Carbonylation iodine-promoted

AJthaugh various propiisals for the ni chani m of methanol homologation exist, the course of the reaction is still not fully understood. This is especially true for the activation of methanol with a concomitant C-0 bond scission. Also, the folc of the iodine promoter and of ligands remains unclear. This situation is controversial to the closely-related carbonylation of methanol to acetic acid with rhodium catalysts, where the oxidative addition of the intermediate methyl iodide to a rhodium (1) is a generally-accepted reaction path [SR]. 

Some insight into the mechanisms of the iodine-promoted carbonylation has been obtained by radioactive tracer techniques [17] and low-temperature NMR spectroscopy [18]. The mechanism involves the formation of HI, which in a series of reactions forms with rhodium a hydrido iodo complex which reacts with ethylene to give an ethyl complex. Carbonylation and reductive elimination yield propionic acid iodide. The acid itself is then obtained after hydrolysis. The rate of carboxylation was reported to be accelerated by the addition of minor amounts of iron, cobalt, or manganese iodide [19]. The rhodium catalyst can be stabilized by triphenyl phosphite [20]. However, it is doubtful whether the ligand itself would meet the requirements of an industrial-scale process. 

A real breakthrough came with the discovery of promoter molecules that could be added to the reaction mixture. One such promoter is Ru(CO)3I2. Its mode of action seems to be in part to act as an I abstractor from [MeIr(CO)2I3] (equation 9.37), which has the effect of accelerating steps c and d. Several other transition metal carbonyl-iodo complexes will work, as will simple iodides of Zn, Cd, and Hg. Studies have shown that the iodinated promoter product is recycled by contributing its extra I- to the process for producing CH3I. 

Acetic anhydride is used in the manufacture of cellulose acetate-based film, cigarette filters, and plastics. Eastman Chemical developed a process that is based on gasification of coal in a Texaco gasifier to make synthesis gas which then is converted to methanol. The methanol is converted to methyl acetate by esterification with acetic acid and then carbonylated. The carbonylation process uses rhodium salt catalysts with ligands and an iodine promoter [30]. 

The first step is the oxidative carbonylation of aniline to form the intermediate diphenyl urea shown in Eq. (38). This reaction is carried out with a noble metal catalyst and an iodine promoter. A palladium catalyst with a sodium iodide promoter has been used successfully. The intermediate, diphenyl urea is oxidatively carbonylated in ethanol in the presence of the palladium catalyst to form ethyl phenyl carbamate (EPC) as shown in Eq. (39). Reactor conditions are 160°C (320°F) and 80 bars (1175 psig). 

Mixed oxides, typically containing bismuth and molybdenum, are used as catalysts and these have been improved over the years so that the conversion of propylene to acrylonitrile is now over 80% per pass through the reactor. HCN is produced in a side reaction and is used mainly to make acetonitrile or methyl methacrylate [23]. Acetic acid and acetic anhydride, which are made in high yields by the carbonylation of methanol and methyl acetate, respectively, using an iodine promoted rhodium catalyst, can now be made in a variable ratio, to match market demand, using the same plant [24]. 

The reduction steps on active Co sites are strongly affected by activated hydrogen transferred from promoter metal particles (Pt and Ru). Several indications for the existence and importance of hetero-bimetallic centers have been obtained.63 [Cp Co(CO)2] in the presence of PEt3 and Mel catalyzes the carbonylation of methanol with initial rates up to 44 mol L 1 h 1 before decaying to a second catalytic phase with rates of 3 mol L 1 h-1.64 HOAc-AcOMe mixtures were prepared by reaction of MeOH with CO in the presence of Co(II) acetate, iodine, and additional Pt or Pd salts, e.g., [(Ph3P)2PdCl2] at 120-80 °C and 160-250 atm.65... 

Nickel carbonyl charged, or formed in the carbonylation reaction mixture, can catalyze the carbonylation of methanol (11). To maintain the activity of the nickel carbonyl catalyst high temperature and pressure are required (12-14). However, certain promoters can maintain an active, soluble, nickel carbonyl species under much milder conditions. The most reactive promoters are phosphines, alkali metal salts, tin compounds, and 2-hydroxypyridine. Reaction rates of 2 to 7 X 10-3(mol/1.sec) can be achieved without the use of high concentration of iodine (Table II). in addition, high reaction rates... 

The products were isolated as esters by reaction of the acylcobalt carbonyls with an alcohol and iodine. In the case of the alkyl halides, carbon monoxide was normally absorbed, but under nitrogen, acylcobalt tricarbonyls must be formed. The reaction with alkyl halides was slow and some isomerization was noted using M-propyl iodide (formation of n-butyrates and isobutyrates). Absence of carbon monoxide promoted the isomerization. Isopropyl iodide gave no reaction. When ethyl a-bromopropionate was used, no isomerization was found at — 25 °C under carbon monoxide, but the isomerized product, diethyl succinate, was the major product at 25° C under carbon monoxide or nitrogen. Under the conditions of the experiments no isomerization of the alkyl halide itself was found. 

Data for aliphatic aldehyde enolisation are very scarce, probably because the enolisation process is often complicated by oxidation and hydration. Nevertheless, the rate constants for base- and acid-catalysed iodination of R R2CHCHO were determined in aqueous chloroacetic acid-chloroacetate ion buffers (Talvik and Hiidmaa, 1968). The results in Table 4 show that alkyl groups R1 and R2 increase the acid-catalysed reactivity in agreement with hyperconjugative and/or inductive effects. This contrasts with aliphatic ketones for which steric interactions are important and even sometimes dominant. Data for base-catalysis are more difficult to interpret since a second a methyl group, from propionaldehyde to isobutyraldehyde, increases the chloroacetate-catalysed rate constant. This might result from a decrease of the a(C—H) bond-promoted hyperconjugative stabilisation of the carbonyl compound... 

A variety of iodine compounds may be used as promoters, including l2. HI, Coil. CH >I, C2H5I, Phi, R4NI. R4PI. Nal. Kl, Csl. and Calj. [24. 29.30. It is interesting to note that the ionic iodides, with the exception of HI, arc inactive when used in combination with phosphines. Possibly, this is due to the enhanced electron density of phosphine-substituted cobalt carbonyls, which would make the nucleopliilic attack of iodide more difficult [24. ... 

A remarkable step change to existing technology has been 1996 the introduction of the Cativa technology by BP Chemicals (now BP Amoco). This process incorporated the first commercial use of iridimn (promoted by iodine, Ru-salt etc.), rather than rhodimn, as a catalyst for methanol carbonylation. The main improvements of the process are much higher reactivity (45 mol L h, Rh 10-15 mol L h ) coupled with low by-product formation and lower energy requirements for the purification of the product acid. 

However, Halcon have now developed a process, catalysed by rhodium (or nickel) with iodine and other promoters, for the carbonylation of methyl acetate (or dimethyl ether) to acetic anhydride. Like the ketene route, this technology fits in well with acetylation processes. 

At present, this process utilizes Co2(CO)s as catalyst with I2 as promoter, or Rhl2(CO) as catalyst with Mel as promoter. Other rhodium carbonyls with the addition of iodine compounds may be utilized. Also, this reaction is catalyzed by nickel and iron carbonyls, although at considerably higher pressures and temperature. The process with the application of the cobalt catalyst is carried out at ca 460 K and 20 MPa, while in the presence of Rhl2(CO)T, at 453 K and 3-4 MPa. 

For a long time it was known that group VIII metal carbonyls are efficient catalysts for carbonylation reactions. In 1996, BP developed a new catalyst system for methanol carbonylation based on iridium (additionally promoted by iodine and Ru-salts), called the Cativa process. Fundamental studies had shown before that the oxidative addition of methyl iodide to iridium is 150-times faster than to rhodium. Thus, in the Cativa process this step is no longer rate determining (as in the case of Rh-based methanol carbonylation). The slowest step in the iridium-cyde is the insertion of CO. This step involves the elimination of iodide and coordination of an additional CO ligand to iridium (Figure 6.15.6). Accordingly, the reaction rate can be described by Eq. (6.15.8) ... 

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