Acidity Control Of Zr-based Metal-organic Framework UiO-66 And Its Catalytic Performance In Esterification Reaction

Cyclohexanol is an important intermediate in the industrial production of caprolactam.The indirect hydration of cyclohexene to prepare cyclohexanol includes two steps,esterification of cyclohexene with formic acid and hydrolysis of cyclohexene formate,of which the first step of the esterification reaction is the key step of the process.The esterification reaction is a typical acid-catalyzed reaction,and the construction of a highly efficient and stable solid acid catalyst is the focus of research.In this paper,the12-coordinated high-stability zirconium-based metal organic framework material UiO-66 is used as the basic carrier,which is acid modified and acid regulated by the method of sulfonic acid functionalization,increasing the number of lattice defect sites and the compounding of heteropolyacid,characterize the structural characteristics of the material after acid adjustment by various means,and evaluate the acid catalytic performance and reuse performance of the modified material in the esterification reaction of cyclohexene with formic acid,and the relationship between the structure,performance and stability of acid catalyst is discussed to provide work accumulation for acid modification of metal organic framework materials.Firstly,using ZrCl₄as the source of zirconium,terephthalic acid?H₂BDC?or 2-sulfonic acid monosodium terephthalate?H₂BDC-SO₃Na?as the linking agent to prepare UiO-66 and sulfonic acid functionalized UiO-66-SO₃H material by the method of thermal solvent,and the specific surface areas of the obtained materials were 1127m²/g and 367m²/g,respectively;UiO-66-SO₃H maintained the octahedral crystal form with that of UiO-66 and the grain size was also consistent with UiO-66,the acid amount of UiO-66-SO₃H reached 2.00mmol/g.UiO-66 catalyzed the esterification of cyclohexene with formic acid,the conversion rate of cyclohexene was 3.37%,the selectivity of cyclohexyl formate was 93.95%,and when UiO-66-SO₃H was used,the conversion rate of cyclohexene was 47.83%and the selectivity of ester was 96.72%.The catalytic activity of UiO-66-SO₃H decreased slightly after repeated use for three times.Secondly,acetic acid was selected as the modulator,ZrCl₄was used as the zirconium source,and H₂BDC-SO₃Na was used as the linking agent,UiO-66-SO₃H with different number of defect sites was prepared by adjusting the added amount of acetic acid.As the added amount of acetic acid increased,the particle size of the material increased from 0.30?m to 2.86?m;the number of defect sites contained in the material was gradually increased by thermogravimetric analysis;the generation of defect sites did not affect the thermal stability of the material,and the final decomposition temperature was maintained at 580?.The physical adsorption characterization showed that the specific surface area of the defect site material increased first and then decreased with the increase of the added amount of acetic acid,and the pore volume gradually increased;the specific surface area of the UiO-66-SO₃H?20?material was up to 1090m²/g,the pore volume was 0.59cc/g.As the amount of acetic acid increased,the acid content of the materials with defect sites increased first and then decreased.The maximum acid content of UiO-66-SO₃H?15?material was2.15mmol/g.The 5 kinds of materials with defect sites were used in the esterification reaction of cyclohexene with formic acid respectively,UiO-66-SO₃H?20?had the best catalytic performance,the conversion rate of cyclohexene was 83.39%,and the selectivity of ester was99.83%.After repeated use,the conversion rate of cyclohexene was reduced to 62.70%,and the selectivity of ester was 98.32%.By comparing the infrared spectroscopy and XRD characterization of the samples stored in different solvents and reaction media for a certain period of time,the results showed that formic acid had a serious destructive effect on the structural stability of the catalyst.Finally,in order to further improve the acidity and stability of the catalyst,phosphotungstic acid?HPW?was loaded in situ on the carriers of UiO-66,UiO-66-SO₃H and UiO-66-SO₃H?20?with defective sites,respectively.The ICP analysis showed that the loading amount of HPW on the three carriers was 14.80%,4.41%and 1.48%.The characterization results showed that the loading of HPW will reduce the specific surface area and pore volume of the carrier itself.The XPS characterization showed that there was an interaction between the-SO₃H group and HPW in the support,and the UV-DRS also proved an interaction between the Zr₆-oxo cluster and HPW,these interactions were conducive to the improvement of the stability of the material;the loading of HPW improved the acid catalytic performance of the material and reduced the used amount of catalyst.When HPW@UiO-66 catalyzed the esterification reaction,the conversion rate of cyclohexene was 41.43%,and the selectivity of the ester was 97.65%;when used HPW@UiO-66-SO₃H,the cyclohexene conversion rate was70.72%and the ester selectivity was 97.72%,showing that the amount of acid and the specific surface area of the material affect its acid catalytic performance.The material of HPW@UiO-66-SO₃H was reused for 4 times,the conversion rate of cyclohexene stabilized above 63.93%with the improved stability.When HPW@UiO-66-SO₃H?20?catalyzed the esterification reaction,the conversion rate of cyclohexene reached 87.65%,and the selectivity of the ester was 97.56%,because of the lattice defects contained in the carrier and the lower loading amount of HPW,the cyclohexene conversion rate fell to 63.68%after the catalyst was reused once,and no improvement in stability.