Uncatalyzed esterification of biomass-derived carboxylic acids

To shift from a petroleum-based to a biomass-based economy will require the development not only of biofuels, but also of biorenewable replacements for petroleum-derived chemicals. In this regard, environmentally friendly biomass-derived esters may serve as alternatives to fossil-derived chemicals such as toxic halogenated solvents and glycol ethers. Therefore, esterification of various carboxylic acids that find significant applications in the chemical, pharmaceutical, petrochemical, food, and cosmetic industries has been initiated by the chemical industry.

At atmospheric condition, esterification is a reversible reaction limited by the low equilibrium conversion and slow reaction rate, and has recently been performed with excess alcohol to shift the equilibrium conversion. Heterogeneous or homogeneous acid catalysts are used to achieve acceptable reaction rates. The conventional acid-catalyzed process has been extensively developed; but it suffers from problems associated with the generation of side reactions, corrosion of equipment, expensive purification procedures, long reaction times and discharge of acidic wastes. Various attempts on esterification of carboxylic acids with ethanol have previously addressed important issues concerning product distribution, catalyst activity, and kinetics of acid-catalyzed esterification at lower reaction temperatures, but kinetics of uncatalyzed esterification at elevated reaction temperatures are still very limited. It is thus of great interest from a practical viewpoint that more information such as kinetic and thermodynamic parameters are required to develop a possible esterification process without using any catalyst.

In this work, therefore, a fundamental study on the uncatalyzed esterification of different aliphatic carboxylic acids with stoichiometric amounts of ethanol was undertaken to examine the possibility of converting the biomass-derived carboxylic acids to ethyl esters and to determine the kinetic and thermodynamic parameters for the uncatalyzed esterification. Experiments were conducted with isothermal batch reactors at temperatures ranging from 298 K to 623 K. A 2nd-order reversible kinetics rate expression was used to fit the experimental data. The thermodynamic and kinetic values estimated were found to vary for different esterification systems studied. The dependence of Keq on temperature for esterification of short-chain and long-chain carboxylic acids varied. Despite the nonlinearity of the Van't Hoff plot for esterification of linoleic acid, the Arrhenius and Eyring plots were linear. Two thermodynamic paths were developed for estimating the equilibrium conversions, and the theoretical values compared well with the experimental results reported in this study.

Additional experiments performed to assess the corrosive and catalytic influences of metallic materials on esterification reaction indicated Inconel 625 alloy, nickel wire and stainless steel materials have potential corrosion problems on the uncatalyzed esterification reaction at elevated reaction. However, tantalum and grade 5 titanium materials showed acceptable level of compatibility for similar reaction conditions, and this can encourage the design of a flow reactor system.