Aluminum polycations

Seredych M, Bandosz TJ, Adsorption of ammonia on graphite oxide / aluminum polycation and graphite oxide/zirconium polyoxycations composites,/ Colloid Interface Sci. 2008, 324 25-35. 

Aluminum polycations are obtained by the controlled hydrolysis of AI(III) solutions the extent of the hydrolysis reaction can be monitored with pH measurements. The products of the hydrolysis reactions, I.e. the successive generation of polycations of Increasing degree of polymerization with Increasing extent of hydrolysis, must be Inferred from non-equilibrium measurements. Thus, equilibria between many polycatlonic species, I.e. [Al2lOH)2] , [A124(0H)5q]" 12, etc., were proposed. More recently, " A1 NMR has... 

Allouche L, Gerardin C, Loiseau T, Ferey G, Taulelle F (2000) Also A giant aluminum polycation. Angew Chem IntT Edition 39 511-514... 

Allouche L, Huguenard C, Taulelle F (2001) 3QMAS of three aluminum polycations space group consistency between NMR and XRD. J Phys Chem Solids 62 1525-1531 Amirbahman A, Gfeller M, Furrer G (2000) Kinetics and mechanism of ligand-promoted decomposition of the Keggin Afs polymer. Geochim Cosmochim Acta 64 911-919 Baes CF, Mesmer RE (1976) The Hydrolysis of Cations. John-Wiley, New York... 

Bottero, J.Y., Tchoubar, D., Axelos, M.A.V., Quienne, P. and Fiessinger, F. (1990). Hoc-culation of sihea colloids with hydroxy aluminum polycations. Relation between floe structure and aggregation mechanisms. Langmuir, 6, 596-602. 

Parker, D. R., and P. M. Bertsch. 1992a. Formation of the A113 tridecameric aluminum polycation under diverse synthesis conditions. Environmental Science and Technology 26, no. 5 914-921. doi 10.1021/es00029a007. 

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. 

Oades, J.M. (1984) Interactions of polycations of aluminum and iron with clays. Clays Clay Min. 32 49-57... 

The charge per aluminum atom is -i-3 for the mononuclear species on the left-hand side of the equation, but only -i-2 for the dinuclear species on the right-hand side. Thus, polycation formation reduces the overall positive charge of the species by -l-l per M. 

Characterization of aluminum chlorohydrate has revealed a predominance (about 88%) of Ai polymer with the balance being monomers and smaller polycations (20). The highly charged Al 3 surrounded by Q counterions is self-stabilized by repulsion forces, preventing association and subsequent aluminum hydroxide precipitation (21). Thus, aluminum chlorohydrate solutions remain clear and free of precipitate after years of storage at room temperature. Aqueous dilution or an increase in pH to 5—6, however, results in rapid degradation and formation of gibbsite [14762-49-3] an insoluble aluminum hydroxide polymorph (8). 

Wood T.E., Siedle A.R., Hill J.R., Skarjune R.P., Goodbrake C.J. Hydrolysis of aluminum—are all gels created equal Mater. Res. Soc. Symp. Proc. 1990 180 97-116 Yamabi S., Imai H. Crystalline phase control for titanium dioxide films by direct deposition in aqueous solutions. Chem. Mater. 2002 14 609-614 Yamaguchi T., Fujita T., Takusagawa N., Kitajima K. Complex formation of highly polymerized hydroxoaluminum polycations with synthetic expandable fluorine mica. Nippon Kagaku Kaishi 1996 307-310... 

Actually, the drop of pH is related to more complex reactions and species. Thus, in more sophisticated models, several hydrolysis reactions and metal chloride formation are taken into account but the selection of species and reactions is somewhat different from model to model. Oldfield and Sutton [94] and Watson and Postlethwaite [2] considered only hydroxides as the product of cation hydrolysis. Sharland [96] introduced simple metallic chlorides. The most complete set of species and reactions has been used by Bernhardsson et al. [4], which made available the thermodynamic data of a large number of species, including several iron, nickel, chromium, and molybdenum polycations as well as metal chlorides and hydroxychlorides. Gartland [19] used a more limited set of species (Table 10.3) selected among the Bernhardsson data. According to their experimental results, Hebert and Alkire [95] included Al(OH) " as the hydrolysis product in their model of the crevice corrosion of aluminum alloys. 

Under stronger alkalinization, chromium forms a series of increasingly condensed polycations [3], and then a hydrated hydroxide gel Cr(OH)3(bH2)3 [4]. Different behavior is observed for iron(lll) and aluminum(lll). Aluminum forms the stable hydroxide AlfOH), (gibbsite, bayerite) [5], whereas the Fe(OH)3 hydroxide has never been identified- It transforms very rapidly to an oxyhydroxide through spontaneous dehydration [6j. 

Chromium forms small polycations, usually kinetically stable [11,13], whereas iron polycations are very labile and form colloids very easily [14-17]. Among these three elements, only aluminum is able to form the very peculiar AI13 polycation (Figure 3.2) [18-20]. 

This hist step ensures the stability of the cation. It is further favored by heating the h = 2.6 aluminum solution around 80 C. Under such conditions, it is possible to probe the polycation by NMR [20]. Distortion of the AlO octahedra is significant, as indicated by the very large peak on the MAS-NMR spectrum of the polycation at solid state (Figure 3.5). 

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. 

McCauley has, however, not investigated the structure of the pillaring species further in order to substantiate this suggestion and no such polymeric ion has so far been reported in the literature. Investigations of hydrothermally treated mixtures of aluminum chlorohydrate and rare earth salts are required in order to establish the structure of this polycation. 

---