- Email: [email protected]
IR and TPD studies of the interaction of alkenes with Cu+ sites in CuNaY and CuNaX zeolites of various Cu content. The heterogeneity of Cu+ sites
Journal of Molecular Structure 794 (2006) 261–264 www.elsevier.com/locate/molstruc
IR and TPD studies of the interaction of alkenes with CuC sites in CuNaY and CuNaX zeolites of various Cu content. The heterogeneity of CuC sites J. Datka, E. Kukulska-Zaja¸c, P. Kozyra * Faculty of Chemistry, Jagiellonian University, ul. Ingardena 3, 30-060 Krako´w, Poland Received 10 November 2005; received in revised form 16 February 2006; accepted 16 February 2006 Available online 17 April 2006
Abstract CuC ions in zeolites activate organic molecules containing p electrons by p back donation, which results in a distinct weakening of multiple bonds. In this study, we followed the activation of alkenes (ethene and propene) by CuC ions in CuY and CuX zeolites of various Cu content. We also studied the strength of bonding of alkenes to CuC ions. IR studies have shown that there are two kinds of CuC sites of various electron donor properties. We suppose that they could be attributed to the presence of CuC ions of various number of oxygen atoms surrounding the cation. IR studies have shown that Cu ions introduced into Y and X zeolites in the first-order (at low Cu content) form CuC ions of stronger electron donor properties (i.e. activate alkenes to larger extend) than Cu ions introduced in the next order (at higher Cu content). IR and TPD studies of alkenes desorption evidenced that CuC ions of stronger electron donor properties bond alkenes stronger than less electron donor ones. It suggests that p back donation has more important contribution to the strength of bonding alkenes to cation than p donation. q 2006 Elsevier B.V. All rights reserved. Keywords: IR spectroscopy; TPD; Cu–zeolites; alkenes activation
1. Introduction Our study concerns the activation of alkene molecules by CuC ions in zeolites. Since, the early reports by Iwamoto et al. [1–4] on the activity of CuZSM-5 in the decomposition of nitrogen oxides, copper containing zeolites attracted a great deal of attention. DFT calculations showed [5–9] that the high activity of CuC in CuZSM-5 could be related to the high energy of the HOMO orbital of CuC in MFI and to the p back donation of electrons from the d orbitals of Cu to p* antibonding orbitals of NO, resulting in a distinct weakening and dissociation of the NaO bond. The role of zeolite was the partial neutralization of the electrical charge of CuC: the charge decreased from C1 to C0.3 when CuC was placed in a cluster simulating a fragment of the MFI structure and therefore in an increase of the HOMO energy. According to Goursot et al. [9], zeolitic framework acts as a reservoir of electrons – the negative charge transferred to adsorbed NO
* Corresponding author. Tel.: C48 12 6632081; fax: C48 12 6340515. E-mail address: [email protected] (P. Kozyra).
0022-2860/$ - see front matter q 2006 Elsevier B.V. All rights reserved. doi:10.1016/j.molstruc.2006.02.043
comes mostly from the framework with only a small variation in the charge on the CuC ion. Apart from the activity in the ‘denox’ reaction, Cucontaining zeolites were also found to be active catalysts in some reactions of organic molecules. Therefore, we have undertaken the IR studies and DFT calculations concerning the activation of organic molecules by CuC ions in zeolites. Our earlier studies have shown that CuC ions were able to activate organic molecules which similarly as NO contain p electrons [10–13]. IR studies evidenced a distinct weakening of the CaC bond in alkenes (DnCaCZ78–115 cmK1), very important weakening of triple bond in acetylene (DnCbCz170 cmK1), less pronounced weakening of CaO bond in acetone (DnCa0z40 cmK1), and small weakening of C–C in benzene (DnZ 13 cmK1). DFT calculations evidenced that, similarly as in the case of NO, the activation of organic molecule was realized by p back donation of d electrons of CuC to p* antibonding orbitals of a given molecule. As a result, alkenes and benzene acquired the negative charge (K0.05 to K0.09), whereas CuC became more positive (from C0.32 to C0.36). Part of the charge transmitted to molecule came from CuC itself and part from the zeolite framework which acted as ‘electron reservoir’ [9]. The molecules of acetylene and acetone received positive charge when interacting with zeolitic CuC(C0.03 and C0.15,
J. Datka et al. / Journal of Molecular Structure 794 (2006) 261–264
2. Experimental Zeolites CuNaY (Si/AlZ2.56) and CuNaX (Si/AlZ1.3) were prepared from parent sodium forms synthesized in the Department of Chemical Technology, Jagiellonian University and the Institute of Industrial Chemistry (Warsaw). Na-forms A 0.05 ethene
3. Results and discussion The IR band of CaC in the spectra of ethene and propene sorbed in CuNaY and CuNaX zeolites of various Cu content are presented in Fig. 1. The band at 1520–1550 cmK1 of the double bond interacting with CuC is present in all the spectra. This band is red shifted by 80–120 cmK1 from the position typical of unperturbed molecule (z1600–1650 cmK1). These frequency shifts were distinctly larger, than those observed if ethane and propene interacted with NaC cations in zeolites NaY and NaX (1–15 cmK1; spectra not shown). All the spectra show that in zeolites of higher CuC content (Cu/AlO0.2) the CaC band shows two maxima. They will be denoted as a high frequency (h.f.) and a low frequency (l.f.) maxima. If alkenes are sorbed in CuY and CuX zeolites of low CuC content (Cu/AlZ0.11) the l.f. band is either the only band or a band dominant in the doublet. All these results indicate the existence in both CuY and CuX zeolites of two kinds of CuC sites of various ability of the activation of double bonds. This result agrees with the conclusion derived from the studies of ethene and acetylene B
a
Absorbance
1532
1539
1542
a
a 0.01
0.06
1551
0.08
b
0.03 CuY
0.02
propene 0.10
1537 1545
0.04
Absorbance
were treated with Cu(CH3COO)2 solution at 350 K. Following ion exchange, the samples were washed with distilled water and dried in air at 370 K. The exchange degrees (Cu/Al) were 0.11 and 0.26 for CuNaY and 0.11 and 0.22 for CuNaX. Ethene (UCAR—99.5%), propene (Fluka—99.9%), and CO (PRAXAIR 3.5) were used in our experiments. For IR studies, the Cu–zeolites were pressed into thin wafers and activated in situ in IR cell at 730 K at vacuum for 1 h. IR spectra were recorded by BRUKER IFS 48 spectrometer equipped with a MCT detector. Spectral resolution was 2 cmK1. TPD experiments were realized upon the sorption of ethene (the amount equal to the amount of CuC ions) at room temperature. Desorption was carried out at vacuum with heating rate 5 K/min. Desorbing ethene was detected with a PFEIFFER PRISMA QMS 200 mass spectrometer.
b
CuY
0.04
1540
respectively). In these cases, the charge was transmitted from the molecule via CuC to the zeolite framework. The electric charge on the cation did not change and zeolite framework acted again as ‘electron reservoir’ but in this case the framework received the electrons. The double and triple bond stretching modes, which were IR inactive in free ethene and acetylene, became active in IR spectra if molecules interacting with CuC in zeolites, indicating the loss of molecule symmetry. As mentioned, this study concerns the activation of alkenes by CuC ions in CuNaY and CuNaX zeolites. Our earlier studies [10], as well as the studies of Hu¨bner [14–16] of ethene sorption in zeolite CuY and acetylene sorption in CuX and CuY, evidenced that the CaC band interacting with CuC (around 1540 cmK1) composes of two maxima. In the case of other alkenes [10], this band was asymmetric, suggesting that it also composed of several maxima. These results suggest that at least two kinds of CuC sites of various ability of double and triple bond activation exist in CuY and CuX zeolites. This study was undertaken to get more information on the heterogeneity of CuC sites in CuY and CuX zeolites and to know whether the proportion between these two kinds of CuC sites depends on Cu content. Another interesting problem was the study of desorption of alkenes from CuC and the role of p donation and p back donation to the strength of bonding of alkenes to CuC. This problem was investigated by following the desorpion of alkenes from CuY and CuX zeolites by IR and TPD. Ethene and propene was sorbed in zeolites CuNaY of Cu/AlZ0.11 and 0.26 as well as in CuNaX of Cu/AlZ0.11 and 0.22.
1548
262
a
0.02
b
b CuX
CuX 0.00
0.00 1560
1550
1540
1530
Wavenumber (cm–1)
1520
1560
1545
Wavenumber
1530
1515
(cm–1)
Fig. 1. A. IR spectra of ethene sorbed in CuNaY (Cu/AlZ0.26 (a) and 0.11 (b)) and CuNaX (Cu/AlZ0.22 (a) and 0.11 (b)) zeolites of various Cu content. B. IR spectra of propene sorbed in CuNaY (Cu/AlZ0.26 (a) and 0.11 (b)) and CuNaX (Cu/AlZ0.22 (a) and 0.11 (b)) zeolites of various Cu content.
0.01 0.00 1560
1545
1530
0.03
263
1540
increasing desorption temperature
0.02
propene/CuX 1548
0.03
B 0.04 Absorbance
1542
propene/CuY
1551
Absorbance
A 0.04
increasing desorption temperature
J. Datka et al. / Journal of Molecular Structure 794 (2006) 261–264
0.02 0.01 0.00
1515
1560
Wavenumber (cm–1)
1545
1530
1515
Wavenumber (cm–1)
Fig. 2. A. IR spectra recorded during propene desorption from CuNaY zeolite at increasing temperature. B. IR spectra recorded during propene desorption from CuNaX zeolite at increasing temperature.
adsorption in zeolite CuY by Hu¨bner et al. [13–16] who interpreted these sites as CuC in various positions and therefore of various number of oxygen atoms surrounding the cation. The presence of two kinds of CuC sites of various electron donor properties has been also confirmed in the studies of CO sorption in zeolite CuY [16–20]. The comparison of the results of alkenes sorption in CuY and CuX zeolites of various Cu content suggests that Cu ions firstly introduced into Y and X zeolites (at low Cu content) form sites of stronger electron donor properties, i.e. of higher DnCaC giving the l.f. CaC band. On the other hand, CuC ions introduced in the second-order (at higher Cu content) form less electron donor sites of lower DnCaC values, giving the h.f. CaC band. The strength of alkenes to CuC bonding was studied in the desorption experiments. The amount of alkenes (propene or ethene) sufficient to cover all CuC sites was sorbed at room temperature. The alkenes were consequently desorbed in vacuum at increasing temperature (300–400 K). The spectra recorded during propene desorption from CuY and CuX zeolites are presented in Fig. 2. For both zeolites, the h.f. band of CaC stretching decreased at lower temperature than the l.f. one. Similar results were obtained for ethene. All these results indicate that CuC ions which are more electron donor (characterized by the l.f. CaC band) bond alkenes more strongly than the less electron donor ones. As discussed earlier, both p donation (donation of electrons from the molecule to the electron donor adsorption site) and p
B 1537
ethene/CuY
a 0.010
1545
Absorbance
0.015
MS Signal [10–10 A]
A
back donation (donation of electrons from the CuC site to the alkene molecule) contribute to the strength of alkenes to CuC bonding. The more electron donor the adsorption site is, the weaker is p donation and stronger p back donation. The fact that alkenes are bonded stronger to the more electron donor CuC sites suggests that in the case of alkenes, p back donation has more important contribution to the strength of alkenes to CuC bonding. The results obtained for alkenes desorption are in the opposition to those obtained for CO desorption from CuY and CuX zeolites [20]. CO bonded to more electron donor CuC sites (represented by the l.f. CO bands) was more weakly bonded than that bonded to less electron donor sites. This was evidenced by the fact [20] that the l.f. CO band diminished as the first during the desorption at elevated temperature. Comparing the results obtained at the experiments of alkenes and CO desorption from both CuY and CuX zeolites it can be concluded that while for alkenes the donation of electrons from CuC to the molecule (p back donation) has more important impact to the strength of alkenes to CuC bonding, for CO the donation of electrons from the molecule to CuC(s donation) is more important. More information on the strength of alkenes to CuC bonding was obtained in TPD studies of ethene desorption from CuY zeolite. Two series of experiments were performed: in one series all the CuC ions, whereas in the second one only z30% of CuC ions bonded ethene. The CaC band is presented in Fig. 3A and TPD diagrams in Fig. 3B (spectra a
c
0.005
b
b
6
a 4
b 2
0.000 1560
1550
1540
Wavenumber
1530
(cm–1)
1520
0 300
350
400
450
500
Temperature [K]
Fig. 3. A. IR spectra of ethene sorbed in CuNaY zeolite ((a)100% coverage of CuC, (b) z30% coverage of CuC, (c) the same as spectrum b heated to 370 K). B. TPD diagrams of ethene desorption from CuNaY zeolite ((a) 100% coverage of CuC, (b) z30% coverage of CuC).
264
J. Datka et al. / Journal of Molecular Structure 794 (2006) 261–264
and b). If all CuC ions bonded ethene molecules (curve a), the desorption started at room temperature already, the corresponding TPD peak is rather broad and shows maximum at about 400 K. On the other hand, if only 30% of CuC ions bonded ethene the desorption started at z370 K, the TPD peak is more narrow and shows maximium at about 450 K. All these results can be interpreted as follows. CuY zeolite contains CuC sites which bond ethene with various strength, and the alkene desorbs from less strongly bonding sites (less electron donor ones) at lower temperature than from the stronger bonding ones (more electron donor). If only small amount of ethene was adsorbed, it bonded with CuC sites of various electron donor properties without choosing the ones the most favorable energetically. The rearrangement of ethene between the sites bonding less and more strongly occurs at higher temperature. The molecules bonded to less strongly adsorbing sites desorb and readsorb on more strongly bonding ones without leaving the zeolite cavities. This is well seen in the IR spectra (Fig. 3A, spectra b and c): the l.f. band increases at the expense of the h.f. one when the sample was heated from room temperature to z370 K. TPD results evidenced therefore (similarly as IR results) the presence of CuC sites of various strength of alkenes bonding. The interaction of alkenes with CuC cations in zeolites resulted in withdrawing of cation from oxygen rings (ref. 10) similar, but smaller effects were observed in the case adsorbed CO [21]. The withdrawing of cation results in some weakening of the interaction of CuC with framework oxygens and smaller effect of neutralization of positive charge of cation. Generally, all the three partners: CuC, zeolite framework and alkene molecule act as one system. The fact, that two CaC bands are present (see Figs. 1–3) suggests that two kinds of such ‘systems’ exist in our zeolites CuY and CuX. 4. Conclusions 1. Two kinds of CuC sites of different electron donor properties exist in CuY and CuX zeolites. Most probably, they are ions of various number of oxygen atoms surrounding the cation. More electron donor sites activate alkenes stronger by p back donation which results in the larger Dn. Therefore, the IR band of CaC stretching of alkenes interacting with CuC composes of two maxima. 2. Cu cations introduced into zeolite in the first-order (at low Cu content) form CuC ions of stronger electron donor properties, and of higher DnCaC values than ions introduced afterwards (at higher Cu content).
3. Both IR and TPD experiments evidenced that alkenes bond stronger to the more electron donor CuC sites. It suggests that p back donation has more important contribution to the strength of alkenes to CuC bonding than p donation. This situation is in the opposition to the CO adsorption experiments, as for CO, the p back donation has less important impact to the strength of CO to CuC bonding.
Acknowledgements This study was sponsored by Ministry of Scientific Research and Informational Technology (grant no. 3 T09A 006 27). References [1] M. Iwamoto, H. Furokawa, Y. Mine, F. Uemura, S. Mikuriya, S. Kagawa, J. Chem. Soc., Chem. Commun. (1986) 1272. [2] M. Iwamoto, S. Yakoo, K. Sakai, S.J. Kagawa, J. Chem. Soc., Faraday Trans. 77 (1981) 1629. [3] M. Iwamoto, H. Yachiro, Y. Mine, S. Kagawa, Chem. Lett. (1989) 213. [4] M. Iwamoto, H. Yachiro, T. Kutsuno, S. Bunyu, S. Kagawa, Bull. Chem. Soc. Jpn 62 (1989) 583. [5] E. Brocławik, J. Datka, B. Gil, W. Piskorz, P. Kozyra, Top. Catal. 11/12 (2000) 335. [6] E. Brocławik, J. Datka, B. Gil, P. Kozyra, Zeolites and mesoporous materials at the dawn of the 21st century, Studies in Surface Science and Catalysis, vol. 135, Elsevier, New York, NY, 2001. pp. 13–15. [7] E. Brocławik, J. Datka, B. Gil, P. Kozyra, in: R. Aiello, G. Giordano, F. Testa (Eds.), Impact of zeolites and other porous materials on the new technologies at the beginning of the new millennium Studies in Surface Science and Catalysis, vol. 142, Elsevier, New York, NY, 2002, p. 1971. [8] J. Datka, E. Kukulska-Zaja˛c, P. Kozyra, Catal. Today 90 (2004) 109. [9] A. Goursot, B. Coq, F. Fajula, J. Catal. 216 (2003) 324–332. [10] J. Datka, E. Kukulska-Zaja˛c, J. Phys. Chem. B 108 (2004) 11760. [11] J. Datka, P. Kozyra, E. Kukulska-Zaja˛c, W. Kobyzewa, Catal. Today 101 (2005) 117. [12] J. Datka, E. Kukulska-Zaja˛c, W. Kobyzewa, Catal. Today 101 (2005) 123. [13] E. Brocławik, P. Kozyra, J. Datka, Comptes Rendus Chimie 8 (2005) 491. [14] G. Hu¨bner, E. Roduner, Zeolites and mesoporous materials at the dawn of the 21st century, Studies in Surface Science and Catalysis, vol. 135, Elsevier, New York, NY, 2001. pp. 9–12. [15] G. Hu¨bner, G. Rauhut, H. Stoll, E. Roduner, Phys. Chem. Chem. Phys. 4 (2002) 1073. [16] G. Hu¨bner, G. Rauhut, H. Stoll, E. Roduner, J. Phys. Chem. 107 (2003) 8568. [17] G.T. Palomino, S. Bordiga, A. Zecchina, G.L. Marra, C. Lamberti, J. Phys. Chem. B 104 (2000) 8641. [18] V.Yu. Borovkov, H.G. Karge, J. Chem. Soc., Faraday Trans. 191 (1995) 2035. [19] V.Yu. Borovkov, M. Jiang, Y. Fu, J. Phys. Chem. B 103 (1999) 5010. [20] J. Datka, P. Kozyra, J. Mol. Struct. 774 (2005) 991. [21] J. Sarkany, J. Mol. Struct. 410-411 (1997) 145.
IR and TPD studies of the interaction of alkenes with CuC sites in CuNaY and CuNaX zeolites of various Cu content. The heterogeneity of CuC sites J. Datka, E. Kukulska-Zaja¸c, P. Kozyra * Faculty of Chemistry, Jagiellonian University, ul. Ingardena 3, 30-060 Krako´w, Poland Received 10 November 2005; received in revised form 16 February 2006; accepted 16 February 2006 Available online 17 April 2006
Abstract CuC ions in zeolites activate organic molecules containing p electrons by p back donation, which results in a distinct weakening of multiple bonds. In this study, we followed the activation of alkenes (ethene and propene) by CuC ions in CuY and CuX zeolites of various Cu content. We also studied the strength of bonding of alkenes to CuC ions. IR studies have shown that there are two kinds of CuC sites of various electron donor properties. We suppose that they could be attributed to the presence of CuC ions of various number of oxygen atoms surrounding the cation. IR studies have shown that Cu ions introduced into Y and X zeolites in the first-order (at low Cu content) form CuC ions of stronger electron donor properties (i.e. activate alkenes to larger extend) than Cu ions introduced in the next order (at higher Cu content). IR and TPD studies of alkenes desorption evidenced that CuC ions of stronger electron donor properties bond alkenes stronger than less electron donor ones. It suggests that p back donation has more important contribution to the strength of bonding alkenes to cation than p donation. q 2006 Elsevier B.V. All rights reserved. Keywords: IR spectroscopy; TPD; Cu–zeolites; alkenes activation
1. Introduction Our study concerns the activation of alkene molecules by CuC ions in zeolites. Since, the early reports by Iwamoto et al. [1–4] on the activity of CuZSM-5 in the decomposition of nitrogen oxides, copper containing zeolites attracted a great deal of attention. DFT calculations showed [5–9] that the high activity of CuC in CuZSM-5 could be related to the high energy of the HOMO orbital of CuC in MFI and to the p back donation of electrons from the d orbitals of Cu to p* antibonding orbitals of NO, resulting in a distinct weakening and dissociation of the NaO bond. The role of zeolite was the partial neutralization of the electrical charge of CuC: the charge decreased from C1 to C0.3 when CuC was placed in a cluster simulating a fragment of the MFI structure and therefore in an increase of the HOMO energy. According to Goursot et al. [9], zeolitic framework acts as a reservoir of electrons – the negative charge transferred to adsorbed NO
* Corresponding author. Tel.: C48 12 6632081; fax: C48 12 6340515. E-mail address: [email protected] (P. Kozyra).
0022-2860/$ - see front matter q 2006 Elsevier B.V. All rights reserved. doi:10.1016/j.molstruc.2006.02.043
comes mostly from the framework with only a small variation in the charge on the CuC ion. Apart from the activity in the ‘denox’ reaction, Cucontaining zeolites were also found to be active catalysts in some reactions of organic molecules. Therefore, we have undertaken the IR studies and DFT calculations concerning the activation of organic molecules by CuC ions in zeolites. Our earlier studies have shown that CuC ions were able to activate organic molecules which similarly as NO contain p electrons [10–13]. IR studies evidenced a distinct weakening of the CaC bond in alkenes (DnCaCZ78–115 cmK1), very important weakening of triple bond in acetylene (DnCbCz170 cmK1), less pronounced weakening of CaO bond in acetone (DnCa0z40 cmK1), and small weakening of C–C in benzene (DnZ 13 cmK1). DFT calculations evidenced that, similarly as in the case of NO, the activation of organic molecule was realized by p back donation of d electrons of CuC to p* antibonding orbitals of a given molecule. As a result, alkenes and benzene acquired the negative charge (K0.05 to K0.09), whereas CuC became more positive (from C0.32 to C0.36). Part of the charge transmitted to molecule came from CuC itself and part from the zeolite framework which acted as ‘electron reservoir’ [9]. The molecules of acetylene and acetone received positive charge when interacting with zeolitic CuC(C0.03 and C0.15,
J. Datka et al. / Journal of Molecular Structure 794 (2006) 261–264
2. Experimental Zeolites CuNaY (Si/AlZ2.56) and CuNaX (Si/AlZ1.3) were prepared from parent sodium forms synthesized in the Department of Chemical Technology, Jagiellonian University and the Institute of Industrial Chemistry (Warsaw). Na-forms A 0.05 ethene
3. Results and discussion The IR band of CaC in the spectra of ethene and propene sorbed in CuNaY and CuNaX zeolites of various Cu content are presented in Fig. 1. The band at 1520–1550 cmK1 of the double bond interacting with CuC is present in all the spectra. This band is red shifted by 80–120 cmK1 from the position typical of unperturbed molecule (z1600–1650 cmK1). These frequency shifts were distinctly larger, than those observed if ethane and propene interacted with NaC cations in zeolites NaY and NaX (1–15 cmK1; spectra not shown). All the spectra show that in zeolites of higher CuC content (Cu/AlO0.2) the CaC band shows two maxima. They will be denoted as a high frequency (h.f.) and a low frequency (l.f.) maxima. If alkenes are sorbed in CuY and CuX zeolites of low CuC content (Cu/AlZ0.11) the l.f. band is either the only band or a band dominant in the doublet. All these results indicate the existence in both CuY and CuX zeolites of two kinds of CuC sites of various ability of the activation of double bonds. This result agrees with the conclusion derived from the studies of ethene and acetylene B
a
Absorbance
1532
1539
1542
a
a 0.01
0.06
1551
0.08
b
0.03 CuY
0.02
propene 0.10
1537 1545
0.04
Absorbance
were treated with Cu(CH3COO)2 solution at 350 K. Following ion exchange, the samples were washed with distilled water and dried in air at 370 K. The exchange degrees (Cu/Al) were 0.11 and 0.26 for CuNaY and 0.11 and 0.22 for CuNaX. Ethene (UCAR—99.5%), propene (Fluka—99.9%), and CO (PRAXAIR 3.5) were used in our experiments. For IR studies, the Cu–zeolites were pressed into thin wafers and activated in situ in IR cell at 730 K at vacuum for 1 h. IR spectra were recorded by BRUKER IFS 48 spectrometer equipped with a MCT detector. Spectral resolution was 2 cmK1. TPD experiments were realized upon the sorption of ethene (the amount equal to the amount of CuC ions) at room temperature. Desorption was carried out at vacuum with heating rate 5 K/min. Desorbing ethene was detected with a PFEIFFER PRISMA QMS 200 mass spectrometer.
b
CuY
0.04
1540
respectively). In these cases, the charge was transmitted from the molecule via CuC to the zeolite framework. The electric charge on the cation did not change and zeolite framework acted again as ‘electron reservoir’ but in this case the framework received the electrons. The double and triple bond stretching modes, which were IR inactive in free ethene and acetylene, became active in IR spectra if molecules interacting with CuC in zeolites, indicating the loss of molecule symmetry. As mentioned, this study concerns the activation of alkenes by CuC ions in CuNaY and CuNaX zeolites. Our earlier studies [10], as well as the studies of Hu¨bner [14–16] of ethene sorption in zeolite CuY and acetylene sorption in CuX and CuY, evidenced that the CaC band interacting with CuC (around 1540 cmK1) composes of two maxima. In the case of other alkenes [10], this band was asymmetric, suggesting that it also composed of several maxima. These results suggest that at least two kinds of CuC sites of various ability of double and triple bond activation exist in CuY and CuX zeolites. This study was undertaken to get more information on the heterogeneity of CuC sites in CuY and CuX zeolites and to know whether the proportion between these two kinds of CuC sites depends on Cu content. Another interesting problem was the study of desorption of alkenes from CuC and the role of p donation and p back donation to the strength of bonding of alkenes to CuC. This problem was investigated by following the desorpion of alkenes from CuY and CuX zeolites by IR and TPD. Ethene and propene was sorbed in zeolites CuNaY of Cu/AlZ0.11 and 0.26 as well as in CuNaX of Cu/AlZ0.11 and 0.22.
1548
262
a
0.02
b
b CuX
CuX 0.00
0.00 1560
1550
1540
1530
Wavenumber (cm–1)
1520
1560
1545
Wavenumber
1530
1515
(cm–1)
Fig. 1. A. IR spectra of ethene sorbed in CuNaY (Cu/AlZ0.26 (a) and 0.11 (b)) and CuNaX (Cu/AlZ0.22 (a) and 0.11 (b)) zeolites of various Cu content. B. IR spectra of propene sorbed in CuNaY (Cu/AlZ0.26 (a) and 0.11 (b)) and CuNaX (Cu/AlZ0.22 (a) and 0.11 (b)) zeolites of various Cu content.
0.01 0.00 1560
1545
1530
0.03
263
1540
increasing desorption temperature
0.02
propene/CuX 1548
0.03
B 0.04 Absorbance
1542
propene/CuY
1551
Absorbance
A 0.04
increasing desorption temperature
J. Datka et al. / Journal of Molecular Structure 794 (2006) 261–264
0.02 0.01 0.00
1515
1560
Wavenumber (cm–1)
1545
1530
1515
Wavenumber (cm–1)
Fig. 2. A. IR spectra recorded during propene desorption from CuNaY zeolite at increasing temperature. B. IR spectra recorded during propene desorption from CuNaX zeolite at increasing temperature.
adsorption in zeolite CuY by Hu¨bner et al. [13–16] who interpreted these sites as CuC in various positions and therefore of various number of oxygen atoms surrounding the cation. The presence of two kinds of CuC sites of various electron donor properties has been also confirmed in the studies of CO sorption in zeolite CuY [16–20]. The comparison of the results of alkenes sorption in CuY and CuX zeolites of various Cu content suggests that Cu ions firstly introduced into Y and X zeolites (at low Cu content) form sites of stronger electron donor properties, i.e. of higher DnCaC giving the l.f. CaC band. On the other hand, CuC ions introduced in the second-order (at higher Cu content) form less electron donor sites of lower DnCaC values, giving the h.f. CaC band. The strength of alkenes to CuC bonding was studied in the desorption experiments. The amount of alkenes (propene or ethene) sufficient to cover all CuC sites was sorbed at room temperature. The alkenes were consequently desorbed in vacuum at increasing temperature (300–400 K). The spectra recorded during propene desorption from CuY and CuX zeolites are presented in Fig. 2. For both zeolites, the h.f. band of CaC stretching decreased at lower temperature than the l.f. one. Similar results were obtained for ethene. All these results indicate that CuC ions which are more electron donor (characterized by the l.f. CaC band) bond alkenes more strongly than the less electron donor ones. As discussed earlier, both p donation (donation of electrons from the molecule to the electron donor adsorption site) and p
B 1537
ethene/CuY
a 0.010
1545
Absorbance
0.015
MS Signal [10–10 A]
A
back donation (donation of electrons from the CuC site to the alkene molecule) contribute to the strength of alkenes to CuC bonding. The more electron donor the adsorption site is, the weaker is p donation and stronger p back donation. The fact that alkenes are bonded stronger to the more electron donor CuC sites suggests that in the case of alkenes, p back donation has more important contribution to the strength of alkenes to CuC bonding. The results obtained for alkenes desorption are in the opposition to those obtained for CO desorption from CuY and CuX zeolites [20]. CO bonded to more electron donor CuC sites (represented by the l.f. CO bands) was more weakly bonded than that bonded to less electron donor sites. This was evidenced by the fact [20] that the l.f. CO band diminished as the first during the desorption at elevated temperature. Comparing the results obtained at the experiments of alkenes and CO desorption from both CuY and CuX zeolites it can be concluded that while for alkenes the donation of electrons from CuC to the molecule (p back donation) has more important impact to the strength of alkenes to CuC bonding, for CO the donation of electrons from the molecule to CuC(s donation) is more important. More information on the strength of alkenes to CuC bonding was obtained in TPD studies of ethene desorption from CuY zeolite. Two series of experiments were performed: in one series all the CuC ions, whereas in the second one only z30% of CuC ions bonded ethene. The CaC band is presented in Fig. 3A and TPD diagrams in Fig. 3B (spectra a
c
0.005
b
b
6
a 4
b 2
0.000 1560
1550
1540
Wavenumber
1530
(cm–1)
1520
0 300
350
400
450
500
Temperature [K]
Fig. 3. A. IR spectra of ethene sorbed in CuNaY zeolite ((a)100% coverage of CuC, (b) z30% coverage of CuC, (c) the same as spectrum b heated to 370 K). B. TPD diagrams of ethene desorption from CuNaY zeolite ((a) 100% coverage of CuC, (b) z30% coverage of CuC).
264
J. Datka et al. / Journal of Molecular Structure 794 (2006) 261–264
and b). If all CuC ions bonded ethene molecules (curve a), the desorption started at room temperature already, the corresponding TPD peak is rather broad and shows maximum at about 400 K. On the other hand, if only 30% of CuC ions bonded ethene the desorption started at z370 K, the TPD peak is more narrow and shows maximium at about 450 K. All these results can be interpreted as follows. CuY zeolite contains CuC sites which bond ethene with various strength, and the alkene desorbs from less strongly bonding sites (less electron donor ones) at lower temperature than from the stronger bonding ones (more electron donor). If only small amount of ethene was adsorbed, it bonded with CuC sites of various electron donor properties without choosing the ones the most favorable energetically. The rearrangement of ethene between the sites bonding less and more strongly occurs at higher temperature. The molecules bonded to less strongly adsorbing sites desorb and readsorb on more strongly bonding ones without leaving the zeolite cavities. This is well seen in the IR spectra (Fig. 3A, spectra b and c): the l.f. band increases at the expense of the h.f. one when the sample was heated from room temperature to z370 K. TPD results evidenced therefore (similarly as IR results) the presence of CuC sites of various strength of alkenes bonding. The interaction of alkenes with CuC cations in zeolites resulted in withdrawing of cation from oxygen rings (ref. 10) similar, but smaller effects were observed in the case adsorbed CO [21]. The withdrawing of cation results in some weakening of the interaction of CuC with framework oxygens and smaller effect of neutralization of positive charge of cation. Generally, all the three partners: CuC, zeolite framework and alkene molecule act as one system. The fact, that two CaC bands are present (see Figs. 1–3) suggests that two kinds of such ‘systems’ exist in our zeolites CuY and CuX. 4. Conclusions 1. Two kinds of CuC sites of different electron donor properties exist in CuY and CuX zeolites. Most probably, they are ions of various number of oxygen atoms surrounding the cation. More electron donor sites activate alkenes stronger by p back donation which results in the larger Dn. Therefore, the IR band of CaC stretching of alkenes interacting with CuC composes of two maxima. 2. Cu cations introduced into zeolite in the first-order (at low Cu content) form CuC ions of stronger electron donor properties, and of higher DnCaC values than ions introduced afterwards (at higher Cu content).
3. Both IR and TPD experiments evidenced that alkenes bond stronger to the more electron donor CuC sites. It suggests that p back donation has more important contribution to the strength of alkenes to CuC bonding than p donation. This situation is in the opposition to the CO adsorption experiments, as for CO, the p back donation has less important impact to the strength of CO to CuC bonding.
Acknowledgements This study was sponsored by Ministry of Scientific Research and Informational Technology (grant no. 3 T09A 006 27). References [1] M. Iwamoto, H. Furokawa, Y. Mine, F. Uemura, S. Mikuriya, S. Kagawa, J. Chem. Soc., Chem. Commun. (1986) 1272. [2] M. Iwamoto, S. Yakoo, K. Sakai, S.J. Kagawa, J. Chem. Soc., Faraday Trans. 77 (1981) 1629. [3] M. Iwamoto, H. Yachiro, Y. Mine, S. Kagawa, Chem. Lett. (1989) 213. [4] M. Iwamoto, H. Yachiro, T. Kutsuno, S. Bunyu, S. Kagawa, Bull. Chem. Soc. Jpn 62 (1989) 583. [5] E. Brocławik, J. Datka, B. Gil, W. Piskorz, P. Kozyra, Top. Catal. 11/12 (2000) 335. [6] E. Brocławik, J. Datka, B. Gil, P. Kozyra, Zeolites and mesoporous materials at the dawn of the 21st century, Studies in Surface Science and Catalysis, vol. 135, Elsevier, New York, NY, 2001. pp. 13–15. [7] E. Brocławik, J. Datka, B. Gil, P. Kozyra, in: R. Aiello, G. Giordano, F. Testa (Eds.), Impact of zeolites and other porous materials on the new technologies at the beginning of the new millennium Studies in Surface Science and Catalysis, vol. 142, Elsevier, New York, NY, 2002, p. 1971. [8] J. Datka, E. Kukulska-Zaja˛c, P. Kozyra, Catal. Today 90 (2004) 109. [9] A. Goursot, B. Coq, F. Fajula, J. Catal. 216 (2003) 324–332. [10] J. Datka, E. Kukulska-Zaja˛c, J. Phys. Chem. B 108 (2004) 11760. [11] J. Datka, P. Kozyra, E. Kukulska-Zaja˛c, W. Kobyzewa, Catal. Today 101 (2005) 117. [12] J. Datka, E. Kukulska-Zaja˛c, W. Kobyzewa, Catal. Today 101 (2005) 123. [13] E. Brocławik, P. Kozyra, J. Datka, Comptes Rendus Chimie 8 (2005) 491. [14] G. Hu¨bner, E. Roduner, Zeolites and mesoporous materials at the dawn of the 21st century, Studies in Surface Science and Catalysis, vol. 135, Elsevier, New York, NY, 2001. pp. 9–12. [15] G. Hu¨bner, G. Rauhut, H. Stoll, E. Roduner, Phys. Chem. Chem. Phys. 4 (2002) 1073. [16] G. Hu¨bner, G. Rauhut, H. Stoll, E. Roduner, J. Phys. Chem. 107 (2003) 8568. [17] G.T. Palomino, S. Bordiga, A. Zecchina, G.L. Marra, C. Lamberti, J. Phys. Chem. B 104 (2000) 8641. [18] V.Yu. Borovkov, H.G. Karge, J. Chem. Soc., Faraday Trans. 191 (1995) 2035. [19] V.Yu. Borovkov, M. Jiang, Y. Fu, J. Phys. Chem. B 103 (1999) 5010. [20] J. Datka, P. Kozyra, J. Mol. Struct. 774 (2005) 991. [21] J. Sarkany, J. Mol. Struct. 410-411 (1997) 145.