(741e) Selective Preparation of Pentenoic Acids Via Ring Opening of g-Valerolactone

Author: Xinlei Huang and Jesse Q. Bond

Lignocellulose is an attractive source of carbon because of its accessibility, its abundance, and its cost-effectiveness. Considering current fuel prices, it is presently best suited as a precursor to poly-functional intermediates rather than liquid alkanes. Along these lines, C₅ alkene carboxylic acids, namely pentenoic acids (PEA), can be prepared by acid-catalyzed ring opening of γ-valerolactone (GVL); unfortunately, these are challenging targets to prepare in good yield. Kinetically, GVL ring opening is relatively facile over various Bronsted and Lewis-acidic solid oxides; however, it is also endothermic (≈ 50 kJ mol^-1) and equilibrium limited to <10% yield below about 300°C. One strategy for improving PEA yields is to increase reaction temperature; however, above 300°C, irreversible side reactions, primarily decarboxylation of PEA to form butene, become kinetically accessible, which severely limits attainable yields of pentenoic acids. We propose that it is possible to improve selectivity toward PEA by finding an acidic catalyst that facilitates ring opening but minimizes C-C scission via decarboxylation. The balance between selectivity toward ring opening and C-C scission will generally depend on the nature of the acid catalyst and the residence time in the reactor. Here, we aim to identify both catalysts and reactor operation conditions that maximize selectivity toward pentenoic acids. In particular, we examine the performance of various Bronsted and Lewis acidic solid acids (SiO₂-Al₂O₃, g-Al₂O₃, TiO₂, and ZrO₂) during GVL ring opening. Since selectivity is a strong function of feed conversion in this system, PEA yields are determined over a wide range of contact times using packed bed reactors. Acid site densities were quantified using titration methods. Specifically, temperature programmed desorption (TPD) of isopropylamine was used to determine Bronsted site densities, and Fourier-transform infrared (FTIR) spectra of adsorbed pyridine were used to quantify the relative populations of Bronsted and Lewis sites. By analyzing trends in the conversion of GVL and selectivity toward PEA over catalysts with various types of surface acidity, we identify catalysts and operating conditions that maximize PEA yields and correlate shifts in selectivity with the nature of surface acidity.

Key Words: γ-Valerolactone, ring opening, catalyst activity, characterization, silica alumina, gamma alumina, titanium oxide and Zirconium dioxide