- Email: [email protected]
Relationship between acid-strength and framework aluminum content in dealuminated mordenites
Relationship between acid-strength and framework aluminum content in dealuminated mordenites Helmut Stach and Jochen J~inchen
Central Institute of Physical Chemistry, Berlin, Germany Calorimetric measurements of ammonia at 423 K on dealuminated mordenites with Si/AI ratios between 7 and 48 show a distinct dependence of the number of very strong acid sites on the content of the framework aluminum. Plots of these numbers against the molar fraction of aluminum yield a volcano-shaped curve with an abscissa of 0.096, which corresponds to a calculated value of Barthomeuf, thus verifying her theoretical concept. Calorimetric measurements of Klyachko et al. are in good coincidence with the given data. Keywords: Dealumination; calorimetric measurements; Br6nsted acidity; topological AI density
INTRODUCTION The concept of the acidity in zeolites is still not fully understood. To describe the acidic behavior, it is necessary to determine the nature, the strength, and the number of acid sites. Many methods have been used to get this information (see the excellent reviews in Refs. 1-4), but only the calorimetric measurement of the chemisorption heats of basic molecules at elevated temperatures is able to yield quantitative results concerning the number and the strength of the acid sites. Other methods (e.g., i.r. or n.m.r. spectroscopy) are necessary to reveal the nature of these sites. To study the acidity of often catalytically used zeolites, we measured the chemisorption of ammonia on dealuminated HY- and H-mordenites and HZSM5 zeolites with a large range of different Si/AI ratios. Here, we present the results for the H-mordenites; the other zeolite types will be found elsewhere. 5
EXPERIMENTAL The aluminum-deficient mordenites were prepared by refluxing the Na-mordenite in solutions of hydrochloric acid. 6 T h e Al content of the framework was calculated from the chemically determined bulk Si/Al ratio, in combination with 27A1 MAS n.m.r, spectra, and the Al site distribution was derived from 29Si MAS n.m.r, measurements. 6 Differential heats of adsorption of ammonia were measured at 423 and 473 K by using a Calvet-type microcalorimeter connected to the adsorption unit. All samples were Address reprint requests to Prof. Stach at the Central Institute of Physical Chemistry, Rudower Chaussee 5, 0-1199 Berlin, Germany. Received 8 May 1991; accepted 22 July 1991 (~) 1992 Butterworth-Heinemann 152
ZEOLITES, 1992, Vol 12, February
evacuated for 24 h at 653 K. The total acid amounts of the H-mordenites were additionally received from temperature-programmed desorption of ammonia and from ammonium-exchange-capacity measurements. All methods used are described in more detail in Ref. 7. RESULTS AND DISCUSSION T h e number of the acidic sites were derived from the curves of the chemisorption heats. It has been shown that the heats of ammonia adsorption Q on the acid sites exceed the value of 80 kJ/mol s'9 and that every ammonia molecule interacts with only one of these sites. So, the total number of acid sites is found as the amounts of ammonia adsorbed at Q > 80 kJ/mol. From the heat curves showing a nearly stepwise decrease 6 of the heat of chemisorption with increasing amount adsorbed, it may be concluded that the adsorption sites are heterogeneous. Therefore, four groups of sites with different strengths of chemisorption and, hence, acidity were chosen: N2 > 80; 80 < N~ < 100; 100 < N~ < 120; N2 > 120 kJ/mol. The corresponding concentration of sites with different acid strength are given in Figure 1. They are plotted against the number of aluminum atoms in the elementary cell. Curve 1 reflects the dependence of the total number of acid sites on the framework aluminum atoms. It is seen that the density of the acid sites raises linearly with an increasing number of aluminum atoms. T h e results of the t.p.d, and ammoniumexchange experiments confirm the linear dependence mentioned. For the sake of clarity, they were omitted in Figure I, but will be presented in Ref. 6. This dependence is not unexpected and was found as well by other authors.~°ArIn contrast to this
Acid strength and framework AI in dealuminated mordenites: H. Stach and J. Janchen
behavior, the curves of the density of the stronger acid sites show a distinct maximum. The abscissa of the maximum corresponds to an A1 content of 4.6/u.c. and a Si/A1 ratio of 9.5. This follows from curves 3 and 4 in Figure1. Included also are results of Klyachko et a1.12 received from calorimetric measurements of the ammonia chemisorption on dealuminated H-mordenites. Although the calorimetric experiments were performed at a higher temperature of measurement (compared with our experimental conditions), and the dealuminated mordenites were from different origin, the coincidence of our data (curve 2) with those of Klyachko et al. ]~ (filled circles) is fair. Furthermore, the data from the literature assure the existence of a maximum with the abscissa mentioned. In the catalytic o-xylene conversion on dealuminated H-mordenites, maxima of the rates of isomerization and disproportionation were found for a mordenite with a Si/AI ratio of about 9.4,1° comparable to the Si/AI ratio of the maximum of the strong acidic sites. Comparing the curves of the concentration of sites differing in acid strength of H-mordenites (given in Figure 1) with those found in dealuminated HY zeolites,s'~ ~ it is seen that in the latter case a maximum is also found with an abscissa at a different Si/A1 ratio. We have to conclude, therefore, that the structure and not the AI content of the framework determines the acid strengths of the zeolites investigated. To investigate the influence of the zeolite structure on the acidity, Barthomeuf 13 considered and calculated the topological aluminum density TDAI for different zeolites. She started from the topological
H-Mordenite /
,," / /
,,,' /
0
/
/ /
//11I
E E 0
z
////
0
2
4
6
8
A[ / u.c. Figure 1 Distribution of the acid strengths in dependence on the aluminum content of the framework in dealuminated H-mordenites [number of the acid sites Na with Q > 80 (A); 80 < N~ < 100 (~7); 100 < N~ < 120 if-I) and N~ > 120 kJ/tool (O); (0) data from Ref. 13
0.8
~O E E O
"2
0.6
0.4
Z
0.2
I
0
I
0.1
i
0.2
ALIAI+Si Figure 2 Concentration of the very strong acid sites in dependence on the aluminum molar fraction in dealuminated H-mordenites. The vertical dashed line corresponds to the m-value of the synthesized mordenite, not being dealuminated; our results (O); data from Ref. 13 (O)
density of the T atoms (TD2_s) and pointed out that a limit value of TDAI (lim TDA0 exists, which should be the same for any zeolite structure. Because of the different TD2_5 values of the different zeolites, the corresponding limit value of the aluminum molar fraction m = A1/Si + A1 (lim mNNN) is not the same, but depends on the density of the T atoms of the framework. Below the lim mNNN, no AI atom has an another AI atom as a next-nearest neighbor and therefore all acid sites show a strong acidity, i.e., a high acid strength. Above the lim mNNN, there are in the second coordination sphere A1 atoms, with the consequence that not all BrOnsted sites have a strong acidity (although the total number of acid sites is still rising with rising number of AI atoms in the unit cell; see Figure 1), and their concentration decreases with increasing AI content of the framework. Barthomeuf calculated the lim mNNN value for mordenite (and other zeolites) in advance and found that it corresponds to lim mNNN = 0.096, which is equal to a Si/AI ratio of 9.4. In Figure2 are plotted our results concerning the measured concentration of very strong acid sites (Q > 120 kJ/mol) against the aluminum molar fraction m of the dealuminated mordenites. Included are results given by Klyachko et al. 9 Their calorimetric measurements of ammonia chemis0rption were performed at 573 K with mordenites of different origin (including synthesized and dealuminated). The agreement between the results of the two sets of measurements is remarkable. The abscissa value of the maximum corresponds to mNNN = 0.096, which is also in good agreement with the predicted one o f m = 0.096.12 We conclude from the results of our calorimetric measurements that the concept of the aluminum topological density (Barthomeuf) is very advantageous for the study of the zeolitic acidity. A further
ZEOLITES, 1992, Vol 12, February
153
Acid strength and framework AI in dealuminated mordenites: H. Stach and J. J~nchen
still more important result is the experimental verification of the model of isolated a l u m i n u m atoms as the sites, o f the strong Br6nsted acidity. H,~5
REFERENCES 1 Jacobs, P.A. Carboniogenic Activity of Zeofites, Elsevier, Amsterdam, 1977 2 Rabo, J.A. and Gajda, G.J. CataL Rev. 1990, 31,385 3 Barthomeuf, D. Mater. Chem. Phys. 1987, 17, 49 4 Dwyer, J. and O'Malley, P.J. Keynotes in Energy-Related Catalysis, Studies in Surface Science and Catalysis, Vol. 37, Elsevier, Amsterdam, 1988, p. 5 5 St,ach, H., Lohse, U. and J~nchen, J. Zeofites, submitted 6 Stach, H., Zibrowius, B., J~nchen, J., Parlitz, B. and Jers-
154
ZEOLITES, 1992, Vol 12, February
chkewitz, H.-G. J. Phys. Chem., submitted 7 Lohse, U., Parlitz, B. and Patzelova, V. J. Phys. Chem. 1989, 93, 3677 8 Stach, H., Wendt, R., Lohse, U., J~nchen, J. and Spindler, H. Catal. Today 1988, 3, 431 9 Klyachko, A.L., Brueva, T.R., Mishin, I.V., Kapustin, E.I. and Rubinstein, A.M. Acta Phys. Chem. (Szeged) 1978, 24, 183 10 Sawa, M., Niwa, M., and Murakami, Y. Zeolites 1990, 10, 535 11 Mishin, I.V., Bremer, H. and Wendlandt, K.P. Catalysis on Zeolites (Eds. D. Kallo and Ch.M. Minachev) Budapest, 1988, p. 250 12 Klyachko, A.L. and Mishin, I.V. Neftechimija 1990, 30, 339 13 Barthomeuf, D. Catalysis 1987, Elsevier, Amsterdam, 1988, p. 177 14 Dempsey, E. J. Catal. 1974, 33, 497 15 Pine, L.A., Maher, P.J. and Wachter, W.A.J. Catal. 1984, 85, 476
Central Institute of Physical Chemistry, Berlin, Germany Calorimetric measurements of ammonia at 423 K on dealuminated mordenites with Si/AI ratios between 7 and 48 show a distinct dependence of the number of very strong acid sites on the content of the framework aluminum. Plots of these numbers against the molar fraction of aluminum yield a volcano-shaped curve with an abscissa of 0.096, which corresponds to a calculated value of Barthomeuf, thus verifying her theoretical concept. Calorimetric measurements of Klyachko et al. are in good coincidence with the given data. Keywords: Dealumination; calorimetric measurements; Br6nsted acidity; topological AI density
INTRODUCTION The concept of the acidity in zeolites is still not fully understood. To describe the acidic behavior, it is necessary to determine the nature, the strength, and the number of acid sites. Many methods have been used to get this information (see the excellent reviews in Refs. 1-4), but only the calorimetric measurement of the chemisorption heats of basic molecules at elevated temperatures is able to yield quantitative results concerning the number and the strength of the acid sites. Other methods (e.g., i.r. or n.m.r. spectroscopy) are necessary to reveal the nature of these sites. To study the acidity of often catalytically used zeolites, we measured the chemisorption of ammonia on dealuminated HY- and H-mordenites and HZSM5 zeolites with a large range of different Si/AI ratios. Here, we present the results for the H-mordenites; the other zeolite types will be found elsewhere. 5
EXPERIMENTAL The aluminum-deficient mordenites were prepared by refluxing the Na-mordenite in solutions of hydrochloric acid. 6 T h e Al content of the framework was calculated from the chemically determined bulk Si/Al ratio, in combination with 27A1 MAS n.m.r, spectra, and the Al site distribution was derived from 29Si MAS n.m.r, measurements. 6 Differential heats of adsorption of ammonia were measured at 423 and 473 K by using a Calvet-type microcalorimeter connected to the adsorption unit. All samples were Address reprint requests to Prof. Stach at the Central Institute of Physical Chemistry, Rudower Chaussee 5, 0-1199 Berlin, Germany. Received 8 May 1991; accepted 22 July 1991 (~) 1992 Butterworth-Heinemann 152
ZEOLITES, 1992, Vol 12, February
evacuated for 24 h at 653 K. The total acid amounts of the H-mordenites were additionally received from temperature-programmed desorption of ammonia and from ammonium-exchange-capacity measurements. All methods used are described in more detail in Ref. 7. RESULTS AND DISCUSSION T h e number of the acidic sites were derived from the curves of the chemisorption heats. It has been shown that the heats of ammonia adsorption Q on the acid sites exceed the value of 80 kJ/mol s'9 and that every ammonia molecule interacts with only one of these sites. So, the total number of acid sites is found as the amounts of ammonia adsorbed at Q > 80 kJ/mol. From the heat curves showing a nearly stepwise decrease 6 of the heat of chemisorption with increasing amount adsorbed, it may be concluded that the adsorption sites are heterogeneous. Therefore, four groups of sites with different strengths of chemisorption and, hence, acidity were chosen: N2 > 80; 80 < N~ < 100; 100 < N~ < 120; N2 > 120 kJ/mol. The corresponding concentration of sites with different acid strength are given in Figure 1. They are plotted against the number of aluminum atoms in the elementary cell. Curve 1 reflects the dependence of the total number of acid sites on the framework aluminum atoms. It is seen that the density of the acid sites raises linearly with an increasing number of aluminum atoms. T h e results of the t.p.d, and ammoniumexchange experiments confirm the linear dependence mentioned. For the sake of clarity, they were omitted in Figure I, but will be presented in Ref. 6. This dependence is not unexpected and was found as well by other authors.~°ArIn contrast to this
Acid strength and framework AI in dealuminated mordenites: H. Stach and J. Janchen
behavior, the curves of the density of the stronger acid sites show a distinct maximum. The abscissa of the maximum corresponds to an A1 content of 4.6/u.c. and a Si/A1 ratio of 9.5. This follows from curves 3 and 4 in Figure1. Included also are results of Klyachko et a1.12 received from calorimetric measurements of the ammonia chemisorption on dealuminated H-mordenites. Although the calorimetric experiments were performed at a higher temperature of measurement (compared with our experimental conditions), and the dealuminated mordenites were from different origin, the coincidence of our data (curve 2) with those of Klyachko et al. ]~ (filled circles) is fair. Furthermore, the data from the literature assure the existence of a maximum with the abscissa mentioned. In the catalytic o-xylene conversion on dealuminated H-mordenites, maxima of the rates of isomerization and disproportionation were found for a mordenite with a Si/AI ratio of about 9.4,1° comparable to the Si/AI ratio of the maximum of the strong acidic sites. Comparing the curves of the concentration of sites differing in acid strength of H-mordenites (given in Figure 1) with those found in dealuminated HY zeolites,s'~ ~ it is seen that in the latter case a maximum is also found with an abscissa at a different Si/A1 ratio. We have to conclude, therefore, that the structure and not the AI content of the framework determines the acid strengths of the zeolites investigated. To investigate the influence of the zeolite structure on the acidity, Barthomeuf 13 considered and calculated the topological aluminum density TDAI for different zeolites. She started from the topological
H-Mordenite /
,," / /
,,,' /
0
/
/ /
//11I
E E 0
z
////
0
2
4
6
8
A[ / u.c. Figure 1 Distribution of the acid strengths in dependence on the aluminum content of the framework in dealuminated H-mordenites [number of the acid sites Na with Q > 80 (A); 80 < N~ < 100 (~7); 100 < N~ < 120 if-I) and N~ > 120 kJ/tool (O); (0) data from Ref. 13
0.8
~O E E O
"2
0.6
0.4
Z
0.2
I
0
I
0.1
i
0.2
ALIAI+Si Figure 2 Concentration of the very strong acid sites in dependence on the aluminum molar fraction in dealuminated H-mordenites. The vertical dashed line corresponds to the m-value of the synthesized mordenite, not being dealuminated; our results (O); data from Ref. 13 (O)
density of the T atoms (TD2_s) and pointed out that a limit value of TDAI (lim TDA0 exists, which should be the same for any zeolite structure. Because of the different TD2_5 values of the different zeolites, the corresponding limit value of the aluminum molar fraction m = A1/Si + A1 (lim mNNN) is not the same, but depends on the density of the T atoms of the framework. Below the lim mNNN, no AI atom has an another AI atom as a next-nearest neighbor and therefore all acid sites show a strong acidity, i.e., a high acid strength. Above the lim mNNN, there are in the second coordination sphere A1 atoms, with the consequence that not all BrOnsted sites have a strong acidity (although the total number of acid sites is still rising with rising number of AI atoms in the unit cell; see Figure 1), and their concentration decreases with increasing AI content of the framework. Barthomeuf calculated the lim mNNN value for mordenite (and other zeolites) in advance and found that it corresponds to lim mNNN = 0.096, which is equal to a Si/AI ratio of 9.4. In Figure2 are plotted our results concerning the measured concentration of very strong acid sites (Q > 120 kJ/mol) against the aluminum molar fraction m of the dealuminated mordenites. Included are results given by Klyachko et al. 9 Their calorimetric measurements of ammonia chemis0rption were performed at 573 K with mordenites of different origin (including synthesized and dealuminated). The agreement between the results of the two sets of measurements is remarkable. The abscissa value of the maximum corresponds to mNNN = 0.096, which is also in good agreement with the predicted one o f m = 0.096.12 We conclude from the results of our calorimetric measurements that the concept of the aluminum topological density (Barthomeuf) is very advantageous for the study of the zeolitic acidity. A further
ZEOLITES, 1992, Vol 12, February
153
Acid strength and framework AI in dealuminated mordenites: H. Stach and J. J~nchen
still more important result is the experimental verification of the model of isolated a l u m i n u m atoms as the sites, o f the strong Br6nsted acidity. H,~5
REFERENCES 1 Jacobs, P.A. Carboniogenic Activity of Zeofites, Elsevier, Amsterdam, 1977 2 Rabo, J.A. and Gajda, G.J. CataL Rev. 1990, 31,385 3 Barthomeuf, D. Mater. Chem. Phys. 1987, 17, 49 4 Dwyer, J. and O'Malley, P.J. Keynotes in Energy-Related Catalysis, Studies in Surface Science and Catalysis, Vol. 37, Elsevier, Amsterdam, 1988, p. 5 5 St,ach, H., Lohse, U. and J~nchen, J. Zeofites, submitted 6 Stach, H., Zibrowius, B., J~nchen, J., Parlitz, B. and Jers-
154
ZEOLITES, 1992, Vol 12, February
chkewitz, H.-G. J. Phys. Chem., submitted 7 Lohse, U., Parlitz, B. and Patzelova, V. J. Phys. Chem. 1989, 93, 3677 8 Stach, H., Wendt, R., Lohse, U., J~nchen, J. and Spindler, H. Catal. Today 1988, 3, 431 9 Klyachko, A.L., Brueva, T.R., Mishin, I.V., Kapustin, E.I. and Rubinstein, A.M. Acta Phys. Chem. (Szeged) 1978, 24, 183 10 Sawa, M., Niwa, M., and Murakami, Y. Zeolites 1990, 10, 535 11 Mishin, I.V., Bremer, H. and Wendlandt, K.P. Catalysis on Zeolites (Eds. D. Kallo and Ch.M. Minachev) Budapest, 1988, p. 250 12 Klyachko, A.L. and Mishin, I.V. Neftechimija 1990, 30, 339 13 Barthomeuf, D. Catalysis 1987, Elsevier, Amsterdam, 1988, p. 177 14 Dempsey, E. J. Catal. 1974, 33, 497 15 Pine, L.A., Maher, P.J. and Wachter, W.A.J. Catal. 1984, 85, 476