Mechanistic studies of the catalytic dehydration of isobutyl alcohol on NaH-ZSM-5

Abstract

Using in situ FTIR and GC kinetic studies, we have examined the mechanism of dehydration of isobutyl alcohol to butene on well characterized ZSM-5 zeolite (number of active sites determined by various methods). Dehydration takes place on Brønsted-acid sites. The kinetics of water evolution from butanol is followed by in situ FTIR and both the rate constant and the activation energy of water evolution are estimated [k₂=k₀₂ exp (–E₂/RT), where k₀₂= 2 × 10⁹ s^–1 and E₂= 19 ± 3 kcal mol^–1]. At low temperatures (45–70 °C), elimination of water is accompanied by simultaneous formation of isobutyl ether (which at the given temperatures is adsorbed and desorbed with difficulty, but is able to form inside the channels). At higher temperatures (125 °C), there is a shift in the equilibria of various reaction steps, resulting in the formation of butene. This butene may desorb into the gas phase with traces of ether (in conditions of excess alcohol, flow GC experiments) or form oligomers which remain adsorbed in the zeolite (no excess of alcohol, static IR experiments). The measured rate constant and activation energy [k₄=k₀₄ exp(–E₄/RT), where k₀₄= 3 × 10¹⁴ s^–1 and E₄= 32 ± 2 kcal mol^–1] for butene formation are effective values, containing contributions from several reaction steps, which explains the rather high value of E₄.