Selectivity of the First Two Glycerol Dehydrogenation Steps Determined Using Scaling Relationships
Journal article, 2021
Glycerol is a byproduct of biodiesel production and an abundant feedstock that can be used for the synthesis of high-value chemicals. There are many approaches for glycerol valorization, but, due to the complicated reaction mechanism, controlling which products are produced is challenging. Here, we describe glycerol's chemical selectivity for different metallic catalysts using descriptors for carbon (mainly *C, *CH2OH) and oxygen (mainly *O, CH3O*). The quality of these descriptors and the weighted combinations thereof are validated based on their fit, via linear regression, to the binding energies of all reaction intermediates generated in the first two glycerol dehydrogenation steps on a number of close-packed Ru, Co, Rh, Ir, Ni, Pd, Pt, Cu, Ag, and Au surfaces. We show that *CH2OH is a better descriptor than *C for the studied carbon-bound intermediates, which is attributed to the observation that the adjacent *OH group interacts with the surface. This leads to a negative oxygen dependence, which can be generalized to similar alcohol-derived adsorbates. Furthermore, we show that CH3O* is a better oxygen descriptor than *0 for the studied intermediates. This is mainly attributed to the difference between the single and double bonds, as we show that *OH is closer to the accuracy of CH3O*. Multilinear regression with different combinations of *C, *O, and *OH is comparable in accuracy to that of *CH2OH and CH3O*. Scaling relationships are used to determine the selectivity map for glycerol dehydrogenation. The results show that the first dehydrogenation is selective toward two different intermediates (one bonded via the secondary carbon and the other via the secondary oxygen) depending on the relative bond strength of the carbon and oxygen descriptors. The second dehydrogenation step results in five intermediates, again depending primarily on the relative bond strength of carbon and oxygen to the surface. The selectivity maps can be used together with kinetic considerations and experimental data to find catalyst candidates for glycerol dehydrogenation.