Study On The 4-dimethylaminopyridine(DMAP) Covanlently Immobilized On Polyhydroxy Branching Silica Carriers

4-Dimethylaminopyridine(DMAP),as a small molecule super-nucleophilic catalyst,is widely used in many reactions such as acylation,Darkin-West,methylation,Baylis-Hillman,tribenzylation and polymerization reaction.DMAP has the advantages of high catalytic efficiency and mild reaction conditions.However,it is difficult to separate from the homogeneous system,which brings more problems to subsequent processing.Therefore,it is necessary to prepare heterogeneous DMAP catalysts.In the early stage of our laboratory,potassium iodide was used as an accelerator to avoid the use of hazardous chemicals-Na H.A molecular sieve carrier-supported DMAP catalyst was prepared,but the immobilized amount of the catalyst was limited by the number of hydroxyl groups on the surface of silica.In this paper,in order to increase the hydroxyl content on the surface of the silicon-based carriers,and then increase the catalyst immobilizing amount.Glycerol and sorbitol were selected as branching agents.After silane coupling,epoxy-alcohol addition and N-alkylation reaction,DMAP was covalently immobilized on the branched silicon-based carrier to obtain branched immobilized DMAP.The key synthetic reactions were systematically investigated,and the activity of the two catalysts were examined.Glycerol reacted with the epoxy group of the coupled silica gel to form a polyhydroxy branched end.Then,the hydroxyl group was chloropropylated with epichlorohydrin,and 4-methylaminopyridine(MAP)was used as the precursor to immobilized DMAP on the carriers through N-Alkylation.Results showed that when the molar ratio of glycerol to epoxy group was 2: 1,the reaction temperature was 50 ℃,the reaction time was 8 h,and the stirring rate was 500 rpm,the highest hydroxyl content was 3.2 mmol/g.Under the optimal reaction conditions,the maximum immobilization of glycerol branched DMAP could reach 1.9 mmol/g.The catalyst was characterized by Fourier transform infrared(FT-IR),scanning electron microscope(SEM),and Brunauer emmett teller(BET).Results confirmed that DMAP was successfully immobilized on the suface of silica.The immobilization progress was carried out by using sorbitol with a higher hydroxyl content to substitute for glycerin as a branching agent.The results showed that when the molar ratio of glycerol to epoxy group was 2: 1,the reaction temperature was 50 °C,the reaction time was 8 h,and the stirring rate was 500 rpm,the hydroxyl content was as high as 6.4 mmol/g.Under above optimal reaction conditions for sorb itol branching,the immobilizing amount of DMAP could reach 3.6 mmol/g.The catalyst was characterized by Fourier transform infrared(FT-IR),scanning electron microscope(SEM),and Brunauer emmett teller(BET).Results confirmed that DMAP was successfully immobilized on the suface of silica.The acylation reaction was used as an indicator reaction to analyze the activity of the two branched immobilized DMAP.The activity of the immobilized DMAP on sorbitol branched silica gel was higher.Then the effect of different carriers contained nano-silica(NS),silica gel(MSP),and molecular sieve(MCM-41)on the loading of DMAP were studied.Finally,the activity and stability of these three catalysts were investigated,and the nano-silica was the best carrier.