Heterogeneous catalysis
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Better understanding of the molecular science of heterogeneous catalysts is an important part of research efforts in catalysis.
Heterogeneous catalysts, which are in a different phase from that of the reactants, offer certain advantages compared with
homogeneous catalysts, which are in the same phase as the reactants. Typically solids, heterogeneous catalysts can be more
easily separated and recovered in a product stream and therefore lend themselves to continuous chemical processing. They are
also generally more tolerant of extreme operating conditions than homogeneous catalysts (5).
A challenge in heterogeneous catalysis, however, is that the materials used as heterogeneous catalysts can contain a distribution
of particles and pores of different sizes. To better understand the function of the active site of a heterogeneous catalyst,
three key elements need to be considered, according to a recent report by the Dutch National Research School Combination Catalysis
Controlled by Chemical Design (NRSC-Catalysis). NRSC-Catalysis represents scientists from various disciplines from nine Dutch
universities: the University of Amsterdam (UvA), University Amsterdam (VA), Delft University of Technology, Eindhoven University
of Technology, University of Nijmegen, University of Groningen, Leiden University, Utrecht University, and the University
of Twente. NRSC-Catalysis issued the report to highlight the advances in catalyst design and development made by the group
and to offer a perspective on future research efforts (5).
Three elements that need to be considered in heterogeneous catalyst development are: the chemical nature of the active center
(0.1-nm length scale) that controls the bond-breaking and bond-making action of the catalyst; the local environment such as
its hydrophilic or hydrophobic nature and the stereochemistry that is affected by processes on nanometer-length scales; and
the accessibility of active centers that affect local concentrations and the rate of transport of molecules (5).
Researchers at the University of California at Berkeley recently reported on a method for applying heterogeneous catalysts
to known homogeneous catalytic reactions through the design and synthesis of electrophilic platinum nanoparticles (6). These
nanoparticles were selectively oxidized by a hypervalent iodine species. They catalyzed a range of p-bond activation reactions
previously only catalyzed through homogeneous processes (6). The importance of this method is that it shows the potential
of using nanoparticles to develop novel catalytic reactions that were previously not accessible through heterogeneous catalysis
(6, 7). The researchers also reported that the platinum polyamidoamine dendrimer-capped nanoparticles showed better activity
and recyclability compared with larger, polymer-capped analogs (6, 7).
A widely used class of heterogeneous catalysts are zeolites, which are microporous crystalline solids with well-defined structures.
They are used as solid-acid catalysts in petrochemical processing. They are also a helpful tool for better understanding the
effects of the environment of the catalytically active site, such as the pore wall, solvents, ligands, and the interaction
with adsorbates. The NRSC-Catalysis report points out that various molecular-synthesis techniques can be used to activate
zeolites for other reactions such as selective oxidation or dehydrogenation (5).
One interesting application is the development of selective oxidation catalysts by incorporating single-site silsesquioxane
clusters connected through a polymeric matrix embedded in the channels of a porous inorganic support. Silsesquioxane clusters
are small molecular complexes of silica or alumina, which contain reactive-single site atoms with similar reactivity as the
inorganic support of a heterogeneous catalyst (5).
The NRSC-Catalysis program is also developing new designs for microfluidics reactor systems, which can be used for producing
well-defined nanoparticles that can be used in heterogeneous catalysts (6). The design of catalysts, carriers, and reactors
are interrelated, and studying these interrelationships is an important research area of NRSC-Catalysis. For example, researchers
at the University of Nijmegen have built nano-sized reactor architectures with the goal of constructing cell-like assemblies
from amphiphilic block copolymers. They developed a procedure to encapsulate enzymes at specific locations in these capsule
and have used these systems to perform cascade reactions (5).
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