Reactivity, Selectivity, and Stability of Zeolite‐Based Catalysts for Methane Dehydroaromatization

Abstract

strong C-H bonds complicates the selective conversion of methane to higher hydrocarbons and oxygenates. Currently, the main industrial method to use methane as a material feedstock involves the transformation to syngas by steam reforming, followed by processes such as methanol synthesis and Fischer-Tropsch synthesis to obtain liquid fuels and other useful chemicals. A disadvantage of this indirect technology is the high capital cost of reforming plants, making it only economically attractive at a very large scale. An efficient direct process for the conversion of methane to higher hydrocarbons or oxygenates remains one of the "holy grails" of chemistry. [6,7] The approaches to directly convert methane can be categorized into oxidative and non-oxidative methods. Oxidative processes are often met with low product selectivity because of the higher reactivity of targeted products as compared to methane itself, resulting in overoxidation to CO 2. Non-oxidative methods are generally more atom-efficient, because, in addition to valuable hydrocarbon products, pure CO xfree hydrogen gas (H 2) is obtained, which can be used for instance in fuel cells. [8] Among several alternatives, including non-oxidative coupling of methane to ethylene and deep methane dehydrogenation to solid carbon and hydrogen, [9-12] methane dehydroaromatization (MDA) remains one of the most promising non-oxidative reactions for the direct valorization of natural gas. MDA involves the selective conversion of methane to a mixture of easily transportable aromatics, that is, benzene (predominantly), naphthalene, and toluene. As all other non-oxidative methane conversion reactions, MDA is a highly endothermic process: 6CH C H 9H 532 kJ mol Non-oxidative dehydroaromatization is arguably the most promising process for the direct upgrading of cheap and abundant methane to liquid hydrocarbons. This reaction has not been commercialized yet because of the suboptimal activity and swift deactivation of benchmark Mo-zeolite catalysts. This progress report represents an elaboration on the recent developments in understanding of zeolite-based catalytic materials for high-temperature non-oxidative dehydroaromatization of methane. It is specifically focused on recent studies, relevant to the materials chemistry and elucidating i) the structure of active species in working catalysts; ii) the complex molecular pathways underlying the mechanism of selective conversion of methane to benzene; iii) structure, evolution and role of coke species; and iv) process intensification strategies to improve the deactivation resistance and overall performance of the catalysts. Finally, unsolved challenges in this field of research are outlined and an outlook is provided on promising directions toward improving the activity, stability, and selectivity of methane dehydroaromatization catalysts.

Identification of Kinetic and Spectroscopic Signatures of Copper Sites for Direct Oxidation of Methane to Methanol

“…Thei nterest in the direct conversion of methane to commodities is continuously growing. [1][2][3][4] Thel ow price of natural gas and its abundance are drivers to find novel routes of methane conversion to functional molecules,which can be inserted in the existing production chains.T he major directions in the direct one-step functionalization of methane are oxidative coupling, [5] non-oxidative aromatization, [6,7] dry reforming, [8] and oxidation to methanol. [9][10][11][12][13][14][15][16][17][18][19][20][21][22] Operation of truly catalytic and continuous conversion of methane to methanol requires either the use of expensive oxidants,s uch as hydrogen peroxide and nitrous oxide, [23,24] or extremely diluted oxygen feedstock to avoid over-oxidation to CO x , [25][26][27] which is thermodynamically favored.…”

Dual Active Sites on Molybdenum/ZSM‐5 Catalyst for Methane Dehydroaromatization: Insights from Solid‐State NMR Spectroscopy

“…The commercial application of the MDA process is hampered by the rapid deactivation of catalysts [7–9] . Considerable research effort has been invested in improving the MDA catalyst and process with focus on the understanding of the active sites and reaction mechanism [10, 11] …”

Introducing Methane Activation

“…Since then, various catalysts based on metal ions (Mo, Fe, Zn, Cr, V, W, etc.) dispersed on zeolites (e.g., ZSM-5, MCM-22, ZSM-8, MCM-49, and mordenite) have been developed and tested to improve MDR performance [44][45][46][47]. Detailed catalyst summary and design concept, as well as the suggested mechanism, will be elucidated in Sect.…”