Angelo Vaccari

Preparation and catalytic properties of cationic and anionic clays

Abstract The structure, main properties and preparation methods of cationic and anionic clays are reviewed in light of the versatility and potential of these materials. Recently reported catalytic applications in some fields of high industrial and scientific relevance (organic or fine chemistry, environmental catalysis, etc.) are also summarized. Clays exhibit many profitable features (such as low cost, wide range of preparation variables, use in catalytic amounts, ease of set-up and work-up, gain in yield and/or selectivity, etc.) which may be useful tools in the move towards establishing environmentally friendly technologies. Furthermore, the preparation and properties of pillared layer cationic or anionic clays are also discussed, considering that the upgrading of these materials by the pillaring process opens new and interesting perspectives, on account also of possible shape selective effects.

Catalysts for low-temperature methanol synthesis. Preparation of Cu-Zn-Al mixed oxides via hydrotalcite-like precursors

Cu-Zn-Al-mixed oxides are well known as catalysts for methanol synthesis at low temperature. An in-depth investigation into the nature and characteristics of a ternary compound (Mg/sub 6/Al/sub 2/(OH)/sub 16/CO/sub 3/ x 4H/sub 2/O) and its evolution with the calcination temperature until the formation of the oxide phases was carried out. X-ray patterns of the pure hydrotalcite-like phase calcinated at different temperatures (90/sup 0/-45/sup 0/C) and also ESR spectra at this range were reported. After calcination at 350/sup 0/C for 24 hours, the total decomposition of the precursors, with the formation of CuO and ZnO was revealed by X-ray diffraction. There was no evidence of the presence of free Al/sub 2/O/sub 3/. 4 figures, 2 tables.

Fundamentals in the preparation of heterogeneous catalysts

The preparation of heterogeneous catalysts has by now lost all empirical aspects. The approach is scientific and involves a wide number of specific competencies of solid state chemistry, analytical chemistry, physical chemistry, kinetics, rheology, etc. The fundamental aspects in the preparation of heterogeneous catalysts starting from catalyst design up to the catalyst in its final form are briefly reviewed, with focus on the key factors in each preparation step and the main differences between laboratory and industrial scale preparations. The main properties of monolithic catalysts and their preparation methods are also reported, considering their relevance for environmental applications.

Partial oxidation of methane. Effect of reaction parameters and catalyst composition on the thermal profile and heat distribution

The partial oxidation of methane was carried out using new catalysts obtained by calcination and reduction of Mg/Al hydrotalcite-type precursors, containing Rh, Ni or Rh/Ni. The preparation methodology ensured high dispersion of the metal and activity in the partial oxidation of methane. To study the activity, the reaction in autothermal conditions was investigated using IR thermography to monitor the surface temperature. This technique made it possible to monitor the thermal profile of the catalytic bed and the gas/solid heat distribution, following its changes with residence time and reagent concentration. Furthermore, it allowed the diffusion limits to be estimated as a function of the initial ratio between total and partial oxidation of methane. The surface temperature profile was found to be very dependent on the catalyst composition and controlled by the Ni oxidation. For the reforming reactions the outlet composition was compared with the data at equilibrium, calculated at the outlet surface temperature in all reaction conditions.

Synthesis and thermal evolution of hydrotalcite-type compounds containing noble metals

Abstract M/Mg/Al hydrotalcite-type (HT) anionic clays [or layered double hydroxides (LDHs)] (M2+/M3+ atomic ratio≈2.5; M=Rh3+, Ir3+, Ru3+, Pd2+ and Pt2+) with the amount of M ranging from 0.04% to 5.0% (as atomic ratio) were successfully prepared by co-precipitation in an aqueous solution at pH 10.0, although for the Pt-containing sample about a 20% loss of the noble metal ions occurred. Key factors in the synthesis were the ionic radius and preferential coordination, the latter playing a more important role. Calcination of the HT precursors at 923 K gave rise mainly to cubic mixed oxides, with significant increases in surface area. As a function of their nature and amount, the noble metal ions were also detected as segregated oxide or metal side-phases, with the following scale of stability: Rh>Ir>Ru>Pd>Pt. This segregation was only partial, as evidenced by both quantitative XRD analysis and the crystallographic parameters of the mixed oxides. Calcination of the HT precursors at 1173 K formed mainly MgO- and spinel-type phases, with significant increases in the amount of segregated oxide or metal side-phases and decreases in surface area, which, however, were always higher than 45 m2/g. On the basis of the crystallographic parameters, it was concluded that the not segregated noble metal ions were always present inside the spinel-type phases. The scale of stability inside these phases was the same already observed for the samples obtained at 923 K and may be related to the oxidation state and the preferential structure of the noble metal ions.

Structure and reactivity of high-surface-area Ni/Mg/Al mixed oxides

High-surface-area Ni/Mg/Al mixed oxides are obtained by calcination up to about 1023 K of hydrotalcite-type (HT) anionic clay precursors. The samples exhibit unusual pore size distributions and low reducibility of the Ni2+ ions. On the basis of the XRD data, TPR profiles and values of the apparent activation energy of the reduction reaction (before and after NaOH treatment), the low reducibility of the Ni-rich samples may be attributed to the presence of a surface spinel-type phase, which decreases the accessibility of the surface to the reducing mixture and/or makes the generation of nickel nuclei at the surface difficult. For Ni/Mg ratios < 1 the reduction of the Ni2+ ions is further hindered by the presence of a fraction of Al3+ ions inside the oxide phases. Higher calcination temperatures destroy any particular properties related to the nature of the precursors, giving rise to segregation of stoichiometric spinel and oxide or oxide solid solution. For the Ni-rich samples, this increases the reducibility of the main fraction of the Ni2+ ions, which behaves like free NiO. The Mg-rich samples show a reduced reducibility due to the formation of NiO/MgO solid solutions. The validity of the structural model previously proposed is discussed as a function of the composition and the calcination temperature.

Catalysts for low-temperature methanol synthesis. II: Catalytic behavior of Cu/Zn/Al mixed oxides

Abstract The aim of this work was to study the catalytic behavior of Cu/Zn/Al mixed oxides active in low-pressure methanol synthesis. The maximum activity was obtained for different Cu Zn ratios as a function of the gas mixture composition. At values higher than 10.0%, the aluminum showed an inhibitor effect with both the mixtures. The determination of the copper surface area by N 2 O decomposition showed that during the reaction the catalysts were in a partially oxidized state. Furthermore, three different copper-containing species were identified in the spent catalysts: metallic copper, CuO, and copper not detectable by XRD analysis, the latter being probably the copper inside or related to the ZnO. While no correlation existed between the catalytic activity and only one of these species, a good fit was observed both with the sum and the product of the amount of CuO and undetected copper. Therefore, both a form of easily reoxidizable copper and the copper related to the ZnO have been suggested as active species, even though it was not possible to determine if they have similar activities or must be present contemporaneously.

Electrocatalytic properties of nickel(II) hydrotalcite-type anionic clay: application to methanol and ethanol oxidation

Abstract Glassy carbon electrodes modified with a [Ni/Al–Cl] hydrotalcite-type anionic clay have been studied with respect to the electrochemical oxidation of the Ni(II) centres. The oxidation of Ni(II) is quasi-reversible and the presence in the hydrotalcite lattice of nickel with mixed oxidation states increases the conductivity of the coverage strongly. It has been verified that these nickel redox systems may act as redox mediators for the oxidation of alcohol substrates (methanol and ethanol have been tested) which are not oxidised at the bare electrode in the potential window available. For both alcohols the height of the current responses in linear sweep voltammetry is roughly linear with varying concentration in solution up to a limit of ca. 3000 ppm, with a detection limit of 3–4 ppm.

Chromium-free catalysts for selective vapor phase hydrogenation of maleic anhydride to γ-butyrolactone

Abstract The replacement of chromite catalysts in the selective vapor phase hydrogenation of maleic anhydride (MA) to γ-butyrolactone (GBL), useful intermediate in the production of chlorine-free solvents, was investigated. In order to avoid the interferences related to physical dishomogeneity of the precursors, Cu/Zn/M catalysts (38:38:24, atomic ratio percent, where M = Cr, Al or Ga) obtained by thermal decomposition of hydrotalcite-type anionic clays were used. The Al-containing catalyst exhibited better performances than the Cr-containing catalyst, while with gallium no significant differences in comparison to aluminum were observed, capable of justifying the significantly higher cost. Cu/Zn/Al catalysts showed complete MA conversion and high yields in GBL, with maximum values when the Al-content was about 17% (as atomic ratio) and the Cu/Zn ratio fell between 0.5 and 1.0. Furthermore, better carbon balances were observed for these last catalysts, which may be attributed to lower light hydrocarbon synthesis and tar formation.

Catalytic partial oxidation and CO2-reforming on Rh- and Ni-based catalysts obtained from hydrotalcite-type precursors

Abstract The catalytic activity of two catalysts (Ni10Mg61Al29 and Rh1Mg71Al28, as atomic ratio percent) obtained by calcination and reduction of hydrotalcite-type (HT) precursors were investigated in catalytic partial oxidation (CPO) and CO2-reforming of methane at low contact times. Characterization of the samples after different thermal treatments showed that the preparation via HT precursors represents a simple and reproducible way to obtain catalysts, characterized by highly dispersed metallic crystallites stabilized inside an inert matrix at high surface area. The CPO tests were carried out at various feed compositions, reaction temperatures and contact times, with, however relevant differences between the oven temperature and those measured. The best catalytic performances were found at high temperature, approaching thermodynamic equilibrium. When the amount of reaction heat was decreased by using a diluted mixture, the specific effect of contact time was observed and attributed to the role of steam- and CO2-reforming consecutive reactions. CO2-reforming tests at low contact times confirmed the high activity of the catalysts obtained from HT precursors, with a higher activity of nickel towards CO2 and of rhodium towards methane, the latter associated with a higher selectivity in H2 due to a lower tendency to form water. These results help to explain the CPO data, that suggest a higher activity of rhodium in selective CPO of methane. Finally, it is worth noting that both catalysts did not show any structural and/or physical modifications after the catalytic tests, with, furthermore, no deactivation phenomena due to coke formation in the CO2-reforming reaction.