Abstract
MXenes are a new family of two-dimensional carbides and/or nitrides. Their 2D surfaces are typically terminated by O, OH and/or F atoms. Here we show that Ti3C2T x —the most studied compound of the MXene family—is a good acid catalyst, thanks to the surface acid functionalities. We demonstrate this by applying Ti3C2T x in the epoxide ring-opening reaction of styrene oxide (SO) and its isomerization in the liquid phase. Modifying the MXene surface changes the catalytic activity and selectivity. By oxidizing the surface, we succeeded in controlling the type and number of acid sites and thereby improving the yield of the mono-alkylated product to >80%. Characterisation studies show that a thin oxide layer, which forms directly on the Ti3C2T x surface, is essential for catalysing the SO ring-opening. We hypothesize that two kinds of acid sites are responsible for this catalysis: In the MXene, strong acid sites (both Lewis and Brønsted) catalyse both the ring-opening and the isomerization reactions, while in the Mxene–TiO2 composite weaker acid sites catalyse only the ring-opening reaction, increasing the selectivity to the mono-alkylated product.
Highlights
MXenes are getting increased attention as novel two-dimensional (2D) materials
MXenes are denoted by Mn+1XnTx, where T stands for a terminating functional group that depends on the delamination conditions [2,3,4,5]
By studying the reaction kinetics and mechanism, we show that the kinetics and selectivity in epoxide ring opening can be controlled and boosted by modifying the surface of Ti3C2Tx, which influences the number and type of acid sites
Summary
MXenes are getting increased attention as novel two-dimensional (2D) materials. They are prepared by delaminating MAX phases, which are atomically layered solids where M is an early transition metal, A is a group IIIA/IVA element, and X is carbon or nitrogen [1]. We explore the Ti3C2Tx MXene and its derivatives as catalysts for acid-catalysed reactions using the styrene oxide (SO) ring opening reaction as a model reaction (equation (1)). Homogeneous and heterogeneous catalysts such as Lewis acids, Brønsted acids and mineral acids were used previously for the alcoholysis of epoxides Many of these catalysts are difficult to separate and recycle and some need harsh reaction conditions to reach high conversions [37,38,39,40,41,42,43]. By studying the reaction kinetics and mechanism, we show that the kinetics and selectivity in epoxide ring opening can be controlled and boosted by modifying the surface of Ti3C2Tx, which influences the number and type of acid sites. The increase, depends on the presence of Ti3C2Tx multilayers
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