Ionic Liquids IV Not Just Solvents Anymore - American Chemical Society

Chapter 9

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Carbonylation in Ionic Liquids Using Vapor-Takeoff Reactors Gerald C . Tustin, Regina M . Moncier, and Joseph R. Zoeller Eastman Chemical Company, P.O. Box 1972, Kingsport, TN 37662

Ionic liquids allow the carbonylation of lower molecular weight alcohols and esters be operated in a continuous vapor takeoff mode resulting in higher reaction rates, with fewer units of operation, and negligible catalyst loss. For example, when the Rh catalyzed carbonylation of methanol to acetic acid is conducted in an ionic liquid, the process can be operated in a continuous vapor a take-off mode of operation, while still demonstrating improved reactor production rates (approaching 25 mol/L-h) and high methanol conversion (up to 100%) and no Rh loss.

Worldwide demand for acetic acid exceeds 6.9 million metric tons per year (2). The majority of acetic acid consumed in the world is made by the carbonylation of methanol, a process which has been extensively reviewed (3), and commercially the best systems employ a catalyst system composed of methyl iodide and a combination of either rhodium and lithium (4) or iridium and ruthenium (5). For an industrial process, the process operates under modest pressure (450-500 psig, 30-35 atm.) and temperature (175-195°C) while demonstrating very high conversions (99-100% methanol conversion) and selectivities (99-100% based on methanol, ca. 97% on CO), and excellent rates (>10 mol/l-h and possibly higher with specialized equipment.) While these processes are well established and superior to any prior process, there are some key issues still faced by anyone commercially practicing this technology. Specifically, key issues are:

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© 2007 American Chemical Society

In Ionic Liquids IV; Brennecke, J., et al.; ACS Symposium Series; American Chemical Society: Washington, DC, 2007.

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Product/Catalyst Separation. Currently, the product is flashed from the catalyst by transferring to a separation unit and releasing the pressure. The flash is normally conducted adiabatically, or at best with only a moderate amount of heat addition, to avoid catalyst precipitation. This requires a separate unit of operation, only allows partial product removal per pass (implying extensive recycle streams), and represents a bottleneck in the process. Heat Removal. The reaction is highly exothermic ( A H o ca. -28.8 kcal/mol). Whereas much of the heat is removed in the adiabatic flash in the current operation, the reaction is subject to rapid exotherms and the high heat of reaction has hampered the development of vapor phase processes where heat removal is difficult. Water Removal. Most commercial processes use 4-5% water, but water is difficult and expensive to remove from the product, reduces the depth of flash in the adiabatic operation, and leads to C O yield losses as the C O is converted to C 0 and hydrogen by water gas shift. =

2 9 8

C

2

Similar problems, especially catalyst separation and heat removal, are encountered in related carbonylations such as the commercially practiced carbonylation of methyl acetate to acetic anhydride (3) and the potentially useful carbonylation of ethanol to propionic acid. A process that could circumvent some or all of these issues would be very useful in commercial practice. While normally one would address the issue of product/catalyst separation using heterogeneous catalysts, the application of heterogeneous catalysts has been hampered by leaching when operated in the liquid phase. When operating in the vapor phase where leaching is reduced, one encounters either poor catalyst activity or difficulties with heat removal when active catalysts are found (6). While several methods around this problem have been under active examination in our laboratories (1,6,7), we have found that ionic liquids can play a particularly useful role since they permit the development of a useful vapor take-off reactor, which effectively resolves the issues of catalyst/product separation and heat removal while offering rate and cost advantages.

Development of Vapor Take-off Reactors with Ionic Liquids Vapor take-off reactors, which combine the reaction and separation steps into the same vessel by distilling the product continuously from the reaction medium, are common in industrial practice. When operating vapor take-off reactors one needs a high boiling, non-reactive solvent or starting material and operating conditions are constrained to temperature and pressure regimes where the product can be distilled or entrained from the reactor. The first and obvious advantage is that there is a capital savings when the reaction and separation are



130 combined into a single unit of operation. However, there are further potential advantages. In cases where a catalyst is unstable outside the reaction environment or where leaching may be an issue, vapor take-off reactors can be useful in maintaining the catalyst. In the specific case of methanol carbonylation, catalyst decomposition and precipitation occurs during separation as the catalyst is heated in the absence of stabilizing C O and iodide. However in a vapor take-off reactor the Rh catalyst is always in the presence of stabilizing carbon monoxide and iodide. Additionally, since heat transport in liquids is more efficient, when compared to heterogeneous catalysts employing vapor-solid reactions, it is possible to operate vapor take-off reactor at faster rates without losing temperature control. Since ionic liquids are non-volatile, normally thermally stable, normally chemically inert, and effective heat transfer fluids they should represent ideal vapor take-off solvents regardless of the application. However, iodide based ionic liquids, which are easily generated by simple alkylation of the parent phosphine or amine with an alkyl iodide, were found to be especially suitable for the carbonylation of methanol with rhodium catalysts. When used in methanol carbonylation, the rhodium catalysts were not only very soluble in ionic liquids, but were stabilized by the iodide in the ionic liquid. The continued presence of C O in vapor take-off reactors and inherent higher catalyst stability in ionic liquids eliminated both catalyst precipitation and any Rh volatilization. This allowed operation at higher Rh concentrations (and therefore higher rates) than are achievable in the commercial liquid phase reactors without catalyst losses. Further, Rh was inherently more reactive in iodide based ionic liquids. When compared in batch reactions, Rh catalysts were found to be ca. 1.5X more active in iodide based ionic liquids than they are in conventional operations which use acetic acid solutions of rhodium and lithium. The only potential downfall was the virtual insolubility of C O in most ionic liquids. In testing vapor take-off reactors, most of the initial work was conducted in a microreactor which consisted of an unstirred, gas stripped reactor which is visually depicted in Figure 1, but which is described in detail in the experimental section and in earlier work (1). This represents a classic design for vapor take off reactors and agitation is accomplished in these units by the combination of gas addition and the boiling liquid. In practice, upon the introduction of reactants the liquid swelled to ca. 2 X its original volume as starting material and products became dissolved in the ionic liquid. Figure 2 depicts the results of an extended run with l-butyl-3-methyl-dimethylimidazolium iodide (BMIMI). As shown, the catalyst was stable over an extended period of time while operating at high conversion, at high selectivity, and at rates comparable to, or better than, currently operated in commercial processes. While the primary focus was on rhodium catalyzed methanol carbonylation, the unstirred vapor take-off reactor was used to demonstrate the usefulness of vapor takeoff reactors in several additional carbonylation reactions, such as the



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Figure 1. Diagram of Unstirred, Vapor Take-off Reactor.

Figure 2 Continuous Vapor Take-Off Carbonylation of Methanol with Mel using BMIM ionic liquid. (Conditions: Feed; 2.8:1:0.14 CO:MeOH:MeI; Catalyst: 0.32 mmolRh in 10 mL BMIMI; 190°C, 200psig; Space Velocity: Varied: 153, 302, 455 h' ) (Reproduced from reference 1 with permission of author.) 1



132 carbonylation of methyl acetate to acetic anhydride and ethanol to propionate derivatives. (See Table I. A l l tests were performed with the same molar catalyst levels and volume (10 mL) as our Rh runs.) In addition, several additional methanol catalysts were demonstrated including Pd and Ir-Ru systems. (See Table I.) The palladium example is noteworthy in that, while Pd is normally a poor methanol carbonylation catalyst, it has been shown that Pd can operate in the presence of a large excess of iodide ions (8), a situation very consistent with an ionic liquid media. When examined in the vapor take-off mode where Pd is maintained in a very large excess of iodide, the methanol carbonylation with Pd catalysts, while not as fast as those with Rh, was surprisingly facile and could be operated for long periods of time with no noticeable deactivation. Regarding the Ir catalyst and its design, it has been well established that excess iodide inhibits the Ir-Ru catalyst system (5). To bind the excess iodide, the ionic liquid was formed from a mixture of butyltridodecylphosphonium iodide (BTDPI) and zinc iodide to form butyltridoceylphosphonium triiodozincate as the ionic liquid. (The choice of the butyltridoceylphosphonium cation was based on the solubility of the resultant triiodozincate.) This led to an effective catalyst for the carbonylation of methanol with Ir-Ru based catalysts.

Table I. Results of carbonylation with other substrates and catalysts in the unstirred vapor phase reactor.

Catalyst

Ionic Liquid

Conversion (based on)

Product 1 (Rate)

Product 2 (Rate)

EtOH

Rh

BMIMI

66% (EtOH)

EtC0 Et (1.3 mol/L-h)

Methyl Acetate

Rh

BMIMI

21% (MeOAc)

MeOH

Pd

BMIMI

59% (MeOH)

MeOH

Ru-Ir-Li (5:1:1)

92% (MeOH)

MeOH

Co

BTDPI + 2ZnI2 BMIMI

MeOH

Ni/ [MePPh f iodide

BMIMI

21% (MeOH)

EtC0 H (1.2 mol/L-h) Ac 0 (1.4 mol/L-h) AcOH (2.4 mol/L-h) AcOH (4 mol/L-h) AcOH (0.03 mol/L-h) AcOH (0.02 mol/L-h)

Feed

3

35% (MeOH)

2

2

-

2

MeOAc (3.6 mol/L-h) MeOAc (5 mol/L-h) MeOAc (0.09 mol/L-h) MeOAc (0.09 mol/L-h)



133 Nickel and cobalt are also known to be active catalysts for the homogeneous carbonylation of methanol, although they normally are less active than Rh or Ir and normally operate at high pressure (3). While they showed some marginal activity in the vapor take-off reactor, the activity was still very low. It is apparent that high iodide environments can not substitute for high C O pressures normally associated with N i or Co catalysts. Several additional ionic liquids were tested using Rh based catalysts in the unstirred vapor take-off reactor. Each of the ionic liquids tested demonstrated differing rates and different optimal operating conditions. Some exemplary results are shown in Table II, however the variable performance of these differing ionic liquids exposed an inherent difficulty in using ionic liquids for carbonylation. When performing carbonylations in ionic liquids, the viscosity is normally high and the carbon monoxide solubility very low. Further, both properties can vary significantly with the nature of the ionic liquid. The combination of high viscosity and low solubility often leads to mass transfer problems when conducting gas-liquid biphasic catalytic process. In the unstirred vapor take-off reactor, this represented a potentially serious limitation. To test for mass transfer effects, an alternative reactor design was employed in which a standard stirred 300 mL autoclave was fitted with a vapor take-off outlet connected to a high pressure condenser and a collection vessel. (This represented a 10X scale up in reactor volume. None of the concentrations were changed.) Normally in homogeneous catalysis, i f mass transfer is an issue one will see a rate effect with increasing stirring speed. As can be seen in Figure 3, which depicts the rate as a function of stirrer speed, there was a significant effect based on stirrer speed which implies a significant mass transfer barrier. A t high stirrer speeds, the relative rates in the differing ionic liquids changed and

Table II, Summary of best behavior for several ionic liquids in unstirred vapor take-off reactor.

Temp, °C Pressure, psig Rate (Mol HOAc/L-h) % MeOH Com. % CO Conv. HOAc/MeOAc Wt%H20

BMIMI 180 210 24.6 93.6 29 2.5 7.4

Ionic Liquid BTDPI 210 210 9.8 97.9 77 3.0 5.3

MTBPI 220 210 7.9 100 41 38 0.58

MTBPI = methyltributylphosphonium iodide


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eventually reached a plateau. The final rate order (MTBPI>BMIMI>BTDPI) is inversely related to the melt viscosities (MTBPI = 23-38 cp; B M I M I = 36-45 cp; BTDPI = 50-54 cp) for the three ionic liquids which further supports the importance of mass transfer in these reactions since higher viscosities result in lower diffusion rates and reduced mass transfer rates.

In commercial operation, the most desirable reactions would have very high (>99%) methanol conversion, high rates, high AcOH/MeOAc ratios, and low water content. In Table III, results are shown for three ionic liquids operated at the high methanol conversions which would be required for useful operation. The highest rate process was obtained with M T B P I (methyltributylphosphonium iodide) and when optimized for the key variables, the Rh/MTBPI systems displayed rates of 17 mol/L-h at 100% methanol conversion and provided a product which had an AcOH/MeOAc ratio of 18 with only 1.1 wt.% water present in the effluent when operated at 225-230°C and 225 psig. The combination of low water content and high AcOH/MeOAc ratio translates into a simpler, lower cost product separation than conventional processes since there is less material to be recycled to the reactors. The ionic liquids tested in this study demonstrated stable operation over the >300 hours of reaction time examined in the course of this study. N M R examination of the recovered solutions using the preferred M T B P I ionic liquid indicated that there was no detectable exchange of alkyl groups (monitored up to


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Table III. Rh catalyzed reactions in stirred vapor takeoff reactor.

Temp, °C Pressure, psig Rate (Mol HOAdL-h) VoMeOHConv. % CO Com". Liquid Recycle HOAc/MeOAc Product Wt % H20 Product wt % Met Product wt%MeOAc ProductWtP/oAcOrf

b

BMIMI 190 135 12.5 99.9 75 Large 4.8 3.4 17 16 64

Ionic Liquid BTDPI 220 225 9.7 99.8 74 small 16.6 1.2 16 6 74

MTBPI 230 225 18.9 99.3 75 medium 6.5 3.0 17 13 68

(a) CO:MeOH 1.33:1; (b) Wt % values are raw GC data for condensed liquid product; BMIMI operated near dew point.

NOTE:

200 hrs.) Further, when stripped of any reaction products (evacuation at 90°C, 99% methanol conversion) with: (1) no catalyst loss or deactivation over 300 hrs of operation, (2) no ionic liquid decomposition over 300 hours of operation, (3) and a purer reactor effluent (

Ionic Liquids IV Not Just Solvents Anymore - American Chemical Society

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