D. Cornet

Surface properties of sulfided MoAl2O3 catalysts

Three types of measurement have been successively performed on a series of sulfided MoAl2O3 catalysts with initial contents ranging from 3 to 21 wt% MoO3, namely (i) catalytic activity for thiophene hydrogenolysis and propene hydrogenation reactions (420 °C, 1 atm total pressure), (ii) dynamic oxygen chemisorption at 60 °C, and (iii) selective dissolution of molybdenum in a basic medium. Correlation between the three sets of data brings a new approach to the active surface species. They reveal that catalysts activated under thiophene bear different Mo species according to the loading. It is suggested that the molybdenum ions are grouped as polythiomolybdate clusters bound to the alumina support. The isolated polyions characterized in the less concentrated catalysts tend to polymerize further at higher Mo contents. Oxygen is a good probe molecule for titration of HDS active sites associated with the smaller Mo units. The stoichiometry of the polythiomolybdate entities cannot be precisely established, but it is clearly a function of the sulfiding conditions.

Nickel and molybdenum sulfides loaded into zeolites: Activity for catalytic hydrogenation

Stabilized HY zeolites containing nickel or molybdenum, or the combination NiMo, as well as a dealuminated zeolite containing NiMo, were submitted to sulfiding. The ability of these solids to catalyze benzene hydrogenation was tested. The sulfide phases in the zeolites were further characterized by their capacity for oxygen and carbon monoxide chemisorptions, and by infrared spectra of adsorbed CO. Sulfidation of Mo or NiMo in the zeolites is incomplete, ranging from 50 to 90%. Lowest sulfur levels are observed after reacting unsaturated hydrocarbons under a high hydrogen pressure. Activity for benzene hydrogenation is a sensitive test of the catalytic properties and suggests the formation of a NiMoS phase inside the zeolite. The synergy observed between Ni and Mo is confirmed by the appearance of a band at 2086 cm−1 in the IR spectrum of adsorbed CO. Side reactions are also observed in the course of benzene conversion. Extra-lattice aluminum favors selectivity toward hydrogenation.

On the promoting effect in sulfided NiMo/Al2O3 catalysts as studied by chemisorption

Two series of thiophene-sulfided NiMo/Al2O3 catalysts, with constant molybdenum content (7 and 16 wt% MoO3, respectively) and variable nickel concentration have been characterized by their thiophene hydrodesulfurization activities and their chemisorption capacities measured in situ. Carbon monoxide adsorption was performed at 0 °C and oxygen uptake at 60 °C. Both adsorbate molecules exhibit similar behavior in the series and are thought to be good probes for anion vacancies. For the first series of catalysts, the characterizations distinguish three concentration ranges according to the Ni/Mo ratio. The main enhancement of activity is observed in the range 0.2 < Ni/Mo < 0.6 with a simultaneous slight increase in gas consumption. Comparison of the data with similar characterizations of Ni/Al2O3 catalysts indicates that the promoting effect is not associated with an excess of anion vacancies. The whole set of samples leads to three different kinds of correlation between activity and chemisorption, thus revealing different actions of nickel ions on the molybdenum-based catalyst. For the second series, chemisorption results are not clearly influenced by the Ni content.

Isomerization of n-butenes into isobutene over fluorinated aluminas: Influence of experimental conditions upon selectivity

When 1-butene is converted over a fluorinated alumina catalyst, the theoretical yield of isobutene is lowered by a number of side reactions: dimerization into C8, transformation into C3 and C5, hydrogen transfer and coking. The amount of the byproducts depends upon temperature, feed composition, contact time and time-on-stream. Conditions ensuring a high isobutene selectivity together with a slow catalyst deactivation are derived.

Preparation and stabilization of high specific area zirconia carriers

Abstract Zirconia samples have been prepared by precipitation from an aqueous solution of zirconyl nitrate followed by calcination in flowing air at temperatures up to 970 K. The textural properties (SBET and pore size distribution) of these zirconia powders were very sensitive to the activation procedure. Calcination in carefully controlled conditions at 770 K yielded crystallized zirconia with surface area (SBET ≈ 130 m2 g−1) and porosity suitable for use as a catalyst carrier. Zirconia samples doped with yttrium, nickel or aluminium were obtained by impregnation of the amorphous hydrous oxide. For solids calcined at 770–970 K, the surface area and thermal stability were improved, but the porosity was lower.

Catalytic properties of nickel molybdenum sulphide supported on zirconia

Abstract Zirconia has been investigated as a support material for Mo and NiMo sulphide catalysts. Thiophene HDS studies (1 atm, 400°C) reveal that the ZrO 2 -supported Mo catalysts are twice as active as the corresponding Mo/Al 2 O 3 catalysts. The promoting effect brought by nickel, however, is much smaller than expected in comparison with alumina supported catalysts; the use of zirconia induces higher hydrogenation selectivity. It is suggested that the promoter ions interact with both the Mo sulphide slab and the zirconia carrier, which results in a change in properties of the mixed catalytic sites. The increased selectivity towards hydrogenation may be an important advantage when dealing with hydrodenitrogenation processes. Indeed, in catalytic tests carried out at 70 atm, 350°C, 2,6-diethylaniline reacts more rapidly over zirconia catalysts ; similarly, the hydrogenation route is well developed in the complex reaction scheme of quinoline. However the inhibiting effect of quinoline (or 1,2,3,4-tetrahydroquinoline) upon the reactivity of alkylanilines is as strong on zirconia supported as on alumina supported catalysts. Nevertheless, the beneficial effect of the zirconia carrier is observed in the HDN of the less reactive phenanthridine molecule (140 atm, 340°C), and also in the HDN of real feed in a pilot test.

HYDROCONVERSION OF N-NONANE CATALYZED BY PDHY ZEOLITES. I, INFLUENCE OF CATALYST PRETREATMENT

Abstract Pd/HY catalysts reduced at temperatures ranging from 573 to 873 K were compared in respect of the hydroconversion of n-nonane performed at 473 K and 0.1 MPa. The catalyst reduced at 573 K had a stable activity and a high selectivity for isomerization, but samples reduced at 773 K or above initially favoured cracking at the expense of isomerization. The evolution of the reaction mechanism with the temperature of reduction is related to a change in the dispersion of palladium revealed by TEM.

Surface phases in sulfided NiMo/Al2O3 catalysts

The promoting action of nickel on sulfided Mo catalysts has been examined on two series of thiophene-sulfided NiMo/Al2O3 catalysts with constant Mo content (7 and 16% MoO3, respectively) and variable Ni concentration. The catalysts are characterized by their activity for thiophene HDS, by the fraction of Ni or Mo which can be extracted in an acidic or basic medium, and by their sulfur content. Comparison with “one-component” Ni/Al2O3 and Mo/Al2O3 catalysts shows that the dissolution of molybdenum is unaffected by the presence of nickel; by contrast, molybdenum lowers the non-extractable fraction of nickel, and thus partially hampers the migration of the promoter ions into the alumina matrix. The changes in the catalytic activity are discussed according to the amount of “free” (extractable) nickel NiF: three different roles of nickel are found and related to the nature of the surface phases. In the main promotion range of the first series (0.1 < NiF/Mo < 0.6), the results are consistent with increasing formation of a “NiMoS” phase which consists of small Mo entities associating almost two promoter atoms to three molybde num atoms. For the high-Mo series, interaction between the Mo units leads to a different NiMoS association in larger patches, which limits at a lower NiF/Mo ratio the favorable effect of added nickel. Higher Ni content results for both series in bulk Ni3S2, unsuitable for further development in activity. At low NiF/Mo ratio, the catalyst structure is not elucidated.

Carbon monoxide and oxygen chemisorption and functionalities of sulphided Ni-W/Al2O3 hydrotreating catalysts

Abstract Carbon monoxide and oxygen chemisorption was used to investigate the nature and number of surface sites present over sulphided Ni-W/Al2O3 catalysts with compositions varying over a wide range (0–36% WO3; 0–8% NiO). The gas uptakes were measured by the pulse method at 273 K (for CO) and 333 K (for O2). The binding of CO on the surface was further studied by FTIR spectroscopy. The significance of the results in terms of the different functions of the catalysts, namely hydrogenolysis of C-S and C-N bonds and hydrogenation, is discussed. In the pulse experiments, the amounts of CO chemisorbed on the single-component Ni or W/Al2O3 catalysts are negligible. On the promoted catalysts, the amounts chemisorbed are much more important: they can be related to the activities for both C-S and C-N hydrogenolysis, and thus follow the promoter effect. The correlation accounts for the effects of sulphiding, calcination temperature and deactivation with running time, all of which modify the number of active sites, but not their intrinsic activity. The IR spectra of CO adsorbed on the promoted catalysts are characterized by a band at 2090 cm−1, which persists after evacuation, whereas the bands recorded over unpromoted Ni or W sulphide do not persist under these conditions. The 2090 cm−1 band is assigned to CO adsorbed on highly reduces W ions influenced by neighbouring Ni ions, and the intensity of the signal is a function of both catalyst composition and sulphiding procedure. In addition, a new band at 1830 cm−1 is recorded over the tungsten-rich promoted catalysts. Oxygen chemisorption measurements provide complementary data. The oxygen uptake measures the number of promoter (Ni) ions in the Ni-W phase, but does not account for all of the HDS sites as CO adsorption is not completely hampered by pre-adsorbed oxygen. However, it appears that oxygen is able to reach the sites responsible for hydrogenation of the pyridine ring. Thus, combining CO and O2 chemisorption measurements provides a satisfactory correlation with the hydrogenation activity. The nature of the sites remains unclear, however.

Optimization of the composition of Ni-W/A1203 hydrotreating catalysts using model molecules and real feedstock conversion studies

Abstract Alumina supported Ni-W catalysts with contents of components varying over a wide range were systematically studied in order to optimize a catalyst for further industrial use, mainly for HDN reactions. The catalytic activities were determined in different reactions representative of the hydrotreatment process, viz., piperidine HDN, biphenyl and pyridine hydrogenations, thiophene and dibenzothiphene HDS. The promoting effect of Ni is always observed but the amplitude of the synergy is greatly dependent on the type of molecule to be treated. The nature of the active sites responsible for the catalytic activities is discussed in terms of the mixed phase model. The selected most active catalyst for performing HDN reactions was tested in real feedstock conversions and compared with conventional nickel-tungsten catalysts.