Preparation And Characterization Of Novel Acid-Base Bifunctional Ionic Liquids And Their Catalytic Performance For N-Valeraldehyde Self-Condensation

As a novel green solvent or catalyst,ionic liquid has been widely used in organic synthesis,extraction and separation,electrochemistry,nanomaterials preparation and so on.Stimulated by the acid-base bifunctional catalysts which usually show excellent synergesticlly catalytic performance,we prepared two kinds of acid-base bifunctional ionic liquids(ABFILs):one with an acid center and a base center on a same cation,and another with an acid center and a base center separately on an anion and a cation.The structures,acid properties and base properties of the synthesized ABFILs were characterized and determined.And then the catalytic performance of ABFILs for n-valeraldehyde self-condensation reaction was evaluated.The first kind of ABFIL([CH3C00-Zn-03S-bim-CH2CH2NH2]C1,AMSFIL)with zinc atom as the acid center and an amino group as the base center on an imidazole cation was prepared.The structure of AMSFIL was confirmed with the help of 1H NMR,FT-IR,HPLC-MS and elemental analysis.The acid amount and base amount of AMSFIL were measured by acid-base titration.The catalytic performance of AMSFIL for n-valeraldehyde self-condensation reaction was evaluated,and the suitable reaction conditions were obtained as follows:a weight percentage of AMSFIL=15wt%,a reaction temperature=140? and a reaction time=8 h.Under the above reaction conditions,the yield and selectivity of 2-propyl-2-heptenal were 80.7%and 84.5%,respectively.The reason for the poor reusability of AMSFIL is probably due to its interaction with some organic substaces existing in the reaction system based on the result of 1H NMR analysis.The second kind of ABFIL with an acid center(zinc atom or lead atom)and a base center(piperidine group or diethylamino group)separately on an anion and a cation was prepared.In order to shorten the preparation time of intermediates[PEmim]Cl-HCl and[DEAEmim]Cl·HCl,a microwave technique and the orthogonal experimental design were utilized to optimize the synthesis conditions for[PEmim]Cl-HCl.The result showed that the preparation time decreased from 24 h to 30 min without lowering the yield of[PEmim]Cl-HCl.The catalytic performance of different ABFILs on n-valeraldehyde self-condensation reaction was evaluated and the suitable[PEmim]Cl-0.5(CH3COO)2Zn was determined.Using[PEmim]Cl-0.5(CH3COO)2Zn as the catalyst,the suitable reaction conditions for n-valeraldehyde self-condensation reaction were obtained as follows:a weight percentage of[PEmim]Cl-0.5(CH3COO)2Zn=15wt%,a reaction temperature=120?and a reaction time=8 h.Under the above reaction conditions,the yield and selectivity of 2-propyl-2-heptenal were 82.0%and 86.6%,respectively.After being treated before the next recycle,the recovered[PEmim]Cl-0.5(CH3COO)2Zn was effectively reused up to five cycles without much loss in its activity.Additionally,the reaction products obtained from[PEmim]Cl-0.5(CH3COO)2Zn catalyzed n-valeraldehyde self-condensation were indentified by GC-MS and the synergistically catalytic mechanism of n-valeraldehyde self-condensation catalyzed by[PEmim]Cl-0.5(CH3COO)2Zn was conjectured.Finally,the synergistically catalytic effect of[PEmim]Cl-0.5(CH3COO)2Zn on the n-valeraldehyde self-condensation was further confirmed by DFT calculation.