Bifunctional catalysts with versatile zeolites enable unprecedented para-xylene productivity for syngas conversion under mild conditions

Abstract

The direct conversion of syngas to aromatics has received considerable attention. The development of a highly active and stable catalyst with tunable selectivity for the synthesis of aromatics remains a great challenge, notwithstanding the understanding of the reaction mechanism. In this study, we fabricated a series of high-performance bifunctional catalysts comprising CoMnAl catalysts, which contain Co 2 C nanoprisms as the active phase, and versatile HZSM-5@silicalite-1 zeolites, with a hollow nanostructure and a scaly shell, for the transformation of syngas into aromatics. The catalysts achieve a CO conversion >70% under mild reaction conditions (280°C, 2 MPa), with ultrahigh stability. The selectivity toward aromatics is higher than 63% with <2.9% selectivity toward CH 4 , and the para -xylene (PX) fraction of the aromatics produced reaches 34.7%, leading to unprecedentedly high space-time yield of PX. Furthermore, the reaction pathway and the intermediates involved in the formation of the aromatic products are also determined. • ZSM-5@silicalite-1 zeolite with a hollow nanostructure and a scaly shell is prepared • CoMnAl/HZSM-5@S1 composite catalyst shows high activity, selectivity, and stability • Versatile HZSM-5 zeolite enables unprecedentedly high yield of para -xylene at 280°C • Oxygen-containing aromatic compounds play a crucial role in aromatics formation The direct transformation of syngas to aromatics, especially para -xylene (PX), has received considerable attention and can be achieved using tandem catalysts through methanol or olefin intermediates. However, it remains a great challenge to develop efficient catalysts with high catalytic activity, selectivity, and stability. We fabricated nanosized HZSM-5 with a hollow structure to enhance the diffusion of the intermediates and products, improving both the CO conversion and the aromatic selectivity. The addition of a scaly silicalite-1 layer grown epitaxially on the HZSM-5 crystals effectively inhibits side reactions from occurring without affecting the diffusion property of the zeolites; thus, the conversion of syngas to PX is improved, and catalyst shows ultrahigh stability. In addition to the development of a high-performing catalyst with industrial application potential, this work significantly improves our understanding of the complex reaction network of syngas to aromatics. To develop a highly active and stable catalyst with tunable selectivity for syngas to aromatics, especially para -xylene, versatile zeolites with a hollow nanostructure and a scaly shell were fabricated and combined with CoMnAl catalysts, with Co carbide nanoprisms as the active phase. The combined catalyst exhibited ultrahigh catalytic activity, selectivity, and stability under mild reaction conditions. Oxygen-containing aromatic compounds produced through the coupling of CO with olefins or higher oxygenates play a crucial role in the formation of aromatics.