Synthesis of aluminum phosphate molecular sieves using cobalticinium hydroxide

MICROPOROUS MATERIALS ELSEVIER

Microporous Materials 3 (1995) 489 495

Synthesis of aluminum phosphate molecular sieves using cobalticinium hydroxide K.J. Balkus, Jr. *'1, A.G. Gabrielov, S. Shepelev Department of Chemistry, UniversiO, of Texas at Dallas, Richardson, TX 75083 0688, USA Received 31 May 1994: accepted 7 July 1994

Abstract

The metal complex bis(cyclopentadienyl)cobalt(III) hydroxide, Cp2CoOH, was found to be a template ['or the synthesis of A1PO4-5 and A1PO4-16 molecular sieves. The metal complexes are included in the molecular sieves and are not removed by solvent extraction or ion-exchange methods. The molecular sieves were characterized by X-ray diffraction (XRD), Fourier transform infrared (FTIR) spectroscopy, ultraviolet visible (UV-VIS) detection and scanning electron microscopy (SEM) as well as elemental analysis. Kevwords: Molecular sieves: AIPO4-5; A1PO4-16: Cobalticinium ion: Template

1. Introduction

The incorporation of metal complexes during the synthesis of molecular sieves has been shown to be a viable method for the preparation of zeolite ship-in-a-bottle complexes [1 ]. In a few cases, the metal complex may also function as a structuredirecting agent during crystallization. The metal complex bis(cyclopentadienyl)cobalt(III) ion, Cp2Co ~, has been reported to be a template for nonasil [2,3] in both hydroxide and fluoride media as well as for the isostructural ZSM-51 [4]. The NON topology consists of interconnected cages composed of five- and six-membered rings [5]. The CpzCo + complexes are easily accommodated inside the ellipsoidal [58612] cages of nonasil but are completely encapsulated. The nonasil [58612] cages appear to resemble the Cp2Co + guest molecules in size, shape and symmetry, unlike most of * Corresponding author. NSF Presidential Young Investigator 1991---1996. 0927-6513/95/$9.50 5) 1995 Elsevier Science B.V. All rights reserved SSDI 0927-6513(94)00057-3

the flexible organic templates with which it is often difficult to make a connection between template and molecular sieve structure. Therefore, this fairly rigid metal complex, Cp2Co ~, may prove to be a useful molecule for studying structure,-directing effects during molecular sieve synthesis. As part of this effort we have explored the synthesis of A1PO4 molecular sieves in the presence of cobalticinium hydroxide. In this paper we report the synthesis of A1PO4-5 as well as AIPO4-16 using Cp21CoOH as the template. AIPO4-16 like nonasil is a cage-type molecular sieve, whereas AIPO4-5 has a onedimensional channel structure [6]. It may not be surprising that AIPO4-5 can be formed, since a multitude of organic templates lead to this structure. Nevertheless, CpzCo + has now been shown to form both channel- and cage-type molecular sieves. There was previously only one known template for AIPO4-16, namely quinuclidene which is similar in size to Cp2Co +. The AIPO4 molecular sieves containing the cobalt complex were characterized by XRD, FTIR, UV VIS and SEM as well as by elemental analysis.

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2. Experimental Solutions of bis (cyclopentadienyl)cobalt(III) hydroxide were prepared from cobalticinium hexafluorophosphate (Aldrich) as previously described [2]. A1PO4 molecular sieves were prepared using an AlzO3/PzOs/CpzCo+/H20 molar ratio of 1 : 1 : 1:40 in the starting gel. In a typical synthesis the aluminum phosphate gel was prepared by mixing 4.24 g of Catapal B alumina (Vista) with 4.26 ml of 85% H3PO4 (Fisher) and 1.54 ml of deionized water. After stirring for 2 h, 19.9 ml of an aqueous 1.53 M Cp2CoOH solution were added dropwise to the A1PO4 mixture with vigorous stirring. The gel was aged at room temperature for 2 h, then transferred into a PTFElined pressure reactor (Parr) and heated under static conditions at various temperature and times as shown in Table 1. After heating under autogeneous pressure, the mixture was cooled to room temperature and diluted with 200 ml of deionized water. The resulting yellow crystals were purified by centrifugation and/or sedimentation, washed with deionized water, suction-filtered through a nitrocellulose membrane (2 lain) and dried at 90°C for 15 h. Samples of A1PO4-5 molecular sieves containing tripropylamine (TPA) as a template were prepared using previously described molecular ratios [7]. X-Ray powder diffraction patterns were Table 1 Results for the synthesis of AIPO 4 molecular sieves using Cp2CoOH Sample

Temperature

Time

(°c)

(h)

1" 2a 3

150 150 150

48 96 144

4

150

192

5

160

48

6

170

48

Phase

Co

A1PO4-5 AIPO4-5 A1PO4-5 AIPO4-H3 (traces) A1PO4-5 AIPO4-H3 (traces) AIPO4-5 A1PO4-16 AIPO4-5 A1PO4-16

3.62 3.62

(%, w/w)

"Water adsorption capacity ~ 17% after 320°C.

recorded on a Scintag XDS 2000 diffractometer using CuKe radiation with a chopper increment of 0.01 and a scan rate of 1 deg min -1. CaF 2 was used as an internal standard. Electronic spectra of the molecular sieves were obtained from samples prepared as nujol mulls between quartz plates using a Hitachi U-2000 UV-VIS spectrophotometer. Mid-IR spectra were obtained from KBr pellets using a Mattson 2025 FTIR spectrophotometer. Scanning electron micrographs were obtained at Texas Instruments (Dallas, TX, USA). Elemental analyses were performed by Galbraith Laboratories (Knoxville, TN, USA).

3. Results Using the A1PO4 gel composition described above at 150°C, the metal complex Cp2CoOH leads to pure phase A1PO4-5 as seen from Table 1. Although, the crystallization time has not been optimized, highly crystalline A1PO4-5 was obtained after 48 h. This is in contrast to A1PO4-5 synthesized with TPA as the template where less than 15 h were required to obtain comparable crystallinity. Further heating of the gel mixture beyond four days resulted in trace quantities of A1PO4-H3. This is a small-pore molecular sieve that could not accommodate the metal complex [6]. The appearance of AIPO4-H3 is not unreasonable since this phase does not require an organic template to form. Increasing the synthesis temperature to 160 or 170°C results in formation of A1PO4-16 together with varying amounts of A1PO4-5. Pure A1PO4-16 has not been obtained, yet these results are encouraging given the apparent overwhelming propensity to form AIPO4-5. In contrast, pure silicoaluminum phosphate SAPO-16 was prepared using CpzCoOH whereas SAPO-5 was not observed [8]. The A1PO4-5 and A1PO4-16 samples were bright yellow in color which is a result of the occluded metal complexes. The complexes could not be extracted by washing with water or organic solvents. There was also no evidence of ionexchange properties, either cation or anion. The pure-phase A1PO4-5 samples 1 and 2 both contained 3.62% (w/w) Co which corresponds to ~ 1 complex per unit cell taking into account a water

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K.J. Balkus, Jr. et al./Microporous Materials 3 ( 1995 ,~489-495

content of ~ 17% as shown in Table 1. The water content is based on the adsorption capacity after heating at 320°C which is well below the decomposition temperature (~373°C) of the metal complex. Phase identification and purity were determined by X-ray powder diffraction. Fig. 1 shows the XRD patterns for pure A1PO4-5 containing TPA (A) and Cp2CoOH (B) as well as the A1PO4-5-AIPO4-16 mixture (C). There are clearly some differences in relative intensities of the XRD patterns for the different A1PO4-5 preparations with suppression of the 110 reflection, in the case of the complex-containing molecular sieve, being the most notable. The 110 reflection is normally less intense than the 200 reflection for assynthesized A1PO4-5 as shown in pattern A but not to the extent of pattern B. Another interesting feature is the apparent resolution of the 212 and 311 reflections at a d-spacing of 3.07 A for the cobalticinium-modified A1PO4-5. The lack of phase purity in the case of AIPO4-16 makes it difficult to evaluate any differences. The lattice parameters for AIPO4-5 and A1PO4-16 also reflect the differences between the organic and inorganic guest molecules as shown in Table 2. The unit cell parameters a (b) for the A1PO4-5 samples prepared using CpzCoOH are lower than for the TPA analog which results in a smaller unit cell volume. In contrast, the cell volume of A1PO4-16 containing

'5'

''fo ....

15 . . . . . . .

~5 . . . . . .

~

s'

Table 2 Lattice parameters a for synthesized AIPO4 samples Sample

a (,~,)

c (A.)

V(A 3)

A1PO4-5 TPA AIPO4-5-CpzCoOH A1PO4-16-quinuclideneb A1PO4-16-Cp2CoOH

13.68 13.58 13.38 13.48

8.44 8.44

1368 1348 2397 2452

a Standard deviation less than 0.01. u Ref. [9].

Cp2CoOH is greater than that reported for the same phase prepared using quinuclidene [9]. The A1PO4-5 prepared using cobalticinium ion form bundles of quasi-hexagonal-shaped crystals as illustrated by the scanning electron micrographs in Figs. 2 and 3. The morphology of A1PO4-5 seems to vary with the nature of the template but in general there always seems to be some ~exagonal features to the crystals. Since the crystallizations were carried out under static conditions there may be temperature gradients within the reactor resulting in different crystal morphologies. Fig i 4 shows some A1PO4-5 crystals collected from the reactor walls where the average temperature lmay be slightly higher. These crystals appear thinner than the bulk of the sample (Fig. 2) with edges that are serrated. In contrast, Fig. 5 shows the tertrahedral shape of the A1PO4-16 crystals prepared ~sing the metal complex template. This is nearly !the same type of morphology obtained when A1PO4-16 is

"

Fig. 1. XRD patterns for (A) A1PO4-5-TPA, (BJ AIPO4-5-Cp2CoOH and (C) AIPO4-16 (dotted peaks) A1PO4-5 mixture containing Cp2CoOH.

Fig. 2. Scanning electron micrograph of AIPO4-5 crystals in sample 1.

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K.J. Balkus, Jr. et al./Microporous Materials 3 (1995) 489 495

Fig. 3. Scanning electron micrograph of A1PO4-5 crystals in sample 1 at higher magnification.

Fig. 5. Scanning electron micrograph of A1PO4-16 crystals in sample 5.

Fig. 4. Scanning electron micrograph of A1PO4-5crystals from reactor wall.

crystallized with quinuclidene [9]. The XRD pattern and crystal morphology for SAPO-16 and A1PO4-l 6 containing the metal complex are nearly the same which provides further support for the identification of A1PO4-16 in the mixture [8]. Both the FTIR and electronic spectra provide evidence for the incorporation of metal complexes in A1PO4-5. The FTIR spectra of the A1PO4-5

prepared with TPA (spectrum A) and with Cp2CoOH (spectrum B) are presented in Fig. 6. There are no discernible differences in the IR bands associated with the A1PO4-5 lattice that results from using either template. The observable bands at 1420 and 868 cm -1 can be assigned to the occluded complex. Fig. 7 shows the C-H stretch for CpzCoPF 6 at 3128cm -1 and for the A1PO4-5-included complex at 3106 cm 1. The C-H bands associated with the Cp rings are fairly sensitive to the nature of the charge balancing anions. For example, nonasil-encapsulated Cp2Co + exhibits C-H bands at 3140 and 3105cm -1 which we assigned to OH- and F salts, respectively [2]. In AIPOa-5-A1PO4-16 mixtures the 3106 cm-1 band may be associated with a phosphate salt. Efforts to prepare a phosphate salt outside the molecular sieves has resulted in a mixture of complexes which all exhibit red-shifted Vc_H bands. These phosphate salts also exhibited very low solubility. The UV-VIS spectra shown in Fig. 8 indicate the presence of the complex. There is a red shift ( 10 rim) for those bands arising from the complex which was also noted for the nonasile n t r a p p e d CpzCo + complexes [2]. The molecular sieves do not appear to impart any special stability

493

K.J. Balkus, Jr. et al./ Microporous Materials 3 (1995i 489-495

/ r

N S m

t t

/-

A

c

[B

----T

:1600

I

I

~.400

1200

I

±000

I

800

I

'

600

wavenL~mbePs

I

[

3{500

I

[

3400

[

I

3200

I

3000

WaveNumlDer's

Fig. 6. FTIR spectra in the region 400-1750cm -1 for as-synthesized A1PO4-5 containing (A) TPA and (B) Cp2CoOH (sample 1 ).

Fig. 7. FTIR spectra in the region 3000 3700cm ~ for (A) AIPO4-5 containing CpzCoOH (sample I) and (B) Cp2CoPF 6.

to the encapsulated metal complex as decomposition to presumably cobalt oxide occurs upon calcination (>400°C). This behavior is quite similar to that noted for the nonasil-containing cobalticinium ion [2].

channel of A1PO4-5 which was obtained b y fixing the lattice atom positions and then minimizing the complex in the channel using MM2 parameters. The metal complex (--~5.17 x4.86 A) could easily fit in the channel in any number of orientations but appears to prefer lining up along the principal rotation axis. Although, there is no obvious translation of symmetry elements, the complex does have a cylindrical shape that fits well in the channel. It is not clear why the metal complex cannot be removed from the AIPO4-5 molecular sieves. Whatever balances the molecular charge may immobilize the complex. In the case of A1PO4-16, the resistance of the complex to extraction is understandable since the cobalticinium ion forms a ship-in-a-bottle complex inside the molecu-

4. Discussion

The synthesis of both channel-type (A1PO4-5) and cage-type (nonasil, AIPO4-16) molecular sieves using Cp2CoOH as a template was unexpected. However, there are so many different organics that form A1PO4-5 that it may not be surprising that this phase was formed [6]. Fig. 9 shows a calculated Cp2Co + molecule positioned in a 7.3-]~

494

K~J. Balkus, Jr. et al./Microporous Materials 3 (1995) 489-495

1 I

I

'\ \

\

\

\

0 c

~ - - ~

x160

0

<

200

L400

300

500

Wavelength, nm Fig. 8. UV-VIS spectra for (A) an aqueous Cp2CoOH solution and (B) a nujol mull of AIPO4-5 containing Cp2CoOH.

Fig. 9. Cp2Co + in a channel of A1POr5 calculated using CAChe Molecular Modeling and the crystallographic parameters in Ref. [10].

lar sieve, which has only four- and six-membered rings. Fig. 10 shows Cp2Co + calculated in the A1PO4-16 cage along with a space-filling model of quinuclidene for a comparison of size and shape. The calculated electron density surface volume of Cp2Co + ( ~ 107.6 ,&3) is similar to that of quinuclidene (~91.4 A3). Interestingly, quinuclidene also forms the all-silica clathrate octadecasil and SAPO-16 which are isostructural (AST) with A1PO4-16 [9]. Apparently, the similarity in size and shape between the two templates does not extend to the SiO2 composition since Cp2Co + has so far produced only the NON topology and not octadecasil. Interestingly, the combination of Si, AI and P in the gel results in the formation of pure SAPO-16 [8]. Considering that only one other template was previously known for A1PO4-16 it is encouraging that Cp2CoOH forms this phase. This suggests that we may be able to exploit the unique structure-directing properties of this metal complex and its derivatives in synthesis of new materials.

K.J. Balkus, Jr. et al./Microporous Materials 3 (1995) 489-495

495

Fig. 10. Cp2Co + in a cage of A1PO4-16 calculated using CAChe Molecular Modeling and the crystallographic parameters in Ref. [9]. A space filling model of quinuclidene is shown for size and shape comparison,

5. Conclusions

References

Cp2CoOH was found to be a template for the synthesis of A1PO4-5 and A1PO4-16 molecular sieves. The metal complexes are incorporated into the molecular sieves and not removable. Based on IR and UV-VIS spectra the A1PO4-5-included complexes appear intact. Further characterization of the A1PO4-16 guest molecules awaits preparation of a pure-phase sample. It is clear that cobalticinium ion can affect the crystallization of molecular sieves with several different topologies. We are currently attempting to evaluate the scope of these properties by varying further the framework composition.

[1] K.J. Balkus, Jr. and A.G. Gabrielov, in N. Herron and D. Corbin (Eds.), Inclusion Chemistry with Zeolites, Nanoscale Materials by Design, Kluwer, Dordrecht, 1994, in press (and references therein). [2] K.J. Balkus. Jr. and S. Shepelev, Microporous Mater., 1 (1993) 383. [3] K.J. Balkus, Jr. and S. Shepelev, Petrol. Preprints, 38 (1993) 512. [4] E.W. Valyocsik, U.S. Pat., 4568654 (1986). [5] D. Morler, N. Dehnbostel, H.H. Eulert, H. Gies and F. Liebau, J. Inclus. Phenom. Mol. Recog. Chem.. 4 (1986) 339. [6] R. Szostak, Handbook of Molecular Sieves, Van Nostrand Reinhold, New York, NY, 1992. [7] S.T. Wilson, Stud. Surf Sci. Catal., 58 (1991) 137. [8] K.J. Balkus, Jr. and A.G. Gabrielov, in preparation. [9] J.M. Bennett and R.M. Kirchner, Zeolites, 11 ( 1991 ) 502. [10] J.M. Bennett, J.P. Cohen, E.M. Flanigen, J.J. Pluth and J.V. Smith, 4m. Chem. Soc. Syrup. Ser., 218(1983) 109.

Acknowledgement We would like to thank the Robert A. Welch Foundation for support of this research.