Arnaldo Faro

The main catalytic challenges in GTL (gas-to-liquids) processes

In the present review the main catalytic challenges for GTL processes are discussed. It is considered that GTL comprises three main catalytic areas, namely synthesis gas generation, Fischer–Tropsch synthesis and upgrade. Each one is analysed and the main characteristics of traditional and innovative catalysts are presented. For syngas generation, steam methane reforming, non-catalytic partial oxidation, two-step reforming, autothermal reforming and catalytic partial oxidation of methane are discussed. For Fischer–Tropsch, we highlight the role of nanocatalysis, hybrid zeolite-containing catalysts, diffusion limitations and selectivity to high molecular weight hydrocarbons. Also, new reactors technologies such as micro reactors are presented. Finally, special attention is paid to the main upgrade steps (Hydrocracking and Hydroisomerisation/Dewaxing), the new mechanisms of isomerisation being discussed for bifunctional zeolitic catalysts.

Total oxidation of toluene over calcined trimetallic hydrotalcites type catalysts.

Two trimetallic ZnCuAl and MnCuAl hydrotalcites have been successfully synthesized by a co-precipitation method. The manganese based material was identified as a new hydrotalcite phase. Both lamellar precursors were calcined at 450 and 600 degrees C and the resulting catalysts were tested on reaction of total oxidation of toluene. The solids were characterized by X-ray diffraction, thermal analysis, atomic absorption spectroscopy, Fourier transformed infrared spectroscopy, N(2) adsorption and H(2) temperature-programmed reduction. It was found that ZnCuAl materials are composed of copper and zinc oxides supported on alumina; while MnCuAl ones comprise basically spinel phases, which were not completely identified. The catalytic behavior of the calcined samples showed that Mn hydrotalcite calcined at 450 degrees C exhibited the best catalytic performance that corresponds to 100% toluene conversion into CO(2) at about 300 degrees C.

Surface acidic properties of alumina-supported niobia prepared by chemical vapour deposition and hydrolysis of niobium pentachloride

Abstract Niobia–aluminas were prepared by chemical vapour deposition at 150°C of niobium pentachloride on the surface of γ-aluminas calcined at different temperatures and with controlled degrees of hydration, followed by hydrolysis with water vapour at 150°C and a thermal treatment with steam at 440°C aimed at removing surface chloride contamination. The samples were characterised with respect to chemical composition, surface area, acidity by temperature-programmed desorption of ammonia, nature of acid sites by infrared spectroscopy of adsorbed pyridine and catalytic activity at 370°C in the dealkylation of cumene. The results showed that, for each alumina calcination temperature, the catalysts with the lowest niobium content have a higher density of acid sites than the alumina support, but the acidity decreased, within each series with an increase in the niobium content. Comparatively to the TPD results, catalytic activity in cumene dealkylation was much more sensitive to the history and composition of the samples. The niobia-alumina samples were much less active than the alumina support, but this was most likely due to the severe hydrothermal treatment for chlorine removal, since their activity was close to that of an alumina submitted to the same treatment. A strong decrease in the acidic activity was observed with increase in the niobium content. A sample of pure niobium oxide had a much higher activity than the niobia-alumina samples. Bronsted acidic sites could only be observed by the IR spectra of adsorbed pyridine on the surface of the pure niobium oxide sample.

Zirconia–alumina mixing in alumina-supported zirconia prepared by impregnation with solutions of zirconium acetylacetonate

Materials containing 2–20 wt% zirconia supported on γ-alumina were prepared by multiple step incipient wetness impregnation with solutions of zirconium acetylacetonate in benzene. The materials were calcined at 823 K and were characterised by X-ray diffraction, XPS, DRS in the UV-visible region and FTIR in the regions of OH stretching and skeletal vibrations of adsorbed pyridine. The results demonstrated that the zirconium is present as isolated species at the surface and in the bulk of a mixed zirconia–alumina phase formed by migration of the zirconium ions into the subsurface layers of the support. The presence of zirconia at the alumina surface caused the appearance of basic hydroxyl groups bonded to a single zirconium ion, of weak Lewis acidic sites, probably associated with zirconium sites, and a decrease in concentration of stronger Lewis sites associated with the alumina surface.

Transition metal oxides additivated with sulphate or phosphate as catalysts for the cracking of cumene and supports for sulphided nickel-tungsten hydrocracking catalysts

Abstract Oxides of zirconium, titanium, niobium, tin, and a mixed zirconium-tin oxide, additivated with sulphate, were prepared by the adsorption of sulphate from a sulphuric acid solution on the corresponding hydrous oxides, followed by calcination at 773 K. An oxide of niobium, additivated with phosphate, was also prepared by a similar procedure using phosphoric acid. The additivated oxides were characterized with respect to the total acidity by n-butylamine chemisorption using a gravimetric method. The acid site strength distribution was measured by titrating them with n-butylamine using Hammett indicators. The catalytic activities of the materials for the cracking of cumene were determined using a pulse method at 573 K. Nickel and tungsten were supported by the additivated oxides and the resulting catalysts were tested for the hydrocracking of cumene at 673 K and 3.1 MPa pressure, with carbon disulphide as a sulphiding agent. With the exception of the niobium samples, all oxides had their acid site density increased by the additivation. In all cases, except for the sulphated niobia, there was a marked shift in the acid site strength distribution towards the stronger and strongest sites (Ho≤−8,2), observed on the sulphated zirconia and phosphated niobia. These two samples also had activities, selectivities and stabilities, for the conversion of cumene, similar to those obtained with a zeolite containing sample, indicating the Bronsted nature of their acidic sites. Only sulphided nickel-tungsten, supported on sulphated zirconia and titania, and on phosphated niobia, displayed any significant activity in the hydrocracking of cumene at 673 K; but of these, only the phosphated niobia presented a good stability for this reaction. An activation effect, during the first few hours of the reaction, was observed with the catalysts supported by the sulphated titania sample, suggestive of the generation of Bronsted acid sites by a spill-over effect upon exposure to hydrogen.

Mixed-oxide formation during preparation of alumina-supported zirconia: an EXAFS and DFT study

Alumina-supported zirconia catalysts, containing from 2.1 to 20.1 wt% ZrO2 and prepared by multiple incipient wetness impregnation using benzene solutions of zirconium acetylacetonate, were characterised using the EXAFS technique. The results indicated that the supported phase consists of Zr4+ species that do not have Zr as second neighbours, hexacoordinated to oxide anions, suggesting that these ions occupy octahedral positions in the defective spinel structure of the γ-alumina support. In order to test this hypothesis, an idealised unit cell was constructed with the formula ZrAl4O8, with spinel structure. Atomic positions in this cell were optimised using theoretical calculations based on DFT. The smallest energies corresponded to structures where the Zr4+ ions occupy octahedral sites in the spinel structure. Simulations of the EXAFS spectra using this structure provided excellent agreement between the theoretical and experimental spectra.

Effect of support sulphidation on the hydrocracking activity of niobia-supported nickel and molybdenum catalysts

Nickel or molybdenum were supported on niobium oxide, both pure and modified with phosphate. The catalysts and supports were sulphided at 673 K under a 15 mol% H2S in H-2 mixture. It was observed by X-ray photoelectron spectroscopy (XPS) that the sulphur uptake of the catalysts was much in excess to the amount necessary for complete sulphidation of nickel and molybdenum and occurred even with the non-impregnated supports. Simultaneously, a peak attributable to niobium in an oxidation state lower than +5 appeared in the XPS spectra. A good linear correlation was found between the atomic percentage of sulphur, above the one necessary to sulphide the supported metals, and the atomic percentage of niobium in the reduced state, showing that partial reduction-sulphidation of the support occurred during the sulphidation treatment. The slope of the best line through the experimental points was 1.99, indicating that a surface niobium sulphide with stoichiometry close to NbS2 was formed during the treatment. While the reduction-sulphidation of niobium was enhanced by the presence of supported nickel and especially molybdenum, it was inhibited by phosphate. The supports and catalysts were tested in the hydrocracking of cumene at 623 K and 2.7 MPa. When the materials were sulphided under 15% H2S/H-2 at 673 K, a linear correlation was observed between the activity and the extent of reduction-sulphidation of the supports, indicating that the surface niobium sulphides were principally responsible for their acidic activity. However, when the materials were sulphided at 623 K with the feed containing CS2 as a sulphiding agent, before the catalytic test, no such correlation was found and the activity in cumene hydrocracking was much smaller, showing that the lower temperature is insufficient for complete development of the support sulphidation, especially with the molybdenum catalysts. It was observed by XPS that, at 673 K, reduction of the molybdenum to the +4 state during sulphidation is nearly complete with an alumina support, but not with the niobia supports, which suggests the existence of a strong molybdena-niobia interaction

Simultaneous tetralin HDA and dibenzothiophene HDS reactions on NiMo bulk sulphide catalysts obtained from mixed oxides

NiMo bulk sulphide catalysts were obtained from mixed-oxides. The mixed-oxides were obtained by calcining the as-synthesized lamellar precursor with the so-called ϕy structure and (NH4)H2xNi3−x(OH)2(MoO4)2 formula. The corresponding mixed-oxides obtained by calcination were characterized by nitrogen adsorption, XRD, and ICP. Mixtures of α-NiMoO4 and β-NiMoO4 were obtained. A batch reactor was used for CS2/n-hexadecane in situ sulphidation of the mixed-oxides. A mixture of dibenzothiophene and tetralin was used for the liquid phase reaction carried out at 613 K and 70 bar. After the catalytic tests, the bulk sulphide catalysts were characterised by nitrogen physical adsorption, synchrotron light XRD, EXAFS and HR-TEM. The EXAFS simulations are consistent with disordered nickel sulphide particles dispersed in the catalysts. HR-TEM images showed randomly oriented, stacked-layer particles typical of Mo sulphide. The bulk catalysts had larger HDS and HDA activities and selectivities for hydrogenation reactions than alumina supported conventional catalysts containing the same Ni : Mo ratio. A pronounced support effect was observed for both HDS and HDA reactions. The use of the support strongly suppressed both cyclohexylbenzene formation in HDS of DBT and cis-decalin formation in HDA of tetralin. This suggests that similar active sites are involved in the formation of these compounds on the one hand, while another type of site is involved in biphenyl and trans-decalin formation on the other.

Niobia-supported nickel molybdenum catalysts: Characterisation of the oxide form

Nickel, molybdenum and nickel molybdenum catalysts supported on niobia and on alumina were prepared by wet-point impregnation and calcination at 723 K. The catalysts were characterised in their calcined form using nitrogen adsorption (BET method), X-ray diffraction, X-ray fluorescence spectroscopy, diffuse reflectance spectroscopy in the UV visible region, Raman spectroscopy, X-ray photoelectron spectroscopy and temperature-programmed reduction. Pronounced support effects were observed on the nature of the supported species. Thus, on alumina, the molybdenum was highly dispersed and predominantly present as polymolybdate clusters while, on niobia, there was evidence for a strong molybdena-support interaction, possibly with formation of a mixed niobium molybdenum oxide. The alumina support strongly stabilised supported oxidic nickel species towards reduction, in contrast with the niobia support. This was explained by a smaller polarisation of the Ni-O bond in the niobia than in the alumina environment. On alumina, the nickel and molybdenum oxidic species, when simultaneously present in the catalysts, interacted preferentially with each other rather than with the support, whereas the opposite occurred on the niobia support.

Preparo de óxido de nióbio suportado em alumina por deposição química em fase vapor: caracterização por espectroscopia vibracional e termogravimetria

Alumina supported niobium oxide was prepared by chemical vapor deposition (CVD) of NbCl5. The alumina was calcined and pretreated at differents temperatures in order to vary the density of OH groups on the surface which was determined by thermogravimetric analysis. A good correlation was found between the amount of anchored niobium and the total number of anionic sites (oxide and hydroxyl groups) on the surface of the alumina. The infrared spectra on the OH stretching region indicate that OH groups coordinated to at least one tetrahedral aluminum were more reactive towards NbCl5.