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The effects of some rare earth metals on the catalytic properties of reforming catalysts
Journal of the Less-Common
Metals, 148 (1989) 399 - 403
399
THE EFFECTS OF SOME RARE EARTH METALS ON THE CATALYTIC PROPERTIES OF REFORMING CATALYSTS* ZHU GUANGZHONG, Department
LI JIANHUI, SHI QIUJIE and LI FENGYI
of Chemistry, Jiangxi University, Nanchang (China)
(Received July 8, 1988 ; in revised form October 5, 1988)
Summary Some improvements of the Pt/Al,O, catalyst caused by such rare earth metals as yttrium and ytterbium were observed in the conversion of methylcyclopentane (MCP) and n-hexane (nC,). The rare earth metals facilitated the dispersion of ensembles on the catalyst surface, thus favoring aromatization and isomerization. The method of preparation was found to affect the properties of the catalysts, and Pt/Al*Os and PtYb/Al,O, showed a remarkable difference in activity and selectivity after presulfidation.
1. Introduction Bimetallic reforming catalysts, such as PtRe/Al,Os, PtSn/A1203, and PtIr/Al,Os have been widely studied [l - lo], but the effects of rare earth metals on the catalytic properties of reforming catalysts have seldom been investigated. To extend the application of rare earth metals to catalytic reformation, various catalysts were prepared for this study on possible effects of rare earth elements on the catalytic properties of Pt/Al*Os, and various methods of preparation and presulfidation techniques were also investigated. We hope that this study will provide a scientific background for the development of a series of platinum rare earth catalysts.
2. Experimental details 2.1. Materials Catalysts were prepared by conventional incipient wetness impregnation with metallic salt precursors, and dried in air at 110 “C for 4 h before use. Three different procedures were used to prepare the catalysts. The co-supported catalysts, designated PtRE, were prepared by common coimpregnation methods. The physically mixed catalysts, designated Pt + RE, *Paper presented at the 18th Rare Earth Research Conference, September 12 - 16,1988. 0022-5088/89/$3.50
@ Elsevier Sequoia/Printed
Lake Geneva, WI,
in The Netherlands
400
were made by impregnating y-Al,Oa separately with solutions of H,PtCl, and RECls in water first, and then mixing equal amounts of the impregnated supports [ 111. The partly co-supported catalysts, designated PtRE + A1203, were prepared by impregnating +y-Al,Os and then mixing it with unimpregnated support [ 121. The active metal loadings were 0.3 wt.% and all catalysts contained 0.6 wt.% Cl. The catalysts used are presented here as follows: (1) platinum 100 mg 0.3 wt.% Pt/Al,Oa (2) PtYb 100 mg 0.3 wt.% Pt-0.3 wt.% Yb/A1203 (3) PtY 100 mg 0.3 wt.% Pt-0.3 wt.% Y/A1,03 (4) Pt + Yb 50 mg 0.6 wt.% Pt/A120s + 50 mg 0.6 wt.% Yb/Al,Os (5) PtYb + Al,O, 50 mg 0.6 wt.% Pt-0.6 wt.% Yb/A1203 + 50 mg Al,Os 2.2. Apparatus and procedures Catalysts were examined for the conversion of hydrocarbons in hydrogen in a pulse microcatalytic reactor operated at atmospheric pressure and temperatures of 480 and 500 “C. The reaction products were analyzed by an on-line type 100 gas chromatograph, and peak areas were determined using a CDMC-1 integrator. Microstructural characterization was carried out with a JEM 200 cx electron microscope operating at 200 keV. All catalysts were reduced in flowing hydrogen at 500 “C for 2 h in the reactor before injection of the reagents. Where applicable, the catalyst presulfidation was carried out in situ such that sufficient C$ was injected into the reactor to poison the entire surface of the catalyst. The catalysts were tested hourly until the activity and selectivity were unchanged.
3. Results and discussion 3.1. Experiments investigating the effect of rare earth metals We prepared two catalysts for the conversion of methylcyclopentane (MCP). The results are listed in Table 1. The results show that PtY displays a higher selectivity for both isomerization and aromatization. The two catalysts do not differ significantly in the conversion of MCP. As the selectivity patterns of the platinum and TABLE 1 MCP Conversion at 500 “C for Pt and PtY catalysts Catalyst
Pt PtY
Conversion (mol.%) 98.1 95.4
Selectivity (mol.%) Sh
4
41.3 29.3
14.9 23.5
43.8 47.2
Fig. 1. TEM of the platinum and PtY catalysts (X200 000 magnification). (b) PtY.
(a) platinum,
PtY catalysts are markedly different, it is suggested that differences in their geometric structures are responsible for their characteristic performances. To prove this, results of the transmission electron microscopy study of the two catalysts are shown in Fig. 1. It is observed that the ensembles on the surface of PtY are much finer g-rained than those on the platinum catalyst, suggesting that rare earth metals make platinum ensembles on the surface much easier to disperse, in favor of aromatization and isomerization. 3.2. Experiments investigating the effect of preparation methods Table 2 gives the results of the conversion of n-hexane at 480 “C on the three catalysts prepared by different methods. It can be seen that PtYb + A1203 shows the highest selectivity for aromatization, and that the conversion of PtYb f Al,Os is almost as high as that of PtYb. The physically mixed Pt + Yb shows the lowest activity and highest selectivity for hydrogenolysis. The isomerization selectivity remains essentially unchanged for all three catalysts. The differences in activity and selectivity among these catalysts imply that some interaction of platinum with ytterbium is responsible for the characteristic behavior of the catalysts, but details of the interaction remain unknown. Further studies are needed to interpret these differences.
TABLE 2 n-Hexane conversion at 480 “C on PtYb, PtYb + A1203 and Pt + Yb catalysts Catalyst
PtYb PtYb + A1203 Pt + Yb
Conversion (mol.%) 60.5 59.6 51.2
Selectivity
(mol.%)
Sh
si
S,
27.2 22.9 34.5
43.0 41.5 41.8
29.8 35.6 23.1
402
3.3. Experiments investigating the effect of sulfur In Fig. 2, the relative activity as a function of time on-stream of a presulfided platinum catalyst is compared with that of a presulfided PtYb catalyst for the conversion of n-hexane at 480 “C. The relative activity is defined as the ratio of the activity of a presulfided catalyst to the activity of the same catalyst unsulfided. The relative activity of PtYb attains almost loo%, remaining unchanged after 4 h of sulfur poisoning, whereas the platinum catalyst attains a relative activity of only 70% under the same conditions. As shown in Fig. 3, the relative selectivity patterns of the two catalysts display striking differences after presulfidation. Presulfiding the platinum catalyst dramatically reduces the selectivity for hydrogenolysis, increases the selectivity for aromatization, and the selectivity for isomerization remains essentially unchanged. It is interesting to note that the selectivity patterns of the PtYb catalyst remain unchanged upon presulfidation. These results suggest that sulfur atoms can be strongly or weakly held on the surface of the platinum catalyst. The strongly held sulfur, which cannot be removed under the reaction conditions, covers some of the active centers of the platinum catalyst, causing decreases in catalytic activity. The changes in the selectivities observed on the presulfided platinum catalyst can be rationalized by using the argument that the strongly held sulfur divides the bare
Fig. 2. Relative activity us. time on-stream for presulfided platinum and presulfided PtYb (0, platinum; A, PtYb).
*
/
1 Tim.
3 thl
4
5
/
z
3
4
TJ’me(/I)
5
O/2345
7-IWC (h)
Fig. 3. Relative selectivities vs. time on-stream for presulfided platinum and presulfided PtYb (0, platinum;A, PtYb).
403
metal surface into small ensembles which favor structure-insensitive reactions, such as aromatization and isomerization. On the surface of the PtYb catalyst, however, sulfur can be only weakly held, and can easily be removed under the reaction conditions. This may explain why the activity and selectivity of PtYb are not affected by presulfidation.
Acknowledgment We thank Dr. Hu Mei-Sheng for providing
us with the TEM data.
References 1 P. Biloon, J. N. Hellon, H. Verbeek, F. M. Dautgenberg and W. M. H. Sachtler, J. Catal., 63 (1980) 112. 2 F. M. Dantgenberg, J. N. Hellen, P. Bilogen and W. M. H. Sachtler, J. Catal., 63 (1980) 119. 3 R. Burth,J. Cotal., 71 (1981) 348. 4 R. Burth and L. C. Garla, J. Catd., 71 (1981) 360. 5 A. N. Webb, J. Catd., 39 (1975) 485. 6 B. D. McNicol, J. Catal., 46 (1977) 438. 7 P. G. Menon and G. F. Forment,J. Mol. Catal., 25 (1984) 59. 8 L. W. Jossens and E. F. Petersen, J. Catal., 76 (1982) 265. 9 R. W. Coughlin, K. Kawakami and A. Hasan, J. Catal., 88 (1984) 150. 10 B. H. Isaacs and E. E. Petersen, J. Catol., 85 (1984) 8. 11 K. S. Victor, B. B. John and W. M. H. Sachtler, J. Catal., 96 (1985) 371. 12 U.S. Patent, 3 925 196 (1975).
Metals, 148 (1989) 399 - 403
399
THE EFFECTS OF SOME RARE EARTH METALS ON THE CATALYTIC PROPERTIES OF REFORMING CATALYSTS* ZHU GUANGZHONG, Department
LI JIANHUI, SHI QIUJIE and LI FENGYI
of Chemistry, Jiangxi University, Nanchang (China)
(Received July 8, 1988 ; in revised form October 5, 1988)
Summary Some improvements of the Pt/Al,O, catalyst caused by such rare earth metals as yttrium and ytterbium were observed in the conversion of methylcyclopentane (MCP) and n-hexane (nC,). The rare earth metals facilitated the dispersion of ensembles on the catalyst surface, thus favoring aromatization and isomerization. The method of preparation was found to affect the properties of the catalysts, and Pt/Al*Os and PtYb/Al,O, showed a remarkable difference in activity and selectivity after presulfidation.
1. Introduction Bimetallic reforming catalysts, such as PtRe/Al,Os, PtSn/A1203, and PtIr/Al,Os have been widely studied [l - lo], but the effects of rare earth metals on the catalytic properties of reforming catalysts have seldom been investigated. To extend the application of rare earth metals to catalytic reformation, various catalysts were prepared for this study on possible effects of rare earth elements on the catalytic properties of Pt/Al*Os, and various methods of preparation and presulfidation techniques were also investigated. We hope that this study will provide a scientific background for the development of a series of platinum rare earth catalysts.
2. Experimental details 2.1. Materials Catalysts were prepared by conventional incipient wetness impregnation with metallic salt precursors, and dried in air at 110 “C for 4 h before use. Three different procedures were used to prepare the catalysts. The co-supported catalysts, designated PtRE, were prepared by common coimpregnation methods. The physically mixed catalysts, designated Pt + RE, *Paper presented at the 18th Rare Earth Research Conference, September 12 - 16,1988. 0022-5088/89/$3.50
@ Elsevier Sequoia/Printed
Lake Geneva, WI,
in The Netherlands
400
were made by impregnating y-Al,Oa separately with solutions of H,PtCl, and RECls in water first, and then mixing equal amounts of the impregnated supports [ 111. The partly co-supported catalysts, designated PtRE + A1203, were prepared by impregnating +y-Al,Os and then mixing it with unimpregnated support [ 121. The active metal loadings were 0.3 wt.% and all catalysts contained 0.6 wt.% Cl. The catalysts used are presented here as follows: (1) platinum 100 mg 0.3 wt.% Pt/Al,Oa (2) PtYb 100 mg 0.3 wt.% Pt-0.3 wt.% Yb/A1203 (3) PtY 100 mg 0.3 wt.% Pt-0.3 wt.% Y/A1,03 (4) Pt + Yb 50 mg 0.6 wt.% Pt/A120s + 50 mg 0.6 wt.% Yb/Al,Os (5) PtYb + Al,O, 50 mg 0.6 wt.% Pt-0.6 wt.% Yb/A1203 + 50 mg Al,Os 2.2. Apparatus and procedures Catalysts were examined for the conversion of hydrocarbons in hydrogen in a pulse microcatalytic reactor operated at atmospheric pressure and temperatures of 480 and 500 “C. The reaction products were analyzed by an on-line type 100 gas chromatograph, and peak areas were determined using a CDMC-1 integrator. Microstructural characterization was carried out with a JEM 200 cx electron microscope operating at 200 keV. All catalysts were reduced in flowing hydrogen at 500 “C for 2 h in the reactor before injection of the reagents. Where applicable, the catalyst presulfidation was carried out in situ such that sufficient C$ was injected into the reactor to poison the entire surface of the catalyst. The catalysts were tested hourly until the activity and selectivity were unchanged.
3. Results and discussion 3.1. Experiments investigating the effect of rare earth metals We prepared two catalysts for the conversion of methylcyclopentane (MCP). The results are listed in Table 1. The results show that PtY displays a higher selectivity for both isomerization and aromatization. The two catalysts do not differ significantly in the conversion of MCP. As the selectivity patterns of the platinum and TABLE 1 MCP Conversion at 500 “C for Pt and PtY catalysts Catalyst
Pt PtY
Conversion (mol.%) 98.1 95.4
Selectivity (mol.%) Sh
4
41.3 29.3
14.9 23.5
43.8 47.2
Fig. 1. TEM of the platinum and PtY catalysts (X200 000 magnification). (b) PtY.
(a) platinum,
PtY catalysts are markedly different, it is suggested that differences in their geometric structures are responsible for their characteristic performances. To prove this, results of the transmission electron microscopy study of the two catalysts are shown in Fig. 1. It is observed that the ensembles on the surface of PtY are much finer g-rained than those on the platinum catalyst, suggesting that rare earth metals make platinum ensembles on the surface much easier to disperse, in favor of aromatization and isomerization. 3.2. Experiments investigating the effect of preparation methods Table 2 gives the results of the conversion of n-hexane at 480 “C on the three catalysts prepared by different methods. It can be seen that PtYb + A1203 shows the highest selectivity for aromatization, and that the conversion of PtYb f Al,Os is almost as high as that of PtYb. The physically mixed Pt + Yb shows the lowest activity and highest selectivity for hydrogenolysis. The isomerization selectivity remains essentially unchanged for all three catalysts. The differences in activity and selectivity among these catalysts imply that some interaction of platinum with ytterbium is responsible for the characteristic behavior of the catalysts, but details of the interaction remain unknown. Further studies are needed to interpret these differences.
TABLE 2 n-Hexane conversion at 480 “C on PtYb, PtYb + A1203 and Pt + Yb catalysts Catalyst
PtYb PtYb + A1203 Pt + Yb
Conversion (mol.%) 60.5 59.6 51.2
Selectivity
(mol.%)
Sh
si
S,
27.2 22.9 34.5
43.0 41.5 41.8
29.8 35.6 23.1
402
3.3. Experiments investigating the effect of sulfur In Fig. 2, the relative activity as a function of time on-stream of a presulfided platinum catalyst is compared with that of a presulfided PtYb catalyst for the conversion of n-hexane at 480 “C. The relative activity is defined as the ratio of the activity of a presulfided catalyst to the activity of the same catalyst unsulfided. The relative activity of PtYb attains almost loo%, remaining unchanged after 4 h of sulfur poisoning, whereas the platinum catalyst attains a relative activity of only 70% under the same conditions. As shown in Fig. 3, the relative selectivity patterns of the two catalysts display striking differences after presulfidation. Presulfiding the platinum catalyst dramatically reduces the selectivity for hydrogenolysis, increases the selectivity for aromatization, and the selectivity for isomerization remains essentially unchanged. It is interesting to note that the selectivity patterns of the PtYb catalyst remain unchanged upon presulfidation. These results suggest that sulfur atoms can be strongly or weakly held on the surface of the platinum catalyst. The strongly held sulfur, which cannot be removed under the reaction conditions, covers some of the active centers of the platinum catalyst, causing decreases in catalytic activity. The changes in the selectivities observed on the presulfided platinum catalyst can be rationalized by using the argument that the strongly held sulfur divides the bare
Fig. 2. Relative activity us. time on-stream for presulfided platinum and presulfided PtYb (0, platinum; A, PtYb).
*
/
1 Tim.
3 thl
4
5
/
z
3
4
TJ’me(/I)
5
O/2345
7-IWC (h)
Fig. 3. Relative selectivities vs. time on-stream for presulfided platinum and presulfided PtYb (0, platinum;A, PtYb).
403
metal surface into small ensembles which favor structure-insensitive reactions, such as aromatization and isomerization. On the surface of the PtYb catalyst, however, sulfur can be only weakly held, and can easily be removed under the reaction conditions. This may explain why the activity and selectivity of PtYb are not affected by presulfidation.
Acknowledgment We thank Dr. Hu Mei-Sheng for providing
us with the TEM data.
References 1 P. Biloon, J. N. Hellon, H. Verbeek, F. M. Dautgenberg and W. M. H. Sachtler, J. Catal., 63 (1980) 112. 2 F. M. Dantgenberg, J. N. Hellen, P. Bilogen and W. M. H. Sachtler, J. Catal., 63 (1980) 119. 3 R. Burth,J. Cotal., 71 (1981) 348. 4 R. Burth and L. C. Garla, J. Catd., 71 (1981) 360. 5 A. N. Webb, J. Catd., 39 (1975) 485. 6 B. D. McNicol, J. Catal., 46 (1977) 438. 7 P. G. Menon and G. F. Forment,J. Mol. Catal., 25 (1984) 59. 8 L. W. Jossens and E. F. Petersen, J. Catal., 76 (1982) 265. 9 R. W. Coughlin, K. Kawakami and A. Hasan, J. Catal., 88 (1984) 150. 10 B. H. Isaacs and E. E. Petersen, J. Catol., 85 (1984) 8. 11 K. S. Victor, B. B. John and W. M. H. Sachtler, J. Catal., 96 (1985) 371. 12 U.S. Patent, 3 925 196 (1975).