One-pass conversion of syngas to BTX/para-xylene aromatics over tandem oxide-zeolite catalysts based on large crystal size HZSM-5

The one-pass conversion of renewable syngas into aromatics, particularly to BTX (benzene, toluene, xylenes) and para-xylene (PX), represents a promising approach to produce high-value sustainable chemicals from non-fossil resources. In this work, a systematic study of the modification of HZSM-5 zeolite by surface silylation and incorporation of MgO was performed aiming at maximising the BTX and PX selectivity in the syngas-to-aromatics (STA) reaction using tandem Zn-ZrO2+HZSM-5 catalysts following the methanol route. A zeolite with unusually large crystal size was purposedly used to intensify the shape selectivity effect. This zeolite exhibited high BTX selectivity (53.0 % in aromatics) albeit poor stability with TOS. Generation of a proper amount of mesopores via controlled desilication improved the catalyst stability and the aromatics selectivity (from 59.2 % up to 73.2 %) at the cost of lowering the BTX selectivity. Effective passivation of the external Brønsted acid sites was reached by submitting the mesoporous zeolite to four silylation cycles with TEOS. The silylation treatment significantly raised the BTX selectivity to 52.1 % while maintaining high selectivity of total aromatics (67.9 %), but had only a moderate positive effect on PX selectivity. Incorporation of Mg species in the silylated zeolite produced the largest impact on PX selectivity, attaining an outstanding value of 76.8 % in xylenes (41.2 % in aromatics) upon loading 3 wt% Mg.