Dehydrogenative aromatization of saturated aromatic compounds by graphite oxide and molecular sieves

Article abstract of DOI:10.1002/cjoc.201200174

Graphite oxide (GO) has attracted much attention of material and catalysis chemists recently. Here we describe a combination of GO and molecular sieves for the dehydrogenative aromatization. GO prepared through improved Hummers method showed high oxidative activity in this reaction. Partially or fully saturated aromatic compounds were converted to their corresponding dehydrogenated aromatic products with fair to excellent conversions and selectivities. As both GO and molecular sieves are easily available, cheap, lowly toxic and have good tolerance to various functional groups, this reaction provides a facile approach toward aromatic compounds from their saturated precursors. Copyright

Full text of DOI:10.1002/cjoc.201200174

FULL PAPER
DOI: 10.1002/cjoc.201200174
Dehydrogenative Aromatization of Saturated Aromatic Com-
pounds by Graphite Oxide and Molecular Sieves
Zhang, Xuana(张轩)
Xu, Lianga(徐亮)
Wang, Xitaob(王希涛)
Ma, Ning*,a(马宁)
Sun, Feifei*,a(孙菲菲)
a Department of Chemistry, School of Science, Tianjin University, Tianjin 300072, China
b School of Chemical Engineering and Technology, Tianjin University, Tianjin 300072, China
Graphite oxide (GO) has attracted much attention of material and catalysis chemists recently. Here we describe
a combination of GO and molecular sieves for the dehydrogenative aromatization. GO prepared through improved
Hummers method showed high oxidative activity in this reaction. Partially or fully saturated aromatic compounds
were converted to their corresponding dehydrogenated aromatic products with fair to excellent conversions and se-
lectivities. As both GO and molecular sieves are easily available, cheap, lowly toxic and have good tolerance to
various functional groups, this reaction provides a facile approach toward aromatic compounds from their saturated
precursors.
Keywords graphite oxide, molecular sieves, dehydrogenation, oxidation
Introduction
(below 40 ℃) and/or additional KMnO4 was served,
which was often called improved Hummers method.[5]
Recently Tour and co-workers proposed another im-
proved method involving KMnO4, H3PO4 and H2SO4
(Tour Method).[5b] Compared to original Hummers
method, both improved Hummers method and Tour
method produce GO in better yields. Although the pre-
cise chemical structure of GO is the subject of consid-
erable debate, it is commonly believed that various
oxygen-containing functionalities, such as alcohols, ep-
oxides, ketones and carboxylic acids, were incorporated
to the basal planes or the edges of the graphite platelets
during the aforementioned oxidation processes. One of
the well-known models is shown in Figure 1.[4b]
Dehydrogenative aromatization of partially or fully
saturated aromatic compounds is of great significance in
oil refining industry and organic synthesis. The general
methods involve oxidants, such as S, Se, nitrobenzene,
DDQ and peroxide, or the catalysts containing transition
metals, such as Pt, Pd, Rh, Ru, Re and Cu as well as
their metal-ligand complexes.[1] Used in stoichiometric
quantity or in excess, these oxidants are usually toxic
themselves or tend to generate toxic products in dehy-
drogenation process. Although metals are used in a
small amount, metal-catalyzed reactions frequently suf-
fer from high price and toxicity of the metals as well as
metal leakage and possible contamination of the final
product. Hence the exploitation of new cheap, facile and
environmentally benign oxidation system for dehydro-
genative aromatization is still of importance.
Graphite oxide (GO) and graphene oxide (exfoliated
GO platelet) have got more attention recently because
they are usually used as nanomaterials or precursors to
reduced graphene oxide (rGO) and chemically modified
graphene (CMG) materials.[2,3] GO is generally prepared
from graphite by Hummers or Staudenmaier method.[4]
In original Hummers method, graphite was treated with
KMnO4 in the presence of NaNO3 and H2SO4 at 35 ℃
for 0.5 h. Then the mixture was diluted with water
slowly, causing an increasing in temperature to 98 ℃,
and was maintained at this temperature for 15 min.[4a]
To increase the efficiency of this oxidation reaction,
longer reaction time (above 12 h), lower temperature
The reactions of GO with different reductants have
been widely investigated. Treating GO with hydrazine
hydrate produces rGO, which found extensive applica-
tions on nanomaterials.[6] Besides, MeNHNH2,[7a]
HONH2,[7b] NaBH4,[7c] hydroquinone,[7d] Vitamin C,[7e]
[7g]
[7h]
amino acid,[7f] H2
and NaHSO3
were reported to
reduce GO to rGO. Recently, Bielawski et al. first
demonstrated catalytic activity of GO on the oxidation
of alcohols to aldehydes or ketones as well as the hydra-
tion of alkynes to ketones.[8] Accordingly, a new prepa-
ration process for rGO using alcohols as efficient re-
ductant was reported.[9] They also proposed GO-in-
volved auto-tandem oxidation-hydration-aldol coupling
reactions,[10a] C—H oxidations,[10b] oxidation of thiols
and sulfides,[10c] dehydrative polymerization of benzyl
alcohol,[10d] and ring opening polymerization of cyclic
*
E-mail: mntju@tju.edu.cn, sff@tju.edu.cn; Tel.: 0086-022-27403475; Fax: 0086-022-27403475
Received February 22, 2012; accepted April 17, 2012; published online XXXX, 2012.
Chin. J. Chem. 2012, XX, 1—6
© 2012 SIOC, CAS, Shanghai, & WILEY-VCH Verlag GmbH & Co. KGaA, Weinheim
1

Zhang et al.
FULL PAPER
Preparation of GO[5b]
lactones or lactams.[10e] Meanwhile, GO was reported to
catalyze Friedel-Crafts addition of indoles to α,β-un-
saturated ketones or nitro styrenes,[11a] aza-Michael ad-
dition of amines to activated alkenes,[11b] and oxidative
aromatization of 1,4-dihydropyridines.[11c] Carboxyl-
modified graphene oxide was reported to catalyze the
reaction of peroxidase substrate 3,3,5,5-tetramethyl-
benzidine (TMB) with H2O2 and facilitate glucose de-
tection.[11d] Accordingly, we presume that the oxygen-
containing functionalities of GO, such as hydroxyl,
carbonyl and carboxylic groups, can activate sp3-hy-
bridized C—H bonds of partially or fully saturated aro-
matic compounds and facilitate the dehydrogenation
reaction through hydrogen bonds between GO and the
substrates. Moreover, water-absorbing additives like
molecular sieves (MS) might be helpful to the reaction.
Thus, the dehydrogenative aromatization of saturated
aromatic compounds by a combination of GO and MS is
proposed in this report.
A total of 3.0 g graphite powder and 1.5 g NaNO3
was slowly added into 69 mL concentrated H2SO4 with
stirring in a 250 mL beaker. The mixture was cooled to
0 ℃ by an ice bath. Then 9.0 g KMnO4 was added
slowly in portions to keep the reaction temperature be-
low 20 ℃. The ice bath was removed and the mixture
was warmed to 35 ℃ and stirred for 7 h. Additional
9.0 g KMnO4 was added in portions and the reaction
was kept stirring for 12 h at 35 ℃. The reaction mix-
ture was cooled to room temperature and poured into
400 mL ice water. Then 3 mL 30% H2O2 was added, the
suspension was filtered and the solid was stirred with
400 mL 30% HCl for at least 30 min. The mixture was
filtered, washed by deionized water and filtered repeat-
edly until pH value of the filtrate is about 7. Finally GO
was dried to constant weight in vacuum.
Preparation of TGO[5b]
A total of 3.0 g graphite powder was added into a
500 mL flask, and subsequently a 9∶1 mixture of con-
centrated H3PO4 and H2SO4 (360∶40 mL) was added.
The mixed ingredients were cooled to 0 ℃ by an ice
bath, and 18 g KMnO4 was slowly added to keep the
reaction temperature below 20 ℃. Then the reaction
was heated to 50 ℃ and stirred for 12 h. The reaction
mixture was cooled to room temperature and poured to
400 mL ice water, and 3 mL H2O2 was added. Then the
mixture was filtered and washed as described in the
preparation process of GO above (multiple washing,
filtering and vacuum drying).
Experimental
Materials and apparatus
All chemicals were purchased in analytical grade
and used without any further purification. Molecular
sieves were used as received without further treatment.
1H NMR spectra were recorded in CDCl3 on a Bruker
AVANCE III 400 spectrometer (TMS as the internal
standard). Gas chromatography was carried out using a
Varian 3800 instrument with a CP-sil43CB column.
GC-MS was recorded on a Thermo Finnigan TRACE
DSQ apparatus with a DB-5ms column. FT-IR spec-
troscopy was performed using a Bruker ALPHA spec-
trophotometer. The thermal properties of the GO were
characterized by a NETZSCH STA409PC thermo gra-
vimeter, and the measurements were carried out under
air gas over a temperature range of 30—600 ℃ with a
Representative dehydrogenation procedure
To a 7 mL vial was charged 50 mg 1,2,3,4-tetrahy-
droquinoline and measured quantities of GO. Then 50
mg 4 Å molecular sieves, 0.5 mL CDCl3 and a magnetic
stir bar were added. The vial was sealed by a Teflon cap
and heated at 100—150 ℃ for 12 h. After the reaction
was complete, the mixture was cooled to room tem-
perature, diluted with 0.5 mL CDCl3 and filtered
through a thin layer of 300 mesh quartz sands. The fil-
trate was analyzed by 1H NMR, GC-MS and GC.
－
1
ramp rate of 6 ℃•min . Elemental analysis was car-
ried out using an Elementar Vario Micro cube instru-
ment.
OH
HO2C
OH
OH
OH
CO2H
OH
HO2C
OH
O
O
O
OH
O
O
CO2H
O
OH
O
O
HO
OH
OH
HO2C
O
O
HO
OH
OH
CO2H
HO
HO
O
OH
OH
OH
O
HO2C
OH
OH
HO
OH
CO2H
O
O
O
Figure 1 The schematic model of GO monolayer (graphene oxide).
2
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© 2012 SIOC, CAS, Shanghai, & WILEY-VCH Verlag GmbH & Co. KGaA, Weinheim
Chin. J. Chem. 2012, XX, 1—6

Dehydrogenative Aromatization of Saturated Aromatic Compounds
Results and Discussion
tra showed that this species was reduced graphite oxide
and the peaks for the oxygen-containing functional
Initially, dehydrogenation of 1,2,3,4-tetrahydroqui-
noline was conducted. The amount of GO and MS as
well as the reaction temperature have been investigated.
As shown in Table 1, the optimum condition was found
as 200 wt% GO, 50 mg 4 Å MS, 100 ℃. The reaction
was performed for 12 h and the conversion decreased as
the reaction time was shorter. Under these conditions,
tetrahydroquinoline was transformed to quinoline in
more than 99% conversion and no byproduct was de-
groups (3419 cm－ for O—H, 1733 cm－ for C＝O)
were reduced (Figure 2). TG-DSC analysis showed that
GO lost about 10% of weight in air at 160 ℃ (Figure
3), which indicated that the thermo reduction of GO
took place slowly below 160 ℃. Therefore, the recov-
ered reduced graphite oxide (rGO) was probably formed
from the oxidation-reduction reaction with tetrahydro-
quinoline rather than from the thermal decomposition of
GO. Moreover, the recovered rGO showed much less
activity than GO (Entry 13, Table 1). Thus, we can de-
duce that the oxygen-containing functionalities in GO
play important role in dehydrogenation process. Be-
cause the dehydrogenating reaction by the combination
of GO and MS also took place in the atmosphere of ni-
trogen, GO acted mainly as an oxidant in the reaction.
1
1
1
tected by H NMR and GC-MS. GO prepared by im-
proved Hummers method was found superior to that by
original Hummers method (Entries 3, 5 vs. Entry 9),
possibly due to the introducing of more oxygen atoms
into graphite by the former method. In addition, im-
proved Hummers method produces GO in much higher
yield. So this method is more preferable in the reaction.
GO prepared by Tour method showed slightly lower
oxidative activity than that obtained by improved
Hummers method (Entries 3, 5 vs. Entry 8). As such,
GO prepared by improved Hummers method was used
as the oxidant in the reaction, although GO prepared by
original Hummers method showed almost the same ac-
tivity when combined with MS (Entry 10). MS was em-
ployed only to absorb water generated in the reaction
and it did not show oxidative activity in the control re-
action (Entry 11). Moreover, graphite showed much less
activity (Entry 12).
Table 1 Dehydrogenation of 1,2,3,4-tetrahydroquinoline to
quinoline under various conditionsa
Figure 2 FT-IR spectra (KBr) of GO (a) and the recovered
black solid after the reaction of GO with 1,2,3,4-tetrahydroqui-
noline in a weight ratio of 2∶1 at 200 ℃ for 12 h (b).
Entry Reagent
GO amount/wt%
Temp./℃ Conv.b/%
1
GO
200
200
200
100
200
30
100
120
150
100
100
150
150
150
150
150
100
100
100
67
2
GO
90
3
GO
98
4
GO, MS
GO, MS
GO, MS
GO, MS
TGOc
HGOd
78
5
＞99
87
6
7
50
97
8
200
200
97
9
94
10
11
12
HGO, MS 200
MS
＞99
0
graphite
rGOe
200
200
19
13
33
a
A total of 50 mg 1,2,3,4-tetrahydroquinoline and GO was re-
acted with or without 50 mg 4 Å MS in 0.5 mL CDCl3 for 12 h.
Figure 3 TGA and DSW plots of GO.
b
1
c
Conversion determined by H NMR. GO prepared by Tour
d
e
method. GO prepared by original Hummers method. Recov-
ered from Entry 5.
To investigate the scope of the reaction, various hy-
drogenated heterocyclic compounds were screened and
the result is summarized in Table 2 (Entries 1—6). In
most cases, the amount of GO was increased to make
the good conversion and excellent selectivity. Nitrogen-
containing heterocycles such as 6-methyl-1,2,3,4-tetra-
hydronquinoline, 1,2,3,4-tetrahydroisoquinoline and
After the reaction, GO became a black solid, which
was filtered out of the reaction mixtures through a fil-
tering membrane. Additional CDCl3 (about 0.5 mL) was
frequently added to facilitate the filtration. FT-IR spec-
Chin. J. Chem. 2011, XX, 1—6
© 2012 SIOC, CAS, Shanghai, & WILEY-VCH Verlag GmbH & Co. KGaA, Weinheim
www.cjc.wiley-vch.de
3

Zhang et al.
FULL PAPER
Table 2 Dehydrogenation of partially or fully saturated aromatic compountsa
Entry
1
Reactant
Product
Temp./℃
Conv.b/%
Select.b/%
100
150
＞99
97c
N
H
N
H3CO
H3CO
100
150
＞99
2
3
4
＞99c
N
N
H
150
150
＞99
＞99
NH
N
N
N
H
H
5
6
7
8
150
200
200
200
76
42
74
97
O
O
N
N
H3CO
H3CO
CN
CN
CN
9
200
100
40
85 15
100
100d
75c
41 19
44 24
47 22
10
11
12
280
200
59
48
76 24
O
O
O
O
O
13
200
13
O
CHO
CHO
14
15
16
150
200
200
95
36
15
CH3CO
CH3CO
a
b
Conditions: 200 wt% GO, 12 h, 50 mg 4 Å MS and 0.5 mL CDCl3. Conversion and selectivity determined by 1H NMR, GC-MS and
GC. c 50 wt% GO. d 100 wt% GO, 80 ℃.
2,3-dihydroindole afforded the corresponding aromatic
products in above 97% conversions and the byproducts
were not observed (Entries 2—4). However, 2,3-dihy-
drobenzofuran and 5,6,7,8-tetrahydroquinoline only
gave 76% and 42% conversion, respectively (Entries 5,
6).
4
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© 2012 SIOC, CAS, Shanghai, & WILEY-VCH Verlag GmbH & Co. KGaA, Weinheim
Chin. J. Chem. 2012, XX, 1—6

Dehydrogenative Aromatization of Saturated Aromatic Compounds
Conclusions
We then focused our attention on partially or fully
saturated aromatic compounds without heteroatom. Al-
though higher temperature was needed for dehydroge-
nation of tetralin, temperature above 200 ℃ led to
more byproducts. Thus, tetralin was converted to naph-
thalene in 74% conversion under the conditions of
200% GO, 50 mg 4 Å MS, 0.5 mL CDCl3, 200 ℃ and
12 h (Entry 7, Table 2). It is worthy to note that no par-
tially dehydrogenating products, such as 1,2-dihy-
dronaphthalene and 1,4-dihydronaphthalene, were deter-
mined by 1H NMR. The substitution at benzene ring by
electron-donating group, such as methoxy group, facili-
tated the reaction, while substitution of tetralin H-1 by
cyano group made the dehydrogenation difficult (En-
tries 8, 9).
In conclusion, we describe a combination of GO and
molecular sieves for the dehydrogenative aromatization
of fully or partially saturated aromatic compounds. GO
prepared through improved Hummers method showed
higher activity than that prepared by other methods in
this reaction. By this reaction, saturated aromatic com-
pounds were converted to their corresponding dehydro-
genated aromatic products with fair to excellent conver-
sions and selectivities. This method provides a facile
approach toward aromatic compounds, especially nitro-
gen- or oxygen-containing heterocycles, through dehy-
drogenative aromatization from their saturated precur-
sors.
1,2-Dihydronaphthalene underwent disproportion of
dehydrogenation and hydrogenation to yield naphtha-
lene and tetralin with a ratio of about 2∶1 simultane-
ously (Entry 10). We found that more amount of GO led
to higher conversion of the substrate and lower selectiv-
ity of dehydrogenation product. Surprisingly, decalin
was transformed to tetralin and naphthalene in 59%
conversion and about 3∶1 ratio (Entry 11). Hence an
aromatic ring tends to be important but not necessary
for the dehydrogenative aromatization. Then cyclohex-
ene and its derivatives were employed as the substrates
(Entries 12—14). Although cyclohexene and 1-cyclo-
hexene-1,2-dicarboxylic anhydride did not give high
conversions, 3-cyclohexene-1-carbaldehyde was con-
verted to benzaldehyde with excellent conversion. Fur-
thermore, cyclohexylbenzene underwent the dehydro-
genation to give biphenyl in 36% conversion (Entry 15),
and acetyl group derived cyclohexylbenzene only af-
forded the corresponding product in 15% conversion
(Entry 16).
Shortly before our investigation was accomplished,
Bielawski et al. also described the C-H oxidation using
GO.[10b] However, few examples of dehydrogenative
aromatization were reported by them and the conver-
sions were not satisfactory. The reason why our oxida-
tive system is more efficient mainly lies on our usage of
GO prepared by different method and the addition of
MS. The elemental analysis results showed that GO
prepared by improved Hummers method had much less
C/O ratio (approximately 1.18) than that of GO used by
Bielawski et al. through original Hummers method (C/O
ratio 1.94). Thus, improved Hummers method furnishes
GO with more oxygen-containing functionalities than
original Hummers method, which might lead to the
higher oxidative ability of GO. Moreover, water formed
in the reaction was adsorbed in MS, and it also facili-
tated the conversion of substrates. As both GO and MS
are easily available, cheap, lowly toxic[12] and have
good tolerance to various functional groups, this reac-
tion provides a facile approach toward aromatic com-
pounds from their saturated precursors.
Acknowledgement
We thank the National Natural Science Foundation
of China (No. 20802049) for financial support.
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