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Evidence of surface active sites on NaY zeolite by a model reaction
Applied Surface Science 253 (2007) 5688–5691 www.elsevier.com/locate/apsusc
Evidence of surface active sites on NaY zeolite by a model reaction V. Santos a, K. Barthelet a,*, A.A. Quoineaud a, T. Armaroli a, I. Gener b, P. Magnoux b b
a IFP-Lyon, BP3, 69390 Vernaison, France Laboratoire de Catalyse en Chimie Organique, UMR CNRS 6503, Universite´ de Poitiers, 40 Avenue du Recteur Pineau, 86022 Poitiers Cedex, France
Available online 27 December 2006
Abstract Owing to the development of a new test reaction, namely the isomerization of 1-dodecene, it becomes possible to characterize the activity of cationic zeolites under conditions close to those of industrial adsorption and separation processes (temperature around 150–200 8C and liquid phase). Indeed, 1-dodecene is highly active and still in a liquid state at 150 8C. Furthermore, by comparing the reactivity of NaY before and after treatments applied to reduce its activity ((i) passivation of the external surface by deposition of TetraEthylOrthoSiloxane (TEOS) and (ii) washing the zeolite with a basic or neutral solution), we are able to propose a nature and localization for the residual active sites of this zeolite. Indeed, the evolution of the NaYactivity in their function indicates that the active sites are located both at the external and internal surfaces of NaYand that two types of sites can be described: OH groups and structure defects. # 2007 Elsevier B.V. All rights reserved. PACS : 82.30.Qt; 82.33.Jx Keywords: Acidity; NaY; Surface modifications; Test reaction; 1-Dodecene isomerization
1. Introduction Numerous separation or purification processes based either on shape or size selectivity use cationic zeolites as adsorbents. Even if these processes result from adsorption phenomena, premature aging of the adsorbent is often observed. It certainly comes from undesired chemical reactions taking places during the adsorption processes. These phenomena get more pronounced with the solid acidity, i.e. with the number and the strength of protons. Thus, cationic and especially monocationic zeolites should show a very weak acidity but it could be sufficient to induce slow but progressive decrease in the performances of the molecular sieve and consequently of processes. Therefore, it appears important to extend our knowledge about the nature and the origin of this reactivity in order to be able to control it. In literature, zeolite acidity has been widely studied through test reactions and/or physical methods (TPD, microcalorimetry, IR and NMR spectroscopies using or not probe molecules) [1–4]. However, these methods were developed for protonic zeolites [5] and they are not often
* Corresponding author. Tel.: +33 478022096; fax: +33 478022066. E-mail address: [email protected] (K. Barthelet). 0169-4332/$ – see front matter # 2007 Elsevier B.V. All rights reserved. doi:10.1016/j.apsusc.2006.12.044
sensitive enough if transposed to weakly acid solids (quantity and/or strength of sites too small). So, specific methods have been developed but even then the characterization of the acidity is not always satisfying [6,7]. Moreover, they are often carried out under the conditions that are quite different from those of industrial processes: physical methods often necessitate preliminary dehydration of the solid, whereas processes regularly need partially hydrated adsorbents and most of the test reaction occur in the gaseous phase, whereas numerous industrial separations proceed in the liquid phase. Therefore, we developed a new batch model reaction in the liquid phase at 150 8C that will be adapted to characterize the reactivity of zeolites which possess only weak acid sites. Double bond isomerization of olefins was shown as a sensitive reaction to characterize low acidity of materials [8–10]. In this work, we have adapted the olefin isomerization in the liquid phase by choosing 1-dodecene as the model molecule because it is highly active and has a boiling point up to 200 8C. As zeolite, we chose NaY that is quite inert and commercially available. As it exhibits some reactivity, we try to reduce it using different kinds of treatment: deposition of TetraEthylOrthoSiloxane (TEOS) at its external surface and washing by alkaline (NaOH) or neutral (NaCl) solution. By comparing different activities obtained before and after treatment, we will
V. Santos et al. / Applied Surface Science 253 (2007) 5688–5691
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are recorded on a Nexus 1 device with a resolution of 4 cm 1. The powdered samples are pressed into self-supporting pellets of 20 mg/cm2 and activated at 450 8C for 10 h under vacuum on a laboratory-made adsorption bank. Then two spectra are registered on solids, one at room temperature and the other one at the temperature of the liquid nitrogen ( 196 8C). Then, successive pulses of known CO volume are applied at 196 8C and a spectrum is recorded after each pulse.
propose nature and localization for the active sites on NaY zeolite. 2. Experimental section 2.1. Materials NaY zeolite (CBV100, supplied by Zeolysts International) is the main object of this study. This zeolite is tested as received and after modifications (surface passivation and washing). The passivation consists in deposing of silica compound (TEOS) by chemical vapor deposition (CVD) leading to the coverage of the external surface of zeolite by a thin film. The zeolite is pretreated at 550 8C (10 8C/min for 1 h) under air (3 L/h g) and submitted at 150 8C to an up-flow of a mixture of TEOS and nitrogen (3 L/h g) at 58 8C in a cylindrical reactor for 1 h. The film of TEOS deposited at the surface of zeolite is stabilized by zeolite calcinations at 550 8C (10 8C/min) for 4 h (2.5 L/h g). The obtained solid will be noted as NaY-TEOS. In parallel, washing was performed by suspending 10 g of zeolite NaY in 250 mL aqueous solution of NaOH or NaCl at different concentrations between 0.001 and 1 M. After 3 h of stirring at room temperature, the solid is recovered by filtration, washed with deionized water and dried at 100 8C in an oven for 12 h. The resulting samples are referenced NaY-NaOH (X M) and NaY-NaCl (X M), X corresponding to the concentration of the solution.
2.3. Test reaction Three grams of previously activated (heating at 450 8C for 2 h under N2 flow (30 L/h)) NaY zeolite are suspended in 75 g of 1-dodecene (purity: 95%, Aldrich) in a round-bottom flask, 3-neck angled (500 mL). On the flask top, a refrigerant system (refrigeration fluid: water at 20 8C) and a system to remove samples (a 2 mL syringe equipped with a needle) are fixed. The round-bottom flask is placed in a silicon bath kept at 150 8C and the system is stirred at 500 rd/mn (magnetic stirrer). Experiments were carried out under air. The samples of 0.05 mL were removed at regular intervals of time and analyzed by gas phase chromatography (Chromatograph: Agilent 6890 Series GC System 7683 Series Injector; Column PONA, diameter of 200 mm and length of 50 m). 3. Results 3.1. Characterization
2.2. Characterization The results of bulk and surface chemical composition are given in Table 1. The XPS analysis confirms that surface passivation is effective: the ratio Si/Al is higher for the NaYTEOS compared to that of NaY. Moreover, we verify that this treatment does not lead to decrease of the porous volume, nor to narrowing of the pore aperture: the Dubinin’s volume after treatment is 0.340 cm3/g that is really close to the initial value, 0.349 cm3/g, and the adsorption of meta-xylene (MX) occurs sensitively in the same manner before and after passivation proving that the pores are still accessible. By washing NaY with NaOH or NaCl, an excess of sodium is introduced into the zeolite. Indeed, the results of XPS show that the percentage of sodium at the external surface is similar before and after
The bulk chemical composition is measured by X-ray fluorescence (Panalitical PW2404) for Si and Al and by ICPMS for Na. The surface composition is determined by X-ray photoelectron spectroscopy (Spectrometer ESCA KRATOS Axis Ultra). Textural properties (specific surface area and Dubinin’s volume) are characterized using the N2 adsorption isotherm measured at 196 8C with a Micromeretics ASAP 2000 device. Before measurements, the samples are outgassed under vacuum (10 5 Torr) at 500 8C for 12 h. The magic angle spinning (MAS) NMR was recorded with a spectrometer Avance 400 with a spinning speed of 10 kHz. All samples are pre-saturated with water before being analyzed. The IR spectra Table 1 Chemical composition and textural properties of zeolites NaY Si/Al-FXa % Na-ICP-SM (wt%) % Na-XPSb (wt%) % Cl-XPS (wt%) % Al-XPS (wt%) % Si-XPS (wt%) Si/Al(XPS) Aire BET (m2/g) Volume Dubinin (cm3/g) a b
2.6 6.82 13.2 0.15 29.5 11.35 2.50 838 0.349
NaY-TEOS
NaY-NaOH (0.001 M)
NaY-NaOH (0.01 M)
12.3 0.1 30.0 11.5 2.51
13.1 0.1 29.5 11.8 2.40
NaY-NaOH (0.1 M) 6.8
2.61 823 0.340
FX = chemical composition by X-ray fluorescence. XPS = surface composition determined by X-ray photoelectron spectroscopy.
859 0.355
NaY-NaOH (1 M)
NaY-NaCl (0.5 M)
NaY-NaCl (1 M)
7.53 11.55 0.1 30.1 10.8 2.67 823 0.340
7.33 12.5 0.3 29.55 11.6 2.44
6.93 12.7 0.5 29.8 11.4 2.52
5690
V. Santos et al. / Applied Surface Science 253 (2007) 5688–5691
Fig. 1. Comparison of CO region of NaY spectra at different quantities of injected CO.
treatments. So, if, as supposed, sodium cations act as Lewis sites, the number of this kind of sites has been increased but mainly at the internal surface of the zeolite. The porosity does not change a lot during treatments: the value variation is around 2% whatever the applied treatment. All IR spectra of the samples (NaY, NaY-TEOS, NaY-NaOH (0.1 M) and NaY-NaCl (0.5 M)) present just one band in the region of the hydroxyl region (3400–4000 cm 1) around 3750 cm 1 which corresponds to isolated silanols. NaY and NaY-TEOS do not possess any Bro¨nsted sites or their concentration is too low to be detected by IR. After CO adsorption, some modifications occur on the IR spectra. To make them clear, we subtract the initial spectrum (i.e. before the first pulse of CO) from the IR spectra after CO adsorption: thus for each function interacting with CO, a negative band will appear in the region of the initial function and a positive one for the system function-CO. Fig. 1 shows the region of CO groups in the spectra of NaY after adsorption of different quantities of CO. At very low coverage, only two bands at 2175 and 2182 cm 1 corresponding to the interaction between CO and Na+ are detected: NaY possesses sodium cations in two different environments. The band at 2175 cm 1 is well known in literature and the other one at 2182 cm 1 has already been attributed in a previous work. This band at 2182 cm 1 is related to the sites that first interact with CO; indeed, as the coverage by CO increases, it becomes less important compared that one at
Fig. 2. Comparison of CO region of NaY spectra, NaY-TEOS, NaY-NaOH (0.1 M) and NaY-NaCl (0.5 M) at the same pulse.
2175 cm 1. This also signifies that they correspond to the minor sites: only the intensity of the band at 2175 cm 1 increase with CO pulses. For the other samples (NaY-TEOS, NaY-NaOH (0.1 M) and NaY-NaCl (0.5 M), quite similar spectra (Fig. 2) are obtained: they possess the same kind of sites as the initial NaY. However, the appearance of the interaction bands between CO and sodium cations occurs later when NaY is treated (Fig. 2). By 27Al NMR we verify that NaY does not possess any extra-framework aluminum as no peak around 0–10 ppm, characteristic of extra-framework Al(VI), was detected. 3.2. Activity in isomerization of 1-dodecene For several samples the evolutions of the conversion of 1dodecene as a function of the time on stream are presented in Fig. 3. Initially, the conversion increases. After 200–400 min the conversion stabilizes at a quite low value. From the thermodynamical calculations, the final conversion was expected to be close to 100%. As it is not the case, we suppose that the active sites are deactivated during isomerization. This is confirmed by the carbon content on NaY zeolite that is close to 12.0 wt%. The analysis of carbonaceous compounds deposited on the zeolite indicates that they are oxygenated. Their presence is probably a consequence of operating conditions especially of the atmosphere (reactions are
Fig. 3. Conversion of 1-dodecene as a function of time on stream for various NaY samples.
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Fig. 4. Presentation of the different kinds of possible active sites on zeolite.
carried out under air). These phenomena will be discussed in a future publication. The analysis of the reaction media indicates that only 2-dodecene cis and trans are formed: (3-dodecene, 4-dodecene, 5-dodecene and 6-dodecene) do not appear even after a long time of reaction. This is also probably due to the zeolite deactivation. The slope of the tangent of the initial part of the reaction curve reflects the initial activity of the catalyst. NaY possesses a significant activity: about 10 2 mol of reactant can be converted per hour and per gram of catalyst. The same activity was found for NaY-NaOH (0.001 M) which means that this concentration of NaOH was too low to neutralize the active sites present on NaY. But, when NaOH concentration increases, the initial activity strongly decreases (6 10 3 mol/h g for NaY-NaOH (0.01 M) and 8 10 5 mol/h g for NaY-NaOH (0.1 M)) until becoming a negligible for a concentration of 0.5 M while keeping an unchanged porosity. The same behaviour but less pronounced is observed with NaY-NaCl (1.8 10 3 mol/h g for NaY-NaCl (0.5 M) and 1.4 10 3 mol/h g for NaY-NaCl (1 M)). As far NaY-TEOS for which only external sites are removed, the initial activity is seven times lower than that of NaY (1.4 10 3 mol/ h g). The remaining activity means that only a part of active sites is neutralized. 4. Discussion In NaY possible active sites could be sodium (Lewis sites), silanols, surface defects (Fig. 4), or extra-framework aluminium (EFAL species). However, the chosen NaY does not possess any EFAL as it was shown by NMR experiment. As silica and silicalite sample do not exhibit any activity for isomerization of 1-dodecene, silanols groups cannot be responsible for the observed activity. So, the activity of NaY has to be associated with the presence of cations or external surface defects. Both are supposed to act as Lewis sites. However, the activity of zeolite decreases each time after washing it with NaOH or NaCl, whereas the number of sodium cations increases. So, it seems that sodium cations are not active toward 1-dodecene isomerization. The last hypothesis is that there are some Brønsted sites but so few that they cannot be detected by infrared spectroscopy. Because of its route of preparation, NaY-TEOS should possess sites only on its internal surface compared to NaY. The previous works showed that TEOS deposition did not modify the internal surface of Y zeolite but only neutralized external sites. We confirm this by verifying that the Dubinin’s volume remains the same and that
this available porosity is still accessible as the m-xylene diffuses and is adsorbed in the same manner before and after the TEOS-treatment. Consequently, the decrease of reactivity from NaY to TEOS-NaY has to be attributed to the neutralization of active sites located on the external surface. As NaY-TEOS has still some activity, NaY has also active sites on its internal surface or treatment by TEOS does not mask all external active sites. NaY-NaOH shows intermediate activity between those of NaY and NaY-TEOS that signifies that alkaline treatment neutralizes some sites, probably OH groups. So, we have to suppose that NaY possesses some, though none has been seen on the IR spectra. This hypothesis is reinforced by the fact that lowering of the activity of the zeolite becomes more pronounced by increasing the concentration of NaOH solution. After washing NaY with NaCl solution, we also observe decrease of the activity. This confirms the hypothesis of nonactivity of the sodium cations. To explain the influence of such neutral solution, we suppose that the anions will interact with some active sites that are probably structural defects, as it was suggested in literature [11]. This is coherent with the facts that the Si/Al ratio of the surface decreases after the NaCl treatment and that some chloride anions are detected by XPS measurements. In conclusion, this study allows us to confirm the presence of residual active sites on NaY zeolites that seems to be located on the external and internal surfaces. Comparing the information given by each experiment on NaY treated by different methods, we suppose that the active sites are partly OH groups and partly structural defects. References [1] W.E. Farneth, R.J. Gorte, Chem. Rev. 95 (1995) 615. [2] R.J. Gorte, Catal. Lett. 62 (1999) 1. [3] J.D. Wyer, V. Zholobenko, A. Khodakov, S. Bates, M.A. Makarova, Stud. Surf. Sci. Catal. 108 (1997) 2307. [4] E. Brunner, Catal. Today 38 (1997) 361. [5] A. Trunschke, B. Hunger, Top. Catal. 19 (2002) 215. [6] G. Martra, R. Ocule, L. Marchese, G. Centi, S. Coluccia, Catal. Today 73 (2002) 83. [7] V.J. Rao, D.L. Perlstein, R.J. Robbins, P.H. Lakshminarasimhran, H.M. Kao, C.P. Grey, V. Ramamurthy, Chem. Commun. (1998) 269. [8] P.C. Mihindou-Koumba, J.L. Lemberton, G. Perot, M. Guisnet, Nouv. J. Chim. 8 (1984) 31. [9] K. Pitchumani, P.H. Lakshminarasimhran, N. Prevost, D.R. Corbin, V. Ramamurthy, Chem. Commun. (1997) 127. [10] M. Guisnet, Stud. Surf. Sci. Catal. 20 (1985) 283. [11] K.T. Thomson, R.M. Wentzcovitch, J. Chem. Phys. 108 (1998) 8585.
Evidence of surface active sites on NaY zeolite by a model reaction V. Santos a, K. Barthelet a,*, A.A. Quoineaud a, T. Armaroli a, I. Gener b, P. Magnoux b b
a IFP-Lyon, BP3, 69390 Vernaison, France Laboratoire de Catalyse en Chimie Organique, UMR CNRS 6503, Universite´ de Poitiers, 40 Avenue du Recteur Pineau, 86022 Poitiers Cedex, France
Available online 27 December 2006
Abstract Owing to the development of a new test reaction, namely the isomerization of 1-dodecene, it becomes possible to characterize the activity of cationic zeolites under conditions close to those of industrial adsorption and separation processes (temperature around 150–200 8C and liquid phase). Indeed, 1-dodecene is highly active and still in a liquid state at 150 8C. Furthermore, by comparing the reactivity of NaY before and after treatments applied to reduce its activity ((i) passivation of the external surface by deposition of TetraEthylOrthoSiloxane (TEOS) and (ii) washing the zeolite with a basic or neutral solution), we are able to propose a nature and localization for the residual active sites of this zeolite. Indeed, the evolution of the NaYactivity in their function indicates that the active sites are located both at the external and internal surfaces of NaYand that two types of sites can be described: OH groups and structure defects. # 2007 Elsevier B.V. All rights reserved. PACS : 82.30.Qt; 82.33.Jx Keywords: Acidity; NaY; Surface modifications; Test reaction; 1-Dodecene isomerization
1. Introduction Numerous separation or purification processes based either on shape or size selectivity use cationic zeolites as adsorbents. Even if these processes result from adsorption phenomena, premature aging of the adsorbent is often observed. It certainly comes from undesired chemical reactions taking places during the adsorption processes. These phenomena get more pronounced with the solid acidity, i.e. with the number and the strength of protons. Thus, cationic and especially monocationic zeolites should show a very weak acidity but it could be sufficient to induce slow but progressive decrease in the performances of the molecular sieve and consequently of processes. Therefore, it appears important to extend our knowledge about the nature and the origin of this reactivity in order to be able to control it. In literature, zeolite acidity has been widely studied through test reactions and/or physical methods (TPD, microcalorimetry, IR and NMR spectroscopies using or not probe molecules) [1–4]. However, these methods were developed for protonic zeolites [5] and they are not often
* Corresponding author. Tel.: +33 478022096; fax: +33 478022066. E-mail address: [email protected] (K. Barthelet). 0169-4332/$ – see front matter # 2007 Elsevier B.V. All rights reserved. doi:10.1016/j.apsusc.2006.12.044
sensitive enough if transposed to weakly acid solids (quantity and/or strength of sites too small). So, specific methods have been developed but even then the characterization of the acidity is not always satisfying [6,7]. Moreover, they are often carried out under the conditions that are quite different from those of industrial processes: physical methods often necessitate preliminary dehydration of the solid, whereas processes regularly need partially hydrated adsorbents and most of the test reaction occur in the gaseous phase, whereas numerous industrial separations proceed in the liquid phase. Therefore, we developed a new batch model reaction in the liquid phase at 150 8C that will be adapted to characterize the reactivity of zeolites which possess only weak acid sites. Double bond isomerization of olefins was shown as a sensitive reaction to characterize low acidity of materials [8–10]. In this work, we have adapted the olefin isomerization in the liquid phase by choosing 1-dodecene as the model molecule because it is highly active and has a boiling point up to 200 8C. As zeolite, we chose NaY that is quite inert and commercially available. As it exhibits some reactivity, we try to reduce it using different kinds of treatment: deposition of TetraEthylOrthoSiloxane (TEOS) at its external surface and washing by alkaline (NaOH) or neutral (NaCl) solution. By comparing different activities obtained before and after treatment, we will
V. Santos et al. / Applied Surface Science 253 (2007) 5688–5691
5689
are recorded on a Nexus 1 device with a resolution of 4 cm 1. The powdered samples are pressed into self-supporting pellets of 20 mg/cm2 and activated at 450 8C for 10 h under vacuum on a laboratory-made adsorption bank. Then two spectra are registered on solids, one at room temperature and the other one at the temperature of the liquid nitrogen ( 196 8C). Then, successive pulses of known CO volume are applied at 196 8C and a spectrum is recorded after each pulse.
propose nature and localization for the active sites on NaY zeolite. 2. Experimental section 2.1. Materials NaY zeolite (CBV100, supplied by Zeolysts International) is the main object of this study. This zeolite is tested as received and after modifications (surface passivation and washing). The passivation consists in deposing of silica compound (TEOS) by chemical vapor deposition (CVD) leading to the coverage of the external surface of zeolite by a thin film. The zeolite is pretreated at 550 8C (10 8C/min for 1 h) under air (3 L/h g) and submitted at 150 8C to an up-flow of a mixture of TEOS and nitrogen (3 L/h g) at 58 8C in a cylindrical reactor for 1 h. The film of TEOS deposited at the surface of zeolite is stabilized by zeolite calcinations at 550 8C (10 8C/min) for 4 h (2.5 L/h g). The obtained solid will be noted as NaY-TEOS. In parallel, washing was performed by suspending 10 g of zeolite NaY in 250 mL aqueous solution of NaOH or NaCl at different concentrations between 0.001 and 1 M. After 3 h of stirring at room temperature, the solid is recovered by filtration, washed with deionized water and dried at 100 8C in an oven for 12 h. The resulting samples are referenced NaY-NaOH (X M) and NaY-NaCl (X M), X corresponding to the concentration of the solution.
2.3. Test reaction Three grams of previously activated (heating at 450 8C for 2 h under N2 flow (30 L/h)) NaY zeolite are suspended in 75 g of 1-dodecene (purity: 95%, Aldrich) in a round-bottom flask, 3-neck angled (500 mL). On the flask top, a refrigerant system (refrigeration fluid: water at 20 8C) and a system to remove samples (a 2 mL syringe equipped with a needle) are fixed. The round-bottom flask is placed in a silicon bath kept at 150 8C and the system is stirred at 500 rd/mn (magnetic stirrer). Experiments were carried out under air. The samples of 0.05 mL were removed at regular intervals of time and analyzed by gas phase chromatography (Chromatograph: Agilent 6890 Series GC System 7683 Series Injector; Column PONA, diameter of 200 mm and length of 50 m). 3. Results 3.1. Characterization
2.2. Characterization The results of bulk and surface chemical composition are given in Table 1. The XPS analysis confirms that surface passivation is effective: the ratio Si/Al is higher for the NaYTEOS compared to that of NaY. Moreover, we verify that this treatment does not lead to decrease of the porous volume, nor to narrowing of the pore aperture: the Dubinin’s volume after treatment is 0.340 cm3/g that is really close to the initial value, 0.349 cm3/g, and the adsorption of meta-xylene (MX) occurs sensitively in the same manner before and after passivation proving that the pores are still accessible. By washing NaY with NaOH or NaCl, an excess of sodium is introduced into the zeolite. Indeed, the results of XPS show that the percentage of sodium at the external surface is similar before and after
The bulk chemical composition is measured by X-ray fluorescence (Panalitical PW2404) for Si and Al and by ICPMS for Na. The surface composition is determined by X-ray photoelectron spectroscopy (Spectrometer ESCA KRATOS Axis Ultra). Textural properties (specific surface area and Dubinin’s volume) are characterized using the N2 adsorption isotherm measured at 196 8C with a Micromeretics ASAP 2000 device. Before measurements, the samples are outgassed under vacuum (10 5 Torr) at 500 8C for 12 h. The magic angle spinning (MAS) NMR was recorded with a spectrometer Avance 400 with a spinning speed of 10 kHz. All samples are pre-saturated with water before being analyzed. The IR spectra Table 1 Chemical composition and textural properties of zeolites NaY Si/Al-FXa % Na-ICP-SM (wt%) % Na-XPSb (wt%) % Cl-XPS (wt%) % Al-XPS (wt%) % Si-XPS (wt%) Si/Al(XPS) Aire BET (m2/g) Volume Dubinin (cm3/g) a b
2.6 6.82 13.2 0.15 29.5 11.35 2.50 838 0.349
NaY-TEOS
NaY-NaOH (0.001 M)
NaY-NaOH (0.01 M)
12.3 0.1 30.0 11.5 2.51
13.1 0.1 29.5 11.8 2.40
NaY-NaOH (0.1 M) 6.8
2.61 823 0.340
FX = chemical composition by X-ray fluorescence. XPS = surface composition determined by X-ray photoelectron spectroscopy.
859 0.355
NaY-NaOH (1 M)
NaY-NaCl (0.5 M)
NaY-NaCl (1 M)
7.53 11.55 0.1 30.1 10.8 2.67 823 0.340
7.33 12.5 0.3 29.55 11.6 2.44
6.93 12.7 0.5 29.8 11.4 2.52
5690
V. Santos et al. / Applied Surface Science 253 (2007) 5688–5691
Fig. 1. Comparison of CO region of NaY spectra at different quantities of injected CO.
treatments. So, if, as supposed, sodium cations act as Lewis sites, the number of this kind of sites has been increased but mainly at the internal surface of the zeolite. The porosity does not change a lot during treatments: the value variation is around 2% whatever the applied treatment. All IR spectra of the samples (NaY, NaY-TEOS, NaY-NaOH (0.1 M) and NaY-NaCl (0.5 M)) present just one band in the region of the hydroxyl region (3400–4000 cm 1) around 3750 cm 1 which corresponds to isolated silanols. NaY and NaY-TEOS do not possess any Bro¨nsted sites or their concentration is too low to be detected by IR. After CO adsorption, some modifications occur on the IR spectra. To make them clear, we subtract the initial spectrum (i.e. before the first pulse of CO) from the IR spectra after CO adsorption: thus for each function interacting with CO, a negative band will appear in the region of the initial function and a positive one for the system function-CO. Fig. 1 shows the region of CO groups in the spectra of NaY after adsorption of different quantities of CO. At very low coverage, only two bands at 2175 and 2182 cm 1 corresponding to the interaction between CO and Na+ are detected: NaY possesses sodium cations in two different environments. The band at 2175 cm 1 is well known in literature and the other one at 2182 cm 1 has already been attributed in a previous work. This band at 2182 cm 1 is related to the sites that first interact with CO; indeed, as the coverage by CO increases, it becomes less important compared that one at
Fig. 2. Comparison of CO region of NaY spectra, NaY-TEOS, NaY-NaOH (0.1 M) and NaY-NaCl (0.5 M) at the same pulse.
2175 cm 1. This also signifies that they correspond to the minor sites: only the intensity of the band at 2175 cm 1 increase with CO pulses. For the other samples (NaY-TEOS, NaY-NaOH (0.1 M) and NaY-NaCl (0.5 M), quite similar spectra (Fig. 2) are obtained: they possess the same kind of sites as the initial NaY. However, the appearance of the interaction bands between CO and sodium cations occurs later when NaY is treated (Fig. 2). By 27Al NMR we verify that NaY does not possess any extra-framework aluminum as no peak around 0–10 ppm, characteristic of extra-framework Al(VI), was detected. 3.2. Activity in isomerization of 1-dodecene For several samples the evolutions of the conversion of 1dodecene as a function of the time on stream are presented in Fig. 3. Initially, the conversion increases. After 200–400 min the conversion stabilizes at a quite low value. From the thermodynamical calculations, the final conversion was expected to be close to 100%. As it is not the case, we suppose that the active sites are deactivated during isomerization. This is confirmed by the carbon content on NaY zeolite that is close to 12.0 wt%. The analysis of carbonaceous compounds deposited on the zeolite indicates that they are oxygenated. Their presence is probably a consequence of operating conditions especially of the atmosphere (reactions are
Fig. 3. Conversion of 1-dodecene as a function of time on stream for various NaY samples.
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5691
Fig. 4. Presentation of the different kinds of possible active sites on zeolite.
carried out under air). These phenomena will be discussed in a future publication. The analysis of the reaction media indicates that only 2-dodecene cis and trans are formed: (3-dodecene, 4-dodecene, 5-dodecene and 6-dodecene) do not appear even after a long time of reaction. This is also probably due to the zeolite deactivation. The slope of the tangent of the initial part of the reaction curve reflects the initial activity of the catalyst. NaY possesses a significant activity: about 10 2 mol of reactant can be converted per hour and per gram of catalyst. The same activity was found for NaY-NaOH (0.001 M) which means that this concentration of NaOH was too low to neutralize the active sites present on NaY. But, when NaOH concentration increases, the initial activity strongly decreases (6 10 3 mol/h g for NaY-NaOH (0.01 M) and 8 10 5 mol/h g for NaY-NaOH (0.1 M)) until becoming a negligible for a concentration of 0.5 M while keeping an unchanged porosity. The same behaviour but less pronounced is observed with NaY-NaCl (1.8 10 3 mol/h g for NaY-NaCl (0.5 M) and 1.4 10 3 mol/h g for NaY-NaCl (1 M)). As far NaY-TEOS for which only external sites are removed, the initial activity is seven times lower than that of NaY (1.4 10 3 mol/ h g). The remaining activity means that only a part of active sites is neutralized. 4. Discussion In NaY possible active sites could be sodium (Lewis sites), silanols, surface defects (Fig. 4), or extra-framework aluminium (EFAL species). However, the chosen NaY does not possess any EFAL as it was shown by NMR experiment. As silica and silicalite sample do not exhibit any activity for isomerization of 1-dodecene, silanols groups cannot be responsible for the observed activity. So, the activity of NaY has to be associated with the presence of cations or external surface defects. Both are supposed to act as Lewis sites. However, the activity of zeolite decreases each time after washing it with NaOH or NaCl, whereas the number of sodium cations increases. So, it seems that sodium cations are not active toward 1-dodecene isomerization. The last hypothesis is that there are some Brønsted sites but so few that they cannot be detected by infrared spectroscopy. Because of its route of preparation, NaY-TEOS should possess sites only on its internal surface compared to NaY. The previous works showed that TEOS deposition did not modify the internal surface of Y zeolite but only neutralized external sites. We confirm this by verifying that the Dubinin’s volume remains the same and that
this available porosity is still accessible as the m-xylene diffuses and is adsorbed in the same manner before and after the TEOS-treatment. Consequently, the decrease of reactivity from NaY to TEOS-NaY has to be attributed to the neutralization of active sites located on the external surface. As NaY-TEOS has still some activity, NaY has also active sites on its internal surface or treatment by TEOS does not mask all external active sites. NaY-NaOH shows intermediate activity between those of NaY and NaY-TEOS that signifies that alkaline treatment neutralizes some sites, probably OH groups. So, we have to suppose that NaY possesses some, though none has been seen on the IR spectra. This hypothesis is reinforced by the fact that lowering of the activity of the zeolite becomes more pronounced by increasing the concentration of NaOH solution. After washing NaY with NaCl solution, we also observe decrease of the activity. This confirms the hypothesis of nonactivity of the sodium cations. To explain the influence of such neutral solution, we suppose that the anions will interact with some active sites that are probably structural defects, as it was suggested in literature [11]. This is coherent with the facts that the Si/Al ratio of the surface decreases after the NaCl treatment and that some chloride anions are detected by XPS measurements. In conclusion, this study allows us to confirm the presence of residual active sites on NaY zeolites that seems to be located on the external and internal surfaces. Comparing the information given by each experiment on NaY treated by different methods, we suppose that the active sites are partly OH groups and partly structural defects. References [1] W.E. Farneth, R.J. Gorte, Chem. Rev. 95 (1995) 615. [2] R.J. Gorte, Catal. Lett. 62 (1999) 1. [3] J.D. Wyer, V. Zholobenko, A. Khodakov, S. Bates, M.A. Makarova, Stud. Surf. Sci. Catal. 108 (1997) 2307. [4] E. Brunner, Catal. Today 38 (1997) 361. [5] A. Trunschke, B. Hunger, Top. Catal. 19 (2002) 215. [6] G. Martra, R. Ocule, L. Marchese, G. Centi, S. Coluccia, Catal. Today 73 (2002) 83. [7] V.J. Rao, D.L. Perlstein, R.J. Robbins, P.H. Lakshminarasimhran, H.M. Kao, C.P. Grey, V. Ramamurthy, Chem. Commun. (1998) 269. [8] P.C. Mihindou-Koumba, J.L. Lemberton, G. Perot, M. Guisnet, Nouv. J. Chim. 8 (1984) 31. [9] K. Pitchumani, P.H. Lakshminarasimhran, N. Prevost, D.R. Corbin, V. Ramamurthy, Chem. Commun. (1997) 127. [10] M. Guisnet, Stud. Surf. Sci. Catal. 20 (1985) 283. [11] K.T. Thomson, R.M. Wentzcovitch, J. Chem. Phys. 108 (1998) 8585.