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SiO2 catalysts in their behaviour in CO hydrogenation
KMNAL
OF
MOLECULAR CATALYSIS Journal of Molecular Catalysis 88 ( 1994) 213-222
ELSEVIER
Effect of the method of preparing Ru-Mo/Si02 catalysts in their behaviour in CO hydrogenation M. Josefina Perez Zurita, Irene S. Henriquez, Mireya R. Goldwasser, M.Luisa Cubeiro Centro de Catdlisis, Petroleo y Petroquimica. Escuela de Quimica, Facultad de Ciencias. Universidad Central de Venezuela, Apartado Postal 47102, Caracas, Venezuela
Geoffrey C. Bond* Department of Chemistry, Brunei University, Oxbridge, VB8 3PH, UK (Received June 10, 1993; accepted October 5, 1993)
Abstract Sequential impregnation of SiOZ by RuC13 and then by (N&) 6M07024, followed by drying and reduction in H, at 623 K, afforded catalysts in which, according to their benzene hydrogenation activity, the Ru particles were progressively covered by MO-containing species, the effect being greatest when RuCl, was introduced first. With catalysts of this type, in CO hydrogenation, oxygenate formation occurred at low MO content but was eliminated as the MO concentration was increased. When the MO salt was applied first to the support, oxygenate formation was greater and did not diminish significantly on raising the MO content. A semi-quantitative model is developed in which oxygenates are formed at Ru-MO sites; the hydrocarbon-forming process is not however much inhibited by the presence of MO, and may take place on MO species covering the Ru particles. Key words: carbon monoxide; hydrogenation;
molybdenum;
ruthenium:
silica
1. Introduction Much effort has been devoted to the synthesis of higher alcohols from CO+H2. two strategies in particular having been explored: ( 1) the addition to methanol-forming metals, particularly Cu, of components to promote chain-growth, and (2) the modification of hydrocarbon-forming metals, particularly Ru, by adding components that suppress CO *Corresponding
author. Fax. ( + 44-895) 256844.
0304-5102/94/$07.00 0 1994 Elsevier Science B.V. All rights reserved SSDI0304-5102(93)E0261-E
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dissociation and allow formation of higher oxygenates. There is a quite extensive literature concerning the modification of the properties of Ru in Fischer-Tropsch synthesis by changing the particle size [ l] or support [ 21 or using a transitional metal oxide [ 31 or other unusual material [4] as support. Alternatively, and more usefully, the properties of the metal can be adjusted by placing it on a conventional ‘inactive’ support and adding extra components in low concentration [ 5-101. In this way, the metal particle size and loading, the type and concentration of the modifier and the type of support can be independently altered, and the system fine-tuned to give the desired outturn: basic promoters as well as transition metal compounds can readily be incorporated [ 6,7]. However, despite this large effort, only a few papers [l-8] mention the formation of oxygenates by Ru catalysts. R&pillared clay catalysts form alcohols [ 41, but the more interesting papers are those by Inoue et al. [ 6,7] on Ru-Mo-Na/A1203 catalysts and those by Chen [ 31 who claimed that Ru-K/MOO, gave a high yield of straight-chain alcohols; this important observation seems to have been largely ignored. Foley et al. [ 51, in an extensive study of the promotion of Ru and Rh catalysts for Fischer-Tropsch synthesis, found some alcohol formation on a Ru-W/A1203 catalyst, although the amount was inferior to that given by Rh-Mo/Al,O, and Rh-W/Al*O,. Nevertheless, there is a clear suggestion from the literature that it is possible to modify the behaviour of Ru, to encourage the formation of higher oxygenates. Although models have been proposed [ 111 to explain the effect of promoters, there have been few systematic studies of how the method of preparation affects the resulting catalyst structure, and techniques for the rational construction of specific geometries are still in their infancy. We now report on a short investigation designed to show how the concentration of the promoter (MO) and the sequence in which the metal (Ru) and promoter are added to the support (SiO,) affect catalyst performance.
2. Experimental 2.1. Catalyst preparation and characterization Catalysts were prepared using Davidson 62 SiOZ (Grace) (355 m* g- ‘), RuC1,*xH20 (Aldrich) and ( NI-L,)&Io~O~~( Riedel-de-Haen) . For series A, Si02 was first impregnated with an aqueous solution of RuCl, by the incipient wetness method to give a nominal Ru content of 3%; the resultant wet powder was dried overnight at 383 K. Portions of this material were re-impregnated with solutions of the MO salt to give nominal MO contents of 1,3 and 5%, followed by overnight drying at 383 K. For series B, the sequence of impregnations was reversed. Catalysts are identified by codes which show the impregnation sequence and MO content: thus RuMo5 signifies the series A preparation having 5% MO. Materials containing 1 and 5% MO only were also prepared. The use of the element symbol implies nothing about its actual oxidation state. Catalysts were pre-reduced in situ with HZ for 4 h at 623 K. In one experiment, the Ru/ Si02 precursor was heated for 4 h at 623 K in N, before testing in the CO + H2 reaction. Chemical analyses were performed with an X-ray fluorescence analyser (KEVEX 0.700
M.J. Perez Zurita et al. /Journal of Molecular Catalysis 88 (1994) 213-222
215
Xes Control) and surface areas were measured by a single-point BET procedure (Micromcritics Rapid Surface Area Analyser 2200). CO chemisorption isotherms were measured in a glass high-vacuum system by the doubleisotherm method. Samples ( = 0.5 g) were reduced in situ in flowing Hz (60 cm3 min- ‘> for 3 h at 623 K and then evacuated for 1 h at the same temperature. Quoted values of CO uptake are those found at 100 Torr equilibrium pressure. 2.2. Catalytic tests Benzene hydrogenation was carried out at 403 K in a flow system at atmospheric pressure, using a mixture of 72.5 Torr benzene and 687.5 Torr Hz at a total flow-rate of 3.6 1 hh’. Activity for CO hydrogenation was measured at high pressure in a fixed-bed Aow microreactor; 1 g catalyst was used. Conditions were as follows: space velocity, 1246 cm3 g-’ h- ‘; pressure 3.45 MPa (500 psi) ; 573 K. The feed gas was a 1: 1 CO: H2 mixture with 5% N2 as internal standard. Gaseous components (CO, CO*, Nz CH,) were analysed by a Hewlett-Packard 3392A GC fitted with a CTR-1 column which comprised two concentric columns; the inner (‘/a in. diameter) contained molecular sieve, while the outer ( 1/4 in. diameter) had a mixture of Porapaks. Hydrocarbons were analysed on a Perkin Elmer Sygma 3B GC fitted with a 1.5 m Durapack n-octanelPorasi1 C column. Liquids collected after 24 h reaction were analysed by a Hewlett-Packard GC fitted with a Pona column, and having a mass spectrometer as detector.
3. Results 3.1. Chemical and physical characterization
The analysed Ru and MO contents, the BET surface areas and the CO uptakes at 100 Torr pressure are shown in Table 1. The compositions are close to, but as is usual with the impregnation method, are a little lower than, the nominal values. BET areas for the materials containing either Ru or MO are only a little smaller than that of the support (355 mz g- ‘); however when both were present together the area decreased as the MO content increased, more so for series A than for series B. Table 1 Chemical analysis, BET surface areas and CO uptakes Catalyst code
Area/m’ g-’
Ru/%
MO/%
CO uptake/m01 g-’ X IO4
Ru Mol Mo5 RuMol Ruh403 RuMoS MolRu Mo5Ru
346 333 330 313 253 199 318 290
2.71
0.95 4.69 0.93 2.58 4.87 0.95 4.69
1.39 1.31 0.95 0.54
2.67 2.69 2.78 2.47 2.85
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CO uptakes were in general very small; in the case of the series B catalysts, they fell progressively as the MO content was raised. 3.2. Benzene hydrogenation This reaction was used to give a measure of the free metallic surface under reaction conditions: it was chosen for its simplicity (single product, structure insensitivity), but all catalysts readily lost activity, presumably because of the formation of a carbonaceous deposit. Conversions were measured at various times up to 150 min, and initial values were estimated from plots of reciprocal conversion vs. time. Fig. 1 shows three of the sets of results: there was little variation in the relative rates of deactivation (compare the Ru and RuMo5 catalysts), although some of the catalysts (e.g. RuMo3 and Mo5Ru) appeared to stabilise in the 100-150 min period. Addition of MO however had a marked effect in suppressing hydrogenation activity, especially for the series A catalysts (Fig. 2) ; the effect was particularly noticeable when comparing RuMo5 and MoSRu, the latter being 25-30 times more active. 3.3. Hydrogenation of carbon monoxide In this reaction, CO conversion on pre-reduced Ru/SiOz at 573 K decreased with timeon-stream as shown in Fig. 3, reaching an almost steady state after about 6 h. The N,pretreated sample of the same material, presumably starting as RuOJSi02, increased in activity with time (see also Fig. 3), attaining after 9 h a constant CO conversion only slightly below that given by the pre-reduced sample. Evidently the Ru02 was slowly reduced by the reaction mixture, but we cannot tell whether any deactivation occurred at the same time. Full product analyses were performed with each Ru-containing catalyst, gas-phase analysis being made after 7-10 h on steam (shorter times for Mol and Mo5). The results are shown in Table 2; no liquid fraction was formed with RuMo5 or with Mol or Mo5. In spite of marked differences in product selectivities, activities did not vary greatly (except for the
t I min Fig. 1. Deactivation of catalysts for benzene hydrogenation at 403 K: (0)
Ru/Si02; (0)
RuMo5; (A) MoSRu.
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1.5 h 1.0 ,-’ . g 0.5 .!! 0, > 0 z 2-0.5 4 -1.0
0
1
2
3
4
5
Fig. 2. Dependence of percentage conversion for benzene hydrogenation on MO concentration, time and (A, A) after 150 min on stream; (0, A) series A; (0, A) series B.
0. 0
5 Time-on-stream
Fig, 3. Change of conversion with time-on-stream catalyst; ((3) catalyst preheated in N2
for CO hydrogenation
(0, 0) at zero
10 I h using Ru/Si02
catalyst: (0)
pre-reduced
Mol and Mo5 catalysts), CO conversions being 22-5 1%. With the series A catalysts, the first addition of MO suppressed the formation of lower hydrocarbons (C,-C,) ,decreased the alkene/alkane ratio and markedly increased the C5 + and oxygenates selectivities. Except for the alkene/alkane ratio, these trends were not progressive, and the catalysts RuMo3 and RuMoS behaved in some respects similarly to the pure Ru/SiO,, although there were differences in the yields of the C r4 hydrocarbons. The COz selectivity rose continuously with MO content. Reversal of the sequence of adding the components to the support produced clear effects (see also Table 2 for the B series catalysts). The smallest amount of Mo gave rise to changes that were broadly similar to those seen in series A (marked increased in CO2 selectivity, decrease in Cl4 selectivities) , but the increase in the higher hydrocarbons noted before did
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Table 2 Effect of MO concentration
on products of CO hydrogenation
Catalyst Ru
RuMol
RuMo3
RuMo5
Mo5
MolRu
Mo5Ru
CO conv.% r/h
36.5 7
35.9 7
21.9 9
32.6 10
7.6 3 78.2
51.3 7 34.6
36.3 7 28.9
CO,/% C,l% c,-C,/ % &+I% alkene/alkane total liquid” organic phase” aqueous phase” c7+/o/o oxygenates/%
11.6 21.9 54.4 6.5 1.95 4.8 2.0 2.8 5.6 0
23.9 13.4 24.3 17.3 1.29 5.3 0.7 4.6 6.2 14.9
31.5 25.2 29.5 5.1 0.08 2.3 0.3 2.0 4.2 4.5
39.8 30.8 21.6 7.7 0.29 -b
21.2 0 0 _b
16.7 23.5 4.3 0.19 6.3 1.1 5.2 1.15 19.9
15.3 21.6 6.4 0.36 4.0 1.2 2.8 10.1 17.7
-
“Amount in grams. bNo liquid products formed.
not now occur. The alkene/alkane ratio was low and there was a high (20%) oxygenates selectivity. Most significantly, in all major respects essentially the same behaviour was found with MoSRu: only the C,+ selectivity was not in line.
4. Discussion It is desirable to try to construct a model which offers a qualitative interpretation of our results: more cannot be expected on the basis of a comparatively few experiments. There are very many variations possible for the construction of a modified metal catalyst, and a full understanding of how the final structure develops would require detailed analysis of the precursor at all stages. In the method adopted here, we have deliberately introduced the second component before the first had been calcined or reduced, with the purpose of maximising the interaction between them. Thus in series A, the RuCl, was presumably still present as such, or as some simple derivative, held to the SiO, surface by ion exchange, when the heptamolybdate was introduced: some partial calcination to RuOZ during drying is not however ruled out. During impregnation of the heptamolybdate and subsequent drying, the Keggin structure is likely to have dissociated, with the formation of Moos- ions attached to the SO2 surface. Although on the basis of this work we cannot explore in any detail the evolution of the catalysts from the precursors, certain observations stand out in great clarity. ( 1) Both the methods used resulted in effective modification of the Ru by the MO. (2) The two methods led to structures that were in some respects different. Further interpretation requires some discussion of the applicability and significance of the methods used for characterization. The small size of the CO uptakes (Table 1) may be due to the inhibiting effect of retained Cl-, and, although the values respond roughly as
M.J. Perez Zurira et al. /Journal of Molecular Catalysis 88 (1994) 213-222
219
might be expected, there is no direct correlation either with benzene hydrogenation rates or with any of the characteristics of the CO + Hz reaction. As noted above, the former reaction was used to estimate qualitatively the free Ru surface: although this reaction has the reputation of being structure-insensitive, this is not necessarily so for all metals and for all particle size ranges, so that quantitative use of the results in Fig. 2 is scarcely warranted. The possibility of some electronic modification of the Ru surface atoms by adjacent oxyMO complexes, as has often been suggested in discussions of the ‘strong metal-support interaction’ [ 121, cannot be eliminated. Lastly we must bear in mind that the reactants and products in CO hydrogenation (not forgetting HzO) may create active centres unlike those present immediately after reduction, such as might determine the benzene hydrogenation activities. In other systems there is clear evidence of dramatic changes in product selectivities with time-on-stream, these being attributed to structural changes induced by the reactants r131. Nevertheless, the results in Fig. 2 provide some pointers to the post-reduction structures. ( 1) With series A catalysts, both initial and final activities decrease smoothly with increasing MO content, and most of the free Ru surface seems to have been eliminated at the 3% MO loading. Evidently, after reduction, Mo-containing species effectively block the Ru surface. (2) However, with series B catalysts, the rate of activity decline as the MO content is raised is much less, and indeed to start with, there is no effect as revealed by the initial activities. At the 5% MO loading, the series B catalyst is 25-30 times more active than that of series A. We may confidently conclude that in the post-reduction the structures in series B the Ru particles are much less fully covered by MO-containing species than is the case for series A. In the case of CO hydrogenation, there is no corresponding decrease in rate with increasing MO concentration (Table 2), although as described above the product selectivities are considerably altered. Our interpretation focuses on the major and systematic changes in the oxygenates selectivities. The sites needed to produce these are wholly absent from the pure Ru/SiO,, but are abundantly present in both series of catalysts when only 1% MO has been added. Indeed in series A the further increase in MO concentration removes these centres, presumably because of progressive and excessive coverage of the Ru particles by the MO species. However in series B a somewhat higher concentration of the oxygenate-forming sites occurs when 1% MO is added, and this is essentially maintained in Mo5Ru. The simplest interpretation would by that the series B procedure leads to Ru particles either on and amidst a partial layer of oxy-MO species attached to the Si02 surface, while the series A procedure gives chiefly Ru particles under such a layer. Benzene hydrogenation proceeds on unobstructed Ru atoms, but oxygenates are formed at Ru-MO sites which may be at the periphery of Ru particles or at isolated MO species decorating their surface. This idea has often been proposed in the context of other promoted or modified metal catalysts [ 11,121. The diagram (Fig. 4) shows in a semi-quantitative way how the concentration of these sites and the extent of free Ru surface may change with MO content for the two series. This model however leaves several questions unresolved. ( 1) The overall rate of CO hydrogenation is not much changed as the MO species cover the Ru particles; in series A catalysts, the C1 selectivity increased, and with both series the C,-C, selectivity was depressed, at high MO loadings. (2) CO2 formation was high with all modified catalysts, and its yield increased with MO content in series A. Catalyst Mo5 gave much CO2 (Mol
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M.J. Perez Zurita et al. /Journal of Molecular Catalysis 88 (1994) 213-222
Fig. 4. Schematic representation of the growth of coverage of Rn particles by MO-containing species as the MO concentration is increased, in the two methods of catalyst preparation. Unshaded areas represent MOO, or similar oxy-MO species: dark areas indicate locations of reduced MO ions, forming part of the active centre for oxygenate synthesis.
under the same conditions gave only 53% COz). (3) Except for RuMol, the alkeneialkane ratio is low ( < 0.4) for all modified catalysts. It is difficult to avoid the conclusion that the hydrocarbon-forming reaction can proceed on the MO component, perhaps particularly on reduced oxy-MO species (e.g. MOO:+ ) covering the Ru particles. The high CO* yields in these cases may be due to the reduction of Mo6’ to lower valent species: certainly not all the COP observed can derive from the reaction proper. This concept could also account for the low alkene/alkane ratios; evidently the hydrocarbons are not (in the modified catalysts) formed on normal Ru surface, where the ratio is about 2. Redox processes on the MO species may be an effective means for hydrogenating adsorbed alkyl groups.
5. Conclusions Modification of Ru/SiO, by Mo-containing species is readily achieved, although the sequence of impregnation of the precursors and the concentration of MO are both important factors in achieving reasonable selectivity to oxygenates. We believe that the best selectivity occurs when the surface of Ru particles is partially covered by oxo-MO species in a way which maximises the concentration of Ru-MO pair-sites. The number of large ensembles of free Ru atoms needs to be minimised if hydrocarbon formation is to be suppressed, but it also seems that this process may perhaps occur on top of oxo-MO species activated by underlying Ru atoms. The way in which transition-metal cations (in this case, probably Mo4+ ) can facilitate formation of oxygenates has been discussed many times before [ 111; such ions may also draw charge from neighbouring Ru atoms, giving them positive character. Analogies are seen with the “strong metal-support interaction” phenomenon, especially as it relates to the activation of C-O bonds [ 121.
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221
6. Acknowledgement We are grateful to the British Council for the provision of funds which made our collaboration possible.
7. References [ l] K.J. Smith and R.C. Everson, J. Catal., 99 (1986) 349. [2] C.S. Kellner and A.T. Bell, J. Catal., 71 (1981); Y. Doi, H. Miyake and K. Soga, J. Chem. Sot. Chem. commun., (1987) 347. [3] Yu-Wen Chen, I. Chin. Inst. Chem. Eng., 17 (1986) 13; Can. J. Chem. Eng., 64 (1986) 875. [4] T.S. Pinnavia, M. Rameswaran, E.D. Dimotakis, E.P. Giannelis and E.G. Rightor, Faraday Discuss. Chem. Sot., 87 (1989) 227. [.5] H.C. Foley, A.J. Hong, J.S. Brinen, L.F. Allard and A.J. Garratt-Reed, Appl. Catal., 61 (1990) 351. [6] M. moue, T. Miyake, Y. Takegami and T. Jnui, Appl. Catal., 29 ( 1987) 285. 171 M. moue, T. Miyake, S. Yonezawa, D. Medhanavyn, Y. Takegami and T. Inui, J. Mol. Catal., 45 (1988) 111. [8] A. Juan and D.E. Damiani, J. Catal., 137 ( 1992); Catal. Today, 15 (1992) 469. [9] Yu-Wen Chen and J.G. Goodwin Jr., React. Kinet. Catal. Lett., 26 (1984) 453. [ 101 T. Mori, A. Miyamoto, N. Takahashi, M. Fukagaya, T. Hattori and Y. Murakami, J. Phys. Chem., 90 ( 1986) 5197. [ 1 l] V. Ponec, Catal. Today, 12 (1992) 227. [ 121 M.A. Vannice, Catal. Today, 12 ( 1992) 255. [ 131 G.C. Bond and D.G. Richards, Appl. Catal., 28 (1986) 303.
OF
MOLECULAR CATALYSIS Journal of Molecular Catalysis 88 ( 1994) 213-222
ELSEVIER
Effect of the method of preparing Ru-Mo/Si02 catalysts in their behaviour in CO hydrogenation M. Josefina Perez Zurita, Irene S. Henriquez, Mireya R. Goldwasser, M.Luisa Cubeiro Centro de Catdlisis, Petroleo y Petroquimica. Escuela de Quimica, Facultad de Ciencias. Universidad Central de Venezuela, Apartado Postal 47102, Caracas, Venezuela
Geoffrey C. Bond* Department of Chemistry, Brunei University, Oxbridge, VB8 3PH, UK (Received June 10, 1993; accepted October 5, 1993)
Abstract Sequential impregnation of SiOZ by RuC13 and then by (N&) 6M07024, followed by drying and reduction in H, at 623 K, afforded catalysts in which, according to their benzene hydrogenation activity, the Ru particles were progressively covered by MO-containing species, the effect being greatest when RuCl, was introduced first. With catalysts of this type, in CO hydrogenation, oxygenate formation occurred at low MO content but was eliminated as the MO concentration was increased. When the MO salt was applied first to the support, oxygenate formation was greater and did not diminish significantly on raising the MO content. A semi-quantitative model is developed in which oxygenates are formed at Ru-MO sites; the hydrocarbon-forming process is not however much inhibited by the presence of MO, and may take place on MO species covering the Ru particles. Key words: carbon monoxide; hydrogenation;
molybdenum;
ruthenium:
silica
1. Introduction Much effort has been devoted to the synthesis of higher alcohols from CO+H2. two strategies in particular having been explored: ( 1) the addition to methanol-forming metals, particularly Cu, of components to promote chain-growth, and (2) the modification of hydrocarbon-forming metals, particularly Ru, by adding components that suppress CO *Corresponding
author. Fax. ( + 44-895) 256844.
0304-5102/94/$07.00 0 1994 Elsevier Science B.V. All rights reserved SSDI0304-5102(93)E0261-E
214
M.J. Perez. Zurita et al. /Journal of Molecular Catalysis 88 (1994) 213-222
dissociation and allow formation of higher oxygenates. There is a quite extensive literature concerning the modification of the properties of Ru in Fischer-Tropsch synthesis by changing the particle size [ l] or support [ 21 or using a transitional metal oxide [ 31 or other unusual material [4] as support. Alternatively, and more usefully, the properties of the metal can be adjusted by placing it on a conventional ‘inactive’ support and adding extra components in low concentration [ 5-101. In this way, the metal particle size and loading, the type and concentration of the modifier and the type of support can be independently altered, and the system fine-tuned to give the desired outturn: basic promoters as well as transition metal compounds can readily be incorporated [ 6,7]. However, despite this large effort, only a few papers [l-8] mention the formation of oxygenates by Ru catalysts. R&pillared clay catalysts form alcohols [ 41, but the more interesting papers are those by Inoue et al. [ 6,7] on Ru-Mo-Na/A1203 catalysts and those by Chen [ 31 who claimed that Ru-K/MOO, gave a high yield of straight-chain alcohols; this important observation seems to have been largely ignored. Foley et al. [ 51, in an extensive study of the promotion of Ru and Rh catalysts for Fischer-Tropsch synthesis, found some alcohol formation on a Ru-W/A1203 catalyst, although the amount was inferior to that given by Rh-Mo/Al,O, and Rh-W/Al*O,. Nevertheless, there is a clear suggestion from the literature that it is possible to modify the behaviour of Ru, to encourage the formation of higher oxygenates. Although models have been proposed [ 111 to explain the effect of promoters, there have been few systematic studies of how the method of preparation affects the resulting catalyst structure, and techniques for the rational construction of specific geometries are still in their infancy. We now report on a short investigation designed to show how the concentration of the promoter (MO) and the sequence in which the metal (Ru) and promoter are added to the support (SiO,) affect catalyst performance.
2. Experimental 2.1. Catalyst preparation and characterization Catalysts were prepared using Davidson 62 SiOZ (Grace) (355 m* g- ‘), RuC1,*xH20 (Aldrich) and ( NI-L,)&Io~O~~( Riedel-de-Haen) . For series A, Si02 was first impregnated with an aqueous solution of RuCl, by the incipient wetness method to give a nominal Ru content of 3%; the resultant wet powder was dried overnight at 383 K. Portions of this material were re-impregnated with solutions of the MO salt to give nominal MO contents of 1,3 and 5%, followed by overnight drying at 383 K. For series B, the sequence of impregnations was reversed. Catalysts are identified by codes which show the impregnation sequence and MO content: thus RuMo5 signifies the series A preparation having 5% MO. Materials containing 1 and 5% MO only were also prepared. The use of the element symbol implies nothing about its actual oxidation state. Catalysts were pre-reduced in situ with HZ for 4 h at 623 K. In one experiment, the Ru/ Si02 precursor was heated for 4 h at 623 K in N, before testing in the CO + H2 reaction. Chemical analyses were performed with an X-ray fluorescence analyser (KEVEX 0.700
M.J. Perez Zurita et al. /Journal of Molecular Catalysis 88 (1994) 213-222
215
Xes Control) and surface areas were measured by a single-point BET procedure (Micromcritics Rapid Surface Area Analyser 2200). CO chemisorption isotherms were measured in a glass high-vacuum system by the doubleisotherm method. Samples ( = 0.5 g) were reduced in situ in flowing Hz (60 cm3 min- ‘> for 3 h at 623 K and then evacuated for 1 h at the same temperature. Quoted values of CO uptake are those found at 100 Torr equilibrium pressure. 2.2. Catalytic tests Benzene hydrogenation was carried out at 403 K in a flow system at atmospheric pressure, using a mixture of 72.5 Torr benzene and 687.5 Torr Hz at a total flow-rate of 3.6 1 hh’. Activity for CO hydrogenation was measured at high pressure in a fixed-bed Aow microreactor; 1 g catalyst was used. Conditions were as follows: space velocity, 1246 cm3 g-’ h- ‘; pressure 3.45 MPa (500 psi) ; 573 K. The feed gas was a 1: 1 CO: H2 mixture with 5% N2 as internal standard. Gaseous components (CO, CO*, Nz CH,) were analysed by a Hewlett-Packard 3392A GC fitted with a CTR-1 column which comprised two concentric columns; the inner (‘/a in. diameter) contained molecular sieve, while the outer ( 1/4 in. diameter) had a mixture of Porapaks. Hydrocarbons were analysed on a Perkin Elmer Sygma 3B GC fitted with a 1.5 m Durapack n-octanelPorasi1 C column. Liquids collected after 24 h reaction were analysed by a Hewlett-Packard GC fitted with a Pona column, and having a mass spectrometer as detector.
3. Results 3.1. Chemical and physical characterization
The analysed Ru and MO contents, the BET surface areas and the CO uptakes at 100 Torr pressure are shown in Table 1. The compositions are close to, but as is usual with the impregnation method, are a little lower than, the nominal values. BET areas for the materials containing either Ru or MO are only a little smaller than that of the support (355 mz g- ‘); however when both were present together the area decreased as the MO content increased, more so for series A than for series B. Table 1 Chemical analysis, BET surface areas and CO uptakes Catalyst code
Area/m’ g-’
Ru/%
MO/%
CO uptake/m01 g-’ X IO4
Ru Mol Mo5 RuMol Ruh403 RuMoS MolRu Mo5Ru
346 333 330 313 253 199 318 290
2.71
0.95 4.69 0.93 2.58 4.87 0.95 4.69
1.39 1.31 0.95 0.54
2.67 2.69 2.78 2.47 2.85
216
M.J. Perez Zurita et al. /Journal of Molecular Catalysis 88 (1994) 213-222
CO uptakes were in general very small; in the case of the series B catalysts, they fell progressively as the MO content was raised. 3.2. Benzene hydrogenation This reaction was used to give a measure of the free metallic surface under reaction conditions: it was chosen for its simplicity (single product, structure insensitivity), but all catalysts readily lost activity, presumably because of the formation of a carbonaceous deposit. Conversions were measured at various times up to 150 min, and initial values were estimated from plots of reciprocal conversion vs. time. Fig. 1 shows three of the sets of results: there was little variation in the relative rates of deactivation (compare the Ru and RuMo5 catalysts), although some of the catalysts (e.g. RuMo3 and Mo5Ru) appeared to stabilise in the 100-150 min period. Addition of MO however had a marked effect in suppressing hydrogenation activity, especially for the series A catalysts (Fig. 2) ; the effect was particularly noticeable when comparing RuMo5 and MoSRu, the latter being 25-30 times more active. 3.3. Hydrogenation of carbon monoxide In this reaction, CO conversion on pre-reduced Ru/SiOz at 573 K decreased with timeon-stream as shown in Fig. 3, reaching an almost steady state after about 6 h. The N,pretreated sample of the same material, presumably starting as RuOJSi02, increased in activity with time (see also Fig. 3), attaining after 9 h a constant CO conversion only slightly below that given by the pre-reduced sample. Evidently the Ru02 was slowly reduced by the reaction mixture, but we cannot tell whether any deactivation occurred at the same time. Full product analyses were performed with each Ru-containing catalyst, gas-phase analysis being made after 7-10 h on steam (shorter times for Mol and Mo5). The results are shown in Table 2; no liquid fraction was formed with RuMo5 or with Mol or Mo5. In spite of marked differences in product selectivities, activities did not vary greatly (except for the
t I min Fig. 1. Deactivation of catalysts for benzene hydrogenation at 403 K: (0)
Ru/Si02; (0)
RuMo5; (A) MoSRu.
217
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1.5 h 1.0 ,-’ . g 0.5 .!! 0, > 0 z 2-0.5 4 -1.0
0
1
2
3
4
5
Fig. 2. Dependence of percentage conversion for benzene hydrogenation on MO concentration, time and (A, A) after 150 min on stream; (0, A) series A; (0, A) series B.
0. 0
5 Time-on-stream
Fig, 3. Change of conversion with time-on-stream catalyst; ((3) catalyst preheated in N2
for CO hydrogenation
(0, 0) at zero
10 I h using Ru/Si02
catalyst: (0)
pre-reduced
Mol and Mo5 catalysts), CO conversions being 22-5 1%. With the series A catalysts, the first addition of MO suppressed the formation of lower hydrocarbons (C,-C,) ,decreased the alkene/alkane ratio and markedly increased the C5 + and oxygenates selectivities. Except for the alkene/alkane ratio, these trends were not progressive, and the catalysts RuMo3 and RuMoS behaved in some respects similarly to the pure Ru/SiO,, although there were differences in the yields of the C r4 hydrocarbons. The COz selectivity rose continuously with MO content. Reversal of the sequence of adding the components to the support produced clear effects (see also Table 2 for the B series catalysts). The smallest amount of Mo gave rise to changes that were broadly similar to those seen in series A (marked increased in CO2 selectivity, decrease in Cl4 selectivities) , but the increase in the higher hydrocarbons noted before did
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M.J. Perez Zurita et al. /Journal of Molecular Catalysis 88 (1994) 213-222
Table 2 Effect of MO concentration
on products of CO hydrogenation
Catalyst Ru
RuMol
RuMo3
RuMo5
Mo5
MolRu
Mo5Ru
CO conv.% r/h
36.5 7
35.9 7
21.9 9
32.6 10
7.6 3 78.2
51.3 7 34.6
36.3 7 28.9
CO,/% C,l% c,-C,/ % &+I% alkene/alkane total liquid” organic phase” aqueous phase” c7+/o/o oxygenates/%
11.6 21.9 54.4 6.5 1.95 4.8 2.0 2.8 5.6 0
23.9 13.4 24.3 17.3 1.29 5.3 0.7 4.6 6.2 14.9
31.5 25.2 29.5 5.1 0.08 2.3 0.3 2.0 4.2 4.5
39.8 30.8 21.6 7.7 0.29 -b
21.2 0 0 _b
16.7 23.5 4.3 0.19 6.3 1.1 5.2 1.15 19.9
15.3 21.6 6.4 0.36 4.0 1.2 2.8 10.1 17.7
-
“Amount in grams. bNo liquid products formed.
not now occur. The alkene/alkane ratio was low and there was a high (20%) oxygenates selectivity. Most significantly, in all major respects essentially the same behaviour was found with MoSRu: only the C,+ selectivity was not in line.
4. Discussion It is desirable to try to construct a model which offers a qualitative interpretation of our results: more cannot be expected on the basis of a comparatively few experiments. There are very many variations possible for the construction of a modified metal catalyst, and a full understanding of how the final structure develops would require detailed analysis of the precursor at all stages. In the method adopted here, we have deliberately introduced the second component before the first had been calcined or reduced, with the purpose of maximising the interaction between them. Thus in series A, the RuCl, was presumably still present as such, or as some simple derivative, held to the SiO, surface by ion exchange, when the heptamolybdate was introduced: some partial calcination to RuOZ during drying is not however ruled out. During impregnation of the heptamolybdate and subsequent drying, the Keggin structure is likely to have dissociated, with the formation of Moos- ions attached to the SO2 surface. Although on the basis of this work we cannot explore in any detail the evolution of the catalysts from the precursors, certain observations stand out in great clarity. ( 1) Both the methods used resulted in effective modification of the Ru by the MO. (2) The two methods led to structures that were in some respects different. Further interpretation requires some discussion of the applicability and significance of the methods used for characterization. The small size of the CO uptakes (Table 1) may be due to the inhibiting effect of retained Cl-, and, although the values respond roughly as
M.J. Perez Zurira et al. /Journal of Molecular Catalysis 88 (1994) 213-222
219
might be expected, there is no direct correlation either with benzene hydrogenation rates or with any of the characteristics of the CO + Hz reaction. As noted above, the former reaction was used to estimate qualitatively the free Ru surface: although this reaction has the reputation of being structure-insensitive, this is not necessarily so for all metals and for all particle size ranges, so that quantitative use of the results in Fig. 2 is scarcely warranted. The possibility of some electronic modification of the Ru surface atoms by adjacent oxyMO complexes, as has often been suggested in discussions of the ‘strong metal-support interaction’ [ 121, cannot be eliminated. Lastly we must bear in mind that the reactants and products in CO hydrogenation (not forgetting HzO) may create active centres unlike those present immediately after reduction, such as might determine the benzene hydrogenation activities. In other systems there is clear evidence of dramatic changes in product selectivities with time-on-stream, these being attributed to structural changes induced by the reactants r131. Nevertheless, the results in Fig. 2 provide some pointers to the post-reduction structures. ( 1) With series A catalysts, both initial and final activities decrease smoothly with increasing MO content, and most of the free Ru surface seems to have been eliminated at the 3% MO loading. Evidently, after reduction, Mo-containing species effectively block the Ru surface. (2) However, with series B catalysts, the rate of activity decline as the MO content is raised is much less, and indeed to start with, there is no effect as revealed by the initial activities. At the 5% MO loading, the series B catalyst is 25-30 times more active than that of series A. We may confidently conclude that in the post-reduction the structures in series B the Ru particles are much less fully covered by MO-containing species than is the case for series A. In the case of CO hydrogenation, there is no corresponding decrease in rate with increasing MO concentration (Table 2), although as described above the product selectivities are considerably altered. Our interpretation focuses on the major and systematic changes in the oxygenates selectivities. The sites needed to produce these are wholly absent from the pure Ru/SiO,, but are abundantly present in both series of catalysts when only 1% MO has been added. Indeed in series A the further increase in MO concentration removes these centres, presumably because of progressive and excessive coverage of the Ru particles by the MO species. However in series B a somewhat higher concentration of the oxygenate-forming sites occurs when 1% MO is added, and this is essentially maintained in Mo5Ru. The simplest interpretation would by that the series B procedure leads to Ru particles either on and amidst a partial layer of oxy-MO species attached to the Si02 surface, while the series A procedure gives chiefly Ru particles under such a layer. Benzene hydrogenation proceeds on unobstructed Ru atoms, but oxygenates are formed at Ru-MO sites which may be at the periphery of Ru particles or at isolated MO species decorating their surface. This idea has often been proposed in the context of other promoted or modified metal catalysts [ 11,121. The diagram (Fig. 4) shows in a semi-quantitative way how the concentration of these sites and the extent of free Ru surface may change with MO content for the two series. This model however leaves several questions unresolved. ( 1) The overall rate of CO hydrogenation is not much changed as the MO species cover the Ru particles; in series A catalysts, the C1 selectivity increased, and with both series the C,-C, selectivity was depressed, at high MO loadings. (2) CO2 formation was high with all modified catalysts, and its yield increased with MO content in series A. Catalyst Mo5 gave much CO2 (Mol
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M.J. Perez Zurita et al. /Journal of Molecular Catalysis 88 (1994) 213-222
Fig. 4. Schematic representation of the growth of coverage of Rn particles by MO-containing species as the MO concentration is increased, in the two methods of catalyst preparation. Unshaded areas represent MOO, or similar oxy-MO species: dark areas indicate locations of reduced MO ions, forming part of the active centre for oxygenate synthesis.
under the same conditions gave only 53% COz). (3) Except for RuMol, the alkeneialkane ratio is low ( < 0.4) for all modified catalysts. It is difficult to avoid the conclusion that the hydrocarbon-forming reaction can proceed on the MO component, perhaps particularly on reduced oxy-MO species (e.g. MOO:+ ) covering the Ru particles. The high CO* yields in these cases may be due to the reduction of Mo6’ to lower valent species: certainly not all the COP observed can derive from the reaction proper. This concept could also account for the low alkene/alkane ratios; evidently the hydrocarbons are not (in the modified catalysts) formed on normal Ru surface, where the ratio is about 2. Redox processes on the MO species may be an effective means for hydrogenating adsorbed alkyl groups.
5. Conclusions Modification of Ru/SiO, by Mo-containing species is readily achieved, although the sequence of impregnation of the precursors and the concentration of MO are both important factors in achieving reasonable selectivity to oxygenates. We believe that the best selectivity occurs when the surface of Ru particles is partially covered by oxo-MO species in a way which maximises the concentration of Ru-MO pair-sites. The number of large ensembles of free Ru atoms needs to be minimised if hydrocarbon formation is to be suppressed, but it also seems that this process may perhaps occur on top of oxo-MO species activated by underlying Ru atoms. The way in which transition-metal cations (in this case, probably Mo4+ ) can facilitate formation of oxygenates has been discussed many times before [ 111; such ions may also draw charge from neighbouring Ru atoms, giving them positive character. Analogies are seen with the “strong metal-support interaction” phenomenon, especially as it relates to the activation of C-O bonds [ 121.
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6. Acknowledgement We are grateful to the British Council for the provision of funds which made our collaboration possible.
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