P-Xylene selectivity enhancement in methanol toluene alkylation by separation of catalysis function and shape-selective function

Abstract

Catalyst reactivity and selectivity always compromise with each other in most reactions. HZSM-5 catalyst for the methanol toluene alkylation reaction suffers from active site loss when modified by elements such as Si, P, Mg and B to attain high p-xylene selectivity, incurring problems of low toluene conversion and rapid catalyst deactivation. In the present study, the separation of catalysis function and shape-selective function was achieved by epitaxial growing a Silicate-1 layer over HZSM-5 crystal to create an HZSM-5@S-1 catalyst. HZSM-5@S-1 was shown to possess less weak acid sites but more strong acid sites than the parent HZSM-5. With reduced contact time, the catalyst showed increased p-xylene selectivity up to 88% but no obvious degradation in toluene conversion. The enhancement of Silicate-1 shell to PX selectivity was less effective when applied to low Si/Al ratio ZSM-5 samples, due to the re-incorporation of aluminium leached from low Si/Al ratio parental ZSM-5 samples into the newly formed Silicate-1 framework. Benefiting from retaining most of the strong acid sites of HZSM-5, the catalyst showed high toluene conversion with increasing WHSV without apparent deactivation over a 130hours service life test.