Inspiration from Nature for Heterogeneous Catalysis

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.

Triple the fun: tris(ferrocenyl)arene-based gold(i) complexes for redox-switchable catalysis

The modular syntheses of C₃-symmetric tris(ferrocenyl)arene-based tris-phosphanes and their homotrinuclear gold(i) complexes are reported. Choosing the arene core allows fine-tuning of the exact oxidation potentials and thus tailoring of the electrochemical response. The tris[chloridogold(i)] complexes were investigated in the catalytic ring-closing isomerisation of N-(2-propyn-1-yl)benzamide, showing cooperative behaviour vs. a mononuclear chloridogold(i) complex. Adding one, two, or three equivalents of 1,1′-diacetylferrocenium[tetrakis(perfluoro-tert-butoxy)aluminate] as an oxidant during the catalytic reaction (in situ) resulted in a distinct, stepwise influence on the resulting catalytic rates. Isolation of the oxidised species is possible, and using them as (pre-)catalysts (ex situ oxidation) confirmed the activity trend. Proving the intactness of the P–Au–Cl motif during oxidation, the tri-oxidised benzene-based complex has been structurally characterised.

Trinuclear gold(i) complexes of C₃-symmetric tris(ferrocenyl)arene-based tris-phosphanes with four accessible oxidation states catalyse the ring-closing isomerisation of N-(2-propyn-1-yl)benzamide with different rates depending on their redox state.

Cooperative Surface-Particle Catalysis: The Role of the "Active Doughnut" in Catalytic Oxidation

We consider the factors that govern the activity of bifunctional catalysts comprised of active particles supported on active surfaces. Such catalysts are interesting because the adsorption and diffusion steps, which are often discounted in "conventional" catalytic scenarios, play a key role here. We present an intuitive model, the so-called "active doughnut" concept, defining an active catalytic region around the supported particles. This simple model explains the role of adsorption and diffusion steps in cascade catalytic cycles for active particles supported on active surfaces. The concept has two important practical implications. First, the reaction rate is no longer proportional to the number of active sites, but rather to the number of "communicative" active sites-those available to the reaction intermediates during their respective lifetimes. Second, it generates an important testable prediction concerning the dependence of the total reaction rate on the particle size. With these tools at hand, we examine six experimental examples of catalytic oxidation from the literature, and show that the active doughnut concept gives valuable insight even when detailed mechanistic information is hard to come by.

Aerosol-Assisted Rapid Fabrication of a Heterogeneous Organopalladium Catalyst with Hierarchical Bimodal Pores

Heterogeneous organometallic catalysts with well-defined active sites and hierarchical pores hold tremendous promise for efficient and eco-friendly chemical processes. However, the simple and scalable preparation of these materials remains difficult to date, which has hampered a more broad application scope. Herein, we reported a low-cost, rapid, and scalable aerosol-assisted assembly approach for the synthesis of a well-defined PdDPP (PdCl₂(PPh₂(CH₂)₂))_ complex-containing benzene-bridged organosilica-based catalyst with a hierarchical bimodal micro-macroporous structure. This novel material was realized by using Pd(II) organometallic silane (Pd[PPh₂(CH₂)₂Si(OEt)₃]₂Cl₂) as the active species, organosilane 1,4-bis(triethoxysilyl)benzene (Ph[Si(OEt)₃]₂) as the silicate scaffold and the surfactant cetyltrimethylammonium bromide and the inorganic salt NaCl as the dual templates on a home-built aerosol spraying-instrument. Multiple techniques including X-ray photoelectron spectroscopy and solid-state NMR spectra revealed that the organopalladium complex with a well-defined molecular configuration of major trans model and minor cis model existed in the phenyl-functionalized silica material. As expected, it efficiently promoted a variety of important carbon-carbon cross-coupling transformations including Tsuji-Trost, Sonogashira, and Suzuki reactions in pure water without the assistance of any additives. In comparison with the homogeneous catalyst PdCl₂(PPh₂CH₃)₂, it even exhibited enhanced activity and selectivity in some cases owing to the unique confinement effect and the shape selectivity generated from the hierarchical porous structure. Meanwhile, it was easily recycled and reused eight times without the loss of its activity.

Nature-inspired optimization of hierarchical porous media for catalytic and separation processes

Nature-inspired structuring at the meso-scale: broad macropores separate the mesoporous catalyst grains.