Direct dehydrogenation of methanol to formaldehyde over ZnO–SiO2-based catalysts†

Direct dehydrogenation of methanol to formaldehyde and hydrogen is a “dream reaction” requiring catalysts, which are not only active in this highly endothermic reaction but also stable under harsh reaction conditions. Previous reports showed that materials with Zn₂SiO₄ exhibit a relatively high activity along with considerable long-time stability. However, neither detailed information on the physicochemical properties of such zinc silicates nor information on deactivation mechanisms was provided and discussed. In this study, the Zn : Si ratio has been varied to obtain different phases of zinc silicate and to investigate their specific activities in the methanol dehydrogenation reaction. Amorphous ZnO and SiO₂, as well as crystalline phases of zinc oxide and zinc silicate, viz. α-Zn₂SiO₄ and β-Zn₂SiO₄, were present in almost all materials in different concentrations. The β-Zn₂SiO₄ phase was found to be relatively unstable in methanol dehydrogenation similar to ZnO, which is readily reduced to metallic Zn. Since detailed material characterization was not reported in studies before, the catalytic role of different phases present in zinc silicate materials for the target reaction remained unclear. Some aspects of this role are addressed within this work with a focus on α-Zn₂SiO₄ and its potential as a catalyst for direct methanol dehydrogenation.