Pillaring agents

The possibility of pillaring smectites with various other large polyhydroxides (e.g. of titanium and various transition metals) is under active investigation. For example, the preparation of thermally stable TiPILCs (e.g. with BET areas > 300 m2 g at 600°C) has been described by Sterte (1991) and characterized by Bernier et al. (1991). The use of different pillaring agents offers the prospect of the development of... 

Many cation-exchanged clays are suitable for the production of metal-oxide-pillared clays. The hydrolysis of the cation helps the pillaring step, so, at first, the aluminum ion was applied as a pillaring agent. Later, other elements were also used, for example, zirconia chromium iron transition metal elements and some lantanoids, organometallic complexes, surfactants, and polymers. 

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 . 

Clays containing hydrated metal cations or organic-based cations collapse upon moderate heating due to the thermal instability of these pillaring agents (10-16). Using polynuclear hydroxy metal cations such as [Ali304(0H)24(H20)i2l stable porous clay materials can be made (1-9). However, the number of metals that form suitable polymeric species is limited. 

The anionic polyoxometalates comprise a much larger class of metal oxide-based pillaring agents (18). It was proposed that hydrotaldtes (also referred to as layered double hydroxides, or LDHs) would be suitable host materials for pillaring with polyoxometalates. Subsequently, a route for synthesizing isopol3rmetalate-pillared hydrotalcites via organic-anion-pillared precursors was developed (19-20). 

The preparation and properties of smectites pillared with A1 hydroxy oligomers have been extensively investigated. Most of the work has been performed on montmoriUonite and hectorite, which have been intercalated with a great variety of pillaring agents and oUgocations. 

Another approach to designing shape-selective heterogeneous oxidation catalysts was to use redox metal oxides as the pillaring agents in the preparation of pillared clays. These redox pillared clays have been used for a number of selective oxidations. Chromium pillared montmorillonite (Cr-PILC) is an effective catalyst for the selective oxidation of alcohols with tert-butyl hydroperoxide. 7 Primary aliphatic and aromatic alcohols are oxidized to the aldehydes in very good yields. Secondary alcohols are selectively oxidized in the presence of a primary hydroxy group of a diol to give keto alcohols in excellent yields (Eqn. 21.12). 2... 

Chiral catalysis using zeolites has been considered, however, intercalation of chiral metal chelates or the use of asymmetric organic pillaring agents may be a new way of introducing such selectivity. Iwai and coworkers have used asymmetric organic pillars, (R)- or (S)- phenylethylammonium cations in several layered phases and have observed some molecidar recognition Certainly further work is needed in this rather different definition of selective catalysis. 

J. Valverde, P. Sanchez, F. Dorado, C. Molina, and A. Romero, Influence of the Synthesis Conditions on the Preparation of Titanium-pillared Clays Using Hydrolyzed Titanium Ethoxide as the Pillared Agent. Microporous Mesoporous Mater., 2002, 54, 155-165. 

Figure 4. XRD-analysis of Mg,Al-Cl LDH s before (A) and after (B) ion exchange of the BINAP complex via normal exchange (1), exchange in the presence of a swelling agent (2) and in the presence of a pillaring agent (3).

In case of the exchange in presence of a pillaring agent, there is an obvious shift of the doos reflection towards larger interlamellar spacing (up to 32 A) In this case, intercalation of the complex will occur at least partially. 

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