A Comparative Study of n -Butane Isomerization over H-Beta and H-ZSM-5 Zeolites at Low Temperatures: Effects of Acid Pro
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A Comparative Study of n-Butane Isomerization over H-Beta and H-ZSM-5 Zeolites at Low Temperatures: Effects of Acid Properties and Pore Structures Wenfang Zhang1 · Pengzhao Wang2 · Chaohe Yang1 · Chunyi Li1 Received: 25 September 2018 / Accepted: 23 January 2019 © Springer Science+Business Media, LLC, part of Springer Nature 2019
Abstract The influences of acidic properties and pore structures of H-Beta and H-ZSM-5 zeolites on the reaction properties of n-butane isomerization at low temperatures were investigated. The results showed that bimolecular pathway of n-butane conversion predominates over H-ZSM-5 zeolites, while the monomolecular and bimolecular pathways occur simultaneously over H-Beta zeolites. The conversion rate of n-butane strongly relies on the amount of strong Brønsted acid sites regardless of zeolite topology. However, the topology of zeolites crucially determines the products distribution, and the density of strong Brønsted acid sites plays a secondary role. The cavities of zeolites, formed in the intersections of channels, provide the places for the bimolecular reaction. The formation of trimethyl C8 intermediates is spatially restricted in the narrow channel intersections of H-ZSM-5 zeolites, resulting in higher contribution of n-butane disproportionation reaction. In addition, the narrow pore channels of H-ZSM-5 zeolite limit the monomolecular isomerization of n-butane molecules and affect the diffusion of heavier products (pentane) produced from bimolecular reaction, leading to the severe secondary reaction and high selectivity to propane. In contrast, the pore channels of H-Beta zeolite allow the monomolecular isomerization of n-butane and the deposition of coke. Graphical Abstract The topology of zeolites crucially determines the products distribution.
Keywords Zeolites · Acidity · Topology · n-Butane isomerization
Electronic supplementary material The online version of this article (https://doi.org/10.1007/s10562-019-02683-0) contains supplementary material, which is available to authorized users. * Chunyi Li [email protected] Extended author information available on the last page of the article
1 Introduction In recent years, n-butane isomerization has become an attractive research subject with the increasing demand of isobutane in the production of alkylation gasoline, which fits well with the specifications of reformulated gasoline [1–7].
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Except for commercial application, the reaction mechanism of n-butane skeletal isomerization over acid catalysts is of interest [8–10]. There have been large amount of investigations attempt to find and develop a zeolite catalyst that has available acidity and shape selectivity for n-butane isomerization [11–16]. Within those studies, different reaction mechanisms of n-butane isomerization over zeolites are proposed. For instance, many studies reported that n-butane is converted via bimolecular pathway over Mordenite, involving the formation of C8 carbenium ions [17–19]. However, some researchers pr
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