Insight into the direct conversion of methane to methanol on modified ZIF-204 from the perspective of DFT-based calculations.

Direct oxidation of methane over oxo-doped ZIF-204, a bio-mimetic metal-organic framework, is investigated under first-principles calculations based on density functional theory. In the pristine ZIF-204, the tetrahedral methane molecule anchors to an open monocopper site via the so-called η2 configuration with a physisorption energy of 0.24 eV. This weak binding arises from an electrostatic interaction between the negative charge of carbon in the methane molecule and the positive Cu2+ cation in the framework. In the modified ZIF-204, the doped oxo species is stabilized at the axial position of a CuN4-base square pyramid at a distance of 2.06 Å. The dative covalent bond between Cu and oxo is responsible for the formation energy of 1.06 eV. With the presence of the oxo group, the presenting of electrons in the O_pz orbital accounts for the adsorption of methane via hydrogen bonding with an adsorption energy of 0.30 eV. The methane oxidation can occur via either a concerted direct oxo insertion mechanism or a hydrogen-atom abstraction radical rebound mechanism. Calculations on transition-state barriers show that reactions via the concerted direct oxo insertion mechanism can happen without energy barriers. Concerning the hydrogen-atom abstraction radical rebound mechanism, the C-H bond dissociation of the CH4 molecule is barrierless, but the C-O bond recombination to form the CH3OH molecule occurs through a low barrier of 0.16 eV. These predictions suggest the modified ZIF-204 is a promising catalyst for methane oxidization.

Some remarks on the malaria and anopheline mosquitoes in Quang Binh province based on the data of the surveys in the third quater, 2004

All of 8 villages of Lam Thuy and Ngan Thuy communes are high malaria endemic areas. The slide positive rates (SPR) in Lam Thuy and Ngan Thuy are 11.56% and 36.20% respectively; P. falciparum positive rates are 72.50% and 67.80%; Parasite positive rates in children at 1-5 years old are 45.00% and 32.20%. Habit of sleeping without bed net of the local population and unfectioning microscopic points have made the malaria situation higher in these two communes. Anopheline component in Km 14 village, Ngan Thuy commune, Le Thuy district is abundant (19 species): with the presentation of the two main malaria vectors An.dirus and An.minimus. The landscape in this area is suitable for the production and development of two main malaria vectors as well as other anopheline mosquitoes