Khanindra Kalita

Highly efficient CO ₂ and (bi)carbonate hydrogenation to formate in water was achieved by exploiting the cooperative interaction of the two ruthenium centers in a series of water‐soluble diruthenium complexes [{( η ⁶ ‐arene)RuCl} ₂ ( μ ‐ κ ² : κ ² ‐benztetraimd)] ²⁺ containing the bridging meta ‐/ para ‐bis‐imidazole methane‐based ligands ( m ‐/ p ‐benztetraimd). Our findings revealed the pivotal role of the ligand structure in tuning the catalytic activity, where the catalyst p ‐C3 [{( η ⁶ ‐ p ‐cymene)RuCl} ₂ ( μ ‐ κ ² : κ ² ‐ p ‐benztetraimd)] ²⁺ outperformed others. Consequently, a high formate yield (2.73 mmol) with a turnover number (TON) of 1092 (1524 in 24 h) and a turnover frequency (TOF) of 137 h ⁻¹ was achieved over the catalyst p ‐C3 at 40 bar p CO ₂ / p H ₂ (1:3) at 80°C. Advantageously, the catalyst p ‐C3 also efficiently hydrogenated potassium bicarbonate and potassium carbonate to formate. We further utilized the optimized process for demonstrating the conversion of CO ₂ captured (via direct air capture) to formate through an integrated carbon capture and CO ₂ hydrogenation process. Moreover, the catalyst p ‐C3 exhibited appreciable recyclability over multiple catalytic cycles for the reversible CO ₂ /(bi)carbonate hydrogenation and formate dehydrogenation process. The key reaction intermediates, such as hydride species formed by treating the complex p ‐C3 with H ₂ gas, revealed the role of such intermediates in achieving high catalytic activity for CO ₂ hydrogenation over the diruthenium catalysts.

Hydrogen is considered as a potential alternative and sustainable energy carrier, but its safe storage and transportation are still challenging due to its low volumetric energy density. Notably, C1-based substrates, methanol and formaldehyde, containing high hydrogen contents of 12.5 wt% and 6.7 wt%, respectively, can release hydrogen on demand in the presence of a suitable catalyst. Advantageously, both methanol and aqueous formaldehyde are liquid at room temperature, and hence can be stored and transported considerably more safely than hydrogen gas. Moreover, these C1-based substrates can be produced from biomass waste and can also be regenerated from CO₂, a greenhouse gas_. In this review, the recent progress in hydrogen production from methanol and formaldehyde over noble to non-noble metal complex-based molecular transition metal catalysts is extensively reviewed. This review also focuses on the critical role of the structure–activity relationship of the catalyst in the dehydrogenation pathway.