formulation of powders into mechanically stable objects to withstand a variety of process conditions (elevated pressure, gas/liquid flow, mixing); Linker codes: BDC – benzene-1,4-dicarboxylic acid; BTB – 1,3,5-benzenetribenzoate; MIM – 2-methyl imidazole; MIC – 4-methyl-5-imidazolecarboxaldehyde; BTC – benzene-1,3,5-tricarboxylic acid; DHBDC – 2,5-dihydroxy-1,4-benzenedicarboxylic acid; BPDC – biphenyl-4,4′-dicarboxylic acid; and FA – formic acid. Binder codes: PVA – polyvinyl alcohol; SB – pseudoboehmite; and PVB – polyvinyl butyral. “—” not specified. a Used as an additive to improve thermal conductivity. Y. H. Hu and L. Zhang, Amorphization of metal–organic framework MOF-5 at unusually low applied pressure, Phys. Rev. B: Condens. Matter Mater. Phys., 2010, 81, 174103, DOI: 10.1103/PhysRevB.81.174103. Bakytzhan Yeskendir received his MSc in Chemistry and Spectroscopy in 2018 within the framework of the Advanced Spectroscopy in Chemistry Master Program funded by Erasmus Mundus. He is now pursuing his PhD in materials science in the fields of synthesis, characterization, upscaling and shaping of materials for application in catalysis and gas adsorption, with special interest in the design of catalysts and adsorbents based on Metal–Organic Frameworks and zeolites, as collaborating work between the MATCAT group led by Pr. Sébastien Royer at the Université de Lille and the LMCPA led by Pr. Christian Courtois at UPHF. M. Mon, R. Bruno, J. Ferrando-Soria, D. Armentano and E. Pardo, Metal–organic framework technologies for water remediation: towards a sustainable ecosystem, J. Mater. Chem. A, 2018, 6, 4912–4947, 10.1039/C8TA00264A.

The paste formulation is crucial and requires special attention. Indeed, mixing of the parent powder with a liquid should yield a paste with suitable rheological properties to enable extrusion. There are many aspects which define the flow behavior such as the size and shape of the powder particles, their chemical properties, etc. Overall, the paste viscosity is dictated by the liquid content and can be decreased upon increasing the total liquid/solid ratio. More viscous pastes might require higher pressures for displacement within an extruder; however, unlike pelletization, extrusion does not affect as much the compaction of the particles as they are suspended in a liquid. Besides, in some cases the flowability, plasticity, or ability of the paste to withstand deformation upon extrusion can be enhanced by adding plasticizers. These are typical organic compounds based on cellulose or polyalcohols which facilitate the formation of the overall network. Generally, they are removed from the final extrudate composition by calcination. N. Heymans, S. Vaesen and G. De Weireld, A complete procedure for acidic gas separation by adsorption on MIL-53 (Al), Microporous Mesoporous Mater., 2012, 154, 93–99, DOI: 10.1016/j.micromeso.2011.10.020.Peterson et al. 47 performed another study on HKUST-1 to examine the evolution of its physical and chemical properties. Thus, the authors applied pressures of 1000 psi (∼7 MPa) and 10 000 psi (∼69 MPa). While the crystal structure was globally preserved, compressed HKUST-1 exhibited broader reflections as well as high signal-to-noise ratios on the XRD patterns. This suggests partial framework damage. Consequently, there was a certain decrease in BET surface area, from 1698 m 2 g −1 for the powder to 892 m 2 g −1 for the pellets made at ∼69 MPa. These values are somewhat different from the ones reported by Kim et al., 48 who stated that above 10 MPa the HKUST-1 framework underwent structural degradation. At the same time, Dhainaut et al. 49 reported a low (15%) loss in BET surface area for HKUST-1, reaching 1091 m 2 g −1 upon densification at 121 MPa. Besides, they showed that addition of 2 wt% of a binder (graphite) slightly improved the mechanical stability of HKUST-1 pellets without significant loss of BET surface area. They explained this relatively small loss as due to the presence of the remaining solvent within the framework, acting as a scaffold during compression, as well as the slow compression speed applied to the powder bed. J. Y. Choi, R. Huang, F. J. Uribe-romo, H. K. Chae, K. S. Park, Z. Ni, A. P. Co, M. O. Keeffe and O. M. Yaghi, Exceptional chemical and thermal stability of zeolitic imidazolate frameworks, Proc. Natl. Acad. Sci. U. S. A., 2006, 103, 10186–10191, DOI: 10.1073/pnas.0602439103. As with granulation, extrusion implies the addition of binders to ensure sufficient mechanical strength to extrudates by developing cross-linking forces between the individual particles. Therefore, the choice of binder and its content are governed by the same principles as in the case of granulation and pelletization. Namely, the binder should provide sufficient mechanical stability with the minimum loss of intrinsic physical and chemical properties of the parent powder. Thus, zeolites extruded with clays and boehmite as binders and cellulose-based plasticizers experienced certain alterations of textural and chemical properties as well as a clear enhancement of mechanical resistance. 77 Two step, continuous flow spray-drying method, dense structures Garzon-Tovar et al. 138 also reported the spray-drying of a series of MOFs with high-nuclearity. To do so, they combined continuous flow and spray-drying methods ( Fig. 16b and c). As in the case of Mitsuka et al., 137 the former is beneficial to initiate the nucleation step, while the latter favors the crystal growth. Thus, the so-called spray-drying continuous flow-assisted synthesis was applied to produce spherical microbeads of UiO-66 and its derivatives.

Ligand codes: BTC – benzene-1,3,5-tricarboxylic acid; BDC – benzene-1,4-dicarboxylic acid; FDC – 2,5-furandicarboxylic acid; TazBz – 3,3′,5,5′-azobenzenetetracarboxylate; and MIM – 2-methyl imidazole. Binder codes: PVA – polyvinyl alcohol; PVB – polyvinyl butyral; MRA – mesoporous ρ-alumina; and HPC – hydroxypropyl cellulose. “—” not specified. Following spinodal decomposition, which is also a phase separation method, Hara et al. 155 prepared UiO-66_NH 2-based monolithic materials with a trimodal pore structure. For that, all MOF precursors were dissolved into DMF along with poly(propylene glycol) (PPG) at 60 °C, and the clear solution was sealed in a hydrophobic glass tube kept at 80 °C. After 12 hours, hydrophilic UiO-66_NH 2 MOF mismatched growth occurred, as well as phase separation with the hydrophobic PPG. After washing with solvent, PPG was evacuated from the monolithic solid, leading to the formation of macropores whose diameter, between 0.9 and 1.8 μm, can be controlled by the amount of PPG. The XRD patterns displayed a few broad reflections, with 2 θ positions comparable to those of the simulated UiO-66. The structural properties of the MOF were proven by FT-IR spectroscopy, yielding a spectrum comparable to that of standard UiO-66_NH 2 powder. All samples presented specific surface areas between 712 and 749 m 2 g −1, further underlining the presence of a microporous network, while interparticular mesoporosity could also be deduced from N 2 sorption isotherms at higher relative pressure. Indeed, the TEM images showed particles with sizes below 50 nm. Uniaxial compression tests demonstrated that these monoliths presented a maximal compressive strength of 2.5 MPa. Interestingly, the authors showed that addition of acetic acid, a known modulator accelerating the crystallization, allowed obtaining larger mesopores. Alternatively, a post-shaping solvothermal treatment also allowed controlling the final size of the mesopores following the secondary growth of the MOF crystals. A. Dailly and E. Poirier, Environmental Science Evaluation of an industrial pilot scale densified MOF-177 adsorbent as an on-board hydrogen storage medium, Energy Environ. Sci., 2011, 4, 3527–3534, 10.1039/C1EE01426A. Avci-Camur et al. 141 continued exploiting the spray-drying technique for the synthesis of MOFs, targeting the UiO-66 family and more specifically UiO-66-NH 2 by the combined continuous-flow spray-drying method under aqueous conditions. For this purpose, the authors used water-soluble ZrOCl 2·8H 2O and 2-aminoterephthalic acid as the metal-precursor and the ligand, respectively. In this work specific stress was given to the use of a modulator, the acetic acid. Generally, the application of monotopic acids such as hydrochloric, formic and acetic acids facilitates the formation/crystallization of the UiO-family of MOFs. 142 Accordingly, it was shown that an increase in the acid concentration caused significant changes in textural properties. Thus, the UiO-66-NH 2 prepared with 14% acetic acid in the feed solution yielded microbeads with a S BET of 840 m 2 g −1 when spray-dried at T coil = 90 °C, T in = 150 °C, flow rate = 336 mL min −1 and feed rate = 2.4 mL min −1. However, at elevated (56%) concentrations of the acid, the S BET significantly increased up to 1036 m 2 g −1 under the same operating conditions. It should be noted that a further increase (70%) in the acid content led to a partial loss in crystallinity viewed as a decrease in reflection intensities in the XRD pattern as well as a loss in S BET down to 655 m 2 g −1. This suggests a competition between the modulator and the ligand for coordination with the metal clusters and therefore subsequent structural collapse upon exceeding occupation of the clusters by the modulator. The optimal acid concentration was found to be 30%. At this value, the spray-dried UiO-66-NH 2 yielded microbeads with a size distribution of 4–10 μm ( Fig. 16e) and exhibiting the UiO-66 structure according to XRD results. Besides, the S BET value, 1261 m 2 g −1, lies in the range of non-functionalized UiO-66 made via the solvothermal route with DMF, and is much higher than that of the spray-dried UiO-66-NH 2 prepared by Garzon-Tovar et al. ( S BET = 752 m 2 g −1). 138 Finally, the same protocol was applied to the Zr-fumarate MOF. The corresponding information is given in Table 14.

Dhainaut et al. 49 reported a detailed study of the effect of compression on the textural properties of some of the most studied MOFs including UiO-66, UiO-66-NH 2 and UiO-67. They found that the impact of pressing UiO-66 and UiO-66-NH 2 was in line with the pressure applied as their textural properties decreased accordingly. Thus, they reported a 26% decrease in UiO-66-NH 2 BET surface area upon compression at 164 MPa, which is in good agreement with the results reported by Peterson et al. 51 Interestingly, it was found that the UiO-67 structure started collapsing upon compression above 63 MPa, while at 82 MPa it lost ∼80% of its initial surface area (2034 vs. 397 m 2 g −1). Based on their results, they proposed to limit the compression to a final bulk density that represents at most 80% of the crystal density of the related MOF. V. Finsy, H. Verelst, L. Alaerts, D. E. De Vos, P. A. Jacobs, G. V. Baron and J. F. M. Denayer, Pore-Filling-Dependent Selectivity Effects in the Vapor-Phase Separation of Xylene Isomers on the Metal−Organic Framework MIL-47, J. Am. Chem. Soc., 2008, 130, 7110–7118, DOI: 10.1021/ja800686c. As confirmed by XRD, the crystal structure remained intact upon granulation. The presence of the binder was assumed as a secondary plate-like phase was observed in the SEM images. Consequently, there was an evident impact on the textural properties of the UiO-66 granules brought about by the binder. Namely, the specific surface area decreased to 674 m 2 g −1, which represents 50% of the SSA of the parent powder. Accordingly, the total pore volume decreased from 0.56 to 0.34 cm 3 g −1. In agreement with that, the hydrogen uptake similarly experienced a coherent decrease, from 1.54 cm 3 g −1 for the UiO-66 powder to 0.85 cm 3 g −1 obtained for the granules. Importantly, the authors provided data on the mechanical stability of the granulated UiO-66 based on non-conventional drop tests. Thus, no breakage was observed when dropping the granules on a steel surface from 0.5 m height after 70 consecutive drops. Moreover, attrition tests revealed that only 5% of the initial granule weight ended up as “fines”, after 60 min of tumbling at 25 rpm and further sieving. This suggested a considerable mechanical stability of the shaped granules. PVA and PVB binders Another class of binders largely used for wet granulation is polyalcohols, such as polyvinyl alcohol (PVA), and their derivatives, including polyvinyl butyral (PVB). The former was used in a study by Hindocha et al. 74 who formulated three MOFs (Cu-BTC (HKUST-1), CPO-27 and MIL-100) into spherical granules. The typical procedure implied pre-mixing 1 g of MOF powder with 2 wt% PVA followed by granulation upon addition of 0.25 mL of water. This formulation yielded spheres of 0.3–1.0 mm on average after sieving. As suggested by XRD results, this procedure had a considerable impact on the HKUST-1 framework, as the granules presented a pattern combining several mixed phases which were absent for the parent powder. In agreement with that, the shaped material showed a considerable decrease in specific surface area upon granulation, from 1605 to 147 m 2 g −1 for the parent powder and the granules, respectively. Consequently, this material, losing its MOF structure, was not able to retain a similar ammonia adsorption capacity, reaching only 19 mg g −1, while the parent powder could adsorb up to 105 mg g −1 under the same conditions (500 ppm ammonia, 40% RH). Thus, using water to shape HKUST-1 following wet granulation cannot be considered as an appropriate method. a Attrition tests were performed by rotating a cylinder containing a baffle and the shaped UiO-66-COOH at 60 rpm for 30 min. The percentage corresponds to the total mass of the fine particles – less than 425 μm – after sieving.

Tian et al. 160 showed the possibility to form ZIF-8 monoliths without using binders nor high pressures. For this, they immersed a newly-formed ZIF-8 powder into an ethanolic solution containing dissolved precursors (Zn-nitrate and methylimidazole), and the solid product was recovered via centrifugation. The authors outlined the importance of drying the solid at room temperature so that it retained its monolithic shape. Besides, when extra amounts of precursors were added, mechanically stable monoliths were formed due to the extension of polymerization reactions. The mechanical resistance was assessed by measuring the elastic modulus (7 GPa) and hardness (0.6 GPa) by nanoindentation. The thus-formed monoliths retained the original crystal structure of ZIF-8 as well as high specific surface area, up to 1395 m 2 g −1. Fig. 4 Typical wet granulation equipment: a high shear-rate mixer (Maschinenfabrik Gustav Eirich GmbH & Co KG), also referred to as a granulating pan (a) with an adjustable speed and direction of rotation; and a disc pelletizer (ERWEKA GmbH) also referred to as a rolling machine (b) with a controllable speed and inclination angle. Schematic representation of the wet granulation process: (c) mixing; (d) wetting and nucleation; (e) growth; and (f) spherization by attrition and breakage. X. Fang, B. Zong and S. Mao, Metal–Organic Framework-Based Sensors for Environmental Contaminant Sensing, Nano-Micro Lett., 2018, 10, 64, DOI: 10.1007/s40820-018-0218-0. The thus-shaped objects were found to exhibit XRD patterns similar to a MOF-5 degraded by humidity. This was attributed to the formulation procedure prior to printing rather than to the printing procedure itself. Nevertheless, these 3D printed objects were shown to have the ability to adsorb H 2 despite the complex polymer environment. On the other hand, the CPO-27 and MIL-100 frameworks proved to be more stable under the applied conditions, as the granules’ diffractograms yielded matching patterns with their powder counterparts. The MIL-100 granules presented only a slight decrease in SSA ( S BET = 1172 m 2 g −1), which is in the range of 2% loss as compared to the parent powder ( S BET = 1212 m 2 g −1), consistent with the initial amount of the binder. Surprisingly, the CPO-27 granules exhibited a considerable increase in specific surface area ( S BET = 1319 m 2 g −1) as compared to S BET = 937 m 2 g −1 of the as-synthesized CPO-27. This phenomenon was stated to be unclear by the authors.The same approach was also applied to shape MIL-100 by Martins et al. 69 In a typical shaping procedure, the parent MIL-100 powder was mixed with 10 wt% silica as a binder in a rolling machine. During mixing, water and ethanol were periodically sprayed on the blend to facilitate the agglomeration of individual particles. Eventually, the granules were isolated and dried at 100 °C to remove the residual solvents. This procedure resulted in semi-spherical granules with an average size of 1.0–3.0 mm ( Fig. 5b), presenting a micropore volume of 0.58 cm 3 g −1 and a specific surface area of 1568 m 2 g −1, which is in agreement with Kim et al. 68 The beads were further applied to ethane/propane and ethylene/propane gas mixture separation. The results suggested preferential C 3H 8 adsorption over C 2H 6 and C 2H 4. This remained the case when the temperature was varied, highlighting the potential of the MIL-100 granules for C 2/C 3 separation following pressure-swing adsorption (PSA). Moreover, lab-scale vacuum-swing adsorption (VSA) experiments starting from a 0.30 ethane/0.70 propane mixture, at 50 °C and 150 kPa, were conducted. The MIL-100 granules yielded an ethane-rich stream with a purity of 99.5% and a recovery of 86.7%, as well as a propane-rich stream with a purity of 99.4% and a recovery of 97.0%. The same VSA experiment starting from a 0.30 ethylene/0.70 propane mixture resulted in an ethylene-rich stream with a purity of 100% and a recovery of 75.8%, as well as a propane-rich stream with a purity of 94.7% and a recovery of 100%. The obtained results show that MOFs such as MIL-100 adequately shaped are highly promising for industrial separation processes. Mesoporous ρ-alumina (MRA) Another class of inorganic binders was first probed by Valekar et al. 57 for granulating a series of MOFs. They produced granules of MIL-100, MIL-101, UiO-66 and UiO-66-NH 2 by mixing pre-defined amounts of MOF powders with 5–20 wt% mesoporous ρ-alumina (MRA) in a rolling machine. During mixing, the blend was sprayed with water to facilitate particle agglomeration. The thus-produced granules were further sieved and rounded in a rolling machine. Finally, spheres with sizes of 2.0–2.5 mm were isolated and dried at 110 °C for 12 h ( Fig. 5c–f). In 2014 Ahmed et al. 156 proposed a different method for MOF shaping based on controlled freezing. According to it, a MOF powder in suspension can be shaped into monoliths upon controlled freezing of the solvent with its subsequent elimination via freeze-drying. The authors applied this methodology to obtain Cu-based HKUST-1 monoliths. For this, the MOF precursors were dissolved in DMSO and left for 24 h at 80 °C. After that, the solution was frozen in liquid nitrogen for 1 min and placed into a freeze-dryer to sublime the solvent. This procedure yielded highly crystalline HKUST-1 monoliths as confirmed by XRD. Moreover, the specific surface area was 870 m 2 g −1 with characteristics of both micropores and mesopores, as visible from the N 2 physisorption isotherms. Additionally, as shown by Hg intrusion, the monoliths exhibited macropores with diameters around 0.4 and 10 μm. Importantly, these macropores generated upon ice-templating were oriented in one particular direction due to the orientational growth of ice crystals during freezing. Lastly, the authors showed that the size of these macropores could be varied by altering the freezing temperature. Thus, upon freezing at 5 °C the macropores were two times bigger (∼50 μm) than the macropores generated upon freezing at −80 and −20 °C (32 and 25 μm, respectively). M. A. Moreira, J. C. Santos, A. F. P. Ferreira, U. Müller, N. Trukhan, J. M. Loureiro and A. E. Rodrigue