Atomically dispersed calcium as solid strong base catalyst with high activity and stability

Solid strong base catalysts are highly attractive for diverse reactions owing to their advantages of neglectable corrosion, facile separation, and environmental friendliness. However, their widespread applications are impeded by basic components aggregation and low stability. In this work, we fabricate single calcium atoms on graphene (denoted as Ca₁/G) by use of a redox strategy for the first time, producing solid strong base catalyst with high activity and stability. The precursor Ca(NO₃)₂ is first reduced to CaO at 400 ℃ by the support graphene, forming CaO/G with conventional basic sites, and the subsequent reduction at 850 ℃ results in the generation of Ca₁/G with atomically dispersed Ca. Various characterizations reveal that Ca single atoms are anchored on graphene in tetra-coordination (Ca-C₂-N₂) where N is in situ doped from Ca(NO₃)₂. The atomically dispersed Ca, along with their anchoring on the support, endow Ca₁/G with high activity and stability toward the transesterification reaction of ethylene carbonate with methanol. The turnover frequency value reaches 128.0 h^-1 on Ca₁/G, which is much higher than the traditional counterpart CaO/G and various reported solid strong bases (2.9-46.2 h^-1). Moreover, the activity of Ca₁/G is well maintained during 5 cycles, while 60% of activity is lost for the conventional analogue CaO/G due to the leaching of Ca.