S. Marengo

On the role of phosphate anion in the MoO3-Al2O3 based catalysts

The incorporation of phosphate ion into -A12O3 was studied by chromatographic techniques. Phosphate ion strongly interacts with alumina through a mechanism in-volving surface basic sites; it competes with molybdate in the adsorption onto alumina and, when it is the first to be added, it limits the amount of Mo(V1) which can be adsorbed.The study of the phase transitions of alumina in the range 673-1323 K evidenced a stabilizing effect of phosphate. As phosphorus content increases, the isomerization activity of -A12O3 decreases in parallel with the adsorption of Mo(VI). In the Mo03-A12O3 systems, the hydrogen activation properties are strongly promoted by the presence of phosphate. These results are rationalized by hypothesizing the presence on y-alumina of strained Al-O-Al groups which have the character of Lewis conjugated acid-base sites and are responsible for adsorption properties, nucleation of a-alumina and catalytic activity. The role of phosphate is unique in stabilizing and/or deactivating these strained sites.

Spectroscopic study of the structure of chromium(VI), molybdenum(VI) and tungsten(VI) oxospecies anchored on aluminium oxide

Diffuse reflectance in the ultraviolet–visible and Raman spectra indicate that CrVI anchored to the Al2O3 surface by ‘pulse-’ and ‘step-’ addition methods is in a monomeric form both before and after activation. Monomeric MoVI and WVI oxospecies are also the most probable products on both freshly exchanged and activated dilute samples. On more concentrated samples spectral modifications occur which can be understood without invoking polymerization effects.

Bauxite-based catalysts in heavy crude oil hydrotreating

Abstract In hydrotreating heavy crudes, shale oils and coal liquefaction products, the cost of the catalyst is expected to be a critical factor in the process economy. This is due to non-volatile metal components in the feedstocks which cause high catalyst consumption. In a systematic study of the use of natural bauxite as carrier for low cost hydrotreating catalysts, the catalytic performance of bauxite-based catalysts in hydrotreating a 60% reduced crude from the Adriatic Sea region has been determined in a trickle-bed reactor. The following systems have been considered: MoO 3 /bauxite, WO 3 /bauxite, CoO-MoO 3 /bauxite, NiO-MoO 3 bauxite, NiO-WO 3 /bauxite. Data on a commercial catalyst,containing 4 and 12 wt % of CoO and MoO 3 respectively, are included for comparison. For maximum surface dispersion of the active components, Mo(VI) and W(VI) were incorporated on bauxite by anionic exchange. The Mo(VI) and W(VI) contents of the catalysts (6.6 wt % MoO 3 and 11.6 wt % WO 3 ) correspond to the chemisorption capacity of the natural material (after thermal activation). The catalytic properties of the bauxite-based catalysts resemble those of alumina-based catalysts. The synergistic effect of cobalt and nickel on molybdenum and tungsten in hydrodesulfurization (HDS) is evident. Tungsten-based catalysts are less active than molybdenum-based systems, in accordance with literature data. The lower activity of all bauxite-based catalysts, compared with the commercial CoO-MoO 3 -a1umina system, is attributed to differences in composition and surface properties. All of the catalysts examined promoted the production of a low boiling fraction in similar amounts, i.e., all catalytic systems show similar cracking activity. However, hydrogen uptake, as measured by the increase of the H/C ratio of the product, is substantially different and suggests large differences in the hydrogen-ating activity of the catalysts. Activity in hydrodenitrogenation (HDN) is lower than in HDS. Also the nitrogen removal properties of bauxite-based catalysts are comparable with the alumina-based systems. Differences in the kinetic steps involved in the HDS and HDN processes have been proposed; the experimental data indicate that the nitrogen removal process takes place through a number of kinetically important steps and this is consistent with the reaction network proposed for the HDN of model compounds. In metal removal, bauxite-based catalysts exhibit about the same activity as the commercial catalyst; this result is attributable to the relevant role played in this reaction by the pore structure of the support. These data show that, by the use of appropriate evaluation criteria, natural minerals, such as bauxites, can be selected to prepare efficient hydrotreating catalysts and that the activity and selectivity of these systems can be, to a certain extent, adjusted by a proper choice of chemical composition.

Promotion effects in methanol synthesis over MgO-supported Fe-Ir catalysts prepared from mixed-metal clusters

Abstract Methanol synthesis over bimetallic FeIr/MgO catalysts prepared from mixed-metal carbonyl clusters, was studied with the aim of clarifying the role of the Fe/Ir ratio in the precursor and the influence of process parameters. Upon impregnation, the molecular precursors are physisorbed on the magnesia surface. Active catalysts are obtained by H 2 treatment of these systems at 623 K. The presence of iron in the precursor favors the formation of surface sites exhibiting marked bifunctional character, as shown by effective production of both methanol and methane in the CO hydrogenation. This promotion is strongly influenced by reaction conditions, as a consequence of the dependence of kinetic parameters on the Fe loading. Upon raising the reaction pressure, the overall CO conversion rate increases dramatically and product distribution also changes to a significant extent, methanol being the most favored product. With Fe:Ir ratio of 1:1, high methanol productivity (above 100 mol MeOH/mol Ir .h) was obtained at 6.1 MPa and 523 K, with 91% oxygenates selectivity.

Methanol from synthesis gas over cluster-derived FeIr/MgO catalysts

Abstract Cluster-derived FeIr/MgO catalysts prepared from [Et 4 N] 2 [Fe 2 Ir 4 (CO) 16 ] and [Et 4 N] 2 [Fe 2 Ir 2 (CO) 12 ] exhibit high activity in methanol synthesis from CO + H 2 . The catalyst performance improves significantly upon raising the Fe:Ir ratio in the precursor from 2:4 to 2:2; with the latter sample, at 488–523 K and 6.1 MPa, methanol productivities up to 100 mol/mol i r h are obtained with selectivity around 90%. Preliminary extended X-ray absorption fine structure characterization suggests that the formation of active sites upon activation in hydrogen at 623 K, is related to the presence of highly dispersed iridium particles stabilized on the surface by interfacial iron.

A Study of the Chemisorption of Chromium(VI), Molybdenum(VI) and Tungsten (VI) Onto γ-Alumina

In the pulse and step addition of ammonium dichromate, ammonium paramolybdate and ammonium paratungstate solutions to γ-alumina a large increase of the pH was observed in the liquid phase which suggests a common mechanism of interaction with the basic sites of γ-alumina. This mechanism involves the decondensation of the isopolyanion in the presence of alumina followed by the chemisorption of monomeric species. The most relevant difference between the three metal ions is the limiting amount of the chemisorbed oxospecies on γ-alumina: CrO 3 3. 28; MoO 3 7. 72; WO 3 19. 2 wt. %. The electronic and Raman spectra of the chemisorbed molybdenum(VI) and tungsten(VI) at low coverage can be interpreted as due to oxospecies in tetrahedral co-ordination. The relevance of the adsorption phenomena in the preparation of the commercial catalyst is finally pointed out.

Studies under transient conditions of CO hydrogenation over Rh catalysts using an automatized microreactor

Abstract CO hydrogenation over supported Rh catalysts was studied in an automatized microreactor by methods of parameter-programmed reaction. It was evidenced how process variables and catalyst composition modify the reaction pathway and the product distribution. Molybdenum enhances the overall reaction rate, but favors to a larger extent methanol formation and the water-gas shift reaction (WGSR); the latter prevails at higher temperature and with longer residence time. Alcohol yield is promoted by addition of small amounts of CO 2 to the feed via a reverse WGSR, but is suppressed by high concentration of CO 2 .

Surface Chemistry of Zirconia-Supported Rhodium Carbonyl Clusters

CO hydrogenation catalyst precursors prepared by adsorbing Rh4(CO)12 and Mo(CO)6 on ZrO2 were studied by Fourier transform infrared photoacoustic spectroscopy and by temperature-programmed desorption. Rh4(CO)12/ZrO2 evolves, at temperature near ambient, to a system dominated by Rh1(CO)2 species through a mechanism involving CO dissociation. The small Rh crystallites formed by reduction of the sample at 523 K are disrupted and converted to the same Rh1 species when exposed to CO at 305 K. Mo enhances CO dissociation and favours the oxidative interaction of surface hydroxyls with Rh0 species at about 500 K; the promotion effect of Mo in CO hydrogenation is discussed in connection with these results.

Experimental studies of transient thermal effects during catalytic oxidation in a packed-bed reactor

The dynamic phenomena associated with the rhodium-catalyzed oxidation of carbon monoxide, methane and propane have been studied by in-situ infrared thermography. High-resolution temperature maps of the reacting catalyst revealed the mobility of the reaction front during ignition and extinction of the CO oxidation, and the development of thermokinetic oscillations. The catalytic oxidation of methane and propane produced weaker dynamics. Chemisorption and kinetic experiments suggest that the competitive adsorption of the reactants and the occurrence of self-inhibition, represent key factors in the development of the observed transient effects.

Investigation on Adsorption and Acid-Base Properties of Solid Catalysts by Infrared Thermography

Publisher Summary The infrared imaging system (AGA Thermovision 782) supplies a signal proportional to the radiation (wavelength 3.5-5.6 μm) that impinges onto the detector; the signal is converted into a temperature value through calibration parameters. By scanning the target 25 times per second and processing the data by a PC, the system yields black/white or colored thermal maps of the sample in real time. Infrared thermography is utilized in a great variety of fields, including the chemical industry that applies this technique in energy saving and routine inspections on commercial plants. Only a few examples can be found in the literature of the bench-scale studies of catalytic reactions by thermography. This chapter describes the evaluation of the adsorption properties of catalysts and presents qualitative and quantitative results obtained with different materials studied in gas–solid and gas–liquid adsorption systems.