| The aqueous solution chemistry of Al?III?is an important subject in the fields of environmental chemistry and geochemistry.It is also a research and exploration hotspot in many academic fields.Although several of polyaluminum compounds formed by hydrolysis have been widely used as antiperspirants,mordants,deodorants,surfactants,catalysts and flocculants in water treatment,however,it is very difficult to obtain pure polyaluminum compound crystals by evaporating or adding a crystallizing reagent to capture the dominant polyaluminum species,because the hydrolyzation and polymerization process is quite complex,a large number of polyaluminum species exist in the hydrolyzed solution simultaneously,and their distributions constantly change with temperature,chemical composition of the solution,aging time and other conditions.Due to the shortage of structural information,a unified understanding for the hydrolyzation and polymerization process of aluminum salts and the evolution law of aluminum species has not yet reached.Therefore,it is of great significance for the study of the formation and transformation mechanisms of polyaluminum species in aqueous solution by accurately controlling to synthesize more polyaluminum compounds and solving their structures.After many years of unremitting exploration,an effective method,by which the crystals of a target polyaluminum compound can be accurately obtained at certain temperature,was found by our group,that is,the phase diagram control method.In this paper,the single crystals of three kinds of polyaluminum compounds were successfully cultivated by adding different concentration of Na2SO4 and2,6-naphthalenedisulfonate disodium?NDS?solution into asame spontaneously hydrolyzed aluminum solution with[Al]:[SO4]and[Al]:[NDS]in a certain percentage.The results of X-ray single crystal structure analysis show that Crystal 1obtained by adding 0.5mol/L Na2SO4 capturing reagent and then slowly evaporated for one year is a new polyaluminum compound,whose structure formula is[Al8?OH?14?H2O?18]Cl2?SO4?4·14H2O,belonging to the triclinic system P—1—space group.The structure of the polyaluminum cation is similar to that in Al8 sulfate([Al8?OH?14?H2O?18]?SO4?5·16H2O)obtained from a spontaneously hydrolyzed aluminum sulfate solution reported by literature?Although the kinds and quantities of the counterions and the number of intersticial watermolecules are completely different?,four AlO6 octahedra linked together by edge-sharing formed a"four Al plank",and then another four AlO6 octahedra suspended onto the perihery of the"four Al plank"with two upper and another two lower by sharing two neighboring corners forming a"Plank-suspengding"structure?P-Al8?.This is the first polyaluminum species captured by Na2SO4 from a spontaneously hydrolyzed Al solution and composed of AlO6 octahedra only without an AlO4 tetrahedral core.It indicates that more aluminum species with a"Plank-suspending"structure can be captured from a spontaneously hydrolyzed Al solution by adjusting the concentration and amount of Na2SO4 solution.The Crystals?Crystal 2?respectively obtained by adding high concentration Na2SO4 solution at 50?and standing at room temperaturehave same unit cell parameters and they are in conformity with those reported by our group in S-Al32 sulfate chloride([Al32O8?OH?60?H2O?28?SO4?2]?SO4?7Cl2×30H2O)that was obtained by adding 0.24mol/L Na2SO4 into a NaOH forcedly hydrolyzed Al Cl3 solution and aged for 8h at 150?(In which the polycation is composed of two AlO4 tetrahedral cores,thirty AlO6 octahedral shells,and two SO42-that participated in the coordination,formed a core-shell structure),indicating that hydrolyzing forcedly and aging at high temperature is not the only condition for producing an Al O4 tetrahedral core,which can be completely induced out under a mild condition?50??by adding concentrated Na2SO4 solution even from a spontaneously hydrolyzed solution in which the AlO4 tetrahedron does ont exist,forming a polyaluminum species that has a core-shell structure,it is of great significance for communicating the relationship between spontaneously and forcedly hydrolyzing.The structure of Crystal 3([Al30.5O8?OH?61.5?H2O?22]?NDS?4Cl6×37H2O)obtained by adding NDS and slowly crystallized at room temperature is similar to[Al30.5O8?OH?61.5?H2O?22]?NDS?4Cl6×36H2O that was reported by Chanjuan Li from our group two years ago(In which the polycation is made up of two AlO4 tetrahedral cores,twenty eight AlO6 octahedral shells,and two AlO5 triangle bipyramids?occupancy 1/4?connected by edge-sharing,forming an infinite core-shell chain structure with Al30.5 as repeating unit).Due to an obvious improvement in crystal quality,all the acceptance indexes for crystal structure analysis results have reached to the requirements for publishing a paper,and the related supporting information such as XRD,EDX,and TGA results are ready,a paper is writing.By comparing the structural characteristics of the three kinds of polyaluminum species(P-Al8,S-Al32and Al30.5)obtained by adding same capturing reagent with different concentration and different capturing reagents into a same spontaneously hydrolyzed Al solution,it indicates that the concentration of a capture agent and the size of its anion can directly influence the structure of polyaluminum species in the obtained polyaluminum compound crystals and the mode of hydrolyzation and polymerization.In the process of drawing the phase diagram of the AlBr3-H2O-Al2O3 system at25?,AlBr3 solution was spontaneously hydrolyzed with Al powder.The result of X-ray structure analysis for Crystal 4 grown under low basification condition shows that its structural formula is[Al13?OH?24?H2O?24]Br15·13H2O,it belongs to monoclinic system P21/c space group.The structure of polyaluminum cation[Al13?OH?24?H2O?24]15+is similar to those appearing in Al13 chloride and nitrate reported by literatures and in Al133 iodide synthesized by our group(Only a XRD pattern of Al13 bromide was recorded in PDF 27-6,its structure has not yet solved),an AlO6 octahedral core is surrounded by a"hexameric ring"that are composed by six edge-shared AlO6 octahedra formed a"seven Al plank"through a edge-sharing manner,and then another six octahedral AlO6 units are suspended onto the perihery of the hexameric ring upper and lower alternately by sharing two neighboring corners forming a"Plank-suspengding"structure(P-Al13),the six suspended Al O6 octahedra have an average inward turn angle of 21.4°in this bromide,which is slightly larger than 21.1°in the chloride,but smaller than 26.0°in the iodide and 26.2°in the nitrate.It shows that the turn angles of the outer octahedra gradually increase with the increase of the volume of the anions,which indicates that the spatial orientation of the outer AlO6 octahedra in P-Al13 structure can be adjusted"freely"by changing the turn angle according to the size of the counterion.The discovery of P-Al13 bormide also proves the universality of existing this kind of"Plank-suspending"Al13 species and a new member was added to the crystallographic database.In addition,27Al NMR was used to monitor the existing Al species in the solution before and after crystallization and all the crystalline products were characterized by XRD,EDX,CA?chemical analysis?and TGA.Furthermore,under the guidance of the concept having the solid-liquid equilibrium adequately reached,in this paper,the phase diagram of AlCl3-H2O-Al2O3system at 50°C has been perfected and thoroughly completed with a combination of wet slag method and synthesis system method.The previous static evaporation was changed as dynamic evaporation with continuously stirring and the solid-liquid equilibrium time was extended,moreover,whether a solid-liquid equilibrium is really reached,was judged by a criterion that the composition of liquid is no longer changed with time.As a result,the problem of insufficient solid-liquid equilibrium existed in drawing phase diagrams in the past was solved.A large number of experimental data have confirmed that the phase diagram determined in this paper is more accurate and reasonable,and it provides a more reliable basis for designing the synthesis routes and preparing Al9Cl6?OH?21·18H2O and Al13Cl15?OH?24·37H2O in a single-phase isolation manner at this temperature. |
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