A practical concept for catalytic carbonylations using carbon dioxide

The study addresses the challenge of safely and efficiently using carbon dioxide (CO2) as a C1 feedstock by converting it to carbon monoxide (CO) via the reverse water-gas shift (RWGS) and immediately using the CO in catalytic carbonylation reactions. CO2 is thermodynamically stable, making its reduction challenging. Traditional reductants (silanes, metals) generate waste, while H2-based reductions can yield useful products (formic acid, methanol, methane) with water as the only by-product. Selective CO production from CO2/H2 (RWGS) is industrially attractive because CO is a key building block for numerous carbonylation processes (e.g., hydroformylation, acetic acid synthesis), but CO handling is hazardous, and logistics limit its use. Prior RWGS catalysts often struggle with either low CO2 conversion or poor CO selectivity, and Cu-based systems can sinter at high temperatures. The research question is whether a robust, selective, and stable heterogeneous catalyst can generate CO from CO2/H2 continuously and be directly coupled, without gas purification, to key carbonylation reactions under mild conditions, thereby providing a practical and safer alternative to handling CO gas.

The authors situate their work within several relevant domains: (1) CO2 utilization as a renewable C1 source in synthesis and polymers; (2) RWGS catalysis over precious metals (Rh, Pt, Pd, Au) and 3d metals (Ni, Cu, Fe) on oxides, where achieving both high CO2 conversion and high CO selectivity remains challenging; many Cu-based catalysts show low-to-moderate CO selectivity with competing methanol/methane formation, and sintering at high temperature reduces performance. (3) CO surrogates (formates, aldehydes, metal carbonyls, Cogen, SilaCOgen) and two-chamber systems allow safer carbonylations but add steps and waste. (4) Prior combined CO2-to-CO generation and immediate carbonylation typically used homogeneous catalysts (e.g., Ru3(CO)12, Rh(acac)(CO)2, [RhCl(CO)2]2) under high pressures and temperatures, or silacarboxylate approaches; electrochemical CO2 reduction to CO has limited generation rates. The gap is a practical, heterogeneous, additive-free RWGS system that stably generates sufficient CO for direct, continuous carbonylations at mild conditions without gas purification.

Step I: CO generation via RWGS

• Catalyst preparation: Cu-based materials prepared by precipitation hydrothermal method (PHM) and conventional impregnation method (CIM). Supports: SiO2 (chosen for inertness and potential ligand effects), Al2O3, and carbon black. Optimal material: 10Cu@SiO2-PHM.
• Reactor and conditions: Continuous-flow fixed-bed reactor; tests between 200–400 °C. Standard conditions for Table 1: 300 mg catalyst; gas feed H2/CO2 = 3:1 at 100 NmL/min; GHSV = 15,000 h−1; T = 400 °C; P = 10 bar. Effects of temperature (250–400 °C) and pressure studied; stability assessed at 400 °C, 20 bar, CO2/H2 = 1:3 for 100 h on-stream.
• Characterization: ICP for composition; N2 physisorption (BET) for surface area, pore volume/diameter; XRD for phase identification (Cu2O vs CuO/Cu2O mixtures); H2-TPR for reducibility and dispersion; TEM/STEM-HAADF with EDXS and FFT/IFFT for particle size/morphology (Cu nanoparticles 1–10 nm, some polycrystalline aggregates); XPS (Cu 2p3/2) for oxidation states (Cu(I)/Cu(II) in fresh; predominantly Cu after reaction); in situ FTIR under CO2/H2 at 400 °C to monitor CO formation; room-temperature CO adsorption IR (strong band at ~2131 cm−1 for active PHM sample).

Step II: Direct follow-up carbonylation reactions using generated CO (without purification; presence of CO2/H2O tolerated). Flow setup couples RWGS outlet directly to reaction vessels.

• Hydroformylation of alkenes: Catalyst Rh(acac)(CO)2 (0.1 mol%) with 6-DPPon ligand (0.4 mol%) in THF (20 mL); 25 °C; 20 h; ambient pressure. Substrates include 1-octene, 1-heptene, 1-dodecene, 3,7-dimethylocten-1-en-7-ol, vinylsilanes, allyl benzene, eugenol, styrenes.
• Alkoxycarbonylation of alkynes (methoxycarbonylation): Drent-type Pd system: Pd(acac)2 (1 mol%), Ph2PPy (4 mol%), PTSA·H2O (8 mol%) in MeOH (10 mL); 25 °C; 20 h; ambient pressure. Substrates: phenylacetylene, substituted arylacetylenes, aliphatic alkynes bearing chloro or cyano groups.
• Aminocarbonylation of aryl halides: Catalyst [Pd(dba)2] (1 mol%) with PPh3 (2 mol%) and NEt3 (2 equiv) in toluene (10 mL); 80 °C; 20 h; ambient pressure. Substrates: aryl iodides (e.g., 4-iodoanisole) with secondary and primary amines (e.g., piperidine).
• Operational notes: CO recycling not implemented in current setup; could be realized with an added reactor/compressor. Water formed in RWGS did not adversely affect carbonylations.

• RWGS performance: SiO2-supported Cu catalysts outperformed Al2O3 and carbon supports for selective CO production. Best catalyst 10Cu@SiO2-PHM achieved CO2 conversion of 27% with 99% CO selectivity at 400 °C (H2/CO2 = 3:1, GHSV 15,000 h−1, 10 bar). At 250 °C, 10Cu@SiO2-PHM gave 10.4% conversion with 97% selectivity. Methanol formation was negligible across temperatures; CH4 detected only in trace amounts (1–2%) at higher temperature. Pressure had little effect on overall conversion at 400 °C.
• Catalyst structure–activity: PHM-prepared catalyst showed only Cu2O by XRD (vs CuO/Cu2O in CIM), smaller well-dispersed crystallites, a single TPR reduction feature indicating uniform dispersion, and stronger CO adsorption IR band (~2131 cm−1). XPS indicated fresh catalyst contains Cu(I)/Cu(II) with reduction to predominantly Cu under reaction; CIM sample showed stronger Cu(II)–support interaction and lower activity. H2 consumption data: inactive 10Cu@SiO2-CIM ~1290.8 µmol/g vs active 10Cu@SiO2-PHM ~627.97 µmol/g, consistent with different Cu speciation.
• Stability: 10Cu@SiO2-PHM showed no deactivation over 100 h on-stream at 400 °C and 20 bar; long-term reuse up to 6 months reported. Spent catalyst analysis confirmed stable Cu phase.
• Direct carbonylations using generated CO: • Hydroformylation: 1-octene → n-nonanal 92% yield; 1-heptene → n-heptanal quantitative; 1-dodecene → n-tridecanal 90% yield with >99:1 linear/branched; 3,7-dimethylocten-1-en-7-ol → hydroxy aldehyde 98% yield at rt and ambient pressure (vs 85 bar industrially); silyl-substituted olefin → silyl aldehyde quantitative; allyl benzene and eugenol → aldehydes in 79–>99% yields; styrene derivatives fully converted to aldehydes in 94–99% yield with branched selectivity. Acrylates and acrylonitrile were unreactive under these mild conditions. • Alkoxycarbonylation (methoxycarbonylation of alkynes): Phenylacetylene → branched acrylate in quantitative yield with >99% branched selectivity using Pd(acac)2/Ph2PPy/PTSA at 25 °C; other aryl and aliphatic alkynes (chloro, cyano substituents) afforded acrylates in 85–>99% yields. • Aminocarbonylation: 4-iodoanisole and related aryl iodides with piperidine gave amides in 90–94% yields using [Pd(dba)2]/PPh3; linear secondary and primary amines furnished amides in good yields. Exclusive monocarbonylation observed, avoiding dicarbonylation common in related protocols.
• The presence of water from RWGS did not affect carbonylations. All carbonylations proceeded at mild conditions (ambient pressure, low temperature) with rates comparable to homogeneous CO-based benchmarks.

The study demonstrates a practical two-step cascade: a robust heterogeneous Cu@SiO2-PHM catalyst generates CO selectively from CO2/H2, and the CO stream is directly used in hydroformylation, alkoxycarbonylation, and aminocarbonylation without purification. This addresses safety and logistics concerns of handling CO by producing it on demand from benign feedstocks. The structural features of the PHM-prepared catalyst (Cu(I)-rich surface, small well-dispersed particles, favorable reducibility, strong CO adsorption) correlate with superior RWGS activity and selectivity. Continuous operation with stable performance over 100 h (and months-long recyclability) underscores practical viability. The successful coupling under ambient pressure and low temperature to deliver valuable aldehydes, acrylates, and amides in high yields and selectivities shows that RWGS-derived CO can effectively replace bottled CO in key transformations. The tolerance to water simplifies operation and reduces process complexity. Overall, the findings validate a mini-plant/two-chamber concept for integrating CO2 valorization with downstream synthesis, advancing greener carbonylation chemistry.

A hydrothermally prepared heterogeneous copper catalyst (10Cu@SiO2-PHM) enables highly selective, stable CO generation from CO2/H2 in continuous flow. The on-demand CO can be directly fed into diverse carbonylation reactions (hydroformylation, alkoxycarbonylation, aminocarbonylation) to afford functionalized aldehydes, esters, and amides in high yields and selectivities under mild, ambient-pressure conditions, while tolerating water. This provides a safer, cost-effective alternative to using and transporting CO, facilitating continuous syntheses relevant to drug discovery and fine chemicals. Future work could focus on implementing CO recycling via compressors/additional reactors, expanding substrate scope (e.g., activating less reactive alkenes like acrylates/acrylonitrile under mild conditions), further increasing CO2 conversion at lower temperatures, and scaling the integrated system for industrial applications.

• Current CO2-to-CO conversion with 10Cu@SiO2-PHM, while highly selective (≥98–99%), reaches moderate conversion (~27%) at 400 °C, with traces of CH4 at higher temperature; achieving higher conversion at lower temperatures remains a challenge.
• The present setup does not recycle unused CO; integrating gas compression or additional reactors would improve atom economy.
• Substrate scope limitations under mild conditions were noted (e.g., acrylates and acrylonitrile were unreactive in hydroformylation).
• Conventional CIM-prepared Cu/SiO2 catalysts were significantly less active/selective, indicating sensitivity to preparation method and potential scalability considerations for PHM synthesis.