Article pubs.acs.org/IC
Structure and Properties of Sodium Enneaborate, Na2[B8O11(OH)4]·B(OH)3·2H2O Doinita Neiner,† Yulia V. Sevryugina,‡ Larry S. Harrower,† and David M. Schubert*,†,§ †
U.S. Borax Inc., Rio Tinto Borates, Greenwood Village, Colorado 80111, United States Department of Chemistry, Texas Christian University, Fort Worth, Texas 76129, United States
‡
S Supporting Information *
ABSTRACT: Millions of tons of sodium borates are used annually by global industries in diverse applications important to modern society. The Na2O−B2O3−H2O phase diagram in the 0−100 °C temperature range contains 13 unique hydrated crystalline sodium borates, including five important industrial products. Structures were previously reported for each of these except for that having the highest boron content, known as sodium enneaborate, Na4B18O29·11H2O or 2Na2O·9B2O3· 11H2O (1). Here we report the single-crystal structure of 1, revealing the structural formula Na2[B8O11(OH)4]·B(OH)3· 2H2O, and describe some of its properties and relationships to other sodium borates. The structure of 1 features linear polyborate chains composed of the repeating [B8O11(OH)4]2− fundamental building blocks with interstitial water and boric acid molecules integrated by extensive H bonding. Interstitial sodium cations occur in groups of four with interatomic distances of 3.7830(6) and 3.7932(8) Å. Upon heating, 1 initially becomes amorphous and then crystallizes as α-Na2B8O13 along with amorphous B2O3. Notably, α-Na2B8O13 contains octaborate fundamental building blocks that are topologically equivalent to those in 1. Compound 1 crystallizes in the monoclinic space group P21/n with a = 10.2130(8) Å, b = 12.940(1) Å, c = 12.457(1) Å, β = 93.070(2)°, V = 1644.0(2) Å3, and Z = 2.
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INTRODUCTION Sodium borates comprise the most important family of boron compounds in terms of intensity of industrial use. Millions of tons of sodium borates are utilized annually by global industries in the manufacture of numerous goods important to modern society, including glasses, ceramics, agricultural products, and energy-saving and durable building materials. Sodium tetraborate pentahydrate, Na2B4O5(OH)4·3H2O, often called borax pentahydrate and written as Na2B4O7·5H2O, is produced in the largest volume of all refined boron compounds, with annual production well exceeding one million tons. It is transported to markets around the world by trucks, railcars, and ocean-going bulk freight ships that carry tens of thousands of tons per voyage. Other industrial sodium borates produced on scales ranging from hundreds to tens of thousands of tons annually include the crystalline compounds Na2B4O5(OH)4·8H2O, also known as borax decahydrate (Na2B4O7·10H2O), sodium pentaborate, NaB5O6(OH)4·3H2O (NaB5O8·5H2O), and the sodium metaborates NaB(OH) 4 and NaB(OH) 4 ·2H 2 O (NaBO 2 ·2H 2 O and NaBO 2 ·4H 2 O). Anhydrous borax, Na2B4O5, and disodium octaborate tetrahydrate, Na2B8O13· 4H2O, are industrially important sodium borates that are noncrystalline as articles of commerce.1,2 Sodium borates can be formulated as oxides of composition aNa2O·bB2O3·cH2O, where the value Q, defined as the molar ratio b/a, relates to many properties relevant to their applications. This ratio also roughly correlates with theoretical © 2017 American Chemical Society
B2O3 content, a measure of the commercial value of industrial borates. Several studies of the phase relationships and solubility isotherms in the Na2O−B2O3−H2O system have been published.3,4 Studies published prior to 1967 were often inaccurate because the long equilibration times needed to establish some phase boundaries were not accounted for. The most accepted version of this phase diagram contains 13 unique crystalline sodium borates in the 0−100 °C temperature range, given in Table 1.3 Other hydrated sodium borates obtained under hydrothermal and high-pressure conditions have been reported, but such conditions are generally not practical for large-scale industrial production. Known as sodium enneaborate, 1 has the semiempirical formula Na4B18O29·11H2O, which is often written in resolved oxide form as 2Na2O·9B2O3·11H2O (Q = 4.5). It contains 20.5 wt % B, the highest boron content of any sodium borate occurring in the 0−100 °C region of the Na2O−B2O3−H2O system. It was discovered a half century ago by Nelson P. Nies of the Rio Tinto Borates company during his extensive studies of this system.3 It occurs in a relatively small crystallization field in a composition region of roughly 27−33% B2O3 and 4−9% Na2O. This field was confirmed and further defined in later studies of this system, including that of Kocher.4 Except for a minor hydrate of disodium monoborate, 1 is the only sodium Received: March 30, 2017 Published: May 26, 2017 7175
DOI: 10.1021/acs.inorgchem.7b00823 Inorg. Chem. 2017, 56, 7175−7181
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Inorganic Chemistry Table 1. Crystalline Sodium Borates Found in the 0−100 °C Na2O−B2O3−H2O System oxide formula
Qa
B2O3 (wt %)
common name
structural formula
2Na2O·B2O3·5H2O 2Na2O·B2O3·H2O Na2O·B2O3·8H2Oc Na2O·B2O3·4H2Oc 3Na2O·3B2O3·2H2O Na2O·2B2O3·10H2Oc Na2O·2B2O3·5H2Oc Na2O·2B2O3·4H2O 2Na2O·5B2O3·7H2O 2Na2O·5B2O3·5H2O Na2O·3B2O3·4H2O 2Na2O·9B2O3·11H2O (1) Na2O·5B2O3·10H2Oc
0.5 0.5 1.0 1.0 1.0 2.0 2.0 2.0 2.5 2.5 3.0 4.5 5.0
24.54 32.90 25.25 34.18 48.48 36.51 47.80 50.95 58.19 61.92 60.91 66.05 58.98
disodium monoborate 2·1·5 disodium monoborate 2·1·1 sodium metaborate 8-mold sodium metaborate 4-mold trisodium triborate 3·3·2b borax decahydrate (tincal) borax pentahydrate (tincalconite) synthetic kernite Suhr’s borate (ezcurrite) Auger’s borate (nasinite) synthetic ameghinite sodium enneaborate sodium pentaborate (sborgite)
Na2BO2OH·2H2O4 Na2BO2OH5 Na[B(OH)4]·2H2O6 Na[B(OH)4]7 Na3[B3O3(OH)2]b,,8 Na2[B4O5(OH)4]·8H2O9−11 Na2[B4O5(OH)4]·3H2O12,13 Na2[B4O6(OH)2]·3H2O14 Na2[B5O8(OH)3]·2H2O15 Na2[B5O8(OH)]·2H2O16 Na[B3O3(OH)4]17 Na2[B8O11(OH)4]·B(OH)3·2H2O Na[B5O6(OH)4]·3H2O18
Q is the molar ratio b/a for the oxide formula aNa2O·bB2O3·cH2O. bReferred to in earlier literature as Na2O·B2O3·H2O. cCommercial industrial borate. dCommon commercial name based on oxide formula. a
supernatant liquid. The liquid fraction was decanted from 1 and produced crystals of sodium pentaborate upon cooling. The remaining crystals of 1 were washed with a small amount of water to remove adhering liquor. Acid−base titration confirmed the oxide composition of the resulting product as 13% Na2O, 66% B2O3, and 21% H2O by difference (2Na2O·9B2O3·11H2O). Crystallography. Crystallographic data were collected using a Bruker diffractometer equipped with a Photon 100 CMOS detector and a Mo-target X-ray tube (λ = 0.71073 Å). A colorless block crystal of 1 having dimensions 0.14 × 0.17 × 0.35 mm was used for X-ray data collection with a scan width of 0.5° and exposure time of 10 s/frame. Data were integrated, scaled, and evaluated for space group determination using the Bruker APEX2 suite 20 (integration, SAINT;21 multiscan scaling, SADABS22) and XPREP. Starting models were generated using SHELXT.23 Final least-squares refinement was carried out in SHELXL2014.24 All non-hydrogen atoms were refined with anisotropic thermal parameters. All hydrogen atoms were located in difference Fourier maps and refined individually.
borate found in the nonhydrothermal temperature range that was not previously structurally characterized. Borate compounds contain trigonal BO3 and tetrahedral BO4 units that may link by corner sharing to form rings, cages, chains, sheets, and networks that arrange in a multitude of ways, providing a rich structural chemistry that frequently features six-membered B3O3 rings as a common structural motif.19 The compositional and structural diversity of borates provides the basis for their wide range of industrially useful properties emerging from an interplay between electrondeficient three-coordinate boron and electronically satisfied four-coordinate boron. Here we describe the structure of 1, which illustrates these principles, along with its thermal and spectroscopic properties.
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EXPERIMENTAL SECTION
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Materials and Methods. Boric acid, borax pentahydrate, and sodium pentaborate were products of U.S. Borax Inc., Rio Tinto Borates. Thermal analysis was done using a TA Instruments SDT 600Q TGA/DSC analyzer under flowing nitrogen at a scan rate of 5 °C/min. Infrared spectra were collected with a Bruker Alpha FT-IR spectrometer equipped with a single-reflection diamond ATR module. SEM micrographs were captured using a JEOL JEM-2500SE scanning electron microscope operated at 200 kV at the Colorado School of Mines microscopy user facility using powder samples affixed to carbon tape mounted on an Al stub. Powder X-ray diffraction data were collected on a Phillips X’Pert diffractometer equipped with a graphite monochromator using Cu Kα radiation (λ = 1.5418 Å). Solid-state 11B NMR data were collected on a Varian 500 MHz spectrometer at the Pacific Northwest National Laboratory in Richland, WA, using a 5 mm ZrO rotor with a spin rate of 20 kHz with 1H decoupling and chemical shifts referenced to BF3·OEt2 (δ 0 ppm) at 20 °C. Bulk Synthesis of 1. In a typical experiment, boric acid (618.6 g, 10.0 mol) was added to a slurry of borax pentahydrate (581.3 g, 2.0 mol) in 650 mL of water at 95 °C along with 25 g of 1 as seed. The mixture was maintained with stirring for 5 h. The resulting viscous slurry was filtered while hot. A preheated Büchner funnel was used in this step to keep the slurry hot until it was substantially dewatered, as cooling results in solidification to a solid block owing to crystallization of sodium pentaborate from adhering liquor. Preparation of Single Crystals of 1 for Crystallography. Single crystals of 1 suitable for crystallographic work were prepared by the solid-state reaction of boric acid, sodium pentaborate, and sodium hydroxide. A mixture of boric acid (15.39 g, 0.25 mol), NaOH (3.32 g, 0.083 mol), and sodium pentaborate (73.48 g, 0.25 mol) was placed in a glass pressure tube with ca. 1 mL of H2O and maintained at 110 °C for 3 days. Crystals of 1 formed at the bottom of the tube below a clear
RESULTS AND DISCUSSION Synthetic Aspects. Sodium enneaborate (1) is readily prepared under nonhydrothermal conditions by crystallization from solutions of appropriate stoichiometric compositions. Prerequisite for the formation of 1 is high equivalent Na2O and B2O3 solution concentrations only achieved at somewhat elevated temperatures. Nies et al. showed that 1 crystallizes at temperatures as low as 75 °C along with minor amounts of borax pentahydrate and forms as a pure phase at 95 °C.3 The required components can be provided using combinations of boric acid, boric oxide, sodium borates, and sodium hydroxide in appropriate equivalent B2O3/Na2O molar ratios. Reaction times and yields depend on the amount of seed added, with shorter reaction times ensuing from increased seed addition. Synthesis of 1 from concentrated aqueous slurries made from mixtures of sodium hydroxide and boric acid, as given by eq 1, is a suitable method. Since borax pentahydrate and boric acid are the most abundant and economical of the industrial borates, eq 2 also represents an expedient route to 1. 4NaOH + 18B(OH)3 → Na4B18O29 · 11H 2O + 7H 2O (1)
2Na 2B4O7 ·5H 2O + 10B(OH)3 → Na4B18O29 ·11H 2O + 14H 2O 7176
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DOI: 10.1021/acs.inorgchem.7b00823 Inorg. Chem. 2017, 56, 7175−7181
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Inorganic Chemistry Compound 1 also crystallizes from concentrated solutions of sodium pentaborate held for an extended time near the boiling point, especially if a small amount of 1 is added as seed.3 In addition, 1 is known to crystallize from concentrated solutions of the industrial borate disodium octaborate tetrahydrate, Na2B8O13·4H2O, when they are maintained at elevated temperatures for prolonged periods, especially if seed is present. Under nonhydrothermal conditions 1 typically forms as a microcrystalline powder. In addition to preparing bulk samples of microcrystalline 1 at temperatures below 100 °C, we employed mild hydrothermal conditions of 110 °C and autogenous pressure to accelerate formation of single crystals suitable for X-ray crystallography. An SEM micrograph of crystals of 1 formed under these conditions is shown in Figure 1.
Each B3O3 ring contains one tetrahedral and two trigonal boron atoms. Two of the rings (A and B) share a common tetrahedral boron atom to form a B5O6 unit, which is further linked to ring C by an exocyclic bridging oxygen atom. The average BΔ−O distances are 1.369(1) Å and average B□−O distances are 1.469(1) Å, and the B−O bonds in the exocyclic B−O−B linkage between rings B and C are 1.477(1) Å for B□−O and 1.370(1) Å for BΔ−O. These distances are consistent with typical boron−oxygen bonds around three- and four-coordinate boron centers.27 The B3O3 rings in borate compounds usually deviate from planarity resulting from ring strain. Ring A exhibits an average deviation of 0.112 Å from the mean plane defined by the six ring atoms. Rings B and C are more planar with average deviations from the mean planes of the six ring atoms of 0.041 and 0.037 Å, respectively. Deviations from planarity for all three rings are primarily a result of out-of-plane displacements of the tetrahedral boron atoms. The angle between the mean planes of B3O3 rings A and C is 19.2°, and the angle between the planes of rings B and C is 85.9°. Parallel polyborate chains disseminate in a zigzag manner along the crystallographic b direction and are interconnected by an extensive network of H bonding interactions supported by interstitial water and boric acid molecules (O−H···O distances and angles 1.82−2.57 Å and 159−177°) and coordinated sodium cations. The asymmetric unit of 1 is shown in Figure 3.
Figure 1. SEM micrograph of sodium enneaborate (1) crystals formed under mild hydrothermal conditions.
Structure Description. The structural formula of 1 was found to be Na2[B8O11(OH)4]·B(OH)3·2H2O, which is equivalent to the semiempirical formula Na4B18O29·11H2O and oxide formula 2Na2O·9B2O3·11H2O traditionally used to describe this compound. The structure of 1 features 1D polyborate chains based on repeating [B 8 O 11 (OH) 4 ] 2− octaborate fundamental building blocks (FBBs), as shown in Figure 2, with interstitial sodium cations and boric acid and water molecules. The octaborate FBB in 1 falls into the category of an inoborate having the Christ and Clark descriptor ∞ 1 { 5: ( 4Δ + T ) + 3 : ( 2Δ + T ) } a n d B u r n s d e s c r i p t o r 6Δ2□:⟨2Δ□⟩−⟨2Δ□⟩⟨2Δ□⟩, where each FBB contains six trigonal (Δ) and two tetrahedral (T or □) boron centers arranged into three B3O3 rings, labeled A−C in Figure 2.25,26
Figure 3. Asymmetric unit of 1.
To describe the hydrogen-bonding patterns in 1, three types of polyborate H bond acceptor oxygens are defined, as shown in Figure 4: Oα, oxygen bonded to 4-coordinate boron atoms; Oβ, hydroxyl oxygen; Oγ, oxygen lying between two threecoordinate boron atoms. One bonding pattern is a 1D chain of
Figure 2. Polyborate chain of 1 showing three repeating [B8O11(OH)4]2− FBBs.
Figure 4. Labeling scheme of H bond acceptor oxygen atoms in the polyborate FBB of 1. 7177
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Inorganic Chemistry H bonding and sodium coordination interactions running along the crystallographic c direction formed by alternating boric acid molecules, sodium cations, water molecules, and polyborate moieties (O(16)BA···Na(1)···OW−HW···O(9)α−B(4)−O(5)α···H(18)BA−O(18)BA−B(9)BA−O(16A)BA, where subscripts BA and W denote boric acid and water, respectively). In this assembly, boric acid and water molecules both donate H bonds to different Oα atoms of the pentaborate subunit with bond distances and angles of 1.82(2) Å and 178° for O(18)BA− H(18)BA···O5α and 2.07(2) Å and 159° for Ow−Hw···O(9)α. Additionally, H(17)BA forms an H bond (1.89(2) Å and 170°) with Oα bridging the pentaborate and triborate subunits. In the crystallographic a direction, boric acid and one of the water molecules are centers of H bonding networks. In addition to boric acid O(16) coordination to Na(1), all other oxygen and hydrogen atoms are involved in H bonding interactions with distances in the 1.82(2)−1.89(2) Å range and angles in the 165−178° range. An unusually long H bonding contact of 2.03(2) Å occurs between boric acid hydrogen and the polyborate O(13)γ acceptor with an O(16)−H(16)···O(13)γ angle of 170°. This H bonding interaction to polyborate Oγ acceptor is weak, as expected on the basis of the lower electron accepting character of this oxygen in comparison to Oα. While O(19) water oxygen coordinates to and bridges between Na(1) and Na(2), its attached hydrogen atoms link O(9)α of one polyborate chain and O(12)β of another chain at distances of 2.07(2) and 1.94(2) Å, respectively. The latter is the unique contact formed by the polyborate Oβ acceptor. Polyborate hydroxyl hydrogens also participate in H bonding with boric acid and water molecules at distances in the 1.86(2)−2.17(2) Å range. Notably, the nonmetal borates [H3NCH2CHCH3NH3][B8O11(OH)4]·H2O (2), [Cd(TREN){B8O11(OH)4}] (3; TREN = tris(2-aminoethyl)amine) and [Zn(en)2{B8O11(OH)4}] (4; en = ethylenediamine) also contain the [B8O11(OH)4]2− FBB but feature different polymerization modes.28,29 The 1D chains in 1 are composed of alternating triborate and pentaborate substructures, whereas 2 and 3 feature 1D chains composed only of triborate substructures with pentaborate substructures pendant to the chains. Compound 4 also contains an octaborate FBB composed of triborate and pentaborate substructures but exhibits chains having different connectivity than 1-3. The octaborate FBB in 1 is formally produced by condensation of triborate and pentaborate monoanions, which are abundant species in aqueous borate solutions in the intermediate pH region, as shown in Scheme 1.1,2 Further condensation of the presumed intermediate anion [B8O10(OH)6]2− leads to the extended polyborate chains in 1. We first described the insular [B8O10(OH)6]2− anion, shown in Figure 5a, in the nonmetal borate [H3N(CH2)7NH3][B8O10(OH)6]·2B(OH)3 (5), which readily crystallizes from aqueous solutions of 1,7-diaminoheptane with excess boric
Figure 5. (a) Insular [B8O10(OH)6]2− anion in 5, and (b) isomeric insular anion in 6.30,31
acid.30 An isomer of the [B8O10(OH)6]2− anion having completely different topology, shown in Figure 5b, was found in the compound [Co(en)3][B5O6(OH)4][B8O10(OH)6)]· 5H2O (6), as reported by the Beckett group.31 Further condensation of the [B8O11(OH)4]∞2− 1D chains in 1 leads to topologically equivalent [B8O13]2− units found in several previously reported anhydrous 3D network borates.32 Thus, the polyborate chains in 1 can be regarded as representing an intermediate stage of condensation between the fully hydrated insular anion [B8O10(OH)6]2− and fully anhydrous [B8O13]2− networks. Each dianionic octaborate FBB is charge balanced by two crystallographically nonequivalent sodium cations. A distinctive feature of the structure is the presence of sodium cations in groups of four edge-sharing polyhedra arranged as Na(1)··· Na(2)···Na(2)···Na(1), as shown in Figure 6, with like cations related by an inversion center. Within each sodium tetramer, the Na+···Na+ distances are similar to those in elemental sodium (3.82 Å) and previously reported rectangular Na44+ clusters (3.539(5) and 3.728(6) Å).33 The Na(1)−Na(2) and Na(2)−Na(2) distances are 3.7830(6) and 3.7932(8) Å,
Scheme 1. Condensation of Pentaborate and Triborate Monoanions To Form the Insular [B8O10(OH)6]2− Anion
Figure 6. Arrangement of and coordination environment around sodium cations in 1. 7178
DOI: 10.1021/acs.inorgchem.7b00823 Inorg. Chem. 2017, 56, 7175−7181
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Inorganic Chemistry respectively. Each sodium cation has close contacts with polyborate, boric acid, and interstitial water oxygen atoms in the range of 2.3264(8)−2.9000(8) Å. Na(1) has seven close contacts with oxygen atoms with a mean Na(1)−O distance of 2.5421(8) Å. Na(2) has six close contacts with oxygen with a mean Na(2)−O distance of 2.4242(8) Å. The Na(1) and Na(2) cations are bridged by the oxygen of a water molecule, O(19). Sodium tetramers aligned along polyborate chains contribute to the formation of a 3D supramolecular network, as shown in Figure 7.
Figure 9. TGA/DSC plot for 1 in the 20−1000 °C range under nitrogen.
transition in the DSC scan, largely in the 200−250 °C range (Figure 9, point 1), corresponding to water loss involving initial liberation of interstitial water molecules followed by condensation of boric acid and polyborate B−OH groups. Water loss of 22 wt % leads to an amorphous anhydrous product of composition Na4B18O11 (2Na2O·9B2O3), as described by eq 3. Powder X-ray diffraction patterns for 1 before and after heating to various temperatures are shown in Figure 10. Figure 7. View of 1 displaying two [B8O11(OH)4]∞2− chains together with interstitial water, boric acid, and sodium cations (yellow).
2Na 2[B8O11(OH)4 ]·B(OH)3 ·2H 2O 1 500 ° C
⎯⎯⎯⎯⎯⎯→ Na4B18O29(amorph) + 11H 2O
The solid-state 11B NMR spectrum of 1, shown in Figure 8, exhibits peaks corresponding to several chemically inequivalent
(3)
Figure 10. PXRD plots for 1 as prepared (a) and after heating to 500 °C (b), 700 °C (c), and 1000 °C (d). Figure 8. MAS 11B{1H} NMR spectrum of sodium enneaborate (1).
trigonal and tetrahedral boron atoms. The resonance at 2.2 ppm is assigned to the tetrahedral boron of the polyborate chain, and the broad duplet-like signals at 9.9 and 15.7 ppm (uncorrected quadrupolar shift) are ascribed to trigonal boron atoms of the polyborate chain and boric acid.34−36 Anhydrous cesium enneaborate, which contains similar B3O3 rings having trigonal and tetrahedral boron atoms linked into a different polyborate structure, displays a similar 11B NMR spectrum.37 Thermal Properties. The thermal behavior of 1 was studied by TGA and DSC analysis. The combined TGA/DSC plot for 1 is shown in Figure 9. The compound continuously loses weight when it is heated in the 100−500 °C temperature range. This weight loss is associated with an endothermic
The PXRD pattern obtained after heating 1 to 500 °C (Figure 10b) indicates an amorphous material. The DSC scan shows exothermic events occurring in the 550−600 °C temperature range (Figure 9, points 2), which correspond to crystallization of anhydrous sodium octaborate, α-Na2B8O13 (7), a previously characterized phase in the anhydrous Na2O− B2O3 system, as seen in the PXRD scan shown in Figure 10c.38 This decomposition is described by eq 4. A final endothermic event observed above 800 °C in the DSC scan (Figure 9, point 3) corresponds to melting of 7. 7179
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Inorganic Chemistry
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CONCLUSIONS Sodium enneaborate (1), long known only by the formulas Na4B18O29·11H2O and 2Na2O·9B2O3·11H2O, was structurally characterized, revealing the structural formula Na2[B8O11(OH)4]∞·B(OH)3·2H2O. The structure features 1D polyborate chains of [B8O11(OH)4]2− repeating fundamental building blocks with interstitial sodium cations and boric acid and water molecules integrated by extensive hydrogen bonding. Each octaborate FBB contains three B3O3 rings having one tetrahedral and two trigonal-planar boron atoms each. The 1D chains in 1 can be regarded as a partially condensed form of the previously reported insular [B8O11(OH)6]2− anion.30,31 Sodium enneaborate has the highest boron content of the sodium borates that form under industrially practical mild nonhydrothermal conditions. Other previously described borates that form under more extreme conditions contain 3D networks based on the [B8O13]2− unit,32 which can be regarded as a further condensed form of the [B8O11(OH)4]∞2− 1D chains in 1. When it is heated to 500 °C, 1 decomposes to amorphous Na4B18O29, and on further heating above 600 °C, it crystallizes as α-Na2B8O13. The latter preserves the topological structure of the octaborate FBB in unheated 1.
Na4B18O29(amorph) > 600 ° C
⎯⎯⎯⎯⎯⎯⎯⎯→ α ‐Na 2B8O13 + B2O3(amorph) 7
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Figure 11 shows the PXRD pattern exhibited by 1 after heating to 700 °C in comparison to the pattern simulated from
Figure 11. (a) PXRD pattern simulated from single-crystal data for αNa2B8O13 (7) and (b) pattern observed after heating 1 to 700 °C.
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the single-crystal data for 7.38 It is notable that 7 contains an octaborate FBB (Figure 12) which is topologically equivalent to that in 1. A β-form of anhydrous metal octaborate also exists for sodium and silver.39
ASSOCIATED CONTENT
S Supporting Information *
The Supporting Information is available free of charge on the ACS Publications website at DOI: 10.1021/acs.inorgchem.7b00823. Crystallographic data for 1 (PDF) Accession Codes
CCDC 1550843 contains the supplementary crystallographic data for this paper. These data can be obtained free of charge via www.ccdc.cam.ac.uk/data_request/cif, or by emailing data_
[email protected], or by contacting The Cambridge Crystallographic Data Centre, 12 Union Road, Cambridge CB2 1EZ, UK; fax: +44 1223 336033.
Figure 12. Polyborate FBB found in α-Na2B8O13 (7).
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The FTIR spectrum of 1, shown in Figure 13, confirms that heating 1 to 500 °C results in loss of water and hydroxyl
AUTHOR INFORMATION
Corresponding Author
*E-mail for D.M.S.:
[email protected]. ORCID
David M. Schubert: 0000-0003-2142-4825 Present Address §
D.M.S.: AvidChem LLC, Lone Tree, Colorado USA.
Notes
The authors declare no competing financial interest.
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ACKNOWLEDGMENTS We thank Dr. John Linehan of the Pacific Northwest National Laboratory, Richland, WA, for running the 11B NMR experiment.
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Figure 13. FTIR spectra for as-prepared 1 (a) and spectra after heating to 500 °C (b), 700 °C (c), and 1000 °C (d).
REFERENCES
(1) Schubert, D. M. Boron Oxides, Boric Acid, and Borates. KirkOthmer Encyclopedia of Chemical Technology; Wiley: Hoboken, NJ, online Apr 15, 2011; pp 1−68. DOI: 10.1002/ 0471238961.0215181519130920.a01.pub3. (2) Schubert, D. M. Boric Oxide, Boric Acid, and Borates. Ullmann’s Encyclipedia of Chemical Technology; Wiley-VCH: Weinheim, Germany, online Feb. 10, 2015; pp 1−32. DOI: 10.1002/ 14356007.a04_263.pub2.
groups. The spectrum is consistent with PXRD data, which show a loss of crystallinity above 500 °C. Local structure features in the 500−1500 cm−1 region are associated with the structural building blocks of both crystalline 1 and 8. 7180
DOI: 10.1021/acs.inorgchem.7b00823 Inorg. Chem. 2017, 56, 7175−7181
Article
Inorganic Chemistry
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DOI: 10.1021/acs.inorgchem.7b00823 Inorg. Chem. 2017, 56, 7175−7181
