Catalyst – solid sorbent – or support therefor: product or process – Zeolite or clay – including gallium analogs – Clay
Patent
1998-04-15
2000-02-08
Dunn, Thomas
Catalyst, solid sorbent, or support therefor: product or process
Zeolite or clay, including gallium analogs
Clay
502 60, 502 62, 502 80, 502 85, B01J 2116
Patent
active
060228210
DESCRIPTION:
BRIEF SUMMARY
FIELD OF THE INVENTION
Kaolin clays, composed of one sheet of tetrahedrally co-ordinated Si and one sheet of octahedrally co-ordinated Al, have a restricted capacity to intercalate as opposed to 2:1 structures of montmorillonites. This limited capacity to accommodate compounds from solution has meant little use of these abundant minerals as catalyst, sorbents or as porous materials, while montmorillonites have seen a continuous growth. The capacity of kaolins to intercalate can be enhanced by repeated expansion and contraction of layers, a process which modifies the atomic scale structure of these minerals. The newly derived capacity of these clays makes them useful precursors for preparation of adsorbents, catalysts, and porous materials.
BACKGROUND OF INVENTION
Clay minerals, also known as hydrous layer silicates, are part of a larger family of minerals called phyllosilicates or layer silicates. A unit layer of a given clay mineral is typically composed of a two dimensional arrangement of tetrahedral and octahedral sheets, each with specific elemental composition. The tetrahedral sheet may have composition of T.sub.2 O.sub.5 where T, the tetrahedral cation, is Si, Al and/or Fe, and the octahedral sheet commonly contains cations such as Al, Fe and Mg. A tetrahedral sheet containing only Si.sup.4+ cations is electrically neutral, as are Mg-octahedral sheets and Al octahedral sheets with one third vacant sites. Both tetrahedral and octahedral sheets can have cationic substitutions resulting in a net negative charge which is balanced by interlayer cations. The type of sheets in a unit layer, degree of substitutions, and stacking of layers vary greatly, and determine the type and economic usefulness of a given clay mineral.
Clay minerals can be broadly classified into 2:1 and 1:1 type on the basis of type of sheets in a unit layer. The 2:1 type clay minerals are composed of one octahedral sheet sandwiched between two tetrahedral sheets (FIG. 1). The upper tetrahedral sheet is inverted so that apical oxygens point down and are shared by the octahedral sheet. When all the sites in the tetrahedral sheet are occupied by Si and all sites in the octahedral sheet are occupied by Mg or two-thirds of the sites are occupied by Al, the resulting layers are electrically neutral. In nature, these conditions are exemplified by talc and pyrophyllite, respectively. Electrically neutral layers of these minerals are coupled by weak dipolar and van der Waals forces. There is no driving force for attraction of interlayer cations or intercalating compounds. Therefore, these minerals have little use as exchangers or adsorbents.
In contrast to talc and pyrophyllite, layers in mica bear a net negative charge of 1e per Si.sub.8 O.sub.20 unit due to excessive cationic substitutions in the tetrahedral sheet. The resultant net negative charge is balanced by interlayer K.sup.+, which is coordinated to the hexagonal array of oxygen atoms on either side of the interlayer space. The interlayer K strongly binds the layers and resist intercalation and exchange processes for these minerals.
While pyrophyllite-talc group and mica group minerals represent the extreme cases with either no or excessive charge, the smectite group represent 2:1 clay minerals with an intermediate level of charge, usually ranging from 0.2 to 0.6 e per Si.sub.8 O.sub.20. Whilst the net negative charge on smectites is sufficient to hold exchangeable cations and a provide driving force for intercalation, it is not too excessive and localised, as in micas, to resist swelling of layers. The interlayer charge in smectites is usually balanced by intercalated alkaline earth or alkali metals which can be readily exchanged with desired cations.
Some smectites (such as beidellite and nontronite) derive their charge from substitutions in tetrahedral sheets, while others derive their charge from substitutions in octahedral sheets. The layer charge in tetrahedrally charged smectites is relatively localised, which results in greater three dimensional order and -poorer intercalat
REFERENCES:
patent: 3271323 (1966-09-01), Whittemore
patent: 5672555 (1997-09-01), Maxwell et al.
Richard MacKinnon Ian Donald
Singh Balbir
Dunn Thomas
The University of Queensland
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