Temperature-Dependent Diels-Alder Cycloaddition on Polyoxometalate-Supported Single-Atom Catalysts M1/PTA (M=Mn, Fe, Co, Ru, Rh, Pd, Os, Ir and Pt; PTA=[PW1240]3-)

Diels-Alder (D-A) reaction shaped both the art and science of total synthesis to some degree, while the effect of temperature on D-A activity over polyoxometalates supported single-atom catalysts (SACs) has been infrequently studied and simulated using theoretical calculations. Herein, cycloaddition of 1,3-butadiene (C4H6) and ethylene (C2H4) was employed as a model D-A reaction. The multitudes of SACs M-1/PTA (M=Mn, Fe, Co, Ru, Rh, Pd, Os, Ir and Pt; PTA=[PW12O40](3-)) were examined by DFT-M06l computations to understand the reaction mechanism on a molecular level. The adsorption energies of reactant and product, and activation energy barriers for all the studied SAC systems have the same variation trends with the temperature variations. Considering that the adsorption for C4H6 is always stronger than that of C2H4 in all the studied systems, the initial adsorption configurations is the M-1/PTA SACs adsorbed one C4H6 molecule. Three SACs, namely the Co-1/PTA and Rh-1/PTA at 100 K, Rh-1/PTA at 300 K were identified, which show predominant catalytic activity and the corresponding activation energy barriers are 4.21, 8.51 and 5.11 kcal mol(-1), respectively. The bonding interaction between adsorbate C4H6 and SACs arises from the occupied molecular orbitals (MOs) with a mixture of pi orbitals of C4H6 and d atomic orbitals of the metal single atom. These theoretical calculations give new guidelines to develop high catalytic activity and cost-effective SACs towards the D-A reaction.