Catalysis Letters, Vol.41, No.1-2, 83-87, 1996
Improvement of Para-Selectivity in Methylation of Toluene on Various MFI-Type Metallosilicate Catalysts
MFI-type metallosilicates incorporating various transition metal elements were synthesized by the rapid crystallization method. Compared with H-ZSM-5, these metallosilicates possess fewer acid sites and weaker acidic strength. In the methylation of toluene with methanol, protonated metallosilicates exhibited lower conversion of toluene, higher para-selectivity and higher yield of paraxylene than H-ZSM-5. Distributions of trimethylbenzene and tetramethylbenzene isomers produced in toluene methylation indicate that the effective pore size (openings and/or channels) of metallosilicates may be smaller than that of ZSM-5, which is more advantageous for formation of the smallest isomers in trimethylbenzenes and tetramethylbenzenes. The relation between the para selectivity in toluene methylation and the isomerization activity in para-xylene conversion suggests that the para-selectivity could occur exclusively, when isomerization of para-xylene to other isomers could be completely prevented. Therefore, the improvement in para-selectivity on MFI-type metallosilicates should be attributed to both the reduction in effective pore size and the decrease of strong acid sites, on which produced para-xylene would be isomerized to ortho- and meta-xylenes.