Identification of Intermediates during the Hydration of Na8[AlSiO4]6

Article pubs.acs.org/JPCA

Cite This: J. Phys. Chem. A XXXX, XXX, XXX−XXX

Identification of Intermediates during the Hydration of Na8[AlSiO4]6(BH4)2: A Combined Theoretical and Experimental Approach Alexander G. Schneider,† Lars Schomborg,‡ Claus H. Rüscher,‡ and Thomas Bredow*,† †

Mulliken Center for Theoretical Chemistry, Institut für Physikalische und Theoretische Chemie, Rheinische Friedrich-Wilhelms-Universität Bonn, Beringstrasse 4-6, D-53115 Bonn, Germany ‡ Institut für Mineralogie, Leibniz Universität Hannover, Callinstrasse 3, D-30167 Hannover, Germany S Supporting Information *

ABSTRACT: Tetrahydroborate sodalites have been discussed as possible materials for reversible hydrogen storage. In order to access the suitability of Na8[AlSiO4]6(BH4)2, its reaction with water was investigated theoretically and experimentally. Density functional theory (DFT) calculations at the generalized gradient approximation (GGA) level were performed to identify the reaction intermediates. We compared experimental IR spectra and 11B NMR chemical shifts with theoretical results for selected molecules in the sodalite cage. Furthermore, the free energies of reaction of the intermediates with respect to Na8[AlSiO4]6(BH4)2, gaseous water, and molecular hydrogen at different temperatures were also calculated.

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Na8[AlSiO4]6(BH4)2 (BH4−SOD, cf. Figure S1) was suggested for a stepwise reaction of hydrogen release.11 The capability of the sodalite structure for incorporating ions of different sizes can be explained by its large flexibility through the tilt mechanism and the tetragonal distortion of the Si/AlO4 tetrahedra.12,13 BH4−SOD was synthesized by Buhl et al. by a low-temperature hydrothermal method. Structurally Cl− is substituted by BH4− in Na8[AlSiO4]6Cl2.11,14 The reaction of BH4−SOD with water yields molecular hydrogen and boron− oxygen species (cf. eq 1).15,16 At higher temperatures a dehydration process has been observed (cf. eq 2).11 The species shown in eqs 1 and 2 and in Figure 1 were suggested by Buhl et al. both for the thermal reaction of BH4−SOD with water and for the dehydration of Na8[AlSiO4]6(B(OH)4)2 (B(OH)4− SOD).11,15

INTRODUCTION Hydrogen storage has become an important field in research, since the conventional energy sources are limited and are environmentally not benign. Hydrogen provides a large amount of energy per mass of 142 MJ/kg.1 Another advantage of hydrogen-based energy technology is that hydrogen and energy production can be physically and timely separated, since hydrogen can be stored and transported to power plants. Various methods of hydrogen storage have been suggested: compression, liquefaction, physisorption, complex hydrides, and metallic hydrides. The manageable tanks and volumes for compression and liquefaction of hydrogen are however limited. For the complex hydrides and metallic hydrides the dynamics of the hydrogen-releasing process is a problem.2 One of the most prominent complex hydrides employed for hydrogen storage is NaBH4, which forms NaB(OH)4(aq) and molecular hydrogen in a fast reaction with water.3,4 This reaction is however not reversible, and the byproducts must be extracted from the fuel mixture to be regenerated.5 Therefore, the development of other potent hydrogen storage materials is of particular interest for future power economy. Among other compounds zeolites have come into focus of research during the past two decades. For example, the absorption of hydrogen in several kinds of zeolites was investigated by Li et al.,6 Rosi et al.,7 and Yang et al.8 To investigate the mechanism of hydrogen release and thereby the possible improvement of reloadability of NaBH4 within microporous solids, we consider hydrogen-containing sodalites (SODs) as a model system. These SODs are possibly suitable for reversibly storing and releasing hydrogen and have a simple crystal structure.6,9,10 In particular, © XXXX American Chemical Society

BH4 − + H 2O → H3BOH− + H 2 H3BOH− + H 2O → H 2B(OH)2− + H 2 H3BOH− → H 2BO− + H 2 (side product) H 2B(OH)2− + H 2O → HB(OH)3− + H 2 HB(OH)3− + H 2O → B(OH)4 − + H 2

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B(OH)4 − → OB(OH)2− + H 2O OB(OH)2− → BO2− + H 2O

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Received: January 26, 2018 Revised: February 22, 2018 Published: March 8, 2018 A

DOI: 10.1021/acs.jpca.8b00898 J. Phys. Chem. A XXXX, XXX, XXX−XXX

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The Journal of Physical Chemistry A

twenty eight scans were accumulated at a MAS rotation frequency of 12.5 kHz. Tetramethylsilane was used as reference standard for 1H. 11B-MAS NMR measurements were carried out at 128.38 MHz using a second Bruker standard 4 mm MAS probe which had a ceramic stator which is boron free with sample spinning at 12.5 kHz. Chemical shifts are given relative to BF3·Et2O. For the 11B-MAS NMR experiments, a short single pulse duration of 0.6 μs has been used to ensure homogeneous excitation of the central and all satellite transitions. A recycle delay of 1 s has been used, and 6000 scans have been accumulated. Measuring parameters are given in Table 1. Table 1. Measuring Parameters of MAS-NMR Figure 1. Reaction intermediates of BH4−SOD hydration and B(OH)4−SOD dehydration reactions as suggested by Buhl et al.:11,15 boron, pink; hydrogen, white; oxygen, red.

parameter rotor speed (vrot) no. of scans accumulated (NS) recycle delay (D1) pulse duration (P1)

It is reasonable to assume that B(OH)4−SOD is the final product of BH4−SOD hydration. For a better understanding of the underlying reaction steps it seems however necessary to further investigate the reaction intermediates, which could be identified spectroscopically. Therefore, in the present work we compare experimental IR spectra and 11B NMR chemical shifts with calculated results obtained at DFT level for selected intercalated species. The starting material BH4−SOD was synthesized as described in our previous study.17 In the present study the same theoretical setup was used that has been recently applied to BH4−SOD17 and B(OH)4−SOD.18 The crystalline-orbital program CRYSTAL1419,20 was used for optimizations and frequency calculations. The thermodynamical stability of each hypothetical intermediate is checked on the basis of calculated free energies of reaction at different temperatures with respect to BH4−SOD, gaseous hydrogen, and water. Additionally, the Vienna ab initio simulation package (VASP version 5.3.321−25) was used for chemical shift calculations. The paper is organized as follows: in the next sections the details of the experimental and computational setups are described. In the following section calculated vibration spectra and chemical shifts for selected species are compared to the experimental data. On the basis of this comparison a modified reaction scheme is proposed. Finally, the results are summarized and an outlook is provided.

1

H-MAS NMR

11

B-MAS NMR

12.5 kHz 128 10 s 4.0 μs

12.5 kHz 6000 1s 0.6 μs

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COMPUTATIONAL DETAILS Basic Details. The same accuracy parameters, functional and basis sets for geometry optimization, and frequency calculations were used as in our previous study of B(OH)4− SOD,18 the PWGGA26 functional and CRYSTAL standard basis sets (cf. Table 2). The overlap and penetration threshold Table 2. CRYSTAL Standard Basis Sets Used for the Calculations atom

basis set

Na B O H Al Si

8-511G28 6-21G*29 6-31d130 5-11G*31 85-11G*32 86-311G**33

for Coulomb integrals, overlap threshold for HF exchange integrals, and pseudo-overlap were set to 10−9 and 10−18, respectively. A Monkhorst−Pack net, the k-point mesh, which defines the grid density for numerical integration in reciprocal lattice, of 8 × 8 × 8 was employed. The Anderson method was used for accelerating SCF convergence.27 For the Kohn−Sham matrix mixing a weighting factor (α) of 0.5 was used. All plots were made with gnuplot,51 and all calculated structure drawings were obtained with Jmol.38 Geometry Optimization. For all structure optimizations the experimental atomic positions of silicon, aluminum, and cage-forming oxygen of B(OH)4−SOD at 350 K (P43̅ n with a = 9.0534 Å)18,34 were used as a starting point. In order to study the spectroscopic properties of possible intermediates, one or several B(OH)4− groups were replaced by a molecule M, denoted as M−SOD in the following. The primitive unit cell (PUC) of B(OH)4−SOD was employed for most calculations. It contains two B(OH)4− species and two cages, respectively. The positions of the enclosed molecules were manually set. Standard values were assumed for initial bond distances and angles. B−O distances of about 1.5 Å and O−H distances of about 0.9 Å were set. O−B−O angles of about 120° for trigonal, about 109° for tetrahedral, and about 180° for 2-fold

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EXPERIMENTAL DETAILS The FTIR measurements were carried out using a Bruker Vertex 80v spectrometer. The samples were analyzed in the mid-infrared (MIR) range between 370 and 5000 cm−1 with a resolution of 2 cm−1 and 32 scans using the KBr method. Typically about 1 mg of the sample was diluted in 199 mg of potassium bromide to form a solid pellet. All samples were annealed in a water loaded with flowing N2 for 2 h at the corresponding temperature. Details of the setup have been described in our previous studies.17,18 All NMR measurements were carried out using a superconducting Bruker ASX 400 WB FT-NMR spectrometer with a standard Dewar configuration in the absence of proton decoupling at room temperature. The 1H-MAS NMR spectra were obtained at 400.13 MHz using a standard Bruker 4 mm MAS probe with a boron nitride stator. Typical conditions were pulse lengths of 4.0 μs and a 10 s recycle delay. One hundred B


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Table 3. Measured 11B NMR Chemical Shifts during the Hydration of BH4−SOD at Different Temperatures (in ppm) and Integrated Intensities (in %) of the Total Area in Parentheses

coordinated boron atoms were used. We used several starting orientations and structures in the geometry optimizations to identify the most stable one. To maintain the cubic lattice as it is observed experimentally,13 the optimization of the lattice constants was performed under symmetry restrictions. An an exemple, the optimized atomic positions and lattice constant of OB(OH)2− and BO2−SOD are given in Tables S2 and S3 in the Supporting Information. Frequency Calculations. Frequency calculations were performed for the optimized structures. The intensities of the IR signals were calculated with a coupled perturbed Kohn− Sham (CPKS) approach.35−37 Through visualization with the program Jmol38 the modes could be related to different types. In the IR spectra shown below all modes are given, but they are only labeled if their intensity is larger than 50 km/mol. In all spectra the highest calculated intensity was normalized to the experimental mode of BH4−SOD with the highest relative absorbance. The standard free energy of reaction (ΔRG°T) with respect to BH4−SOD, H2, and H2O was calculated as ΔR GT ° =

∑ vAGT ° A

chemical shift [ppm] (integrated intensities [%]) signal

293 K

a b c d e

523 K

573 K

17.0 (0.1) 16.6 (2.9)

17.0 (0.9) 16.6 (4.5)

1.8 (0.1) −0.4 (0.3)

1.8 (0.8) −0.4 (0.7)

673 K 17.0 16.4 11.0 1.8 −0.5

(4.1) (9.2) (2.0) (3.5) (2.2)

The calculated 11B NMR chemical shifts of the intermediates suggested by Buhl et al.11,15 (cf. eqs 1 and 2) are given in Table 4. Table 4. Calculated 11B NMR Chemical Shifts of the Different Suggested Intermediates11,15 of the Hydration of BH4−SOD (in ppm)

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where GT° denotes the standard free energy at temperature T of a compound and vA are the stoichiometric coefficients, with negative signs for reactants and positive signs for products (cf. eqs 1 and 2). Details of the calculation of GT° are given in the Supporting Information. No supercells were employed in the frequency calculations due to the large computational cost. It is assumed that the unit cell is large enough to cover the most important phonons for G°T. Chemical Shift. The chemical shifts of the M−SODs were referenced to BF3O(Et)2 as in the experiments. The 11B NMR chemical shifts of the CRYSTAL14-optimized structures were calculated with VASP using the PAW (projector augmented wave) method39,40 and the PBE41 functional. The computational setup was the same as in our previous study of B(OH)4−SOD.18 The number of bands (NBANDS) was set to 144 or 168, respectively (cf. Table S1 in Supporting Information), for the cells and to 40 for BF3O(Et)2. A Monkhorst−Pack grid of 2 × 2 × 2 and a well-converged cutoff energy of 700 eV were used. For all elements standard PAW pseudopotentials were employed, but for the element boron a harder PAW pseudopotential (B_h) was used.39,40 The 1 H NMR chemical shifts were referenced to BH4−SOD.

M

chemical shift [ppm]

BH4− H3BOH H2B(OH)2 H2BO HB(OH)3 B(OH)4 OB(OH)2 BO2

−59.3 −20.9 −4.5 39.9 −2.0 1.218 16.4 8.6

Since it has been shown by Sethio et al.42 that calculated chemical shifts for pure boron hydride molecules agree within 10 ppm to the experiments and by de Wijs et al.43 that average deviations of chemical shifts obtained with DFT methods from experiment may be up to 11 ppm, we tolerate this maximum deviation in all following assignments. By comparing Tables 3 and 4 it can be seen that the 11B NMR chemical shifts of H3BOH− and H2BO−SOD are not in the region of any experimental signal, which means that these species are not formed during the hydration process or their amount is so low that it is not detectable. Therefore, these compounds could be excluded as stable intermediates and are not further discussed. The calculated chemical shifts for H2B(OH)2− and HB(OH)3−SOD fit to signal e. The previously calculated chemical shift of B(OH)4−SOD18 fits to signal d. The OB(OH)2−SOD is in the region of the two signals a and b. Since OB(OH)2− gives rise to only one signal, only one of the experimentally observed signals a and b could be assigned to OB(OH)2−SOD and one is undetermined. The calculated chemical shift of 8.6 ppm for BO2−SOD fits to signal c at 11 ppm. At this point, based on the calculated 11B NMR chemical shifts and the increasing integrated intensities of the dehydration intermediates with increasing temperature, we could confirm the following intermediates

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RESULTS AND DISCUSSION Simple Dehydration Intermediates. In the experimental 11 B NMR spectra it is observed that annealing at 523, 573, and 673 K reveals significant signal intensities between 25 and −10 ppm (cf. Figures S2 and S3 in Supporting Information). The chemical shifts obtained by spectral analysis, signals a−e, are given in Table 3. Only signal b was fitted by a quadrupolar signal shape. For clarity, we do not consider the BH4−SOD signal around −50 ppm and the satellite signals at about 50 and −150 ppm. The signals a, b, d, and e appear already at 523 K and change by less than 0.2 ppm with increasing temperature, whereas the integrated intensities of the signals increase. Signal c first occurs at 673 K, and the signal b has the largest integrated intensity at this temperature, which is a hint for a dehydration process. C


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stretching mode of BH4−, which has been anharmonically corrected in the calculations.17,45 One can see from Figure 2 that the intensities of BH4− modes and νs,as(H2O) decrease with increasing temperature, while the intensities of signals B, C, and F increase. Signal F is hidden by the broad water peak. The signals A, D, and E first occur at 673 K. Signal B is split into two peaks at 1290 and 1325 cm−1. Signal E is also a doublet, with an intensity relation of 4:1 (cf. Figure S5 in Supporting Information). Signals C and D are close together and only differ by 18 cm−1 (cf. Table 5), while D is split into four peaks (cf. Figure S6 in Supporting Information). Signal C is difficult to assign due to superimposition with CO32− vibrations (measured at 1440 cm−1) and the possible formation of B2O346 or NaBO247 outside the sodalite cages. The calculated frequencies of the suggested intermediates in eq 4 are given in Table 6. It has to be noted that the isotope

BH4 − + 2H 2O → H 2B(OH)2− (e) + 2H 2 H 2B(OH)2− + H 2O → HB(OH)3− (e) + H 2 HB(OH)3− + H 2O → B(OH)4 − (d) + H 2 B(OH)4 − → OB(OH)2− (a , b) + H 2O OB(OH)2− → BO2− (c) + H 2O

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Since it has been shown by Neugebauer et al.44 that the average deviations of vibration frequencies calculated at the GGA DFT level are up to 27 cm−1, we tolerate this maximum deviation in all of the following assignments. The experimental IR spectra of BH4−SOD at room temperature (black) and after annealing at different temperatures in wet N2 atmosphere are shown in Figure S4 in the Supporting Information. The same stepwise reaction as described previously by Buhl et al. takes place.11,15 It also can be seen that the modes below 1100 cm−1 only slightly change during the reaction. These modes arise from the cage-forming atoms, which means that the cage does not react during the heating process but only the enclosed molecule. Thus, we treat only those modes which are larger than 1100 cm−1 in the IR spectra (cf. Figure 2 and Table 5).

Table 6. Calculated Vibration Frequencies (cm−1) of the Intermediates11,15 Suggested in Eq 4 of the Hydration of BH4−SODa wavenumber [cm−1]

M BH4 H2B(OH)2 HB(OH)3 B(OH)4 OB(OH)2 11 BO2 10 BO2

1109; 1145; 1116; 1187; 1192; 1996 2069

2218; 1238; 1165; 1191; 1559;

2278; 2398 1296; 2104; 2225; 3648−3675 1226; 2176; 3625−3638 3655−3694 1563; 3501−3673

a

No overtones and combination modes are given for intermediates. With anharmonically corrected ν(B−H).

effect for boron is not detectable in the experimental spectra due to the resolution of the spectra measured at room temperature, which results in broader bands as obtained for B(OH)4−SOD.18 However, for the antisymmetric stretching mode of BO2−SOD (see below) this effect is directly observed for the isolated BO2− (cf. Table 6) as reported by Morgan and Staats.48 One can see from Tables 5 and 6 and Figure S7 in Supporting Information that the higher lying calculated stretching modes ν(B−H) of H2B(OH)2−SOD at 2225 cm−1 would be covered by the overtone 2ν4 of BH4−. However, the lower lying stretching mode of H2B(OH)2−SOD (2104 cm−1) differs by 135 cm−1 from 2ν4, which is the only signal in this region and should therefore be visible. The same result is obtained for the stretching mode of HB(OH)3−SOD at 2176 cm−1 (cf. Figure S8 in Supporting Information), which differs by 63 cm−1 from 2ν4 and should be detectable in the experimental spectrum. Thus, we conclude that neither H2B(OH)2−SOD nor HB(OH)3−SOD is formed during the hydration of BH4−SOD or only in a very small amount, consistent with the low integrated intensities of the NMR signals. Figures 3 and S9 in the Supporting Information and Tables 5 and 6 show that the O−H stretching modes could be assigned to signal F of annealed BH4−SOD as mentioned in our previous study.18 Possibly due to the small amount of B(OH)4− formed at the beginning of the reaction (as concluded by the integrated intensities of the 11B NMR spectra, cf. Table 3), the calculated deformation modes (δ(B(OH)4)) at 1187 and 1191 cm−1 are not detectable at lower temperatures and are covered by signal A at 673 K.

Figure 2. Measured IR spectra (relative absorbance) of BH4−SOD (black) and annealed BH4−SOD in wet N2 atmosphere (red, 523 K; green, 573 K; blue, 673 K).

Table 5. Wavenumbers of Labeled Signals in the Experimental IR Spectra of BH4−SOD and Annealed BH4− SOD (in cm−1) signal

wavenumber [cm−1]

ν4 A B C D δ(H2O) E 2ν4 ν3 ν2 + ν4 νs,as(H2O) F

1134 1225 1290/1325 1447 1465/1493/1511/1556 ∼1630−1782 1963/2036 2239 2287 2387 ∼3000−3700 ∼3500−3700

As discussed in our previous study,17 ν4 is a bending mode, 2ν4 its overtone, ν2 + ν4 is a combination mode of ν4 and the Raman active bending mode ν2, and ν3 is the antisymmetric D


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Figure 3. Comparison of measured IR signals of annealed BH4−SOD (black) and calculated IR signals of M−SODs (red).

Figure 4. Suggested enclosed molecules: boron, pink; hydrogen, white; oxygen, red.

The calculated O−H stretching modes of OB(OH)2−SOD could be assigned to signal F (cf. Figure S10 and Tables 5 and 6). The calculated signals at about 1560 cm−1 are close to the highest shoulder of signal D. If the NMR signal a is assigned to OB(OH)2−SOD and signal d is assigned to B(OH)4−SOD (cf. Table 3), the signal intensities suggest that both intermediates are formed in similar amounts. This means that for OB(OH)2 the same conclusion could be made as for B(OH)4−SOD (vide supra): due to the small concentration at the early stages of the reaction the calculated deformation mode at 1192 cm−1 is not detectable and at 673 K it is covered by signal A. If alternatively the NMR signal b was assigned to OB(OH)2, its amount would be much larger and then the signals at 1192 cm−1 should be detectable. Since this is not the case, we are left with no assignment for signal b in the 11B NMR spectra (cf. Table 3) at the present stage. As mentioned above, the measured IR signal E consists of two peaks with an intensity ratio of about 4:1 (cf. Figure S5 in Supporting Information), which corresponds to the natural isotope distribution of 11B and 10B (cf. Figures 3 and S11 in Supporting Information). The frequencies calculated for BO2− SOD fit to signal E, if the isotope effect for boron is considered. Since no other signals were found in this region, we tolerate a deviation of 33 cm−1 in this case. Thus, we can assign every calculated signal only for BO2− SOD (cf. Figure 3). However, since these signals also appear in the dehydration of B(OH)4−SOD,16 the B(OH)4−SOD has also to be formed. Anhydrides. Since none of the monomer species discussed in the previous section could account for IR signals A, B, C, and D and for 11B NMR signal b, we investigated the possibility of formation of polyborates that can be formed starting from B(OH)4− in the sodalite cages. Due to the known equilibrium between B(OH)3 and B(OH)4− in solution and the observed formation of polyborates,49,50 we tested the M species shown in Figure 4. Here we focus on dimers, since it is very unlikely that larger polymers fit into the cage. The species are formally anhydrides (B2O7H62−, AH2−), dianhydrides (B2O6H42−, DAH2−), and trianhydrides (B2O5H22−, TAH2−) of B(OH)4− dimers. All calculated 11B NMR chemical shifts and IR frequencies of the intermediates shown in Figure 4 are given in Tables 7 and 8. In our models for B(OH)3, which could be formed directly before or after the formation of B(OH)4−, we placed a separated OH− within the same cage for charge balance. Since only a part of BH4− reacts, we occupied one cage with B(OH)3

Table 7. Calculated 11B NMR Chemical Shifts of Suggested Products of the Reaction of BH4−SOD (in ppm) M

chemical shift [ppm]

BH4/B(OH)3·OH AH DAH TAH

−59.0/14.7 0.7/0.9 0.2/15.9 14.1/17.2

and OH− and the other with BH4− (BH4/B(OH)3·OH−SOD). One can see from Tables 3 and 7 that the computed 11B NMR chemical shift of this compound (14.7 ppm) fits to signal b (16.4−16.6 ppm). In the IR spectra (cf. Figures 3 and S12 in Supporting Information) the calculated antisymmetric stretching (νas) and deformation modes (δ) at 1310 and 1458 cm−1 fit to the measured signals B (1290, 1325 cm−1) and C (1447 cm−1), respectively. Here the O−H stretching modes are hidden by the broad water peak and ν2 + ν4, respectively. Additionally, an isolated OH− group is confirmed by the calculated (−0.5 ppm) and measured (about −2.5 ppm) 1H NMR chemical shift relative to the BH4−SOD. The first step of the formation of a polyborate starting from B(OH)4− is an anhydride of the dimer (AH2−), which is shown in Figure 4. This dimer is located in one cage, while the other cage is empty (AH−SOD). Due to the different geometrical environment of the two boron atoms, two slightly different 11B NMR chemical shifts are observed (cf. Table 7). Since these two signals (0.7 and 0.9 ppm) differ only by 0.2 ppm we could assign them both to d (1.8 ppm) or e (−0.4, − 0.5 ppm). The IR spectrum (cf. Figures 3 and S13 in Supporting Information) shows deformation modes (δ(B(OH)3, δ(B2O(OH)6; 1201, 1244 cm−1) that fit to signal A (1225 cm−1) and two modes (δ(B2O(OH)6); 1295, 1335 cm−1) that fit to signal B (1290, 1325 cm−1). The O−H stretching modes are in the region of the broad water peak. Signal B could therefore be assigned to B(OH)3 and AH2−. Due to the small intensities of signals d, e, and A below 673 K (cf. Table 3), the assignment of AH−SOD as a possible intermediate is not certain. The 11B NMR chemical shifts calculated for the dianhydride DAH−SOD (0.2, 15.9 ppm) fit to signal b (16.4, 16.6 ppm) for the 3-fold-coordinated boron and to e (−0.4, 0.5 ppm) for the 4-fold-coordinated boron (cf. Tables 3 and 7). The calculated IR signal at 1544 cm−1 (cf. Figures 3 and S14 in Supporting Information) fits to the shoulder of signal D (1556 cm−1), and the deformation mode at around 1210 cm−1 fits to signal A (1225 cm−1). The calculated ν(OH) are within the broad E


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The Journal of Physical Chemistry A Table 8. Calculated Vibration Frequencies of the Intermediates Shown in Figure 4 (in cm−1)a

a

M

wavenumber [cm−1]

BH4/B(OH)3·OH AH DAH TAH

1107; 1123; 1310; 1458; 2258−2276; 2385; 3306−3651 1141−1201; 1244; 1295; 1335; 3335−3584 1112; 1145; 1188; 1209; 1544; 3435−3596 1123; 1128; 1187; 1241; 1490; 1543; 3349−3425

ν3 is anharmonically corrected.

region of measured νs,as(H2O). For these reasons we conclude that the dianhydride is formed during the dehydration of B(OH)4−SOD. The next dehydration step is the formation of an trianhydride (TAH) (TAH−SOD; cf. Figure 4). One can see from Tables 3 and 7 that the calculated 11B NMR chemical shifts (14.1, 17.2 ppm) fit to the measured signals a (17.0) and b (16.6, 16.4 ppm). From Figures 3 and S15 in the Supporting Information one can see that the calculated IR frequencies (1241, 1490, 1543 cm−1) fit to signals A (1225 cm−1) and D (1465, 1493, 1511, 1556 cm−1), respectively. The appearance of signal b could therefore be explained by two compounds: BH4/ B(OH)3·OH−SOD and TAH−SOD. If TAH−SOD is further dehydrated the already confirmed BO2−SOD is formed. It is also noteworthy that the relative intensity of 11B NMR signals d and e is inverted from 523 to 573 K (cf. Table 3). This confirms our interpretation that the formation of AH−SOD or DAH−SOD is following after B(OH)4−SOD formation. Free Energies of Reaction. In Figure 5 it is shown that the suggested stepwise pathway of the dehydration is compatible

overcome an activation barrier, so that the condensation reaction is slowed down and the other two SODs becomes detectable. Between 500 and 600 K the DAH−SOD and above 600 K BO2−SOD are most stable. The TAH−SOD is less stable than the DAH−SOD below 650 K and less stable than BO2−SOD above 550 K. Thus, the formation of TAH2− is thermodynamically not favored. It is rather a metastable intermediate in the dehydration of DAH2− to 2 BO2−. On the basis of the spectroscopic and thermodynamic results we give the following suggestion for the reaction sequence of BH4−SOD hydration (eq 5) BH4 − + 4H 2O → B(OH)3 (b , B , C , F ) + OH− + 4H 2 B(OH)3 + OH− → B(OH)−4 (d , F ) 2B(OH)4 − → AH2 − (d , e , A , B) + H 2O AH2 − → DAH2 − (b , e , A , D , F ) + H 2O DAH2 − → TAH2 − (a , b , A , D , F ) + H 2O TAH2 − → 2BO−2 (c , E) + H 2O (5)

The optimized atomic positions and lattice constants of all compounds are given in Tables S4−S8 in the Supporting Information.

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CONCLUSION AND OUTLOOK

The temperature-dependent reaction of Na8[AlSiO4]6(BH4)2 with water was studied theoretically and experimentally. Possible intermediates were identified by calculations of IR spectra and 11B NMR chemical shifts and comparison to measured spectra. It was possible to assign all measured signals to chemical species. The previously suggested BHn(OH)4−n− species with n = 1−3 could be excluded. The calculated free energy of reaction at different temperatures was used to determine the energetic sequence of reaction products. The first step is a hydration of BH4− leading to the formation of B(OH)4−, which is in an equilibrium with neutral B(OH)3 and OH−. At higher temperatures B−O species are assumed to migrate from one cage to another, and dimer formation of B(OH)4− species takes place within one cage of the sodalite. With further increasing temperature this dimer releases water and forms anhydrides (AH). The final product at 673 K is an isolated BO2− in each cage. All reaction steps are exergonic with respect to Na8[AlSiO4]6(BH4)2. The thermodynamically stable species are AH2− below 500 K, DAH2− between 500 and 600 K, and BO2− above 600 K. All other species that could be identified spectroscopically are metastable. Reactions leading to more stable species involve the migration of B−O species from another cage, most probably through open six-membered rings in the sodalite cage due to Na defects. In further studies the migration of the molecules

Figure 5. Calculated standard free energies of reaction at different temperatures for suggested SODs with respect to BH4−SOD, gaseous water, and hydrogen.

with the calculated standard free energies of reaction. All free energies of reaction are given relative to the BH4−SOD as reactant. The least exergonic step is the formation of BH4/ B(OH)3·OH−SOD with an increase of the free energy of reaction with increasing temperature. The next exergonic step is the formation of B(OH)4−. In this case we occupied one cage with B(OH)4− and the other with BH4− as for BH4/B(OH)3· OH−SOD, since only a part of BH4− reacts. The most stable SOD below 500 K is the AH−SOD, so this compound should be formed during heating to 523 K and the BH4/B(OH)3·OH− SOD and BH4/B(OH)4−SOD should not be detectable in any spectra. However, since one B(OH)4− has to migrate into a neighboring cage for the formation of the anhydride, it must F


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from one cage to another will be investigated in order to obtain the corresponding activation barriers.

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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.jpca.8b00898. Experimental IR spectra and 11B NMR spectra of reaction of BH4−SOD; table of optimized atomic positions in fractional units of BH4/B(OH)3·OH−, BH4/B(OH)4−, OB(OH)2, AH−, DAH−, TAH−, and BO2−SOD; calculated IR spectra of H2B(OH)2−, HB(OH)3−SOD, BH4/B(OH)3·OH−, B(OH)4, OB(OH)2, AH−, DAH−, TAH−, and BO2−SOD (PDF)

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AUTHOR INFORMATION

Corresponding Author

*E-mail: [email protected]. Phone: +49 (0)228 733839. Fax: +49 (0)228 739064. ORCID

Alexander G. Schneider: 0000-0002-2950-171X Notes

The authors declare no competing financial interest.

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ACKNOWLEDGMENTS This work was supported by the Deutsche Forschungsgemeinschaft (DFG) within the project “Transport and reaction properties of new Boron-hydride-hydrate-oxide sodalites” (BR1768/8-1 and RU764/6-1). We thank M. Fechtelkord for the NMR measurements at the Institut fü r Geologie, Mineralogie und Geophysik of the Ruhr-Universität Bochum.

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REFERENCES

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Identification of Intermediates during the Hydration of Na8[AlSiO4]6

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