Infrared spectroscopic studies of the reactions of alcohols over group IVB metal oxide catalysts. Part 2.—Methanol over TiO2, ZrO2and HfO2

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Abstract
Infrared spectroscopy has been used to analyse the gas-phase reaction products, and the related adsorbed species, obtained from the decomposition of methanol over TiO2, ZrO2 or HfO2 catalysts after consecutive 10 min heating periods at 50 °C intervals between room temperature and 400 °C. Somewhat different results were obtained over a 500 °C calcined TiO2 catalyst (hydroxylated anatase surface) and an 800 °C calcined one (dehydroxylated rutile surface). The ZrO2 and HfO2 catalysts had dehydroxylated surfaces, were monoclinic in crystal structure, and gave products similar to those from TiO2(800). The principal initial surface species were several different methoxides, which were replaced by formate ions at higher temperatures. The dehydroxylated TiO2, ZrO2 and HfO2 catalysts were more selective for the promotion of the dehydrogenation reaction forming methyl formate (doubtless derived from the dimerization of formaldehyde) and ultimately CO, than for the dehydration reaction forming dimethyl ether. Methoxide formation was caused by a condensation reaction with the elimination of water on the hydroxylated TiO2 surface or by reaction with Ti4+O2– acid–base pairs on the dehydroxylated surfaces. In all cases more than one type of methoxide species giving different νC–O absorption bands were present on a given surface. It is tentatively concluded that on ZrO2 the methoxide group giving a band at 1163 cm–1, and attributed to a monodentate methoxide co-ordinated to only one metal ion, decomposed to give the dehydrogenation products. Both the hydroxylated and dehydroxylated TiO2 catalysts gave rise to methane as a final product, which is clearly derived from the decomposition of the dimethyl ether. Spectroscopic evidence suggests that the reaction involves the catalytic reduction of the ether, to give the alkane plus water, by hydrogen generated by the parallel dehydrogenation reaction.

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