Angewandte
Chemie
DOI: 10.1002/anie.200904185
Microwave Effects
Microwave Chemistry in Silicon Carbide Reaction Vials: Separating
Thermal from Nonthermal Effects**
David Obermayer, Bernhard Gutmann, and C. Oliver Kappe*
The use of microwave energy to enhance chemical reactions is
growing at a rapid rate; new and innovative applications in
organic and peptide synthesis, polymer chemistry, material
sciences, nanotechnology, and biochemical processes are
carbide is a strongly microwave-absorbing chemically inert
ceramic material that can be utilized at extremely high
temperatures owing to its high melting point ( ꢀ 27008C) and
[7]
very low thermal expansion coefficient. Microwave irradi-
ation induces a flow of electrons in the semiconducting SiC
that heats the reaction vessel very efficiently through
[1]
being reported continuously. In many instances, microwave
irradiation has been shown to dramatically reduce processing
times, increase product yields, and enhance product purities
or material properties over that reported for conventionally
[
7]
resistance (ohmic) heating mechanisms. We have speculated
that because of the high microwave absorbtivity of SiC, any
material (i.e. a reaction mixture) contained within the vial will
be effectively shielded from the electromagnetic field. This
hypothesis was confirmed by heating experiments using
solvents of vastly different microwave absorbtivity (loss
[
1]
processed experiments. Regardless of the relatively large
body of published work in this field, there is still considerable
controversy on the exact reasons why microwave irradiation
is able to enhance chemical processes. In particular, there is
an ongoing debate in the scientific community whether the
observed enhancements are the result of purely thermal/
kinetic effects as a consequence of the rapid heating and high
bulk reaction temperatures that can be attained using micro-
[2,3]
tangent, tand)
at constant microwave power in which the
heating profiles attained in a standard Pyrex vial were
compared with the profiles in the SiC vessel. For this purpose
a custom-made 10 mL reaction vial made out of sintered SiC
of the exact same geometry as a standard microwave-trans-
parent Pyrex microwave process vial was utilized (see
Figure S1 in the Supporting Information). While the heating
profiles obtained with the Pyrex vials followed the expected
trend and were correlated to the tand value of the solvent
(Figure 1a), the heating of solvents in the SiC vessel was
apparently not related to their microwave absorbtivity, but
rather dependent on other parameters such as specific heat
capacity, viscosity, and heat-transfer coefficients (Figure 1b).
The fact that in the SiC vial nearly microwave-transparent
hexane (tand = 0.02) is heated at the same rate as the strongly
absorbing EtOH (tand = 0.941) (Figure 1b) clearly indicates
that the microwave field intensity inside the SiC vial must be
extremely low, and that heating occurs in essence by means of
conventional heat-transfer mechanisms and not by dielectric
[
2]
wave dielectric heating, or whether they are related to
selective interactions of the electromagnetic field with
specific substrate molecules, reagents, or catalysts not con-
nected to a macroscopic bulk temperature effect (so-called
[3,4]
“
specific” or “nonthermal” microwave effects).
Indeed,
there is experimental evidence that certain chemical trans-
formations, when carried out at the same measured reaction
temperature using either microwave or conventional heating,
lead to different results in terms of product distribution
[
3–5]
(
selectivity) and/or yield.
Herein we describe technology that makes it possible to
rapidly evaluate whether an observed enhancement seen in a
microwave-assisted chemical transformation is the result of a
purely thermal phenomenon, or whether specific/nonthermal
microwave effects are involved. Key to this method is the use
of a reaction vessel made out of silicon carbide (SiC), in
combination with a single-mode microwave reactor that
allows simultaneous temperature monitoring by external
[
2]
heating effects. This was further corroborated by immersing
a Hg electrodeless discharge lamp (EDL, see Figure S2 in the
[
9]
Supporting Information) into the SiC vial. Even when a
300 W magnetron output power was applied it was not
possible to induce a gas discharge. In contrast, in a standard
Pyrex vial 1–5 W of microwave power was sufficient to trigger
gas discharge causing the emission of UV/Vis irradiation in
[
6]
infrared (IR) and internal fiber-optic probes (FO). Silicon
[
*] Mag. D. Obermayer, Mag. B. Gutmann, Prof. Dr. C. O. Kappe
Christian Doppler Laboratory for Microwave Chemistry (CDLMC)
and Institute of Chemistry
[9]
these EDLs (see Figure S2 in the Supporting Information).
Karl-Franzens-University Graz
Heinrichstrasse 28, 8010 Graz (Austria)
Fax: (+43)316-380-9840
E-mail: oliver.kappe@uni-graz.at
Homepage: http://www.maos.net
A direct comparison of the heating profiles obtained in
Pyrex and SiC vials for each of the four solvents discussed
above (Figure 1) demonstrates that—with the exception of
the strongly microwave-absorbing ionic liquid [bmim]PF —
6
the solvents are generally heated at the same rate if not faster
in the SiC vial as in the Pyrex vial, in particular in the high-
temperature range (see Figure S3 in the Supporting Informa-
tion). Similar to a standard microwave heating experiment,
the use of higher power levels in the SiC vial leads to a more
rapid heating of the solvent/reaction mixture (see Figure S4 in
the Supporting Information). Importantly, heating experi-
[
**] This work was supported by a grant from the Christian Doppler
Society (CDG). We acknowledge Anton Paar GmbH for the
provision of the Monowave microwave reactor and technical
support and thank Dr. Jennifer M. Kremsner for early contributions
to this work.
Angew. Chem. Int. Ed. 2009, 48, 8321 –8324
ꢀ 2009 Wiley-VCH Verlag GmbH & Co. KGaA, Weinheim
8321