Density Functional Theory Study of Cu-ZSM-5-Catalyzed C-H Bond Activation: The Importance of Active Centers

In the present contribution, we investigate the catalytic cycle of the methane activation reaction over several copper sites by density functional theory calculation. Our results demonstrate that the C-H bond activation step is the rate-limiting step for all investigated copper sites. After C-H bond cleavage, different types of copper sites show different mechanisms for methane-to-methanol conversion. For the dioxygen copper sites, after the CH4 activation step, the CH3 radical is prone to react with the hydroxide group on the copper sites and form methanol directly. For the monooxygen copper sites, the CH3 radical is energetically favorable to adsorb on monooxygen copper sites, resulting in a stable methoxide. Then, a water molecule would react with the methoxide and form methanol with a negligible energy barrier (2.8 kcal/mol over [CuOCH3] and 3.3 kcal/mol over [Cu2OCH3]). Our results demonstrate that [CuO] sites are the most active for the C-H bond activation of methane, but their formation is predicted to be difficult from both the O-2 and N2O methods. However, [Cu2O] sites can be readily formed through both the O-2 and N2O methods, and they exhibit competitive activity for C-H activation. Increasing the [Cu2O] concentration in Cu-based zeolites is proposed as a feasible way to enhance the catalytic activity of Cu-ZSM-5.