Selective hydrogenation via precise hydrogen bond interactions on catalytic scaffolds

Enzyme catalysis, characterized by precise active site environments and weak interactions with substrates, has long inspired the design of heterogeneous catalysts. Mimicking this precision remains a significant challenge in creating artificial analogs with tailored spatial structures and electronic properties. Researchers have explored various approaches, such as modifying metal-organic frameworks (MOFs) to alter hydrophobicity and enhance substrate concentration, and engineering single-atom catalysts to control the local coordination and electronic state of the catalytic center. However, precisely controlling weak interactions, particularly hydrogen bonds crucial in low-barrier hydrogenations, remains challenging due to the lack of structural precision in traditional solid catalysts. This work focuses on designing catalysts with precisely controlled hydrophobic or hydrophilic environments to form hydrogen bonds with H-acceptor substrates. The chosen platform is hyper-crosslinked porous polymers (HCPs), offering advantages such as well-defined functional groups, high surface area, and structural stability. HCPs provide a versatile platform for modifying the active site environment by controlling the density and distribution of functional groups, unlike traditional activated carbon materials with inherent heterogeneity of functional groups. Previous research demonstrated the use of POPs as effective mimics of enzyme-inspired catalysts through encapsulating metal nanocrystals or incorporating functional groups similar to solvents employed in reactions, which provided a “solid solvent” effect. This study aims to synthesize HCPs with nitrogen (N) atoms for anchoring metal nanoparticles and either methyl (CH3) or hydroxyl (OH) groups to precisely control the active site environment's hydrophilicity/hydrophobicity. Through a combination of spectroscopic techniques, reaction studies with various substrates, and density functional theory (DFT) calculations, the research will evaluate the effects of hydrogen bonding on substrate adsorption, hydrogenation rates, and selectivity.

The literature review extensively cites prior work demonstrating the impact of active site microenvironment engineering in catalysis. Examples include the modification of MOFs with poly(dimethylsiloxane) (PDMS) to enhance hydrophobicity and catalytic activity, and the engineering of single-atom catalysts through local coordination and electronic state control. The importance of hydrogen bonding in enzyme catalysis, particularly low-barrier hydrogenations, is highlighted with reference to specific examples like human transketolase. However, the lack of precise structural control in traditional solid catalysts to mimic these weak interactions is emphasized as a major hurdle. The review also covers recent advancements in porous organic polymers (POPs) as tunable catalytic materials. The work of Cargnello's group on encapsulating metal nanocrystals in amine-based POPs, and Xiao and Ma's synthesis of POPs mimicking high-boiling solvents to enhance fructose dehydrogenation, are presented as examples showcasing POPs' potential as enzyme-inspired catalysts. This background sets the stage for the current research which aims to leverage the precise control offered by HCPs to achieve selective catalysis through hydrogen bond interactions.

The study employed a two-pronged approach: synthesis and characterization of HCP catalysts, and catalytic performance evaluation with various substrates. HCPs with -OH groups (HCP-OH) were synthesized via Friedel-Crafts alkylation using phenol as the monomer and triphenylamine for metal nanoparticle anchoring. HCPs with -CH3 groups (HCP-CH3) served as controls, synthesized similarly using toluene. Thorough characterization employed solid-state 13C nuclear magnetic resonance (CP/MAS 13C-NMR), Fourier-transform infrared spectroscopy (FT-IR), scanning electron microscopy (SEM), focused ion beam (FIB) 3D reconstruction, high-angle annular dark-field scanning transmission electron microscopy (HAADF-STEM) elemental mapping, and nitrogen adsorption isotherms to confirm the structural integrity, functional group presence, porosity, and metal nanoparticle distribution. Wettability tests confirmed the hydrophilic nature of HCP-OH and hydrophobic nature of HCP-CH3. Iridium nanoparticles were introduced via impregnation and reduction. The stability of both catalysts was assessed using thermogravimetric analysis (TGA). Catalytic performance was evaluated using a Parr reactor under controlled conditions (temperature, pressure, and substrate concentration) for various substrates including toluene, furfural, and 2-methylfuran. The impact of the active site environment on adsorption was analyzed through adsorption isotherms. In situ diffuse reflectance infrared Fourier transform spectroscopy (DRIFTS) was used to investigate furfural adsorption on both catalysts and identify various binding modes. Concentration-dependent 1H-NMR was employed to confirm hydrogen bond formation between furfural and the OH groups of the HCP-OH catalyst. DFT calculations using zigzag and armchair graphene ribbons as models were performed to elucidate the effect of OH and CH3 groups on furfural adsorption and the interaction energies. The effect of OH density was studied by varying the phenol-to-toluene ratio in the HCP synthesis. The particle size effect of Ir-HCP-OH on catalytic activity was investigated by varying the Ir loading. Finally, the selectivity control aspect was explored using cinnamaldehyde hydrogenation, and the recyclability of the Ir-HCP-OH catalyst was tested via repeated reaction cycles. In addition to these methods, experiments were also performed using Pd and Pt nanoparticles, and a range of complementary characterization techniques such as XPS and CO adsorption Drifts-Ir were applied.

The study demonstrated a strong correlation between the active site environment and catalytic performance. HCP-OH catalysts, exhibiting a hydrophilic environment, significantly enhanced the hydrogenation rate of carbonyl-containing substrates like furfural, showcasing the importance of hydrogen bond interactions. Conversely, HCP-CH3 catalysts promoted the hydrogenation of less polar substrates like toluene. This substrate specificity underscored the impact of the active site environment beyond simple substrate adsorption. Detailed characterization confirmed the successful synthesis of HCPs with the desired functional groups and the homogeneous distribution of Ir nanoparticles. Adsorption isotherms revealed a significantly higher affinity of furfural for HCP-OH compared to HCP-CH3, further supporting the role of hydrogen bonding in substrate adsorption. In situ DRIFTS studies revealed multiple furfural binding modes, with a greater proportion of the η1 conformation (C=O group interaction) on HCP-OH, indicating stronger C=O group interaction. Concentration-dependent 1H-NMR definitively confirmed hydrogen bond formation between furfural and the OH groups in HCP-OH, quantifying the interaction energy. DFT calculations corroborated the experimental findings, showing stronger furfural adsorption on OH groups due to hydrogen bonding, which also weakens the C=O bond. The OH density was shown to directly influence the catalytic performance. Further, the study revealed a particle size dependence, indicating a synergistic effect between Ir edge sites and nearby OH groups. Importantly, the hydrogen bond interaction enabled selectivity control in the hydrogenation of multifunctional substrates, favoring C=O hydrogenation over C=C hydrogenation in cinnamaldehyde. The recyclability tests indicated the catalyst's heterogeneous nature with some loss of activity after multiple cycles potentially due to Ir nanoparticle sintering.

The findings directly address the research question by demonstrating precise control over the catalytic activity and selectivity through manipulation of hydrogen bond interactions in the active site environment. The results strongly suggest that the functional groups' influence goes beyond simple substrate adsorption, actively participating in catalytic promotion and selectivity control by partially activating the C=O bond. This mechanism contrasts with previous methods that primarily focused on enhancing substrate adsorption. The success in achieving both rate and selectivity control through this bio-inspired approach has significant implications for designing advanced catalysts for selective hydrogenation reactions. The observed substrate specificity and the quantitative analysis of hydrogen bonding interaction energies provide valuable insights into the design principles for future catalysts. This strategy's efficacy is not limited to POPs, as shown by preliminary experiments with zeolite materials exhibiting similar hydrogen bonding effects, highlighting its wider applicability across various porous materials. The synergistic effect of the metal nanoparticles and the hydroxyl groups is a critical observation, prompting further investigation into designing catalysts with optimized proximity and density of these functional groups.

This study introduces a novel strategy for designing bio-inspired catalysts with precisely tuned active site environments for selective hydrogenation. The synthesis of HCPs with controlled OH and CH3 groups enabled the modulation of hydrogen bonding interactions, leading to substrate-specific catalytic performance. The observed enhancement in reaction rates, selectivity, and the elucidation of the underlying mechanism through spectroscopic and computational techniques provide a valuable blueprint for future catalyst development. Further research could explore expanding the range of functional groups, optimizing the spatial arrangement of active sites and functional groups, and testing this strategy with different metal nanoparticles and reaction systems.

The recyclability of the Ir-HCP-OH catalyst exhibited some limitations due to the observed sintering of Ir nanoparticles after multiple reaction cycles. Although the catalyst demonstrated stability in initial tests, prolonged use resulted in a decrease in catalytic activity. This sintering effect could be mitigated through techniques like redispersion or encapsulation strategies. Furthermore, the DFT calculations used simplified models of the HCPs, focusing on functional group interactions rather than the full complex structure. While the simplified models provided valuable insights, future work should incorporate more realistic models of the amorphous HCP structure to improve the accuracy of theoretical predictions.