Role of the water content of clear synthesis solutions on the thickness of silicalite layers grown on porous alpha-alumina supports

In situ hydrothermal synthesis of silicalite layers on rnacroporous alpha-Al2O3 disks were performed from a batch composition of 6.5Na(2)O:25SiO(2):xH(2)O:6.9TPABr by varying the water content of a clear crystallization solution from 500 to 2000 moles at 200 degreesC. As the water content of the clear solution was increased, continuous and thinner silicalite layers formed from smaller crystals were obtained and the layer quality was improved. The silicalite layer which was synthesized from the most concentrated batch was formed from crystals with an average size of 100 mum and had a thickness of 36 mum. The layer thickness and average crystal size decreased to about 8 and 7 mum, respectively, when the batch containing 1400 moles of water was used for synthesis. The results showed that the layer thickness on porous alumina supports could be adjusted by varying the water content of the synthesis solution. As-synthesized membranes were impermeable to N-2 after a single crystallization step. The membranes were tested for possible pinholes and microcracks by measuring single gas permeations of N-2 and SF6 after calcination at 500 degreesC by a slow and step-wise calcination procedure. The calcined membranes showed an N-2 permeance of 16.3 mmoV(bar m(2) s) and an ideal selectivity of 1630 for N-2/SF6 at room temperature. The high selectivity showed, that the silicalite layer is free of large interzeolitic pores and the applied calcination procedure is, suitable for TPA removal without forming microcracks

Synthesis of submicron silicalite-1 crystals from clear solutions

Discrete and monodisperse submicron crystals of silicalite-l with a mean size of 0.3 mum were synthesized from clear crystallization solutions. The effects of silica content, alkalinity of batch and the nature of silica source on the product yield, crystal morphology and particle size distribution were investigated. The crystal shape was sphere-like or hexagonal twinned disks when silicic acid was the silica source. Change of silica source to sodium silicate solution leads to the formation of rounded-edged-hexahedron crystals. Silica content of batch does not influence crystal morphology. Alkalinity of clear crystallization solution has a strong effect both on the particle size distribution and yield of product. Increase of alkalinity caused bimodal particle size distribution and decrease of yield

Influence of nature of silica source on template-free synthesis of ZSM-5

Synthesis of ZSM-5 from template-free batches which preceded the preparation of template-free ZSM-5 layers on porous supports was studied to ascertain the effect of nature of silica source on the purity of template-free ZSM-5. Silicic acid and two colloidal silica sols were used as silica sources to prepare the template-free batches with a molar composition of 6.5Na(2)O:Al2O3:80SiO(2):3196H(2)O. One of the colloidal silica sols contained methanol as stabilizer while the other did not. The product purity and rate of crystallization increased when colloidal silica sols were used as silica source, however, use of silicie acid led to low purity and slow crystallization rate. The methanol in the colloidal silicic sol appeared to act as template to promote the crystallization and was occluded in the resultant ZSM-5 pores. The dissolution of the meta-stable ZSM-5 phase and formation of quartz was observed regardless of the nature of the silica source in case of prolonging the crystallization time more than 90 h at 200 degreesC

COMPOSITION VARIATIONS OF LATTICE-PARAMETERS OF TLIN(SE1-XTEX)2, TLIN(SE1-XSX)2, AND TLIN1-XGAXSE2 MIXED-CRYSTALS

TlInSe2 and TlInTe2 crystallize in a tetragonal cell (space group 14/mcm) (Muller et al.). A primitive cell of these ternary crystals contains two formula units. All the Tl atoms in these compounds are univalent. The In atoms are trivalent being in a tetrahedral environment consisting of Se or Te atoms. InSe4 or InTe4 fragments of a unit cell form the chains extended along c-axis coinciding with [001] directions. Such chains are bonded to each other by Tl atoms

CRYSTALLIZATION KINETICS OF SILICALITE .1. USE OF TETRAPROPYLAMMONIUM BROMIDE AND CHLORIDE AS TEMPLATES

Silicalite-1, the aluminum-free end number of the ZSM-5 zeolites, was synthesized from a batch composition of 3.25 Na2O . 40.0 SiO2 . 552 H2O . 2.00 TPA by using tetrapropylammonium bromide (TPA-Br) or chloride (TPA-Cl) as templates in the temperature range of 100 to 175-degrees-C in a batch system. Conversion of silica in the starting batch composition into silicalite-1 in the product was followed quantitatively. The activation energies of nucleation and crystallization were determined as 37.2 and 66.5 kJ/mol, respectively. The use of TPA-Cl as the template instead of TPA-Br results in longer induction period for crystallization to start and a larger crystal size in product

Crystallization field study for the formation of single phase sodium clinoptilolite: Batch composition, seed and temperature effects

Although clinoptilolite is the most abundant zeolite mineral in nature, its synthesis under laboratory conditions has been difficult. A partial crystallization field study was done for the synthesis of clinoptilolite based on a nominal batch composition of 2.1Na(2)O:Al2O3:10Si(2)O:110H(2)O to delineate the limits of composition and temperature within which sodium clinoptilolite can be produced as a single phase in high yields. Effects of temperature, SiO2/Al2O3 molar ratio in the batch composition, and the use of different raw materials in batch preparation were studied. The need for the use of seed crystals for reproducible synthesis of clinoptilolite was established. (c) 2009 WILEY-VCH Verlag GmbH & Co. KGaA, Weinhei

Crystallization field and rate study for the formation of single phase sodium-potassium and potassium clinoptilolite

Reproducible synthesis of clinoptilolite as the single crystalline phase was achieved in the narrow crystallization field at or around the nominal batch composition of 2.1(Na2O+K2O):Al2O3:10SiO(2):110H(2)O at 140 degrees C. Clinoptilolites of high purity were crystallized in the pure sodium or mixed sodium-potassium cation systems. Partial replacement of hydroxyl anions with the salts of carbonates or chlorides also yielded clinoptilolite as the single crystalline phase although at lower crystallization rates. (C) 2011 WILEY-VCH Verlag GmbH & Co. KGaA, Weinhei

The synthesis and sodium-silver ion exchange of sodalites

Aluminosilicate sodalites of three different types-chlorosodalite, basic sodalite, and oxalate sodalite-have been synthesized in their sodium forms using hydrothermal synthesis. The syntheses were investigated in terms of their relative rates of crystallization and final crystallinities using powder X-ray diffraction. These sodalites were subsequently loaded with silver by aqueous ion exchange. The exchange was studied primarily by monitoring the solution phase. Rate of exchange and equilibrium behavior have been established over the temperature range of 25-80degreesC for all three types. Extreme selectivity toward silver over sodium was observed, with little dependence on temperature. Both sodium and silver sodalites were characterized by powder diffraction, and crystallographic data for silver sodalites have been derived using Rietveld refinement. Electron micrographs of sodalites have been used for investigations of crystallite morphology and size. Silver sodalites have been observed to manifest various optical properties including photochromic, barochromic, thermochromic, and fluorescent behavior

Crystallization of silicalite-1 from clear synthesis solutions: Effect of template concentration on crystallization rate and crystal size

Synthesis of silicalite-1 powders and membranes from initially clear solutions with different tetrapropylammonium hydroxide or bromide concentrations was studied. While tetrapropylammonium bromide acts only as template, tetrapropylammonium hydroxide provides both the template and hydroxyl ions to the synthesis medium. The effects of template and hydroxyl ion concentration on the product yield, crystallization rate and crystal size were investigated. Pure and highly crystalline silicalite-1 was obtained with all compositions. The nucleation time decreases from 100 h to 20 h and the crystal size decreases from 3.5 mu m to 0.35 mu m as the template amount x is increased from 5 to 30 moles at a batch composition of 80SiO(2 center dot)xTPAOH center dot 1500H(2)O at 95 degrees C. Yield of silicalite-1 passes through a maximum at intermediate TPA concentration. (C) 2009 WILEY-VCH Verlag GmbH & Co. KGaA, Weinhei

Pozzolanic activity of clinoptilolite: A comparative study with silica fume, fly ash and a non-zeolitic natural pozzolan

WOS: 000274869600006Pozzolanic activity of clinoptilolite, the most common natural zeolite mineral, was studied in comparison to silica fume, fly ash and a non-zeolitic natural pozzolan. Chemical, mineralogical and physical characterizations of the materials were considered in comparative evaluations. Pozzolanic activity of the natural zeolite was evaluated with various test methods including electrical conductivity of lime-pozzolan suspensions; and free lime content, compressive strength and pore size distribution of hardened lime-pozzolan pastes. The results showed that the clinoptilolite possessed a high lime-pozzolan reactivity that was comparable to silica fume and was higher than fly ash and a non-zeolitic natural pozzolan. The high reactivity of the clinoptilolite is attributable to its specific surface area and reactive SiO(2) content. Relatively poor strength contribution of clinoptilolite in spite of high pozzolanic activity can be attributable to larger pore size distribution of the hardened zeolite-lime product compared to the lime-fly ash system. (C) 2009 Elsevier Ltd. All rights reserved.Scientific and Technological Research Council of Turkey [104M393]This study was funded by The Scientific and Technological Research Council of Turkey under Project Number: 104M393. The instrumental analyses in this study were carried out in Middle East Technical University Central Laboratory. Acknowledgment is given to Emeritus Professor P. Kumar Mehta from the Civil and Environmental Engineering Department at the University of California, Berkeley, for his helpful comments during the investigation