| Methyl formate is the "building block" for synthesizing formic acid, formamides, and pesticides etc, and it plays an irreplaceable role in the chemical production. In recent years, the production capacity of methanol which is the important raw material for synthesizing methyl formate is excess in domesticmarket, and thus it has important practical significance to convert methanol to methyl formate. As a green route, the photocatalytic conversion of methanol to methyl formate has become a hot research topic, and it provides a new sight on the chemical production and the scientific research.In this study, ZnO is used as the main raw material, which has the similar band structure with TiO2, to prepare the catalyst with excellent photocatalytic performance for the conversion of methanol to methyl formate. The main work includes two parts of catalyst screen and catalyst optimization. First of all, the number and distribution of surface hydroxyl on ZnO have been modulated by doping chemicals with abundant surface hydroxyl such as SiO2, Al2O3 or CuZnAl hydrotalcite (LDH). Then the photocatalysts have been prepared by loading copper species on the modified ZnO and the photocatalytic performance for selective oxidation of methanol to methyl formate has been investigated on the catalysts., The catalyst of copper supported on the nanocomposites of CuZnAl layered double hydrotalcite (LDH) and ZnO (Cu/H-ZnO) has been screened out as the best catalyst for the reaction. Second, the impact of calcination temperature of the support and the content, of Cu, Zn and Al on the photocatalytic performance have has been investigated for the series of Cu/H-ZnO catalysts. And the phase, texture, morphology, band structure, distribution of surface species and their valence variations of catalysts have been characterized by the X-ray diffraction (XRD), the specific surface area and pore structure (BET), the transmission electron microscope (TEM), the UV-vis diffuse reflection spectroscopy (UV-Vis DRS), the H2 temperature programmed reduction (TPR), the Fourier transform infrared spectroscopy (FTIR) and the X-ray photoelectron spectroscopy (XPS). The effects of kinetics factors of the light intensity, the methanol partial pressure and the oxygen partial pressure on the catalytic performance have been investigated and the reaction mechanism for the photocatalytic conversion of methanol to methyl formate has been studied further. The reaction condition is as following:UV light irradiation,26℃-45℃ of the reaction temperatureand atmospheric pressure,2 of the ratio of methanol/oxygen,1 vol% of methanol contentin the feed gas balanced with nitrogen gas. The main content and results of the research are as follows:1、The nanocomposites of ZnO modified by SiO2, A12O3 CuZnAl LDH carriers have been prepared and used as supports of three kinds of catalysts Cu/ZnO-SiO2, Cu/ZnO-A12O3 and Cu/H-ZnO, respectively, by loading Cu with the wet chemical reduction method. The catalyst of Cu/H-ZnO is screened out as the best catalyst by after the investigation of the catalytic activity characterization of the catalyst.2、The optimized catalyst calcination temperature and element contents for the catalyst Cu/H-ZnO, which are 700℃ of the calcination temperature,7% of the Cu content and 5% of the Al content, have been determined after investigation the influence of the calcination temperature, Al content and Cu content on the catalytic performance3、The variation of copper species in different stages of catalyst 7Cu/H-ZnO-5(7) has been investigated, forund that the two forms of independent CuO and Cu species in hydrotalcite structure exist in the as-prepared catalyst before reaction. During the reaction, CuO is reduced to metal Cu under the irradiation by UV light, but the copper in hydrotalcite structure is not reduced. Exposed in air for ten days after reaction, the metallic copper is re-oxidized to CuO. After calcination, most of the hydrotalcite in the catalyst is decomposed, the particle size of CuO increases and the catalytic activity was greatly decreased.4^ The influences of kinetics factors on the catalytic performance of Cu/H-ZnO have been investigated for the conversion of methanol to methyl formate. It has been found that the optimized ratio of CH3OH to O2 is 2:1, the optimized content of methanol is 1 vol% and the optimized content of oxygen is 0.5 vol%. The methanol conversion increases with strengthening the light intensity, but the methyl formate selectivity is almost not influenced by light intensity. The methanol conversion is less affected by the reaction temperature, and the methyl formate selectivity slowly decreased with increasing the reaction temperature.5、The results show that CuO in the catalyst was reduced to Cu under UV lightirradiation, and the heterojunction is formed at the interface of Cu, ZnO and CuZnAl hydrotalcite. The hydroxyls are provided at side of heterojunction near hydrotalcite, which can react with methanol molecule to give rise to methoxy, and the holes that have the redox potential more positive than that of CH3O-/CH2O2- can be produced at the sites of the heterojunction close to the side of ZnO under UV irradiation. The photoexcited holes can oxidize methoxy to coordinated formaldehyde, and the coordinated formaldehyde reacts with the neighboring methoxy to give rise to methyl formate. Metal Cu forms Ohmic contact/Schottky barrier with the semiconductor at the contact interface, which transfer pohoexcited electrons in the conduction band from the valence band of heterojunction. Oxygen molecules can be dissociatively chemisorbed on the negatively charged metal Cu surfaces, which can spillover to recover the oxygen vacancies on the support. |
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