Dynamic Disproportionation Shapes Ga Sites in Silica-Supported Propane Dehydrogenation Catalysts

Ga is a key component of commercial propane dehydrogenation (PDH) catalysts. The regeneration of those catalysts involves oxidation and reduction cycles, the latter typically under H2 at about 550–650 °C. However, the structural evolution of Ga3+ sites under those strongly reducing conditions is underexplored. Here, we monitored the transformations of Ga3+ sites dispersed on a SiO2 support under H2 at 650 °C. The results show that this treatment generates metastable ºSiOGa1+ sites that disproportionate to Ga0 and Ga3+ sites. The formed Ga0 species react with surface silanols, regenerating ºSiOGa1+ sites, and with water formed during the reduction of Ga3+ to Ga1+, ultimately yielding large crystalline particles of k-Ga2O3. In addition, an overall loss of Ga from the catalyst is observed and attributed to the formation of Ga0 and volatile Ga2O. The Lewis acidity of dispersed Ga3+ decreases when it interacts with weakly Lewis-basic ºSiOGa1+ motifs. All of these processes associated with ºSiOGa1+ sites decrease the number of dispersed Ga3+ sites and are therefore detrimental for PDH activity, except that dispersed Ga3+ sites formed from disproportionation of ºSiOGa1+ sites likely have increased strain, beneficial for PDH activity. While H2 treatment alone has a negative impact on the catalyst’s PDH activity and propene selectivity, an oxidative air treatment of the reduced catalyst terminates disproportionation dynamics by oxidizing Ga0 and increases the Ga-based activity relative to both the reduced and fresh catalysts. These findings suggest that the disproportionation of Ga1+ species under an H2 atmosphere appreciably affects gallium speciation, loading, and catalytic performance.