Acidity enhancement through synergy of penta- and tetra-coordinated aluminum species in amorphous silica networks

Abstract

Amorphous silica-aluminas (ASAs) are widely used in acid-catalyzed C-H activation reactions and biomass conversions in large scale, which can be promoted by increasing the strength of surface Brønsted acid sites (BAS). Here, we demonstrate the first observation on a synergistic effect caused by two neighboring Al centers interacting with the same silanol group in flame-made ASAs with high Al content. The two close Al centers decrease the electron density on the silanol oxygen and thereby enhance its acidity, which is comparable to that of dealuminated zeolites, while ASAs with small or moderate Al contents provide mainly moderate acidity, much lower than that of zeolites. The ASAs with enhanced acidity exhibit outstanding performances in C-H bond activation of benzene and glucose dehydration to 5-hydroxymethylfurfural, simultaneously with an excellent calcination stability and resistance to leaching, and they offer an interesting potential for a wide range of acid and multifunctional catalysis.

Engineering the Distinct Structure Interface of Subnano-alumina Domains on Silica for Acidic Amorphous Silica–Alumina toward Biorefining

“…However, the EFAl species generated on zeolite surfaces can easily leach out in liquid-phase reactions, resulting in severe activity loss. 9 , 10 …”

“… 10 − 12 In ASAs, the formation of BAS has been widely accepted to originate from an Al atom flexibly coordinated to a neighboring silanol oxygen atom ( Scheme 1 b) 13 − 15 or via aluminic pseudobridging silanol (PBS-Al, Scheme 1 c, d) at the interface between silica and alumina. 10 , 16 , 17 Zeolite-like bridging Si(OH)Al or replacement of an Al atom by Si in Scheme 1 c (PBS-Si) are also proposed for BAS formation by theoretical calculation studies but lack direct spectral evidence yet. 18 Moreover, alumina domains on ASAs are associated with the formation of Lewis acid sites (LAS).…”

“… 19 These properties render ASAs ideal bifunctional Brønsted–Lewis acidic catalysts with versatile acid strengths, high stability, and lower energy barriers compared to zeolites. 10 , 11 …”

Multigraded Heterojunction Hole Extraction Layer of ZIF‐CoxZn1−x on Co3O4/TiO2 Skeleton for a New Photoanode Architecture in Water Oxidation

“…Due to the sustainable energy‐environment challenge, the development of new clean energy from renewable sources has become increasingly important. [ 1–3 ] Solar energy is the most naturally abundant, clean, and sustainable resource. Fujishima and Honda first reported that TiO 2 can achieve effective photoelectrochemical (PEC) water splitting, [ 4 ] which provides an alternative way to produce sustainable hydrogen.…”

Dual Active Sites on Molybdenum/ZSM‐5 Catalyst for Methane Dehydroaromatization: Insights from Solid‐State NMR Spectroscopy

“…Solid‐state NMR spectroscopy has become a critical tool for characterizing the active sites on heterogeneous catalysts, capable of providing detailed information on correlation and connectivity of distinct nuclei in a local structure [34–38] . In this work, we established a fundamental understanding of the active sites on Mo/ZSM‐5 for MDA reaction.…”