Geometric Response of Silanol-Anchored Co(II) Site Regulates Propane C–H Activation Barrier in Silicalite-1

Abstract Zeolite-confined isolated cobalt catalysts exhibit promising performance in propane dehydrogenation (PDH), while their catalytic performance varies in the orders of magnitude despite most studies claim isolated Co sites are active sites. It is believed that these sites are stabilized by silanols generated by framework defects, which are normally considered as anchoring site. The present study shows that separation between silanol groups has a significant impact on the PDH activity of isolated cobalt sites. Here, a Co–O2 motif, in which Co(II) is anchored by two isolated internal silanols with varying separations, is employed to explore how the relative arrangement of silanol anchoring sites influences PDH performance. By varying the separation between the two silanols, the O–Co–O angle (θ) was tuned from bent to nearly linear configurations. The calculated C–H activation barriers show a positive correlation with the O–Co–O angle of the propane-adsorbed state, θC3H8*. Distortion/interaction (Activation Strain) analysis reveals that this barrier trend mainly arises from the distortion penalty required to reach the transition state. Smaller-θ sites require less angular contraction of the Co–O2 motif and enable stronger Co-centered charge reorganization during C–H cleavage. Experimental comparison provides qualitative support for this geometry-reactivity relationship: Co/S-1 catalysts with silanol-nest environments, which are associated with smaller-θ Co–O2 geometries in model structures, exhibit higher PDH performance and lower apparent activation energy than terminal-silanol samples. These results identify θ as a physically meaningful descriptor that links the geometric response of silanol-anchored Co(II) sites to propane C–H activation.