Microporous materials, hydrothermal

There are, however, two limitations associated with preparation and application of zeolite based catalysts. First, hydrothermal syntheses Umit the extent to which zeolites can be tailored with respect to intended appUcation. Many recipes involving metals that are interesting in terms of catalysis lead to disruption of the balance needed for template-directed pore formation rather than phase separation that produces macroscopic domains of zeoUte and metal oxide without incorporating the metal into the zeohte. When this happens, the benefits of catalysis in confined chambers are lost. Second, hydrothermal synthesis of zeoHtic, silicate based soHds is also currently Hmited to microporous materials. While the wonderfully useful molecular sieving abihty is derived precisely from this property, it also Hmits the sizes of substrates that can access catalyst sites as weU as mass transfer rates of substrates and products to and from internal active sites. 

The hydrothermal method has been employed in recent years to synthesize a variety of solids that include aluminium phosphates (ALPOs) and other microporous transition-metal phosphates and transition-metal polychalcogenides (Davis Lobo, 1992 Haushalter Mundi, 1992 Liao Kanatzidis, 1990, 1992). Unlike zeolites, synthesis of ALPOs requires acidic or mildly basic conditions and no alkali metal cations. A typical synthetic mixture for making ALPO consists of alumina, H3PO4, water and an organic material such as a quaternary ammonium salt or an amine. The hydrothermal reaction occurs around 373-573 K. The use of fluoride ions, instead of hydroxide ions as mineralizer, allows synthesis of novel microporous materials under acidic conditions (Estermann et al, 1991 Ferey et ai, 1994). 

The recent descriptions of the ALPO-n, SAPO-n and MeAPO-n families of microporous materials illustrate that hydrothermal syntheses can afford a wide and diverse range of four-coordinate framework structures based on nearregular tetrahedra [1,2]. As building blocks, octahedra and tetrahedra can also be combined, in various proportions, into a variety of structure types [3,4]. Reflecting the conditions used for conventional synthesis [3,4], most of these structures are condensed, with little accessible pore volume. There are, however, examples of both synthetic [5-7] and natural materials [8-11] that have microporous crystalline structures. Further, the formation chemistry of silicates and aluminosilicates [12,13] illustrates that the more open structures are generally produced under relatively mild conditions. Open octahedral-tetrahedral structures with large pore systems might therefore also be accessible under appropriate low temperature hydrothermal conditions. 

Zeolites and related aluminosilicates constitute a vital family of microporous materials with immense applications in catalysis, sorption and separation processes [1-3]. The discovery of aluminophosphates is an important landmark in the science of these materials [4], All these materials are, in general, synthesized under hydrothermal conditions by making use of template molecules [2]. The template molecules are usually organic amines and they are involved in the formation of these framework structures in more ways than one. While it is difficult to pinpoint the exact manner in which the amines participate or direct the formation of these inorganic structures, it is generally believed that their size and shape are crucial in determining the pore structure. In recent years, a variety of open-framework structures formed by divalent metal phosphates... 

The synthesis of the compound Ba5Nb303F18(HF2) [101] is remarkable since the HF2 entity from the solution is maintained. One major advantage of the hydrothermal procedure is the range of usable reaction conditions. Gels, well known in zeolite synthesis, have been used successfully in the preparation of KAIPO4F [104], while the addition of amines to the reaction mixture has provided access to microporous materials. Mesoporous and microporous materials as active materials... 

These kinds of microporous materials have been consecutively synthesized, mainly under hydrothermal conditions. Table 6 gives some materials synthesized in the recent years. The basic framework units involving Ge are also exhibited. 

Even though crystalline microporous materials include those with pore size between 10 and 20 A (called extra-large pore materials), few of them have a pore size within this range. This limits the applications of microporous materials to small molecules. There has always been a desire to increase the pore size of a crystalline material to more than 10 A while maintaining adequate thermal or hydrothermal stability required for various applications. Recent advances in chalcogenide and metal-organic framework materials have shown much promise for the preparation of extra-large pore materials. 

For example, in order to synthesize a MCM-41-type material at room temperature a low pH (about 8.5) is essential, while a micropore material is obtained at pH=ll (12). By contrast when the MCM-41 synthesis is performed by hydrothermal treatment, the pH... 

Recently, renewed attention has been given to so-called soft chemistry methods of synthesis of new metastable materials [9]. The synthesis of new microporous materials containing transition metals in the framework is of growing interest due to the expected catalytic redox properties [10]. The microporous titanium(IV) silicates [11] discovered have already proven the concept by showing very good catalytic activities and are widely used nowadays [12]. Similarly, hydrothermally synthesized titanium phosphates with open-finmework or layered structures are attracting attention as potential materials with similar properties [13]. 

Various redox metals, including Ti, V, Cr, Mn, Fe, Co, Cu, Zn, As, Zr and Sn, have been incorporated into microporous materials such as silicalites through hydrothermal synthesis by the addition of the respective cations to the synthesis gel. The disadvantages of this method include the time-consuming optimization of synthesis procedure for each metal-zeolite combination and the necessity of A1 for crystallization of certain structures. The presence of A1 leads to Bronsted acidity... 

The discrimination of protonated Si sites by CP MAS was also used by Yang and Kirkpatrick (1989) in a study of the hydrothermal decomposition of albite and sodium aluminosilicate glass, and rhyolitic glass (Yang and Kirkpatrick 1990), and as a means of differentiating between the various sites in acid-treated montmorillonites (Tkac et al. 1994). CP MAS NMR has also been used to identify a Si site at -100 ppm in a microporous material derived from acid-leached metakaolinite as the unit Si(0Si)30H (Okada et al. 2000). 

Nnhn H, Rossbach J (2000) LINAC-based short wavelength FELs The challenges to be overcome to produce the nltimate X-ray source-the X-ray laser. Synchrot Radial News 13 18-32 OTfare D, Evans ISO, Francis R, Price S, O Brien S (1998a) The use of in situ powder diffraction in the study of intercalation and hydrothermal reaction kinetics. Mater Sci Forum 278 367-378 OHare D, Evans ISO, Francis RJ, Halasyamani PS, Norby P, Hanson J (1998b) Time-resolved, in situ X-ray diffraction studies of the hydrothermal syntheses of microporous materials. Microporous Mesoporous Mat 21 253-262... 

Hydrothermal synthesis of microporous compounds in the presence of fluoride source refers to the hydrothermal or solvothermal crystallization of aluminosilicate zeolites or microporous aluminophosphate such as AlP04-n series in the presence of a fluoride source. The successful introduction of fluoride ion into the hydrothermal or solvothermal synthesis of microporous materials paves the way for the introduction of other complex-ion or chelation agent s to the hydrothermal crystallization of microporous compounds. 

NaOH was involved in the synthesis. After systematic studies, W.Q. Pang and S.L. Qiu developed a general approach to growing large single crystals of zeolites and related microporous materials from fluorine ion synthesis systems. Later, J.L. Guth and W.Q. Pang expanded the fluoride source hydrothermal synthesis approach to the synthesis of micro-porous aluminophosphates and other metal phosphates. 

Although crystallization does occur at low temperatures, hydrothermal methods are often used to speed up the reaction by raising the temperature to 160-250 C. Numerous templating agents have been used in the formulation of zeolite materials. Microporous materials (highly crystalline, pores <12 A) often use organic bases such as TEA (tetraethyl-ammonium), while mesoporous materials (larger pores but lower crystallinity, pores >12 A) often use liquid crystal templates ... 

This work deals with the synthesis of monooctylamines by ammonia alkylation with octanol-1 in gaseous phase using various catalysts. These microporous materials were prepared by the hydrothermal method. Y-faujasite and ZSM-5 supports were exchanged by lead and uranyl ions at different concentrations in order to increase their surface acidity necessary for reaction mechanism. The obtained results show that the use of these catalysts results in the formation of primary amines. Monooctylamines selectivities of 90 % were obtained in the present work. It was observed that when SAPO-34 is used, the trioctylamine isomer could be formed in the external surface of the catalyst. 

Carbon/silica adsorbents or carbosils have been prepared by both conventional and microwave heating. These materials were produced by pyrolysis of CH2CI2 on microporous silica gel surfaces for 30 min to 6 h at 550 °C. The resultant materials were hydrothermally treated with steam or liquid water using either a conventional autoclave or a microwave unit. As with the clay materials, hydrothermal treatment using microwave irradiation leads to a significant increase of surface area and total pore volume of the carbosils, as compared to conventional methods. 

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