Selective Synthesis of Benzene, Toluene, and Xylenes from Syngas

Abstract

Although direct synthesis of aromatics from synthesis gas (syngas) using a bifunctional catalyst composed of metal oxide and zeolite (OX–ZEO) has attracted extensive attention, the selectivity of benzene, toluene, and xylenes (BTX) remains a challenge. Herein, we show that a hybrid dual bed by packing a zeolite bed below the MnCr–ZSM-5 composite significantly enhances BTX formation. For instance, the BTX fraction in aromatics reaches over 80% when the second bed is beta zeolite and rises to 88% if an ultrastable Y zeolite (USY) is used. Studies show that the zeolite structure of three-dimensional (3D) channels of the 12-membered ring plays an important role in facilitating the BTX formation. Passivation of the external acid sites of zeolites further enhances the BTX selectivity. However, too much silica on the surface of ZSM-5 could narrow the pore opening and is detrimental for the diffusion of BTX, resulting in a lowered BTX selectivity. This hybrid catalyst system provides a promising strategy for direct production of BTX from syngas.