Confined Cu Single Sites in ZSM-5 for Photocatalytic Hydroxylation of Benzene to Phenol

Zeolites with band-like charge transport properties have exhibited their potential activities in sensing, optics, and electronics. Herein, a precisely designed Cu@ZSM-5 catalyst is presented with an ultra-wide bandgap of 4.27 eV, showing excellent photocatalytic activity in hydroxylation of benzene with benzene conversion 27.9% and phenol selectivity 97.6%. The SXRD and Rietveld refinement results illustrate that Cu@ZSM-5 has an average of 0.8 Cu atoms per unit cell and the single Cu atoms located in the cross-section of the sinusoidal and straight channels. XANES and EXAFS further demonstrate that the Cu atoms have an oxidation state of +2, coordinated with three OMFI-framework atoms and one ─OH group. Detailed characterizations demonstrate that the Cu@ZSM-5 with tailored bandgap is able to enhance the photoinduced electron-hole separation and hence promote selective hydroxylation of benzene to phenol via the superoxide radical route. This work may open a new way for designing electrically conductive zeolite-supported photocatalysts.

The Overlooked Potential of Sulfated Zirconia: Reexamining Solid Superacidity Toward the Controlled Depolymerization of Polyolefins

Closed-loop recycling via an efficient chemical process can help alleviate the global plastic waste crisis. However, conventional depolymerization methods for polyolefins, which compose more than 50% of plastics, demand high temperatures and pressures, employ precious noble metals, and/or yield complex mixtures of products limited to single-use fuels or oils. Superacidic forms of sulfated zirconia (SZrO) with Hammet Acidity Functions (H0) ≤ - 12 (i.e., stronger than 100% H2SO4) are industrially deployed heterogeneous catalysts capable of activating hydrocarbons under mild conditions and are shown to decompose polyolefins at temperatures near 200 °C and ambient pressure. Additionally, confinement of active sites in porous supports is known to radically increase selectivity, coking and sintering resistance, and acid site activity, presenting a possible approach to low-energy polyolefin depolymerization. However, a critical examination of the literature on SZrO led us to a surprising conclusion: despite 40 years of catalytic study, engineering, and industrial use, the surface chemistry of SZrO is poorly understood. Ostensibly spurred by SZrO's impressive catalytic activity, the application-driven study of SZrO has resulted in deleterious ambiguity in requisite synthetic conditions for superacidity and insufficient characterization of acidity, porosity, and active site structure. This ambiguity has produced significant knowledge gaps surrounding the synthesis, structure, and mechanisms of hydrocarbon activation for optimized SZrO, stunting the potential of this catalyst in olefin cracking and other industrially relevant reactions, such as isomerization, esterification, and alkylation. Toward mitigating these long extant issues, we herein identify and highlight these current shortcomings and knowledge gaps, propose explicit guidelines for characterization of and reporting on characterization of solid acidity, and discuss the potential of pore-confined superacids in the efficient and selective depolymerization of polyolefins.

Surface acidity of tin dioxide nanomaterials revealed with 31P solid-state NMR spectroscopy and DFT calculations

Tin dioxide (SnO₂) nanomaterials are important acid catalysts. It is therefore crucial to obtain details about the surface acidic properties in order to develop structure–property relationships. Herein, we apply ³¹P solid-state NMR spectroscopy combined with a trimethylphosphine (TMP) probe molecule, to study the facet-dependent acidity of SnO₂ nanosheets and nanoshuttles. With the help of density functional theory calculations, we show that the tin cations exposed on the surfaces are Lewis acid sites and their acid strengths rely on surface geometries. As a result, the (001), (101), (110), and (100) facets can be differentiated by the ³¹P NMR shifts of adsorbed TMP molecules, and their fractions in different nanomaterials can be extracted according to deconvoluted ³¹P NMR resonances. The results provide new insights on nanosized oxide acid catalysts.

Facet-dependent acidity of SnO₂ nanosheets and nanoshuttles is revealed with TMP-assisted ³¹P solid-state NMR spectroscopy and DFT calculations.