| Terrestrial biomass is mainly in the form of sugars,which are stored in sugar crops,cellulose and starch in the form of monosaccharides or polysaccharides.These biomass sugars can be completely hydrolyzed to glucose,and many high value-added chemical raw materials are prepared,such as 5-hmf,levulinic acid,gamma-pentolides,lactic acid and methyl lactate,etc.Lactic acid and methyl lactate are important platform compounds derived from biomass,which have broad application prospects.Lactic acid can be selectively converted into green solvents,fine chemicals,general chemicals and fuel precursors.At present,saccharidase fermentation is used in the industrial preparation of lactic acid.However,biological methods have some disadvantages,such as high enzyme price,slow reaction speed,low space yield and difficult purification.These problems can be effectively solved by the directly chemical conversion of carbohydrates from biomass into lactic acid or its derivatives,such as methyl lactate.In this study,we intend to develop supported catalysts with high activity for the directly chemical conversion of glucose to lactic acid.Firstly,the mechanism of Yb(?)-catalzyed the conversion of glucose to lactic acid was studied by theoretical calculation method.At the same time,the effects of anionic ligands(Cl-and OTf)on the catalytic performance were studied by experimental method.Secondly,mesoporous silica with different pore structure was prepared by calcination method and solvent method,and then supported Yb(OTf)3 catalyst was prepared.The effects of pore structure and silica hydroxyl content on the immobilization capacity and catalytic performance of Yb(OTf)3 catalysts were investigated.Thirdly,the effects of co-condensation and chemical grafting on the catalytic performance of supported Yb(OTf)3 catalysts were studied.The reaction mechanism was discussed based on the experimental results.The effects of reaction conditions such as temperature,time and catalyst dosage on the yield of lactic acid were studied and the stability of the catalyst was investigated.Fourthly,Sn/SBA-15 was synthesized in situ by copolycondensation method and used as a carrier to prepare supported bifunctional Yb(OTf)3 catalyst.The catalytic activity of the catalyst in lactic acid conversion from glucose was studied.The reaction mechanism was discussed based on the experimental results.The effects of reaction conditions such as temperature,time and catalyst dosage on the yield of lactic acid were studied and the stability of the catalyst was investigated.The catalytic performance of supported bifunctional Yb(OTf)3 catalyst in the conversion of glucose to methyl lactate was studied.The reaction mechanism was discussed based on the experimental results.The effects of reaction conditions such as temperature,time and catalyst dosage on the yield of lactic acid were studied and the stability of the catalyst was investigated.Specific research works and main conclusions are summarized as follows:1.The mechanism of the conversion of glucose to lactic acid catalyzed by Yb(OTf)3 was revealed by the combination of DFT theoretical calculations and experiments.The reason why Yb(OTf)3 has higher catalytic activity than YbC13 was successfully explained.Quantum chemistry DFT theoretical calculation results show that Yb(?)catalyzed the conversion of glucose to lactic acid mainly consists of three steps:first,the conversion of glucose into fructose based on proton transfer mechanism,then the decomposition of fructose into two trioses such as glyceraldehyde based on anti-aldol condensation mechanism,and finally,the conversion of glyceraldehyde into lactic acid through multi-step reaction;and also proved that Yb(?)catalyzed the conversion of glucose to lactic acid.The reaction barrier is obviously lower than that without catalyst.Catalytic experiments and calculations of NBO charge and LUMO-HOMO orbital energy levels confirm that the catalytic activity of Yb(OTf)3 is higher than YbCl3,which is due to the stronger electron absorption ability of trifluoromethyl sulfonate(OTf).2.The preparation of SBA-15 mesoporous silica by solvent method was screened and confirmed and the homogeneous Yb(OTf)3 catalyst was prepared base on SBA-15 mesoporous silica.The catalytic activity of the catalyst was high in the conversion of lactic acid from glucose.The yield of lactic acid could reach 42.35%after 60 minutes reaction at 190?.Four kinds of mesoporous silica based on MCM-41 and SBA-15 were prepared by solvent method and calcination method,respectively.The content of silicon hydroxyl on the surface of mesoporous silica prepared by solvent method is higher than that by calcination method,so the amount of sulfonic acid group and Yb(OTf)3 immobilized on the surface of mesoporous silica prepared by solvent method is also more.Therefore,the corresponding catalytic activity is higher,and the corresponding yield of lactic acid is about 2%higher than that of roasting method.The pore size of MCM-41 support is smaller,and the pore is blocked during the immobilization of Yb(OTf)3 catalyst.Therefore,the immobilization capacity and catalytic activity of the corresponding catalyst Yb(OTf)3 are smaller than those of SBA-15.The experimental results of catalytic performance showed that the yields of lactic acid were 14.86%and 16.41%for MCM-41 catalysts prepared by calcination method and solvent method respectively,while the yields of lactic acid for SBA-15 catalysts were 38.22%and 42.35%respectively.3.By optimizing the preparation conditions of SBA-15 supported Yb(OTf)3 catalyst,its catalytic activity was improved.The yield of lactic acid reached 58.05%at 190? for 60 minutes.The experimental results of catalytic performance confirmed that glyceraldehyde was the key intermediate in the conversion of glucose to lactic acid.Both grafting and co-condensation methods can realize the sulfonic group functionalization of SBA-15.The Yb(OTf)3 of the sulfonic group functionalized SBA-15 prepared by grafting method has larger immobilization capacity and more regular pore structure.The more regular pore structure is beneficial to the diffusion of reactants,so the corresponding catalysts have higher activity.By adjusting the parameters of n(S)/n(Si)and n(Yb)/n(S),the immobilization capacity of Yb(OTf)3 was increased and the activity of the catalyst was improved.The experimental results of catalytic performance confirmed that the pathway of conversion of glucose to lactic acid by fructose and glyceraldehyde or to HMF by dehydration was competitive reaction.The nucleophilic attack of Yb(OTf)3 on carbonyl oxygen atoms makes the formation of glyceraldehyde by breaking of the C3-C4 bond in fructose easier and then the selectivity of lactic acid was improved.4.Immobilizied bifunctional Yb(OTf)3 catalyst was prepared by doping Sn in SBA-15 framework,which improved the activity of the catalyst.The yield of lactic acid reached 70.65%at 190 0C for 60 minutes.The synergistic catalytic mechanism of Yb(OTf)3 and Sn was evaluated by characterization and catalytic performance experiments.The analysis results of NH3-TPD and HPLC showed that Sn was successfully introduced into the framework of gel materials,and the incorporation of Sn increased the acidity of Yb(OTf)2-SO3-SBA-15.The results of HPLC analysis showed that Sn mainly catalyzed the isomerization of triose to lactic acid,while Yb(OTf)3 catalyzed the reverse aldol condensation and decomposition of glucose.The synergistic catalytic effect of Yb(OTf)3 and Sn increased the yield of lactic acid in the reaction.The experimental results of catalytic performance showed that the conversion of glucose and the yield of lactic acid increased with the increase of temperature,time and catalyst dosage.5.In the methanol reaction system,methyl lactate was prepared by one-step method from glucose,and a high yield of methyl lactate was obtained,which shortened the process of purifying high-purity lactic acid by esterification.In the reaction system of methanol as solvent,solid Yb(OTf)3 catalyst immobilized on Sn/SBA-15 can effectively catalyze the conversion of glucose to methyl lactate.The conversion of glucose and the yield of methyl lactate increased with the increase of temperature,time and catalyst dosage.The yield of methyl lactate could reach 74.7%at 190? for 180 minutes.GC-MS analysis showed that glyceraldehyde was the key intermediate in the conversion of glucose to methyl lactate.The synergistic catalytic effect of Yb(OTf)3 and Sn in supported bifunctional Yb(OTf)3 catalyst improved the selectivity and yield of methyl lactate. |
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