Zeolites literature analysis

The main calorimetric studies on adsorption of water and ammonia on TS-1 and silicalite-1 have been reported by Bobs et al. [64,83,84,86], while other contributions came from the Auroux group [92] and Janchen et al. [93]. Cor-ma s group has investigated the interaction of water on zeolite [39]. The most important conclusion from the available literature is that calorimetric data require a very careful analysis, as probe molecules interact both with the silanols of the internal hydroxyl nests (see Sect. 3.8) and with Ti(lV) species. 

The analysis of the literature data shows that zeolites modified with nobel metals are among perspective catalysts for this process. The main drawbacks related to these catalysts are rather low efficiency and selectivity. The low efficiency is connected with intracrystalline diffusion limitations in zeolitic porous system. Thus, the effectiveness factor for transformation of n-alkanes over mordenite calculated basing on Thiele model pointed that only 30% of zeolitic pore system are involved in the catalytic reaction [1], On the other hand, lower selectivity in the case of longer alkanes is due to their easier cracking in comparison to shorter alkanes. 

The tetraethylammonium-Beta (TEA-3) zeolites used in this work have been synthesized following the procedure described in the literature (5). Samples with Si/Al ratios between 7 and 106 (as measured by chemical analysis) and crystallite sizes in the range of 0.2-0.9 ym (as measured by scanning microscopy) were obtained. The H-form of these zeolites was prepared in the following way the TEA-3 samples were heated at 550 C for 3 hours by slowly increasing the calcination temperature (5°C min l), with one-hour intermediate steps at 350 and 450 C. After this treatment all TEA molecules had been removed from the zeolite (as monitored by IR spectroscopy). In a second step, the zeolite was exchanged with 1 M ammonium acetate solution and then heated at 550°C for 3 hours as described. 

CATALYST PREPARATION ZSM-5 was prepared according to procedures discussed in the literature (8.9) and identified as such by X-ray diffraction analysis. Ultrastable Y zeolite was obtained from the... 

Table II gives the unit cell composition of the 10 of our as synthesized samples for which an accurate analysis was possible. Also are included and adapted 3 other Nu-10 samples for which an analysis was available in the literature (samples 14, 15, 18). We also included the composition of silica-ZSM-22, another zeolite possessing the same framework topology as Nu-10 (12), that was synthesized in presence of diethylamine (DA) under veiy particular conditions (12).

A moment analysis of Si MAS spectra of zeolites is shown to provide direct information on the number of second-nearest-neighbor Al-Al pairs. Monte Carlo computer calculations are described of randomized A1 distributions in zeolite frameworks, under restrictions of Loewenstein s and Dempsey s rules. The method is applied to a hypothetical square planar lattice which allows the various A1 distribution patterns to be visualized in simple displays, and to the zeolite X and Y framework. The results are compared with experimental data taken from the literature. 

The number of studies on TS-1, and also on other Ti-zeolites, rose exponentially since the first one was published in 1986 and a vast literature now exists [4], A few review articles deal with the synthesis, the characterization and, to a lesser extent, with the catalysis of such materials [5-9]. It is the aim of this chapter to focus on the last issue, providing a comprehensive picture of the variety of oxidations performed, a critical analysis of the problems and solutions envisaged in the use of hydrogen peroxide and an in-depth discussion of mechanistic aspects. 

For microporous materials there is considerable debate on how to interpret surface area data due to the presence of micropores. Usually BET data are reported when one purchases a zeolite from a commercial vendor. The specific value of the BET measurement may give some indication of the relative surface area of the zeolite. Several experimental methods have been developed to collect BET data and many different equations have been used to model adsorption data. There are several discussions in the literature on various methods that have been used to analyze adsorption data by Broekhoff, et al.2 and Masthan et al. There is considerable debate on which method is the best for measuring BET data, however, there is good consensus that pore size distributions can be measured accurately with little problem in data analysis. 

It has been shown that the relatively broad band around 1910 cm, shown in literature (10) for NO adsorbed on Cu zeolites of various structures, consists from several individual bands as indicated by the spectra analysis, described above and depicted in Fig. 3. When NO adsorption is employed for the Cu ion siting characterization, it is necessary to prevent partial reduction of the readily reducible Cu sites. The reduced Cu ions are easily detected in the spectrum as Cu" nitrosyl complexes (Fig. 4B, bands at 1835, 1811, and 1775 cm for Cu-ZSM-5). If no Cu nitrosyl complexes are detected, the spectra correspond to Cu -NO mononitrosyl complexes of all the accessible Cu sites. The integrated intensity of the individual IR bands of Cu -NO can be used for the semiquantitative evaluation of the individual Cu sites, when no bands originate from the mutual interaction of adsorbed NO species, and under assumption that the extinction coefficients of all the adsorbed NO are similar. However, there is a problem of the hidden Cu sites, which cannot be solved by the specific NO adsorption. 

The results of the analysis agree well with the literature studies (Park et al, 2000) the product recovery increases as the zeolite fraction increase while purity passes through a maximum. In addition, it is noticed that this effect is more important at lower pressures. 

In the preceding chapter it had already been discussed that it is less the synthesis itself which may be the bottleneck in high-throughput zeolite science but rather the analysis of the solids formed in a high-throughput program. There are several standard characterization techniques which are typically employed to characterize zeolitic materials. These include powder XRD for phase identification, X-ray fluorescence analysis (XRF) or atomic absorption spectrometry to analyze elemental composition, sorption analysis to study the pore system, IR-speclroscopy, typically using adsorbed probe molecules to characterize the acid sites, NMR spectroscopy and many others. For some of these techniques parallelized solutions have been developed and described in the literature, other properties are more difficult to assess in a parallelized or even a fast sequential fashion. 

Molecular sieves belonging to alumino phosphate with an AEL structure and its derivatives were prepared by hydrothermal synthesis as per the literature [10]. Composition of these samples was established by wet chemical analysis methods by using atomic absorption (Hitachi, Japan ) and inductively coupled plasma spectroscopy ( Jubin Yuan, France ). Different degrees of alkali exchanged Y type zeolites were also prepared from Linde NaY by the ion-exchange procedure. Characterization of the catalysts and the experiments are carried out as discussed in the publication [11]. 

L Acidic properties are most important to obtain high activity and yield as, for example, dealuminated H-Faiyasite (Si/Al=15) and H-beta (Si/Al=25). Such a behaviour is in dose agreement with the literature concerning Hie increase in the acidity of zeolites through partial dealumination, i.e. the number of protonic species is decreased but their strength is reinforced. Characterization of BrOnsted and Lewis acidity of zeolites is relatively well documented (13), particularly in the gas phsise where it is possible to differentiate between the two types of acids from e analysis of products formed as, for example, in deamination of sec-butylamine... 

Samples of tridimensional microporous zinc phosphates with fully connected framework with structures type FAU and CZP have been prepared. The influence of some synthesis parameters on the nature of the crystalline phases obtained, such as crystallization time and temperature, concentration of the different reactives or pH, are discussed. Physicochemical characterization of the pure samples has been performed by different techniques ICP, XRD, C and P MAS-NMR, TG, TG-MS, in-situ thermal XRD analysis and SEM. These materials were tested in the Knoevenagel condensation of different esters and benzaldehyde. They have demonstrated a good selectivity and a higher activity than the basic zeolites previously described in the literature with these reactions. 

Zeolite dehydration, its reversibility, water substitution for other molecules, having dimensions not larger than the characteristic pore openings of the material, interactions between host structure and guest molecule, surface properties (acidity and adsorption capability), the occurrence of solid-solid phase transitions, thermal stability, and so on, are all research subjects in which thermal analysis can make an invaluable contribution. Thermal analysis has been used extensively since its appearance to determine some of the peculiar properties of the zeolites, so that several hundred papers on this subject are present in the technical literature. 

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