Size and shape selectivity

Future development of SAM-based analytical technology requires expansion of the size and shape selectivity of template stmctures, as well as introduction of advanced chemical and optical gating mechanisms. An important contribution of SAMs is in miniaturization of analytical instmmentation. This use may in turn have considerable importance in the biomedical analytical area, where miniature analytical probes will be introduced into the body and target-specific organs or even cell clusters. Advances in high resolution spatial patterning of SAMs open the way for such technologies (268,352). 

The intention of this chapter is to provide a general survey on the preparative methodologies for the size- and shape-selective synthesis of metallic nanoparticles that have emerged from the benches of chemical basic research during the last few decades and become established as practical standard protocols. Industrial scale-up, however, has only just started to test the economic viability of these procedures and to determine whether they can meet the challenges of a number of very specific applications. The commercial manufacture of such thermodynamically extremely unstable nanoparticles in defined sizes and shapes on the kilo-scale is still confronted by a number of major problems and it remains to be seen how these can be solved. 

Size and Shape Selective Synthesis of Metal Nanoparticles by Seed-Mediated Method and the Catalytic Activity of Growing Microelectrodes (GME) and Fully Grown Microelectrodes (FGME)... 

Size and Shape Selective Synthesis of Metal Nanoparticles... 

Generally, stable and well-dispersed metal NPs have been prepared in ILs by the simple reduction of the M(I-IV) complexes or thermal decomposition of the organometallic precursors in the formal zero oxidation state. Recently, other methods such as the phase transfer of preformed NPs in water or organic solvents to the IL and the bombardment of bulk metal precursors with deposition on the ILs have been reported. However, one of the greatest challenges in the NPs field is to synthesize reproducibly metal NPs with control of the size and shape. Selected studies of the preparation of metal NPs in ILs that, in some cases, provide NPs with different sizes and shapes are considered in this section. 

Ratios of turnover frequencies (TOFs) corresponding to the degree of enhancement of the reaction rates by the imprinting revealed that the imprinted Rh-dimer catalyst (10) showed size and shape selectivities for the alkenes as shown in Figure 8.10. Selectivity for the alkene hydrogenation on the Rh2imp catalyst (10) depends on the size and shape of the template cavity as reaction site in... 

Supramolecular concepts involved in the size- and shape-selective aspects of the channels and cavities of zeolites are used to control the selectivity of reactions of species produced by photoexcitation of molecules encapsulated within zeolites. The photochemistry of ketones in zeolites has been extensively studied. Photoexcitation of ketones adsorbed on zeolites at room temperature produces radical species by the Norrish type 1 reaction. A geminate (born together) radical pair is initially produced by photolysis of the ketone, and the control of the reaction products of such radicals is determined by the initial supramolecular structure... 

Z. (1996) Chemical liquid deposition zeolites with controlled pore-opening size and shape-selective separation of isomers. Ind. Eng. Chem. Res., 35, 430. 

Mohanty, S. and McCormick, A.V. (1999) Prospects for principles of size and shape selective separations using zeolites. Chem. Eng. J., 74, 1-14. 

For samples photolyzed on ZSM-5 zeolite, the product distributions of 31 and 32 are dramatically different from those photolyzed in homogeneous solutions. First, the rearrangement products were totally suppressed. Second, diphenylethane 39 resulted from coupling of benzyl radical was not found. Only phenol 38 and toluene were detected. In contrast, photolyses of 33 and 34 on ZSM-5 follow strikingly different pathways. Both photo-Fries rearrangement 36 and 37 and decarbonylation products 35 and 39 were formed. These results can be understood from consideration of size- and shape-selective sorption combined with restriction on the mobility of the substrates and reaction intermediates imposed by the pentasil pore system. 

In the present section, analogies and similarities will be noted between enzymes and heterogeneous catalysts in the concept of the active site and metal-protein/metal-support analogies the possession of size and shape selectivity the similarity or identity in kinetics between the two processes the use of electrochemical organization on a molecular or supramolecular level the possibility of... 

The fully protonated H-ZSM-5 had a constraint index of 10.8 in agreement with the literature values. The modified Na, H-ZSM-5 had a constraint index of 1.1 which is typical of a larger pore zeolite with a pore diameter greater than 6A. The modified zeolite reacts with the substrates primarily at external catalytic sites and in a non-discriminating manner. Size and shape selectivity are not major factors since few catalytic sites are located internally. 

Zeolite size and shape selectivity due to its rigid pores and channels... 

Figure 4.14 describes three systems where recognition is size- and shape-selective [ref. 9]. In one system, two [1,3] hexagons assembled with a receptor in the presence of [1,4] or [1,2,3] hexagons (Fig. 4.14). In a second system, two ligand-receptor pairs (2a and la 2b and lb) formed in the presence of each other. The pattern of the hydrophobic faces on the ligands and receptors was chiral, and the receptors and ligands assembled in a way that juxtaposed enantiomeric chiral faces. In a third system, one [1,4] hexagon selectively assembled with a receptor. Receptors that selectively assembled two or three [1,4] hexagons were also fabricated.

The pore structure, such as pore geometry and pore size, of mesoporous materials is one of the key parameters for practical applications, especially those dependent on size and shape selectivity and ready access to porosity. Considerable efforts have been devoted to control the pore structure of mesoporous materials. 

In addition to the importance of the M41S materials for size- and shape-selective applications, these materials have been also regarded as a suitable mesoporous model adsorbent for testing theoretical predictions of pore condensation. Pore condensation represents a first order phase transition from a gas-like state to a liquid-like state of a pore fluid in presence of a bulk fluid reservoir, which occurs at a pressure p less than the saturation pressure po at gas-liquid coexistence of the bulk fluid [6,7]. In this sense pore condensation can be regarded as a shifted gas-liquid bulk phase transition due to confinement of a fluid to a pore. Recent work has shown that in fact the complete phase diagram of the confined fluid is shifted to lower temperature and higher mean density as compared with the bulk coexistence curve [e.g., 8,9]. 

Rh, the shape of the template cavity, and the cavity wall all provide input for size- and shape-selective hydrogenation of alkenes [59]. 

Zeolites are crystalline aluminosilicate solid materials consisting of networks with pores of molecular dimensions. Because of the periodicity of the solid network and the nanometer size of the pores, zeolites have an unprecedented resolution (atomic size and shape selectivity) in a number of industrial applications, including catalysis, adsorption, and ion exchange. Currently, zeolites are also being explored as hosts for growth of nanowires, quantum dots, and sensors. 

A very good review article based on a panel study of status, future research needs, and opportunities for porous sorbent materials was published several years ago. It was pointed out that very significant advances have been made in tailoring the porosity of porous sorbent materials in terms of size and shape selectivity. Relatively little progress has been achieved in terms of chemoselectivity of sorbents based on specific interactions between adsorbate molecules and functional groups in the sorbents. Incorporation of active sites into sorbents is of high priority in the development of sorbents. 

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