Catalyst Site Selection via Control over Noncovalent Interactions in Self-Assembled Monolayers

Abstract

One strategy for controlling selectivity in surface-catalyzed reactions is to precisely control the types of surface sites available for reaction. Here, we show that such control can be achieved on Pd/Al 2 O 3 catalysts modified by alkanethiol selfassembled monolayers (SAMs) by changing the length of the modifier's alkyl tail. Density functional theory (DFT) calculations show that thiolates with short alkyl chains preferentially bind to undercoordinated Pd step sites, but that adsorption on (111) terrace sites is more favorable at higher chain lengths due to greater stabilization by van der Waals interactions. Linear alkanethiol SAMs with chain lengths ranging from six to eighteen carbon atoms were deposited on Pd/Al 2 O 3 catalysts to probe this predicted effect experimentally. Infrared spectroscopy measurements conducted after CO adsorption confirmed that increases in alkyl chain length resulted in increasing specificity in poisoning of terrace sites. The catalysts were also evaluated for furfuryl alcohol hydrogenation, a structure-sensitive probe reaction.Selectivity to the desired hydrodeoxygenation to methylfuran increased from <20% to >60% as chain length increased from six to eighteen carbons, consistent with increasing efficiency for thiolate blocking of Pd terrace sites. DFT models demonstrate that the presence of thiolates strongly suppressed decarbonylation reactions, and that step sites surrounded by thiolates could still be active for hydrodeoxygenation. This work demonstrates that site availability, and thus catalyst selectivity, can be tuned by changing the architecture of SAM precursors.

Carboxylic acid-modified metal oxide catalyst for selectivity-tunable aerobic ammoxidation

“…Medlin’s group has successfully improved the selectivity of the hydrogenation of 1-epoxy-3-butene to 1-epoxybutane by modifying the conventional Pd/Al 2 O 3 catalyst with alkanethiol 13 . Later, a series of pioneering studies were reported on self-assembled thiol and amine modifiers that enhanced the chemoselective reaction of bifunctional compounds, for example, the chemoselective nitrostyrene reduction 14 , hydrogenation of cinnamaldehyde to cinnamyl alcohol 15 , 16 , hydrogenation of the aldehyde moiety of furfural to produce furfuryl alcohol and methylfuran 17 – 19 , and, more recently, hydrogenation of acetophenone to phenylethanol 20 . Although a variety of organic modification strategies have successfully improved the chemoselective reactivity, the application of this strategy has been limited to the confined catalyst field of metallic nanoparticles, especially noble metal nanoparticles 21 .…”

Directing Reaction Pathways through Controlled Reactant Binding at Pd–TiO2 Interfaces

“…[9,10] Ap romising approach is to employ inverted systems in which the metal oxide is deposited as afilm onto the metal nanoparticles (NPs). [22][23][24][25][26][27][28] Here,w er eport on am ethod for controlling the encapsulation of Pd NPs with nanoporous TiO 2 to produce interfacial sites where accessible conformations of aromatics are restricted. [22][23][24][25][26][27][28] Here,w er eport on am ethod for controlling the encapsulation of Pd NPs with nanoporous TiO 2 to produce interfacial sites where accessible conformations of aromatics are restricted.…”

“…While encapsulated structures have been applied to enhance the stability of metal NPs [11][12][13][14] or improve the selectivity and/or activity for selective oxidation, [15][16][17] hydrogenation, [18][19][20] and decarbonylation, [21] HDO provides an important probe reaction for testing their utility for reactions that occur at metal/oxide interfaces.M oreover,t he unique geometry of the sites in an encapsulated structure could result in an ew way to control interfacial properties via biasing the reactant binding orientations.F or aromatic alcohols,flat-lying adsorbates are prone to decarbonylation and ring hydrogenation while upright adsorption geometries favor HDO. [22][23][24][25][26][27][28] Here,w er eport on am ethod for controlling the encapsulation of Pd NPs with nanoporous TiO 2 to produce interfacial sites where accessible conformations of aromatics are restricted. Ther esulting catalysts exhibited high selectivity toward HDO with improved activity during reactions of furfuryl alcohol, benzyl alcohol, furfural and m-cresol.…”

Reaction mechanisms at the homogeneous–heterogeneous frontier: insights from first-principles studies on ligand-decorated metal nanoparticles

“…125,126 Janik, Medlin, and coworkers reported the effect of alkanethiol-modified Pd catalysts for the selective hydrogenation of furfuryl alcohol to methylfuran (Scheme 11a). 127 DFT studies suggested that conversion of furfural alcohol to either methylfuran via hydrodeoxygenation (HDO) or furan via decarbonylation (DC) are comparable for clean Pd (111) and Pd(221) surfaces. On the contrary, methanethiol-coated Pd(221) surfaces created a confined environment that induces the vertical adsorption of the substrate and precluded the co-adsorption of DC intermediates, thus increasing the selectivity towards methylfuran via HDO (Scheme 11b).…”