EXAFS

 

 

Mezoporous catalytical molecular sieves: Silicalite-1

Related publications:
1) N. Novak Tušar, N. Zabukovec Logar, I. Arčon, F. Thibault-Starzyk, A. Ristić, N. Rajić, Venčeslav Kaučič, Chemistry of Materials 15 (2003) 4745-4750
2) N. Novak Tušar, N. Zabukovec Logar, I. Arčon, F. Thiboult-Starzky, V. Kaučič, Croatica Chemica Acta, 74 (2001) 837-849
3) A. Barbier, A.Tuel, I. Arčon, A. Kodre, G. Antonin Martin, Journal of Catalysis Vol. 200 (2001) 106-116

Abstract

Mn- and Co-containing silicalite-1 (MnS-1 and CoS-1) were synthesized hydrothermally. Template-free samples were obtained by calcination at 550°C for 6 hours in oxygen flow. X-ray diffraction, elemental, thermogravimetric and cation exchange analyses suggested a possible in-corporation of Mn and Co cations into the framework positions of the silicalite-1. The valence state and local environment of both cations in the samples were examined by XANES and EXAFS analyses. Results show that Mn and Co cations isomorphously substitute silicon in the silicalite-1 framework only if they are in trivalent oxidation state. Divalent cations are incorpo-rated on extra-framework positions. In the template-free MnS-1 sample all Mn cations are in tri-valent state and substitute Si in the framework, forming a distorted and coordinatively unsatu-rated 3-fold symmetry, which is characteristic for Lewis acid sites. In the template-free CoS-1 sample only part of Co cations are in trivalent state substituting silicon in the framework.

Introduction

Zeolites with aluminosilicate framework are crystalline oxides comprising corner-sharing TO4 tetrahedra (T=Si, Al) [1]. They possess regular pore system with diameters in the range from 0.3-1.4 nm and act as sieves at molecular level.

Silicalite-1 is an aluminium-free zeolite with the MFI structural topology [1]. Modification of silicalite-1 by isomorphous substitution of silicon by transition-metal elements enhances its catalytic activity. Incorporation of tetravalent (e.g. Ti, V) transition metals into the framework of silicalite-1 results in novel molecular sieves with selective oxidation properties [2]. Incorporation of trivalent (e.g. Cr, Fe) transition metals into the framework of silicalite-1 results in high-quality inorganic membranes used for catalytic membrane reactors [3]. So far all attempts of incorpora-tion of divalent transition metals (e.g. Mn, Co) lead to the formation of the silicalite-1 with diva-lent cations on extra-framework positions [4].
In this paper we report on the isomorphous substitution of silicon in the framework of sili-calite-1 by manganese and cobalt cations. Mn and Co K-edge XANES analysis is used to probe directly the valence state of the transition metal cations in the as-synthesized and template-free samples, while their local environment and therewith the site of the incorporation is examined by EXAFS.

Experimental

Manganese-containing silicalite-1 (MnS-1) was syntesized hydrothermally using tetraethyl-ammonium hydroxide (TEAOH) as a structure-directing agent (template) [5]. Cobalt-containing silicalite-1 (CoS-1) was synthesized by similar procedure using tetrapropylammonium hydroxide (TPAOH) as a template. Template-free products were prepared by calcination at 550°C for 6 hours in oxygen flow. Cation exchange was performed at room temperature by stirring the solid sample in a 1M solution of NaCl for 24 hours with solid/liquid mass ratio 1/10. X-ray powder diffraction (XRPD) patterns of the products were collected on a Siemens D-5000 diffractometer with CuKa radiation. Thermogravimetric analysis (TGA) was performed on a TA 2000 thermal analyzer in static air. Elemental analysis was carried out using an EDXS (energy dispersive X-ray spectroscopy) anaysis within the LINK ISIS 300 system, attached to the scanning electron microscope JEOL JSM 5800.

X-ray absorption spectra of MnS-1, CoS-1 and reference samples in the respective energy regions of the Mn and Co K-edge were measured at E4 beamline of HASYLAB synchrotron fa-cility at DESY in Hamburg. The beamline provided a focused beam from an Au-coated torroidal mirror and a Si(111) double crystal monochromator with about 1 eV resolution at 7 keV. Har-monics were effectively eliminated by a plane Au-coated mirror, and by a slight detuning of the monochromator crystals, keeping the intensity at 60% of the rocking curve with the beam stabili-zation feedback control. Powder samples were prepared on multiple layers of adhesive tape. Sev-eral layers were stacked to obtain optimal attenuation above the K-edge of the investigated ele-ment. Reference spectra were measured under the same conditions on empty tapes without the sample. The standard stepping progression within [-250 eV .. 1000 eV] region of the Mn and Co K-edge was adopted for EXAFS spectra with an integration time of 4 s/step. Exact energy cali-bration was established with the simultaneous absorption measurements on Mn and Co metal.

Results and discussion

The as-synthesized MnS-1 and CoS-1 were identified from XRPD patterns as single crystalline phase products with MFI structure. Elemental and thermogravimetric analyses of the as-synthesized and template-free MnS-1 and CoS-1 suggested that 0.5 % Si was isomorphously sub-stituted by Mn and Co, respectively. Cation exchange procedure revealed that there were no ex-changeable cationic sites in the template-free MnS-1, while some exchangeable cationic sites were present in the template-free CoS-1 sample. This indicated that all incorporated manganese atoms in the template-free sample isomorphously substituted framework silicon in the silicalite-1, while the Co substitution in the template-free CoS-1 sample sample was only partial.

To determine the valence state of the transition metal cations in the as-synthesized and cal-cined samples we examined Co and Mn K-edge energy shifts in the absorption spectra of the samples. The precise energy position of the edge was taken at the edge inflection point. A linear relation between the edge shift and the oxidation state was established for the atoms with the same type of ligands [6-9]. For manganese atoms coordinated to oxygen atoms a shift of 3.5 eV per oxidation state was found [7], while for cobalt atoms shifts of 1.5 - 3 eV per valence were reported [9].

Fig. 1. Normalized Mn K-edge XANES spectra of the as-synthesized and template-free MnS-1 and Mn reference samples Mn metal, Mn2+O, K3[Mn3+(C2O4)3].3H2O, and Mn4+O2. The spectra are displaced vertically for clarity. The zero energy is taken at the first inflection point of the Mn K-edge in the spectrum of Mn metal (6539.0 eV).

Fig. 2. Normalized Co K-edge XANES spectra of the as-synthesized and template-free CoS-1 and Co reference samples Co metal and Co2+(CH3COO)2ˇ4H2O. The spectra are displaced vertically for clarity. The zero energy is taken at the first inflection point of the Co K-edge in the spectrum of Co metal (7709.0 eV).

 

 

 

 

 

 

 

 

 

 

 

 

 

In Fig. 1 the Mn XANES spectra of the as-synthesized and template-free MnS-1 sample to-gether with the spectra of the reference manganese compounds Mn2+O, K3[Mn3+(C2O4)3].3H2O, and Mn4+O2 with known oxidation states are shown. The edge shift in the as-synthesized MnS-1 is the same as in the Mn2+O compound, indicating that the average oxidation state of manganese in the as-synthesized MnS-1 is 2+. The Mn K-edge in the template-free MnS-1 sample is shifted and coincides with the edge shift in the K3[Mn3+(C2O4)3].3H2O compound. We can thus conclude that during calcination all Mn+2 cations oxidize to Mn+3.

The Co XANES spectra (Fig. 2) reveal completely different behaviour of Co cations in the CoS-1 samples. The same energy position of Co K-edge is found in both as-synthesized and tem-plate-free CoS-1 samples, demonstrating that the valence state of the incorporated Co cations does not change during the process of calcination. Furthermore, the Co K-edge in the CoS-1 samples is shifted for about 1 eV relative to the edge in the reference Co2+ acetate sample, which indicates the presence of Co2+ and Co3+ cations in the silicalite samples.
Mn and Co K-edge EXAFS spectra of the as-synthesized and template-free MnS-1 and CoS-1 samples were quantitatively analyzed for the coordination number, distance, and Debye-Waller factor of the nearest coordination shells of neighbor atoms using the University of Wash-ington UWXAFS package [10] and FEFF6 code [11]. The amplitude reduction factor (So2 = 0.80) is kept fixed in the fits of all spectra. Its value is obtained from previous experimental results [12]. Fourier transforms of Mn and Co EXAFS spectra are shown in Figs. 3 and 4 together with best-fit EXAFS models. Complete lists of best-fit parameters are given in Tables I and II.

Fig. 3. Fourier transforms of the k3-weighted Mn EXAFS spectra of the as-synthesized and template-free MnS-1 sample, calculated in the k range of 5-12 A-1 (solid line - ex-periment, dotted line - EXAFS model).

Fig. 4. Fourier transforms of the k2-weighted Co EXAFS spectra of the as-synthesized and template-free CoS-1 sample, calculated in the k range of 3.5-12 A-1 (solid line - ex-periment, dotted line - EXAFS model).

 

 

 

 

 

 

 

 

 

 

 

Two shells of neighbours are found around Mn cations in the as-synthesized MnS-1 sam-ple. The fit in the R range from 1.2 A to 3.5 A shows that the first shell is composed of four oxy-gen atoms at 2.18 A, while the second shell comprises about two Mn atoms at 3.35 A. The pres-ence of Mn-O-Mn links indicates that Mn cations (divalent as shown by XANES) are located at extraframework sites, most probably in the form of metal-oxo species coordinated to the frame-work, but simultaneously bearing extra-framework oxygen atoms [7].

Significantly different local Mn neighborhood (within the same R range) is found in the template-free MnS-1 sample. Mn is coordinated to three oxygen atoms in the first coordination shell, two of them at a shorter distance of 1.93 A and one at a longer distance of 2.15 A. The short distance of 1.93 A is consistent with the average tetrahedral Mn+3-O distance of 1.93(4) A reported for MnAsO4 [13]. In the second coordination shell two oxygen atoms were identified at 2.81 A and 3.04 A. Additionally, at a larger distance of about 3.5 A a presence of silicon atoms is indicated. Taking into account XANES results we can conclude that Mn3+ cations substitute Si in the framework of the template-free MnS-1, forming a distorted and coordination-wise unsatu-rated 3-fold symmetry, which is characteristic for Lewis acid sites (manganese framework sites) [14].

In modeling Co EXAFS spectra, the fit in the R range 1.3 A – 3.5 A shows that in the as-synthesized sample cobalt atoms are coordinated to four oxygen atoms at 2.10 A, while the sec-ond shell is composed of Co and O atoms. The local Co neighborhood in the template-free sam-ple is slightly different (Fig. 4, Table II) but the Co-O-Co links are retained. Combining Co cation exchange, XANES and EXAFS results, we may conclude that the template-free CoS-1 sample contains two Co species (Co2+ and Co3+), where trivalent cobalt cations substitute silicon in the framework sites while divalent Co cations are located at extra-framework sites in the form of metal-oxo species.
In conclusion, the results show that Mn and Co cations substitute silicon in the silicalite-1 framework isomorphously only if they are in the trivalent oxidation state. Divalent cations incor-porate into the silicalite-1 on the extra-framework positions.

Acknowledgment
We acknowledge the support by the Slovenian Ministry of Education, Science and Sport through the research program P0-0516-0104 and the project Z2-3457-0104, by Internationales Buero BMBF (Germany), and by the IHP-Contract HPRI-CT-1999-00040 of the European Commis-sion. Advice on beamline operation by Konstantin Klementiev of HASYLAB is gratefully ac-knowledged.

Table 1. Structural parameters of the nearest coordination shells around Mn atom in as-synthesized and template-free MnS-1: type of neighbor atom, average number N, distance R, and Debye-Waller factor s2. Uncertainties in the last digit (estimated by UWXAFS fitting program) are given in the parentheses.

 

Table 2. Structural parameters of the nearest coordination shells around Co atom in as-synthesized and template-free CoS-1: type of neighbor atom, average number N, distance R, and Debye-Waller factor s2. Uncertainties in the last digit (estimated by UWXAFS fitting program) are given in the parentheses.

 

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E-mail:iztok.arcon@p-ng.si Last change: 02-Jun-2006