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J. Phys. Chem. C 2007, 111, 1474-1479
Analysis of Toluene Adsorption on Na-Form Zeolite with a Temperature-Programmed Desorption Method Ryosuke Yoshimoto, Kazuya Hara, Kazu Okumura, Naonobu Katada,* and Miki Niwa Department of Materials Science, Faculty of Engineering, Tottori UniVersity, 4-101 Koyama-cho Minami, Tottori 680-8552, Japan ReceiVed: September 12, 2006; In Final Form: NoVember 9, 2006
Temperature-programmed desorption (TPD) of toluene was carried out on various zeolites, and theoretically analyzed to determine thermodynamic parameters. The TPD process was identified as the case of equilibrium control, showing that the analytical method, the same as that for ammonia TPD, can be applied to the toluene TPD to calculate the adsorption heat. It was observed that approximately one toluene molecule was adsorbed on one Na+ cation for MFI and BEA. The heat of toluene adsorption on Na+ was MOR > MFI > BEA > FAU, similar to the heat of ammonia adsorption on the corresponding H-form zeolite. A linear relationship was observed between both adsorption heats. It suggests that the adsorption of toluene on Na+ is controlled by the electron-withdrawing nature of the ion exchange site, as well as the Brønsted acid strength of the H-form zeolite.
Introduction Adsorption of such an aromatic compound as toluene on zeolites gathers attention from multiple viewpoints. Zeolites are widely utilized as solid acid catalysts. For understanding the catalysis of zeolites, analysis of two parameters, acid amount and strength, must be important. There are several candidates such as CO, N2, Ar, and aromatic hydrocarbon as nucleophilic probes for acidity evaluation of solids including zeolites.1-10 These compounds are physically adsorbed on acid sites. However, adsorption of these compounds on such a cation as Na+ stronger than that on H+ in zeolite sometimes disturbs the analysis of acidity of a practical catalyst usually containing multiple components; in the case of aromatic hydrocarbons, an alkaline cation acts as a Lewis acid to form a strong interaction with π electrons.11-14 In contrast, a typically basic molecule, ammonia is chemisorbed on acid sites more strongly than on nonacidic sites, and therefore we can count the number of acid sites (acid amount) through the number of strongly adsorbed ammonia molecules.15 Frequently, different conclusions are derived from experiments based on different probe molecules, and therefore, there is a controversy about the validity of the use of these probes. Among the different methods, ammonia TPD (temperatureprogrammed desorption)16 has advantages in measuring the important parameters, i.e., acid amount and strength. Especially with the development of the water vapor treatment technique17 and analysis theory,18,19 nowadays, one can measure the number of acid sites and the heat of ammonia adsorption easily with high reproducibility.20 There is, however, one question that is not negligible but cannot be solved simply; the adsorption of ammonia, as well as adsorption of other molecules, can be affected by steric hindrance or, in contrast, the “confinement effect” in a small cavity: the former can weaken the adsorption, while the latter can enhance it.21,22 These steric effects can disturb the estimation * Address correspondence to this author. Phone: +81-857-31-5684. E-mail:
[email protected].
of intrinsic acid strength from the adsorption heat of a probe, even if it has been measured exactly. From this sense, it is proposed that a small alkaline cation, e.g., Na+, should be an ideal base probe for the acidity evaluation of a solid surface to avoid any steric effects. As mentioned above, some molecules are strongly adsorbed on Na+ in zeolites, and it may be possible to study the nature of the interaction between the acid site (or ion-exchange site) and Na+ through the adsorption behavior of those molecules. In this study, we analyze the behavior of toluene adsorption mainly on Na-containing zeolites in detail by means of TPD. Thermodynamic parameters concerning the adsorption of aromatic compounds on zeolites have been collected by direct equilibrium measurements23-25 and calorimetry.26 To obtain the same parameters, this study proposes the effective use of TPD, which is easy and reliable for systematic study on many zeolites, as supported by the presence of many attempts with TPD as a tool to study the adsorption of aromatic compounds on zeolites for various applications as mentioned below.9,11,12,27-34 For this purpose, we introduce a theoretical analysis of toluene TPD, which has been established in the ammonia TPD18 as above. The TPD process is classified into three cases, kinetics control, equilibrium control with free readsorption, and diffusion control;16 experimental classification of toluene TPD on zeolites has been carried out in this study for the first time. Application of this method to various zeolites clarifies the factors controlling the amount and strength of toluene adsorption. Finally, we show the relationship between the adsorption strength of toluene on Na+ and that of ammonia on H+ in order to discuss what is shown by these adsorption strengths. The adsorption of aromatic compound is also a subject of research on diffusion of molecules in micropores,35 because the molecular size is similar to the pore size. For this purpose, theoretical treatment has been established.36 Moreover, the adsorption of an aromatic compound can be a tool for probing the cation position in very narrow pores.37 The present study aims to obtain a principle on fundamental interaction between
10.1021/jp065966x CCC: $37.00 © 2007 American Chemical Society Published on Web 12/24/2006
Toluene Adsorption on Na-Zeolites
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TABLE 1: Thermodynamic Parameters Determined by TPD of Toluene over Various Zeolites amount of desorbed toluene/mol kg-1 + a
+ b
heat of adsorptionlkJ mol-1
sample
[Al]
a
[Na ]
[H ]
high-temp peak
low-temp peak
high-temp peak
low-temp peak
Na-MFI (24) HNa-MFI (24) HNa-MFI (24) HNa-MFI (24) H-MFI (24) H-MFI (90) Na-MOR Na-BEA Na-FAU H-BEA
1.4 1.4 1.4 1.4 1.4 0.37 1.6 1.4 4.8 1.4
1.2 0.80 0.56 0.37 0.05 0.37 1.4 0.80 4.5 0.01
0.2 0.6 0.8 1.0 1.4 0.00 0.2 0.6 0.3 1.39
1.0-1.1c 0.51 0.46 0.41 0.00 0.35 0.34 0.84 3.6 0.00
0.13-0.23c 0.05 0.06 0.13 0.80 0.16 0.00 0.76 0.00 1.4
120-122c 122 119 115
88-90c 100 100 100 92 85
a
b
+
117 134 120 95
86 80
c
Based on ICP. Calculated from [Al]-[Na ]. Reproducibility and accuracy were confirmed by two experiments.
the aromatic compound and surface species in the region less affected by such steric effects. Apart from the above viewpoints, adsorption of such a hydrocarbon compound as toluene, itself, gathers attention. Nowadays, efforts are being made to control the emission of unburned hydrocarbon and NOx from automobile engines during the “cold-start period” when the catalyst temperature has not reached a suitably high level. It is promising for removal of hydrocarbons to store them at a low temperature and release to send them to a combustion catalyst after it is heated.30,38 Also for removal of NOx, storage of hydrocarbons as a reductant is useful.39,40 For these purposes, zeolites in various forms are proposed.30,39,40 The adsorption behavior of toluene, which is one of the main components of exhaust gas in the cold-start period from a gasoline engine, has been studied from this viewpoint.30,31,33-38,41 In addition, toluene is a typical VOC (volatile organic compound), and removal of it from the atmosphere by catalytic combustion or adsorption is being attempted widely. Also from this viewpoint, the adsorption behavior is being studied.23,32,42 One purpose of this study is development of a quantitative analysis method based on TPD of toluene to contribute to these fields, where most of the above studies have utilized TPD of aromatic hydrocarbons only qualitatively. Experimental Section Catalysis Preparation. Na-ZSM-5 (a commercial sample of Na-MFI (24) from Tosoh Corporation, Si/Al2 ) 23.8, and a reference catalyst Na-MFI (90) from the Catalysis Society of Japan, JRC-Z5-90NA (1), Si/Al2 ) 90), H-beta (PQ corporation, Si/Al2 ) 20; H-BEA), Na-mordenite (JRC-Z-M20, reference catalyst supplied from the Catalysts Society of Japan, Si/Al2 ) 20; Na-MOR), and Na-Y (Catalysts and Chemicals Ind., Co. Ltd., Si/Al2 ) 5.1; Na-FAU) were employed for TPD measurements. H-MFI (24) was prepared by ion exchange of Na-MFI (90) with an aqueous solution of ammonium nitrate at 353 K for 4 h, followed by calcination in an N2 flow at 773 K for 4 h. HNa-MFI (24) was prepared by the same method by varying the amount of added ammonium nitrate. Na-BEA was prepared by ion exchange of H-BEA with an aqueous solution of sodium nitrate in conditions the same as above. The chemical composition of zeolite was determined by ICP (inductively coupled plasma) spectroscopy (Rigaku CIROS CCD) after dissolution in hydrofluoric acid (Table 1). Temperature-Programmed Desorption of Toluene. Sample powder (0.1-0.5 g) was pretreated in a Pyrex tube (i.d. 10 mm) at 773 K for 1 h in a helium flow. Toluene vapor (3.8 kPa) was introduced to the sample at room temperature for 1 h, followed by evacuation for 0.5 h. The TPD of toluene was measured in a helium flow (100-400 cm-3 min-1, atmospheric pressure) at
Figure 1. Toluene TPD profiles on Na-MFI (24) at different W/F ratios: (a) W ) 0.1 g, F ) 400 cm3 min-1; (b1 and b2) W ) 0.1 g, F ) 200 cm3 min-1; (c) W ) 0.1 g, F ) 100 cm3 min-1; (d) W ) 0.3 g, F ) 200 cm3 min-1; and (e) W ) 0.5 g, F ) 200 cm3 min-1. The experiments at W ) 0.1 g, F ) 200 cm3 min-1 were repeated as (b1) and (b2) under the same conditions.
a ramping rate of 5 K min-1 with a mass spectrometer (Pfeiffer Vacuum QMS2000M2), using a fragment at m/e 91. Simultaneously, signals at m/e 15, 78, and 92 were recorded. Results and Discussion Thermodynamic Analysis of Toluene TPD. During all TPD experiments, it is considered that only toluene was desorbed from the zeolite, because signals at m/e 15, 91, and 92 were observed to show peaks with the same shapes and positions; m/e 15, 91, and 92 must show CH3+, C6H5CH2+, and C6H5CH3+ ions, respectively, formed in the mass spectrometer from toluene. No peak at m/e 78 was observed, indicating that no benzene was formed by such a side reaction as disproportionation of toluene. From these results, we can conclude that toluene was reversibly adsorbed and desorbed on zeolites without considerable side reaction. Toluene TPD profiles were recorded on Na-MFI (24) at different W/F ratios as shown in Figure 1. A desorption peak was observed at high temperatures (hereafter high-temperature peak, 500-650 K) in all cases, and toluene concentration in the gas phase at the peak maximum increased with increasing the W/F ratio. At the same time, the peak maximum shifted to higher temperature with increasing the W/F ratio. In addition, a peak was observed at a low temperature (hereafter lowtemperature peak, ca. 390 K) only at high W/F ratios. It is speculated that the low-temperature peak appeared only at high W/F ratios because the adsorption equilibrium constant was low. In contrast, the high-temperature peak can be analyzed quantitatively as follows. As stated above, the TPD process of a vapor on a solid is classified into three cases: (A) kinetics control, (B) equilibrium control, and (C) diffusion control. As explained in the Support-
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Yoshimoto et al.
Figure 2. Relationship between ln Tm - ln(A0W/F) and 1/Tm on Na-MFI (24).
ing Information, if a linear and positive relationship is observed between ln Tm - ln A0W/F and 1/Tm in multiple experiments on one sample with varying W/F, (B) the equilibrium must control the TPD process; Tm, A0, W, and F mean temperature at the peak maximum (K), number of the adsorption sites per unit weight of the tested sample (mol kg-1), weight of the tested zeolite (kg), and flow rate of the carrier gas (m3 s-1), respectively. If Tm is independent of W/F, (A) kinetics or (C) diffusion should control the process. The dependence of Tm on W/F shown in Figure 1 contradicts (A) and (C). As shown in Figure 2, a linear and positive relationship was observed between ln Tm - ln A0W/F and 1/Tm, indicating that the TPD of toluene on zeolite is classified into the equilibrium control (B), as well as ammonia TPD on various zeolites18,20 and non-zeolitic solid acids.43 An attempt of thermodynamic analysis of toluene TPD on mesoporous aluminosilicate has been reported, but it assumed the kinetic control without experimental confirmation.29 The present study is the first case in which the TPD of toluene has been classified based on the experiments. From the slope and intercept of this plot, the entropy change with respect to the adsorption (standard adsorption entropy ∆S°, J K-1 mol-1) can be calculated based on the following equation derived in the Supporting Information (it is noteworthy that distribution and temperature dependence of enthalpy and entropy changes are ignored in this study)
ln Tm - ln
A0W ∆H° β(1 - θm)2(∆H° - RTm) + ln ) F RTm P°e∆S°/R
(1)
where ∆H° is the standard adsorption enthalpy (J mol-1), R is the gas constant (8.314 J K-1 mol-1), β is the heating rate (K s-1), θm is the coverage at the peak maximum, and P° is the pressure of the standard conditions (1.013 × 105 Pa). From the experimental results shown in Figure 2, ∆S° is calculated to be 148 J K-1 mol-1. It should consist of entropy changes due to the phase transformation [∆Strans] and the mixing [∆Smix(T)]. The latter term is calculated to be about 60 J K-1 mol-1 from the gaseous composition at the peak maximum,18 and the former is hence estimated to be ca. 90 J K-1 mol-1. Trouton’s rule states that the entropy change with respect to the liquid vaporization is approximately constant (in many cases 80-110 J K-1 mol-1, and 87.3 J K-1 mol-1 for toluene) for various materials, showing that the entropy change is mainly determined by the free volume of a gas molecule.44 The entropy change for desorption can also be determined by the free volume of the gas molecule. Therefore, the entropy change due to the phase transformation estimated by the present study (ca. 90 J K-1 mol-1), which agrees with the vaporization entropy change shown by Trouton’s rule, supports the validity of the present
Figure 3. Toluene TPD curves simulated based on the assumption of 120 (A), 122 (B), and 124 kJ mol-1 (C) of the heat of toluene adsorption. The experimental curve on Na-MFI (24) is drawn as a thick line.
analysis. A similar entropy change was observed for the adsorption of ammonia on various solid acids.18,20,43 From these findings, we can assume eq 2, and a constant ∆Strans ) ca. 90 J K-1 mol-1 for analysis of toluene TPD on various zeolites. TPD curves were simulated with varying ∆H° as shown in Figure 3, and for this example, ∆H° is estimated to be 122 kJ mol-1, because this value gives the curve fitted best to the experimental curve
Cg ) -
βA0W dθ θ P° -∆H°/RT ∆S°/R ) e e F dT 1 - θ RT
(2)
where Cg is the concentration of toluene in the gas phase (mol m-3), T is the temperature (K), and θ is the coverage on the adsorption site; see the Supporting Information for the derivation of this equation. In Figure 3, the curves simulated based on the assumptions with 2 kJ mol-1 larger or smaller than ∆H° are also drawn. It is apparent that the simulated curves do not agree with the experimental curve. Therefore, the accuracy for the estimation of ∆H° should be within Na-MFI (122) > Na-BEA (120) > Na-FAU (95). This sequence is the same as that of ammonia adsorption heat on the corresponding H-form zeolite. From Figure 8, one can find a linear relationship between the adsorption heat of toluene on Na+ and that of ammonia on H+ in the corresponding zeolite.20,18,45-48 This linear relationship simultaneously indicates the following principles. (1) The heat of toluene adsorption on Na+ reflects the Lewis acid strength of the Na+ cation on zeolite, whereas the heat of ammonia adsorption on H+ shows the Brønsted acid strength. Both of them show intrinsic acidity but are not significantly affected by the steric effect; this is speculated from the facts that both molecules are so different in size but show same sequence in the adsorption heat. (2) The adsorption heat of toluene on Na+ is controlled by the strength of the acid-base interaction between Na+ and zeolite. Stronger adsorption of toluene is induced by stronger Lewis acidity of Na+, which results from the stronger electron withdrawing nature of the ion exchange site. (3) The same electron withdrawing nature causes the Brønsted acidity of H+. (4) In such a small pore as 8MR in MOR, exceptionally the steric hindrance suppresses the strong adsorption of toluene. We have clarified that the ammonia adsorption heat on H-form zeolite was controlled mainly by the crystal structure.20,47 This means that bond angle and distance control the electronic nature of the ion exchange site. It is proposed that the adsorption strength of the aromatic hydrocarbon is also controlled by the bond angle and distance in the zeolite framework. From the present results, Na-MFI (24) is predicted to have the highest ability as a hydrocarbon adsorbent, because both the adsorption amount and strength were relatively high among the tested Na-form zeolites. We have found that addition (physical mixing) of Na-MFI (24) as a hydrocarbon storage agent to a supported Pd catalyst significantly enhanced the catalytic activity for reduction of NO by toluene at a relatively low temperature; the effect of addition of Na-MFI (24) was superior to that of other zeolites.39,40 In the course of the mixing of Na-MFI (24) and Pd/H3W12PO40/SiO2, a portion of the Pd migrated into Na-MFI. As a result, partial oxidation of adsorbed toluene took place over Pd/Na-MFI to give such an efficient
The apparent desorption rate of toluene on zeolites was controlled by equilibrium in the conditions utilized for TPD. By applying the theoretical equation, the heat of toluene adsorption on zeolites can be calculated. The adsorption heat was higher on Na+ than on H+. Almost 1:1 adsorption of the toluene molecule was observed on the Na+ cation. The heat of toluene adsorption on Na+ was in the order MOR > MFI > BEA > FAU. This order was the same as that of the ammonia adsorption heat on H+; a linear relationship was obtained between both adsorption heats. The adsorption of toluene on Na+ is controlled by acid-base interaction between Na+ and the ion exchange site. The electron-withdrawing nature of the ion exchange site is considered to be determined by the bond angle and the distance of the zeolite framework, as well as the Brønsted acid strength of the H-form zeolite. The high adsorption capacity and adsorption heat on Na-MFI explain well the high performance as a hydrocarbon storage agent when added to an NO reduction catalyst. Acknowledgment. The present work is partly supported by Grant-in-Aid for Scientific Research (KAKENHI) (A) 18206082, (C) 17560681, in Priority Area “Molecular Nano Dynamics”, and in Young Scientists (B) (18760584) from the Ministry of Education, Culture, Sports, Science and Technology. Supporting Information Available: Experimental derivations. This material is available free of charge via the Internet at http://pubs.acs.org. References and Notes (1) Kno¨zinger, H.; Ratnasamy, P. Catal. ReV.-Sci. Eng. 1978, 17, 31. (2) Wakabayashi, F.; Kondo, J.; Wada, A.; Domen, K.; Hirose, C. J. Phys. Chem. 1993, 97, 10761. (3) Geobaldo, F.; Lamberti, C.; Ricchiardi, G.; Bordiga, S.; Zecchina, A.; Palomino, G. T.; Area´n, C. O. J. Phys. Chem. 1995, 99, 11167. (4) Corma, A. Catal. ReV. 1995, 95, 559. (5) Szanyi, J.; Paffett, M. T. Microporous Mater. 1996, 7, 201. (6) Kustov, L. M. Top. Catal. 1997, 4, 131. (7) Zecchina, A.; Lamberti, C.; Bordiga, S. Catal. Today 1998, 41, 169. (8) Matsuhashi, H.; Tanaka, T.; Arata, K. J. Phys. Chem. B 2001, 105, 9669. (9) Sivasankar, N.; Vasudevan, S. J. Phys. Chem. B 2004, 108, 11585. (10) Gribov, E. N.; Cocina, D.; Spoto, G.; Bordiga, S.; Ricchiardi, G.; Zecchina, A. Phys. Chem. Chem. Phys. 2006, 8, 1186. (11) Choudhary, V. R.; Srinivasan, K. R.; Singh, A. P. Zeolites 1990, 10, 16. (12) Otremba, M.; Zajdel, W. React. Kinet. Catal. Lett. 1993, 51, 481. (13) Su, B.-L.; Norberg, V. Langmuir 2000, 16, 6020. (14) Su, B.-L.; Norberg, V.; Martens, J. A. Langmuir 2001, 17, 1267. (15) Niwa, M.; Iwamoto, M.; Segawa, K. Bull. Chem. Soc. Jpn. 1986, 59, 3735. (16) Cvetanovic´, R. J.; Amenomiya, Y. AdV. Catal. 1967, 17, 103. (17) Igi, H.; Katada, N.; Niwa, M. In Proceedings of the 12th International Zeolite Conference; Treacy, M. M. J., Marcus, B. K., Bisher, M. E., Higgins, J. B., Eds.; Materials Research Society: Warrendale, 1999; Vol. 4, p 2643. (18) Niwa, M.; Katada, N.; Sawa, M.; Murakami, Y. J. Phys. Chem. 1995, 99, 8812. (19) Katada, N.; Igi, H.; Kim, J.-H.; Niwa, M. J. Phys. Chem. B 1997, 101, 5969. (20) Katada, N.; Niwa, M. Catal. SurVeys Asia 2004, 8, 161.
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