(132b) Using Microkinetic Analysis to Predict Product Selectivity during Propionic Acid Hydrodeoxygenation over Supported Pt and Ru Catalysis

Recent industrial expansion in the biotechnology sector has raised interest in using succinic acid as an oxygenated platform for commodity chemicals. Most notably, succinic acid serves as a biogenic alternative to maleic anhydride, which is the typical industrial precursor to gamma-butyrolactone, butanediol, and tetrahydrofuran. Each of these can alternatively be prepared by partial hydrodeoxygenation (HDO) of succinic acid; however, each target is susceptible to deep hydrogenation/hydrogenolysis, and selectivity control is nontrivial. The central challenge in partial hydrogenation of succinic acid is that carboxylic acid reduction is difficult relative to the array of secondary and tertiary reactions that occur under similar conditions. These include aldehyde decarbonylation, alcohol hydrogenolysis, alkane hydrogenolysis, and methanation, which broadly result in the formation of low-value carbon oxides and light alkanes.

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 “promoter” 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 reactions—all of which are generally expected to occur under typical reaction conditions—it 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 pathways—such 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.