(132b) Using Microkinetic Analysis to Predict Product Selectivity during Propionic Acid Hydrodeoxygenation over Supported Pt and Ru Catalysis
Although noble metals, such as Pt and Ru, show good activity in hydrodeoxygenation, they generally also activate secondary and tertiary reaction pathways, resulting in the formation of undesired fragmentation and deep hydrogenation products. A popular method for tuning their selectivity is to pair these noble metals with oxophilic âpromoterâ metals, such as Sn. Unfortunately, our present understanding of how secondary metals impact reaction kinetics to modulate selectivity is murky, and the design of such materials remains largely alchemical. A more rational framework for material synthesis requires an elementary understanding of reaction kinetics governing both carboxylic acid HDO and its myriad competing pathways. Furthermore, we must understand how the addition of promoter metals will perturb this free energy landscape to enhance (or detract from) monometallic selectivity. The goal of the present study is to establish kinetic expectations for carboxylic acid HDO over monometallic Pt and Ru, which provides the necessary foundation for subsequent discussion of how the addition of secondary metals impacts observed selectivity.
As a model system, we focus on propionic acid HDO and attempt to reconcile mechanistic descriptions of HDO chemistry with experimentally observed product formation rates through microkinetic analysis. Because the carboxylic acid HDO occurs alongside multiple side reactionsâall of which are generally expected to occur under typical reaction conditionsâit is exceedingly difficult to infer rates of primary reactions that consume carboxylic acids. In general, we are only able to directly quantify reaction rates for terminal pathwaysâsuch as methanation and alkane hydrogenolysis. Accordingly, we first consider the elementary kinetics of these terminal pathways and subsequently apply these insights as we build toward a quantitative description of carboxylic acid HDO.