Porosil

Besides stmctural variety, chemical diversity has also increased. Pure silicon fonns of zeolite ZSM-5 and ZSM-11, designated silicalite-l [19] and silicahte-2 [20], have been synthesised. A number of other pure silicon analogues of zeolites, called porosils, are known [21]. Various chemical elements other than silicon or aluminium have been incoriDorated into zeolite lattice stmctures [22, 23]. Most important among those from an applications point of view are the incoriDoration of titanium, cobalt, and iron for oxidation catalysts, boron for acid strength variation, and gallium for dehydrogenation/aromatization reactions. In some cases it remains questionable, however, whether incoriDoration into the zeolite lattice stmcture has really occurred. 

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... 

Popova and colleagues47 carried out TLC of oxidation products of 4,4 -dinitrodiphenyl sulphide (the sulphoxide and sulphone) on silica gel + a fluorescent indicator, using hexane-acetone-benzene-methanol(60 36 10 l) as solvent mixture. Morris130 performed GLC and TLC of dimethyl sulphoxide. For the latter, he applied a 6% solution of the sample in methanol to silica gel and developed with methanol-ammonia solution(200 3), visualizing with 2% aqueous Co11 thiocyanate-methanol(2 1). HPLC separations of chiral mixtures of sulphoxides have been carried out. Thus Pirkle and coworkers131-132 reported separations of alkyl 2,4-dinitrophenyl sulphoxides and some others on a silica-gel (Porosil)-bonded chiral fluoroalcoholic stationary phase, with the structure ... 

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. 

If one examines the evolution of new zeolite structures over the past decade the most interesting discoveries have been made with high silica compositions. Many of these phases can be prepared in essentially all silica forms. Purists would prefer to classify such molecular sieves as organosilicates or porosils (1), in part because the physical properties differ from more classical low Si/Al ratio zeolites. In particular, the high silica zeolites tend to be more thermally stable and chemically robust. Additionally, the higher the Si/Al ratio the more hydrophobic the zeolite. These features are desirable for catalysts that may be used in catalytic processes such as cracking (3). 

The vibrational relaxation of the B, A, A, and X states is mainly caused by the collisions with the surroundings and was found to be completed after less than 2 ps for room temperature. The strong influence of the surroundings on this process could be demonstrated by polarization dependent measurements on iodine embedded in single crystals of the DDR porosil. At room temperature the relaxation time was found to be longer than 4 ps for iodine molecules oriented parallel to the crystal c axis. 

Materials with very high Si/Al ratios (tending to infinity) are called all-silica molecular sieves, zeosils or porosils. If any aluminium is present, non-framework cations such as alkaline or alkaline earth metals... 

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). 

The zeolite, porosil or A1P0 is the "host" and the zeolitic component the "guest". The host-guest complex is a solution, amenable at equilibrium to solution thermodynamics (14,15), and the host-guest relationship thereby described is one of the most important in the chemistry of porous crystals because, without the zeolitic guest,... 

For porosils the natural choice of the gram molecule (or mole) in host-guest solutions is Sio, and for comparability among all porosils and zeolites the mole will therefore be taken as M A1 Si. o where 0 < x < 0.5 and M is an equivalent of cations. 

Because many of the organic molecules which have served as guests have molecular volumes substantially larger than water their values of V are less than that of water. Values of V and some structural information are given in Table VIII for several porosils and AlPO s. 

TABLE VIII. Values of V for some Porosils and AlPO s... 

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]. 

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. 

Fig. 4. Porosil structures (oxygen atoms omitted) that have so far been synthesized with organometallic complexes as SDAs a) NON topology, b) AST topology, c) DOH topology, d) ZSM-48, e) UTD-1.

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

In the structures of several other microporous solids, including porosils, AlPOs and GaPOs, that were synthesized by the fluoride method, the fluoride anion was found in a different location than in [Co(cp)2]-NON, i.e., not directly bonded to one of the framework atoms. Instead, it occupies the center of a double four-ring unit. An example is another porosil of AST structure, namely quinuclidinium-AST [53]. Therefore, it has been supposed that the F" anion acts as a co-template, stabilizing structures that contain double four-ring units [23]. In agreement with this idea, we find that AST is only formed in fluoride-containing syntheses, and not in fluoride-fi-ee preparations. 

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]. 

A first indication of successful incorporation of intact organometallic SDAs is the colour of the porosil crystals. All the organometallic SDAs described in this work are yellow. Correspondingly, in normal light, porosil crystals from successful syntheses also appear yellow (the behaviour under polarized light is more peculiar, see below). UV-Vis spectra quantify this visual impression of colour [42, 56],... 

Structure) analyses [42, 43], When spectra from the organometallic/porosil composites are compared to the corresponding spectra of the pure hexafluorophosphate salts of the organometallic cations or their solutions, certain minor deviations are often observed. These can be explained by interactions of the organometallic cations with the silica host. All spectral data thus agree with the presence of intact organometallic SDAs in the pores. 

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