María A. Volpe

On the nature of highly dispersed vanadium oxide catalysts: effect of the support on the structure of VOx species

The nature of VOx specie supported on TiO2, γ-Al2O3 and SiO2 is analyzed. The plausibility of different VOx specie (mono, di, pyrovanadates) is evaluated by means of a theoretical study of the extended Huckel (EH) type. Theoretical results are compared with published characterization data and discussion is performed taking account both results. The interaction of VOx with the support and the structure of supported surface vanadia are influenced by the composition of the oxide carrier. On SiO2, only the monovanadate species are formed. On γ-Al2O3, dimers seem to be the more plausible supported form of VOx. On TiO2, monomers and dimers are present; however, the latter species being the most abundant.

Preparation of Pd–Pb/α-Al2O3 catalysts for selective hydrogenation using PbBu4: the role of metal–support boundary atoms and the formation of a stable surface complex

Pd–Pb/α-Al2O3 catalysts were prepared by reacting PbBu4 with supported palladium samples derived from Pd(AcAc)2, both in the presence and absence of hydrogen. The amount of lead fixed depends mainly on the concentration of palladium on the metal–support boundary. In the presence of hydrogen, all butyl groups are released during the anchoring process. When the Pd/α-Al2O3 was reduced and then purged with nitrogen, two butyl groups remained attached to the lead atom and a stable surface complex was formed. The analysis of gaseous products evolved during the PbBu4–Pd/α-Al2O3 interaction and subsequent temperature‐programmed reaction experiments indicate that a (]‐L)2–Pb(Bu)3 complex was obtained. Upon reduction at 573 K, the Pd–Pb/α-Al2O3 catalysts became very selective for the hydrogenation of acetylene in the presence of ethylene.

A combined theoretical and experimental study of supported vanadium oxide catalysts

Abstract In order to investigate the nature of VOx species supported on SiO2, γ-Al2O3 and TiO2, a combination of a theoretical study and an experimental characterization is carried out. A molecular orbital approach of the extended Huckel type is performed to measure the plausibility of different postulated VOx species (monovanadates, divanadates, polyvanadates, etc.). The experimental characterization is performed by TPR, oxygen uptake measurements and FTIR of adsorbed pyridine. Both theoretical and experimental results indicate that the structure of VOx and its interaction with the support are strongly influenced by the nature of the oxide carrier. On TiO2 and γ-Al2O3 dimers are the most abundant species, while on SiO2 only monomers are formed.

Preparation and characterization of Pd-Pb catalysts for selective hydrogenation

Abstract The preparation of Pd/α-Al2O3 and Pd-Pb/α-Al2O3 catalysts using Pd-acetylacetonate and tetrabutyl-lead (TBPb) has been studied. In the monometallic catalyst the precursor decomposition in air leads to a well dispersed Pd phase. Reacting the Pd/α-Al2O3 (0.19%) catalyst with TBPb in the presence of adsorbed hydrogen, produces one mol of hydrocarbons (mainly butane) per mol of Pb fixed, suggesting the formation of a surface intermediate; Pd-Pb(C4H8)3. In the absence of hydrogen, the reaction is slower and the evolution of hydrocarbons (mainly butenes) decreases by a factor of 2. As the amount of Pb fixed by both procedures is the same, Pb/Pd3 = 1.3, other surface compounds must be involved. Characterization by H2 and CO chemisorption and FTIR spectroscopy of adsorbed CO shows that Pb addition produces a dilution of the surface Pd atoms, preventing the dissociative adsorption of H2 and inducing the adsorption of CO in the linear form. The XPS spectra of Pd/α-Al2O3 and Pd-Pb/α-Al2O3 (0.19 %– 0.27%) indicate a Pb 4f/Pd 3d ratio of 1.4 and a decrease in the Pd 3d/Al 2s intensity ratio with Pb addition. These results suggest that Pb is well dispersed and in close contact with the Pd surface.

A discussion of a mechanism for isomerization of n-butane on sulfated zirconia

Although a huge amount of experimental work has been done, there is no clear picture about the superacidity in sulfated zirconia (SZ). Some authors think that sulfated zirconia is not superacid at all. Moreover, the structure of active site in n-butane isomerization is still now under controversy. This work tries to give some light about the mechanism of n-butane isomerization, taking into account experimental and theoretical works from others and from our own. MM2 and DFT simple calculations were performed in order to clarify the nature of the supported active species in sulfated zirconia. It is clear that a penta-coordinated species, monomeric or even dimeric, would be responsible for forming the most active sites at reaction conditions of n-butane isomerization. This idea is the key of this article. Different studies demonstrated that no superacidity is present before contact with n-butane. The sites formed at reaction conditions allow the n-butane dehydrogenation and its isomerization. A bimolecular mechanism of n-butane isomerization on penta-coordinated sites is proposed to occur. Several steps are discussed.

Palladium–vanadium interaction in binary supported catalysts

Abstract We have studied the activity and selectivity of Pd/γ-Al2O3, VOx/γ-Al2O3 and Pd–VOx/γ-Al2O3 catalysts for the decomposition of NO and the reduction of NO with CO. Pd–VOx/γ-Al2O3 catalysts were prepared by anchoring Pd(AcAc)2 on VOx/γ-Al2O3. Characterization of the binary samples by hydrogen chemisorption and TPR measurements indicated that the reduction of VOx is enhanced by a close contact with palladium and that partially reduced vanadia decorate noble metal particles. This palladium–vanadium interaction alters the catalytic properties of palladium: the activity for NO decomposition is higher for the binary sample and, for the NO–CO reaction, both the activity and the selectivity to N2 increase when vanadium is in contact with palladium.

Pyrolysis of sunflower seed hulls for obtaining bio-oils

Bio-oils from pyrolysis of as received sunflower seed hulls (SSH), hulls previously washed with acid (SSHA) and hulls submitted to a mushroom enzymatic attack (BSSH) were analyzed. The concentration of lignin, hemicellulose and cellulose varied with the pre-treatment. The liquid corresponding to SSH presented a relatively high concentration of acetic acid and a high instability to storage. The bio-oil from SSHA showed a high concentration of furfural and an appreciable amount of levoglucosenone. Lignin was degraded upon enzymatic activity, for this reason BSSH led to the highest yield of bio-oil, with relative high concentration of acetic acid and stability to storage.

Catalytic and catalytic free process for cellulose conversion: fast pyrolysis and microwave induced pyrolysis studies

The catalytic and non-catalytic pyrolysis of microcrystalline cellulose and phosphoric acid pre-treated cellulose was investigated. The thermal processes were carried out applying two different methodologies: conventional fast pyrolysis and microwave-induced pyrolysis. For the catalytic experiments different catalysts were evaluated: CeO2, Nb2O5, SiO2, high surface area SiO2, Si-MCM-48 and Al-Fe-MCM-48. In all cases the liquid fraction was evaluated by quantifying the yields of anhydrosugars (mainly levoglucosan, levoglucosenone and 1,4:3,6-dianhydro-α-d-glucopyranose) and aromatic hydrocarbons. In the reaction of microcrystalline cellulose levoglucosan was the main product, while levoglucosenone was predominant in the pyrolysis of phosphoric acid pre-treated cellulose. Catalysts improved the fraction of bio-oil and the product distribution depended on the nature of catalytic materials as well as the starting cellulose. On the other hand, the microwave induced pyrolysis favored the formation of char at expenses of liquid fraction. In this case levoglucosenone and other anhydrosugars in conjunction with furan compounds were the main products.

A combined theoretical and experimental study of VOx/γ-Al2O3 catalyst

Abstract In order to investigate the nature of VO x supported on γ-Al 2 O 3 a combination of a theoretical study and experimental characterisation is carried out. A Molecular Orbital approach of the Extended Huckel type is performed to calculate the formation and the stabilisation energies of postulated VO x species. The evaluation of these energies indicates that pyrovanadates and divanadates are plausible species. Experimental characterisation of VO x /γ-Al 2 O 3 catalysts was performed by TPR and Oxygen Uptake Measurements to corroborate theoretical models. Experimental results also indicate that VO x /γ-Al 2 O 3 catalyst with low vanadium loading should be envisaged as supported vanadium dimers.

Valorization of Rhizoclonium sp. algae via pyrolysis and catalytic pyrolysis

The valorization of Rhizoclonium sp. algae through pyrolysis for obtaining bio-oils is studied in this work. The reaction is carried out at 400°C, at high contact time. The bio-oil has a practical yield of 35% and is rich in phytol. Besides, it is simpler than the corresponding to lignocellulosic biomass due to the absence of phenolic compounds. This property leads to a bio-oil relatively stable to storage. In addition, heterogeneous catalysts (Al-Fe/MCM-41, SBA-15 and Cu/SBA-15), in contact with algae during pyrolysis, are analyzed. The general trend is that the catalysts decrease the concentration of fatty alcohols and other high molecular weight products, since their mild acidity sites promote degradation reactions. Thus, the amount of light products increases upon the use of the catalysts. Particularly, acetol concentration in the bio-oils obtained from the catalytic pyrolysis with SBA-15 and Cu/SBA-15 is notably high.