On-chip catalysis using a lithographically fabricated glass microreactor—the
dehydration of alcohols using sulfated zirconia
Natalie G. Wilson and Tom McCreedy*
Department of Chemistry, University of Hull, Hull, UK HU6 7RX. E-mail: T.McCreedy@chem.hull.ac.uk
Received (in Cambridge, UK) 2nd March 2000, Accepted 22nd March 2000
Published on the Web 13th April 2000
9
A heated microreactor has been fabricated for heterogeneous
catalysis, employing channels microfabricated in glass using
photolithography and wet chemical etching; the demonstra-
tion reaction is the dehydration of alcohols.
methoxymethylation of alcohols. It has also been reported for
the dehydration of alcohols to the associated alkene, with the
added advantage that the reaction conditions are significantly
milder than those required for other acid catalysts, such as
silica-alumina. In a previous paper,10 a 1 mm diameter reactor
was reported to offer conversion efficiencies of only 3% for the
conversion of hexanol to hexene, however in this paper,
conversion efficiencies approaching 100% are reported (com-
pared to the industrial process that only gives ca. 30%
conversion).
There remains growing interest in miniaturised systems for
chemical analysis and synthesis. These chemical analysis
devices are commonly referred to as micro total analytical
systems (mTAS), and have been reported for a diverse range of
1
applications. Hadd et al. employed laser induced fluorescence
detection to determine a number of acetycholinesterase in-
hibitors after electrophoretic separation on a glass chip. A
polymeric microchip was employed for the analysis of PCR
products of the hepatitis C virus,2 again laser induced
fluorescence detection was employed. Fibre optically linked
spectrophotometric detection of analytes on chip provides a
convenient detection method for on-chip measurements, includ-
Sulfated zirconia (ZrO
2 4
/SO 22) catalyst (MEL Chemicals,
Swinton, Manchester, UK) was first activated by heating to
600 °C for 1 h in a microwave furnace (CEM). The microreactor
used with syringe pumping was constructed from two plates.
The base plate was produced by photolithography and wet
chemical etching. The mask was produced in house and used to
transfer the pattern onto glass coated with chrome and photo
resist layers (Alignrite, Bridgend, South Wales). After etching
3
ing the determination of nitrite via the formation of an azo dye,
4
and orthophosphate via the molybdenum blue reaction. The
3
in 1% HF·NH (aq) for 1 h at 70 °C, the remaining photo-resist
integration of microfabricated devices with mass spectrometry
offers a potentially powerful analytical tool, for example in the
rapid analysis of proteolytic digests. While electrophoretic
processes prove vital for separations on chip, electroosmotic
flow offers a very convenient pumping mechanism for fluidic
manipulation, such as for moving solutions through micro-
machined filters.6
and chrome was removed. The channel dimensions were 200
mm wide by 80 mm deep and 30 mm long (in a ‘Z’ shaped
configuration). The top plate was prepared from Sylgard 184
(ISL, West Midlands, UK), a polydimethylsiloxane (PDMS)
resin and cast in a mould containing inserts to form the
reservoirs. After baking for 1 h at 100 °C, the resin was released
from the mould and the inserts removed. The mating face was
then coated with a thin layer of PDMS and activated catalyst
dusted over the surface; it was then baked at 100 °C for 1 h. The
effect of this step was to give a high surface area for the catalyst
reactor, but with the catalyst firmly immobilised. When these
two surfaces were clamped together, the effect was to produce
a reactor with one wall of the channel being catalytically active.
The in situ heater was fabricated from Nichrome wire (Ni90/
Cr10) 0.25 mm o.d. (Goodfellows, Cambridge, UK) im-
mobilised in the PDMS top plate. The heating wire was inserted
into the mould near to the mating face before the liquid PDMS
was poured into the mould. The PDMS was stable to heating up
to 175 °C (this temperature was not exceeded so no data exists
beyond this point). Heating was achieved by a potentiostat
(0–270 V) and monitored via a digital thermometer (Jencons
model 2003) with the temperature probe (RS components)
located close to the reaction channel. Where electroosmotic
flow was used as a pumping method, a 2 cm long tube (0.5 mm
i.d.) was inserted into the base plate, butting up to the start of the
reaction channel. On-chip electroosmotic pumping is at present
unreliable owing to vapourisation of the ethanol on the heated
chip, however, this limitation is being overcome with the aid of
a heat sink.
5
However, while great interest exists in mTAS, the application
of similar devices to synthetic applications is deriving much
interest. One only has to look through the proceedings of recent
conferences (such as IMRET 3, Frankfurt, April 1999) to see the
potential applications of such devices. These include highly
exothermic reactions, e.g. the direct fluorination of aromatic
compounds, the in situ generation of hazardous compounds, e.g.
phosgene, and rapid energy transfer systems, e.g. fuel pumps.
However, one area of interest to us is on chip heterogeneous
catalysis. This represents an important area in organic synthesis,
and there are many potential applications for miniaturised
synthetic reactors able to utilise small amounts of catalysts in
conjunction with very limited reaction volumes. This latter
feature permits very rapid mass transfer and control of reaction
conditions. In addition, the potential to employ electroosmotic
pumping with such devices represents a very attractive feature,
permitting reagents to be pumped by a device with no moving
parts. Within the Hull group, much extensive work is on-going
in the area of microreactors employing catalyst mediation, but
the work reported here is the first in a heated system.
Miniaturised catalytic devices can be used to conveniently gain
kinetic and thermodynamic data for reactions. They are also
convenient for screening new reactions and catalysts. The
screening can either be with respect to the catalyst, i.e. to
determine which catalysts show activity, or with respect to the
reaction, i.e. which reaction shows promise on a certain
catalyst.
The hexan-1-ol was obtained from Avocado Research
Chemicals (Heysham, UK), while other alcohols were obtained
from supplies bottled in-house. Alcohols were purged with
nitrogen for 10 min prior to use in order to minimise coking in
the reactor; a frequent problem at high temperatures when trace
oxygen is present. Pumping was achieved using a syringe pump
(Baby Bee, BAS) or an electroosmotic pump fabricated as
previously reported.10
Here, we report the dehydration of hexanol to hexene, and
ethanol to ethene (with some degree of cracking). The catalyst
used was sulfated zirconia (zirconia treated with sulfuric acid).
This catalyst is known as a super acid catalyst with well
documented reactions including the conversion of methanol to
hydrocarbons,7 isobutane/but-1-ene alkylation,8 and the
The effect of temperature on the dehydration of hexan-1-ol to
hexene has been studied in prior work, and an optimum
conversion occurred at a temperature of 155–160 °C. This was
DOI: 10.1039/b001705l
Chem. Commun., 2000, 733–734
This journal is © The Royal Society of Chemistry 2000
733
Wilson, Natalie G.
