Enhanced Catalytic Activity of (DMSO)₂PtCl₂ for the Methane Oxidation in the SO₃-H₂SO₄ System

Among the various methane activation reactions, oleum-mediated (SO₃-H₂SO₄) methane oxidation to methyl bisulfate (MBS), a methanol precursor, is one of the best methods in terms of methane conversion and product selectivity. In this report, we investigated the effect of the catalyst concentration on the MBS yield using the catalyst systems (bpym)PtCl₂, K₂PtCl₄, and (DMSO)₂PtCl₂ at 180 °C for 3 h. (bpym)PtCl₂ showed a very high stability as well as a high MBS yield of over 84%, but its catalytic activity expressed in terms of turnovers for 3 h was in the range 50-500. K₂PtCl₄ showed very high catalyst turnovers of over 17 000 at a low catalyst concentration; however, it deactivated rapidly to PtCl₂ which prevents achieving a high MBS yield that was achieved by (bpym)PtCl₂. However, (DMSO)₂PtCl₂ showed an enhanced catalytic performance for both the MBS yield and the turnovers. An MBS yield of over 84% with a selectivity of 94% was obtained at a catalyst concentration of 3.0 mM, and its turnovers reached over 19 000 at a low catalyst concentration. This higher catalytic performance of (DMSO)₂PtCl₂ compared to the other chloride-ligated Pt compounds is due to the DMSO ligand on the Pt, which increases the solubility of the Pt species in oleum. Furthermore, as the DFT study revealed, the low dissociation energy of DMSO from the Pt center can facilitate the coordination of methane on the Pt reaction center. Although (DMSO)₂PtCl₂ was deactivated to PtCl₂ after the reaction like the other Pt compounds with the chloride ligand, it can be reactivated to some extent by adding DMSO.