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O. M. Yaghi and H. Li, Hydrothermal Synthesis of a Metal–Organic Framework Containing Large Rectangular Channels, J. Am. Chem. Soc., 1995, 117, 10401–10402, DOI: 10.1021/ja00146a033. An aqueous spray-drying synthesis of the Zn-imidazole ZIF-8 was done by Tanaka et al. 134 In a typical synthesis, an aqueous suspension containing Zn-acetate and 2-methylimidazole was spray-dried at T in = 150 °C and a feed rate of 5 mL min −1. These conditions yielded dense spherical particles with an average size of 3.9 μm as confirmed by SEM and TEM. However, the XRD results suggested the formation of an unknown phase different from that of the original ZIF-8. Moreover, the product poorly adsorbed nitrogen as revealed by N 2 sorption measurements. Notably, the authors observed the coordination of dissolved species and therefore the solution turning into a suspension right before spraying. The authors explained this phenomenon as due to the hindrance of crystallization created by acetic acid, a by-product originating from the Zn-precursor. The presence of the acid in the as-synthesized product was demonstrated by means of FTIR spectroscopy and TGA. Accordingly, during the spray-drying process, the as-released acetic acid caused a rearrangement of Zn-(2-methylimidazole) bonds, leading to the amorphization of the final product due to the incomplete coordination of the ligands around the metal. Interestingly, the presence of non-coordinated ligands was similarly evidenced by TGA. However, redispersing the spray-dried particles in an alcohol enabled the recrystallization and thus the formation of the targeted ZIF-8 framework. Interestingly, the size of the alcohol molecule influenced the size of the nanocrystals: specifically, the longer the carbon chain the larger the nanocrystals. However, the microbead size remained in the same range. Upon recrystallization, the product yielded an XRD pattern characteristic of ZIF-8 with a S BET of 1440 m 2 g −1, which is consistent with the results published elsewhere. 135 Surprisingly, once these ZIF-8 microbeads were redispersed in an alcoholic solution, they undergo a transition from dense to hollow superstructures. Hence, the recrystallization process is fed by gradually dissolving the amorphous by-product from the surface to the core of the microbeads. 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). 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.
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. Therefore, each of the shaping techniques provides unique features to the final objects in terms of size and appearance for a defined application. This review will focus on conventional shaping techniques such as granulation, pelletization, extrusion, and spray-drying and challenges associated upon formulation of MOF powders. This will also include the 3D printing method as it can be referred to as a type of extrusion with controlled deposition of the forming paste in three dimensions in space. Therefore, 3D printing allows shaping powders with desired shapes and dimensions for a wide variety of applications. They will be as well discussed in the corresponding section along with the challenges related to the formulation of MOF powders. Besides, a separate section will be dedicated to the so-called non-conventional techniques which include freeze granulation, ice templating and biopolymer precipitation. Membranes and coatings, and sol–gel-based monoliths have been excluded from this review on purpose as they have been recently reviewed. 19 Ligand codes: 1,3,5-BTC – benzene-1,3,5-tricarboxylic acid; 1,2,4-BTC – benzene-1,2,4-tricarboxylic acid; BDC – benzene-1,4-dicarboxylic acid; CA – citric acid; and MIM – 2-methyl imidazole. Binder codes: MSE – methoxy-siloxane ether; PVA – polyvinyl alcohol; PVC – polyvinyl chloride; KH570 – 3(trimethoxysilyl)propyl methacrylate; and MC – methyl cellulose. Plasticizer codes: MHPC – methyl hydroxyl propyl cellulose and DMF – N, N-dimethylformamide. “—” not specified. a Measured by Hg intrusion. 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). 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.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. 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. 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. 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.
R. V. Jasra, B. Tyagi, Y. M. Badheka, V. N. Choudary and T. S. G. Bhat, Effect of Clay Binder on Sorption and Catalytic Properties of Zeolite Pellets, Ind. Eng. Chem. Res., 2003, 42, 3263–3272, DOI: 10.1021/ie010953l. 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. Z. R. Herm, R. Krishna and J. R. Long, CO 2/CH 4, CH 4/H 2 and CO 2/CH 4/H 2 separations at high pressures using Mg 2(dobdc), Microporous Mesoporous Mater., 2012, 151, 481–487, DOI: 10.1016/j.micromeso.2011.09.004.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. Among other studies on ZIF-8 densification, there is a study by Bazer-Bachi et al. 39 (who also densified SIM-1). The authors applied a wide range of pressures and showed that the crystallinity of ZIF-8 was preserved upon compression up to ∼230 MPa. At the same time, the loss in BET surface area was about 11%, with the ZIF-8 pellet reaching 1278 m 2 g −1, while the pristine ZIF-8 powder exhibited 1433 m 2 g −1. Noteworthily, these results are in good agreement with the ones reported by Ribeiro et al. 37 and Chapman et al. 38 Upon compression, SIM-1 demonstrated a similar trend with a 28% drop in surface area (516 vs. 370 m 2 g −1) at a decent pressure of ∼400 MPa while preserving its framework topology according to its XRD pattern. Moreover, spray-drying allows the direct synthesis of various materials. 128 In 2002, du Fresne von Hohenesche et al. 129 successfully prepared MCM-41 spherical microbeads with a defined arrangement of macro- and mesopores with the help of a spray-dryer. Since then, the same approach has been used for preparation of other types of porous materials, 130 allowing spray-drying to be considered as a tool for simultaneous synthesis and shaping.
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