Emmanuelle Schulz

Comparative inhibiting effect of polycondensed aromatics and nitrogen compounds on the hydrodesulfurization of alkyldibenzothiophenes

The influence of anthracene, phenanthrene, fluorene, acridine and carbazole on the transformation of 4,6-dimethyldibenzothiophene on a NiMo/Al2O3 catalyst was studied. Reactivities have been evaluated under increasing concentrations of inhibitors and negative effect of some polycondensed ring aromatic compounds has been shown to be in the same order of magnitude than that of N-containing compounds. Moreover, such inhibition was proposed to be due to some partially hydrogenated molecule and conclusions relative to the practice of deep hydrodesulfurization (HDS) were drawn.

Effect of experimental parameters on the relative reactivity of dibenzothiophene and 4-methyldibenzothiophene

Abstract Numerous studies concerning the hydrodesulfurization of crude oil have already been realized and have shown that substituted dibenzothiophenes, and especially the 4-methyldibenzothiophene and the 4,6-dimethyldibenzothiophene are refractory molecules of the process. The reaction mechanism of their transformation is still discussed. In this work, we have chosen to carry out the transformation of dibenzothiophene and 4-methyldibenzothiophene on various catalysts and under different experimental conditions. Attention has been particularly paid to the effect of H2S on the two different reaction pathways: the DDS (direct desulfurization) and the HDS (hydrogenation followed by hydrogenolysis) routes. The relative importance between the two routes is greatly modified by the presence of H2S: the DDS route appears to be more inhibited by this compound. An original approach to hydrodesulfurization, the competitive catalytic reaction kinetics, was also used in this study: it demonstrates that the dibenzothiophene and the 4-methyldibenzothiophene have the same adsorption equilibrium constant and suggests that the molecules are adsorbed flat on the catalyst surface. The lower reactivity of the 4-methyldibenzothiophene can then be due to a steric hindrance of the methyl group during the approach of the hydrogen molecule for the hydrogenolysis reaction.

‘Diam-BINAP’; a highly efficient monomer for the synthesis of heterogeneous enantioselective catalysts

Abstract The synthesis of a monomeric analogue of BINAP, diam-BINAP, is described. The polyaddition of this monomer with 2,6-diisocyanato toluene gave the corresponding oligomer, poly-NAP, with a polymerization degree of 8. The ruthenium complex of this polymer proved to be a very efficient heterogeneous catalyst for the hydrogenation of β-ketoesters (99% ee, 0.1 mol% of catalyst). Furthermore the catalyst could be easily reused four times by simple filtration without loss of activity or enantioselectivity.

Palladium-catalyzed synthesis of oligo(alkylthiophenes)

A new palladium-catalyzed polymerization of 2-bromo-3-octylthiophene is described giving rise to oligo(3-octylthiophenes) with good yields. A study was carried out in order to optimize the polymer regioregularity (reaching 90%) and to increase the molecular weight (up to dodecamers). A mechanism is proposed with a coordination between palladium and sulfur, in a modified Heck mechanism.

Recent progress in asymmetric heterogeneous catalysis: use of polymer-supported catalysts

Abstract The asymmetric heterogeneous catalytic reduction of carbonyl bonds by hydrogen transfer reduction or hydrogenation by means of molecular hydrogen as well as the asymmetric allylic substitution of allylic acetates are reported. In order to perform these reactions, new polymer-supported catalysts were employed. These polymers are either a Merrifield resin with a chiral pendent ligand or a chiral main chain polymer with ureas, thioureas and a diphosphine as functional groups. Comparison of the results obtained in these heterogeneous asymmetric reactions was made with those obtained in the homogeneous ones.

Hydrodesulfurization of 4-methyl-dibenzothiophene: a detailed mechanistic study

A mechanism for the transformation of alkyl-dibenzothiophenes by hydrodesulfurization (HDS) on a NiMo catalyst is proposed. 4-Methyl-dibenzothiophene (4-MDBT) (1) has been chosen as a model for this study because this non-symmetrical molecule leads to the formation of a great variety of intermediates. Thanks to the synthesis of these compounds, a detailed study of the reaction course by the observation of the relative concentrations of intermediate products was realized. The results are consistent with adsorption of the molecules via the aromatic system, parallel to the catalyst surface, as proposed previously. The formation of a dihydrogenated intermediate is followed by successive hydrogenation or elimination steps. The rates of these reactions are dependent on the location of the methyl substituent and hence on the generated sterical hindrance. The relative amount observed in each compound is in total accordance with this mechanistic proposal.

Towards the preparation of modified chiral electrodes for heterogeneous asymmetric catalysis: synthesis and electrochemical properties of (S,S)-5,5′-bis-[3-(3-methyl-pentyl)-thiophen-2-yl]-[2,2′]-bipyridine

Abstract The aim of this work is the electrochemical synthesis of a new type of material possessing the properties of organic conductive polymers along with those of organometallic complexes, for their use in asymmetric heterogeneous catalysis. The synthesis of ( S,S )-5,5′-bis-[3-(3-methyl-pentyl)-thiophen-2-yl]-[2,2′]-bipyridine is described in nine steps with good yields. This chiral synthon has been successfully used as ligand in the homogeneous asymmetric catalytic reduction of acetophenone by hydride transfer with various transition metals. Its electro(homo- or co-)polymerization failed until now, probably due to the electron deficiency of this monomer.

Charge transfer complexes between tetranitrofluorenone and polyaromatic compounds from gasoil: a combined DFT and experimental study

A new method has been recently developed for the hydrodesulfurization of gasoils based on the formation of charge transfer complexes. This method is highly selective towards the removal of alkyldibenzothiophenes from other aromatic compounds present in gasoils. Total energy calculations based on density-functional theory have been performed to study the complexation phenomenon and clarify its high selectivity. DFT calculations improve the understanding of the process and they are also useful for the design of more efficient acceptor derivatives for the desulfurization process. Five charge transfer complexes between a given acceptor and five representative donor compounds present in gasoils have been studied. Three main criteria have been used to characterize the optimized complexes: their complexation energy, their geometry and their electronic structure. The decomposition of total energy showed that the main contribution to the complexation energy comes from the Pauli repulsion and the electrostatic interactions. Hence the charge transfer complexes are not only ruled by charge transfer but also by electrostatic contributions. However, the selectivity of the complexation phenomenon is controlled by the orbital interactions.

Heterogeneous catalytic hydrogenation of olefinic substrates by poly-NAP

With our previously described poly-NAP, various olefinic substrates were reduced with selectivities comparable to those obtained by BINAP. For substrates which contained a methyl ester, the selectivities were higher than those observed for their carboxylic acid analogues.

A new green catalytic method for biaryl cross-coupling and oligothiophene synthesis

Lintroduction de groupements aryles fonctionnalises sur des composes thiopheniques est le plus souvent realisee en utilisant des quantites stœchiometriques de composes organometalliques. Nous proposons une arylation directe de thiophenes, actives par une reaction de type Heck, avec un systeme catalytique compose de Pd(OAc) 2 , de n-Bu 4 NBr et dune hase. Cette nouvelle methode, plus propre, a ete appliquee a differents thiophenes substitues en positions 2 et 3, et une etude des parametres de la reaction a ete effectuee. A partir des resultats obtenus, nous avons etendu cette methode « verte » a la synthese doligothiophenes. Nous avons ainsi polymerise divers 2-halogeno-3-alkylthiophenes, avec de bons rendements en utilisant le meme systeme catalytique, et nous avons obtenu des oligomeres fonctionnalises et regioreguliers.