J. Phys. Chem. 1995, 99, 6937-6943 6937

Small Pore Aluminum Phosphate Molecular Sieves with Chabazite Structure: Incorporation of Cobalt in the Structures -34 and -44

Sunil Ashtekar; Satyanarayana V. V. Chilukuri,* A. M. Prakash,+ C. S. Harendranath,* and Dipak K. Chakrabarty*$+ Solid State Laboratory, Department of Chemistry, and Regional Sophisticated Instrumentation Centre, Indian Institute of Technology, Bombay 400076, India

Received: September 12, 1994@

CoAPO and CoSAPO with -34 and -44 framework structures have been synthesized. The presence of cobalt in the framework has been established by a number of experimental techniques. Distribution of cobalt in the crystal was heterogeneous for the CoAPSOs. Using electron microprobe analysis, it was observed that cobalt is homogeneously distributed in the CoAPOs while in the CoSAPOs it has higher concentration near the center of the crystal that progressively decreases toward the edges.

Introduction

It is well-known that metal ions can substitute aluminum in many AlP04-n molecular sieve framework structures. Co2+ can enter into the framework only at the aluminum site.* The presence of cobalt has been reported in a number of Alp04 structures such as -5, -11, -14, -18, -25, -34, and -44.3-12 Some of these structures can accept cobalt only when silicon is also present at the T sites.

The structures -34 and -44 with silicon in the framework (SAPO-34 and SAPO-44) crystallize in the chabazite (a small pore zeolite) structure and are of particular interest as catalysts, as they have been found to have high selectivity for ethene in the methanol dehydration reaction.13-16 The mechanism of silicon substitution in these two structures has been discussed by Ashtekar et al.17 It has been reported by Das et a1.I2 that the substitution of Co2+ in AlP04-5 generates acid sites that are different from those present in SAPO-5. If this trend is followed by other CoAPOs, cobalt-substituted aluminum phos- phate molecular sieves will be interesting materials as solid acid catalysts. It has been recently reported that Co2+ in the tetrahedral framework site of a molecular sieve can be reversibly oxidized to Co3+ that opens up the possibility of using these materials as redox catalysts.11 In view of such possibilities, we have undertaken a systematic study of the transition metal containing aluminum phosphate molecular sieves with different framework structures. The results on the two small pore structures -34 and -44 will be reported here.

Experimental Section

Synthesis. Tetraethylammonium hydroxide is generally used for the synthesis of the structure -34. We have successfully synthesized SAPO-34 by using a cheaper template m~rpholine.'~ For SAPO-44, cyclohexylamine is generally used as the template. We have used morpholine and cyclohexylamine for the synthesis of the cobalt containing samples with structures -34 and -44, respectively.

CoAPOs were synthesized by the following method. Pseudo- boehmite and orthophosphoric acid were dissolved in water to which cobalt sulfate solution was added with vigorous stimng.

+ Solid State Laboratory, Department of Chemistry, Indian Institute of

* Regional Sophisticated Instrumentation Centre, Indian Institute of

@ Abstract published in Advance ACS Absrmcrs, April 1, 1995.

Technology.

Technology.

0022-3654/95/2099-6937$09.00/0

TABLE 1: Gel Composition and Synthesis Conditions reaction condition

gel composition temp time samDle COO A1201 PZOS Si02 T" H20 DH ("(2) (h) . . - . . , . .

COAPO-34 0.4 0.8 1.0 2.7 60 6.1 180 72 CoSAPO-34/1 0.03 0.985 1.0 0.6 2.7 60 7.1 200 48 CoSAPO-34/2 0.1 0.95 1.0 0.6 2.7 60 6.8 200 48 COAPO-44 0.4 0.8 1.0 1.9 60 5.5 180 72 CoSAPO-44/1 0.03 0.985 1.0 0.6 1.9 60 6.1 190 48 CoSAPO-44/2 0.1 0.95 1.0 0.6 1.9 60 5.9 190 48

T = template.

This was followed by the addition of the template. In the case of the CoSAPOs, a solution of fumed silica in the template was added instead of the template alone. The resulting gel was heated in a stainless steel autoclave under autogeneous pressure. The gel composition and synthesis conditions are given in Table 1. CoSAPOs of two different compositions each were prepared. The crystals were filtered, washed with water, and dried at 110 "C for 12 h.

Characterization. X-ray powder diffraction patterns were recorded on a JEOL/JSM-8040 instrument using Cu K a radiation with a nickel filter. Crystals were observed under a Reichert MeF3 A optical microscope. Scanning electron microscopy and high-resolution electron microprobe analysis were carried out using a Cameca-Su-SEM-PROBE analytical scanning electron microscope. X-ray line profile analysis was carried out using an inclined wavelength dispersive spectrometer, and quantitative microprobe analysis was performed using a Kevex Energy dispersive system. A highly regulated electron beam having a probe size '0.2 pm was used for microprobe analysis.

Differential thermal (DTA) and thermogravimetric (TG) analyses were carried out in air on a Dupont 9900 thermal analyzer at a heating rate of 10 "C/min.

Infrared spectra as KBr disks were recorded using a Nicolet 170 SX FT-IR spectrometer.

MAS NMR spectra were recorded on a Varian VXR-300s spectrometer with a Doty Scientific CP-MAS probe. The frequencies were 78.15, 121.41, and 59.59 MHz for 27Al, 31P, and 29Si, respectively. Forty-five degree pulses were used for all measurements with repetition times of 3 s for 27Al and 10 s for 31P and 29Si. Data were acquired at a MAS speed of 4.5 KHz. Aluminum nitrate in water, 85% phosphoric acid, and tetramethylsilane were employed as references.

1995 American Chemical Society

6938 J. Phys. Chem., Vol. 99, No. 18, 1995

-

Ashtekar et al.

I 1 1 - - I 2

t 10 1'5 20 25 30 35

-213 - Figure 1. X-ray powder diffraction pattems of as-synthesized samples: (a) CoAPO-34; (b) CoAPO-44.

TABLE 2: Samples from Temperature-Programmed Desorption of Ammonia

Chemical Composition and Acidity of the

acidity (P + si)/ (-0Ug)

sample Co A1 P Si (A1 + Co) moderate strong COAPO-34 0.141 0.362 0.496 COSAPO-3411 0.011 0.495 0.372 0.123 0.98 0.86 0.82 CoSAPO-34/2 0.035 0.452 0.387 0.126 1.05 0.86 0.78 COAPO-44 0.119 0.406 0.474 0.96 COSAPO-4411 0.012 0.490 0.381 0.116 0.99 0.79 0.83 CoSAP0-44/2 0.029 0.463 0.391 0.116 1.03 0.80 0.72

Chemical analysis of the samples was carried out after calcination at 500 "C. The samples were dissolved in aqua regia for analyzing aluminum and phosphorus. The undissolved portion was fused with lithium metaborate and subsequently dissolved in dilute nitric acid. Analysis was done on an atomic emission spectrometer with an ICP source (Labtam Plasma Lab 8440).

The acidities of the calcined samples were determined by temperature-programmed desorption of ammonia at a heating rate of 10 "Chin . The as-synthesized samples were first calcined by raising the temperature at 1 "Ch in up to 450 "C, and the sample was held at this temperature for 12 h.

UV-visible diffused reflectance spectra of the solid samples were recorded on a Shimadzu-260 UVNis spectrophotometer.

Results and Discussion

Synthesis. The CoAPOs could be crystallized in pure form only at 180 "C. At lower temperatures, pure phases could not be obtained even on prolonged heating for 7 days. Both the CoAPOs and CoSAPOs needed a template to aluminum ratio greater than 2 in order to avoid crystallization of impurity phases.

The compositions of the as-synthesized samples are given in Table 2. If it is assumed that cobalt can occupy only the aluminum sites and silicon can go to the phosphorus sites only, the (A1 + Co):(P + Si) ratio should be unity. As seen from Table 2, this is about correct in all the samples except CoAPO- 44, which had a higher amount of Co + Al. This suggests the

TABLE 3: Results of Thermal Analysis water template no. of loss loss template DTA peak

sample (wt. %) (wt. %) molecules1u.c. temp ("C)

COAPO-34 COSAPO-3411 COSAPO-3412 SAPO-34 COAPO-44 COSAPO-4411 COSAPO-4412 SAPO-44

4.5 11.3 4.5 16.5 4.5 17.0 4.8 17.5 1.4 19.3 3 19.0 2 19.5 2.8 19.5

3.62 403,502,670 5.26 417, 577 5.52 43 1,586 5.6 430,610 (ref 17) 5.71 477,574,660 5.39 450,572 5.55 488,560 5.4 450,580 (ref 17)

TABLE 4: IR Band Positions in Framework Vibration Region (cm-')

sample asym stretch sym stretch D-6-R T-0 bend COAPO-34 1123 734

713

COSAPO-3412 1116 742 1044

COAPO-44 1080 738 706 677

COS APO-4412 1109 742

688 1019

623 562 497

418 634 526 562 479

439 64 1 504 5 94 482

425 64 1 526 576 479

425

presence of extraframework cobalt. The presence of ex- traframework cobalt is possible in the other samples also, though to a lesser extent.

X-ray Diffraction. The X-ray powder diffraction pattems of CoSAPO-34 and CoSAPO-44 were very much similar to those of the respective SAPOs with the (101) and (211) lines having highest intensity." Although the line positions for CoAPO-34 and CoAPO-44 were very close to those of the respective SAPOs, the intensity pattern underwent a very drastic change (Figure 1). In these samples, the (101) line was several times more intense than the rest of the XRD lines, which may suggest that the cobalt ions preferentially occupied this plane in the crystal. Since the amount of cobalt in the CoSAPOs was rather low, it was not expected to have much effect on the XRD line positions. The experimental results are in agreement with this. Heating of as-synthesized CoAPO-34 changed the structure to that of crystoballite. Detemplation of CoAPO-44 with a small loss in crystallinity was possible by slowly raising the temper- ature, whereas rapid heating produced an amorphous material.

Optical and Electron Microscopy. The crystals of CoAPOs were intense blue and could be clearly seen under the optical microscope. CoSAPOs with low cobalt content had crystals that were blue at the center and colorless at the crystal boundaries. By increasing the amount of cobalt in the CoSA- POs, the blue color intensified, but the edges remained colorless (Figure 2). XRD did not show any second phase in these samples. This suggests a heterogeneous distribution of the Co2+ ions in the crystal. At the begining of crystallization when the Co2+ concentration in the gel was high, more cobalt was incorporated and the concentration of cobalt in the crystal fell as the crystal continued to grow. Figure 3 shows the X-ray line profile of CoAPO and CoSAPO crystals. It can be seen that the concentration of cobalt increased as the beam moved from the edge to the center of the crystal in the case of CoSAPO, but for CoAPO, this remained constant throughout the crystal. For a typical CoSAPO-44 crystal, the cobalt concentration showed a sixfold increase in going from the edge to the center. No such change was observed for the CoAPO. The results of microprobe analysis are thus in agreement with the observations under the optical microscope. This suggests that there exists

Alp04 Molecular Sieves with Chabazite Structure J. Phys. Chem., Vol. 99, No. 18, 1995 6939

Figure 2. Optical photographs: (a) CoAPO-34; (b) CoSAPO-34/2; (c) CoAPO-44; (d) CoSAPO-44/2.

the possibility of composition variation not only at different regions of the same crystals but also from crystal to crystal.

Thermal Analyses. DTA-TG-DTG curves for CoAPO-34 and CoAPO-44 are shown in Figure 4. The initial mass loss observed was due to the removal of water. The removal of the template took place in two steps. It is interesting to note that, in the case of CoAPO-44, more of the templates are strongly held as compared to that in SAPO-44.17 The small exotherm without any corresponding mass loss at about 660 "C was possibly due to structural transformation. It has been found that CoAPO-34 on heating at 450 "C underwent transformation to crystoballite irrespective of the heating rate used. CoAPO- 44, on the other hand, became amorphous on rapid heating, but if the temperature was slowly raised at the rate of 1 "C/min to 450 "C, detemplation without structural collapse was possible to achieve, although there was some loss of crystallinity. Detemplation was carried out by heating the sample initially in nitrogen and finally by heating in air, as it was noticed that heating in nitrogen alone did not remove all the organic molecules. Heating in air oxidized the organic molecules generating heat. This possibly gave rise to high temperature in the sample, leading to its structural collapse. This could be avoided if heating was carried out by slowly raising the temperature.

The results of CoAPO-34 and CoAPO-44 were similar to those of the corresponding SAPO samples except that CoAPO- 34 had a lower number of template molecules per unit cell as compared to SAPO-34 and CoSAPO-34. The same thing was reported by Goepper et a1.,18 who used tetraethylammonium

hydroxide as the template. The results of thermal analyses have been summarized in Table 3.

Infrared Spectra. Infrared spectra of the structures -34 and -44 in the framework vibration region are shown in Figures 5 and 6, respectively. The assignment has been made based on the chabazite structure19 (Table 4). The spectra were quite similar to those of SAPO-34 and SAPO-44, re~pective1y.l~ Unlike the case in the SAPOS and the CoSAPOs, the symmetric stretching vibration near 740 cm-l was split for CoAPOs. We assign the lower wavenumber band to the presence of cobalt that increases the average AVCo-0 distance.

MAS NMR. Figure 7 shows the 31P and 27Al MAS NMR spectra of CoSAPO-34. The spectra were identical with those of SAPO-34 reported earlier17 except that the cobalt-containing samples showed prominent spinning side bands due to the presence of paramagnetic cobalt ions. The band at -27.7 ppm is due to the P(4A1) environment.20.21 The peak at 37 ppm in the 27Al spectrum represents tetrahedral aluminum. The second peak at 9 ppm was due to additional coordination perhaps by water molecules.21 The MAS NMR peaks for CoSAPO-44 appeared at -28.4 ppm for 31P and at 35 and 8.4 ppm for 27Al (Figure 8). These spectra were similar to those of SAPO-44.17

The 29Si MAS NMR spectra of CoSAPO-34 are interesting (Figure 9a,b). They have spectral lines at -90.8, -95.1, and -109.5 ppm that can be assigned to Si(4Al), Si(3Al), and Si- (OAl), r e s p e ~ t i v e l y . ~ ~ . ~ ~ A very weak line at -98 ppm is due to Si(2Al). The most intense peak at -90.8 ppm suggests that most of the silicon atoms are isolated from each other, but the other two peaks indicate that some small silica patches too are

6940 J. Phys. Chem., Vol. 99, No. 18, 1995 Ashtekar et al.

Figure 3. Scanning electron micrographs with X-ray line profiles of cobalt: (a) CoAPO-34; (b) CoSAPO-34/2; (c) CoAPO-44; (d) CoSAPO-44/2.

formed. There is no extra peak due to the presence of neighboring cobalt; even the peak positions are identical with those in SAPO-34.17 This could be due to the very low concentration of cobalt present in the framework.

The 29Si MAS NMR spectrum of CoSAPO-44 is similar to that of SAPO-44.I7 It mainly shows the Si(4Al) environments and silica patches are rather few (Figure 9c,d). These observa- tions are identical with our earlier observation on the NMR spectra of SAPO-44.17

Temperature-Programmed Desorption. Figure 10 shows the temperature-programmed desorption (TPD) profiles of the precalcined samples. The first peak at 160 "C was due to ammonia associated with surface hydroxyl groups and can be ignored. In the CoSAPOs, the peak appearing near 440 "C is related to structural acidity. The patterns are essentially similar to those shown by SAPO-34 and SAPO-44 reported earlier.I7 TPD results did not identify any desorption peak specifically due to cobalt in the framework. Since the amount of substituted cobalt is very small, the number of acid sites was essentially controlled by the amount of silicon, which was not much different from one sample to another (Table 2). Marchese et al.24 reported a detailed DRIFT spectroscopic study of SAPO- 34 that established a quantitative relation between the oxonium

ions formed by exposing SAPO-34 to water vapor and the amount of silicon present.

TPD of CoAPO-44 was somewhat different. This had only one peak due to structural acidity at 350 "C, and this is certainly due to substitution of cobalt. Chen and Thomas had reported an ammonia desorption peak for CoAPO-18 at 430 OC.I0 Their studies on DRIFT spectra showed that the Bronsted acidity of this material is related to hydroxyls bridging a P and a metal atom. It is possible to assume a similar hydroxyl bridge between P and Co in CoAPO-44 accounting for its acidity. No TPD peak was shown by CoAPO-34, and X-ray diffraction has shown that this structure changed to crystoballite during calcination. The CoAPO-44, on the other hand, did not change structurally on slow calcination although some loss of crystallinity was noticed.

Electronic Spectra. Using EXAFS, Chen et al.9 demon- strated that Co2+ is tetrahedrally coordinated in CoAPO-5 and -1 1. In the absence of EXAFS facilities, electronic spectra possibly provide the most conclusive evidence of Co2+ being present at the tetrahedral framework site. Schoonheydt et al.25 reported a detailed study of the UV-visible spectrum of CoAPO-5. Figure 11 shows the electronic absorption spectra of CoSAPO-34/2 and CoSAPO-44/2. All the as-synthesized

AlP04 Molecular Sieves with Chabazite Structure J. Phys. Chem., Vol. 99, No. 18, 1995 6941

-0 1bO 260 300 LbO 5bO 660 700 800 960 11 Trmmrature ('a

0.20

035 f E U -

0.10 p E

E

.. 0

0.05 ~

I - 0.00 E c

-0.05 0

Temperature ( ' ~ 1

Figure 4. TG, DTG, and DTA curves of the as-synthesized samples: (a) CoAPO-34; (b) CoAPO-44.

Figure 5. IR spectra of as-synthesized samples: (a) CoAPO-34; (b) cosAPo-34/2.

samples showed absorption in the 500-650 nm region. This was due to the 4A2 - T*(P) transitions in tetrahedral Co2+, clearly indicating Co2+ being present in the framework of these materials in tetrahedral coord ina t i~n .~~ Splitting of the band

1"" o 1 6 0 1440 1260 io80 960 720 si0 2 Wovenumbers

io

Figure 6. IR spectra of as-synthesized samples: (a) CoAPO-44; (b) COSAPO-4412.

10 -10 -30 -50 -70 1 L O 100 G O 20 -20 -60 ppm

Figure 7. 31P and 27Al MAS NMR spectra of CoSAPO-34: (a) COSAPO-3411 ; (b) CoSAPO-34/2.

m .a 0

n 2 : 31P

J,LJL 10 -10 -30 -50 3 0 140 100 GO 20 20 40 ppm

Figure 8. 31P and 27Al MAS NMR spectra of CoSAPO-44: (a)

arises due to various transitions to the doublet states that gain in intensity due to spin-orbit coupling.

Besides the band due to tetrahedral Co2+, the spectra showed a band around 330 nm which was fairly intense for CoSApO- 3412. This band could not be assigned to CoZf in any other coordination. Octahedral Co3+ was expected to give a band in this region with one additional band around 5 0 0 nm. It is not clear from this whether Co3+ occupies a framework site with additional coordination with waterltemplate molecules or it arises from extraframework cobalt. The intensity of the 330 nm peak relative to the tetrahedral Co2+ peak was very weak

COSAPO-4411; (b) CoSAPO-44/2.

6942 J. Phys. Chem., Vol. 99, No. 18, 1995 Ashtekar et al.

C 0 SAPO-3 CoSAPOdL I I

' " I CoSA PO-4 L / 2

-40 -GO -80 -100 -120 -140 - L O -60 -a0 -100 -120 -NO P P ~

Figure 9. 29Si MAS NMR spectra of (a) CoSAPO-34/1; (b) CoSAPO- 34/2; (c) CoSAPO-44/1; (d) CoSAPO-44/2.

Temperature ('C) Figure 10. Temperature-programmed desorption spectra of am- monia: (a) CoSAPO-34/1; (b) CoSAPO-34/2; (c) CoSAPO-44/1; (d) CoSAPO-44/2; (e) CoAF'O-44.

for CoAPO-34 and CoAPO-44 because of a large concentration of Coz+ in the tetrahedral framework site. When cobalt was extracted from the as-synthesized CoSAPO-34/2, it was found that nearly 28% of the cobalt present could be removed with 2 M ammonium acetate solution. The corresponding figures for CoAPO-34 and CoSAPO-34/1 were 24% and 9.4%, respec- tively. It should, however, be kept in mind that the extracted cobalt may come not only from the extraframework cobalt but also from removal of cobalt from the framework. It could be seen that, even after extracting the cobalt, the peak around 330 nm persisted. Hence, this absorption band could not be due to extraframework cobalt.

05

z

a 1 L

4

0 250 350 550 750 350 550 750

Wavelength ( n m l

Figure 11. Diffuse reflectance spectra of CoSAPO-3412 and CoSAPO- 44/2 samples: (a) as-synthesized; (b) calcined and rehydrated; (c) reduced at 450 "C.

When the CoSAPO samples were calcined in air, the intensity of the framework Co3+ peaks increased at the expense of the tetrahedral Co2+ peak (Figure 11). The sample at the same time changed to light green. The blue color was regained by reducing the sample in hydrogen, which partly restored the tetrahedral Co2+ signal. These results were qualitatively similar to what has been reported for CoAPO-5 and CoAPO- 1 1 by others. 1925926

From these results, it appears that some of the cobalt incorpo- rated in the framework was in the f 3 state and that they acquire additional coordination. The tetrahedral framework Co2+ can be reversibly oxidized to Co3+, and this may have important consequences in their use as redox catalysts.

While this manuscript was being written, a paper appeared by Kurshev et al.,*' who claimed that cobalt in the AlP04-5 framework does not undergo an oxidation-reduction cycle and that the color change is possibly due to change in the coordination of Co2+. We believe that this point needs further investigation by multiple experimental methods.

Conclusions

CoAPO and CoSAPO with -34 and -44 framework structures have been synthesized. They have been characterized by XRD, FT-IR, thermal, and chemical analyses and MAS NMR. Temperature-programmed desorption of ammonia showed the presence of two peaks due to structural acidity for CoSAPO samples, and this pattern was nearly identical with what had been obtained for SAPO-34 and SAPO-44. TPD of CoAPO- 44 showed a desorption peak different from those shown by the CoSAPOs.

Using electron microprobe analysis it was observed that cobalt was homogeneously distributed in the CoAPOs while in CoSAPOs it had a higher concentration near the center of the crystal that decreased toward the edges.

Acknowledgment. This work was funded by a research grant from CSIR, New Delhi. S.A. and A.M.P. are grateful to the CSIR for the award of research fellowships.

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