- Email: [email protected]
Al2O3 catalysts
Studies in Surface Science and Catalysis 130 A. Corma, F.V, Melo, S.Mendioroz and J.L.G. Fierro (Editors) 9 2000 Elsevier Science B.V. All fights reserved.
2813
Characterization of reduced and sulphided Ru-V/AI203 catalysts C.E. Scott, J. Guevara, A. Scaffidi, E. Escalona, C. Bolivar C., M.J. P6rez-Zurita and J. Goldwasser. Centro de Cathlisis Petr61eo y Petroquimica. Universidad Central de Venezuela, Facultad de Ciencias, Escuela de Quimica. Apartado Postal 47102- Los Chaguaramos- Caracas 1020AVenezuela. e-mail: [email protected] ABSTRACT Ru-V/AI203 catalysts were prepared impregnating the support with ammonium vanadate (0-6 wt%V), followed by a calcination process. The solids were then impregnated with ruthenium chloride (Ru 6 wt%). One portion of the solids was dried (non-calcined, ne catalysts) and the other proportion was calcined (calcined, e catalysts). The catalysts were characterised by X-ray diffraction, specific surface area (BET), HE chemisorption and temperature programmed reduction. Hydrodesulphurization of thi0phene at atmospheric pressure, on the sulphided catalysts, was performed as the catalytic test. X ray diffraction results show the formation of a RuO2 segregated phase upon calcination, while TPR experiments revealed only one reduction peak for the ne catalysts and more than one for the e ones. These results suggest that only one surface specie is present in the ne catalysts, which we propose is a mixed Ru-V specie. HDS of thiophene increases, with the V content, for the ne catalysts, but decreases for the e catalysts. These catalytic data seem to show that the proposed mixed Ru-V specie, in the ne catalysts, promotes the formation of more active sites in the sulphided catalysts. Also, it was shown that the presence of Ru in the Ru-V catalysts facilitates de reduction of V. 1. INTRODUCTION Due to the new environmental regulations there is no doubt that hydrotreatment is of great importance for the refining industry. The need for processing heavier petroleum fractions into light distillates as a result of the decline in fuel oil demand, together with stricter environmental legislation regarding the maximum content of sulphur, nitrogen and aromatics in fuels, have made hydroprocessing increasingly difficult. Conventional Co(Ni)Mo(W) supported on alumina hydrotreating catalysts are not sufficiently effective for an exhaustive heteroatom removal from heavy petroleum fractions. One possible way to overcome these difficulties is to develop new, more active and selective, hydrotreating catalysts. Ru based catalysts exhibit interesting properties in hydrotreatment, and when a second metal, like Mo, is added the catalytic activity is enhanced [1-6]. It has been also shown that the calcination step has an important effect on the catalytic properties of Ru catalysts [ 1,2,7]. In the present work we have studied alumina supported V -Ru catalysts.
2814
2. EXPERIMENTAL 2.1. Catalysts preparation Catalysts were prepared by successive impregnation of alumina (~/-A1203, 210 mt. 2 g-l). First with an ammonium vanadate (Riedel-de-Harn, 98.5 %) solution (50 cm3), the excess of liquid was rota-evaporated at 60 ~ The solid was then dried at 150 ~ for 12 h. and calcined at 500 ~ for 3 h. Samples of the solids thus obtained were then impregnated, by the incipient wetness method, with Ruthenium Chloride hydrate (Aldrich) solutions. Solids were then dried at room temperature for 24 h., and 150 ~ for 12 h. These will be referred as non-calcined (nc) catalysts. A portion of each sample was calcined at 500 ~ for 3 h (c catalysts).
2.2. Catalysts characterisation Temperature program reduction, were performed in a Micromeretics TPD/TPD 2900 apparatus. 50 mg. of the catalyst was treated in a H2(9.9 v%)/Ar mixture flowing at 40 cm 3 sl. The samples were heated at 0.166 ~ s"l. X ray diffraction patterns were obtained in a Phillips 1840 instrument. Equipped with a Co anode. N2 adsorption experiments (BET), were performed on a Micromeritics flow sorb 2300 system. 80 mg. of each solid was used. The catalysts were dried at 150 at ~ for 1.5 h. to eliminate any moisture, and then a N2(30%v)/He(70%v) mixture was used as the adsorbent. H2 chemisorption experiments were done in a conventional BET. Catalysts samples of approximately 0.2 g were placed in a glass microreactor and reduced in situ with pure H2 (1 cm 3 s"l) for 3 h at 500 ~ A detailed procedure can be found in reference 7.
2.3. Catalytic reaction Hydrodesulfurization (HDS) of thiophene, in a continuos flow system at atmospheric pressure, was carried out as the catalytic test. Catalysts were pre-sulphided in a H2S(15%v)/H2 stream. The temperature was increased at a rate of 0.0833 ~ sl, up to 400 ~ and then kept at this temperature for 4 h. Thiophene HDS was performed using a liquid feed composed of 10% v of thiophene in nheptane (2.7x10 -4 cm 3 sl ) and H2 (0.25 cm 3 sl ) at 280 ~ The system was covered with a heating mantle (150 ~ in order to avoid any condensation of the reaction products. These were injected to a Perking Elmer (AutoSystem XL) gas chromatograph equipped with an Flame Ionisation Detector and a capillary column (metilsilicone, 30 m). The only reaction products detected were n-butane, 1-butene, cis-2-butene and trans-2-butene. 3. RESULTS AND DISCUSSION BET surface areas and catalysts compositions are shown in Table 1. It can be seen that surface areas are very similar for all the catalysts. However, it is found that there is a slight decrease in surface area as the metal content increases, which is quite normal since the supported metals can plug some smaller pores of the support. Also, there is a tendency for non calcined catalysts to have areas a little lower than the corresponding Calcined ones, and this could be explained if during calcination some agglomeration of metal particles can occur, which would re-open some of the initially plugged pores.
2815 TABLE 1 Catalysts nominal compositio n and surface area (BET). Catalyst Ru6V0 Ru6V 1 Ru6V2 Ru6V4 Ru6V6 Ru0V6
V (wt%) Ru 0 1 2 4 6 6 e: calcined; ne:non-calcined
(wt%) 6 6 6 6 6 0
........ S,,,,,urfaceArea (m 2 g-l) c nc 176 172 170 168 166 163 161 157 160 155 ---
X ray diffraction patterns are shown in Figure 1. It can be seen that upon calcination the Ru containing catalysts present lines that correspond to the formation of RuO2 (JCPDS file number 40-1290). For the others only lines due to y-A1203 (JCPDS file 10-425) were observed. Clearly, calcination produces the formation of a RuO2 segregated phase that can be readily observed by X ray diffraction, indicating crystals of relatively large sizes. However, the presence of V in the catalysts produces broader RuO2 lines. Obviously, the presence of V is having an effect on the formation of the RuO2 crystals. H2 chemisorption results are shown in Table 2. We observe that, as expected, calcination of the catalysts produces a decrease in dispersion (H2 chemisorption). Also, for both series of catalysts, the dispersion decreases as the content of V in the catalyst is increased. For the calcined catalysts the dispersions are slightly lower than for the parent non-calcined catalysts. The decrease in dispersion with increasing amounts of V is similar to that previously observed for Ru-Mo catalysts as the Mo content in the catalyst increases [3,8]. TPR of calcined and non-calcined catalysts are totally different (see Table 2). For the Ru calcined catalyst we see a main reduction peak with a maximum in 252~ and for the noncalcined only one peak is observed which is situated at 150 ~ . According to our X ray diffraction results the peak a 252 ~ in the calcined catalyst, should correspond to the segregated RuO2, while the reduction peak in the non-calcined catalyst must be due to the reduction of RuC13 dispersed on the alumina support. V on alumina catalyst shows one peak at 517 ~ which is due to the reduction of V § to V§ For the Ru-V calcined catalyst we observe two main reduction peaks, one at 254-270 ~ (which is near the value found for the Ru calcined catalyst) a second one that shifts from 545 ~ (for l%V) to 350 ~ (for 6%V). The second peak is assigned to the reduction of V. If we compare the reduction temperature of the V for the Ru-V catalysts with the V catalyst, we observed that V is more easily reduced in the presence of Ru. Thus, the reduction of V is facilitated, either by Ru (spillover effect) or by the presence of bulk V205 as V loading is increased, or both. For non-calcined catalysts only one reduction peak that monotonically shifts from 150 ~ (for Ru) to 202 ~ for the catalyst with 6% V, was observed. This change can be due to the formation of a Ru-V mixed site. The mixed Ru-V site would be similar to the one proposed for Ru-Mo (i.e. Ru-MoOx). In fact for reduced Ru-Mo catalysts we have proposed the formation of mixed sites according to TPR, infrared spectroscopy of adsorbed CO and electron spin resonance spectroscopy[8]. In the mixed site there are Ru-V bimetallic species like those evidence by Choi et. all [10], on the basis of EXAFS and XANES studies, in reduced Pt-Mo/A1203 catalysts. Mixed sites can also be present in calcined catalysts, but in low proportion (a small peak a 200 ~ is observed for calcined catalysts). Apparently the formation of bimetallic particles is hindered by the fact
2816 that Ru is partially segregated in the form of RuO2 (evidenced by our XRD results) when the catalysts are calcined. I
I
I
I
I
'I
RuO2
'I
'I
'I R u O 2
I
I
I
I
I
I
AI203
'
,
,
I
I
,"
I
I
RuO2
ai c (D
,
c
a
b
80
Degrees
20
40
Figure 1. X ray diffraction patterns for Ru-V/AI203 catalysts, a) Ru6V0c; b) Ru6V0nc; c) Ru0V6c; d) Ru0V6nc; e) Ru6V6c; f) Ru6V6nc; g)y-A1203
TABLE 2. H 2 Chemisorption and Temperature Programmed Reduction for Ru-V/A1203 catalysts H/Ru* Temperature at Peak Maximum (~ Catalyst C nc c nc Ru6V0 0.35 0.45 220(s), 252 150 Ru6V1 0.29 0.37 200(sm), 2|0(sm), 254, 545 159 Ru6V2 0.19 0.23 200(sm), 265,492 165 Ru6V4 0.18 0.22 200(sm), 258, 404 188 Ru6V6 0.14 0.19 200(sm),270,350 202 .... Ru0V6 Did not show ~ adsorption ......... 517 s= shoulder; sm = small; * Determined by hydrogen chemisorption.
The results for thiophene HDS and surface areas are shown in Figure 2. Once again, a different behaviour can be observed when RuV/AI203 catalysts are calcined. For RuV(nc) catalysts HDS activity increases with increasing amount of V, while it decreases for the RuV(c) catalysts. The V/A1203 showed a conversion of 3.2 %. The Ru-V calcined catalysts with 4 and 6% of V are even less active than the catalyst containing only V. The differences can not be assigned to different surfaces areas (SA) since both series of catalysts have practically the same SA, nor to a particle sintering, given that dispersions (for
2817
reduced catalysts with the same V content) are very similar (see H2 chemisorption in table 2). The only possible explanation is that the Ru and V surface species present in the nc catalysts, which are the responsible for the formation of the Ru-V mixed site in reduced catalysts, also promote active sites for the HDS of thiophene. On calcination the formation of RuO2 hinders the formation of the HDS sites. In Figure 3, catalyst selectivity for the C4 products (n-butane, 1-butene, cis-225 butene and trans-2-butene) is shown. Even though the experiments were not done at the same conversion level, the obtained _o conversion were not too different. Thus, c for non-calcined catalysts conversion are o ~vcl Q between 8 and 20 %, and for calcined r 10 o catalysts between 2.3 and 9 %. Taking .1r Q. _o into account the limitations previously ~s mentioned, some important observations can be drawn from Figure 3. The main 1 result is that for V containing catalysts, 0 2 4 6 product distributions are fairly similar. V content/wt % This would be an indication that the type Figure 2. Thiophene HDS on Ru-V/AI203 of sites are similar and that the changing catalysts. factor is the number of sites.
.........................................................................
4550[40 _~
35
=o
30
DRu6V0nc
-
mRu6Vlnc
1
BBRu6Vlc
~, 9
mRu6V2c 25
g
1 BRu6VOc
.
mRu6V2nc
20
mRu6V4c
15
mRu6V4nc
10
-
mRu6V6c
5
mRu6V6nc lmRu0V6
1-Butene
n-Butane Reaction
t-2-Butene
c-2-Butene
Product
Figure 3. Product distribution for the HDS of thiophene on the sulphided catalysts. This observation is consistent with the idea of the formation of mixed Ru-V sites and that these sites promote the active HDS species. Mixed Ru-V sites are also present, but in lower amounts, in the calcined catalysts, as was shown by our TPR experiments.
2818 4. CONCLUSIONS For RuV/A1203 catalysts, the formation of mixed Ru-V species has been evidenced by temperature programmed reduction experiments. This species promotes the formation more active sites, than Ru or V on their own, in the sulphided catalysts. Thus, Ru-V non-calcined catalysts shows higher thiophene HDS activity, than the calcined counterparts. The formation of the mixed Ru-V species is hindered when catalysts are calcined due to the formation of a RuO2 segregated phase. Also, it was shown that the presence of Ru in the Ru-V catalysts facilitates de reduction of V. 5. ACKNOWLEDGEMENT The authors are grateful to CONICIT, through grant G-97000658, for the financial support. REFERENCES 1. P.C.H. Mitchell and C.E. Scott., Bull. Soc. Chim. Belg., 93(1984)619. 2. P.C.H. Mitchell, C.E. Scott, J.P. Bonelle and J.G. Grimblot., J. Catal., 107(1987)482. 3. C.E. Scott, T. Romero, E. Lepore, M. Arruebarrena, P. Betancourt, C. Bolivar, M.J. P&ezZurita, P. Marcano and J. Goldwasser., Appl. Catal. A.,125(1995)71. 4. J. Shabtai, Q. Guohe, K. Balusami, N.K. Nag and F.E. Massoth. J. Catal. 113(1988)206. 5. A.S. Hirschon, R.B. Wilson and R.M. Laine. App. Catal. 34(1983)311. 6. C. Geantet. S. G6b/516s, J.A. de los Reyes, M. Cattenot, M. Vrinat and M. Breysse. Catal. Today. 10(1991 ).665 7. P. Betancourt, A. Rives, R. Hubaut, C.E. Scott, J. Goldwasser, App. Catal. 170(1998)307. 8. C.E. Scott, P. Betancourt, M.J. P&ez-Zurita, C. Bolivar and J. Goldwasser. App. Catal. In press. 9. R.L.C. Bonn6, A.D. Van Langeveld and J.A. Moulijn. J. Catal., 154(1995)75. 10. S.H. Choi and J.S. Lee. J. Catal, 167(1997)364.
2813
Characterization of reduced and sulphided Ru-V/AI203 catalysts C.E. Scott, J. Guevara, A. Scaffidi, E. Escalona, C. Bolivar C., M.J. P6rez-Zurita and J. Goldwasser. Centro de Cathlisis Petr61eo y Petroquimica. Universidad Central de Venezuela, Facultad de Ciencias, Escuela de Quimica. Apartado Postal 47102- Los Chaguaramos- Caracas 1020AVenezuela. e-mail: [email protected] ABSTRACT Ru-V/AI203 catalysts were prepared impregnating the support with ammonium vanadate (0-6 wt%V), followed by a calcination process. The solids were then impregnated with ruthenium chloride (Ru 6 wt%). One portion of the solids was dried (non-calcined, ne catalysts) and the other proportion was calcined (calcined, e catalysts). The catalysts were characterised by X-ray diffraction, specific surface area (BET), HE chemisorption and temperature programmed reduction. Hydrodesulphurization of thi0phene at atmospheric pressure, on the sulphided catalysts, was performed as the catalytic test. X ray diffraction results show the formation of a RuO2 segregated phase upon calcination, while TPR experiments revealed only one reduction peak for the ne catalysts and more than one for the e ones. These results suggest that only one surface specie is present in the ne catalysts, which we propose is a mixed Ru-V specie. HDS of thiophene increases, with the V content, for the ne catalysts, but decreases for the e catalysts. These catalytic data seem to show that the proposed mixed Ru-V specie, in the ne catalysts, promotes the formation of more active sites in the sulphided catalysts. Also, it was shown that the presence of Ru in the Ru-V catalysts facilitates de reduction of V. 1. INTRODUCTION Due to the new environmental regulations there is no doubt that hydrotreatment is of great importance for the refining industry. The need for processing heavier petroleum fractions into light distillates as a result of the decline in fuel oil demand, together with stricter environmental legislation regarding the maximum content of sulphur, nitrogen and aromatics in fuels, have made hydroprocessing increasingly difficult. Conventional Co(Ni)Mo(W) supported on alumina hydrotreating catalysts are not sufficiently effective for an exhaustive heteroatom removal from heavy petroleum fractions. One possible way to overcome these difficulties is to develop new, more active and selective, hydrotreating catalysts. Ru based catalysts exhibit interesting properties in hydrotreatment, and when a second metal, like Mo, is added the catalytic activity is enhanced [1-6]. It has been also shown that the calcination step has an important effect on the catalytic properties of Ru catalysts [ 1,2,7]. In the present work we have studied alumina supported V -Ru catalysts.
2814
2. EXPERIMENTAL 2.1. Catalysts preparation Catalysts were prepared by successive impregnation of alumina (~/-A1203, 210 mt. 2 g-l). First with an ammonium vanadate (Riedel-de-Harn, 98.5 %) solution (50 cm3), the excess of liquid was rota-evaporated at 60 ~ The solid was then dried at 150 ~ for 12 h. and calcined at 500 ~ for 3 h. Samples of the solids thus obtained were then impregnated, by the incipient wetness method, with Ruthenium Chloride hydrate (Aldrich) solutions. Solids were then dried at room temperature for 24 h., and 150 ~ for 12 h. These will be referred as non-calcined (nc) catalysts. A portion of each sample was calcined at 500 ~ for 3 h (c catalysts).
2.2. Catalysts characterisation Temperature program reduction, were performed in a Micromeretics TPD/TPD 2900 apparatus. 50 mg. of the catalyst was treated in a H2(9.9 v%)/Ar mixture flowing at 40 cm 3 sl. The samples were heated at 0.166 ~ s"l. X ray diffraction patterns were obtained in a Phillips 1840 instrument. Equipped with a Co anode. N2 adsorption experiments (BET), were performed on a Micromeritics flow sorb 2300 system. 80 mg. of each solid was used. The catalysts were dried at 150 at ~ for 1.5 h. to eliminate any moisture, and then a N2(30%v)/He(70%v) mixture was used as the adsorbent. H2 chemisorption experiments were done in a conventional BET. Catalysts samples of approximately 0.2 g were placed in a glass microreactor and reduced in situ with pure H2 (1 cm 3 s"l) for 3 h at 500 ~ A detailed procedure can be found in reference 7.
2.3. Catalytic reaction Hydrodesulfurization (HDS) of thiophene, in a continuos flow system at atmospheric pressure, was carried out as the catalytic test. Catalysts were pre-sulphided in a H2S(15%v)/H2 stream. The temperature was increased at a rate of 0.0833 ~ sl, up to 400 ~ and then kept at this temperature for 4 h. Thiophene HDS was performed using a liquid feed composed of 10% v of thiophene in nheptane (2.7x10 -4 cm 3 sl ) and H2 (0.25 cm 3 sl ) at 280 ~ The system was covered with a heating mantle (150 ~ in order to avoid any condensation of the reaction products. These were injected to a Perking Elmer (AutoSystem XL) gas chromatograph equipped with an Flame Ionisation Detector and a capillary column (metilsilicone, 30 m). The only reaction products detected were n-butane, 1-butene, cis-2-butene and trans-2-butene. 3. RESULTS AND DISCUSSION BET surface areas and catalysts compositions are shown in Table 1. It can be seen that surface areas are very similar for all the catalysts. However, it is found that there is a slight decrease in surface area as the metal content increases, which is quite normal since the supported metals can plug some smaller pores of the support. Also, there is a tendency for non calcined catalysts to have areas a little lower than the corresponding Calcined ones, and this could be explained if during calcination some agglomeration of metal particles can occur, which would re-open some of the initially plugged pores.
2815 TABLE 1 Catalysts nominal compositio n and surface area (BET). Catalyst Ru6V0 Ru6V 1 Ru6V2 Ru6V4 Ru6V6 Ru0V6
V (wt%) Ru 0 1 2 4 6 6 e: calcined; ne:non-calcined
(wt%) 6 6 6 6 6 0
........ S,,,,,urfaceArea (m 2 g-l) c nc 176 172 170 168 166 163 161 157 160 155 ---
X ray diffraction patterns are shown in Figure 1. It can be seen that upon calcination the Ru containing catalysts present lines that correspond to the formation of RuO2 (JCPDS file number 40-1290). For the others only lines due to y-A1203 (JCPDS file 10-425) were observed. Clearly, calcination produces the formation of a RuO2 segregated phase that can be readily observed by X ray diffraction, indicating crystals of relatively large sizes. However, the presence of V in the catalysts produces broader RuO2 lines. Obviously, the presence of V is having an effect on the formation of the RuO2 crystals. H2 chemisorption results are shown in Table 2. We observe that, as expected, calcination of the catalysts produces a decrease in dispersion (H2 chemisorption). Also, for both series of catalysts, the dispersion decreases as the content of V in the catalyst is increased. For the calcined catalysts the dispersions are slightly lower than for the parent non-calcined catalysts. The decrease in dispersion with increasing amounts of V is similar to that previously observed for Ru-Mo catalysts as the Mo content in the catalyst increases [3,8]. TPR of calcined and non-calcined catalysts are totally different (see Table 2). For the Ru calcined catalyst we see a main reduction peak with a maximum in 252~ and for the noncalcined only one peak is observed which is situated at 150 ~ . According to our X ray diffraction results the peak a 252 ~ in the calcined catalyst, should correspond to the segregated RuO2, while the reduction peak in the non-calcined catalyst must be due to the reduction of RuC13 dispersed on the alumina support. V on alumina catalyst shows one peak at 517 ~ which is due to the reduction of V § to V§ For the Ru-V calcined catalyst we observe two main reduction peaks, one at 254-270 ~ (which is near the value found for the Ru calcined catalyst) a second one that shifts from 545 ~ (for l%V) to 350 ~ (for 6%V). The second peak is assigned to the reduction of V. If we compare the reduction temperature of the V for the Ru-V catalysts with the V catalyst, we observed that V is more easily reduced in the presence of Ru. Thus, the reduction of V is facilitated, either by Ru (spillover effect) or by the presence of bulk V205 as V loading is increased, or both. For non-calcined catalysts only one reduction peak that monotonically shifts from 150 ~ (for Ru) to 202 ~ for the catalyst with 6% V, was observed. This change can be due to the formation of a Ru-V mixed site. The mixed Ru-V site would be similar to the one proposed for Ru-Mo (i.e. Ru-MoOx). In fact for reduced Ru-Mo catalysts we have proposed the formation of mixed sites according to TPR, infrared spectroscopy of adsorbed CO and electron spin resonance spectroscopy[8]. In the mixed site there are Ru-V bimetallic species like those evidence by Choi et. all [10], on the basis of EXAFS and XANES studies, in reduced Pt-Mo/A1203 catalysts. Mixed sites can also be present in calcined catalysts, but in low proportion (a small peak a 200 ~ is observed for calcined catalysts). Apparently the formation of bimetallic particles is hindered by the fact
2816 that Ru is partially segregated in the form of RuO2 (evidenced by our XRD results) when the catalysts are calcined. I
I
I
I
I
'I
RuO2
'I
'I
'I R u O 2
I
I
I
I
I
I
AI203
'
,
,
I
I
,"
I
I
RuO2
ai c (D
,
c
a
b
80
Degrees
20
40
Figure 1. X ray diffraction patterns for Ru-V/AI203 catalysts, a) Ru6V0c; b) Ru6V0nc; c) Ru0V6c; d) Ru0V6nc; e) Ru6V6c; f) Ru6V6nc; g)y-A1203
TABLE 2. H 2 Chemisorption and Temperature Programmed Reduction for Ru-V/A1203 catalysts H/Ru* Temperature at Peak Maximum (~ Catalyst C nc c nc Ru6V0 0.35 0.45 220(s), 252 150 Ru6V1 0.29 0.37 200(sm), 2|0(sm), 254, 545 159 Ru6V2 0.19 0.23 200(sm), 265,492 165 Ru6V4 0.18 0.22 200(sm), 258, 404 188 Ru6V6 0.14 0.19 200(sm),270,350 202 .... Ru0V6 Did not show ~ adsorption ......... 517 s= shoulder; sm = small; * Determined by hydrogen chemisorption.
The results for thiophene HDS and surface areas are shown in Figure 2. Once again, a different behaviour can be observed when RuV/AI203 catalysts are calcined. For RuV(nc) catalysts HDS activity increases with increasing amount of V, while it decreases for the RuV(c) catalysts. The V/A1203 showed a conversion of 3.2 %. The Ru-V calcined catalysts with 4 and 6% of V are even less active than the catalyst containing only V. The differences can not be assigned to different surfaces areas (SA) since both series of catalysts have practically the same SA, nor to a particle sintering, given that dispersions (for
2817
reduced catalysts with the same V content) are very similar (see H2 chemisorption in table 2). The only possible explanation is that the Ru and V surface species present in the nc catalysts, which are the responsible for the formation of the Ru-V mixed site in reduced catalysts, also promote active sites for the HDS of thiophene. On calcination the formation of RuO2 hinders the formation of the HDS sites. In Figure 3, catalyst selectivity for the C4 products (n-butane, 1-butene, cis-225 butene and trans-2-butene) is shown. Even though the experiments were not done at the same conversion level, the obtained _o conversion were not too different. Thus, c for non-calcined catalysts conversion are o ~vcl Q between 8 and 20 %, and for calcined r 10 o catalysts between 2.3 and 9 %. Taking .1r Q. _o into account the limitations previously ~s mentioned, some important observations can be drawn from Figure 3. The main 1 result is that for V containing catalysts, 0 2 4 6 product distributions are fairly similar. V content/wt % This would be an indication that the type Figure 2. Thiophene HDS on Ru-V/AI203 of sites are similar and that the changing catalysts. factor is the number of sites.
.........................................................................
4550[40 _~
35
=o
30
DRu6V0nc
-
mRu6Vlnc
1
BBRu6Vlc
~, 9
mRu6V2c 25
g
1 BRu6VOc
.
mRu6V2nc
20
mRu6V4c
15
mRu6V4nc
10
-
mRu6V6c
5
mRu6V6nc lmRu0V6
1-Butene
n-Butane Reaction
t-2-Butene
c-2-Butene
Product
Figure 3. Product distribution for the HDS of thiophene on the sulphided catalysts. This observation is consistent with the idea of the formation of mixed Ru-V sites and that these sites promote the active HDS species. Mixed Ru-V sites are also present, but in lower amounts, in the calcined catalysts, as was shown by our TPR experiments.
2818 4. CONCLUSIONS For RuV/A1203 catalysts, the formation of mixed Ru-V species has been evidenced by temperature programmed reduction experiments. This species promotes the formation more active sites, than Ru or V on their own, in the sulphided catalysts. Thus, Ru-V non-calcined catalysts shows higher thiophene HDS activity, than the calcined counterparts. The formation of the mixed Ru-V species is hindered when catalysts are calcined due to the formation of a RuO2 segregated phase. Also, it was shown that the presence of Ru in the Ru-V catalysts facilitates de reduction of V. 5. ACKNOWLEDGEMENT The authors are grateful to CONICIT, through grant G-97000658, for the financial support. REFERENCES 1. P.C.H. Mitchell and C.E. Scott., Bull. Soc. Chim. Belg., 93(1984)619. 2. P.C.H. Mitchell, C.E. Scott, J.P. Bonelle and J.G. Grimblot., J. Catal., 107(1987)482. 3. C.E. Scott, T. Romero, E. Lepore, M. Arruebarrena, P. Betancourt, C. Bolivar, M.J. P&ezZurita, P. Marcano and J. Goldwasser., Appl. Catal. A.,125(1995)71. 4. J. Shabtai, Q. Guohe, K. Balusami, N.K. Nag and F.E. Massoth. J. Catal. 113(1988)206. 5. A.S. Hirschon, R.B. Wilson and R.M. Laine. App. Catal. 34(1983)311. 6. C. Geantet. S. G6b/516s, J.A. de los Reyes, M. Cattenot, M. Vrinat and M. Breysse. Catal. Today. 10(1991 ).665 7. P. Betancourt, A. Rives, R. Hubaut, C.E. Scott, J. Goldwasser, App. Catal. 170(1998)307. 8. C.E. Scott, P. Betancourt, M.J. P&ez-Zurita, C. Bolivar and J. Goldwasser. App. Catal. In press. 9. R.L.C. Bonn6, A.D. Van Langeveld and J.A. Moulijn. J. Catal., 154(1995)75. 10. S.H. Choi and J.S. Lee. J. Catal, 167(1997)364.