Bio-derived platform chemicals can serve as renewable alternatives to petroleum-based analogs. For example, maleic anhydrideâpresently synthesized in the petrochemical industry by direct oxidation of butaneâis currently the main industrial source for the production of oxygenated commodity chemicals like gamma-butyrolactone, 1,4-butanediol, and tetrahydrofuran. A potential alternative to maleic anhydride is bio-based succinic acid, which can be produced by anaerobic fermentation of sugars. It can be readily converted into any of the typical maleic anhydride derivatives via partial hydrodeoxygenation (HDO) of one or both of its carboxylic acid groups. The required transformations can be carried out under H2
atmospheres over Group VIII metals, but selectivity control is challenging. Pt and Ru generally have good HDO activity, and they can facilitate the desired reduction and reductive deoxygenation of carboxylic acids; however, they also facilitate a host of other primary, secondary, and tertiary reactions that consume additional H2
and diminish selectivity toward the intended targets of partial HDO. Such chemistries include C-C hydrogenolysis, C-O hydrogenolysis; decarbonylation; decarboxylation; and CO methanation. The key challenge is that most of these sequential reactions are facile relative to initial conversion of the carboxylic acid; as such, carboxylic acid HDO over monometallic catalysts typically leads to a spectrum of low-value fragmentation products. It is widely reported that the addition of oxophilic âpromoter metals,â such as Sn can be used to tune the selectivity of Pt and Ru during the HDO of carboxylic acids; however, their mechanism of action is poorly understood. A more rational synthetic framework requires an elementary understanding of the reaction kinetics that govern carboxylic acid HDO and its many competing pathways.
In order to build foundational knowledge, the goal of the present study is to map reaction networks and quantify reaction kinetics during propionic acid HDO over monometallic Pt and Ru. In particular, our effort focuses on extracting the rates of primary pathways that consume carboxylic acidsâsuch as hydrogenation, decarbonylation, and decarboxylationâfrom species production rates that reflect net contributions of primary, secondary, and tertiary reactions. In this study, we examine trends in selectivity and species production rates as a function of contact time on supported Pt and Ru catalysts. Data are subsequently fit to a semi-empirical kinetic model that captures the sequential nature of carboxylic acid HDO and the evolution of product distribution in integral packed-bed reactors. This treatment allows one to predict rates of primary reaction pathways in the zero-conversion limit, which facilitates subsequent microkinetic analysis and provides an experimental benchmark for recent computational studies that have reported detailed mechanisms and elementary kinetic parameters for carboxylic acid HDO [1-2]. We expect that trends in propionic acid HDO are universal in carboxylic acid processing; thus, outcomes from this analysis can inform catalyst design strategies to improve selectivity during succinic acid upgrading.
 J. Lu, M. Faheem, S. Behtash, A.Heyden. J. Catal., 324 (2015), pp (14-24).
 K. Lugo-Jose, J. R. Monnier, A. Heyden, C. T. Williams*, Catal. Sci. Tech. 4, 3909-3916 (2014).