Tetra-n-heptyl ammonium tetrafluoroborate: Synthesis, phase equilibrium with CO2 and pressure swing absorption for carbon capture

Abstract

Based on the analyses of the interaction energy with CO2, COSMO calculation and melting temperature, a quaternary ammonium salt, tetra-n-heptyl ammonium tetrafluoroborate ([thepAm][BF4]), was synthesized using [thepAm][Br] and NaBF4 as the raw materials. The as-synthesized [thepAm][BF4] was characterized by FTIR and NMR. The solid–liquid–gas (SLG) phase equilibrium line of the [thepAm][BF4]–CO2 system was determined by high pressure differential scanning calorimeter, indicating it was more stable than [thepAm][Br] because of higher melting temperatures of [thepAm][BF4] in pressurized CO2. The solubility data of CO2 in [thepAm][BF4] at 313.2, 323.2 and 333.2K up to 15MPa were measured by using the high-pressure quartz spring approach. The Peng-Robinson equation of state with the van der Waals-1 mixing rules was employed to calculate the solubility and SLG phase equilibrium data. Results showed high molar fractions of CO2 in [thepAm][BF4] at high pressures (e.g., molar fraction of 0.88 at 10.0MPa and 313.2K). A cyclic absorption–desorption procedure and a pressure swing absorption process were proposed and tested for CO2 capture/separation from the H2, CO and CO2 mixture by using [thepAm][BF4] as the absorbent, verifying its selective capture of CO2 over H2 and CO. The study may open a new way for CO2 capture.