Mechanical mixtures of Me (Ni, Pd) Ce oxides and silica-supported heteropolyacids: Role and optimal concentration of each active species in n-hexane isomerization

Catalytic properties of silica-supported heteropolyacids (HPA) in a mechanical mixture with reduced Me-Ce oxides (Me = Ni, Pd) in n-hexane isomerization are studied. The role of each component of the mixed oxides (Ce and, typically, Ni and Pd) and their optimum content has been illuminated: cerium is not only beneficial for eliminating or preventing coke deposition but is also effective for maintaining the Keggin structure of the highly-organized HPA during the reaction and probably allows a better dispersion of the second metal species. Nickel and palladium, present as Ni⁰ and Pd⁰, reinforce the activation of the alkane, which is difficult to obtain by means of a direct attack by an acid, and, thus, enhance noticeably the activity of the catalyst. The best mechanical mixtures are obtained with 30–70 wt % NiCeO-HPW/SiO₂ and 50–50 wt % Pd_0.1CeO-HPW/SiO₂. These mixtures have the highest efficiency for a Ni/(Ni + W) atomic ratio of 0.66 and a Pd/(Pd + W) ratio of 0.40, respectively. Finally, the conversion of n-hexane is in the order HPW > HSiW > HBW, which seems to be consistent with the order of their acid strength as per the literature, but the isomerization selectivity appears to be slightly higher on HSiW. Published in Russian in Kinetika i Kataliz, 2006, Vol. 47, No. 1, pp. 24–28. The text was submitted by the authors in English.     

A short proof for a generalization of Vizing's theorem

For a simple graph of maximum degree Δ, it is always possible to color the edges with Δ + 1 colors (Vizing); furthermore, if the set of vertices of maximum degree is independent, Δ colors suffice (Fournier). In this article, we give a short constructive proof of an extension of these results to multigraphs. Instead of considering several color interchanges along alternating chains (Vizing, Gupta), using counting arguments (Ehrenfeucht, Faber, Kierstead), or improving nonvalid colorings with Fournier's Lemma, the method of proof consists of using one single easy transformation, called “sequential recoloring”, to augment a partial k-coloring of the edges.     

Rate of Convergence of a Stochastic Particle System for the Smoluchowski Coagulation Equation

By continuing the probabilistic approach of Deaconu et al. (2001), we derive a stochastic particle approximation for the Smoluchowski coagulation equations. A convergence result for this model is obtained. Under quite stringent hypothesis we obtain a central limit theorem associated with our convergence. In spite of these restrictive technical assumptions, the rate of convergence result is interesting because it is the first obtained in this direction and seems to hold numerically under weaker hypothesis. This result answers a question closely connected to the Open Problem 16 formulated by Aldous (1999).     

Oligomerisations electro-assistees-I : Un reexamen de la reduction de la chalcone en milieu aprotique

The electroreduction of 1–3 diphenylpropenone (chalcone) in anhydrous DMF, on a mercury pool, leads to a new trimer compound or some classical hydrodimers. The former was obtained with TBA⁺ as a counter ion and its configuration completely solved. The latter were obtained in the presence of Li⁺, Cr³⁺ or Mn²⁺ and their structures were determined unambiguously for the first time. In all cases the influence of the counter ion on the distribution of products was reported.     

On the preparation of carbohydrate-protein conjugates using the traceless Staudinger ligation

[reaction: see text] The nature of a linker used for preparing glycoconjugate vaccines is of utmost importance as it may lead to immunogenic biomolecules. We report the conjugation of carbohydrate haptens to protein carriers leading to potential vaccines using the traceless Staudinger ligation. The ligation relies on the selective transfer of a phosphane substituent to an azide to form a native amide bond in the final product upon release of an oxidized phosphane byproduct. We designed new phosphino-functionalized cross-linkers suitable for protein carrier derivatization. We evaluated their utility in preparing conjugates using both synthetic and purified bacterial carbohydrates. The use of a borane-protected phosphane which is deprotected at the time of the ligation reaction led to the best results observed thus far in terms of stability toward oxidation and reactivity.     

Determination of atrazine, its degradation products and metolachlor in runoff water and sediments using solid-phase extraction

In order to determine the fate of the herbicides atrazine (as well as some of its degradation products) and metolachlor in water and sediments, a method was developed to extract and analyse these compounds. The two matrices were separated completely by centrifugation followed by filtration using nylon filters (0.45 mum). Sediments were extracted with a mixture of methanol-0.1N hydrochloric acid (50:50, v/v) using a wrist-action shaker. Filtered water and extracts of sediments were adjusted to pH 4, then concentrated and purified onto two solid-phase extraction cartridges using in tandem C(18) bonded phase column atop sulfonic acid bonded column (SCX). Atrazine, deethylatrazine, deisopropylatrazine and metolachlor retained by the C(18) column were eluted with ethyl acetate. Chlorodiaminotriazine and hydroxyatrazine retained by the SCX column were eluted with a 50:50 (v/v) acetonitrile-0.1M Na(2) HPO(4) aqueous solution (pH 8.5). The extracts were quantified by high performance liquid chromatography with diode array detector (HPLC-DAD) and by gas chromatography with nitrogen-phosphorus detector (GC-NPD). Overall percent recoveries were about 75% and detection limits were between 0.05 and 0.15 microg/l., and 0.5 and 1.5 microg/kg for water and sediments, respectively.     

Investigation of the reactivity of arylamines, organo-hydrazines and tolylisocyanate towards [PW12-xMxO40]n- Keggin anions

Reaction of K7[A,alpha-PW9Mo2O39] with Na2MoO4.2H2O in a mixture of water/dioxane/hydrochloric acid and further precipitation with (Bu4N)Br provided (Bu4N)3[A,alpha-PW9Mo3O40](3). Analogous reaction with K7-xNax[alpha-PW11O39] is an alternative to the synthesis of (Bu4N)3[alpha-PW11O39{MoVIO}]2. Multinuclear NMR and ESI mass spectrometry have been used to interpret the reaction of (Bu4N)x[alpha-PW11O39{ReO}](x=3 1; x=4 1I), (Bu4N)x[alpha-PW11O39{MoO}](x=3 2; x=4 2I) and (Bu4N)3[A,alpha-PW9Mo3O40]3 by organohydrazines, arylamines, tolylisocyanate and tetraphenylphosphine imide.