Microporous Materials and Zeolites

The mesoscopic domain of real catalysts is mostly covered by the typical catalysis periodicals, such as Applied Catalysis, the Journal of Catalysis, Catalysis Letters, Topics in Catalysis, Catalysis Today, Microporous Materials and Zeolites, although occasionally articles also appear in Journal of Physical Chemistry and Physical Chemistry-Chemical Physics, and many others. 

The mechanisms of diffusion in these two systems (gas and liquid) are different and unrelated diffusion in gases is the result of the collision process, whereas that in liquids is an activated process (Bird et al., 1960). Diffusion in microp-orous materials is neither gaseous nor liquid diffusion. The closest case for such diffusion is surface diffusion, where molecules hop within the surface force field (see review by Kapoor et al., 1989b). Pick s law is used for both application (in modeling of adsorption processes) and experimental measurement of diffusion. Extensive reviews are available on diffusion in microporous materials and zeolites (Karger and Ruthven, 1992 Do, 1998). A lucid discussion on the nonlinear, and in some cases peculiar, phenomena in zeolite diffusion was given... 

The physical and chemical activation processes have been generally employed to prepare the porous carbons.18"35 However, the pore structures are not easily controlled by the activation processes and the size of the pores generated by the activation processes is limited to the micropore range only. Recently, much attention has been paid to the synthesis of meso/macroporous carbons with various pore structures and pore size distributions (PSD) by using various types of such inorganic templates as silica materials and zeolites.17,36 55... 

Shape-selective effects may occur whenever the pore size of a microporous catalyst is in the same range as the diameter of the molecules or transition states involved in the reacting system. Common microporous materials are zeolites and related materials (aluminophospates, pillared clays, etc.) which possess a regular crystal lattice together with a well defined pore size. According to Weisz [111] and Csicsery [27], shape-selective effects may be classified into three types (Fig. 25). 

Further variation of the stmctural and catalytic properties of four-coimected tetrahedral frameworks is obtained by the substitution of silicon or metal cations,giving materials known as SAPO s and MeAPO s, respectively. More than twenty metal aluminophosphate frameworks have been identified with Mg, Mn, Fe, Co, or Zn substituents. These give the possibility of framework redox activity (e.g. Fe +/Fe +) in catalysis as well as the usual Bronsted acidity. For further information about zeolitic and microporous phosphate frameworks see Porous Inorganic Materials and Zeolites) and recent reviews. ... 

Phosphates having these types of open structures can act as shape-selective acid catalysts, for example, for the cracking and isomerization of hydrocarbons. For examples of lamellar materials, see Section 5.3 and see Intercalation Chemistry). Microporous catalysts are described above and in (see Porous Inorganic Materials and Zeolites). Mesoporous AlPO materials have larger pores within a matrix of amorphous A1P04. ... 

In the context of microporous and mesoporous materials, lUPAC has provided a variety of recommendations for nomenclature and characterization of porous materals, that can be found in the literature. Microporosity should not be based on structural data but on adsorption data. Sorption by materials that show Type 1 isotherms is an indication of a microporous material. Pore size distributions less than 20 A are related to microporous materials like zeolites. Materials having pores between 20 A and 500 A are refered to as mesoporous materials. Materials that have pores larger than 500 A are refered to as macroporous. 

In a word, ion-exchange with the assistance of microwaves is feasible, convenient, and fast. It can reach a higher exchange degree than can traditional methods and make the inaccessible ions in traditional methods exchangeable. This method is especially appropriate for the laboratory preparation of ion-exchanged zeolite molecular sieves. The microwave technique is very successful in the synthesis of microporous crystals, modification of the properties of zeolites, secondary synthesis of microporous materials, and the preparation of ultra-fine particles and films, and has attracted the wide interest of chemists in the field of molecular sieves. 

These experiments confirmed that IR spectroscopy indeed provides a promising means for the investigation of the uptake of binary mixtures into microporous materials and its kinetics. This successful experiment prompted us to start a systematic study on the adsorption and adsorption kinetics of some aromatics in zeolites by Fourier transform IR spectroscopy (FTIR) and FTIR microscopy or, more precisely, IR micro -spectroscopy. This topic is dealt with in Sect. 2, where the generally employed apparatuses and procedures are also described. In this context, it should be mentioned that micro-FTIR spectroscopy was also applied by Schiith and coworkers in studying the adsorption of guest molecules in microporous soHds [ 12-14]. 

However, the equation is invalid, for instance, for microporous materials and combinations of micro-, meso-, and macropores, such as zeolites, activated charcoals, and several others. Therefore, anew standard isotherm was proposed, known as f-plot. In this case the adsorption isotherms can be represented by a single curve. 

With the exception of direct condensation of metal vapors in micropores, the preparation of metal clusters involves at least two steps, viz., the loading of a metal precursor in the microporous material and the decomposition or reduction of the precursor yielding metal clusters. These two steps are highly interdependent and the second is often crucial for governing the final metal dispersion. The different strategies of metal cluster preparation will be examined in this section, and detailed examples of preparations for specific zeolites and metals will be given in Sect. 3. 

Numerical values for solid diffusivities D,j in adsorbents are sparse and disperse. Moreover, they may be strongly dependent on the adsorbed phase concentration of solute. Hence, locally conducted experiments and interpretation must be used to a great extent. Summaries of available data for surface diffusivities in activated carbon and other adsorbent materials and for micropore diffusivities in zeolites are given in Ruthven, Yang, Suzuki, and Karger and Ruthven (gen. refs.). 

Volume 84 Zeolites and Related Microporous Materials State of the Art 1994. 

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