A practical concept for catalytic carbonylations using carbon dioxide

The increasing atmospheric CO2 levels are a major contributor to global warming, highlighting the urgent need for CO2 utilization strategies. Converting CO2 into valuable chemicals offers a promising solution. This research focuses on developing a catalytic process that utilizes CO2 as a C1 feedstock for carbonylations, reactions crucial in producing various chemicals including pharmaceuticals and agrochemicals. Traditional carbonylations rely on highly toxic and hazardous carbon monoxide (CO), posing significant safety and handling challenges, including transportation difficulties as evidenced by past industrial incidents. Therefore, there is a strong demand for alternative, safer methods to generate CO. This paper explores the direct reduction of CO2 with H2 to generate CO in situ for subsequent carbonylation reactions, eliminating the need to handle and transport gaseous CO. The challenge lies in finding a catalyst that achieves high selectivity for CO formation, avoiding the generation of side products such as methane or methanol. Furthermore, the integration of CO generation and its immediate use in carbonylation within a single, continuous process is a key objective. This would increase efficiency and practicality by avoiding the isolation and purification of the intermediary CO.

Previous research has explored various approaches to CO2 reduction, including the use of strong reductants like silanes or metals. However, these methods often generate significant amounts of waste. The reverse water-gas shift (RWGS) reaction, reducing CO2 with H2 to produce CO and water, is a more environmentally friendly approach. While precious metals (Rh, Pt, Pd, Au) and 3d-metals (Ni, Cu, Fe) supported on oxides have shown catalytic activity in the RWGS reaction, achieving high CO2 conversion with high CO selectivity remains a challenge. Cu-based catalysts are particularly attractive due to their cost-effectiveness, but often show low to moderate CO selectivity, producing substantial amounts of methanol and/or methane as byproducts. Multicomponent catalysts have been investigated to enhance selectivity, with careful control over particle size and dispersion crucial for optimizing activity. High temperatures favor CO formation in the endothermic RWGS reaction; however, many Cu-based catalysts are susceptible to deactivation at elevated temperatures due to sintering. The use of CO surrogates like formates, aldehydes, or metal carbonyls has been explored, but direct CO generation from CO2 followed by immediate use in carbonylation reactions has been largely under-investigated. Limited prior work involves separate reactors (two-chamber systems) for CO generation and carbonylation, adding complexity and inefficiency. This work aims to overcome these limitations by developing a continuous flow system for both CO generation and its direct utilization in carbonylations.

The researchers synthesized several novel Cu-based catalysts using precipitation hydrothermal (PHM) and conventional impregnation (CIM) methods. Silica was chosen as the support material for its inertness and ability to act as both a carrier and ligand. The catalytic activity of these materials was evaluated in a continuous flow reactor for CO2 hydrogenation to CO at temperatures ranging from 200 to 400 °C. Detailed characterization techniques were employed to analyze the catalysts, including ICP (Inductively Coupled Plasma), BET (Brunauer-Emmett-Teller) surface area analysis, XRD (X-ray diffraction), TPR (Temperature-Programmed Reduction), TEM (Transmission Electron Microscopy), XPS (X-ray photoelectron spectroscopy), and CO-IR (Infrared spectroscopy). The optimal catalyst, 10Cu@SiO2-PHM, was identified based on its superior activity and CO selectivity. The stability of this catalyst was tested in continuous operation over an extended period. Subsequently, the in situ generated CO from the optimized catalyst was directly used in various carbonylation reactions without intermediate purification: hydroformylation (using Rh(acac)(CO)2/6-DPPon as the catalyst), alkoxycarbonylation (using Pd(acac)2/Ph2PPy/PTSA·H2O), and aminocarbonylation (using Pd(dba)2/PPh3). A wide range of substrates (olefins, alkynes, aryl halides) were tested to assess the scope and limitations of this methodology. Product yields and selectivities were determined using standard analytical techniques like gas chromatography and NMR spectroscopy. The impact of the water produced in the RWGS reaction on the subsequent carbonylation steps was also investigated.

The study revealed that 10Cu@SiO2-PHM prepared by the PHM method exhibited significantly higher catalytic activity (27% CO2 conversion, 99% CO selectivity) compared to other Cu-based catalysts, including those prepared by CIM or using different supports (Al2O3, carbon). Catalyst characterization using XRD, TEM, and XPS indicated that the high activity of 10Cu@SiO2-PHM was attributed to the presence of highly dispersed, small Cu nanoparticles (1-10 nm), and a mixture of Cu(I) and Cu(II) species. In situ IR spectroscopy confirmed the high CO production rate and selectivity of the optimal catalyst. The 10Cu@SiO2-PHM catalyst showed remarkable stability, maintaining its activity and selectivity for over 100 hours of continuous operation at 400 °C and 20 bar. The in-situ generated CO was successfully used in several industrially relevant carbonylation reactions. Hydroformylation of various olefins produced linear aldehydes in high yields (90-99%), with excellent regioselectivity observed in some cases. Alkoxycarbonylation of alkynes afforded acrylates with high yields (85-99%) and selectivity. Aminocarbonylation of aryl halides resulted in benzamides with good to excellent yields (90-94%). Notably, the reactions proceeded efficiently under ambient conditions (ambient pressure, low temperature) and the water byproduct from the RWGS reaction did not affect the subsequent carbonylation processes. The reaction rates under mild conditions were comparable to those achieved with conventional homogeneous catalysts using pure CO.

The findings demonstrate the feasibility of using a continuous flow system to perform carbonylations using CO2 and H2 as a safe and efficient CO surrogate. The high selectivity of the 10Cu@SiO2-PHM catalyst for CO generation, coupled with its stability and ease of use, makes this process attractive. The broad substrate scope showcased in the carbonylation steps highlights the versatility of this approach. The mild reaction conditions employed reduce energy consumption and improve sustainability compared to traditional methods. These results are significant because they provide a practical and environmentally friendly alternative to conventional carbonylation methodologies, reducing reliance on toxic CO and contributing to a more sustainable chemical industry.

This research successfully demonstrated a practical concept for catalytic carbonylations using CO2 and H2 as a safe CO source. The newly developed heterogeneous copper catalyst (10Cu@SiO2-PHM) enabled the selective on-demand generation of CO, which was directly utilized in hydroformylation, alkoxycarbonylation, and aminocarbonylation reactions with high yields and selectivities under mild reaction conditions. This approach offers a promising pathway toward sustainable chemical production and merits further investigations to optimize the process and expand its application to a wider range of carbonylations.

While the 10Cu@SiO2-PHM catalyst showed excellent performance, further optimization might be needed to improve CO2 conversion efficiency. The current setup does not allow for recycling of the CO generated; however, modifications to enable CO recycling are possible. The current method primarily focuses on carbonylations; further research could explore extending this approach to other CO2-based transformations. The scope of substrates successfully employed in carbonylation reactions could be further expanded.