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 (Bu 4 P + ) and tetrabutylammonium (Bu 4 N + ) salts were treated with 6M HCl at 80°C for 2–12h. Elemental analysis revealed higher resistance of Bu 4 P-S and Bu 4 N-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. Bu 4 N-S decomposed in slightly lower extent than Bu 4 P-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 Bu 4 N-S showed onset temperature of near 200°C, significantly higher temperatures, above 300°C, was found for Bu 4 P-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 Bu 4 N-S and Bu 4 P-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 Bu 4 N-S (above 380°C) than for Bu 4 P-S (above 500°C). Highlights Tetrabutyl-ammonium and -phosphonium montmorillonites were treated with HCl. Bu 4 P-S was slightly more susceptible to decomposition in HCl than Bu 4 N-S. TG and IR of evolved gasses showed higher thermal stability of Bu 4 P-S than Bu 4 N-S. Surfactants present on acid-treated samples were released at higher temperature. Variable temperature NIR spectra of solids proved higher stability of Bu 4 P-S. Graphical abstract [DISPLAY OMISSION]
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