Search

Abstract: Ring-opening reactions of decalin and methylcyclohexane (MCH) over bifunctional catalysts (1.2Ir/WO3/Al2O3) were investigated. A series of catalysts containing up to 5.3at. W/nm2 and 1.2wt.% Ir was prepared. The acidity of the solids was monitored by low-temperature CO adsorption followed by infrared spectroscopy. Characterization of the Ir metal phase was performed by H2 chemisorption and X-ray diffraction. The activity and product selectivity patterns obtained for the decalin ring-opening reaction were compared with those observed for MCH. For both naphthenes, ring contraction precedes ring opening, suggesting a similar ring-opening mechanism. Kinetic modeling based on the proposed reaction network allowed the determination of the activation energies and initial rates. Based on the yields and products distribution obtained for the decalin reaction, the potential for improvement of the cetane number is discussed. [Copyright &y& Elsevier]

Abstract: Ring-opening reactions of decalin and MCH were studied over monofunctional acid (WO3/Al2O3) and metal (Ir/Al2O3) catalysts containing, respectively, up to 5.3 at. W/nm2 and 1.8wt% Ir. The catalysts were characterized by X-ray diffraction, Raman spectroscopy, low-temperature CO adsorption followed by infrared spectroscopy, and H2 chemisorption. A reaction network was proposed for both molecules and used to determine the kinetic parameters. Kinetic modeling allowed relating characterization results and catalytic performance. For WO3/Al2O3 catalysts, ring contraction precedes ring opening of both molecules. The evolution of ring contraction activity was consistent with the development of relatively strong Brønsted acid sites. Ring opening occurs according to a classic acid mechanism. For Ir/Al2O3 catalysts, only direct ring opening was observed. Ring opening proceeds mostly via dicarbene mechanism. Analysis of products indicated that monofunctional metal catalysts are better suited than acid solids for upgrading LCO. [Copyright &y& Elsevier]

Abstract: The present paper investigates the development of Ir-based catalysts supported on tungstated aluminas for the ring opening reaction of naphthenic molecules using methylcyclohexane (MCH) as a model compound. A series of tungstated aluminas WO x /Al2O3 containing up to 6.0at.W/nm2 was prepared. Ir-based catalysts containing 1.2wt% were obtained by impregnation of these solids. The performance of Ir/WO3/Al2O3 catalysts for methyl cyclohexane ring opening was found to be similar to that observed over the corresponding zirconia-based system indicating that the metal–acid balance (and not the nature of the support) is the key parameter for optimum performance. [ABSTRACT FROM AUTHOR]

Abstract: Tungstated zirconias consist typically of WO x species deposited on zirconia support. Additives like Si can be added to stabilize the catalyst surface area. The aim of this work is to identify the location of the Si (surface or bulk) as a function of W addition. Zirconia doped with 1.2wt% Si remained purely tetragonal and exhibited a high specific surface area (150m2/g) following calcination at 1023K. Infrared quantification of the silanol band at 3745cm−1 indicated that Si is essentially present at the surface. The 29Si NMR chemical shift of the zirconia is characteristic of isolated silicium. This result is corroborated with infrared by the absence of the structural band ascribed to Si–O–Si vibrations. NMR and infrared results indicated that W addition brings about a progressive evolution of the Si environment towards that of silica-like species. These results can be tentatively explained by a ligand enhanced dissolution of the support on W deposition. Si leached from the surface will then aggregate, forming small SiO2-like domains on the zirconia. [Copyright &y& Elsevier]

Abstract: The present paper is the second part of a systematic study of the influence of W and Ir loading on the activity of Ir/WO3/ZrO2–SiO2 catalysts for the ring-opening reaction of naphthenic molecules using methylcyclohexane (MCH) as a model compound. A series of Si-stabilized tungstated zirconias, WO x /ZrO2–SiO2, containing up to 3.5 atom W/nm2, was prepared. Ir-based catalysts containing up to 1.2 wt% were obtained by impregnation of these solids. Characterization of the metal dispersion and catalyst acidity was described in a previous article. The objective of the present study was to determine the best metal/acid balance for optimal performance of Ir/WO x /ZrO2–SiO2 catalysts in the ring-opening reaction of MCH. Monofunctional (acid WO x /ZrO2–SiO2 or metal Ir/ZrO2–SiO2) and bifunctional (Ir/WO3/ZrO2–SiO2) catalysts were examined. Based on the analysis of the yields and products distributions, a reaction network was proposed, and kinetic data (e.g., activation energies, initial rates) were calculated. Correlations between characterization results obtained earlier (e.g., acidity, dispersion) and catalytic performance are also reported. The monofunctional acid catalysts WO x /ZrO2–SiO2 showed a low selectivity for ring opening. The ring-contraction activity developed for W surface density above a threshold value of 1 atom W/nm2. This was attributed to the appearance and the development of a relatively strong Brönsted acidity monitored by infrared measurements. MCH ring contraction and C5 naphthene ring opening occur according to a classic acid mechanism. For low conversions, the monofunctional metal catalysts Ir/ZrO2–SiO2 exhibited significant selectivity for ring opening that decreased with increasing conversion. Because of the lack of ring-contraction products, the observed activity was attributed to the direct ring opening of the MCH. Ring opening and cracking occur according to a dicarbene mechanism. The study of MCH conversion on Ir/WO x /ZrO2–SiO2 catalysts indicated that MCH ring contraction to alkylcyclopentanes occurs before ring opening. The best yields for ring opening were obtained with the 1.2% Ir/WO x /ZrO2 (1.5 atom of W/nm2). Further increases in W surface density led to a decrease in the indirect ring-opening yield, attributed to a decrease in Ir dispersion. For bifunctional metal/acid catalysts, analysis of the mechanism is less straightforward. The activation energy for C6 ring contraction and indirect C6 ring opening is a function of the metal/acid ratio. For high ratios, indirect ring opening occurs essentially over metallic sites. A decrease in the metal/acid ratio enhances the contribution of acid mechanism. [Copyright &y& Elsevier]

Using in situ 13C MAS NMR, it has been demonstrated that propane is carbonylated with carbon monoxide to produce isobutyraldehyde and isobutyric acid at 100–150 °C on sulfated zirconia. Isobutyraldehyde represents the intermediate reaction product, which is almost completely converted into the acid at 150 °C. The selective formation of isobutyraldehyde at low temperatures on SZ indicates that principally the secondary C&z.sbnd;H bond of propane is activated, rather than its C&z.sbnd;C bond. This pathway of the alkane activation can be realized either on Lewis acid sites of SZ or by direct formylation by the formyl cation formed as equilibrated species from the formate. Infrared spectroscopy provides evidence that the sulfate groups of SZ are responsible for the aldehyde oxidation. The formation of surface dithionate species as a result of sulfur reduction is suggested. These dithionate species are readily reconverted into sulfate groups by an oxidizing treatment in an O2 atmosphere. [Copyright &y& Elsevier]