Adsorption studies of pharmaceutical feed impurities on Pd/C catalysts fail to predict activity in liquid phase hydrogenation

Insight into the thermodynamics of liquid phase adsorption of nitrogen-containing compounds, which may serve as catalyst poisons for Pd/C, is important for the optimization of such catalytic processes in pharmaceutical industry. Here, the adsorption behaviors of six nitrogenous adsorbates pyridine, 3-methylquinoline, 3-methylisoquinoline, 4-methylquinoline, Hünig's base, and dibutylamine were examined by means of the Langmuir adsorption isotherm model. Equilibrium adsorption constants (K) varied from 141 L·mol⁻¹ for 3-methylquinoline with pK a = 5.17 to 2271 L·mol⁻¹ for dibutylamine with pK a = 11.39, indicating a strong dependence of adsorption affinity on pK a . The monolayer binding capacity far surpassed the availability of active palladium sites implying that the support played a significant role in equilibrium adsorption. Indeed, computation of a sampling of adsorbate binding energies for Pd(111) showed no correlation to measured adsorption constants, while adsorbate binding energies for R-COOH sites on the support showed a notable correlation. On the other hand, in a comparison of 3-methylquinoline to ethyl di-isopropyl amine as potential poisons, the suppression of reaction rate for styrene hydrogenation was strongly correlated to the computed adsorbate binding energy for Pd(111) and not correlated to the equilibrium binding constant overall. This observation further supports the conclusion that R-COOH sites play a significant role in adsorption equilibrium but experimentally determined equilibrium binding constants (via the Langmuir adsorption model) are insufficient to provide insight into catalyst poisoning. Rather analysis of reaction rate perturbations are recommended as essential for both practical and scientific examination of the role of catalyst poisons. These results offer important information on the adsorption mechanisms of nitrogen-containing compounds on Pd/C catalysts, which can be used in the design of more efficient catalytic systems for pharmaceutical use. • Equilibrium adsorption data of six amines on Pd/C catalyst fit to Langmuir adsorption isotherm • Catalyst activity is not well-correlated to experimental K values, consistent with non-catalytic adsorption sites contributing to adsorption equilibrium • Correlation of adsorption strength with pKa was established • A DFT model generates binding energies for Pd(111) that correlate with observed reactivity trends