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

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.

Asymmetric Elimination Reaction on Chiral Metal Surfaces

The production of enantiopure materials and molecules is of uttermost relevance in research and industry in numerous contexts, ranging from nonlinear optics to asymmetric synthesis. In the context of the latter, dehalogenation, which is an essential reaction step for a broad class of chemical reactions, is investigated; specifically, dehalogenation of prochiral 5-bromo-7-methylbenz(a)anthracene (BMA) on prototypical, chiral, intermetallic PdGa{111} surfaces under ultrahigh vacuum conditions. Asymmetric halogen elimination is demonstrated by combining temperature-programmed X-ray photoelectron spectroscopy, scanning probe microscopy, and density functional theory. On the PdGa{111} surfaces, the difference in debromination temperatures for the two BMA surface enantiomers amounts up to an unprecedented 46 K. The significant dependence of the dehalogenation temperature of the BMA surface enantiomers on the atomic termination of the PdGa{111} surfaces implies that the ensemble effect is pronounced in this reaction step. These findings evidence enantiospecific control and hence promote intrinsically chiral crystals for asymmetric on-surface synthesis.

Rotation in an Enantiospecific Self-Assembled Array of Molecular Raffle Wheels

Tailored nano-spaces can control enantioselective adsorption and molecular motion. We report on the spontaneous assembly of a dynamic system-a rigid kagome network with each pore occupied by a guest molecule-employing solely 2,6-bis(1H-pyrazol-1-yl)pyridine-4-carboxylic acid on Ag(111). The network cavity snugly hosts the chemically modified guest, bestows enantiomorphic adsorption and allows selective rotational motions. Temperature-dependent scanning tunnelling microscopy studies revealed distinct anchoring orientations of the guest unit switching with a 0.95 eV thermal barrier. H-bonding between the guest and the host transiently stabilises the rotating guest, as the flapper on a raffle wheel. Density functional theory investigations unravel the detailed molecular pirouette of the guest and how the energy landscape is determined by H-bond formation and breakage. The origin of the guest's enantiodirected, dynamic anchoring lies in the specific interplay of the kagome network and the silver surface.

Influence of N-Substituents on the Adsorption Geometry of OH-Functionalized Chiral N-Heterocyclic Carbenes

Adsorption of chiral molecules on heterogeneous catalysts is a simple approach for inducing an asymmetric environment to enable enantioselective reactivity. Although the concept of chiral induction is straightforward, its practical utilization is far from simple, and only a few examples toward the successful chiral induction by surface anchoring of asymmetric modifiers have been demonstrated so far. Elucidating the factors that lead to successful chiral induction is therefore a crucial step for understanding the mechanism by which chirality is transferred. Herein, we identify the adsorption geometry of OH-functionalized N-heterocyclic carbenes (NHCs), which are chemical analogues to chiral modifiers that successfully promoted α-arylation reactions once anchored on Pd nanoparticles. Polarized near-edge X-ray absorption fine structure (NEXAFS) measurements on Pd(111) revealed that NHCs that were associated with low enantioselectivity were characterized with a well-ordered structure, in which the imidazole ring was vertically positioned and the OH-functionalized side arms were flat-lying. OH-functionalized NHCs that were associated with high enantioselectivity revealed a disordered/flexible adsorption geometry, which potentially enabled better interaction between the OH group and the prochiral reactant.

Asymmetric azide-alkyne Huisgen cycloaddition on chiral metal surfaces

Achieving fundamental understanding of enantioselective heterogeneous synthesis is marred by the permanent presence of multitudinous arrangements of catalytically active sites in real catalysts. In this study, we address this issue by using structurally comparatively simple, well-defined, and chiral intermetallic PdGa{111} surfaces as catalytic substrates. We demonstrate the impact of chirality transfer and ensemble effect for the thermally activated azide-alkyne Huisgen cycloaddition between 3-(4-azidophenyl)propionic acid and 9-ethynylphenanthrene on these threefold symmetric intermetallic surfaces under ultrahigh vacuum conditions. Specifically, we encounter a dominating ensemble effect for this reaction as on the Pd₃-terminated PdGa{111} surfaces no stable heterocoupled structures are created, while on the Pd₁-terminated PdGa{111} surfaces, the cycloaddition proceeds regioselectively. Moreover, we observe chirality transfer from the substrate to the reaction products, as they are formed enantioselectively on the Pd₁-terminated PdGa{111} surfaces. Our results evidence a determinant ensemble effect and the immense potential of PdGa as asymmetric heterogeneous catalyst.

2D Ising Model for Adsorption-induced Enantiopurification of Racemates

Mechanisms for the spontaneous transformation of achiral chemical systems into states of enantiomeric purity have important ramifications in modern pharmacology and potential relevance to the origins of homochirality in life on Earth. Such mechanisms for enantiopurification are needed for production of chiral pharmaceuticals and other bioactive compounds. Previously proposed chemical mechanisms leading from achiral systems to near homochirality are initiated by a symmetry-breaking step resulting in a minor excess of one enantiomer via statistical fluctuations in enantiomer concentrations. Subsequent irreversible processes then amplify the majority enantiomer concentration while simultaneously suppressing minority enantiomer production. Herein, equilibrium adsorption of amino acid enantiomer mixtures onto chiral and achiral surfaces reveals amplification of surface enantiomeric excess relative to the gas phase; i. e. enantiopurification of chiral adsorbates by adsorption. This adsorption-induced amplification of enantiomeric excess is shown to be well-describe by the 2D Ising model. More importantly, the 2D-Ising model predicts formation of homochiral monolayers from adsorption of racemic mixtures or prochiral molecules on achiral surfaces; i. e. enantiopurification with no apparent chiral driving force.

Adsorption geometry and self-assembling of chiral modifier (R)-(+)-1-(1-naphthylethylamine) on Pt(111)

A mechanistic study on interaction of a chiral modifier - (R)-(+)-1-(1-naphthylethylamine) (R-NEA) - with a single crystalline Pt(111) surface is reported. The details of the adsorption geometry of individual R-NEA molecules and their intermolecular interactions are addressed by combination of infrared reflection absorption spectroscopy (IRAS) and scanning tunneling microscopy (STM). The spectroscopic observations suggest that the molecules are tilted with respect to the underlying metal substrate with the long axis of the naphthyl ring being parallel and the short axis tilted with respect to the surface. In the medium coverage range, formation of directed 3-5 membered chains was observed by STM for the first time, which points to intermolecular bonding between individual molecules and might account for an unusual tilted adsorption geometry deduced from the IR spectra. Based on the STM images revealing the atomic structure of the Pt grid close to the R-NEA chains, we propose the adsorption configuration of NEA fitting both the IRAS and STM data. The obtained results suggest that this strong intermolecular interaction energetically stabilizes the tilted adsorption geometry of the naphthyl ring, which otherwise would be expected to lie flat on the metal to maximize the dispersive interactions. At the coverage close to saturation, R-NEA builds a self-assembled overlayer with hexagonal symmetry, exhibiting intermolecular distances larger than in the directed chains.

Understanding Enantioselective Interactions by Pulling Apart Molecular Rotor Complexes

Enantioselective interactions underpin many important phenomena from biological mechanisms to chemical catalysis. In this regard, there is great interest in understanding these effects at the molecular level. Surfaces provide a platform for these studies and aid in the long-term goal of designing heterogeneous enantiospecific interfaces. Herein we report a model system consisting of molecular rotors, one intrinsically chiral (propylene oxide) and one that becomes chiral when adsorbed on a surface (propene). Scanning tunneling microscopy (STM) measurements enable the chirality of each individual molecule to be directly visualized, and density functional theory based calculations are performed to rationalize the chiral time-averaged appearance of the molecular rotors. While there are no attractive intermolecular interactions between the molecular species themselves, when mixed together there is a strong preference for the formation of 1:1 heteromolecular pairs. We demonstrate that STM tip-induced molecular manipulations can be used to assemble these complexes, examine the chirality of each species, and thereby interrogate if their interactions are enantioselective. A statistical analysis of this data reveals that intrinsically chiral propylene oxide preferentially binds one of the enantiomers of propene with a 3:2 ratio, thereby demonstrating that the surface chirality of small nonchiral molecules can be directed with a chiral modifier. As such, this investigation sheds light onto previously reported ensemble studies in which chirally seeded layers of molecules that are achiral in the gas phase can lead to an amplification of enantioselective adsorption.

Molecular beam/infrared reflection-absorption spectroscopy apparatus for probing heterogeneously catalyzed reactions on functionalized and nanostructured model surfaces

A new custom-designed ultrahigh vacuum (UHV) apparatus combining molecular beam techniques and in situ surface spectroscopy for reactivity measurements on complex nanostructured model surfaces is described. It has been specifically designed to study the mechanisms, kinetics, and dynamics of heterogeneously catalyzed reactions over well-defined model catalysts consisting of metal nanoparticles supported on thin oxide films epitaxially grown on metal single crystals. The reactivity studies can be performed in a broad pressure range starting from UHV up to the ambient pressure conditions. The UHV system includes (i) a preparation chamber providing the experimental techniques required for the preparation and structural characterization of single-crystal based model catalysts such as oxide supported metal particles or ordered oxide surfaces and (ii) the reaction chamber containing three molecular beams-two effusive and one supersonic, which are crossed at the same point on the sample surface, infrared reflection-absorption spectroscopy for the detection of surface-adsorbed species, and quadrupole mass spectrometry for gas phase analysis. The supersonic beam is generated in a pulsed supersonic expansion and can be modulated via a variable duty-cycle chopper. The effusive beams are produced by newly developed compact differentially pumped sources based on multichannel glass capillary arrays. Both effusive sources can be modulated by a vacuum-motor driven chopper and are capable of providing high flux and high purity beams. The apparatus contains an ambient pressure cell, which is connected to the preparation chamber via an in situ sample transfer system and provides an experimental possibility to study the reactivity of well-defined nanostructured model catalysts in a broad range of pressure conditions-up to ambient pressure-with the gas phase analysis based on gas chromatography. Additionally, a dedicated deposition chamber is connected to the preparation chamber, which is employed for the in situ functionalization of model surfaces with large organic molecules serving as promoters or modifiers of chemical reactions. We present a general overview of the apparatus as well as a description of the individual components and their interplay. The results of the test measurements involving the most important components are presented and discussed.

Revealing Catalytically Relevant Surface Species by Kinetic Isotope Effect Spectroscopy: H-Bonding to Ester Carbonyl of trans-Ethyl Pyruvate Controls Enantioselectivity on a Cinchona-Modified Pt Catalyst

Monitoring active surface species on an operating technical catalyst is a challenging task due to the presence of multiple different adsorption sites and the abundance of bulk species. In this work, kinetic isotope effect (KIE) spectroscopy is introduced to capture the signals of catalytically relevant hydrogenation species from the IR spectroscopic detection in attenuated total reflection mode. The catalytic interface formed between a cinchona-modified Pt/Al₂O₃ catalyst and the solvent toluene is sensitively probed directly at the rate limiting step(s) during the asymmetric hydrogenation of ethyl pyruvate by measuring the effects of substituting H₂ by D₂ kinetically and spectroscopically in the same operando experiment. The application of KIE spectroscopy provides unprecedented molecular level insight into the structure of the diastereomeric intermediate surface complex and the phenomenon rate enhancement, which revolutionizes our understanding of chirally modified metal catalysts.

Chirality in adsorption on solid surfaces

In the present review we survey the main advances made in recent years on the understanding of chemical chirality at solid surfaces. Chirality is an important topic, made particularly relevant by the homochiral nature of the biochemistry of life on Earth, and many chiral chemical reactions involve solid surfaces. Here we start our discussion with a description of surface chirality and of the different ways that chirality can be bestowed on solid surfaces. We then expand on the studies carried out to date to understand the adsorption of chiral compounds at a molecular level. We summarize the work published on the adsorption of pure enantiomers, of enantiomeric mixtures, and of prochiral molecules on chiral and achiral model surfaces, especially on well-defined metal single crystals but also on other flat substrates such as highly ordered pyrolytic graphite. Several phenomena are identified, including surface reconstruction and chiral imprinting upon adsorption of chiral agents, and the enhancement or suppression of enantioselectivity seen in some cases upon adsorption of enantiomixtures of chiral compounds. The possibility of enhancing the enantiopurity of adsorbed layers upon the addition of chiral seeds and the so-called "sergeants and soldiers" phenomenon are presented. Examples are provided where the chiral behavior has been associated with either thermodynamic or kinetic driving forces. Two main approaches to the creation of enantioselective surface sites are discussed, namely, via the formation of supramolecular chiral ensembles made out of small chiral adsorbates, and by adsorption of more complex chiral molecules capable of providing suitable chiral environments for reactants by themselves, via the formation of individual adsorbate:modifier adducts on the surface. Finally, a discussion is offered on the additional effects generated by the presence of the liquid phase often required in practical applications such as enantioselective crystallization, chiral chromatography, and enantioselective catalysis.

Supramolecular Corrals on Surfaces Resulting from Aromatic Interactions of Nonplanar Triazoles

Interaction forces between aromatic moieties, often referred to as π-π interactions, are an important element in stabilizing complex supramolecular structures. For supramolecular self-assembly occurring on surfaces, where aromatic moieties are typically forced to adsorb coplanar with the surface, the possible role of intermolecular aromatic interactions is much less explored. Here, we report on unusual, ring-shaped supramolecular corral surface structures resulting from adsorption of a molecule with nonplanar structure, allowing for intermolecular aromatic interactions. The discrete corral structures are observed using high-resolution scanning tunneling microscopy, and the energetic driving forces for their formation are elucidated using density functional theory calculations and Monte Carlo simulations. The individual corrals involve between 11 and 18 molecules bound through triazole moieties to a ring-shaped ensemble of bridge site positions on (111) surfaces of copper, silver, or gold. The curvature required to form the corrals is identified to result from the angle dependence of aromatic interactions between molecular phenanthrene moieties. The study provides detailed quantitative insights into triazole-surface and aromatic interactions and illustrates how they may be used to drive surface supramolecular self-assembly.

Structure and Dynamics of Individual Diastereomeric Complexes on Platinum: Surface Studies Related to Heterogeneous Enantioselective Catalysis

The modification of heterogeneous catalysts through the chemisorption of chiral molecules is a method to create catalytic sites for enantioselective surface reactions. The chiral molecule is called a chiral modifier by analogy to the terms chiral auxiliary or chiral ligand used in homogeneous asymmetric catalysis. While there has been progress in understanding how chirality transfer occurs, the intrinsic difficulties in determining enantioselective reaction mechanisms are compounded by the multisite nature of heterogeneous catalysts and by the challenges facing stereospecific surface analysis. However, molecular descriptions have now emerged that are sufficiently detailed to herald rapid advances in the area. The driving force for the development of heterogeneous enantioselective catalysts stems, at the minimum, from the practical advantages they might offer over their homogeneous counterparts in terms of process scalability and catalyst reusability. The broader rewards from their study lie in the insights gained on factors controlling selectivity in heterogeneous catalysis. Reactions on surfaces to produce a desired enantiomer in high excess are particularly challenging since at room temperature, barrier differences as low as ∼2 kcal/mol between pathways to R and S products are sufficient to yield an enantiomeric ratio (er) of 90:10. Such small energy differences are comparable to weak interadsorbate interaction energies and are much smaller than chemisorption or even most physisorption energies. In this Account, we describe combined experimental and theoretical surface studies of individual diastereomeric complexes formed between chiral modifiers and prochiral reactants on the Pt(111) surface. Our work is inspired by the catalysis literature on the enantioselective hydrogenation of activated ketones on cinchona-modified Pt catalysts. Using scanning tunneling microscopy (STM) measurements and density functional theory (DFT) calculations, we probe the structures and relative abundances of non-covalently bonded complexes formed between three representative prochiral molecules and (R)-(+)-1-(1-naphthyl)ethylamine ((R)-NEA). All three prochiral molecules, 2,2,2-trifluoroacetophenone (TFAP), ketopantolactone (KPL), and methyl 3,3,3-trifluoropyruvate (MTFP), are found to form multiple complexation configurations around the ethylamine group of chemisorbed (R)-NEA. The principal intermolecular interaction is NH···O H-bonding. In each case, submolecularly resolved STM images permit the determination of the prochiral ratio (pr), pro-R to pro-S, proper to specific locations around the ethylamine group. The overall pr observed in experiments on large ensembles of KPL-(R)-NEA complexes is close to the er reported in the literature for the hydrogenation of KPL to pantolactone on (R)-NEA-modified Pt catalysts at 1 bar H₂. The results of independent DFT and STM studies are merged to determine the geometries of the most abundant complexation configurations. The structures reveal the hierarchy of chemisorption and sometimes multiple H-bonding interactions operating in complexes. In particular, privileged complexes formed by KPL and MTFP reveal the participation of secondary CH···O interactions in stereocontrol. State-specific STM measurements on individual TFAP-(R)-NEA complexes show that complexation states interconvert through processes including prochiral inversion. The state-specific information on structure, prochirality, dynamics, and energy barriers delivered by the combination of DFT and STM provides insight on how to design better chiral modifiers.

Chiral segregation driven by a dynamical response of the adsorption footprint to the local adsorption environment: bitartrate on Cu(110)

Bitartrate, a strongly bound chiral modifier, is able to restructure its adsorption footprint on Cu(110) in response to local adsorbates.

Enhanced hydrogenation activity and diastereomeric interactions of methyl pyruvate co-adsorbed with R-1-(1-naphthyl)ethylamine on Pd(111)

Unmodified racemic sites on heterogeneous chiral catalysts reduce their overall enantioselectivity, but this effect is mitigated in the Orito reaction (methyl pyruvate (MP) hydrogenation to methyl lactate) by an increased hydrogenation reactivity. Here, this effect is explored on a R-1-(1-naphthyl)ethylamine (NEA)-modified Pd(111) model catalyst where temperature-programmed desorption experiments reveal that NEA accelerates the rates of both MP hydrogenation and H/D exchange. NEA+MP docking complexes are imaged using scanning tunnelling microscopy supplemented by density functional theory calculations to allow the most stable docking complexes to be identified. The results show that diastereomeric interactions between NEA and MP occur predominantly by binding of the C=C of the enol tautomer of MP to the surface, while simultaneously optimizing C=O····H₂N hydrogen-bonding interactions. The combination of chiral-NEA driven diastereomeric docking with a tautomeric preference enhances the hydrogenation activity since C=C bonds hydrogenate more easily than C=O bonds thus providing a rationale for the catalytic observations.

Achiral sites on a catalyst can result in formation of racemic product, though this can be avoided where the chiral sites lead to enhanced reaction rates. Here, the authors report that for the hydrogenation of methyl pyruvate the chiral modifier enhances the hydrogenation reactivity by favouring the more reactive enol tautomer.

Asymmetric hydrogenation of α-ketoesters on the Pt(111) surface

A novel process is developed to prepare highly efficient single site Pt(111) hexagonal nanocrystals on a carbon support for the asymmetric hydrogenation of α-ketoesters.

Simple rules and the emergence of complexity in surface chirality

Surface chirality arising from self-organized molecular monolayers may manifest both a handedness and footedness, leading to a dual level of chiral expression.

Cellulose Nanocrystals as Chiral Inducers: Enantioselective Catalysis and Transmission Electron Microscopy 3D Characterization

Cellulose nanocrystals (CNCs), derived from cellulose, provide us with an opportunity to devise more sustainable solutions to current technological challenges. Enantioselective catalysis, especially heterogeneous, is the preferred method for the synthesis of pure chiral molecules in the fine chemical industries. Cellulose has been long sought as a chiral inducer in enantioselective catalysis. We report herein an unprecedentedly high enantiomeric excess (ee) for Pd patches deposited onto CNCs used as catalysts for the hydrogenation of prochiral ketones in water at room temperature and 4 bar H2. Our system, where CNCs acted as support and sole chiral source, achieved an ee of 65% with 100% conversions. Cryo-electron microscopy, high-resolution transmission electron microscopy, and tomography were used for the first time to study the 3D structure of a metal functionalized CNC hybrid. It established the presence of sub-nanometer-thick Pd patches at the surface of CNCs and provided insight into the chiral induction mechanism.

Correlated bifunctionality in heterogeneous catalysts: selective tethering of cinchonidine next to supported Pt nanoparticles

A strategy has been devised to add molecular functionality to heterogeneous catalysts in a spatially correlated fashion.

Chirally-modified metal surfaces: energetics of interaction with chiral molecules

Imparting chirality to non-chiral metal surfaces by adsorption of chiral modifiers is a highly promising route to create effective heterogeneously catalyzed processes for the production of enantiopure pharmaceuticals.

Single-chiral-catalytic-surface-sites: STM and DFT study of stereodirecting complexes formed between (R)-1-(1-naphthyl)ethylamine and ketopantolactone on Pt(111)

Revealing the set of most stable bimolecular complexes formed by a prochiral molecule and a chiral modifier on Pt(111).

Adsorption and stability of chiral modifiers based on 1-(1-naphthyl)-ethylamine for Pt catalysed heterogeneous asymmetric hydrogenations

Catalytic and in situ ATR-IR spectroscopic investigations provide mechanistic insight relevant to heterogeneous asymmetric hydrogenation on Pt catalyst using naphthylethylamine-based modifiers.

Synthesis of homochiral tris-indanyl molecular rods

By Ti-mediated alkyne trimerization and subsequent Sonogashira and Ohira–Bestman reactions, homochiral molecular rod molecules were prepared for surface self-assembly studies.

Affecting surface chirality via multicomponent adsorption of chiral and achiral molecules

Here we report on the apparent reduction in surface chirality upon co-assembling a chiral and achiral molecule into a physisorbed self-assembled monolayer at the liquid/solid interface as revealed by scanning tunneling microscopy (STM).