The objective was mainly to overcome the limitations of MOFs in terms of mechanical properties, chemical stability and processability while imparting novel functionality catalytic activity, etc.) and hierarchical porosity. These composites were considered for numerous applications including gas/liquid adsorption and separation, (photo)catalysis, biomedicine, energy storage, conversion and so on. The performance of such composites depends strongly on their microstructural and physico-chemical properties which are mainly driven by the chemical strategies used to design and process such composites. In this perspective article, we propose to cover this topic and provide a useful survey of recent progress in the synthesis and design of MOFs–carbon material composites. This article will describe the development of composites with increasing complexity in terms of porous architecture, spatial structuration and organisation, and functionality.In the past few years, metal organic frameworks (MOFs) have been assembled with (bio)polymers and a series of carbon-based materials (graphene, graphene oxide, carbon nanotubes, carbon quantum dots,  The article covers the survey of recent progress in the design of MOFs–polymers and MOF–carbon material composites with increasing complexity in terms of porous architecture, spatial structuration, organisation, and functionality. Indeed, numerous biominerals such as pearls, sea urchin spines, oyster shells or corals are well-known inorganic/organic composites and one of their remarkable features lies in their hierarchical ordered structure spanning from the nano- to macroscopic scale.1,2 Rational synthesis methods based on the co-assembly of inorganic colloids  with self-assembled organic superstructures or templates were explored to prepare composite materials with tailored and complex morphology, high porosity and hierarchy. The design of such composites with a high level of morphological complexity, hierarchical dimensionality and porosity, was relevant to the development of novel types of heterogeneous catalysts, separation membranes, biomedical implants or drug delivery nanocarriers.1,2In the past few decades, one of the most prominent areas of research in materials chemistry has been the exploration of novel bio-inspired strategies to prepare inorganic and hybrid materials with controlled sizes, morphologies and defined texture.3,4 Their highly ordered pore structures, large surface areas and chemical functionalities make these materials highly promising for a wide range of applications, including gas storage and separation, catalysis, sensing, energy storage and biomedicine among others.5–9 However, MOFs still face certain limitations that greatly hinder their transfer and implementation in real industrial applications. A large number of MOFs exhibit limited chemical and thermal stability and thus a poor resistance to water. This can result in the irreversible degradation of their frameworks when exposed to humid air. Additionally, conventional MOF synthesis methods often produce polydisperse powder samples. The low mechanical properties of MOF powders, along with their poor processability and handling properties as well as their low mass/heat transfer rate significantly impede the development of MOF-based applications.10 As an example, MOFs have been mainly processed in the forms of pellets, granules, beads, fibres and membranes but in many cases, the accessibility to the MOF porosity is lost after shaping.10 A high pressure drop can also be observed for densely packed MOF powders within an adsorption column.10 In catalysis, the use of MOF powders hampers the catalyst recycling. To address these issues, various strategies have been explored, primarily involving the integration of MOFs with different supporting materials, such as (bio)polymers or carbon-based materials   with the desirable characteristics of the supporting matrix . Moreover, this approach can also enhance the stability, impart electron conductivity, and extend the porosity of MOFs in the meso- or macroporous regime.11–16 The hybridization of MOFs with carbon-based materials has resulted so far in a wide range of composites that have found applications in different fields including gas adsorption and separation, sensing, water purification, catalysis, and biotechnology.11–16 The high performance of such composites in those applications is required to optimise the MOF loading and the accessible porosity while limiting the presence of interfacial defects. The physico-chemical properties of such composites are thus strongly driven by the control over the spatial distribution and compartmentalisation of MOFs within organic matrices and the MOF/carbon material interfaces. This has been extensively described in the processing of MOF–polymer membranes.11,12 Therefore, the microstructural properties of such composites greatly depend on how the components are spatially integrated, and this is determined by the synthetic strategies employed to construct such MOF-based composites.In the continuous quest for novel multifunctional composites, such strategies were expanded to the combination of organic materials with metal–organic frameworks (MOFs). MOFs are a class of porous and crystalline hybrid materials constructed by assembling metal-based nodes and organic polydentate ligands.s, etc.) .11–13 Thetc.). Our contribution is not intended to focus on the performance of MOF-based composites in a certain field of application  as it was extensively reviewed elsewhere.17–21 In this perspective, we aim to provide an overview and critical discussion of the different synthetic strategies that were deployed for the fabrication of MOF–polymer and MOF–carbon material composites. These synthesis approaches will be divided into five parts, taking advantage of the structural and physico-chemical properties of MOFs including their high porosity, functionality of the framework and the surface chemistry of the particles. The first two parts will be dedicated to composites prepared through the insertion or formation of the carbon-based component either in the pores or in the framework of MOFs. Then, the third and fourth parts will describe the integration of preformed MOFs into organic matrices without altering the MOF porosity. The surface engineering of MOF is the key point of this approach and will be outlined in the third section. The fifth part deals with the design of hierarchically porous MOF/carbon material composites. Note that the cohesion and stability of such composites strongly depend on their interfacial properties. Composites with MOFs incorporated in organic matrices or encapsulating carbon entities are mainly based on electrostatic, van der Waals interactions or hydrogen bonds. However, polymers could also be integrated into the MOF framework through coordination bonds or covalently grafted at the MOF surface. This perspective also aims to emphasise that the design of highly performing composites requires control over the MOF particle size and morphology, the nanostructuration and spatial organisation of components at different length scales as well as the hierarchical porosity in the micro-, meso- and/or macroporous regimes. Thus, this article intends to address the main advances achieved in the design of such hierarchically porous MOF–carbon material composites in the fourth and fifth sections. We expect that this review will provide synthetic guidance for developing the next generation of advanced MOF-based composites.This perspective article aims to describe recent developments in the design of composites combining MOFs with (bio)polymers or carbon materials  is a stepwise synthesis approach that consists of synthesising separately the CNEs and MOFs followed by their mixing in solution at room temperature (RT)11,23,24  which involved the concomitant nucleation and growth of MOFs with the insertion of CNEs  with preformed CNEs and then the synthetic protocol is carried out under the same conditions as for the pure MOF synthesis. This method is not restrained by the MOF porosity and similarly to the ship-in-bottle approach, it allows the entrapment and stabilization of CNEs in a 3D assembly of MOF crystals. The presence of functional groups  at the surface of CNEs can also act as nucleation sites of the MOFs thereby leading to a high spatial distribution of CNEs within the composite.3,11,24 However, in some cases, these capping groups can also compete with the organic ligands for metal coordination, thereby affecting the nucleation and growth of the MOFs. Each strategy, either the ship-in-bottle or bottle-around-ship strategy has its advantages and limitations and was thus selected depending on the MOF/CNE system.Carbon nanoentities (CNEs) such as fullerene, carbon quantum dots or graphene quantum dots were encapsulated in the pores of MOFs. This approach aims to enhance the chemical stability of CNEs and prevent their aggregation which is detrimental to their physicochemical properties .11,23,24 . This me of CNEs .11,23,2422,24 CQDs are a class of carbon nanomaterials with a size lower than 10 nm, consisting of a polyaromatic core stabilised through surface functionalisation.25 Their combination of properties such as tunable fluorescence emission, unique photoluminescence stability, good electron conductivity, simple preparation, low cost and good biocompatibility make these nanomaterials very interesting for different fields including sensing, (photo)catalysis and bioimaging.22,24,25 In the past few years, a variety of Ti, Zr, Cu or Fe-based MOFs have been explored for photocatalytic application.5,26,27 Upon light excitation, the organic linker of the MOF can act as a light absorber and the MOF can undergo a photo-induced charge separation with electrons populating the inorganic building units (IBUs) and holes populating the organic linker.26,27 However, their photocatalytic efficiency is yet far from optimal. One major issue is the large band gap of MOFs that restricts their photoresponse to UV light or short-wavelength visible light (<500 nm), thereby largely limiting their applications under the solar spectrum.26,27 In addition, most MOF photocatalysts suffer from severe charge recombination.26,27 As CQDs can act as both photosensitizers and electron receptors, the combination of MOFs with CQDs was thus explored to improve their photocatalytic efficiency for different reactions such as the degradation of organic pollutants or the CO2 reduction reaction (CO2RR).18,24,28,29 S. Li et al. synthesised two types of composites by assembling the porous Zr amino-terephthalate MOF (i.e. UiO-66(Zr)-NH2) and 3 nm diameter CQDs through either the ship-in-bottle or bottle-around-ship strategy -NH2 nanoparticles (NPs) whose external surface was decorated with CQDs, the in situ approach allowed the entrapment of CQDs in the cavities of UiO-66(Zr)-NH2 NPs -NH2 for the CO2RR. It was shown that the CQD-embedded UiO-66(Zr)-NH2 photocatalysts outperform both the CQD-decorated UiO-66(Zr)-NH2 and the pristine MOF for the CO2RR .30Among CNEs, carbon quantum dots (CQDs) were combined with MOFs and the resulting porous CQDs–MOF composites were mainly investigated for applications in photocatalysis and optical detection.strategy .30 Depen-NH2 NPs .30 This he CO2RR .30 Chargunctions .30 Besid22 A large number of luminescent MOFs have been reported for applications in sensing, white-light-emitting diodes (LEDs) and biomedicine.22,31–33 In particular, a range of MOFs was developed as luminescent detection platforms for various target analyses, such as ions, organic molecules, gases, explosives, humidity, pH and temperature.22,31–33 These materials have thus shown great promise for the construction of luminescent sensors with a high selectivity and sensitivity. In addition to the luminescence of the MOF itself, various luminescent guests  can also be encapsulated in the MOF pores to expand their photonic functionality.22 Among luminescent guests, CQDs present a high photonic quantum yield and high photoluminescence stability that were exploited for the building up of fluorescent probes and sensors.22 However, as reported for fluorescent dyes, they easily aggregate in solution and this results in fluorescence quenching.34,35 The dispersion of CQDs within MOF matrices was thus explored to avoid this fluorescence aggregation quenching, regulate the fluorescent properties of CQDs or potentially synergistically combine the fluorescent properties of both MOFs and CQDs through adequate host–guest interactions.22 Ma et al. and J. Yang et al. prepared a fluorescence sensor by the in situ method, allowing the confinement of CQDs in the cavities of the zeolitic imidazolate framework ZIF-8(Zn).35,36 This CQDs@ZIF-8 that combined the yellow fluorescence of CQDs with the blue fluorescence of ZIF-8 took advantage of this dual-emission and was thus used for the ratiometric fluorescence detection of Cu2+.35 A high diversity of dual-emission fluorescent probes was prepared similarly by the in situ method enabling the encapsulation of luminescent guests (preformed CQDs and lanthanide ions) into the MOF pores during the MOF crystallization.22 Recently, another approach took advantage of the good thermal stability of a few MOFs. CQDs were formed through the thermal post-synthetic treatment of MOFs loaded with organic molecules (glucose).37 This strategy was extended to the non-activated ZIF-8 containing an excess of free imidazolate and its thermal treatment led to the growth of 4 nm sized CQDs in the cavities of ZIF-8  was encapsulated in the pores of a chiral MOF by the in situ strategy.39 Although C60 can be described as a highly symmetric truncated icosahedron with the absence of any chirality, the circular dichroism spectrum of the composite showed an intense chiral signal in the absorption region of C60.39 A series of characterization studies and DFT calculations showed that this unprecedented chiral induction of C60 is mediated by strong host–guest interactions, resulting in molecular orbital hybridisation between the chiral MOF and C60.39 This chiral induction approach was not only of high interest to impart chirality into inherently symmetric molecules but it can also deepen our fundamental knowledge concerning the spontaneous symmetric breaking mechanism.S.-W. Lo 5,9 As presented in the next section, polymers could also be encapsulated in the MOF pores while they were also integrated as part of the MOF framework.It is worth noting that numerous composites were similarly prepared through the confinement of a wide range of inorganic and biological nanoentities and such composites have found numerous applications in (bio)catalysis, biomedicine, (bio)sensing and so on.12,21,40 A mutual recognition between MOFs and polymers as a result of attractive interactions at the MOF/polymer interface can influence both the structure and physicochemical properties of MOFs and polymers. This part will cover the synthesis of functional composites through the integration of polymers either in the framework or the pores of MOFs  with those of polymers  but also to impart novel features to the composite as a result of synergism between both components. of MOFs . We will42 The strategy of converting a non-porous, flexible and amorphous polymer into a porous and three-dimensional crystalline network is entropically challenging since the packing and entanglements of polymer chains generally lead to polymer materials with low crystallinity and porosity. Moreover, the formation of MOFs with high crystallinity is generally favoured by using small and rigid organic linkers. Interestingly, this class of novel porous hybrid materials termed polyMOFs was able to inherit the complementary features of both polymers (good film-forming properties and processability) and MOFs .21,42 To achieve this objective, a series of pbdc-xa polyethers containing benzene dicarboxylic acid (bdc) as repeating units connected by different methylene spacers (x = 5–8 carbon atoms) were synthesised by step-growth polymerisation and their hydrothermal reaction with Zn2+ salts produced a series of polyMOFs (i.e. Zn-pbdc-xa) isostructural to MOF-5.42 The bdc units served as organic ligands of the Zn-pbdc-xa. The crystallinity of such materials strongly depends on the length of the methylene spacer, the crystallinity being higher for x = 7–8.42 In addition, the hydrophilic–hydrophobic balance of such materials could be tuned by varying the spacer length. As a consequence, polyMOFs prepared with x = 7–8 were significantly more hydrophobic and chemically stable towards moisture than the parent MOF-5.42 While the pure pbdc-xa polymer ligands are non-porous, Zn-pbdc-8a and -7a exhibited a high residual BET area (856 and 1104 m2 g−1 for x = 7 and 8 respectively vs. 2960 m2 g−1 for the pristine MOF-5).42 The CO2 adsorption capacity of Zn-pbdc-7a and -8a is significantly higher than that of the pure MOF-5 and this may be explained by strong interactions between CO2 and the polyMOFs as a result of the confinement effect of CO2 molecules in the reduced porosity of such polyMOFs.42 In an attempt to extend the design of polyMOFs to other MOF architectures, new types of polyMOFs were developed by combining the pbdc-xa polyethers with co-ligands bpy  and bpe ethane).43 Note that these bpy and bpe ligands were previously used for the construction of pillared square grid MOFs such as [Zn2(BME-bdc)2(bpy)]n (MOF 1). This co-assembly of pbdc-xa with different “pillaring” pyridine co-ligands (pby and bpe) and metal cations (Zn2+ and Cu2+) successfully led to the formation of polyMOFs isostructural to pillared square grid based MOFs.43 This study clearly showed the potentiality of the polymer ligand approach to enlarge the family of polyMOFs. This was further demonstrated through the design of polyMOFs based on the chemically robust UiO-66(Zr) MOF.44 While pure UiO-66 crystals generally present an octahedral shape, the polyUiO-66 materials were shown to present a different interlaced morphology and a hierarchical porosity that were certainly imparted by the well-defined pbdc-xa ligands.44 Indeed, pbdc-xa were prepared by acyclic diene methathesis polymerisation (ADMET) and were thus characterised by a higher molecular weight and an improved dispersity in comparison to previous polymer ligands.44 Interestingly, it was shown that polyUiO-66 materials were not only obtained from homopolymers but were also formed from different block copolymers (random blocks and end-blocks) and statistical copolymers comprising coordinating pbdc blocks and non-coordinating poly(ethylene) (PEG) blocks ).50 In comparison to other polymerisation routes, RAFT offers the advantages of a higher control over the composition, molecular weight and dispersity of polymers.50 The use of well-defined RAFT-based polymers not only allowed the design of polyMOF-5 with tunable microstructural and textural properties but also showed that increasing the polymer dispersity favoured the polyMOF formation.50 Therefore, the synthesis of polyMOFs by using RAFT-based polymers with low-dispersity required the co-addition of H2bdc for the crystallization of polyMOFs.50The design of MOFs by using polymers as organic ligands was initially pioneered by Cohen and coworkers in 2015 .42 The s) blocks . The cry domains .46 This gle step .48 In MOlisation .48 It waEG chain .48 Note 12,21,40 The spontaneous polymer insertion into MOFs has been reported from the melting state or the solution state.40 Different polymers in the melt state such as PEG, linear alkanes or poly(methylpropylsilane) were included in the nanochannels of MOFs.51–53 This insertion proceeds presumably through the extension of polymer chains from coil conformation. It is thermodynamically favoured since the entropic loss upon polymer uncoiling is compensated by enthalpy gain as a result of attractive electrostatic and van der Waals interactions between the MOF network and the polymers.40 Polymer insertion into MOF pores also takes place under diluted conditions as a result of a competitive adsorption process between solvents and polymers in MOFs. This solvent-mediated polymer insertion into MOFs was equally reported for biomacromolecules such as proteins, DNA and single-stranded RNA. The main objective of protein inclusion into MOFs is to stabilize the enzymes and promote their catalytic activity.54,55 It is worth noting that this approach requires a size matching between the polymer chains and MOF pores since only polymers having a gyration diameter much lower than the MOF cavities are likely to enter the pores. Therefore, another strategy involving the post-synthetic polymerisation of monomers in the MOF pores was also extensively deployed /polymer membranes for the development of ratiometric luminescence thermometers.56 Ln-MOFs suffer from different shortcomings such as poor processability and very low chemical stability due to the lability of the Ln–O coordination bonds in aqueous solutions and acid/alkaline media. A mixed-lanthanide MOF (i.e. EuxTbx1−BABDC x = 0.0025) with the BABDC organic linker terephthalic acid) bearing polymerizable sites was thus synthesised and then copolymerised with butyl methacrylate monomers under UV and in the presence of a photo-initiator.56 This photopolymerisation process that occurs at RT under solvent-free conditions, allowed the formation of free-standing membranes with good mechanical properties. Remarkably, this MOF-polymer membrane retained the photoluminescence properties of the pristine EuxTbx1−BABDC MOF and behaved as an excellent luminescent thermometer capable of sensing temperature in the 90–240 K range.56 In contrast to the strong degradation of the pristine MOF upon water exposure, this membrane combined chemical stability and its performance in temperature sensing was not altered under harsh conditions such as high humidity and acid/alkaline media.56 D. Zhao et al. took advantage of the temperature-dependent coil-to globular conformational change of poly(N-isopropylacrylamide) (PNIPAM) to obtain thermo-responsive MOF–polymer adsorbents towards water  and PNIPAM with different PNIPAM loadings.57 First, the synthesis and activation of MIL-101(Cr) were followed by the encapsulation of NIPAM in the pores. The free-radical polymerization of NIPAM was carried out using azobisisobutyronitrile (AIBN) as the initiator.57 The full characterisation of these composites by multiple techniques was consistent with the accommodation of the polymer chains in the mesopores of MIL-101(Cr) while molecular modelling could reveal that the confined polymer chains are preferentially located in the large mesopores and mostly interact with the MOF backbone through hydrogen bonds.57 The water adsorption properties of such MIL-101(Cr)/PNIPAM composites were characterised by recording sorption isotherms at 25 °C and 90% relative humidity (RH). Notably, the water uptake of the MIL-101(Cr)/PNIPAM-1 composite with the lowest PNIPAM content was higher than that of the other composites due to a partial clogging of the MOF pores by the polymer chains upon increasing the PNIPAM loading.57 The adsorption performance of MIL-101(Cr)/PNIPAM-1 was finally tested in a humidity chamber at 96% RH and 25 °C. Remarkably, the MIL-101(Cr)/PNIPAM-1 composite was able to capture approximately 440 wt% of water under those conditions while the parent MIL-101(Cr) and pure PNIPAM only exhibited 110 and 74 wt% water uptakes respectively (57 The low water capacity of the pure PNIPAM and MIL-101(Cr) can be respectively attributed to the non-porous character of the polymer and the hydrophobic nature of the MOF. Finally, the phase transition of PNIPAM in MIL-101(Cr)/PNIPAM-1 was shown to trigger the release of about 98% of the captured water under mild conditions (40% RH and 40 °C)   were selected due to their high efficiency as catalysts for the hydrolytic degradation of CWAs. In the first step, a polymerisation initiator , ACPA) was post-synthetically attached to the Zr-oxo clusters and in the second step, the free-radical polymerisation of vinyl monomers was carried out using these ACPA modified MOFs  catalytic sites of a Cu MOF -1,3,5-triazine) to prepare a novel Cu-TDPAT/polyazodianiline(PAD) catalyst for biomass conversion.60 After the loading of 4,4′-azodianiline monomers in the MOF pores, their subsequent reaction with CO2 molecules was catalysed by Cu2+ centres of the MOF and this led to the formation of short oligomeric chains confined in the pores of the MOF.60 Such oligomers consisted of azodianiline moieties bridged by urea linkages. Such composites were used as a catalytic support for the immobilisation and stabilisation of Pd NPs.60 This ternary Cu-TDPAT/PAD/Pd catalyst showed a high catalytic activity for the reductive amination of levulinic acid with a high recyclability.60 Such performance was partially explained by the dual stabilising effect of the polymer on both the MOF and Pd NPs. In numerous polymer-inside composites such as those presented in this part, the polymers were demonstrated to enhance the physico-chemical properties of MOFs  or impart novel functions . We would like also to stress that polymers can also alter the phase diagram of MOFs. This was previously reported for flexible MOFs. This class of MOFs is characterised by structural transformations between different metastable phases that can be triggered by different stimuli .61 This dynamic response of MOFs not only offers advantages in the selective adsorption of analytes for separation applications but also paves the way for the development of efficient sensing and delivery devices. The work reported by T. Uemura et al. illustrates the possible impact of polymer guests on the flexibility behaviour of MOFs.62 The flexible [Co22(bpy)]n can exist in two forms: the activated MOF is a non-porous material that can be converted to a porous analogue upon solvent uptake. It has been shown that the incorporation of polystyrene in the MOF pores has stabilized the open phase of the MOF.62 The introduction of polystyrene into a Zn pillar-layered MOF has also been shown to alter the phase transformation of the MOF upon thermal treatment.63 Under those conditions the pure MOF evolved to a dense phase as a result of the pillar removal. In contrast, the thermal annealing of the MOF/polystyrene composites led to the formation of a turbostratic phase in which the Zn square-grid layers are threaded by polystyrene chains.63 These examples show that the assembly of MOFs with polymers can either increase the thermodynamic stability of MOFs or induce the formation of novel phases. A synergism between MOFs and polymers was thus observed in numerous polymer-inside composites presented in this section. The polymers not only impart mechanical properties and processability but could also enhance the chemical stability of MOFs and alter their dynamic and transport properties. Another class of MOF-based composites was developed relying on the integration of preformed MOF particles in different matrices as developed in the next two sections.Numerous studies reported the construction of functional composites through the integration of polymers in the pores of MOFs. A wide diversity of strategies was explored such as the infiltration of polymers in the MOF pores or the polymerisation of monomers from initiator-functionalized ligands within MOFs .12,21,40deployed . This geds water .57 Upon ectively .57 The ld 40 °C) .57 In ads (CWAs) .58 Theseied MOFs .58 The itimulant .58 Otherin situ polymerisation of macromolecules in the pores of MOFs, as described in the precedent section is a reliable approach that led to the formation of functional composites. However, the inherent decrease in the surface area of MOF can have a detrimental effect on the transport or catalytic properties of MOF-based composites. Therefore, numerous composites based on MOFs were prepared through the blending of preformed MOF particles in different matrices ((bio)polymers, graphene, graphene oxides, carbon nitrides, etc)  (PEG)) was envisaged in biomedicine to improve their colloidal stability in physiological media and the pharmacokinetic profile of drugs.71–74 However, in spite of the improvements achieved in these pioneering studies, this strategy often involved physical adsorption of polymers and thus precluded their strong attachment at the surface of MOF NPs, thereby leading to a low or non-controlled amount of polymer, a partial release of polymers in solution or a possible partial accommodation of polymer chains in the pores of MOFs. Therefore, a selective covalent grafting of a thin organic or polymeric layer at the outer surface of MOFs with a good control of the surface density of functional groups was considered as the most appealing surface modification of MOF NPs. Note that this surface modification of MOFs is challenging since it should be restricted to the external surface of MOFs while keeping intact the internal surface of MOFs and preserving their crystallinity and porosity. However, the difficulty relies also on the surface chemistry of MOFs which is inherently heterogeneous as a result of the presence of organic and inorganic building units, vacant coordination sites, surface defects and coordinating solvent molecules.74 Two covalent routes namely grafting-to and grafting-from, were extensively explored for the synthesis of polymer-grafted MOFs.12,21 The principle of the grafting-to approach consists of using presynthesised polymers with chain-end functional groups able to react with complementary reactive groups present at the MOF surface. This strategy thus took advantage of covalent interactions involving externally exposed functional sites of MOFs such as coordination unsaturated sites (CUS) or functional groups of the organic linker.75 Terminal phosphate-modified biomolecules such as cyclodextrins, nucleotides or lipids were covalently attached at the surface of MOF NPs through metal–phosphate coordination bonds, leading in some cases to a significant improvement of the colloidal stability of MOF NPs.76–79 Another strategy consists of introducing active unsaturated or nucleophilic functions in the organic linker that can be coupled to specific functions of polymers through carbodiimide-mediated amidation or click chemistry.80–82 As an example, R. S. Forgan and coworkers reported a selective two-step PEGylation approach of MOF NPs.82 UiO-66(Zr) NPs were first synthesised with azide functionalised modulators to control their particle size and their surface functionality. The covalent coupling of azide-functionalised UiO-66(Zr) NPs with propargyl-PEG chains was then performed by “click” chemistry through copper(i)-catalysed azide-alkyne cycloaddition.82 This PEG surface modification of UiO-66 NPs not only conferred an improved chemical and colloidal stability under physiological medium conditions but was also able to enhance the drug release kinetics and induced a pH-responsive release of the drug.82 Our group reported the synthesis of PEGylated ZIF-8 NPs through the Graftfast strategy.83 Graftfast® also termed the diazonium-induced anchoring process is a versatile functionalisation approach that was developed to covalently graft a wide range of polymers with a defined thickness onto a large series of materials .84,85 Moreover, this simple and single-step process occurs at RT and in water without any pre-modification of the MOF. It can thus be considered as a biocompatible, sustainable and scalable polymerization approach. Acryl-PEG chains of different lengths (480 Da and 5 kDa) were grafted onto the ZIF-8 external surface using GraftFast  that have shown high performance in isopropanol dehydration by pervaporation.83 PEGylated MIL-100(Fe) NPs were also prepared by GraftFast and have shown high chemical and colloidal stability in biofluids while exhibiting a high adsorption capacity of drugs.86 In general, the grafting-to approach has the advantage of using polymers that were fully characterised before their reaction with MOFs but can lead to polymer-grafted MOF NPs with a low polymer density. Alternatively, the robust “grafting from” polymerization route was also developed to grow polymer chains from pre-tethered active sites at the MOF surface.87,88 Such a method involves the reactivity of an initiator-carrying linker from which the polymer is grown. This method has the advantage of producing MOFs grafted with highly dense polymer brushes but their characterization is a complicated process requiring the degradation of the modified particles and the subsequent isolation and characterisationof the polymers. Moreover, in numerous cases, this strategy cannot prevent the inevitable polymer insertion in the MOF pores as a result of the presence of the initiator located on the MOF framework. A third type of polymer grafting strategy referred to as “grafting through” was also used and consisted of functionalizing the MOF surface with a monomeric group that was then incorporated into a growing polymer chain. Recently, nylon 6,6 was covalently attached to UiO-66-NH2 under mild conditions.89 Although nylon 6,6 is one of the most widely used polymers, especially in the textile industry, its covalent coupling with MOFs under mild conditions was rarely reported. In this example, a post-synthetic modification of UiO-66-NH2 with adipolyl chloride followed by an interfacial polymerisation step with the hexamethylene diamine monomers led to the incorporation of the MOF into the growing polyamide fibers.89 Such composites presented a good catalytic activity for the degradation of a chemical warfare stimulant. Although this interfacial polymerization is a mild and scalable method, it is only limited to amine-based MOFs and cannot be applied to any MOF structure. In most of the previous studies, the grafting-from and grafting-through strategies were mainly applied to radical polymerizable polymers  that are not suitable to address several MOF/polymer compatibility issues. Ideally, a good physicochemical matching between MOFs and a polymer matrix could be achieved by using polymer grafts that are chemically similar to the polymer matrix. However, in the field of gas separation, MMMs are mainly based on a limited number of condensation polymers such as polyimide (PI), polysulfone, polycarbonate and polymers of intrinsic microporosity (PIM) and the surface modification of MOFs with these polymers is still highly challenging so far. The group of T. Li reported different generalizable and non-covalent strategies to modify the surface of MOFs.90–92 In a first approach, a series of MOF–polymer composites was prepared through surface-initiated atom transfer radical polymerisation (SI-ATRP).90 However, in contrast to the traditional SI-ATRP process, the methodology followed by T. Li et al. did not imply the pre-anchoring of the ATRP initiator on the MOF surface but relies on the deposition of a macroinitiator at the surface of MOFs NPs (90 A random copolymer bearing multiple carboxylic acid groups and bromoisobutyrate (BiB) functional groups was thus coated at the surface of MOF NPs. The presence of the carboxylic acid group promoted the formation of inter-chain hydrogen bond cross-linking at the surface of MOFs , ZIF-8(Zn), ZIF-67(Co), MIL-96(Al), and MIL-101(Cr)), thereby demonstrating the generalisation of this approach. Moreover, through a judicious selection of the monomer, it was possible to finely tune the hydrophobic–hydrophilic balance of the MOF.90 Another approach reported by T. Li et al. relies on a coordinative crosslinking between MOFs modified by a metal–organic nanocapsule (MONC) and polymers  that were shown to coordinatively crosslink a large series of polymers through their exposed Cu open metal sites.93 Compared to covalent grafting, this coordinative crosslinking approach is more generalizable since Cu sites can interact with any polar coordinative functions such as hydroxyl, carboxylate, sulfonyl, etc. For that purpose, the surface of UiO-66-NH2 (Zr) NPs was first decorated with PgC5Cu as a result of attractive electrostatic interactions.92 The mixture of the PgC5Cu modified MOF with a polyimide (6FDA-DAM) under moderate heating induced the cross-linking of the polymer chains at the surface of MOF leading to the formation of a 10 nm thick polymer coating.92 This method did not alter the porosity of UiO-66-NH2 as the large size of PgC5Cu prevented it from entering the MOF pores. Moreover, this strategy could be extended to other MOFs  and also other condensation polymers commonly used in gas separation .92 As previously explained in this section, the integration of MOFs into condensation polymers is still facing compatibility issues and this strategy was revealed efficient for this class of polymers. The dispersion of polymer grafted MOFs into polymer matrices was finally assessed by TEM observation on ultramicrotomed samples  framework as the covalent anchoring points to graft PI brushes at the MOF surface.91 However, this method has the disadvantage of significantly reducing the MOF porosity which is not always desirable depending on the targeted application. In another approach reported by J. Rzayev et al., the interfacial properties of MOFs with PI were improved through the post-synthetic ligand exchange between MOF crystals and a poly(amic acid) (PAA).94 Half of the repeating units of this polymer contained the bdc units that are commonly used as an organic linker in MOF structures. A PAA/MOF-5 composite was easily formed by mixing a suspension of MOF-5 NPs with a solution of PAA in DMF.94 The hybridisation process first involves the exchange between surface MOF ligands and the PAA chains, leading to the coating of PAA chains at the surface of MOF-5 NPs. The interaction of polymer chains with different MOF crystals was prone to induce a cross-linking of MOF crystals through a PAA matrix.94 This approach allowed the preparation of highly porous composites with a high MOF loading exceeding 80 wt%. The rate of the reaction was shown to strongly depend on the number of exchangeable ligand sites at the MOF surface and thus on the MOF particle diameter.94 Moreover, this process was successfully extended to ZIF-8(Zn) but unfortunately, the synthesis of composites failed for UiO-66, presumably as a result of the high stability of the Zr–ligand bond and the low exchange rate with PAA.94The insertion or es, etc) . The most matrix . These traftFast .83 This al bonds . The proal bonds .83 The fMOFs NPs .90 A ran of MOFs . Subsequmposites .90 This polymers .92 Indee samples .92 As exposition .92 In anTo conclude this section, a high diversity of synthetic strategies was reported to functionalize the outer surface of MOF particles and optimize the compatibility and loading of MOFs with polymers and carbon-based materials. etc. For the vast majority of the composites reported so far, their microstructure can be described as a random distribution and orientation of MOF crystals in the host matrices. However, the spatial organisation and nanostructuration of MOF-based composites can strongly impact their physicochemical properties in adsorption, catalysis and detection. As an example in the domain of MMMs for gas separation, isotropic MOF fillers are often randomly oriented in the polymer matrix and this can present a detrimental effect on the MOF loading and the gas transport properties.64 Indeed, for numerous MOF crystals, the channels/pores are not parallel to the gas diffusion direction. The hierarchy of MOF-based composites at the scale of porosity, morphology and composition is thus crucial for their performance in applications. This section will be devoted to the approaches proposed to control the morphology, spatial distribution and organization of MOF NPs within the host matrices.As described in the precedent section, valuable strategies for MOF surface functionalisation were developed to enhance the interfacial properties of MOFs with different matrices such as polymers, graphene, graphene oxides, i.e. 1,2-dipalmitoyl-sn-glycero-3-galloyl, DPGG) (95 This strategy was successfully applied to a large series of imidazolate or carboxylate-based MOFs (i.e. ZIF-8(Zn), ZIF-67(Co), UiO-66(Zr), MIL-88A(Fe), HKUST-1(Cu), MIL-101(Cr), CPP-3(In), and MIL-96(Al)). According to PXRD and N2 porosimetry, the crystallinity and porosity of MOFs are preserved after surface functionalization.95 As a result of the very high colloidal stability of the DPGG-modified MOF particles in the non-polar solvent toluene, MOF monolayers could be processed through evaporation-induced interfacial self-assembly  and 3D arrays can allow the integration of MOFs into well-defined periodic structures such as metamaterials, plasmonic or photonic-based sensors. As an example, it was possible to form a monolayer of ZIF-8 particles organized into a close-packed hexagonal array with a 〈111〉 crystal orientation.95 Compared to other MOF assemblies prepared by drop-casting, ZIF-8 NPs were more densely packed as a result of the clustering of hydrophobic MOF particles at the toluene/air interface. After toluene evaporation, the array was transferred to a water surface, allowing the processing of the freestanding film. Similarly, a hexagonal close-packing array of UiO-66 particles was obtained (95 Freestanding ZIF-8 films supported on the poly(methylmethacrylate) (PMMA) support were also prepared by assembling DPGG-modified ZIF-8 with lipid-soluble PMMA added to the toluene/MOF solution.95 This assembly into 2D MOF arrays resulted from the phase separation of hydrophobic MOFs at the solvent/air and polymer/air interfaces.The group of C. J. Brinker reported a versatile functionalisation of the outer surface of MOFs through a phase transfer process involving the strong coordination of the surface metal centres of MOFs by the galloyl head groups of a lipid molecule   left.95 assembly  right.95obtained  right.9565 Such membrane configuration with a very thin selective layer (less than 1 μm) is required to optimize the membrane performance since the porous polymer layer can increase the gas flux and thus the gas permeability while the MOF–polymer layer can enhance the selectivity of the membrane.65 One of the main methods to manufacture asymmetric membranes is the phase inversion method. In this protocol, a MOF–polymer dispersion is initially cast onto a support and then immersed in a solvent inducing the precipitation of the polymer.65 As an example, Zhu et al. prepared asymmetric MMMs by assembling functionalised MIL-53(Al) particles and polyetherimide Ultem® 1000 polymer matrix.96 The post-functionalisation of MIL-53(Al) with aminosilane was required to enhance the compatibility between the MOF and the polymer.96It can also be of great interest to prepare MOF/carbon material composites in which MOF particles are heterogeneously distributed allowing the processing of asymmetric composites with a composition gradient. Such an approach has been deeply explored for the preparation of asymmetric MMMs for gas separation. These membranes typically consist of thin selective MOF–polymer layers supported on a nonselective porous polymer support.97 These membranes were prepared through a coordination-driven replacement procedure which consisted of using a preformed ionically cross-linked poly(ionic liquid) (PIL) membrane as a template and precursor for the processing of HKUST/PIL membranes   particles embedded in the PVDF (PVDF = poly(vinylidene fluoride)) host matrix  crystals (98 The high flexibility of the MIL-88A(Fe) upon water adsorption was herein exploited to induce actuation in the etched MIL-88A(Fe)/PVDF films.98 Indeed, such patterned composite films exhibited a tunable self-folding response driven by water adsorption. Such responsive composite films could be used as autonomous soft mechanical devices in different applications such as micromanipulation and robotics.98Hierarchy in the MMMs was not only created in their porous structure but also in the spatial distribution and size of MOF NPs. Indeed, the group of J. Yuan reported asymmetric HKUST-based MMMs exhibiting a gradient profile in their pore size, the diameter and distribution of MOF particles along the cross-section of the membranes.embranes .97 First (H3btc)  left. ThL chains  left.97 membrane  right.97t matrix .98 The s vapours .98 Discrcrystals .98 The h99 First, two MOF–polymer dispersions were deposited side-by-side at two different locations of the same substrate, leading to the formation of a continuous film after the solvent evaporation  to the MOF/PVDF suspension prior to casting of the MMM.99 To avoid any significant volume variation of the first layer, a pre-swelling of the PVDF within the first layer was required prior to the deposition of the second layer. This strategy led to the processing of two-layer and three-layer MOF MMMs with a vertical distribution of MOFs within the single PVDF matrix.99 Finally, the potential of mixed and layered MMMs as catalytic membrane reactors for chemical transformations was finally evidenced.99The introduction of different MOF particles within a single polymer matrix is also a valuable approach for the spatial compartmentalisation of different MOF catalysts within a single catalytic membrane. Such membranes may be used in flow system reactors for cascade reactions or as a multifunctional catalyst able to degrade a variety of different species in an industrial waste stream. In this context, the group of S. M. Cohen explored different strategies to prepare multi-MOF membranes with different microstructures and spatial distribution of MOF NPs .99 Firstporation  top. Thetwo MOFs  middle. pensions  bottom.964,100 This strategy thus relies on the design of thin MOF nanosheets prior to their assembly with polymers. Top-down exfoliation and bottom-up strategies were reported for the synthesis of MOF nanosheets.64 Top-down exfoliation involves mainly the delamination of bulk layer-structured MOFs through different methods including sonication-assisted exfoliation, and mechanical or chemical exfoliation. These methods are thus restricted to layered-structured MOFs and the resulting exfoliated nano-sheets present generally structural defects as a result of the strong external forces applied during the exfoliation process. Moreover, since the exfoliation step is quite complex, costly and time-consuming, the large scale production of MOF nano-sheets is impeded by this strategy. In contrast, the direct synthesis of MOF nano-sheets by bottom-up approaches is highly suitable for their large-scale production.64 Various bottom-up synthesis methods were reported including interfacial synthesis, sonication-assisted synthesis, three-layer synthesis or surfactant-assisted synthesis. One of the pioneering studies in this field was reported by J. Gascon and coworkers who developed Cu-bdc nanosheets through a three-layer synthesis.101 Cu-bdc nanosheets with a lateral dimension of 0.5–4 μm and a thickness of 5–25 nm were prepared by using three solvent layers containing a mixture of two miscible solvents of different ratios (DMF and acetonitrile).101 The nucleation and growth of Cu-bdc nanosheets took place at the interface between one solution containing the Cu source and another containing the H2bdc organic linker.101 Cu-bdc was selected as the parent bulk MOF exhibiting a layered structure. These Cu-bdc nano-sheets were then combined with the Matrimid polyimide for the processing of MMMs (i.e. ns-Cubdc@PI).101 As clearly shown by tomographic focused ion beam scanning electron microscopy (FIB-SEM) images, the Cu-bdc nano-sheets were homogeneously dispersed and remarkably, they were preferably oriented parallel to the basal plane of the membrane, thereby exposing their pore system in the direction of the gas flux.101 This configuration significantly enhances the spatial distribution of such nanoparticles in the polymer matrix in comparison to MMMs prepared by using isotropic Cu-bdc nanocrystals (i.e. nc-Cubdc@PI). As a consequence of this oriented microstructure, the CO2/CH4 separation selectivity of ns-Cubdc@PI MMMs is 30–80% higher than that of the pure polymer membrane and 60–140% higher than that of nc-Cubdc@PI MMMs.101 Similarly, MMMs based on 4 wt% Cu-bdc nano-sheets and highly permeable polyimides (6FDA-DAM and PIM-1) have shown a significant increase of the CO2/CH4 selectivity in comparison to the pure polymers but at the expense of a strong decrease in CO2 permeability.102,103 It was thus highly desirable to design other MOF nano-sheet fillers with inherently enhanced gas separation performance. Recently, the group of M. Eddaoudi developed a series of highly efficient MMMs based on (001)-oriented AlFFIVE-1-Ni nanosheets with a MOF loading up to 60 wt% for the CO2/H2S/CH4 separation (104 AlFFIVE-1-Ni exhibits a three-periodic framework constructed by assembling a square-grid layer based on Ni2+ cations and the pyrazine ligand connected by [AlF5(H2O)]2− anions in the third direction  nano-sheets with a lateral dimension of 0.5–4 μm and a thickness of 20–50 nm ]2− unit, moderate temperature and choice of solvents) of the synthesis.104 MMMs based on AlFFIVE-1-Ni nano-sheets and 6FDA-DAM were then prepared by solvent casting under conditions of slow solvent evaporation, thereby inducing the in-plane alignment of nanosheets in the polymer matrix  was successfully reported through a one-step modulated synthesis in solution.107 Such nano-sheets were used for the preparation of ultrathin (20 nm) and highly oriented NUS-8 membranes in a large area (>200 cm2) that demonstrated a high performance for liquid separation and desalination.107 Note that oriented and polycrystalline MOF membranes (without any polymer) were also extensively studied in the past few years but this topic is not in the scope of the present article and can be found in a relevant review.64In the past few years, different groups have developed highly oriented and layered composites by tuning the morphology of MOF nanocrystals. As an example, several MMMs composed of 2D MOF nanosheets with a pore structure parallel to the gas pathway were shown to exhibit excellent gas separation performance.paration .104 AlFFirection .105,106 20–50 nm .104 Thisr matrix . This rer matrix .104 This108 This PCP (labelled MIL-178(Fe)) exhibited interesting properties for CO2 separation such as high hydrothermal stability, ultra-micropores and polar OH groups acting as CO2 adsorption sites.108 This material was synthesised as sub-micrometer-sized platelets through an easily-scalable and environmentally friendly protocol without using any toxic reactant.108 MIL-178(Fe)/Pebax®-3533 MMMs with a MIL-178(Fe) loading up to 25 wt% have shown an enhanced CO2/N2 separation performance in comparison to the pure Pebax®-3533. Such properties are likely due to the homogeneous dispersion and preferential orientation of MIL-178(Fe) fillers in the polymer matrix, the absence of any interfacial defects, the increase of the CO2 solubility and the lower crystallinity degree of the Pebax matrix in comparison to the pure polymer.108Our group has recently reported MMMs exhibiting a layered microstructure due to the preferential orientation of porous coordination polymer (PCP) platelets.in situ synthetic approach in which the nucleation and growth of the MOF occurred in the presence of the organic template. Diverse organic templates such as (bio)polymers, graphene or graphene oxide (GO) were previously used to template MOF materials since they possess a large variety of coordination functions and can spontaneously self-assemble into different aggregates such as micelles, nanostructures or mesophases. GO is the oxidised form of graphene bearing different oxygen functional groups such as epoxy and hydroxy groups on the basal plane and carboxylic acid functions on the edges of the sheet. As an example, X. Zhang and coworkers reported highly oriented and ultrathin MOF membranes consisting of 2D nanosheets supported on a porous tubular substrate.109 A thin layer of GO was employed to control and guide the oriented growth of MOF nanosheets, due to attractive interactions at the MOF/GO interface.109 Graphene-derived materials were also shown to direct the growth of 1D MOF nanostructures such as nanowires (NWs) and nanotubes. The pioneering work reported by M. Jahan and coworkers demonstrated that the in situ formation of MOF-5(Zn) in the presence of benzoic acid functionalised graphene (BFG) led to composite nanowires.110 Each single NW consisted of stacked MOF-5 nanorods intercalated by BFG nanosheets regularly distributed along the NW as shown by TEM and micro-Raman spectroscopy.110 The BFG sheets did not only act as a structure-directing agent of MOF-5 favouring the growth of MOF-5 in a direction perpendicular to the BFG basal but they were also integrated as a component of the NW.110 Our group has also reported the synthesis of Al3+ dicarboxylate MOF (that is MIL-69(Al)) NWs by using GO nanoscrolls as a structure agent  NWs were synthesised in situ under reflux by mixing the precursors of MIL-69(Al) with GO as a co-reactant. This MIL-69(Al)/GO composite consisted of an assembly of GO nanoscrolls and MIL-69(Al) NWs with lengths up to 2 μm and an average diameter of 70 nm as shown by combining multiple techniques  (111 According to selected-area electron diffraction (SAED) (3+ octahedra in the structure of MIL-69(Al).111 By coupling various ex situ characterisation tools and molecular modelling, it was shown that the anisotropic growth of these MOF NWs occurred in the inner cavity of the GO nanoscrolls and was certainly driven by covalent interactions between surface functional groups of MIL-69 (Al) and oxygen groups of GO (i.e. hydroxy or carboxylic acid functions).111 This material thus resulted from a mutual recognition between GO and the MOF: GO guided the 1D growth of the MOF while the MOF induced the structuring of GO. A mechanism for the formation of this composite was proposed involving mainly three steps: (i) the nucleation of MIL-69(Al) NPs and the concomitant rolling-up of GO sheets, (ii) the growth of primary small aligned MIL-69(Al) NWs in the inner cavity of GO nanoscrolls and (iii) the lateral fusion of MIL-69(Al) NWs into one single crystal MIL-69(Al) NW through the Ostwald ripening process and/or oriented attachment mechanism /GO composite exhibited a semi-conducting behaviour although MIL-69(Al) is an insulating material. This indicated that the distribution of MOF NWs within GO did not disrupt the carrier traveling through the extended conjugated network in the GO sheets. This strategy of using GO nanoscrolls as MOF templates can thus be considered as a valuable process to homogeneously disperse MOF nanocrystals into GO matrices without any MOF aggregation while imparting good electron transport properties. Note that other carbon-based materials were used to template the formation of 1D nanostructures and superstructures based on MOFs and the reader can find a scope of this topic in a recent review.112 A large diversity of MOF/GO composites were prepared to circumvent a few drawbacks of MOFs including poor electron or thermal conductivity, low processability as well as a moderate chemical or thermal stability for some of them.112 Such composites were thus explored for different applications including supercapacitors, batteries, gas storage and catalysis. As an example, C. Serre and co-workers developed MOF/GO composites as adsorbents for CO2 capture.113 Although numerous MOFs present very interesting CO2 adsorption properties, they suffer from low thermal conductivity and thus slow desorption kinetics. This is critical for their application in the thermal swing adsorption process which requires a uniform and fast heating of the adsorbent during the desorption step.113 In this work, MOF/GO composites were thus explored for CO2 capture by microwave swing adsorption (MSA). Indeed, it was previously demonstrated that microwave irradiation can induce a faster CO2 release compared to conventional heating. MOF/GO composites were thus designed to combine the good CO2 adsorption properties of MOFs with the high electrical conductivity of GO. The Ti(iv) bisphosphonate MOF (i.e. MIL-91(Ti)) was selected due to its high CO2/N2 selectivity, good CO2 adsorption capacity and high hydrothermal stability.114 Two types of structurally different MIL-91(Ti)/GO composites with different MIL-91(Ti) loadings were prepared. First, MIL-91(Ti)/GO composites were prepared by a post-synthetic route involving the assembly of preformed MIL-91(Ti) NPs with GO sheets while a second series of composites were prepared using a one-step in situ approach.113 The objective was to compare the impact of the microstructure of such composites on their physicochemical properties. Only MIL-91(Ti)/GO in situ composites with a MOF loading as low as 5 wt% were very promising for CO2 capture by MSA.113 Indeed, such composites combined a high porosity, a high CO2 adsorption capacity and a semi-conducting behaviour while the MIL-91(Ti)/GO post-synthesis composites and the pristine MIL-91(Ti) were insulating. This indicated that both types of composites exhibited a distinct microstructure in which the MIL-91(Ti) NPs are spatially differently arranged on the GO sheets.113 According to the characterisation of the MIL-91(Ti)/GO in situ composite by combining XPS and TEM, it could be assumed that the nucleation and growth of MIL-91(Ti) NPs in such composites took place on the sp3 regions of GO sheets bearing coordinative oxygen functions that could interact with the Ti centres of the MOF.113 This nanostructuration of MIL-91(Ti) NPs on the GO sheets did not interfere with the electron hopping of GO taking place between the sp2 regions. This work highlights the importance of the strategy used to design a MOF-based composite.Alternatively, the control of the MOF nanocrystal morphology was achieved by using organic templates that not only guided the crystallisation of the MOF NPs but also imparted processability and mechanical properties to the composites. Such composites were often prepared by the re agent .111 We tn (SAED)  right, iechanism .111 We h49 It is noteworthy that the dimension of MOF NPs is determined by a complex interplay between equilibria such as the organic linker deprotonation, the modulator deprotonation as well as the metal complexation.115 At high concentrations in the reaction medium, the addition of a molecular modulator can limit the deprotonation of the organic linker by decreasing the pH and its competition with the organic linker for metal coordination can allow a kinetic control of the MOF crystallisation, thereby leading to MOFs with large particle size.49,115 Alternatively, its introduction in the reaction medium can also reduce the size of MOF particles by acting as a surface capping agent.49,115 Numerous examples have shown the efficiency of monocarboxylic acids to control the diameter of UiO-type MOFs, and the addition of such molecules affects also the amount of structural defects, the porosity and eventually the hydrophobic–hydrophilic balance of such MOFs, thereby altering their adsorption or catalytic properties.116 Similarly, it was reported that polymers and biopolymers could exert the role of coordination modulators in the morphogenesis of MOFs as explained previously.48 Very recently, we have shown that gelatin can act as a modulator for the formation of UiO-66(Zr) NPs.117 Such MOF NPs were prepared by using an in situ approach in which the nucleation and growth of MOF NPs occurred in the presence of gelatin.117 Monodisperse and well-defined octahedral UiO-66(Zr) crystals with a diameter of 350 nm were synthesized in the presence of gelatin  could exert a remarkable role of crystal modulator and could induce the formation of MOF mesocrystals  consisted of one poly(methacrylic acid) block containing complexing carboxylic acid functions prone to coordinate metal ions and one solvating poly(ethylene oxide) block that can trigger mesoscale assembly.120 The effect of this polymer was studied with a 3D MOF, [Zn2(bdc)2dabco] 122 that can crystallise into a hexagonal metastable kinetic phase 1 h or a tetragonal thermodynamic phase 1t. Owing to its inherent metastable character, the control of both the dimension and morphology of 1 h has been rarely reported so far. Anisotropic hexagonal rod mesocrystals (mc-1h) with an average length and diameter of 2.9 and 0.53 μm were thus formed by adding PEO68-b-PMAA8 as a co-reactant into the reaction medium of bulk 1t 2dabco]. According to the time-dependent TEM and SEM experiments, the growth mechanism of mc-1h was proposed . Initially, the first generation of composites consisted of randomly distributed MOF particles within the matrices with a low to moderate amount. Through an adequate functionalization of MOF particles, MOF-based composites could be processed with a high MOF loading (>30 wt%). More recently, a second generation of hierarchically structured composites was developed through precise control over the loading, morphology and spatial positioning of MOF particles within the matrices. These materials were mainly explored in gas/liquid separation and catalysis. The next section will be focused on hierarchically porous MOF composites.To conclude this section, a high diversity of MOF-based composites were prepared through the blending of MOF particles into a wide range of matrices ((bio)polymers, graphene, graphene oxide, carbon nanotubes, 123,124 In addition to their intrinsic microporosity, these HP MOF frameworks show the peculiarity of containing an array of meso- and/or macropores in their framework. The construction of MOFs with intrinsic hierarchical porosity is outside the scope of this article and the reader can refer to recent reviews.123–125 In comparison to pure HP MOFs, the assembling of MOFs with (bio)polymers or carbon-based materials can introduce multiple inter-connected porosities over lengths in the micro-, meso-and macroporous regimes that can be located in the MOF framework, in the carbon-based matrix or at the MOF/carbon material interfaces. Different complex fluids polymer solutions, micellar solutions, colloidal suspensions, gels, foams and emulsions) were used not only to process MOF based composites in different shapes  but also to impart a hierarchical porosity (126 These complex fluids often present non-Newtonian fluid properties under an applied deformation (e.g. shear) including viscoelasticity, yield-stress, and shear-thinning viscosity that could be exploited for the processing of composites.MOFs have shown great potential in applications related to gas/liquid separations, sensing or catalysis but these applications are generally limited to the adsorption, conversion or detection of small molecules as the majority of these materials are microporous. This range of pore size may decrease mass-transportation rates, limit the diffusion of substrates to catalytically active sites and finally prevent the processing of MOF composites embedding large components such as nanoparticles or proteins. To extend the porosity of MOFs in the mesoporous regime (pore size 2–50 nm), hierarchically porous (HP) MOFs have gained significant attention in the past few years.porosity .126 Thes127 These composites were prepared through a non-solvent-induced phase separation. In this procedure, acetone as a non-solvent of PVA was added to an aqueous suspension containing PVA and preformed MOF particles (i.e. MIL-101(Cr) or Basolite A 520 ).127 Once the liquid–liquid phase separation was completed, it was possible to obtain a series of MIL-101(Cr)/PVA and Alfum/PVA composites with different MOF loadings.127 Composites with a MOF loading up to 80 wt% presented a high crystallinity and porosity according to PXRD and N2 porosimetry. For MIL-101(Cr)/PVA, the mesoporosity of the neat MOF was accessible to a large extent and almost 80% of the MIL-101(Cr) water uptake was retained.127 In contrast, Alfum/PVA composites lost partly the microporosity inherent in the MOF but exhibited mesopores presumably located at the MOF/PVA interface. Such mesopores led to an increase in the water adsorption capacity in comparison to the pure Alfum.127As an example, C. Janiak and coworkers developed MOF/PVA  monoliths for applications in the field of adsorption heat pumps and thermally driven adsorption chillers.128,129 In addition to the intrinsic porosity of the MOFs, additional pores can be formed either at the biopolymer/MOF interfaces or in the biopolymer matrix. Biomacromolecules have also the advantages of presenting a high density of functional groups on their backbone and self-assembly properties for which no synthetic polymers are currently available. Moreover, their large abundance, non-fossil origin, good biocompatibility and biodegradability are key features for the development of “green” composites. Composite hydrogels were synthesised through the concomitant gelation of alginate and the in situ formation of MOFs.130,131 Alginate is a polysaccharide and a linear copolymer consisting of random sequences or blocks of 1,4-linked β-d-mannuronate (M) and α-l-guluronate (G block) residues. It is well known that a sol–gel transition of this biopolymer can be triggered by adding divalent or trivalent cations to an alginate aqueous solution. The exchange of the Na+ cations by divalent cations such as Ca2+ on the G blocks can induce a cross-linking of the alginate chains. MOF–alginate composites were thus prepared by a two-step procedure.130 First, alginate hydrogels were prepared by cross-linking the biopolymer with different divalent or trivalent cations  used as precursors of MOFs (i.e. HKUST-1(Cu), ZIF-67(Co), ZIF-8(Zn), and MIL-100(Fe) respectively). These hydrogels were post-synthetically treated with the organic linker of MOFs, leading to a series of MOF/alginate hydrogels.130 These composites could be processed as spherical beads by adding droplets of alginate aqueous solutions into the metal aqueous solutions. Although these composites presented a good MOF loading, they suffered from moderate porosity as a result of the penetration of alginate chains in the MOF pores.130 Moreover, ZIF-8 and ZIF-67-based composites had low chemical stability in water due to the low affinity of the Zn2+ and Co2+ cations for alginate.130 An alternative synthetic approach of the HKUST-1/alginate hydrogel was recently explored for which the btc3− ligand was added to the Ca2+ exchanged alginate beads prior to the supply of Cu2+ cations.131 In comparison to the first strategy , the HKUST-1/alginate hydrogel presented a higher crystallinity due to the larger amount of HKUST-1 and an enhanced distribution of MOF particles in the alginate matrix.131 This composite has shown a good performance for the selective capture of dye. Among biopolymers, gelatin is one of the most widely used in industry due to its low cost and non-toxicity. Moreover, a wide range of nanocomposites were prepared by assembling gelatin with different inorganic materials oxides, and hydroxyapatite) and demonstrated a high efficiency in the field of regenerative medicine, biosensing or drug vectorization.132,133 The combination of MOFs with gelatin was recently explored for different bio-applications including tissue engineering, wound healing or drug delivery.119,134,135 The design of such MOF-gelatin hydrogels was mainly carried out through the blending of preformed MOF particles into either functionalized gelatin-based scaffolds or physical gelatin gels.119,134,135 Although such composites have shown promising properties, they present severe shortcomings such as low porosity and low MOF content which largely limits their practical application.119,134,135 Moreover, most MOF/gelatin hydrogels were synthesised by using MOFs with moderate stability while the processing of such composites with chemically robust MOFs is quite scarce. In this context, our group has reported a novel approach to design a series of chemically stable and highly porous MOFs–gelatin nanocomposites by taking advantage of the thermo-reversible behaviour of gelatin and a liquid–liquid phase separation, namely the coacervation process  or polyelectrolytes (complex coacervation) to the gelatin solution. In our work, the simple coacervation of gelatin was induced by adding ethanol (nonsolvent) to an aqueous solution of gelatin NPs previously reported141 was thus carried out by reacting the precursors of the MOFs (i.e. {Zr6} oxo cluster and the bdc2− ligand) with gelatin aqueous solution and ethanol   as well as the Zr4+ fumarate MOF (i.e. MOF-801) , homogeneous MOF distribution in the gelatin matrix and excellent MOF/gelatin interfacial properties  .117 Thisoperties  bottom.1nditions  bottom.1124,126 Such technologies were classically used for the processing of inorganic and organic macroporous aerogels.142 The principle of supercritical drying is to heat a wet gel at a temperature and pressure exceeding the critical temperature Tc and pressure Pc of the liquid entrapped  in the gel porosity. The liquid is thus transformed to a supercritical fluid with a low surface tension and this drying allows preservation of the porous texture of the wet material quite intact by avoiding the pore collapse phenomenon.142 Alternatively, freeze-drying was also widely employed to prepare hierarchically porous MOF-based composites. In this technology, the solvent entrapped in the gel is first frozen and thereafter dried by sublimation. The frozen solvent acts as a porogen to generate large pores in the material.142,143 A wide range of aerogels were developed by different groups through the combination of MOFs with diverse (bio)polymers or carbon-based materials .124,126,144,145 Their synthesis is generally based on the formation of a hybrid gel combining the MOF and carbon material followed by its supercritical drying or freeze drying. The hybrid gel can be prepared either by mixing preformed MOF particles or through the in situ synthesis of MOFs. In comparison to the direct mixing, the one-step in situ synthesis generally led to an enhanced distribution of MOF particles in the gel as previously explained. As an example, S. Kaskel and coworkers reported the synthesis of chitin–MOF composites with a hierarchical porous microstructure.146 Chitin is a chemically stable natural polysaccharide currently used for adsorption, filtration and water cleaning as well as bio-applications. Porous chitinous fibres extracted from a marine sponge were used as scaffolds to induce the in situ formation of the HKUST-1 MOF.146 This chitinous skeleton was thus exposed to a mixture of Cu2+ salt and the btc3− ligand under physicochemical conditions similar to the RT synthesis of the pure HKUST-1.146 This step was followed by a supercritical drying with CO2 as supercritical fluid. According to PXRD and SEM, it was shown that HKUST-1 crystals were formed inside the hollow chitin fibres.146 HKUST-1/chitin presented a high MOF loading (53 wt%) and a remarkable BET surface area (800 m2 g−1 for the composite vs. 18 m2 g−1 for the pure chitin) due to the presence of the MOF micropores as well as an array of inter-particle mesopores and macropores.146 This hierarchical porosity confers to this material good gas transport properties and a high adsorption capacity of ammonia.146 The group of C. Li developed aerogels by assembling TEMPO-oxidised cellulose nanofibrils  and divalent metal MOFs (ZIF-8 (Zn), ZIF-67(Co) and HKUST-1(Cu)).147 These materials combined the microporosity of MOF crystals with the bimodal meso- and macroporosities, processability and low density of cellulose aerogels.147 The advantage of using cellulose nanofibrils is related to their high surface area and the high density of carboxylate groups that can act as strong binding sites for MOF nucleation and growth. The preparation of such aerogels is based on the sequential processes of cellulose gelation, in situ formation of MOFs and freeze drying.147 As previously reported for alginate hydrogels, the sol–gel transition of cellulose proceeds similarly through cation-driven cross-linking of cellulose. The metal-chelated cellulose hydrogels were solvent-exchanged with methanol and finally exposed to the organic ligand, thereby leading to the nucleation and growth of MOFs.147 After freeze-drying, the MOF/cellulose aerogels exhibited a fibrous and multimodal porous structure.147 Due to their good mechanical properties, this composite could be shaped in different forms such as monoliths, tablets, etc… depending on the target application. They have shown a high adsorption kinetics and adsorption capacity for organic dyes.147 One of the main limitations of the preparation of aerogels through the in situ MOF synthesis is that it essentially applies to MOFs that can be obtained under mild ambient conditions. Indeed, such conditions are compatible with the chemical stability and gelation of the carbon based matrix. In general, solvothermal conditions used for the synthesis of MOFs are not applicable since they can lead to degradation and/or precipitation of the carbon based material. Moreover, this strategy is also restricted to chemically robust MOFs that are stable in the presence of the solvents (water and ethanol) used for the gelation of the (bio)polymer.To extend the porosity of MOF-based composites in the meso- and macroporous regimes, MOFs/(bio)polymers and MOF/GO gels were dried by either supercritical drying or freeze-drying.in situ formation of MOFs was carried out by mixing preformed aerogels of graphene, graphene oxide or biopolymers with the metal and ligand precursors of the MOF.124,126 One key point is to control the growth of MOF particles that can seriously impact the textural properties of the carbon aerogels. As an example, the group of H. -C. Zhou used preformed macroporous melamine foam (MF) as a scaffold to induce the one-pot in situ synthesis of a series of porphyrin PCN-224 MOFs -porphyrin (TCPP) ligand (4+ and TCPP(M) precursors as well as benzoic acid and MF  crystals (148 These PCN-224(M)/MF  composites combined a high MOF loading, high crystallinity, hierarchical meso-/macroporosities, high chemical stability and good mechanical properties.148 They were finally tested for the catalytic epoxidation of cholesteryl esters in order to evaluate the impact of their hierarchical texture on their catalytic performance. PCN-224(Ru)/MF outperformed the other composites for this reaction as well as the pure PCN-224(Ru).148 PCN-224(Ru)/MF exhibited high catalytic activity for the epoxidation of cholesteryl esters with yields of more than 90% owing to the enhanced diffusion of substrates in its macroporous network. This composite could be recycled more than 6 times with good stability in its microstructural and catalytic properties.148To circumvent such issues, the integration of MOFs into preformed 3D foams or aerogels was also envisaged to impart macroporosity in MOF based composites. The 224 MOFs .148 Thes) ligand  right. Nd and MF .148 Remacrystals  bottom.1142 It was also used to synthesise hierarchical porous MOF-based composites. The principle of this method is to form the solid around the droplets which are then removed by solvent evaporation, thereby leading to a porous material.142 Two kinds of emulsions were used: “water-in-oil” (W/O) emulsion in which water droplets are dispersed in an oil continuous phase or “oil-in-water” (O/W) emulsion in which oil droplets are dispersed in a water continuous phase. Highly interconnected porous MOFs/polymer monoliths and beads were prepared by using high internal phase emulsions (HIPEs) as templates.149–151 HIPE is an emulsion, characterised by a high volume of the internal droplet phase exceeding 74.05 v/v%.142 To enhance the thermodynamic stability of O/W emulsions, surfactants are generally added at concentrations allowing the formation of lyotropic mesophases. Foams termed polyHIPES can be formed through the polymerisation of the continuous phase and removal of the droplets in the dispersed phase. PolyHIPES can also be prepared by using Pickering emulsions that are emulsions stabilised by nanoparticles.152 The group of D. Bradshaw has shown that O/W Pickering emulsions stabilised by ZIF-8 NPs at the O/W interface could be used for the preparation of ZIF-8-polymer composite microcapsules.153 Such emulsions were formed by the application of shear force to a mixture of dodecane and an aqueous colloidal suspension of ZIF-8. This step was followed by the polymerisation of the dispersed phase at 65 °C since the oil phase contained the styrene and divinylbenzene monomers and initiator.153 A cross-linked polystyrene (PS) membrane was formed at the interface between the MOF NPs and dodecane due to the poor solubility of PS in dodecane  and MIL-101(Cr). It was shown that the capsule formation was driven by the PS phase separation that was dependent on the hydrophilic/hydrophobic balance of the MOF and the nature of the oil phase.153 According to SEM, the PS membrane presented a hierarchical and asymmetric porous structure that consisted of a disordered network of interconnected meso- and macropores covered by a denser polymer skin.153 The good efficiency of the ZIF-8@PS microcapsule for the capture of oil red dye resulted from a synergistic combination of a microporous MOF and a hierarchical porous polymer membrane.153 Then, D. Bradshaw and coworkers developed nanostructured and hierarchical porous ZIF-8/polyHIPES composites by inducing the polymerisation of the continuous phase in MOF-stabilized Pickering W/O emulsions , MIL-101(Cr) and UiO-66(Zr)), thereby showing the versatility of this approach. These composites could be processed in different forms  as a result of their good mechanical properties  and the combined implementation of processing and/or drying techniques. Moreover, these strategies not only produce composites with a hierarchical porosity but also an adequate shaping for applications in catalysis, gas/liquid separation, etc.In the different examples described in this last section, different strategies were deployed to impart multiple porosities over lengths in the micro, meso to macroporous ranges in MOF composites. Such approaches took advantage of the possible association of MOFs with different complex fluids . Different strategies were also explored to integrate polymers either in the framework or in the pores of MOFs. The design of polyMOFs relied on the integration of polymeric dicarboxylate ligands as part of the MOF lattice. In alternative approaches, the well-defined porous structure of MOFs was used as an environment for polymerisation reactions of a wide range of monomers. A high diversity of polymer-inside composites was thus prepared through the growth of polymers from pre-tethered active sites located at the MOF backbone. In numerous cases, the attractive interactions and close communication between both MOFs and polymers have allowed improvement of the chemical stability of MOFs and control of the MOF dynamics (flexibility) while imparting also novel functionalities including electron/proton conductivity, processability, mechanical or stimuli responsive properties among others. The design of composites was also extensively explored by restricting the assembly of carbon materials and (bio)polymers on the surface of MOF particles, with the aim of preserving the porosity of MOFs. However, numerous composites suffered from poor interfacial properties which led to different shortcomings such as high MOF aggregation, possible phase separation, structural defects or low porosity. Among the strategies envisaged to enhance the physico-chemical matching between MOFs and carbon materials, numerous functionalisation strategies of the MOF surface were concomitantly developed as described in this perspective article. The in situ formation of MOF in the presence of the carbon material was also shown to enhance the spatial distribution and structuration of MOF particles in the carbon-based matrix while also controlling the morphology of MOF particles through coordination modulation or template effects. One major difficulty in this area arises from the inherent complexity and heterogeneity of the surface chemistry of MOFs which is characterised by the presence of the organic linker, the IBU, vacant coordination sites, surface defects and coordinating solvent molecules. A computational approach based on molecular dynamics (MD) simulations was successfully developed by the group of G. Maurin to gain insight into the structure of MOF/polymer interfaces57,155 but the deployment of experimental tools was comparatively quite limited to different microscopies . Recently, the interactions of MOFs with different polymers within MMMs were described by combining solid-state NMR and MD simulations.156 It should be of interest to extend this approach to other MOF-based composites while exploring the combination of different experimental tools able to characterise the surface of MOFs or MOF-based composites such as X-ray photoelectron spectroscopy (XPS), dynamic nuclear polarization (DNP) surface enhanced NMR or multiple electron microscopies . In addition, the correlation between synthetic parameters  and the resulting microstructure/composition of MOF-based composites is often challenging to predict, especially for composites prepared through the bottle-around-ship strategy (in situ approach). As a result, the synthesis of MOF-based composites is typically achieved through trial and error rather than a systematic rational approach. However, “in situ” composites result generally from a complex mutual recognition between MOFs and carbon materials: the organic material can drive the nucleation and growth of the MOF while conversely, MOF crystal surfaces can direct the self-assembly processes and structuring of the carbon material. An in-depth understanding of such a complex interplay between both components would be very helpful to control and scale up the production of composites using reliable manufacturing protocols. Notably, significant progress has been made in understanding the formation mechanisms of MOFs through in situ and time-resolved studies, employing a range of experimental techniques  as well as simulation techniques.157 A similar combination of advanced characterisation and simulation techniques in time-resolved and in situ studies would undoubtedly provide valuable insights into the formation of MOF/carbon material composites. Recently, the design of MOFs/carbon material composites with a hierarchical dimensionality, composition and porosity was also reported. However, the design of such composites is still in its infancy since these materials were often characterized by one type of structural hierarchy. Inspired by the amazing complex and hierarchical architecture of biominerals, a wide diversity of inorganic and organic materials  with a hierarchy at multiple levels of porosity, composition, morphology and structure were developed for different applications .121 Therefore, it would be highly desirable to synthesise MOF/carbon materials with a high level of morphological complexity and multiple levels of structural hierarchy since this approach would be relevant to the design of novel types of catalyst supports, separation membranes, biomaterials in regenerative medicine, etc. This required also a multi-scale and advanced characterisation of such materials to evaluate the impact of such complex microstructures on their physico-chemical properties  and performance for various applications. In particular, the pore size distribution, and connectivity of the micro-, meso and macroporosities of these hierarchically porous materials should be characterised by combining different experimental tools . Another strategy to increase the complexity and functionality of composites relied on the assembly of MOFs with a variety of bioentities including cells, bacteria, yeasts or viruses.158,159 The nascent field of using MOFs for the preparation of living materials aims to exploit the unique properties of cells including stimuli-responsiveness, biocatalytic activity and self-repair. Although extensively explored with other materials, this biomimetic approach is still in its early stages of development with MOFs and MOFs have not yet reached the maturity of other immobilisation matrices such as silica or clays. Moreover, the assembly of MOFs with bioentities could be coupled with advanced processing strategies such as microfluidics. Given their operational characteristics, including simplicity, small sizes, short processing times, continuous operation, and improved transport properties, diverse microfluidic platforms  were recently used for the preparation of MOF particles, MOF hollow spheres, and MOF/polymer membranes as well as hierarchical yolk/shell MOFs/polymer composites.160–162 It could be envisioned to expand this approach for the production of a wider range of hierarchical MOF/(bio)organic material composites.This perspective showed the diversity of synthetic strategies used to assemble MOFs with carbon-based materials. The development of a wide range of MOF-based composites took advantage of the structural and functional modularity of MOFs. In the first approach, the high and well-defined porosity of MOFs was exploited to encapsulate preformed carbon-based nanoentities (CQDs, fullerene, K. D. and S. D. were involved in the literature search and the preparation of figures. S. D. compiled the list of references. N. S. conceptualized and wrote the perspective. Critical inputs were added by K. D., S. D. and E. D. All authors discussed the perspective. E. D. and N. S. reviewed and edited the final version of the perspective.There are no conflicts to declare.