Hydrothermal stability and catalytic

Due their enhanced textural properties, hydrothermal-stability, and catalytic properties, the experimental silico-aluminates are promising alternative candidates for application as matrix additives to FCC catalysts for the efficient upgrading of bottoms into higher quality gasoline and LPG with high olefinicity. 

Zeolites though possess high hydrothermal stability and enhanced catalytic activity, the merits are not fully exploited for industrial applications because micropores hinder the diffusion of bulkier molecules. This led to the development of new kind of silicious materials like that of MCM and SBA with large pore volumes which can be accessed by bulkier molecules. But these suffer with the limitation of week catalytic activity and poor hydrothermal stability which hampers their use for various industrial applications... 

Zeolites are crystalline aluminosilicates with a regular pore structure. These materials have been used in major catalytic processes for a number of years. The application using the largest quantities of zeolites is FCC [102]. The zeolites with significant cracking activity are dealuminated Y zeolites that exhibit greatly increased hydrothermal stability, and are accordingly called ultrastable Y zeolites (USY), ZSM-5 (alternatively known as MFI), mordenite, offretite, and erionite [103]. 

The CeMCM-41 material studied had much higher quality than the corresponding MCM-41 sample synthesized under the same conditions. While both materials exhibited analogous adsorption properties with respect to nitrogen, their interaction with n-butylamine was different. Thermogravimetric analysis of w-butylamine thermodesorption showed that CeMCM-41 possessed medium and strong acid sites in contrast to the pure silica MCM-41, the acidity of which was negligible. Thus, incorporation of cerium to MCM-41 seems to improve its hydrothermal stability and enhance the adsorption and catalytic properties. 

In our previous paper we have reported that silica MCM-41 exhibits a large amount of reversible adsorption, high thermal and hydrothermal stabilities, and little catalytic acidity and is an excellent adsorbent in PSA process for recovery of 2-propanol and toluene vapors [1]. Here we present the results of PSA of butanone on silica MCM-41 and discuss the effect of pretreatment temperatures on adsorption properties of MCM-41. 

There are many difficulties involved in the application of pure nanozeolites as catalytic materials, such as the aggregation of nanozeolites, their low thermal and hydrothermal stabilities, and the difficulties in regeneration, filtration, and recycling. Hence, how to develop nanozeolites into new catalytic materials with application values in industry is of particular interest. Here, we use /i-type (BEA) nanozeolite catalytic material as an example to introduce this topic. 

The modification of zeolites mainly relies on secondary synthesis methods. The aim of modification is to reprocess the zeolites using suitable techniques to improve the properties and functions such as (1) acidity, (2) thermal and hydrothermal stability, (3) catalytic performance such as redox catalytic and coordination catalytic properties, etc., (4) channel structures, (5) surface properties and microporous frameworks and charge-balancing ions. Modification is also called secondary synthesis and can lead to new properties that cannot be achieved through direct synthesis. Let us consider the case of faujasite (FAU), the main component of the cracking catalyst, and its catalytic performance (represented by the catalytic activity K/K Std for n-hexane cracking). From Table 6.1 it is seen that the secondary synthesis affects the catalytic performance to a considerable degree. 

The big advantage of the ordered mesoporous material is that it has a high surface area and a large pore volume. The big disadvantage may be that its wall is amorphous, and is not ordered at the atomic level. This results in its poor performances low hydrothermal stability and low catalytic acid strength, although there are many methods available to improve those performances. Simultaneously, the amorphous wall makes the modification chemistry on the wall much easier, which allowed the various wall compositions to be synthesized successfully. In contrast, the extension of zeolite framework compositions is difficult because of the limits of their critical crystalline structures at the atomic level. 

A hierarchically macro-/meso-/microporous structured catalyst, Hp-ZSM, has been recently reported by. Li et al. [173]. The catalyst was synthesized using cetyltrimethylammonium bromide (CTAB) and TBAOH as the meso- and micropore templates. Ethanol was used to generate macropores, probably via an ethanol-in-water microemulsion mechanism. The hierarchical porous zeolite shows higher hydrothermal stability and sustained higher catalytic activity than either the amorphous aluminosilicate ZSM-5 or meso-ZSM-5 catalysts used in the reactions involving large molecules. 

These materials are characterized by great thermal and hydrothermal stability and show a number of useful catalytic properties, including the ability to catalyze the conversion of methanol to gasoline range hydrocarbons without excessive coke formation. 

The Si/Al ratio plays a significant role, since the aluminum atom is directly related to the acidic site. Dealumination processes can promote porous structure modifications, which may improve some interesting properties of zeolites, like thermal and hydrothermal stability, acidity, catalytic activity, resistance to aging and low coking rate, and matter transfer. However, a severe dealumination may also cause a loss of crystallinity [47]. 

Spinel ferrite nanoparticles have been intensively studied in the recent years, because of their typical ferromagnetic properties, low conductivity, high electrochemical stability and catalytic behavior. These materials are widely used in large-scale applications (i) in electric and electronic devices, (ii) in H2O, CO2 and alcohols decomposition and in CO and CH4 oxidation [1, 2]. Several routes are used for the preparation of NiFe204 catalysts such as co-precipitation, hydrothermal, sol gel, combustion [3-6] etc. However, the structural and textural properties of ferrite spinel are strongly influenced by the preparation methodology used in their synthesis and may influence the catalytic activity of these materials when used as catalysts. Hence, the effect of the preparation method on the surface acid-basic properties and therefore on the catalytic activity is a very interesting subject. 

MicrocrystalUne zeolites such as beta zeolite suffer from calcination. The crystallinity is decreased and the framework can be notably dealuminated by the steam generated [175]. Potential Br0nsted catalytic sites are lost and heteroatoms migrate to extra-framework positions, leading to a decrease in catalytic performance. Nanocrystals and ultrafine zeolite particles display aggregation issues, difficulties in regeneration, and low thermal and hydrothermal stabilities. Therefore, calcination is sometimes not the optimal protocol to activate such systems. Application of zeolites for coatings, patterned thin-films, and membranes usually is associated with defects and cracks upon template removal. 

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... 

The preparation methods of aluminum-deficient zeolites are reviewed. These methods are divided in three categories (a) thermal or hydrothermal dealumination (b) chemical dea-lumination and (c) combination of thermal and chemical dealumination. The preparation of aluminum-deficient Y and mordenite zeolites is discussed. The structure and physico-chemical characteristics of aluminum-deficient zeolites are reviewed. Results obtained with some of the more modern methods of investigation are presented. The structure, stability, sorption properties, infrared spectra, acid strength distribution and catalytic properties of these zeolites are discussed. 

The characteristics of aluminophosphate molecular sieves include a univariant framework composition with Al/P = 1, a high degree of structural diversity and a wide range of pore sizes and volumes, exceeding the pore sizes known previously in zeolite molecular sieves with the VPI-5 18-membered ring material. They are neutral frameworks and therefore have nil ion-exchange capacity or acidic catalytic properties. Their surface selectivity is mildly hydrophilic. They exhibit excellent thermal and hydrothermal stability, up to 1000 °C (thermal) and 600 °C (steam). 

The introduction of silicon into hypothetical phosphorus sites produces negatively charged frameworks with cation-exchange properties and weak to mild acidic catalytic properties. Again, as in the case of the aluminophosphate molecular sieves, they exhibit excellent thermal and hydrothermal stability. 

The spectrum of adsorption pore sizes and pore volumes and the hydrophilic surface selectivity of the MeAPOs are similar to those described for the SAPOs. The observed catalytic properties vary from weakly to strongly acidic and are both metal- and structure-dependent. The thermal and hydrothermal stability of the MeAPO materials is somewhat less than that of the AIPO4 and SAPO molecular sieves. 

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