10988
J. Phys. Chem. 1995, 99, 10988-10994
Electron Spin Resonance and Electron Spin Echo Modulation Study of Ni(1) in Silicoaluminophosphate Type 5: Adsorbate Interactions and Evidence for the Framework Incorporation of Ni(1) Martin Hartmann, Naoto Azuma, and Larry Kevan* Department of Chemistry, University of Houston, Houston, Texas 77204-5641 Received: March 15, 1995; In Final Form: May 9, 1995@
The various Ni(1) species formed by reduction and adsorbate interactions in silicoaluminophosphate-5(SAPO5 ) , in which (a) Ni(I1) was incorporated into framework positions by incorporation of Ni(I1) into the SAPO-5 synthesis mixture (NiAPSO-5) and (b) Ni(I1) was incorporated into extraframework positions by partial ion exchange of H+ in H-SAPO-5 by Ni2+ (NiH-SAPO-5), were studied by electron spin resonance (ESR) and electron spin echo modulation (ESEM). After dehydration at temperatures above 673 K one Ni(1) species is observed in NiAPSO-5 and in NiH-SAPO-5. Two distinct Ni(1) species, assigned as isolated Ni(1) and Ni(I)-(H&, are also observed in hydrogen-reduced NiAPSO-5 and NiH-SAPO-5. The difference between Ni(1) in NiH-SAPO-5 and NiAPSO-5 can be shown by their 31Pmodulation. Adsorption of D2O on both types of materials at 77 K leads to the formation of a Ni(I)-(D20), complex at low temperature; at room temperature the complex decomposes with water decomposition. Ammonia adsorption on NiH-SAPO-5 leads to two different ammonia complexes Ni(I)-(ND3)4 with hyperfine splitting constants of 7.4 and 14.4 G, respectively, whereas in NiAPSO-5 only the Ni(I)-(ND3)4 complex with a 7.4 G splitting is seen. Ni(1) complexes with methanol show differences in the ESR g parameters and in the ESEM between NiAPSO-5 and NiH-SAPO-5. The adsorption of ethylene onto NiH-SAPO-5 produces a Ni(I)-(C2D4)1 complex compared to a Ni(I)-(C2D4)2 species formed in NiAPSO-5. The contrasting ESR and ESEM characteristics of Ni(1) in NiAPSO-5 and NiH-SAPO-5 suggest that Ni is indeed in a framework site in NiAPSO-5. The effect of an increase of the channel size between SAPO- 11 (10-ring channel) and SAPO-5 (12-ring channel) is seen by an increase in the number of coordinated methanol and ethylene molecules in SAPO-5.
Introduction The aluminophosphates (AlPO4-n) and silicoaluminophosphates (SAPO-n) belong to a class of microporous materials,'q2 which are potential candidates in the ongoing search for new efficient catalysts. The modification of these materials by isomorphous replacement of framework atoms by transition metal ions or by incorporation of such ions into extraframework positions is of potential significance for specific catalytic reactions. The catalytic activities of such materials depend on the oxidation state, location, and dispersion of the transition metal ion and the properties of the supporting molecular sieves. The catalytic importance of nickel-modified molecular sieves has prompted numerous studies into the physical and chemical nature of incorporated nickel species in microporous materials. It has been shown that Ni(1) ions can be active sites in catalytic reactions such as acetylene cyclomerization and ethylene and propylene olig~merization.~-~ Ni(1) ions formed by reduction of Ni(II) ions can be stabilized in zeolites, silica,6-1i and in silicoaluminophosphates.12~13 Recently, we reported the successful incorporation and characterization of Ni(I1) into the framework of SAPO-11,i4which has 10-ring channels. In this study we have synthesized NiAPSO-5 where Ni(I1) is added to the synthesis mixture and report spectroscopic evidence for Ni incorporationinto the SAPO-5 framework. The SAPO-5 molecular sieve structure is composed of 4-ring, 6-ring, and 12-ring straight channels, which are interconnected by 6-ring 'Abstract published in Advance ACS Abstrucrs, June 15, 1995.
0022-365419512099-10988$09.0010
windows. The NiAPSO-5 materials have the AlP04-5 structureI5 except that the framework phosphorus (P) tetrahedral sites are partially substituted by silicon (Si). This substitution produces a negative framework charge, which is, after template removal, belanced by Hf. The Ni-exchanged product is denoted NiH-SAPO-5 in contrast to the synthesized materials NiAPSO5 , in which Ni is in a framework site. Electron spin echo modulation (ESEM) spectroscopy can supplement electron spin resonance (ESR) spectroscopy to ascertain the environment of an incorporated paramagnetic transition metal ion and especially its adsorbate interaction.16 This is the first investigation comparing Ni in ion-exchanged and framework sites of the SAPO-5 structure. Experimental Section Synthesis. As-synthesized NiAPSO-5 was prepared by hydrothermal reaction with Ni(OCOCH3)2*4H20 (Aldrich) in the synthesis mixture for SAPO-5." A typical synthesis is performed as follows: 5.94 g of Catapal alumina (Vista) was stirred in 48.7 g of deionized water for 10 min to obtain a homogeneous mixture. Then 11.53 g of 85 wt % H3P04 (Mallinckrodt) was added drop by drop while stirring the mixture for about 2 h. Then 0.78 g of fumed silica (Cab-0Sil) was added, and the mixture was stirred for an additional 30 min. Finally 0.50 g of nickel acetate and 7.59 g of the template triethylamine were added drop by drop to the mixture. The gel was aged with stimng at room temperature overnight. Then 1.96 g of HF (-51%) (Aldrich) was added to the entire gel, and it was stirred for an additional hour. The gel composition was 1.5Et3N:0.04Ni0:0.26Si02:Al~O~:P20~:HF: 0 1995 American Chemical Society
J. Phys. Chem., Vol. 99, No. 27, 1995 10989
Ni(1) in Silicoaluminophosphate Type 5
4-
TABLE 1: XRD Data for NiAAPSO-5 and H-SAPO4 ComDared to the Literature NiAPSO-5 (this work) H-SAPO-5 (this work) H-SAPO-5 (ref 20) ~~~~
~~
dlA
100lIo
dlA
100lIo
dlA
100/Ia
11.9 6.84 5.93 4.48 4.24 3.96 3.60 3.42 3.08 2.96 2.66 2.58
100 10 17 36 19 49 3 18
11.9 6.88 5.94 4.49 4.23 3.97 3.60 3.43 3.08 2.98 2.66 2.59
100 14 18 33 37 55 8 23 14 12 6 11
11.8 6.86 5.91 4.46 4.21 3.96 3.59 3.43 3.07 2.96 2.66 2.59
100 12 26 61 53 77 5 30 17 19 5 16
6 11 3 10
70H20. The reaction mixture was placed in a stainless steel pressure vessel (90 mL intemal volume) lined with Teflon and heated in an oven at 473 K for 24 h. Calcined NiAPSO-5 was prepared by heating as-synthesized samples in oxygen at 873 K, during which the material tumed from violet to white. The chemical composition of the calcined sample was Ho.o4(Nio.olSio.ozAlo.soOPo.4,)Oz based on electron probe microanalysis. H-SAPO-5 was synthesized and calcined as described e l ~ e w h e r e . ' ~ NiH-SAPOJ .'~ was prepared by solid state ion exchange with NiC12 at 600 "C for 12 h.I3 The chemical composition of NiH-SAPOJ based on electron probe microanalysis was Ho.olNio.ol( S ~ O . O ~ A ~ O . ~ O PThe O . ~ nickel ~)OZ. content of NiH-SAPO-5 seems to be the upper limit for the ion exchange with respect to the silicon content of these samples. Sample Treatment and Measurement. For ESR and ESEM measurements, calcined samples were loaded into 3 mm 0.d. by 2 mm i.d. Suprasil quartz tubes which were sealed and gradually heated in vacuum (< hPa) to 773 K for 24 h (thermal reduction). In another reduction method the samples were dehydrated as described above and then contacted with 500 hPa (500 mbar) of dry oxygen and subsequently heated to 773 K for 6 h. After oxidation with static oxygen the color of the samples is still white. In order to produce Ni(I), the samples are reduced by dry hydrogen under static conditions (100 hPa (100 mbar) of Hz, 573 K) for only 1 h to avoid formation of metallic nickel (hydrogen reduction). In order to prepare Ni(1) complexes with various adsorbates, the hydrogen-reduced samples were evacuated at 573 K for 10 min and then exposed to the room temperature vapor pressure of D20 (Aldrich Chemical), CH30D, CD30H (Stohler Isotope Chemical), 20 and 100 hPa of ND3 (MSD), and 40 hPa of C2D4 (Cambridge Isotope Laboratories). The color of all samples did not change during these procedures. It is crucial for the success of these preparations to avoid any contamination with water, because Ni(1) is very reactive to only traces of HzO. 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 K with a Bruker ESP 380 pulsed ESR spectrometer. Three pulse echoes were measured by using a (nl2-z-nl2-T-nl2) pulse sequence as a function of time T to obtain the time domain spectrum. To minimize 27Almodulation from zeolitic aluminum in measurements of phosphorus modulation, the t value is fixed at 0.27 ps.18 The phsophorus and deuterium modulations were analyzed by a spherical approximation for powder samples in terms of N nuclei at distance R with an isotropic hyperfine coupling Aiso.I9 The
NiH-SAPO-5
v-
gAI1 2.470
gAL= 2.101
ge = 1.93
(b)
NiAPSO-5
2000
,'g = 2.516 gAy-2.111
go = 1.935
Figure 1. ESR spectra at 77 K of (a) NIH-SAPO-5 (b) and NiAPSO-5 after dehydration for 24 h at 773 K.
best fit simulation for an ESEM signal is found by varying the parameters until the sum of the squared residuals is minimized. Results and Discussion Powder X-ray Diffraction. The structures of as-synthesized NiAPSO-5 and H-SAPO-5 were characterized by powder X-ray diffraction (XRD). Table 1 shows the powder XRD pattem of synthesized NiAPSO-5 and H-SAPO-5. The X R D patterns agree reasonably with the patent literatureZoto identify the product. Note that the intensity of the peak with d = 4.24 A for NiAPO-5 is significantly lower than in H-SAPO-5. This has previously been found in NiAPO-5 and MgAPO-5 in comparison to AlPo4-5.2' As in the H-SAPO-1 l/NiAPSO-ll system,I4 the structure is not detectably changed by nickel incorporation. As in NiAPSO- 11, the violet color of the as-synthesized NiAPSO-5 tums white after calcination with only slight changes in the XRD powder pattem. Electron Spin Resonance. NiH-SAPO-5 and NiAPSO-5 do not show any ESR signal at 77 K. Thus, the Ni species exist in the form of Ni(I1). Figure 1 shows ESR spectra of dehydrated (a) NiAPSO-5 and (b) NiH-SAPO-5. Dehydration of the samples at this temperature leads to the formation of three species, denoted A, A', and B. Species A and A' with an axially symmetric g tensor are ascribed to isolated Ni(1) produced by reduction of Ni(I1) by desorbing waterI3 or hydroxyl groups.'2 The g values for species A (gl = 2.478 and gll= 2.101) and A' (gl = 2.516 and gll = 2.111) are similar to those reported for Ni(1) in NiCa-X,2z NiCa-Y,6 and NiAPSO-11.I4 Species B with g = 1.93 can also be Seen in H-SAPO-5,'3 so it is tentatively assigned to a nonspecific framework defect, which is produced during water or hydroxyl group desorption. Figure 2 shows the ESR spectra of NiAPSO-5 and NiHSAPO-5 after hydrogen reduction at 573 K. Three distinct Ni(I) species, denoted A, A', and C (Figure 2a,c), are observed in NiAPSO-5 and NiH-SAPO-5 in the presence of hydrogen gas. The g values of species A and A' are the same as these of isolated Ni(1) observed in the dehydrated samples. The g values are also almost identical to those for isolated Ni(1) in NiHSAPO-11 and NiAPSO-11.I4 This species remains after hydrogen outgassing at room temperature (Figure 2b,d). On the other hand, species C is lost after evacuation of the sample at room temperature (Figure 2b,d) but is readily regenerated by subsequent exposure to dry hydrogen. Thus, species C is assigned to Ni(I)-(H& It is noteworthy that after oxidation and subsequent hydrogen reduction species B cannot be seen in NiAPSO-5 or in NiH-SAPO-5.
Hartmann et al.
10990 J. Phys. Chem., Vol. 99, No. 27, 1995
NiH-SAPO5
g*,=2.101"
: 9 = 2.061
1
I AI
gp ~ 2 . 1 5 0
I
200 G
g,:
I
2.335
02-
NiH-SAPO-5 H, redud
x5
9;
= 2.051
Y ' f
NiAPS0-5
(d'
T
Figure 2. ESR spectra at 77 K of (a) NiAPSO-5 after hydrogen reduction for 1 h at 573 K, (b) NiAPSO-5 after 10 min evacuation of
(a) at room temperature, (c) NiH-SAPO-5 after hydrogen reduction for 1 h at 573 K, and (d) NiH-SAPO-5 after 10 min evacuation of (c) at room temperature. '.OR
s
........... ......... ................................_ .."._..-............
-.....
1.0
v)
z p
mod.
NiAPSO.5 I
0.8
0.2
I I
0
."......................................
2
1
3
"............
I . . . . . .
4
5
T, PS
Figure 3. Experimental (-) and simulated three-pulse ESEM spectra showing 31Pmodulation for (a) NiH-SAPO-5 and (b) NiAPSO-5 after hydrogen reduction and subsequent 10 min evacuation at room temperature. Spectra recorded at 4 K with r = 0.27ps to suppress 27Almodulation. See Table 2 for simulation parameters. (.a*)
TABLE 2: Simulation Parameters of 31PESEM of Ni(1) in NLH-SAPO-5 and NiAPSO-5 sample shell N Rlnm AiJMHz 8.8 0.5 1 0.1 NiAPSO-5 1 2 24.5 0.97 0 NiH-SAPO-5
1
5.2
0.33
2
10.5
0.72
0.56 0.01
ESEM measurements of 31Pmodulation of Ni(1) species A and A' produced by hydrogen reduction in NiH-SAPO-5 q d NiAPSO-5 show significant differences between these two species (Table 2) although they have similar ESR parameters. Figure 3 shows the experimental and simulated three-pulse 31P ESEM pattem recorded at t = 0.27 ps to suppress the zeolitic 27Almodulation. There is a significant difference in the 31P modulation pattem between NiH-SAPO-5 and NiAPSO-5. Simulation of the 31Pmodulation for Ni(1) in NiH-SAPO-5 shows 5.2 nearest phosphorus atoms at 0.33 nm, which is
-
100 G
'It
gDi= 2.061
gDi= 2.051
Figure 4. ESR spectra at 77K of (a) NiH-SAPO-5 with adsorbed D20 at 77 K and (b) NiH-SAPO-5 with adsorbed D2O at 77 K after warming to room temperature for 5 min and (c) NiAPSO-5 with adsorbed D2O at 77 K. consistent with a Ni(1) location in the center of a hexagonal prism site (SI). Since Ni(1) in NiAPSO-5 is in a framework site, it can replace P or A1 in the original alternating A1-P framework. The simulation data in Table 2 showing 8.8 nearest P atoms at a distance of 0.51 nm are more consistent with replacement of a P atom in the original framework this is also consistent with the chemical analysis. If Ni replaces Al, the nearest phosphorus distance would be 0.314 nm, which is calculated as follows. The Ni-0 distance in NiAPSO-5 is close to 0.198 nm. Assuming a P-0 bond distance of 0.154 nm and a bond angle of 126°,23the distance is calculated to be 0.314 nm, which is a little more than half of the distance found in our experiments. Thus, the 31Pmodulation gives evidence for Ni incorporation into a P position of the framework. Adsorbate Interactions. In order to study adsorbate interactions with Ni(1) in NiH-SAPOJ and NiAPSO-5, the materials were reduced with hydrogen. Before an adsorbate is added, the hydrogen-reduced samples were evacuated at 573 K for 10 min. The adsorbate was then exposed to the sample for about 5 min at room temperature, and then the sample was quenched to 77 K. Adsorption of D20. The adsorption of a small amount of DzO at 77 K on NiH-SAPOJ showing species A leads to the formation of species D with a rhombic g tensor (Figure 4a). Somewhat similar g values (gl = 2.16, g2 = 2.09, and g3 = 2.055) have been assigned to Ni(I)-(O& in NiCa-Y zeolites.' Increasing the temperature from 77 K to room temperature for 5 min leads to a significant intensity decrease of signals A and D accompanied by the formation of an 0 2 - radical species with gll = 2.009 and g l = 1.999. This substantiates the oxidation of Ni(1) by water, but this oxidation does not occur at 77 K as indicated by the absence of the 0 2 - signal at that temperature initially. So we assign species D to a Ni(I)-(DzO), complex. However, the ESE for species D observed at gl = 2.15 and g3 = 2.051 showed no deuterium modulation, which makes this assignment tentative.
Ni(1) in Silicoaluminophosphate Type 5
J. Phys. Chem., Vol. 99, No. 27, 1995 10991 NiAPSO-SIND,
go,= 2.073
lOOG
NiH-SAPO-5
v
7.4 G
+ CH30D
NiH-SAPO-SIND,
100 G
(d’
OH,, c 2.574 QHI = 2.063
gEI = 2.073
Figure 5. ESR spectra at 77 K after hydrogen reduction and subsequent adsorption of 100 hPa of ammonia-& on (a) NiAPSO-5 and (b) NiHSAPO-5 and of 50 hPA of ammonia on (c) NiH-SAPO-5. Adsorption of water on NiAPSO-5 showing species A’ also results in the immediate formation of species D’ at 77 K, which vanishes on warming the sample to room temperature for a short time. Similiar results were previously found for the NiAPSO1l/NiH-SAPO-ll system,I4 showing that Ni(1) is not stable in the presence of DzO in these SAPO systems. Adsorption of Ammonia& Figure 5 shows the ESR spectra of ND3 (100 hPa) adsorbed on NiAPSO-5 and NiHSAPO-5. The ESR signal of the Ni(I)(ND3),, complex in NiAPSO-5 has gli = 2.325 and gL = 2.073 (species E) (Figure 5a). A previously reported Ni(I)(ND3)ncomplex on silica at 77 K with similar g values was assigned to Ni(1) in an octahedral environment.’O The same type of spectrum was obtained in NiCa-X with an initial low Ni(I1) concentration (2.7 Ni(II)/ unit In NiH-SAPOJ species E can also be seen at low ND3 pressures (20 hPa) (Figure 5b), but a stronger signal F with gl = 2.034, gz = 2.204, and g3 = 2.634 is superimposed on species E at high ammonia pressures (100 hPa) (Figure 5c). Similar g components have been found in NaCa-X with a higher Ni content (8.1 Ni(II)/unit The nine visible I4N hyperfine splitting lines on species F with a splitting constant of 14.4 G show an interaction of four equivalent nitrogens from ND3 with Ni(1). Species F is not seen in NiAPSO-5 even at high ammonia concentrations. In the case of species E nine hyperfine lines are also detected, but the hyperfine splitting constant is only 7.4 G. This suggests more distant nitrogen nuclei than in species F. Adsorption of Methanol. The ESR spectra recorded after CH30D adsorption at room temperature on NiH-SAPO-5 and NiAPSO-5 are shown in Figure 6. In NiH-SAPO-5 species H is assigned with gll = 2.574 and g l = 2.063. In contrast, NiAPSO-5 shows a sharp signal K with gll = 2.385 and g l = 2.132. Since the g values of species H and K differ from those of Ni(1) species A and A‘, methanol coordination is indicated which is supported by ESEM data. The experimental and simulated three-pulse ESEM spectra for NiH-SAPO-5 and NiAPSO-5 with adsorbed methanol (CH3OD and CD30H) are
Figure 6. ESR spectra at 77 K after hydrogen reduction and subsequent methanol adsorption on (a) NiAPSO-5 and (b) NiH-SAPO-5. shown in Figure 7. Simulations with a two-shell model indicate the interaction of Ni(1) with three methanol molecules with a different geometry in NiAPSO-5 compared to NiH-SAPOJ. The data for the simulations are summarized in Table 3. In NiAPSO-5 with adsorbed CH30D, Ni(1) interacts with two deuteriums, i.e., two molecules of methanol, at a short distance of 0.25 nm which indicates an H-0 bond orientation as shown in Figure 8a. A third CH30D is indirectly coordinated with a Ni(1)-D distance of 0.35 nm. The H-0 bond orientation geometry is typical for a negative ion rather than a positive ion and supports that Ni(1) is in a framework site which is locally negative.’4b The ESEM data for CD30H are consistent with the structure in Figure 8a, showing six deuteriums at a distance of 0.33 nm for the two directly coordinated molecules and three deuteriums with a longer coordination distance (0.36 nm) for the indirectly coordinated molecule. The coordination of the indirectly coordinated methanol molecule must be oriented in such a way that its methyl group is closer to the Ni(1) center to give a Ni(1)-D distance of 0.35 nm for CH3OD and 0.36 nm for CD30H as shown in Figure 8a. The methanol coodination geometry for NiH-SAPO-5I3 is quite different as shown in Figure 8b. Two methanols are directly coordinated to Ni(1) at a Ni(1)-D distance of 0.29 nm for CH30D. One indirectly coordinated CH30D molecule is located at a Ni(1)-D distance of 0.31 nm. This methanol coordination geometry is typical for exposed cations in ion-exchange sites. Similar coordination geometries have been reported for Cu(11) in Cu(II)-exchanged zeolite rhoz4and CuH-SAPO-5.” In contrast to these results in the SAPO-5 systems, in NiH-SAPO11 and NiAPSO-11 Ni(1) is only coordinated to one methanol molecule,I4 showing that the increase of a 10-ring channel in SAPO-11 to a 12-ring channel in SAPO-5 increases the methanol coordination number. Adsorption of Ethylene. Figure 9 shows the ESR spectrum at 77 K after the adsorption of 40 hPa of C2D4 at room temperature on (a) NiAPSO-5 and (b) NiH-SAPO-5 containing Ni(1). Ni(1) species A and A’ are still quite prominent as seen by the g l feature, so only a fraction of Ni(1) actually coordinates with ethylene. New species L and M are observed and assigned to Ni(I)(C2D& complexes. Similar ethylene complexes with
Hartmann et al.
10992 J. Phys. Chem., Vol. 99, No. 27, 1995
NiH-SAPO-5 / CH30D
F 1.0 f 3 m 0.8
a
d
t:z O.= 5 0.4 Iz 8 o.2 Y
1
2
3 f.
4
1
5
w
3
2
4
5
T, lu
Figure 7. Experimental (-) and simulated (.* three-pulse ESEM spectra at 4 K showing 2H modulation for NiAPSO-5 at g = 2.06 after hydrogen reduction, evacuation, and (a) CH30D and (b) CD3OH adsorption and for NiH-SAPO-5 at g = 2.13 after the same pretreatment and (c) CH3OD and (d) CD30H adsorption. The first interpulse time z was 28 ps to suppress 27Almodulation. See Table 4 for simulation parameters. a)
TABLE 3: ESR g Values of Ni(1) Species in NIH-SAPO-5 and NiAPSO-5 matrix adsorbate assignment glI 2.478 NiH-SAPO-5 2.516 NiAPSO-5 NiH-SAPO-5 NiAPSO-5 NiH-SAPO-5 NiAPSOJ NiH-SAPO-5 NiAPSO-5 NiH-SAPO-5 NiAPSO-5 NiH-SAPO-5 NiH-SAPO-5 NiAPSO-5 NiH-SAPO-5 NiAPSO-5 NiH-SAPO-5 NiAPSO-5 . a
2.335 2.335 2.48 1 2.519 2.150 2.150 2.634 2.325 2.325 2.574 2.385 2.705 2.623
g1
2.101
species
1.930
A A' B
1.935
B
2.071 2.071
C C A A' D
2.1 11
2.111
2.111 2.061, 2.051 2.061, 2.051 2.204, 2.034 2.073 2.073 2.063 2.132 2.489, 1.959 2.492, 1.996
D' E F G H K L M
Determined from the I4N hyperfine splitting.
rhombic symmetry are found in NiCa-X,22 NiLa-Y,25 and NiH-SAPO-11.I4 This complex formation is confirmed by the ESEM data with adsorbed C2D4 performed at g = 1.96 (Figure 10). In NiH-SAPO-5 Ni(1) shows interaction with four equivalent deuteriums at a distance of 0.35 nm (Table 4). The suggested coordination geometry consistent with this shows that one ethylene is coordinated with its molecular plane perpendicular to a line toward the nickel atom (Figure 8d). This is consistent with metal ion n-bonding to the ethylene molecule as expected for an exposed metal ion in an ion-exchange site. A similar coordination was suggested for Ni(1) in NiH-SAPO11l 4 with a Ni(I1)-D distance of 0.38 nm. The NiAPSO-5 ESEM data (Table 4) show a different coordination geometry for C2D4 with Ni(1). Four deuteriums are 0.31 nm from Ni(I), and four more deuteriums are 0.55 nm
from Ni(1). This suggests the geometry shown in Figure 8c of two C2D4 molecules interacting with one Ni(1). The geometry shown indicates that x-bonding is not present. This coordination is chosen in accordance to previous data on NiAPSO-11, showing two deuteriums at a distance of 0.27 nm and two deuteriums at 0.47 nm.14 In NiAPSO-11 and NiAPSO-5 the obtained distances for the more distant D nuclei are in excellent agreement with the known ethylene structure, assuming a weak a-bonding with one CH2 group of the ethylene. Since this type of bonding is only present in NiAPSO-5 and NiAPSO-11, it is assumed that n-bonding is not possible due to steric hindrance, which is evidence for Ni(1) being in a framework site. The coordination of two ethylene molecules to Ni(1) in NiAPSO-5 in contrast to only one in NiAPSO-11 l 4 is consistent with a larger coordination number for the larger channel in SAPO-5.
J. Phys. Chem., Vol. 99, No. 27, 1995 10993
Ni(1) in Silicoaluminophosphate Type 5
'*.&
NH-SAPO-5 I methanol
NIH-SAPO-5 I ethylene
TABLE 4: Simulation Parameters of 2HESEM of Ni(1) in NiH-SAPO-5 and NMPSO-5 sample
NIAPSO-5 I ethylene
NiAPSO-5 I methanol
H
-i ,
0
NiAPSO-5 NiH-SAPO-5 NiAPSO-5 NiH-SAPO-5 NiAPSO-5
adsorbate shell N Rlnm AiflHz DzO DzO ND3 ND3 CH30D
NiH-SAPO-5
CH3OD
NiH-SAPOJ
CD3OH
NiAPSO-5
CD3OH
NiAPSO-5
C2D4
NiH-SAPO-5
CZD4
molecules
a a
a a 1 2 1 2 1 2 1 2 1 2 1
2 1 2 1 6 3 6
3 4 4 4
0.25 0.35 0.29 0.31 0.34 0.40 0.33 0.36 0.31 0.55 0.35
0.15
0 0.43 0.10 0.21 0 0.08
0 0.09 0 0.22
2 1 2 1 2 1 2 1 2 1
H
Figure 8. Schematic diagram of Ni(1) coordinated to methanol in (a) NiH-SAPO-5 and (b) NiAPSO-5 and to ethylene in (c) NiH-SAPO-5 and (d) NiAPSO-5.
-
200 G
1
'
Figure 9. ESR spectra at 77 K of (a) NiAPSO-5 and (b) NiH-SAPO-5 after hydrogen reduction and subsequent adsorption of CzD4.
a
Modulation not visible.
NiAPSO-5. Although Ni(1) has almost the same g values, differences can clearly be detected by 31Pnuclear modulation, showing different locations in SAPO-5. A Ni(I)-(H& species is also produced in both systems, but this species in only stable in the presence of hydrogen. Ni(1) species, identified as Ni(I)-(D20)n, are produced in NiAPSO-5 and NiH-SAPO-5 after adsorption of water at low temperature; at room temperature this species decomposes to generate 02-, showing the Ni(1)catalyzed decomposition of water. Two different Ni(1) ammonia complexes can be found in NiH-SAPO-5, showing a coordination of four ammonia molecules to the paramagnetic center with I4N splitting constants of 7.4 and 14.4 G. In NiAPSO-5 only a Ni(I)-(ND3)4 complex with a 7.4 G I4N splitting is observed. The Ni(1) complex generated in NiH-SAPO-5 after methanol adsorption differs by electron spin resonance from that of NiAPSO-5. Electron spin echo modulation shows a different coordination geometry in the two systems which is consistent with Ni(1) framework incorporationin NiAPSO-5. The adsorption of C2D4 on Ni(1) produced a Ni(I)-(CzD& complex in NiH-SAPOJ and a Ni(I)-(CzD& complex in NiAPSO-5. The different ESR spectra and the contrasting coordination properties from ESEM data show that the local environment of Ni(1) in synthesized NiAPSO-5 is different from that in ion-exchanged NiH-SAPO-5. The Ni(1) in NiH-SAPO-5 is clearly in an ionexchange position, whereas the results support that Ni(1) occupies a framework position in NiAPSO-5. Acknowledgment. This research was supported by the National Science Foundation and the Robert A. Welch Foundation. References and Notes
0
1
2
4
3
5
TP b
Figure 10. Experimental
(-) and simulated *) three-pulse ESEM spectra at 4 K showing ZH modulation of ethylene adsorption on (a) NiH-SAPO-5 and (b) NiAPSO-5. Spectra were recorded at g = 1.96 for species K and L. See Table 4 for simulation parameters. 6
Conclusions This work shows comparative ESR studies of Ni(1) reduction and adsorbate interaction between synthesized NiAPSO-5 and ion-exchanged NiH-SAPO-5 in order to differentiate the Ni(I) location in these two preparations. Thermal and hydrogen reduction produces isolated Ni(1) in both NiH-SAPO-5 and
(1) Wilson, S. T.; Lok, B. M.; Flanigan, E.M. U.S.Patent 4 3 10 440, 1982. (2) Wilson, S.T.; Lok, B. M.; Messina, C. A.; Cannon, T. R.; Flanigan, E. M. J. Am. Chem. SOC. 1982, 104, 1146. (3) Kazansky, V. B.; Elev, I. V.; Shelimov, B. N. J . Mol. Catal. 1983, 21, 265. (4) Bonneviot, L.; Olivier, D.; Che, M. J . Mol. Catal. 1983, 2 1 , 415. ( 5 ) Moller, B. W.; Kemball, C.; Leach, H. F. J . Chem. Soc., Faraday Trans. 1 1983, 79, 453. (6) Rabo, J. A.; Angell, C. L.; Kasai, P. H.; Shomaker, V. Discuss. Faraday SOC. 1966,41, 328. (7) Garbowski, E. D.; Vedrine, J. C. Chem. Phys. Lett. 1977,48,550. (8) Olivier, D.; Richard, M.; Che, M.; Bozon-Verduraz, F.; Clarkson, R. B. J . Phys. Chem. 1980, 84, 420. (9) Olivier, D.; Richard, M.; Che, M. Chem. Phys. Lett. 1978, 60, 77. (10) Elev., I. V.; Pershin, A. N.; Shelimov, B. N.; Kazansky, V. B. Kinet. Katal. 1982, 23, 936. (1 1) Garbowski, E. D.; Mathieu, M. V.; Primet, M. Chem. Phys. Lett. 1977, 49, 247.
10994 J. Phys. Chem., Vol. 99, No. 27, I995 (12) Azuma, N.; Kevan, L. J . Phys. Chem. 1995, 99, 5083. (13) Azuma, N.; Hartmann, M.; Kevan, L. J . Phys. Chem. 1995, 99, 6670. (14) (a) Azuma, N.; Lee, C. W.; Kevan, L. J . Phys. Chem. 1994, 98, 1221. (b) Azuma, N.; Lee, C. W.; Zamadics, M.; Kevan, L. In Zeolites and Related Microporous Materials: State of the Art 1994; Weitkamp, J., Karge, H. G., Pfeiffer, H., Holderich, W., Eds.; Studies in Surface Science and Catalysis; Elsevier; Amsterdam 1994; Vol. 84, pp 805-812. (15) Flanigan, E. M.; Lok, B. M.; Patton, R. L.; Wilson, S. T. In New Developments Zeolite Science and Technology; Murakami, Y., Iijma, A,, Ward, J. W., Eds.; Proceedings of the 7th International Zeolite Conference; Elsevier: Amsterdam, 1986; pp 103- 112. (16) Kevan, L. Ace. Chem. Res. 1987, 20, 1. (17) Chen, X.; Kevan, L. J . Am. Chem. Soc. 1991, 113, 2861. (18) Fenter, P.; Eisenberger, P.; Li, J.; Camillione 111, N.; Bemasek, S.; Scoles, G.; Ramanarayanan, T. A,; Liang, K. S. Langmuir 1991, 95,4777.
Hartmann et al. (19) Kevan, L. In Time Domain Electron Spin Resonance; Kevan, L., Schwartz, R. N., Eds.; Wiley: New York, 1979; Chapter 8. (20) Lok, B. M.; Messina, C . A.; Patton, R. L.; Gajek, R. T.; Cannan, T. R.; Flanigan, E.M. U.S.Patent 4 440 871, 1984. (21) Xu, Y.; Maddox, P. J.; Thomas, J. M. Polyhedron 1989, 8, 819. (22) Michalik, J.; Narayana, M.; Kevan, L. J . Phys Chem. 1984, 88, 5236. (23) Xu, Y.; Couves, J. W.; Jones, R. H.; Catlow, C. R. A.; Greaves, N. G.; Chen, J.; Thomas, J. M. J . Phys. Chem. Solids 1991, 52, 1229. (24) Anderson, M. W.; Kevan, L. J . Phys. Chem. 1986, 90, 6552. (25) Schoonheydt, R. A,; Vaesen, J.; Leeman, H. J . Phys. Chem. 1989, 93, 1515.
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