ISOMERIZATION OF XYLENES ON BOROALUMINOSILICATE CATALYSTS WITH THE ZSM-5 STRUCTURE, SYNTHESIZED IN NONALKALINE MEDIA

We examined catalytic isomerization of xylenes at 300 and 445-degrees- C on zeolite [Si, B, Al]-ZSM-5, prepared under non-alkaline conditions , on amorphous SiO2 - AlO3 and on the conventional zeolite H-ZSM-5. Th e reaction, known to proceed on Bronsted acid sites, is an excellent m onitor of shape selectivity. Because of fast coking, the catalysts wer e examined in the pulse mode. The results, displayed using the Wei-Pra ter triangle plots, show that reaction paths on boroaluminosilicates l ie in the region where diffusion effects are present, and that shape s electivity contributes to the enhanced formation of p-xylene. The esti mated Thiele modulus is greater-than-or-equal-to 10 for boroaluminosil icates, zero for SiO2 - Al2O3, and has an intermediate value for conve ntional H-ZSM-5. Boroaluminosilicate catalysts prepared under non-alka line conditions are highly shape selective in the isomerization of xyl enes.