Mechanistic Insight into Ethanol Dehydration over SAPO-34 Zeolite by Solid-state NMR Spectroscopy

Zeotype-Confined Frustrated Lewis Pair and Its Role in Catalyzing Hydrogenation

Recent theoretical studies predicted that the frustrated Lewis pair (FLP) formed by carbonaceous species confined in zeolites/zeotypes can activate H-H and C-H bonds. However, there still lacks experimental evidence and understanding on the role of FLP in the hydrogenation reaction. Herein, we combined experiments and density functional theory (DFT) calculations to demonstrate that the Brønsted acid sites with weak acid strength can transfer H+ to the confined carbonaceous species to form Si-O--Al as a Lewis base and carbocation as a Lewis acid. They are electrostatically attracted and sterically repelled, thus, forming FLP sites. We present for the first time experimental evidence and a general principle about the formation of FLP sites inside zeotypes and investigated the effect of the topology and the acid strength on the FLP formation. FLP sites are active in ethylene hydrogenation, and their activity is inversely correlated with their parent Brønsted acid strength. FLP derived from weaker Brønsted acid sites promotes C2H4 adsorption and H2 activation, thus enhancing hydrogenation. This work not only provides mechanistic insights into the origin of olefin hydrogenation over metal-free zeolites/zeotypes but also offers guidance for further development of high-performance zeolite/zeotype-based catalysts and heterogeneous FLP catalysts.

Advanced solid-state NMR spectroscopy and its applications in zeolite chemistry

Solid-state NMR spectroscopy (ssNMR) can provide details about the structure, host-guest/guest-guest interactions and dynamic behavior of materials at atomic length scales. A crucial use of ssNMR is for the characterization of zeolite catalysts that are extensively employed in industrial catalytic processes. This review aims to spotlight the recent advancements in ssNMR spectroscopy and its application to zeolite chemistry. We first review the current ssNMR methods and techniques that are relevant to characterize zeolite catalysts, including advanced multinuclear and multidimensional experiments, in situ NMR techniques and hyperpolarization methods. Of these, the methodology development on half-integer quadrupolar nuclei is emphasized, which represent about two-thirds of stable NMR-active nuclei and are widely present in catalytic materials. Subsequently, we introduce the recent progress in understanding zeolite chemistry with the aid of these ssNMR methods and techniques, with a specific focus on the investigation of zeolite framework structures, zeolite crystallization mechanisms, surface active/acidic sites, host-guest/guest-guest interactions, and catalytic reaction mechanisms.

Water-Induced Micro-Hydrophobic Effect Regulates Benzene Methylation in Zeolite

Water is a ubiquitous component in heterogeneous catalysis over zeolites and can significantly influence the catalyst performance. However, the detailed mechanism insights into zeolite-catalyzed reactions under microscale aqueous environment remain elusive. Here, using multiple dimensional solid-state NMR experiments coupled with ultrahigh magic angle spinning technique and theoretical simulations, we establish a fundamental understanding of the role of water in benzene methylation over ZSM-5 zeolite under water vapor conditions. We show that water competes with benzene for the active sites of zeolite and facilitates the bimolecular reaction mechanism. The growth of water clusters induces a micro-hydrophobic effect in zeolite pores, which reorients benzene molecules and drives their interactions with surface methoxy species (SMS) on zeolite. We identify the formation and evolution of active SMS-Benzene complexes in a microscale aqueous environment and demonstrate that their accumulation in zeolite pores boosts benzene conversion and methylation.

Exclusive SAPO-seeded synthesis of ZK-5 zeolite for selective synthesis of methylamines

ZK-5 zeolite with superior selectivity for monomethylamine (MMA) plus dimethylamine (DMA) is fast synthesized using the exclusive silicoaluminophosphate SAPO-34 seed and K+ and Na+ cations.