Preparation of monosize silica spheres and their crystalline stack

doi:10.1016/S0927-7757(98)00276-3

Colloids and Surfaces A: Physicochemical and Engineering Aspects

Volume 142, Issue 1, 30 November 1998, Pages 59-63

https://doi.org/10.1016/S0927-7757(98)00276-3 Get rights and content

Abstract

Monosize silica spheres, 0.1∼3 μm in diameter, were prepared through the hydrolysis and condensation of tetraethylorthosilicate (TEOS) in aqueous ethanol or methanol solutions with ammonia as catalyst. A suspension of the monosize silica, dispersed in EtOH, H₂O or a mixture of them, was allowed to sediment, and an ordered, crystalline phase was formed due to an electrostatic repulsion between the silica spheres. Through replacing the H₂O or EtOH, in a step wise manner, with another solvent, which is miscible with H₂O and EtOH, the Zeta potential on the surface of the silica spheres was reduced, until an ordered closed packed SiO₂ material was formed. Because of the monodispersity of the silica spheres, the ordered closed packed SiO₂ has a uniform pore size, which can be calculated from the monosize particle's diameter, and can be changed arbitrarily by selecting the size of the monosize silica constructing the SiO₂ materials.

Introduction

Organic materials with uniform pore diameter are very useful in the field of catalysis and membrane separation. The pore size of catalysts has a significant effect on the performance of the catalysts as in the case of hydrocracking of heavy oil 1, 2and the selective oxidation of butane and o-oxylene to maleic and phthalic anhydride, respectively 3, 4. The performance of membrane separation (especially the ultrafiltration membrane separation) depends mainly on the membrane's pore size and its distribution. A uniform-pore-size membrane is not only the desired membrane for use, but also the perfect means to study the transport phenomena in the membrane separation 5, 6. As a consequence, many researchers studied the preparation of pore size materials through hydrothermal treatment [7], thermal sintering 8, 9, and careful control of preparation conditions in the sol–gel synthesis process 6, 10. So far, however, a real uniform-pore-size catalyst has not been prepared. In order to study the relationship between the performance of catalysts or membranes and their pore size, one has to use the average pore diameter. Because the effects of different pore size on the performance of catalysts and membranes do not obey average addition, it is difficult to study in theory, on the basis of the average pore diameter, the effects of pore size on the catalytic mechanism and transport behavior. It is the needs and wishes of the catalysis and membrane separation researchers to obtain uniform-pore-size catalysts.

This paper reports on the synthesis of monosize silica spheres and the tailor-making of uniform-pore-size SiO₂ materials using the monosize silica spheres as the constructing part.

Section snippets

The synthesis of monosize silica spheres

The monosize silica spheres, from which the uniform pore size SiO₂ materials were made, can be prepared by a one-step process (Stöber process) [11]and the seeded growth process [12]. The overall reaction can be written as follows: $Si(OEt)_{4} +4H_{2} O=Si(OH)_{4} +4EtOH$ $n Si(OH)_{4} =n Si(OH)_{4} +2n H_{2} O .$

The following chemical reagents were used in the experiment: tetraethylorthosilicate (TEOS), reagent grade; dehydrated ethanol and methanol, anlaytical purity; redistilled water; dioxane, analytical purity; ammonium

Preparation of silica crystalline stack

Treating the monosize silica spheres by the hydrothermal process or by calcination at 700–900°C according to the size of the monodisperse silica can make the micropore in the silica disappear, so that the BET specific surface area of the treated silica is identical to the silica outside surface area calculated by the diameter of the silica [14]. Because of the monodispersity of the silica spheres, the pore size of their ordered close-packed materials is uniform, and a desired pore size can be

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Preparation of monosize silica spheres and their crystalline stack

Abstract

Introduction

Section snippets

The synthesis of monosize silica spheres

Preparation of silica crystalline stack

J. Colloid Interface Sci.

J. Catal.

Appl. Catal. A

J. Colloid Interface Sci.

J. Colloid Interface Sci.

Can. J. Chem. Eng.

Ind. Eng. Chem. Res.