DOI:
10.1039/B007223K
(Paper)
Analyst, 2001,
126, 21-23
Microfabricated reactors for on-chip heterogeneous
catalysis
Received 6th September 2000, Accepted 24th November 2000
First published on 18th December 2000
Abstract
Microfabricated devices constructed from glass and
polydimethylsiloxane with integral heaters are described, which can be used
for heterogeneous catalysis reactions. Sulfated zirconia is used as the
catalyst in an open channel reactor, with either a syringe pump or
electroosmotic flow being used to deliver the reactants. The results
clearly demonstrate that very high conversion efficiencies are possible,
however, the thermodynamics of the reactions are the same as in bulk
systems. Ethanol and hexanol are dehydrated to ethene and hexene,
respectively, with conversion efficiencies approaching 100%, and the
esterification of ethanol is investigated. Yields of approximately 30%
ethyl acetate are obtained by gas chromatographic analysis. This is the
first time such a method for fabricating a catalyst micro reactor has been
reported, yet it demonstrates sufficient robustness and resistance to
leakage. The use of electroosmotic flow in a heated catalyst reactor is a
significant advancement in reactor design.
Introduction
Miniaturised systems for chemical analysis and synthesis represent a
rapidly developing research area, encompassing chemical and biological
applications. Commercial systems are already available, most notably for
DNA analysis. The chemical analysis devices are commonly referred to as
micro total analytical systems (µTAS), and have been reported for a
diverse range of applications. Laser induced fluorescence detection has
proved to offer very sensitive detection for chip-based separations; such
as reported by Hadd et al.1 who
determined a number of acetycholinesterase inhibitors after electrophoretic
separation on a glass chip, and a polymer chip2 that was employed for the analysis of PCR products
of the hepatitis C virus. Spectrophotometric detection of analytes using
fibre-optic connections provides a convenient quantification method for
on-chip measurements, including the determination of nitrite via
the formation of an azo dye,3 and
orthophosphate via the Molybdenum Blue reaction.4 Electrochemical detection of analytes has been
reported by a number of workers, such as Wang et al.5 for the detection of nitroaromatic explosives and
catecholamines. There are numerous reports of microchips employing mass
spectrometric detection, this offers a potentially powerful analytical
tool, e.g., for rapid analysis of proteolytic digests.6 While electrophoretic processes prove vital for
on-chip separations, electroosmotic flow offers a very convenient pumping
mechanism for fluidic manipulation, such as for moving solutions through
micromachined filters.7However, the application of microfabricated devices to synthetic
applications is deriving similar interest, one only has to look through the
proceedings of recent conferences (such as IMRET 3 (1999) or IMRET 4
(2000)) to see the potential 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 clear benefits in developing miniaturised synthetic reactors
utilising minimum catalyst 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. 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.
Within the Hull group, extensive work is on-going in the area of micro
reactors employing catalyst mediation, but the work reported in this paper
is the first in a heated system. This paper expands on the developments we
reported earlier,8 and shows further
reaction details and additional reactions. We report the relationship
between electroosmotic flow rate and carbon number for alcohols, the
dehydration of hexanol to hexene and ethanol to ethene (with some degree of
cracking), and the esterification of ethanol. 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,9
isobutane/but-1-ene alkylation,10 and the
methoxymethylation of alcohols.11 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,12 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 (compared with the industrial process that only gives
approximately 30% conversion for the dehydration of hexanol). In the
context of this work, conversion efficiency is the amount of product made
compared with the amount of starting material. Therefore a 100%
‘yield’ would equate to 1 mol of starting material being
completely converted to the product(s).
Experimental
All the alcohols were bottled in-house except for hexan-1-ol (Avocado
Research Chemicals, Heysham, UK). Prior to use in the catalyst reactor,
ethanol and hexanol were purged for 10 min with nitrogen to prevent coking;
a frequently encountered problem at high temperatures when trace oxygen is
present. The ethanoic acid and sodium tetraborate (Fischer Scientific Ltd,
Loughborough, UK) were used without further purification. The sulfated
zirconia (ZrO2–SO42−) catalyst
(MEL Chemicals, Swinton, Manchester, UK) was first activated by heating to
600 °C for 1 h in a microwave furnace (CEM). The polydimethylsiloxane
(PDMS) was obtained as Sylgard 184 (ISL, West Midlands, UK), and required
mixing and vacuum degassing for 20 min prior to use. The Nichrome wire
(Goodfellows, Cambridge, UK) was obtained without insulation. The Crown
white glass (Alignrite, Bridgend, South Wales) used for the fabrication of
the chips was obtained pre-patterned, and the etchant was prepared from
AnalaR hydrofluoric acid (BDH) and 98% ammonium fluoride (Lancaster
Chemicals, UK). The Superwhite glass for the top plate was obtained from
Instrument glass (Enfield, UK). Pumping was achieved using a syringe pump
(Baby Bee, BAS, Cheshire, UK) or an electroosmotic pump fabricated as
previously reported.12Chip fabrication
The chips were fabricated from either glass alone or glass and PDMS. The
latter combination was found to be most versatile since it permitted the
catalyst to be easily immobilised on one wall of the channel and to allow
incorporation of the heater element above. In this work, the base plate
containing the channels was fabricated from glass, however, PDMS would be
as applicable for many applications.The micro reactors were constructed from two plates. The base plate was
produced by photolithography and wet chemical etching of Crown white glass
that had been pre-patterned and developed by Alignrite. The patterned glass
was hard baked at 120 °C for 24 h prior to etching for 1 h in 1%
HF–NH4F at 70 °C. Then, the remaining photoresist and
chrome was removed. Two channel designs were used. The first consisted of
two intersecting channels, each 20 mm long, 200 µm wide and 80
µm deep forming a T-pattern. In the second design, the channel
dimensions were 200 µm wide by 80 µm deep and 30 mm long in a
Z-configuration. A schematic diagram of the channel layout can be seen in
Fig. 1. The top plate for the T-chip
consisted of a 17 mm thick block of Superwhite glass drilled with three 3
mm reservoirs to match the end of the channels. This was thermally bonded
at 570 °C for 3 h.13 The top plate for
the Z-chip was prepared from Sylgard 184 resin, 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 this surface was clamped onto
the Z-chip, 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 (Ni–Cr 90∶10) 0.25 mm diameter
carefully formed into a serpentine shape and immobilised in the PDMS top
plate and covering the channels. In the finished chips, the heating element
was 2 mm from the catalyst surface and it heated 15 mm of catalyst coated
channel. 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 180 °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 id) was inserted into the base plate, butting up to
the start of the reaction channel. This was filled with a porous
frit12 to act as the pump. Platinum
electrodes were located at the front and rear of the tube to provide the
potential, and a strip of copper was placed 1 mm away to act as a heat
sink. On-chip electroosmotic pumping is at present very difficult to
operate reliably due to vaporisation of the ethanol on the heated chip,
however, this limitation has been overcome here with the aid of the copper
heat sink. The two halves of the composite chip were held together by
pressure exerted from two rigid Perspex plates, clamped together using four
bolts. The bolts were tightened sufficiently to prevent leaks but without
distortion to the soft polymer top plate.
 |
| Fig. 1 The layout of the microreactor channels on the T- and Z-chips. The
dotted circles show the positions of the reservoirs. | |
Results and discussions
Electroosmotic flow study of alcohols
The glass T-chip was used for this study, with 0.5 mm diameter platinum
electrodes inserted into two reservoirs. Initially, the chip was filled
with 20 mM disodium tetraborate buffer. A constant voltage of 695 V (HVPS,
Farnell, Leeds, UK) was applied between the electrodes, resulting in a
field strength of 230 V cm−1. The voltage was applied for
a period of 10 min, after which time the volume of solution moved between
the two reservoirs was measured. This was repeated three times with fresh
solutions, then the alcohols were studied sequentially. The results can be
seen in Table 1, and clearly show an
overall decrease in flow rate with increasing carbon number, hexanol does
not move by EOF. When a graph of carbon number versus flow rate is
plotted, a linear relationship is obtained for
C1–C4, with y = −0.78x
+ 3.95 (r2 = −0.9941) being the equation of the
line. It is interesting to note that hydrodynamic flow can also occur in
the chip for the lower chain alcohols (decreasing rapidly with chain
length), arising due to developing unequal heights in the reservoirs from
the EOF. It is apparent that it is in the opposite direction to the
electroosmotic flow, and the figures quoted do not include compensation for
the effect of hydrodynamic flow.
Table 1 The electroosmotic flow of five alcohols through the T-chip (n
= 3). Sodium tetraborate is included only for comparative purposes
Solvent | Flow rate/µl min−1 | σ(n
−
1) |
---|
20 mM sodium tetraborate | 3.6 | 0.06 |
Methanol | 3.2 | 0.10 |
Ethanol | 2.3 | 0.10 |
Propan-2-ol | 1.7 | 0.10 |
Butan-1-ol | 0.8 | 0.06 |
Hexan-1-ol | 0.0 | 0.0 |
Catalysis reactions
All the catalyst reactions were performed with the PDMS micro reactor,
since it offered the advantages of a straightforward catalyst
immobilisation method and operated with open channels thus reducing the
resistance to flow.Dehydration of hexanol
Initial studies of the influence of temperature on dehydration
efficiency using a larger reactor12 found
the optimum to be 155 °C, which was in close agreement with literature
values for this reaction on the industrial scale. When an optimisation
study was carried out with the micro reactor, an identical result was
obtained. The influence of temperature was significant, and reducing the
temperature by 10 °C lead to a significant decrease in conversion
efficiency. Operating at a temperature of 155 °C, the hexan-1-ol was
pumped through the reactor chip at a flow rate of 3 µL
min−1 using a syringe pump. The products were collected
and analysed by gas chromatography using a Porapak Q column at 220 °C.
During a typical run time of 3 h, samples were collected every 30 min. From
the gas chromatogram, a hexanol to hexene conversion of between 85 and 95%
could be determined. No additional products were detected. The conversion
efficiency is favourable compared with that observed with large scale
reactors, where 30% is common. The reactor was used constantly over three
working days, and no degradation of the performance was observed. However,
without the thorough removal of oxygen, rapid coking occurred leading to a
reduction in conversion to 10% in under 10 min. The reproducibility between
different catalyst top plates was better than 95%, this was due to the
reproducible production method for the catalyst top plate and to the fact
that the channel plate was reusable (however, different channel plates were
identical if taken from the same batch). In addition, the use of a torque
wrench to tighten the bolts on the clamp resulted in a highly reproducible
system.Dehydration of ethanol
Ethanol has been pumped by both electroosmotic flow and by means of a
syringe pump. Initially, ethanol was pumped through the chip at a flow rate
of 3 µL min−1 using a syringe pump and a reaction
temperature of 155 °C (from an optimisation study similar to that for
hexanol). The products were collected every 30 min and analysed by gas
chromatography using a Porapak Q column at 50 °C. Only trace amounts of
ethanol were detected, with most of the feedstock converted to ethene
(68%), ethane (16%), and methane (15%). The electroosmotic pump produced
repeatable flow rates of 0.9–1.1 µL min−1 at a
field strength of 200 V cm−1, giving a significantly
greater residence time for ethanol in the reactor. The effect of this could
clearly be seen in the GC results obtained where the only detectable
product was methane. This indicates that the reaction has gone beyond
dehydration to complete cracking of the feedstock. Trace amounts of
methanol were also detected. The ability to almost totally crack ethanol to
methane may prove very important to the production of chemical feedstocks
and energy production, for although the present system is energy intensive,
developments of further miniaturised systems may reduce energy demand.Esterification of ethanoic acid
The esterification of an alcohol and carboxylic acid has historically
been performed using homogeneous catalysis (i.e., the addition of
concentrated sulfuric acid to a mixture of ethanol and ethanoic acid).
Since the reaction is an equilibrium, the reaction can reverse in presence
of the catalyst, so using a solid catalyst should limit the reverse
reaction. This concept was examined using the catalyst reactor for the
production of ethyl acetate from ethanol and ethanoic acid. A 1 : 1 v/v
mixture of ethanol and ethanoic acid was pumped through the Z-reactor by a
syringe pump at a flow rate of 2 µL min−1 over a
series of reaction temperatures from room temperature to 180 °C, and
the production of ethyl acetate was determined by gas chromatography. The
results can be seen in Fig. 2, and clearly
show that there is an activation temperature below which no reaction
occurs. The reactor was used constantly over a 5 day period, and no
degradation was seen in performance. Also, coking was not observed, a usual
occurrence with this catalyst. |
| Fig. 2 The effect of temperature on yield of ethyl acetate (each data point is
obtained from a mean of three 1 h experiments performed in a random
order). | |
Conclusions
We have demonstrated the use of a micro reactor for heterogeneous
catalysis. We have shown that highly efficient reactions are possible
without the need for high pressure devices, and that polymeric reactors are
suitable for quite demanding reactions. Perhaps of great significance is
that while micro reactors have great potential, as far as catalytic
reactions are concerned the thermodynamics of reaction, i.e.,
activation temperature, is independent of the scale of the reaction.Acknowledgements
The authors would like to express their thanks to Mel Chemicals for
supplying the sulfated zirconia catalyst, and to Terry Aspinwall and Mike
Bailey for technical help with the chip production. NGW was funded by the
University of Hull.References
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