Heterogeneous Silica-Supported Ruthenium Phosphine Catalysts for Selective Formic Acid Decomposition

The water-soluble ruthenium(II)–mTPPTS complex (mTPPTS= meta-trisulfonated triphenylphosphine) efficiently catalyzes the generation of hydrogen from formic acid in aqueous solution. This catalyst has been immobilized in a hydrophilic silica sup- port to facilitate mobile portable applications. It was used in diluted formic acid solutions, in which the recycling of a liquid phase catalyst is problematic. A series of heterogenized catalysts have been prepared by the reaction of the ruthenium(II)– mTPPTS dimer and silica functionalized with diphenylphos- phine groups via alkyl chains. The length of the alkylene chain separating the silica from the diphenylphosphine group has been varied in these catalysts—all of them catalyzing the decomposition of formic acid. The catalysts were easily separated from the solution and reused. The optimized catalytic system based on MCM41-Si-(CH2)2PPh2/Ru-mTPPTS demonstrated an activity and stability comparable to those of the homogeneous catalyst : a turnover frequency of 2780 h-1 was obtained at 1108C, and no ruthenium leaching was detected after turnover numbers of 71000.