CO2 separation using composites consisting of 1-butyl-3-methylimidazolium tetrafluoroborate/CdO/1-aminopyridinium iodide

1-Aminopyridinium iodide (iodine salt) was used in CO2 separation composites consisting of CdO and 1-butyl-3-methylimidazolium tetrafluoroborate (BMIM+BF4−). Using iodine salt, the separation performance was largely improved. The CO2/N2 selectivity was 64.6 and the permeance of CO2 gas was 22.6 GPU, which was about twice that of BMIM+BF4−/CdO composites without addition of iodine salt. These results were due to the both effect of iodine salt on the transport of the N2 molecules by the cyclic ring compound and the promoting transport of CO2 molecules by the amine groups. Moreover, the oxide layer on the surface of the CdO could enhance the CO2 solubility, resulting in the enhancement of separation performance. The mechanical and chemical properties were measured using SEM, Raman, TGA and FT-IR. The cross-section of coated membranes was confirmed by SEM. The coordinative interactions of iodine salts with BMIM+BF4−/CdO composite were observed by Raman.

In our previous study, we have studied the CO 2 permeance and CO 2 /N 2 selectivity by preparing membranes by adding various metal oxides to ionic liquids. For example, for the BMIMBF 4 /ZnO composite membrane, the CO 2 permeance was 101 GPU and the CO 2 /N 2 selectivity was 42.1 24 . The ZnO oxide layer has a strong affinity for CO 2 and has been shown to influence the increase in solubility of CO 2 . The BMIMBF 4 /CdO composite membrane had CO 2 /N 2 selectivity of 32.5 and a CO 2 permeance of 57.1 GPU 25 . As a result, when the metal oxide was introduced, the oxide layer could improve the CO 2 solubility, which has influenced the CO 2 transport by free ions in the ionic liquid.
In this study, BMIM + BF 4 − /CdO composite membrane was used with an iodine salt containing amine group in metal oxide doped membrane. For these membranes, it was expected that the amine group in iodine salt will interact with CO 2 molecules for solubility enhancement. Furthermore, the CdO nanoparticles have strong affinity for CO 2 capable of enhancing the transport. It was also thought that the synergistic influence of the cyclic ring effect in iodine salt plays a role as barriers for N 2 transport, resulting in the increase of CO 2 /N 2 selectivity. Fig. 1, the morphology was examined by SEM and used to investigate the average thickness of the coating solution on the polysulfone microporous support. The average pore size of neat polysulfone was 0.1 μm and pore of surface state was also observed as microporous structure. SEM image showed that the polysulfone support was similar to the sponge like structure and the thickness of selective layer was about 8.7 μm.

SEM images analysis. As shown in
Previous studies have shown that the CO 2 /N 2 selectivity in the 1/0.007 in the composite membrane of BMIM + BF 4 − /CdO on polymer support with finger-like structure was 32.5 25 . In this study, the shape of the cross-section of the polysulfone support was sponge like structure.

TEM images analysis.
To investigate the CdO nanoparticles in BMIM + BF 4 − /CdO/iodine salt composite, TEM was observed as shown in Fig. 2. TEM image showed that the average size of generated CdO nanoparticles were ranged from 100 to 200 nm and the aggregation phenomena was observed. Figure 3 showed the separation of CO 2 /N 2 using a BMIM + BF 4 − ionic liquid containing CdO particles and iodine salt. The weight ratio of BMIM + BF 4 − /CdO was fixed at 1:0.007 and membranes with increasing mole ratio of iodine salt were tested at room temperature using a single gas (CO 2 and N 2 ). These experiments were tested three times and the single gas permeances measured for CO 2 and N 2 were described in Fig. 3(a). As the mole ratio of iodine salt increased, the permeance of CO 2 increased to 0.05 mole ratio of salts and decreased above that ratio due to the aggregation phenomena of iodine in composite. These aggregation phenomena of iodine salts prevented the gas molecules from being transported through membrane, diminishing the permeance. Thus, CO 2 permeance decreased with increasing iodine salt to 0.05 mole ratio of salts. Thus, the best separation performance was observed at 0.05 mole ratio of iodine salts as shown in Fig. 3

Separation performance.
The single gas permeance and selectivity of the composite membranes: 1/0.007 BMIM + BF 4 − /CdO and 1/0.007/0.05 BMIM + BF 4 − /CdO/iodine salt membranes were compared as shown in Table 1. Table 1 indicated that BMIM + BF 4 − /CdO composite membranes showed the CO 2 permeance of 22.9 GPU and the selectivity of 17.6 (CO 2 /N 2 ) while the CO 2 permeance of 22.6 GPU and the selectivity of 64.6 for BMIM + BF 4 − /CdO/iodine salt composite membranes. These results were attributable to that the iodine salt acts as a hindrance to the moving of gas molecules, resulting in the decrease in overall permeance ('barrier effect'). However, the transport of CO 2 could be accelerated by the amine group of iodine salt. In addition, the interaction between the oxide layer formed from the dissociated CdO nanoparticles and the CO 2 molecule also could improve the solubility of CO 2 .
Raman analysis. Raman was measured to investigate the interaction of iodine salt molecule in BMIM + BF 4 − / CdO composite. The Raman spectrum BMIM + BF 4 − /CdO (1/0.007) was described in Fig. 4(a), and the addition of 0.05 mol of iodine salt was shown in Fig. 4(b). The three different ionic species for various BF 4 − states such as ionic aggregates, ion pairs, and free ions were observed at 777, 770, and 765 cm −1 , respectively 26 . Table 2      FT-IR analysis. Figure 6 showed the FT-IR spectra for the neat BMIM + BF 4 − and BMIM + BF 4 − /CdO/iodine salt composites. The C-H stretching band the alkyl group of neat BMIM + BF 4 − was known to be observed at 2966 cm −1 27 . However, when iodine salt was added to neat BMIM + BF 4 − , it shifted from 2966 to 2962 cm −1 , due to the new coordinative interaction.
As shown in Fig. 7, with the new combination of BMIM + and iodine salt, the existing C-H stretching band was weakened. This also indicated BF 4 − became more free ions when iodine salt was added to BMIM + BF 4 − /CdO in the raman spectra.

Conclusions
We have succeeded in preparing high selective carbon dioxide membranes consisting of BMIM + BF 4 − /CdO/ iodine salt composite to facilitate the CO 2 transport for high separation performance. The features and interactions in BMIM + BF 4 − /CdO/iodine salt composites were characterized by SEM, Raman, TGA, and FT-IR. The separation performance of BMIM + BF 4 − /CdO/iodine salt composite membrane was significantly increased compared to BMIM + BF 4 − /CdO composite. When CdO and iodine salt were incorporated into BMIM + BF 4 − , the CO 2 / N 2 selectivity was 64.6 and the permeance of CO 2 molecules was 22.6 GPU. These results were due to the both
Preparation of membranes. The membranes were prepared utilizing BMIM + BF 4 − , cadmium oxide, iodine salt and ethanol. As first step, the cadmium oxide was sonicated to be dispersed in ethanol for 5 minutes. Then, BMIM + BF 4 − and iodine salt were added to the ethanol mixture with cadmium oxide dispersed. The solution was heated at 85 °C for 24 hours to evaporate the ethanol. Then, solution was coated onto a polysulfone microporous support and cast using a RK control coater (Model K202, Control Coater RK Print-Coat Instruments Ltd., UK). The best performance of BMIM + BF 4 − /CdO/iodine salt was observed at 1/0.007/0.05 (The ratio of BMIM + BF 4 − / CdO was described by weight ratio while BMIM + BF 4 − /iodine salt was mole ratio).

Gas separation experiments.
The all gas flow rates represented by gas permeance were determined using a bubble flow meter at the steady-state. Gas flow rates were measured with a mass flow meter at an upstream with