Construction Of Z-scheme CeO_2-x/Bi₂SiO₅ Heterojunction And Its Photocatalytic Performance In Oleic Acid Esterification

Due to the non-renewable nature of traditional fossil fuels and environmental pollution,researchers are urgently needed to explore new environmentally friendly renewable energy sources.Among them,biodiesel has attracted much attention because of its non-toxic,biodegradable,clean emissions and other environmental advantages,its availability and renewable ability,while having good engine starting performance and suitable fuel characteristics.Biodiesel is an ideal renewable energy to petroleum-derived diesel,which is conducive to alleviating the global energy crisis,reducing environmental pollution and promoting sustainable development.Methyl oleate is widely used because it can be directly used as biodiesel.There are many methods for synthesizing biodiesel,and the traditional methods mainly include acid or alkali catalysis,biological enzyme method,engineering microalgae method,supercritical method,but there are disadvantages such as catalyst is not easy to separate,high product purification cost,strong equipment corrosiveness,environmental unfriendliness,difficult to reuse,large capital investment,and maintenance difficulties.Therefore,there is an urgent need to find a new way to avoid the problem.In recent years,photocatalysis has been used as a new research method in biodiesel synthesis.Based on this,the photocatalytic esterification method was selected to prepare biodiesel.Oleic acid is an unsaturated fatty acid,and Bi₂Si O₅ was used as a photocatalyst to undergo esterification with methanol for generate methyl oleate;a single Bi₂Si O₅ has problems such as wide band gap and low utilization rate of visible light.In this paper,Ce O_2-x/Bi₂Si O₅composites were prepared by hydrothermal-calcination method to form Z-scheme heterojunction,which improved the utilization rate of photogenerated electron-hole pairs.The main research contents and results are as follows:Firstly,the preparation of 3D flower spherical Bi₂SiO₅ via a hydrothermal method and its photocatalytic oleic acid esterification reaction.3D flower spherical Bi₂Si O₅ was prepared and used for the photocatalytic esterification of oleic acid with methanol under simulated sunlight irradiation.The yield of methyl oleate was quantitatively analyzed by ¹H NMR,and the calculation process is simple and the results are accurate and reliable.The effects of four factors,including alcohol-oil molar ratio,catalyst dosage,reaction time and reaction temperature,on the photocatalytic esterification reaction were mainly investigated.The results show that the reaction time and reaction temperature have a greater influence on the yield of methyl oleate,while the amount of catalyst has the least impact on it.Under simulated sunlight irradiation,the optimum reaction conditions:12:1 alcohol to oil ratio,5 wt.%catalyst,70°C and 6 h,the yield of methyl oleate was 28.8%.The results of the ¹H NMR analysis showed that the selectivity of methyl oleate was as high as 100%,and the yield of methyl oleate after three cycles of the catalyst could still reach 26.8%,which was highly stable.ESR tests have shown the formation of hydroxyl radicals（·OH）and superoxy radicals（·O₂^-）under light conditions.Based on the fact that Bi₂Si O₅ was first used in photocatalytic esterification reaction,in order to explain the reason for its catalytic activity,the photocatalytic esterification reaction mechanism was proposed.Secondly,the Z-scheme CeO_2-x/Bi₂SiO₅ heterojunction with rich oxygen vacancies and Ce⁴⁺/Ce³⁺redox centers synergistically enhance the photocatalytic oleic acid esterification reaction.The oxygen-rich Ce O_2-x/Bi₂Si O₅ composite photocatalyst was prepared by hydrothermal-calcination method,and the esterification reaction between oleic acid and methanol was catalyzed under simulated sunlight to prepare methyl oleate.The physicochemical and optical properties of Ce O_2-x/Bi₂Si O₅ catalysts were characterized by XRD,XPS,SEM,TEM（HRTEM）,N₂ physisorption,NH₃-TPD,UV-Vis DRS,photocurrent and EPR,and were analyzed by FT-IR analysis for reaction products.The research results show that after the addition of cerium source,the specific surface area and strong acid sites is increased,the photoresponse range is red-shifted,and the photocurrent and the oxygen vacancies is enhanced.XPS proved that Ce coexisted in the form of+3 and+4 valence states in the Ce O_2-x/Bi₂Si O₅ composite photocatalyst.Under the reaction conditions of 12:1 alcohol to oil ratio,catalyst dosage of 5 wt.%,reaction temperature of 70°C,and light reaction time of6 h,the yield of methyl oleate was 77.8%,and the yield of methyl oleate of the three-cycle catalyst reached 74.6%,which had good stability,and the reaction mechanism of photocatalytic oleate esterification of Z-scheme Ce O_2-x/Bi₂Si O₅ heterojunction was proposed.In summary,a single Bi₂SiO₅ was modified to form a CeO_2-x/Bi₂SiO₅ composite material through the introduction of a cerium source,and the yield of photocatalytic esterification reaction was improved under the synergistic action of oxygen vacancy,Z-type heterojunction,Ce⁴⁺/Ce³⁺redox centers.The research content provides unique insights and ideas for the design of catalyst systems used in the preparation of biodiesel by photocatalytic esterification,and provides a novel electron transfer pathway for the mechanism of photocatalytic esterification,which has guiding significance.