User:BrxBrx/methane dehydroaromatization

Methane dehydroaromatization reactions are a class of industrially important reactions in which methane is catalytically converted non-oxidatively into economically valuable alkenes and aromatic compounds. In contrast to processes that require partial oxidation of methane to methanol or other oxidized materials, or through the subsequent cracking of syngas, non-oxidative direct conversion is more attractive due to high selectivity to the target product, and increased atom efficiency. While the current methane to higher hydrocarbon process is dominated by the indirect conversion routes, direct conversion, for the reasons of potential economy, and efficiency, are an active research area. Industrially, methane dehydroaromatization process is chiefly carried out by zeolite supported catalysts, however, routes involving Frustrated Lewis pairs and photocatalysis are also scientifically prominent.

C-H bond activation
Carbon–hydrogen bond functionalization reactions are extremely attractive industrially because of the possibility of building larger organic molecules direction. However, due to the strength of the C-H bond, breaking this bond selectively in the general case is very difficult - and thus for the majority of the history of organic chemistry, carbon-carbon couplings are done through more reactive intermediates. The C-H bond in methane in particular has a very high enthalpy, and consequently, breaking it without forming an oxidized product has been difficult. Historically, this has been exploited, by the partial oxidation of methane over catalysts to form methanol or formaldehyde(cite Caballero), or by simply converting it into higher alkanes through the various Gas to liquids processes - which yields a variety of higher alkanes and alcohols.(cite Höök)

Zeolite Supported Catalysts
The first reported process for methane dehydroaromatization was reported in 1993, with a molybdenum modified zeolite HZSM-5, under which methane, when passed over the zeolite at elevated temperatures, in the absence of oxygen, was converted to aromatics, chiefly benzene, and hydrogen gas(cite wang). Since then, a number of other zeolites have been reported to have the same reactive properties, providided their pore size is of similar size (near 5.6 angstroms), and have been modified molybdenum, rhenium, tungsten, cobalt, gallium and other similar metals. The mechanism is believed to be a bifunctional catalyst. In the most studied molybdenum modified catalysts, Molybdenum trioxide centers are reduced by methane to form a molybdenum carbide material of uncertain composition, $$\mathrm{Mo_x C_y}$$. (cite Vollmer). These molybdenum carbide centers then catalyse the methane C-H bond activation, through a variety of proposed mechanisms, forming reactive $$\mathrm{C_2 H_y}, y\leq 4$$ intermediates that are adsorbed to the carbide, or are gas phase in the zeolite channels. Finally, these intermediates are oligomerized and cyclized by an Bronsted acid-catalyzed reaction to form benzene and higher aromatics, on the acidic sites of the zeolite. The main mechanism for catalyst deactivation is through the accumulation of elemental carbon, forming a coke-like substance, which is accelerated at high temperatures.