Efficient Depolymerization And Conversion Of Lignocellulose And Inhibition Of Carbon Deposition Products

Lignocellulosic biomass is abundant and has a short cycle.The utilization of lignocellulose to replace petroleum resources is one of the important approaches to solve problems of energy and environment.Pretreatment is a key step during the process for lignocellulose conversion.Improving pretreatment efficiency is extremely important for the subsequent conversion of lignocellulose.At present,the traditional method widely used is the dilute inorganic acid pretreatment.However,there are some problems such as severe corrosion of equipment and difficulty in recovery or disposal of waste acid.Therefore,it is necessary to develop new pretreatment methods or to improve the traditional pretreatment methods.In addition,many researchers have been puzzled by the problem of carbon deposition in the catalytic conversion of lignocellulose.Up to now,there is still lack of understanding on the level of carbon deposition.The mechanism of carbon deposition is significant to improve the conversion efficiency of lignocellulose.It is also hot topic for efficient isomerization of glucose to fructose.Glucose is efficient isomerized to fructose by inhibiting carbon deposition,which is beneficial to the high value utilization of fructose.In this paper,our objective was to invesitigate the oxalic acid recycling,mechanism of carbon deposition and high efficiency isomerization of glucose to fructose under the condition of inhibiting carbon deposition during the process of lignocellulose conversion.After the improvement of our previous research in dilute oxalic acid pretreatment,the hydrolysis efficiency of hemicellulose was improved.And the dissolved oxalic acid was also recycled.In view of the different degrees of carbon deposition in the pretreatment of dilute oxalic acid,the mechanism of carbon deposition was explored.The catalytic effect of trithylamine on the isomerization of glucose to fructose was dicussed at low reaction temperature under the condition of efficiently inhibiting carbon depostioin form.The main research results were as follows:（1）Considering the problem of organic acid recovery in the pretreatment of lignocellulose,a new approach was proposed for the recovery and reuse of oxalic acid in the pretreatment of corncob.The optimum reaction parameters of oxalic acid pretreatment were studied with xylose as the main product.Recycling of oxalic acid was dicussed based on the difference in solubility of xylose and oxalic acid in ethanol.The results showed that the yield of xylose was85.0%when the reaction temperature and time were 140 ^oC,2.5 h,and the concentration of oxalic acid was 150 mmol/L.Morever,at the optmiezed pretreatment condition,the oxalic acid used in the pretreatment was recovered by using ethanol.The pretreatment with recycled oxalic acid was carried out 5 times,the yields of xylose were 46.7-62.3%,and the total sugar contents were 178.93-269.76 mg/g.（2）Concerning the carbon deposition problem in the process of biomass conversion,it was studied by controlling the reaction conditions for the relationship between the pseudo-lignin,humin and acid-induced hydrothermal carbon and the formation mechanism of three substances.The reaction pathways were explored for these three substances in the initial stage of formation.The results showed that it was similar for formation conditions,microscopic morphology and chemical structure of pseudo-lignin,humins and acid-induced hydrothermal carbon.The acidity of the system was enhanced due to substances generated via oxidation of furfural,thus the rates of polycondensation were accelerated.However,polycondensation of oxidized products was difficult to achieve.When the carbon source was furfural,possible reaction pathways of the carbon deposition product were direct condensation of furfural and condensation aftre ring-cleavage of furfural.When the carbon source was xylose,possible reaction pathways were condensation between the isomerized xylose and the furfural/ring-cleavage products of furfural.（3）In consideration of the problem of solid catalyst deactivation caused by carbon deposition,a new method was proposed for the isomerization of glucose to fructose using trimethylamine as catalyst at low temperature to effectively inhibit carbon deposition.The effects of reaction temperature and time were investigated as well as substrate concentration on yield and selectivity of the isomerization of glucose to fructose.The product was verified by ¹³C NMR.The catalytic processes of trimethylamine and NaOH were comparatively studied.It was found that at lower temperatures and longer reaction times,the isomerization of glucose by triethylamine tended to achieve high yield and selectivity of fructose,and could effectively inhibit carbon deposition at high substrate concentration.When the concentration of triethylamine was 0.1 M,the reaction temperature and time were 60 ^oC and 12/28 h,and the glucose concentration was 1/2 M,the yield and selectivity of fructose were 38.6/35.1 and80.1/76.1%,respectively.Compared with NaOH,triethylamine could act as a buffer to provide a stable alkaline environment during the catalytic process,which was beneficial to the isomerization of glucose to fructose.