Die Kristallstruktur des Natriumoxohydroxoaluminathydrates Na₂[Al₂O₃(OH)₂] · 1,5 H₂O

Abstract

Die Kristallstruktur des Natriumoxohydroxoaluminathydrates Na2[Al2O3(OH)2] · 1,5 H2O – bisher in der Literatur als Na2O · Al2O3 · 2,5 H2O bzw. Na2O · Al2O3 · 3 H2O bezeichnet – wird bestimmt. Die Einkristallröntgenstrukturanalyse (tetragonal, Raumgruppe P‐421m, a = 10,522(1) Å, c = 5,330(1) Å, Z = 4) ergibt eine aus AlO3/2(OH)‐Tetraedern aufgebaute, polymere Schichtstruktur. Die zwischen den Schichten angeordneten Na+‐Ionen bilden mit O2−‐Ionen von AlOAl‐Brücken, AlOH‐Gruppen und von Kristallwasser NaO6‐Oktaeder, die über gemeinsame Flächen zu tetrameren Baugruppen verknüpft sind. 27Al‐ und 23Na‐MAS‐NMR‐Messungen bestätigen die Kristallstrukturanalyse. Die Beziehungen zwischen dem Kristallisationsverhalten der Verbindung und der Konstitution der Aluminatanionen in der entsprechenden Natriumaluminatlösung bzw. in der kristallinen Verbindung werden diskutiert.

Raman, IR, and 27Al-MAS-NMR Spectroscopic Studies of Sodium (Hydroxy)Aluminates

“…The MAS-NMR signals of four-coordinate aluminum compounds (according to combined MAS-NM R and single-crystal X-ray diffraction data) occur between 55 and 80 ppm. 14,16 Despite claims to the contrar y, 20 no evidence has been found among aluminate compounds for anything other than sixand four-coordination geometry.…”

“…The composition of the solids was calculated on the assumption that water accounted for the remaining mass. 13,16 The supernatant from preparation I was stored in a Pyrex glass bottle at room temperature. After several days, well-de® ned needle-like crystals appeared.…”

“…However, the chemical composition of our product was determined to be 2Na 2 O´Al 2 O 3´5 H 2 O, which is equivalent to one of the products, NaAl(OH ) 5 , described by Raizman et al, 20 according to whom the 27 Al-MAS-NM R spectrum was consistent with pentahedrally coordinated alum inum. However, on the basis of single-crystal XRD and 27 Al-MAS-NM R observations, Weinberger et al 16 argued convincingly for tetrahedral aluminum coordination geometry in a compound with the com position Na 2 [Al 2 O 3 (OH) 2 ] 1.5H 2 O.…”

Intermediate Species in the Crystallization of Sodium Aluminate Hydroxy Hydrates

“…Such observations indicate that the Al 3+ coordination change occurs either on the NSA crystal surface or during a solid-state amorphous-to-crystalline transition but not in solution. Given that the isolation of Na 2 Al 2 O­(OH) 6 has eluded many others exploring the Na 2 O/Al 2 O 3 /H 2 O phase space, − it is reasonable to infer from the instability of Na 2 Al 2 O­(OH) 6 that it may exist as a reactive intermediate during solid-state crystallization, linking the soluble tetrahedral aluminates under highly alkaline conditions to the monomeric octahedra in NSA. Further isolation of crystallization pathways, including the prospect of solid-state transformation, is underway to better understand Al 3+ coordination change mechanisms in caustic environments relevant to myriad process applications.…”

“…One potentially important species is the μ-oxo aluminate dimer (Al 2 O­(OH) 6 2– ), which can form by oxolation in solution and, in concentrated potassium and rubidium hydroxide, leads directly to crystallization of M 2 Al 2 O­(OH) 6 salts (M = K + or Rb + ), principally comprised of this dimer motif. ,,− However, crucially, in the sodium system, no comparable sodium aluminate dimer crystal structure has been found despite thorough exploration of the Na 2 O/Al 2 O 3 /H 2 O phase diagram. − Analysis of the formation, reactivity, and dynamics of Al 2 O­(OH) 6 2– in solution is challenging because of rapid exchange of Al 3+ ion speciation through hydrolysis and condensation during nuclear magnetic resonance (NMR) spectroscopy, ,,,, degenerate Raman-active vibrational modes, nonmonotonic trends in pre-edge features in X-ray absorption spectroscopy, and multimodal relaxation processes in both dielectric spectroscopy and quasielastic neutron scattering . Among such techniques, the most diagnostic is Raman spectroscopy, which clearly shows that vibrational modes of sodium aluminate solutions contain a component similar to that of K 2 Al 2 O­(OH) 6 . ,, …”