Entropy enthalpy compensation in the SO2 absorption reaction and the optimal capture temperature range with amine alkyl siloxane as absorbent

Abstract

In the present work, a kind of amine alkyl siloxane N-butyl-N-((1,1,3-trimethyl-3,3-diphenyldisiloxaneyl)methyl)butan-1-amine (BTDMBA) was synthesized and used to capture SO2. Its thermal decomposition temperature is 160℃, which is comparable with that of Ionic liquids (ILs), but its viscosity is only 125 mPa∙s at 25℃ after the absorption of SO2, which is much lower than that of ILs. The reaction mechanism is that BTDMBA reacted with SO2 to form charge transfer complex BTDMBA∙SO2. The reaction enthalpy and entropy change are −83.95 kJ/mol and −240.48 J·mol−1·K−1, respectively, which are more negative than that of ILs. By comparing the enthalpy and entropy change of amine alkyl siloxane and absorbents reported in literatures, an unexpected entropy enthalpy compensation phenomenon was discovered and the isokinetic temperature was found to be 371 K. Based on this phenomenon and the optimization objective function, it was also found that with the decrease of entropy change, the optimal absorption and desorption temperature difference increased initially and decreased afterwards. The maximum of entropy changes is −201 J·mol−1·K−1 for absorbent to achieve recycle absorption capacity 0.79 mol/mol under the condition of absorption pressure 0.002 bar and desorption pressure 1 bar.