Effects of Microwaves and Microwave Frequency on the Selectivity of Sorption for Binary Mixtures on Oxides

Effects of Microwaves and Microwave Frequency on the Selectivity of Sorption for BinaryMixtures on Oxides

Steven J. Vallee and William Curtis Conner*Chemical Engineering Department, UniVersity of Massachusetts, Amherst, Massachusetts 01003

ReceiVed: January 12, 2008; ReVised Manuscript ReceiVed: May 5, 2008

In adsorption systems, the absorption of microwave energy depends on the properties of the adsorbates,adsorbents, and of the surface properties at the interface where adsorption occurs. Heating these systemsusing microwaves may lead to sorption behavior that is different from conventional heating. Sorptionexperiments were carried out using a dual-adsorbate flow adsorption system measuring changes in the amountadsorbed with conventional heating and using microwave heating at 2.45 and 5.8 GHz. In the case of adsorbatesmethanol and cyclohexane, microwave heating caused the methanol to desorb almost twice as much asconventional heating. The adsorption selectivity as a function of microwave frequency was examined for acase in which the adsorbates have an opposite dependence of permittivity with frequency (2-propanol had agreater permittivity than acetone at 2.45 GHz, and acetone had a greater permittivity than 2-propanol at 5.8GHz). Differences in the adsorption selectivity were not as great as expected based on the bulk liquidpermittivities of the adsorbates due to the miscibility of the components. Also, the permittivities of the adsorbatesin the adsorbed phase at low surface coverage may be different than that of their respective bulk liquids.

Introduction

Previous research shows that microwave energy uniquelyinfluences sorption on oxides.1,2 Microwave energy may selec-tively heat the adsorbent surface and/or the adsorbed phase ofthe adsorbate. The gas phase and bulk solid phase may be at alower temperature than that required for desorption by conven-tional heating. The temperature at the surface where sorptionoccurs is “effectively” greater than the measured solid or gastemperature, since oxides have a low permittivity and arerelatively transparent to microwaves.2

Often, different adsorbates have sufficiently different capa-bilities for adsorbing microwave energy (permittivities), resultingin different local temperatures and different sorption selectivitiesin the presence of microwaves.1 The permittivity of a materialis also a function of microwave frequency. In a binarycomponent adsorption system, if the oxide adsorbent that islargely transparent to microwaves, and the adsorbates have afrequency dependence such that one component has a greaterpermittivity at one microwave frequency and the other com-ponent a greater permittivity at another microwave frequency,changing the microwave frequency should influence the selec-tivity of adsorption.

In this research, sorption experiments were carried out usinga dual-component flow adsorption system. The flow passesthrough a glass reactor bed packed with oxide adsorbent(silicalite zeolite or Aerosil 200 Silica), and the effluent streamfrom the reactor is analyzed by a mass spectrometer to determineits composition. For conventional heating experiments, thereactor bed is heated by heating tape wrapped along the outsideof the reactor. For experiments with microwave heating, thereactor passes through a section of specially constructedwaveguide, using microwaves at 2.45 or 5.8 GHz.

The absorption of microwave energy and conversion of theenergy into heat is dependent on a property of the medium, the

imaginary part of the permittivity, ε′′ . To study the effects ofmicrowave frequency on adsorption selectivity, the adsorbatepair 2-propanol and acetone was used. From figure 1 theimaginary part of the bulk liquid permittivity for 2-propanol isgreater at 2.45 GHz (3.15:1 ratio), and the bulk liquid permit-tivity for acetone is greater at 5.8 GHz (1.71:1 ratio). Microwaveheating at 2.45 GHz should selectively desorb more 2-propanol,whereas microwave heating at 5.8 GHz should selectively desorbmore acetone. From these experiments, the influence of micro-wave energy and microwave frequency on sorption selectivitywas studied and compared to conventional heating.

Background. The absorption of microwave energy by amedium is dependent on a property of the medium called itspermittivity, ε, which is divided into real and imaginary parts,(eq 1).3

ε) ε′- jε″ (1)

This is often expressed relative to the permittivity of freespace, ε0, and the loss tangent, tan δ, the conductivity, σ, andthe angular frequency ω by eqs 2 and 3.3,4

* To whom correspondence should be addressed. E-mail: [email protected]. Phone: (413) 545-0316. Fax: (413) 545-1647.

Figure 1. Frequency dependence of permittivity for bulk liquids of2-propanol and acetone.

J. Phys. Chem. C 2008, 112, 15483–15489 15483

10.1021/jp800295k CCC: $40.75 2008 American Chemical SocietyPublished on Web 09/06/2008

ε′ ) εrεo (2)

tan δ) (ωε″ + σ) ⁄ (ωε′) (3)

The ability of a molecule to be polarized by an electric fieldis expressed by the real part of the permittivity.5 The imaginarypart of the permittivity accounts for loss in the medium that isconverted to heat.6

Microwaves will have less influence on materials with a lowerpermittivity. The permittivity of a medium is also a function ofthe microwave frequency.

The average power dissipated in a volume of a medium isgiven by Poynting’s Theorem,

Pl ) (12)ωε″|E|2 (4)

where ω is the angular frequency, |E| is the magnitude of theelectric field, and ε′′ is the imaginary part of the permittivity ofthe dielectric material.3

The permittivities of adsorbed species may be different thanthe permittivities of their respective bulk liquids, and may alsomodify the dielectric properties of the surface where theyadsorb.10-12 In the presence of two or more sorbates withdifferent permittivities, since permittivity is a function ofmicrowave frequency,13,14 the microwave frequency also willinfluence sorption.15

Experimental Section

Apparatus. An apparatus has been constructed for measure-ments of the amount adsorbed on a packed bed for twoadsorbates. This can be heated conventionally or placed througha waveguide to be heated with microwaves at 2.45 or 5.8 GHz.The apparatus is shown in Figure 2.

A helium carrier gas is divided into three streams. A flowcontroller is attached to control the flow of each of the threestreams. Two streams each have a flow controller (Tylan FC-260), and those streams each go through a bubbler that may befilled with any adsorbate with an appropriate vapor pressure.The third stream with a flow controller (Edwards) is used asthe helium diluent stream. The three streams are then combinedand can be fed to or bypass the reactor.

The reactor is made of glass tubing that is 15 mm in diameterand 55 cm long. Two fiber optic temperature probes (Neoptixmodel T1) entering through a septum from each end of thereactor are used to monitor the temperature. One is placed inthe center of the adsorbent bed, and in the effluent gas phasefrom the reactor.

About 3 g of adsorbent is used. On top of the adsorbent is asmall amount of glass wool to hold the adsorbent in place. Then,3 mm diameter glass beads are placed on top of the bed todistribute the flow. Another small amount of glass wool is placedabove the glass beads to hold them in place. The reactor maybe placed going through a specially constructed section ofwaveguide, or wrapped with heating tape.

The gas phase effluent from the reactor is sampled using atwo stage pumping system. Some of the gas phase exiting thereactor passes through a needle valve to a section of tubingconnected to a rough pump (Pfeiffer model Duo 2.5A), thenthrough another needle valve to the section to which the turbopump (Alcatel model cfv 100) and another rough pump (Alcatelmodel M2004A) are attached, as well as the mass spectrometer(Extorr model XT200M).

Some experiments were carried out with the effluent gas fromthe reactor being condensed in a liquid nitrogen trap. The trappedliquid was then analyzed using by GC/MS (gas chromatograph

(HP 5890)/mass spectrometry (HP 5989A)) in order to determineif there were any reactions taking place.

Adsorbents and Adsorbates. The silica used in the experi-ments was Aerosil 200 fumed silica from Degussa, and wascalcined at 385 °C. To prevent plugging of the glass frit in thereactor, the silica was pressed to 5000 psi, and broken up intosmall chunks (about 1-2 mm in size). Samples were pretreatedwith a flow of helium and heated to a temperature above 100°C to remove any water adsorbed on the sample. The silicalitezeolite (Si/Al > 1000) used was from Union Carbide, lot no.961884061002-S, and was calcined at 720 °C.

Permittivities were measured with a Hewlett-Packard 8510Network Analyzer, using a 3/4 in. diameter probe connected tothe instrument by a shielded coaxial cable at room temperature.Liquid permittivities were measured by immersing the probetip in the liquid far away from the container walls. Powderswere measured by hand packing the powder to about 1 in.thickness and placing the probe on top of the packed powder.Permittivities were measured at from 0.5-18 GHz and at 22°C, and are shown at 2.45 GHz in Table 1.16,17

Procedure. The surface area of the Aerosil 200 silica wascalculated using a multipoint BET method to be 183 m2/g. Thesurface area of the silicalite was calculated using a multipointBET method to be 369 m2/g; although, the BET surface areasfor zeolites are not meaningful since the pores are filled atpressures below which the BET theory is valid.2

Figure 2. Multi-component sorption system.

TABLE 1: Measured Permittivities of Materials at 2.45GHz and 22 °C16,17a

2.45 GHz 5.8 GHz

material ε′ ε′′ ε′ ε′′

Aerosil 200 silica 1.4 -0.1 1.5 0.0silicalite 2.4 0.0 2.4 0.1acetone 21.9 1.0 21.6 3.1isopropanol 18.3 3.2 3.8 1.8methanol 23.0 13.8 12.1 12.2cyclohexane 2.0 0.1 2.3 0.1benzene NA NA NA NA

a Values are ( 0.1.

TABLE 2: Change in Amount Adsorbed for Methanol andCyclohexane on Silicalite

Silicalite

conventional 2.45 GHz

temperature 107.8 59.5

change in amount adsorbed w/ heating (molecules/silicalite unit cell)methanol -8.46 -6.89cyclohexane 2.79 1.38

15484 J. Phys. Chem. C, Vol. 112, No. 39, 2008 Vallee and Conner

Single component isotherms were obtained at room temper-ature using the volumetric system for each adsorbent/adsorbatepair. The resulting isotherms and partial pressure present in theflow adsorption system allowed the quantification of the amountadsorbed for a single component while using the flow adsorptionsystem. To quantify the amount adsorbed when more than onecomponent was present, experiments were carried by flowingone adsorbate, allow it to come to steady state, and then addthe second adsorbate and measure the change in adsorption ofthe first component due to the second. This was done byintegrating the changes in the mass spectrometer signal. Thisallows the amount adsorbed as well to be calculated in the flowadsorption system when two adsorbates are present. Experimentswere then carried out in the flow adsorption system with twocomponents adsorbing, and then heating the system withconventional heating or microwave at 2.45 or 5.8 GHz andmeasuring the resulting adsorption behavior by integrating thechanges to the mass spectrometer signal for each component.

When measuring the amount adsorbed in these experimentsthe error is typically about 10%, which is typical of physicaladsorption measurements. There is a time lag between the timea change in temperature occurs and when a change in amountadsorbed is measured. This difference is due to the time it takesfor the adsorbates to flow from the reactor to the massspectrometer.

Results

Experiments were carried out with low surface area glassbeads loaded in the place of the adsorbent bed in the reactor.Experiments were carried out with only helium flowing throughthe reactor with conventional heating, acetone and 2-propanolwith conventional heating, acetone and 2-propanol with micro-wave heating at 2.45 GHz and 120 W, methanol and benzenewith conventional heating, and methanol and benzene withmicrowave heating at 2.45 GHz and 120 W. In all cases, theresults showed that the partial pressures did not change uponheating the reactor, since there was very little adsorption takingplace on the low surface area glass beads in the reactor.

To test the validity of the results, experiments were performedthat were comparable to those done by Turner.1 In addition toobserving the trends of the adsorption of methanol and cyclo-hexane in the presence of microwaves, the change in amountadsorbed was quantified. The adsorption of methanol andcyclohexane on silicalite with conventional heating was alsostudied. The selectivity of adsorption for a multicomponentadsorption system cannot be determined solely from the heatof adsorption of the individual components.

However, due to the very slow time scale for adsorption ofcyclohexane within silicalite, a single component isotherm forthe adsorption of cyclohexane on silicalite was not attainable(D)7.3 × 10-16 m2/s at 50 °C and 1 molecule per unit cellloading18). Therefore, the actual surface coverage could not becalculated; but the change in amount adsorbed could still befound by integrating the mass spectrometer signal as describedin the experimental section.

The changes in amount adsorbed due to conventional heating(Figure 3) and microwave heating at 2.45 GHz and 120 W(Figure 4) are shown. The first series for each component isthe change in amount adsorbed upon heating, and the second isthe change in the amount adsorbed upon cooling. The resultsfor methanol and cyclohexane on silicalite show a change inmolecules adsorbed/silicalite unit cell, since the surface coveragecould not be calculated for competitive adsorption of cyclo-hexane in these studies.

For the conventional heating experiments, a rheostat and wasused to control the heating, and for the experiments usingmicrowave heating, the heating was controlled by using a fixedmicrowave power. This led to a difference in temperaturebetween experiments. In comparing the changes in amountadsorbed between the experiment with microwave heating andthe one with conventional heating, the amount adsorbed mustbe adjusted for the differences in the change in temperature inthe experiments. It was assumed that the heat of adsorption wasconstant with respect to surface coverage (similar to a Langmuir-type isotherm), only physical adsorption was taking place, andthat the change in the amount adsorbed due to a change intemperature is proportional to A exp(-∆Hads/RT); where it isassumed that the pre-exponential factor A is not a function oftemperature. The change in amount adsorbed due to heatingwas adjusted to what would be expected at 112.6 °C, since thiswas the highest temperature measured during any of theexperiments. The measured bed temperature might not be thesame as the effective surface temperature during sorption.

Tables with the changes in amount adsorbed both before andafter adjusting for the temperature are shown. In both methodsof heating, methanol desorbs and cyclohexane adsorbs. FromTable 3, after adjusting for the temperature difference, withmicrowave heating the methanol is desorbed to a much greaterextent. This is due to the large permittivity difference betweenmethanol and cyclohexane. The bulk liquid permittivities ratiois 275:1 for methanol to cyclohexane.

Competitive adsorption experiments with methanol andbenzene on silicalite with conventional heating were performed.Results were compared to the previous work, and used to helpfurther simulations by the Auerbach research group at Umass.19,20

The changes in the amount adsorbed are shown in Figure 5.

Figure 3. Methanol and cyclohexane on Silicalite with conventionalheating.

Figure 4. Methanol and cyclohexane on Silicalite with microwaveheating at 2.45 GHz and120W.

Selectivity of Sorption for Binary Mixtures on Oxides J. Phys. Chem. C, Vol. 112, No. 39, 2008 15485

The amount adsorbed did not have to be adjusted for anydifferences in temperature (the adjustments in volume adsorbedfor the other experiments were adjusted to match this temper-ature).

The changes in the amount adsorbed in Table 4 below areexpressed in molecules/silicalite unit cell to compare to theexperiments with methanol and cyclohexane on silicalite (top),and in the ratios of the amount adsorbed during heating to thatbefore heating (bottom) to compare to the other experiments.

The results for the adsorption of methanol and benzene withconventional heating were similar to that of methanol andcyclohexane; however, since the difference in the heat ofadsorption of methanol and benzene is smaller than that ofmethanol and cyclohexane, the differences in the amountadsorbed were not as great. Comparing the values in Table 3and Table 4 (top) shows this difference. Also, as opposed tomethanol and cyclohexane (which are immiscible), methanoland benzene are miscible. Miscible components may lead to asingle adsorbed phase with an intermediate permittivity.

The components do not quite return to the same steady stateafter heating has taken place. After the reactor temperature againreaches room temperature, the amount adsorbed for bothcomponents have increased, so some change in the adsorbent

must have taken place; or there is some degree of chemisorptionor a surface reaction was taking place. Methanol can react withsurface hydroxyl groups to methoxylate the surface.2,21 Thisinfluences the surface properties for adsorption. There was noevidence of any chemical reaction taking place in the form ofnew molecules being observed from the online mass spectrom-eter. The adsorbate molecules that react with the surface mustbe bound there. There were no abnormal temperature effectsobserved from the fiber optic temperature probe in the adsorbentbed.

The bulk of the experimental work was involving theadsorbate pair of acetone and 2-propanol. This was because itwas hypothesized that since the bulk permittivity of 2-propanolis greater than acetone at 2.45 GHz, and the bulk permittivityof acetone is greater than 2-propanol at 5.8 GHz, there shouldbe a change in adsorption selectivity due to heating with themicrowaves at different frequencies. The adsorption of acetoneand 2-propanol was studied on both Aerosil 200 silica andsilicalite.

The literature values of the heats of adsorption vary foracetone and 2-propanol on Aerosil and there is some overlapin the literature values shown in Table 5. From the experiments2-propanol has a higher heat of adsorption on Aerosil thanacetone due to greater changes in the bed temperature whenindividual components were fed to the reactor. This is expecteddue to the greater amount of hydrogen bonding of the 2-propanolto the surface hydroxyl groups.

Changes in the amount adsorbed due to conventional heatingare show in Figure 6. As the reactor is heated, the 2-propanoldesorbs. The acetone also shows a small amount of desorptioninitially, but once more of the 2-propanol desorbs, there is moresurface available and the acetone adsorbs.

The adsorptions of acetone and 2-propanol on Aerosil 200with microwave heating at 2.45 GHz and 120 W (Figure 7)and 5.8 GHz and 20 W (Figure 8) are shown.

For the adsorption of acetone and isopropanol on Aerosil 200at the flow conditions used in these experiments, the total relativesurface coverage was calculated to be 1.2 monolayers at roomtemperature.

TABLE 3: Change in Amount Adsorbed for Methanol andCyclohexane on Silicalite, Adjusted for Changes inTemperature

Silicalite

conventional 2.45 GHz

change in amount adsorbed w/ heating (molecules/silicalite unit cell)methanol -8.84 -15.12cyclohexane 2.91 3.03

Figure 5. Methanol and benzene on silicalite with conventionalheating.

TABLE 4: Change in Amount Adsorbed for Methanol andBenzene on Silicalite, in Molecules/Silicalite Unit Cell (Top)and Ratios of the Amount Adsorbed (Bottom)

Silicalite

conventionaltemperature 112.6

change in amount adsorbed w/ heating(molecules/silicalite unit cell)

methanol -3.89benzene 3.04

SilicaliteconventionalTh2/Th1

methanol 0.70benzene 1.49

TABLE 5: Literature Values for Heats of Adsorption

Aerosil 200Hads (kJ/mol)

silicaliteHads (kJ/mol)

acetone 50.2,22 56.1,23 59.224 67.025,26

2-propanol 56.4,23 60.52 45.5,26 47.12

methanol 50.223 43.027

cyclohexane 30.32 63.028

benzene 40.623 52.0-57.829

Figure 6. Acetone and 2-propanol on Aerosil 200 silica withconventional heating.


In comparing the amount adsorbed for these experiments thetemperatures at which the reactor was heated to during eachexperiment were different. This must be taken into account andwas done in a manner similar to that as described for methanoland cyclohexane.

Tables 6 and 7 show the changes in surface coverage due toheating with and without adjusting for the differences intemperature. Th2/Th1 is the amount adsorbed at steady stateduring heating divided by the amount adsorbed at steady statebefore heating.

The ratios of the amount adsorbed at steady state duringheating (after adjusting for the differences in temperature) tothat before heating are compared.

The use of microwaves did not change which componentdesorbs upon heating compared to conventional heating. Asignificant portion of the microwave energy may be absorbedby the adsorbent and then transferred to the adsorbed phase,leading to results similar to that of conventional heating. Theratio of the bulk permittivity of 2-propanol to acetone is only3.15 (compared to a factor of 275 for methanol to cyclohexane)at 2.45 GHz.

Based on the bulk permittivity frequency dependence,microwave heating at 5.8 GHz should cause the acetone todesorb and the 2-propanol to adsorb. The ratio of the bulk liquidpermittivities for acetone to 2-propanol is at 5.8 GHz is 1.71.At 5.8 GHz the 2-propanol was still desorbed and the acetoneadsorbed. Since the surface coverage is low, the dielectricproperties of the adsorbates are not the same as the respectivebulk liquid permittivities, and it is expected to be lower thanthe measured bulk permittivities. The adsorption selectivitymight also depend on the frequency dependence of the permit-tivity of the surface, especially for Aerosil 200 due to the surfacehydroxyl groups. Unlike the adsorbate pair of methanol andcyclohexane (which are immiscible), acetone and 2-propanolare miscible as bulk liquids. If the adsorbed phase is alsomiscible, the adsorbed phase containing both species mightbehave as if it were a solution with a single permittivity that isintermediate to both acetone and 2-propanol. The amount ofenergy absorbed by the adsorbed phase when exposed tomicrowaves would still be a function of microwave frequency;however, the microwave frequency would no longer directlyinfluence the adsorption selectivity if the multicomponentadsorbed phase exhibited a single average permittivity.

The adsorption of acetone and 2-propanol on silicalite withconventional heating is shown in figure 9. In contrast to theprevious case on Aerosil, acetone has a greater heat of adsorptionthan 2-propanol on silicalite.

As the reactor heating is turned off and the reactor cools, theacetone and 2-propanol should return to the same surfacecoverage as the steady state before the reactor was heated ifonly physical adsorption is taking place. This is approximatelytrue for 2-propanol but not for acetone in this case. Fromintegrating the changes in the amount of acetone present in thesystem, acetone must have a higher surface coverage for thesame flow conditions and temperature after the reactor had beenheated. There was no evidence of any chemical reaction takingplace in the form of new molecules being observed with theonline mass spectrometer; however, the acetone and 2-propanolboth are fragmented in the mass spectrometer and it is thefragmentation peaks that are tracked to quantify the adsorption,and the fragmentation may mask new products. There were noabnormal temperature effects observed from the fiber optic

Figure 7. Acetone and 2-propanol on Aerosil 200 with microwaveheating at 2.45 GHz and 120 W.

Figure 8. Acetone and 2-propanol on Aerosil 200 with microwaveheating at 5.8 GHz and 20 W.

TABLE 6: Ratio of the Amount Adsorbed for Acetone and2-Propanol on Aerosil

Aerosil

conventional 2.45 GHz 5.8 GHz

temperature 86.3 59.6 42.0

Th2/Th1 Th2/Th1 Th2/Th1acetone 1.10 1.14 1.172-propanol 0.68 0.85 0.94

TABLE 7: Ratio of the Amount Adsorbed for Acetone and2-Propanol on Aerosil with the Amount Adsorbed duringHeating Adjusted for Temperature Differences

Aerosil



Figure 9. Acetone and 2-propanol on Silicalite with conventionalheating.


temperature probe in the adsorbent bed. In order to examinewhether there was any reaction was taking place, the experi-mental setup was modified to put a liquid nitrogen trap on theeffluent stream to condense it. The condensate was studied byGC/MS, but no components besides the adsorbates were found.The adsorbate molecules that react with the surface are likelybound there. This could also change the surface properties overextended reuse of the silicalite; however, we did not havesufficient time to fully examine this. The silicalite adsorbentwas periodically recalcined.

The adsorptions of acetone and 2-propanol on silicalite withmicrowave heating at 2.45 GHz and 120 W (Figure 10) and5.8 GHz and 20 W (Figure 11) are shown. In comparing thechange in surface coverage for these experiments, however, thetemperatures at which the reactor was heated to during eachexperiment were different. This must be taken into account andwas done in a manner similar to that as described before; thechange in the amount adsorbed due to a change in temperatureis proportional to e(-∆Hads/RT). The ratios of the amount adsorbedat steady state during heating (after adjusting for the differencesin temperature) to that before heating that are compared.

Tables 8 and 9 show the changes in surface coverage due toheating with and without adjusting for the differences intemperature. Th2/Th1 is the amount adsorbed at steady stateduring heating divided by the amount adsorbed at steady statebefore heating.

The use of microwaves did not change which componentdesorbs upon heating compared to conventional heating. Theacetone did seem to absorb more microwave energy at 5.8 GHz,but not to as great an extent as expected. The reasons for thisare similar to those on Aerosil.

From previous work with Aerosil and silicalite,2 since almostnone of the microwave energy is adsorbed by the surface ofthe silicalite adsorbent, larger differences between conventionalheating and microwave heating would be evident using silicalite.Aerosil absorbs some of the microwave energy at the surfaceof the adsorbent due to the surface hydroxyl groups present,which leads to adsorption behavior that is more similar toconventional heating. Comparing Tables 6-9 shows differencesin the adsorbents, Aerosil 200 and silicalite. In the case ofconventional heating and the case of microwave heating onAerosil, the 2-propanol desorbs and acetone adsorbs. In the caseon silicalite with microwave heating, however, both componentsdesorb.

Conclusions

Experiments were performed to study the effect of microwavefrequency on adsorption selectivity, and the differences betweenmicrowave and conventional heating. From the results, thefollowing conclusions can be made:

1) In the case of methanol and cyclohexane, microwaveheating caused the methanol to desorb almost twice as muchas conventional heating. This was because the methanol had amuch greater permittivity (the bulk liquid permittivities have a275:1 ratio).

2) The selectivity for desorption of methanol and benzeneusing conventional heating was about half-that of methanol andcyclohexane. This is mostly due to the smaller in the heat ofadsorption of benzene than cyclohexane.

3) The adsorbent Aerosil 200 absorbed a significant amountof microwave energy. This energy was then transferred to theadsorbates and contributed to a heating mechanism that wassimilar to conventional heating. This makes it difficult todistinguish the effects of microwave heating compared toconventional heating.

4) The frequency dependence of the adsorption of acetoneand 2-propanol was not as selective as expected based on thebulk permittivity differences (ratios 2-propanol/acetone 3.15:1at 2.45 GHz and 2-propanol/acetone 1:1.71 at 5.8 GHz), asshown in Tables 7 and 9. The smaller than expected change inadsorption selectivity with microwave frequency might beattributed to the miscibility of acetone and 2-propanol. If theadsorbed phase is also miscible, the adsorbed phase containingboth species might behave as if it were a solution with a singlepermittivity that is intermediate to both acetone and 2-propanol.Since the surface coverage was low, the dielectric properties ofthe adsorbates might not be the same as the respective bulkliquid permittivities, and are expected to be lower than themeasured bulk permittivities. Also, the frequency dependenceof the adsorbed phases may be different than that of the bulk

Figure 10. Acetone and 2-propanol on Silicalite: Microwave heatingat 2.45 GHz and 120 W.

Figure 11. Acetone and 2-propanol on Silicalite: Microwave heatingat 5.8 GHz and 20 W.

TABLE 8: Ratio of the Amount Adsorbed upon Heating forAcetone and 2-Propanol on Silicalite

Silicalite

Conventional 2.45 GHz 5.8 GHz

temperature 70.0 61.2 73.2


TABLE 9: Ratio of the Amount Adsorbed upon Heating forAcetone and 2-Propanol on Silicalite, Adjusted forTemperature Differences

Silicalite




liquids. It might also depend on the frequency dependence ofthe permittivity of the surface, especially for Aerosil 200 dueto the presence of surface hydroxyl groups.

5) In some cases there was a change in the amount adsorbedbefore the reactor was heated compared to the amount adsorbedafter the reactor was heated and cooled. In these cases, theremust be some degree of chemisorption or a reaction taking place,to alter the surface characteristics. Determining the extent ofthis reaction was beyond the scope of this work, as no reactionwas expected.

One cannot predict what will be selectively desorbed due toa change in temperature based solely on the heats of adsorptionof the adsorbates. The influence of microwave frequency wasnot significant for changing selectivity; however, the desorptionefficiency appeared greater at 5.8 GHz than at 2.45 GHz onsilicalite. This effect may be due to an inherent efficiency ofmicrowaves to influence interfacial interactions in nonresonantapplications wherein the exposure can vary in intensity withtime.30

Acknowledgment. Funding was provided by a NSF NIRTNanoscale Interdisciplinary Research Team grant. Thanks toProf. Robert Laurence, Prof. Sigfrid Yngvesson, Prof. ScottAuerbach, and Geoff Tompsett for their discussions. Thanks toGerald Ling for having assembled the piping and wiring forthe flow controllers on the multicomponent flow adsorptionsystem. Thanks to Karl Hammond for writing the computerprograms that collect the mass spectrometer data using labviewand convert the data into an excel-compatible format. Thanksto Fan Lu and Kyu-Ho Lee for providing permittivitymeasurements.

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Effects of Microwaves and Microwave Frequency on the Selectivity of Sorption for Binary Mixtures on Oxides