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Silicon speciation in environmental, biological and industrial matrices is of considerable importance due to its wide use in many consumer and personal care products and industry. In addition, the entry of silicones in various compartments like wastes, soils, air and water highlights the need to perform exposure studies, toxicological surveys and to measure negative effects. Due to possible contamination and trace level presence of silicon compounds, challenges to determination, identification and quantification are presented. The principal species of concern include siloxanes, silanols, silanediols and silanes. State of the art of analytical methods for total silicon determination and silicon speciation are established. Atomic spectroscopic methods are mainly used to measure total Si at trace concentration levels. On the opposite, hyphenated techniques are performed for Si speciation. Particular attention is paid to chromatographic methods coupled to sensitive and selective detectors (MS, AED and ICP) allowing structural information. Liquid and gas chromatography emerge as the most widespread separation techniques. However, other procedures such as MS, NMR, IR and XRF enable a better knowledge of these species. The potential and limitations of hyphenated techniques are highlighted, particularly concerning sensitivity and selectivity. Furthermore, potential sources of contamination and analytical artifacts in silicon determination are reviewed.

A detailed kinetic model was proposed to analyze experimental data obtained from indole hydrodenitrogenation (HDN) over γ-Al2O3 and amorphous silica–alumina (ASA)-supported NiMo catalysts. The goal was to investigate the support acidity effects on indole HDN and compare with a recent study on quinoline HDN. Similarly to quinoline HDN, indole HDN occurred via hydrogenation of the aromatic ring, followed by N-ring opening and exocyclic C–N bond breaking. The high support acidity of NiMo­(P)/ASA exhibited a promoting effect for N-removal steps and adsorption of nitrogen compounds. However, in contrast to quinoline HDN, it did not clearly induce a positive effect for the hydrogenation step. The acidic function of ASA also favored the formation of byproducts such as toluene, cyclohexane, dimer, and trimer of indole. Catalytic conversion of a quinoline and indole mixture revealed a strong inhibiting effect of quinoline on indole HDN, whereas the inhibiting effect of indole on quinoline HDN was weak. The inhibition was weaker over NiMo­(P)/Al2O3 than over NiMo­(P)/ASA. This result is in agreement with a relative ranking of apparent adsorption constants of quinoline, indole, and their products on NiMo­(P)/Al2O3 and ASA.

A kinetic study of the hydrodenitrogenation of quinoline is performed in a batch reactor, over a NiMo(P)/γ-Al 2 O 3 sulfide catalyst, in the range of temperature of 340−360 °C and concentration of 1−2 wt % of quinoline. Liquid−vapor mass transfer is considered in the reactor model, and the kinetic expression using Langmuir−Hinshelwood model considers competitive adsorption of reactants, products, and solvents. The activation energies of every elementary reaction and adsorption enthalpies of nitrogen compounds are calculated. The kinetic modeling shows that the hydrogenation of 1,2,3,4tetrahydroquinoline into decahydroquinoline is the rate-determining step of the principal reaction pathway. The self-inhibition effect due to competitive adsorption of nitrogen-containing compounds is confirmed. The adsorption constants of nitrogen compounds decrease in the order saturated amines > NH 3 > aromatic amines, showing that their adsorption strength is related to the basicity of molecules. Moreover, the kinetic model is validated by an additional experiment using ammonia as an inhibitor.

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