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Interaction between Ni and V with USHY and rare earth HY zeolite during hydrothermal deactivation
Applied Catalysis A: General 286 (2005) 196–201 www.elsevier.com/locate/apcata
Interaction between Ni and V with USHY and rare earth HY zeolite during hydrothermal deactivation Alyne S. Escobar a, Marcelo M. Pereira a, Ricardo D.M. Pimenta b, Lam Y. Lau b, Henrique S. Cerqueira c,* a
Universidade Federal do Rio de Janeiro, Centro de Tecnologia, Instituto de Quı´mica, Departamento de Inorgaˆnica. Cidade Universita´ria, Rio de Janeiro, 21949-900 RJ, Brazil b Petrobras, Centro de Pesquisas e Desenvolvimento Leopoldo A. Miguez de Mello (Cenpes), Pesquisa e Desenvolvimento do Abastecimento, Tecnologia em FCC. Ilha do Funda˜o, Av.Jequitiba´ 950, Rio de Janeiro, 21941-598 RJ, Brazil c Petrobras, Centro de Pesquisas e Desenvolvimento Leopoldo A. Miguez de Mello (Cenpes), Pesquisa e Desenvolvimento em Ga´s, Energia e Desenvolvimento Sustenta´vel. Ilha do Funda˜o, Av Jequitiba´ 950, Rio de Janeiro, 21941-598 RJ, Brazil Received 28 December 2004; received in revised form 24 February 2005; accepted 1 March 2005 Available online 26 April 2005
Abstract The effect of Ni and V in the hydrothermal deactivation of an ultrastable Y zeolite with and without rare earth elements is presented and discussed. Although in the presence of steam, vanadium itself is the main factor responsible for zeolite loss of surface area, for the rare earth containing sample the presence of nickel increased the vanadium effect. Temperature-programmed reduction of these zeolites further support this observation by showing an increased amount of mobile readily reducible vanadium species while nickel and rare earth interact. The same deleterious effect of nickel was also observed in rare earth containing FCC catalysts after hydrothermal deactivation. # 2005 Elsevier B.V. All rights reserved. Keywords: Y zeolite; Nickel; Vanadium; Deactivation; Rare earth
1. Introduction During their lifetime in the catalytic cracking (FCC) process, the catalyst becomes contaminated with various amounts of metals present in the feedstock. Poisons such as nickel, vanadium and iron are present in significant amounts in heavy gas oils and residue feedstocks, being responsible for an undesirable increase in both hydrogen and coke yields. The metal containing molecules undergo thermal decomposition when the feedstock is contacted with the hot catalyst in the base of the riser reactor. After catalytic reactions, the catalyst is stripped with steam and directed to the regenerator, where the coke is burned in the presence of air at temperatures close to 710 8C. Under those conditions, part of the FCC catalyst – especially the Y zeolite – is permanently deactivated. * Corresponding author. Tel.: +552138656635; fax: +552138656626. E-mail address: [email protected] (H.S. Cerqueira). 0926-860X/$ – see front matter # 2005 Elsevier B.V. All rights reserved. doi:10.1016/j.apcata.2005.03.002
Many studies in the literature have already discussed the effect of vanadium on fluid catalytic cracking catalysts. Vanadium may deactivate the zeolite either temporarily, adsorbing and neutralizing acid sites [1] or permanently, due to the attack of vanadic acid to the zeolite structure or reacts with rare earth cations [1,2]. The higher the temperature used in the hydrothermal treatment, the higher the loss of activity caused by vanadium. Although the vanadium pentoxide is mobile, it was previously observed by imaging secondary ion mass spectrometry that nickel presents a low mobility, remaining in the area where it was deposited, making it a good measure to determine the age of individual FCC catalyst particles [3]. A traditional way to simulate equilibrium FCC catalysts performance was proposed by Mitchell and consisted of an incipient wetness impregnation of nickel and vanadium compounds, followed by a hydrothermal treatment [4]. Despite the differences obtained for the nickel distributions over the catalyst surface, the activity was in line with
A.S. Escobar et al. / Applied Catalysis A: General 286 (2005) 196–201
equilibrium catalyst. Efforts were made in the development of new laboratory deactivation procedures in order to better simulate the metals distribution and oxidation state. Examples of such methods are the cyclic propylene steaming (CPS) where after deposition of metals by incipient wetness technique, the catalyst is submitted to reduction–oxidation cycles using propylene as the reducing medium [5,6]. A more sophisticated approach is the cyclicdeactivation (CD) procedure, where a fresh catalyst is submitted to repeated cycles of cracking with vacuum gas oil spiked with metals and regeneration [7]. In short, it is important to note that there are different laboratory methods of introduction of metal onto a catalyst, which in turn may affect not only the metal support interaction but also the metal–metal interaction as well [8]. Although there are many works discussing the effect of metals under FCC conditions, the published results are controversial about the interaction between nickel and vanadium with rare earth elements. Using impregnation methods on catalysts, Chester [9] reported that nickel has a four times higher activity than vanadium for hydrogen and coke make, but observed no synergism between those two metals. Yang et al. [10,11] working with metal impregnation onto pure zeolites, reported that nickel could reduce the damage to the Y zeolite caused by vanadium. However, this previous study was made using a high level of vanadium in the range of 7500–10,000 ppm, which was introduced first to the zeolite. On the other hand, temperature-programmed reduction (TPR) has been shown to be sensitive to the state of Ni and V supported on typical FCC catalysts in this concentration range [12–15]. In general, V oxide dispersed on different oxide supports could be reduced from 350 to 600 8C [16]. On FCC catalyst, the reduction temperature was found to lie in a higher range, from 510 to 650 8C [12,13]. Bayraktar and Kugler [12] further reported a reduction peak at 690 8C. For supported Ni species, the reduction temperature was found to be higher. For example, Cheng et al. found two bands, one around 680 8C and the other at 800–880 8C which the authors attributed to supported NiO and Ni spinels, respectively [15]. In the present paper, a study was made using simultaneous introduction of V and Ni in an USHY with and without rare earth elements. Lower levels of both V and Ni compared to previously published data were employed and the hydrothermal deactivation was carried out at different water partial pressures and times. Temperatureprogrammed reduction was carried out in order to further clarify the interaction between Ni, V and rare earth components.
2. Experimental The NaY zeolite was prepared at CENPES, following a procedure described elsewhere [17]. The USHY zeolite was
197
Table 1 Characterization of the zeolite samples Sample
SiO2 (wt.%)
Al2O3 (wt.%)
RE2O3 (wt.%)
USHY REHY
72.6 64.2
22.2 20.0
0.5 11.8
prepared by ion-exchange of an aqueous suspension of Nazeolite. In a 2 l beaker, 750 ml of deionized water were added drop wise to 500 g of Na-zeolite under stirring. A solution of NH4NO3 (2 M) was added (750 ml) and kept at 70 8C for 90 min. After this, it was filtered under vacuum and the remaining zeolite was washed with hot water. The exchanged zeolite sample was kept for 720 min in an oven and then at room temperature. The ammonium-exchanged zeolite was steamed at 100% vapor for 60 min. The rare earth containing zeolite was obtained by post-exchange with an RECl3 solution at room temperature and pH 4.0, followed by a calcination at 600 8C for 3 h. Both zeolites were post-exchanged with NH4 with a similar procedure as described above, in order to reduce sodium. The two samples presented a final sodium content close to 1.0% (w/w). The composition of the ultra-stable Y (USHY) and REHY zeolites are presented in Table 1. No pre-treatment was needed before metals impregnation. The wet point was determined using a burette to add deionized water to 2 g of the sample until a change in the free flowing properties was observed. A value of approximately 0.55 ml g 1 was obtained for both zeolites. For metals impregnations, solutions of nickel 2+ and vanadyl octoates were diluted by toluene to achieve the volume corresponding to the wet point of sample. After homogenization for 60 min, the solvent was removed by means of a rotating evaporator. The zeolite was then calcined at 600 8C for 3 h. The samples containing both Ni and V were prepared by co-impregnation. Two commercial samples of rare earth containing FCC catalysts currently used at two Petrobras’ FCC units processing heavy gas oil were also submitted to the same impregnation procedure. Characteristics of the FCC catalysts are presented in Table 2. Ion-exchanged zeolite samples were also prepared, by the addition of aqueous solutions of VOSO4 and/or Ni(NO3)2 to the USHY or REHY suspension at 80 8C. After 1 h at this temperature, the system was washed with deionized water and then calcined at 600 8C for 2 h. For hydrothermal deactivation, samples were placed in crucibles and exposed to a steam atmosphere inside the furnace that can condition 12 samples simultaneously. The steam partial pressure was controlled by means of a water vapor saturator, through which an air stream passes continuously. In all tests, the furnace temperature was set to 800 8C. The absence of temperature profiles inside the furnace was checked with a sample of REHY placed in four different positions. The variation in the surface area was smaller than 3%.
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Table 2 Characterization of the FCC catalyst samples Sample
SiO2 (wt.%)
Al2O3 (wt.%)
RE2O3 (wt.%)
Na2O (wt.%)
Surface area (m2 g 1)
Micropore volume (cm3 g 1)
Catalyst A Catalyst B
55.8 57.1
39.3 38.7
2.52 2.19
0.25 0.34
385 255
0.121 0.079
The surface area of the deactivated samples was measured by nitrogen adsorption at 196 8C in the Gemini III 2375 equipment. Prior to the analysis, the samples were calcined at 600 8C for 1 h. The temperature-programmed reduction was carried out in a unit constructed in the laboratory. A sample dried overnight in oven was further dried in situ by a flow of argon at 0.5 ml s 1 at 400 8C for 0.5 h. The reduction was carried out using 1.6 mol% hydrogen in argon, with a flow of 0.5 ml s 1 through the reactor, with reduction temperature raised from room temperature up to 1000 8C, at a rate of 10 8C min 1.
3. Results and discussion In the absence of metals and with 33% of steam, there is a small decrease in the surface area of the zeolites (Fig. 1A), with a minor difference between the samples with and without rare earth elements. On the other hand, when the atmosphere is changed to 100% steam, an important decrease is observed for the USHY sample, the REHY results being comparable to the results with 33% steam (Fig. 1B). This was expected due to the known ability of RE elements in stabilizing the zeolite unit cell size [18]. Samples containing only Ni (3000 ppm) were submitted to deactivation under 100% steam. The results (Fig. 2A) were of the same magnitude of samples without metals, indicating that Ni alone has no effect on the zeolite surface area loss. Using V (3000 ppm) instead of Ni an important decrease in the area of zeolite samples occurred (Fig. 2B), resulting in surface area retentions of 59% for REHY and as low as 5% for USHY after 10 h treatment. This can be
explained due to interactions between V2O5 and rare earth components, forming low melting points RE-vanadates that can incorporate RE ions, destroying the zeolite structure [1,19]. A nice evidence of this interaction between vanadium and RE elements could be obtained by luminescence studies [20]. Results concerning a sample containing both metals (3000 ppm V and 1000 ppm Ni) treated in a 100% steam atmosphere are shown in Fig. 3. A further decrease in the surface area of the REHY was observed (compared to Fig. 2B) resulting in 44% surface area retention after 10 h treatment. This further decrease was not observed for the USHY sample. The same trend occurred when this sample was treated with 33% steam, but the effect was much lower. This combined effect is contrary to what was previously reported in the literature with RE exchanged Y zeolite [10]. In that work, the metal level was much higher (7500 ppm V and 3500 ppm Ni) and the metal impregnation was carried out in multiple steps, starting with vanadium. To confirm the combined effect Ni–V–RE, another set of REHY samples was prepared and submitted to the same protocol described above with impregnation and hydrothermal deactivation. Table 3 shows that the combined effect nickel and vanadium was observed again. In this case, a small effect of nickel alone was observed, resulting in a reduction of the REHY surface area from 605 to 527 m2 g 1. The RE elements exchanged on the zeolite framework could be affected by the deposition of Ni. In order to verify this hypothesis, new samples were prepared with nickel and vanadium exchanged in the zeolite framework. Table 4 shows that the Ni only results were similar to the previous ones, suggesting that for the impregnated catalysts, nickel is
Fig. 1. Surface area vs. time without metals and (A) 33% steam and (B) 100% steam: (*) USHY zeolite; (*) REHY zeolite.
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Fig. 2. Surface area vs. time with 100% steam and (A) 3000 ppm Ni and (B) 3000 ppm V: (*) USHY zeolite and (*) REHY zeolite.
Fig. 3. Surface area vs. time with 3000 ppm V, 1000 ppm Ni and 100% steam for (*) USHY zeolite and (*) REHY zeolite.
Fig. 4. TPR profiles of: (1) Ni-USHY, (2) Ni-V–REHY, (3) Ni–REHY, (4) V–REHY and (5) REHY.
Table 3 Total and micropore area for nickel and vanadium introduced by coimpregnation on REHY
probably partially exchanged into zeolite, displacing the exchanged rare earth and interacting with them. This interaction may be responsible for a reduction in the zeolite structure stability. To furnish a better interpretation of the effect of Ni on the passivation of V by rare earth species, one could examine the temperature-programmed reduction (TPR) profiles of the samples studied in Table 4, where Ni and V were exchanged in the zeolite. The profiles are shown in Fig. 4.
Ni (ppm)
V (ppm)
Surface area (m2 g 1)
Micropore volume (cm3 g 1)
– 1720 – 1010
– – 5930 5720
605 527 494 209
0.28 0.24 0.22 0.08
Table 4 Total and micropore area for nickel and vanadium exchanged in the REHY Ni exchange (ppm)
V exchange (ppm)
Surface area (m2 g 1)
Micropore volume (cm3 g 1)
– 5000 – 2400
– – 4000 4000
605 546 327 220
0.27 0.23 0.14 0.09
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Table 5 Effect of order of introduction of Ni and V onto FCC catalyst A: % of surface area after deactivation with 100% steam at 800 8C for 5 h First metal (ppm)
Second metal (ppm)
RE2O3 (%)
Surface area retention (%)
V (3000) Ni (1000) V (3000) Ni (1000) + V (3000)
– V (3000) Ni (1000) –
2.85 2.85 2.85 2.85
45 47 42 45
The TPR profile of REHY (Fig. 4, line 5) showed a relatively flat line, consistent with little reduction possible for the components of the sample. The V–REHY sample (Fig. 4, line 4) showed low broad absorptions band and two maximum could be observed at 470 and 650 8C. These positions were intermediate between that of vanadium oxides on various oxide supports (350–600 8C) and that of V on FCC catalysts (510–650 8C) [12–16]. The Ni–REHY sample showed a band around 630 8C (Fig. 4, line 3). In analogy to studies of similar range of Ni concentration on FCC catalysts [12], it is possible to interpret them as dispersed NiO. Most interesting is the TPR profile of Ni–V–REHY (Fig. 4, line 2). In this case, a very distinct peak could be seen in temperatures around 580 8C. This absorption was higher in intensity than those observed in the Ni–REHY or V– REHY samples (line 3 and 4). Since V–REHY contained a V to Ni ratio of 3 and the position of the peak is much lower than 630 8C, this absorption could be attributed to reduction of supported vanadium species. Thus, the presence of Ni in the zeolite promoted the formation of a significantly larger amount of reducible V oxide species. The TPR results and the surface area retention of the samples gave the same consistent picture. Vanadium was generally passivated by rare earth present in the zeolite or the FCC catalysts. However, when one sequentially introduced nickel and then vanadium onto a REHY, nickel–rare earth interaction took place and the passivation action of Vanadium by rare earth elements was attenuated. More readily reducible, mobile vanadium species were observed in the TPR measurements and these species caused more damage to the zeolites during hydrothermal deactivation. A further factor that may also contribute to de-stabilization of the lattice in the case of Ni exchanged samples was Ni substitution of rare earth in the exchanged sites, decreasing the stabilization effect of the rare earths towards dealumination and collapse. Although no characterization technique was used to evidence this latter possibility, a small decrease in the surface area was observed for the samples containing only Ni when compared to the pure zeolite samples (Tables 3 and 4). Even though the above picture was consistent by itself, the combined effect of Ni and V to further destabilize REHY was opposite to what was previously observed by detailed study of Yang et al. [10]. We find several factors that may contribute to the differences observed. First, Yang et al. used a zeolite of rather low crystallinity, the fresh zeolites has a
Table 6 Combined effect of Ni and V sequentially introduced onto FCC catalyst B after deactivation with 100% steam at 800 8C for 5 h Ni (ppm)
V (ppm)
Additional RE2O3 (wt.%)
Surface area retention (%)
1000 5000 1000 5000 1000 5000 1000 5000
1000 1000 5000 5000 1000 1000 5000 5000
1 1 1 1 5 5 5 5
50.4 29.0 49.1 16.0 52.6 48.8 50.3 35.6
low BET area of 438 m2 g 1 instead of the commonly observed values above 600 m2 g 1 for this type of zeolite. The X-ray spectra shown on the fresh sample also gave indication of the presence of amorphous material [10]. Next, the amount of vanadium deposited was a factor of two higher then the commonly encountered range of practical catalysts (7500–10,000 ppm). As a consequence, the deactivated area of the zeolites was very low and a lot of non-framework alumina was being generated. Thirdly, the authors introduced vanadium onto the zeolites before nickel instead of co-impregnation or Ni before V as most part of the present work. In the present work, the effect of the metal introduction order onto a FCC catalyst was also studied (Table 5). Compared to the catalyst where we deposited only V, the presence of Ni did not seem to affect much the surface area retention. In this case the Ni and V seemed to be independent. Combining these observations, one could postulate that the Ni–V–RE interaction on a zeolitic support is a complex competition between different components. In the case of a support that contains components that strongly interact with Ni, such as an FCC catalyst, Ni-Rare earth interaction was inhibited. This could be attributed to the presence of an active matrix or a zeolite containing large amount of amorphous silica or silica-alumina; in this case the passivation of vanadium by rare earth elements will not be affected by the presence of Ni. Anchored nickel might further form stable compounds with vanadium species, attenuating their destructive effect on zeolites as proposed by Yang et al. [10]. However, in systems where Ni–RE interaction was easy, the introduction of Ni decreased the amount of rare earth component that could be used to
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passivate vanadium, resulting to a system more susceptible to vanadium attack. As a further support of this hypothesis, we introduced rare earth onto a standard catalyst by first impregnating rare earth chloride solution and then adding ammonia onto the system in excess, forming free precipitated rare earth oxides upon calcination [21]. Then Ni and V were introduced onto these modified catalysts. Very clear deleterious interaction between Ni and V can be observed in these catalysts containing two different levels of RE elements when submitted to hydrothermal deactivation in the same conditions, as shown in Table 6.
4. Conclusions A negative effect on the texture retention was observed when both Ni and V were introduced by Mitchell impregnation on either rare earth containing HY zeolites or FCC catalysts. A lower zeolite surface area was observed for the samples containing both Ni and V when compared to samples containing only Ni or only V. TPR data support this observation, showing more mobile reducible vanadium on the zeolite. This effect, contrary to various indications in the literature, was attributed to an interaction of Ni with rare earth component, reducing its capacity to passivate vanadium. This observation put an alert on the complex interaction between the metal–metal and metal–support on a working catalyst. For catalysts presenting a strong Ni-support interaction, the Ni-rare earth interaction discussed here may be much attenuated. Instead, anchored Ni species can further interact with mobile vanadium species decreasing its negative effect on the other components of the catalyst. Naturally, these interactions will be affected by the mode of introduction of the metals. The effect observed due to
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Mitchell impregnation followed by calcination could not be simply translated to other cyclic methods of simulation of equilibrium catalyst behavior and also to the metals effect in industrial practice. In these later cases, Ni and V will be partially reduced during part of each operating cycle. Moreover, their facility in being reduced could also be affected by the proximity of each other.
References [1] C.A. Trujillo, U.N. Uribe, P.-P. Knops-Gerrits, L.A. Oviedo, P.A. Jacobs, J. Catal. 168 (1997) 1. [2] F.J. Elvin, Oil Gas J. 43 (1987) 42. [3] E.L. Kugler, P.D. Leta, J. Catal. 109 (1988) 387. [4] B.R. Mitchell, Ind. Eng. Chem. Prod. Res. Dev. 19 (1980) 209. [5] L.T. Boock, T.F. Petti, J.A. Rudesill, ACS Div. Petrol. Chem. 40 (1995) 421–426. [6] D. Wallenstein, R.H. Harding, J.R.D. Nee, L.T. Boock, Appl. Catal. A 204 (2000) 89. [7] L.A. Geritsen, H.N.J. Wijngaards, J. Verwoert, P. O’Connor, Catal. Today 11 (1991) 61. [8] P. O’Connor, J.P.J. Verlaan, S.J. Yanik, Catal. Today 43 (1998) 305. [9] A.W. Chester, Ind. Eng. Chem. Res. 26 (1987) 863. [10] S.-J. Yang, Y.-W. Chen, C. Li, Appl. Catal. 177 (1994) 109. [11] S.-J. Yang, Y.-W. Chen, C. Li, Zeolites 15 (1995) 77. [12] O. Bayraktar, E. Kugler, Appl. Catal. A 260 (2004) 125. [13] V. Cadet, F. Raatz, J. Lynch, C. Marcilly, A model study, Proc. 8th Int. Zeolite Conf., 1989, 1377–1386. [14] V. Cadet, F. Raatz, J. Lynch, C. Marcilly, Appl. Catal. 68 (1991) 263. [15] W.-C. Cheng, M.V. Juskelis, W. Sua´ rez, Appl. Catal. A 103 (1993) 87. [16] J. Kijenski, A. Baiker, M. Glinski, P. Dollenmeier, A. Wokaun, J. Catal. 101 (1968) 1. [17] G. Kriechbaum, H. Strack, (A.G. Degussa), DE 3,538,416 A1 (1987). [18] J. Biswas, I.E. Maxwell, Appl. Catal. 63 (1990) 197. [19] R. Pompe, S. Jara´ s, N.-G. Vannerberg, Appl. Catal. 13 (1984) 171. [20] G.L. Baugis, H.F. Brito, W. de Oliveira, F.R. de Castro, E.F. SousaAguiar, Micropor. Mesopor. Mater. 49 (2001) 179. [21] H.W. Beck, C.F. Lochow, C.W. Nibert, Ashland Oil Inc., US patent 4,515,683 (1985).
Interaction between Ni and V with USHY and rare earth HY zeolite during hydrothermal deactivation Alyne S. Escobar a, Marcelo M. Pereira a, Ricardo D.M. Pimenta b, Lam Y. Lau b, Henrique S. Cerqueira c,* a
Universidade Federal do Rio de Janeiro, Centro de Tecnologia, Instituto de Quı´mica, Departamento de Inorgaˆnica. Cidade Universita´ria, Rio de Janeiro, 21949-900 RJ, Brazil b Petrobras, Centro de Pesquisas e Desenvolvimento Leopoldo A. Miguez de Mello (Cenpes), Pesquisa e Desenvolvimento do Abastecimento, Tecnologia em FCC. Ilha do Funda˜o, Av.Jequitiba´ 950, Rio de Janeiro, 21941-598 RJ, Brazil c Petrobras, Centro de Pesquisas e Desenvolvimento Leopoldo A. Miguez de Mello (Cenpes), Pesquisa e Desenvolvimento em Ga´s, Energia e Desenvolvimento Sustenta´vel. Ilha do Funda˜o, Av Jequitiba´ 950, Rio de Janeiro, 21941-598 RJ, Brazil Received 28 December 2004; received in revised form 24 February 2005; accepted 1 March 2005 Available online 26 April 2005
Abstract The effect of Ni and V in the hydrothermal deactivation of an ultrastable Y zeolite with and without rare earth elements is presented and discussed. Although in the presence of steam, vanadium itself is the main factor responsible for zeolite loss of surface area, for the rare earth containing sample the presence of nickel increased the vanadium effect. Temperature-programmed reduction of these zeolites further support this observation by showing an increased amount of mobile readily reducible vanadium species while nickel and rare earth interact. The same deleterious effect of nickel was also observed in rare earth containing FCC catalysts after hydrothermal deactivation. # 2005 Elsevier B.V. All rights reserved. Keywords: Y zeolite; Nickel; Vanadium; Deactivation; Rare earth
1. Introduction During their lifetime in the catalytic cracking (FCC) process, the catalyst becomes contaminated with various amounts of metals present in the feedstock. Poisons such as nickel, vanadium and iron are present in significant amounts in heavy gas oils and residue feedstocks, being responsible for an undesirable increase in both hydrogen and coke yields. The metal containing molecules undergo thermal decomposition when the feedstock is contacted with the hot catalyst in the base of the riser reactor. After catalytic reactions, the catalyst is stripped with steam and directed to the regenerator, where the coke is burned in the presence of air at temperatures close to 710 8C. Under those conditions, part of the FCC catalyst – especially the Y zeolite – is permanently deactivated. * Corresponding author. Tel.: +552138656635; fax: +552138656626. E-mail address: [email protected] (H.S. Cerqueira). 0926-860X/$ – see front matter # 2005 Elsevier B.V. All rights reserved. doi:10.1016/j.apcata.2005.03.002
Many studies in the literature have already discussed the effect of vanadium on fluid catalytic cracking catalysts. Vanadium may deactivate the zeolite either temporarily, adsorbing and neutralizing acid sites [1] or permanently, due to the attack of vanadic acid to the zeolite structure or reacts with rare earth cations [1,2]. The higher the temperature used in the hydrothermal treatment, the higher the loss of activity caused by vanadium. Although the vanadium pentoxide is mobile, it was previously observed by imaging secondary ion mass spectrometry that nickel presents a low mobility, remaining in the area where it was deposited, making it a good measure to determine the age of individual FCC catalyst particles [3]. A traditional way to simulate equilibrium FCC catalysts performance was proposed by Mitchell and consisted of an incipient wetness impregnation of nickel and vanadium compounds, followed by a hydrothermal treatment [4]. Despite the differences obtained for the nickel distributions over the catalyst surface, the activity was in line with
A.S. Escobar et al. / Applied Catalysis A: General 286 (2005) 196–201
equilibrium catalyst. Efforts were made in the development of new laboratory deactivation procedures in order to better simulate the metals distribution and oxidation state. Examples of such methods are the cyclic propylene steaming (CPS) where after deposition of metals by incipient wetness technique, the catalyst is submitted to reduction–oxidation cycles using propylene as the reducing medium [5,6]. A more sophisticated approach is the cyclicdeactivation (CD) procedure, where a fresh catalyst is submitted to repeated cycles of cracking with vacuum gas oil spiked with metals and regeneration [7]. In short, it is important to note that there are different laboratory methods of introduction of metal onto a catalyst, which in turn may affect not only the metal support interaction but also the metal–metal interaction as well [8]. Although there are many works discussing the effect of metals under FCC conditions, the published results are controversial about the interaction between nickel and vanadium with rare earth elements. Using impregnation methods on catalysts, Chester [9] reported that nickel has a four times higher activity than vanadium for hydrogen and coke make, but observed no synergism between those two metals. Yang et al. [10,11] working with metal impregnation onto pure zeolites, reported that nickel could reduce the damage to the Y zeolite caused by vanadium. However, this previous study was made using a high level of vanadium in the range of 7500–10,000 ppm, which was introduced first to the zeolite. On the other hand, temperature-programmed reduction (TPR) has been shown to be sensitive to the state of Ni and V supported on typical FCC catalysts in this concentration range [12–15]. In general, V oxide dispersed on different oxide supports could be reduced from 350 to 600 8C [16]. On FCC catalyst, the reduction temperature was found to lie in a higher range, from 510 to 650 8C [12,13]. Bayraktar and Kugler [12] further reported a reduction peak at 690 8C. For supported Ni species, the reduction temperature was found to be higher. For example, Cheng et al. found two bands, one around 680 8C and the other at 800–880 8C which the authors attributed to supported NiO and Ni spinels, respectively [15]. In the present paper, a study was made using simultaneous introduction of V and Ni in an USHY with and without rare earth elements. Lower levels of both V and Ni compared to previously published data were employed and the hydrothermal deactivation was carried out at different water partial pressures and times. Temperatureprogrammed reduction was carried out in order to further clarify the interaction between Ni, V and rare earth components.
2. Experimental The NaY zeolite was prepared at CENPES, following a procedure described elsewhere [17]. The USHY zeolite was
197
Table 1 Characterization of the zeolite samples Sample
SiO2 (wt.%)
Al2O3 (wt.%)
RE2O3 (wt.%)
USHY REHY
72.6 64.2
22.2 20.0
0.5 11.8
prepared by ion-exchange of an aqueous suspension of Nazeolite. In a 2 l beaker, 750 ml of deionized water were added drop wise to 500 g of Na-zeolite under stirring. A solution of NH4NO3 (2 M) was added (750 ml) and kept at 70 8C for 90 min. After this, it was filtered under vacuum and the remaining zeolite was washed with hot water. The exchanged zeolite sample was kept for 720 min in an oven and then at room temperature. The ammonium-exchanged zeolite was steamed at 100% vapor for 60 min. The rare earth containing zeolite was obtained by post-exchange with an RECl3 solution at room temperature and pH 4.0, followed by a calcination at 600 8C for 3 h. Both zeolites were post-exchanged with NH4 with a similar procedure as described above, in order to reduce sodium. The two samples presented a final sodium content close to 1.0% (w/w). The composition of the ultra-stable Y (USHY) and REHY zeolites are presented in Table 1. No pre-treatment was needed before metals impregnation. The wet point was determined using a burette to add deionized water to 2 g of the sample until a change in the free flowing properties was observed. A value of approximately 0.55 ml g 1 was obtained for both zeolites. For metals impregnations, solutions of nickel 2+ and vanadyl octoates were diluted by toluene to achieve the volume corresponding to the wet point of sample. After homogenization for 60 min, the solvent was removed by means of a rotating evaporator. The zeolite was then calcined at 600 8C for 3 h. The samples containing both Ni and V were prepared by co-impregnation. Two commercial samples of rare earth containing FCC catalysts currently used at two Petrobras’ FCC units processing heavy gas oil were also submitted to the same impregnation procedure. Characteristics of the FCC catalysts are presented in Table 2. Ion-exchanged zeolite samples were also prepared, by the addition of aqueous solutions of VOSO4 and/or Ni(NO3)2 to the USHY or REHY suspension at 80 8C. After 1 h at this temperature, the system was washed with deionized water and then calcined at 600 8C for 2 h. For hydrothermal deactivation, samples were placed in crucibles and exposed to a steam atmosphere inside the furnace that can condition 12 samples simultaneously. The steam partial pressure was controlled by means of a water vapor saturator, through which an air stream passes continuously. In all tests, the furnace temperature was set to 800 8C. The absence of temperature profiles inside the furnace was checked with a sample of REHY placed in four different positions. The variation in the surface area was smaller than 3%.
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Table 2 Characterization of the FCC catalyst samples Sample
SiO2 (wt.%)
Al2O3 (wt.%)
RE2O3 (wt.%)
Na2O (wt.%)
Surface area (m2 g 1)
Micropore volume (cm3 g 1)
Catalyst A Catalyst B
55.8 57.1
39.3 38.7
2.52 2.19
0.25 0.34
385 255
0.121 0.079
The surface area of the deactivated samples was measured by nitrogen adsorption at 196 8C in the Gemini III 2375 equipment. Prior to the analysis, the samples were calcined at 600 8C for 1 h. The temperature-programmed reduction was carried out in a unit constructed in the laboratory. A sample dried overnight in oven was further dried in situ by a flow of argon at 0.5 ml s 1 at 400 8C for 0.5 h. The reduction was carried out using 1.6 mol% hydrogen in argon, with a flow of 0.5 ml s 1 through the reactor, with reduction temperature raised from room temperature up to 1000 8C, at a rate of 10 8C min 1.
3. Results and discussion In the absence of metals and with 33% of steam, there is a small decrease in the surface area of the zeolites (Fig. 1A), with a minor difference between the samples with and without rare earth elements. On the other hand, when the atmosphere is changed to 100% steam, an important decrease is observed for the USHY sample, the REHY results being comparable to the results with 33% steam (Fig. 1B). This was expected due to the known ability of RE elements in stabilizing the zeolite unit cell size [18]. Samples containing only Ni (3000 ppm) were submitted to deactivation under 100% steam. The results (Fig. 2A) were of the same magnitude of samples without metals, indicating that Ni alone has no effect on the zeolite surface area loss. Using V (3000 ppm) instead of Ni an important decrease in the area of zeolite samples occurred (Fig. 2B), resulting in surface area retentions of 59% for REHY and as low as 5% for USHY after 10 h treatment. This can be
explained due to interactions between V2O5 and rare earth components, forming low melting points RE-vanadates that can incorporate RE ions, destroying the zeolite structure [1,19]. A nice evidence of this interaction between vanadium and RE elements could be obtained by luminescence studies [20]. Results concerning a sample containing both metals (3000 ppm V and 1000 ppm Ni) treated in a 100% steam atmosphere are shown in Fig. 3. A further decrease in the surface area of the REHY was observed (compared to Fig. 2B) resulting in 44% surface area retention after 10 h treatment. This further decrease was not observed for the USHY sample. The same trend occurred when this sample was treated with 33% steam, but the effect was much lower. This combined effect is contrary to what was previously reported in the literature with RE exchanged Y zeolite [10]. In that work, the metal level was much higher (7500 ppm V and 3500 ppm Ni) and the metal impregnation was carried out in multiple steps, starting with vanadium. To confirm the combined effect Ni–V–RE, another set of REHY samples was prepared and submitted to the same protocol described above with impregnation and hydrothermal deactivation. Table 3 shows that the combined effect nickel and vanadium was observed again. In this case, a small effect of nickel alone was observed, resulting in a reduction of the REHY surface area from 605 to 527 m2 g 1. The RE elements exchanged on the zeolite framework could be affected by the deposition of Ni. In order to verify this hypothesis, new samples were prepared with nickel and vanadium exchanged in the zeolite framework. Table 4 shows that the Ni only results were similar to the previous ones, suggesting that for the impregnated catalysts, nickel is
Fig. 1. Surface area vs. time without metals and (A) 33% steam and (B) 100% steam: (*) USHY zeolite; (*) REHY zeolite.
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Fig. 2. Surface area vs. time with 100% steam and (A) 3000 ppm Ni and (B) 3000 ppm V: (*) USHY zeolite and (*) REHY zeolite.
Fig. 3. Surface area vs. time with 3000 ppm V, 1000 ppm Ni and 100% steam for (*) USHY zeolite and (*) REHY zeolite.
Fig. 4. TPR profiles of: (1) Ni-USHY, (2) Ni-V–REHY, (3) Ni–REHY, (4) V–REHY and (5) REHY.
Table 3 Total and micropore area for nickel and vanadium introduced by coimpregnation on REHY
probably partially exchanged into zeolite, displacing the exchanged rare earth and interacting with them. This interaction may be responsible for a reduction in the zeolite structure stability. To furnish a better interpretation of the effect of Ni on the passivation of V by rare earth species, one could examine the temperature-programmed reduction (TPR) profiles of the samples studied in Table 4, where Ni and V were exchanged in the zeolite. The profiles are shown in Fig. 4.
Ni (ppm)
V (ppm)
Surface area (m2 g 1)
Micropore volume (cm3 g 1)
– 1720 – 1010
– – 5930 5720
605 527 494 209
0.28 0.24 0.22 0.08
Table 4 Total and micropore area for nickel and vanadium exchanged in the REHY Ni exchange (ppm)
V exchange (ppm)
Surface area (m2 g 1)
Micropore volume (cm3 g 1)
– 5000 – 2400
– – 4000 4000
605 546 327 220
0.27 0.23 0.14 0.09
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Table 5 Effect of order of introduction of Ni and V onto FCC catalyst A: % of surface area after deactivation with 100% steam at 800 8C for 5 h First metal (ppm)
Second metal (ppm)
RE2O3 (%)
Surface area retention (%)
V (3000) Ni (1000) V (3000) Ni (1000) + V (3000)
– V (3000) Ni (1000) –
2.85 2.85 2.85 2.85
45 47 42 45
The TPR profile of REHY (Fig. 4, line 5) showed a relatively flat line, consistent with little reduction possible for the components of the sample. The V–REHY sample (Fig. 4, line 4) showed low broad absorptions band and two maximum could be observed at 470 and 650 8C. These positions were intermediate between that of vanadium oxides on various oxide supports (350–600 8C) and that of V on FCC catalysts (510–650 8C) [12–16]. The Ni–REHY sample showed a band around 630 8C (Fig. 4, line 3). In analogy to studies of similar range of Ni concentration on FCC catalysts [12], it is possible to interpret them as dispersed NiO. Most interesting is the TPR profile of Ni–V–REHY (Fig. 4, line 2). In this case, a very distinct peak could be seen in temperatures around 580 8C. This absorption was higher in intensity than those observed in the Ni–REHY or V– REHY samples (line 3 and 4). Since V–REHY contained a V to Ni ratio of 3 and the position of the peak is much lower than 630 8C, this absorption could be attributed to reduction of supported vanadium species. Thus, the presence of Ni in the zeolite promoted the formation of a significantly larger amount of reducible V oxide species. The TPR results and the surface area retention of the samples gave the same consistent picture. Vanadium was generally passivated by rare earth present in the zeolite or the FCC catalysts. However, when one sequentially introduced nickel and then vanadium onto a REHY, nickel–rare earth interaction took place and the passivation action of Vanadium by rare earth elements was attenuated. More readily reducible, mobile vanadium species were observed in the TPR measurements and these species caused more damage to the zeolites during hydrothermal deactivation. A further factor that may also contribute to de-stabilization of the lattice in the case of Ni exchanged samples was Ni substitution of rare earth in the exchanged sites, decreasing the stabilization effect of the rare earths towards dealumination and collapse. Although no characterization technique was used to evidence this latter possibility, a small decrease in the surface area was observed for the samples containing only Ni when compared to the pure zeolite samples (Tables 3 and 4). Even though the above picture was consistent by itself, the combined effect of Ni and V to further destabilize REHY was opposite to what was previously observed by detailed study of Yang et al. [10]. We find several factors that may contribute to the differences observed. First, Yang et al. used a zeolite of rather low crystallinity, the fresh zeolites has a
Table 6 Combined effect of Ni and V sequentially introduced onto FCC catalyst B after deactivation with 100% steam at 800 8C for 5 h Ni (ppm)
V (ppm)
Additional RE2O3 (wt.%)
Surface area retention (%)
1000 5000 1000 5000 1000 5000 1000 5000
1000 1000 5000 5000 1000 1000 5000 5000
1 1 1 1 5 5 5 5
50.4 29.0 49.1 16.0 52.6 48.8 50.3 35.6
low BET area of 438 m2 g 1 instead of the commonly observed values above 600 m2 g 1 for this type of zeolite. The X-ray spectra shown on the fresh sample also gave indication of the presence of amorphous material [10]. Next, the amount of vanadium deposited was a factor of two higher then the commonly encountered range of practical catalysts (7500–10,000 ppm). As a consequence, the deactivated area of the zeolites was very low and a lot of non-framework alumina was being generated. Thirdly, the authors introduced vanadium onto the zeolites before nickel instead of co-impregnation or Ni before V as most part of the present work. In the present work, the effect of the metal introduction order onto a FCC catalyst was also studied (Table 5). Compared to the catalyst where we deposited only V, the presence of Ni did not seem to affect much the surface area retention. In this case the Ni and V seemed to be independent. Combining these observations, one could postulate that the Ni–V–RE interaction on a zeolitic support is a complex competition between different components. In the case of a support that contains components that strongly interact with Ni, such as an FCC catalyst, Ni-Rare earth interaction was inhibited. This could be attributed to the presence of an active matrix or a zeolite containing large amount of amorphous silica or silica-alumina; in this case the passivation of vanadium by rare earth elements will not be affected by the presence of Ni. Anchored nickel might further form stable compounds with vanadium species, attenuating their destructive effect on zeolites as proposed by Yang et al. [10]. However, in systems where Ni–RE interaction was easy, the introduction of Ni decreased the amount of rare earth component that could be used to
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passivate vanadium, resulting to a system more susceptible to vanadium attack. As a further support of this hypothesis, we introduced rare earth onto a standard catalyst by first impregnating rare earth chloride solution and then adding ammonia onto the system in excess, forming free precipitated rare earth oxides upon calcination [21]. Then Ni and V were introduced onto these modified catalysts. Very clear deleterious interaction between Ni and V can be observed in these catalysts containing two different levels of RE elements when submitted to hydrothermal deactivation in the same conditions, as shown in Table 6.
4. Conclusions A negative effect on the texture retention was observed when both Ni and V were introduced by Mitchell impregnation on either rare earth containing HY zeolites or FCC catalysts. A lower zeolite surface area was observed for the samples containing both Ni and V when compared to samples containing only Ni or only V. TPR data support this observation, showing more mobile reducible vanadium on the zeolite. This effect, contrary to various indications in the literature, was attributed to an interaction of Ni with rare earth component, reducing its capacity to passivate vanadium. This observation put an alert on the complex interaction between the metal–metal and metal–support on a working catalyst. For catalysts presenting a strong Ni-support interaction, the Ni-rare earth interaction discussed here may be much attenuated. Instead, anchored Ni species can further interact with mobile vanadium species decreasing its negative effect on the other components of the catalyst. Naturally, these interactions will be affected by the mode of introduction of the metals. The effect observed due to
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Mitchell impregnation followed by calcination could not be simply translated to other cyclic methods of simulation of equilibrium catalyst behavior and also to the metals effect in industrial practice. In these later cases, Ni and V will be partially reduced during part of each operating cycle. Moreover, their facility in being reduced could also be affected by the proximity of each other.
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