Clays, as catalysts

Use of natural clays as catalyst greatly improved cracking efficiency. 

The interest in mineral-promoted organic reactions stems from the need to understand the fate of pesticides in soils and pollutants in sedimentary environments (8), petrogenesis (20-27), humification (19, 28, 29), the origin and evolution of life (1, 30), the use of clays as catalysts in industrial processes (31-37), in pharmaceutical applications (3), and as pigments and fillers in paper, plastic, and rubber (37). 

Perfect chemoselectivity (100% of dithioacetals) was also achieved with kaolinitic clays as catalysts for protection of various aldehydes (aliphatic, aromatic, heteroaromatic, a,(I-unsaturated) with 1,2-ethanedithiol <1996TL4605>. 

The first successful catalytic cracking process was the Houdry process, announced in 1933 (132) and commercialized in 1936 (172). This was a fixed-bed process employing, at first, an activated bentonite clay as catalyst. It had been known previously that certain types of decolorizing clays catalyzed the decomposition of hydrocarbon oils (165,188), but a carbonaceous deposit rapidly accumulated on the clay and seriously impaired its activity. During his early work in France, between 1927 and 1930, Houdry found that catalyst activity could be maintained at a satisfactory level by carefully burning off the carbonaceous deposit, or coke, at frequent intervals before the concentration became high enough to interfere seriously with the desired catalytic reactions. 

Substitutions. Etherification of phenols with alkylating agents, a base, and phase transfer catalyst in dry media under microwave irradiation can be an effective method. For preparation of alkyl azides from alkyl bromides one can use surfactant pillared clays as catalyst. ... 

The Claisen rearrangement can be effectively catalyzed by Lewis acids, Bronsted acids, bases, Rh(I) and Pt(0) complexes as well as by silica . Several reviews were published recently in which the application of zeolites and acid-treated clays as catalysts for the Claisen rearrangement was described Thus, it was shown that the rearrangement conditions for phenolic allyl ethers can be dramatically milder if this reaction is carried out by thermolysis of a substrate immobilized on the surface of previously annealed silica gel for chromatography. For example, the thermolysis of ether 159 on silica gel (in a 159 Si02 ratio of 1 10 w/w) at 70°C gives the phenol 160 in 95% yield after 3.5 hours (equation 70). An additional example is shown in equation 71. ... 

In most studies on the use of clays as catalysts or catafyst supports, the surface acidity of the clays has been considered a determinant of their catalytic activity [9,10]. All the supports (of lamellar and zeolitic structures) have acid centers, whose strength and number varied depending on the peculiar structure of the synthesized constituent materials [11]. 

Most applications for clays as catalysts have been in add catalysed reactions1,2 but as an understanding of clays has developed, they have been used as the basis for a large number of more varied catalysts.3,4... 

Cornelis, A., Gerstmans, A., Laszlo, R, Mathy, A., and Zieba, 1.1990. Friedel-Crafts acylations with modified clays as catalysts. Chem. Lett. 6 103-109. 

The aromatic acylation with carboxylic acids (Eq. 3) instead of acid anhydrides and acyl chlorides has attracted interest, because it is an environmentally benign reaction, resulting in the formation of water as the only by-product. It has been attempted with zeolites and clays as catalysts. Heteropoly acids have proved to be more active catalysts for this... 

A multicomponent reaction has been described for assembly of quinoxaline derivatives using montmorillonite KIO clay as catalyst under MW irradiation conditions (Scheme 8.82) [195]. 

Use of natural clays as catalyst greatly improved cracking efficiency and was used in the Houdry process... 

Figueras, F., 1988. Pillared Clays as Catalysts. Catalysis Reviews-Science and Engineering, 30(3) 457. 

Polyolefins, which are normally defined as polymers based on alkene-1 monomers or a-olefins, are the most widely used group of thermoplastic polymers today. The use of many different coordination catalysts has been reported for the production of polyolefin/clay nanocomposites. The methods of in situ synthesis of polyolefin/clay nanocomposite by coordination catalysts mostly depends on the role of clay and can be divided into three categories (1) clay as pol5nner filler, (2) clay as catalyst or cocatalyst support, and (3) Clay acts as a cocatalyst for coordination polymerization. 

Reddy et al. synthesized 5-hydroxyindole derivatives via the Nenitzescu reaction by coupling of aniline, ethyl acetoacetate, and p-benzoquinone, in 1,2-dichloroethane, using montmorillonite KSF clay as catalyst (Scheme 18) [87]. The reaction was also performed using acyclic and cyclic 1,3-diketones and with aryl and alkyl amines. Among several catalysts tested in this reaction, montmorillonite KSF clay proved to be the best catalyst leading to the highest yields in a shorter reaction time. 

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