Derek Creaser

Aging effects on the nucleation and crystallization kinetics of colloidal TPA-silicalite-1

Abstract A direct experimental method, involving a two-stage varying-temperature synthesis, was utilized to investigate the effects of aging on the nucleation kinetics for the synthesis of nanosized TPA-silicalite-1 with various silica sources, including tetraethoxysilane (TEOS) and amorphous silica Ludox TM. For TEOS aging had only a mild influence on the nucleation and crystallization kinetics, whereas for Ludox TM dramatic changes occurred. After extended aging periods, the nucleation kinetics for syntheses with Ludox TM became similar to those for syntheses with TEOS, leading to increasing similarities in the properties of the products of the crystallization. Independent of aging time and silica source, the nucleation processes occurred over a substantial period of time, extending over the induction period, but were completed before crystal growth was detected. With increased aging, the nucleation period was gradually decreased and tended to be completed earlier in the induction period. Raman spectra revealed that with Ludox TM, aging enhanced the interaction between TPA + and the silica species, leading to an increase in the concentration of precursor species for nucleation, which in turn accelerated the nucleation rate.

The nucleation period for crystallization of colloidal TPA-silicalite-1 with varying silica source

The effect of varying silica source on the nucleation and crystallization of TPA-silicalite-1 was investigated. A direct experimental method, involving a two-stage varying-temperature synthesis, was used to determine the nucleation period for colloidal crystals of TPA-silicalite-1 with different silica sources, including tetraethoxysilane (TEOS) and amorphous silica (Ludox TM and Ludox LS). For syntheses performed at 60°C with TEOS as silica source, the duration of the nucleation was about 72 h, and a very rapid increase in the crystal population occurred during the initial crystallization time. However, with the amorphous silica sources (Ludox TM or Ludox LS), the duration of the nucleation period was extended to about 120 h, and the nucleation profile consisted of a self-accelerating nucleation rate at the beginning of the nucleation period. The two-stage synthesis method could be used to determine the nucleation profile for the various silica sources. However, this technique overestimated the crystal concentration at the earliest stage of nucleation with amorphous silica. The use of amorphous silica gave rise to a broader crystal size distribution compared to that of TEOS. However, it was found that for both TEOS and amorphous silica the vast majority of the nucleation occurred during an induction period when little or no crystal growth was observed. In addition, Raman spectroscopy revealed structural differences between Ludox TM and Ludox LS which may account for differences in the nucleation processes observed for these two amorphous silicas.

Preparation and evaluation of thin ZSM-5 membranes synthesized in the absence of organic template molecules

Porous α-alumina supports with a pore size of 100 nm were seeded with colloidal TPA-silicalite-1 crystals with a size of 120 nm. The seeded supports were calcined and treated in a synthesis solutio ...

Application of the seed-film method for the preparation of structured molecular sieve catalysts

Abstract The seed-film method has been applied for the preparation of various materials of potential interest as structured molecular sieve catalysts. The method has proven to be very flexible and allows for the reproducible preparation of a number of molecular sieve–substrate combinations as well as the control of the materials’ properties of importance in catalytic applications such as zeolite loading, film thickness, film density and crystal orientation. The preparation of thin molecular sieve films on ceramic foams, α-alumina pellets and porous alumina supports as well as various metal surfaces is described. The preparation of zoned coatings with a compositional gradient is also discussed.

Transient study of oxidative dehydrogenation of propane

Abstract Kinetics and the mechanism of the oxidative dehydrogenation of propane were investigated using various transient techniques. Results support a redox reaction mechanism in which propane and intermediate products react with lattice oxygen, reducing the catalyst surface, which is reoxidized by gas-phase O 2 . Partial reduction of the catalyst occurs during the start-up to a steady state. Successive pulsing with C 3 H 8 reduced V 5+ in the magnesium ortho -vanadate phase to V 2+ . Carbon-containing species were observed upon interruption of the reaction, although only minute amounts were formed. Cycling increases the amount of the carbon deposited, but this carbon is reactive and most of it is oxidized in the succeeding O 2 pulse. Temperature-programmed oxidation (TPO) experiments on the catalyst used in steady-state operation revealed mainly strongly bound carbonaceous matter on the catalyst, but this carbon deposition did not affect catalyst activity. Thus, adsorbed oxygen is an important source of total combustion. Our experiments show, however, that lattice oxygen also produces total oxidation. Propene selectivity of the reaction in the absence of gas-phase O 2 was superior to steady-state selectivity, at the same propane conversion. Propene selectivity could be further improved by increasing the degree of reduction of the catalyst.

Cyclic operation of the oxidative dehydrogenation of propane

The cyclic operation of the oxidative dehydrogenation of propane over a V-Mg-O catalyst by the alternate feeding of propane and oxygen gas mixtures was investigated. Generally, the response of the ...

Oxygen partial pressure effects on the oxidative dehydrogenation of propane

Abstract The effect of the gas-phase oxygen partial pressure for the oxidative dehydrogenation of propane over a V–Mg–O catalyst was investigated to explore reasons for improved propene yields when dehydrogenation was carried out under periodic operation. Steady-state experiments were performed in which the oxygen partial pressure was varied over a large range at constant propane partial pressures. In addition, the catalyst mass was varied to control the propane conversion so that selectivity and yield could be compared at a constant propane conversion. At the same propane conversion, propene selectivity increased as the partial pressure of oxygen decreased. Thus, propene yields can be improved at steady state by employing low oxygen partial pressures. It is likely that this is the reason for the higher propene yields observed under unsteady-state or periodic operation. It appears that the reaction mechanism must consist of more than one pathway for production of carbon oxides.

Zoned MFI films by seeding

The synthesis and characterization of zoned MFI films consisting of intergrown TPA-silicalite-1 and ZSM-5 crystals are described.

Crystal population balance model for nucleation and growth of colloidal TPA- silicalite-1

Abstract A model describing the crystallization of colloidal TPA-silicalite-1 is developed and tested. The nucleation rate is approximated from experimental data. Crystal growth is surface- reaction limited and the Gibbs-Thomson effect suppresses growth of small crystals, resulting in an induction period. Slow crystal growth during the nucleation period results in the narrow crystal size distribution typically observed for colloidal TPA-silicalite-1 syntheses. 123

20-P-09-Structured zeolite ZSM-5 coatings on ceramic packing materials

Publisher Summary This chapter discusses structured zeolite ZSM-5 coatings on ceramic packing materials. Homogeneous coatings of zeolite ZSM-5 are prepared by the seed film method on porous ceramic foams and on alumina spheres. The zeolite is predominately present in the form of a film on the support surface rather than as aggregated crystals on the surface. The results from gas adsorption and scanning electron microscopy (SEM) analysis indicate that the entire surface of the foams is successfully covered with a 450 nm film. A 500 nm film is formed on the external surface and in pores close to the external surface of the spheres. Zeolite is not formed on the internal surface of the alumina spheres. Aluminum leaching from the foams is observed; however, it did not seem to have any detrimental effects on the substrates.