Study On The Modification Of ZSM-5 Catalysts And Their Application In The Dehydration Of Lactic Acid To Acrylic Acid

In our previous study, it is found that alkaline-treated ZSM-5 has rather high selectivity for acrylic acid and is considered as one of the most potential catalysts. In this paper, all our work is based on the alkaline-treated ZSM-5 and is to improve selectivity for acrylic acid and stability of catalysts(1) NaOH solutions were first used to treat H-ZSM-5 zeolites to eliminate its strong acidic sites which are preferable for the formation of acetaldehyde. The resulted catalysts were used in the dehydration reaction of lactic acid to produce acrylic acid. Data collected from the reaction shows that ZSM-5 catalyst treated by 0.5mol/L NaOH solution has the best catalytic performance:the conversion of lactic acid is up to 95.8% and lactic acid selectivity is as high as 67.7%. The reason may be that the structure of the modified ZSM-5 is basically integral and the ratio of the amount of medium acidic sites to weak acidic sites is high.(2) After being treated by NaOH, ZSM-5 catalyst was treated by different sodium phosphates in order to further regulate its surface acid and base and the results indicates that Na2HPO4 was the most efficient material. Then concentration of Na2HPO4 solutions was taken into consideration and we found that the optimum concentration of Na2HPO4 is 0.5mol/L. At last, three reaction parameters were investigated and results revealed the optimum condition is at the reaction temperature of 350? with 30wt% lactic acid and 0.08mL/min flow rate of lactic acid and 35mL/min flow rate of carrier gas. Under the optimum reaction conditions, lactic acid conversion was 96.9% and selectivity for acrylic acid was 77.9%. After operating for 52h, conversion of lactic acid is still 88% and acrylic acid selectivity is 65%.(3) In order to further improve the stability of catalyst, we loaded other metal salts on the catalysts before Na2HPO4 impregnation. The influence of different salts, loading and calcination temperature were investigated and results indicate that the best salt is Ca(NO5)2 and the optimum loading and calcination temperature were 4% and 400?, respectively. The stability of the catalyst was also tested and it is found that lactic acid conversion was 96.6% and selectivity for acrylic acid was 79%; after operation for 60h, we still have 90% lactic acid conversion and 70% acrylic acid selectivity.