Understanding the Hydrocracking of Polycyclic Aromatic Hydrocarbons within FAU Zeolites: Hydrogen Splitting Catalyzed by the Frustrated Lewis Pair

To understand the hydrocracking of aromatic rings in the petrochemical industry, the first step, hydrogen splitting on catalytic centers, has been studied based on density functional theory. Using ab initio thermodynamic analysis, the Lewis acid site (LAS) composed of a single extraframework-Al 3+ atom is predicted to be most stable mononuclear extraframework aluminum species during high-temperature calcination. Based on unbiased ab initio molecular dynamics (AIMD) simulations, this LAS is found to be the best catalytic center for hydrogen splitting, where hydrogen can be split spontaneously, resulting in the stable tetrahydroanthracen-1-ylium cation. With the help of AIMD and metadynamics, free energy landscapes characterizing hydrogen splitting processes on five different catalytic centers, viz., single extraframework-Al 3+ atom, two molecular frustrated Lewis pair (FLP) sites, and two ionic FLP sites, have been examined. Each hydrogen splitting occurs via a single-step concerted mechanism. Two molecular FLP sites exhibit higher splitting reactivity with lower free energy barriers. Further charge analysis reveals that the reactivity of FLP sites is strongly affected by the local electric field generated by the hydride acceptor and proton acceptor within FLP sites.