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Al layered double hydroxides and their adsorption for CO2
Accepted Manuscript Title: Structural and electronic analysis of Li/Al layered double hydroxides and their adsorption for CO2 Authors: Xin-Juan Hou, Huiquan Li, Peng He, Zhenhua Sun, Shaopeng Li PII: DOI: Reference:
S0169-4332(17)31223-0 http://dx.doi.org/doi:10.1016/j.apsusc.2017.04.187 APSUSC 35868
To appear in:
APSUSC
Received date: Revised date: Accepted date:
18-1-2017 17-4-2017 22-4-2017
Please cite this article as: Xin-Juan Hou, Huiquan Li, Peng He, Zhenhua Sun, Shaopeng Li, Structural and electronic analysis of Li/Al layered double hydroxides and their adsorption for CO2, Applied Surface Sciencehttp://dx.doi.org/10.1016/j.apsusc.2017.04.187 This is a PDF file of an unedited manuscript that has been accepted for publication. As a service to our customers we are providing this early version of the manuscript. The manuscript will undergo copyediting, typesetting, and review of the resulting proof before it is published in its final form. Please note that during the production process errors may be discovered which could affect the content, and all legal disclaimers that apply to the journal pertain.
Structural and electronic analysis of Li/Al layered double hydroxides and their adsorption for CO2
Xin-Juan Houa,b, Huiquan Lia,b*, Peng Hea, Zhenhua Suna, Shaopeng Lia, a
Key Laboratory of Green Process and Engineering, National Engineering Laboratory for Hydrometallurgical Cleaner Production Technology, Institute of Process Engineering, Chinese Academy of Sciences, Beijing 100190,China b
University of Chinese Academy of Sciences, Beijing 100149,China
Submitted to: Applied Surface Science Submission Date: 2017-4-17
*Corresponding author E-mail address: [email protected] 1
Highlights (1) The effects of different monovalent and bivalent anions on the expansion, interlayer structure, and interaction of anions or water with layers are confirmed. (2) It is verified that the interactions among mineral surfaces, anions, water and CO2 are dominated by hydrogen bond, electrostatic interaction, and van der Waals.
ABSTRACT The most stable structures and electronic properties of different Li/Al layered double hydroxides models (i.e., Li/Al-X, X=F-, Cl-, Br-, OH-, NO3-, CO32-, SO42-) and their hydrates, the adsorption of CO2 on Li/Al-X (X=Cl-, NO3-, CO32-) were ascertained by means of density functional theory. Results revealed that the planes of NO3- and CO32- are parallel with the layers in dehydrated state, although the plane of NO3becomes vertical with the layers upon the introduction of water molecules. Electronic density analysis suggested that SO42- and CO32- significantly strengthens the reducibility of the Li/Al layered double hydroxides. The distribution of the frontier orbitals indicated the high reactivity of the anions and hydroxyl groups of the layers. The orders of the predicted stability are F- Cl- Br- >NO3- for the monovalent anions and SO42- CO32- for the divalent anions. The calculated adsorption energies of CO2 in Li/Al-X (X = Cl-, NO3-, CO3-) supported the experimental observation that Li/Al-CO3 exhibits higher CO2 capture capacity than Li/Al-NO3 and Li/Al-Cl. Non-covalent interaction analysis indicated that the interactions among mineral surfaces, anions, water and CO2 are dominated by H-bonds, electrostatic interactions, and van der Waals forces. In addition, radial distribution functions were applied to provide insight for the interaction of water or CO2 with carbonate ion and hydroxyl layers.
Keywords: Li/Al layered double hydroxides,CO2, Non-covalent interaction analysis,Density functional theory 2
1. Introduction Layered double
hydroxides
(LDHs)
exhibit
excellent
and
permanent
anion-exchange capability and are used as ideal catalysts 1, excellent adsorbent materials for various anion species and pollutants 2–5, hosts for drugs and biomolecules 6–8 and for CO2 sequestration 9–13. In Li/Al LDHs, Li+ cations occupy vacant octahedral sites within Al(OH)3 layers, resulting in a positively charged layer; the interlayer and the surface anions ensure the charge balance. LiAl2(OH)6+ layers are span along the c-axis. In the corresponding hydrates, interlayer water molecules form a hydrogen bond network with hydroxide groups and anions. The intercalation behavior and effects of different anions on Li/Al LDHs are important in many processes and applications. For example, carbonate impurities must be controlled and reduced when LDHs are used as additives in polymers or for drug delivery 11. Fogg et al. 14 performed time-resolved in situ X-ray diffraction analysis to measure the intercalation rate of lithium salts (LiX, X = Cl-, Br-, NO3-, OH-, SO42-) into gibbsite; the half-lives of the reaction follow the order SO42- OH- Cl- Br- NO3-. Isupov 15 studied the structures of Li/Al LDHs with Cl-, Br-, OH-, and NO3- anions and confirmed the position of Li+ and anions through single crystal investigations; the reactivity of lithium salts with gibbsite exhibits the following order: LiC1 LiNO3 Li2SO4 > LiBr > LiI. Hou et al. 16 employed XRD and NMR methods to analyze the effects of hydration state on the structure and dynamical behavior of interlayer anions and factors controlling the expansion behavior of Li/Al and Mg/Al LDH phases containing SO42–, CO32–, F–, Cl–, Br–, OH–, and NO3–; the studied phases can be grouped into three types according to the extent of expansion of the interlayer space 16. Williams et al. 17 examined the factors influencing staging during anion-exchange intercalation into hydrated Li/Al-X (X=Cl, Br, and NO3); staging was found to be strongly correlated with anion exchange rate. Tarasov et al. 18 evaluated the de-intercalation kinetics of Li+ cations, Cl-, Br-, NO3-, and SO42from their hydrates. The de-intercalation rate exhibited the following order: NO3- Br3
Cl- ; for SO42-, rapid de-intercalation only achieved 40% completion 18. Nagendran et al. 19 combined experimental and theoretical methods to verify that the molecular plane of nitrate ion in dehydrated Li/Al-NO3 is parallel to the metal hydroxide layer; they also confirmed that the nitrate ion reorients itself upon rehydration, such that its molecular plane is inclined to the metal hydroxide layer. Moreover, structural analysis was
conducted
to
examine
the
interplay
between
Coulomb
force
and
hydrogen-bonding interactions in hydrated Li/Al-X (X = Br-, Cl-) 20. Fogg et al. 21 applied molecular mechanics to rationalize the structures of Li/Al-X (X = Br-, Cl-, NO3-). Hou et al. 22 performed molecular dynamics simulation to gain insights into the structural, dynamical, and energy aspects of LiAl2(OH)6ClnH2O. Although several studies have reported on LDH-derived CO2 adsorbents, the majority of which have focused on conventional Mg/Al LDHs. Huang et al. 9 investigated the performance of Li/Al LDHs intercalated with CO32-, Cl-, and NO3- anions as novel CO2 adsorbents; they found that Li/Al-CO3 demonstrated higher CO2 capture capacity than those of Li/Al-NO3 and Li/Al-Cl. The properties of LDHs are strongly influenced by interlayer inorganic anions and their structural arrangement. As such, the effects of inorganic anions (e.g., halogen anions, hydroxyls, nitrates, carbonates, and sulfates) on the structures and thermal behavior of LDHs must be accurately determined. For Li/Al LDHs, no systemic theoretical study has explored the influence of different anions on the expansion, interlayer structure, and interaction of anions or water with layers. The CO2 adsorption mechanism of Li/Al LDHs also remains unclear. This work aims to achieve an overall representation of intercalated Li/Al LDHs at the molecular level. An explicit theoretical investigation is performed to clarify the intercalation behavior of different anions, namely, F–, Cl–, Br–, OH–, NO3–, CO32–, and SO42–, in the layers. The effects of water molecules and the adsorption mode of CO2 in Li/Al LDHs are also analyzed.
2 Model and calculations 4
2.1. Computational Method The unit cells of Li/Al-X (X = F-, Cl-, Br-, OH-, NO3-, CO32-, SO42-, Fig. 1), their hydrates (Li/Al-X-H), and CO2 adsorbed on Li/Al-X (M = Cl-, NO3-, CO32-)(Li/Al-MCO2) were optimized by P1 representation using CASTEP program 23. Generalized gradient approximation (GGA) for the exchange-correlation potential (PBE) [24] is performed for all compounds. In all GGA/PBE calculations, the method proposed by Grimme was used for DFT-D corrections [25]. The threshold values of the convergence criteria were 1.010−5 eV/atom for energy, 0.03 eV/Å for the maximum force, 0.001 Å for the maximum displacement, and 0.05GPa for maximum stress. The Monkhorst–Pack scheme k-points grid sampling was set at 3 × 3 × 1 for Li/Al-X or Li/Al-X-H (X = F-, Cl-, Br-, OH-, NO3-) , 3 × 2× 1 for Li/Al-X or Li/Al-X-H (X =CO32-, SO42-). The plane-wave basis set energy cutoff was set at 340 eV; ultrasoft pseudopotentials were used for all chemical elements. In geometry optimization of the compounds, all atoms and unit cell parameters are relaxed. The calculated cell parameters for these compounds are summarized in Table 1. Interaction analysis and figure plotting were performed by non-covalent interaction (NCI) analysis to investigate the interaction mechanism of different anions and water in the layered structures. The adsorption mode of CO2 in Li/Al LDHs was also examined. 2.2. Interaction Energy (1) The interaction energies of different ions and the Li+ cation with the host layers are calculated to confirm the ion exchange ability of different Li/Al-X models. With Li/Al-Cl and Li/Al-Cl-H as examples, the interaction energies of anions (IEx or IEX-H) or Li+ (IELi or IELi-H) with host layers can be defined as follows: IEX= ELi/Al-Cl-(Elayer-Cl+ECl) IEX-h= ELi/Al-Cl-H-(Elayer-Cl-H+ECl-H) IELi= ELi/Al-Cl-(Elayer-Li+ELi) IELi-h= ELi/Al-Cl-H-(Elayer-Li-H+ELi-H) ELi/Al-Cl and ELi/Al-Cl-H denote the energies of the optimized Li-Al-Cl and its hydrate, respectively. Elayer-Cl and Elayer-Cl-H represent the single-point energies of the 5
optimized Li/Al-Cl and Li/Al-Cl-H structures without the Cl- anion, respectively. ECl and ECl-H are the single-point energies of Cl- in the cell with cell parameters same to those of the optimized Li/Al-Cl and Li/Al-Cl-H. Elayer-Li and Elayer-Li-H are the single-point energies of the optimized Li/Al-Cl and Li/Al-Cl-H structures without the Li+ anion. ELi and ELi-H indicate the single-point energies of Cl- in the cell with cell parameters same to those of the optimized Li/Al-Cl and Li/Al-Cl-H. (2) De-intercalation energy (Ede-x-H) is used to obtain information on the reversible intercalation/de-intercalation cycles of Li/Al-X-H. Take Li/Al-Cl-H as example, Ede-Cl is defined as follows: Ede-Cl= Egibbsite-H+ELi-Cl-ELi/Al-Cl-H ELi/Al-Cl-H denotes the energy of the fully optimized Li/Al-Cl-H. Egibbsite-H and ELi-Cl represent the energies of the optimized hydrated gibbsite and Li-Cl with the fixed cell parameters of the optimized Li/Al-Cl-H, respectively. (3) Hydration energy (EH) is calculated to predict the effects of different anions on the hydration ability of Li/Al LDH. For Li/Al-Cl-H, hydration energy (EH) is defined as follows: EH =ELi/Al-Cl-H – (ELi/Al-Cl + nEw ) ELi/Al-Cl-H is the energy of the optimized Li/Al-Cl-H; ELi/Al-Cl is the energy of the optimized Li/Al-Cl; and Ew is the energy of the optimized single water molecule in a cell with the same cell parameters of the optimized Li/Al-Cl-H. The variable n denotes the number of water molecules at the Li/Al-Cl-H layers. (4) The adsorption energy of CO2 (AECO2 ) in Li/Al-X (X = Cl-, NO3-, CO32-) can be defined as AECO2= ELi/Al-X-CO2-( ELi/Al-X+ECO2) ELi/Al-X-CO2 is the energy of the optimized Li/Al-X-CO2, and ECO2 is the energy of the optimized CO2 located in the cell of the optimized Li/Al-X structure. 2.3. Non-covalent interaction analysis NCI analysis was performed to investigate the interaction among LDH layer, anions, water, and CO2 by examining the reduced electron density gradient derived from the electron density and its first derivative 26. The reduced electron density gradients (RDG) can be expressed as follows: 6
RDG(r)=
𝟏
(𝐫)
𝟐(𝟑𝛑𝟐 )𝟏/𝟑
(𝐫)𝟒/𝟑
Weak interactions exist in regions with low electron density and low RDG values. Different types of interactions (attraction and repulsion) can be distinguished when the density is multiplied by the sign of the second density Hessian eigenvalue (2). This method visualizes NCI by plotting the electron density with respect to the RDG. On the NCI surface, red signifies steric effects, blue indicates strong attraction (e.g., hydrogen bonding between two atoms), and green denotes weak van der Waals interactions. The relevant interaction analysis and figure plotting were performed using Multiwfn program [27]. 2.4. Molecular Dynamics Simulation Molecular dynamics (MD) simulations were performed using canonical ensemble conditions with carbonate ion, water or carbon dioxide simulated as adsorbates on the LDH layer. The atomic partial charges obtained by GGA/PBE method were adopted. In the MD simulation, the cell parameters were fixed and the heavy atoms of LDH layer were frozen except for H atom, while carbonate ion, water or carbon dioxide were full relaxed. The adsorption behavior was modeled using the Universal force field (UFF) [28]. The columbic interactions were calculated by using the Ewald summation method [29] while the van der Waals interactions were evaluated within a cut-off radius of 18.5Å. The system was calculated in an NVT ensemble for 2ns. The radial distribution function [30], which is one of the most important structural quantities characterizing a system, was analyzed based on MD simulation results.
3. Results and discussion 3.1. Structural relaxation Table 1 shows the structural parameters of the relaxed computational cells of all models. The validity of the computational structures was verified by comparing the predicted structures of Li/Al-X (X = Cl, Br, NO3, CO3) with the structures described in experimental studies. The calculated lattice parameters are consistent with the 7
experimental data [21, 31–32]. The calculated cell volumes of Li/Al-X (X = F, Cl, Br) increased as the ionic radii increased. The calculated lattice parameters of all Li/Al-X models indicated that the cell was enlarged by water molecules. The extent of the cell expansion was influenced by intercalation. For Li/Al-X, the layer spacing was significantly influenced by SO42-; slightly expanded by Cl-, Br-, and NO3-; and not expanded by F-, OH-, and CO32-. The optimized results verified the experimental observation 16 that the tetrahedral oxyanions induce a more significant expanding effect on LDHs than planar oxyanions or single atom ions. In this work, we did not consider the effect of different number of water molecules for the d-spacing of LDHs. Only two water molecules are located in the layer spaces of unit cell as shown in Fig.1, namely, each layer of unit cell possess one water molecule. It can be deducted that that the layer spacing of Li/Al-F-H and Li/Al-OH-H and Li/Al-CO3-H are smallest, while the layer spacing of Li/Al-Cl-H and Li/Al-Br-H are slightly expandable and those of Li/Al-NO3-H and Li/Al-SO4-H are significantly expandable. This observation is qualitatively coincident with the experimental results 16. The charges of anions are listed in Table 1. Population analysis revealed that that the direction of electron transfer direction was from anions toward metal hydroxyl layers and water molecules. The charge of SO42- remained invariant in both hydrated and dehydrated states. Cl-, Br-, CO32-, and NO3- became more ionic after water molecules were removed, suggesting that the charges of electrons transfer are less pronounced during dehydration. Fig. 1 depicts the structure of Li/Al-Cl and its hydrates down the c-axis, which is vertical to the metal hydroxide layer; the figure clearly shows the arrangements of the anions and cations. In the optimized structure of Li/Al-X (X = F, Cl, Br), halogen ions are located on the edge of the unit cell and midway between Al(OH)3 layers, the lithium ions are located in the Al(OH)3 layer, which is consistent with the X-ray diffraction pattern 31. Both halogen and lithium ions located in the center of the hole formed an octahedral Al(OH)3 and were parallel to the c-axis, forming an alternating Li+…X-…Li+… chain. The calculated distances of Li from the nearby F-, Cl-, and Br- anions are 2.982, 3.574, and 3.751 Å, respectively. In the optimized 8
hydrated structures of Li/Al-X (X = F, Cl, Br), water molecules introduced a positional disorder in the placement of halogen ions [Figs. 2(a)–2(c)]. Comparison with the dehydrated Li/Al-X (X=F, Cl, Br) showed no evident changes in the location of Li+; that is, Li+ ions are parallel to the c-axis, forming an alternating Li+…Li+…chain, whereas halogen cations move away from the Li+…Li+… chain. The calculated closest FH, ClH and BrH contacts in Li/Al-X (X=F, Cl, Br) are 1.802, 2.283, and 2.437 Å, respectively, suggesting that hydrogen bonding interactions with F- ions and hydroxyl groups in Al(OH)3 layers are stronger than those of Cl- and Br- ions. The calculated ClH and LiCl distances in Li/Al-Cl are 2.283 and 3.574 Å, respectively, which are consistent with the experimental values of 2.30 and 3.57Å 31. In the hydrated Li/Al-X-H (X = F, Cl, Br), the calculated closest FH distance with hydroxyl groups and interlayer water are 1.654 and 1.675 Å, respectively, which have corresponding values of 2.061 and 2.021 Å in Li/Al-Cl-H and 2.188 and 2.039 Å in Li/Al-Br-H. These results verified the experimental observation 16 that Cl– was stabilized by hydrogen bonding from both hydroxyl groups and interlayer water. In Li/Al-OH and hydrated Li/Al-OH-H, the calculated distances between the O atom of the OH- anion and the H atom from the hydroxyl layers are 1.733 and 1.645 Å, respectively, indicating the presence of strong hydrogen bonds. As shown in Fig. 2, water molecules in the hydrated Li-Al-X(X = F, Cl, Br and OH) are in parallel layers because such arrangement optimizes the interaction between water molecules and surrounding anions or hydroxyl groups. CO32- anions in Li/Al-CO3 and its hydrate are almost parallel with the hydroxyl layers. As such, three oxygen atoms of the CO32- anions simultaneously form strong hydrogen bonds with the hydroxyl groups of two side layers, creating the largest interactions between anions and LDH layers. In the hydrated Li/Al-CO3-H (Fig. 2), both carbonate and water molecules are almost parallel with the hydroxide layers. Thus, the water molecules formed a strong hydrogen bond with the hydrogen atom of two side layers and the oxygen atom of carbonate [Fig. 3(d)]. According to the TGA data, the hydroxyl groups in the main layer exhibited a strong interaction with the interlayer water and carbonate, in contrast to that in Li/Al-Cl 11. 9
Experiments 15, 33 showed that the nitrate ion in the dehydrated phase of Li/Al-NO3 is intercalated with its molecular plane parallel to the metal hydroxide layer. In the optimized hydrated Li/Al-NO3 [Fig. 2(f)], water molecules are located on one side of the metal hydroxide layer and induce the nitrate plane to become almost perpendicular with the hydroxide layers; as such, two nitroxide atoms formed strong hydrogen bonds with the hydroxyl layers, and one nitroxide atom formed a strong hydrogen bond with nearby water molecules [Fig. 3(f)]. Our theoretical calculation verified the experimental observation 16 that the interlayer NO3– groups are oriented with their C3 axes at high angles to the basal plane, in contrast to their flat-lying orientation in Mg/Al-NO3 32. The interlayer separation of the optimized Li-Al-SO4 is approximately 8.7 Å, which is comparable with the experimental value of 8.8 Å 21. Two O atoms of SO42interact with the upper hydroxyl layer, and the two other O atoms interact with the lower hydroxyl layer; this phenomenon is consistent with previous MD modeling, in which SO42– groups are oriented with their C2 axes perpendicular to the basal plane 21. With the existence of water molecules, the other possible orientation of SO42[Fig. 2(g)] is that three oxygen atoms are directed toward one layer, resulting in a trigonal pyramidal orientation. In the hydrated Li/Al-SO4, water molecules formed hydrogen bonds with the oxygen of SO4- and the H atoms of the hydroxide layers [Fig. 3(g)]. Fig. 4 shows the optimized geometries of Li/Al-X (X = Cl, NO3, CO3) with adsorbed CO2 (Li/Al-X-CO2). In Li/Al-Cl-CO2 [Fig. 4(a)], CO2 molecules are parallel with the metal hydroxide layers, and the Cl- anion and CO2 molecules are located near the top and bottom layers, respectively. Cl- anions and the O atoms of CO2 formed hydrogen bonds with the hydrogen atoms of nearby hydroxide layers [Fig. 4(a’)). Electrostatic attraction existed between the Cl- anion and the carbon of CO2 at a distance of 3.1 Å. In the optimized Li/Al-NO3-CO2 [Fig. 4(b)], both the NO3- anion and CO2 are slightly sloped toward the c-axis, causing one O atom of CO2 (OCO2) and two O atoms of NO3- to form hydrogen bonds with the hydroxyl layers. Electrostatic attraction also existed between the other O atom of CO2 and the N atom of NO3- [Fig. 10
4(b’)]. In the optimized Li/Al-CO3-CO2 [Fig. 4(c)], the plane of CO32- is slightly sloped toward the layers to maintain balance between one hydrogen bond of OCO3with the top layer and two hydrogen bonds of the other OCO3- atoms with the bottom layers. The two OCO2 atoms simultaneously formed hydrogen bonds with both the “up” and “down” layers, indicating that the CO2 molecules in Li/Al-CO3 are more stable than those in the Li/Al-Cl and Li/Al-NO3 layers [Fig. 4(c’)]. 3.2. Effects of anions on interactions Table 2 presents the interaction energies between Li+ and LDH in hydrated and dehydrated states (IELi or IELi-H). The average interaction energies between Li+ and LDH containing different anions are almost the same, suggesting that anion type did not evidently affect the interaction between the Li+ cation and layers. Water molecules slightly increased the absolute value of IELi. The absolute values of IEx in Table 2 indicate that the stability of the monovalent anions in the layers follows the order F- OH- Cl- Br- > NO3-, while the stability of divalent anions is SO42- CO32-. The stability of different anions in Li/Al layers is consistent with that in Mg/Al layers 34. The Mulliken charges of Cl- and Br- obviously increased in the hydrated than that in the dehydrated LDH. Comparable absolute values were observed for CO32-. The charge of NO3- slightly increased, whereas those of OH-, F-, and SO42remained almost invariant. The change in the Mulliken charges also suggested that the water molecules exerted a stronger influence on the Cl- and Br- anions than on the other anions. In the presence of water, the stabilities of Cl- and Br- are increase that mainly caused by the more negative electrons in Cl- and Br- in the hydrated Li/Al LDHs promoted electrostatic interaction. In both dehydrated and hydrated Li/Al LDHs containing monovalent anions, the interaction between NO3- and metal hydroxyl layers is the lowest. These results confirmed that the halogen anion exhibits a higher affinity to LDHs than NO3-, and fluoride ions can be incorporated into the interlayer space of LDH throughout the ion exchange reaction 35. The hydrated energies of different Li/Al-X (Table 3) indicated that OH- and NO3facilitated the easier adsorption of water molecules on LDH compared with that of LDH containing halogen anions. Tarasov et al. 18 investigated the de-intercalation 11
of anions and Li+ from Li/Al LDHs in water; the results suggested that LDHs spontaneously lose both Li and anionic ions to yield crystalline Al(OH)3. As shown in Table 3, the calculated de-intercalation energies of LiX with monovalent anions observed the following sequence: F- Cl- Br- > OH- NO3-; this sequence is an inverse relationship to the reaction rate 18. The adsorption energies of CO2 in Li/Al-X with X = Cl, NO3, CO3 are 38.4, 8.1, and -5.4 kJ mol-1, which supported the experimental finding 9 that Li/Al-CO3 exhibited a higher CO2 capture capacity than those of Li/Al-NO3 and Li/Al-Cl. As shown in Fig. 4, in Li/Al-Cl-CO2 and Li/Al-NO3-CO2, Cl- and NO3- were preferred to be located at one side of the hydroxyl layer, indicating that the hydrogen bonds between Cl- and NO3- and metal hydroxyl layers were less than that of CO32simultaneously forming hydrogen bonds with both the upper and lower hydroxyl layers. 3.3. Band structure and density of states The electronic structures of Li-Al LDHs provide key information regarding the physical and chemical properties of these materials. The densities of state (DOS) were predicted based on the optimized cells. The Fermi energy level was set to 0 eV. The states near the Fermi level correspond to the electronic structure that can be easily transferred. Comparison of the DOS and PDOS of Li/Al LDHs (Fig. 5 and Figs 1S– 6S) revealed that the orbital contributions from the hydroxyl layers dominate the overall shapes of the total DOS. The calculation results revealed that the top of the low-valence band (VB) of Li/Al LDHs containing halogen anions are mainly contributed by O2p and halogen 2p (Figs. S2–S9); the bottom of the conduction band (CB) is mainly dominated by Al2p. For Li/Al-Br, Br2p also contribute to CB. For Li/Al-OH, Li/Al-NO3, Li/Al-CO3, and Li/Al-SO4, the O2p of the hydroxyl layers and the anions dominates the VB. As shown in Fig. S1-S6, the DOS of O2p in LDH and their hydrates are higher than those of Al2p. This result showed that some electrons from Al2p transferred to VB and became part of the Al and O interaction. The Li 2sp states slightly contribute to the VB, suggesting that Li is highly ionized. As shown in Fig. 6, the VB of their hydrated compounds are also mainly contributed by the 2p 12
orbital of hydroxyl O, anions, and water. Moreover, water molecules obviously decreased the electron density in the VB. The top of the VB and the bottom of the CB are usually associated with reactivity; that is, a system with a small or no band gap is more reactive than a system with large band gap. The NO3-, SO42- and CO32- anions are responsible for decreasing the band gap in the corresponding LDH. These results suggested that NO3-, SO42-, and CO32- can considerably strengthen the reducibility of Li/Al LDH. Bandwidth is primarily determined by the extent to which orbitals on neighboring atoms overlap, which in turn can predict the nature of chemical bonding. For Li/Al-X (X = NO3-, CO32-, SO42-) and their hydrates, the VB widths are larger than those of Li/Al-X (X = F-, Cl-, Br-, OH-) and their hydrates. This finding indicates that the inter-atomic orbital overlaps in Li/Al-X (X = NO3-, CO32-, SO42-) and their hydrate compounds are weaker than those of Li/Al-X (X = F-, Cl-, Br-, OH-). 3.4. Orbital analysis The highest occupied molecular orbital (HOMO) and lowest unoccupied orbital (LUMO) provide insights into the potential reactivity of a system, given that a chemical reaction requires the excitation of electrons from their Fermi levels to the lowest conduction band. The HOMOs and LUMOs of Li/Al-X (Fig. 7) were examined to determine changes in their chemical reactivity. The HOMO and LUMO of Li/Al-X (X = Cl-, Br-, OH-, NO3-) are mainly located at the anions. For Li/Al-X (X = CO32-, SO42-), the HOMO and LUMO are located at the anions and –OH of the layers, respectively. For Li/Al-F, the HOMO is mainly located at –OH, and LUMO is located at F-. The distribution of the frontier orbital indicated that Li+ cations are very difficult for further reaction, whereas the anions and –OH of the layers exhibit high reactivity. 3.5. Radial distribution function We have calculated the radial distribution (RDF), g(r), to give the probability of finding a particle in the distance r from another particle 30. From the aspect of peak distance and peak sharpness of the RDFs present in Fig 8, it can be concluded that for Li/Al-CO3-CO2 at 298K, Ocarbonate-CCO2 has peak distance at 2.78Å which mainly 13
contributed by the electrostatic interaction; oxygen atom of carbonate and CO2 have stronger interaction with hydroxyl layers at peak distance about 2.38 and 2.42 Å, respectively. For Li/Al-CO3-H at 298K, the peak distance of oxygen atom of carbonate with hydrogen atoms of water or hydroxyl layers are about 2.35 Å, while that of oxygen atom and hydrogen atoms of hydroxyl layers is about 2.25 Å To determine whether the increase in temperature leads to strengthening of the interaction between adsorbates and hydroxyl layers, RDF calculation were performed on 298K and 348K. As shown in Fig.8, the RDFs of Li/Al-CO3-CO2 have more sharp peaks and move to lower peak distance with increasing temperature which means the interactions are more significant. For Li/Al-CO3-H, increasing temperature has no obviously effect for the peak of Ocarbonate-Ow and Ocarbonate-Hlayer but increasing the probability of finding water molecules near hydroxyl layers.
4. Conclusions In this work, the effects of different monovalent and bivalent anions on the electronic structures and properties of Li/Al LDHs were investigated by DFT method. This study explored the structural parameters, DOS, frontier orbital, hydration energies, and interaction energies of metal ions; de-intercalated energies; and the interaction mode among mineral surfaces, anions, and CO2. Water molecules in Li/Al-X (X = F-, Cl-, Br-, OH-) are parallel with the hydroxyl layers and located near the middle part between the upper and lower layers with the anions. In Li/Al-X (X = CO32-, SO42-), the plane of water molecules are vertical to the hydroxyl layers and located near one side of the layer. NCI analysis intuitively indicated that the interactions among mineral surfaces, anions, and CO2 are dominated by hydrogen bond, electrostatic interaction, and van der Waals. The stability of monovalent anions in the layers follows the order F- Cl- Br- > NO3-, while the stability of the divalent anions is SO42- CO32-. The calculated adsorption energies of CO2 in Li/Al-X (X = Cl, NO3, CO3) verified that Li/Al-CO3 exhibits a higher CO2 capture capacity than those of Li/Al-NO3 and Li/Al-Cl for stronger hydrogen bonds between CO2 and layers. The hydrated energies indicated that Li/Al-OH more easily 14
adsorbed water molecules than LDHs intercalated by other monovalent anions. The calculated de-intercalation energy of Li-Al LDHs with monovalent anions follows the order F- Cl- Br- > OH- NO3- .
Acknowledgments We would like to thank the NSFC (Grant No. 51304184) for its financial support. The results described in this paper are obtained on the Deepcomp7000 of Supercomputing Center in the Computer Network Information Center of the Chinese Academy of Sciences.
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capture: Structure evolution and regeneration. Ind. Eng. Chem. Res. 45(2006)7504-7509. [11] E. Conterosito, L. Palin, D. Antonioli, D. Viterbo, E. Mugnaioli, U. Kolb, L. Perioli, M. Milanesio, V. Gianotti, Structural characterisation of complex layered double hydroxides and TGA-GC-MS study on thermal response and carbonate contamination in nitrate- and organic-exchanged hydrotalcites. Chem. Eur. J. 21(2015)14975-14985. [12] Q. Wang, Y.S. Gao, J.Z. Luo, Z.Y. Zhong, A. Borgna, Z.H. Guo, D. O'Hare, Synthesis of nano-sized spherical Mg3Al-CO3 layered double hydroxide as a high-temperature CO2 adsorbent. RSC advances, 3(2013) 3414-3420. [13] S. Li, Y.X. Shi, Y. Yang, Y. Zheng, N.S. Cai, High-performance CO2 adsorbent from inter layer potassium-promoted stearate-pillared hydrotalcite precursors. Energy & Fuels, 27(2013) 5352-5358. 14 A. M. Fogg, D. O’Hare, Study of the intercalation of lithium salt in gibbsite using time-resolved in situ X-ray diffraction, Chem. Mater. 11(1999)1771-1775. 15 V. P. Isupov, Intercalation compounds of alumium hydroxide, J. Struct. Chem., 40(1999) 672-685. 16 X. Hou, D. L. Bish, S. L. Wamg, C. T. Johnston, R. J. Kirkpatrick, Hydration, expansion, structure, and dynamics of layered double hydroxides. American Mineralogist, 88(2003)167–179. 17 G. R. Williams, D. O’Hare, Factors influencing staging during anion-exchange intercalation into [LiAl2(OH)6]XmH2O (X= Cl-, Br-, NO3-)，Chem. Mater. 17(2005) 2632-2640. 18 K. A. Tarasov, V. P. Isupov, L. E. Chupakhina, D. O'Hare, A time resolved, in-situ X-ray diffraction study of the de-intercalation of anions and lithium cations from [LiAl2(OH)6]nX qH2O (X = Cl-, Br-, NO3-, SO42-). J. Mater. Chem. 14(2004)1443-1447. 19 S. Nagendran, G. Periyasamya, P. V. Kamath, Structure models for the hydrated and dehydrated nitrate-intercalated layered double hydroxide of Li and Al, Dalton Trans., 45(2016) 18324-18332. 20 S. Nagendran, P. V. Kamath, Structure of the chloride- and bromide-intercalated layered double hydroxides of Li and Al- interplay of coulombic and hydrogen-bonding interactions in the interlayer gallery. Eur. J. Inorg. Chem. 26(2013) 4686–4693. 21 A. M. Fogg, A. L. Rohl, G. M. Parkinson, D. O’Hare, Predicting guest orientations in layered double hydroxide intercalates. Chem. Mater., 11(1999)1194-1200. 22 X. Hou, A. G. Kalinichev, R. J. Kirkpatrick, Interlayer structure and dynamics of Cl--LiAl2-layered double hydroxide:
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First-principles simulation: ideas, illustrations and the CASTEP code. J. Phys.: Condens.
Mat. 14(2002) 2717-2744.
16
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17
Fig. 1. Optimized structure of Li/Al-Cl (Al, magenta; O, red; Li, blue; Cl, cyan).
18
(a) Li/Al-F-H
(b) Li/Al-Cl-H
(c) Li/Al-Br-H
(d) Li/Al-OH-H
CO3
(e) Li/Al-CO3-H
(f) Li/Al-NO3-H
(g) Li/Al-SO4-H
(h) Li/Al-SO4-H
Fig.2. Optimized structures of Li/Al-X-H (X=F-, Cl-, Br-, OH-, NO3-, CO32-, SO42-) (Al, magenta; O, red; Li, blue; H,white).
19
(a) Li/Al-F-H
(b) Li/Al-Cl-H
(c) Li/Al-Br-H
(d) Li/Al-OH-H
(e) Li/Al-CO3-H
(f) Li/Al-NO3-H
(g) Li/Al-SO4-H
(h) Li/Al-SO4-H
Fig. 3. Gradient isosurfaces (s=0.30au) for optimized structures of Li/Al-X-H (X=F-, Cl-, Br-, OH-,
NO3-, CO32-, SO42-).
20
(a) Li/Al-Cl-CO2
(a’) Li/Al-Cl-CO2
(b) Li/Al-NO3-CO2
(b’) Li/Al-NO3-CO2
(c’) Li/Al-CO3-CO2
(c) Li/Al-CO3-CO2
Fig. 4. Optimized structures of Li/Al-X-CO2 (X= Cl-, NO3-, CO32-)(Al, magenta; O, red; C, grey; Si, orange; and Na, green) (a)-(c) and corresponding gradient isosurfaces (s=0.30au) (a’)-(c’).
21
(a) Li/Al-F
(b) Li/Al-Cl
(c) Li/Al-Br
(d) Li/Al-OH
(e) Li/Al-NO3
(f) Li/Al-CO3
(g) Li/Al-SO4 Fig. 5. DOS of Li/Al-X (X=F-, Cl-, Br-, OH-, NO3-, CO32-, SO42-).
22
(a) Li/Al-F-H
(b) Li/Al-Cl-H
(c) Li/Al-Br-H
(d) Li/Al-OH-H
(e) Li/Al-NO3-H
(f) Li/Al-CO3-H
(g) Li/Al-SO4 Fig. 6. DOS of Li/Al-X-H (X=F-, Cl-, Br-, OH-, NO3-, CO32-, SO42-). 23
(a) Li/Al-F HOMO
(a’) Li/Al-F LUMO
(c) Li/Al-Br HOMO
(c’) Li/Al-Br LUMO
(e) Li/Al-NO3 HOMO
(e’)Li/Al-NO3 LUMO
(g) Li/Al-SO4 HOMO
(b)Li/Al-Cl HOMO
(d)Li/Al-OH HOMO
(b’) Li/Al-Cl LUMO
(d’) Li/Al-OH LUMO
(f)Li/Al-CO3 HOMO (f’) Li/Al-CO3 LUMO
(g’)Li/Al-SO4 LUMO
Fig. 7. Frontier orbital distributions for Li/Al-X (X=F-, Cl-, Br-, OH-, NO3-, CO32-, SO42-). 24
(a)
(b) Fig. 8. RDFs for Li/Al-CO3-CO2 (a) and Li/Al-CO3-H (b).
25
Table 1 DFT-optimized cell parameters of Li/-X and Li/Al-X-H. Model
a(Å)
b(Å)
(°)
c(Å)
(°)
(°)
V0(Å3) Charges in anion
Li/Al-F
5.120
5.120
11.930
90.0
90.0
120.0 270.9
-0.66
Li/Al-Cl
5.131
5.131
14.298
90.0
90.0
120.0 326.1
-0.66 a
5.100
5.100
14.299
90.0
90.0
120.0 322.1
5.136
5.136
15.004
90.0
90.0
120.0 342.8
5.100a
5.100
14.946
90.0
90.0
120.0 336.6a
5.071b
5.071
15.474
90.0
90.0
120.0
Li/Al-OH
5.118
5.118
12.372
90.0
90.0
119.8 281.2
-0.56
Li/Al-CO3
5.165
8.801
14.097
89.5
82.0
90.1
634.7
-1.36 -1.00
Li/Al-Br
-0.52
c
5.086
8.809
7.758*2
5.159
5.152
15.531
82.4
84.3
119.9 348.0
5.109a
5.109
14.374
90.0
90.0
120.0 324.9a
5.070b
5.070
14.154
90.0
90.0
120.0
Li/Al-SO4
5.145
8.828
18.579
117.4 87.9
89.8
748.6
-1.42
Li/Al-F-H
5.126
5.121
14.635
90.0
90.0
120.0 330.0
-0.68
Li/Al-Cl-H
5.208
5.148
15.258
90.0
90.0
120.0 354.4
-0.60
Li/Al-NO3
102.62
a
5.096
5.096
15.292
90.0
90.0
120.0 344.0
Li/Al-Br-H
5.191
5.179
15.581
90.0
90.0
118.7 367.3
-0.35
Li/Al-OH-H
5.079
5.079
14.528
90.0
90.0
90.0
330.0
-0.57
Li/Al-CO3-H
5.166
8.774
14.250
92.0
85.2
90.0
643.2
-1.33
Li/Al-NO3-H
5.128
5.106
20.395
97.9
79.2
119.4 456.4
-0.77
Li/Al-SO4-H
5.159
8.810
17.348
81.1
91.9
90.1
778.6
-1.42
Li/Al-Cl-CO2
5.175
5.240
15.744
90.1
89.7
120.2 368.8
-0.66
Li/Al-CO3-CO2
5.171
8.800
14.600
84.5
82.0
90.1
656.8
-1.40
Li/Al-NO3-CO2
5.143
5.195
20.696
86.5
85.5
120.2 236.7
-0.76
a
Ref.31 b Ref.21 c Ref.36
26
Table 2 Interaction energies between Li+ cation and layers (IELi and IELi-H in kJ mol–1), different anions and layers of Li/Al-X (IEX and IEX-H in kJ mol–1) and their hydrates. IELi
IEX
IELi-H
IEX-H
Li/Al-F
236.8
-101.3
Li/Al-F-H
248.4
-85.5
Li/Al-Cl
239.8
-84.1
Li/Al-Cl-H
252.4
-90.3
Li/Al-Br
242.6
-74.5
Li/Al-Br-H
252.3
-86.9
Li/Al-OH
236.9
-87.9
Li/Al-OH-H
255.0
-73.1
Li/Al-NO3
258.2
-52.3
Li/Al-NO3-H
256.0
-36.4
Li/Al-CO3
241.9
-143.5
Li/Al-CO3-H
247.3
-137.4
Li/Al-SO4
240.8
-99.1
Li/Al-SO4-H
246.6
-89.0
Table 3 Hydrated energies of Li/Al-X-H (Eh in kJ mol–1) and de-intercalation energies of Li-X from Li/Al-X-H (Ede-X in kJ mol–1). Eh
Ede-X
Li/Al-F-H
-24.4
315.0
Li/Al-Cl-H
-19.1
263.2
Li/Al-Br-H
-7.5
249.1
Li/Al-OH-H
-48.3
183.1
Li/Al-NO3-H
-33.4
92.1
Li/Al-CO3-H
-48.6
198.6
Li/Al-SO4-H
-42.6
203.8
27
S0169-4332(17)31223-0 http://dx.doi.org/doi:10.1016/j.apsusc.2017.04.187 APSUSC 35868
To appear in:
APSUSC
Received date: Revised date: Accepted date:
18-1-2017 17-4-2017 22-4-2017
Please cite this article as: Xin-Juan Hou, Huiquan Li, Peng He, Zhenhua Sun, Shaopeng Li, Structural and electronic analysis of Li/Al layered double hydroxides and their adsorption for CO2, Applied Surface Sciencehttp://dx.doi.org/10.1016/j.apsusc.2017.04.187 This is a PDF file of an unedited manuscript that has been accepted for publication. As a service to our customers we are providing this early version of the manuscript. The manuscript will undergo copyediting, typesetting, and review of the resulting proof before it is published in its final form. Please note that during the production process errors may be discovered which could affect the content, and all legal disclaimers that apply to the journal pertain.
Structural and electronic analysis of Li/Al layered double hydroxides and their adsorption for CO2
Xin-Juan Houa,b, Huiquan Lia,b*, Peng Hea, Zhenhua Suna, Shaopeng Lia, a
Key Laboratory of Green Process and Engineering, National Engineering Laboratory for Hydrometallurgical Cleaner Production Technology, Institute of Process Engineering, Chinese Academy of Sciences, Beijing 100190,China b
University of Chinese Academy of Sciences, Beijing 100149,China
Submitted to: Applied Surface Science Submission Date: 2017-4-17
*Corresponding author E-mail address: [email protected] 1
Highlights (1) The effects of different monovalent and bivalent anions on the expansion, interlayer structure, and interaction of anions or water with layers are confirmed. (2) It is verified that the interactions among mineral surfaces, anions, water and CO2 are dominated by hydrogen bond, electrostatic interaction, and van der Waals.
ABSTRACT The most stable structures and electronic properties of different Li/Al layered double hydroxides models (i.e., Li/Al-X, X=F-, Cl-, Br-, OH-, NO3-, CO32-, SO42-) and their hydrates, the adsorption of CO2 on Li/Al-X (X=Cl-, NO3-, CO32-) were ascertained by means of density functional theory. Results revealed that the planes of NO3- and CO32- are parallel with the layers in dehydrated state, although the plane of NO3becomes vertical with the layers upon the introduction of water molecules. Electronic density analysis suggested that SO42- and CO32- significantly strengthens the reducibility of the Li/Al layered double hydroxides. The distribution of the frontier orbitals indicated the high reactivity of the anions and hydroxyl groups of the layers. The orders of the predicted stability are F- Cl- Br- >NO3- for the monovalent anions and SO42- CO32- for the divalent anions. The calculated adsorption energies of CO2 in Li/Al-X (X = Cl-, NO3-, CO3-) supported the experimental observation that Li/Al-CO3 exhibits higher CO2 capture capacity than Li/Al-NO3 and Li/Al-Cl. Non-covalent interaction analysis indicated that the interactions among mineral surfaces, anions, water and CO2 are dominated by H-bonds, electrostatic interactions, and van der Waals forces. In addition, radial distribution functions were applied to provide insight for the interaction of water or CO2 with carbonate ion and hydroxyl layers.
Keywords: Li/Al layered double hydroxides,CO2, Non-covalent interaction analysis,Density functional theory 2
1. Introduction Layered double
hydroxides
(LDHs)
exhibit
excellent
and
permanent
anion-exchange capability and are used as ideal catalysts 1, excellent adsorbent materials for various anion species and pollutants 2–5, hosts for drugs and biomolecules 6–8 and for CO2 sequestration 9–13. In Li/Al LDHs, Li+ cations occupy vacant octahedral sites within Al(OH)3 layers, resulting in a positively charged layer; the interlayer and the surface anions ensure the charge balance. LiAl2(OH)6+ layers are span along the c-axis. In the corresponding hydrates, interlayer water molecules form a hydrogen bond network with hydroxide groups and anions. The intercalation behavior and effects of different anions on Li/Al LDHs are important in many processes and applications. For example, carbonate impurities must be controlled and reduced when LDHs are used as additives in polymers or for drug delivery 11. Fogg et al. 14 performed time-resolved in situ X-ray diffraction analysis to measure the intercalation rate of lithium salts (LiX, X = Cl-, Br-, NO3-, OH-, SO42-) into gibbsite; the half-lives of the reaction follow the order SO42- OH- Cl- Br- NO3-. Isupov 15 studied the structures of Li/Al LDHs with Cl-, Br-, OH-, and NO3- anions and confirmed the position of Li+ and anions through single crystal investigations; the reactivity of lithium salts with gibbsite exhibits the following order: LiC1 LiNO3 Li2SO4 > LiBr > LiI. Hou et al. 16 employed XRD and NMR methods to analyze the effects of hydration state on the structure and dynamical behavior of interlayer anions and factors controlling the expansion behavior of Li/Al and Mg/Al LDH phases containing SO42–, CO32–, F–, Cl–, Br–, OH–, and NO3–; the studied phases can be grouped into three types according to the extent of expansion of the interlayer space 16. Williams et al. 17 examined the factors influencing staging during anion-exchange intercalation into hydrated Li/Al-X (X=Cl, Br, and NO3); staging was found to be strongly correlated with anion exchange rate. Tarasov et al. 18 evaluated the de-intercalation kinetics of Li+ cations, Cl-, Br-, NO3-, and SO42from their hydrates. The de-intercalation rate exhibited the following order: NO3- Br3
Cl- ; for SO42-, rapid de-intercalation only achieved 40% completion 18. Nagendran et al. 19 combined experimental and theoretical methods to verify that the molecular plane of nitrate ion in dehydrated Li/Al-NO3 is parallel to the metal hydroxide layer; they also confirmed that the nitrate ion reorients itself upon rehydration, such that its molecular plane is inclined to the metal hydroxide layer. Moreover, structural analysis was
conducted
to
examine
the
interplay
between
Coulomb
force
and
hydrogen-bonding interactions in hydrated Li/Al-X (X = Br-, Cl-) 20. Fogg et al. 21 applied molecular mechanics to rationalize the structures of Li/Al-X (X = Br-, Cl-, NO3-). Hou et al. 22 performed molecular dynamics simulation to gain insights into the structural, dynamical, and energy aspects of LiAl2(OH)6ClnH2O. Although several studies have reported on LDH-derived CO2 adsorbents, the majority of which have focused on conventional Mg/Al LDHs. Huang et al. 9 investigated the performance of Li/Al LDHs intercalated with CO32-, Cl-, and NO3- anions as novel CO2 adsorbents; they found that Li/Al-CO3 demonstrated higher CO2 capture capacity than those of Li/Al-NO3 and Li/Al-Cl. The properties of LDHs are strongly influenced by interlayer inorganic anions and their structural arrangement. As such, the effects of inorganic anions (e.g., halogen anions, hydroxyls, nitrates, carbonates, and sulfates) on the structures and thermal behavior of LDHs must be accurately determined. For Li/Al LDHs, no systemic theoretical study has explored the influence of different anions on the expansion, interlayer structure, and interaction of anions or water with layers. The CO2 adsorption mechanism of Li/Al LDHs also remains unclear. This work aims to achieve an overall representation of intercalated Li/Al LDHs at the molecular level. An explicit theoretical investigation is performed to clarify the intercalation behavior of different anions, namely, F–, Cl–, Br–, OH–, NO3–, CO32–, and SO42–, in the layers. The effects of water molecules and the adsorption mode of CO2 in Li/Al LDHs are also analyzed.
2 Model and calculations 4
2.1. Computational Method The unit cells of Li/Al-X (X = F-, Cl-, Br-, OH-, NO3-, CO32-, SO42-, Fig. 1), their hydrates (Li/Al-X-H), and CO2 adsorbed on Li/Al-X (M = Cl-, NO3-, CO32-)(Li/Al-MCO2) were optimized by P1 representation using CASTEP program 23. Generalized gradient approximation (GGA) for the exchange-correlation potential (PBE) [24] is performed for all compounds. In all GGA/PBE calculations, the method proposed by Grimme was used for DFT-D corrections [25]. The threshold values of the convergence criteria were 1.010−5 eV/atom for energy, 0.03 eV/Å for the maximum force, 0.001 Å for the maximum displacement, and 0.05GPa for maximum stress. The Monkhorst–Pack scheme k-points grid sampling was set at 3 × 3 × 1 for Li/Al-X or Li/Al-X-H (X = F-, Cl-, Br-, OH-, NO3-) , 3 × 2× 1 for Li/Al-X or Li/Al-X-H (X =CO32-, SO42-). The plane-wave basis set energy cutoff was set at 340 eV; ultrasoft pseudopotentials were used for all chemical elements. In geometry optimization of the compounds, all atoms and unit cell parameters are relaxed. The calculated cell parameters for these compounds are summarized in Table 1. Interaction analysis and figure plotting were performed by non-covalent interaction (NCI) analysis to investigate the interaction mechanism of different anions and water in the layered structures. The adsorption mode of CO2 in Li/Al LDHs was also examined. 2.2. Interaction Energy (1) The interaction energies of different ions and the Li+ cation with the host layers are calculated to confirm the ion exchange ability of different Li/Al-X models. With Li/Al-Cl and Li/Al-Cl-H as examples, the interaction energies of anions (IEx or IEX-H) or Li+ (IELi or IELi-H) with host layers can be defined as follows: IEX= ELi/Al-Cl-(Elayer-Cl+ECl) IEX-h= ELi/Al-Cl-H-(Elayer-Cl-H+ECl-H) IELi= ELi/Al-Cl-(Elayer-Li+ELi) IELi-h= ELi/Al-Cl-H-(Elayer-Li-H+ELi-H) ELi/Al-Cl and ELi/Al-Cl-H denote the energies of the optimized Li-Al-Cl and its hydrate, respectively. Elayer-Cl and Elayer-Cl-H represent the single-point energies of the 5
optimized Li/Al-Cl and Li/Al-Cl-H structures without the Cl- anion, respectively. ECl and ECl-H are the single-point energies of Cl- in the cell with cell parameters same to those of the optimized Li/Al-Cl and Li/Al-Cl-H. Elayer-Li and Elayer-Li-H are the single-point energies of the optimized Li/Al-Cl and Li/Al-Cl-H structures without the Li+ anion. ELi and ELi-H indicate the single-point energies of Cl- in the cell with cell parameters same to those of the optimized Li/Al-Cl and Li/Al-Cl-H. (2) De-intercalation energy (Ede-x-H) is used to obtain information on the reversible intercalation/de-intercalation cycles of Li/Al-X-H. Take Li/Al-Cl-H as example, Ede-Cl is defined as follows: Ede-Cl= Egibbsite-H+ELi-Cl-ELi/Al-Cl-H ELi/Al-Cl-H denotes the energy of the fully optimized Li/Al-Cl-H. Egibbsite-H and ELi-Cl represent the energies of the optimized hydrated gibbsite and Li-Cl with the fixed cell parameters of the optimized Li/Al-Cl-H, respectively. (3) Hydration energy (EH) is calculated to predict the effects of different anions on the hydration ability of Li/Al LDH. For Li/Al-Cl-H, hydration energy (EH) is defined as follows: EH =ELi/Al-Cl-H – (ELi/Al-Cl + nEw ) ELi/Al-Cl-H is the energy of the optimized Li/Al-Cl-H; ELi/Al-Cl is the energy of the optimized Li/Al-Cl; and Ew is the energy of the optimized single water molecule in a cell with the same cell parameters of the optimized Li/Al-Cl-H. The variable n denotes the number of water molecules at the Li/Al-Cl-H layers. (4) The adsorption energy of CO2 (AECO2 ) in Li/Al-X (X = Cl-, NO3-, CO32-) can be defined as AECO2= ELi/Al-X-CO2-( ELi/Al-X+ECO2) ELi/Al-X-CO2 is the energy of the optimized Li/Al-X-CO2, and ECO2 is the energy of the optimized CO2 located in the cell of the optimized Li/Al-X structure. 2.3. Non-covalent interaction analysis NCI analysis was performed to investigate the interaction among LDH layer, anions, water, and CO2 by examining the reduced electron density gradient derived from the electron density and its first derivative 26. The reduced electron density gradients (RDG) can be expressed as follows: 6
RDG(r)=
𝟏
(𝐫)
𝟐(𝟑𝛑𝟐 )𝟏/𝟑
(𝐫)𝟒/𝟑
Weak interactions exist in regions with low electron density and low RDG values. Different types of interactions (attraction and repulsion) can be distinguished when the density is multiplied by the sign of the second density Hessian eigenvalue (2). This method visualizes NCI by plotting the electron density with respect to the RDG. On the NCI surface, red signifies steric effects, blue indicates strong attraction (e.g., hydrogen bonding between two atoms), and green denotes weak van der Waals interactions. The relevant interaction analysis and figure plotting were performed using Multiwfn program [27]. 2.4. Molecular Dynamics Simulation Molecular dynamics (MD) simulations were performed using canonical ensemble conditions with carbonate ion, water or carbon dioxide simulated as adsorbates on the LDH layer. The atomic partial charges obtained by GGA/PBE method were adopted. In the MD simulation, the cell parameters were fixed and the heavy atoms of LDH layer were frozen except for H atom, while carbonate ion, water or carbon dioxide were full relaxed. The adsorption behavior was modeled using the Universal force field (UFF) [28]. The columbic interactions were calculated by using the Ewald summation method [29] while the van der Waals interactions were evaluated within a cut-off radius of 18.5Å. The system was calculated in an NVT ensemble for 2ns. The radial distribution function [30], which is one of the most important structural quantities characterizing a system, was analyzed based on MD simulation results.
3. Results and discussion 3.1. Structural relaxation Table 1 shows the structural parameters of the relaxed computational cells of all models. The validity of the computational structures was verified by comparing the predicted structures of Li/Al-X (X = Cl, Br, NO3, CO3) with the structures described in experimental studies. The calculated lattice parameters are consistent with the 7
experimental data [21, 31–32]. The calculated cell volumes of Li/Al-X (X = F, Cl, Br) increased as the ionic radii increased. The calculated lattice parameters of all Li/Al-X models indicated that the cell was enlarged by water molecules. The extent of the cell expansion was influenced by intercalation. For Li/Al-X, the layer spacing was significantly influenced by SO42-; slightly expanded by Cl-, Br-, and NO3-; and not expanded by F-, OH-, and CO32-. The optimized results verified the experimental observation 16 that the tetrahedral oxyanions induce a more significant expanding effect on LDHs than planar oxyanions or single atom ions. In this work, we did not consider the effect of different number of water molecules for the d-spacing of LDHs. Only two water molecules are located in the layer spaces of unit cell as shown in Fig.1, namely, each layer of unit cell possess one water molecule. It can be deducted that that the layer spacing of Li/Al-F-H and Li/Al-OH-H and Li/Al-CO3-H are smallest, while the layer spacing of Li/Al-Cl-H and Li/Al-Br-H are slightly expandable and those of Li/Al-NO3-H and Li/Al-SO4-H are significantly expandable. This observation is qualitatively coincident with the experimental results 16. The charges of anions are listed in Table 1. Population analysis revealed that that the direction of electron transfer direction was from anions toward metal hydroxyl layers and water molecules. The charge of SO42- remained invariant in both hydrated and dehydrated states. Cl-, Br-, CO32-, and NO3- became more ionic after water molecules were removed, suggesting that the charges of electrons transfer are less pronounced during dehydration. Fig. 1 depicts the structure of Li/Al-Cl and its hydrates down the c-axis, which is vertical to the metal hydroxide layer; the figure clearly shows the arrangements of the anions and cations. In the optimized structure of Li/Al-X (X = F, Cl, Br), halogen ions are located on the edge of the unit cell and midway between Al(OH)3 layers, the lithium ions are located in the Al(OH)3 layer, which is consistent with the X-ray diffraction pattern 31. Both halogen and lithium ions located in the center of the hole formed an octahedral Al(OH)3 and were parallel to the c-axis, forming an alternating Li+…X-…Li+… chain. The calculated distances of Li from the nearby F-, Cl-, and Br- anions are 2.982, 3.574, and 3.751 Å, respectively. In the optimized 8
hydrated structures of Li/Al-X (X = F, Cl, Br), water molecules introduced a positional disorder in the placement of halogen ions [Figs. 2(a)–2(c)]. Comparison with the dehydrated Li/Al-X (X=F, Cl, Br) showed no evident changes in the location of Li+; that is, Li+ ions are parallel to the c-axis, forming an alternating Li+…Li+…chain, whereas halogen cations move away from the Li+…Li+… chain. The calculated closest FH, ClH and BrH contacts in Li/Al-X (X=F, Cl, Br) are 1.802, 2.283, and 2.437 Å, respectively, suggesting that hydrogen bonding interactions with F- ions and hydroxyl groups in Al(OH)3 layers are stronger than those of Cl- and Br- ions. The calculated ClH and LiCl distances in Li/Al-Cl are 2.283 and 3.574 Å, respectively, which are consistent with the experimental values of 2.30 and 3.57Å 31. In the hydrated Li/Al-X-H (X = F, Cl, Br), the calculated closest FH distance with hydroxyl groups and interlayer water are 1.654 and 1.675 Å, respectively, which have corresponding values of 2.061 and 2.021 Å in Li/Al-Cl-H and 2.188 and 2.039 Å in Li/Al-Br-H. These results verified the experimental observation 16 that Cl– was stabilized by hydrogen bonding from both hydroxyl groups and interlayer water. In Li/Al-OH and hydrated Li/Al-OH-H, the calculated distances between the O atom of the OH- anion and the H atom from the hydroxyl layers are 1.733 and 1.645 Å, respectively, indicating the presence of strong hydrogen bonds. As shown in Fig. 2, water molecules in the hydrated Li-Al-X(X = F, Cl, Br and OH) are in parallel layers because such arrangement optimizes the interaction between water molecules and surrounding anions or hydroxyl groups. CO32- anions in Li/Al-CO3 and its hydrate are almost parallel with the hydroxyl layers. As such, three oxygen atoms of the CO32- anions simultaneously form strong hydrogen bonds with the hydroxyl groups of two side layers, creating the largest interactions between anions and LDH layers. In the hydrated Li/Al-CO3-H (Fig. 2), both carbonate and water molecules are almost parallel with the hydroxide layers. Thus, the water molecules formed a strong hydrogen bond with the hydrogen atom of two side layers and the oxygen atom of carbonate [Fig. 3(d)]. According to the TGA data, the hydroxyl groups in the main layer exhibited a strong interaction with the interlayer water and carbonate, in contrast to that in Li/Al-Cl 11. 9
Experiments 15, 33 showed that the nitrate ion in the dehydrated phase of Li/Al-NO3 is intercalated with its molecular plane parallel to the metal hydroxide layer. In the optimized hydrated Li/Al-NO3 [Fig. 2(f)], water molecules are located on one side of the metal hydroxide layer and induce the nitrate plane to become almost perpendicular with the hydroxide layers; as such, two nitroxide atoms formed strong hydrogen bonds with the hydroxyl layers, and one nitroxide atom formed a strong hydrogen bond with nearby water molecules [Fig. 3(f)]. Our theoretical calculation verified the experimental observation 16 that the interlayer NO3– groups are oriented with their C3 axes at high angles to the basal plane, in contrast to their flat-lying orientation in Mg/Al-NO3 32. The interlayer separation of the optimized Li-Al-SO4 is approximately 8.7 Å, which is comparable with the experimental value of 8.8 Å 21. Two O atoms of SO42interact with the upper hydroxyl layer, and the two other O atoms interact with the lower hydroxyl layer; this phenomenon is consistent with previous MD modeling, in which SO42– groups are oriented with their C2 axes perpendicular to the basal plane 21. With the existence of water molecules, the other possible orientation of SO42[Fig. 2(g)] is that three oxygen atoms are directed toward one layer, resulting in a trigonal pyramidal orientation. In the hydrated Li/Al-SO4, water molecules formed hydrogen bonds with the oxygen of SO4- and the H atoms of the hydroxide layers [Fig. 3(g)]. Fig. 4 shows the optimized geometries of Li/Al-X (X = Cl, NO3, CO3) with adsorbed CO2 (Li/Al-X-CO2). In Li/Al-Cl-CO2 [Fig. 4(a)], CO2 molecules are parallel with the metal hydroxide layers, and the Cl- anion and CO2 molecules are located near the top and bottom layers, respectively. Cl- anions and the O atoms of CO2 formed hydrogen bonds with the hydrogen atoms of nearby hydroxide layers [Fig. 4(a’)). Electrostatic attraction existed between the Cl- anion and the carbon of CO2 at a distance of 3.1 Å. In the optimized Li/Al-NO3-CO2 [Fig. 4(b)], both the NO3- anion and CO2 are slightly sloped toward the c-axis, causing one O atom of CO2 (OCO2) and two O atoms of NO3- to form hydrogen bonds with the hydroxyl layers. Electrostatic attraction also existed between the other O atom of CO2 and the N atom of NO3- [Fig. 10
4(b’)]. In the optimized Li/Al-CO3-CO2 [Fig. 4(c)], the plane of CO32- is slightly sloped toward the layers to maintain balance between one hydrogen bond of OCO3with the top layer and two hydrogen bonds of the other OCO3- atoms with the bottom layers. The two OCO2 atoms simultaneously formed hydrogen bonds with both the “up” and “down” layers, indicating that the CO2 molecules in Li/Al-CO3 are more stable than those in the Li/Al-Cl and Li/Al-NO3 layers [Fig. 4(c’)]. 3.2. Effects of anions on interactions Table 2 presents the interaction energies between Li+ and LDH in hydrated and dehydrated states (IELi or IELi-H). The average interaction energies between Li+ and LDH containing different anions are almost the same, suggesting that anion type did not evidently affect the interaction between the Li+ cation and layers. Water molecules slightly increased the absolute value of IELi. The absolute values of IEx in Table 2 indicate that the stability of the monovalent anions in the layers follows the order F- OH- Cl- Br- > NO3-, while the stability of divalent anions is SO42- CO32-. The stability of different anions in Li/Al layers is consistent with that in Mg/Al layers 34. The Mulliken charges of Cl- and Br- obviously increased in the hydrated than that in the dehydrated LDH. Comparable absolute values were observed for CO32-. The charge of NO3- slightly increased, whereas those of OH-, F-, and SO42remained almost invariant. The change in the Mulliken charges also suggested that the water molecules exerted a stronger influence on the Cl- and Br- anions than on the other anions. In the presence of water, the stabilities of Cl- and Br- are increase that mainly caused by the more negative electrons in Cl- and Br- in the hydrated Li/Al LDHs promoted electrostatic interaction. In both dehydrated and hydrated Li/Al LDHs containing monovalent anions, the interaction between NO3- and metal hydroxyl layers is the lowest. These results confirmed that the halogen anion exhibits a higher affinity to LDHs than NO3-, and fluoride ions can be incorporated into the interlayer space of LDH throughout the ion exchange reaction 35. The hydrated energies of different Li/Al-X (Table 3) indicated that OH- and NO3facilitated the easier adsorption of water molecules on LDH compared with that of LDH containing halogen anions. Tarasov et al. 18 investigated the de-intercalation 11
of anions and Li+ from Li/Al LDHs in water; the results suggested that LDHs spontaneously lose both Li and anionic ions to yield crystalline Al(OH)3. As shown in Table 3, the calculated de-intercalation energies of LiX with monovalent anions observed the following sequence: F- Cl- Br- > OH- NO3-; this sequence is an inverse relationship to the reaction rate 18. The adsorption energies of CO2 in Li/Al-X with X = Cl, NO3, CO3 are 38.4, 8.1, and -5.4 kJ mol-1, which supported the experimental finding 9 that Li/Al-CO3 exhibited a higher CO2 capture capacity than those of Li/Al-NO3 and Li/Al-Cl. As shown in Fig. 4, in Li/Al-Cl-CO2 and Li/Al-NO3-CO2, Cl- and NO3- were preferred to be located at one side of the hydroxyl layer, indicating that the hydrogen bonds between Cl- and NO3- and metal hydroxyl layers were less than that of CO32simultaneously forming hydrogen bonds with both the upper and lower hydroxyl layers. 3.3. Band structure and density of states The electronic structures of Li-Al LDHs provide key information regarding the physical and chemical properties of these materials. The densities of state (DOS) were predicted based on the optimized cells. The Fermi energy level was set to 0 eV. The states near the Fermi level correspond to the electronic structure that can be easily transferred. Comparison of the DOS and PDOS of Li/Al LDHs (Fig. 5 and Figs 1S– 6S) revealed that the orbital contributions from the hydroxyl layers dominate the overall shapes of the total DOS. The calculation results revealed that the top of the low-valence band (VB) of Li/Al LDHs containing halogen anions are mainly contributed by O2p and halogen 2p (Figs. S2–S9); the bottom of the conduction band (CB) is mainly dominated by Al2p. For Li/Al-Br, Br2p also contribute to CB. For Li/Al-OH, Li/Al-NO3, Li/Al-CO3, and Li/Al-SO4, the O2p of the hydroxyl layers and the anions dominates the VB. As shown in Fig. S1-S6, the DOS of O2p in LDH and their hydrates are higher than those of Al2p. This result showed that some electrons from Al2p transferred to VB and became part of the Al and O interaction. The Li 2sp states slightly contribute to the VB, suggesting that Li is highly ionized. As shown in Fig. 6, the VB of their hydrated compounds are also mainly contributed by the 2p 12
orbital of hydroxyl O, anions, and water. Moreover, water molecules obviously decreased the electron density in the VB. The top of the VB and the bottom of the CB are usually associated with reactivity; that is, a system with a small or no band gap is more reactive than a system with large band gap. The NO3-, SO42- and CO32- anions are responsible for decreasing the band gap in the corresponding LDH. These results suggested that NO3-, SO42-, and CO32- can considerably strengthen the reducibility of Li/Al LDH. Bandwidth is primarily determined by the extent to which orbitals on neighboring atoms overlap, which in turn can predict the nature of chemical bonding. For Li/Al-X (X = NO3-, CO32-, SO42-) and their hydrates, the VB widths are larger than those of Li/Al-X (X = F-, Cl-, Br-, OH-) and their hydrates. This finding indicates that the inter-atomic orbital overlaps in Li/Al-X (X = NO3-, CO32-, SO42-) and their hydrate compounds are weaker than those of Li/Al-X (X = F-, Cl-, Br-, OH-). 3.4. Orbital analysis The highest occupied molecular orbital (HOMO) and lowest unoccupied orbital (LUMO) provide insights into the potential reactivity of a system, given that a chemical reaction requires the excitation of electrons from their Fermi levels to the lowest conduction band. The HOMOs and LUMOs of Li/Al-X (Fig. 7) were examined to determine changes in their chemical reactivity. The HOMO and LUMO of Li/Al-X (X = Cl-, Br-, OH-, NO3-) are mainly located at the anions. For Li/Al-X (X = CO32-, SO42-), the HOMO and LUMO are located at the anions and –OH of the layers, respectively. For Li/Al-F, the HOMO is mainly located at –OH, and LUMO is located at F-. The distribution of the frontier orbital indicated that Li+ cations are very difficult for further reaction, whereas the anions and –OH of the layers exhibit high reactivity. 3.5. Radial distribution function We have calculated the radial distribution (RDF), g(r), to give the probability of finding a particle in the distance r from another particle 30. From the aspect of peak distance and peak sharpness of the RDFs present in Fig 8, it can be concluded that for Li/Al-CO3-CO2 at 298K, Ocarbonate-CCO2 has peak distance at 2.78Å which mainly 13
contributed by the electrostatic interaction; oxygen atom of carbonate and CO2 have stronger interaction with hydroxyl layers at peak distance about 2.38 and 2.42 Å, respectively. For Li/Al-CO3-H at 298K, the peak distance of oxygen atom of carbonate with hydrogen atoms of water or hydroxyl layers are about 2.35 Å, while that of oxygen atom and hydrogen atoms of hydroxyl layers is about 2.25 Å To determine whether the increase in temperature leads to strengthening of the interaction between adsorbates and hydroxyl layers, RDF calculation were performed on 298K and 348K. As shown in Fig.8, the RDFs of Li/Al-CO3-CO2 have more sharp peaks and move to lower peak distance with increasing temperature which means the interactions are more significant. For Li/Al-CO3-H, increasing temperature has no obviously effect for the peak of Ocarbonate-Ow and Ocarbonate-Hlayer but increasing the probability of finding water molecules near hydroxyl layers.
4. Conclusions In this work, the effects of different monovalent and bivalent anions on the electronic structures and properties of Li/Al LDHs were investigated by DFT method. This study explored the structural parameters, DOS, frontier orbital, hydration energies, and interaction energies of metal ions; de-intercalated energies; and the interaction mode among mineral surfaces, anions, and CO2. Water molecules in Li/Al-X (X = F-, Cl-, Br-, OH-) are parallel with the hydroxyl layers and located near the middle part between the upper and lower layers with the anions. In Li/Al-X (X = CO32-, SO42-), the plane of water molecules are vertical to the hydroxyl layers and located near one side of the layer. NCI analysis intuitively indicated that the interactions among mineral surfaces, anions, and CO2 are dominated by hydrogen bond, electrostatic interaction, and van der Waals. The stability of monovalent anions in the layers follows the order F- Cl- Br- > NO3-, while the stability of the divalent anions is SO42- CO32-. The calculated adsorption energies of CO2 in Li/Al-X (X = Cl, NO3, CO3) verified that Li/Al-CO3 exhibits a higher CO2 capture capacity than those of Li/Al-NO3 and Li/Al-Cl for stronger hydrogen bonds between CO2 and layers. The hydrated energies indicated that Li/Al-OH more easily 14
adsorbed water molecules than LDHs intercalated by other monovalent anions. The calculated de-intercalation energy of Li-Al LDHs with monovalent anions follows the order F- Cl- Br- > OH- NO3- .
Acknowledgments We would like to thank the NSFC (Grant No. 51304184) for its financial support. The results described in this paper are obtained on the Deepcomp7000 of Supercomputing Center in the Computer Network Information Center of the Chinese Academy of Sciences.
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17
Fig. 1. Optimized structure of Li/Al-Cl (Al, magenta; O, red; Li, blue; Cl, cyan).
18
(a) Li/Al-F-H
(b) Li/Al-Cl-H
(c) Li/Al-Br-H
(d) Li/Al-OH-H
CO3
(e) Li/Al-CO3-H
(f) Li/Al-NO3-H
(g) Li/Al-SO4-H
(h) Li/Al-SO4-H
Fig.2. Optimized structures of Li/Al-X-H (X=F-, Cl-, Br-, OH-, NO3-, CO32-, SO42-) (Al, magenta; O, red; Li, blue; H,white).
19
(a) Li/Al-F-H
(b) Li/Al-Cl-H
(c) Li/Al-Br-H
(d) Li/Al-OH-H
(e) Li/Al-CO3-H
(f) Li/Al-NO3-H
(g) Li/Al-SO4-H
(h) Li/Al-SO4-H
Fig. 3. Gradient isosurfaces (s=0.30au) for optimized structures of Li/Al-X-H (X=F-, Cl-, Br-, OH-,
NO3-, CO32-, SO42-).
20
(a) Li/Al-Cl-CO2
(a’) Li/Al-Cl-CO2
(b) Li/Al-NO3-CO2
(b’) Li/Al-NO3-CO2
(c’) Li/Al-CO3-CO2
(c) Li/Al-CO3-CO2
Fig. 4. Optimized structures of Li/Al-X-CO2 (X= Cl-, NO3-, CO32-)(Al, magenta; O, red; C, grey; Si, orange; and Na, green) (a)-(c) and corresponding gradient isosurfaces (s=0.30au) (a’)-(c’).
21
(a) Li/Al-F
(b) Li/Al-Cl
(c) Li/Al-Br
(d) Li/Al-OH
(e) Li/Al-NO3
(f) Li/Al-CO3
(g) Li/Al-SO4 Fig. 5. DOS of Li/Al-X (X=F-, Cl-, Br-, OH-, NO3-, CO32-, SO42-).
22
(a) Li/Al-F-H
(b) Li/Al-Cl-H
(c) Li/Al-Br-H
(d) Li/Al-OH-H
(e) Li/Al-NO3-H
(f) Li/Al-CO3-H
(g) Li/Al-SO4 Fig. 6. DOS of Li/Al-X-H (X=F-, Cl-, Br-, OH-, NO3-, CO32-, SO42-). 23
(a) Li/Al-F HOMO
(a’) Li/Al-F LUMO
(c) Li/Al-Br HOMO
(c’) Li/Al-Br LUMO
(e) Li/Al-NO3 HOMO
(e’)Li/Al-NO3 LUMO
(g) Li/Al-SO4 HOMO
(b)Li/Al-Cl HOMO
(d)Li/Al-OH HOMO
(b’) Li/Al-Cl LUMO
(d’) Li/Al-OH LUMO
(f)Li/Al-CO3 HOMO (f’) Li/Al-CO3 LUMO
(g’)Li/Al-SO4 LUMO
Fig. 7. Frontier orbital distributions for Li/Al-X (X=F-, Cl-, Br-, OH-, NO3-, CO32-, SO42-). 24
(a)
(b) Fig. 8. RDFs for Li/Al-CO3-CO2 (a) and Li/Al-CO3-H (b).
25
Table 1 DFT-optimized cell parameters of Li/-X and Li/Al-X-H. Model
a(Å)
b(Å)
(°)
c(Å)
(°)
(°)
V0(Å3) Charges in anion
Li/Al-F
5.120
5.120
11.930
90.0
90.0
120.0 270.9
-0.66
Li/Al-Cl
5.131
5.131
14.298
90.0
90.0
120.0 326.1
-0.66 a
5.100
5.100
14.299
90.0
90.0
120.0 322.1
5.136
5.136
15.004
90.0
90.0
120.0 342.8
5.100a
5.100
14.946
90.0
90.0
120.0 336.6a
5.071b
5.071
15.474
90.0
90.0
120.0
Li/Al-OH
5.118
5.118
12.372
90.0
90.0
119.8 281.2
-0.56
Li/Al-CO3
5.165
8.801
14.097
89.5
82.0
90.1
634.7
-1.36 -1.00
Li/Al-Br
-0.52
c
5.086
8.809
7.758*2
5.159
5.152
15.531
82.4
84.3
119.9 348.0
5.109a
5.109
14.374
90.0
90.0
120.0 324.9a
5.070b
5.070
14.154
90.0
90.0
120.0
Li/Al-SO4
5.145
8.828
18.579
117.4 87.9
89.8
748.6
-1.42
Li/Al-F-H
5.126
5.121
14.635
90.0
90.0
120.0 330.0
-0.68
Li/Al-Cl-H
5.208
5.148
15.258
90.0
90.0
120.0 354.4
-0.60
Li/Al-NO3
102.62
a
5.096
5.096
15.292
90.0
90.0
120.0 344.0
Li/Al-Br-H
5.191
5.179
15.581
90.0
90.0
118.7 367.3
-0.35
Li/Al-OH-H
5.079
5.079
14.528
90.0
90.0
90.0
330.0
-0.57
Li/Al-CO3-H
5.166
8.774
14.250
92.0
85.2
90.0
643.2
-1.33
Li/Al-NO3-H
5.128
5.106
20.395
97.9
79.2
119.4 456.4
-0.77
Li/Al-SO4-H
5.159
8.810
17.348
81.1
91.9
90.1
778.6
-1.42
Li/Al-Cl-CO2
5.175
5.240
15.744
90.1
89.7
120.2 368.8
-0.66
Li/Al-CO3-CO2
5.171
8.800
14.600
84.5
82.0
90.1
656.8
-1.40
Li/Al-NO3-CO2
5.143
5.195
20.696
86.5
85.5
120.2 236.7
-0.76
a
Ref.31 b Ref.21 c Ref.36
26
Table 2 Interaction energies between Li+ cation and layers (IELi and IELi-H in kJ mol–1), different anions and layers of Li/Al-X (IEX and IEX-H in kJ mol–1) and their hydrates. IELi
IEX
IELi-H
IEX-H
Li/Al-F
236.8
-101.3
Li/Al-F-H
248.4
-85.5
Li/Al-Cl
239.8
-84.1
Li/Al-Cl-H
252.4
-90.3
Li/Al-Br
242.6
-74.5
Li/Al-Br-H
252.3
-86.9
Li/Al-OH
236.9
-87.9
Li/Al-OH-H
255.0
-73.1
Li/Al-NO3
258.2
-52.3
Li/Al-NO3-H
256.0
-36.4
Li/Al-CO3
241.9
-143.5
Li/Al-CO3-H
247.3
-137.4
Li/Al-SO4
240.8
-99.1
Li/Al-SO4-H
246.6
-89.0
Table 3 Hydrated energies of Li/Al-X-H (Eh in kJ mol–1) and de-intercalation energies of Li-X from Li/Al-X-H (Ede-X in kJ mol–1). Eh
Ede-X
Li/Al-F-H
-24.4
315.0
Li/Al-Cl-H
-19.1
263.2
Li/Al-Br-H
-7.5
249.1
Li/Al-OH-H
-48.3
183.1
Li/Al-NO3-H
-33.4
92.1
Li/Al-CO3-H
-48.6
198.6
Li/Al-SO4-H
-42.6
203.8
27