This journal is c The Royal Society of Chemistry 2012 Catal. Sci. Technol.

Cite this: DOI: 10.1039/c2cy20462b

Sulfated zirconia: an efficient solid acid catalyst for esterification of

myristic acid with short chain alcoholsw

K. Saravanan, Beena Tyagi* and H. C. Bajaj

Received 11th June 2012, Accepted 20th July 2012

DOI: 10.1039/c2cy20462b

Sulfated zirconia (SZ) catalysts prepared by a two-step sol–gel method and calcined at 600–700 1C

were evaluated for esterification of myristic acid with methanol using varied acid to alcohol ratio,

reaction temperature and catalyst concentration. An exceptionally small concentration of SZ

catalysts (0.125–0.5 wt% to acid) exhibited 98–100% conversion of myristic acid with methanol at

60 1C after 5 h. The conversion was decreased with an increase in the alkyl chain of alcohol from

methanol to butanol, however, similar conversion was achieved by increasing the reaction

temperature to 90 1C. The ester formation was selective, irrespective of alcohol and other reaction

variables. The calcination temperature has strong influence on the structural, textural and acidic

features and thus on the activity and re-usability of SZ catalysts. The reaction is sensitive to

moisture present in methanol or reaction mixture. The studied reaction is Bronsted acid catalyzed

and the SZ catalyst having higher number of Bronsted acid sites was re-used successfully without

significant loss in activity; whereas the SZ catalyst having lower number of Bronsted acid sites

showed a decrease in activity (B28%) after five reaction cycles. The results clearly indicated the

necessity of higher number of Bronsted acid sites for better performance and recycling of the SZ

catalysts for esterification of myristic acid with methanol under the conditions studied.

1. Introduction

Myristic acid is a saturated fatty acid (n-tetradecanoic acid,

C14H28O2) naturally occurring in nutmeg Myristica fragrans.

It is also found in palm kernel oil, coconut oil, butter fat and

animal fats, which are used to produce fatty acid alkyl esters

(FAAEs), used as biodiesel.1 It increases low density lipoprotein

cholesterol making it one of the most hypercholesterolemic of

the saturated fatty acids.2 Alkyl myristate of short chain

alcohols namely methanol, ethanol, propanol and butanol

are used in perfumes, flavors, cosmetics, personal care

products, food additives, detergents, soaps, lubricants for

textiles, plasticizers etc.3 Besides, methyl myristate has been

approved as a new active constituent in veterinary chemical

products by Australian pesticides and veterinary medicines

authority4 and ethyl myristate is used as a marker of excessive

ethanol consumption that can be isolated from the hair of an

alcoholic individual.5

Conventionally, FAAEs have been prepared by homo-

geneous acid catalyzed esterification of the corresponding acid

with an alcohol or homogeneous base catalyzed transesterifi-

cation of oils and fats. Industrial esterification processes are

carried out in the presence of homogeneous Bronsted acid

catalysts such as sulfuric, p-toluene sulfonic or phosphoric

acid.6 To overcome the disadvantages associated with homo-

geneous catalysts, a number of solid acid catalysts such as

inorganic metal oxides, heteropoly acids and sulfonic acid

based resins etc. are being studied to produce FAAEs via

esterification of fatty acids (FAs) or transesterification of oil

with alcohols.6,7 Among metal oxide based solid acid catalysts,

sulfated zirconia (SZ) has exhibited significant activity in both

the reactions.6–13 Kiss et al.9 have screened various solid acids

including zeolites, ion exchange resins and metal oxides for the

esterification of lauric acid with different alcohols and found

SZ as the most active solid acid catalyst; zeolites were not

suitable due to their microporous nature having diffusion

limitations of large fatty acid molecules, whereas, ion

exchange resins have low thermal stability. SZ is a potential

solid acid catalyst for alkane isomerization at mild temperature14

and many other commercially important organic transforma-

tions due to its strong acid properties.15–17

Few studies have been reported for the esterification of

myristic acid using SZ18,19 and niobia20 metal oxide catalysts.

Zeolites21 and acidic ion exchange resins namely Amberlyst-15

and silica based resin having sulfonic acid groups22 were not

Discipline of Inorganic Materials and Catalysis, Council of Scientificand Industrial Research (CSIR), Central Salt and Marine ChemicalsResearch Institute (CSMCRI), G. B. Marg, Bhavnagar, Gujarat 364 002,India. E-mail: btyagi@csmcri.org; Fax: +91-278-2566970w Electronic supplementary information (ESI) available: PXRD pattern,TG/DTA, nitrogen sorption isotherms, IR spectra, DRIFT spectra fromRT to 450 1C, pH vs. time after stirring the SZ-600 catalyst in water andmethanol and comparison with other acid catalysts. See DOI: 10.1039/c2cy20462b

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found effective as compared with metal salts or homogenous

p-TSA. Among various metal salts, zirconium sulfate showed

significant activity.21 MCM-48 supported tungstophosphoric

acid23 and ZrOCl2�8H2O24 have been studied with long chain

alcohols such as cetyl alcohol in sc-CO2 and other organic

solvents. However, most of these studies have been done for

various long chain saturated and unsaturated FAs or along

with triglycerides at high temperature (120–180 1C)18,19,23,24

in the presence of higher concentration of solid catalysts

(5–15 wt%).19–21 To the best of our knowledge, no detailed

study has been reported for the esterification of myristic

acid with methanol and other short chain alcohols over SZ

catalysts.

We have studied SZ catalysts for various reactions mainly

acetylation,25a,b isomerization,25c,d Pechmann,25e,f condensation26a

and recently esterification of caprylic acid.26b Herein, we

report a systematic detailed study of the esterification of

myristic acid with methanol at lower temperature in the

presence of small concentration of SZ catalysts. The effect of

various reaction parameters such as acid to alcohol ratio,

catalyst concentration and reaction temperature along with

other short chain alcohols namely ethanol, n-propanol and

n-butanol has been investigated. The effect of calcination

temperature on the acidity and activity, re-usability and

deactivation of SZ catalysts has also been addressed. In our

earlier report25c we have observed that selectivity of the

isomerized products can be varied depending upon the acidity

of SZ catalysts in terms of Bronsted (B) to Lewis (L) acid site

concentration ratio. Herein, we have found that the acidic

properties in terms of Bronsted acid site concentration, B/L

ratio and total surface acidity play a major role in the activity

and deactivation of SZ catalysts for esterification of myristic

acid with methanol under the conditions studied. The present

study gives novel insight into the influence of the acidic

properties on the catalytic performance of fresh and re-used

SZ catalysts and the use of a very small amount of the catalyst

resulting in maximum conversion of acid at lower temperature.

2. Results and discussion

2.1 Catalyst characterization

The partial characterization of SZ catalysts has been reported

in our previous paper.26a In the context of the present work

few salient features are mentioned briefly to understand the

structure–activity relationship.

A powder X-ray diffraction (PXRD) pattern of SZ catalysts

showed the presence of tetragonal crystalline phase after

calcination at 600–700 1C. SZ-600 was less crystalline and

the crystallinity of the samples increased with increasing

calcination temperature (Fig. S1, ESIw). The catalysts were

found to have nano-crystallite size in the range of 11–16 nm

(Table 1). The bulk sulfur content before calcination was

3.7 wt%, which successively decreased after calcination at

600–700 1C (Table 1). The TG/DTA profile (Fig. S2, ESIw)of the catalyst also confirmed the crystallization of the SZ

sample at 4600 1C.

A TEM micrograph of SZ-600 (Fig. 1) showed the aggre-

gates of SZ crystallites varying from 6.9 to 10.2 nm with an

average crystallite size of 7.3 nm, which was in agreement with

crystallite size calculated by PXRD. The high-resolution TEM

image (inset, Fig. 1) exhibited the lattice fringes, the width

between two fringes (B2.86 A) also agreed with d-spacing

(B2.93 A) of characteristic peaks of tetragonal phase of

zirconia (2y = 30.22) shown by PXRD.

BET surface area and pore volume were found to decrease

with an increase in calcination temperature; however SZ-700

showed higher pore diameter due to collapsing of pore walls at

higher temperature (Table 1). The N2 adsorption isotherms

(Fig. S3, ESIw) showed the increase in adsorption at higher

relative pressure indicating the presence of larger size of meso-

pores in the samples.27 The inflection point at P/Po =B0.4–0.5

Table 1 Characterization of sulfated zirconia catalysts

CatalystCrystallitesizea (nm)

Sulfura

(wt%)SBET

(m2 g�1)Average porevolume (cm3 g�1)

BJH porediameter (nm)

Cyclohexanolconversion (%)

NH3-TPD DRIFTb

Temp.(1C)

Acid sites(mmol g�1)

Total acid sites(mmol g�1)a

B acid site(%T cm�1)

B/Lratio

SZ-600 11 2.6 75 0.12 6.5 91 719 2.50 2.5 3690 2.07SZ-650 15 1.04 74 0.09 5.7 87 116 0.80 0.99 2939 1.66

564 0.19SZ-700 16 0.87 54 0.08 7.1 85 112 0.38 0.67 2738 1.21

400 0.07627 0.14793 0.08

a Data from ref. 26a. b B and L = Bronsted and Lewis acid sites, at 150 1C.

Fig. 1 TEM image of the SZ-600 catalyst at low resolution (inset)

high resolution.

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was not sharp, which reflected that the pores have varied pore

size distribution (Fig. 2).

IR-spectra of the catalysts showed the presence of sulfate

groups in the range of 1245–900 cm�1 (Fig. S4, ESIw). SZ-600showed the presence of a broad intense peak showing

shoulders at 1230, 1138, 1040 and 990 cm�1, however, highly

crystalline SZ-650 and SZ-700 clearly exhibited the peaks at

1230–1245, 1136–1142, 1047–1056 and 988 cm�1, which are

assigned to asymmetric and symmetric stretching frequencies

of S]O and S–O bonds and are characteristic of inorganic

chelating bidentate sulfate. The partially ionic nature of the

S]O bond is responsible for the Bronsted acid sites in sulfated

zirconia.28 The intensity of the sulfate peaks was reduced in

SZ-700 due to loss of sulfate species at higher calcination

temperature.

The dehydration of cyclohexanol over SZ-600 showed 91%

conversion of cyclohexanol with selective formation of cyclo-

hexene indicating the presence of higher Bronsted acidity in

SZ-600, which slightly decreased in SZ-650 and SZ-700

(Table 1). Similarly, the total surface acidity analyzed

by NH3-TPD showed the highest number of acid sites

(2.5 mmol g�1) in SZ-600, which decreased ominously in

SZ-650 and SZ-700 (Table 1). The acidity in sulfated zirconia

is attributed to the presence of sulfate groups; as SZ-600 has

the highest sulfur content, it showed the highest number of

acid sites, the decrease in sulfur content with increasing

calcination temperature resulted in a decrease in acidity.

Furthermore, NH3 desorption occurred at relatively higher

temperature in the SZ-600, indicating the presence of only

strong acid sites (719 1C), whereas SZ-650 and SZ-700 showed

the presence of weak (112–116 1C), moderate (400–500 1C) to

strong (4600 1C) acid sites (Table 1).

The DRIFT spectra of pyridine adsorbed SZ catalysts

exhibited the characteristic peaks of pyridinium ions (Bronsted

acid sites) at 1540 cm–1 and of covalently bonded pyridine

(Lewis acid sites) at 1441 cm–1 (Fig. 3) along with peaks at

1616 cm–1 and 1488 cm–1 representing the Bronsted and total

acid sites respectively.25e,29 Both acid sites were observed to be

strong enough as they were present even after heating at 450 1C;

though the intensity of the peaks was decreased after succes-

sive heating (Fig. S5, ESIw). SZ-600 and SZ-650 showed

intense peaks for Bronsted acid sites, whereas, in SZ-700 the

peak for the Lewis acid site was intense. The Bronsted acid site

concentration and B/L ratio, calculated from the characteristic

peak area at 150 1C, were in the descending order from SZ-600

to SZ-700 (Table 1).

2.2 Catalytic activity

Initially, esterification of myristic acid was studied with

methanol over the SZ-600 catalyst to optimize acid to alcohol

ratio, catalyst concentration and temperature to achieve the

maximum conversion of myristic acid and selectivity for

methyl myristate.

2.2.1 Effect of acid to alcohol ratio and catalyst concen-

tration. The reaction was scrutinized using an acid to methanol

molar ratio of 1 : 5 to 1 : 20 over catalyst concentration in the

range of 0.125–0.5 wt% to acid. The varying concentration of

the catalyst (0.125–0.5 wt%) resulted in 98–100% conversion

of myristic acid having 100% selectivity for methyl myristate

using an acid to methanol molar ratio of 1 : 20 at 60 1C after

7 h (Table 2). By decreasing the acid to methanol molar ratio

to 1 : 10, the catalyst showed 47% conversion with 0.125 wt%

of catalyst, which enhanced upon increasing the catalyst

concentration. By further decreasing the acid to methanol

ratio to 1 : 5, 82% conversion was observed with 0.5 wt%

catalyst concentration. The results indicated the requirement

Fig. 2 Pore size distribution profiles of SZ catalysts calcined at

different temperatures.

Fig. 3 DRIFT spectra of SZ catalysts after pyridine desorption at

150 oC.

Table 2 Esterification of myristic acid with methanol over variedconcentrations of SZ catalystsa

Catalyst (wt%)

Conversion (wt%)

Methyl myristate (wt%)A B

0.5 498 (82*) 498 1000.25 74 498 1000.125 47 498 100

a Reaction conditions: temperature = 60 1C; time = 7 h. A –

acid :methanol = 1 :10; B – acid :methanol = 1 :20; *acid :methanol =

1 :5.

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of higher acid to alcohol ratio to obtain complete conversion

of FAs as also reported in the literature.6

Theoretically, esterification reaction requires one mole of

alcohol for one mole of acid; however, in practice, a molar

ratio higher than stoichiometric ratio is needed to complete the

reaction having acceptable reaction rates. The higher amount

of methanol shifted the equilibrium to the right side, thus

achieving the maximum conversion even with lower catalyst

amount (0.125 wt%). On the other hand, higher concentration

(0.5 wt%) of the catalyst increased the availability of acid sites,

which favour the accessibility of more number of reactant

molecules to these acid sites and thus enhanced the conversion

in the presence of lower acid to alcohol ratio. Though a very

high acid to alcohol ratio has been used to achieve maximum

acid conversion of FAs, e.g., 1 : 50,30 1 : 4031 to 1 : 10011 and

1 : 20032 in the presence of HPA, SZ and WZ catalysts,

respectively; lower amount of alcohol is more desirable in

the industrial synthesis of FAAEs, and thus we preferred an

acid to methanol ratio of 1 : 10 along with 0.5 wt% of catalyst

(protocol A) for rest of the reactions. However, for compar-

ison, we have also studied few reactions with an acid to

methanol ratio of 1 : 20 in the presence of 0.125 wt% of

catalyst (protocol B). It is worth to be noted here that the

amount of SZ catalyst used was exceptionally small (0.125–0.5

wt% to myristic acid, i.e., acid : SZ (wt/wt) = 800–200 : 1) and

to the best of our knowledge, not reported earlier. Similar

good activity was also found for caprylic acid with 0.5 wt% of

catalyst.26b

2.2.2 Effect of temperature. The reaction was carried out in

the temperature range of 40–60 1C with both sets of reaction

variables as stated above. The reaction having (A) variables

showed 69% conversion of myristic acid at room temperature

(32 1C) after 7 h, which increased at 50 1C and remained

constant upon further increasing the temperature (Table 3).

Whereas, the reaction having (B) variables showed a gradual

increase in the conversion from 81% to 98% by increasing the

temperature from 40 to 60 1C. The availability of higher

concentration of the catalyst in the (A) reaction significantly

enhanced the interaction between the acid sites and the

reactants by increasing the temperature from 40 to 50 1C.

However, the lower concentration of the catalyst in the (B)

reaction was further diluted in the presence of higher amount

of methanol and thus resulted in a gradual enhancement in the

conversion with an increase in the temperature from 40 to 60 1C.

To optimize the reaction temperature, we have studied the

reaction kinetics both at 50 and 60 1C. Though the final

conversion of myristic acid was similar (98–100%) after 7 h

and remained steady afterwards both at 50 and 60 1C (Fig. 4),

the rate of increase in conversion was higher at 60 1C. There-

fore, 60 1C temperature was chosen as optimum reaction

temperature for rest of the studies.

2.2.3 Effect of calcination temperature. To study the influ-

ence of calcination temperature on the activity of SZ catalysts,

all three samples that calcined in the range of 600–700 1C have

been evaluated under both the reaction variables as stated

above (Table 4). SZ-600 and SZ-650 catalysts resulted in

similar conversion under (A) reaction variables, however,

SZ-700 showed a significant decrease in the conversion

(61%). The acidity of SZ catalysts is ascribed to the presence

of sulfate species which undergo thermal decomposition dur-

ing the calcination and therefore the loss of sulfate species with

increasing calcination temperature resulted in the decrease in

the acidity and thus the activity of the catalyst. Though SZ-

650 has lower sulfur content, Bronsted acidity, total surface

acidity and weak to moderate acid sites as compared to SZ-600

(Table 1); its catalytic activity for the studied reaction was not

affected. It may be due to the presence of minimum required

Table 3 Esterification of myristic acid with methanol at differentreaction temperaturesa

Temperature (1C)

Conversion (wt%)

A B

32 69 —40 71 8150 498 8755 498 9160 498 498

a Reaction conditions: A – acid :methanol = 1 : 10 and catalyst =

0.5 wt%; B – acid :methanol = 1 : 20 and catalyst = 0.125 wt%;

Reaction Time = 7 h.

Fig. 4 Conversion of myristic acid with time at (a) 50 1C and

(b) 60 1C, (c) effect of fresh methanol (at 60 1C). Reaction conditions:

acid :methanol = 1 : 10; SZ = 0.5 wt%.

Table 4 Esterification of myristic acid with methanol over SZcatalysts calcined at different temperaturesa

Catalyst

Conversion (wt%)

A B

SZ-600 498 498SZ-650 498 80SZ-700 61 58Zr-400 28 —Zr-600 23 —

a Reaction conditions: temperature = 60 1C; time = 7 h. A –

acid :methanol = 1 : 10 & catalyst = 0.5 wt%. B – acid :methanol =

1 : 20 & catalyst = 0.125 wt%.

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acidity for the studied reaction along with the highly crystal-

line nature, higher BET surface area and pore volume of

SZ-650 (Table 1), which may play a significant role by making

the reaction facile and active.

Under (B) reaction variables, decrease in conversion was

observed over both SZ-650 and SZ-700 catalysts (Table 4).

Though, the SZ-650 catalyst showed similar conversion to

SZ-600 under (A) reaction conditions, it reduced to 80%

under (B) reaction conditions. Because, its lower acidity and

Bronsted acid site concentration (compared to SZ-600) were

sufficient to achieve 498% conversion when its concentration

was higher in reaction (A), however, with lower concentration

along with higher amount of methanol in reaction (B), the

number of acid sites available was insufficient to interact with

the reactant molecules and thus to obtain complete conversion

of acid. The decreasing catalytic activities of SZ catalysts were

in good agreement with their decreasing Bronsted acid site

concentration as well as B/L ratio and total surface acidity

with an increase in calcination temperature (Table 1). These

results clearly revealed that the acidity and activity of a SZ

catalyst are strongly influenced by the temperature at which it

has been calcined before undergoing the reaction along with

the reaction parameters. The pure ZrO2 sample, calcined at

400–600 1C, showed only 23–28% conversion of myristic acid

(Table 4) thus confirming the enhanced catalytic activity of SZ

catalysts.

2.2.4 Effect of alcohols. The reaction was carried out with

different short chain alcohols namely ethanol, n-propanol and

n-butanol over SZ-600 at (A) reaction parameters. Methanol

and ethanol having smaller carbon number resulted in similar

conversion (98%) at 60 1C after 7 h, however the conversion

was decreased successively with propanol (71%) and butanol

(69%) due to the inductive effect of increased carbon chain of

alcohol (Fig. 5).

A similar trend was found with caprylic acid26b and also has

been reported for SZ catalyzed transesterification of triglycerides10

and esterification of carboxylic acids of different chain lengths

with methanol using sulfuric acid and nafion–silica composite33

suggesting that the lower reaction rates are due to steric

hindrance effects of the larger alkyl chains either of alcohols

or acids. However, in the present study the reduced conversion

of myristic acid with propanol and butanol was enhanced by

increasing the reaction temperature to 90 1C. The results

clearly envisage that methanol having a small carbon chain

is more economical for the esterification of fatty acid owing to

its lower boiling point that consumes lower thermal energy

and also in terms of comparatively inexpensive. The selectivity

for alkyl myristate was 100% irrespective of alcohol and

temperature. As excess of alcohol may lead to dehydration

or etherification in the presence of acid catalyst, the selectivity

of ester was further confirmed by checking the formation of

side products during the reaction of methanol with a SZ-600

catalyst at 60 1C for 5–10 h. No dehydrated or ether product

was detected by GC analysis under the conditions studied.

2.3 Re-usability of SZ catalysts

The used SZ-600 and SZ-650 catalysts were recovered from the

reaction mixture, washed with methanol, dried and activated

at 450 1C for 2 h before their re-use for further reaction cycle

under the similar reaction conditions (A).

The SZ-600 catalyst exhibited similar activity for myristic

acid esterification till five reaction cycles without showing any

significant decrease in the conversion (only a slight decrease to

94%). However, the activity of SZ-650 was decreased to 86%

after one re-cycle, which further decreased (71%) after five

reaction cycles (Fig. 6). The sulfur content of the re-activated

SZ-600 and SZ-650 catalysts after five reaction cycles was

decreased; 1.84 wt% and 0.604 wt%, respectively, compared

to fresh catalysts. However, IR spectra of re-activated cata-

lysts were found to be similar to the fresh ones (Fig. 7).

Furthermore, the absence of any adsorbed organic species in

the IR spectra of re-activated SZ-600 and SZ-650 catalysts

indicated that the deactivation of the catalyst did not occur

due to the deposition of the product molecules on the active

acid sites of the catalyst.

Fig. 5 Esterification of myristic acid with different alcohols over the

SZ-600 catalyst. Reaction conditions: acid :methanol = 1 : 10; SZ =

0.5 wt%. Fig. 6 Re-usability of SZ-600 and SZ-650 catalysts after different runs.

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We tried to find out the possible reasons for decreased

activity of the SZ-650 catalyst as deactivation of a SZ catalyst

is supposed to be its major disadvantage though having strong

super acidity.

2.4 Deactivation of SZ catalysts

The successive decrease in the activity of SZ catalysts may be

due to (i) deactivation of active acid sites by water molecules

formed during the esterification reaction or/and (ii) leaching of

SO42� species from the catalyst in polar alcohol medium

during the course of reaction. Both possibilities have been

debated in the following sections.

2.4.1 Presence of water. We have checked the effect of

moisture present in methanol by repeating the reaction using a

fresh bottle of methanol of same purity. The initial conversion

of myristic acid was found to be significantly higher (B1.6 times)

compared to the previous reaction studied under the similar

reaction conditions and B100% conversion was achieved

after 5 h instead of 7 h and was steady afterwards (Fig. 4c).

The results indicated the sensitivity of the esterification reac-

tion of myristic acid towards the atmospheric moisture present

in alcohol. For rest of the studies, the fresh methanol was used

with precautions.

The effect of water was also studied by adding a small

amount of water (2000 ppm) to the reaction mixture. The

reaction showed lower conversion (90–91%) of myristic acid

after 5–7 h (Fig. 8). Water has higher affinity to interact

with the active acid sites and thus lowers the interaction of

methanol with active sites resulting in a decrease in the

conversion of acid. A similar retarding effect of water has

been observed in homogeneous acid catalyzed esterification of

FAs and also base catalyzed transesterification of triglycerides,

however, acid catalyzed esterification is more tolerant to water

compared to the latter.34 Kiss et al.35 have successfully used a

reactive distillation process to remove the water, however, the

reaction was done at higher temperature (130–160 1C).

The results indicated the sensitivity of the esterification reac-

tion of FAs towards the water molecules present in the

reaction system.

2.4.2 Leaching test. The leaching of SO42� species of SZ

catalysts especially in aqueous/alcoholic medium is a subject

of controversy, as some studies reported the hydrolysis of SZ

in pure aqueous medium36 but deactivation of SZ catalysts did

not occur by leaching of SO42� groups in the presence of small

amount of water in organic phase,9,36 whereas other reports

claimed the leaching of active SO42� groups from the catalyst

surface and transformed into H2SO4/HSO4�/SO4

2� ions in the

presence of alcohol10,11 or water.37

We have studied the leaching of SO42� ions of the catalyst

by employing two methods. (i) In one experiment, it was tested

during the esterification reaction. After 10 min of the reaction,

the catalyst was removed from the reaction mixture; and the

reaction was continued with the remaining solution without

having any catalyst. The plot of concentration of myristic acid

with time (Fig. 9) clearly showed an insignificant decrease in

concentration of myristic acid after removal of the catalyst as

Fig. 7 IR spectra of (i) fresh and (ii) re-activated (after 5 cycles) (a)

SZ-600 and (b) SZ-650 catalysts.Fig. 8 Effect of addition of water on the conversion of myristic acid

with methanol. Reaction conditions: acid :methanol = 1 : 10; SZ =

0.5 wt%

Fig. 9 Concentration of myristic acid (%) without catalyst,

(J) catalyst removed after 10 min and (m) with catalyst. Reaction

conditions: acid :methanol = 1 : 10; SZ = 0.5 wt%

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compared to a linear decrease in myristic acid concentration in

the presence of catalyst. The autocatalysis (a blank test in the

absence of catalyst), under the similar reaction conditions,

showed nil conversion till 45 min and increased from 2.99%

(at 60 min) to 4.87% after 120 min (19% after 7 h, not shown

in figure). The initial rate of autocatalysis was significantly

lower (0.5 mmol min�1) as compared to the catalytic reaction

(12.9 mmol min�1). The rate after removal of the catalyst

(2.4 mmol min�1) was observed to be five times lower than

the reaction done in the presence of catalyst. These results

showed that no significant leaching of SO42� ions of SZ

catalysts occur under the experimental conditions studied.

(ii) In a second experiment, a fresh SZ-600 catalyst was

stirred in methanol and distilled water separately (25 ml of

each) and pH was measured (Toshniwal, CL54) during 24 h.

The pH of the reaction mixture was found to decrease

significantly after 1 h of stirring both in methanol and water,

however, it was not much affected further till 24 h (Fig. S6,

ESIw). Initially, the pH of the reaction mixture was higher in

water (4.3) compared to methanol (2.7), however, the decrease

in pH was observed in a similar range (38.4% in water and

38.6% in methanol) after 1 h of stirring.

It appeared from the decrease in pH by treatment with

water or methanol that the sulfate groups may be hydrolyzed

in the presence of –OH groups on the catalyst surface9 or the

sulfate groups may be leached out during the initial first

hour,10 which may not be strongly bonded with the zirconia

surface; however strongly bonded sulfate groups did not leach

out till further 24 h of stirring. To further confirm this, the

SZ-700 catalyst having less sulfur content and highly crystal-

line nature was checked for the same study, which also showed

a decrease in pH from 4.74 to 3.63 during the first hour of

stirring in water. The pure ZrO2 sample having no sulfate

species also showed a decrease in pH from 5.76 to 4.88–5.04

during 1–2 hours of stirring in water, though it was lower

compared to SZ. Therefore, the decrease in pH (as also

reported by others) does not seem to be purely due to the

leaching of sulfate species; it may be due to the hydrolysis of

sulfate species and also due to the acidic metal oxide matrix.

2.5 Reaction rate

The kinetic profile of esterification of myristic acid with fresh

methanol (Fig. 10a) showed the continuous decrease in

concentration of myristic acid with time. The myristic acid

concentration [CA] was calculated by subtracting the con-

sumed concentration from the initial concentration of myristic

acid. A fast linear decrease in myristic acid concentration was

observed till 60 min. The reaction rate (n) was determined by

applying linear analysis to the plot of decreasing [CA] with

time by the equation, n = �d[CA]/dt. The initial rate was

12.9 mmol min�1. Turnover frequency (TOF, moles of fatty

acid converted per mole of acid site concentration per hour) at

100% conversion was 4.44 per hour.

The kinetic experiments with o10 min reaction time were

performed to have a view on apparent induction period of the

reaction. The kinetic profile in Fig. 10b shows the conversion

of myristic acid within 1 min of the reaction, a fast decrease in

[CA] between 1 and 2 min, which extended to 4 min and after

that it became slow. It is to be noted that while performing the

reaction, the catalyst was added after mixing the acid and

methanol by stirring at ambient temperature. This time was

taken as zero time. After that the reaction mixture was heated

at the desired temperature (60 1C), which was already main-

tained at 60 1C (to avoid the shooting of temperature (B4–5 1C)

beyond 60 1C that normally occurs during the heating of the

system). This was kept in mind because of low boiling point of

methanol (64–65 1C). Thus, the apparent induction period of the

studied reaction is just the heating of the system; it will be slightly

extended during the heating of the system to the desired tem-

perature. The initial reaction rate of this period was determined

to be significantly higher (38.8 mmol min�1).

2.6 Comparison of catalytic performance with other acids

Based on literature reports9,11–13,36 we have focused on SZ in the

present study. However, we have also carried out the reaction

with concentrated H2SO4 and heterogeneous ion exchange resin

Amberlyst-15 to have a comparable view on the catalytic activity.

The results (Fig. S7, ESIw) showed the activity in the order of

Fig. 10 Esterification of myristic acid with fresh methanol (a) conversion/concentration of myristic acid with time (b) apparent induction period.

Reaction conditions: acid :methanol = 1 : 10; SZ = 0.5 wt%

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H2SO4 4 SZ 4 Amberlyst-15. Concentrated H2SO4, used as a

benchmark catalyst, is not eco-friendly and cannot be readily

re-cycled. To overcome the disadvantages associated with

liquid acid catalysts, heterogeneous catalysts are the sustain-

able alternatives, though having lower catalytic performance.

The initial rate of esterification reaction was slow with SZ

catalysts (12.9 mmol min�1) as compared to conc. H2SO4

(14–15 mmol min�1), the final conversion after 2 h was in a

comparable range. These results clearly showed the potential

of SZ solid acid catalyst near to the benchmark catalyst.

Amberlyst-15 showed significantly lower activity (5–6 mmol min�1)

under the reaction conditions studied and was comparable with

esterification of oleic acid with methanol at 80 1C.38

A comparison of catalytic activity of SZ used in the present

study with reported studies of SZ or other solid acid catalysts

gave fairly good performance (716 mmol min�1 g�1) as com-

pared to reported SZ (125 mmol min�1 g�1) and SZ incorpo-

rated on SBA-15 (175 mmol min�1 g�1) for methyl palmate

and laurate formation at 68 1C.39 TOF of the present reaction

was also found to be higher (4.44 per hour) as compared to

SZ (1.9–2.5 per hour) and SZ incorporated on SBA-15

(2.6–3.4 per hour) for methyl palmate and laurate (calculated

from data given in ref. 39). Similarly, the activity was observed

to be higher (716 mmol min�1 g�1) as compared to other SZ

(150 mmol min�1 g�1) and sugar catalyst (478 mmol min�1 g�1)

for methyl oleate formation at 80 1C.38 The conversion rates

were also observed in a similar range for esterification of lauric

acid with 2-ethyl hexanol by the SZ (SSZr-1) catalyst, studied

at 170 1C with 10 wt% of catalyst.8 The authors reported

similar results for esterification of lauric and myristic acids

with iso-propanol. TOF values of the reported reactions may

be higher. A high reaction rate for esterification of myristic

acid with methanol was observed over SZ18 (75–85% conver-

sion in 5 min) at 22 bar in a Parr reactor at 120–170 1C. The

high temperature and pressure are important factors to signifi-

cantly enhance the reaction rates besides the catalyst properties.

Therefore, a proper comparison of catalytic activity, though

normalized with acid site concentration, can only be viable

when the reactions have been performed at comparable reaction

parameters such as temperature, catalyst amount and acid to

alcohol ratio9,18,40,41 along with the carbon chain length of

alcohol8,20,41 and carboxylic33/fatty acid.19,20

2.7 Structure–activity co-relation

The co-relation between acidic features in terms of total

surface acidity, Bronsted acidity, Bronsted acid site concen-

tration, B/L ratio and the activity of all three SZ catalysts

reveals that esterification of myristic acid with short chain

alcohols is mainly Bronsted acid catalyzed reaction. The

activity decreases with a decrease in Bronsted acidity in either

terms. Furthermore, a SZ catalyst with lower number of weak

and moderate acid sites is also able to demonstrate the

esterification activity similar to a SZ catalyst having compara-

tively higher number of strong acid sites; however, the latter is

requisite to retain the original activity of re-used SZ catalysts

along with a higher B/L ratio. The difference in the activity of

re-used catalysts clearly revealed that re-usability of SZ cata-

lysts strongly depends on the acidity of the catalysts.

3. Experimental

3.1 Materials

Zirconium n-propoxide (70 wt% in n-propanol) was procured

from Sigma-Aldrich, Germany; n-propanol (99.5%), n-butanol

(99.5%), aqueous ammonia solution (25%) and concentrated

sulfuric acid (98%) were procured from s.d. Fine chemicals,

India. Methanol (99%) was purchased from Ranbaxy, India

and Merck, India. Myristic acid (99.5%) and ethanol (99.9%)

were procured from Spectrochem Pvt. Ltd., India.

3.2 Catalyst synthesis and characterization

A sulfated zirconia catalyst was prepared using a two-step

sol-gel technique as described previously.26a In a typical

synthesis procedure, Zr(OPr)4 was hydrolyzed by aqueous

ammonia and the obtained gel was treated with H2SO4 after

oven drying at 120 1C for 12 h. The sulfated powder was

calcined at 600–700 1C for 4 h and designated SZ-T, where T

represents the calcination temperature.

PXRD patterns were obtained using a Philips X’pert

diffractometer. The crystallite size was determined from the

characteristic peak of tetragonal phase (2y = 30.22) using the

Scherrer equation,42 crystallite size = Kl/W cos y, where K is

the Scherrer constant (0.9), l = 1.5406 A (CuKa radiation),

W = Wb � Ws; Wb is the broadened profile width of the

experimental sample and Ws is the standard profile width of

the reference silicon sample and y is the angle of diffraction.

TEM micrographs were obtained using a JEOL JEM 2100

transmission electron microscope by dispersing the catalyst

sample in ethanol by sonication and deposited on a Cu grid

coated with carbon film. IR spectra were recorded on a Perkin

Elmer GX spectrophotometer. The bulk sulfur present before

and after calcination at different temperatures was analyzed by

ICP emission spectroscopy using Perkin-Elmer, Optima 2000

DV. BET surface area, pore volume and pore diameter were

calculated from nitrogen sorption isotherms after pre-activation

at 120 1C for 4 h at �196 1C on ASAP 2010, Micromeritics.

NH3-TPD was used to estimate the total surface acidity, i.e.

strength and number of acid sites present in the catalysts using

Micromeritics Pulse Chemisorb 2720 as described earlier.26a

Vapor phase cyclohexanol dehydration in a fixed bed reactor

was used as a model reaction to assess the Bronsted acidity of

the catalysts.26b Diffuse Reflectance FT-IR spectroscopy

(DRIFT) was used to differentiate between Bronsted and

Lewis acidity of pyridine adsorbed SZ samples using Perkin

Elmer GX equipped with DRIFT Selector accessory (Graseby

Specac, P/N 19900 series).25e The spectra were recorded at

room temperature (B27 1C) to 450 1C after holding at each

temperature for 10 min, thus allowing sufficient time for

pyridine desorption.

3.3 Catalytic activity

Esterification of myristic acid was carried out in a liquid phase

batch reactor.26b In a typical reaction procedure, required

amounts of acid, alcohol, and catalyst were taken in a round

bottom flask and the suspension was magnetically stirred

(600 rpm) in an oil bath maintained at constant temperature

(�1 1C) in the temperature range of 40–60 1C. The reactions

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This journal is c The Royal Society of Chemistry 2012 Catal. Sci. Technol.

were performed in duplicate with �2% variations in conver-

sion. The samples were withdrawn at regular intervals and the

reaction product was analyzed using a gas chromatograph (HP

6890) having a DB-225 capillary column (20 m length, 100 mmdiameter and 0.10 mm film thickness) and a FID detector.

4. Conclusions

The acidity and activity of a SZ catalyst for esterification of

myristic acid with methanol and other short chain alcohols are

strongly influenced by its calcination temperature along with

the reaction parameters mainly acid to alcohol ratio, catalyst

concentration and reaction temperature. Among the structural,

textural and acidic properties of the catalyst, the acidic

features play a major role in its activity and re-usability

behaviour for the studied reaction.

The present study reveals a very remarkable result that the

SZ-catalyst having higher number of Bronsted acid sites could

be re-used for five reaction cycles without significant loss in

activity for myristic acid esterification, whereas the SZ-catalyst

having less number of acid sites showed decrease in activity

after five reaction cycles. This explicitly confirms the require-

ment of more number of Bronsted acid sites for better

re-usability of SZ catalysts for esterification of myristic acid.

The present study concludes that the requirement of a very

small concentration of the catalyst yielding maximum conver-

sion and selective formation of the ester at lower temperature

within reasonable reaction time makes SZ an appealing

catalyst for the synthesis of FAAEs. In addition, eco-friendly,

recyclable SZ catalysts may find wide applications in reactions

where conc. H2SO4 is currently being used as a catalyst.

Acknowledgements

The authors are thankful to CSIR Network Programme on

Inorganic Materials for Diverse Application and to Analytical

Science discipline for catalyst characterization analysis. The

authors are also thankful to Dr Ram S. Shukla for helping in

initial reaction rate determination.

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