Chemical equilibria and particle morphology of boehmite (AlOOH) in sub and supercritical water

doi:10.1016/S0378-3812(99)00118-1

Fluid Phase Equilibria

Volumes 158–160, June 1999, Pages 733-742

https://doi.org/10.1016/S0378-3812(99)00118-1 Get rights and content

Abstract

In a recent study, we proposed a supercritical water crystallization method for production of metal oxide particles. Around the critical point, the morphology of boehmite (AlOOH) particles varied greatly with the reaction temperature, pressure and concentration of aqueous aluminum nitrate solution. In this study, the relationship between the morphologies of particles obtained and the chemical species in solution is discussed. For estimation of chemical species concentrations, evaluation of equilibrium constants of the hydrothermal reactions around the critical point is required. For this, a model based on the Gibbs energy change by temperature, solvent effects and ion–ion interactions is employed. The solvent effect was calculated by the Born equation. The effect of ion–ion interaction was calculated by the extended Debye–Huckel equation. Using this model, the distribution of chemical species for the AlOOH system (Al³⁺, Al(OH)²⁺, Al(OH)₂⁺, Al(OH)₃, Al(OH)₄⁻, NO₃⁻) in subcritical (350°C, 30 MPa) and supercritical water (400°C, 30 MPa) was estimated. The particle morphology seems to be determined by selective adsorption of positive charged species, Al(OH)²⁺, on the negatively charged faces of AlOOH crystal.

Introduction

Supercritical water has been shown to provide a unique environment for material treatment and material processing. We recently proposed a supercritical water crystallization method for production of metal oxide particles 1, 2. We found that the morphology of boehmite (AlOOH) particles could be controlled by varying the reaction temperature, reaction pressure and aluminum nitrate solution concentration [2]. Particle morphology was greatly affected by reaction atmosphere and chemical species concentration and their distribution, temperature, pressure, and solvent properties.

Understanding chemical equilibria (disassociation, dissolution) at hydrothermal conditions is very important for a number of organic and inorganic applications. Thermodynamic properties of the hydrothermal systems have been studied in fields of geochemistry for many years. The density model of Marshall and Franck [3] has been widely applied for data correlation since it is simple and requires limited thermodynamic data. Anderson et al. [4] proposed a revised density model for the calculation of the equilibrium constant at high temperature and pressures of reactions in aqueous solution: $ln K=P_{1} +P_{2} /T+P_{3} /T ln ρ$ where P₁, P₂, and P₃ are the system specific parameters which can be determined by thermodynamics parameters (enthalpy, heat capacity, expansion coefficient of water, density of water) at the standard state (1 bar, 25°C). Castet et al. [5] measured the solubility of AlOOH under hydrothermal conditions (90–350°C) and applied the density model for that system. Their system could be correlated with Eq. (1)and the proper chemical equilibrium equations. However, these empirical equations cannot be applied to reactions around the critical point, where the solvent effect is considered to vary greatly.

Xiang and Johnston [6] have studied the equilibrium constant K_BHA for a reaction between an organic acid (β-naphthol) and a base (OH-ion) in supercritical water. They employed a model for the equilibrium constant that was described as a function of temperature, density of water and dielectric constant of water, as follows: $ln K_{BHA} =− Δ G^{0} (T_{0},ρ_{0}) RT_{0} + ∂ln K_{BHA} ∂ (1/T)_{ρ_{0}} 1 T − 1 T_{0} + 83,549 T 1 ε − 1 ε_{0} 1 R_{OH^{−}} * − 1 R_{A^{−}} * .$ They described Eq. (2)by the solvent effect on the chemical potential of chemical species. The dielectric constant of water around the critical point decreased drastically with increasing temperature. The chemical potential of ionic species is affected by the change of dielectric constant of solvent. This solvent effect was described by Born equation and is represented by the third term in Eq. (2).

In this work, the objective was (i) to model the chemical reaction equilibria in supercritical water and (ii) to clarify the role of the chemical species for particle morphology in supercritical water crystallization. We propose a simple estimation model for the chemical equilibria of the hydrothermal reactions. Using this model, the distribution of chemical species for the hydrothermal AlOOH system (Al(OH)_x^+3−x (x=0–4), NO₃⁻) was calculated and relationship between morphologies of obtained experimentally and the chemical species in solution is discussed.

Section snippets

Estimation model

The Gibbs energy change of chemical species, i, in the (hydrothermal) reaction can be described as a sum of several contributions to the standard Gibbs energy, as follows: $Δ G_{i} (T,ε,I)= Δ G_{i} °(T_{0},ε_{0},I_{0})+ Δ G_{temp,i} °(T_{0} →T,ε_{0},I_{0})+ Δ G_{solv,i} °(T,ε_{0} →ε,I_{0})+ Δ G_{inter,i} °(T,ε,I_{0} →I)$ where the second and third terms present the effect of temperature and the solvent effect, respectively. The fourth term accounts for the effect of interactions among ions in systems. The first two terms can be calculated using the

Experiments

Starting materials were prepared by dissolving aluminum nitrate (Al(NO₃)₃·9H₂O: Wako) in deionized water.

Experiments were performed in a flow type reactor shown in Fig. 2. The prepared aluminum nitrate aqueous solution was fed by a high pressure pump at a rate of 4 cm³/min. The solution was directly mixed with supercritical water fed by an another line at a tee mixer. Flow rate of supercritical water was 8 cm³/min at room temperature. To avoid heating the metal salt before reaction the solution

Particle morphologies

Fig. 3 shows TEM photographs of particle obtained in a various conditions. Morphologies of particles obtained were rhombic (Fig. 3a), hexagonal (Fig. 3b) and long hexagonal plates (Fig. 3c). From comparison of these figures, two features among these shapes were found. First, is that an obtuse angle of rhombic particle was about 105° and its degree of angle was the same as that of hexagonal plate. Second is that both rhombic particles and hexagonal plates had a notch. These observations mean

Conclusion

In this study, equilibrium constants of hydrothermal reactions for aqueous AlOOH systems around the critical point of water were evaluated by a model based on the Gibbs energy change by temperature, solvent effects and ion–ion interactions. The influence of the dielectric constant of the solvent is accounted for by Born's equation. The chemical species distribution in supercritical water crystallization was calculated from the model. The particle morphology could be related to the chemical

Nomenclature

A, B, C, D	Constants in Eq. (4)
A_D	Function of temperature and dielectric constant
C_i	Concentration of chemical species, mol/kg
C_p°	Heat capacity at reference state, J/(mol K)⁻¹
G	Gibbs energy, J/mol
H	Enthalpy, J/mol
I	Ion strength
K_S0, K_S1, K_S2, K_S3, K_S4	Equilibrium constant of dissolving AlOOH
K₁, K₁, K₃, K₄	Equilibrium constant of dissociation of aluminum hydroxides
K_BHA	Equilibrium constant of acid (HA)–base (A⁻) reaction
P₁, P₂, P₃	Parameters in Eq. (1)
R	Gas constant
$R_{OH^{−}}$ , $R_{A^{−}}$	Born radii of OH⁻ ion and base, A⁻

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Chemical equilibria and particle morphology of boehmite (AlOOH) in sub and supercritical water

Abstract

Introduction

Section snippets

Estimation model

Experiments

Particle morphologies

Conclusion

Nomenclature

Geochim. Cosmochim. Acta

Geochim. Cosmochim. Acta

Geochim. Cosmochim. Acta

Geochim. Cosmochim. Acta

J. Am. Ceram. Soc.