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Cupric Ion Location and Adsorbate Interactions in Cu(II) Exchanged Erionite-like SAPO-17 Molecular Sieve A. M. Prakash and Larry Kevan* Department of Chemistry, University of Houston, Houston, Texas 77204-5641 Received April 18, 1997. In Final Form: July 25, 1997X Erionite-like silicoaluminophosphate molecular sieve SAPO-17 has been synthesized using cyclohexylamine as the organic structure directing molecule. The location of Cu(II) ions exchanged into H-SAPO-17 and the interaction of Cu(II) ions with water, ammonia, methanol, and ethylene have been investigated by electron spin resonance (ESR) and electron spin echo modulation (ESEM) techniques. Simulation parameters of the 31P ESEM spectrum suggest that the hydrated Cu(II) complex, CuII(H2O)6, is located in the erionite supercage. During dehydration some of the bare Cu(II) ions migrate into the smaller cancrinite cage. Room temperature adsorption of D2O on CuH-SAPO-17 after dehydration at 723 K and subsequent O2 treatment and evacuation at the same temperature (activation) forms a Cu-aquo complex with axially symmetric ESR parameters (g|| ) 2.391 and A|| ) 0.0142 cm-1). This complex is suggested to be CuII(D2O)3 based on 2D ESEM data. Adsorption of ND3 on activated CuH-SAPO-17 produces CuII(ND3)3 with axially symmetric ESR parameters (g|| ) 2.342 and A|| ) 0.0184 cm-1). When CD3OH is adsorbed on CuH-SAPO-17, the complex formed with ESR parameters g|| ) 2.327 and A|| ) 0.0125 cm-1 is found to contain two methanol molecules coordinating directly with the metal ion. Adsorption of C2D4 on activated CuH-SAPO-17 gives an ESR signal characterized by orthorhombic g-values and the complex is suggested to be CuII(C2D4)1 based on ESEM data.
Introduction Zeolites are among the most important catalysts for a wide variety of chemical reactions. Transition metal exchanged zeolites can serve as bifunctional catalysts in a variety of chemical reactions such as hydroisomerization and hydrocracking, hydrocarbon conversion, aromatization of alkenes, and CO hydrogenation.1 In particular, Cu-exchanged zeolites are well-known in oxidation reactions such as propylene to acrolein (CH2dCHCHO).2 Zeolite-supported Cu(II) catalysts have been found to be highly efficient in NOx decomposition to meet the environmental demand to reduce NOx in exhaust emissions and in power plant stack gases.3 Since the discovery of aluminophosphate-based molecular sieves in the early 1980s, several reactions previously studied on zeolitebased catalysts have been investigated for this new class of materials.4 Aluminophosphates provide a new array of molecular sieve structures and compositions. While some of the structures are analogous to naturally occurring zeolites, most of them are novel. Cu-exchanged SAPO molecular sieves have been reported to be thermostable catalysts for selective reduction of NOx with hydrocarbons.5 Of the various catalysts tested including SAPO-5, SAPO11, SAPO-34, beta, USY, and ZSM-5, Cu-SAPO-34 was found to exhibit the highest activity for NO reduction at temperatures above 973 K. The silicoaluminophosphate SAPO-17 is a little studied molecular sieve although it is structurally analogous to Abstract published in Advance ACS Abstracts, September 15, 1997. X
(1) Sachtler, W. M. H.; Zhang, Z. Adv. Catal. 1993, 39, 129. (2) (a) Ben Taarit, Y.; Che, M. In Catalysis by Zeolites; Imelik, B., Naccache, C., Ben Taarit, Y., Vedrine, C., Couduvier, G., Praliaud, H., Eds.; Studies in Surface Science and Catalysis; Elsevier: Amsterdam, 1980; Vol. 5, pp 167-193. (b)Mochida, I.; Hayata, S.; Kato, A.; Seiyama, T. J. Catal. 1971, 23, 31. (3) Li, Y.; Hall K. J. Catal. 1991, 129, 202. (4) Rabo, J. A.; Pellet, R. J.; Coughlin, P. K.; Shamshoun, E. S. In Zeolites as Catalysts, Sorbents and Detergent Builders: Applications and Innovations; Karge, H. G., Weitkamp, J., Eds.; Studies in Surface Science and Catalysis; Elsevier: Amsterdam, 1989; Vol. 46, p 1. (5) Ishihara, T.; Kagawa, M.; Hadama, F.; Takita, Y. In Zeolites and related Microporous Materials: State of the Art 1994; Weitkamp, J., Karge, H. G., Pfeifer, H., Holderich, W., Eds.; Studies in Surface Science and Catalysis; Elsevier: Amsterdam, 1994; Vol. 84, pp 1493-1500.
S0743-7463(97)00402-2 CCC: $14.00
the commercially important zeolite, erionite. The aluminosilicate erionite is a useful catalyst for shape selective hydrocarbon cracking and also for selective production of lower olefins from methanol.6 SAPO-17 has also been shown to exhibit interesting adsorptive and catalytic properties.7 The behavior of transition metals in SAPO17 has not been reported previously. Until now, only a few studies have dealt with SAPO-17 mainly because of the difficulty of preparing this structure type in pure form. SAPO-17 is found to crystallize in the presence of both quinuclidine and cyclohexylamine.8,9 Whereas SAPO-35 (LEV) is found to be a common impurity in the synthesis with quinuclidine, SAPO-5 (AFI), SAPO-34 (CHA), and SAPO-44 (CHA) are found to crystallize along with SAPO17 from a gel containing cyclohexylamine. Recently, the synthesis and mechanism of silicon substitution in SAPO17 crystallized in both the absence and the presence of HF have been studied.8,10 Since the catalytic properties of cation-exchanged zeolites and other molecular sieves are strongly dependent on the nature and location of ions, its accessibility, and coordination with ligands,11 it is important to elucidate the location of the ions within the molecular sieve structure and the number and orientation of the molecules coordinated to them. Electron spin resonance (ESR) and electron spin echo modulation (ESEM) spectroscopies have been effectively used to probe Cu(II) ion location and its (6) Kalies, H.; Roessner, F.; Karge, H. G.; Steinberg, K.-H. In Zeolite Chemistry and Catalysis; Jacobs, P. A., Jaeger, N. I., Kubelkova, L., Wichterlova, B., Eds.; Studies in Surface Science Catalysis; Elsevier: Amsterdam, 1991; Vol. 69, pp 425-434. (7) (a) Richter, M.; Roost, U.; Lohse, U. J. Chem. Soc., Chem. Commun. 1993, 1616. (b) Richter, M.; Ehrhardt, K.; Roost, U.; Kosslick, H.; Parlitz, B. In Zeolites and Related Microporous Materials: State of the Art 1994; Weitkamp, J., Karge, H. G., Pfeifer, H., Holderich, W., Eds.; Studies in Surface Science and Catalysis ; Elsevier: Amsterdam, 1994; Vol.. 84, pp 1285-1292. (c) Chen, J.; Wright, P. A.; Natarajan, S.; Thomas, J. M. Ibid. pp 1731-1737. (8) Lohse, U.; Loffler, E.; Kosche, K.; Janchen, J.; Parlitz, B. Zeolites 1993, 13, 549. (9) Lok, B. M.; Messina, C. A.; Patton, R. L.; Gajek, R. T.; Cannan, T. R.; Flanigen E. M. U.S. Patent 4 440 871, 1984. (10) Lohse, U.; Jancke, K.; Loffler, E.; Schaller, T. Cryst. Res. Technol. 1994, 29, 237. (11) Smith J. V. In Zeolite Chemistry and Catalysis; Rabo, J. A., Ed.; American Chemical Society: Washington, DC, 1976; Chapter 1.
© 1997 American Chemical Society
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coordination geometry with adsorbate molecules in zeolites and other molecular sieves.12,13 While ESR can be used to deduce the local symmetry of the transition metal ions, analysis of the ESEM signals yields the number of surrounding interacting nuclei and their interaction distance. In transition metal modified SAPO molecular sieves, analysis of the ESEM signal due to 31P nuclei from the framework often yields direct information about the location of the metal ion. In the present paper we describe the synthesis of SAPO-17 and the results of our ESR/ ESEM study on the location and coordination geometry of Cu(II) exchanged into SAPO-17 molecular sieves. Experimental Section Preparation. SAPO-17 was prepared hydrothermally using cyclohexylamine as the organic template. A sample of AlPO4-17 was also prepared following the procedure reported elsewhere.14 The following chemicals were used without further purification: orthophosphoric acid (85%, Mallinckrodt), aluminum isopropoxide (Aldrich, 98%), fumed silica (Sigma), and cyclohexylamine (99%, Aldrich). Syntheses were carried out in 100 cm3 stainless steel reactors lined with poly(tetrafluoroethylene) at autogenous pressure without agitation. On the basis of preliminary experiments, the following molar composition was optimized for the preparation of SAPO-17: 1.0 Al2O3: 1.0 P2O5: 0.1 SiO2: 1.0 CHA: 61 H2O. In a typical synthesis, 40.85 g of aluminum isopropoxide was mixed with 80 g of H2O while stirring. Phosphoric acid (23.05 g) was added dropwise to this slurry followed by 10 g of water. This was followed by the addition 0.6 g of fumed silica and 10 g of H2O. After homogenizing the gel, 9.9 g of cyclohexylamine was added slowly to the gel followed by 5 g of H2O. About 200 mg of AlPO4-17 mixed with 5 g of H2O was added to the gel as seed crystals. The final gel was thoroughly stirred before charging into the autoclaves. The pH of the gel at this stage was 6.2. Crystallization was done at 473 K for 42 h. After crystallization the product was separated from the mother liquor, washed with water, and dried at 373 K overnight. H-SAPO-17 was prepared by heating as-synthesized material slowly to 823 K in O2 and maintaining at this temperature for 16 h for removal of the organic matter. CuH-SAPO-17, where Cu(II) ions exist in extra framework position in the structure of SAPO-17, was prepared by stirring 1 g of H-SAPO-17 with 20 mL of 1 mM CuCl2 and 80 mL of H2O at 330 K for 24 h. The exchanged material was washed several times with water and dried at 330 K overnight. Sample Treatment and Measurements. Powder X-ray diffraction (XRD) patterns were recorded on a Siemens D5000 X-ray diffractometer using Cu KR radiation. Chemical analysis of the samples was carried out by electron microprobe analysis on a Jeol JXA-8600 spectrometer. For ESR and ESEM measurements, the exchanged samples were loaded into 3 mm o.d. by 2 mm i.d. Suprasil quartz tubes and evacuated to a final pressure of 10-4 Torr at 295 K overnight. To study the behavior of the copper as a function of hydration, the samples were heated under vacuum from 295 to 673 K at regular intervals. For each interval, the temperature was raised slowly and held at that temperature for 16 h. Then ESR spectra were measured at 77 K to study the change in ESR behavior by this thermal treatment. To study the reduction behavior of CuH-SAPO-17, the samples were dehydrated at 673 K and then contacted with 1 atm of O2 at 673 K for 16 h followed by evacuation at the same temperature (activation). The activated samples were contacted with 100 Torr of dry hydrogen at various temperatures for varying duration before ESR measurements. In order to prepare Cu(II) complexes with various adsorbates the activated samples were exposed to the room temperature vapor pressure of D2O (Aldrich Chemical), CD3OH (Stohler Isotope Chemicals), 150 Torr C2D4 (Cambridge Isotope Laboratories), 100 Torr 15NH3, and 90 Torr ND3 (Stohler Isotope Chemicals). These samples with adsorbates were sealed (12) Ichikawa, T.; Kevan L. J. Am. Chem. Soc. 1981, 103, 5355. (13) Lee, C. W.; Chen, X.; Kevan L. J. Phys. Chem. 1992, 96, 357. (14) Wilson, S. T.; Lok, B. M.; Messina, C. A.; Cannan, T. R.; Flanigen, E. M. In Intrazeolite Chemistry; Stucky, G. D., Dwyer, F. G., Eds.; ACS Symp. Ser. 218; American Chemical Society: Washington, DC, 1983; pp 79-106.
Figure 1. X-ray powder diffraction pattern for as-synthesized SAPO-17. Inset shows the structural model for as-synthesized SAPO-17. and kept at room temperature for 24 h before ESR and ESEM measurements. ESR spectra were recorded with a Bruker ESP-300 X-band spectrometer at 77 K. The magnetic field was calibrated with a Varian E-500 gaussmeter. The microwave frequency was measured by a Hewlett- Packard HP 5342A frequency counter. ESEM spectra were measured at 4.5 K with a Bruker ESP 380 pulsed ESR spectrometer. Three pulse echoes were measured by using a π/2-τ-π/2-T-π/2 pulse sequence as a function of time T to obtain the time domain spectrum. To minimize 27Al, 29Si, and 1H modulations in measurements of phosphorus and deuterium modulations, the τ value was selected accordingly depending on the magnetic field position. The phosphorus and deuterium modulations were analyzed by a spherical approximation, which assumes a random distribution of nuclei around a paramagnetic center, for powder samples in terms of N nuclei at distance R with an isotopic hyperfine coupling Aiso.15 The best fit simulation of an ESEM signal is found by varying the parameters until the sum of the squared residuals is minimized. The validity of the spherical approximation for powder samples has been well documented.15b The parameters are typically determined to the nearest integral N, ( 0.1 Å for R and ( 10% for Aiso. It has been found in many studies that the molecular dipole of polar adsorbates points toward a metal ion in an ion-exchange position. This usually enables one to determine the number of equivalently coordinated partially deuterated molecules from the number of nearest neighbor deuteriums measured. Also, olefins adapt a π-bonded configuration with such a metal ion, so again the number of coordinated molecules can be determined.
Results Samples were characterized by XRD. Figure 1 shows the powder XRD pattern of SAPO-17 in its as-synthesized form. This pattern, both in intensity and in line position, matches well with the patterns reported for structure type 17.8 Unlike a previous report,9 lines due to additonal phases were not observed. The structure has a hexagonal unit cell with lattice parameters a ) 13.23 Å, b ) 13.23 Å, c ) 14.77 Å, and γ ) 120°. Practically no loss in crystallinity was observed when the as-synthesized samples were heated at 823 K for 16 h to remove the organic template. The chemical composition of CuHSAPO-17 was estimated by electron probe microanalysis. The idealized chemical composition of this sample is Cu0.003H0.017(Si0.02Al0.50P0.48)O2. This corresponds to 7 Si and 1 Cu ions per 10 unit cells. (15) (a) Kevan, L. In Time Domain Electron Spin Resonance; Kevan, L., Schwartz, R. N., Eds.; Wiley: New York, 1979; Chapter 8. (b)Anderson, M. W.; Kevan, L. J. Chem. Phys. 1987, 87, 1.
Cupric Ion Location and Adsorbate Interactions
Figure 2. ESR spectra at 77 K of CuH-SAPO-17: (a) fresh; (b) after evacuation at 295 K for 16 h; (c) after dehydration at 373 K for 16 h; (d) after dehydration at 473 K for 16 h; and (e) after dehydration at 673 K for 16 h.
The observed framework chemical composition of SAPO17 suggests isolated Si atom substitution for P. In SAPO17 samples of low silicon content, isolated silicon substitution for P has been confirmed independently by 29Si MAS NMR.8 The ESR spectrum of hydrated CuH-SAPO-17 is shown in Figure 2a. The spectral parameters are g|| ) 2.397 , A|| ) 0.0142 cm-1 and g⊥ ) 2.074 and correspond to hexaaquocopper(II) complex.16 This sample was then evacuated at 395 K for 16 h. Evacuation at room temperature starts removing water ligands, and the corresponding ESR spectrum appears to show two Cu(II) species (Figure 2b). Upon dehydration of the sample at higher temperatures, two Cu(II) species A1 and A2 are visible in the corresponding spectra (Figure 2c). The g parameters of species A1 and A2 are g|| ) 2.360, A|| ) 0.0156 cm-1 and g|| ) 2.323, A|| ) 0.0173 cm-1, respectively. When the sample is dehydrated at higher temperatures, the intensity of the spectrum reduces substantially, especially at temperatures above 373 K, as can be seen from parts d and e of Figure 2. Dehydration at 673 K produces a new Cu(II) species B also. In Cu(II)-exchanged mordenite, it has been reported that dehydration at temperatures above 573 K causes a reduction by half of the ESR signal due to Cu(II).17 This loss in intensity of Cu(II) signal was attributed to reduction of Cu(II) to Cu(I) by residual water with water decomposition. The following reaction mechanism has been proposed for the reduction of Cu(II) to Cu(I) by residual water where M represents the molecular (16) Herman, R. G. Inorg. Chem. 1979, 18, 995. (17) Kasai, P. H.; Bishop, R. J., Jr. J. Phys. Chem. 1977, 81, 1527.
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Figure 3. ESR spectra at 77 K of CuH-SAPO-17: (a) after O2 treatment of a dehydrated sample at 673 K for 16 h and subsequent evacuation; (b) after H2 treatment of an activated sample at 295 K for 16 h; (c) after H2 treatment of an activated sample at 673 K for 16 h; and (d) after D2O adsorption on an activated sample at 295 K.
sieve.17 -(n - 2)H2O
[2Cu(II), nH2O]M 9 8 [2Cu(II), 2H+, 2OH-] M T > 573 K f [2Cu(I), 2H+]M + H2Ov + (1/2)O2v The decomposition of water has been observed in other transition metal ion exchanged zeolites also. Further reduction of Cu(I) to Cu(0) is not probable. Such reduction would generate metallic Cu(0)n clusters which are expected to produce a several hundred gauss broad ESR line as we have observed for metallic nickel clusters, but no such broad line is observed. Also, a dehydrated sample regains its original Cu(II) ESR intensity upon room temperature exposure to H2O or D2O consistent with oxidation of Cu(I) to Cu(II) taking place. When a sample of CuH-SAPO-17, previously dehydrated at 673 K, is treated with O2 at 673 K, an ESR spectrum is observed similar to that after dehydration at 373 K (Figure 3a). The effect of dehydration at 373 K under evacuation is primarily to remove water ligands from CuII(H2O)6 to form dehydrated Cu(II) ions. Dehydration at higher temperature reduces some of the Cu(II) ions to Cu(I). Further O2 treatment oxidizes Cu(I) to Cu(II). Therefore dehydration of CuH-SAPO-17 at 373 K (Figure 2c) and O2 treatment of a sample which has been dehydrated at 673 K (Figure 3a) generate the same final state with similar ESR spectra. The reduction behavior of CuH-SAPO-17 in the presence of H2 was studied. An ESR spectrum similar to that
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Prakash and Kevan Table 1. ESR Parameters at 77 K for Cu(II) Ions in CuH-SAPO-17 after Various Treatments treatment fresh dehydrated H2 /673 K D2O ND3 C2D4 CD3OH a
Figure 4. ESR spectra at 77 K of CuH-SAPO-17: (a) ND3 adsorption on an activated sample at 295 K; (b) 15NH adsorption on an activated sample at 295 K; (c) 3 C2D4 adsorption on an activated sample at 295 K; (d) CD3OH adsorption on an activated sample at 295 K.
after after after after
observed on an activated sample was observed when the activated sample was treated with hydrogen at 295 K for 16 h (Figure 3b). However, when a hydrogen-treated sample is heated to higher temperatures, the intensity of the signal reduces and at 673 K the spectrum seems to consist of a single Cu(II) species C. The loss in intensity is attributed to the reduction of some of the Cu(II) ions to Cu(I) in the presence of hydrogen. The ESR parameters of Cu(II) species C are g|| ) 2.380 and A|| ) 0.0150 cm -1, which are different from those of Cu(II) species A1 and A2. When D2O is adsorbed on an activated sample, an ESR signal similar to that observed on a hydrated sample is obtained (Figure 3d). The ESR parameters of Cu(II) species G observed after D2O adsorption are g|| ) 2.391 and A|| ) 0.0142 cm -1. ESR spectra recorded for CuHSAPO-17 after adsorbing ND3, 15NH3, C2D4, and CD3OH are given in Figure 4. When ND3 is adsorbed at 295 K on an activated sample of CuH-SAPO-17, a new Cu(II) species D with axially symmetric g characteristics (g|| ) 2.342 and A|| ) 0.0184 cm-1) is observed. Adsorption of 15NH (15N has a nuclear spin of 1/2) produces a single 3 species H with g|| ) 2.253 and A|| ) 0.0181 cm-1 as shown in Figure 4b. The spectrum also reveals four superhyperfine lines with intensity ratios of 1:3:3:1 and A⊥ ) 0.0018 cm-1 at g⊥ of species H. This suggests that three equivalent 15 N nuclei are directly coordinated to Cu(II). When deuterated ethylene is adsorbed on activated CuH-SAPO17, the major signal observed is due to a new species E with orthorhombic g characteristics (g1 ) 2.395, g2 ) 2.156, and g3 ) 2.042). Cu(II) species E is superimposed on
species A A1 A2 C G D E F
assignment CuII(H2O)6 Cu(II) Cu(II) CuII(H2)n CuII(D2O)3 CuII(ND3)3 CuII(C2D4)1 CuII(CD3OH)2
g||
A|| (cm-1)
g⊥
2.397 2.360 2.323 2.380 2.391 2.342 2.395 2.327
0.0142 0.0156 0.0173 0.0150 0.0142 0.0184 0.0041 0.0125
2.074 a a a 2.074 2.136 2.156, 2.042 2.000
Not resolved.
Cu(II) species A1 and A2 observed in dehydrated samples. Species E was distinguished from the other species by the time dependent variation of the ESR intensity of the various species present. Immediately after the adsorption of ethylene onto CuH-SAPO-17, the intensity of species E is considerably lower than for the other species. However, after 24 h, the intensity of species E increases significantly with a concomitant decrease in the intensity of species A1 and A2. Thereafter no noticeable change in the ESR intensities of these species is observed after annealing at room temperature for several days. This suggests that a fraction of the Cu(II) ions are situated at sites inaccessible to the ethylene molecules. Cations locating at sites within the cancrinite cage are not accessible to ethylene molecules as the kinetic diameter of ethylene is larger than the six-ring window diameter. The Cu(II)-ethylene interaction does not seem sufficient to cause Cu(II) to migrate from the cancrinite cage to the supercage. The rhombic character of species E is unusual. In other Cu-exchanged SAPO materials, ethylene adsorption normally yields a species with axial g parameters.13 When CD3OH is adsorbed on an activated sample of CuHSAPO-17, a new Cu(II) species F with axial g parameters (g|| ) 2.327, A|| ) 0.0125 cm-1, and g⊥ ) 2.000) is observed. The ESR parameters of the various Cu(II) species investigated in this study are summarized in Table 1. Three-pulse ESEM spectra were recorded at the magnetic field corresponding to g⊥ of various Cu(II) species. 31 P (I ) 1/2, ν ) 6.038 MHz at 3500 G, 100% abundance), 1H (I ) 1/ , ν ) 14.902, 99.98%), and 2D (I ) 1,ν ) 2.288 2 MHz, 99.8%) nuclei were investigated by spin-echo modulation. For a particular nucleus, the delay between the first and second pulses (τ) was selected so as to minimize modulation from other magnetic nuclei present in the system.15 Figure 5 shows the ESEM spectrum of hydrated CuH-SAPO-17 from framework 31P nuclei at a magnetic field corresponding to g⊥ of Cu(II) species A (Figure 2). Simulation of the spectrum shows three nearest neighbor phosphorus atoms at a distance of 4.7 Å . This sample shows strong 1H modulation consistent with the hydrated Cu(II) ions present. The 31P modulation observed for a dehydrated sample is somewhat similar to that recorded for a hydrated sample (Figure 6). Since the spectrum contains contributions from Cu(II) species A1 and A2, a reasonable simulation was not successful. As found previously, the decay of the echo intensity in a dehydrated sample is slower than that in a hydrated sample. In Figure 7 is given the experimental and simulated 2D ESEM spectra of CuH-SAPO-17 after adsorbing D2O. The magnetic field was set at g⊥ of Cu(II) species G (Figure 3e). Simulation of the spectrum gives six deuterium nuclei at a distance of 2.8 Å and another six nuclei at a distance of 3.8 Å. These values are consistent with three D2O molecules coordinated directly with Cu(II) and another three molecules interacting from a farther distance. Figure 8 shows the experimental and simulated 2D ESEM
Cupric Ion Location and Adsorbate Interactions
Figure 5. Experimental (;) and simulated (‚‚‚) three pulse 31P ESEM spectra of hydrated CuH-SAPO-17 at magnetic fields corresponding to g⊥ of Cu(II) species A.
Figure 6. Experimental three pulse 31P ESEM spectrum of dehydrated CuH-SAPO-17 at the magnetic fields corresponding to g⊥ of Cu(II) species A1 and A2.
Figure 7. Experimental (;) and simulated (‚‚‚) three pulse 2 D ESEM spectra of CuH-SAPO-17 after D O adsorption. The 2 spectrum was recorded at the magnetic field corresponding to g⊥ of Cu(II) species G.
spectra of CuH-SAPO-17 after ND3 adsorption. The magnetic field was set at g⊥ of Cu(II) species D (Figure 4a). Simulation of the spectrum gives nine deuteriums at a distance of 2.9 Å. These parameters are consistent with three ND3 molecules directly coordinated with the Cu(II) ions. In Figure 9 is given the experimental and
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Figure 8. Experimental (;) and simulated (‚‚‚) three-pulse 2 D ESEM spectra of CuH-SAPO-17 after ND3 adsorption. The spectrum was recorded at the magnetic field corresponding to g⊥ of Cu(II) species D.
Figure 9. Experimental (;) and simulated (‚‚‚) three-pulse 2D ESEM spectra of CuH-SAPO-17 after C D adsorption. The 2 4 spectrum was recorded at the magnetic field corresponding to g3 of Cu(II) species E.
simulated 2D ESEM spectra observed for CuH-SAPO-17 after adsorbing C2D4. The field was set at g3 ) 2.042 of Cu(II) species E (Figure 4b). The species due to bare Cu(II) also has its g⊥ at this field. It should be noted that this species is that fraction of bare Cu(II) ions not having direct coordination with C2D4. Some contribution of this species to the deuterium modulation is possible. Simulation parameters indicate one molecule of ethylene coordinating with Cu(II) species E. Figure 10 shows the experimental and simulated 2D ESEM spectra observed for CuH-SAPO-17 after adsorbing CD3OH. The field was set at g⊥ of Cu(II) species F (Figure 4c). The spectrum is simulated by six deuteriums interacting at a distance of 3.7 Å and three deuteriums interacting at a distance of 4.7 Å. These parameters are interpreted in terms of two methanol molecules directly coordinated with the Cu(II) ions and a third methanol interacting more weakly at a farther distance. Discussion SAPO-17 Structure. SAPO-17 is a small pore molecular sieve with eight-membered ring pore openings. The structure of calcined SAPO-17 is analogous to the
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Figure 10. Experimental (;) and simulated (‚‚‚) three pulse 2 D ESEM spectra of CuH-SAPO-17 after CD3OH adsorption. The spectrum was recorded at the magnetic field corresponding to g⊥ of Cu(II) species F.
Figure 11. Structural model of calcined SAPO-17. Also shown are possible cation positions. See text for description of the cation positions.
natural zeolite erionite.18 A structural model of calcined SAPO-17 is given in Figure 11. The structure contains three dimensional channels perpedicular to the [001] direction. The structure possesses hexagonal symmetry with lattice parameters a ) 13.3 Å and c ) 15.3 Å. Polyhedral cages observed in SAPO-17 are double hexagonal prisms, cancrinite cages, and supercages (erionite cage). The erionite cages are quite large (6.3 × 13 Å) with eight-membered windows of dimension 3.6 × 5.1 Å. The structures of as-synthesized AlPO-17 and SAPO-17 are slightly different from that of erionite. As-synthesized SAPO-17 belongs to the subclass AlPO4 hydroxide with specific Al atoms from opposite six-ring windows of the (18) Pluth, J. J.; Smith, J. V.; Bennett, J. M. Acta Crystallogr. 1986, C42, 283.
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cancrinite cage linked through a hydroxyl bridge. Thus, both tetrahedral and trigonal bipyramidal Al sites are present in the framework. When calcined, the hydroxyl groups are removed and the structure attains the framework topology of zeolite erionite. When a calcined sample is exposed to moisture, some framework aluminum coordinates with additional water molecules and transforms that site into aluminum dihydrate. The chemical composition of CuH-SAPO-17 suggests that Si atoms substitute for framework phosphorus sites. Such a substitution of Si for P generates a net framework negative charge which is balanced by protonated templating species in the as-synthesized form and by H+ in the calcined form. During ion-exchange with Cu2+, some of these H+ ions are replaced by Cu(H2O)62+ ions. The water ligated Cu(II) ions are positioned in the vicinity of a Si ion within the large supercage. Due to size constraints offered by the six-ring window, it is unlikely that hydrated copper ions occupy positions within cancrinite cages or double hexagonal prisms. In calcined SAPO-17, several cation sites can be identified. On the basis of the structural model of SAPO17 and by analogy with the cation positions in the analogous zeolite erionite,19 possible cation sites in SAPO17 are shown in Figure 11. Site S I is the center of a double hexagonal prism. Site S II is the center of a sixring window joining a cancrinite cage and a double hexagonal prism. Site S II′ is site S II displaced toward the center of a cancrinite cage. Site S III is the center of a six-ring window joining a cancrinite cage and a supercage. Site S III′ and site S III* are site S III displaced toward the center of a cancrinite cage and a supercage, respectively. Site S IV is the center of a six-ring joining two supercages. Site S IV* is S IV displaced toward the center of a supercage. In natural zeolites, cations are found to occupy selected sites depending on the cation type.19 Na+ ions normally occupy site S IV , while K+ ions occupy site S II′. Smaller cations like Mg2+ and Ca2+ are found to occupy site S I. Hydrated Cu(II) Complexes. The ESR parameters of fresh CuH-SAPO-17 have been previously assigned to hexacoordinated CuII(H2O)62+.16 This assignment is supported by both 31P and 1H ESEM spectra observed for this sample. The 4.7 Å distance to the nearest neighbor P nuclei is too long for direct coordination between framework oxygen and the Cu(II) ions. Direct coordination between framework oxygen and the Cu(II) ions would mean a nearest neighbor distance of about 3.2 Å between the Cu(II) ions and framework phosphorus. It is likely that the Cu(II) complex occupies sites near Si ion during ion exchange. The framework chemical composition of SAPO-17 suggests that Si ions substitute for framework P sites. The 31P ESEM simulation parameters suggest that the aquoligated Cu(II) complex is situated near the center of an eight-ring window in which one of the P sites is occupied by Si. The complex may interact with the framework by hydrogen bonding between the water ligands and framework oxygen. Only one species A is observed in hydrated CuH-SAPO17. The presence of two species A1 and A2 with ESR parameter different from that of species A in dehydrated CuH-SAPO-17 indicates that some of the Cu ions lose their aquo ligands and migrate to new locations. Migration of metal ions from supercages and larger channels to smaller ones during dehydration is a common phenomena in zeolites and other molecular sieves.1 In low and (19) Mortier, W. J. Compilation of Extra Framework Sites in Zeolites; Structure Commission of the International Zeolite Association; Butterworth: Surrey, 1982; pp 17-18.
Cupric Ion Location and Adsorbate Interactions
intermediate silica zeolites, the high negative charge density in the small cavities and channels provides the main driving force for the migration of multivalent transition metal ions from larger channels or supercages to smaller ones. In zeolite X and Y, metal ions initially located within the large supercage migrate to a smaller sodalite cage during dehydration. Similarly in SAPO-5 and SAPO-11 channel-type molecular sieves, metal ions initially located in the large 12-ring or 10-ring channels migrate to smaller 6-ring channels during dehydration.20 In SAPO molecular sieves, the negative framework charge is generated by the substitution of Si for framework P sites. According to chemical analysis one unit cell of SAPO-17 contains a maximum of one Si ion substituting for a P site. The specific location of Si ions within a unit cell may influence the migration of Cu(II) ions during dehydration. In dehydrated CuH-SAPO-17, 31P ESEM was not effective in identifying the location of Cu(II) species because of the superposition of the two species A1 and A2. In a recent study on CuH-SAPO-18 the single Cu species observed after dehydration has similar ESR parameters to species A1 in CuH-SAPO-17. On the basis of ESEM results, this species was assigned to Cu(II) ions located within the pear-shaped cavity near a six-ring window coordinating with three framework oxygens.21 Structurally similar sites have been suggested for Cu(II) ions after dehydration in several other Cu-exchanged zeolites and SAPO molecular sieves. In SAPO-17, sites S III* and S IV* inside the supercage and sites S II′ and S III′ inside the cancrinite cage seem to be the most suitable sites for cation location. The effect of hydrogen at high temperature on an activated sample is 2-fold. The considerable loss in intensity of the ESR signal (Figure 3b) suggests that a fraction of the Cu(II) ions is reduced to Cu(I). Cu(II) species C observed after high temperature H2 treatment has different ESR parameters than either Cu(II) species A1 or A2 observed after activation. Thus, Cu(II) species C is assigned to a CuII-(H2)n complex. The effect of adsorbing D2O on an activated sample is to generate an ESR signal similar to that observed for an orginally hydrated sample. The parameters are characteristic of octahedral Cu(II).13 2D ESEM results show that there are three D2O molecules directly coordinated to Cu(II). A possible geometry of this complex is one in which the Cu(II) ions coordinate to three oxygens of D2O and three oxygens from the framework. Three more D2O molecules are interacting indirectly from the opposite side of the six-ring window. Four different Cu(II) species corresponding to different degrees of water coordination have been identified in zeolites and other molecular sieves.22 For example, in zeolite A distorted octahedral (CuII(Oz)3(D2O)3), trigonal bipyramidal CuII(Oz)3(D2O)2, and distorted tetrahedral (CuII(Oz)3(D2O)1) complexes have been observed, depending on the type of cocation present and on the pretreatment of the sample. Here Oz represents framework oxygen. In Cu(II)-exchanged X zeolite trigonal, trigonal bipyramidal, and octahedral Cu(II) species have been observed. In CuH-SAPO-34, which has the chabazite topology, Cu(II) ions directly coordinate with three water molecules forming a distorted octahedral complex.23 Cu(II)-Adsorbate Complexes. As in the case with adsorbed D2O only one Cu(II) species is observed with (20) Lee, C. W.; Chen, X.; Kevan, L. Catal. Lett. 1992, 15, 75. (21) Wasowicz, T.; Kim, S. J.; Hong, S. B.; Kevan, L. J. Phys. Chem. 1996, 100, 15954. (22) Kevan L.; Narayana, M. In Intrazeolite Chemistry; Stuckey, G. D., Dwyer, F. G., Eds.; ACS Symp. Ser. 218; American Chemical Society: Washington, DC, 1983; pp 283-299. (23) Zamadics, M.; Chen, X.; Kevan L. J. Phys. Chem. 1992, 96, 2652.
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adsorbed ND3 or 15NH3 on CuH-SAPO-17. On the basis of the ESEM results a possible coordination geometry of this complex is one in which Cu(II) coordinates with three ammonia ligands and three framework oxygens. More than one Cu-ammonia complex differing in the number of ammonia ligands to Cu(II) have been observed in molecular sieves. A square planar CuII(ND3)4 complex has been observed in CuH-Y, CuH-SAPO-18, and CuHSAPO-11.16,20,21 CuH-SAPO-17 coordinates three ammonia molecules as also found for CuH-SAPO-5, CuHSAPO-34, and CuH-SAPO-37.23,24 With adsorbed C2D4, CuH-SAPO-17 shows entirely different ESR spectra in comparison to other materials. In general, Cu-exchanged zeolites and SAPO materials show an axially symmetric ESR spectrum after ethylene adsorption.12,24 Contrary to more polar molecules, relatively nonpolar ethylene only coordinates to relatively exposed Cu(II) ions and part of the Cu(II) does not coordinate with ethylene. The kinetic diameter of ethylene (∼4 Å) is too large for it to enter the cancrinite cage for which the largest window is a six-ring with an effective diameter of ∼2.5 Å. This also supports that during dehydration a fraction of the Cu(II) ions migrate to the cancrinite cage and are then not available to coordinate with ethylene. Simulation parameters for the 2D ESEM pattern suggest that one ethylene molecule coordinates weakly with the Cu(II) ions. A π bond between ethylene and a metal ion in which the molecular plane of ethylene is oriented perpendicular to the line toward the metal ion is suggested for this complex.25 Unlike the case of ethylene, only one Cu(II) species is observed after methanol adsorption indicating that all of the Cu(II) is accessible to interact with methanol. The possible coordination geometry of Cu(II) species F after CD3OH adsorption is one in which the Cu(II) directly coordinates two methanol molecules and interacts with another methanol at a further distance. Due to its large kinetic size, methanol cannot enter into the cancrinite cage. However, because of its polar nature, methanol can pull Cu(II) ions from the cancrinite cage into the supercage to form complexes. The absence of Cu(II) species A1 and A2 in the ESR spectrum after methanol adsorption can be explained in this way. The number of methanols coordinating with Cu(II) again varies among molecular sieve structures. Similar geometries have been suggested for Cu-exchanged SAPO-5 and SAPO-11 materials.13,20 When methanol is adsorbed on CuNa-A and CuK-A, only one molecule coordinates with Cu ion.12 However, in CuNa-X and CuK-X, no direct coordination between Cu(II) and methanol is observed.26 This is because in dehydrated X zeolite, Cu(II) occupies a hexagonal prism site S I. Because of the high negative charge associated with the hexagonal prism, the electrostatic interaction between Cu(II) and methanol is not sufficient to pull Cu(II) ions into the supercage. On the other hand, in the analogous SAPO material, CuH-SAPO-37, where Cu(II) ions are located in the supercage, three molecules of methanol coordinate with the Cu(II) ions.24 It is clear that several factors including the structure types, the framework charge, the presence of cocations, and the pretreatment conditions determine the location of Cu(II) ions and their coordination geometry with adsorbates in molecular sieves. A direct comparison between two different structure types for the location and adsorbate interactions of a metal ion is significant for possible catalytic applications. Our 31P ESEM modulation (24) Zamadics, M.; Kevan, L. J. Phys. Chem. 1993, 97, 10102. (25) Kevan, L. Acc. Chem. Res. 1987, 20, 1. (26) Ichikawa, T.; Kevan, L. J. Am. Chem. Soc. 1993, 105, 402.
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clearly indicates that the specific location of Cu(II) ions in hydrated CuH-SAPO-17 is in the erionite supercage. Other SAPO materials do not contain a similar supercage. Also in SAPO-17, Cu(II) coordination to ethylene has rather different ESR parameters than it does in other SAPO structures such as SAPO-5, SAPO-11 and SAPO34. Thus, the behavior of a transition metal ion exchanged into a particular molecular sieve is somewhat specific to that material and justifies the studies of transition metal ions in a wide range of molecular sieve materials. Conclusions SAPO-17 molecular sieve has been synthesized with cyclohexylamine. Using ESR and ESEM spectroscopic methods the location and adsorbate interactions of Cu(II) ions exchanged into H-SAPO-17 has been investigated. In hydrated CuH-SAPO-17, Cu(II) ions exist as Cu(H2O)62+ and are located at sites inside the supercage. High temperature dehydration under vacuum reduces most of the Cu(II) ions to Cu(I). Dehydration, oxidation, and subsequent evacuation at 673 K (activation) creates two Cu(II) species due to the migration of some of the Cu(II) ions into the smaller cancrinite cage. Addition of hydrogen to an activated sample has no effect at room temperature,
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but at higher temperature a significant fraction of the Cu(II) reduces to Cu(I) while the remaining fraction appears to form a CuII(H2)n complex. Adsorption of D2O on activated CuH-SAPO-17 generates a CuII(Oz)3(D2O)3 complex based on ESEM results. The ESR parameters of this species is similar to the CuII(H2O)6 complex observed in freshly prepared hydrated material. Unlike other zeolite and SAPO materials, adsorption of ND3 on activated CuH-SAPO-17 generates a species suggested to be CuII(Oz)3(ND3)3 based on ESEM data. With methanol, activated CuH-SAPO-17 forms CuII(Oz)3(CD3OH)2. With adsorbed C2D4, activated CuH-SAPO-17 shows an ESR signal with rhombic g characteristics which is somewhat unusual and is assigned to CuII(Oz)3(C2D4)1. The fact that a fraction of the Cu(II) ions does not react with ethylene supports the idea that during activation some of the Cu(II) ions present in the large supercage migrate to sites which are inaccessible to relatively large molecules. Acknowledgment. This research was supported by the National Science Foundation and the Robert A. Welch Foundation. LA970402T
