Acid-assisted catalytic oxidation of benzyl alcohol by NOx with dioxygen

Article abstract of DOI:10.1016/j.catcom.2010.06.012

Efficient transformation of alcohols to the corresponding aldehydes is an important process in organic synthesis and industry. In the absence of transition metal ion, an acid-activated NO_x has been explored for the catalytic oxidation of benzyl

Full text of DOI:10.1016/j.catcom.2010.06.012

Catalysis Communications 11 (2010) 1189–1192
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Catalysis Communications
journal homepage: www.elsevier.com/locate/catcom
Acid-assisted catalytic oxidation of benzyl alcohol by NOx with dioxygen
Xuebin Sheng a,b, Hong Ma a, Chen Chen a, Jin Gao a, Guochuan Yin c, Jie Xu a,
⁎
a
State Key Laboratory of Catalysis, Dalian Institute of Chemical Physics, Chinese Academy of Sciences, 457 Zhongshan Road, Dalian, 116023, China
b
Graduate University of Chinese Academy of Sciences, Beijing 100049, China
c
School of Chemistry and Chemical Engineering, Huazhong University of Science and Technology, Wuhan 430074, China
a r t i c l e i n f o
a b s t r a c t
Article history:
Efficient transformation of alcohols to the corresponding aldehydes is an important process in organic synthesis
and industry. In the absence of transition metal ion, an acid-activated NOx has been explored for the catalytic
oxidation of benzyl alcohol. Under theoptimalconditions, 95% yield of benzylaldehyde could beachieved atroom
temperature with dioxygen as the terminal oxidant. The reaction mechanism was investigated through UV–
visible spectrometry and cyclic voltammetry, and the key active species for oxidation have been assigned to the
nitrosonium cation.
Received 18 March 2010
Received in revised form 13 June 2010
Accepted 21 June 2010
Available online 1 July 2010
Keywords:
Alcohol oxidation
Dioxygen
© 2010 Elsevier B.V. All rights reserved.
Cyclic voltammetry
Nitrosonium cation
1. Introduction
been developed for catalytic oxidation of benzyl alcohol, and 95% yield
of aldehyde could be achieved at room temperature.
Oxidation plays a key role in laboratory synthesis and industrial
manufacture in which the stoichiometric oxidants like dichromate,
persulfate, permanganate and nitrite are traditionally used [1]. With the
increasing concerns about environmental issues, developing new
methods to use cheap and clean dioxygen as the terminal oxidant and
even without the toxic heavy metal ions has been attracting more and
more attention than ever [2]. Because dioxygen exists in the inactive
triplet ground state, its efficient activation is crucial for its successful
utility in catalytic process. In recent years, NOx has attracted
considerable attention as an efficient oxygen activator through coupling
with p-benzoquinone/Pd [3], 2,2,6,6-tetramethyl-piperidyl-1-oxyl
(TEMPO)/Br2 [4], hypervalent iodine(V)/Br2 [5], [bis(acetoxy)iodo]-
benzene/TEMPO [6] and TEMPO/FeCl3 [7] in versatile oxidations. In
these laboratories, we have also successfully developed a few efficient
oxygen activators including 2,3-dichloro-5,6-dicyano-benzoquinone/
NO2 for the oxidative dehydrogenation of dihydroarenes and vanadyl
sulfate/NO2 for alcohol oxidation [8,9]. In addition to the above systems
for using NOx as dioxygen activator, stoichiometric NOx has also been
applied in the oxidation of 1,4-dihydropyridines to the corresponding
pyridine derivatives, and in formation of carbonyl compound from
alcohols, in which NOx was activated by certain acids to in situ generate
the high active nitrosonium cation intermediate (NO+) [10–12]. In this
communication, a simple inorganic acid/NaNO2/dioxygen system has
2. Experimental
Analytical pure chemicals were used as received without further
purification. Benzyl nitrite was synthesized through nitrosation of
benzyl alcohol according to the literature [13].
The oxidation experiments were carried out in a 10 mL polytetra-
fluoroethylene autoclave. After a certain amount of acid, 2 mmol of
benzyl alcohol and 2 mL of CH2Cl2 were added in the autoclave,
10 mol% NaNO2 were added with stirring. Then, the autoclave was
sealed immediately and charged with dioxygen (0.5 MPa), and then,
the reaction mixture was stirred at 20 °C for 2 h. The products were
analyzed by GC, and further confirmed by GC–MS. The isolated yield
was obtained through silica gel column.
The UV–visible experiments were carried out with a Shimadzu UV-
2550 spectrophotometer. The electrochemical measurements were
performed with a PARSTAT 2273 potentiostat/galvanostat with Ag/AgCl
electrode as reference electrode, a glassy carbon disc electrode (3 mm)
as working electrode, and a platinum wire as counter electrode.
3. Results
3.1. Influence of different acids
Although NaNO2/acid has been generally used as an efficient
stoichiometric oxidant in organic synthesis [11,12], its catalytic
performance is scarcely investigated. Initially, 10 mol% NaNO2 together
with 30 mol% different acids were employed to catalyze benzyl alcohol
⁎
Corresponding author. Tel./fax: +86 411 84379245.
E-mail address: xujie@dicp.ac.cn (J. Xu).
1566-7367/$ – see front matter © 2010 Elsevier B.V. All rights reserved.
doi:10.1016/j.catcom.2010.06.012

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X. Sheng et al. / Catalysis Communications 11 (2010) 1189–1192
Table 1
Influence of different acids on benzyl alcohol oxidation.a
Entry
Acid
Conversion
(%)
Yield (%)
Benzyl aldehyde
Benzyl nitrite
Other
1
2
3
4
5
6
H3BO3
CH3COOH
H3PO4
HOCOCOOH
H2SO4
HClO4
2
2
5
26
27
28
16
99
96
97 (95)
98 (94)
74
73
72
84
0
0
0
0
0
0
0
0
1
4
3
2
5
84
55
100
100
b
7
H2SO4
H2SO4
8b,c
a
Reaction conditions: 2 mmol benzyl alcohol, 0.6 mmol acid, 0.2 mmol NaNO2, 2 mL
CH2Cl2, 0.5 MPa O2, 20 °C, 2 h.
b
0.8 mmol acid, isolated yield to benzyl aldehyde is in bracket.
1 h.
c
Fig. 2. Influence of H2SO4 concentration on benzyl alcohol oxidation.
oxidation using dioxygen as oxidant (0.5 MPa) at 20 °C (Table 1). For the
added acid, one third reacts with NaNO2 to generate NOx, and the
remains could activate NOx to generate the active intermediate for
oxidation. When the weak/medium acids were used in reaction, the
conversions were below 10% with 16–28% selectivity to benzyl aldehyde
(Entries 1–4 in Table 1), and the dominant product was benzyl nitrite,
suggestingthatthese weak/medium acids are inefficient to activate NOx.
When the strong acids like H2SO4 and HClO4 were applied, 84% and 55%
conversion could be achieved with 99% and 96% selectivity to benzyl
aldehyde, respectively. Significantly, in the case of using 40 mol% H2SO4
and 10 mol% NaNO2, 100% conversion of alcohol could be achieved with
94% isolated yield of benzyl aldehyde in 1 h, or 95% yield in 2 h. In the
blank experiments, H2SO4 or HClO4 alone leads to minor aldehyde
formation, supporting that the presence of NaNO2 is crucial for the
efficient catalytic oxidation.
3.3. Influence of acid concentration
Due to the formation of water in the oxidation process, the
concentration of H2SO4 would gradually decrease with the reaction
proceeding. Thus, the influence of the initial acid concentration was
investigated, and the results are presented in Fig. 2. When 25–60%
H2SO4 was used, only 5–9% conversion could be obtained with benzyl
nitrite as the major product, which is quite similar with those from the
weak/medium acids. When the initial concentration of H2SO4 increased
from 60% to 98%, the conversion could be sharply improved from 9% to
100% with the selectivity changing from 35% to 97% for aldehyde,
supporting that the oxidation of benzyl alcohol is apparently acid
concentration dependent.
3.4. UV–visible spectrometry and cyclic voltammetry study on the
solution of NaNO2 in H2SO4
3.2. Influence of acid amount
In line with the catalytic efficiency, the influence of H2SO4
concentration on the NO+ formation has been directly evidenced by
the UV–visible spectrometry. The maximum absorbance of the UV
spectrum is below 200 nm for NaNO2 in aqueous H2SO4 with
concentration less than 60%, which is characteristic of HNO2, whereas
the maximum absorbance shifts to the range of 214–350 nm when
NaNO2 was dissolved into 60–98% H2SO4, indicating the appearance of
the NO+ species [14].
The redox behavior of NaNO2 in H2SO4 was studied by cyclic
voltammetry. In Fig. 3, graph a shows no redox signal in 20% H2SO4. The
oxidation signal of NO to NO+ occurs in 50% and 60% H2SO4, however,
the corresponding reduction signal of NO+ is invisible in 50% H2SO4, and
insignificant in 60% H2SO4 (graphs b and c). In literatures, the absence of
the NO+ reduction signal has been attributed to the rapid hydrolysis of
the NO+ species [15–18]. In 80% H2SO4 (graph d), the redox response of
the NO+ species is evidently close to a reversible process, implicating
that the hydrolysis of the NO+ species becomes ignorable [16,19].
Furthermore, the rising reduction current indicates the increase of the
NO+ concentration with the increase of acid concentration.
The influence of H2SO4 usage is displayed in Fig. 1. When 10 or
20 mol% H2SO4 was used in reaction, the catalytic efficiency was
apparently low, and a large amount of benzyl nitrite was formed as well
as those in the weak/medium acids. When loading 30 mol% H2SO4, the
formation of benzyl nitrite was minimized, and 96% selectivity to benzyl
aldehyde was achieved with 84% conversion. Complete conversion of
benzylalcohol could be achieved with 40 mol% H2SO4, suggesting that in
comparison with stoichiometric oxidation, 60% of H2SO4 and 90% of
NaNO2 could be saved through the catalytic process developed here.
4. Discussion
4.1. The confirmation of the active species
When sodium nitrite was added to the H2SO4 solution, the release
of N2O3 was observed. Although NOx alone can catalyze the oxidation
of certain organic compounds like olefin and hydroquinone with
dioxygen at room temperature, the compounds are limited to those
capable of inducing the disproportionation of NOx to generate the
active NO+ species [20,21], and NOx alone is apparently incapable of
oxidizing relatively stable substrates like benzyl alcohol. In the case of
Fig. 1. Influence of H2SO4 amount on benzyl alcohol oxidation.

X. Sheng et al. / Catalysis Communications 11 (2010) 1189–1192
1191
Fig. 3. Cyclic voltammograms of NaNO2 in H2SO4 solution, mass concentration of H2SO4 solution: a, 20%; b, 50%; c, 60%; d, 80%.
N2O3 as the stoichiometric oxidant, a strong acid was generally
employed as catalyst to generate the active NO+ species for alcohol
oxidations [10]. In addition, the NO+ species was even proposed as
the key active species in the oxidations with stoichiometric NaNO2
[11,12]. Because neither H2SO4 nor NOx alone is capable of oxidizing
benzyl alcohol in the present study, and the formation of the NO+
species from H2SO4 and NaNO2 has been detected by UV–visible and
cyclic voltammogrametry at room temperature, accordingly, the NO+
species could be assigned as the key active species in this H2SO4/
NaNO2 catalyzed alcohol oxidation.
mation of benzyl nitrite to aldehyde was very slow with 50% H2SO4, and
hydrolysis of benzyl nitrite to benzyl alcohol dominated. These results
strongly suggest that the participation of the NO+ species is essential for
the transformation of benzyl nitrite to the corresponding aldehyde.
Thus, a plausible mechanism has been proposed for this inorganic
acid/NaNO2 catalyzed alcohol oxidation (Scheme 1). In the catalytic
cycle, reaction of NaNO2 with acid under oxygen generates NOx which
can be converted to the active NO+ species with proton (H+). One NO+
reacts with benzyl alcohol to generate benzyl nitrite with releasing H+;
then, oxidation of benzyl nitrite in the participation of another NO+
provides the aldehyde product with releasing NO. The reduced NO is re-
oxidized by oxygen to NOx which finally regenerates the NO+ species in
the presence of acid.
Furthermore, the proposal of the NO+ as the active species could
well interpret the reaction data presented above. The formation of NO+
is closely related with the strength of the acids. In the weak/medium
acids, the proton (H+) concentration is limited, which leads to the
shortage of the NO+ cation in the catalytic solution and the failure of
efficient oxidation (entries 1–4 in Table 1), whereas much NO+ species
could be generated in strong acids [22], thus benzyl alcohol was
selectively oxidized to benzyl aldehyde. In addition, the formation of the
NO+ species is remarkably influenced by acid concentration. The UV–
visible and cyclic voltammetric observations indicate that it seldom
exists in H2SO4 with concentrations below 60%. In line with these, the
efficient oxidation could be only observed at H2SO4 beyond 60%.
Furthermore, the increase of acid loading is helpful to resist its dilution
during reaction, thus it provides a higher conversion of substrate (Fig. 1).
5. Conclusions
In summary, this work demonstrated a successful example of acid-
activated NOx as the efficient dioxygen activator for oxidizing alcohol to
the corresponding aldehyde, in which the NO+ species has been
proposed as the key active species in the catalytic cycle. The mechanistic
information revealed here advices that the formation of the NO+ species
4.2. Formation of benzyl aldehyde
Since benzyl nitrite was found to be the dominant product when
weak/medium acid (Table 1) or diluted strong acid (Fig. 2) was used, the
formation of the benzyl nitrite has been suspected to be one step in
aldehyde formation in catalytic cycle as well as in stoichiometric oxidation
with N2O3 [10]. To test this postulation, 0.5 g of benzyl nitrite was
dissolved into 5 mL of CH2Cl2 with 0.1 mL 80% H2SO4 catalyst. After
stirring under nitrogen protection for 0.5 h at room temperature, benzyl
nitrite was completely converted, and 45% benzyl aldehyde was obtained
with 55% benzyl alcohol, meanwhile, NO was released quickly,
implicating that the participation of another NO+ cation is essential for
the oxidation of benzyl nitrite to the corresponding aldehyde (the NO+
cation was generated through hydrolysis of benzyl nitrite). When the
transformation was carried out in air, benzyl nitrite was quantitatively
converted into benzyl aldehyde within 0.5 h. In contrast, the transfor-
Scheme 1. Proposed catalytic cycle for benzyl alcohol oxidation by in situ generated NO+
species.

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X. Sheng et al. / Catalysis Communications 11 (2010) 1189–1192
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Acknowledgement
This work was financially supported by the National Natural
Science Foundation of China (20736010 and 20803074).
Appendix A. Supplementary data
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