Aqueous Chemistry of Metal Cations: Hydrolysis and Condensation (Lesson 1) PDF

Aqueous Chemistry of Metal Cations: Hydrolysis and Condensation The sol-gel synthesis of metal oxides can be performed via the hydrolysis and condensation of metal cations in aqueous solutions. Sol-gel method is a technology of producing m...

Aqueous Chemistry of Metal Cations: Hydrolysis and Condensation The sol-gel synthesis of metal oxides can be performed via the hydrolysis and condensation of metal cations in aqueous solutions. Sol-gel method is a technology of producing materials in a convenient and cost-effective way for industrial uses. This inorganic route is cheap and reliable : green chemistry and soft chemistry route (low temperatures and wet chemistry) Sol-gel procedures involved different processes such as hydrolysis, polymerization, drying and densification allowing the the development of nano-structured materials for a wide range of applications. The so-called Partial Charge Model provides a useful guide to describe and predict hydrolysis and condensation reactions in aqueous solutions. 1- Introduction 2- The Partial Charge Model 3- Condensation : mechanism 4- Olation and oxolation 5- Application to silica and hybrid sol gel materials: hydrolysis and alcoxolation of precursors, examples 6- Some other examples of applications of sol-gel technology (if I've got time) 1  1- sol-gel chemistry: bottom-up synthesis Gel is a liquid system with a semi-solid character because of In a liquid a three-dimensional phase, a sol is solid network. a stable Colloidal gels are suspension of translucent and colloidal solid porous : colloidal particles. Monomers in solution : - Percolation of particles : wet gel formation dispersion of tiny wide variety of precursors particles are interconnected to form the porous network : percolation Silica Xerogel Silica Aerogel 2 Different ways of shaping : 3 Exemples of densified xerogels : Orange silica gel 4 - Silica aerogels from supercritical drying : the structure of the humid gel is preserved as supercritical fluids have no capillary tension and no pore collapse has occurred. !!!! However, very brittle structure... 5 Caution: drying is not so easy and the “how to do “ is very important Monoliths may shrink during the densification step, and this can lead to surface cracking brought on by capillary forces : 6  Sol-gel precursors undergo chemical reactions with water. One of the most efficient models used to predict those reactions is the partial charge model (PCM) expressed in terms of the electronegativity of atoms or molecules It is based on the electrical interactions between the partial electric charges, δ(i) or δ(C) , carried by each atom (i) and molecule (C) and the electronegativity of the molecule. Indeed all the atoms in a molecule display the electronegativity because the transfer of electrical charges continues until all atoms the molecules reach an equilibrium where they all have the same electronegativity. The electronegativity of an isolated atom is therefore defined as Chemical Hardness is defined as : chemical hardness is defined as the resistance towards electron cloud polarization The hardness and electronegativity are linked by the following relationship : 7 Thus : where χ0i is the Allred-Rochow electronegativity and η0i the Allred-Rochow hardness. In the case of a complex molecule Cz+ composed of several elements, we must consider its total charge z, or formal charge, which is defined as : Considering that electronic transfers between atoms will lead to an equalization of electronegativities (average electronegativity). At equilibrium i = moy   i*  1,36 z The average electronegativity: moy  moy  i 1  i  i* 8 The average electronegativity: moy   i*  1,36 z  moy   i*  moy  i and i  1 1,36  i*  i  i* 9 Exemple pour des hexaaquo complexes : The charge of H2O ligand is positive: This is an outgoing group not nucleophilic (metallic cation is positive) with the oxidation state of the metal Deprotonation (OH) = (O) + (H) =-1 Nucleophilic groups : require to start reaction of condensation 10  The Formation of Hydroxo Ligands: A hydroxo ligand is formed when the metal cation is an acid and when water acts as a Lewis base. This corresponds to the following reaction: the following reaction for h consecutive loss of protons: Addition of a base In this mechanism, a free OHnucleophilic anion attacks one of the hydrogen atoms of one of the water molecules in the first solvation shell of the metal M. 11 Since hydrogen carries a positive partial charge (δ(H) > 0), an electron charge transfer occurs between the incoming OH- ion and the original metal complex. Consequently, the partial charge, δ(H2O), of the H2O group composed of the incoming OH- ion and the attacked H atom increases until it becomes null, δ(H2O) = 0. When this state is reached, an independent water molecule leaves the metal complex. Such deprotonation reactions occur as long as, for an H2O group ligand, δ(O)free water < δ(O)complex < 0. 12  Formation of oxo group : formation of anions CrO42- or MnO4- Oxo ligand are a strong nucleophilic ligand but it is a poor leaving group so pure oxo complexes can’t start condensation The condensation requires the formation of hydroxo ligands 13 While the metals which have acidic oxides form hydroxo ligands Those which have basic oxides are characterized by the formation of oxo ligands, CrO42-. For such oxo complexe, a hydroxo ligand can still be produced with an acid by attack of a free H+ ion on the nucleophilic oxygen of an oxo group: addition of acid. 14 According to the partial charge model, the electronegativity of any complex C = [M(OH)h(OH2)N-h](z-h)+ can be calculated from its formal charge. For an aquo-hydroxo complex, the number h = hydroxo ligands present in an aquo-hydroxo complex is obtained from this relation: The most acidic and the most basic forms of some metal cations as calculated by the partial charge model: 15 h = 0 complexe aquo 0