David T. Hayhurst

Diffusion of C1 to C5 normal paraffins in silicalite

Diffusion coefficients were measured for the C 1 to C 5 normal paraffins in silicalite using a molecular sieve membrane. The coefficients were measured at 334 K and were found to range from 1.07 × 10 −6 cm 2 /s for methane to 2.42 × 10 −8 cm 2 /s for n-pentane. For each of the gases tested, the diffusion coefficients were found to be independent of diffusant concentration over the concentration range tested. Values of the diffusion coefficients were found to correlate directly with the carbon number of the diffusant.

Effect of hydroxide on growth rate and morphology in silicalite synthesis

Crystallization kinetics and crystal morphology were determined for silicalite crystallized from two similar reaction batch mixtures. The batch compositions studied were xNa 2 O + 8 TPABr + 100 SiO 2 + 1000 H 2 O and xTPA 2 O + (8 − 2x)TPABr + 100 SiO 2 + 1000 H 2 O, where x was varied from 0.5 to 4.0. For the first mixture, sodium hydroxide was used as the base, while for the second reaction mixture, TPAOH was the alkali source. Reactions were carried out at 170°C in unstirred, Teflon-lined autoclaves. For all runs, silicalite was the only phase found to crystallize. Crystal morphology of silicalite produced from batches with different hydroxide contents were found to differ considerably. As the alkalinity of the reaction mixture was reduced, the aspect ratio (length/width) of the crystals increased from 0.9 for x = 4 to 6.7 for x = 0.5. For synthesis made without sodium, both nucleation and crystallization occurred more rapidly. From these results, some insights into the crystallization mechanism were proposed.

Compositional and kinetic studies on the crystallization of ETS-10 in the presence of various organics

Abstract An extensive compositional study was conducted in the presence of various tetraalkylammonium salts and ethanolamine (EA) to investigate the effects of organics on the crystallization and purity of ETS-10 and to calculate the activation energy. The results suggest that the addition of organic additives enhances the crystallization and purity of ETS-10. It also indicates that pure ETS-10 is hardly formed in the absence of potassium. It might be attributed to the fact that hydrated Na + cation is too large to be accommodated in the adjacent site to Ti within the framework to form the channel structure of highly disordered ETS-10, requiring an appropriate ratio of Na + to K + . Kinetic studies were also conducted at 463, 473 and 483 K, respectively. To interpret the data, a modified Avrami–Erofeev equation was adopted and the activation energies for nucleation, transition and crystal growth stages were calculated. The results show that the total activation energies increase in the order tetramethylammonium, tetraethylammonium, tetrapropylammonium, EA and no organic.

Study on the rapid crystallization of ETS-4 and ETS-10

Abstract The synthesis of ETS-10 was carried out at 200°C using a 2 3 factorial method to optimize the overall composition of reaction mixture to produce pure ETS-10 within a short time. The results show that ETS-10 with a high crystallinity and well-defined morphology was successfully synthesized at high water content and low alkalinity within 18 h of reaction. The results also show that the effect of water on crystal size and morphology was not significant. A kinetic study on ETS-10 was performed using the optimum composition and the apparent activation energy was calculated as 66.78 kJ/mol. In addition, a kinetic study on ETS-4 was also performed and the apparent activation energy was calculated as 14.47 kJ/mol.

Synthesis of hydrothermally stable mcm-41 with initial adjustment of ph and direct addition of naf

Abstract The synthesis of a hydrothermally stable MCM-41 was conducted at 100°C for 24 h in a 15 ml Teflon-lined Morey type autoclaves. Results show that a hydrothermally stable MCM-41 with long range ordering of its hexagonal array can easily be prepared. With addition of an optimum amount of NaF and with appropriate pH adjustment, an as-synthesized MCM-41 shows excellent hydrothermal stability up to 433 K. XRD, magic angle spinning 29 Si NMR, N 2 -adsorption and TEM data show well-ordered structure. It was found that the addition of NaF and the order of mixing are critical in the formation of this hydrothermally stable MCM-41.

Intracrystalline diffusion of benzene in silicalite: effect of structural heterogeneity

The sorption kinetics of benzene in silicalite have been measured gravimetrically using large silicalite crystals of sizes 350 µm × 105 µm × 105 µm and 270 µm × 70 µm × 70 µm in the temperature range 283–343 K. Experiments were performed under conditions that ensured isothermal operation with intracrystalline diffusion control. An analytical expression for the Darkens correction factor (δ In P/δ In a) was derived based on the Hill–de-Boer equation and was used to determine the variation of corrected diffusivity with concentration. The transport diffusivity varied significantly with the adsorbed-phase concentration but the corrected diffusivity was found to be essentially independent of the concentration. However, at temperatures below the surface transition temperature, a maximum in corrected diffusivity was observed at an adsorbed-phase concentration of four molecules per unit cell (uc). This maximum is the direct result of the type IV isotherm exhibited by the silicalite–benzene system below the surface transition temperature. It is proposed that the increase in corrected diffusivity at the critical adsorbed-phase concentration of 4 molecules uc–1 arises from reorientation of benzene molecules resulting in a much more efficient packing in the pore channel system.

GAS ADSORPTION BY SOME NATURAL ZEOLITES

Abstract Several natural zeolite mined in the United States were tested for their effective equilibrium adsorption capacities at 25°C using SO2, CO2 and NH3 as the adsorbates. Adsorption isotherms were found to give excellent correlation with the Freundlich equation. Effective pore diameters were also determined.

The Effects of Varying the Tetraalkylammonium Template on High-Silica Zeolite Synthesis

Abstract Synthesis of high silica zeolites in the presence of TMABr, TEABr, TPABr and TBABr is reported. The tetraalkyl ammonium cation was found to affect the structure-type, growth rate and yield of the product zeolite. Nucleation rate is, however, found to be controlled by the sodium concentration. An exception was noted for systems containing large amounts TMABr, where large TMA concentrations resulted in the formation of ZSM-39. It is concluded that only sodium and the small TMA cation has the ability to form different SBU. Complete crystallization diagrams are presented for each of the synthesis.

Parameters Affecting the Growth of Large Silicalite Crystals

A statistical method was employed to optimize the synthesis of large silicalite crystals. Using a starting reactant batch composition of 10Na 2 O-100Si 2 O.249H 2 O-ll TPABr, a factorial method was used to adjust the contents of NaOH and TPABr in the synthesis mixture in order to maximize the crystal size. It was found that lesser amounts of NaOH and TPABr favored the growth of large crystals. Crystals up to a size of 400×80×80μm were identified, with the optimum reaction batch mixture being 2.25Na 2 O-100Si 2 O-2832H 2 O-5.22TPA. Additional experiments were performed using KOH, CsOH and sodium salts to replace NaOH in the batch formulation. Products from these runs were found to be well-crystallized, uniform material with crystal lengths of up to 200μm.

Synthesis of titanium molecular sieve ETS-10 and ETS-4

Summary In this paper the crystallisation kinetics of the large and small pored crystalline titanium molecular sieve ETS-10 and ETS-4 are reported. These ETS materials have an open structure, both with tetrahedral and octahedral primary building units. The effect of varying the single components of the reaction mixture, as reported in the patent literature on the kinetic parameters, on the gel preparation and on the properties of the final products is discussed.