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A series of cerium-based UiO-66 was obtained via hydrothermal and sonochemical methods, using the same quantities of reagents (cerium ammonium nitrate (CAN), terephthalic acid (H2BDC)) and solvents) in each synthesis. The impact of synthesis method and metal to linker ratio on the structural and textural properties of obtained UiO-66(Ce), as well as their composition in terms of Ce4+/Ce3+ ratio, structure defects resulting from missing linker, and CO2 adsorption capacity was discussed. By using typical characterization techniques and methods, such as XRD, N2 and CO2 sorption, TGA, XPS, and SEM, it was shown that the agitation of reacting mixture during synthesis (caused by stirring or ultrasounds) allows to obtain structures that have more developed surfaces and fewer linker defects than when MOF was obtained in static conditions. The specific surface area was found to be of minor importance in the context of CO2 adsorption than the contribution of Ce3+ ions that were associated with the concentration of linker defects.

Direct hydrogenation of CO 2 to methanol is an interesting method to recycle CO 2 emitted e.g., during combustion of fossil fuels. However, it is a challenging process because both the selectivity to methanol and its production are low. The metal-organic frameworks are relatively new class of materials with a potential to be used as catalysts or catalysts supports, also in the reaction of MeOH production. Among many interesting structures, the UiO-66 draws significant attention owing to its chemical and thermal stability, developed surface area, and the possibility of tuning its properties e.g., by exchanging the zirconium in the nodes to other metal cations. In this work we discuss-for the first time-the performance of Cu supported on UiO-66(Ce/Zr) in CO 2 hydrogenation to MeOH. We show the impact of the composition of UiO-66-based catalysts, and the character of Cu-Zr and Cu-Ce interactions on MeOH production and MeOH selectivity during test carried out for 25 h at T = 200 • C and p = 1.8 MPa. Significant increase of selectivity to MeOH was noticed after exchanging half of Zr 4+ cations with Ce 4+ ; however, no change in MeOH production occurred. It was found that the Cu-Ce coexistence in the UiO-66-based catalytic system reduced the selectivity to MeOH when compared to Cu/UiO-66(Zr), which was ascribed to lower concentration of Cu 0 active sites in Cu/UiO-66(Ce/Zr), and this was caused by oxygen spill-over between Cu 0 and Ce 4+ , and thus, the oxidation of the former. The impact of reaction conditions on the structure stability of tested catalyst was also determined.Catalysts 2020, 10, 39 2 of 17Synthesis of MeOH is an exothermic reaction that leads to the reduction of the number of molecules; therefore, according to the Le Chatelier's principle, the increase of the pressure and decrease of temperature will favor product formation. However, CO 2 is a chemically inert molecule and its activation requires some increase in reaction temperature. The process is usually carried out over Cu/ZnO/Al 2 O 3 catalyst at temperatures between 230 and 280 • C and at elevated pressure of 50-120 bar [1][2][3][4][5]. The active site for MeOH production from CO 2 consists of Cu steps decorated with Zn atoms. Copper, when alone, interacts with CO 2 poorly; hence, the presence of Zn is necessary to enhance its adsorption and speed up its conversion to methanol [6].MeOH production via CO 2 hydrogenation is a challenging process since the selectivity of the reaction is low and a number of unnecessary and undesired by-products is formed. At higher temperatures the CO formation is favored, so the RWGS reaction (Equation (3)) can occur during MeOH synthesis. That reaction consumes hydrogen and lowers alcohol production. Besides, both the MeOH synthesis (Equation (2)) and RWGS reaction (Equation (3)) produce H 2 O that leads to catalyst deactivation by inhibiting the active metal [7].Direct hydrogenation of CO 2 to obtain methanol is an interesting method to recycle the CO 2 produced, e.g., during combustion of fossil fuels; therefore,...

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