Study On Photocatalytic Performance And Deactivation Mechanism Of BiOCl For Direct Dehydrogenation Of Methanol

The route from coal via methanol to important chemicals is an important approach in the field of clean utilization of coal.The photocatalytic direct dehydrogenation of methanol to methyl formate and other important chemicals is a new green synthetic method.It can not only consume methanol to alleviate the pressure of its excess capacity,but also achieve economic benefits,and thus has certain practical significance and research value.BiOCl is a kind of semiconductor with unique layered structure.Its application in the direct dehydrogenation of methanol under anaerobic conditions is quite innovative,which has expanded the research scope of photocatalytic organic synthesis and enriched the photocatalytic theory of organic synthesis.So the study has certain scientific significance.BiOCl catalysts were prepared by the hydrothermal method,and the influence of solvent on the structure and properties of BiOCl catalyst was investigated.the phases,optical properties,surface hydroxyls,morphology and element valence of the catalyst were characterized by the X-ray diffraction?XRD?,ultraviolet-visible diffuse reflection?UV-Vis?,Fourier transform infrared spectroscopy?FT-IR?,transmission electron microscopy?TEM?and X-ray photoelectron spectroscopy?XPS?.The performance of the photocatalyst for the direct dehydrogenation of methanol was investigated,and the deactivation of the catalyst was investigated from three aspects:irradiation only by ultraviolet light,irradiation by ultraviolet light in methanol atmosphere and H₂ reduction.The deactivation mechanism of the catalyst was clarified by comparing the changes of the structure and properties of the catalyst before and after the reaction.The specific findings are as follows:1.The structure and morphology as well as the photocatalytic performance of the catalyst for the dehydrogenation of methanol are significantly affected by the solvent.The main products of the reaction are dimethoxymethane,methyl formate and hydrogen.The BiOCl catalyst prepared with HNO₃ as solvent has the best activity.The methanol conversion of each catalyst increased at first and then decreased rapidly.The remarkable deactivation of the catalyst could be observed within 2-3hours.2.The structure of BiOCl cannot be changed by solely irradiation of ultraviolet light or reduction in low concentration H₂ atmosphere,but some bismuth species in the catalyst can be reduced to metallic bismuth by ultraviolet light irradiation in methanol atmosphere.The reason is that photoelectrons can be trapped by Bi³⁺in BiOCl to give rise to Bi,Cl combines with H to form HCl,and O released from BiOCl participates in the formation of dimethoxymethane?DMM?and methyl formate?MF?,which leads to the collapse of the BiOCl structure and deactivation of the catalyst.3.The stability and catalytic activity of the catalyst can be improved by introducing oxygen into the feed gas.The increase of oxygen concentration can stabilize the structure of catalyst,prolong the life of catalyst and enhance the conversion of methanol.But too much oxygen can lead to deep oxidation of methanol to produce large amounts of CO₂,decreasing the selectivity of dimethoxymethane and methyl formate.