Controlling crystal growth of platy porous materials as potential substrates for effects pigments

Abstract

The mesoporous silica SBA-15 (SBA = Santa Barbara) has been prepared with a hexagonal platy morphology, particle sizes 8 µm × 0.5 µm thick, of interest as a substrate for effects pigments. Starting from a literature preparation [Park et al. Chem. Com. 2006. 4131] the synthetic variables of reactions temperature, time, concentrations and additives have been optimised. Characterisation (SEM, HRSEM, TEM, N₂ porosimetry) indicated the product was SBA-15 with a BET surface area of 635 m²/g. Calcination resulted in a pore volume of 1.17 cm³g⁻¹, whereas extraction gives a pore volume of 1.04 cm³g⁻¹, indicating an extraction efficiency of 88.5%. A simple DOE (DOE = Design of Experiment) was carried out to examine the interaction between dissolution temperature and hydrolysis time. A general trend was seen, indicating that conditions which favoured uniform plates also discouraged intergrowth. The optimum product formed from the DOE had a dissolution temperature of 5°C and a hydrolysis time of 60 minutes. It had an average size of 3.5 µm × 0.4 µm (aspect ratio 8.7). In addition, the literature synthesis of the crystalline layered silicate Na-RUB-18 (RUB = Ruhr Universiteit Bochum) has successfully been repeated, giving rectangular plates 5.4 µm × 0.7 µm (aspect ratio 7.7). Crystallinity curves were constructed of the standard reaction and seeded reaction, and it was found the majority of the product was formed within the first 20 days of the reaction. Using seeds did reduce the nucleation time but did not decrease the overall time required for the crystallisation to become complete. Agitating the mixture through stirring or tumbling prevented the reaction from occurring. This material was then rendered slightly porous (0.09 cm³/g) through exchanging the sodium ion with CTA⁺ ion (CTA⁺ = cetyltrimethylammonium) and then calcining the product. Finally, the synthetic method for the synthesis of a crystalline platy magnesioaluminophosphate prepared previously in the Wright group as part of a mixture, [Mg₁Al₃P₄O₁₆]PPh₄⁺(PPh₄⁺(TPP) = tetraphenylphosphonium), has been simplified, giving a pure solid consisting of hexagonal plates 30 µm × 2 µm suitable for further characterisation. The final preparation includes two organic additives, TPP, which acts as the ‘template’, and tetrabutylammonium hydroxide (TBA) which is used to control the reaction pH to 6.8.Comparisons of experimental XRD patterns and patterns predicted from the structure of the material, derived from single crystal X-ray diffraction data indicated ordering of TPP⁺ cations within the interlayer region. The additional diffraction peaks observed can be accounted for by a doubling of the unit cell derived from single crystal diffraction [a = 16.48 (600) b= 10.76 (180) c=9.32 (340) Sp. Gp. P2(1)/c β= 100.81° (16)]. This layered structure could also be prepared as a silicoaluminophosphate (SAPO), and EDX data confirmed the presence of silicon in the structure. Attempts were made to exchange the TPP ion with CTA ions but this was unsuccessful.