Micropores of a zeolite

In order to reduce the reaction temperature of acid catalysed alkane conversion reactions one can reduce the temperature by replacing carbonium formation by a route via the carbenium ion by protonation of alkenes generated by metal-catalysed (group 8-10 metals, e.g. Pt, Pd) dehydrogenation of alkanes. The metals can be readily dispersed in the micropores of a zeolite. A lowering of the reaction temperature is especially useful for alkane isomerization. A low temperature favours the branched product and inhibits consecutive reactions. 

When a molecule adsorbs on the siliceous part of the micropore of a zeolite, the main interaction it experiences is a dispersive van der Waals-type interaction. This is due to the dominant interaction with the large polarizable oxygen atoms that make up the zeolite framework. For example, the interaction between a hydrocarbon CH3 or CH2 group and the siliceous framework typically results in an interaction energy that is on the order of 5-10 kJ/mol. These electrostatic interactions are small. 

The reactivity of a zeolite activated by ion exchange with a soft Lewis acid cation will be examined in detail by following C-H bond activation over a Zn + ion. We compare the results with those for the reactivity of the ZnOZn " " oxycation, often also formed in experimental systems during the ion exchange-reaction of Zn + into zeolites. As an introduction to Chapter 7 on biocatalysis, we will also discuss the hydrolysis of acetonitrile by a Zn + ion exchanged into the micropore of a zeolite. A comparison will be made with the reactivity of Ga+ and polarization effects due to a hard Lewis acid such as Mg " ". 

In addition to these statistical-mechanical effects, the medium often modifies the potential energy surface of reacting molecules. This is not only important in solvents, where it leads to solvation effects, but also in solids. When reactions occur in the narrow micropores of a zeolite, steric constraints of the zeolite channel will favor reaction paths that demand the least space. 

The traditional definition of a zeolite refers to microporous, crystalline, hydrated aluminosilicates with a tliree-dimensional framework consisting of comer-linked SiO or AlO tetrahedra, although today the definition is used in a much broader sense, comprising microporous crystalline solids containing a variety of elements as tetrahedral building units. The aluminosilicate-based zeolites are represented by the empirical fonmila... 

The micro-, meso- and macro-sized pores of a zeolite impact the catalyhc and separation properhes. Based on lUPAC terminology micropores are defined by pore sizes smaller than 2nm, mesopores are between 2 and 50 nm and pores greater than 50 nm are referred to as macropores. 

Zeolite Structures These are crystalline, microporous solids that contain cavities and channels of molecular dimensions (3 A to 10A) and sometimes are called molecular sieves. Zeolites are used principally in catalysis, separation, purification, and ion exchange The fundamental building block of a zeolite is a tetrahedron of four oxygen atoms surrounding a central silicon atom (i.e.. (Si04)4-). From the fundamental unit, numerous combinations of secondary building units (polygons) can be formed. The corners of these polyhedra may he Si or A1 atoms.2... 

The isomorphous substitution of T atoms by other elements produces novel hybrid atom molecular sieves with interesting properties. In the early 1980s, the synthesis of a zeolite material where titanium was included in the MFI framework of silicalite, that is, in the aluminum-free form of ZSM-5, was reported. The name given to the obtained material was titanium silicalite (TS-1) [27], This material was synthesized in a tetrapropylammonium hydroxide (TPAOH) system substantially free of metal cations. A material containing low levels (up to about 2.5 atom %) of titanium substituted into the tetrahedral positions of the MFI framework of silicalite was obtained [28], TS-1 has been shown to be a very good oxidation catalyst, mainly in combination with a peroxide, and is currently in commercial use. It is used in epoxidations and related reactions. TS-1, additionally an active and selective catalyst, is the first genuine Ti-containing microporous crystalline material. 

The catalysts used for cracking before the 1960s were amorphous [Si-Al] catalysts. The replacement of these catalysts by faujasite zeolites was a big step forward in the oil refining industry, which led to an increase in the production of gasoline [20], The acid catalyst, currently used in FCC units, is generally composed of 5-40 wt % of 1-5 pm crystals of the H-Y zeolite included in a porous particle composed of an active matrix, which in turn is composed of amorphous alumina, silica, or [Si-Al] and a binder. The porous particle allows the diffusion of the reactants and products of the cracking reaction to and from the micropores of the zeolite [10]. 

The major characteristic of a zeolitic material that distinguishes it from a non-zeolitic one is its microporous structure, due to the presence of interconnected channels. This implies that, whereas the interfacial energy is a negligible quantity for large crystallites of non-porous materials, this is no longer more the case for a microporous system. 

TTeterogeneous catalysts are usually high-area porous materials which may be amorphous or crystalline. An important aspect of all such materials is the rapidity with which reactant molecules reach active sites and products leave these sites. Apart from flow in gas or liquid phase, there may be surface migration into and from micropores, whether in amorphous catalysts or in crystalline ones, such as the zeolites. It is still an open question how important such migration processes are as ratecontrolling steps. However, it seems likely that active sites deep in a porous crystal will be less important than sites near the surface because many more unit diffusion steps will be needed to transport molecules to and from deeply buried sites. As corollaries, one would expect that only a limited volume fraction of a crystal of a zeolite such as sieve Y is catalytically effective, and that for best performance crystals in the catalyst support should be well exposed and as small as possible, in order to provide the largest surface-to-volume ratio. 

Adsorption plays an important role in permeation through microporous media. The selectivity of a zeolitic membrane at low temperatures is largely determined by differences in adsorption between species, as was shown in Section II. Moreover, the surface concentration, which is related to the partial pressure by the adsorption isotherm, plays an important role in the models for zeolitic diffusion. Finally, the thermodynamic factor from Eq. (17) is related to the adsorption isotherm. 

FIGURE 6.9 Microporous membrane structures (a) resulting from packing and sintering of ceramic nanoparticles and (b) ultramicroporous channels in the crystalline structure of a zeolite. 

Recent work in Versailles and Santa Barbara has led to the synthesis of several nanoporous nickel(II) phosphates. A zeolitic nickel(II) phosphate, VSB-1 (Versailles/Santa Barbara-1), was prepared under simple hydrothermal conditions [22] and has a unidimensional pore system delineated by 24 NiO and PO4 poly-hedra with a free diameter of approximately 0.9 nm (Figure 18.7). It becomes microporous on calcination in air at 350 °C, yielding BET surface areas up to 160 m g and is stable in air to approximately 500 °C. The surface area appears low compared with aluminosilicate zeolites, but the density of VSB-1 is twice that of a zeolite and the channel walls are particularly thick. VSB-1 can be prepared in both ammonium and potassium forms, and exhibits ion-exchange properties that lead, for example, to the formation of the lithium and sodium derivatives. Other cations (e.g. Mn, Fe, Co, and Zn) can be substituted for Ni in VSB-1, up to a level as high as 30 atomic%. The parent compound shows canted antiferromagnetic order at Tn = 10.5 K with 6 = —71 K on doping with Fe, Tn increases to 20 K and 6 decreases to —108 K. 

I. Kiricsi, S. Shimizu, Y. Kiyozumi, M. Toba, S. Niwa, and E Mizukami, Catalytic Activity of a Zeolite Disc Synthesized through Solid-state Reactions. Microporous Mesoporous Mater., 1998, 21, 453 159. 

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