Preparation of a novel sulfonated carbon catalyst for the etherification of isopentene with methanol to produce tert-amyl methyl ether

Article abstract of DOI:10.1016/j.catcom.2010.03.001

A novel method for the preparation of acid carbon catalyst from glucose and 4-hydroxybenzenesulfonic acid (p-HBSA) was reported. Glucose was first hydrolyzed to hydroxymethylfurfural that reacted with p-HBSA (a phenol compound) to form a phenolic (PF)-like resin containing -SO₃H groups. The resin was carbonized and sulfonated further in concentrated sulfuric acid at 443 K to form an amorphous carbon material with a surface area of 22 m²/g. This acidic carbon contained 3.1 mmol/g of strong surface acid sites (-SO₃H), and thus exhibited good catalytic activity for the etherification of isopentene with methanol to produce tert-amyl methyl ether.

Full text of DOI:10.1016/j.catcom.2010.03.001

Catalysis Communications 11 (2010) 824–828
Contents lists available at ScienceDirect
Catalysis Communications
journal homepage: www.elsevier.com/locate/catcom
Preparation of a novel sulfonated carbon catalyst for the etherification of isopentene
with methanol to produce tert-amyl methyl ether
Yu Zhao, Hezhi Wang, Yupei Zhao, Jianyi Shen ⁎
Lab of Mesoscopic Chemistry, School of Chemistry and Chemical Engineering, Nanjing University, Nanjing 210093, China
a r t i c l e i n f o
a b s t r a c t
Article history:
Received 8 January 2010
Received in revised form 28 February 2010
Accepted 2 March 2010
Available online 8 March 2010
A novel method for the preparation of acid carbon catalyst from glucose and 4-hydroxybenzenesulfonic acid
(
(
p-HBSA) was reported. Glucose was first hydrolyzed to hydroxymethylfurfural that reacted with p-HBSA
a phenol compound) to form a phenolic (PF)-like resin containing –SO H groups. The resin was carbonized and
3
sulfonated further in concentrated sulfuric acid at 443 K to form an amorphous carbon material with a surface
2
area of 22 m /g. This acidic carbon contained 3.1 mmol/g of strong surface acid sites (–SO
good catalytic activity for the etherification of isopentene with methanol to produce tert-amyl methyl ether.
2010 Elsevier B.V. All rights reserved.
3
H), and thus exhibited
Keywords:
Acidic carbon
©
Hydrolysis of glucose
4-hydroxybenzenesulfonic acid (p-HBSA)
Phenolic resin containing sulfonic groups
Carbonization
Etherification reaction
1
. Introduction
hydrolyzed into hydroxymethylfurfural (HMF) catalyzed by an acid and
then the resulted HMF was polymerized into a resin, which could be
subsequently carbonized to mesoporous carbon materials. The same
method might be used for the preparation of acidic carbon materials
with some appropriate modifications.
The acid catalyzed reactions are important in chemical industry [1].
Sulfuric acid is frequently used for various acid catalyzed reactions, such
as alkylation, esterification, isomerization, hydration and etherification
[
2]. However, the problems of pollution and separation are always there
Accordingly, a new strategy for the synthesis of acidic carbon materials
was proposed in this work. Glucose and 4-hydroxybenzenesulfonic acid
(p-HBSA) were used as the raw materials. Glucose was first hydrolyzed
into HMF which reacted with p-HBSA (a phenol compound) to form a
when sulfuric acid and other inorganic acids are used as catalysts. Thus,
enormous efforts have been made in the academic and industrial
communitiesinorder to findsolid acid catalysts toreplace them. Various
solid acids have been studied, including proton exchanged resins [3],
3
phenolic (PF)-like resin. Since the use of p-HBSA, great amount of –SO H
2
−
solid superacids (e.g., SO
4
2
/ZrO ) [4] and sulfonated carbon materials
groups were introduced into the PF-like resin. The resin was then
carbonized and further sulfonated in concentrated sulfuric acid, resulting
in an acid carbon enriched with surface sulfonic groups. This synthesis
process is schematically shown in Fig. 1.
Since no high temperature treatment process was involved in this
new method, the surface area of and surface sulfonic groups on the
acidic carbon thus prepared would not be very low. The catalytic
activity of this acidic carbon material was evaluated by the reaction of
etherification of isopentene with methanol to produce tert-amyl
methyl ether (TAME). TAME is an important additive in gasoline [11].
This reaction is typically catalyzed by Brønsted acids catalysts such as
sulfuric acid and proton exchanged resin [12].
[5]. The acidic resins are widely used as catalysts in industry, but they
cannot be used at temperatures higher than 423 K and may swell in an
organic solvent resulting in the crush of catalyst bed. On the other hand,
acidic carbon catalysts do not swell and exhibited much better thermal
stability. Recently, Hara, et al. prepared a series of acidic carbon catalysts
through the direct carbonization of raw materials such as sugar [6,7] and
cellulose [8,9] followed by the sulfonation of resulted carbons, and
found that such acidic carbon materials exhibited good catalytic
behavior. Their work promoted the development of acidic carbon
catalysts. However, the acidic carbons prepared by Hara et al. possessed
2
low surface areas (∼2 m /g).
Recently, a method was developed in our group for the synthesis of
mesoporous carbons from carbohydrates such as fructose, glucose,
sucrose and so on [10]. The carbohydrates were found to be first
2. Experimental
2.1. Preparation of the catalyst
In a typical synthesis, 10 g glucose and 20 g p-HBSA (65% aqueous
⁎
Corresponding author. Tel./fax: +86 25 83594305.
E-mail address: jyshen@nju.edu.cn (J. Shen).
solution) were dissolved in 30 ml de-ionized water in a beaker to form
1566-7367/$ – see front matter © 2010 Elsevier B.V. All rights reserved.
doi:10.1016/j.catcom.2010.03.001

Y. Zhao et al. / Catalysis Communications 11 (2010) 824–828
825
Fig. 1. Schematic diagram for the synthesis of acidic carbon materials from glucose and 4-hydroxybenzenesulfonic acid.
a clear brownish red solution. The solution was heated at 358 K under
stirring for about 2 h during which water was simultaneously
evaporated and a black viscous paste was formed. The paste was
then cured at 303 K for 1 h to form a black solid (resin). It was washed
with de-ionized water for several times until the filtrate was neutral
and no sulfate ions were detested, and then dried at 353 K. The resin
thus prepared was denoted as HS-R, and further heated in
concentrated sulfuric acid (98%) at 443 K for 12 h, during which
carbonization and sulfonation processes occurred. Afterwards, the
carbonized sample was washed thoroughly by de-ionized water, and
dried at 353 K overnight. The acidic carbon thus prepared was
denoted as HS-C.
capacitance manometer for precise pressure measurements. Samples
were evacuated at 353 K for 12 h before the measurements at 353 K.
The thermogravimetry-mass spectrometry (TG-MS) coupling
technique employing Netzsch STA 449C TG-DSC and Pfeiffer Thermo-
star Mass Spectrometer was used to probe the surface function groups
and their stabilities. The quartz capillary tube was used as the
interface between the thermal analyzer and quardrupole. The
different m/z mass numbers were monitored in the multiple ion
detection (MID) mode. The measurement was carried out under
flowing N with a linear ramp of temperature from room temperature
2
to 873 K at a rate of 10 K/min.
FTIR spectra were obtained on a Thermo Fisher Scientific Nicolet-
iS10 spectrometer. Samples were palletized with KBr.
For comparison, an acidic carbon (denoted as Hara-C) was also
prepared according to the method reported by Hara, et al. [6]. In this
method, glucose was carbonized in N
2
at 673 K for 15 h. The carbonized
2.3. Catalytic tests
sample was ground to a powder and then heated in sulfuric acid at423 K
for 15 h. Afterwards, the black powder was washed thoroughly and
dried at 353 K.
The etherification of isopentene with methanol to produce TAME
was carried out in an autoclave equipped with a magnetic stirrer. The
reaction was performed at 353 K and 0.3 MPa (autogenous pressure)
with the alkene to alcohol ratio of 1:1. Specifically, a 100 ml Teflon-lined
autoclave was charged with 10 g isopentene, 4.57 g methanol, 35.43 g
solvent (toluene) and 0.5 g catalyst. The reactor was sealed and purged
2
.2. Catalyst characterizations
2
Surface areas were measured by the adsorption of N at 77.3 K
using a Micromeritics ASAP 2000 surface area analyzer and the
2
with N for 3 times. It was then heated to 353 K and remained at the
specific areas were calculated according to the Brunauer–Emmett–
temperature for 20 h under stirring. The reaction products were
analyzed by a gas chromatography equipped with a SE-30 capillary
column and a FID detector.
Teller (BET) equation. Samples were degassed in flowing N
83 K before the measurements.
X-ray diffraction (XRD) patterns were collected on a Philips X'Pert
Pro powder diffractometer using a Ni-filtered Cu Kα radiation
λ=0.15418 nm). The 2θ scans covered the range of 10 to 80° with
2
for 5 h at
3
3. Results and discussion
(
a step of 0.02°.
3.1. Textural and structural properties
The density of surface acid sites was measured by a neutralization
titration method described in [13]. In short, the sample was added
into an aqueous solution of NaCl (in excess), and HCl formed due to
Table 1 shows the surface areas and acid densities of catalysts. It is
seen that both the surface area and acid density were significantly
higher for HS-C than for Hara-C. The Hara-C was prepared according
to Hara et al. and possessed similar surface area and acid density as
they reported [6]. The sample HS-R possessed a low surface area
+
the exchange of Na with proton on sulfonic groups was titrated by a
standard solution of NaOH. In this way, the density of strong surface
acid sites or the number of sulfonic groups was determined. On the
other hand, when the sample was added into a standard solution of
NaOH (in excess), all the surface acid sites could be reacted with
NaOH and the remaining NaOH was titrated by a standard solution of
HCl. In this way, the total number of surface acid sites was measured.
The microcalorimetric adsorption of ammonia was performed by
using a Setaram Tian–Calvet C-80 heat-flux microcalorimeter,
connected to a gas-handling system equipped with a Baratron
2
(1 m /g). When it was treated in sulfuric acid at 443 K, the acidic
carbon material (HS-C) was formed with the reactions of carboniza-
tion and sulfonation. The HS-C thus formed exhibited a surface area of
2
22 m /g.
Fig. 2 shows the XRD patterns of HS-C and HS-R. The HS-R exhibited
a broad and weak peak in the range of 10°–30°, a typical XRD pattern
for a polymer. This diffraction peak was significantly enhanced after it

826
Y. Zhao et al. / Catalysis Communications 11 (2010) 824–828
Table 1
Surface area and acid density of the catalysts.
Catalysts
Acid density (mmol/g)
SO
Surface area
2
(m /g)
–
3
H
Total
HS-C
HS-R
Hara-C
3.1
1.8
0.9
6.3
4.2
1.7
22
1
1
H
D001
2
SO
4
20.4
4.7
20.4
4.7
–
12
Note: D001 is a proton exchanged styrene resin.
was treated in concentrated sulfuric acid at 443 K, indicating the
formation of a carbonized material. In fact, the carbonization of organic
compounds in sulfuric acid has been documented [14]. The diffraction
peak round 24° belongs to the C (002) face. Since the peak is narrow for
crystalline carbon (the full width at half maximum (FWHM) is
1
7
.85°± 0.05°), while it is broad for amorphous carbon (FWHM is
.00°± 0.50°) [15], the XRD pattern of HS-C shown in Fig. 2 clearly
showed that it was an amorphous carbon.
3
.2. Surface acidity
The surface acid densities of the samples determined by the
titration method are presented in Table 1. The data showed that the
sample HS-R possessed 1.8 mmol/g strong acid sites, indicating the
polymerization of HMF and p-HBSA for the formation of a resin
containing sulfonic groups. The total acid density reached 4.2 mmol/g
3
for HS-R, indicating the presence of acidic sites other than –SO H in
the resin. These acidic sites might be hydroxyl groups on p-HBSA
polymerized into the resin. After treated in sulfuric acid at 443 K, HS-R
was converted into HS-C, which possessed sulfonic groups of
3
.1 mmol/g. Apparently, the sulfonation reaction occurred simulta-
neously with the carbonization process of HS-R.
Microcalorimetric adsorption of NH was also used to determine
the surface acidity of HS-C and compared with that of Hara-C. Fig. 3(a)
shows the isotherms for the adsorption of NH over the two samples.
Apparently, the uptake of NH was significantly higher on HS-C than
3
3
Fig. 3. Microcalorimetric adsorption of NH at 353 K over HS-C and Hara-C.
3
3
on Hara-C, indicating more acidic sites on HS-C than on Hara-C,
agreeing with the results of titration. Fig. 3(b) shows the heat of
Hara-C from the dehydration of sulfonic groups. Without the
endothermic process, the initial heat should be the highest, and the
surface acidity might be descried by the initial heat and coverage for
the adsorption of ammonia. Owing to the endothermic process, the
initial heat could not be used to judge the strength of acidic sites.
However, the results in Fig. 3(b) seemed to indicate that the strength
of strong acidic sites due to sulfonic groups was stronger on Hara-C
adsorption versus coverage for the adsorption of NH
was low for the adsorption of NH over the two carbon samples. The
heat increased and passed a maximum with the increase of coverage.
Since it is known that the adsorption of NH on the pyrosulfuric group
is an endothermic process [16], the low initial heat indicated the
endothermic process for the adsorption of NH on the carbon samples,
3
. The initial heat
3
3
3
than on HR-C. This was probably due to the high density of –SO
3
H on
H/Ǻ ) that it might be taken as
4
). On the other hand, the density of
2
and thus some pyrosulfuric groups might be formed on the HR-C and
Hara-C, which was so high (5.4 –SO
being similar to oleum (SO –H SO
H was much lower on HS-C (0.85 –SO
3
3
2
2
–SO
3
3
H/Ǻ ).
3.3. Thermal stability of surface functional groups
The surface sulfonic groups were decomposed upon the heat
treatment, releasing SO
detected by TG-MS. Fig. 4 shows the results. It is seen that SO
evolve around 453 K and a great amount of SO was evolved until 823 K.
SO was evolved simultaneously, but was much less than SO . These
results clearly indicated the presence of a great amount of –SO H on the
surface of HS-C, which were stable at the temperatures not higher than
53 K. The presence of –SO H was also confirmed by FTIR results (see
Fig. S1 in Supplementary material). A band around 1032 cm
clearly seen for –SO H groups on HS-C [17]. In addition, a great amount
of CO (m/z=44) was detected at the temperatures higher than 423 K,
2
(m/z=64) and SO
3
(m/z=80) that could be
2
began to
2
3
2
3
4
3
−
1
was
3
2
demonstrating the presence of a substantial amount of –COOH groups
on the surface of HS-C, which might be attributed to the weak surface
acid sites.
Fig. 2. X-ray diffraction patterns of HS-R and HS-C.

Y. Zhao et al. / Catalysis Communications 11 (2010) 824–828
827
Fig. 5. Conversion of isopentene and rate of formation of TAME (TOF) for the reaction of
isopentene with methanol at 353 K after 20 h.
Fig. 4. Evolution of SO
the TG-MS measurement.
2 3 2 2
, SO and CO from HS-C when it was linearly heated in N during
respectively). The FTIR spectrum of HS-C after the three cycles of use
−
1
for the reaction showed that the band around 1032 cm
3
for –SO H
remained (see Fig. S1 in Supplementary material), indicating the
stability of HS-C under the conditions employed for the reaction.
3
.4. Catalytic activity
The catalysts were used for the etherification of isopentene with
4
. Conclusions
methanol to produce TAME. This is a typical Brønsted acid catalyzed
reaction, in which the mechanism has been documented [18]. Under
the reaction conditions employed in this work, TAME was the only
product and no by-products were detected. Fig. 5 compares the
3
A resin containing a great amount of –SO H groups was obtained
by heating glucose with 4-hydroxybenzenesulfonic acid (p-HBSA) at
53 K, since glucose was first converted to hydroxymethylfurfural
HMF) which was then polymerized with p-HBSA to form the resin.
3
(
catalytic activity of H
styrene resin D001. The conversion of isopentene was found to be
2.4, 55.2, 38.3 and 49.6% with the catalysts H SO , HS-C, Hara-C and
2 4
SO , HS-C, Hara-C and a proton exchanged
The resin was carbonized and further sulfonated in concentrated
6
2
4
sulfuric acid (98%) at 443 K to obtain an acidic carbon material (HS-C)
D001, respectively. Sulfuric acid was the most active for the reaction.
2
with the surface area of 22 m /g and –SO
3
H density of 3.1 mmol/g.
The HS-C exhibited high activity for the etherification reaction.
This acidic carbon exhibited excellent catalytic activity and stability
for the etherification of isopentene with methanol to produce TAME,
due to its relatively high surface area and density of sulfonic groups.
The new method seemed better than the sulfonation of directly
carbonized carbohydrates for the preparation of acidic carbon
catalysts.
2 4
Although it was slightly less active than H SO , it was significantly
more active than Hara-C. The high activity of HS-C might be due to its
high density of strong acid sites and high surface area. Since the
amount of a catalyst used for the reaction was 0.5 g, the number of
protons related to –SO
0.2, 1.55, 0.45 and 2.35 mmol, and thus the rate in TOF (turnover
frequency) for the production of TAME was correspondingly calcu-
3
H groups in the reactor was calculated to be
1
−
1
Acknowledgements
lated to be 44, 254, 607 and 150 h
D001, respectively. Thus, protons in HS-C and Hara-C seemed much
more active than those in liquid H SO and D001 for the etherification
reaction, indicating the structure and acidic strength of sulfonic
groups in the two acidic carbons might be different from those in
sulfuric acid and proton exchanged resin.
2 4
for H SO , HS-C, Hara-C and
This work was supported by the MSTC (2005CB221406), the NSFC
2
4
(
20673055) and the Jiangsu Province of China (BE2009145).
Appendix A. Supplementary data
The catalytic stability of HS-C was also studied. It was used
successively for three cycles of the reaction and the conversion of
isopentene was found to be nearly the same (55.2, 55.9 and 54.3%,
Supplementary data associated with this article can be found, in
the online version, at doi:10.1016/j.catcom.2010.03.001.

828
Y. Zhao et al. / Catalysis Communications 11 (2010) 824–828
[
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