the interparticlespace allows for multilayer adsorption and capillary condensation.Type III isotherms belong to nonpororus or macroporous adsorbents,as well, but in this case the interaction with the adsorbate is veryweak. Type IV isotherms are typical of mesoporous solids. Type IV(a)isotherms present a hysteresis loop related to capillary condensationin the pores and are given by solids in which the opening of the poreexceeds a certain value, which depends on the nature of the adsorbate.Hysteresis is a concept that refers to the case in which adsorptionand desorption follow a different path in the isotherm plot. TypeIV(b) isotherms belong to solids having smaller mesopores and cylindricalor conical pores with closed ends, in which capillary condensationdoes not result in a hysteresis phenomenon. A final plateau or inflectionpoint is typical in this type of isotherm. Type V isotherms are seenin micro- and mesoporous adsorbents in cases where the sorbent–sorbateinteraction is weak. In this sense, their low-pressure regime is similarto that of type III isotherms. At higher pressures, pore filling occurs,and the adsorbate–adsorbate interactions prevail, thus leadingto a steep increase in the adsorbed amount. Hysteresis is typicalin these isotherms. Type VI isotherms are given by highly uniformnonporous surfaces, in which layer by layer adsorption is distinguishable.278 As zeolitesare the protagonist adsorbents of this review, some of the most relevantmodels used to describe type I isotherms will be briefly presentedhere, together with their base assumptions and shortcomings. The mathematicalexpressions of these models are presented in The underlying thermodynamics that give rise to these isothermshapes have been reproduced via a variety of models. These modelsare usually specific to one or several isotherm types and are widelyused for process design.279Q is the adsorbed amount of an adsorbate; Qs is the adsorption capacity at saturation; b is the equilibrium constant; and P isthe pressure. At low pressure/concentration, the Langmuir isothermbecomes analogous to Henry’s law, i.e., Θ = bP, and thus, from fitting this model to low pressure/concentrationdata, the Henry constant and subsequently the adsorption enthalpyat low coverage can be obtained. The Henry constant remains one ofthe most important parameters for process design.249In an adsorbate–adsorbent system, the simplestcase thatcan be thought of is one where the surface presents a homogeneousenergy distribution; there are no adsorbate–adsorbate interactions;and only one molecule can be adsorbed per adsorption site (monolayeradsorption). These are the basic assumptions of the Langmuir modelfor adsorption. This model is not precise for the description of zeolites,mainly because they are adsorbents with an energetically heterogeneoussurface, but it is still a good starting point to fit type I isothermsin general. The Langmuir adsorption isotherm is presented in 286 such as the Toth, Unilan, and Sips models, each presenting differentapproaches to simulate an energy distribution. These models tend tobe a better fit for zeolite adsorption isotherms. The Toth model284 accounts for surface heterogeneity by meansof a parameter n which modifies the shape of theisotherm, allowing for higher loading values at low pressure as comparedto the Langmuir model. It is based on the fact that a solid presentinga heterogeneous surface will have a larger capacity at low pressurethan one with a homogeneous surface. When n = 1,the model converges to the Langmuir model, i.e., homogeneous surface,and it behaves correctly throughout the whole pressure range. TheSips and Unilan models were both derived by Sips.282 The Sips model assumes a Gaussian-like distribution of the energy of the adsorptionsites and presents incorrect behavior at low pressure, as it doesnot follow Henry’s law. The Unilan model assumes a uniformdistribution of energies between two given limit values. For an infinitesimalenergy range, it converges to the Langmuir model, and for wide energydistributions, it tends to the square isotherm characteristic of irreversibleadsorption. It behaves correctly throughout the whole pressure range.Another frequently used way to account for surface heterogeneity isby applying the Dual-Site Langmuir model, which is a simple extensionof the Langmuir model, in which two adsorption sites with differentmaximum loading and/or Henry constants are present. Nonetheless, thephysical meaning of the parameters in cases where there are not twodistinct adsorption sites is lost. The semiempirical Dubinin–Radushkevichand Dubinin–Astakhov models are applicable for condensablevapors, with the latter empirically taking into account surface heterogeneity.285 These models assume a micropore filling mechanism,and thus, the adsorption capacity is a function of the pore volumeand the molar volume of the adsorbate. Another model that takes thecondensability of the adsorbate into account and, thus, the multilayeradsorption phenomenon is the Brunauer–Emmett–Tellerisotherm model.287 This model is only validfor a reduced pressure range, namely, 0.05–0.35 P/P0 (even lower in some cases),288 depending on the nature of the material, andis based on the assumption of a flat homogeneous surface, which isfar from the reality in zeolites. However, it is one of the most widelyused models for a comparison of surface area. An in-depth study ofthe thermodynamic implications of the models presented can be foundin ref at a defined temperature T and pressure P ,292 and other models independentor derived from the above.295 The Langmuir-based models and the IAST assume homogeneous surfaces,no lateral interactions between adsorbate molecules, and no size dependence,which make them poorly applicable to some systems involving adsorbentswith heterogeneous surfaces or limited pore size, such as zeolites.Therefore, there is a need for said modified and new models, whichintroduce the effect of energy distribution, pore size distribution,or size effect. Mixture selectivities calculated from any of the mentionedmodels need to be contrasted with experimental data prior to theiruse for process design, especially if their applicability to a specificsystem has not been proved yet.296Obviously, pure component selectivitiesare a very rough approximationof the real selectivity in the case of competitive adsorption. Themeasurement of multicomponent isotherms is complex, and very few researchersand technicians have access to the needed equipment.297 Therefore,the absolute value of enthalpy of physisorption is generally lower(≤50 kJ/mol) than that of chemisorption (≥50 kJ/mol).It is common that an industrial adsorptive separation process usingzeolites preferably involves physisorption instead of chemisorption,whereas a catalytic process involves chemisorption and further reaction.Note that adsorption is one of the necessary steps in any heterogeneouscatalytic process. Physisorption phenomena are always exothermic,as the entropy decreases. This means that, at a constant pressure,the adsorbed amount will decrease with increasing temperature.Adsorption can be physical (physisorption) or chemical (chemisorption),depending on the strength of the interaction between the adsorbateand the adsorbent surface. The intermolecular forces that are involvedin physisorption include interaction between induced or permanentdipoles and/or quadrupoles, while in chemisorption there is a changein the electronic structure of the adsorbent and the adsorbate andthe formation of a chemical bond.qst is the negative adsorption enthalpy and ispositive. There are several ways to determine the experimental qst, which may be direct  or indirect,based on isotherm measurements at different temperatures and Clausius–Clapeyron’seq  may be characterized for acertain adsorbate–adsorbent pair. Intracrystalline diffusioncan be described by several models specific to the geometry of thecrystals and the nature of the system. For spherical particles, Crank’ssolution to the transient diffusion equation stands 4.3004wheJ is the flux of matter; D is the diffusion coefficient; and grad  c is the concentration gradient. The diffusion coefficientmay vary with both P and T. In themesopore range, the adsorbent–adsorbate interactions tend tobe more frequent than the adsorbate–adsorbate interactions,and Knudsen diffusion is observed, in which the flux can be describedby a Fickian relation with the diffusivity depending on the pore sizeand the temperature of the adsorbate molecules. In zeolites and othermicroporous solids, micropore diffusion is the most important contributionto the kinetics of adsorption, although macropore diffusion may alsoplay a part, as the intercrystalline space resembles macroporosity.The contribution of intercrystalline diffusion will be relevant especiallyfor adsorbents with a small crystal size.In macropores, the adsorbate–adsorbate interactionsarepredominant, and thus diffusion takes place via the molecular diffusionmechanism, which can be mathematically described by Fick’sfirst law of diffusion 5.5where 164 ZSM-5, in which the presence of kaolinite as a binder wasfound to present a detrimental effect to diffusion, whereas aluminaor silica binders did not;165 5A, wherethe presence of a binder partially blocks the adsorption of hydrocarbons;301 or mordenite, where pore blockage by a silicabinder was also observed.302 To keep thebenefits of shaped adsorbents, while overcoming these transport limitations,different strategies have been developed, such as the production ofbinderless or hierarchically porous adsorbent particles.303With regard to anadsorptive separation process, diffusion is adecisive factor as to whether adsorption occurs at an acceptable rateor not. Industrial adsorbents are usually conformed into aggregatesof different sizes and shapes containing not only the adsorbent butalso other components, mostly inert, such as the binder see 2.3.4. Th49 Kinetic selectivity and molecular sieving are achieved only on materialsthat present a well-defined and narrow pore size distribution.From the point of view of the separationmechanism, adsorptionkinetics can determine how the separation is carried out. In processesthat take advantage of a thermodynamic selectivity, fast adsorptionkinetics, i.e., large diffusivities and mass transfer coefficients,are necessary. However, in the case that one of the components ofthe mixture that is to be separated diffuses much faster than others,a kinetically controlled separation may be feasible, and low diffusivitiesof the slower diffusing species are desirable. The extreme case, wheresome components of the mixture enter the pores and others are toobig for entering the pores and being adsorbed, receives the name ofmolecular sieving. This phenomenon is very representative of zeolites,to the point that they have been referred to as molecular sieves fora long time.299304 and pulsed-field gradient nuclear magneticresonance (PFG NMR),306 also considered adequate forassessing diffusion at a microscopic level. A variety of elementaryadsorption steps have been observed for different types of zeolites,mostly involving jumps between specific adsorption sites. For a deeperinsight into these processes, the reader may consult the extensivediscussion presented by Kärger et al. in ref  involved are short compared to the length scales needed forthe study of the overall diffusion process. In other words, a largenumber of elementary steps result in diffusion. Elementary adsorptionsteps may be assessed by molecular dynamics simulations and experimentaltechniques like quasi-elastic neutron scattering (QENS). in ref . Some ge307 butane or butene isomers inside zeolite 5A,308 and xylenes in X-type zeolites.309Intracage jumps, where the adsorbate jumps from oneadsorption site to another found in the same cavity. It is prone tooccur more frequently when the molecule’s critical diameteris comparable to the window size and the cavity presents several adsorptionsites. This motion is therefore representative of many zeolite–adsorbatesystems, for instance, methane inside zeolite 4A,310Cage-to-cage jumps,where the adsorbate jumps from oneadsorption site found in a cavity to another found in a neighboringcavity. This elementary adsorption step competes with intracage jumpsand is the one that leads to overall diffusion. The required activationenergy for these jumps to take place is comparably higher than thatof intracage jumps and usually more so the smaller the size of thewindow relative to the molecular diameter.312 nuclear magneticresonance (NMR), more specifically PFG NMR,318 and light diffraction (interference microscopy) and absorption .323Microscopic diffusion processes arestudied at a scalewhere the adsorbate–adsorbent system is homogeneous, i.e.,inside a single particle of the adsorbent, typically of the orderof μm. The techniques that allow the study of microscopic diffusionprocesses are referred to as microscopic and are mainly based on neutronscattering, more specifically QENS,326 or frequency response.328Macroscopic diffusion processes arestudied at a scalethat encompasses a large number of adsorbent particles and the spacebetween them. These processes are studied by techniques such as uptake/desorptionrate measurements, zero length column (ZLC) chromatography,The kinetics of adsorption can be characterized at three differentlevels, i.e., elementary adsorption steps and microscopic and macroscopicdiffusion processes.D or diffusional time constants D/r2 are provided. In most ofthese, diffusioncoefficients are obtained from macroscopic measurements . The ratio of diffusion coefficients of a given adsorbate pairon the corresponding material at a defined temperature and pressuregives their kinetic separation factor 329In some publications on adsorption on zeolites, diffusioncoefficients vity see 6.6where rred see 7.32973.1.3330 Screening of real or theoretical adsorbentsfor the capture of CO2,333 separation of alkaneisomers,336 separation of alkenes from alkenes,337 and other separations343 and also the study of mixtures adsorption can be approached withcomputational tools. Frequently, the study of thermodynamical propertiesand observables, such as adsorption isotherms, Henry constants, heatsof adsorption, is carried out via Monte Carlo simulations, which arebased on statistical mechanics.346 These can also be usedto determine the pore volume and size distribution and surface areaof adsorbents whose structure is known. On the other hand, moleculardynamics simulations are widely used for computing transport propertiesof adsorbate–adsorbent systems. Trajectories of the adsorbatein the pores can be simulated and recorded and parameters, such asmean square displacements and diffusivities, can be obtained. Fora more detailed review of computational techniques for the study ofadsorption on zeolites, please refer to ref , vacuumpressure swing adsorption (VPSA), electric swing adsorption (ESA),and rapid PSA (RPSA) have been developed or used in combination withtypical TSA and PSA.351Industrialgas adsorption processes use a technology named swing adsorption,in which the adsorbent bed is subjected to cycling conditions of pressureor temperature, thus giving rise to pressure swing adsorption (PSA)or temperature  swing adsorption (TSA). PSA is mostly usedin bulk separations, where the component to be separated represents>10% of the stream to be processed, whereas TSA is preferably usedin purification applications, i.e., where the component to be removedis present at concentrations <10% .7 The processefficiency is highly dependent on the interplay between adsorbentproperties and process design, which allows for the use of differentadsorbents for the same separation.298The conceptual scheme of a swing adsorption process is relativelysimple see 9. A mini10 The adsorbent productivityis the amount of feed processed per unit time and amount of adsorbentand can be expressed referring to a specific component of the mixture.The product purity refers to a certain component and equals its molarfraction  in the volume-averagedproduct obtained throughout a certain step in the process. The productrecovery also refers to a specific component and equals the amountof that component present in the product divided by the amount ofthat component present in the feed that has been processed.Important parameters which help describe the performanceof a swingadsorption process are the product purity, product recovery, and adsorbentproductivity.Another important parameter in the selection of the adsorbent fora swing adsorption process is its working capacity, which is definedas the difference between the adsorbed amount at the end of the adsorptionstep and the adsorbed amount at the end of the desorption step andcan be estimated from its adsorption isotherms. For TSA and PSA processes,a simplified graphical explanation of the working capacity is providedin As can be seen, the working capacity depends notonly on the equilibriumadsorption capacity of the adsorbent but also on the isotherm shape.High adsorption capacities are desired, as they will decrease therequired quantity of adsorbent. Isotherms with a moderate affinitytoward the adsorbate (neither too steep nor too flat) will also favora large working capacity.3.2.27 Simulated moving bed adsorption overcomes some of the technicaldisadvantages of TMB by not involving motion of the adsorbent particlesbut instead a switching of inlet and outlet ports controlled by arotary valve patented by Broughton and Gerhold for UOP in 1957.352 Some of the separations relevant to this reviewuse SMB technology and zeolites as the adsorbent, such as the separationof normal from isoparaffins or the separation of xylenes.7Adsorptive separation processes may be carried out in the liquidphase as well. Techniques, such as continuous countercurrent operation) and simulated moving bed(SMB) present the highest efficiencies.283 In a case where two adsorbatesA and B are separated using SMB techniques and the strength of adsorptiondecreases according to A > D > B, the sections areSection I, where desorption of A takes place, is foundbetween the inlet of the pure eluent D and the outlet of the extract(A+D). The incoming eluent D displaces A, and part of this A-richextract leaves the adsorption bed. The fluid phase flowing into thenext section contains mostly D, but also A.Section II, where desorption of B takes place, is foundbetween the outlet of the extract (A+D) and the inlet of the feed(A+B). Component B is displaced by A and D, both of which adsorb morestrongly. The fluid phase flowing into the next section contains thethree components, A, B, and D.SectionIII, where A is adsorbed and B and D are displaced,is found between the feed inlet and the outlet of the raffinate (B+D).The fluid phase flowing into the next section contains B and D, andpart of it is withdrawn as the B-rich raffinate.Section IV is found between the outlet of raffinate(B+D) and the inlet of additional eluent D. The fluid phase flowinginto the next section contains mostly D.A simplified scheme of an SMB process isdepicted in The direction of flow of the liquid phase is coupledwith the rotationof the inlet and outlet ports and goes from Section I to Section IV.The composition of the liquid phase is depicted in 3.2.3Breakthrough experiments are a powerfultool for researchers, as they can provide information on the adsorbent–adsorbatesystem at conditions close to the industrial case. The separationof multicomponent mixtures can be studied, and insight on relevantthermodynamic and even kinetic parameters of the system can be calculated.In these experiments, the adsorbent is placed in a fixed bed orcolumn, and after activation under inert gas flow and/or at high temperature,it is exposed to a flow of an adsorptive or mixture of adsorptives.The concentration/molar flow at the exit of the column is recordedagainst time see 12. From 354 Real mixture selectivities canbe calculated according to yi is the molar fraction of component “i” in the fluid phase.The adsorbed amount can be calculated by applyingmass balance.47 In order to separate the components of a mixture, differences intheir physical and/or chemical properties are exploited, and dependingon which property the separation is based on, various types of processes/techniquesare distinguished, such as distillation, extraction, crystallization,absorption, adsorption, and membrane separations.Separationprocesses are essential in the chemical industry, asmany valuable compounds need to be extracted or purified from mixtures.Thermodynamic separations are performed at equilibrium,and their effectiveness relies on differences in the interaction strengthof the adsorbates.Kinetic separationsare performed away from equilibrium,and their effectiveness relies on differences in the adsorption rateof the adsorbates.Molecular sievingseparations are exclusive to nanoporousadsorbents and imply size and/or shape exclusion of some componentsof the mixture from the pores. It can be understood as an extremecase of kinetic separation.Adsorption-basedseparation processes can rely on different mechanismsto achieve selectivity:The separations relevant to this review are directedtoward separatingfluid mixtures. The compounds involved in these separations and relevantproperties thereof are listed in 4.1366 The compositions of these streamsis as follows:2, i.e., syngas of C4+ hydrocarbons and is saturated with H2O.ROG typically contains 65–90% HHydrogenis primarily (>95%) produced in refineries, as a major componentinsteam methane reforming off-gas and refinery off-gas .2 product; however, it may also be optimized to produce ammonia synthesisgas (3:1 mixture of H2 and N2).370 Additionally, the process can be designed to produce a secondaryproduct stream containing >99% CO2 for its sequestrationor use  HCCS, see 4.6.2. ThThe purification of hydrogen from SMROGor ROG is carried out bydifferent means, depending on the desired product composition andpurity and the intended use of the waste stream. The two types oftechnology that have been implemented industrially are PSA and membranes.367 PSA units use multiple columns(4 to 12) to achieve high product purities (>99.999%)371 and are based on the selective adsorption ofthe other components of the mixture, as H2 tends to interactpoorly with the adsorbents used . Processes directed to producing only H2 frequentlyuse various adsorbents in different layers in the same bed, in orderto optimize the adsorption–desorption cycle. Examples of layeredbeds include combinations of activated carbon and zeolite 5A,375 activated carbon and zeolites X and Y,376 or activated carbon and silica gel.377 Processes directed to the obtention of bothH2 and CO2 use combinations of adsorbents indifferent beds, such as activated carbon and zeolites,378 or add systems for CO2 capture priorto or after the primary H2 PSA.379 To date, zeolite 5A still remains the most competitive adsorbentfor this application.381 The order in which the adsorbentsare placed, the interplay between the adsorbents, and the interplaybetween adsorbents and process design are crucial for proper operation.PSA technology is used in 85% of the hydrogen production facilitiesglobally.2 purification by PSA is directedtoward:367Developing RPSA (rapid PSA) processes that allow costreduction.Improving the mass transfercoefficients in currentand potential adsorbents by shaping them into monoliths and sheets.2 is separated from the reaction medium simultaneouslyto its production,thus displacing equilibrium. High-temperature adsorbents are underdevelopment for this technology.382Sorption-enhanced SMR processes, in whichCOCurrently, research in H4.22, D2, and T2, is of current interest to the industryand represents an especially challenging case of separation.342 Deuterium (in the form of heavy water) is usedas a neutron moderator in chemical reactors, as an isotopic tracer,and for the production of deuterated chemicals and drugs.384 Both deuterium and tritium areraw materials for fusion energy technologies, which are under intenseresearch.387 The production of deuterium and tritiumand the removal of tritium from nuclear waste388 are processes that require the separation of these isotopesfrom mixtures or compounds containing them.Theseparation of the isotopes of hydrogen, i.e., H389 Mature technologies for the separation of hydrogenisotopes, especially deuterium from hydrogen, include cryogenic distillationand electrolysis of heavy water coupled to the Girdler–Sulfideprocess,391 both of which are highly energy demanding. Other methods that havebeen studied are thermal diffusion, membrane technology, adsorption,chromatography,392 combinations of chromatographyand cryogenic distillation,393 combinedelectrolysis catalytic exchange,396 and quantum sieving.397Hydrogen isotopespresent very similar physical and chemical properties see 2, which 404 According to these studies, the heavier isotopes were more stronglyadsorbed than the lighter isotopes, mainly due to their larger heatsof adsorption, and no remarkable influence of the pore size was observed,not even in microporous adsorbents.403 This thermodynamic preference toward the heavierisotope is still of interest to researchers, and new materials withimproved separation prospects are being discovered, especially MOFs406 and zeolites.412 Commercial and ion-exchanged zeolites of types A, X, and Y havebeen most frequently studied for this purpose, and seemingly the cationswith the higher charge density lead to the largest thermodynamic selectivities.Trapdoor phenomena in zeolites have been described as well, in whicha D2-sensitive Cs-exchanged chabazite can separate D2 at low concentrations.260Separation-oriented adsorption studies of hydrogen isotopesonactivated carbons, silicas, and zeolites have been reported sincethe 1930s by several authors.397 Since then, numerous studiesfeaturing different kinds of adsorbents, i.e., carbon nanomaterialsand carbon molecular sieves,417 boron nitride nanomaterials,418 MOFs and COFs,422 POCs,383 and zeolites,424 have been carried out. In the case of zeolites and zeotypes, allof the proposed materials present small pores, i.e., 8-rings and minimumpore openings below 4.1 Å, preferably below 3.3 Å.342In the mid 1990s, the term “quantum sieving” wasproposed by Beenakker et al. to denote the quantum effect that ariseswhen the difference between pore size and adsorbate size is closeto the de Broglie wavelength of the adsorbate .4.312 This is not surprising, as theirname ultimately stems from their ability to reversibly adsorb water.19 Natural and synthetic low-silica zeolites aresignificantly hydrophilic due to their charge dispersion, i.e., negativelycharged framework and extraframework cations.16 Water molecules interact strongly with the zeolites’ highlypolar surface thanks to their dipole and quadrupole moments.12 Zeolites of type A  and X (13X) have been widely used for the drying of gasand liquid streams, mainly in TSA processes but in the case of 13Xalso in PSA processes.58 Even membranes of NaA zeolitehave been proposed for drying of industrially relevant streams.425 Their ability to selectively adsorb low concentrationsof water from mixtures at temperatures above ambient has helped themreplace the previously used active alumina and silica adsorbents.426 Indeed, drying is one of the earliest applications where zeoliteswere successfully used as adsorbents.431 Milton patented the use of zeolite 4A in a TSA process for dryingnatural gas428 and the use of both 5A and13X for simultaneous drying and sweetening of natural gas.429 He also proposed the use of type X and A zeolitesfor drying vapor streams.431 Zeolite 3A, whichcan act as a molecular sieve toward water, has been implemented inthe drying of olefins,54 thus avoiding possible oligomerizationinside the pores of the adsorbent. This molecular sieving effect hasproven practical, as well, for drying mixtures which contain otherpolar compounds, such as alcohols or even the ethanol/water azeotrope.434 An RPSA drying process for natural gas thatuses 3A zeolite as the adsorbent has been recently patented by ExxonMobil.436 Drying of streams containing highly acidic gases has been achievedby using high-silica mordenite and chabazite, which present sufficientchemical stability toward acids.298 Furthermore,at a laboratory scale, beads of zeolites 3A and 4A are nonregenerativelyused for drying of organic solvents.252Zeolites arewidely used in drying applications at the industrial and laboratoryscale.4.4438 In 1959, Milton patented differenttype A exchanged zeolites17 and observedthe size exclusion of N2 at low temperatures. He showedinterest in further exploring this approach. Nonetheless, a thermodynamicallycontrolled separation based on selective adsorption of N2 at close to ambient temperatures became the method of choice.16 Since the 1960s, different exchanged type Xzeolites have been tested for carrying out this separation in PSAprocesses, with the focus on the obtention of pure oxygen. Three patentsassigned to Union Carbide Corporation were issued in 1964 on thistopic, in which the separation is achieved using zeolites of typesX, Y, and L  atlow temperatures,439 Sr2+-,Ba2+-, and Ni2+-exchanged X zeolites at ambienttemperature,440 or Li+-exchangedX at ambient temperature.437 In the threepatents, a pore size above 4 Å is pointed at as an importantfactor to enhance mass transfer of N2. The interactionsof the quadrupole of N2 with the cations are consideredthe basis of the selectivity of these adsorbents. A notable effortwas put into developing better adsorbents for air separation in thefollowing years, in most cases still pulling the thread of alkali-or earth-alkali-exchanged type X zeolites.443 In 1989, Chao from UOP patented LiLSX (Lithium Low Silica X) zeolite,which presented extraordinary N2/O2 selectivityand N2 working capacity.438 Chaocarried a systematic study on the Si/Al ratio and the extent of Liexchange, which allowed for his discovery. He concluded that low Si/Alratios and Li-exchange percentages above 80% yielded materials withthe best selectivities , was patented to carry out the separation of oxygen fromair based on the molecular sieving of oxygen at room temperature,4.5450 Currently, the most widely employedmethod for nitrogen removal from natural gas is cryogenic distillation,which is highly energy demanding. Alternative methods that are beingresearched are based on adsorption and membranes and can be selectivetoward either N2 or CH4.The separation of nitrogen from methane is important in landfillgas, natural gas, and biogas processing, as nitrogen needs to be removedto increase the heating value of the mixture and meet specificationsfor transport through pipelines (<3%) and as liquefied naturalgas (<1%).452 However, methane-selective pressure swing adsorption processes presentthe disadvantage that methane is present in these mixtures at a muchhigher concentration than nitrogen, and thus, the required bed sizewould be much larger. On the other hand, the fact that N2 is smaller (3.64 Å) than CH4 (3.76 Å) allowsfor a nitrogen-selective separation under kinetic control and evenby molecular sieving.Most known adsorbentspreferentially adsorb methane over nitrogen due to the larger polarizabilityof the former see 2.298,451453 but the hydrophilicity ofthis material andthe high temperatures needed for its activation rendered it impractical.454 Natural clinoptilolites in their original andcalcium-exchanged forms have been studied as adsorbents that kineticallydistinguish between nitrogen and methane.455 Clinoptilolite in its magnesium-exchanged form was patented by Chaofor its use in a PSA unit to separate N2 from CH4.456 Titanosilicate materials developedby Engelhard Corporation (now BASF), such as ETS-4, CTS-1, Ba-ETS-4,and Sr-ETS-4, have been demonstrated to be excellent adsorbents forthis application at a small scale.459 They have been commercializedunder the name Molecular Gate and consist of a mixed octahedral–tetrahedrallycoordinated framework with 8-ring openings, the size of which canbe tailored by ion-exchange and thermal treatment.460 Ion exchange can improve thethermal stability, and with it the reusability, of these titanosilicates.459 All the mentioned materials present minimumpore openings between 3 and 4 Å, which account for the kineticdiscrimination of N2 from CH4.Zeolite 4A was patented for this purposeby Habgood,4.62) is a ubiquitous compound, found mainly inthe gaseous state on the Earth. Its separation from mixtures withmethane, nitrogen, water, hydrogen, etc., is a very active researchtopic.461 Mixtures of industrial interest, where CO2 is sought to be removed, are classified according to the reasonfor interest and listed below:Methane-containing mixtures with intended use as fuel:4 (30–98%), and the other components are present in variable amounts.464 It can appear associated with an oil deposit or nonassociated.Natural gas, a fossil fuel where the main componentis usually CH2, N2, lighthydrocarbons, H2S, and SO2.468Coalbed methane, which is fossil methanefound alongwith coal, with a methane content of 50–99%, typically above80%, and variable amounts of CO4 and CO2 as the major components and a considerable amountof N2 and H2O.470Landfill gas and biogas and renewablefuels derivedfrom fermentation of residues and biomass, which contain CHHydrogen containing mixtures, suchas steam methanereforming off-gas and refinery off gas see 4.1.2.Mixtureswhere the main objective is to capture theCO2 (70–75%),CO2 (3–30%), H2O (5–7%), and residualO2 (3–4%), with minor amounts of CO and nitrogenand sulfur oxides.472 Oxyfuel technology is a special case, in which the fuel is burntin the presence of oxygen instead of air, and thus, nitrogen is notpresent in the flue gases (CO2 content >90%473).Postcombustion streams, also called flue gases, whichare the byproduct of combustion processes for energy production ingeneral and also importantly in cement and metallurgy industries.The composition of this stream is mainly N2 is only a lesser component(416 ppm in August 2021474).Ambient air,where COCarbondioxide  can be obtained from fossil or renewable (biogas and landfill gas)sources, and its major use is as a fuel, its global electric powergeneration share being 23% in 2018 and with expectations of growthin the coming decades.476 It also serves asa starting material in some petrochemical processes, such as methanereforming for syngas and/or hydrogen production.479Methane  or coal . Biogasand landfill gas are produced in anaerobic digestion processes ofanthropogenic waste, which take place in sewage plants (wastewater)or landfills (solid waste), respectively.450 The state-of-the-arttechniques for CO2 removal from natural gas include absorptionin chemical, physical, or hybrid solvents, adsorption, membranes,cryogenic distillation, and methanation.482 Absorption in aqueous alkanolamines involves the formation of acarbamate upon flowing the gas through the amine solution, and itis the traditionally preferred method for large-scale facilities.450 After this process, the treated gas is saturated with water andwill require drying. The amine solution needs to be regenerated torelease the acid gases (CO2 and H2S), a stepwhich is highly energy intensive. The high capital and operation costsinherent to this technology make it impracticable for small-scalefacilities and remote deposits. In the search for more optimal andenvironmentallly friendly processes, also applicable to medium- andsmall-scale facilities, other separation techniques are under considerationand research. Adsorption and membrane technologies have the potential to be much less energyintensive and to reduce the operation costs significantly.484As suggested above, the removal of carbon dioxide is centraltothe upgrading process, as it is frequently a major component of thesemixtures see 4.6, and 486 Out of these, zeolites, titanosilicates,carbon molecular sieves, and metal-based adsorbents have found commercialapplication.449 However, the search forbetter adsorbents that can further improve the efficiency and economyof the process is still a very active research field.PSA processes for carbondioxide separation from methane have beenstudied on a wide range of materials, such as zeolites, metal–organicframeworks (MOFs), carbon molecular sieves, activated carbons, porouspolymers, or amine-impregnated mesoporous silica.488 as well as silicalite, mordenite,489 natural clinoptilolite,490 andtitanosilicate ETS-4.494 There are different ways that zeolitic adsorbents may be selectivetoward CO2. One is by exploiting differences in the interactionstrength, i.e., the heat of adsorption, of the two adsorbates. Anotherpossibility involves discriminating between their sizes, e.g., molecularsieving. More complex mechanisms, such as trapdoor mechanisms andguest-induced structure flexibility, are also interesting, as theyusually lead to extremely high selectivities  CO2. Furthermore, natural gas and the other addressed gas mixturesfrequently contain a certain amount of water, which will also stronglyadsorb on these highly polar (and thus hydrophilic) zeolites. It maybe the case that this simultaneous removal of water and CO2 is intended, but this depends greatly on the specific process conditions.497 In most cases, this competitive adsorption is undesired, and itis also noteworthy that water not only will compete with CO2 in the adsorption process but also may favor its chemisorption andthe formation of bicarbonate-like species.499Selectivity onlow silica (highly polar) zeolites, such as A, X,and Y zeolites, is achieved by the first mechanism. CO492 the efficiency of the process is based on molecular sieving. Theright pore size of the adsorbent is selected by controlled thermaltreatment. In coherence with this approach, part of the current researchon this separation is directed toward finding materials with a lowersurface polarity, and thus a lower heat of adsorption of CO2, that maintain a high CO2/CH4 selectivityas well as a large working capacity thanks to their pore size andtopology. These parameters can be tuned by proper selection of thestructure and composition.501 Promising values of CO2/CH4 selectivity and heat of adsorption of CO2 have been obtained using medium-, high-, and pure-silica LTA,501 RHO,273 MWF, PWN502 (and other RHO-related structures), FAU,483 CHA,503 AEI, STT, and RRO504 zeolites and aluminophosphates (AlPOs) andsilicoaluminophosphates (SAPOs) with CHA, RHO, AFX, AFN, and AEI structures.508 Some recent patents aim in this direction, as well, presenting materials,such as SSZ-45 (EEI structure), zeolite RHO, and ITQ-55.511 Out of these new-generation zeolitic adsorbents, structures featuringsmall pores (8-rings) stand out, as they maximize the intrinsic structuralselectivity. This seems reasonable, as they present similar pore diameters(ca. 3–4 Å) than the kinetic diameters of CO2 (3.3 Å) and CH4 (3.8 Å). Note that, despitebeing promising, these materials are relatively expensive to obtain,due to the need of an OSDA, which hinders their commercial development.In the case of ETS-4 and related MolecularGate technology,2 adsorption.515 These structural changes and trapdoor effects do not take placeupon CH4 adsorption, which leads to extremely high selectivities.Similar but more severe trapdoor effects upon CO2 adsorptionare observed in low silica aluminosilicate CHA materials containingpotassium and prepared in the absence of an OSDA.516 Nonetheless, said CHA materialsoperate poorly under the presence of water, which is a major issuefor industrial implementation.516 Merlinoiteis another zeolite which presents guest-induced flexibility upon CO2 adsorption and very promising CO2/CH4 selectivities.518 Additionally, Hong et al. have reported its OSDA-free synthesis,making it a very promising and potentially cheap CO2 adsorbent.264In the case of RHO and RHO-related structures, the selectivityis enhanced by structural changes in the framework and in cation relocationupon CO2 and its separationfrom CH4. The Si/Al ratios have been included as well asthe estimated framework negative charge, in order to rationalize thedifferent types of adsorbents as high-, medium-, or low-polarity materials.55 The estimated framework negative charge is aparameter introduced by our group in a previous publication152 in order to be able to compare zeolites withAlPOs and SAPOs, i.e., materials which are not based on silica. Valuesbelow 0.17 correspond to low polarity materials, while values above0.33 correspond to highly polar materials. Loadings at 1 bar, workingcapacities between 5 and 1 bar, ideal selectivities of CO2 over CH4 at 1 and 5 bar, and isosteric heats of adsorptionat low CO2 loadings 2 partial pressures at the inletof a CO2 removal unit are 0.4–35 bar,519 most frequently above 1 bar. Thus, pressures from 1–10 barare typical for the feed.5202 and CH4 taking place. The highest selectivities come from materialsthat present trapdoor phenomena, especially those with RHO, PAU, andMWF structures. Zeolite 13X (or NaX) also presents very high selectivities.However, except for Na,Cs-RHO, the working capacities on most of thesematerials tend to be low, due to the isotherm reaching saturationat low pressures, and the heats of adsorption present relatively highvalues, indicating a highly energy-demanding regeneration. Lower polarityFAU, CHA, and LTA materials present good selectivities, intermediateworking capacities, and moderate heats of adsorption in the rangeof 27–33 kJ/mol, described as optimal for a thermodynamic separation.483 Pure silica zeolites and AlPOs with small porespresent the largest working capacities and lowest CO2 heatsof adsorption, and some of them, such as Si-RUB-41, SAPO-34, SAPO-56,or M-DNL-6, present selectivities comparable to the ones of higherpolarity materials.Data obtained from breakthrough experiments of mixturesresultin systematically higher selectivities than the ones obtained fromthe pure-component isotherms for the same material. This could bedue to competitive adsorption phenomena of CO4.6.22) occurs from both natural and anthropogenic sources,with anthropogenic contributions being by far larger than natural ones . The anthropogenic CO2 emissions havebeen rapidly increasing for the last 50 years, and they surpass largelythe amount of CO2 that the biosphere can reabsorb.524 Furthermore, there is a clear correlation betweenthese greenhouse gas emissions  and climate change, and therefore, there isan urgent need to mitigate their effect.Carbon dioxide is a necessarystrategy to reduce CO2 emissions and its concentrationin the atmosphere and to mitigate climate change.5Anthropogenic carbondioxide emissions can be prevented and countered following differentstrategies, such as optimizing the use of energy, reducing carbonintensity by switching to renewable energy sources, or enhancing itssequestration.2 removal needs to be performed anyway.527 Still, a large effort needs to be put into the development and deploymentof more CCS facilities.5CCS technologies have been implemented in differentindustriesand deal with mixtures of diverse nature, such as natural gas, steammethane reforming off-gas (precombustion stream), flue gases (postcombustionstream), and ambient air. A total of 19 large-scale CCS facilitieswere in operation in 2019, out of which 10 have been implemented innatural gas upgrading, 3 in hydrogen production (precombustion), 2in fertilizer production, and 2 in power generation (postcombustion).It is noteworthy that most CCS operating facilities have been implementedin industries where CO2 removal technique for postcombustionstreams is chemical absorption with aqueous amines.529 The flue gas of processes that use oxy-fueltechnology consists mostly only of H2O and CO2, which allows for an easy separation of the former by condensation.530 In the case of precombustion, swing adsorptionprocesses are state of the art (see 532 Direct air capture (DAC) isstill under development, due to the difficulty of separating CO2 from an ultradilute source.534 In fact, techniquesinvolving high interaction energies ,such as chemical absorption and adsorption, are needed to captureCO2 from air and subsequently, a large energy input isneeded for its recovery, which renders DAC a controversial approach.535 However, this technology is currently experiencing a rapid growth6 and close to 20 small scale facilities are alreadyoperational, with the first large-scale DAC facility being plannedfor 2024.536 As CO2 removalfrom precombustion streams and natural gas has already been discussedin previous sections , it is not surprising that 13X is relatively adequatefor this application. High adsorption capacity is needed at pressuresclose to ambient, along with high CO2/N2 selectivities.Furthermore, compared with chemisorbents, the energy required forits regeneration will be low. Nevertheless, its hydrophilicity isa great disadvantage that needs to be overcome with process modifications,527 such as a drying step prior to the CO2/N2 separation.553In what refers to zeolites and zeolite-type adsorbents , most research has focused on commercialtype A, X, and Y zeolites, out of which zeolite 13X (NaX) is considereda benchmark adsorbent for postcombustion CO559 A large effort is being put into finding optimal adsorbents, andoptimal process configurations via computational screening of existingand theoretical adsorbents for the capture of carbon dioxide fromflue gases.560 However, one frequent drawback of noncommercial adsorbentsis the difficulty in scaling up their production from both technicaland economical perspectives.331Other materials with lower polarity,such as high- and pure-silicazeolites, AlPOs, and SAPOs, minimize water adsorption and the energyrequired for regeneration, while keeping promising selectivities andworking capacities in some cases.2 from N2 is provided in 507 NaTEA-PST-20,275 Li-SSZ-13–6, Na-SSZ-13–6 (CHA),555 or SAPO-56 (AFX)507 present promising values of selectivities (>10) and working capacities(ca. 0.5 mmol/g) at relevant conditions, comparable with those oftraditional 5A521 and 13X522 zeolites. Some AlPO materials with GIS and SIV structureare also predicted to be promising candidates.333Alist of zeolitic adsorbents and relevant parameters for the separationof CO4.72, there are other gaseous compounds presentin industrial streams that can act as pollutants of the environment,such as CO, NH3, NO2, NO, H2S, andSO2.562 Most of these are byproductsof industrial processes, especially combustion and chemical production,but some may as well be important starting materials . In any case, their emission to the atmosphere needs to be minimized.Together with CO2-permeable membranes or adsorbentsselective toward the other components of the mixture, e.g., zeolite5A. Further separation of these components may be done by condensation/stripping/distillationsteps.562 The separation of H2 and CO has been studied, as well, on zeolitic MFI-type membranes.564 Removal of CO from other mixtures, such as air or nitrogen, forits use as an inert may be done by Ag+-exchanged zeolites.250Carbon monoxide is a highly toxic compound that is produced alongwith hydrogen in steam-reforming processes and is mainly used as araw material for chemical and metallurgic production. The incompletecombustion of fuels also leads to the formation of CO. Removal/recoveryof CO is part of the process of hydrogen production, as presentedin 566 additionally, other techniques,such as membranes and adsorption are being studied.577 Zeolites have been known to adsorb ammonia since 1896, when Friedelpublished a work on the reversible adsorption of vapors in these materials.27 Since then, some studies on the adsorption ofammonia by zeolites have been issued.580 However, the large heat of adsorption of ammonia on traditionalaluminosilicate zeolites is a drawback to their application,581 as it renders the hypothetical process notcompetitive with state-of-the-art techniques. Hence, it is not surprisingthat most attention has been put on MOFs regarding this separationin the last years.577 Nonetheless, several works study highly efficient zeolitic membranesof high silica MFI,570 and it has been recently evidencedthat pure silica LTA and FAU zeolites are promising adsorbents forcarrying out the removal of ammonia through a swing adsorption method.571Ammonia is a highly toxic and reactive gas that is producedata very large scale and is used as a starting material in the productionof fertilizers and chemicals. State-of-the-art techniques to preventammonia emissions to the atmosphere include water scrubbing and biologicalfiltration;582 Its separation from these mixtures is essential for further usethereof, both as fuels and/or as starting materials, as it may causecorrosion and plugging of the equipment. Moreover, sulfur in its reducedstate acts as a poison to catalysts containing noble metals, whichmay be used in refining and petrochemical processes  at usualbed regeneration conditions (TSA) according to the reaction:5832S on these materials is lower in comparisonwith traditional zeolites, and H2S/CH4 selectivitiesare still high, thus making regeneration easier without a loss inefficiency. Promising candidates from these studies are pure silicaCHA and MFI zeolites.589 Titanosilicate ETS-2 has been proposed asa highly selective H2S sorbent, as well.591Hydrogen sulfide is a very toxic andcorrosive gas that appearsas a contaminant in fossil fuels  and is also produced in the decomposition of organic matter (biogas).sses see 2.4.2. Th2, frequentlyreferredto as NOx) and sulfur oxide (SO2) are undesired byproducts of combustion processes and especiallyof coal and transportation fuels.594 They are toxic compoundsthat account for serious environmental problems, like smog and acidicrain, and also negatively affect human health. Their emissions aremainly prevented by proper treatment and selection of the fuel andby correctly setting the temperature and conditions of the combustionprocess. For instance, the development of the three-way catalyticconverter for automotives has contributed to a decrease of the NOx emissions through the method known as selectivecatalytic reduction (SCR),595 and the shift fromcoal toward other fuels for energy production has helped reduce SO2 emissions. Further control of SO2 emissions fromcoal burners can be done by a variety of well-implemented methods,which may be classified as regenerable and nonregenerable and usuallyinvolve the revalorization of the captured sulfur.596 Simultaneous catalytic removal and transformation of SO2 and NOx has been also been studied andapplied.598 There is, however, still anongoing interest in developing adsorption processes with improvedenergetics for the removal of these compounds.Nitrogen oxides ,CaA (5A), and NaX (13X), thus showing that zeolites could be potentialSO2 adsorbents.54 Adsorptionof SO2 was studied on hydrophobic zeolites DAY , silicalite-1, HZSM-5, and mordenite,612 and the conclusions of these works point at silicalite-1 being asuitable adsorbent for SO2 removal from flue gases. Kopaçet al. studied the adsorption of SO2 on commercial zeolitesby means of pulse chromatography at temperatures above 250 °Cand found that the affinity toward sulfur dioxide decreased in theorder 4A > 13X > 5A.614 Gupta et al. foundzeolite 5Apromising for trace removal of sulfur dioxide at 70 °C.615 Srinivasan et al. studied a set of zeolitessynthesized from fly ash and found that the largest SO2 adsorption capacity at room temperature was given by a mixture ofsodalite and analcime.616 A computationalscreening of zeolitic structures carried out by Matito-Martos et al.suggested that zeolites with JRY, NAT, AFY, FAU, and SBE structurescould be potentially applicable at certain conditions.339 Systematic studies dealing with SO2, NO, CO2, and N2 mixtures were carried outby Deng and co-workers, showing that zeolite 5A performed better thanzeolite 13X in adsorbing the pollutants, especially SO2.618Being mostly based onchemical sorption, the current methods forSO4.8619 They are produced mainly in catalytic cracking, steam cracking,thermal cracking, MTO, and catalytic dehydrogenation processes,622 along with other hydrocarbons. The separation of light olefins fromthe other products is performed by cryogenic distillation. Due totheir close boiling points  are important raw materialsfor the production of polymers and chemicals .ints see 2, the se624 These ways of separatingolefins from paraffins may rely on differences in the physical and/orchemical properties of said molecules –NaX is another materialpresenting a high ethene/ethane selectivity (ca. 5) and a low workingcapacity between 0.5 and 1 bar (ca. 0.3 mmol/g).629 The high heat of adsorption in these cases not only leadsto low VSA/PSA working capacities but also makes regeneration moreenergy-demanding. Furthermore, the strongly adsorbed olefin can oligomerizeinside of such adsorbents, giving rise to pore obstruction.Adsorbents thatcontain Ag and Cu, such as supported silicas andaluminas, exchanged zeolites (AgY), and several MOFs, may be thermodynamicallyselective toward the olefin thanks to a π-complexation mechanism.623 Aluminosilicate zeolites with FAU and LTA structures,more specifically, 13X,631 4A,634 LiNaA,635 and 5A,637 along with some exchanged titanosilicates of types ETS-10638 and ETS-4639 haveachieved high to very high selectivities (5–70) toward etheneand propene, although their working capacities in the range of 0.5–1bar are low (<0.3 mmol/g). Zeolite 4A additionally presents a markedpropene/propane kinetic selectivity.633 MOFs such as M-MOF-74 ,642 M2(m-dobdc) ,643 and NOTT-300644 are promisingfor ethene-selective separations,624 inwhich they present high to very high thermodynamic selectivities towardthe olefins 3Agpresents large selectivities and working capacities for both C2 (645 MOF MIL-101-Cr-SO3Ag presents ahigh selectivity (ca. 10) toward ethene and a notably high workingcapacity (0.9 mmol/g).646 However, thehigh production cost and reduced long-term stability of most of thesematerials hinder their deployment at a larger scale.Thermodynamic selectivity may, as well, stem from physical interactionswith the adsorbent. Adsorbents with a polar surface are selectivetoward the olefin, which is the most frequent situation.624 It is of high interest to achieve high workingcapacities and selectivities using paraffin-selective adsorbents,as these represent the lesser amount of the steam cracker productstream, and thus, their separation would require smaller adsorbentinventory and enable the direct production of a highly pure olefinstream.650 At the same time, by selectivelyadsorbing the alkane, the risk of olefin oligomerization is avoided.Severalnonpolar MOFs have been reported to be thermodynamicallyselective toward ethane over ethene, although the selectivities arerelatively low so far (<5) in comparison to ethene selective adsorbents.651 Various pure- and high-silica zeolites and aluminophosphate-basedmaterials presenting 8-rings, i.e., ITQ-29 (LTA), DD3R (DDR), ITQ-12(ITW), Si-CHA (CHA), ITQ-3 (ITE), ITQ-32 (IHW), AlPO-34 (CHA), andNa-SAPO-17 (ERI), present extraordinarily high (700 – 46 000)propene/propane kinetic selectivities.658 Similarly, zeolite ITQ-55, a small-pore zeolite with extremely narrowpores, presents a very high ethene/ethane kinetic selectivity,272 which derives from an ethene adsorption-drivenchange in the framework structure. Further advantages of these pure-silicamaterials is that no reactions of the olefins will take place insidetheir nonpolar surface, and their hydrophobicity will prevent competitiveadsorption of water and other relatively polar molecules. MOF [Ca(C4O4)(H2O)] presents an unprecedentedmolecular sieving effect, in which it adsorbs selectively only etheneand completely excludes ethane.659 OtherMOFs, such as ZIF-8,660 ZIF-67 (ZIF = zeoliticimidazolate framework),661 Zn(ox)0.5(trz), and Zn(ox)0.5(atrz) ,662 presentkinetic selectivities of the order of 102–103 of propene over propane. These are probably not as high asfor zeolites due to the higher flexibility of MOFs in general.Whereas thermodynamically selective adsorbents have received themost attention, it is kinetically selective and molecular sievingadsorbents that present the largest selectivities.cis-2-butene, trans-2-butene, isobutene(2-methylpropene), and 1,3-butadiene. They are coproduced along withethene and propene in the steam cracking of naphta and as a byproductin FCC.664 The increasingdemand of the olefins of this fraction, especially isobutene (45%)and butadiene (40%), has led to targeting their production via otherprocesses,364 such as MTO, low and highseverity steam cracking,664 catalytic dehydrogenationof butane, or oxidative dehydrogenation of butene.665 Said processes yield products that consist of mixturesof C4 olefins and paraffins in various proportions.665 Therefore, the separation of the C4 fraction is necessary for thefurther use of several components. This separation cannot be carriedout via simple distillation due to the close boiling points of thedifferent species , 1-butene, cies see 2. Extractert-butanol or methyl tert-butylether. The remaining linear butenes and butane are separatedby either extractive distillation or molecular sieving (the referencecontaining information regarding said molecular sieve could not beaccessed by the authors of this review).664Butadiene polimerizes readily under a wide set of conditions, andthus it must be handled and kept in the absence of oxygen and acids.It is separated first via extractive distillation or liquid–liquidextraction. Then, further extractive distillation may be used to separatebutanes from butenes. Isobutene is separated by reactive distillationin the forms of 364 The C4 olefins maintain thetypical olefin interactions with metals (such as π- and/or electrostaticinteractions) and also general trends, such as larger diffusivitythereof compared to the analogous paraffins if the pore size is adequate.Nevertheless, new levels of hierarchy arise, and each and every oneof the C4 isomers behaves differently upon adsorption on microporousframeworks.Adsorptive separation of the components of the C4 fractionhasbeen studied on different materials, such as zeolites, mesoporoussilicas and MOFs.666 butthis process is not used currently, as it does not match the presentpurity requirements of butadiene.363 Anadsorption process for the separation of butenes from butanes on 13Xusing a C5 or C6 paraffin as a desorbent has been patented by Kimet al. recently.668 The separation of the otherC4 components under thermodynamic control has been studied on commerciallyavailable FAU (12-rings) and MFI (10-rings) zeolites, such as 13X,669 NaY,670 ZSM-5, andsilicalite-1.671 The adsorption on 13Xfollows the order cis-2-butene > 1-butene > trans-2-butene > butane.669 OnNaY, the same order is kept (isobutene > cis-2-butene> 1-butene > trans-2-butene > butane),670 which indicates that enhanced interactionsare established by isobutene, followed by cis-2-butene.MFI-structured materials, such as ZSM-5 and silicalite-1, presenta limited diffusivity of the branched compounds in comparison to thelinear compounds.673 Furthermore, MFI materials adsorblarger quantities of 1-butene than n-butane.671The separation of butadiene from the other C4 componentsusingzeolite 13X was patented in 1976,675 Further selectivity was observedfor NaY zeolites of Si/Al ratios between 6 and 15, in which butadienewas adsorbed at higher amounts.675 ZeoliteCu(I)Y was studied by the same authors,676 and better butadiene/1-butane selectivities than on AgY were found,with the additional advantage of Cu(I) not being sensitive to thepresence of contaminants such as H2 or H2S.Interactions with transition and noblemetal cations, such as Agand Cu, have been exploited to achieve high selectivities toward butadiene.Zeolite AgY presents a large selectivity of 1-butene and butadieneover butane.364 In this case, the preferredlinear compounds tend to be 1,3-butadiene and trans-2-butene, and the least adsorbed one is butane. Isobutane and isobuteneare usually excluded from small-pore zeolites. Pure- and high-silicazeolites with CHA, DDR,679 IHW,657 RRO,680 and AlPOs and SAPOs with ERIstructure681 have been studied for thisapplication. Si-CHA, AlPO-34, DD3R, and SAPO-17 are considered tobe promising materials,682 and Si-CHA and AlPO-34were patented for ExxonMobil in the 2000s.678However, similarly to the C2 and C3 fractions, it is kineticallyselective low polarity small-pore zeolitic adsorbents that appearto be more promising.4.9683 Nonetheless, some of these components, such as benzene, aromatics,and olefins, are restricted due to their environmental and/or healthhazards.684 This leaves branched paraffinsas the component of choice to meet ON specifications. Hydroisomerizationof straight run naphtha (mostly linear C4–C10 paraffins) isan effective method of obtaining higher ON components for the gasolineblend. These are reacted with hydrogen in the presence of a highlyactive supported metal hydrogenation catalyst to yield the desiredmultibranched products. Due to equilibrium limitations, low temperaturesare needed in order to minimize hydrocracking.685 A strategy which prevents hydrocrackingfrom taking place and thus increases the yield and productivity ofthe unit includes separation of the branched products from the effluentand recycling of the linear and monobranched hydrocarbons to the headof the unit.687 The separation of linear frombranched isomers is done by adsorption using zeolite 5A as the adsorbent.This zeolite has been implemented in liquid- (SMB) and vapor-phase(VSA/TSA) hydrocarbon separation processes since the 1960s.690 In fact, molecular sieving separations oflinear and branched paraffins were one of the first major industrialsuccesses of zeolites.691 Other zeolites, such as X, Y,and ZSM-5, have been commercialized for this purpose, as well.695Gasoline is a liquid hydrocarbon mixture whichconsists mainly of hydrocarbons in the C4–C12 fractions andis one of the most widely used fuels. The octane number (ON) is ameasure of the performance of the gasoline upon combustion in an internalcombustion engine (ON of ca. 100 is desired), and it is regulatedby official institutions. The ON of gasoline depends on its composition,in which some of its components, such as branched paraffins, aromatics,or olefins, increase the ON of the mixture.704 Other zeolites with diverse structures, such as AFI,705 AEL, ATO, BEA, FAU, FER,696 ATS, CFI,707 EUO, MWW, NES,709 MEL, MRE, and MTT,703 have been patentedfor this separation as well, but none of them clearly surpasses Si-MFI.13 Recently, pure silica zeolites Si-STW710 and EMM-17711 andMOF Fe2(BDP)3 ,712 have been proposed as promising adsorbentsfor this separation. Zeolite Si-STW is superior to Si-MFI at ambienttemperature in both adsorption capacity (ca. 30% increase) and kineticselectivity toward the linear and monobranched compounds in the C5fraction. Further selectivity between dibranched compounds was found,in which quaternary C-atom-containing isomers of the C5 and C6 fractionswere completely excluded.710 EMM-17 canbe used to separate C6 isomers at ambient temperature as determinedfrom breakthrough experiments. The separation is kinetically controlled,with n-hexane and 2,2-dimethylbutane being the fastest-and slowest-diffusing isomers, respectively.711 Fe2(BDP)3 presented slightly better performance(0.54 mol/L) in terms of the 92 RON productivity than top-performingzeolites with MWW (<0.52 mol/L) and MFI (0.51 mol/L) structures.712However, if applied at the exit of the hydroisomerizationunit, the ideal target is the separation of linear and monobranchedhydrocarbons from multibranched ones, as it would increase the efficiencyof the whole process by recycling both low-octane linear and monobranchedhydrocarbons to the head of the unit and yield a multibranched product-enrichedraffinate with a high ON. Several materials have been studied andpatented for this purpose, out of which silicalite-1 (Si-MFI) andother materials with MFI structure have been most frequently considered.4.10n-paraffins) results in a product rich in benzene, toluene, xylenes(dimethylbenzenes), and ethylbenzene,204 frequently referred to as BTX. There are several possible isomersof xylene, i.e., methaxylene, orthoxylene, and paraxylene, also referredto as m-xylene, o-xylene, and p-xylene  maybe further separated in a super fractionation unit, where orthoxyleneis separated from the other three components.206 However, the very close boiling points of these components  is lower than that of the pure components,due to the eutectic mixture that results.713 Another way of separating these compounds is via adsorption and,more specifically, via molecular sieving, taking advantage of theirdifferent sizes.544 Paraxylene is the smallest ofthe three isomers, and thus, it may be adsorbed on a material withthe right pore size, while the other components of the mixture maybe excluded.Benzene and toluene are easily separated by distillation.Xylenes and ethylbenzene , and more recently 2-methylpropane(isobutanol) can be produced from renewable resources via biologicalpathways and serve as biofuels and starting materials for the productionof important chemicals.719 For instance, producing light olefins from bioalcohols by dehydrationis a highly interesting approach,721 for whichsome processes have been already developed, such as Axen, Total, andIFPEN’s ATOL process.722Environmental concern and the future shortage ofpetroleum-derived products have boosted the research and productionof renewable fuels and chemicals. Short-chain alcohols, such as ethanol,1-butanol . The resulting aqueous fermentate contains 8–14vol % of ethanol, which needs to be separated for further use. Currently,its separation involves at least two steps, the first of which consistsof distillation and leads to the formation of a distillate with amaximum ethanol content of 95.5 wt %, due to the ethanol–waterazeotrope.727 This process has been intermittently used throughout the years toobtain 1-butanol and/or acetone and is currently of great practicalinterest.729 Differentstrains of bacteria of the class clostridia, e.g., Clostridiumacetobutylicum and clostridium beijerinckii, can perform this fermentation. The process is carried out in anaerobicconditions, and the product consists of a diluted aqueous solution(< 3wt %) of acetone, butanol, and ethanol in a 3:6:1 molar ratio,respectively.730 Along with the liquidproducts, some CO2 and H2 are produced in thefermentation.Biobutanol, chemically speaking, 1-butanol,is an excellent biofuelwith analogous properties to gasoline, and it serves as a platformmolecule for the production of important chemicals, such as butenes.1-Butanol can be produced from fermentation of starch and sugars,in what is known as the ABE  fermentation,first patented by Chaim Weizmann  in the 1910s.731 Alternatively, it can be carried out following different methods,such as extraction, gas stripping, pervaporation, or adsorption, outof which the last two seem the most promising.732 Many studies on liquid-phase adsorptive separation of butanol fromthe ABE product have been carried out using a variety of adsorbents,730 such as activated carbons,737 polymeric resins,741 zeolites,747 or MOFs.737 In 2014 Abdehagh et al. proposedthe combination of gas stripping and adsorption  as an effective recovery method.732The recovery of 1-butanol from the fermentationbroth was originallycarried out by distillation. However, mainly due to its low concentrationin the product, this turns out to be a highly energy-intensive methodand requires a high-energy integration and capital cost.750 zeolites,754 and MOFs.756 The effect of CO2 as a carriergas has been considered in some of these works.756 As can be seen, zeolites have been used as adsorbents for liquid-and vapor-phase separations, with silicalite-1 being the most frequentlystudied material, probably due to it being the first pure silica zeoliteavailable.758 Cavity-like zeolites, such aspure silica LTA (Si-LTA) and SAPO-34, i.e., CHA-structured SAPO, wereused in combination by van der Perre et al. to achieve an unprecedentedlyhigh recovery and purity of 1-butanol.751The isolation of 1-butanol from the fermentation broth usingvapor-phaseadsorption on microporous materials has since been studied by differentgroups on activated carbons,761 Isobutanol is a very important platformmolecule that can be used as a starting material in the productionof olefins (butenes) and aromatics first and polymers and gasolineadditives (MTBE) finally.762 The recoveryof bioisobutanol from the aqueous phase is currently done by gas strippingand solvent extraction764 or by flash evaporation,followed by condensation and distillation.765 but alternative methods, such as extractive distillation using K2CO3766 and adsorption,have been proposed. In the case of adsorption, a pure silica Betazeolite has been used for the vapor-phase recovery of isobutanol fromits mixtures with ethanol and water, achieving high selectivitiesand promising separation performance.767Renewable isobutanol (bioisobutanol) can be produced via fermentativeand nonfermentative methods, which have been developed only recently.5p-xylene .Zeolitesare microporous crystalline materials widely used as catalysts,adsorbents, and ion exchangers. The main characteristics, preparationmethods, and applications have been presented. The basic conceptsregarding adsorption and separation processes on zeolites have beensummarized. The relevance of zeolites as adsorbents has been examined,and the most important applications of zeolites in adsorption andseparation science have been reviewed. Zeolites and zeolite-like materialsare state-of-the-art adsorbents for applications, such as the purificationof hydrogen from SMR (zeolite 5A), the drying of industrial streams(zeolites 3A and 4A), the small- to intermediate-scale air separation(LiLSX), the separation of nitrogen from methane , theseparation of linear hydrocarbons from isomer mixtures (5A), and theseparation of Overall, a trend can be seen where well-established zeolitic adsorbentsare kept for ongoing industrial processes, and new zeolitic materialsare studied for emerging adsorptive separations, such as the separationof hydrogen isotopes, carbon dioxide capture, separation of olefins,removal of pollutants, and separation of bioalcohols. In the caseof the well-established zeolitic adsorbents, most of the researchis directed toward process improvement. Process intensification shouldbe considered for adsorptive separations using zeolites, e.g., bystudying tandem processes, where adsorption and reaction are coupled.Two major advantages can be obtained from such approaches, the firstbeing an equilibrium displacement of the reaction (sorption-enhancedreaction) and the second better heat integration of the whole process.Adsorbent conformation to improve process dynamics and mass transferis another research topic which deserves careful consideration.2/CH4 is carriedout by ETS-4 materials at a small scale, yet the challenge to replacecryogenic distillation at a large scale remains. Research should bedirected toward finding materials with high enough selectivities andimproved regenerability.The separation of N2 fromN2 or CH4 have been carried out using zeolitesof different types. In the case of CO2/CH4,new studies should be directed toward low polarity small pore zeolites,which prevent water competition and improve process performance. Inthe case of CO2/N2, the use of medium Si/Alratio small pore zeolites is encouraged. Molecular sieving and trapdoorphenomena should be exploited while keeping minimal competition withwater and high regenerability.The widely studiedseparations of COThe olefin/paraffinseparation is one of the most importantindustrial separations and one that is still carried out via distillation,which is a highly energy-demanding process, due to the close boilingpoints of the compounds to be separated. The highest kinetic separationfactors of olefins over paraffins have been found using small porepure-silica zeolites as adsorbents, which makes them very promising.Recent studies on hydrocarbon separationsby pure-silicamedium pore zeolites open up the possibility of separating linearand monobrached from dibranched hydrocarbons, including olefins.The combination of gas stripping and adsorptionforthe recovery of alcohols from fermentation broths has resulted ina very efficient approach. Pure silica zeolites pose as a promisingalternative to the currently implemented methods, which are mostlybased on distillation. Lowering the energy required for these separationsopens the door to the truly sustainable use of bioalcohols as biofuelsand chemical feedstocks. In this sense, the research focus shouldbe put into testing a wider set of pure-silica adsorbents and on improvingthe regeneration procedure.The highlychallenging separation of hydrogen isotopeshas been tried by many, and the success ratio has been low to themoment. Nonetheless, it is important that new adsorbents and membranescontinue to be evaluated for this application. Adsorbents with poressmaller than 3 Å are the most obvious choice for pursuing a quantumsieving-based separation.On the other hand, the emerging applications each present specificresearch questions and needs:2 adsorption,where in addition small pore zeolites presenting gating effects arereceiving the most attention. Small pore zeolites featuring extremelysmall pore openings (< 3 Å) are the most relevant for hydrogenisomer separation, CO2 capture, and ethylene/ethane separation.As can be seen, pure silica zeolites have gained importanceforhydrocarbon and bioalcohol separations, where undesired reactionsneed to be avoided, and the hydrophobicity prevents competition withwater adsorption. This is also true for COCommercial availability has been and still is one of the primaryreasons why certain adsorbents are tested for new applications. Withthe progressive addition of more zeolitic materials to the spectrumof commercial absorbents, it will be easier for adsorption researchersto acquire and test these. Nonetheless, the input from zeolite synthesisresearch groups is especially valuable, as the technology and know-howneeded to produce new and existing zeolites that are not commerciallyavailable is not accessible to everybody. Thus, it is not surprisingthat joint research by zeolite and adsorption experts has resultedin some of the most promising discoveries in the field of zeoliticadsorbents.The active search for new structures and new compositionalvariantsof existing structures is mostly driven by previous observations onmaterials sharing similar properties. However, the trial-and-errorapproach is not always truly successful in producing a scientificbreakthrough but rather gradual improvements or purely nonappliedknowledge. Therefore, the identification of promising structures fora certain separation can be greatly accelerated by computational screening,which despite the simplifications needed correctly predicts generaltrends. Additionally, these computational studies serve as a majordriving force to achieve the synthesis of new materials. We also firmlybelieve that the enormous quantity of accumulated adsorption datacomprises a solid starting point for data-mining approaches, whichhave proven successful in the field of zeolite synthesis.Asmentioned throughout the review, a major drawback of new adsorbentsis their production and scaling-up cost. In some cases, the addedvalue of the product of a separation accounts for the high cost ofthe sorbent, provided that the regenerability is sufficiently good.However, in most cases adsorbents that are very promising in the laboratorynever make it to the next stage of practical development. These issuescan be addressed by aiming toward robust, hydrophobic adsorbents whichcan be produced without the need of an expensive OSDA.