J.L. Lemberton

Mechanistic considerations on the involvement of dihydrointermediates in the hydrodesulfurization of dibenzothiophene-type compounds over molybdenum sulfide catalysts

The mechanism of the hydrodesulfurization of dibenzothiophene-type compounds involving their dihydroderivatives as intermediates was examined in the light of previous experimental results dealing with the effect of cobalt and nickel promoters on the selectivity of the reaction. It is suggested that the orientation of the hydrodesulfurization reaction regarding the so-called “direct desulfurization” or “hydrogenation” pathways depends on various factors: the distribution of the dihydrointermediates (two of them only among nine possible isomers can lead directly from dibenzothiophene to biphenyl); the availability of dissociated hydrogen on the catalytic centers on which they adsorb and the basicity of the sulfur anions associated to the catalytic centers.

Mechanisms of n-decane hydrocracking on a sulfided NiW on silica-alumina catalyst

The hydrocracking of n-decane was carried out on a sulfided NiW/silica-alumina catalyst and for comparison on a sulfided NiW/USHY zeolite catalyst (fixed-bed reactor, 380 °C, 6 MPa total pressure, presence of sulfur and nitrogen-containing compounds in the feed). As could be expected from the weak acidity of the silica-alumina support, hence from the high ratio between the hydrogenating and the acid functions, at high n-decane conversion NiW/silica-alumina was found more selective than NiW/zeolite for the formation of isomerization products. The difference in selectivity between the two catalysts was even more significant at low n-decane conversion, due to a direct cracking of n-decane which occurred on NiW/silica-alumina in addition to the classical bifunctional process. This reaction was shown to take place on the sulfided NiW phase, possibly through abstraction of a proton of the molecule by a basic surface sulfur atom.

Catalytic isomerization of ethylenic hydrocarbons: XVIII. Effect of drying and reaction temperature on the isomerization of deuterated butenes over magnesium oxide

The isomerization of selectively deuterated cis-2-butenes was investigated at 60 and 250 °C on magnesium oxide predried between 300 and 550 °C. A rise in the drying temperature increases the number of active sites, but modifies neither their nature, nor their strength. The working temperature has a marked influence on the reactions observed. At 60 °C, magnesium oxide catalyzes only double-bond migration without exchange of hydrogen between the olefin and the catalyst; at 250 °C, a multiple exchange of hydrogen occurs during double-bond migration. In both cases the active sites are basic O2− ions associated with weak acid Mg2+ ions, but adjacent OH groups participate in the reaction at 250 °C. Some strong basic O2− ions are also active at 250 °C, but they deactivate very rapidly: on these sites, double-bond migration and cis-trans isomerization, both without exchange, are observed.

Hydrogenation of tetralin on a sulfided ruthenium on KY zeolite catalyst. Effect of the sulfidation method

Abstract Hydrogenation of tetralin was used to modelize the hydrogenation of aromatic compounds in gasoils. This reaction was carried out under a 5 MPa hydrogen pressure, at 300°C, in the presence of hydrogen sulfide. Before reaction, the Ru on KY zeolite catalyst was presulfided in situ at 400°C either by a H 2 S/H 2 mixture, or by dimethyl disulfide in n-heptane. The catalyst sulfided by dimethyl disulfide was 3 times less active than that sulfided by H 2 S, due to coking during sulfidation. Another type of carbonaceous deposit was formed during the tetralin reaction, the nature and the amount of this deposit was not depending on the sulfidation method.

Catalytic properties of nickel molybdenum sulphides supported on nickel and magnesium aluminates

Abstract The hydrodesulphurization (HDS) of thiophene at atmospheric pressure, hydrodenitrogenation (HDN) of 2,6 diethylaniline (DEA) and of 1,2,3,4 tetrahydroquinoline (THQ) at 70 atm in a dynamic flow microreactor - these molecules being present alone or mixed in the feed - and finally HDN of mixtures of quinoline (Q) at 70 atm or of phenanthridine (Ph) at 140 atm with DEA in a batch reactor were the test reactions used to compare the performances of (Ni)MoS catalysts supported on MgAl 2 O 4 and NiAl 2 O 4 with a classical NiMoS/γ Al 2 O 3 commercial catalyst. Some of the catalysts supported on NiAl 2 O 4 appeared to be superior to the commercial one, whereas those supported on MgAl 2 O 4 were found to be less active. HDS of thiophene, as well as the amount of carbon monoxide adsorbed at 0°C by sulphided catalysts permitted the prediction of the order of HDN activity of the catalysts for the conversion of some nitrogen-containing molecules. A strong inhibiting effect of molecules like Q, THQ or Ph (or some of their products formed during the process) on the HDN of DEA has been found and attributed to the competitive adsorption of these molecules on the supported active phase.

Formation of sulfur-containing compounds under fluid catalytic cracking reaction conditions

The main sulfur-containing compounds in FCC gasoline are thiophene and thiophene derivatives with short alkyl chain (methyl or dimethylthiophenes). The aim of this work was to measure the reactivity of thiophene derivatives under FCC conditions and more specifically, to examine whether these products could be formed from the long alkyl chain thiophenes present in the feed. Therefore, we studied the product distribution obtained by catalytic cracking of thiophene, and of different alkylthiophenes over a FCC commercial catalyst. Ethyl and n-hexylthiophenes, were reactive under condition similar to the of the FCC, whereas thiophene and methylthiophene were not. However, the desulfurization of n-hexylthiophenes occurred to a significant extent, while this reaction was hardly observed with 2-ethylthiophene.

Study on the role of platinum in PtMo/Al2O3 for hydrodesulfurization of dibenzothiophene

Hydrodesulfurization (HDS) of dibenzothiophene (DBT) over PtMo/Al2O3 was studied in the presence of H2S. Pt was found to be an effective promoter for this reaction and the Pt-containing catalyst was sulfur tolerance. Data of the H2-D2 exchange revealed that both the amount of retained hydrogen and the initial rate of H-D exchange on the sulfided catalyst were increased by the introduction of a slight amount of Pt into the catalyst, aorresponded very well to the catalytic activities for HDS. The reduction and sulfidation of Mo sites was also facilitated very much by Pt, which was found by the results of the pulse reduction and sulfidation technique. It is proposed that, during the pretreatment of PtMo/Al2O3, H2 is first dissociated into hydrogen atoms on Pt sites, which the spillover onto Mo sites, providing a large amount of coordinately unsaturated Mo sites for the HDS reaction, and resulting in the higher HDS activity than the unpromoted one.

Hydrogenation of tetralin over a sulfided ruthenium on Y zeolite catalyst: comparison with a sulfided NiMo on alumina catalyst

Hydrogenation of tetralin at 300°C, under hydrogen pressure, and in the presence of hydrogen sulfide was used to modelize the hydrogenation of aromatics in gasoils. The activity for this reaction of a sulfided ruthenium on dealuminated Y zeolite catalyst (RuKYd), was found to be much higher than that of a sulfided NiMo on alumina. Moreover, the activity of the RuKYd catalyst was dependent very much on the sulfidation method: owing to a very significant carbon deposition initiated by the zeolite acid sites, the catalyst sulfided by dimethyldisulfide in solution in n-heptane was less active than when sulfided by a H2S/H2 mixture. This effect was not observed in the case of the NiMo/alumina catalyst, which is not acidic. On the other hand, whatever the catalyst, the presence of n-heptane as a solvent during the hydrogenation test decreased significantly the conversion of tetralin. This was due to the competitive adsorptions between the two molecules.

Selective promoting effect on alkyldibenzothiophenes hydrodesulfurization pathways

Dibenzothiophene hydrodesulfurization occurs through two parallel reactions: direct desulfurization (DDS) yielding biphenyl, and hydrogenation (HYD) leading to tetrahydrodibenzothiophene with subsequent hydrodesulfurization into, cyclohexylbenzene. Introduction of Ni or Co to the Mo system enhances the global DBT conversion. However, the promoting effect is more important for the DDS reaction (higher than 60) than for the HYD reaction (about 3-4). 4,6-Dimethyldibenzothiophene presents a lower reactivity compared to dibenzothiophene, due to the inhibition of the DDS route. The promoting effect is related to changes in the catalytic sites or active surface species. IR studies of adsorbed pyridine or 2,6-dimethylpyridine indicated that the promoter addition increases the strength of the Lewis acid sites and the number of Bronsted sites. However, the effect of the promoter could also be attributed to an increase in the basicity of certain sulfur anions.