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.