Porosity of zeolites

The BET methodology must be very carefully applied in order to get correct results [2,4,5,20], The methodology is really valuable in cases were the sorbates do not penetrate in the primary porosity, and, subsequently, the adsorption process occurs only in the outer surface. Consequently, the BET equation is really valid for the surface area analysis of nonporous and mesoporous materials consisting of pores of wide pore diameters. However, this is not truly valid in the case of micropo-rous adsorbents [20], as long as the BET theory describes surface recovery, and adsorption in the primary porosity of zeolites is volume filling [2],... 

Finally, the mechanism of the process can be reduced to the dissolution of fly ash glass or ciystalline zeohte, in the case of natural pozzolana. There is an opinion that the pozzolanas composed of zeolites are more reactive than the vitreous ones [27], This can be related to the high porosity of zeolites and their ion exchange ability [41], Therefore the bonding of calcium ions occurs veiy quickly and pozzolana is transformed into the alumina-siUca gel. 

Ribeiro FR (1993) Adaptation of the porosity of zeolites for shape selective reactions. Cattil Lett 22 107-121 Rodriguez-Gonzalez L, Franke ME, Simon U (2005) Electrical detection of different amines with proton-conductive H-ZSM-5. Stud Surf Sci Catal 158 2049-2056... 

Any crystalline material displaying characteristics similar to those of zeolites can be described as a zeotype. These characteristics include having a similar strnctnre and the well-documented porosity of zeolites. 

For example, the observed trends of variations such as one between porosity and CEC and another between porosity and specific gravity are exhibited by Fig. 2.2a. It can be noticed that there is negligible change in specific gravity with increase in porosity of zeolites (viz., Analcime, Mordenite, Philipsite, Clinoptilolite, Erionite, Heulandite and Chabazite), whereas, the trend of variation in CEC is initially decreasing with increasing porosity up to 34 %. Beyond this, there is reversion in... 

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). 

On this basis the porosity and surface composition of a number of silicas and zeolites were varied systematically to maximize retention of the isothizolinone structures. For the sake of clarity, data is represented here for only four silicas (Table 1) and three zeolites (Table 2). Silicas 1 and 3 differ in their pore dimensions, these being ca. 20 A and 180 A respectively. Silicas 2 and 4, their counterparts, have been calcined to optimise the number and distribution of isolated silanol sites. Zeolites 1 and 2 are the Na- and H- forms of zeolite-Y respectively. Zeolite 3 is the H-Y zeolite after subjecting to steam calcination, thereby substantially increasing the proportion of Si Al in the structure. The minimum pore dimensions of these materials were around 15 A, selected on the basis that energy-minimized structures obtained by molecular modelling predict the widest dimension of the bulkiest biocide (OIT) to be ca. 13 A, thereby allowing entry to the pore network. 

It corresponds to the cobalt initially exchanged into the HMOR porosity. Nevertheless, a fraction of cobalt oxide - Co304 - is produced after calcination, as previously seen in the case of Cat I, on the surface of zeolite grains. 

Determination of zeolite closed porosity in (ID) channel systems (AFI and MTW types ). 

We have observed large variations in the sorption capacities of zeolite samples characterized by (ID) channel systems, as for instance AFI (AIPO4-5 zeolite) and MTW (ZSM-12 zeolite) architectural framework types. Indeed, for such unconnected micropore networks, point defects or chemisorbed impurities can annihilate a huge number of sorption sites. Detailed analysis, by neutron diffraction of the structural properties of the sorbed phase / host zeolite system, has pointed out clear evidence of closed porosity existence. Percentage of such an enclosed porosity has been determined. 

In view of catalytic potential applications, there is a need for a convenient means of characterization of the porosity of new catalyst materials in order to quickly target the potential industrial catalytic applications of the studied catalysts. The use of model test reactions is a characterization tool of first choice, since this method has been very successful with zeolites where it precisely reflects shape-selectivity effects imposed by the porous structure of tested materials. Adsorption of probe molecules is another attractive approach. Both types of approaches will be presented in this work. The methodology developed in this work on zeolites Beta, USY and silica-alumina may be appropriate for determination of accessible mesoporosity in other types of dealuminated zeolites as well as in hierarchical materials presenting combinations of various types of pores. 

Zeolite samples (NaY. Na-mordenite and Na-ZSM-5) were prepared in Research Institute for Petroleum and Hydrocarbon Gases in Bratislava. A mesoporous alumina, the carrier for reforming catalyst was used. Porosity of pure mesoporous alumina evaluated by t-plot method did not show the presence of micropores within the range of accuracy of 0.001 cm3/g. Mixtures of zeolites with mesoporous alumina were prepared on the base of dried samples in 5% steps. The prepared mixtures of alumina with zeolite were homogenized in vibration mill. 

The N2 adsorption-desorption isotherms of dried chitosan gel and chitosan-zeolite composites are reported in Figure 4 (a). Dried chitosan gels present a surface area lower than 5 m2 g"1 and virtually no porosity, the evaporation of water having brought about the coalescence of the polymer fibrils. The composites with a small amount of zeolites (less than 8 % for the zeolite X composite) present a type 4 isotherm leaning towards... 

Full catalyst formulations consist of zeolite, metal and a binder, which provides a matrix to contain the metal and zeolite, as well as allowing the composite to be shaped and have strength for handling. The catalyst particle shape, size and porosity can impact the diffusion properties. These can be important in facile reactions such as xylene isomerization, where diffusion of reactants and products may become rate-limiting. The binder properties and chemistry are also key features, as the binder may supply sites for metal clusters and affect coke formation during the process. The binders often used for these catalysts include alumina, silica and mixtures of other refractory oxides. 

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