Designing Sites in Heterogeneous Catalysis: Are We Reaching Selectivities Competitive With Those of Homogeneous Catalysts?

Abstract

A critical review of different prominent nanotechnologies adapted to catalysis is provided, with focus on how they contribute to the improvement of selectivity in heterogeneous catalysis. Ways to modify catalytic sites range from the use of the reversible or irreversible adsorption of molecular modifiers to the immobilization or tethering of homogeneous catalysts and the development of well-defined catalytic sites on solid surfaces. The latter covers methods for the dispersion of single-atom sites within solid supports as well as the use of complex nanostructures, and it includes the post-modification of materials via processes such as silylation and atomic layer deposition. All these methodologies exhibit both advantages and limitations, but all offer new avenues for the design of catalysts for specific applications. Because of the high cost of most nanotechnologies and the fact that the resulting materials may exhibit limited thermal or chemical stability, they may be best aimed at improving the selective synthesis of high value-added chemicals, to be incorporated in organic synthesis schemes, but other applications are being explored as well to address problems in energy production, for instance, and to design greener chemical processes. The details of each of these approaches are discussed, and representative examples are provided. We conclude with some general remarks on the future of this field.

Formation of 2D Self-Assembled Layers of Allyl Cyanide on Pd(111)

“…71−73 Specifically, for cyano-containing compounds (mostly, higher mass organic molecules), a large body of experimental work was previously published, in which the combination of long-and short-range interactions was put forward to explain the experimental results. 28,31,56−61 Thus, Gottardi et al 57 observed a hexagonal closed-packed phase forming upon the adsorption of a triarylamine derivate 4,4,8,8,12,12-hexamethyl-4H,8H,12H-benzo [1,9]quinolizino- [3,4,5,6,7-defg]acridine-2,6,10-tricarbonitrile on Au(111). The authors explained the experimentally observed patterns by a combination of (i) long-range attractive dipolar coupling of the terminal CN-groups of two adjacent molecules (CN•••CN bond) and (ii) short-range hydrogen bonding between the nitrogen atom of the CN-group and an H atom of the phenyl ring in close proximity.…”

“…The authors explained the experimentally observed patterns by a combination of (i) long-range attractive dipolar coupling of the terminal CN-groups of two adjacent molecules (CN•••CN bond) and (ii) short-range hydrogen bonding between the nitrogen atom of the CN-group and an H atom of the phenyl ring in close proximity. Stoḧr et al 31 showed the formation of fully segregated domains of pure enantiomers formed upon adsorption of the racemic mixture of 6,13-dicyano [7]helicene on Cu(111). As a driving force for the chiral recognition in enantiopure domains, the authors put forward a combination of (i) short�range hydrogen bonding between an aromatic hydrogen atom and the adjacent CN-group of another molecule and (ii) long-range attractive CN•••CN dipolar coupling.…”

“…Functionalization of catalytically active metal surfaces with 2D layers of organic compounds has been successfully employed in heterogeneous catalysis to significantly improve selectivity in multipathway reactions of hydrocarbons. − Indeed, competing reactions, which usually exhibit only small differences in the activation barriers, are difficult to control by tuning solely the reactant–metal interactions because a catalytic surface showing high activity for the desired reaction route might also greatly accelerate the unwanted reaction paths. To overcome this problem, additional interaction between the reactant and a specific active site can be introduced, which could selectively enable the preferred reaction pathway.…”

“…Such specific sites can be created by functionalization of the metal surface with a 2D layer of organic ligand adsorbates, exhibiting strong lateral interactions with the reactants. − By virtue of these reactant–ligand lateral interactions, the surface reaction can be directed toward the desired route, and the unwanted reactions become inhibited. The approach based on the lateral interactions between a reactant and a coadsorbed 2D ligand layer is currently being developed for numerous multipathway hydrogenation reactions, ,− such as selective hydrogenation of acetophenone, − trans-stilbene and phenylacetylene hydrogenation over Pd and Rh nanoparticles functionalized with N-heterocyclic carbens, , selective hydrogenation 1-epoxy-3-butene on Pd nanoparticles precovered with n -alkanethiol, hydrogenation of acetophenone to phenylethanol over Pt nanoparticles functionalized with proline, and chemoselective hydrogenation of the CO bond in α,β-unsaturated aldehydes over Pt 3 Co nanocrystals precovered with an oleyamine array …”

Homogeneous {Ti2Ni} Heterotrinuclear Catalyst for Ethylene Polymerization and Copolymerization

“…Two or more metal centers synergically act in the active site of several enzymes, thus improving kinetics and imparting high selectivity [1,2]. Based on this mechanism of action, multinuclear cooperative catalysis has been applied to many processes, including coordination polymerization, olefin hydrogenation, hydroformylation, cycloaddition, and epoxidation [3][4][5]. In olefin polymerization reactions catalyzed by multinuclear transition metal complexes, cooperative effects significantly enhance the molecular weight, chain branch density, and comonomer enchainment selectivity of the products compared to the reactions catalyzed by analogous mononuclear catalysts [6,7].…”