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Fig. 3 BET SSA as a function of applied pressure during pelletization (left): ( ) – ZIF-8 by Ribeiro et al., 37 ( ) – ZIF-8 by Bazer-Buchi et al., 39 ( ) – UiO-66-NH 2 by Peterson et al., 51 ( ) – UiO-66-NH 2 by Dhainaut et al., 49 ( ) – HKUST-1 by Bazer-Buchi et al., 39 ( ) – HKUST-1 by Dhainaut et al., 49 and ( ) – HKUST-1 by Alcañiz-Monge et al. 25 BET surface area as a function of bulk density (right): ( ) – MOF-177 by Zacharia et al., 32 ( ) – MIL-101 by Ardelean et al., 41 and ( ) – MOF-5 by Purewall et al. 28

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. Being a MOF undeniably implies responsibility and one of my favorite may well be passing on the passion and art to the next generation. My role at EHL plays an important part in this and I am thrilled every time I see students’ eyes light up when accomplishing something they never thought they were capable of.”concludesMeilleur Ouvrier de France Chef Fabien Pairon Under the same conditions, the UiO-66 framework proved to be more stable toward high pressures. 47 Upon compression up to 69 MPa, the BET surface area of the pellet reached 1080 m 2 g −1, which is identical to that of the parent powder. Therefore, when tested for octane adsorption, the UiO-66 pellet compressed at ∼69 MPa demonstrated a saturation loading comparable to its powder counterpart (2.1 vs. 2.5 mmol g −1, respectively).Further reports on shaping ZIF-8 via DIW include the work by Lefevere et al., 112 who managed to formulate the MOF with a blend of inorganic and organic binders. The former was added to improve the mechanical stability of the shaped objects, and the latter to enhance the rheological properties of the paste. Typically, the parent ZIF-8 powder (66.7 wt%) was mixed with bentonite (16.7 wt%) and methylcellulose (16.7 wt%) with a subsequent addition of water and mixing to form a homogeneous printable paste. Once homogenized, it was further loaded into a 50 mL syringe and extruded through 250 and 600 μm diameter nozzles in a layer-by-layer fashion at room temperature ( Fig. 11g and h). A further study done by Boix et al. 143 in Maspoch's group led to the incorporation of inorganic nanoparticles into UiO-66 microbeads. The process followed the same sequence of steps, including the formation of primary nuclei in the flow reactor at 115 °C with a feed rate of 2.4 mL min −1, an inlet temperature of 180 °C, and a flow rate of 336 mL min −1. The thus-formed UiO-66 microbeads exhibited an average size of 1.5 ± 1.0 μm with a crystalline framework corresponding to the UiO-66. Additionally, the beads had a high surface area ( S BET = 945 m 2 g −1), which was slightly below than the one obtained by Garzon-Tovar et al. ( S BET = 1106 m 2 g −1) 138 following the same method. The difference might be attributed to the Zr-source used in each case: zirconium propoxide and zirconium tetrachloride, respectively. Interestingly, the UiO-66 itself was shown to be active towards the adsorption of toxic heavy metals such as Cr( VI) and As( V) with removal efficiencies of 99 and 45%, respectively. However, once functionalized with thiol (–SH) groups and doped with CeO 2 nanoparticles, it became active and efficient towards heavier metal species including Cd( II), Cu( II), Pb( II) and Hg( II) with removal efficiencies of 87, 99, 99 and 98%, respectively. Importantly, CeO 2-doped UiO-66-(SH) 2 microbeads retained their removal efficiency after 10 adsorption/desorption cycles in a continuous flow column, making them appropriate for further developments as water-purifying adsorbents. Liang et al. 149 studied the shaping of a Ti-based MIL-125 MOF with chitosan as a binding biopolymer into spherical beads. They first mixed chitosan and an FeCl 3 solution, followed by the addition of the MOF. Once well mixed, a 3% Na 5P 3O 10 solution was added dropwise to initiate the cross-linking step ( Fig. 17g). The thus-formed beads were recovered, washed and dried. The authors showed that such a formulation had no impact on the crystal structure nor the framework composition as confirmed by XRD, FTIR spectroscopy and XPS analyses. Therefore, the beads exhibited a consequent capacity for the removal of Pb( II) species, with only an ∼12% decrease in efficiency (from 100 to 88 mg g −1) after five consecutive cycles.

Finally, other less-popular techniques have been successfully applied for shaping MOFs, among which have been reviewed the so-called molecular gastronomy, ice-templating (also called freeze-casting), and phase separation (also called spinodal decomposition). These three techniques presented very low impact on the physicochemical properties of the MOFs applied and are therefore worth investigating more in detail. It should be noted, however, that ice-templating and phase separation both involve the creation of a second level of porosity macrosized (>50 nm) following the replication of ice crystals and polymers, respectively. The MOF title is really unique. It carries an important historical legacy and recognizes work approaching perfection. It is a true honor to receive recognition for one'speers and country. Today, I proudly represent and further with my best abilities the values of professional excellence, innovation and transmission.” explains Meilleur Ouvrier de France ChefChristian Segui What is the competition about? S. S.-Y. Chui, S. M.-F. Lo, J. P. H. Charmant, A. G. Orpen and I. D. Williams, A Chemically Functionalizable Nanoporous Material [Cu 3(TMA) 2(H 2O) 3]n, Science, 1999, 283, 1148–1151, DOI: 10.1126/science.283.5405.1148. 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.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. As is evident from Table 13, both extrusion and DIW can yield shaped objects while preserving the textural properties of pristine MOFs. The loss in surface area in the case of the 3D printed objects is somewhat higher than the binder content which is due to the presence of the plasticizer. The final printed objects have a developed network of millimeter-sized channels thanks to the layer-by-layer deposition upon printing. This can significantly improve the diffusion of gas or liquid within the shaped objects. Spray-drying The spray-drying technique has been known for over a century. As the name suggests, it is used to dry powder-like materials in a continuous fashion. Compared with ovens and conveyor belts, spray-dryers allow treating several tons per hour of wet product. Over the past few decades, the application potential of spray-drying has considerably expanded and now includes the food industry, 125 the pharmaceutical industry, 126 and many others. 127 G. Férey, C. Mellot-Draznieks, C. Serre, F. Millange, J. Dutour, S. Surblé and I. Margiolaki, A Chromium Terephthalate-Based Solid with Unusually Large Pore Volumes and Surface Area, Science, 2005, 309, 2040–2042, DOI: 10.1126/science.1116275. It should be noted that there are two types of granulation processes distinguished in the literature: wet and dry granulation. Dry granulation is applied when powders are incompatible with the use of solvents. Typically, it implies the compression of a parent powder at high pressures followed by mild crushing and sieving. Mainly, this process resembles and is typically subsequent to pelletization. Therefore, it was described in the previous part.

Recently, 3D printing has been applied to a large number of structured adsorbents and catalysts. Thus, Al 2O 3 was shown to be printable into monoliths exhibiting high catalytic efficiency as well as good recyclability. 95 Zeolites 13X and 5A have also been printed into monoliths for CO 2 removal purposes, 96 while a 3D-printed zeolite (ZSM-5) has been probed for CO 2, CH 4 and N 2 separation. Among the other printed structures can be found carbons, 97 amorphous aluminosilicates 98 and other classes of adsorbents. 99 Compaction itself serves as a source of reinforcement; however, sometimes the use of binders to enhance the mechanical stability of pellets is of particular interest. Binders are usually classified into organic binders such as starch, cellulose and polyvinyl alcohol (PVA) and inorganic binders such as clays, silica and graphite. 20 They facilitate bonding of individual particles by generating a link between them. As an example, it was shown that zeolites X and Y could be pelletized using bentonite as the binder, 21 and kaolinite could be employed to bind ZSM-5 zeolite crystals together. 22 In both cases there is an alteration of both the physical and chemical properties of the final materials compared to the pristine zeolites. C. Perego and P. Villa, Catalyst preparation methods, Catal. Today, 1997, 34, 281–305, DOI: 10.1016/S0920-5861(96)00055-7. J. J. Purewal, D. Liu, J. Yang, A. Sudik, D. J. Siegel, S. Maurer and U. Mu, Increased volumetric hydrogen uptake of MOF-5 by powder densification, Int. J. Hydrogen Energy, 2012, 37, 2723–2727, DOI: 10.1016/j.ijhydene.2011.03.002.R. Bingre, B. Louis and P. Nguyen, An Overview on Zeolite Shaping Technology and Solutions to Overcome Diffusion Limitations, Catalysts, 2018, 8, 163, DOI: 10.3390/catal8040163. Granulation is the last industrially-mature technology reviewed herein, and allows producing millimeter-sized grains. Two types of granulation techniques are typically discussed: wet granulation, when powders are aggregated in a high-shear rate mixer in the presence of a solvent; and dry granulation, when grains are obtained from a previously shaped object either mildly crushed and sieved, or spheronized. Due to higher stresses applied, the dry granulation implies more severe losses in the initial physicochemical properties of the MOFs, while the wet granulation has a less pronounced effect and therefore might be more adequate. Especially, replacing water with another solvent with a lower surface tension is highly beneficial.