Synthesis and characterization of Co-containing zeolites of MFI structure

Studies in Surface Science and Catalysis 140 A. Gamba, C. Colella and S. Coluccia (Editors) 9 2001 Elsevier Science B.V. All rights reserved.

353

Synthesis and characterization of Co-containing zeolites of MFI structure E. Nigro a, F. Testa a, R. Aiello a, P.Lentz b, A. Fonseca b, A. Oszko e, P. Fejes d, A. Kukovecz d, I. Kiricsi d, and J. B.Nagy b aDipartimento di Ing. Chimica e dei Materiali, Universit~t della Calabria, 87030 Rende (Cs), Italy bLaboratoire de RMN, Facult6s Universitaires Notre-Dame de la Paix, 5000 Namur, Belgium CInstitute of Solid State and Radiochemistry, University of Szeged, 6720 Szeged Hungary ~ Chemistry Department, University of Szeged, 6720 Szeged Hungary Co-containing zeolites of MFI structure were synthesized using alkaline media. The orthorhombic-monoclinic symmetry transition suggests that at least the Co(II) ions also occupy tetrahedral framework positions. The XPS data clearly show that the samples contain both framework tetrahedral and extraframework octahedral Co(II) ions at ion exchange positions. The diffuse reflectance UV-visible spectra show unambiguously the presence of tetrahedral Co(II) ions in the structure. 1. INTRODUCTION The isomorphous substitution of silicon within the zeolitic framework is an important problem and a challenge for elements different from aluminium. Although the introduction of boron, gallium, or iron is relatively easy and well documented [ 1], few studies are devoted to the introduction of Co(II) into the framework of zeolites [2]. As both the framework and the extraframework Co-species seem to be active in catalysis [3], it is of paramount importance to synthesize and well characterize Co-containing zeolites [4]. We report in this study the hydrothermal synthesis of Co-silicate of MFI structure in alkaline media. The samples were characterized by XRD, chemical analysis, thermal analysis, 27A1-NMR, diffuse reflectance UV-visible spectroscopy and XPS spectroscopy. 2. EXPERIMENTAL Two different gels were prepared. For the first series gels (series A) using sodium silicate the final composition were: 35SiO2-xNa20-yCo(CH3COO)24H20-3.4TPABr-8.4H2SO4-808H20 with x=l 1, 12.9 and 15 and y-0.5, 1, 1.5 and 2. The second type of gels B were prepared using also AI(OH)3 (Pfaltz & Bauer) and their global compositions were 35SiO2-xNazO0.5Co(CH3COO)2-0.5A1203-3.4TPABr-808H20 with x=3, 6, 9 and 12. The gels, after complete homogenization, were put in PTFE-lined 25 cm 3 stainless-steel auoclaves. The Cocontaining samples were obtained by hydrothermal synthesis at 170~ after two days. After quenching the autoclaves, the products were recovered, filtered, washed with distilled water and finally dried a 80~ overnight. The samples were characterized by various physicochemical techniques such as XRD, chemical and thermal analysis, XPS, diffuse reflectance UV-visible spectroscopy and 27A1-NMR spectroscopy.

354

Sample

~ V ~

8

I

I

i

I

10

20

30

40

(20) degrees Fig. 1. XRD patterns of samples 1, 8 and 19 (see Table 1). 3. RESULTS AND DISCUSSION Fig. 1 illustrates some XRD pattems at various Co content. It can be seen that the material of sample n~ has a well crystalline MFI structure of orthorhombic symmetry. Sample n ~ 8 shows the presence of some amorphous phase as well. Indeed, these samples contain a rather high Co-content where most of the Co(II) ions are certainly extra-framework ions. In the first series A, where the Co- MFI samples were obtained from sodium silicate source the crystallinity of the samples decreases both with the increase of alkalinity (x) and with the increase of Co-content (y) in the gel. The decrease of the crystallinity with increasing alkalinity is also observed in series B in presence of aluminium. However, sample n ~ 19 shows a high crystallinity (Fig. 1). Note, that high alkalinity is required for the Co-ZSM5 synthesis. Indeed, when no NaOH was added to the reaction mixture, no crystalline material was obtained after two days of synthesis. In addition, the initial gel was pink showing the presence of octahedrally coordinated Co, while for the gels prepared with high Na20 content, the gels were all blue having the charecteristic colour of Co(OH)4 ions.

355 Table 1 Chemical composition of the as-made Co-containing MFI samples a) Samples obtained from gels A of composition 35SiO2-xNa20-yCo(CH3COO)z3.4TPABr-8.4HzSO4-808H20 at 170~ after two days of synthesis Sample

x

y

1 2 7 8 11 12

11 11 12.9 12.9 15 15

0.5 1 1.5 2 1.5 2

Co/u.c. Na/u.c. H20/u.c TPA/u.c. 2.7 3.8 6.6 12.7 5.6 10

8.3 7.3 9 16.5 10.5 15.6

0 7.3 9.3 7.0 7.4 8.3

3.6 3.6 3.5 3.4 3.8 3.1

TPA/u.c. T(~ LT a HT b 1.8-402 1.8-456 2.2-395 1.4-456 1.9-408 1.6-454 1.8-398 1.6-456 406(450) c 406(450) c

b) Samples obtained from gels B of composition 35SiO2-0.5Co(CH3COO)2-0.5A1203xNazO-3.4TPABr-808H20 at 170~ after two days of synthesis. Sample x Co/u.c. A1/u.c. Na/u.c. H20/u.c. TPA/u.c. 18 3 2.0 2.4 5.6 8.5 3.4 19 6 2.0 3.1 2.8 9.2 3.4 20 9 1.8 3.2 2.0 10.3 3.2 21 12 1.6 4.2 1.6 9.8 3.0 a.) LT:low temperature peak; b.) HT: high temperature peak; c.) shoulder

T(~ 461 470 471 469

The chemical analysis by Atomic Absorption yielded the values of Co/u.c. and Na/u.c., while the thermal analysis led to the TPA/u.c. and H20/u.c. (Table 1). The Co/u.c. values are rather high and suggest that the greater part of the Co is extraframework. These values increase with increasing Co-content of the gel (Table l a). The Na/u.c. values are also rather high and suggest that most of the Co (II) ions are extraframework. Indeed, in the case of A1-ZSM-5 samples, only some 6.9 Na/u.c. was found for an A1 content of 8.3 A1/u.c. [5] (Table l a and b). The HzO/u.c. varies in a random manner and does not seem to be linked to any Co/u.c. or Na/u.c. variation. In the (Co, A1)-ZSM-5 samples (series B), the Co/u.c. values are rather low with respect to the Co-ZSM-5 samples (Table l b). In addition, the Co/u.c. values are decreasing with increasing A1/u.c. values of the samples. It is rather well known that A1 is more easily incorporated into the MFI lattice and its presence disfavours the incorporation of Co in the samples. Note that the 27A1-NMR spectra show the presence of both framework tetrahedral (in large amount) and extra-framework octahedral (in small amount) aluminium species. The Na/u.c. values decrease with increasing A1/u.c. showing that in these samples Na+cations are not preferentially linked to the negative charge created by the A1 in the tetrahedral lattice. The HzO/u.c. values are close to 9.5 and do not vary much as a function of the A1- or the Cocontent. The DSC results are very revealing on the possible incorporation of Co in the tetrahedral framework. Fig. 2 illustrates the DSC curves of three samples. When the framework Co content is low (samples of series A) two peaks characterize the decomposition of the occluded TPA + ions. The low temperature peak (LT) at ca 400~ and the high temperature peak (HT) at ca 455~ The temperature of decomposition is not influenced by the formal Co/u.c values.

356 The TPA/u.c. does not vary much as a function of Co/u.c. and is close to an average value of 3.7/u.c. Approximately half of it is decomposed at low temperature and the other half at high temperature (Table 1a).

Sample

1

E X O

E N D O 19

.

.

.

.

.

.

.

.

.

.

.

.

.

.

.

.

260 36o 460 560 660 760 860 Temperature, ~ Fig. 2. DSC curves of samples 1, 8, and 19 (see Table 1). Only at high Co content (samples 8 and 12) there is some decrease in the TPA/u.c. values suggesting the incorporation of some of Co in the samples. The (Co,A1)-ZSM-5 samples are once again more revealing (Table l b). The TPA/u.c. decreases with increasing Al/u.c. as it was previously reported [5, 6]. In addition, only one d.t.a, peak was observed at already 2.4 A1/u.c. suggesting that some of the Co is also incorporated in the structure. Diffuse reflectance (DR) spectroscopy of Co 2§ permits the observation of d-d transitions in the near infrared and visible region and charge transfer (CT) transitions in the ultraviolet region. Co a§ is the only c o m m o n d 7 ion and because of its stereochemistry the respective

357 spectra have been extensively studied [4]. The two representative c a s e s : C o 2+ ion in octahedral (Oh) and tetrahedral (Th) [4] crystal field can be interpreted in the same way as the octahedral d 2 ion (e.g. V 3+) and octahedral d 3 ion (e.g. Cr3+), respectively. (For the corresponding Tanabe-Sugano diagrams see in [4]). As all d-d transitions for octahedral complexes possessing symmetry centre are symmetry-forbidden, these bands are in the spectra exceptionally weak. Thus, the DR spectra of zeolites, ZSM-5 zeolites in particular, with framework (FW) and extra-framework (EFW) Co 2+ content are dominated by - intense O 2- ~ Co 2+ charge transfer transitions, characteristic essentially for Co 2+ in Th coordination around 200-230 nm; - the most intense Th transition, v3 [ corresponding to 4Az(e4t23 ) =::>4T](P)] around 590-630 nm and a weaker one, v2, which is already in the near infrared (1400-1500 nm). (The third d-d transition, Vl, is also in the infrared region [2000-3300 nm], but its examination is hindered by overlying vibrational bands).

0.2 ~ ~..,i r.~

=

o~.-i

=

d~ ,.Q

200

300

400

500

600

700

800

Wavelength(nm)

Fig. 3. Diffuse reflectance UV-VIS spectra of samples No. 1 (a), 18 (b) and 21 (c) recorded at a resolution of 2 nm.

358 Table 2 Visible diffuse reflectance spectra of selected as-made Co-ZSM-5 samples 480 nm Co Octa

Band Intensities (a.u,) 530nm+590nm+640nm Co Tetra

340nm Co(III)

1

11

21

25

18 20 21

8 0 17

17 19 20

13 46 26

Sample . . . . .

Fig. 3 exhibits in the 460-700 nm visible region (from blue to red) at least 4 badly resolved bands (at 486, 530, 584 and 643 nm), of which the last three are Th d-d transitions. This triplet is indicative for high-spin (d 7) Co 2§ in tetrahedral crystal field. The nearly invisible 357 nm band is supposed to be due to high-spin Co 3+ in unknown coordination [3]. Table 2 compares the relative intensities of the 480 nm band (octahedral Co(II)), the sum of 530, 590 and 640 nm bands (tetrahedral Co(II)) and the 340 nm band (Co(Ill)). The values are in arbitrary units and only their variations have a meaning for the state of Co ions in the ZSM-5 samples. The octahedral Co(II) ions should be extraframework ions, the tetrahedral Co(II) ions could be due to both framework and extraframework species and the Co(II) ions are considered essentially extra-framework secies. It can be seen that tetrahedral Co(II) ions are present in all studied samples in roughly similar amounts. The amount of extraframework Co(II) ions are similar in samples 1 and 21, while sample 18 and 20 show only a rather small amount. The amount of extraframework Co(Ill) ions is low in sample 18, somewhat higher in samples 1 and 21 and rather high in sample 20. No systemic variations could be detected as a function of the formal Co/u.c. content (Table 1). XPS or electron spectroscopy for chemical analysis (ESCA) is used for characterizing surface species. In the case of Co z+ the 2p electron transitions are observed which, after ionisation, become separated into two levels: 2p3/2 and 2pl/2. The 2pl/2 peak is shifted upwards in energy by about 15.7 eV. Each peak has one, more or less intense, shoulder, a so called satellite, thus, we see not two, but rather four peaks (3 peaks and 1 strong shoulder) in the XP spectrum. The appearance of these "excess" peaks is due to multiplet splitting of the respective electron orbitals. Table 3 XPS data of selected Co-ZSM-5 samples Sample Reference Co/ZSM-5 1 8 12 21

Binding energies (eV) Co (2p3/2) Co (2pl/2) 780.9 796.8 780.95 796.95 781.4 797.25 781.05 797.1 781.05 797.1

Satellites (eV) 785.9 785.95 786.75 786.6 786.0

802.85 803.05 803.35 803.35 803.2

359

Co(2P3p)

Co(2Pl/z)

A

/ \

Sample

6~

/A

Z

/ i\

\

4 i

i

/

f

! I

I

i

i

/

9

\./

/

2-

t

/

/

1

0

i

770

i

!

.......... t'"

780 Binding

i

790 energy

......

i .....

I ....

i

....

800

81.0

(eV)

Figure 4. XPS spectra of Co(2p3/2) and Co(2pl/2) of samples n ~ 1, 8 and 21.

The paper deals with two series of Co-ZSM-5 samples: a). (n o 1 to 12, cf. Table 1) is characterised by variable x/y ratios in the slurry. The 2 nd series, b)., contained both Co 2+ and A13+ and they were charged in a constant Co/A1 = 0.5 ratio. It would have been very informative to take the XP spectra of each sample, however, the well known general trend of such syntheses in alkaline media (resulting only in partial substitution) and the registered UVVIS spectra (see above) revealed unequivocally that it is impossible to attain complete incorporation of Co 2+, thus, only three samples (n ~ 1,8 and 21) have been chosen for closer examination. In a previous study we succeeded to detect a slight, but unmistakable 1.9 eV difference in the locations of 2p3/2 XP signals of FW and EFW Fe 3+ ions in heat-treated Fe-ZSM-5 samples [7]. Even though the theme was not treated in the literature so far, it was deemed to be interesting to try to find a small shift for FW and EFW Co 2+ too. This would have been indicative of mixed coordination, i.e. the presence of both tetrahedral and octahedral Co ions.

360 The XP spectra of the C O 2+ photoelectron region for the Co-ZSM-5 samples n ~ 1, 8, 12 and 21 are given in Fig. 4. Table 3 provides information on the binding energies of the Co(2p3/2), Co(2pl/2) (with respect to Si(2p)) and of the satellites. If we add a fourth Co-ZSM-5 sample to the selected ones (synthesized from magadiite in the presence of Co-pyrocatecholate by P. Fejes) (in Table 3 marked as reference, maybe it is not an overstatement that the increase of 2+ Co content (cf. Table 1.) brings about a little upward shift in the location of the 2p3/2 peaks which do not attain 781.5 eV, typical for free Co 2+, even at sample n ~ 8, bearing the largest 2+ 2+ 9 Co content (12 Co /u.c. correspondmg to Si/Co = 6.56). It is believed that the binding energies of core level electrons for Co 2§ in tetrahedral coordination (similar to the respective F e3+levels) are smaller than those in octahedral symmetry and in mixed coordination the spectrometer, being not able to resolve the two peaks (separated by a few tenths of eV), registers only one peak at the weighted average of the two binding energies. This manifests itself as an upward energy shift when the percentage of octahedral Co 2§ increases with the cobalt content. 4. CONCLUSIONS The various physicochemical techniques used showed unambiguously that part of the Co(II) ions is incorporated into the tetrahedral framework positions. However, despite the joint effort of the different techniques no quantitation of the tetrahedral Co(II) ions was possible. 5. ACKNOWLEDGMENT The present work is a part of a project coordinated by A. Zecchina and cofinanced by the Italian MURST (Cofin 98, Area 03). This work was carried out with the financial support of the Regione Calabria (POP 97/99). The authors thank the Belgian Program on Inter University Poles of Attraction initiated by the Belgian State, Prime Minister's Office for Scientific Technical and Cultural Affairs (OSTC-PAI-IUAP No. 4/10 or Reduced Dimensionality Systems) for financial support. REFERENCES 1. R. Szostak, Molecular Sieves. Principles of Synthesis and Identification, Van Nostrand Reinhold, New York, 1989, p. 205. 2. T. Inui, J.-B. Kim and T. Takeguchi, Zeolites, 17 (1997) 354. 3. K. Kagawa, Y. Ichikawa, S. Iwamoto and T. Inui, Microporous and Mesoporous Mater., 25(1998) 15. 4. A. Verberckmoes, B. M. Weckhuysen and R.A. Schoonheydt, Microporous Mesoporous Mater., 22 (1998) 165. 5. G. Debras, A. Gourgue, J. B.Nagy and G. De Clippeleir, Zeolites, 5 (1985) 377. 6. R. Aiello, F. Crea, E. Nigro, F. Testa, R. Mostowicz, A. Fonseca and J. B.Nagy, Microporous Mesoporous Mater. 28 (1999) 241. 7. P. Fejes, J. B.Nagy, J. Halfisz and A. Oszk6, Appl. Catal. A: General, 175 (1998) 89.