ORIGINAL PAPER

Esterification of caprylic acid with alcohol over nano-crystallinesulfated zirconia

K. Saravanan • Beena Tyagi • H. C. Bajaj

Received: 8 November 2011 / Accepted: 27 December 2011 / Published online: 6 January 2012

� Springer Science+Business Media, LLC 2012

Abstract The catalytic activity of nano-crystalline sul-

fated zirconia catalyst, prepared by sol–gel method and

characterized by various analytical tools, was evaluated for

the esterification of caprylic acid with different short chain

alcohols. The lower concentration of catalyst (0.5 wt%)

exhibited 96–98% conversion of caprylic acid with meth-

anol and 100% selectivity for methyl caprylate at 60 �C.

The conversion was decreased with increasing carbon

chain of alcohols namely with ethanol, n-propanol and

n-butanol at 60 �C but increased significantly (91–98%)

at higher reaction temperature. The selectivity for respec-

tive alkyl caprylate was observed to be 100% irrespective

of the alcohol used. The activity of the catalyst was slightly

decreased with successive five reaction cycles due to the

water formed during the reaction.

Keywords Caprylic acid � Esterification � Fatty acid

methyl ester � Sulfated zirconia � Solid acid catalyst

1 Introduction

Alkyl esters have important industrial applications in a

variety of products, e.g., emulsifying agents for foods,

cosmetics; lubricants for plastics, paint and ink; perfumery

and flavor chemicals; pharmaceuticals, textile and in agri-

cultural industry, etc. [1]. Conventionally, alkyl esters have

been prepared by the esterification of corresponding acid

with an alcohol in presence of homogenous acid catalyst.

Caprylic acid (n-octanoic acid, C8H16O2) is a saturated

fatty acid and one of the constituents of coconut and palm

kernel oil [2], which are used to produce biodiesel by

transesterification in presence of base catalyst. Alkyl ca-

prylates are also used in perfumery and in the manufacture

of dyes [3].

Industrial esterification processes are carried out in

presence of conventional Bronsted acid catalysts such as

sulfuric, p-toluene sulfonic or phosphoric acid [4]. There is

a strong global demand to replace these homogenous cat-

alysts with eco-friendly solid catalysts, which are active

under mild conditions, can be easily recovered and reused.

Kiss et al. [5] have screened various solid acids including

zeolites, ion exchange resin and metal oxides for the

esterification of lauric acid (C12H24O2) with different

alcohols at 130–180 �C and found sulfated zirconia (SZ) as

the most active solid acid catalyst. SZ is well known for its

strong acidity, which is attributed to sulfate ions [6]. Few

studies have been reported for the esterification of caprylic

acid with various short chain alcohols using metal oxides

such as SZ, TiZ, WZ, AlZ [7–10], niobia [11], nafion/silica

composite [12] and sulfonated carbon [13] catalysts.

Besides, enzymatic catalyzed esterification of the same has

also been studied [14]. However, most of these studies

have been done at high temperature (75–200 �C) [7–10] or

in presence of higher concentration of catalyst (1–15 wt%)

[5, 10, 11] or longer reaction time (24 h) [12] is required to

achieve maximum conversion of fatty acid. Furuta et al. [7]

have found high activity of SZ for esterification of caprylic

acid under continuous flow reactor at higher temperature

([150 �C), however remarkably low activity was observed

at \120 �C.

The present study reports the esterification of caprylic

acid with methanol at 60 �C to methyl caprylate in

K. Saravanan � B. Tyagi (&) � H. C. Bajaj

Discipline of Inorganic Materials and Catalysis, Central Salt

and Marine Chemicals Research Institute, Council of Scientific

and Industrial Research (CSIR), G B Marg,

Bhavnagar 364 002, Gujarat, India

e-mail: btyagi@csmcri.org

123

J Sol-Gel Sci Technol (2012) 62:13–17

DOI 10.1007/s10971-011-2671-9

presence of small concentration of SZ solid acid catalyst.

The study also reports the influence of reaction temperature

on the conversion of acid to respective alkyl caprylate with

other short chain alcohols such as ethanol, n-propanol and

n-butanol.

2 Experimental

2.1 Materials

Zirconium n-propoxide, Zr(OPr)4 was procured from

Sigma-Aldrich, Germany; concentrated sulfuric acid

(98%), n-propanol (99%) and n-butanol (98%) were pro-

cured from s.d. Fine chemicals, India; aqueous ammonia

solution (25%) from Rankem, India; Caprylic acid (99.5%)

and ethanol (99.9%) were purchased from Spectrochem

Pvt. Ltd., India.

2.2 Catalyst synthesis

Nano-crystalline sulfated zirconia catalyst was prepared by

sol–gel method [15]. In a typical synthesis procedure,

Zr(OPr)4 was hydrolyzed by aqueous ammonia followed

by the treatment with H2SO4 (1 N). The sulfated powder

was oven dried at 120 �C for 12 h followed by calcination

at 600 �C for 4 h.

2.3 Catalyst characterization

The crystalline characterization of sulfated zirconia cata-

lyst was done by powder X-ray diffractometer (Philips

X’pert) using CuKa radiation (k = 1.5405 A). The crys-

tallite size of tetragonal phase of zirconia was determined

from the characteristic peak (2h = 30.22 for the (111)

reflection) using Scherrer formula with a shape factor

(K) of 0.9 [16]. The bulk sulfur (wt%) present in catalyst

before and after calcination at 600 �C was analyzed by

inductively coupled plasma (ICP) emission spectroscopy

using a Perkin-Elmer, Optima 2000 DV spectrometer.

Specific surface area and pore size of the sample were

determined from nitrogen adsorption–desorption isotherms

at -196 �C (ASAP 2010, Micromeritics, USA) using BET

equation and BJH adsorption respectively [17].

The total surface acidity of the catalyst was measured by

temperature programmed desorption (TPD) of NH3 (Mi-

cromeritics Pulse Chemisorb 2720). In a typical procedure,

a mixture of 10% NH3 and He gas was passed for 30 min at

40 �C over the sample (in situ activated at 120 �C for 2 h).

The excess physisorbed NH3 was flushed out for 10 min

with pure He gas flow. The sample was then heated at a

rate of 10 �C min-1 up to 800 �C and volume of desorbed

NH3 was measured.

Vapor phase cyclohexanol dehydration to cyclohexene

in a fixed bed reactor was used to evaluate the Bronsted

acidity of the catalyst. Cyclohexanol (2 ml) was delivered

by a syringe pump injector (Cole Parmer, 74900 series)

with a flow rate of 1 ml h-1 under N2 at 175 �C over the

catalyst sample (0.2 g) packed in a reactor bed. Product

samples were collected after 1 h and analyzed with a

Hewlett–Packard gas chromatogram (HP 6890) having FID

detector. The conversion of cyclohexanol and selectivity

for cyclohexene was calculated as follows:

Conversion of cyclohexanol (wt%Þ¼ 100� Initial wt%� Final wt%½ �=Initial wt%

Selectivity for cyclohexene (wt%Þ¼ 100� GC peak area% of cyclohexene½ �=

R peak area of total products

2.4 Catalytic activity for esterification of caprylic acid

with alcohol to synthesize alkyl caprylate

Esterification of caprylic acid with alcohol was carried out

in a liquid phase batch reactor to evaluate the catalytic

activity of sulfated zirconia sample. In a typical reaction

procedure, required amount of caprylic acid, alcohol (acid :

alcohol molar ratio = 1:10) and catalyst (0.5 wt%) were

taken in a round bottom flask and the suspension was

magnetically stirred (600 rpm) in oil bath maintaining

constant temperature (±1 �C) in the temperature range of

60–110 �C. The reaction kinetics was monitored by with-

drawing small amounts of the reaction mixture at 1 h

intervals and analyzed using a gas chromatograph (HP

6890) having a DB-225 capillary column and FID detector.

The conversion of caprylic acid and selectivity for alkyl

ester was calculated by GC peak area as below:

Conversion of caprylic acid (wt%Þ¼ ½ðInitial wt%� Final wt%Þ=Initial wt%� � 100

Selectivity for alkyl ester(wt%޼�GC peak area% of alkyl ester=

R peak area of total products�� 100

3 Results and discussion

3.1 Catalyst characterization

Powder X-ray diffraction (PXRD) pattern of sulfated zirconia

catalyst, calcined at 600 �C, showed the presence of tetrago-

nal crystalline phase (2h = 30.21, 35.27, 50.21, 60.31)

(Fig. 1). The sample was observed to be less crystalline due to

the presence of higher sulfur content (Table 1); as higher

thermal energy is required for dehydroxylation during crys-

tallization in the presence of sulfate groups that enhances the

14 J Sol-Gel Sci Technol (2012) 62:13–17

123

crystallization temperature of the sample [18]. The sample

was found to have nano-crystallite size of 11 nm (Table 1).

The bulk sulfur present in sulfated zirconia sample was

3.7 wt% before calcination, which decreased to 2.6 wt%

after calcination at 600 �C (Table 1).

N2 adsorption isotherm (Fig. 2) of the sample was found

to be of type IV, which is characteristic of mesoporous

solids [17], having the narrow pore size distribution

(Fig. 2) and average pore diameter of 7.6 nm (Table 1).

The sample was found to have low BET surface area

(28 m2/g) and pore volume (0.05 cm3/g).

NH3-TPD results showed that NH3 desorption occurred

at 719 �C indicating the presence of stronger acid sites.

The total number of acid sites was found to be 2.5 mmol/g

(Table 1). The vapor phase dehydration of cyclohexanol to

cyclohexene resulted into 91% conversion of cyclohexanol

with 100% selectivity for cyclohexene. It indicated the

presence of higher Bronsted acidity in the prepared sulfated

zirconia catalyst (Table 1).

3.2 Esterification of caprylic acid

The esterification of caprylic acid with methanol was car-

ried out in presence of sulfated zirconia catalyst in a liquid

phase batch reactor at 60 �C (b.p. methanol *65 �C). GC

analysis of the reaction mixture showed the selective for-

mation (100%) of methyl caprylate.

The reaction kinetic study showed an exponential

increase in the conversion of caprylic acid from 10 min

(37%) to 4 h (90%) followed by a linear increase till 7 h

and remained steady afterwards (Fig. 3). The maximum

conversion of caprylic acid was 96–98% after 7 h. The

earlier studies reported in the literature showed the higher

conversion of caprylic acid either at higher temperature or

using higher amount of the catalyst or after longer reaction

10 20 30 40 50 60 70 80

a

b

Inte

nsit

y (a

.u)

2 Theta (degree)

Fig. 1 Powder X-ray diffraction pattern of nano-crystalline sulfated

zirconia (a) before and (b) after calcination at 600 �C

Table 1 Characterization of nano-crystalline sulfated zirconia catalyst

Crystallite

size (nm)

BET surface

area (m2/g)

Average pore

volume (cm3/g)

Average pore

diameter (nm)

Sulfur

(wt%)

Acid sites

(mmol/g)

Cyclohexanol

conversion (%)

11 28 0.05 7.6 2.60 2.5 91

0.0 0.2 0.4 0.6 0.8 1.0

0

10

20

30

40

50

60

70

3 6 9 12 15 18 21 24 27 30

0.000

0.002

0.004

0.006

0.008

Por

e vo

lum

e (c

m3 /g

)

Pore diameter (nm)

Vo

lum

e A

dso

rbed

cm

³/g

ST

P

Relative Pressure (P/Po)

Fig. 2 N2 adsorption–desorption isotherm, (inset) Pore size distribu-

tion of nano-crystalline sulfated zirconia

0 100 200 300 400 500 60030

40

50

60

70

80

90

100

Con

vers

ion

(wt.

%)

Time (min)

Fig. 3 Esterification of caprylic acid with methano1 over sulfated

zirconia. Reaction conditions: caprylic acid: methano1 molar

ratio = l:10; catalyst: 0.5 wt%, temperature: 60 �C

J Sol-Gel Sci Technol (2012) 62:13–17 15

123

time [Table 2]. Besides caprylic acid, higher conversion of

other fatty acids has also been reported using 2–75 wt% of

different solid acid catalysts [19].

The difference in the activity of various catalysts occurs

due to the variation in their structural, textural and acidic

features, which strongly depends on the preparation methods.

Different reaction parameters also lead to different catalytic

behaviour. To the best of our knowledge, this is the first report

of using minimum concentration (0.5 wt%) of sulfated zir-

conia catalyst resulting 98% conversion of caprylic acid

within reasonable reaction time period (7 h) at 60 �C.

3.2.1 Effect of alcohols

The esterification of caprylic acid with various alcohols

namely ethanol, n-propanol and n-butanol showed the suc-

cessive decrease in the conversion values (88–67%) with

increasing carbon chain of alcohols (Fig. 4). This is due to the

inductive effect of increased carbon chain of alcohol. The

electron donating ability of alkyl group towards the hydroxyl

group increases with increasing the alkyl chain of alcohol thus

lowers the hydroxylation and limiting the electrophilic attack

by the acid. When the reaction was studied at higher tem-

perature in reflux conditions, near to the boiling point tem-

perature of respective alcohol (75–110 �C), the conversion

was found to increase significantly (91–98%) (Fig. 4). The

selectivity for respective alkyl caprylate was observed to be

100% irrespective of the alcohol used.

3.2.2 Reusability of SZ catalyst

To study the reusability of SZ catalyst, the used catalyst

was recovered from the reaction mixture, washed with

methanol, dried and activated at 450 �C for 2 h before its

re-use for further reaction cycle. The activity of SZ catalyst

was found to slightly decrease with successive reaction

cycles and the conversion of caprylic acid was 81% after

five cycles (Table 3). The decrease in the activity may be

due to the deactivation of the catalyst by the presence of

water, a co-product formed during the esterification reac-

tion. Water having the higher dielectric constant (80 at

20 �C) than methanol (33 at 20 �C) has higher affinity to

interact with the active acid sites and thus lowers the

interaction of methanol with active sites resulting into a

decrease in conversion of acid.

3.2.3 Mechanism

Esterification of acid with alcohol may be catalyzed by

both Bronsted and Lewis acids. SZ catalyst has both

Bronsted (H?) and Lewis acid sites (Zr?) as shown in

Scheme 1, therefore, both acid sites may take part in the

reaction. As the catalyst was having higher Bronsted

acidity, the studied esterification is supposed to be mainly

Table 2 Esterification of caprylic acid with alcohol using different

solid acid catalysts

Catalyst Catalyst

amount

Temp (�C),

Time (h)

Max.

conversion

(%)

Reference

SZ, Nafion,

Amberlyst

1–3 wt% 120–180,

1–5

*80 [5]

SZ, SSn 4 g 150, 20 *100 [6]

WZA, SZA,

STO

4 g 200, 20 *100 [7]

TiZ, AlZ 4 g 175, 20 *100 [8]

SZ, WZ, TiZ 8 wt% 75, 4 *10–60 [9]

Niobia 15 wt% 65, 3 *98 [10]

Nafion/silica 2.42 wt% 60, 24 *60 [11]

Sulfonated

carbon

2.5 wt% 60, 1 *20 [12]

SZ 0.5 wt% 60, 7 98 Present

results

MeOH EtOH n-PrOH n-BuOH0

20

40

60

80

100

Con

vers

ion

(wt.

%)

110 °C90 °C

75 °C

60 °C

Fig. 4 Esterification of caprylic acid with various alcohols at

different temperatures over sulfated zirconia. Reaction conditions:

caprylic acid: methano1 molar ratio = l:10; catalyst: 0.5 wt%

Table 3 Conversion (%) of caprylic acid over re-generated sulfated

zirconia catalyst

Cycles Conversion (%)

1 98

2 89

3 88

4 84

5 81

Reaction conditions: caprylic acid: methano1 molar ratio = l:10;

catalyst: 0.5 wt%, temperature: 60 �C, time: 7 h

16 J Sol-Gel Sci Technol (2012) 62:13–17

123

Bronsted acid catalyzed reaction, wherein H? transfers

from the Bronsted acid sites of catalyst to carbonyl oxygen

of the acid, which is nucleophillic attacked by oxygen of

alcohol; subsequent deprotonation and loss of water forms

the ester product (Scheme 1).

4 Conclusions

Sulfated zirconia catalyst, prepared by sol–gel method, was

found to have nano-crystalline size of 11 nm. It showed the

presence of higher Bronsted and total surface acidity;

though the crystallinity and BET surface area was low. The

small concentration of the catalyst showed encouraging

activity for esterification of caprylic acid with different

alcohols. With methanol 96–98% conversion of caprylic

acid and 100% selectivity for methyl caprylate was

observed at 60 �C in presence of 0.5 wt% catalyst con-

centration. The conversion was found to decrease

(88–67%) with increasing carbon chain of alcohols, i.e.,

ethanol, n-propanol and n-butanol without affecting

the selectivity for respective alkyl caprylate. However, the

conversion was increased (91–98%) by increasing the

reaction temperature near to the boiling point of the alco-

hol. The activity of the catalyst was found to slightly

decrease with successive five reaction cycles, which may

be due to the water molecules formed during the esterifi-

cation reaction as a co-product. The higher sulfur content,

Bronsted and total acidity of sulfated zirconia seems to be

responsible for its high activity for the studied reaction.

The detailed study for varied reaction parameters, effect

of calcination on the crystallinity, acidity and activity of

the catalyst, deactivation of sulfated zirconia catalyst in

presence of water and esterification of higher carbon chain

fatty acids are the subject of ongoing investigations in our

laboratory.

Acknowledgments Authors are thankful to CSIR Network Pro-

gramme on Inorganic material for diverse application and to Ana-

lytical science discipline for catalyst characterization analysis.

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OZr

OZr

OZr+

O

O

S

O

OO

HO-H

Brönsted

Lewis

C7H15-C-OH

Caprylic acid

OH+

SZ

C7H15-C-OH

OH

SZ

ROHC7H15-C-OH

OH

SZ

OHR

C7H15-C-OH2

OH

SZ

OR

-H2OC7H15-C

O

OR

C7H15-COOR

Alkyl caprylate H

SZ

SZ

Scheme 1 Plausible structure of SZ catalyst and mechanistic repre-

sentation of esterification of caprylic acid with alcohol to synthesize

alkyl caprylate using Bronsted acid sites of SZ

J Sol-Gel Sci Technol (2012) 62:13–17 17

123