Study On Levulinic Acid Aldol Self-condensation Catalyzed By Acid-functionalized Ionic Liquids

Levulinic acid（LA）is one of the most valuable biomass platform chemicals.LA aldol self-condensation can increase the carbon chains and thus produce a mixture of C10～C15 oxygen-containing compounds which can be used not only as fuel additives and surfactants but also as liquid biofuels after hydrodeoxygenation.Therefore,study on LA aldol self-condensation has both important academic value and potential application prospect.In this paper,the reaction network of LA self-condensation catalyzed by [HSO₃-b-mim]CF₃SO₃ was established first and then the catalytic performance of four acid-functionalized ionic liquids on the LA self-condensation was evaluated.Firstly,a sulfonic functionalized ionic liquid of[HSO₃-b-mim]CF₃SO₃ was prepared and used to catalyze LA self-condensation at a reaction temperature of 170℃and a reaction time of 6 h.The structures of some major products（C9～C15）in the reaction system were determined by means of HPLC-MS analysis and the related possible reactions were presumed.On this basis,a reaction network of LA self-condensation catalyzed by[HSO₃-b-mim]CF₃SO₃ was established.The analysis results showed that the carbon number distribution of the products in this system was in a range of C5～C21.Among them,C10 products with a molecular weight of 214have two structures,i.e,one is linear structure:（Z）-3-acetyl-4-methylhept-3-enedioic acid and（E）-4-methyl-6-oxonon-4-enedioic acid,and the other is cyclic structure:3-（2-methyl-5-oxotetrahydrofuran-2-yl）-4-oxopentanoicacidand5-（2-methyl-5-oxotetrahydrofuran-2-yl）-4-oxopentanoic acid.Other C10 products with a molecular weight of 196 have only cyclic structure,5-（（2-methyl-5-oxotetrahydrofuran-2-yl）methyl）furan-2（3H）-one,5-methyl-4-（2-methyl-5-oxotetrahydrofuran-2-yl）furan-2（3H）-oneand（E）-4-methyl-5-（5-oxo-4,5-dihydrofuran-2-yl）pent-4-enoic acid.Then two sulfonic acid functionalized ionic liquids（[HSO₃-b-mim]CF₃SO₃ and[HSO₃-b-N（Et）₃]CF₃SO₃）and two carboxylic acid functionalized ionic liquids（[c-m-mim]BF₄ and[c-m-N（Et）₃]BF₄）were prepared.Their structures,acidity and acid strength were analyzed by means of IR spectrum analysis,acid-base titration and Hammett indicator combined with UV-vis spectrum analysis respectively.In addition,the catalytic performance of the prepared ionic liquids for LA self-condensation was evaluated.The results showed that acid amount of the ionic liquids was negatively correlated with C10 yield,while acid strength of the ionic liquids was positively correlated with LA conversion rate and negatively correlated with C9～C15 yield.The carboxylic acid functionalized ionic liquid of[c-m-N（Et）₃]BF₄ showed better catalytic performance.The effects of solvent type and amount,catalyst amount,reaction temperature,and reaction time on LA self-condensation catalyzed by[c-m-N（Et）₃]BF₄were investigated.The suitable reaction conditions were determined as follows:a catalyst amount of 10 wt.%,a reaction temperature of 130℃,a reaction time of 6 h and ether solvent amount of 27 m L.Under these conditions,the conversion of LA was49.1%and the yield and selectivity of C10 were 10.7%and 21.8%,respectively.The recovered ionic liquid could be reused four times and its catalytic performance was substantially unchanged.