Organometallic catalysts, entrapment

In the 1990s a large variety of organometallic catalysts were physically entrapped in leach-proof silica gels showing enhanced activity and... 

The sol-gel approach has been used in catalysis in four main directions preparation of inorganic oxide catalysts " entrapment of metal microcrystallites polymerization of trialkoxysilyl derivatives of metal ligands and by direct, straightforward entrapment of organometallic catalysts in sol-gel matrices ". In this Section we concentrate on the latter and compare it with covalent attachment of trialkoxy derivatives, which is fully described in Section IV. As a representative example we consider the case of ion-pair catalysts " and, in particular, RhCls/Aliquat 336 and RhCl3/[Me3N(CH2)3... 

The Entrapment of Organometallic Catalysts within Sol-Gel Materials 1965 p-Toluenesulfonic acid silica ... 

The next classical pair of destructive steps that was addressed were oxidations and reductions. Here, the oxidant, pyridinium dichromate, and an H2 reduction catalyst RhCl[P(C6H5)3]3 were entrapped in separate silica sol-gel matrices [24], and with these entrapped reagent and catalyst, several different sequences of one-pot redox reaction pairs were carried out - up to four reactions in one pot - without their mutual destruction and with no need for separation steps one of these sequences is shown in Figure 31.10. More mutual destructive combinations are possible, and the last but not least mentioned here is the two-step reaction with a biocatalyst - an enzyme - and an organometallic catalyst, again... 

Entrapment of organometallic catalysts in sol-gel matrices opens the way to work with these catalysts in solvents not tailored originally for the catalysts. A simple example was the entrapment of several chiral catalytic complexes (Ru-BINAP, Rh-DIOP, and Rh—BPPM) [25] in order to perform enantioselective hydrogenation of itaconic acid. The catalysts are hydrophobic, and itaconic acid is water soluble however, after entrapment the catalytic reaction was made possible in water, by the dispersion of the catalytically doped sol-gel powder in water. More sophisticated approaches also involved emulsions, and here are several examples. 

In Section 31.6 we mentioned the enantioselective reduction of itaconic acid by a number of entrapped chiral organometallic catalysts [25]. A follow-up and major improvement of that study was reported by Volovych et al. [30]. These authors hydrogenated itaconic acid with sol-gel-entrapped Rh complexed with (2S,4S)-l-tert-butoxycarbonyl-4-diphenylphosphino-2-(diphe-nylphosphinomethyl)pyrrolidine catalyst in methanol solutions. The immobilization process was carried out with different sol-gel precursors TMOS, TEOS, triethoxyphenylsilane PhSi(OEt)3/TMOS, and trimethoxy (octyl)silane OcSi(OMe)3/TMOS. The choice of the precursor was found to influence the enantioselectivity and the rate of the reaction. The immobilized catalyst could be recovered and recycled several times under N2 atmosphere. About 90-99% ee was achieved for the hydrogenation of itaconic acid to (S)-(+)-2-methyl succinic acid. 

The same authors compared catalysts prepared from these precursors and [Ru(BINAP)Cl2]2 adsorbed on MCM-41 (with 26 and 37 A pores) and an amorphous mesoporous silica (with 68 A pores) all treated with combinations of SiPh2Cl2 and Si(CH2)3X (X = NH2, CO2H). Catalysts were also prepared in which the organometallic precursors were immobilized by entrapment into silica (using sol-gel techniques). This is one of the few studies in which the performance of chiral phosphine catalysts immobilized by covalent and noncovalent procedures are compared directly. The materials were examined as catalysts for the hydrogenation of sodium a-acetamidocinnamate and of a-acetamidocinnamic acid under similar conditions to those used for the catalysts on unmodified MCM-41. The catalysts... 

Different immobilization methods were applied for Jacobsen s catalyst. The entrapment of the organometallic complex in the supercages of the dealuminated zeolite was achieved without noticeable loss of activity and selectivity. The immobilized catalysts were reusable and did not leach. For the oxidation of (-)-a-pinene the system used only O2 at RT instead of sodium hypochloride at 0 °C. There was a disadvantage in the use of pivalic aldehyde for oxygen transformation via the corresponding peracid. This results in the formation of pivalic acid, which has to be separated from the reaction mixture. 

A variety of metal cluster compounds have been chemically bound on amorphous metal oxides and entrapped inside zeolite cages by new preparative tools such as surface organometallic chemistry and the so-called ship-in-bottle technique. They oflier much promise as molecular precursors for rational preparation of tailored metal catalysts having a uniform distribution of discrete metal-bimetallic ensembles, namely, organometallics which are active for catalytic reactions. They also provide advantages as metal precursors to achieve higher metal dispersions and well-managed metal... 

Obviously, the catalytic future of zeolite-occluded clusters and organometallics is two-fold. They can be used as coordination-type catalysts or act as zeolite-entrapped metal precursors. In both cases, the zeolite will provide its solid solvent property, its bifunctional capability (in a broad sense, either its acidic or macroanionic properties), and its unique shape-selective properties. 

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