Kinetics of Citraconic Anhydride Formation via Condensation of Formaldehyde and Succinates Dushyant Shekhawat, Kirthivasan Nagarajan, ² James E. Jackson, ² and Dennis J. Miller* Department of Chemical Engineering, Michigan State UniVersity, East Lansing, Michigan 48824, U.S.A. Abstract: Formation of citraconic anhydride via condensation of succinic acid and its derivatives with formaldehyde is carried out over γ-alumina catalyst in a continuous fixed-bed reactor. Dimethyl succinate and Formalin (37 wt % formaldehyde, 10 wt % methanol in water) are the preferred feed materials for the reaction; catalyst activity is sustained with Formalin relative to that with other formaldehyde sources such as trioxane or Formcel, because the water in Formalin inhibits coke formation. With this feed combination, a total citraconate yield of 31% of theoretical with 72% selectivity is achieved at a weight hour space velocity of 0.9 kg of succinate/kg of catalyst/h, a succinate to formaldehyde molar feed ratio of 1:2, and a temperature of 380 °C. The reaction is free from mass transfer limitations at these conditions. A kinetic model is presented that describes product distributions and reactant conversion as a function of space velocity and temperature. The reaction system is part of an overall process to produce itaconic acid from renewable resource-based succinic acid. I. Introduction Succinic acid (1,4-butanedioic acid) and its alkyl esters are reactive species that find applications in industrial and consumer products and as intermediates for specialty and fine chemicals production. 1,2 Recent advances in fermentation technologies for succinic acid production, 3-6 arising both from genetic modification of the microorganism and im- proved separations, have led to yields of succinic acid as high as 1.1 kg of succinic acid/kg of glucose (with CO 2 incorporation) and have nearly eliminated acetic acid as a coproduct. 6 A significant decrease in the price of the acid as a raw material is thus expected as the manufacturing technology matures, making it attractive as a feedstock for biomass-based chemical production. The Stobbe condensation, 7 discovered in 1894, offers a potentially efficient route for the production of substituted succinates via condensation of esters of succinic acid with aldehydes and ketones. The reaction is essentially unique to succinic acid esters and generally takes place in alcohol solution in the presence of a strong base (alkoxide or sodium hydride) to give the half-ester as a product. 8 The classic Stobbe condensation does not occur to any practical extent when formaldehyde is used as the aldehyde. However, the vapor-phase catalytic (heterogeneous) con- densation of succinic acid, succinic anhydride, or alkyl succinate esters with formaldehyde does take place to give citraconic anhydride (CAN), an isomer of itaconic anhydride (Scheme 1). Several patents describe catalytic routes to CAN formation from succinates, 9-12 with transient yields as high as 70% of theoretical claimed. 9 The motivation for CAN formation is to produce itaconic acid via hydrolysis of CAN to citraconic acid and isomerization to itaconic acid. Itaconic acid is a carboxylated analogue of the important monomer methacrylic acid and as such is able to take part in addition polymerization, giving polymers with many free carboxylic acid groups that confer advantageous wettability and ion- exchange properties. 13 Itaconic acid is currently produced commercially (8 × 10 6 kg/yr) by the fermentation of glucose using Aspergillus terreus. 13 This fungal fermentation is carried out in batch processes requiring dilute solutions (∼10 wt % glucose) and extended processing times (6-10 days per batch). Itaconic acid yields are on the order of 50-60% of theoretical; this process results in an itaconic acid selling price of ∼$2/lb * To whom correspondence should be addressed. Department of Chemical Engineering, 2527 Engineering Building, Michigan State University, East Lansing, MI 48824. Telephone: (517) 353-3928. Fax: (517) 432-1105. E-mail: millerd@egr.msu.edu. ² Department of Chemistry, Michigan State University, East Lansing, MI 48824. 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