Silicalite, hydrophobic zeolitic silica

The most-tested hydrophobic zeolite in adsorption of organic compounds from water solutions is silicalite-1 [89], This material is a molecular sieve with an MFI structure composed of pure silica. The MFI framework has a 10-MR channel system with elliptical pores having diameters of 5.2 x 5.7 A [83], Additionally, other zeolites, as the all-silica P-zeolite [216] which possesses a three-dimensional, 12-numbered ring, interconnected channel system with pore diameters of 7.1 x 7.3 A [83] have been used in the elimination of methyl tert-butyl ether (MTBE) from water solutions [88],... 

In 1978, the same year that the structure of ZSM-5 was first described, Flanigen and her co-workers reported the synthesis, structure and properties of a new hydrophobic crystalline silica molecular sieve (Flanigen et al., 1978). The new material, named Silicalite (now generally called Silicalite-I), has a remarkably similar channel structure to that of ZSM-5 but contains no aluminium. It was pointed out by the Union Carbide scientists that, unlike the aluminium-containing zeolites, Silicalite has no cation exchange properties and consequently exhibits a low affinity for water. In addition, it was reported to be unreactive to most acids (but not HF) and stable in air to over 1100°C. 

Table 7.1 lists the typical sorbents used their uses as well as strengths and weaknesses. The four major commercial adsorbents are the following zeolite molecular sieves (zms), activated alumina, silica gel, and activated carbon. The surfaces of activated alumina and most molecular-sieve zeolites are hydrophilic, and will preferentially adsorb water over organic molecules. Silicalite, which is a hydrophobic zeolite, is the main exception. Activated carbon, on the other hand, preferentially adsorbs organic and non-polar or weakly polar compounds over water. The surface of silica gel is somewhere in between these limits and has affinity for both water and organics. Detailed information about each of these classes of adsorbents can be found in Refs. [1,4, 6, 7]. 

Four main types of porous silica adsorbents have been identified compacts of pyrogenic powders, precipitated silicas, silica gels, and zeolitic silicas. The importance of porosity relative to the adsorptive properties of each group is reviewed, with particular reference to the adsorption of nitrogen, argon, and water vapor. The differences in size and specificity of these adsorptive molecules may be exploited to explore the surface properties of each grade of silica. A notable feature cf Silicalite I, which is the best known of the zeolitic silicas, is its remarkable hydrophobic character. Furthermore, the uniform tubular pore structure of this microporous silica is responsible for other highly distinctive properties. 

It is well known that high-silica zeolites such as silicalite are hydrophobic. Addition of hexane to ZSM-5 does not affect the NMR signal from water, but addition of butanol has a very marked influence. This indicates that butanol displaces water from the intracrystalline space to the outer surface of the zeolite particles. 

The surface character of the AlPO molecular sieves differs from that of the silica molecular sieves even though both framework types are neutral with no extra-framework cations. The molecular sieve silicalite is hydrophobic and the AlPO molecular sieves are moderately hydrophilic. Zeolites are hydrophilic due to the interaction of the dipole of the Hz0 molecule with the electrostatic fields of the anionic aluminosilicate framework and the balancing nonframework cations. The hydrophilicity of the AlPOi, materials is apparently due to the difference in electronegativity between Al(1.5) and P(2.1). Neither mechanism is possible with silica molecular sieves. The AlPOi, molecular sieves do exhibit less affinity for HzO than the hydrophilic zeolites such as Type A and Type X. 

A new family of crystalline molecular sieves, 2) having aluminophosphate frameworks was synthesized. Strict alternation of A1 and P on the tetrahedral nodes yields neutral oxygens in contrast to the aluminosilicate zeolites, and non-framework cations are not needed for charge balance. Whereas a microporous silica (silicalite, 3 ) with neutral oxygens is hydrophobic, the aluminophosphate sieves are moderatley hydrophilic. 

A number of microporous polymorphs of crystalline silica can now be prepared. One procedure is to attempt the dealumination of a readily available zeolite another approach involves direct synthesis, for example, of ZSM-5, in the form of Silicalite I (31). The considerable amount of recent interest shown in these Al-free zeolites (or porotectosilicates) has been stimulated by the uniformity of their channel structures and their hydrophobic nature. 

The Si/Al ratio in a zeolite is never less than 1.0 but there is no upper limit and pure silica analogs of some of the zeolite structures have been prepared. The adsorptive properties show a systematic transition from the aluminum-rich sieves, which have very high affinities for water and other polar molecules, to the microporous silicas such as silicalite which are essentially hydrophobic and adsorb n-paraffins in preference to water. The transition from hydrophilic to hydrophobic normally occurs at a Si/Al ratio of between 8 and 10. By appropriate choice of framework structure, Si/Al ratio and cationic... 

Defective Silicalite is an all-silica MFI zeolite, which is non-hydrophobic and weakly acidic as a consequence of the abundant polar defects (Si-OH nests) generated by the stmcture to compensate the Si atoms vacancies [7]. IR spectra in the vqh stretching frequency region give a clear evidence of the presence of Si-OH nests, which are characterized by a different geometrical arrangement according to the synthesis procedure and/or post-synthesis treatments, as illustrated in ref. [36]. 

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