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Comparisons between Measured and Calculated Properties in the Microwave Heating of Na-A, K-A, and Na,Ca-A Zeolites Tatsuo Ohgushi,* Yuya Sakai, Yoshimichi Adachi, and Hirohisa Satoh Department of Materials Science, Toyohashi UniVersity of Technology, Tempaku-cho Toyohashi 441-8580, Japan ReceiVed: January 05, 2009; ReVised Manuscript ReceiVed: March 09, 2009
For three dehydrated zeolite A samples, heated behaviors in microwave (f ) 2.45 GHz) irradiation were measured. In addition, the relative energy absorption ratios for microwaves among the three zeolites were theoretically calculated as a function of temperature, and properties obtained by the calculations and measurements were compared with each other. During the irradiation under every condition, the heated temperatures were in the order of Na12-A > K12-A > Na4Ca4-A. The magnitude of the energy absorption ratio was the same as the order of the heated temperature. It was found that the absorption ratio calculated by the present method can correctly reflect the heated behaviors of zeolites in microwave irradiation. The dielectric relaxation loss (relax II) that appeared in the higher frequency side had a decisively important influence on the absorption of microwaves. A zeolite with a lower activation energy of cation movement (or more mobile cations) was more easily heated by microwaves. Introduction Microwaves can quickly, widely, uniformly, and internally heat substances. Because of these properties, microwaves have also been widely applied to the synthesis, ion exchange, modification, and reactivation of zeolites.1-10 When zeolites are directly irradiated in states without solution or solvent, several zeolites are easily and efficiently heated to a glowing temperature.4,11-13 Because microwave heating is a dielectric heating, responses of substances to microwaves depend on their dielectric properties (permittivity, ε′, and dielectric loss, ε′′). An energy absorption ratio of a substance for microwaves is expressed by a simple equation containing ε′ and ε′′.14 Hence, if a dielectric loss of a substance is related to behaviors of a particular component in the substance, then the properties of the substance during microwave heating can be also related to the behaviors of particular components, leading to an elucidation of the mechanism of microwave heating. In previous studies, dielectric spectra of Na-A zeolites (LTAs) were measured and analyzed in detail, and the dielectric losses in the zeolites were attributed to movements of the Na+ ion on site 3 (Na+/S3).15-17 From comparisons of the heated behaviors and dielectric properties between Na-A with Na+/S3 and Na,Ca-A without cation/S3, the importance of cation/S3 of zeolite A during microwave heating was elucidated.18 In the study, an important, new method was tried; values of ε′ and ε′′ (tan δ ) ε′′/ε′) at the microwave frequency for Na-A and Na,Ca-A were estimated, relative energy absorption ratios of the zeolites for microwaves were calculated by using the values, and the ratios were compared with the heated behaviors of zeolites in microwave irradiation. The ratios were consistent with the behaviors. On the basis of the results, the mechanism of microwave heating of zeolite A was confirmed from a theoretical standpoint. Although the theoretical method was useful to elucidate the mechanism, there were some problems in applying the method: * To whom correspondence should be addressed. E-mail: ohgushi@ tutms.tut.ac.jp. Phone: +81-532-44-6796. Fax: +532-48-5833.
(1) The dielectric data of Na,Ca-A were inadequate in terms of quality and quantity, and the absorption ratio could not be calculated for Na,Ca-A with sufficient accuracy. (2) Because there were, by a lucky chance, large differences in both the measured behaviors and calculated properties between Na-A and Na,Ca-A, a satisfactory coincidence was obtained in the comparison between the measurement and the calculation for the zeolites, regardless of less accuracy. Due to this situation, we could not find a limit of the accuracy of the calculations. Because there were no accurate and well analyzed data other than those of Na-A, such calculation could not be carried out, except for Na-A. Therefore, to confirm the applicability and reliability of the calculations, it was necessary to obtain more practical examples. Recently, dielectric data of K-A were obtained and analyzed in detail.19 The accuracy of the data and analyses was comparable to those of Na-A.15 Hence, a close comparison of the absorption ratio between K-A and Na-A becomes possible now. Because K-A has a cation on S3 (K+/S3), as does Na-A, K-A will be easily heated by microwaves, and the difference in the heated behaviors between K-A and Na-A will be much smaller than that between Na-A and Na,Ca-A. Therefore, the comparison between the measured behaviors and the calculated properties for the microwave heating of Na-A, K-A, and Na,Ca-A will be significant for assessing the applicability and reliability of the calculations, especially for the comparison between Na-A and K-A. In the present study, we aim to compare the calculated absorption ratios with the heated behaviors for K-A, Na-A, and Na,Ca-A and assess the applicability and reliability of the calculations. Experimental Section Materials. The starting material was a commercial (Tosoh Co.) 4A zeolite powder without binder. It was repeatedly treated with a 0.2 M KCl solution at 353 K, and in the last two treatments, it was soaked in the 0.02 M KCl solution for 10 days at room temperature to obtain a true equilibrium cation distribution. The treated zeolite was filtered and briefly washed
10.1021/jp900080x CCC: $40.75 2009 American Chemical Society Published on Web 04/15/2009
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with pure water on the filter. The resultant zeolite was analyzed by atomic absorption spectroscopy, and its chemical composition was determined to be K11.6Na0.1(AlO2)11.9(SiO2)12.1 per (pseudo) unit cell. This sample was used in the present study and is designated as K12-A in this paper. Na-A and Na,Ca-A zeolites were the same batches as those used in the previous study.18 The chemical compositions of Na-A and Na,Ca-A were Na12.0(AlO2)12.0(SiO2)12.0 and Na4.0Ca4.0(AlO2)12.0(SiO2)12.0, respectively. The former and latter are designated as Na12-A and Na4Ca4-A, respectively. Microwave Heating. For K12-A, Na12-A, and Na4Ca4-A, heated curves by microwave irradiation were measured as a relationship of heated temperature versus radiation time. Because a heated curve strongly depends on the hydration degree of the zeolite, the curve was measured after full dehydration to avoid an influence of adsorbed water. The dehydration of the sample was carried out as follows. The hydrated zeolite was gradually heated to 573 K in a conventional electric oven; the heated zeolite was taken out of the oven and immediately placed into a high-purity aluminum vessel. The vessel was set in a holder of thermal insulation made of aluminum fiber, and the holder was transferred to the conventional electric oven at 573 K and heated for 1 h. The holder with the vessel was heated up to 673 K and kept at that temperature for 1 h. The treatment through this stage was for dehydration of the sample. After the dehydration treatment, the temperature of the electric oven was adjusted to a preheated temperature, Tpr (573 K e Tpr e 673 K), and the holder was kept at Tpr for 1 h. After it was removed from the electric oven, the holder was set in a box of thermal insulation made of aluminum fiber, and the box was transferred into a microwave oven. The irradiation with microwaves was begun 1 min after it was removed from the electric oven. The temperature of the irradiated zeolite was measured by inserting a thermocouple into the zeolite immediately after irradiation. The microwave oven used was a commercial model used for cooking (Sharp Co., model RE-LA 1-W) with a frequency of 2.45 GHz. Results Microwave Heating. The heated curves measured under common conditions were compared among the three zeolites. Depending on the conditions, the heated curves changed. However, under every condition examined and at every moment, Na12-A reached the highest temperature and Na4Ca4-A stayed at the lowest temperature. Representative results are shown in Figure 1. It was found from the results that the heated temperature during the microwave irradiation is in the order of Na12-A > K12-A > Na4Ca4-A in every condition. Calculation of Absorption Ratio. The energy absorption ratio, K, of a substance for microwaves is given by the equation14
K ) C√ε′ tan δ or
Cε ′′ /√ε′
Figure 1. Heated curves for dehydrated zeolite A: 0, Na12-A; O, K12A; 4, Na4Ca4-A. The irradiation power of the microwaves was 1000 W, and the amount of sample was 6.8 cm3. (a) Tpr ) 573 K. (b) Tpr ) 673 K.
in this study, the term, called the relative energy absorption ratio and expressed as Kr, is calculated for three zeolites. In K12-A, two relaxation processes were measured in the dielectric spectra in the ranges of 385 e T/K e 531 and 2 < log(f/Hz) < 7.19 Because the two relaxations fairly overlapped each other, the relaxations were separated from each other to determine the properties of each relaxation by using the following equations. For a single relaxation, ε′, ε′′, and tan δ at an applied alternating field of a certain frequency f have values expressed by the following equations20
{
1 ε ′ ) ε∞ + (εs - ε∞) 1 2
and
cosh βx + cos sin
βπ 2
cosh βx + cos
tan δ )
(1)
where ε′, ε′′, and tan δ are the values of permittivity, dielectric loss, and dielectric loss tangent at microwave frequency (2.45 GHz), respectively, and C is a constant depending on irradiation conditions. When substances are irradiated under common conditions, the factor C is considered to be a common value for every substance. In such a case, a difference in the absorption ratio for microwaves among the substances stems from the difference of the term ε′ tan δ (or ε′′/ε′). It is difficult to estimate accurately the value of the factor C for various cases (depending on vessel size, vessel shape, packing density of the sample, etc.). However, the value of ε′(tan δ) (or ε′′/ε′) can be calculated by using results obtained in the studies. Hence,
{
1 ε ′′ ) (εs - ε∞) 2
sinh βx
ε′′ ε′
βπ 2
βπ 2
}
}
(2)
(3)
(4)
with x ) ln (f/fm) and where εs and ε∞ are the values of ε′ at f ) 0 and ∞, respectively; fm and βπ are the frequency at the maximum value, ε′′m, of ε′′ and a central angle of a circular arc in a Cole-Cole plot, respectively. In the low-frequency region of the spectra, a low-frequency dispersion (l.f.d.) was measured and its contributions, ε′L and ε′′L, to ε′ and ε′′, respectively, are expressed by
ε′L ) and
A fm
(5)
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ε″L )
B fm
Ohgushi et al.
(6)
where A, B, and m are constants and m has a value of 0 < m < 1. In the previous study, the spectra measured for the dehydrated K12-A were analyzed by using eqs 2-6 and each relaxation was separated from each other (the relaxations in the low- and highfrequency sides are named relax I and II, respectively).19 In the analyses, parameters needed to calculate ε′ and ε′′ were determined for a spectrum measured at each temperature, and their temperature dependences were also determined.19 The value of each parameter at temperature T was determined by the following equations
ln fmI ) 24.7 - 7.69 × 103/T εsI ) 74.3 - 9.33 × 103/T ε∞I ) 14.1 + 4.67 × 103/T
(7)
βI ) 0.64 + 0.085 × 103/T and
ln fmII ) 27.9 - 7.37 × 103/T εsII ) 17.9 + 3.44 × 103/T ε∞II ) 12.8
(8)
βII ) 1.03 - 0.13 × 103/T where the quantity associated with relax I (II) is marked with a subscript I (II). The parameters for the l.f.d. were also determined as
log A ) 7.65 - 2.36 × 103/T log B ) 6.93 - 1.82 × 103/T and m ) 0.85
(9)
Because the values of the parameters at a particular temperature are obtained from the above equations, the values of ε′, ε′′, and tan δ at that temperature can be calculated as a function of f; namely, the spectra of ε′, ε′′, and tan δ at T are obtained. The spectra calculated at 474 K are shown in Figure 2 and compared with the measured spectra. It is found from the comparisons that the calculated spectra have sufficient accuracy in the measured frequency range. If the above equations are applied outside of the examined temperature and frequency ranges, one can calculate the values of ε′, ε′′, and tan δ in the higher temperature and frequency ranges. The examples of calculations for the range of log(f/Hz) e 10 at 900 K are shown in Figure 2. From the calculations, one can easily obtain the values of ε′, ε′′, and tan δ at f ) 2.45 GHz. Because similar calculations can be carried out at various temperatures, the values of ε′, ε′′, and tan δ at f ) 2.45 GHz can be determined as a function of T. When the determined values are inserted into eq 1, Kr can be obtained as a function of T, and the relationship is illustrated in Figure 3. The curves of Kr for Na12-A and Na4Ca4-A are also given in the figure. In the previous studies, the former was obtained by a method similar to that described above but the latter was obtained by a less exact manner because of the less accurate dielectric data.18 The value of Kr for Na12-A is the largest at any temperature and that for Na4Ca4-A is the smallest at any temperature. The results in the figure indicate that the heated temperatures during the microwave irradiation should be in the order of Na12-A > K12-A > Na4Ca4-A.
Figure 2. Calculated dielectric spectra of the dehydrated K12-A zeolite: (a), ε′; (b), ε′′; (c), tan δ. Lines and symbols shown in each panel: O, measured point; dashed line, relax I; dotted line, relax II; dasheddotted line, l.f.d.; and solid line, calculated spectrum. The calculated spectra on the left and right sides are at 474 and 900 K, respectively. The vertical line at log(f/Hz) ) 9.39 indicates a position of f ) 2.45 GHz.
Figure 3. Relative energy absorption ratio of dehydrated zeolite A for microwaves: (1), Na12-A; (2), K12-A; (3), Na4Ca4-A.
Discussion Na12-A reached the highest temperature at every moment during microwave irradiation under the common conditions among the three zeolites, and the heated temperature was always in the order of Na12-A > K12-A > Na4Ca4-A under every condition examined, as shown in Figure 1. In addition, in the theoretical calculations, the magnitude of the (relative) energy absorption ratio for microwaves was in the order of Na12-A >
Microwave Heating of Na-A, K-A, and Na,Ca-A Zeolites
J. Phys. Chem. C, Vol. 113, No. 19, 2009 8209 TABLE 2: Comparison of Activation Energies
Figure 4. Temperature dependence of ε′′ at 2.45 GHz for dehydrated zeolite A: (1), Na12-A; (2), K12-A; (3), Na4Ca4-A.
TABLE 1: Variations of Dielectric Properties at 2.45 GHz with Temperature K12-A
573 K
673 K
773 K
873 K
973 K
ε′ ε′′ tan δ (ε′′/ε′) Kr(ε′ tan δ)
12.8 0.064 0.0050 0.018
12.9 0.261 0.0203 0.0728
13.0 0.774 0.0598 0.215
13.2 1.816 0.1374 0.500
13.9 3.448 0.2485 0.926
K12-A > Na4Ca4-A over the whole temperature range examined, as shown in Figure 3. This result means that the absorption ratio calculated by the present method can correctly reflect the heated behaviors of zeolites in microwave irradiation. Once a zeolite is heated to a middle temperature by microwaves, its temperature accelerates further and results in a thermal runaway, as shown in Figure 1. Such phenomena have been frequently measured in microwave heating.11,13,18 The measured property can be well-explained by the temperature dependences of Kr for Na12-A and K12-A; the absorption ratios of substances become larger as the temperature rises, and hence, their heated rates accelerate. As can be seen from the spectra in Figure 2, the value of ε′′ at 2.45 GHz greatly increases as the temperature rises but that of ε′ does not show a large increase until the temperature is K12-A > Na4Ca4-A. The energy absorption ratio and dielectric loss at f ) 2.45 GHz calculated by the present method were in the same order with that of the heated temperatures and correctly reflected the heated behaviors of the zeolites in the microwave irradiation. The increase of the absorption ratio with raising the temperature was almost attributed to the increase of ε′′. The dielectric relaxation loss (relax II) that appeared in the higher frequency side had a decisively important influence on the absorption of microwaves. The zeolite with a lower activation energy of cation movement (or more mobile cations) was more easily heated by microwaves. The (relative) absorption ratios of Na-A and K-A were calculated by using dielectric analytical results with sufficient accuracy to explain the heated behaviors. This is the first practical example of obtaining such accurate calculations and comparing the calculated results with the measured behaviors. References and Notes (1) Chu, P.; Dwyer, F. G.; Vartuli, J. C. United States Patent 4,778,666, 1988.
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(2) Komarneni, S.; D’Arrigo, M. C.; Leonelli, C.; Pellacani, G. C.; Katsuki, H. J. Am. Ceram. Soc. 1998, 81, 3041. (3) Kuroda, Y.; Okamoto, T.; Kumashiro, R.; Yoshikawa, Y.; Nagao, M. Chem. Commun. 2002, 1758. (4) Ohgushi, T.; Nagae, M. J. Porous Mater. 2003, 10, 139. (5) Celer, E. B.; Jaroniec, M. J. Am. Chem. Soc. 2006, 128, 14408. (6) Romero, M. D.; Ovejero, G.; Uguina, M. A.; Rodriguez, A.; Gomez, J. M. Microporous Mesoporous Mater. 2007, 98, 317. (7) Zhu, G.; Li, Y.; Zhou, H.; Liu, J.; Yang, W. Mater. Lett. 2008, 62, 4357. (8) Youssef, H.; Ibrahim, D.; Komarneni, S. Microporous Mesoporous Mater. 2008, 115, 527. (9) Jiang, T.; Shen, W.; Tang, Y.; Zhao, Q.; Li, M.; Yin, H. Appl. Surf. Sci. 2008, 254, 4797. (10) Gonzalez, M. D.; Cesteros, Y.; Salagre, P.; Medina, F.; Sueriras, J. E. Microporous Mesoporous Mater. 2009, 118, 341.
Ohgushi et al. (11) Komarneni, S.; Roy, R. Mater. Lett. 1986, 4, 107. (12) Whittington, B. I.; Milestone, N. B. Zeolites 1992, 12, 815. (13) Ohgushi, T.; Komarneni, S.; Bhalla, A. S. J. Porous Mater. 2001, 8, 23. (14) MacDowell, J. F. Am. Ceram. Soc. Bull. 1984, 63, 282. (15) Ohgushi, T.; Ishimaru, K. Phys. Chem. Chem. Phys. 2001, 3, 3229. (16) Ohgushi, T.; Ishimaru, K. 13th International Zeolite Conference, Montpellier, France, July 8-13, 2001, paper 14-P-06. (17) Ohgushi, T. J. Phys. Chem. C 2007, 111, 4688. (18) Ohgushi, T.; Numata, T. J. Porous Mater. 2003, 10, 207. (19) Ohgushi, T.; Ishimaru, K.; Adachi, Y. J. Phys. Chem. C 2009, 113, 2468. (20) Cole, K. S.; Cole, R. H. J. Chem. Phys. 1941, 9, 341. (21) Ohgushi, T.; Sato, S. J. Solid State Chem. 1990, 87, 95.
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