Salim Nassreddine

Thiotolerant Ir/SiO2–Al2O3 bifunctional catalysts: effect of support acidity on tetralin hydroconversion

Tetralin hydroconversion over supported iridium catalysts has been investigated under a pressure of 4 MPa in the presence of H2S in a continuous high-pressure gas-phase microreactor. Decalin, naphthalene, ring-contraction bicyclic products and one-ring-opening products are formed. A screening of silica, alumina and amorphous silica–alumina (ASA) supports demonstrates that only ASA provides thiotolerance and ring opening/contraction selectivity to iridium nanoparticles. By testing Ir/ASA catalysts with various silica–alumina ratios but similar Ir particle size (1.5 nm), it is shown that the intermediate concentration of silica (40 wt%) leads to the highest activity and selectivity, in correlation to the Bronsted acidity measured by infrared spectroscopy of adsorbed pyridine.

Operando study of iridium acetylacetonate decomposition on amorphous silica–alumina for bifunctional catalyst preparation

The decomposition of iridium acetylacetonate Ir(acac)(3) impregnated on amorphous silica-alumina (ASA) has been investigated by combined thermogravimetry-differential thermal analysis-mass spectrometry (TG-DTA-MS) and by in situ X-ray diffraction (XRD). The resulting Ir/ASA hydrotreating catalysts have also been characterized by transmission electron microscopy (TEM). The effects of heating treatments under oxidative, reductive or inert gas flows are compared with each other and with similar experiments on ASA-supported acetylacetone (acacH). It is shown that Ir(acac)(3) undergoes exothermic combustion during calcination in air, leading to agglomerated IrO(2) particles. Conversely, direct reduction involves hydrogenolysis of the acac followed by hydrogenation of the ligand residues to alkanes and water. These two processes are catalyzed by Ir clusters, the gradual growth of which is followed in situ by XRD. The resulting nanoparticles are highly and homogeneously dispersed.

Mechanism of Tetralin Ring Opening and Contraction over Bifunctional Ir/SiO2-Al2O3 Catalysts

The development of cleaner fuels from conventional resources requires the finding of new hydrotreatment processes able to improve the combustion performances of fuels and limit undesirable emissions. In the context of gas oil upgrading by selective ring opening, we have investigated the hydroconversion of tetralin over iridium nanoparticles supported on amorphous silica-alumina. The conversion of tetralin leads to hydrogenation, ring-contraction, and ring-opening products. The selectivity to ring-opening/-contraction products (ROCPs) increases linearly with the acid-metal site ratio and can be tuned by modifying the metal loading, the metal nanoparticle size, or the support composition. From the combination of catalytic tests at variable conversion and the products identification by two-dimensional gas chromatography, a mechanistic reaction scheme has been established. Aromatic ROCPs are formed through purely acidic steps, whereas the formation of saturated ROCPs mostly involves bifunctional reaction steps. Iridium-catalyzed hydrogenolysis appears to be a minor pathway with respect to iridium-catalyzed hydrogenation and Brønsted acid catalyzed isomerization.