Kinetics and Adsorption Calculations: Insights into the MgO-Catalyzed Detoxification of Simulants of Organophosphorus Biocides

We report the targeted decomposition of the organophosphate methyl paraoxon by means of its transesterification with 1-propanol catalyzed by magnesium oxide. Catalyst characterization by energy dispersive X-ray fluorescence (EDXRF), nitrogen adsorption/desorption measurements (BET and BJH methods), and temperature programmed desorption of CO2 (CO2-TPD) showed that the employed MgO presents properties favorable for the methyl paraoxon adsorption and for the transesterification to occur. A thorough kinetic investigation showed that rate enhancements up to 3 x 106-fold can be achieved in comparison with the spontaneous propanolysis of the substrate, and that the material can be used in additional cycles without loss of catalytic activity, being the catalyst recovery attained through a simple washing procedure. Adsorption energies for 1-propanol and methyl paraoxon onto a MgO model surface were obtained by density functional theory calculations, which showed that the latter displays a stronger affinity with the catalyst surface, and that the reaction should proceed with methyl paraoxon and 1-propanol molecules juxtapositioned at adjacent Mg2+ sites, with nucleophilic and electrophilic centers c.a. 2.4 A distant from each other. Additionally, MgO also promoted rate enhancements up to 5 x 104-fold in the propanolysis of a further range of representative phosphate triesters, and in most of the cases the final transesterified products are trialkyl phosphates structurally related to a family of flame-retardants. The results thus provide insights into the development of novel systems for the targeted conversion of organophosphorus compounds into value-added products by employing simple, highly efficient, and low cost metal oxide catalysts.