Esterification of formic acid, acrylic acid and methacrylic acid with cyclohexene in batch and distillation column reactors: ion-exchange resins as catalysts

ELSEVIER

Reactive & Functional Polymers

REACTIVE & FUNCTIONAL POLYMERS

28 (1996) 263-278

Esterification of formic acid, acrylic acid and methacrylic acid with cyclohexene in batch and distillation column reactors: ion-exchange resins as catalysts Basudeb Saha, Man Mohan Sharma * University Depanment of Chemical Technology (Autonomous), Matunga, Bombay 400 019, India Received

2 June

1995; revised version

accepted

25 September

1995

Abstract Esterification

of formic

of cation-exchange catalyst

loading,

conditions. studied.

types of catalyst,

The reaction

Experiments

to compare

Effect of various

temperature,

of formic

column

of formic reactor

acid in aqueous solutions (DCR),

packed

acid, location

in batch mode

for the selective esterification

and in distillation

of formic

to optimise

in aqueous solutions, catalyst, p-toluene

of anhydrous

to cyclohexyl

with an acidic

size,

the reaction

with cyclohexene

sulphonic

esterification

formate,

ion-exchange

was

acid (p-TSA),

between

by reaction

acrylic acid

column

between

reactor

acid with cyclohexene

with cyclohexene,

resin catalyst was studied;

amount of catalyst in the reboiler,

of the feed point, etc., was studied. A comparison

cyclohexene

was studied

out in the presence catalyst particle

has been reported.

variables such as feed flow rate, molar ratio of reactants, with

concentrations

catalyst. A comparison

acid with cyclohexene

was carried

e.g. speed of agitation,

out in the presence of homogeneous

results with the heterogeneous

The conversion

acid with cyclohexene

parameters,

mole ratio of the reactants,

acid, with different

were carried

and that of methacrylic distillation

acid, acrylic acid and methacrylic

resins as catalysts.

concentration

the rate of esterification

has been made. An

from a mixture

of formic

attempt

in a

the effect

of

of formic

of formic

acid

was also made

acid and acetic acid in a

DCR. Keywords: Esterification;

Cation-exchange

ene; Cyclohexyl

Cyclohexyl

formate;

resin; Formic acid; Acrylic

acrylate; Cyclohexyl

1. Introduction The esterification of carboxylic acid with olefins in the presence of cation-exchange resins has considerable academic and industrial importance. Both homogeneous and heterogeneous catalysts have been widely used for this purpose. Some of the advantages of using ion-exchange * Corresponding author.

methacrylate;

acid; Methacrylic Cyclohexyl

acid; Acetic

acid; Cyclohex-

acetate; Reactive distillation

resins as catalysts in esterification reactions are ease of separation of the catalyst from the reaction mixture, milder reaction conditions, higher yield and selectivity towards the ester, higher catalytic activity, catalyst reusability etc. The direct esterification of cyclohexene with carboxylic acid is becoming encouraging due to the availability of cyclohexene on a large scale at attractive prices. The esters of carboxylic acids with cyclohexene are of commercial importance

1381-5148/96/$15.00 0 1996 Elsevier Science B.V. All rights reserved. SSDI 1381-5148(95)00092-5

264

B. Saha, M.M. Sharma I Reactive & Functional

because of its diverse uses. In the esterification reaction between formic acid and cyclohexene, the resulting ester, namely, cyclohexyl formate can be used as a dispersant for paints or inks. Acrylic and methacrylic esters are used exclusively for the production of polymers. The polymers are used mainly for coating materials, paints, adhesives, and binders for leather, paper and textiles. Moreover, these cyclohexyl esters can be easily hydrolysed to cyclohexanol, which is of great importance in chemical industry; the most important use of cyclohexanol is via dehydrogenation to cyclohexanone, which is a raw material for nylon-6. The cyclohexanolcyclohexanone mixture can be oxidised to adipic acid with nitric acid. Cyclohexanol is also used for the manufacture of cyclohexyl and dicyclohexyl phthalates which are used as plasticizers. The selective hydrogenation of benzene to cyclohexene has been reported [l]. It was thought that cyclohexanol can be advantageously manufactured from cyclohexene either via direct hydration or via esterification with formic acid or acrylic acid or methacrylic acid; these cyclohexyl esters can be catalytically hydrolysed to cyclohexanol and the corresponding acid can be recycled. Cyclohexyl acrylate/methacrylate can be transesterified with methanol, ethanol, IZbutanol, 2-ethylhexanol, etc., to give cyclohexanol and the corresponding acrylate/methacrylate esters which are in great demand. This route via esterification would be encouraging since the hydration route encounters difficulties in view of immiscibility of cyclohexene and water. This route was followed by Chakrabarti and Sharma [2] to produce cyclohexanol from cyclohexene via cyclohexyl acetate with cationexchange resins as catalysts. The anhydrous esterification of acetic acid with styrene in the presence of ion-exchange resins as catalysts was also investigated by Chakrabarti and Sharma

[31* In this work a detailed study has been presented on the synthesis of cyclohexyl esters, namely, cyclohexyl formate/acrylate/methacrylate using cation-exchange resins as catalysts. Es-

Polymers 28 (1996) 263-278

terification of formic acid with cyclohexene in a distillation column reactor has been performed. An attempt has also been made for the selective esterification of formic acid, from a mixture of formic acid and acetic acid, with cyclohexene, in a distillation column reactor. 2. Previous studies There is practically no literature on the esterification of formic acid, acrylic acid and methacrylic acid with cyclohexene using ionexchange resins as catalysts. However, there is some information on the esterification of cyclohexanol with formic acid with ion-exchange resins as catalysts. Haran et al. [4] have studied the esterification of cyclohexanol with formic acid using Amberlite IR-120 as catalyst. Shanxin [5] has reported that cation exchanger 001X7 can be used instead of HzS04 as esterification catalyst for the preparation of cyclohexyl formate, cyclohexyl acetate, and cyclohexyl butyrate. Kuusk and Faingol’d [6] have reported that esters can be prepared by the reaction of acrylic acid with an excess of cyclohexene, 1-hexene, 1-octene, 1-decene, 1-dodecene, 2-octene at 333-383 K using 95% HzSO~ as catalyst. lnoe et al. have claimed that cyclohexyl acrylate can be prepared with good yield in a short time by reacting acrylic acid with cyclohexene in the presence of hetero polyacids containing MO oxides and W oxides as main components [7,8]. A convenient use of polyphosphoric acid in the esterification reaction between (meth)acrylic acid and different (cycle)alkenes has been reported by Mebah et al. [9]. In view of the industrial importance of these esterification reactions, it was thought desirable to conduct a systematic investigation to optimise the reaction conditions with emphasis on the selectivity towards the desired product. The suitability of commercially available solid acid catalysts such as macroporous sulphonic acid resin (Indion 130 and Amberlyst 15), gelular or microreticular cation-exchange resin (Amberlite IR 120) and acid-treated montmorillonite clay (Engelhard F 24) was assessed.

B. Saha, M.M. Sharma I Reactive & Functional Polymers 28 (1996) 263-278

3. Experimental 3.1. Materials The raw materials used in the present work were of very high purity (>99%). Formic acid was obtained from S.d. fine Chem., India; acrylic acid and methacrylic acid were obtained from Pidilite Industries Ltd., India. Cyclohexene was prepared in high purity by dehydration of cyclohexanol using 85% orthophosphoric acid (S.d. fine Chem., India) as catalyst. The prepared cyclohexene was redistilled to obtain the desired (>99%) purity. Indion 130 was obtained from Ion Exchange India Ltd and Amberlyst15 and Amberlite IR-120 were obtained from Fluka, Switzerland. Clay catalyst, Engelhard F24, was obtained from Engelhard, U.S.A. Cationexchange resin and clay catalysts were washed with acetone and dried at 373 K under vacuum (l-2 mm Hg) for 6 h before use. The physical properties of the catalysts are listed in Table 1. 3.2. Apparatus and procedure In the batch mode of operation, all the esterification reactions were carried out in a 1 x 10m4 rr$ autoclave (Parr Instruments, U.S.A.) with an internal diameter of 0.05 m. The temperature was maintained at fl K of the desired value with the help of an in-built proportionalintegral-derivative (PID) controller. A fourbladed pitched turbine impeller was employed

265

for agitation. In a typical experiment, 50 g of cyclohexene and formiciacryliclmethacrylic acid (in the required molar ratio) and 2.5 g of catalyst were taken in the reactor. The temperature was raised to the desired value taking care to agitate only the liquid and not suspending the catalyst. The agitation was increased sufficiently to suspend the catalyst particles completely when the desired temperature was reached. This was noted as the starting time of the reaction. Samples were withdrawn at regular intervals and analysed. Some of the reactions between cyclohexene and formic acid/acetic acid were performed in a packed distillation column reactor (DCR), shown in Fig. 1. The column consisted of three distinct parts, (i) non-reactive stripping section, (ii) catalytic packed section, and (iii) nonreactive enriching section. The height and diameter of each section of the column were 400 mm and 25.4 mm, respectively. The reboiler was heated by a heating mantle and the column was heated by an electric blanket and was properly insulated. The temperature was maintained at fl K of the desired value using an external electronic on-off controller. The condenser was water cooled. The reactants were fed through PTFE tubes to the DCR controlling the flow with independent control valves and the flow rates of the feeds were monitored volumetrically. The catalytic section of the column was filled with a combination of Raschig rings having an average diameter of 5 mm and 70 g of

Table I Physical properties of different catalysts Physical property

Amberlyst-15 (macroreticular)

Indian 130 (macroreticular)

Engelhard F-24 (acid-treated clay)

Amberlite IR-120 (gelular)

Shape Size (mm; min. 90%) Internal surface area (m*/g) Weight capacity (meq/g) Crosslinking density (% DVB) Porosity (vol%) Temperature stability (K)

beads 0.5 55.0 4.15 20-25 36.0 293

beads 0.55 _a 4.8 _a _a 293

granular 0.66 350.0 0.3 _b 32.0 _a

beads 0.5 _a 4.4 8 no permanent porosity 293

a Data not available. b Not applicable.

B. Saha, M.M. Sharma /Reactive & Functional Polymers 28 (1996) 263-278

266

e

nriching section

Counter current mode: II = Formic acid I3 = Cyclohexene Cocurrant mode: I, = Formic acid I2 = Cyclohexene

WHP was used for the analysis. The injector and the detector temperatures were kept at 573 K and the oven temperature was programmed appropriately for different systems. Nitrogen was passed as the carrier gas and its flow rate was maintained at 0.5 ml/s. The reaction samples were washed with excess water to remove unreacted acids and the organic layer was dried over anhydrous sodium sulphate before injecting into the gas chromatograph. The material balance was checked by determining the saponification value of the ester. 4. Results and discussion 4.1. Elimination of mass-transfer limitations (batch mode) 4.1.1. Effect of speed of agitation

Fig. 1. Schematic

diagram

of distillation

column reactor.

Indion 130 cation-exchange resin having an average particle size of 0.5 mm. The feeds were introduced at room temperature. In a typical experiment, formic acid was fed to the catalyst bed from the top and cyclohexene was fed from the bottom to achieve maximum conversion. The column was operated at total reflux to prevent losses of cyclohexene which is the most volatile component in this system. The reaction mixture was continuously removed from the bottom and analysed. 3.3. Analysis Analysis of the samples was done in a gas chromatograph (PE 8500) with a flame ionisation detector. A stainless steel column 2 m long having an internal diameter of 2 mm and packed with 10% OV-17 supported on chromosorb-

Preliminary experiments showed that there was no effect of the speed of agitation in the range of 12 to 20 rps on the overall rate of reaction. Hence, all the esterification reactions were carried out at 18.33 rps, ensuring that there was no external mass transfer resistances. It was also confirmed that no attrition of the resin particle took place under the experimental conditions employed for this work. 4.1.2. Effect ofparticle size For a specified catalyst loading, there was no effect of the variation in the particle size from 300 pm to 600 pm on the rate of the reaction. This endorses that intraparticle diffusional resistances of the reactant in the ion-exchange resin are not important. In the presence of polar reactant, swelling occurs leading to the easy accessibility of the reactants to the active sites and free mobility of all the components. It has been shown by a number of investigators that for reactions such as hydration of isobutylene and etherification of isobutylene with methanol, the microgel effectiveness factor for macroporous resins is unity [lo-121. For clay catalyst also, the particle size (in the range employed) had no effect on the rate of reaction.

B. Saha, MM. Sharma I Reactive & Functional Polymers 28 (I 996) 263-278

of the hydroxyl group of the acids to give the corresponding ester [13]. On the other hand, in the presence of aqueous solution, the carbonium ion may be attacked by the lone pair of electrons of the oxygen atom of water molecule to form cyclohexanol. The electrophile can also add with another cyclohexyl cation to form cyclohexene dimer. The reaction network and the mechanism of the reaction are shown in Fig. 2a and b, respectively.

Esteriticatwn reacts

+ 0

II O-C-R

Catun exchrge rerm

R R-C-O-U

where.R = -H. CUZ= CH-. CW= $C”J

Simultaneous hydratum and estenticatlon reactwn

c) +

Hi

OH

1

-

0

267

FI

O-C-R

Cycbkxyl ester

n

(b) Fig. 2. (a) Neovork of reaction. (b) Mechanism of reaction.

4.2. Mechanism of reaction In the presence of cation-exchange resin, which acts as a proton donor, it is assumed that all these reported esterification reactions involve addition of proton to the cyclohexene unit, which forms carbonium ion like intermediate or transition state. The latter is simultaneously attacked by the lone pair of electrons of the oxygen atom

4.3. Esterification of formic acid with cyclohexene (batch mode) 4.3.1. Effect of catalyst loading The effect of catalyst loading was studied with 1: 1 mole ratio of cyclohexene to 90% formic acid at 353 K using Indion 130 catalyst. The rate of reaction increased with an increase in the catalyst loading in the range 5 to 15% (w/w). With an increase in the catalyst loading, more sulphonic acid groups are available, leading to the formation of higher numbers of carbonium ions per unit time, which in turn increases the rate of reaction. A marked increase in the conversion of cyclohexene was observed with the increase in catalyst loading from 5% to 10%. However, with an increase in the catalyst loading from 10% to 15%, no enhancement in the rate of reaction was observed. This can be attributed to the fact that beyond a certain catalyst loading, there exists an excess of catalyst sites than actually required by the reactant molecules and hence there is levelling off of the reaction rate [14]. The effect of catalyst loading on the conversion of cyclohexene is shown in Fig. 3. 4.3.2. Effect of catalyst Although formic acid is the strongest (p& = 3.74) of the simple unsubstituted carboxylic acids, its strength in the reaction medium is not sufficient enough to catalyse the reaction in the absence of catalyst. Hence, five different catalysts were used to study the effect of catalyst in the reaction of 90% formic acid and cyclohexene (1: 1 mole ratio) at 358 K. Acid-treated clay,

B. Saha, M.M. Shama I Reactive & Functional Polymers 28 (1996) 263-278

268

Time

0

(hour)

2

6

Time ‘(hour)

Tern erature: 358 K cata Yyst: Indim 130 Mole ratio (cycloherene : formic acid) 1:l Concentration of formic acid : 90%

Fig. 3. Effect of catalyst acid with cyclohexene.

loading

on the esterification

of formic

Engelhard F-24, gelular cation-exchange resin, Amberlite IR-120, and homogeneous catalyst pTSA showed very poor catalytic activity for the reaction as compared to Amberlyst-15 and Indion 130 (Fig. 4). The conversion of cyclohexene in the presence of Amberlite IR-120 was -13% in 6 h, while that in the presence of Engelhard F-24 was -10%. Indion-130 and Amberlyst-15 (both macroporous type of ion-exchange resin) have higher capacity than Engelhard F-24 and hence the lower catalytic activity of the latter was observed. For gelular Amberlite IR-120 resin, “swelling” is necessary for the catalytic activity of the catalyst to be appreciable [15]. It is possible that the reactants do not “swell” the catalyst appreciably and thus showed lower catalytic activity. Moreover, the lesser exchange capacity (meqig) of Amberlite IR 120 compared to macroporous resins, Indion 130 and Amberlyst 15, may also reduce the conversion of cyclohexene in the esterification reaction. An activation of the catalyst occurred with increasing reaction time for gelular and clay catalysts. This may be due to the progress in the swelling of the catalyst.

Fig. 4. Effect of catalyst with cyclohexene.

on the esterification

of formic

acid

4.3.3. Effect of temperature The temperature was varied from 343-373 K to study its effect on the rate of esterification of 90% formic acid with cyclohexene at 1: 1 mole ratio using Indion 130 as catalyst. The results are shown in Fig. 5. The conversion of cyclohexene to formate as well as the selectivity towards the ester increased with an increase in temperature from 343 to 358 K, but the selectivity towards cyclohexyl formate decreased with an increase in temperature from 358 to 373 K. This is because of the formation of small amount of cyclohexene dimer along with cyclohexanol at 373 K. Since at 358 K, the highest selectivity towards cyclohexyl formate was obtained, subsequent experiments were conducted at 358 K. 4.3.4. Effect of mole ratio of cyclohexene to formic acid The mole ratio of cyclohexene to 90% formic acid was varied from 1: 1 to 1: 5 to study the selectivity of cyclohexyl formate with Indion 130 as catalyst (loading 10% w/w). Fig. 6 shows that

B. Saha, M.M. Sharma I Reactive & Functional Polymers 28 (1996) 263-278

269

&~&ATemp. 358 K GQJ!JDTemp. 363 K W Temp. 343 K

0

1

2

TiZe

(tour) 5

6

7

Time (hour)

Tern erature: 350 K Cata Pyst: Indion 130 Catalyst loading 1On (w/w) Concentration of formic acid : 90s

CetalyBt: Indian 130 catalyst loadin : ma (w/n) Mole ratio (cyc‘fohexene : formic acid) 1:l Concentration of formic acid : 90s Fig. 5. Effect acid with

of temperature

on the esterification

of formic

cyclohexene.

with an increase in the mole ratio of formic acid to cyclohexene from 1: 1 to 1: 5, the conversion of cyclohexene after 7 h of reaction increased from 49.7% to 62.4%, but the selectivity towards cyclohexyl formate decreased from 95.4% to 90.1%. An increase in the molar ratio results in higher concentration of formic acid in the resin phase which appears to have increased the rate of the reaction. With an increase in the mole ratio of formic acid to cyclohexene, the amount of water in the reaction mixture increases which seems to have decreased the selectivity towards cycloheql formate because of the formation of cyclohexanol.

Fig. 6. Effect

of mole ratio

on the esterification

of formic

acid

with cyclohexene.

version of cyclohexene as well as the selectivity towards the cyclohexyl formate increased. With a decrease in the concentration of formic acid, the concentration of water increases which appears to have increased the formation of cyclohexanol, which in turn reduces the selectivity towards cyclohexyl formate. Moreover, in the presence of water molecules, four sulphonic acid groups of the resin are attached to one water molecule, which may also decrease the catalytic activity of the resin [16]. The results are given in Table 2.

Table 2

4.3.5. Effect of concentration of formic acid The concentration of formic acid was varied to assess the effect of acid concentration on the equilibrium conversion as well as the selectivity towards ester. All the runs were studied at 358 K using Indion 130 catalyst at 10% (w/w) catalyst loading with 1: 1 mole ratio of the reactants. The result showed that with an increase in the concentration of formic acid, the con-

Effect

of concentration

of formic

acid on the

conversion

of

cyclohexene Concentration formic

of

acid (%)

Conversion cyclohexene

of (%)

Selectivity

towards

cyclohexyl

formate

85

34.98

93.84

90

49.76

95.43

98

62.32

Reaction conditions: 10% (w/w); formic

95.43

Catalyst:

temperature:

acid: 1 : 1; period

(%)

358

lndion

K; mole

of reaction:

130; ratio 7 h.

catalyst

loading:

of cyclohexene

to

270

B. Saha. MM.

Sharma I Reactive I_? Functional

Poiymets

28 (1996)

263-278

4.4. Esterification of acrylic acid with cyclohexene (batch mode) 4.4.1. Effect qf catalyst loading

The effect of catalyst loading on the esterification of acrylic acid with cyclohexene was studied with 1: 1 mole ratio of the reactants at 358 K with Indion 130 as catalyst using 100 ppm of hydroquinone monomethyl ether as polymerisation inhibitor. At different catalyst loadings, i.e., at 5% and 10% (w/w), no significant change in the conversion of cyclohexene was realised. The conversion of cyclohexene were 51.2% and 52.3% at 5% and 10% catalyst loading, respectively. This could be because of the fact that above a certain catalyst loading, there exists an excess of catalyst sites than actually required by the reactant molecules and hence there is no further enhancement in the rate of reaction

P41. 4.4.2. Effect qf mole ratio The anhydrous esterification of acrylic acid with cyclohexene was studied at different mole ratios of the reactants. At 358 K, 1: 1 mole ratio of cyclohexene to acrylic acid, with Indion 130 cation-exchange resin (catalyst loading 5%) with 100 ppm of hydroquinone monomethyl ether as polymerisation inhibitor, the conversion of cyclohexene was found to be 52% with 100% selectivity towards cyclohexyl acrylate after 5 h of reaction. With 1: 2 mole ratio of the reactants, under otherwise identical conditions, the conversion of cyclohexene increased to 72.1% without any change in the selectivity towards ester. The same reaction was carried out with 1: 4 mole ratio of the reactants under otherwise similar conditions. In this case, the conversion of cyclohexene increased to 92.7% without affecting the selectivity of cyclohexyl acrylate. The results are shown in Fig. 7. 4.4.3. Effect of temperature The effect of temperature was studied from the temperature range of 343 to 373 K with Indion 130 cation-exchange resin (loading 5%) at

;

90

2g

tlo

‘;O 70 2

eo

%

50 fi .2 40 : $ 30 cl s 20 R

Mole ratio of AASHE 4:l 10 0

0

1

2 Time3

@I-)~

5

Tern erature: 358 K Catayst: P Imlion 130 Catalyst loading: 5s (w/w)

Fig. 7. Effect of mole ratio on the esterification with cyclohexene.

of acrylic acid

1: 4 mole ratio of cyclohexene to acrylic acid using 100 ppm of hydroquinone monomethyl ether as polymerisation inhibitor. The result showed that the conversion of cyclohexene increased with an increase in temperature. It was found that the increase in the conversion of cyclohexene was very low when the temperature was varied from 358 to 373 K, whereas a marked increase in the conversion of cyclohexene was observed when the temperature was increased from 343 to 358 K. The maximum observed conversion of cyclohexene at 373 K, was 97.7% and in all cases the selectivity towards the ester was 100%. The results are plotted in Fig. 8. 4.4.4. Effect of catalyst Macroporous cation-exchange resins Indion 130 and Amberlyst 15, gelular cation-exchange resin Amberlite IR-120 and homogeneous catalyst p-TSA were used to verify the effect of different types of catalyst for the esterification of acrylic acid with cyclohexene at 373 K with 4: 1 mole ratio of acrylic acid to cyclohexene using

B. Saha, M.M. Sharma I Reactive & Functional Polymers 28 (1996) 263-278

MMD Temperature 4LU&Temperature %#.# Temperature

343 356 373

00000 ~AAAA &Q&0 W

K K K

0

Mole ratio (Cyclohexemxacrylic Catalyst: Indian 130 Catalyst loading: 5% (n/w)

Fig. 8. Effect of temperature acid with cyclohexene.

acid)

on the esterification

1:4

of acrylic

100 ppm of hydroquinone monomethyl ether as polymerisation inhibitor at otherwise same acid equiv/l of catalyst. The observed conversion of cyclohexene with Indion 130, Amberlyst 15, Amberlite IR 120 and p-TSA was 95.8%, 92.9%, 52.7% and 39.5%, respectively, after 5 h of reaction, as shown in Fig. 9. This may be due to the higher capacity of Indion 130 and Amberlyst 15 compared to other catalysts. Such low value of the conversion with homogeneous catalyst, pTSA, can be explained by the hypothesis of Ancillotti et al. [18]. The hydrogen ion concentration inside the resin is of the order of -3 equiv/l. In homogeneous catalysis at such high levels, the reaction rates are not proportional to the acid molar concentration but follow Hammetttype acidity function (HO). The HO value of Amberlyst 15 is -2.2, whereas for p-TSA it is +0.55. Since the acidity functions increase with an exponent order greater than one with respect to the hydrogen ion concentration and in a homogeneous system although the acid equivalents are the same, the hydrogen ion concentration is

1

211

Indion 130 Amberlyst 15 Amberlite IR 120 p-TSA

2

5

Mole ratio (cyclohexene:acrylic Tern erature: 373 K Cata Pyst loading: 5% (n/n)

Fig. 9. Effect of catalyst with cyclohexene.

on the esterification

6

acid)

1:4

of acrylic

acid

very low as it is distributed in the entire volume of the reactant, the acidity function is very low. Hence, the reaction rate is very low with homogeneous catalyst. The conversion of cyclohexene was substantially reduced in the presence of Amberlite IR-120 compared to macroporous resins. This may be because of the less susceptibility of the reactants to swell the resin, which in turn reduces the catalytic activity of the gelular resin. 4.4.5. Effect of moisture At low concentration of water in the resin phase, four sulphonic acid groups attached to one water molecule [16]. Thus, the availability of the active sites decreases resulting in a sharp decrease in the rate of reaction. Again, water molecule permeates freely inside the pore of ion exchangers resulting in adsorption of water in the resin matrix [17]. In this case, with the addition of 1% (w/w) water, the rate of reaction was affected drastically and the equilibrium was not attained even after 7 h of reaction.

B. Saha, M.M. Shama I Reactive & Functional Polymers 28 (1996) 263-278

212

0

0

2

Tim:

6

(hr)

Time (hr)

Mole ratio (cyclohexene:methacrylic Catalyst: Indion 130 Temperature: 373 K Fig.

10. Effect

methacrylic

of

catalyst

acid with

loading

on

6

the

acid)

esterification

1:4

of

cyclohexene.

4.5. Esterification of methacrylic acid with cyclohexene (batch mode) 4.5.1. Effect of catalyst loading The effect of catalyst loading on the esterification of methacrylic acid with cyclohexene was studied with Indion 130 catalyst at 373 K with 4 : 1 mole ratio of the reactants using 100 ppm of hydroquinone monomethyl ether as polymerisation inhibitor. Fig. 10 shows that considerable increase in the conversion of cyclohexene was observed when the catalyst loading was increased from 2.5% to 5%, but no significant enhancement in the conversion of cyclohexene was observed for an increase in the catalyst loading from 5% to 10%. The reason for this effect has been discussed in Section 4.4.1. 4.5.2. Effect of catalyst The effect of different types of catalyst, e.g. Indion 130, Amberlyst 15, Engelhard F-24 and p-TSA, was studied at 373 K, at methacrylic acid to cyclohexene mole ratio of 4 : 1 using 100 ppm of hydroquinone monomethyl ether as polymeri-

Mole ratio (cyclohexene:methacrylic Catalyst loadm :3% (w/w) Temperature: A Fig. 11. Effect acid with

of catalyst

on the esterification

I

acid) 1:4

of methacrylic

cyclohexene.

sation inhibitor. The conversion of cyclohexene for Indion 130, Amberlyst 15, Engelhard F-24 and p-TSA was 90.6%, 85.6%, 51.2% and 33.8%, respectively, after 6 h of reaction with 100% selectivity towards the ester. The reason may be the same as discussed in Section 4.4.4. The results are given in Fig. 11. 4.5.3. Effect of mole ratio of methacrylic acid to cyclohexene Esterification of methacrylic acid with cyclohexene was carried out with Indion 130 catalyst (loading 5%) at 373 K, at various mole ratios of the reactants using hydroquinone monomethyl ether as the polymerisation inhibitor. With an increase in the mole ratio of methacrylic acid to cyclohexene from 2 : 1 to 4 : 1, the conversion of cyclohexene increased from 51.8% to 90.6% after 6 h of reaction without any change in the selectivity towards cyclohexyl methacrylate. 4.54. Effect of temperature Reactions were conducted between methacrylic acid and cyclohexene at 4: 1 mole ratio

B. Saha, M.M. Sharma I Reactive & Functional Polymers 28 (1996) 263-278

0

2

8

6

Tim:

(hr)

Mole ratio (cyclohexene:methacrplic Catalyst: Indion-130 Cat&at loading: 5s (n/w)

Fig. 12. Effect of temperature methacrylic acid with cyclohexene.

273

acid)

Mole ratio (cyclohexene:acrylic/methacrylic Tern erature: 358 K Cata Pyst: Indion-130 Catalyst loading: 5s (r/w)

1:4

on the esterification

of

using Indion 130 catalyst (loading 5%) at a temperature range of 343-373 K with hydroquinone monomethyl ether as polymerisation inhibitor. The conversion of cyclohexene increased with an increase in temperature. After 6 h of reaction, the maximum observed conversion of cyclohexene at 373 K was 90.6%, whereas at 343 K, the conversion of cyclohexene was reduced to 34.5% with 100% selectivity towards the ester in both the cases. The results are shown in Fig. 12.

Fig. 13. Comparison of the rate of esterification acrylic acid and methacrylic acid with cyclohexene.

acid)

1:4

between

lower conversion of cyclohexene with methacrylic acid may be due to the presence of bulkier - CH3 group in the a-carbon atom of methacrylic acid, which by virtue of its +I effect may decrease the acidity as well as the reactivity of methacrylic acid compared to acrylic acid. The higher acidity of acrylic acid compared to methacrylic acid is also confirmed from their p& values which are 4.26 and 4.66, respectively. However, the selectivity towards the ester in both the cases was 100%. The results are shown in Fig. 13.

4.6. Comparison of anhydrous esterification between acrylic acid and that of methaclylic acid with qclohexene

4.7. Esterification of formic acid with cyclohexene in distillation column reactor (DCR)

Anhydrous esterification between acrylic acid /methacrylic acid with cyclohexene was studied separately at 358 K using Indion 130 catalyst (catalyst loading 5%) with 4: 1 mole ratio of the reactants using 100 ppm of hydroquinone monomethyl ether as polymerisation inhibitor. The conversion of cyclohexene with acrylic acid and methacrylic acid were -92% and -58%, respectively, after 5 h of reaction. The reason for the

Reactive distillation has received considerable attention in recent years as an advance over the conventional processes where the conversion is limited by unfavourable chemical equilibrium. The main advantages of this process relative to the conventional alternatives are the possibility of carrying out equilibrium limited reactions towards completion and the simultaneous removal of the product from the reaction mixture in a

214

B. Saha, M.M. Sharma I Reactive & Functional

single unit, which in turn reduces reactor and recycle cost [19]. Distillation column reactors have been considered since 1921 to carry out esterification reactions [20]. In recent years, reactive distillation has been considered for the separation of close boiling mixtures [21,22], and for the production of methyl-tert-butyl ether (MTBE), an important anti-knock agent to replace tetraethyl lead in gasoline [23], and for the manufacture of high-purity methyl acetate [24-261, and cumene [27]. Reactive distillation columns have also been found in processes, e.g. in the production of condensation polymers [28], and in the removal of acid gases from light hydrocarbon mixtures [29]. An exhaustive survey of the literature on reactive distillation is given by Doherty and Buzad [30]. The recovery of dilute acetic acid by esterification in a packed chemorectification column with an acidic organic polymerisation catalyst was studied by Neumann and Sasson [31]. Very recently, Sharma [32] has mentioned some interesting and industrially potentially useful reactions, e.g. 80-85% formic acid with cyclohexene, where cyclohexene can be converted to cyclohexanol through an ester and subsequent hydrolysis in a distillation column reactor. This idea has promoted us to study the esterification reaction of formic acid with cyclohexene in detail in a DCR. As the esterification reactions of carboxylic acid with alkene, such as the reaction of formic acid-cyclohexene, are reversible, it is difficult to achieve the quantitative conversions without shifting the reaction in the forward direction by removing the product from the reaction mixture. In this process, the ester, namely, cyclohexyl formate has got the highest boiling point among all the components and thus it was removed from the bottom. Since cyclohexene is the lowest boiling component among all these, the column was operated at total reflux to increase its conversion. Five parameters were evaluated to study the conversion of cyclohexene and the selectivity towards cyclohexyl formate. They were feed flow rate of the reactants, molar ratio of the reactants feed, effect of catalyst in

Polymers 28 (1996) 263-278

Table 3 Effect of input flow rate on the conversion DCR 85% formic acid, feed

Cyclohexene, feed (molis)

Conversion cyclohexene

4.12 2.74 1.64 1.64

58.59 60.43 47.61 -a

of cyclohexene

of (%)

in

Selectivity of formate (%)

(molis) 5.15 4.12 2.06 2.06

x x x x

1O-5 1O-5 10-s 1O-4

x x x x

lO-5 1O-5 1O-5 1O-4

85.11 83.92 81.03 _a

Reaction conditions: Concentration of formic acid: 85%; feed temperature -298 K, temperature of the reboiler -363 K; temperature of the column -358 K; period of reaction: 3 h. BAt this feed Bow rate the column was flooded.

the reboiler, location of the feed point and the concentration of formic acid in the feed. 4.7.1. Efect of feed flow rate The effect of the input flow rate is shown in Table 3. Generally, in typical continuous reactors with monotonic rate expression, the conversion increases with decreasing flow rates of the reactants. However, in distillation column reactor, there are limitations in the operating feed flow rates, since the reactor also acts as a distillation column. At very high flow rates, there is possibility of flooding of the column and hence no effective separation will occur. On the other hand, at very low flow rates, most of the reactants are present in the vapour phase, causing the reaction to be extremely slow. Small amount of cyclohexanol was also formed which in turn reduces the selectivity towards cyclohexyl formate. 4.7.2. Effect of molar ratio of the reactants feed The effect of molar feed ratio on the conversion of cyclohexene is summarised in Table 4. As the mole ratio of formic acid to cyclohexene increased, the conversion of cyclohexene was enhanced resulting in lower selectivity towards cyclohexyl formate with lower purity of the ester. With an increase in mole ratio of the reactants, the amount of formic acid increases, which in turn increases the equilibrium conversion of cyclohexene. But the purity of cyclohexyl formate decreased since unreacted formic acid being a

B. Saha, MM. Sharma I Reactive & Functional Polymers 28 (1996) 263-278

275

Table 4 Effect of molar feed ratio on the conversion of cyclohexene in DCR Molar feed ratio (formic acid to cyclohexene)

98% formic acid, feed (mol/s)

Cyclohexene, feed (mobs)

Conversion of cyclohexene (%)

Selectivity towards cyclohexyl formate (%)

1.1: 1 1.5: 1 4:l

4.53 X 10-S 4.12 x 1O-5 10.98 x 1O-5

4.12 x 1O-5 2.74 x 1O-5 2.74 x lo@

82.3 88.9 95.7

96.6 96.5 95.2

Reaction conditions:

temperature

Concentration of formic acid: 98%; feed temperature of the column -358 K; period of reaction: 3 h.

high boiling component the distillation unit.

resides at the bottom of

4.7.3. Effect of the amount of catalyst in the reboiler In DCR, the product (cyclohexyl formate) of the reaction mixture was removed from the reboiler. Small amount of unconverted cyclohexene was also present in the product stream when the catalyst was not present in the reboiler. Hence, in order to check whether the unconverted cyclohexene reacts further with formic acid to give cyclohexyl formate, few runs were carried out by adding catalyst to the reboiler. It should be noted here that when the effect of catalyst in the reboiler was studied, the catalytic packed section was also provided in the DCR. The effect of the amount of catalyst in the reboiler-reactor is shown in Table 5. The result showed that the effect of catalyst has little impact on the conversion of cyclohexene in DCR. With an increase in the catalyst loading in the reboiler, the conversion of formic acid Table 5 Effect of the amount of catalyst in the reboiler on the conversion of cyclohexene in DCR Catalyst in the reboiler (g)

Conversion of cyclohexene (%)

Selectivity of formate (%)

0 5 10

88.9 91.3 92.4

96.5 96.6 96.7

-298

K temperature

of the reboiler -363

K;

increases slightly because of higher equilibrium conversion of cyclohexene. 4.7.4. Eflect of the location of feed point The effect of the location of the feed point was studied by introducing the feed in the reactor in cocurrent and counter-current fashion. The result revealed that the conversion of cyclohexene as well as the purity of the ester were enhanced when the feed inlet was changed from cocurrent to counter-current manner. For the optimum operation of the column, there should be a maximum area of contact between the reactants so that the whole catalytic section of the column can be used as a reactor. Since the formic acid has got higher boiling point than cyclohexene, it should enter into the column from the top to achieve maximum conversion. The reverse should be valid for cyclohexanol. When the feed entered into the column in cocurrent manner, the product purity was affected because of the presence of unconverted formic acid at the bottom of the distillation unit. The results are given in Table 6. Table 6 Effect of the location of feed point on the conversion cyclohexene in DCR

of

Mode of operation

Conversion of cyclohexene (%)

Selectivity towards formate (%)

Cocurrent Counter-current

49.8 60.4

89.8 91.5

Reaction conditions: Mole ratio of formic acid: cyclohexene

Reaction conditions: Mole ratio of formic acid: cyclohexene

1.5: 1; concentration of formic acid: 98%; flow rate of formic acid: 4.12 x 10m5 mol/s; flow rate of cyclohexene: 2.74 x 10-j mol/s; feed temperature -298 K; temperature of the reboiler -363 K; temperature of the column -358 K; period of reaction: 3 h.

1.5 : 1; concentration of formic acid: 85%; flow rate of formic acid: 4.12 x lo@ mobs; flow rate of cyclohexene: 2.74 x 10V5 mobs; feed temperature -298 K, temperature of the reboiler -363 K; temperature of the column -358 K, period of reaction: 3 h.

216 Table 7 Effect of concentration

B. Saha, M.M. Shanna I Reactive & Functional Polymers 28 (1996) 263-278

of formic

acid on the conversion

of cyclohexene

in DCR

Concentration of formic acid (%)

Formic acid, feed (mol/s)

Cyclohexene, feed (molis)

Conversion cyclohexene

85 98

4.12 x 1O-5 4.12 x lo-’

2.74 x 1O-5 2.74 x lo-’

60.4 88.9

Reaction conditions: Mole ratio of formic acid: cyclohexene 1.5 : 1; feed temperature temperature of the column -358 K; period of reaction: 3 h.

4.7.5. Effect of concentration of formic acid

The effect of the dilution of formic acid was significant on the conversion of cyclohexene in distillation column reactor. The results are reported in Table 7. It has been found that the conversion of cyclohexene as well as the selectivity towards cyclohexyl formate increased markedly with an increase in the formic acid concentration from 85% to 98% under otherwise identical conditions. 4.8. Comparison of the rate of esterification of formic acid with cyclohexene in batch mode and in distillation column reactor

The continuous esterification between formic acid and cyclohexene in distillation column reactor showed much higher conversion of cyclohexene compared to the same in the batch mode of operation. In the batch mode, at 1: 1.5 mole ratio of cyclohexene to 85% formic acid, the conversion of cyclohexene was -36%, while in DCR, the conversion of cyclohexene was -60% under otherwise identical conditions. Similarly, at 1: 1.5 mole ratio of cyclohexene to 98% formic acid in batch mode, the conversion of cyclohexene was -62%, whereas the same in distillation column reactor showed -89% conversion of cyclohexene. 4.9. Selective esterification of formic acid with cyclohexene from the mixture of formic acid and acetic acid The selective esterification of formic acid with cyclohexene was studied in distillation column reactor taking a mixture of formic acid and acetic

of (%)

Selectivity cyclohexyl

towards formate

(%)

91.5 96.5 -298

K, temperature

of the reboiler

-363

K;

acid as the feed. A mixture of 98% formic acid and glacial acetic acid (1: 1 mole ratio) was taken as a feed with cyclohexene at molar feed ratio of 1: 1.1. The result showed that 85% conversion of cyclohexene was achieved with the selectivity of cyclohexyl formate and cyclohexyl acetate of -64% and -3O%, respectively. This may be due to the higher reactivity of formic acid compared to acetic acid in the esterification reaction. The reactivity of any acid in the esterification reaction depends on its acidity. The smaller inductive effect of hydrogen atom in comparison to an alkyl group (-CHs) leads to formic acid (p& = 3.74) being a considerably stronger acid than acetic acid (p& = 4.77) and in fact the strongest of the simple unsubstituted carboxylic acids [33]. So, from a mixture of formic acid and acetic acid, the former can be selectively esterified to obtain cyclohexyl formate and can be separated from the reaction mixture. Small amount of cyclohexanol was also formed due to the presence of water in 98% formic acid. The results are reported in Table 8. Another reaction was carried out with 2 : 1 mole ratio of formic acid to acetic acid as feed with cyclohexene under otherwise identical conditions. Table 8 shows that the conversion of cyclohexene increased to 87.4% and the selectivity towards cyclohexyl formate also increased to -78% from -64%. Because of the difference in the boiling points of cyclohexyl formate (-438 K) and cyclohexyl acetate (-446 K), the product mixture can be separated and subsequently hydrolysed to give cyclohexanol and the corresponding acids. So, if the feed contains higher mole ratio of formic acid to acetic acid, effective separation of the acid mixture can be achieved by carrying

B. Saha, M.M. Sharma I Reactive & Functional Polymers 28 (1996) 263-278 Table 8 Selective

esterification

of formic

acid with cyclohexene

from the mixture

Mixture of 98% formic acid and glacial acetic acid, feed (molis)

Mole ratio of formic acid to acetic acid

Cyclohexene, feed (mol/s)

Conversion cyclohexene

6.04 x 1O-5 6.04 x 1O-5

I:1 2:l

5.49 x 10-S 5.49 x 10-Z

85.0 87.4

Reaction conditions: Feed temperature of reaction:

-298

K; temperature

of acetic acid and formic of (%)

Selectivity cyclohexyl

towards formate

64.4 78.2

of the reboiler

-363

K; temperature

277

acid in DCR

(%)

Selectivity cyclohexyl

towards acetate (%)

30.0 14.6 of the column

-358

K; period

3 h.

out the reaction with cyclohexene in a DCR because of the higher selectivity towards cyclohexyl formate compared to cycloheql acetate. 5. Conclusions The esterification of formic acid/acrylic acid/ methacrylic acid with cyclohexene can be conveniently carried out with macroporous cationexchange resins, Indion 130 and Amberlyst 15, as catalysts. The optimum conditions for selectively obtaining the esters have been delineated. The rate of esterification between acrylic acid and cyclohexene was significantly higher than that with methacrylic acid. The conversion of formic acid to cyclohexyl formate by esterification with cyclohexene in a DCR packed with macroporous cation-exchange resin catalyst was found to give higher conversion of cyclohexene than that in the batch mode. Selective esterification of formic acid with cyclohexene in a DCR can be adopted for the separation of the same from a mixture of formic acid and acetic acid. Acknowledgement The financial support given to B.S. during this work in the form of a Senior Research Fellowship by the University Grants Commission, New Delhi is gratefully acknowledged. References [l] Y. Fukuoka and H. Nagahara, vision of Petroleum Chemistry,

Presented before the DiInc., American Chemical

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