Stability and Thermodynamics of the PtCl_2 Type Catalyst for Activating Methane to Methanol: A Computational Study

Stimulated by the report of high-yield, low-temperature catalytic conversion of methane to methyl bisulfate (Periana et al., Science 1998, 280, 560), we studied the relative stability and reaction mechanism of the Pt(NH_3)_2Cl_2 and Pt(bpym)Cl_2 complexes in concentrated sulfuric acid. We find that the mechanism involves a series of steps beginning with C−H activation to form an intermediate ion-pair Pt(II)−CH_4 methane complex prior to forming a Pt(II)−CH_3 complex. Our calculated relative activation barriers for C−H activation are in good agreement with experimentally observed H/D ratios. Subsequent oxidation to a Pt(IV) complex can occur with reduction of SO_3. Release of methyl bisulfate regenerates the Pt(II) catalyst. Our calculations indicate that for the bipyrimidine system C−H activation prefers electrophilic substitution, whereas for the ammine system oxidation addition is more favorable. We find that the oxidation step (the rate-determining step) is more favorable for the ammine catalyst, suggesting higher activity than the bipyrimidine catalyst. However, we find that in sulfuric acid the ammine complex is unstable, while the bipyrimidine catalyst is stable. Bipyrimidine acts as a “proton sink”, allowing the protonated form of the ligand to remain bound to Pt in concentrated sulfuric acid. These results are consistent with the observed behavior of the catalysts, suggesting that computational approaches may be useful in seeking modified catalysts that would be more economically feasible