Tuning the siting of aluminum in ZSM-11 zeolite and regulating its catalytic performance in the conversion of methanol to olefins

The catalytic performance of acid zeolite in the conversion of methanol to olefins (MTO) is closely related to the location of aluminum (Al) in the lattice sites; however, it is still a great challenge to finely tune the siting of framework Al and thereon purposefully regulate the performance of zeolite catalyst. In this work, adding proper alkali metal ions in the synthesis gel was used to alter the Al siting in ZSM-11 zeolite and the effect of Al siting on the catalytic performance of ZSM-11 in MTO was then investigated. The results indicate that the addition of Na+ and/or Li+ in the synthesis gel can elevate the fraction of Al sited in the intersection cavity of ZSM-11, whereas in the alkali metal ions free synthesis system, an increase in the Si/Al ratio leads to a more prominent decrease of Al sited in the intersection cavity and a relative enrichment of Al distributed in the straight channel. The variation of Al siting has a significant influence on the catalytic performance of ZSM-11 in MTO, which can be well explained by the dual-cycle hydrocarbons pool (HCP) mechanism. More Al atoms located in the straight channel can enhance the alkene-based cycle that produces more propene and butene; in contrast, an increase in the fraction of Al in the intersection cavity can then promote the aromatic-based cycle and facilitate the formation of ethene and aromatics. This work provides an efficient method to finely tune the siting of Al in ZSM-11 zeolite lattices and controllably regulate its catalytic performance in MTO; moreover, the insight shown in this work is also helpful for a better understanding on the relationship between the Al siting/acid sites distribution and the performance of a zeolite catalyst. (C) 2019 Elsevier Inc. All rights reserved.