Sensors and Actuators B 105 (2005) 516–520

Efficiency, monitoring and control of microwave heating withina continuous flow capillary reactor

Ping He, Stephen J. Haswell∗, Paul D. I. FletcherDepartment of Chemistry, University of Hull, Hull HU67RX, UK

Received 27 April 2004; received in revised form 5 July 2004; accepted 22 July 2004Available online 25 September 2004

Abstract

We describe a method of monitoring temperature within a continuous flow capillary reactor (800�m, outer diameter 1.2 mm and totallength 138 mm) by measuring the change in electrical conductivity of a solution undergoing heating due to interaction with microwave energy.The method has been used to determine the extent of microwave heating as a function of liquid flow rate for solvents of differing microwaveabsorption properties. Deposition of gold metal on the outside surface of a glass capillary reactor was found to improve the efficiency of them has beenu©

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icrowave heating process. The alkylation of 2-pyridone with benzyl bromide, performed in DMF solution containing 0.01N NaBr,sed to illustrate the suitability of the proposed methodology for monitoring the reaction temperature.2004 Elsevier B.V. All rights reserved.

eywords:Continuous flow capillary reactor; Microwave heating; Organic synthesis; Thermal measurement

. Introduction

In recent years, the use of microwave-based heating in or-anic synthesis has proved to be a popular methodology[1]

eading to numerous examples in which significant reduc-ions in reaction times and enhancements in product conver-ions and selectivity are possible[2–4]. Interestingly, mostf the work reported to date has been a batch rather thanow through technique and the opportunity of combiningicrowave heating with a capillary based flow reactor couldffer an attractive route to high throughput reaction evalu-tions. In developing such methodology, however, two im-ortant issues need to be address. Firstly obtaining a direct

emperature measurement from within a reacting solution,hich can be difficult using conventional methods such as

hermocouples and optical probes and secondly achievingontrollable efficient heating of a capillary reactor using aicrowave field. Whilst the use of current monitoring has

∗ Corresponding author. Tel.: +44 1482 465469; fax: 44 1482 466416.

previously been used to determine the average tempeof a buffer in electrokinetic systems[5,6], its application tohydrodynamically pumped microwave heated capillarytem has not been reported. In addition to current mement a number of optical temperature monitoring technihave been reported, in which thermal measurements oring have been obtained through the use of a temperadependent fluorophores added to the system[5,6]. Whilstthis approach overcomes the spatial limitation of therent measurement approach, which will only give an avedetermination of temperature, the necessity to imagerophore emission from a reaction medium and their remfrom products would be a severe disadvantage for chesynthesis applications.

In this paper, we describe an electrical conductivity (method for in situ temperature monitoring within a gocoated capillary flow reactor under microwave irradiatThe method has been used to measure the microwaveing characteristics of different solvents as a function ofrate within a capillary flow reactor and to monitor therm

E-mail address:s.j.haswell@hull.ac.uk (S.J. Haswell). conditions during a reaction. To illustrate the relevance of

925-4005/$ – see front matter © 2004 Elsevier B.V. All rights reserved.oi:10.1016/j.snb.2004.07.013

P. He et al. / Sensors and Actuators B 105 (2005) 516–520 517

Fig. 1. Schematic diagram of the capillary flow reactor fitted within themicrowave cavity showing the positions of the IR temperature sensor andthe electrodes 1–4 used to monitor the different temperature values.

this proposed methodology, the alkylation of 2-pyridone withbenzyl bromide has been used.

2. Experimental

The experimental set-up is shown schematically inFig. 1.The flow reactor consisted of a U-shaped glass capillary of in-ner diameter 800�m, outer diameter 1.2 mm and total length138 mm. The capillary was mounted within the cavity of aDiscover microwave system from CEM. This provided mi-crowave radiation of 2.45 GHz and total power in the range0–300 W, which was incident on the lower section of theU-shaped capillary. The microwave cavity was fitted withan infrared sensor, which was aligned so as to monitor thetemperature of the external surface of the lower part of theU-shaped capillary. The capillary was connected via two,two-way connectors and PTFE tubing to an external syringepump (Harvard, model PHD 2000). The two-way connec-tors were fitted with two Pt wire electrodes (0.4 mm outerdiameter), which are labelled 1–4 inFig. 1. Measurementof the conductivity between electrodes 1 and 2 was usedto determine the temperature of the inlet tube section, be-tween electrodes 3 and 4 for the outlet section and betweenelectrodes 2 and 4 for the average temperature of the mainU e dif-f Kerr6 and1 val-u ownc g tot ts oft 1%a cord-i of thec

cor-r were

measured as a function of temperature using a PTI-18 con-ductivity meter fitted with a conventional dip-cell. Solutionswere thermostatted to±0.1◦C using a Grant LTD6 circula-tory thermostat.

Water was purified by reverse osmosis and by pas-sage through a Milli-Q reagent water system. The solventsdimethylformamide (DMF, Lancaster, 99%) andN-methylformamide (NMF, Lancaster, 99%), NaBr (Fisher Chemi-cals, 99%) and KCl (BDH, AnalaR grade) were used withoutfurther purification. Immediately prior to microwave heatingexperiments, test solutions were degassed by briefly boilingunder reduced pressure with ultrasound treatment. This wasdone to avoid bubble formation within the capillary.

The outer surface of the capillaries were coated with metalusing a SEMPREP 2 Sputter Coater (Nanotech Ltd.). The ar-eas not requiring coating were masked off with tape beforeplacing on a water-cooled table within the vacuum chamber.Metal was sputtered in an atmosphere of argon at a pressureof 200 mTorr and an HT current of 20 mA being applied tothe metal target. These settings were kept constant for allthe coatings described here. The coating thickness was con-trolled at the time of sputtering; for example, 90 s sputteringwas estimated to yield a thickness of 10–12 nm and 180 sproduced a thickness of 20–22 nm. Both sides of the capil-laries were coated by sputtering once, turning the capillaryo

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-shaped section. The conductance values between therent electrode pairs were measured using a Wayne430A precision component analyser operating at 40 Hz0 V. Conductance values were converted to conductivityes using calibration measurements (with solutions of knonductivity values) of the cell constants correspondinhe different capillary sections. Repeated measuremenhe cell constants were found to be reproducible withinnd showed good agreement with values calculated ac

ng to the measured lengths and cross-sectional areasapillary sections.

In order to convert measured conductivities into theesponding temperatures, the solution conductivities

ver and sputtering a second time.For the alkyltion reaction, 2-pyridone solutions (0.5

ancaster, 98+%) was premixed with benzyl bromide (0.ancaster, 99%) in a DMF solution containing 0.01N Nnd pumped using a syringe pump (Harvard PHD 2

hrough the U-capillary reactor heated by microwave irrtion. The residence times of the solutions within the heaone defined by the gold coating were directly measureiming the movement of the liquid front during first fillinhe products from each reaction run were collected in aial for a period of 90 s, weighed and a known amount ofecane (10�l, Aldrich, 99+%) was added as an internal stard. Samples were treated with distilled water to remnreacted 2-pyridone and extracted with dichloromethFisher, AR). The extract was then washed three timesistilled water, collected and dried over MgSO4 (Fisher9+%). Samples were then analysed for benzyl bromidelkylation product using a GC instrument (Shimadzu G7A) equipped with a capillary column (CP SIL 8 CB, 30

ength, Chrompack). Pressure of carrier gas (helium)00 kPa and injector temperature was set to 280◦C. The GColumn temperature was held initially at 70◦C for 4 min,amped at 20◦C/min to reach 240◦C, which was then heor 12 min.

. Results and discussion

Initial measurements were made to establish a suitablrowave absorbent film that could easily be deposited outside surface of the glass capillary reactor. FromTable 1, it

518 P. He et al. / Sensors and Actuators B 105 (2005) 516–520

Table 1Temperatures measured on the external capillary surface using the IR sensorfor different metal films under MW irradiation

Metal Sensor temperature (◦C) Resistivity (�� cm)

Carbon 66 1375Platinum 98 10.6Gold >200 2.3

NMF is used as solvent and flow rate 0.1 ml min−1. The thicknesses of metalfilms were all 20 nm. MW power was 50 W and heating time 20 s.

can be seen that, for the same MW power and film thickness,the measured temperatures based on the integrated instru-ment IR sensor were in the order Au > Pt > C. This rankingsequence correlates with the electrical conductivities of thematerials, i.e., the more conducting the better the MW ab-sorption and the higher the temperature reached. Therefore,gold was selected for producing heating patch films.

Plots of conductivity versus temperature for 0.01 M NaBrsolutions in water, DMF and NMF are shown inFig. 2. Overthis temperature range, the relationship is virtually linear andthe fitted values of slopes and intercepts of the plots were usedto convert measured conductivity values to the correspondingtemperatures.

Conductance values recorded for a liquid flowing in thecapillary during microwave irradiation were converted toconductivity (using the measured cell constants) and then totemperature (using the calibration data ofFig. 2). As notedpreviously, conductance measured between electrodes 1 and2 gives the inlet temperature, electrodes 3 and 4 give the out-let temperature and electrodes 2 and 4 provide the averageliquid temperature over the length of capillary within the mi-crowave cavity. An additional temperature value provided bythe IR sensor reading corresponds to the temperature on theexternal surface of the lowest part of the capillary.

Steady-state values of the four different temperatures arep a-t erw of the

F aBrs

Fig. 3. Plots of temperature (average, outlet, inlet and sensor) vs. liquidflow rate for (a) DMF, (b) water and (c) NMF in uncoated capillaries with amicrowave power of 50 W.

different plots can be explained as follows. Firstly, the in-let temperature is unaffected by the microwaves and simplyremains constant at the external, ambient temperature. Thetemperature of the outlet is low at slow flow rates as the liquidcools in the long travel time between the microwave irradia-tion zone and the outlet. Increasing the flow rate correspondsto less time for cooling and also less exposure time to theincident microwaves. The net result of these competing ef-fects is that the outlet temperature passes through a maximumvalue with increasing flow rate. The average temperature

lotted versus liquid flow rate for 0.01 M NaBr in DMF, wer and NMF inFig. 3(a–c). The incident microwave powas 50 W. For each solvent, the characteristic shapes

ig. 2. Calibration plots of conductivity vs. temperature for 10 mM Nolutions in the three solvents.

P. He et al. / Sensors and Actuators B 105 (2005) 516–520 519

(i.e., the average over the capillary length between elec-trodes 2 and 4) behaves similarly except that the flow ratecorresponding to the maximum temperature is shifted. Thesensor temperature, measured in the microwave zone, de-creases monotonically with increasing flow rate due to thedecrease in exposure time of the flowing liquid to the incidentmicrowaves.

For fixed sample geometry and microwave incident in-tensity, the heating rate of a sample depends on the densityρ, heat capacityCp and the dielectric lossε′′ of the sampleaccording to[7]:

dT

dt∝ ε′′

ρCp(1)

For the three solvents shown inFig. 3, the densities andheat capacities are similar and hence, to a first approxima-tion, the relative heating rates are proportional toε′′. For amicrowave frequency of 2.45 GHz, the values ofε′′ at roomtemperature are approximately 6, 9 and 75 for DMF, waterand NMF, respectively[8–10]. FromFig. 3, the correspond-ing values of maximum temperature rise (�Tmax is taken tobe the maximum average temperature minus the inlet tem-perature) are 8, 12 and 37◦C, respectively. It can be seenthat the microwave heating correlates withε′′, highly polarsolvents such as NMF couple strongly with microwaves andh MF.H icha ting( wer)i atao Wi thet

r mi-c MF,t ep -f y ofc MFt . For

TA illary re

H powerperatu

R 0M 50/69M 0/69

R M) and e capillr

Fig. 4. Temperature (average, outlet, inlet and sensor) vs. liquid flow ratefor NMF with microwave power of 10 W in a capillary coated with a goldfilm of 20 nm thickness and 50 mm length.

non-polar solvents showing virtually no microwave absorp-tion at 2.45 GHz, the heating efficiency will be zero.

Microwave absorption efficiency can be greatly improvedby coating the outer surface of the capillary with gold.Fig. 4shows the temperature plots versus flow rate for NMF withina capillary coated with gold (20 nm thickness, coated length50 mm) with a microwave power of 10 W. In the absence ofgold coating (Fig. 3a), the�Tmaxis approximately 37◦C for amicrowave power of 50 W. FromFig. 4, the�Tmax is approx-imately 47◦C for a microwave power of only 10 W. From themeasured temperature value, the gold coating has increasedthe microwave power absorption efficiency to approximately10%.

The alkylation reaction of 2-pyridone with benzyl bromidewas carried out to domenstrate the suitability of the proposedmethodology to monitor online the reaction temperature un-der microwave heating in a continuous flow capillary reactor.FromTable 2it can be seen that whilst no product was de-tected at room temperature, under the same flow conditionsand with microwave irradiation, with and without the pres-ence of a gold film, product was produced. Measurement ofthe average reaction temperature, determined by the conduc-tivity measured between electrodes 2 and 4, was found to give

eating much more than weakly polar solvents such as Dowever, it is important to note that even for solvents, whbsorb microwaves very strongly, the efficiency of the heai.e., the power absorbed relative to the total incident pon this type of micro-flow system is very low. From the df Fig. 3for NMF, a maximum of only approximately 0.5

s converted into heat in the liquid, i.e., only about 1% ofotal microwave power.

Higher temperatures can be achieved by using higherowave powers. For the relatively-weakly absorbing Dhe�Tmax increases from 8◦C at 50 W incident microwavower to 16◦C at 100 W and 21◦C at 150 W. For the dif

erent powers, it remains true that the overall efficienconverting the microwave energy to heat is lower for Dhan for the stronger absorbing NMF (discussed above)

able 2lkylation of 2-pyridone with benzyl bromide in a continuous flow cap

eating method Flow (ml min−1)/residence time (s)

MWtem

T 0.05/48 0/2W heating, no Au coating 0.05/48 2W heating plus gold coatinga 0.05/48 22

eaction mixture consisting of 2-pyridone (0.5 M), benzyl bromide (0.5eactor.a Gold coating was of 10 nm thickness and 5 cm length.

actor under microwave (MW) heating

(W)/IR sensorre (◦C)

Measured temperature (◦C) Yield (%)

Outlet Average

22 22 091 70 19

92 70 29

NaBr (0.01 M) in a DMF solvent was pumped continuously through thary

520 P. He et al. / Sensors and Actuators B 105 (2005) 516–520

a similar value to that obtained using the IR sensor probablythe external capillary surface. However, the outlet tempera-ture, determined by the conductivity measured between elec-trodes 3 and 4, indicated that the actual temperatue within thereaction zone inside the capillary was higher than determinedby the IR sensor or the average conductivity measurements.The presence of a localised hot zone within the capillaryreactor, particularly when using the gold film, is confirmedby a higher yield of product even when no apparent differ-ence in the average (sensor and current) was observed due tosignificant thermal loss between the reaction zone and outlet.Whilst this result demonstrates the limitation of the proposedtechnique to obtain spatial measuerements, the technique wasfound to be comparable in usefulness to the current IR sen-sor and offered the opportunity to monitor an output valuewithin the capillary reactor, which is not curently possiblewith existing methodology.

4. Conclusions

Conductivity measurements of electrolyte solutions canbe used to monitor the mean temperatures of the solvent andreaction matrix within different sections of a capillary undermicrowave irradiation. The temperatures achievable are de-pendent on the microwave power, the flow rate and the mag-n fors GHzm yp-i ne thee canb cies,a nts int

Acknowledgements

We thank Ms. J. Halder, University of Hull for the prepa-ration of the metal coatings and the Engineering and PhysicalSciences research Council, UK for funding.

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he product yield.

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