A highly active bimetallic supported Rh–Co hydroformylation catalyst prepared from RhCl3 and Co2(CO)8

The preparation of a highly active bimetallic SiO₂‐supported Rh–Co catalyst from RhCl₃ and Co₂(CO)₈ (Rh:Co= 1 : 3 atomic ratio) has been studied by IR spectroscopy and ethylene hydroformylation, etc. Two steps are involved in the preparative process: (1) surface‐mediated synthesis of Rh⁺(CO)₂/SiO₂ from calcined RhCl₃/SiO₂; (2) impregnation of Rh⁺(CO)₂/SiO₂ with a Co₂(CO)₈ solution followed by H₂ reduction at 623 K. The IR results of reductive carbonylation of calcined RhCl₃/SiO₂ have been compared to those of uncalcined RhCl₃/SiO₂ at 373 K. In situ IR observations, extraction results and elemental analysis suggest that approximately 50% of RhCl₃ are transformed to Rh₂O₃ on the SiO₂ surface and that calcined RhCl₃/SiO₂ is converted to a mixture of [Rh(CO)₂Cl]₂ and [Rh(CO)₂O₂ (O_s: surface oxygen) under CO at 373 K. When this SiO₂‐supported mixture was submitted to impregnation with a Co₂(CO)₈ solution at room temperature, IR study and elemental analysis show that [Rh(CO)₂Cl]₂ reacts easily with Co₂(CO)₈ on the surface to give RhCo₃(CO)₁₂, whereas [Rh(CO)₂O₂ does not react with Co₂(CO)₈. Catalytic study in steady‐state ethylene hydroformylation shows that a catalyst thus derived is more active than a catalyst derived from RhCo₃(CO)₁₂/SiO₂ and a catalyst derived by coimpregnation of [Rh(CO)₂Cl]₂ and Co₂(CO)₈ on SiO₂. This result suggests that the high rhodium dispersion of [Rh(CO)₂O₂ plays a crucial role in the formation of highly dispersed bimetallic Rh–Co sites. This revised version was published online in July 2006 with corrections to the Cover Date.     

SiO2‐supported bimetallic Rh–Co catalysts derived from [Rh(CO)2Cl]2 and cobalt carbonyls

According to the results of IR characterization and catalytic study in ethylene hydroformylation, bimetallic Rh–Co catalysts can be efficiently prepared from [Rh(CO)₂Cl]₂ and cobalt carbonyls by co‐impregnation on SiO₂. The reaction of Co₂(CO)₈ with [Rh(CO)₂Cl]₂ (Rh : Co = 1 : 3 atomic ratio) gives rapidly RhCo₃(CO)₁₂ on the surface of SiO₂. Although Co₄(CO)₁₂ is not reactive with [Rh(CO)₂Cl]₂ on SiO₂ to form directly RhCo₃(CO)₁₂, an equivalent bimetallic catalyst can be easily obtained from ([Rh(CO)₂Cl]₂ + Co₄(CO)₁₂)/SiO₂ or its derivative (Rh⁺ + Co²⁺)/SiO₂ (Rh : Co = 1 : 3 atomic ratio) under reducing conditions. This revised version was published online in July 2006 with corrections to the Cover Date.     

Surface-grafted triruthenium ketenylidene clusters and their catalytic activity in co exchange and hydroformylation of ethylene

A triruthenium ketenylidene cluster [PPN]₂[Ru₃(CO)₉(CCO)] was deposited on MgO, SiO₂, and SiO₂-Al₂O₃, and the nature of surface species on the oxides were studied by an IR spectroscopic study along with catalytic performances in¹³CO exchange reaction and hydroformylation of ethylene. The IR study suggested the stoichiometric protonation of [Ru₃(CO)₉(CCO)]^2-with surface hydroxyl groups on SiO₂ and SiO₂-Al₂O₃ to give [HRu₃(CO)₉(CCO)]^– and H₂Ru₃(CO)₉(CCO), respectively. H₂Ru₃(CO)₉(CCO)/SiO_2– Al₂O₃ was active for¹³CO exchange reaction, while [Ru₃(CO)₉(CCO)]^2–/MgO showed high activity and selectivity toward propanol in hydroformylation of ethylene.     

Ethylene hydroformylation with the silica-supported K2[Rh12(CO)30] cluster: evidence for vapor-phase cluster catalysis

The SiO₂-supported [Rh₁₂(CO)₃₀]^2– cluster, as K⁺ salt, is a stable and active catalyst for the heterogeneous hydroformylation of ethylene at atmospheric pressure.     

Use of catalytic reactions to probe Mg-Al mixed oxide surfaces

The atmospheric hydroformylations of ethylene and propylene were investigated over SiO₂-supported Rh₄(CO)₁₂, Co₂(CO)₈, Rh₂Co₂(CO)₁₂ and RhCo₃(CO)₁₂-derived catalysts. The bimetal cluster-derived catalysts showed excellent activities for the formation of oxygenates. In situ IR study on partially dehydroxylated SiO₂-supported RhCo₃(CO)₁₂ suggested that the bimetal cluster framework may be preserved after decarbonylation under H₂ at 623 K and may be recarbonylated at room temperature. A strong physisorption of RhCo₃ (CO)₁₂ on SiO₂ is proposed, due to a nucleophilic attack of surface oxygen on the Co atoms, which promotes a metal-support interaction and thus stabilizes the bimetal cluster framework. A subcarbonyl bimetal cluster is thought to be the actual catalytic species on the surface.     

FTIR study of the rearrangement of adsorbed CO species on Al2O3-supported rhodium catalysts

The adsorption of CO at low temperatures (130–293 K) has been investigated on Rh/Al₂O₃ catalysts of low (0.001–1 wt%) Rh loadings by means of Fourier transform infrared spectroscopy. The surface structure of Rh produced at different reduction temperatures (573 and 1173 K) was shock-cooled to 130 K, where the addition of CO caused the appearance of the band due to bridge-bonded CO ((Rh⁰)₂–CO) on all samples. The appearance of the bands due to gem-dicarbonyl (Rh⁺(CO)₂) and linearly bonded CO (Rh_x–CO) depended on the Rh content and the reduction temperature of the catalysts. The positions and the integrated absorbances of the symmetric and asymmetric stretchings of the Rh⁺(CO)₂ changed with temperature. On the basis of the above findings the rearrangement of the adsorbed CO species (indirectly that of surface Rh) is discussed. This revised version was published online in July 2006 with corrections to the Cover Date.     

Redox chemistry of highly dispersed rhodium in zeolite NaY

NaY zeolite exchanged with [Rh(NH₃)₅Cl]²⁺ ions have been studied using temperature programmed oxidation (TPO), temperature programmed reduction (TPR), and Fourier transformed infrared spectroscopy. The TPO profiles show that ammine ligands in NaY encaged [Rh(NH₃)₅Cl]²⁺ are destroyed above 300 °C, whereas the Rh precursor ion remains intact after calcination at 200 °C. TPR profiles in conjunction with the CO_ads IR spectra show that the reducibility of Rh by H₂ is largely controlled by the concentration of the surface protons, i.e. Rh³⁺+H₂Rh⁺+2H⁺ Rh⁺ + 1/2H₂Rh⁰+H⁺ In the presence of ammonia, the protons are neutralized and Rh³⁺ is reduced to Rh⁰. However, reduction remains incomplete when the concentration of protons is high. The ammonia was provided either by NH₃ admission or by conservation of ammine ligands by controlled calcination. CO adsorption does not lead to reoxidation of Rh⁰ particles to Rh⁺ ions.     

Studies on the interaction between ruthenium and cobalt in supported catalysts in favor of hydroformylation

The interaction between ruthenium and cobalt atoms in SiO₂‐supported catalysts prepared from various precursors by H₂ treatment at 350 °C has been studied by ethylene hydroformylation, temperature‐programmed reduction (TPR) technique and IR spectroscopy. Incorporation of cobalt with ruthenium gives a catalyst with remarkably enhanced hydroformylation activity with respect to those of monometallic catalysts, irrespective of the ruthenium and cobalt precursors used. The synergistic effect of ruthenium and cobalt on the catalysis is consistent with TPR and IR results. TPR analysis shows regularly a promoted reduction of cobalt due to the “hydrogen spillover” effect, which indicates that ruthenium and cobalt atoms are in intimate contact in the catalysts. CO adsorption IR study demonstrates a strong decrease of CO chemisorption on Ru in the presence of cobalt, proposing that ruthenium and cobalt atoms interact on the SiO₂ surface to form Ru–Co bimetallic particles. The results suggest that the catalysts thus obtained contain Ru–Co bimetallic particles, at least atoms of the two metals in intimate contact. However, in situ surface IR spectra of ethylene hydroformylation exhibit little modification by the presence of cobalt on Ru/SiO₂. This revised version was published online in August 2006 with corrections to the Cover Date.     

On the kinetics of CO oxidation by O2 over RhI(CO)2 catalytic species anchored to a zeolitic support

The reactivity of Rh^I(CO)₂ towards CO oxidation was studied on a model Rh(0.7 wt%)/HY material. The kinetic results show that Rh^I(CO)₂ exhibit a fairly low activity. It is therefore suggested that the catalytic species responsible for the enhanced activity of Rh/Ce_0.68Zr_0.32O₂ [Manuel et al., J. Catal. 224 (2004) 269] would rather be electron-deficient Rh clusters (Rh _n ^δ+ ).     

EXAFS characterization and selective olefin hydroformylation on Rh4, Rh2Co2 and RhCo3 carbonyl clusters attached to tris-hydroxymethylphosphine-grafted silica

Rh_4-xCo_x(CO)₁₂ (x = 0, 2, 3) are attached by carbonyl substitution to THP (tris-hydroxymethylphospine)-grafted silica keeping their cluster frameworks. They have been characterized by Rh K-edge EXAFS (extended X-ray absorption fine structure) and Fourier transform IR spectroscopy. They exhibited high catalytic activity with > 98% selectivity in gas phase hydroformylation of ethene and propene to give aldehydes under mild conditions (40 kPa and 300–373 K).On leave from Research Center, Arakawa Chemical Industries, LTD, Tsurumi, Osaka 538, Japan.On leave from Department of Chemistry, National University of Literal, Santiago del Estero, 2829-3000, Santa Fe, Argentina.     

Olefin hydroformylation and selective hydrogenation of acetaldehyde on Mo-promoted Rh/SiO2 catalysts derived from metal salt and heteronuclear cluster precursors

Molybdenum promoted Rh/SiO₂ catalysts have been prepared by using the heteronuclear cluster (C₅H₅)₃RhMo₂(CO)₅ as well as metal salt precursors. The promoting effect of molybdenum has been studied for the hydroformylation of ethene and propene and the hydrogenation of acetaldehyde. It has been found that molybdenum, especially on the cluster-derived catalyst, increases both the hydrogenation and the hydroformylation rate of the olefins. No specific influence on the CO insertion reaction could be obtained. As an explanation, the promotion of the initial step to form intermediate surface alkyl groups has been proposed as the rate determining step for ethene hydroformylation. The promotion of the alcohol formation by bimetallic centers having Rh and Mo in close vicinity has been supported by the results of the hydrogenation of acetaldehyde.     

Organometallics derived (Pd + Ln)/SiO2 catalysts for the reactions of synthesis gas conversion

C₃H₆ hydroformylation and CH₃OH synthesis on organometallics derived (Pd + Ln)/ SiO₂ and Pd/SiO₂ catalysts have been studied. The activity and selectivity towards methanol in CO + H₂ reaction were observed to increase for all the modified catalysts while both the hydroformylation activity and selectivity towards oxygenates in C₃H₆ hydroformylation decreased for the catalysts in comparison to those of Pd/SiO₂. The FTIR, TPD data and characteristic catalytic properties of the catalysts studied allow to suggest that C₃H₆ hydroformylation on (Pd + Ln)/SiO₂ catalysts occurs on monometallic Pd clusters without participation of mixed active sites and CO complexes activated thereon.     

Synergy of ruthenium and cobalt in SiO2-supported catalysts on ethylene hydroformylation

Dynamic hydroformylation of ethylene at atmospheric pressure and 150°C has been studied in a fixed bed reactor over ruthenium- and cobalt-containing SiO₂-supported catalysts (1% Ru loading). Any combination of ruthenium and cobalt precursors leads to significant improvement of hydroformylation activity with respect to those of monometallic catalysts. The optimal atomic ratio of Co:Ru is estimated to be 3:1 for ideal catalytic activity. A catalyst derived from Ru₃(CO)₁₂ and Co₂(CO)₈ is most active. A catalyst derived from metal carbonyls is generally more active than a catalyst prepared from metal salts. Metal chlorides retard the preparation of active catalysts in most cases. The catalysts studied exhibit fairly good catalytic stability. The determined rate enhancement of ethylene hydroformylation suggests a synergy of ruthenium and cobalt, which is understood as catalysis by bimetallic particles or ruthenium and cobalt monometallic particles in intimate contact. The synergy causes high ethylene hydrogenation activity while giving enhanced ethylene hydroformylation activity. Meanwhile, the potential of the ruthenium-based catalysts is evaluated from both catalytic performances and cost by comparison with the corresponding rhodium-based ones.     

Probing the bimetallic RhCo3 cluster preserved on SiO2 after thermal treatment under O2 by hydroformylation

RhCo₃(CO)₁₂/SiO₂, after decarbonylation under atmospheric O₂ at 623 K, exhibits excellent catalytic performances in atmospheric ethylene hydroformylation at 423 K, which is consistent with the corresponding catalysis by the bimetallic cluster catalyst RhCo₃/SiO₂.     

From Electron Density Flow Towards Activation: Benzene Interacting with Cu(I) and Ag(I) Sites in ZSM-5. DFT Modeling

The effects of support pretreatment with nC₁–C₅ alcohols on the performance of Rh–Mn–Li/SiO₂ catalyst in the synthesis of C₂-oxygenates from syngas have been investigated by CO hydrogenation reaction, transmission electron microscopy (TEM), pulse adsorption of CO and H₂, and Fourier Transform infrared (FT-IR) spectroscopy. The catalysts prepared from the pretreated silica supports exhibited higher space time yields of C₂-oxygenates (STY_C2-oxy) and selectivities towards C₂-oxygenates (S_C2-oxy) than that prepared from the untreated silica support. The enhancement in the hydrophobicity of the pretreated silica supports would be favorable for increasing Rh dispersion and ratio of Rh⁺/Rh⁰ sites, therefore increasing the number of active sites, especially the active sites for CO insertion. Such variations are responsible for the improvements in the catalytic performance of the Rh–Mn–Li/SiO₂ catalyst.     

Conversion of syngas to C2+ oxygenates over Rh-based/SiO2 catalyst: The promoting effect of Fe

The effect of Fe promoter on the catalytic properties of Rh–Mn–Li/SiO₂ catalyst for CO hydrogenation was investigated. The catalysts were comprehensively characterized by means of X-ray diffraction (XRD), N₂ adsorption–desorption, temperature programmed reduction (TPR), temperature programmed desorption (TPD), temperature programmed surface reaction (TPSR), and diffuse reflectance infrared Fourier transform spectroscopy (DRIFTS). Activity testing results showed that low loading of Fe (≤0.1 wt%) improved the reactivity and yield of C₂⁺ oxygenates; however, the opposite effect appeared at the high values of Fe (>0.1 wt%). Characterization results suggested that the addition of Fe strengthened the Rh–Mn interaction and increased the desorption/transformation rate of adsorbed CO, which could be responsible for the increase of CO conversion. But on the other hand, the existence of Fe might deposit over the Rh surface, and decreased the number of active sites, resulting in the decrease of CO conversion when the Fe amount was excessive. The selectivity to C₂⁺ oxygenates varied inversely with the reducibility of Rh oxide species. Moreover, it is proposed that the transformation of dicarbonyl Rh⁺(CO)₂ into H–Rh–CO is favorable for the formation of C₂⁺ oxygenates, and the hydrogenation ability of Fe can increase the hydrogenation of acetaldehyde to ethanol.     

Comparative Study on the Mechanisms of Partial Oxidation of Methane to Syngas Over Rhodium Supported on SiO2 and γ-Al2O3

A comparative study on the mechanisms of the partial oxidation of methane (POM) to syngas over SiO₂- and -Al₂O₃-supported Rh catalysts was carried out using in situ time-resolved FTIR spectroscopy to follow the primary products of POM reaction over the catalysts. Experiments of catalytic performance evaluation and temperature-programmed reduction (TPR) characterization of the catalysts, as well as the in situ FTIR spectroscopic study using CO to probe the oxidation state of Rh species over the catalysts were performed. It was found that the direct oxidation of CH₄ to syngas is the main pathway of the POM reaction over Rh/SiO₂ catalysts, while the combustion--reforming mechanism is the dominant pathway of syngas formation over Rh/-Al₂O₃ catalysts. The results of TPR characterization indicate that Rh supported on -Al₂O₃ is more difficult to reduce than Rh supported on SiO₂. The IR experiments of CO adsorption over Rh/SiO₂ and Rh/-Al₂O₃ after the POM reaction reveal that the surface of the Rh/-Al₂O₃ catalyst contains more partially oxidized rhodium (Rh⁺) species as compared to the Rh/SiO₂ catalyst. These results suggest that the significant difference in the mechanisms of the POM reaction over Rh/SiO₂ and Rh/-Al₂O₃ catalysts can be related to the difference in the surface concentration of O^2- species over the catalysts under the reaction conditions mainly due to the difference in oxygen affinity of the Rh species on the two supports.     

Structure and catalytic reactivity of Rh oxides

Using a combination of experimental and theoretical techniques, we show that a thin RhO₂ surface oxide film forms prior to the bulk Rh₂O₃ corundum oxide on all close-packed single crystal Rh surfaces. Based on previous reports, we argue that the RhO₂ surface oxide also forms on vicinal Rh surfaces as well as on Rh nanoparticles. The detailed structure of this film was previously determined using UHV based techniques and density functional theory. In the present paper, we also examine the structure of the bulk Rh₂O₃ corundum oxide using surface X-ray diffraction. Being armed with this structural information, we have explored the CO oxidation reaction over Rh(1 1 1), Rh(1 0 0) and Pt₂₅Rh₇₅(1 0 0) at realistic pressures using in situ surface X-ray diffraction and online mass spectrometry. In all three cases we find that an increase of the CO₂ production coincides with the formation of the thin RhO₂ surface oxide film. In the case of Pt₂₅Rh₇₅(1 0 0), our measurements demonstrate that the formation of bulk Rh₂O₃ corundum oxide poisons the reaction, and argue that this is also valid for all other Rh surfaces. Our study implies that the CO oxidation reaction over Rh surfaces at realistic conditions is insensitive to the exact Rh substrate orientation, but is rather governed by the formation of a specific surface oxide phase.     

Infrared spectroscopic detection of Rh(1) by CO in supported Rh catalyst

The detection limit of Rh(1) in the Rh/Al₂O₃ catalyst in a form of Rh¹(CO)₂ was determined by FTIR spectroscopy. It was demonstrated that at least 0.5 g Rh, corresponding to 0.005 wt% of Rh, can be identified in this way. During synthesis gas conversion the predominant surface species is Rh _x -CO, but a detectable amount of Rh(1) exists on the catalyst up to 473 K.This laboratory is a part of the Center for Catalysis, Surface and Material Science at the University of Szeged.     

Phenoxaphosphino‐Modified Xantphos‐Type Ligands in the Rhodium‐Catalysed Hydroformylation of Internal and Terminal Alkenes

The solubility of the modifying ligand is an important parameter for the efficiency of a rhodium‐catalysed hydroformylation system. A facile synthetic procedure for the preparation of well‐defined xanthene‐type ligands was developed in order to study the influence of alkyl substituents at the 2‐, and 7‐positions of the 9,9‐dimethylxanthene backbone and at the 2‐, and 8‐positions of the phenoxaphosphino moiety of ligands 1 – 16 on solubility in toluene and the influence of these substituents on the performance of the ligands in the rhodium‐catalysed hydroformylation. An increase in solubility from 2.3 mmol⋅L⁻¹ to >495 mmol⋅L⁻¹ was observed from the least soluble to the most soluble ligand. A solubility of at least 58 mmol⋅L⁻¹ was estimated to be sufficient for a large‐scale application of these ligands in hydroformylation. Highly active and selective catalysts for the rhodium‐catalysed hydroformylation of 1‐octene and trans‐2‐octene to nonanal, and for the hydroformylation of 2‐pentene to hexanal were obtained by employing these ligands. Average rates of >1600 (mol aldehyde) × (mol Rh)⁻¹×h⁻¹ {conditions: p(CO/H₂) = 20 bar, T = 353 K, [Rh] = 1 mM, [alkene] = 637 mM} and excellent regio‐selectivities of up to 99% toward the linear product were obtained when 1‐octene was used as substrate. For internal olefins average rates of >145 (mol aldehyde)×(mol Rh)⁻¹×h⁻¹ {p(CO/H₂) = 3.6–10 bar, T = 393 K, [Rh] = 1 mM, [alkene] = 640–928 mM} and high regio‐selectivities up to 91% toward the linear product were obtained.