Dehydration of n-Butanol on Zeolite H-ZSM-5 and Amorphous Aluminosilicate: Detailed Mechanistic Study and the Effect of Pore Confinement

Article abstract of DOI:10.1006/jcat.1994.1270

This study of the catalytic dehydration of n-butanol on zeolite H-ZSM-5 and amorphous aluminosilicate confirms the reaction scheme proposed earlier by the authors for isobutanol dehydration. The rate constant for n-butanol dehydration on H-ZSM-5 (determined from in situ FTIR kinetic studies by monitoring the growth of the water deformation peak at 1640 cm^-1) is shown to be the true dehydration rate constant (1.7*10^-4 s^-1 at 100 deg C). On the other hand, the rate constants determined from GC steady-state kinetic studies (temperature interval 105-185 deg C) are effective ones, giving activation energies of 22+/-2 kcal/mol and 33+/-2 kcal/mol for complete dehydration and dehydration to butene only, respectively. By studying the dehydration reaction under different conditions (flow and static reactors, steady-state and non-steady-state regimes) and on samples with rather similar acid strengths but different porous systems (H-ZSM-5-microporous channels with diameter ca. 5.5 Angstroem, and amorphous aluminosilicate-pores of average diameter ca. 50 Angstroem), it was shown that depending on the concentration of butanol in the immediate vicinity of the active alkoxide intermediate <*>-OC4H9, different reaction paths are utilized. High concentrations of alcohol favor ether formation, whereas low ones favour butene. This also explains the so-called "stop effect" observed in GC experiments, where an increase in the rate of butene formation occurs when the flow of alcohol is stopped and replaced with a flow of pure helium. Here, decreasing the concentration of alcohol in the micropores results in more of the alkoxide intermediate transforming to butene rather than to ether (which was the case at steady state).