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The equilibrium constants for dimerization of three different carboxylic acids in near-critical and supercritical carbon dioxide and ethane at various temperatures and densities are presented. Fourier transform infrared spectroscopy was used to determine the equilibrium constants by examining the CdO stretching frequencies of the monomer and dimer of the acids in solution. The equilibrium constants for formic acid, propionic acid, and trifluoroacetic acid decrease with temperature for both ethane and CO 2 solvents and strongly depend on CO 2 solvent density. The modified lattice fluid hydrogen-bonding (MLFHB) model was used to interpret this density dependence of the equilibrium constants. The density dependence provides evidence for specific solvent-solute interactions between CO 2 and the carboxylic acid functional group, which is attributed to strong Lewis acid-Lewis base interactions. Since carboxylic acids contain both a Lewis acid and a Lewis base moiety, the examination of a strong acid such as trifluoroacetic acid provides insight as to whether CO 2 acts as a Lewis acid or as a Lewis base in these specific interactions. Ab initio computations are also presented and indicate that CO 2 interacts strongly with the carboxylic acids.

Agricultural byproducts are a major source of biomass for biofuel/bioenergy conversion. The southeastern U.S. produces a great amount of nutshells from pecan, walnut, and peanut processing. In this study, walnut shells were selected as a representative agricultural byproduct, and a hydrothermal process catalyzed by both bases and acid was applied to convert the walnut shells into liquefied organic compounds. Conversion rate, major organic products, and their distribution were measured under different concentration of bases (0−1.0 M) and reaction temperature (200−300 °C, corresponding to a pressure range of 1.5−8.6 MPa). An increase in base concentration (KOH and Na2CO3 ) or reaction temperature generally resulted in higher conversation rates and was more favorable to the generation of organic compounds of lower molecular weights. HCl as a catalyst promoted the generation of levulinic acid, but the conversion rates were very low. Major compounds from hydrothermal process catalyzed by bases were phenol derivatives. Small amounts of cyclopenten derivatives and C12−18 fatty acids were detected. The effects of reaction conditions on the distribution of products were characterized by the relative abundance of each compound group categorized based on the GC retention time.

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