Controlling Single-Atom Sites Using Phosphonic Acids

Olefin hydroformylation is a significant homogeneously catalyzed industrial process of aldehyde synthesis. Despite the efficiency of homogeneous catalysts, their recovery is a major and costly challenge, ultimately leading to the net loss of finite, expensive noble metals. Single-atom catalysts, composed of an atomically dispersed noble metal across a metal oxide support, combine the uniformity and defined active sites of homogeneous catalysts with the advantageous catalyst recovery of heterogeneous catalysts. This project was focused on the fundamental aspects of controlling single-atom sites using phosphonic acid self-assembled monolayers (SAMs) to steer the product selectivity of ethylene hydroformylation and hydrogenation. Phosphonic acids with differing tail group functionalities were deposited onto an atomically dispersed rhodium on titanium oxide catalyst. Catalyst surfaces were characterized by diffuse reflectance infrared Fourier transform spectroscopy (DRIFTS) both before and after CO adsorption to confirm the success of the phosphonic acid depositions and to probe rhodium site availability following SAM deposition. The simultaneous hydroformylation and hydrogenation of ethylene was utilized as a gas-phase probe reaction to evaluate the ability of phosphonic acid SAMs to steer single-atom rhodium product selectivity. Findings from this study show that, in comparison to the uncoated single-atom rhodium catalyst, the presence of phosphonic acid SAMs on the catalyst surface increased product selectivity of hydroformylation while decreasing that of hydrogenation despite some apparent rhodium site blocking caused by the SAMs. Additionally, it was found that aliphatic versus aromatic phosphonic acid SAMs, while causing similar rhodium site blocking, resulted in differing product selectivity, displaying the ability of phosphonic acids with varying tail group functionalities to influence single-atom rhodium sites.