A Study On The Selective Catalytic Oxidation Of Biomass To Formic Acid With Oxygen Using Heteropoly Acid Catalysts

Formic acid（FA）is an important bulk chemical.In recent years,FA has also been considered a promising hydrogen energy carrier.The current FA production relies on fossil feedstocks.With the diminishing fossil resources and the growing concerns about environmental issues,it is appealing to develop technologies of FA production from renewable bioresources.Catalytic aerobic oxidation is a promising strategy,in which vanadium-substituted heteropoly acids（HPA-n,n is the vanadium substitution number）shows excellent catalytic performance.However,side reactions limit the selectivity of FA.This work investigated the rate relationships among various reactions of biomass-derived carbohydrates in the HPA-2 catalytic system and their effects on product distribution.Besides,the mechanism of over-oxidation of biomass-derived carbohydrates in the HPA-n catalytic systems and its inhibition by organic additives was revealed,based on which two methods were developed to selectively oxidize biomass into FA.The main research contents and conclusions are as follows.（1）The rate relationships among the reduction of HPA-n(HPA-n_ox),the re-oxidation of reduced HPA-n(HPA-n_red)and acid-catalyzed reactions of carbohydrates were investigated.The redox kinetics of HPA-2 were studied separately,and their consistency with the kinetics of catalytic aerobic oxidation verified the Mars-van Krevelen mechanism.The kinetic parameters indicate that the rate constant of the HPA-2_ox reduction by glucose increases faster than that of the HPA-2_red re-oxidation with the increase of temperature,so higher O₂ pressures are required at higher temperatures to maintain the rate matching relationship between them for high oxidative activity of HPA-2.As for glucose,oxidation and acid-catalyzed decomposition compete with each other.High O₂ pressures are required at high temperatures to maintain the oxidative activity of the catalyst,thus maintaining the rate of oxidation much higher than that of acid-catalyzed decomposition.As for cellulose,high temperatures are required to promote the hydrolysis,the rate-determining step,in order to accelerate the FA generation,which alliviates the over-oxidation of FA caused by too long residence time in the catalytic system.（2）The mechanism of inhibiting over-oxidation by additives in the HPA-n（n=2-6）catalytic system was studied.Methanol,ethanol,isopropanol,tert-butanol and dimethyl sulfoxide as additives could obviously inhibit over-oxidation,and they all belong to hydroxyl radical（·OH）scavengers.Controlled experiments and characterization of radicals demonstrate that·OH is formed in the HPA-n_red re-oxidation step.Differences in product selectivity with different oxidative species and model reactions elucidate that·OH can oxidize aldehyde groups of carbohydrates to carboxyl groups,which are converted into CO₂ in the oxidative C-C bond cleavage.·OH scavengers can eliminate the negative influence of·OH and thus saccharides can be selectively converted into FA with HPA-2.When the isopropanol dosage was twice the amount consumed to scavenge all·OH,the yield of FA and its isopropyl ester reached 92.2%from glucose.For other HPA-n catalytic systems and various carbohydrate and vicinal diol substrates,the addition of isopropanol can also significantly inhibited over-oxidation and improved the selectivity of FA.（3）Based on the mechanism of inhibiting over-oxidation by additives,selective oxidation of biomass to FA coupled with low-energy formaldehyde（FAld）production from methanol was developed.Controlled experiments and characterization of radicals demonstrate the synergistic effect that biomass oxidation activates the methanol oxidation by boosting the catalytic redox cycle to form·OH and methanol scavenges·OH to inhibit the over-oxidation of biomass.Under the optimized conditions,cellulose and corn stalk were almost completely converted with the FA yields of 76.4%and 59.3%,respectively,while the conversions of methanol were enhanced by 124%and52%comparing with the separate catalytic oxidation,respectively.The coupling catalytic oxidations of water-soluble biomass and methanol can be operated at a low temperature of 90 ^oC,in which the yields of biomass-derived FA were as high as 88.6%and the conversion efficiency of methanol was13-16 times higher.The recycling stability of HPA-5 and a good result of the high-concentration experiment exhibit a promising application of the coupling catalytic system.（4）Based on the mechanism of over-oxidation of carbohydrates in the HPA-n（n=2-6）catalytic system,a chemical looping oxidation method was developed that the oxidation of biomass by HPA-n_ox and the re-oxidation of HPA-n_red by O₂ are separately conducted,in order to avoid the over-oxidation caused by·OH.Compared with the catalytic aerobic oxidation,the chemical looping oxidation of glucose achieved a much higher selectivity of FA.Therein,HPA-5 performed the highest selectivity of FA among HPA-n,and the yield of FA reached 95.4%at the HPA-5 dosage of 1.75-time stoichiometric amount.With sufficient HPA-5,the acid-catalyzed decomposition of glucose and the oxidation of saccharide intermediates to saccharic acids are weakened,so the selectivity of glucose oxidation into FA can be further enhanced.By observing the product evolution with reaction time at a low temperature and model reactions,it has been revealed that glucose undergoes successive C1-C2 bond oxidative cleavage to gradually generate FA.The recycling stability of HPA-5 as an oxygen carrier and good results of the oxidations of different biomasses exhibit a promising application of the chemical looping oxidation method.