Stable Mo/HZSM-5 methane dehydroaromatization catalysts optimized for high-temperature calcination-regeneration

Nikolay Kosinov,Ferdy J.A.G Coumans,Guanna Li,Evgeny Uslamin,Brahim Mezari,Alexandra S.G Wijpkema,Evgeny A Pidko,Emiel J.M Hensen

doi:10.1016/j.jcat.2016.12.006

Nikolay Kosinov, Ferdy J.A.G Coumans + Show 6 more

Open Access

https://doi.org/10.1016/j.jcat.2016.12.006

Copy DOI

Journal: Journal of Catalysis	Publication Date: Jan 11, 2017
Citations: 148	License type: cc-by

Affiliation: Eindhoven University of Technology

Abstract

Dehydroaromatization of methane is a promising reaction to directly convert methane into aromatics and hydrogen. The main drawback of this reaction is the rapid deactivation of the Mo/HZSM-5 catalyst due to coking. Regeneration at high reaction temperature by air calcination is not possible due to extensive dealumination of the zeolite. We investigated the structural and textural stability of HZSM-5 as a function of the Mo loading in air at high temperature (550–700°C) and demonstrated that lowering the Mo loading below 2wt% greatly improves the oxidative stability of Mo/HZSM-5. At low Mo loading (1–2wt% Mo), Mo is predominantly in the zeolite micropores as cationic mono- and dinuclear Mo-oxo complexes irrespective of the calcination temperature. At higher loading, most of the initially aggregated Mo-oxide at the external surface is dispersed into the micropores upon calcination above 550°C, resulting in reaction of mobile MoO3 species with framework Al, aluminum molybdate formation and irreversible damage to the zeolite framework. A DFT-based free energy analysis indicates that water formation from reaction of MoO3 with Brønsted acid sites and high concentration of Mo during MoO3 migration causes aluminum molybdate formation. The high oxidative stability of Mo/HZSM-5 with low Mo loading makes them suitable candidates for a novel isothermal (700°C) reaction – air regeneration protocol of methane dehydroaromatization. Whereas a 5wt% Mo/HZSM-5 rapidly lost its initial activity, an optimized 2wt% Mo/HZSM-5 catalyst retained more than 50% of its initial activity after 100 reaction-regeneration cycles (1week) with a substantially improved total aromatics yield.

Highlights

Abundant reserves of natural gas together with improved technologies of its production make it important to find efficient ways to convert methane to more valuable hydrocarbons such as liquid fuels and chemical building blocks
Whereas a 5 wt% Mo/HZSM-5 rapidly lost its initial activity, an optimized 2 wt% Mo/ HZSM-5 catalyst retained more than 50% of its initial activity after 100 reaction-regeneration cycles (1 week) with a substantially improved total aromatics yield
We investigated the thermal stability of the Mo/HZSM-5 samples by thermogravimetric analysis (TGA)

Summary

Introduction

Abundant reserves of natural gas together with improved technologies of its production make it important to find efficient ways to convert methane to more valuable hydrocarbons such as liquid fuels and chemical building blocks. Coke is a mixture of heavy carbonaceous deposits of pregraphitic nature, which block the zeolite micropores and the active MoOxCy centers, leading to the inevitable decline in catalytic performance [25,26,27] It is relatively high rate of deactivation of Mo/HZSM-5 catalysts in this reaction that hinders its commercialization. The novel insight provided by the combined experimental and computational efforts is that at low Mo content the zeolite composite is much more stable during air calcination at 700 °C than at high Mo content This opens the possibility to calcine the partially coked catalyst at reaction temperature and, in this way, to improve drastically the catalytic stability. This strategy holds great promise for the development of a scalable methane dehydroaromatization process based on in situ regeneration of the catalyst

Catalyst preparation

Characterization

Catalytic activity measurements

Catalyst characterization

Catalytic activity

Conclusions