J. Phys. Chem. C 2008, 112, 15483–15489
15483
Effects of Microwaves and Microwave Frequency on the Selectivity of Sorption for Binary Mixtures 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 systems using microwaves may lead to sorption behavior that is different from conventional heating. Sorption experiments were carried out using a dual-adsorbate flow adsorption system measuring changes in the amount adsorbed with conventional heating and using microwave heating at 2.45 and 5.8 GHz. In the case of adsorbates methanol and cyclohexane, microwave heating caused the methanol to desorb almost twice as much as conventional heating. The adsorption selectivity as a function of microwave frequency was examined for a case in which the adsorbates have an opposite dependence of permittivity with frequency (2-propanol had a greater permittivity than acetone at 2.45 GHz, and acetone had a greater permittivity than 2-propanol at 5.8 GHz). Differences in the adsorption selectivity were not as great as expected based on the bulk liquid permittivities of the adsorbates due to the miscibility of the components. Also, the permittivities of the adsorbates in the adsorbed phase at low surface coverage may be different than that of their respective bulk liquids. Introduction Previous research shows that microwave energy uniquely influences sorption on oxides.1,2 Microwave energy may selectively heat the adsorbent surface and/or the adsorbed phase of the adsorbate. The gas phase and bulk solid phase may be at a lower temperature than that required for desorption by conventional heating. The temperature at the surface where sorption occurs is “effectively” greater than the measured solid or gas temperature, since oxides have a low permittivity and are relatively transparent to microwaves.2 Often, different adsorbates have sufficiently different capabilities for adsorbing microwave energy (permittivities), resulting in different local temperatures and different sorption selectivities in the presence of microwaves.1 The permittivity of a material is also a function of microwave frequency. In a binary component adsorption system, if the oxide adsorbent that is largely transparent to microwaves, and the adsorbates have a frequency dependence such that one component has a greater permittivity at one microwave frequency and the other component a greater permittivity at another microwave frequency, changing the microwave frequency should influence the selectivity of adsorption. In this research, sorption experiments were carried out using a dual-component flow adsorption system. The flow passes through a glass reactor bed packed with oxide adsorbent (silicalite zeolite or Aerosil 200 Silica), and the effluent stream from the reactor is analyzed by a mass spectrometer to determine its composition. For conventional heating experiments, the reactor bed is heated by heating tape wrapped along the outside of the reactor. For experiments with microwave heating, the reactor passes through a section of specially constructed waveguide, using microwaves at 2.45 or 5.8 GHz. The absorption of microwave energy and conversion of the energy into heat is dependent on a property of the medium, the * To whom correspondence should be addressed. E-mail: wconner@ ecs.umass.edu. Phone: (413) 545-0316. Fax: (413) 545-1647.
Figure 1. Frequency dependence of permittivity for bulk liquids of 2-propanol and acetone.
imaginary part of the permittivity, ε′′. To study the effects of microwave frequency on adsorption selectivity, the adsorbate pair 2-propanol and acetone was used. From figure 1 the imaginary part of the bulk liquid permittivity for 2-propanol is greater at 2.45 GHz (3.15:1 ratio), and the bulk liquid permittivity for acetone is greater at 5.8 GHz (1.71:1 ratio). Microwave heating at 2.45 GHz should selectively desorb more 2-propanol, whereas microwave heating at 5.8 GHz should selectively desorb more acetone. From these experiments, the influence of microwave energy and microwave frequency on sorption selectivity was studied and compared to conventional heating. Background. The absorption of microwave energy by a medium is dependent on a property of the medium called its permittivity, 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 free space, ε0, and the loss tangent, tan δ, the conductivity, σ, and the angular frequency ω by eqs 2 and 3.3,4
10.1021/jp800295k CCC: $40.75 2008 American Chemical Society Published on Web 09/06/2008
15484 J. Phys. Chem. C, Vol. 112, No. 39, 2008
Vallee and Conner
ε′ ) εrεo
(2)
tan δ ) (ωε″ + σ) ⁄ (ωε′)
(3)
The ability of a molecule to be polarized by an electric field is expressed by the real part of the permittivity.5 The imaginary part of the permittivity accounts for loss in the medium that is converted to heat.6 Microwaves will have less influence on materials with a lower permittivity. The permittivity of a medium is also a function of the microwave frequency. The average power dissipated in a volume of a medium is given by Poynting’s Theorem,
Pl )
( 21 )ωε″|E|
2
(4)
where ω is the angular frequency, |E| is the magnitude of the electric field, and ε′′ is the imaginary part of the permittivity of the dielectric material.3 The permittivities of adsorbed species may be different than the permittivities of their respective bulk liquids, and may also modify the dielectric properties of the surface where they adsorb.10-12 In the presence of two or more sorbates with different permittivities, since permittivity is a function of microwave frequency,13,14 the microwave frequency also will influence sorption.15 Experimental Section Apparatus. An apparatus has been constructed for measurements of the amount adsorbed on a packed bed for two adsorbates. This can be heated conventionally or placed through a 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 flow controller is attached to control the flow of each of the three streams. Two streams each have a flow controller (Tylan FC260), and those streams each go through a bubbler that may be filled with any adsorbate with an appropriate vapor pressure. The third stream with a flow controller (Edwards) is used as the helium diluent stream. The three streams are then combined and can be fed to or bypass the reactor. The reactor is made of glass tubing that is 15 mm in diameter and 55 cm long. Two fiber optic temperature probes (Neoptix model T1) entering through a septum from each end of the reactor are used to monitor the temperature. One is placed in the center of the adsorbent bed, and in the effluent gas phase from the reactor. About 3 g of adsorbent is used. On top of the adsorbent is a small amount of glass wool to hold the adsorbent in place. Then, 3 mm diameter glass beads are placed on top of the bed to distribute the flow. Another small amount of glass wool is placed above the glass beads to hold them in place. The reactor may be placed going through a specially constructed section of waveguide, or wrapped with heating tape. The gas phase effluent from the reactor is sampled using a two stage pumping system. Some of the gas phase exiting the reactor passes through a needle valve to a section of tubing connected to a rough pump (Pfeiffer model Duo 2.5A), then through another needle valve to the section to which the turbo pump (Alcatel model cfv 100) and another rough pump (Alcatel model M2004A) are attached, as well as the mass spectrometer (Extorr model XT200M). Some experiments were carried out with the effluent gas from the reactor being condensed in a liquid nitrogen trap. The trapped liquid was then analyzed using by GC/MS (gas chromatograph
Figure 2. Multi-component sorption system.
TABLE 1: Measured Permittivities of Materials at 2.45 GHz and 22 °C16,17a 2.45 GHz
5.8 GHz
material
ε′
ε′′
ε′
ε′′
Aerosil 200 silica silicalite acetone isopropanol methanol cyclohexane benzene
1.4 2.4 21.9 18.3 23.0 2.0 NA
-0.1 0.0 1.0 3.2 13.8 0.1 NA
1.5 2.4 21.6 3.8 12.1 2.3 NA
0.0 0.1 3.1 1.8 12.2 0.1 NA
a
Values are ( 0.1.
TABLE 2: Change in Amount Adsorbed for Methanol and Cyclohexane on Silicalite Silicalite temperature
conventional
2.45 GHz
107.8
59.5
change in amount adsorbed w/ heating (molecules/silicalite unit cell) methanol -8.46 -6.89 cyclohexane 2.79 1.38
(HP 5890)/mass spectrometry (HP 5989A)) in order to determine if there were any reactions taking place. Adsorbents and Adsorbates. The silica used in the experiments was Aerosil 200 fumed silica from Degussa, and was calcined at 385 °C. To prevent plugging of the glass frit in the reactor, the silica was pressed to 5000 psi, and broken up into small chunks (about 1-2 mm in size). Samples were pretreated with a flow of helium and heated to a temperature above 100 °C to remove any water adsorbed on the sample. The silicalite zeolite (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 8510 Network Analyzer, using a 3/4 in. diameter probe connected to the instrument by a shielded coaxial cable at room temperature. Liquid permittivities were measured by immersing the probe tip in the liquid far away from the container walls. Powders were 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 was calculated using a multipoint BET method to be 183 m2/g. The surface area of the silicalite was calculated using a multipoint BET method to be 369 m2/g; although, the BET surface areas for zeolites are not meaningful since the pores are filled at pressures below which the BET theory is valid.2
Selectivity of Sorption for Binary Mixtures on Oxides Single component isotherms were obtained at room temperature using the volumetric system for each adsorbent/adsorbate pair. The resulting isotherms and partial pressure present in the flow adsorption system allowed the quantification of the amount adsorbed for a single component while using the flow adsorption system. To quantify the amount adsorbed when more than one component was present, experiments were carried by flowing one adsorbate, allow it to come to steady state, and then add the second adsorbate and measure the change in adsorption of the first component due to the second. This was done by integrating the changes in the mass spectrometer signal. This allows the amount adsorbed as well to be calculated in the flow adsorption system when two adsorbates are present. Experiments were then carried out in the flow adsorption system with two components adsorbing, and then heating the system with conventional heating or microwave at 2.45 or 5.8 GHz and measuring the resulting adsorption behavior by integrating the changes to the mass spectrometer signal for each component. When measuring the amount adsorbed in these experiments the error is typically about 10%, which is typical of physical adsorption measurements. There is a time lag between the time a change in temperature occurs and when a change in amount adsorbed is measured. This difference is due to the time it takes for the adsorbates to flow from the reactor to the mass spectrometer.
J. Phys. Chem. C, Vol. 112, No. 39, 2008 15485
Figure 3. Methanol and cyclohexane on Silicalite with conventional heating.
Results Experiments were carried out with low surface area glass beads loaded in the place of the adsorbent bed in the reactor. Experiments were carried out with only helium flowing through the reactor with conventional heating, acetone and 2-propanol with conventional heating, acetone and 2-propanol with microwave heating at 2.45 GHz and 120 W, methanol and benzene with conventional heating, and methanol and benzene with microwave heating at 2.45 GHz and 120 W. In all cases, the results showed that the partial pressures did not change upon heating the reactor, since there was very little adsorption taking place on the low surface area glass beads in the reactor. To test the validity of the results, experiments were performed that were comparable to those done by Turner.1 In addition to observing the trends of the adsorption of methanol and cyclohexane in the presence of microwaves, the change in amount adsorbed was quantified. The adsorption of methanol and cyclohexane on silicalite with conventional heating was also studied. The selectivity of adsorption for a multicomponent adsorption system cannot be determined solely from the heat of adsorption of the individual components. However, due to the very slow time scale for adsorption of cyclohexane within silicalite, a single component isotherm for the adsorption of cyclohexane on silicalite was not attainable (D)7.3 × 10-16 m2/s at 50 °C and 1 molecule per unit cell loading18). Therefore, the actual surface coverage could not be calculated; but the change in amount adsorbed could still be found by integrating the mass spectrometer signal as described in 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 is the change in amount adsorbed upon heating, and the second is the change in the amount adsorbed upon cooling. The results for methanol and cyclohexane on silicalite show a change in molecules adsorbed/silicalite unit cell, since the surface coverage could not be calculated for competitive adsorption of cyclohexane in these studies.
Figure 4. Methanol and cyclohexane on Silicalite with microwave heating at 2.45 GHz and120W.
For the conventional heating experiments, a rheostat and was used to control the heating, and for the experiments using microwave heating, the heating was controlled by using a fixed microwave power. This led to a difference in temperature between experiments. In comparing the changes in amount adsorbed between the experiment with microwave heating and the one with conventional heating, the amount adsorbed must be adjusted for the differences in the change in temperature in the experiments. It was assumed that the heat of adsorption was constant with respect to surface coverage (similar to a Langmuirtype isotherm), only physical adsorption was taking place, and that the change in the amount adsorbed due to a change in temperature is proportional to A exp(-∆Hads/RT); where it is assumed that the pre-exponential factor A is not a function of temperature. The change in amount adsorbed due to heating was adjusted to what would be expected at 112.6 °C, since this was the highest temperature measured during any of the experiments. The measured bed temperature might not be the same as the effective surface temperature during sorption. Tables with the changes in amount adsorbed both before and after adjusting for the temperature are shown. In both methods of heating, methanol desorbs and cyclohexane adsorbs. From Table 3, after adjusting for the temperature difference, with microwave heating the methanol is desorbed to a much greater extent. This is due to the large permittivity difference between methanol and cyclohexane. The bulk liquid permittivities ratio is 275:1 for methanol to cyclohexane. Competitive adsorption experiments with methanol and benzene on silicalite with conventional heating were performed. Results were compared to the previous work, and used to help further simulations by the Auerbach research group at Umass.19,20 The changes in the amount adsorbed are shown in Figure 5.
15486 J. Phys. Chem. C, Vol. 112, No. 39, 2008 TABLE 3: Change in Amount Adsorbed for Methanol and Cyclohexane on Silicalite, Adjusted for Changes in Temperature Silicalite conventional
2.45 GHz
change in amount adsorbed w/ heating (molecules/silicalite unit cell) methanol -8.84 -15.12 cyclohexane 2.91 3.03
The amount adsorbed did not have to be adjusted for any differences in temperature (the adjustments in volume adsorbed for the other experiments were adjusted to match this temperature). The changes in the amount adsorbed in Table 4 below are expressed in molecules/silicalite unit cell to compare to the experiments with methanol and cyclohexane on silicalite (top), and in the ratios of the amount adsorbed during heating to that before heating (bottom) to compare to the other experiments. The results for the adsorption of methanol and benzene with conventional heating were similar to that of methanol and cyclohexane; however, since the difference in the heat of adsorption of methanol and benzene is smaller than that of methanol and cyclohexane, the differences in the amount adsorbed were not as great. Comparing the values in Table 3 and Table 4 (top) shows this difference. Also, as opposed to methanol and cyclohexane (which are immiscible), methanol and benzene are miscible. Miscible components may lead to a single adsorbed phase with an intermediate permittivity. The components do not quite return to the same steady state after heating has taken place. After the reactor temperature again reaches room temperature, the amount adsorbed for both components have increased, so some change in the adsorbent
Figure 5. Methanol and benzene on silicalite with conventional heating.
Vallee and Conner TABLE 5: Literature Values for Heats of Adsorption Aerosil 200 Hads (kJ/mol) acetone 2-propanol methanol cyclohexane benzene
50.2,22 56.1,23 59.224 56.4,23 60.52 50.223 30.32 40.623
silicalite Hads (kJ/mol) 67.025,26 45.5,26 47.12 43.027 63.028 52.0-57.829
must have taken place; or there is some degree of chemisorption or a surface reaction was taking place. Methanol can react with surface hydroxyl groups to methoxylate the surface.2,21 This influences the surface properties for adsorption. There was no evidence of any chemical reaction taking place in the form of new molecules being observed from the online mass spectrometer. The adsorbate molecules that react with the surface must be bound there. There were no abnormal temperature effects observed from the fiber optic temperature probe in the adsorbent bed. The bulk of the experimental work was involving the adsorbate pair of acetone and 2-propanol. This was because it was hypothesized that since the bulk permittivity of 2-propanol is greater than acetone at 2.45 GHz, and the bulk permittivity of acetone is greater than 2-propanol at 5.8 GHz, there should be a change in adsorption selectivity due to heating with the microwaves at different frequencies. The adsorption of acetone and 2-propanol was studied on both Aerosil 200 silica and silicalite. The literature values of the heats of adsorption vary for acetone and 2-propanol on Aerosil and there is some overlap in the literature values shown in Table 5. From the experiments 2-propanol has a higher heat of adsorption on Aerosil than acetone due to greater changes in the bed temperature when individual components were fed to the reactor. This is expected due to the greater amount of hydrogen bonding of the 2-propanol to the surface hydroxyl groups. Changes in the amount adsorbed due to conventional heating are show in Figure 6. As the reactor is heated, the 2-propanol desorbs. The acetone also shows a small amount of desorption initially, but once more of the 2-propanol desorbs, there is more surface available and the acetone adsorbs. The adsorptions of acetone and 2-propanol on Aerosil 200 with 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 200 at the flow conditions used in these experiments, the total relative surface coverage was calculated to be 1.2 monolayers at room temperature.
TABLE 4: Change in Amount Adsorbed for Methanol and Benzene on Silicalite, in Molecules/Silicalite Unit Cell (Top) and Ratios of the Amount Adsorbed (Bottom) Silicalite temperature methanol benzene
methanol benzene
conventional 112.6 change in amount adsorbed w/ heating (molecules/silicalite unit cell) -3.89 3.04 Silicalite conventional Th2/Th1 0.70 1.49
Figure 6. Acetone and 2-propanol on Aerosil 200 silica with conventional heating.
Selectivity of Sorption for Binary Mixtures on Oxides
Figure 7. Acetone and 2-propanol on Aerosil 200 with microwave heating at 2.45 GHz and 120 W.
Figure 8. Acetone and 2-propanol on Aerosil 200 with microwave heating at 5.8 GHz and 20 W.
TABLE 6: Ratio of the Amount Adsorbed for Acetone and 2-Propanol on Aerosil Aerosil conventional
2.45 GHz
5.8 GHz
temperature
86.3
59.6
42.0
acetone 2-propanol
Th2/Th1 1.10 0.68
Th2/Th1 1.14 0.85
Th2/Th1 1.17 0.94
TABLE 7: Ratio of the Amount Adsorbed for Acetone and 2-Propanol on Aerosil with the Amount Adsorbed during Heating Adjusted for Temperature Differences Aerosil
acetone 2-propanol
conventional
2.45 GHz
5.8 GHz
Th2/Th1 1.14 0.57
Th2/Th1 1.30 0.68
Th2/Th1 1.73 0.73
In comparing the amount adsorbed for these experiments the temperatures at which the reactor was heated to during each experiment were different. This must be taken into account and was done in a manner similar to that as described for methanol and cyclohexane. Tables 6 and 7 show the changes in surface coverage due to heating with and without adjusting for the differences in temperature. Th2/Th1 is the amount adsorbed at steady state during heating divided by the amount adsorbed at steady state before heating. The ratios of the amount adsorbed at steady state during heating (after adjusting for the differences in temperature) to that before heating are compared.
J. Phys. Chem. C, Vol. 112, No. 39, 2008 15487
Figure 9. Acetone and 2-propanol on Silicalite with conventional heating.
The use of microwaves did not change which component desorbs upon heating compared to conventional heating. A significant portion of the microwave energy may be absorbed by the adsorbent and then transferred to the adsorbed phase, leading to results similar to that of conventional heating. The ratio of the bulk permittivity of 2-propanol to acetone is only 3.15 (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 to desorb and the 2-propanol to adsorb. The ratio of the bulk liquid permittivities 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 acetone adsorbed. Since the surface coverage is low, the dielectric properties of the adsorbates are not the same as the respective bulk liquid permittivities, and it is expected to be lower than the measured bulk permittivities. The adsorption selectivity might also depend on the frequency dependence of the permittivity of the surface, especially for Aerosil 200 due to the surface hydroxyl groups. Unlike the adsorbate pair of methanol and cyclohexane (which are immiscible), acetone and 2-propanol are miscible as bulk liquids. If the adsorbed phase is also miscible, the adsorbed phase containing both species might behave as if it were a solution with a single permittivity that is intermediate to both acetone and 2-propanol. The amount of energy absorbed by the adsorbed phase when exposed to microwaves would still be a function of microwave frequency; however, the microwave frequency would no longer directly influence the adsorption selectivity if the multicomponent adsorbed phase exhibited a single average permittivity. The adsorption of acetone and 2-propanol on silicalite with conventional heating is shown in figure 9. In contrast to the previous case on Aerosil, acetone has a greater heat of adsorption than 2-propanol on silicalite. As the reactor heating is turned off and the reactor cools, the acetone and 2-propanol should return to the same surface coverage as the steady state before the reactor was heated if only physical adsorption is taking place. This is approximately true for 2-propanol but not for acetone in this case. From integrating the changes in the amount of acetone present in the system, acetone must have a higher surface coverage for the same flow conditions and temperature after the reactor had been heated. There was no evidence of any chemical reaction taking place in the form of new molecules being observed with the online mass spectrometer; however, the acetone and 2-propanol both are fragmented in the mass spectrometer and it is the fragmentation peaks that are tracked to quantify the adsorption, and the fragmentation may mask new products. There were no abnormal temperature effects observed from the fiber optic
15488 J. Phys. Chem. C, Vol. 112, No. 39, 2008
Vallee and Conner TABLE 9: Ratio of the Amount Adsorbed upon Heating for Acetone and 2-Propanol on Silicalite, Adjusted for Temperature Differences Silicalite
acetone 2-propanol
Figure 10. Acetone and 2-propanol on Silicalite: Microwave heating at 2.45 GHz and 120 W.
Figure 11. Acetone and 2-propanol on Silicalite: Microwave heating at 5.8 GHz and 20 W.
TABLE 8: Ratio of the Amount Adsorbed upon Heating for Acetone and 2-Propanol on Silicalite Silicalite Conventional
2.45 GHz
5.8 GHz
temperature
70.0
61.2
73.2
acetone 2-propanol
Th2/Th1 1.04 0.96
Th2/Th1 0.98 0.91
Th2/Th1 0.92 0.82
temperature probe in the adsorbent bed. In order to examine whether there was any reaction was taking place, the experimental setup was modified to put a liquid nitrogen trap on the effluent stream to condense it. The condensate was studied by GC/MS, but no components besides the adsorbates were found. The adsorbate molecules that react with the surface are likely bound there. This could also change the surface properties over extended reuse of the silicalite; however, we did not have sufficient time to fully examine this. The silicalite adsorbent was periodically recalcined. The adsorptions of acetone and 2-propanol on silicalite with microwave heating at 2.45 GHz and 120 W (Figure 10) and 5.8 GHz and 20 W (Figure 11) are shown. In comparing the change in surface coverage for these experiments, however, the temperatures at which the reactor was heated to during each experiment were different. This must be taken into account and was done in a manner similar to that as described before; the change in the amount adsorbed due to a change in temperature is proportional to e(-∆Hads/RT). The ratios of the amount adsorbed at steady state during heating (after adjusting for the differences in temperature) to that before heating that are compared. Tables 8 and 9 show the changes in surface coverage due to heating with and without adjusting for the differences in temperature. Th2/Th1 is the amount adsorbed at steady state during heating divided by the amount adsorbed at steady state before heating.
conventional
2.45 GHz
5.8 GHz
Th2/Th1 1.08 0.93
Th2/Th1 0.95 0.82
Th2/Th1 0.86 0.71
The use of microwaves did not change which component desorbs upon heating compared to conventional heating. The acetone did seem to absorb more microwave energy at 5.8 GHz, but not to as great an extent as expected. The reasons for this are similar to those on Aerosil. From previous work with Aerosil and silicalite,2 since almost none of the microwave energy is adsorbed by the surface of the silicalite adsorbent, larger differences between conventional heating and microwave heating would be evident using silicalite. Aerosil absorbs some of the microwave energy at the surface of the adsorbent due to the surface hydroxyl groups present, which leads to adsorption behavior that is more similar to conventional heating. Comparing Tables 6-9 shows differences in the adsorbents, Aerosil 200 and silicalite. In the case of conventional heating and the case of microwave heating on Aerosil, the 2-propanol desorbs and acetone adsorbs. In the case on silicalite with microwave heating, however, both components desorb. Conclusions Experiments were performed to study the effect of microwave frequency on adsorption selectivity, and the differences between microwave and conventional heating. From the results, the following conclusions can be made: 1) In the case of methanol and cyclohexane, microwave heating caused the methanol to desorb almost twice as much as conventional heating. This was because the methanol had a much greater permittivity (the bulk liquid permittivities have a 275:1 ratio). 2) The selectivity for desorption of methanol and benzene using conventional heating was about half-that of methanol and cyclohexane. This is mostly due to the smaller in the heat of adsorption of benzene than cyclohexane. 3) The adsorbent Aerosil 200 absorbed a significant amount of microwave energy. This energy was then transferred to the adsorbates and contributed to a heating mechanism that was similar to conventional heating. This makes it difficult to distinguish the effects of microwave heating compared to conventional heating. 4) The frequency dependence of the adsorption of acetone and 2-propanol was not as selective as expected based on the bulk permittivity differences (ratios 2-propanol/acetone 3.15:1 at 2.45 GHz and 2-propanol/acetone 1:1.71 at 5.8 GHz), as shown in Tables 7 and 9. The smaller than expected change in adsorption selectivity with microwave frequency might be attributed to the miscibility of acetone and 2-propanol. If the adsorbed phase is also miscible, the adsorbed phase containing both species might behave as if it were a solution with a single permittivity that is intermediate to both acetone and 2-propanol. Since the surface coverage was low, the dielectric properties of the adsorbates might not be the same as the respective bulk liquid permittivities, and are expected to be lower than the measured bulk permittivities. Also, the frequency dependence of the adsorbed phases may be different than that of the bulk
Selectivity of Sorption for Binary Mixtures on Oxides liquids. It might also depend on the frequency dependence of the permittivity of the surface, especially for Aerosil 200 due to the presence of surface hydroxyl groups. 5) In some cases there was a change in the amount adsorbed before the reactor was heated compared to the amount adsorbed after the reactor was heated and cooled. In these cases, there must be some degree of chemisorption or a reaction taking place, to alter the surface characteristics. Determining the extent of this reaction was beyond the scope of this work, as no reaction was expected. One cannot predict what will be selectively desorbed due to a change in temperature based solely on the heats of adsorption of the adsorbates. The influence of microwave frequency was not significant for changing selectivity; however, the desorption efficiency appeared greater at 5.8 GHz than at 2.45 GHz on silicalite. This effect may be due to an inherent efficiency of microwaves to influence interfacial interactions in nonresonant applications wherein the exposure can vary in intensity with time.30 Acknowledgment. Funding was provided by a NSF NIRT Nanoscale Interdisciplinary Research Team grant. Thanks to Prof. Robert Laurence, Prof. Sigfrid Yngvesson, Prof. Scott Auerbach, and Geoff Tompsett for their discussions. Thanks to Gerald Ling for having assembled the piping and wiring for the flow controllers on the multicomponent flow adsorption system. Thanks to Karl Hammond for writing the computer programs that collect the mass spectrometer data using labview and convert the data into an excel-compatible format. Thanks to Fan Lu and Kyu-Ho Lee for providing permittivity measurements. References and Notes (1) Turner, M. D.; Laurence, R. L.; Conner, W. C. AIChE J. 2000, 46, 758. (2) Vallee, S. J.; Conner, W. C. J. Phys. Chem. B. 2006, 110, 15459. (3) Pozar, D. M. MicrowaVe Engineering; John Wiley and Sons, Inc.: Toronto, 1998. (4) Ulaby, F. T. Fundamentals of Applied Electromagnetics; Prentice Hall: Upper Saddle River, NJ, 2001.
J. Phys. Chem. C, Vol. 112, No. 39, 2008 15489 (5) Galema, S. A. Chem. Soc. ReV. 1997, 26, 233. (6) Stuerga; Gaillard, P. J. MicrowaVe Power Electromagn. Energy 1996, 31, 87. (7) Gregg, S. J.; Sing, K. W. Adsorption, Surface Area and Porosity; Academic Press Inc.: San Diego, 1982. (8) Fogler, H. S. Elements of Chemical Reaction Engineering, 3rd ed.; Prentice Hall PTR: Saddle River, NJ, 1999. (9) Tester, J. W.; Model, M. Thermodynamics and Its Applications, 3rd ed.; Prentice Hall: Upper Saddle River, NJ, 1997. (10) Gruermeur, R.; Jacolin, C. Surf. Sci. 1994, 315, 323. (11) Kobayashi, S.; Kim, Y.; Kenmizaki, C.; Kushiyama, S.; Mizuno, K. Chem. Lett. 1996, 769. (12) Turner, M. D.; Laurence, R. L.; Yngvesson, K. S.; Conner, W. C. Catal. Lett. 2000, 71, 133. (13) Barthel, J.; Bachhuber, K.; Buchner, R.; Gill, J. B.; Kleebauer, M. Chem. Phys. Lett. 1990, 167, 62. (14) Barthel, J.; Bachhuber, K.; Buchner, R.; Hetzenauer, H. Chem. Phys. Lett. 1990, 165, 369. (15) Lopez, A.; Diamy, E.; Legrand, A.; Fraissard, J. Stud. Surf. Sci. Catal. 2004, 154B, 1866. (16) Lee, K. Experiments using a automatic network spectrum analyzer to measure permittivity versus frequency, University of Massachusetts, 2003. (17) Lu, F. Experiments using a automatic network spectrum analyzer to measure permittivity versus frequency, University of Massachusetts, Amherst, 2004. (18) Duan, L.-H.; Song, L.-J.; Zhang, X.-T.; Tang, K.; Dia, Z.-H.; Sun, Z.-L. Chin. J. Chem. 2006, 24, 961. (19) Blanco, C.; Auerbach, S. M. J. Am. Chem. Soc. 2002, 124, 6250. (20) Turaga, S.; Auerbach, S. Abstracts Paper Am. Chem. Soc. 2002, 223, 406. (21) Meier, M. Studies on Desorption Under the Use of Microwave Energy in a 1 Meter Column. Independent Study, University of Massachusetts, 2001. (22) Hair, M. L.“The Molecular Nature of Adsorption on Silica Surfaces,” Xerox Corporation. (23) Bardina, I. A.; Kovaleva, N. V.; Nikitin, Y. S. Russ. J. Phys. Chem. 1999, 74, 421. (24) Kokunova, E. Y.; Lanin, S. N.; Nikitin, Y. S.; Shoniya, N. K. Russ. J. Phys. Chem. 1992, 67, 1508. (25) Lee, C.; Gorte, R. J.; White, D. J. Phys. Chem. 1996, 100, 18515. (26) Gorte, R. J.; White, D. Topics Catal. 1997, 14, 57. (27) Thamm, H. J. Chem. Soc., Faraday Trans. I 1989, 85, 1. (28) Celia, L.; Cavalcante; Ruthven, D. Ind. Eng. Chem. Res. 1995, 34, 177. (29) Klemm, E.; Wang, J.; Emig, G. Microporous Mesoporous Mater. 1998, 26, 11. (30) Tompsett, G.; Conner, W. C.; Yngvesson, K. S. ChemPhysChem 2006, 7, 296.
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