In situ encapsulation of ultrasmall MoO3 nanoparticles into beta zeolite for oxidative desulfurization

Abstract

Oxidative desulfurization is an appealing alternative technology to eliminate organic sulfides in fuel oil in response to environmental and economic requirements. The supported metal oxide catalysts generally possess low catalytic activity and/or deactivation due to the loss and sintering of their metal oxides during the reaction or removal of carbon deposition at elevated temperature. In this work, beta zeolite-fixed MoO₃ (MoO₃@beta) was hydrothermally synthesized via structural reconstruction. A series of characterizations confirmed that MoO₃ nanoparticles with an ultrasmall crystal size of 1.5–2.5 nm were fixed into the beta zeolite. Owing to the small particle size of MoO₃, the MoO₃@beta material exhibited nearly three times higher catalytic activity, weight-based space–time-yield, and active site-based turnover frequency than an impregnated MoO₃/beta sample with MoO₃ in an extra-framework position during the process of oxidative desulfurization with the oxidant H₂O₂. As for the oxidant molecular O₂, MoO₃@beta also possessed superior catalytic activity in a deep eutectic solvent under mild reaction conditions of 90 °C and ambient pressure, regardless of the model oil or actual fuel oil. Besides, excellent reusability was observed for the MoO₃@beta catalyst via the elimination of carbon deposition through simple calcination, resulting from its unique feature to resist the loss and sintering of MoO₃ fixed in the zeolite. More importantly, the catalytic mechanism of MoO₃@beta was also revealed. This work opens a new avenue for effectively synthesizing zeolite-fixed metal oxide and establishing a green heterogeneous reaction system for catalytic oxidations.