Preparation And Catalytic Activities In Esterification Of Levulinic Acid Of Functionalized UiO-66 Catalysts

Due to its excellent stability,tunable defect and accessible functionalization,Zirconium terephthalate UiO-66 type metal organic frameworks（MOFs）has been a promising catalyst in many catalytic reactions.Levulinic acid is one of the most important twelve platform compounds.Due to mainly dominated by Br?nsted acid sites,esterification of levulinic acid with ethanol was chosen as a test reaction to better understand the catalytic activity sites in UiO-66 and further to guide the design and modification of UiO-66 in the future.A series of UiO-66-X（OA）samples（X stands for the mole ratio of H₂BDC and ZrCl₄,X=0.3,0.5,1.0,1.5 and 2.0）were prepared by changing the molar ratio of H₂BDC and ZrCl₄ with octanoic acid as a modulator.The result showed that UiO-66 with 0.5 of H₂BDC/Zr ratio possessed the highest specific surface area（1266 m²/g）,the largest pore volume（0.93 cm³/g）and the highest defect density（1.61 per node）.The influence of defects on the catalytic performance was investigated in the esterification reaction of levulinic acid and ethanol.The results showed that UiO-66-0.5（OA）has the highest catalytic activity.The yield of ethyl levulinate could be up to 77%with 1:15 of molar ratio（levulinic acid:ethanol）,1.8 mol%of catalyst amount with respect to levulinic acid,78℃of reaction temperature and 8 h of reaction time.A series of UiO-66-0.5（M）samples with different defect numbers（M stands for the modulator,M=acetic acid,octanoic acid,lauric acid and myristic acid）were prepared with 0.5 of H₂BDC/Zr molar ratio.The results of sample characterization showed that the specific surface area,pore volume and defect number of the sample could be greatly increased by properly extending the chain length of the acid modulator.However,the structure of UiO-66 would be destroyed if the alkyl chain in modulators is too long.In addition,the IR results showed the hydroxyl peak of the UiO-66-0.5（LA）sample activated at 80℃for 12 h disappeared gradually.When the synthesized material was used to catalyze the esterification of levulinic acid and ethanol,the un-activated UiO-66-0.5（LA）has the highest catalytic activity.This is attributed to the highest defect density（2.1 per node）endowed by the proper chain length of lauric acid and the cooperative effect between Lewis acid sites provided by unsaturated Zr and Br?nsted acid sites given by the hydroxyl.UiO-66-NH₂ was prepared with octanoic acid as the modulator under the condition of 0.5 of H₂BDC/Zr molar ratio.The sulfonic group was introduced by the method of post-synthesis modification（PSM）.Finally,HPW@UiO-66-NH-RSO₃H with strong Br?nsted acid sites was prepared by loading H₃PW₁₂O₄₀（HPW）.The results showed that the supported H₃PW₁₂O₄₀ was dispersed on the host.In addition,the introduction of H₃PW₁₂O₄₀and sulfonic acid group improves the catalytic activity of the material.Compared with the non-functional samples,the yield of ethyl levulinate has a significant increase.When HPW@UiO-66-NH-RSO₃H was used as catalysts,the yield of ethyl levulinate can reach 89%with 1:15 of molar ratio（levulinic acid:ethanol）,0.8 mol%of catalyst amount with respect to levulinic acid,78℃of reaction temperature and 8 h of reaction time.The catalytic activity had no obvious decreased after four times of catalyst circulation.