Prearation Of Glycolic Acid By Carbonylation Of Formaldehyde

The traditional process of formaldehyde preparation of glycolic acid uses sulfuric acid and hydrofluoric acid as catalysts,which seriously corrodes the equipment and the subsequent process is complicated.The research focus of formaldehyde carbonylation process is to develop new catalysts that are easy to separate and environmental protection.In this paper,several series of strong acid catalysts were prepared,their catalytic performance was investigated,the reaction conditions were optimized,and the catalytic reaction mechanism of phosphotungstic acid was explored.Firstly,the catalytic properties of four sulfonic acid resins were investigated.Amberlyst-15 catalysts modified with Co²⁺,Fe³⁺,Ni²⁺ and Sn⁴⁺were prepared by selecting Amberlyst-15 catalysts with the highest catalytic activity as the carrier.FT-IR and XPS showed that metal ions had been supported in the resins.The performance of Fe3+ modified Amberlyst-15 catalyst is better,the conversion of formaldehyde is 83.60%,and the yield of methyl glycolate（MG）is 62.64%.In order to increase the yield of MG,the catalytic performance of phosphotungstic acid,phosphomolybdic acid,silicotungstic acid and silicomolybonic acid was investigated.The results showed that phosphotungstic acid had the best catalytic performance,the conversion rate of formaldehyde was 99.06%,and the yield of MG was 75.54%.Then,the influence of reaction conditions on MG yield under phosphotungstic acid catalyst was investigated.It was found that high temperature and too long reaction time would lead to the polymerization of the product.When the mass ratio of phosphotungstic acid to formaldehyde was greater than 0.4,increasing the amount of phosphotungstic acid could not improve the product yield,but increasing the CO pressure could improve the MG yield.The optimized reaction conditions were as follows: the mass ratio of phosphotungstic acid to formaldehyde was 0.5,the temperature was 160 ℃,the time was 2 h and the CO pressure was 10 MPa.Under the optimized reaction conditions,the formaldehyde conversion rate was 99.36%and the MG yield was 79.51%.In order to facilitate the separation of catalysts,phosphotungstic acid catalysts supported by four molecular sieves,including HZSM-5,were prepared.FT-IR and XRD showed that phosphotungstic acid was successfully supported on molecular sieves.Compared with the catalytic performance of phosphotungstic acid supported by different molecular sieves,HZSM-5 was the best carrier.The effects of Si/Al ratio and phosphotungstic acid loading on the catalytic performance of supported phosphotungstic acid were investigated.It was found that the catalytic performance of HZSM-5 with Si/Al ratio of 38 and40% phosphotungstic acid loading was the highest,and the yield of MG was60.89%.Due to the low catalytic activity of phosphotungstic acid on molecular siolites,phosphotungstic acid catalyst was prepared.The types and contents of cations were studied.The results show that（NH4）1PW has very high catalytic activity,formaldehyde conversion rate is 96.24%,MG yield is 70.53%,and the catalytic performance of（NH4）1PW has no obvious change after five uses.The results showed that ammonium phosphotungstate catalyzed formaldehyde carbonylation with high activity and stability.Finally,the reaction mechanism of formaldehyde carbonylation process was explored.The change of product composition after the activation of formaldehyde was inhibited by water and methanol.It was concluded that formaldehyde carbonylation reaction was carried out by carbocation mechanism,and the reaction products were MG,methyl methoxyacetate（MMAc）,dimethyl glycol（DMDG）and some oligomers.In order to better understand the process of side reaction,the product composition without CO was investigated,combined with the Tyshchenko reaction mechanism,it was shown that formaldehyde can form methyl formate（MF）through semi-acetal structure intermediate.Finally,the complete transformation path of formaldehyde was obtained.The transformation relationship showed that inhibiting formaldehyde disproportionation reaction and product polymerization could improve the yield of MG.