Zeolites internal surface area

Zeolite lattices have a network of very small pores. The pore di uneter of nearly all of today s FCC zeolite is approximately 8.0 angstroms (°A). These small openings, with an internal surface area of roughly 600 square... 

There are four widely accepted theories of shape selectivity reactant shape selectivity (RSS), product shape selectivity (PSS), transition state selectivity (TSS) (Figure 12.2), and concentration effect all of them are based on the hypothesis that the reactions occur within the zeolite micropores only. As indicated earlier, this hypothesis is often verified, the external surface area of the commonly used zeolites being much lower (one to two orders of magnitude) than their internal surface area. ... 

Zeolite crystal size can be a critical performance parameter in case of reactions with intracrystalline diffusion limitations. Minimizing diffusion limitations is possible through use of nano-zeolites. However, it should be noted that, due to the high ratio of external to internal surface area nano-zeolites may enhance reactions that are catalyzed in the pore mouths relative to reactions for which the transition states are within the zeolite channels. A 1.0 (xm spherical zeolite crystal has an external surface area of approximately 3 m /g, no more than about 1% of the BET surface area typically measured for zeolites. However, if the crystal diameter were to be reduced to 0.1 (xm, then the external surface area becomes closer to about 10% of the BET surface area [41]. For example, the increased 1,2-DMCP 1,3-DMCP ratio observed with decreased crystallite size over bifunctional SAPO-11 catalyst during methylcyclohexane ring contraction was attributed to the increased role of the external surface in promoting non-shape selective reactions [65]. 

The windows to the channels thus form a three-dimensional sieve with mesh widths between about 300 and 1000 pm, thus the well-known name molecular sieve for these crystalline aluminosilicates. Zeolites thus have large internal surface areas and high sorption capacities for molecules small enough to pass through the window into the cavities. They can be used to separate mixtures such as straight-chain and branched-chain hydrocarbons. 

Zeolites possess an enormous internal surface and a system of pores and channels. Many relevant properties of zeolites such as acidity, composition, area of the internal surface, and geometry of the pore and channel system can be modified by dispersing oxides or salts on the zeolite internal surface. Such modification would certainly cause the catalytic behavior of the zeolite to alter. 

Since the mordenite type zeolite has two dimensional pore structure with nonintersecting parallel channels, the internal surface area of the catalyst may be easily blocked by the adsorption of reactants as well as by the deposition of deactivating agents. To confirm the speculation that the adsorbed reactants can block the pores, the change in surface... 

The reversible Type I isotherm (Type I isotherms are sometimes referred to as Langmuir isotherms, but this nomenclature is not recommended) is concave to the p/pa axis and na approaches a limiting value as p/p° — 1. Type I isotherms are given by microporous solids having relatively small external surfaces (e.g. activated carbons, molecular sieve zeolites and certain porous oxides), the limiting uptake being governed by the accessible microporc volume rather than by the internal surface area. 

It has been concluded that, for most cases, catalysis over zeolites occurs within the intracrystalline voids. Strong supporting evidence for this was provided by Weisz (71), who compared the rates of dehydration of 2-butanol over Linde lOX and 5A zeolites at relatively high temperatures and low conversion. The rate constant per unit volume of 5A was 1/lOO-l/lOOOth that for lOX, a magnitude consistent with the ratio of available surface areas for the external area of 1-5/x-sized 5A crystals and for lOX, where the internal surface area was available to the alcohol. The strong driving force for occlusion within the intracrystalline zeolite voids is exemplified by the rapid adsorption kinetics and rectangular adsorption isotherms observed for molecules whose dimensions are not close to those of the entry pores. 

Many reactions over zeolites don t seem to require strong acidity. They are probably utilizing the polarity of the crystal lattice, and taking advantage of the tremendous reactant-concentrating effect of the vast, membrane-like internal surface area of the zeolite. Perhaps we even can look at zeolites as rugged, primitive proto-enzymes. Of course, they... 

The physical meaning of any energetic correlation with either framework density or molar volume is not obvious. Some of the most important structural characteristics of zeolites are their high porosity and high internal surface area. They are nanomaterials... 

Figure 7. AHtran vs. internal surface area for a-quartz, a-ciistobalite, and seventeen zeolites using a probe-atom radius of 0.96 A. The solid regression line includes all 19 data points, while the dashed regression line excludes a-quartz and a-cristobalite. ( ) Piccione et al. () Navrotsky et al. ...

Molodetsky, 1, Navrotsky A, Lajavardi M, Brune A (1998) The energetics of cubic zirconia from solution calorimetry of yttria- and calcia-stabilized zirconia. Z Physik Chem 207 59-65 Molodetsky 1, Navrotsky A, Paskowitz MJ, Leppert VJ, Risbud SH (2000) Energetics of X-ray-amorphous zirconia and the role of surface energy in its formation. J Non-Crystalline Solids 262 106-113 Moloy EC, Davila LP, Shackelford JF, Navrotsky A (2001) High-silica zeolites a relationship between energetics and internal surface area. Microporous Mesoporous Materials (submitted)... 

Mass-transfer Effects. - Zeolites possess a large internal surface area and are necessarily subject to mass-transfer effects, although there have been relatively few studies of these. Swabb and Gates observed that for H-mordenite at low temperatures (155°C), rate was independent of crystallite size for methanol dehydration, but at higher temperature rate variation was consistent with a Thiele model. 

A schematic picture of different t5q)es of pores is given in Fig. 9.1 and of main types of pore shapes in Fig. 9.2. In single crystal zeolites the pore characteristics are an intrinsic property of the crystalline lattice [3] but in zeolite membranes other pore types also occur. As can be seen from Fig. 9.1, isolated pores and dead ends do not contribute to the permeation under steady conditions. With adsorbing gases, dead end pores can contribute however in transient measurements [1,2,3]. Dead ends do also contribute to the porosity as measured by adsorption techniques but do not contribute to the effective porosity in permeation. Pore shapes are channel-like or slit-shaped. Pore constrictions are important for flow resistance, especially when capillary condensation and surface diffusion phenomena occur in systems with a relatively large internal surface area. 

The catalytic importance of zeolites from an industrial standpoint resides both in the ability to subtly tailor their properties to described characteristics and in the consequent high activities and selectivities( l). These last two attributes are primarily a result of the large internal surface area of the zeolites and their microporosity, respectively. The aspect of tailoring zeolites to desired characteristics demands an intimate knowledge of both their structural and chemical properties. 

A defining feature of zeolites is their regular intracrystalline network of pores and chamiels of subnanometre dimensions that, depending on precise composition, result in internal surface areas of up to c. lOOOm /g. The general formula for a zeolite is ... 

The size and shape of the void spaces within the zeolite depends on the particular material selected. Figure 6.17 shows a representative structure of this type. This structure gives the material an enormous internal surface area (typically hundreds of square meters per gram), access to which is restricted by the size of the apertures between the pores (typically 0.3-0.8 nm in diameter). Zeolites have been coated onto surfaces in a number of ways. Probably the simplest approach... 

However it must be borne in mind that many examples of more complex surfaces are known. Clays and zeolites show huge internal surface area, with a variety of layers and pore sizes. The whole field of heterogeneous catalysis depends on nano-structured surfaces of massive area, which certainly can not be thought of as planar. Grain boundaries may be thought of as internal surfaces or homo-interfaces [27] and are perhaps similar to the solid-liquid interface during melting. 

Activated charcoal is a finely divided form of amorphous carbon and is manufactured from organic materials (e.g. peat, wood) by heating in the presence of reagents that promote both oxidation and dehydration. Activated charcoal possesses a pore structure with a large internal surface area microporous materials exhibit pores <2nm wide, macroporous refers to activated charcoals with a pore size >50nm, and mesoporous materials fall in between these extremes. The largest internal surface areas are found for microporous materials (>700m g ). The ability of the hydrophobic surface to adsorb small molecules is the key to the widespread applications of activated charcoal. (Comparisons should be made with the porous structures and applications of zeolites see Sections 13.9 and 26.6.)... 

Most heterogeneous catalysts exist in the form of microporous solids. The catalysts are usually produced in the shape of spheres, cylinders, or monoliths, such as those shown in Figure 1. The internal surface area is typically 10-10 m /g. Catalysis occurs either on the surface of the microporous solid, as in the case of zeolites, or on the surface of microdomains of active material dispersed inside the microporous solid, as in supported metals, oxides, sulfides, etc. In either case, the high internal surface area of the microporous solid is used to obtain a high concentration per unit volume of catalytically active centers. 

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