Mechanism and Side-reactions of the Monsanto Process

It is important to note that the two organic reactions (1) and (2) involving iodide are quantitative. As a result, all of the iodide in the system occurs as a methyl iodide and, due to reaction (1), the rate of the catalytic cycle is independent of the methanol concentration [89]. 

One of the main drawbacks of the process is the loss of the expensive metal due to the formation of inactive Rh(III) species which, in areas of low CO pressure at the end of the reaction, precipitate as Rhis. The inactive species [Rhl4(CO)2] can be produced by reaction of compounds 43 and 44 with HI according to  

In the Monsanto process this problem is solved by keeping the water content relatively high, because it inhibits Rh precipitation by regenerating the Rh(I)-ac-tive species from the labile [Rhl4(CO)2r  

H2 can also accomplish this goal (6) and it can be generated in the reactor at high water concentrations through the water-gas shift reaction (7). In the absence of water, added H2 could also perform this role, as it does in the related Eastman acetic anhydride process [90, 91]. 

In fact, the presence of water is controversial it is needed to maintain the rhodium in the active Rh(I) form, but there are also several problems associated with keeping it at high concentrations  

---