The adsorption equilibrium and regeneration efficiency of n-hexane, methyl ethyl ketone (MEK), and toluene under supercritical CO2 on an activated carbon fiber were experimentally determined. When the densities of the supercritical fluids were fixed at 0.32, 0.45, and 0.62 g/cm3, the Langmuir model agreed with the experimental isotherms at the temperatures 308, 318, and 328 K. As the concentration of adsorptive in supercritical CO2 was kept constant, the crossover of the equilibrium loading at different temperatures was observed at relatively high pressure and the crossover pressure of MEK was lower than that of n-hexane. In both cases, the crossover pressure increased with an increase in concentration. Also, the activated carbon fiber loaded with toluene and MEK was regenerated by supercritical CO2 in the range of 0.69−0.90 g/cm3. Desorption became favorable with an increase in pressure, and there was an optimal temperature at every pressure condition. However, the regeneration efficiency increased with temperature at the same density of supercritical CO2. In the case of the MEK/toluene mixture, the difference in the desorption rate and desorption amount between two adsorbates was decreased as the pressure was increased. A one-parameter mathematical model assuming linear desorption kinetics matched the experimental data using an activated carbon fiber.

No abstract

Bimetallic catalysts have been the subject of numerous studies during the last decades (Sachtler et al., 1961). Particularly, it has been interesting when the metal itself active in a catalytic reaction was modified or promoted with a second metal that also had activity in the same reaction. Addition of a small amount of a second metal is a way to increase the selectivity of the system to alkenes (Sarkany et al., 1995; Monteiro et al., 1995). Both the hydrogen availability and the strength of diene complexation can be modified by the addition of a second metal, thereby improving the selectivity of alkene formation and extending the lifetime of the catalyst. The bimetallic catalysts allow a larger degree of freedom to tune the reaction sites for optimum activity and selectivity. The consequent changes in activity and selectivity are explained in terms of different ensemble requirements for paralleVconsecutive reaction paths and also in terms of the so-called ligand effect. As the holes in the d-band of transition metals were assumed to be responsible for adsorption and catalysis by these metals, a filling of these holes by alloying would drastically influence the adsorption capacity of the alloy (Koschel et al., 2000). It is obvious that the changes in selectivity upon alloying cannot simply be accounted for by a decrease in the number of surface atoms exposed to the reactants, and that changes in geometry of the reactive sites and changes in intrinsic activity of the metal surface atoms have to be considered. The electron deficiency character of the metallic centres would increase the strength of their interaction with the electron rich substrates, hindering the subsequent hydrogenation process. As a consequence the metal centres would be covered with the electron-rich substrate, obstructing the adsorption of hydrogen and favouring the isomerization reaction of the 1 -butene with respect to the hydrogenation reaction.On the basis of the above consideration y-alumina supported Cu-Ni catalysts were prepared with the aim of taking advantage of the combination of the properties of the support with those of a bimetallic system. One of the interesting points of this combination i s that Ni and Cu have similar geometric, but very different electronic properties. Both metals have an fcc crystal structure with lattice constants of 3.52 A (nickel) and 3.61 A (copper), respectively. However, their electronic properties are very different. Copper, being a coinage metal, has a completely filled 3d band and therefore no d-states at the Fermi level; in contrast, nickel, being a transition metal, has a partially filled 3d band with a high density of states at the Fermi level. This difference in the electronic structure is Addition of a second metal often improves the selectivity of a supported catalyst for the hydrogenation of 1,3-butadiene. Catalysts containing 15 wt96 Ni and varying amounts of Cu were prepared and characterized by TPR, XRD and XPS. The Cu-Ni interaction affects the reduction behavior of the catalysts. TPR res...

No abstract

No abstract