Hydrothermal carbon enriched with sulfonic and carboxyl groups as an efficient solid acid catalyst for butanolysis of furfuryl alcohol

Article abstract of DOI:10.1016/j.catcom.2019.02.016

Carbonaceous material (GC-PTSA-AC) functionalized with both high density of SO₃H and COOH groups was prepared by one-step hydrothermal carbonization of glucose with p-toluenesulfonic acid and acrylic acid. This novel carbon could be used directly for the alcoholysis of furfuryl alcohol and n-butanol without any post-modification, and it was found to be more efficient than the monofunctional hydrothermal carbon only decorated with SO₃H or COOH groups. The reason was attributed to the larger amount of COOH groups on GC-PTSA-AC, cooperating with the SO₃H active sites to facilitate the butanolysis reaction.

Full text of DOI:10.1016/j.catcom.2019.02.016

Accepted Manuscript
Hydrothermal carbon enriched with sulfonic and carboxyl groups
as an efficient solid acid catalyst for butanolysis of furfuryl
alcohol
Jinfan Yang, Hongyu Zhang, Zhifeng Ao, Sufeng Zhang
PII:
S1566-7367(19)30050-0
DOI:
Reference:
https://doi.org/10.1016/j.catcom.2019.02.016
CATCOM 5627
To appear in:
Catalysis Communications
Received date:
Revised date:
Accepted date:
11 December 2018
19 February 2019
19 February 2019
Please cite this article as: J. Yang, H. Zhang, Z. Ao, et al., Hydrothermal carbon enriched
with sulfonic and carboxyl groups as an efficient solid acid catalyst for butanolysis of
furfuryl alcohol, Catalysis Communications, https://doi.org/10.1016/j.catcom.2019.02.016
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ACCEPTED MANUSCRIPT
Hydrothermal carbon enriched with sulfonic and carboxyl groups as an
efficient solid acid catalyst for butanolysis of furfuryl alcohol
Jinfan Yang,a* Hongyu Zhang,a Zhifeng Ao,a Sufeng Zhang,a*
a Shaanxi Provincial Key Laboratory of Papermaking Technology and Specialty
Paper Development, Key Laboratory of Paper Based Functional Materials of China
National Light Industry, Shaanxi University of Science & Technology, Xi’an
710021, China
*Corresponding author: Dr. Jinfan Yang, Prof. Sufeng Zhang
College of Bioresources Chemical and Materials Engineering
Shaanxi University of Science & Technology
Xi’an, Shaanxi, 710021, China
Tel.: +86-029-86168575;
Fax: + 86-029-86168575;
E-mail: yangjinfan@sust.edu.cn; sufengzhang@sust.edu.cn

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Abstract
Carbonaceous material (GC-PTSA-AC) functionalized with both high density of
SO3H and COOH groups was prepared by one-step hydrothermal carbonization of
glucose with p-toluenesulfonic acid and acrylic acid. This novel carbon could be used
directly for the alcoholysis of furfuryl alcohol and n-butanol without any post-
modification, and it was found to be more efficient than the monofunctional
hydrothermal carbon only decorated with SO3H or COOH groups. The reason was
attributed to the larger amount of COOH groups on GC-PTSA-AC, cooperating with
the SO3H active sites to facilitate the butanolysis reaction.
Key words: Hydrothermal carbon; Solid acid; Butanolysis; Synergic effect.

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1. Introduction
The efficient transformation of renewable biomass into fuels and valuable
chemicals has attracted considerable interest in green chemistry [1-3]. Furfural is the
hydrolysis product of hemicellulose, which can be further hydrogenated to furfuryl
alcohol (FAL) over copper-based catalysts [4]. Alkyl levulinates, an important
downstream derivative of FAL, has been identified by the U.S. Department of Energy
as one of the top ten biorefinery candidates [5]. In particular, n-butyl levulinates (BL)
is a versatile chemical with numerous applications as solvent, flavor, and plasticizer
[6]. Moreover, BL has been proved to be a more promising fuel additive than ethyl
levulinate, as it possesses better miscibility in diesel fuel at low temperature [7]. The
acid-catalyzed alcoholysis of FAL is the common route to produce alkyl levulinates.
Substitution of homogeneous acids by heterogeneous acids is recommended to avoid
handling corrosion and environmental problems. To date, various solid acid catalysts
including zeolites [8,9], resins [10], metal oxides [11,12], aluminosilicate [13],
modified mesoporous silica [5,14], graphene [15], supported heteropoly acids [16],
organic-inorganic hybrid solid acid [17], and ionic liquids [18] have been explored for
the synthesis of alkyl levulinates. Nevertheless, the high cost and tedious preparation
protocol are major obstacles to the wide applications of these solid acids. Recently
aluminum salts were found to be cheap and efficient catalysts for catalyzing
alcoholysis, but the separation of the catalysts was difficult [19,20]. Consequently, it
is desirable to develop an easy-to-prepare, economical, and efficient solid acid
catalyst for production of BL.

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Carbonaceous solid acid catalysts, mainly derived from biomass, have gained great
attention in recent years as a novel type of “green catalysts”. They are generally
prepared by high-temperature pyrolysis (>400 ℃) followed by sulfonation with
concentrated sulfuric acid [21-23]. However, due to harsh carbonization conditions
and emission of harmful gases, this synthetic route is not energy-effective and
ecofriendly. In this case, more facile and environmentally benign processes for the
preparation of carbonaceous materials by in-situ functionalization are essential for the
catalysis. Hydrothermal carbonization provides a sustainable synthetic method using
mild conditions (<250 ℃), self-generated pressures, and water as the carbonization
medium [24-26]. It is well-known that the oxygenated groups have been assumed to
be the active centers in many acid-catalyzed reactions [27], whereas the raw
hydrothermal carbon (HTC) only possesses small quantity of carboxyl and phenolic
groups, therefore, much attention has been paid to in-situ functionalization on the
HTC matrix to prepare promising solid acid catalysts. For instance, Titirici et al. [24]
showed that carboxyl groups could be enriched on HTC by commixing acrylic acid
and glucose in the carbonization process. Wang et al. [28] and Qi et al. [29]
synthesized sulfonated HTC by the hydrothermal carbonization of glucose with p-
toluenesulfonic acid or sulfosalicylic acid. These modified HTCs exhibited good
catalytic activities for esterification, hydrolysis and Beckmann rearrangement [28-30].
Motivated by the aforementioned ideas, we found that many research efforts have
concentrated on preparing monofunctional HTCs by merely introducing sulfonic or
carboxyl groups to the carbon matrix, and the interactions of different functional

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groups in catalytic reactions are less discussed.
Herein, a method to synthesize bifunctional HTCs enriched with both sulfonic and
carboxyl groups was presented by hydrothermal carbonization of glucose in presence
of two co-monomers. This carbon was characterized and first employed to catalyze
the butanolysis of FAL to produce the versatile chemical BL. The results showed that
this carbonaceous solid acid displayed better activity than those of HTC only
functionalized with SO3H or COOH groups.
2. Experimental
2.1. Catalyst preparation
The bifunctional carbon was prepared as follows. 5 g glucose, 5 g p-toluenesulfonic
acid (PTSA), and 5 g acrylic acid (AC) were dissolved in 50 mL deionized water. The
mixture was placed in a stainless-steel autoclave and treated at 180 °C for one day.
The resulting product was filtered, washed with water and ethanol, and dried
overnight. The sample was denoted as GC-PTSA-AC. The control sample GC was
prepared in the same way without adding co-monomers. As a comparison, 5 g PTSA
or 5 g AC was added into the glucose solution before it was transferred to the
autoclave, the following procedure was the same with that described above, and the
resulting carbonaceous materials were denoted as GC-PTSA or GC-AC. Hβ zeolite
(SiO2/Al2O3=25) were provided by Nankai University. Amberlyst-15 resin was
purchased from Sigma-Aldrich. Sulfonated active carbon was prepared according to
the literature [23].
2.2. Catalyst characterization

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The powder X-ray diffraction patterns were collected by PANalytical X’Pert-Pro
diffractometer using Cu Kα radiation source (λ = 0.1541 nm). The SEM images were
recorded by VEGA3 TESCAN scanning electron microscope. Elemental analysis was
determined by Elementar vario EL b apparatus. XPS analysis was conducted with an
ESCALAB 250Xi spectrometer using Al Kα X-ray source. FT-IR was recorded by a
VERTEX 70 with the KBr pellet. NH3-TPD was conducted with a Micromeritics
AutoChem II 2920 Automated Catalyst Characterization System coupling with a mass
spectrometry detector. The acidity densities of the carbon materials were measured by
the Boehm titration method [29].
2.3. Activity test
Typically, 0.196 g FAL, 5.92 g n-butanol and 0.1 g catalyst were added into a Parr
batch reactor. The reaction mixture was stirred and heated at 120 °C after introduction
of N2. The liquid sample was analyzed by HP6890 gas chromatograph equipped with
a HP-INNOWAX capillary column and a flame ionization detector. The intermediates
were identified by mass spectrometry coupled with gas chromatography.
3. Results and discussion
3.1. Catalyst characterization
The formation of GC-PTSA-AC catalyst involved the dehydration of glucose into
the key intermediate 5-(hydroxymethyl)-2-furaldehyde) as the first step and
subsequent polymerization and carbonization as a second step [24]. During this
process, PTSA and AC could not only react with the intermediates to introduce SO3H
and COOH groups to the carbon matrix, but also work as acids to catalyze the

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dehydration of glucose. The XRD patterns of GC-PTSA-AC and GC catalysts are
shown in Fig. 1. Both samples exhibited a broad C (002) diffraction peak at 10–30°
and a weak C (101) diffraction peak at 35–50°, which were assigned to the amorphous
carbon composed of aromatic carbon sheets oriented in a considerably random
fashion [21]. Meanwhile, GC-PTSA-AC displayed a stronger diffraction intensity,
indicating a higher aromatization degree than GC. The elemental analysis (Table S1)
also supports this view, as the carbon content increased from 62.21% to 65.64% after
modification with co-monomers. The SEM images (Fig. S1) showed that upon
hydrothermal carbonization of glucose, micrometer sized (2–4 μm) carbon spheres
with smooth surfaces were obtained. The addition of co-monomers induced a change
in the particle morphology, that is, the diameter of the carbon spheres increased to 3–5
μm and the carbon surface was no longer smooth.
The FT-IR spectra of GC-PTSA-AC and GC catalysts are shown in Fig. 2. The
distinct bands at 1043, 1173 and 1396 cm-1 on GC-PTSA-AC corresponded to
O=S=O stretching vibration in SO3H groups [31], indicating SO3H groups have been
successfully incorporated into GC-PTSA-AC due to the participation of PTSA in the
hydrothermal process. Meanwhile, both catalysts possessed the bands at 1616, 1700
and 3425 cm-1, which were assigned to the stretching vibration of C=C in aromatic
carbons, C=O in COOH groups, O-H in COOH and phenolic OH groups, respectively
[31]. It is worth noting that GC-PTSA-AC had a stronger adsorption peak at 1700 cm-
1 in comparison with GC, highlighting its higher content of COOH groups. The
quantitative results of various functional groups were summarized in Table 1. The

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SO3H density was calculated according to the sulfur content in the catalyst
composition, since the sulfur component was only in the form of SO3H groups with
S2p binding energy of 168.7 eV (Fig. S2). The density of COOH and phenolic OH
groups were determined by Boehm titration. As we can see, COOH and phenolic OH
groups were generated upon hydrothermal carbonization of pure glucose without
additives, but the density of COOH groups was only 0.71 mmol g-1. The addition of
AC or PTSA as a co-monomer to the glucose solution successfully incorporated
COOH or SO3H group to the carbon, leading an obvious increase in COOH content
(from 0.71 to 1.28 mmol g-1) or SO3H content (from 0 to 0.51 mmol g-1). When the
two co-monomers were carbonized with glucose together, the as-prepared GC-PTSA-
AC was found to possess the largest content of SO3H and COOH groups at the same
time. The NH3-TPD result shows that GC-PTSA-AC catalyst possessed both weak
and strong acid sites on the carbon surface, because two desorption peaks around
200 °C and 530 °C were observed as depicted in Fig. 3. The former resulted from the
interaction of NH3 with incompletely formed carbon sheets and COOH groups, and
the latter was due to the desorption on SO3H groups [32]. The missing peak at
elevated temperature on GC catalyst indicates the participation of co-monomers in the
hydrothermal process could improve the acid strength of conventional HTC.
3.2. Activity
The catalytic activities of GC-PTSA-AC as well as the control samples were
evaluated for the butanolysis of FAL into BL (Scheme S1). All experiments were
conducted at 120 °C for 4 h with n-butanol to FAL molar ratio of 40. 2-

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butoxymethylfuran (BMF) was identified as the main intermediate according to GC-
MS results, and the by-product dibutyl ether resulted from the non-productive
consumption of n-butanol (Fig. S3), the phenomenon was in accordance with previous
reports [14, 16]. Trace of humins derived from the polymerization of FAL was also
obtained on these carbonaceous catalysts (Table S2). The reason for low humins yield
was that the amount of soluble humin precusors was excluded from the amount of
solid residue by washing with ethanol and a large excess of water, besides, the low
concentration of FAL suppressed the humins formation. As shown in Table 2, GC and
GC-AC samples exhibited negligible activity for alcoholysis, indicating that weak
acidic COOH groups and phenolic OH groups on these samples could not effectively
catalyze this reaction. Conversely, upon adding PTSA as co-monomer, GC-PTSA
sample exhibited a good activity with 82.4% BL yield, which was owing to the
increased acidity provided by SO3H groups. A comparative test was carried out under
the catalysis of PTSA, acetic acid and phenol which represents the SO3H, COOH and
OH groups on GC-PTSA, respectively. Both acetic acid and phenol were inactive,
whereas PTSA exhibited near complete conversion of FAL with 75.0% yield of BL.
The above results unambiguously revealed that only SO3H groups were the intrinsic
active sites for the alcoholysis. It is worth mentioning that up to 16.0% yield of
humins (Table S2) was obtained in the reaction catalyzed by PTSA, decreasing the
reaction temperature could suppress the formation of humins and increase the BL
yield to 94.2%. However, homogeneous PTSA suffer from disadvantages such as
corrosion, difficulty in the separation and acid-waste generation, leading to it

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uncompetitive with respect to the carbonaceous solid acids.
In addition, compared to GC-PTSA sample, GC-PTSA-AC displayed a further
increment of activity with a complete conversion of FAL and 91.0% yield of BL, even
though both carbons possessed similar SO3H densities. The consumption rate of FAL
and the formation rate of BL on GC-PTSA-AC were observed to be faster than those
on GC-PTSA as well (Fig. S4). This result implies that the SO3H density was not the
only factor to influence catalytic activity. As mentioned in Table 1, GC-PTSA-AC
was found to possess higher density of COOH groups than GC-PTSA, suggesting that
the presence of COOH groups are crucial for the conversion of FAL to BL. The
hypothesis was confirmed by the adsorption test. GC-PTSA-AC exhibited stronger
adsorption capacity of FAL in comparison with GC-PTSA (1.72 v.s. 1.08 g g-1, Table
S3). The phenomenon can be rationalized by the higher content of COOH groups on
GC-PTSA-AC, because they can adsorb hydroxyl group of FAL through forming
hydrogen bonds, thus improving the accessibility of the FAL molecules to SO3H
active sites and promoting the alcoholysis reaction. Moreover, the order of the
adsorption capacity of different carbons (GC-PTSA-AC > GC-AC > GC-PTSA, Table
S3) agreed with the order of their COOH density, further indicating the COOH group
could improve the affinity of catalyst towards FAL. Consequently, the remarkable
activity of GC-PTSA-AC was attributed to the synergic effect of SO3H and COOH
groups.
The product yield as a function of reaction time over GC-PTSA-AC is
demonstrated in Fig. 4. The reactant was almost completely consumed in 1 h, whereas

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only 50.0% yield of BL and 45.8% yield of BMF were formed respectively. As the
reaction proceeded, the BL yield increased gradually at the same time a decrease in
the BMF yield was observed. It indicates that the butanolysis of FAL occurred
through the formation of BMF as intermediate which further converted to BL as the
reaction progressed. Increasing the temperature or catalyst amount could facilitate the
conversion of FAL and BMF to final product, the maximum yield of BL was obtained
at 120 °C with 0.1 g catalyst (Fig. S5 and S6). Besides, the FAL alcoholysis with
other alcohols (such as methanol, ethanol or n-propanol) over GC-PTSA-AC catalysts
could also achieve satisfactory product yields (> 91.0%, Table S4), indicating its great
versatility.
The catalytic performance of some typical solid acid catalysts, including Hβ
zeolite, Amberlyst-15, and sulfonated active carbon, were also evaluated under the
same conditions (Table S5). Hβ zeolite gave a low BL yield of 40.3%, which was
likely due to its weak acidity as the maximum NH3 desorption temperature was only
375 °C (Fig. S7). Amberlyst-15, a commercial resin with high SO3H density (4.8
mmol g-1), displayed satisfactory activity with 86.2% BL yield. Compared to
Amberlyst-15, GC-PTSA-AC catalyst had lower SO3H density, however, it displayed
higher catalytic activity, further indicating that the presence of COOH groups in GC-
PTSA-AC were crucial for the alcoholysis. Sulfonated active carbon displayed a
moderate activity with 61.0% BL yield. This catalyst had a low SO3H density (0.31
mmol g-1) based on the elemental analysis, thereby the active sites were supposed to
be insufficient to convert the FAL and BMF to final product.

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3.3. Catalyst stability
The reusability of catalyst is vital to evaluate the efficiency of catalyst. Fig. 5
shows that 100.0% FAL conversion and 87.0% yield of BL were still obtained after
four times cycles, indicating a good catalytic stability of GC-PTSA-AC catalyst. In
order to exclude the possibility of homogeneous catalysis, the catalyst was filtered out
under hot conditions and the reaction was continued with the filtrate under same
conditions. The result (Fig. S8) shows that the reaction stopped when the catalyst was
removed, and the reaction resumed with a similar rate after re-adding the catalyst. It
proves that the leaching of SO3H active species was negligible. FT-IR and element
analysis further demonstrated that the sulfur content in the spent catalyst was almost
constant (Fig. S9 and Table S6). The high stability of GC-PTSA-AC can be ascribed
to the high density of COOH group on the carbon surface, as the electron-
withdrawing COOH groups are supposed to increase the electron density between
carbon and sulfur atoms, resulting in the tight attachment of SO3H groups to carbon
framework [33].
According to previous literatures, ionic liquid [MIMBS]3PW12O40 could gave 88%
BL yield via alcoholysis for 12 h, and the activity decreased obviously with 65% BL
yield in the second run [34]. Aluminosilicate Al-TUD-1 only gave 60% BL yield at
140 °C [13]. Up to 96% yield of BL could be obtained on SO3H/SBA-15 while the
catalyst reusability was not reported [5]. Commercial α-Fe2O3 gave 86% yield of BL
at a high temperature of 250 °C [11]. Zn1TPA/Nb2O5 was found to be an active and
stable catalyst for the butanolysis, yielding 94% BL after four cycles, nevertheless, the

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catalyst preparation was costly and complicated [16]. In this work, a cheap
carbonaceous catalyst GC-PTSA-AC could be prepared by one-step facile
hydrothermal carbonization of glucose and organic acids. It possessed great
advantages with respect to economy, activity and reusability.
4. Conclusion
A novel carbonaceous material enriched with abundant SO3H and COOH groups
was synthesized via one-step hydrothermal carbonization of glucose with two organic
co-monomers (PTSA and AC) under mild conditions. The carbon exhibited good
catalytic activity and stability for butanolysis of FAL. The superior performance
resulted from the synergic effect of SO3H and COOH groups on the carbon surface.
Because SO3H groups played the role of active sites, and the high density of COOH
groups improved the affinity of carbonaceous material towards FAL.
Acknowledgements
This work was supported by Natural Science Foundation Research Project of Shaanxi
Province (No. 2017JQ2015), Scientific Research Foundation of Shaanxi University of
Science and Technology (No. 2015BJ-27).

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Figure Captions
Fig. 1. XRD patterns of GC-PTSA-AC and GC catalysts.
Fig. 2. FT-IR spectra of GC-PTSA-AC and GC catalysts.
Fig. 3. NH3-TPD results of GC-PTSA-AC and GC catalysts.
Fig. 4. The conversion and product yield as a function of reaction time over GC-
PTSA-AC catalyst. Reaction conditions: 0.196 g FAL, 5.92 g n-butanol, 0.1 g
catalyst, 120 °C.
Fig. 5. Stability test of GC-PTSA-AC catalyst. Reaction conditions: 0.196 g FAL,
5.92 g n-butanol, 0.1 g catalyst, 120 °C.

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Fig. 1

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Fig. 2

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Fig. 3

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Fig. 4

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Fig. 5

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Table 1. Acidity distributions of different carbonaceous materials.
S
SO3H
COOH
Ar-OH
Total acid
densityd
(mmol g-1)
1.86
Samples
contenta density
densityb
densityc
(wt %)
(mmol g-1)
(mmol g-1) (mmol g-1)
GC
-
-
0.71
1.28
0.80
1.33
1.15
1.17
1.20
1.18
GC-AC
GC-PTSA
-
-
2.45
1.66
0.51
0.53
2.51
GC-PTSA-AC 1.68
3.04
a Based on elemental analysis.
b Obtained by subtracting concentration of sulfonic groups from titration results with
NaHCO3.
c Obtained by subtracting titration results using NaHCO3 from titration results of
NaOH.
d Obtained by titration with NaOH.

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Table 2. Catalytic performance of FAL alcoholysis to BL over different catalystsa.
Entry
Catalyst
Conversion
(%)
Yield (%)
BL
0
BMF
0.8
1
2
3
4
5
GC
5.2
GC-AC
GC-PTSA
GC-PTSA-AC
PTSA
6.0
0
1.7
94.5
82.4
91.0
8.0
100.0
100.0
4.2
75.0 / 94.2b 3.6 / 1.5b
a Reaction conditions: 0.196 g FAL, 5.92 g n-butanol, 0.1 g catayst, 120 °C, 4 h.
b 80 °C.

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Highlights
 A novel carbon catalyst possessed both high density of SO3H and COOH groups.
 It exhibited good activity and stability for butanolysis of furfuryl alcohol.
 COOH groups cooperated with SO3H active sites to facilitate the reaction.