C–H activation in strongly acidic media. The co-catalytic effect of the reaction mediumw

Selective low temperature conversion of methane to methanol remains as a grand challenge for chemical technology. The top five highest yielding systems utilize Pt, Hg, Au, I, and Pd catalysts and a concentrated sulfuric acid reaction medium (acetic acid in the case of Pd). The most promising system to date uses Pt(bpym)Cl2 as the catalyst and concentrated H2SO4 as both the solvent and the oxidant (Fig. 1). 2a The oxy-functionalized product is a mixture of methyl bisulfate, protonated methanol and methanol depending on the concentration of the sulfuric acid solvent. The methyl bisulfate and protonated methanol can be readily hydrolyzed to methanol. Despite excellent one pass yields of 470% and selectivity of 490% at 220 1C, this reaction [developed at Catalytica by Periana et al. in 1998] has not been commercialized (Scheme 1). It was shown by Periana et al. that the methanol yield and selectivity were excellent in highly concentrated sulfuric acid (102% = 9% SO3 dissolved in H2SO4) but as the reaction proceeded the water product led to dramatically decreased rates for concentrations lower than 96%. H–D exchange of CH4 in D2SO4 (Fig. 2) was found to proceed at lower temperature (150 1C) than methanol formation, and the rate of the H–D exchange was found to be strongly dependent on the acidity of the solvent. Most interesting was that under conditions where H–D exchange was observed without formation of methanol, the methane reactant molecules became multiply deuterated. This observation implies that there is a barrier to coordinating CH4 with the catalyst to form an ion pair complex, followed by a smaller barrier for activating this complex to form a M–CH3 adduct, as suggested by Kua et al. Herein we use QM to show how the acid plays an integral part of the reaction mechanism,z explaining the strong acid dependence of the catalyst. Prior studies proposed that the role of the strong acid solvent is to generate the bisulfate platinum complex 1, from which the bisulfate is more readily replaced by methane. At lower solvent acidities, the higher concentrations of water would replace the bisulfate with H2O, leading to a new, more stable, ground state 2, which would increase the barrier to reach the uptake transition state. This original proposal assumed that the aquo complex reacts via the same transition state as the bisulfate complex. Since this assumption was not tested, we used QM to calculate the barriers for displacing bisulfate and water, respectively. We find that replacing water (4ts) has a barrier 1.8 kcal mol 1 lower than replacing bisulfate (3ts) (Fig. 3). This means that the sulfuric acid must be involved in some other way than in the previous explanation. Fig. 1 Catalytica system by Periana et al. (ref. 2a).