Optimization Of Conditions And Kinetics Of The Vanillin Reaction Prepared By Glyoxylate Method

Vanillin,an important raw material of aroma,has played an important role in the food industry and improved the flavors of many products.With the gradual increase of the application field of vanillin,the demand for vanillin in various industries worldwide is also increasing year by year,and the annual production rate of the world is about 18000 T/year at present.At present,chemical synthesis is the most mainstream production method for the synthesis of vanillin in the world,which has the advantages of mature process,many raw materials and easy availability,among which the glyoxylate method is one of the most common synthesis methods in chemical synthesis,the first step of which intermediates are obtained by reacting guaiacol with glyoxylic acid under basic conditions,The second step passes the resulting intermediate through oxidation to give the product vanillin.3-Methoxy-4-hydroxymandelic acid is an important intermediate in the production of vanillin by this process,while its derivatives are also of great use in various areas of daily life.The desired product vanillin was obtained by chemical oxidation of 3-Methoxy-4-hydroxymandelic acid with Cu SO4 under heating conditions.There are a variety of reaction conditions affecting the yield in the process of preparing the intermediate 3-Methoxy-4-hydroxymandelic acid,and the effect of different reaction conditions on the selectivity of individual condensation products was roughly explored by controlling variables.Studies have shown that the changes in acid-base sites created by zeolites and molecular sieves based on their different Si-aluminum ratios have distinct effects on the selectivity of condensation reaction products.Therefore,Al³⁺as a typical zwitterionic main group metal ion and Zn²⁺as a typical zwitterionic side group metal ion were selected to be investigated as catalysts for the guaiacol glyoxylate condensation reaction.In which the addition of Al³⁺improved the selectivity of 3-methoxy-4-hydroxyepi to around 91%.Therefore,to improve the reaction selectivity of the key intermediates and useful byproducts in guaiacol preparation of Vanillin,the catalytic mechanism of Al³⁺on the reaction of guaiacol with glyoxylic acid was investigated.Reaction kinetics studies revealed that upon Al³⁺addition,the reaction rate decreased from 1.32×10^-4 L/（mol·h）decreased to 1.26×10^-4L/（mol·h）,the activation energy is improved to 17.11 k J/mol from the original 16.38 k J/mol,which is more beneficial to improve the selectivity of the product.To further explore the effect on product selectivity,thermogravimetric analysis of guaiacol in the presence of Al³⁺suggested that there might be different binding modes of guaiacol to Al³⁺.In situ Fourier transform infrared spectroscopy was used to investigate the multiple interactions of Al³⁺with healed wood phenol in the presence of Al-O and Al...H,which suppress the generation of double condensation by-products.Moreover,the chemical shift changes of guaiacol in ¹H NMR with the increase of Al³⁺suggested that Al³⁺changed the electron cloud distribution of guaiacol by affecting the ortho position of the phenolic hydroxyl group,and then affected the selectivity of the target products.Combined with the above research means,a possible catalytic mechanism was proposed:the simultaneous interaction of Al³⁺with the ortho and para hydrogen atoms of guaiacol hydroxyl group inhibited the further reaction of 3-methoxy-4-hydroxy-mandelic acid and 2-hydroxy-3-methoxy-mandelic acid with glyoxylic acid,thereby improving the selectivity of the target products.In the subsequent preparation of vanillin reaction using the first step condensation product,Cu SO₄·5H₂O was selected as catalyst to study the effects of different p H,reaction temperature,and different Cu SO₄·5H₂O contents on vanillin yield and selectivity.Among them,we focused on the effect of the molar amount of Cu SO₄·5H₂O and maintaining the constant p H on the yield and selectivity of vanillin,the yield of vanillin could reach about 97%;And it was demonstrated that vanillin undergoes complete decarboxylation under basic conditions.