Methionine bound to Pd/γ-Al2O3 catalysts studied by solid-state 13C NMR

Abstract

The chemisorption and breakdown of methionine (Met) adsorbed on Pd/γ-Al2O3 catalysts were investigated by solid-state NMR. 13C-enriched Met (ca. 0.4 mg) impregnated onto γ-Al2O3 or Pd/γ-Al2O3 gives NMR spectra with characteristic features of binding to γ-Al2O3, to Pd nanoparticles, and oxidative or reductive breakdown of Met. The SCH3 groups of Met showed characteristic changes in chemical shift on γ-Al2O3 (13 ppm) vs. Pd (19 ppm), providing strong evidence for preferential binding to Pd, while the NC carbon generates a small resonance at 96 ppm assigned to a distinct nonprotonated species bound to O or Pd. Additionally, NMR shows that the SCH3 groups of Met are mobile on γ-Al2O3 but immobilized by binding to Pd particles; on small Pd particles (ca. 4 nm), the NCH groups undergo large-amplitude motions. In a reducing environment, Met breaks down by C-S bond cleavage followed by formation of C2-C4 organic acids. The SCH3 signal shifts to 22 ppm, which is likely the signature of the principal species responsible for strong catalyst inhibition. These experiments demonstrate that solid-state magic-angle spinning NMR of 13C-enriched Met can be a sensitive probe to investigate catalyst surfaces and characterize catalyst inhibition both before reaction and postmortem.