27Al NMR Chemical Shifts and Relative Stabilities of Aqueous

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Article Cite This: ACS Earth Space Chem. 2019, 3, 1353−1361

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27

Al NMR Chemical Shifts and Relative Stabilities of Aqueous Monomeric Al3+ Hydrolytic Species with Different Coordination Structures

Shaonan Dong, Wenjing Shi, Jing Zhang, and Shuping Bi* School of Chemistry and Chemical Engineering, State Key Laboratory of Coordination Chemistry of China & Key Laboratory of MOE for Life Science, Nanjing University, Nanjing 210023, China Downloaded via UNIV OF SOUTHERN INDIANA on July 19, 2019 at 22:48:52 (UTC). See https://pubs.acs.org/sharingguidelines for options on how to legitimately share published articles.

S Supporting Information *

ABSTRACT: In this study, the 27Al NMR chemical shifts and relative stabilities of monomeric Al3+ hydrolytic species with different coordination structures in aqueous solution are systematically investigated by using the density functional theory quantum chemical cluster model (DFT-CM) at the B3LYP/6311+G(d,p) level. The main work includes: the static configurations of 20 possible existing monomeric Al3+ hydrolytic species from Al3+ to Al(OH)4− are optimized, and their 27Al NMR shieldings are calculated; the dehydration reaction pathways for typical monomeric Al3+ hydrolytic species are modeled, and the dominant forms of the intermediate hydrolytic species of Al(OH)2+, Al(OH)2+, and Al(OH)30 are analyzed based on the Gibbs free energy changes of the dehydration reactions. The important role of the tetracoordinated Al(H2O)(OH)30 in the formation mechanism of the polynuclear Keggin-Al13 is discussed. This work provides valuable references for further studying the formation and transformation mechanisms of the aqueous monomeric and polymeric Al species. KEYWORDS: aluminum ion, hydrolytic species, coordination structure, aqueous solution, density functional theory

1. INTRODUCTION The hydrolysis of the monomeric aluminum ion (Al3+) in aqueous solution is a fundamental process in its transforming into other Al forms such as hydroxopolynuclear Al clusters and Al colloidal minerals. To elucidate the dominating structures of the monomeric Al3+ hydrolytic species at different hydrolysis reaction stages is of great importance for many aspects such as industrial applications, for examples, the synthesis of catalytic materials, flocculant behaviors in wastewater treatments, as well as relating environmental science.1 From solution pH 3 to 7, the hexacoordinated Al(H2O)63+ undergoes a series of deprotonation and dehydration reactions and converts into the tetracoordinate Al(OH)4−, while the complex problem for the coordination structures of the intermediate hydrolytic species of Al(OH)2+, Al(OH)2+, and Al(OH)30 is unsolved.2 Due to that varieties of Al3+ species may coexist in a narrow pH range and that polymerization and precipitation easily occur at high Al concentrations, it is difficult to use traditional experimental methods such as 27Al NMR, X-ray diffraction, or potentiometry to determine the existing forms of the intermediate hydrolytic species.2 Therefore, researchers turned to theoretical methods and have performed a large majority of studies on the coordination structures of the aqueous monomeric Al3+ hydrolytic species. 3 However, because of the different calculation methods used, there are big differences between © 2019 American Chemical Society

the reported dominant forms and their coordination numbers (CNs) for the intermediate hydrolytic products in the literature. For example, for the first hydrolysis product Al(OH)2+, it is reported that its dominant form may be hexa-4−9 or pentacoordinated,2,10−12 or both two coordination structures coexist;13,14 for the second hydrolysis product Al(OH)2+, the hexa-,6,8,9,15,16 penta-,12 or tetracoordinated13 configurations or their coexistence4,5,14 have been reported to be dominant in the literature; for the third hydrolysis product Al(OH)30, it is considered that the penta-6,16 or tetracoordinated3,9,12,13,17−19 or both coordination forms5,20 are dominant, and there are even studies suggesting that Al(OH)30 exists in the tricoordination form.14,21 These inconsistent results have significantly hindered further understanding of the hydrolysis-polymerization mechanisms of Al forms in aqueous solution. In order to solve this problem with the theoretical modeling way, a comprehensive investigation of all possible existing forms of the aqueous monomeric Al3+ hydrolytic species with a suitable calculation method and solvation model is needed. In this work, the density functional theory quantum Received: Revised: Accepted: Published: 1353

April 18, 2019 May 27, 2019 May 30, 2019 May 30, 2019 DOI: 10.1021/acsearthspacechem.9b00102 ACS Earth Space Chem. 2019, 3, 1353−1361

Article

ACS Earth and Space Chemistry

Figure 1. Twenty different coordination structures of monomeric Al3+ hydrolytic species.

chemical cluster model (DFT-CM) method22,23 and the gas phase-supermolecule-polarizable continuum model (GP-SMPCM)24,25 are employed to systematically study the 27Al NMR chemical shifts and more importantly the relative stabilities of the aqueous monomeric Al3+ hydrolytic species with different coordination structures. The main works include: to optimize the static configurations of 20 possible existing coordination forms of the hydrolytic species and calculate their 27Al NMR chemical shifts; to model dehydration reaction pathways for typical coordination structures of the hydrolytic species and analyze the favorable existing forms for each hydrolysis products; and to discuss the important role of tetracoordinated Al(H 2 O)(OH) 3 0 in the formation mechanism of the polynuclear Keggin-Al13.

supermolecule (GP-SM) clusters are constructed by adding explicit solvent water molecules around the gas phase clusters of the species, in order to consider the short-range interactions between the solutes and the surrounding solvent hydrogenbonding networks in the second coordination shells.24,25 Referring to our previous report,6 12, 13, and 14 explicit solvent waters are added for the hexa-, penta-, and tetracoordinated configurations, respectively (Al(H2O)m(OH)n(3−n)·xH2O, m + n = 6, 5, and 4; m + n + x = 18). All GP-SM clusters are optimized in the polarizable continuum model (PCM) to consider the outer coordination shells and bulk solvents.26,27 The 27Al NMR shieldings σ of the optimized static configurations are calculated in PCM using the gauge-independent atomic orbital (GIAO) method.28 Al(H2O)63+ is taken as reference, and the chemical shifts for other hydrolytic species are calculated using δ = σref − σ, where σref and σ are the shieldings for the reference and target species, respectively. 2.2. Kinetic Dehydration Reaction Pathways. The dehydration reaction pathways for the hexacoordinated Al(H 2 O) 6 3+ , Al(H 2 O) 5 (OH) 2+ , Al(H 2 O) 4 (OH) 2 + , Al(H2O)3(OH)30, and Al(H2O)2(OH)4− and the pentacoordinated Al(H2O)4(OH)2+, Al(H2O)3(OH)2+, Al(H2O)2(OH)30, and Al(H2O)(OH)4− configurations are modeled. For the Al(H 2 O) 5 (OH) 2+ , cis-Al(H 2 O) 4 (OH) 2 + , and mer-Al(H2 O) 3(OH) 30 configurations, there are two different dehydration reaction sites (cis and trans to OH−), and the reaction pathways at both sites are modeled. The numbers (Nm′) and arrangements of the explicit solvent water molecules added in the second coordination shells of the reaction species are determined by adding explicit solvent waters incrementally at different locations until the energy barriers for the favorable pathways converge.11,23,29

2. COMPUTATIONAL METHODS 2.1. 27Al NMR Chemical Shifts of Static Configurations. The optimized static configurations are the hexa-, penta-, and tetracoordinated structures (CN = 6, 5, and 4) of the Al3+, Al(OH)2+, Al(OH)2+, Al(OH)30, and Al(OH)4− species, including the hexacoordinated Al(H2O)63+, Al(H2O)5(OH)2+, Al(H2O)4(OH)2+, Al(H2O)3(OH)30, and Al(H2O)2(OH)4−, the pentacoordinated Al(H2O)53+, Al(H2O)4(OH)2+, Al(H2O)3(OH)2+, Al(H2O)2(OH)30, and Al(H2O)(OH)4−, as well as the tetracoordinated Al(H2O)43+, Al(H2O)3(OH)2+, Al(H2O)2(OH)2+, Al(H2O)(OH)30, and Al(OH)4−. The Al(H2O)4(OH)2+, Al(H2O)3(OH)2+, and Al(H2O)2(OH)4− species are divided into cis and trans configurations according to the relative positions of the OH− and H2O ligands, while similarly the Al(H2O)3(OH)30 and Al(H2O)2(OH)30 species are divided into fac or mer configurations. The 20 configurations are listed in Figure 1. In optimizing the static configurations, the gas phase1354

DOI: 10.1021/acsearthspacechem.9b00102 ACS Earth Space Chem. 2019, 3, 1353−1361

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ACS Earth and Space Chemistry Table 1.

27

Al NMR Shieldings σ and Relative Chemical Shifts δ of Monomeric Al3+ Hydrolytic Species (ppm)a B3LYP/6311+G(d,p)

no.

species

B3LYP/def2TZVP

ωB97X-D/6311+G(d,p)

HF/6311+G(d,p)

σ

δ

σ

δ

σ

δ

σ

δ

1 2 3 4

Al(H2O)6 ·12H2O Al(H2O)53+·13H2O Al(H2O)43+·14H2O Al(H2O)5(OH)2+·12H2O

572.7 538.0 503.7 567.5

0.0 34.7 69.0 5.2

569.8 535.9 500.9 564.9

0.0 33.9 68.9 4.9

580.3 547.0 513.0 575.1

0.0 33.3 67.3 5.2

609.0 578.4 543.9 604.1

0.0 30.6 65.1 4.9

5 6 7 8 9 10 11 12 13 14 15 16 17 18 19 20

Al(H2O)4(OH)2+·13H2O Al(H2O)3(OH)2+·14H2O cis-Al(H2O)4(OH)2+·12H2O trans-Al(H2O)4(OH)2+·12H2O cis-Al(H2O)3(OH)2+·13H2O trans-Al(H2O)3(OH)2+·13H2O Al(H2O)2(OH)2+·14H2O fac-Al(H2O)3(OH)30·12H2O mer-Al(H2O)3(OH)30·12H2O fac-Al(H2O)2(OH)30·13H2O mer-Al(H2O)2(OH)30·13H2O Al(H2O)(OH)30·14H2O cis-Al(H2O)2(OH)4−·12H2O trans-Al(H2O)2(OH)4−·12H2O Al(H2O)(OH)4−·13H2O Al(OH)4−·14H2O

533.0 495.6 560.2 557.2 528.6 530.8 495.0 554.9 553.3 525.5 522.1 495.2 548.8 552.1 515.8 490.3

39.7 77.1 12.5 15.5 44.1 41.9 77.7 17.8 19.4 47.2 50.6 77.5 23.9 20.6 56.9 82.4

530.9 492.7 557.9 555.6 526.2 528.4 491.7 552.8 551.5 522.9 519.6 491.6 547.0 550.4 512.9 486.5

38.9 77.1 11.9 14.2 43.6 41.4 78.1 17.0 18.3 46.9 50.2 78.2 22.8 19.4 56.9 83.3

541.9 505.6 567.9 565.6 537.2 539.6 504.7 563.1 561.9 534.1 530.5 504.2 557.7 560.7 524.6 499.4

38.4 74.7 12.4 14.7 43.1 40.7 75.6 17.2 18.4 46.2 49.8 76.1 22.6 19.6 55.7 80.9

574.0 538.0 597.9 595.9 569.4 572.3 536.5 593.6 592.7 566.7 562.5 535.6 589.2 592.2 557.9 532.7

35.0 71.0 11.1 13.1 39.6 36.7 72.5 15.4 16.3 42.3 46.5 73.4 19.8 16.8 51.1 76.3

3+

literature values δ(expt)

δ(calcd, literature)

0

0

0.34,36 3.537

1,9 8.8,43 16.443

3.737

27.044 15,9 24.944

8.538b

279

3638b

48,9 469

∼6538b

70,9 73.7,18 77.818

80,36 79.937

76.8,18 78,9 79.7,45 88.3,18 89.243

a Items in bold indicate pentacoordinated configurations, and items in italic indicate tetracoordinated configurations. Al(H2O)63+ is reference. b27Al MAS NMR results for amorphous Al(OH)30(am) solid.

dral, trigonal-bipyramidal, and tetrahedral, respectively (for detailed configurations and structural parameters, see S1 in the Supporting Information). Table 1 suggests that the calculated δ values of the monomeric Al3+ hydrolytic species are similar for the B3LYP/6-311+G(d,p), B3LYP/def2-TZVP, and ωB97X-D/6-311+G(d,p) methods and are slightly smaller for the HF/6-311+G(d,p) method. In general, the calculated 27 Al NMR shifts are consistent with the calculated values in the literature. The δ values are clearly distributed in three regions. Taking the B3LYP/6-311+G(d,p) results for instance: the δ values of the hexacoordinated configurations are in the range of 0−24 ppm, the δ values of the pentacoordinated configurations are in the range of 35−57 ppm, and the δ values of the tetracoordinated configurations are in the range of 69−82 ppm. The δ values are consistent with the experimental and theoretical calculation results from literature reports. Obviously, the δ values of the monomeric Al3+ hydrolytic species are closely related to the CN of the species. The δ values are also affected by the number of the inner-shell OH− ligands. When the CN is the same, the δ values increase with increasing numbers of OH− ligands. Since Al(H2O)63+, Al(H2O)53+, and Al(H2O)43+ have no OH− ligand in their inner coordination shells and their δ values are 0, 34.7, and 69.0 ppm, respectively, it is proposed that the distribution of the δ values falling into three regions is mainly caused by the difference in the CNs of the species, rather than the difference in the numbers of the OH− ligands. In 27Al NMR experimental studies, Perry and Shafran assigned δ = 0.34 ppm for Al(OH)2+ and Faust et al. assigned 3.5 and 3.7 ppm for Al(OH)2+ and Al(OH)2+, respectively.36,37 All these measured chemical shifts are in the upfield region, close to those calculated values for the hexacoordinated Al(H2O)5(OH)2+ (δ = 5.2 ppm) and Al(H2O)4(OH)2+ (δ = 12.5 and 15.5 ppm) configurations in Table 1. Currently, there

All DFT-CM calculations were performed using the Gaussian 03 suite of programs.30 The structural optimizations and frequency analysis of the static configurations and the reaction species (reactants (R), transition states (TS), and products (P)) in the kinetic dehydration reactions are carried out at the B3LYP/6-311+G(d, p) level of theory,31,32 which has been proved suitable for modeling the aqueous monomeric Al species.7,11,29 The 27Al NMR shielding calculations are performed with the B3LYP/6-311+G(d,p), B3LYP/def2TZVP, ωB97X-D/6-311+G(d,p), and HF/6-311+G(d,p) methods (the B3LYP/def2-TZVP and ωB97X-D/6-311+G(d,p) calculations are performed using Gaussian 09, revision B.01). In modeling the kinetic dehydration reaction pathways, the TS structures with one imaginary frequency are obtained from Berny TS optimizations,30 while the stable R and P structures without imaginary frequency are obtained by further optimizing the final structures achieved from intrinsic reaction coordinate (IRC) calculations.33 The aqueous Gibbs free energies G of the dehydration reaction species at 298.15 K are obtained on the basis of the single-point energies of the GPSM clusters calculated using the MP2/6-311+G(d,p) method, by adding zero-point energies, thermal and entropy corrections, MP2-PCM solvation energies, and DFT-dispersion corrections.34 The UAKS radii35 and a dielectric constant of ε = 78.39 are used for water in the PCM calculations.

3. RESULTS AND DISCUSSION 3.1. 27Al NMR Chemical Shifts of Monomeric Al3+ Hydrolytic Species with Different Coordination Structures. Table 1 lists the 27Al NMR shieldings σ and relative chemical shifts δ of the 20 different coordination structures of the monomeric Al3+ hydrolytic species. The optimized hexa-, penta-, and tetracoordinated configurations are near octahe1355

DOI: 10.1021/acsearthspacechem.9b00102 ACS Earth Space Chem. 2019, 3, 1353−1361

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ACS Earth and Space Chemistry

Figure 2. Optimized TS structures for the dehydration processes of the monomeric Al3+ hydrolytic species (Nm′ = 6) (no. 1−14).

is no available experimental δ data for aqueous Al(OH)30. In a solid-state 27Al MAS NMR study of amorphous Al(OH)30(am), Isobe et al. observed hexa-, penta-, and tetracoordinations of O atoms for Al, and the 27Al NMR peaks for the AlO6, AlO5, and AlO4 subunits are at 8.5, 36 and ∼65 ppm, respectively.38 They suggested that the AlO5 and AlO4 coordinations are characteristics of amorphous Al(OH)30(am).38 In some minerals containing AlO5 units, the pentacoordinated AlO5 are shown to give 27Al NMR chemical shifts of 30−42 ppm using solidstate NMR spectroscopy,39−41 and the range is approximately in agreement with our calculated δ for the pentacoordinated monomeric Al3+ hydrolytic species (34.7−50.6 ppm for Al(H2O)53+, Al(H2O)4(OH)2+, Al(H2O)3(OH)2+, and Al-

(H2O)2(OH)30). Furrer et al. observed peaks near δ = 35 ppm in the 27Al MAS NMR spectra of Al oxyhydroxide flocs formed in natural streams, and this is also the characteristic chemical shift for pentacoordinated AlO5.42 3.2. Dehydration Reactions and Relative Stabilities of Monomeric Al3+ Hydrolytic Species with Different Coordination Structures. In modeling the dehydration reaction pathways for the typical monomeric Al3+ hydrolytic species, the testing results of incrementally adding explicit solvent waters suggest that adding six explicit solvent waters (Nm′ = 6) can lead to the convergence of the dehydration reaction energy barriers (for details, see S2.1 in the Supporting Information). A total of 14 dehydration reaction pathways for 1356

DOI: 10.1021/acsearthspacechem.9b00102 ACS Earth Space Chem. 2019, 3, 1353−1361

Article

ACS Earth and Space Chemistry Al(H 2 O) 6 3+ , Al(H 2 O) 5 (OH) 2+ , Al(H 2 O) 4 (OH) 2+ , Al(H2O)4(OH)2+, Al(H2O)3(OH)2+, Al(H2O)3(OH)30, and Al(H2O)2(OH)30 configurations are obtained (no. 1−14), and the TS structures are listed in Figure 2 (R and P structures are shown in S2.2 in the Supporting Information). The 14 dehydration pathways have similar water dissociation processes. The leaving waters are originally located in the inner coordination shell of Al3+, and they gradually remove away from the inner shell as the dehydration proceeds, resulting in the decrease in CN of Al 3+ . After dehydration, the hexacoordinated reactants transform into near trigonalbipyramidal pentacoordinated products (in the no. 10 pathway, the dehydration of fac-Al(H2O)3(OH)30 leads to the formation of a tetracoordinated product since an innershell coordinated water removes together with the leaving water), while the pentacoordinated reactants transform into tetrahedral products. In R structures, the distances between the leaving waters and the central Al3+ are r(Al−OH2)leaving = 2.0− 2.3 Å. In TS structures, these distances lengthen to 2.3−2.9 Å, and the leaving waters form 1−2 hydrogen bonds with innershell OH−, H2O ligands, or second-shell solvent waters. In P structures, the leaving waters are located in the second coordination shells with r(Al−OH2)leaving = 3.4−4.1 Å. For all dehydration reaction pathways, the changes of the sum of the inner-shell Al−O bond lengths Σr(Al−O)I from R to TS are positive values, confirming the dissociative character of the dehydration processes.46 Table 2 lists the activation Gibbs free energies ΔG298,a⧧ and the reaction Gibbs free energies ΔG298,r⧧ for the 14

and 6.4−9.8 kJ/mol, respectively (nos. 2, 3, 5−7, 10−12), while for the pentacoordinated Al(H 2 O) 4 (OH)2+ , Al(H2O)3(OH)2+, and Al(H2O)2(OH)30 configurations, the ΔG298,a⧧ values are 41.2, 8.5−10.8, and 3.1−4.3 kJ/mol, respectively (nos. 4, 8, 9, 13, 14). Obviously, the more the inner-shell OH− ligands that are present, the lower the dehydration energy barriers, and the easier the dissociation of the inner-shell coordinated waters. For the Al(H2O)5(OH)2+ and cis-Al(H2O)4(OH)2+ configurations, the dissociation of the waters trans to the OH− ligands are ∼10 kJ/mol lower in ΔG298,a⧧ than the cis waters (nos. 2, 3, 5, 6), reflecting the trans labilizing effects of the OH− ligands. While for the merAl(H2O)3(OH)30 configuration, the ΔG298,a⧧ values for the cis and trans waters are close to each other (6.4 and 9.8 kJ/mol, respectively, nos. 11, 12). These two ΔG298,a⧧ values are not large, and thus, the trans labilizing effect of OH− is not apparent. In the dehydration reaction pathway for Al(H2O)63+, the Gibbs free energy of the pentacoordinated product is 64.8 kJ/ mol higher than that of the reactant (no. 1), reflecting the high stability of Al(H2O)63+. For the first hydrolysis product Al(OH)2+, after the hexacoordinated Al(H2O)5(OH)2+ dehydrates, the Gibbs free energy of the reaction system increases by 22.8−43.3 kJ/mol (nos. 2, 3), and the dehydration of the pentacoordinated Al(H2O)4(OH)2+ further increase the Gibbs free energy of the reaction system by 42.3 kJ/mol (no. 4), indicating that the hexacoordinated Al(H2O)5(OH)2+ is apparently more stable than the penta- or tetracoordinated configurations. For the second hydrolysis product Al(OH)2+, the hexacoordinated Al(H2O)4(OH)2+ has cis- and transconfigurations. Figure 3 shows that the trans-Al(H2O)4(OH)2+ configuration is lower in energy by ∼20 kJ/mol than the cisAl(H2O)4(OH)2+ configuration (no. 5−7). In their dehydration reaction pathways, the pentacoordinated products have higher Gibbs free energies by >16.9 kJ/mol than the transAl(H2O)4(OH)2+ configuration (no. 5−7). After the pentacoordinated cis- and trans-Al(H2O)3(OH)2+ configurations dehydrate into tetracoordinated products, the system energies increase by 6.8−8.8 kJ/mol (nos. 8, 9). The results indicate that the hexacoordinated trans-Al(H2O)4(OH)2+ configuration is most stable. For the third hydrolysis product Al(OH)30, the hexacoordinated Al(H2O)3(OH)30 has fac- and mer- configurations. Figure 3 shows that the fac-Al(H2O)3 (OH)30 configuration is lower in energy than the mer-Al(H2O)3(OH)30 configuration by ∼15 kJ/mol (nos. 10−12). The facAl(H2O)3(OH)30 dehydrates and transforms into tetracoordinated Al(H2O)(OH)30, with the system energy decreases by 3.3 kJ/mol (No. 10). The mer-Al(H2O)3(OH)30 (cis to OH) dehydration product is a fac-Al(H2O)2(OH)30 configuration, with a Gibbs free energy close to the hexacoordinated facAl(H2O)3(OH)30 (no. 11). The mer-Al(H2O)3(OH)30 (trans to OH) dehydration product is a mer-Al(H2O)2(OH)30 configuration, with a Gibbs free energy ∼12 kJ/mol lower than the hexacoordinated fac-Al(H2O)3(OH)30 (no. 12). In the dehydration reactions of the pentacoordinated fac- and mer-Al(H2O)2(OH)30, the two R configurations are close to each other, and the tetracoordinated dehydration products Al(H2O)(OH)30 have slightly lower Gibbs free energies than the pentacoordinated reactants (nos. 13, 14). Thus, the pentacoordinated fac- and mer-Al(H2O)2(OH)30 as well as the tetracoordinated Al(H2O)(OH)30 are relative stable configurations and can convert into each other, while the hexacoordinated fac-Al(H2O)3(OH)30 may also exist in

Table 2. Gibbs Free Energy Parameters for the Dehydration Processes of the Monomeric Al3+ Hydrolytic Speciesa no. 1 2 3 4 5 6 7 8 9 10 11 12 13 14

ΔG298,a⧧ (=GTS − GR) (kJ/mol)

species Al(H2O)63+ Al(H2O)5(OH)2+ (cis to OH) Al(H2O)5(OH)2+ (trans to OH) Al(H2O)4(OH)2+ cis-Al(H2O)4(OH)2+ (cis to OH) cis-Al(H2O)4(OH)2+ (trans to OH) trans-Al(H2O)4(OH)2+ cis-Al(H2O)3(OH)2+ trans-Al(H2O)3(OH)2+ fac-Al(H2O)3(OH)30 mer-Al(H2O)3(OH)30 (cis to OH) mer-Al(H2O)3(OH)30 (trans to OH) fac-Al(H2O)2(OH)30 mer-Al(H2O)2(OH)30

ΔG298,r⧧ (=GP − GR) (kJ/mol)

71.0 (71.2,47 72.348) 47.1 (47.249)

64.8 43.3

37.0

22.8

41.2 26.9

42.3 13.7

16.8

2.5

34.8 10.8 8.5 9.6 6.4

16.9 6.8 8.8 −3.3 −13.9

9.8

−20.7

3.1 4.3

−5.8 −0.8

a

In parentheses are the experimental data.

dehydration pathways, and Figure 3 shows the relative Gibbs free energy curves. For Al(H2O)63+, ΔG298,a⧧ = 71.0 kJ/mol (no. 1), in agreement with the 17O NMR measured activation Gibbs free energy 71−72 kJ/mol for the water-exchange reaction of Al(H2O)63+.47,48 For the hexacoordinated Al(H2O)5(OH)2+, Al(H2O)4(OH)2+, and Al(H2O)3(OH)30 configurations, the ΔG298,a⧧ values are 37.0−47.1, 16.8−34.8 1357

DOI: 10.1021/acsearthspacechem.9b00102 ACS Earth Space Chem. 2019, 3, 1353−1361

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ACS Earth and Space Chemistry

Figure 3. Relative Gibbs free energy curves for the dehydration processes of the monomeric Al3+ hydrolytic species (For isomers with multiple dehydration reaction pathways, the R structures with lowest Gibbs free energies are taken as references. In the no. 10 pathway, the dehydration product is tetracoordinated.)

aqueous solution in small amount. The dehydration reaction pathways for the hexa- and pentacoordinated Al(OH)4− species are not found. During searching the TS structures for these pathways, the leaving waters or inner-shell coordinated waters move into the second shell spontaneously to form tetracoordinated configurations, and finally the TS structures can not be located (see S2.2 in the Supporting Information). This result proves that the hexacoordinated Al(H2O)2(OH)4− and the pentacoordinated Al(H2O)(OH)4− are very likely to dehydrate in aqueous solution, and the tetracoordinated Al(OH)4− is the dominant configuration. In summary, the hydrolysis of Al3+ in aqueous solution starts from the hexacoordinated Al(H2O)63+, the first hydrolysis product Al(OH) 2+ mainly exists in the form of the hexacoordinated Al(H2O)5(OH)2+, the second hydrolysis product Al(OH) 2 + mainly exists in the form of the hexacoordinated trans-Al(H2O)4(OH)2+, the third hydrolysis product Al(OH) 30 mainly exists in the form of the pentacoordinated fac- and mer-Al(H2O)2(OH)30 as well as the tetracoordinated Al(H2O)(OH)30, while the hexacoordinated fac-Al(H2O)3(OH)30 may also exist in small amounts, the fourth hydrolysis product Al(OH)4− mainly exists in the tetracoordinated configuration. Both the Al(OH)2+ and Al(OH)2+ species mainly exist in hexacoordinated configurations, in agreement with traditional views,1 and also consistent with our previous studies.7,15 Al(OH)30 is the only species with multiple favorable coordination structures among the four hydrolysis products. The hexa-, penta-, and tetracoordinated configurations of Al(OH)30 may coexist in aqueous solution, similar to the observation by Isobe et al. that the AlO6, AlO5 and AlO4 units coexist in the amorphous Al(OH)30(am) solid.38 3.3. Exploration of Role of the Tetracoordinated Al(H2O)(OH)30 in the Formation Mechanism of KegginAl13. The result that Al(OH)30 mainly exists in the form of penta- and tetracoordinated configurations in aqueous solution has important significance for studying the formation mechanisms of hydroxyl polynuclear Al clusters such as Keggin-AlO4Al12(OH)24(H2O)127+ (Keggin-Al13). Keggin-Al13 is a good flocculant, and it is widely used in the synthesis of catalysts, adhesives, and other materials.50−52 Keggin-Al13

contains one highly symmetry tetrahedral AlO4 and 12 octahedral AlO6 units, and it can form under the experimental conditions of fast base titration, moderate base neutralization and the natural conditions.50 Previous studies considered that the formation of Keggin-Al13 requires the Al(OH)4− ion as a precursor, and the Al(OH)4− ion is formed in the local over alkaline regions such as at the solute-base precipitate interface and is protected from the acidic environments by the gelatinous Al(OH)3.53,54 While this viewpoint can explain the formation mechanism of Keggin-Al13 under fast base titration conditions, it is not applicable in moderate reaction conditions; i.e., under a moderate base titration rate and a vigorous stirring condition, the acidity and basicity of the solution are uniform without local over alkaline regions. The moderate natural condition of mixing acidic effluent from old mines and acidic soil (pH = 2.3) into waters with a higher pH (pH = 7.7) also causes the formation of Keggin-Al13.42 The interpretation of Al(OH)4− as a precursor is not applicable for the formation of Keggin-Al13 under moderate base neutralization and the natural conditions. It is proposed that, under fast base titration conditions, the precursor for Keggin-Al13 formation may be the Al(OH)4− ion formed in local over alkaline regions, while under moderate conditions, the precursor for Keggin-Al13 formation and the source of AlO4 can be considered to be the tetracoordinated Al(H2O)(OH)30. In the effective pH 4−6 range for Keggin-Al13 formation, the neutral Al(OH)30 is dominant and easy to form.55 At a high Al(OH)30 concentration, the Al(OH)30 with different coordination structures (hexa-, penta-, or tetracoordinated) would aggregate into low-molecular-weight amorphous [Al(OH)3]n0(aq) sol. In the process of sol aging, the coordinated H2O in tetracoordinated Al(H2O)(OH)30 slowly ionizes and releases H+ and then forms the sol hydroxyl species [Al(OH)3]n−10·Al(OH)4−(aq), where the formed Al(OH)4− is protected by the sol from the neutralization by H+ in the acidic solution.18 At the same time, the monomeric Al3+ hydrolytic species polymerizes and forms other oligomeric species such as Al2 and Al3. Then, the Keggin-Al13 forms from condensation of several oligomers around the Al(OH)4− ion that comes from the [Al(OH)3]n−10·Al(OH)4−(aq). Furrer et al. found the existence of AlO5 in the flocs formed from aggregation of 1358

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ACS Earth and Space Chemistry

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Keggin-Al13 in natural streams,42 and this is just the evidence for the involvement of the amorphous [Al(OH)3]n0(aq) sol in the formation of Keggin-Al13. Thus, the formation mechanisms of Keggin-Al13 under fast base titration and moderate reaction conditions are proposed to be different. The reaction equations of t h e t w o me c h a n i s m s c a n b e w r i t t e n as 4 [Al3(OH)7(H2O)6]2+ + Al(OH)4− → [AlO4Al12(OH)24(H2O)12]7+ + 16H2O for fast base titration and 4[Al 3 (OH) 7 (H 2 O) 6 ] 2 + + Al(H 2 O)(OH) 3 → [AlO4Al12(OH)24(H2O)12]7+ + 16H2O + H+ for moderate base neutralization and natural condition.

Article

ASSOCIATED CONTENT

S Supporting Information *

The Supporting Information is available free of charge on the ACS Publications website at DOI: 10.1021/acsearthspacechem.9b00102. Static optimized configurations of monomeric Al3+ hydrolytic species; results for kinetic dehydration reactions of monomeric Al 3+ hydrolytic species; Cartesian coordinates of the static optimized configurations and dehydration reaction species (Å) (PDF)

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AUTHOR INFORMATION

Corresponding Author

4. CONCLUSION

*Phone: 011-86-025-86205840. Fax: +011-86-025-83317761. E-mail: [email protected].

In this work, the DFT-CM method is used to systematically investigate the 27Al NMR chemical shifts and relative stabilities of aqueous monomeric Al3+ hydrolytic species with different coordination structures. The results show: (1) The 27Al NMR chemical shifts δ of the monomeric Al3+ hydrolytic species fall in three regions: δ = 0−24 ppm for hexacoordinated configurations, δ = 35−57 ppm for pentacoordinated configurations, and δ = 69−82 ppm for tetracoordinated configurations. The δ values are determined by the Al innershell coordination numbers and are also influenced by the number of the OH− ligands. (2) In aqueous solution, the monomeric Al3+ hydrolysis starts from Al(H2O)63+, and the dominant forms of the first and second hydrolysis products are the hexacoordinated Al(H 2 O) 5 (OH) 2+ and trans-Al(H2O)4(OH)2+ configurations. The third hydrolysis product Al(OH)30 mainly exists in the form of the pentacoordinated fac- and mer-Al(H2O)2(OH)30 as well as the tetracoordinated Al(H 2 O)(OH) 3 0 , while the hexacoordinated fac-Al(H2O)3(OH)30 may also exist in small amounts. The fourth hydrolysis product Al(OH)4− mainly exists in the tetracoordinated configuration. (3) It is proposed that, under fast base titration conditions, the precursor for Keggin-Al13 formation may be the Al(OH)4− ion formed in local over alkaline regions, while under moderate conditions, the tetracoordinated Al(H2O)(OH)30 in the amorphous [Al(OH)3]n0(aq) sol can be considered as the precursor for Keggin-Al13 formation. This study clarifies the dominant existing forms and transformation processes of the monomeric hydrolytic species in the four stages of Al3+ hydrolysis reactions, and provides important reference for further study on the hydrolysis-polymerization and transformation mechanisms of aqueous Al species. As an extension of future research, the critical issues such as the proton and hydroxyl transfer mechanism in the Al3+ hydrolysis reactions,56−58 the interaction between the dehydration and deprotonation processes, and the effect of pH on the distributions of the Al3+ hydrolysis products can be explored on the basis of our calculation results. The method used in this study to determine the favorable dehydration reactions of the hydrolytic Al3+ species with appropriate solute−solvent interactions (named as the “one-by-one” method)59,60 is expected to combine with molecular dynamics simulation methods in future studies.61 Thus, the dehydration and proton transfer pathways in the Al3+ hydrolysis reaction systems can be comprehensively investigated with appropriate and representative solvent structures.

ORCID

Shuping Bi: 0000-0003-4437-7769 Notes

The authors declare no competing financial interest.

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ACKNOWLEDGMENTS This project is supported by the National Natural Science Foundation of China (No. 21177054). We are grateful to the High Performance Computing Center of Nanjing University for doing the numerical calculations in this paper on its Blade cluster system.

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REFERENCES

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