Synthesis porosil

The prime requirement for a molecule to act as an SDA in a porosil synthesis is stability under the hydrothermal synthesis conditions. Although these are very mild compared to conventional high-temperature solid-state syntheses, they are harsh enough to destroy labile organic compounds. In the usual procedure, a reaction mixture consists of a reactive silica source (amorphous silica, e.g, fumed or precipitated silica, silica gel), a mineralizer (e g. NaOH), the SDA and water. Alkaline mineralizers such as NaOH raise the pH to 12 or higher These strongly alkaline solutions are treated at elevated temperatures (up to 200°C) for several days. 

Synthesis parameters 02-0-02 02-P-08 Synthesis, particulate precursors Synthesis, pH effect Synthesis, phosphates, substituted Synthesis, phosphates, Ti Synthesis, phosphates, W Synthesis, porosils Synthesis, rare earth silicates Synthesis, SAPO-11... 

Since every SDA is distinct in its properties, the influence of the SDA on the structure type formed during the porosil synthesis is obvious. The vast amount of experimental data on the synthesis of porosils available in the literature allows the deduction of a number of empirical synthesis rules [20,36,37,38,39, 40]. Very little is known of atomistic details of the synthesis mechanism [41,42]. 

Alternative Synthesis Routes Used for Porosil Synthesis... 

Gies FI, Marler B and Werthmann U 1998 Synthesis of porosils crystalline nanoporous silicas with cage- and channel-like void structures Moiecuiar Sieves Science and Technoiogy vo 1, ed FI G Karge and J Weitkamp (Berlin Springer) pp 35-64... 

U. (1998) Synthesis of porosils Crystalline nanoporous silicas with cage-and channel-like void structures in Molecular Sieves Science and Technology, vol. 1 (eds H.G.Karge and ). Weitkamp), Springer, Heidelberg, pp. 35-64. 

To succeed in synthesis it is essential during growth to stabilise the open structure by inclusion of quest molecules. This requirement has a thermodynamic origin which has been developed and applied to formation of zeolites, porosils and AlPO s. 

Water and hydroxyl ion are the classic mineralisers in hydrothermal synthesis, firstly because aqueous alkali dissolves amphoteric oxides and so promotes mobility and mixing of molecular and ionic species as a pre-requisite for reaction. A second vital role is that of molecular water which (see below) stabilises aluminous zeolites by filling channels and cavities. This role can be shared or taken over by organic molecules (e.g. in porosils, silica-rich zeolites or AlPO s), and by salts (e.g. in scapolites, sodalite and cancrinite). 

Keywords Materials / Microporous Solids / Organometallic complexes / Porosils / Structure-directed Synthesis... 

Similar to the porosils, the dense, thermodynamically stable Si02 modification a-quartz is also prepared under hydrothermal conditions. However, in the industrial process for the production of quartz, the temperatures are rather high (around 400°C). In this process, NaOH is added as a mineralizer to the aqueous solution to promote dissolution of the silica precursor. The reaction mixtures for the preparation of porosils and other zeotype materials also generally contain a mineralizer, but the reaction conditions are much milder. Synthesis temperatures are below 200°C, typically between 140 and 180°C. Some zeolites can even be prepared from aqueous solutions under reflux at normal pressure. These mild synthesis conditions provide the kinetic control necessary to form metastable products [5-9]. 

Table 1. Synthesis conditions and crystallographic characterization of porosils synthesized in our group by using organometallic cations as SDAs.

In addition to the results on porosils given above, the cobalticinium cation has also been applied to the synthesis of microporous solids of other host compositions. Balkus et al. have described their results for the synthesis of open-framework aluminophosphates, [54] and Kallus et al. have determined the structure of a cobalticinium-containing gallium phosphate. This compound, however, is not a true zeotype since it does not possess a three-dimensional framework structure [55]. 

The porosils have appeared only recently upon the scene [lOj. They include the silica end-products of synthesis of zeolites richer and richer in silica, such as silicalites I and II, and also other species which do not always have a zeolite counterpart. They can be sub-divided into clathrasils in which openings between intracrystalline cavities are too small for molecule migration and zeosils in which these openings are adequate for molecule diffusion. Examples of each sub-division are ... 

The number and structural diversity of porosils should increase substantially as research into their synthesis continues. Interest could centre upon the zeosils as very stable, hydrophobic molecular sieves and catalyst carriers. Their synthesis requires at least partial occupation of channels or cavities by stabilising guest molecules. 

A variety of porous 3-dimensional (3-D) 4-connected AIPO4 nets have been synthesised, some of which have the same topologies as their zeolite counterparts while others have novel topologies [13, 14]. As with porosils and silica-rich zeolites, basic organic species are involved in their synthesis, to occupy channels and cavities as space fillers and stabilisers. AlPO s have... 

P-41 - The influence of concentration on the structure-directing effects of diethylenetriamine in the synthesis of porosils... 

Various parameters are known to influence the kinetically controlled synthesis of silica-rich zeolites and related compounds such as porosils. In syntheses where diethylenetriamine is acting as a structure-directing agent (SDA), different porosil structures can crystallize nonasil, ZSM-48, silicalite-1 and an unknown structure. The type of porosil obtained is found to depend upon the concentration of the SDA, i.e. upon the compositional ratio between diethylenetriamine and water in the synthesis gel. The results can be rationalized on the basis of the structural arrangements of the diethylenetriamine molecules in the porosil formed. These arrangements are influenced by the competition between intramolecular hydrogen bonds within individual diethylenetriamine molecules, by the ability to form intermolecular hydrogen bonds between the molecules of the SDA and by the solvation of these molecules by water. 

Gies. H. Marler. B. The structure controlling role of 24. organic templates for the synthesis of porosils in the system Si02/template/H20. Zeolites 1992. 12,42-49. 

Synthesis of Porosils Crystalline Nanoporous Silicas with Cage- and Channel-Like Void Structures. 35... 

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