Aluminosilicates Zeolites

ZeoUtes are another group of compounds that have generated some interest (Rolison 1990 Walcarius 1999 Kulprathipanja 2010). Chemically, they are represented by the empirical formula 

Framework Cationic form Formula of typical unit cell Window Effective channel diameter (nm) 

It was established that the ratio of silicon to aluminum atoms in the lattice, xly, is a very important parameter for zeolite characterization. The more aluminum-based units present in the zeolite lattice, i.e., the higher y, the more cations are needed for charge compensation. As a consequence, zeolites with high aluminum content are highly polar materials with excellent ion-exchange capacity and potential for high ion conductivity. This means that by changing the framework Si/Al ratio of the 

Their surface and structural properties can easily be modified, which makes them ideal candidates for the selective adsorption of various volatile hydrocarbons and small organic molecules (Pejcic et al. 2007). For example, various post-synthesis steps were developed to modify the zeolite chemically by incorporating catalytically active metal clusters such as Pt, Fe, or Cu, or by anchoring organic dyes within the pore strncture. 

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Bibby D M, Milestone N B and Aldridge L P 1979 Silicalite-2 a silica analogue of the aluminosilicate zeolite ZSM-11 Nature 280 664-5... 

Guth J-L and Kessler H 1999 Synthesis of aluminosilicate zeolites and related silica-based materials Catalysis and Zeolites, Fundamentals and Applications ed J Weitkamp and L Puppe (Berlin Springer) pp 1-52... 

Zeolite A [1318-02-1] Zeolite (aluminosilicate) Zeolite catalysts Zeolite L Zeolites... 

Volume 33 Synthesisof High-silica Aluminosilicate Zeolites edited by P.A. Jacobs and J.A. Martens... 

J. A. Martens, Synthesis of High-Silica Aluminosilicate Zeolites, Elsevier, Amsterdam, 1987, 390 pp. M. L. Occelli and H. E. Robson (eds.). Zeolite Synthesis, ACS Symposium Series No. 398, 1989, 664 pp. J. Klinowski and P. J. Barrie (eds.) Recent Advances in Zeolite Science, Elsevier, Amsterdam, 1990, 310 pp. G. V. Tsitsishvili, T. G. Andronikashvili, G. M. Kirov and L. D. Filizova, Natural Zeolites, Ellis Horwood, Chichester, 1990, 274 pp. 

X-Ray irradiation of quartz or silica particles induces an electron-trap lattice defect accompanied by a parallel increase in cytotoxicity (Davies, 1968). Aluminosilicate zeolites and clays (Laszlo, 1987) have been shown by electron spin resonance (e.s.r.) studies to involve free-radical intermediates in their catalytic activity. Generation of free radicals in solids may also occur by physical scission of chemical bonds and the consequent formation of dangling bonds , as exemplified by the freshly fractured theory of silicosis (Wright, 1950 Fubini et al., 1991). The entrapment of long-lived metastable free radicals has been shown to occur in the tar of cigarette smoke (Pryor, 1987). 

The data points in Fig. 6 show a slow decrease of Cq for aluminosilicate zeolites with increasing proton affinity of the probe molecule. Experimental data of methanol were not included in the illustration because of possible proton hopping which... 

Aluminosilicates, see Organic syntheses, using aluminosilicates Zeolites specific compounds... 

Molecular sieves are porous aluminosilicates (zeolites) or carbon solids that contain pores of molecular dimensions which can exhibit seleaivity according to the size of the gas molecule. The most extensive study on carbon molecular sieve membranes is the one by Koresh and Soffer (1980,1987). Bird and Trimm (1983) also described the performance of carbon molecular sieve membranes, but they were unable to prepare a continuous membrane. Koresh and Soffer (1980) prepared hollow-fiber carbon molecular sieves, with pores dimensions between 0.3 and 2.0 run radius (see Chapter 2). 

The past nearly six decades have seen a chronological progression in molecular sieve materials from the aluminosilicate zeolites to microporous silica polymorphs, microporous aluminophosphate-based polymorphs, metallosilicate and metaHo-phosphate compositions, octahedral-tetrahedral frameworks, mesoporous molecular sieves and most recently hybrid metal organic frameworks (MOFs). A brief discussion of the historical progression is reviewed here. For a more detailed description prior to 2001 the reader is referred to [1]. The robustness of the field is evident from the fact that publications and patents are steadily increasing each year. 

The early evolution of aluminosilicate zeolites, in the 1950s to 1970s, is summarized in Table 1.2, based on increasing framework Si/Al composition. The four somewhat arbitrary categories are (i) low , (ii) intermediate , (hi) high silica zeolites and (iv) silica molecular sieves. 

Table 1.4 lists some of the major new structures reported in the 1990s. Interestingly, as organic SDAs tended to dominate discovery of new frameworks, there were no new aluminum-rich synthetic zeoUtes reported in either the 1980s or the 1990s. The new aluminosilicate structures were all high silica or pure sihca in composition. It awaited the 2000s for new aluminosilicate zeolite materials with low to medium Si/Al to be reported (see below). 

Jacobs, P.A. and Martens, G.A. (1987) Synthesis in High-Silica Aluminosilicate Zeolites, Elsevier, Amsterdam, p. 321. 

In a typical aluminosilicate zeolite synthesis, sodium aluminate is dissolved in water along with some fraction of the additional sodium hydroxide that is needed in the reactant mixture. Separately, sodium siHcate is mixed with the remainder of the sodium hydroxide. The two solutions are combined using the required mix order and agitation level, resulting in the initial zeolite gel. In some cases this initial gel is aged at an intermediate temperature for a time to allow evolution of... 

Rollmann, L.D. (1976) Manufacmre of crystalline aluminosilicate zeolites. US Patent 3,939,246. 

The windows to the channels thus form a three-dimensional sieve with mesh widths between about 300 and 1000 pm, thus the well-known name molecular sieve for these crystalline aluminosilicates. Zeolites thus have large internal surface areas and high sorption capacities for molecules small enough to pass through the window into the cavities. They can be used to separate mixtures such as straight-chain and branched-chain hydrocarbons. 

Oxides of transition metals can act as acid-base or redox catalysts. Oxides of non-transition metals (AI2O3, SiOj) are, however, good acid-base catalysts. There is a large family of aluminosilicate zeolitic acids (e.g. H -ZSM-5, H-mordenite). Micropor-ous aluminium phosphates (ALPOs) can be modified to yield acidic SAPOs (Si replaces... 

In 1993, UOP commercialized an improved Pt-based catalyst, 1-210. This catalyst is based on a molecular sieve, but not an aluminosilicate zeolite. UOP claims that yields are about 10% better than those for 1-9 catalyst. EB to xylenes conversion is about 22—25% with a Cg aromatics per pass loss of about 1.2—1.5%. As discussed below, UOP s Isomar process can also use zeolite catalysts which convert EB to benzene rather than to xylenes. UOP has licensed over 40 Isomar units. 

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