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.