Gas-phase catalytic dehydrogenation of methanol to formaldehyde over ZnO/SiO2 based catalysts, zeolites, and phosphates

A series of ZnO/SiO2 based catalysts modified by El-oxides (El: In, La, Ce, Cr, Mn, Fe, Cu, Ag), zeolites (Na-ZSM-5, NaZSM-5(B), Cu-ZSM-5, Zn-X), sodium carbonate-doped phosphates of zinc and magnesium (NaZnP, NaMgP), and phosphorus-added silica-supported copper catalyst (Cu-P/SiO2) were prepared and comparatively tested for the selective gas-phase dehydrogenation of methanol to formaldehyde. The selectivity of catalysts was found to be dependent on the acid-base and redox properties, which were affected by their synthesis and calcination. Among the tested catalysts, the highest selectivity to formaldehyde is exhibited by NaZnP and Cu-P/SiO2; nevertheless, their activity is low and they are prone to rapid deactivation. Although Na-ZSM-5, Zn-X and Cu-ZSM-5 possess promising selectivity, they suffer from lack of stable performance due to 'coking' reactions. On the other hand, ZnO/SiO2 based catalysts proved to possess superior long-term stability and higher yields when compared to zeolites or phosphates. At isoconversional conditions, the selectivity of ZnO/ SiO2 catalysts can be improved by incorporation of Fe oxide, while the addition of other oxides (Cu, La, Ag, In) does not show the same influence. However, at the same operating conditions, the modification with the mentioned oxides alters the catalyst activity (Cu and La enhance, while In and Ag suppress the ZnO/SiO2 activity). The modification with Ce-oxide, as well as with sodium carbonate, increases the dehydrogenation activity of the ZnO/SiO2 catalyst. Nevertheless, Ce and Na ions favour consecutive reaction to CO, and are therefore to be avoided while preparing a selective catalyst. (C) 1997 Elsevier Science B.V.