Synthesis and Isolation of Methyl Acetate through Heterogeneous Catalysis withLiquid-Liquid Extraction

Susanne Lux,* Thomas Winkler, and Matthaus Siebenhofer

Institute of Chemical Engineering and EnVironmental Technology, Graz UniVersity of Technology, Inffeldgasse25 C, 8010 Graz, Austria

Formation of azeotropic mixtures may cause difficulties in separation. Synthesis and isolation of methyl acetateis a representative example for formation of azeotropic product mixtures. In this project chemical reactionengineering in batch and plug flow reactor, as well as the combination of synthesis with extractive isolationof methyl acetate from the multi component mixture methanol/acetic acid/methyl acetate/water, wereinvestigated. Reaction paths were discussed in order to avoid or bypass the low boiling azeotropic mixturesof methyl acetate with water and methanol. The esterification reaction was heterogeneously catalyzed withcation exchange resins. For extractive separation of methyl acetate aliphatic hydrocarbons of the C8 to C10

class were used. The project provides the basic strategy for process design of esterification processes.In the concept of process intensification, alternative process routes in methyl acetate synthesis and isolationwere discussed.

1. Introduction

Within the scope of “process intensification” approaches forsignificant improvement of procedural plants and processes arefocused. According to the definition of Stankiewicz and Moul-ijn,1 any chemical engineering development that leads to asubstantially smaller, cleaner, and more energy-efficient technol-ogy is regarded as process intensification. New technologiesand new design of existing processes are under investigation.The whole field can be divided into two main areas which areprocess-intensifying equipment, such as novel reactors orintensive mixing, and process-intensifying methods, such ashybrid separations as well as integration of reaction andseparation.1 With these process-intensifying means limitationsof energy and mass transport are targeted and reaction kineticsare fully utilized.2 Both areas are taken into account in thisresearch project, experimentally based on the example ofsynthesis and isolation of methyl acetate according to the overallreaction equation:

Esterification of acetic acid with methanol, yielding the mainproducts methyl acetate and water, represents the class of liquid-phase equilibrium reactions in which conversion of the reactantsis determined by the thermodynamic equilibrium. Non catalyzedconversion of the alcohol and the carboxylic acid is slow. Inthe absence of catalysts equilibrium will not be obtained within49 days at a temperature of 40 °C and ambient pressure.3 Estersynthesis is therefore conducted with acidic catalysts.4 Catalysisof the esterification of acetic acid with methanol has been widelyinvestigated. The reaction has been studied using bothhomogeneous3,5,6 and heterogeneous catalysts.6-10 Styrene-based cation exchange resins with sulfonic acids as functionalgroups provide high catalytic activity in esterification.11

In chemical industry the formation of azeotropic mixturesresults in challenging separation. Stringent efforts for productisolation are required. The production of methyl acetate through

esterification of acetic acid with methanol is a representativeexample for processes with difficult separation due to formationof azeotropic product mixtures. Methyl acetate forms binarylow boiling azeotropic mixtures with methanol (19 wt %methanol at 54 °C) and water (3.5 wt % of water at 56.6 °C).12,13

This physical property is a major obstacle for simple productisolation and plant design.

State of the art industrial production of methyl acetate is basedon the Eastman Kodak process. Methyl acetate is producedthrough countercurrent contact of glacial acetic acid andmethanol in a single reactive distillation column. The columnconsists of an extractive distillation section and a methyl acetate/acetic acid rectification section.14,15

In this research project a different approach for synthesis andisolation of methyl acetate, covering the effect of stoichiometryof reactants and implementation of liquid-liquid extraction onavoidance of azeotrope formation, is investigated. The synthesisreaction was systematically investigated in order to avoid theformation of the methyl acetate/methanol azeotropic mixture.Product separation was combined with extractive isolation ofmethyl acetate from the multi component mixture methanol/acetic acid/methyl acetate/water. From solvent screening ali-phatic hydrocarbons of the C8 to C10 class were identified asappropriate solvents capable of overcoming the methyl acetate/water azeotrope. The esterification reaction was heterogeneouslycatalyzed with the cation exchange resins Lewatit K1461,Lewatit K2620, Lewatit K2621, Lewatit K2629, and LewatitK2431 in the H+-form. The resins differ in specific load of activesubstituents and water retention according to the specificationin Table 1. The resins are strongly acidic resins with sulfonicacid substituents as functional groups. Cation exchange resinsoffer distinct advantages in catalytic synthesis reactions com-pared with homogeneous catalysts. Phase separation can eitherbase on gravity or sieving, corrosion does not play a significantrole, manufacture of packings and fixed bed as well as fluidizedbed operation offer a broad variety for technical and techno-logical process design. Key limitation of cation exchange resinsis the upper operation temperature limit, which is typically below120 °C.16,17 This temperature limit does not limit technicalapplicability of resin based catalysts because the synthesisreaction can be carried out with sufficient rate at temperatures

* To whom correspondence should be addressed. Phone: +43 3168737476. Fax: +43 316 8737469. E-mail: susanne.lux@tugraz.at.

CH3OH + CH3COOH 798H+

CH3COOCH3 + H2O

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10.1021/ie1005433 2010 American Chemical SocietyPublished on Web 08/05/2010

below 120 °C. Nevertheless styrene-based catalysts offerinteresting features for process design based on differentadsorption/desorption properties of substances involved.

2. Experimental Section

2.1. Materials. Methanol (MeOH,g99.9 wt %, Roth), aceticacid (HOAc, g99.7 wt %, T.J. Baker) and methyl acetate(MeOAc,g99 wt %, Riedel-de Haen) were used without furthertreatment or purification. For extractive separation of methylacetate the solvent n-decane (100%, Haltermann Products) wasused.

The strongly acidic cation exchange resins Lewatit K1461,Lewatit K2620, Lewatit K2621, Lewatit K2629, and LewatitK2431 were supplied from Lanxess. Lewatit K2620, LewatitK2621, Lewatit K2629, and Lewatit K2431 are macroreticularresins with cross-linked polystyrene matrix and sulfonic acidsubstituents as functional groups. Lewatit K1461 is a gel typeresin. The wet resins were delivered in the H+-form. The resinswere either applied as delivered or prepared by drying orswelling with acetic acid. Preparation started with washing withdeionized water and methanol in order to remove impurities.The resins were then dried (24 h under vacuum at T ) 105°C), and saturated with acetic acid for 8 h. Drying at highertemperatures above T ) 105 °C will damage the catalyst dueto desulfonization of the polystyrene matrix. The resins are heat-sensitive at temperatures above 125-140 °C.

The main properties of the cation exchange resins are listedin Table 1. The ion exchange capacity of the resins wasdetermined by neutralization with sodium hydroxide. Afterwashing with deionized water and separation of the washedresins, the aqueous sodium hydroxide solution was titrated withhydrochloric acid. The total exchange capacity of wet resins ineq/dm3 resin compares well with the values given by themanufacturer in Table 1. The total capacities expressed inequivalents (eq) per kilogram of dry resin were calculated.

2.2. Experimental Setup. Investigation of the esterificationreaction was carried out in batch mode in a stirred tank reactorand in a continuous plug flow reactor. The temperaturecontrolled batch reactor was equipped with a stirrer, thermom-eter, and a reflux condenser for condensation of volatilecompounds. The reaction temperature for the liquid-phasereaction was varied between 20 and 50 °C. Reaction temperaturewas kept constant within (0.5 °C. Results are representativelyreported for 40 and 50 °C. The effect of resin preparation onconversion was investigated in batch mode. In the plug flowreactor the reaction was carried out with cation exchange resinwhich was swollen in acetic acid for 8 h before usage.

In batch mode acetic acid and the cation exchange resin wereheated in the reactor until the reaction temperature remained

constant. After adding preheated methanol the esterificationreaction started. Stirrer speed set point was 500 rpm. Accordingto Xu and Chuang,7 who investigated kinetics of esterificationof acetic acid with methanol catalyzed by Amberlyst 15,production rate of methyl acetate is independent of stirrer speed.Consequently, external mass transfer resistance is not ratedetermining. The cation exchange resins used show similarfeatures as Amberlyst 15. Therefore the conclusion can beextended to this work.

The isothermal plug flow reactor (inner diameter: 8 mm,length: 265 mm) was packed with acid saturated cation exchangeresin. Due to swelling the reactor cannot be packed with drycatalyst. Liquid feed methanol and acetic acid were premixedand metered with a HPLC-pump (Milton Roy CM 4000 multiplesolvent delivery system). The temperature of the feed mixturesof acetic acid and methanol was adjusted in the feed line. Themixtures were metered with flow rates from 0.2 to 1.5 cm3/min. For condensation of volatile compounds a heat exchangerwas installed behind the plug flow reactor (Figure 1).

Liquid-liquid extraction experiments were carried out intemperature controlled shaking funnels at T ) 20 °C. A solvent/water ratio of Vorg:Vaqu ) 1:1 was applied. Extractive isolationof methyl acetate can be directly combined with synthesis inthe batch reactor and has to be applied in series when synthesisof methyl acetate is carried out in the plug flow reactor. Forcombination of the synthesis reaction with liquid-liquid extrac-tion the reaction mixture is directly overlaid with the solventin the batch reactor.

2.3. Analysis. A HP 5890 Series II gas chromatographequipped with an automatic injector and FID was used toquantify the concentration of methanol, acetic acid and methylacetate of the samples. For dilution of the samples tetrahydro-furan (THF, g99.9 wt %, Roth) was used. A 60 m × 0.32 mm×0.5 µm DB Wax column (J&W Scientific) was used toseparate the reaction mixture. The samples were injected at 250°C injection port temperature. The detector was operated at 280°C. For determination of the water content in the liquid samplesKarl Fischer titration was used. The titration equipment(Radiometer Copenhagen) was equipped with a VIT 90 VideoTitrator, Abu 93 Triburet and SAM 90 Sample Station.

3. Results and Discussion

Investigations in batch mode provided basic rate data forinvestigation of the esterification reaction in continuous mode.The effect of catalyst amount and water content of the catalystresin was investigated. Esterification in the plug flow reactorwas needed for process modeling and for collecting data forsubsequent downstream-processing. For modeling isolation ofmethyl acetate, liquid-liquid extraction was based on theexperimentally determined equilibrium data, and product separa-tion by distillation was needed for validation of literature data.13

As opposed to the reactive distillation process, liquid-phaseesterification had to be carried out at moderate temperature

Table 1. Specification of Lewatit K1461, Lewatit K2620, LewatitK2621 Lewatit K2629, and Lewatit K2431 from Lanxess AG andTotal Capacity of the Dry Resinsa

LewatitK1461

LewatitK2431

LewatitK2620

LewatitK2621

LewatitK2629

total capacity(min eq/dm3 resin)

1.8 1.2 1.8 1.4 1.6

total capacity(eq/kg dry resin)

5.05 5.30 5.26 5.00 4.85

water retention (wt%) 47-53 60-65 50-55 57-63 50-55particle size (mm) 0.62 ( 0.05 0.5-1.6 - 0.4-1.25 0.4-1.25surface area (m2/g) 25 33 40 40pore volume (cm3/g) 0.35 0.45 0.6 0.3pore diameter (nm) 40 41 65 65

a The capacity is expressed in equivalents (eq) per cubic decimeter(dm3) of wet settled resins.

Figure 1. Schematic of the experimental set up for continuous operation;MeOH: Methanol, HOAc: Acetic acid.

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(below boiling of methanol and methyl acetate) at 40 and 50°C, according to the aim of this project of esterification atambient pressure with subsequent separation through liquid-liquidextraction.

Dehydration of methanol and formation of dimethyl ether andwater can be a major competitive reaction to esterification.According to Song et al.8 the reaction is not significant at lowtemperatures and pressures even with high catalyst concentra-tions. In the temperature range covered in this work theformation of dimethyl ether was not observed.

Esterification of acetic acid with methanol is a reversiblesecond order reaction.3,9 Both pseudohomogeneous and hetero-geneous adsorption based models have been used for modelingreaction kinetics of heterogeneously catalyzed esterification.8-10

3.1. Variation of the Catalyst Concentration. The con-centration of the catalyst Lewatit K2621 was varied between50 and 180 cm3/dm3 reaction mixture in the batch reactor. Thereaction temperature was kept constant at 50 °C. Figure 2 showsthe effect of the amount of catalyst on the formation of methylacetate in the solution. Increasing the concentration of LewatitK2621 from 50 cm3/dm3 to 140 cm3/dm3 increases the reactionrate of methyl acetate formation. No further increase in ratewas observed with increasing catalyst concentration to 180 cm3/dm3.

3.2. Effect of Water Content of the Cation ExchangeResin. Cation exchange resins have high affinity to polarcomponents. The adsorption of acetic acid, methanol, methylacetate and water on various cation exchange resins has beeninvestigated by several researchers.8,9,18 The ability of ionexchange resins to selectively adsorb components of a liquidmixture may increase the resin’s usability as a selective catalyst.Due to their affinity to polar components cation exchange resinsparticularly adsorb water. Adsorption of polar componentsallows the resin to swell. With more polar components swellingof the resin is higher and therefore the accessibility of the activesites improves.16 Chakrabarti and Sharma16 also reported thatin presence of a polar reaction medium, the polymer-boundsSO3H groups are more dissociated leading to minor acidityof the sulfonic acid resin.

The effect of swelling water on the rate of formation ofmethyl acetate was investigated with the catalyst Lewatit K2621.The catalytic activity of dry resin, water saturated resin and resinwithout pretreatment was compared. Figure 3 shows thedependency of the rate of methyl acetate formation on the watercontent of the cation exchange resin.

Dry resins show the highest rate of reaction since water isremoved from the reaction mixture by adsorption.

3.3. Comparison of Macroreticular Resins and GelType Resins. Due to water retention the reaction rate of methylacetate formation is significantly higher when esterification iscatalyzed with dry resin than with water saturated resin.

For comparison of the catalytic effect of various Lewatitresins, both macroreticular resins (Lewatit K2620, LewatitK2621, Lewatit K2629, and Lewatit K2431) and the gel typeresin Lewatit K1461 were investigated. The concentration ofactive sites was 0.17 eq/dm3 reaction mixture.

As shown in Figure 4 no difference in methyl acetateformation was observed for macroreticular resins as well as gel

Figure 3. Effect of water content of Lewatit K2621 on the rate of reactionof formation of methyl acetate; cLewatit ) 140 cm3/dm3 reaction mixture,cMeOH,0 ) cHOAc,0 ) 10 mol/dm3, cMeOAc,0 ) 0, T ) 50 °C (uncertainty ofdata is below 1%).

Figure 4. Methyl acetate formation with Lewatit K2621, Lewatit K2620,Lewatit K1461, Lewatit K2629, and Lewatit K2431; cLewatit ) 0.17 eq/dm3

reaction mixture, cMeOH,0 ) cHOAc,0 ) 10 mol/dm3, cMeOAc,0 ) cH2O,0 ) 0, T) 50 °C (error interval of data is below 1%).

Figure 5. Concentration vs residence time of methanol, methyl acetate andwater for esterification in the packed bed plug flow reactor; catalyst: LewatitK2620, feed ratio of acetic acid and methanol cHOAc,0:cMeOH,0 ) 2:1, cH2O,0

) 0.33 mol/dm3, T ) 40 °C.

Figure 2. Effect of the amount of catalyst on the formation of methyl acetatein batch mode; the catalyst Lewatit K2621 was used without pretreatment,cMeOH,0 ) cHOAc,0 ) 10 mol/dm3, cMeOAc,0 ) 0, T ) 50 °C (error interval ofdata is below 1%).

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type resins. The activity of the catalyst Lewatit K2431 deviatessignificantly indicating the effect of the high water retentioncapacity of this resin on the rate of reaction.

3.4. Continuous Operation. Conversion of esterificationreactions is limited by the equilibrium composition. When thefeed concentration of one reactant is increased conversion ofthe limiting reactant is increased too. This effect was expectedto help overcome the methyl acetate/methanol azeotropicmixture by metering acetic acid in excess. The stoichiometricratio of acetic acid to methanol in the feed was varied and theeffect on conversion of methanol was recorded. As expectedfrom thermodynamics the azeotropic mixture of methyl acetateand methanol can be passed by with acetic acid in excess.

When investigating the residence time curves in the packedbed plug flow reactor with acid saturated catalyst, water retentioncan be quantified. Due to water retention, temporary overswingof methyl acetate on the reactor outlet is observed. Water andmethanol replace acetic acid in the resin resulting in retardationof their break through curve compared with methyl acetate. Thecation exchange resin does not show any tendency to adsorbmethyl acetate. Figure 5 shows the effect of acetic acid/methanol-stoichiometry of cHOAc,0:cMeOH,0 ) 2:1 in the feed onthe formation of methyl acetate as well as retardation of waterdue to adsorption, when swelling the catalyst Lewatit K2620in acetic acid prior to the experiment.

The mean residence times of methyl acetate, methanol andwater for different molar ratios of acetic acid and methanol inthe feed are given in Table 2. The difference in mean residencetime of methyl acetate and water confirms adsorptive substitutionof acetic acid with water in the cation exchange resin.

3.5. Solvent Screening. For combination of esterificationwith extractive separation of methyl acetate, the solvent andthe catalyst must not interact. Several hydrocarbon basedsolvents, including aliphatic hydrocarbons, alcohols, aldehydes,and esters were compared. Their capability for methyl acetateextraction from the multi component reaction mixture acetic

acid/methanol/methyl acetate/water was investigated. Extractionof methanol was not considered because of complete conversionwith acetic acid in excess. Liquid-liquid extraction of methylacetate from the three component system acetic acid/methylacetate/water was investigated. According to Figure 6 aliphatichydrocarbons perform best in liquid-liquid extraction of methylacetate. Low molecular weight aliphatic hydrocarbons cannotbe used due to higher water solubility. n-Decane is an appropri-ate solvent for combination of chemical reaction and extractionbecause of low water solubility and sufficient solubility ofmethyl acetate as well as significant boiling point difference.

Complete distillative separation of methyl acetate fromn-decane is possible. The binary mixture does not form anazeotropic mixture. Acetic acid does not negatively affectdistillative separation. Methanol, present in traces only becauseof complete conversion at high stoichiometric ratio of aceticacid and methanol in the feed, does not have a negative effecton product quality. Water, separated from the product mixtureby liquid-liquid extraction, does not limit the product quality.Even single step batch distillation of the extract phase resultsin methyl acetate concentration of 96.2 wt %.

4. Conclusion

An alternative approach to state of the art synthesis of methylacetate through heterogeneous catalysis of esterification of aceticacid with methanol was investigated. With acetic acid instoichiometric excess the low boiling azeotropic mixture ofmethyl acetate and methanol can be avoided. Reactor designcan be based on catalyst activity at complete water saturation.Methyl acetate can be isolated from the reaction mixture byliquid-liquid extraction with aliphatic solvents such as n-decane.With both measures, excess acetic acid in the feed as well asselective liquid-liquid extraction of the product, formation ofthe low boiling azeotropic mixtures of methyl acetate withmethanol and water can be avoided. High grade methyl acetatecan finally be isolated.

Acknowledgment

We acknowledge Lanxess AG for supplying the catalystsamples.

Literature Cited

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Table 2. Mean Residence Times of Methyl Acetate, Methanol andWater for Molar Ratios of Acetic Acid to Methanol in the Feed ofcHOAc,0:cMeOH,0 ) 2:1, 4:1, and 8:1; T ) 40 °C

molar ratio of acetic acid and methanol in the feed

2:1 4:1 8:1

methyl acetate 12.2 min 13.2 min 13.5 minmethanol 17.2 min 19.0 min 26.5 minwater 19.0 min 20.7 min 31.6 min

Figure 6. Comparison of results of solvent screening for extractive separation of methyl acetate from acetic acid/methanol acetate mixtures; T ) 18 °C.19

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ReceiVed for reView March 8, 2010ReVised manuscript receiVed July 20, 2010

Accepted July 22, 2010

IE1005433

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