MECHANISTIC STUDY OF SEC-BUTYL ALCOHOL DEHYDRATION ON ZEOLITE H-ZSM-5AND AMORPHOUS ALUMINOSILICATE

The dehydration of sec-butyl alcohol has been studied by in situ FTIR and gas-chromatographic (GC) kinetic methods in the range 60-140-degre es-C on zeolite H-ZSM-5 and amorphous aluminosilicate (AAS) samples wi th a well characterized number and strength of Bronsted acid sites. Un der flow conditions (GC kinetic studies), the reaction yields butenes [but-1-ene, (Z)- and (E)-but-2-ene] and water, with an activation ener gy of 40 +/- 1 kcal mol-1 determined from steady-state data. Under non -steady-state conditions, the so-called 'stop effect' is observed: nam ely, an increase in the rate of butene evolution (as compared with tha t 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 followe d by the appearance and growth of a peak for adsorbed water (water def ormation peak at 1640 cm-1). The rate constant determined from the kin etics of water formation in the FTIR experiments (1.1 X 10(-3) s-1 at 70-degrees-C) is found to be 400 times as high as the rate constant ca lculated from GC steady-state kinetic data. All these anomalous phenom ena observed under flow conditions (the low rate of reaction, the high activation energy and the 'stop effect') can be explained by the slow ing down of dehydration under these conditions as a result of the reve rse reaction, i.e. the hydration of the product butene with product wa ter. When the zeolite pores are free from physically adsorbed reactant s (in the FTIR experiments or during the 'stop effect'), the extent of the reverse reaction decreases and the rate of butene formation incre ases. On AAS, which has acid sites of similar strength, but which has a much more open surface (average pore diameter ca. 50 angstrom compar ed with 5.5 angstrom for ZSM-5), similar effects are observed, but the y are much less pronounced. This probably arises from the lower reacta nt concentration in the AAS at steady state and hence, a lower concent ration of water in the vicinity of the active sites.