Ali Çulfaz

Sorption properties of silicalite-1 of pure silica form : the influence of sorption history on sorption kinetics of critically sized molecules

Abstract Sorption properties of silicalite-1 samples of pure silica form for some parfinic and aromatic hydrocarbons of different molecular sizes were investigated. It was observed that sorption rates and apparent equilibrium capacities for critically sized molecules, m -xylene and o -xylene, depend on the sorption history of the sample. Sorption of m - and o -xylene proceeded extremely slowly on fresh samples of silicalite-1, whereas sorption rates of these molecules on silicalite-1 samples that had passed through a previous p -xylene sorption/regeneration step were significantly faster and equilibrium capacities reached about 92–95% of sorption volume of silicalite-1 when sufficient time was allowed. Uptake curves for m - and o -xylene were of the sigmoid-type, indicative of lattice distortions of silicalite framework. The results of this work, along with some recent, independent studies involving n.m.r. MAS and XRD methotds strongly suggest that sorption of some critically sized molecules causes subtle lattice distortions of high-silica ZSM-5 zeolites, modifying their sorptive behavior.

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:Al 2 O 3 :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 silicic acid led to low purity and slow crystallization rate. The methanol in the colloidal silica 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°C.

Synthesis of Submicron Silicalite-1 Crystals from Clear Solutions

Discrete and monodisperse submicron crystals of silicalite-1 with a mean size of 0.3 μ m 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.

Counterdiffusion of Liquid Hydrocarbon Pairs in Ion-Exchanged Forms of Zeolite X

Abstract Adsorptive counterdiffusion rates of liquid benzene, ethyl benzene, 1,3,5-trimethyl benzene, and 1-methyl naphthalene into Na+, K+, Cs+, and Ca2+ ion exchanged forms of zeolite X saturated with cyclohexane were measured. The diffusivity of the adsorbing hydrocarbon decreases as the effective pore size of the zeolite is decreased by ion exchange. The effective pore size in CaX and CsX was so small as not to allow the adsorptive counterdiffusion of 1,3,5-trimethyl benzene and 1-methyl naphthalene. Diffusion took place only after the cations in the pore mouths were displaced to other cation sites in the structure under the influence of the adsorbing hydrocarbon molecules, and thus the rate of counterdiffusion appeared to increase with time.

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 25-80°C 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.

EFFECT OF SODA CONCENTRATION ON THE MORPHOLOGY OF MFI-TYPE ZEOLITE MEMBRANES

MFI-type zeolite membranes were hydrothermally synthesized as a thin layer on macroporous α-alumina discs coated with 1µm MFI-type seed crystals as seed. The effect of soda (Na2O) concentration on the morphology of the membranes was investigated. Soda concentration, and thus the hydroxyl ion concentration, was changed between 0.25 and 6.5 mol/mol gel. At low soda concentrations the crystals forming the layer exhibited mostly (h0h)/c-axis orientation, but at high soda concentrations a membrane layer formed from randomly oriented crystals. The membranes showed high H2/n-butane ideal selectivity of 476 and 36 at 25°C and 150°C, respectively.

Sorption of Sulfur Dioxide on Hydrogen Form and Aluminum-Deficient Mordenites

Abstract The SO2 sorption characteristics of a commercial H-mordenite and an aluminum-deficient mordenite with a molar SiO2/Al2O3 ratio of 75, prepared from the H-form, were investigated. The sorption isotherms and the reduced sorptive diffusivities were determined in the temperature range 298–358 K and at pressures up to 0.2 atm. It was found that the partially irreversible SO2 sorption on the H-form became completely reversible by the removal of structural aluminum, while the absolute sorption capacity was decreased by 10–40% and the rate of sorption was improved by 50%.

Sorption properties of synthetic ferrierite

Limiting sorption volumes of the hydrogen form of a synthetic ferrierite sample were determined for sorbates--water, methanol, propane, hexane, and o-, m-, p-xylenes--at 298 K and at a relative pressure of P/P o = 0.5 using a gravimetric sorption apparatus. Methanol, which has a kinetic diameter significantly smaller than the pore dimensions, was sorbed relatively rapidly and by the greatest amount. The sorption rates of other sorbates were very slow and for some it took several days for equilibrium to be established. The results indicated that, although their kinetic diameters are similar, the adsorption behavior of propane and hexane are quite different. Hexane is able to fill only ten-membered ring channels whereas propane is able to fill both ten- and eight-membered ring channels, at least partially. The inability of the hexane molecules to be sorbed into eight-membered ring channels is attributed to chain length effect. Xylene isomers are sorbed by the H-ferrierite but to smaller extents, consistent with the significantly larger kinetic diameters of these molecules compared to the reported pore dimensions of ferrierite. Among the xylene isomers, p-xylene, with the smallest kinetic diameter of three isomers, is sorbed to a greater extent than either o-xylene or m-xylene.