Effects of multivariate linker substitution, metal binding, and reactor conditions on the catalytic activity of a Pd-functionalized MOF for olefin hydrogenation

We investigated the performance of zinc-based metal-organic framework (MOP) catalysts that were postsynthetically modified with the homogeneous palladium catalyst Pd(CH3CN)(2)Cl-2 for the hydrogenation of propylene in a packed-bed, tubular microreactor. The catalytic conversion and apparent reaction kinetics were analyzed across a range of metal loadings, reactant flow rates, feed concentrations, and reactor temperatures. The catalyst's deactivation in the presence of a common palladium catalyst poison, carbon monoxide, was also examined. The catalytic conversion was optimal at moderate metal loadings, stoichiometric excess of hydrogen, and relatively mild temperatures. The activity depended strongly on reactant feed composition but showed no dependence on total flow rate, indicating a diffusion-limited process. To investigate the effects of intra-particle diffusion limitations, internal diffusion coefficients for propylene in the MOF catalysts were measured with pulsed field gradient nuclear magnetic resonance (PFG NMR) and were incorporated into the kinetics analysis. Using these coefficients to compute effectiveness factors for heterogeneous catalytic reactions, diffusion-limited artifacts were accounted for to obtain intrinsic rate constants and activation energies from apparent kinetics data. The average intrinsic activation energy was found to be 51(6) kJ/mol. The MOF catalyst was also found to be reversible under carbon monoxide poisoning, suggesting a weak binding mechanism. (C) 2014 Elsevier B.V. All rights reserved.