Bifunctional catalyst MoSx@H-Beta for highly selective conversion of CO2 to C2–6 hydrocarbons

Abstract

We report here a bifunctional catalyst MoS_x@H-Beta for highly selective conversion of CO₂ to C_2–6 hydrocarbons. The catalyst is prepared using a two-step solid reaction method. Firstly, MoO_x@H-Beta, with MoO_x⁺ clusters at the exchangeable site of the zeolite, is obtained by sublimating and reacting MoO₃ with zeolitic H⁺ sites during heating. Then, MoO_x@H-Beta is converted into MoS_x@H-Beta through a vulcanization reaction, by heating the mixture of MoO_x@H-Beta and sulphur powder in flowing 5 vol% H₂/N₂. During the vulcanization of molybdenum oxide clusters, the zeolitic H⁺ sites are recovered. Two types of active sites, i.e., molybdenum sulfide clusters and the zeolitic H⁺ sites, are intimately encapsulated in channels of the MoS_x@H-Beta catalyst. Over the catalyst, more than 75% of selectivity to C_2–6 hydrocarbons in organic products at ∼13% of CO₂ conversion is obtained under the optimal reaction conditions of 300 °C, 4 MPa, CO₂/H₂ = 1/3, and 1600 mL g⁻¹ h⁻¹. The characterization results indicate that CO₂ is firstly hydrogenated into intermediate CH₃OH over the MoS_x sites, which is subsequently converted into C_2–6 hydrocarbons over the adjacent Brønsted acid sites in channels of H-Beta zeolite. The MoS_x clusters are highly dispersed and occupied in channels of H-Beta zeolite, exhibiting an excellent stability during 200 h on stream. The cooperative mechanism between molybdenum sulfide clusters and Brønsted acid sites of H-Beta zeolite constitutes the nature of a bifunctional catalyst, demonstrated by theoretical calculations, well accounting for its high catalytic performance.