Yuhua Duan

CO2 capture properties of lithium silicates with different ratios of Li2O/SiO2: An ab initio thermodynamic and experimental approach

The lithium silicates have attracted scientific interest due to their potential use as high-temperature sorbents for CO2 capture. The electronic properties and thermodynamic stabilities of lithium silicates with different Li2O/SiO2 ratios (Li2O, Li8SiO6, Li4SiO4, Li6Si2O7, Li2SiO3, Li2Si2O5, Li2Si3O7, and α-SiO2) have been investigated by combining first-principles density functional theory with lattice phonon dynamics. All these lithium silicates examined are insulators with band-gaps larger than 4.5 eV. By decreasing the Li2O/SiO2 ratio, the first valence bandwidth of the corresponding lithium silicate increases. Additionally, by decreasing the Li2O/SiO2 ratio, the vibrational frequencies of the corresponding lithium silicates shift to higher frequencies. Based on the calculated energetic information, their CO2 absorption capabilities were extensively analyzed through thermodynamic investigations on these absorption reactions. We found that by increasing the Li2O/SiO2 ratio when going from Li2Si3O7 to Li8SiO6, the corresponding lithium silicates have higher CO2 capture capacity, higher turnover temperatures and heats of reaction, and require higher energy inputs for regeneration. Based on our experimentally measured isotherms of the CO2 chemisorption by lithium silicates, we found that the CO2 capture reactions are two-stage processes: (1) a superficial reaction to form the external shell composed of Li2CO3 and a metal oxide or lithium silicate secondary phase and (2) lithium diffusion from bulk to the surface with a simultaneous diffusion of CO2 into the shell to continue the CO2 chemisorption process. The second stage is the rate determining step for the capture process. By changing the mixing ratio of Li2O and SiO2, we can obtain different lithium silicate solids which exhibit different thermodynamic behaviors. Based on our results, three mixing scenarios are discussed to provide general guidelines for designing new CO2 sorbents to fit practical needs.

Amino acid-functionalized ionic liquid solid sorbents for post-combustion carbon capture.

Amino acid ionic liquids (AAILs) are potential green substitutes of aqueous amine solutions for carbon dioxide (CO2) capture. However, the viscous nature of AAILs greatly hinders their further development in CO2 capture applications. In this contribution, 1-ethyl-3-methylimidazolium lysine ([EMIM][Lys]) was synthesized and immobilized into a porous poly(methyl methacrylate) (PMMA) microsphere support for post-combustion CO2 capture. The [EMIM][Lys] exhibited good thermal stability and could be facilely immobilized into porous microspheres. Significantly, the [EMIM][Lys]-PMMA sorbents retained their porous structure after [EMIM][Lys] loading and exhibited fast kinetics. When exposed to CO2 at 40 °C, [EMIM][Lys]-PMMA sorbent exhibited the highest CO2 capacity compared to other counterparts studied and achieved a capacity of 0.87 mol/(mol AAIL) or 1.67 mmol/(g sorbent). The capture process may be characterized by two stages: CO2 adsorption on the surface of sorbent and CO2 diffusion into sorbent for further adsorption. The calculated activation energies of the two-stage CO2 sorption were 4.1 and 4.3 kJ/mol, respectively, indicating that, overall, the CO2 can easily adsorb onto this sorbent. Furthermore, multiple cycle tests indicated that the developed sorbents had good long-term stability. The developed sorbent may be a promising candidate for post-combustion CO2 capture.

CO2 capture properties of alkaline earth metal oxides and hydroxides: A combined density functional theory and lattice phonon dynamics study

By combining density functional theory and lattice phonon dynamics, the thermodynamic properties of CO(2) absorption/desorption reactions with alkaline earth metal oxides MO and hydroxides M(OH)(2) (where M=Be,Mg,Ca,Sr,Ba) are analyzed. The heats of reaction and the chemical potential changes of these solids upon CO(2) capture reactions have been calculated and used to evaluate the energy costs. Relative to CaO, a widely used system in practical applications, MgO and Mg(OH)(2) systems were found to be better candidates for CO(2) sorbent applications due to their lower operating temperatures (600-700 K). In the presence of H(2)O, MgCO(3) can be regenerated into Mg(OH)(2) at low temperatures or into MgO at high temperatures. This transition temperature depends not only on the CO(2) pressure but also on the H(2)O pressure. Based on our calculated results and by comparing with available experimental data, we propose a general computational search methodology which can be used as a general scheme for screening a large number of solids for use as CO(2) sorbents.

Immobilization of amino acid ionic liquids into nanoporous microspheres as robust sorbents for CO2 capture

Supported nanoporous microspheres immobilized with amino acid ionic liquids (AAILs) as robust sorbents were developed for CO2 capture. AAILs could be facilely immobilized into porous support materials. The developed sorbents exhibited fast kinetics as well as good sorption capacity, and can be regenerated and reused. The presented strategy may pave the way for developing AAIL-functionalized sorbents with high capacity and fast CO2 transport kinetics.

Water steam effect during high CO2 chemisorption in lithium cuprate (Li2CuO2) at moderate temperatures: experimental and theoretical evidence

Li2CuO2 was evaluated as a CO2 captor at moderate temperatures, using water vapor in the gas flow. Different water vapor sorption experiments were performed using N2 or CO2 as carrier gases. If N2 was used as carrier gas, it was evidenced that Li2CuO2 is able to trap water physically and chemically, producing in the second case Li–OH superficial species. Moreover, when CO2 was used as carrier gas, Li2CuO2 continued trapping water, as in the previous case, but in this case CO2 was mainly trapped, forming Li2CO3 and CuO phases. Additionally, the microstructure changes importantly when CO2 and H2O are chemically trapped in Li2CuO2. Li2CO3 and CuO seemed to segregate changing the morphology and the specific surface area. The Li2CuO2 sample was able to capture up to 6.7 mmoles of CO2 per gram of ceramic at 80 °C, a considerably high CO2 amount. Furthermore, all these experiments were theoretically supported by different thermodynamic calculations. Experimental and theoretical results show that H2O acts as a catalytic intermediate, diminishing the activation energy of the whole CO2 chemisorption process. Therefore, the presence of water vapor strongly favored the CO2 chemisorption on Li2CuO2 at moderate temperatures (30–80 °C).

Regeneration mechanisms of high-lithium content zirconates as CO2 capture sorbents: experimental measurements and theoretical investigations

By combining TGA and XRD measurements with theoretical calculations of the capture of CO2 by lithium-rich zirconates (Li8ZrO6 and Li6Zr2O7), it has been demonstrated that the primary regeneration product during absorption/desorption cycling is in the form of Li2ZrO3. During absorption/desorption cycles, lithium-rich zirconates will be consumed and will not be regenerated. This result indicates that among known lithium zirconates, Li2ZrO3 is the best sorbent for CO2 capture.

Bifunctional application of sodium cobaltate as a catalyst and captor through CO oxidation and subsequent CO2 chemisorption processes

The potential bifunctional mechanism of sodium cobaltate (NaCoO2) in the catalysis of CO oxidation and subsequent CO2 chemisorption was systematically analysed. Different catalytic and gravimetric experiments were performed dynamically and isothermally at multiple temperatures. Initially, the CO oxidation process was evaluated using a catalytic reactor connected to a gas chromatograph. Once the production of CO2 was confirmed, its chemisorption capacity with NaCoO2 was studied gravimetrically. Catalytic and gravimetric analysis products were studied by XRD, FTIR and SEM to elucidate the double reaction mechanism. Sodium cobaltate exhibited interesting catalytic properties over a wide temperature range, although the NaCoO2 crystalline structure and chemical composition changed during the CO2 capture process. Furthermore, all the experiments were theoretically supported by first-principles density functional theory thermodynamic calculations. The calculated thermodynamic properties of the CO oxidation and CO2 capture reactions with NaCoO2 under different oxidation conditions were in good agreement with the experimental measurements.

Density functional theory studies on the electronic, structural, phonon dynamical and thermo-stability properties of bicarbonates MHCO3, M = Li, Na, K

The structural, electronic, phonon dispersion and thermodynamic properties of MHCO(3) (M = Li, Na, K) solids were investigated using density functional theory. The calculated bulk properties for both their ambient and the high-pressure phases are in good agreement with available experimental measurements. Solid phase LiHCO(3) has not yet been observed experimentally. We have predicted several possible crystal structures for LiHCO(3) using crystallographic database searching and prototype electrostatic ground state modeling. Our total energy and phonon free energy (F(PH)) calculations predict that LiHCO(3) will be stable under suitable conditions of temperature and partial pressures of CO(2) and H(2)O. Our calculations indicate that the [Formula: see text] groups in LiHCO(3) and NaHCO(3) form an infinite chain structure through O⋯H⋯O hydrogen bonds. In contrast, the [Formula: see text] anions form dimers, [Formula: see text], connected through double hydrogen bonds in all phases of KHCO(3). Based on density functional perturbation theory, the Born effective charge tensor of each atom type was obtained for all phases of the bicarbonates. Their phonon dispersions with the longitudinal optical-transverse optical splitting were also investigated. Based on lattice phonon dynamics study, the infrared spectra and the thermodynamic properties of these bicarbonates were obtained. Over the temperature range 0-900 K, the F(PH) and the entropies (S) of MHCO(3) (M =Li, Na, K) systems vary as F(PH)(LiHCO(3)) > F(PH)(NaHCO(3)) > F(PH)(KHCO(3)) and S(KHCO(3)) > S(NaHCO(3)) > S(LiHCO(3)), respectively, in agreement with the available experimental data. Analysis of the predicted thermodynamics of the CO(2) capture reactions indicates that the carbonate/bicarbonate transition reactions for Na and K could be used for CO(2) capture technology, in agreement with experiments.

First-principles study on the electronic, optical and thermodynamic properties of ABO3 (A = La,Sr, B = Fe,Co) perovskites

The electronic, optical and thermodynamic properties of ABO3 (A = La,Sr, B = Fe,Co) perovskites are investigated using first-principles calculations. The obtained results indicate that SrCoO3 and SrFeO3 are metals, while LaCoO3 and LaFeO3 are insulators and all of them exhibit strong hybridization of the Fe/Co-3d and O-2p orbitals. By correlating the energy band structures with the peaks of the imaginary part of the dielectric function, we obtained the origin of each electron excitation to provide information about the active bands for the corresponding optical transitions observed in the experiment. Moreover, the Debye temperatures θD obtained from the phonon frequencies are comparable to the available data. Finally, the thermodynamic properties of the Helmholtz free energy F, entropy S, and constant-volume heat capacity Cv are investigated based on the phonon spectra.

Molecular Dynamics Study of the Bulk and Interface Properties of Frother and Oil with Saltwater and Air

For water treatment purposes, the separation processes involving surfactants and crude oil at seawater-air interfaces are of importance for the chemical and energy industries. Little progress has been made in understanding the nanoscale phenomena of surfactants on oily saltwater-air interfaces. This work focuses on using molecular dynamics with a united-atom force field to simulate the interface of linear alkane oil, saltwater, and air with three surfactant frothers: methyl isobutyl carbinol (MIBC), terpineol, and ethyl glycol butyl ether. For each frother, although the calculated diffusivities and viscosities are lower than the expected experimental values, our results show that diffusivity trends between each frother agree with experiments but the method cannot be applied for viscosity. Binary combinations of liquid (frother or saltwater)-air and liquid-liquid interfaces are equilibrated to study the density profiles and interfacial tensions. The calculated surface tensions of the frother-air interfaces are like that of oil-air, but lower than that of saltwater-air. Only the MIBC-air and terpineol-air interfaces agreed with our experimental measurements. For the frother-saltwater interfaces, the calculated results showed that terpineol has interfacial tensions higher than those of MIBC-saltwater. The simulated results indicate that the frother-oil systems underwent mixing such that the density profiles depicted large interfacial thicknesses.