Background: In recent years CO2 utilization and conversion of CO2 into the value added
chemicals have attracted great attention of leading research groups. However, CO2 is widely accepted
as reagent for the synthesis of carbonates and other important chemicals such as methanol, aspirin,
formic acid etc, but CO2 reduction is still a major challenge for the scientific community as they are
extremely thermodynamically stable. In this report, we are offering the synthesis of silica-tethered ruthenium
catalyst (SRUC) for the hydrogenation of CO2 to formic acid.
Methods: The SIRUC catalytic system was synthesized by a multistep grafting process using iminophophine
ligand tethered to mesoporus SBA-15 inorganic support. After activating the SRUC catalyst with hydrogen gas,
it was applied as hydrogenating catalyst for CO2 gas. It is worth noted that, SBA-15 was synthesized as per reported protocol
and the important IV- type isotherm was recorded while performing N2 physisorption analysis.
Results: The catalytic efficiency of silica-tethered Ru catalysts was screened for the hydrogenation of CO2 to formic acid.
The hydrogenation reaction was carried out in 100 mL autoclave with triethyl amine (NEt3) and water under the pressure
of CO2 and hydrogen gas (40 bar, CO2: H2=1:1) at 75°C. The formation of formic acid (or formate) was calculated
through 1H NMR. As per the experimental data, it was clearly observed that catalyst system only works effectively with
phosphine ligands and offered the formic acid with significant TON/TOF value opposite to SRUC-4A catalytic system.
Higher catalytic activity in terms of TON/TOF value was obtained with SRUC-1A material carries the bidentate
phosphine ligand at 100°C.
Conclusion: In this manuscript, we reported a new protocol to synthesize mesoporous silica-tethered Ru complexes
(SRUC 1A-4A). Among these, materials, SRUC 1A was found and effective heterogeneous catalyst for the selective CO2
hydrogenation reaction to obtain formic acid under normal reaction condition. In terms of catalyst recycling, this catalytic
system was found highly active in catalyst recycling test up to 6 cycles without any significant loss of TON value of formic
acid. In parallel, we also performed the filtration experiment and the obtained results were found in good agreement
with recycling test results.