Studies On The Catalytic Dehydroxylation Of Tartaric Acid To Succinic Acid By Halogen

Biomass is the only renewable organic carbon resources in nature,and efficient utilization of biomass plays an essencial role in establishing a sustainable chemical society.However,due to the existence of abundant hydroxyl groups in biomass,in partitular cellulose and hemicellulose,the activation of C-O bonds and removal of OH groups is a key issue for the conversion of biomass to chemicals and fuels,which is also challenging.In this thesis,the catalytic dehydroxylation of biomass-based tartaric acid to succinic acid is selected as a target reaction,and an efficient catalytic system combining halogen and supported noble metal catalyst has been developed.This catalytic system exhibits high efficiency in the cleavage of C-O bonds in tartaric acid.The function of catalyst,key factors influencing the catalytic performance and reaction mechanism were studied in the thesis,and following conclusions were obtained:(1)The catalytic performances of different halogen have been investigated for the conversion of tartaric acid to succinic acid in H2.As compared to the reaction without any catalyst,the presence of halogen not only promoted the conversion of tartaric acid,but also showed enhanced capability for the C-O bond activation.The catalytic performances of halogen for dehydroxylation increased in the order of Cl<Br<I,whereas the strength of C-X bond(X = Cl,Br,I)decreased in the same order.This suggests that the halogen with relatively lower C-X bond energy is more beneficial to the activation of C-O bond and is more readily removed after substitution for the C-O bond.Kinetic studies show that the reaction involves the esterification of tartaric acid with acetic acid to form 2,3-diacetyl-tartaric acid intermediate and subsequent halogenation and hydrogenation to produce succinic acid.When the reaction was performed under the reaction as follows:tartaric acid 0.5 mmol,n(I):n(-OH)= 5,acetic acid 6 ml,P(H2)= 4 MPa,T =433 K,t= 4,a yield of succinic acid could reach 80%.(2)Although halogen can catalyze the dehydroxylation of tartaric acid to succinic acid,excess amount of the catalyst is required.By combining halogen with a supported metal catalyst,we constructed an efficient catalytic system for the conversion of tartaric acid,which could not only reduce the use of halogen,but also improve the catalytic performance.Pt nanoparticles have been loaded on different supports(i.e.,NbOPO4,SiO2,TiO2 and CNT)and investigated;it was found that the combination of Pt/NbOPO4 and I2(or HI)showed superiror catalytic performance.Under the optimized reaction conditions,succinic acid yield could be more than 90%;moreover,the amount of halogen used in the reaction was reduced to 10%of that without any metal-based catalyst.Pt/NbOPO4 catalyst is stable.After recycling for several times,the catalyst remained its high activity and selectivity of succinic acid.The characterization results show that the excellent catalytic performance of Pt/NbOPO4 is correlated to the strong acidity of the support and the outstanding ability of metal Pt for hydrogen activation.Detailed studies on the reaction mechanism showed that the main reaction pathway included of the esterification of dihydroxy of tartaric acid with acetic acid,halogenation and dehalogenation by hydrogen.At the same time,this route was also accompanied by the reaction of tartaric acid to form malic acid intermediate followed by dehydroxylation of mailic acid to succinic acid.The rate-limiting step is the activation of C-O with halogen and halogenation of the esterified intermediates.We speculate that the strong acidity of Pt/NbOPO4 catalyst and its ability to dissociate hydrogen facilitate the acitivation of halogens,the halogenation reaction and then dehalogenation of the reactant intermediates,thus enhancing the dehydroxylation of tartaric acid and reducing the use of halogen.