Structural Features And Evolution Mechanisms Of Two New Al₃₀ And Al₁₃ Species

The species structure of aluminum salt hydrolysates is not only the key to study the chemistry of aluminum solution,but also the key to reveal the flocculation mechanism of Al-series flocculants in water pollution control chemistry,and the key to fully understand the transport and transformation law of aluminum in environmental chemistry to effectively prevent aluminum pollution.It is also the key to understand the biological metabolism of aluminum in biochemistry to effectively prevent aluminum poisoning.However,the hydrolysis process of aluminum（III）in aqueous solution is complicated and variable,and the species distribution changes with the external environment time to time.Due to the shortage of polyaluminum species structural information,it has still not been reached to a consensus,although it has been studied for a century that how the monomeric aluminum is evolved into aluminum hydroxide in solution.Therefore,obtaining more intermediate products and extracting effective structural information from them is the key to study the formation and evolution mechanism of aluminum species in solution.To solve the problem,our group has developed a systematic research method consisting of separately isolating（or capturing）polyaluminum species from a hydrolyzed Al solution under the direction of the phase diagram rules,solving their crystal structures,and speculating on their mechanisms of formation and evolution.In this paper,the basification degree（B）of H₂O and D₂O aqueous solution of AlCl₃ was respectively adjusted by KOH and KOD,then aged for a period of time to prepare a forcedly hydrolyzed stock solution of H₂O and D₂O aqueous solution of sodium 2,6-naphthalenedisulfonate（2,6-NDS）were added to the stock solution to capture the polyaluminum species,and two single crystals were respectively obtained.The results of X-ray structure analyses show that both of the new compounds belong to the triclinic system P1^—space group,but their compositions,structures and unit cell parameters are different.The former’s structural formula is[Al₃₀O₈（OH）₅₈（H₂O）₂₄]（2,6-NDS）₈?50H₂O;The latter’s structural formula is[Al₃₀O₈（OD）₆₀（D₂O）_22.25]（2,6-NDS）₇?60.75D₂O.The structural formulas of the polyaluminum cations in the above two crystals are[Al₃₀O₈（OH）₅₆(OH_1.5)₄（H₂O）₂₂]¹⁶⁺(labeled as H₂Al₃₀)and[Al₃₀O₈（OD）₆₀（D₂O）_22.25]¹⁴⁺(labeled as（D₂O）_1/4─Al₃₀),they can be regarded as the products that"Al₃₀"([Al₃₀O₈（OH）₅₆（H₂O）₂₆]¹⁸⁺)reported in literature is further hydrolyzed to different degrees.Both H₂Al₃₀ and（D₂O）_1/4─Al₃₀ are the results that a hydrolyzation is happened on the positions where the local basification degrees are the most unequalized in"Al₃₀"to reduce the level of local basification degree unequalization,which is fully in line with the expectation of the Local Basification Degree Symmetrical Equalization（LBDSE）principle（hydrolyzing and polymerizing reactions always occur on the positions where the local basification degrees in symmetry are the most unequalized）proposed by our group,so the correctness of the LBDSE principle was further proved.In this paper,an AlCl₃ solution was forcedly hydrolyzed with NaOH solution to prepare a stock solution with a basification degree.An equal volume of sodium 2,7-naphthalenedisulfonate（2,7-NDS）solution was added to the stock solution,two months later colorless flaky single crystals were grown.X-ray structural analysis results show that the structural formula of the obtained new compound is[（2,7-NDS）Na₂Al₁₃O₄（OH）₂₄（H₂O）₁₈][Al₁₃O₄（OH）₂₄（H₂O）₁₂]（2,7-NDS）₇?31H₂O,belonging to the triclinic system P^—1space group.There are two polyaluminum species in this compound,one is[（2,7-NDS）Na₂Al₁₃O₄（OH）₂₄-（H₂O）₁₈]⁷⁺(labeled as 2,7-NDS─Na₂─δ-K─Al₁₃),the other is[Al₁₃O₄（OH）₂₄（H₂O）₁₂]⁷⁺(labeled asδ-K─Al₁₃).The structure of 2,7-NDS─Na₂─δ-K─Al₁₃ is obviously different from the Na─δ-K─Al₁₃ reported in literature.The NaO₆ octahedral cap in the latter is put on（edge-shared connection）the triangular pit outside the Al₃ unit rotated by 60°inδ-K─Al₁₃;however,one of the two NaO₆ octahedral caps in the former is still put on the Al₃ unit rotated by 60°,but is connected to one of the three AlO₆ octahedra of the Al₃ unit by face-sharing,and the other NaO₆ octahedral cap coordinated by a 2,7-NDS anion with replacement of a H₂O molecule is linked to an adjacent unrotated Al₃ unit by edge-sharing.This is not only another example that naphthalenedisulfonate participates in the coordination of polyaluminum species,but also a new discovery that?-K─Al₁₃ and2,7-NDS─Na₂─?-K─Al₁₃ simultaneously coexist in a same compound,reflecting the complexity in the interaction between NaO₆ and?-K─Al₁₃ and the diversity in the connection ways between NaO₆ and?-K─Al₁₃.The structure ofδ-K─Al₁₃ is equivalent to the result that one of the four Al₃ units connected to the AlO₄ tetrahedron by vertice-sharing in?-K─Al₁₃ is rotated by 60°around the line connecting its center and the shared vertice.This pureδ-K─Al₁₃ uncapped with any NaO₆ cap is discovered for the first time in the structures of polyaluminum species.This discovery is of great significance for obtaining a deep recognizing and understanding of the process and mechanism in which how?-K─Al₁₃ is ultimately transformed into?-K─Al₁₃ via?-,γ-and?-K─Al₁₃.The successful preparation of the three single crystals described above,further proves the superiority and applicability of sodium naphthalenedisulfonate as a crystalline agent for capturing the polyaluminum species,it is also very important for obtaining a deep understanding of the interaction rules between sodium naphthalenedisulfonate and polyaluminum species,and the formation mechanisms of?-K─Al₁₃,Al₂₆ and Al₃₀.In addition,the solid-liquid equilibrium phase diagram of Al₂（SO₄）₃-Al（OH）₃-H₂O system at 100°C is completed by a combination of the wet slag method and the synthetic system method,which not only provides a control basis for designing synthetic routes to prepare 7Al₂（SO₄）₃?6Al（OH）₃?78H₂O and Al₂O₃?SO₃?6H₂O（new phases）which can be separately isolated at this temperature and have not been recorded in the PDF database,but also lays a solid foundation for constructing a quaternary tetrahedral phase diagram by adding a capture agent component in the ternary system.