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Insights into the reaction mechanism of n-hexane dehydroaromatization to benzene over gallium embedded HZSM-5: effect of H2 incorporated on active sites

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Abstract

The catalytic dehydroaromatization of alkanes to aromatics has attracted considerable attention from the scientific community, because it can be used for the upgrading of low-cost alkanes into high added-value aromatics, such as benzene, toluene, and xylene (BTX). In this context, we report the reaction mechanism of n-hexane dehydroaromatization to benzene over two different reduced gallium species embedded in HZSM-5, including univalent Ga+ embedded in HZSM-5 (Ga/HZSM-5) and dihydrido gallium complex (GaH2+) embedded in HZSM-5 (GaH2/HZSM-5) by using the M06-2X/6-31G(d,p) level of calculation. The reaction proceeds by following two main steps: (i) the dehydrogenation of hexane to haxa-1,3,5-triene; (ii) the dehydroaromatization of haxa-1,3,5-triene to benzene. For the univalent Ga+ embedded in HZSM-5, the first step of the hexane dehydrogenation is considered to be the rate-determining step, which requires a high activation energy of 76.6 kcal mol−1. In strong contrast to this, in the case of the GaH2/HZSM-5 catalyst the rate determining step is found to be the second hydrogen abstraction from n-hexane with a lower activation barrier of 11.1 kcal mol−1. The reaction is therefore preferentially taking place over the GaH2/HZSM-5 catalyst. These observations clearly confirm the existence of a dihydrido gallium complex (GaH2+) as one of the most active species for the dehydroaromatization of alkanes and it is obtained in the presence of hydrogen in the catalytic system. This example opens up perspectives for a better understanding of the effect of active species on the catalytic reaction.

Graphical abstract: Insights into the reaction mechanism of n-hexane dehydroaromatization to benzene over gallium embedded HZSM-5: effect of H2 incorporated on active sites

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Publication details

The article was received on 17 Sep 2018, accepted on 19 Nov 2018 and first published on 19 Nov 2018


Article type: Paper
DOI: 10.1039/C8CP05864D
Citation: Phys. Chem. Chem. Phys., 2019, Advance Article
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    Insights into the reaction mechanism of n-hexane dehydroaromatization to benzene over gallium embedded HZSM-5: effect of H2 incorporated on active sites

    A. Thivasasith, T. Maihom, S. Pengpanich, J. Limtrakul and C. Wattanakit, Phys. Chem. Chem. Phys., 2019, Advance Article , DOI: 10.1039/C8CP05864D

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