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.

A novel thermoregulated phase-transfer catalysis system for chiral nano-Pt-catalyzed asymmetric hydrogenation

The chiral ionic liquid CILTPT-MS [CH3(OCH2CH2)16CD]+[CH3SO3]− (CD = cinchonidine) is found to exhibit cloud point character. Due to this cloud point character, a CILTPT-MS-stabilized chiral nano-Pt catalyst is prepared and utilized to realize transfer in a H2O–1-pentanol biphasic system by simply changing the temperature. Therefore, a fluorine-free, thermoregulated phase-transfer catalysis system is developed and applied to the asymmetric hydrogenation of α-ketoesters with enantiomeric excess of >99% and excellent conversion. In addition, the chiral nano-Pt catalyst can be easily separated and reused efficiently.

A thermoregulated phase transfer chiral Pt nanocatalyst for enantioselective hydrogenation of α-ketoesters

An efficient and recyclable thermoregulated phase transfer chiral Pt nanocatalyst was developed and applied to the enantioselective hydrogenation of α-ketoesters.

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.

Pt nanoparticles stabilized by thermosensitive polymer as effective and recyclable catalysts for the asymmetric hydrogenation of ethyl pyruvate

Thermosensitive polymer stabilized Pt nanocatalysts were firstly applied in asymmetric hydrogenation and exhibited excellent catalytic activity and durability.

Platinum functionalized multiwall carbon nanotube composites as recyclable catalyst for highly efficient asymmetric hydrogenation of methyl pyruvate

A highly efficient platinum nanoparticle functionalized carbon nanocomposite is demonstrated to be a recyclable heterogeneous catalyst for asymmetric hydrogenation of methyl pyruvate.