Svetlana Mintova

Controlling the preferred orientation in silicalite-1 films synthesized by seeding

Abstract Single crystal silicon (100) wafers were seeded with colloidal silicalite-1 crystals and hydrothermally treated in a precursor solution to grow thin silicalite-1 films. A total of 28 experiments in eight series were investigated with SEM and XRD to evaluate the preferred orientation of the crystals constituting the films. The investigated parameters in the film formation process were seed crystal size, amount of adsorbed seed crystals and film thickness after hydrothermal treatment of the seeded substrates. In thin films, most of the crystalline material is oriented with the b-axis perpendicular to the substrate surface. In thick films, most of the crystalline material is oriented with the a-axis perpendicular to the substrate surface. The change in preferred orientation with film thickness is faster when small seeds are used. The amount of adsorbed seeds has a larger influence on the preferred orientation when large seeds are used. A mechanism explaining these trends is proposed. The choice of size and coverage of seeds can be used to control the preferred orientation of the crystals in a film of given thickness within certain limitations.

Preparation and characterization of hollow fibers of silicalite-1

Hollow fibers consisting solely of silicalite-1 were prepared by a novel method consisting of four basic steps. Carbon fibers were first surface modified to facilitate adsorption of colloidal seed crystals of silicalite-1. Such particles were then adsorbed as a monolayer on the fiber surface and induced to grow into a continuous film of intergrown crystals of silicalite-1. Finally, the carbon fiber used as a template was removed by calcination in air, yielding hollow fibers of silicalite-1. Hollow fibers were prepared in two experimental series employing different synthesis conditions resulting in two different modes of growth. The first experimental series was performed with a clear homogeneous TPAOH/silica solution crystallizing at 100°C, and the second series with a NaOH-TPABr/silica gel crystallizing at 150°C. Materials produced in the first series showed a smooth and homogeneous surface and a relatively uniform fiber wall with a final thickness of 1.1 μm. In these materials the film was formed by crystals growing outward from the surface of the carbon fiber. Materials produced in the second series were less uniform with a final thickness of the fiber wall of about 3.2 μm. The crystal growth in these materials appeared to take place both in an inward and outward direction starting from the seed crystals adsorbed on the carbon fiber. The hollow fibers prepared were characterized by SEM, XRD, FT i.r., and nitrogen adsorption measurements.

Thin molecular sieve films on noble metal substrates

A novel method whereby dense thin microporous films less than 300 nm thick may be prepared on noble metal substrates is exemplified by the deposition of silicalite-1 on a gold surface in a three-step process. The gold surface is modified with a silane coupling agent, gamma-mercaptopropyltrimethoxysilane, after which the silane is hydrolyzed to yield a negatively charged interface. Positively charged colloidal silicalite-1 crystals are readily adsorbed as a monolayer upon this surface and are induced to crystallize further in a separate hydrothermal treatment step to yield a dense thin silicalite-1 film.

ZSM-5 films prepared from template free precursors

Thin continuous films of zeolite ZSM-5 were synthesized on quartz substrates. The substrates were first surface modified and covered by a monolayer of colloidal silicalite-1 seed crystals. These crystals were grown into continuous films with thicknesses in the range 230–3500 nm by hydrothermal treatment in a synthesis gel free from organic templates. The preferential orientation of the crystals constituting the film was initially one with thec-axis close to parallel to the substrate surface. During the course of crystallization this orientation changed to one with most of the crystals having the c-axes directed approximately 35° from perpendicular to the substrate surface. A mechanism explaining this behavior is proposed. The final thickness of the film was controlled by the synthesis time but also by the addition of seed crystals to the synthesis gel. Films prepared according to this method may be of great value for the development of zeolite based membranes.

Deposition of continuous silicalite-1 films on inorganic fibers

Silicalite-fiber composites were prepared by a new method for the synthesis of molecular sieve films. The method consists of two steps: in the first one, the surface charge of the support is revers ...

Synthesis of titanium silicate ETS-10: The effect of tetrametylammonium on the crystallization kinetics

Abstract Titanium silicate ETS-10 was synthesized from titanosilicate gels in the presence of an organic template—tetrametylammonium (TMA) chloride—at temperatures of 160, 180, and 200°C. The kinetic analysis of the crystallization of ETS-10 permits us to follow the role of the used template during the consecutive steps of the process. The presence of TMA diminishes the nucleation time and acclerates the crystal growth. The incorporation of TMA in the channel system of titanium silicate changes significantly the crystal morphology of ETS-10.

Adhesivity of molecular sieve films on metal substrates

Films of silicalite-1, zeolite Y, and SAPO-5 were deposited on steel and copper substrates. The film adhesivity, thermal stability, and morphology of the zeolite films were studied. It is shown that the main factors determining the zeolite film mechanical properties are the interactions at the substrate-gel interface. On the other hand, the mechanism of film formation depends on the type of the initial gel and the substrate surface activity. In addition, the results obtained indicate that the substrate plastic deformation, i.e., the high density of substrate surface defects, is one of the dominant parameters for good film adhesivity and thermal stability.

Influence of metal substrate properties on the kinetics of zeolite film formation

The primary stages of zeolite Y and silicalite-1 film formation on both untreated and plastically deformed copper substrates are considered from the viewpoint of the film growth kinetics and morphology. Experimental results indicate a satisfactory agreement between the kinetic and profilometric data.

Growth of silicalite-1 films on gold substrates

The crystallisation of thin continuous silicalite-1 films on modified gold surfaces prepared at 100°C in clear precursor solutions has been studied by grazing angle X-ray diffractometry, infrared spectroscopy, dynamic light scattering, scanning electron microscopy, Kr adsorption and ellipsometry. The gold surfaces were pre-treated with a silane coupling agent, gamma-mercaptopropyltrimethoxysilane, and hydrolysed in an acidic solution, pH<4, to create a modified negatively charged interface on which positively charged colloidal silicalite-1 seed crystals were adsorbed. The colloidal seed crystals are believed to be preferentially adsorbed with their {010} face parallel to the Au surface. Continued growth of the oriented seed crystals in a following hydrothermal step results in the formation of a continuous thin oriented crystalline film. The mechanism for the formation of silicalite-1 films on Au is discussed.

Investigation of the ion-exchanged forms of the microporous titanosilicate K2TiSi3O9·H2O

The microporous titanosilicate K2TiSi3O9·H2O was subjected to ion exchange by the other alkaline cations and NH+4. The material can be totally exchanged by NH+4 and to different extents with the alkaline cations. The substitution of potassium leads to changes in the physicochemical properties of the material, like the thermal stability and water adsorption capacity as well as the unit cell parameters. The framework of the material is very flexible and needs stabilization provided by the cation–water complexes situated in the intracrystalline volume. Rietveld refinement showed that the two cationic positions, M1 and M2, are not equally exchangeable by the different cations. Small highly hydrated cations occupy preferentially the eight-membered ring channel, while the large cations occupy the seven-sided window.