Ind. Eng. Chem. Res. 1997, 36, 3505-3512
3505
Reaction and Spectroscopic Studies of Sodium Salt Catalysts for Lactic Acid Conversion Man S. Tam,† Garry C. Gunter,† Radu Craciun,‡ Dennis J. Miller,*,† and James E. Jackson‡ Departments of Chemical Engineering and Chemistry, Michigan State University, East Lansing, Michigan 48824-1226
Catalytic conversion of lactic acid to 2,3-pentanedione over sodium salts and base on low surface area silica support has been studied. Yield and selectivity toward 2,3-pentanedione are optimal at around 300 °C, 3-4 s residence time, and 0.5 MPa total pressure. Anions of initial salt catalysts used do not participate in lactic acid condensation to 2,3-pentanedione once steadystate conditions have been achieved; instead, sodium lactate has been identified by postreaction FTIR spectroscopy as the primary, stable species on the support during reaction. Sodium lactate is believed to be an intermediate in 2,3-pentanedione formation. Conversion of a sodium salt to sodium lactate is greatest when the salt used has a low melting point and a volatile conjugate acid; the extent of conversion depends weakly on reaction time and temperature within experimental conditions. At high temperature (∼350 °C), sodium lactate decomposes to sodium propanoate and sodium acetate, which may explain reduced 2,3-pentanedione yields at higher temperatures. Introduction Although the production rate (50 million lb/year) of lactic acid is still moderate (Kroschwiz and Howe-Grant, 1992), this bifunctional, biomass-derived renewable feedstock has great potential for production of commodity and specialty chemicals. The production of acrylic acid, propylene glycol, dilactide, and poly(lactic acid) from lactic acid via catalytic conversion has been the focus of many studies (Kroschwiz and Howe-Grant, 1992; Lipinsky and Sinclair, 1986). Advances in fermentation technologies, driven by the growing demand for biodegradable polymers and the opening of new conversion pathways to chemicals, are leading toward lower production costs and greater availability of lactic acid (McCoy, 1996; Amrane and Prigent, 1996; Silva and Yang, 1995). Previous studies of lactic acid conversion have identified sodium phosphate as an active catalyst for the dehydration to acrylic acid (Holmen, 1958; Paparizos et al., 1988; Sawicki, 1988). Catalytic conversion to acrylic acid and acetaldehyde in supercritical and near-critical water have been examined in detail by Mok et al. (1989), and by Lira and McCrackin (1993); the latter achieved 58% selectivity to acrylic acid in the presence of sodium phosphate at 310 bars and 360 °C. Lactic acid has also been observed to undergo reduction to propanoic acid through fermentation and catalytic processes; a 64% propanoic acid yield with a total conversion of 99% at 350 °C and 1 atm was achieved by Velenyi and Dolhyj (1987) using a mixed metal oxide catalyst (Mo5Cu4SnOx) on a silica-alumina support. Studies conducted in our laboratory on vapor phase conversion of lactic acid at 0.5 MPa over sodium phosphate catalysts led to the discovery of a condensation pathway to 2,3-pentanedione from lactic acid (Gunter et al., 1994). 2,3-Pentanedione is used in industry as a flavoring agent, and it has potential as a * To whom all correspondence should be addressed. Phone: (517) 353-3928. FAX: (517) 432-1105. E-mail: millerd@ egr.msu.edu. † Department of Chemical Engineering. ‡ Department of Chemistry. S0888-5885(97)00014-6 CCC: $14.00
photoinitiator, biodegradable solvent, and feedstock for production of duroquinone (von Pechmann, 1888) and pyrazines (Akiyama et al., 1978). It is currently produced at $50-80/lb through the oxidation of methyl propyl ketone in excess NaNO2 and diluted HCl or via extraction from dairy waste (Furia and Bellanca, 1975). Our inexpensive route to 2,3-pentanedione from lactic acid is extremely attractive economically and is being developed for commercial production, with 2,3-pentanedione production costs projected to be $5-10/lb. The effects of support material and the sodium salt on the yields of the major products including acrylic acid, propanoic acid, acetaldehyde, and 2,3-pentanedione from lactic acid have been investigated (Langford et al., 1995), and optimum conditions for 2,3-pentanedione and acrylic acid formation over sodium nitrate have been identified (Wadley et al., 1997). Low temperatures (
