Confirmation Of Non-shared Hydroxyl(oxygen)coordination Sites In Various Polyaluminum Species And Determination Of Tetrahedral Phase Diagram Of Polyaluminium Suifate System At 75℃

The structure and evolution law of the hydrolyzation and polymerization species of aluminum salt is not only the key to explore the genesis of aluminum ore in geochemistry,the regular pattern of aluminum migration and transformation in environmental chemistry and the action mechanism of aluminum-based coagulants in water treatment chemistry,but also the key to search for the pathology and toxicology of aluminum metabolism in biochemistry.However,due to the influences of hydrolysis mode,temperature,pressure,basification degree,solution’s chemical composition and other conditions,the structure and distribution of the hydrolyzed and polymerized species of Al³⁺ions in aqueous solution are complex and changeable,so it is difficult to isolate a single-phase polyaluminum compound and know its exact structure.Therefore,up to now the formation and evolution mechanism of polyaluminum species have not reached to a consensus,because of a serious shortage of their structural information.In order to separate out a polyaluminum species in the form of single-phase crystal from a hydrolyzed solution with much coexisted species,in this paper,the single crystals of five polyaluminum compounds were grown by adding Na₂SO₄,Al₂（SO₄）₃,Na₂（2,6-NDS）and Na₂（2,7-NDS）（NDS represents naphthalene disulfonate）to NaOH or KOH forcedly hydrolyzed or Alpowder spontaneously hydrolyzed AlCl₃solution for catching the polyaluminum species in it.In order to objectively determine the exact structure of each polyaluminum species,including the positions of all H atoms,in this paper,the objective positions of all H atoms on those polyaluminum cations are successfully found from the difference Fourier diagrams by collecting hundreds of thousands of high-resolution single crystal diffraction data at 100K low temperature and the location of non-shared hydroxyl（oxygen）coordination sites are unambiguously confirmed.In order to explore the crystallization mechanism of adding capture agent in the hydrolyzed solution,change casual catching to targeted catching,improve the capture efficiency and realize controlled synthesis,this paper took the most used Na₂SO₄ capture agent in literatures as the fourth component,designed and determined an isothermal solid-liquid equilibrated tetrahedral phase diagram of Na₂SO₄-Al₂（SO₄）₃-Al（OH）₃-H₂O system at 75℃under atmospheric pressure,to obtain the three-dimensional single-phase crystallization regions of various sodium containing or sodium free polyaluminum sulfates that can be separated out at this temperature,and provide a phase-diagram theoretical basis for designing a scientific and reasonable crystallization route,controllably synthesizing these polyaluminum compounds and solving their crystal structures.In this paper,AlCl₃ solution was forcedly hydrolyzed with NaOH solution and aged at 60℃for 7 days prepared a stock solution with basification degree（B=[OH]/[Al]）of 2.5,then Na₂SO₄ and Al₂（SO₄）₃ solution were added respectively.After being placed at room temperature for two months,colorless and transparent single crystals were grown in both cases.X-ray structure analysis shows that the structural formula of the former is[AlO₄Al₁₂（OH）₂₅（H₂O）₁₁][SO₄]₃·17H₂O.The structure of the polyaluminum species[AlO₄Al₁₂（OH）₂₅（H₂O）₁₁]⁶⁺(K-Al₁₃⁶⁺)in it is similar to the famous K-Al₁₃([AlO₄Al₁₂（OH）₂₄（H₂O）₁₂]⁷⁺),in which an AlO₄ tetrahedron is surrounded by twelve AlO₆ octahedra through vertex-sharing,but its charges are balanced by three sulfate ions only（X-ray fluorescence spectrum analysis shows that it does not contain chlorine and sodium）,indicating that it brings only six positive charges,it is equivalent to the result that one of the 12 coordination water molecules in K-Al₁₃ continues to hydrolyze and remove a proton.Specifically,which coordination water becomes the non-shared hydroxyl group,it can be easily identified from the difference Fourier diagram based on 50984 high-resolution diffraction data at 100K.It is found that as a hydrogen bond donor the only H atom connected to O31 atom extends to an O atom of a sulfate ion,and as a hydrogen bond acceptor two pairs of lone electrons of O31 atom extend to the H atoms of two coordinated H₂O molecules on adjacent K-Al₁₃⁶⁺,forming a very reasonable hydrogen bond giving and accepting relationship around it.These structural information made up for the defect that Johansson first reported that the aluminum species[AlO₄Al₁₂（OH）₂₅（H₂O）₁₁]⁶⁺in13Al₂O₃·6SO₃·x H₂O（x≈79）has Keggin structure and brings six positive charges,but the diffraction data obtained by Weissenberg photography at that time could not give the position of H atom,so the site of non-shared hydroxyl group could not be confirmed.It is also proved that the composition of the compound is 13Al₂O₃·6SO₃·81H₂O,and the structural details of the polyaluminum species in it are improved.The structural formula of the latter is[Al₃₂O₈（OH）₆₀（H₂O）₂₈（SO₄）₂][SO₄]₇[Cl]₂·20H₂O.The structure of polycation[Al₃₂O₈（OH）₆₀（H₂O）₂₈（SO₄）₂]¹⁶⁺(S-Al₃₂)in it is similar to that of S-Al₃₂in[Al₃₂O₈（OH）₆₀（H₂O）₂₈（SO₄）₂][SO₄]₇[Cl]₂·30H₂O reported by our research group in2011,which is equivalent to that one of the four groups of Al₃O₁₃ units connected with AlO₄ tetrahedron in K-Al₁₃ rotated 60°relative to the other three groups changing the edge-shared connection into vertex-shared connection forming aδ-K-Al₁₃,and then an AlO₆ octahedral“cap”is put on the Al₃O₁₃ unit forming an Al-δ-K-Al₁₃.After that,two Al-δ-K-Al₁₃ are connected together by two AlO₆ octahedra through sharing respective four vertices in the manner of“head to head”forming an“Al₃₀”,two AlO₆octahedra are respectively suspended on two pairs of non-shared coordination sites(one from Alcap and the other from the Al₃O₁₃ unit in each pair)in both sides of its waist forming an Al₃₂.Finally,two of the eight coordination water molecules on the two suspended AlO₆ octahedra are respectively replaced with two SO₄ tetrahedra by vertex-sharing forming the centrosymmetric S-Al₃₂,so for balancing its charges besides the seven sulfate ions there are two chloride ions.The 20 interstitial water molecules are 10 less than the results published by our research group in 2011,so it is a new polyaluminum compound,which is due to the different unit cell parameters and volumes of the compounds obtained by different synthetic methods.In 2011,it was caught by adding Na₂SO₄ after aging at 150℃for 8 hours under hydrothermal condition,in this paper,it was caught by adding Al₂（SO₄）₃ after aging at 60℃for 7days under atmospheric pressure,indicating that S-Al₃₂ species can also be formed under mild conditions,which provides a new clue for the study of its formation mechanism.In this paper,AlCl₃ solution was also spontaneously hydrolyzed with Alpowder,and a hydrolyzed solution with B=1.42 was prepared,after aged at 100℃for 24 hours and then aged at 170℃for 4 days under hydrothermal condition,Na₂（2,7-NDS）solution were added to the solution that was filtered because it is slight turbid.After being placed at room temperature for half a year,light yellow single crystals were grown.The result of X-ray structure analysis shows that its structural formula is[Al₃₀O₈（OH）₅₈（H₂O）₂₄]（2,7-NDS）₈·53H₂O,which belongs to the P1-space group of triclinic system.The structure of polycation[Al₃₀O₈（OH）₅₈（H₂O）₂₄]¹⁶⁺(H₂Al₃₀)is similar to the“Al₃₀”mentioned above,but its charges are balanced by eight 2,7-NDS^2-anions only（EDS analysis shows that it does not contain chlorine）,indicating that it has only 16 positive charges,which is equivalent to the result that the two coordinated H₂O molecules on the two Alcaps in“Al₃₀”([Al₃₀O₈（OH）₅₆（H₂O）₂₆]¹⁸⁺)reported by Abeysinghe et al.were hydrolyzed and two protons were removed becoming two non-shared hydroxyl groups.Which two coordination water molecules become non-shared hydroxyl groups,it can be easily identified from the difference Fourier diagram based on 141318 high-resolution diffraction data at 100K.It is found that as a hydrogen bond donor the only H atom connected to O33 atom on the Alcap extends to an O atom of interstitial water,and as a hydrogen bond acceptor a pair of lone electrons of O33 atom extend to the H atom of the only coordination water O34 on its adjacent Al₃O₁₃ unit,forming a very reasonable hydrogen bond giving and accepting relationship around it.The confirmation of the structure of H₂Al₃₀ species in this polyaluminum compound,especially the confirmation of the non-shared hydroxyl coordination sites,has again verified the scientificity and applicability of the Local Basification Degree Symmetrical Equalization principle proposed by our research group in predicting and explaining the hydrolytic active sites of polyaluminum species.In this paper,Na₂（2,6-NDS）solution were respectively added to spontaneously hydrolyzed AlCl₃-Al（OH）₃-H₂O system（B=0.6,aged at 50℃for 24 hours）and forcedly hydrolyzed AlCl₃-NaOH-H₂O system（B=1.2,aged at 80℃for 24 hours）.After being placed at 0℃for one month,colorless and transparent lamellar single crystals were grown in both cases.The result of X-ray structure analysis shows that the polyaluminum compounds obtained by two methods belong to the same P1-space group of triclinic system,and have the same unit cell parameters,which is the same compound,and its structural formula is[Al₃₀O₈（OH）₆₀（H₂O）₂₂][2,6-NDS]₆[Cl]₂·49H₂O.It is considered that in addition to six 2,6-NDS^2-groups,there are two Cl^-ions in the anions.Qualitative analysis of X-ray fluorescence spectrum also proves the existence of chlorine.It shows that the polycation[Al₃₀O₈（OH）₆₀（H₂O）₂₂]¹⁴⁺(Al₃₀)has only 14charges,which is equivalent to the result of the H₂Al₃₀ mentioned above is furtherly hydrolyzed and two protons are removed.This can be easily recognized from the difference Fourier diagram based on 224025 high-resolution diffraction data at 100K,the two protons are removed from the two coordination H₂O molecules on the two rotated Al₃O₁₃ unis.Thus,each of the four non-shared coordination O atoms that respectively locates on the two Alcaps and the two adjacent Al₃O₁₃ units is only connected to one H atom,forming a reasonable hydrogen bond giving and accepting relationship with the periphery.The successful synthesis of this polyaluminum compound indicates that the products obtained by different hydrolysis methods can be the same,besides,the spontaneous hydrolysis is actually connected with the forced hydrolysis each other.This discovery provides an important clue for the unified understanding of the internal relationship between the spontaneous hydrolysis and the forced hydrolysis.In this paper,an AlCl₃ solution was forcedly hydrolyzed with KOH and aged at80℃for 24 hours forming a stock solution with B=1.4,then,Na₂（2,6-NDS）solution was added to catch the polyaluminum species in it.After being placed at room temperature for one year light yellow block single crystals were grown.The result of X-ray structure analysis shows that its structural formula is[Al₃₀O₁₀（OH）₅₆（H₂O）₂₄][2,6-NDS]₇·66H₂O.During the structural analysis,it is found that there are only seven 2,6-NDS^2-anions balancing its charges（EDS analysis shows that it does not contain chlorine and sodium）,indicating that the polycation[Al₃₀O₁₀（OH）₅₆（H₂O）₂₄]¹⁴⁺(μ₁-O^2--Al₃₀)brings 14 charges only,which is equivalent to the result of the H₂Al₃₀ mentioned above is furtherly hydrolyzed and two protons are removed.However,its formation process is different from the above-mentioned Al₃₀.From the difference Fourier diagram based on 199774 high-resolution diffraction data at 100K,it is found that the two protons are furtherly removed from the two non-sharedμ₁-OH^-ligands on the two Alcaps,making it changed from non-shared hydroxyl ligand to non-shared oxygen anion ligand(μ₁-O^2-).Therefore,as a hydrogen bond acceptor its three pairs of lone electrons extends to one H atom of the coordination water molecule on the Al₃O₁₃ unit and two H atoms of two interstitial water molecules respectively,forming a very reasonable hydrogen bond relationship with them.It is the first time for finding that a non-shared coordination water is completely hydrolyzed becoming an non-shared oxygen negative ion on an polyaluminum species in the environment of medium basification degree.This discovery shows that subtle differences in preparation methods can also cause some differences in hydrolysis sites and species structures,reflecting the flexible diversity of H₂Al₃₀ hydrolysis manners,which enlightens us that it is necessary to design,determine and plot the tetrahedral phase diagram of Al₂（NDS）₃-AlCl₃-Al（OH）₃-H₂O system,to explain the internal cause of this difference by phase diagram theory.In this paper,KOH solution was used to depolymerize the samples,and the content of each element was determined by solution X-ray fluorescence spectrometry method.The tetrahedral coordinate system was converted into three-dimensional rectangular coordinate system,and an isothermal solid-liquid equilibrated tetrahedral phase diagram of Na₂SO₄-Al₂（SO₄）₃-Al（OH）₃-H₂O system at 75℃under atmospheric pressure was determined and plotted.The three-dimensional single-phase crystallization regions of NaAl₃（SO₄）₂（OH）₆,NaAl（SO₄）₂·6H₂O and Al（OH）₃,the two-phase co-saturation lines of NaAl₃（SO₄）₂（OH）₆ with NaAl（SO₄）₂·6H₂O,Na₂SO₄ and Al（OH）₃ respectively,and the three-phase co-saturation point of NaAl₃（SO₄）₂（OH）₆,Al（OH）₃ and Na₂SO₄ were successfully determined by selecting an appropriate reactants adding cross-section,calculating the adding points and adding scientifically and accurately.They provide a phase-diagram theoretical basis for scientifically and reasonably designing the crystallization routes,controlling the syntheses of polyaluminum compounds such as NaAl₃（SO₄）₂（OH）₆,and solving their crystal structures.The crystal structure of NaAl₃（SO₄）₂（OH）₆ is solved by Rietveld full-profile fitting method based on its powder XRD data,and the result is consistent with the structure reported by Tang Youqi and Zhou Gongdu in 1957.