Designation Of Novel Heteropolyacid Catalysts In The Catalytic Synthesis Of Levulinic Acid Esters

Due to the increasing depletion of fossil feedstocks,biomass finds increasing attention as one of analternative renewable energy source.Levulinic acid esters?LAEs?are biomass-based platform molecule able to improve the cold flow properties of biodiesel and to serve as oxygenate additives for diesel fuels and gasoline.Their good performance is attributed to moderate flow properties at low temperature.Different technologies forthe production of LAEs are listed below: esterification of levulinic acid?LA?with alcohols,alcoholysisof carbohydrates?monosaccharide,disaccharide,polysaccharide?,alcoholysisof furfural alcohol.However,the yield of LAEs for esterification of LA is limited by the thermodynamic equilibrium.As a result,esterification is conducted in the presence of a great excess of alcohol in order to shift the equilibrium towards products.Consequently,LAEs are formed in low concentration in alcohols as substrate and solvent causing addition costs for product separation.There are many other shortcomingssuch as high reaction temperature,complicated conversion steps,etc.,which do not accord with the standards of the "green chemistry" concept.Only scares data on the reaction of alpha-Angelica lactone?alpha-AL?with alcohols is currently available,Manzer and Palkovits et al.have reported the production of LAEs based on alpha-AL over commercial solid acid catalyst such as Amberlyst-36.However,all the reported protocols were implemented under anhydrous conditions,e.g.under inert gas atmosphere in a Schlenk line,as a special precaution to prevent rehydration of alpha-AL to LA.The required precautions potentially restrict the industrial application of such reaction conditions.In this regard,novel synthesis route of LAEs by an equimolar addition reaction of an alcohol with alpha-AL over heteropolyacid catalysts under open system are first proposed.Analysis of the reaction network suggests a transformation of alpha-AL with alcohol to yield intermediate pseudo-Levulinic acid esters?p-LAEs?with further conversion toLAEs by transesterification process over catalyst with sufficient acid strength.?Model Reaction?The correlation between catalytic activity and the acidic properties of catalysts are investigated based on model reaction.?1?Number of Keggin/Dawson type molecule HPAs were evaluated as homogeneous catalysts for the alpha-AL addition with BuOH,Dawson-type HPAs H₆P₂W₁₈O₆₂?D-HPW?demonstrated an outstanding catalytic activity attributed to its highBr?nsted acidity.A series of cesium exchanged Dawson-type HPAs [Cs_nH_6-nP₂W₁₈O₆₂?n=1-6?] was synthesized by ion-exchange method,of which H₆P₂W₁₈O₆₂ and cesium chloride?CsCl?as the precursors.The structure of Cs_nH_6-nP₂W₁₈O₆₂?n=1-6?was confirmed by a series of characterization methods and it was used as catalysts for the reaction of alpha-AL with n-butanol forming butyl levulinate?BL?.The exchange of protons by cesium cations leads to decreasing total acidic content?activity?/increasing heterogenety of the HPAs at room temperature.Considering the heterogenety/activityof the acid catalyst Cs_nH_6-nP₂W₁₈O₆₂?n=1-6?,Cs3H3P2W18O62 was the best candidate.The yield of BL reached 79.6% under a moderate temperature of 75? at 1.5 h over catalyst amount?15 wt% = 93.75 mg?.Cs3H3P2W18O62 could be successfully reused 5 times without significant loss of activity.?2?A series of temperature-responsivecholine exchanged Dawson-type HPAs?HOCH2CH2N?CH3?3?nH?6-n?P2W18O62 [abbreviated as Ch_nH_6-nP₂W₁₈O₆₂?n=1-6?,Choline cation abbreviated as Ch+] was synthesized,of which H₆P₂W₁₈O₆₂ and choline?ChCl?as the precursors.The structure of Ch_nH_6-nP₂W₁₈O₆₂?n=1-6?was confirmed by a series of characterization methods.Importantly,it could precipitate upon cooling the reaction mixture to room temperature as heterogeneous catalyst facilitating catalyst separation and recycling;it also could be soluble in reaction mixture under 75? as homogeneous catalyst facilitating reaction rate.Ch_nH_6-nP₂W₁₈O₆₂?n=1-6?used as temperature-responsive catalysts for the reaction of alpha-AL with n-butanol forming BL.Optimizing the catalyst composition for the proposed reaction requires a compromise between catalytic activity and solubility.In thisregard,Ch2H4P2W18O62 was the appropriate catalyst.The yield of BL reached 79.4% at full conversion of alpha-AL at a moderate temperature of 75? at 1 h over catalyst amount?20 wt% = 125.12 mg?.Ch_nH_6-nP₂W₁₈O₆₂?n=1-6?could be successfully reused 5 times without significant loss of activity.?3?Cs_nH_6-nP₂W₁₈O₆₂?n=1-6?and Ch_nH_6-nP₂W₁₈O₆₂?n=1-6?were synthesized by adjusting the counter ion of H₆P₂W₁₈O₆₂?D-HPW?with CsCl and ChCl in chapter 2 and chapter 3,exhibiting only Br?nsted acidity/not so high mechanical stabolity.The immobilization of D-HPW onto Lewis acidic mesoporous ZrO₂/SiO₂?m-SiZr?support by wet impregnation method to prepare D-HPW/m-SiZr could be a solution,D-HPW/m-SiZr exhibited strong Br?nsted acidity,moderate Lewis acidity and strong mechanical stability.The structure of D-HPW/m-SiZr was confirmed by a series of characterization techniques,D-HPW/m-SiZr showed excellent activity for the model reaction.Under optimized reaction condition: alpha-AL?0.625 g,6.375 mmol?,BuOH?0.473 g,6.375 mmol?,75?,1 h,D-HPW/m-SiZr?5 wt%=31.28 mg?,the yield of BL achieves 91.1%.D-HPW/m-SiZrcould be successfully reused 6 times without significant loss of activity.?4?In order to test the role of Br?nsted\Lewis acidity in the transformation from alpha-AL with different alcohol to various LAEs?modelreaction?.The Lewis acidity was introduced into Keggin-type H3PW12O40 by incorparating Lewis metal into its primary structure?PW12O403-?.1:11 metal mono-substituted Keggin-type HPAs?HnPW11MO40,M=Lewis acidic metal TiIV,CuII,AlIII,SnIV,FeIII,CrIII,ZrIV and ZnII?was synthesized.The influence of these Lewis metals on dual Br?nsted\Lewisacidic strength of catalyst was investigated systematically,incorporation of Lewis metal would enhance the total acidity facilitating the transformation from alpha-AL with different alcohol to various LAEs.Among all these HPAs,H₅PW₁₁TiO₄₀ displayed best activity.Calcination treatment to H₅PW₁₁TiO₄₀ made it insoluble in reaction mixture which confirmed its heterogeneous performance in model reaction.Different alphatic alcohol could be reacted with alpha-AL to produce series of LAEs based on super acidity strength of H₅PW₁₁TiO₄₀.Under optimized reaction condition: alpha-AL?0.625 g,6.375 mmol?,BuOH?0.473 g,6.375 mmol?,75?,10 min,H₅PW₁₁TiO₄₀?1 wt%=6.256 mg?,the yield of BL achieves 96.3%.Furthermore,H₅PW₁₁TiO₄₀ did not suffer from leaching anddeactivation in 5 reaction runs.