Monsanto conditions

The reaction chemistry of the rhodium-catalyzed methanol carbonylation process under Monsanto conditions has been investigated extensively [6-8, 10, 12, 21, 26-29] (cf Section 2.1.2.1.1). The overall reaction kinetics are first order in both rhodium catalyst and methyl iodide promoter. The reaction is zero order in methanol and zero order in carbon monoxide partial pressure above 2 atm (eq. (6)) [27]. The kinetics agree well with the basic mechanism common to the three carbonylation reactions (see Section 2.1.2.1.1 and Tables 1 and 2). 

Most solution-cast composite membranes are prepared by a technique pioneered at UOP (35). In this technique, a polymer solution is cast directly onto the microporous support film. The support film must be clean, defect-free, and very finely microporous, to prevent penetration of the coating solution into the pores. If these conditions are met, the support can be coated with a Hquid layer 50—100 p.m thick, which after evaporation leaves a thin permselective film, 0.5—2 pm thick. This technique was used to form the Monsanto Prism gas separation membranes (6) and at Membrane Technology and Research to form pervaporation and organic vapor—air separation membranes (36,37) (Fig. 16). 

Biphenyl has been produced commercially in the United States since 1926, mainly by The Dow Chemical Co., Monsanto Co., and Sun Oil Co. Currently, Dow, Monsanto, and Koch Chemical Co. are the principal biphenyl producers, with lesser amounts coming from Sybron Corp. and Chemol, Inc. With the exception of Monsanto, the above suppHers recover biphenyl from high boiler fractions that accompany the hydrodealkylation of toluene [108-88-3] to benzene (6). Hydrodealkylation of alkylbenzenes, usually toluene, C Hg, is an important source of benzene, C H, in the United States. Numerous hydrodealkylation (HDA) processes have been developed. Most have the common feature that toluene or other alkylbenzene plus hydrogen is passed under pressure through a tubular reactor at high temperature (34). Methane and benzene are the principal products formed. Dealkylation conditions are sufficiently severe to cause some dehydrocondensation of benzene and toluene molecules. 

Therefore, if processability is to be measured on a regular basis, it would be extremely useful if a piece of equipment was available that could measure the dynamic properties under realistic operating conditions. Fortunately, one piece of test equipment has been developed, which is commercially available, the RPA 2000 (Monsanto Co.), which may meet the requirements. A considerable number of investigations have been reported on the RPA 2000 [2J, that support the view that it may meet the requirements of an instrument that measures both polymer and compound processability. The work to date identifies differences in polymers and compounds. However, it is important to relate those differences to processing characteristics in the manufacturing environment. 

In the Monsanto/Lummus Crest process (Figure 10-3), fresh ethylbenzene with recycled unconverted ethylbenzene are mixed with superheated steam. The steam acts as a heating medium and as a diluent. The endothermic reaction is carried out in multiple radial bed reactors filled with proprietary catalysts. Radial beds minimize pressure drops across the reactor. A simulation and optimization of styrene plant based on the Lummus Monsanto process has been done by Sundaram et al. Yields could be predicted, and with the help of an optimizer, the best operating conditions can be found. Figure 10-4 shows the effect of steam-to-EB ratio, temperature, and pressure on the equilibrium conversion of ethylbenzene. Alternative routes for producing styrene have been sought. One approach is to dimerize butadiene to 4-vinyl-1-cyclohexene, followed by catalytic dehydrogenation to styrene ... 

Tests have been conducted with Monsanto high barrier nitrile resins using the common food simulating solvents (Table X) plus some typical beverages. Conditioning times and temperatures were based on applicable FDA regulations and guidelines (16). 

It is now nearly 40 years since the introduction by Monsanto of a rhodium-catalysed process for the production of acetic acid by carbonylation of methanol [1]. The so-called Monsanto process became the dominant method for manufacture of acetic acid and is one of the most successful examples of the commercial application of homogeneous catalysis. The rhodium-catalysed process was preceded by a cobalt-based system developed by BASF [2,3], which suffered from significantly lower selectivity and the necessity for much harsher conditions of temperature and pressure. Although the rhodium-catalysed system has much better activity and selectivity, the search has continued in recent years for new catalysts which improve efficiency even further. The strategies employed have involved either modifications to the rhodium-based system or the replacement of rhodium by another metal, in particular iridium. This chapter will describe some of the important recent advances in both rhodium- and iridium-catalysed methanol carbonylation. Particular emphasis will be placed on the fundamental organometallic chemistry and mechanistic understanding of these processes. 

It was discovered by Monsanto that methanol carbonylation could be promoted by an iridium/iodide catalyst [1]. However, Monsanto chose to commercialise the rhodium-based process due to its higher activity under the conditions used. Nevertheless, considerable mechanistic studies were conducted into the iridium-catalysed process, revealing a catalytic mechanism with similar key features but some important differences to the rhodium system [60]. 

Monsanto developed the rhodium-catalysed process for the carbonylation of methanol to produce acetic acid in the late sixties. It is a large-scale operation employing a rhodium/iodide catalyst converting methanol and carbon monoxide into acetic acid. At standard conditions the reaction is thermodynamically allowed,... 

The rate equation for the Monsanto process under process conditions reads v = k.[RhI2(CO)2-].[CH3I]... 

In many applications acetic acid is used as the anhydride and the synthesis of the latter is therefore equally important. In the 1970 s Halcon (now Eastman) and Hoechst (now Celanese) developed a process for the conversion of methyl acetate and carbon monoxide to acetic anhydride. The process has been on stream since 1983 and with an annual production of several 100,000 tons, together with some 10-20% acetic acid. The reaction is carried out under similar conditions as the Monsanto process, and also uses methyl iodide as the "activator" for the methyl group. 

Five organophosphorus pesticides were chosen that could be iso-thermally and simultaneously analyzed by gas chromatography using an N-P TSD detector. They are all currently commercially used and exhibit a wide range of physicochemical properties (Table I). Also influencing the choice of these pesticides was the fact that volatilization data measured from soil and water under controlled laboratory conditions are scarce for methyl parathion, parathion, and diazinon (14-17), and are not available for malathion and mevinphos. Technical mevinphos (60% E-isomer, Shell Development Co.), diazinon (87.2%, Ciba-Geigy Corp.), and malathion (93.3%, American Cyanamld), and analytical grade methyl parathion (99%, Monsanto) and parathion (98%, Stauffer Chemical Co.) were used. 

In 1986, BP Chemicals became the owners of the Monsanto technology. They subsequently also developed their own Cativa process, aimounced in 1996, carbonylation of MeOH to AcOH catalysed by Ir and Mel and promoted with specific metal iodides [8]. As with the improvements in the original Monsanto Rh process, Cativa had benefits such as improved catalyst stability and more favorable operating conditions [9]. 

The original work by Monsanto identified [Rh(CO)2l2] as the major Rh spedes under their process conditions and reaction of Mel with this complex as rate controlling for the process [3]. However, the proposed primary product of oxidative addition of Mel to [Rh(CO)2l2r, [RhMe(CO)2l3] , was not observed in early work. Forster [10] studied the reaction of Mel with [Rh(CO)2l2] by IR and found that it gave the acyl complex [Rh(C(0)Me)(C0)l3] , which was also isolated and characterized in the solid state as a dimer. The reaction could be followed quantitatively but the observed rate constant would be antidpated to be a composite of the rate constants for the formation of the intermediate [RhMe(CO)2l3r and its further reaction to [Rh(C(0)Me)(C0)l3] (Eq. (15)) and (Eq. (16)). 

In fact, Monsanto opted to favor the elimination of Acl and Forster demonstrated the elimination of Acl from the complex [Rh(C(0)Me)(C0)2l3] , though under anhydrous conditions. 

The Rh catalysed carbonylation of MeOH to AcOH was studied at Monsanto by HP IR under working reaction conditions using a short path length transmission cell coupled to a stirred reactor [12]. The presence of [Rh(CO)2l2] as the principal Rh species was generally noted. Consistent with the model studies and the kinetics of the carbonylation reaction, which tended to first order in total Rh and Mel, the rate controlling step was of course the reaction of [Rh(CO)2l2r with Mel. 

An interesting reaction of methyl formate is its isomerization to give acetic acid. Based on patent literature, a number of companies have recently reinvestigated this isomerization which has been known for over 30 years ( ). It is unlikely that it can compete with the Monsanto process however, since it doesn t need pure CO and may be operable at milder reaction conditions, some potential may be seen. Combining isomerization to acetic acid and decarbonylation to methanol and CO, could provide a direct synthesis for acetic anhydride starting directly from methyl formate (Equation 13). 

Temporal analysis of products (TAP) reactor systems enable fast transient experiments in the millisecond time regime and include mass spectrometer sampling ability. In a typical TAP experiment, sharp pulses shorter than 2 milliseconds, e.g. a Dirac Pulse, are used to study reactions of a catalyst in its working state and elucidate information on surface reactions. The TAP set-up uses quadrupole mass spectrometers without a separation capillary to provide fast quantitative analysis of the effluent. TAP experiments are considered the link between high vacuum molecular beam investigations and atmospheric pressure packed bed kinetic studies. The TAP reactor was developed by John T. Gleaves and co-workers at Monsanto in the mid 1980 s. The first version had the entire system under vacuum conditions and a schematic is shown in Fig. 3. The first review of TAP reactors systems was published in 1988. 

The preparation of acetic acid represents a special case. Olah and coworkers as well as Hogeveen and coworkers have demonstrated that CO can react with methane under superacidic conditions, giving the acetyl cation and by subsequent quenching acetic acid or its derivatives (see Section 7.2.3). Monosubstituted methanes, such as methyl alcohol (or dimethyl ether), can be carbonylated to acetic acid.115 Similarly, methyl halides undergo acid-catalyzed carbonylation.115,116 Whereas the acid-catalyzed reactions can be considered as analogs of the Koch reaction, an efficient Rh-catalyzed carbonylation of methyl alcohol in the presence of iodine (thus in situ forming methyl iodide) was developed by Monsanto and became the dominant industrial process (see Section 7.2.4). 

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