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Green Chemistry by Nanocatalysis

Vivek Polshettiwar

8 October 2013

Catalysis lies at the heart of chemical processes that lead to a variety of chemical products and synthetic materials. This can be highlighted by the fact that about >80-85% of synthetic materials and commercial chemical products see at least one catalyst at some point of their synthesis. This means, the synthesis of many useful household products such as medicines, detergents, polymeric fibers, perfumes, fuels, paints, lubricants, and a myriad of other value-added chemical products essential to humans would have been neither possible nor feasible in the absence of catalysts.

Catalysts are chemical substances that enable the ("smooth") transformation of fine chemicals into value-added chemical products or synthetic materials. Catalysts are traditionally divided into two major groups based on the type of phase of the catalyst is in relative to the catalytic reaction mixtures, i.e., homogeneous or heterogeneous catalysts. Homogeneous catalysts are those that exist in in the same phase as the reactants. They are generally soluble organic or organometallic complexes and often give chemo-, regio- and stereo-selective products. However, they are relatively difficult to separate from reaction mixtures for reuse at the end of reactions. On the other hand, there are solid or insoluble catalysts, also called heterogeneous catalysts. In many instances, the solid catalysts contain homogeneous catalysts supported on neutral or catalytic-active solid support materials such as porous silica or alumina. These types of catalysts are easily separable and reusable at the end of reactions; however, they often give relatively poor reaction yields, compared to many of their homogeneous counterparts. Thus, the catalyst which can bridge homogeneous and heterogeneous system is needed.

The fields of nanotechnology have been unquestionably thriving over the last two or so decades. One area of immediate impacts that nanotechnology has long had has been in the field of catalysis.Nanocatalysis can thus be simply defined as the use of nanoscale materials in catalysis. Nanocatalysts are highly active (like homogeneous system) and also easy to recover/reuse (like heterogeneous system). In other words, nanocatalysts exhibit quasi-homogeneous or quasi-heterogeneous catalytic properties, and thus allow for rapid and selective chemical transformations, with excellent product yield and ease of catalyst separation and recovery. It has also made the greening of chemistry possible. In addition to the size, nanocatalysts are extremely shape sensitive and their catalytic efficiency and selectivity dramatically depend on their shape and morphology.

Nanocatalysis can help design catalysts with excellent activity, greater selectivity, and high stability. Their properties can easily be tuned by tailoring the size, shape, and morphology of the particular nanomaterial. In the Polshettiwar's nano-catalysis laboratory, they are synthesizing various nano-materials (silica, metal oxides, metals, MOF) with specific shapes, sizes and morphologies and then evaluating their use as a nano-catalyst for the development of sustainable protocols for various challenging reactions like C-H activation, C-C coupling, oxidation, metathesis, hydrogenolysis, hydrogenation, CO2 capture and conversion to fine chemicals, photocatalysis and environmental remediation. A guiding hypothesis is that access to highly reactive catalytic sites can be controlled by varying shape and morphology of nanomaterials.

Further References