Catalyst rhodium

Rhodium Catalysts. - The hydroformylation of propene with a Rh/triphenyl-phosphine catalyst is now an established industrial process which will consume over a million tonnes per annum of propene when all licensed plants are operational. Most of the product n-butyraldehyde is converted to 2-ethylhexanol for plasticiser applications. The process is also applicable to the hydroformylation of C2, C4, and C5 alkenes. The process is remarkable for the long lifetime of the Rh catalyst but by-products are formed which deactivate the catalyst and have to be removed. The formation of triphenyl-phosphine oxide, benzaldehyde, and propyldiphenylphosphine under hydroformylation conditions has been investigated where benzaldehyde is produced by or /zo-metallation of triphenylphosphine followed by CO insertion and P-C bond cleavage and propyldiphenylphosphine was assumed to result from reaction of propene with the co-ordinated diphenylphosphine group remaining after benzaldehyde formation. The same authors have also studied the kinetics of the formation of heavy by-products which are dependent on 

Gregorio, G. Montrasi,M. Tampieri, P. Cavalieri d Oro,G. Pagani, and A. Andreeta, Symp. Rhodium Homogeneous Catal. [ProcJ, 1978, p. 121 also Chem. Ind. (Milan), 1980, 62, 389. 

The hydroformylation of various alkenes by Rh catalysts has been well covered in recent reviews and will not be discussed here. Modification of the catalytic activity of Rh by ligand variation or by addition of promoters 

Asymmetric hydroformylations with the chiral ligand DIOP (1) and variations on this structure have also been studied. Although optical 

Butadiene is polymerized by rhodium compounds in aqueous or alcoholic solution [178]. It is generally accepted that the active species is a TT-allyl rhodium complex of low valency [28, 179] which is not rapidly terminated by reaction with water or alcohol. No clear kinetic pattern was observed in the earlier papers but a recent investigation [180] has shown the rate and molecular weight data to be accommodated by a scheme involving monomer transfer and physical immobilization of the active centres in precipitated polymer. In the initial stages the polymerization is first order in rhodium and, at constant monomer concentration, is (pseudo) zero order E = 14.8 kcal mole ). This is followed by a declining rate which is almost independent of temperature. Molecular weights rise slowly to a maximum value with time (ca. 4000 after 22 h at 70°C). 

From the linear relationship between 1/P and 1/ [M] o [M] all the rhodium initiates polymerization and [C ]/[Rh] = 2 (Fig. 21) hence the slow rate must be the result of a slow propagation reaction. The existence of a monomer transfer reaction is shown by a linear relationship between 1/P and l/[M]o. The effect of emulsifiers, known to be important in accelerating the rate and rather specific in behaviour [181], were considered to stabilize the active centres against termination. 

There are few reports on the use of rhodium for methanol homologation, although this metal is best suited for the related methanol carbonylation. The extreme selectivity of rhodium for carbonylation is noteworthy and even when a 1 1 H]/CO mixture Ls applied, selective formation of acetic acid occurs and virtually no hydrogenated by products, such as ethanol or acetaldehyde, arc detected [68], 

Accordingly, Dcluzarche et a/, observed the formation of acetates, rather than of ethanol, with catalysts such as Rh4(CO)i and RhA(C0)t , although a syngas rich in hydrogen was used 26.  

Using the catalyst system known from the Monsanto process, Dumas et at. have been able to direct the reaction towards ethanol formation using syngas mixtures extremely rich in hydrogen [87]. As is shown in Table XII, no acetic acid and only minor amounts of acetates are formed at an H3/CO ratio of 60. Ethanol and acetaldehyde aie the main products along with considerable amounts of methyl ethyl ether. Unfortunately, the Dumas c/ at. based the yields and conversion on carbon monoxide and not on methanol. This makes the data of this interesting process difficult to compare with those of other catalyst systems. 

A mechanism via methyliodidc formation, oxidative addition to rhodium, CO insertion, oxidative addition of hydrogen, and elimination of the aldehyde 

Meihanol homplogatton with ihodium cataly ts with syngas rich in hydrogen li7  

Catalytic oxygenation of olefins to ketones 3.2.2.1. Rhodium catalysts 

The reactivity of coordinated superoxo ligand is in some cases sufficient to abstract an H-atom from a hydrocarbon, as in the case of a cyclooctene-rhodiumCI)-superoxo complex [152-154]  

The actual transfer of the dioxygen ligand to the coordinated olefin has been demonstrated for [(diene)Rh02]2 complexes with diene = 

Oxidation of styrene by 0 is catalyzed by the dimeric rhodium(I) complex [ RhCl(C2H )2)2] HO 0 [156]. Typically the reaction 

Terminal olefins are oxygenated to methyl ketones as main products in the presence of RhCl(Ph P) or RhCl(Ph2P)2(02 catalyst, in dry 

Before turning to specific results we will have a look at the properties of rhodium(II) acetates/carboxamidates as catalysts for reactions with diazocompounds as the substrates via carbenoid intermediates. Rhodium(II) has a d7 electron configuration, forming the lantern type dimers with bridging carboxylates. The single electrons in the respective dz2 orbitals form an electron 

The rhodium dimer has two Lewis acidic sites and thus the catalyst could coordinate to two substrate molecules under saturation kinetics, which would make the Michaelis-Menten plots complicated. This does not happen and the second site becomes less acidic once the other site is occupied by the substrate. What does happen, though, is that other Lewis bases compete with the substrate, as might be expected. The ligand dissociation reaction may be part of the rate equation of the process. Coordination of one Lewis base reduces already the activity of the catalyst. The solvent of choice is often anhydrous dichloromethane. The polar group may also be part of one of the substrates and in this instance one cannot avoid inhibition. 

Knowledge of patents claiming cobalt catalysts for the conversion of H2/CO mixtures to ethylene glycol (31-33) appears to have led to initial investigation of rhodium catalysts for this reaction at Union Carbide (27, 85-87). Early experiments by Pruett and Walker at pressures of about 3000 atm indicated that the activity of rhodium was notably greater than that found for cobalt. Several other potential catalyst precursors, including compounds of Sn, Ru, Pd, Pt, Cu, Cr, Mn, Ir, and Pb, were screened for activity under pressures of about 1500 atm and found not to produce detectable 

Examples from the patent literature, as shown in Expts. B and C of Table VI, are generally found to contain possible secondary products in much lower proportions. These examples illustrate selectivities to the two major primary products, methanol and ethylene glycol, of greater than 90%. The higher proportion of probable secondary products in Expt. A would appear to be a result of carrying out this reaction to very high product concentrations the volume of liquid products formed during the experiment is approximately equal to the initial volume of solvent. Results of experi- 

The reaction rates in this system are presumably first-order in catalyst concentration, as implied by the scaling of product formation rates proportionately to rhodium concentration (90, 92, 93). Responses to several other reaction variables may be found in both the open and patent literature. Fahey has reported studies of catalyst activity at several pressures in tet-raglyme solvent with 2-hydroxypyridine promoter at 230°C (43). He finds that the rate to total products is proportional to the pressure taken to the 3.3 power. A large pressure dependence is also evident in the results shown in Table VII. Analysis of these results indicates that the rate of ethylene glycol formation is greater than third-order in pressure (exponents of 3.2-3.5), and that for methanol formation somewhat less (exponents of 2.3-2.8). The pressure dependence of the total product formation rate is close to third-order. A possible complicating factor in the above comparisons is the increased loss of soluble rhodium species in the lower-pressure experiments, as seen in Table VII. Experiments similar to those of Fahey have also been 

Solvent Pressure (atm) Ethylene glycol rate (hr- ) Methanol rate (hr-1) Rhodium recovery (%)f 

Before proceeding to a more detailed description of the effects of various solvents and promoters on catalyst activity and stability, it should be noted that the responses described above are possibly, or even probably, influenced by solvents and promoters. The responses shown, however, appear to be generally characteristic of these rhodium-containing systems. It is apparent that the rate of product formation is significantly accelerated by increases in reaction temperature. Higher temperatures, however, can bring about catalyst instability unless the pressure is simultaneously increased. Higher 

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]. 

Similarly, the adsorption of Rli4(CO)i2 on y-Al203 followed by an oxidative fragmentation produced dicarbonyl rhodium (1) species that have been characterized by EXAFS [100]. 

The effects of ammonium chromate, ammonium dichromate, ammonium molybdate, and ammonium tungstate as additives on the catalytic performance of rhodium catalysts for the hydroformylation of Cg-oleftns and 1-dodecene were investigated. Modification of the rhodium catalyst with ammonium salts resulted in moderate increase in aldehyde yields and decrease in rhodium losses in the distillation process for the separation of products from the catalyst [11]. 

Hydroformylation of 3 at 80 °C and 80bar CO H2/1 1 pressure using 5 mol% RhCl(PPh3)3 as the catalyst precursor was found to give an isomeric mixture of4 and 

5 quantitatively (Equation 7.4), which can be converted into the corresponding alcohols [12]. 

The naturally occurring cinchona alkaloids, cinchonidine, quinine, and quinidine, were hydroformylated selectively to the corresponding terminal aldehyde derivatives with 87, 71, and 85% isolated yields, respectively, using Rh(CO)2(acac)/tetrapho-sphite/5 catalyst system at 90 °C and 20bar CO H2 = 1 1 in toluene [15]. 

The hydroformylation of vegetable oils (soybean, high oleic safflower, safflower, and linseed) using Rh(CO)2(acac) as the catalyst precursor in the presence of PPhs or (PhOlsP was studied. The ligand (PhO)3P resulted in a lesser reactivity compared to TPP in contrast to the rates of bulky phosphite ligands reported in the literature [16]. 

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Some examples of the use of a temporary additional site of coordination have been published. Burk and Feaster have transformed a series of ketones into hydrazones capable of chelating to a rhodium catalyst (Scheme 4.7). Upon coordination, enanti os elective hydrogenation of the hydrazone is feasible, yielding N-aroylhydrazines in up to 97% ee. Finally, the hydrazines were transformed into amines by treatment with Sml2. 

Similar activation takes place in the carbonylation of dimethyl ether to methyl acetate in superacidic solution. Whereas acetic acid and acetates are made nearly exclusively using Wilkinson s rhodium catalyst, a sensitive system necessitating carefully controlled conditions and use of large amounts of the expensive rhodium triphenylphosphine complex, ready superacidic carbonylation of dimethyl ether has significant advantages. 

On catalytic hydrogenation over a rhodium catalyst the compound shown gave a mixture containing as 1 ten butyl 4 methylcyclohexane (88%) and trans 1 ten butyl 4 methylcyclo hexane (12%) With this stereochemical result in mind consider the reactions in (a) and (b)... 

CO, and methanol react in the first step in the presence of cobalt carbonyl catalyst and pyridine [110-86-1] to produce methyl pentenoates. A similar second step, but at lower pressure and higher temperature with rhodium catalyst, produces dimethyl adipate [627-93-0]. This is then hydrolyzed to give adipic acid and methanol (135), which is recovered for recycle. Many variations to this basic process exist. Examples are ARCO s palladium/copper-catalyzed oxycarbonylation process (136—138), and Monsanto s palladium and quinone [106-51-4] process, which uses oxygen to reoxidize the by-product... 

Process Technology. In a typical oxo process, primary alcohols are produced from monoolefins in two steps. In the first stage, the olefin, hydrogen, and carbon monoxide [630-08-0] react in the presence of a cobalt or rhodium catalyst to form aldehydes, which are hydrogenated in the second step to the alcohols. 

Rhodium Ca.ta.lysts. Rhodium carbonyl catalysts for olefin hydroformylation are more active than cobalt carbonyls and can be appHed at lower temperatures and pressures (14). Rhodium hydrocarbonyl [75506-18-2] HRh(CO)4, results in lower -butyraldehyde [123-72-8] to isobutyraldehyde [78-84-2] ratios from propylene [115-07-17, C H, than does cobalt hydrocarbonyl, ie, 50/50 vs 80/20. Ligand-modified rhodium catalysts, HRh(CO)2L2 or HRh(CO)L2, afford /iso-ratios as high as 92/8 the ligand is generally a tertiary phosphine. The rhodium catalyst process was developed joindy by Union Carbide Chemicals, Johnson-Matthey, and Davy Powergas and has been Hcensed to several companies. It is particulady suited to propylene conversion to -butyraldehyde for 2-ethylhexanol production in that by-product isobutyraldehyde is minimized. 

The acetic anhydride process employs a homogeneous rhodium catalyst system for reaction of carbon monoxide with methyl acetate (36). The plant has capacity to coproduce approximately 545,000 t/yr of acetic anhydride, and 150,000 t/yr of acetic acid. One of the many challenges faced in operation of this plant is recovery of the expensive rhodium metal catalyst. Without a high recovery of the catalyst metal, the process would be uneconomical to operate. 

Other Methods. A variety of other methods have been studied, including phenol hydroxylation by N2O with HZSM-5 as catalyst (69), selective access to resorcinol from 5-methyloxohexanoate in the presence of Pd/C (70), cyclotrimerization of carbon monoxide and ethylene to form hydroquinone in the presence of rhodium catalysts (71), the electrochemical oxidation of benzene to hydroquinone and -benzoquinone (72), the air oxidation of phenol to catechol in the presence of a stoichiometric CuCl and Cu(0) catalyst (73), and the isomerization of dihydroxybenzenes on HZSM-5 catalysts (74). 

This process is one of the three commercially practiced processes for the production of acetic anhydride. The other two are the oxidation of acetaldehyde [75-07-0] and the carbonylation of methyl acetate [79-20-9] in the presence of a rhodium catalyst (coal gasification technology, Halcon process) (77). The latter process was put into operation by Tennessee Eastman in 1983. In the United States the total acetic anhydride production has been reported to be in the order of 1000 metric tons. 

MMA and MAA can be produced from ethylene [74-85-1/ as a feedstock via propanol, propionic acid, or methyl propionate as intermediates. Propanal may be prepared by hydroformylation of ethylene over cobalt or rhodium catalysts. The propanal then reacts in the Hquid phase with formaldehyde in the... 

The search for catalyst systems which could effect the 0x0 reaction under milder conditions and produce higher yields of the desired aldehyde resulted in processes utilizing rhodium. Oxo capacity built since the mid-1970s, both in the United States and elsewhere, has largely employed tertiary phosphine-modified rhodium catalysts. For example, over 50% of the world s butyraldehyde (qv) is produced by the LP Oxo process, technology Hcensed by Union Carbide Corporation and Davy Process Technology. 

Ligand-Modified Rhodium Process. The triphenylphosphine-modified rhodium oxo process, termed the LP Oxo process, is the industry standard for the hydroformylation of ethylene and propylene as of this writing (ca 1995). It employs a triphenylphosphine [603-35-0] (TPP) (1) modified rhodium catalyst. The process operates at low (0.7—3 MPa (100—450 psi)) pressures and low (80—120°C) temperatures. Suitable sources of rhodium are the alkanoate, 2,4-pentanedionate, or nitrate. A low (60—80 kPa (8.7—11.6 psi)) CO partial pressure and high (10—12%) TPP concentration are critical to obtaining a high (eg, 10 1) normal-to-branched aldehyde ratio. The process, first commercialized in 1976 by Union Carbide Corporation in Ponce, Puerto Rico, has been ficensed worldwide by Union Carbide Corporation and Davy Process Technology. 

Other Rhodium Processes. Unmodified rhodium catalysts, eg, 1 14(00)22 [19584-30-6] have high hydroformylation activity but low selectivity to normal aldehydes. 

Meth5l-l,3-propanediol is produced as a by-product. The hydroformylation reaction employs a rhodium catalyst having a large excess of TPP (1) and an equimolar (to rhodium) amount of 1,4-diphenylphosphinobutane (DPPB) (4). Aqueous extraction/decantation is also used in this reaction as an alternative means of product/catalyst separation. 

High enantioselectivities and regioselectivities have been obtained using both mono- and 1,2-disubstituted prochinal olefins employing chiral phosphine phosphite (33,34) modified rhodium catalysts. For example, i7j -2-butene ia the presence of rhodium and (12) (33) gave (3)-2-meth5ibutanal ia an optical yield of 82% at a turnover number of 9.84. ... 

Efficient enantioselective asymmetric hydrogenation of prochiral ketones and olefins has been accompHshed under mild reaction conditions at low (0.01— 0.001 mol %) catalyst concentrations using rhodium catalysts containing chiral ligands (140,141). Practical synthesis of several optically active natural... 

The most common oxidation states, corresponding electronic configurations, and coordination geometries of iridium are +1 (t5 ) usually square plane although some five-coordinate complexes are known, and +3 (t7 ) and +4 (t5 ), both octahedral. Compounds ia every oxidation state between —1 and +6 (<5 ) are known. Iridium compounds are used primarily to model more active rhodium catalysts. 

A major step in the production of nitric acid [7697-37-2] (qv) is the catalytic oxidation of ammonia to nitric acid and water. Very short contact times on a platinum—rhodium catalyst at temperatures above 650°C are required. 

Conventional triorganophosphite ligands, such as triphenylphosphite, form highly active hydroformylation catalysts (95—99) however, they suffer from poor durabiUty because of decomposition. Diorganophosphite-modified rhodium catalysts (94,100,101), have overcome this stabiUty deficiency and provide a low pressure, rhodium catalyzed process for the hydroformylation of low reactivity olefins, thus making lower cost amyl alcohols from butenes readily accessible. The new diorganophosphite-modified rhodium catalysts increase hydroformylation rates by more than 100 times and provide selectivities not available with standard phosphine catalysts. For example, hydroformylation of 2-butene with l,l -biphenyl-2,2 -diyl... 

A synthesis of optically active citroneUal uses myrcene (7), which is produced from P-piaene. Reaction of diethylamine with myrcene gives A/,A/-diethylgeranyl- and nerylamines. Treatment of the aHyUc amines with a homogeneous chiral rhodium catalyst causes isomerization and also induces asymmetry to give the chiral enamines, which can be readily hydrolyzed to (+)-citroneUal (151). 

About 86% of Hoechst s butanal is produced with the Rhc )ne-Poulenc water-soluble rhodium catalyst the remainder is stiU based on cobalt. 

The original catalysts for this process were iodide-promoted cobalt catalysts, but high temperatures and high pressures (493 K and 48 MPa) were required to achieve yields of up to 60% (34,35). In contrast, the iodide-promoted, homogeneous rhodium catalyst operates at 448—468 K and pressures of 3 MPa. These conditions dramatically lower the specifications for pressure vessels. Yields of 99% acetic acid based on methanol are readily attained (see Acetic acid Catalysis). 

With Unsaturated Compounds. The reaction of unsaturated organic compounds with carbon monoxide and molecules containing an active hydrogen atom leads to a variety of interesting organic products. The hydroformylation reaction is the most important member of this class of reactions. When the hydroformylation reaction of ethylene takes place in an aqueous medium, diethyl ketone [96-22-0] is obtained as the principal product instead of propionaldehyde [123-38-6] (59). Ethylene, carbon monoxide, and water also yield propionic acid [79-09-4] under mild conditions (448—468 K and 3—7 MPa or 30—70 atm) using cobalt or rhodium catalysts containing bromide or iodide (60,61). 

Rhodium catalyst is used to convert linear alpha-olefins to heptanoic and pelargonic acids (see Carboxylic acids, manufacture). These acids can also be made from the ozonolysis of oleic acid, as done by the Henkel Corp. Emery Group, or by steam cracking methyl ricinoleate, a by-product of the manufacture of nylon-11, an Atochem process in France (4). Neoacids are derived from isobutylene and nonene (4) (see Carboxylic acids, trialkylacetic acids). 

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