Synthesis Of Acid Functional MIL-101(Cr) And Its Catalytic Performance In Esterification Of Cyclohexene With Formic Acid

Cyclohexanol was prepared by the indirect hydration of cyclohexene with formic acid as entrainer.Cyclohexylformate was synthesized first,and then cyclohexanol was produced by the hydrolysis of ester.This process not only has low potential safety hazard,but also overcomes the thermodynamic limitation of cyclohexene direct hydration process,which is of great research value.Since cyclohexylformate is easily hydrolyzed to cyclohexanol,and the esterification reaction of cyclohexene with formic acid is a typical acid-catalyzed reaction,so finding a high-efficient solid acid catalyst is the key to realize the indirect hydration of cyclohexene to cyclohexanol.In this thesis,a series of sulfonic acid functionalized MIL-101?Cr?samples were prepared by using metal-organic framework materials MIL-101?Cr?with high thermal and chemical stability as the carrier,and the catalytic performance of acid was further improved by compounding heteropolyacids.Some characterization methods were used to analyze the structure of the synthesized samples,and their catalytic performance in the esterification reaction of cyclohexene with formic acid was evaluated.The aim was to provide effective technological accumulation for developing of solid acid catalysts for the indirect hydration of cyclohexene.Firstly,the metal organic framework material MIL-101?Cr?was prepared by hydrothermal synthesis method using Cr?NO₃?₃·9H₂O as chromium source,terephthalic acid as organic ligand and HF or NaAC as mineralizers.The characterization results show that the specific surface of the samples prepared by the HF and NaAC as mineralizer was 2416 and 2397 m²/g,respectively,and the pore volume of the samples prepared by the HF and NaAC as mineralizer was 1.348 and1.354 cm³/g respectively.The thermal stability of MIL-101?Cr?_HFF is higher.MIL-101?Cr?only contains the Lewis acid sites formed by the unsaturated coordination of Cr metal,which has low acid activity and cannot effectively catalyze the esterification reaction.Secondly,a series of sulfonic acid functional MIL-101?Cr?were prepared by one-step synthesis method using Cr?NO₃?₃·9H₂O or CrO₃ as chromium source,2-sulfoterephthalic acid monosodium salt as monoligand or 2-sulfoterephthalic acid monosodium salt and terephthalic acid with the same molar ratio as double ligand,respectively,using HF,NaAC or HCl as mineralizer.The results indicates that the acid amount of MIL-101-SO₃H_HClCl synthesized by CrO₃ as chromium source,HCl as mineralizer and 2-sulfoterephthalic acid monosodium salt as single-ligand was 0.82mmol.g^-1.It shows the best catalytic effect in the esterification of cyclohexene with formic acid with a conversion of 63.97%and a selectivity of 97.61%for cyclohexylformate,and it could be reused three times.Using HCl as mineralizer,MIL-101-SO₃H-bdc_HClCl prepared by 2-sulfoterephthalic acid monosodium salt and terephthalic acid as dual-ligand had better catalytic performance than MIL-101-SO₃H_HFF and MIL-101-SO₃H_NaACaAC synthesized by 2-sulfoterephthalic acid monosodium salt as single-ligand.Its cyclohexene conversion was 44.75%,the selectivity of cyclohexylformate was 96.64%.In order to study the influence of the ratio of sulfonic acid ligand and terephthalic acid ligand on the structure stability and acidity of the sulfonic acid functional MIL-101?Cr?,the sulfonic acid functional MIL-101?Cr?samples with molar fractions of 1,0.5,0.25 and 0.1 were synthesized using CrO₃ as chromium source and HCl as mineralizer.The results revealed that the acid amount of sulfonic acid functional MIL-101?Cr?gradually increased,with the increase of molar amount of sulfonic ligand.MIL-101-SO₃H?1?with molar fraction of 1 still performed best catalytic activity.The conversion of MIL-101-SO₃H?0.25?in the sample prepared by the double ligand has little change during the recovery of 5 times,and the reusability performance is better,indicating that the structure is relatively more stable.Finally,in order to improve the catalytic activity and stability of sulfonic acid functional MIL-101?Cr?,the phosphotungstic acid was supported on MIL-101-SO₃H by in-situ synthesis method and impregnation method,respectively.And the influence of the loading amount of phosphotungstic acid and the supporting method on the structure and performance of catalyst were investigated.The results showed that the HPW@MIL-101-SO₃H?0.5?sample synthesized by in-situ synthesis method under the loading of 0.5g/2g sulfonic acid ligand had the highest acid amount and the best catalytic performance.The cyclohexene conversion reached 80.20%and the selectivity of cyclohexylformate was 98.45%.No significant decrease in activity was observed after 5 times of reuse.The strong interaction between the phosphotungstic acid and sulfonic acid group was beneficial to the improvement of the structure stability of the samples.