Silica-supported Rhodium Catalysts Research Articles

Promoted Hydroformylation of Formaldehyde By Electronic Metal–Support Interactions in N-Group Functionalized Silica Supported Rhodium Catalyst

The influence of surface modification of SiO2 carrier by grafting method and its effects for Rh supporting was investigated on formaldehyde hydroformylation reaction. The relationship between the structure of catalysts and their performance was characterized in detail by X-ray diffraction, N2 adsorption–desorption, temperature programmed reduction with H2, X-ray photoelectron spectroscopy, and in situ infrared. After grafting the amino organic functional group, the surface circumstance of SiO2 was altered, and moreover, sites available for metal loading was enriched on the carrier that distribute a high dispersion of Rh species Under the same Rh loading, the Rh@N–SiO2 catalyst exhibits better catalytic activity than pristine SiO2 supported Rh catalysts. Spectroscopic evidences suggest the presence of an electronic perturbation between Rh and modified SiO2 which promotes the hydroformylation of formaldehyde by maintaining Rh at proper Rh0/(Rh3+ + Rhδ+) ratio. Diagram of the effect of surface modified with amino group on metal rhodium loading.

Selective hydrogenation of phenol to cyclohexanone by SiO2-supported rhodium nanoparticles under mild conditions

A silica-supported rhodium catalyst for the selective hydrogenation of phenol to cyclohexanone under mild conditions has been developed. As the Rh concentration on the catalyst increased from 0.5 to 15 wt%, the conversion (at phenol/Rh mole ratio 100/1) dropped whereas the initial selectivity to cyclohexanone increased. The direct hydrogenation to cyclohexanol occurred in parallel with partial hydrogenation to cyclohexanone. The negative correlation between selectivity and Rh dispersion suggests that direct hydrogenation occurs at low coordination sites whereas dissociation of phenol to phenoxy followed by hydrogenation to cyclohexanone takes place at higher coordinated terrace sites. DFT calculations revealed that the activation barrier for O–H bond cleavage is lower for phenol adsorbed on a Rh(1 1 1) flat surface than on small particles. By blocking the low coordination edge and step sites through grafting with (3-mercaptopropyl)trimethoxysilane, the cyclohexanone selectivity was improved from 82 to 93% at 100% conversion. The catalyst is active at room temperature and 1 atm H2 pressure and can be easily activated by in-situ reduction.

Effect of pore structure on catalytic properties of mesoporous silica supported rhodium catalysts for the hydrogenation of cinnamaldehyde

Rhodium particles for catalyzing the hydrogenation were effectively confined in the mesopores of MCM-41 and MSU-H silica by using supercritical CO2 as impregnation solvent. It was found that the calcination was an important step to control the size of Rh particles, and its influence was strongly dependent on the pore size distribution of mesoporous silica support. The prepared catalysts were characterized by X-ray diffraction (XRD), nitrogen adsorption, and transmission electron microscopy (TEM). While irregularly big Rh particles were often found on MCM-41 with smaller pore size (2.7nm) during calcination at 873K, homogeneous and small Rh particles were maintained inside MSU-H with relatively larger pore size (8.4nm). The hydrogenation of cinnamaldehyde using the prepared catalysts was carried out in supercritical CO2. Rh/MSU-H which was calcined at 673K provided the highest catalytic activity among the studied conditions. The results indicated that the catalyst on the support with larger pore size and smaller size of Rh particles might be desirable to attain higher catalytic activity.

Synthesis of Chiral Phthalides Using a Silica-Supported Rhodium Catalyst

Key words hollow-shell nanospheres - organorhodium catalysis - tandem reduction–lactonization - heterogeneous catalysis

Comparative study on stability and coke deposition over supported Rh and FePO4 catalysts for oxy-bromination of methane

Rhodium- and iron phosphate-based catalysts are by far the most promising catalysts for oxy-bromination of methane (OBM) reaction. However, most literature reported either Rh- or FePO4-based catalysts, and the results were rarely studied in a uniform environmental condition. In this report, comparative study was conducted on silica- and silicon carbide-supported rhodium and iron phosphate catalysts with the main focuses on stability performance and coke deposition. The catalytic results demonstrated that the stability of both Rh- and FePO4-based catalysts was greatly influenced by the supports used, and silicon carbide-supported catalysts showed much better anti-coking ability as compared with silica-supported ones. Temperature-programmed oxidation over the used catalysts further indicated that the coke formation mechanisms were completely different between silica-supported rhodium and iron phosphate catalysts. While cokes might be caused by condensation of CH2Br2 over supported iron phosphate, methane decomposition might be the reason for coke formation over silica-supported rhodium catalyst. These findings might pave the way for designing highly efficient and stable catalysts of the OBM reaction.

Mechanism of the hydrogenolysis of ethers over silica-supported rhodium catalyst modified with rhenium oxide

The Rh–ReO x /SiO 2 (Re/Rh = 0.5) exhibited high activity in the hydrogenolysis of ethers with an OH group. The C O bond neighboring CH 2OH group was selectively dissociated: The hydrogenolysis of tetrahydro-5-methyl-2-furfuryl alcohol and 2-isopropoxyethannol gave 1,5-hexanediol and ethanol + isopropanol, respectively. This tendency suggests the regioselective C O dissociation mechanism via anion intermediate formed by the attack of hydride and the subsequent protonation of the anion.

Effect of pore structure of amine-functionalized mesoporous silica-supported rhodium catalysts on 1-octene hydroformylation

Amine-functionalized mesoporous silicas with different pore sizes (MCM-41, SBA-15 and amorphous silica) were prepared using the post-synthesis method. Subsequently, rhodium was immobilized on the aminated mesoporous silica materials in order to be evaluated as a heterogeneous catalyst for 1-octene hydroformylation. Two kinds of amine compounds, namely ( N(β-aminoethyl) γ-aminopropylmethyldimethoxysilane (AEAPMDMS) and 3-aminopropyltriethoxysilane (APTES) were compared as functional groups for the immobilization of the rhodium complex. Three kinds of rhodium-immobilized mesoporous silicas whose pore structure differs from one another, such as MCM-41 (pore size; 2.5–2.7 nm, ordered hexagonal pore structure), SBA-15 (pore size; 4.2–4.5 nm, ordered hexagonal pore structure) and amorphous silica (pore size; 8.8–9.2 nm, worm-like structure) were selected to elucidate the effect of the pore structure on the 1-octene hydroformylation. The larger pore and ordered pore structure would be favorable in terms of total aldehyde yield and activity. In addition, AEAPMDMS, which has two nitrogen atoms, was superior to APTES as a functional agent in the 1-octene hydroformylation due to its stronger electron-donating effect toward the Rh complex. Among the synthesized catalysts, SBA-15/AEAPMDMS/Rh represented the highest yield of aldehyde in the 1-octene hydroformylation at about 48%.

Silica-Supported Rhodium Catalyst for Hydrosilylation of Alkenes

Rhodium siloxide immobilized on silica support was prepared and used for hydro­silylation of alkenes as a recyclable catalyst. Thus, [Rh(µ-OSiMe3)(cod)]2 reacted with aerosil silica (Aerosil 200) in hexane or benzene at r.t. for 24 h to give the rhodium siloxide immobilized on silica support. Hydrosilylation of alkenes with hepta­methylhydrotrisiloxane was carried out with the heterogeneous Rh siloxide catalyst (0.01 mol% Rh) to afford the corresponding hydrosilylation products in 88-97% yield (4 examples; TOF = 147-162/min). The catalyst was reused 15-20 times without significant loss of catalytic activity.

Catalysis of the Heck-type reaction of alkenes with arylboronic acids by silica-supported rhodium: an efficient phosphine-free reusable catalytic protocol

A 3-aminopropyl-functionalized silica-supported rhodium(0) catalyst (SiO2–Rh0) was prepared and employed in the Heck-type coupling of arylboronic acids and alkenes, affording good-to-excellent yields of substituted alkenes; the catalyst was recovered by filtration and reused for several cycles.

Time distribution of adsorption entropy of gases on heterogeneous surfaces by reversed-flow gas chromatography

The reversed-flow gas chromatography (RF-GC) technique has been applied to measure the adsorption entropy over time, when gaseous pentane is adsorbed on the surface of two solids (γ-alumina and a silica supported rhodium catalyst) at 393.15 and 413.15 K, respectively. Utilizing experimental chromatographic data, this novel methodology also permits the simultaneous measurement of the local adsorption energy, ɛ, local equilibrium adsorbed concentration, c s * , and local adsorption isotherm, θ( p, T, ɛ) in a time resolved way. In contrast with other inverse gas chromatographic methods, which determine the standard entropy at zero surface coverage, the present method operates over a wide range of surface coverage taking into account not only the adsorbate–adsorbent interaction, but also the adsorbate–adsorbate interaction. One of the most interesting observations of the present work is the fact that the interaction of n-pentane is spontaneous on the Rh/SiO 2 catalyst for a very short time interval compared to that on γ-Al 2O 3. This can explain the different kinetic behavior of each particular gas–solid system, and it can be attributed to the fact that large amounts of n-C 5H 12 are present on the active sites of the Rh/SiO 2 catalyst compared to those on γ-Al 2O 3, as the local equilibrium adsorbed concentration values, c s * , indicate.

The Effect of Rh Particle Size on the Catalytic Performance of Porous Silica supported Rhodium Catalysts for CO Hydrogenation

Abstract Silica-supported Rh catalysts with different Rh particle dimensions were investigated for CO hydrogenation. The catalysts were characterized by various techniques such as TEM, H2-TPR and N2 adsorption to study the catalyst morphology, the size distributions of Rh particles and the silica pores. It was found that the distribution and the size of Rh particles were affected by the silica pores, and the metal grains were enclosed in the pores of the support, and thereby their growth was limited. The catalytic activity and selectivity to C2-oxygenates for CO hydrogenation were found to be significantly controlled by the Rh particle sizes, and the higher activity and selectivity to C2-oxygenates were obtained over bigger Rh particles, within the range of the reported particle sizes.

Gas chromatographic investigation of the competition between mass transfer and kinetics on a solid catalyst

The reversed-flow gas chromatography (RF-GC) method is used to investigate the competition between mass transfer and kinetics in heterogeneous catalysis. The well-studied dissociative adsorption of carbon monoxide over a silica supported rhodium catalyst at various temperatures is used as model system. The Thiele-type modulus Φ s and the effectiveness factor η are calculated for both adsorbate (CO) and product (CO 2), from the experimental chromatographic peaks. The values experimentally found are similar to those predicted theoretically and give interesting information for the mechanism of the interaction of carbon monoxide with the catalyst studied.

Study of CO Dissociative Adsorption over Pt and Rh Catalysts by Inverse Gas Chromatography

Carbon monoxide dissociative adsorption was studied over silica-supported platinum, rhodium and Pt-Rh alloy catalysts, by Reversed Flow-Gas Chromatography. Using appropriate mathematical analysis physicochemical quantities such as fractional catalytic conversions of CO to CO2, as well as rate constants for the adsorption, desorption and surface reaction, describing the dissociative adsorption of CO, were determined. From the variation of the above parameters against the nature of the studied catalysts (Rh content) useful conclusions concerning the mechanism of CO dissociative adsorption were extracted.

Characterization and catalytic performances of alkali-metal promoted Rh/SiO2 catalysts for propene hydroformylation

The influence of the cationic promoters (M=Li,Na,K,Rb,Cs) on silica-supported rhodium catalysts was examined in the hydroformylation of propene at 413 K and atmospheric pressure. The catalysts were prepared by aqueous co-impregnation of RhCl3·3H2O and alkali-metal chloride. The rhodium to promoter molar ratio was 1. A pre-treatment cycle under O2/H2/O2 at 673 K was carried out on the catalyst before the admission of the hydroformylation mixture. By in situ Rh K-edge extended X-ray adsorption fine-structure (EXAFS) characterization of the activated catalyst, two different Rh and alkali-metal oxide phases, highly dispersed and in intimate contact with each other, were detected on the support surface. Under catalytic conditions, small Rh metal particles are formed, with a mean diameter of 3.2 nm, as detected by HRTEM. The activity in aldehyde formation depends on the alkali-metal promoter in the following order: Li>Na>K>Rb⪢Cs. This behavior parallels the trend of the polarizing power of the cation within the Group 1.

Influence of residual chloride ions in the CO hydrogenation over Rh/SiO2 catalysts

Silica-supported rhodium catalysts prepared from nitrate and chloride precursors were tested in the CO+H 2 reaction. The ex -chloride catalyst showed a lower 1-olefin/ n -paraffin ratio. From the H 2 TPD results, it is suggested that residual chloride ions favour H 2 spillover from the Rh metal to the SiO 2 surface, thus leading to new active sites where olefins are hydrogenated to the corresponding paraffins. Silica-supported rhodium catalysts prepared from nitrate and chloride precursors were tested in the CO hydrogenation reaction. Similar reaction rate and selectivities to the different product families were found for both catalysts, however the ex -chloride catalyst showed a lower 1-olefin/ n -paraffin ratio. Characterisation by X-ray photoelectron spectroscopy (XPS), transmission electron microscopy (TEM), H 2 chemisorption and Fourier transform infrared spectroscopy (FTIR) of CO chemisorbed did not evidence disparate features between ex -nitrate and ex -chloride samples. However, a much higher H 2 desorption was found in the ex -chloride catalyst according to the hydrogen temperature-programmed desorption (TPD) studies. Residual chloride species seems to be involved in the H 2 adsorption on silica enhancing the spillover of hydrogen from metal particles to the silica support. It is suggested that spilt-H atoms can create active sites on the silica surface where olefins can be hydrogenated to the corresponding paraffins.

An Inverse Gas Chromatographic Instrumentation for the Study of Carbon Monoxide's Adsorption on Rh/SiO2 Catalyst, Under Hydrogen‐Rich Conditions

A new and simple methodology for the study of the adsorption of carbon monoxide over a silica supported rhodium catalyst, referring to a wide range of hydrogen‐rich atmosphere (25%–75% H2), in experimental conditions compatible with the operation of fuel‐cells is presented. Experimentally, it is based on an inverse gas chromatographic technique known as reversed flow gas chromatography (RFGC), which is combined with the appropriate mathematical analysis which permits the measurement of adsorption, desorption, and surface reaction rate constants, as well as local adsorption energies, local isotherms, and surface diffusion coefficients. The values of the parameters of the present work vary with the hydrogen amount into the H2‐rich gas atmosphere and they are comparable to those obtained by other experimental, as well as theoretical techniques.

1H MAS NMR characterization of hydrogen over silica-supported rhodium catalyst

Hydrogen species in both SiO2 and Rh/SiO2catalysts pretreated in different atmospheres (H2, O2, helium or air) at different temperatures (773 or 973 K) were investigated by means of1H MAS NMR. In SiO2 and O2-pretreated catalysts, a series of downfield signals at ∼7.0, 3.8–4.0, 2.0 and 1.5–1.0 were detected. The first two signals can be attributed to strongly adsorbed and physisorbed water and the others to terminal silanol (SiOH) and SiOH under the screening of oxygen vacancies in SiO2lattice, respectively. Besides the above signals, both upfield signal at ∼−110 and downfield signals at 3.0 and 0.0 were also detected in H2-pretreated catalyst, respectively. The upfield signal at ∼−110 originated from the dissociative adsorption of H2 over rhodium and was found to consist of both the contributions of reversible and irreversible hydrogen. There also probably existed another dissociatively adsorbed hydrogen over rhodium, which was known to be β hydrogen and in a unique form of “delocalized hydrogen”. It was presumed that the β hydrogen had an upfield shift of ca. −20–−50, though its1H NMR signals, which, having been masked by the spinning sidebands of Si-OH, failed to be directly detected out. The downfield signal at 3.0 was assigned to spillover hydrogen weakly bound by the bridge oxygen of SiO2. Another downfield signal at 0.0 was assigned to hydrogen held in the oxygen vacancies of SiO2 (Si-H species), suffering from the screening of trapped electrons. Both the spillover hydrogen and the Si-H resulted from the migration of the reversible hydrogen and the β hydrogen from rhodium to SiO2 in the close vicinity. It was proved that the above migration of hydrogen was preferred to occur at higher temperature than at lower temperature.

Ethylene hydroformylation and carbon monoxide hydrogenation over modified and unmodified silica supported rhodium catalysts

Ethylene hydroformylation and carbon monoxide hydrogenation (leading to methanol and C 2-oxygenates) over Rh/SiO 2 catalysts share several important common mechanistic features, namely, CO insertion and metal–carbon (acyl or alkyl) bond hydrogenation. However, these processes are differentiated in that the CO hydrogenation also requires an initial CO dissociation before catalysis can proceed. In this study, the catalytic response to changes in particle size and to the addition of metal additives was studied to elucidate the differences in the two processes. In the hydroformylation process, both hydroformylation and hydrogenation of ethylene occurred concurrently. The desirable hydroformylation was enhanced over fine Rh particles with maximum activity observed at a particle diameter of 3.5 nm and hydrogenation was favored over large particles. CO hydrogenation was favored by larger particles. These results suggest that hydroformylation occurs at the edge and corner Rh sites, but that the key step in CO hydrogenation is different from that in hydroformylation and occurs on the surface. The addition of group II–VIII metal oxides, such as MoO 3, Sc 2O 3, TiO 2, V 2O 5, and Mn 2O 3, which are expected to enhance CO dissociation, leads to increased rates in CO hydrogenation, but only served to slow the hydroformylation process slightly without any effect on the selectivity. Similar comparisons using basic metals, such as the alkali and alkaline earths, which should enhance selectivity for insertion of CO over hydrogenation, increased the selectivity for the hydroformylation over hydrogenation as expected, although catalytic activity was reduced. Similarly, the selectivity toward organic oxygenates (a reflection of the degree of CO insertion) in CO hydrogenation was also increased.

Preparation method for supported metal catalysts using w/o microemulsion: Study on immobilization conditions of metal particles by hydrolysis of alkoxide

It was previously found that the silica-supported rhodium catalyst prepared using water-in-oil microemulsion had rhodium particles partly, or wholly, embedded in silica. In this work, consequently, we investigated the effect of hydrolysis conditions of tetraethylorthosilicate, employed as the source of silica, on the atomic ratio of surface rhodium in contact with the gas phase, to total surface rhodium of nanoparticles. This ratio is denoted as R in this paper. R became higher when the catalyst was prepared under the following hydrolysis conditions: a shorter hydrolysis time and a smaller amount of tetraethylorthosilicate. On the other hand, R showed the minimum value when the water content in the preparation solution was 33 vol%. From these results, it is demonstrated that it was important to form silica as early as possible in hydrolysis of TEOS in order to increase R values. In addition, the effect of R on the catalytic behavior in CO hydrogenation was investigated. At R values below 30%, the turnover frequencies increased with a decrease in R.

Size control of rhodium particles of silica-supported catalysts using water-in-oil microemulsion

Effects of components of water-in-oil microemulsions on rhodium particle sizes of silica-supported rhodium catalysts were investigated in the catalyst preparation method using microemulsion. In the case of the microemulsion of polyoxyethylene(23)dodecyl ether/ n-alcohols/RhCl 3 aq., the rhodium particle size increased from 3.4 to 5.0 nm as the specific permittivity of the organic solvent increased. The chain length of hydrophilic group of polyoxyethylene- p-nonylphenyl ether ( n = 5 to 15) employed as surfactants had an effect on the rhodium particle size where the rhodium size ranged between 2.0 and 3.6 nm. The rhodium particle size was 1.5 nm in the case of sodium bis(2-ethylhexyl) sulfocuccinate and this value was found to be the smallest. These results could be interpreted in terms of the adsorption of the surfactant on rhodium-hydrazine particle surface.