Controlling the Evolution of Active Molybdenum Carbide by Moderating the Acidity of Mo/HMCM-22 Catalyst in Methane Dehyd
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Controlling the Evolution of Active Molybdenum Carbide by Moderating the Acidity of Mo/HMCM‑22 Catalyst in Methane Dehydroaromatization Sourabh Mishra1,2 · M. Ali Haider2 · K. K. Pant1 Received: 8 March 2020 / Accepted: 19 May 2020 © Springer Science+Business Media, LLC, part of Springer Nature 2020
Abstract Effect of framework Brønsted Acidity by varying SiO2/Al2O3 ratio (SAR) of HMCM-22 zeolite is studied with respect to the formation of active molybdenum carbide and its anchoring over zeolite (HMCM-22) channels of Mo/HMCM-22 catalyst tested for methane dehydroaromatization (MDA) reaction. For this purpose, HMCM-22 is synthesized by conventional methods with varying SAR (30, 40 & 55) and is studied for MDA reaction with 5 wt% Mo loading. XRD, BET, NH3-TPD, H2-TPR, 27Al MAS NMR, Raman spectroscopy and XPS techniques are used to characterize 5Mo/HMCM-22 catalyst having different SAR. XPS analysis of carburized 5Mo/HMCM-22 (SAR-30, 40 & 55) catalyst confirms that higher content of molybdenum carbide (Mo2C) forms over HMCM-22 channels at SAR-30 as compared to SAR-40 and SAR-55 due to effective binding of initial MoOx species. Interaction of initial MoOx species with HMCM-22 zeolite framework is analyzed using 27Al MAS NMR and Raman spectroscopic studies which confirm that MoOx species bind differently at SAR-30, 40 and 55 which affects the catalytic performance. Lower reducibility of M oOx species at SAR-30 confirms that M oOx species strongly interact with HMCM-22 channels at SAR-30 as confirmed by H2-TPR study. Maximum transformation of MoOx species into active molybdenum carbide over HMCM-22 at lower SAR (30) during carburization results in higher activity of 5Mo/HMCM-22 (SAR-30) catalyst with lower coke content.
* M. Ali Haider [email protected] * K. K. Pant [email protected] 1
Catalytic Reaction Engineering Lab, Department of Chemical Engineering, Indian Institute of Technology Delhi, Hauz Khas, New Delhi, India
Renewable Energy and Chemicals Lab, Department of Chemical Engineering, Indian Institute of Technology Delhi, Hauz Khas, New Delhi, India
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Graphic Abstract
Keywords Methane dehydroaromatization · Molybdenum carbide · MoOx anchoring · Brønsted acidity
1 Introduction In an attempt to convert natural gas to chemicals, direct methane dehydroaromatization (MDA) in non-oxidative conditions is considered to be an important route to produce aromatic compounds [1, 2]. In 1993, Wang et al. have reported the activity of Mo/HZSM-5 catalyst for the MDA process showing high selectivity (> 90%) for benzene at around 700 °C and atmospheric pressure [3]. Afterwards, several studies have directed efforts towards understanding the nature and evolution of active Mo species and role of zeolite support in the MDA reaction [4]. Since equilibrium conversion of methane (~ 11%) to aromatics has been observed low at 700 °C, prolong operations are desirable in the MDA process. In this attempt, Mo/HZSM-5 catalyst has shown severe coke formation in the
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