In situ IR spectroscopic study of the adsorption behaviour of ethylbenzene and diethylbenzenes related to ethylbenzene disproportionation over HY zeolite

First, adsorption of ethylbenzene (EB) from the gas phase was studied mainly on NaY using a constant volume-variable pressure apparatus varying both temperature and sorbate concentration. For comparison, experiments were also conducted with HY under conditions where the reaction of the sorbates was negligible. From the set of isotherms of EB for NaY sorption isosteres were estimated and from them isosteric heats of adsorption determined. The latter were found to decrease from ca. 95 to ca. 89 kJ/mol, with increasing the loading from 0.05 to 0.5 mmol/g. The heats of adsorption of o-, m- and p-diethylbenzene (DEB) on NaY were similar, increasing slightly in the order: p-DEB <m-DEB <o-DEB, which corresponds to the sequence of the electrostatic contributions to the sorption interactions. Comparison of the isotherms of the diethylbenzenes and ethylbenzene clearly shows that the three isomers are more strongly sorbed in faujasite than EB. The reaction of ethylbenzene disproportionation over HY was carried out in a cell inserted into an IR spectrophotometer; a continuous flow mode was used. During the experiment the concentrations of the reactants and products were monitored simultaneously in the zeolite pores and in the effluent gas by means of in situ Fourier transform IR spectroscopy (FTIR) and gas chromatography, respectively. The analysis of the composition of the gas phase and the sorbate clearly shows that the product diethylbenzene is preferentially sorbed in the zeolite, compared with the reactant ethylbenzene. Thus, it replaces the latter from a fraction of its sorption sites. The differences in the heats of sorption of ethylbenzene and diethylbenzenes, estimated from the sorption and catalytic experiments, are in good agreement with earlier results and with data from the literature. (C) 1998 Elsevier Science B.V. All rights reserved.