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