Micro- and Mesoporous Solids

Siliceous MEL-type (silicalite-2) zeolite nanocrystals with average particle size of less than 100 nm have been investigated by and Si MAS NMR.  

The dealumination of zeolite MCM-22 by calcination or hydrothermal treatment has been investigated by conventional multinuclear solid-state NMR and checked by ultrahigh-field NMR experiments (19.6 T) with a fast spin rate (19.1 kHz). The presence or variation of different species, sueh as silanol nest, 4-coordinated framework, 6-coordinated and 5-coordinated extraframework aluminium, during or after dealumination has been detected, and their 

Using Al MAS and Al MQ MAS NMR, dealumination of zeolite H- 3 has been observed at specific T-positions in the framework. Al MQ MAS NMR was able to resolve aluminium in the Ti and Tj positions from the other (T3-T9) positions in the framework. A quantitative analysis of the Al MQ MAS NMR spectra has shown that aluminium atoms in positions Ti and T3 resist dealumination and do not adopt an octahedral coordination. It has also been shown that a heat treatment of NH4-P gives a single type of fairly symmetric framework octahedral aluminium, which can be reconverted into framework tetrahedral aluminium by ammonia treatment. 

Highly crystalline microporous titano-alumino-silicate ETAS-10 materials with different framework aluminium contents (Al/Ti molar ratio 0.1-0.48) have been studied by Si MAS NMR.  

Epitaxial FAU zeolite films have been prepared on micrometresized EMT support crystals and have been characterized using Si MAS NMR.  

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More recently, bis(oxazoline)-metal complexes supported in micro- and mesoporous solids have been used as catalysts of hetero-Diels-Alder and ene reactions. 

Combination of usual characterization techniques (N2 and Hg adsorption, TEM), catalytic test and liquid-phase breakthrough technique is a useful methodology to give complete information on micro- and mesoporous solids. 

A good deal of information presented in this chapter has been adapted from textbooks and monographs. For further reading, all of them can be recommended. They contain enormous amounts of additional information and give excellent reviews not only on all topics related to catalysis on micro- and mesoporous solids but also on catalysis and on porous solids in general. 

This study consists in verifying the coherence of a few commonly used analysis methods of nitrogen adsorption-desorption isotherms. These methods were tested on model samples obtained by mechanically mixing two micro- and mesoporous solids respectively with known mass proportions. Although the individual analysis methods may lead to discrepancies in the interpretation of the isotherms, their systematic comparison allows drawing a coherent picture of the porous texture. 

The aim of this work is to test and to compare the performances of various nitrogen adsorption-desorption isotherms analysis methods. These models were applied to model samples obtained by mechanically mixing two micro- and mesoporous solids respectively in perfectly known proportions. The relevant morphological characteristics of the porous texture of the mixtures, such as the specific surface and volume, are physically additive. A criterion that allows determining the reliability of the analysis methods tested is thus to check the linearity of the relation between a given parameter and the weight percentage of the pure solids. 

Various examples of solid-state NMR applications are collected in the final Section 4. This section is divided into 13 subsections depending on the type of the material studied (4.1) organic solids (4.2) inclusion compounds (4.3) amino acids and peptides (4.4) proteins (4.5) pharmaceutical and biomedical applications (4.6) polymers (4.7) carbonaceous materials (4.8) organometallic and coordination compounds (4.9) glasses and amorphous solids (4.10) micro- and mesoporous solids (4.11) surface science and catalysis, and (4.12) inorganic and other related solids. 

The parent zeolites, MOR and BEA, show reversible type-I adsorption/desorption isotherm with a steep rise at pipe, < 0.01, typical for microporous solid while the recrystallized exhibit rather sharp steps at pipe, 0.35, corresponding to the existence of uniform mesopores (typical for MCM-41 phase). According to BJH calculation, the size of the mesopores was about 3.0 nm. The contribution of micro- and mesopores in recrystallized materials was adjusted by variation of alkalinity during recrystallization procedure [2] (Table 1). The formation of mesopores resulted in significant increase of pore volumes of the samples upon recrystallization. 

Gas adsorption (physisorption) is one of the most frequently used characterization methods for micro- and mesoporous materials. It provides information on the pore volume, the specific surface area, the pore size distribution, and heat of adsorption of a given material. The basic principle of the methods is simple interaction of molecules in a gas phase (adsorptive) with the surface of a sohd phase (adsorbent). Owing to van der Waals (London) forces, a film of adsorbed molecules (adsorbate) forms on the surface of the solid upon incremental increase of the partial pressure of the gas. The amount of gas molecules that are adsorbed by the solid is detected. This allows the analysis of surface and pore properties. Knowing the space occupied by one adsorbed molecule, Ag, and the number of gas molecules in the adsorbed layer next to the surface of the solid, (monolayer capacity of a given mass of adsorbent) allows for the calculation of the specific surface area, As, of the solid by simply multiplying the number of the adsorbed molecules per weight unit of solid with the space required by one gas molecule ... 

The unique properties of zeolites and other micro- or mesoporous solids that may favour their application to fine chemical synthesis are (1) the compatibility between the size and shape of their channels or cavities with the size of the reactants and/or products (generally referred to as molecular shape selectivity) that may direct the reaction away from the thermodynamically favoured route (2) the occurrence of confinement effects increasing the concentration of reactants near the catalytic sites and (3) the ability to tune their catalytic properties (acidic, basic, or other) via various treatments as described in this Volume. 

Figure 10 groups the parameters according to geometry, bulk defects, surface phenomena, and extrinsic modifications. The geometry of a catalyst particle is given by its size, its habitus (meaning the anisotropy or deviation from a spherical shape), and by its pore system. Only for micro-and mesoporous samples is XRD a sensitive tool to determine the pore architecture (Chen et al., 2005 Davidson, 2002 Li and Kim, 2005 Liu et al., 2002 Ohare et al., 1998). In many solids that are more compact than most catalysts, only secondary effects are related to the pores. 

Presented are the examinations of the multifimctional mineral-earbon and zeolite-carbon sorbents prepared from kaolinite with an admixture of carbonaceous materials industrial waste deposits, municipal sewage sludge and cellulose. The mixture of raw materials was thermally and hydrothermally pretreated in order to facilitate their specific structure. The parameters of capillary structure (micro and mesopores) were determined. For examinations of porous structure the mereury porosimetry method was used. In order to evaluate the solid phase transformation during the each step of sorbent preparation the SEM observation with quantitative X-ray mieroanalysis were made. 

In Table 2 the textural properties of all the composites heat-treated at 150°, 500°C and 850°C are presented. The sample designation is the same as that used for the raw materials with the addition of the letter m to indicate that the results refer to monolith composites. The total pore volume is the sum of the micro- and mesopore volumes (0-2 nm and 2-50 nm) calculated from the corresponding nitrogen adsorption/desorption isotherms, and the macroporosity (50 nm - 100 pm) determined from MIP, respectively. The threshold diameter was that at which in the MIP analysis there was a sudden upswing in the cumulative volume curve where a large part of the porous network became filled. This pore size can be considered as that which controls any transport phenomena through the solid sample. 

Amorphous silica-alumina materials represent an important class of porous inorganic solids which have not long-range order and usually have a wide distribution of the pore size, in the micro and mesopore region. They show outstanding catalytic behaviours in several acid catalysed reactions (5, 6). 

Micro- and mesoporous materials containing niobium in the framework or extra framework positions were studied in the oxidation of dibutyl sulphide with H2O2. Leaching of Nb from the solid to the liquid phase was considered. Some of the catalysts prepared via the impregnation with Nb-salts show some leaching of Nb to the liquid phase and the oxidation partially occurs homogeneously in the liquid phase. The reaction proceeds mainly on the catalyst surface when the mesoporous molecular sieves containing Nb in the framework are used. 

NANOSTRUCTURED SOLIDS MICRO- AND MESOPOROUS MATERIALS AND POLYMER NANOCOMPOSITES... 

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