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. 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. Fig. 9 Schematic representation of the 3D printing process via the Direct Ink Writing (DIW) method. Extrusion is another classical technique which is especially used to produce extrudates and honeycombs for catalytic converters. When it is applied to MOFs, limited impact on the structural and textural properties can be observed for most MOFs, due to lower pressures and shear forces applied. Extrusion requires, however, finely controlling the formulation and related rheological properties of the extruded paste. Advantageously, extrusion can also be used for the direct preparation of MOF objects starting from precursors (reactive extrusion). The latter is of particular interest as it allows limiting or avoiding completely the toxic solvents traditionally used for the synthesis of MOF powders. At the same time, reactive extrusion implies a continuous process with high potential space time yields. While this approach might not be applicable to all MOF structures, the reactive extrusion presents several advantages over more conventional methods such as solvo/hydrothermal or microwave-assisted syntheses of MOFs. On the other hand, these conventional methods remain better in terms of obtained crystallinity and surface area for most MOF structures.

For instance, the authors used copper hydroxide and trimesic acid mixed with methanol as a feed material to produce HKUST-1. Upon extrusion at room temperature, the product was washed with ethanol and dried to yield HKUST-1 extrudates with a specific surface area of 1738 m 2 g −1 and a crystal structure expected for this MOF. Furthermore, the authors showed that ZIF-8 extrudates can be made by both single and twin screw extrusion processes. For this, they used a blend of zinc carbonate and 2-methylimidazole with no solvent added and extruded it at 200 °C. In both cases, the processes yielded a crystalline product with the ZIF-8 topology and high surface areas: 1604 m 2 g −1 (twin screw) and 1750 m 2 g −1 (single screw). Lastly, the authors produced a highly crystalline Al-fumarate with a BET surface area of 1010 m 2 g −1 by extruding a mixture of Al-sulfate, fumaric acid and sodium hydroxide at 150 °C. It is worth noting that this approach enables the production of MOFs with decent space-time yields (STY) as single and twin screw extrusions are continuous processes. Interestingly, they also prepared MIL-100 pellets following the pelletization method and compared the thus formed bodies with the granules in terms of NH 3 adsorption. The latter exhibited higher adsorption capacity at 25 °C (4.4 vs. 3.6 mmol g −1), suggesting that upon pelletization, the parent powder underwent more drastic structural and textural changes as compared to granulation. This was supported by XRD and N 2 physisorption measurements. In 2015, Crawford et al. 92 described the mechanochemical synthesis of MOFs using a twin screw extruder (TSE) ( Fig. 7g), thus combining synthesis and shaping in one step. Indeed, the rotating screws composed of different zones (conveying, shearing, kneading) displace the starting solid MOF precursors along the heated barrel with good control over the residence time, and the mixing duration and intensity. Hence, through the combination of shearing and compression forces, solid-state reactions between the precursors can be obtained. Ideally, upon reaching the exit port, the product is formed and it is further drawn through a die into extrudates. Of note, the controllable heating of the barrel allows better control over the reaction conditions as compared to conventional milling approaches. R. Zacharia, D. Cossement, L. Lafi and R. Chahine, Volumetric hydrogen sorption capacity of monoliths prepared by mechanical densification of MOF-177, J. Mater. Chem., 2010, 20, 2145–2151, 10.1039/B922991D.The craftsmanship exhibition has been held every three to four years and showcases different professions including florists, carpenters, butchers, jewelry makers, and of course pastry chefs, to name just a few. Along with the title, winners receive a medal and state diploma. Less than 10,000 people have ever won the right to call themselves one of the Best Craftsmen in France. MOF winners retain their title for life. Many choose to join the National Society of Meilleur Ouvrier de France, a sort of alumni society for winners. The title is taken so seriously that sporting such a collar fraudulently is a crime punishable by prison-time. Our servers are getting hit pretty hard right now. To continue shopping, enter the characters as they are shown 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.

Y. Zhao, Z. Song, X. Li, Q. Sun, N. Cheng, S. Lawes and X. Sun, Metal organic frameworks for energy storage and conversion, Energy Storage Mater., 2016, 2, 35–62, DOI: 10.1016/j.ensm.2015.11.005. A. Dhakshinamoorthy, Z. Li and H. Garcia, Catalysis and photocatalysis by metal organic frameworks, Chem. Soc. Rev., 2018, 47, 8134–8172, 10.1039/C8CS00256H. Pelletization has been regarded for long as the “reference” technology for comparing shaping techniques. Obviously, when applying forces in the range of the ones applied industrially to zeolites and activated carbons, the MOF structures typically collapse which directly impact their final performances. However, recent developments show that when applying mild compression, along with the presence of a scaffolding compound (solvent or other), the collapsing is greatly limited. Moreover, binderless pelletization is possible owing to MOF crystals’ binding ability. Therefore, pelletization offers some of the best compromises between mechanical stability, and volumetric and gravimetric uptakes, making it a leading technique for mobility applications. To further limit the effect of compression on MOFs while reaching high mechanical resistance, isostatic compression could be advantageously used.Finally, Lawson et al. 111 studied the post-printing crystallization of HKUST-1 starting from a gel containing all precursors. In this case, a mixture of bentonite (21 wt%), methylcellulose (2 wt%) and PVA (6 wt%) was used to obtain satisfactory rheological properties. The as-printed grids presented a fair replication of the initial model, and they were further placed in a convection oven at 120 °C for 20 hours to induce crystallization of the MOF. The resulting material presented a S BET of 500 m 2 g −1, slightly higher than that of a comparative solid directly 3D-printed starting from the HKUST-1 powder (470 m 2 g −1). While the solids were extensively washed with acetone, residual DMF was observed by FTIR spectroscopy as characterized by a band at 2100 cm −1. Finally, the CO 2 capacities of both solids at 25 °C were compared. While the solid prepared from the HKUST-1 powder presented a CO 2 capacity 50% higher (2.1 mmol g −1 against 1.4 mmol g −1), which is not in line with their respective S BET, the solid obtained by growing HKUST-1 crystals on the as-printed solid displayed enhanced mass transfer kinetics (diffusivity × 10 8 (cm 2 s −1): 8.75 against 5.25). This was attributed to the presence of a larger extent of mesopores ( V meso (cm 3 g −1 STP) = 0.16 against 0.09). 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.