Mechanistic study of sec-butyl alcohol dehydration on zeolite H-ZSM-5 and amorphous aluminosilicate

Abstract

The dehydration of sec-butyl alcohol has been studied by in situ FTIR and gas-chromatographic (GC) kinetic methods in the range 60–140 °C on zeolite H-ZSM-5 and amorphous aluminosilicate (AAS) samples with a well characterized number and strength of Brønsted acid sited. Under flow conditions (GC kinetic studies), the reaction yields butenes [but-1-ene, (Z)- and (E)-but-2-ene] and water, with an activation energy of 40 ± 1 kcal mol^–1 determined form, steady-state data. Under non-steady-state conditions, the so-called ‘stop effect’ is observed: namely, an increase in the rate of butene evolution (as compared with that at steady state) when the flow of alcohol into the reactor is halted. The course of dehydration on H-ZSM-5 in a static IR cell was followed by the appearance and growth of a peak for adsorbed water (water deformation peak at 1640 cm^–1). The rate constant determined from the kinetics of water formation in the FTIR experiments (1.1 × 10^–3 s^–1 at 70 °C) is found to be 400 times as high as the rate constant calculated from GC steady-state kinetic data. All these anomalous phenomena observed under flow conditions (the low rate of reaction, the high activation energy and the ‘stop effect’) can be explained by the slowing down of dehydration under these conditions as a result of the reverse reaction, i.e. the hydration of the product butene with product water. When the zeolite pores are free from physically adsorbed reactants (in the FTIR experiments or during the ‘stop effect’), the extent of the reverse reaction decreases and the rate of butene formation increases. On AAS, which has acid sites of similar strength, but which has a much more open surface (average pore diameter ca. 50 Å compared with 5.5 Å for ZSM-5), similar effects are observed, but they are much less pronounced. This probably arises from the lower reactant concentration in the AAS at steady state and hence, a lower concentration of water in the vicinity of the active sites.