Silica-supported Rh Research Articles

Mesoporous silica-supported rhodium complexes alongside organic functional groups for catalysing the 1,4-addition reaction of arylboronic acid in water

Mesoporous silica-supported Rh complexes alongside organic functional groups enable 1,4-addition reaction in water.

Atomic-Scale Observation of the Metal-Promoter Interaction in Rh-Based Syngas-Upgrading Catalysts.

The direct conversion of syngas to ethanol, typically using promoted Rh catalysts, is a cornerstone reaction in CO2 utilization and hydrogen storage technologies. A rational catalyst development requires a detailed structural understanding of the activated catalyst and the role of promoters in driving chemoselectivity. Herein, we report a comprehensive atomic-scale study of metal-promoter interactions in silica-supported Rh, Rh-Mn, and Rh-Mn-Fe catalysts by aberration-corrected (AC) TEM. While the catalytic reaction leads to the formation of a Rh carbide phase in the Rh-Mn/SiO2 catalyst, the addition of Fe results in the formation of bimetallic Rh-Fe alloys, which further improves the selectivity and prevents the carbide formation. In all promoted catalysts, Mn is present as an oxide decorating the metal particles. Based on the atomic insight obtained, structural and electronic modifications induced by promoters are revealed and a basis for refined theoretical models is provided.

Influence of a Co-immobilized Tertiary Amine on the Structure and Reactivity of a Rh Complex: Accelerating Effect on Heterogeneous Hydrosilylation

Co-immobilization of a Rh complex and a tertiary amine on the same silica surface enhances the hydrosilylation reaction of olefins. In our previous study, the silica-supported Rh complex-tertiary amine catalyst (SiO2/Rh-NEt2) showed higher activity than a silica-supported Rh complex (SiO2/Rh) alone. Herein, the influence of the amine on the local structure of the Rh complex was evaluated by DNP-enhanced solid-state 15N NMR spectroscopy and Rh K-edge XAFS measured at 20 K. The amine distorts the structure of the cyclooctadiene (COD) ligand in the Rh complex immobilized on the silica surface. The reactivity of the COD ligand with hydrosilane was confirmed by the in situ FTIR spectra, and it was found that the presence of an amine facilitates COD dissociation from the Rh center. This phenomenon accelerates the ligand exchange process in Rh-catalyzed hydrosilylation.

Rh-catalyzed 1,4-addition reactions of arylboronic acids accelerated by co-immobilized tertiary amine in silica mesopores

Mesoporous silica-supported Rh complex catalysts were prepared by simple silane-coupling, followed by complexation, and characterized by FT-IR, SEM, Rh K-edge XAFS, and elemental analysis. Local structures of the Rh complexes in each sample were almost similar to those of a nonporous silica-supported diaminorhodium complex. Co-immobilization of a tertiary amine on the same silica surface induced slight changes to the Rh complex structure in the case of the support with smaller pores. The prepared catalysts showed high activity for the 1,4-addition reaction of phenylboronic acids. Co-immobilization of the tertiary amine increased the reaction rate by more than 7-fold, with turnover number of nearly 8500. The catalytic performance achieved with this novel system is with much higher than that reported previously with a nonporous silica-supported catalyst. The mesoporous silica-supported Rh complex-tertiary amine showed a wide substrate scope, including unsaturated ketones and nitriles. This co-immobilized tertiary amine may activate phenylboronic acid to enhance its reactivity in the transmetalation step with Rh-OH species.

Insights into structure and dynamics of (Mn,Fe)Ox-promoted Rh nanoparticles.

The mutual interaction between Rh nanoparticles and manganese/iron oxide promoters in silica-supported Rh catalysts for the hydrogenation of CO to higher alcohols was analyzed by applying a combination of integral techniques including temperature-programmed reduction (TPR), X-ray photoelectron spectroscopy (XPS), X-ray absorption spectroscopy (XAS) and Fourier transform infrared (FTIR) spectroscopy with local analysis by using high angle annular dark-field scanning transmission electron microscopy (HAADF-STEM) in combination with energy dispersive X-ray spectroscopy (EDX). The promoted catalysts show reduced CO adsorption capacity as evidenced through FTIR spectroscopy, which is attributed to a perforated core-shell structure of the Rh nano-particles in accordance with the microstructural analysis from electron microscopy. Iron and manganese occur in low formal oxidation states between 2+ and zero in the reduced catalysts as shown by using TPR and XAS. Infrared spectroscopy measured in diffuse reflectance at reaction temperature and pressure indicates that partial coverage of the Rh particles is maintained at reaction temperature under operation and that the remaining accessible metal adsorption sites might be catalytically less relevant because the hydrogenation of adsorbed carbonyl species at 523 K and 30 bar hydrogen essentially failed. It is concluded that Rh0 is poisoned due to the adsorption of CO under the reaction conditions of CO hydrogenation. The active sites are associated either with a (Mn,Fe)Ox (x < 0.25) phase or species at the interface between Rh and its co-catalyst (Mn,Fe)Ox.

The Role of Sodium in Tuning Product Distribution in Syngas Conversion by Rh Catalysts

Alkali metal oxides commonly exist as impurities or promoters in syngas conversion catalysts and can significantly influence the activity and selectivity towards higher oxygenate products. In this study, we investigate the effects of sodium oxide on silica-supported Rh catalysts by experimentally introducing different amounts of sodium and monitoring the change in reactivity and CO adsorption behavior. The experimental results combined with density functional theory (DFT) calculations show that sodium selectively blocks step/defect sites on Rh surfaces, leading to reduced activity but higher C2 oxygenate selectivity. DFT calculations also suggest that sodium present on Rh terrace sites can facilitate CO dissociation, potentially increasing C2 oxygenate production. The overall activity and selectivity toward various products can be changed significantly based on the degree of site blocking by the added sodium.

Investigation of inherent differences between oxide supports in heterogeneous catalysis in the absence of structural variations

Oxide supports in heterogeneous catalysts can profoundly influence the catalytic properties of the metal or alloy active phase. Supports prepared by conventional methods can have not only different chemical properties but also different structural properties depending on the oxide. These structural differences can in turn affect the dispersion, distribution, and accessibility of the active phase. In this study, we apply an atomically-controlled synthesis method to isolate the effects arising from the different chemical properties of three oxide supports – titania, alumina and silica – used in Rh catalysts for syngas conversion reactions. We perform atomic layer deposition of titania and alumina to chemically modify the surface of silica gel, without changing its structural characteristics such as surface area and porosity. An inverse catalyst structure is also fabricated by depositing titania and alumina as overlayers onto silica-supported Rh. Titania is found to increase syngas conversion activity and higher hydrocarbon selectivity as both a support layer and an overlayer. An alumina support layer increases Rh nanoparticle dispersion and activity whereas an alumina overlayer decreases the activity of Rh. No significant increase in higher oxygenate selectivity was observed with either titania or alumina.

2+2+2] Cycloaddition Using a Silica-Supported Rh–NHC Catalyst

Key words cycloaddition - rhodium - N-heterocyclic carbenes - silica support

A Selective Functionalized Mesoporous Silica-Supported Rh Catalyst for Effective 1-Octene Hydroformylation

Through different functionalization methods, three kinds of Rh-immobilized mesoporous silicas have successfully been prepared to investigate catalytic behavior, including yield and the linear/branched ratio of aldehyde (L/B) in 1-octene hydroformylation. A conventional post grafting method and two kinds of selective bifunctionalized methods for modification of the mesoporous silica have been applied for this purpose. A relatively high L/B (> 2.0) was effectively achieved using Rh-immobilized inner pores in the MCM-41 support due to the confinement effects of the Rh complex in the nanospace. Moreover, the Rh-immobilized MCM-41 catalyst, passivated with trimethylchlorosilane (TMCS) only on the external surface, showed fairly good yields of the aldehyde (> 40%).

The kinetics of gas-phase propene hydroformylation over a supported ionic liquid-phase (SILP) rhodium catalyst

An investigation of the kinetics of propene hydroformylation in the gas phase has been conducted over a silica-supported Rh–sulfoxantphos complex stabilized by the ionic liquid [bmim][OctSO4]. The reaction temperature was found to have a strong effect on the kinetics of n- and iso-butanal formation. For both products, it was observed that increasing the temperature decreased the apparent activation energy, altered the reaction orders with respect to reactants, and decreased the molar ratio of n- to iso-butanal. The observed changes in the kinetics are discussed in terms of the generally accepted mechanism for olefin hydroformylation and are attributed to a change in the rate-determining step (RDS). It is concluded that at low temperature, the RDS is alkene insertion into an Rh–H bond but becomes the oxidative addition of H2 at high temperature. The change in the RDS is rationalized in terms of a change in the elementary step with the largest Gibbs free energy of activation (ΔG‡). A greater loss in entropy for the oxidative addition of H2 over alkene insertion causes the ΔG‡ of the oxidative addition to be greater than the ΔG‡ of alkene insertion at high temperature.

Hydrodesulfurization properties of rhodium phosphide: Comparison with rhodium metal and sulfide catalysts

Silica-supported rhodium phosphide (Rh 2P/SiO 2) catalysts were prepared and characterized by X-ray diffraction (XRD), transmission electron microscopy (TEM), 31P solid-state NMR spectroscopy, X-ray photoelectron spectroscopy (XPS), and chemisorption measurements. XRD and TEM analysis of a 5 wt.% Rh 2P/SiO 2 catalyst confirmed the presence of well-dispersed Rh 2P crystallites on the silica support having an average crystallite size of 10 nm. NMR spectroscopy showed unsupported and silica-supported Rh 2P to be metallic and XPS spectroscopy yielded a surface composition of Rh 1.94P 1.00 that is similar to that expected from the bulk stoichiometry. The 5 wt.% Rh 2P/SiO 2 catalyst exhibited a higher dibenzothiophene (DBT) hydrodesulfurization (HDS) activity than did Rh/SiO 2 and sulfided Rh/SiO 2 catalysts having a similar Rh loading and was also more active than a commercial Ni Mo/Al 2O 3 catalyst. The Rh 2P/SiO 2 catalyst showed excellent stability over a 100 h DBT HDS activity measurement and was more S tolerant than the Rh/SiO 2 catalyst. The Rh 2P/SiO 2 catalyst strongly favored the hydrogenation pathway for DBT HDS, while the Rh/SiO 2 and sulfided Rh/SiO 2 catalysts favored the direct desulfurization pathway.

Ab Initio Study of CO Hydrogenation to Oxygenates on Reduced Rh Terraces and Stepped Surfaces

Previously reported syngas conversion experiments on silica-supported Rh nanoparticles show that CO conversion and oxygenate selectivity vary as a function of nanoparticle size. Theoretical studies in the literature have examined the effect of steps on CO dissociation, but structure sensitivity for C1 and C2 oxygenates has not been systematically investigated. In this study, density functional theory-based reaction energetics and kinetics for C−H, C−C, C−O, and O−H bond formation on flat Rh(111) and stepped Rh(211) surfaces are reported and compared. Multiple paths for methanol and ethanol formation are considered to ascertain the lowest energy pathways. Nearly an identical methanol formation route via CO → CHO → CH2O → CH3O → CH3OH is found to be favored on both Rh terrace and (211) sites. CO insertion into CH2 is deduced to be the precursor for C2 oxygenate formation irrespective of site structure. Ethanol formation pathways, however, are determined to be markedly different on flat and stepped Rh surfac...

CO hydrogenation on lanthana and vanadia doubly promoted Rh/SiO 2 catalysts

This paper reports on a study of the combined promoting effect of La and V oxides for ethanol formation during CO hydrogenation on silica-supported Rh catalysts. Non-promoted and La and/or V oxide promoted Rh/SiO 2 catalysts were prepared by sequential or co-impregnation methods and characterized by TEM, CO chemisorption and FT-IR. Their catalytic properties for CO hydrogenation were investigated using a differential fixed bed reactor at 230 °C and 1.8 atm. It was found that, compared to non-promoted Rh/SiO 2, the singly promoted catalysts, Rh–La/SiO 2 and Rh–V/SiO 2, showed improved reactivity (3×) and better ethanol selectivities. However, the doubly promoted Rh–La–V/SiO 2 catalysts exhibited even higher activity (9×) and selectivity for ethanol and other C 2+ oxygenates, with the selectivity of total C 2+ oxygenates >30% at these low pressure reaction conditions. The better performance of the Rh–La–V/SiO 2 catalysts appears to be due to a synergistic promoting effect of the combined lanthana and vanadia additions through intimate contact with Rh. Use of just more of each promoter by itself was not able to produce the enhanced catalytic performance.

Hydrogenation of Hindered Ketones with a Silica-Supported Rh complex

Hydrogenation of Hindered Ketones with a Silica-Supported Rh complex

Inverse gas chromatographic investigation of the active sites related to CO adsorption over Rh/SiO 2 catalysts in excess of hydrogen

In the present work, the novel methodology of reversed-flow gas chromatography (RF-GC) is extended in a topic of contemporary scientific and technological interest, such as the effect of hydrogen in the “ topography” of the active sites related to CO adsorption. This study concerns CO adsorption on a silica-supported Rh catalyst, at 90 °C. The topography of the catalyst in the absence of hydrogen consists of both randomly and islands of CO bound over chemisorbed CO molecules. In contrast, under H 2-rich conditions, the observed topography is almost entirely patchwise ascribed to long-range lateral attractions between adsorbate molecules. In excess of hydrogen, CO adsorption is shifted at higher lateral attractions values which correspond to weaker adsorbate–adsorbent interactions and lower surface coverage. This provides an indication of a H 2-induced desorption, which can be attributed to the formation of an H–CO complex desorbing from the catalyst surface below the temperature required for CO desorption, in the absence of H 2, and it may explain the well-known enhancement of the rate of selective CO oxidation in excess of hydrogen by H 2.

Interaction of Pt and Rh nanoparticles with ceria supports: Ring opening of methylcyclobutane and CO hydrogenation after reduction at 373–723 K

The catalytic properties of a Pt (4%) and a Rh (2.5%) catalyst on a low-surface area ceria support were determined as a function of hydrogen reduction in the temperature range between 373 and 723 K and compared to those of silica-supported Rh (3%) subjected to equivalent treatments. Two reactions were studied: the ring opening of methylcyclobutane (MCB) as representative for structure-sensitive hydrocarbon reactions and the hydrogenation of CO as an example for C O bond activation. After reduction in the low- and mid-temperature range the rates of either reaction decrease significantly on ceria-supported Pt and Rh whereas they are hardly affected on Rh–silica. Annealing in vacuum at 723 K before the reduction step has a beneficial effect on the reaction rates but annealing after reduction leads to a general activity decrease. Accompanying ex situ high-resolution electron microscopy (HREM) investigations largely exclude particle decoration and formation of noble metal–Ce intermetallic bonds as possible effects of metal-support interaction at low and medium reduction temperatures. Taking also into account parallel surface science studies it is concluded that the observed activity decrease originates mainly from electronic perturbations at the interface between the metal nanoparticles and the increasingly reduced ceria support.

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CO2 hydrogenation was carried out over alkali metals such as Li, Na and K promoted silica-supported Rh–Co catalysts (Rh–Co–M/SiO2). Na additive was most effective of the three alkali metals for ethanol formation over 5 wt% Rh–Co(1 : 1)/SiO2. The added amount of Na influenced product selectivity as well as CO2 conversion. The highest ethanol selectivity was obtained over 5 wt% Rh–Co–Na (1 : 1 : 0.5)/SiO2 catalyst. The mechanism of promotion effect of Na added amount was investigated by means of in situ FT-IR observation during reaction. The variation of Na added amount changed the ratio of Rh2–(CO)3 and bridged type adsorbed CO species to linear type one. This finding suggested that the amount of Na affected on the hydrogenation ability of Rh–Co/SiO2 catalysts, leading to the difference in CO2 hydrogenation activity.

CO 2 hydrogenation reactivity and structure of Rh/SiO 2 catalysts prepared from acetate, chloride and nitrate precursors

Silica-supported Rh catalysts (Rh/SiO 2) were prepared from acetate, chloride and nitrate precursors by an impregnation method and were applied to CO 2 hydrogenation reaction. CO 2 conversion over the catalyst prepared from chloride precursor was lower than that over acetate or nitrate one, because of fewer active sites on catalysts, as estimated by H 2 chemisorption. The main product was CO over the catalysts prepared from acetate and nitrate, but it was CH 4 over the catalyst prepared from chloride precursor. Characterization of catalysts by TEM, FT-IR and XPS was carried out in order to elucidate the effect of metal precursor on the CO 2 hydrogenation reactivity. The results of XPS showed that the O atomic ratio to Rh on surface hydroxyl groups increased in the order: chloride<nitrate<acetate precursor. The ratio of hydroxyl groups to Rh particles on SiO 2 surface was expected to have a significant influence on the reactivity.

Methane conversion by various metal, metal oxide and metal carbide catalysts

The progress in the field of methane conversion into higher hydrocarbons including aromatics and oxygenated compounds in the recent five years will be reviewed shortly, together with a new type of the methane conversion reaction with carbon monoxide at lower temperatures (600–700 K) by supported group VIII metal catalysts. Benzene was formed selectively among hydrocarbons in the CH4–CO reaction over silica-supported Rh, Ru, Pd and Os catalysts under atmospheric pressure. Both CH4 and CO were required for benzene formation, and only ethane and ethylene were formed besides benzene. The amount of C3–C5 hydrocarbons was negligible, which suggests that a completely different mechanism from the CO–H2 reaction may be operating over these catalysts despite of the similarity in the reaction conditions with the CO–H2 reaction. The mechanism of benzene formation was studied deeply by means of kinetical investigation as well as infrared spectroscopy and isotopic tracer method in connection with that of CO hydrogenation.

Infrared Spectroscopy Study of Aldehydes Adsorbed on Rh−Sn Bimetallic Systems: Selective Activation of Aldehydes by Tin

Infrared spectra of propionaldehyde was studied over silica-supported Rh−Sn bimetallic catalysts. Two absorption bands of the carbonyl group were observed at 1670 and 1720 cm-1, and the aldehyde hydrogen (−CHO) was also observed at 2748 and 2848 cm-1 over the Sn/Rh/SiO2 catalysts on which propionaldehyde was preadsorbed. One of the absorption bands of carbonyl groups at 1670 cm-1 readily disappeared by contact with H2, whereas the other band at 1720 cm-1 remained. The band of the aldehyde hydrogen also disappeared by the contact with H2. The absorption band at 1670 cm-1 was assigned to a donating-on-top η1 adsorbed species that were bound to tin atoms with oxygen atoms of carbonyl groups. The band observed at 1720 cm-1 was assigned to a species weakly adsorbed on the catalyst surface. The intensity of the band at 1670 cm-1 was increased with an Sn/Rh ratio up to unity and then was gradually decreased with the Sn/Rh ratio of the catalysts. The reduction temperature at 573 K yielded a maximum intensity rati...