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Using in situ FTlR 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 Brsnsted-acid sites. The kinetics of water evolution from butanol is followed by in situ FTlR and both the rate constant and the activation energy of water evolution are estimated [k, = k , , exp (-€,/R77, where k , , = 2 x lo9 s-' and E, = 19 3 kcal mol-'1. At low temperatures (45-7OoC), 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 [k, = k , , exp(-€,/RT), where k , , = 3 x lo1, s-' and E, = 32 2 kcal mol-'1 for butene formation are effective values, containing contributions from several reaction steps, which explains the rather high value of E,.We have already reported the start of a series of studies devoted to shape-selectivity and pore-confinement effects in the dehydration of C, alcohols on zeolites, in particular on H-ZSM-5.'"vb The molecular dimensions of C, alcohols are rather similar to the dimensions of the channels in ZSM-5,2 and so this choice of reaction and reactants is particularly suitable. Furthermore, using the four butanol isomers, it is possible to modify systematically the molecular structure and rtxtivity of the reactant. The aim of the current work is to elucidate the mechanism of isobutyl alcohol dehydration on a molecular level, and in particular, to clarify how confinement of reactants, transition/intermediate complexes and products in a channel system of molecular dimensions, influence the rate and the reaction pathway. In order to do this, it is essential to: (1) thoroughly characterize the active centres of the zeolite, (2) separate the diffusion and kinetic regimes of reaction and (3) determine the mechanism of molecular conversion at the active centres. High-silica type ZSM-5 zeolites are highly active catalysts for the conversion of alcohol^.^ In recent years, most studies have concentrated on the conversion of methanol to hydroc a r b o n~.~.~ Here, interest has mainly focused on the formation of the first C-C bond, formation of higher olefins and aromatics, and the influence of the zeolite shape selectivity on these final products. Even so, the mechanism of reaction is still under debate. The initial dehydration reaction has received less attention, other than the observation of rapid formation of dimethyl ether under reaction conditions. Less work has been carried out on higher alcohols. There are stud...

Adsorption of methane and ethane as molecular probes on cationic forms of high-silica zeolites has been investigated by diffuse reflectance infrared spectroscopy. In addition to physically adsorbed species of the alkanes, molecules perturbed via an interaction with the cations were observed. Such a polarization results in a low-frequency shift of the antisymmetric stretching vibrations of methane molecules. A linear correlation was found between the extent of CH, polarization and the electrostatic field of the cation.

The sorption of krypton in Linde Molecular Sieve 13X has been studied over the temperature range 25 to -150°C. Analysis of these isotherms show that the heat of sorption is constant, when k0.35, rising, because of mutual sorbate interactions, to a maximum at 0 = 0.75. Occlusion beyond 0 = 0.75 is believed to involve the development of a second layer of sorbed molecules with a much reduced heat of sorption. If only the monolayer region is considered both K, derived from the isotherm equation K = exp (-2). and the differential thermal entropy are constant over nearly the whole range of coverage indicating that this system is a near perfect example of localized sorption on a homogeneous surface with mutual interactions between sorbate molecules.