Enhancing Rate of Selective Hydrogenation Catalysts By Modulation of Pd Crystallite Diameter

The purpose of this research is to study the structure-activity relationship of supported metal catalysts. In particular, in the hydrodeoxygenation of furfural to 2-methylfurn (MF) the use of traditional catalysts produce a wide range of undesirable products in addition to MF such as tetrahydrofuran, furan, and butanes. In general, the product obtained from furfural conversion depends on the adsorption geometry of furfural on the active surface.¹ The selectivity of a catalyst for MF may be increased by directing the adsorption of furfural to more favorable surface species. One method of accomplishing this is through the use of self-assembled monolayers (SAM’s). Previous work has demonstrated that an octadecanethiol coated Pd/Al₂O₃ catalyst is highly selective for MF. This coating is believe to work by blocking furfural access to terrace sites on the (100) and (111) FCC crystal lattice.^2-3 Instead, the surfacing coating directs furfural adsorption to the corner and edge palladium atoms on the lattice where furfural is preferentially converted into MF. This work demonstrates that the size of the supported palladium crystallites has a significant effect on the material’s selectivity for MF. A supported Pd/Al₂O₃catalyst with an average crystallite diameter of ~2 nm achieves an initial MF selectivity approaching 30%; an average crystallite diameter of ~8 nm achieves an initial MF selectivity approaching 10%; and commercial Pd catalysts, which have average crystallite diameters >20 nm, achieve a MF selectivity less than 1%. These results are encouraging in their support of the notion that the physical structure of catalysts may be fine-tuned to produce substantial improvements in a catalyst’s activity.

1] Sitthisa, S.; Wei, A.; Resasco, D.E. Selective conversion of furfural to methylfuran over silica-supported Ni – Fe bimetallic catalysts. Journal of Catalysis 2011(284), pp 90-101.

2] S.H. Pang, C.A. Schoenbaum, D.K. Schwartz, J.W. Medlin. (2013). Directing reaction pathways by catalyst active-site selection using self-assembled monolayers. Nat. Commun. Vol. 4, 2448.

3] Lear, T.; et al. The application of infrared spectroscopy to probe the surface of alumina-supported palladium catalysts. J. Chem. Phys. 2005 (123), 174706.