PhD thesis title: Silica nanoparticles from South African coal fly ash derived sodium silicate solutions

Abstract:

Silica nanoparticles are one of the most frequently used inorganic support materials for heterogeneous catalyst immobilization. Their high thermal and chemical resistance, and their good sorption properties provided by their high surface area and porous structure, tailor their suitability as catalyst support. The presence of high concentrations of hydroxyl groups at the surface of silica nanoparticles facilitates catalyst attachment. Commercially prepared silica nanoparticles such as MCM-41 and SBA-15 are frequently used support materials. The production and application of ultra-pure silica nanoparticles produced from coal fly ash (CFA), as alternative catalyst support in the oxidation of veratryl alcohol was investigated in this study.

The production of amorphous mesoporous silica (SiO₂) nanoparticles can be achieved using sodium silicate (Na₂SiO₃) solutions prepared from South African CFA. CFA is a by-product from the combustion of pulverised coal in thermoelectric power stations. South Africa’s public electricity utility, Eskom, consumes about 120 million tons of coal per annum, which produces 34 million tons of CFA. About 7 % of South African CFA is recycled, mostly in the cement and construction industry. Due to its high silicon and aluminium content, CFA could represent a valuable secondary resource for purified silica and alumina, provided suitable economically-viable processes can be developed.

The first part of this study compared two processes for the preparation of Na₂SiO₃ solutions from CFA. The first process, hereafter called sequential acid-alkaline leaching (SAAL), is a two-stage process, which involves (i) a H₂SO₄ leaching step for the preferential extraction of reactive aluminium over silicon, followed by (ii) the preferential extraction of silicon over aluminium from the resulting residues using NaOH. The second process is a direct alkaline leaching (DAL) process, which consists of a single-stage elemental extraction from ash using NaOH, i.e. without the preceding acid leaching step used in SAAL. The reason for comparing these two processes was to test whether the removal of reactive aluminium from CFA, prior to silicon extraction for the production of intermediate Na₂SiO₃ solutions, is beneficial to SiO₂ nanoparticles synthesis. The two processes generated Na₂SiO₃ solutions with identical pH (11.8), similar silicon (10.2-10.3 g/L), iron (ca. 200 mg/L) and potassium (ca. 800 mg/L) content, and low calcium concentrations (≤ 29 mg/L). However, the inclusion of the acid leaching step in the SAAL process yielded a Na₂SiO₃ solution with substantially lower aluminium (166 mg/L vs 1158 mg/L).

The Na₂SiO₃ solutions obtained from the SAAL and DAL processes were used as silica precursors to synthesise sub-200 nm amorphous mesoporous silica nanoparticles via a sol-gel method using polyethylene glycol (PEG) as surfactant and sulphuric acid as catalyst. Synthesised silica nanoparticles were characterised by a high level of purity (up to 99.3 wt. % SiO₂). Removal of reactive aluminium from CFA prior to silicon extraction during the production of Na₂SiO₃ solutions slightly improved the purity of the SiO₂ nanoparticle products. Varying the surfactant (CTAB, CPC, SLS and PEG) used as dispersing agent improved the purity (99.9 wt. %), specific surface area (741-785 m² g^-1), pore volume (0.40 cm³ g^-1) and dispersion of the synthesised silica nanoparticles when CTAB or CPC were used. The physical properties of the obtained silica nanoparticles compared well to those of commercial silica nanoparticles (MCM-41 and SBA-15).

The suitability of the SiO₂ nanoparticles prepared from CFA as catalyst supports was demonstrated by testing them as catalyst support for a copper(II) 2-[(E)-(propylimino)methyl] phenol complex, used as catalyst precursor in the oxidation of veratryl alcohol with tert-butyl hydrogen peroxide as oxidant. The functionalized copper complex was successfully immobilized onto the silica nanoparticle supports and the resulting catalyst had the ability of oxidizing veratryl alcohol to veratrylaldehyde and veratric acid. The degree of conversion of veratryl alcohol and the selectivity of the catalyst was influenced by the physical properties of the silica nanoparticle support. The performance of the CFA-derived supports compared favourably to that of commercial MCM-41 and SBA-15 in the specific catalyst process investigated.

This thesis successfully demonstrates the synthesis of amorphous mesoporous silica nanoparticles from sodium silicate solutions, which were prepared from a South African classified coal fly ash. It further validates the application of the silica nanoparticles as catalyst support in heterogeneous catalysis, proving that these products can potentially be employed as alternative to commercially available mesoporous silica.