Sol-Gel Chemistry and Molecular Sieve Synthesis

J. Livage    Chimie de la Matière Condensée, Université Pierre et Marie Curie 4 place Jussieu-75252 Paris-France

Sol-gel chemistry provides a new approach to the preparation of oxide materials [1]. Starting from a solution, a solid network is progressively formed via inorganic polymerization reactions. The term sol-gel chemistry could actually be used in a broader sense to describe the synthesis of inorganic oxides by wet chemistry methods such as precipitation, coprecipitation or hydrothermal synthesis. Two routes are currently used depending on the nature of the molecular precursor. The inorganic route with metal salts in aqueous solutions and the metal-organic route with metal alkoxides in organic solvents. In both cases the reaction is initiated via hydrolysis in order to get reactive M-OH groups. This reaction can be simply performed by adding water to an alkoxide or by changing the pH of an aqueous solution [2]. Condensation then occurs leading to the formation of metal-oxygen-metal bonds. Condensed species are progressively formed from the solution leading to oligomers, oxopolymers, colloids, gels or precipitates. Oxopolymers and colloidal particles give rise to sols which can be shaped, gelled, dried and densified in order to get powders, films, fibres or monolithic glasses [3].

This paper describes the basic chemical reactions involved in sol-gel syntheses from both inorganic and metal-organic precursors.

1 METAL CATIONS IN AQUEOUS SOLUTIONS

Molecular sieves are usually prepared via hydrothermal methods from aqueous solutions [4]. This route has already been used for a long time in industry for the synthesis of catalysts and ceramic powders. Literature provides many data describing the hydrolysis of metal cations in dilute solutions [5] but very little is known about the formation of polynuclear species at concentrations greater than about 1 mol.l- 1, although these conditions are generally relevant to the synthesis of solid phases. One of the main problem arises from the fact that water behaves both as a ligand and a solvent. A large number of oligomeric species are formed simultaneously. They are in rapid exchange equilibria and it is not obvious to predict which one would nucleate the solid phase. The key parameter is usually the pH of the aqueous solutions but anions, cations or templates are often added in order to obtain the desired product [4][6].

The whole synthesis occurs in an aqueous medium in which water exhibits very specific properties, both as a molecule and as a solvent.

The water molecule has a high dipolar moment, μ = 1.84 Debye, and liquid water exhibits a high dielectric constant (ε = 80). Water is therefore a good solvent for most ionic compounds. It breaks polar bonds (ionic dissociation) and dipolar water molecules solvate both cations and anions.

The water molecule is a Lewis base via its 3a1 molecular orbital. It reacts with metal cations M2 + (Lewis acids) via acid-base reactions giving OH2, OH- or O2 - ligands. These hydrolysis equilibria are responsible for the behavior of metal cations toward condensation. It is therefore very important to know the chemical nature of aqueous species as a function of parameters such as pH, temperature or concentration.

The Partial Charge Model

The Partial Charge Model (PCM) will be used as a guide to describe the aqueous chemistry of metal cations [7]. It is based on the electronegativity equalization principle stated by R.T. Sanderson as follows: “when two or more atoms initially different in electronegativity combine, they adjust to the same intermediate electronegativity in the compound” [8]. The main consequence is that both the electronegativity χx of a given atom X and its partial charge δx vary when the atom is chemically combined. These two parameters must be related. A linear relationship is usually assumed as follows:

x=χx0+ηxδx

where ηx is the hardness of atom X as introduced by Pearson. Hardness is related to the softness σx = 1/ηx which provides a measure of the polarisability of the electronic cloud around X. Softness increases with the size of the electronic cloud, i.e. with the radius r of X. Therefore hardness varies as 1/r. According to the Allred-Rochow scale, electronegativity is proportional to Zeff/r2[9]. Hardness may then be approximated as:

=k√χ0

where k is a constant that depends on the electronegativity scale, k = 1.36 when Pauling electronegativities are expressed in the frame of Allred-Rochow′s model (Table 1).

The total charge “z” of a given chemical species is equal to the sum of the partial charges of all individual atoms z = Σδi. This together with equations (1) and (2) leads to the following expressions for:

meanelectronegativityχ=Σi√χi0+1.36zΣi1/√χi0

thepartialchargeδi=χ−χi0/1.36√χi0

q.4canalsobewritenasδi=σiχ−χi0

σi=1.36√χi0−1

The Partial Charge Model provides an easy way to work out the mean electronegativity of chemical species and the charge distribution on each atom. In the case of the water molecule for instance equation (3) and (5) lead to χ(H2O) = 2.49, δH ≈ + 0.2 and δO ≈ − 0.4

Hydrolysis of metal cations

The word “hydrolysis” is used here to describe those reactions of metal cations with water that liberate protons and produce hydroxy or oxy species [5]. In aqueous solutions this reaction results from the solvation of positively charged cations by dipolar water molecules. It leads to the formation of [M(OH2)N]z + species. Water is a Lewis base and the formation of a M ⇐ OH2 bond with the metal cation (Lewis acid) draws electrons away from the bonding σ molecular orbital of the water molecule. This electron transfer weakens the O-H bond and coordinated water molecules behave as stronger acids than the solvent water molecules. Spontaneous deprotonation then takes place as follows:

OH2NZ+hH2O⇒MOHhOH2N−hz−h++hH3O+

Where “h” is called the hydrolysis ratio. It indicates how many protons have been removed from the solvation sphere of the metal cation. The acidity of coordinated water molecules increases as the electron transfer within the M-O bond increases. In dilute solutions this leads to a whole set of more or less deprotonated species ranging from aquocations [M(OH2)N]z + (h = 0) to neutral hydroxydes [M(OH)z]0 (h = z) or even oxyanions [MON′](2N′-z)- corresponding to the case when all protons have been removed from the coordination sphere of the metal. The electron transfer within the M ⇐ OH2 bond increases with the oxidation state of the metal cation Mz + and coordinated water molecules become more acidic as z increases (Table 2).

A convenient rule-of-thumb shows that the hydrolysis ratio h of a given precursor [M(OH)h(OH2)N-h](z-h)+ mainly depends on two parameters, the pH of the solution and the oxidation state of the metal cation Mz +. A charge-pH diagram can then be drawn in order to show which aqueous species predominate (Fig.1). Two lines corresponding to h = 1 and h = 2 N-1 respectively separate three domains in which H2O, OH or O2 - ligands are observed [10].

Determination of the hydrolysis ratio

In very dilute aqueous solutions, metal cations exhibit several hydrolyzed monomeric species in the pH range 0-14. The problem is then to know whether it is possible to predict the chemical nature of these aqueous species at a given pH.

Following the electronegativity equalization principle it can be stated that deprotonation (eq.7) goes on until the electronegativity χh of hydrolyzed...