Zeolites and Other Aluminosilicates

Dordmieux-Morin et al. (1991) using H MAS NMR spectroscopy showed that partly dehydrated silica-alumina and strongly dealuminated HY zeolite have similar properties with respect to surface hydroxyls. However, there is a fundamental difference between the dehydration processes of crystalline and amorphous samples due to the differences in the surface structure and OH group location. 

Beutel et al. (2001) studied the interactions of phenol with zeolite NaX using solid-state H and Si MAS and Si CP-MAS NMR spectroscopy. The H MAS NMR spectrum of NaX degassed at 400°C included peaks at 5h= 1.24,2.05,4.05, and 5.11 ppm. The signal at 2.05 ppm is characteristic of silanol protons at the external surface of the zeolite. On NaY zeolite (Si/Al=2.4), a peak at 5h= 1.6 ppm was found after pretreatment in air and in vacuum at 400°C and assigned to traces of water adsorbed on cations. Therefore, the peak at 8h=1.24 ppm was attributed to free hydroxyl protons of water molecules strongly adsorbed on Na+ ions. The peaks at great 8h values could be attributed to water bound to different active sites. 

Nuclear Magnetic Resonance Studies of Interfacial Phenomena 

FIGURE 2.94 MAS NMR spectra of titanoaluminosilicate ETAS-10 with different Al/Ti ratios (curves 1-5). (Adapted from Rocha and Anderson, Eur. J. Inorg. Chem., 801, 2000) and aluminosilicate zeolite ZK-5 (curve 6) at Si/Al = 2.2 and from J. Mol. Struct., 441, Nakata, S., Tanaka, Y., Asaoka, S., and Nakamura, M., Recent advances in applications of multinuclear solid-state NMR to heterogeneous catalysis and inorganic materials, 267-281,1998, Copyright 1998, with permission from Elsevier.) 

Kneller et al. (2003) studied dealuminated zeolites NaY and mordenite (MOR) characterized by Al and Si MAS NMR and temperature dependent Xe NMR. However, they did not use the Xe NMR spectra for quantitative structural characterization of the zeolites because very low xenon loadings were used to analyze the surface structure but not the PSD. 

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Pores of different shape are present in many solid materials zeolites and other aluminosilicates are outstanding examples. When the pore is very small there is no more distinction between superfieial and bulk molecules of the liquid. Nature (and human ingenuity) offers examples over the eomplete range of situations. For all the systems we have here introduced (quite important in prineiple and for practical reasons), there is no need of... 

Whilst solid acids such as zeolites and other aluminosilicates have been the subject of considerable study in the context of alkane chemistry (e.g. catalytic cracking and alkene isomerisation), the design of more versatile materials which can be used in the fine chemical industry has been less thoroughly researched. However, the need for the production of solid acids to replace traditional protonic acids such as hydrogen fluoride, phosphoric and sulfuric acids in liquid phase processes is an increasingly important goal. Some progress has been made in this area and the commercial product Envirocat EPIC [19] provides an excellent example of a... 

Cation emitters The alkali metal zeolites, and other alkali metal aluminosilicates, are efficient emitters of alkali metal cations. The cation emitters have been known for a much longer time than the anion emitters, but the anion emitters are better understood from a chemical perspective hence they are discussed here. Both types of emitters, however, can be scaled up in intensity readily to be used for the primary ion guns in static SIMS instruments. Ion beams of 50 pA to 1 nA focused to a 1-mm spot size are routinely produced by using these emitters. These emitters are primarily used in SIMS guns, as opposed to being used for isotope ratio analyses. 

Tab. 18.2. Some important applications of aluminosilicate zeolites and other nanoporous materials.

The singular porosity of MOFs allows for a significant redox conductivity that, in contrast with zeolites and other microporous aluminosilicates, can involve all units of the material. This is the case of Cu-- and ZrF -based MOFs with terephtalic acid in these materials, both metal centers and organic units are potentially electroactive in contact with suitable electrolytes. 

Beyond the relevance in this particular study, the peroxy-like defects can be reaction intermediates in other zeolites-based oxidations in industrial applications. On the basis of present results, it can be argued that more effective oxidizing media can be obtained by modifying zeolites and mesoporous aluminosilicates in order to allow an easier formation of peroxy-like structures. In this respect, the presented data may suggest a possible activation mechanism of the inert triplet state of dioxygen in the cavities of nitrite sodalite to the more reactive singlet O2 [26]. 

As examples of aluminosilicate materials, it is difficult to draw a clear demarcation between zeolites and other ionic materials. Zeolites are unusual in that the relevant catalysis appears to occur within the well-defined cage and channel structure of the material. Their catalytic activity appears primarily to arise because of their Bronsted acidity, while their control over selectivity arises primarily from geometrical considerations of which molecules (and transition states) can be accommodated within the structure. 

This section summarizes the most studied organic reactions catalyzed by zeolites and mesoporous aluminosilicates useful in the synthesis of flavoring compoxmds, in perfumery, and other industrial fields. Acetalization reactions of aldehydes/ketones and the intramolecular hydroalkylation of xmsatxxrated alcohols or tandem reactions such as Claisen rearrangement and subsequent intramolecxxlar hydroxyalkylation of olefins are some of the noted reactions. 

One of the major criticisms of MOFs with respect to their use as heterogeneous catalysts is their lower crystal stability compared with that of zeolites and other porous aluminosilicates [21]. In fact, it is well known that some of the first MOFs reported such as MOF-5 are notoriously instable and the crystal structure tends to collapse on storage under ambient conditions or during the reaction [22]. This lack of stability seems to be relatively common for Zn- and Cu-containing MOFs. For instance, Cu3(BTC)2 (BTC 1,3,5-benzenetricarboxylate) is unstable in the presence of thiols, probably due to the strong... 

Posttreatment fines migration is quite common in sandstone acidizing. It may be difficult to avoid in many cases. The reaction of HE with clays and other aluminosilicate minerals and quartz can release undissolved fines. Also, new fines may be generated as a result of partial reaction with high-surface-area minerals, particularly clays and certain zeolites, in which they more rarely occur. 

Molecular sieves are an adsorbent that is produced by the dehydration of naturally occurring or synthetic zeolites (crystalline alkali-metal aluminosilicates). The dehydration leaves inter-crystalline cavities into which normal paraffin molecules are selectively retained and other molecules are excluded. This process is used to remove normal paraffins from gasoline fuels for improved combustion. Molecular sieves are used to manufacture high-purity solvents. 

Most of the papers available on desilication are devoted to the alkaline treatment of ZSM-5 zeolites. Since a large number of zeolites consist of aluminosilicate frameworks and framework Al plays a crucial role in the mesoporosity development during desilication, this methodology should be suitable for extrapolation to other... 

A ground mixture of iron(III) nitrate and HZSM-5 zeolite, termed zeofen , has also been used both, in dichloromethane solution and in solid state under MW irradiation conditions [101]. It has been suggested that the zeolite aids the reproducibility of the reaction but any other aluminosilicate support would probably be equally effective. Recent studies point out attractive alternatives that do not employ any of the solid supports in such oxidations with nitrate salts [102]. 

Zeolites, which are aluminosilicates that can be regarded as being derived from AI2O3 and SiC>2, function as acidic catalysts in much the same way (Section 7.3). In addition, they catalyze isomerization, cracking, alkylation, and other organic reactions. A structurally related class of micro-porous materials based on aluminum phosphate (AIPO4) has also been developed (Section 7.7) like zeolites, they have cavities and channels at the molecular level and can function as shape-selective catalysts. 

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