Investigation of methanol conversion over high-Si beta zeolites and the reaction mechanism of their high propene selectivity

Author(s): Xuebin Zhao ¹ ^, ² ^, ³ ^, ⁴ ^, ⁵ , Linying Wang ¹ ^, ² ^, ³ ^, ⁴ ^, ⁵ , Jinzhe Li ¹ ^, ² ^, ³ ^, ⁴ ^, ⁵ , Shutao Xu ¹ ^, ² ^, ³ ^, ⁴ ^, ⁵ , Wenna Zhang ¹ ^, ² ^, ³ ^, ⁴ ^, ⁵ , Yingxu Wei ¹ ^, ² ^, ³ ^, ⁴ ^, ⁵ , Xinwen Guo ⁶ ^, ⁷ ^, ⁸ ^, ⁹ ^, ¹⁰ , Peng Tian ¹ ^, ² ^, ³ ^, ⁴ ^, ⁵ , Zhongmin Liu ¹ ^, ² ^, ³ ^, ⁴ ^, ⁵

Publication date (Electronic): 2017

Journal: Catalysis Science & Technology

Publisher: Royal Society of Chemistry (RSC)

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Abstract

Large pore high-Si beta zeolites (Si/Al = 136 to 340) were synthesized by a HF-assisted method, and their catalytic performance for the conversion of methanol to propene was explored.

Abstract

Large pore high-Si beta zeolites (Si/Al = 136 to 340) were synthesized by a HF-assisted method, and their catalytic performance for the conversion of methanol to propene was explored. It is demonstrated that beta zeolites with low acid density facilitate the achievement of high propene selectivity and a high propene/ethene ratio. The HF dosage in the synthesis has great influence on the Al distribution in the framework, as evidenced by ²⁷Al MAS NMR and ²⁷Al MQ MAS NMR spectroscopy, which may influence the acidity and microstructure of acid sites and lead to a remarkable catalytic lifespan. A HF/SiO ₂ ratio of 0.45 is found to facilitate the synthesis of high-Si beta enriched with Al atoms located at the T9 sites; this helps the catalyst show the longest lifetime, with a propene selectivity of 49.7–58.3% at 550 °C and WHSV = 2 h ⁻¹. With the aid of ¹²C/ ¹³C-methanol switch experiments, we elucidated that the olefin-based mechanism dominates the reaction and contributes to the formation of ethene, propene, and higher olefins. Moreover, two phenol compounds are identified in the coke species, which have not been observed previously and have been found to be detrimental to the reaction.

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Methanol to Olefins (MTO): From Fundamentals to Commercialization

Zhongmin Liu, Peng Tian, Mao Ye … (2015)

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The mechanism of methanol to hydrocarbon catalysis.

Weiguo Song, Warren D Marcus, J Haw … (2003)

The process of converting methanol to hydrocarbons on the aluminosilicate zeolite HZSM-5 was originally developed as a route from natural gas to synthetic gasoline. Using other microporous catalysts that are selective for light olefins, methanol-to-olefin (MTO) catalysis may soon become central to the conversion of natural gas to polyolefins. The mechanism of methanol conversion proved to be an intellectually challenging problem; 25 years of fundamental study produced at least 20 distinct mechanisms, but most did not account for either the primary products or a kinetic induction period. Recent experimental and theoretical work has firmly established that methanol and dimethyl ether react on cyclic organic species contained in the cages or channels of the inorganic host. These organic reaction centers act as scaffolds for the assembly of light olefins so as to avoid the high high-energy intermediates required by all "direct" mechanisms. The rate of formation of the initial reaction centers, and hence the duration of the kinetic induction period, can be governed by impurity species. Secondary reactions of primary olefin products strongly reflect the topology and acid strength of the microporous catalyst. Reaction centers form continuously through some secondary pathways, and they age into polycyclic aromatic hydrocarbons, eventually deactivating the catalyst. It proves useful to consider each cage (or channel) with its included organic and inorganic species as a supramolecule that can react to form various species. This view allows us to identify structure-activity and structure selectivity relationships and to modify the catalyst with degrees of freedom that are more reminiscent of homogeneous catalysis than heterogeneous catalysis.