Methanol rhodium zeolites

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... 

Carbonylation of organic substrates was investigated using these well defined complexes. These carbonyl compounds exhibited catalytic properties in the carbonylation of organic substrates. In particular methanol carbonylation to methyl acetate in the gas phase was successfully attempted. Mechanistic and kinetic studies of this reaction over rhodium and iridium zeolites showed the similarities between the homogeneous and the zeolite mediated reactions. Aromatic ni-tro compounds were also converted to aromatic isocyanates using similar catalytic systems. The mechanistic aspect of this reaction will be also examined. 

For many catalysts, the major component is the active material. Examples of such unsupported catalysts are the aluminosilicates and zeolites used for cracking petroleum fractions. One of the most widely used unsupported metal catalysts is the precious metal gauze as used, for example, in the oxidation of ammonia to nitric oxide in nitric acid plants. A very fast rate is needed to obtain the necessary selectivity to nitric oxide, so a low metal surface area and a short contact time are used. These gauze s are woven from fine wires (0.075 mm in diameter) of platinum alloy, usually platinum-rhodium. Several layers of these gauze s, which may be up to 3 m in diameter, are used. The methanol oxidation to formaldehyde is another process in which an unsupported metal catalyst is used, but here metallic silver is used in the form of a bed of granules. 

By selecting (he proper reaction conditions, ccdialt and rhodium catalysts prove active for the direct hydrogenation of the intermediate di dehyde (0 ethylene glycol [44). Glycolaldehyde is also formed with a zeolite/NaOH catalyst [451, Monsanto has patented a route to ethylene ycol, which starts with methanol (Equation (21)) [46]. 

Lefebvre et al. (170) have conducted the high pressure CO + H2 reaction (30 atm, 503-523 K) over Rh-NaY catalysts. Whatever the rhodium precursors [e.g., Rh -NaY and Rh (CO)2-NaY], the reaction data were similar. This is in agreement with the fact that all the precursors were ultimately converted to Rh6(CO),6 under catalytic conditions. The external Rh crystals deposited on the zeolite surface exhibit significant activity for hydrocarbons, mainly methane, whereas the carbonyl clusters gave lower conversion to hydrocarbons with a small amount of oxygenates such as methanol and ethanol. 

An important requirement for all homogeneous catalytic processes is that the dissolved catalyst must be separated from the liquid product and recycled to the reactor without significant catalyst loss the need is acute when the metal is as expensive as rhodium. One approach to aid this separation process is to immobilize (anchor) the soluble catalyst on a solid support in order to confine the catalyst to the reactor and overcome the need for a catalyst recycle step. A number of types of solid support have been employed to anchor rhodium catalysts for use in methanol carbon-ylation with liquid- or gas-phase reactants. These were reviewed by Howard et al. in 1993 [8] and include activated carbon, inorganic oxides, zeolites, and a range of polymeric materials. 

Carbonylation and decarbonylation reactions of alkyl complexes in catalytic cycles have been reviewed . A full account of the carbonylation and homologation of formic and other carboxylic acid esters catalysed by Ru/CO/I systems at 200 C and 150-200 atm CO/H2 has appeared. In a novel reaction, cyclobutanones are converted to disiloxycyclopentenes with hydrosilane and CO in the presence of cobalt carbonyl (reaction 4) . The oxidative addition of Mel to [Rh(CO)2l2] in aprotic solvents (MeOH, CHCI3, THF, MeOAc), the rate determining step in carbonylation of methyl acetate and methyl halides, is promoted by iodides, such as Bu jN+I", and bases (eg 1-methylimidazole) . A further kinetic study of rhodium catalysed methanol carbonylation has appeared . The carbonylation of methanol by catalysts prepared by deposition of Rh complexes on silica alumina or zeolites is comparable with the homogeneous analogue . 

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