Pergamon Chemical En tineerin~j Science Vol. 52, No. 19. pp. 3369 3381, 1997 1997 Elsevier Science Ltd. All rights reserved Printed in Great Britain PII: S0009-25Ug(97)UU139---fi 00o9 250997 $17.00 + o,(x) Development of a kinetic model for the esterification of acetic acid with methanol in the presence of a homogeneous acid catalyst Robert R6nnback,* Tapio Salmi,** Antti Vuori, * Heikki Haario) Juha Lehtonen,* Anna Sundqvist ~ and Esko Tirronen * • Laboratory of Industrial Chemistry, ~,bo Akademi, FIN-20500 Turku, Finland; • Kemira Agro OY, FIN-02270 Espoo, Finland; °'Kemira Engineering OY, FIN-00100 Helsinki. Finland; ~Profmath OY, FIN-00150 Helsinki, Finland (Received 14 October 1996; in revised form 25 March 1997; accepted l l April 1997) Abstract--The esterification kinetics of acetic acid with methanol in the presence of hydrogen iodide as a homogeneous acid catalyst was studied with isothermal batch experiments at 30-60~C. The catalyst concentration was varied between 0.05 and 10.0 wt%. The experiments revealed that besides the main reaction, the esterification of acetic acid, a side reaction appeared: the catalyst, hydrogen iodide, was esterified by methanol to methyl iodide. Plausible reaction mechanisms for methyl acetate and methyl iodide formation were proposed. The rate-determining step in the acetic acid esterification was assumed to be the nucleophilic attack of methanol to the carbenium ion formed through proton donation to acetic acid, whereas the rate-determining step in the hydrogen iodide esterification was presumed to be the substitution of the iodide to protonated methanol. Hydrogen iodide and acetic acid act as proton donors; thus the protolysis equilibria of acetic acid and hydrogen iodide were included in the mecha- nism. Rate equations, concentration-based as well as activity-based with UNIFAC activity coefficient estimations, were derived, and the kinetic and equilibrium parameters included in the rate equations were estimated from experimental data with regression analysis. Simulation of the models with the estimated parameters revealed that the rate equations predict correctly the experimental trends in the acid catalyzed esterification. ~C 1997 Elsevier Science Ltd Keywords: Esterification; kinetics; homogeneous catalysis. INTRODUCTION Esterification of a carboxylic acid with an alcohol, 0 0 It + R'OH ~ - tl + H20 RC-OH RC-OR' represents a very old and well-known category of homogeneous liquid-phase reactions. For instance, it can be mentioned that Berthelot and Saint-Gilles studied the equilibrium of acetic acid and ethanol in 1862 (Berthelot et al., 1862). The esters of carboxylic acids have later got an enormous practical import- ance, for example, millions of tons of polyesters are produced via the reaction of dicarboxylic acids with diols and a wide variety of mono- and di-esters are used in the production of fine and speciality chem- icals, such as pharmaceuticals, herbicides, pesticides and fragrances. The esterification reaction is a homogenous liquid- phase process, where the limiting conversion of the *Corresponding author. reactants is determined by the equilibrium. Typically the equilibrium constants of esterification reactions have values ~ 1-10, which implies that considerable amounts of reactants exist in the equilibrium mixture. This problem is in practice surmounted by continuous removal of the product --especially water-- from the reaction mixture through distillation. Furthermore, the esterification reactions are very slow; it requires typically several days to attain the equilibrium in the absence of a catalyst. Therefore, the reaction rate is enhanced with an added catalyst. Homogeneous and heterogeneous acids act catalytically in the esterifica- tion, because the initiating step in the reaction mecha- nism is the protonation of the carboxylic acid, i.e. RCOOH +H + ~ RCOOH2 ~. Efficient homogeneous catalysts are mineral acids, such as H2SO4, HC1, HI and strong organic acids, such as HCOOH. As heterogeneous catalysts the ion- exchange resins containing sulphonic acid (-SO3H) groups are active in the esterification. The acids used as catalysts often undergo side reactions, since they 3369
`
`IPR2015-00171
`Exhibit 1058
`
`000001
`
`

`
`3370
`
`R. Ronnback er :1].
`
`calculated from stoichiometric relations due to lack of
`analytical accuracy.
`
`QUALITATIVE RESULTS
`
`Besides the catalyzed kinetic experiments two addi-
`tional experiments were performed: an experiment in
`the absence of the catalyst and an experiment with the
`catalyst and methanol in the absence of acetic acid.
`The former experiment confirmed that
`the un-
`catalyzed reaction is negligible in current conditions.
`The results depicted in Fig. 1 indicate that a reaction
`time of 49 days is not suflicient to reach the equilib-
`rium conditions: the ratio,
`
`CCH3COOCH3 ' cH2O
`KC =4
`cCH3COOH ' CcH3oH
`
`was 4.5 after 28 days, 5.9 after 42 days and 6.3 after 49
`days
`The experiment with methanol (25 mol) and hydro-
`gen iodide (115 g, 10 wt %) indicated that some es-
`terification of methanol takes place; the concentration
`of methyl iodide formed in the reactor was about
`4 wt %. The kinetics of this reaction remained, how-
`ever, obscure because of the scattering in the primary
`data:
`all
`the samples
`taken at
`reaction times
`15-240 min contained about 3-4 wt % methyl iodide
`which may be due to either a very rapid reaction or an
`insufficient termination of the reaction in the samples.
`The primary data from the kinetic experiments at
`30—60°C, with catalyst concentrations varying from
`005-100 wt % are presented in Figs. 2-5. The results
`show the increase of the reaction rate as a function of
`
`the temperature and the amount of catalyst. The reac-
`tion is rather slow: with the lower concentrations of
`
`the total reaction time (3 h) was not
`the catalyst,
`sufficient to reach the equilibrium composition. At
`higher catalyst concentrations (5 and 10 wt %) it
`seemed diflicult to terminate the reaction in the sample
`fast enough before the analysis. Therefore an error,
`especially in the beginning of the reaction, where the
`reactants have reacted further, is very probable.
`
`70
`
`3370 can in principle be esterified by the alcohols being present in the system. In spite of the huge amount of accumulated know- ledge concerning the synthesis of different esters and the catalytic effects of numerous agents, many funda- mental problems still need to be elucidated. For the optimization of an industrial esterification process the reaction kinetics should be well-determined, because the design of a large-scale reactor should be based on rate equations, which include the effects of the cata- lysts and the side reactions appearing in the system. An appropriate rate equation should be based on the true mechanism, including the elementary steps of the main and side reactions as well as the dissociation equilibria of the acids. The rate equation should also predict the correct equilibrium composition, which concerns the role of activity coefficients in the rate equation. The endeavor of the present work is to develop a systematic approach to the modelling of esterifica- tion kinetics of carboxylic acids in the presence of an inorganic acid as a homogeneous catalyst. The follow- ing aspects will be included in the modelling: the elementary steps of the main (carboxylic acid esterifi- cation) and the side (esterification of the catalyst) reactions, the protolysis equilibria of the acids and the influence of the activity coefficients on the equilibria. A classical esterification process, namely the reaction between acetic acid with methanol, has been chosen as the demonstration system. EXPERIMENTAL The kinetic experiments were carried out in a batch reactor (glass, RC-1, 2000 ml). The reagents (meth- anol, > 99.5 wt %; acetic acid, 100 wt %; hydrogen iodide, 70 wt % in aq.) were of laboratory grade and they were used as received. Equivalent amounts (10 tool) of methanol and acetic acid were mixed in the reaction vessel and after the temperature had been stabilized the first sample for analysis was withdrawn. The reaction was initiated by adding the catalyst, hydrogen iodide. The reaction temperatures were 30, 40, 50 and 60°C. The catalyst concentration was varied from 0.05 wt % to 10.0 wt %. The reaction in the analytical samples was extinguished by diluting the sample with acetonitril. The total volume of the diluted sample was 50 ml and the samples were kept in darkness. The samples were analyzed with gas chromatogra- phy (GC), Hewlett-Packard 5790A, HP-INNOWAX column at 45°C (3 min) 15°C/min 200°C (1 rain) and high pressure liquid chromatography (HPLC), Merch Hitachi, Sperisorb 10 C18, 25 cm x 4.6 mm I.D. The eluent was 6.25% ACN and 94.75% water with 4 g/l dibuthylaminine flowing at 2 ml/min. UV detec- tion at 215 nm was used. The injection volumes for GC and HPLC were 1 and 5 pl, respectively. The concentrations of acetic acid, methanol and methyl acetate were directly obtained from the analysis. For some experiments, the concentration of methanol was R. R6nnback et al. calculated from stoichiometric relations due to lack of analytical accuracy. QUALITATIVE RESULTS Besides the catalyzed kinetic experiments two addi- tional experiments were performed: an experiment in the absence of the catalyst and an experiment with the catalyst and methanol in the absence of acetic acid. The former experiment confirmed that the un- catalyzed reaction is negligible in current conditions. The results depicted in Fig. 1 indicate that a reaction time of 49 days is not sufficient to reach the equilib- rium conditions: the ratio, CCH3COOCH 3 • CH20 Kc : CCH3COOH " CCH30H was 4.5 after 28 days, 5.9 after 42 days and 6.3 after 49 days. The experiment with methanol (25 mol) and hydro- gen iodide (115 g, 10 wt %) indicated that some es- terification of methanol takes place; the concentration of methyl iodide formed in the reactor was about 4 wt %. The kinetics of this reaction remained, how- ever, obscure because of the scattering in the primary data: all the samples taken at reaction times 15-240 min contained about 3-4 wt % methyl iodide which may be due to either a very rapid reaction or an insufficient termination of the reaction in the samples. The primary data from the kinetic experiments at 30-60°C, with catalyst concentrations varying from 0.05-10.0 wt % are presented in Figs. 2-5. The results show the increase of the reaction rate as a function of the temperature and the amount of catalyst. The reac- tion is rather slow: with the lower concentrations of the catalyst, the total reaction time (3 h) was not sufficient to reach the equilibrium composition. At higher catalyst concentrations (5 and 10 wt %) it seemed difficult to terminate the reaction in the sample fast enough before the analysis. Therefore an error, especially in the beginning of the reaction, where the reactants have reacted further, is very probable. 70 i i ; ~ i ! 60 .............. i .............. i .............. ! ............... j .............. j .............. ~" 50 ................................................ "~" ~ ............................. ~o i ~'~I ~ : ! _,~,,_. H20 "~ 40 ,.#~. .......... J ............ J ............ J .............. i .............. "-~" AcOMo 7. , it,! ] i i i --~ AcOH .~_ 30 ...... ~-~...-] .......... i ............ i ............. l .............. + MeOH 20 ...... ; ............. ~-.~ '~ ~--m~ qim- ._:...~ .............. 10- .~r~ .......................... , ....................... ~ .............. 0, ............ i ....................... i .............. ! ............ ! ............ i ! i ! * i i 0 10 20 30 40 50 60 time [days] Fig. 1. Esterification of acetic acid with methanol without catalyst at 40°C.
`
`can in principle be esterified by the alcohols being
`present in the system.
`In spite of the huge amount of accumulated know-
`ledge concerning the synthesis of different esters and
`the catalytic effects of numerous agents, many funda-
`mental problems still need to be elucidated. For the
`optimization of an industrial esterification process the
`reaction kinetics should be well-determined, because
`the design of a large-scale reactor should be based on
`rate equations, which include the effects of the cata-
`lysts and the side reactions appearing in the system.
`An appropriate rate equation should be based on the
`true mechanism, including the elementary steps of the
`main and side reactions as well as the dissociation
`
`equilibria of the acids. The rate equation should also
`predict the correct equilibrium composition, which
`concerns the role of activity coeflicients in the rate
`equation.
`The endeavor of the present work is to develop
`a systematic approach to the modelling of esterifica-
`tion kinetics of carboxylic acids in the presence of an
`inorganic acid as a homogeneous catalyst. The follow-
`ing aspects will be included in the modelling:
`the
`elementary steps of the main (carboxylic acid esterifi-
`cation) and the side (esterification of the catalyst)
`reactions, the protolysis equilibria of the acids and the
`influence of the activity coefiicients on the equilibria.
`A classical esterification process, namely the reaction
`between acetic acid with methanol, has been chosen as
`the demonstration system.
`
`EXPERIMENTAL
`
`The kinetic experiments were carried out in a batch
`reactor (glass, RC-1, 2000 ml). The reagents (meth-
`anol, > 99.5 wt %; acetic acid, 100 wt %; hydrogen
`iodide, 70 wt % in aq.) were of laboratory grade and
`they were used as
`received. Equivalent amounts
`(10 mol) of methanol and acetic acid were mixed in the
`reaction vessel and after the temperature had been
`stabilized the first sample for analysis was withdrawn.
`The reaction was initiated by adding the catalyst,
`hydrogen iodide. The reaction temperatures were 30,
`40, 50 and 60°C. The catalyst concentration was
`varied from 0.05 wt % to 10.0 wt %. The reaction in
`
`the analytical samples was extinguished by diluting
`the sample with acetonitril. The total volume of the
`diluted sample was 50 ml and the samples were kept
`in darkness.
`
`The samples were analyzed with gas chromatogra-
`phy (GC), Hewlett-Packard 5790A, HP-INNOWAX
`column at 45°C (3 min)—l5"C/min — 200°C (1 min)
`and high pressure liquid chromatography (HPLC),
`Merch Hitachi, Sperisorb 10 C18, 25 cm X 4.6 mm I.D.
`The eluent was 6.25% ACN and 94.75% water with
`
`4 g/l dibuthylaminine flowing at 2 ml/min. UV detec-
`tion at 215 nm was used. The injection volumes for
`GC and HPLC were 1 and 5 ,ul, respectively. The
`concentrations of acetic acid, methanol and methyl
`acetate were directly obtained from the analysis. For
`some experiments, the concentration of methanol was
`
`60 -
`
`E 50
`in
`‘*3’ 40
`
`E 30
`§
`g 20
`10
`
`U 5
`
`0
`
`--v-- H20
`—A--AcOMe
`
`_ —-—AcOH
`-0- MeOH
`
`0
`
`1 0
`
`20
`
`30
`time [days]
`
`40
`
`50
`
`60
`
`Fig. 1. Esterification of acetic acid with methanol without
`catalyst at 40°C.
`
`000002
`
`000002
`
`

`
`Esterification kinetics of acetic acid
`
`3371
`
`30 “C
`
`(b)
`
`40 "C
`
`UIO
`
`-5-O
`
`[0,
`
`
`
`concentration[weight-%]
`
`(d)
`
`time [h]
`
`60 “C
`
`4.)O
`O
`concentration[weight-%] 58
`
`Esterification kinetics of acetic acid 3371 (a) 70 30 °C 60 .so eaa '~ 40 g 30 20 Q § ~o v ........ '~ .......................... ! ............................ ~ ............................ I ................. i ............................. i .......................... ~ .......................... ' i I ........ ............. ......... i ...................... .................... i ............................ i ............................ i .......................... , i i ! : 1 2 3 4 time [h] (c) 70 50 "C 60 .................................................. ! ......................................................... - .so ...... ,'~ ............................................ i ............................ { ............................. = "-,-..... ! ! i "~__~ 40 J ""' .......................... ~7~: ............. ----~i ............. ............ !,=. "- 30 i .......... • 20 10 0 ~ ~ ! .... I .... I .... i .... 0 I 2 3 4 time [hi model MeOH ----- AcOH ........ MeOAc (b) (d) 70 • 40 °C ,v.~ i 50 -t........_..,.....~_! ....................... ~ ........................ i .......................... J ~0 -~ .......................... = ......................... ! .......... ~ ~.=~.~' ..................... 3O ] "~~~'~":i i ...................... ')0 -4 .................. " ........................ m-t/ ............ ,. .............. ~ .......... ........... 0 4 ........................... i ......................... i .............. ........ t .... i .... o 2 3 time [h] 60 °C 70 ~~ i 60 .......................... i ........................... ~ ........................ ~ ......................... 5o ................ ! ............................ i .......................... !i ......................... "+" ' ..................... ~.~Y,:2: ............ ! .......................... ! .......................... 2o77 .......................... ,o Iili ................. i' ........... iii ........... i ................ i ......... i .............. o ............................ ! .............................. ~ ............................. i .......................... 0 2 3 4 time [h] experimental data • MeOH • AcOH • MeOAc Fig. 2. Kinetics of the esterification of acetic acid with methanol. The catalyst concentration: 0.2 wt % The model: concentration-based rate equations with temperature-dependent parameters. MECHANISM AND KINETICS Two overall reactions are considered in the kinetic model: esterification of acetic acid with methanol and esterification of hydrogen iodide to methyl iodide. The overall reactions can be written as follows: CH3COOH + CH3OH ~H3OL O CH3C ~ + H20 OCH 3 (1) HI + CH3OH ~H3OL CH3I + H20. (2) The second reaction is a side reaction which destroys the catalyst. The hydronium ions acting as catalytic agents in the esterification are created through protolysis of HI and CH3COOH; the protolysis is enabled by the presence of water in the system (all experiments were commenced with some water present in the begin- ning in the liquid phase). The protolysis equilibria are (aI) HI + H20~,~-I- + H30 + (3) (aII) CH3COOH + HzO~.-~-CH3COO- + H3 O+ (4) The equilibria are both rapid, but the acid strengths of CH3COOH and HI are very different: CH3COOH is a rather weak acid (pKcn3coon = 4.75 in aq. at 25°C, Weast et al., 1986), whereas HI is a very strong acid (pKm < -7 in aq. at 25°C, Bailar et al., 1973). To obtain rate equations for the esterification reac- tions (1) and (2), a detailed knowledge of the underly- ing mechanisms is necessary. For the acid-catalyzed esterification of carboxylic acids with alcohols, the following mechanism has been proposed (Streitweiser
`
`VI0
`
`JR0
`
`
`
`Lo.)0
`
`
`
`-—IQ.aJQCC
`
`4>0
`
`(21)
`
`
`
`concentration(weight—%]
`
`
`
`concentration[weight—‘7r]
`
`time [h]
`model
`
`MeOH
`
`——- ACOH
`
`. . . . . . ..
`
`time [h]
`
`experimental data
`0 MeOH
`
`I
`
`AcOH
`
`‘ MeOAc
`
`Fig. 2. Kinetics of the esterification of acetic acid with methanol. The catalyst concentration: 0.2 wt "/0 The
`model: concentration-based rate equations with temperature-dependent parameters.
`
`MECHANISM AND KINETICS
`
`Two overall reactions are considered in the kinetic
`model: esterification of acetic acid with methanol and
`
`commenced with some water present in the begin-
`ning in the liquid phase). The protolysis equilibria
`are
`
`esterification of hydrogen iodide to methyl iodide.
`The overall reactions can be written as follows:
`
`(al)
`
`HI + H2021’ + H3O+
`
`(3)
`
`cH,cooH + CH3OH
`
`HI + cH,oH
`
`
`3
`1-1 0*
`
`
`1130*
`
`’/
`0
`CH3C\
`
`OCH:
`
`+ H20
`(1)
`
`CH,1 + 11,0.
`
`(2)
`
`The second reaction is a side reaction which destroys
`the catalyst.
`The hydronium ions acting as catalytic agents in
`the esterification are created through protolysis of H1
`and CH3COOH;
`the protolysis is enabled by the
`presence of water in the system (all experiments were
`
`(all) CH3COOH + H2O:_‘CH3COO' + H30‘ (4)
`
`The equilibria are both rapid, but the acid strengths
`of CH3COOH and HI are very different: CH3COOl-I
`is a rather weak acid (pKcH,Co0H = 4.75 in aq.
`at 25°C, Weast et al., 1986), whereas H1 is a very
`strong acid (pKH, < -7 in aq. at 25°C, Bailar et al.,
`1973).
`To obtain rate equations for the esterification reac-
`tions (1) and (2), a detailed knowledge of the underly-
`ing mechanisms is necessary. For the acid—catalyzed
`esterification of carboxylic acids with alcohols, the
`following mechanism has been proposed (Streitweiser
`
`000003
`
`000003
`
`

`
`3372
`
`R. Rénnback et al.
`
`«PO
`
`LIIC
`
`—-l\)DJoooo
`
`
`
`I
`......~....._l.,.......t._.-.._.l...._l_.m._._.._.._.._.
`
`.-.-.._--.-_._ ._..-.-.._._.L.__.-_.--...-._fi_....._._.-..._.._.
`
`model
`
`MeOH
`
`—-° AcOH
`
`. . . - . . o.
`
`experimental data
`O MeOH
`
`ACQH
`
`u
`
`‘
`
`Fig. 3(a)—(d). Kinetics of the esterification of acetic acid with methanol. The catalyst concentration:
`0.2 wt %. The model: activity-based rate equations with temperature-dependent parameters.
`
`
`
`concentration[weight-%]
`
`(C)
`
`§
`J____'..
`..°_°H.)
`
`E E EE 8I
`
`: 8
`
`3372 R. R6nnback et al. (a) 30 °C 7O 60 ~ ....................... 1~ .............................................................................................. , , -~ ! ! I ~', 50 .................. ~--.%-~:: ................. ~ ............................... f ............................... ~o .~ 40 ................................ ). ............................................................................................... I g3o ~ ........................................ .~, i .................. 20 ............................... F'""v= ........... :?":': .......................... T .............................. e, • ..... : .. ......... I ! i 10 -..~,::: ................. ~ ............................... ~ ............................... ~ ............................. / I i i .... I .... i .... : .... 0 1 2 time [h] (b) 40 °C 70 i 5o ................. , ........................... T ............................... i ................................ ~o~ ............. ~ ......... i ................................ i ............................... i ................................ ] T ....... +_ ) ~0 -~ ............................... ! ................................ .L.....:.:===:=~ ............................... 30~: ......................... i .............................. 20 ..... ~ ............................. T ............................. ..." ................................ -...." i i i .... i .... J .... i .... 0 I 2 3 4 time [h] (c) 70 6O .~ 5o etO 40 • ~ 3o _~ 2o 8 1o 50 °C i ! i ~" ............... I ....................... t ......................... t ..................... \ i i i .... ?.: ..... k _ i ' ~-~_ i ............... T ...................... I ................... .................... t"'-'~:::~::= ........ t ........................ I ............... I ...... -~' • ~--- -/- ............ <:=::'-":'.~ ......... ~ ............... L ............. ,..-. -->,,f , - .'" " l .-.~~,- .......... / 1 t "-__ [ ...... I .......... T ......... V .............. I [ I .... i .... I .... i .... 0 1 2 3 4 time [hl (d) 60 °C 70, I J so4..\. .................... ~.. ........................ t ............................. I ............................... ~o¢-......'.....'~#~--..~i:.=::::::::: ............ ~ ............................ ..... ~....- <::___+ ,o. .................. F=::: ..... + ....................... 20- ~ .......................... ,o. ! ................ 1~ ............... 1 ....................... -t--- . /---- i .- -- 0 1 2 3 4 time [h] model experimental data MeOH • MeOH ----- AcOH • AcOH ........ MeOAc • MeOAc Fig. 3(a)-(d). Kinetics of the esterification of acetic acid with methanol. The catalyst concentration: 0.2 wt %. The model: activity-based rate equations with temperature-dependent parameters. et al., 1992): O - OH+ II + H3 O+ ~ II + H20 CH 3 -- C -- OH CH 3- C -- OH (5) OH ~O I ~H H+ + CH3OH _ ~ CH3--C-OH (6) CH3--C--OH I + HO CH 3 OH OH I I CH3--C--OH +H20 ~ - CH3__C_OH+ H30 + I I HO*CH 3 OCH3 (7) OH OH I I + CH 3 --C- OH + H30 + ~ ~ CH3--C-OI'I2 + H20 I OCH 3 OCH3 (8) CH 3 H: + OH + ~ + H20 OCH3 CH3-C-OCH3 (9) OH + O CH II + H20 _ ~ )1 + I-I30 +. 3-- C-OCH 3 CI~--C-OCH 3 (10)
`
`+HO
`2
`
`(5)
`
`(6)
`
`et al., 1992):
`O
`ll
`CH3--C '- OH
`
`
`+HO+''‘‘‘‘‘
`3
`
`01'“
`ll
`CH3-C - OH
`
`OH
`,,
`I
`CH —C-OH+HO
`I
`3
`3
`0C1-13
`
`(|)H
`+
`CH —C-OH2 +HO
`'
`3
`2
`OCH3
`(8)
`
`OH‘
`(II
`CH3‘C- OH
`
`
`
`+ CH,OH
`
`(Im
`CH3—$—OH
`H01-CH3
`
`*OH
`EH
`CH3‘ I ‘(BHX w‘-‘=‘ CH _g_OCH + H20
`QC]-[3
`3
`3
`
`(9)
`
`OH
`OH
`
`CH —<':--oH+H,o—-CH —c—oH+Ho*
`3
`I
`3
`I
`3
`Ho*cH,
`OCH;
`
`+
`
`(RH
`CH3_ C_
`
`+1-1%:
`2
`
`3
`
`ii)
`
`(7)
`
`000004
`
`+
`
`0*
`
`['13
`
`(10)
`
`000004
`
`

`
`(a)
`
`0.05 weight-%
`
`(b)
`
`0.20 weight-%
`
`Esterification kinetics of acetic acid
`
`3373
`
`[weight—%]
`concentration
`
`0
`
`1
`
`2
`
`time [h]
`
`3
`
`4
`
`(c)
`
`1.0 weight-%
`
`[weight-%]
`concentration
`
`(d)
`
`Esterification kinetics of acetic acid 3373 (a) 0.05 weight-% 7060 L..~ .............................................. ~ .......................... - ......................... t "'--~ ! ! ! 50 ................... ; ..................... -T~:.::.~z:.z._~ ~. ........................ 30 ........ .~ .............................. i .......................... ~i ................ 20 g 10 ............... 0 ............................................. .... i .... i ........ o 1 2 time [hi (h) 70 0.20 weight-% ,o i .... .... 40 ............................................... ' ........... 30 ~ ................... :::::::! .............. 20 ' i :::'" ":: '; .......... ~-~" 10 .......... : i . 0 ........................ 2 ........................................... ~ ................ i .... i .... ~ .... i .... 0 1 2 3 4 time [hi (c) 1.0 weight-% 70 6O .~ 50 '~ 40 30 O 7 2o l0 8 0 ....]..~ .. ..................... ; ............................ ~ ................... .~ ................................. • ......... i ............................ i ............................ • /4 "%...~ i i ........ 7: ............. ~' -- ,r " ~ ~ -.,~:;.]' ............................ i ............................ 0 2 3 4 time [hi (d) 70 6O 50 " 4o g 30 7 20 8 10 o 0 5.0 weight-% i'" " ..; .......... i ............................. i ......................... i ....................... ~ . . i a -~-~..&_ ,:o 0 1 2 3 time [hi model experimental data MeOH • MeOH ----- AcOH • AcOH ........ MeOAc • MeOAc Fig. 4. Kinetics of the esterification of acetic acid with methanol at 40'C. The model: concentration based rate equations. The nucleophilic substitution, step (6), is generally believed to be rate determining, whereas the proton donation step (5) as well as the subsequent steps (7)-(10) are assumed to be rapid. Consequently, steps (6)--(10) can in the kinetic treatment be lumped to a pseudo-step and the simplified mechanism becomes (I) O ,OH CH3C ~t + H30+ ~ ~ CH3C + + H20 OH "OH (ll) (II) CH3 C÷ _OH O + CH3OH ~ ~ CH3C ~ + H3 O+. OH OCH 3 The speed of the rate-determining step (rds) (12) can thus be expressed as r2 = k2 " CA" CCH3Ott -- k 2 " CE" CH3()~ (13) where A = CH3C(OH)~- and E = CH3COOCH3. The concentration of the intermediate (A) is obtained after applying the quasi-equilibrium hypothesis on the rapid step (11): KI _ CA " CH20 (14) CCH3COO H " CH30 + The rate equation is rewritten as r 2 = K lk 2/--//CcH~COOH " CCH~O. \CH20/L - Kl(kz/k-2)J (12J (15)
`
`[weight-%]
`concentration
`[weight—%]
`concenuation
`
`time [h]
`
`5.0 weight-%
`
`time [h]
`
`model
`
`MeOH
`
`-—— - ACOH
`...... .. MeOAc
`
`time [h]
`
`experimental data
`0 MeOH
`
`I AcOH
`A MCOAC
`
`Fig. 4. Kinetics of the esterification of acetic acid with methanol at 40"C. The model: concentration based
`rate equations.
`
`is generally
`The nucleophilic substitution, step (6),
`believed to be rate determining, whereas the proton
`donation step (5) as well as the subsequent steps
`(7)—(10) are assumed to be rapid. Consequently,
`steps (6)—-(10) can in the kinetic treatment be lumped
`to a pseudo-step and the simplified mechanism
`becomes
`
`(1)
`
`,,0
`CH3C\
`OH
`
`,OH
`+ H3O+: cH3c* \
`
`OH
`
`+ H20
`
`+ ,0H
`(U) CH3C \
`
`OH
`
`40
`+ CH3OH wt‘ CH_,C\
`
`OCH3
`
`(11)
`
`+ 1130*.
`
`(12)
`
`The speed of the rate-determining step (rds) (12) can
`thus be expressed as
`
`’‘2 = k2 ' (‘A ‘ ('(‘H,oH — k——2 ' (‘E ' CH30‘
`
`(13)
`
`where A = CH3C(OH)2+ and E = CH3COOCH3.
`The concentration of the intermediate (A) is obtained
`after applying the quasi-equilibrium hypothesis on
`the rapid step (11):
`
`K1:
`
`CA ' (‘H20
`CCHJCOOH ' CH3o *
`
`(14)
`
`The rate equation is rewritten as
`
`k
`V2 = K1 2
`
`C1130‘
`,
`Cnzo
`
`(‘E ' (‘H10
`_
`_
`(CH3COOH ' Cc:-13011 — ifi_
`K1( 2/I»--2)
`(15)
`
`000005
`
`000005
`
`

`
`(a)
`
`0.05 weight-%
`
`(b)
`
`0.20 weight-%
`
`R. Ronnback et al.
`
`time [h]
`
`[weight-%]
`concentration
`[weight-%]
`concentration
`
`(d)
`
`5.0 weight-%
`
`[weight-%]
`concentration
`[weight-%]
`concentration
`
`3374 R. R6nnback et al. (a) 70 60 50 "~ 40 & g 30 20 = l0 8 (c) 70 60 50 "~ 40 30 i 10 0 0.05 weight-% i ........................... i ... 72.~--.~.~-L,~ ..................... i,, ................. i ...... d. .................................................... 2.~::..:::" ........... ;'::::':'}: ................. ...... i ............ 2 ........... i "..;::::"':: ........... i ........................... t ........................... ~ ........................ 3 time [hl (b) 70 60 50 '~4o 30 '=~ 20 10 8 0 0.20 weight-% i :212i ............................ T .............................. ! ............................... ...... i i -./:: ......................... i ................................ " ............................................................. ............................. . l .... i .... i ........ 0 1 2 4 time [hi 1.0 weight-% (d) 5.0 weight-% .............................................................................................. ! ............................... 60 ............................... i'""""15Z2;;'5;!.'.I"5;;:;.;5::::;::::::!'~ ............................. ............................ i ............................... ~ ....................... ,L...i ............................... ~ 50 ...... .'.'--';: .............................................. ; ............................. T ............................... • "-<!-~ ..................... i .............................. i .............................. } 40 ;, ...................... L ................................ t .............................. , ............................ - i _~........'../;;:...:......~7L'2...., ..... .._..~ ........................... ,~ ............................ ] ~ 30 ~f"" ~~ .......... i .............................. i .............................. i .......................... I .-" ! .~-- : ..... i j "--~J ~ J i ,: i ~ .......................... ~--! ......................... -I ~ ~0 , , , .................... _~ ............................. ~ ......................... i .............................. 1 o ............ i ............................ i .................. i i j i ........... . ........... 0 2 3 4 0 1 2 time [h] time [hi model experimental data MeOH • MeOH ----- AcOH • AcOH ........ MeOAc • MeOAc Fig. 5. Kinetics of the esterification of acetic acid with methanol at 40"C. The model: activity based rate equations. The product Kl(k2/k-2 ) equals the equilibrium con- stant of the overall reaction, Kin. In addition, the product Klk2 is denoted by a lumped constant k~. Thus, we obtain for r2, ., fCH~o?~f r 2 = K 2 |--J |CCH3COOH • CCH3OH \ CH20 ,//\ cc c.~o~ KE2 / (16) The catalytic contribution depends on the protolysis equilibria (3) and (4) and on the side reaction (2). A two-step mechanism has been proposed (Streit- weiser et al., 1992) for the side reaction (2), (VII) CH3OH + H30 + ~.~-CH3OH~- + H20 (17) (VIII) CH3OH~ + 1- ~---CH3I + H20. (18) The protolysis step (17) is assumed to be rapid, where- as the second step is rate determining. The velocity of step (18) can be written as r8 = ks • Ccn3ou2 " CI -- k-8" CCH31 ' CH20. (19) The concentration of CH3OH~- is calculated from the quasi-equilibrium: K.7 CCH3OH2 "CH20 (20) CCH3OH " CH30~ Then the rate equation becomes r 8 = k 8 • g 7 • CCH3OH • CH30+ CH20 • C 1 -- k-8 " CCH31 "CH20 (21)
`
`time [h]
`
`(c)
`
`1.0 weight-%
`
`experimental data
`
`0 Me0H
`
`I
`
`ACOH
`
`A MeOAc
`
`Fig. 5. Kinetics of the esterification of acetic acid with methanol at 40°C. The model: activity based
`rate equations.
`
`The product K1(k2/k_2) equals the equilibrium con-
`stant of the overall reaction, K52. In addition, the
`product K,k2 is denoted by a lumped constant kg.
`Thus, we obtain for r2,
`
`k,
`V2 = 2
`
`CH3O‘
`)
`(H10
`
`CE'CHZo
`_
`_
`cCH3C00H €cH3oH *“T
`K132
`
`(16)
`
`The catalytic contribution depends on the protolysis
`equilibria (3) and (4) and on the side reaction (2).
`A two-step mechanism has been proposed (Streit-
`weiser et al., 1992) for the side reaction (2),
`
`(VII) CH3OH + H3O+ .—_‘CH3OH2" + H20
`
`(VIII)
`
`CI-I3OH{ +1’ :—‘CH3I + H20.
`
`(17)
`
`(18)