Rhodium carbonyl iodide catalyst, carbonylation

Ethylene Carbonylation. The classical rhodium catalyzed carbonylation of ethylene to propionic acid (Eqn. 1) used ethyl iodide or HI as a co-catalyst (6). In the presence of excess ethylene and CO the process could proceed further to propionic anhydride (Eqn. 2). While additional products, such as ethyl propionate (EtC02Et), diethyl ketone (DEK), and ethanol were possible (See Eqns. 3-5), the only byproduct obtained when using a rhodium-alkyl iodide catalyst was small amounts (ca. 1-1.5%) of ethyl propionate. (See Eqns. 3-5.)... 

Unfortunately, when the carbonylation of ethylene with a rhodium-ethyl iodide catalyst was operated in ionic liquid media generated the product mixture now contained a significant amoimt of EtC02Et (15-35%). (See Table 37.1.) Unless this selectivity issue was resolved, the carbonylation of ethylene in ionic liquids would have been imtenable. 

In 1992, about 6.5 billion lb acetic acid was produced worldwide, of which about 3.6 billion lb was produced in the United States [1]. The current commercial processes for its production include oxidation of ethanol (acetaldehyde), oxidation of butane-butene mixture or naphtha, and carbonylation of methanol or methyl acetate. These are catalytic processes. The last, liquid-phase carbonylation of methanol using a rhodium and iodide catalyst, has become the dominant process since its introduction in the late 1960s, and accounted for about half the production of acetic acid in the United States [2]. That represents a conversion of 1.5 x 106 ton per year of methanol into 2.8 x 106 ton per year of acetic acid. In the United States, 80% of actual plant operation capacity is based on this technology [3]. The reaction is thermodynamically favorable [4], and the theoretical conversion is practicalty 100% at 389 K ... 

The carbonylation of methanol was developed by Monsanto in the late 1960s. It is a large-scale operation employing a rhodium/iodide catalyst converting methanol and carbon monoxide into acetic acid. An older method involves the same carbonylation reaction carried out with a cobalt catalyst (see Section 9.3.2.4). For many years the Monsanto process has been the most attractive route for the preparation of acetic acid, but in recent years the iridium-based CATIVA process, developed by BP, has come on stream (see Section 9.3.2) ... 

Meanwhile, Wacker Chemie developed the palladium-copper-catalyzed oxidative hydration of ethylene to acetaldehyde. In 1965 BASF described a high-pressure process for the carbonylation of methanol to acetic acid using an iodide-promoted cobalt catalyst (/, 2), and then in 1968, Paulik and Roth of Monsanto Company announced the discovery of a low-pressure carbonylation of methanol using an iodide-promoted rhodium or iridium catalyst (J). In 1970 Monsanto started up a large plant based on the rhodium catalyst. 

It was discovered by Monsanto that methanol carbonylation could be promoted by an iridium/iodide catalyst [1]. However, Monsanto chose to commercialise the rhodium-based process due to its higher activity under the conditions used. Nevertheless, considerable mechanistic studies were conducted into the iridium-catalysed process, revealing a catalytic mechanism with similar key features but some important differences to the rhodium system [60]. 

Monsanto developed the rhodium-catalysed process for the carbonylation of methanol to produce acetic acid in the late sixties. It is a large-scale operation employing a rhodium/iodide catalyst converting methanol and carbon monoxide into acetic acid. At standard conditions the reaction is thermodynamically allowed,... 

In addition to the successful reductive carbonylation systems utilizing the rhodium or palladium catalysts described above, a nonnoble metal system has been developed (27). When methyl acetate or dimethyl ether was treated with carbon monoxide and hydrogen in the presence of an iodide compound, a trivalent phosphorous or nitrogen promoter, and a nickel-molybdenum or nickel-tungsten catalyst, EDA was formed. The catalytst is generated in the reaction mixture by addition of appropriate metallic complexes, such as 5 1 combination of bis(triphenylphosphine)-nickel dicarbonyl to molybdenum carbonyl. These same catalyst systems have proven effective as a rhodium replacement in methyl acetate carbonylations (28). Though the rates of EDA formation are slower than with the noble metals, the major advantage is the relative inexpense of catalytic materials. Chemistry virtually identical to noble-metal catalysis probably occurs since reaction profiles are very similar by products include acetic anhydride, acetaldehyde, and methane, with ethanol in trace quantities. 

Two homogeneous metal complex water-gas shift catalyst systems have recently appeared 98, 99). The more active of these comes from our Rochester laboratory (99, 99a). It is composed of rhodium carbonyl iodide under CO in an acetic acid solution of hydriodic acid and water. The catalyst system is active at less than 95°C and less than 1 atm CO pressure. Catalysis of the water-gas shift reaction has been unequivocally established by monitoring the CO reactant and the H2 and C02 products by gas chromatography The amount of CO consumed matches closely with the amounts of H2 and C02 product evolved throughout the reaction (99). Mass spectrometry confirms the identity of the C02 and H2 products. The reaction conditions have not yet been optimized, but efficiencies of 9 cycles/day have been recorded at 90°C under 0.5 atm of CO. Appropriate control experiments have been carried out, and have established the necessity of both strong acid and iodide. In addition, a reaction carried out with labeled 13CO yielded the same amount of label in the C02 product, ruling out any possible contribution of acetic acid decomposition to C02 production (99). 

The catalytic activity of the methanol carbonylation is very dependent on the nature of the iodide promoter, and different chemistry appears to follow using HI or Nal in this regard (72). However, under otherwise identical conditions, the catalytic activity increased in the order Nal < CH3I < HI. Contrary to what is observed for the rhodium/iodide catalyst, Braca et al. did not consider CH3I to be directly involved in the catalytic carbonylation cycle (70-73). This conclusion is based on the observation that CH3I was not carbonylated under their reaction conditions. Instead, because of the necessity of a proton supplier and the promoting effect of Nal, these authors... 

Acetica A process for making acetic acid by the heterogeneous carbonylation of methanol in a bubble column reactor. The catalyst is a rhodium carbonyl iodide, anchored by ion-pairing to a polyvinyl pyridine resin. Developed by Chiyoda Corporation and UOP and first described in 1998. Licensed to Guizhou Crystal Organic Chemical Group, China, in 2002 one plant was under construction in 2005. 

Anionic metal complexes, for example [Rh(CO)2l2], can be exchanged onto the anion-exchange resin Dowex 1-X8. The supported rhodium carbonyl iodide complex functions as an immobilized methanol Carbonylation catalyst. Metal complexes of the water-soluble phosphine TPPTS and its monosulfonated analog have also been exchanged onto anion-exchange resins. The pendant sulfonate groups provide the electrostatic attraction to the support. 

In the low-water AO technology [23], the major function of the iodide salts is to stabilize the rhodium carbonyl catalyst complexes from precipitation as insoluble rhodium triiodide (RhD [5c]. Lithium iodide (Lil) is the preferred salt. The iodide salts also promote catalyst activity (see below). However, the key factor that con-... 

Prior to these investigations by HCC the promotional effect of iodide on the oxidative addition of Mel was investigated by others [9, 39, 40]. Foster demonstrated that the rate enhancement of this reaction in anhydrous medium was attributable to increased nucleophilicity of the rhodium catalyst with added iodide. The rationale for this observation was the generation of an anionic rhodium carbonyl complex, [Rh(CO)2l(L)]. Generation of this species was observed only with iodide added to certain neutral Rh species. No rate enhancement occurred with iodide added to the anionic complex, [Rh(CO)2l2] [39]. Similarly, in solvents with a high water concentration, iodide salts exhibited no rate enhancement in the presence of [Rh(CO)2l2] [11]. Maitlis and co-workers, in more recent investigations, reported a promotional effect of iodide in aprotic solvents on the oxidative addition of CH3I on [Rh(CO)2l2] [9a, 9c]. 

The efficacy of an iridium/iodide catalyst for methanol carbonylation was discovered by Monsanto at the same time as their development of the process using the rhodium/iodide catalyst [5]. Mechanistic investigations by Forster employing in situ HPIR spectroscopy revealed additional complexity compared to the rhodium system [115]. In particular, the carbonylation rate and catalyst speciation were found to show a more complicated dependence on process variables, and three distinct regimes of catalyst behavior were identified. At relatively low concentrations of Mel, H20, and ionic iodide, a neutral iridium (I) complex [Ir(CO)sI] was found to dominate, and the catalytic reaction was inhibited by increasing the CO partial pressure. Addition of small amounts of a quaternary ammonium iodide salt caused the dominant iridium species to become an Ir(III) methyl complex, [Ir(CO)2l3Me]. Under these conditions, the rate... 

It is not surprising that homogeneous WGS catalysts are in two categories—those which operate in acidic and those in basic media. Of the acid-based systems, the most active are the rhodium carbonyl iodide combinations ", the PtCl4 /SnCl3 preparation, and the system based on the ruthenium carbonyl cluster catalyst precursors Ru3(CO)i2 and H4Ru4(CO),2 . The Rh carbonyl iodide system under more vigorous conditions (185°C, 23 atm ) shows a catalytic rate of 400 tumovers/h. 

This reaction is carried out over rhodium carbonyls as catalyst using HI as a promoter. Acetic anhydride is produced from the carbonylation of methylacetate over lithium-iodide-promoted rhodium catalyst ... 

The carbonylation is carried out in the liquid phase with a rhodium-iodide catalyst. 

The commercial production of acetic acid by the carbonylation of methanol is carried out continuously in bubble-column reactors at 170 190°C and 20 0 atmospheres using a soluble rhodium catalyst with methyl iodide as a promoter. Tests in small stirred reactors showed that the reaction rate is proportional to the product of the rhodium and iodide concentrations but is independent of the methanol concentration and the partial pressure of carbon monoxide for partial pressures greater than about 2 atmospheres... 

Methanol carbonylation with rhodium iodide catalysts gives acetic acid directly. Exercise 3.4... 

Rhodium carbonyl/iodide systems are used industrially in the carbonylation of methanol to acetic acid. Initial work to elucidate the reaction mechanism was carried out by Forster and Dekleva at Monsanto. Using HP-IR, Maitlis determined that [Rh(CO)2l2] is the predominant species under reaction conditions, and confirmed that oxidative addition of methyl iodide to this species is rate determining.The reaction of this key intermediate with methyl iodide was also investigated, and compared to the corresponding iridium species. While the iridium system underwent oxidative addition to form an alkyl complex much more rapidly than rhodium, it was found that subsequent migratory insertion to afford the acyl intermediate was much faster for rhodium due to the inherent instability of the alkyirhodium complex. Fontaine et at also observed [Rh(CO)2l2] as the catalyst resting state for the conversion of methyl formate to methyl acetate or acetaldehyde. 

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