(561d) Effect of Alkanethiol Functionalization On Surface Reactivity and Selectivity

The interaction of organic adsorbates is known to play a significant role in a large number of chemical reactions[1], yet the design of catalytic systems to exploit these effects on a fundamental level to improve selectivity are lacking. Alkanethiols represent a possible platform for the modification of catalytic surfaces through their ability to form well ordered monolayers on the surface of metals such as palladium[2]. We have modified the surface of a supported palladium catalyst with an alkanethiol self-assembled monolayer (SAM) to create a near-surface hydrophobic environment to be highly selective toward the hydrogenation of the alkyl functionality of a molecule over oxygenate functionalities.

Catalysts containing 5 wt% palladium on alumina (Sigma) were exposed to unsaturated oxygenates such as crotonaldehyde (2-butenal) with and without the presence of hydrogen in a packed bed reactor. Catalyst rate and selectivity was compared to cases where the catalyst was coated with alkanethiols such as hexanethiol by immersing the catalyst in a 20mM solution of thiols in ethanol for a period of 2 days and then drying in helium. Although the rate of crotonaldehyde hydrogenation was significantly reduced, the thiol coated catalyst reacted the carbon ? carbon double bond in crotonaldehyde over the aldehyde functionality with high selectivity, even at high conversion, whereas the uncoated palladium catalyst produced the complete hydrogenation product, butanol.

The effect of alkanethiol coatings on palladium reactivity and selectivity was also evaluated using metal-insulator-semiconductor sensors. MIS sensors consist of a layer of a catalytic gate metal (e.g., Pd) and a semiconductor separated by a dielectric (e.g., SiO2). Hydrogen can diffuse from the metal surface to the metal-insulator interface and cause a shift in the capacitance-voltage curve which can be measured for many decades of gas phase partial pressure. Additional analytes can react on the surface of the sensor, altering the hydrogen absorption dynamics and enabling the detection of many compounds other than hydrogen. MIS sensors are highly sensitive to hydrogen, so they are excellent for evaluating the ability of unsaturated hydrocarbons to react on the catalytic metal surface. In this work, we present results of surface functionalization with alkanethiols for the selective detection and reaction of acetylene over ethylene and the detection of unsaturated adsorbates such as crotonaldehyde and acrylic acid.

References

1. Zaera, F. J Phys Chem B 106, 4043 (2002).

2. Love, C.J., Wolfe, D.B., Haasch, R., Chabinyc, M.L., Paul, K.E., Whitesides, G.W., and Nuzzo, R.G. J Am Chem Soc 125, 2597 (2003).