The emerging technology of Integrating fermentation and catalysis for the biorenewable production of chemicals is essentially a step towards sustainability. The complex reaction environments, wherein catalysts, solvents, and biogenic impurities (amino acids and proteins from fermentation media) interact, pose a key challenge in this line. To understand such reaction systems, catalytic transfer hydrogenation of 6-amyl-α-pyrone (6PP) was chosen as a probe (Figure 1.a). The hydrogenation was performed in the liquid phase under mild temperature/pressure conditions using a 10% Pd/C catalyst. Almost complete conversion of 6PP was observed in10 mins in cyclohexane, with 79% DDL yield. We observed a strong correlation of catalyst activity with solvent dielectric constant and solvent accessible surface area (SASA) (Figure 1b). A major challenge observed was the reduced catalyst activity and altered product selectivity in the presence of biogenic impurities. Trace quantities of impurities such as methionine (Met), cysteine (Cys) was found to cause significant deactivation. The 6PP conversion dropped to 77% and 64% in Met and Cys' presence, respectively. To develop insights into the mechanism of deactivation, Density Functional Theory (DFT) simulations were employed. Methionine bound through sulfur atom in its most stable adsorption mode with a binding energy=-186 kJ/mol, and the C-S bond breaks to SCH<sub>3</sub> species on the surface with an activation barrier Ea=136 kJ/mol (Figure 1.e-f). It further dissociates into sulfur atoms accompanied by an activation barrier of Ea=136 kJ/mol and -141 kJ/mol reaction energy. A plausible mechanism for the dissociation of Met on Pd {111} surface was proposed, as shown in the reaction energy diagram (Figure 1.f). Our previous study on Ni and Ni-Au alloy surfaces explains how alloying effectively control catalyst deactivation (Figure 1.g). The alloy surfaces had lower binding energies than the pure metal leading to weaker binding of biogenic impurities.

Understanding the challenges in complex reaction environments for the production of biorenewable chemicals

ACS Spring Meeting 2021

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