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32Electron-acceptor Centers of Alkali Metal Modified Alumina Studied by the Anthraquinone as a Probe Molecule
M.V. Burova1,*, A.V. Fionov1, M. Bonora2, A. Lund2, V.V. Lunin1
1 Chemical Department, Moscow State University, Moscow, Russia 2 Department of Physics and Measurement Technology IFM, Linköping University
Linköping, Sweden * E-mail: mvburova@mail.ru
Received November 18, 2006 Revised January 17, 2007 Accepted January 29, 2007
Volume 9, No. 1, pages 13-17, 2007
http://mrsej.ksu.ru
Electron-acceptor Centers of Alkali Metal Modified Alumina Studied by the Anthraquinone as a Probe Molecule
M.V. Burova1*, A.V. Fionov1, M. Bonora2, A. Lund2, V.V. Lunin1
1Chemical Department, M.V. Lomonosov Moscow State University, 119992, Leninskie Gory, Moscow, Russia 2Department of Physics and Measurement Technology IFM, Linköping University, S-581 83,
Linköping, Sweden * E-mail: mvburova@mail.ru
Anthraquinone paramagnetic complexes have been used to study electron-acceptor centers on the surface of alumina modified with alkali metal ions (Li+, Na+, K+). Complexes have been characterized by CW EPR, ENDOR and pulse EPR (HYSCORE) techniques. It has been shown that alkali metal ions decreased the strength of electron-acceptor centers due to the inductive effect. As a result the concentration of anthraquinone complex with two Lewis acid sites (LAS) decreased, but the concentration of complex with one LAS increased with the increasing of alkali metal content. At large alkali metal concentration other kinds of anthraquinone paramagnetic complexes are formed, that are anthrasemiquinone weakly bound with LAS as well as anthrasemiquinone ion pair with alkali metal cation (lithium).
PACS: 68.47.Gh, 33.35.+r, 33.40.+f, 31.30.Gs
Keywords: EPR, ENDOR, HYSCORE, electron-acceptor centers, paramagnetic complexes of anthraquinone, alumina, aluminates
1. Introduction
Alkali metals are often used for the modification of alumina catalysts, and also of Al2O3-based catalysts [1-2]. Their presence on the surface could strongly affect the acid-base properties and, hence, the catalytic properties. The main goal of this work was to study how a modifying with alkali Li+, Na+, K+ cations can influence the electron-acceptor sites on the surface of alumina. We applied the method of paramagnetic complexes of probe molecules for investigation. We chose 9,10-anthraquinone as a probe molecule.
2. Results and Discussion
After the adsorption of anthraquinone on the surface of studied samples the EPR spectra appeared (Fig. 1). Three different types of spectra have been found:
1. The 11-component spectrum with line intensities 1:2:3:4:5:6:5:4:3:2:1, g = 2.0036, average splitting between components 7.4 ± 0.2 G. This spectrum was analogous to those observed previously on the alumina [3] and alumina modified by LiAl5O8, MgAl2O4 and boric acid [4,5], and corresponded to the paramagnetic anthraquinone complex with two equivalent aluminum ions (spin of 27Al is 5/2).
2. The 6-component spectrum with equal intensities, g = 2.0036, average splitting between components 9.0±0.2 G. This spectrum was analogous to those observed previously on the various aluminas [3], Al2O3-ZrO2 system [6], and corresponded to the paramagnetic anthraquinone complex with one aluminum ion.
3. The narrow spectrum, g = 2.0036, peak to peak line width 8.0±0.2 G. The nature of this spectrum was not clear before because of the absence of hyperfine splitting (h.f.s). The nature of this spectrum is discussed in the present investigation.
In the row of modifying cations Li+, Na+, K+ in the equal concentration (0.9 mmol/g Al2O3) the shape of the spectra noticeably changed. The contribution of 11-component spectrum decreased in this row, while the contribution of narrow spectrum increased. Thus, the spectra of complexes on the surface of y-Al2O3 and of Li-modified sample were mainly similar and exhibit h.f.s. of 11-component, for a sample with Na the contribution of the 6-component spectrum and of narrow spectrum in the middle appeared, and for K-modified sample these two spectra became dominant. The same changes in the spectrum shapes took place, when the Li+ concentration increased up to the 4 mmol/g [7]. Taking into account the mechanism of formation of the paramagnetic anthraquinone complex [4], the appearance of the 6-component EPR spectrum is explained, most likely by a considerable enhancement of the basicity of alkali metal modified alumina. In this case, it can be assumed that the interaction of anthraquinone with Lewis acid sites (LAS) affords only a complex with one coordinatively unsaturated Al3+ cation, because the strength of electron-acceptor sites is not strong enough for an interaction of this complex with an additional Al3+ cation. The decrease of the strength of the LAS is also responsible, probably, for the appearance of a narrow spectrum, which could be attributed to anthrasemiquinone weakly bounded with the LAS.
MHz
Fig. 1 EPR spectra of anthraquinone adsorbed on the Fig. 2 ENDOR spectra of anthraquinone adsorbed on the
surface of the: 1-y-Al2O3; 2-Li+/Al2O3; 3-Na+/Al2O3; surface of the: 1-y-Al2O3; 2-Li+/Al2O3; 3-
4-K+/Al2O3. Na+/Al2O3; 4-K+/Al2O3.
The ENDOR spectra of the anthraquinone complexes of the samples, modified with alkali metal ions were in good agreement with EPR spectra (Fig. 2). The two broad lines, denoted by 27Ala, split by twice the 27Al Zeeman frequency 2vI = 7.8 MHz in the applied magnetic field and centered at 10.5 MHz, attributed to the 11-line EPR spectra and were observed for complexes with y-Al2O3 and with Li+/Al2O3. In the same row of modifiers (Li+, Na+, K+) the intensity of lines corresponding to the large proton coupling about 8-9 MHz (two broad lines centered at 14,9 MHz -
Zeeman frequency of 1H) increased. As it was shown earlier [5] this pair of lines was attributed to the anthraquinone radical, which gave narrow spectrum. According to the obtained data it has been shown that modification with alkali metal led to decrease of the strength of electron-acceptor sites because of their inductive effect (electronic influence) and did not block the acid centers.
For more detailed investigation of the alkali metal influence on the electron-acceptor centers we studied aluminates LiAl5O8 and a-LiAlO2, as a model compounds with known structure. LiAl5O8 has a spinel-like structure with aluminum ions distributed between octahedral and tetrahedral positions. The three-coordinated aluminum ions formation is possible after the removal of the terminal OH-groups from tetrahedral coordinated aluminum ions (they seem to play a role of Lewis acid sites on the alumina surface). In the structure of a-LiAlO2 there are only octahedral aluminum ions. The spectra of the complexes on the surface of aluminates were similar to the spectra of the complexes on alumina with large lithium concentration (4 mmol/g) [7] and consisted of 6-component spectrum and of a narrow spectrum (Fig. 3). When the temperature of anthraquinone adsorption increased from 120oC to 200oC the concentration of single line spectrum decreased and the contribution of 6-component spectrum rose. This fact means that the complex with narrow spectrum is less stable, than the complex with one aluminum. In the ENDOR spectra of aluminates (Fig. 4) a matrix line from lithium revealed. The intensity of this line was higher for the complexes with a-LiAlO2 samples due to higher lithium content. With the help of Q-band EPR it was shown, that narrow spectrum for aluminates was a superposition of two spectra, one of them could be attributed to anthrasemiquinone weakly bound with the LAS and another was supposed to be concerned with anthrasemiquinone ion pair with lithium cation.
vh
MHz
Fig. 3 EPR spectra of anthraquinone adsorbed on the surface Fig. 4 ENDOR spectra of anthraquinone adsorbed on the
of the: 1- LiAl5O8 at 120oC; 2- LiAl5O8 at 200oC; 3- a- surface of the: 1- LiAl5O8 at 120oC; 2- LiAl5O8 at
LiAlO2 at 120oC; 4- a-LiAlO2 at 200oC. 200oC; 3- a-LiAlO2 at 120oC; 4- a-LiAlO2 at
200oC.
We managed to obtain distinct HYSCORE spectra of anthraquinone complexes on the surface of lithium aluminates. The HYSCORE spectra of anthraquinone adsorbed on a-LiAlO2 are depicted on the Fig. 5-6. They both clearly showed the presence of the pair of cross-peaks, centered on the Zeeman frequency of 7Li (5.7 MHz in the field of 3450 G). Thus with the help of HYSCORE technique we showed the presence of h.f.s. from the lithium in the spectra, that was not revealed in the X-band EPR. Thus in the case of aluminates one of the narrow spectra (detected by Q-band EPR) could be attributed to the anthrasemiquinone ion pair with lithium.
It has been shown that alkali metal ions decreased the strength of electron-acceptor centers due to the inductive effect. As a result the concentration of anthraquinone complex with two Lewis acid sites (LAS) decreased, when the concentration of complex with one LAS increased with the increasing of alkali metal content. At large alkali metal concentration other kinds of anthraquinone paramagnetic complexes are formed, that are anthrasemiquinone weakly bound with LAS as well as anthrasemiquinone ion pair with alkali metal cation (lithium).
Fig. 5 HYSCORE spectra of anthraquinone adsorbed on a- Fig. 6 HYSCORE spectra of anthraquinone adsorbed
LiAlO2,obtained at the maximum of the EPR signal at on a-LiAlO2, obtained at the maximum of the
t = 180 ns. signal at t = 200 ns. EPR
Acknowledgement
This work was supported by the Federal Agency on the Science and Innovations (State Contract 02.451.11.7012 of August 29, 2005), the Concil on Grants of the President Of the Russian Federation (Program of State Support for Leading Scientific Schools of the Russian Federation, Grant RI-112/001/056) and the Russian Foundation for Basic Research (Project No. 06-03-32830a).
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