Growth silicalite

For elevated gravity reactions, these yields increased to over 55% based on total silica in the initial reaction mixture. A discussion of how elevated gravity affects nucleation, growth, yield and crystal size of silicalite is presented. 

FIGURE 10.9 Scanning electron microscopy (SEM) analysis of silicalite membranes over nonporous alumina substrates after 20 h of secondary growth synthesis conditions. Left top view. Right cross section. 

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. 

Schoeman BJ, Erdem-Senatalar A, Hedlund J, and Sterte J. The growth of sub-micron films of TPA-silicalite-1 on single crystal silicon wafers from low-temperature clear solutions. Zeolites 1997 19(l) 21-28. 

The band at about 2162 cm appears together with the perturbation of surface silanol groups, that is diminishing of the band at about 3750 cm" and growth of that of hydroxyls perturbed by adsorbed CO molecules, and is due, thus, to molecules hydrogen bonded to the surface silanol groups. This kind of specific CO adsorption is the only one for pure silica samples like amorphous aerosil [18] or silicalite, and could be detected spectroscopically for most of the other silica-containing systems. 

The synthesis and characterisation of silicalite-1 membranes on porous alumina ceramic supports have been described here. The growth of the silicalite-1 membrane could be optimised by controlling the hydrothermal synthesis conditions. It has been shown that by controlling the synthesis conditions it is possible to optimise the growth and structure of silicalite-1 membranes. Thus at lower synthesis temperatures (150 °C), the growth of silicalite inside the macro-pores of the ceramic support is favoured. At higher temperatures (190 °C), thick, well crystallised zeolite layers develop from the surface of the support. A more stable membrane is... 

Normally, increase of the crystal growth rate caused by an increase of crystallization temperature is much higher than that caused by an increase of the nucleation rate. Thus, big crystals could be obtained at high temperature in a short crystallization time (e.g., NaX, Silicalite-I). Crystallization temperature can affect the morphology of the crystals as well because the activation energy of the crystal faces is related to crystallization temperature. 

The procedure of Zhdanov and Samulevich enables the calculation of isothermal nucleation rate profiles from determinations of growth rate and crystal size distribution [16,82]. Originally implemented in analyses of zeolite Na-A [83] and Na-X [82] crystallisation, the method has subsequently been applied to other zeolite systems, including silicalite [84,85]. If it is supposed that all the crystals in a batch have the same (known) growth rate behaviour, the total growth time of each crystal can be calculated. Assuming also that the nuclcation point for each crystal can be obtained by linear extrapolation to zero time, the nucleation profile for the whole batch can be determined from their final sizes. 

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 ].

Porous materials can also be coated with zeolite films by direct synthesis. For example, microcellular SiOC ceramic foams in the form of monoliths were coated on their cell walls with thin films of silicalite-1 and ZSM-5 using a concentrated precursor solution for in situ hydrothermal growth (Fig. 9).[62] The zeolite-coated monoliths show a bimodal pore system and are thermally stable to at least 600 °C. A related strategy is based on the conversion of macroporous Vycor borosilicate glass beads, having pores of about 100 nm, to MFI-type zeolite-containing beads retaining the same macroscopic shape.[63] This conversion was achieved by hydrothermal treatment with an aluminium source and a template such as TPABr. 

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. 

Effect of NaOH, TPAOH, and TPABr Concentration on the Growth Rate and Morphology of Silicalite-1... 

The objective of this study was to systematically vary the concentrations of NaOH, TPAOH and TPABr in the synthesis of silicalite-1 and to evaluate how these changes affect the crystal size, morphology and growth rate. A simple reaction system was... 

Figure 4. Model for the growth of silicalite from D5R (adapted from Ref. 25, 28)...

Results presented in this report represent a comprehensive, systematic study on the effects of NaOH, TPAOH and TPABr addition on the synthesis of silicalite-1. From these data, the following conclusions may be drawn. First, the length to width (aspect) ratio of silicalite increases with decreasing alkalinity of the reaction mixture. This ratio is in reasonable agreement with the growth rate ratios for the crystal length (R ) and width (Rw). 

---