COMMUNICATION
Continuous hydrogenation reactions in a tube reactor packed with Pd/C
Nungruethai Yoswathananont, Kohei Nitta, Yumi Nishiuchi and Masaaki Sato*
Received (in Cambridge, UK) 2nd July 2004, Accepted 4th October 2004
First published as an Advance Article on the web 19th November 2004
DOI: 10.1039/b410014j
maintain this condition, at the inlet of the column, the flow of
the substrate solution was controlled to 0.038 ml min21 by an
HPLC pump and the flow of the hydrogen gas was adjusted to
the same 0.038 ml min21 by regulating the hydrogen pressure
at around 2.5 MPa. This hydrogen gas flow rate corresponds to
0.95 ml min21 at 1.0 atm, 0.1 MPa. At the outlet of the column,
the product solution and the excess amount of the hydrogen gas
came out to the atmospheric pressure. Therefore, the pressure drop
between the inlet and the outlet of the column was 2.4 MPa,
indicating that the gas–liquid flows vary inside the reactor. The
hydrogen conversion within the flow reactor corresponded to the
product formation, since the hydrogen dissolved in the solution
was only a small amount compared with the product.6
Continuous flow of the substrate solution and hydrogen gas
through a tube reactor packed with Pd/C catalyst brings about
a highly reactive and efficient hydrogenation system, which
converts 4-cyanobenzaldehyde to the benzyl alcohol derivatives
at 25 uC, and at 90 uC, the cyano group becomes reduced to
give the corresponding amine and toluene derivatives within
2 min.
We wish to report an efficient hydrogenation reaction in a
continuous microflow system by the use of a novel gas–liquid–
solid tube reactor. It is well known that hydrogenation reactions
play important roles in the chemical and pharmaceutical
industries.1 In a conventional hydrogenation reaction with a batch
reactor, catalyst such as palladium on carbon (Pd/C) is suspended
in a solution, where substrate molecules are dissolved and
hydrogen gas is bubbled through.2 This heterogeneous hydro-
genation has been utilized in many organic syntheses because of its
simplicity since no other than hydrogen gas is needed for the
reducing agent. A disadvantage of this gas–liquid–solid reaction,
however, is probably slow mass transfers between different phases.
To increase the reaction rate, an autoclave has often been used in
the research laboratory and the hydrogenation reactions are
carried out at elevated pressures. In industry, however, the
reactions at high pressures using a batch reactor should be
preferably avoided for safety and economical reasons.
The substrate solution and the hydrogen gas are introduced to
the tube column packed with Pd/C. They travel through very
narrow channels, which are formed between and within the Pd/C
porous particles having the diameter of 20 ¡ 10 microns. The
pressure drop dissipates some energy in the liquid and gas flows
inside the channels. Consequently, the stagnant layer of the carbon
support catalyst particles will be very thin giving rise to enhanced
mass transfers (gas–liquid and liquid–solid) relative to conven-
tional batch reaction systems, although there is no way to quantify
the hydrodynamics in the catalyst bed where liquid and gas flow
through is completely unknown. In the case of homogeneous
catalysts, the hydrogenation reaction itself will be fast,7 but it
usually needs additional procedures to separate the product from
the catalyst. Consequently, our flow system running through
narrow channels formed in the Pd/C packed column, where mass
transfer limitations are reduced and a separation procedure is
unnecessary, is thought to give an efficient hydrogenation reaction.
Attempts to immobilize Pd catalyst on the channel wall and to
carry out hydrogenation reactions in gas8 or gas–liquid9 phases
have been reported in some flow systems under atmospheric or
moderate pressures. To achieve high efficiencies, it was shown that
micro-sized channels were essential for the flow reaction since the
substrates should diffuse to the wall. These fine reactors needed
expensive, complex, and multi-step fabrication, but the catalysts
were not easily reactivated or renewed.
In recent years, flow reactors with channel space in the
micrometer range, which are called microreactors,3 were reported
to have advantages over exothermic reactions such as direct
fluorination of organic compounds with elemental fluorine.4 Using
the microreactor, the hazardous gas–liquid reaction was found to
be conducted in high yield with safety.
Here, we show efficient hydrogenation reactions with the use of
a gas–liquid–solid microflow system under elevated pressure
(Fig. 1). Both substrate solution and hydrogen gas are introduced
continuously into one end of a fine column packed with Pd/C5
(5 wt% Pd). From the other end, the hydrogenated product
solution emerges continuously. The residence time in the column
was only 2 min. The column consists of a stainless steel tube
(6.3 mm outer diameter, 1.0 mm inner diameter, 25 cm length)
with two filters at both ends so as to hold the Pd/C within the
column. The substrate solution was mixed with the hydrogen gas
in the T-shaped mixer. A transparent Teflon tube was set between
the T-shaped mixer and the tube column to observe the gas–liquid
flow. A plug flow, alternate gas and liquid layers, was formed in
the Teflon tube region. Maximum efficient hydrogenation took
place when the plug flow was controlled to have almost the same
volume of gas and liquid layers at the transparent Teflon tube. To
Our flow reactor, on the contrary, had a simple structure, which
made the packing and renewing the Pd/C quite easy. In addition,
the hydrogenation reactions could be carried out at high pressure
conditions simply by increasing the flow rates of the liquid and the
hydrogen gas, as there was a pressure drop in the Pd/C packed
column. These flow conditions under elevated pressure were
thought to facilitate the hydrogenation reaction by improving and
circumventing diffusion steps.
The packing of the Pd/C into the stainless steel column with a
filter at the bottom was performed by filling the Pd/C in methanol,
a slurry solution, from the top at a flow rate of 0.18 ml min21. The
*masasato@ms.cias.osakafu-u.ac.jp
40 | Chem. Commun., 2005, 40–42
This journal is ß The Royal Society of Chemistry 2005
Yoswathananont, Nungruethai
