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DRIFTS study of methanol adsorption on Mg–Al hydrotalcite catalysts for the transesterification of vegetable oils
Catalysis Communications 17 (2012) 189–193
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Short Communication
DRIFTS study of methanol adsorption on Mg–Al hydrotalcite catalysts for the transesterification of vegetable oils A. Navajas a, G. Arzamendi a, F. Romero-Sarria b, M.A. Centeno b, J.A. Odriozola b, L.M. Gandía a,⁎ a b
Departamento de Química Aplicada, Edificio de los Acebos, Universidad Pública de Navarra, Campus de Arrosadía s/n, E-31006 Pamplona, Spain Instituto de Ciencia de Materiales de Sevilla, Centro Mixto CSIC-Universidad de Sevilla, Avda. Américo Vespucio 49, E-41092 Sevilla, Spain
a r t i c l e
i n f o
Article history: Received 14 September 2011 Received in revised form 31 October 2011 Accepted 3 November 2011 Available online 12 November 2011 Keywords: Biodiesel DRIFTS Hydrotalcite Methanolysis Transesterification
a b s t r a c t Mg–Al hydrotalcites rehydrated after calcination are promising catalysts for the methanolysis of vegetable oils. To gain insight into the basis of their catalytic action, the adsorption of methanol over some commercial Mg–Al hydrotalcites was studied by diffuse reflectance infrared Fourier transform spectroscopy (DRIFTS). Different species formed after methanol adsorption were identified, being the total quantity of methoxy species related to the basic character of the sample. A linear correlation between the amount of adsorbed monodentate methoxy species and the catalytic activity in the biodiesel production was found. Therefore, it is proposed that these species are the mainly involved in the transesterification reaction. © 2011 Elsevier B.V. All rights reserved.
1. Introduction Hydrotalcites, or Mg–Al hydroxycarbonates, are layered double hydroxides with general formula [Mg2 +nAl3 +m (OH)2(n + m)]m +[CO32 −]m/ 2·yH2O. Their structure consists of brucite-like layers where the partial substitution of Mg2 + cations by Al3 + induces a net positive charge that is compensated with anions (typically carbonates) in the interlayer space [1–4]. Water molecules are located in the interlayer space as well. Thermal decomposition of these compounds leads to a mixture of Mg and Al oxides with basic properties that are active catalysts for a number of reactions such as transesterification for biodiesel synthesis [5–8]. These oxides can recover the original lamellar structure of hydrotalcites when exposed to liquid water or steam. In this case, both hydroxide (OH−) anions and water are present in the interlayer space. The rehydrated hydrotalcites have been also considered as catalysts for transesterification reactions due to their basic properties associated to the presence of Brønsted basic sites [8–10]. In a recent work by our group [8], the catalytic performance for the methanolysis of sunflower oil of a series of commercial hydrotalcites was investigated. It was found that after calcination at several temperatures (350–700 °C), only a sample mainly consisting of MgO but containing some Al (4.2 wt.%) was significantly active. This catalyst yielded 50% oil conversion after 24 h of reaction at 60 °C, methanol/oil molar ratio of 12 and 2 wt.% of catalyst referred to the oil. Rehydration of the calcined solids in boiling water was adopted as activation
⁎ Corresponding author. Tel.: + 34 948 169 605; fax: + 34 948 169 606. E-mail address: [email protected] (L.M. Gandía). 1566-7367/$ – see front matter © 2011 Elsevier B.V. All rights reserved. doi:10.1016/j.catcom.2011.11.005
treatment. Whereas Mg(OH)2 obtained from MgO was not active, the rehydrated hydrotalcites with the highest Mg/Al molar ratios (1.9 and 2.3) showed a significantly improved activity. A reasonably good correlation was found between the catalytic performance and the basic properties of the solids characterized by means of Hammett indicators, titration with benzoic acid and CO2-TPD. As concerns the catalysts stability, no Mg or Al leaching took place during the catalytic tests; however, the rehydrated hydrotalcites resulted almost irreversibly deactivated, probably due to strong adsorption of triglycerides on their surface. In contrast, the calcined sample containing 4.2 wt.% Al could be reused, yielding 68% of the original sunflower oil conversion. According to Yu and Schmidt [11] it is also possible that the loss of activity of the hydrotalcites is due to the loss of interlayer water molecules. The contact between a protic molecule RH and a sufficiently basic surface provokes its deprotonation generating an anionic intermediate. According to Chizallet et al. [12] the value of the equilibrium constant of this reaction is a measure of two different factors: i) the ability of the surface O2 − ions to share an electron pair to stabilize the proton and ii) that of the surface metallic cation to accept the electron pair from the anionic intermediate stabilizing it. In this work, methanol has been chosen as probe molecule because its adsorption may be doubly interesting. The dissociation of methanol generates OH and methoxy species so the quantitative analysis of the ability of the solids to dissociate methanol will indicate the basic character of their surfaces. On the other hand, studying the methoxy species formed may also provide interesting information about the methanolysis of vegetable oils given that methoxy species have been proposed to participate in this process in the presence of basic catalysts. In this context, to gain insight into the basis of the catalytic activity of hydrotalcites for methanolysis reactions, a diffuse reflectance
A. Navajas et al. / Catalysis Communications 17 (2012) 189–193
infrared Fourier transform spectroscopy (DRIFTS) study of the adsorption of methanol over a selected series of commercial Mg–Al hydrotalcites has been carried out. Although the literature on the use of hydrotalcites as catalysts for the synthesis of biodiesel is abundant [8–11, 13–15], there is a lack of studies oriented towards the identification of the active species and relating their formation with the catalyst properties in order to obtain information helpful for the preparation of more effective catalysts. In contrast to the conventional transmission infrared spectroscopy, DRIFTS is based on the analysis of not the transmitted radiation but the information contained in the radiation reflected by the sample [16]; for this reason, this technique is very sensitive to adsorbed species and useful to investigate them.
0.5 a. u 1640 1430
3643
1000
3700
Absorbance (a. u.)
190
3516 3440 3480 3500
HT100 HT70 HT60
2. Experimental Commercial hydrotalcites, PURAL® MG 61 HT and MG 70 HT (CONDEA/Sasol Germany GmbH) were employed as catalyst precursors. The Mg/Al molar ratios of these materials were 1.9 and 2.3, respectively. Additionally, the commercial solid PURAL® MG 100, mainly consisting of Mg(OH)2 but containing 4.2 wt.% Al was also included in this study. The as-received materials were referred to as HTx where x = 61, 70 or 100. Two activation methods have been considered; the first one was calcination for 6 h under static air at 500 °C using a heating rate of 1.5 °C/min. Calcined solids will be referred to as HTx-ca. The second activation treatment was rehydration after calcination by immersion into boiling deionized water under stirring for 30 min and then drying at 65 °C in vacuum. This treatment was applied only to the calcined HT70-ca and HT61-ca solids to recover the original hydrotalcite structure. The rehydrated solids will be referred to as HT70-rh and HT61-rh. Transesterification reactions were carried out in a Radleys Carousel Tornado IS6 system with mechanical stirring. The reactions were performed with refined sunflower oil at atmospheric pressure, 60 °C, methanol/oil molar ratio of 12 and 2 wt.% of catalyst referred to the oil. Reaction samples were analyzed by size exclusion chromatography with differential refractive index detector at room temperature. More details can be found elsewhere [8, 17]. DRIFT images were collected for samples placed in a controlled DRIFT chamber (Spectra-Tech 101) with SeZn windows that was coupled to a Thermo Nicolet Nexus infrared spectrometer using KBr optics and a MCT/B detector working at liquid nitrogen temperature. The sample was placed inside the chamber without packing or dilution. Spectra were obtained by co-adding 64 scans at 4 cm− 1 resolution. In order to eliminate H2O and CO2 adsorbed on the solids surface, the sample cell temperature was first raised up to 300 °C and kept at this value for 1 h under flowing He (30 cm3 STP/min). Methanol adsorption was carried out at 100 °C for 15 min using a stream of He saturated with methanol vapor at room temperature. Next, pure He was passed through the sample cell and DRIFT spectra were collected at different temperatures. The spectrum obtained for an Al mirror was used as background. 3. Results and discussion 3.1. Original catalysts The DRIFT spectra of the as received samples recorded at 100 °C under a flow of He are presented in Fig. 1. Two main regions can be distinguished: i) Between 3000 and 3800 cm − 1 the stretching mode of the surface hydroxyls groups and those corresponding to adsorbed water molecules are observed. The presence of hydrogen bonded species provokes a broadening and shift of the hydroxyls groups signals resulting in a broad band centered at around 3300 cm − 1. The shift suffered by the band corresponding to a given hydroxyl group after the formation of a hydrogen bond is proportional to the bond strength [18].
4000
3500
2000
1500
1000
Wavenumbers (cm-1) Fig. 1. FTIR spectra of the as received, HT100, HT70 and HT61 samples.
ii) The region 1800–1200 cm − 1 where bands due to the bending mode of water molecules (1655 cm − 1) and carbonate species (1400 cm − 1) in the interlayer spaces are detected. The positions of these bands are strongly affected by the Mg/Al ratio of the hydrotalcites [19]. It can be observed in Fig. 1 that the 3800–3000 cm − 1 region in the spectrum of the HT100 sample is markedly different from those in the spectra of HT70 and HT61. The spectrum of the sample with the highest magnesium content (HT100) shows a sharp band at 3700 cm − 1, and others at 3643, 3516 and 3440 cm − 1. According to literature data, the band at 3700 cm − 1 corresponds to the stretching vibration mode of hydroxyls groups located on the corners/edges of the MgO structure. The position of this band is highly dependent on the water coverage shifting to 3740 cm − 1 for very low water contents [19, 20]. This indicates that in our case the sample is not completely dehydrated. The band detected at 3643 cm − 1 is due to hydroxyls groups on the extended faces of the MgO structure, while those at 3516 and 3440 cm − 1 are assigned to water molecules hydrogenbonded to different hydroxyls groups. The total vanishing of the bands at 3516 and 3440 cm − 1 during the thermal treatment of the sample (results not shown) supports this attribution as well as the fact that the band at 1640 cm − 1 (bending mode of water molecules) is also eliminated upon heating the sample. It should be remarked that this band (1640 cm − 1) shows a shoulder at higher wavenumbers that indicates the presence of water molecules strongly interacting with a cation [21]. This suggests that at least a part of the Al content (4.3 wt.%) of HT100 is not incorporated into the structure. As stated before, marked differences are observed, mainly in the hydroxyls' region when Al 3 + is incorporated into the samples. Well-defined bands due to ν(OH) modes are absent in the samples containing Al; only broad bands centered at 3480 and 3500 cm − 1 are observed for the HT70 and HT61 samples, respectively. These bands indicate the presence of water molecules hydrogen-bonded to the surface hydroxyls groups. The difference between their positions (20 cm − 1) points out that the water molecules are strongly bonded to the surface in the case of the HT70 sample. This fact may be explained considering that the strong ionic character of the Mg\O bond brings about a high covalency of the O\H bond. The main consequence is that the hydrogen atoms of the OH groups are not able to participate in very strong hydrogen bonds with other polar molecules. The presence of certain quantities of aluminum modifies the covalency of the metal–oxygen bond and consequently, the formation of hydrogen bonds is favored. The positions of these bands, closely related to the hydrogen bond strength, depend on the Mg/Al ratio of the sample [21]. By inspecting Fig. 1, one can conclude
A. Navajas et al. / Catalysis Communications 17 (2012) 189–193
that after a thermal treatment, the hydration degree of the samples containing Al is higher than that of the HT100 sample, and that the hydrogen bonds formed are weaker in the sample with the higher Al content. In the second region, for all the three samples, the bending vibration of water molecules appears around 1640 cm − 1, while the carbonate species show a typical vibration around 1430 cm − 1, which is also strongly dependent on the Mg/Al ratio. It can be also found bands associated to the δ(OH) mode. On the whole, one can say that the formation of a hydrogen bond between a hydroxyl group and water molecules provokes a shift to higher frequencies of the δ(OH) vibration and a shift to lower frequencies of the ν(OH) band. The δ(OH) mode may be shifted up to values as high as 1300 cm − 1 according to some authors [5, 19, 22]. 3.2. Spectra after methanol adsorption The species formed after the adsorption of methanol on this type of compounds as well as their thermal evolution are illustrated using the spectra of the HT100-ca sample that are included in Fig. 2. In order to clearly see the formed species, the difference DRIFT spectra taking as reference the pre-heated sample are shown. The spectrum a was recorded at 100 °C in presence of a flow of He saturated in methanol. After that, the sample was purged with a flow of pure He maintaining constant the temperature (100 °C, spectrum b) and then, the sample was heated up to 300 °C (spectra c to f). Two main regions may be distinguished: absorptions in the 3100–2800 cm − 1 range that correspond to the ν(CH3) mode of methoxy species and/or undissociated methanol [23] and the bands between 1200 and 1000 cm − 1 that are due to the ν(OC) vibration mode of methanol and/or methoxy species becoming from the dissociative adsorption of the alcohol [24]. The region about 1400 cm − 1 has been omitted because in this region bands due to the δ(CH3) overlap with that corresponding to carbonate species that are modified during the thermal treatment [19, 25] making very difficult to extract any conclusion. It is observed that the saturation of the surface at 100 °C (Fig. 2, spectrum a) provokes the decreasing of the hydroxyls groups with vibration band at 3740 cm − 1, indicating that an interaction by hydrogen bond is produced with this type of hydroxyls (in the corners and edges). Moreover, new hydroxyls groups appear at 3767 cm − 1, indicating that the solid is able to dissociate methanol due to the basic character of the surface. In the 3100–2800 cm − 1 region, a set of intense bands is detected when the spectrum is recorded in the presence of methanol. The elimination of methanol from the gas 1,2 1060
0.2 a. u
2910
1,0 1030
0,6 0,4
1010
1090
2855
1115
2795
3760
Absorbance (a. u.)
0,8
(a)
0,2
191
flow provokes the vanishing of the bands corresponding to methanol in gas phase and well defined features are now detected at 2910, 2855 and 2795 cm − 1. These bands are attributed to the νs(CH3) and ν(CH3)a modes of adsorbed species [12]. In the 1200–1000 cm − 1 region, ν(CO) modes are observed. In Fig. 2 (a), bands corresponding to gaseous methanol overlapping with those corresponding to the adsorbed species can be found. The evolution as temperature increases allows us to ascribe the bands at 1030 and 1010 cm − 1 to methanol in gas phase while those vibration modes at 1060, 1090 and 1115 cm − 1 to adsorbed species. Bensitel et al. [24] studied by FTIR the adsorption of methanol on MgO and reported a detailed description of the bands formed. They found three absorption bands at 1115, 1090 and 1060 cm − 1 that were ascribed to three different adsorbed species named I, II and H, respectively, that are illustrated in Fig. 3. The band at 1060 cm − 1 corresponds to undissociated methanol reversibly adsorbed through coordination to surface Mg 2 + and unsaturated O 2 − ions; it was called H species. The bands at 1115 and 1090 cm − 1 correspond to two methoxy species named species I and II, respectively (see Fig. 3), resulting from the breaking of the O\H bond of the methanol molecule. Species I shows a monodentate linkage through a bond between the oxygen atom and a Mg 2 + surface ion. In the case of species II the linkage is bidentate, through the oxygen atom bonded to two magnesium ions. The thermal evolution of the surface (Fig. 2, spectra b to f) confirms this attribution. It is observed that species H are the firstly desorbed from the surface as the temperature increases and then, species I that at 200 °C are almost completely vanished. Species II remain on the surface even at 300 °C, but their ν(OC) is slightly shifted from 1090 to 1098 cm − 1. According to previously published data, this shift is due to the modification of the surface after desorption of species I [26]. Since methoxy species are likely involved in the methanolysis reaction, it may be interesting to study the type (and relative quantities) of these species formed on the most active catalysts for biodiesel synthesis (HT100-ca, HT70-rh and HT61-rh in our case). As reported previously [8], the original samples resulted inactive, but an activation pretreatment (calcination for HT100 and rehydration for HT70-ca and HT61-ca) considerably improved the catalytic activity. In Fig. 4, the spectra registered at 100 °C in a flow of He after methanol adsorption are presented for the as received (black lines) and activated (red lines) samples. In all the cases, the pretreatment leads to an increase of the total area of the bands in this IR region. Moreover, the total area decreases as the Al content increases. According to literature data [26], the molar absorption coefficient of methoxy species type I and II are very similar (about 6.1 cm·μmol − 1). Therefore, we can suppose that a measure of the total area in this region should be related with the activity in biodiesel production if methoxy species are the active reaction intermediates. However, taking into account that not all the species are equally labile (as evidenced Fig. 2), one can think that the participation of the several species is also different. In order to clarify this point, the deconvolution of the spectra corresponding to the surface saturated with methanol of the three most active samples was carried out and the area corresponding to each species, measured. The deconvoluted spectra are included in Fig.4. As previously observed for the HT100-ca sample (Fig. 4, A), three bands are distinguished at 1118, 1094 and 1060 cm − 1 attributed to
3740
0,0
CH3
-0,2
O- H
1098 (f)
-0,4 3500
3000
1200
1150
1100
CH3
1050
1000
Mg
O
CH3
O
H
Mg
O
O Mg
Mg
-1
Wavenumber (cm )
species H Fig. 2. Difference spectra at 100 °C after methanol adsorption (a) and in a flow of He at 100 °C (b), 150 °C (c), 200 °C (d), 250 °C (e) and 300 °C (f).
species I
species II
Fig. 3. Adsorbed methanol species on MgO according to Bensitel et al. [24].
192
A. Navajas et al. / Catalysis Communications 17 (2012) 189–193 1118
(C)
1094
0.02 a.u
1076
(B)
0.02 a.u
1030
Sample
AI
AII
AH
AI + AII
HT100-ca HT70-rh HT61-rh
5.22 1.61 (1.11b) 1.66
9.68 0.88 0.85
3.55 0.64
14.90 3.60 2.51
a b
(A)
0.2 a.u
1250
1200
1150
1100
1050
1000
950
Wavenumber (cm-1) Fig. 4. Difference spectra in a flow of He at 100 °C after saturation with methanol for the as received (black line) and activated (red line) HT100-ca (A), HT70-rh (B) and HT61-rh (C) solids.
species I, II and H (see Fig. 3), respectively. For the HT70-rh sample, with a higher Al content, besides these bands, two new features at 1076 and 1030 cm − 1 appear that have been observed previously upon methanol adsorption on mixed oxides of aluminum [25] and were attributed to monodentade methoxy species. As for the sample with the highest Al content (HT61-rh), the molecular species at 1060 cm − 1 and the bands previously attributed to the presence of aluminum are not observed. Only two bands at 1118 and 1094 cm − 1 (slightly shifted to higher frequencies) are visible on the spectrum. The explication of this fact is found in the structure of these compounds. Indeed, Bellotto et al. [27] studied the structural modifications occurring in hydrotalcite compound during thermal treatments and concluded that the dehydration degree of the sample is the main factor governing the process. In a first step (below 200 °C), in which the hydrotalcite is partially dehydrated, a rearrangement of the octahedral brucite-type layers is produced with the migration of the trivalent cations to tetrahedral sites in the interlayer. When the total dehydration of the sample is raised, a structure consisting in regular oxygen closed packed network with a disordered cation distribution in the interstices is formed. In our case, as explained before, the Mg/Al ratio modules the degree of hydration of the solid and the hydrogen bond strength. Samples HT70-rh and HT61-rh subjected to a similar thermal treatment result in solids with different hydration degrees and therefore, different structures. The sample HT61-rh is comparatively most dehydrated than HT70-rh and a more disordered Al 3 + distribution can be expected. This means that the probability of finding adjacent Al 3 + sites is higher for the HT61-rh sample, being the formation of bidentate methoxy species most probable. This explains the absence of the bands at 1030 and 1076 cm − 1 in its spectrum and the slight shift of the IR band corresponding to bidentate species. The absence of the band at 1060 cm − 1 in the spectrum of the HT61-rh sample is related with the modifications induced by Al in the Mg\O bond. Oxygen in MgO is a strong base of Lewis (due to the ionic character of the Mg\O bond) and the named H species, in which a hydrogen bond is formed with the hydrogen from methanol, are easily formed (Fig. 3). The introduction of aluminum provokes the decreasing of the donor ability of the oxygen, which results in the no formation of these species. According to the definition of the surface basicity explained in the introduction, one can relate the total amount of methoxy species formed with the basic character of the surface, which must be reasonably matched with the areas of the bands of the methoxy species I and
Areas are expressed per mg of solid. Contribution of the Al sites.
II even considering the difficulties associated to DRIFTS quantification [28, 29]. In Table 1, the areas (A) of the IR bands corresponding to species I, II and H are given. The area corresponding to monodentade species on Al for the HT70-rh sample is given in brackets. A correlation between the total basicity of the samples and their catalytic activity was not found. This fact may be explained by the different stability of the methoxy species. As shown in Fig. 2, the type II methoxy species require very high temperatures to disappear, and even at 300 °C, they are still adsorbed on the surface. It is not likely that these very stable species are involved in the transesterification reaction at 60 °C. On the other hand, a good correlation between the oil conversion during biodiesel production and the area corresponding to type I methoxy species (AI) on the three samples has been found, as shown in Fig. 5. Conversions correspond to the data obtained after 24 h of reaction at 60 °C, methanol/sunflower oil molar ratio of 12 and 2 wt.% of catalyst referred to the oil [8]. The observed trend for both the area of the type I methoxy species and the conversion data suggest that the more labile species I are involved in the methanolysis reaction. The mechanistic aspects of the methanolysis of triglycerides have been reviewed by Di Serio et al. [30] and Lotero et al. [31, 32]. In the presence of basic catalysts the reaction goes through the activation of methanol to form a nucleophilic species that attacks the electrophilic carbon atom of the ester groups of the glycerides [33]. The results of this work point to the participation of the adsorbed monodentate methoxy (I) species on the basis of the linear relation shown in Fig. 5. In contrast, the adsorbed bidentate methoxy species (II) seem not be involved in the reaction due to their high stability, although on the basis of the present study their participation cannot be completely discarded. This aspect was previously suggested by Bailly et al. [34] who proposed that methoxy species that can be stabilized on the solid surface could also be less reactive. From these findings it seems reasonable that the reaction mechanism is of the Eley–Rideal 55 50 45
Oil conversion (%)
Absorbance (a.u)
1060
Table 1 Integrated areas (A) of the characteristic bands of species I, II and H in the 1160–1000 cm− 1 region after methanol adsorption at 100 °Ca.
40 35 30 25 20 15 10 1
2
3
4
5
Integrated area of methoxy (I) (a.u) Fig. 5. Correlation between the sunflower oil conversion during the methanolysis reaction and the integrated areas of the band corresponding to methoxy (I) species for samples HT100-ca, HT70-rh and HT61-rh.
A. Navajas et al. / Catalysis Communications 17 (2012) 189–193
type, involving adsorbed monodentate methoxy species and triglycerides, diglycerides and monoglycerides from the liquid phase, although the reaction between adsorbed methoxy species and adsorbed ester molecules cannot be discarded [11]. In conclusion, methanol adsorption on hydrotalcites yields different species that may be detected in the 1200–1000 cm− 1 IR region. The total amount of methoxy species formed after methanol adsorption is related to the basicity of the sample. However, mainly the adsorbed monodentate methoxy species (I) participate in the biodiesel production. Moreover, it has been detected a strong dependence of the basic properties of the solids on their Al content, being also responsible for the type and quantity of the adsorbed methanol species formed. Acknowledgments Financial support by the Spanish Ministry of Science and Innovation (TRACE: TRA2009_0265_02, ENE2009-14522-C05-01 and ENE2009-14522-C05-03) is gratefully acknowledged. This work has been co-financed by FEDER funds from European Union. References [1] F. Cavani, F. Trifirò, A. Vaccari, Catalysis Today 11 (1991) 173–301. [2] V. Rives, Materials Chemistry and Physics 75 (2002) 19–25. [3] S. Abelló, F. Medina, D. Tichit, J. Pérez-Ramírez, J.C. Groen, J.E. Sueiras, P. Salagre, Y. Cesteros, Chemistry - A European Journal 11 (2005) 728–739. [4] D.P. Debecker, E.M. Gaigneaux, G. Busca, Chemistry - A European Journal 15 (2009) 3920–3935. [5] H. Zeng, Z. Feng, X. Deng, Y. Li, Fuel 87 (2008) 3071–3076. [6] W. Xie, H. Peng, L. Chen, Journal of Molecular Catalysis A 246 (2006) 24–32. [7] D.G. Cantrell, L.J. Gillie, A.F. Lee, K. Wilson, Applied Catalysis A 287 (2005) 183–190. [8] A. Navajas, I. Campo, G. Arzamendi, W.Y. Hernández, L.F. Bobadilla, M.A. Centeno, J.A. Odriozola, L.M. Gandía, Applied Catalysis B: Environmental 100 (2010) 299–309. [9] Y. Xi, R.J. Davis, Journal of Catalysis 254 (2008) 190–197. [10] Y. Xi, R.J. Davis, Journal of Catalysis 268 (2009) 307–317. [11] K. Yu, J.R. Schmidt, Journal of Physical Chemistry C 115 (2011) 1887–1898.
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Catalysis Communications journal homepage: www.elsevier.com/locate/catcom
Short Communication
DRIFTS study of methanol adsorption on Mg–Al hydrotalcite catalysts for the transesterification of vegetable oils A. Navajas a, G. Arzamendi a, F. Romero-Sarria b, M.A. Centeno b, J.A. Odriozola b, L.M. Gandía a,⁎ a b
Departamento de Química Aplicada, Edificio de los Acebos, Universidad Pública de Navarra, Campus de Arrosadía s/n, E-31006 Pamplona, Spain Instituto de Ciencia de Materiales de Sevilla, Centro Mixto CSIC-Universidad de Sevilla, Avda. Américo Vespucio 49, E-41092 Sevilla, Spain
a r t i c l e
i n f o
Article history: Received 14 September 2011 Received in revised form 31 October 2011 Accepted 3 November 2011 Available online 12 November 2011 Keywords: Biodiesel DRIFTS Hydrotalcite Methanolysis Transesterification
a b s t r a c t Mg–Al hydrotalcites rehydrated after calcination are promising catalysts for the methanolysis of vegetable oils. To gain insight into the basis of their catalytic action, the adsorption of methanol over some commercial Mg–Al hydrotalcites was studied by diffuse reflectance infrared Fourier transform spectroscopy (DRIFTS). Different species formed after methanol adsorption were identified, being the total quantity of methoxy species related to the basic character of the sample. A linear correlation between the amount of adsorbed monodentate methoxy species and the catalytic activity in the biodiesel production was found. Therefore, it is proposed that these species are the mainly involved in the transesterification reaction. © 2011 Elsevier B.V. All rights reserved.
1. Introduction Hydrotalcites, or Mg–Al hydroxycarbonates, are layered double hydroxides with general formula [Mg2 +nAl3 +m (OH)2(n + m)]m +[CO32 −]m/ 2·yH2O. Their structure consists of brucite-like layers where the partial substitution of Mg2 + cations by Al3 + induces a net positive charge that is compensated with anions (typically carbonates) in the interlayer space [1–4]. Water molecules are located in the interlayer space as well. Thermal decomposition of these compounds leads to a mixture of Mg and Al oxides with basic properties that are active catalysts for a number of reactions such as transesterification for biodiesel synthesis [5–8]. These oxides can recover the original lamellar structure of hydrotalcites when exposed to liquid water or steam. In this case, both hydroxide (OH−) anions and water are present in the interlayer space. The rehydrated hydrotalcites have been also considered as catalysts for transesterification reactions due to their basic properties associated to the presence of Brønsted basic sites [8–10]. In a recent work by our group [8], the catalytic performance for the methanolysis of sunflower oil of a series of commercial hydrotalcites was investigated. It was found that after calcination at several temperatures (350–700 °C), only a sample mainly consisting of MgO but containing some Al (4.2 wt.%) was significantly active. This catalyst yielded 50% oil conversion after 24 h of reaction at 60 °C, methanol/oil molar ratio of 12 and 2 wt.% of catalyst referred to the oil. Rehydration of the calcined solids in boiling water was adopted as activation
⁎ Corresponding author. Tel.: + 34 948 169 605; fax: + 34 948 169 606. E-mail address: [email protected] (L.M. Gandía). 1566-7367/$ – see front matter © 2011 Elsevier B.V. All rights reserved. doi:10.1016/j.catcom.2011.11.005
treatment. Whereas Mg(OH)2 obtained from MgO was not active, the rehydrated hydrotalcites with the highest Mg/Al molar ratios (1.9 and 2.3) showed a significantly improved activity. A reasonably good correlation was found between the catalytic performance and the basic properties of the solids characterized by means of Hammett indicators, titration with benzoic acid and CO2-TPD. As concerns the catalysts stability, no Mg or Al leaching took place during the catalytic tests; however, the rehydrated hydrotalcites resulted almost irreversibly deactivated, probably due to strong adsorption of triglycerides on their surface. In contrast, the calcined sample containing 4.2 wt.% Al could be reused, yielding 68% of the original sunflower oil conversion. According to Yu and Schmidt [11] it is also possible that the loss of activity of the hydrotalcites is due to the loss of interlayer water molecules. The contact between a protic molecule RH and a sufficiently basic surface provokes its deprotonation generating an anionic intermediate. According to Chizallet et al. [12] the value of the equilibrium constant of this reaction is a measure of two different factors: i) the ability of the surface O2 − ions to share an electron pair to stabilize the proton and ii) that of the surface metallic cation to accept the electron pair from the anionic intermediate stabilizing it. In this work, methanol has been chosen as probe molecule because its adsorption may be doubly interesting. The dissociation of methanol generates OH and methoxy species so the quantitative analysis of the ability of the solids to dissociate methanol will indicate the basic character of their surfaces. On the other hand, studying the methoxy species formed may also provide interesting information about the methanolysis of vegetable oils given that methoxy species have been proposed to participate in this process in the presence of basic catalysts. In this context, to gain insight into the basis of the catalytic activity of hydrotalcites for methanolysis reactions, a diffuse reflectance
A. Navajas et al. / Catalysis Communications 17 (2012) 189–193
infrared Fourier transform spectroscopy (DRIFTS) study of the adsorption of methanol over a selected series of commercial Mg–Al hydrotalcites has been carried out. Although the literature on the use of hydrotalcites as catalysts for the synthesis of biodiesel is abundant [8–11, 13–15], there is a lack of studies oriented towards the identification of the active species and relating their formation with the catalyst properties in order to obtain information helpful for the preparation of more effective catalysts. In contrast to the conventional transmission infrared spectroscopy, DRIFTS is based on the analysis of not the transmitted radiation but the information contained in the radiation reflected by the sample [16]; for this reason, this technique is very sensitive to adsorbed species and useful to investigate them.
0.5 a. u 1640 1430
3643
1000
3700
Absorbance (a. u.)
190
3516 3440 3480 3500
HT100 HT70 HT60
2. Experimental Commercial hydrotalcites, PURAL® MG 61 HT and MG 70 HT (CONDEA/Sasol Germany GmbH) were employed as catalyst precursors. The Mg/Al molar ratios of these materials were 1.9 and 2.3, respectively. Additionally, the commercial solid PURAL® MG 100, mainly consisting of Mg(OH)2 but containing 4.2 wt.% Al was also included in this study. The as-received materials were referred to as HTx where x = 61, 70 or 100. Two activation methods have been considered; the first one was calcination for 6 h under static air at 500 °C using a heating rate of 1.5 °C/min. Calcined solids will be referred to as HTx-ca. The second activation treatment was rehydration after calcination by immersion into boiling deionized water under stirring for 30 min and then drying at 65 °C in vacuum. This treatment was applied only to the calcined HT70-ca and HT61-ca solids to recover the original hydrotalcite structure. The rehydrated solids will be referred to as HT70-rh and HT61-rh. Transesterification reactions were carried out in a Radleys Carousel Tornado IS6 system with mechanical stirring. The reactions were performed with refined sunflower oil at atmospheric pressure, 60 °C, methanol/oil molar ratio of 12 and 2 wt.% of catalyst referred to the oil. Reaction samples were analyzed by size exclusion chromatography with differential refractive index detector at room temperature. More details can be found elsewhere [8, 17]. DRIFT images were collected for samples placed in a controlled DRIFT chamber (Spectra-Tech 101) with SeZn windows that was coupled to a Thermo Nicolet Nexus infrared spectrometer using KBr optics and a MCT/B detector working at liquid nitrogen temperature. The sample was placed inside the chamber without packing or dilution. Spectra were obtained by co-adding 64 scans at 4 cm− 1 resolution. In order to eliminate H2O and CO2 adsorbed on the solids surface, the sample cell temperature was first raised up to 300 °C and kept at this value for 1 h under flowing He (30 cm3 STP/min). Methanol adsorption was carried out at 100 °C for 15 min using a stream of He saturated with methanol vapor at room temperature. Next, pure He was passed through the sample cell and DRIFT spectra were collected at different temperatures. The spectrum obtained for an Al mirror was used as background. 3. Results and discussion 3.1. Original catalysts The DRIFT spectra of the as received samples recorded at 100 °C under a flow of He are presented in Fig. 1. Two main regions can be distinguished: i) Between 3000 and 3800 cm − 1 the stretching mode of the surface hydroxyls groups and those corresponding to adsorbed water molecules are observed. The presence of hydrogen bonded species provokes a broadening and shift of the hydroxyls groups signals resulting in a broad band centered at around 3300 cm − 1. The shift suffered by the band corresponding to a given hydroxyl group after the formation of a hydrogen bond is proportional to the bond strength [18].
4000
3500
2000
1500
1000
Wavenumbers (cm-1) Fig. 1. FTIR spectra of the as received, HT100, HT70 and HT61 samples.
ii) The region 1800–1200 cm − 1 where bands due to the bending mode of water molecules (1655 cm − 1) and carbonate species (1400 cm − 1) in the interlayer spaces are detected. The positions of these bands are strongly affected by the Mg/Al ratio of the hydrotalcites [19]. It can be observed in Fig. 1 that the 3800–3000 cm − 1 region in the spectrum of the HT100 sample is markedly different from those in the spectra of HT70 and HT61. The spectrum of the sample with the highest magnesium content (HT100) shows a sharp band at 3700 cm − 1, and others at 3643, 3516 and 3440 cm − 1. According to literature data, the band at 3700 cm − 1 corresponds to the stretching vibration mode of hydroxyls groups located on the corners/edges of the MgO structure. The position of this band is highly dependent on the water coverage shifting to 3740 cm − 1 for very low water contents [19, 20]. This indicates that in our case the sample is not completely dehydrated. The band detected at 3643 cm − 1 is due to hydroxyls groups on the extended faces of the MgO structure, while those at 3516 and 3440 cm − 1 are assigned to water molecules hydrogenbonded to different hydroxyls groups. The total vanishing of the bands at 3516 and 3440 cm − 1 during the thermal treatment of the sample (results not shown) supports this attribution as well as the fact that the band at 1640 cm − 1 (bending mode of water molecules) is also eliminated upon heating the sample. It should be remarked that this band (1640 cm − 1) shows a shoulder at higher wavenumbers that indicates the presence of water molecules strongly interacting with a cation [21]. This suggests that at least a part of the Al content (4.3 wt.%) of HT100 is not incorporated into the structure. As stated before, marked differences are observed, mainly in the hydroxyls' region when Al 3 + is incorporated into the samples. Well-defined bands due to ν(OH) modes are absent in the samples containing Al; only broad bands centered at 3480 and 3500 cm − 1 are observed for the HT70 and HT61 samples, respectively. These bands indicate the presence of water molecules hydrogen-bonded to the surface hydroxyls groups. The difference between their positions (20 cm − 1) points out that the water molecules are strongly bonded to the surface in the case of the HT70 sample. This fact may be explained considering that the strong ionic character of the Mg\O bond brings about a high covalency of the O\H bond. The main consequence is that the hydrogen atoms of the OH groups are not able to participate in very strong hydrogen bonds with other polar molecules. The presence of certain quantities of aluminum modifies the covalency of the metal–oxygen bond and consequently, the formation of hydrogen bonds is favored. The positions of these bands, closely related to the hydrogen bond strength, depend on the Mg/Al ratio of the sample [21]. By inspecting Fig. 1, one can conclude
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that after a thermal treatment, the hydration degree of the samples containing Al is higher than that of the HT100 sample, and that the hydrogen bonds formed are weaker in the sample with the higher Al content. In the second region, for all the three samples, the bending vibration of water molecules appears around 1640 cm − 1, while the carbonate species show a typical vibration around 1430 cm − 1, which is also strongly dependent on the Mg/Al ratio. It can be also found bands associated to the δ(OH) mode. On the whole, one can say that the formation of a hydrogen bond between a hydroxyl group and water molecules provokes a shift to higher frequencies of the δ(OH) vibration and a shift to lower frequencies of the ν(OH) band. The δ(OH) mode may be shifted up to values as high as 1300 cm − 1 according to some authors [5, 19, 22]. 3.2. Spectra after methanol adsorption The species formed after the adsorption of methanol on this type of compounds as well as their thermal evolution are illustrated using the spectra of the HT100-ca sample that are included in Fig. 2. In order to clearly see the formed species, the difference DRIFT spectra taking as reference the pre-heated sample are shown. The spectrum a was recorded at 100 °C in presence of a flow of He saturated in methanol. After that, the sample was purged with a flow of pure He maintaining constant the temperature (100 °C, spectrum b) and then, the sample was heated up to 300 °C (spectra c to f). Two main regions may be distinguished: absorptions in the 3100–2800 cm − 1 range that correspond to the ν(CH3) mode of methoxy species and/or undissociated methanol [23] and the bands between 1200 and 1000 cm − 1 that are due to the ν(OC) vibration mode of methanol and/or methoxy species becoming from the dissociative adsorption of the alcohol [24]. The region about 1400 cm − 1 has been omitted because in this region bands due to the δ(CH3) overlap with that corresponding to carbonate species that are modified during the thermal treatment [19, 25] making very difficult to extract any conclusion. It is observed that the saturation of the surface at 100 °C (Fig. 2, spectrum a) provokes the decreasing of the hydroxyls groups with vibration band at 3740 cm − 1, indicating that an interaction by hydrogen bond is produced with this type of hydroxyls (in the corners and edges). Moreover, new hydroxyls groups appear at 3767 cm − 1, indicating that the solid is able to dissociate methanol due to the basic character of the surface. In the 3100–2800 cm − 1 region, a set of intense bands is detected when the spectrum is recorded in the presence of methanol. The elimination of methanol from the gas 1,2 1060
0.2 a. u
2910
1,0 1030
0,6 0,4
1010
1090
2855
1115
2795
3760
Absorbance (a. u.)
0,8
(a)
0,2
191
flow provokes the vanishing of the bands corresponding to methanol in gas phase and well defined features are now detected at 2910, 2855 and 2795 cm − 1. These bands are attributed to the νs(CH3) and ν(CH3)a modes of adsorbed species [12]. In the 1200–1000 cm − 1 region, ν(CO) modes are observed. In Fig. 2 (a), bands corresponding to gaseous methanol overlapping with those corresponding to the adsorbed species can be found. The evolution as temperature increases allows us to ascribe the bands at 1030 and 1010 cm − 1 to methanol in gas phase while those vibration modes at 1060, 1090 and 1115 cm − 1 to adsorbed species. Bensitel et al. [24] studied by FTIR the adsorption of methanol on MgO and reported a detailed description of the bands formed. They found three absorption bands at 1115, 1090 and 1060 cm − 1 that were ascribed to three different adsorbed species named I, II and H, respectively, that are illustrated in Fig. 3. The band at 1060 cm − 1 corresponds to undissociated methanol reversibly adsorbed through coordination to surface Mg 2 + and unsaturated O 2 − ions; it was called H species. The bands at 1115 and 1090 cm − 1 correspond to two methoxy species named species I and II, respectively (see Fig. 3), resulting from the breaking of the O\H bond of the methanol molecule. Species I shows a monodentate linkage through a bond between the oxygen atom and a Mg 2 + surface ion. In the case of species II the linkage is bidentate, through the oxygen atom bonded to two magnesium ions. The thermal evolution of the surface (Fig. 2, spectra b to f) confirms this attribution. It is observed that species H are the firstly desorbed from the surface as the temperature increases and then, species I that at 200 °C are almost completely vanished. Species II remain on the surface even at 300 °C, but their ν(OC) is slightly shifted from 1090 to 1098 cm − 1. According to previously published data, this shift is due to the modification of the surface after desorption of species I [26]. Since methoxy species are likely involved in the methanolysis reaction, it may be interesting to study the type (and relative quantities) of these species formed on the most active catalysts for biodiesel synthesis (HT100-ca, HT70-rh and HT61-rh in our case). As reported previously [8], the original samples resulted inactive, but an activation pretreatment (calcination for HT100 and rehydration for HT70-ca and HT61-ca) considerably improved the catalytic activity. In Fig. 4, the spectra registered at 100 °C in a flow of He after methanol adsorption are presented for the as received (black lines) and activated (red lines) samples. In all the cases, the pretreatment leads to an increase of the total area of the bands in this IR region. Moreover, the total area decreases as the Al content increases. According to literature data [26], the molar absorption coefficient of methoxy species type I and II are very similar (about 6.1 cm·μmol − 1). Therefore, we can suppose that a measure of the total area in this region should be related with the activity in biodiesel production if methoxy species are the active reaction intermediates. However, taking into account that not all the species are equally labile (as evidenced Fig. 2), one can think that the participation of the several species is also different. In order to clarify this point, the deconvolution of the spectra corresponding to the surface saturated with methanol of the three most active samples was carried out and the area corresponding to each species, measured. The deconvoluted spectra are included in Fig.4. As previously observed for the HT100-ca sample (Fig. 4, A), three bands are distinguished at 1118, 1094 and 1060 cm − 1 attributed to
3740
0,0
CH3
-0,2
O- H
1098 (f)
-0,4 3500
3000
1200
1150
1100
CH3
1050
1000
Mg
O
CH3
O
H
Mg
O
O Mg
Mg
-1
Wavenumber (cm )
species H Fig. 2. Difference spectra at 100 °C after methanol adsorption (a) and in a flow of He at 100 °C (b), 150 °C (c), 200 °C (d), 250 °C (e) and 300 °C (f).
species I
species II
Fig. 3. Adsorbed methanol species on MgO according to Bensitel et al. [24].
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A. Navajas et al. / Catalysis Communications 17 (2012) 189–193 1118
(C)
1094
0.02 a.u
1076
(B)
0.02 a.u
1030
Sample
AI
AII
AH
AI + AII
HT100-ca HT70-rh HT61-rh
5.22 1.61 (1.11b) 1.66
9.68 0.88 0.85
3.55 0.64
14.90 3.60 2.51
a b
(A)
0.2 a.u
1250
1200
1150
1100
1050
1000
950
Wavenumber (cm-1) Fig. 4. Difference spectra in a flow of He at 100 °C after saturation with methanol for the as received (black line) and activated (red line) HT100-ca (A), HT70-rh (B) and HT61-rh (C) solids.
species I, II and H (see Fig. 3), respectively. For the HT70-rh sample, with a higher Al content, besides these bands, two new features at 1076 and 1030 cm − 1 appear that have been observed previously upon methanol adsorption on mixed oxides of aluminum [25] and were attributed to monodentade methoxy species. As for the sample with the highest Al content (HT61-rh), the molecular species at 1060 cm − 1 and the bands previously attributed to the presence of aluminum are not observed. Only two bands at 1118 and 1094 cm − 1 (slightly shifted to higher frequencies) are visible on the spectrum. The explication of this fact is found in the structure of these compounds. Indeed, Bellotto et al. [27] studied the structural modifications occurring in hydrotalcite compound during thermal treatments and concluded that the dehydration degree of the sample is the main factor governing the process. In a first step (below 200 °C), in which the hydrotalcite is partially dehydrated, a rearrangement of the octahedral brucite-type layers is produced with the migration of the trivalent cations to tetrahedral sites in the interlayer. When the total dehydration of the sample is raised, a structure consisting in regular oxygen closed packed network with a disordered cation distribution in the interstices is formed. In our case, as explained before, the Mg/Al ratio modules the degree of hydration of the solid and the hydrogen bond strength. Samples HT70-rh and HT61-rh subjected to a similar thermal treatment result in solids with different hydration degrees and therefore, different structures. The sample HT61-rh is comparatively most dehydrated than HT70-rh and a more disordered Al 3 + distribution can be expected. This means that the probability of finding adjacent Al 3 + sites is higher for the HT61-rh sample, being the formation of bidentate methoxy species most probable. This explains the absence of the bands at 1030 and 1076 cm − 1 in its spectrum and the slight shift of the IR band corresponding to bidentate species. The absence of the band at 1060 cm − 1 in the spectrum of the HT61-rh sample is related with the modifications induced by Al in the Mg\O bond. Oxygen in MgO is a strong base of Lewis (due to the ionic character of the Mg\O bond) and the named H species, in which a hydrogen bond is formed with the hydrogen from methanol, are easily formed (Fig. 3). The introduction of aluminum provokes the decreasing of the donor ability of the oxygen, which results in the no formation of these species. According to the definition of the surface basicity explained in the introduction, one can relate the total amount of methoxy species formed with the basic character of the surface, which must be reasonably matched with the areas of the bands of the methoxy species I and
Areas are expressed per mg of solid. Contribution of the Al sites.
II even considering the difficulties associated to DRIFTS quantification [28, 29]. In Table 1, the areas (A) of the IR bands corresponding to species I, II and H are given. The area corresponding to monodentade species on Al for the HT70-rh sample is given in brackets. A correlation between the total basicity of the samples and their catalytic activity was not found. This fact may be explained by the different stability of the methoxy species. As shown in Fig. 2, the type II methoxy species require very high temperatures to disappear, and even at 300 °C, they are still adsorbed on the surface. It is not likely that these very stable species are involved in the transesterification reaction at 60 °C. On the other hand, a good correlation between the oil conversion during biodiesel production and the area corresponding to type I methoxy species (AI) on the three samples has been found, as shown in Fig. 5. Conversions correspond to the data obtained after 24 h of reaction at 60 °C, methanol/sunflower oil molar ratio of 12 and 2 wt.% of catalyst referred to the oil [8]. The observed trend for both the area of the type I methoxy species and the conversion data suggest that the more labile species I are involved in the methanolysis reaction. The mechanistic aspects of the methanolysis of triglycerides have been reviewed by Di Serio et al. [30] and Lotero et al. [31, 32]. In the presence of basic catalysts the reaction goes through the activation of methanol to form a nucleophilic species that attacks the electrophilic carbon atom of the ester groups of the glycerides [33]. The results of this work point to the participation of the adsorbed monodentate methoxy (I) species on the basis of the linear relation shown in Fig. 5. In contrast, the adsorbed bidentate methoxy species (II) seem not be involved in the reaction due to their high stability, although on the basis of the present study their participation cannot be completely discarded. This aspect was previously suggested by Bailly et al. [34] who proposed that methoxy species that can be stabilized on the solid surface could also be less reactive. From these findings it seems reasonable that the reaction mechanism is of the Eley–Rideal 55 50 45
Oil conversion (%)
Absorbance (a.u)
1060
Table 1 Integrated areas (A) of the characteristic bands of species I, II and H in the 1160–1000 cm− 1 region after methanol adsorption at 100 °Ca.
40 35 30 25 20 15 10 1
2
3
4
5
Integrated area of methoxy (I) (a.u) Fig. 5. Correlation between the sunflower oil conversion during the methanolysis reaction and the integrated areas of the band corresponding to methoxy (I) species for samples HT100-ca, HT70-rh and HT61-rh.
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type, involving adsorbed monodentate methoxy species and triglycerides, diglycerides and monoglycerides from the liquid phase, although the reaction between adsorbed methoxy species and adsorbed ester molecules cannot be discarded [11]. In conclusion, methanol adsorption on hydrotalcites yields different species that may be detected in the 1200–1000 cm− 1 IR region. The total amount of methoxy species formed after methanol adsorption is related to the basicity of the sample. However, mainly the adsorbed monodentate methoxy species (I) participate in the biodiesel production. Moreover, it has been detected a strong dependence of the basic properties of the solids on their Al content, being also responsible for the type and quantity of the adsorbed methanol species formed. Acknowledgments Financial support by the Spanish Ministry of Science and Innovation (TRACE: TRA2009_0265_02, ENE2009-14522-C05-01 and ENE2009-14522-C05-03) is gratefully acknowledged. This work has been co-financed by FEDER funds from European Union. References [1] F. Cavani, F. Trifirò, A. Vaccari, Catalysis Today 11 (1991) 173–301. [2] V. Rives, Materials Chemistry and Physics 75 (2002) 19–25. [3] S. Abelló, F. Medina, D. Tichit, J. Pérez-Ramírez, J.C. Groen, J.E. Sueiras, P. Salagre, Y. Cesteros, Chemistry - A European Journal 11 (2005) 728–739. [4] D.P. Debecker, E.M. Gaigneaux, G. Busca, Chemistry - A European Journal 15 (2009) 3920–3935. [5] H. Zeng, Z. Feng, X. Deng, Y. Li, Fuel 87 (2008) 3071–3076. [6] W. Xie, H. Peng, L. Chen, Journal of Molecular Catalysis A 246 (2006) 24–32. [7] D.G. Cantrell, L.J. Gillie, A.F. Lee, K. Wilson, Applied Catalysis A 287 (2005) 183–190. [8] A. Navajas, I. Campo, G. Arzamendi, W.Y. Hernández, L.F. Bobadilla, M.A. Centeno, J.A. Odriozola, L.M. Gandía, Applied Catalysis B: Environmental 100 (2010) 299–309. [9] Y. Xi, R.J. Davis, Journal of Catalysis 254 (2008) 190–197. [10] Y. Xi, R.J. Davis, Journal of Catalysis 268 (2009) 307–317. [11] K. Yu, J.R. Schmidt, Journal of Physical Chemistry C 115 (2011) 1887–1898.
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