(715f) Enhanced Deoxygenation on Bifunctional Pd/Al2O3 Modified with Phosphonate Self-Assembled Monolayers

The modification of Pd/Al₂O₃ with self-assembled monolayers (SAMs) deposited from phosphonic acids for improving deoxygenation was investigated. In this study the approach was to control the near-surface environment of the metal-support interface by applying SAMs to the catalyst support. More specifically, the goal was to produce a bifunctional deoxygenation catalyst via the introduction of carboxylic acid functionality in the SAM tails. Phosphonic acids containing either alkyl or carboxylic acid tails were deposited onto Pd/Al₂O₃ from solution, then annealed to covalently bind the SAMs to Al₂O₃ before washing to remove excess phosphonic acid. Fourier-transform infrared spectroscopy (FT-IR) was used to confirm the presence and identity of the different SAMs. The uncoated and SAM-coated catalysts were then tested in the gas-phase deoxygenation of different pyrolysis-oil components, such as furfuryl alcohol, under hydrogenation conditions. At the conditions tested, uncoated Pd/Al₂O₃ exhibited low selectivity (<5%) toward the deoxygenation product, 2-methylfuran, and primarily performed ring hydrogenation. When modified with phosphonate SAMs, a significant improvement in deoxygenation selectivity can be observed, regardless of the tail group tested. Though, when comparing SAMs of similar size and coverage, those having carboxylic acid tails show greater rates of deoxygenation than those having alkyl tails. Modification with phosphonate SAMs typically resulted in an increase of the overall catalyst activity as well, so that the observed rate of deoxygenation was improved by more than an order of magnitude with particular SAMs. FT-IR following the adsorption of pyridine was used to confirm the introduction of Brønsted acidity by phosphonate SAMs, and to compare the relative acid strengths. The results of this work suggest that preparation of bifunctional catalysts with organic monolayers may be a useful way to tune catalyst activity and selectivity, particularly given the ease of preparation and the high stability of the SAMs.