Mechanistic Investigations for Electrocatalytic Oxidation of Furfural Using Density Functional Theory

Mechanistic Investigations for Electrocatalytic Oxidation of Furfural Using Density Functional Theory Naveen Agrawal,a Li Gong,a,b Alex Roman,c Lesli Mark, c Will Medlin,c Adam Holewinski,c Michael J. Janika aDepartment of Chemical Engineering, The Pennsylvania State University, University Park, PA 16802, USA. b College of Environment, Zhejiang University of Technology, Hangzhou 310014, China. cDepartment of Chemical and Biological Engineering, University of Colorado, Boulder, CO 80309, USA. Furfural is a promising biomass-derived synthetic platform species with numerous chemical outlets[1]. Furfural oxidation could provide an alternative to fossil feedstock production of furanones, furoic acid, maleic anhydride (MAN), maleic acid (MA) and succinic acid (SA) [2], but scalable production routes remain elusive due to poor selectivity and energy efficiency. Most previous studies of furfural partial oxidation to acid anhydrides and dicarboxylic acids utilized either oxygen or hydrogen peroxide as the oxidant together with heterogeneous catalysts. Driving reactions with electric potential, such as the electro-oxidation of furfural, could be a promising green method using electricity derived from solar, wind and geothermal sources. Electro-oxidation may offer different reaction paths and selectivity control not offered in chemical oxidation, motivating our mechanistic analysis. We have applied density functional theory (DFT) to investigate the electrocatalytic oxidation mechanism on the Pt (111) surface, and validated the mechanistic insights with comparison to both low-pressure TPD and HREELS studies as well as electrochemical flow cell experiments. The potential-dependent reaction free energy profiles for furfural electrocatalytic oxidation to furoic acid, succinic acid, maleic acid, and maleic anhydride are reported. After comparing several possible furfural oxidation paths, we conclude that the electro-oxidation of furfural preferentially proceeds to furoic acid, with further oxidation slowed by difficult C-C bond dissociation. HREELS studies confirmed the presence of furoic acid like intermediate (furoate) during furfuryl alcohol oxidation on an oxygen covered Pt (111) surface, which might represent the Pt (111) surface under oxidative conditions [3]. Oxidation beyond furoic acid can proceed to succinic acid via 2(3H)-furanone as an intermediate and to maleic acid and maleic anhydride via 2(5H)-furanone as an intermediate. The rate of these processes is likely limited by the decarboxylation of furoic acid. DFT analysis of elementary step thermodynamics and kinetics suggest that the selectivity between furoic acid, succinic acid, maleic acid, or other oxidized products is tunable by varying the electrode potential. Initial electrochemical flow cell experimental results show furoic acid as the most significant product (> 80 % selectivity) at 0.9 V-RHE on a Pt electrode, in agreement with DFT results. These results broaden our fundamental understanding into electrocatalytic oxidation of furfural, which is applicable in upgrading renewable biomass derivatives. References: [1] R. Mariscal, P. Maireles-Torres, M. Ojeda, I. Sádaba, M. López Granados, Furfural: a renewable and versatile platform molecule for the synthesis of chemicals and fuels, Energy Environ. Sci., 9 (2016) 1144-1189. [2] G. Centi, F. Trifiro, J.R. Ebner, V.M. Franchetti, Mechanistic aspects of maleic-anhydride synthesis from C4-hydrocarbons over phosphorus vanadium-oxide, Chem. Rev., 88 (1988) 55-80. [3] Hansen, H. A., Rossmeisl, J., & Nørskov, J. K. (2008). Surface Pourbaix diagrams and oxygen reduction activity of Pt, Ag and Ni (111) surfaces studied by DFT. Physical Chemistry Chemical Physics, 10(25), 3722-3730.