Rh Catalysts Research Articles

Metal-catalyzed divergent synthesis from ylides with 3-arylbenzo[d][1,2,3]triazin-4(3H)-ones.

The present work reveals a new metal-catalyzed synthetic reaction involving 1,2,3-benzotriazinones with carbonyl sulfoxonium ylide and iodonium ylide, resulting in divergent products. Within this catalytic system, 3-phenylbenzo[d][1,2,3]triazin-4(3H)-one derivatives undergo C-H alkylation processes facilitated by a Cp*Rh(III) catalyst when combined with a carbonyl sulfoxonium ylide. On the other hand, when iodonium ylide substrates are used, they undergo an alkenylation reaction facilitated by a Cp*Ir(III) catalyst. In addition, hydrazone products are produced by synthesizing iodonium ylide substrates with the use of a copper catalyst. These transformations demonstrate mild reaction conditions, a wide range of substrates, and excellent compatibility with various functional groups. The strategy and tactics utilized have been effectively implemented on a significant scale.

Integrated electrochemical CO2 reduction and hydroformylation.

The development of integrated multi-catalyst processes has become of high interest to transform abundant feedstocks or environmental pollutants to commodity chemicals in a one pot, one pass fashion. Specifically, CO2 poses a large environmental burden and would thus be a desirable, relatively abundant C1 source in multi-step synthetic chemistry. Herein we disclose the synthesis of aldehydes from CO2via the integration of electrochemical CO2 reduction (CO2RR) and hydroformylation, taking advantage of the typically unwanted concomitant hydrogen evolution (HER) to generate the necessary CO and H2 needed for hydroformylation. Though typical hydroformylation catalysts based on Rh would be deactivated under CO2RR conditions, we circumvent this limitation by spatially segregating our CO2RR and hydroformylation systems in a vial-in-vial reactor, while allowing CO and H2 transport between catalyst sites. In this manner, 97% aldehyde yield from CO2RR and styrene was achieved selectively using a classic homogeneous hydroformylation catalyst in HRh(CO)(PPh3)3, and 43% aldehyde yield was obtained using a heterogenized version of this Rh catalyst onto mesoporous silica. This work not only repurposes undesired HER in CO2RR and prepares aldehydes from CO2 without added H2, but expands the scope of processes that transform feedstocks all the way to commodity chemicals in a one pass manner.

CO Oxidation over Ligand Coordinated Single Site Rh Catalyst: Identification of Active Complex

Single atom catalysis has evolved as a promising strategy to enhance atom utilization efficiency, lower reaction temperatures, and control reaction pathways in heterogeneous catalytic reactions. An important challenge using supported...

Hydrogenolysis of isosorbide to diols and triols over a heterogeneous SiO2-supported Rh catalyst

SiO2-supported Rh (Rh/SiO2) was an effective and reusable heterogeneous catalyst for the hydrogenolysis of isosorbide to diols and triols, and 58% total yield of diols and triols was achieved.

Diffusion-Restricted Cation Exchange Derived Rhodium Nanoparticles for Hydrazine Assisted Hydrogen Production

Water splitting using renewable energy is a promising hydrogen production method without carbon emission. However, oxygen evolution reaction still suffers from its large overpotential and sluggish kinetics. Thus, alternative oxidation reactions rather than oxygen evolution reaction, such as ammonia, alcohols and hydrazine oxidation reaction are developed for hydrogen production. Rh is one of the most promising catalysts for electrochemical hydrazine splitting that can promote hydrogen evolution reaction on the cathode, which is a much more energy-saving way to generate hydrogen gas than water splitting. Unfortunately, Rh is also one of the most expensive novel metals on the market. Nevertheless, only a few studies have considered the amount of used Rh. In this study, the diffusion-restricted cation exchange (CE) process is suggested as an effective method to reduce the mass of inactive Rh for enhanced mass activity. By immersing the NiOOH substrate in the Rh3+ aqueous solution, Rh3+ atoms are easily exchanged with Ni3+ atoms in the NiOOH lattice on the surface, and the RhOOH forms on the outermost layer. Then, the RhOOH compounds are reduced into metallic rhodium by an electrochemical reduction process, resulting in fine Rh nanoparticles smaller than 2 nm. Due to the suppression of Rh aggregation, a doubled mass activity for electrocatalytic hydrazine oxidation reaction is attained compared to that of conventional electrodeposited Rh catalysts. As a result, the proposed CE-derived Rh catalyst shows stability over 36 hours under the two-electrode hydrazine splitting system.

The mono and bimetallic catalysts for hydrogenation of PAO lubricants

AbstractUsing halloysite as a natural support and palladium and rhodium salts, one bimetallic and two monometallic catalysts were prepared through wet‐impregnation method. The catalysts were characterized via TGA, XRD, FTIR, EDS and elemental mapping analyses and their activity for hydrogenation of poly‐alpha‐olefin oil was examined and compared. The results indicated that among the three catalysts, halloysite supported Pd nanoparticles and bimetallic halloysite supported Pd−Rh catalyst exhibited the highest and lowest activity, respectively. To clarify the origin of the observed difference in the activity of the catalyst, they have been further characterized via ICP and TEM analyses. The result indicated that the nature of the metallic species affected the loading and dispersion of the formed metallic nanoparticles. In the case of halloysite supported Pd nanoparticles, while metal loading was the highest, the formed particles were fine and well‐dispersed. It is whilst that in the case of bimetallic catalyst aggregation of particles was observed which was correlated to its lower efficiency. Optimization of the reaction variables via response surface methodology was also conducted using Pd/HNT catalyst. It was shown that using 5 wt.% catalyst at 130 °C under H2 gas with pressure of 8 bar, hydrogenation led to 97 % yield in 7 h. The catalyst was also reusable for 6 consecutive runs with negligible loss of activity.

Efficient Interfacial Sites between Metallic and Oxidized Cobalt for Propene Hydroformylation.

Currently, the hydroformylation of short olefins is operated almost exclusively by using Rh catalysts. Considering the high cost and scarcity of rhodium resources, it is important to develop non-noble metal catalysts toward hydroformylation. Herein, we report an efficient cobalt-based catalyst rich in interfacial sites between metallic and oxidized cobalt species for the hydroformylation of short olefin, propene, under a moderate syngas pressure. The catalyst exhibited a high specific activity of 252 mol molCo-1 h-1 in toluene under 2 bar of propene and 40 bar of CO/H2 mixed gas (CO/H2 = 1:1) at 160 °C. According to mechanistic studies, the interface of metallic and oxidized cobalt species promoted the adsorption of CO and propene. Moreover, the interfacial sites lowered the energy barrier for CO* hydrogenation and C-C coupling compared with metallic cobalt.

Development and full system testing of novel co‐impregnated La0.20Sr0.25Ca0.45TiO3 anodes for commercial combined heat and power units

AbstractOver the past decade, the University of St Andrews and HEXIS AG have engaged in a highly successful collaborative project aiming to develop and upscale La0.20Sr0.25Ca0.45TiO3 (LSCTA‐) anode “backbone” microstructures, impregnated with Ce0.80Gd0.20O1.90 (CG20) and metallic electrocatalysts, providing direct benefits in terms of performance and stability over the current state‐of‐the‐art (SoA) Ni‐based cermet solid oxide fuel cell (SOFC) anodes.Here, we present a brief overview of previous work performed in this research project, including short‐term, durability, and poison testing of small‐scale (1 cm2 area) SOFCs and upscaling to full‐sized HEXIS SOFCs (100 cm2 area) in short stacks. Subsequently, recent results from short stack testing of SOFCs containing LSCTA‐ anodes with a variety of metallic catalyst components (Fe, Mn, Ni, Pd, Pt, Rh, or Ru) will be presented, indicating that only SOFCs containing the Rh catalyst provide comparable degradation rates to the SoA Ni/cerium gadolinium oxide anode, as well as tolerance to harsh overload conditions (which is not exhibited by SoA anodes). Finally, results from full system testing (60 cells within a 1.5 kW electrical power output HEXIS Leonardo FC40A micro‐combined heat and power unit), will be outlined, demonstrating the robust and durable nature of these novel oxide electrodes, in addition to their potential for commercialization.

Modeling of CO Oxidation by Diffusion of Oxygen Atoms in Ceria-Zirconia Particulates in a Three-Way Catalyst Particle Membrane Filter

In order to design a microporous membrane filter comprising Three-way Catalyst (TWC) particles with a size distribution of 1 to 2 microns, isothermal CO oxidation experiments and numerical simulations were conducted to investigate the transport of oxygen atoms within primary Ceria-Zirconia (CZ) particulates. These spherical TWC particles were fabricated through the agglomeration of primary CZ and alumina particulates, incorporating Pd and Rh catalysts. By comparing experimental CO2 emissions with simulation results over time, a temperature-dependent diffusion coefficient was determined. The simulation results reveal that the effective distance of oxygen atom transport within CZ particulates, heterogeneously distributed in the spherical TWC particle, is limited to approximately 100 nm from the surface of agglomerated spherical TWC particles within a temperature range of 175 to 225°C.

Continuous-Flow Enantioselective Hydroacylations under Heterogeneous Chiral Rhodium Catalysts.

Transition metal-catalyzed enantioselective C-H bond functionalizations have become efficient methods for the synthesis of complex optically active molecules. Heterogeneous catalysts for this chemistry remain largely unexplored despite the advantages they offer in terms of ease of separation and reuse of catalysts. Herein, we report the development of heterogeneous chiral Rh catalysts for continuous-flow enantioselective hydroacylations. Heterogeneous catalysts could be prepared simply by mixing supports and Rh complexes. The prepared catalysts exhibited excellent activity and enantioselectivity affording optically active ketones in quantitative yields with 99 % ee's. Under the optimized reaction conditions, a turnover number >300 was achieved without the leaching of Rh species. The catalysts exhibited a wide substrate scope and in sequential-flow reactions with other heterogeneous catalysts, the syntheses of biologically active molecules and functional materials were demonstrated.

Continuous‐Flow Enantioselective Hydroacylations under Heterogeneous Chiral Rhodium Catalysts

AbstractTransition metal‐catalyzed enantioselective C−H bond functionalizations have become efficient methods for the synthesis of complex optically active molecules. Heterogeneous catalysts for this chemistry remain largely unexplored despite the advantages they offer in terms of ease of separation and reuse of catalysts. Herein, we report the development of heterogeneous chiral Rh catalysts for continuous‐flow enantioselective hydroacylations. Heterogeneous catalysts could be prepared simply by mixing supports and Rh complexes. The prepared catalysts exhibited excellent activity and enantioselectivity affording optically active ketones in quantitative yields with 99 % ee's. Under the optimized reaction conditions, a turnover number >300 was achieved without the leaching of Rh species. The catalysts exhibited a wide substrate scope and in sequential‐flow reactions with other heterogeneous catalysts, the syntheses of biologically active molecules and functional materials were demonstrated.

Chemo selective C-H alkylation of isoquinolones with maleimides: A combined experimental and computational case study

A comparative study of C8-alkylation of isoquinolone with maleimides is reported under different catalytical systems. Among different catalytical systems, Ru and Rh catalysts were found to be excellent in providing C8-alkylated products in good yields, and complete selectivity was achieved using Ru-catalyst. Density functional theory (DFT) investigations suggest a reaction mechanism involving three major steps: C-H activation at C8 position of the isoquinolone, maleimide migratory insertion into the metal-aryl bond, and protonation by the acetic acid. Non-covalent interaction (NCI) analysis was also executed to understand the reaction feasibility and selectivity pattern. Experimental mechanistic studies combined with DFT calculations and NCI analysis provide in-depth information to unravel the mystery behind the selective formation of alkylated products. The current catalytic methodology is further elaborated with a broad substrate scope and late-stage modification.

Architecture of urchin-like TiO2 integrated ultrasmall Rh nanoparticles with oxygen vacancy-reinforced electronic metal-support interaction for boosting hydrogen production from ammonia borane hydrolysis

Metal oxide-supported Rh nanoparticles (NPs) can provide abundant catalytically active sites for hydrogen evolution from ammonia borane hydrolysis (ABH). Considering the scarcity and high cost, developing effective strategies to reduce the utilization of Rh and simultaneously improving its catalytic activity is absolutely needed. Here, we develop ultrafine Rh NPs anchored on oxygen vacancies (VO)-bearing urchin-like TiO2 (U-TiO2). VO can facilitate the formation of electron-rich metallic Rh surface and boost the H2O dissociation, leading to highly efficient hydrogen generation from ABH without alkali promoters. Noticeably, the temperature-controlled annealing of the Rh/U-TiO2 precursor at 450 °C in reducing atmosphere generates the maximum VO concentration. This arouses strong electronic metal-support interaction between the Vo and Rh NPs to yield electron-rich metallic Rh surface. The obtained Rh/UT-450Vo exhibits a high initial turnover frequency of 1729 min−1 for ABH without alkali promoters, outperforming that of the contrast samples and most of previously developed Rh catalysts. This finding offers an effective strategy to fabricate high-efficiency metal oxide-supported metal NPs toward catalytic hydrogen production.

Rhodium‐Catalyzed Asymmetric Transfer Hydrogenation of Heterocyclic Diaryl Ketones: Facile Access to Key Intermediate of Baloxavir†

Comprehensive SummaryTransition metal‐catalyzed asymmetric transfer hydrogenation has been proven to be a powerful approach for the synthesis of chiral alcohols. Herein, a highly efficient and enantioselective transfer hydrogenation of dibenzoheptaheterocyclic ketones catalyzed by an arene‐tethered TsDPEN‐based Rh(III) catalyst has been successfully developed, and a variety of dibenzoheptaheterocyclic ketones were reduced by a 1/1 mixture of formic acid and DBU (1,8‐diazabicyclo[5.4.0]undec‐7‐ene) with high yields and enantioselectivities. With this method, the asymmetric reduction of 7,8‐difluorodibenzo[b,e]thiepin‐11(6H)‐one has been realized, providing the key intermediate of baloxavir marboxil with >99% yield and >99% ee at a substrate/catalyst molar ratio of 1000.

A New Ammonia Kinetic Model in Ru-Catalyzed Steam-Reforming Reaction Containing N2 in Natural Gas

Hydrogen for building fuel cells is primarily produced by natural-gas steam-reforming reactions. Pipeline-transported natural gas in Europe and North America used to contain about 1% to 5% N2, which reacts with H2 in steam-reforming reactions to form NH3. In the case of Ru, one of the catalysts used in natural-gas steam-reforming reactions, the activity of the NH3-formation reaction is higher than that of Ni and Rh catalysts. Reforming gas containing NH3 is known to poison Pt catalysts in Polymer Electrolyte Membrane Fuel Cells (PEMFCs) and also poison catalysts in preferential oxidation (PROX). In this study, Langmuir–Hinshelwood-based models of the NH3-formation reaction considering H2 and CO were proposed and compared with a simplified form of the Temkin–Pyzhev model for NH3-formation rate. The kinetic parameters of each model were optimized by performing multi-objective function optimization on the experimental results using a tube-type reactor and the numerical results of a plug-flow one-dimension simple SR (steam-reforming) reactor.

Divergent Synthesis of Chelating Aziridines and Cyclic(Alkyl)(Amino)Carbenes (CAACs) from Pyridyl-Tethered Robust Azomethine Ylides.

Azomethine ylides are typically in situ generated synthons for making N-heterocycles through cycloaddition reactions. But an offbeat aspect about them is the isomeric nature of aldiminium-based azomethine ylides and (alkyl/aryl)(amino)carbenes, interconvertible by a formal 1,3-H+ transfer. Herein, two thermally robust azomethine ylides with a N-appended picolyl sidearm are isolated, which cyclize to py aziridines at 80 °C but unprecedentedly result N-pico CAAC-CuCl (CAAC=cyclic(alkyl)(amino)carbene) complexes when heated with CuCl at merely 60 °C. The pendant Npy , as revealed by computational analysis, plays a crucial role in this unusual 1,3-H+ shift using a deprotonation-protonation sequence, as well as in placing the CuCl at the carbenic site in tandem. The softer nature of Cu(I) is also critical. Chelating CAACs are rare and one with a N-tethered additional donor is priorly unknown. Both N-pico CAAC and py aziridine are bidentate chelators giving highly active cationic Rh(I) catalysts for hydrosilylating unactivated olefins by Et3 SiH. Notably, the py aziridine-Rh(I) is superior than the N-pico CAAC-Rh(I) catalyst.

Divergent Synthesis of Chelating Aziridines and Cyclic(Alkyl)(Amino)Carbenes (CAACs) from Pyridyl‐Tethered Robust Azomethine Ylides

AbstractAzomethine ylides are typically in situ generated synthons for making N‐heterocycles through cycloaddition reactions. But an offbeat aspect about them is the isomeric nature of aldiminium‐based azomethine ylides and (alkyl/aryl)(amino)carbenes, interconvertible by a formal 1,3‐H+ transfer. Herein, two thermally robust azomethine ylides with a N‐appended picolyl sidearm are isolated, which cyclize to pyaziridines at 80 °C but unprecedentedly result N−picoCAAC‐CuCl (CAAC=cyclic(alkyl)(amino)carbene) complexes when heated with CuCl at merely 60 °C. The pendant Npy, as revealed by computational analysis, plays a crucial role in this unusual 1,3‐H+ shift using a deprotonation‐protonation sequence, as well as in placing the CuCl at the carbenic site in tandem. The softer nature of Cu(I) is also critical. Chelating CAACs are rare and one with a N‐tethered additional donor is priorly unknown. Both N‐picoCAAC and pyaziridine are bidentate chelators giving highly active cationic Rh(I) catalysts for hydrosilylating unactivated olefins by Et3SiH. Notably, the pyaziridine‐Rh(I) is superior than the N‐picoCAAC‐Rh(I) catalyst.

Rh(II)-Catalyzed Si-H Insertion with Nosyl-hydrazone-Protected Aryl Donor Diazo Compounds.

Dirhodium(II,II) paddlewheel catalysts were evaluated in silyl-hydrogen insertion reactions of aryl diazo compounds generated from o-nosyl hydrazones. The high reactivity of aryl diazo compounds necessitates their in situ generation from sulfonyl-protected hydrazones. Herein, we describe our efforts to evaluate this transformation utilizing Rh(II) catalysts, including those with tethered, axially coordinating ligands. The heteroleptic catalyst, Rh2(OAc)3(2-OX), provided the highest yield of silanes when dioxane was the solvent.

Promotional Effect of Ag on the Catalytic Decomposition of N2O in the Presence of O2 over the Al2O3-Supported Rh Catalyst

Promotional Effect of Ag on the Catalytic Decomposition of N₂O in the Presence of O₂ over the Al₂O₃-Supported Rh Catalyst

A New Process for Efficient Recovery of Rhodium from Spent Carbonyl Rhodium Catalyst by Microreactor.

Triphenylphosphine acetylacetone carbonyl rhodium (ROPAC) is an important catalyst in the petrochemical industry, and its deactivated waste catalyst holds significant value for recovery. This study focuses on the existing forms of rhodium (Rh) in waste catalysts and the current status of traditional processes. A green, efficient, and continuous recovery technique was developed using a sealed stainless steel microchannel reactor. The influence of reaction temperature, reaction time, and phase ratio on the Rh recovery rate was investigated, and the process parameters were optimized using response surface methodology (RSM). The results indicate that the magnitude of the impact on the Rh recovery rate follows the order: reaction temperature > reaction time > phase ratio. The optimized process parameters were determined as follows: a reaction time of 29 min, a reaction temperature of 110 °C, and a phase ratio of 1:1, with a corresponding maximum recovery rate of Rh of 66.06%. Furthermore, secondary treatment was performed on the organic phase after primary recovery using the same process conditions, resulting in an overall Rh recovery rate of 95.6%, indicating satisfactory recovery efficiency. Moreover, the application of FTIR and ICP-OES analysis provided definitive evidence that the oxidative dissociation of the rhodium-phosphine chemical bond by H2O2 within ROPAC leads to the conversion of Rh+ into Rh3+. Subsequently, Rh forms chloroaquorhodium (III) complexes that enter the aqueous phase, enabling effective recovery of Rh.