K.P. Vijayalakshmi

Mechanistic outlook on thermal degradation of 1,3-dialkyl imidazolium ionic liquids and organoclays

The thermal degradation mechanisms of ionic liquids (ILs) 1-butyl-3-methylimidazolium chloride (BMImCl) and 1-butyl-3-methylimidazolium tetrafluoroborate (BMImBF4) have been established using pyrolysis-GC-MS (Py-GC-MS) and B3LYP/6-311+G(d,p) level of density functional theory (DFT). BMImCl decompose through a bimolecular nucleophilic substitution (SN2) while BMImBF4 exhibit SN2 along with a competitive E2 elimination pathway. Activation energy parameters obtained using Kissinger–Akahira–Sunose method and Ozawa–Flynn–Wall method is compared with the computed activation barriers. The montmorillonite based organoclay prepared using these ionic liquids exchange only the cation part ([BMIm]+) into the clay gallery leading to an expansion of d-spacing from 12.08 to 13.64 A. The organoclay showed the maximum decomposition at 462 °C in the TGA experiment and the decomposition products were identified as methyl imidazole and 1-butene using Py-GC-MS. DFT studies employing a model compound Si(OH)3O− suggested a mechanism involving an imidazole-2-ylidine (carbene) intermediate for the decomposition of [BMIm]+ in the clay. Theoretical results were further supported by 13C NMR analysis of IL in presence of colloidal silica which showed a characteristic carbene NMR signal at 187.6 ppm.

Imidazolium based energetic ionic liquids for monopropellant applications: a theoretical study

A large variety of 1-ethyl-3-methylimidazolium ([EMIm]+) based energetic ionic liquids (ILs) have been studied via their ion pair ([EMIm]+[X]−) formation using high accuracy G3MP2 method and density functional theory (DFT) methods M06L, M05-2X, M06-2X and B3LYP. The selected X− includes nitrogen rich derivatives of tetrazolate and triazolate, dinitramine, dicyanamide as well as conventional anions BF4− and PF6−. The nitrogen enrichment in the system produces energetic ionic liquids (EILs) which showed comparable and in some cases superior thermochemical, fluid and specific impulse (Isp) properties than conventional ionic liquids. The binding energy values for [EMIm]+[X]− are in the range 336–400 kJ mol−1 at DFT levels while the atomization procedure used to compute their heat of formation (ΔfH°) at the G3MP2 level produced results in very close agreement with available experimental data (maximum deviation < 5%). The ΔfH° of conventional ILs is negative whereas that of EILs (167–559 kJ mol−1) confirmed their high energy state. The predicted Isp of all EILs are slightly lower compared to hydrazine in monopropellant systems whereas a significant increase in Isp is observed with the addition of hydroxyl ammonium nitrate (HAN). A good linear correlation between Isp and the wt% of (N + O) content of the EIL is also observed. Our results suggest that imidazolium based energetic ionic liquids have attractive thermochemical properties for use as green substitutes to hazardous hydrazine for monopropellant application in spacecraft technology.

Supramolecular β-cyclodextrin–aniline system: a new class of amine on solid support for carbon dioxide capture with high amine efficiency

Carbon dioxide adsorption on a supramolecular system of aniline encapsulated into a β-cyclodextrin (β-CD) cavity is studied for the first time. The molecular level distribution of an amine on a solid support is achieved in the present system. The amine group of aniline is oriented towards the wider rim of β-CD and is favorably exposed to the gas stream, which results in its maximum utilization for CO2 adsorption. The mechanism of the chemisorption is through the formation of bicarbonate with the primary amino group and the water molecule present inside the β-CD cavity, as evidenced from the high amine efficiency (0.85 mol CO2 per mol of nitrogen) with dry carbon dioxide gas, which is close to the maximum reported value. The formation of C6H5–NH3+⋯HCO3− inside the β-CD cavity is supported by molecular modeling and confirmed by NMR and Raman spectroscopy studies.

1,3-Dialkylimidazolium modified clay sorbents for perchlorate removal from water

Sodium montmorillonite clays modified using 1-alkyl-3-methylimidazolium based ionic liquids with varying alkyl chain length viz. C4, C6, C8, C10 and C16 were used for perchlorate adsorption from water. Pristine MMT showed negligible adsorption whereas ionic liquid modified clays showed an increase in adsorption with increase in chain length of the exchanged cation. 1-Hexadecyl-3-methylimidazolium modified clay (C16-clay) with a d-spacing of 18.55 A showed a maximum adsorption of 0.16 mmol g−1 of clay. The d-spacing of the C16-clay decreased on adsorption of perchlorate to 13.70 A without a change in the composition of the modified clay, as confirmed by CHN analysis. Raman spectroscopic studies substantiated the conformational change from gauche to trans for the imidazolium cations on perchlorate adsorption. The adsorption followed the Freundlich isotherm with pseudo second order kinetics. The modified clays were thermally stable (<200 °C) and regenerated by heating to 175 ± 5 °C in air and 95% regenerability was observed.

Inter molecular azide–diisocyanate coupling: new insights for energetic solid propellants

Hydroxyl terminated azide binders can undergo a spurious reaction with diisocyanates to form tetrazoline-5-one via an inter molecular 1,3-dipolar cycloaddition reaction apart from urethane/allophanate groups which has been overlooked. This has serious implications on solid propellants. The computed activation barrier using density functional theory (DFT) for urethane formation reaction is 28.4 kJ mol−1 and that for tetrazoline-5-one formation reaction is 108.0 kJ mol−1. DFT studies reveal that the rate limiting step of the reaction is 1,3-dipolar cycloaddition between azide and isocyanate. A dual cure was observed in the temperature ranges 42–77 °C and 78–146 °C by differential scanning calorimetry (DSC) and rheological studies, confirming multiple reactions. Tetrazoline-5-one formation was confirmed by Fourier transform infrared spectroscopy (FTIR) and solid state nuclear magnetic resonance spectroscopy (NMR).