A Reaction Mechanism of Methane Coupling on a Silica-Supported Single-Site Tantalum Catalyst

Density functional
theory calculations were utilized to study the
reaction mechanisms of nonoxidative coupling of methane (NOCM) occurring
on a silica-supported single-site tantalum (Ta) catalyst. Two catalytic
cycles, namely, catalytic cycles A (CCA) and B (CCB), as well as other
competing pathways, were investigated by exploring the potential energy
surfaces for the reactions of interest. The supported methyltantalum
[(SiO3)2Ta–CH3] and
tantalum hydride [(SiO3)2Ta–H]
catalyzed the reaction of NOCM through CCA and CCB, respectively.
CCA and CCB comprise five and six elementary steps, respectively.
The two rate-determining states for both catalytic cycles were elucidated.
The turnover number of methane conversion catalyzed by the supported
methyltantalum was about 105 larger than that catalyzed
by the supported tantalum hydride. This large difference indicates
that the former species is predominantly responsible for the conversion
of methane to ethane.