Rhodium processes

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. 

Products of Cobalt and Modified Rhodium Processes for Butyraldehyde... 

Raw Material Requirements for Conventional Cobalt and Modified Rhodium Process" (103)... 

Of the three catalytic systems so far recognized as being capable of giving fast reaction rates for methanol carbonylation—namely, iodide-promoted cobalt, rhodium, and iridium—two are operated commercially on a large scale. The cobalt and rhodium processes manifest some marked differences in the reaction area (4) (see Table I). The lower reactivity of the cobalt system requires high reaction temperatures. Very high partial pressures of carbon monoxide are then required in the cobalt system to... 

In contrast to the rhodium process the most abundant iridium species, the catalyst resting state, in the BP process is not the lr(l) iodide, but the product of the oxidative addition of Mel to this complex. 

The rhodium process is highly selective and operates under mild reaction pressure, 400 to 1000 psig. However, because of the high price of rhodium, an efficient recovery of the catalyst is essential. An expensive rhodium recovery section is an integral part of any new acetic acid plant (15-16). This can be a substantial financial burden, especially in a smaller plant. 

The reaction effluent of the rhodium process is subjected to flash distillation under mild conditions in order to maintain the activity of the sensitive catalyst and to contain the costly metal in a limited section of the plant. The effluent of the nickel process is subjected directly to distillation of the products with considerable savings. 

Several economic evaluations of the nickel process as compared with the rhodium process, made by Halcon and by independent consultants, agreed that there is savings of about Iq/lb of acetic acid in favor of the nickel process. The difference is derived essentially from utility savings due to the higher reaction rate, simplified separation and lower water concentration. The nickel process consumes less than half the energy needed for the rhodium-catalyzed process. The catalyst inventory, and the equipment needed for its recovery contribute to the higher cost of production in the rhodium case. 

The high selectivity and mild conditions make the rhodium process more attractive than the cobalt one for the manufacture of n-butyraldehyde. The high cost of rhodium makes near-complete catalyst recovery a must for the commercial viability of the process. As we shall see, this has been achieved by developing an elegant separation method based on water-soluble phosphines. 

The rhodium process has been in commercial use since 1976. In the interim the increasing importance ot environmental, health and safety considerations in industrial processes make the use of less-extreme reaction conditions, catalyst longevity and the minimization of side-product formation of crucial importance. 

The Shell process is a variant of the cobalt-catalyzed process in which phosphine-modified catalysts of the type [HCo(CO)j(PR3)] are used. Such catalysts, which are stable at low pressures, favor the hydrogenation of the initially formed aldehydes, so that the main products are oxo alcohols. However, a disadvantage is the lower catalyst activity and increased extent of side reactions, especially the hydrogenation of the olefin starting material. The superiority of the low-pressure rhodium process can be seen from the process data listed in Table 3-3. 

The costs of the rhodium process are, however, higher owing to the required work up, catalyst recycling, and corrosion problems. Therefore, intensive research is being carried out to develop heterogeneous rhodium catalysts. However, this has so far been thwarted by the low stabihty of die catalysts. 

Since Shell s report on the use of phosphines in this process [3], many industries started applying phosphine ligands in the rhodium process as well [52]. While alkylphosphines are the ligands of choice for cobalt, they lead to slow catalysis when applied in rhodium catalysis. In the mid-sixties the work of Wilkinson showed that arylphosphines should be used for rhodium and that even at very mild conditions very active catalysts can be obtained [9]. 

Both factors together with the reduced fixed costs and the usage of an own technology (no license fees in case of Celanese plants) makes the RCH/RP process ca. 10 % cheaper in manufacturing costs (costs for ligand synthesis already included) compared to classical rhodium process applying a homogeneous phosphine-modified catalyst. 

While reaction rates for cobalt catalysts depend on the methanol concentration and partial pressure of carbon monoxide, the rates with rhodium catalysts are independent of both reactant and product concentrations. The reaction mechanism for cobalt catalyst in methanol carbonylation is similar to hydroformyla-tion but is different for rhodium catalysts. The catalyst intermediate in the rhodium process is Rh(CO)2l2 and has been identified at 100°C and 6 atm by infrared spectroscopy. Reaction with methyl iodide in the rate-determining step then forms a methyl rhodium complex that rapidly gives an acyl complex (CH3CORh(CO)l3)2 from reaction with the high carbon monoxide content in the reactor. The complex decomposes after reaction with water to produce acetic acid, and with simultaneous regeneration of the catalyst ... 

Thus, as expected the reaction in Scheme 9.10 (66 67) is much faster for iridium. In and of itself this does not mean that the iridium catalyst is therefore faster than the rhodium catalyst As we have learned before, the reductive elimination may be slower for iridium. Apparently, this is not the case. Migration is now the slowest step (67 69) [126]. In contrast to the rhodium process, the most abundant iridium species, the catalyst resting state, in the BP process is not 66, but the product of the oxidative addition of Mel to this complex, 67. 

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