Rhodium-catalyzed processes

Since 1960, the Hquid-phase oxidation of ethylene has been the process of choice for the manufacture of acetaldehyde. There is, however, stiU some commercial production by the partial oxidation of ethyl alcohol and hydration of acetylene. The economics of the various processes are strongly dependent on the prices of the feedstocks. Acetaldehyde is also formed as a coproduct in the high temperature oxidation of butane. A more recently developed rhodium catalyzed process produces acetaldehyde from synthesis gas as a coproduct with ethyl alcohol and acetic acid (83—94). 

From Synthesis Gas. A rhodium-catalyzed process capable of converting synthesis gas directly into acetaldehyde in a single step has been reported (83,84). 

The stringency of the conditions employed in the unmodified cobalt 0x0 process leads to formation of heavy trimer esters and acetals (2). Although largely supplanted by low pressure ligand-modified rhodium-catalyzed processes, the unmodified cobalt 0x0 process is stiU employed in some instances for propylene to give a low, eg, - 3.3-3.5 1 isomer ratio product mix, and for low reactivity mixed and/or branched-olefin feedstocks, eg, propylene trimers from the polygas reaction, to produce isodecanol plasticizer alcohol. 

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... 

In the mid-1980s, Ruhrchemie (now Hoechst) converted its oxo capacity to a proprietary water soluble rhodium catalyzed process (27,28), a technology developed jointly with Rhc ne-Poulenc. Product separation in this process is by decantation. Isomer ratios of n- to isobutyraldehyde of about 20 1 are obtained. 

Mitsubishi Chemical uses a proprietary medium pressure rhodium-catalyzed process (29) in some plants which operate at 90—120°C and 5—10 MPa (725—1450 psi), and gives isomer ratios of about 4 1. 

In rhodium hydroformylations, highly efficient separation and recovery of catalyst becomes imperative, because of the very expensive nature of the catalyst. Any loss, by trace contamination of product, leakage, or otherwise, of an amount of rhodium equivalent to 1-2 parts per million (ppm) of aldehyde product, would be economically severe. The criticalness of this feature has contributed to some pessimism regarding the use of rhodium in large hydroformylation plants (63). However, recent successful commercialization of rhodium-catalyzed processes has proved that with relatively simple process schemes losses are not a significant economic factor (103, 104). 

Starting from longer-chain olefins means high boiling points of the products, and hence distillation at higher temperatures and vacuum. These demanding conditions cannot be met by the modern rhodium-catalyzed processes. 

In 1975 Kuntz has described that the complexes formed from various rhodium-containing precursors and the sulfonated phosphines, TPPDS (2) or TPPTS (3) were active catalysts of hydroformylafion of propene and 1-hexene [15,33] in aqueous/organic biphasic systems with virtually complete retention of rhodium in the aqueous phase. The development of this fundamental discovery into a large scale industrial operation, known these days as the Ruhrchemie-Rhone Poulenc (RCH-RP) process for hydroformylation of propene, demanded intensive research efforts [21,28]. Tire final result of these is characterized by the data in Table 4.2 in comparison with cobalt- or rhodium-catalyzed processes taking place in homogeneous organic phases. 

Padwa has shown that rhodium-catalyzed oxidation of indolyl carbamate 67 employing either Phl(OAc)2 or Phl=0 follows a path similar to that of the D-aUal carbamate (Scheme 17.26) [95]. In principle, indole attack of the putative rhodium-nitrene generates zwitterion 68, which is trapped subsequently by an exogenous nucleophile. Spiro-oxazolidinone products (for example, 69) are isolated as single diastereomers in yields ranging from 50 to 85%. As an intriguing aside, Padwa has found that certain carbamates react with Phl=0 in the absence of any metal catalyst to furnish oxazoHdinone products. This result may have implications for the mechanism of the rhodium-catalyzed process, although it should be noted that control experiments by Espino and Du Bois confirm the essential role of the metal catalyst for C-H amination [57]. 

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. 

Several differences between the cobalt- and rhodium-catalyzed processes are noteworthy with regard to mechanism. Although there is a strong dependence in the cobalt system of the ethylene glycol/methanol ratio on temperature, CO partial pressure, and H2 partial pressure, these dependences are much lower for the rhodium catalyst. Details of the product-forming steps are therefore perhaps quite different in the two systems. It is postulated for the cobalt system that the same catalyst produces all of the primary products, but there seems to be no indication of such behavior for the rhodium system. Indeed, the multiplicity of rhodium species possibly present during catalysis and the complex dependence on promoters make it... 

It has been found that in the rhodium-catalyzed process a slow loss of catalytic activity occurs and this is partly due to the presence of the more basic PPh2Prn, formed according to equation (61) and involving ortho-metallated rhodium intermediates such as (73).305... 

As mentioned in the previous section, the carbonylation of methanol to acetic acid is an important industrial process. Whereas the [Co2(CO)s]-catalyzed, iodide-promoted reaction developed by BASF requires pressures of the order of 50 MPa, the Monsanto rhodium-catalyzed synthesis, which is also iodide promoted and which was discovered by Roth and co-workers, can be operated even at normal pressure, though somewhat higher pressures are used in the production units.4,1-413 The rhodium-catalyzed process gives a methanol conversion to acetic acid of 99%, against 90% for the cobalt reaction. The mechanism of the Monsanto process has been studied by Forster.414 The anionic complex m-[RhI2(CO)2]- (95) initiates the catalytic cycle, which is shown in Scheme 26. 

Union Carbide, Johnson Matthey and Davy McKee have jointly developed a low pressure, two-step rhodium catalyzed process which takes place at around 110°C and 100-300 psi (3 ) The rhodium complex HRh(CO)(P/Tj)3 is stable at typical reaction temperatures and pressures. The rhodium concentration in the reaction mix is usually around 0.02 wt%. 

Studies aimed at the comparison of Rh(II) and Cu(II)-catalyzed onium ylide reactions of diazoketones 449 using 3 mol% of the former and 15 mol% of the latter led to the conclusion that the copper-catalyzed process provides the better yields and selectivities for [1,2]-rearrangement products 450 (Fig. 107) [490, 491]. In the rhodium-catalyzed process, 1,5-C-H insertion may compete. The diastereo-selectivity with both catalysts is in some cases similar, in others the Rh-catalyzed process is more selective. Analogous reactions of acetal 451 provided a mixture of stereoisomers 452a and 452b at the benzylidene position, supporting a stepwise process. The authors proposed that the involved intermediate was either a 1,6-biradical or the corresponding ion pair. 

The cobalt and rhodium catalysts have one important difference between their respective mechanisms. Unlike in the rhodium-catalyzed process, there is no oxidative addition or reductive elimination step in the cobalt-catalyzed hy-droformylation reaction. This is reminiscent of the mechanistic difference between rhodium- and cobalt-based carbonylation reactions (see Section 4.2.3). The basic mechanism is well established on the basis of in situ IR spectroscopy, kinetic and theoretical analysis of individual reaction steps, and structural characterization of model complexes. 

Mankind has produced acetic acid for many thousand years but the traditional and green fermentation methods cannot provide the large amounts of acetic acid that are required by today s society. As early as 1960 a 100% atom efficient cobalt-catalyzed industrial synthesis of acetic acid was introduced by BASF, shortly afterwards followed by the Monsanto rhodium-catalyzed low-pressure acetic acid process (Scheme 5.36) the name explains one of the advantages of the rhodium-catalyzed process over the cobalt-catalyzed one [61, 67]. These processes are rather similar and consist of two catalytic cycles. An activation of methanol as methyl iodide, which is catalytic, since the HI is recaptured by hydrolysis of acetyl iodide to the final product after its release from the transition metal catalyst, starts the process. The transition metal catalyst reacts with methyl iodide in an oxidative addition, then catalyzes the carbonylation via a migration of the methyl group, the "insertion reaction". Subsequent reductive elimination releases the acetyl iodide. While both processes are, on paper, 100%... 

As far as large ale industrial application is concerned, the major Reppepropionic acid from ethylene. This was carried out by a BASF process (plant of about 30000 l/a) under 240 atm at 2 0 C with NKCO) as the catalyst. It also appears that Monsanto has a rhodium-catalyzed process on stream, which operates under milder conditions. 

Cyclic enyne-imines 27 have also been used as precursors to a series of fused pyrroles 28 in a rhodium-catalyzed process with suitable alkenes involving the (2-pyrrolyl)carbenoid intermediates 29 <03OL2615>. 

The rhodium-catalyzed process gives a methanol conversion to acetic acid of 99%, against 90% for the cobalt reaction. The mechanism of the Monsanto process has been studied by Forster. The anionic complex c -[Rhl2(CO)2] (95) initiates the catalytic cycle, which is shown in Scheme 26. 

Since 1979, numerous reviews have appeared on the kinetics, mechanisms, and process chemistry of the metal-catalyzed methanol carbonylation reaction [11, 14-20], especially the Monsanto rhodium-catalyzed process. In this section, the traditional process chemistry as patented by Monsanto is discussed, with emphasis on some of the significant improvements that Monsanto s licensee, Celanese Chemicals (CC) has contributed to the technology. The iridium-based methanol carbonylation process recently commercialized by BP Chemicals Ltd. (BP) will be discussed also. 

The broad development of homogeneously catalysed syntheses of commercial anhydride manufacturing is directly related to the process developments of several companies. In particular, Tennesee Eastman developed a rhodium-catalyzed process based on syngas [47]. 

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