Study On Carbon-loaded Metals For Catalyzing The Conversion Of Carbohydrates To Lactic Acid And Diols

Lactic acid and diols are important platform compounds for production of functional chemicals and materials.The conversion of carbohydrates into platform compounds such as lactic acid and diols can reduce the dependence on petrochemical-based resources and promote sustainable development of human society.Although numerous chemical catalytic systems have been exploited with the aim of converting the carbohydrates to lactic acid and diols,previously developed catalysts suffer from the defects of requiring rare metals（e.g.,lanthanide）,long preparation periods（e.g.,molecular sieve loaded metal Sn,etc.）,and difficult separation of catalyst from products（e.g.,homogeneous Pb（NO₃）₂,etc.）.To overcome these shortcomings,in this thesis,two carbon-based catalysts were synthesized and used to catalyze the conversion of carbohydrates to lactic acid and diols.The catalysts were characterized and measured by chemisorption（NH₃-TPD）,X-ray photoelectron spectroscopy（XPS）,X-ray diffraction（XRD）,thermogravimetric analysis（TG）,and physical adsorption instrument.The catalytic processes and conversion pathways of two catalysts for converting carbohydrates（glucose,xylose,cellulose,xylan,etc.）into lactic acid and diols（ethylene glycol,propylene glycol,etc.）was preliminary explorated by comparing the reaction pathways of model compounds.The details of the study and the results are as follows.First,catalysts（Ni@C）towards the loading of nickel acetate on carbon black（C）were prepared by an impregnation-calcination method.The effects of the preparation conditions（metal salt type,metal salt impregnation amount,carrier type,nitrogen heterocyclic ligand,calcination method,calcination temperature and calcination time）on the activity of Ni@C was investigated.The results showed that the optimum catalyst preparation conditions were as follows:nickel acetate tetrahydrate as the metal salt,conductive carbon black（VULCAN@XC72R）as the carrier,the mass ratio of nickel acetate tetrahydrate to conductive carbon black with 1:1.8,calcination temperature of800℃,calcination time of 2 h,calcination atmosphere of Ar at 20 m L/min.The optimum reaction conditions were as follows:reaction temperature 220℃,reaction time 2 h,N₂ pressure 2 MPa,glucose addition 0.05 g,Ni@C addition 0.1 g,deionised water 10 m L.Under these conditions,the conversion of glucose was 100%and the yield（C mol%）of lactic acid was 49%.Second,Ni@C also showed convenient catalytic property when pentose（xylose）was used as the reaction substrate.Under optimal reaction conditions（reaction temperature:240℃,reaction time:1 h,substrate addition:0.05 g,Ni@C addition:0.1g,deionised water:10 ml）,a xylose conversion rate of 100%was achieved while the lactic acid yield was 47%.Under the same reaction conditions,the yields of lactic acid were 31%and 22%for xylan and microcrystalline cellulose as substrate.In addition,XRD and XPS characterisation showed that Ni in Ni@C presented both Ni⁰ and Ni²⁺valence forms,which contribute to the Lewis acid of this catalyst.Meanwhile,BET and TG results showed that Ni@C possesses mesoporous structure and is stable under high temperatures.The intermediates such as fructose,1,3-dihydroxyacetone and glyceraldehyde were selceted as model compounds to study the reaction pathways of Ni@C catalyzed carbohydrate conversion to lactic acid.Combining the reaction results of the above model compounds,the reaction pathways of Ni@C catalyzed conversion of carbohydrates to lactic acid under hydrothermal conditions mainly include:1)glucose transform to fructose via isomerization;2)fructose undergoes a reverse aldol reaction to produce 1,3-dihydroxyacetone and glycerol aldehyde;3)1,3-dihydroxyacetone and glyceraldehyde are converted into lactic acid through dehydration and hydrogen shift reactions,etc.When the reaction substrate is changed to xylose,the reaction path is similar to that of glucose,i.e.,xylose is mainly converted to lactic acid through isomerization,retro-aldol,and hydrogen shift reactions.Fianlly,the catalyst PA-Ni@C was prepared through loading phosphotungstic acid（PA）on Ni@C.The preferred catalyst preparation conditions were:the mass ratio of PA to Ni@C with 1:30,calcination temperature of 300℃,calcination time of 5 h and no venting（calcination atmosphere was the air existed in the tube furnace）.For PA-Ni@C catalyzed conversion of xylose to diols,the xylose conversion and the diol yield reached 100%and 56%（1,2-propanediol,39%;1,3-Propylene glycol,5%;1,2-butanediol,12%）respectively,under the reaction conditions of reaction temperature of 225℃,reaction time of 1 h,xylose addition of 0.05 g,PA-Ni@C addition of 0.05 g,H₂ pressure of 4 MPa and 10 m L of deionized water.In addition,XRD and XPS characterisation revealed that owing to the Keggin structure of partial phosphotungstic acid in PA-Ni@C the catalyst,PA-Ni@C not only acquired Br?nsted acidity,but also enhanced its initial Lewis acid property.As the addition of phosphotungstic acid did not cause the change of the valence state of Ni in the catalyst,PA-Ni@C possessed Br?nsted acid,Lewis acid and hydrogenation reduction properties.PA-Ni@C can catalyse not only the isomerisation and retro-aldol reactions of glucose and xylose,but also the hydrogenation reduction of intermediates such as pyruvaldehyde to diols such as 1,2-propanediol under H₂ atmosphere.Based on the reaction pathways of intermediates such as glyceraldehyde,1,3-dihydroxyacetone and glycolaldehyde as model compounds,PA-Ni@C-catalyzed conversion of xylose to diols mainly involves:1)the retro-aldol reaction of xylose to glyceraldehyde,1,3-dihydroxyacetone and glycolaldehyde;2)the conversion of glyceraldehyde and 1,3-dihydroxyacetone to 1,2-propanediol via hydrodeoxidation reaction;3)Some of the glycolaldehydes are directly reduced to ethylene glycol,and some of them are transformed to erythrose via aldol reactions,which are further transformed to 1,2-butanediol through successive hydrodeoxidation reactions.Meanwhile,erythrose can further condense with glycolaldehyde to form glucose.The glucose may isomerise to fructose,which can be converted to glyceraldehyde,1,3-dihydroxyacetone and finally reduced to 1,2-propanediol.For polysaccharides such as cellulose and xylan,they are first hydrolyzed to glucose and xylose,and the subsequent reactions follow the same pathway as above.Overall,in this thesis,two carbon-based catalysts（Ni@C and PA-Ni@C）were prepared,which can efficiently catalyze the conversion of carbohydrates to platform compounds such as lactic acid and diols under inert and reduction atmospheres,respectively.Combined with characterisations of XPS,XRD,NH₃-TPD,BET,TG,and SEM-EDS,the reaction pathways of Ni@C and PA-Ni@C for catalyzing the hydrothermal conversions of carbohydrates to lactic acid and diols were initially explored,providing a theoretical basis for the high value-added refinery of carbohydrates in lignocellulosic biomass.