INSITU IR SPECTROSCOPIC STUDY OF THE SURFACE SPECIES DURING METHYLATION OF TOLUENE OVER HZSM-5

The alkylation of toluene with methanol over HZSM-5 zeolites was studied in a continuously stirred tank reactor equipped with CaF2 windows, which allows the monitoring of the catalyst and the adsorption complexes under reaction conditions by means of IR spectroscopy. The reactor effluent was analyzed by means of gas chromatography. The experiments were carried out in a continuous flow mode between 373 and 673 K. The selectivity to p-xylene changed in this temperature interval from 30% (at 473 K) to 55% (at 673 K). In contrast to these observations, the products adsorbed in the HZSM-5 pores were mainly m- and o-xylene as well as trimethylbenzenes. The results suggest that a subtle balance between the intrinsic rate of alkylation, the rate of isomerization, and the rate of diffusion of the products determines the selectivity observed. © 1991.