Journal
MICROPOROUS AND MESOPOROUS MATERIALS
Volume 341, Issue -, Pages -Publisher
ELSEVIER
DOI: 10.1016/j.micromeso.2022.112051
Keywords
Methanol to olefins; ZSM-11 zeolite; Acid sites distribution; Halogen anions; Aluminum siting; Zeolite framework
Categories
Funding
- National Key R&D Program of China [2018YFB0604802, 2020YFB0606402]
- National Natural Science Foundation of China [21991092, U1910203, 21875275]
- Strategic Priority Research Program of the Chinese Academy of Sciences (CAS) [XDA21020500]
- Natural Science Foundation of Shanxi Province of China [201901D211581]
- Youth Innovation Promotion Association CAS [2021172]
- Autonomous research project of State Key Laboratory of Coal Conversion, Institute of Coal Chemistry, CAS [2020BWZ004]
- Young Talent Training Program of State Key Laboratory of Coal Conversion, Institute of Coal Chemistry, CAS [2021BWZ003]
- Excellent doctoral student award and subsidy program of Shanxi Province [BK2018001]
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Purposefully regulating the spatial distribution of acid sites in a zeolite catalyst can effectively enhance its performance in the conversion of methanol to olefins (MTO). The use of different sodium halides in the synthesis of ZSM-11 zeolites was found to influence the location and distribution of acid sites, which in turn affected the catalytic performance in MTO. The presence of Br- or I- anions in the synthesis gel led to a uniform distribution of Al species and improved catalytic stability and selectivity in MTO.
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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