Heterogeneous catalysts are ubiquitous in industry but suffer from surface heterogeneity and complex structures, hindering mechanism elucidation and atom efficiency. Single-atom catalysts (SACs) offer improved control over the active center's environment, enabling structure-property relationships and mechanistic understanding. However, identifying the precise active site remains challenging. Recent advances in SACs for biomass conversion include various noble metal catalysts (Ru, Pd, Pt) supported on different matrices. These studies demonstrated enhanced activity compared to nanoparticle catalysts. However, non-noble metal-based SACs are more desirable due to cost and abundance. Iron, in particular, is abundant and environmentally friendly, making it an attractive alternative. Previous work explored Fe-catalyzed catalytic transfer hydrogenation (CTH) of furfural (FF), a key biomass derivative, but suffered from low intrinsic activity and poorly defined active sites, resulting in low turnover frequencies (TOFs). This research addresses these limitations by developing a highly active single-atom iron catalyst for the CTH of FF.

The literature review highlights the existing body of work on single-atom catalysts for biomass conversion. Several examples of noble metal-based SACs are presented, showcasing their superior activity compared to nanoparticle counterparts in various reactions such as formic acid decomposition, selective hydrogenation of vanillin, and reductive catalytic fractionation of lignocellulose. While these studies establish the potential of SACs, they focus mainly on noble metals. The use of non-noble transition metals, such as iron, is discussed as a cost-effective and sustainable alternative. Previous research on iron-based catalysts for CTH of furfural is reviewed, noting the challenges associated with low activity and ill-defined active sites. The research context is established by showing that while some Fe-based catalysts have been reported, their activity is typically low and measured under harsh conditions. The low TOF (<10 h⁻¹) of conventional iron nanoparticle catalysts further motivates the search for more efficient catalysts.

The authors synthesized single-atom iron catalysts using a saturated adsorption strategy. This involved using iron nitrate as the iron precursor, a smaller salt compared to those used in previous studies (e.g., Fe(acac)3), and adsorbing it onto ZIF-8 to achieve ultra-low Fe loadings (<0.1 wt%). A two-step synthesis route was also explored to fine-tune the Fe loading. The catalysts were characterized extensively using various techniques including: scanning electron microscopy (SEM), transmission electron microscopy (TEM), high-angle annular dark-field scanning transmission electron microscopy (HAADF-STEM), energy dispersive spectroscopy (EDS), nitrogen adsorption measurements, X-ray diffraction (XRD), X-ray photoelectron spectroscopy (XPS), X-ray absorption near-edge structure (XANES), extended X-ray absorption fine-structure (EXAFS) spectroscopy, Raman spectroscopy, thermogravimetric analysis (TGA), NH3-TPD and CO2-TPD. The catalytic performance was evaluated using CTH of furfural with isopropanol as the hydrogen donor, and the reaction products were analyzed using gas chromatography (GC) and gas chromatography-mass spectrometry (GC-MS). Density functional theory (DFT) calculations were performed to elucidate the reaction mechanism. The DFT calculations involved periodic models of Fe(II)-pIN3, Fe(II)-pIN4, and Zn(II)-pIN, based on structures identified through XANES simulations. Microkinetic modeling (MKM) was also employed using Chemkin to simulate the reactions on the different active site models. Isotopic labeling experiments with deuterated isopropanol were conducted to determine the reaction mechanism. The recyclability of the catalyst was also investigated.

The Fe-ZIF-8-800 catalyst exhibited outstanding performance in the CTH of furfural, achieving 99.6% conversion and 96.9% selectivity to furfuryl alcohol at 120 °C in 6 hours. The TOF of this catalyst (1882 h⁻¹ at 120 °C) was significantly higher than those of other Fe-based catalysts (2-4 orders of magnitude). Isotopic labeling experiments confirmed an intermolecular hydride transfer mechanism consistent with the Meerwein-Ponndorf-Verley (MPV) pathway. DFT calculations and XANES/EXAFS analyses identified the active site as pyrrolic Fe(II)-PIN3. Crucially, the study highlights the importance of the flexible geometry of the Fe(II)-PIN3 site, which allows for simultaneous coordination of the substrate and solvent molecules. This is in contrast to the rigid Fe(II)-PIN4 site, which shows significantly lower activity due to steric hindrance. The catalyst demonstrated good chemoselectivity for a range of substrates but showed lower activity for substrates with acidic hydroxyl groups. Recyclability studies revealed that the activity decreased after repeated use but could be fully restored by adding fresh iron precursor and re-pyrolyzing. The superior performance is attributed to the unique structural properties and the synergistic interaction between the Fe atoms and N-C sites in the ZIF-8 derived support, especially its flexibility. The relationship between pKa of hydroxyl groups and CTH yields indicates that substrates with stronger acidic H atoms will show weaker activity, consistent with their interactions with Fe-N sites. The study provides significant evidence that the Fe(II)-PIN3 site is responsible for the superior catalytic activity, based on DFT calculations, including TOF calculations, suggesting that it is about six orders of magnitude more active than the alternative sites.

The findings address the research question by demonstrating the synthesis of a highly active single-atom iron catalyst for CTH of furfural. The superior activity is attributed to the unique structural characteristics of the active site (Fe(II)-PIN3) and its flexibility. The results challenge the commonly held assumption that Fe(II)-N4 is the active site in Fe-N-C catalysts and highlights the significance of structural flexibility in heterogeneous catalysis. This work contributes to the broader field by providing a novel synthesis strategy for creating highly active SACs and offering valuable insights into the relationship between active site structure and catalytic performance. The identification of the Fe(II)-PIN3 site as the active center and the demonstration of its superior activity have important implications for the design of efficient and sustainable catalysts for biomass conversion.

This work successfully synthesized a highly active single-atom iron catalyst for catalytic transfer hydrogenation with an ultra-low Fe loading, achieving a TOF that surpasses previously reported Fe catalysts by several orders of magnitude. The active site, identified as a flexible Fe(II)-PIN3 structure, was shown to be crucial for the high catalytic activity via a combination of isotopic labeling and multiscale simulations. The unique properties of this catalyst pave the way for the development of efficient and sustainable alternatives to noble metal catalysts for biomass valorization. Future research could explore different supports to further optimize the catalyst's activity and stability and extend the catalytic applications to other relevant reactions.

The study primarily focused on the CTH of furfural and a limited set of substrates. The long-term stability and reusability could be further investigated under more demanding industrial conditions. While DFT calculations provided valuable mechanistic insights, experimental validation of specific intermediates and transition states would strengthen the conclusions. The effect of defects on the overall catalytic performance requires further investigation to fully understand the structure-function relationship.