Synthesis of Zeolite Catalysts Containing Zn and Ga for Aromatization of n-Hexane

Abstract

The aromatization of light alkanes including ethane, propane, and n-hexane is an important process for producing value-added aromatic compounds, especially benzene, toluene, and xylenes (BTX), because the aromatization process can be adapted to use natural gas, to meet the increased demands for raw chemicals in the petrochemical industry in recent decades. For the aromatization of alkanes, various metal-loaded zeolite catalysts based on ZSM-51)~14), MCM-2215), ZSM-1116), and L zeolite17)~21) have been extensively developed. In particular, Znor Ga-containing ZSM-5 provides high catalytic activities and selectivities for aromatic compounds because of the combination of the dehydrogenation over the metal species and Brønsted acidity of the ZSM-5 zeolite1),3),5),6),10). Furthermore, the metal species are highly dispersed on ZSM-5 through the replacement of protons on ZSM-5 with the metal species by ion exchange and reducing-oxidizing treatment to provide active sites such as cationic metal species instead of bulk ZnO and Ga2O322)~25). Since the aromatization of alkanes fundamentally proceeds via the dehydrogenation of alkanes, it is important to increase the number of active metal sites for improving the catalytic performances. The incorporation of metal species into the framework of zeolite through the hydrothermal synthesis leads to the formation of highly dispersed and isolated metal species in the zeolite. In addition, the incorporation of trivalent Ga species, instead of Al species, into the zeolite framework generates Brønsted acid sites with weaker acidity than those generated by the incorporation of Al species24), resulting in a bifunctional catalyst for the aromatization of alkanes26)~30). We previously found that Zn-containing MFI zeolite synthesized by the hydrothermal treatment in the absence of Al species exhibited a high catalytic activity for the dehydrogenation of n-butane to produce 1,3-butadiene31). Introducing Zn species into the zeolite would form isolated active Zn sites with high dehydrogenation activity. Therefore, it is expected that the formation of both isolated Zn and Ga sites in zeolite would improve the catalytic performances of zeolite catalysts for the aromatization (dehydrocyclization) of alkanes to selectively produce BTX. Here we report the direct synthesis of MFI-type zeo[Research Note]