Hydrothermal Zeolite Synthesis

The first inverse of the ship-in-bottle synthesis, in which the zeolite cage is constructed around a porphyrin template, was reported by Li and Zhan. " The Fe(II) and Mn(II) derivatives of tetra(Af,A, Af-trimethylanilinium)porphyrin and H2(TMPyP) were used as cationic templates in zeolite hydrothermal synthesis. Attempts to incorporate an anionic porphyrin species, metallo-tetra(4-sulfunatophenyl)por-phyrin, were unsuccessful and trials with neutral TPP species resulted in trace encapsulation. High activity for the catalytic oxidation of cyclohexene by terf-butyl hydroperoxide was reported. 

Zeolites. A large and growing industrial use of aluminum hydroxide and sodium alurninate is the manufacture of synthetic zeoHtes (see Molecular sieves). ZeoHtes are aluminosiHcates with Si/Al ratios between 1 and infinity. There are 40 natural, and over 100 synthetic, zeoHtes. AH the synthetic stmctures are made by relatively low (100—150°C) temperature, high pH hydrothermal synthesis. For example the manufacture of the industriaHy important zeoHtes A, X, and Y is generaHy carried out by mixing sodium alurninate and sodium sHicate solutions to form a sodium alurninosiHcate gel. Gel-aging under hydrothermal conditions crystallizes the final product. In special cases, a small amount of seed crystal is used to control the synthesis. 

Preparation of Mono-Dispersed MFI-type Zeolite Nanoerystals via Hydrothermal Synthesis in a Water/Surfactant/Oil Solution... 

The zeolite nanocrystals have attracted the considerable attention of many researchers [1-5]. The syntheses of several types of zeolites with different nanometer sizes, such as silicalite-1, ZSM-5, A-type and Y-type, have been reported. Recently, micellar solutions or surfactant-containing solutions have been used for the preparation of zeolite nanoerystals [4,5], We have also successMIy prepared silicalite nanoerystals via hydrothermal synthesis using surfactants. In this study, we demonstrate a method for preparing mono-dispersed silicalite nanoerystals in a solution consisting of surfiictants, organic solvents and water. 

In order to prepare ZSM-5 zeolite nanocrystals, an A1 source of aluminium isopropoxide was added into solution A, and hydrothermal synthesis of the solution A containing Si and A1 sources was carried out in an 0-15/cyclohexane solution at 120 degree C for 50 h. Figures 4 show ac-NHj-TPD spectra and a SEM photograph of the ZSM-5 zeolite nanocrystals. Nanocrystals with a diameter of approximately 150 nm were observed, and the NH3-TPD spectrum showed desorption of NHj above 600 K, indicating that the nanocrystals possessed strong acid sites. 

The same periodic structures can also be formed from alternating AIO4 and PO4 tetrahedra the resulting aluminophosphates are not called zeolites but AlPOs. Zeolites are made by hydrothermal synthesis under pressure in autoclaves, in the presence of template molecules such as tetramethylammonium, which act as structure directing agents. 

Aza macrocycles have also been used as templating agents in the formation of zinc and other metal cation-containing aluminophosphates via hydrothermal synthesis.705 The zeolite-like structures have been formed in the presence of cyclam- and hexaaza-based macrocycles. 

Cancrinites are one of the rarest members of the feldspathoid group, classified as such due to its low silicon content. However, cancrinite is also classified as a zeolite, due to its open pore structure, which confers molecular sieve properties [1], Likewise, variable sodium carbonate and NaOH concentrations in the hydrothermal synthesis of cancrinite could direct the synthesis of the intermediate phase or the disordered cancrinite formation [2], The intermediate phase is described as a phase between cancrinite and sodalite [3], The disordered cancrinite is an intermediate phase which is much closer to the cancrinite structure than sodalite structure [2],... 

Zeolites are formed by crystallization at temperatures between 80 and 200 °C from aqueous alkaline solutions of silica and alumina gels in a process referred to as hydrothermal synthesis.15,19 A considerable amount is known about the mechanism of the crystallization process, however, no rational procedure, similar to organic synthetic procedures, to make a specifically designed zeolite topology is available. The products obtained are sensitive functions of the reaction conditions (composition of gel, reaction time, order of mixing, gel aging, etc.) and are kinetically controlled. Nevertheless, reproducible procedures have been devised to make bulk quantities of zeolites. Procedures for post-synthetic modifications have also been described.20 22... 

Cundy, C.S. and Cox, P.A. (2003). The hydrothermal synthesis of zeolites history and development from the earliest days to the present time. Chem, Rev. 103, 663-701... 

Zeolite synthesis around a metal complex was introduced by Balkus [40]. The metal complex is added to the zeolite synthesis mixture and is incorporated into the zeolite structure during the zeolite synthesis. Of course, this procedure is only applicable when the metal complex is soluble in the synthesis mixture and can withstand the hydrothermal synthesis conditions. Another requirement is that the zeolite structure-directing agent added must be removable by a milder... 

A broad range of high silica and pure silica molecular sieves have been synthesized by employing hydrothermal synthesis in fluoride media at low H2O concentration, near neutral pH and alkali-free [81]. The significant new pure silica zeolite, lTQ-29, a structural analog of zeolite A (LTA) was reported by Corma at al [82]. Unlike the highly hydrophilic zeolite A, the ITQ-29 is hydrophobic. 

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. 

Present data do not justify the attribution of this V species to a real substitutional V site in the zeolite framework, because the amount of these V sites is very low and at present the degree of incorporation of these sites in the zeolite cannot be extended. It is therefore reasonable to assume that these V sites form at defect sites, possibly hydroxyl nests, the formation of which may be enhanced by the presence of V during hydrothermal synthesis, in agreement with Rigutto and van Bekkum (5). 

It is well known that the elements in framework of zeolite molecular sieves greatly influence the properties and behaviors of these materials [1-3], The introduction of heteroatoms into the framework has become one of most active fields in study of zeolites. The investigations were mostly focused on the methods to introduce heteroatoms into the framework (for examples, hydrothermal synthesis and post-synthesis), the mechanisms for incorporations, the effect of heteroatoms on the acid-base properties and the catalytic features of modified samples [1-10]. Relatively less attention was paid to the effect of treatment process on the porous properties of samples although the incorporation of heteroatoms, especially by the so-called post-synthesis, frequently changes the distribution of pore size. Recently, we incorporated Al, Ga and B atoms into zeolites (3 by the post-synthesis in an alkaline medium named alumination, galliation and boronation, respectively. It was found that different trivalent elements inserted into the [3 framework at quite different level. The heteroatoms with unsuitable atom size and poor stability in framework were less introduced, leading to that a considerable amount of framework silicon were dissolved under the action of base and the mesopores in zeolite crystal were developed. As a typical case, the boronation of zeolites (3 and the accompanied formation of mesopores are reported in the present paper. 

The synthesis of titanium-containing beta zeolite has been carried out by direct hydrothermal synthesis and by secondary synthesis. In the direct synthesis with aluminum salts and titanium and silicon alkoxides, it is necessary to operate in the absence of alkalies, since these cause the formation of an amorphous precipitate containing Ti02 and Si02. Evidence for the substitution... 

Zeolite-supported Re catalysts have been synthesized by chemical vapor deposition (CVD) ofMTO (CH3Re03) (3) on various zeolites such as HZSM-5, H-Beta, H-USY and H-Mordenite. HZSM-5 samples with different A1 contents were prepared by a hydrothermal synthesis method. For comparison, conventional impregnated catalysts were also prepared by an impregnation method using an aqueous solution of NH4Re04. All catalysts were pretreated at 673 K in a flow of He before use as catalyst. 

The other way to introduce heterometals is their isomorphous substitution for Si in the framework, in a similar manner to the isomorphous substitution of Al. The heteroatoms should be tetrahedral (T) atoms. In hydrothermal synthesis, the type and amount of T atom, other than Si, that may be incorporated into the zeolite framework are restricted due to solubility and specific chemical behavior of the T-atom precursors in the synthesis mixture. Breck has reviewed the early literature where Ga, P and Ge ions were potentially incorporated into a few zeolite structures via a primary synthesis route [9]. However, until the late 1970s, exchangeable cations and other extraframework species had been the primary focus of researchers. 

Cundy, C. S. and Cox, P. A. The hydrothermal synthesis of zeolites precursors, intermediates and reaction mechanism, Microporous Mesoporous Mater., 2005,82, 1-78. 

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