Crystalline zeolite

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. 

Acid-treated clays were the first catalysts used in catalytic cracking processes, but have been replaced by synthetic amorphous silica-alumina, which is more active and stable. Incorporating zeolites (crystalline alumina-silica) with the silica/alumina catalyst improves selectivity towards aromatics. These catalysts have both Fewis and Bronsted acid sites that promote carbonium ion formation. An important structural feature of zeolites is the presence of holes in the crystal lattice, which are formed by the silica-alumina tetrahedra. Each tetrahedron is made of four oxygen anions with either an aluminum or a silicon cation in the center. Each oxygen anion with a -2 oxidation state is shared between either two silicon, two aluminum, or an aluminum and a silicon cation. 

This paper is concerned with the synthesis of Y zeolite with Si02/Al203 ratio of 4.5 from kaolin taken in Yen Bai-Vietnam and their catal3dic activity for the cracking of n-heptane. The synthesized sample (NaYl) showed the Y zeolite crystallinity of 53% and PI zeolite crystallinity of 32%, and exhibited good thermal stability up to 880 C. The activity and the stability of HYl turned out to be lower than those of standard sample (HYs), but the toluene selectivity was higher. The conversion of n-heptane to toluene might be due to the metal oxide impurities, which was present in the raw materials and this indicates the potential application of this zeolite for the conversion of n-parafRn to aromatics. 

Nowadays synthesis of mesoporous materials with zeolite character has been suggested to overcome the problems of week catalytic activity and poor hydrothermal stability of highly silicious materials. So different approaches for the synthesis of this new generation of bimodal porous materials have been described in the literature like dealumination [4] or desilication [5], use of various carbon forms as templates like carbon black, carbon aerosols, mesoporous carbon or carbon replicas [6] have been applied. These mesoporous zeolites potentially improve the efficiency of zeolitic catalysis via increase in external surface area, accessibility of large molecules due to the mesoporosity and hydrothermal stability due to zeolitic crystalline walls. During past few years various research groups emphasized the importance of the synthesis of siliceous materials with micro- and mesoporosity [7-9]. Microwave synthesis had... 

Reaction with chelating agents. Such reactions have been used primarily for partial dealumination of Y zeolites. In 1968, Kerr (8,21) reported the preparation of aluminum-deficient Y zeolites by extraction of aluminum from the framework with EDTA. Using this method, up to about 50 percent of the aluminum atoms was removed from the zeolite in the form of a water soluble chelate, without any appreciable loss in zeolite crystallinity. Later work (22) has shown that about 80 percent of framework aluminum can be removed with EDTA, while the zeolite maintains about 60 to 70 percent of its initial crystallinity. Beaumont and Barthomeuf (23-25) used acetylacetone and several amino-acid-derived chelating agents for the extraction of aluminum from Y zeolites. Dealumination of Y zeolites with tartaric acid has also been reported (26). A mechanism for the removal of framework aluminum by EDTA has been proposed by Kerr (8). It involves the hydrolysis of Si-O-Al bonds, similar to the scheme in Figure 1A, followed by formation of a soluble chelate between cationic, non-framework aluminum and EDTA. 

This approach is commonly employed for bound zeolites. In this manner the relative amount of crystalline zeolite present can be determined. As usually implemented, the method provides a number that is the ratio of intensities for peaks in the diffraction pattern of the sample of interest to the intensity of the same peaks in the pattern of a reference zeolite. Since the intensity ratio is often expressed as a percentage, it is commonly referred to as percent zeolite crystallinity. The better terminology is relative amount of crystalline zeolite compared to a specific reference. 

The vapor adsorption of Mo(CO),5 in an EMT zeolite followed by nitration with ammonia under a thermal treatment proved to be an appropriate method to introduce molybdenum oxynitrides into the zeolite, preserving the zeolite crystallinity and giving a homogeneous distribution of the molybdenum species in the zeolite [35]. 

In a series of experimental runs on virgin commercial catalysts and sieves then available and some of our experimental catalysts, we quickly learned that a catalyst impregnated with vanadium, and subjected to high temperatures in steam and air deactivated rapidly. Vanadium, especially in the +5 valence state, rapidly deactivated a catalyst by destroying zeolite crystallinity (Figure 17). In the presence of sodium, the deactivation rate of vanadium was even more severe. (lA-17)... 

The present study shows that only the youngest, most active fractions are density-separated on the basis of activity for coking from isobutene. For the majority of the separated fractions, the gradual increase in skeletal density with increasing age (Ni level) appears to be associated with the gradual loss of zeolite crystallinity that occurs with increasing time in the unit. 

The heavier portion of the fresh and laboratory steamed catalyst (Float B) exhibits a decreased micropore volume with respect to the lighter portion (Float A), suggesting that the density fractionation within each catalyst reflects the presence of a finite range of zeolite crystallinities. Particularly for the fresh catalyst, this inhomogeneity is small (Table VIII). 

Loss of micropore area/zeolite crystallinity is a slow process in comparison with dealumination. Following an initial decline in micropore area of 17% upon catalyst addition (Tables IV, VI), zeolite micropore area/crystallinity decreases by only an additional 3% over the next 26 days (Fraction B, Table IV,... 

Catalyst Activity and Selectivity Losses Follow the Loss in Zeolite Crystallinity... 

Zeolites (crystalline aluminosilicates) supporting oxidizing metals (Fe or Cu) have been introduced by Garcia and co-workers [106]. In particular, they showed that FeMCM-41 affords binaphthol from 2-naphthol as virtually the only product. Unfortunately, this compound is trapped in the zeolite pores and the isolated yields are lower than expected. 

A new type of mesoporous silica has been prepared via a dual-template, thiee-step hydrothermal-flocculation-steaming (HFS) synthesis procedure. This material showed 780 mVg of BET surface area and 0.6 ml/g of primaiy mesopores narrowly distributed around 4.2 nm. More importantly however, is that it showed short-range MFI zeolite crystallinity as demonstrated by FTIR and XRD analysis, and hydrophobicity as demonstrated by water and n-hexane adsorption. 

L. Moscou Generally, weakly acidic ion exchange resins have been used in order to prevent loss of zeolite crystallinity. This method enables one to exchange at least 75% of the sodium in Y-type zeolites. Thermal stability of these H-Na-Y zeolites is between 700° and 800°C. 

At 1 mbar thiophene pressure at room temperature the amount of adsorbed thiophene decreased with increasing metal (Ni or Co) loading of the zeolites (Figure 2). Differences in adsorption capacity may reflect differences in zeolite crystallinity, empty pore volume and density of adsorbate thiophene. Differences in crystallinity were not found by XRD. The pore volume of supercages of oxidic Ni(X)NaY zeolites are identical according to xenon adsorption measurements [7]. The low adsorption... 

XRD experiments indicated that the expected loss of the zeolite crystallinity during the reaction cannot he responsible for the activity decay. Application of the stabilized Y-zeolites and. nickel... 

Zeotypes comprise a variety of different framework compositions. The most prominent group are the zeolites, crystalline aluminosilicates containing channel-like voids with apertures from 4 to ca. 15 A. Zeolites have important uses as catalysts and catalyst supports, as ion-exchangers and in separation processes. Aluminosilicates with cage-like voids are called clathralites in analogy to clathrate hydrates [19]. 

Zeolites, crystalline alumina silicates with open regular structure, oiFer unusual opportunities for carrying out catal3rtic studies. Their well-defined crystalline structure and their regular pore distribution permit a better description of the surface than that offered by alumina-silica gel catalysts. Consequently, in recent years they have been the object of many scientific investigations. In addition, zeolites of a variety of types have shown highly desirable properties in industrial catalysis (1.2). This chapter is a review of the work on zeolites carried out at Princeton University and is not intended to be an exhaustive review of the topic. 

Zeolites Crystalline alumosilicates, three-dimensional network of silica and alumina with adjusted silica to alumina ratio Hydrophobic or hydrophilic depending on the silica/alumina ratio, cation exchanger, Br0nsted and Lewis acidity... 

The pore diameter, pore volume, framework density, and secondary building units (SBU) of the typical microporous zeolites crystallized from this system are listed in Table 3.3 for further study of the correlation between crystallization temperature and the structures of the corresponding microporous zeolitic crystallines. 

Zeolites, crystalline solids with regular, molecular-scale pores that incorporate catalytic groups, exert unique catalytic influence because of the molecular-sieving properties of their pore structures and the solvent-like character of their narrow pores. 

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