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

[EN] 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 Br & Oslash;nsted 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.