Confined Carbon Mediating Dehydroaromatization of Methane over Mo/ZSM‐5

Abstract

Non-oxidative dehydroaromatization of methane (MDA) is apromising catalytic process for direct valorization of natural gas to liquid hydrocarbons.T he application of this reaction in practical technology is hindered by al acko f understanding about the mechanism and nature of the active sites in benchmark zeolite-based Mo/ZSM-5 catalysts,w hich precludes the solution of problems such as rapid catalyst deactivation. By applying spectroscopya nd microscopy, it is shown that the active centers in Mo/ZSM-5 are partially reduced single-atom Mo sites stabilized by the zeolite framework. By combining ap ulse reaction techniquew ith isotope labeling of methane,M DA is shown to be governed by ah ydrocarbon pool mechanism in whichb enzenei sd erived from secondary reactions of confined polyaromatic carbon species with the initial products of methane activation.The abundance of natural gas reserves calls for the development of an efficient conversion technology to upgrade its principal component, methane,i nto easily transportable chemicals.[1] Several catalytic technologies,w hich could replace the current indirect route involving an expensive synthesis gas generation step,a re being considered. Broadly, we can distinguish between oxidative and non-oxidative direct routes.[2] Among the non-oxidative approaches,c atalytic methane dehydroaromatization (MDA) is one of the most promising methods.A fter the initial reports on MDA almost three decades ago, [3] as ubstantial body of literature has appeared.[4] Thei ndustrial implementation of the MDA process is mainly hindered by rapid catalyst deactivation caused by the deposition of ac arbonaceous material that blocks the catalytically active sites. [5] Although there have been remarkable achievements in regeneration procedures, [6] developing astable MDAcatalyst is still required to arrive at acommercial process.Aprogress in this direction is seriously hampered by limited understanding of the active sites in the benchmark Mo/ZSM-5 catalyst and the mechanism of methane conversion to benzene and hydrogen. Despite considerable debate on the nature of the active phase,t here is ag rowing consensus that the active sites are confined as highly dispersed Mo species by the zeolite pores in working Mo/ZSM-5 catalysts and that Mo 2 Cn anoparticles on the external surface are inactive. [7] Concerning the reaction mechanism, most reports support ab ifunctional pathway in which methane is activated and coupled to ethylene over Mocarbide species,followed by ethylene aromatization over the zeolite Brønsted acid sites. [8] Important challenges in gaining insight into these aspects are the high reaction temperature at which the MDAreaction takes place and its transient nature,w hich involves rapid activation and deactivation stages when the fresh Mo/ZSM-5 catalyst is exposed to am ethane feed. These factors complicate operando spectroscopy and kinetic investigations.A valuable approach in this regard is to increase the temporal resolution by pulsing the reactant over the catalyst an...

Progress in Developing a Structure‐Activity Relationship for the Direct Aromatization of Methane

“…In some sense, the metal‐zeolite system is comparable to e. g. an organometallic complex grafted on silica, where silanol OH groups serve as the anchor, while an acidic OH group serves this purpose on a zeolite . Many studies suggest that only the Mo species anchored to the framework Al through oxygen bridges are actually able to activate methane ,,. This is consistent with what has been proposed for the methane to methanol reaction, and for SCR of N 2 O …”

“…Similarities can also be expected with the Methanol to Hydrocarbons (MTH) process, for which a hydrocarbon pool mechanism is widely accepted . Indeed a hydrocarbon pool was recently also proposed to play a crucial role in MDA, although it was suggested to proceed via aromatic radicals instead of carbocations . A series of dehydrogenation, cracking, isomerization, oligomerization, dehydrocyclization, β‐scission, protolysis and hydride transfer reactions has to be considered .…”

“…A couple of indirect methods are available to characterize the dispersion of the metal, probing how many acid sites are replaced by a metal. This was effectively achieved by H/D exchange,,, 1 H NMR,, 27 Al NMR, NH 3 ‐TPD,,, and probing adsorbed molecules by FTIR,. CH 4 ‐TPR and H 2 ‐TPR were also used to study the ease of reduction of the Mo species present on the zeolite.…”

“…In addition, since the active species form at reaction conditions, operando characterization is necessary to spot them. It was by o perando X‐ray Absorption Spectroscopy (XAS) studies that the evolution of the Mo oxidation state during activation was first confirmed ,,. Another issue relates to the time‐resolution.…”

“…Therefore experiments need to be designed carefully to properly follow the evolution of the active phase. This has been achieved by using a pulsed reaction technique ,. This powerful technique also in combination with quasi‐in‐situ studies enables the preparation of samples during particular phases of active site formation and allows studying the development of the active site with a variety of techniques ,.…”