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A combined EPR and NMR study of oxidation sites in dealuminated mordenites
H.G. Karge and J. Weitkamp (Eds.) Zeolite Science 1994: Recent Progress and Discussions Studies in Surface Science and Catalysis, Vol. 98 9 1995 Elsevier Science B.V. All rights reserved.
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A COMBINED EPR AND NMR STUDY OF OXIDATION SITES IN DEALUMINATED MORDENITES. G. Harvey Estermann, Laboratorium f0r Technische Chemie, ETH-Zentrum, CH-8092 Z0rich. R. Crockett and E. Roduner, Physikalisch-chemisches Institut der Universit&t ZOrich, CH-8057 ZUrich
Summary The radical cation of 2,5-dimethyl-2,4-hexadiene (1) has been observed after adsorption of 2,5-dimethyl-3-hexene (2), and 2,5-dimethyl-l,5-hexadiene (3) onto dealuminated mordenites. The intensity of the EPR signal correlated with the presence of extra-framework aluminium as observed by 27AI MASNMR. When the extra-framework aluminium was removed by mineral acid leaching or chelation by oxalic acid, only a very weak EPR signal was detected. The conversion of alkenes (2) and (3) to diene (1) was also shown by G.C. analysis to be dependant on the type of mordenite. The generation of radical cations in these mordenites is dependant on the presence of Lewis acid sites located on extra-framework aluminium, although cationic reactions due to Bronsted acid sites often remain the major reaction pathway. Introduction EPR has often been used as a technique to identify intermediates in zeolite catalysis 1, although it remains unclear, which species within the zeolite is responsible for the generation of radical cations 2. One possibility is that the extra-framework aluminium, frequently present in dealuminated zeolites, forms Lewis acid sites, which act as electron acceptors 3. Experimental HM/NH, dV03: Na-mordenite (MOR) (PM-1, CU Chemie Uetikon AG) was exchanged
twice with 1M NH4NO3 and calcined at 700~ in air for ca. 15 h. HM/HN03: Na-MOR was treated first with 1M HCI and then with 6N HNO3 under reflux for 3h. The zeolite was calcined at 700~ for 1 h. The HNO3 treatment was repeated and finally the zeolite heated to 700~ for ca. 15 h. HM/OX: Na-MOR was exchanged with 1M NH4NO3 and then with 2M oxalic acid 3 times and calcined at 550~ for 8h. All zeolites were characterised by XRD and elemental analysis. For EPR and G.C. measurements HM/NH4NO3/A, HM/HNO3, and HM/OX were heated in air, and under vacuum, HM/NH4NO3/B, to 700~ for 15 h. EPR measurements were made at room temp. on a Bruker ESP 300 spectrometer. For G.C. the products were distilled off at room temp. and ca. 5x10-6 mbar. The 27AI, 29Si and 1H MASNMR measurements were recorded on a Bruker AMX400.
Results and Discussion HM/NHdNO3 Following adsorption of diene (3) onto HM/NH4NO3/A, the EPR spectrum of the radical cation of diene (1) was observed. The same signal was
?4
measured after adsorption of hexene (2), and the parent diene (1). In all cases the EPR signal was strong. No EPR signal could be detected after adsorption onto HM/NH4NO3/B. It was thus concluded that oxygen was required to generate the necessary oxidation sites. The Si:AI ratio only increased from 5.8 to 6.4 after treatment but the 27AI NMR spectrum showed that a large proportion of the AI was present as extra-framework species. 1H NMR of the dehydrated sample showed the presence of some Bronsted acid sites. G.C. analysis of the diene (1) showed that no reaction had taken place beyond the formation of a radical cation, the alkene (2) and diene (3) were converted to diene (1) to a small extent on both mordenites (table 1).
2
1
3
Table I Percentage of diene (11 formed after 30 min on mordenites.
diene (1) aikene (2) diene (3)
HM/NH4NO3/A
HM/NH4NO3/B
HM/HNO3
HM/OX
no reaction 0.5% 1%
no reaction <0.02% 4%
no reaction 0.2% 31%
43%
HM/HNO3 The intensity of the EPR spectrum obtained after adsorption of diene (1) onto HM/HNO3 was only 1.5% of that measured above. Acid leaching increased the Si:AI ratio to 63 and the 27AI NMR spectrum showed that the remaining aluminium was present both as framework and extra-framework species. 1H NMR showed that, as above, some Br~nsted acid sites were present. The alkene (2) underwent very little reaction (table 1). However, diene (3) reacted to (1) to a much greater extent. This is probably a Br~nsted acid catalysed reaction, and indicates that the acid sites are stronger or more numerous in the mordenite treated with HNO3. HM/OX This mordenite (Si:AI 51) gave similar results as HM/HNO3 indicating that the presence of extra-framework aluminium is important, and not the method of removal. Conclusion
These results indicate that the generation of radical cations is closely correlated with the presence of a large amount of extra-framework aluminium which has been previously treated at high temperature in air, resulting in the formation of Lewis acid sites. 1: R. Crockett and E. Roduner, J. Chem. Soc. Perkin Trans. 2, 1503, 1993. 2: S. Shih, J. Catal., 79, 390, 1983. 3: R.D. Shannon et al, J. Phys. Chem., 89, 4778, 1985.
73
A COMBINED EPR AND NMR STUDY OF OXIDATION SITES IN DEALUMINATED MORDENITES. G. Harvey Estermann, Laboratorium f0r Technische Chemie, ETH-Zentrum, CH-8092 Z0rich. R. Crockett and E. Roduner, Physikalisch-chemisches Institut der Universit&t ZOrich, CH-8057 ZUrich
Summary The radical cation of 2,5-dimethyl-2,4-hexadiene (1) has been observed after adsorption of 2,5-dimethyl-3-hexene (2), and 2,5-dimethyl-l,5-hexadiene (3) onto dealuminated mordenites. The intensity of the EPR signal correlated with the presence of extra-framework aluminium as observed by 27AI MASNMR. When the extra-framework aluminium was removed by mineral acid leaching or chelation by oxalic acid, only a very weak EPR signal was detected. The conversion of alkenes (2) and (3) to diene (1) was also shown by G.C. analysis to be dependant on the type of mordenite. The generation of radical cations in these mordenites is dependant on the presence of Lewis acid sites located on extra-framework aluminium, although cationic reactions due to Bronsted acid sites often remain the major reaction pathway. Introduction EPR has often been used as a technique to identify intermediates in zeolite catalysis 1, although it remains unclear, which species within the zeolite is responsible for the generation of radical cations 2. One possibility is that the extra-framework aluminium, frequently present in dealuminated zeolites, forms Lewis acid sites, which act as electron acceptors 3. Experimental HM/NH, dV03: Na-mordenite (MOR) (PM-1, CU Chemie Uetikon AG) was exchanged
twice with 1M NH4NO3 and calcined at 700~ in air for ca. 15 h. HM/HN03: Na-MOR was treated first with 1M HCI and then with 6N HNO3 under reflux for 3h. The zeolite was calcined at 700~ for 1 h. The HNO3 treatment was repeated and finally the zeolite heated to 700~ for ca. 15 h. HM/OX: Na-MOR was exchanged with 1M NH4NO3 and then with 2M oxalic acid 3 times and calcined at 550~ for 8h. All zeolites were characterised by XRD and elemental analysis. For EPR and G.C. measurements HM/NH4NO3/A, HM/HNO3, and HM/OX were heated in air, and under vacuum, HM/NH4NO3/B, to 700~ for 15 h. EPR measurements were made at room temp. on a Bruker ESP 300 spectrometer. For G.C. the products were distilled off at room temp. and ca. 5x10-6 mbar. The 27AI, 29Si and 1H MASNMR measurements were recorded on a Bruker AMX400.
Results and Discussion HM/NHdNO3 Following adsorption of diene (3) onto HM/NH4NO3/A, the EPR spectrum of the radical cation of diene (1) was observed. The same signal was
?4
measured after adsorption of hexene (2), and the parent diene (1). In all cases the EPR signal was strong. No EPR signal could be detected after adsorption onto HM/NH4NO3/B. It was thus concluded that oxygen was required to generate the necessary oxidation sites. The Si:AI ratio only increased from 5.8 to 6.4 after treatment but the 27AI NMR spectrum showed that a large proportion of the AI was present as extra-framework species. 1H NMR of the dehydrated sample showed the presence of some Bronsted acid sites. G.C. analysis of the diene (1) showed that no reaction had taken place beyond the formation of a radical cation, the alkene (2) and diene (3) were converted to diene (1) to a small extent on both mordenites (table 1).
2
1
3
Table I Percentage of diene (11 formed after 30 min on mordenites.
diene (1) aikene (2) diene (3)
HM/NH4NO3/A
HM/NH4NO3/B
HM/HNO3
HM/OX
no reaction 0.5% 1%
no reaction <0.02% 4%
no reaction 0.2% 31%
43%
HM/HNO3 The intensity of the EPR spectrum obtained after adsorption of diene (1) onto HM/HNO3 was only 1.5% of that measured above. Acid leaching increased the Si:AI ratio to 63 and the 27AI NMR spectrum showed that the remaining aluminium was present both as framework and extra-framework species. 1H NMR showed that, as above, some Br~nsted acid sites were present. The alkene (2) underwent very little reaction (table 1). However, diene (3) reacted to (1) to a much greater extent. This is probably a Br~nsted acid catalysed reaction, and indicates that the acid sites are stronger or more numerous in the mordenite treated with HNO3. HM/OX This mordenite (Si:AI 51) gave similar results as HM/HNO3 indicating that the presence of extra-framework aluminium is important, and not the method of removal. Conclusion
These results indicate that the generation of radical cations is closely correlated with the presence of a large amount of extra-framework aluminium which has been previously treated at high temperature in air, resulting in the formation of Lewis acid sites. 1: R. Crockett and E. Roduner, J. Chem. Soc. Perkin Trans. 2, 1503, 1993. 2: S. Shih, J. Catal., 79, 390, 1983. 3: R.D. Shannon et al, J. Phys. Chem., 89, 4778, 1985.