Regio-, Stereo-, and Enantioselectivity in Hydrocarbon Conversion on Metal Surfaces

Selectivity is one of the most important criteria for the design of new catalytic processes. More selective catalysis could be both cheaper and greener because it does not waste reactants, does not require expensive separation procedures, and generates fewer toxic byproducts. Traditionally, control of selectivity in heterogeneous catalysis has been hampered by both a lack of understanding of the molecular details that define such selectivity and the limited range of synthetic tools available to make low catalysts with the specific properties required. However, progress in surface science as well as in nanotechnology and self-assembly are providing greater molecular understanding and a wider synthetic range to address these limitations. In this Account, we describe our studies using model systems to pinpoint the mechanistic factors that define selectivity in a number of increasingly subtle hydrocarbon dehydrogenation and hydrogenation reactions. The first examples show how the electronic properties of a metal surface affect the regioselectivity of hydrogen elimination from alkyl species adsorbed on that surface. Nickel preferentially promotes the extraction of hydrogen atoms from the carbon directly bonded to the surface, a step that leads to undesirable cracking reactions, whereas platinum allows for dehydrogenation farther down the hydrocarbon chain, facilitating a more desirable isomerization processes. In a second set of examples, we address the issue of selectivity in alkene isomerizations involving either double-bond migrations or cis-trans interconversions. In those reactions, the key mechanistic steps require hydrogen abstraction from a beta-carbon of the hydrocarbon chain (the second when counting away from the surface), and selectivity is defined by steric considerations around the different hydrogens available at those positions. We observed that close-packed surfaces of platinum have the unique ability to promote the thermodynamically unfavorable but highly desirable conversion of trans-alkenes to their cis counterparts, and we prepared new shape-controlled catalysts to take advantage of that valuable behavior. Finally, we discuss the more subtle issue of enantioselectivity. Hydrogenation of prochiral reactants such as asymmetric ketones can produce chiral compounds, but regular metal catalysts are achiral and therefore yield racemic mixtures. Fortunately, the adsorption of chiral modifiers onto a catalytic surface can bestow chirality on it. With cinchona alkaloids, individual molecules can provide the required chiral environment on the surface for such enantioselectivity. Simpler molecules may also bestow chirality on surfaces, even if that may require their assembly into chiral supramolecular structures held together by the surface. In both cases, a specific surface chiral site is produced with the help of molecular adsorbates. The examples discussed in this Account highlight the need to design and prepare heterogeneous catalysts with sophisticated surface sites in order to promote reactions selectively. Perhaps more importantly, they also hint at some of the tools available to accomplish that task.