To verify the inherent formation mechanism of the prepared DBU-2EG multimer absorbent and absorption mechanism for H2S, NMR experiments were performed at 25℃. 1H-NMR of DBU-2EG was shown in Figure 3(a). A peak at 5.14 ppm was attributed to active H. When the concentration of DBU-2EG and CDCl3(V (DBU-2EG): V (CDCl3)) increased from 6% to 26%, the chemical shifts of active H moved from 4.67 ppm to 5.30 ppm (see Figure S1 and Table S2), indicating that the interaction between DBU and EG was weakened under the existence of CDCl3, which demonstrated the combination of DBU and EG via H-bond. In comparison with the 1H-NMR spectra of EG and DBU (see Figure S2), the characteristic chemical shifts in DBU-2EG moved slightly from 3.71, 3.17, 3.09 and 3.06 ppm to 3.55, 3.13, 3.12 and 3.10 ppm, respectively, indicating that the interaction between
in DBU and -OH in EG enhance shielding effect, while the interaction between
in DBU and -OH in EG reduce this effect. This evidence had shown that both N in DBU interacts with EG molecules. Figure 3(b) showed that an additional broad peak packet at 4.26 ppm was observed after absorption of H2S, which was attributed to the interaction between H in H2S and O of in DBU-2EG. The corresponding signal of active H disappears compared with Figure 3(a). At the same time, the1H-NMR signals of the DBU-2EG have slightly deviated towards the low field (Supplementary 5 spectroscopic data). This means that there is an interaction between H in H2S and O in DBU-2EG, weakening the electronic shielding effect. Meanwhile, the absorption of H2S by DBU-2EG belonged to physical absorption, which was inconsistent with the results of DFT calculations. The 13C NMR analysis was observed (see Figure S3).
The thermal stability of the DBU-2EG multimer absorbent and DBU-2EG-4H2S complexes were analyzed utilizing the TG-DTA test under the N2 atmosphere. As shown in Figure 3(c), with the temperature rising from 30 to 74.2 ℃ (line (a)), the DBU-2EG multimer absorbent had endothermic behavior for the gradual weakening of the interaction between DBU and EG. When the temperature is 74.2 ℃, the DBU-2EG multimer absorbent has an obvious exothermic, indicating that the intermolecular H-bonds in the DBU-2EG begin to break. As the temperature increased, DBU and EG compounds gradually volatilize and had a significant mass loss from 103.5 ℃ (line (b)). No obvious mass loss was observed over 162 ℃, which indicates that the DBU-2EG multimer absorbent is completely volatilized. In Figure 3(d), the DBU-2EG-4H2S complexes had an obvious weight loss with the temperature ranging from 30 to 56 ℃ (line (a)), indicating that the H2S was released from the complexes with an exothermic process. Then the process became endothermal from 56 to 66.5 ℃ (line (b)) corresponding to the gradual weakening of the interaction between DBU and EG, which is similar to DBU-2EG thermal stability analysis. However, the dissociation temperature of H-bonds between DBU and EG in the DBU-2EG-4H2S is slightly reduced compared with DBU-2EG, which implied that H-bonds between DBU and EG became weaker after absorbing H2S. With the heating temperature increasing from 66.5 to 103.5 ℃ (line (c)), the DBU-2EG multimer absorbent gradually loses mass. In addition, the mass loss at low temperatures signifies that the absorption of H2S in DBU-2EG belongs to the physical absorption process. When the temperature exceeds 103.5 ℃, the process presented an exothermic process, which means that DBU-2EG began to be volatilized. Therefore, the desorption temperature of 60 ℃ can be adopted in the absorbent regeneration process.
Viscosity is a fundamental physiochemical property of absorbents. It can greatly affect the mass transfer and mixing performances of the system, thus influencing the reaction rate. In addition, the energy consumption of absorbent transportation and the foaming property of absorbents is also determined by their viscosity. Therefore, we carry out the viscosity analysis of multimer absorbent at different temperatures. As shown in Figure 3(e), the viscosity of the DBU-2EG multimer absorbent gradually decreases with increasing temperature, and its viscosity value is 17.66 mPa·s at 20 ℃ and 8.7 mPa·s at 45 ℃, respectively, indicating that the viscosity of DBU-2EG multimer absorbent is significantly lower than some functionalized ILs for H2S absorption.36