Authors:María Baez, Adriana Robinsohn, María Legaspi, Mónica. Hedrera, and Beatriz Fernández
High-performance thin-layer chromatography has been used to follow the synthesis of some quinoxalinone derivatives by biocatalysis in a one-pot reaction, as an alternative route to the poor
reaction. The progress of the one-pot reactions can be followed by HPTLC without previous isolation of the different intermediates arising from them. Biocatalytic synthesis of quinoxalinone derivatives leads to an irreversible reaction which gives a single product of high purity and yields in excess of 90–95%.
Authors:T. Tanaka, M. Mukai, T. Maeda, J. Matsumoto, H. Ogawa, Zhentang Li, Xudong Wang, Zhiwen Fan, Liangtian Guo, and Cunli Liu
Migration experiments with 237Np(V) and 241Am(III) have been performed using a column system, packed with loess, taken from Shanxi, China. The adsorption mechanism of 237Np and 241Am on the loess was examined by a chemical extraction method. Most of 237Np was adsorbed on the influent edge of the column, and the adsorbtion was mainly controlled by surface complexation. However, the migration of 237Np in loess media could be roughly evaluated by the distribution coefficient. In the case of 241Am, particulate, the 241Am species was formed in the influent solution and moved in the column. The 241Am adsorbed on loess was controlled by irreversible reactions. The migration behavior of particulate 241Am in loess media could be expressed by the filtration theory.
Adsorption experiments were performed to measure distribution coefficients of237Np(V),238Pu(IV) and241Am(III) for sedimentary sequential chemical extraction of the adsorbed radionuclides was carried out with water, CaCl2, KCl, NH2OH−HCl, K-oxalate and H2O2 solutions, to elucidate their dominant sorption mechanisms. The distribution coefficient of237Np was two orders of magnitude smaller than that of238Pu and241Am. Most of237Np adsorbed was extracted with CaCl2 solution and its sorption was controlled by a reversible ion exchange reaction. The adsorbed238Pu was mainly extracted with NH2OH−HCl+K-oxalate solution and its sorption was possibly controlled by irreversible reactions.
Authors:Everton Fernando Zanoelo and Cristina Benincá
The oxidation of 5-O-caffeoylquinic acid (5-CQA) was investigated in the temperature range from 161 to 188 °C. The kinetic experiments were carried out in small batch reactors made of glass, where a very thin (1.1 μm) fixed bed of powder 5-CQA was in contact with air at approximately 91.4 kPa. Aqueous 5-CQA solutions were prepared with the solid found in the reactors at different reaction times and used to measure the decrease of 5-CQA concentrations with a spectrophotometer operated at 323 nm. A shrinking core model was applied to describe the non-catalytic heterogeneous reaction between 5-CQA and oxygen. It basically assumes mass transfer of O2 from bulk air to the outer surface of nonporous particles of solid 5-CQA, where an irreversible reaction with no formation of solid products takes place. The proposed model correctly reproduced the obtained kinetic results in all the investigated operating conditions when an exponential reduction of external area of solid with reaction time was taken into account. Arrhenius-type expressions successfully described the dependence of the calculated global reaction and shrinking rate constants on temperature. An activation energy close to 77046 J mol−1 was found for the slow combustion reaction of 5-CQA.
The thermal reactivity of the naturally occurring silicates jadeite, NaAl[Si2O6], and nephrite, a variety of actinolite, Ca2 (Mg,Fe)5[(OH,F)Si4O11]2, have been investigated by thermogravimetric and thermomechanical analysis as well as temperature-dependent X-ray diffraction
and analytical electron microscopy. TG shows that nephrite undergoes a weight loss at around 900°C. Mass spectrometry reveals
that this irreversible reaction corresponds to the evolution of H2O, and XRD shows that a phase related to diopside CaMg[Si2O6] is formed. Jadeite does not undergo and observable weight changes up to 1000°C. Thermomechanical analysis indicates a reversible
phase transition at about 950°C. Temperature-dependent X-ray diffraction shows that jadeite is again present on cooling (peak
temperature: 1000°C), but that this is accompared by an additional unidentified phase. The mechanism of this process is not
yet clear although it has been observed in several samples from different origins and with different metal impurities.
irreversiblereactions in geochemical processes involving minerals and aqueous solutions. II. Applications. Geochimica Cosmochimica Acta. 33. 455–481.
Hess, P. C. 1966. Phase equilibria of some minerals in the K 2 O–Na2O–Al2O 3 -SiO
Authors:Djalma Souza, Teresa Eligio Castillo, and Rubén Jésus Sánchez Rodríguez
of the several elementary mechanisms, as well as the combinations of these mechanisms. In general, the dependence of E a on the fraction of mass conversion is associated with the occurrence of parallel, consecutive, and irreversiblereactions on the
reactions occur simultaneously. However, with the assumption that biomass undergoes a thermal degradation in any particular but clearly defined temperature zone as a single step irreversiblereaction of n th order, Eq. 4 can be used to express the