Bis ( (2-ethylhexyl)oxy)benzo[1,2-b 4,5-b’]dithiophene-2,6-diyl)bis (trimethylstannane) MOF Product Moreira et al. 52 demonstrated the reverse selectivity of UiO-66 towards liquid-phase separation of xylene isomers. Indeed, the obtained results suggested o-xylene selectivities of 1.8 and 2.4 with respect to m- and p-xylene, at 40 °C with n-heptane as the eluent. Besides, the authors showed that the selectivities were retained upon compression, meaning that no major modification of the pore network took place upon compression. Interestingly, the authors stated that at low concentrations the selectivity values of UiO-66 were comparable to the ones previously reported for MIL-53. However, the latter failed to separate m- and p-isomers unlike UiO-66. 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.

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.

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.

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. As in the case of extrusion, the paste formulation is a crucial step in 3D printing and should yield a final composition with appropriate rheological properties. Apart from the parent powder and a liquid, the paste is also composed of a binder and a plasticizer. The former provides adequate mechanical resistance to the final 3D objects, while the latter improves the flowability and plasticity of the paste to be printed. One of the major differences is the printing nozzle: while the die in extruders can reach sizes up to a few centimeters, in 3D printers the nozzle (or needle) is typically smaller than millimeters in diameter. Such a thin nozzle allows designing objects with complex geometries that would be challenging to obtain via a conventional method. The process parameters entirely depend on the initial powder to be shaped. Mainly, the pressure applied on it should be carefully selected to avoid the complete destruction of the crystal structure (amorphization) and therefore loss of intrinsic properties. Additionally, the rate of pressure increase should be adequate for the same reason. 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. Though the hospitality industry is just one of many sectors participating in the MOF competition, the chef and pastry chef categories have become more widely publicized in recent years. MOF chefs wear have the designation emblazoned on their chef coats via a tri-colored collar and the MOF logo, serving as a very visible representation of the fact that they've achieved a fantastically high level of excellence. If you see a date below the MOF logo, that's the year he or she won the MOF title.A mixture of PVA and PVB was used as a binder in the study by Chanut et al. 71 The authors first mixed 5 g of MOF powder with a 3 wt% polymer blend, followed by periodical spraying of ethanol for a total of 50 mL to cause primary particle agglomeration. Upon sieving, a fraction with sizes between 1.3 and 1.7 mm ( Fig. 5h) was rounded using a rolling device to achieve the final shape. Eventually, the spheres were dried at 110 °C for 12 h to remove the residual ethanol. 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? Ligand codes: BTC – benzene-1,3,5-tricarboxylic acid; BDC – benzene-1,4-dicarboxylic acid; BPTC – biphenyl-3,3′,5,5′-tetracarboxylic acid; BTB – 1,3,5-tris(4-carboxyphenyl)benzene; DHBDC – 2,5-dihydroxy-1,4-benzenedicarboxylic acid; FA – fumaric acid; MIM – 2-methyl imidazole; IP – isophthalate; BPY – 4,4′-bipyridine; NDC – naphthalenedicarboxylic acid; and L – 1 H-pyrazole-4-carboxylic acid. Spray-dryer codes: B-290 – B-290 Mini Spray Dryer (Büchi Labortechnik); SD-1000 – SD-1000 Spray Dryer (Tokyo Rikakikai); and AF-88 – AF-88 Laboratory Spray Dryer (AFIND Scientific Instrument Co. Ltd). “—” not specified. 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.

In particular, they performed a full study on the production of UiO-66 spherical microbeads. First of all, they found that introducing a continuous flow reactor before the spray-dryer is a crucial step as the application of the spray-dryer alone yielded an amorphous product, as reported by Mitsuka et al. 137 Moreover, the sole use of a continuous flow reactor without the spray-dryer gave rise to the product with a low yield (12%) as well as poor textural properties ( S BET = 708 m 2 g −1). Thus, the combination of both was required. The authors further showed that the optimal conditions for the synthesis of UiO-66 and its derivatives included an inlet temperature of 180 °C, a flow rate of 336 mL min −1 and a feed rate of 2.4 mL min −1. The chosen inlet temperature corresponds to the minimum temperature required for the complete evaporation of the solvent, DMF. The UiO-66 microbeads produced under these conditions yielded dense spherical superstructures with an average size of 4.3 ± 2.6 μm composed of individual nanoparticle aggregates ( Fig. 16d). XRD confirmed the presence of the UiO-66 structure. In terms of textural properties, the spray-dried microbeads exhibited pronounced porosity with a high S BET of 1106 m 2 g −1, which is in good agreement with that of UiO-66 produced by classical solvothermal methods (1150–1250 m 2 g −1). 139,140 For MOFs, the primary goal of using binders is to enhance the mechanical stability of the granules. For this purpose, compounds capable of creating decently strong bonds with the surfaces of MOFs are preferred as binders. As an example, this implies cohesion of MOF particles via hydrogen bonding, and therefore, compounds possessing multiple functional groups (mainly –OH) are beneficial. This includes alcohols (polyvinyl), sugars (sucrose, cellulose), esters (hydroxypropyl cellulose) and others. However, upon shaping they cannot be removed due to the limited thermal stability of MOFs.According to XRD results, the printed objects retained the original crystal structure of HKUST-1 upon formulation. A certain peak broadening was observed for all materials, suggesting small MOF crystals. Indeed, as confirmed by SEM, the shaped objects were composed of HKUST-1 crystals with sizes in the 20–50 nm range. However, a significant decrease of the S BET was measured, from 1850 m 2 g −1 for the parent powder to 1134 m 2 g −1 for the 3D-printed solids. As no binder was present, this decrease might be ascribed to the partial collapse of the HKUST-1 framework.