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Isomorphic Substitutions in Clay Materials and Adsorption of Metal Ions onto External Surfaces: A DFT Investigation Qian Wang, Chang Zhu, Jiena Yun, and Gang Yang* College of Resources and Environment & Chongqing Key Laboratory of Soil Multi-scale Interfacial Process, Southwest University, Chongqing 400715, China S Supporting Information *

ABSTRACT: Clay materials show particular adsorption performances while the mechanisms and influencing factors remain largely elusive, which are presently tackled by DFT calculations. Isomorphic substitutions pervasive in clay materials may bring about critical impacts on the adsorption processes. Octahedral substitutions in montmorillonite occur preferentially at specific T sites (referred to as octahedral or tetrahedral sites) or evenly at different T sites, which depend strongly on the ionic radii of heteroatoms. Structural perturbations resulting from isomorphic substitutions and substitution energies generally increase with ionic radii, except Be2+/Al3+ substitutions that cause structural collapses. Octahedral rather than tetrahedral substitutions are preferred, and, compared with montmorillonite, octahedral substitutions in hectorite are more facile while tetrahedral substitutions are more difficult; furthermore, hectorite exhibits superior adsorption performances. The second heteroatoms are always more difficult to incorporate, and their locations depend significantly on the first heteroatoms. After analyzing the adsorption structures and energies, the various factors affecting the adsorption performances (identity of heteroatoms, crystallographically distinct T sites, structural alterations, quantity of negative charges, distance from charge centers to metal ions and source of negative charges) are discussed, with their respective contributions being estimated. Quantity of negative charges is the foremost factor that controls the adsorption performances, while other factors in certain circumstances can also play a critical role. Results are beneficial to understand the particular adsorption behaviors of clay materials. higher charges. Although more structurally complex, UO22+ adsorbed onto kaolinite and rutile can produce resembling surface complexes as alkali cations through the U6+ center.12−14 Molecular dynamics simulations with the explicit water models were also performed to study the adsorption of K+ onto mica surfaces,22,23 finding that K+ ions are situated at the hexagonal rings of tetrahedral sheets and construct stable inner-sphere complexes as in the case of dry conditions (usually the choice of quantum chemical calculations). Zeolites are mainly aluminosilicates as a large portion of clay materials and extensive literatures showed that isomorphic substitutions in zeolites are preferred in specific sites, which further bring about profound impacts on the following adsorption and catalysis processes.24−28 In clay materials, the octahedral and tetrahedral sites are collectively referred to as T sites, similar to the condition of zeolites. Two types of octahedral Al atoms of different chemical environments were detected in MMT and assigned as T1 and T2 sites,29,30 see Figure 1a and the Supporting Information (S1: Structural parameters for two octahedral T sites in MMT). The 27Al 5Q

1. INTRODUCTION Isomorphic substitutions are pervasive in almost all types of clay materials and largely responsible for their applications in adsorption, catalysis, ceramics, pharmaceuticals, and a number of other fields.1,2 Owing to isomorphic substitutions, strong surface complexes are constructed between clay materials and adsorbed metal ions that further promote the preservation of water and nutrients, management of organic contaminants, and catalysis of chemical reactions,3−5 while a molecular-level understanding remains lacking within this context, such as the distribution of different heteroatoms, adsorption structures and energies of metal ions as well as respective contributions of the various influencing factors on the adsorption processes. Quantum chemical modeling has gained a great deal of valuable information otherwise inaccessible with regard to the microscopic structures of clay materials and the adsorption behaviors of cations, water and organic substances.6−21 BrionesJurado and Agacino-Valdes6 demonstrated that Al3+/Si4+ rather than Mg2+/Al3+ substitutions in montmorillonite (MMT) have larger adsorption energies for proton (H+) and hence correspond to the inferior Brönsted acidity. Adsorption of Li+, Na+, K+, Mg2+, and Ca2+ onto Al3+/Si4+-substituted MMT models was investigated by Shi et al.,8 and their binding affinities were found to increase with smaller ionic radii and © 2017 American Chemical Society

Received: April 13, 2017 Revised: October 13, 2017 Published: November 14, 2017 26722

DOI: 10.1021/acs.jpcc.7b03488 J. Phys. Chem. C 2017, 121, 26722−26732

Article

The Journal of Physical Chemistry C

alteration of clay structures can affect isomorphic substitutions and following adsorption performances, and to what extent. HCT and MMT are structurally close to each other and both of them belong to smectites. Then, the adsorption structures and energies of K+ on the external surfaces of all the above MMT and HCT models were systematically investigated. Considering other models such as pristine and deprotonated MMT (vide post), the various factors affecting the adsorption performances of metal ions were inspected and their respective contributions were estimated, which conduce greatly to the understanding of the particular adsorption processes within clay materials.

Figure 1. Models for (a) MMT labeling two crystallographically different Al sites and (b) HCT. Si, Al, Mg, O, and H atoms are displayed in yellow, purple, green, red. and white, respectively.

2. COMPUTATIONAL DETAILS 2.1. Models. In line with previous literatures,32−35 the crystallographic structure reported by Viani et al.29 was used for MMT. As shown in Figure 1a, the model of pristine MMT (Si24Al12O96H60) contains a total of 192 atoms, where an octahedral sheet is sandwiched by two tetrahedral sheets and each sheet has 12 Si4+ (or Al3+) sites mimicking the 2:1-type clay materials. Two crystallographically distinct octahedral Al sites that were differentiated by 27Al 5Q MAS NMR spectra30 were referred to as T1 and T2 sites. Except for boundary O and H atoms, all atomic coordinates in MMT models in addition to adsorbents were allowed to relax fully during structural optimizations. MMT models with no (pristine), mono, and double substitutions carry 0, −1, and −2 charges, respectively. With regard to double substitutions, all possible combinations were considered, producing a total of 10 configurations; see the details in section 3.5. A hexagonal ring of the tetrahedral SiO4 sheet that is known to be the arena for adsorption of metal ions8,21,36,37 was kept intact. The boundary O atoms of pristine MMT were saturated by H atoms that were directed along the bond vectors of what should have been the next lattice atoms, and the O−H distances were adjusted to 1.000 Å. HCT is a 2:1 clay material and also belongs to smectites as MMT. The main structural difference between MMT and HCT lies in that HCT is a trioctahedral smectite while MMT is a dioctahedral smectite. The model of pristine HCT (Si24Mg10O84H52) was taken from the crystallographic structure

MAS NMR spectra indicated that the isotopic chemical shifts (quadrupolar products) of octahedral T1 and T2 sites are 5.8 ppm (2.6 MHz) and 6.2 ppm (3.0 MHz), respectively.30 As aforementioned, isomorphic substitutions are prevalent in clay materials and regulate significantly the physicochemical properties and adsorption performances,1−5 while scant reports are available regarding to the distribution of heteroatoms in clay materials, possibility for the occurrence of double substitutions, adsorption of metal ions and respective contributions for the various influencing factors during the adsorption processes, which will be tackled in this work by density functional theory (DFT) calculations. DFT calculations for isomorphic substitutions in MMT were organized as follows: (1) Distribution of a variety of heteroatoms (M2+ = Be2+, Ni2+, Mg2+, Cu2+, Zn2+, Co2+, Fe2+, Mn2+, Ca2+) at crystallographically different T sites of the octahedral sheet in MMT. (2) Isomorphic substitutions of a number of heteroatoms (M3+ = B3+, Al3+, Ga3+, Fe3+, In3+) at the tetrahedral sheet of MMT. (3) Double substitutions at the octahedral (OHOH), tetrahedral (THTH) and both (OHTH) sheets of MMT, considering all potential combinations. Isomorphic substitutions are considerable in 2:1-type clay materials and the M2+:Al3+ ratios in MMT can reach as high as 1:3.31 (4) Isomorphic substitutions at the tetrahedral and octahedral sheets of hectorite (HCT), to inspect whether the

Table 1. Average M−O distances (R, Å), Mean-Square Deviations (Θ), Substitution Energies (Esub, kcal/mol), and Relative Energies (RE, kcal/mol) for M2+/Al3+- and M3+/Si4+-Substituted MMT Models T1

T2

M2+ or M3+a

R

Θ

Esub

REb

R

Θ

Esub

Be2+ (0.41)c Ni2+ (0.83) Mg2+ (0.86) Cu2+ (0.87) Zn2+ (0.88) Co2+ (0.89) Fe2+ (0.92) Mn2+ (0.97) Ca2+ (1.14) B3+ (0.25) Al3+ (0.53) Ga3+ (0.61) Fe3+ (0.63) In3+ (0.76)

1.763 2.068 2.059 2.092 2.097 2.090 2.112 2.139 2.235 1.495 1.752 1.836 1.860 2.062

28.33 8.08 8.37 8.41 8.49 8.51 8.87 9.03 10.19 1.01 1.47 2.04 3.28 6.86

572.5 587.0 695.5 618.3 657.6 615.3 653.8 706.9 813.1 654.3 1099.4 1085.5 1125.5 1317.8

−1.4 −0.6 1.5 −2.5 0.1 0.1 −0.1 1.3 3.3

1.863 2.067 2.061 2.086 2.094 2.089 2.108 2.136 2.227

27.50 8.88 9.09 9.19 9.45 9.19 9.40 9.73 10.30

573.9 587.6 694.1 620.9 657.5 615.2 653.9 705.6 809.8

a

Ionic radii of M2+ and M3+ are given in parentheses; bFor octahedral substitutions, T2 sites are used as energy benchmark; cFour Be−O distances at T1 site and five Be−O distances at T2 site are taken into account, respectively. 26723


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Figure 2. Local structures of pristine MMT model at T1 site as well as those with Mg2+/Al3+, Be2+/Al3+ and Fe2+/Al3+ substitutions. Si, Al, Mg, Be, Fe, O, and H atoms are visualized in yellow, purple, green, cyan, dark gray, red, and white, respectively. The six T1-bonded O atoms are numbered successively as 1−6, and distances are given in Å.

Figure 3. Local structures of pristine MMT model at T2 site as well as those with Mg2+/Al3+, Be2+/Al3+ and Fe2+/Al3+ substitutions. Si, Al, Mg, Be, Fe, O, and H atoms are visualized in yellow, purple, green, cyan, dark gray, red and white, respectively. The six T2-bonded O atoms are numbered successively as 1−6, and distances are given in Å.

reported by Breu et al.38 and contains two tetrahedral sheets and one sandwiching octahedral sheet resulting in a total of 170 atoms, see Figure 1b. Metal ions were then adsorbed on the external surfaces of the various MMT and HCT models. 2.2. Methods. All calculations were conducted using the DMOL3 module of Materials Studio software packages (version 7.0) installed in the National Supercomputing Center in Shenzhen.39,40 The BP exchange-correlation functional,41,42 as testified to be accurate especially for systems containing heavy atoms such as Fe2+, Ni2+, Mn2+, Cu2+, Co2+, Zn2+, and Ga3+ presently investigated,43,44 was employed in combination

with the double numerical plus polarization (DNP) basis set. Owing to the significance of relativistic effects,45,46 the inner electrons for elements with atomic numbers beyond 20 were represented by effective core potentials (ECP). Structural optimizations were completed when the self-consistent density, total energy, atomic force, and atomic displacement fall below 10−6 Ha, 10−5 Ha, 0.002 Ha/Å, and 0.005 Å, respectively. Then the optimized structures were verified to be energy minima showing no negative frequencies. Different multiplicities were considered for Fe3+, Ni2+, Mn2+, Cu2+, and Co2+, and models with lower electronic energies were reported in the text, i.e., 26724


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The Journal of Physical Chemistry C sextet for Fe3+, triplet for Ni2+, sextet for Mn2+, doublet for Cu2+, and quartet for Co2+.

from Be2+ to Ni2+ (4.2 kcal/mol for a radius span of 0.42 Å) is apparently less than that from Ni2+ to Ca2+ (about 224.1 kcal/ mol for a radius span of 0.31 Å), in that the beneficial effect of smaller ionic radius for Be2+ is counteracted substantially by the serious distortion of octahedral configurations. With regard to different M2+/Al3+ substitutions, the relative energies for T1 vs T2 sites equal −1.4, −0.6, 1.5, −2.5, 0.1, 0.1, −0.1, 1.3, and 3.3 kcal/mol for Be2+, Ni2+, Mg2+, Cu2+, Zn2+, Co2+, Fe2+, Mn2+, and Ca2+, respectively (Table 1). Zn2+, Co2+, and Fe2+ have no clear site preference and structures at two octahedral T sites are nearly energy-degenerate. Instead, the other M2+ heteroatoms are not distributed randomly: Be2+, Ni2+, and Cu2+ are preferential at T1 site while Mg2+, Mn2+, and Ca2+ at T2 site. In general, the distribution of heteroatoms at two octahedral T sites is strongly dependent on their ionic radii: heteroatoms with smaller ionic radii prefer to T1 site while with larger ionic radii prefer to T2 site. 3.2. K+ Adsorption in MMT with Octahedral Substitutions. K+ ions are adsorbed at the hexagonal rings of tetrahedral sheets in MMT,8,21,36,37 and their local coordination environments are close for all M2+/Al3+ substitutions, irrespective of different M2+ heteroatoms and octahedral T sites. Interactions of K+ with OS atoms (OS stands for surface-O atoms) are reinforced substantially by isomorphic substitutions (Figure 4), and for all M2+/Al3+ substitutions, there exist three

3. RESULTS AND DISCUSSION 3.1. Octahedral Substitutions in MMT. Two crystallographically distinct T sites in the octahedral sheet of MMT (Figure 1a) have been identified by NMR experiments,30 while their structural perturbations and site preferences during the incorporation of heteroatoms remain elusive. The structural deviations of octahedral sites from regular octahedron can be characterized by the mean-square deviations (ΘO),24,28,47,48 ΘO =

1 12

12

∑ (αi − α̅ )2 i=1

(1)

where αi and α̅ stand for the ith θ(O−T−O) angle and average of 12 θ(O−T−O) angles, respectively. The average Al−O bond distances in pristine MMT are 1.928 and 1.930 Å, respectively, for T1 and T2 sites, which agree with the XRD results (1.912 Å).49 The M−O distances at both octahedral T sites ascend as Be2+ < Mg2+ < Ni2+ < Cu2+ ≈ Co2+ < Zn2+ < Fe2+ < Mn2+ < Ca2+, consistent with the trend of ionic radii (Table 1 and Figures 2, 3). The calculated M−O distances are also in line with the EXAFS data available, e.g., approximately 2.067 vs 2.05 Å for Ni−O bonds and 2.095 vs 2.08 Å for Zn−O bonds, respectively.50,51 The M−O distances may vary slightly for two octahedral T sites, e.g., Ca−O bonds are averaged at 2.235 and 2.227 Å respectively for T1 and T2 sites. All heteroatoms except Be2+ maintain the octahedral configurations as in pristine MMT. The deviations from regular octahedron, as reflected by ΘO values, increase monotonically with ionic radii, and Mg2+ that is often found in natural MMT corresponds to relatively small ΘO values. Be2+ is an exception and has elongated distances with O4 and O5 atoms at T1 site while with O3 atom at T2 site, probably due to that its orbitals available are fewer than those of other heteroatoms (e.g., 3d orbitals for Mg2+). As a result, the octahedral configurations are distorted seriously for Be2+/Al3+ substitutions, where the ΘO values reach as large as 27.50−28.33 and are 3 times more than those of Mg2+/Al3+ substitutions. The M2+/Al3+ substitution energies (Esub) are defined similarly as in zeolites,24−28 Esub = [E(M‐MMT) + E(Al3 +)] − [E(MMT) + E(M2 +)]

(2)

where E(MMT) and E(M-MMT) are denoted as the electronic energies of pristine and M2+/Al3+-substituted MMT models, respectively. The M2+/Al3+ substitution energies (Esub) for both T1 and T2 sites arise as Be2+ < Ni2+ < Co2+ < Cu2+ < Fe2+ < Zn2+ < Mg2+ < Mn2+ < Ca2+ (Table 1), and the positive Esub values are in line with the substitutions in zeolites that are also energetically costly.28,47 The Esub trend remains the same when calculated using hydrated ions; see the Supporting Information (S2: Calculation of substitution energies using hydrated ions). Heteroatoms with larger ionic radii are more difficult to incorporate, and the d-block transition metal ions such as Ni2+ and Cu2+ have close ionic radii as Mg2+ while are more favorable to incorporate, probably due to that their more flexible electronic configurations are beneficial to interact with the surrounding O atoms in the octahedral sheet. Albeit Be2+/ Al3+ substitutions are the most favorable, the Esub increment

Figure 4. Local structures for K+ adsorption on the surfaces of (a) pristine MMT model as well as those with (b) Mg2+/Al3+ substitution at T1 site, (c) Mg2+/Al3+ substitution at T2 site and (d) Al3+/Si4+ substitution. Si, Al, Mg, O, and K atoms are visualized in yellow, purple, green, red, and dark green, respectively. Distances are given in Å.

K−OS bonds with distances below 3.0 Å indicating stronger interactions than the others. The K−OS distances can be more affected by choice of octahedral T sites than identity of heteroatoms (Table 2); e.g., the average K−OS distances of Ca2+/Al3+-substitutions are 2.827 and 2.802 Å, respectively, for T1 and T2 sites, while the change of heteroatoms produces a smaller K−OS difference (≤0.009 Å). The adsorption energies of K+ on pristine and M2+/Al3+-substituted MMT models (Ead) are written as 26725


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The Journal of Physical Chemistry C Table 2. Average K−OS Distances (RK−Os, Å), Charge Transfers (qtr, |e|), and Adsorption Energies (Ead, kcal/mol) for the Adsorption of K+ on the M2+/Al3+- and M3+/Si4+Substituted MMT Models T1 M

2+

or M

3+

RK−Os

Be2+ Ni2+ Mg2+ Cu2+ Zn2+ Co2+ Fe2+ Mn2+ Ca2+ B3+ Al3+ Ga3+ Fe3+ In3+

a

2.824 2.823 2.825 2.832 2.824 2.827 2.827 2.826 2.827 2.794 2.794 2.784 2.776 2.798

T2

Ead

qtr

RK−Osa

−91.4 −89.3 −90.7 −89.6 −90.2 −91.2 −85.8 −87.6 −91.9 −98.1 −97.8 −98.4 −94.6 −92.5

0.145 0.147 0.146 0.144 0.147 0.147 0.146 0.147 0.146 0.168 0.157 0.132 0.120 0.073

2.811 2.810 2.811 2.808 2.809 2.808 2.808 2.807 2.802

Ead

qtr

−92.5 −90.8 −91.4 −91.7 −91.3 −93.0 −87.7 −88.5 −92.0

0.145 0.147 0.147 0.148 0.147 0.147 0.146 0.147 0.145

a

The three nearest O atoms have been considered for the calculation of K−OS distances.

Ead = E(K‐MMT) − [E(MMT) + E(K+)]

(3) +

Ead = E(K‐M‐MMT) − [E(K‐MMT) + E(K )]

Figure 5. Local structures of MMT models with B3+/Si4+, Al3+/Si4+ and In3+/Si4+ substitutions. Si, B, Al, In, O, and H atoms are visualized in yellow, pink, purple, blue, red, and white, respectively. The four Mbonded O atoms are numbered successively as 1−4, and distances are given in Å.

(4)

where E(K-MMT) and E(K-M-MMT) stand for the electronic energies of pristine and M2+/Al3+-substituted MMT models with adsorption of K+, respectively. The adsorption energies of K+ on substituted MMT models (Ead) show dependence on the identity of M2+ heteroatoms and vary within −85.8 to −91.9 kcal/mol and −87.7 to −93.0 kcal/ mol, respectively, for T1 and T2 sites (Table 2), which are almost twice as that of pristine MMT (−45.4 kcal/mol). Hence, the adsorption of metal ions is fundamentally promoted by octahedral substitutions. For each heteroatom, the Ead value of T2 substitutions is slightly larger than that of T1 substitutions (Table 2), due to the shorter distance of K+ with T2 site rather than with T1 site (about 5.961 vs 6.563 Å) as corroborated subsequently. Charge transfers occur from K+ to M-MMT and according to Mulliken population analyses are 0.144−0.148 |e| for the various M2+/Al3+ substitutions, which are close to each other and agree well with the results of adsorption energies. 3.3. Tetrahedral Substitutions in MMT. Similar to the condition of octahedral sites, the structural deviations of tetrahedral sites from tetrahedron are characterized by the mean-square deviations (ΘT),24,28,47,48 ΘT =

1 6

result, the [InO4] configuration deviates significantly from regular tetrahedron (ΘT = 6.86), which is prejudicial to structural stability. The ΘT values are, in general, obviously less than the ΘO values, an indication of the superior adaptability of tetrahedral rather than octahedral configurations caused by isomorphic substitutions. The M3+/Si4+ substitution energies in MMT (Esub) are calculated as, Esub = [E(M‐MMT) + E(Si4 +)] − [E(MMT) + E(M3 +)] (6) 3+

The M /Si substitution energies (Esub) increase in the order of B3+ < Ga3+ < Al3+ < Fe3+ < In3+ (Table 1) and show a nearly consistent trend as ionic radii. Whether for smaller or for larger cations, the incorporation processes are energetically costly and the difficulty for substitutions increase with ionic radii. Although with less structural perturbations as reflected by the ΘT and ΘO data, M3+/Si4+ substitution energies are often much larger than those of M2+/Al3+, which coincides with the fact that isomorphic substitutions in MMT occur principally at octahedral sheets. 3.4. K+ Adsorption in MMT with Tetrahedral Substitutions. The adsorption structure of K+ on Al3+/Si4+substituted MMT is shown in Figure 4d. The shortest K−OS distance equals 2.645 Å and is significantly less than the corresponding values of Mg2+/Al3+ substitutions (2.766 Å for T1 site and 2.762 Å for T2 site). The same trends are detected for the average K−OS distances that 2.794, 2.794, 2.784, 2.776, and 2.798 Å respectively for B3+, Al3+, Ga3+, Fe3+, and In3+, suggesting the enhanced interactions of K+ with M3+/Si4+rather than M2+/Al3+-substituted models (Table 2). With regard to Al3+/Si4+ substitutions, Shi et al.8 reported that the K−OS distance is 2.775 Å, which is consistent with the present

6

∑ (αi − α̅ )2 i=1

4+

(5)

The Si−O bonds in pristine MMT are calculated at around 1.634 Å and show slight elongations as compared to XRD data (1.618 Å).49 B3+ incorporation causes a contraction of T−O bonds that are stabilized at approximately 1.495 Å, and the [BO4] configuration closely represents a regular tetrahedron (ΘT = 1.01), while incorporation of larger heteroatoms elongates the T−O bonds that further squeeze adjacent lattice atoms and result in more structural distortions (Table 1 and Figure 5). The T−O distances increase in a direct proportion of ionic radii, and in the case of In3+ reach as large as 2.062 Å. As a 26726


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The Journal of Physical Chemistry C results. The K+ adsorption energies of B3+/Si4+, Al3+/Si4+, Ga3+/ Si4+, Fe3+/Si4+ and In3+/Si4+ substitutions (Ead), calculated according to eq 4, amount to −98.1, −97.8, −98.4, −94.6, and −92.5 kcal/mol, respectively. Therefore, clay materials with tetrahedral substitutions produce superior adsorption performances for metal ions than with octahedral substitutions, in line with the results of proton (H+) adsorption.6 Charge transfers from K+ to MMT, based on Mulliken population analyses, are 0.168, 0.157, 0.132, 0.120, and 0.073 |e| respectively for B3+/ Si4+, Al3+/Si4+, Ga3+/Si4+, Fe3+/Si4+ and In3+/Si4+ substitutions and have nearly the identical sequence as adsorption energies (Ead), clearly indicating that stronger adsorption is in favor of charge transfers. 3.5. Double Substitutions in MMT. Isomorphic substitutions are known to be abundant in MMT31 and it is necessary to investigate double substitutions. The octahedral and tetrahedral sites of different positions are labeled in Figure 6a, and the various combinations of double substitutions

Mg2+/Al3+ substitutions in Mg-MMT: Esub2 A = [E(2Mg‐MMT) + E(Al3 +)] − [E(Mg‐MMT) + E(Mg 2 +)] 2+

Mg /Al

3+

(7)

substitutions in Al-MMT:

Esub2 B = [E(Mg‐MMT) + E(Al3 +)] − [E(Al‐MMT) + E(Mg 2 +)] 3+

Al /Si

4+

(8)

substitutions in Mg-MMT:

Esub2 = [E(2Al‐MMT) + E(Si4 +)] C

− [E(Al‐MMT) + E(Al3 +)]

(9)

Al3+/Si4+ substitutions in Al-MMT: Esub2 D = [E(Mg‐Al‐MMT) + E(Si4 +)] − [E(Mg‐MMT) + E(Al3 +)]

(10)

where E(2Mg-MMT), E(2Al-MMT) and E(Mg−Al-MMT) are the electronic energies of MMT with double substitutions of OHOH, THTH and OHTH types, respectively. The energy differences between the second and first substitutions are ΔEsub = Esub2 − Esub

(11)

The second substitution energies (Esub2) show dependence on the first heteroatoms, and the ΔEsub values state that incorporation of the second heteroatoms is obviously more difficult than that of the first heteroatoms (Table 3). Meanwhile, the distribution of the second heteroatoms is affected by the occupancy of the first heteroatoms; e.g., when the first Mg2+ occupies T1a site, the preferential sites for residing the second Mg2+ follow as T1d > T1e > T2c. Relative energies for double substitutions may vary considerably according to the distribution of two heteroatoms, Table 3. Substitution Energies for the Second Heteroatoms (Esub2, kcal/mol), Their Differences from the First Substitution Energies (ΔEsub, kcal/mol), and Relative Energies (RE, kcal/mol) for the Double Substituted MMT Models as Well as Average K−OS Distances (RK−Os, Å), Charge Transfers (qtr, |e|), and Adsorption Energies (Ead, kcal/mol) for the Adsorption of K+ on these MMT Models

Figure 6. Local structures for (a) pristine MMT model as well as those with the optimal double substitutions: (b) Two Mg 2+ /Al 3+ substitutions at the octahedral sites (OHOH), (c) Two Al3+/Si4+ substitutions at the tetrahedral sites (THTH), and (d) one Mg2+/Al3+ substitution at the octahedral site and one Al3+/Si4+ substitution at the tetrahedral site (OHTH). In (a), the Si atoms in the hexagonal ring at the tetrahedral sheet are numbered successively as Sio, Sip, Siq, Sir, Sis, and Sit while the Al atoms in a similar ring at the octahedral sheet are numbered successively as Al1a, Al1b, Al2c, Al1d, Al1e, and Al2f. The Si, Al, Mg, and O atoms are visualized in yellow, purple, green, and red, respectively.

first

second

Esub2

ΔEsub

REa

RK−Os

Ead

qtr

Mg1a

Mg1d Mg1e Mg2c Mg1a Mg2f Alp Alq Alr Als Als Alt Mg1a Mg2c

738.6 740.8 741.2 742.7 736.9 1146.1 1142.9 1145.4 1147.2 1148.2 1143.6 743.3 742.9

43.1 45.2 47.2 47.2 42.8 46.7 43.5 46.1 47.8 48.8 44.2 47.8 48.8

3.1 5.4 5.8 5.8 0 3.1 0 3.4 5.1 4.6 0 5.1 4.6

2.770 2.772 2.782 2.782 2.765 2.742 2.735 2.759 2.760 2.761 2.759 2.760 2.761

−138.0 −138.1 −135.7 −135.7 −136.9 −147.3 −149.3 −143.4 −143.4 −144.9 −144.0 −143.4 −144.9

0.177 0.178 0.166 0.166 0.168 0.183 0.186 0.179 0.179 0.179 0.175 0.179 0.179

Mg2c

obeying the Loewenstein rule52 have been considered, resulting in a total of ten independent configurations that are denoted as Mg1 a +Mg1d, Mg1 a +Mg1e , Mg1 a +Mg2 c , Mg2 c +Mg2 f for octahedral substitutions (OHOH type), Alt+Alp, Alt+Alq for tetrahedral substitutions (THTH type) and Mg1a+Alr, Mg1a+Als, Mg2c+Als, and Mg2c+Alt for substitutions at both sheets (OHTH type). Take the first substitution at Al1a site for example. The second substitution can occur at Al1 d (Mg1a+Mg1d), Al1e (Mg1a+Mg1e), Al2c (Mg1a+Mg2c), Sir (Mg1a+Alr) or Sis (Mg1a+Als). Note that Al1d and Al1e are both Al1 sites while have different distances away from Al1a. The substitution energies of the second heteroatoms (Esub2) are defined according to the first heteroatoms,

Alt Mg1a Mg2c Als

a Mg2c+Mg2f, Alq+Alt, and Mg2c+Alt are used as energy benchmarks for double substitutions at the octahedral (OHOH), tetrahedral (THTH), and both (OHTH) sheets, respectively.

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OHTH type > −135.7 to −138.1 kcal/mol for OHOH type, in line with the trends of K−OS distances and charge transfers based on Mulliken population analyses (0.183−0.186 |e| for THTH type >0.175−0.179 |e| for OHTH type >0.166−0.178 |e| for OHOH type); see Table 3. The average distances between K+ and charge centers decline as THTH type > OHTH type > OHOH type, corroborating that the closer to metal ions, the larger the adsorption energies. The THTH type results in superior adsorption performances, consistent with the results of mono substitutions. 3.7. Isomorphic Substitutions in HCT and K + Adsorption. Structural optimizations are performed for pristine and substituted HCT as well as following K+adsorption models and results are listed in Table 4. For both

and the lowest-energy configurations are Mg2c+Mg2f, Alt+Alq, and Mg2c+Alt for the OHOH, THTH, and OHTH types, respectively (Table 3 and Figure 6). Priority is generally given to two T sites with larger distances;25,28,53,54 e.g., for the O H O H type, relative stabilities (distances of two Mg 2+ heteroatoms) rank as Mg2c+Mg2f (6.031 Å) > Mg1a+Mg1d (5.927 Å) > Mg1a+Mg2c (5.191 Å) > Mg1a+Mg1e (5.183 Å). 3.6. Adsorption of K+ onto MMT with Double Substitutions. Adsorption of K+ on doubly substituted MMT models is investigated and the local structures corresponding to the optimal OHOH (Mg2c+Mg2f), THTH (Alt+Alq), and OHTH (Mg2c+Alt) types are given in Figure 7.

Table 4. Average M−O Distances (R, Å), Mean-Square Deviations (Θ), and Substitution Energies (Esub, kcal/mol) for the M+/Mg2+- and M3+/Si4+-Substituted HCT Models as Well as Average K−OS Distances (RK−Os, Å), Charge Transfers (qtr, |e|), and Adsorption Energies (Ead, kcal/mol) for the Adsorption of K+ on these HCT models

The average K−OS distances exhibit a clear trend as 2.765− 2.782 Å for OHOH type >2.759−2.761 Å for OHTH type >2.735−2.742 Å for THTH type (Table 3). In all cases, the K− OS distances of double substitutions are less than those of mono substitutions (both octahedral and tetrahedral, see Table 2) suggesting the reinforced adsorption. The adsorption energies of K+ onto these MMT models (Ead) are defined similarly as in eq 4, Ead = E(K‐2Mg‐MMT) − [E(2Mg‐MMT) + E(K+)] (12) +

Ead = E(K‐Mg‐Al‐MMT) − [E(Mg‐Al‐MMT) + E(K )] (13)

Ead = E(K‐2Al‐MMT) − [E(2Al‐MMT) + E(K )]

R

Θ

Esub

RK−Os

Ead

qtr

Li+ Na+ B3+ Al3+ Ga3+

2.121 2.234 1.498 1.751 1.832

5.08 5.89 3.39 3.95 6.03

364.6 407.0 670.3 1112.4 1099.4

2.891 2.884 2.811 2.835 2.826

−96.9 −96.3 −103.6 −104.3 −106.6

0.155 0.166 0.162 0.163 0.142

octahedral and tetrahedral substitutions, the M−O distances are raised with ionic radii, which are in line with the results of MMT (Table 4). As indicated by the ΘO and ΘT values that are calculated using eqs 1 and 5, isomorphic substitutions cause relatively mild structural perturbations in HCT models, and for specific M3+/Si4+ substitution, the ΘT values of HCT are larger than those of MMT, which is due to more deviations from regular tetrahedron for pristine HCT (ΘT = 2.60) than pristine MMT (ΘT = 0.78). The K−OS distances are close for all substitutions and become slightly shorter in the case of tetrahedral rather than octahedral substitutions, suggesting the enhanced interactions and agreeing with the results of MMT. The octahedral substitution energies (Esub) in HCT are apparently less than those of MMT while those of tetrahedral substitutions are slightly larger (Tables 1 and 4); e.g., the B3+/ Si4+ substitution energies are calculated at 670.3 and 654.3 kcal/mol in HCT and MMT, respectively. In consequence, as compared to MMT, incorporation of heteroatoms in HCT occurs more facilely at the octahedral sheet while becomes a little more difficult at the tetrahedral sheet, in agreement with the ΘO and ΘT analyses. The K+-adsorption structures of HCT (Figure 8) show divergences from those of MMT: The three OS atoms that are the closest to K+ are adjacent in HCT instead of apart in MMT, due to structural differences between two clay materials. As a result, K+ ions in HCT deviate somewhat from the center of hexagonal rings. The adsorption energies (Ead) are −96.9, −96.3, −103.6, −104.3, and −106.6 kcal/mol, respectively, for Li+/Mg2+-, Na+/Mg2+-, B3+/Si4+-, Al3+/Si4+-, and Ga3+/Si4+substituted HCT models (Table 4). Adsorption is enhanced pronouncedly by isomorphic substitutions (−48.5 kcal/mol for pristine HCT), and tetrahedral substitutions produce stronger adsorption than octahedral substitutions, which are consistent with the results of MMT. The adsorption energies of metal ions

Figure 7. Local structures for K+ adsorption on the doubly substituted MMT models: (a) Two Mg2+/Al3+ substitutions at the octahedral sites (OHOH), (b) two Al3+/Si4+ substitutions at the tetrahedral sites (THTH), and (c) one Mg2+/Al3+ substitution at the octahedral site and one Al3+/Si4+ substitution at the tetrahedral site (OHTH). Si, Al, Mg, O, and K atoms are visualized in yellow, purple, green, red, and dark green, respectively. Distances are given in Å.

+

M+ or M3+

(14)

where E(K-2Mg-MMT), E(K−Mg−Al-MMT), and E(K-2AlMMT) stand for the electronic energies of K+-adsorbed MMT models with the OHOH-, OHTH-, and THTH-type double substitutions, respectively. The adsorption energies (Ead) decline as −147.3 to −149.3 kcal/mol for THTH type > −143.0 to −144.9 kcal/mol for 26728


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Figure 8. Local structures for K+ adsorption on (a) pristine HCT model as well as those with (b) Li+/Mg2+, (c) Na+/Mg2+, (d) B3+/Si4+, (e) Al3+/ Si4+, and (f) Ga3+/Si4+ substitutions. Si, Al, Mg, Li, Na, B, Ga, O, and K atoms are visualized in yellow, purple, green, orange, brown, pink, dark blue, red, and dark green, respectively. Distances are given in Å.

are noticeably different for MMT and HCT, and HCT shows superior adsorption performances, probably due to the different chemical environments of hexagonal rings in these two clay materials that further result in the distinct adsorption structures and energies for K+ ions; see more details in the Supporting Information (S5: Structural differences between MMT and HCT). 3.8. Factors Affecting the Adsorption of Metal Ions. So far, we have made an elaborate discussion on the isomorphic substitutions in clay materials and the following adsorption of metal ions. The adsorption performances of K+ are affected by a number of factors, and because of mutual entanglements some approximations have to be taken when estimating their contributions: (A) Identity of heteroatoms. A number of heteroatoms have been isomorphically substituted into the framework of MMT, and the |Ead| differences caused by the various heteroatoms are not more than 6.1, 5.3, and 5.9 kcal/mol, respectively, for octahedral T1, T2 and tetrahedral sites (Table 2), which are very close to each other. (B) Crystallographically distinct T sites. Distribution at two octahedral T sites of MMT shows dependence on the identity of heteroatoms, while the |Ead| values are close for two octahedral T sites and differ not more than 2.1 kcal/mol (Table 2), in line with their similar surface charge features (Figure 9b and c). (C) Structural alterations. Structural alterations between MMT and HCT cause the divergence of adsorption performances and the influences are assessed by the |Ead| differences averaged over the various heteroatoms, which are 6.2 and 8.5 kcal/mol respectively for the octahedral and tetrahedral T sites (Tables 2 and 4). (D) Quantity of negative charges. MMT carries a plethora of negative charges, and whether for octahedral or tetrahedral substitutions, the Ead values of mono and double substitutions are almost twice and triple as those of pristine models,

Figure 9. Molecular electrostatic potential maps projected on the tetrahedral surfaces of MMT models where metal ions are to be adsorbed: (a) Pristine; (b) Mg2+/Al3+ substitution at T1 site; (c) Mg2+/Al3+ substitution at T2 site; (d) deprotonation in the vicinity of T2 site of octahedral sheet; (e) Mg2+/Al3+ double substitutions; (f) Al3+/Si4+ substitution, where the most negative regions are shown in red.

respectively (Figure S1 and Tables 2, 3), in line with the amounts of negative surface charges as reflected in Figure 9. The critical role of charge quantity during the adsorption processes is corroborated by the results with negative charges originating from deprotonation of bridging hydroxyls at the octahedral sheet (Figure S1) and of H+ adsorption given in the Supporting Information (S4: H+ adsorption in MMT). With consideration of all sources of negative charges, the |Ead| enhancements per negative charge are 44.3−52.4 kcal/mol during the adsorption of K+ ions. (E) Distance from charge centers to metal ions. Two tetrahedral sheets in MMT are linked by an octahedral sheet, and owing to the enriched negative surface charges (Figure 9f), the tetrahedral sheet with Al3+/Si4+ substitutions adjacent to K+ has a larger Ead value than the other tetrahedral sheet (−97.8 vs −87.5 kcal/mol). Hence, the |Ead| reduction per layer spacing is 26729


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4. CONCLUSIONS Isomorphic substitutions are pervasive in clay materials and can bring about critical impacts on the adsorption, catalysis and other processes. With use of DFT calculations, isomorphic substitutions in MMT and HCT as well as following adsorption processes of metal ions have been addressed at a molecular level. Isomorphic substitutions in MMT, whether at octahedral or tetrahedral T sites, cause structural perturbations that have almost the same trends as ionic radii. Be2+ is an exception due to structural collapses. Octahedral substitutions in MMT occur preferentially at T1 site, or at T2 site, or evenly at two T sites, which are strongly dependent on the ionic radii of heteroatoms. Incorporation of heteroatoms is energetically costly, and substitution energies at both octahedral and tetrahedral sheets increase with ionic radii. Tetrahedral substitutions are apparently more energetically costly than octahedral substitutions, consistent with the experimental observations. For double substitutions, the second heteroatoms are always more difficult to incorporate and their locations depend heavily on the first heteroatoms. Optimal configurations are given for all types (OHOH, THTH, and OHTH). Isomorphic substitutions in HCT cause resembling structural perturbations and substitution energies as in MMT, while divergences are detected such as larger perturbations for tetrahedral substitutions in HCT. As compared to MMT, incorporation of heteroatoms in HCT is more facile at the octahedral sheet while more difficult at the tetrahedral sheet; in addition, HCT shows superior adsorption performances. Six factors (identity of heteroatoms, crystallographically distinct T sites, structural alterations, quantity of negative charges, distance from charge centers to metal ions, and source of negative charges) can affect the adsorption of metal ions on clay materials and their respective contributions have been estimated. Quantity of negative charges is the foremost factor that controls the adsorption performances. In all cases, crystallographically distinct T sites exert a minor influence, quite different from the situation in zeolites. Other factors may play a critical role in certain circumstances (see the text for details); e.g., for clay materials with variable charges, the adsorption performances can be pronouncedly altered by pH changes.

estimated to be 5.1 kcal/mol. Albeit the middle octahedral sheet encounters Mg2+/Al3+ substitution instead, its Ead value (−91.0 kcal/mol averaged over T1 and T2 sites) accords well with this estimation. (F) Source of negative charges. The Ead values are calculated to be −91.0 (averaged over T1 and T2 sites) and −90.4 kcal/ mol for MMT models with Mg2+/Al3+ substitutions and deprotonation of bridging hydroxyls at the octahedral sheet, where the |Ead| difference for two charge sources is merely 0.6 kcal/mol and shows agreement with their similar surface charge features (Figure 9). Considering Al3+/Si4+ substitutions, the | Ead| differences caused by the various charge sources will not exceed 2.3 kcal/mol (effect of layer spacing, i.e., 5.1 kcal/mol, has been deducted). So far, the respective contributions of six factors that affect the adsorption of metal ions on clay surfaces have been evaluated, with their maxima being plotted in Figure 10. In all

Figure 10. Largest extents to affect the adsorption energies of K+ ions onto MMT surfaces (max|ΔEad|, kcal/mol) have been estimated for the various influencing factors, where the contribution of structural alterations are derived by comparisons with HCT.

cases, crystallographically distinct T sites exert a minor influence, which is quite different from the scenario in zeolites.24−28 Quantity of negative charges is the foremost factor that differentiates the adsorption performances for different clay materials. In addition, for clay materials with variable charges, the adsorption of metal ions will be significantly altered by external conditions such pH, as supported by experimental observations that the enhanced adsorption of metal ions on clay materials at higher pH values is attributed to the increase of negative charges.55,56 Adsorption can be also affected by the other factors but to a less extent. For different clay particles with close charge quantities, the adsorption performances of metal ions will not differ substantially, and structural alterations may be the primary factor (Figure 10). Even two structurally similar clay materials (e.g., MMT and HCT presently studied) exhibit noticeably different adsorption performances, and hence larger contributions of structural alterations are anticipated for two more structurally diverse clay materials. For two clay particles of varying sizes, distances of charge centers and metal ions can diverge greatly and therefore are a significant factor to affect the adsorption performances.

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ASSOCIATED CONTENT

S Supporting Information *

The Supporting Information is available free of charge on the ACS Publications website at DOI: 10.1021/acs.jpcc.7b03488. Structural parameters for two octahedral T sites in MMT; calculation of substitution energies using hydrated ions; influences of charge sources on the adsorption performances; H+ adsorption in MMT; structural differences between MMT and HCT; atomic coordinates for optimized structures (PDF)

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AUTHOR INFORMATION

Corresponding Author

*E-mail: [email protected]. Phone: 086-023-68251504. Fax: 086-023-68250444. ORCID

Gang Yang: 0000-0003-1032-6840 26730


Article

The Journal of Physical Chemistry C Notes

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The authors declare no competing financial interest.

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ACKNOWLEDGMENTS We gratefully acknowledged the financial supports from the National Natural Science Foundation of China (No. 21473137) and Natural Science Foundation Project of CQ CSTC, China (cstc2017jcyjAX0145). The National Supercomputing Center in Shenzhen is thanked for computational resources.

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