Rhodium carbides

Chlorine remains on the surface even upon heating till 527 °C, a temperature at which large rhodium particles are formed. Moreover, dihydrogen reacts with Rh (C0)2/Ti02 at 27 °C to form a monocarbonyl intermediate species, presumably Rh(H)(CO) and to remove partially chlorine from the surface. Further heating at 152 °C under H2 leads to incomplete CO removal, as evidenced by some graphitic carbon and rhodium carbide, both detected on the surface. 

Fig. 7. A chiral rhodium carbide cluster (49) and its K5 subgraph (50). The atoms chosen as the vertices of 50 have been numbered in 49 (the Rh —C bonds have been neglected for clarity)...

Tan, H., Liao, M., Balasubramanian, K. Electronic states and potential energy surfaces of rhodium carbide (RhC), Chem. Phys. Lett. 280 (1997) 423 29. 

Cobalt and rhodium also form a series of high nuclearity carbido-cluster compounds. Contrasting with carbido compounds of the iron group which have a clear tendency to put the carbide atom in octahedral cavities, carbides of the group 9 often place it in trigonal prismatic cavities. As shown in scheme in Fig. 3.12, the parent compound [Co6(CO)i5C] of a series of encapsulated car-bido-cobalt species may be prepared by the reaction of Co3(CO)9CCl with [Co(CO)4]. The same scheme also describes some carbido-cobalt cluster interconversions. Rhodium carbide clusters are in general similar to the cobalt ones. 

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 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. 

Propane, 1-propanol, and heavy ends (the last are made by aldol condensation) are minor by-products of the hydroformylation step. A number of transition-metal carbonyls (qv), eg, Co, Fe, Ni, Rh, and Ir, have been used to cataly2e the oxo reaction, but cobalt and rhodium are the only economically practical choices. In the United States, Texas Eastman, Union Carbide, and Hoechst Celanese make 1-propanol by oxo technology (11). Texas Eastman, which had used conventional cobalt oxo technology with an HCo(CO)4 catalyst, switched to a phosphine-modified Rh catalyst ia 1989 (11) (see Oxo process). In Europe, 1-propanol is made by Hoechst AG and BASE AG (12). 

Cobalt carbonyls are the oldest catalysts for hydroformylation and they have been used in industry for many years. They are used either as unmodified carbonyls, or modified with alkylphosphines (Shell process). For propene hydroformylation, they have been replaced by rhodium (Union Carbide, Mitsubishi, Ruhrchemie-Rhone Poulenc). For higher alkenes, cobalt is still the catalyst of choice. Internal alkenes can be used as the substrate as cobalt has a propensity for causing isomerization under a pressure of CO and high preference for the formation of linear aldehydes. Recently a new process was introduced for the hydroformylation of ethene oxide using a cobalt catalyst modified with a diphosphine. In the following we will focus on relevant complexes that have been identified and recently reported reactions of interest. 

With reference to the homogeneous catalyst systems thus far reported for the synthesis of hydrocarbons/chemicals from carbon monoxide and hydrogen, only the anionic rhodium systems of Union Carbide show any appreciable shift activity. With neutral species of the type M3(CO)12 (M = Ru or Os), only small quantities of carbon dioxide are produced under the synthesis conditions (57). 

SHOP [Shell Higher Olefins Process] A process for producing a-olefins by oligomerizing ethylene, using a proprietary rhodium/phosphine catalyst. The a-olefins can then be iso-merized to internal olefins as required. Invented by W. Keim in the Institut fur Technische Chemie und Petrolchemie, Aachen, in the 1970s. The first plant was built in Geismar, LA, in 1979 the second in Stanlow, Cheshire, in 1982. Licensed worldwide by a consortium of Union Carbide, Davy-McKee, and Johnson Matthey. 

Niobium and rhodium cluster anions have been prepared by laser vaporization and the reactions with benzene studied by FT-ICR/MS (58). The reactions of the anions and similar cations have been compared. With few exceptions the predominant reaction of the niobium cluster anions and cations was the total dehydrogenation of benzene to form the metal carbide cluster, [Nb C6]-. The Nb19 species, both anion and cation, reacted with benzene to form the coordinated species Nb 9C6I I6p as the predominant product ion. The Nb22 ions also formed some of the addition complex but the Nb2o Nb2i, and all the other higher clusters, formed the carbide ions, Nb C6. ... 

The observations were rationalized by the initial formation of a collision complex. The energy produced in the interaction to form the complex (probably a n complex) must be distributed throughout the atoms of the cluster. Niobium is known to form stable strongly bonded carbides, whereas rhodium forms less stable... 

Organometallic compounds, 14 550-551 25 71. See also Organometallics carbides contrasted, 4 648 as initiators, 14 256-257 iridium, 19 649-650 molybdenum(III), 17 27 osmium, 19 642-643 palladium, 19 652 platinum, 19 656-657 reaction with carbonyl groups, 10 505-506 rhodium, 19 645-646 ruthenium, 19 639 sodium in manufacture of, 22 777 titanium(IV), 25 105-120 Organometallic fullerene derivatives,... 

As a result, the second-generation processes used rhodium as the metal. The first rhodium-catalysed, ligand-modified process came on stream in 1974 (Celanese) and more were to follow in 1976 (Union Carbide Corporation) and in 1978 (Mitsubishi Chemical Corporation), all using triphenylphosphine (tpp). The UCC (now Dow) process has been licensed to many other users and it is... 

LPO process. Propene hydroformylation can be done with a rhodium triphenylphosphine catalyst giving a linearity ranging from 60 to 96 % depending on the phosphine concentration. At very high phosphine concentration the rate is low, but the linearity achieves its maximum value. The commercial process (Union Carbide Corporation, now Dow Chemicals) operates presumably around 30 bar, at 120 °C, at high triphenylphosphine concentrations, and linearities around 92%. The estimated turnover frequency is in the order of only 300 mol(product).mol 1 (Rh).h Low ligand... 

After the initial demonstration of stoichiometric nucleophilic attack on 7i-allyl ligands, catalytic allylic substitution reactions were pursued. In 1970, groups from Union Carbide [3, 4], Shell Oil [5], and Toray Industries [6] published or patented examples of catalytic allylic substitution. All three groups reported allylic amination with palladium catalysts. The Toray Industries report also demonstrated the exchange of aryl ether and ester leaving groups, and the patent from Shell Oil includes catalysts based on rhodium and platinum. 

Fig. 35. Skeletal structures of the rhodium-carbonyl carbide complexes Rh8(CO)i9C and [Rhis(CO)28C2]- (7).

The above studies concerning rhodium deposition provide evidences of the crucial importance of surface chemistry on the final quality of the deposit. Purity can be controlled by addition of reactive components that assist the expected loss of ligands, which otherwise would leave contaminants such as halides, carbides or oxides on the deposits. 

Figure 6 Union Carbide chiral bis-phosphite-rhodium complexes used as catalysts for the asymmetric hydroformylation.

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... 

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