Hydroformylation Catalysts Research Articles

Balanced Rh+‐Rh0 Sites over Rh Clusters Enhance Heterogeneous Hydroformylation of Aldehyde

Controlling the metal geometric and electronic structure is of significance in developing efficient catalysts for heterogeneous hydroformylation. This study examines the structural sizes of Rh and Rh+‐Rh0 distribution to construct a highly active catalyst for formaldehyde hydroformylation. The active sites for hydroformylation require several Rhn atoms, while single‐atom Rh can solely catalyze hydrogenation. The highest activity was achieved on Rh nanoclusters (0.95 nm), giving a TOF of 191 h‐1 and selectivity of 82% for glycolaldehyde formation. The tunability of the electronic properties of Rh nanoclusters and the synergistic interaction between Rh+ and Rh0 are essential for enhanced activity. Pseudo‐in situ FT‐IR analysis elucidated that formaldehyde adsorbed on Rh nanoclusters prefers to produce glycolaldehyde via hydroformylation, while formaldehyde adsorbed on isolated Rhδ+ sites tends to form methanol via hydrogenation. This study provides a new insight into the design of heterogeneous catalysts and guidance for understanding the reaction mechanism for aldehydes/olefins hydroformylation.

Effective hollow Rh@H-S-1 catalyst for hydroformylation of 1-hexene

A hollow zeolite encapsulated Rh catalyst Rh@H-S-1 with an “eggshell” structure was prepared using the method of organic alkali treatment and recrystallization. Compared with catalysts with directly encapsulated Rh@S-1 and surface impregnated Rh/S-1, this catalyst exhibited both superior activity and a higher TOF value in hydroformylation of 1-hexene due to the high dispersion of Rh and high diffusion. The Rh element did not need to be reduced in the preparation, and it was in situ reduced by the syngas. The content of lower valence Rhδ+ gradually increased with the reaction procedure and played as an active center. The low oxidized state Rh+ or Rhδ+ in the form of Rh(CO)2 are proposed as active sites in hydroformylation, which were generated under the synergistic effect between CO and H2. Importantly, the hollow encapsulated catalyst effectively decreased Rh leaching to 0.1 ppm and showed better stability compared with the impregnated Rh/S-1 catalyst.

Influence of support on Rh-Co bimetallic catalysts for ethylene hydroformylation

Bimetallic Rh- and Co-based catalysts are promising materials for the heterogenous ethylene hydroformylation reaction. Here, the influence of a mesoporous silica (SBA-15) support on Rh and Co bimetallic interactions was investigated through a comparison with silica gel and gamma-alumina supports. The bimetallic catalyst supported on mesoporous silica (RhCo3/SBA-15) showed the best C3 oxygenate yield among the three bimetallic catalysts. In-situ vibrational studies suggested moderate binding of the gem-dicarbonyl and Rh(CO)(C2H4) intermediates due to this bimetallic interaction that led to improved hydroformylation performance. Kinetic studies revealed a lower hydroformylation barrier for the bimetallic compared to a Rh monometallic catalyst, and in-situ X-ray absorption spectroscopy investigations showed clear Rh-Co alloy formation on RhCo3/SBA-15. The bimetallic enhancement effect from the interaction with the mesoporous silica support shown here can be further optimized to design olefin hydroformylation catalysts.

Heterogeneous Bisphosphine Ligand-Encapsulated Rh Catalysts for Heterogeneous Hydroformylation of Alkenes with CO2

Heterogeneous Bisphosphine Ligand-Encapsulated Rh Catalysts for Heterogeneous Hydroformylation of Alkenes with CO₂

A highly active phosphine oxides-containing porous organic polymer supported Co catalyst for hydroformylation of 2-octene

The development of earth-abundant catalysts with high activity and selectivity is important yet challenging in hydroformylation. In this work, a stable and efficient phosphine-oxide group containing porous organic polymer was afforded, which served as a ligand and a support to fabricate a heterogeneous Co catalyst (Co/POL-POPh3) for hydroformylation of 2-octene under mild condition. A 2-octene conversion of 94.6 % and at least three recycles were achieved using Co/POL-POPh3 in the hydroformylation with a 51.1 % aldehyde selectivity under 423 K and 3.0 MPa. The experimental and characterization results elucidated that the highly dispersed Co ions was coordinated with the phosphine-oxide groups in the polymer, which was beneficial for the 2-octene isomerization and in turn led to the excellent performance for synthesis of oxygenates. Furthermore, a reaction mechanism was also suggested for 2-octene hydroformylation over the Co/POL-POPh3 catalyst. The current study delivers valuable insights into the adjustment of the catalytic efficiency of non-previous metal catalysts in the heterogeneous hydroformylation of internal olefins.

Elucidation of hemilabile-coordination-induced tunable regioselectivity in single-site Rh-catalyzed heterogeneous hydroformylation

Revealing key factors that modulate the regioselectivity in heterogeneous hydroformylation requires identifying and monitoring the dynamic evolution of the truly active center under real reaction conditions. However, unambiguous in situ characterizations are still lacking. Herein, we elaborately construct a series of Rh-POPs catalysts for propylene hydroformylation which exhibited tunable regioselectivity. Multi-technique approaches reveal the unique microenvironment of the diverse HRh(CO)(PPh3-frame)2 sites with distinct P-Rh-P bite angles ranging from 90° to 120° and 158° to 168°, respectively. In situ time-resolved XAFS, FT-IR, and quasi-in situ Solid-state NMR experiments combined with DFT calculations explain the dynamic evolution of the electronic and coordinate state of the distinct active sites induced by hemilabile PPh3-frame ligands and further disclose the regulatory mechanism of regioselectivity. These state-of-the-art techniques and multiscale analysis advance the understanding of how hemilabile coordination influences regioselectivity and will provide a new thought to modulate the regioselectivity in future industrial processes.

Robust, scalable, and highly selective spirocyclic catalysts for industrial hydroformylation and isomerization-hydroformylation.

Hydroformylation (HF) or isomerization-hydroformylation (ISO-HF) represents the most direct and practical route for producing aldehydes on an industrial scale. To resolve the issues of low activity, low linear/branched (l/b) ratio, and low stability in HF and ISO-HF, we herein reported a class of spirocyclic diphosphites. Notably, the ligand termed O-SDPhite afforded excellent catalytic activity and regioselectivity for the HF of various olefins. Excellent l/b ratio and an unprecedented turnover number of up to 17,620,000 were achieved. O-SDPhite was also found to be effective in the regioselective ISO-HF of the industrially related cheap and abundant C4 Raffinates to n-valeraldehyde produced on a multimillion-ton scale. The reaction with O-SDPhite, superior to that of benchmark Biphephos, was continuously operated for 41 days and afforded an average 38.6 l/b ratio (31 days and 14.7 l/b ratio for Biphephos).

Rational solvent selection for the preparation of industrial monolithic supported liquid-phase (SLP) olefin hydroformylation catalyst

Heterogenisation of the hydroformylation (HyFo) process converting C4 olefins and syngas (CO/H2) to n-pentanal can substantially reduce its carbon footprint by simplifying downstream processing. Current upscaling efforts include constructing a pilot plant containing a Rh-diphosphite-stabilized supported liquid-phase (SLP) catalyst. The air-sensitive nature of the catalyst requires in-situ impregnation of the porous support material with the liquid phase components (Rh-precursor, ligand, stabiliser) entailing the use of large quantities of an organic solvent. Environment, health, and safety (EHS) as well as regulatory aspects prompt the search for an alternative to dichloromethane (DCM), which was used as standard solvent for lab-scale studies. To obtain alternatives to DCM, the replacement solvent methyl tert-butyl ether (MTBE) and in-silico screening of more than 5000 solvent candidates has been performed covering boiling points and solubility predictions by COSMO-RS as well as carcinogenicity- and mutagenicity predictions by quantitative structure–activity relationship (QSAR) models. The final list of solvent candidates contained unsaturated compounds (alkenes, alkynes) in addition to ether- and ester-functionalized​ solvents, which are typically recommended as DCM replacements. Sufficiently high solubilities of ligand and stabiliser to prepare the desired SLP catalyst were experimentally determined in the following solvents: methyl propionate (MP), dimethoxymethane (DMM), MTBE, tetrahydrofuran (THF) and DCM for benchmarking, and a monolithic SLP catalyst prepared with MTBE demonstrated to perform equally well in the HyFo reaction of 1-butene as the benchmark catalyst prepared using DCM solvent. Overall, the study demonstrates a practical approach for selecting solvents with a greener profile for making catalyst systems applicable for large-scale production.

Regioselective hydroformylation of propene catalysed by rhodium-zeolite.

Hydroformylation is an industrial process for the production of aldehydes from alkenes1,2. Regioselective hydroformylation of propene to high-value n-butanal is particularly important, owing to a wide range of bulk applications of n-butanal in the manufacture of various necessities in human daily life3. Supported rhodium (Rh) hydroformylation catalysts, which often excel in catalyst recyclability, ease of separation and adaptability for continuous-flow processes, have been greatly exploited4. Nonetheless, they usually consist of rotationally flexible and sterically unconstrained Rh hydride dicarbonyl centres, only affording limited regioselectivity to n-butanal5-8. Here we show that proper encapsulation of Rh species comprising Rh(I)-gem-dicarbonyl centres within a MEL zeolite framework allows the breaking of the above model. The optimized catalyst exhibits more than99% regioselectivity to n-butanal and more than99% selectivity to aldehydes at a product formation turnover frequency (TOF) of 6,500 h-1, surpassing the performance of all heterogeneous and most homogeneous catalysts developed so far. Our comprehensive studies show that the zeolite framework can act as a scaffold to steer the reaction pathway of the intermediates confined in the space between the zeolite framework and Rh centres towards the exclusive formation of n-butanal.

Synthesis and Characterization of Co(II) Substituted Keggin‐Type Polyoxometalates as Novel Catalysts for the Hydroformylation of 1‐Hexene in a Thermomorphic Solvent System

AbstractPolyoxometalates (POMs) are a unique class of metal oxides, being of huge interest for the catalysis society. CoII‐substituted phosphomolybdate POMs (namely H7[PCoMo11O40], H11[PCo2Mo10O40] and H15[PCo3Mo9O40]) have been successfully synthesized for the first time. The structure of the new CoII substituted POMs was solved by single‐crystal X‐ray diffraction as well as vibrational (FT‐IR and Raman) and nuclear magnetic/electron paramagnetic resonance (NMR and EPR) spectroscopy and found to be of the Keggin‐type. The Co‐POMs were then applied as molecular catalysts for the hydroformylation of 1‐hexene in a thermomorphic solvent mixture of water and 1‐butanol. This is the very first example of a 1st row transition metal POM acting as a hydroformylation catalyst achieving 46 % heptanal yield. The best results were obtained using H11[PCo2Mo10O40] as a catalyst at 140 °C, 150 bar CO/H2 1 : 2, and a reaction time of 4 h, paving the way for new POM‐based catalysts in hydroformylation reactions.

Containing Nitrogen and Phosphorus Porous Organic Polymer-Supported Rhodium Catalyst for Hydroformylation of Olefins

Containing Nitrogen and Phosphorus Porous Organic Polymer-Supported Rhodium Catalyst for Hydroformylation of Olefins

K-promoted Ru-based heterogeneous catalysts for ethylene hydroformylation

Hydroformylation is a prototypical C-C coupling reaction of olefins and syngas to produce valuable higher aldehydes. In this study, we investigated the promotional effect of potassium (K) on Ru-based heterogeneous ethylene hydroformylation. The propanal selectivity could reach up to 84.9% at an ethylene (C2H4) conversion of 10.7 % with the addition of K as a promoter. Furthermore, the formation of ethane and the second hydrogenation of olefins were effectively suppressed. Structural characterization results suggested that K promoter could enhance the dispersion of Ru species and increase the electronic density of Ru active sites, thus improving CO adsorption and dissociation. The synergistic effect of Ru and K created a C*-rich and H*-lean local chemical environment, facilitating the formation of aldehydes while effectively reducing the hydrogenation of olefins. This work may provide new insights for development promising Ru-based heterogeneous catalysts for hydroformylation.

Activity promotion of Rh1Cax/Al2O3 single-atom catalyst in 1-octene hydroformylation: a volcano curve exists between Ca adding ratio and catalytic activity

Activity promotion of Rh1Cax/Al2O3 single-atom catalyst in 1-octene hydroformylation: a volcano curve exists between Ca adding ratio and catalytic activity

Unravelling structure sensitivity in heterogeneous hydroformylation of aldehyde over Rh

Delineating the geometrical and electronic structure of supported metal is significant for developing heterogeneous hydroformylation catalysts. This study modulated the configuration and charge distribution of Rh to construct highly efficient Rh@SiO2 catalysts for formaldehyde hydroformylation. The structure sensitivity of Rh species reveals that single-atom Rh can solely catalyze hydrogenation, while Rh aggregates are active for hydroformylation. Moderate Rh nanoclusters (0.98 nm) offered the highest TOF (183 h−1) and selectivity (90.8 %) of glycolaldehyde. Electronic effects demonstrate that synergy between Rh0 and Rh+ sites on the cluster is essential to enhancing the reaction. The Rh0 species function as the adsorption/activation sites for H2 and formaldehyde, and the Rh+ sites act to coordinate PPh3 and CO to form Rhδ+(CO)x(PPh3)y active species. This study provides new insights into the design of heterogeneous catalysts and guidance for understanding the structure sensitivity of active metal and hydroformylation mechanism.

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.

K-promoted cobalt-based catalysts for heterogeneous hydroformylation

K-promoted Co/SiO2 catalysts were developed for heterogeneous olefin hydroformylation. Through various characterization techniques, the promotional effect of the K promoter in enhancing oxygenate selectivity was investigated.

Theoretical study on hydroformylation catalyzed by cationic cobalt(II) complexes.

Hydroformylation is one of the most important homogeneous reactions in industrial production. Herein, a density functional theory (DFT) method was employed to investigate two proposed reaction mechanisms of hydroformylation catalyzed by cationic cobalt(II) complexes, the carbonyl dissociative mechanism and the associative mechanism. The calculated results showed that the heterolytic H2 activation is the rate-determining step for both the dissociative mechanism and the associative mechanism, with energy barriers of 26.8 kcal mol-1 and 40.5 kcal mol-1, respectively. Meanwhile, the regioselectivity, the spin multiplicity of the catalyst and the substituent effects on the reaction were also investigated. The most stable cobalt(II) catalyst has a doublet state and the linear aldehyde is the dominant product. In addition, it was found that the energy barrier of the reaction decreased when the electron density of the Co center of the catalyst was increased by changing the ligand. The catalytic activity of the catalyst was proposed to be the best when the PEt2 group of the ligand is replaced by the P(tBu)2 group. This study might not only provide new insights for hydroformylation catalyzed by cobalt but also facilitate theory-guided design of novel transition metal catalysts for hydroformylation.

Understanding the role of supported Rh atoms and clusters during hydroformylation and CO hydrogenation reactions with in situ/operando XAS and DRIFT spectroscopy.

Supported Rh single-atoms and clusters on CeO2, MgO, and ZrO2 were investigated as catalysts for hydroformylation of ethylene to propionaldehyde and CO hydrogenation to methanol/ethanol with in situ/operando diffuse reflectance infrared Fourier transform spectroscopy (DRIFTS) and X-ray absorption spectroscopy (XAS). Under hydroformylation reaction conditions, operando spectroscopic investigations unravel the presence of both single atoms and clusters and detected at first propanal and then methanol. We find that the formation of methanol is associated with CO hydrogenation over Rh clusters which was further confirmed under CO hydrogenation conditions at elevated pressure. The activity of catalysts synthesized via a precipitation (PP) method over various supports towards the hydroformylation reaction follows the order: Rh/ZrO2 > Rh/CeO2 > Rh/MgO. Comparing Rh/CeO2 catalysts synthesized via different methods, catalysts prepared by flame spray pyrolysis (FSP) showed catalytic activity for the hydroformylation reaction at lower temperatures (413 K), whereas catalysts prepared by wet impregnation (WI) showed the highest stability. These results not only provide fundamental insights into the atomistic level of industrially relevant reactions but also pave the way for a rational design of new catalysts in the future.

Substrate scope driven optimization of an encapsulated hydroformylation catalyst.

Caged complexes can provide impressive selective catalysts. Due to the complex shapes of such caged catalysts, however, the level of selectivity control of a single substrate cannot be extrapolated to other substrates. Herein, the substrate scope using 41 terminal alkene substrates is investigated in the hydroformylation reaction with an encapsulated rhodium catalyst [Rh(H)(CO)3(P(mPy3(ZnTPP)3))] (CAT1). For all substrates, the amount of branched products formed was higher with CAT1 than with the unencapsulated reference catalyst [Rh(H)(CO)2(P(mPy3))2] (CAT2) (linear/branched ratio between 2.14 and 0.12 for CAT1 and linear/branched ratio between 6.22 and 0.59 for CAT2). Interestingly, the level of cage induced selectivity depends strongly on the substrate structure that is converted. Analysis of the substrate scope combined with DFT calculations suggests that noncovalent interactions between the substrate moieties and cage walls play a key role in controlling the regioselectivity. Consequently, these supramolecular interactions were further optimized by replacing the ZnTPP building block with a zinc porphyrin analog that contained OiPr substituents on the meta position of the aryl rings. The resulting caged catalyst, CAT4, converted substrates with even higher branched selectivity.

Heterogeneous Phosphine-Containing Hydroformylation Catalysts Based on Modified Porous Organic Framework

A series of heterogeneous phosphine-containing rhodium hydroformylation catalysts based on porous aromatic frameworks (PAFs) were prepared. The catalysts PAF30-MDEA-TPPTS-Rh (TPPTS is trisodium triphenylphosphine-3,3',3''-trisulfonate, MDEA is methyldiethanolamine fragment) and PAF-30-Im-TPPTS-Rh (Im is imidazole fragment) showed the highest stability in repeated use in 1-hexene hydroformylation. The catalyst PAF-30-MDEA-TPPTS-Rh before and after use in hydroformylation was characterized by elemental C,N,H,S analysis, inductively coupled plasma atomic absorption spectroscopy, low-temperature nitrogen adsorption–desorption, transmission electron microscopy, IR spectroscopy, and X-ray photoelectron spectroscopy. The effect of temperature, pressure, and solvent on the course of hydroformylation in the presence of PAF-30-MDEA-TPPTS-Rh was studied. The catalyst is active in hydroformylation of a series of unsaturated compounds, including functionalized substrates and olefins with internal double bond.