Aquo-aluminum

Oppm is due to hexa aquo aluminum species, (b) Si MAS NMR ofY zeolites subjected to calcination (bottom spectrum), steaming and calcination (middle spectrum) and steaming and acid wash (top spectrum). Steaming and calcination results in increase of Q OAI) species at the expense of other species clearly demonstrating the dealumina-tion of the material. 

Akitt JW, Farthing A (1981) Aluminum-27 nuclear magnetic resonance studies of the hydrolysis of aluminum(lll). Part 4 Hydrolysis using sodium carbonate. J Chem Soc Dalton Trans 1981 1617-1623 Akitt JW, Greenwood NN, Khandelwal BL, Lester GD (1972) Al nuclear magnetic resonance studies of the hydrolysis and polymerization of the hexa-aquo-aluminum(III) cation. J Chem Soc Dalton Trans 1972 604-610... 

The mechanism of complex formation for organic-aluminum chelates probably involves a dissociative mechanism, as the aquo-aluminum aqueous complex involves a full octahedral coordination shell (Hewkin and Prince... 

Fig. 6. Ball and stick model of the fully coordinated aquo-aluminum complex in an octahedral geometry...

An associative pathway would involve a hepta-coordinate transition state complex, which is unlikely if only based on steric criteria. In a study of salicylate-aluminum complexes at 25 °C, Secco and Venturini (1974) proposed that the first step is a fast ion-pair association of the anion and the aquo-aluminum coordination complex, i.e. ... 

Alternatively, several workers have shown that not only is the soluble, zero-charged hydrolysis product considerably more surface active than the free (aquo) ion but also a polymeric charged or uncharged hydrolysis product may be formed at the solid-liquid interface at conditions well below saturation or precipitation in solution. Hall (5) has considered the coagulation of kaolinite by aluminum (III) and concluded that surface precipitates related to hydrated aluminum hydroxide control the adsorption-coagulation behavior. Similarly Healy and Jellett (6) have postulated that the polymeric, soluble, uncharged Zn(OH)2 polymer can be nucleated catalytically at ZnO-H20 interfaces and will flocculate the colloidal ZnO via a bridging mechanism. 

The Al3+ ion shows a propensity to suffer hydrolysis which can produce the formation of various mononuclear hydroxo complexes. At pH around 7.0 it often precipitates as Al(OH)3, which redissolves leading to the formation of aluminate, Al(OH)4. After this, polynuclear complexes such as Al2(OH)2, Al3(OH)4] and Al13(OH)32 can also form. These species can produce, in a very slow process, oligomeric complexes, which are necessary precursors of the macromolecular polymer [Al(OH)3]n [16, 17]. In Fig. 2 we can observe the distribution of the Al3+ aquo ion and the species for 100 mM and 10 pM total aluminum. The maximum concentration of Al(OH)3 is reached at about neutral pH. At pH > 7.0 the predominant species is aluminate ion A OH). ... 

The hydrogen atom of the aquo group of (XII) is acidic the complex reacts with sodium hydroxide to yield an ill-characterized salt (10), and will condense with other acidic hydroxyl groups. Thus Owen and Kenney (268) have synthesized a series of aryloxy aluminum derivatives by the condensation of (XII) with various phenols. The complexes formed with phenol, p-phenylphenol, and p-methoxyphenol—(XIV), (XV), and (XVI)... 

The aquo and aluminate ions have already been noted. Aluminum acetate has a structure similar to that of the basic acetates of Cr3+, Fe3+ and other M3+ ions and probably contains the basic unit [Al30(00CMe)6(H20)3]+, where the H20 molecules can dissociate to form OH or can be replaced by other ligands. 

According to their amphoteric nature, aluminum hydroxides have their minimum solubdity in water at a close-to-neutral pH value, that is, in the pH range of 6—7. At a pH larger than 8.5, the solubihty increases, and the [A1(0H)4] anion is formed. At a pH smaller than 4, the aquo cation [A1(0H2)6] is reported to be predominant, whereas at a pH between 4 and 6, its dissociated form [Al(OH2)5(OH)] prevails (36,37), likely both with less tightly bound water molecules in the secondary hydration shell. However, the speciation of aluminum in aqueous environments is very complicated. Depending on the conditions, such as pH, concentration (or hydrolysis ratio), and anions present, a variety of polynuclear species can be found in solution. In pure water, polynuclear species include dimers... 

Aluminum is a hard trivalent ion, and is usually found as an aquo- or hydroxy-complex or hydrous oxide solid (Martell and Motekaitis 1989). Aluminum can act as a complexing agent of other metals, or as a metal center to other inorganic and organic ligands (Buffle 1990). As the hexa-coordinated hydrolyzed cation, aluminum is octahedrally coordinated by six waters (Fig. 6), and as such acts as a multiprotic acid (Stumm and Morgan 1981). 

Table 6. Aluminum-27 Chemical Shifts in Mixed Octahedral Aquo Complexes AI(H20)s " ...

Iron polycations are not as well known as chromium or aluminum polycations because of the lability of ferric complexes. Only a few polycations (dimers, trimers) have been characterized in acidic solutions (pH < 1.5) [39]. [Fe2(OH)2] and [Fc20] dimers are present in organic complexes such as L3(H20)Fe(OH>2-Fc(OH2)L3 and LjFeOFeLs, where the L3 ligand is a tridentate picolinate and L5 a tridentate amine [16,40,41]. Other polydentate ligands, such as proteins, are able to stabilize many polynuclear iron complexes [42-45]. The existence of lire aquo complexes [(H20)4Fe2(0H)2(0H2)4] " and [(H20)5Fe20(0H2)s] is very probable in spite of the lack of structural data. 

Chromium(lII) differs from aluminum and iron in its exceptional chemical inertia. The fast addition of a base in a solution of Cr + ions also causes the formation of a gel owing to cancellation of the charge on the aquo complexes or polycations. Various hydrated hydroxides ate formed, and they are made of aggregates of species that previously existed in solution. 

The addition of a base to the monomer [Cr(OH2)6] causes the precipitation of the grey-blue hydrated hydroxide Cr(OH)3(OH2)3, in which Cr(OH)3(OH2)3 octahedra are connected by double [H302] bridges formed by hydrogen bonds between hydroxo and aquo ligands [21,25]. The solid contains planar hexagons similar to those observed in aluminum hydroxides Al(OH)3 (Figure 3.13). 

In addition to the application of highly reactive aluminum compounds (halides), the aluminum enrichment of ZSM-5 zeolite with "aquo species from AI2O3" has been also reported [12]. This involves the hydrothermal treatment of ZSM-5 and active alumina mixtures. Presximably, the hydrous alumina species migrate into the zeolite, ultimately assuming sites as framework lattice T cations. The importance of the presence of a liquid water phase is emphasized, because no catalytic evidence for aluminum incorporation is observed by treatment in dry steam. Here again, the amount of tetrahedral aluminum is determined by Al MAS NMR, indicating the presence of additional aluminum in the lattice following the treatment. 

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