Transformations of Formaldehyde Molecules in Cu−ZSM-5 Zeolites

J. Phys. Chem. C 2007, 111, 3471-3475

3471

Transformations of Formaldehyde Molecules in Cu-ZSM-5 Zeolites Ewa Kukulska-Zaja¸ c and Jerzy Datka* Faculty of Chemistry, Jagiellonian UniVersity, 30-060 Krako´ w, Ingardena 3, Poland ReceiVed: October 13, 2006; In Final Form: January 3, 2007

The activation of CdO bond in formaldehyde molecule was followed by IR spectroscopy. IR studies showed that the interaction of formaldehyde with Cu+ weakens of the CdO bond and results in a red shift of the CdO IR band (∆ν ) 56 cm-1). The activation of the CdO bond is the result of π back-donation of d electrons of Cu+ to π* antibonding orbitals of the molecule. Experiments on coadsorption of formaldehyde and CO on the same Cu* showed that formaldehyde acts as electron donor to the Cu+-CO system, resulting in stronger π back-donation to antibonding orbitals of CO. IR studies showed that formaldehyde molecules sorbed in zeolite Cu-ZSM-5 are oxidized to formate ions. Cu2+ ions, which survived the self-reduction to Cu+ during the pretreatment of zeolite under vacuum, were oxidant. Such Cu2+ ions are reduced to Cu+ during the reaction with formaldehyde molecules.

Introduction The present paper concerns the ability of Cu+ ions in zeolite Cu-ZSM-5 to activate the molecules of formaldehyde and the transformation of formaldehyde sorbed in zeolite. Since the early reports by Iwamoto et al. [e.g., see refs 1 and 2] on the activity of Cu-ZSM-5 in the decomposition of nitrogen oxides, coppercontaining zeolites attracted a great deal of attention. Density functional theory (DFT) calculations showed [e.g., see refs 3-5] that the high activity of Cu+ in Cu-ZSM-5 could be related to the high energy of the HOMO orbital of Cu+ in MFI and to the π back-donation of electrons from the d orbitals of Cu to π* antibonding orbitals of NO, resulting in a distinct weakening and dissociation of the NdO bond. The role of zeolite was to partially neutralize the electrical charge of Cu+: the charge decreased from +1 to +0.3 when Cu+ 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.,6 the zeolitic framework acts as a reservoir of electronssthe negative charge transferred to NO comes mostly from the framework with only a small variation in the charge on the Cu+ ion. According to Berthomieu et al.,7 the zeolite framework plays also a role of multipositional ligand for Cu+ ions. In addition to the activity in the “denox” reaction, Cucontaining zeolites were also found to be active catalysts in some reactions of organic molecules (e.g., see refs 8-19). Recently, we undertook IR studies and DFT calculations of the activation of multiple bonds in organic molecules. Both IR studies and calculations showed20-24 an important weakening of the CdC bond in alkenes (∆ν ) 78-115 cm-1) and of the CtC bond in acetylene (∆ν ) 168 cm-1), less important weakening of the CdO bond in acetone (∆ν ) 39 cm-1), and slight weakening of the C-C bond in benzene (∆ν ) 13 cm-1), the LUMO orbital of which has only a slightly antibonding character. The DFT calculations evidenced that this was the result of π back-donation of d electrons of Cu+ to antibonding π* orbitals of organic molecules. This study was undertaken to follow the activation of formaldehyde molecule by Cu+ ions in zeolite Cu-ZSM-5 * Corresponding author. E-mail: [email protected].

by IR spectroscopy, as well as further reactions of formaldehyde sorbed in zeolite Cu-ZSM-5. In our study Cu+ ions were obtained by heat treatment under vacuum of zeolite Cu-ZSM-5 obtained by ionic exchange of zeolite Na-ZSM-5 with Cu2+ salt. It is well-known that such a treatment results in the self-reduction of Cu2+ to Cu+ accompanied by oxygen evolution.7 It was evidenced by various experimental methods, including electron paramagnetic resonance (EPR)25-27 and IR.25,28 The combination of IR and EPR experiments25 showed a “parallel” decrease of the EPR signal of Cu2+ and an increase of the IR band of CO bonded to Cu+ in the samples of Cu-ZSM-5 vacuum-treated at increasing temperatures. The most important effect was observed between 350 and 450 K. Experimental Section Zeolites Cu-ZSM-5 and NaCu-ZSM-5 of various Cu contents were prepared from a parent Na-ZSM-5 (Si/Al ) 35) synthesized at the Institute of Industrial Chemistry (Warszawa) by treatment with Cu(CH3COO)2 solutions of various concentrations at 350 K. After ion exchange the samples were washed with distilled water and dried in air at 370 K. The exchange degrees (Cu/Al) were 0.45 and 0.20 for Cu-ZSM-5 and NaCuZSM-5 respectively. Before the IR experiments the zeolites were activated in situ in an IR cell under vacuum at 720 K. In some experiments H-ZSM-5 was used. It was obtained by the ionic exchange from parent Na-forms with NH4Cl, which was followed by the activation in situ in an IR cell at 870 K. The exchange degree (H/Al) was 0.95. Gaseous formaldehyde was produced by heating (at ca. 370 K) paraldehyde (Aldrich). Oxygen and CO (PRAXAIR) were also used. IR spectra were recorded with a Bruker IFS 48 spectrometer equipped with an MCT detector. The spectral resolution was 2 cm-1. The decomposition of formate ions formed in zeolite Cu-ZSM-5 was followed by temperature-programmed desorption-IR (TPD-IR) experiments in which the gaseous products were studied with a Pfeifer Prisma QMS 200 mass spectrometer.

10.1021/jp066732g CCC: $37.00 © 2007 American Chemical Society Published on Web 02/02/2007

3472 J. Phys. Chem. C, Vol. 111, No. 8, 2007

Figure 1. IR spectra of formaldehyde: gaseous (a) and sorbed in zeolites Na-ZSM-5 (b), H-ZSM-5 (c), and Cu-ZSM-5 (d).

Results and Discussion Interaction of Formaldehyde with Cu+ Ions: IR Results. The IR spectra of gaseous formaldehyde and formaldehyde sorbed in zeolites Na-ZSM-5, H-ZSM-5, and Cu-ZSM-5 are presented in Figure 1. The spectrum of gaseous formaldehyde (spectrum a) shows a distinct band of CdO stretching vibration at 1745 cm-1, as well as a less intense band of CH2 scissoring at 1501 cm-1. The interaction of formaldehyde with Na+ ions in zeolite Na-ZSM-5 (spectrum b) results in a small red shift of the CdO band to 1728 cm-1, and the interaction with SiOH-Al groups (by hydrogen bonding)sin a further red shift of the CdO band to 1714 cm-1 (spectrum c). The sorption of formaldehyde in zeolite Cu-ZSM-5 (spectrum d) results in the appearance of both of the above-mentioned bands, because some Na+ ions are still present even when 90% of the Na+ ions have been exchanged, and Si-OH-Al groups were produced by the hydrolysis of Cu2+. The third band at 1689 cm-1 can be assigned to CdO interacting with Cu+ cations. The frequency of this band is lower by 56 cm-1 than in free molecules. This frequency shift (∆ν ) 56 cm-1) is higher than for the CdO group in acetone interacting with Cu+ (∆ν ) 39 cm-1 22). The CdO bond weakening in formaldehyde indicates CdO bond activation, which is more important for formaldehyde than for acetone. The spectra recorded at the sorption of increasing amounts of formaldehyde in zeolite Cu-ZSM-5 are presented in Figure 2A, and the spectra recorded at the desorption of formaldehyde by evacuation at increasing temperatures (in the range from room temperature to 370 K) are presented in Figure 2B. The spectra shown in Figure 2A suggest that formaldehyde molecules react with all three kinds of adsorption sites (Na+, SiOH-Al, and Cu+) without choosing the most energetically favorable. However, the desorption experiments showed that the bands of formaldehyde bonded to Na+ and Si-OH-Al (the bands at 1728 and 1714 cm-1) decrease at lower temperatures than the band of formaldehyde bonded to Cu+ (1589 cm-1). This proves that Cu+ ions bond with formaldehyde stronger than Na+ and Si-OH-Al groups. Similar results were observed in the case of acetone.22 Coadsorption of Formaldehyde and CO. The DFT calculations29 showed that the interaction of formaldehyde with Cu+ cations resulted in a net flow of electrons from the molecule via Cu+ toward zeolite framework. It was therefore interesting to know how this electron transfer would affect the interaction

Kukulska-Zaja¸ c and Datka of Cu+ with CO (CO has often been used as a probe molecule for the electron donor properties of Cu+). To follow the coadsorption of formaldehyde and CO, formaldehyde was sorbed at room temperature, and subsequently doses of CO were introduced into the cell. The spectra of CO sorbed in zeolite Cu-ZSM-5 “solo” without formaldehyde and with formaldehyde are presented in Figure 3 (spectra a and b). The CO band is shifted from 2157 cm-1 (without formaldehyde) to 2136 cm-1 if both formaldehyde and CO are bonded to the same Cu+ cation. The ∆ν value (∆ν ) 21 cm-1) for formaldehyde is lower than the corresponding frequency shift if acetone (∆ν ) 27 cm-1) and CO interacted with the same Cu+ cation (Figure 3, spectrum c, taken from our previous study30). The obtained results can be interpreted as follows. The Cu+, the zeolite framework, the molecules of formaldehyde, and CO can be considered as one system. In this system CO plays the role of electron acceptor and formaldehyde, of electron donor. The flow of electrons from formaldehyde toward CO (via Cu+ cation) weakens the CtO bond because of electron donation to π* antibonding orbitals of CO. This effect is weaker for formaldehyde than for acetone because formaldehyde is a weaker electron donor than acetone (lower energy of HOMO orbital, -6.35 and -5.76 eV, respectively). Oxidation of Formaldehyde in Zeolite Cu-ZSM-5. The spectrum of formaldehyde sorbed in zeolite Cu-ZSM-5 in addition to the bands of CdO interacting with Na+, Si-OHAl, and Cu+ (1728, 1714, and 1689 cm-1) shows also another band at 1578 cm-1 (Figure 1, spectrum d). This band grows with the contact time and with the temperature (Figure 4). The 1578 cm-1 band grows together with two weaker bands at 1371 and 1363 cm-1. These three bands can, according to Kno¨zinger at al.31,32 and Miller et al.,33 be attributed to antisymmetric and symmetric stretching of COO- (1578 and 1363 cm-1, respectively) and to C-H bending in formate ions (1371 cm-1). The spectrum of solid sodium formate (in KBrsFigure 4, spectrum e) also shows the bands 1361 and 1603 cm-1 of frequency similar to the product of formaldehyde transformation in zeolite Cu-ZSM-5. The formation of formate ions indicates that formaldehyde molecules are oxidized in zeolite Cu-ZSM-5. The oxidation of formaldehyde results in a decrease of formaldehyde bands, but according to Figure 4 the bands of formaldehyde bonded to Na+ and Si-OH-Al (1728 and 1714 cm-1) decrease in the first order, whereas the band of formaldehyde bonded to Cu+ (1689 cm-1) practically does not change. This indicates that relatively strong bonding of formaldehyde to Cu+ slows down their oxidation. The formate ions decompose at evacuation above 430 K, their IR bands decrease (spectra not shown), and above 570 K there are no more formate ions. The products of the decomposition of formate ions were followed in TPD-IR experiments, in which the all the reactions were carried out in the IR cell and were followed by IR spectroscopy, and the products of formate ions decomposition were additionally analyzed by mass spectrometry. The formate ions were formed first by the reaction at 370 K of formaldehyde at 370 K sorbed in zeolite Cu-ZSM5; as much as possible of nonreacted formaldehyde was subsequently removed by the evacuation at the same temperature. TPD procedure was next performed with the heating rate of 5 K/min. The TPD diagram (Figure 5) showed a peak of CO2 at 470 K and a less important peak of hydrogen. It may therefore be concluded that CO2 and H2 are the main products of formate ion decomposition. We suppose that formate ions are formed in zeolite CuZSM-5 by the oxidation of formaldehyde by Cu2+ ions which

Formaldehyde Molecules in Cu-ZSM-5 Zeolites

J. Phys. Chem. C, Vol. 111, No. 8, 2007 3473

Figure 2. IR spectra recorded at the sorption of increasing amounts of formaldehyde (A) and at the desorption of formaldehyde at increasing temperatures (B) (from room temperature to 350 K).

Figure 3. IR spectra of CO sorbed in zeolite Cu-ZSM-5 solo (a) and together with formaldehyde (b) and acetone (c).

Figure 4. IR spectra of formaldehyde sorbed in Cu-ZSM-5 (Cu/Al ) 45) at room temperature (a), after a few minutes of contact (b), and at 370 K (c). Spectra of formaldehyde sorbed in Cu-ZSM-5 (Si/Al ) 0.20) at 370 K (d) and of solid sodium formate (in KBr) (e).

Figure 5. TPD diagram of the decomposition of formate ions in zeolite Cu-ZSM-5.

did not undergo the self-reduction during the activation of zeolite. The presence of this Cu2+ in zeolite Cu-ZSM-5 activated at vacuum was confirmed in the IR experiments of NO sorption at low temperature (ca. 170 K). The experiments were performed at such a low temperature in order to avoid

further oxidation of Cu+ to Cu2+ by NO. The spectrum of NO sorbed at 170 K in zeolite in Cu-ZSM-5 (Figure 6A , spectrum a) shows two intensive bands at 1824 and 1730 cm-1 of the asymmetric and symmetric stretching of dinitrosyls Cu+(NO)2 and a weaker band at 1895 cm-1, of NO bonded to Cu2+. The evidence that Cu2+ ions oxidize formaldehyde to formate ions may be the fact that they are consumed in the reaction with formaldehyde: the spectra of NO sorbed in Cu-ZSM-5 upon the formation of formate ions and their subsequent decomposition show a distinct decrease of the Cu2+-NO band (Figure 6A, spectrum b). The role of Cu2+ ions in the formation of formate ions was also shown in the experiment in which formaldehyde was sorbed in freshly activated zeolite Cu-ZSM-5 (Figure 6B, spectrum a) and in zeolite in which formate ions were formed and subsequently decomposed (Figure 6B, spectrum b). In this second case, i.e., if Cu2+ ions were consumed, the formate ions were not produced. We suppose that the Cu2+ ions which oxidize formaldehyde to formate ions are in the form of CuO (or other oxide-like species). This CuO can be formed by the hydrolysis of Cu2+ introduced into zeolite by ion exchange) when the zeolite is heated under vacuum during the pretreatment. Some water molecules present inside zeolitic channels desorb, but some others react with Cu2+ ion: Cu2+ + H2O f CuOH+ + H+. Protons form bridging Si-OH-Al groups (seen in IR spectras spectra not shown); CuOH+ condenses to form CuO or other oxide-like species. The presence of CuO in zeolite Cu-ZSM-5 was evidenced by Beutel et al.26 Spectrum d in Figure 4 was recorded upon the sorption of formaldehyde followed by heating to 370 K in zeolite CuZSM-5 of low Cu content (Cu/Al ) 0.20) in which most of the Cu is expected to be in the form of isolated Cu+ ions and the contribution of CuO is expected to be lower than in Cu-ZSM-5 of higher Cu content (of Cu/Al ) 0.45). In this spectrum only the bands of formaldehyde bonded to Na+, Si-OH-Al, and Cu+ are present, and no bands of formate ions are seen. This suggests that CuO clusters play an important role in formaldehyde oxidation. The concentration of Cu2+ ions in zeolite Cu-ZSM-5 may be increased if Cu+ is oxidized by O2. Therefore, we studied the oxidation of formaldehyde to formate ions in Cu-ZSM-5 which was previously treated with O2. The oxidation was realized by O2 treatment at 720 K followed by evacuation at 470 K. The evidence that Cu+ was really oxidized to Cu2+ under such conditions was obtained in the experiments of the sorption of CO and NO (used as probe molecules) and by the analysis of the spectrum recorded in a “transmission window” (9001000 cm-1). The spectra of CO sorbed show a distinct decrease of the Cu+-CO band at 2157 cm-1 and a distinct increase of

Kukulska-Zaja¸ c and Datka

3474 J. Phys. Chem. C, Vol. 111, No. 8, 2007

Figure 6. (A) Spectra of NO sorbed at 170 K in zeolite Cu-ZSM-5 freshly activated (a) and zeolite in which formate ions were formed by the reaction of formaldehyde and next decomposed by the evacuation at 570 K (b). (B) Spectra of formate ions formed upon the sorption of formaldehyde and subsequent heating at 370 K in zeolite freshly activated (a) and zeolite in which formate ions were formed and subsequently decomposed (b).

Figure 7. Spectra of CO sorbed at room temperature (A) and NO sorbed at 170 K (B), spectra in the “transmission window” (C), and spectra of formate ions formed by the reaction of formaldehyde (D) in zeolite Cu-ZSM-5 freshly activated (a), oxidized by oxygen (b), and zeolite in which formate ions were formed and next decomposed (c).

the Cu2+-NO band at 1895 cm-1 (Figure 7A,B, spectra a,b), as well as by a shift of the 970 cm-1 band typical of an oxygen ring deformed by Cu+ to 930 cm-1 (typical of an oxygen ring deformed by Cu2+sFigure 7C). The spectra of formate ions created by the sorption of formaldehyde in Cu-ZSM-5 not oxidized (pretreated at vacuum) and zeolite oxidized by O2 are presented in Figure 7D. The intensities of formate ion bands is higher in zeolites Cu-ZSM-5 oxidized again illustrating the role Cu2+ plays in the formation of formate ions. The Cu2+ ions produced by the oxidation of Cu+ are reduced to Cu+ during the reaction with formaldehyde. The band of Cu+CO increases and that of Cu2+-NO decreases (Figure 7A,B, spectra c), and the spectrum in the transmission window (Figure 7C) becomes again typical of an oxygen ring deformed by Cu+. Therefore the oxidation of formaldehyde sorbed in zeolite CuZSM-5 is realized by the oxidation of Cu+ to Cu2+ which subsequently oxidizes formaldehyde. It may be supposed that

the mechanism of the oxidation of other organic molecules in Cu-containing zeolites, which are catalysts for numerous reactions of oxidation of organic molecules, is similar. Conclusions 1. IR studies showed that the interaction of formaldehyde with Cu+ ions in zeolite Cu-ZSM-5 results in an activation and weakening of the CdO bond. The CdO band shifts to lower frequency by 56 cm-1. The activation of the CdO bond results from the π back-donation of d electrons of Cu+ to π* orbitals of formaldehyde. 2. The experiments of coadsorption of formaldehyde and CO on the same Cu+ proved that formaldehyde acts as an electron donor to the Cu* + CO system, which results in stronger π back-donation to antibonding orbitals of CO. 3. Formaldehyde molecules sorbed in zeolite Cu-ZSM-5 are oxidized to formate ions (IR bands at 1363, 1371, and 1578

Formaldehyde Molecules in Cu-ZSM-5 Zeolites cm-1). IR experiments showed that Cu2+ ions which survived the self-reduction to Cu+ during the pretreatment at vacuum may be oxidant for formaldehyde oxidation. Such Cu2+ ions are reduced to Cu+ during the reaction with formaldehyde. Acknowledgment. This study was sponsored by the Polish Ministry of Scientific Research and Informational Technology (Grant No. 3 T 09A 006 27). References and Notes (1) Iwamoto, M.; Furokawa, H.; Mine, Y.; Uemura, F.; Mikuriya, S.; Kagawa, S. J. Chem. Soc., Chem. Commun. 1986, 1272. (2) Iwamoto, M.; Yakoo, S.; Sakai, K.; Kagawa, S. J. J. Chem. Soc., Faraday Trans. 1981, 77, 1629. (3) Brocławik, E.; Datka, J.; Gil, B.; Piskorz, W.; Kozyra, P. Top. Catal. 2000, 11/12, 335. (4) Brocławik, E.; Datka, J.; Gil, B.; Kozyra, P. In Studies in Surface Science and Cataysis; Aiello, R., Giordano, G., Testa, F., Eds.; Elsevier: Oxford, U.K., 2002; Vol. 142, p 1971. (5) Datka, J.; Kukulska-Zaja¸ c, E.; Kozyra, P. Catal. Today 2004, 109, 90. (6) Goursot, A.; Coq, B.; Fajula, F. J. Catal. 2003, 216, 324. (7) Berthomieu, D.; Jardiller, N.; Delahay, G.; Coq, B.; Goursot, A. Catal. Today 2005, 110, 294. (8) Velu, S.; Wang, L.; Okazaki, M.; Tomura, S. Microporous Mesoporous Mater. 2002, 54, 113. (9) Kumar, N.; Nieminen, V.; Lindfors, L. E.; Salmi, T.; Murzin, D. Y.; Laine, E.; Heikkila¨, T. Catal. Lett. 2002, 78, 105. (10) Nieminen, V.; Kumar, N.; Pa¨iva¨rinta, J.; Datka, J.; Hotokka, M.; Laine, E.; Salmi, T.; Murzin, D. Y. Microporous Mesoporous Mater. 2003, 60, 159. (11) Arnold, U.; da Cruz, R. S.; Mandelli, D.; Schuchardt, U. In Studies in Surface Science and Catalysis; Galarneau, A., Di Renzo, F., Fajula, F., Vedrine, J., Eds.; Elsevier: Oxford, U.K., 2001; Vol. 135, p 27P06. (12) Batista, M. S.; Urquieta-Gonzales, E. A. In Studies in Surface Science and Catalysis; Galarneau, A., Di Renzo, F., Fajula, F., Vedrine, J., Eds.; Elsevier: Oxford, U.K., 2001; Vol. 135, p 27P12. (13) Antunes, A. P.; Silva, J. M.; Ribeiro, M. F.; Ribeiro, F. R.; Magnoux, P.; Guisnet, M. In Studies in Surface Science Catalysis;

J. Phys. Chem. C, Vol. 111, No. 8, 2007 3475 Galarneau, A., Di Renzo, F., Fajula, F., Vedrine, J., Eds.; Elsevier: Oxford, U.K., 2001; Vol. 135, p 30P23. (14) Kubota, T.; Kumada, F.; Tominaga, H. Int. Chem. Eng. 1973, 13, 539. (15) Arai, H.; Yamashiro, T.; Kubo, T.; Tominaga, H. Bull. Jpn. Pet. Inst. 1976, 18, 39. (16) Espeel, P. H.; de Peuter, G.; Tielen, M. C.; Jacobs, P. A. J. Phys. Chem. 1994, 98, 11588. (17) Minachev, Kh.; Kharlamov, V. V. Acta Phys. Chem. 1985, 31, 497. (18) Mochida, I.; Hayata, S.; Kato, A.; Seiyama, T. T. J. Catal. 1970, 19, 405. (19) Piffer, R.; Fo¨rster, H.; Niemann, W. Catal. Today 1991, 9, 491. (20) Datka, J.; Kukulska-Zaja¸ c, E. J. Phys. Chem. B 2004, 108, 11760. (21) Datka, J.; Kukulska-Zaja¸ c, E.; Kobyzewa, W. Catal. Today 2005, 101, 123. (22) Datka, J.; Kozyra, P.; Kukulska-Zaja¸ c, E.; Kobyzewa, W. Catal. Today 2005, 101, 117. (23) Kukulska-Zajac, E.; Kozyra, P.; Datka, J. Appl. Catal. 2006, 307, 46. (24) Broclawik, E.; Rejmak, P.; Kozyra, P.; Datka, J. Catal. Today 2006, 114, 162. (25) Gil, B.; Datka, J.; Witkowski, S.; Sojka, Z.; Broclawik, E. In Studies in Surface Science and Catalysis; Corma, A., et al., Eds.; Elsevier: Oxford, U.K., 2000; Vol. 130, p 3249. (26) Beutel, T.; Sarkany, J.; Lei, G.-D.; Yan, J. Y.; Sachtler, W. M. H. J. Phys. Chem. 1996, 845. (27) Kuroda, Y.; Kumashiro, R.; Nagao, M. Appl. Surf. Sci. 2002, 96, 408. (28) Brocławik, E.; Datka, J.; Gil, B. In Studies in Surface Science Catalysis; Kiricsi, I., Pal-Borbely, G., Nagy, J. B., Karge, H. G., Eds.; Elsevier: Oxford, U.K., 1999; Vol. 125, p 603. (29) Kozyra, P.; Datka, J. Unpublished results. (30) Datka, J.; Kukulska-Zaja¸ c, E.; Kobyzewa, W. Catal. Today 2006, 114, 169. (31) Solymosi, F.; Kno¨zinger, H. J. Catal. 1990, 122, 166. (32) Kantchewa, M.; Albano, E. V.; Ertl, G.; Kno¨zinger, H. Appl. Catal. 1983, 8, 71. (33) Miller, G. J.; Rochester, C. H.; Waught, K. C. J. Catal. 1995, 155, 52.