Mechanism and Control of Saponite Synthesis from a Self-Assembling Nanocrystalline Precursor.

ABSTRACT

Saponite is a clay mineral of the smectite group that finds applications in the chemical industry as a catalyst or catalyst precursor as well as in nanocomposites used for structural or catalytic applications. Saponite of controlled composition, crystallinity, particle size, and morphology would be highly beneficial to industry; however, such materials are not found in a sufficiently pure form in nature. Synthetic methods to produce saponite with specific properties are currently lacking as the understanding of the mechanisms controlling its formation, crystalline properties and particle morphology, is limited. Understanding the saponite formation mechanism is crucial for the development of a highly tuned and controlled synthesis leading to materials with specific properties. Here, we report a new chemical reaction mechanism explaining the nucleation and kinetics of saponite growth at different pHs, at 95-100 °C, and under the influence of pH-modifying additives explored via a combination of X-ray scattering methods and infrared spectroscopy. Our results show that the main factor affecting the nucleation and growth kinetics of saponite is the pH, which has a particularly significant impact on the rate of initial nucleation. Non-uniform reactivity of the aluminosilicate gel also significantly affects saponite growth kinetics and causes a change in the rate-determining step as seen in graphical abstract. The most crystalline saponite is obtained when the nucleation is suppressed by a low initial pH (<7), but the reaction is performed at a higher pH of about 9. The stacking of the saponite sheets can be further improved by a separate postsynthesis treatment with an alkali (NaOH) solution. A simple, ambient pressure method for synthesizing a highly crystalline saponite is proposed that could be easily upscaled for industrial purposes.