Secondary Synthesis Hydrothermal

The hydrothermal synthesis system for secondary scale screening is based on multi-well-tumbled autoclaves modules [49]. Each module contains 16 PTFE vials each vial has an internal volume of 7 mL. The reactors can operate up to 200 °C. Many chemical and physical factors can affect the final hydrothermal synthesis outcome, making it a good application for high-throughput methods. In addition to the gel ingredients, these factors include the order of reagent addition, gel mix- 

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Niobium and titanium incorporation in a molecular sieve can be achieved either by hydrothermal synthesis (direct synthesis) or by post-synthesis modification (secondary synthesis). The grafting method has shown promise for developing active oxidation catalyst in a simple and convenient way. Recently, the grafting of metallocene complexes onto mesoporous silica has been reported as alternate route to the synthesis of an active epoxidation catalyst [21]. Further the control of active sites, the specific removal of organic material (template or surfactant) occluded within mesoporous molecular sieves during synthesis can also be important and useful to develop an active epoxidation catalyst. Thermal method is quite often used to eliminate organic species from porous materials. However, several techniques such as supercritical fluid extraction (SFE) and plasma [22], ozone treatment [23], ion exchange [24-26] are also reported. 

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... 

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. 

Dealumination and silicon-enrichment reaction of (NH SiF6 (AHFS) with zeolites Besides the hydrothermal method for preparation of ultra-stable Y zeolite (USY), Breck and Skeels[23] in 1983 invented a new secondary synthesis method for silicon-enriched zeolites. This method uses an ammonium hexafluorosilicate solution to remove the aluminum atoms from the framework structure of Y zeolite to the solution, and to insert silicon atoms back into the Al-removal vacancies in the framework so as to form a more or less perfect Y zeolite with a high Si/Al ratio. In comparison with the USY prepared by the hydrothermal method, the framework silicon-enriched Y zeolite obtained through the current technique possesses fewer framework hydroxyl vacancies, and the resulting zeolite has an ideal crystal lattice, and hence higher structural stability. Meanwhile, there... 

Borosilicates have been prepared via hydrothermal synthesis in alkaline solutions (1.16-24). Alternatively, synthesis has been successful from neutral or slightly acidic media in the presence of fluoride anions (22). Borosilicate molecular sieves have been prepared through secondary synthesis techniques as reported by Derouane, et al. (26.), in which the aluminosilicate ZSM-5 was treated with boron trichloride to replace aluminum with boron in tetrahedral sites. 

In this work the primary pore structure of the beads was varied according to a hydrothermal treatment procedure (step 2). In order to control the secondary pore structure of the agglomerates, the primary particle size has to be adjusted. This can be carried out varying synthesis parameters (step 1) like the concentration of ammonia and water [4], the chain length of the alcohol [5] and the synthesis temperature [6],... 

Ex Situ Hydrothermal Synthesis or Secondary Growth Methods.274... 

One-step direct synthesis of the highly stable mesoporous silica-based material is also possible. MMS-H[209] has a structure analogous to that of MCM-48 but which contains zeolite building units. A mixture of CTAB and Brij30 was used as template for the mesopores. The use of TPAOH without the assistance of NaOH helps to introduce zeolite secondary building units, as well as the direct formation of acidity after removal of the template. This material was also found to possess superior thermal, hydrothermal, steam, and mechanical stabilities. 

For operating the secondary growth, the seeded support is hydrothermally treated in a diluted synthesis mixture (with or without template) to grow the seed crystals, resulting in a compact zeolite film. A simple steaming of the seed layer can also partially consolidate the zeolite membrane by local dissolution and recrystallization processes in the seed layer. The films are finally dried or calcined if an SDA was used. 

Due to their pore diameters, less than 1 nm, the application of zeolites in catalytic processes is limited. On the other hand, mesoporous molecular sieves such as MCM-41 and MCM-48 with pore diameters up to 10 nm [1], have insufficient thermal and hydrothermal stability. To overcome these restrictions many efforts were imdertaken to combine tihe catalytic activity and stability of microporous zeolites with the better accessibility on the active sites of mesoporous molecular sieves [2]. The majority of the studies have been focused to the transformation of the amorphous pore walls of mesoporous molecular sieves into crystalline microporous zeolites by secondary crystallization [3], the mesostructuration of zeolite precursors [4] or the synthesis of a zeolite using porous carbons as cast [5]. The first step to develop... 

Alteration mineral assemblages characterize the many varied hydrothermal-chemical reactions that occur in active geothermal systems. The effect of chemical and temperature conditions on the occurrence of secondary minerals is well known, based on field observation and mineral synthesis laboratory experiments. It is less clear, however, how specific chemical species become enriched in one part of an active hydrothermal system. 

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