Organic Modifiers Promote Furfuryl Alcohol Ring Hydrogenation via Surface Hydrogen-Bonding Interactions

Abstract

Interactions between surface adsorbed species can affect catalyst reactivity, and thus, the ability to tune these interactions is of considerable importance. Deposition of organic modifiers provides one method of intentionally introducing controllable surface interactions onto catalyst surfaces. In this study, Pd/Al2O3 catalysts were modified with either thiol or phosphonic acid (PA) ligands and tested in the hydrogenation of furanic species. The thiol modifiers were found to inhibit ring hydrogenation (RH) activity, with the degree of inhibition trending with the thiol surface coverage. This suggests that thiols do not strongly interact with the reactants and simply serve to block active sites on the Pd surface. PAs, on the other hand, were found to enhance RH when furfuryl alcohol (FA) was used as the reactant. Density functional theory calculations suggested that this enhancement was due to hydrogen-bonding interactions between FA-derived surface intermediates and PA modifiers. Here, installation of hydrogen-bonding groups on the Pd surface served to preferentially stabilize RH product states. Furthermore, the promotional effect on the RH of FA was observed to be greater when a higher-coverage PA was used, providing a rate more than twice that of the unmodified Pd/Al2O3. The results of this work suggest that organic ligands can be designed to impart tunable surface interactions on heterogeneous catalysts, providing an additional method of controlling catalytic performance.

Altering Linear Scaling Relationships on Metal Catalysts via Ligand–Adsorbate Hydrogen Bonding

“…Specifically, the optimal δ of 0.09 eV resulted in a 3.7-fold increase in r D and a 7-fold increase in rate selectivity compared to the unshifted system (δ = 0 eV). Experimental studies which utilized organic ligands with hydrogen-bonding functionality have shown similar improvements; for instance, Coan et al attributed a ∼ fourfold increase in the turnover frequency for furfuryl alcohol ring hydrogenation to stabilization via H-bonding ligands . Based on our results, this rate improvement could be further optimized through tuning of H-bond strength.…”

“…Experimental studies which utilized organic ligands with hydrogen-bonding functionality have shown similar improvements; for instance, Coan et al attributed a ∼ fourfold increase in the turnover frequency for furfuryl alcohol ring hydrogenation to stabilization via H-bonding ligands. 29 Based on our results, this rate improvement could be further optimized through tuning of H-bond strength. It is important to note that site blocking effects from the ligands, which were observed in Coan's study, were not taken into account in our calculations of reaction rates.…”

“…A theoretical study of nitrogen reduction on Ru by Ortuño et al showed that decorating the Ru surface with fluorinated ionic liquids causes the formation of H-bonds that stabilize N 2 H on the surface, driving selectivity away from hydrogen evolution . Recent experimental studies have demonstrated the use of organic ligands with hydrogen-bonding functionality to promote the binding of specific adsorbates for both furanic ring hydrogenation and direct H 2 O 2 synthesis reactions. , However, these experimental studies lack direct evidence of H-bond formation and instead depend on comparisons between functionalized and unfunctionalized ligands in order to draw conclusions. Additionally, these studies leave open the question of whether H-bonding ligands can be used to systematically alter LSRs in a predictable manner.…”

Controlling Heterogeneous Catalysis with Organic Monolayers on Metal Oxides

“…It is important to note that deposition of SAMs onto oxide supports can be accompanied by some deposition on metals, particularly oxophilic metals such as Ni . In some cases this modification can be beneficial, but deposition strategies to avoid metal modification are available . These include deposition of metals onto oxide surfaces with the ligands already present as well as the “protection” of metal sites with removable site blockers. , In general, determination of the organic ligand distribution on supported metal catalysts is most straightforward using STEM-EDS if the metal, oxide, and ligands all contain elements with distinct characteristic X-rays within detection range.…”

Reactivity of Pd–MO2 encapsulated catalytic systems for CO oxidation

“…For this reason, measurements for all rates were conducted at D 2 conversions below 50%, and the reverse reaction corresponding to HD dissociation was neglected. To estimate apparent dispersions, H 2 /D 2 exchange rates were measured, at a conversion of 50%, for the control supported Pd catalyst with a known dispersion of 42% (obtained from CO chemisorption 53 ). These measurements were extrapolated to estimate the quantity of exposed surface Pd atoms for all catalysts, which was then used to calculate percent dispersion and metal surface area.…”