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

Abstract

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)(2)PtCl2 at 180 degrees C for 3 h. (bpym)PtC(l)2 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)(2)PtCl2 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)(2)PtCl2 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)(2)PtCl2 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.