Investigations of the industrial potential of mechanochemistry for continuous processing

Mechanochemical reactions stimulated by mechanical forces and energy facilitates environmentally benign, sustainable chemical syntheses in the absence of toxic solvents. The industrial viability of mechanochemical processing is hindered by the lack of mechanistic understanding for mechanochemical synthesis, and thus, the mechanics, energetics and chemical kinetics of mechanically stimulated reactions are the focus of this thesis.

A novel experimental approach was developed to assess the mechanics and energetics of mixer mill apparatus typically employed for mechanochemical synthesis. Real time and in situ monitoring were executed by high speed imaging to evaluate the collision properties and energetics which govern the mill process. The dissipation of energy was assessed as a function of mill frequency. The rate of energy transfer in Retsch mixer mill apparatus at milling frequencies 15 hertz and 25 hertz was 0.9 J/s and 2.5 J/s, respectively. The energy dissipated during inelastic collisions in milling apparatus is consumed for homogenisation, comminution and mechanochemical reaction of powders within the mill vial.

The reaction kinetics for the mechanochemical synthesis of hydroxyapatite were explored to validate the rate of energy transfer observed for the mixer mill process. A solid state characterisation study was employed to assess the reaction mechanism and reaction yield for the solid state synthesis of hydroxyapatite in a Retsch mixer mill. The reaction kinetics were assessed by graphical methods applying the Arrhenius Law. A two-step mechanism was observed for the solid state synthesis of hydroxyapatite. The synthesis of calcium phosphate hydroxyapatite transpires through a precursor mechanism, with the presence of metastable intermediate octacalcium phosphate observed prior to synthesis of hydroxyapatite. The rate of energy consumed was 1.5 J/s during mechanical treatment at 25 hertz.

In addition, the application of twin screw extrusion for continuous mechanochemical synthesis was explored for the synthesis of MOF HKUST-1. The reactivity of MOF HKUST-1 was assessed in milling and extrusion apparatus. Enhanced mass transfer by diffusion and accelerated rates of reaction were realised for liquid assisted synthesis in the presence of minute amounts of solvent.