Selectivity methyl acetate carbonylation

The catalyst system for the modem methyl acetate carbonylation process involves rhodium chloride trihydrate [13569-65-8]y methyl iodide [74-88-4], chromium metal powder, and an alumina support or a nickel carbonyl complex with triphenylphosphine, methyl iodide, and chromium hexacarbonyl (34). The use of nitrogen-heterocyclic complexes and rhodium chloride is disclosed in one European patent (35). In another, the alumina catalyst support is treated with an organosilicon compound having either a terminal organophosphine or similar ligands and rhodium or a similar noble metal (36). Such a catalyst enabled methyl acetate carbonylation at 200°C under about 20 MPa (2900 psi) carbon monoxide, with a space-time yield of 140 g anhydride per g rhodium per hour. Conversion was 42.8% with 97.5% selectivity. A homogeneous catalyst system for methyl acetate carbonylation has also been disclosed (37). A description of another synthesis is given where anhydride conversion is about 30%, with 95% selectivity. The reaction occurs at 445 K under 11 MPa partial pressure of carbon monoxide (37). A process based on a montmorillonite support with nickel chloride coordinated with imidazole has been developed (38). Other related processes for carbonylation to yield anhydride are also available (39,40). 

Remarkably, the cycloaddition of acrolein at the intermediate cobaltacycle selectively occurs at the carbonyl-, rather than at the C,C double bond, to give a vinylpyrane. In this cycloaddition, methyl acetate stabilizes the cpCo complex (87MI6) [Eq.(37)]. 

While the direct carbonylation is well accepted by industry, the reductive and oxidative carbonylations are still in the research and development stage. Using Texaco technology (j, 7/ ) the combined synthesis of ethene and ethanol is feasible via homologation of acids according to Figure 3. Ethene can also be obtained from the reductive carbonylation of methyl acetate to ethyl acetate followed by pyrolysis (2 ). Both routes, so far, lack selectivity. 

Methyl acetate is the principal by-product in the reductive carbonylation of methanol. As indicated in Table I, decreasing the Hp/CO increases the methyl acetate selectivity. In the limit of pure CO, methyl acetate is obtained in 90-95% selectivity. 

Thus, the overall acetaldehyde selectivity approaches 98%. The utility of methyl acetate as an alternative feedstock has been previously illustrated by the reported carbonylation to acetic anhydride ( ) and homologation ( ) to ethyl acetate via reaction with synthesis gas. 

Concurrent with acetic anhydride formation is the reduction of the metal-acyl species selectively to acetaldehyde. Unlike many other soluble metal catalysts (e.g. Co, Ru), no further reduction of the aldehyde to ethanol occurs. The mechanism of acetaldehyde formation in this process is likely identical to the conversion of alkyl halides to aldehydes with one additional carbon catalyzed by palladium (equation 14) (18). This reaction occurs with CO/H2 utilizing Pd(PPh )2Cl2 as a catalyst precursor. The suggested catalytic species is (PPh3)2 Pd(CO) (18). This reaction is likely occurring in the reductive carbonylation of methyl acetate, with methyl iodide (i.e. RX) being continuously generated. 

It was found that a nickel-activated carbon catalyst was effective for vapor phase carbonylation of dimethyl ether and methyl acetate under pressurized conditions in the presence of an iodide promoter. Methyl acetate was formed from dimethyl ether with a yield of 34% and a selectivity of 80% at 250 C and 40 atm, while acetic anhydride was synthesized from methyl acetate with a yield of 12% and a selectivity of 64% at 250 C and 51 atm. In both reactions, high pressure and high CO partial pressure favored the formation of the desired product. In spite of the reaction occurring under water-free conditions, a fairly large amount of acetic acid was formed in the carbonylation of methyl acetate. The route of acetic acid formation is discussed. A molybdenum-activated carbon catalyst was found to catalyze the carbonylation of dimethyl ether and methyl acetate. 

The synthesis of acetic acid (AcOH) from methanol (MeOH) and carbon monoxide has been performed industrially in the liquid phase using a rhodium complex catalyst and an iodide promoter ( 4). The selectivity to acetic acid is more than 99% under mild conditions (175 C, 28 atm). The homogeneous rhodium catalyst is also effective for the synthesis of acetic anhydride (Ac O) by the carbonylation of dimethyl ether (DME) or methyl acetate (AcOMe) (5-13). However, rhodium is one of the most expensive metals, and its proved reserves are quite limited. It is highly desirable, therefore, to develop a new catalyst as a substitute for rhodium. 

Table I shows the effects of Mel/DME and CO/DME ratios in the feed gas on product yields. With increasing Mel/DME ratio both methyl acetate yield and selectivity increased. The yield of methyl acetate increased with an increase in the CO/DME ratio whereas its selectivity decreased. In the case of methanol carbonylation on Ni/A.C. catalyst, the product yield and selectivity were strongly affected by CO/MeOH ratio but not by Mel/MeOH ratio (14-16). The promoting effect of methyl iodide on the methanol carbonylation reached a maximum at a very low partial pressure, that is 0.1 atm or lower. However, both CO/DME and Mel/DME ratios were important for regulating the product yield and selectivity of the dimethyl ether carbonylation. This suggests that the two steps, namely, the dissociative adsorption of methyl iodide on nickel (Equation 4) and the insertion of CO (Equation 5) are slow in the case of dimethyl ether reaction.

Figure 9 shows the product yields as a function of operational pressure in the carbonylation of methyl acetate. The yield of acetic anhydride increased monotonically with increasing pressure, while that of methane was almost unchanged. The yield of acetic acid increased up to 30 atm and then decreased above that pressure. Acetic anhydride was formed with a yield of 15% and a selectivity of 83% at 45 atm, indicating that high operational pressure was favorable for the selective formation of acetic anhydride on the Mo/A.C. catalyst. 

Operational Factors Controlling Rate and Selectivity of Carbonylation. In Figures 5 and 6 are shown the effects of reaction temperature and of CO/MeOH feed gas ratio on catalytic performances. Methanol conversion increased monotonically with an increase in the temperature and was 99% at 300 C. The yield of methyl acetate reached a maximum level at 250 C and then decreased. Acetic acid yield increased with increasing temperature and was 95% at 300 C. It should be noted that the yield of DME was 2.7% or less and that its yield was almost zero at 300 C. As already pointed out by the present authors, DME and methyl acetate are converted successively to methyl acetate and acetic acid, respectively (6,2) ... 

These species show different promoting effects on the activity and selectivity of the homologation of methyl acetate with CO + H2 (carbonylation to acetic acid, homologation to ethyl acetate and hydrogenation to methane) (5). 

Acetic Acid. Carbonylation of methanol is the most important reaction in the production of acetic acid.189-192 BASF developed a process applying C0I2 in the liquid phase under extreme reaction conditions (250°C, 650 atm).122 193 The Monsanto low-pressure process, in contrast, uses a more active catalyst combining a rhodium compound, a phosphine, and an iodine compound (in the form of HI, Mel, or T2).122 194—196 Methanol diluted with water to suppress the formation of methyl acetate is reacted under mild conditions (150-200°C, 33-65 atm) to produce acetic acid with 99% selectivity at 100% conversion. 

The Kinetics of Methanol Carbonylation Over RhX, RhY and IrY zeolites Carbonylation of methanol proceeds readily at atmospheric pressure under mild temperature conditions 150°-180°C. This reaction ZCH OH + CO - CH COOCH + HjO produces mainly methyl acetate and water. Acetic acid was detected at high conversions and high temperatures. Traces of dimethyl ether could also form. In most cases the selectivity to methyl acetate was at least 90% in presence of the iodide promotor. 

The carbonylation of methanol to acetic acid and methyl acetate, and the carbonylation of the latter to acetic anhydride, was found by W. Reppe at BASF in the 1940s, using iodide-promoted cobalt salts as catalyst precursors. This process required very high pressure (600 bar) as well as high temperatures (230°C) and gave ca. 90% selectivity for acetic acid. 

Ueda et al. (37) have proposed magnesium oxide catalyst modified with a transition metal ion (M/MgO) for the vinylation of methyl propionate and acetonitrile. Acetonitrile is vinylated to acrylonitrile selectively (94% selectivity at about 10% conversion) over Cr/MgO catalysts at 350 C in the absence of oxygen. The selectivity for the vinylation of methyl propionate over Mn/MgO catalysts is not different from the value obtained with Ti -TSM in the presence of oxygen. The catalyst system, however, is not effective for the reaction of acetic acid. We conducted the reaction of acetonitrile and methanol over Ti -TSM in the presence of oxygen, and found that the vinylation does not take place but the hydrolysis to acetic acid and subsequent esterification with methanol into methyl acetate proceed preferentially. It is likely that Ti -TSM is an appropriate catalyst for the vinylation of carbonyl compounds and M/MgO is appropriate for the vinylation of nitriles. 

Low-water operation can be accomplished with modifications to the process which include significant changes in the catalyst system [23]. The main catalytic cycle for high-water methanol carbonylation is still operative in the low-water process (see Section 2.1.2.1.1), but at low water concentration two other catalytic cycles influence the carbonylation rate. The incorporation of an inorganic or organic iodide as a catalyst co-promoter and stabilizer allows operation at optimum methyl acetate and water concentrations in the reactor. Carbonylation rates comparable with those realized previously at high water concentration (ca. 10 molar) are demonstrated at low reaction water concentrations (less than ca. 4 molar) in laboratory, pilot plant, and commercial units, with beneficial catalyst stability and product selectivity [23]. With this proprietary AO technology, the methanol carbonylation unit capacity at the Celanese Clear Lake (TX) facility has increased from 270 X 10 metric tons per year since start-up in 1978 to 1200 X 10 metric tons acetic acid per year in 2001 with very low capital investment [33]. This unit capacity includes a methanol-carbonylation acetic acid expansion of 200 X 10 metric tons per year in 2000 [33]. 

The resulting heterocycles in the complex may be further reduced or desilylated (either in the complex or after demetallation). Further synthetic potential exists in the use of the primary products, obtained by cobalt-mediated cycloadditions, as synthons in organic chemistry. For example, indole derivatives have been co-cyclized at the j/ -Cp-cobalt catalyst to give 4a,9a-dihydro-9//-carbazoles or, after oxidation, precursors for strychnine [50]. Remarkably, the cycloaddition of acrolein in the presence of a small amount of methyl acetate occurs at the carbonyl, rather than at the C=C double bond, to give vinylpyran selectively (eq. (19)) [48]. 

Liquid phase carbonylation of methanol to acetic acid with a rhodium complex catalyst is a well known process (ref. 1). The authors have found that group 8 metals supported on carbonaceous materials exhibit excellent activity for the vapor phase carbonylation of methanol in the presence of iodide promoter(ref. 5). Especially, a nickel on active carbon catalyst gave acetic acid and methyl acetate with the selectivity of 95% or higher at 100% methanol conversion under 10 atm and 250 °C. In the present study it has been found that a small amount of hydrogen which is always contained in the commercially available CO and requires much cost for being removed completely, accelerates greatly the carbonylation reaction. 

Copper-containing mordenite catalysts have also been reported to be active for carbonylation of vapor-phase methanol [170]. Initially, the predominant reaction products were hydrocarbons resulting from methanol-to-gasoline chemistry, but after about 6 h on stream at 350 °C the selectivity of the catalyst changed to give acetic acid as the main product. A recent investigation was carried out with in situ IR and solid-state NMR spectroscopies to probe the mechanism by detecting surface-bound species. The rate of carbonylation was found to be enhanced by the presence of copper sites (compared to the metal-free system), and formation of methyl acetate was favored by preferential adsorption of CO and dimethyl ether on copper sites [171],... 

Isomerization of methyl formate to acetic acid is a well-known reports in the patent literature date back to 1929. With a Co-iodide catalyst the reaction is carried out at 160° and 10.5 MPa CO . The selectivity to acetic acid is >95%. The best reported productivities are obtained with a Rh-Lil catalyst. In this case, the reaction is carried out at 180°C and 2.75 MPa with 99% conversion and near quantitative yield of acetic acid. The mechanism of the reaction involves initial cleavage of methyl formate by Lil. CH3I, obtained in the cleavage reaction, is carbonylated to acetyl iodide via the same catalytic chemistry observed in CH3OH carbonylation. The key to making acetic acid is that the mixed anhydride CHjCfOlOCfOlH is unstable and thermally decomposes to acetic acid and CO at the reaction conditions. 

Acetic Anhydride by the Carbonylation Process. The methyl acetate reaction takes place at 175°C (350 F) and 26 bars (380 psig) pressure. Conversion of methyl acetate to acetic anhydride is approximately 75% and selectivity to anhydride is greater than 95%. 

The effect of total pressure on the carbonylation of methanol was also studied. In the range of 7.5-12.5 bar the pressure had a positive effect on the carbonylation reaction, increasing both the rate of acetic acid and of methyl acetate formation Figure 8). At higher pressures (above 12.5 bar), the increase in rate of ester formation became slower while the rate of acetic acid started to decrease. A total pressure of 11-12.5 bar looks to be the optimal for the carbonylation of methanol over the H[Rh(CO)2l2]/CDB catalyst, it is not needed to run the reaction at higher pressure. The selectivity towards acetic acid reached the best values even at lower pressure, at around 10 bar. 

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