Recent Progress on Structure of Aqueous Polyborate Solutions

The crystal structure of metal borates has been extensively investigated by X-ray and neutron diffraction,but,the structure of aqueous polyborate solutions are still largely unknown.Over the last decade,our group has focused on studying the structure of complex aqueous polyborate solutions of Li,Na,K,Rb,Cs,and Mg using synchrotron radiation X-ray scattering (XRS),EXAFS,Raman,NMR,and DFT,as well as determining the density,conductivity and pH of such solutions.Polyborate species distributions were calculated using pH measurements,and the main species in the solution have been confirmed by NMR and Raman spectra.For alkali-metal metaborates,the dominant species is always B(OH)4^- in a wide range of concentration,while the presence of others species is negligible.For alkali metal tetraborates,when concentration is in the extreme low range,only B(OH)3 and B(OH)4^- are present in these solutions.As the total boron concentration increases,B(OH)3 and B(OH)4^- polycondensated to form more complex oligomers.Of them,while B4O5(OH)4^2- in the tetraborate solutions is the main species,B(OH)3 ,B(OH)4^-,and B3O3(OH)4^- are minor species,and B3O3 (OH)5^2- and B5O6 OH)4^- are present only in negligible amounts.As solution continues to concentrate,B4O5(OH)4^2- eventually becomes the dominant species,which is consistent with the congruent compound M2B4O7·nH2O (M=Li,Na,K,Rb,Cs) in the system M2O-B2O3-H2O (M=Li,Na,K,Rb,Cs).For alkali metal pentaborates,B(OH)3 and B(OH)4^- are the main species at low concentrations.The species distribution,Raman and NMR spectroscopy results verified that the dominant species in concentrated pentaborate solutions with Li and Na is pentaborate B5O6 OH)4^-,but it is surprising that the main species with K,Rb,and Cs is always the triborate monoanion B3O3(OH)4^-.Although all M[B5O6 OH)4 ]·nH2O (M=Li,Na,K,Rb,Cs) are all congruent compounds in the system M2O-B2O3 -H2O (M=Li,Na,K,Rb,Cs),the main species in aqueous solutions are quite different because of various cation hydration distance (d),hydration number (CN),and configuration,especially charge (Z).For bivalent Mg^2+,three borate minerals,namely,Inderite (2MgO ·3B2O3 ·15H2O),Hungchaoite (MgO ·2B2O3 ·9H2O),and Mcallisterite (MgO ·3B2O3 ·7.5H2O),exist in the system MgO-B2O3 -H2O at 298K.Inderite is a congruent compound,but Hungchaoite and Mcallisterite are incongruent compounds.The species distribution and Raman spectra demonstrat that the predominant species in all the solutions with magnesium borates is bivalent triborate B 3O3(OH)5^2-,while the subordinate species are B(OH)4^- at a low B2O3 /MgO ratio and B(OH)3 at a high B2O3 /MgO ratio,and the other anions are negligible,as the high Z of the borate anion must match that of Mg 2+.The disappearance of divalent B4O5(OH)4^2- is in agreement with its incongruent nature.The DFT and XRS results showed that tetrahedral Li(H2O)4^+(d=0.20 nm,CN=4),octahedral Na(H2O) 6 ^+(d=0.236 nm,CN=6),and Mg(H2O)6^2+(d=0.210 nm,CN=6) in the first hydration shell belong to Platonic polyhedra.However,K(H2O)+ 8 (d=0.28 nm,CN=8,XRS and DFT),Rb(H2O)8^+(d=0.293 nm,CN=7.7-8.2,EXAFS),and Cs(H2O)8^+(d=0.320-0.326 nm,CN=7.6-7.9,EXAFS) are inclined Voronoi polyhedra.Therefore,the effects of cation Z and d on the structure of polyborates in aqueous solutions are deterministic,while the effect of hydrated-cation symmetry is secondary.Their hydrolysis order was:Mg>Li>Na>K>Rb>Cs,in step with hydration power.Among them,Mg^2+ and Li^+ have a strong tendency towards hydrolysis,but Na^+ scarcely hydrolyzes,especially Rb^+ and Cs^+ have a little protonation.X-ray scattering of aqueous alkaline sodium borohydride solutions confirmed that dihydrogen bonds exist in an aqueous solution.The four distinct features of dihydrogen bonds in aqueous solution-unidirectionality,divaricativity,multicentricity,and multidentativity have been also described here in brief.

Volumetric and Transport Properties of Aqueous NaB（OH）4 Solutions

Density, pH, viscosity, conductivity and the Raman spectra of aqueous NaB（OH）4 solutions precisely measured as functions of concentration at different temperatures （293.15, 298.15, 303.15, 313.15 and 323.15 K） are presented. Polyborate distributions in aqueous NaB（OH）4 solution were calculated, covering all the concentration range, B（OH）4 is the most dominant species, other polyborate anions are less than 5.0%. The volumetric and the transport properties were discussed in detail, both of these properties indicate that B（OH）4 behaves as a struc- ture-disordered anion.

A Raman Investigation on the Boron form Occurring in Salt Lake Brine during Evaporation

Boron resources are abundant in the Da Qaidam Salt Lake of the Qaidamu basin in China, which has generated significant attention due to the presence of polyborate species in brine from this lake. In this study, Raman spectroscopy was used to investigate the existing form of boron in brine during evaporation. MgO·2B2O3 -H2O, MgO·2B2O3 -MgCl2 -H2O, and MgO·2B2O3 -MgSO4 - H2O solutions were also studied to determine the influence of boron concentration, pH, and electrolytes on the borate speciation from brine. The mononborates B(OH)3 and B(OH)4^- were found to be the only forms present in natural salt lake brine. Brine evaporation promoted the formation of the polyborate anions B3O3(OH)4^-,B5O6(OH)4^-, and B6O7(OH)6^2- and also promoted the disappearance of the B(OH)4^- ion from brine at boron concentrations of more than 11 g/L B2O3. The pentaborate ion B5O6(OH)4^- was sensitive to the solution pH and appeared only at pH values less than 8.0. Meanwhile, the hexaborate ion B6O7(OH)6^2- was observed to be more dependent on the electrolyte magnesium chloride due to its special properties, such as promoting boron accumulation, lowering solution pH, and also its strong affinity for water molecules, which were all beneficial for the polymerization of borate ions in brine. Interaction mechanisms between polyborate anions during evaporation are also proposed herein.ake;evaporation;polyborate species;interaction mechanism.

Synthesis and Structure Description of a New Tetradecaborate [H_3N(CH_2)_2NH_3]_2[B_(14)O_(20)(OH)_6]

A new tetradecaborate, [H3N（CH2）2NH3]2[B14O20（OH）6]（1）, was synthesized by a boric acid ＇flux＇ method, and characterized by X-ray powder diffraction, ICP, IR, TGA and DTA analyses. Single-crystal X-ray diffraction analysis showed that it crystallized in the monoclinic space group P1（No.2） with a=0.85092（10） nm, b=0.88678（11） nm, c=1.00724（13） nm, α=95.099（2）°, β=96.950（2）°, γ=116.845（2）°, V=0.66425（14） nm^3, Z=2, R1=0.0625[1〉2σ（I）], and WRE=0.1172（all data）. The molecule structure of compound 1 comprises the isolated polyanion clusters [B14O20（OH）6]^4- and diprotonated ethylenediamine molecules. Each polyanion cluster contains an 8-ring enclosed by six B3O3 3-ring units. Adjacent clusters are further connected with each other through strong hydrogen-bonding interactions of N（O）-H…O to form a three dimensional（3D） supramolecular structure with 2D pseudo channels along the [010] and [001] directions. The diprotonated ethylenediamine molecules located in the channels interact with the host through extensive hydrogen-bonds. Compound 1 presents few example containing isolated polyborate anions synthesized by a boric acid ＇flux＇ method.