Novel routes to designer silicas: studies of the decomposition of (M+)2[Si(C6 H4O2)3]·xH2O. Importance of M+ identity of the kinetics of oligomerisation and the structural characteristics of the silicas produced

Abstract

Silica is an important industrial chemical but the mechanisms involved in its formation from aqueous precursors remain far from well understood. In this study we have prepared a range of silicon catecholate salts where M⁺ is Li⁺, Na⁺, K⁺, NH₄⁺, Et₃NH⁺ and followed complex breakdown and silica oligomerisation under approximately neutral pH conditions by ¹H solution NMR and a molybdenum blue colorimetric method. The silicas produced have been analysed by electron microscopy and gas adsorption methods to give information on particle sizes and their distribution, surface areas and porosity. The oligomerisation reaction is complex but it is possible to identify time periods over which principally dimers, trimers and higher molecular weight oligomers are formed. In the first case, no loss of orthosilicic acid from solution is measured; in the second case, apparent third-order kinetics with respect to loss of orthosilicic acid from solution are observed; in the third case, a reversible first-order reaction is observed and information is obtained for both polymerisation (k₊) and depolymerisation (k_–) processes. Once ca. 95% of the available silicon has been polymerised, the only process occurring is Ostwald ripening which cannot be measured reliably by the colorimetric method. The reaction rates and solubility of the soluble silicon species were found to vary with cation identity. Values for the apparent third-order section of the oligomerisation reaction gave rate constants with values between 9.66 × 10^–6 and 5.02 × 10^–5 l² mmol^–2 s^–1 for Li⁺ > Na⁺, NH₄⁺ > Et₃NH⁺ > K⁺. Rate constants for the addition of one orthosilicic acid molecule to oligomers gave k₊ between 1.27 × 10^–3 and 2.74 × 10^–4 s^–1, with Li⁺ > Na⁺ > Et₃NH⁺ > K⁺ > NH₄⁺. An activation energy of 58 kJ mol^–1 for this process was obtained for the K⁺ salt. The change in (k₊+k_–) between the various Group 1 cations was found to vary linearly with the radius of the hydrated metal ions, but the situation was more complex for the NR₃H⁺(R = H, Et) cations. The solubility of orthosilicic acid in the presence of NR₃H⁺(R = H, Et) cations was lower than in the presence of Group 1 cations. This led to the production of higher levels of silica from the ammonium and triethylammonium salts.

TEM and CRYOTEM were used to identify the size and nature of aggregates formed during the silica polymerisation reaction. From 10 min to 1 h of reaction time, material built up from ca. 1 nm particles was detected. After 3 h of reaction, particles were of the order of 2–3 nm in diameter and small aggregates of 5 + particles could be observed. Electron microscopy (TEM and SEM) of silica samples taken at the end of the reaction (ca. 7 days) showed that precipitation in the presence of counter-ions of increasing ionic radius resulted in an increase in average particle size from 2 to 20 nm in diameter and an increase in average aggregate size from 57 to 71 nm in diameter. Surface area data showed a reduction in measured surface area from 558 m² g^–1 for Li⁺ to 237 m² g^–1 for Et₃NH⁺. Silica precipitated in the presence of lithium ions had spherical particles and spherical pores, and silica precipitated in the presence of triethylammonium ions had irregular shaped particles and slit shaped pores. The change in pore shape was progressive as the size of the counter-ion was increased.

It is clear that counter-ion identity has a very significant effect on the kinetics of oligomer formation and, consequently, on the nature of the silica produced. The behaviour of the precipitating systems containing the Group 1 metal cations is related to the size of the hydrated metal cation. For the two cations based on ammonia, the arguments are more complex and involve size, hydrogen bonding and hydrophobicity effects.