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Investigation of alternative support materials for monometallic and bimetallic catalysts of CO hydrogenation
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 rights reserved.
3723
Investigation of alternative support materials for monometallic and bimetallic catalysts of CO hydrogenation T. Btilbtil a, A. i. l~lia , A. E. Aksoylu b and Z. i. Onsana(*) aDepartment of Chemical Engineering, Bo~azigi University, 80815 Bebek, Istanbul, Turkey bDepartment of Chemical Engineering, University of Porto, 4099 Porto Codex, Portugal Various Ni and Ni-Mo catalysts were prepared by using activated carbon and carboncovered alumina a~ alternative support materials9 Their performance in CO hydrogenation was compared with alumina-supported catalysts with similar Ni and Ni-Mo loadings. 1. INTRODUCTION A large part of the raw materials used in the chemical industry consist of lower olefins which are at present produced from petroleum. An alternative route is the conversion of coal into synthesis gas (CO+H2), liquefied fuels and lower olefins via the Fischer-Tropsch reaction9 Fischer-Tropsch synthesis (FTS) is a promising option for the production of chemicals from CO feedstock; however, an increase in the yield of lower hydrocarbons over the AndersonSchulz-Flory distribution requires the design and development of new, highly selective catalysts [ 1, 2]. Catalytic CO hydrogenation over supported transition metals has been studied for many years. A number of recent studies indicate that the FTS activities and hydrocarbon selectivities of Co, Ni, Fe, Ru and Mo can vary over orders of magnitude depending on the type of catalyst support used [3 ]. Alumina is the most widely used support material for commercial Co, Ni, Mo catalysts9 Notable features of alumina supports are their ability to disperse up to 20 wt% of the active metal phase and good mechanical properties. The strong metal-support interactions that occur with alumina at low metal loadings may impede desirable reactions. Moreover, Co and Ni may interact with the alumina to form COA1204 or NiAI204, or they can occupy octahedral or tetrahedral sites inside the alumina lattice, both of which render them catalytically inactive [4] 9 Carbon supports have also been used with classical FT metals; carbons are considered to be inert with very weak metal-support interactions [5, 6]. Results reported in the literature show that carbon is a promising support for CO hydrogenation [6-12]. However, the rather low density, extensive microporosity and poor mechanical strength of carbon materials constitute important drawbacks in their use as catalyst supports in industrial applications.
*Corresponding author. Fax. (+90 212) 2872460; e-mail: [email protected]; Research grants: Bo~azigi University Research Fund projects 99A504 and DPT-97K120640.
3724 Since both alumina and carbon have advantages and disadvantages, a new support material that merges the attributes of both materials is worth exploring. If the alumina surface is coated with a thin layer of carbon, the resulting support may inherit the favorable physical properties of alumina while also keeping the hydrogenation activity and reduced coking propensity associated with carbon [4]. In this study, various monometallic Ni and bimetallic Ni-Mo catalysts were prepared by using activated carbon (AC) and carbon-covered alumina (CCA) as support material, (a) to investigate catalyst preparation and FT reaction conditions and (b) to compare their catalytic performance in CO hydrogenation with the corresponding alumina supported Ni and Ni-Mo catalysts that were characterized previously [13-15]. 2. EXPERIMENTAL
Two AC supported monometallic catalysts with 5 wt% and 10 wt% Ni loading and two AC supported bimetallic catalysts with 5wt%Ni-5wt%Mo and 10wt%Ni-5wt%Mo loading were prepared by impregnation and coimpregnation respectively, using the incipient wetness technique. The commercially available activated carbon support NORIT ROX 0.8 was ground to 250-344 ~tm particle size, dried at 378 K for 16 h and impregnated with aqueous nickel nitrate and ammonium heptamolybdate solutions under vacuum in an ultrasound mixer to ensure uniform distribution of the active components. The catalysts were then dried at 378 K for 16 h, subjected to heat treatment at 673 K under helium for 2 h and reduced at 623 K under hydrogen for 14 h. The CCA support was prepared by pyrolyzing propene at 673 K [16]. Approximately 4 g of ~{-A1203 with 250-344 gm particle size was placed in a 10 mm i.d. stainless steel reactor and heated up to 673 K at a heating rate of 10 K/min under 100 cm3/min (NTP) nitrogen flow. The gas flow was then switched either to a propene-helium mixture or to pure propene. The pyrolysis reaction was conducted for 2 h with a pure propene stream flowing at 100 cm3/min (NTP) or for 6 h with a 15 vol% propene-helium mixture at the same flowrate in the preparation of CCA1 and CCA2 respectively. The CCA support was cooled down to ambient temperature under nitrogen flow, taken out from the reactor and dried at 378 K for 16 h. Two CCA1 supported bimetallic catalysts with 5wt%Ni-5wt%Mo and 10wt%Ni-5wt%Mo loading and one CCA2 supported 10wt%Ni-5wt%Mo catalyst were then prepared by coimpregnation to incipient wetness in the same way as the AC supported catalysts. The total surface areas (TSA) of the samples were measured by nitrogen adsorption from He-N2 mixtures using a multipoint technique in a Micromeritics Flowsorb II-2300 system. The metal surface areas (MSA) of the reduced samples were determined by irreversible CO adsorption using a chromatographic pulse technique in a Hewlett Packard 5890 Series II gas chromatograph fitted with a TCD and a 60 cm long 3 mm i.d. stainless steel column packed with catalyst particles. The experimental and calculational procedures for MSA determination in monometallic and bimetallic catalysts have been described previously [ 13, 17]. CO hydrogenation tests were carried out at atmospheric pressure in the 498-623 K temperature range, using an externally heated 3.5 mm i.d. stainless steel fixed-bed down-flow microreactor containing 270+2 mg of catalyst sample. All reactant gas flowrates were controlled by Omega mass flow controllers, and the reduction of the fresh catalyst sample was
3725
carried out in situ before each run. Blank tests showed that the AC or the CCA support alone was not catalytically active under the reaction conditions. 3. RESULTS AND DISCUSSION
AC supported Ni and Ni-Mo catalysts and CCA supported Ni-Mo catalysts each having two different combinations of metal loading were tested by CO hydrogenation to study the metal-support interaction by comparing their catalytic performance with y-A1203 supported catalysts having similar Ni or Ni-Mo content. A space velocity of 6.2 cm3/g-s was used in the reaction experiments to guarantee low conversion levels and the absence of transport effects. The results of CO hydrogenation tests conducted in the 498-623 K temperature range with 10 mol% CO concentration and H2/CO = 2 in the feed are presented, and the effect of support material is discussed in terms of activity, specific activity and C2-C3 hydrocarbons selectivity. 3.1. Effect of support material on catalyst physical properties The metal loadings of the AC and CCA supported samples and the results obtained from total surface area and metal surface area measurements are presented in Table 1 together with the values reported for A1203 supported Ni and Ni-Mo catalysts [ 13]. The TSA values for both A1203 and AC supported catalyst samples decrease with increasing total metal loading, indicating the filling up of the pores in the supports. The TSA of the CCA support is expected to be lower than that of the A1203 support alone, since the A1203 used in CCA preparation was the same and the weight gain after carbon coating with pure propene was approximately 6%. The MSA of both the A1203 and the AC supported Ni-Mo samples show an almost linear increase with the doubling of the Ni content. It has been reported that the MoO• species do not chemisorb CO, and a reduction temperature of 623 K is rather low for the molybdenum oxides [14]. A comparison of the MSA values for Ni/A1203 and Ni-Mo/Al203 samples does, however, indicate the textural promotion by MoOx species leading to increases in Ni particle dispersion[ 13]. Metal surface areas for 5wt%Ni-5wt%Mo/AC and 10wt%Ni-5wt%Mo/AC are well below those for 5wt%Ni-5wt%Mo/A1203 and 10wt%Ni-5wt%Mo/A1203 respectively, despite the much larger total surface areas provided by the AC support. There are two possible explanations for the MSA values of AC-supported samples: (i) the Ni sites anchored in the meso-micro pores of AC can easily be entrapped by the blockage of pore mouths during reduction and thus become inaccessible; this explanation is supported by the fact that oxygen
Table 1 Catalyst composition and physical properties AC Support ]CCA1 Support Metal (wt %) I y-A1203Support [7] [ Ni Mo TSA (+2%) MSA (+3%) TSA (+2%) MSA (+3%) MSA (+3%) (mZ/g) (m2/g) (m2/g) (m2/g) (m2/g) 0 5 5 10 10
0 0 5 0 5
218 193 189 186 166
0 0.57 1.03 1.60 2.17
958 856 830 829 761
0 nd 0.48 nd 0.88
0 nd nd nd 1.99
3726 bearing groups, which are the anchoring sites for the metallic species, are well distributed on the interior sites of AC grains, and (ii) the number of anchoring sites for Ni and Mo precursors on the AC support are fewer; it is reported that the number of these oxygen bearing surface groups leading to higher dispersion can be increased when industrial AC supports are oxidized prior to the impregnation step [18, 19]. On the other hand, since alumina is dominant in determining the physical properties of CCA supports, they do not possess high microporosity and are free from the blockage problem encountered with AC supports. The MSA value determined for 10wt%Ni-5wt%Mo/CCA1 is close to that measured for the corresponding 10wt%Ni-5wt%Mo/Al203 sample, indicating a significant difference between the new composite support CCA and AC and showing that the effect of the lower TSA of CCA (compared to A1203) on the MSA of the active metal is rather limited.
3.2. Effect of support on activity, specific activity and selectivity The performance of Ni and Ni-Mo catalysts supported on AC, ~,-A1203 and CCA are discussed and compared in terms of activity (total hydrocarbon production), specific activity (total hydrocarbon production per metal surface area) and C2-C3 hydrocarbon selectivity. The characterization of AC supported monometallic Ni and bimetallic Ni-Mo catalysts was carried out first. In this case, CO hydrogenation was studied at five different temperatures in the 498-623 K range, using monometallic catalysts with 5wt% and 10wt% Ni loading and bimetallic catalysts with 5wt%Ni-5wt%Mo and 10wt%Ni-5wt%Mo loading. The effect of temperature on activity is presented in Figure 1. Methane production starts at about 573 K on monometallic Ni catalysts, and small quantities of ethane appear at 623 K. Introduction of Mo has a distinct effect on both activity and product distribution in that methane and some ethane are produced even at 498 K on the bimetallic Ni-Mo catalysts. 10wt%Ni-5wt% Mo/AC even starts to produce some propane at 623 K. This increase in activity may be explained in
0
300
"x~ 250 i
_.i~5wt% Ni
05
--m--lOwt% Ni ._ 5wt%Ni_5wtO/oM~
~ 200 .
"~ 150
x _IOwtO/oNi,Swt%Mo
/
J
/
.~_~, 100 9=>
~ [ "
o
<
50
0 [ 480
_
"
. . . . .
.
. . 520
.
.
. 560 Temperature, K
600
640
Fig. 1. Effect of temperature on the activity of AC supported catalysts terms of the stabilization of active Ni sites by the presence of MoOx species as well as by the electronic interaction between Ni and MoO• which enhances the polymerization activity of the Ni sites leading to C2 and some C3 production. When compared with similar A1203 supported
3727 Ni and Ni-Mo catalysts [14, 15], both the activities and the specific activities obtained with the AC support are much lower in the 498-548 K temperature range. The fall in both the total activity and the site activity may be a result of the extensive microporosity and the very high TSA of the AC support, both of which tend to decrease the proximity of the active sites and hence the synergetic Ni-MoOx interaction. Moreover, the decrease in activity may partly be caused by the acidity of the support which can reduce CO chemisorption and thus lower catalyst activity [20] and by the sulphur content of the NORIT ROX support, reported to reach 0.7 wt%, which can lead to poisoning of the active Ni centers. A comparison of the activity, specific activity and C2-C3 hydrocarbon selectivity of Ni-Mo catalysts on different supports at the reaction temperature of 548 K is presented in Table 2, which shows that the C2-C3 Table 2 Effect of support on activity, specific activity and C2-C3 selectivity (548 K, 2 h on stream) Support [ 5 wt%Ni-5 wt%Mo I 10 wt%Ni-5 wt%Mo A SA C2-C3 Sel. A SA C2-C3 Sel. AC 59 123 0.07 74 84 0.05 A1203 [8] 252 245 0.23 454 209 0.17 CCA1 442 nd 0.33 919 462 0.13 A: activity, gmol/g-s (x100); SA: specific activity, ~tmol/m2-s (x100)
selectivity also suffers from the diminished level of the synergetic interaction since the production of C2+ hydrocarbons requires higher polymerization activity on the Ni sites. The total activity, specific activity and C2-C3 hydrocarbon selectivity of the CCA1 supported Ni-Mo catalysts are higher than even those of the A1203 supported catalysts. This can be explained by the inertness of the carbon layer formed leading to enhanced interaction between the Ni and MoO• species by excluding the Ni-A1203 interaction that can weaken the synergetic interaction between the metallic sites. Furthermore, there is the advantage provided by the moderate total surface area similar to A1203, allowing the Ni and MoOx sites to be close enough for the bimetallic interaction It must also be noted that the zero sulphur content of the carbon layer formed by the pyrolysis of research grade propene eliminates the possibility of sulphur poisoning, thus contributing to the higher total activities and site activities observed on the CCA1 supported samples. A pronounced decrease is observed in the C2-C3 selectivity of CCA1 supported catalysts with increasing Ni content when compared to the corresponding A1203 supported samples. The rather high site activity of 10wt%Ni-5wt%Mo catalyst may be one of the reasons for this observation, and the increase in the polymerization activity of the Ni sites may be veiled by the drastic increase in their methanation activity. Another reaction experiment conducted over 10wt%Ni-5wt%Mo supported on CCA2, prepared by using a 15 vol% propene-helium mixture in the pyrolysis step, aimed to compare the effect of different preparation procedures on carbon deposition, activity and selectivity. The weight gain of the A1203 support was somewhat lower in the case of CCA2, and this catalyst exhibited lower total activity than 10wt%Ni-5wt%Mo/CCA1, 7.58 gmol/g-s (xl00 for comparison with Table 2), but much higher C2-C3 selectivity, 0.39. A comparison of the results obtained on CCA1 and CCA2 supported Ni-Mo catalyst indicates that the preparation
3728 procedure for carbon-covered alumina can be manipulated according to the particular activityselectivity characteristics targeted.
4. CONCLUSIONS
The metal surface areas measured for AC supported Ni-Mo catalysts are well below the values for the A1203 supported catalysts, despite the extensive total surface area provided by the AC support, while the MSA determined for the corresponding CCA supported catalyst is close to that of the Ni-Mo/AI203 sample. When compared with similar A1203 supported Ni and Ni-Mo catalysts, both the activities and specific activities of the AC supported samples are much lower, while the total activity, specific activity and C2-C3 hydrocarbon selectivity of the CCA supported Ni-Mo catalysts are higher. The results of this study indicate that the CCA support can be tailored to obtain the desired activity-selectivity characteristics by adjusting the preparation procedure. Acknowledgement. The contributions and suggestions of Dr. A. N. Akin and M. D6nmez are gratefully acknowledged. REFERENCES
1. M. Janardanarao, Ind. Eng. Chem. Res., 29 (1990) 1735. 2. R. Snel, Catal. Rev. Sci. Eng., 29 (1987) 361. 3. L. Guczi (ed), New Trends in CO Activation, Studies in Surface Science and Catalysis, Vol. 64, pp. 159- , Elsevier, 1991. 4. P.M. Boorman, K. Chong, R. A. Kydd, and J. M. Lewis, J. Catal., 128 (1991) 537. 5. J. Van Doom, P. Staugaard and J. A. Moulijn, Appl. Catal., 48 (1988) 253. 6. J.W. Dun, E. Gulari and K. Y. S. Ng, Appl. Catal., 15 (1985) 247. 7. A.P.B. Sommen, F. Stoop, K. van der Wiele, Appl. Catal., 14 (1985) 277. 8. F. Rodriguez-Reinoso, J. D. Lopez-Gonzales, C. Moreno-Castilla, A. Guerrero-Ruiz, I. Rodriguez-Ramos, Fuel, 63 (1984) 1089. 9. F. Rodriguez-Reinoso, I. Rodriguez-Ramos, A. Guerrero-Ruiz, J. D. Lopez-Gonzales, Appl. Catal., 21 (1986) 251. 10. J. Barrault, A. Guillemiout, J. C. Achard, V. Paul-Boncour, A. Percheron-Guegan, Appl. Catal., 21 (1986) 307. 11. J. Venter, M. Kaminsky, G. L. Geoffroy, M. A. Vannice, J. Catal., 103 (1987) 450. 12..J. Venter, M. Kaminsky, G. L. Geoffroy, M. A. Vannice, J. Catal., 105 (1987) 155. 13. A. E. Aksoylu, Z. Mlslrh and Z. i. Onsan, Appl. Catal. A, 168 (1998) 385. 14. A. E. Aksoylu and Z. i. Onsan, Appl. Catal. A, 168 (1998) 399. 15. A. E. Aksoylu and Z. i. Onsan, Ind. Eng. Chem. Res., 37-6 (1998) 2397. 16. S. L. Butterworth and A. W. Scaroni, Appl. Catal., 16 (1985) 375. 17. A. E. Aksoylu and Z. i. Onsan, Appl. Catal. A, 164 (1997) 1. 18. J. L. Figueiredo, M. F.R. Pereira, M. M.A. Freitas, J. J. M. Orfao, Carbon, 37 (1999) 1379. 19. A. E. Aksoylu, M. M. A. Freitas, J. L. Figueiredo, Appl. Catal. A, in press. 20. S. Wang and G. Q. M. Lu, Carbon, 36-3 (1998) 283.
3723
Investigation of alternative support materials for monometallic and bimetallic catalysts of CO hydrogenation T. Btilbtil a, A. i. l~lia , A. E. Aksoylu b and Z. i. Onsana(*) aDepartment of Chemical Engineering, Bo~azigi University, 80815 Bebek, Istanbul, Turkey bDepartment of Chemical Engineering, University of Porto, 4099 Porto Codex, Portugal Various Ni and Ni-Mo catalysts were prepared by using activated carbon and carboncovered alumina a~ alternative support materials9 Their performance in CO hydrogenation was compared with alumina-supported catalysts with similar Ni and Ni-Mo loadings. 1. INTRODUCTION A large part of the raw materials used in the chemical industry consist of lower olefins which are at present produced from petroleum. An alternative route is the conversion of coal into synthesis gas (CO+H2), liquefied fuels and lower olefins via the Fischer-Tropsch reaction9 Fischer-Tropsch synthesis (FTS) is a promising option for the production of chemicals from CO feedstock; however, an increase in the yield of lower hydrocarbons over the AndersonSchulz-Flory distribution requires the design and development of new, highly selective catalysts [ 1, 2]. Catalytic CO hydrogenation over supported transition metals has been studied for many years. A number of recent studies indicate that the FTS activities and hydrocarbon selectivities of Co, Ni, Fe, Ru and Mo can vary over orders of magnitude depending on the type of catalyst support used [3 ]. Alumina is the most widely used support material for commercial Co, Ni, Mo catalysts9 Notable features of alumina supports are their ability to disperse up to 20 wt% of the active metal phase and good mechanical properties. The strong metal-support interactions that occur with alumina at low metal loadings may impede desirable reactions. Moreover, Co and Ni may interact with the alumina to form COA1204 or NiAI204, or they can occupy octahedral or tetrahedral sites inside the alumina lattice, both of which render them catalytically inactive [4] 9 Carbon supports have also been used with classical FT metals; carbons are considered to be inert with very weak metal-support interactions [5, 6]. Results reported in the literature show that carbon is a promising support for CO hydrogenation [6-12]. However, the rather low density, extensive microporosity and poor mechanical strength of carbon materials constitute important drawbacks in their use as catalyst supports in industrial applications.
*Corresponding author. Fax. (+90 212) 2872460; e-mail: [email protected]; Research grants: Bo~azigi University Research Fund projects 99A504 and DPT-97K120640.
3724 Since both alumina and carbon have advantages and disadvantages, a new support material that merges the attributes of both materials is worth exploring. If the alumina surface is coated with a thin layer of carbon, the resulting support may inherit the favorable physical properties of alumina while also keeping the hydrogenation activity and reduced coking propensity associated with carbon [4]. In this study, various monometallic Ni and bimetallic Ni-Mo catalysts were prepared by using activated carbon (AC) and carbon-covered alumina (CCA) as support material, (a) to investigate catalyst preparation and FT reaction conditions and (b) to compare their catalytic performance in CO hydrogenation with the corresponding alumina supported Ni and Ni-Mo catalysts that were characterized previously [13-15]. 2. EXPERIMENTAL
Two AC supported monometallic catalysts with 5 wt% and 10 wt% Ni loading and two AC supported bimetallic catalysts with 5wt%Ni-5wt%Mo and 10wt%Ni-5wt%Mo loading were prepared by impregnation and coimpregnation respectively, using the incipient wetness technique. The commercially available activated carbon support NORIT ROX 0.8 was ground to 250-344 ~tm particle size, dried at 378 K for 16 h and impregnated with aqueous nickel nitrate and ammonium heptamolybdate solutions under vacuum in an ultrasound mixer to ensure uniform distribution of the active components. The catalysts were then dried at 378 K for 16 h, subjected to heat treatment at 673 K under helium for 2 h and reduced at 623 K under hydrogen for 14 h. The CCA support was prepared by pyrolyzing propene at 673 K [16]. Approximately 4 g of ~{-A1203 with 250-344 gm particle size was placed in a 10 mm i.d. stainless steel reactor and heated up to 673 K at a heating rate of 10 K/min under 100 cm3/min (NTP) nitrogen flow. The gas flow was then switched either to a propene-helium mixture or to pure propene. The pyrolysis reaction was conducted for 2 h with a pure propene stream flowing at 100 cm3/min (NTP) or for 6 h with a 15 vol% propene-helium mixture at the same flowrate in the preparation of CCA1 and CCA2 respectively. The CCA support was cooled down to ambient temperature under nitrogen flow, taken out from the reactor and dried at 378 K for 16 h. Two CCA1 supported bimetallic catalysts with 5wt%Ni-5wt%Mo and 10wt%Ni-5wt%Mo loading and one CCA2 supported 10wt%Ni-5wt%Mo catalyst were then prepared by coimpregnation to incipient wetness in the same way as the AC supported catalysts. The total surface areas (TSA) of the samples were measured by nitrogen adsorption from He-N2 mixtures using a multipoint technique in a Micromeritics Flowsorb II-2300 system. The metal surface areas (MSA) of the reduced samples were determined by irreversible CO adsorption using a chromatographic pulse technique in a Hewlett Packard 5890 Series II gas chromatograph fitted with a TCD and a 60 cm long 3 mm i.d. stainless steel column packed with catalyst particles. The experimental and calculational procedures for MSA determination in monometallic and bimetallic catalysts have been described previously [ 13, 17]. CO hydrogenation tests were carried out at atmospheric pressure in the 498-623 K temperature range, using an externally heated 3.5 mm i.d. stainless steel fixed-bed down-flow microreactor containing 270+2 mg of catalyst sample. All reactant gas flowrates were controlled by Omega mass flow controllers, and the reduction of the fresh catalyst sample was
3725
carried out in situ before each run. Blank tests showed that the AC or the CCA support alone was not catalytically active under the reaction conditions. 3. RESULTS AND DISCUSSION
AC supported Ni and Ni-Mo catalysts and CCA supported Ni-Mo catalysts each having two different combinations of metal loading were tested by CO hydrogenation to study the metal-support interaction by comparing their catalytic performance with y-A1203 supported catalysts having similar Ni or Ni-Mo content. A space velocity of 6.2 cm3/g-s was used in the reaction experiments to guarantee low conversion levels and the absence of transport effects. The results of CO hydrogenation tests conducted in the 498-623 K temperature range with 10 mol% CO concentration and H2/CO = 2 in the feed are presented, and the effect of support material is discussed in terms of activity, specific activity and C2-C3 hydrocarbons selectivity. 3.1. Effect of support material on catalyst physical properties The metal loadings of the AC and CCA supported samples and the results obtained from total surface area and metal surface area measurements are presented in Table 1 together with the values reported for A1203 supported Ni and Ni-Mo catalysts [ 13]. The TSA values for both A1203 and AC supported catalyst samples decrease with increasing total metal loading, indicating the filling up of the pores in the supports. The TSA of the CCA support is expected to be lower than that of the A1203 support alone, since the A1203 used in CCA preparation was the same and the weight gain after carbon coating with pure propene was approximately 6%. The MSA of both the A1203 and the AC supported Ni-Mo samples show an almost linear increase with the doubling of the Ni content. It has been reported that the MoO• species do not chemisorb CO, and a reduction temperature of 623 K is rather low for the molybdenum oxides [14]. A comparison of the MSA values for Ni/A1203 and Ni-Mo/Al203 samples does, however, indicate the textural promotion by MoOx species leading to increases in Ni particle dispersion[ 13]. Metal surface areas for 5wt%Ni-5wt%Mo/AC and 10wt%Ni-5wt%Mo/AC are well below those for 5wt%Ni-5wt%Mo/A1203 and 10wt%Ni-5wt%Mo/A1203 respectively, despite the much larger total surface areas provided by the AC support. There are two possible explanations for the MSA values of AC-supported samples: (i) the Ni sites anchored in the meso-micro pores of AC can easily be entrapped by the blockage of pore mouths during reduction and thus become inaccessible; this explanation is supported by the fact that oxygen
Table 1 Catalyst composition and physical properties AC Support ]CCA1 Support Metal (wt %) I y-A1203Support [7] [ Ni Mo TSA (+2%) MSA (+3%) TSA (+2%) MSA (+3%) MSA (+3%) (mZ/g) (m2/g) (m2/g) (m2/g) (m2/g) 0 5 5 10 10
0 0 5 0 5
218 193 189 186 166
0 0.57 1.03 1.60 2.17
958 856 830 829 761
0 nd 0.48 nd 0.88
0 nd nd nd 1.99
3726 bearing groups, which are the anchoring sites for the metallic species, are well distributed on the interior sites of AC grains, and (ii) the number of anchoring sites for Ni and Mo precursors on the AC support are fewer; it is reported that the number of these oxygen bearing surface groups leading to higher dispersion can be increased when industrial AC supports are oxidized prior to the impregnation step [18, 19]. On the other hand, since alumina is dominant in determining the physical properties of CCA supports, they do not possess high microporosity and are free from the blockage problem encountered with AC supports. The MSA value determined for 10wt%Ni-5wt%Mo/CCA1 is close to that measured for the corresponding 10wt%Ni-5wt%Mo/Al203 sample, indicating a significant difference between the new composite support CCA and AC and showing that the effect of the lower TSA of CCA (compared to A1203) on the MSA of the active metal is rather limited.
3.2. Effect of support on activity, specific activity and selectivity The performance of Ni and Ni-Mo catalysts supported on AC, ~,-A1203 and CCA are discussed and compared in terms of activity (total hydrocarbon production), specific activity (total hydrocarbon production per metal surface area) and C2-C3 hydrocarbon selectivity. The characterization of AC supported monometallic Ni and bimetallic Ni-Mo catalysts was carried out first. In this case, CO hydrogenation was studied at five different temperatures in the 498-623 K range, using monometallic catalysts with 5wt% and 10wt% Ni loading and bimetallic catalysts with 5wt%Ni-5wt%Mo and 10wt%Ni-5wt%Mo loading. The effect of temperature on activity is presented in Figure 1. Methane production starts at about 573 K on monometallic Ni catalysts, and small quantities of ethane appear at 623 K. Introduction of Mo has a distinct effect on both activity and product distribution in that methane and some ethane are produced even at 498 K on the bimetallic Ni-Mo catalysts. 10wt%Ni-5wt% Mo/AC even starts to produce some propane at 623 K. This increase in activity may be explained in
0
300
"x~ 250 i
_.i~5wt% Ni
05
--m--lOwt% Ni ._ 5wt%Ni_5wtO/oM~
~ 200 .
"~ 150
x _IOwtO/oNi,Swt%Mo
/
J
/
.~_~, 100 9=>
~ [ "
o
<
50
0 [ 480
_
"
. . . . .
.
. . 520
.
.
. 560 Temperature, K
600
640
Fig. 1. Effect of temperature on the activity of AC supported catalysts terms of the stabilization of active Ni sites by the presence of MoOx species as well as by the electronic interaction between Ni and MoO• which enhances the polymerization activity of the Ni sites leading to C2 and some C3 production. When compared with similar A1203 supported
3727 Ni and Ni-Mo catalysts [14, 15], both the activities and the specific activities obtained with the AC support are much lower in the 498-548 K temperature range. The fall in both the total activity and the site activity may be a result of the extensive microporosity and the very high TSA of the AC support, both of which tend to decrease the proximity of the active sites and hence the synergetic Ni-MoOx interaction. Moreover, the decrease in activity may partly be caused by the acidity of the support which can reduce CO chemisorption and thus lower catalyst activity [20] and by the sulphur content of the NORIT ROX support, reported to reach 0.7 wt%, which can lead to poisoning of the active Ni centers. A comparison of the activity, specific activity and C2-C3 hydrocarbon selectivity of Ni-Mo catalysts on different supports at the reaction temperature of 548 K is presented in Table 2, which shows that the C2-C3 Table 2 Effect of support on activity, specific activity and C2-C3 selectivity (548 K, 2 h on stream) Support [ 5 wt%Ni-5 wt%Mo I 10 wt%Ni-5 wt%Mo A SA C2-C3 Sel. A SA C2-C3 Sel. AC 59 123 0.07 74 84 0.05 A1203 [8] 252 245 0.23 454 209 0.17 CCA1 442 nd 0.33 919 462 0.13 A: activity, gmol/g-s (x100); SA: specific activity, ~tmol/m2-s (x100)
selectivity also suffers from the diminished level of the synergetic interaction since the production of C2+ hydrocarbons requires higher polymerization activity on the Ni sites. The total activity, specific activity and C2-C3 hydrocarbon selectivity of the CCA1 supported Ni-Mo catalysts are higher than even those of the A1203 supported catalysts. This can be explained by the inertness of the carbon layer formed leading to enhanced interaction between the Ni and MoO• species by excluding the Ni-A1203 interaction that can weaken the synergetic interaction between the metallic sites. Furthermore, there is the advantage provided by the moderate total surface area similar to A1203, allowing the Ni and MoOx sites to be close enough for the bimetallic interaction It must also be noted that the zero sulphur content of the carbon layer formed by the pyrolysis of research grade propene eliminates the possibility of sulphur poisoning, thus contributing to the higher total activities and site activities observed on the CCA1 supported samples. A pronounced decrease is observed in the C2-C3 selectivity of CCA1 supported catalysts with increasing Ni content when compared to the corresponding A1203 supported samples. The rather high site activity of 10wt%Ni-5wt%Mo catalyst may be one of the reasons for this observation, and the increase in the polymerization activity of the Ni sites may be veiled by the drastic increase in their methanation activity. Another reaction experiment conducted over 10wt%Ni-5wt%Mo supported on CCA2, prepared by using a 15 vol% propene-helium mixture in the pyrolysis step, aimed to compare the effect of different preparation procedures on carbon deposition, activity and selectivity. The weight gain of the A1203 support was somewhat lower in the case of CCA2, and this catalyst exhibited lower total activity than 10wt%Ni-5wt%Mo/CCA1, 7.58 gmol/g-s (xl00 for comparison with Table 2), but much higher C2-C3 selectivity, 0.39. A comparison of the results obtained on CCA1 and CCA2 supported Ni-Mo catalyst indicates that the preparation
3728 procedure for carbon-covered alumina can be manipulated according to the particular activityselectivity characteristics targeted.
4. CONCLUSIONS
The metal surface areas measured for AC supported Ni-Mo catalysts are well below the values for the A1203 supported catalysts, despite the extensive total surface area provided by the AC support, while the MSA determined for the corresponding CCA supported catalyst is close to that of the Ni-Mo/AI203 sample. When compared with similar A1203 supported Ni and Ni-Mo catalysts, both the activities and specific activities of the AC supported samples are much lower, while the total activity, specific activity and C2-C3 hydrocarbon selectivity of the CCA supported Ni-Mo catalysts are higher. The results of this study indicate that the CCA support can be tailored to obtain the desired activity-selectivity characteristics by adjusting the preparation procedure. Acknowledgement. The contributions and suggestions of Dr. A. N. Akin and M. D6nmez are gratefully acknowledged. REFERENCES
1. M. Janardanarao, Ind. Eng. Chem. Res., 29 (1990) 1735. 2. R. Snel, Catal. Rev. Sci. Eng., 29 (1987) 361. 3. L. Guczi (ed), New Trends in CO Activation, Studies in Surface Science and Catalysis, Vol. 64, pp. 159- , Elsevier, 1991. 4. P.M. Boorman, K. Chong, R. A. Kydd, and J. M. Lewis, J. Catal., 128 (1991) 537. 5. J. Van Doom, P. Staugaard and J. A. Moulijn, Appl. Catal., 48 (1988) 253. 6. J.W. Dun, E. Gulari and K. Y. S. Ng, Appl. Catal., 15 (1985) 247. 7. A.P.B. Sommen, F. Stoop, K. van der Wiele, Appl. Catal., 14 (1985) 277. 8. F. Rodriguez-Reinoso, J. D. Lopez-Gonzales, C. Moreno-Castilla, A. Guerrero-Ruiz, I. Rodriguez-Ramos, Fuel, 63 (1984) 1089. 9. F. Rodriguez-Reinoso, I. Rodriguez-Ramos, A. Guerrero-Ruiz, J. D. Lopez-Gonzales, Appl. Catal., 21 (1986) 251. 10. J. Barrault, A. Guillemiout, J. C. Achard, V. Paul-Boncour, A. Percheron-Guegan, Appl. Catal., 21 (1986) 307. 11. J. Venter, M. Kaminsky, G. L. Geoffroy, M. A. Vannice, J. Catal., 103 (1987) 450. 12..J. Venter, M. Kaminsky, G. L. Geoffroy, M. A. Vannice, J. Catal., 105 (1987) 155. 13. A. E. Aksoylu, Z. Mlslrh and Z. i. Onsan, Appl. Catal. A, 168 (1998) 385. 14. A. E. Aksoylu and Z. i. Onsan, Appl. Catal. A, 168 (1998) 399. 15. A. E. Aksoylu and Z. i. Onsan, Ind. Eng. Chem. Res., 37-6 (1998) 2397. 16. S. L. Butterworth and A. W. Scaroni, Appl. Catal., 16 (1985) 375. 17. A. E. Aksoylu and Z. i. Onsan, Appl. Catal. A, 164 (1997) 1. 18. J. L. Figueiredo, M. F.R. Pereira, M. M.A. Freitas, J. J. M. Orfao, Carbon, 37 (1999) 1379. 19. A. E. Aksoylu, M. M. A. Freitas, J. L. Figueiredo, Appl. Catal. A, in press. 20. S. Wang and G. Q. M. Lu, Carbon, 36-3 (1998) 283.