A Green Route To Acrylic Acid And Methyl Acrylate By An Aldol Condensation Reaction Over HZSM-35 Catalysts

Acrylic acid and methyl acrylate,widely used in the manufacture of paintings,coatings,carbon fibers and adhesives,are currently produced by two-step oxidation of propylene.Considering the structure of primary energy in China,rich in coal,less oil and less gas,there is high demand to develop a novel and green route focusing on the coal resources for acrylic acid and methyl acrylate synthesis.This dissertation focuses on one-step aldol condensation reaction of dimethoxymethane and methyl acetate which can be derived readily from coal resources to produce acrylic acid and methyl acrylate.The route to produce acrylic acid and methyl acrylate,catalyzed by H-form molecular sieves,provides a new way to use the coal resources effectively and cleanly.Catalytic performances of aldol condensation reaction have been studied over H-form molecular sieves with different structures.Among those catalysts,HZSM-35 molecular sieve with 10-ring channels exhibited high reactivity and selectivity,proving to be a potential industrial catalyst for aldol condensation reaction.HZSM-35 molecular sieve with different acid amounts were prepared using Cs incipient impregnation.The effect of the acidic properties on aldol condensation performance was significant for HZSM-35 molecular sieve.Acid sites have been proved to be indispensable in aldol condensation reaction of dimethoxymethane and methyl acetate.The higher acrylic acid and methyl acrylate in product distribution was obtained over HZSM-35 molecular sieve with higher acid amount.The HZSM-35 molecular sieve demonstrated the best performance with the acrylic acid and methyl acrylate percent in product distribution of 31.5%at 100%dimethoxymethane conversion and 71.1%methyl acetate conversion.The appropriate reaction temperature of 400 ℃,the suitable gas hourly space velocity of 3600 h-1,SiO2/Al2O3 ratio of 30 and the optimal molar ratio of dimethoxymethane to methyl acetate of 2 were essential for the aldol condensation reaction catalyzed by HZSM-35 molecular sieve.Reaction network of dimethoxymethane and methyl acetate has been fabricated for HZSM-35 molecular sieve supported by in situ mass spectroscopy and DRIFTS analysis.By using different feeding mode,dimethoxymethane was observed to decompose into formaldehyde and dimethyl ether over HZSM-35 molecular sieve,and then formaldehyde reacted with methyl acetate to produce acrylic acid and methyl acrylate via aldol condensation reaction.Simultaneously,both precursors(dimethoxymethane and methyl acetate)and products(formaldehyde,methanol,methyl formate and so on)participated in series of by-reactions,such as hydrolysis,disproportionation,decomposition,dehydration,finally forming a complex reaction system.The acidic properties of HZSM-35 molecular sieve have been deeply studied via IR and NH3-TPD analysis.Catalytic performance of aldol condensation reaction for HZSM-35 molecular sieves with different acidic properties showed that both Bronsted acid and Lewis acid were active sites for aldol condensation reaction.HZSM-35 molecular sieves with different Bronsted acid concentrations have been prepared using sodium ion exchange method.The higher acrylic acid and methyl acrylate percent in product distribution was formed with higher Bronsted acid concentrations.TG,nitrogen adsorption-desorption,NH3-TPD and GC-MS analysis helped to reveal that Bronsted acid played an important role in the formation of coke over HZSM-35 molecular sieve.The retained organic species in the spent HZSM-35 molecular sieve included benzene,naphthalene,and phenanthrene derivatives.Amount of naphthalene and phenanthrene derivatives increased with the Bronsted acid concentration.It was found that coke was mainly formed in the initial period of aldol condensation reaction(about 120 minutes)with a weight of 6.7 wt%,holding 80%of the whole coke.Coke over HZSM-35 molecular sieve was mainly derived from the methanol to hydrocarbon reaction(MTH).