Research On Metal Salts Catalysts For The Conversion Of Carbohydrates And Their Derivatives Into Platform Molecules

Carbohydrates is one of the most important existing biomass resources.Selective conversion of carbohydrates to a variety of platform molecules,including furfural,5-hydroxymethylfurfural(HMF),levulinic acid(LA),alkyl levulinates and so on,offers an alternative path to produce fuel and chemicals from fossil oil.Among all these platform molecules,alkyl levulinates and furfural are two of the most attractive platform molecules that have wide applications for the production of flavor,materials,drugs,fuel additives,and so on.Developing cost-effective and highly efficient pathways to obtain alkyl levulinates and furfural from inexpensive biomass-derivered carbonhydrates is of great importance for the sustainability of future chemical production.For this purpose,inexpensive metal salts were selected as catalysts for the conversion of various carbohydrates and their derivates to alkyl levuliates and furfural in this study.Firstly,a new strategy for the efficient conversion of furfuryl alcohol to alkyl levulinates using some inexpensive metal salts under microwave irradiation was developed.It was found that the conversion is strongly dependent on the Br?nsted acidity of the catalyst.The highest methyl levulinate yield of 80.6% was achieved over Al2(SO4)3 catalyst for its excellent Br?nsted acidity in methanol solvent.Compared with the conventional heating method,microwave irradiation was proved to improve the reaction rate dramatically,while it had no effect on the selectivity of products.The Al2(SO4)3 also demonstrated excellent catalytical activity for the alcoholysis of furfuryl alcohol in different solvents to its synthesize corresponding esters.In addition,the stability and recyclability of Al2(SO4)3 catalyst was proved to be excellent after six-runs.Finally,an integrated reaction pathway was proposed by tracing the progress of the reaction.Secondly,metal salt catalysts were employed for the esterification reaction of levulinic acid,a hydrolytic product of glucose.A high methyl levulinate yield up to 99.4% was achieved with the full conversion of levulinic acid.Moreover,Al2(SO4)3 showed remarkable catalytic activity for the esterification of simulated carbohydrate hydrolysate,which would make it a potential catalyst for the practical application of direct conversion of carbohydrate hydrolysate without further dewater process.The results of the dielectric properties of the reaction system indicated that Al2(SO4)3 was hydrolyzed or alcoholized in methanol during the conversion progress.Based on the above results,a facile and high-yield synthesis of alkyl levulinates directly from cellulose over the metal salt aluminium sulfate under microwave irradiation was developed.The highest methyl levulinate yield of 70.6% was obtained at 180 oC with a short reaction time of 40 minutes.The introduction of some water to the reaction system could not only increase the rate of cellulose conversion,but also switch the reaction pathway via more reactive intermediate glucose.Moreover,the Al2(SO4)3 also showed good compatibility for the conversion of other carbohydrates and raw lignocellulosic biomass to methyl levulinate with product yields from 62.7% to 83.4%.Finally,the conversion of pentose into furfural using metal salts under microwave irradiation in a biphasic system was studied.Particularly,xylan was used as the model compound.The effect of catalyst type,reaction temperature,amount of catalyst loading,the volume ratio of organic phase to water phase and solid-liquid ratio were investigated and optimized.Comparative study between Al2(SO4)3 and Al Cl3 suggested that Al2(SO4)3 was not only be a highly active catalyst for this reaction,but also an excellent phase separation promoter of the binary phase.Four frequently-used organic solvents were also employed for the xylan conversion,and the results showed that ?-valerlactone exhibited great ability to promote the reaction rate and furfural selectivity,which was due to some of its inherent advantages,such as good dielectric properties and excellent dissolving capacity for the substrates and catalytic site.Further study on the reaction mechanism indicated the plausible pathway for the conversion of xylan into furfural as follow: initially,xylan was hydrolyzed to xylose catalyzed by Br?nsted acid sites,which was further transformed to xylulose by Lewis-acid-catalyzed isomerization.Subsequently,generated xylulose was quickly converted into target product furfural over Br?nsted acid sites again.And the result from catalyst characterization indicated that the [Al(OH)2(H2O)4] + generated from the deep hydrolysis of hexacoordinated complexes was the major Lewis acidic specie.In summary,this thesis developed a green and highly efficient catalytic system for the conversion of carbohydrates and their derivatives into high-grade platform molecules.The reaction pathways and catalytic activity were also studied comprehensively.The findings of this thesis could provide theoretical guidance and reference for the further industrial scale production of alkyl levulinates and furfural.