1 Introduction
Hydrogen sulfide (H2S) is a colorless toxic gas with
corrosive, flammable and rotten egg odor characteristics, and it
commonly exists in natural gas, refinery gas, biomass syngas, coal gas
and so on. For H2S is highly toxic to the human body,
direct discharge is not allowed. Therefore, the gas containing
H2S needs to be separated, burned and then desulfurized
before discharge. Commonly, it is an economic and green route to remove
H2S to an ultra-low level from the feed gas without a
post-desulfurization process.1 For example, natural
gas usually undergoes purification technologies for the removal
H2S below 20 mg/m3 before use.
Therefore, how to efficiently remove H2S has become a
research focus. Common gas purification technologies include absorption,
adsorption, membrane separation and cryogenic
distillation.2 Among them, the absorption method with
the advantages of fast reaction rate, high desulfurization efficiency,
high sulfur capacity and small size has dominated gas separation
processes.3
For the absorption process, absorbents are the main factor determining
the absorption effect.2 Researchers have developed
various types of absorbents (e.g. Poly (ethylene glycol) dimethyl ether
(PEGDME),4 methanol,5alkanolamine6 and ionic liquids
(ILs)7, et al). According to the function mechanism
between gas and absorbent molecule, absorbents can be broadly classified
into chemical and physical categories. At present, chemical absorbents
have been widely used in industry. Since alkanolamine solutions have
first been developed for the absorption process to remove acidic
components from gas in 1930,8 alkanolamines including
monoethanolamine (MEA),3 diethanolamine
(DEA),9 methyldiethanolamine
(MDEA),10 diisopropanolamine
(DIPA),11 2-Amino-2-methyl-1-propanol
(AMP)12 and their mixtures have been developed
successively. However, due to the strong bonding force between chemical
absorbent and H2S formed during the absorption process,
the desorption process temperature is usually high with the vaporization
of water, resulting in a high operating cost.10
Physical absorbents have the advantage of easier regeneration at a lower
temperature, almost no quality loss and non-corrosive equipment compared
with chemical absorbents. Physical absorbents such as methanol,
morpholine, PEGDME and N-methyl-2-pyrrolidone (NMP) have been used in
the industrial H2S removal
processes.4,5,13 Nevertheless, as a result of the slow
absorption rate of physical solvent, it is difficult to remove
H2S deeply and rapidly under normal temperature and
pressure. Therefore, the physical absorption of H2S
usually is carried out under higher pressure and low
temperature,14,15 which increases the equipment
investment and operating cost. Therefore, it will be transformative to
develop a physical absorbent with a similar absorption effect to a
chemical absorbent.
Nonaqueous solvents, including ILs, deep-eutectic solvents (DESs),
phase-change absorbents, etc, are a kind of absorbents that are expected
to achieve this aim. Lee and Jesús et al16 have
demonstrated that ILs with special functions for H2S
absorption could be designed by adjusting the length and branch of the
alkyl chain or the type of precursor anion. Phase-change absorbent has
been developed by Zhao et al17 to improve the
absorption of H2S and reduce the energy consumption of
its regeneration process. DESs (e.g. 1-ethyl-3-methylimidazolium
chloride ([Emim]Cl) + imidazole) have also been developed by Liu et
al18 for the selective absorption of
H2S from the mixed gas, which has a high absorption
efficiency for H2S but low selectivity to
CO2. Multimer solvents, aggregated via H-bonds between
solvent molecules, exist widely in nature and have been used as
absorbent.19-21 However, pre-study on multimer mainly
focuses on the properties analysis of multimer as a solvent, less on the
use of multimer for the absorption of the acidic components. It is still
a big challenge to design efficient physical absorbents.
Quantum chemical calculation
(QCC), using a form of intermolecular force,22,23dipole moment,24 affinity,25 etc to
analyze the interaction between solvent and solute molecules. Therefore,
it has been used as an efficient and low-cost method for absorbent
design and absorption mechanism study. For example, the surface charge
density distribution can be used to guide and design the preparation of
Ibuprofen cooling crystallization solvent and Menschutkin reaction
solvent.26 Wu et al found that the prepared protic
ionic liquids (PILs) and DESs with a strong interaction with
H2S molecules obtained by QCC have a good selective
absorption performance of H2S.27,28Conceptual density functional theory analysis showed that electron
affinity and electrophilicity could better describe the mechanism of
extraction desulfurization capacity. Thus, QCC can be used to study the
relationship between solvents and H2S at the molecular
level. Using this method combined with experiment,
1,8-Diazabicyclo-[5.4.0]-undec-7-ene (DBU) as organic superbases has
been used to synthesize switch solvents and PILs29-31with the molar ratio of DBU and pyrazole, imidazole, 1,2,3-1H-triazole,
1,2,4-1H-triazole and 1-hexanol at 1:1, they showed high absorption rate
for H2S and CO2. However, there are two
nitrogen atoms in the molecular structure of DBU, and the 1:1 molar
ratio mentioned above may not be the best, the whether there are more
active sites for H2S absorption.
In this work, we designed a novel double hydrogen bonds (H-bonds)
DBU-2EG multimer absorbent with high absorption solubility for
H2S. QCC was adopted to study the formation mechanism of
DBU-2EG synthesized by DBU and EG, and found a new structure of double
H-bonds DBU-2EG multimer absorbent which has a good absorption
performance for H2S. The interaction mechanism between
DBU-2EG and H2S also was explored. Furthermore, we
carried out experiments to prove the QCC results. This work developed a
novel high-efficient physical absorbent for H2S and
provide an efficient strategy to design a physical absorbent.