Potassium as the best alkali metal promoter in boosting the hydrogenation activity of Ru/MgO for aromatic LOHC molecules by facilitated heterolytic H2 adsorption

Abstract

Alkali metals (AM) are frequently used as promoters in a number of reactions. In aromatic hydrogenation over Ru/MgO, their basicity may promote heterolytic H2 adsorption, thereby boosting reaction kinetics. Herein, Ru/AM/MgO catalysts (AM: Na, K, and Cs) with different AM/Ru molar ratios are prepared and evaluated in the hydrogenation of toluene and benzyltoluene. The activity exhibits a volcano-type relationship with the AM/Ru molar ratio, where the optimal AM content decreases in the order Na+ > K+ > Cs+ and the AM showing the best activity is in the sequence Cs+ < Na+ < K+. These results originate from the heterolytic H2 adsorption induced by charge transfer from AM to proximate Ru atoms at the Ru-MgO interface. The degree of charge transfer depends on the AM's ionic radius and electronegativity affecting surface mobility and charge donation ability. Consequently, K+ is the best AM promoter in Ru/MgOcatalyzed aromatic hydrogenation.