| Fatty acid esters were widely applied in food,light industry because of their low corrosiveness and high stability.In addition,fatty acid esters(e.g.,fatty acid methyl esters)were also a renewable and green fuel.Thus,development of preparation of fatty acid esters is very important.Though traditional homogeneous acidic catalysts had high catalytic activity,their non-recyclable character had resulted in a waste and environmental pollution.As the novel porous materials,metal-organic frameworks(MOFs)were hot topic in the research among hetergenous catalysts.In this paper,the common organic acids in laboratory,p-toluenesulfonic acid and methanesulfonic acid were supported on UiO-66 to obtain a new supported catalyst.Then they had been applied into esterification of fatty acids to obtain fatty acid ester products.And the catalyst-recovery characteristics were also studied.Firstly,UiO-66 was synthesized through solvothermal method,then characterized by XRD,FT-IR and nitrogen adsorption-desorption experiments.The esterification of palmitic acid and methanol was used as the model reaction.Using p-toluenesulfonic acid and methanesulfonic acid as catalysts,esterification reaction was carried out by direct mixing in situ with UiO-66.The experimental results showed that the higher amount of UiO-66 added,the more amount of methanol adsorped by UiO-66,which led to less yield of methyl palmitate.When addition amount of UiO-66 was 25 mg with amount of palmitic acid was 2 mmol,reaction temperature was 100 °C,reaction time was 2 h,molar ratio of methanol to palmitic acid was 8:1,the yield of methyl palmitate were 97.0% and 96.8%.p-Toluenesulfonic acid and methanesulfonic acid was supported on UiO-66 after the reaction through FT-IR analysis.Recovery experiments were carried out under the same reaction conditions.The yield of palmitic acid were 80.0 % and 84.6 % after using for three times.UiO-66 had strong stability through XRD analysis.p-Toluenesulfonic acid and methanesulfonic acid could be supported on UiO-66 after the reaction through directly mixing.Secondly,p-toluenesulfonic acid(PTSA)and methanesulfonic acid(MSA)were supported on UiO-66 by impregnation method.The new catalysts were named PTSA/UiO-66 and MSA/UiO-66.The average amount of PTSA on UiO-66 was 258.9±5.6 mg/g,0.95 mol/g after 5 h impregnation through elemental analysis.The average content of MSA supported on UiO-66 was larger than PTSA,and was 247.1±2.1 mg/g,2.57 mol/g.The esterification of palmitic acid and methanol as a model reaction.When the amount of palmitic acid was 2 mmol,the reaction temperature was 100 °C,the reaction time was 4 h,the amount of catalyst was 25 g/mol,and the molar ratio of methanol to palmitic acid was 8: 1,the yield of PTSA/UiO-66 and MSA/UiO-66 to catalyze the methyl palmitate were 97.1% and 97.0%,respectively.The yield of methyl palmitate catalyzed by PTSA/UiO-66 and MSA/UiO-66 were reduced to 84.3 % and 96.9 % after 10 times use when methanol was pulled away first.After 10 times use,the content of PTSA and MSA acid in UiO-66 were 213.4 mg/g and 199.0 mg/g.According to the characterization of PTSA/UiO-66 and MSA/UiO-66 after 10 times use,PTSA/UiO-66 and MSA/UiO-66 had strong stability.Then,MSA/UiO-66 was used as the catalyst in esterification of palmitic acid with n-butanol to obtain butyl palmitate.The effects of reaction temperature,reaction time,catalyst dosage and molar ratio of alcohol to acid on the conversion of butyl palmitate were investigated.When the reaction temperature was 100 °C,the reaction time was 2 h,the catalyst dosage was 25 g/mol,the molar ratio of n-butanol to palmitic acid was 3:1,the conversion of palmitic acid was 99.2%.After 10 recovery experiments,the yield of palmitic acid decreased to 87.2%.And the MSA in UiO-66 was 204.7 mg/g.Under the conditions of reaction temperature was 100 °C,reaction time was 2 h,catalyst dosage was 25 g/mol and molar ratio of n-decanol to palmitic acid was 2:1,the conversion of palmitic acid was 91.4%.Moreover,the conversion of palmitic acid had almost no attenuation after 10 times use,while the content of MSA in UiO-66 was 209.3 mg/g.This experimental results showed that alcohols with lower polarity can reduce MSA leaching,which enhance the recovery effect.Finally,MSA/UiO-66 was used as catalyst in the esterification of mixed fat acids with n-butanol.The effects of reaction temperature,reaction time,catalyst dosage and molar ratio of alcohol to fat acids were investigated.The results showed that the conversion of mixed fatty acids was 98.0 % under the conditions of reaction temperature was 120 °C,reaction time was 2 h,catalyst dosage was 6 wt% and molar ratio of alcohol to acid was 3:1.The conversion of mixed acids was reduced to 90.1 % after 5 times use.XRD analysis of MSA/UiO-66 showed that the crystal structure of UiO-66 had almost no change. |
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