Titanate nanotubes-bonded organosulfonic acid as solid acid catalyst for synthesis of butyl levulinate

Article abstract of DOI:10.1039/c7ra12994g

In this study, titanate nanotubes-bonded organosulfonic acid (TNTs-SO₃H) was prepared and employed as an efficient heterogeneous catalyst for esterification of levulinic acid with n-butanol. Two reaction products pseudo n-butyl levulinate (pseudo-BL) and n-butyl levulinate (BL) were detected by GC-MS. The catalyst showed 86.8% conversion of levulinic acid with 99.7% selectivity towards n-butyl levulinate. TNTs-SO₃H exhibited strong acidic sites and high stability even after seven cycles of usage and would be well expected to be a potential candidate for alkyl levulinate production.

Full text of DOI:10.1039/c7ra12994g

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Titanate nanotubes-bonded organosulfonic acid as
solid acid catalyst for synthesis of butyl levulinate†
Cite this: RSC Adv., 2018, 8, 3657
ab
a
a
a
Dabo Jiang,a Xianxiang Liu,
*
Yiping Chen and Dulin Yin
Shuolin Zhou,
In this study, titanate nanotubes-bonded organosulfonic acid (TNTs–SO3H) was prepared and employed as
an efficient heterogeneous catalyst for esterification of levulinic acid with n-butanol. Two reaction products
pseudo n-butyl levulinate (pseudo-BL) and n-butyl levulinate (BL) were detected by GC-MS. The catalyst
showed 86.8% conversion of levulinic acid with 99.7% selectivity towards n-butyl levulinate. TNTs–SO3H
exhibited strong acidic sites and high stability even after seven cycles of usage and would be well
expected to be a potential candidate for alkyl levulinate production.
Received 3rd December 2017
Accepted 2nd January 2018
DOI: 10.1039/c7ra12994g
rsc.li/rsc-advances
outstanding chemical and physical properties.15–17 In addition,
TNTs can be easily obtained by hydrothermal treatment of TiO2
1. Introduction
particles (TNPs) in NaOH aqueous solution, followed by a facile
post-treatment.18 To the best of our knowledge, till date, there
are very few reports of titanate nanotubes as an acid-based
catalyst support.19 Although sulfate-TiO2 nanotubes used as
catalysts have exhibited high activity for the esterication of
acetic acid,20 the challenge of stability occurs in the presence of
water due to the detachment of active SO42ꢀ ions, which leads to
activity loss during recycling of the catalysts. Titanate nano-
tubes, on the other hand, possess large surface area, abundant
surface hydroxyl groups and stable tubular structures, which
advantageously make them a versatile candidate in the catalysis
eld. It was found that the potential to modify TiO2 nanotubes
to incorporate organosulfonic acid groups will open new
opportunities for their use as acid-based catalysts in a variety of
reactions.
In this paper, titanate nanotubes-bonded organosulfonic
acid was prepared by chemical graing-oxidation method using
(3-mercaptopropyl)trimethoxysilane as precursor and TiO2
nanotubes as support (Scheme 1) and then used as catalyst for
the esterication reaction of levulinic acid with n-butyl alcohol.
The effects of different reaction parameters on the catalytic
activity of the catalyst and the reusability of the catalyst aer
several reaction cycles were evaluated. The as-prepared TNTs–
SO3H catalyst was expected to be a good candidate for alkyl
levulinate production from biomass-derived platform
molecules.
Biomass has received a great deal of attention due to the
possibility of becoming an alternative source for the sustainable
production of chemicals and fuels.1 Levulinic acid (LA), an
important biomass-derived platform molecule, has been used
for preparing numerous important products in chemical
industries.2,3 Among them, extensive studies have been focused
on the production of levulinic esters.4,5 In particular, n-butyl
levulinate is a FDA-approved avor and fragrance, and it also
nds application in the solvent and plasticizer sectors.6
Traditional acid catalysts such as H2SO4 and p-toluene-
sulfonic acid are applied for the synthesis of n-butyl levulinate
via esterication of LA with n-butyl alcohol.7 However, it is well
known that the mineral acid-catalyzed homogeneous processes
suffer from severe corrosion, waste and safety problems, which
make these catalysts unsuitable for use. Thus, it would be highly
desirable to develop a recyclable heterogeneous catalyst.8
Recently, various solid catalysts, for example heteropolyacid
supported on acid-treated clay,9 ammonium and silver co-
doped phosphotungstic acid,10 modied H-ZSM-5,11 pyr-
azinium polyoxometalate,12 proved to be effective for esteri-
cation reaction. The development of a new and effective solid
acid catalyst for the synthesis of biomass-derived compounds is
an active ongoing research area.13,14
Over the past few years, TiO2 nanotubes (TNTs) have shown
numerous promising applications, for instance in photo-
catalysis, catalyst support and catalyst owing to some
aNational & Local United Engineering Laboratory for New Petrochemical Materials &
Fine Utilization of Resources, College of Chemistry and Chemical Engineering, Hunan
2. Experimental
Normal University, Changsha 410081, P. R. China. E-mail: dulinyin@126.com; Fax: 2.1 Catalyst preparation
+86-731-8872576; Tel: +86-731-8872576
bChangsha Normal University, Changsha 410100, P. R. China
TiO2 nanotubes were synthesized using
a hydrothermal
synthesis approach described in literature.18 To prepare TNTs–
SO3H, 4 mL of (3-mercaptopropyl)trimethoxysilane was added
drop-wise to a three-necked round-bottom ask containing
† Electronic supplementary information (ESI) available: The GC-MS spectra of the
reaction products, TEM image and FT-IR spectra of the spent catalyst. See DOI:
10.1039/c7ra12994g
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Scheme 1 Synthesis strategy for the preparation of TNTs–SO3H catalyst.
1.0 g of TNTs and 15 mL of dry toluene, and the reaction MS, a Shimadzu GC system equipped with a capillary (30 m
mixture was reuxed for 24 h under continuous mechanical length, 0.32 mm internal diameter, 0.25 mm lm thickness).
stirring. Then, the resultant product was ltered and washed Mass spectrometer conditions were the following: ionization
with acetone. Next, the obtained powder was oxidized with mode: EI, electron energy 70 eV, interface temperature: 523 K,
10 wt% H2O2 in methanol (10 mL) at 298 K for 24 h, followed by ion source temperature: 473 K, mass scan range: 40–640 m/z,
treatment with 1 mol Lꢀ1 H2SO4 for complete protonation. solvent delay 3.0 min. The ow rate of the carrier gas (helium)
Finally, the mixture was ltered and washed with H2O and was 1.0 mL minꢀ1. A split ratio of 1 : 50 was used for the
acetone to obtain the nal TNTs–SO3H catalyst.
injection of 0.2 mL of the solutions. The NIST05S.LIB library was
used for the mass spectrum analysis.
2.2 Characterization
The samples were dispersed in ethanol and poured onto grids
for observation. The morphologies of the samples were
investigated using transmission electron microscopy (TEM)
with an acceleration voltage of 200 kV (JEOL 2100, Japan). N2
adsorption–desorption isotherms were recorded with an ASAP
2400 physisorption instrument made by Micromeritics
Corporation (UNITED STATES). Prior to measurement, all
samples were dried under vacuum at 333 K for 24 h. Pore size
distribution was calculated by the BJH method. Fourier
transform infrared spectra (FT-IR) of the samples were
collected by the KBr pellet technique on a Nicolet 370 infrared
spectrophotometer in the range 500–4000 cmꢀ1. Thermo
gravimetric analysis (TGA) curves were recorded in air ow on
a Netzsch Model STA 409PC instrument with a heating rate of
20 K minꢀ1 from room temperature to 973 K using a-Al2O3 as
the standard material. The acidity strength of catalyst was
determined by Hammett acidity function method combined
with UV-visible spectroscopy (Shanghai Tianmei Co. Ltd,
China). 4-Nitroaniline was chosen as the basic indicator and
CCl4 was chosen as the solvent.
3. Results and discussion
3.1 Characterization of catalyst
The morphologies of the TNTs and TNTs–SO3H were investi-
gated by employing TEM. As shown in Fig. 1(a), the TNTs reveal
a needlelike shape and can be up to several microns in length;
each tube tends to have a diameter of approximately 10 nm.
Fig. 1(b) shows a typical TEM image of the TNTs–SO3H. It is
2.3 Catalytic testing
In a typical run, the experimental procedure was as follows:
1 mmol of LA and 5 mmol of n-butanol together with catalyst
(5 wt% of LA) were added to a round bottom ask and heated in
an oil bath. At periodic intervals, 0.2 mL of the reaction solution
was withdrawn, centrifuged, and analyzed using an Agilent
6890N gas chromatograph (GC). The gas chromatograph was
equipped with a HP-5 capillary column (30 m length, 0.32 mm
internal diameter, 0.25 mm lm thickness) and a ame ioniza-
tion detector (FID). The injection port temperature was 523 K,
the oven temperature was 453 K, and the detector temperature
Fig. 1 TEM images of (a) TNTs and (b) TNTs–SO3H; the insets are the
was 523 K. Degradation intermediates were identied by GC- corresponding photographs.
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attributed to Si–O stretching vibrations. The band at
1374.5 cmꢀ1 corresponds to the stretching frequency of S]O in
SO3H, which is consistent with the reported IR spectra for
–SO3H.22
The thermal stability of TNTs and TNTs–SO3H was
conrmed by TGA as displayed in Fig. 4. The TGA curve of TNTs
shows an initial weight loss (4 wt%) below 373 K, which corre-
sponds to the loss of the physically adsorbed water. At approx-
imately 425 K, the TG curve was slightly inected, possibly
corresponding to the dehydration of interlayer water.23 In the
TGA curve of TNTs–SO3H a mass loss of approximately 8%
weight occurred between 473 and 823 K, corresponding to the
loss of the covalently bound organic group. Simultaneously,
from the TGA, it is understood that TNTs–SO3H has greater
thermal stability, conrming that it can be used in organic
reactions in the range of 353–473 K.
Fig. 2 Nitrogen gas adsorption–desorption isotherms and pore size
distribution curves of TNTs and TNTs–SO3H.
The acidity of the catalyst can be effectively evaluated by
determining the Hammett acidity functions (H0) using UV-
visible spectroscopy. The H0 can be expressed as:
H0 ¼ pK(I)aq + log([I]s/[IH+]s)
observed that the tubular structure of TNTs does not change
aer modication.
Fig. 2 presents the nitrogen adsorption and desorption
isotherms and the corresponding Barrett–Joyner–Halenda (BJH)
pore size distribution curves of the TNTs and TNTs–SO3H. As
shown, it can be observed that the nitrogen adsorption
isotherms do not change aer modication. The as-prepared
TNTs–SO3H have a high specic surface area and pore volume
of 217 ꢁ 8 m2 gꢀ1 and 0.56 cm3 gꢀ1, respectively. In addition, the
obtained pore size distribution plot of TNTs–SO3H shows
a narrow microporous distribution with an average pore diam-
eter of 4.8 nm, which are in good accordance with TEM results.
Fig. 3 shows the FT-IR spectra of the pure TNTs and TNTs–
SO3H samples. The FT-IR characterization of TNTs is consistent
with those reported in the literature.21 The peaks at 3376.9 cmꢀ1
and 1627.7 cmꢀ1 correspond to the –OH stretching and the
bending vibrations of the adsorbed water respectively. In the
case of TNTs–SO3H, C–H stretching vibrations of silylating
agent was observed at 2925 cmꢀ1 and 2825 cmꢀ1. Furthermore,
the bands at 1040.3 cmꢀ1 and 899.6 cmꢀ1 can be collectively
where ‘I’ represents the indicator base (4-nitroaniline), and [I]s
and [IH+]s are the molar concentrations of the unprotonated
and protonated forms of the indicator, respectively. The pK(I)aq
value of 4-nitroaniline is 0.99.
The UV-vis spectra of 4-nitroaniline with the concentration of
1.40 ꢂ 10ꢀ4 mol Lꢀ1 in CCl4 solution is displayed in Fig. 5. The
maximal peak was observed at 329 nm and the corresponding
absorbance value was 1.725. Aer addition of TNTs–SO3H, the
absorbance value of the unprotonated form of the indicator was
decreased to 0.718, indicating that the indicator is partially in the
form of [IH+]s. According to the Lambert–Beer's law, the value of
[I]s/[IH+]s can be determined and calculated. TNTs–SO3H showed
H0 value of 0.84. The total acid density of the catalyst determined
by NaOH titration was around 0.50 mmol gꢀ1
.
3.2 Catalytic activity
In the present study, the reaction products, n-butyl levulinate
(BL) and pseudo n-butyl levulinate (pseudo-BL), were detected
Fig. 3 FT-IR spectra of TNTs and TNTs–SO3H.
Fig. 4 TGA profile of TNTs and TNTs–SO3H.
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effectively. As for the TNTs–SO3H, it exhibits higher catalytic
activity (Table 1, entries 6 and 7).
It is worth noting that the conversion of LA improves to 48.2%
aer 4 h with 97% selectivity of n-BL. This is due to the graed
accessibility of the SO3H sites in catalyst, which is able to facili-
tate the reaction. Moreover, the dispersing property of catalyst in
reaction media was improved. In view of the relative dimensions
of LA and pseudo-BL, the pore openings of TNTs–SO3H should
allow these molecules to freely diffuse inside the pore system of
the TNTs–SO3H and reach both internal and external active sites.
Overall, the trend in conversion of LA over the prepared catalyst
samples is as follows: TNTs–SO3H > TNTs > TNPs.
3.3 Effects of reaction conditions
Fig. 5 Absorption spectra of 4-nitroaniline and TNTs–SO3H in CCl4
solution.
The inuence of reaction temperature and time on LA esteri-
cation over TNTs–SO3H was further investigated and the results
are shown in Fig. 6. At a lower temperature (373 K), the
conversion of LA is 38.6% aer 4 h of reaction. The conversion
increased to 48.2% and 59.3% on carrying out the reaction at
383 and 393 K respectively. The obtained results show that at
high reaction temperature, the reaction had a high conversion.
The selectivity of BL increases on increasing the reaction time
from 0.5 to 4 h, whereas the selectivity of pseudo-BL initially
increases and then decreases with reaction time. This reveals
that the pseudo-BL is an intermediate product during the
reaction process. At 383 and 393 K, the selectivity of BL
improves to 97% and 99.3%, respectively. This is probably due
to the fact that the efficient collisions between the reactions
provide sufficient energy for pseudo-BL conversion into BL at
high temperature, and BL selectivity is therefore improved.
by GC-MS; the corresponding GC-MS spectra are shown in
Fig. S1.† Similar results have recently been found by Ciptonu-
groho et al.24 The catalytic performances of all the materials, i.e.,
TNPs, TNTs and TNTs–SO3H, were investigated for LA conver-
sion and the results are shown in Table 1. Blank experiment was
performed without addition of any catalyst, in which the
conversation of LA was calculated to be 6.3% (Table 1, entry 1).
In contrast, the presence of catalysts resulted in higher product
yields than that corresponding to the blank reaction reference,
revealing the necessity of an acid catalyst to carry out this
reaction. For TNPs, 11.0% LA conversion with 44.6% BL selec-
tivity is obtained (Table 1, entry 2). This may be because TNPs as
a mild Lewis acid catalysts catalyze this reaction.25 The
conversion of LA is 15.4% with 64.9% BL selectivity when
catalyzed by TNTs (Table 1, entry 3). The enhanced reaction
activity may be ascribed to the TNTs with open-tubular struc-
ture, providing more active sites. However, when TNTs are used
as catalyst and the reaction time is further increased to 6 h, the
conversion of LA only reaches 25.5% (Table 1, entry 4). More-
over, TNPs–SO3H obtained from TNPs using the same prepa-
ration method was used as a catalyst, resulting in a quantitative
LA conversion (Table 1, entry 5). This implies that an acid
catalyst with a moderate density and the acid strength of the
acidic sites might be required to allow this reaction to proceed
Table 1 Catalytic activity over different catalyst samplesa
Selectivity/%
Entry Catalyst
Time/h LA Conversion/% BL
Pseudo-BL
1
2
3
4
5
6
7
—
TNPs
TNTs
TNTs
TNPs–SO3H
TNTs–SO3H
TNTs–SO3H
2
2
2
6
2
2
4
6.3
11.0
15.4
25.5
27.9
37.5
48.2
36.8 63.2
44.6 55.4
64.9 35.1
80.4 19.6
81.5 18.5
84.4 15.6
97.0
3.0
a
Fig. 6 Change of LA conversion (a) and BL selectivity (b) as function of time
over TNTs–SO3H catalyst at different reaction temperatures. Experimental
conditions: 1 mmol of LA, 5 mmol of n-butanol, 5 wt% catalyst loading.
Experimental conditions: 1 mmol of LA, 5 mmol of n-butanol, 5 wt%
catalyst loading, 383 K.
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We also investigated the effect of catalyst amount at 393 K.
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LA conversion and BL selectivity rise to 86.8% and 99.7% by
applying 10 wt% of TNTs–SO3H. The increase in both LA
conversion and BL selectivity with an increasing catalyst dosage
would be attributed to an increase in the availability and
number of catalytically active sites.
3.4 Reusability of catalyst
The catalytic stability of the TNTs–SO3H was evaluated in the
reaction by performing consecutive batch reactions at 393 K
with alcohol acid molar ratio 5 : 1 and 10 wt% of catalyst
loading. Aer each catalytic cycle, the catalyst was recovered by
ltration, washed with acetone and dried overnight at 333 K
before reuse. As shown in Fig. 7, it is evident that the catalytic
activity of TNTs–SO3H was practically the same even aer 7
consecutive cycles with around 80% conversion of LA. In addi-
tion, the selectivity of BL on recycled catalyst was almost equal
to that over the fresh catalyst. TGA analysis of seven-times used
and fresh catalyst are very similar (Fig. 2), evidencing that the
–SO3H groups were essentially preserved. In addition, TEM
image and FT-IR spectra (Fig. S2 and S3 in the ESI†) of the spent
catalyst aer repeated experiments show that the TNTs–SO3H
structure and surface groups have no noticeable change. Even
though a slight activity decay was observed, which might be due
to mass loss of the catalyst during several recycling runs, LA
conversion decreases only from 86.8% to 81.2% in 7 cycles. It
can be concluded that the catalyst can be reused without
considerable loss in activity.
Scheme 2 Proposed possible mechanism for levulinic acid esterifi-
cation with n-butyl alcohol.
to that recently reported by Al-shaal et al.26 and Ciptonugroho
et al.24 The enol form of LA can undergo intramolecular lacto-
nization to afford a-angelica lactone, followed by the electro-
philic addition of n-butanol to obtain the intermediate product
pseudo-BL. Subsequently, the protonation of pseudo-BL, in
turn, facilitates another nucleophilic addition and ring opening
occurs resulting in BL.
4. Conclusion
Herein, titanate nanotubes-bonded organosulfonic acid as
a novel solid acid catalyst was successfully prepared, and it
exhibited good catalytic performance for esterication of
biomass-derived LA with n-butanol. A high conversion of LA and
desired product (BL) selectivity were achieved under relatively
mild conditions. It was noted that the catalyst retains its good
catalytic performance even aer seven consecutive reaction
cycles. These results indicated that TNTs–SO3H nanocatalysts
are useful as eco-friendly and ideal materials for esterication
and biomass-based production.
Furthermore, in order to investigate the heterogeneity of the
catalytic reaction, a hot ltration experiment was run. Aer 2 h,
the catalyst was separated by centrifugation and the ltrate was
further stirred for another 12 h at 393 K. It was found that no
product was detected, indicating that the catalytic reaction is
predominantly heterogeneous.
3.5 Proposal for a possible mechanism
According to above-obtained results, the mechanism proposed
for the conversion of LA to BL is shown in Scheme 2. It is similar
Conflicts of interest
There are no conicts to declare.
Acknowledgements
This work was supported by the National Natural Science
Foundation of China (Grant No. 21606082, 21776068) and
Scientic Research Fund of Hunan Provincial Education
Department (Grant No. 15C0816, 16C0114).
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Fig. 7 Stability of the TNTs–SO3H on the esterification of LA with n-
butanol.
582.
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