Study Of Catalytic Oxidation Of Cellulose To Formic Acid With Iron-based Catalyst System

At present,the majority of energy and organic chemicals are based on the fossil fuels.However,the non-renewability of fossil fuels and the environmental pollution caused by fossil fuels is gradually attracting much attention.Therefore,the research for alternatives of fossil fuels has become an urgent study topic.Biomass is the most abundant clean-renewable energy in nature.The reasonable use of biomass is of great significance for improving various social issues caused by the excessive use of fossil fuels.Formic acid?FA?is an important and valuable chemical.Currently,industrial production methods are mainly based on non-renewable fossil fuel routes,which exists obvious problems.Therefore,it is of great significance to develop a method for the efficient preparation of FA from biomass as raw material.The research on the preparation of FA from biomass via oxidation has made some progress in recent years.The existing catalysts with high-efficiency for the preparation of FA from biomass are mainly vanadium-based catalysts,such as vanadium-containing heteropolyacids,VOSO₄,NaVO₃ and the like.It is well known that vanadium-containing compounds are not only unfriendly to the environment because of toxicity,but also expensive.Therefore,it is important to find an environmentally friendly and inexpensive catalyst to replace the existing vanadium-based catalyst.Iron as an important transition metal,not only has rich reserves,low prices,relatively non-toxic,but also has outstanding catalytic performance.Iron catalyzed reactions are seen as a green,environmentally friendly,and sustainable metal catalyzed reaction.In this paper,the catalytic oxidation of cellulose with iron-based catalyst system was studied.The main contents and conclusions are as follows.1.In order to improve the performance of NaVO₃/H₂SO₄ catalyst system to increase FA yield,we added a small amount of different kinds of transition metal chlorides as additives to NaVO₃/H₂SO₄ system.By screening different kinds of transition metal chlorides,we found that FeCl₃ had a synergistic effect with NaV03,which showed a better performance.The FA yield was improved from 59.0%in NaVO₃/H₂SO₄ system to 66.3%in FeCl₃-NaVO₃-H₂SO₄ system.Then we investigated the effects of reaction time,H₂SO₄ amount and FeCl₃:NaVO₃ mass ratio on the oxidation of cellulose with FeCl₃-NaVO₃-H₂SO₄ system.The yield of FA in the FeCl₃-NaVO₃-H₂SO₄ system can reach up to 71.4%under the optimum reaction conditions.The FA yield was significantly higher than that in NaVO₃/H₂SO₄ system.2.The oxidation of cellulose was studied in FeCl₃-H₂SO₄ system with molecular oxygen as oxidant.The effects of reaction conditions such as FeCl₃ concentration,H₂SO₄ concentration,temperature and time on the cellulose conversion and products yields were investigated.It was found that the main product was FA with a very small amount of by-products of acetic acid?AA?and glycolic acid?GOA?,and the gas product was CO2.The yield of FA could reach up to 51.2%under the optimum conditions.The reaction conditions can affect the stability of FA.With the increase of FeCl₃ concentration,H₂SO₄ concentration,temperature and reaction time,FA stability was gradually decreased.In the catalyst system,the effect of FeCl₃ is to promote the catalytic oxidation of cellulose and improve the selectivity of FA.The effect of H₂SO₄ is mainly to promote the hydrolysis of cellulose and the selectivity to AA.With an increase in FeCl₃ concentration increased,FA yield was promoted and AA yield was inhibited.Increasing H₂SO₄ concentration firstly promotes the catalytic oxidation of cellulose to product FA,but if sulfuric acid concentration was too high,the by-product AA was generated.After reaction,liquid phase products?FA,AA,GOA?could be completely separated from the aqueous solution by the extraction of butanone.The aqueous solution contained the catalyst was used for the next cycle.Its catalytic performance remained almost unchanged after reuse for four times.3.Through ¹³C NMR technique and model compounds oxidation,we speculated a possible transformation pathway of cellulose in FeCl₃-H₂SO₄ system.Cellulose is first converted into D-glucose?in equilibrium with D-mannose?,which is followed by two parallel reactions:retro-aldol condensation and hydrolysis.Retro-aldol condensation leads to C3-C4 bond cleavage,producing D-glyceraldehyde and 1,3-dihydroxyacetone.These two C3 intermediates are further oxidized to FA and glycolaldehyde,which is further oxidized to GOA.GOA remains as final product with partially degradation to FA and CO2.D-glyceraldehyde can also be dehydrated to methylglyoxal,which is further oxidized to FA,AA and C02.Hydrolysis results in the formation of levulinic acid,which yields a large amount of AA,as the main pathway for AA formation.4.The cycling mechanism of the catalyst system was proposed.FeCl₃ is first hydrolyzed to form Fe³⁺ in the strong acid aqueous solution,and then the active Fe³⁺--O species is formed under the condition of oxygen.Fe3--O further oxidizes the small molecule carbohydrate such as D-glucose,which is produced from cellulose by hydrolyzation,into FA,AA,GOA and CO2,and then Fe³⁺--O species is reduced to Fe2+--O.In this system Fe2+--O is oxidized to Fe3--O by oxygen,and the catalytic oxidation reaction is continued.