Catalysts exchanged clay

This i>aper describes two broad types of intercalation conpomds which are based on graphite on the one hand and sheet aluminosilicate (clay) hosts on the other. Taken together these provide a rich veiriety of examples of heterogeneously catalysed reactions. Appropriately cation exchanged clays, for example, act as efficient catalysts for a number of commercially important proton catalysed reactions (10-13) (see Table I). Graphite intercalates, whilst also capable of... 

Alumlmm-catlon-exchanged clays, IR spectra, V77f Alumlnum-Cu catalyst... 

Bakke et al. (1982) have shown how montmorillonite catalyses chlorination and nitration of toluene nitration leads to 56 % para and 41 % ortho derivative compared to approximately 40 % para and 60 % ortho derivatives in the absence of the catalyst. Montmorillonite clays have an acidity comparable to nitric acid / sulphuric acid mixtures and the use of iron-exchanged material (Clayfen) gives a remarkable improvement in the para, ortho ratio in the nitration of phenols. The nitration of estrones, which is relevant in making various estrogenic drugs, can be improved in a remarkable way by using molecular engineered layer structures (MELS), while a reduction in the cost by a factor of six has been indicated. With a Clayfen type catalyst, it seems possible to manipulate the para, ortho ratio drastically for a variety of substrates and this should be useful in the manufacture of fine chemicals. In principle, such catalysts may approach biomimetic chemistry our ability to predict selectivity is very limited. 

Zinc chloride exchanged clay catalysts have been reported to be highly active for the Friedel-Crafts alkylation and acylation reactions these are commercially sold by Contract Catalysts under the name Envirocats. These are montmorillonite catalysts modified by ZnCU and FeCli. Some of the reported examples of Friedel-Crafts reactions are given below there are claims that some of the processes are commercially practised. 

The positively charged phosphonium ligand was intercalated in the hectorite and was used to catalyze olefin hydroformylation.156 Cu(II)-exchanged clays were tested as catalysts in the cyclopropanation reaction of styrene with... 

Figure 2. Schematic representation of electron transfer from an aromatic compound to O2 with a Cu-exchanged clay as the catalyst and the formation of polymers (Reaction A) and hydrogen peroxide (Reaction B).

Kawai, Onaka, and Izumi also demonstrated a sequential Michael-aldol addition using aluminum cation exchanged clay montmorillonite as catalyst however, the stereochemistry of the product was not ascertained (78). 

The Diels-Alder reaction has been shown to be subject to catalysis by a wide range of solid catalysts (see Chapter 4 for some examples). Acidic mesoporous aluminosilicates can be used to catalyse selective Diels-Alder reactions such as that between cyclopentadiene with methyl acrylate. The zinc-exchanged version of the material is particularly effective and compares well to other more established solid acids such as the ion-exchanged clay Zn2+-K10 as well as homogeneous catalysts such as boron trifluoride (Table 2.7).50... 

Many papers have been published on the catalytic activity of ion-exchanged clays. Ballantine et al. refluxed hex-l-ene with a Cu2+-exchanged montmor-illonite and produced a branched chain symmetrical ether (Reaction 3).21 The clay acted both as a Bronsted acid catalyst as well as providing water molecules for the reaction. 

Table 3. Acylation of mesitylene by benzoyl chloride with either supported ( ) or exchanged clay catalysts ( ) ( activation temperature) [56].

Thanks to their low cost and easy availability, many metal-exchanged clays have been patented as efficient solid catalysts in Friedel-Crafts acylation reactions. A great number of arylketones is prepared by electrophilic acylation of arenes with anhydrides in the presence of ion-exchanged clays at 150°C-250°C. Thus, for example, aluminum-enriched mica promotes the reaction of BAN with mcto-xylene at reflux for 4 h in 99% yield. 

The examples described here show that metal-exchanged clays can be easily prepared and efficiently utilized in the electrophilic acylation of different aromatic compounds carrying various functionalities. However, in our opinion, more studies are needed to give convincing evidence of the efficient reusability and the real heterogeneity of these catalysts. 

Choudary, B. M., Sateesh, M., Lakshmi Kantam, M., Ranganath, K. V. S., and Raghavan, K. V. 2001. Selective acetylation of 5-numbered aromatic heterocycle compounds using metal-exchanged clay catalysts. Catd. Lett. 7G ... 

Among the non-caicined ciays some cations, nameiy Cu(ii), H(l), Cr(lll), Ti(IV) and Ce(IV), deserve particular consideration. When Cu(ll) and H(l) exchanged clays are used as catalysts the reaction stops at low conversions. 

In 1999, an attempt was made to synthesize a polyolefin in clay gallery. Specifically, clay was ion-exchanged using tetradecylammonium ions, and this ion-exchanged clay and a paUadium-based complex of the Brookhart-type were mixed and conditioned in toluene to polymerize ethylene. The interlayer distance was initially 1.99 nm. This increased to 2.76 nm after a palladium catalyst was added. It was confirmed that the X-ray peak disappeared 24 hours after ethylene was introduced [40]. 

TABLE 5 Benzoylation of Mesitylene by Benzoyl Chloride Using Impregnated and Exchanged Clay Catalysts at 433 K... 

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