Birch reduction of benzene in a low-temperature plasma

Article abstract of DOI:10.1002/anie.200805256

(Chemical Equation Presented) Selective and specific dihydrogenation of benzene and other arenes has been observed in a low-temperature helium plasma. A surface Birch reduction mechanism has been proposed in which benzene molecules adsorbed on the dischar

Full text of DOI:10.1002/anie.200805256

Angewandte
Chemie
DOI: 10.1002/anie.200805256
Surface Reactions
Birch Reduction of Benzene in a Low-Temperature Plasma**
Na Na, Yu Xia, Zhenli Zhu, Xinrong Zhang,* and R. Graham Cooks*
We describe the observation of the highly selective, albeit
modest-yielding, dihydrogenation of benzene and other
arenes in a low-temperature plasma (LTP) of helium at
atmospheric pressure. The reduction of benzene is of great
interest as a model for the hydrogenation of aromatic
compounds.[1] Benzene reduction has been achieved by
Figure 1. Schematic view of the experimental setup consisting of an
atmospheric pressure low-temperature plasma (LTP) ion source
configured to provide a dielectric barrier discharge (DBD) and to
allow product ions and neutral species to be swept into a mass
spectrometer.
using various heterogeneous and homogeneous chemical
systems in the solid, liquid, and gas phases.[2] Full reduction
to cyclohexane (C6H12) is the usual result of benzene hydro-
genation, because of the high thermodynamic stability of this
product. Other than the sodium/alcohol Birch reduction[3] and
some well-controlled metal-catalyzed reductions,[4,5] the par-
tial reduction of benzene to cyclohexadiene with high
selectivity is very challenging. Unlike metal-catalyzed hydro-
genations, in which molecular hydrogen is used as the
hydrogen source, in the Birch reduction an alcohol functions
as the proton donor while the alkali metal acts as an electron
donor, with cyclohexadienes being formed as the reduction
products. Partial reduction of nitrogen-containing aromatic
compounds has been encountered in the gas phase,[6,7]
however, there are no reports of gas-phase dihydrogenation
of benzene that give significant yields or selectivity.[8]
Figure 1 shows the experimental setup used for conduct-
ing the plasma processing of organic compounds (for details
see the Supporting Information). Headspace vapors of
samples are carried by the discharge gas (helium) through a
discharge chamber, where a helium plasma is sustained by
dielectric barrier discharge[9] at atmospheric pressure. The
resulting ionic species are introduced into the mass spec-
trometer by on-line mass analysis (that is, no ionization source
is used except for the plasma system itself).
Typical LTP mass spectra derived from benzene, toluene,
and naphthalene are shown in Figure 2A–C. The major signal
in the spectrum of each of these arenes has a mass 2 Da above
that of the reactants. Interestingly, neither molecular radical
+
cations (MC ) nor the protonated molecules ([M+H]+), which
are normally detected using atmospheric pressure ionization,
+
are observed. Besides the [M+2]C signals, lower abundance
ions with masses 16 Da and 32 Da higher than the molecular
mass of the reactant are also present, indicating that oxidation
accompanies reduction. The neutral products formed from
the LTP treatment of benzene were collected cryogenically
and analyzed off-line by EI/GC/MS and tandem mass
spectrometry (for details see the Supporting Information).
These data show that no benzene remains after passage
though the discharge chamber. The major products include
1,3- and 1,4-cyclohexadiene (C6H8), phenol (C6H5OH), and
+
biphenyl (C12H10). Clearly, the [M+2]C signal corresponds to
+
dihydrogenation of the benzene ring ([M+2H]C ) and the
+
+
[M+16]C signal is due to oxygen addition ([M+O]C ).
Compounds such as 1,4-cyclohexadiene, 4-methyl-1,3-dioxo-
lane, and pyridine were also subjected to LTP, and the
corresponding spectra are shown in Figure 2D–F. The major
+
ionic species are the molecular ions (MC , [MÀH]+, [M+H]+),
[*] N. Na, Z. Zhu, Prof. X. Zhang
Department of Chemistry
similar to what has been reported for glow-discharge ioniza-
Tsinghua University
Beijing 100084 (P.R. China)
Fax: (86)10-62782485
E-mail: xrzhang@mail.chem.tsinghua.edu.cn
N. Na, Y. Xia, Prof. R. G. Cooks
Department of Chemistry and
Center for Analytical Instrumentation Development
Purdue University, West Lafayette, IN 47907 (USA)
Fax: (+1)765-494-9421
E-mail: cooks@purdue.edu
[**] Y.X. and R.G.C. acknowledge support from the US National Science
Foundation (CHE04-12782). N.N acknowledges the China Scholar-
ship Council ([2007]3020). Z.Z. and X.Z. acknowledge support from
the National Natural Science Foundation of China (20535020). We
thank Dr. Karl Wood, Campus-Wide Mass Spectrometry Center at
Purdue University, for GC-MS analysis.
Supporting information for this article is available on the WWW
under http://dx.doi.org/10.1002/anie.200805256.
Figure 2. LTP mass spectra of: A) benzene, B) toluene, C) naphthalene,
D) 1,4-cyclohexadiene, E) 4-methyl-1,3-dioxolane, and F) pyridine.
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tion[10] and atmospheric pressure chemical ionization
(APCI).[11]
A certain degree of oxidation is also observed for these
compounds, but no signal corresponding to a reduced species
is detected. Molecules with a variety of functional types were
also subjected to LTP and their product ions were analyzed by
mass spectrometry (Table S1 in the Supporting Information).
Similar to the results shown in Figure 2, reduction by
dihydrogenation was observed for arenes but not for other
types of compounds, even those having functional groups
susceptible to reduction. It is also remarkable that under the
LTP conditions, the reduction of benzene stops after the
addition of two hydrogen atoms to form cyclohexadiene
products (C6H8), in spite of the fact that complete hydro-
genation is thermodynamically more favorable once the
aromatic system is destroyed.[12]
Figure 3. LTP mass spectra of benzene with: A) a discharge chamber
containing a silicon wafer surface exposed to the plasma, and B) a
À
discharge chamber containing an Si H-modified silicon wafer exposed
to the plasma.
To elucidate the reaction mechanism, efforts were
directed at finding the source of the added hydrogen.
Hydrogen atoms from benzene, trace molecules in the
environment, and adsorbate on the discharge surfaces are
all possible contributors to the reduction. The possibility of
benzene itself acting as the hydrogen source was precluded by
analyzing the LTP mass spectrum of hexadeuterobenzene
(C6D6, Mr = 84). If hydrogen was provided from benzene, the
reduction product ions should appear at m/z 88 (addition of
two deuterium atoms). However, the base signal occurs at
m/z 85 with ions at m/z 86 and 84 of much lower abundances,
but no signal at m/z 88 (Figure S2 in the Supporting informa-
tion). The observation of abundant ions of m/z 85 is most
likely due to back exchange of deuterium to hydrogen from
hydrogen was the terminating group.[14] The corresponding
mass spectrum of benzene after LTP treatment is shown in
Figure 3B. As expected, the intensity of the MC ions of
+
benzene (m/z 78) decreases significantly, while ions corre-
+
sponding to the reduction product [M+2H]C (m/z 80)
become the base signal. The above results indicate that
benzene reduction involves hydrogen atoms covalently bound
to the discharge surface.
A surface-assisted benzene reduction pathway is pro-
posed, as shown in Scheme 1. The efficient diffusion of
electrons from the plasma to the surface means the discharge
surface is negatively charged.[15] Electrons with low binding
energies (ca. 1 eV) are adsorbed onto the discharge surface
and can diffuse across it or recombine with positively charged
entities. In the first step of the reduction, the adsorbed
benzene molecule is proposed to capture an adsorbed
electron in its vicinity on the discharge surface, thereby
giving rise to an adsorbed benzene radical anion intermediate.
The proposed next step for the reduction of benzene involves
charge neutralization by reaction of the benzene radical anion
with a positively charged group on the surface, which provides
a hydrogen atom for the first proton addition step. One
+
the initially formed dihydrogenation product ([C6D6+2H]C )
at m/z 86. The possibility of hydrogen arising from chemicals
in the surrounding environment such as water and methanol
(a common solvent) is highly unlikely. This is evident from the
fact that after feeding deuterated water or methanol into the
plasma system during the LTP treatment of benzene, the
dominant signal was still at m/z 80 (Figure S3 in the Support-
ing Information). The chance that surface physisorbed water
may contribute was also ruled out by using properly baked
discharge surfaces.
Based on the above results, the only remaining hydrogen
source is the discharge surface, which is made of glass. Since
glass is terminated by silicon-bound OH groups,[13] it can
potentially provide hydrogen atoms for reduction. To test this
possibility, two thin sections of silicon wafer were inserted
into the discharge chamber to cover the glass slides which
comprise the surfaces at the top and bottom of the chamber.
In this case, the glass slides still functioned as a dielectric
barrier to maintain a stable plasma, but it is the silicon wafer
with a surface terminated with silicon atoms that is exposed to
the plasma. The corresponding LTP mass spectrum of
benzene is shown in Figure 3A. The base signal is now the
molecular ion of benzene at m/z 78, with the reduction
product at m/z 80 appearing in relatively lower abundance.
Since the entire area of the discharge chamber was not
covered by silicon, the exposed glass surface might contribute
to the observation of ions at m/z 80. To confirm the role of
covalently bound surface hydrogen atoms in the reduction of
benzene, the silicon wafer surface was modified so that
possible donating entity in the case of the glass surface is
+
À
(···Si OC···H) . The addition of a second electron followed by
another proton will form the cyclohexadiene product, which
could be collected as such or ionized in the plasma to yield the
+
observed [M+2H]C ions. GC/MS analysis of the benzene
reduction products showed 1,4-cyclohexadiene as the major
reduction product, in a yield 2.5 times greater than that of the
co-product 1,3-cyclohexadiene. It is likely that some 1,3-
cyclohexadiene is formed from 1,4-cyclohexadiene as a result
of isomerization promoted by its greater stability.[16] The
Scheme 1. Mechanism for the surface-assisted reduction of benzene in
an LTP. Benzene molecules are adsorbed (ads.) on the discharge
surface during reduction and this is followed by desorption of the
reduction products into the gas phase.
2018
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further reduction of 1,4-cyclohexadiene is not favorable
because of its low reactivity toward electron addition,[16,17]
which results in dihydrogenation being the main reduction
pathway. Efforts have been made to identify the proposed
reaction intermediates—including benzene radical anions,
cyclohexadienyl radicals, and cyclohexadienyl anions. How-
ever, their unstable nature precludes analysis in the gas phase;
this is in contrast to recent studies of solution-phase reaction
intermediates by mass spectrometry.[18–20] Clearly, further
experimental and theoretical investigations are needed to
verify the proposed mechanism. We also note the similarities
between reduction in the LTP plasma and the conditions
employed in the Birch reduction. Both reactions are initiated
by the addition of a single electron and followed by
protonation, with 1,4-cyclohexadiene observed as the major
product. In both cases only arenes can be reduced effectively,
while weak aromatic systems including pyridine and furan or
other functional groups are less prone to reduction. Substan-
tial differences in the reaction environment exist, especially
the fact that unlike the Birch reduction, the present system is
heterogeneous and the addition of proton donors cannot
directly influence the reaction, as shown by the results of
deuterated methanol or water addition.
Another most interesting phenomenon in the plasma
treatment of benzene is that oxidation accompanies reduction.
As shown in Figure 2A, ions occur at m/z 94 and 110,and were
identified by MS/MS as phenol and dihydroxybenzene, respec-
tively. The formation of phenol by benzene oxidation has been
observed under different atmospheric ionization conditions. It
is believed that the addition of an O(3P) atom to the benzene
ring followed by 1,2-hydrogen transfer results in the production
of phenol.[21] Adding oxygen gas (5% volume) to the helium
plasma, however, reduced the extent of phenol production (as
judged from the abundance of ions at m/z 94), and dramatically
increased another signal at m/z 97 (Figure S4 in the Supporting
Information). The identity of the ions at m/z 97 was interro-
gated by adding 18O2 to the LTP and conducting tandem mass
spectrometry studies (Figure S4 in the Supporting Informa-
tion). Its composition is most likely C6H9O+; however, the
structure of this ion has not been identified since several
possible conformers can interconvert under ion-trap collision-
induced dissociation (CID) conditions. The formation of
C6H9O+ ions at a higher oxygen concentration likely involved
subsequent oxidation of the benzene reduction product (C6H8;
for details see the Supporting Information). The effect of the
discharge gas flow rate on benzene redox reactions in the LTP
system was also evaluated. It is interesting to note that with an
increase in the helium gas flow rate, the relative abundances of
the reduction product at m/z 80 and the oxidation product at
m/z 97 are increased, while that of the oxidation product at
m/z 94 is decreased (Figure S5 in the Supporting Information).
The possible contributing factors are discussed in the Support-
ing Information.
from surface hydroxy groups (in the case of glass). This
heterogeneous reduction, similar to the homogeneous Birch
reduction, is unique in its high selectivity for arenes and its
exclusive formation of dihydrogen reduction products. The
simultaneous benzene oxidation is believed to be due to gas-
phase reactions induced by oxidative radicals such as triplet O
and OHC derived from oxygen or water in the discharge
environment. The overall yield of the benzene dihydrogenation
products under the current experimental conditions is limited
because of the occurrence of other competitive reactions such
as oxidation and dimerization. Future studies are needed to
optimize the reduction of benzene in a plasma system for the
reaction to be applicable in synthesis. The surface Birch
reduction reaction exemplifies the rich but little explored
chemistry that occurs at the discharge surface in a plasma. It
also has longer term potential for synthetic chemistry, consid-
ering the simplicity and low cost of implementing LTP, the
rapid reactions, and the easy sample handling processes.
Received: October 27, 2008
Published online: February 4, 2009
Keywords: Birch reduction · mass spectrometry ·
.
plasma chemistry · redox chemistry · surface reactions
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