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

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/MgO-catalyzed aromatic hydrogenation.