Zeolites pure-silica

Sefcik J, Demiralp E, Cagin T, Goddard WA (2002) Dynamic charge equilibration-morse stretch force field application to energetics of pure silica zeolites. J Comput Chem 23(16) 1507-1514... 

Keywords pure silica zeolites, solid state NMR. 

Pure silica zeolites or molecular sieves are metastable with regards to the thermodynamic stable polymorph at ambient conditions, a-quartz. However, they are... 

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. 

Burkett, S. L. and Davis, M. E. Mechanism of structure direction in the synthesis of pure-silica zeolites. 2. Hydrophobic hydration and structural specificity, Chem. Mater., 1995, 7, 1453-1463. 

Although some pure silica zeolites (notably the Silicalites) are known, it is not possible to obtain a zeolitic alumina. Indeed, according to Loewenstein s rule, to avoid any direct A1—O—A1 linkage, the permitted Si/Al ratio is at least 1.0 (i.e. y > x). 

M. J. Mora-Fonz, C. R. A. Catlow and D. W. Lewis, The role of solvation and pH in the nucleation of pure silica zeolites in Studies in Surface Science and Catalysis Molecular Sieves From basic research to Industrial Applications parts A and B, 2005, 158, 295. 

On the basis of these structural features, it is easy to understand why zeolites constructed by Si and A1 cannot have extra-large pores. Nevertheless, pure-silica zeolites with 14-membered rings, i.e. CIT-5 and UTD-1, have been synthesized recently, and further investigation into crystallization mechanisms in combination with the vast experimental data available and with theoretical simulation and computation may help us to rationally design and synthesize extra-large microporous aluminosilicate molecular sieves with special channels such as multidimensionally interconnected and chiral ones. 

In addition to pure-silica zeolites, titaniumsilicates such as TS-1 and TS-2, Ti-BEA, borosilicates, zincosilicates, and microporous aluminophosphates can be synthesized by using the DGC (SAC) method as well/1061... 

Figure 4.37 SEM images of pure-silica-zeolite spin-on films. Top views of films at second-stage synthesis times of a) 8 h, b) 10 h, c) 69 h. d) Cross-sectional view of a film with a second-stage synthesis time of 69 h. Reproduced with permission from [138], Copyright (2004) Wiley-VCH...

They successfully prepared pure-silica zeolite (PSZ) MFI films by in-situ, seeded growth, and spin-on methods. b-Oriented in-situ films yielded k values of 2.7 and elastic modulus (E) values of 30-40 GPa.[1521 Spin-on MFI films that incorporated y-cyclodextrin... 

Figure 4.44 Elastic modulus versus dielectric constant for amorphous porous silicas and pure-silica zeolites. The open circles and solid data-fitting line correspond to experimental data taken from Xu et at. Reproduced with permission from [150]. Copyright (2006) Wiley-VCH...

Over the past 10 years, more crystallization systems supporting the solid-phase mechanism have been reported. Remarkable examples included Tsapatsis s study on the crystallization of zeolite L by using High Resolution Transmission Electron Microscope (HRTEM) technique in 1996 [23] Serrano s study on the crystallization of TS-1 by using a couple of spectral techniques in 1996[24] and on the crystallization of pure silica zeolite beta under the presence of F [25] and Uguina s study on the crystallization of TS-2 by using multiple techniques.[26] The commonly used dry-gel synthesis of zeolites in recent years (DGC and SAC, see Section 3.2.5, Sub-section Dry Gel Conversion in Chapter 3) partially confirms the rationality of the solid-phase mechanism. 

PA. Barrett, M.A. Camblor, A. Corma, R.H. Jones, and L.A. Villaescusa, Synthesis and Structure of as-prepared ITQ-4, a Large Pore Pure Silica Zeolite The Role and Location of Fluoride Anions and Organic Cations. J. Phys. Chem. B, 1998, 102, 4147-4155. 

Fig. 2 (A) SEM top view of fo-oriented pure-silica-zeolite (PSZ) MFI monocrystal-thick film (B) cross-sectional view of the sample in (A) after slight polishing (C) schematic illustration of molecular sieving in h-oriented PSZ MFI monocrystal-thick film. A distance was intentionally kept between the MFI film and the electrode so that the sieving behavior can be illustrated more clearly. (From Ref...

Wang, Z. Wang, H. Mitra, A. Huang, L. Yan, Y. Pure-silica zeolite low-k dielectric thin films by spin-on process. Stud. Surf. Sci. Cata. 2001, 135, 3217-3224. 

Li, S. Yan, Y. Organic-functionalized pure-silica-zeolite MFI ultra low-k films. Chem. Mater. 2005, 77 (7), 1851-1854. 

The well-defined porous structure of zeolitic materials makes these materials true shape-selective molecular sieves. The presence of charge-compensating cations such as alkaline and alkaline earth cations, protons, etc. within the inorganic frameworks adds ion exchange and catalytic properties. Moreover, the hydrophobic nature of pure silica zeolites or the hydrophilic nature of aluminosilicates makes these solids useful as specific adsorbents of organic molecules or water in the gas or liquid phase. 

In the relentless quest for ever faster computer circuitry, the dielectric constant of the insulating layers between conductors on the chip is becoming a major issue. This constant should be as small as possible while the mechanical properties of the dielectric material must withstand the subsequent processing steps and ensure the integrity of the computer microprocessor. Nanoscale zeolite crystals, particularly pure silica zeolites, have been proposed as candidates for thin films with low dielectric constant (low k). As an example, suspensions of nanoscale crystals of the pure zeolite silicalite-1 (MFI-typc) were used for spin-on deposition of thin dielectric layers.[101] The as-deposited films were subsequently calcined at 450 °C in order to remove organic molecules and to consolidate the films. The authors report low dielectric constants (although the adsorption of humidity must be controlled) and satisfactory mechanical properties of their films. 

---