Crystal silicalite

Zeolite single crystals may serve as zeolite membrane models. We mention the early elegant work of Hayhurst and Paravar [10] on an oriented large silicalite-1 crystal embedded in epoxy resin. 

In a more recent membrane model study [11] a large (twinned) silicalite-1 crystal (100 100 300 p.m) was embedded in an epoxy matrix, using an aluminium gasket as a support. Polishing improved the crystal surface exposure. Mcropore diffiisitivities were measured for benzene, toluene and the xylenes. 

This work has been extended towards arrays of individual oriented large silicalite-1 crystals in a gas tight matrix (Figure 3). 

Thus Geus et al. studied oriented silicalite-1 crystals in an expoxy matrix on perforated metal with each crystal fully covering one hole of the metal support [12], Geus also investigated [13] monolayers of silicalite-1 crystals on macroporous alumina covered with a thin clay film. Zeolite surface coverages of 75% were achieved (Figure 4). 

The permeation examples that are given here are based on own work and have been published in different articles [61-66]. The membrane used is of the asymmetric type and consists of a 40 pm thick layer of intergrown silicalite-1 crystals on a 3 mm thick layer of highly porous sintered stainless steel. The geometric surface area amounts to 3 cm. Stainless steel has the advantage of easy mounting in all types of equipment which facilitates practical application compared to ceramic supports. 

O. Pachtova, M. Kocirik, A. Zikanova, B. Bernauer, S. Miachon, and J.-A. Dalmon, A Comparative Study of Template Removal from Silicalite-1 Crystals in Pyrolytic and Oxidizing Regimes. Microporous Mesoporous Mater., 2002, 55, 285-296. 

In order to avoid the infiltration of seeds in the support and to develop ultra-thin membranes (typically 500 nm thick) with a high permeability, a masking techniques has been recently developed in Lulea University [111]. A solution of poly methyl methacrylate (PMMA) in acetone was applied and dried on the support top surface. The interior of the support was subsequently filled with wax and the protective PMMA layer was dissolved in acetone. The masked support was then seeded with a monolayer of silicalite-1 crystals before being submitted to the classical hydrothermal and calcination steps. 

Fig. 9. SEM images of the fracture surface of SiOC microcellular foam coated with silicalite-1 crystals synthesized at 150 °C (a, b) and 125 °C (c, d).[62]...

After establishing that the use of promoter drastically reduces the synthesis time of zeolites, efforts were made to find out whether synthesis temperature can be reduced using these promoters. Silicalite -1 (Si-MFI) was chosen for study because of its simplicity and the requirement of less crystallization time, the effect of promoter (NaH2P04) on the crystallization time of Silicalite-1 at various temperatures, ranging from 363-463 K, shows that the reduction in both, the crystallization time at a particular temperature as well as faster crystallization at relatively lower temperature (363 - 363 K) in the presence of promoter (vis-a-vis in its absence) could be accomplished. For example, the time taken at 343, 358 and 373 K was 36, 14 and 8 h, respectively, in the presence of promoter NaH2P04. In the absence of promoter, while at 343 K no crystalline material was obtained even after 5 days, Silicalite-1 crystallized after ca. 48 and 32 h at 358 and 373 K, respectively. 

A recent study on interactions at low temperature (20-60 C) of H2 0, and D2O with TS-1 adds new information related to this argument [45].The Interaction of H20 with large Sillcalite-1 crystals (>1 mm) does not produce variation on 0-MAS-NMR spectra while the interaction of H20 with small Silicalite-1 crystals (-... 

With the TEX-PEP technique experiments on the diffusion and adsorption of mixture of n-hexane/2-methylpentane in large silicalite-1 crystals have been performed. By modeling the experimental tracer exchange curves values of intracrystalline diffusion coefficient and adsorption constant were obtained. Slight preference for the adsorption of /t-hexane was found. Diffusivity of -hexane sharply decreases with increasing fraction of its isomer, since the last one occupies channel intersections thus blocking zeolite network. 

Fig. 34 Microscopic images of a typical silicalite-1 crystal in two different orientations. The hourglass structure is made visible by using the shearing mechanism of the microscope. The length scale in the x,y, and z directions is shown in micrometers...

Fig. 35 Schematic representations of the internal structiu-e of silicalite-1 crystals a according to [73-75] b according to [33]...

Figure 36 compares the actual measurements of transient concentration profiles during molecular uptake by a silicalite-1 crystal as displayed in Fig. 34 with uptake simulations. The diagrams show the integrals of concentration in the x direction as a function of z (this is in longitudinal extension of the... 

The Silicalite-1 crystals used in these experiments were highly crystalline and "coffin shaped with dimensions of260 x 40 x 40 pm. ... 

Every analytical instrument has its own restriction on its use for analysis so, a truly in situ measurement under identical synthesis condition may not be always possible. In other words, often compromises have to be made. However, it is possible to work around this problem by adapting the reaction conditions. Examples include the synthesis of sihcahte-l at room temperature [23], or by looking at the reverse process, that is, the dissolution of silicalite-1 crystals [27]. Much insight could also be obtained by these approaches. Here, however, we choose to focus on measurements where the analysis technique has been adapted to suit the hydrothermal reaction conditions, as this is the topic of this chapter. 

Several mixed matrix membranes made of silicalite-1 crystals of different size and shape embedded in Teflon AF 1600, Teflon AF 2400 and Hyflon AD 60X have been prepared. The gas permeation properties of the membranes have been tested with pure gases, and some experiments with n-C4Hio/CH4 mixtures have also been carried out on silicalite-l/Teflon AF 2400 membranes. 

Figure 6.2 Droplet of water on a layer of fluorophilic 80 nm silicalite-1 crystals...

The hybrid membranes are designated as follows e.g. AD60 1500 42 indicates a membrane containing Hyflon AD 60 and 1500nm silicalite-1 crystals, in which the mass of the zeolite is 42% of the total AF24 dC5 40 indicates instead a membrane containing Teflon AF 2400 and MFl dC5 aystals, in which the mass of the zeolite is 40% of the... 

The data regarding Teflon AF1600 mixed matrix membranes therefore seem to indicate that, when the zeolite particles are small, the gas flow runs mainly around them, whereas with larger particle sizes the overall resistance increases, and the gas molecules have a higher probability to cross the membrane surface and pass through the zeolites. For this reasons some membranes have been prepared with silicalite-1 crystals of about 1.5 pm and with dC5 MFL... 

Defect-free membranes comprising zeolites and amorphous glassy perfluoropolymers can be prepared by modifying the surface of the filler. The pure gas permeation experiments of a series of Teflon AF 1600 membranes with various amounts of 80 and 350nm silicalite-1 crystals cannot be interpreted on the basis of the Maxwell model, but are compatible with a model in which a barrier to transport exists on the zeolite surface and a lower density polymer layer surrounds the crystals. With a small zeolite size (80nm) the low density layers around the crystals may coalesce and form percolation paths of lesser resistance and less selectivity. Silicalite-1 crystals improve the CO2/CH4 selectivity of Hyflon AD60X, and drive the N2/CH4 selectivity beyond the Robeson s upper bound. It also turns out that the presence of silicaUte-l crystals, like fumed silica, promote the inversion of the methane/butane selectivity of Teflon AF2400 in mixed gas experiments. 

In composite materials coatings of laterally oriented silicalite-1 crystals are assumed to be a (shape) selective component in catalysis. A catalytic membrane is obtained if a noble metal coating is applied after growth of the crystal layer, which is self-supporting or bonded to a meso-porous support. Also the catalytic site can be applied onto the support before the in situ growth of the zeolite layer. 

Figure 5 AFM picture of lateraify oriented silicalite-1 crystals on a Fe Oy layer, image size is 5 x 5 /im.

Figure 6 HRSEM photograph of silicalite-1 crystals grown on a continuous layer of FeyOy.

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. 

Table 7. Comparable studies from the literature, regarding the crystallization of ultra-small silicalite-1 crystals from clear solutions [59,60] ...

Combining ZIF-8 437 silicalite-1 in poly-sulfone Udel P-3500 (ZIF-8/S1C-PSF MMM) did not improve the separation results compared to either SIC-PSF or ZIF-8-PSF alone in MMMs for CO2/CH4 and CO2/N2 gas mixtures. The relatively large silicalite-1 crystals probably could not be intercalated between small ZIF-8 particles. ZIF-8 alone produced the highest increase of CO2 permeability, which can be attributed to its textural properties and its small particle size, which however gave poorly dispersed aggregates. For O2/N2 and H2/CH4 gas separation, a ZIF-8-PSF MMM produced the best selectivity-permeability results compared to a Cu-BTC or SIC-PSF MMM. This may be due to an increase in free volume (as suggested for ZIF-8-polyimide MMMs) " together with an efficient molecular separation effect (based on diffusion differences) because of the small pore aperture window of ZIF-8 (3.4 A, 0.34 nm) compared to Cu-BTC (6 A, 0.6 nm) and SIC (5.5 A, 0.55 nm). ... 

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