Efficient [Fe-Imidazole@SiO₂] Nanohybrids for Catalytic H₂ Production from Formic Acid

Three imidazole-based hybrid materials, coded as IGOPS, IPS and impyridine@SiO₂ nanohybrids, were prepared via the covalent immobilization of N-ligands onto a mesoporous nano-SiO₂ matrix for H₂ generation from formic acid (FA). BET and HRTEM demonstrated that the immobilization of the imidazole derivative onto SiO₂ has a significant effect on the SSA, average pore volume, and particle size distribution. In the context of FA dehydrogenation, their catalytic activity (TONs, TOFs), stability, and reusability were assessed. Additionally, the homologous homogeneous counterparts were evaluated for comparison purposes. Mapping the redox potential of solution E_h vs. SHE revealed that poly-phosphine PP₃ plays an essential role in FA dehydrogenation. On the basis of performance and stability, [Fe²⁺/IGOPS/PP₃] demonstrated superior activity compared to other heterogeneous catalysts, producing 9.82 L of gases (VH₂ + CO₂) with TONs = 31,778, albeit with low recyclability. In contrast, [Fe²⁺/IPS/PP₃] showed the highest stability, retaining considerable performance after three consecutive uses. With VH₂ + CO₂ = 7.8 L, [Fe²⁺/impyridine@SiO₂/PP₃] activity decreased, and it was no longer recyclable. However, the homogeneous equivalent of [Fe²⁺/impyridine/PP₃] was completely inactive. Raman, FT/IR, and UV/Vis spectroscopy demonstrated that the reduced recyclability of [Fe²⁺/IGOPS/PP₃] and [Fe²⁺/impyridine@SiO₂/PP₃] nanohybrids is due to the reductive cleavage of their C-O-C bonds during catalysis. An alternative grafting procedure is proposed, applying here to the grafting of IPS, resulting in its higher stability. The accumulation of water derived from substrate's feeding causes the inhibition of catalysis. In the case of [Fe²⁺-imidazole@SiO₂] nanohybrids, simple washing and drying result in their re-activation, overcoming the water inhibition. Thus, the low-cost imidazole-based nanohybrids IGOPS and IPS are capable of forming [Fe²⁺/IGOPS/PP₃] and [Fe²⁺/IPS/PP₃] heterogeneous catalytic systems with high stability and performance for FA dehydrogenation.