Acidity And Pore Structure Regulation Of UiO-66 And Its Catalytic Esterification Of Cyclohexene With Cyclohexanecarboxylic Acid

Cyclohexanol is an important organic chemical intermediate.The preparation of cyclohexanol by indirect hydration of cyclohexene is divided into two steps.Firstly,cyclohexene is esterified with formic acid,and then cyclohexyl formate is hydrolyzed to obtain cyclohexanol and formic acid.The key step in this process is the esterification of alkene and acid,which is a typical acid catalyzed reaction.Previous studies have shown that formic acid can cause great damage to the structure of metal organic frameworks（MOFs）catalysts.Therefore,cyclohexanecarboxylic acid is selected to replace formic acid in the esterification of cyclohexene.Cyclohexanecarboxylic acid has weak polarity and is insoluble in water,it is expected to maintain the structural stability of MOFs while avoiding the generation of dilute acid aqueous solution in subsequent hydrolysis reaction and the resulting high separation energy consumption.The weak acidity of cyclohexanecarboxylic acid and the non polarity of reaction system put forward higher requirements for esterification catalyst.In this paper,the optimized Zr based metal organic framework material Ui O-66 was selected as the carrier,which was modified by encapsulation of phosphotungstic acid（HPW）and sulfonic acid functionalization.The pore structure was controlled by lattice defects,and the effect of surface hydrophobic modification on the catalytic performance was investigated.The aim was to construct an efficient and stable catalytic system for the esterification of cyclohexene and cyclohexanecarboxylic acid.Firstly,three kinds of Ui O-66 materials were synthesized by solvothermal method,using Zr Cl₄as zirconium source,terephthalic acid（H₂BDC）as ligand,formic acid and hydrochloric acid as modulators,respectively.The results show that the specific surface area of Ui O-66-1synthesized with formic acid as modulator is 1259 m²/g,the total pore volume is 0.64 cm³/g,the pore size distribution is 0.8 nm and 1.06 nm,and the crystal form is octahedral,which is superior to Ui O-66-2 synthesized with hydrochloric acid as modulator and Ui O-66-0synthesized without modulator.Taking phosphotungstic acid（HPW）as the active component,the optimum conditions for the esterification of cyclohexene with cyclohexanecarboxylic acid were investigated.Under the conditions of the molar ratio of cyclohexene to cyclohexanecarboxylic acid of 1:3,reaction temperature of 140℃,reaction time of 6 h,2%of the total amount of reaction solution,the conversion of cyclohexene was 64.20%,and the selectivity of cyclohexanecarboxylic acid was 88.78%.Secondly,HPW supported Ui O-66 catalyst was prepared by in-situ synthesis method.The structure of the supported catalyst was characterized by various means.The infrared spectrum showed that HPW was encapsulated in the pore of the support.The XRD spectrum showed that the HPW loading had no effect on the structure of the support;The results of N₂physical adsorption showed that the specific surface area and pore volume of HPW loaded support decreased accordingly.ICP characterization determined the mass fraction of HPW in the materials of 20%HPW@Ui O-66-1,30%HPW@Ui O-66-1,20%HPW@Ui O-66-2 and30%HPW@Ui O-66-2 was 18.40%,25.80%and 13.70%,9.30%respectively,indicating that Ui O-66-1 carrier was more suitable for packaging HPW.HPW@Ui O-66 showed no catalytic activity in the esterification of cyclohexene with cyclohexanecarboxylic acid,which indicated that the reactants could not contact the HPW encapsulated in the pore,and the pore effect of the catalyst had a significant effect on the esterification.Thirdly,sulfonic acid functionalized mesoporous Ui O-66-SO₃H_defwith lattice defects was prepared by hydrothermal method with acetic acid as the modifier.The structure and properties of which were compared with that of Ui O-66-SO₃H without defects.Ui O-66-SO₃H did not show catalytic activity in the esterification system of cyclohexene and cyclohexanecarboxylic acid,but Ui O-66-SO₃H_defcould effectively catalyze the esterification reaction under the appropriate reaction conditions:molar ratio of alkene and acid 1:3,reaction temperature 150℃,reaction time 8 h,the amount of catalyst 3%of the total amount of reaction liquid,and the conversion of cyclohexene was 50.54%,The selectivity was 86.56%.The conversion of cyclohexene still reached 45.41%after five times of repeated use of Ui O-66-SO₃H_def,and the structure of the recovered catalyst remained unchanged,indicating that Ui O-66-SO₃H_defhad good stability in the reaction system.Finally,because cyclohexene and cyclohexanecarboxylic acid are hydrophobic substances,and Ui O-66 is a typical hydrophilic material,the effect of surface hydrophobic modification of Ui O-66 on esterification was investigated.The contact angle of Ui O-66-1increased rapidly from 25°to 151°after hydrophobic modification by polydimethylsiloxane（PDMS）through chemical vapor deposition method.The mixture of HPW and Ui O-66-1 can not effectively catalyze the esterification,but the mixture of HPW and hydrophobically modified Ui O-66-1 can greatly improve the conversion of esterification.The catalytic activity of HPW impregnated on Ui O-66-1 was lower than that of HPW impregnated on hydrophobically modified Ui O-66-1.The conversion of cyclohexene was 42.88%,and the selectivity of cyclohexyl cyclohexate was 91.62%.The results showed that the surface hydrophobic modification of the catalyst could improve the esterification efficiency of cyclohexene and cyclohexanecarboxylic acid.