(532cl) Enhancing Epoxidation Activity with Hydrogen Peroxide over Highly Dispersed Tantalum Incorporated Mesoporous Silicates

Epoxides are valuable precursors in producing polymers, and pharmaceutical additives. Supported transition metal oxide catalysts (Nb, Ti, Zr, Ta, W, Mo) have been investigated in the selective epoxidation of cyclohexene using aqueous H₂O₂. However, many of them are poisoned by the coordination of the water with the active sites, thereby reducing the epoxide selectivity. Robust catalysts that maximize the H₂O₂ utilization and epoxide selectivity continue to be of interest. Although Ta(V) oxide has less propensity to coordinate water due to the presence of an extra anionic coordination site, its intrinsic properties have not been explored. In this work, we demonstrate that higher catalytic activity is achieved (>95% epoxide selectivity, TOF) when Ta is incorporated into mesoporous silicates such as KIT-6 and TUD-1 synthesized via one-pot methods.

The catalytic performance evaluated under similar reaction conditions revealed that compared to other metals, Ta (2 wt%) loadings display higher and stable (10 h) catalytic activity (96 % selectivity) with high H₂O₂ utilization efficiency (80%). Further, ex situ DR UV-Vis (Figure 1), XPS, Raman, and EXAFS studies reveal that the isolated Ta^V species are dominant at lower loadings and mixed oligomeric species (Ta-O-Ta) are evident with increasing metal loadings. Moreover, dominant Lewis acid sites were identified from Py IR experiments. Such an enhancement in the activity is due to the favorable coordination of the Ta^V sites which tunes the electronic environment to stabilize the epoxide intermediates and selectively catalyze the epoxidation pathway.

Our results clearly show that catalyst synthesis techniques that maximize the Ta utilization by forming the isolated sites are essential to maximizing the epoxidation TOFs. The influence of metal loadings, reaction conditions, H₂O₂ utilization, along with detailed catalyst characterization results (TEM, DR UV-Vis, XPS, in situ Py-IR, EXAFS, TPR-H₂, Raman, and ¹⁵N Pyridine NMR) will be presented to provide structure-activity relationships.