Selective conversion of syngas to aromatics over metal oxide/HZSM-5 catalyst by matching the activity between CO hydrogenation and aromatization

Syngas conversion to aromatics has been studied over metal oxide/HZSM-5 bifunctional catalyst. In this reaction, CO is activated on metal oxide and hydrogenated to C-1/C-2 intermediate. The formation of aromatics involves C-C coupling, cycfization and dehydrogenation on acid sites of HZSM-5. The matching between CO hydrogenation and aromatization activity is key to the overall catalytic performance. High aromatic selectivity of up to 82.5% may be achieved over various metal oxide/HZSM-5 catalyst among which Cr2O3/HZSM-5 is the best in activity and aromatic selectivity. The CO activation and hydrogenation over Cr2O3 are facilitated by doping of Zn. The effect of Zn doping is revealed by systematic study on phase, morphology, reducibility and oxygen defect concentration. To enhance CO conversion and to prevent excessive hydrogenation, the strict control of Zn/Cr ratio and the phase of Zn species is required. The activity of HZSM-5 may be altered by changing Si/Al ratio and zeolite/oxide ratio and a moderate acid strength and density is necessary to balance the competition between aromatization and side reactions. Furthermore, the stability of the optimal catalyst 1Zn-3Cr/HZSM-5(18) is demonstrated with no significant deactivation after 100 h on stream.