The effect of alkanethiol monolayers coverage on Pd-catalyzed benzyl alcohol hydrodeoxygenation
Chih-Heng Lien and William Medlin*
Department of Chemical and Biological Engineering, University of Colorado Boulder, Jennie Smoly Caruthers Biotechnology Building 596 UCB, Boulder, CO 80309-0596, USA
Benzyl alcohol is a useful probe molecule for biomass-derived oxygenates that are promising sources for renewable feedstocks for chemicals and fuels. Because of the various adsorption configurations available to benzyl alcohol on Pd surfaces, it can undergo multiple reactions to a variety of products including toluene, benzaldehyde and benzene1. Thus, controlling selectivity toward desired products is critical for avoiding separation costs and wastes, two of the main challenges in developing biomass-derived products. One promising strategy for improving selectivity is to control the ensembles of available surface sites, and thus restrict the adsorbed conformations of reactive intermediates by thiol monolayers2. In this study, mixed alkanethiolates with variable ratio of 1-octadecanethiol (C18) to 1-adamantanethiol (AT) were employed to control the catalyst performance and improve the selectivity through restricting the conformation of adsorbed benzyl alcohol on Pd catalysts.
On uncoated Pd catalysts, after the reaction was stable, the dominant product from benzyl alcohol was benzene and the selectivity to toluene was less than 22%. Deposition of sparser AT monolayers improved the selectivity to a smaller extent, but resulted in higher reaction rate than the uncoated catalyst, which is attributed to weaker interactions of the phenyl ring with the surface and reduced coke formation from benzene decomposition on Pd surface3. After the C18 modification, toluene selectivity was remarkably improved to 85% and the production of benzene was not observed with the cost of a large decrease in reaction rate. Additionally, with near full conversion, toluene selectivity can reach 100% without any benzene and benzaldehyde production. For mixed thiol monolayers, toluene selectivity increases, and benzene selectivity and turnover frequency decreases with C18 fractional coverage on Pd surface, which indicates that the catalyst performance is freely controlled by the ratio of C18 to AT on the surface. Inductively coupled plasma mass spectrometry (ICP-MS) and diffuse reflectance infrared fourier transfer spectroscopy (DRIFTS) spectra of the carbon monoxide adsorption on Pd/Al2O3 were respectively used to confirm the thiolate coverage and further characterize the available sites on a catalyst surface. The DRIFTS showed the surface density of thiols changes the availability of active site on Pd surface, which causes selectively shuts down decarbonylation while still allowing hydrodeoxygenation.
1. Pang, S. H.; Román, A. M.; Medlin, J. W., Adsorption Orientation-Induced Selectivity Control of Reactions of Benzyl Alcohol on Pd(111). The Journal of Physical Chemistry C 2012, 116, 13654-13660.
2. Marshall, S. T.; O’Brien, M.; Oetter, B.; Corpuz, A.; Richards, R. M.; Schwartz, D. K.; Medlin, J. W., Controlled selectivity for palladium catalysts using self-assembled monolayers. Nature material 2010, 9.
3. Lien, C.-H.; Medlin, J. W., Promotion of Activity and Selectivity by Alkanethiol Monolayers for Pd-Catalyzed Benzyl Alcohol Hydrodeoxygenation. The Journal of Physical Chemistry C 2014, 118, 23783-23789.