Controlled Synthesis Of MOFs Heterogeneous Catalyst And Catalytic Performances In Esterification For Biodiesel Production

The cost of biodiesel production can be greatly reduced by using waste cooking oil as raw material.Due to free fatty acids and water,it is necessary to use acid catalyst to catalyze transesterification/esterification reaction to prepare biodiesel.However,homogeneous acid catalyst dissolves in the reaction system,which can not be recycled,causing serious environmental pollution.The solid acid catalyst effectively solves this problem.However,the poor catalytic stability of ordinary solid acids restricts their application in industry.Therefore,in this study,based on the Metal Organic Frameworks（MOFs）and their derivatives,stable and efficient solid acid catalysts were synthesized by the post-functionalization method,which were used to catalyze the esterification of oleic acid and methanol to prepare biodiesel.At the same time,Thermal gravimetric analysis（TG）,X-ray powder diffraction analysis（XRD）,Fourier transform infrared spectroscopy analysis（FTIR）,Pyridine FTIR spectroscopy analysis（Py-FTIR）,Scanning electron microscopy（SEM）,Nitrogen adsorption-desorption,Hammett titration and Boehm titration were used to elucidate the structure-activity relationship between the structural characteristics and the catalytic activity,revealing the mechanism of catalytic esterification reaction.（1）Firstly,with dilute sulfuric acid as the sulfonating agent,sulfonated solid acid MSH was obtained by using the defect site in MIL-100（Fe）to support sulfonic groups.The influence of the preparation factors such as dilute sulfuric acid concentration（C_s）,dilute sulfuric acid volume（V_s）,sulfonation temperature（T_s）and sulfonation time（t_s）were obtained by the esterification reaction of methanol and oleic acid.The results showed that high sulfonation temperature would lead to oxidation of-SO₃H reducing the acid content of the catalyst.The optimal conditions for preparation of the catalyst were C_s=0.9 mol/L,t_s=10h,T_s=160℃,V_s=25 m L.The esterification conversion of 95.86%can be achieved at catalytic dosage of 8 wt.%,molar ratio of methanol to oleic acid of 8:1,reaction temperature of 70℃,reaction time of 2 h.After repeated use for 6 times,the catalytic activity of MSH did not significantly decrease,indicating good stability of MSH.（2）Using Co-doped MIL-100（Fe）as the carrier,solid acid catalyst was prepared by supporting（NH₄）₂SO₄.Compared with MIL-100（Fe）,Co can improve the stability of sulfated metal oxides and reduce the loss of active sites.The effects of（NH₄）₂SO₄ loading amount（M_n）and calcination temperature（T_n）on the catalytic activity were investigated.The results show that the optimal conditions are（NH₄）₂SO₄ loading amount of 1:1 and calcination temperature of 400℃.The prepared catalyst（MS（Fe/Co））was used to catalyze the esterification of oleic acid and methanol.The esterification conversion of 92.56%can be achieved at the conditions of catalytic dosage of 8 wt.%,molar ratio of methanol to oleic acid of 10:1,reaction temperature of 70℃,reaction time of 2 h.After repeated use for 4times,the catalytic activity of MS（Fe/Co）did not significantly decrease.Using kinetic model of solid-liquid-liquid heterogeneous catalytic esterification reaction,the activation energy and pre-exponential factor was calculated as E_a=52.13 k J/mol and A=4.44×10⁵ min^-1,respectively.（3）Using UiO-66 as the carrier,the high stability UiO-66 was functionalized with non-volatile and low corrosive p-toluene sulfonic acid（PTSA）to obtain high efficiency and stability solid acid catalyst.The effects of the preparation factors,such as the mass ratio of PTSA to UiO-66（M_P）,the concentration of PTSA（C_P）and the impregnation temperature（T_P）were investigated obtaining the optimal preparation conditions as M_p=1,C_p=0.5 mol/L,T_p=110℃.The maximum esterification conversion was 91.3%under the catalyst amount of 8wt.%,molar ratio of methanol to oleic acid of 12,reaction temperature of 70℃and reaction time of 2 h.Under this condition,the catalyst can be reused for 4 cycles without significant loss of activity.The calculated activation energy of the esterification reaction is E_a=37.55k J/mol,and the pre-exponential factor is A=30.13 min^-1,indicating that UH can effectively reduce the activation energy and promote the reaction process.