Highly-selective solvent-free catalytic isomerization of α-pinene to camphene over reusable titanate nanotubes

Article abstract of DOI:10.1039/d0ra01093f

Titanate nanotubes, prepared by the hydrothermal reconstitution and modification with hydrochloric acid, were tested as solid acid catalysts in the isomerization of α-pinene under solvent free conditions. The results showed that titanate nanotubes have be

Full text of DOI:10.1039/d0ra01093f

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Highly-selective solvent-free catalytic
isomerization of a-pinene to camphene over
Cite this: RSC Adv., 2020, 10, 10606
reusable titanate nanotubes
a
b
a
a
a
Geng Huang, Shuolin Zhou,
Jian Liu, Shengpei Su* and Dulin Yin*
Titanate nanotubes, prepared by the hydrothermal reconstitution and modification with hydrochloric acid,
were tested as solid acid catalysts in the isomerization of a-pinene under solvent free conditions. The results
showed that titanate nanotubes have better catalytic properties than titanium dioxide nanoparticles, and the
camphene was the main product for a-pinene isomerization. The effects of several reaction variables, such
as reaction temperature, catalyst dosage, and reaction time, on the conversion of a-pinene and the
selectivity to camphene were examined. The highest conversion was up to 97.8% with selectivity to
camphene of 78.5% under the mild reaction conditions, and the catalyst also showed outstanding
reusability after four runs. It is proposed that appropriate surface acidic sites and opened nanotubular
structures are mainly responsible for the excellent catalytic performance of titanate nanotubes materials.
Received 5th February 2020
Accepted 9th March 2020
DOI: 10.1039/d0ra01093f
rsc.li/rsc-advances
pinene. However, there is still a challenge to develop new
catalysts which posse higher activity and selectivity to
Introduction
The production of high value-added chemicals from biomass camphene during the a-pinene isomerization process.
1
has attracted a lot of attention. a-Pinene extracted from resin
In recent years, titanium dioxide nanoparticles (TNPs) were
tapping, wood pulp papermaking and cellulose production is an worked as efficient catalyst for the acceleration of various
1
3
inexpensive and important essential oil, which is widely used in organic reactions including Knoevenagel condensations,
14
the synthesis of various ne chemicals. Many research works Friedel–Cras alkylation of indoles, the aldol condensation of
indicated that a-pinene can be rearranged to give several furfural, Mannich synthesis of b-aminocarbonyls, and
isomeric products in the presence of an acid catalyst. Among synthesis of tetrahydrobenzo[b]pyran derivatives, a-amino-
several isomers, camphene is especially important substrate in phosphonates and polyhydroquinoline derivatives. However,
the production of fragrance materials, acrylates, terpene-phenol the weak acidity sites on the surface of TNPs are become a major
15
16
17
18
19
2
resins and other chemical derivatives. Camphene is also a key drawback that limits the catalytic applications of TNPs in
intermediate from a-pinene for the production of isobornyl organic reactions. It has been reported that the reaction rate on
esters and camphor which are extensively used as mildew titanium dioxide nanoparticles is low for the a-pinene isomer-
2
0
proong and insecticidal agents in our daily life.
Camphene is generally obtained by the isomerization of a- with acidic groups is an efficient method to increase the cata-
ization to camphene. Surface modication of TiO materials
2
23,24
pinene using acidic catalysts. In recent years, the use of lytic activity.
heterogeneous catalyst is
shi toward green and inuencing the activity of acid catalysts. Among titanium
environmental-friendly technologies. Various solid acid cata- dioxide materials, one dimensional titanate nanotubes (TNTs)
Besides, textural properties are usually a factor
a
3,4
5
lysts, such as zeolites,
iron-modied zeolites, sulfated have novel properties such as large surface area and tubular
21
6
7
zirconia, modied layered aluminosilicates, Al-incorporated structure that increase the number of active sites. TNTs can be
8
9
10
MCM-41, MSU-S mesoporous materials, W
2
O
3
/Al
2
O
3
,
phos- synthesized in large quantities via the hydrothermal treatment
powder. The main hollow tunnel
11
photungstic heteropoly acids, Ti-SBA-15 (ref. 12 and 13), and using commercial TiO
2
12
ionic liquids have been employed for the isomerization of a- structures are formed aer the acid washing step that removing
+
the Na ion to form Ti–OH bonds. This means that the surface
a
acidity can be modied to tune the catalytic activity of TNTs
National & Local Joint Engineering Laboratory for New Petro-chemical Materials and
Fine Utilization of Resources, Key Laboratory of Resource Fine-Processing and during the acid washing process.
Advanced Materials of Hunan Province, Key Laboratory of Chemical Biology and
Traditional Chinese Medicine Research (Ministry of Education of China), College of
Chemistry and Chemical Engineering, Hunan Normal University, Changsha,
In the present work, titanate nanotubes were modied by
different acids as solid catalysts to improve the isomerization of
a-pinene to camphene (Scheme 1). Repetition reuses were
conducted in order to suit the catalyst's potential application.
4
10081, P. R. China. E-mail: sushengpei@yahoo.com; dulinyin@126.com; Fax: +86-
31-88872531; Tel: +86-731-88872530
7
b
College of Education, Changsha Normal University, Changsha, 410100, P. R. China
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a thermostatic oil bath at atmospheric pressure. Once the
reaction nish, the products were taken, centrifugated and
analyzed quantitatively by gas chromatography. The products of
the isomerization were identied by GC-MS analyses.
Scheme 1 The isomerization of a-pinene over titanate nanotubes.
Results and discussion
Catalytic performance of modied TNTs in isomerization
Experimental
Materials
The catalytic performance of TNPs and different TNTs was eval-
uated in the isomerization of a-pinene under solvent free
conditions, and the results have been summarized in Table 1. It
TiO
purchased from Aladdin and were used as received. Other
chemical reagents, such as NaOH, HCl, H SO , HNO , hexane,
2
nanoparticles (anatase, >99.9%) and a-pinene were
was found that a-pinene conversion was almost zero for 6 h at
ꢁ
1
00 C in the blank experiment (Table 1, Entry 1), and the similar
2
4
3
result was obtained for reaction using the commercial TNPs
Table 1, Entry 2). However, when modied TNTs materials were
and ethanol were of analytical grade and used without further
purication.
(
used as catalyst, an increase on the conversion of a-pinene was
observed (Table 1, Entry 3–5). It was found that the highest
conversion of a-pinene was 42.8% with 64.8% highest camphene
selectivity when catalyzed by TNTs-Cl for only 120 min (Table 1,
Entry 5). From the adsorption of pyridine, the surface acidity of
modied TNTs was quantied in Table 1. The amount of acidity
on the sample surface is related to the kind of acid agent. This
means that the acid catalytic sites can be modied by mineral
acid in the washing step. The improvement on the catalytic
performance of TNTs can be explained as TNTs has Lewis acids
Preparation of catalyst
TNTs were synthesized by the hydrothermal method in NaOH
solution using commercial TNPs as Ti source referring to the
previous work with some modications. 3 g TNPs were
uniformly dispersed in 120 mL of 10 mol L NaOH aqueous
solution, followed by hydrothermal treatment at 150 C in
22
ꢀ1
ꢁ
a Teon-lined autoclave for 24 h. The white powders were
ꢀ
1
+
washed thoroughly with distilled water and 0.1 mol L [H ] HCl
aqueous solution. Then, the suspension was centrifugated and
the solid sample was washed with distilled water until the pH of
the rinsing solution close to 7, and subsequently dried over-
24
sites and Brønsted acid sites, and the strength distribution of
acid sites are also adjusted by modifying the surface with
different mineral acid. Lewis acid sites were mainly promoted
24
ꢁ
when HCl was used in the washing step. Interestingly, we found
that the major product aer the reaction was camphene, thus
suggesting the appropriate surface acidic sites of TNTs-Cl play
a determinant role in the selective isomerization reaction for
enlarging the four number ring of a-pinene to dicyclic camphene.
night under vacuum at 60 C, the product denoted as TNTs-Cl.
Similarly, TNTs-SO
4
and TNTs-NO
3
were prepared with
ꢀ
1
0.1 mol L
H
2
SO
4
and HNO aqueous solution during the acid
3
washing process, respectively.
The amount of acid sites of TNTs-NO
0
3
and TNTs-SO
.58 mmol g , respectively. It is deduced that the stronger
Brønsted acidic sites exist on the surface of TNTs-SO and TNTs-
4
are 0.46 and
Characterization
ꢀ
1
Prior to measurement, all samples were dried under vacuum.
Fourier transform infrared spectrum (FT-IR) was obtained by the
KBr pellet technique on a Nicolet 370 infrared spectrophotometer
in the range 400–4000 cm . The crystalline structure of TNTs
was determined by X-ray diffraction (XRD) spectroscopy using
4
NO , so more monocyclic isomers, such as limonene and terpi-
3
nolene are formed from cracking the four number ring of a-
pinene. The higher conversion of a-pinene was obtained aer 6 h
and is 77.7% with 76.2% of selectivity for camphene over TNTs-Cl
ꢀ
1
ꢀ
Bruker diffractometer with Cu Ka wavelength (l ¼ 1.5418 A) and
(Table 1, Entry 6).
ꢁ
diffraction angle (2q) ranging from 10–90 . The microscopic
morphology of TNTs was investigated by using transmission
scanning microscopy (TEM, JEOL 2100, Japan) and scanning
electron microscope (SEM, Carl Zeiss Sigma-HD, Germany). The
Table
isomerization
1 Catalytic performance of modified TNTs in a-pinene
a
N
2
adsorption–desorption isotherms was recorded with an ASAP
2
400 physisorption instrument made by Micromeritics Corpora-
Acidity (mmol
Conversion Selectivity
ꢀ1
Entry Catalyst
g
)
Time (min) (%)
(%)
tion (United States). Surface acidity of sample was determined
from the adsorbed pyridine in hexane solution by ultraviolet
spectrum (Shimadzu UV-1900, Japan).
1
2
3
4
5
6
—
360
240
120
120
120
360
—
—
23
TNPs
TNTs-NO
TNTs-SO
TNTs-Cl
TNTs-Cl
—
1.3
70.6
52.3
46.8
64.8
76.2
3
0.46
0.58
0.83
15.2
18.7
42.8
77.7
Catalytic testing
4
The isomerization reaction was carried out in a 5 mL round
bottom ask equipped with a condenser. In a typical run,
a
Reaction conditions:
1
mL a-pinene, 0.05
g
catalyst, reaction
0.050 g catalyst and 1.0 mL of a-pinene were added to the round
ꢁ
temperature 100 C.
bottomed ask and stirred for 5 min, and heated in
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Catalyst characterization
Modied TNTs materials were identied by FT-IR analysis, and
the corresponding FT-IR spectra are depicted in Fig. 1. The FT-
IR spectra of the three samples were similar. For TNTs samples,
ꢀ
1
the broad peak present at around 3214–3600 cm
assigned the surface –OH stretching vibrations. The peak at
can be
ꢀ1
1638 cm
arises from the O–H deformation vibration of
ꢀ
1
H–O–H as well as Ti–OH bonds. The band at 1384 cm can be
25
attributed to the Ti–O–H deformation vibration. In addition,
the peak around 477–486 cm in the spectrum could be same
ꢀ
1
26
to interpret as the crystal lattice vibration of TiO
The XRD patterns of the TNTs-Cl, TNTs-SO
samples were recorded, and the results are presented in Fig. 2.
6
octahedra.
4
and TNT-NO
3
Fig. 2 XRD patterns of modified TNTs.
ꢁ
ꢁ
ꢁ
ꢁ
The peaks at 2q ¼ 10.8 , 24.5 , 28.6 and 48.5 which are still
consistent with the characteristic diffraction signals of titanate
30
facilitate the contact of the substrate molecules with the active
sites during the catalytic reaction process.
nanotubes, the main structure is not changed by the acid
agent. Furthermore, the XRD patterns of the TNTs-Cl and TNTs-
SO showed a little change in the peaks.
4
TEM image of TNTs-Cl is shown in Fig. 3. It can be observed Effect of isomerization condition
from Fig. 3a that the as-prepared titanium dioxide nanotubes
To check the variables such as reaction temperature, catalyst
are interlaced with each other, and nanotubes can be several
micrometers in length. TNTs-Cl showed the nanotubes with
openings at the ends. Titanium dioxide nanotubes have
a hollow tubular structure with an internal diameter of about
dosage, and reaction time effect on the results of the reaction,
a series of single-factor experiment were performed using the
best TNTs-Cl catalyst. Firstly, the conversion of a-pinene and
the selectivity of camphene were investigated as a function of
9
nm, which is consistent with the reported literature. The
temperature at a catalyst amount of 0.05 g between 90 and
morphology and structure of TNTs-Cl were further surveyed by
SEM. It can be seen from the Fig. 3b that it is consistent with the
above TEM observations.
The textural property of TNTs-Cl was further investigated
2
using N adsorption–desorption isotherms, and the results are
ꢁ
1
30 C, and the results are presented in Fig. 5.
It can be seen that an increase of the reaction temperature
caused a signicant increase in the conversion of a-pinene. For
ꢁ
the lower temperatures of 90 C, the values of a-pinene
conversion were quite low and were 19.0%. The higher
present in Fig. 4. TNTs-Cl has the type IV isotherms based on
IUPAC classication, which is characteristic of the mesoporous
materials. TNPs are non porous materials. The surface area
conversion of a-pinene was obtained when the reaction
ꢁ
temperature reached 100 C. It was observed that a greater
2
value of TNTs-Cl was calculated by BET method to be 295 m
g
ꢀ1
3
ꢀ1
. Besides, the pore volume of TNTs-Cl was 0.646 cm g . At
the same time, the results of pore size distribution of TNTs-Cl
by BJH method, which can support the observation of TEM
image. It can be clearly seen that the hydrothermal reconstitu-
tion method and post-modication method can be used to
transform nanoparticles into nanotubes, which increases the
surface area of the material. The opened hollow structure can
Fig. 1 FT-IR spectra of modified TNTs.
Fig. 3 TEM image (a) and SEM photographs (b) of TNTs-Cl.
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Fig. 7 Effect of reaction time on the a-pinene conversion and
camphene selectivity. Reaction conditions: 1 mL a-pinene, 0.05 g
ꢁ
TNTs-Cl, reaction temperature 120 C.
Fig. 4 Nitrogen adsorption–desorption isotherms for TNTs-Cl.
explained by camphene transformed into monocyclic isomer
products at relatively high temperature.
The effects of the catalyst dosage on the isomerization were
further investigated, and the results are summarized in Fig. 6. It
can be obviously seen that an increase in the catalyst amount
resulted in the better catalytic performance in term of a-pinene
conversion. However, camphene selectivity remarkably
decreased with the catalyst amount increase. This may be
caused by the large amount of acidic active centers available,
thus leading to higher production of monocyclic products. This
phenomenon is good agreement with results previously
27
published.
The effects of reaction time on the isomerization were
investigated, and results are presented in Fig. 7. The conversion
of a-pinene increased with reaction time. However, the reaction
rate was very slow aer 60 min as if pseudo-equilibrium was
reached. The conversion of a-pinene remained 97.8% and the
prolongation of the reaction time did not seem to increase
obviously the system's activity. It is worth noting that camphene
selectivity increases with the reaction time. Aer 90 min of
reaction, camphene selectivity remained approximately
unchanged, keeping at 78.5%.
Fig. 5 Effect of the temperature in the catalytic performance of TNTs-
Cl. Reaction conditions: 1 mL a-pinene, 0.05 g TNTs-Cl, reaction time
1
20 min.
increase in the values of a-pinene conversion, which rose from
ꢁ
ꢁ
4
0.2% (100 C) to 98.7% (110 C). The highest selectivity of
ꢁ
camphene was amounted to ca. 78.5% at 120 C. The reaction
temperature was further increased, while the selectivity of
camphene was lowered. The result can be most probably
Fig. 6 Effect of the catalyst dosage on the a-pinene conversion and Fig. 8 Recycling of TNTs-Cl for the isomerization of a-pinene.
camphene selectivity. Reaction conditions: 1 mL a-pinene, reaction Reaction conditions: 1 mL a-pinene, reaction time 120 min, 0.05 g
ꢁ
ꢁ
time 120 min, reaction temperature 120 C.
TNTs-Cl, reaction temperature 120 C.
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Table 2 Comparison of TNTs-Cl with reported catalyst
Cat./a-pinene
(wt%)
Conv.
(%)
Camphene sele.
(%)
ꢁ
Catalyst
Temp. ( C)
Time (h)
Ref.
Zeolites beta
70
80
0.5
1
1
2
2
2
1.6
8
4
5
6
91.4
61.0
90.0
88.2
73.0
98.0
85.0
93.7
97.6
60.0
97.8
43.9
65.0
50.0
67.4
55.0
34.0
54.3
65.6
64.8
65.0
78.5
3
27
11
28
10
29
7
2
4
0Ga-41
PW12
Sulfated zirconia
/Al
3.5
0.6
1.5
1
5
1
17
—
15
5.8
H
3
O40/SiO
2
100
120
150
150
140
155
140
140
120
W
2
O
3
2
O
3
Sulfated ZrO
Illite clay
2
–SiO
2
3+
Fe -loaded clinoptilolite
HO S–(CH –Net ]Cl–ZnCl
[
3
2
)
3
3
2
12
30
Ti-SBA-15
TNTs-Cl
2
This work
Catalyst stability
Acknowledgements
The stability of TNTs-Cl was also evaluated during the isomeri-
zation process. Aer each run, TNTs-Cl was easily removed from
the reaction mixture by centrifugation. The catalyst was washed
and centrifugated repeatedly with hexane and ethanol to remove
the residues. Then, the catalyst was reused in the next run. Fig. 8
shows that the catalyst exhibits fairly outstanding stability. The
conversion of a-pinene was maintained 94% with 71.4%
camphene selectivity aer four runs. While supplemental addi-
tion of 10% TNTs-Cl dosage for the mass lost in recovering, the
conversion and selectivity in h are comparable to the rst run.
This indicates that the catalytic activity of TNTs-Cl catalyst was
not much affected on recycling. The catalytic activities of various
catalysts that have been studied in the previous literatures are
summarized in Table 2. The catalysts with stronger acid sites,
This work was supported by the National Natural Science
Foundation of China (Grant No. 21776068, 21606082), the Key
Scientic Research Fund of Hunan Provincial Education
Department (Grant No. 19A035) and Collaborative Innovation
Center of New Chemical Technologies for Environmental
Benignity and Efficient Resource Utilization.
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isomerization of a-pinene to camphene.
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