Seed silicalite

From our earlier experiences, we have learned that the zeolite seed hydrothermal approach is rather difficult to reproduce. Consequently, a dvee-step-synthesis procedure involving the preparation of zeolite nanoprecursors (NPs) by a short hydrothermal step, the flocculation of these NPs using a sui ctant, and the steaming of the NPs/surfactant composite to produce the final material was developed. We have recently demonstrated that aggregates of less than 30 nm silicalite nanocrystals can be prepared from this procedure. We further discovered that the nature of the as-collected NPs was very much dependent on the stirring time of NPs/CTAMeBr flocculants. Under identical steaming condition, the 3 h-stirred NPs were converted into nanocrystals of silicalite-1, whereas. 

Based on these observations, Wang and Caruso [237] have described an effective method for the fabrication of robust zeolitic membranes with three-dimensional interconnected macroporous (1.2 pm in diameter) stmctures from mesoporous silica spheres previously seeded with silicalite-1 nanoparticles subjected to a conventional hydrothermal treatment. Subsequently, the zeolite membrane modification via the layer-by-layer electrostatic assembly of polyelectrolytes and catalase on the 3D macroporous stmcture results in a biomacromolecule-functionalized macroporous zeolitic membrane bioreactor suitable for biocatalysts investigations. The enzyme-modified membranes exhibit enhanced reaction stability and also display enzyme activities (for H2O2 decomposition) three orders of magnitude higher than their nonporous planar film counterparts assembled on silica substrates. Therefore, the potential of such structures as bioreactors is enormous. 

Coating the support with the seeds is a critical task. Different strategies are proposed in the literature. The supports can be seeded by simple contact (deep coating for a few minutes) with a suspension of zeolite crystals at an appropriate pH, and subsequent washing to keep only a surface monolayer [51], Fig. 2 shows a thick layer of silicalite-1 seeds on (XAI2O3 support after 3 h contact with the seed suspension. 

Fig. 2. aAl203 support (200 nm pore size) covered with a thick and close pack layer of silicalite-1 seeds (3 h contact time) and dried for 6 hours at 155 °C [101 ]. 

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. 5. FESEM observations of the MFI membranes prepared at different temperatures, by MW-assisted secondary growth, from a layer of MW derived silicalite-I seeds [101 ].

A detailed study of the growth process and the structural evolution of silicalite-1 (MFI) films was undertaken with the aid of grazing incidence synchrotron X-ray diffraction. [65] The diffraction data of the adsorbed and grown zeolite films at different incident and exit angles reflect the distribution of the crystal orientation along the film thickness. The films were prepared via assisted adsorption of nanoscale MFI seed crystals, followed by calcination and subsequent hydrothermal synthesis on the seed layers. The adsorbed (multi-) layer of seed crystals consists of randomly oriented crystals. With progressing hydrothermal growth, the film surface becomes smoother and a preferred crystal orientation with the b-axis close to vertical to the substrate develops. 

Supported zeolite Y, silicalite-1, A and a composite of silicalite-1 on A can be synthesised in a membrane configuration in a reproducible manner. Synthesis techniques using seeds were applied in the membrane preparation. The double layer type membrane has great potential in the bifunctional operation in one integrated unit. 

All the above mentioned high perm-selectivity of zeolite membranes can be attributed to the selective sorption into the membranes. Satisfactory performance can be obtained by defective zeolite membranes. Xylene isomers separation by zeolite membranes compared with polymeric membranes are summarized in Table 15.4. As shown, zeolite membranes showed much higher isomer separation performances than that of polymeric membranes. Specially, Lai et al. [41] prepared b-oriented silicalite-1 zeolite membrane by a secondary growth method with a b-oriented seed layer and use of trimer-TPA as a template in the secondary growth step. The membrane offers p-xylene permeance of 34.3 x 10 kg/m. h with p- to o-xylene separation factor of up to 500. Recently, Yuan et al. [42] prepared siUcalite-1 zeolite membrane by a template-free secondary growth method. The synthesized membrane showed excellent performance for pervaporation separation of xylene isomers at low temperature (50°C). 

The orientation of crystal axes in a silicalite membrane can be controlled by a two-step process in which different structure-directing agents are used for (1) the growth of seed crystals, which lie down on the porous support, and (2) their subsequent growth to give an oriented membrane. 

FIGURE 11.11 AFM topographic and SEM images from the external surface of silicalite layers over seeded nonporous alumina substrates after synthesis time of (a and b) 35 min and (c and d) 50 min. 

Several types of zeolite membranes such as A-type, Y-type, silicalite, ZSM-5, etc. have been developed, and have been applied mainly to gas and pervapo-ration separations. Kumakiri et al. [43] prepared A-type zeolite membranes by hydrothermal synthesis with seed growth, and applied these to the reverse osmosis separation of water/ethanol mixtures. The zeolite A membrane showed a rejection of 40% and a permeate flux of 0.06 kg m h for 10 wt% ethanol at a pressure difference of 1.5 MPa, while a permeate flux of 0.8 kgm h and a separation factor of 80 were obtained in PV. 

Fig. 10. SEM photomicrograph (at lOOOX) of silicalite seed crystals (the largest crystals present) and the new population of silicalite crystals formed by initial breeding. The porcupine morphology is illustrated by the small crystals growing out of the seed crystal surfaces. Crystals grown in the NH J-silicalite system by Gonthier [68], using the composition reported in [5]...

Most of the mentioned problems can, however, be overcome by addition of small amount of seed crystals (ZSM-5, silicalite-1) in the TPA+-free reaction mixture [6, 27, 35, 38, 40, 45, 50], Seed induction synthesis is a well developed strategy which could not only shorten the duration of synthesis, but also control the product properties [51] addition of seed crystals results in the formation of zeolite ZSM-5 with high degree of crystallinity and a narrow size distribution at short synthesis times [38,40]. Such method has been used for the synthesis of zeolites with various framework topologies [52]. Recently, small sized zeolites were obtained fastly, using this approach [53]. This method, although old, is still under developing. 

Batch oxide molar chemical composition of the reaction mixture (hydrogel) for the synthesis was 1.0 AI2O3/IOO Si02/28 Na20/4000 H2O. A series of silicalite-1 nanocrystals having different mean diameters (90, 180, 220, 260 and 690 nm Fig. 2) were prep>ared by synthesis from clear solution and used as seeds for the further growth of ZSM-5 nanocrystals. 

Fig. 4. SEM images of ZSM-5 samples synthesized with using 4 wt. % of 90 nm (a), 180 nm (b), 220 nm (c) and 690 nm (d) silicalite-1 seed crystals. The scale bars in all the images are of 2 pm. (Adopted from Ref. [50] with permission of Publisher.)...

Fig. 10 shows that the fraction of the small single particles (ZSM-5 crystals formed by growth of silicalite-1 seed crystals) having the size 400 - 600 nm increases with increasing alkalinity. A, of the reaction mixture and that, for a given batch alkalinity, the fraction of the single 400 - 600 nm particles increases with increasing value of B. 

Fig. 14. Particle size distributions by volume of the products obtained by hydrothermal treatment (at 483 K for 2 h) of the reaction mixtures having aged at room temperature for 0 (A), 3 (B) and 48 h (C) in the presence of silicalite-1 seed crystals. (Adopted from Ref. [55] with permission of Publisher)...

Fig. 21 displays the PSD curves (by number) of the 260 nm sdicalite-l seed crystals (Fig. 21A) and that of the ZSM-5 zeolites obtained using same type of seeds (Fig. 21B). It can be observed that both curves possess almost the same shape and trend. The only difference is the size of crystals. Such phenomenon, although simple, clearly revealed the fact that the total number of crystals during the process of crystallization remains imchanged. It can be deduced that the nucleation process is completely suppressed under the current synthesis condition (SDA-free system with the presence of seed crystals), further proves the only occurrence of the growth of ZSM-5 zeolites on the surface of silicalite-1 seeds.

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