Thermal stability of tetrabutyl-phosphonium and -ammonium exchanged montmorillonite: Influence of acid treatment

Abstract

Thermal stability of acid-treated organo-montmorillonites was investigated by thermogravimetric analysis (TA) coupled with infrared (IR) spectroscopic analysis of evolved gasses, and by in-situ measurement of near-IR (NIR) spectra of solid samples. The organoclays prepared from Na-form of SAz-1 montmorillonite (Na-S) and tetrabutylphosphonium (Bu4P+) and tetrabutylammonium (Bu4N+) salts were treated with 6M HCl at 80°C for 2–12h. Elemental analysis revealed higher resistance of Bu4P-S and Bu4N-S to decomposition compared to Na-S, leaving 44 and 52% of the octahedral Mg and/or Al in the solid reaction products after 12h treatment. Bu4N-S decomposed in slightly lower extent than Bu4P-S, pointing thus to the influence of the type of cation headgroup on its stability in HCl. The TG/DTG profiles of both samples showed pronounced mass loss due to decomposition of organic cations within temperature 200–800°C. While Bu4N-S showed onset temperature of near 200°C, significantly higher temperatures, above 300°C, was found for Bu4P-S. The intensities of the absorption bands related to CH vibrations of aliphatic hydrocarbons in the IR spectra reached maxima at 277°C and 446°C for Bu4N-S and Bu4P-S, respectively. The mass loss attributed to the organic cation decomposition for acid-treated samples dropped down below 50% of its initial value due to the leaching of organic phase. Moreover, the onset temperature of organic phase release was shifted to higher temperature indicating that only more strongly held cations remained in the samples. NIR spectra showed that the intensity decrease of the first 2νCH overtones band due to organic cation release began at lower temperature for Bu4N-S (above 380°C) than for Bu4P-S (above 500°C).