The Preparation Of Heteropoly Acid Functionalized SBA-15 Silicas And Their Applications In Organic Catalysis

Heteropolyacids(HPAs)are an important class of multifunctional inorganic catalysts,which have been used extensively in organic synthesis.HP As process many attractive features for synthetic chemists,such as strong Bronsted acidity,high redox potential,non-corrosiveness,as well as environmental benign.Unfortunately,HP As are very soluble in polar solvents,such as water,alcohols,ketones,etc,which means HP As exhibit some difficulties for product separation and catalyst recovery in these catalytic systems.HP As are insoluble in non-polar solvents,and therefore they can be considered as heterogeneous catalysts.However,a main disadvantage is their very low surface area(<10 m2/g),which greatly effects their catalysis efficiency.Over the past decade,the heterogenization of HPAs,together with their applications in heterogeneous organic catalysis has been an attractive research area in organic synthesis.In this thesis,we firstly incorporated HP As into the mesoporous silica matrix(SBA-15)by a direct sol-gel technique to obtain HP As-functionalized SBA-15 silicas(HP A@SB A-15).And then these materials were tested as heterogeneous acid catalysts in the ring-opening reaction of epoxides and N-tosyl aziridines,as well as tandem Meinwald/acetalization reactions of epoxides.The contents were as follows:Part 1.Three HP As-functionalized SBA-15 silicas(HPW@SBA-15,HSiW@SBA-15,HPMo@SBA-15)containing HPW、HSiW and HPMo were directly synthesized by a sol-gel technique.Then the structures and acidities of these materials were characterized by XRD、TEM、BET and acid-base potentiometric titration method.The results showed that the pore structures of HPW@SBA-15,HSiW@SBA-15,HPMo@SBA-15 were similar to that of pure SBA-15,with pore sizes of 5.88-6.05 nm,surface areas of 790-948 m2/g and pore volumes of 1.16-1.43 cm3/g.The results of acid-base potentiometric titrations also showed that the acid sites amounts of these materials were in range of 0.62-0.73 mmol/g,and the acid strength decreased in the sequence of HPW@SBA-15>HSiW@SBA-15>HPMo@SBA-15,which was according to that of parent HP As.Part 2.The catalytic activities of three kinds of HPA@SBA-15 were investigated by using the ring-opening of styrene oxide with methanol as a model reaction.It was found that the ring-opening reaction could proceed at room temperature under the catalysis of each one of HPA@SBA-15 and HPW@SBA-15 exhibited much better catalytic activity than HSiW@SBA-15 and HPMo@SBA-15 with a TOF reaching to 3.97 min-1.It was attributed to its strongest acidity.Furthermore,the generality of HPW@SBA-15 in this ring-opening reaction was tested.The results showed that,by employing HPW@SBA-15 as acid catalyst,a wide range of aryl and alkyl epoxides could react with primary alcohol,secondary alcohol,propargyl alcohol and water at room temperature and corresponding ring-opening products were obtained in 85-99%yields.Part 3.Under the catalysis of HPW@SBA-15 and in non-polar solvents,aryl epoxides could proceed with Meinwald rearrangement to form corresponding aldehydes at room temperature.Especially,a quantitative yield could be resulted with THF as reaction medium.Moreover,the catalyst HPW@SBA-15 also could sequentially catalyze the transformation of the formed aldehydes(Meinwald rearrangement products)to corresponding acetals by using triethyl orthoformate,ethanediol or ethanedithiol as acetalization reagents.This tandem Meinwald rearrangement/acetalization reaction could be carried out in one-pot at room temperature and various kinds of acetals could be directly obtained from aryl epoxides and corresponding acetalization reagents in 85-98%total yields.Part 4.The catalysis activities of HPW@SBA-15,HSiW@SBA-15 and HPMo@SBA-15 in the ring-opening of aziridines withO-nucleophiles were studied by using the reaction of 2-phenyl-1-tosylaziridine and methanol as a model reaction.Similarly,HPW@SBA-15 exhibited best catalysis activity in this reaction with a TOF of 3.29 min-1.Finally,under the catalysis of HPW@SBA-15,a wide range of aryl and alkyl N-tosylaziridines react with alcohols,acetic acid and water at room temperature to form corresponding ring-opening products in 92-98%yields.Noteworthy,in all above tested reactions,the heterogeneous catalyst HPW@SBA-15 could be easily isolated from reaction mixture by filtration.After simply washing with solvent,the recovered catalyst could be recycled for many times without any obvious loss of catalysis activity.