Communications
DOI: 10.1002/anie.201005092
Continuous-Flow Reactors
Real-Time Product Switching Using a Twin Catalyst System for the
Hydrogenation of Furfural in Supercritical CO **
2
James G. Stevens, Richard A. Bourne,* Martyn V. Twigg, and Martyn Poliakoff*
There is an on-going debate over the relative merits of
continuous and batch processing for the production of fine
and speciality chemicals. Flow reactors have an inherently
smaller volume than the corresponding batch reactor for a
given production rate. Owing to this smaller volume, flow
reactors can be operated above ambient conditions more
safely than batch processes. Flow reactors have been used in
organic synthesis where scale-up is possible using parallel
reactors and combinatorial libraries are synthesized using
divergent or convergent strategies with multireactor
promoting different reactions, three different products could
be obtained from the same substrate, depending on whether
the first, the second, or both reactors were heated. Further-
more if, as in this paper, the first catalyst could produce two
different products depending on its temperature, five differ-
ent products could be obtained by the correct combination of
temperatures in the two reactors. We now demonstrate this
concept in practice using furfural as the feedstock.
Furfural (1; Scheme 1) is obtained from the acid-catalyzed
dehydration of a wide variety of renewable feedstocks, such as
oats and rice husks. It is a potential platform chemical because
[
1–3]
setups.
Reactions can be finely tuned much more easily in
continuous flow than in batch mode because flow reactors
can be operated under a wider range of conditions than batch
reactors, for example, elevated temperatures and pressures,
and various reactant stoichiometries by the adjustment of
flow rates. However, despite many advances in continuous
processing in the past decade, batch processing still predom-
inates in the fine-chemicals and pharmaceutical industries,
largely because the same batch reactor can be used for many
different conversions.
Nevertheless, there is one thing that batch reactors cannot
easily do, namely to switch back and forth between different
products. Such “real-time” switching could potentially enable
chemical industries to respond more rapidly to changing
market demand for products, enhancing profitability and
reducing reactor downtime.
Scheme 1. Compounds of interest in the hydrogenation of furfural:
furfural (1), furfuryl alcohol (2), 2-methylfuran (3), tetrahydrofurfuryl
alcohol (4), methyltetrahydrofuran (5), and furan (6).
it is easily converted into a range of useful compounds.
Furfuryl alcohol (2) and 2-methylfuran (3) can be produced in
various yields by hydrogenating 1 using copper-containing
catalysts. Traditionally copper chromite is used at atmos-
pheric pressure which allows the continuous production of 2
Herein we demonstrate that such switching can be
achieved remarkably simply for a set of reactions in super-
[5,6]
critical carbon dioxide (scCO ). We show that five different
or 3 depending on the reaction temperature.
2
products can be obtained in high yields from a single
feedstock, merely by switching reaction conditions—almost
like getting drinks from a vending machine.
Chromium-free catalysts such as copper on carbon or
silica supports have also been reported for the hydrogenation
[4]
[7–10]
of furfural to 2.
Relatively few studies have been
Consider a flow system consisting of two reactors in
tandem, such that each reactor operates only when heated.
Then, if the two reactors contain different catalysts capable of
published on the selective hydrogenation of 2 to tetrahydro-
furfuryl alcohol (4), although Chen et al. have reported high
yields of 4 from 2 using supported nickel catalysts. A wide
[
11]
range of metal-supported catalysts have been used for the
[
12]
hydrogenation of 3, including ruthenium,
palladium or
Palladium has been
reported as a catalyst for the decarbonylation of furfural to
[
*] J. G. Stevens, Dr. R. A. Bourne, Prof. M. Poliakoff
School of Chemistry, University of Nottingham
Nottingham, NG7 2RD (UK)
[
13]
[14]
platinum,
and nickel catalysts.
[
15,16]
Fax: (+44)115-951-3058
6 in high yield at temperatures above 2508C.
E-mail: richard.bourne@nottingham.ac.uk
Homepage: http://www.nottingham.ac.uk/supercritical
Supercritical CO2 is a highly effective medium for
reactions with permanent gases such as H because they are
2
fully miscible with scCO . Mass-transfer limitations are
2
Dr. M. V. Twigg
Johnson Matthey, Orchard Laboratories
Orchard Road, Royston, Hertfordshire, SG8 5HE (UK)
reduced in scCO and the viscosity is relatively low thus
2
[17,18]
further enhancing hydrogenation reactions.
Furfural (1)
is a promising candidate for hydrogenation in scCO because
2
[
**] We thank the EPSRC and the Thomas Swan & Co. Ltd. for support.
We also thank Dr. S. K. Ross for scientific discussions and M. Dellar,
M. Guyler, D. Litchfield, R. Wilson, and P. Fields for their technical
support at the University of Nottingham.
it is highly soluble in scCO2 and recent research has
demonstrated the potential for using scCO in the production
2
[
19]
of 1 from biomass.
8856
ꢀ 2010 Wiley-VCH Verlag GmbH & Co. KGaA, Weinheim
Angew. Chem. Int. Ed. 2010, 49, 8856 –8859