Rhodium/alumina catalysts, carbon

Alkali moderation of supported precious metal catalysts reduces secondary amine formation and generation of ammonia (18). Ammonia in the reaction medium inhibits Rh, but not Ru precious metal catalyst. More secondary amine results from use of more polar protic solvents, CH OH > C2H5OH > Lithium hydroxide is the most effective alkah promoter (19), reducing secondary amine formation and hydrogenolysis. The general order of catalyst procUvity toward secondary amine formation is Pt > Pd Ru > Rh (20). Rhodium s catalyst support contribution to secondary amine formation decreases ia the order carbon > alumina > barium carbonate > barium sulfate > calcium carbonate. 

The direct reduction of gallic acid described here illustrates the virtue of the rhodium-on-alumina catalyst to achieve the perhydrogenation of polyhydroxylated aromatic compounds with minimal attendant hydrogenolysis. A closely related hydrogenation, that of pyrogallol, to yield a dihydro intermediate, and also the direct reduction of pyrogallol with palladium-on-stron-tium carbonate to afford the all ci5-pyrogallitol (1,2,3-cyclohex-anetriol) have been reported. ... 

Galwey AK, Bettany DG, Mortimer M (2006) Kinetic compensation effects observed during oxidation of carbon monoxide on y-alumina supported palladium, platinum, and rhodium metal catalysts toward a mechanistic explanation. Int J Chem Kin 38 689... 

The influence of the support is undoubted and spillover was further confirmed by the excess of hydrogen chemisorbed by a mechanical mixture of unsupported alloy and TJ-A1203 above that calculated from the known values for the separate components. It was also observed that the chemisorption was slower on the supported than on the unsupported metal and that the greater part of the adsorbate was held reversibly no comment could be made on the possible mediation by traces of water. On the other hand, spillover from platinum-rhenium onto alumina appears to be inhibited for ratios Re/(Pt Re) > 0.6. In an infrared investigation of isocyanate complexes formed between nitric oxide and carbon monoxide, on the surface of rhodium-titania and rhodium-silica catalysts, it seems that the number of complexes exceeded the number of rhodium surface atoms.The supports have a pronounced effect on the location of the isocyanate bond and on the stability of the complexes, with some suggestion of spillover. 

A palladium derivative of an oligo-p-phenylenetereph-thamide performed better as a catalyst for the reduction of phenylacetylene to ethylbenzene than palladium on carbon or silica or alumina catalysts.207 A rhodium(I) complex of the polyamide (5.48) was used to catalyze the addition of silanes to 1,3-dienes with good regio- and stereoselectivity.208 Rhodium is one of the metals that is so expensive that losses must be kept at an absolute minimum. 

The catalyst system for the modem methyl acetate carbonylation process involves rhodium chloride trihydrate [13569-65-8]y methyl iodide [74-88-4], chromium metal powder, and an alumina support or a nickel carbonyl complex with triphenylphosphine, methyl iodide, and chromium hexacarbonyl (34). The use of nitrogen-heterocyclic complexes and rhodium chloride is disclosed in one European patent (35). In another, the alumina catalyst support is treated with an organosilicon compound having either a terminal organophosphine or similar ligands and rhodium or a similar noble metal (36). Such a catalyst enabled methyl acetate carbonylation at 200°C under about 20 MPa (2900 psi) carbon monoxide, with a space-time yield of 140 g anhydride per g rhodium per hour. Conversion was 42.8% with 97.5% selectivity. A homogeneous catalyst system for methyl acetate carbonylation has also been disclosed (37). A description of another synthesis is given where anhydride conversion is about 30%, with 95% selectivity. The reaction occurs at 445 K under 11 MPa partial pressure of carbon monoxide (37). A process based on a montmorillonite support with nickel chloride coordinated with imidazole has been developed (38). Other related processes for carbonylation to yield anhydride are also available (39,40). 

The results obtained show that the most active metal is Rhodium, particularly when supported on alumina, with carbon and graphite giving lower activity catalysts, which consequently give different selectivities. Rh/C, particularly in hexane solvent gave the highest selectivities to cis-cyclohexanol. Of the other metals evaluated (Pt, Pd, Ru, Ir) only Pt showed significant activity when used in hexane solvent, but this gave the reverse selectivity to the Rh/C catalyst. 

Catalytic reductions have been carried out under an extremely wide range of reaction conditions. Temperatures of 20 C to over 300 C have been described. Pressures from atmospheric to several thousand pounds have been used. Catal3rsts have included nickel, copper, cobalt, chromium, iron, tin, silver, platinum, palladium, rhodium, molybdenum, tungsten, titanium and many others. They have been used as free metals, in finely divided form for enhanced activity, or as compounds (such as oxides or sulfides). Catalysts have been used singly and in combination, also on carriers, such as alumina, magnesia, carbon, silica, pumice, clays, earths, barium sulfate, etc., or in unsupported form. Reactions have been carried out with organic solvents, without solvents, and in water dispersion. Finally, various additives, such as sodium acetate, sodium hydroxide, sulfuric acid, ammonia, carbon monoxide, and others, have been used for special purposes. It is obvious that conditions must be varied from case to case to obtain optimum economics, yield, and quality. 

Heterooeneous catalysts. In 1967 Hardy and Bennett reported the conversion o-f aromatic mononitro compounds, to isocyanates by CO, catalysed by palladium or rhodium on alumina or carbon, with dry FeCl as co-catal yst 027] ... 

Alumina foams have been directly impregnated for propane CPO and OSR [13, 14, 40] to yield 0.01 wt.% rhodium. The catalyst on the foam body, which was 15 mm in diameter, 7 mm long and contained 400 cells per square inch (84% porosity), showed optimum performance at an oven temperature of700 °C and good stability under CPO conditions (C O = 0.8), even though a remaining hot spot of more than 200 K was observed in the foam. Under OSR conditions (C 0 = 0.5 and steam to carbon ratio = 1) only a 150 K hot spot was observed. However, the catalyst deactivated more rapidly, maybe due to the increase in byproduct formation. Complete homogeneous conversion was observed at an oven temperature of 800 °C... 

Rhodium (5%) on carbon is a better catalyst for the reduction of the isomeric pyridinecarboxylic acids, their esters, and amides to the corresponding piperidine analogues than is rhodium on alumina. Reductions are generally slow, but yields are very satisfactory. Catalytic reductions of pyridylalkane... 

The SR of methanol is typically performed in the temperature range 523-573 K. Cu/ZnO catalysts are most frequently used, often supported on alumina. The use of palladium- and rhodium-containing catalysts has also been reported [115], in addition to some promoters [116]. The main by-product is carbon monoxide. To minimize the concentration of CO in the product stream, the reaction is performed using a stoichiometry of steam to methanol of > 1. The conversion of CO with H2O is known as the water gas shift (WGS) reaction and is addressed later [Eq. (15.9)]. Detailed information about the reaction path of methanol SR on Cu/ZnO catalysts and the active sites of Cu/ZnO catalysts together with details of the Pd/ZnO system can be found elsewhere [116]. 

Wanat et al. investigated methanol partial oxidation over various rhodium containing catalysts on ceramic monoliths, namely rhodium/alumina, rhodium/ceria, rhodium/ruthenium and rhodium/cobalt catalysts [195]. The rhodium/ceria sample performed best. Full methanol conversion was achieved at reaction temperatures exceeding 550 °C and with O/C ratios of from 0.66 to 1.0. Owing to the high reaction temperature, carbon monoxide selectivity was high, exceeding 70%. No by-products were observed except for methane. 

Cost. The catalytically active component(s) in many supported catalysts are expensive metals. By using a catalyst in which the active component is but a very small fraction of the weight of the total catalyst, lower costs can be achieved. As an example, hydrogenation of an aromatic nucleus requires the use of rhenium, rhodium, or mthenium. This can be accomplished with as fittie as 0.5 wt % of the metal finely dispersed on alumina or activated carbon. Furthermore, it is almost always easier to recover the metal from a spent supported catalyst bed than to attempt to separate a finely divided metal from a liquid product stream. If recovery is efficient, the actual cost of the catalyst is the time value of the cost of the metal less processing expenses, assuming a nondeclining market value for the metal. Precious metals used in catalytic processes are often leased. 

Hydrogenation. Hydrogenation is one of the oldest and most widely used appHcations for supported catalysts, and much has been written in this field (55—57). Metals useflil in hydrogenation include cobalt, copper, nickel, palladium, platinum, rhenium, rhodium, mthenium, and silver, and there are numerous catalysts available for various specific appHcations. Most hydrogenation catalysts rely on extremely fine dispersions of the active metal on activated carbon, alumina, siHca-alumina, 2eoHtes, kieselguhr, or inert salts, such as barium sulfate. 

Rapoport s findings have been confirmed in the authors laboratory where the actions of carbon-supported catalysts (5% metal) derived from ruthenium, rhodium, palladium, osmium, iridium, and platinum, on pyridine, have been examined. At atmospheric pressure, at the boiling point of pyridine, and at a pyridine-to-catalyst ratio of 8 1, only palladium was active in bringing about the formation of 2,2 -bipyridine. It w as also found that different preparations of palladium-on-carbon varied widely in efficiency (yield 0.05-0.39 gm of 2,2 -bipyridine per gram of catalyst), but the factors responsible for this variation are not knowm. Palladium-on-alumina was found to be inferior to the carbon-supported preparations and gave only traces of bipyridine,... 

The most widely used method for adding the elements of hydrogen to carbon-carbon double bonds is catalytic hydrogenation. Except for very sterically hindered alkenes, this reaction usually proceeds rapidly and cleanly. The most common catalysts are various forms of transition metals, particularly platinum, palladium, rhodium, ruthenium, and nickel. Both the metals as finely dispersed solids or adsorbed on inert supports such as carbon or alumina (heterogeneous catalysts) and certain soluble complexes of these metals (homogeneous catalysts) exhibit catalytic activity. Depending upon conditions and catalyst, other functional groups are also subject to reduction under these conditions. 

Palladium gave the highest activity of all the platinum group metals evaluated platinum, rhodium and ruthenium exhibited very poor activity. The choice of support was also demonstrated to be very important the activated carbon supported Pd catalyst showed a nearly fourfold increase in activity than did Pd supported on alumina. 

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