The escalating global CO₂ emissions necessitate significant reductions to mitigate climate change. The utilization of CO₂ as a sustainable C1 building block offers a promising solution. However, CO₂'s thermodynamic stability and kinetic inertness present challenges requiring highly reactive nucleophiles or transition-metal catalysts for activation. While metal-catalyzed reactions dominate the field, organocatalysis is also actively pursued, employing catalysts such as N-heterocyclic carbenes (NHCs), TBD, thiazolium carbene, and 1,3,2-diazaphosphatrane (NHP-H). The use of inexpensive reducing agents, like polymethylhydrosilane, has also been explored. Metallic silicon, a readily available and inexpensive alternative to hydrogen or hydrosilanes, presents an attractive option, particularly considering the growing volume of silicon waste from solar panel production. Previous research has demonstrated the reduction of CO₂ to formic acid and methanol using silicon powder; however, the detailed mechanism of fluoride catalysis in CO₂ reductive functionalization, particularly concerning the behavior of fluoride within the silicon particles, remains unclear. This study aims to address this gap by investigating the efficient synthesis of formamides from CO₂ using waste silicon and fluoride catalysis, elucidating the reaction mechanism through spectroscopic and isotopic experiments.

Numerous studies have focused on the conversion of CO2 into valuable chemicals. Metal-catalyzed reactions are prevalent, but organocatalysis is gaining traction due to its potential for cost-effectiveness and environmental benefits. Organocatalysts like N-heterocyclic carbenes (NHCs), 1,5,7-triazabicyclo[4.4.0]dec-5-ene (TBD), thiazolium carbenes, and 1,3,2-diazaphosphatranes (NHP-H) have shown catalytic activity comparable to metal-based catalysts. The use of less expensive reducing agents, such as polymethylhydrosilane, has been investigated. However, the use of metallic silicon as a reducing agent for CO2 functionalization, particularly in the context of formamide synthesis, is relatively unexplored. Previous studies have shown the reduction of CO2 to formic acid and methanol using silicon powder, but the detailed role of fluoride catalysis and its interaction with silicon remain unclear.

The researchers used silicon powder obtained from recycled solar panels. Various fluoride catalysts were tested, with tetrabutylammonium fluoride (TBAF) showing superior performance. The reaction involved mixing the silicon powder, amine substrate, TBAF, water, and CO₂ in a suitable solvent (NMP, DMSO, DMA, DMF). The reaction conditions (temperature, pressure, solvent) were optimized to maximize formamide yield. Spectroscopic techniques, including in situ Fourier transform infrared (FTIR) spectroscopy, X-ray photoelectron spectroscopy (XPS), X-ray diffraction (XRD), and N₂ adsorption/desorption analyses, were employed to investigate the reaction mechanism and characterize the silicon material before and after the reaction. Isotopic labeling experiments using ¹³CO₂ and D₂O were conducted to determine the source of carbon and hydrogen atoms in the formamide product. Time-course analysis was also performed to study the reaction kinetics and intermediate formation. Specifically, XPS analysis was conducted on fresh and recovered silicon powder to investigate the oxidation states of silicon and the distribution of fluoride ions. FTIR spectroscopy was used to identify reaction intermediates such as Si-H species. XRD was used to assess the crystallinity of the silicon powder. Nitrogen adsorption-desorption isotherms and BJH analysis were used to determine the surface area and pore size distribution of the silicon material.

The study demonstrated the successful synthesis of formamides from CO₂ using recycled silicon powder as the reducing agent. TBAF proved to be the most effective catalyst among those tested. A wide range of amines were successfully converted to their corresponding formamides with high yields (>99% in optimized conditions). The reaction required both water and CO₂ to proceed. The reaction showed a strong solvent dependence, with aprotic polar solvents containing C=O or S=O bonds (DMSO, DMA, NMP, DMF) exhibiting high activity. In situ FTIR spectroscopy provided evidence for the formation of Si-H intermediates, which subsequently reacted with CO₂. XPS analysis revealed the oxidation of silicon atoms both on the surface and within the silicon particles during the catalytic reaction, confirmed by the absence of the Si(0) peak and the presence of a Si(4+) peak in the recovered silicon. XRD patterns indicated a decrease in crystallinity of the silicon powder after the reaction, consistent with oxidation to amorphous silica. Isotopic labeling experiments confirmed that the carbon atom in the formamide formyl group originates from CO₂, while the hydrogen atom originates from H₂O. The surface area of the recovered silicon powder increased significantly after the reaction, with the formation of mesopores observed via BJH analysis, indicating a restructuring of the silicon material during the reaction. Time course analysis indicated a stepwise reaction mechanism involving formic acid as an intermediate.

The findings demonstrate a novel and sustainable method for CO₂ conversion to value-added chemicals using readily available and inexpensive materials. The use of recycled silicon waste not only reduces environmental impact but also adds economic value to waste materials. The high yields and broad substrate scope achieved in the formamide synthesis showcase the potential of this approach. The detailed mechanistic insights obtained from spectroscopic and isotopic experiments provide a foundation for further optimizing the reaction conditions and expanding its applicability to other reductive transformations. The formation of mesoporous structures in the recovered silicon suggests potential improvements in the catalyst design through material engineering.

This study successfully demonstrated the synthesis of formamides from CO₂ using recycled silicon powder as a reducing agent and TBAF as a catalyst. The high yields and broad substrate scope highlight the potential of this environmentally friendly and economically viable approach. Future research could focus on further optimization of the reaction conditions, exploring different catalyst systems, and expanding the scope of the reaction to other substrates. The formation of mesoporous silicon during the reaction warrants further investigation for its potential application in other catalytic processes.

While the study achieved high yields under optimized conditions, the reaction's efficiency might be affected by the quality and particle size of the recycled silicon powder. The scalability and long-term stability of the catalyst system under industrial conditions need to be further investigated. The study focused on formamide synthesis, and the applicability of this methodology to other reductive functionalization reactions requires further exploration.