Rhodium zeolite

A RhNaY (1 wt.% Rh) catalyst was prepared by ion exchange using an aqueous solution of [Rh(NH3)6]Cl3 (226, 229), followed by treatment with a CO/H2 (1/1) mixture at 130°C and 80 atm pressure. The catalyst so formed was observed to have good activity and high total aldehyde selectivity ( 95%) for the liquid-phase hydroformylation of hexene-1. Some typical results are shown in Table VI, which indicate that the normal/iso aldehyde product ratio is similar to that obtained with homogeneous rhodium carbonyl catalysts (223). 

The catalyst was examined (226) before and after the pretreatment with CO/H2 by means of infrared (IR) spectroscopy. IR bands characteristic of terminal and bridging carbonyl groups associated with rhodium carbonyl clusters were observed following this pretreatment of the catalyst. However, these spectra were apparently not identical to those of the known rhodium clusters, Rh4(CO)12 or Rh6(CO)16. The differences were associated with the positions of the bands due to bridging carbonyl groups. It is conceivable that 

The authors also claimed that the zeolite catalyst exhibited unusually high selectivity to dialdehydes (60%) in the hydroformylation of 1,5-hexadiene. Unfortunately, comparative data obtained under similar conditions with homogeneous rhodium carbonyl catalysts were not presented. 

A patent (230) to Atlantic Richfield Co. claims that hydride platinum group metal carbonyl complexes such as ClRh(PPh3)3 supported on zeolites, for example, NaY, are suitable catalysts for the hydroformylation of low molecular weight olefins. However, since the bulky metal complex cannot diffuse into the inner pores of the zeolite it must simply be adsorbed on the external surface of the support. This is consistent with the rather poor catalyst stability which was attributed to leaching of the active species from the support. 

The synthesis of methane from synthesis gas (CO/H2) is of considerable importance in the production of substitute natural gas (SNG) i.e., 

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For hydroformylation over cobalt and rhodium zeolites the active species have not been defined. However, in the case of RhNaY the in situ formation of a rhodium carbonyl cluster has been identified (226) by infrared spectroscopy. Interestingly, this cluster appears to be different from known compounds such as Rh4(CO)12 and Rh6(CO)16. This does suggest that alternative carbonyl clusters may possibly be formed in zeolites due to the spatial restrictions of the intracrystalline cavities. The mechanism of hydroformylation in these zeolites is probably similar to that known for homogeneous catalysis. 

This intricate behaviour was reflected in the competitive carbonylation of the methyl and ethyl radicals when EtI + MeOH and Mel + EtOH mixtures were reacted. Ethyl acetate but also trace methyl acetate were produced. No ethyl propionate was detected on the one hand and essentially methyl acetate and lower amounts of propionic acid, methyl propionate and ethyl acetate were formed (25) using rhodium zeolites indicating that various exchange reactions were proceeding with high enough rates so as to result in a different effective feed composition. These side reactions reveal the importance of the polarity of the medium as well as the nature of the transition metal. 

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... 

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... 

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... 

The first pathway proposed by Iizuka and Lunsford [17] considers the reduction of nitric oxide by CO over rhodium/Y-zeolite. It leads only to N20 as follows ... 

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. 

T. J. McCarthy, G.-D. Lei, and W. M. H. Sachtler, Methylcyclopentane conversion catalysis over zeolite Y encaged rhodium a test for the metal-proton adduct model, J. Catal. 159, 90-98... 

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