Reversible sulfur dioxide capture by amino acids containing a single amino group at low sulfur dioxide concentrations.

SO2, an air pollutant, is harmful to human health and causes air pollution; therefore, numerous studies have focused on the development of SO2 control technologies. Although limestone- and ammonia-based absorbents have been widely used in wet desulfurization, they are difficult to regenerate and do not enable the recycling of SO2, which is a useful resource. Recently, amino acids have attracted attention as reversible SO2 absorbents because they are eco-friendly and have excellent reactivity with SO2, as well as high regeneration performance. Glycine, L-alanine, ß-alanine, 4-aminobutyric acid, 5-aminovaleric acid, and 6-aminohexanoic acid were analyzed to investigate the relationship between SO2 absorption and the amino acid molecular structure using the simulated actual flue gas (200 ppmv SO2 + 13% CO2 in N2 balance). The SO2 absorption of amino acids (with the molecular structure of glycine and alkyl chains of various lengths) improved as the alkyl chain length increased, possibly owing to a decrease in the inductive effect in the molecular structure of the amino acid. Furthermore, 13C-nuclear magnetic resonance spectroscopy was conducted to analyze the SO2 absorption reaction mechanism (including the possible generation of irreversible species), and experiments involving a number of consecutive absorption-desorption cycles were used to confirm the reusability of the amino acids. The tested amino acids exhibited higher cyclic capacities compared to those of deep eutectic solvents and ionic liquids reported in the literature, thereby exhibiting excellent potential as SO2 absorbents. Thus, this study can guide the future design and development of eco-friendly SO2 absorbents.

Discovery of High-Performing Metal-Organic Frameworks for On-Board Methane Storage and Delivery via LNG-ANG Coupling: High-Throughput Screening, Machine Learning, and Experimental Validation.

Liquefied natural gas (LNG) gasification coupled with adsorbed natural gas (ANG) charging (LNG-ANG coupling) is an emerging strategy for efficient delivery of natural gas. However, the potential of LNG-ANG to attain the advanced research projects agency-energy (ARPA-E) target for onboard methane storage has not been fully investigated. In this work, large-scale computational screening is performed for 5446 metal-organic frameworks (MOFs), and over 193 MOFs whose methane working capacities exceed the target (315 cm3 (STP) cm-3 ) are identified. Furthermore, structure-performance relationships are realized under the LNG-ANG condition using a machine learning method. Additional molecular dynamics simulations are conducted to investigate the effects of the structural changes during temperature and pressure swings, further narrowing down the materials, and two synthetic targets are identified. The synthesized DUT-23(Cu) and DUT-23(Co) show higher working capacities (≈373 cm3 (STP) cm-3 ) than that of any other porous material under ANG or LNG-ANG conditions, and excellent stability during cyclic LNG-ANG operation.

Selective Sulfur Dioxide Absorption from Simulated Flue Gas Using Various Aqueous Alkali Solutions in a Polypropylene Hollow Fiber Membrane Contactor: Removal Efficiency and Use of Sulfur Dioxide.

Hollow fiber membrane contactors (HFMCs) provide a large specific surface area. Thus, their significantly reduced volume provides an advantage compared to the conventional gas-liquid contactor. In this study, the selective removal efficiency of flue gas, in which sulfur oxide (SO2) and carbon dioxide (CO2) coexist, was measured using a polypropylene (PP) HFMC with such advantages. To increase the selective removal efficiency of SO2, experiments were conducted using various alkaline absorbents. As a result, with 0.05 M ammonia solution, the removal efficiency of 95% or more was exhibited with continuous operation for 100 h or more. We confirmed that the absorbent saturated by the once-through mode was aqueous ammonium sulfate ((NH4)2SO4) solution and could be used as a fertilizer without additional processing.

Observation of Olefin/Paraffin Selectivity in Azo Compound and Its Application into a Metal-Organic Framework.

Olefin/paraffin separation is an important and challenging issue because the two molecules have similar physicochemical properties. Although a couple of olefin adsorbents have been developed by introducing inorganic nanoparticles into metal-organic frameworks (MOFs), there has been no study on the development of an olefin adsorbent by introducing a certain organic functional group into a MOF. In this study, we posited that azo compounds could offer olefin/paraffin selectivity. We have revealed using first-principles calculations that the simplest aromatic azo compound (azobenzene, Azob) has an unusual propylene/propane selectivity due to special electrostatic interactions between Azob and propylene molecules. On the basis of this interesting discovery, we have synthesized a novel propylene adsorbent, MIL-101(Cr)_DAA, by grafting 4,4'-diaminoazobenzene (DAA) into open metal sites in a mesoporous MIL-101(Cr). Remarkably, MIL-101(Cr)_DAA exhibited enhanced propylene/propane selectivity as well as considerably higher propylene heat of adsorption compared to pristine MIL-101(Cr) while maintaining the high working capacity of MIL-101(Cr). This clearly indicates that azo compounds when introduced into MOFs can provide propylene selectivity. Moreover, MIL-101(Cr)_DAA showed good C3H6/C3H8 separation and easy regeneration performances from packed-bed breakthrough experiments and retained its propylene adsorption capacity even after exposure to air for 12 h. As far as we know, this is the first study that improves the olefin selectivity of MOF by postsynthetically introducing an organic functional group.