ZrO2-SiO2 mixed oxides xerogel and aerogel as solid acid catalysts for solvent free isomerization of α-pinene and dehydration of 4-methyl-2-pentanol

Article abstract of DOI:10.1007/s10562-011-0602-6

Sulfated and non-sulfated ZrO₂-SiO₂ mixed oxide xerogel and aerogel samples having varied Zr/Si molar ratio were evaluated as solid acid catalysts for the isomerization of α-pinene and dehydration of 4-methyl-2-pentanol. Sulfation resulted into enhancement in the catalytic activity of both xerogel and aerogel samples towards the studied reactions. For example, sulfated catalysts showed 86-98% conversion of α-pinene and 8-35% conversion of 4-methyl-2-pentanol. The selectivity data for camphene and limonene indicated the requirement of moderate acidity. The correlation with cyclohexanol dehydration showed that isomerization of α-pinene is a Bronsted acid catalyzed reaction. The relationship of 4-methyl-2-pentanol conversion with acid site density and sulfur per unit area was found to be linear.

Full text of DOI:10.1007/s10562-011-0602-6

Catal Lett (2011) 141:1164–1170
DOI 10.1007/s10562-011-0602-6
ZrO2–SiO2 Mixed Oxides Xerogel and Aerogel as Solid Acid
Catalysts for Solvent Free Isomerization of a-Pinene
and Dehydration of 4-Methyl-2-Pentanol
•
Kalpesh B. Sidhpuria Beena Tyagi
•
Raksh V. Jasra
Received: 20 December 2010 / Accepted: 9 April 2011 / Published online: 26 April 2011
Ó Springer Science+Business Media, LLC 2011
Abstract Sulfated and non-sulfated ZrO2–SiO2 mixed
oxide xerogel and aerogel samples having varied Zr/Si
molar ratio were evaluated as solid acid catalysts for the
isomerization of a-pinene and dehydration of 4-methyl-
2-pentanol. Sulfation resulted into enhancement in the
catalytic activity of both xerogel and aerogel samples
towards the studied reactions. For example, sulfated cata-
lysts showed 86–98% conversion of a-pinene and 8–35%
conversion of 4-methyl-2-pentanol. The selectivity data for
camphene and limonene indicated the requirement of
moderate acidity. The correlation with cyclohexanol
dehydration showed that isomerization of a-pinene is a
Brønsted acid catalyzed reaction. The relationship of
4-methyl-2-pentanol conversion with acid site density and
sulfur per unit area was found to be linear.
Keywords ZrO2–SiO2 mixed oxides ꢀ Xerogels ꢀ
Aerogels ꢀ Pinene isomerization ꢀ Alcohol dehydration
1 Introduction
Zirconia as a catalyst or catalytic support has received
considerable attention in recent years [1, 2]. However,
relatively low surface area and weak acidity limited its
extensive application in catalysis [3, 4]. In order to improve
its surface area, zirconia can be combined with some
materials possessing high surface area such as silica. Mixed
metal oxides have been widely investigated since they have
interesting catalytic properties [5–8]. The presence of a
foreign element in the matrix of a pure metal oxide can
greatly modify the structural, textural, acid–base and cat-
alytic properties [9–11]. ZrO2–SiO2 mixed oxides have
strong surface acidity and could be useful for diverse acid
catalyzed reactions due to enhanced acidity as compared to
less acidic individual zirconia or silica [11, 12]. The acidity
of mixed oxides may further be modified by the use of
sulfation [5, 10, 13]. The enhanced surface acidity and
catalytic activity resulting from sulfation is extensively
studied in the case of pure zirconia [11]. Influence of
sulfation on the acidic properties of ceria–zirconia [10],
zirconia–silica [5, 9, 13] and titania–silica [6, 14] mixed
oxides has also been reported.
Electronic supplementary material The online version of this
article (doi:10.1007/s10562-011-0602-6) contains supplementary
material, which is available to authorized users.
K. B. Sidhpuria ꢀ B. Tyagi (&) ꢀ R. V. Jasra (&)
Discipline of Inorganic Materials and Catalysis, Central Salt
and Marine Chemicals Research Institute (CSMCRI), Council
of Scientific and Industrial Research (CSIR), G. B. Marg,
Bhavnagar 364002, Gujarat, India
e-mail: btyagi@csmcri.org
R. V. Jasra
e-mail: rakshvir.jasra@ril.com
K. B. Sidhpuria
e-mail: kalpesh@ua.pt
Present Address:
K. B. Sidhpuria
CICECO-Department of Chemistry, University of Aveiro,
Campus de Santiago, P-3810-193 Aveiro, Portugal
Present Address:
R. V. Jasra
Reliance Technology Group, Reliance Industries Limited,
Vadodara Manufacturing Division, Vadodara 391346,
Gujarat, India
Monoterpenes like a-pinene are economically impor-
tant compounds, which are extensively used in the
123

Xerogel and Aerogel as Solid Acid Catalysts
1165
pharmaceutical, cosmetic, food, fragrance and fine chemi-
cal industries [15]. a-pinene undergoes acid-catalyzed
isomerization reaction resulting into valuable products
such as camphene, limonene, mono-, bi- and tricyclic
hydrocarbons, p-cymene, and oligomers [16]. Camphene is
used in the production of camphor and limonene is widely
employed in the synthesis of oxygenated compounds of
perfumery industry [17]. The selectivity of various prod-
ucts strongly depends on the structural, textural and cata-
lytic features of the catalyst along with the reaction
conditions [18, 19]. Various solid acid catalysts such as
zeolites, clays, zirconia and sulfated zirconia have been
studied for the isomerization of a-pinene [20–25]. How-
ever, the selective isomerization of a-pinene to the desired
isomers is still required.
sulfated using 1 N H2SO4 followed by calcination at
600 °C and are designated as SX-ZSi-R and SA-ZSi-R,
respectively.
2.2 Characterization
Sulfated and non-sulfated ZrO2–SiO2 mixed xerogel and
aerogel samples, before and after calcination were ana-
lyzed using X-ray powder diffractometer (Philips X’pert)
˚
using CuKa radiation (k = 1.5405 A). Specific surface
area, pore volume and pore size distributions of samples
were determined from nitrogen adsorption–desorption
isotherms at 77.4 K (ASAP 2010, Micromeritics, USA).
The bulk sulfur (wt%) loading in sulfated ZrO2:SiO2
aerogel and xerogel samples before and after calcination at
600 °C was analyzed by CHNS/O elemental analyzer
(Perkin–Elmer, 2400, Sr II, USA). Total surface acidity
measurement by Temperature Programmed Desorption
(TPD) of NH3 was carried out using Micromeritics Pulse
Chemisorb 2720 instrument [13].
Dehydration of alcohol is commercially important
reaction to produce alkene as well as to identify acid and
basic sites in heterogeneous catalysts [26, 27]. Isopropanol
[28], 1-butanol [29], 4-methyl-2-pentanol [30, 31] and
cyclohexanol [20, 32] are the most common alcohols
studied for the dehydration reactions. Hexenes and skeletal
rearranged products are reported during 4-methyl-2-pent-
anol dehydration [31]. Among these, hexenes such as
4-methyl-1-pentene and 4-methyl-2-pentene are major
products, in which former has commercial values and is
useful starting material for manufacturing thermoplastic
polymers.
2.3 Catalytic Activity for a-Pinene Isomerization
Typically, a-pinene (2 mL) and catalyst (0.1 g) mixture
was heated at 150 °C under constant magnetic stirring for
2 h. Precautions were taken to maintain the temperature
and to check the vapor loss of the products. The reaction
products were analyzed by a gas chromatograph-mass
spectrometer (Shimadzu, GC–MS QP 2010, Japan) with
Petrocol DH 50.2 capillary column, programmed oven and
ion source at 200 °C using He as a carrier gas. The con-
version and selectivity values were calculated as below:
Recently, we have studied the effect of Zr/Si molar
ratio, thermal and super critical drying methods, and sul-
fation on the structural, textural and catalytic properties of
ZrO2–SiO2 mixed xerogel and aerogel samples [13]. In the
present study, we have evaluated the catalytic activity of
these materials for solvent free liquid phase isomerization
of a-pinene and 4-methyl-2-pentanol dehydration. To the
best of our knowledge, no report has been found so far in
literature about isomerization of a-pinene and dehydration
of 4-methyl-2-pentanol using sulfated or non-sulfated
ZrO2–SiO2 mixed xerogels and aerogels.
Conversion of a-pineneðwt %Þ
¼ 100 ꢁ ½Initial wt % ꢂ Final wt %ꢃ=½Initial wt %ꢃ
Selectivity for camphene (wt%) = 100 9 [GC peak area
% of camphene]/[R Total peak area of all the products].
2.4 Catalytic Activity for 4-Methyl-2-Pentanol
Dehydration
2 Experimental
Dehydration of 4-methyl-2-pentanol was studied in a fixed
bed reactor. The catalyst (0.2 g) was packed in a glass
reactor bed and activated in situ at 450 °C for 2 h under N2
flow. 4-methyl-2-pentanol (2 mL) was delivered by a syr-
inge pump injector (Cole Parmer, 74900 series) with a flow
rate of 1 mL h-1 under N2 (15 mL min-1) at 145 °C. The
product samples were collected after 1 h and analyzed by a
gas chromatograph (HP 6890) having HP5 capillary col-
umn, FID detector and programmed oven and N2 as carrier
gas. The conversion of 4-methyl-2-pentanol and selectivity
for each product was calculated following the procedure
similar to that described above for a-pinene.
2.1 Catalyst Synthesis
ZrO2–SiO2 mixed oxides having varied molar ratio of Zr to
Si (1:1, 1:2 and 2:1, respectively) have been prepared and
characterized as reported earlier [13]. ZrO2–SiO2 xerogel
samples obtained after conventional thermal drying fol-
lowed by calcination at 600 °C are designated as X-ZSi-R
and aerogel samples were obtained after supercritical
drying using n-propanol as a solvent are designated as
A-ZSi-R (where, R = 1:1, 1:2 and 2:1 representing ZrO2:
SiO2 molar ratio). Both xerogel and aerogel samples were
123

1166
K. B. Sidhpuria et al.
3 Results and Discussion
3.1 Catalyst Characterization
mainly into bicyclic camphene, tricyclene and monocyclic
limonene and a-terpinene under the experimental condi-
tions studied. The reaction parameters namely temperature,
time and substrate to catalyst ratio for the isomerization of
a-pinene were optimized over one of the catalysts, i.e.,
A-ZSi-1:1. Initially, the reaction was carried out at differ-
ent temperatures in the range of 110–150 °C (Fig. 1). The
conversion of a-pinene was found to increase from 14 to
36% with increasing the reaction temperature from 110 to
150 °C. There was no significant variation observed in
camphene (*36%) and a-terpinene (*2%) selectivity.
However, limonene selectivity was found to increase from
36 to 45% with an increase in the temperature from 110 to
150 °C. It suggested that formation of limonene was more
affected by temperature changes than those of the forma-
tion of camphene and a-terpinene.
The kinetic study was carried out at 150 °C over
A-ZSi-1:1 catalyst (Fig. 2). It was observed that conversion
of a-pinene increased from 26 to 36% till 90 min and
remained nearly constant between 120 and 180 min.
Hence, we have taken 120 min as optimized reaction time
for further study. No significant variation was observed in
camphene and tricyclene selectivity with increase in time.
However, the formation of limonene increased from 39 to
45% with increasing time from 10 to 120 min, whiles other
products such as terpinolene, iso-terpinolene, c-terpinene,
etc., were observed to decrease from 26 to 17% with
increasing reaction time.
The detailed characterization of sulfated and non-sulfated
ZrO2–SiO2 mixed xerogel and aerogel samples have been
mentioned in our earlier report [13]. It was observed that
the ZrO2:SiO2 molar ratio and drying method has signifi-
cant effect on the structural and textural properties in terms
of crystallinity, surface area, pore volume and pore diam-
eter. Xerogel samples were found to be amorphous in
nature indicating the uniform mixing and homogeneity of
mixed oxides, whereas, supercritical drying at high tem-
perature and pressure resulted into crystalline aerogel
samples having predominantly tetragonal zirconia. Both
xerogels and aerogels, though having different structural
and textural features, were found to have the total number
of acid sites per unit surface area (0.0021–0.0029 mmol
NH3 m-2) and the catalytic activity for cyclohexanol
conversion (31–41 wt%) in the similar range (Table 1).
Sulfation resulted into remarkable enhancement of cata-
lytic activity for cyclohexanol conversion (91–99%) for
both sulfated xerogel and aerogel samples, though the
surface area and pore volume decreased significantly.
Sulfated xerogel samples showed higher acid sites density
(0.0074–0.013 mmol NH3 m-2) compared to sulfated aero-
gel samples (0.0027–0.0082 mmol NH3 m-2) (Table 1)
due to the higher sulfur loading in xerogel samples (5.10 to
7.57 wt%) compared to aerogel samples (1.58 to 2.65 wt%)
[13] and therefore resulted into higher cyclohexanol con-
version ([99%).
The effect of substrate to catalyst ratio was studied in
the range of 10–40 by varying the amount of catalyst
and by keeping a-pinene amount constant (2.0 g) over
A-ZSi-1:1 catalyst at 150 °C. The conversion of a-pinene
was observed to decrease by increasing substrate to catalyst
ratio from 10 to 40. The maximum conversion of a-pinene
(53%) was observed with substrate to catalyst ratio of 10,
which decreased to 36 and 34% with substrate to catalyst
3.2 Isomerization of a-Pinene
The isomerization of a-pinene over sulfated and non-
sulfated ZrO2–SiO2 xerogel and aerogel samples resulted
Table 1 Characterization
Sample
BET surface Acid sites conc. Acid sites/Surface
area (m2/g)
% S/Surface
Cyclohexanol
of sulfated and non-sulfated
ZrO2–SiO2 xerogel and aerogel
catalysts [13]
(mmol NH3/g)
area (mmol NH3/m2) area (% S/m2) conversion (%)
X-ZSi-1:1
X-ZSi-1:2
X-ZSi-2:1
266
300
290
0.78
0.67
0.78
1.20
1.19
1.64
1.31
1.46
0.93
0.99
0.78
1.31
0.0029
0.0022
0.0027
0.0114
0.0074
0.013
–
39
31
–
–
41
SX-ZSi-1:1 105
SX-ZSi-1:2 160
SX-ZSi-2:1 126
0.048
0.039
0.060
–
[99
93
[99
34
A-ZSi-1:1
A-ZSi-1:2
A-ZSi-2:1
475
678
335
0.0028
0.0021
0.0028
0.0051
0.0027
0.0082
–
32
–
35
SA-ZSi-1:1 194
SA-ZSi-1:2 285
SA-ZSi-2:1 160
0.011
0.005
0.016
92
91
97
123

Xerogel and Aerogel as Solid Acid Catalysts
1167
camphene and monocyclic limonene, a-terpinene and
terpinolene. Pure zirconia (ZrO2) and sulfated zirconia
(SO4–ZrO2) samples prepared by sol–gel method were also
studied for comparison.
45
40
35
30
25
20
15
10
5
ZrO2 showed no activity for the isomerization of a-
pinene, while SO4–ZrO2 resulted into 100% conversion of
a-pinene, however, the selectivity for the desired products
such as camphene (24%) and limonene (nil) was less and
the formation of side products was more (62%). It sug-
gested the necessity of the moderate acidity to obtain
the desired products during the isomerization of a-pinene.
ZrO2–SiO2 xerogel samples with varied molar ratio
showed the conversion of a-pinene in the wide range of
8–79%, while aerogel samples showed the conversion in
the range of 36–56%. However, the selectivity for
camphene (32–36%) and limonene (34–45%) was observed
in the similar range for both xerogel and aerogel samples.
The increase in the conversion of a-pinene compared to
ZrO2 and increase in the selectivities for the desired
products compared to SO4–ZrO2 observed over ZrO2–SiO2
samples indicated that the presence of a foreign element in
the matrix of a pure metal oxide can significantly modify
the catalytic properties.
conversion
Camphene
Alpha-Terpinene
Limonene
Others
0
110
120
130
140
150
Temperature (oC)
Fig. 1 Isomerization of a-pinene over A-ZSi-1:1 catalyst at different
temperatures. Reaction conditions Catalyst = 0.1 g, a-Pinene =
2.0 g, time = 2 h, (others: terpinolene, iso-terpinolene, c-terpinene,
etc.)
ratio 20 and 40, respectively. However, camphene selec-
tivity was found to be maximum (36%) with substrate to
catalyst ratio of 20 and was lower (32%) with substrate to
catalyst ratio of 10 and 40. Similarly, limonene selectivity
was also observed to increase from 41 to 45% with an
increase in substrate to catalyst ratio from 10 to 20 and
decreased to 39% on further increasing the substrate to
catalyst ratio to 40. Hence, substrate to catalyst ratio of 20
was selected as optimized ratio on the basis of the selec-
tivity of the desired products rather than the conversion of
a-pinene.
Sulfated ZrO2–SiO2 samples showed significant enhance-
ment in the conversion of a-pinene (86–98%). Among the
products, formation of tricyclene was observed to be sig-
nificantly increased over sulfated samples (14–36%) com-
pared to non-sulfated samples (1–2%). On the contrary,
formation of camphene and limonene was higher over non-
sulfated samples. The formation of a-terpinene could not be
observed linear. It indicated that formation of camphene and
limonene requires moderate acidity, whereas tricyclene and
a-terpinene need higher acidity.
The isomerization of a-pinene over a series of sulfated
and non-sulfated ZrO2–SiO2 mixed xerogel and aerogel
samples (Table 2) showed the formation of mainly bicyclic
50
40
a
b
45
38
40
35
30
25
36
34
32
30
28
26
24
22
20
20
Camphene
Tricyclene
15
Alpha-Terpinene
Limonene
Others
10
5
0
0
20 40 60 80 100 120 140 160 180 200
0
20
40
60
80
100
120
Time (min)
Time (min)
Fig. 2 a Variation in a-Pinene conversion and b various products formation with time over A-ZSi-1:1 catalyst. Reaction conditions
Catalyst = 0.1 g, a-pinene = 2.0 g, temperature = 150 °C, time = 2 h (others: terpinolene, iso-terpinolene, c-terpinene, etc.)
123

1168
K. B. Sidhpuria et al.
Table 2 Isomerization of a-pinene and selectivity for various products over sulfated and non-sulfated ZrO2–SiO2 xerogel and aerogel catalysts
Catalyst
% Conversion
% Selectivity
Camphene
Tricyclene
a-Terpinene
Limonene
Others
ZrO2
Nil
100
8
–
–
–
–
–
SO4–ZrO2
X-ZSi-1:1
X-ZSi-1:2
X-ZSi-2:1
SX-ZSi-1:1
SX-ZSi-1:2
SX-ZSi-2:1
A-ZSi-1:1
A-ZSi-1:2
A-ZSi-2:1
SA-ZSi-1:1
SA-ZSi-1:2
SA-ZSi-2:1
24
32
32
35
24
31
29
36
33
32
32
34
28
14
–
–
–
62
34
19
22
19
27
29
16
20
24
16
34
23
–
34
42
34
26
3
29
79
89
93
93
36
56
39
93
98
86
1
6
2
7
23
17
18
1
8
22
22
2
2
45
42
40
31
2
1
4
1
3
19
14
36
2
16
10
3
Reaction conditions Catalyst = 0.1 g, a-Pinene = 2.0 g, temperature = 150 °C, time = 2 h (others: terpinolene, iso-terpinolene, c-terpinene,
etc.)
The formation of various mono-, bi- and tricyclic
products by the isomerization of a-pinene occurred through
the carbocation formation by the interaction of the proton
of the acid catalyst with the double bond of a-pinene [17].
It is reported that in the presence of high acidic conditions,
camphene and isomeric bi- and tricyclic compounds are
transformed to monocyclic compounds [17]. This may be
the reason that we found lower camphene selectivity with
sulfated samples having higher acid site density compared
to non-sulfated samples having lower acid site density. It
further confirmed with sulfated xerogel and aerogel sam-
ples having varied acidity. For example, sulfated xerogel
samples having higher sulfur and acid site density (Table 1
and Sect. 3.1) resulted into 24 to 31% of camphene
selectivity, whereas, sulfated aerogel samples having lower
sulfur and acid site density resulted into higher camphene
selectivity (28 to 34%). These results were in good
agreement with Encormier et al. [23], who observed that
low sulfur of SO4–ZrO2 catalyst favoured the camphene
formation.
sulfated xerogel and aerogel samples (Fig. 3) clearly
showed that isomerization of a-pinene is a Brønsted acid
catalyzed reaction. Similar result was also observed using
acid treated montmorillonite clays [20].
3.3 Dehydration of 4-Methyl-2-Pentanol
The dehydration of 4-methyl-2-pentanol over ZrO2–SiO2
samples leads to a mixture of 4-methyl-1-pentene and
4-methyl-2-pentene; along with skeletal isomers of C6-
alkenes and dehydrogenation to 4-methyl-2-pentanone
110
Deahydration of cyclohexanol;
Isomerization of α-pinene
100
90
80
70
60
50
40
30
20
10
0
The formation of other side products was greatly
reduced in sulfated ZrO2–SiO2 samples (16–34%) com-
pared to pure SO4–ZrO2 (62%), though the values for the
conversion of a-pinene and camphene selectivity were
comparable. Non-sulfated ZrO2–SiO2 samples also showed
the reduced formation of other side products. It clearly
suggested that the acidity of the catalyst and the selectivity
for the desired products can be tunable with mixed oxides
as compared to pure metal oxides.
Catalyst samples
Fig. 3 Correlation between conversion values of a-pinene isomeri-
zation and cyclohexanol dehydration reaction over sulfated
ZrO2–SiO2 xerogel and aerogel samples
Furthermore, the correlation between the conversion of
a-pinene and cyclohexanol dehydration reaction over
123

Xerogel and Aerogel as Solid Acid Catalysts
1169
Table 3 Conversion (%) of 4-methyl-2-pentanol dehydration and selectivity (%) for various products over sulfated and non-sulfated ZrO2–SiO2
xerogel and aerogel catalysts
Samples
% Conversion
% Selectivity
4-Methyl-1-pentene
4-Methyl-2-pentene
C6 Alkene isomers
4-Methyl-2-pentanone (MIBK)
X-ZSi-1:1
X- ZSi-1:2
X- ZSi-2:1
A-ZSi-1:1
A-ZSi-1:2
A-ZSi-2:1
SX-ZSi-1:1
SX-ZSi-1:2
SX-ZSi-2:1
SA-ZSi-1:1
SA-ZSi-1:2
SA-ZSi-2:1
1
55
51
60
–
45
49
33
–
–
–
–
7
–
–
–
–
–
7
–
5
7
1
–
2
–
Nil
Nil
Nil
25
8
–
–
–
–
–
–
–
68
75
67
36
36
42
–
32
13
12
49
32
33
12
14
15
27
18
35
7
4
14
Reaction conditions Catalyst 0.2 g, reaction temperature = 145 °C under N2, vaporizer temperature = 150 °C, flow rate = 1 mL/h
0.014
0.012
0.010
0.008
0.006
0.004
0.002
0.07
0.06
0.05
0.04
0.03
0.02
0.01
0.00
a
b
Sulfated Xerogel
Sulfated Aerogel
Sulfated Xerogel
Sulfated Aerogel
0
5
10
15
20
25
30
35
40
0
5
10 15 20 25 30 35 40
% Conversion
% Conversion
Fig. 4 Correlation of 4-methyl-2-pentanol conversion with a acid site density and b % S per unit surface area of ZrO2–SiO2 sulfated xerogel and
aerogel
(Table 3). The xerogel samples showed a very low conversion
(1–2%) of 4-methyl-2-pentanol, whereas, aerogel samples did
not show any activity for 4-methyl-2-pentanol conversion.
Sulfated samples showed enhanced activity in terms of
increased conversion of 4-methyl-2-pentanol (8–35%) and
selectivity for 4-methyl-1-pentene (67–75%). Silica rich
sulfated xerogel sample (SX-ZSi-1:2) showed the minimum
conversion of 8%. The conversion was found to increase
from 8 to 35% with increase in Zr content. Sulfated aerogel
samples showed lower conversion of 4-methyl-2-pentanol
(4–14%) and selectivity for 4-methyl-1-pentene (36–42%)
as compared to sulfated xerogel samples. However, sulfated
aerogel samples also showed the similar trend of lower
conversion with silica rich samples (4%), which increased
(7 to 14%) with increasing Zr content, without showing
significant effect on the selectivity for 4-methyl-1-pentene
(36–42%). 4-methyl-2-pentene (12–14%) and other
C6-alkene isomers (12–32%) along with a little dehydro-
genated product i.e., 4-methyl-2-pentanone (7%) were also
found with both sulfated xerogel and aerogel samples,
however, sulfated aerogel samples showed higher amount
of these side products as compared to sulfated xerogel
samples (Table 3). The conversion of 4-methyl-2-pentanol
increased linearly with increasing the amount of acid sites
and sulfur per unit area (Fig. 4). Higher alcohol conversion
was observed in xerogels samples compared to aerogel
samples due to the presence of higher sulfur and thus higher
acid site density in sulfated xerogel samples compared to
aerogel samples. These results suggested that higher surface
acidity is required to achieve significant conversion of
4-methyl-2-pentanol as well as for higher selectivity for the
desired product, i.e., 4-methyl-1-pentene.
123

1170
K. B. Sidhpuria et al.
References
It is reported that during secondary alcohol dehydration,
alcohol molecule first gets adsorbed on the surface of metal
oxide catalysts followed by a two-step mechanism
involving intermediate carbocation formation (E1), a con-
certed pathway (E2) or via carbanion formation mechanism
(E1cB), which resulted into the different products such as
1-alkene, 2-alkene etc. [31, 33]. E1cB mechanism favors
the formation of 1-alkene (Hofmann projection), while E1
and concerted E2 mechanisms result into 2-alkene (Say-
tzeff projection) as a major product. The results of the
present study showed that sulfated xerogels and aerogels
samples formed 4-methyl-1-pentene in the range of
67–75% and 36–42%, respectively. Additionally, the sig-
nificant formation of 4-methyl-2-pentene (12–27%) and
slight formation of 4-methyl-2-pentanone (5–7%) is
observed. However, it is difficult to unambiguously
comment on the operating mechanism based on the data
collected in the present study.
1. Tanabe K (1985) Mater Chem Phys 13:347
2. Wu NL, Wang SY, Rusakova IA (1999) Science 285:1375
3. Anderson JA, Fergusson C, Rodriguez-Ramos I, Guerrero-Ruiz A
(2000) J Catal 192:344
4. Courty P, Marcilly C, Delmon B, Jacobs PA, Poncelet G (eds)
(1976) Preparation of catalysts. Elsevier Press, Amsterdam, p 119
5. Yang H, Lu R, Shen L, Song L, Zhao J, Wang Z, Wang L (2003)
Mater Lett 57:2572
´
6. Rodrıguez Avendan˜o RG, De Los Reyes JA, Viveros T, Montoya
De La Fuente JA (2009) Catal Today 148:12
7. Tanabe K, Misono M, Ono Y, Hattori H (1989) In: Delmon B,
Yates JT (eds) Studies in surface science and catalysis, vol. 51.
Elsevier, Amsterdam
8. Gervasini A, Carniti P, Auroux A (2005) Thermochimica Acta
434:42
9. Wu Z, Zhao Y, Liu D (2004) Microporous Mesoporous Mater
68:127
10. Azambre B, Zenboury L, Weber JV, Burg P (2010) Appl Surf Sci
256:4570
11. Akkari R, Ghorbel A, Essayem N, Figueras F (2007) Appl Catal
A 328:43
12. Zhang Y, Pan L, Gao C, Wang Y, Zhao Y (2010) J Sol Gel Sci
Technol 56:27
13. Tyagi B, Sidhpuria KB, Shaik B, Jasra RV (2010) J Porous Mater
17:699
14. Jung SM, Dupont O, Grange R (2001) Appl Catal A 208:393
4 Conclusions
Sulfated and non-sulfated ZrO2–SiO2 mixed xerogel and
aerogel samples having varied Zr/Si molar ratios were
evaluated as heterogeneous acid catalysts for solvent free
isomerization of a-pinene and dehydration of 4-methyl-
2-pentanol. Sulfated samples showed significant enhance-
ment in the catalytic activity for both the reactions studied.
Sulfated samples exhibited higher conversion of a-pinene
(86–98%) compared to non-sulfated samples; however, the
selectivity for camphene and limonene were comparable or
slightly lower indicating that the formation of camphene
and limonene requires moderate acidity. The results clearly
suggested that the acidity of the catalyst and the selectivity
for the desired products can be tunable with ZrO2–SiO2
mixed oxides as compared to pure metal oxides. The cor-
relation with cyclohexanol dehydration showed that isom-
erization of a-pinene is a Brønsted acid catalyzed reaction.
For 4-methyl-2-pentanol dehydration, ZrO2–SiO2 mixed
xerogel and aerogel catalysts resulted into lower conver-
sion (4–35%); sulfated xerogel samples showed higher
conversion and selectivity for 4-methyl-1-pentene com-
pared to sulfated aerogel samples. The correlation of
4-methyl-2-pentanol conversion with acid site density and
sulfur per unit area was found to be linear, which suggested
that higher surface acidity is required to achieve significant
conversion of 4-methyl-2-pentanol as well as for higher
selectivity for the desired product, i.e., 4-methyl-1-pentene.
¨
15. Akpolat O, Gu¨ndu¨z G, Ozkan F, Bes¸u¨n N (2004) Appl Catal A
265:11
16. McGraw GW, Hemingway RW, Ingram LL Jr, Canady CS,
McGraw WB (1999) Environ Sci Technol 33:4029
´
´
17. Chimal-Valencia O, Robau-Sanchez A, Collins-Martınez V,
´
Aguilar-Elguezabal A (2004) Bioresour Technol 93:119
¨
18. Besu¨n N, Ozkan F, Gu¨ndu¨z G (2002) Appl Catal A 224:285
19. Roberge DM, Buhl D, Niederer JP, Holderich W (2001) Appl
Catal A 215:111
20. Yadav MK, Chudasama CD, Jasra RV (2004) J Mol Catal A
216:51
21. Allahverdiev A, Irandoust S, Murzin DY (1999) J Catal 185:352
´
´
22. Lopez C, Machado F, Rodrıguez K, Arias D, Mendez B,
Hasegawa M (1999) Catal Lett 62:221
23. Encormier MA, Wilson K, Lee AF (2003) J Catal 215:57
24. Grzona LM, Comelli NA, Masini O, Ponzi EN, Ponzi M (2000)
React Kinet Catal Lett 69:271
25. Ecormier MA, Lee AF, Wilson K (2005) Microporous Meso-
porous Mater 80:301
26. Linnekoski JA, Krause AOI, Holmen A, Kjetsa M, Moljord K
(1998) Appl Catal A 174:1
´
´ ´
27. Manrıquez ME, Lopez T, Gomez R, Navarrete J (2004) J Mol
Catal A 220:229
´
28. Corma A, Fornes V, Rey F (1994) J Catal 148:205
29. Berteau P, Delmon B, Dallons JL, van Gysel A (1991) Appl Catal
70:307
30. Cutrufello MG, Ferino I, Solinas V, Primavera A, Trovarelli A,
Auroux A, Picciau C (1999) Phys Chem Chem Phys 1:3369
31. Ferino I, Casula MF, Corrias A, Cutrufello MG, Monaci R,
Paschina G (2000) Phys Chem Chem Phys 2:1847
32. Sidhpuria KB, Parikh PA, Bahadur P, Tyagi B, Jasra RV (2009)
Catal Today 141:12
33. Winterbottom JM (1981) In: Kemball C (ed) Catalysis (specialist
periodical reports) vol 4. The Chemical Society, London, 141
Acknowledgments Authors are thankful to CSIR Network Pro-
gramme on Catalysis and Analytical Science Discipline for providing
instrumental analysis. KBS is further thankful to FCT (SFRH/BPD/
44803/2008).
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