Citronellal cyclisation over heteropoly acid supported on modified montmorillonite catalyst: effects of acidity and pore structure on catalytic activity

Article abstract of DOI:10.1007/s11164-017-3237-4

Citronellal cyclisation to isopulegol is an important intermediate step in the production of menthol. Several heteropoly acids (PTA) supported on modified montmorillonite (MM) catalysts were synthesized and then tested in cyclisation reactions. The prepared samples were characterized by XRD, ICP-OES, FTIR, N₂ sorption, NH₃-TPD, pyridine adsorption, amine titration and FE-SEM techniques. Effects of post-treatment were studied on montmorillonite pore structure, acidity and catalytic activity. The catalytic activity and isopulegol selectivity improved with acid-treatment and PTA loading. The amount of Lewis acidity of montmorillonite was enhanced with acid-treatment and PTA impregnation. In cyclisation, highest catalytic activity (31.87?mmol?cat?g^?1?min^?1) was achieved with 96% isopulegol yield in the use of 20% PTA-MM catalyst. The highest catalytic activity and selectivity were obtained in the presence of higher acidity and strong Lewis acidic sites, whereas effects of pore structure blockage seemed minor. The catalytic activity further decreased with the loss of active acidic sites (L and B) due to PTA decomposition with calcination at a higher temperature.

Full text of DOI:10.1007/s11164-017-3237-4

Res Chem Intermed
https://doi.org/10.1007/s11164-017-3237-4
Citronellal cyclisation over heteropoly acid supported
on modifed montmorillonite catalyst: efects of acidity
and pore structure on catalytic activity
Abdul Karim Shah1,2 · Sungsoo Park1 · Hassnain Abbas Khan3 ·
Umair Hassan Bhatti4 · Praveen Kumar1 · Abdul Waheed Bhutto2 ·
Yeung Ho Park1
Received: 28 December 2016 / Accepted: 14 December 2017
© Springer Science+Business Media B.V., part of Springer Nature 2018
Abstract Citronellal cyclisation to isopulegol is an important intermediate step
in the production of menthol. Several heteropoly acids (PTA) supported on modi-
fed montmorillonite (MM) catalysts were synthesized and then tested in cyclisation
reactions. The prepared samples were characterized by XRD, ICP-OES, FTIR, N2
sorption, NH3-TPD, pyridine adsorption, amine titration and FE-SEM techniques.
Efects of post-treatment were studied on montmorillonite pore structure, acidity
and catalytic activity. The catalytic activity and isopulegol selectivity improved with
acid-treatment and PTA loading. The amount of Lewis acidity of montmorillonite
was enhanced with acid-treatment and PTA impregnation. In cyclisation, highest
catalytic activity (31.87 mmol cat g−1 min−1) was achieved with 96% isopulegol
yield in the use of 20% PTA-MM catalyst. The highest catalytic activity and selec-
tivity were obtained in the presence of higher acidity and strong Lewis acidic sites,
whereas efects of pore structure blockage seemed minor. The catalytic activity fur-
ther decreased with the loss of active acidic sites (L and B) due to PTA decomposi-
tion with calcination at a higher temperature.
Electronic supplementary material The online version of this article (https://doi.org/10.1007/
s11164-017-3237-4) contains supplementary material, which is available to authorized users.
*
Abdul Karim Shah
drakshah1214@gmail.com; karim@hanyang.ac.kr
1
2
3
4
Department of Fusion Chemical Engineering, Hanyang University, Sangnok-su, Ansan,
South Korea
Department of Chemical Engineering, Dawood University of Engineering and Technology,
Karachi, Sindh, Pakistan
Clean Energy and Chemical Engineering, University of Science and Technology, Daejeon,
South Korea
Green Energy Process Laboratory, Korea Institute of Energy Research, University of Science
and Technology, Daejeon, South Korea
1 3

A. K. Shah et al.
Keywords Acidity · Citronellal cyclisation · Heteropoly acids · Isopulegol ·
Montmorillonite · Pore structure
Abbreviations
SBET
Vtotal
Vmeso
Ds
Specifc surface area
Total pore volume
Mesopore volume
Stereo-selectivity of ( ) isopulegol
Montmorillonite
MT
MM
PTA
Acid-treated montmorillonite
Phosphotungstic acid
PTA-MM Heteropoly acid supported acid-treated montmorillonite
L.A
Lewis acid sites
B.A
Brønsted acid sites
BET
FTIR
Brunauer–Emmet–Teller
Fourier transforms infrared spectroscopy
ICP-OES Inductively coupled plasma optical emission spectrometry
XRD
X-ray difraction
Introduction
Citronellal and isopulegol are important precursors for the synthesis of menthol,
which has important applications in pharmaceuticals, cosmetics, fragrances and the
tobacco industry [1–3]. The acid catalyzed cyclisation of citronellal to isopulegol
(Scheme 1) is an important part of the synthetic route in the production of menthol.
Scheme 1 Cyclisation of ( ) citronellal to isopulegol, followed by the hydrogenation to menthols
1 3

Citronellal cyclisation over heteropoly acid supported on…
Diferent stereoisomers of isopulegol such as ( )-isopulegol, ( )-neo-isopulegol,
( )-iso-isopulegol, and ( )-neoiso-isopulegol (Scheme 1) are formed during cycli-
sation of citronellal. Further hydrogenation of these isopulegol isomers lead to
four pairs of enantiomers: ( )-menthol, ( )-neo-menthol, ( )-iso-menthol, and
( )-neoiso-menthol. Among these isomers, (−)-menthol has a peppermint odor and
exerts a cooling efect.
The cyclisation of citronellal has already been studied over various homogeneous
and heterogeneous catalysts. In previously published reports, diferent homogeneous
catalysts such as scandium trifuoromethane sulfonate, Lewis acids such as ZnCl2,
ZnBr2, and ZnI2, and molybdenum and tungsten complexes were found to be active
in the reaction. Among these catalysts, ZnBr2 showed a high selectivity for (−)-isop-
ulegol, 94%, and has been used at industrial scale [1, 4]. Similarly, scandium trif-
luoromethane sulfonate was found to be an efcient catalyst for selective cyclisation
of citronellal to isopulegol under sub-ambient temperatures. At – 78 °C, a yield of
isopulegol of about 95% was obtained. In contrast, the yield at room temperature
was only 58% because of the consecutive reaction of the formed isopulegol with cit-
ronellal [5]. Recently, Takasago International Corporation obtained 100% selectiv-
ity for isopulegol over substituted tris(2,6-diarylphenoxy)aluminum catalyst, but, the
bulky ligand group resulted in decreases in isopulegol selectivity and formed other
isomers of isopulegol. While homogenous catalysts ofer advantages, there are some
drawbacks such as difculty in catalyst recovery, toxicity, weak stability and dis-
posal problems. On the basis of homogeneous catalyst problems, a number of solid
acid heterogeneous catalysts (Lewis or Brønsted acidic in nature) have been applied
in this reaction for a getting higher selectivity of isopulegol [12]. These include
CMS [6], hydrous zirconia [1], zeolites and mesoporous materials [7], solid Lewis
acids [8], HPW-SiO2 [9], inorganic fuorides [10] and HPW-MCM-41 [11], MCM-
22, mordenite, alumina, mixed oxides of SiO2–TiO2, SiO2–ZrO2 and SiO2–Al2O3
and metal cation-exchanged montmorillonite, but some of these seem less active and
selective in this reaction [10, 13]. In cyclisation, catalytic activity and isopulegol
selectivity appear to be more dependent on Lewis or Brønsted acidity of the cata-
lyst. Cyclisation has also been carried out over SiO2 at 25 °C with 83% yield in 2 h
[14]. A previous study suggested that Lewis acidic catalysts are more efcient in the
synthesis of isopulegol, but a huge quantity of catalyst was used. Fuentes et al. [15]
used diferent zeolites in cyclisation and found a correlation of activity with strength
and pore size of Brønsted acid sites. Zeolites of small pore size resulted in low activ-
ity, whereas, zeolites of larger pore size caused a decrease in selectivity because of
more access to Brønsted acid sites, which facilitates more dehydration and cracking
of isopulegol. However, Zr-exchanged montmorillonite was found to be more ef-
cient in citronellal cyclisation and gave 91% yield within 24 h with less formation of
side products [16]. In addition, pore structure, acidity and nature of acidic sites have
a key efect on catalytic activity and selectivity in cyclisation. Still, the nature of
the acidic sites have remained unclear, as the activity and selectivity have been cor-
related either with Lewis or Brønsted acidity. It might be worthwhile to utilize more
acidic catalysts such as a heteropoly acid (HPA). HPAs have Keggin structures,
strong redox nature and super acidic properties [13, 17–21]. For this investigation,
some catalysts of diferent acidic natures were synthesized using heteropoly acids.
1 3

A. K. Shah et al.
Phosphotungstic acid (PTA) is the most stable among all HPAs and is commonly
used for acid catalysis, since it possesses the highest Brønsted acidity [22–24]. In
homogenous liquid phase catalysis, the advantages of the HPAs are unique due to
their volatility, high acidity, low corrosiveness, and fexibility [25]. Their industrial
applications, however, have been limited due to their low surface area (1–10 m2 g−1)
and recovery problems [13, 26–28].
Montmorillonite clays, micro-layered silicates, can be used as a diferent support
for HPA since they are used as inorganic supports for reagents or catalysts in numer-
ous organic reactions [29–33]. Montmorillonite has been modifed with diferent
metals and applied to diferent reactions [18, 34–42]. It might be worthwhile to
treat montmorillonite with acid to remove extraneous materials from the inside and
improve mass transfer and acidity. Therefore, fne and uniform dispersion of PTA on
acid-treated montmorillonite will increase acidity and catalytic activity. From the
literature, we found that HPAs supported on acid-treated montmorillonite have been
applied to esterifcation of acetic acid with sec-butanol, esterifcation of acetic acid
with butanol [43] and dehydration of dilute bio-ethanol [44]. This type of catalytic
system, however, has not yet been utilized in the citronellal cyclisation reaction. The
novelty of this work is to resolve mass transfer limitations and recovery problems of
montmorillonite and heteropoly acid, respectively, through acid-treatment and PTA
impregnation.
In this research work, we tried to investigate major changes in pore structural and
acidic (acidity strength and Lewis/Brønsted acid sites) characteristics of montmoril-
lonite after acid-treatment and heteropoly acid impregnation, and we further investi-
gate their correlation with catalytic activity and selectivity in citronellal cyclisation.
In this regard, we prepared heterogeneous acid catalysts through acid-treatment of
montmorillonite, HPA impregnation, and calcination temperature. The active and
selective catalyst was prepared especially for cyclisation reaction. In comparison
with our prepared catalysts, commercially available catalysts such as Al2O3, USY,
H-MCM-41, and ZSM-5 have been tested analytically and compared.
Experimental
Materials and methods
Phosphotungstic acid hydrate, one of the HPAs (PTA, H3PW12O40·nH2O), montmo-
rillonite (K10), alumina neutral, pyridine anhydrous (99.8%), ( ) citronellal (≥ 95%
pure) and hydrochloric acid solution (4 M HCl in H2O) were supplied by Merck.
Benzene, nitrobenzene (99.5%) and methanol (99.8%) were purchased from Dae-
Jung Chemicals and used as received without further purifcation. For comparative
study, USY (CBV 780, Si/Al = 80) and ZSM-5 (CBV-5524G with Si/Al = 50) were
purchased from Zeolyst International USA. MCM-41 (Si/Al = 5) was purchased
from Nankai Plant, China.
1 3

Citronellal cyclisation over heteropoly acid supported on…
Catalyst preparation
A series of catalysts having 5, 10, 20 and 30% loadings of PTA on the acid-treated
support were synthesized in three steps: (1) the preparation of the support by an
acid treatment of montmorillonite, (2) impregnation of PTA on the support and
(3) thermal treatment of the catalyst. The original montmorillonite (which will be
designated as ‘MT’) was treated with 4 M HCl (20 mL g−1) and heated to 80 °C
for 2 h under refux conditions. The acid-treated montmorillonite was then washed
and neutralized with distilled water, dried overnight and calcined at 200 °C for 2 h
(the result of which will be designated as ‘MM’). The diferent loadings (5–30%) of
heteropoly acids (PTA) were then impregnated on the acid-treated montmorillonite
‘MM’ in methanol by wet impregnation [13, 44]. The resulting solids were evapo-
rated and dried at 60 °C for 12 h. (These catalysts were labeled as x%-PTA-MM
catalyst, where x is the PTA loading amount). Some of the prepared catalysts were
further calcined at other high temperatures (200–750 °C) in order to determine their
catalytic behavior. These catalysts were designated as ‘wt%PTA-MM-y’, whereas; y
shows calcination temperature.
Catalyst characterization
XRD patterns of all samples were collected through Powdered X-ray difraction (D/
Max 2200 Rigaku) using Cu Kα radiation at a fxed accelerating voltage (40 kV) and
current (100 mA) within the 2θ range of 5°–60°. ICP-OES was used to determine
the elemental composition of samples with HPA deposition amount using an Optima
8000. For elemental analysis, a 0.1 g quantity of each sample was dissolved in 1 mL
of 36% HCl, 1 mL of 70% HNO3, 0.5 mL of 60% HF and 4 mL of H2O. After
being complete dissolved, HF was neutralized with 2 g of boric acid. Afterwards,
this digested mixture was further diluted with water to get the desired concentra-
tion (1 ppm) [45]. The surface area, pore volume, and pore size distribution were
measured by nitrogen adsorption using a Micrometrics Tri-Star 3020. Before the
measurement, all samples were degassed at 200 °C for 3 h under vacuum conditions,
and micropore volume of all samples was determined by the t-plot method. For fnd-
ing changes in surface morphology of samples, SEM images were collected through
a feld emission scanning electron microscope FE-SEM, MIRA-3 Tescan, South
Korea. Before this analysis, all samples were prepared on graphite carrier and then
coated with a 3 mm layer of platinum. During SEM analysis, the SEM–EDX charac-
terization was done through FE-SEM, MIRA-3, equipped with EDX-micro analyzer
EX-350. The acidity of samples was measured by NH3-Temperature-Programmed
Desorption (TPD) using an Auto-Chem 2920, Micrometrics, USA. Adsorption
of NH3 was performed at 50 °C temperature with a fow rate of 20 cm3 min−1 for
45 min, followed by helium purging for 1.5 h at room temperature in order to remove
the physisorbed NH3. The desorption process was recorded in the temperature range
of 50–900 °C at a heating rate of 10 °C min−1 under a helium fow (20 cm3 min−1),
and the evolved NH3 was monitored by TCD. In comparison with NH3-TPD data,
the acidity amount of samples was determined through the amine titration technique
1 3

A. K. Shah et al.
as described in procedure [46] using 0.1 N butyl amine (in dry benzene) solution as
a weak base and dimethyl yellow as an indicator. Each catalyst sample (0.1 g) was
dissolved in 9 mL dry benzene and 2–3 drops of indicator were added in the cata-
lyst solution. The dissolved catalyst solution was titrated with 0.1 N n-butyl amine
solution until the color changed from pink to yellow. The solution was continuously
stirred for 4 h and titrated again with butyl amine to achive complete saturation.
Total volume (mL) of the consumed titrant was recorded, and the acidity amount
was calculated. For the measurement of acidic sites of samples, a KBr mixed FTIR
pellet (55 mg) was prepared using a hydraulic pressing machine (11 tons pressure,
3 min). The pellet was fxed inside a stainless steel IR cell and dehydrated at 300 °C
for 2 h under a vacuum system using 5 °C min−1 temperature ramp. The IR cell
was then connected with the FTIR equipment in order to collect a visible spectrum
of the sample using 600–4000 cm−1, 8 cm−1 optical resolution and co-addition of
32 scans through FTIR equipment. Next, pyridine vapors were introduced into the
IR cell until pyridine pressure became 10 kPa. Pyridine adsorption was performed
at 100 °C for 2 h. After complete pyridine adsorption, physisorbed pyridine was
removed at 150 °C under vacuum conditions. FTIR apparatus was connected for
measurement of the IR spectrum under similar conditions [47]. The quantitative
amount of acidic sites was calculated using the following formula:
C(L) = 1.42 ∗ IA(LA) ∗ R2∕Wcat
,
C(B) = 1.88 ∗ IA(BA) ∗ R2∕Wcat
where C(L) and C(B) show concentration of Lewis and Brønsted acid sites, respec-
tively, and IA (L) and IA (B) indicate integrated surface areas of Lewis and Brøn-
sted acid sites peaks (1440 and 1540 cm−1), respectively. R is the radius of the pellet
and Wcat is the total amount of catalyst which was used in the preparation of the KBr
pellet.
Catalytic cyclisation of citronellal
The cyclisation of the citronellal reaction was carried out in a glass reactor consist-
ing of a 10 mL glass vial, a rubber septum, a magnet and a plastic cap. A specifc
amount of chemicals, such as 4.5 mmol of ( ) citronellal (≥ 95%), 0.2 mL nitroben-
zene (internal standard), 5 mL benzene solvent and 5.5 mg catalyst, were poured
into glass vial and the cap tightened. Before the start of the reaction, a silicon oil
bath system was heated and its temperature was maintained up to 40 °C. All glass
reactors were fxed inside the heating bath. The reaction proceeded for 1 h and reac-
tion sampling was done at diferent times (3–60 min) in order to analyze the cata-
lytic conversion of citronellal. GC analysis of reaction samples was done by chiral
column (Cyclodex-B, length 60 m, diameter 0.254 mm and flm thickness 0.25 µm).
Helium was used as the carrier gas. The products were separated by the following
temperature program: 95 °C (1 min), 0.3 °C min−1, 120 °C (1 min), 15 °C min−1 and
220 °C (1 min) with a split ratio of 100:1. The detector and injector temperatures
1 3

Citronellal cyclisation over heteropoly acid supported on…
were 280 and 250 °C, respectively. The reaction products and isomers were further
identifed by GC–MS with a triple axis detector (Agilent, Model No. 7890A) and
GC chemical standards.
Results and discussion
Textural and acidic properties
During acid-treatment of montmorillonite (MT), some amount of Al was removed
from the montmorillonite framework in accordance with ICP-OES data, and then
more siliceous support (MM) was prepared (Table 1), which resulted in more
enhancements in mesopore surface area. This enhancement in support might be
related to the removal of some of the aluminum from octahedral sites of montmoril-
lonite [48, 49]. The specifc amount of PTA was doped on ‘MM’ support as well,
but less than actual loading (Table 1). The XRD crystalline structure of ‘MT’ was
maintained even after acid-treatment and PTA impregnation, and minor changes in
XRD pattern were seen (Fig. 1). Some XRD peaks of heteropoly acid (2θ = 10°,
25°, 37° and 60°) were seen on the ‘MM’ surface after PTA deposition, but intensi-
ties of PTA were rather low, which might be due to high dispersion of PTA over the
support (Fig. 1).
FTIR analysis
In the FTIR spectra, bulk PTA shows the characteristic IR bands at 1071 cm−1
(P–O in central tetrahedral), 975 cm−1 (terminal W = O), 813 cm−1 and 900 cm−1
(W–O–W) associated with the asymmetric vibrations of the Keggin polyanions
(Fig. 2). After PTA was deposited on the MM support, some of the characteristic
Keggin bands were observed at 812, 900 and 986 cm−1 , as well as other bands,
merged with those of montmorillonite, producing a single broad IR peak (Fig. 2).
The changes in IR spectrum of ‘MM’ indicate successful deposition of PTA on the
‘MM’ surface.
BET analysis
The mesopore surface area (164–188 m2 g−1), mesopore volume (0.24–0.29 cm3 g−1)
and mesopore size (3.8–5.5 nm) of ‘MT’ increased after acid-treatment (Table 1).
Signifcant changes in the pore structure of the ‘MT’ were seen, which might be
due to Al removal from the main framework of montmorillonite (Fig. 3). However,
the increase in PTA loading on the support (MM) caused a decrease in the specifc
mesopore surface area (188–88 m2 g−1) and mesopore volume (0.29–0.17 cm3 g−1),
respectively. High mesopore surface area of the support (MM) might be related to
the removal of some of the aluminum from octahedral sites of montmorillonite. The
mesopore surface area and mesopore volume decreased with PTA loading, which
may be due to the blockage of pores by PTA molecules. The pore size of ‘MT’
1 3

A. K. Shah et al.
1 3

Citronellal cyclisation over heteropoly acid supported on…
Fig. 1 XRD patterns of a MT, b
MM and c PTA-MM samples
PTA
Quartz
(c)
(b)
(a)
10
20
30
40
50
60
70
2-theta (deg)
1600
1500
(e)
(d)
(c)
3616
OH
812
1400
900
1300
(b)
(a)
986
1200
1100
975
1071
900
813
1000
900
400
800 1200 1600 2000 2400 2800 3200 3600
Wavenumber (cm-1)
Fig. 2 FTIR spectra of a pure PTA, b MM, c 10% PTA-MM, d 20% PTA-MM and e 30% PTA-MM.
This graph shows deposition of Keggin structure components (IR bands at 800, 894, 980 and 1080 cm−1
on the acid-treated montmorillonite surface
)
improved with acid treatment and PTA loading (Table 1). The pore size distribution
(BJH) of all samples show that average pore sizes of ‘MT’ and other samples are in
the range of 3.8–7.6 nm, suggesting that catalysts lie in the mesoporous region in
the presence of micropores (Table 1). The adsorption–desorption isotherms of MM
and supported PTA catalysts, shown in Fig. 3, have a hysteresis loop of isotherms
type I and IV, which shows the characteristic of the micro-mesoporous material. In
addition, these results are in agreement with adsorption data which shows that the
pore volume, as well as mesopore volume, of the ‘MT’ was enhanced with acid-
treatment, but lessened with PTA loading.
1 3

A. K. Shah et al.
Fig. 3 N2-adsorption desorp-
tion isotherms of prepared
samples
MT
MM
10% PTA-MM
20% PTA-MM
30% PTA-MM
200
150
100
50
0
0.0
0.2
0.4
0.6
0.8
1.0
Relative pressure p/po)
SEM analysis
In Fig. 4, FE-SEM images indicate that montmorillonite is a layered clay consisting
of octahedral aluminate modules and tetrahedral silicate modules [1]. After success-
ful impregnation of PTA, the surface morphology of MM changed and layers were
deteriorated; no more thin and long layers were seen. Furthermore, FE-SEM–EDX
shows the elemental composition of MT, which contains Si, Al and Mg, whereas the
intensity of Si and Al decreased after acid treatment as shown in Online Resource 2.
The acidity strength of samples was determined by NH3-TPD and amine titration
techniques, respectively, as shown in Table 1. The original montmorillonite ‘MT’
contains a specifc amount of acidity. Further acidity strength of ‘MT’ was enhanced
(0.31–1.40 µmol g−1 cat) with acid treatment and PTA loading. However, simi-
lar observations were seen in acidity enhancement as compared to amine titration
Fig. 4 SEM images of a MM, b 20%PTA-MM samples
1 3

Citronellal cyclisation over heteropoly acid supported on…
analysis data (Table 1). The analysis revealed that acid-treatment and PTA impreg-
nation treatments were found to be more efective in enhancing acidity strength of
‘MT’.
Pyridine analysis
Pyridine adsorption spectroscopy was used to determine the nature and strength of
acidic sites [50], as shown in Table 1 and Fig. 5. Bands in the range of 1440–1454,
1480–1500 and 1540–1640 cm−1 indicate the presence of both hydrogen-bonded
and co-coordinately bonded pyridine, respectively. ‘MT’ contains medium Lewis
and weak Brønsted acidic sites. After acid treatment of MT, the number of Lewis
acid sites increased with a decrease in the number of Brønsted acid sites. However,
the number of Lewis acid sites further increased with increases in PTA amount.
After PTA loading, an increase in acidity was seen, designated by a slight shift of
the bands to high wavenumbers and an increase in the absorbance of the 1485 and
1540 cm−1 band. In addition, a weak band centered at 1540 cm−1is suggestive of
Brønsted acidic sites for PTA supported samples. However, with a further increase
in PTA loading above 20%, the number of Brønsted acid sites increased with a
decrease in Lewis acid sites (Table 1; Fig. 5).
Similarly, the number of Lewis and Brønsted acid sites sharply decreased with
the use of higher calcination temperature (Table 3). Similarly, the peaks of water
adsorbed (1640 cm−1) and hydroxyl groups (3640 cm−1) increased with PTA load-
ing (Fig. 2), and further decreased with increase in calcination temperature (Online
Resource 7). This might be due to dehydration or dehydroxylation of the catalyst.
Furthermore, total acidity amount (1.32–0.153 mmol g−1 cat) of 20% PTA-MM
decreased with an increase in calcination temperature due to loss of PTA molecules
(Table 3 and Online Resource 6). The mesopore surface area (145–49 m2 g−1) of
20% PTA-MM catalysts rigorously decreased with increases in pore size (Table 3).
These data are in agreement with XRD and FTIR spectra, which show some signs
Fig. 5 Pyridine adsorbed IR
L.A
spectra of a MT, b MM, c 10%
PTA-MM, d 20% PTA-MM and
e 30% PTA-MM catalysts
L.A
B+L
B.A
B.A
(e)
(d)
(c)
(b)
(a)
1400
1450
1500
1550
1600
1650
Wavenumber ( )
cm-1
1 3

A. K. Shah et al.
of PTA decomposition. More data about calcination temperature efects on textural,
acidic and catalytic properties are provided in Online Resources 5–8. The detailed
experimental observation reveals that high calcination temperature is not suitable for
a getting highly active and selective catalyst for the cyclisation reaction.
Catalytic activity
Table 2 shows the conversion of ( ) citronellal and the selectivity towards isop-
ulegol over the prepared samples. The catalyst amount was found to be within 0.8%
for the reaction. The diferent isomers were recognized by GC grade chemicals
and GC-MSD. All the heteropoly acid supported catalysts gave ( )-isopulegol as
the predominant isomer, followed by ( )-neo-isopulegol, ( )-iso-isopulegol, and
( )-neoiso-isopulegol (Table 2).
The catalytic citronellal cyclisation reaction was conducted in glass type reactors,
and reaction performance of several catalysts was evaluated. In the frst instance,
phosphotungstic acid hydrate (PTA) was tested, which gave 41% conversion with
53% selectivity within 1 h. It was highly soluble in benzene solvent and its recov-
ery seemed difcult. The montmorillonite (MT) sample exhibited catalytic char-
acteristics that resulted in 54% citronellal conversion with 70% isopulegol produc-
tion within 1 h. Further, the catalytic activity (72.4%) and selectivity (81%) were
enhanced drastically when the MT sample was treated with HCl (Table 2). The poor
catalytic performance of the MT sample might be related to the narrow pore struc-
ture and weak acid sites (Lewis or Brønsted) (Table 1). It has been observed that
pore structure and Lewis acid site strength of the MT sample was improved after
Table 2 Results of citronellal cyclization on various PTA supported catalysts
Catalyst type
Catalytic
Conversionb (%) Yieldc (%) Selectivity (%) Isomers
Ds (%)
activitya
distribution
(mmol g−1
,
(%)d
min−1
)
2a 2b 2c 2a
PTA
MT
MM
6.8
7.6
9.99
41
21.7
37.8
58.6
84.7
89.6
95.5
94
53
70
81
93
94
96
94
35 13
52 14
64 12
75 15
75 16
78 15
77 16
5
4
5
3
3
2
1
66
74
79
81
80
82
82
54.12
72.4
91.1
95.3
100
100
5% PTA-MM 13.13
10% PTA-MM 17.38
20% PTA-MM 31.87
30% PTA-MM 29.6
aDetermined by total moles of citronellal * total conversion * molecular weight of citronellal divided by
100 * catalyst amount * reaction time
bReaction conditions: 4.275 mmol ( ) citronellal, 5 mL C6H6, 0.2 mL C6H5NO2, 5.5 mg catalyst, 40 °C,
1 h
cYield of isopulegol was calculated by total conversion multiplied by overall isopulegol selectivity (2a–
2d) divided by 100
d( ) isopulegol (2a): ( ) neo-isopulegol (2b), ( ) iso-isopulegol (2c)
1 3

Citronellal cyclisation over heteropoly acid supported on…
acid treatment, resulting in enhancement in catalytic activity and selectivity. When
PTA was impregnated on the MM support, higher catalytic activity and selectivity
were achieved than in MM and MT samples. The 5–30% PTA-MM catalysts were
very active with 91–100% conversion of citronellal, and their initial catalytic activ-
ity was about 13.13–31.87 mmol cat g−1 min−1. Furthermore, the selectivity over
these catalysts was very high, 93–96%, with only a small fraction of the isopulegol
undergoing successive dimerization to ethers. The initial reaction rate of these cata-
lysts was very high up to 3 min, then their catalytic activity rate decreased with reac-
tion time (Fig. 6).
However, when a PTA amount was impregnated on MM, the amount of Lewis
acidity increased, leading to high catalytic activity and selectivity. Among these
samples, 20% PTA-MM catalyst was found to be a more active and selective catalyst
for citronellal cyclisation (100% conversion and 96% selectivity) (Table 2).
The 20% PTA supported MM catalyst retained high selectivity and catalytic activ-
ity, which could be due to the micro-mesoporous nature of catalyst and the presence
of strong Lewis acidity combined with weak Brønsted acidity [1]. The incorpora-
tion of PTA on the MM surface changes its acidic property. Indeed, when MT was
modifed through acid-treatment and PTA deposition, the resulting material, 20%
PTA-MM, was catalytically active for the cyclisation reaction (Table 2), even though
it possessed low mesopore surface area. This catalyst prepared by PTA impregnation
under normal drying (60 °C) was highly active in the synthesis of isopulegol. The
increase in Lewis acidity was a key factor for enhancing catalytic activity and selec-
tivity. When 30% PTA was impregnated on the MM support, the catalytic activ-
ity (29.6 mmol cat g−1 min−1) and selectivity (94%) decreased slightly compared to
the 20% PTA-MM sample, because ofa decrease in mesopore surface area and an
increase in Brønsted acidity. About 6% of by-products were formed, which might be
due to dehydration and cracking of isopulegol.
After fnding optimum loading of PTA, 20% PTA-MM catalyst was found to be
more active and selective for cyclisation, so this catalyst was calcined at diferent
calcination temperatures (200–750 °C) in order to determine the catalytic activity
behavior with respect to their acidic properties (Table 3). The catalyst which was
Fig. 6 Reaction performance of
solid acid catalysts in citronel-
lal cyclisation reaction. This
reaction was performed at 40 °C
using 0.7 g citronellal, 5 mL
Benzene and 5.5 mg catalyst
100
80
60
40
20
0
MT
MM
10% PTA-MM
20% PTA-MM
30% PTA-MM
0
5
10 15 20 25 30 35 40 45 50 55 60 65
Reaction time (min)
1 3

A. K. Shah et al.
1 3

Citronellal cyclisation over heteropoly acid supported on…
calcined at 200 °C showed a good catalytic activity (29.2 mmol cat g−1 min−1)
and selectivity (95%), but less than that of 20% PTA-MM that was dried at 60 °C.
This decreasing efect was due to a slight decrease in mesopore surface area, acid-
ity, Lewis and Brønsted acid sites, because of dehydration. This decreasing trend
seemed proportional with an increase of calcination temperature up to 750 °C. How-
ever, the catalytic activity and selectivity further decreased with increases in calci-
nation temperature because of dehydroxylation and PTA decomposition [51]. More
loss of mesopore surface area, acidity or acidic sites results in greater decreases in
catalytic activity and selectivity.
Experimental data suggest that high acidity strength, strong Lewis and weak
Brønsted acid sites can be considered to be the main tools for a achieving higher
catalytic activity and selectivity. However, acidity strength of MT and Lewis acid
site intensity increased after acid treatment and PTA impregnation. The catalytic
activity and selectivity of MT was enhanced (7.6–31.87 mmol g−1 min−1) with more
amelioration in acidity strength (0.31–1.35 µmol g−1 cat). The acidity strength and
quantitative number of acid sites (L or B) have a direct relation to each other in this
reaction. Strong Lewis acid sites are considered to be efective for getting high isop-
ulegol selectivity in cyclisation [1]. Experimental data reveal that enhancement in
surface area and pore volume results in a positive contribution to catalytic activity,
whereas the severe blockage of pore structure of the catalyst has a negative impact
on catalytic performance in the cyclisation reaction. The linear connectivity rela-
tionship of catalytic activity with acidity strength, Lewis/Brønsted acid sites ratio
and mesopore volume are shown in Fig. 7.
In order to check the reusability of the optimum catalyst (20% PTA-MM) after
reaction, it was fltered with a membrane flter, further washed with 50 mL benzene
solvent and introduced into the reactor with fresh reaction mixture. Citronellal con-
version was 100% in the frst run, then decreased to 95% in the second run, further
3.5
Catalytic activity*10
3.0
2.5
2.0
1.5
1.0
0.5
0.0
Acidity
L/B ratio
Pore Volume
MT
MM
10%PTA-MM 20%PTA-MM 30%PTA-MM
Catalysts
Fig. 7 Linear connectivity relationship of catalytic activity with acidity strength, L/B ratio and pore vol-
ume characteristics
1 3

A. K. Shah et al.
Fig. 8 Recyclability test of 20% PTA-MM catalyst in citronellal cyclisation
Table 4 Comparative study of PTA-MM catalyst with other solid acid catalysts
Catalyst
Conversiona (%) Selectivity (%) Acidity (mmol g−1
Amine titrationb NH3-TPDc
)
Smesod (m2 g−1
)
Al2O3
7.0
46
95
88
90
96
0
0
107
1137
135
47
MCM-41 (Si/Al = 5) 68
USY (Si/Al = 80) 78
ZSM-5 (Si/Al = 50) 25
20% PTA-MM 100
0.16
0.44
0.65
1.32
0.65
1.14
1.25
1.35
145
aReaction conditions: 4.275 mmol ( )citronellal, 5 mL C6H6, 0.2 mL C6H5NO2, 5.5 mg catalyst, T
40 °C, 30 min
bDetermined by amine titration technique
cDetermined by NH3-TPD technique
dDetermined by BJH method
decreased to 89% in the third run and fnally decreased up to 80% in the ffth run
(Fig. 8). It seems that the activity of the catalyst decreased only slightly with the
number of cycles, whereas the loss of catalyst during fltration was under observa-
tion. Little variation was seen in isopulegol production during the recyclability test.
Our investigation was extended in order to determine the individual roles of
Lewis and Brønsted acid sites in cyclisation. An Al2O3 sample was used in citron-
ellal cyclisation in order to evaluate its catalytic performance. However, it showed
negligible catalytic activity in citronellal cyclisation (7% conversion and 46% selec-
tivity within 1 h). This poor performance might be related mainly to its weak acid-
ity (Table 4). Furthermore, the pyridine IR studies show that only weak Lewis acid
sites were present on the alumina surface. However, individual Lewis acid sites
seemed inefective for a complete cyclisation and higher isopulegol yield forma-
tion. For comparison, a mesoporous siliceous material such as MCM-41 was used
1 3

Citronellal cyclisation over heteropoly acid supported on…
in cyclisation in order to determine the efect of mesoporosity on cyclisation. It gave
68% conversion and 95% isopulegol selectivity within 1 h; so this good performance
might be related to the presence of medium Lewis acid sites (acidity) with weaker
Brønsted sites, as can be seen clearly in Table 4 and Fig. 9. From reaction data, it is
clear that higher mesopore surface area (1137 m2 g−1) had no direct efect on cata-
lytic activity as compared with other samples. We investigated further to determine
the efects of strong Brønsted acidic sites through pyridine adsorption on catalytic
activity and isopulegol selectivity. Ultra stable zeolite Y (USY) and ZSM-5 zeolites
of diferent pore structured materials were used in this reaction. The USY showed
good catalytic activity (conv. 78%) and selectivity (88%) when compared with the
ZSM-5 catalyst (conv. 25% and selectivity 90%), because USY contains medium
intensity of Lewis sites with a higher mesopore surface area (135 m2 g−1) as com-
pared to ZSM-5 (47 m2 g−1). In the use of USY catalyst, about 12% of the total isop-
ulegol amount was cracked or dehydrated into other by-products, due to the presence
of strong Brønsted acid sites. ZSM-5 showed poorer catalytic activity than USY,
even though it contained a higher amount of acidity. So, this poor performance of
ZSM-5 might be due to the presence of weak Lewis and strong Brønsted acid sites
and a small mesopore surface area. Experimental data and previous work [1] suggest
that citronellal cyclisation to isopulegol occurs through protonation–deprotonation
mechanism with the help of strong Lewis and weak Brønsted acid sites. When cit-
ronellal molecule binds with Lewis acid site via the aldehyde oxygen. This brings
the citronellal into an orientation favourable for ring closure through an intramolec-
ular carbonyl-ene (C–C) reaction. In the transition state, protonation of the oxygen
occurs via a neighbouring Brønsted hydroxyl group, together with an abstraction of
hydrogen from the isopropyl group followed by ring closure to form isopulegol.
In comparison, PTA-MM catalyst was found to be more active and selective
than other catalysts, because of high acidity strength, strong Lewis and weak Brøn-
sted acid sites. This study suggests that mesopore surface area and pore size might
have negligible efects in the cyclisation reaction, whereas high acidity strength
and strong Lewis acid sites in the presence of weak Brønsted acid sites have major
Fig. 9 Pyridine adsorbed IR
(e)
spectra of a PTA-MM, b ZSM-
5, c USY, d MCM-41 and e
Al2O3
(d)
(c)
(b)
(a)
1400
1450
1500
1550
1600
1650
Wavenumber (cm-1)
1 3

A. K. Shah et al.
efects on catalytic activity and selectivity. However, the catalytic activity increased
with an increase in the number of Lewis acid sites, but decreased with an increase
in the number of Brønsted acidic sites with loss of Lewis acid sites. Furthermore,
strong Lewis and weak Brønsted acid sites are necessary to promote efcient cit-
ronellal cyclisation. In addition, the mere presence of Lewis acid sites on catalyst
surface seemed insufcient for achieving complete conversion of citronellal to isop-
ulegol in cyclisation. Continuing this reaction for a long time, some by-products
were formed. This side product formation was possible through dehydration, etheri-
fcation or cracking of isopulegol over strong acidic sites of the catalyst.
Conclusion
In this research, efcient solid acid catalysts were prepared through acid-treatment
of montmorillonite (MT) and impregnation of a heteropoly acid, H3PW12O40 (PTA)
and then were applied in citronellal cyclisation for isopulegol synthesis. The cat-
alysts were characterized by XRD, ICP-OES, FTIR, N2 sorption, NH3-TPD, pyri-
dine adsorption, amine titration and FE-SEM techniques. After acid-treatment of
montmorillonite, the catalytic activity and selectivity improved due to increases in
Lewis acidity and mesopore surface area. The amount of acidity, Lewis, and Brøn-
sted acidic sites was enhanced with an increase in PTA loading. The catalytic activ-
ity and isopulegol selectivity signifcantly improved with an increase in acidity and
Lewis acidic sites (with PTA loading), whereas the efect of pore structure block-
age in citronellal conversion seemed minor. In the cyclisation reaction, 20% PTA
loading on modifed montmorillonite was found to be the optimum for achieving
higher catalytic activity in citronellal cyclisation (100% conversion, 96% yield, and
initial catalytic activity 31.87 mmol cat mg−1 min−1). In addition, the highest isop-
ulegol selectivity (96%) was achieved in to the presence of strong Lewis and weak
Brønsted acid sites on the catalyst surface. The catalytic activity decreased with 30%
PTA loading on montmorillonite due to an increase in the number of Brønsted acidic
sites and blockage of pore structure. The total acidity and mesopore surface area
decreased with increases in calcination temperature (200–750 °C), which seemed to
be due to PTA decomposition. Thermal treatment at 60 °C was found to be the best
condition for the preparation of an active and selective catalyst after post-treatment
of MT.
Acknowledgements This research was conducted through a mutual collaboration of Higher Education
Commission Pakistan and Hanyang University, South Korea.
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