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Lewis acid properties of alumina based catalysts: study by paramagnetic complexes of probe molecules
Surface Science 507–510 (2002) 74–81 www.elsevier.com/locate/susc
Lewis acid properties of alumina based catalysts: study by paramagnetic complexes of probe molecules Alexander V. Fionov
*
Department of Chemistry, Moscow State University, Leninskije Gory, Moscow 119899, Russia
Abstract Lewis acid properties of LiAl5 O8 /Al2 O3 (2 wt.% Li) and MgAl2 O4 /Al2 O3 (3 wt.% Mg) catalysts were studied by EPR of adsorbed probe molecules––anthraquinone and 2,2,6,6-tetramethylpiperidine-N-oxyl (TEMPO). The lesser (in comparison with c-Al2 O3 ) concentration and the strength of Lewis acid sites (LAS) formed on the surface of aluminate layer has been shown. The stability of this layer plays important role in the change of Lewis acid properties during the calcination of modified alumina. The lithium aluminate layer was stable at used calcination temperature, 773 K, meanwhile magnesium aluminate layer observed only at calcination temperature below 723 K. The increase of the calcination temperature to 773 K caused the segregation of MgAl2 O4 on the surface resulted in the release of alumina surface and recovery of the Lewis acid properties. The differences in the LAS manifestations towards TEMPO and anthraquinone was discussed. The mechanism of the formation of anthraquinone paramagnetic complexes with LAS––three-coordinated aluminum ions––was proposed. This mechanism includes the formation of anthrasemiquinone, and then––anthrasemiquinone ion pair or triple ion. Fragments like –O–Alþ –O– play the role of cations in these ion pairs and triple ions. Proposed mechanism can also be applied for the consideration of similar anthraquinone paramagnetic complexes on the surface of gallium oxide containing systems. Ó 2002 Elsevier Science B.V. All rights reserved. Keywords: Electron spin resonance; Physical adsorption; Chemisorption; Aluminum oxide
1. Introduction The information on the surface acidity is very important for the complete characterization of alumina-based catalysts and supports. It is known that transitional aluminas (for example, c-Al2 O3 ) have an appreciable Lewis acidity. As it has been proved [1], coordinatively unsaturated aluminum ions play a role of Lewis acid sites (LAS) on the alumina surface. The strength and concentration
*
Tel.: +7-095-9393278; fax: +7-095-9394575. E-mail address: fi[email protected] (A.V. Fionov).
of these sites can be changed with the help of various dopants. In particular, metal cations of I, II and III groups are used as dopants for aluminabased catalysts [2–6]. The lithium [7–9] and magnesium-modified alumina [10,11] are considered as promising catalysts and supports. Nevertheless the mechanism of the modifying and the structure of LAS on the surface are under discussion up to now. EPR of paramagnetic complexes of probe molecules (in particular, 2,2,6,6-tetramethylpiperidineN-oxyl (TEMPO) and anthraquinone) has been used previously for the characterization of Lewis and Bronsted acid sites of alumina, gallia, zirconia
0039-6028/02/$ - see front matter Ó 2002 Elsevier Science B.V. All rights reserved. PII: S 0 0 3 9 - 6 0 2 8 ( 0 2 ) 0 1 1 7 8 - 0
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and other oxide catalysts [12–15]. Probe molecules form stable paramagnetic complexes with these sites on the surface. The analysis of magnetic resonance parameters of these species enables to get the information on the nature, concentration, strength and mutual arrangement of the sites. Recently this method was used for the study of alumina, modified with Naþ , Ca2þ , Sr2þ and Ba2þ cations [16,17]. Taking into account the interest to lithium- and magnesium-modified aluminas, the study of similar systems was performed by means of EPR of paramagnetic complexes of probe molecules–– anthraquinone and TEMPO. The 2 wt.% Li and 3 wt.% Mg loadings were chosen as frequently used values, which are quite high but do not have significant influence on the surface area.
could be possible to study the surface segregation process, observed in the system MgAl2 O4 /Al2 O3 . TEMPO was adsorbed under vacuum at room temperature from the gaseous phase, according to [12]. Anthraquinone was adsorbed according to the method described in [17]. The final temperature of anthraquinone adsorption was 473 K. EPR spectra were recorded using X-band spectrometer RE-1306. g-values were determined with a reference to diphenylpicrylhydrazyl standard (g ¼ 2:0036). Spin concentrations were measured by double integration of the spectrum and comparison with calibrated sample (carbonized dextrose) using a reference sample (Cr3þ in ruby). EPR spectra were recorded at room temperature, or at 77 K (for measurements of the parallel hyperfine splitting (h.f.s.) constant on nitrogen, AN k , of adsorbed TEMPO).
2. Experimental
3. Results and discussion
The samples LiAl5 O8 /Al2 O3 (2 wt.% Li) and MgAl2 O4 /Al2 O3 (3 wt.% Mg) were prepared by the impregnation of commercial c-Al2 O3 (trademark A-1 (Russia), specific surface area 180 10 m2 /g) with LiNO3 or Mg(NO3 )2 aqueous solution, followed by drying at 393 K and calcination. Lithium containing sample was calcined at 773 K, 4 h. Magnesium containing sample was calcined at 623 or 773 K, 4 h. The specific surface area of samples was measured on the gasometer GKH-1 (Russia) by nitrogen adsorption at 77 K and desorption at room temperature using the desorption peak area. The gas mixture 6 vol.% N2 in helium has been used for these measurements. Phase composition of the samples was determined by X-ray analysis with CuKa -radiation on the diffractometer DRON-3M. Before the adsorption of probe molecules all samples were activated for 2 h at desired temperature in the air and then for 2 h at the same temperature under vacuum 103 Pa. The activation temperature was 743 K for all samples, excluding two samples of magnesium-modified alumina, calcined at 623 K. These two samples had the activation temperatures 673 and 723 K, so that it
3.1. EPR spectra of adsorbed anthraquinone Specific surface of all samples decreased slightly after the modifying, but was enough high (160 10 m2 /g instead of 180 10 m2 /g for initial alumina). X-ray phase analysis of modified alumina samples showed the presence of the low-crystalline LiAl5 O8 or MgAl2 O4 phases besides the c-alumina one. After the adsorption of anthraquinone on the surface of studied samples the EPR spectra appeared. 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 ¼ 2:0036, a ¼ 7:4 0:2 G. This spectrum was analogous to those observed previously on the alumina [12] and alumina modified by boric acid [18,19], and corresponded to the paramagnetic anthraquinone complex with two equivalent aluminum ions (spin of 27 Al is 5=2). 2. The 6-component spectrum with equal intensities, g ¼ 2:0036, a ¼ 9:0 0:2 G. This spectrum was also analogous to those observed previously
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Table 1 Anthraquinone paramagnetic complexes on alumina, modified with lithium and magnesium cations Sample
Tcalcination (K)
Tactivation (K)
Total concentration of the anthraquinone paramagnetic complexes (N 1016 sites/m2 )
EPR spectrum shapea
c-Al2 O3 2 wt.% Liþ /Al2 O3 3 wt.% Mg2þ /Al2 O3 3 wt.% Mg2þ /Al2 O3 3 wt.% Mg2þ /Al2 O3
– 773 623 623 773
743 743 673 723 743
14 7.5 0.8 2.5 13.8
11 6þS 6 6 11
a Abbreviations: 11––eleven-component spectrum of anthraquinone complex with two Al3þ ions; 6––six-component spectrum of anthraquinone complex with one Al3þ ions; S––single line spectrum (about 10% of total concentration of anthraquinone complexes).
on the various aluminas [12], alumina modified with Naþ and Ca2þ [16], Sr2þ and Ba2þ [17], Al2 O3 –ZrO2 system [14], and corresponded to the paramagnetic anthraquinone complex with one aluminum ion. 3. Single line spectrum g ¼ 2:0036, peak-to-peak line width 8:0 0:2 G. The nature of this spectrum was not clear because of the absence of h.f.s. The obtained data are summarized in the Table 1. There was only 11-component spectrum of adsorbed anthraquinone on the surface of c-Al2 O3 (Fig. 1a). Significant dipole–dipole broadening of the EPR spectrum showed high local concentrations of paramagnetic species. This result was similar to previous data [12]. It was supposed that anthraquinone molecule interacts with two LAS (threecoordinated aluminum ions) which are disposed at the distance about 0.53 nm (this is the distance between carbonyl groups of the anthraquinone molecule) [12]. These data enable us to conclude that LAS on c-Al2 O3 are disposed regularly at high local concentrations. This consideration is in a good agreement with the ideas developed by Knozinger and Ratnasami [1] about the disposition of three-coordinated aluminum ions formed after the dehydroxylation of (1 1 1)A and (1 1 0)C planes of alumina surface. Adsorption of the anthraquinone resulted in the appearance of other type of EPR spectrum on the lithium modified sample. This spectrum was interpreted as a superposition of 6-component and single line spectra (Fig. 1b). The concentration of 6-component spectrum was enough high (up to
Fig. 1. EPR spectra (registered at room temperature) of paramagnetic anthraquinone complexes on the surface of: (a) calumina, (b) 2 wt.% Liþ /Al2 O3 , (c–e) 3 wt.% Mg2þ /Al2 O3 . Calcination temperature/activation temperature: (a) –/743 K; (b) 773 K/743 K; (c) 623 K/673 K; (d) 623 K/723 K; (e) 773 K/ 743 K. Temperature of the anthraquinone adsorption 473 K.
7 1016 spin/m2 ). Taking into account similar anthraquinone paramagnetic complex formation
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on the surface of pure LiAl5 O8 sample [20], data on lithium modified alumina can be explained by the lithium aluminate layer formation, LiAl5 O8 . This substance has a spinel-like structure, and aluminum ions are distributed between tetrahedral and octahedral positions in the LiAl5 O8 bulk [21]. The three-coordinated aluminum ions formation is possible after the removal of the terminal OHgroups from tetrahedral coordinated aluminum ions, like in the case of c-alumina. Nevertheless an anthraquinone cannot interact with two aluminum ions because of different surface structure of LiAl5 O8 and to produce 11-component spectrum. The nature of single line spectrum is not clear. Apparently this is also paramagnetic species which is formed after anthraquinone adsorption. But its concentration was not dominant (about 10% from the total concentration of anthraquinone complexes). In the case of magnesium modified alumina the 6-component spectrum formed at low calcination temperatures (Fig. 1c and d). This result can be explained also by aluminate layer formation, MgAl2 O4 . The concentration of three-coordinated aluminum ions on the surface of this layer is lower than in the case of LiAl5 O8 layer. It can be associated with the absence of aluminum ions in tetrahedral positions in ideal MgAl2 O4 structure [22]. The calcination of magnesium modified alumina at 773 K caused the surprised results. The EPR spectrum of adsorbed anthraquinone changed to 11-component one (Fig. 1e) simultaneously with the increase of the paramagnetic complex concentration up to initial one (Table 1). Taking into account the relative increase of the crystallinity of MgAl2 O4 (showed by the X-ray patterns), the segregation of MgAl2 O4 on the surface could be proposed. As a result, the alumina surface, which has been covered by the modifier at low calcination temperature, becomes accessible after the MgAl2 O4 segregation. 3.2. EPR spectra of adsorbed 2,2,6,6-tetramethylpiperidine-N-oxyl TEMPO is known as a paramagnetic probe which can be used for the study of Lewis and
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Bronsted acid sites. The value of AN k is known to be used as a scale for evaluating the electronpair acceptor strength of acid sites. For example, the value of AN k (0.2 G) of TEMPO in toluene solution is 34.6 G, on the silica surface is 38.0 G, in H3 PO4 (10%) is 39.5 G [23]. For TEMPO complexes with Lewis acids the AN k also increases with the increase of electron acceptor strength of the site. For strongest LAS on alumina surface AN k ¼ 45 0:5 G. Also, EPR spectrum of TEMPO complex with LAS on alumina has additional h.f.s. from 27 Al nucleus. It is supposed that TEMPO interacts with three-coordinated aluminum ions [12]. This model has been confirmed by experimental data (the similarity of TEMPO complexes on the alumina surface and with AlCl3 in anhydrous solutions) as well as quantumchemical calculations [24]. There were similar models proposed on the basis of IR spectroscopy of probe molecules (pyridine, carbon monoxide and others) [1,25]. The quantitative data on the concentration of LAS, determined by EPR of TEMPO, are consistent with similar data obtained by EPR of adsorbed anthraquinone as well as IR spectroscopy of probe molecules and other methods. The use of TEMPO has significant advantage due to the unambiguous determination of the nature of Lewis acid site in the case of h.f.s. from the nucleus of acceptor cation. After the adsorption of TEMPO on the surface of alumina (used in this work) the well known multicomponent EPR spectrum of the TEMPO complex with coordinatively unsaturated aluminum ion was observed. H.f.s. of this spectrum corresponded to the interaction of unpaired electron with one 14 N and one 27 Al nuclei. This result is consistent with literature data [12]. After the adsorption of TEMPO on the lithium modified alumina the three line EPR spectrum (h.f.s. only on 14 N nucleus) was observed. AN k value for this spectrum (40 0:2 G) could be assigned to the interaction of TEMPO molecule with strong Bronsted acid site or with relatively weak Lewis acid site (in comparison with the alumina surface). The same spectrum observed previously after TEMPO adsorption on the LiAl5 O8 surface [20]. Taking into account the stability of adsorbed TEMPO (which could not be observed in the
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presence of the Bronsted acidity [23]) and the unambiguous presence of coordinatively unsaturated aluminum ions (determined with the help of anthraquinone adsorption), it is possible to conclude that TEMPO as well as an anthraquinone interacts with LAS––three-coordinated aluminum ions formed on the LiAl5 O8 layer. The absence of 27 Al h.f.s. in the EPR spectrum of TEMPO complex with these sites can be explained by larger distance between >N–O group and Al atom, because, as it was shown by quantum-chemical calculations [26], the spin density on Al appears at the distance about 0.2 nm and less. At the distances 0.25 nm and more (which are reasonable for weaker sites) it is possible to observe increased spin density on the nitrogen atom (in comparison with free nitroxide molecule) with the absence of h.f.s. from aluminum. Data on the decrease of the strength of LAS are in accordance with observations which were made by de Miguel et al. [7] with the help of IR spectroscopy of carbon monoxide adsorbed on lithium modified alumina. The three line spectrum with AN k ¼ 40 0:5 G has been registered also after TEMPO adsorption on the magnesium modified alumina, calcined at low temperatures. The segregation of MgAl2 O4 (after calcination at 773 K) resulted in the significant increase of the multicomponent (h.f.s. from 14 N and 27 Al) spectrum impact. So, data on the surface properties obtained by means of adsorbed TEMPO and anthraquinone are consistent and complementary.
3.3. Nature of the paramagnetic complexes of probe molecules on the oxide surfaces It is reasonable to discuss the nature of paramagnetic complexes of probe molecules. In particular, there is very different manifestation of LAS on the alumina in the EPR spectra of the TEMPO and anthraquinone. Magnetic resonance parameters of TEMPO are sensitive to the acid site strength. TEMPO molecule coordinates with Lewis (or Bronsted) acid site like common donor molecule (for example, carbon monoxide, pyridine, ammonia, etc.). The changes
in the EPR spectrum correlate with the strength of the interaction. Magnetic resonance parameters of anthraquinone complexes are not sensitive to the strength of LAS. In particular, both 11-component and 6component spectra of anthraquinone complexes do not depend on the nature of studied catalyst. If the formation of the anthraquinone complexes is possible then it produces the same EPR spectra. The difference between samples is the concentration of anthraquinone complexes and the ratio between 11-component and 6-component spectra. It is interesting that on the different gallium oxides [13] as well as on the Ga2 O3 –ZrO2 [14] and Ga2 O3 –Al2 O3 [15] systems the anthraquinone adsorption also resulted in the formation of paramagnetic complexes with two or one gallium cations (7- or 4-component spectrum respectively, because spin of 69 Ga as well as 71 Ga is 3=2). Properties of such complexes are similar to the properties of the anthraquinone complexes with aluminum ion(s), that are the necessity of the heating to prepare the complexes, the unstability of the complexes towards adsorption of bases (water et al.) and the similarity of the h.f.s. constants of 7 (or 4) component spectra of different samples. Therefore the anthraquinone complex formation process seems to be a chemisorption process, when the configuration of adsorption site and adsorbate can change. As a result, the same stable paramagnetic complex structure is achieved. Also it should be taken into account that the anthraquinone complex formation process cannot be described as usual complex formation, like in the case of TEMPO. The anthraquinone molecules as well as LAS are diamagnetic and the formation of paramagnetic species requires one-electron transfer with the cation-radical or anion-radical formation. Up to now the anthraquinone (AQ) complex formation on the alumina surface was considered to occur in two steps [12]: 1. The transfer of one electron from AQ molecule to the surface resulted in the cation-radical formation, AQþ . 2. The interaction of AQþ with one or two LAS.
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This mechanism has two drawbacks: On the one hand, as it was considered by Eberson[27], the anthraquinone is very resistant towards oxidation ðE0 AQþ =AQÞ 2:25 VÞ. And so-called ‘‘cation-radical’’ of AQ, which can be prepared in the strong acid media, is diprotonated anthrasemiquinone, ðAQH2 Þþ , as a matter of fact. On the other hand, improbably that AQþ , which is already positively charged, will be able to interact with LAS, with next electron density transfer from anthraquinone to acceptor cation. It is more reasonable in this way to consider the possibility of the AQþ interaction with electron donor sites. But experimental data showed unambiguous presence of Al3þ ions (one or two) in the structure of the complex. Nevertheless this model was used because the observed EPR spectrum (as well as infrared spectrum) could not be assigned to anthrasemiquinone or diprotonated anthrasemiquinone [12]. The detailed analysis of literature data showed that there are complexes of anthrasemiquinone which are very similar to the complexes on alumina surface. In the excellent work made by Chen and Hirota [28] the triple ions (Mþ AQ Mþ ), ion pairs (Mþ AQ ) and alcohol-solvated ion pairs (Mþ AQ HOR) where M ¼ Li, Na or K, were described. There are similar features between triple ions and anthraquinone complex with two Al3þ (Ga3þ ) ions as well as between ion pairs (or alcohol-solvated ion pairs) and anthraquinone complex with one Al3þ (Ga3þ ) ion. In particular: 1. There is the equivalence of two nuclei of metal in the anthraquinone triple ions and in the complex with two Al3þ (Ga3þ ) ions. 2. From the one hand, there is an equilibrium between anthraquinone triple ions and ion pairs. This equilibrium shifts to triple ions with the increase of the temperature (from 178 to 298 K). From the other hand, the formation of anthraquinone complexes with two Al3þ (Ga3þ ) ions requires a heating up to 473 K. The complexes with one Al3þ (Ga3þ ) ion forms as intermediate species during this heating.
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3. The addition of the alcohol (i-C3 H7 OH) causes the transformation of triple ions to alcohol solvated ion pairs. Also, the adsorption of donor molecules (for example, H2 O) causes reversible transformation of the anthraquinone paramagnetic complex with two Al3þ ions to the complex with one Al3þ ion and then to the neutral anthraquinone [29]. Taking into account these data as well as 100% conversion of adsorbed AQ into paramagnetic state [30] and the activation energy for AQ complex formation (12 kcal/mol [30]), the next mechanism of AQ paramagnetic complex formation on the alumina surface can be proposed (Fig. 2). The process can be divided into three stages: 1. Fig. 2a. Formation of anthrasemiquinone as a result of the one-electron transfer from electron-donor center of the surface to the AQ molecule. It is difficult to say exactly about the nature of this center. Probably this center is negative oxygen ion as showed in Fig. 2a. The unpaired electron localized on surface oxygen atom is not observed by EPR, probably due to very short spin–lattice relaxation time, or for another reason. 2. Fig. 2b. Anthrasemiquinone reacts with threecoordinated aluminum ion, to produce paramagnetic complex with one Al3þ . This process requires the break of one bond and the formation of another bond. This is a reason for the presence of an activation energy. Another >C–O group can interact with neighboring OH group as shown in the Fig. 2b. 3. Fig. 2c. During more prolonged heating the AQ molecule can ‘‘find’’ another neighboring LAS and interact with it. All stages are reversible. The adsorption of water can shift equilibrium from right to left due to the stronger bonding with LAS, in comparison with anthraquinone molecule. The heating of the sample and evacuation of water shifts equilibrium from left to right, in conformity with experimental data [12]. It is clear on the basis of the proposed mechanism why the data on the quantity of LAS measured by the maximum AQ paramagnetic complex
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Fig. 2. Proposed mechanism of the formation of the anthraquinone paramagnetic complexes with LAS of the alumina surface.
concentration (taking into account the number of LAS in the complex), are in line with data obtained by TEMPO adsorption or by other probe molecules. So, the same LAS (three-coordinated aluminum ions) can interact either with oxygen atoms of anthraquinone, or with other probe molecules. But the anthraquinone adsorption results in the formation of two possible complexes with definite structures. This is the reason why the LAS, which are different in strength towards TEMPO [12] or carbon monoxide [31], are uniform towards AQ. Evidently, similar mechanism can be proposed for the formation of anthraquinone paramagnetic complexes with three-coordinated gallium cations on gallium oxide, Ga2 O3 –ZrO2 and Ga2 O3 –Al2 O3 . 4. Conclusions It was shown that Lewis acid properties of the alumina modified by lithium or magnesium cations are determined by the aluminate layer (LiAl5 O8 or MgAl2 O4 ) formation. A stability of this layer play simportant role in the change of Lewis acid properties during the calcination of modified alumina.
It should be concluded that the interaction of anthraquinone with LAS is completely different from the interaction of other common probe molecules (for example, TEMPO) with the same centers. The interaction of anthraquinone with LAS requires the break and formation of chemical bonds, but the interaction of other common donor molecules with LAS is only formation of donoracceptor bond. The paramagnetic complex of anthraquinone with two aluminum ions should be considered as the structure like to triple ion of anthrasemiquinone with two cations. Two fragments like –O– Alþ –O– play the role of cations in these triple ions. The paramagnetic complex of anthraquinone with one aluminum ion should be considered as the ion pair of anthrasemiquinone and one fragment like –O–Alþ –O–. Therefore, the anthraquinone adsorption is a convenient tool for the measurements of the total concentration of LAS. Additional information from EPR spectra of adsorbed anthraquinone can be obtained about the regularities in the LAS disposition on the surface.
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Similar mechanism of the formation of anthraquinone complex with two or one threecoordinated gallium ions can be proposed for the explanation of known data on the Lewis acid properties of gallium oxide containing systems. TEMPO probe is supplementary to the anthraquinone one, and can be used for the evaluation of the strength of LAS. In the case of relatively weak coordinatively unsaturated aluminum ions on LiAl5 O8 surface, when there is no h.f.s. from the nucleus of acceptor cation in the TEMPO complex, with the help of adsorbed anthraquinone data it is possible to determine unambiguously the nature of acceptor cation (aluminum). Therefore, the use of both probes, anthraquinone and TEMPO, is quite informative method to study Lewis acidity of alumina-based catalysts. Acknowledgements Author would like to express a great gratitude to Elena V. Lunina (1940–1999), who was one of the founders of the EPR method of paramagnetic probe molecules and educated many scientists, including the author. Author acknowledges RFBR (grant 98-0332129a) and INTAS (grant YSF 00-252) for financial support. Author is very grateful to Prof. Anders Lund (Linkoping University) for helpful discussions. Author is grateful to Anara O. Turakulova, Elena N. Chinennikova and Galina P. Murav’yova (Department of Chemistry, Moscow State University) for the help with some experiments. Author is grateful to Goar L. Markaryan (Department of Chemistry, Moscow State University) for useful comments on this article. References [1] H. Knozinger, P. Ratnasamy, Catal. Rev. Sci. Eng. 17 (1978) 31. [2] M. Ziolek, J. Kujawa, J. Czyzniewska, I. Nowak, A. Aboulayt, O. Saur, J.C. Lavalley, Appl. Catal. A. 171 (1998) 109. [3] V. Bolis, G. Magnacca, C. Morterra, Res. Chem. Intermed. 25 (1) (1999) 25. [4] Y. Ono, T. Baba, Catal. Today 38 (1997) 321.
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Lewis acid properties of alumina based catalysts: study by paramagnetic complexes of probe molecules Alexander V. Fionov
*
Department of Chemistry, Moscow State University, Leninskije Gory, Moscow 119899, Russia
Abstract Lewis acid properties of LiAl5 O8 /Al2 O3 (2 wt.% Li) and MgAl2 O4 /Al2 O3 (3 wt.% Mg) catalysts were studied by EPR of adsorbed probe molecules––anthraquinone and 2,2,6,6-tetramethylpiperidine-N-oxyl (TEMPO). The lesser (in comparison with c-Al2 O3 ) concentration and the strength of Lewis acid sites (LAS) formed on the surface of aluminate layer has been shown. The stability of this layer plays important role in the change of Lewis acid properties during the calcination of modified alumina. The lithium aluminate layer was stable at used calcination temperature, 773 K, meanwhile magnesium aluminate layer observed only at calcination temperature below 723 K. The increase of the calcination temperature to 773 K caused the segregation of MgAl2 O4 on the surface resulted in the release of alumina surface and recovery of the Lewis acid properties. The differences in the LAS manifestations towards TEMPO and anthraquinone was discussed. The mechanism of the formation of anthraquinone paramagnetic complexes with LAS––three-coordinated aluminum ions––was proposed. This mechanism includes the formation of anthrasemiquinone, and then––anthrasemiquinone ion pair or triple ion. Fragments like –O–Alþ –O– play the role of cations in these ion pairs and triple ions. Proposed mechanism can also be applied for the consideration of similar anthraquinone paramagnetic complexes on the surface of gallium oxide containing systems. Ó 2002 Elsevier Science B.V. All rights reserved. Keywords: Electron spin resonance; Physical adsorption; Chemisorption; Aluminum oxide
1. Introduction The information on the surface acidity is very important for the complete characterization of alumina-based catalysts and supports. It is known that transitional aluminas (for example, c-Al2 O3 ) have an appreciable Lewis acidity. As it has been proved [1], coordinatively unsaturated aluminum ions play a role of Lewis acid sites (LAS) on the alumina surface. The strength and concentration
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of these sites can be changed with the help of various dopants. In particular, metal cations of I, II and III groups are used as dopants for aluminabased catalysts [2–6]. The lithium [7–9] and magnesium-modified alumina [10,11] are considered as promising catalysts and supports. Nevertheless the mechanism of the modifying and the structure of LAS on the surface are under discussion up to now. EPR of paramagnetic complexes of probe molecules (in particular, 2,2,6,6-tetramethylpiperidineN-oxyl (TEMPO) and anthraquinone) has been used previously for the characterization of Lewis and Bronsted acid sites of alumina, gallia, zirconia
0039-6028/02/$ - see front matter Ó 2002 Elsevier Science B.V. All rights reserved. PII: S 0 0 3 9 - 6 0 2 8 ( 0 2 ) 0 1 1 7 8 - 0
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and other oxide catalysts [12–15]. Probe molecules form stable paramagnetic complexes with these sites on the surface. The analysis of magnetic resonance parameters of these species enables to get the information on the nature, concentration, strength and mutual arrangement of the sites. Recently this method was used for the study of alumina, modified with Naþ , Ca2þ , Sr2þ and Ba2þ cations [16,17]. Taking into account the interest to lithium- and magnesium-modified aluminas, the study of similar systems was performed by means of EPR of paramagnetic complexes of probe molecules–– anthraquinone and TEMPO. The 2 wt.% Li and 3 wt.% Mg loadings were chosen as frequently used values, which are quite high but do not have significant influence on the surface area.
could be possible to study the surface segregation process, observed in the system MgAl2 O4 /Al2 O3 . TEMPO was adsorbed under vacuum at room temperature from the gaseous phase, according to [12]. Anthraquinone was adsorbed according to the method described in [17]. The final temperature of anthraquinone adsorption was 473 K. EPR spectra were recorded using X-band spectrometer RE-1306. g-values were determined with a reference to diphenylpicrylhydrazyl standard (g ¼ 2:0036). Spin concentrations were measured by double integration of the spectrum and comparison with calibrated sample (carbonized dextrose) using a reference sample (Cr3þ in ruby). EPR spectra were recorded at room temperature, or at 77 K (for measurements of the parallel hyperfine splitting (h.f.s.) constant on nitrogen, AN k , of adsorbed TEMPO).
2. Experimental
3. Results and discussion
The samples LiAl5 O8 /Al2 O3 (2 wt.% Li) and MgAl2 O4 /Al2 O3 (3 wt.% Mg) were prepared by the impregnation of commercial c-Al2 O3 (trademark A-1 (Russia), specific surface area 180 10 m2 /g) with LiNO3 or Mg(NO3 )2 aqueous solution, followed by drying at 393 K and calcination. Lithium containing sample was calcined at 773 K, 4 h. Magnesium containing sample was calcined at 623 or 773 K, 4 h. The specific surface area of samples was measured on the gasometer GKH-1 (Russia) by nitrogen adsorption at 77 K and desorption at room temperature using the desorption peak area. The gas mixture 6 vol.% N2 in helium has been used for these measurements. Phase composition of the samples was determined by X-ray analysis with CuKa -radiation on the diffractometer DRON-3M. Before the adsorption of probe molecules all samples were activated for 2 h at desired temperature in the air and then for 2 h at the same temperature under vacuum 103 Pa. The activation temperature was 743 K for all samples, excluding two samples of magnesium-modified alumina, calcined at 623 K. These two samples had the activation temperatures 673 and 723 K, so that it
3.1. EPR spectra of adsorbed anthraquinone Specific surface of all samples decreased slightly after the modifying, but was enough high (160 10 m2 /g instead of 180 10 m2 /g for initial alumina). X-ray phase analysis of modified alumina samples showed the presence of the low-crystalline LiAl5 O8 or MgAl2 O4 phases besides the c-alumina one. After the adsorption of anthraquinone on the surface of studied samples the EPR spectra appeared. 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 ¼ 2:0036, a ¼ 7:4 0:2 G. This spectrum was analogous to those observed previously on the alumina [12] and alumina modified by boric acid [18,19], and corresponded to the paramagnetic anthraquinone complex with two equivalent aluminum ions (spin of 27 Al is 5=2). 2. The 6-component spectrum with equal intensities, g ¼ 2:0036, a ¼ 9:0 0:2 G. This spectrum was also analogous to those observed previously
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Table 1 Anthraquinone paramagnetic complexes on alumina, modified with lithium and magnesium cations Sample
Tcalcination (K)
Tactivation (K)
Total concentration of the anthraquinone paramagnetic complexes (N 1016 sites/m2 )
EPR spectrum shapea
c-Al2 O3 2 wt.% Liþ /Al2 O3 3 wt.% Mg2þ /Al2 O3 3 wt.% Mg2þ /Al2 O3 3 wt.% Mg2þ /Al2 O3
– 773 623 623 773
743 743 673 723 743
14 7.5 0.8 2.5 13.8
11 6þS 6 6 11
a Abbreviations: 11––eleven-component spectrum of anthraquinone complex with two Al3þ ions; 6––six-component spectrum of anthraquinone complex with one Al3þ ions; S––single line spectrum (about 10% of total concentration of anthraquinone complexes).
on the various aluminas [12], alumina modified with Naþ and Ca2þ [16], Sr2þ and Ba2þ [17], Al2 O3 –ZrO2 system [14], and corresponded to the paramagnetic anthraquinone complex with one aluminum ion. 3. Single line spectrum g ¼ 2:0036, peak-to-peak line width 8:0 0:2 G. The nature of this spectrum was not clear because of the absence of h.f.s. The obtained data are summarized in the Table 1. There was only 11-component spectrum of adsorbed anthraquinone on the surface of c-Al2 O3 (Fig. 1a). Significant dipole–dipole broadening of the EPR spectrum showed high local concentrations of paramagnetic species. This result was similar to previous data [12]. It was supposed that anthraquinone molecule interacts with two LAS (threecoordinated aluminum ions) which are disposed at the distance about 0.53 nm (this is the distance between carbonyl groups of the anthraquinone molecule) [12]. These data enable us to conclude that LAS on c-Al2 O3 are disposed regularly at high local concentrations. This consideration is in a good agreement with the ideas developed by Knozinger and Ratnasami [1] about the disposition of three-coordinated aluminum ions formed after the dehydroxylation of (1 1 1)A and (1 1 0)C planes of alumina surface. Adsorption of the anthraquinone resulted in the appearance of other type of EPR spectrum on the lithium modified sample. This spectrum was interpreted as a superposition of 6-component and single line spectra (Fig. 1b). The concentration of 6-component spectrum was enough high (up to
Fig. 1. EPR spectra (registered at room temperature) of paramagnetic anthraquinone complexes on the surface of: (a) calumina, (b) 2 wt.% Liþ /Al2 O3 , (c–e) 3 wt.% Mg2þ /Al2 O3 . Calcination temperature/activation temperature: (a) –/743 K; (b) 773 K/743 K; (c) 623 K/673 K; (d) 623 K/723 K; (e) 773 K/ 743 K. Temperature of the anthraquinone adsorption 473 K.
7 1016 spin/m2 ). Taking into account similar anthraquinone paramagnetic complex formation
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on the surface of pure LiAl5 O8 sample [20], data on lithium modified alumina can be explained by the lithium aluminate layer formation, LiAl5 O8 . This substance has a spinel-like structure, and aluminum ions are distributed between tetrahedral and octahedral positions in the LiAl5 O8 bulk [21]. The three-coordinated aluminum ions formation is possible after the removal of the terminal OHgroups from tetrahedral coordinated aluminum ions, like in the case of c-alumina. Nevertheless an anthraquinone cannot interact with two aluminum ions because of different surface structure of LiAl5 O8 and to produce 11-component spectrum. The nature of single line spectrum is not clear. Apparently this is also paramagnetic species which is formed after anthraquinone adsorption. But its concentration was not dominant (about 10% from the total concentration of anthraquinone complexes). In the case of magnesium modified alumina the 6-component spectrum formed at low calcination temperatures (Fig. 1c and d). This result can be explained also by aluminate layer formation, MgAl2 O4 . The concentration of three-coordinated aluminum ions on the surface of this layer is lower than in the case of LiAl5 O8 layer. It can be associated with the absence of aluminum ions in tetrahedral positions in ideal MgAl2 O4 structure [22]. The calcination of magnesium modified alumina at 773 K caused the surprised results. The EPR spectrum of adsorbed anthraquinone changed to 11-component one (Fig. 1e) simultaneously with the increase of the paramagnetic complex concentration up to initial one (Table 1). Taking into account the relative increase of the crystallinity of MgAl2 O4 (showed by the X-ray patterns), the segregation of MgAl2 O4 on the surface could be proposed. As a result, the alumina surface, which has been covered by the modifier at low calcination temperature, becomes accessible after the MgAl2 O4 segregation. 3.2. EPR spectra of adsorbed 2,2,6,6-tetramethylpiperidine-N-oxyl TEMPO is known as a paramagnetic probe which can be used for the study of Lewis and
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Bronsted acid sites. The value of AN k is known to be used as a scale for evaluating the electronpair acceptor strength of acid sites. For example, the value of AN k (0.2 G) of TEMPO in toluene solution is 34.6 G, on the silica surface is 38.0 G, in H3 PO4 (10%) is 39.5 G [23]. For TEMPO complexes with Lewis acids the AN k also increases with the increase of electron acceptor strength of the site. For strongest LAS on alumina surface AN k ¼ 45 0:5 G. Also, EPR spectrum of TEMPO complex with LAS on alumina has additional h.f.s. from 27 Al nucleus. It is supposed that TEMPO interacts with three-coordinated aluminum ions [12]. This model has been confirmed by experimental data (the similarity of TEMPO complexes on the alumina surface and with AlCl3 in anhydrous solutions) as well as quantumchemical calculations [24]. There were similar models proposed on the basis of IR spectroscopy of probe molecules (pyridine, carbon monoxide and others) [1,25]. The quantitative data on the concentration of LAS, determined by EPR of TEMPO, are consistent with similar data obtained by EPR of adsorbed anthraquinone as well as IR spectroscopy of probe molecules and other methods. The use of TEMPO has significant advantage due to the unambiguous determination of the nature of Lewis acid site in the case of h.f.s. from the nucleus of acceptor cation. After the adsorption of TEMPO on the surface of alumina (used in this work) the well known multicomponent EPR spectrum of the TEMPO complex with coordinatively unsaturated aluminum ion was observed. H.f.s. of this spectrum corresponded to the interaction of unpaired electron with one 14 N and one 27 Al nuclei. This result is consistent with literature data [12]. After the adsorption of TEMPO on the lithium modified alumina the three line EPR spectrum (h.f.s. only on 14 N nucleus) was observed. AN k value for this spectrum (40 0:2 G) could be assigned to the interaction of TEMPO molecule with strong Bronsted acid site or with relatively weak Lewis acid site (in comparison with the alumina surface). The same spectrum observed previously after TEMPO adsorption on the LiAl5 O8 surface [20]. Taking into account the stability of adsorbed TEMPO (which could not be observed in the
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presence of the Bronsted acidity [23]) and the unambiguous presence of coordinatively unsaturated aluminum ions (determined with the help of anthraquinone adsorption), it is possible to conclude that TEMPO as well as an anthraquinone interacts with LAS––three-coordinated aluminum ions formed on the LiAl5 O8 layer. The absence of 27 Al h.f.s. in the EPR spectrum of TEMPO complex with these sites can be explained by larger distance between >N–O group and Al atom, because, as it was shown by quantum-chemical calculations [26], the spin density on Al appears at the distance about 0.2 nm and less. At the distances 0.25 nm and more (which are reasonable for weaker sites) it is possible to observe increased spin density on the nitrogen atom (in comparison with free nitroxide molecule) with the absence of h.f.s. from aluminum. Data on the decrease of the strength of LAS are in accordance with observations which were made by de Miguel et al. [7] with the help of IR spectroscopy of carbon monoxide adsorbed on lithium modified alumina. The three line spectrum with AN k ¼ 40 0:5 G has been registered also after TEMPO adsorption on the magnesium modified alumina, calcined at low temperatures. The segregation of MgAl2 O4 (after calcination at 773 K) resulted in the significant increase of the multicomponent (h.f.s. from 14 N and 27 Al) spectrum impact. So, data on the surface properties obtained by means of adsorbed TEMPO and anthraquinone are consistent and complementary.
3.3. Nature of the paramagnetic complexes of probe molecules on the oxide surfaces It is reasonable to discuss the nature of paramagnetic complexes of probe molecules. In particular, there is very different manifestation of LAS on the alumina in the EPR spectra of the TEMPO and anthraquinone. Magnetic resonance parameters of TEMPO are sensitive to the acid site strength. TEMPO molecule coordinates with Lewis (or Bronsted) acid site like common donor molecule (for example, carbon monoxide, pyridine, ammonia, etc.). The changes
in the EPR spectrum correlate with the strength of the interaction. Magnetic resonance parameters of anthraquinone complexes are not sensitive to the strength of LAS. In particular, both 11-component and 6component spectra of anthraquinone complexes do not depend on the nature of studied catalyst. If the formation of the anthraquinone complexes is possible then it produces the same EPR spectra. The difference between samples is the concentration of anthraquinone complexes and the ratio between 11-component and 6-component spectra. It is interesting that on the different gallium oxides [13] as well as on the Ga2 O3 –ZrO2 [14] and Ga2 O3 –Al2 O3 [15] systems the anthraquinone adsorption also resulted in the formation of paramagnetic complexes with two or one gallium cations (7- or 4-component spectrum respectively, because spin of 69 Ga as well as 71 Ga is 3=2). Properties of such complexes are similar to the properties of the anthraquinone complexes with aluminum ion(s), that are the necessity of the heating to prepare the complexes, the unstability of the complexes towards adsorption of bases (water et al.) and the similarity of the h.f.s. constants of 7 (or 4) component spectra of different samples. Therefore the anthraquinone complex formation process seems to be a chemisorption process, when the configuration of adsorption site and adsorbate can change. As a result, the same stable paramagnetic complex structure is achieved. Also it should be taken into account that the anthraquinone complex formation process cannot be described as usual complex formation, like in the case of TEMPO. The anthraquinone molecules as well as LAS are diamagnetic and the formation of paramagnetic species requires one-electron transfer with the cation-radical or anion-radical formation. Up to now the anthraquinone (AQ) complex formation on the alumina surface was considered to occur in two steps [12]: 1. The transfer of one electron from AQ molecule to the surface resulted in the cation-radical formation, AQþ . 2. The interaction of AQþ with one or two LAS.
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This mechanism has two drawbacks: On the one hand, as it was considered by Eberson[27], the anthraquinone is very resistant towards oxidation ðE0 AQþ =AQÞ 2:25 VÞ. And so-called ‘‘cation-radical’’ of AQ, which can be prepared in the strong acid media, is diprotonated anthrasemiquinone, ðAQH2 Þþ , as a matter of fact. On the other hand, improbably that AQþ , which is already positively charged, will be able to interact with LAS, with next electron density transfer from anthraquinone to acceptor cation. It is more reasonable in this way to consider the possibility of the AQþ interaction with electron donor sites. But experimental data showed unambiguous presence of Al3þ ions (one or two) in the structure of the complex. Nevertheless this model was used because the observed EPR spectrum (as well as infrared spectrum) could not be assigned to anthrasemiquinone or diprotonated anthrasemiquinone [12]. The detailed analysis of literature data showed that there are complexes of anthrasemiquinone which are very similar to the complexes on alumina surface. In the excellent work made by Chen and Hirota [28] the triple ions (Mþ AQ Mþ ), ion pairs (Mþ AQ ) and alcohol-solvated ion pairs (Mþ AQ HOR) where M ¼ Li, Na or K, were described. There are similar features between triple ions and anthraquinone complex with two Al3þ (Ga3þ ) ions as well as between ion pairs (or alcohol-solvated ion pairs) and anthraquinone complex with one Al3þ (Ga3þ ) ion. In particular: 1. There is the equivalence of two nuclei of metal in the anthraquinone triple ions and in the complex with two Al3þ (Ga3þ ) ions. 2. From the one hand, there is an equilibrium between anthraquinone triple ions and ion pairs. This equilibrium shifts to triple ions with the increase of the temperature (from 178 to 298 K). From the other hand, the formation of anthraquinone complexes with two Al3þ (Ga3þ ) ions requires a heating up to 473 K. The complexes with one Al3þ (Ga3þ ) ion forms as intermediate species during this heating.
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3. The addition of the alcohol (i-C3 H7 OH) causes the transformation of triple ions to alcohol solvated ion pairs. Also, the adsorption of donor molecules (for example, H2 O) causes reversible transformation of the anthraquinone paramagnetic complex with two Al3þ ions to the complex with one Al3þ ion and then to the neutral anthraquinone [29]. Taking into account these data as well as 100% conversion of adsorbed AQ into paramagnetic state [30] and the activation energy for AQ complex formation (12 kcal/mol [30]), the next mechanism of AQ paramagnetic complex formation on the alumina surface can be proposed (Fig. 2). The process can be divided into three stages: 1. Fig. 2a. Formation of anthrasemiquinone as a result of the one-electron transfer from electron-donor center of the surface to the AQ molecule. It is difficult to say exactly about the nature of this center. Probably this center is negative oxygen ion as showed in Fig. 2a. The unpaired electron localized on surface oxygen atom is not observed by EPR, probably due to very short spin–lattice relaxation time, or for another reason. 2. Fig. 2b. Anthrasemiquinone reacts with threecoordinated aluminum ion, to produce paramagnetic complex with one Al3þ . This process requires the break of one bond and the formation of another bond. This is a reason for the presence of an activation energy. Another >C–O group can interact with neighboring OH group as shown in the Fig. 2b. 3. Fig. 2c. During more prolonged heating the AQ molecule can ‘‘find’’ another neighboring LAS and interact with it. All stages are reversible. The adsorption of water can shift equilibrium from right to left due to the stronger bonding with LAS, in comparison with anthraquinone molecule. The heating of the sample and evacuation of water shifts equilibrium from left to right, in conformity with experimental data [12]. It is clear on the basis of the proposed mechanism why the data on the quantity of LAS measured by the maximum AQ paramagnetic complex
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Fig. 2. Proposed mechanism of the formation of the anthraquinone paramagnetic complexes with LAS of the alumina surface.
concentration (taking into account the number of LAS in the complex), are in line with data obtained by TEMPO adsorption or by other probe molecules. So, the same LAS (three-coordinated aluminum ions) can interact either with oxygen atoms of anthraquinone, or with other probe molecules. But the anthraquinone adsorption results in the formation of two possible complexes with definite structures. This is the reason why the LAS, which are different in strength towards TEMPO [12] or carbon monoxide [31], are uniform towards AQ. Evidently, similar mechanism can be proposed for the formation of anthraquinone paramagnetic complexes with three-coordinated gallium cations on gallium oxide, Ga2 O3 –ZrO2 and Ga2 O3 –Al2 O3 . 4. Conclusions It was shown that Lewis acid properties of the alumina modified by lithium or magnesium cations are determined by the aluminate layer (LiAl5 O8 or MgAl2 O4 ) formation. A stability of this layer play simportant role in the change of Lewis acid properties during the calcination of modified alumina.
It should be concluded that the interaction of anthraquinone with LAS is completely different from the interaction of other common probe molecules (for example, TEMPO) with the same centers. The interaction of anthraquinone with LAS requires the break and formation of chemical bonds, but the interaction of other common donor molecules with LAS is only formation of donoracceptor bond. The paramagnetic complex of anthraquinone with two aluminum ions should be considered as the structure like to triple ion of anthrasemiquinone with two cations. Two fragments like –O– Alþ –O– play the role of cations in these triple ions. The paramagnetic complex of anthraquinone with one aluminum ion should be considered as the ion pair of anthrasemiquinone and one fragment like –O–Alþ –O–. Therefore, the anthraquinone adsorption is a convenient tool for the measurements of the total concentration of LAS. Additional information from EPR spectra of adsorbed anthraquinone can be obtained about the regularities in the LAS disposition on the surface.
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Similar mechanism of the formation of anthraquinone complex with two or one threecoordinated gallium ions can be proposed for the explanation of known data on the Lewis acid properties of gallium oxide containing systems. TEMPO probe is supplementary to the anthraquinone one, and can be used for the evaluation of the strength of LAS. In the case of relatively weak coordinatively unsaturated aluminum ions on LiAl5 O8 surface, when there is no h.f.s. from the nucleus of acceptor cation in the TEMPO complex, with the help of adsorbed anthraquinone data it is possible to determine unambiguously the nature of acceptor cation (aluminum). Therefore, the use of both probes, anthraquinone and TEMPO, is quite informative method to study Lewis acidity of alumina-based catalysts. Acknowledgements Author would like to express a great gratitude to Elena V. Lunina (1940–1999), who was one of the founders of the EPR method of paramagnetic probe molecules and educated many scientists, including the author. Author acknowledges RFBR (grant 98-0332129a) and INTAS (grant YSF 00-252) for financial support. Author is very grateful to Prof. Anders Lund (Linkoping University) for helpful discussions. Author is grateful to Anara O. Turakulova, Elena N. Chinennikova and Galina P. Murav’yova (Department of Chemistry, Moscow State University) for the help with some experiments. Author is grateful to Goar L. Markaryan (Department of Chemistry, Moscow State University) for useful comments on this article. References [1] H. Knozinger, P. Ratnasamy, Catal. Rev. Sci. Eng. 17 (1978) 31. [2] M. Ziolek, J. Kujawa, J. Czyzniewska, I. Nowak, A. Aboulayt, O. Saur, J.C. Lavalley, Appl. Catal. A. 171 (1998) 109. [3] V. Bolis, G. Magnacca, C. Morterra, Res. Chem. Intermed. 25 (1) (1999) 25. [4] Y. Ono, T. Baba, Catal. Today 38 (1997) 321.
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