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The effect of cerium introduction on vanadium-USY catalysts
Studies in Surface Science and Catalysis 143 E. Gaigneaux et al. (Editors) 9 2002 Elsevier Science B.V. All rights reserved.
T h e effect of c e r i u m introduction on v a n a d i u m - U S Y
915
catalysts
C.R. Moreira 1, M. Schmal z* and M.M. Pereira 1 1 Institute of Chemistry, Federal University of Rio de Janeiro (UFRJ), Ilha do Fund~o, CT - Bloco A, sala 637, CEP: 21941-590, Rio de Janeiro, RJ, Brazil, [email protected]. z Chemical Engineering P r o g r a m - COPPE/NUCAT/UFRJ, [email protected]
USY modified by cerium and vanadium tolerance of theses catalysts were studied. Three methodologies were used for cerium introduction: aqueous precipitation at 25 and 90~ wetness impregnation and ion exchange. The results suggest that the ratio of total cerium and cerium exchanged is very different for each catalyst, decreasing from impregnated to exchanged cerium introduction methodologies. These differences become much more evident in the presence of vanadium. All characterization, DRS in situ, DRX, TPR, BET after steam and cracking activity are in agreement that cerium and vanadium interacted. For cerium located on zeolite it is not possible to detect vanadium in the oxidation state V § by DRS in situ. On the other hand, increasing cerium into the framework the vanadium in oxidation state V § increased. In this way, the impregnation methodology leads to the most vanadium tolerance. 1. I N T R O D U C T I O N The fluid catalytic cracking unit (FCCU) is used for vacuum distillates and residues into olefinic gases. The great demand in processing heavy feedstocks and the high amounts of metals in Brazilian oils, forced to develop novel catalysts that are more resistant to metal contamination. Since the FCCU is a cyclic process, the catalyst passes through reduction and oxidation conditions. Indeed, the reductive atmosphere and coke observed in the riser favor the deactivation and reduce the life time of the catalysts. The burn off in the regenerator releases steam, CO, NOx, SOx, and other compounds. The catalyst is recycled in the reaction-regeneration (reduction-oxidation) between 10000-50000 times and, therefore, the change in the metal environment is very complex, affecting the metal oxidation state, which is an important parameter. Although the reaction conditions are well known in the literature, there are few reports concerning the environment of reduced vanadium species. Vanadium is the most important deactivation compound in FCC catalysts. In steam atmosphere, the zeolite framework is completely destroyed, and therefore the rate of make-up catalyst in the unit is very high [1,2]. The low melting point of V205 (690~ and the possibility of formation of acid species, as reported in the literature, are responsible for this deleterious effect [3,4]. The control of vanadium
916 migration and oxidation state are important to preserve the FCC catalyst. Rare earth elements have been used as metal traps, but there are still fundamental questions concerning the rare earth zeolite interactions and vanadium oxidation states. The objective is to study the effect of cerium introduced in USY zeolite and the resistance of these modified catalysts to vanadium in steam. 2. E X P E R I M E N T A L
2.1. Catalyst preparation The ultra-stable (USY) zeolite (SAR=13) was exchanged twice with a NHnNO3 aqueous solution at 343 K for 1 h, reducing the sodium content to less than 0.5% [5]. After a calcination at 873 K for 2 h in a muffle, the zeolite (HUSY) was modified by addition of cerium using three different methods: precipitation of an aqueous solution of cerium (III) chloride at 298K(PP25) and 363K (PP90); by wetness impregnation (IMP), and ion exchange (EX) as reference. A 1.5 M solution of cerium (III) chloride was poured together with a 1 M ammonium hydroxide solution in the HUSY suspension at pH 8, forming a precipitate of a cerium (III) hydroxide which, after calcination at 873K for 2 h, was transformed in cerium(IV) oxide. In the second procedure, the required amount of cerium (III) chloride was dissolved in ethanol and added to the zeolite at 268K. After drying overnight, it was calcined in a muffle at 873K for 2 h. In the ion exchange method, an aqueous solution of cerium (III) chloride was added to a HUSY suspension at 353K. After 1 h at this temperature, the system was washed with deionized-water and then calcined at 873K for 2 h. In the impregnation method, vanadium was introduced using vanadyl octanoate in toluene, followed by drying and calcination at 873K [6]. All the catalysts were submitted to a hydrothermal treatment for 3 h at 1073K using water at partial pressure of 0.3 atm. The metal contents were determined by Inductive Coupled Plasma Atomic Emission Spectroscopy (ICP-AES, Perkin Elmer 1000), after dissolving 100 mg of the sample in 5 drops and 5 ml of hot HNO3 and HF, respectively.
2.2. Textural properties The BET surface areas of the catalysts were determined in a Gemini 2360 Micromeritrics equipment. Samples were first calcined at 873K for 1 h and then transferred at high temperature to a vacuum unit at 50 mTorr for 1 h and cooled to 473K. Finally, the sample was transferred to the equipment for N2 adsorption. Isotherms were taken in a range of relative pressure of 0.06 to 0.21atm.
2.3. Temperature Programmed Reduction (TPR) The TPR analysis was carried out in continuous flow system, as described elsewhere [7]. Before reduction the catalyst was heated with an argon flux at 873K for 2 h to eliminate water. After cooling to 373K, the catalyst was reduced in a 1.53% H2/Ar flow (30 ml/min) at 10K/min up to 1273K. The hydrogen consumption was measured using a
917 thermal conductivity detector.
2.4. X-ray diffraction (DRX) The X-ray diffraction patterns were obtained using a Rigaku Miniflex equipment with Cu K a radiation of 154.18 pm. The angular interval scans were carried out over the range 14-35~ in 0.5 ~ steps and counts of 1.5 s per step.
2.5. Cracking activity The catalysts were evaluated by measuring the activity and the conversion with time on stream by using cracking of cyclohexane. Before reaction, the catalysts were reduced under flowing 10% Hz/Nz at 60 ml/min, rising the temperature at 10K/min up to 773K and held at this temperature for 5 min, according to the literature [8]. The reaction was performed at 703K, using a saturator with cyclohexane at a constant temperature of 284K, and pure hydrogen as carrier gas (20 ml/min). The reaction products were analysed by on-line chromatography (Shimadzu GC-17 A) with a packed Chrompack column (60 m length and 0.32 nm diameter) at 453K.
2.6. UV-VIS Spectroscopy in diffuse reflection mode (DRS) ~in sitm> The analysis was performed in a Varian Cary 5 spectrophotometer equipped with a Harrick diffuse reflectance chamber. The first spectra were taken after drying at 473K for 1 h, then after a reduction at 773K for 1 h passing a 20% H2/N2 flow at 10K/min. The spectra were taken at room temperature in the range 200-2000 nm, using zeolite HUSY as reference [9]. 3. RESULTS
3.1. Cerium-zeolite catalysts The preparation method and the main properties are presented in Table 1. The loss of surface area is presented as the ratio of BET area after and before the hydrothermal treatment. The surface area of the samples prepared by ion exchange and impregnation, respectively, EX and IMP, are very similar to the surface area of zeolite HUSY. However, the precipitated catalysts (PP25 and PP90) presented a lower surface area, which can probably be attributed to some blocking of pores. After hydrothermal treatment with steam, the loss of surface area of the reference zeolite was 28%, while the loss of surface area of the catalysts containing cerium was not more than 21%. The PP25 sample presented a small loss of surface area, and was therefore more resistant. The EX and IMP catalysts presented similar losses of surface area, around 20-21%. The reduction profiles catalysts are presented in Fig. 1. The PP25 and EX catalysts exhibited similar reduction profiles with a broad peak with a maximum at 836K and at 786K. On the other hand, the PP90 sample showed a profile with two reduction peaks at 897K and at 1059K. Note that the temperature of precipitation is an important parameter in this method. The impregnated (IMP) sample showed a different behavior with no significant hydrogen uptake.
918 Table 1. Properties of cerium-zeolite catalysts and the lost of area after hydrothermal deactivation by steaming. Catalyst Ce Method of cerium introduction BET area Loss of area (%) (m2/g) after steam (%) HUSY 648 28 EX 2.2 Ion exchange 631 20 PP25 3.0 Precipitation T = 25~ 612 12 PP90 3.5 Precipitation T = 90~ 609 17 ................!M P 2,0 ~ Wetness Impregnation 639 .......................................21
(o)
__ 624~
563~
~~--~--._.__..~_..
(b)
513oc
(a)
26o
~o
~
Temperature(~
~o
,doo
Fig. 1. Temperature Programmed Reduction (TPR) profiles [8]" a) EX; b) PP25; c) PP90, d) IMP.
3.2. Vanadium-cerium-zeolite catalysts Table 2 presents the main properties of these samples, the loss of surface area after hydrothermal treatment and the surface area after a TPR analysis. The H2 consumption was calculated based on the metal content (~mol H2/~tmol Ce + V). In addition, a ratio of extrapolated and observed H2 uptake from the TPR profile (H2 ext/obs) was calculated, taking in account the H2 consumption of isolated cerium and vanadium oxides in the bimetallic system, in order to see if interaction between both occurred or not, or if they are isolated particles. As observed, there is a loss of the surface area after steam treatment, which markedly depends on cerium addition method. The PP25V catalyst after steam treatment led to a higher damage of the zeolite structure. Although the TPR results of the PP25V and EXV samples indicate some similarity, the BET surface areas suggest noteworthy differences in their morphology, due to the cerium location. Indeed, the impregnated sample (IMP) was the only one that protected the zeolite structure and presented the lowest loss of surface area (25%).
919 Table 2. Properties of cerium-vanadium-zeolite catalysts including: BET area after TPR measurement, H2 consumption during TPR analysis and cracking activity of cyclohexane. Catalysts V Loss of area Area after TPR ~mol H2/ C6H12 Ha (ppm) after steam (mZ/g) ~mol Ce + V (ext/obs) (ext/obs) (%) activity HUSYV EXV PP25V PP90V IMPV
3000 2000 3900 2600 2000
33 31 45 28 25
455 33 239
1.15 0.78 0.48 0.57 0.28
4.23 3.35 2.82 2.75
0.5 1.0 1.2 3.2
However, the BET surface area of the catalysts after TPR experiment presented a drastic crystalline damage of the EXV catalyst and some crystallinity retention for the IMPV catalyst. These results agreed with the X-ray diffraction pattern after TPR measurements, as shown in Fig. 2. The crystalline difference for both catalysts was 20 %. Vanadium reduction was strongly affected on the modified zeolites. The amount of Hz consumption in the TPR analysis related to the metal content (~tmol Hz/~tmol Ce + V) decreased largely. Moreover, the ratio of the extrapolated and observed Ha consumption (Hz ext/obs) was higher on the IMPV catalyst compared to the others. This behavior was supported by the UV-VIS spectroscopy data (Figs. 2 and 3), showing similarity of both profiles, the dry catalyst and after the reduction treatment. Both did not present absorption bands attributed to d-d transition in the 800-1800 nm region. These results suggest that almost all the vanadium is in the oxidation state V +5 in the IMPV catalysts. On the other hand, the ratio H2 (ext/obs) < 1 of the EXV catalyst indicates an increase of the amount of hydrogen consumption. The UV-VIS spectra on this catalyst (Fig. 3) support this result, showing a large band in the of d-d transitions region. According to the literature [3,10], the peak at 800 nm corresponds to the octahedral vanadium species in oxidation state V +4. The catalytic activity, C6Hlz (ext/obs), decreases depending on the preparation method (EXV > IMPV) of the catalysts, in opposition to the increase of H2 (ext/obs) ratio. The highest activity observed on the EXV catalyst indicates that, in this case, the acid sites of the zeolite were better protected by the presence of rare earth than on the IMPV catalyst. The activity of the cyclohexane cracking are presented in Table 2. The C6H12 (ext/obs) activity is presented as the ratio between the extrapolated and the observed activity. The extrapolated activity was calculated assuming isolated metals without interaction, while the observed value was obtained experimentally, meaning that the extrapolated activity is the sum of the isolated activity, pondered to the metal content in the catalyst.
920 0,10.
~3.
..-,~ 0,05
(~ o,1 am
(a) c~o 800
1000
1200
1400
V~de-cJh(n~
1630
18~0
2[]30
i 800
,
i 1000
,
! 1330
,
! 1400
,
i 1600
,
i 1800
, 2000
Figs. 2. UV-VIS Spectroscopy in situ after Fig. 3. UV-VIS Spectroscopy in situ after dry (a) and reduction (b) treatment for dry (a) and reduction (b) treatment for IMPVcatalyst. EXV catalysts. 4. DISCUSSION 4.1. State of cerium in the zeolite The results have shown that the introduction of a rare earth element in the zeolite really depends on the preparation method that directs the location of this metal in the zeolite. As expected, when cerium oxide is introduced by impregnation, the surface area decreases due to formation of particles blocking the pores. On the other hand, the ion exchange method would favor the introduction of cerium in the zeolite framework and, therefore, it does not affect the surface area but favors the dispersion of cerium. Noteworthy is the influence of the temperature in the precipitation method during the introduction of cerium in the zeolite. As seen, the BET surface area of the modified zeolites with cerium at 298K (PP25) and 363K (PP90), decreases slightly compared to HUSY, or to ion exchanged catalysts(EX). Therefore, the best method is when cerium is introduced in the zeolite framework and, therefore, a better dispersion of cerium is expected. The TPR results have shown very similar profiles for EX and PP25 catalysts. It supports the previous results that cerium on PP25 catalyst was exchanged in the framework. However, in this case, after steam treatment the destruction in the zeolite framework was very large. It suggests that the cerium environment in the PP25 catalyst is different compared to the EX catalyst. On the other hand, the reduction of the PP90 catalyst showed a profile which is very similar to the reduction of cerium (IV) oxide, and this suggests the presence of superficial cerium species and bulk species [11,12,13]. Therefore, there are larger cerium aggregates on the external surface of the zeolite and
921 isolated cerium species. On the contrary, the impregnation of cerium evidences the reduction of external particles and the existence of different easily reducible cerium bulk species. The presence of different cerium species would probably influence the vanadium environment and the catalytic behavior of these catalysts containing both elements. Indeed, the Hz(ext/obs) ratio which is a measure of the reduction degree and therefore indicates if there is an interaction with the zeolite or between cerium and vanadium, exhibited different values, depending on the way of introduction and species formation. The catalyst treated with steam, EXV, presented a low H2 (ext/obs) ratio, which indicates a better reduction. On the other hand, the impregnated catalyst (IMPV) presented a high H2 (ext/obs) ratio, and thus low reduction. This could explain the indication that an interaction occurred during the treatment, with the formation of bimetallic or alloys or even the formation of aluminum silicate-metal interaction. DRS measurements support the TPR results. The impregnated catalysts and steam treated (IMPV) did not show the presence of V +4 after the reduction. Probably, the hydrogen consumption in the TPR profile is due (a) to the reduction of cerium. The band (b) in the d-d transition can be attributed to the formation of alloys like cerium vanadate, according to the literature [14]. Baugis et al. [15] reported that the presence of vanadate with rare earth decreases the diffusion of vanadium in the zeolite structure [14]. The existence of these compounds Fig 4. X-ray diffraction patterns of a) EXV may affect the oxidation state, the and b) IMPV catalysts after TPR dispersion, morphology and location measurements. of cerium species in the catalyst. The DRS spectrum of the EXV catalyst after reduction showed the presence of vanadium in V § oxidation state. Based on thermodynamics redox results, it is expected that when vanadium and cerium present some interaction, the last one should present an easier reduction. The reduction of cerium is favored because of its higher potential (1.64 eV), that should maintain vanadium in the V § oxidation state [16]. Therefore, the formation of rare earth vanadate is favored. On the IMPV catalyst, where probably cerium is dispersed over the zeolite, the reduction process would be preserved. On the other hand, for the EXV catalyst, the reduction of cerium exchanged in the presence of vanadium leads to an easier reduction of both components. But it is not possible here to distinguish and to quantify the formation of V +4 and cerium in a Ce +3 oxidation state. The DRX diffractograms after TPR suggest a possible model of crystalline destruction of the catalysts using their reduction potentials. The highest crystalline damage is observed on the IMPV catalyst compared to the catalyst containing exchanged cerium (EXV) that is completely amorphous (Fig. 2). This sustains the proposed model that the introduction of cerium by wetness impregnation leads to more cerium species outside the
922 zeolite structure. If cerium species are exchanged in the framework, the catalyst should stay amorphous like the other one, due to the formation of vanadates inside the zeolite framework. The cracking activity suggests that the amount of active sites of the zeolite poisoned by vanadium depends on the cerium location in the zeolite. The higher the activity value of C6H12 (ext/obs), the lower is the poisoning effect of vanadium. (Table 2). Therefore, the most active catalysts are those that protect the zeolite better, and this was observed on those catalysts that contain exchanged cerium, resulting in a higher proximity between cerium and vanadium. This is because the cerium species exchanged in the acid sites would not allow that vanadium interact with the acid sites, keeping vanadium close to them. On the other hand, on the IMPV catalyst, part of the actives sites of the zeolite would be affected by vanadium. The literature has reported methodologies to quantify the oxidation state of vanadium species using TPR [ 17] and EPR/DRS [10]. In summary, this work shows for the first time a marked influence of cerium species on the vanadium reducibility. Since rare earth elements in FCC catalysts depend on different preparation morphologies, it is necessary to develop a model to quantify the oxidation state of vanadium. 4.2. Resistance of zeolite under steam
The BET results after steaming show that the catalysts containing cerium exhibited higher hydrothermal stability. The literature reported that rare earth exchanged in zeolites enhance the thermal and hydrothermal stability [5]. After the introduction of vanadium, it is possible to verify that the catalyst containing cerium introduced by impregnation (IMPV) protected the zeolite structure. Therefore, the protection depends on the dispersion of cerium species on the zeolite surface, decreasing the vanadium mobility. However, the mechanism which explains the damage of zeolite by vanadium is unclear [1,3]. The higher cerium contact increases the framework damage. Probably the steam effect on PP25V catalyst could explain the non-homogeneous distribution of cerium exchange in the framework as it is in the EXV catalyst. The heterogeneous cerium distribution leads to a high local damage and a higher effect of steam. In this way, it is possible to observe that the preservation of the zeolite structure depends very much on the cerium location in the catalyst. Probably vanadium introduction first localizes vanadium outside the zeolite framework, as expected in real cracking catalyst, which increases the probability of formation of cerium-vanadium compounds on the IMP catalyst. 5. CONCLUSIONS The different ways of cerium introduction lead to different morphology and location on the zeolite. The ratio of total cerium and cerium exchanged into zeolite decreases from cerium impregnated to cerium exchanged. The precipitated catalysts lead to intermediary systems between IMP and EX catalysts, showing exchanged and superficial cerium species. After vanadium introduction, different behaviors in the vanadium reduction/oxidation capacity were observed depending on the cerium location and the
923 morphology, but cerium exchanged and dispersed on zeolite presented high interaction with vanadium. The last modified zeolite did not show, after the reduction treatment, vanadium in low oxidation state, by in situ UV-Vis spectroscopy. On the other hand, V +4 was observed when cerium was exchanged in the zeolite. Finally, cerium provides better thermal resistance to the zeolite and, after hydrothermal treatment with and without vanadium, the crystallinity of the zeolite depends on the cerium species in the catalyst. Zeolite modified with cerium are good models compounds to study the vanadium oxidation state. ACKNOWLEDGMENT
CNPQ and CTPETRO are gratefully acknowledged for financial support. REFERENCES o
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9. 10. 11. 12. 13. 14. 15. 16. 17.
M. Torrealba, and M.R. Goldwasser, Appl. Catal. A: General, 90 (1992) 35. J. Biswas and I.E. Maxwell, Appl. Catal., 63, (1990) 197. C.A. Trujillo, C.A. et al., J. Catal., 168 (1997) 1. G. Martino, Stud. Surf. Sci. Catal., 130 (2000) 83. M.L. Occelli, Catal. Rev.-Sci. Eng., 33 (3-4) (1991) 241. B.R. Mitchell, Ind. Eng. Chem. Res. Dev., 19, (1980) 209. L.T. Santos et al, Stud. Surf. Sci. Catal., 139 (2001) 343. J. Abbot, J. Catal., 123 (1990) 383. M.A. Bafiares, M.A. et al., S. Surf. Sci. Catal., 130 (2000) 3125. G. Catana et aL, Phys. Chem. B, 102 (1998)8005. A. Piras, A. Trovarelli and G. Dolcetti, Appl. Catal. B: Environ., 28, (2000) L77 B. Ernst, L. Hilaire and A. Kiennemann, Catal. Today, 50, (1999) 413. F. Giordano, J. Catal., 193 (2000) 273. R. Zhuo, F. Wang and W. Wu, in: 215th National Meeting American Chemical Society, Dallas, 1998 A. G.L. Baugis et al., in: 11~ Congresso Brasileiro de Catfilise e 1~ Congresso de Catfilise do Mercosul, 2 (2001) 916. J.G. Nery et al., Zeolites, 18 (1997) 44. E.F. Souza-Aguiar et al, Zeolites, 15 (1995) 620.
T h e effect of c e r i u m introduction on v a n a d i u m - U S Y
915
catalysts
C.R. Moreira 1, M. Schmal z* and M.M. Pereira 1 1 Institute of Chemistry, Federal University of Rio de Janeiro (UFRJ), Ilha do Fund~o, CT - Bloco A, sala 637, CEP: 21941-590, Rio de Janeiro, RJ, Brazil, [email protected]. z Chemical Engineering P r o g r a m - COPPE/NUCAT/UFRJ, [email protected]
USY modified by cerium and vanadium tolerance of theses catalysts were studied. Three methodologies were used for cerium introduction: aqueous precipitation at 25 and 90~ wetness impregnation and ion exchange. The results suggest that the ratio of total cerium and cerium exchanged is very different for each catalyst, decreasing from impregnated to exchanged cerium introduction methodologies. These differences become much more evident in the presence of vanadium. All characterization, DRS in situ, DRX, TPR, BET after steam and cracking activity are in agreement that cerium and vanadium interacted. For cerium located on zeolite it is not possible to detect vanadium in the oxidation state V § by DRS in situ. On the other hand, increasing cerium into the framework the vanadium in oxidation state V § increased. In this way, the impregnation methodology leads to the most vanadium tolerance. 1. I N T R O D U C T I O N The fluid catalytic cracking unit (FCCU) is used for vacuum distillates and residues into olefinic gases. The great demand in processing heavy feedstocks and the high amounts of metals in Brazilian oils, forced to develop novel catalysts that are more resistant to metal contamination. Since the FCCU is a cyclic process, the catalyst passes through reduction and oxidation conditions. Indeed, the reductive atmosphere and coke observed in the riser favor the deactivation and reduce the life time of the catalysts. The burn off in the regenerator releases steam, CO, NOx, SOx, and other compounds. The catalyst is recycled in the reaction-regeneration (reduction-oxidation) between 10000-50000 times and, therefore, the change in the metal environment is very complex, affecting the metal oxidation state, which is an important parameter. Although the reaction conditions are well known in the literature, there are few reports concerning the environment of reduced vanadium species. Vanadium is the most important deactivation compound in FCC catalysts. In steam atmosphere, the zeolite framework is completely destroyed, and therefore the rate of make-up catalyst in the unit is very high [1,2]. The low melting point of V205 (690~ and the possibility of formation of acid species, as reported in the literature, are responsible for this deleterious effect [3,4]. The control of vanadium
916 migration and oxidation state are important to preserve the FCC catalyst. Rare earth elements have been used as metal traps, but there are still fundamental questions concerning the rare earth zeolite interactions and vanadium oxidation states. The objective is to study the effect of cerium introduced in USY zeolite and the resistance of these modified catalysts to vanadium in steam. 2. E X P E R I M E N T A L
2.1. Catalyst preparation The ultra-stable (USY) zeolite (SAR=13) was exchanged twice with a NHnNO3 aqueous solution at 343 K for 1 h, reducing the sodium content to less than 0.5% [5]. After a calcination at 873 K for 2 h in a muffle, the zeolite (HUSY) was modified by addition of cerium using three different methods: precipitation of an aqueous solution of cerium (III) chloride at 298K(PP25) and 363K (PP90); by wetness impregnation (IMP), and ion exchange (EX) as reference. A 1.5 M solution of cerium (III) chloride was poured together with a 1 M ammonium hydroxide solution in the HUSY suspension at pH 8, forming a precipitate of a cerium (III) hydroxide which, after calcination at 873K for 2 h, was transformed in cerium(IV) oxide. In the second procedure, the required amount of cerium (III) chloride was dissolved in ethanol and added to the zeolite at 268K. After drying overnight, it was calcined in a muffle at 873K for 2 h. In the ion exchange method, an aqueous solution of cerium (III) chloride was added to a HUSY suspension at 353K. After 1 h at this temperature, the system was washed with deionized-water and then calcined at 873K for 2 h. In the impregnation method, vanadium was introduced using vanadyl octanoate in toluene, followed by drying and calcination at 873K [6]. All the catalysts were submitted to a hydrothermal treatment for 3 h at 1073K using water at partial pressure of 0.3 atm. The metal contents were determined by Inductive Coupled Plasma Atomic Emission Spectroscopy (ICP-AES, Perkin Elmer 1000), after dissolving 100 mg of the sample in 5 drops and 5 ml of hot HNO3 and HF, respectively.
2.2. Textural properties The BET surface areas of the catalysts were determined in a Gemini 2360 Micromeritrics equipment. Samples were first calcined at 873K for 1 h and then transferred at high temperature to a vacuum unit at 50 mTorr for 1 h and cooled to 473K. Finally, the sample was transferred to the equipment for N2 adsorption. Isotherms were taken in a range of relative pressure of 0.06 to 0.21atm.
2.3. Temperature Programmed Reduction (TPR) The TPR analysis was carried out in continuous flow system, as described elsewhere [7]. Before reduction the catalyst was heated with an argon flux at 873K for 2 h to eliminate water. After cooling to 373K, the catalyst was reduced in a 1.53% H2/Ar flow (30 ml/min) at 10K/min up to 1273K. The hydrogen consumption was measured using a
917 thermal conductivity detector.
2.4. X-ray diffraction (DRX) The X-ray diffraction patterns were obtained using a Rigaku Miniflex equipment with Cu K a radiation of 154.18 pm. The angular interval scans were carried out over the range 14-35~ in 0.5 ~ steps and counts of 1.5 s per step.
2.5. Cracking activity The catalysts were evaluated by measuring the activity and the conversion with time on stream by using cracking of cyclohexane. Before reaction, the catalysts were reduced under flowing 10% Hz/Nz at 60 ml/min, rising the temperature at 10K/min up to 773K and held at this temperature for 5 min, according to the literature [8]. The reaction was performed at 703K, using a saturator with cyclohexane at a constant temperature of 284K, and pure hydrogen as carrier gas (20 ml/min). The reaction products were analysed by on-line chromatography (Shimadzu GC-17 A) with a packed Chrompack column (60 m length and 0.32 nm diameter) at 453K.
2.6. UV-VIS Spectroscopy in diffuse reflection mode (DRS) ~in sitm> The analysis was performed in a Varian Cary 5 spectrophotometer equipped with a Harrick diffuse reflectance chamber. The first spectra were taken after drying at 473K for 1 h, then after a reduction at 773K for 1 h passing a 20% H2/N2 flow at 10K/min. The spectra were taken at room temperature in the range 200-2000 nm, using zeolite HUSY as reference [9]. 3. RESULTS
3.1. Cerium-zeolite catalysts The preparation method and the main properties are presented in Table 1. The loss of surface area is presented as the ratio of BET area after and before the hydrothermal treatment. The surface area of the samples prepared by ion exchange and impregnation, respectively, EX and IMP, are very similar to the surface area of zeolite HUSY. However, the precipitated catalysts (PP25 and PP90) presented a lower surface area, which can probably be attributed to some blocking of pores. After hydrothermal treatment with steam, the loss of surface area of the reference zeolite was 28%, while the loss of surface area of the catalysts containing cerium was not more than 21%. The PP25 sample presented a small loss of surface area, and was therefore more resistant. The EX and IMP catalysts presented similar losses of surface area, around 20-21%. The reduction profiles catalysts are presented in Fig. 1. The PP25 and EX catalysts exhibited similar reduction profiles with a broad peak with a maximum at 836K and at 786K. On the other hand, the PP90 sample showed a profile with two reduction peaks at 897K and at 1059K. Note that the temperature of precipitation is an important parameter in this method. The impregnated (IMP) sample showed a different behavior with no significant hydrogen uptake.
918 Table 1. Properties of cerium-zeolite catalysts and the lost of area after hydrothermal deactivation by steaming. Catalyst Ce Method of cerium introduction BET area Loss of area (%) (m2/g) after steam (%) HUSY 648 28 EX 2.2 Ion exchange 631 20 PP25 3.0 Precipitation T = 25~ 612 12 PP90 3.5 Precipitation T = 90~ 609 17 ................!M P 2,0 ~ Wetness Impregnation 639 .......................................21
(o)
__ 624~
563~
~~--~--._.__..~_..
(b)
513oc
(a)
26o
~o
~
Temperature(~
~o
,doo
Fig. 1. Temperature Programmed Reduction (TPR) profiles [8]" a) EX; b) PP25; c) PP90, d) IMP.
3.2. Vanadium-cerium-zeolite catalysts Table 2 presents the main properties of these samples, the loss of surface area after hydrothermal treatment and the surface area after a TPR analysis. The H2 consumption was calculated based on the metal content (~mol H2/~tmol Ce + V). In addition, a ratio of extrapolated and observed H2 uptake from the TPR profile (H2 ext/obs) was calculated, taking in account the H2 consumption of isolated cerium and vanadium oxides in the bimetallic system, in order to see if interaction between both occurred or not, or if they are isolated particles. As observed, there is a loss of the surface area after steam treatment, which markedly depends on cerium addition method. The PP25V catalyst after steam treatment led to a higher damage of the zeolite structure. Although the TPR results of the PP25V and EXV samples indicate some similarity, the BET surface areas suggest noteworthy differences in their morphology, due to the cerium location. Indeed, the impregnated sample (IMP) was the only one that protected the zeolite structure and presented the lowest loss of surface area (25%).
919 Table 2. Properties of cerium-vanadium-zeolite catalysts including: BET area after TPR measurement, H2 consumption during TPR analysis and cracking activity of cyclohexane. Catalysts V Loss of area Area after TPR ~mol H2/ C6H12 Ha (ppm) after steam (mZ/g) ~mol Ce + V (ext/obs) (ext/obs) (%) activity HUSYV EXV PP25V PP90V IMPV
3000 2000 3900 2600 2000
33 31 45 28 25
455 33 239
1.15 0.78 0.48 0.57 0.28
4.23 3.35 2.82 2.75
0.5 1.0 1.2 3.2
However, the BET surface area of the catalysts after TPR experiment presented a drastic crystalline damage of the EXV catalyst and some crystallinity retention for the IMPV catalyst. These results agreed with the X-ray diffraction pattern after TPR measurements, as shown in Fig. 2. The crystalline difference for both catalysts was 20 %. Vanadium reduction was strongly affected on the modified zeolites. The amount of Hz consumption in the TPR analysis related to the metal content (~tmol Hz/~tmol Ce + V) decreased largely. Moreover, the ratio of the extrapolated and observed Ha consumption (Hz ext/obs) was higher on the IMPV catalyst compared to the others. This behavior was supported by the UV-VIS spectroscopy data (Figs. 2 and 3), showing similarity of both profiles, the dry catalyst and after the reduction treatment. Both did not present absorption bands attributed to d-d transition in the 800-1800 nm region. These results suggest that almost all the vanadium is in the oxidation state V +5 in the IMPV catalysts. On the other hand, the ratio H2 (ext/obs) < 1 of the EXV catalyst indicates an increase of the amount of hydrogen consumption. The UV-VIS spectra on this catalyst (Fig. 3) support this result, showing a large band in the of d-d transitions region. According to the literature [3,10], the peak at 800 nm corresponds to the octahedral vanadium species in oxidation state V +4. The catalytic activity, C6Hlz (ext/obs), decreases depending on the preparation method (EXV > IMPV) of the catalysts, in opposition to the increase of H2 (ext/obs) ratio. The highest activity observed on the EXV catalyst indicates that, in this case, the acid sites of the zeolite were better protected by the presence of rare earth than on the IMPV catalyst. The activity of the cyclohexane cracking are presented in Table 2. The C6H12 (ext/obs) activity is presented as the ratio between the extrapolated and the observed activity. The extrapolated activity was calculated assuming isolated metals without interaction, while the observed value was obtained experimentally, meaning that the extrapolated activity is the sum of the isolated activity, pondered to the metal content in the catalyst.
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Figs. 2. UV-VIS Spectroscopy in situ after Fig. 3. UV-VIS Spectroscopy in situ after dry (a) and reduction (b) treatment for dry (a) and reduction (b) treatment for IMPVcatalyst. EXV catalysts. 4. DISCUSSION 4.1. State of cerium in the zeolite The results have shown that the introduction of a rare earth element in the zeolite really depends on the preparation method that directs the location of this metal in the zeolite. As expected, when cerium oxide is introduced by impregnation, the surface area decreases due to formation of particles blocking the pores. On the other hand, the ion exchange method would favor the introduction of cerium in the zeolite framework and, therefore, it does not affect the surface area but favors the dispersion of cerium. Noteworthy is the influence of the temperature in the precipitation method during the introduction of cerium in the zeolite. As seen, the BET surface area of the modified zeolites with cerium at 298K (PP25) and 363K (PP90), decreases slightly compared to HUSY, or to ion exchanged catalysts(EX). Therefore, the best method is when cerium is introduced in the zeolite framework and, therefore, a better dispersion of cerium is expected. The TPR results have shown very similar profiles for EX and PP25 catalysts. It supports the previous results that cerium on PP25 catalyst was exchanged in the framework. However, in this case, after steam treatment the destruction in the zeolite framework was very large. It suggests that the cerium environment in the PP25 catalyst is different compared to the EX catalyst. On the other hand, the reduction of the PP90 catalyst showed a profile which is very similar to the reduction of cerium (IV) oxide, and this suggests the presence of superficial cerium species and bulk species [11,12,13]. Therefore, there are larger cerium aggregates on the external surface of the zeolite and
921 isolated cerium species. On the contrary, the impregnation of cerium evidences the reduction of external particles and the existence of different easily reducible cerium bulk species. The presence of different cerium species would probably influence the vanadium environment and the catalytic behavior of these catalysts containing both elements. Indeed, the Hz(ext/obs) ratio which is a measure of the reduction degree and therefore indicates if there is an interaction with the zeolite or between cerium and vanadium, exhibited different values, depending on the way of introduction and species formation. The catalyst treated with steam, EXV, presented a low H2 (ext/obs) ratio, which indicates a better reduction. On the other hand, the impregnated catalyst (IMPV) presented a high H2 (ext/obs) ratio, and thus low reduction. This could explain the indication that an interaction occurred during the treatment, with the formation of bimetallic or alloys or even the formation of aluminum silicate-metal interaction. DRS measurements support the TPR results. The impregnated catalysts and steam treated (IMPV) did not show the presence of V +4 after the reduction. Probably, the hydrogen consumption in the TPR profile is due (a) to the reduction of cerium. The band (b) in the d-d transition can be attributed to the formation of alloys like cerium vanadate, according to the literature [14]. Baugis et al. [15] reported that the presence of vanadate with rare earth decreases the diffusion of vanadium in the zeolite structure [14]. The existence of these compounds Fig 4. X-ray diffraction patterns of a) EXV may affect the oxidation state, the and b) IMPV catalysts after TPR dispersion, morphology and location measurements. of cerium species in the catalyst. The DRS spectrum of the EXV catalyst after reduction showed the presence of vanadium in V § oxidation state. Based on thermodynamics redox results, it is expected that when vanadium and cerium present some interaction, the last one should present an easier reduction. The reduction of cerium is favored because of its higher potential (1.64 eV), that should maintain vanadium in the V § oxidation state [16]. Therefore, the formation of rare earth vanadate is favored. On the IMPV catalyst, where probably cerium is dispersed over the zeolite, the reduction process would be preserved. On the other hand, for the EXV catalyst, the reduction of cerium exchanged in the presence of vanadium leads to an easier reduction of both components. But it is not possible here to distinguish and to quantify the formation of V +4 and cerium in a Ce +3 oxidation state. The DRX diffractograms after TPR suggest a possible model of crystalline destruction of the catalysts using their reduction potentials. The highest crystalline damage is observed on the IMPV catalyst compared to the catalyst containing exchanged cerium (EXV) that is completely amorphous (Fig. 2). This sustains the proposed model that the introduction of cerium by wetness impregnation leads to more cerium species outside the
922 zeolite structure. If cerium species are exchanged in the framework, the catalyst should stay amorphous like the other one, due to the formation of vanadates inside the zeolite framework. The cracking activity suggests that the amount of active sites of the zeolite poisoned by vanadium depends on the cerium location in the zeolite. The higher the activity value of C6H12 (ext/obs), the lower is the poisoning effect of vanadium. (Table 2). Therefore, the most active catalysts are those that protect the zeolite better, and this was observed on those catalysts that contain exchanged cerium, resulting in a higher proximity between cerium and vanadium. This is because the cerium species exchanged in the acid sites would not allow that vanadium interact with the acid sites, keeping vanadium close to them. On the other hand, on the IMPV catalyst, part of the actives sites of the zeolite would be affected by vanadium. The literature has reported methodologies to quantify the oxidation state of vanadium species using TPR [ 17] and EPR/DRS [10]. In summary, this work shows for the first time a marked influence of cerium species on the vanadium reducibility. Since rare earth elements in FCC catalysts depend on different preparation morphologies, it is necessary to develop a model to quantify the oxidation state of vanadium. 4.2. Resistance of zeolite under steam
The BET results after steaming show that the catalysts containing cerium exhibited higher hydrothermal stability. The literature reported that rare earth exchanged in zeolites enhance the thermal and hydrothermal stability [5]. After the introduction of vanadium, it is possible to verify that the catalyst containing cerium introduced by impregnation (IMPV) protected the zeolite structure. Therefore, the protection depends on the dispersion of cerium species on the zeolite surface, decreasing the vanadium mobility. However, the mechanism which explains the damage of zeolite by vanadium is unclear [1,3]. The higher cerium contact increases the framework damage. Probably the steam effect on PP25V catalyst could explain the non-homogeneous distribution of cerium exchange in the framework as it is in the EXV catalyst. The heterogeneous cerium distribution leads to a high local damage and a higher effect of steam. In this way, it is possible to observe that the preservation of the zeolite structure depends very much on the cerium location in the catalyst. Probably vanadium introduction first localizes vanadium outside the zeolite framework, as expected in real cracking catalyst, which increases the probability of formation of cerium-vanadium compounds on the IMP catalyst. 5. CONCLUSIONS The different ways of cerium introduction lead to different morphology and location on the zeolite. The ratio of total cerium and cerium exchanged into zeolite decreases from cerium impregnated to cerium exchanged. The precipitated catalysts lead to intermediary systems between IMP and EX catalysts, showing exchanged and superficial cerium species. After vanadium introduction, different behaviors in the vanadium reduction/oxidation capacity were observed depending on the cerium location and the
923 morphology, but cerium exchanged and dispersed on zeolite presented high interaction with vanadium. The last modified zeolite did not show, after the reduction treatment, vanadium in low oxidation state, by in situ UV-Vis spectroscopy. On the other hand, V +4 was observed when cerium was exchanged in the zeolite. Finally, cerium provides better thermal resistance to the zeolite and, after hydrothermal treatment with and without vanadium, the crystallinity of the zeolite depends on the cerium species in the catalyst. Zeolite modified with cerium are good models compounds to study the vanadium oxidation state. ACKNOWLEDGMENT
CNPQ and CTPETRO are gratefully acknowledged for financial support. REFERENCES o
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