On reaction pathways in the conversion of methanol to hydrocarbons on HZSM-5

The Role of Non-Framework Lewis Acidic Al Species of Alkali-Treated HZSM-5 in Methanol Aromatization

“…C3 + hydrocarbons and ethylene profiles were quite similar; their desorption temperatures were quite close to each other in the temperature range (250-400 °C) of an autocatalysis system [34][35][36]. Ethylene is a tracer of the arene cycle, while C3 + hydrocarbons are produced through the alkene cycle [36][37][38]. The closely-neighboring desorption temperatures of C3 + hydrocarbons and ethylene emphasize that arene and alkene cycled proceed concurrently and are highly interwined [39].…”

“…The desorption peak of DME increased with the following trend: HZ-D (230 °C) < HZ (249 °C) < HZ-DA (256 °C), in line with the Brønsted acidity observed from NH3-TPD. C3 + hydrocarbons and ethylene profiles were quite similar; their desorption temperatures were quite close to each other in the temperature range (250-400 °C) of an autocatalysis system [34][35][36]. Ethylene is a tracer of the arene cycle, while C3 + hydrocarbons are produced through the alkene cycle [36][37][38].…”

“…The low-temperature responses should be related to the contributions of ionized fragments (m/e = 16 and 2) of methanol [40] because the formation of methane and the desorption of dihydrogen are thermodynamically unfavorable below 300 °C [41,42]. Methane originates from the reduction of chemisorbed methoxy species through the hydride transfer [36,42], while dihydrogen should be produced from the combination of two chemisorbed hydrides [41]. Accordingly, the desorption behavior of methane and dihydrogen is associated with hydride mobility.…”

“…That is, the higher the Lewis acid concentration, the more mobile hydride can exist. Thus, aromatic molecules are subsequently formed via the hydride transfer of olefin precursors [36]; therefore, the trend of hydride mobility may be considered as the indication of the aromatization Desorbed methanol was detected mostly below 200 • C, indicating an operating condition in which methanol conversion had not completed and side reaction should be suppressed [32]. DME was observed in the range from 150 • C to 300 • C, a so-called pre-inition phase of methanol conversion [33], which can be used to interpret the bonding strength of the Brønsted acid [32].…”

Complex relationship between SAPO framework topology, content and distribution of Si and catalytic behaviour in the MTO reaction

“…The complex reaction and deactivation mechanism of MTH transformation has been widely studied. 3,[11][12][13][14][15] According to the proposed dual cycle hydrocarbon pool mechanism, 16 the process is an autocatalytic reaction and both alkene and aromatic intermediates are involved forming two inter-related cycles in which the methylation and cracking or dealkylation reactions lead to the formation of light alkenes. In a further step, the reaction between methanol and the hydrocarbon intermediates or the combination of such intermediates would lead to the formation of large molecules that are not able to diffuse out of the catalyst and keep retained in the porous structure forming carbonaceous deposits (coke) that may block the access of methanol to the active centres or hinder the diffusion of reaction products, resulting in the deactivation of the catalysts.…”