Preparation And Application Of High-load 4-Dimethylaminopyridine(DMAP) Catalyst On Nano-silica

As a small molecule catalyst,4-dimethylaminopyridine is widely used in the acylation reaction of alcohols and amines;the conditions required for the reaction are relatively mild and the actual catalytic efficiency is high,especially for the compounds with high steric hindrance and low activity prominent.However,it has disadvantages such as difficulty in separating from the reaction system,affecting product quality,incapability of recycling or continuous operation used in the free state.Therefore,carrier supported DMAP catalyst has an important application significance.Based on the preliminary research in the laboratory,this article explored ways to reduce the mass transfer resistance,increase the loading capacity,and prepare high-activity supported DMAP catalysts through the selection of nanocarriers and the branched covalent bonding method.The main findings are as follows:Using spherical silica gel as a carrier,a branching covalent bonding method to increase the loading of DMAP was studied.Through surface epoxy functionalization,polyhydroxy branching agent amplification of hydroxyl,chloroalkylation conversion and N-alkylation to carry out high loading of DMAP.The effects of polyhydroxy branching agents sorbitol,pentaerythritol,and xylitol on amplifying surface hydroxyl groups and thereby increasing the loading of DMAP were studied.As a branching agent,sorbitol showed high catalytic activity.Optimized the epoxy-alcohol addition reaction conditions of the polyhydroxy branching agent to amplify the surface hydroxyl groups.When the molar ratio of the branching agent to the epoxy group is 2.5:1,55 ℃,500 rpm,8 h,the hydroxyl content is 6.56 mmol/g.Furthermore,explore the optimal reaction conditions of methylaminopyridine(MAP)N-alkylation to generate loaded DMAP,the molar ratio of KI to MAP is 1:1,the molar ratio of K2CO3 to MAP is 1.5:1,130 ℃,20 h,500 rpm conditions The loading of DMAP reached 3.86 mmol/g.Characterized by thermogravimetry(TG-DTG),infrared(FT-IR)and specific surface area(BET),DMAP is loaded onto the surface of spherical silica gel by means of covalent bonding.Based on the determination of the branched covalent bonding method,the influence of the type of support on the performance of the supported catalyst was studied.Three typical carriers of molecular sieve,spherical silica gel and nano-silica were selected.The corresponding surface hydroxyl groups after sorbitol branching were 7.53,6.56,and 6.04 mmol/g respectively;the loading amount of DMAP after N-chloroalkylation was respectively 4.31,3.86,3.37 mmol/g;taking the acetylation reaction of α-tocopherol as an indicator,the reaction evaluation shows that the corresponding catalytic activities are 12.63,13.81,15.99 mmol/h·g catalyst,and nanosilica as the carrier achieved the best results.TG-DTG,FT-IR,and specific surface area(BET)characterization verified that DMAP was covalently bound to the surface of the carrier,and the vitality retention rate after 10 cycles of all the three kinds of supported DMAP was greater than 95.5%.Using sorbitol as the branching agent and nano-silica as the carrier,the effect of superimposing branching to amplify surface hydroxyl groups to increase DMAP loading was studied.The surface hydroxyl group after the primary branching and amplification is then epoxy-coupled with a silane coupling agent for the second time,and then the branching and amplifying hydroxyl groups,chloroalkylation conversion,and N-alkylation loading DMAP are superimposed.The surface hydroxyl content of primary branching and superimposed branching were 6.04 and 8.89 mmol/g respectively;the corresponding DMAP loadings were 3.37 and 5.17 mmol/g respectively;the catalytic activity of α-tocopherol acetylation was 15.99 and 24.97 mmol/ h·g catalyst;after 10 cycles of reuse,the activity retention rate is 94.9%,and the superimposed branching significantly improves the DMAP loading capacity of nano-silica and the catalyst performance.The prepared high-load DMAP catalyst was applied to the synthesis of vitamin E succinate and menthol acetyl ester,and both showed excellent catalytic performance.