Research On Microwave-assisted Dehydration Of Lactic Acid To Acrylic Acid

Acrylic acid and acrylates are very important basic materials and polymer monomers of modern organic chemical industry, but acrylic acid production has been greatly affected by the increasing shortage of petroleum, and it is particularly important to investigate how to produce acrylic acid from renewable resources. Lactic acid production by fermentation from renewable resources is a mature process, hence, dehydrating lactic acid to acrylic acid might be an effective alternative for acrylic acid production, and it is one of the most likely routes to replace petroleum-based acrylic acid. But the yield of acrylic acid is low, and the energy consumption is high when using lactic acid as feedstock. Many researchers have paid more attention to microwave heating because of its excellent features, such as fast heating rate, uniform heating, efficient energy conservation, and improved selectivity of product.In this paper, based on the previous research,we use microwave heating to replace the conventional heating, and use calcium sulfate as the main catalyst, copper sulfate, disodium hydrogen phosphate, potassium dihydrogen phosphate as promoters, lactic acid as starting material, nitrogen as carrier gas. All reactions were performed in a vertical, down-flow fixed bed reactor, and we studied the effects of calcination temperature, carrier gas flow rate, lactic acid flow rate, lactic acid concentration, reaction temperature, and reaction time. We characterized the properties of catalyst by XRD, FTIR, NH3-TPD, TA, and so on. The results indicated that the dehydration reactions were mainly catalyzed by Lewis-acid sites. We also speculated the reaction mechanism of dehydration. The optimum conditions were: the mass ratio of components in the catalyst was CaSO4: CuSO4: Na2HPO4: KH2PO4=150:8.8:2.5:1.2, calcination temperature 370°C, lactic acid concentration 25%, lactic acid flow rate 0.05mL/min, nitrogen flow rate 20mL/min, reaction temperature 298°C. At these conditions, we can obtain the highest acrylic acid yield 50.3%, and the conversion of lactic acid was 100%, while acrylic acid yield was 19.1% at 300°C with traditional heating in references. It illustrated that microwave heating can promote the catalytic reaction and improve the product selectivity compared with conventional heating. Simultaneously, it was found that acrylic acid yield can be improved to 65.8% when disodium hydrogen phosphate was used as the only promoter. This paper can lay an important theoretical and technical basis for large-scale production of acrylic acid from fermentation lactic acid.