Pores in zeolites

The pores in zeolite-carbon adsorbents exhibited dual hydrophobic-hydrophilic character. 

Because of the small pores in zeolitic catalysts, reaction rates may be controlled by rates of diffusion of reactants and products. Beecher, Voorhies, and Eberly (4) studied hydrocracking of mixtures of n-decane and Decalin with mordenite catalysts impregnated with palladium. They found that acid leaching of the mordenite produces an aluminum-deficient structure of significantly higher catalytic activity. At least part of this improvement appears to be caused by the decrease in diffusional resistance. They observed that with this type of catalyst, the effective catalyst pore diameter appears to be smaller than calculated owing to the strong interaction or adsorption of hydrocarbon molecules on the pore walls. 

Pillaring is another intercalation reaction that enables synthesis of metastable oxide material [38], Pillaring refers to intercalation of robust, thermally stable, molecular species that prop the layers apart and convert the two-dimensional interlayer space into micropores of molecular dimensions, similar to the pores in zeolites. Smectite clays [38], layered a-Zr(HPO )j, a-MoOj [39], perovskites [40] and double hydroxides (LDHs) [41] have all been pillared by cationic/anionic species such as alkylammonium ions, polyoxocations (e.g. Alj304(0H)24(H20)j2 " ) and isopoly and heteropolyanions (e.g. and PVjWgO / ). 

Finally, catalysis is not confined to well-defined. Miller-indexed metal surfaces. One area of recent interest is in the use of certain minerals to catalyze reactions. Some aluminosilicates—minerals with mixed alumina and silica structures—have pores in which molecules can enter and react catalytically [Figure 22.25(a)]. One type of aluminosilicate is called zeolite, shown in Figure 22.25(b). Thanks to the pores in zeolites (which can vary in size and geometry depending on the exact type of zeolite), the properly sized reactant molecules can enter the structure and a particular reaction can be promoted. In fact, it is thought that such minerals are the future of designed catalysts that can be used to promote a preferred chemical reaction—if the pore size is just right. 

FORMATION OF SECONDARY PORES IN ZEOLITES DURING DEALUMINATION INFLUENCE OF THE CRYSTALLOGRAPHIC STRUCTURE AND OF THE Si/Al RATIO... 

The combined use of nitrogen adsorption and CTEM analysis leads to a coherent description of the formation of secondary pores in zeolites during deaiumination by classical techniques (steam and acid leaching treatments). 

In zeolite synthesis (ref. 2) an aqueous mixture containing a silicon source, an aluminum source, an alkali source (usually NaOH) is autoclaved and subjected to hydrothermal treatment. Hydrated Na-ions are then filling the pore system in the as-synthesized zeolite. In the case of relatively high Si/Al zeolites an organic template is required which is usually a tetraalkylammonium compound, applied as the bromide or the hydroxide. 

The importance of quats as structure-directing agents in zeolite synthesis was recently underlined (ref. 5) by the synthesis of the new zeolites SSZ-26 and -33, which combine 10- and 12-ring pores. The templates applied are shown in Figure 3. 

In the above work quats are occluded in zeolite lattices in a molecular way. In the recently disclosed (ref. 8) superwide pore M41S-materials quats arranged in the... 

Many chemical reactions, especially those involving the combination of two molecules, pass through bulky transition states on their way from reactants to products. Carrying out such reactions in the confines of the small tubular pores of zeolites can markedly influence their reaction pathways. This is called transition-state selectivity. Transition-state selectivity is the critical phenomenon in the enhanced selectivity observed for ZSM-5 catalysts in xylene isomerization, a process practiced commercially on a large scale. 

Fig. 7 Methods for ship-in-bottle synthesis of (salen)Mn complexes inside the pores of zeolites...

MOFs can be considered as organic zeolite analogs, as their pore architectures are often reminiscent of those of zeolites a comparison of the physical properties of a series of MOFs and of zeolite NaY has been provided in Table 4.1. Although such coordinative bonds are obviously weaker than the strong covalent Si-O and Al-O bonds in zeolites, the stability of MOF lattices is remarkable, especially when their mainly organic composition is taken into account. Thermal decomposition generally does not start at temperatures below 300 °C [3, 21], and, in some cases. 

The exact nature of the zeolite is determined by the reaction conditions, the silica to alumina ratio and the base used. For example zeolite /3, a class of zeolites with relatively large pores, in the range of 0.7 nm, of which mordenite is an example, are usually made using tetraethylammonium hydroxide as the base. This acts as a template for the formation of 12-membered ring apertures (Figure 4.3). 

The stability of catalyst is one of the most important criteria to evaluate its quality. The influence of time on stream on the conversion of n-heptane at SSO C is shown in Fig. 5. The conversion of n-heptane decreases faster on HYl than on FIYs with time, so the question is Could the formation of coke on the catalyst inhibit diffusion of reactant into the caves and pores of zeolite and decrease the conversion According to Hollander [8], coke was mainly formed at the beginning of the reaction, and the reaction time did not affect the yield of coke. Hence, this decrease might be caused by some impurities introduced during the catalyst synthesis. These impurities could be sintered and cover active sites to make the conversion of n-heptane on HYl decrease faster. 

The pore size of most zeolites is <1.5nm. This microporosity limits their utility in most areas of chemistry, where the molecules used are much larger, and for which mesoporous materials would be necessary. Unfortunately, attempts to use larger template molecules in the zeolite synthesis, an approach which should in theory lead to larger pore size zeolites, have met with very little success. Indeed, some zeolitic materials have been prepared which have mesopores - none of these has ever displayed any real stability and most collapse on attempts to use them. A new methodology was thus required. 

Figure 4.7. Possibilities for the synthesis of Vitamin K3. The small pore titaninm zeolite TS-1 cannot fit the large naphthalene molecule into its pore system, and thus is effective in this transformation. The larger titanium MTS material is capable of interacting with the molecule, and the desired transformation can take place.

As surface area and pore structure are properties of key importance for any catalyst or support material, we will first describe how these properties can be measured. First, it is useful to draw a clear borderline between roughness and porosity. If most features on a surface are deeper than they are wide, then we call the surface porous (Fig. 5.16). Although it is convenient to think about pores in terms of hollow cylinders, one should realize that pores may have all kinds of shapes. The pore system of zeolites consists of microporous channels and cages, whereas the pores of a silica gel support are formed by the interstices between spheres. Alumina and carbon black, on the other hand, have platelet structures, resulting in slit-shaped pores. All support materials may contain micro, meso and macropores (see text box for definitions). 

Figure 5.31 illustrates ho v a zeolite can influence the selectivity of catalytic reactions. In the first case, one of the reactants is excluded because it cannot enter the zeolite. In the second, A reacts to give two produces, B and C, but C is too large to leave the pore. In the third case, the onward reaction of B to C is prohibited, e.g. because the transition state for this step does not fit. 

Zeolites have led to a new phenomenon in heterogeneous catalysis, shape selectivity. It has two aspects (a) formation of an otherwise possible product is blocked because it cannot fit into the pores, and (b) formation of the product is blocked not by (a) but because the transition state in the bimolecular process leading to it cannot fit into the pores. For example, (a) is involved in zeolite catalyzed reactions which favor a para-disubstituted benzene over the ortho and meso. The low rate of deactivation observed in some reactions of hydrocarbons on some zeoUtes has been ascribed to (b) inhibition of bimolecular steps forming coke. 

Zeolites have ordered micropores smaller than 2nm in diameter and are widely used as catalysts and supports in many practical reactions. Some zeolites have solid acidity and show shape-selectivity, which gives crucial effects in the processes of oil refining and petrochemistry. Metal nanoclusters and complexes can be synthesized in zeolites by the ship-in-a-bottle technique (Figure 1) [1,2], and the composite materials have also been applied to catalytic reactions. However, the decline of catalytic activity was often observed due to the diffusion-limitation of substrates or products in the micropores of zeolites. To overcome this drawback, newly developed mesoporous silicas such as FSM-16 [3,4], MCM-41 [5], and SBA-15 [6] have been used as catalyst supports, because they have large pores (2-10 nm) and high surface area (500-1000 m g ) [7,8]. The internal surface of the channels accounts for more than 90% of the surface area of mesoporous silicas. With the help of the new incredible materials, template synthesis of metal nanoclusters inside mesoporous channels is achieved and the nanoclusters give stupendous performances in various applications [9]. In this chapter, nanoclusters include nanoparticles and nanowires, and we focus on the synthesis and catalytic application of noble-metal nanoclusters in mesoporous silicas. 

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