Rhodium Monsanto catalyst system

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,... 

In the mid-1960s, Paulik and Roth at Monsanto Co discovered that rhodium and an iodide promoter were more efficient than cobalt, with selectivities of 99% and 85%, with regard to methanol and CO, respectively. Moreover, the reaction is operated under significantly milder conditions such as 40-50 bar pressure and around 190 °C [8]. Even though rhodium was 1000 times more costly than cobalt at this time, Monsanto decided to develop the rhodium-based catalyst system mainly for the selectivity concerns, and thus for the reduction of the process cost induced by the acetic acid purification, even if it was necessary to maintain a 14% w/w level of water in the reactor to keep the stability of the rhodium catalyst. In addition, Paulik et al. [9] demonstrated that iridium can also catalyze the carbonylation of methanol although at a lower rate. However, it is noteworthy that the catalytic system is more stable, especially in the low partial pressure zones of the industrial unit. 

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),... 

In addition to rhodium-based catalysts, iridium-based eatalysts have also been developed in a process known as the Cativa process. The iridium system follows a cycle similar to the rhodium system in Figure 14-16, beginning with oxidative addition of j CH3I to [Ir(CO)2l2] The first step in the iridium system is much more rapid than in the Monsanto process and the second step is much slower the second step, involving alkyl . migration, is rate determining for the Cativa process. ... 

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... 

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. 

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. 

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). 

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. 

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 Monsanto rhodium catalyst system has been the subject of numerous reviews [6, 10-17], including a very recent one by Haynes [18]. At high water content, typically more than 8% w/w, the overall rate is first order in both the rhodium complex and the methyl iodide reactant and zero order in both methanol and CO reactants. The catalytic cycle, which is usually adopted is shown in Fig. 20.1. The first step is the CH3I oxidative addition reaction to the [Rhl2(CO)2] active... 

The troublesome process of product separation and catalyst recycling in carbonylation reactions using ionic liquids can be considerably simplified by using a solid ionic phase [68,69] or by introducing of an inert solid support [70]. The continuous liquid-phase carbonylation of methanol has been performed using the rhodium carbonyl iodide complex [Rh(CO)2l2] immobilized on a methylpyridinium cation resin [68,69]. The catalytic activity remains constant for the 2000-h operation with virtually no Rh leaching. IL-impregnated silica was used as a solid support for the Monsanto-type catalyst system [Rh(CO)2l2]-BMM [70]. 

It is now nearly 40 years since the introduction by Monsanto of a rhodium-catalysed process for the production of acetic acid by carbonylation of methanol [1]. The so-called Monsanto process became the dominant method for manufacture of acetic acid and is one of the most successful examples of the commercial application of homogeneous catalysis. The rhodium-catalysed process was preceded by a cobalt-based system developed by BASF [2,3], which suffered from significantly lower selectivity and the necessity for much harsher conditions of temperature and pressure. Although the rhodium-catalysed system has much better activity and selectivity, the search has continued in recent years for new catalysts which improve efficiency even further. The strategies employed have involved either modifications to the rhodium-based system or the replacement of rhodium by another metal, in particular iridium. This chapter will describe some of the important recent advances in both rhodium- and iridium-catalysed methanol carbonylation. Particular emphasis will be placed on the fundamental organometallic chemistry and mechanistic understanding of these processes. 

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]. 

The relevance of the water-gas shift reaction in the petrochemical industry has already been discussed (see Section 1.1). The significance of the water-gas shift reaction in homogeneous systems is twofold. First, it plays a crucial role in stabilizing the rhodium catalyst in the Monsanto process. Second, studies carried out in homogeneous systems employing metals other than rhodium have provided useful mechanistic insights into the heterogeneous water-gas shift reaction. We first discuss the catalytic cycle with 4.1 as one of the catalytic intermediates, and then mechanistic results that are available from an iron-based catalytic system. 

Monsanto also discovered significant catalytic activity for iridium/iodide catalysts however, they chose to commercialize the rhodium-based process due to its higher activity under conventional high water conditions. Despite this, detailed mechanistic studies by Forster and his colleagues were undertaken at Monsanto and revealed a catalytic mechanism for iridium which is similar to the rhodium system in many respects, but with additional complexity due to participation of both anionic and neutral complexes (see below). 

The carbonylation of methanol in acetic acid represents an important industrial process, which has been developed by Monsanto Corporation using a homogeneous rhodium complex. Extensive investigations on the rhodium catalytic system have been carried out and Liu et al. [77] have studied the use of PVP-stabilized Rh nanoparticles for this reaction (Scheme 11.10). The stable PVP-Rh colloid presents a lower activity than Monsanto s homogeneous catalyst under the same drastic conditions (140°C, 54bar). However, the colloidal metal catalyst could be reused several times with an increased activity (TON = 19700 cydes/atom Rh), which... 

---