Implications of methanol disproportionation on catalyst lifetime for methanol-to-olefins conversion by HSSZ-13

Abstract

The effects of methanol space velocity and inlet methanol partial pressure on lifetime and selectivity of methanol-toolefins catalysis are examined and interpreted to elucidate reaction parameters and propose intermediates and reactions relevant to catalyst deactivation. The propensity of active centers in HSSZ-13 to turn over for methanol-to-olefins catalysis increases when the methanol partial pressure local to organic co-catalysts confined within the inorganic chabazite cages is lower either by decreasing methanol space velocity or inlet methanol partial pressure. High initial methane selectivity reveals methanol disproportionation, to methane and formaldehyde, a primary reaction, and continual methane formation implicates persistent participation of methanol in bimolecular hydrogen transfer reactions throughout the catalyst lifetime. Methane selectivity correlates positively with inlet methanol partial pressure reflecting enhanced relative rates of formaldehyde formation with increasing methanol partial pressure. Subsequent alkylation reactions of olefins-and aromatics-based CC chain growth carriers by formaldehyde accelerate the relative rates of hydrogen transfer and proliferate, apparently, the precursors mediating transformation of active hydrocarbon pool participants to those inducing catalyst deactivation.

Improving HSAPO‐34 Methanol‐to‐Olefin Turnover Capacity by Seeding the Hydrocarbon Pool

“…Turnover number—the cumulative moles of product carbon formed per mole Brønsted acid sites—is a more rigorous descriptor of reaction progress compared to the more commonly used time‐on‐stream . When plotted against turnover number as a descriptor of reaction progress, ethene and propene yields do not vary with seed loading (Figure ).…”

“…Additionally, significantly higher hydrogen transfer product yields (5x greater) for methanol‐containing feeds compared to pure olefin feeds, and the greater sensitivity of hydrogen transfer product yields to contact time below 100 % methanol conversion than above emphasize the relevance of methanol as a hydride donor during methanol‐to‐hydrocarbons conversion . The hydride abstractor in methanol transfer dehydrogenation steps can either be a surface methoxide or C 2 + alkoxides (Scheme ), with methoxides being the predominant hydride abstractors at low turnover numbers, as evinced by high initial methane selectivities on HSSZ‐13 at 623 K and 6.1–23 kPa methanol, and the much higher contents of CH 4 , CH 3 D, CD 3 H, and CD 4 (93 %) compared to CH 2 D 2 detected when CD 3 covered HSAPO‐34 is pulsed with CH 3 OH at 673 K …”

“…The dual-cycle schematic, used widely in the mechanistic interpretation of MTO data, describes the interconversion between hydrocarbon chain carriers and their role in ethenea nd propene formation, but does not address the transformation of activec hain carriers to inactive ones. Recent work [11][12][13][14][15][16][17][18] has helped elucidate the role of formaldehyde, formed by the transfer dehydrogenationofm ethanol, as an accelerant for catalyst deactivation mediated by the transformation of monocyclic to polycyclic aromatic hydrocarbons. Rates of chain termi-nation relative to propagation necessarily increase with methanol pressure, and hence, decreasing local methanolp ressures by lowering inlet methanolp ressures, [11] operating in ac ontinuous stirred tank configuration (CSTR) insteado faplug flow reactor configuration( PFR), [15] or using dimethyl ether (DME) as feed instead of methanol [17,19] have all been shown to increase catalystl ifetime.…”

“…Recent work [11][12][13][14][15][16][17][18] has helped elucidate the role of formaldehyde, formed by the transfer dehydrogenationofm ethanol, as an accelerant for catalyst deactivation mediated by the transformation of monocyclic to polycyclic aromatic hydrocarbons. Rates of chain termi-nation relative to propagation necessarily increase with methanol pressure, and hence, decreasing local methanolp ressures by lowering inlet methanolp ressures, [11] operating in ac ontinuous stirred tank configuration (CSTR) insteado faplug flow reactor configuration( PFR), [15] or using dimethyl ether (DME) as feed instead of methanol [17,19] have all been shown to increase catalystl ifetime. These effectso fl ocal methanol pressure, in addition to the deleterious effects on catalystlifetimeobserved on co-feeding formaldehyde with methanolo ver HSSZ-13 [11] and HZSM-5 [17] implicate formaldehyde, formed in methanol transfer dehydrogenation events, as crucial forc atalyst deactivation in methanol-to-hydrocarbons conversion.…”

Quantification and Statistical Analysis of Errors Related to the Approximate Description of Active Site Models in Metal‐Exchanged Zeolites

“…1 2 3 4 5 6 7 8 9 10 11 12 13 14 15 16 17 18 19 20 21 22 23 24 25 26 27 28 29 30 31 32 33 34 35 36 37 38 39 40 41 42 43 44 45 46 47 48 49 50 51 52 53 54 55 56 57 Among the monovalent species, protons are most commonly encountered in zeolites. Brønsted acid zeolites have been subject to many experimental and computational studies for various reactions such as methanol to hydrocarbons (MTH) [66][67][68][69][70][71][72] and methanol to dimethyl ether (DME), [73,74] and serve as reference for CÀ H bond activation on other zeolitic active sites. As seen in Table 1, the predominant dissociation pathway for CH 4 on a Brønsted acid site forms a frameworkbound methoxy species and H 2 gas.…”