Textures zeolites

Most of the microporous and mesoporous compounds require the use of structure-directing molecules under hydro(solvo)thermal conditions [14, 15, 171, 172]. A serious handicap is the application of high-temperature calcination to develop their porosity. It usually results in inferior textural and acidic properties, and even full structural collapse occurs in the case of open frameworks, (proto) zeolites containing small-crystalline domains, and mesostructures. These materials can show very interesting properties if their structure could be fully maintained. A principal question is, is there any alternative to calcination. There is a manifested interest to find alternatives to calcination to show the potential of new structures. 

In spite of such positive effects of the presence of ammonia during preparation, the particles sizes remain important on Au/FAU-2, comparable to particles previously described for similar Au/zeolites [1,2]. Noticeably, they are much bigger than expected from the insertion of the Au particles inside the pores. On the contrary, very small gold nanoparticles with a mean diameter of about 2 nm are obtained on the BEA support, that can be due to the textural properties and high external surface area of this support made... 

Nitrogen adsorption/desorption isotherms on Zeolite and V-Mo-zeolite are very similar and close to a type I characteristic of microporous materials, although the V-Mo-catalysts show small hysterisis loop at higher partial pressures, which reveals some intergranular mesoporosity. Table 1 shows that BET surface area, microporous and porous volumes, decrease after the introduction of Molybdenum and vanadium in zeolite indicating a textural alteration probably because of pore blocking by vanadium or molybdenum species either dispersed in the channels or deposited at the outer surface of the zeolite. The effect is far less important for the catalysts issued from ZSM-5. 

Table 1 Chemical analysis and textural properties of V-Mo-zeolites samples.

The chemical compositions of the samples were obtained by ICP in a Varian 715-ES ICP-Optical Emission Spectrometer. Powder X-ray diffraction was performed in a Philips X pert diffractometer using monochromatized CuKa. The crystallinity of the zeolites was obtained from the intensity of the most intense reflection at 23° 20 considering the parent HZ5 sample as 100% crystalline. Textural properties were obtained by nitrogen physisorption at -196°C in a Micromeritics ASAP 2000 equipment. Surface areas were calculated by the B.E.T. approach and the micropore volumes were derived from the corresponding /-plots. Prior to the adsorption measurements the samples were degassed at 400°C and vacuum overnight. 

Catalytic activity of the beta zeolite with enhanced textural properties in the Friedel-Crafts acylation of aromatic compounds... 

Friedel-Crafts acylation is widely used for the production of aromatic ketones applied as intermediates in both fine chemicals and pharmaceutical industries. The reaction is carried out by using conventional homogenous catalysts, which represents significant technical and environmental problems. The present work reports the results obtained in the Friedel-Crafts acylation of aromatic substrates (anisole and 2-methoxynaphthalene) catalyzed by Beta zeolite obtained by crystallization of silanized seeds. This material exhibits hierarchical porosity and enhanced textural properties. For the anisole acylation, the catalytic activity over the conventional Beta zeolite is slightly higher than with the modified Beta material, probably due to the relatively small size of this substrate and the weaker acidity of the last sample. However, the opposite occurred in the acylation of a bulky substrate (2-methoxynaphthalene), with the modified Beta showing a higher conversion. This result is interpreted due to the presence of a hierarchical porosity in this material, which favors the accessibility to the active sites. 

The present work reports the results obtained in the Friedel-Crafts acylation of different aromatic substrates catalyzed by zeolite Beta obtained according to a novel method based on the crystallization of silanized seeds, as a way to perturb the subsequent crystal growth step and to modify the zeolite textural properties [5], The catalytic behavior of this material is compared with that of the conventional Beta zeolite. 

The catalytic activity of hierarchical and conventional Beta zeolites for acylation of 2-MN is displayed in Figure 2(a) The Beta (PHAPTMS) sample shows a superior catalytic activity than the conventional one, due to its enhanced textural properties. In this case, the bulky nature of both substrate and products may cause the existence of diffusional problems inside the zeolitic channels, which are attenuated in the modified Beta sample due to the presence of the hierarchical porosity. Regarding the product distribution (Figure 2(b)), two main products are observed and a third isomer, 8-A,2-MN isomer is produced just in minor amounts. Interestingly, the selectivity towards the desired isomer increases in the material obtained from silanized seeds, reaching values around 75%. Probably, the active sites located on the surface of the secondary porosity are able to catalyze also the formation of 6-A,2-MN by transacylation. However, this reaction is expected to be strongly hindered in the conventional Beta zeolite since it requires the participation of two bulky molecules as reactants. 

The XRD patterns demonstrated that the MCM-22 zeolites were well crystallized and pillars have been created in the MCM-36 sample, respectively. Thus, the last material exhibited a typical intense peak at 29 2°, corresponding to a Aspacing of 4 nm. The textural properties of solids (Table 1) indicated that the pillaring in MCM-36 resulted in increases in BET specific surface area and external surface area compared with the MCM-22 zeolite. 

The effectiveness of zeolites in catalysis and separation can often be improved by the textural and chemical properties of the matrices in which they are imbedded. Chitosan gels issued from renewable resources are already used as supports for the preparation of heterogeneous catalysts in the form of colloids, flakes or gel beads [1, 2], In this study we present several methods for the incorporation of zeolites in chitosan matrices and characterize the synergic effect of the components on the properties of the composite. 

Supported palladium oxide is the most effective catalyst used in total methane oxidation and in catalytic oxidation of VOCs [1-5]. However, the activity of the conventional catalysts is not sufficient [5-6]. Recently, the Pd-zeolite catalysts have attracted considerable attention due to their high and stable CH4 conversion efficiency [4-8]. In this work, the effect of the preparation method, the nature of the charge-balancing cations, the palladium loading and the pre-treatment gas nature on the texture, structure and catalytic activity of the Pd-ZSM-5 solids are investigated. 

The results of chemical analysis show that all samples have similar values of Si/Al ratio and confirm that the recrystallization procedure doesn t lead to any significant changes in chemical composition. On the contrary, the adsorption measurements point to remarkable changes in zeolite texture (Fig.l). 

Many but not all catalysts are porous materials in which most of the surface area is internal. It is sometimes convenient to speak of the structure and texture of such materials. The structure is defined by the distribution in space of the atoms or ions in the material part of the catalyst and, in particular, by the distribution at the surface. The texture is defined by the detailed geometry of the void space in the particles of catalyst. Porosity is a concept related to texture and refers to the pore space in a material. With zeolites, however, much of the porosity is determined by the crystal structure. 

Pyroclastic rocks include scoria and pumices, which show variably por-phyritic textures with dominant leucite phenociysts and minor clinopyrox-ene, some mica, and rare olivine, set in a microcrystalline to glassy matrix that has been generally affected by strong secondaiy processes, with abundant zeolites and clay minerals (Fomaseri et al. 1963 Trigila et al. 1995). Accessory phases include Fe-Ti oxides, apatite and rare garnet. 

A zeolite membrane is a membrane in which the transfer is controlled by the porous structure of the zeolite. Compared to sol-gel membranes, zeolite membranes can present some advantages for CMR applications. The most useful feature is that the pores of zeolites arc in the ultramicroporous range and have a very narrow size distribution (in this case pores are linked to the structure and not to the texture as in sol gel membranes). These characteristics of zeolite... 

Direct Synthesis the crystallization is done in the presence of organic templates (2) such as TBAOH, crown ethers,. ..These syntheses unfortunately suffer from two economic drawbacks, i.e. the long synthesis times (between 3 to 8 days) compared to the classical synthesis (typically 16 hours) and the use of costly, sometimes toxic, and not always recyclable organic molecules Hydrothermal Treatment a post-synthesis treatment widely practiced in the industry because it also modifies the texture (creation of desired mesoporosity) of the zeolite. Such a treatment also takes place in the regenerator of every FCC unit... 

Textural and chemical characterization of NaX zeolite exchanged with Zn(II) ions... 

Immersion calorimetry is a very useful technique for the surface characterization of solids. It has been widely used with for the characterization of microporous solids, mainly microporous carbons [6]. The heat evolved when a given liquid wets a solid can be used to estimate the surface area available for the liquid molecules. Furthermore, specific interactions between the solid surface and the immersion liquid can also be analyzed. The appropriate selection of the immersion liquid can be used to characterize both the textural and the surface chemical properties of porous solids. Additionally, in the case zeolites, the enthalpy of immersion can also be related to the nature of the zeolite framework structure, the type, valence, chemistry and accessibility of the cation, and the extent of ion exchange. This information can be used, together with that provided by other techniques, to have a more complete knowledge of the textural and chemical properties of these materials. 

This communication reports the characterization of a series of Zn(II) exchanged NaX zeolites with the help of several techniques, with the aim of gaining some insight into the textural and chemieal modifications induced by the presence of exchanged Zn(II)... 

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