3110
J . Phys. Chem. 1992, 96, 31 10-31 13
Comparative Spectroscopic Studies on MnSAPO-11, (L)MnH-SAPO-1 1, and (S)MnH-SAPO-11 Molecular Sieves (SAPO = Silicoaluminophosphate) Chul Wee Lee, Xinhua Chen, Guillaume Brouet, and Larry Kevan* Department of Chemistry, University of Houston, Houston, Texas 77204-5641 (Received: September 10, 1991)
Thermogravimetric analysis (TGA), infrared (IR), electron spin resonance (ESR), and electron spin echo modulation (ESEM) spectroscopic techniques have been used to obtain direct evidence for the incorporation of Mn(I1) into the framework positions of the SAPO-I1 (MnSAPO-11) by comparing with those of ion-exchanged samples, (L)MnH-SAPO-11 and (S)MnH-SAPO-11, where the Mn(I1) is in an extraframework position. TGA of as-synthesized MnSAPO-11 shows that the decomposition temperature in the range 400-510 OC of the di-n-propylaminetemplate is 15-20 ‘C higher than that in as-synthesized SAPO-11. ESR shows that at higher content Mn(I1) in MnSAPO-11 (about IO mol % Mn(II)), Mn(I1) ions occupy both framework and extraframework positions. Infrared spectra show that the position of a band at about 3450 cm-I, which is assigned to a hydroxyl group, is shifted by about 10 cm-’ toward lower energy in MnSAPO-11 versus MnH-SAPO-11. The modulation depth of the two-pulse ESE of MnSAPO-11 with adsorbed D 2 0 is deeper than that of (L)- and (S)MnH-SAPO-11 with adsorbed D20. Simulations of three-pulse ESEM of MnSAPO-11 with adsorbed D 2 0 and C H 3 0 D also show evidence for the incorporation of Mn(I1) into a framework position of MnSAPO-I 1 at low Mn(I1) content.
Introduction The syntheses of a series of microporous aluminophosphate (AlPO) and silicoaluminophosphate (SAPO) molecular sieves as well as their framework-substituted (Mn(II), Co(II), Mg(II), Zn(II), and Fe(I1)) analogues have been reported.’-5 Although there are several studies on transition-metal ion substitutions in AIPO-n and SAPO-n compounds, important remaining questions include the following. What is the maximum loading of transition metal ions? Are the transition metal ions incorporated into tetrahedral framework positions? How can one distinguish Mn(I1) in a framework position from an ion-exchanged position? Several studies on CoAPO-5 showed that the Co(I1) is incorporated into tetrahedral framework sites.610 Goldfarb et al.”J2 reported that the majority of the Mn(I1) in MnAPO-5 does not occupy a framework site, while Brouet et al.I3 reported direct evidence for Mn substitution in a framework position in MnA1PO-1 1. Pluth et al.14 showed that Mn(I1) is tetrahedrally coordinated in framework positions of as-synthesized MnAPO-11, [(h.InA19)Plo040.C6H,6N],by single-crystal X-ray diffraction. In this work we present spectroscopic evidence that Mn(I1) is incorporated into a framework position of MnSAPO-11 by comparison with ion-exchanged samples. Experimental Section Syntheses and Sample Treatment. MnSAPO-11 was synthesized by hydrothermal crystallization according to a patent.3 The following chemicals were used without further purification: 85% H3P04(Mallinckrodt), aluminum isopropoxide (Aldrich), LUDOX LS (Du Pont), di-n-propylamine (Aldrich), Mn(CH3CO(1) Lok, B. M.; Vail, L. D.; Flanigen, E. M. Eur. Pat. Appl. E.P. 158,348, 158,975, 161,491, 1985. (2) Flaniaen, E. M.: Lok. B. M.; Patton, R. L.; Wilson, S. T. Eur. Pat. Appl. E.P. fi8,976, 1985. (3) Lok, B. M.; Marcus, B. K.; Flanigen, E. M. Eur. Pat. Appl. E.P. 161,490, 1985. (4) Wilson, T. S . ; Flanigen, E. M. US.Patent 4,567,029, 1986. (5) Lok, B. M.; Messina, C. A.; Patton, R. L.; Gajek, R. T.; Cannan, T. R.; Flanigen. E. M. US. Patent 4,400,871, 1984. (6) Montes, C.; Davis, M . E.; Murray, B.; Narayana, M. J. Phys. Chem. 1990, 94, 6425. (7) Tapp, N. J.; Milestone, N . B.; Wright, L. J. J . Chem. Soc., Chem. Commun. 1985, 1801. (8) Iton, L. E.; Choi, 1.; Desjardins, J. A.; Maroni, V. A. Zeolites 1989, 9, 5 3 5 . (9) Xu, Y.; Maddox, P. J.; Thomas, J. M. Polyhedron 1989, 8, 819. (10) Shiralkar, V. P.; Saldarriage, C. H.; Perez, J. 0.;Clearfield, A,; Chen, M.; Anthony, R. G.; Donahue, J. A. Zeolifes 1989, 9, 474. ( I 1) Goldfarb, D. Zeolites 1989, 9, 509. (12) Levi, Z.; Raitsimring, A. M.; Goldfarb, D. J . Phys. Chem., in press. (13) Brouet, G.;Chen, X.; Kevan, L. J . Phys. Chem. 1991, 95, 4928. (14) Pluth, J. J.;Smith, J. V.; Richardson, W. J., Jr. J . Phys. Chem. 1988, 92. 2734.
0022-3654/92/2096-3 1 10$03.00/0
TABLE I: XRD Data of As-Synthesized MnSAPO-11 ref 3 this work 20 1001/I” 20 1001/1, 8.1 9.5 13.1 15.7 16.2 19.1
36 61 19 36
20.5 21.1 22.2 22.5 22.7 23.2 24.5 24.8 25.0 26.4
45 100 55 52 61 71 13 16 13 26
28.3 28.6 29.5 31.5 32.8 34.2 35.4 35.8 36.3 37.5 37.8 39.4
13 23 13 16 23 16 IO IO IO 13 16 10
42.9 44.8
IO IO
48.8 50.6 54.6
3 10
10 13
IO
8.1 9.5 13.2 15.7 16.3 19.0 19.6 20.5 21.1
22.2
21 36 17 33 12 11 11 45 100
22.6 22.8 23.2
58 62 62 81
24.8
19
26.4 26.7
22 26
28.7 29.5 31.5 32.9 34.2
27 14 19 27 20
35.9 36.6 37.6 37.9 39.1 42.1 42.8 43.8 45.0 47.1 19.0 50.7 54.8
12 13 21 26 9
IO 14
9 15 9 11
15 14
0)2-4H20 (Sigma), and MnCl2.4H20 (Allied Chemical). A solution was prepared by mixing 3.90 mL of H3P04( 8 5 wt %) with 11.O mL of H 2 0 and stirred for 10 min. Finely ground aluminum isopropoxide (1 1.559 g) was added slowly to this solintion while stirring. Then 1.61 g of 30 wt % LUDOX LS and 6.0 mL of H 2 0were added drop by drop with stirring for about 30 min. To this solution, 8.0 mL of 1.52 X M Mn(CH,C00),-4H,O and 4.20 mL of di-n-propylamine were added drop by drop. The solution was aged with stirring at room temperature overnight to form a gel. The reaction mixture with a bulk chemical 0 1992 American Chemical Society
Spectroscopic Studies on SAPO Molecular Sieves composition of 1.08 n-Pr,NH:4.03 X Mn0:0.28 Si02:A1203:P205:50H 2 0 (Le., 100 Mn/Si A1 P Mn r 0.1 mol a) was placed in a stainless steel pressure vessel lined with Teflon and heated in an oven at 215 "C at autogenous pressure for 4 days. Calcined MnSAPO-11 was prepared by heating the as-synthesized sample in air at 550 "Cfor 48 h. The XRD pattern of the as-synthesized and calcined MnSAPO-11 agree reasonably with the patent (see Table I).3 Samples ion-exchanged in the liquid phase (L) and denoted by (L)MnH-SAPO-l 1 were prepared by adding 15 mL of 2 X M Mn(CH3C00)2.4H20and 85 mL of deionized water to 1 g of calcined H-SAPO- 11I s followed by stirring overnight. The sample was then filtered, washed with hot (about 70 "C) distilled water three times and dried in air at room temperature. A second type of sample ion-exchanged in the solid phase (S) and denoted by (S)MnH-SAPO-11 was prepared as follows: A mixture of 0.0075 g of MnCl2.4H20 and 1 g of H-SAPO-11 were ground with a mortar and pestle. The mixture was pressed in a steel die with about 2 tons for 20 min to make wafers of 12-mm diameter and 2.5-mm thickness. Then the wafer was placed in a quartz boat and heated in a muffle furnace at 500 "C in air for 8 h. It has been shown that this solid-state reaction generates Mn(I1) in ion-exchange sites in H-ZSM-5 zeolite. l6 Samples for ESEM experiments were pretreated by evacuation at 350-390 OC overnight and equilibration with D20, CH30D, and C2D4at room temperature overnight before sealing in Suprasil quartz tubes with the sample end immersed in liquid nitrogen. Measurements. The synthetic molecular sieves were examined by powder X-ray diffraction (XRD) with a Philips PW 1840 diffractometer. Thermogravimetric measurements were performed using a Dupont 951 thermal analyzer (heating rate of 10 "C/min). Infrared spectra were obtained with a Nicolet 740 FT-IR spectrophotometer by the KBr pellet method. X-band ESR spectra were recorded a t 77 K with a Varian E-4 spectrometer. The microwave frequency was measured by a Hewlett-Packard HP 5342A frequency counter. ESEM spectra were measured at 4 K on a home-built spectrometer." Two- and three-pulse echoes were recorded by using 9Oo-~-l8O0 and 9O0-r9Oo-T-9O0 pulse sequences, where the echo is measured as a function of T and T, respectively.
The Journal of Physical Chemistry, Vol. 96, No. 7, 1992 3111
+ + +
Results and Discussion There are no significant differences in the XRD patterns between as-synthesized MnSAPO- l l and as-synthesized SAPO- l l , This indicates that MnSAPO-11 possesses the same framework structure as SAPO-1 1. Since, the XRD patterns of SAPO-1 1, AlP04-11, and Mn-SAPO- 11 are expected to be almost the same, it is not easy to determine the presence of SAPO-1 1 and AlP04-l1 phases from the MnSAPO-11 XRD pattern. From a comparison of the XRD patterns of SAPO-1 1 or AlP04-1 1 synthesized according to the literature with that of MnSAPO-11, no peak broadening was seen. But the degree of crystallinity of MnSAPO- 11 is about 86% of that of SAPO-1 1 and AlP04-11, Thus it is not surprising that the XRD does not give definitive information on possible multiple phases present. When Mn(I1) is added during synthesis to form MnSAPO-11, the product is pale violet. However when Mn(I1) is incorporated by liquid- or solid-state ion exchange to form MnH-SAPO-11, the product is white. This color difference indicates different sites for Mn(I1) in the synthesized and ion-exchanged materials. There is a significant color difference depending on the position of Co(I1) which is a tetrahedral framework or ion-exchanged site in C O A P O - ~ . ~ . ~ J * Electron Spin Resonance. At high Mn(I1) content (about 10 mol % Mn(I1)) in MnSAPO-11, the ESR spectrum shows a broad line with a first-derivative width, AH,equal to 510 G. This indicates strong spinspin interaction which broadens any hyperfine (15) Lee, C. W.; Chen, X.; Kevan, L. J. Phys. Chem. 1991, 95, 8626. (16) Beran, S.; Wichterlova, B.; Karge, H. G.J. Chem. SOC.,Faraday Trans. 1990, 17, 3033. (17) Narayana, P.A.; Kevan, L. Magn. Reson. Reu. 1983, 7, 234. (18) Kraushaar-Czarnetzki, B.; Hoogervorst, W. G. M.; Andrea, R. R.; Emeis,C. A.; Stork, W. H. J. J. Chem. SOC.,Faraday Trans. 1991,87, 891.
ange
200G,
after exchange with Na+
g = 2.00 A = 94.5 G
Figure 1. ESR spectra at 77 K of calcined and hydrated MnSAPO-11 (- 10 mol % Mn): (a) original sample and (b) treated with 2 M NaCl at 80 OC and dried at room temperature.
g8r2y)
as-syn SAPO-I I
g
- 0.08
- 0.06
96
$
- o,,o
goo ~
400 T, "C
200
600
i
.-0 W
3
100 98 -
\
96 -
\
I Io4
(bl as-syn as-svn MnSAPO- I I (b) (0.1&I %Mn)
a ._ I
6
A
94 92 90 I
---I
;
880
200
400 T, "C
600
Figure 2. Thermogravimetric analysis curves for (a) as-synthesized SAPO-11 and (b) as-synthesized MnSAPO-11 (0.1 mol 7% Mn).
components so that only a broad line is observed. After treatment with NaCl or CaCl, aqueous solution, the ESR spectrum shows six resolved hyperfine lines with a splitting A = 94.5 G centered at g = 2.00 (see Figure 1). This indicates that the sample contains at least some extraframework Mn(I1) ions which are exchangeable by Na+ or Ca2+. Similar results are found for (L)MnH-SAPO-1 1. Thus ESR does not distinguish between the synthesized and ion-exchanged material. Thermogravimetric Analysis. The TGA curves of as-synthesized SAPO- 1 1 and as-synthesized MnSAPO- 11 are shown i n Figure 2. They show four stages of weight loss at -60, -280, -400, and -510 OC. The first stage of weight loss is due to the desorption of water physically adsorbed on SAPO-l l and MnSAPO-1 1. The other losses are probably due to the decomposition of the organic templating agent, di-n-propylamine. While the temperature of the second loss is similar for both samples, the temperatures of the third and fourth losses for the as-synthesized MnSAPO- 11 are higher than the as-synthesized SAPO-1 1 by about 15-20 "C. The desorption near 280 "C is assigned to the
3112 The Journal of Physical Chemistry, Vol. 96, No. 7, 1992
4000
3400 3200 3000 cm-1 Figure 3. Infrared spectra of (a) (S)MnH-SAPO-11, (b) calcined HSAPO-11, (c) (L)MnH-SAPO-11, and (d) MnSAPO-11 (0.1 mol '% Mn).
3800
Lee et al.
3600
physically adsorbed free di-n-propylamine and the desorptions near 400-510 "Care assigned to the protonated dipropylamine, which indicates the presence of acid sites.I9 The higher decomposition temperature for as-synthesized MnSAPO-11 indicates that the Mn(I1) in MnSAPO-11 possesses a different local electronic environment than does Mn(I1) in ion-exchanged MnH-SAPO-11. While the weight loss of SAPO-1 1 is complete at about 600 O C , that of MnSAPO-11 continues to about 750 O C . This might be due to the presence of some amorphous phase in MnSAPO- 11. Infrared Spectra. Further evidence for the difference of Mn(I1) in the synthesized and ion-exchanged samples comes from infrared spectra in the frequency region 4000-3000 cm-l (Figure 3). While the position of the band at 3600 cm-I ( v l ) does not change, the band at 3450 cm-l ( v 2 ) of MnSAPO-11 is shifted by about 10 cm-I toward lower energy compared to it in MnH-SAPO-11 and calcined H-SAPO- 11. The MnSAPO- 11 framework is regarded as a silicoaluminophosphate matrix which contains Mn(I1) ions isomorphously substituted for P(V), Al(III), or Si(1V) ions. This substitution can result in the formation of new OH groups bridging Mn(I1) and another tetrahedral ion in the framework as found in zeolites.20 IR bands at 3600 and 3450 cm-I have been assigned to -Si--OH-Al- groups in SAPO-5 and SAPO-34, respectively, formed during the isomorphous substitution of Si(1V) for P(V) in a framework psition.21v22 These two bands are close to those observed for the four SAPO-1 1 samples in this work. Actually t h e band at 3450 cm-' is overlapped with a band of the hydroxyl group in water (H-0-H),23 which is physically adsorbed in molecular sieves. The observed shift indicates changes in the OH vibrational mode, which can be influenced by such factors as the coordination number and the hydrate structure. Electron Spin Echo Modulation. The magnitude of the ESE modulation depth is related to the interaction distance R , the (19) Parker, L. M.; Bibby, D. M.; Patterson, J. E. Zeolites 1984,4, 168. ( 2 0 ) Ward, J. W. In Zeolite Chemistry and Catalysis; Rabo, J., Ed.; ACS Monograph 171; American Chemical Society: Washington, DC, 1976; p 11 8. (21) Hedge, S. G.; Ratnasamy, P.; Kustov, L. M.; Kazansky, V. B. Zeolites 1988,8, 137. (22) Zubkov, S. A.; Kustov, L. M.; Kazansky, V. B.; Girnus, 1.; Fricke, R. J. Chem. SOC.,Faraday Trans. 1991, 87, 897. (23) Scargill, D. J. Chem. SOC.(London) 1961, 4444.
z, jls Figure 4. Two-pulse electron spin echo modulation recorded at 4 K of (a) MnSAPO-11 (0.1 mol '% Mn), (b) (S)MnH-SAPO-11, and (c) (L)MnH-SAPO- 11 with adsorbed D20. The normalized deuterium depths.(a/(a + b)) are 0.71, 0.54, and 0.58, respectively.
(a) MnSAPO-1l Q O
0.8
Shell 1 2
a
N
R.
nm
A , MHr 0.24 0.01
025 0.34
2 2
0.6 1 = 0.281 s
..._CALC
z 0 W
I 0
1
2
3
4
5
T, P
(b) MnSAPO-1llCH3OD
U
9
t 2 v)
N 2
R. nm 0.25
A, MHz 0.22
, I:C .-- n"._"OR r"
.... CALC
z 0.4 w I-
z
p O.* Y T, ps Figure 5. Experimental and simulated three-pulse ESEM spectra recorded at 4 K of (a) MnSAPO-11 (0.1 mol % Mn) with adsorbed D20 and (b) MnSAPO-11 (0.1 mol % Mn) with adsorbed CH,OD.
Spectroscopic Studies on SAPO Molecular Sieves
The Journal of Physical Chemistry, Vol. 96, No. 7, 1992 3113
/"
E Z 3
'.o!l
a 0.8
MnSAPO.1 l/C204 N 4
R , nm
A , MHz
0 34
0 24
T = 0 28
ps
.... CALC
0
1
2
5
4
3
T, PS Figure 7. Experimental and simulated three-pulse ESEM spectra recorded at 4 K of MnSAPO-I1 (0.1 mol % Mn) with adsorbed C2D4.
TABLE II: ESEM Parameters for Several Matrices with Adsorbed
C A (b)
F
i 6. Schematic representations of hydrated (a) MnH-SAPO-11 and
(b) MnSAPO-11. From ESEM data, the distances of Mn(I1)-D, and Mn(II)-D2 are 0.25 and 0.34 nm, respectively.
number of interacting nuclei N , and the isotropic hyperfine coupling constant A.24 While the modulation depth for MnSAPO-11 with adsorbed DzO is about 0.71, that for (L)- and (S)MnH-SAPO-11 is about 0.58 and 0.54,respectively (see Figure 4). This indicates that the local environment of Mn(I1) in (S) and (L)MnH-SAPO-1 1 is almost the same and is different for Mn(I1) in MnSAPO-11. This is consistent with the finding that the location of Cu(I1) in (S)CuH-SAPO-1 1 is almost identical to that of Cu(I1) in (L)CuH-SAPO-1 l.25 The crystal structure of hydrated Mn(I1)-exchanged zeolite A has been determined by singlecrystal diffraction.26 Mn(I1) ions are found to be pentacoordinated in a trigonal bipyramidal manner with two axial waters and three framework oxygens. The average distance between Mn(I1) and the oxygens is approximately 0.20 nm, from which the distance between Mn(I1) and the water hydrogens can be calculated to be approximately 0.27 nm. This distance is thus characteristic of direct coordination of Mn(I1) to water. To determine the interaction distance and the number of interacting nuclei, we attempted to measure three-pulse ESEM with MnH-SAPO-11 and MnSAPO-11 samples but only succeeded for MnSAPO-11 with adsorbed DzO. The simulation of the threepulse ESEM data from the MnSAPO-11 sample requires a two-shell model with the following parameters: N 1 = 2,RI = 0.25 nm, A I = 0.24 MHz, and N2 = 2,R2= 0.34nm, A2 = 0.01 MHz, where N denotes the number of deuterons, R is the distance between Mn(I1) and the deuterons, A is the isotropic hyperfine coupling constant, and subscripts 1 and 2 indicate the shell number (see Figure Sa). This two-shell model agrees well with that from the ESEM simulation of MnAPO-11 with adsorbed D20.13 Since R1 is significantly shorter than 0.27 nm, the oxygen end of water cannot coordinate directly with Mn(I1). The most likely Mn(1I) local geometry is with two waters oriented such that one of the deuteriums (DJ is only 0.25nm away from the Mn(I1) while the other deuterium (D2) is 0.34nm away (see Figure 6b). Such a geometry indicates that Mn(I1) is in a negatively charged site, which is evidence for Mn(I1) substituted into a framework site. It should be noted that a D-0 bond orientation has also been indicated for trapped electrons in frozen aqueous glasses.27 Previously we found that the distance between ion-exchanged Cu(I1) and D for (L)CuH-SAPO-11 with adsorbed D 2 0 was 0.28 (24) Kevan. L. In Time Domain Electron Spin Resonance; Kevan, L., Schwartz, R. N., Eds.; Wiley-Interscience: New York, 1978; Chapter 8. (25) Lee, C. W.; Chen, X.;Kevan, L. J. Phys. Chem. 1992, 96, 357. (26) Yanagida, R. Y.;Vance, T. B., Jr.; Seff, K. fnorg. Chem. 1974, 13, 723. (27) Narayana, P. A.; Bowman, M. K.; Kevan, L.; Yudanov, V. F.; Tsvetkov, Y. D.J. Chem. Phys. 1975, 63, 3365.
matrix CuNa-A" CuH-rhob (S)CuH-SAPO-11 MnSAPO-11
N
R,nm
A,MHz
4 4 4 4
0.38 0.36 0.36
0.0 0.0 0.15
28 25
0.34
0.24
this work
"CuNa-A refers to Cu(I1)-exchanged NaA zeolite. ers to Cu(I1)-exchanged H-rho zeolite.
ref
29
CuH-rho ref-
nm.I5 Figure 6 represents the possible geometrical differences of MnH-SAPO-11 and MnSAPO- 11. While ion-exchanged Mn(I1) ions coordinates to three framework oxygen atoms in a six-ring window and to two axial water molecules resulting in a trigonal-bipyramidal geometry (Figure 6a), Mn(I1) MnSAPO-11 is located in a tetrahedral framework site and interacts with two deuteriums (D1) in bond-oriented water. Other supporting evidence for this adsorbate orientation comes from C H 3 0 D adsorption for which the best ESEM simulation corresponds to interaction with two methanol molecules at a Mn(I1)-D distance of 0.25 nm (Figure Sb). If Mn(I1) coordinates to the oxygen end of CH30D, the Mn(I1)-D distance is about 0.27 nm. The shorter interaction distance (0.25nm) between Mn(I1) and D1 is too short for normal coordination in which the metal ion coordinates to the oxygen. Adsorption of C2D4 on MnSAPO-11 results in interaction with four deuteriums at a distance of 0.34 nm (see Figure 7). This is interpreted as coordination of Mn(I1) to one molecule of ethylene with its molecular plane perpendicular to a line toward Mn(I1). Similar observations were made by Anderson et a1.,2* Ichikawa et and Lee et al.25for Cu(I1) ion-exchanged in CuH-rho, CuNa-A zeolite, and CuH-SAPO- 11, respectively (see Table 11).
Conclusions This work shows spectroscopic evidence that Mn(I1) is in a framework position in synthesized MnSAPO- 1 1 by comparison with ion-exchanged MnH-SAPO-11 samples in which Mn(I1) is in an extraframework position. At low Mn(I1) content Mn substitutes into a framework position of MnSAPO-11. The pale violet color of calcined MnSAPO- 1 1, the higher decomposition temperature of the templating agent for as-synthesized MnSAPO-1 1, the shift of the OH vibrational frequency toward lower energy for MnSAPO-11, and the deeper two-pulse ESE modulation depth of for MnSAPO-11 indicate that the local environment of Mn(1I) in MnSAPO-11 is different from that in the ion-exchanged (L)- and (S)MnH-SAPO-11 samples. More direct evidence is found from threepulse ESEM simulations of adsorbate structure for MnSAPO-11 with adsorbed DzO and CH30D. Acknowledgment. This research was supported by the National Science Foundation, the Robert A. Welch Foundation, and the Texas Advanced Research Program. (28) Anderson, M. W.;Kevan, L. J. Phys. Chem. 1986, 90, 6452. (29) Ichikawa, T.; Kevan, L. J. Am. Chem. Soc. 1981, 103, 5355.
