Colloidal silicalite

From nanoslabs to colloidal Silicalite upon heating... 

The water and n-hexane adsorption isotherms of the zeolitic mesoporous materials obtained are compared to that of a 4S0 nm colloidal silicalite-1 in Figure 5. The water adsorption isotherms are distinctively type HI, whereas the n-hexane isotherms are type 1. The lowest water isotherm was for the colloidal silicalite-1, where the first point measured for the n-hexane isotherm was already at 80 mg g. The amount of n-hexane adsorbed reached 2S0 mg g at high pressure, which roughly corresponds to the filling of silicalite-1 micropores. 

The steamed NPs is more hydrophobic then the directly calcined one, but slightly inferior to the colloidal silicalite-1. However, due to its huge mesopore volume, the loading capacity for hexane is much higher than that of colloidal silicalite-1. It could... 

A comparison between thermal and microwave syntheses of colloidal silicalite-1 has provided a clear demonstration of the separate contributions of room-temperature ageing, heating rate and synthesis temperature to the nucleation process. At ageing times up to about 50 days, the product crystal size obtainable from a single synthesis composition is sensitive to reaction temperature and heating rate. After this time, ageing-generated proto-nuclei are so numerous that the normal self-nucleation of the reaction mixture is suppressed and the product crystal size is independent of reaction conditions. There is a limit to the number of crystals which can be nucleated and this is an intrinsic property of the system. 

Takata. Y. Tsuru, T. Yoshioka, T. Asaeda. M. Gas permeation properties of MFI zeolite membranes prepared by the secondary growth of colloidal silicalite and application to the methylation of toluene. Microporous Meso-porous Mater. 2002, 54, 257-268. 

Excellent studies have been performed on the synthesis of ultra-small crystals of MFI-type zeolite [59,60]. In particular the preparation of suspensions of colloidal silicalite-1 crystals (less than 100 nm) with a narrow particle-size distribution have been extensively studied. 

The incorporation of nanofillers, for instance, zeolites, represents a promising strategy for improving membrane performance in PV. Dobrak et al. [123] prepared PDMS composite membranes and investigated the effect of two types of fillers, namely, commercial zeolite silicalite (CBV 3002) and laboratory-made colloidal silicalite-1, on membrane performance in the removal of ethanol from ethanol/water mixtures through PV. Filler incorporation increased membrane stability by cross-linking. Furthermore, the PDMS membrane filled with conunercial zeolites showed a significant increase of selectivity. Incorporation of CBV 3002 fillers into a PDMS composite membrane was also found to enhance the performance in PV tests of toluene removal from water [124]. 

Similarly, Sano et al. [1994] added colloidal silica to a stirred solution of tetrapropylammonium bromide and sodium hydroxide to synthesize a hydrogel on a stainless steel or alumina support with a mean pore diameter of 0.5 to 2 pm. The composite membrane is then dried and heat treated at 500 C for 20 hours to remove the organic amine occluded in the zeolite framework. The silicalite membranes thus obtained are claimed to be free of cracks and pores between grains, thus making the membranes suitable for more demanding applications such as separation of ethanol/water mixtures where the compound molecules are both small. The step of calcination is critical for synthesizing membranes with a high permselectivity. 

Finally, zeolite nanoparticles have been used as building blocks to construct hierarchical self-standing porous stmctures. For example, multilayers of colloidal zeolite crystals have been coated on polystyrene beads with a size of less than 10 p,m [271,272]. Also, silicalite-1 membranes with a thickness ranging from 20 to several millimeters and controlled mesoporosity [273] have been synthesized by the self-assembly of zeolite nanocrystals followed by high-pressure compression and controlled secondary crystal growth via microwave heating. These structures could be useful for separation and catalysis applications. 

Silicalite-1 membranes, supported on porous alumina ceramic discs, have been prepared by two different routes. In the first the zeolite membrane has been formed by in situ hydrothermal synthesis. Secondly a layer has been formed by controlled filtration of zeolite colloids. To optimise membrane stability, conditions have been established in which penetration of zeolite into the support sublayer occurs. The pore structure of these membranes has been characterised by a combination of SEM and Hg-porosimetry. The permeabilities of several gases have been measured together with gas mbeture separation behaviour. 

This novel route involves the retention of colloidal dispersions of zeolites (silicalite-1 described here) onto the surface of macroporous ceramic substrates. Silicalite-1 colloids were synthesised as described by Schoemann [14]. These were characterised by SEM and filtered through ceramic alumina discs. 

Films from the same research group were subsequently characterized with regards to their porosity, showing both zeolite microporosity and textural mesoporosity.[102] The above concept can be extended towards films with different binders, including organic polymers. Thus, two-component films comprised of nanoscale silicalite-1 and acrylic latex were deposited on silicon wafers via spin-coating.[103] In this case, a purified suspension of colloidal zeolites with sizes of 30 or 60 nm were first deposited followed by calcination. In a second step, a layer of acrylic latex was deposited, resulting in layers with dielectric constants between 2.0 and 2.5. 

Silicalite colloidal solutions with approximate diameters of 100 nm were prepared using distilled water, tetrapro-pylammonium hydroxide (TPAOM), and sodium hydroxide. Cylindrical ot-alumina microfiltration membranes (average pore diameter 1 pm, outer diameter 10 mm, iimer diameter 8 mmj were used as substrates for the zeolite membranes. A hydrothermal synthesis was carried out in a solution of distilled water, tetrapropylammo-nium bromide (TPABr), and sodium hydroxide in a molar composition of TPABr/Si02/H20/Na0H = 0.1/1/80/ 0 1 [2,26] Pqj. ZSM-5 membranes, sodium... 

Supercritical CO2 activation of a Naflon membrane prior to zeolite deposition was used to modify its structure. The resultant Nafion-zeolite composite membranes showed a dramatic decrease in methanol permeability (if the colloidal rather than the suspended Fe-silicalite-1 particles were used for the deposition) and a 19-fold higher selectivity compared with either the composite membranes prepared without previous supercritical treatment or the pure commercial Nafion-115 membrane. The method of the in situ synthesis of a zeolite inside the membrane pores was found to be very effective for preparing the composites, giving a sixfold higher selectivity for the composite manbrane compared with the pure Nation membranes (Gribov et al. 2007). 

Schoeman, B. J. 1998. Analysis of the nucleation and growth of TPA-silicalite-1 at elevated temperatures with the emphasis on colloidal stability. Microporous and Mesoporous Materials 22, no. 1-3 9-22. 

Gundy et al. [7] also proposed that silicalite nucleation occurred on, or in, amorphous gel rafts. The evidence for their proposed mechanism was the observation that samples taken at early times contained a proportion of amorphous material, and that optical and electron microscopy indicated a close association of new zeolite crystals and these amorphous particles. The authors concluded that the initial nucleation period was due to a heterogeneous nucleation mechanism, and arose from the presence of macroscopic or colloidal particles in the solution. Nucleation was thought to be a surface-facilitated phenomenon. While their proposed mechanism appears to be slightly different than that of Doktor et al. [57, 58], it nonetheless involved the participation of extraneous material. 

The following recipe was used to prepare a colloidal suspension of TPA-silicalite-1 ... 

Persson AE, Schoeman BJ, Sterte J, Otterstedt JE. The synthesis of discrete colloidal particles of TPA-silicalite-1. Zeolites 1994 14 557. 

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