Ethylene glycol interaction on alkaline earth oxides: A periodic DFT study

The present paper describes the interaction of ethylene glycol CH2OH–CH2OH with alkaline earth oxide basic catalysts MO (M = Mg, Ca, Sr, Ba) from periodic DFT calculations. The geometry of adsorption depends on the nature of the metallic site: on MgO the alcohol groups bind to the metal sites on quasi-top positions, while on CaO, SrO and BaO the molecule is located on bridging positions. The adsorption is exothermic and the strength correlates with the basicity of the alkaline earth oxide, the more basic the substrate, the more exothermic the adsorption energy: MgO < CaO < SrO < BaO. The glycol molecule deprotonates to form surface alkoxy groups bound to the metal sites. The extent of such deprotonation is also correlated to the basicity of MO:MgO (completely protonated) < CaO (partially protonated) < SrO (completely deprotonated) = BaO (completely deprotonated). Defects, modeled for a stepped CaO slab, are found to enhance the strength of the interaction and the deprotonation extent, inducing a different adsorption mode. The step is found to be more reactive than the most basic BaO regular surface, in agreement with experimental observations on glycerol. The implications of these aspects in chemical reactivity are discussed.