Design and Use of Nanostructured Single-Site Heterogeneous Catalysts

Catalysis plays an essential role in almost every facet of our daily lives and its contributions to present day society cannot be overstated. The production of fuels, polymeric materials, nanomaterials, chemicals, foods, and pharmaceuticals all involve catalysis. The role of a catalyst during a reaction can be broken up into three unique steps (binding, transformation, and release) and the entire process is commonly referred to as a catalytic cycle. A catalytic reaction begins when the catalyst chemically combines with a reactant (i.e. binding). The catalyst then transforms the bound reactant into an intermediate which can be converted into other intermediates and finally into products (i.e. transformation). The last step of the catalytic cycle involves the catalyst releasing the product and returning to its initial state (i.e. release). Once the catalyst has returned to its initial state it can combine with another reactant and go through the cycle again. Good catalysts can cycle through a catalytic cycle over and over again. Ideally, a catalytic cycle continues without limit, but in reality, undesired changes can render the catalyst less active with continued use, and so many catalysts must be periodically regenerated or replaced.[1]

Incipient wetness and grafting procedures are fundamentally equivalent in that both procedures deposit metal cations onto the surface of a pre-existing support. A typical incipient wetness procedure begins with the active site precursor (frequently a water soluble metal salt) being dissolved in an aqueous solution. The idea is to add only enough solution to fill the pores of the support which should result in the active site precursor being evenly distributed in the pores of the support. [2] The exact interactions between the active site precursor and the support depend on the metal complex or complexes that are formed in the solution because every complex can interact differently with the surface of the support. [3]

Tethering is similar to incipient wetness and grafting except for one main difference. Similar to incipient wetness and grafting, the active site is covalently bound to the support. The main difference is that the active site is not directly bound to the support in tethering whereas the active site directly binds to the support in incipient wetness and grafting. One of the main advantages of homogeneous molecular catalysts, when they operate under ideal conditions, is that their active sites are spatially well separated from one another, just as they are in constant energetic interaction between each active site and the reactant (substrate). Another advantage, also a direct consequence of spatial isolation and energetic constancy, is that such catalysts are readily amenable to almost all the techniques of characterization as time-resolved NMR, FT-IR, X-Ray absorption and all other spectroscopic (and calorimetric) methods available. Nevertheless, although homogeneous catalysts present, in general, high activity and selectivity values under mild reaction conditions, they show, as a major drawback, a difficult recycling and separation from the products

In this paper we found that nanostructured single-site an assortment of catalysts have the recompense of conventional solid catalysts, in conditions of easy revitalization and recycling, jointly with a separate modified substance and steric environment in the region of the catalytically lively metal site. To utilize of lifeless oxide chains with preferred outline and porosity at nanometric tallness may have a relevant collision on the regio and stereochemistry of the catalytic response.

[1] Gates, B. C.; Huber, G. W.; Marshall, C. L.; Ross, P. N.; Siirola, J.; Wang, Y. Catalysts for emerging energy applications. MRS Bull. 2008, 33, 429-435. [2] Kamfjord, T.; Wester, T. S.; Rytter, E. Supported metallocene catalysts prepared by impregnation of MAO-modified silica by a metallocene/monomer solution. Macromol. Rapid Commun. 1998, 19, 505-509. [3] Brown, G. E., Jr.; Henrich, V. E.; Casey, W. H.; Clark, D. L.; Eggleston, C.; Felmy, A.; Goodman, D. W.; Graetzel, M.; Maciel, G.; McCarthy, M. I.; Nealson, K. H.; Sverjensky, D. A.; Toney, M. F.; Zachara, J. M. Metal Oxide Surfaces and Their Interactions with Aqueous Solutions and Microbial Organisms. Chem. Rev. 1999, 99, 77-174. [4] Michael Edward Peretich, Targeted Synthesis and Characterization of Nanostructured Silicate Building Block Supports and Heterogeneous Catalysts with Tungsten (VI) or Zirconium(IV) Centers-12-2011 [5] http://www.mdpi.com/1420-3049/15/6/3829 [6] D.Uzio, Nano-Structured Heterogeneous Catalysts: One More Step to an Atomic Scale Design of the Active Surface heterogeneous-catalysts-for-the-selective-transformatio [8] http://rspa.royalsocietypublishing.org/content/ 468/2143/1904.full