Uncovering the role of Lewis and Brønsted acid sites in perforated SAPO-34 with an enhanced lifetime in methanol conversion to light olefins

Abstract

Although the micropores of SAPO-34 (the catalyst in the conversion of methanol to light olefins) favor higher light olefins selectivity, they are vulnerable to blockage by coke, which can result in fast catalyst deactivation. To mitigate this drawback, alkali post-treatment was employed as an efficient route for introducing secondary mesopores and improving mass transfer. Mild-treated samples exhibited ∼50% longer lifetime; however, with precise product analysis, slightly lower light olefins selectivity (due to higher alkane formation) relative to the parent catalyst was uncovered. TGA-TPO, followed by GC-MS and ¹³C-NMR tests, revealed the formation of heavier coke species (with a lower H/C ratio content) over the treated samples that can provide extra hydrogen for higher alkane production. Heavier coke formation over treated samples was hypothesized to be related to the alteration in active sites. According to the CD₃CN-FTIR test, it was noticed that Lewis acid sites remained intact while the Brønsted acidity was depleted after alkali treatment. Our density functional theory (DFT) calculations, complemented by recent findings, revealed that methanol adsorption and C–C coupling are more favorable over Brønsted sites (leading to olefin formation), while Lewis acid sites facilitate dehydrogenation and aromatization (resulting in enhanced coke generation). Hence, a meaningful connection was depicted between heavier coke species formation, higher alkane production, and lower light olefins selectivity with a higher Lewis/Brønsted acid site ratio over the treated catalysts.