Oxidative conversion of propane over lithium-promoted magnesia catalyst. I. Kinetics and mechanism

Oxidative conversion of lower alkanes over lithium-promoted magnesia catalysts offers a viable alternative for propene and ethene production. The catalytic performance of propane oxidative dehydrogenation and cracking shows yields up to 50% of olefin (propene and ethene). The reaction kinetics were studied by means of variation of the partial pressures of the reactants as well as by addition of product species to the reaction mixture. The observations can be qualitatively explained with a mechanism including activation of propane on the catalyst generating propyl radicals that undergo a radical-chain mechanism in the gas phase. Alkane activation is rate determining. Oxygen has two functions in the mechanism. First, the presence of small amounts of oxygen influences the radical gas-phase chemistry significantly because the type and concentration of chain propagator radicals are greatly increased. At higher oxygen partial pressures the radical chemistry is only slightly influenced by the increasing oxygen concentration. The second function of oxygen is to facilitate the removal of hydrogen from the surface OH¿ species that are formed during the activation of propane on the catalyst. Carbon dioxide has a strong inhibiting effect on the reaction without changing the product distribution, due to strong adsorption on the site that activates propane

Oxidative conversion of propane over lithium-promoted magnesia catalyst. II. Active site characterization and hydrocarbon activation

Activation of propane over Li/MgO catalyst has been investigated. It is shown that a small fraction of the oxygen ions in Li/MgO catalysts can be removed from the catalyst by reduction treatment in H2 at 600 °C. Catalytic activity of Li/MgO exhibits a strong correlation to the amount of oxygen that is removed. It is proposed that the sites containing removable oxygen are responsible for the activation of propane. About 70 propane molecules were converted after consumption of one such oxygen site, in the absence of gas-phase oxygen, implying a mechanism in which propane molecules are activated on the catalyst resulting in propyl radicals that are released to the gas phase where they undergo chain propagation reactions, resulting in the products observed. The active O site is consumed by conversion into an OH group, as the oxygen is not removed from the catalyst with propane. The oxidative conversion of propane over Li/MgO catalysts follows a mixed heterogeneous-homogeneous radical chemistry where the catalyst acts as an initiator. At low propane partial pressures (0.1 bar), the catalyst surface area to volume ratio of the catalytic reactor does not influence the chain length in the propagation step. At higher propane partial pressures (>0.3 bar), favoring extensive gas-phase reactions, the catalyst affects conversion and selectivity also via quenching and chain termination

Beyond chelation:EDTA tightly binds taq DNA polymerase, MutT and dUTPase and directly inhibits dNTPase activity

Abstract EDTA is commonly used as an efficient chelator of metal ion enzyme cofactors. It is highly soluble, optically inactive and does not interfere with most chemicals used in standard buffers making EDTA a common choice to generate metal-free conditions for biochemical and biophysical investigations. However, the controversy in the literature on metal-free enzyme activities achieved using EDTA or by other means called our attention to a putative effect of EDTA beyond chelation. Here, we show that EDTA competes for the nucleotide binding site of the nucleotide hydrolase dUTPase by developing an interaction network within the active site similar to that of the substrate. To achieve these findings, we applied kinetics and molecular docking techniques using two different dUTPases. Furthermore, we directly measured the binding of EDTA to dUTPases and to two other dNTPases, the Taq polymerase and MutT using isothermal titration calorimetry. EDTA binding proved to be exothermic and mainly enthalpy driven with a submicromolar dissociation constant considerably lower than that of the enzyme:substrate or the Mg:EDTA complexes. Control proteins, including an ATPase, did not interact with EDTA. Our findings indicate that EDTA may act as a selective inhibitor against dNTP hydrolyzing enzymes and urge the rethinking of the utilization of EDTA in enzymatic experiments