A Study on the Conversion of Biomass to Oxygenated Chemicals by Selective Oxidation with Vanadium-based Catalysts

Author:Lu Zuo

Supervisor:wu wei ze


Degree Year:2018





Biomass and bio-based chemicals have been attracting more and more attention due to increasing energy demand and declining fossil fuel reserves.It is significant to synthesize high value-added chemicals using sustainable biomass as feed instead of fossil fuel.To high-efficiently utilize biomass,in this thesis,soluble sugar,cellulose,lignin and corn cob were chosen as studied objects.We combined metal catalysis with oxygen oxidation to study the conversion of biomass into high value-added oxygenated chemicals in hot water.Several new catalytic systems were developed for the selective oxidation of biomass.The key conversion pathways in the reaction were identified,the transformation method from biomass to oxygenated chemicals was structured,and the key technologies and theoretical issues in these processes were studied systematically.The main researches and results are summarized as follows.1.Catalytic oxidation of cellulose to formic acid by O2 in H5PV2Mo10O40-H2SO4 aqueous solution.Cellulose was transformed to formic acid with conversions from 60%to 100%and formic acid yields from 28%to 61%(based on carbon atom in the feedstock),using H2SO4 as an additive to decrease the pH from 1.79 to 0.56.By decreasing pH,the oxidation potential and electron affinity were increased due to the formation of protonation and dissociation of H5PV2Mo10O40,favoring the reduction of the catalyst and oxidation of the substrate.By decreasing pH,the hydrolysis of cellulose was accelerated to form soluble monosaccharides,but acetic acid was formed if pH further decreased.2.Catalytic oxidation of lignin to small-molecule fatty acids and benzene carboxylic acids in H5PV2Mo10O40-H2SO4 aqueous solution.Lignin was transformed to small-molecule fatty acids with a total yield of 40.6 wt%and benzene carboxylic acids with a total yield of 3.1 wt%(based on weight).It has been found that lignin firstly degrades into phenolic or quinoid structure intermediates,which are further oxidized to form carboxylic acid products;oxidation can also occur directly on the phenolic hydroxyl and primary hydroxyl groups in lignin.The oxidation of lignin model compounds was investigated,and the oxidation of different structures of lignin was revealed.3.Catalytic oxidation of corn cob to formic acid and acetic acid by 02 in H5PV2Mo10O40-H2SO4 aqueous solution.The results indicate that corn cob can be transformed with a formic acid yield of 42.5%and an acetic acid yield of 9.1%(based on carbon atom in the feedstock).Hemicellulose is oxidized rapidly followed by partial oxidation of lignin,and cellulose is the most difficult converted part.The products formic acid and acetic acid mainly come from polysaccharides(hemicellulose and cellulose),and pH has a major influence on cellulose.The recycling of catalyst has been demonstrated,and the catalyst shows good reusability.4.Catalytic oxidation of cellulose to formic acid in V(Ⅴ)-Fe(Ⅲ)-H2SO4 aqueous solution.The results indicate that cellulose is transformed to formic acid,and the yield of formic acid can be further improved to 66.8%(based on carbon atom in the feedstock)by using Fe(Ⅲ)as a second metal species.Compared with single use of V(V),the employment of Fe(Ⅲ)alone is not active enough in the oxidative C-C bond cleavage.V(Ⅴ)other than Fe(Ⅲ)is the main active species for selective oxidation of substrates in V(Ⅴ)-Fe(Ⅲ)-H2SO4 aqueous solution.The addition of Fe(Ⅲ)can promote the oxidation of V(Ⅳ)to V(Ⅴ),increase oxidative ability,and inhibit the overoxidation to by-product CO2.5.Catalytic oxidation of biomass to oxygenated chemicals by H5PV2Mo10O40 in methanol/water solution.The results indicate that the conversions of glucose and cellulose in methanol/water solutions achieved total product yields of oxygenated chemicals up to 92.9%and 89.7%(based on carbon atom in the feedstock,carbon in methanol is not included),respectively.A conversion pathway to clarify the mechanism of product formation was proposed.Protecting reactive functional groups of sugars and reaction intermediates via acetalisation and etherification with methanol effectively suppress CO2 formation and remarkably enhance the production of oxygenated chemicals.The effect of solvent composition on catalyst H5PV2Mo10O40 was investigated,and it was found that the pH of the solution decreased but the structural stability of the catalyst increased by the addition of methanol.