Carbonylation promoted catalyst systems

This hypothesis is supported by Chauvin s report (33) on a catalyst derived from (CO)5W=C(OEt)C4H9. This highly stable carbene-W(O) compound does not display catalytic activity for cyclopentene monomer. When mixed in the dark with TiCl4, a slow evolution of 1 equivalent of CO occurs. Subsequent thermal or photochemical activation produces ah extremely efficient catalyst system. Chauvin demonstrated that a high conversion to polypentenamer is obtainable at a W/cyclopentene ratio of 10 li at 5°C. The role of TiCI4 is not well understood nevertheless, it promotes carbonyl displacement which appears to be essential. 

The first catalyst systems were Cu promoted by phosphoric acid and then first row transition metals promoted by iodide. By the 1950s, the related reaction of carbonylation of MeOAc to AC2O was also known (Eq. (2)). 

Many other modifications, particularly of the Rh and Mel catalysed carbonylation of MeOH, have been proposed and some of these have been operated commercially or may have been tested at significant pilot plant scale. These include, for example, the use of phosphine oxide species such as PPh30 [20] as promoters and systems involving immobilizing the Rh on ion exchange resins [21]. Numerous examples of ligand modified catalysts have been described, particularly for Rh, though relatively few complexes have been shown to have any extended lifetime at typical process conditions and none are reported in commercial use [22, 23]. The carbonyl iodides of Ru and Os mentioned above in the context of the Cativa process are also promoters for Rh catalysed carbonylation of MeOH to AcOH [24]. 

As well as characterizing complexes involved in the main catalyst cycles, spectroscopy has contributed to the measurement of the kinetics of these cycles and to byproduct reactions. The major catalyst species present under working conditions of the catalyst systems have been identified for all the systems. Individual reaction steps involving interconversion of catalyst complexes have been isolated and studied in model reactions. IR has been very important in these studies with metal carbonyl species, including the identification of Ru promoter species in MeOH carbonylation. 

In addition to the successful reductive carbonylation systems utilizing the rhodium or palladium catalysts described above, a nonnoble metal system has been developed (27). When methyl acetate or dimethyl ether was treated with carbon monoxide and hydrogen in the presence of an iodide compound, a trivalent phosphorous or nitrogen promoter, and a nickel-molybdenum or nickel-tungsten catalyst, EDA was formed. The catalytst is generated in the reaction mixture by addition of appropriate metallic complexes, such as 5 1 combination of bis(triphenylphosphine)-nickel dicarbonyl to molybdenum carbonyl. These same catalyst systems have proven effective as a rhodium replacement in methyl acetate carbonylations (28). Though the rates of EDA formation are slower than with the noble metals, the major advantage is the relative inexpense of catalytic materials. Chemistry virtually identical to noble-metal catalysis probably occurs since reaction profiles are very similar by products include acetic anhydride, acetaldehyde, and methane, with ethanol in trace quantities. 

Protonation of a carbonyl oxygen rather than the metal may be encouraged in this case by the high coordination number of vanadium. This would then promote halide attack on the carbonyl carbon to yield an intermediate hydroxyhalocarbene, which reacts further to yield the indicated products. This system represents a potential photoassisted water-gas shift catalyst system since H3V(CO)3(diars) upon photolysis with a mercury vapor lamp yields H2, and in the presence of CO regenerates the starting complex HV(CO)4(diars). The feasibility of coupling these two reactions in the same reaction solution remains to be demonstrated. 

Aldehydes are easily hydrogenated to alcohols but ketones are more difficult to reduce because of steric hindrance. Hydrogenolysis is a problem with the catalytic reduction of carbonyls, particularly when linked to aromatic systems. Pd and H2 reduce alkenes faster than carbonyls. Metal catalyst Pt is commonly used for the reduction of carbonyls. For example, the Adams catalyst (Pt02) reduces 2-naphthaldehyde (6.31) to 6.32 in 80% when used with FeCls as a promoter. When excess of the promoter is used the product is 2-methylnaphthalene (6.33), which is also obtained by the reduction of 6.31 with Pd on BaS04 and H2. 

Chromium compounds increase the activity of platinum catalysts by increasing the electron densities of the active sites. jhe addition of ferrous sulfate, which promotes the hydrogenation of carbonyl groups, and zinc acetate, which inhibits the hydrogenation of double bonds, to platinum gives a catalyst system capable of effecting the selective hydrogenation of an unsaturated aldehyde to an unsaturated alcohoP - (Eqn. 11.12). ... 

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]. 

Nearly all catalyst systems which are used for oxidative carbonylations are based on Pd salts and complexes formed from them. They are modified by ligands, such as phosphines and amines, and promoters (for example, halogen, hydrochloric or hydrobromic acid) as well as different types of co-catalysts. In addition, there is a strong dependence on the chosen solvent, which has a great influence on the course of an oxidative carbonylation. The following conditions have to be considered. 

A process for the coproduction of acetic anhydride and acetic acid, which has been operated by BP Chemicals since 1988, uses a quaternary ammonium iodide salt in a role similar to that of Lil [8]. Beneficial effects on rhodium-complex-catalyzed methanol carbonylation have also been found for other additives. For example, phosphine oxides such as Ph3PO enable high catalyst rates at low water concentrations without compromising catalyst stability [40—42]. Similarly, iodocarbonyl complexes of ruthenium and osmium (as used to promote iridium systems, Section 3) are found to enhance the activity of a rhodium catalyst at low water concentrations [43,44]. Other compounds reported to have beneficial effects include phosphate salts [45], transition metal halide salts [46], and oxoacids and heteropolyacids and their salts [47]. 

A number of oxazaphospholidines (69) have found use as ligands in the preparation of chiral catalyst systems which promote a variety of enantioselective reactions. Examples include the use of palladium complexes for the asymmetric carbonylation of a-methylbenzyl bromide <880M59>,... 

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... 

Ragaini and colleagues recently studied the influences of acid additives [20-22]. Using the palladium-phenanthroline catalyst system for the carbonylation of nitrobenzene to methyl phenylcarbamate, the addition of anthraniUc acid [20] or phosphorus acids [21, 22] can accelerate the reaction. Anthranilic acid produced higher activity compared with the use of simple benzoic acid. The 4-amino isomer does not show the same increased activity. Later on, they established an improved catalytic system for the carbonylation of nitrobenzene by adding phosphoms acids as an additive, for the first time yielding activities and catalyst fife in the range necessary for industrial applications. By pafladium-phenanthroline complexes and phosphorus acids as promoters, nitrobenzene was carbonylated to methyl phenylcarbamate with unprecedented reaction rates (TOP up to 6,000/h) and catalyst sta-bUity (TON up to 10 ). The best promoter was phosphoric acid, which is very cheap, nontoxic and easily separable from the reaction products. The catalyst system was also applied to the economically very important dinitrotoluenes reduction. 

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. 

A, A -Dimethylimidazolinium iodide promoters, typically with ZrO(OAc)2 as a cocatalyst, are used in this Rh/1 -based process at 30 bar and 183°C. The role of the promoters and cocatalyst appears to be associated with stabilization of the Rh catalyst system and to accelerate the rate of carbonylation. 

Butanol is prepared commercially by the iron carbonyl-promoted hydro-xymethylation of propylene (Reppe and Vetter, 1953). Iron pentacarbonyl and a tertiary amine serve as a good catalyst system. The active species has been shown to be HFe(CO)4 formed from the metal carbonyl and hydroxide ion (Wada and Matsuda, 1974). 

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. 

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) ... 

Recently, Fringuelli and coworkers reported AlCl3 2THF system as an efficient catalytic system for Diels-Alder reaction of various a, P-unsaturated carbonyls, such as acrylate derivatives, enones, enals, and maleimides (Scheme 6.127) [151]. Since this catalyst system did not promote polymerization of various 1,3-dienes despite a catalytic activity of AICI3 itself for polymerization of dienes, smooth cycloaddition was realized under solvent-free conditions (SFC). 

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