Journal of the Chemical Society, Faraday Transactions 1: Physical Chemistry in Condensed Phases p. 1723 - 1742 (1989)
Update date:2022-08-11
Topics:
Hussein, Gamal A.M.
Sheppard, Norman
Zaki, Mohamed I.
Fahim, Radamis B.
Infrared spectroscopy has been used to analyse the gas-phase reaction products and the related adsorbed species obtained between room temperature and 400 deg 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 deg C, were dehydroxylated.Different results were obtained from a TiO2 sample calcined at 300 deg C, which had a hydroxylated surface.The acidic sites and reactivities of the surfaces of TiO2(300 deg C) and TiO2(800 deg 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 occured either before or simoltaneously 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 degC) than with TiO2(300 deg C).The dehydrogenation reaction was largely quenched by pre-adsorbed pyridine on both TiO2 samples.The TiO2 (300 deg C) catalyst showed the presence of adsorbed propan-2-ol and 2-propoxide groups at room temperature.The dehydroxylated ZrO2, HfO2 and TiO2(800 deg 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 deg 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 deg 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 deg 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
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