Rhodium catalysts carbon monoxide conversion

The process requires two conversion steps. In the first, an olefin plus synthesis gas (carbon monoxide and hydrogen) are reacted over a cobalt or rhodium catalyst to produce two aldehydes, with one being an isomer of the other. 

Finally, it should be mentioned that there is one important commercial application of the organic halide carbonylation. This is in the rhodium and methyl iodide-catalyzed conversion of methanol and carbon monoxide into acetic acid (25). The mechanism of the reaction appears to involve the oxidative addition of methyl iodide to the rhodium(I) catalyst followed by CO insertion and hydrolysis ... 

The carbonylation process incorporates a rhodium salt, lithium iodide, and methyl iodide as primary catalyst components [80]. The active catalyst form is maintained by the lithium iodide promoter and hydrogen in the carbon monoxide feed to the reaction system. Preferred reaction conditions are a temperature of nearly 190 °C and a pressure of 5 MPa H2/CO. The conversion of methyl acetate to acetic anhydride per passage of reactor is between 50 and 75 %. 

Figure 106. Conversion of carbon monoxide, gaseous hydrocarbons and sulfur dioxide reached over diesel oxidation catalysts as a function of the preeious metal formulation at equimolar loading (monolith catalyst with 62 cells cm - dedicated diesel washeoat formulations with platinum, palladium and rhodium at an equimolar loading of 8 mmol I, fresh model gas test at a gas temperature at catalyst inlet of 723 K a space velocity 50000Nil h model gas simulates the exhaust gas composition of an IDl passenger car diesel engine at medium load and speed and contains 100 vol. ppm SO2). Reprinted with permission from ref. [68], C 1991 Society of Automotive Engineers, Inc.

The linear/branched ratio was 7.5 1, which was greater than that found with the control catalyst in solution. Rhodium(III) chloride has been immobilized on a support made by polymerization of vinylpyridine and divinylben-zene in the presence of silica. The best activity for the conversion of methanol to acetic acid by carbon monoxide was obtained after 20% of the pyridine groups were quaternized with methyl iodide. This suggests ionic bonding of a tetra-halorhodate ion to the polymer.211... 

Whereas the cobalt catalyst systems developed by BASF in particular guarantee a methanol ooce-through conversion of 70 per cent and molar yields in relation to alcohol and carbon monoxide better than 5 and 60 per cent, those developed by Monsanto, based on rhodium, offer better performance. Methanol once-through conversion may exceed 90 per cent and molar yields in relation to alcohol and carbon monoxide are between 98 and 99 per cent and 90 per cent respectively. 

The conversion, developed in particular by Union Carbide takes place with a yield of about 70 molar per cent, in. the presence of a supported rhodium base catalyst, at a temperature between 250 and 350°C, and 10 to 30.10tf Pa absolute, with a carbon monoxide once-through-conversion close to 20 per cent... 

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). 

Hydrocarboxylation is the formal addition of hydrogen and a carboxylic group to double or triple bonds to form carboxylic acids or their derivatives. It is achieved by transition metal catalyzed conversion of unsaturated substrates with carbon monoxide in the presence of water, alcohols, or other acidic reagents. Ester formation is also called hydroesterification or hydrocarb(o)alkoxylation . The transition metal catalyst precursors are nickel, iron or cobalt carbonyls or salts of nickel, iron, cobalt, rhodium, palladium, platinum, or other metals4 5. 

Platinum is an effective oxidation catalyst for carbon monoxide and the complete oxidation of hydrocarbons. Palladium also promotes the oxidation of carbon monoxide and hydrocarbons but is more sensitive to poisoning than platinum in the exhaust environment. Both platinum and palladium promote the reduction of nitric oxide but are less effective than rhodium. In addition to the noble metals, three-way catalysts contain the base metal cerium and possibly other additives such as lanthanum, nickel or iron. These base metal additives are believed to improve catalyst performance by extending conversion during the rapid air-fuel ratio perturbations and help to stabilize the alumina support against thermal degradation. 

Burke discloses a two-step process for the conversion of butadiene to adipic acid at high yields [156]. The first step is the hydrocarboxylation of butadiene to form 3-pentenoic acid. The second step is the hydrocarboxylation of 3-pentenoic acid with carbon monoxide and water in the presence of a rhodium-containing catalyst, an iodide promoter, and certain inert solvents such as methylene chloride. The first reaction step gives also a significant by-product of y-valerolactone and a minor by-product of a-methyl-7-butyrolactone. These lactones can be converted to adipic acid by modified catalyst compositions [157-159]. In a related work, pentenic acids or esters are used as the starting intermediates for conversion to adipic acid [160-166]. 

Rhodium was chosen as construction material for the reactor, which served as active catalyst species at the same time. Rhodium has a high thermal conductivity of 120 W/(m K). Twenty three foils carrying 28 channels each of which was sealed by electron beam and laser welding. The stack of foils formed a honeycomb which was pressure resistant up to 30 bar. The maximum operating temperature of the reactor was 1200°C. The feed was preheated to 300° C and then fed to the reactor. The experiments were carried out between ambient pressure and 25 bar at 0/C ratio 1.0. After ignition of the reaction between 550 and 700°C, 1000°C reaction temperature was then achieved within 1 min, and mainly carbon monoxide and hydrogen were formed. Only 62% conversion of methane but 98% conversion of oxygen was achieved at 1190°C. The performance of the reactor deteriorated when the system pressure was increased. By-product and even soot formation then occurred downstream the reactor. 

Lenz et al. [73] described the development of a 3 kW monolithic steam-supported partial oxidation reactor for jet fuel, which was developed to supply a solid oxide fuel cell (SOFC). The prototype reactor was composed of a ceramic honeycomb monolith (400 cpsi) operated between 950 C at the reactor inlet and 700°C at the reactor outlet [74]. The radial temperature gradient amoimted to 50 K which was attributed to inhomogeneous mixing at the reactor inlet. The feed composition corresponded to S/C ratio of 1.75 and O/C ratio of 1.0 at 50 000 h GHSV. Under these conditions, about 12 vol.% of each carbon monoxide and carbon dioxide were detected in the reformate, while methane was below the detection limit. Later, Lenz et al. [74] described a combination of three monolithic reactors coated with platinum/rhodium catalyst switched in series for jet fuel autothermal reforming. An optimum S/C ratio of 1.5 and an optimum O/C ratio of 0.83 were determined. Under these conditions 78.5% efficiency at 50 000 h GHSV was achieved. The conversion did not exceed 92.5%. In the product of these... 

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]. 

The process is operated at 175°C/13-25 atm. Methanol dissolved in aqueous acetic acid is treated with carbon monoxide in the presence of a soluble rhodium compound and an iodide. The nature of the rhodium compound is not critical after an induction period in which conversion to the active complex occurs, similar activity is attained. The selectivity to acetic acid is 99% based on methanol and 90% on CO. The major side reaction is the water gas shift CO + H O On thermodynamic grounds the reaction CH3OH H-CO-1-2H2 CHjCH OH should be favoured, but the catalyst is very specific in promoting only the desired conversion into acetic acid. 

Specchia et al. performed partial oxidation of methane over rhodium/a-alumina fixed catalyst beds for short contact times over a time range of between 10 and 40 ms [64]. With increasing catalyst particle sizes, conversion decreased, which was attributed to transport limitations. Fligher reactor temperature was observed for larger particles and thus more exothermic reactions took place. When increasing the particle size and the weight hourly space velocity (see Section 4.1), the water content in the product increased, while less carbon monoxide was found and carbon dioxide remained at an unchanged low concentration. Similar to the results of Lyubovski discussed above, steam seemed to be a primary product of the reaction. 

Owing to the negative reaction order of carbon monoxide for the preferential oxidation over noble metal based catalysts such as platinum [114,115] and platinum/ rhodium [116], the amount of catalyst required to achieve 90 or 99.9% conversion does not increase significantly [114]. 

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