Promoters iodide salts

Other companies (e.g., Hoechst) have developed a slightly different process in which the water content is low in order to save CO feedstock. In the absence of water it turned out that the catalyst precipitates. Clearly, at low water concentrations the reduction of rhodium(III) back to rhodium(I) is much slower, but the formation of the trivalent rhodium species is reduced in the first place, because the HI content decreases with the water concentration. The water content is kept low by adding part of the methanol in the form of methyl acetate. Indeed, the shift reaction is now suppressed. Stabilization of the rhodium species and lowering of the HI content can be achieved by the addition of iodide salts. High reaction rates and low catalyst usage can be achieved at low reactor water concentration by the introduction of tertiary phosphine oxide additives.8 The kinetics of the title reaction with respect to [MeOH] change if H20 is used as a solvent instead of AcOH.9 Kinetic data for the Rh-catalyzed carbonylation of methanol have been critically analyzed. The discrepancy between the reaction rate constants is due to ignoring the effect of vapor-liquid equilibrium of the iodide promoter.10... 

Iodide and acetate salts increase the rate of reaction of Li [1] with CH3I at 25 °C in acetic acid. The effects of water, LiBF4, and other additives are also reported. Iodide salts also promote catalytic methanol carbonylation at low water concentrations. In the case of Lil promoter, lithium acetate is produced. The promotional effects of iodide and acetate on both the model and catalytic systems are rationalized in terms of iodide or acetate coordination to (1) to yield five-coordinate RhI anions as reactive intermediates for rate-determining reactions with CH3I.11... 

One approach which enables lower water concentrations to be used for rhodium-catalysed methanol carbonylation is the addition of iodide salts, especially lithium iodide, as exemplified by the Hoechst-Celanese Acid Optimisation (AO) technology [30]. Iodide salt promoters allow carbonylation rates to be achieved at low (< 4 M) [H2O] that are comparable with those in the conventional Monsanto process (where [H20] > 10 M) while maintaining catalyst stability. In the absence of an iodide salt promoter, lowering the water concentration would result in a decrease in the proportion of Rh existing as [Rh(CO)2l2] . However, in the iodide-promoted process, a higher concentration of methyl acetate is also employed, which reacts with the other components as shown in Eqs. 3, 7 and 8 ... 

Evidence has been presented that iodide salts can promote the oxidative addition of Mel to [Rh(CO)2l2]"> the rate-determining step in the Rh cycle [12]. The precise mechanism of this promotion remains unclear formation of a highly nucleophilic dianion, [Rh(CO)2l3]2 , has been suggested, although there is no direct spectroscopic evidence for its detection. Possible participation of this dianion has been considered in a theoretical study [23]. An alternative nucleophilic dianion, [Rh(CO)2l2(OAc)]2 , has also been proposed [31,32] on the basis that acetate salts (either added or generated in situ via Eq. 7) can promote carbonylation. Iodide salts have also been found to be effective promoters for the anhydrous carbonylation of methyl acetate to acetic anhydride [33]. In the absence of water, the catalyst cannot be maintained in its active form ([Rh(CO)2l2]") by addition of Lil alone, and some H2 is added to the gas feed to reduce the inactive [Rh(CO)2l4]. ... 

Pseudo-first-order rate constants for carbonylation of [MeIr(CO)2l3]" were obtained from the exponential decay of its high frequency y(CO) band. In PhCl, the reaction rate was found to be independent of CO pressure above a threshold of ca. 3.5 bar. Variable temperature kinetic data (80-122 °C) gave activation parameters AH 152 (+6) kj mol and AS 82 (+17) J mol K The acceleration on addition of methanol is dramatic (e. g. by an estimated factor of 10 at 33 °C for 1% MeOH) and the activation parameters (AH 33 ( 2) kJ mol" and AS -197 (+8) J mol" K at 25% MeOH) are very different. Added iodide salts cause substantial inhibition and the results are interpreted in terms of the mechanism shown in Scheme 3.6 where the alcohol aids dissociation of iodide from [MeIr(CO)2l3] . This enables coordination of CO to give the tricarbonyl, [MeIr(CO)3l2] which undergoes more facile methyl migration (see below). The behavior of the model reaction closely resembles the kinetics of the catalytic carbonylation system. Similar promotion by methanol has also been observed by HP IR for carbonylation of [MeIr(CO)2Cl3] [99]. In the same study it was reported that [MeIr(CO)2Cl3]" reductively eliminates MeCl ca. 30 times slower than elimination of Mel from [MeIr(CO)2l3] (at 93-132 °C in PhCl). 

In the 1970s, Halcon discovered that MeOAc carbonylation to AC2O could be carried out at industrially attractive rates and selectivities by using a Rh catalysed process promoted with Mel and an iodide salt [4], This was developed into a process operated by Eastman [5]. 

At about the same time, BP Chemicals, ivho ivere also licensors of the Monsanto process, developed their oivn process for carbonylation of a mixed MeOH/MeOAc/ H2O feed to AcOH and AC2O using a Rh and Mel catalysed process promoted with a quaternary ammonium iodide salt ([QAS]I), [7]. 

The carbonylation of MeOH catalysed by Rh and Mel and promoted with iodide salts can be operated at lower reactor ]H20] and higher ]MeOAc] than were originally used in commercial plant. The iodide salt overcomes stability issues and higher reaction rates and lower water gas shift rates are obtained. Some formation of reduced C2 species still takes place both as EtCOOH but also acetaldehyde (AcH). The addition of the iodide salt alone extends the region where the overall rate depends only on ]Rh] and ]MeI] to lower ]MeOAc] and ]H20]. 

The carbonylation of MeOH catalysed by Ir and Mel can also be operated at lower reactor ]H20] and higher ]MeOAc] than the original Monsanto process and without issues of catalyst stability. Commercially acceptable rates can be achieved at lower ]MeI] concentrations by using promoters such as carbonyl iodide complexes of Ru and Os or covalent iodides such as Inij or Znl2 ]9]. Ionic iodide salts are potent poisons for the Ir catalysed reaction ]11]. In contrast with the Rh catalysed systems, CH4 and not H2 is co-produced as a gaseous by-product (Eq. (8)). 

The first quantitative studies of the reaction of [Rh(CO)2l2] with Mel were reported by Maitlis and Hickey [30] who used IR to follow the formation of [Rh(C(0)Me)-(CO)l3j in aprotic media and in the presence of iodide salts. They proposed that the promotion of the overall reaction rate was due to the formation of a species such as [Rh(CO)2l3] , which would be a more potent nudeophile towards Mel than [Rh(CO)2l2r, (Eq. (17-18)). 

The effect of these simple iodide salts on the exchange rate can be explained by a similar mechanism to that described in detail above for the same promoters in anhydrous systems. This comprises catalysis of the reaction of HI with MeOAc, probably via MeOH, overall by a common iodide salt effect for all iodide salts and a second pathway for metals such as Li involving the parallel MI/MOAc cycle so that Lil is the most effective catalyst. 

Such an explanation could also hold for the dependence of reaction selectivity (and activity) on the type of cation in the iodide promoter, as shown in Table III, The role of anionic Ru complexes and the effects of various iodide salts in syngas reactions have been elucidated by Dombek et al. (8). 

The iodide promoter effects seen in Fig. 20, and some of the catalyst behavior to be described below, can be partially understood in terms of the ruthenium chemistry involved. Iodide salts have been found (191) to react... 

Although several studies have examined the effects of various promoters and ligands on the methanol homologation reaction, none has identified a system with substantially improved selectivity. However, there are many claims that iodide accelerates the rate of the reaction 62-64). While the possible sources of this enhancement have been discussed in Section IV,B, it should be noted that the systems from which these interpretations were extracted are by no means simple. Qualitative comparisons among the various studies of promoted and unpromoted systems are difficult for the reasons given above, but, in addition, because the variety of forms by which iodine is introduced (e.g., I2, CH3I, or iodide salts) apparently produce different effects (57, 63, 64). Also, many of the systems involve two promoter components (e.g., triphenylphosphine + methyl iodide or tri-p-tolylphosphine + I2X which further complicates the interpretations as to the role(s) of the halide. 

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. 

The basic organometallic reaction cycle for the Rh/I catalyzed carbonylation of methyl acetate is the same as for methanol carbonylation. However some differences arise due to the absence of water in the anhydrous process. As described in Section 4.2.4, the Monsanto acetic acid process employs quite high water concentrations to maintain catalyst stability and activity, since at low water levels the catalyst tends to convert into an inactive Rh(III) form. An alternative strategy, employed in anhydrous methyl acetate carbonylation, is to use iodide salts as promoters/stabilizers. The Eastman process uses a substantial concentration of lithium iodide, whereas a quaternary ammonium iodide is used by BP in their combined acetic acid/anhydride process. The iodide salt is thought to aid catalysis by acting as an alternative source of iodide (in addition to HI) for activation of the methyl acetate substrate (Equation 17) ... 

It is known that iodide salts react with methanol to give methyl iodide which can subsequently react with transition metal complexes [1]. The substitution of hydroxy group with iodide would be promoted by the Lewis acidic cations coordinating to the oxygen atom as illustrated in Scheme 1. 

In the low-water AO technology [23], the major function of the iodide salts is to stabilize the rhodium carbonyl catalyst complexes from precipitation as insoluble rhodium triiodide (RhD [5c]. Lithium iodide (Lil) is the preferred salt. The iodide salts also promote catalyst activity (see below). However, the key factor that con-... 

Prior to these investigations by HCC the promotional effect of iodide on the oxidative addition of Mel was investigated by others [9, 39, 40]. Foster demonstrated that the rate enhancement of this reaction in anhydrous medium was attributable to increased nucleophilicity of the rhodium catalyst with added iodide. The rationale for this observation was the generation of an anionic rhodium carbonyl complex, [Rh(CO)2l(L)]. Generation of this species was observed only with iodide added to certain neutral Rh species. No rate enhancement occurred with iodide added to the anionic complex, [Rh(CO)2l2] [39]. Similarly, in solvents with a high water concentration, iodide salts exhibited no rate enhancement in the presence of [Rh(CO)2l2] [11]. Maitlis and co-workers, in more recent investigations, reported a promotional effect of iodide in aprotic solvents on the oxidative addition of CH3I on [Rh(CO)2l2] [9a, 9c]. 

A number of iodide salts have been proposed as catalyst stabilizers and copromoters, in particular the iodide salts of Main Group IVA, VA, and VIA elements [99-104]. At equivalent iodide catalyst concentrations, these salts appear to have no significant stabilization or promotional benefit over the preferred alkali metal... 

It has been found that l-butyl-3-methylimidazolium iodide salt can serve simultaneously as a specific stabilizer to protect the transition metal complex against deactivation, a promoter to increase the catalytic performance and a reaction medium to recycle the catalyst. 

One approach that enables the use of lower water concentrations for rhodium-complex-catalyzed methanol carbonylation is the addition of iodide salts, as exemplified by the Celanese Acid Optimization (AO Plus) technology [11,33]. A lithium iodide promoter allows carbonylation rates to be achieved that are comparable with those in the conventional Monsanto process—but at significantly lower water concentrations. The AO technology has been implemented to increase productivity at the Celanese facility in Clear Lake, Texas, and in a new 500 kt/a plant in Singapore. 

It has been found that iodide salts can promote the oxidative addition of Mel to [Rh(CO)2I2], the rate-determining step in the cycle of the rhodium-complex-catalyzed methanol carbonylation reaction [20]. 

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]. 

The effect of iodide and acetate on the activity and stability of rhodium catalysts for the conversion of methanol into acetic acid have been studied. Iodide salts at low water concentrations (<2 M) promote the carbonylation of methanol and stabilize the catalyst. Alkali metal iodides react with methylacetate to give methyl iodide and metal acetate the acetate may coordinate to Rh and act as an activator by forming soluble rhodium complexes and by preventing the precipitation of Rhl3. A water-gas shift process may help to increase the steady-state concentration of Rh(I). The labile phosphine oxide complex (57) is in equilibrium with the very active methanol carbonylation catalyst (58) see equation (56). 

Of particular interest is the behavior of iodine. Fission product iodine exists in the salt in the reduced form— iodide —and is not volatile. After proper accounting for precursor transport, this behavior was confirmed in the irradiation tests and during reactor operation. An extensive chemical study showed that iodine can be removed from the salt only by extremely oxidizing conditions that promote iodide to elemental iodine,or by displacement with fluorine under oxidizing conditions. ... 

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