Infrared spectroscopic studies of the reactions of alcohols over group IVB metal oxide catalysts. Part 1.—Propan-2-ol over TiO2, ZrO2 and HfO2

Abstract

Infrared spectroscopy has been used to analyse the gas-phase reaction products and the related adsorbed species obtained between room temperature and 400 °C from the dehydrogenation/dehydration reactions of propan-2-ol over a series of differently calcined catalysts of TiO₂, ZrO₂ and HfO₂. The ZrO₂ and HfO₂ results were independent of the calcination pretreatment, and the surfaces of these oxides, like that from a TiO₂ sample calcined at 800 °C, were dehydroxylated. Different results were obtained from a TiO₂ sample calcined at 300 °C which had a hydroxylated surface. The acidic sites and reactivities of the surfaces of TiO₂(300 °C) and TiO₂(800 °C) were explored by pyridine adsorption and infrared spectroscopy. Only Lewis-acid sites were detected by pyridine.

On raising the reaction temperature, in all cases the dehydrogenation reaction to give acetone occurred either before or simultaneously to the onset of the dehydration reaction to give propene. Acetone production was most pronounced over ZrO₂ and HfO₂ but also occurred more with TiO₂(800 °C) than with TiO₂(300 °C). The dehydrogenation reaction was largely quenched by pre-adsorbed pyridine on both TiO₂ samples. The TiO₂(300 °C) catalyst showed the presence of adsorbed propan-2-ol and 2-propoxide groups at room temperature. The dehydroxylated ZrO₂, HfO₂ and TiO₂(800 °C) samples only showed appreciable amounts of 2-propoxide groups. In each case the 2-propoxide ions occurred in two different forms, presumably formed by adsorption on different types of sites.

Both the acetone and propene products appeared as absorptions from 2-propoxide surface species decreased in intensity, so the latter are clearly reactive species. Gas-phase acetone production was followed by the chemisorption of acetone at a higher temperature. This subsequently decomposed to give surface acetate species, and finally at 400 °C to give CO₂ and methane in the gas phase. Propene did not give rise to adsorbed species, or to further products in the gas phase.

At the higher temperatures, above 300 °C, the reaction was always selective in favour of the dehydration reaction. However, each of the dehydroxylated catalysts showed some selectivity in favour of the dehydrogenation reaction over the earliest temperature range for alcohol decomposition, between 200 and 250 °C.

A discussion is given of possible mechanistic pathways for the production of surface 2-propoxide species and the two types of products, based on the infrared-supported assumption that the different adsorbed forms of 2-propoxide [and possibly adsorbed propan-2-ol on TiO₂(300 °C)] are reactive intermediates.