Controlled Synthesis And Crystal Structure Of Two New Naphthalene Disulfonate Caps Al₁₃ And Sulfuric Acid Al₂ And Evolution Mechanisms Of Al Species

The structures and evolution rules of hydrolyzed and polymerized species of aluminum salt is not only an important scientific problem for revealing the action mechanisms of Al-series flocculants and synthesizing high-efficiency coagulants,but also a significant scientific problem for understanding the transport and transformation law of Al in the global environment and for preventing Al pollution.Due to the complexity of coexisting various polyaluminum species during the hydrolysis of aluminum salts,it becomes difficult to isolate pure polyaluminum compounds therefrom,and it is more difficult to obtain a large enough single crystal for structure analysis.Because the structurally known Al species are very limited,how the Al monomers are evolved into aluminum hydroxides have still not been reached to a consensus.After many years of exploration,our research group has found an effective control method,the solid-liquid equilibrium phase diagram.Under the guidance of the phase diagram,as long as the chemical composition of the hydrolyzed solution is controlled to the single-phase crystalline area of a target polyaluminum compound,pure compound can be isolated or single crystal can be grown by slow evaporation;and by adding a suitable catching agent one of the polyaluminum species can also be crystallized.An AlCl₃ solution was forcedly hydrolyzed by a NaOH solution at 80℃to prepare a stock solution with a basification degree.Same amounts of sodium2,7-naphthalenedisulfonate（2,7-NDS）was added to two identical stock solutions that were suffered and not suffered from to a hydrothermal aging at 100°C for 16h to capture the dominant polyaluminum species,two single crystals（labeled as Crystal 1and 2）were successfully obtained from the two result solutions with different preparation conditions,respectively.X-ray single crystal diffraction structure analysis showed that the Crystal 1 obtained with hydrothermal aging is a new polyaluminum compound,whose structural formula is[（2,7-NDS）NaAlO₄Al₁₂（OH）₂₄（H₂O）₁₄]（2,7-NDS）₃?11.75H₂O,belonging to the monoclinic system P2₁/c space group.The structure of the polyaluminum cation is equivalent to that a NaO₆ octahedral cap on Na─?-K─Al₁₃3 is divided into two halves,one half lefts in situ and the other half puts on a triangular pit outside the adjacent unrotated Al₃ unit with a naphthalenedisulfonateconnectedtoit,forminga Na_0.5（2,7-NDS）AlO₄Al₁₂（OH）₂₄（H₂O）₁₄Na_0.5 unit,and then such units with opposite orientations are connected end to end,eventually forming a"Z"shaped infinite chain structure,labeled as chain─2,7-NDS-Na─?-K─Al₁₃;however,those Na─?-K─Al₁₃species previously reported are all finite structures existing alone.This structural mode in which disordered NaO₆ cap is coordinated by a NDS group and linked each other forming a one-dimensional infinite chain is discovered for the first time,it is great of significance for obtaining a deep understanding of the applicability of sodium naphthalene disulfonate as a crystalline agent for capturing the polyaluminum species,the interaction rules between sodium naphthalenedisulfonate and polyaluminum species,and the formation mechanisms of?-K─Al₁₃,Al₂₆ and Al₃₀species.The Crystal 2 obtained without hydrothermal aging belongs to the monoclinicsystemPcspacegroup,itsstructuralformulais[NaAlO₄Al₁₂（OH）₂₄（H₂O）₁₅]（2,7-NDS）₄?12.75H₂O,in which the structure of the polyaluminumcationissimilartothatofNa─?-K─Al₁₃in[NaAlO₄Al₁₂（OH）₂₄（H₂O）₁₅]（2,6-NDS）₄?16.5H₂O that belongs to the triclinic system P1^—space group and was reported by literature.The structure of Na─?-K─Al₁₃ can be regarded as that three AlO₆ octahedra connected each other by edge-sharing(Al₃unit,which connects to one of the four vertices of the central AlO₄ tetrahedron by vertice-sharing in?-K─Al₁₃)are rotated 60?around the line linked their center and the vertice and then a NaO₆ octahedral cap is put on the triangular pit outside the rotated Al₃ unit.However,the type of counteranion,the crystal system and the space group,and the unit cell parameters are completely different,so it is also a new polyaluminum compound.It means that a new member can be added to the crystallographic database.In this paper,the phase diagram drawing of Al₂（SO₄）₃-Al（OH）₃-H₂O system at75°C was completed by a combination of wet slag method and synthetic system method,which not onlyprovides a control basis for designing synthetic route to prepare[Al₂（OH）₄（H₂O）₄]SO₄ and[Al₂（OH）₂（H₂O）₈]（SO₄）₂?2H₂Othat can be separately isolated at this temperature,but also laysa foundation of constructing a quaternary tetrahedral phase diagram by adding a capture agent component in the ternary system.Under the guidance of the obtained phase diagram,the single crystal of[Al₂（OH）₂（H₂O）₈][SO₄]₂?2H₂O was successfully grown and its crystal structure was solved.The results show that the structure of polyaluminum cation in it is consistent with that of Al₂（two AlO₆ octahedra connected together by edge-sharing）in the same product obtained by the reaction of aluminum powder and sulfuric acid under mercury catalysis reported in literature,indicating that mercury catalysis is not required,as long as the chemical composition of the hydrolyzed solution is controlled to fall into the single-phase crystalline area of[Al₂（OH）₂（H₂O）₈][SO₄]₂?2H₂O,it can be separately isolated,which proves again that the research method of synthesizing polyaluminum compounds under the direction of the phase diagram rules is scientific and high efficiency.