Catalysts rhodium/zeolite

Table 1.4.1 Kinetic parameters for the simultaneous hydroformylation and hydrogenation of propene over a rhodium zeolite catalyst at 7" = 423 K and 1 atm total pressure [from Rode et al., J. Catal., 96 (1985) 563].

The addition of trimethylphosphine to these rhodium/zeolite catalysts destroyed all catalytic activity because the phosphine was small enough to fit into the zeolite cavity and could deactivate all of the rhodium in the catalyst. The bulky tributylphosphine, however, could not enter the cavity and, thereby, only blocked the external rhodium from further reaction. This specific blocking enhanced the selectivity in the hydrogenation of a mixture of cyclopentene and 4-methylcyclohexene over a Rh/ZSM-11 catalyst. After treatment of the catalyst with tributylphosphine to block the external catalytically active sites, only the... 

Zeolitic systems are very active at low temperatures but they also have disadvantages related to their hydrothermal stability and the possibility of inhibition or poisoning by different compounds. These drawbacks drastically limit the industrial applications of these catalysts. Rhodium-supported systems are also active at low temperatures and low N2O concentration, but at high temperatures and in the presence of O2 the noble metal is oxidized. Furthermore, the high cost of Rh may prove to be a limit for industrial applications. 

Various other rhodium catalysts can initiate hydroacylation reactions. Thus, the indenyl complex [075-C9H7)Rh(J72-C2H4)2] is used in intermolecular hydroacylation44. Rhodium zeolites (RhNaX and RhNaY type zeolites) act as bifunctional catalysts for the synthesis of 2-methyl-3-hexanone and 4-heptanone (1 2 ratio) from propene, carbon monoxide and hydrogen53. In this case, the ketones may be formed via hydrocarbonylation (vide supra), however, according to control experiments, rhodium-free zeolites alone catalyze ketone formation from propene and butyraldehyde53. 

Early studies by Scurrell and coll, demonstrated the use of rhodium zeolites as catalysts for the carbonylation of methanol into methyl acetate in the presence of methyl iodide (65). It was hoped that due to their electrostatic field zeolites would effect the direct carbonylation of methanol without the help of the iodide promoter. In fact, as the CH3OH/CH3I ratio increased, increasing amounts of CH4 and CO2 were produced indicating that the reaction... 

By encapsulation of the trialkylphosphine-modified rhodium catalyst into zeolites, the high chemoselectivity was maintained but the Hb ratio in the product alcohol could be increased by as much as 10 times [65]. Anchoring of trialkylphos-phines to carbosilane dendrimers based on polyhedral silsesquioxane (POSS) produced dendrimeric ligands, which in the hydroformylation-hydrogenation of 1-octene preferentially produced linear alcohols. Regioselectivity exceeded that achieved with the low molecular ligand [66]. 

Direct hydroxylation of benzene to phenol could be achieved using zeolite catalysts containing rhodium, platinum, palladium, or irridium. The oxidizing agent is nitrous oxide, which is unavoidable a byproduct from the oxidation of KA oil (see KA oil, this chapter) to adipic acid using nitric acid as the oxidant. 

Encapsulated rhodium complexes were prepared from Rh-exchanged NaY zeolite by complexation with (S)-prolinamide or M-tert-butyl-(S)-prolinamide [73,74]. Although these catalysts showed higher specific activity than their homogeneous counterparts in non-enantioselective hydrogenations, the hydrogenation of prochiral substrates, such as methyl (Z)-acetamidocinnamate [73] or ( )-2-methyl-2-pentenoic acid [74], led to low... 

A somewhat unusual copper catalyst, namely a zeolite in which at least 25% of its rhodium ions had been exchanged by Cu(II), was active in decomposition of ethyl diazoacetate at room temperature 372). In the absence of appropriate reaction partners, diethyl maleate and diethyl fumarate were the major products. The selectivity was a function of the zeolite activation temperature, but the maleate prevailed in all cases. Contrary to the copper salt-catalyzed carbene dimer formation 365), the maleate fumarate ratio was found to be relatively constant at various catalyst concentrations. When Cu(II) was reduced to Cu(I), an improved catalytic activity was observed. 

High nuclearity carbonyls Rh4(CO)i2 and Rhs(CO)i6 have been extensively used as precursors for the preparation of supported rhodium catalysts. Early studies reported the use of a great variety of supports that includes metal oxides [159-166], zeolites [101, 167], polymers [168] and modified-silica surface [169]. 

Since 1981, three-way catalytic systems have been standard in new cars sold in North America.6,280 These systems consist of platinum, palladium, and rhodium catalysts dispersed on an activated alumina layer ( wash-coat ) on a ceramic honeycomb monolith the Pt and Pd serve primarily to catalyze oxidation of the CO and hydrocarbons, and the Rh to catalyze reduction of the NO. These converters operate with a near-stoichiometric air-fuel mix at 400-600 °C higher temperatures may cause the Rh to react with the washcoat. In some designs, the catalyst bed is electrically heated at start-up to avoid the problem of temporarily excessive CO emissions from a cold catalyst. Zeolite-type catalysts containing bound metal atoms or ions (e.g., Cu/ZSM-5) have been proposed as alternatives to systems based on precious metals. 

Sanchez, F., Iglesias, M., Corma, A. and Delpino, C. New rhodium complexes anchored on silica and modified Y-zeolite as efficient catalysts for hydrogenation of olefins, J. Mol. Catal., 1991, 70, 369-379. 

The isomer ratios are often close to the equilibrium composition at the particular reaction temperature, indicating isomerization as well as dimerization catalytic activity. The two most extensively studied zeolite catalysts contain nickel and rhodium, incorporated via ion exchange, and will be discussed separately. 

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