Reaction Kinetics, Mechanisms and Catalysis

Substitution behavior of the labile aqua ligand in four mononuclear ruthenium(II) terpyridyl complexes with different auxiliary N (pyridine) (Ru1), N^N (2,2′-bipyridyl (Ru2), 2′-(2-pyridyl)quinoline (Ru3), 2,2′-biqunoline (Ru4) ligands was investigated using three nucleophiles; thiourea, 1,1-dimethylthiourea and 1,1,3,3-tetramethylthiourea. The effect of concentration and temperature on the substitution behavior of the complexes were studied under pseudo-first order conditions using UV–Vis spectrophotometer. The second order rate constants (k2) of the aqua complexes decreased in the order: Ru4 > Ru3 > Ru1 > Ru2. The results showed that the rate of substitution of the aqua ligand increased with an increase in the π-surface area of the N^N bidentate auxiliary ligands. This is attributable to an upsurge in π-back-donation and electrophilicity of the complexes as the π-extension of the auxiliary ligands increases. Ru2 is less reactive than Ru1 due to the increased steric hinderance introduced by the 2,2′-bipyridyl bidentate auxiliary ligand in Ru2 compared to Ru1 which has two independent trans pyridines. From computational results, it was observed that as the aromatic surface area of the auxiliary ligand increased from Ru1–Ru4, the HOMO–LUMO gap decreased accordingly. Consequently, the chemical softness and electrophilicity of the complexes increased accordingly. This is corroborated by the decrease in pKa values of the complexes as one moves from Ru1 to Ru4. As a result, the nucleophilic attack becomes facile from Ru1 to Ru4. All the reactions follow an associative interchange mechanism as indicated by the positive activation enthalpy and negative activation entropy. The crystal structure of bipyridylterpyridylthiourearuthenium(II) perchlorate show that the substitution product obtained is stable.

The deposition of carbon on catalysts during the partial oxidation of methane to syngas has been investigated in a fluidized bed. It was found that the relative rate of carbon deposition follows the order Ni>>Pd>Pt, Rh. Although the rate of carbon deposition in the fluidized bed was much lower than that in the fixed bed, carbon deposition could still be detected in the fluidized bed if a CH4 /O2 ratio in greater than 2.3 was used.

The effect of water on the modification of acidic properties of Nb2O5 and Nb1.3MnOx catalysts was investigated using the cracking of cumene as model reaction, and compared to the behavior of a HZSM-5 catalyst. Nb1.3MnOx exhibited stronger Lewis acidity than Nb2O5, which translated into a higher selectivity towards α-methylstyrene formed on Lewis acid sites (LAS) by dehydrogenation of cumene. Steam enhanced strongly the conversion of cumene over both Nb-based catalysts. The products distribution on Nb-based catalysts was also deeply modified in the presence of steam, the selectivity towards α-methylstyrene decreasing strongly in favor of benzene, which is formed on Brønsted acid sites (BAS) by dealkylation of cumene. In contrast, the performances of HZSM-5 for cumene cracking and the products distribution were only marginally modified in the presence of steam. A kinetic model based on the elementary steps of the cumene reaction pathways (dealkylation and dehydrogenation) was used to estimate the ratio of LAS to BAS in absence and presence of water over Nb1.3MnOx. The activation energy of the cracking reaction was higher than that of the dehydrogenation reaction. The model described correctly the changes in the catalyst activity induced by addition of ≈2 V% of water, which resulted in a decrease in the [LAS]/[BAS] ratio from approximatively 3 to 1.

A recombinant cutinase from Fusarium solani pisi was immobilized by adsorption on titanosilicates, titanoaluminosilicates, and vanadosilicate, and its activity in the alcoholysis of butyl acetate with hexanol, in organic media (isooctane), was measured as a function of the water content and water activity.

Physico-chemical (ESR, X-ray photoelectron spectroscopy and phase analysis) and catalytic properties of a Zn−Sb−O catalyst have been studied. The distribution of the products of allylic oxidation was shown to depend on the possibility of electron transfer between the surface compounds and the catalyst. A sharp increase in the rate with respect to acrolein in the formation of the antimonate +α-Sb2O4 system was observed.

Triethoxysilyl substituted diphosphines of the novel type (EtO)3Si(CH2)nP(Ph)CH2CH2PPh2 (n=1, 3) have been prepared and used to immobilize rhodium(I) complexes to silica. The complexes were found to be effective catalysts for hydrogenation of 1,3-cyclooctadiene under mild conditions.

Reduction of V2O5/Al2O3 and V2O5/silica-alumina produced Lewis acid sites which could strongly adsorb CO (heat of adsorption: 90 kJ/mol). Such strong acid sites were not formed in the cases of V2O5/SiO2 and V2O5/MgO. V2O5 loaded on Al2O3 interacted with the basic sites of alumina but not with the acidic sites, while the V2O5 on silica-alumina interacted with the acidic sites of the silica-alumina and decreased its acidity.

A bleached softwood kraft pulp was treated with fluorescent whitening agentsTinopal UP and Leucophor AP under different conditions. The kinetics ofadsorption-chemical interaction between optical brighteners and pulp wasdescribed by an exponential kinetic equation. An entropy heterogeneity of thepulp surface related to the active sorption centers, their accessibility and theirspatial orientation was confirmed. The activation energy values obtained werefound independent of the brightener concentration and on the increase of pulpwhiteness. A decisive role of the entropy factors was ascertained.

In the present study, zinc laurate (ZL) was synthesized by precipitation and characterized by X-ray diffraction, Fourier transform infrared spectroscopy and thermal analysis. ZL was applied for monolaurin synthesis by esterification of lauric acid with glycerol using a 23 factorial design to determine the best conditions for monolaurin production. Conversion of nearly to 70% was achieved by using a lauric acid:glycerol molar ratio of 1:1, 2 wt % of catalyst and 140 °C, reaching to 33% of monolaurin selectivity. Statistical analyses at a confidence level of 95% indicated that the decrease in lauric acid:glycerol molar ratio was significant to enhance the monolaurin production. Moreover, the solid was able to be reused, preserving its original structure and the catalytic activity, which indicated the potential use in continuous processes.