物理化学学报2023，Vol.39Issue(9)：53-68,16.DOI:10.3866/PKU.WHXB202212065
均相催化剂催化氧气氧化生物质制备甲酸
Catalytic Oxidation of Biomass to Formic Acid under O2 with Homogeneous Catalysts
摘要
Abstract
Formic acid(FA)is an important chemical for the production of leathers,medicines,preservatives,rubbers,textiles,and other materials.FA is also used as H2 and CO carriers,and as a fuel in fuel cells.Although most commonly synthesized from fossil fuels,FA can also be obtained from more sustainable sources,such as biomass(e.g.,straw,husk,and sawdust).Oxygen and air are affordable and easily available oxidants used for the oxidation of biomass to FA.Because solid biomass is not soluble in water or organic solvents,homogeneous catalysts are preferred for the catalytic oxidation of biomass to FA by O2 in water.It has been demonstrated that homogeneous catalysts,such as vanadium-containing heteropoly acids(HPA),HPA+H2SO4,NaVO3+H2SO4,HPA-containing ionic liquids,VOSO4,NaVO3-FeCl3+H2SO4,and FeCl3+H2SO4,can convert complex biomass substrates to FA with high atom economy using O2 as the oxidant.The reported biomass substrates include model compounds,cellulose,wood,straw,and corncobs.The reaction conditions were summarized to compare the biomass conversions and FA yields.Vanadium-containing catalysts had the highest FA yield at mild conditions(T ≤ 170 ℃ and P(O2)≤ 3 MPa).Both the reaction rate and FA yield were improved by adding H2SO4.This high conversion can be explained by an electron transfer and oxygen transfer(ET-OT)mechanism,where high-valence transition metals(V5+or Fe3+)oxidize biomass to FA and are reduced to low-valence species(V4+or Fe2+).The catalysts are then regenerated by O2.This reaction occurs through C2-C3 and/or C3-C4 bond cleavages via retro-aldol condensation,followed by continued C-C bond cleavages to form FA.Using isotope-labeled D-glucose as substrate,we determined that oxidation occurs via successive C1-C2 bond cleavages;a V5+catalyst reacts with C1-C2 to form a five-membered ring complex,without C-H bond cleavages,followed by oxidation from another V5+species to form FA.The oxidation of solid cellulose occurs through hydrolysis(hydrolysis of cellulose to monosaccharides,and deep hydrolysis of monosaccharides to levulinic acid)and oxidation(monosaccharides to FA and Ievulinic acid to acetic acid)reactions.The catalytic oxidation of monosaccharides and deep hydrolysis steps are competitive,and the reaction rate of the latter increases faster with increasing temperature.However,catalytic oxidation was favored by higher P(O2).The addition of methanol,ethanol and DMSO to the reaction system,and in situ extraction of FA were performed to inhibit CO2 formation.FA was separated by extraction and the catalyst system was reused.A continuous process for producing FA from molasses was established using a three-phase liquid-liquid-gas system with a reaction volume of 2 L.Finally,the limitations and future requirements of this oxidation reaction are discussed:(1)improving separation or in situ conversion of FA;(2)improving homogeneous catalysts for both biomass hydrolysis and catalytic oxidation to FA;(3)studying the impact of ash in biomass,particularly after catalyst reuse;and(4)understanding the mechanism through which organic solvents such as methanol inhibit CO2 formation.关键词
甲酸/生物质/氧气/催化氧化/均相催化剂Key words
Formic acid/Biomass/Oxygen/Catalytic oxidation/Homogeneous catalyst分类
化学引用本文复制引用
侯玉翠,何卓森,任树行,吴卫泽..均相催化剂催化氧气氧化生物质制备甲酸[J].物理化学学报,2023,39(9):53-68,16.基金项目
The project was supported by the National Natural Science Foundation of China(22178017,21706007)and the Long-Term Subsidy Mechanism from the Ministry of Finance and the Ministry of Education of PRC(BUCT).国家自然科学基金(22178017,21706007)与财政部教育部长效机制项目(BUCT)资助 （22178017,21706007）
