11811
J. Phys. Chem. 1993,97, 11811-11814
Electron Spin Resonance and Electron Spin Echo Modulation Studies of Oxomolybdenum Species in Thermally Reduced MoH-SAPO-5 and MoH-SAPO- 11 Silicoaluminophosphate Molecular Sieves: Comparison of Adsorbate Coordination with CuH-SAPO-5 and CuH-SAPO-11 Chul Wee Lee, Thierry Saint-Pierre, Naoto Azuma, and Larry Kevan. Department of Chemistry, University of Houston, Houston, Texas 77204-5641 Received: July 7, 1993; In Final Form: August 25, 19930
Molybdenum-doped H S A P O - 5 and HSAPO-11 were prepared by a solid-state reaction with Moo3 to produce M O H S m - 5 and MoHSAPO- 11. After dehydration, paramagnetic Mo(V) speciesare generatedand detected by electron spin resonance (ESR)with gc > gl > 81. A rhombic ESR signal is observed after adsorption of D20, CD3OH, and CHsOD. Upon 0 2 adsorption, the Mo(V) ESR signal intensity decreases and an 0 2 - radical is formed. The location and coordination geometry of Mo(V) have been determined by three-pulse electron spin echo modulation data and simulations. For MoHSAPO-5, Mo(V) is directly coordinated to three D2O and two CD3OH, but for MoHSAPO-11, Mo(V) is directly coordinated only to two D20 and one CDsOH. The coordination distances are longer and the number of coordinated adsorbates is less than for Cu(I1)-doped H S A P O - 5 and HSAPO-11. These differences are consistent with Mo(V) being an oxomolybdenum species, likely (MoO2)+, in these SAPO materials.
Introduction
It is well-known that molybdenum-loaded catalysts are useful for industrial processes such as Fischer-Tropsch synthesis, epoxidation,and methanation.' Also, oxomolybdenum complexes catalyze a number of biologically important oxo-transfer reactions such as conversion of sulfite to sulfate and xanthine to uric acid.2 Paramagnetic molybdenum species formed by decomposition of MO(CO)~ on silica and alumina' or Y zeolite: (NH&Mo04 on Y zeolite,5 (NH&Moz07 on silica: and MoCl5 on silica7or on ZSM-5 and mordenite zeolites8have been the subject of several electron spin resonance (ESR)studies. The photoreduction of impregnated (NH&Mo207 on silica has also been reportedg Possible geometries of various adsorbates coordinated to paramagnetic molybdenumspecieson silica have also been determined by ESR methods.lOJ1 From those studies it was concluded that Mo(V) has various coordinations depending on the support and the preparation methods and that these determine the catalytic activity. However, there is only one recent study about the structure of Mo(V) in silicoaluminophosphate(SAPO)molecular sieves, specifically SAPO-5.12 In this work, molybdenum-doped HSAPO-5 and HSAPO11were prepared by a high-temperature solid-state reaction with MoO3, and the paramagnetic Mo(V) species were characterized and comparedby ESR and electron spin echo modulation (ESEM) spectroscopy. Coordination differences are observed between SAPO-5and SAPO-1 1 that can be understood on the basis of the different channel sizes of these molecular sieves.
Experimental Section HSAPO-5 and HSAPO-11 were prepared as in previous ~0rks.13~4 Solid-stateReaction of MoO3 and HSAPO-a (a= 5 and 11). The quantities 0.015 g of MoO3 (Baker Chemical) and 0.5 g of HSAPO-n were mixed with a mortar and pestle for about 30 min. The mixture was pressed in a steel die to make a pellet of 12-mm diameter and 2.5-mm thickness. The pellet was put in a quartz boat, heated in a furnace at 600 OC in air for 12 h, and cooled to room temperature slowly. Before and after this solidstate reaction, the sample remains white. Abtract published in Advance ACS Abstracts, October 15, 1993.
0022-365419312097-11811$04.OO/O
TABLE I: ESR Parameters of Mo(V) in H-SAPO-5 and H-SAPO-11 at 77 K MoHSAPO-5
MoH-SAPO- 1 1
treatment
ala
gla,b
ea
dehyd (A)C dehyd (B)d
1.877 1.849 1.899 1.893 1.878' 1 .895d 1.903 1.903
1.952 1.960 1.958,1.932 1.964,1.932 1.95oC 1 .966d 1.943 1.981
1.877 1.851 1.897 1.891 1.878' 1 .894d 1.908 1.903
a"
1.952 1.964 +Dz0 1.951, 1.930 +CD,OH 1.962,1.930 1 .943' +C2D4 1 .96gd +NH3 1.949 1.988 a Estimated uncertainty is i0.003. g l is mcasured at the maximum peakpositionof thederivativesignal inorder tocomparewiththeprevious literature. Major component. Minor component.
k p l e Treatments. For ESR and ESEM studies, samples were treated as follows: (a) Samples were loaded in 3-mm-0.d. by 2-mm4.d. Suprasil quartz tubes and evacuated to a pressure of l(r Torr at various temperatures. (b) After dehydration, adsorbates such as D20, CDsOH, CHsOD, NH3, and C2D4 were adsorbed at their ambient vapor pressures at room temperature. These samples were immersed into liquid nitrogen and scaled. ESR and ESEM Measurements. The ESR spectra were recorded at 77 K on a modified Varian E-4spectrometerinterfafaccd to a Tracor Northern TN- 1710 signal averager. Each spectrum was obtained by multiple scans to achieve a satisfactory signalto-noise ratio. Each acquired spectrum was transferred from the signal averager to an IBM PC/XT compatible computer for analysis and plotting. The magnetic field was calibrated with a Varian E400 gaussmeter. The microwave frequency was measured by a Hewlett-Packard HP 5342A frequency counter. ESEM data were measured at 4 K with a Bruker ESP 380pulsed ESR spectrometer. Three pulse echoes were measured by using a 9O0-s-9Oo-T-9O0 pulse sequence with 7 = 0.27 ps, and the echo intensity was measured as a function of T. The theory and simulation of ESEM are described else~here.'~J~ Results d Discussion Electron Spin Resonance. The paramagnetic molybdenum species formed in MoHSAPO-5 and MoH-SAPO-11 arc quite similar to each other. However, differencesare found in adsorbate Q 1993 American Chemical Society
-
11812 The Journal of Physical Chemistry, Vol. 97, No. 45, 1993
-
Lee et al.
100 G
MoH-SAPO-11 at 77 K
b2
j
6s MoH-SAPO-11 / 0 2
t
Ba
911
-
Figure 1. ESR spectra at 77 K of MoHSAPO-11: (a) after 10 h at 500 OC under vacuum, (b) after adsorption of DzO, (c) after adsorption of C2D4, and (d) after adsorptionof NH3. In (a) two specica are identified (A and B), and their values are correlated to gl values by the dashed lines. In (d) two species are also indicated by their g l values (1 and 2); also, two additional lines are shown by asterisks. The ESR pattern of MoHSAPO-5 is quite similar (see Table I).
Figure 2. ESR spectra at 77 K of MoHSAPO-5 thermally reduced at 500 OC followed by 02 adsorption at room temperature (a) (1 Torr for 2 min) and (c) (1 Torr for 1 min). In (b) and (d) the field wale is expanded.
TABm II:
VIllue~Of 0 2 -
RpdicllLp
~ o ~ y ~ u m f b xsystems ide system
gl
F
o in ~ E2
g3
ref
M 2.018 2.010 2.004 8 geometries. This difference is not easily observable by ESR, 2.017 2.010 2.004 M 8 however. Table I summarizes the g factors in the two SAPO 2.016 2.01 1 2.004 M h materials. 2.021 2.010 2.004 M h 2.016 2.011 2.005 I ( a ) Thermal Reduction. After the solid-state r e a c t i ~ n ~ ' , ~ ~ M 2.021 2.009 2.006 M i between HSAPO-n (n = 5 and 11) and Mo03 the sampleremains 2.021 2.014 2.008 M I white, and no ESR signal is observed. Dehydration near 300 OC 2.020 2.015 2.006 M I for 1 h begins to form a weak ESR signal, and the sample becomes 2.024 2.013 M 2.006 I gray. The ESR signal intensity increases for continued dehy2.017 2.01 1 2.005 M k 2.017 2.011 M 2.006 k dration at higher temperature and longer time. The maximum 2.039 2.015 M 2.01 1 k intensity wasobtained for dehydration at 500 OC for 10 h. Figure 1 shows a typical axially symmetric ESR pattern of Mo(V) with Mordenite. Formed in MoCls-derived sample activated in vacuo. e Formed in MoC15-derived sample reduced in ammonia. Produced by ge> gL > a,where gcis the free electron value of 2.0023, which 0 2 adsorption on activated sample. e Produced by 0 2 adsorption on a is characteristic of Mo(V) signals with oxygen-containing sample reduced by ammonia. IMajor species. 8 From ref 4.* From ref ligands.lOJI 8. From ref 10. This work. From ref 3. The dehydrated samplesshow two components: a major signal withgL = 1.952andgll3 1.877forbothMoHSAPO-Sand-11, molecules. However, nonpolar molecules like CzD4 interact with which is denoted as Mo(A) in Figure la, and a minor component species Mo(B) and not with species Mo(A). The ESR spectra withamoreanisotropiccharacter withg, = 1.964andgll~1.851 of the samples with adsorbed C2D4 are shown in Figure Id. for MoHSAPO-11 and gl = 1.960 and gll = 1.849 for MoHWhen NH3 was adsorbed at room temperature on a dehydrated SAPO-5, which is denoted as Mo(B) in Figure la. Thermally sample, a new Mo(V) spectrum develops with ESR parameters reduced pure MoO3 samples do not show such spectra.19 Based of gL1= 1.981, gll = 1.903 and g12 = 1.943, = 1.903. Also, the Mo(A) and Mo(B) species are assigned on previous two additional hyperfine lines are observed at lower field with a as Mo(V)k and Mo(V)s, where the subscript denotes the 61-G splitting (see Figure Id). This hyperfine is assigned to the coordination number; no Mo(V)k species is observed. and 91Mo (I = 5/2), which are two Mo isotopes 9sMo (I = (b) Adsorbate Interactions. Upon adsorption of D20, CD315.78%and 9.60% abundant, respectively. This hyperfine signal OH, and CHpOD vapor at room temperature, the sample color has been assigned to two low-field perpendicular components by changes slowly from gray to blue-gray. This might indicate the using samples enriched in 9sM0.21 When this NHa-adsorbed partial oxidation of Mo(V) to molybdenum blue, which is known sample is evacuated at 250 OC for 1 h, the signal becomes identical as a mixed valence Mo(V)-Mo(V1) oxide.20 In contrast, there to a fully dehydrated sample as shown in Figure la. is no color change for C2D4 adsorption. These adsorbates are (c) Adsorption of 0 2 . Figure 2 shows the ESR spectra after assumed to act only as ligands based on previous work.llJ* Upon exposing 1 Torr of oxygen for 1 or 2 min to the dehydrated adsorption of polar adsorbates, such as D20, CH30D. and CD3(reduced) sample at room temperature. The Mo(V) ESR signal OH on the dehydrated sample, gll(A) and gll(B) disappear intensity decreaseswith 0 2 adsorption, and simultaneously a new completely, producing an ESR spectrum typical of rhombic signal is produced which has a rhombic g factor. The new signal symmetry with three different gvalues (see Figure 1b,c and Table is assigned to 0 2 - according to other work.8J2 The g tensor components of 0 2 - in several zeolite systems are compared in I). So species Mo(A) and Mo(B) are reactive toward polar
Oxomolybdenum Species in MoHSAPO-5 and - 11
The Journal of Physical Chemistry, Vol. 97, No. 45, 1993 11813
TABLE IIk JBEM Parameters for Mo(V) with Various Adsorbates in HSAPO-5 and H-SAPO-11 for Cu(II) and Mo(V) Saeeies
HSAPO-5 adsorbate
Na
Dz0 CD3OH
6 6 3 2 1
CH3OD
Cu(I1)d d(nm) AC(MHz) 0.30 0.41 0.31 0.28 0.43
0.18 0.03 0.24 0.21 0.00
C2D4
No 6 6. 3
2 1 4
HSAPO-11 MO(V)~ d(nm) AC(MHz) 0.33 0.49 0.37 0.33 0.45 0.36
0.05 0.00 0.05 0.01 0.10 0.10
No 4 6 3 2 1 4
CU(1I)f Rb(nm) AC(MHz) 0.28 0.40 0.33 0.28 0.43 0.36
Na
0.26 0.01 0.24 0.19 0.04 0.11
4 3 3 1 1 4
Mo(V)r Rb(nm) Ac(MHz) 0.33 0.48 0.37 0.30 0.46 0.45
0.01 0.01 0.15 0.05 0.15 0.10
a Number of deuterium nuclei. Distance between Mo(V) (or Cu(I1)) and deuterium; estimated uncertainty is hO.01 nm. Isotropic hyperfine coupling constant; estimated uncertainty is +lo%. Reference 13. e This work./Reference 14. 8 This work.
(A) MoH-SAPO-5/D20
(B)MoH-SAPO-ll/D20 -
c
$ n0
1
2
3
4
5
0
1
2
3
4
5
3
4
5
T, 1 s
T, PS
Figure 3. Experimental and simulated (dashed line) three-pulse ESEM at 4 K at (A) MoHSAPO-5 and (B) MoHSAPO-11 with adsorbed D20 (see Table 111 for simulation parameters).
Table 11. Different ESR signals areobserved for MoHSAPO-5 and MoHSAPO- 11. Electron SpinEcho Modulation. The ESEM simulation results for MoHSAPO-5 and -1 1 are summarized and compared with thoseof Cu(I1)-doped SAPO-5 and -1 1 in Table 111. For a D20adsorbed sample, Mo(V) interacts with three molecules and two molecules of DzO in MoHSAPO-5 and MoHSAPO-11, respectively. These Mo(V)-D distances are longer than those of Cu(I1)-D by 0.03 and 0.05 nm in CuHSAPO-513 and CuHSAPO- 11,I4respectively. The experimentaland simulated threepulse ESEM for MoHSAPO-5 and MoHSAPO-11 with adsorbed D20 are shown in Figure 3. Generally, when a transition-metal ion is directly coordinated to water (DzO), the distances obtained by ESEM agree with X-ray diffraction data. For example, if Cu(I1) is directly coordinated to HzO, the Cu(I1)-H distance is 0.27-0.30 nm as calculated from the Cu-0 distance of X-ray diffraction data and the water molecule geometry.24 ESEM studies show the same distance in zeolites within this range.13J4 However, the ESEM distances for Mo-doped samplesare larger than for Cu(I1) even though the ionic radius of Cu(I1) (0.087 nm) is -0.01 nm larger than that of Mo(V) (0.075nm).z5 It is well-known that molybdenumusually exists as oxomolybdenum ions such as [ M d ] or [O=MO==O].~ Within a restricted spacesuch as a channel in SAPOmolecularsieves, the coordination of such oxomolybdenum species can differ from that of Cu(I1) which usually does not exist as an oxo species. The adsorbate interactions and coordination properties of Cu(II), as a typical cation in SAPO materials, in CuHSAPO-5 and CuHSAPO11 have been studied and compared using ESR and ESEM techniques.13J4The locationof Mo(V) cation in Mo-dopedSAPO is expected to be in a typical cation site in SAPO material, namely, coordinated to a 6-ring in the large channel as found for example for Cu(II).13J4 The distance between Mo and D (Dz0) is longer than for Cu(I1) because oxygen is already coordinated to molybdenum to form the molybdenyl ion. For CD3OH-adsorbedsamples, while Mo(V) is coordinated to two methanols at 0.49 nm and one more methanol at 0.37 nm in
0
1
2
T,
w
Figure 4. Experimental and simulated (dashed line) threepulse ESEM at 4 K of (A) MoHSAPO-5 and (B) MoHSAPO-11 with adsorbed CD3OH and of (C) MoHSAPO-5 and (D) MoHSAPO-11 with adsorbed CH3OD (see Table 111 for simulation parameters).
MoHSAPO-5, Mo(V) is coordinated to only one methanol at These Mo(V)-D distances are also longer than those of Cu(I1)-D by 0.04-0.08nm in Cu(I1)-doped samples. The ESEM simulation for the CH3OD-adsorbed samples is consistent with that of the CD30H-adsorbed ones. The experimental and simulated threepulse ESEM for MoHSAPO-5 and MoHSAPO-11 with adsorbed CD3OH and CH3OD are shown in Figure 4. The different coordination numbers for water and methanol to Mo(V) in SAPO-5 and SAPO-11 are consistent with the different sizes of the 12-ringchannel in SAPO-5 versua the smaller 10-ring channel in SAPO-1 1. Three waters are coordinated to Cu(I1) in SAPO-5, but only two are coordinated in SAPO-1 1. The same is found for Mo(V) in SAPO-5 and SAPO-11. However,there is sufficient space to coordinate three waters even in the 1O-ring channel, so in previous work on Cu(I1) coordination it was proposed that Cu(I1) locates in a more recessed position from the main channel (site II1*) in SAPO-11 than in SAPO-5 (site II2*).14b The same interpretationcan be appliedto the Mo(V) location in SAPO-1 1 and SAPO-5. With methanol, Cu(I1) directly coordinates to only two molecules in both SAPO-1 1 and SAPO-5 (Table 111). And it also indirectly coordinates to one more distant methanol. This is consistent with the larger diameter of methanol (0.32 nm)26 0.48 and 0.37 nm, respectively, in MoH-SAPO-11.
11814 The Journal of Physical Chemistry, Vol. 97, No. 45, 1993
z 3
(A) MoH-SAPO-~/C~D~ 0.8
v)
z
- 0.27 P
8 01 11 . 2.
.
3
.
4
.
5
T, PS
Figure 5. Experimental and simulated (dashed line) three-pulse ESEM at 4 K of (A) MoHSAPO-5 and (B) MoHSAPO-11 with adsorbed C2D4 (seeTable 111 for simulation parameters).
Lec et al. However, subsequent dehydration results in the formation of Mo(V) species which are characterized by ESR. The ESEM results indicate that the Mo(V) species exist as oxomolybdenum ions as either (MOO)'+ or (MOO# (Figure 6). The (Mo(V)OZ)+speciesseemsmore probable becauseof the followingreasons. Since SAPO-5 and -1 1 have a low negative framework charge,a more positively charged specieslike (Mo(V)O)'+ are not as easily stabilized. Similar results were obtained in Pd-exchanged SAPO-5 and SAPO-11 molecular sieve^.^^^^^ Also, no tetrahedrally coordinated Mo species was observed. The possibility of the presence of dioxomolybdenum species was suggested ion is consistent with the p r e v i o ~ s l y . ' ~An ~ ~oxomolybdenum ~ longer coordination distances to water and methanol that are found for Mo(V) versus Cu(I1).
Acknowledgment. This research was supported by the National Science Foundation and the Robert A. Welch Foundation.
Refereacea and N o h
Figure 6. Possible oxomolybdenum species on the surface of SAPO-5 and SAPO-1 1 molecular sieves: (a) monoxomolybdenum species and (b) dioxomolybdenum species.
compared to that of water (0.265 nm).27 However, the Mo(V) coordinationwith methanol is different in SAPO-1 1versus SAPOS; two methanols are directly coordinated in SAPO-5 but only one in SAPO-1 1. It is suggested that this reflects the longer Mo(V)-methanol distances compared to Cu(I1)-methanol and that the Mo(V) is an oxo species which requires more space. Adsorption ofC2D4on MoHSAPO-11 shows interaction with one ethylene at a distance (Mo(V)-D) of 0.45 nm, which is longer by 0.09 nm compared to that of CuH-SAPO-11 with adsorbed CzD4. This is consistent with the other coordination distance differences between Cu(I1) and Mo(V). The experimental and simulated three pulse ESEM patterns for MoH-SAPO-5 and MoH-SAPO-11 with adsorbed C2D4 are shown in Figure 5. It is most interesting that the coordination distance is shorter in SAPO-5. This is consistent with a more recessed position for Mo(V) in SAPO-11 (site III*) than in SAPO-5 (site IIz*) to which the bulky, nonpolar CzD4 cannot approach optimally.
Conclusions A solid-state reaction of Moo3 with HSAPO-5 and -11 molecular sieves generates no paramagnetic Mo(V) species.
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