Spectroscopic and Computational Analyses of Liquid–Liquid Interfacial Reaction Mechanism of Boric Acid Esterification with 2,2,4-Trimethyl-1,3-pentanediol in Boron Extraction Processes

Abstract

Purification of silica solution to chemically remove impurities is a novel approach for preparing solar-grade Si. Complete elimination of boron is necessary, as it significantly affects the semiconductor properties of Si if included. To build an efficient reactor for boron extraction, the mechanism of extraction reaction based on molecular behavior should be well understood. Here, we investigated the liquid−liquid extraction of boron (boric-acid) using 2,2,4-trimethyl-1,3-pentanediol as an extractant, which takes place at the liquid−liquid interface experimentally and theoretically. Raman spectroscopy for the microflow reactor provided the concentration of boric acid after extraction, whereas density functional theory calculation showed the reaction energy profiles of the underlying chemical reactions. Calculation results suggested that H 2 O molecules from the aqueous phase promote extraction by enabling the formation of [BOH-HOH-HOC], a sufficiently stable, nonsteric structure, which stabilizes the transition state and facilitates boric acid esterification. Otherwise, this reaction cannot take place in standard conditions. Raman spectroscopy applied to the extraction process in a microflow reactor supported this conclusion experimentally. These results suggest that the extraction reaction at the liquid−liquid interface is mass transfer-limited. This can help the design of effective reactors to eliminate boron impurity from silica solution.

Review of Optical Thermometry Techniques for Flows at the Microscale towards Their Applicability to Gas Microflows

“…Most of the applications of Raman microscopy to liquid micro-flows are devoted to measuring the amount of substances (or density). Thus, composition profiles in microfluidic devices have been measured for mixing processes [129,130], diffusion through interfaces [131][132][133], or chemical reactions [134][135][136][137]. Roetmann et al [138] and Rinke et al [139] were able to record 2D density maps (Raman "imaging") of mixing miniflows, with subsecond time resolution, by using pulsed (6 ns) excitation.…”