Shape and Size Selection

The depletion interaction, as argued in Sect. 1.2.5, depends on the concentration of the depletion agent and the overlap volume of the depletion zones. For a given concentration of depletant the only variable is the overlap volume, which in turn depends on the size and (see Chap. 2) shape of the colloidal particles. Tuning the 

Strength of the depletion interaction therefore allows to separate particles of different size and shape. For example, the separation of rod-like particles and spheres under the influence of polymers is schematically indicated in Fig. 1.28. 

Unaware of the underlying principle this was first used by Cohen already in 1941 [87] to separate two viruses Tobacco Mosaic Virus and Tobacco Necrosis Virus. Tobacco Mosaic Virus is a rod-Uke vims with a length of 300 nm and diameter of 18 nm and Tobacco Necrosis Vims a spherical virus with a diameter of about 26 nm. Cohen used the polysaceharide heparin as depletant to separate these viruses. Recently, this method to separate eoUoids of different size and shape has gained new impetus. For nano-based teehnologies partieles with a specific size and shape are critical to optimize the nanostrueture-dependent optieal, electrical and magnetic properties. 

While the self-organisation of nearly monodisperse sphetieal eoUoidal particles has been smdied for a long time, the potential of self-assembly of anisometrie colloidal particles (rods and plates) is far from being achieved. Nevertheless important advances have been made. For example, CdSe semieonduetor nanorods have been shown to form nematie liquid erystals [279] that ean potentially be used as functional components in electro-optical devices. Henee a tool for the effeetive separation of anisometrie colloids from a mixture of partieles of different sizes and shapes is highly desirable. 

Recent studies have shown that depletion-induced shape and size seleetion of colloidal particles has the potential to be a powerful enabling method to aehieve this in an effective way. For instance. Park et al. [280] reported the depletion-induced shape and size selection of gold rods and cubes. In Fig. 1.29 we show their transmission electron microscopy (TEM) images of gold rods (length L = ll nm, diameter D = 11 nm) and cubes (size is 20 nm). In (a) the synthesized mixture is shown, (b) depicts the sediment concentrated in golds rods and (c) is an image of the supernatant enriched in cubes. 

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Figure 8.10 Shape and size selectivity of the imprinted Rh-dimer catalyst in alkene hydrogenation.

The selective intercalation of guests into solid hosts offers the potential for application in catalysis and separation science. An excellent case in point is zeolites, which exhibit shape and size selective inclusion properties and are used for an enormous variety of processes [44,45]. Additionally, a munber of layered materials have been reported to possess selective intercalation properties, including layered metal phosphonates [46,47], montmorUlonite [48], magnesium aluminum oxide [49], and layered double hydroxides [50-59]. 

Shape- and Size-Selective Hydrogenation of Alkenes on the Imprinted Rh Dimer Catalyst... 

Primary Shape Selectivity. There are several types of shape and size selectivity in zeolites. First, the reactant molecules may be too large to enter the cavities. A particularly good illustration of this behavior is given by Weisz and co-workers (5). Zeolites A and X were ion exchanged with calcium salts to create acid sites within the zeolite. These acid sites are formed as the water of hydration around the calcium ions hydrolyzes. When these zeolites are contacted with primary and secondary alcohols in the vapor phase, both alcohols dehydrate on CaX but only the primary one reacts on CaA. Since the secondary alcohol is too large to diffuse through the pores of CaA, it can not reach the active sites within the CaA crystals. This kind of selectivity is called reactant shape selectivity and is illustrated in Figure 3. 

We have been investigating these reactions from the standpoint of stereochemically controlling the reaction at the metal site by designing metalloporphyrins with a shape- and size-selective pocket at the metal center. The pockets designed so far are small, and thus... 

Metal nanoparticles housed in zeolites and aluminosilicates can be regarded as arrays of microelectrodes placed in a solid electrolyte having shape and size selectivity. Remarkably, the chemical and electrochemical reactivity of metal nanoparticles differ from those displayed by bulk metals and are modulated by the high ionic strength environment and shape and size restrictions imposed by the host framework. In the other extreme end of the existing possibilities, polymeric structures can be part of the porous materials from electropolymerization procedures as is the case of polyanilines incorporated to microporous materials. The electrochemistry of these types of materials, which will be termed, sensu lato, hybrid materials, will be discussed in Chapter 8. 

Porous materials continue to attract considerable attention because of their wide variety ot scientific and technological applications, such as catalysis, shape- and size-selective absorjition and adsorption, gas storage, and electrode materials. Roth research and applications of porous materials—via electroanalysis, electrosynthesis, sensing, fuel cells, capacitors, electro-optical devices, and other means—heavily rely on electrochemistry. 

Zeolites are crystalline, microporous aluminosilicates with molecular-sized intracrystalline channels and cages. Guest molecules with molecular diameters smaller than zeolites (from 3 to 15 A) can enter the interior of zeoUte crystals (intercalation) giving rise to shape and size selective sorption and, consequently, highly selective reactions. 

Shape and size selectivity are important when molecules approach the critical dimensions of the pore opening. In Table 4.13, the diameters of common hydrocarbons arc listed in order of size, and we expect to find the largest effect in zeolites with pores and cages close to these dimensions. 

There are three possible types of shape and size selectivity effects, as shown in Fig. 4.2S. First, the reactant molecules may be too large to enter the cavities. Comparison of Tables 4.11 and 4.13 shows that all of the molecules access faujasite structures. Only molecules larger than penta-methyl benzene are excluded. Early examples of shape and size selectivity were almost completely limited to small openings and normal versus branched paraffins. For example, n hexane was selectively cracked in the presence of 3-methylpentane over zeolite A catalysts. Other, more subtle, effects may occur. The diffusivity of frart5 butane-2 is 200 times larger than that of cis-butene-2 in zeolite CaA. By adding Pt to CaA, selective hydroge nation of fr[Pg.79]

FlBHrc 4J5. Shape and size selectivity effects to zeolites. 

Zeolites, with their vast possibilities of existing and potential structures, with shape and size selectivity, and with precise control of acid and other... 

Only the first and last are liquids. The stronger acids can catalyze some reactions that the weaker ones cannot (e.g., the alkylation of isobutane by 2-butene). As a rule of thumb, it is probably better to use the weakest acid that will do the job to avoid unwanted side reactions. Many solid acids, such as clays and molecular sieves, are shape- and size-selective, so that this also enters into the decision on... 

R. Cacciapaglia, A. Casnati, L. Mandolini, D. N. Reinhoudt, R. Salvio, A. Sartori, R. Ungaro, Calix[4]arene-based Zn complexes as shape and size-selective catalysts of ester cleavage, J. Org. Chem., 2005, 70, 5398-5402. 

If it can be shown that the photooxidation of hydrocarbons in zeolites is a general method, then the shape and size-selective properties of zeolites may potentially be used to control the selectivity of specific oxidation reactions (2,3). For example, ZSM-5 is an important shape-selective catalyst in many reactions, such as the disproportionation of toluene (4). Para-xylene is the dominant product because the transport of the other isomers, ortho- and meta-xylene, is restricted due to the pore size of ZSM-5. Thus, stereochemical aspects of selective photooxidation reactions may also be influenced by the zeolite and may be used to design environmentally benign processes for the synthesis of industrially useful molecules. 

The shape- and size-selective px)perties of zeolites have been of both fundamental and... 

By the reactions discussed in this manuscript, it is possible to build frameworks which contain aluminum in the presence of quite large numbers of oxygen atoms. For the larger crown ethers this method may allow the placement of more than one AlCl2 ion onto the ring. Cations of some size are envisioned. When these are coupled with aluminoxane anions such as [Aly05Me 5] [10], complexes which have a certain resemblance to zeolites can be foreseen. It remains in later contributions to show that these new compounds can be constructed with shape and size selectivity. 

The calix[5]arene 43, synthesized by Parisi et aL, for example, presents a marked shape and size selectivity even in ISEs for n-alkyl over iso-, sec-, and t rt-alkylammonium salts. It was proven that linear fz-alkyl ammonium salts form endo-complexes with the NH3+ head group interacting with the phenolic oxygen atoms and the alkyl residue with the calixarene cavity, while other branched alkyl ammonium salts generally form -complexes (see review for specific reference). 

Metalloporphyrins are widely and intensely investigated as models of enzymes such as catalase, peroxidases, and P450 cytochromes. The combination of metalloporphyrins as building blocks of porphyrinic MOFs renders them have the property to act as shape- and size-selective heterogeneous catalysts. To date, a number of porphyrinic MOFs were emerged as efficient catalysts in a range of reactions, especially in the oxidation of various... 

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