New Process For Producing Methyl Benzoate

Methyl benzoate（C₈H₈O₂）,traditionally produced by H₂SO₄catalyzed esterification of benzoic acid and methanol,is an important chemical liquid ester.However,the above route has many disadvantages such as serious corrosion of pipes,discharge of acidic sewage,high molar ratio of methanol to benzoic acid and high energy consumption for the separation of products.Therefore,a new process involving preparation of glycerol tribenzoate（GTB）by esterification of benzoic acid and glycerol and step with the sequential transesterification between the formed glycerides and methanol,has been developed in the research to produce highly qualitied methyl benzoate,with valuable GTB as by-products.The thermal stability of glycerol,especially the reaction kinetics of glycerol polymerization,was examined firstly.It is shown that the polymerization of glycerol is a second-order reaction,no matter catalysts are present or not.However,both solid acid（5 wt%Si O₂）and base（0.5wt%KOH）accelerated significantly the reaction rates,due to the reduced apparent activation energy from 95.34 k J/mol to 64.14 and 29.56 k J/mol,respectively.Reaction parameters such as temperature（T）,pressure（p）,stirring rate（w）,time（t）and the molar ratio of glycerol to benzoic acid（R）were systematically evaluated for the esterification between glycerol and benzoic acid.Under the optimized conditions of T=240°C,p=1 atm,w=500 rpm,t=4 h and R=1/0.60,the conversion of benzoic acid was above 99%.Separation of GTB from the reaction mixture via crystallization and the following recrystallization process were also investigated to purify GTB intermediates.The best conditions for separation of GTB contained the addition of 0.10%GTB as seed crystal and the subsequent cooling5°C for 2 h.Further recrystallization process required the addition a double amount of methanol as solvent under the similar conditions.The final yield and purity of GTB were more than 50%and 90%,respectively.Influences of reaction parameters on the catalytic transesterification between glycerides and methanol by KOH were finally studied to optimize the yield of methyl benzoate.Under the conditions of a catalyst dosage of 1.0 wt%,T=60°C,t=4 h and the molar ratio of methanol to glycerides of 3/1,the yield of methyl benzoate reached 97.89%,with a purity no less than 98%.