## 3 Results and Discussion
### MOF Processing  
#### 3.3.1 Pellets  
In this work, pellets of all materials were obtained by a simple extrusion method, whereby a small amount of water is added to the corresponding MOF powder. The resulting paste is then loaded into a syringe and heated, as depicted in Figure 3a. The processing of these materials into pellets were chosen due to two main reasons: it is a simple and inexpensive processing method, which can be advantageous to transpose to industry; and, in terms of engineering in industrial practice MOFs, are required to be pelletized and packed into columns for real gas capture and process studies [37]. Figure 3: Schematic representation of (**a**) pellet preparation process by an extrusion method and (**b**) pellet coating with polymethacrylate polymer (PMMA). Figure 2: Water vapor sorption and desorption isotherms curves for HKUST and MOF-74 series at 25 \({}^{\circ}\)C. Straight lines and dashed lines represent the adsorption and desorption processes, respectively. Depending on the form of extrusion used, the materials can be processed into different sizes and shapes (Figure 4), and in large quantities, effortlessly. The pellets were characterized by powder X-ray diffraction, showing that this process preserves the structural integrity of the materials (Figure S2). Despite the rapid and straightforward pellet preparation method, with time, the lack of binding agent causes the pellets to peel off, presumably due to their dehydration. To circumvent this problem, we used a coating with methyl methacrylate polymer (PMMA), which has already found application in membranes for CO\({}_{2}\) removal in submarines [38]. PMMA coating enables the MOF adsorption of target gas molecules while preventing contact with water molecules. The coating process is straightforward, as depicted in Figure 3b. The pellets are immersed in a PMMA solution in dichloromethane and left to dry at ambient conditions and preserve their integrity for months. Subsequently, we assessed the water stability of uncoated and PMMA coated pellets. As shown in Figure 5, the uncoated pellets immediately disaggregate after immersion. In contrast, the coated pellets preserve their integrity even after one week immersed in distilled water. Pellets were then introduced for 72 h in a desiccator containing a saturated solution of magnesium nitrate or potassium sulfate to simulate exposure to ca. 54% and 98% RH, respectively. At 54% RH, the coated pellets maintain their integrity, while the uncoated pellets already show some degradation (not shown). At 98% RH, the uncoated HKUST and MgMOF-74 pellets were cracked or even completely disaggregated, while the coated pellets bearing showed no visible degradation (Figure 6). Interestingly, the integrity of the CoMOF-74 pellet remains the same at high RH even without PMMA coating. This behavior is attributed to the low water adsorption of Co- and NiMOF-74 (Section 3.2). All coated materials maintain their structural integrity in 98% RH (Figure 7). To better understand the (un)coated pellets' behavior in humid conditions, water vapor isotherms were also measured at 25 \({}^{\circ}\)C for HKUST. As shown in Figure 8, the processing into pellets decreases considerably the water adsorption of HKUST (ca. 10% water uptake at 98% RH). This response can be explained by the reduced contact surface of crystals when compared to the powder form. Interestingly, the coated and uncoated pellets show approximately the same water uptake and similar isotherms. Thus, the coating process only maintains the pellet form and does not change the water uptake. Figure 4: HKUST processed into pellets with different sizes and in the UA (University of Aveiro) words. Figure 5: HKUST-1 pellets (**a**) uncoated and (**b**) PMMA coated, before and after water immersion (for a period of three weeks). Figure 6: (**a**) Uncoated and (**b**) PMMA coated metal–organic frameworks (MOF) pellets under laboratory conditions, and after 72 h in a chamber with 98% relative humidity (RH).