Synthesis of Porous Silicates

Abstract

The issues of importance and future concern in the synthesis of porous silicates and porous materials that contain a large fraction of silica, e.g. zeolites and other crystalline molecular sieves, are reviewed. The thermodynamics of zeolite synthesis are discussed, including a detailed thermodynamic analysis of the synthesis of pure-silica ZSM-5. The kinetics of porous silicate synthesis are reviewed, with particular emphasis on the control of porous structure formation through the use of organic structuredirecting agents. Ordered mesoporous materials are discussed in the context of distinguishing their features from zeolites in order to describe further the unique properties of each class of material. Finally, several unresolved issues in the understanding of the synthesis process are outlined, the resolutions of which would aid in the synthesis of porous silicates by design.

Catalytic cracking performance of alkaline-treated zeolite Beta in the terms of acid sites properties and their accessibility

“…Less extensive desilication of zeolite Beta in presence of TBAOH is assigned to its protective influence on zeolite structure, resulting from the known affinity of quaternary organic cations to surface of zeolites [37,38].…”

Assembly of Zeolites and Crystalline Molecular Sieves

“…6 These results show a clear relationship that as the molar volume is increased (or the framework density is decreased), the enthalpy increases relative to quartz. Molar volume is defined as cm 3 mol −1 SiO 2 , framework density (FD) is the number of T atoms per nm 3 , and the enthalpy is kJ mol −1 SiO 2 . 5,6 The data of Petrovic et al and Piccione et al show that the transformation of amorphous silica to crystalline silica is not hindered significantly in terms of energetics.…”

“…Helmkamp and Davis were the first to do this for the synthesis of pure-silica ZSM-5 using tetrapropylammonium fluoride as the SDA. 3 In a more recent work, Piccione et al investigated the thermodynamic role of the SDA in the molecular sieve synthesis by carefully measuring the interaction energies between the silica frameworks and the enclathrated organic SDAs.…”

“…Ratios of silicon to aluminum are not less than 1 (i.e., there are not two aluminum atoms bonded to the same oxygen atom), and this arrangement is rationalized by Loewenstein's rule that states that isolated negative charges (Al − -O-Si-O-Al − -O-Si) will be more stable than adjacent negative charges (Al − -O-Al − -O-Si-O-Si). 2,3 Silicon can also be replaced with compounds such as phosphorus, but never in a manner to give a positive framework charge. 4 Here, we will focus on the fully tetrahedral networks, particularly because the bulk of the literature on crystalline microporous materials involves them.…”

Síntesis de materiales nanoestructurados de sílice a partir de tensoactivos orgánicos iónicos y no iónicos

“…El pH tiene una influencia profunda sobre la fijación del radical OHen el enlace Si-O, y el fluoruro interrumpe los enlace disiloxi; Guth y colaboradores [92] encontraron que los geles que contenían fluoruro se estabilizan a pH's más bajos, lo que facilitaba la sustitución de Si y Al por otros átomos T. El fluoruro cataliza la polimerización de la sílice, debido a la neutralización electrostática entre los oligómeros adyacentes [93]. Dutta y colaboradores [94] encontraron que la presencia de alcoholes en los geles tienen un profundo efecto cinético en la aceleración de la cristalización, probablemente a través de la organización molecular del H 2 O y de los cationes [95][96][97].…”