Rh Catalysts Research Articles

Rh nanoparticles anchored on phosphorous-doped porous carbon for efficiently catalytic hydroformylation of alkenes

The hydroformylation of alkenes to prepare aldehydes using rhodium-based catalysts has been widely investigated, while homogeneous Rh catalysts always suffer from poor reusability. Herein, a promising solution was proposed to introduce phosphorous heteroatoms into a porous carbon support to fabricate a stable and efficient catalyst (Rh/P-mC). And the obtained Rh/P-mC had high hydroformylation activity under mild reaction conditions. The substrate expansion experiment using different alkene substrates and the catalyst cycling experiment demonstrated the universality and stability of this catalyst. The hydroformylation reaction mechanism was also proposed, and the effect of the interaction between phosphorus and rhodium on the hydroformylation reaction was verified. Thus, this work has great application prospects for the highly efficient catalytic hydroformylation of alkenes and provides an efficient method for the design of high-performance hydroformylation catalysts.

Allene Trifluoroacetoxylation with a 2-Phosphinoimidazole-Derived Bimetallic Rh(II) Catalyst

Allene Trifluoroacetoxylation with a 2-Phosphinoimidazole-Derived Bimetallic Rh(II) Catalyst

Nitrogen‐containing Porous Organic Polymer Supported Rhodium Catalyst for Hydroformylation of Olefins

AbstractThe hydroformylation reaction of olefins with syngas (CO/H2) to manufacture aldehydes is one of the largest homogeneous catalytic processes. However, it is important to extend the use of Rh catalysts promoted by nitrogen ligands to heterogeneous system. In this study, a nitrogen‐containing porous organic polymer supported Rh catalyst (Rh/TImT‐TBPT) was successfully synthesized, as well as showed excellent catalytic activity (TOF=216 h−1) and selectivity of aldehydes (98 %) in the hydroformylation of 1‐octene. Furthermore, the catalytic performance of Rh/TImT‐TBPT in the hydroformylation of various olefins was also investigated, displaying outstanding results. The catalytic material was characterized by FT‐IR, XRD, XPS, TEM, SEM and TGA in detail. The characterization data showed that Rh species were highly dispersed on the support, leading to the good stability. More importantly, the Rh/TImT‐TBPT catalyst could be separated from the reaction system at the end of the hydroformylation, and then reused in the next cycle under optimized conditions.

Catalyst Design and Feature Engineering to Improve Selectivity and Reactivity in Two Simultaneous Cross-Coupling Reactions.

Highly active catalysts are required in numerous industrial fields; therefore, to minimize costs and development time, catalyst design using machine learning has attracted significant attention. This study focused on a reaction system where two types of cross-coupling reactions, namely, Buchwald-Hartwig type cross-coupling (BHCC) and Suzuki-Miyaura type cross-coupling (SMCC) reactions, occur simultaneously. Constructing a machine-learning model that considers all experimental conditions is essential to accurately predict the product yield for both the BHCC and the SMCC reactions. The objective of this study was to establish explanatory variables x that considered all experimental conditions within the reaction system involving simultaneous cross-couplings and to design catalysts that achieve the target yield and the development of novel reactions. To accomplish this, Bayesian optimization was combined with established variables x to design new catalysts and enhance reaction selectivity. Moreover, the catalyst design in this study successfully pioneered new reactions involving Cu, Rh, and Pt catalysts in a reaction system that did not previously react with transition metals other than Ni or Pd.

Atomic cluster expansion for Pt–Rh catalysts: From ab initio to the simulation of nanoclusters in few steps

Insight into structural and thermodynamic properties of nanoparticles is crucial for designing optimal catalysts with enhanced activity and stability. In this work, we present a semi-automated workflow for parameterizing the atomic cluster expansion (ACE) from ab initio data. The main steps of the workflow are the generation of training data from accurate electronic structure calculations, an efficient fitting procedure supported by active learning and uncertainty indication, and a thorough validation. We apply the workflow to the simulation of binary Pt–Rh nanoparticles that are important for catalytic applications. We demonstrate that the Pt–Rh ACE is able to reproduce accurately a broad range of fundamental properties of the elemental metals as well as their compounds while retaining an outstanding computational efficiency. This enables a direct comparison of atomistic simulations to high-resolution experiments.Graphical abstract

Rhodium-Catalyzed Intramolecular Nitrogen Atom Insertion into Arene Rings.

In this study, we describe the direct insertion of an intramolecular nitrogen atom into an aromatic C-C bond. In this transformation, carbamoyl azides are activated by a Rh catalyst and subsequently directly inserted into the C-C bond of an arene ring to access fused azepine products. This transformation is challenging, owing to the existence of a competitive C-H amination pathway. The use of a paddlewheel dirhodium complex Rh2(esp)2 effectively inhibited the undesired C-H insertion. Density functional theory calculations were performed to reveal the reaction mechanism and origin of the chemoselectivity of the Rh-catalyzed reactions. The novel fused azepine products are highly robust and allow for downstream diversification.

Rapid Synthesis of Benzimidazole-Fused Isoindoles by Rh(III)/Ru(II)-Catalyzed [4 + 1] Cascade C-H/N-H Annulation of 2-Arylbenzimidazoles with Alkynoates and Alkynamide.

A Rh(III)-catalyzed [4 + 1] cyclization of 2-arylbenzimidazoles with alkynoates through C-H activation/ortho-alkenylation/intramolecular annulation cascade to obtain benzimidazole-fused isoindoles is reported. The reaction of the Rh catalyst and internal alkyne ester provides benzo[4,5]imidazo[2,1-a]isoindole acetate exclusively. Conversely, internal alkyne amide participates in the annulation process in the presence of a Ru catalyst to provide benzo[4,5]imidazo[2,1-a]isoindole acetamide. The alkyne acts as a C1 synthon and undergoes [4 + 1] cyclization rather than traditional [4 + 2] annulation.

Computational Insights into the Influence of Ligands on Hydrogen Generation with [Cp*Rh] Hydrides.

This work reports a computational investigation of the effect of ancillary ligands on the activity of an Rh catalyst for hydrogen evolution based on the [Cp*Rh] motif (Cp* = η5-pentamethylcyclopentadienyl). Specifically, we investigate why a bipyridyl (bpy) ligand leads to H2 generation but diphenylphosphino-based (dpp) ligands do not. We compare the full ligands to simplified models and systematically vary structural features to ascertain their effect on the reaction energy of each catalytic step. The calculations based on density functional theory show that the main effect on reactivity is the choice of linker atom, followed by its coordination. In particular, P stabilizes the intermediate Rh-hydride species by donating electron density to the Rh, thus inhibiting the reaction toward H2 generation. Conversely, N, a more electron-withdrawing center, favors H2 generation at the price of destabilizing the hydride intermediate, which cannot be isolated experimentally and makes determining the mechanism of this reaction more difficult. We also find that the steric effects of bulky substituents on the main ligand scaffold can lead to large effects on the reactivity, which may be challenging to fine-tune. On the other hand, structural features like the bite angle of the bidentate ligand have a much smaller impact on reactivity. Therefore, we propose that the choice of linker atom is key for the catalytic activity of this species, which can be further fine-tuned by a proper choice of electron-directing groups on the ligand scaffold.

Pd(II) and Rh(I) Catalytic Precursors for Arene Alkenylation: Comparative Evaluation of Reactivity and Mechanism Based on Experimental and Computational Studies.

We combine experimental and computational investigations to compare and understand catalytic arene alkenylation using the Pd(II) and Rh(I) precursors Pd(OAc)2 and [(η2-C2H4)2Rh(μ-OAc)]2 with arene, olefin, and Cu(II) carboxylate at elevated temperatures (>120 °C). Under specific conditions, previous computational and experimental efforts have identified heterotrimetallic cyclic PdCu2(η2-C2H4)3(μ-OPiv)6 and [(η2-C2H4)2Rh(μ-OPiv)2]2(μ-Cu) (OPiv = pivalate) species as likely active catalysts for these processes. Further studies of catalyst speciation suggest a complicated equilibrium between Cu(II)-containing complexes containing one Rh or Pd atom with complexes containing two Rh or Pd atoms. At 120 °C, Rh catalysis produces styrene >20-fold more rapidly than Pd. Also, at 120 °C, Rh is ∼98% selective for styrene formation, while Pd is ∼82% selective. Our studies indicate that Pd catalysis has a higher predilection toward olefin functionalization to form undesired vinyl ester, while Rh catalysis is more selective for arene/olefin coupling. However, at elevated temperatures, Pd converts vinyl ester and arene to vinyl arene, which is proposed to occur through low-valent Pd(0) clusters that are formed in situ. Regardless of arene functionality, the regioselectivity for alkenylation of mono-substituted arenes with the Rh catalyst gives an approximate 2:1 meta/para ratio with minimal ortho C-H activation. In contrast, Pd selectivity is significantly influenced by arene electronics, with electron-rich arenes giving an approximate 1:2:2 ortho/meta/para ratio, while the electron-deficient (α,α,α)-trifluorotoluene gives a 3:1 meta/para ratio with minimal ortho functionalization. Kinetic intermolecular arene ethenylation competition experiments find that Rh reacts most rapidly with benzene, and the rate of mono-substituted arene alkenylation does not correlate with arene electronics. In contrast, with Pd catalysis, electron-rich arenes react more rapidly than benzene, while electron-deficient arenes react less rapidly than benzene. These experimental findings, in combination with computational results, are consistent with the arene C-H activation step for Pd catalysis involving significant η1-arenium character due to Pd-mediated electrophilic aromatic substitution character. In contrast, the mechanism for Rh catalysis is not sensitive to arene-substituent electronics, which we propose indicates less electrophilic aromatic substitution character for the Rh-mediated arene C-H activation.

Cobalt/Rhodium-Catalyzed Diversified Amidation of Benzocyclobutenols via C-C Cleavage under Catalyst and Condition Control.

Cobalt(III) and rhodium(III)-catalyzed regio- and chemoselective amidation of benzocyclobutenols has been realized using dioxazolone as the amidating reagent to afford three classes of C-N-coupled products via β-carbon elimination of the benzocyclobutenol. The Co(III)-catalyzed coupling initially afforded an isolable o-(N-acylamino)arylmethyl ketone, which could further cyclize to the corresponding indole derivatives under condition control. In contrast, efficient stepwise diamidation has been achieved under Rh(III) catalyst control. The chemoselectivities are jointly controlled by the catalyst and reactions conditions.

A comparative investigation of equimolar Ni-, Ru-, Rh- and Pt-based composite structured catalysts for energy-efficient methane reforming

Hydrogen as an optimal fuel for mobile and local fuel cell-based power generators creates a request for effective on-board fossil fuel transformation to hydrogen-rich syngas. Structured catalysts based on carriers with high thermal conductivity can improve fuel conversion and decrease external heat input to a reformer through intensification of temperature distribution in the reforming processes. In this work, carriers of the structured catalysts were made from metal grids consisting of Fe, Cr, and Al alloy. Aiming to compare a catalytic activity per a one active metal atom four Me/Ce0.75Zr0.25O2/Al2O3/FeCrAlloy (Me = Ni, Ru, Pt, and Rh) catalysts with similar molar content of an active metal were prepared. The catalysts were characterized by transmission (TEM) and scanning electron microscopies (SEM), X-Ray diffraction analysis (XRD), and X-Ray photoelectron spectroscopy (XPS) before and after catalytic tests. Investigation of the catalysts’ effectivities in methane steam reforming (MSR) showed that the performances change in the order Rh > Ru > Pt > Ni. The Rh-based structured catalyst provided thermodynamic equilibrium product distribution of MSR products at a relatively low temperature of 600 °C and high flow rates. Hydrogen content in the syngas obtained in MSR over Rh-based structured catalyst was up to 70.3 vol% at the weight hour space velocity (WHSV) of 290 700 Nml⋅gacc−1⋅h−1. Investigation of methane autothermal reforming (MATR) over Pt- and Rh-based structured catalysts at 700 °C approved better performances of the Pt-based catalyst for this process. The syngas obtained in MATR over the Pt-based catalyst contained up to ∼ 40 vol% of H2 and 7.4 vol% of CO with a H2/CO molar ratio of ca. 5.5. The optimal conditions for MATR over the Pt-based catalyst were selected and the catalyst stability in long-term reforming was approved. The developed mathematical model for the description of MATR over Pt-based catalyst correlates well with the experimental data and can be used for scale-up of the process.

Electrochemical Promotion of CO2 Hydrogenation Using Rh Catalysts Supported on O2− Conducting Solid Electrolyte

Electrochemical promotion was used to modify the activity and selectivity of a Rh catalyst electrode in the CO2 hydrogenation reaction. The experiments were carried out in a temperature range of 350–430 °C at ambient pressure and at different CO2 to H2 gas feeding ratios (1:2 to 4:1). The only reaction products observed were CO and CH4, both under open- and closed-circuit conditions. The CH4 formation rate was found to increase with both positive and negative potential or current application. The CO formation rate followed the opposite trend. The selectivity to CH4 increased under high values of hydrogen partial pressure and decreased at high pressures of CO2. The results demonstrate how electrochemical promotion can be used to finely tune activity and selectivity for a reaction of high technical and environmental importance.

Rhodium-Catalyzed Asymmetric Dehydrocoupling: Enantioselective Synthesis of a P-Stereogenic Diphospholane with Mistake-Correcting Diastereoselectivity

Catalytic asymmetric dehydrocoupling of secondary phosphines is a potentially valuable route to enantiomerically enriched P-stereogenic diphosphines for use as ligands or building blocks for chiral bis(phosphines). Rh(diphos*) catalyst precursors converted a rac/meso mixture of PhHP(CH2)3PHPh (1) to the C2-symmetric P-stereogenic anti-diphospholane PhP(CH2)3PPh (2) in up to a 58:42 enantiomeric ratio (er) with complete diastereoselectivity via catalyst-mediated isomerization of the intermediate syn-diphospholane 3 to 2 (mistake correction by conversion of the diastereomer meso-3 to chiral C2-2). NMR studies of catalytic reactions identified the resting state Rh((R,R)-i-Pr-DuPhos)(PhHP(CH2)3PPh) (4) and suggested a proposed mechanism for stereocontrolled P–P bond formation via oxidative addition and reductive elimination steps.

Atomic-Dispersed Rh Enables Efficient Catalytic Nitrile Hydrogenation: Size Effect and Metal-Dependent Effect

The hydrogenation of nitriles to amines, as an atom-economical synthetic route, has attracted wide attention in industrial production. Despite that some catalysts for nitrile hydrogenation have been investigated in recent years, achieving high activity and selectivity in this process remains a major challenge. Here, we fabricated an atomically dispersed Rh catalyst anchored on defect-rich nanodiamond-graphene (ND@G) via a deposition–precipitation strategy (Rh1/ND@G), which showed the highest activity (TOF = 2592 h–1) in the hydrogenation of benzonitrile reaction among all the known catalysts under mild reaction conditions (333 K, 0.6 MPa H2), and delivered high selectivity (>99%) toward secondary amines. Significantly, we prepared the Rh-cluster catalyst (Rhn/ND@G) and a series of atomically dispersed M1 catalysts (M = Rh, Ru, Pd, Ir, and Pt) to understand the size effect and metal-dependent effect in the hydrogenation of benzonitrile. Experimental and theoretical investigations revealed that the Rh1/ND@G catalyst has higher selectivity toward dibenzylamine than Rhn/ND@G, as it is less likely to form benzylamine on the Rh1 active sites. Meanwhile, the difference of adsorption energies (as a catalyst descriptor) between benzylideneimine and benzonitrile was used to understand the activity of benzonitrile hydrogenation influenced by the metal-dependent effect of M1 catalysts. Modulating the size and metal-dependent effects could lead to high catalytic performance, paving the way to design efficient nitrile hydrogenation catalysts.

Dry reforming of methane over supported Rh and Ru catalysts: Effect of the support (Al2O3, TiO2, ZrO2, YSZ) on the activity and reaction pathway

The effect of Al2O3, TiO2, ZrO2 and Yttria-Stabilized Zirconia (YSZ) supports on the activity of the dry reforming of CH4 towards syngas production was investigated for Rh and Ru catalysts. Ruthenium-based catalysts were found to generally be more active than Rh leading H2/CO ratios close to unity above 700 °C. Methane conversion turnover frequency (TOF) over Rh catalysts becomes optimum for Rh/YSZ followed by Rh/TiO2 and Rh/ZrO2, and finally Rh/Al2O3, which exhibited the lowest activity. The performance order is different for Ru catalysts, with TOF being increased in the order Ru/Al2O3<Ru/TiO2<Ru/YSZ < Ru/ZrO2. Time-on-stream (TOS) stability tests showed that Rh/ZrO2 is stable for 30 h, whereas Ru/ZrO2 exhibits a slight decrease of both reactants’ conversions. DRIFTS studies indicated that the reaction proceeds via a bifunctional mechanism involving both the metal and the support in the reaction pathway. The population and adsorption sites of produced carbonyls are strongly influenced by the metal-support combination determining the catalytic activity.

Highly Active and Durable Rh–Mo-Based Catalyst for the NO–CO–C3H6–O2 Reaction Prepared by Using Hybrid Clustering

We developed a method for preparing catalysts by using hybrid clustering to form a high density of metal/oxide interfacial active sites. A Rh–Mo hybrid clustering catalyst was prepared by using a hybrid cluster, [(RhCp*)4Mo4O16] (Cp* = η5-C5Me5), as the precursor. The activities of the Rh–Mo catalysts toward the NO–CO–C3H6–O2 reaction depended on the mixing method (hybrid clustering > coimpregnation ≈ pristine Rh). The hybrid clustering catalyst also exhibited high durability against thermal aging at 1273 K in air. The activity and durability were attributed to the formation of a high-density of Rh/MoOx interfacial sites. The NO reduction mechanism on the hybrid clustering catalyst was different from that on typical Rh catalysts, where the key step is the N–O cleavage of adsorbed NO. The reducibility of the Rh/MoOx interfacial sites contributed to the partial oxidation of C3H6 to form acetate species, which reacted with NO+O2 to form N2 via the adsorbed NCO species. The formation of reduced Rh on Rh4Mo4/Al2O3 was not as essential as that on typical Rh catalysts; this explained the improvement in durability.

Synthesis of Benzo[c]azepine‐1,3(2H)‐diones via C−H Alkylation/Cyclization with α,β‐Unsaturated Acyl Fluorides

AbstractTransition‐metal‐catalyzed directed C−H functionalization reactions are a powerful method to construct N‐heterocycles. However, compared to the formation of five‐ and six‐membered rings, that of seven‐membered rings has been much less explored. Here, the synthesis of benzo[c]azepine‐1,3(2H)‐diones is described, which are benzene‐fused seven‐membered imides, from hydroxamates and α,β‐unsaturated acyl fluorides via C−H activation using a Cp*Rh(III) catalyst. Under mild reaction conditions, this reaction affords benzo[c]azepine‐1,3(2H)‐diones that bear a substituent at the 5‐position.

Catalyst-Controlled Enantioselective and Regiodivergent Addition of Aryl Boron Nucleophiles to N-Alkyl Nicotinate Salts.

Dihydropyridines are versatile building blocks for the synthesis of pyridines, tetrahydropyridines, and piperidines. Addition of nucleophiles to activated pyridinium salts allows synthesis of 1,2-, 1,4-, or 1,6-dihydropyridines; however, this process often leads to a mixture of constitutional isomers. Catalyst-controlled regioselective addition of nucleophiles to pyridiniums has the potential to solve this problem. Herein, we report that the regioselective addition of boron-based nucleophiles to pyridinium salts can be accomplished by the choice of a Rh catalyst.

One-Pot Synthesis of Benzo[c]phenanthridine Alkaloids from 7-Azabenzonorbornadienes and Aryl Nitrones.

An efficient synthesis of benzo[c]phenanthridine alkaloids via a synergistic combination of C-C bond formation and a cycloaromatization reaction is described. Aryl nitrones react with 7-azabenzonorbornadienes in the presence of a Rh(III) catalyst, providing pharmaceutically useful benzo[c]phenanthridine derivatives in good to moderate yields. Using this methodology, highly useful alkaloids such as norfagaronine, norchelerythrine, decarine, norsanguinarine, and nornitidine were prepared in a single step.

Observations of Ethylene-for-CO Ligand Exchanges on a Zeolite-Supported Single-Site Rh Catalyst by X-ray Absorption Spectroscopy

Quick-scanning X-ray absorption fine structure (QXAFS) measurements were used to characterize the exchanges of ethylene and CO ligands in a zeolite HY-supported single-site Rh complex at a sampling rate of 1.0 Hz. The two ligands were reversibly exchanged on the rhodium, with quantitative results determined for the C2H4-for-CO exchange that are consistent with a first-order process. The apparent rate constant for the exchange decreased with increasing temperature. Fourier-transform infrared spectra characterizing the C2H4 sorbed in the zeolite showed that the amount decreased with increasing temperature, consistent with the decrease in the exchange rate with increasing temperature. The results, illustrating the dynamics of ligand exchanges on a single-site supported metal catalyst, demonstrate the broad emerging applicability of the QXAFS technique for characterizing the dynamics of reactive intermediates on catalysts.