J. Phys. Chem. B 1999, 103, 7277-7286
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Electron Spin Resonance and Electron Spin Echo Modulation Studies of Ion-Exchanged NiH-SAPO-17 and NiH-SAPO-35 Molecular Sieves: Comparison with Ion-Exchanged NiH-SAPO-34 Molecular Sieve Marie-Ange Djieugoue, A. M. Prakash, Zhidong Zhu, and Larry Kevan* Department of Chemistry, UniVersity of Houston, Houston, Texas 77204-5641 ReceiVed: May 24, 1999
Erionite-like silicoaluminophosphate molecular sieve SAPO-17 and levyne-like SAPO-35, in which Ni ions were incorporated via solid-state ion-exchange into known extraframework sites, have been studied by electron spin resonance (ESR) and electron spin echo modulation (ESEM). The Ni ion reducibility, location, and interaction with several adsorbates have been investigated. Among these adsorbates, the interaction with nitric oxide was emphasized and compared to that of Ni ion with NO in the previously studied chabazite-like SAPO-34. Room-temperature adsorption of C2D4 on NiH-SAPO-17 after dehydration at 573 K, oxygen treatment at 823 K, evacuation, and subsequent hydrogen treatment at 573 K produces two Ni-ethylene complexes. Carbon monoxide adsorption gives rise to a Ni(I)-(CO)n complex with unresolved 13C hyperfine lines. Following the kinetics of nitric oxide adsorption on NiH-SAPO-17 shows that initially, a Ni(I)(NO)+ complex, a NO radical, and a new species which appears to be another NO species are generated. After a reaction time of 24 h, NO2 is observed. As the adsorption time further increases, NO2 becomes stronger while Ni(I)-(NO)+ decays, and after 5 days only NO2 remains. NO adsorption on NiH-SAPO-35 shows different features. Initially, two Ni(I)-(NO)+ complexes along with a NO radical are seen. As the adsorption time increases, one of the Ni(I)-(NO)+ complexes decreases in intensity while the other one increases, and after a few days only one Ni(I)-(NO)+ complex remains. Simulation of the 31P ESEM spectrum, supplemented by 27Al modulation, suggests that, upon dehydration, Ni ions in NiH-SAPO-17 migrate from the erionite supercage to the smaller cancrinite cage. In dehydrated NiH-SAPO-34 and NiH-SAPO-35, Ni ions remain in the large chabazite and levyne cages, respectively. As a consequence, Ni(II) in NiH-SAPO-17 is less sensitive to reduction by hydrogen than it is in NiH-SAPO-34 and NiH-SAPO-35.
Introduction The catalytic properties of molecular sieve materials can be controlled by incorporation of transition metal ion species leading to specifically tailored catalytic applications of these materials.1 For instance, Ni(I) ions are active sites in catalytic reactions such as ethylene and propylene oligomerization as well as acetylene cyclomerization.2-4 However, application of transition metal ion modified microporous or mesoporous materials in catalytic reactions requires information about the formation of the active species and a detailed characterization of the metal ion environment. In the past, papers reporting studies dealing with the incorporation and adsorbate interaction of transition metals such as copper,5 palladium,6 or nickel7 in SAPO-34, a structural equivalent of zeolite chabazite, have been published. Only very little information may be found in the literature on the levyne-like molecular sieve SAPO-35. Similarly, though SAPO-17 is structurally analogous to the commercially important zeolite erionite, very few studies have been reported on this silicoaluminophosphate, the main reason being the difficulty of successfully synthesizing this material in pure form. SAPO17 is found to crystallize in the presence of both quinuclidine and cyclohexylamine. Whereas SAPO-35 is found to be a common impurity in the synthesis with quinuclidine, SAPO34 is found to crystallize along with SAPO-17 from a gel containing cyclohexylamine. A previous study reported that in the case of a high silicon content in the gel, SAPO-17 was obtained only with the addition of hydrofluoric acid, whereas
without HF, SAPO-34 crystallized.8 Such small pore molecular sieves have shown potential as catalysts in the form-selective cracking of hydrocarbons and for the selective production of lower olefins from methanol.9 Except for copper,10 the study of transition metals incorporated in SAPO-17 has not been reported so far. Several methods have been used to reduce Ni(II) to Ni(I) in microporous materials including thermal reduction or hydrogen reduction at 473-873 K, γ-irradiation at 77 K, and ultraviolet photoreduction in H2 or CO at 77 K. The first method seems to be the most efficient reduction procedure to produce isolated Ni(I), as long as sufficient care is taken to avoid the formation of metallic nickel clusters. Although irradiation techniques prevent nickel cluster formation, they often result either in small Ni(I) yields, as is the case for UV irradiation, or, in the case of γ-irradiation, yield a strong Ni(I) signal accompanied by a very intense signal around g ) 2 due to framework defects which may further disturb the study of nickel interaction with adsorbates. Electron spin resonance (ESR) and electron spin echo modulation (ESEM) spectroscopies have been effectively used to probe Ni ion behavior, location, and coordination geometry with adsorbate molecules in zeolites and other molecular sieves.11 While ESR can be used to deduce the local symmetry of the transition metals, analysis of the ESEM data yields the number of surrounding nuclei along with the interaction distance. In transition metal modified SAPO molecular sieves, analysis
10.1021/jp9916844 CCC: $18.00 © 1999 American Chemical Society Published on Web 08/10/1999
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of the ESEM signal due to 31P nuclei from the framework often yields direct information about the location of the metal ion. The present paper reports a study of Ni incorporated in SAPO17 through solid-state ion exchange. Comparison with data from SAPO-34 and SAPO-35 shows how the structural differences of these small pore materials affect the Ni ion location and the way it behaves toward reducing agents as well as adsorbates. Experimental Section Synthesis. SAPO-17 was synthesized as previously reported using cyclohexylamine as the organic template.10 SAPO-35 was made following a recipe reported earlier.12 The templating agent was hexamethylenimine. H-SAPO-17 and H-SAPO-35 were prepared by heating the as-synthesized materials slowly to 823 K in O2 and maintaining this temperature for 16 h for removal of the organic matter. NiH-SAPO-17 and NiH-SAPO-35, where Ni ions exist in extraframework positions in the structures of SAPO-17 and SAPO-35, were prepared by solid-state ion exchange with NiCl2‚ 6H2O and H-SAPO-17 or H-SAPO-35, respectively, at 823 K for 16 h. After ion exchange, the samples were cooled in ambient air, during which time they became hydrated. Sample Treatment and Measurements. Powder X-ray diffraction (XRD) patterns were recorded on a Siemens D5000 X-ray diffractometer using Cu KR radiation. Chemical analyses of the samples were carried out by electron microprobe analysis on a JEOL JXA-8600 Electron Beam Superprobe operated at a beam voltage of 15 kV and current of 30 nA. Si and O were calibrated with diopsite, CaMgSi2O6, Ni with metallic Ni, Al with anorthite, CaAl2Si2O8, and P with monazite. Prior to measurement, the samples were prepared as pressed pellets to make a dense material with a reasonably smooth surface. The electron beam was defocused to 10 µm diameter to minimize the damage caused to the specimen by heating. Data were collected from three to five randomly chosen regions and averaged to represent the bulk composition. No significant differences were seen between the regions, which indicates a uniform elemental composition over the sample. The precision was 2) reduces more easily than in X zeolite (Si/Al < 1.5) with the same Ni content.30 The structural features of the host zeolite have direct influence on the site location of the paramagnetic species and their coordination geometry with adsorbates. Because of the high tendency of Ni ions to migrate into small zeolite cages, nickel-exchanged Na-Y and Na-X zeolites require high temperature or extended reaction time for Ni(II) reduction.31 In faujasite type zeolites, aqueous solution ion exchange places hydrated Ni ions initially into the supercages. During dehydration, ligands are removed, leaving bare Ni(II) ions that migrate into smaller cages. Ni(II) ions are most stable in a hexagonal prism (site SI) where they can achieve octahedral coordination with framework oxygen ions. This preference of Ni(II) for site SI has been verified by XRD and diffuse reflectance spectroscopy for synthetic NiNa-Y and nickel-exchanged natural faujasite.32-34 For higher Ni loading, Ni occupation of site SI is limited by the number of such sites available and by its charge density which depends on the Si/Al ratio. It has been reported that in Na-Y with a Si/Al ratio of 2.4, out of 16 sites per unit cell a maximum of 12 SI sites are occupied by Ni(II) ions.35 The remaining ions occupy other sites. When previously dehydrated samples are treated with oxygen at sufficiently high temperature, any reduced Ni(I) is reoxidized to Ni(II) by reaction 2.
2Ni(I) + H2O + (1/2)O2 f 2Ni(II) + 2OH-
(2)
The reducibility of a nickel ion in a hexagonal prism is limited by thermodynamic and kinetic factors. Unlike other sites, access of H2 to a Ni(II) ion inside a hexagonal prism is expected to be a highly activated process. At elevated temperatures the sixring window of a hexagonal prism expands to some extent and allows sufficiently fast diffusion of hydrogen to reduce Ni(II) ions in hexagonal prisms. This is evident from the observation that NiCa-Y and NiCa-X require less stringent reduction conditions for producing Ni(I) in comparison to the corresponding NiNa-X and NiNa-Y zeolites.36,30 Ca2+ ion prefers site SI and blocks Ni(II) from entering a hexagonal prism. Other studies concerning the nature of Ni ions in SAPO materials such as SAPO-5, SAPO-11, and SAPO-4137-39,15,16 also suggest similar behavior for Ni(II) with respect to its location and reduction. In NiH-SAPO-5 and NiH-SAPO-11, thermal reduction around 573 K produces Ni(I) ions located near a six-ring window that constitutes a side of 12-ring or 10ring straight channels, respectively. Thermal reduction at 773
7284 J. Phys. Chem. B, Vol. 103, No. 34, 1999 K, however, produces Ni(I) ions at site SI at the center of a hexagonal prism that is part of a six-ring straight channel. Thus, at high temperatures, migration of nickel ions to a hexagonal prism from a main channel occurs. Our experimental results show that Ni(II) is more easily reduced to Ni(I) in NiH-SAPO34 and NiH-SAPO-35 than in NiH-SAPO-17. Since this is probably due to a difference in location of the Ni ions in the samples, we can rule out sites S3* and S4* as possible locations of Ni in NiH-SAPO-17. In fact our results suggest that in SAPO-34 and SAPO-35, after dehydration, the Ni ions are situated at sites inside the chabazite and levyne cages, respectively. A site located inside a supercage is just as accessible to reducing agents as sites inside the chabazite or the levyne cage. Therefore, if Ni were located at sites S3* or S4* in NiH-SAPO17, the reduction conditions should be very similar for all the samples. This is definitely not the case. On the other hand, sites S2′ or S3′ in NiH-SAPO-17 are much more difficult to reach than sites located inside the levyne and chabazite cages. That is why we suggest that the specific location of the Ni ions in SAPO-17 after dehydration is in the cancrinite cage. This difference in location also explains in part the significant ESR differences which are encountered after contacting the samples with some adsorbates. Adsorbate Effects. With adsorbed C2D4, NiH-SAPO-34 (or NiH-SAPO-35), and NiH-SAPO-17 show some similarities but also definite differences. Contrary to more polar molecules, relatively nonpolar ethylene only coordinates to relatively exposed Ni(I) ions. The size of the ethylene molecule (diameter ∼4 Å) should not hinder its diffusion into the ellipsoidal cavity of SAPO-34 (or SAPO-35) in which the Ni ions are located after dehydration. Our results indicate complete reaction of the Ni ions with the alkene and formation of two Ni-ethylene complexes likely located at two different sites within the SAPO34 (or SAPO-35) cage. This is understandable since all the Ni ions are available to complex with ethylene. In SAPO-17, two Ni-ethylene complexes can also be seen but their intensity is much weaker. Our 31P ESEM results indicate that, after dehydration, Ni is likely located in the cancrinite cage. The kinetic diameter of ethylene 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. Therefore, Ni(I) needs to be pulled out of the cancrinite cage in order to react with ethylene. This seems to be what indeed happens despite ethylene being a relatively nonpolar molecule. There seems to be a strong enough driving force to cause Ni(I) in SAPO-17 to migrate from the cancrinite cage to the supercage and interact with ethylene. As a result of this interaction, two Ni-ethylene complexes as well as a Ni-butene complex, a product of ethylene dimerization, are formed. Moreover, unlike in NiH-SAPO-34 and NiHSAPO-35, in NiH-SAPO-17 ethylene appears to further reduce Ni(I) to Ni(0), as evidenced by the broad baseline seen in Figure 3. This phenomenon has been reported in previous studies where it was observed that Ni(I) can be reduced by ethylene or butenes to form metallic nickel clusters, implying that the active nickel cations are unstable under the reaction conditions and are likely converted to atomic Ni which is catalytically inactive for ethylene dimerization.40 It is therefore reasonable to say that NiH-SAPO-34 and NiH-SAPO-35 should be better candidates than NiH-SAPO-17 for potential catalytic applications. Carbon monoxide adsorption on the samples produces a Ni(I)-(CO)3 complex in NiH-SAPO-35 and NiH-SAPO-34, as evidenced by resolved 13C hyperfine splittings. The nonresolved hyperfine lines in NiH-SAPO-17 probably indicate a weaker Ni-CO interaction. Previous studies have shown that, for
Djieugoue et al. adsorption of CO on NiCa-X, Ni(I) ions migrate from inaccessible sodalite cages to accessible supercages.41 In the supercages, Ni(I) can reversibly coordinate with one to three CO molecules to form Ni(I)-(CO), Ni(I)-(CO)2, and Ni(I)(CO)3 complexes. In NiH-SAPO-17, a similar trend may occur since the 31P ESEM data reveal that the location of the Ni(I)(CO)n complex is inside the supercage. Nitric oxide is an odd-electron molecule with its unpaired electron residing in the 2π antibonding orbital. As a result, its ionization potential is only 9.25 eV compared to 15.58 for N2 and 12.2 for O2. It thus readily ionizes to form the nitrosonium ion NO+, which has a full triple bond versus a bond order of 2.5 for NO. Nitric oxide is totally monomeric at room temperature, but significant amounts of dimer are formed in the gas phase at temperatures below about 150 K. Solid and liquid nitric oxide are essentially totally dimerized, but monomeric NO can still be observed at low temperatures in rare gas matrixes or in inert cryogenic solvents. Because NO has an odd electron, its electronic ground state is 2Π, which is split about 120 cm-1 by spin-orbit interaction into 2Π1/2 and 2Π3/2 components. As a result of this small energy separation, the gas-phase infrared is quite different from that of other diatomic molecules. Despite its unpaired electron, the molecule exhibits no paramagnetism in its 2Π1/2 ground state owing to the exact cancellation between the orbital magnetic moment and the spin magnetic moment of the electron. However, the electric field prevailing in the SAPO cavity can lift the degeneracy of the antibonding π* orbitals and allow the odd electron to occupy a nondegenerate π* orbital and be paramagnetic. NO forms a large number of nitrosyl complexes with transition metals, and numerous review articles discussing these have been published.42-44 The proposed mechanism for the interaction of NiH-SAPO17 with NO can be described in four stages. Stage 1.
Ni2+ + 2NO f Ni+fNO+ + NO• (+ X) slow Stage 2.
(2a) Ni2+ + 2NO f Ni+fNO+ + NO• slow (2b) 4NO f N2O + N2O3 fast Stage 3.
N2O3 S NO + NO2 Stage 4.
(4a) N2O3 S NO + NO2 fast (4b) Ni+-NO+ + NO2 f Ni2+ + NO+ + NO2- slow The first stage describes the process leading to the spectrum in Figure 6a. NO adsorption on the sample triggers a slow migration of the Ni2+ ions from the cancrinite cage which is not accessible to NO molecules to the supercage where they readily interact with the latter to form three species: a Ni+NO+ complex, a NO radical, and a very unstable species (X) whose exact identity and the role it plays in the process are still not clearly understood. The only certainty about this species is that it’s also a NO species since it can be seen in the absence of Ni. The second stage leading to Figure 6b can be described with two reactions which are believed to be taking place simultaneously but at different rates; reaction 2a is the slow
Comparison of Ion-Exchanged Molecular Sieves continuation of the first stage with more and more Ni ions leaving the cancrinite cage and entering the supercage to interact with NO. Previous X-ray diffraction studies on Ni-Y showed that Ni ions, which had initially moved from the sodalite cages to the hexagonal prisms upon dehydration, move back to the sodalite cages upon NO addition.32 Reaction 2b on the other hand is a fast process during which NO undergoes a disproportionation reaction and leads to two diamagnetic compounds N2O and N2O3. This was first suggested by Addison and Barrer who conducted an adsorption isotherm study of NO on various zeolites.45 On the basis of the mass balance and the density of the gas evolved, they concluded that, upon adsorption on zeolites, this reaction occurs nearly to completion and that, while the resulting N2O is desorbed, N2O3 stays occluded in zeolites. These two reactions explain why after 30 min of contact the intensity of Ni+-NO+ increases while that of NO• decreases. The third stage is illustrated by the dissociation of N2O3 into NO• and NO2 and is known to occur easily. NO2 can easily be seen in Figure 6c, but the NO• signal, which occurs in the same region is not visible probably because it overlaps with NO2. The fourth stage which leads to Figure 6d consists of two simultaneous reactions which explain the increase in intensity of NO2 (4a) and the decay of the Ni+-NO+ signal (4b). Equation 4b continues to take place until all the Ni+-NO+ is decomposed into Ni2+ and NO+, while NO2 gradually decays (Figure 6e) and disappears completely. When NO reacts with NiH-SAPO-35, the reaction mechanism between Ni(II) and adsorbed NO is suggested to be as follows. Stage 1.
2Ni2+ + 3NO S (Ni+fNO+)L1 + (Ni+fNO+)L2 + NO• Stage 2.
(2a) (Ni+fNO+)L1 f (Ni+fNO+)L2 (2b) (Ni+fNO+)L1 f Ni2+ + NO (2c) Ni2+ + 2NO f (Ni+fNO+)L2 + NO fast The first stage describes the process leading to Figure 7a. During this stage, Ni2+ interacts with NO to form two Ni+fNO+ species (one stable, L2, and one unstable, L1) along with a NO radical. Since in this case all the Ni ions are available and no migration is necessary due to the fact that Ni is situated at a site accessible to NO, the intensity of the Ni+fNO+ species formed after 3 min is much stronger than in NiH-SAPO-17. The instability of species L1 is confirmed in the second stage where it is thought to partly convert into species L2 and partly decompose into Ni2+ and NO. The reason L2 is more stable than L1 is still speculative, but it could be linked to their specific locations. By looking at the SAPO-35 structure, it seems that a species located at sites III′ or IV′ would be less stable than a species situated at site II′ because of the neighboring double six-ring which acts as a partial shield preventing adsorbates such as NO molecules from accessing site II′. In contrast, sites III′ and IV′ are accessible from all directions. Therefore, a tentative assumption is that L1 is located either at site III′ or IV′, whereas L2 is located at site II′. It is obvious that major differences exist in the way Ni reacts with NO in these materials, the most striking ones being the appearance of NO species X and NO2 in SAPO-17. These materials differ mainly in their silicon content and their structure. First, SAPO-34 has the highest silicon content, followed by
J. Phys. Chem. B, Vol. 103, No. 34, 1999 7285 SAPO-35 and then SAPO-17, which has the lowest silicon content. Second, SAPO-34 and SAPO-35 have similar structures with a similar cage size while SAPO-17 exhibits a bigger supercage and cancrinite cages which are not present in either SAPO-34 and SAPO-35. The silicon content is directly linked to the material’s ion exchange capacity. This factor appears to be of minor influence in this study since SAPO-34 and SAPO35 behave similarly despite their different silicon contents. However, in these materials, the size of the main cage and the presence or absence of cancrinite cages seem to be the predominant factors influencing Ni2+ behavior toward adsorbates in general and nitric oxide in particular. These differences clearly emphasize the fact that several factors including the structure type, the framework charge, and the pretreatment conditions determine the location of Ni 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. Thus, the behavior of a transition metal ion exchanged into a particular molecular sieve is somewhat specific to that material and justifies the study of transition metals in a wide range of molecular sieve materials. Conclusions ESR and ESEM spectroscopic methods were used to compare the reducibility, location, and adsorbate interactions of Ni ions exchanged into H-SAPO-35, H-SAPO-34, and H-SAPO-17. Ni was found to behave quite similarly in H-SAPO-35 and H-SAPO-34, and this behavior was in turn found to be quite different from the one in H-SAPO-17. While in hydrated NiHSAPO-35, NiH-SAPO-34, and NiH-SAPO-17, Ni(I) ions are located at sites inside the levyne cage, chabazite cage, and supercage, respectively, in NiH-SAPO-17, high-temperature dehydration under vacuum causes a migration of the Ni ions from the supercage to the smaller cancrinite cages. This makes it more difficult to reduce Ni2+ in NiH-SAPO-17 because Ni2+ is located at less accessible sites than Ni2+ in NiH-SAPO-35 and NiH-SAPO-34. Ni(I) ions in hydrogen-reduced NiHSAPO-35 and NiH-SAPO-34 react with ethylene to form two Ni-ethylene complexes located at two different sites within the SAPO-35 and SAPO-34 cages. In NiH-SAPO-17, only a fraction of the Ni(I) complexes with ethylene to form two Ni(I)-(C2D4)n species and a Ni(I)-(C4D8)n complex resulting from ethylene dimerization. Moreover, Ni(0) clusters are found to appear in NiH-SAPO-17 after ethylene adsorption, indicating some ethylene reduction of Ni(Ι). With adsorbed 13CO, a wellresolved Ni(I)-(CO)3 complex is generated in NiH-SAPO-35 and NiH-SAPO-34, whereas the Ni(I)-(CO)n complex formed in NiH-SAPO-17 shows no resolved 13C hyperfine, indicating a weaker Ni-CO interaction. NO adsorption on oxidized NiHSAPO-17 readily produces Ni(I)-(NO)+ complexes but otherwise follows a different reaction mechanism from NiH-SAPO35. Acknowledgment. This research was supported by the National Science Foundation, the Robert A. Welch Foundation, the Department of Energy, and the University of Houston Energy Laboratory. References and Notes (1) Maxwell, I. E. AdV. Catal. 1982, 31, 1. (2) Kazansky, V. B.; Elev. L. V.; Shelimov, B. N. J. Mol. Catal. 1983, 21, 265. (3) Bonneviot, L.; Olivier, D.; Che, M. J. Mol. Catal. 1983, 21, 415. (4) Moller, B. W.; Kemball, C.; Leach, H. F. J. Chem. Soc., Faraday Trans. 1 1983, 79, 453.
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