Effect of boron incorporation on the structure, products selectivities and lifetime of H-ZSM-5 nanocatalyst designed for application in methanol-to-olefins (MTO) reaction

Selective production of propylene from methanol over high-silica mesoporous ZSM-5 zeolites treated with NaOH and NaOH/tetrapropylammonium hydroxide

“…6 and Table 3, the distinct decrease in the strength of strong acid sites upon alkaline treatments, especially with the NaOH/TPAOH mixture (ATZ-0.4R sample), can alleviate the hydride transfer and cyclization reactions on strong acid sites that convert light olefins to paraffins, aromatics, naphthenes, and higher olefins. Consequently, the coke precursor condensation steps are also attenuated, and therefore deactivation is minimized [13,34,52]. These results indicate that weakening the acid strength of the ZSM-5 zeolites upon alkaline treatments can effectively improve the MTP catalytic lifetime.…”

“…So, the lifetime of the [31,32]. The prominent difference in the catalytic performance of parent and alkali-treated ZSM-5 catalysts can be explained by the difference in the level of porosity and acidity as well as by the strength of the acid sites and accessibility of reactants to these active sites [13,30]. It has been reported that the significant changes in the acidity and the pore architecture of ZSM-5, resulting from the alkali treatment, could influence both the activity and the stability of the ZSM-5 catalyst in the MTH reaction [31][32][33].…”

“…It has been reported that the conversion of methanol to DME takes place mainly on the weak acid sites while the conversion of DME (and methanol) to light olefins occurs mainly on the strong acid sites [13,49]. Based on FT-IR and NMR studies, Campbell et al [50] also indicated that the initial reactivity of methanol (the first C-C bond formation) is controlled by the concentration of Brønsted acid sites, and is not directly ruled by the concentration of extra-framework Al.…”

“…Although, strong acid sites are the dominant active sites for MTP reaction, they also promote hydrogen transfer reaction of olefin to saturated hydrocarbons and aromatics, which are precursors of coke formation, which deactivates the catalysts [13,51]. So, the higher stability of alkalitreated ZSM-5 catalysts compared to the parent one cannot be explained by the increment of acidity.…”

“…Despite these superior results, the relatively small micropores in ZSM-5 zeolites significantly influence the mass transfer of the reactants and products towards and away from the active sites, which would result in relatively easy coke formation and subsequently limit the performance of the industrial catalysts [12,13]. Hence, the modification of the ZSM-5 zeolite to optimize the activity and selectivity in the MTP reaction still remains a key challenge in state-of-the-art catalysis research.…”