Methanol-to-Olefins Catalysis on HSSZ-13 and HSAPO-34 and Its Relationship to Acid Strength

Abstract

Catalyst lifetime and product selectivity of methanol-toolefins (MTO) catalysis on isostructural HSSZ-13 and HSAPO-34, possessing Brønsted acid sites of different acid strengths, are examined and interpreted to elucidate the role of acid strength in MTO catalysis with and without high-pressure H 2 co-feeds. MTO catalysis without H 2 cofeeds on HSSZ-13 results in a shorter catalyst lifetime and higher paraffins-toolefins ratio than on HSAPO-34, plausibly due to faster rates of formaldehyde formation and involvement of formaldehyde in subsequent alkylation reactions that transform active chain carriers to inactive polycyclics on Brønsted acid sites of higher acid strength. Higher reactivities of protons of greater acid strength in catalyzing hydrogenation of hydrocarbons and oxygenates result in a higher increment in lifetime, paraffins selectivity, and methane selectivity on HSSZ-13 than on HSAPO-34 when coprocessing high-pressure H 2 during MTO catalysis as affirmed in independent kinetic studies and density functional theory (DFT) calculations. These results provide mechanistic insights into the critical role of acid strength in influencing catalyst lifetime and product selectivity during MTO catalysis and rationalize, from a mechanistic and kinetic vantage point, why it may be advantageous to use aluminosilicates such as HSSZ-13 in bifunctional catalytic formulations to upgrade syngas mixtures or during MTO catalysis with cofeeds of molecular hydrogen.

MAPO-18 Catalysts for the Methanol to Olefins Process: Influence of Catalyst Acidity in a High-Pressure Syngas (CO and H2) Environment

“…In the recent literature, the hydrogenation rate constants of coke precursors such as formaldehyde and butadiene were measured to be at least 2 orders of magnitude higher than that of propene and ethene in high pressure H 2 studies, and this was proposed to be the origin of the lifetime improvement with limited increase (2–3 times) in paraffin selectivity. 33 , 34 This is not the case for the reaction conditions used here because paraffin selectivity increased dramatically for all catalysts ( Table 2 ). A wider range of reaction conditions, including different CO/CO x feed ratios and lower methanol partial pressure, were then explored with SAPO-18 ( Figure S21 ).…”

“… 33 For the isostructural CHA catalysts, propane selectivity was notably higher than ethane and butanes, and SSZ-13 with stronger acidic strength resulted in higher hydrogenation rate constants than SAPO-34. 34 According to DeLuca et al , the reaction barrier to alkene hydrogenation decreases with an increasing chain length and stabilization of the intermediate carbocation-like species. 55 Comparison of the SSZ-13 zeolite to the isostructural SAPO-34 material further showed a positive relation between the acid strength and hydrogenation rates, and this acid strength effect is more significant than topology (ZSM-5 vs SAPO-34).…”

“… 32 They extended this strategy to other zeolites with CHA, AEI, FER, and BEA topology and isostructural SAPO-34 and SSZ-13. 33 , 34 Upon H 2 co-feeding, the more acidic SSZ-13 not only had a longer lifetime than SAPO-34 but also higher selectivities toward methane and paraffins. The hydrogenation of coke precursors including formaldehyde and 1,3-butadiene was concluded to suppress the production of deactivation-inducing polycyclics thereby improving the catalyst lifetime.…”

Synthesis of Hierarchical Porous SAPO-34 and Its Catalytic Activity for 4,6-Dimethyldibenzothiophene

“…Besides, the hierarchical porous SAPO-34 can also be synthesized by using a hard template method, which often refers to the application of mesoporous carbon particles or carbon nanotubes, and a soft template method, which refers to the use of organo-silane, surfactants, and polymers ( Schmidt et al, 2012 ; Rimaz et al, 2016 ; Varzaneh et al, 2016 ). Although some researchers reported the synthesis of mesoporous SAPO-34 ( Li et al, 2014 ; Shi et al, 2021 ), the expensive long-chain amines used in the synthesis method seriously hindered its industrial application ( Sun et al, 2021 ; Xuan et al, 2021 ). In this paper, hierarchical porous SAPO-34 was synthesized by using diethylamine (DEA) and morpholine as SDA, silicone quaternary ammonium salt {[(C 2 H 5 O) 3 SiC 3 H 6 N + (CH 3 ) 2 C 18 H 37 ]Br} as mesoporous SDA, and phosphoric acid, pseudo boehmite, and silica sol as phosphorus source, aluminum source, and silicon source, respectively.…”

Confinement-Driven “Flexible” Acidity Properties of Porous Zeolite Catalysts with Varied Probe-Assisted Solid-State NMR Spectroscopy

“…Because of unique structural, acidic, and environmentally friendly properties, zeolites have been widely utilized in petrochemical industries during various reaction processes. − A large number of previous studies have demonstrated that both the diverse acidity properties and pore confinement effect play crucial roles in governing the catalytic performances (e.g., activity, selectivity, and reaction mechanism) of zeolite catalysts. Therefore, a deep insight into the roles of structural and acidic properties on catalytic performances and relevant reaction mechanism of zeolite-catalyzed systems would be of great importance for the optimization, modification, and design of more novel efficient zeolite catalysts in the petrochemical industry, thus attracting the sustained attention of academia in verifying the structure–activity correlations of the catalytic systems from both the experimental and theoretical investigations. − …”