Intensified shape selectivity and alkylation reaction for the two-step conversion of methanol aromatization to p-xylene

Abstract

Two-step conversion of methanol to aromatics via light hydrocarbons can significantly improve the conversion stability compared with direct aromatization of methanol, but it remains a challenge to achieve a high p-xylene (PX) selectivity. Herein, silica coating was firstly used to passivate external acid sites of ZSM-5 catalyst for the aromatization of light hydrocarbons by the chemical liquid deposition method. With the increase of SiO2 deposition, the density of the external acid sites of the catalyst was decreased from 0.1 to 0.03 mmol·g−1, which inhibited the surface secondary reactions and increased the PX/X from 34.6% to 60.0%. In view of the fact that the aromatization process in the second step was partly inhibited as methanol was consumed in advance in the upper methanol-to-light hydrocarbons catalyst layer, part of methanol was directly introduced into the lower aromatization catalyst layer to promote the alkylation process during the aromatization, which decreased the toluene selectivity from 34.5% to 14.3% but increased the xylene selectivity from 40.0% to 55.3%. It was also found that an appropriate external acid density was needed for aromatization catalyst to strengthen the alkylation process and improve the selectivity of xylene under the conditions of methanol introduction.