Monsanto-type 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. 

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]. 

Cells of the second type were initially developed by Tinker and Morris at Monsanto [4] and subsequently by Penninger [5]. In these systems, the reaction solution is circulated from the autoclave through an external IR cell of relatively small volume. This arrangement means that the cell can be isolated from the main reaction vessel relatively easily (for example in the event of window failure) thus protecting the spectrometer. Cells of this sort can, in principle, be fitted to plants or pilot plants to monitor liquid streams. However, the circulation of solution from the main reaction vessel through an external cell introduces some potential problems. A pressure drop in the circulation system can lead to release of dissolved gas, which may accumulate between the cell windows and interfere with the spectroscopic measurement. A change in pressure may also influence the catalyst specia-tion, such that the observed spectra may not be truly representative of the bulk reaction solution. 

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