Catalytic benzene coupling on caesium/nanoporous carbon catalysts

Article abstract of DOI:10.1039/a806604c

Caesium/nanoporous carbon materials have a very high affinity for hydrogen, breaking the extremely energetic C-H bond in benzene and promoting its condensation to biphenyl, opening a new class of chemical reactions to heterogeneous catalysis.

Full text of DOI:10.1039/a806604c

Catalytic benzene coupling on caesium/nanoporous carbon catalysts
Mark G. Stevens,a Keith M. Sellers,a Shekhar Subramoneyb and Henry C. Foley*a
a Center for Catalytic Science and Technology, Department of Chemical Engineering, University of Delaware,
Colburn Laboratory, Academy Street, Newark, Delaware 19716, USA. E-mail: foley@che.udel.edu
b Du Pont Company, Experimental Station, PO Box 80228, Wilmington, Delaware 19880-0228, USA
Received (in Bloomington, IN, USA) 24th August 1998, Accepted 19th October 1998
·
Caesium/nanoporous carbon materials have a very high
affinity for hydrogen, breaking the extremely energetic C–H
bond in benzene and promoting its condensation to biphenyl,
opening a new class of chemical reactions to heterogeneous
catalysis.
C12H10 2, which reacts with another benzene radical anion to
propagate the chain and to produce polyphenyl. Finally,
caesium metal alone, when reacted with benzene in THF and
then washed with water, yielded 1,1A,4,4A-tetrahydrobiphe-
nyl.14
In our study, batch reactions‡ of benzene over a 10% Cs/NPC
produced biphenyl and terphenyl products. Fig. 1 displays
results of several experiments at various temperatures. From
these data the apparent first-order activation energy can be
calculated as 34 kJ mol21, indicating a coupling-limited, free-
radical reaction or that the reaction is taking place in the internal
diffusion limited regime. Table 1 displays the typical carbon
selectivity. Gas chromatography measurements of the vapor
phase over the products confirmed the presence of hydrogen.
Control experiments using pure NPC produced no detectable
biphenyl. Finally, long-term experiments (8 g benzene reacted
over 0.5 g 10% Cs/NPC at 450 °C for 72 hours)confirmed the
catalytic nature of the reaction, by acheiving over 5 turnovers,
based on the total molar caesium content.
To confirm the catalyst is not some form of caesium metal
that has leached into the liquid phase, we performed a second set
of experiments (Table 2), in which benzene was converted to
biphenyl in the vapor phase. A mixture of benzene in argon (5
mol%) was circulated over the catalyst at 450 °C and 2 bar.§ By
means of a continuous separation built into this recirculating
reactor, nearly 100% selectivity to biphenyl was achieved at
Graphite intercalation compounds of alkali metals have been
extensively studied1 and shown to have very high affinity for
hydrogen. The compound C24K promotes deuterium exchange
with methane2 and hydrocarbons having acidity equal to or
greater than that of benzene.3 Béguin and Setton4 found C8K
could condense benzene to biphenyl at 298 K. However, these
materials are pyrophoric, have relatively low surface area, and
exfoliate at reaction temperature.5 In a previous paper6 we
revealed caesium entrapped in high-surface-area ( ~ 1000
m2 g21) nanoporous carbon is an excellent catalyst for double-
bond migration in olefins but, unlike alkali-metal graphite
intercalation compounds, is not pyrophoric and is thermally
stable towards desorption up to 773 K.
By preparing nanoporous carbon (NPC) from poly(furfuryl
alcohol)–poly(ethylene glycol) mixtures7–10 we obtain chem-
ically inert solids that contain nanopores,7,8 with sizes distrib-
uted around 0.5 nm, and transport pores11 with sizes distributed
around 10 nm that allow access to catalytic sites contained by
the nanopores.12 Since caesium’s atomic diameter (0.48 nm
covalent) is close to the mean nanopore size of the NPC, the
vapor of elemental caesium is rapidly and strongly adsorbed.6
By this means we have produced materials containing up to
42% caesium by weight.
Through magnetic-susceptibility measurements and electron-
paramagnetic-resonance spectroscopy studies6 of these Cs/NPC
materials, we have shown them to contain unpaired electrons.
Significantly, an identical g factor value of 2.0026 was
obtained† both for the free radicals in the carbon precursor, as
well as those present in material loaded with 15% Cs by weight.
The g-shift is roughly proportional to the mean spin–orbit
coupling constant, which in the case of caesium should be large.
The EPR data, however, are not indicative of such an effect.
Instead the g-value demonstrates that caesium’s orbitals do not
participate; hence, the electrons from caesium are donated to the
carbon. These two facts, along with their demonstrated ability to
promote the double-bond migration in olefins,6 suggest that the
Cs/NPC materials should have an affinity for hydrogen similar
to, or higher than, their purely graphitic analogues. In 1976, at
least two studies3,4 found that potassium graphite would couple
benzene to biphenyl. Béguin and Setton4 found that a proton
donor/acceptor solvent (THF) was required to promote the
reaction and that water was required to extract the biphenyl
product from the alkali-metal graphite. Matsuzaki and cowork-
ers13 performed a detailed study of the polymerization of
benzene over alkali-metal graphite compounds. Based on a
series of experiments, they proposed a polymerization mecha-
Fig. 1 In each experiment 8 g benzene was reacted for three hours over 0.5
g of a 10% Cs by weight catalyst: (2) indicates individual experiments, (-)
indicates average for a given temperature.
Table 1 Average carbon selectivity of reaction at 1–3% conversion of
benzene
Product
Carbon selectivity (%)
Biphenyl
75
4
7
·
nism: two benzene radical anions C6H6 2 form, then couple and
o-Terphenyl
m-Terphenyl
p-Terphenyl
22
lose dihydrogen forming C12H10 the dianion of biphenyl, a
14
species they observed by Raman spectroscopy. This dianion
loses an electron to produce the biphenyl radical anion,
Chem. Commun., 1998, 2679–2680
2679

Table 2 Results of vapor-phase study
Notes and references
Cs0 in
catalyst conversion
(%)
Benzene
† EPR experiments were performed on a Bruker ER-200D electron-
paramagnetic-resonance analyzer at 298 K. The authors wish to thank Dr.
Paul Krusic and Mr. Steven Hill of the DuPont Company for their assistance
with these measurements.
Benzene
charge/ml
Catalyst/
g
Cs°
turnovers
Time/
h
(%)
‡ Batch reactions were performed in a stainless-steel, ‘tubing-bomb’ reactor
(2.54 cm o.d., 1.77 cm i.d. 15 cm long, capped with a hex nut and plug,
internal volume: 12 cm3). Catalyst and benzene were loaded in an argon-
atmosphere glove box. Extreme care was taken to eliminate air and water
contamination of the experiment. The reactor was sealed with a titanium
gasket, removed from the glove box and placed in a fluidized sand bath
maintained at the reaction temperature. After the desired reaction time was
reached, the reactor was quenched in water and the products were analyzed
by gas chromatography, confirmed by mass spectrometry.
§ A recirculating vaporizer/condenser reactor was constructed for the
experiments. Catalyst was loaded in an argon atmosphere glove box.
Extreme care was taken to eliminate air and water contamination of the
experiment. Argon cycled through the system at 1500 sccm in the following
order: bubbled through a temperature controlled (298 K) vessel containing
100 ml of benzene that was dried and deoxygenated over molecular sieves
and lithium ribbon; the benzene rich (5 mol%) argon passed through a flow
controller, a diaphragm pump and to the catalyst bed; the stream was heated
to 723 K and flowed through the catalyst bed (1.5 g of catalyst containing
15% by weight Cs, 0.025 s per pass contact time); the reacted stream then
returned to the bottom of the benzene vessel in which the biphenyl was
trapped due to its low vapor pressure (0.005 bar at 298 K) to repeat the cycle.
Products were analyzed by gas chromatography. No terphenyl was
detected.
¶ The control (untreated catalyst) and benzene treated samples were
prepared for transmission electron microscopic (TEM) studies by deposit-
ing them dry on carbon-coated Cu TEM grids. The grids were examined in
a JEOL 2000FX S/TEM fitted with a Noran energy dispersive spectroscopic
elemental analyzer. The control sample consisted of disordered appearing
carbon with the Cs dispersed uniformly throughout the support. While we
were not able to image individual particles of Cs on the carbon support,
elemental analysis confirmed the presence of Cs everywhere in the sample.
The used catalyst appeared similar to the control sample in microstructure
as well as elemental composition with the Cs dispersed uniformly
throughout the support. However, we observed a small fraction of the
carbon to have ordered to graphitic nanoparticles and microscopic sheets of
graphitic/turbostratic carbon during the benzene reaction process. No Cs
was detected in these ordered pockets of carbon. Based on the TEM
analysis, we estimate the amount of ordered carbon in the benzene-treated
sample to be approximately 5% on a volume basis.
100
50
50
1.5
1.0
1.1
1.0
15.0
14.9
15.8
0.0
2.40
2.60
2.75
0.00
20.43
16.70
15.15
—
96
72
72
96
50
Cs°
+
NPC
2e–
[Cs/NPC]*
+
2e– 2[Cs/NPC]+
2[Cs/NPC]*
2C6H6
• –
+
2C6H6
[C6H6–C6H6]2–
• –
2C6H6
[C6H6–C6H6]2–
[C6H5–C6H5]2–
C6H5–C6H5
+
H2
[C6H5–C6H5]2–
2e–
+
2e–
+
2[Cs/NPC]+
2[Cs/NPC]*
Scheme 1 Plausible catalytic mechanism.
near 2.5% benzene conversion. In this manner 29.8 mmol of
benzene were converted to biphenyl in 96 hours over 1.46 mmol
of caesium supported on NPC. This amounts to more than 20
turnovers based upon the total molar caesium content of the
catalyst. The experiment was repeated twice with essentially the
same result. Again, a control experiment using pure NPC did
not produce detectable levels of biphenyl.
Since Cs/NPC can facilitate the breaking of the C–H bond in
benzene (460 kJ mol21), we conclude that the metal provides,
through its powerful electropositive nature, the electrons
necessary to initiate the process. The carbon mediates the
reaction by acting as the electron repository, stabilizing the
intermediate radical anions. In the mechanism proposed by
Matsuzaki and coworkers,13 hydrogen and electron transfer are
critical steps. The carbon may aid in these steps by promoting
the net redox chemistry. Protons generated in the reaction can
couple to produce dihydrogen with the carbon supplying the
electrons to mediate the process. Additionally, the electron
exchange between neutral benzene and the radical anion is
facile (Ea ≈ 12 kJ mol21),15 and the carbon may stabilize the
intermediate radical anions produced in this chemistry, making
the net chemistry catalytic rather than stoichiometric. A
plausible catalytic mechanism is shown in Scheme 1. Since the
EPR measurements indicate caesium promotes its electron into
the local carbon structure, in essence NPC acts as a macro
radical anion that may be thought of as [Cs+/NPC·2]. From this
vantage point we can begin to see why the material may be so
able to promote demanding redox chemistry.
Finally, we have previously shown that caesium, when
loaded at high ratios (2/1 Cs0/C w/w), promotes the rearrange-
ment of the amorphous NPC to nanotubes, polyhedra and other
crystalline structures;16 transmission electron microscopy¶ of
the catalyst before and after benzene treatment indicates that
approximately 5% (by volume) of the carbon has transformed to
ordered structures. This limited conversion of the amorphous
NPC as compared to previous work16 may be attributed to the
combined effects of lower caesium content and the competitive
reaction with benzene. In conclusion, since the phenyl radical
anion is extremely difficult to produce and especially consider-
ing that deuterium exchange with methane has been shown,2
these results indicate an entirely new class of radical chemistry
may be open to heterogeneous-catalytic exploration in the
future.
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Communication 8/06604C
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Chem. Commun., 1998, 2679–2680