Synthesis and Characterization of Nanostructured Single-Site Heterogeneous Catalysts

In a single-site heterogeneous catalyst (SSHC), active sites are well defined and evenly distributed entities (single sites). They have distinct chemical surroundings, as in conventional homogeneous systems, but they also show all the advantages of heterogeneous systems, in terms of easy separation, recover and recyclability. Single sites are usually (although not necessarily) located over solid supports with high surface area and they show the following general peculiarities: (i) They consist of a limited and defined number of atomic species, i.e. one atom (as in truly ‘single’ sites) or few atoms (ii) They are spatially isolated from each other, (iii) They all have identical energy of interaction between the site itself and a reactant, and (iv) They are structurally well characterized. The use of inorganic oxide supports with selected shape and porosity at a nanometric level may have a relevant impact on the region and stereochemistry of the catalytic reaction. Analogously, by choosing the optimal preparation techniques to obtain spatially isolated and well-characterized active sites, it is possible to achieve performances that are comparable to (or, in the most favorable cases, better than) those obtained with homogeneous systems. Such catalysts are therefore particularly suitable for the transformation of highly-functionalized fine chemicals and some relevant examples where high chemo, region and stereo selectivity are crucial will be described.

Redox-active zeolitic structures, such as titanosilicalite-1 (TS-1), or transition-metal-containing open-structure aluminophosphates (AlPOs) appeared in the 1980s and can be considered as the first examples of SSHC, as they fulfil the main requirements of isolation, uniform distribution and controlled chemical environment of the active sites (Wilson et al. 1982; Weisz et al. 1984; Chen et al. 1994; Marchese et al. 1994; Thomas et al.1994; Corma 1997). During the design and engineering steps that led to these catalytic systems, a major role was played by in situ characterization methods for solid catalysts. Then, starting from the 1990s, owing to the development of silica-based mesoporous molecular sieves, larger reactant molecules, which can enter the pore network, react there and leave it, can be transformed within the inner space of porous solids (Xiao 2005; Yang et al. 2009). Mesoporous silica materials, such as MCM-41, MCM-48, HMS, KIT-6, SBA-15 or SBA-16, are, and outer surfaces have a profusion of pendant silanol groups (generally from 1 to 3 OH groups per square nanometer) that are optimal loci for immobilizing transition metal catalytic centres (Rigutto et al. 2007). Among them, materials with a good hydrothermal stability, such as HMS or SBA-15, owing to a large pore wall thickness and highly condensed frameworks with limited connectivity defects, are preferable, as they can more easily withstand the strong conditions occasionally experienced during synthesis or catalysis (Zhang et al. 2005). Catalytically active single sites can have a structural role, when they are introduced into the framework of spatially ordered materials, via direct synthesis (figure 1). Otherwise, single sites can be added to a pre-existing support by post-synthesis techniques.

In this paper we found that a catalyst is a material that increases the rate of a reaction without being consumed during the reaction. A catalyst increases the reaction rate by lowering the activation energy) of the chemical reaction. Catalysts play a vital role in almost every aspect of our lives and are used in the production of fuels, polymers, chemicals, foods, and pharmaceuticals. One challenge facing the heterogeneous catalysis community is the targeted synthesis of dispersed catalyst ensembles.

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