Monsanto catalyst system

Using a Monsanto catalyst system (Rh/Mel/CO) the carbonylation of methyl acetate has a long induction period in the absence of water and a low reaction rate in comparison with the synthesis of acetic acid under typical reaction conditions. 

The Monsanto catalyst system has been the subject of numerous studies (for leading references see [6-12,16,18]). The rate of the overall carbonylation process is zero order in each of the reactants (MeOH and CO) but first order in the rhodium catalyst and in the methyl iodide cocatalyst,... 

Currently, almost all cumene is produced commercially by two processes ( /) a fixed-bed, kieselguhr-supported phosphoric acid catalyst system developed by UOP and (2) a homogeneous AlCl and hydrogen chloride catalyst system developed by Monsanto. 

Other catalyst systems such as iron V2O5-P2O5 over silica alumina are used for the oxidation. In the Monsanto process (Figure 6-4), n-butane and air are fed to a multitube fixed-bed reactor, which is cooled with molten salt. The catalyst used is a proprietary modified vanadium oxide. The exit gas stream is cooled, and crude maleic anhydride is absorbed then recovered from the solvent in the stripper. Maleic anhydride is further purified using a proprietary solvent purification system. ... 

In the late 1960s, workers at Monsanto began studies into the carbonyla-tion of methanol to acetic acid. The process they developed (9-11), now known worldwide as the Monsanto acetic acid process, is based on an iodide-promoted rhodium catalyst system. Because of the high efficiency and selectivity of the reaction (typical commercial operating conditions are 150-200°C and 30-100 atm, giving selectivities >99% based on CH3OH),... 

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. 

An overview of Monsanto s catalyst system in comparison with other processes is given in Table 1 [23, 80],... 

In the 1990s, BP re-examined the iridium-catalyzed methanol carbonylation chemistry first discovered by Paulik and Roth and later defined in more detail by Forster [20]. The thrust of this research was to identify an improved methanol carbonylation process using Ir as an alternative to Rh. This re-examination by BP led to the development of a low-water iridium-catalyzed process called Cativa [20]. Several advantages were identified in this process over the Rh-catalyzed high-water Monsanto technology. In particular, the Ir catalyst provides high carbonylation rates at low water concentrations with excellent catalyst stability (less prone to precipitation). The catalyst system does not require high levels of iodide salts to stabilize the catalyst. Fewer by-products are formed, such as propionic acid and acetaldehyde condensation products which can lead to low levels of unsaturated aldehydes and heavy alkyl iodides. Also, CO efficiency is improved. 

Whereas the cobalt catalyst systems developed by BASF in particular guarantee a methanol ooce-through conversion of 70 per cent and molar yields in relation to alcohol and carbon monoxide better than 5 and 60 per cent, those developed by Monsanto, based on rhodium, offer better performance. Methanol once-through conversion may exceed 90 per cent and molar yields in relation to alcohol and carbon monoxide are between 98 and 99 per cent and 90 per cent respectively. 

Perhaps the most famous example of the use of asymmetric hydrogenation at scale for the product of an unnatural amino acid is the Monsanto synthesis of L-dopa, a drug used for the treatment of Parkinson s disease (Scheme 9.19). " The methodology with the Knowles catalyst system has been extended to a number of other unnatural amino acids. ... 

The kinetics and mechanism of the carbonylation of methanol to acetic acid using Monsanto s rhodium complex catalyst has been extensively studied. The reaction is first order in both rhodium and CH3I promoter but zero order in CO pressure. It is believed that oxidative addition of CH3I is the rate-controlling step in this process. This is a unique example of designing a catalyst system with commercial viability in which the substrate (methanol) is first converted to CH3I... 

Low pressure rhodium complex catalyst Due to the need for high activity and selectivity under milder operating conditions, a new catalyst system consisting of a soluble rhodium complex catalyst was developed by Monsanto. The carbonylation occurs at milder operating conditions (150-200 °C and 50-70 atm pressure). 

Processes 4 and 5 in Figure 2.13 are more recent developments entailing dimerization of acrylonitrile. Although formally analogous to the Monsanto process they are quite different because they proceed via catalysis rather than electrolysis. One of these processes developed by Halcon [176] involves liquid-phase reaction of acrylonitrile at 30°C under a nitrogen pressure of 0.7 MPa in the presence of a two-component catalyst system. 

The application of SIL catalysis for continuous methanol carbonylation was reported [33]. The authors developed a siHca-SIL rhodium iodide Monsanto-type catalyst system, [BMIM][Rh(C0)2l2]-[BMIM]I-Si02, which used less catalyst material and allowed a simple process design. Compared to conventional and IL-based carbonylation systems, the advantage of this process was without recirculation and pressure change of tlie catalytic system. Moreover, the SIL catalyst exhibited excellent activity and selectivity toward acetyl products in fixed-bed, continuous gas-phase methanol carbonylation at industrially relevant reaction conditions. 

Another example of successful SILP gas-phase reaction is the rhodium-catalyzed carbonylation of methanol [37]. The technical importance of this reaction is indicated by the Monsanto process, the dominant industrial process for the production of acetic acid (and methyl acetate), carried out on a large scale as a homogeneous liquid-phase reaction [38]. Using [Rh(CO)2l2] anions as the catalyticaUy active species, Riisager and coworkers have developed a new silica SILP Monsanto-type catalyst system [39] 21, in which the active rhodium catalyst complex is part of the IL itself. The SILP system was prepared by a one-step impregnation of the silica support using a methanoUc solution of the IL [BMIM]I and the dimeric precursor species [Rh(CO)2l]2, as depicted in Scheme 15.5. 

Successful application of the SILP concept has also been achieved in continuous gas-phase carbonylation of methanol using a Monsanto-type catalyst system that contains the catalytically active species as the IL anion, impressively demonstrating the high efficiency of the thin IL catalyst phase. 

Methanol carbonylation catalyzed by a combination of iridium-carbonyl compounds and iodide additives was first reported by Monsanto in the 1970s. The mechanism of this process was studied by Forster. ° In the 1990s, BP reported an improved catalyst system based on iridium and iodide that included a "promoter," such as [Ru(CO)jy j. These Ir-based Cativa catalysts are about five times more active than the Rh catalysts, more stable in the presence of low amounts of water (5 wt %), and more soluble. In addition, Lr is usually less expensive than Rh. BP not only built new Cativa plants, but were able to convert existing plants containing rhodium catalysts to plants containing iridium Cativa catalysts because of the similarity of the Ir and Rh systems. 

The most effective molybdenum-based oxide catalyst for propane ammoxidation is the Mo-V-Nb-Te-0 catalyst system discovered and patented by Mitsubishi Chemical Corp., Japan, U.S.A. (140). Under single-pass process conditions, acrylonitrile yields of up to 59% are reported, whereas under recycle process feed conditions, the acrylonitrile selectivity is 62% at 25% propane conversion (141). Although the latter results show that the catalyst operates effectively under recycle feed conditions, the catalyst system was originally disclosed for propane ammoxidation under single-pass process conditions. The catalyst was derived from the Mo-V-Nb-0 catalyst developed by Union Carbide Corp. for the selective oxidation of ethane to ethylene and acetic acid (142). The early work by Mitsubishi Chemical Corp. used tellurium as an additive to the Union Carbide catalyst. The yields of acrylonitrile from propane using this catalyst were around 25% with a selectivity to acrylonitrile of 44% (143). The catalyst was also tested for use in a regenerative process mode much like that developed earlier by Monsanto (144) (see above and Fig. 8). Operation under cyclic reduction/reoxidation conditions revealed that the performance of the catalyst improved when it was partially reduced in the reduction cycle of the process. Selectivity to acrylonitrile reached 67%, albeit with propane conversions of less than 10%, since activity in... 

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