The objectives of this study are to prepare ZSM-5 catalysts, to examine the characteristics with XRD, IR and SEM, and to explore a suitable catalyst for the direct alkylation of toluene with ethylene to produce high yield of para-ethyltoluene (PET, a raw material for the manufacture of poly(para-methystyrene)).
The effects of the reaction temperature, time-on-stream, and siliica/alumina ratio on the reaction over a HZSM-5 catalyst were examined. Under the reaction conditions of this study, the conversion of toluene was about 20% and the selectivity of desired para-isomer (p-selectivity) was about 33%, which was close to the thermodynamic equilibrium composition. The catalyst was deactivated quite rapidly in the initial stage of the reaction. However, after about 2 hours, the catalytic activity remained fairly constant while the p-selectivity increased slowly. Catalysts modified with P,B, and Mg showed impreved p-selectivity and decreased conversion. These are probably due to the disappearance of the strong acid sites and the pore plugging which have been proved by the ammonia temperature programmed desorption and surface area measurement.
HZSM-5 catalyst modified with P(1%) showed 37% p-selectivity, which increased to 56% when the catalyst was impregnated again with Mg. This was probably due to the deactivation of the external acid sites, the decrease of the effective pore size, and the tortuosity increase of the channel. Similar effect was observed with the large crystal size catalyst.
In the case of magnesium silicate, coversion was as low as 3.9%, however p-selectivity was as high as 97%. Thus, the magnesium silicate was expected to be an effective catalyst for the ethylation of toluene if the crystallinity could be improved.
