Regulating the distribution of acid sites in ZSM-11 zeolite with different halogen anions to enhance its catalytic performance in the conversion of methanol to olefins
Microporous and Mesoporous Materials(2022)
摘要
Purposive regulation of the spatial location and distribution of acid sites in a zeolite catalyst, though still rather challenging, has proved to be an effective measure to enhance its performance in the conversion of methanol to olefins (MTO). Herein, a series of ZSM-11 zeolites were synthesized using different sodium halides (with tetrabutylammonium hydroxide (TBAOH) as the structure-directing agent); the effect of halogen anions (Cl-, Br- and I-) in the synthesis gel on the Al siting and thereon the spatial location and distribution of acid sites in the ZSM-11 zeolites as well as their relation to the catalytic performance in MTO was elaborately investigated. The results indicate that for the synthesis of high Si/Al ratio ZSM-11 zeolites (Si/Al > 60), the Cl- anions present in the synthesis gel can induce more Al atoms siting on the external surface of ZSM-11 crystals, thereby leading to a short catalytic lifetime in MTO due to the rapid coke deposition. In contrast, larger Br- or I- anions in the synthesis gel, owing to their stronger covalent interaction with the TBA+ cations, may moderate the formation of crystal silicate nuclei and lead to a uniform distribution of Al species in the inner straight channels, which can effectively suppress the surface coke deposition and inhibit the aromatic-based cycle for MTO. As a result, the high Si/Al ratio ZSM-11 zeolites obtained with Br- or I- in the synthesis gel exhibit much better catalytic stability in MTO and higher selectivity to propene and butene, in comparison with the ZSM-11 counterpart synthesized using Cl-. These results help to clarify the relation between the catalytic performance of ZSM-11 in MTO and the location of acid sites and thus bring forward a facile strategy to enhance the catalytic performance of zeolites through purposively regulating the spatial distribution of acid sites.
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关键词
Methanol to olefins,ZSM-11 zeolite,Acid sites distribution,Halogen anions,Aluminum siting,Zeolite framework
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