Esterification Of Cyclohexene With Cyclohexanecarboxylic Acid Catalyzed By Modified Beta Molecular Sieve

Cyclohexanol is an important organic material in petrochemical industry.The preparation of cyclohexanol by indirect hydration of cyclohexene has the advantages of good atomic economy,low energy consumption and mild reaction conditions.How to improve the efficiency of acid-catalyzed esterification is the key problem of indirect hydration process.The current research mainly focuses on the esterification reaction system of cyclohexene with strong polar carboxylic acids such as formic acid and acetic acid.Formic acid and acetic acid corrode the equipment and have high requirements on the structural stability of the catalyst.Moreover,there are problems of high energy consumption for separation of dilute acid aqueous solution in the subsequent hydrolysis reaction.Cyclohexanecarboxylic acid is insoluble in water and has weak polarity.The esterification reaction of cyclohexanecarboxylic acid and cyclohexene can avoid the problems existing in strong polar acid.In this paper,the Beta（Hβ）molecular sieve is adopted as a catalyst,on the basis of determining the appropriate pretreatment conditions,molecular sieves are modified by pore structure regulation and surface hydrophobic modification,and the structure-activity relationship between the structure of modified molecular sieves and their catalytic performance is explored,so as to provide accumulation for the construction of efficient esterification reaction system between cyclohexene and cyclohexanecarboxylic acid.Firstly,the pretreatment conditions of high temperature roasting for Hβmolecular sieve were screened.Through the characterization of XRD diffraction,N₂physical adsorption,vacuum infrared and other means,it was determined that 600°C was the appropriate calcination temperature,and the catalytic performance of Hβmolecular sieve was improved after pretreatment.The reaction conditions for the esterification of cyclohexene with cyclohexanecarboxylic acid catalyzed by Hβmolecular sieve were investigated.Under the optimized reaction conditions,the molar ratio of cyclohexene to cyclohexanecarboxylic acid was 1:3,the amount of catalyst was 3%of the total reaction liquid,under the temperature of140℃for 2 h,the conversion of cyclohexene was 55.06%,and the selectivity of cyclohexyl cyclohexanoate was 74.99%.Secondly,the effects of alkali treatment with Na OH and tetraethylammonium hydroxide（TEAOH）on the pore-enlarging modification of Hβmolecular sieves were investigated respectively.As the concentration of Na OH alkali solution increases,the specific surface area of the molecular sieve decreased significantly,the total pore volume increased gradually,and the proportion of mesopore volume increased significantly,but the structure of molecular sieve was damaged more seriously.When using TEAOH alkali treatment for pore-enlarging,the structure of the zeolite remained more complete,the specific surface area did not change much,the total pore volume increased significantly,and the proportion of micropore volume decreased.The molecular sieve undergoes secondary crystallization during high temperature TEAOH treatment,which can increase the specific surface area and pore volume.The weak acid and medium strong acid contents of the samples decreased after alkali treatment by both methods.When using mixed Na OH and TEAOH alkali treatment for pore-enlarging,which can increase the specific surface area and pore volume.The catalytic activity of the sample treated with Na OH alkali solution is obviously lower than that before modification due to the lower acid content and the decreased specific surface area.The TEAOH treated or mixed Na OH and TEAOH alkali treatment samples performed better than that of Na OH alkali treated samples.The sample treated with mixed alkali solution can improve the selectivity of cyclohexyl cyclohexanoate up to 84.25%.Finally,the hydrophobic modification on the surface of Hβzeolites was carried out by the method of silylation and chemical vapor deposition.The contact angle test showed that the hydrophobic modification effect of the two methods was obvious.The specific surface area and pore volume of the samples after silanization modification were significantly decreased.Reducing the amount of silanization reagent and modification time could reduce the modification degree and the degree of pore blockage.The specific surface area and pore structure of samples modified by chemical vapor deposition method did not change much.Silanization modification caused a significant decrease in acid content due to occupying the acidic sites of the molecular sieve,and the catalytic performance in the esterification reaction also decreased significantly.The catalytic performance of the samples modified by vapor deposition had little change,and the increase of reaction rate was obviously faster than that of Hβmolecular sieve before modification.The TEAOH pore-expanding modified composite silylated hydrophobic-modified samples have significantly higher selectivity to cyclohexyl cyclohexanoate than the silylated-modified samples,and the reaction rate increases linearly with time,indicating the enhancement effect of pore expanding modification and hydrophobic modification on reaction rate.