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 CH adsorption and H 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.