Imogolite

Allophane and Imogolite. AUophane is an amorphous clay that is essentially an amorphous soHd solution of sUica, alumina, and water (82). In allophane less than one-half of the aluminum is held in tetrahedral coordinations and the Si02 to AI2O2 ratio typically varies between 1.3 and 2.0, but values as low as 0.83 have been reported. The typical morphology of allophane is cylindrical (37). AUophane may be associated with haUoysite, smectite minerals, or it may occur as a homogeneous mixture with evansite, an amorphous soHd solution of phosphoms, alumina, and water. Its composition, hydration, and properties vary. Chemical analyses of two allophane samples are given in Table 5. 

Secondary minerals. As weathering of primary minerals proceeds, ions are released into solution, and new minerals are formed. These new minerals, called secondary minerals, include layer silicate clay minerals, carbonates, phosphates, sulfates and sulfides, different hydroxides and oxyhydroxides of Al, Fe, Mn, Ti, and Si, and non-crystalline minerals such as allophane and imogolite. Secondary minerals, such as the clay minerals, may have a specific surface area in the range of 20-800 m /g and up to 1000 m /g in the case of imogolite (Wada, 1985). Surface area is very important because most chemical reactions in soil are surface reactions occurring at the interface of solids and the soil solution. Layer-silicate clays, oxides, and carbonates are the most widespread secondary minerals. 

J. P. Boudot, Relative efficiency of complexed aluminium, non-crystalline A1 hy-droxyde, allophane and imogolite in retarding the biodegradation of citric acid. Geoderma 52 29 (1992). 

Amorphous Aluminosilicates. These occur in soils influenced by volcanic activity and are associated with very high moisture retention and anion fixation, and low to very high pH-dependent CEC. They may also bind organic matter tightly, protecting it against decomposition. Examples are allophane and imogolite. 

Miophane and Imogolite. Allnphanc is an amorphous clay lhal is essentially an amorphous solid solution of silica, alumina, and water. Allophanc has been found most abundanlly in soils and altered volcanic ash. It usually occurs in spherical form but has also been observed in libers. 

Parfitt, R. L., A. R. Fraser, and V. C. Farmer. 1977. Adsorption on hydrous oxides. HI. Fulvic acid and humic acid on goethite, gibbsite and imogolite. Journal of Soil Science 28 289—296. 

Figure 2.18. Proposed mechanism for the catalysis of hydroxylaluminosilicate ions in catechol humification. Reprinted from Liu, C., and Huang, P. M. (2002). Role of hydroxylaluminosilicate ions (proto-imogolite soil) in the formation of humic substances. Org. Geochem. 33, 295-305, with permission from Elsevier.

Yoshinaga, N. and Aomine, S., 1962. Imogolite in some Ando soils. Soil Sci. Plant Nutr. Tokyo), 8 22-29. 

Ishida, T. and Makino, T. (1999) Effects of pH on dielectric relaxation of montmorillon-ite, allophone, and imogolite suspensions, Journal of Colloid and Interface Science 212, 152-161... 

Ishida, T., Makino, T. and Wang, C. (2000) Dielectric-relaxation spectroscopy of kaolin-ite, montmorillonite, allophone, and imogolite under moist conditions, Clays and Clay Minerals 48 Suppl. 1, 75-84... 

AI203Si20 2H20 (imogolite) + 6H+ o 2AI3+ + H4SiO + 3H20 12.006... 

The microporosity of a new tubular silicatelayered silicate nanocomposite formed by the intercalation of imogolite in Na -montmorillonite has been characterized by nitrogen and m-xylene adsorption. The nitrogen adsorption data yielded liquid micropore volume of -0.20 cm g as determined by both the t-plot and the Dubinin-Radusikevich methods. The t-plot provided evidence for a bimodal pore structure which we attributed to intratube and intertube adsorption environments. The m-xylene adsorption data indicated a much smaller liquid pore volume (-0.11 cm g ), most likely due to incomplete filling of intratubular pores by the planar adsorbate. The FTIR spectrum of pyridine adsorbed on the TSLS complex established the presence of both Bronsted and Lewis acid sites. The TSLS complex was shown to be active for the acid-catalyzed dealkylation of cumene at 350 C, but the complex was less reactive than a conventional alumina pillared montmorillonite. 

Imogolite is a naturally occurring aluminosilicate mineral with a unique tunnel-like or tubular structure. The... 

We have been investigating the use of imogolite as a pillaring agent for smectite clays with layer lattice structures ". The regular intercalation of the tubes within the layered host results in the formation of a tubular silicate-layered silicate (TSLS) complex. These new nanocomposite materials may be viewed as pillared clays in which the pillars themselves are microporous. Significantly, the TSLS structure is thermally stable up to 450 C when montmorillonite is selected as the layered host . 

A schematic representation of a TSLS complex is provided in Figure 1. On the basis of preliminary XRD and stochiometric studies, it appears that the imogolite tubes are in van der Waals contact, most likely in a log-jam-like array in the layer silicate galleries. Although the tubes stuff the galleries, two unique adsorption environments are available, namely, the intra-and inter-tube pores designated A and B in Figure 1. 

In the present work we examine the microporosity of a TSLS complex formed from synthetic imogolite and natural montmorillonite. Nitrogen adsorption and desorption isotherms are reported and analyzed in terms of microporous volume and surface area. Also, the adsorption isotherm for an organic adsorbate, m-xylene, is reported. Preliminary FTIR results for the chemisorption of pyridine and catalytic studies of the dealkylation of cumene suggest that TSLS complexes are promising microporous acids for shape selective chemical conversions. 

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