8812
J. Phys. Chem. 1995,99, 8812-8816
Temperature-Programmed Desorption of Ammonia with Readsorption Based on the Derived Theoretical Equation Miki Niwa,*$tNaonobu Katada,? Masahiko Saws: and Yuichi Murakami$ Department of Materials Science, Faculty of Engineering, Tottori University, Koyama-cho, Tottori 680, Japan, and Department of Applied Chemistry, School of Engineering, Furo-cho, Chikusa-ku, Nagoya 464-01, Japan Received: January 3, 1995; In Final Form: March IO, 1995@
Temperature-programmed desorption (TPD) spectra were analyzed on the basis of the theoretical equation of TPD of ammonia with freely occumng readsorption, and two kinds of methods to determine the strength of zeolite acidity were proposed. The change of enthalpy upon desorption (AH) was determined from the derived equation for the relationship between the temperature at the peak maximum and a parameter including the acid amount and the contact time of the carrier gas. The theoretical equation was confirmed experimentally not only from the precise determination of AH but also from an agreement between the values thus measured and those from microcalorimetry. Because the change of entropy upon desorption of ammonia (AS) was almost independent of the zeolite species, the constancy of AS was assumed; thus, AH was measured by a one-point experiment. Examples of determination of AH using this method were described. The proposed constant value of 150 J K-' mol-' of AS was supported by thermodynamic considerations, Trouton's rule and entropy of ammonia. Because the simulated spectra based on the theoretical equation were similar to those obtained experimentally, a narrow distribution of zeolite acidity was expected.
Introduction Knowledge of the acidity of zeolites is important for understanding their activity and selectivity of catalytic reactions. Recently, the number and strength of zeolite acid sites has been measured by various methods, and their relationships with the structure of the zeolite and the environment of tetrahedral aluminum sites in the zeolite matrix have been clarified.'-9 The concept of the zeolite acidity has obviously progressed on the basis of these investigations. Temperature-programmed desorption (TPD) of ammonia is an easy and reproducible method to determine the number of acid sites as well as their strength, so that it is used frequently. However, the method is often incompletely supported by the theory and experiments to clearly determine the parameter of the acidity.I0 Unfortunately, the TPD experiment contains complex behaviors relating to the readsorption and diffusion in the zeolite framework, and it renders full understanding of the TPD spectra difficult. Previous studies on the measurements of acidity by TPD of ammonia could be divided into two types based on the measurement methods. One of the methods was based on the absence of the readsorption of ammonia, where a high degree of vacuum (e.g., less than Torr) was attained, and a mass spectrometer was used to detect the desorbed ammonia.",'2 However, the important criterion of the measurement method, Le., no readsorption of desorbed ammonia, has not been experimentally confirmed yet. Another method is based on freely readsorpted ammonia, where such a carrier gas as He is used, and the desorbed ammonia is detected by a thermal conductivity detector (usual TCD for gas chromat~graphy)~.'~ or HC1 titration detector.2 Niwa et al., as an activity of the Reference Catalyst of Japan, assigned the desorption peak and showed the dependence of the peak maximum temperature upon the experimental condition^.'^ They showed that one of the desorption peaks at a lower temperature could be assigned to
* To whom all correspondence should be addressed. +
@
Tottori University. School of Engineering. Abstract published in Advance ACS Abstracts, May 1, 1995.
0022-365419512099-8812$09.00/0
physically adsorbed or weakly held ammonia, and the higher temperature peak was due to ammonia molecules desorbed from acid sites. Furthermore, it was reported that the TPD of ammonia was controlled by readsorption in the usual experimental conditions of to g min cm-3 of W E . Under the latter conditions, the readsorption of ammonia made the TPD measurements complex, and the strength of acidity could not be measured from the peak maximum temperature. However, Sawa et al. derived a theoretical equation under the readsorption condition from which the strength of the acidity could be determined.14 Subsequently, a short comment on the practical method, Le., the one-point method based on the assumption of the constancy of the entropy change, was reported by the authors.I5 In this paper, further comments on these methods will be made on the basis of the theoretical equation. A comment on the distribution of the acidity will be made on the basis of the simulation of TPD spectra.
Experimental Section
TPD Measurement. TPD was measured using all-glass equipment with vacuum pumps. The cell for TPD which was designed by Cvetanovic and Amenomiya'O was used in the present investigation, and the inside diameter of the cell was 10-12 mm. The zeolite was first evacuated for 1 h at 773 K, and about 100 Torr of ammonia was adsorbed at 373 K. After excess ammonia was pumped off, He carrier gas was allowed to flow into the cell kept at 30-100 Torr. After the baseline was stabilized, the temperature of the bed was increased linearly at a rate of 10 K min-' from 373 K until all the ammonia was desorbed. The ammonia which desorbed was detected by a thermal conductivity detector, and the intensity of the ammonia was calibrated with ammonia gas in a sample loop. All the data were transferred into a personal computer through an AD convertor, saved, and processed. Two desorption peaks were observed, but the lower temperature one at ca. 423 K was ascribable to physically adsorbed ammonia, and the number and strength of the acid sites were determined from the higher temperature peak.
0 1995 American Chemical Society
TPD of Ammonia with Readsorption
J. Phys. Chem., Vol. 99, No. 21, 1995 8813
TABLE 1: AH and AS of Measured Zeolites zeolite"
AH (kJ mol-')
JRC-Z-HMIO JRC-Z-HMI5 JRC-2-HMZO JRC-25-25H JRC-25-70H HF15
150 160 130 132 143
AS (J K-l mol-')
extemal surf areab (m2 E-')
composition
9.1 13.5 14.9 5.4 12.2 14.0
a JRC-2, reference catalyst supplied by Catalysis Society of Japan; HM, mordenite, 25, ZSM-5, HF, ferrierite. Measured by benzene-filled pore method.
Zeolite. All the zeolites were the H-form Reference Catalyst, supplied from the Catalysis Society of Japan, except for H-ferrierite which was provided from Prof. Tsutsumi of Toyohashi University of Technology. The composition and extemal surface area of the zeolites are shown in Table 1. Results and Discussion
c
-EP
1
I
I
o,02i
J
.
Theoretical Equation of the TPD of Ammonia with Freely Occurring Readsorption. As shown previ~usly,'~ the theoretical equation for the TPD of ammonia with freely occurring readsorption could be derived on the basis of the equilibrium between ammonia in the gas phase and ammonia adsorbed "3)
* NH3 + ( 1
(1)
where (NH3) and ( ) are the adsorbed ammonia and the unoccupied acid site in the zeolite. The assumption is valid when ammonia once desorbed could be easily readsorbed, and then we could write the following equation.
g
where Kp is an equilibrium constant at a temperature T between gaseous ammonia of pressure P, (Pa), adsorbed ammonia by which the extent of coverage is 8, and the unoccupied acid site; R and are the gas constant and a pressure at standard conditions [1.013 x lo5 Pa], respectively. Pg can be changed into the concentration of ammonia in the gas phase C, (mol m-3) on the basis of the ideal gas assumption. Because of the material balance of ammonia to enter into and to exit from the zeolite bed, we can write dB FC, = -AoW at
(3)
where the carrier gas flows at a rate of F (m3 s-I) through the zeolite bed of weight W (kg) and acid concentration A0 (mol kg-I). From eqs 2-3, we obtain
(4) where K~ = exp(-
exp(y)
because Kp can be replaced by the enthalpy change of desorption AH (J mol-') and the entropy change of desorption AS (J K-' mol-'). Because the temperature increases in a rate of p (K S-1)
dT=Pdt
K
Then, we obtain the theoretical equation of the TPD spectrum
1 - e p g - 1 - 0 RTC
KP = e -- o -
Temperature i
Figure 1. Simulated spectra of TPD of ammonia on zeolites with assumed constant parameters (W, 1 x m3 s-I; p , kg; F , 1 x 100 Torr; AH, 140 kJ mol-'; AS, 150 J K-' mol-]) and variable parameters of A0 of 0.2 (a), 0.4 (b), 0.6 (c), 0.8 (d), 1.0 (e), and 1.2 (f) mol kg-I.
(6)
Equation 7 shows the change of C, by elevating the temperature of the zeolite bed linearly with time, Le., TPD spectrum. We can calculate C,(i)on the basis of eq 7 and the C,(i+l) from
Plots of C, against T give the simulated spectrum. Figures 1 and 2 show the simulated spectra based on the assumed appropriate values of the parameters, and these were obtained by changing A0 and AH only, respectively, with all other parameters unaltered. Figure 1 shows the shift of temperature at the peak maximum T, (K) to higher temperature with increasing Ao; the dependence of the peak maximum temperature upon the concentration of the acid sites is in agreement with that found previ~usly.'~ Figure 2 shows the change of spectra with varying AH, Because a set of parameters, AS and AH, is assumed, the spectra correspond to those of the acid sites without any distribution of the acid strength. The TPD spectrum on H-mordenite HM20 (Figure 3) can be compared to the simulated spectra, and the similarity is found obviously between them. The similarity between them showed that a broad distribution of the zeolite acidity strength could be disregarded. Under these conditions of the TPD experiment, the spectra do not show a distribution of acidity, although these appear in the broad range of temperature. This is an important finding obtained from the simulation of the TPD spectrum. Distribution of the zeolite acidity, however, will be required to further precisely fit the observed spectra to the simulated one.
Niwa et al.
8814 J. Phys. Chem., Vol. 99, No. 21, 1995 I
-19.6
1
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E
F
-19.7
7x u -20.5 25
~n h E
X
y-
0 "
L -20.6
v
-20.8
Q
Tcmpcrature / K
Figure 2. Simulated spectra of TPD of ammonia on zeolites with assumed constant parameters (W, 1 x m3 s-I; p , kg; F , 1 x 100 Torr; Ao, 0.6 mol kg-'; AS, 150 J K-I mol-I) and variable parameters of A H of 120 (a), 130 (b), 140 (c), 150 (d), and 160 (e) kJ mol-].
T m (K)
1:
Figure 4. Plot of the second term of the right-hand side of the eq 10 against T,, where A S = 150 J K-I mol-'.
E. l 2 0.02
$
4
i C t-
S
0
-
1
q
~
~
f+
,
5
-
4.00
c
J
a
4.00 i I
E
G 3.00 i
-
3.50
/
600 800 Temperature / K
400
1/Tm
Figure 3. TPD spectrum obtained on HM20 with the determined parameters: A H , 145 kJ mol-'; Ao, 1.28 mol kg-I under the m3 s-I; p , 95 experimental conditions; W , 9.8 x kg; F, 1 x Torr.
One-Point Method To Determine the Strength of Zeolite Acidity from the TPD. As shown above, the peak maximum temperature is not simply related to the strength of the zeolite acidity, because T, depends on A0 as well as on AH. Because, at the peak maximum temperature (9)
dCddT = 0
2.20
(KK-')
Figure 5. Plot of the left-hand side of the eq 10 against UT,,,: Ao, 1.17 (HM15), 1.07 (HM20), 0.87 (25H), 0.24 (70H), and 1.16 mol kg- (HF15).
--
r
160-
I
8
a
k
E
E
i?
e
a
we obtain, from eq 8
A,W
( )
AH
+
In T, - In - = - In RTm
[
- e,)2(m - RT,) Poexp(AS/R)
1
A H by calorimetry (kT mol-')
(10)
where 8, is the coverage by ammonia at the peak maximum. This equation shows the dependence of T, on the parameters of the TPD. The parameter 8 , can be determined from the undesorbed portion of the high-temperature peak at the peak maximum. The second term of the right-hand side could be regarded as invariant for the tested zeolite, because no variable parameters were included. Plots of the left-hand side of the eq 10 against UT, therefore gives a straight line with a slope of AHIR from which AH can be measured experimentally. Constancy of the second term of the right-hand side of the eq 10 and the linear plot between the left-hand side of eq 10 and UT, are shown in Figures 4 and 5, respectively. These linear plots are strong evidence of the validity of the present theoretical analysis of the TPD of ammonia on zeolites. When AH is measured, AS can be calculated numerically on the basis of eq
Figure 6. Comparison between AH measured by TPD and microcalorimeter. Several-points method (0)and one-point method (A) of TPD were used.
10. Table 1 shows AH and AS thus determined on various zeolites, and the measured values of AH approximately agreed with those obtained by Tsutsumi et al. from microcalorimetry," as shown in Figure 6. Because the value of AH measured by microcalorimetry depends on the degree of coverage by ammonia, the value plotted here is an averaged value of more than 85 kJ mol-' of AH. Validity of the theoretical equation and the treatment for TPD, mentioned in this investigation, was thereby supported by the calorimetric measurements, another important measurement method of the zeolite acidity. The coincidence of our measured AH with those obtained by the microcalorimetry is direct evidence of the validity of the method. The nearly 1:1 relationship between the number
TPD of Ammonia with Readsorption
J. Phys. Chem., Vol. 99, No. 21, 1995 8815
TABLE 2: AH Measured by One-Point Method Using Different Amounts of Zeolite zeolite JRC-Z-HM10 JRC-Z-HM15
JRC-Z-HM20
AH W (8) (kJ mol-') 0.202 0.302 0.402 0.110 0.141 0.201 0.202 0.300 0.402 0.077 0.141 0.205 0.301 0.400
147 146 148 152 153 153 152 152 152 153 154 154 154 153
zeolite JRC-Z5-25H
JRC-Z5-70H
HF15
AH W (g) (kJ mol-') 0.052 0.101 0.500 1.00 0.202 0.412 0.637 1.00 0.061 0.100 0.149 0.200 0.302
131 132 131 131 133 133 133 133 146 147 147 147 147
of acid sites and the number of A1 atoms in the zeolite framework which has been previously measured by the authors using the TPD of ammonia' also adds support to the measurements of the zeolite acid sites based on the present theory. The change of entropy is almost constant irrespective of the measured zeolite, as shown in Table 1. Provided that AS is constant independent of the kind of zeolite, we can calculate AH on the basis of eq 10 from only a one-point experiment, because we know every parameter except AH. Equation 10 cannot be solved analytically, so it is solved numerically on a personal computer. Table 2 shows AH thus measured by the one-point experiment of TPD, where we assume 150 J K-' mol-' of AS. The weight of the zeolite was changed in the broad range from 0.05 to 1.0 g, and values of AH were obtained precisely for six kinds of zeolite species. A small experimental error is noteworthy, because only 2 kJ mol-' of the error was included. These values are therefore independent of the experimental conditions used in the present study. We propose this method to determine the zeolite acidity strength as a practical method. The constancy of the entropy change is in agreement with Trouton's rule for the liquid vaporization. Trouton's rule states that AS in the liquid vaporization is approximately constant, although liquids are vaporized with different AH at different boiling points. Constant AS can be understood from the theoretical consideration, because AS is associated with the change of entropy of translation due to the change of the free volume of liquid and gas molecules;'* the entropy of the gas and liquid molecule due to the rotational and vibrational energy could be independent of the gas or liquid phase. In other words, AS upon desorption is related to AS due to the translational energy of ammonia molecules in the gas, and the adsorbed phases independent of what the adsorbed state of ammonia is. In addition, an increase in the entropy upon the mixing of ammonia with helium carrier gas AS,,, is also taken into account. Thus,
AS = S(g) - S(ad)
+ ASmix
AS,ix can be written
where x shows the mole fraction of helium and ammonia. Because AS,ix is 40-70 J K-' mol-' in the range of 30-100 Torr of pressure in the cell and 0.001-0.015 mol m-3 of concentration of ammonia CNH~, and AS is 150 J K-' mol-', the difference between S(g) and S(ad) is estimated to be 80110 J K-' mol-'. Because the entropy change upon the
vaporization of ammonia is 98 J K-' mol-', the estimated difference of the entropy is approximately in agreement with the Trouton's constant. The assumed value of the constant entropy change of 150 J K-' mol-' is therefore rationalized by the thermodynamics of the ammonia. However, it is important to note that the change of entropy, 150 J K-' mol-', depends upon the experimental conditions, in particular the flow rate of the carrier gas, because the Asmixis not a constant value. Demmin and Gorte analyzed the TPD on porous materials and proposed some dimensionless parameters to evaluate the experimental condition^.'^ The time lag due to too slow a flow rate or too large a crystal size of the zeolite can be disregarded easily. Two important parameters which determined the readsorption conditions, however, should be evaluated. Under such conditions as too fast a flow rate of the carrier gas or too large size of crystal, the TPD spectrum is influenced by the diffusion of the adsorbate inside the porous materials, and the readsorption of adsorbate occurs significantly. However, the conditions of the present experiment are not included in such readsorption conditions. Usually, the size of the zeolite is enough small. As has been experimentally confirmed, I 3 we identified the strong dependence of the peak maximum temperature upon the W/F. The readsorption of ammonia is governed by the ratio of the adsorption rate to the flow rate of the carrier gas, and the TPD of ammonia is usually included in this condition. On the contrary, the ideal condition of the readsorption, Le., no gradient of the gas concentration along the bed, due to the back-mixing of the carrier gas is not accomplished usually, as they menti~ned.'~ The readsorption condition is, therefore, not fully satisfied with the theory, but it seems that the experimental confirmation of the readsorption is enough to prove the present theory of TPD, as shown above. Therefore, one has to check the experimental conditions of TPD in one's own experiment, when they are far from the present conditions, Le., from to lo-* g min cm-3 of the WE. Finally, a comment on the TPD study by Forni et al. will be made, because they disregarded the readsorption of ammonia and concluded the diffusion of ammonia in the zeolite framework is the most prevailing phenomenon affecting the TPD spectra.20-21From their experimental conditions, it was estimated that the value of W/F ((1-3) x g cm-3 min) was larger, but the peak maximum temperature on HZSM-5 (423 K) was much lower than ours. Therefore, their obtained TPD spectrum was assigned to those of physically-adsorbedammonia desorbed as a lower temperature peak. The lower temperature peak may be controlled by the diffusion of ammonia; however, the problem is another one not relating to the acidity of zeolites. Very recently, Jozefowics et al. reported microcalorimetric measurements of acid sites in HZSM-5 using an ultrahighvacuum system: they reported a homogeneous Brprnsted acid strength distribution, but they indicated the intrinsic difference between the strengths of Brprnsted and Lewis acid sites.22 There is also another paper which claimed the very homogeneous distribution of acid sites in HZSM-5.8 Recent progresses in measurements of calorimetry thus add valuable information about the distribution of the strength of the acidity in zeolites. Our measurements using the TPD of ammonia could not reveal any difference between the acidity strengths of Lewis and Brprnsted acid sites. Information about the distribution of acid sites using the TPD of ammonia could be done using the precise analysis of spectra based on the theoretical consideration.
Acknowledgment. This work was supported by a Grantin-Aid for Developmental Scientific Research from the Ministry of Education, Science and Culture, Japan (No. 06555243).
8816 J. Phys. Chem., Vol. 99, No. 21, 1995
References and Notes (1) Sawa, M.; Niwa, M.; Murakami, Y. Zeolites 1990, 10, 532. (2) Miller, J. T.; Hopkins, P. 9.;Meyers, B. L.; Ray, G. J.; Roginski, R. T.; Zajac, G. W.; Rosenbaum, N. H. J . Catal. 1992, 138, 115. (3) van Niekerk, M. J.; Flectcher, J. C. Q.; O’Connor, C. T. J . Catal. 1992, 138, 150. (4) Stach, H.; Janchen, J.; Jerschkewitz, H.-G.; Lohse, U.; Parlitz, B.; Zibrowius, B.; Hunger, M. J . Phys. Chem. 1992, 96, 8473. (5) Stach, H.; Janchen, J.; Jerschkewitz, H.-G.; Lohse, U.; Parlitz, B.; Zibrowius, B.; Hunger, M. J . Phys. Chem. 1992, 96, 8480. (6) Crocker, M.; Herold, R. H. M.; Sonnemans, M. H. W.; Emeis, C. A.; Wilson, A. E.; van der Moolen, J. N. J . Phys. Chem. 1993, 97, 432. (7) Barthomeuf, D. J . Phys. Chem. 1993, 97, 10092. (8) Parrillo, D. J.; Lee, C.; Gorte, R. J. Appl. Catal., A: Gen. 1994, 110, 67. (9) Haag, W. 0. In Zeolites and Related Microporous Materials: State of the Art 1994; Weitkamp, J., et al., Eds.; Studies in Surface Science and Catalysis 84; Elsevier: Amsterdam, 1994, p 1375.
Niwa et al. (10) Cvetanovic, R. J.; Amenomiya, Y. Adv. Cutul. 1967, 17, 103. (11) Karge, H.; Dondur, V. J . Phys. Chem. 1990, 94, 765. (12) Kim, J. H.; Namba, S.; Yashima, T. Appl. Catal. A: Gen. 1993, 100, 27. (13) Niwa, M.; Iwamoto, M.; Segawa, K. Bull. Chem. SOC. Jpn. 1986, 59, 3735. (14) Sawa, M.; Niwa, M.; Murakami, Y. Zeolites 1990, 10, 307. (15) Sawa, M.; Niwa, M.; Murakami, Y. Zeolites 1991, 11, 94. (16) Reference deleted in revision. (17) Tsutsumi, K.; Nishiyama, K. Thermochim. Acta 1989, 143, 299. (18) Barrow, G. M. Physical Chemistry, 5th ed.; McGraw-Hill: New York, 1988. (19) Demmin, R. A,; Gorte, R. J. J . Catal. 1984, 90, 32. (20) Fomi, L.; Magni, E. J . Catal. 1988, 112, 437. (21) Fomi, L.; Vatti, F. P.; Ortovela, E. Zeolites 1992, 12, 101. (22) Jozefowics, L. C.; Karge, H. G.; Coker, E. N. J . Phys. Chem. 1994, 98, 8053. JP9500044
