Simulating microwave chemistry in a resistance-heated autoclave made of semiconducting silicon carbide ceramic

Article abstract of DOI:10.1002/chem.201303638

"Microwave chemistry" without microwaves: A resistance-heated SiC autoclave can effectively mimic the sealed-vessel capabilities, rapid heating and cooling profiles, and excellent process-control features that are inherent to modern microwave reactors. Applied to synthetic chemistry, the ready-to-assemble small device provides identical results as can be obtained in a (costly) microwave system (see figure). Copyright

Full text of DOI:10.1002/chem.201303638

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DOI: 10.1002/chem.201303638
Simulating Microwave Chemistry in a Resistance-Heated Autoclave Made of
Semiconducting Silicon Carbide Ceramic
[
a]
David Obermayer, Markus Damm, and C. Oliver Kappe*
First described more than two decades ago, microwave
chemistry has matured from a laboratory curiosity to an es-
tablished scientific technique that is used today almost on
a routine basis by chemists in both academia and industry.
The advantages of this nonclassical heating technology for
synthetic applications—ranging from organic, peptide, and
polymer synthesis, to nanomaterials and material sciences
research—are manifold and are well documented in thou-
sands of publications, hundreds of review articles, and sever-
waves. Significantly, this eliminates a lot of the complexity,
cost, and safety issues that are derived from working with
electromagnetic fields and microwave-reactor technology,
and enables to perform high-temperature autoclave-type
“microwave” chemistry in a much smaller and simpler
device.
[7]
Silicon carbide is a chemically inert and mechanically ex-
tremely robust semiconducting ceramic material that can be
utilized at extraordinarily high temperatures due to its high
melting point (ꢀ27008C) and very low thermal-expansion
[1,2]
al books.
In the majority of the reported examples, use of
[4]
sealed-vessel microwave technology has been shown to radi-
cally reduce reaction times and to increase product yields
and product purities (or material properties) compared to
coefficient. As a result of the high thermal conductivity of
SiC, the heat flow through the wall of the SiC reaction
[4,5]
vessel is exceptionally fast in both directions.
Due to the
[1,2]
conventionally processed experiments.
Today, it is com-
extremely high thermal effusivity of SiC, the contents inside
the SiC reaction vial are also heated in a very efficient
monly accepted in the scientific community that the ob-
served effects in general can be rationalized by a purely
bulk temperature phenomenon, resulting from rapid dielec-
tric heating of the reaction mixture to a high-temperature
regime in a sealed-vessel, autoclave-type microwave reac-
[4,5]
manner.
As described herein, a reaction vessel made of
an appropriate SiC sinter ceramic can be readily trans-
formed into a self-heating pressure reactor by applying sur-
face electrodes on the material. Notably, this approach
allows constructing an autoclave system that is not only min-
iaturized, but also responds unusually fast to set-point
changes. Thus, most of the design requirements for an effi-
cient autoclave system, such as temperature and pressure re-
sistance, chemical inertness and rapid heating/cooling cycles
can be directly derived from the rather unique material
properties inherent to SiC. For the current reactor design,
we have employed cylindrical 10 mL SiC reaction vessels
[
3]
tor.
In 2009, we have described a reaction vessel made out of
sintered silicon carbide (SiC) ceramic for use in conjunction
[4]
with a dedicated single-mode microwave reactor. The
strongly microwave absorbing SiC material effectively
shields the contents inside the reaction vessel from the elec-
tromagnetic field while at the same time allows rapid heat-
ing of the reaction mixture due to the extremely high ther-
mal conductivity and effusivity (a measure for the ability to
identical in shape and size to the process vials used in our
[5]
[4,6]
exchange thermal energy with its surroundings) of the ce-
ramic material. In essence, this technology therefore simu-
lates a conductively heated autoclave experiment under
carefully controlled and monitored processing conditions
previous microwave studies
and have modified these ves-
[8]
sels with a thin gold coating (Figure 1). This layer was sub-
sequently etched by a photolithographic method to generate
[8]
a pattern of interdigitating combs. This modification is not
only useful to tune the electrical resistance of the semicon-
ducting reactor material within a wide range, but also to
precisely direct the heat evolution to the desired areas on
the vessel. Reversible contacting was ensured with two care-
fully designed conical electrodes made of soft graphite
[6]
(
temperature, pressure, stirring rate). Herein, we now
demonstrate that the same processing performance and re-
action control can be achieved in a much simpler way by uti-
lizing a SiC autoclave that is heated by standard resistance
(
ohmic) heating principles, therefore not requiring micro-
(
bottom electrode) and
a
noble-metal/graphite-coated
copper contact (Ni/Pd/Au/C on copper; top electrode;
[
a] Dr. D. Obermayer, Dr. M. Damm, Prof. Dr. C. O. Kappe
Christian Doppler Laboratory for Microwave Chemistry (CDLMC)
and Institute of Chemistry, Karl-Franzens-University Graz
Heinrichstrasse 28, 8010 Graz (Austria)
Fax : (+43)(0)316-380-9840
E-mail: oliver.kappe@uni-graz.at
[8]
Figure 1). The gold-coated SiC vial was then turned into
a programmable temperature-controlled autoclave reactor
by connecting it to a low-voltage alternating current source
coupled to a semiconductor switch controlled by a propor-
[8]
tional-integral derivative (PID) controller (Figure 1). To
be able to safely process reaction mixtures above their boil-
ing points, a sealing mechanism (ꢁ2508C and ꢁ24 bar) with
Homepage: http://www.maos.net
Supporting information for this article is available on the WWW
under http://dx.doi.org/10.1002/chem.201303638.
Chem. Eur. J. 2013, 19, 15827 – 15830
ꢀ 2013 Wiley-VCH Verlag GmbH & Co. KGaA, Weinheim
15827

absorbing or fully microwave transparent reaction mix-
[9]
tures. Employing the electrically self-heating SiC reactor,
all solvents/reaction mixtures are heated at very similar
rates, because the microwave absorptivity is irrelevant for
the heating process.
The data presented above make it apparent that the self-
heating SiC autoclave system described herein can rather ef-
fectively mimic the sealed-vessel capabilities, rapid heating,
and cooling profiles, and excellent process-control features
that are inherent to modern microwave reactors. Therefore,
we have evaluated the performance of the new SiC reactor
for a variety of chemical transformations that were previous-
ly shown to significantly benefit from sealed-vessel micro-
wave processing (shortened reaction times, higher yields,
Figure 1. a) Schematic drawing depicting the concept of an electrically
conductive SiC ceramic as integrated reaction-vessel/heating element in
a temperature-controlled pressure-reactor system. Apart from tempera-
ture and pressure monitoring, magnetic stirring and air cooling are inte-
grated into the reactor. b) Image of a 10 mL gold-coated SiC reaction
vessel.
[4,10–14]
better purity profiles).
The chosen thirteen reactions
[
8]
involved both strongly and weakly microwave-absorbing sol-
vents in a temperature range of 140–2508C (at 2–24 bar in-
ternal pressure) with reaction times of 5–120 min
(Scheme 1). Experiments were carried out on a 2–5 mL
scale in both low- and high-boiling solvents and included re-
actions that were either fully homogeneous or involved het-
erogeneous starting materials, reagents, or catalysts requir-
ing efficient magnetic stirring. The synthetic organic trans-
formations included (cyclo)condensations and cycloadditions
an integrated immersion well for online internal tempera-
ture measurement and monitoring of reaction pressure was
[8]
developed (Figure 1).
The thermal agility of the self-heating SiC autoclave reac-
tor becomes apparent in an experiment, in which 3 mL of
water are heated to 2208C within a 1.5 min ramp time
[
4,10]
[10]
(
Figure 2). The rapid heating step is followed by a 10 min
(Scheme 1a–c),
tions (Scheme 1e,f),
a number of transition-metal-catalyzed CÀC bond-forming
rearrangements (Scheme 1d), substitu-
[10]
[11]
hold time at 2208C (at 23 bar internal pressure), in which
the electrical heating power is reduced by the temperature-
power feedback cycle, compensating only for thermal losses
reductions (Scheme 1g),
and
[4,10,12]
reactions (Scheme 1h–k).
In addition, the ring-opening
[13]
(
Figure 2). The reaction mixture/vessel is then cooled to am-
polymerization of e-caprolactone
and the formation of
[
14]
bient conditions within approximately 5 min with the aid of
ZnO microrods were also studied (Scheme 1l and m, re-
[8]
[15]
a radial fan. Notably, due to the high thermal effusivity of
spectively). For the 13 examples presented herein, virtual-
[5]
the SiC material, this cooling step does not require com-
pressed air as generally the case when using Pyrex vessels—
excellent thermal insulators—in standard microwave reac-
ly identical results in terms of conversion, purity profile,
and/or product yields or material properties were obtained
comparing experiments that involved genuine microwave
chemistry (dielectric heating) in Pyrex vials (MW) with
those performed in the resistance-heated SiC reactor (SiC;
Scheme 1).
[
7]
tors. In general, the rapid heating and cooling cycles that
can be achieved with this type of resistance heated SiC auto-
clave device are very reminiscent of the heating/cooling pro-
files obtainable by using single-mode microwave reactors
The high thermal effusivity of SiC does not only allow
a rapid heat transfer from the outside to the inside of the
SiC autoclave, but also in the reverse direction from the
(
for a comparison, see Figures S10–S13 in the Supporting In-
[1,2,7]
formation).
A problem frequently encountered in mi-
[4]
crowave chemistry is the difficulty of heating low microwave
inside to the outside. This characteristic becomes impor-
tant in the case of exothermic processes, in which the re-
leased heat often leads to a thermal overshoot or runaway
when conventional glass (Pyrex) vessels/autoclaves of low
effusivity are used. A case in point is the formation of imi-
dazolium-based ionic liquids by alkylation of 1-methylimida-
[16]
zole with alkyl halides under solvent-free conditions. As
a model transformation, we have performed the synthesis of
[
bmim]Br by alkylation of N-methylimidazol with n-butyl
bromide on an approximately 16 mmol (3 mL overall
[16]
volume) scale (Figure 3).
When the alkylation reaction
was conducted by immersing a sealed Pyrex vessel equipped
with an internal temperature probe and magnetic stirring
into an oil bath preheated to 1008C, a sharp exothermic
onset at around 908C was observed, causing runaway heat-
ing of the reaction mixture to 1458C (Figure 3). Repeating
this experiment in the SiC reactor under otherwise identical
Figure 2. Temperature (T), pressure (p), and electrical power (P) profiles
for heating 3 mL of water to 2208C for 10 min (hold time) in the resist-
ance heated SiC autoclave (Figure 1).
15828
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Chem. Eur. J. 2013, 19, 15827 – 15830

Simulating Microwave Chemistry
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conditions led to an entirely dif-
ferent behavior, in which run-
away heating was not observed,
and the generated heat was effi-
ciently “buffered” and dissipat-
ed to the comparatively cool air
around the silicon carbide vial
[4,16]
(
Figure 3).
As second model reaction,
we have investigated the iron-
catalyzed reduction of nitroben-
zene to aniline with hydrazine
[11]
(
Scheme 1g).
Even
on
a small scale (2 mmol, 2 mL re-
action volume), this reaction is
rather difficult to control by
using standard microwave heat-
[11]
ing in Pyrex vessels.
Run-
away heating of the reaction
mixture above the set target
temperature of 1508C occurs,
reaching temperatures of 1808C
with concomitant rise in inter-
nal reaction pressure to an
[11]
unsafe level of 30 bar.
In
sharp contrast, using the SiC re-
actor under otherwise identical
reaction conditions no runaway
heating occurred, and the re-
duction could be safely pro-
cessed at 1508C and 20 bar
pressure (Figure S16 in the Sup-
porting Information).
Scheme 1. Yields/conversions
for
chemical transformations performed
under sealed-vessel autoclave condi-
tions employing either a single-mode
microwave reactor by using Pyrex ves-
sels (MW), or the resistively heated
SiC reactor (SiC). Magnetic stirring
combined with internal temperature
measurement was used in both cases
for accurate control of reaction param-
eters. Reactions were performed either
directly in the SiC vials or utilizing
a PTFE (polytetrafluoroethylene) inlet
(
0.2 mm wall thickness) to simplify pu-
rification/isolation issues. Yields are
isolated yields, unless stated otherwise.
For further details, see the Supporting
Information. The inset at Scheme 1m
shows a scanning electron micrograph
(
SEM) of the obtained ZnO micro-
rods, similar in morphology to ZnO
material obtained from the corre-
sponding
microwave
experiment
(
Ref. [14]). TBAB=tetrabutylammoni-
um
bromide;
Cy=cyclohexyl;
PivOH=pivalic acid.
Chem. Eur. J. 2013, 19, 15827 – 15830
ꢀ 2013 Wiley-VCH Verlag GmbH & Co. KGaA, Weinheim
www.chemeurj.org
15829

C. O. Kappe et al.
b) C. O. Kappe, A. Stadler, D. Dallinger, Microwaves in Organic
and Medicinal Chemistry, 2nd ed., Wiley-VCH, Weinheim, 2012;
c) Microwave Heating as a Tool for Sustainable Chemistry (Ed.:
N. E. Leadbeater), CRC Press, Boca Raton, 2011.
[
2] For selected recent reviews covering different fields of microwave
chemistry, see: a) S. L. Pedersen, A. P. Tofteng, L. Malik, K. J.
Jensen, Chem. Soc. Rev. 2012, 41, 1826 (solid-phase peptide synthe-
sis); b) M. Baghbanzadeh, L. Carbone, P. D. Cozzoli, C. O. Kappe,
Angew. Chem. 2011, 123, 11510; Angew. Chem. Int. Ed. 2011, 50,
1
1312 (inorganic nanomaterials); c) K. Kempe, C. R. Becer, U. S.
Schubert, Macromolecules 2011, 44, 5825 (polymer chemistry); d) J.
Klinowski, F. A. Almeida Paz, P. Silva, J. Rocha, Dalton Trans. 2011,
4
0, 321 (materials science).
[
3] C. O. Kappe, B. Pieber, D. Dallinger, Angew. Chem. 2013, 125, 1124;
Angew. Chem. Int. Ed. 2013, 52, 1088, and references cited therein.
4] a) D. Obermayer, B. Gutmann, C. O. Kappe, Angew. Chem. 2009,
[
1
21, 8471; Angew. Chem. Int. Ed. 2009, 48, 8321; b) B. Gutmann, D.
Obermayer, B. Reichart, B. Prekodravac, M. Irfan, J. M. Kremsner,
C. O. Kappe, Chem. Eur. J. 2010, 16, 12182.
[
5] The thermal effusivity e of a material is defined as the square root
of the product of its thermal conductivity and volumetric heat ca-
pacity [e=(k1c ) ]. The thermal effusivity of the SiC used in these
p
Figure 3. Internally measured reaction temperatures during the heating
phase in the synthesis of the ionic liquid [bmim]Br in the SiC reactor
0
.5
studies (ROCARS, Ceramtec) is approximately 10 times higher
than that of standard Pyrex glass, the thermal conductivity by
a factor of approximately 100. For more details, see Ref. [4b].
(
SiC) and in a preheated oil-bath by using a sealed Pyrex vessel of identi-
cal geometry tube (Pyrex).
[
6] The SiC vessel technology has proven extremely valuable in investi-
gating the existence/nonexistence of microwave effects in the past
few years, because by simply switching from a microwave transpar-
ent Pyrex to a strongly microwave absorbing SiC vial, any effects of
the electromagnetic field on the reaction mixture can be easily eval-
uated. For more details, see: C. O. Kappe, Acc. Chem. Res. 2013, 46,
In conclusion, we have demonstrated that a comparatively
easy to assemble resistance-heated SiC autoclave can effec-
tively mimic a standard sealed-vessel microwave experi-
ment. Rapid heating and cooling, superheating of reaction
mixtures above their boiling points, and excellent control
over reaction temperature and pressure can all be duplicat-
ed in the SiC reactor, in which the electrically conductive
SiC ceramic serves as integrated reaction-vessel/heating ele-
ment in a temperature-controlled pressure reactor system
1
579.
[7] For an overview of commercially available microwave reactors, see:
C. O. Kappe, A. Stadler, D. Dallinger, Microwaves in Organic and
Medicinal Chemistry, 2nd ed., Wiley-VCH, Weinheim, 2012, Chap-
ter 3, pp. 41–81.
[
8] A detailed description and assembly instruction of the SiC reactor is
presented in the Supporting Information.
(
Figure 1). Notably, this technology is not dependent on the
microwave absorptivity of the reaction mixture and due to
the high thermal conductivity and effusivity of the SiC ce-
ramic also highly exothermic reactions can be safely pro-
cessed in the device. Other benefits of the SiC autoclave in-
clude the high chemical resistance of the SiC ceramic, which
allows the use of corrosive reagents not suitable for process-
ing in standard glass or metal autoclaves. Most important-
ly, chemical reactions that in the past would typically be per-
formed under microwave conditions in a dedicated (and
costly) microwave instrument can now be executed in
a much smaller and simpler device with equal efficiency.
[9] J. M. Kremsner, C. O. Kappe, J. Org. Chem. 2006, 71, 4651, and ref-
erences therein.
10] D. Dallinger, H. Lehmann, J. D. Moseley, A. Stadler, C. O. Kappe,
Org. Process Res. Dev. 2011, 15, 841.
11] a) D. Cantillo, M. Baghbanzadeh, C. O. Kappe, Angew. Chem. 2012,
124, 10337; Angew. Chem. Int. Ed. 2012, 51, 10190; b) D. Cantillo,
M. Mirhosseini Moghaddam, C. O. Kappe, J. Org. Chem. 2013, 78,
4530.
[
[
[15]
[
12] a) M. Baghbanzadeh, C. Pilger, C. O. Kappe, J. Org. Chem. 2011, 76,
1
507; b) M. Baghbanzadeh, C. Pilger, C. O. Kappe, J. Org. Chem.
2
011, 76, 8138.
[13] S. Hayden, M. Damm, C. O. Kappe, Macromol. Chem. Phys. 2013,
14, 423.
2
ˇ
[
14] M. Baghbanzadeh, S. D. Skapin, Z. C. Orel, C. O. Kappe, Chem.
Eur. J. 2012, 18, 5724.
[
15] Sintered SiC is a universally corrosion-resistant material that exhib-
its excellent heat stability and due to its high hardness additionally
reduces wear and erosion. The corrosion resistance of sintered SiC
is far better than of metals, and even aggressive media, such as con-
centrated acids or bases, HF, or chlorine gas, have a negligible
impact in terms of corrosion (see Ref. [4b] for details). The high cor-
rosion resistance of the SiC reactor is exemplified in the fluorine/
chlorine exchange reaction shown in Scheme 1 f. Using Pyrex vessels
under standard microwave conditions leads to significant corrosion
of the glass vessel, therefore complicating work-up and compromis-
ing safety. See the Supporting Information for further details.
16] D. Obermayer, C. O. Kappe, Org. Biomol. Chem. 2010, 8, 114.
Acknowledgements
This work was supported by a grant from the Christian Doppler Re-
search Society (CDG). We thank R. Schwarzl (University of Graz) for
technical support.
Keywords: high-temperature
chemistry
·
microwave
chemistry · resistance heating · silicon carbide · thermal
effusivity
[
[
1] For recent books, see: a) Microwaves in Organic Synthesis, 3rd ed.
Eds.: A. De La Hoz, A. Loupy), Wiley-VCH, Weinheim, 2013;
Received: September 15, 2013
Published online: October 15, 2013
(
15830
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ꢀ 2013 Wiley-VCH Verlag GmbH & Co. KGaA, Weinheim
Chem. Eur. J. 2013, 19, 15827 – 15830