Direct dehydrogenation of methanol to formaldehyde over ZnO–SiO$_2$ -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 Zn$_2$SiO$_4$ 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 SiO$_2$, as well as
crystalline phases of zinc oxide and zinc silicate, viz. α-Zn$_2$SiO$_4$ and β-Zn$_2$SiO$_4$, were present in almost all
materials in different concentrations. The β-Zn$_2$SiO$_4$ 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
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silicate materials for the target reaction remained unclear. Some aspects of this role are addressed within
this work with a focus on α-Zn$_2$SiO$_4$ and its potential as a catalyst for direct methanol dehydrogenation.