J . Chem. Soc., Faraday Trans. I, 1984,80, 1605-1616 Use of Supported Rhodium and Cobalt Carbonyls as Catalysts for the CO+H, Reaction Effect of the Support and the Metal BY ALESSANDRO CERIOTTI, SECONDO MARTINENGO AND LUCIANO ZANDERIGHI Universita di Milano and Centro CNR sui bassi stati di ossidazione, Via G. Venezian 21, 20133 Milano, Italy, AND CLAUDIO TONELLI, ANTONIO I ANNIBELLO AND ALBERTO GIRELLI* Stazione sperimentale per i Combustibili, Viale A. De Gasperi 3, 20097 San Donato Milanese, Italy Received 23rd September, 1983 The use of cobalt and rhodium catalysts derived from carbonyl clusters and supported on A1,0,, Al,O,-ZnO, ZrO, and SiO, for the CO+H, reaction has been investigated. Only Rh-containing catalysts were active in the formation of oxygenated compounds (essentially ethanol).In all the runs the selectivity to methane plus ethanol was > 90%. The ratios of the rates of ethanol and methane formation or the rate of ethanol formation and the CO reaction reach a maximum when equal amounts of Rh and Co are present on the catalyst surface. Cobalt alone does not form oxygenates, but when mixed with rhodium it increases the selectivity to ethanol. In recent years there have appeared a large number of papers on the direct catalytic synthesis of chemicals from carbon monoxide and hydrogen. Of special interest is the selectivity between hydrocarbons and oxygenated compounds. Until now only processes for the synthesis of one-carbon-atom compounds can operate with high selectivity, other processes giving a broad spectrum of product distribution (hydro- carbons and oxygenated compounds).Mixtures of higher hydrocarbons are produced with good yields and the use of zeolites has greatly improved the selectivity for low-molecular-weight hydrocarbons. For oxygenated compounds, on the other hand, the situation is less satisfying and much work has been done on this problem. Oxygenated compounds can be produced from CO+ H, using Fe, Ni, Pd, Os, Ir and Pt carbonyls or salts as catalysts., The synthesis of methanol on simple metal centres has been reported by Rathke,3 who used mononuclear carbonyls of Mn and Co, and by Bradle~,~ who used Cu carbonyls. Methanol is a major product in the synthesis of ethylene glycol using Rh as catalyst and Pruett and Walker5 have tested a variety of metal catalysts, including Co, Ru, Rh, Pd, Ir, Pt, Cu, Mn, Sn and Pb, but only Rh and Co produced polyhydric alcohols.Cobalt carbonyl hydride catalyses the formation of methanol and higher alcohols up to C,, as well as their formates, acetaldehyde etc.g The literature on homogeneous catalysis shows that the Group VIII coordination complexes, when active for the CO + H, reaction, catalyse the formation of oxygenated products, while hydrocarbons are produced when the metal alone is present.’ The only exception is a homogeneous Fischer-Tropsch reaction catalysed by II-~(CO)~,, under particular experimental conditions.8 Among the Group VIII metals, Rh seems to have 16051606 CO+H, REACTION OVER Rh AND Co CARBONYLS the unique ability of being able to produce by heterogeneous catalysis two-carbon-atom compounds with good sele~tivity.~ Supported rhodium catalysts have been extensively studied by Ichikawa,lo who has shown that the activity and selectivity of the catalyst depend on the precursor, the support, the preparation conditions and the heat treatment.Catalysts prepared from cluster carbonyls show remarkable selectivity for alcohol synthesis : supported on MgO, CaO and ZnO they promote methanol formation and on TiO,, ZrO,, La203 and Ce,03 they promote ethanol formation. With SiO, support, the activity and selectivity change with the pretreatment conditions. Rhodium appears to be a promising catalyst for the CO + H, reaction as its activity and selectivity change for different precursors and supports. Other metals such as Ni and Co do not have such properties.The aim of this work is a study of the activity and selectivity of catalysts prepared by supporting Rh and Co carbonyls on different oxides in the reaction between CO and H, for the direct synthesis of ethanol. As shown by Ichikawa,lo this reaction occurs with good selectivity; nevertheless an improvement in the activity and selectivity of the reaction is necessary before a new synthesis process can be developed. Our work is experimental and in some aspects is a development of previous studies of the activation of carbon monoxidell and of alcohol synthesis.12 EXPERIMENTAL PREPARATION OF THE CATALYSTS The supports used were y-Al,O, CK-300 (surface area 200 m2 g-l) and SiO, SIL-3 E (surface area 200 m2 g-') from Akzo Chemie and Al,O,-ZnO (surface area 53 m2 g-l) and ZrO, (surface area 70 m2 g-l) from Strem Chemical Inc.A1,0, doped with BaO was prepared from CK-300. The pellets were crushed and the fraction between 50 and 150 mesh used. The precursors were cobalt, rhodium or mixed Co-Rh dodecacarbonyls [Co,RH,-,(CO),, (n = 4,3,2, O)] prepared according to the 1iterat~re.l~ All the supports were thermally treated (Al,O, at 820 K for 6 h, ZrO, at 770 K for 7.5 h) in an inert atmosphere; after cooling at room temperature a weighed amount of the support was suspended in an anhydrous degassed organic solvent (pentane or toluene) and a solution of a known amount of carbonyl in a suitable anhydrous solvent (pentane or toluene) was added drop-wise to the stirred suspension in an inert atmosphere: the cluster was adsorbed by the support and the solution lost the colour of the carbonyl.When all the carbonyl was added the stirred mixture was left to equilibrate for 2 h and the liquid was then drained off. The low adsorption properties of silica did not allow it to adsorb significant amounts of the carbonyls ; therefore these catalysts were prepared by the pore filling technique with heptane as solvent. The solid was dried by evaporation in uucuo (low2 Torr) at room temperature for 2 h. The catalyst so obtained was kept in a nitrogen atmosphere in a sealed glass tube. Table 1 gives details of the prepared catalysts. CATALYTIC ACTIVITY MEASUREMENTS The catalytic runs were performed in a recirculating reactor system (Temkin type) equipped with a condensation trap (liquid nitrogen or water at 273 K) to collect the liquid products.The stainless-steel tubular reactor was 200 mm long with a diameter of 3 mm; in each run ca. 2-3.5 g of catalyst were used. Repeated runs were performed for each catalyst in the 453-523 K temperature range at a pressure of 1.33 x lo5 N m-2 and with H,: CO feed ratios of 3 : 1 and 3:2. The feed and the gaseous products were analysed by gas chromatography with a normal gas- sampling valve on line. Two columns were used for the analysis of the liquid and gaseous products: Porapak Q S (length 7 m; diameter 1/8 in; programmed temperature 9CL200 "C; 5 "CA. CERIOTTI et al. 1607 Table 1. Summary of the experimental conditions of catalyst preparation metal (wt%) catalyst no. precursor solvent support c o Rh 1 2 9 14 15 16 18 19 20 21 22 25 30 31 32 pentane pentane toluene toluene pentane pentane pentane pentane pentane pentane pentane pentane heptane heptane heptane A1203 A1203 A1203 Al,O,-BaO A1203-ZnO A1203-ZnO Al,O,-ZnO ZrO, ZrO, ZrO, Si02 SiO, Si02 ZQ zro2 1.69 0.77 0.74 0.8 1.87 1.17 0.46 - - 0.48 0.51 1.6 - 1.35 2.75 1.28" 1.39 3.08 0.8 1 1.58 0.2 1 0.28 0.89 1.6 - - - a 5wt% BaO.min-' ; carrier gas H,) for CO, CO, and gaseous hydrocarbons and polyethylene glycol 1500 on Chromosorb W-AW (length 3 m; diameter 1/8 in; programmed temperature 60 "C; carrier gas He) for the oxygenated products. The analytical results of each run were checked by the CO material balance. The catalyst was transferred under the nitrogen atmosphere from the sealed tube to the reactor (using this procedure the supported carbonyl never came into contact with air).Preliminary tests in a nitrogen atmosphere had shown that decomposition of the carbonyl begins at 350 K and is complete at 570 K. Before each catalytic run all the catalysts were treated for 2 h at 495 K in hydrogen; all the supported carbonyls were thus decarbonylated to a stable catalyst with highly dispersed metal. RESULTS A1,0, SUPPORT All the catalysts prepared with this support give only hydrocarbons (Cl-C,) as measurable products with a selectivity to methane > 70%. Plots of initial reaction rates (CO molecule atom-l s-l) of the carbon monoxide (H, : CO = 3 : 1) against T1 for the different catalysts (no. 1, 2, 9 and 14) and the temperature coefficients of the reaction rates are reported in fig.1 and in table 2, respectively. All the catalysts containing Rh have approximately the same temperature coefficient but different activities : the catalyst prepared from Co,Rh,(CO),, (no. 2) is significantly more active than that prepared from Rh,(CO),, (no. 9). When the alumina surface is doped with BaO (no. 14) the activity decreases sharply. The influence of the H,:CO ratio in the feed is shown in fig. 1 for two catalysts (no. 2 and 9): in all cases, increasing the CO: H, ratio in the feed decreases the activity, without any significant change in the temperature coefficient of the rate; this behaviour is common to all the catalysts tested, as discussed later.1608 " I v) 2 1 0 - 3 . E CO+H, REACTION OVER Rh AND Co CARBONYLS I " 1.8 1.9 2 .o 103 KIT Fig.1. Plots of CO reaction rate against T1 for different carbonyls on Al,O,: open symbols, CO: H, = 1 : 3; closed symbols, CO: H, = 2: 3. Type of catalyst: no. 1, 0 ; no. 2, V; no. 9, ; no. 14, A. Table 2. Temperature coefficients for the initial reaction rate on Al,O,-supported carbonyls E/kJ mol-1 catalyst no. H,:CO = 3: 1 H,:CO = 3 : 2 1 2 9 14 34 106 112 119 - 100 114 - Al,03-ZnO SUPPORT The temperature coefficients of the initial reaction rates of carbon monoxide over supported CO,(CO),, (no. 16), Co,Rh,(CO),, (no. 15) and Rh,(CO),, (no. 18) are reported in table 3. The main difference with respect to the y-Al,03 support is the inversion of the activity between Rh and Co, mainly because of the strong variation of the temperature factor on supported cobalt.Moreover, the bimetallic Co,Rh, catalyst (no. 15) has behaviour intermediate between the two monometallic catalysts (no. 16 and 18). Detectable amounts of oxygenated products, mainly ethanol, were obtained with Rh, (no. 18) and Co,Rh, (no. 15) catalysts; the selectivity to ethanol was not greater than a few percent, while the selectivity to methane was at least 70%. By changing the feed composition from H,: CO = 3 : 1 to 3 : 2 the activity decreases without any significant change in the selectivity to oxygenated products.A. CERIOTTI et al. 1609 Table 3. Temperature coefficients for the initial reaction rate on Al,O,-ZnO- supported carbonyls E/kJ mol-' catalyst no. H,:CO = 3: 1 H,:CO = 3:2 15 16 18 I26 155 75 106 85 - 10-2 c( I $ 10-3 E 1 o - ~ \ 1.9 2 .o 2.1 2 .2 Fig. 2. Plots of CO reaction rate against T1 for different carbonyls on ZrO,: open symbols, CO:H, = 1 : 3 ; closed symbols, CO:H, = 2:3. Type of catalyst: no. 19, 0; no. 20, A; no. 21, 0; no. 22, v; no. 25, 0; no. 19+no. 20, 1 : 1 , *; no. 19+no. 21, 1:3, +. 1 0 3 K/T ZrO, SUPPORT Plots of the initial reaction rates of the carbon monoxide against T-l are shown in fig. 2 and the temperature coefficients are given in table 4. The Rh, catalyst (no. 21) is more active than the Co, catalyst (no. 19); the activities of the bimetallic catalysts Co,Rh, (no. 20) and Co,Rh (no. 25) are similar and intermediate between those of Rh and Co. Also with these catalysts a decrease in activity has been observed on changing the feed composition from H, : CO = 3 : 1 to 3 : 2.53 FAR 11610 CO + H, REACTION OVER Rh AND Co CARBONYLS Table 4. Temperature coefficients, E/kJ mol-l, for the initial reaction rates on ZrO,-supported carbonyls co methane ethanol catalyst H,:CO H,:CO H,:CO H,:CO H,:CO H,:CO no. = 3:l = 3:2 = 3:l = 3:2 = 3:l = 3:2 ~~ - - - 77 - 19 83 20 112 131 139 141 96 77 21 145 146 162 157 91 84 22 143 25 110 162 124 162 91 - 162 - - - - 19+21 = 1:l 103 90 117 108 54 78 19+21 = 1:3 101 132 117 141 50 80 In the fig. 2 the activities of the bimetallic catalysts Co,Rh, (no. 20) and Co,Rh (no. 25) are compared with that of a mechanical mixture of the catalysts 19 and 21 in 1 : 1 and 1 : 3 ratios; the Co,Rh (no. 25), Co,Rh, (no. 20) and 1 : 1 and 1 : 3 mechanical mixtures of CO, (no. 19) and Rh, (no.21) have practically the same activity. In all cases the activity decreases with increasing partial pressure of CO. To study the influence of the rhodium surface concentration on the activity and selectivity, a catalyst (no. 22) containing 7.5 times less rhodium than the reference catalyst (no. 21) was tested. The initial reaction rate of CO was four times lower without any change in the temperature coefficient (fig. 3). The decrease of the activity is due only to the decrease of the pre-exponential factor of the kinetic constant. Unlike the previous catalysts, all the ZrO, catalysts containing rhodium give measurable amounts of ethanol and other detectable oxygenated products. The amount of methane and ethanol together constitutes > 90% of the products; acetaldehyde does not exceed 1 %.Neither methanol nor C, oxygenates were found in the reaction products. The lack of oxygenated C, compounds, particularly methanol, was confirmed by gas-chromatography mass-spectroscopic analysis. With the most active catalysts the selectivity to ethanol decreases sharply on raising the temperature from 493 to 523 K, while the total CO conversion increases. For this reason only the runs at lower temperature have been considered in the analysis of the ethanol data. The initial rates of ethanol and methane formation as a function of temperature are reported in fig. 3 for Rh and Co-Rh catalysts and the temperature coefficients are given in table 4. The rate of ethanol formation on a bimetallic catalyst is slightly lower than on a monometallic Rh, catalyst (no.21); by decreasing the surface concentration of Rh, (no. 22) the reaction rate decreases. The methane formation rate is higher on monometallic Rh catalysts than on all the others; on bimetallic catalysts the formation rate decreases on increasing cobalt content, this decrease being greater than the decrease in the ethanol formation rate. The values of the temperature coefficients of methane are greater than those measured on the other catalysts and 43 kJ greater than those observed for the formation of ethanol. These high values suggest that methane is formed by CO dissociation ; in comparison with ethanol formation, this reaction is not energetically favoured Ecause of its high activation energy.A. CERIOTTI et al. 161 1 - I v) -..- + E 1 0-' 1 I I 10 -3 " I v1 .t l 0-l 2 10- 1.9 2 .o 2 . 1 103 KIT 2 . 2 Fig. 3. Plots of reaction rate of methane and of ethanol formation on different ZrO, catalysts against T1 (CO:H, = 1 :3). Type of catalyst: no. 20, A; no. 21, 0; no. 22, [7 ; no. 25, 0 ; no. 19+no. 21, 1:1, *;no. 19+no. 21, 1:3, +. SiO, SUPPORT The activity data are shown in fig. 4. The cobalt catalysts are more active than the rhodium and bimetallic catalysts; the latter have an activity a little higher than rhodium. Small amounts of oxygenated compound are formed, mainly ethanol, acetaldehyde and dimethylether. The selectivity to ethanol was 2-5 %. The presence of dimethylether suggests the formation of methanol or of some of its precursors. The values of the temperature coefficients (table 5 ) are similar to those found with Al,O, and Al,O,-ZnO.53-21612 10-5 CO+H, REACTION OVER Rh AND Co CARBONYLS \ \ 0 '\ 1.9 2 .o 2.1 2 . 2 1 0 3 KIT Fig. 4. Plots of CO reaction rate against T1 for different carbonyls on SiO, (CO: H, = 1 : 3). Type of catalyst: no. 30, A; no. 31, 0; no. 32, 0. Table 5. Temperature coefficients for the initial reaction rate on Si0,-supported carbonyls cat a1 ys t E/kJ mol-l no. H,:CO = 3: 1 30 104 31 108 32 123 DISCUSSION Clearly, cobalt and rhodium catalysts behave differently : rhodium catalysts are in general more active for carbon monoxide conversion than the cobalt catalysts. For instance, the reaction rates at 493 K and H,:CO = 3:l are 1.15 x (molecule CO s-l atom-') on Rh/Al,O,, 1.4 x on Rh/ZrO, and 4 x on Rh/SiO,; under the same experimental conditions the cobalt catalysts give the following values: 2.5 x on Co/Al,O,, 3.5 x 10+ on Co/Al,O,-ZnO, 2.5 x on Co/ZrO, and 4.4 x on Co/SiO,.The low activity of the Co/Al,O,- on Rh/Al,O,-ZnO, 2.2 xA. CERIOTTI et al. 1613 Rh, RhcCo, Rh,CO, R h Coj Co, Fig. 5. Selectivity to ethanol as a function of Rh:Co ratio on ZrO,. Open symbols, T = 473 K; closed symbols, T = 493 K. Circles, H, : CO = 3 : 1 ; triangles, H,: CO = 3 : 2. ZnO catalyst suggests an interaction between cobalt and ZnO, for instance the formation of a weakly active or inactive catalytic phase. The influence of the support on the selectivity of Rh catalysts is emphasized by the formation of oxygenated products with ZrO, support but only hydrocarbons with A1,0,.Very small amounts of oxygenated compounds are obtained on Rh/SiO, and on Rh/Al,O,-ZnO; in the latter case this is probably due to the direct action of the zinc oxide, In all the experiments, except those using SiO,, methanol was not detected; the results of chromatographic analysis were confirmed in some cases by mass-spectro- scopic analysis. The absence of methanol in the reaction products is one of the more surprising results obtained with the rhodium catalysts. Moreover, the only products present in significant amounts in all the runs were methane and ethanol. The support has a strong influence on the selectivity of Rh, while it has no influence on the selectivity of Co. The activity of bimetallic Co-Rh catalysts is intermediate between that of mono- metallic Co or Rh catalysts, with a decrease in methane formation on increasing the amount of Co; there is also a decrease in ethanol formation but to a lesser extent.1614 CO + H, REACTION OVER Rh AND Co CARBONYLS Maximum selectivity to ethanol is obtained for a 1 : 1 Co: Rh ratio (fig.5). It is difficult to explain the different catalytic behaviour of cobalt and rhodium on the basis of metal-support interactions or differences in the colligative properties alone ; the work functions of the two metals are 4.12-4.35 and 4.52 eV, respectively, and the first ionization potentials are 7.8 1 and 7.7, respectively. Therefore it appears more reasonable to ascribe the different activity and selectivity to the reactivity of the two metals towards the reacting molecules. Both metals can easily dissociate dihydrogen with the formation of active hydrogen on the surface.On the other hand, experimental data and theoretical calculations point to dissociative chemisorption of CO on cobalt16 and non-dissociative chemisorption on rh0dium.l’ By simple theoretical calculations Miyazakil* has shown that the energy of desorption of CO adsorbed on cobalt is similar to the energy of dissociation, while the energy of desorption is significantly lower on rhodium. This different reactivity of the two metals towards chemisorbed CO is due mainly to the metal-carbon and metal-oxygen bond energies, which are greater for cobalt than for rhodium. Moreover, should the metal-support interaction be strong with cobalt, the localisation of a negative charge on the metal would favour the dissociation of CO.The invariance of the selectivity to hydrocarbon formation with support should be ascribed to this specific reactivity of cobalt. As far as the activity and selectivity of rhodium are concerned our results fal! within the wide range of published data on the reduction of CO. An interesting catalytic feature of rhodium is the variability of its behaviour in the various catalysts tested. While some discussion could arise about the dissociation of CO under our conditions, it is reasonable to assume that the selectivity to oxygenated compounds is related to an active undissociated form of chemisorbed CO. Probably the first reaction step is the formation of formyl (M-CH=O) and/or hydroxymethyl (M-CH,OH) species.It is known that formyl species can be formed by the reaction of a carbonyl group with a hydridic hydrogenlg or by addition of a hydrogen atom to a stretched CO molecule. As no hydride can be formed on the surface of the catalyst, the second reaction path is more likely. In this case the chemisorbed CO must interact with a neighbouring active centre so as to lower the carbon-oxygen bond order to a value that allows the addition of a chemisorbed hydrogen atom. Metal formyl compounds are knownz0 but they are usually unstable. Thermodynamic data on the metal formyl formation reaction are not available; however, one can reasonably assume that it is thermodynamically unfavoured and that the formyl species is present on the surface only as an active intermediate that decomposes or reacts quickly with hydrogen to give more stable surface compounds such as methylene or hydroxymethyl groups.The lack of spectroscopic evidence for the presence of metal formyl species is indirect confirmation of their thermodynamic instability. The formation of methyl species from dissociated CO or hydroxymethyl species is promoted by a positive 6+ charge on the Rh atoms, as happens with highly dispersed Rh on ZrO, and A1,0,.21 Three reaction paths are possible for hydroxymethyl species : hydrogenation to methanol (or methane), dehydroxylation to methylene species, promoted by Bronsted surface acidity, and migration to carbon monoxide adsorbed on the same metal centre. Both the support and the morphology of the dispersed rhodium particles must control one or more reaction pathway. Therefore it is possible that dehydroxylation prevails with small rhodium metal particles dispersed on acidic alumina, producing only hydrocarbons, as has been found previously.16 With larger rhodium particles, where the boundary effects are smaller, or with less acidic alumina both the otherA.CERIOTTI et af. 1615 reaction pathways may occur. Both of them are also possible with rhodium supported on basic or neutral solids. The problem now arises of selectivity to methanol or to ethanol. The published datals indicate that the hydrogenation of CO over supported Rh catalysts prepared by decomposition of Rh carbonyls produces primarily methanol on basic oxides (MgO, ZnO, Be0 and CaO) and primarily ethanol on neutral supports (La,O,, ZrO,, TiO,, Tho, and CeO,). This confirms the change of product distribution in the different chemical environments of the supported rhodium.While methanol is formed by direct hydrogenation of the hydroxymethyl species, for ethanol the prior formation of a carbon-carbon bond must be considered. Let us assume, like many others,,, that a carbon+arbon bond is formed by the migration of a methyl group on chemisorbed carbon monoxide with formation of an acyl species, whose partial hydrogenation would give ethanol. 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