Active Site Entropy of Atomically Dispersed Rh/Al2O3 Catalysts Dictates Activity for Ethylene Hydroformylation

Abstract

The interpretation of catalytic kinetics on supported metal catalysts typically assumes that catalytic cycles occur on static active site structures apart from local rearrangement in the coordination environment. It has been reported that certain atomically dispersed metal active sites (e.g., Cu/Chabazite zeolites and Rh/γ-Al2O3) may be mobile under relevant reaction conditions, suggesting that the active sites themselves have entropy that could be relevant to apparent reaction kinetics. Here, we systematically modify the mobility (degrees of freedom or entropy) of atomically dispersed Rhodium gem-dicarbonyls, Rh(CO)2, supported on γ-Al2O3 through functionalization of the support with straight-chain alkyl-phosphonic acids of different tail lengths ranging from 1 (methyl) to 16 carbons (hexadecyl). The restricted mobility of Rh(CO)2 results in up to a 120 °C decrease in the required temperature for CO desorption from Rh(CO)2 and 1000× increase in turn over frequency for propanal formation via ethylene hydroformylation [where Rh(CO)2 is the most abundant surface intermediate] as compared to unfunctionalized Rh/γ-Al2O3. Eyring analysis suggests that the promoted rates of CO desorption and hydroformylation are due primarily to changes in apparent activation entropy [ΔΔS‡ of up to 60 J/(mol·K)], where restricted mobility of Rh(CO)2 promotes the attempt frequency of CO desorption, which is a kinetically relevant step in hydroformylation. Further, the dependence of Rh(CO)2 reactivity on alkyl phosphonic acid tail length suggests that interactions between phosphonic acid tails far from the active site modify the rigidity of the self-assembled monolayers, such that longer tails better restricted the mobility of Rh(CO)2. This work suggests that active site entropy can influence reaction kinetics on heterogeneous catalysts when changes in active site mobility are coupled to reaction coordinates and further that controlling active site entropy can be an effective design approach to increase catalytic performance.