Gold Stability and Diffusion in the Au/TS‑1 Catalyst

The structure and mobility of gold nanoclusters entrained in the titanosilicate zeolite TS-1 were investigated using ab initio molecular dynamics to characterize the stability and diffusion of catalytic gold sites for propylene epoxidation. Gold incorporated inside TS-1 has higher reaction efficiency than gold on the external surface of TS-1 crystallites, but direct characterization of intrapore gold under reaction conditions is difficult given the signal attenuation caused by occlusion of Au particles within TS-1 crystallites. Computational approaches, based on Periodic Density Functional Theory, can provide fundamental insight into the structure and stability of intrapore Au clusters at relevant process conditions. We find that the straight and sinusoidal channels of TS-1 cause restructuring of gold clusters, which exposes undercoordinated gold atoms that strongly bind dioxygen. These clusters are stable in pore intersections, and there is minimal interaction between gold and titanium cations incorporated in the framework. The size and shape of confined gold clusters will consequently affect epoxidation kinetics. Gold diffusion coefficients calculated with transition state theory decrease rapidly with gold cluster size (Dmonomer = 8.9 × 10–9 m2s–1, Dtetramer = 2.2 × 10–15 m2s–1) and are driven by both enthalpic and entropic activation barriers due to cluster restructuring and loss of rotational degrees of freedom at diffusion transition states. These data indicate the importance of considering the effects of gold cluster size and mobility on the performance of microporous epoxidation catalysts. Further, the results suggest broader insights related to transition metal incorporation in microporous catalysts based on the observed metal cluster mobility.