Recent Progress on Structure of Aqueous Polyborate Solutions
投稿时间:2019-05-20  
DOI:10.12119/j.yhyj.201902002
中文关键词:  
英文关键词:Solution structure  polyborate  X-ray scattering  dihydrogen bond  EXAFS
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作者单位
FANG Chun-hui Key Laboratory of Comprehensive and Highly Efficient Utilization of Salt Lake Resources,Qinghai Institute of Salt Lakes,Chinese Academy of Sciences,Xining,810008,China
Key Laboratory of Salt Lake Resources Chemistry of Qinghai Province,Xining,810008,China 〖TPz 
FANG Yan Key Laboratory of Comprehensive and Highly Efficient Utilization of Salt Lake Resources,Qinghai Institute of Salt Lakes,Chinese Academy of Sciences,Xining,810008,China
Key Laboratory of Salt Lake Resources Chemistry of Qinghai Province,Xining,810008,China 〖TPz 
ZHOU Yong-quan Key Laboratory of Comprehensive and Highly Efficient Utilization of Salt Lake Resources,Qinghai Institute of Salt Lakes,Chinese Academy of Sciences,Xining,810008,China
Key Laboratory of Salt Lake Resources Chemistry of Qinghai Province,Xining,810008,China 〖TPz 
ZHU Fa-yan Key Laboratory of Comprehensive and Highly Efficient Utilization of Salt Lake Resources,Qinghai Institute of Salt Lakes,Chinese Academy of Sciences,Xining,810008,China
Key Laboratory of Salt Lake Resources Chemistry of Qinghai Province,Xining,810008,China 〖TPz 
LIU Hong-yan Key Laboratory of Comprehensive and Highly Efficient Utilization of Salt Lake Resources,Qinghai Institute of Salt Lakes,Chinese Academy of Sciences,Xining,810008,China
Key Laboratory of Salt Lake Resources Chemistry of Qinghai Province,Xining,810008,China 〖TPz 
ZHANG Wen-qian Key Laboratory of Comprehensive and Highly Efficient Utilization of Salt Lake Resources,Qinghai Institute of Salt Lakes,Chinese Academy of Sciences,Xining,810008,China
Key Laboratory of Salt Lake Resources Chemistry of Qinghai Province,Xining,810008,China 〖TPz 
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中文摘要:
      
英文摘要:
      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)-4in 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)2-4 in the tetraborate solutions is the main species,B(OH)3,B(OH)-4,and B3O3(OH)-4 are minor species,and B3O3(OH)2-5 and B5O6OH)-4 are present only in negligible amounts.As solution continues to concentrate,B4O5(OH)2-4 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 B5O6OH)-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[B5O6OH)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 Mg2+,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 B3O3(OH)2-5,while the subordinate species are B(OH)-4 at a low B2O3/MgO ratio and B(OH)3at a high B2O3/MgO ratio,and the other anions are negligible,as the high Z of the borate anion must match that of Mg2+.The disappearance of divalent B4O5(OH)2-4 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)2+6(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,Mg2+ 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.
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