P. Galli

Catalytic activity of zirconium phosphate and some derived phases in the dehydration of alcohols and isomerization of butenes

Abstract The catalytic activity of α-Zr(HPO 4 ) 2 · H 2 O prepared by different methods and of phases derived from it by heating between 200 and 1100 °C or by ion exchange with Na + , Cs + , or Ag + , has been investigated by means of different acid-catalyzed test reactions, namely, isopropanol, 1- or 2-butanol dehydration, and 1-butene isomerization. The active centers of both Zr(HPO 4 ) 2 and ZrP 2 O 7 phases are mainly the surface Bronsted sites, as indicated by the strong decrease or annihilation of their catalytic activity after surface Cs + poisoning. An explanation of the low residual activity detected for some samples is given. As deduced from the products of 1-butene isomerization, the acidic sites are generally of medium strength. However, on heating between 350 and 700 °C, when partial or total condensation of hydrogen phosphate to P-O-P groups occurs (with progressive formation of the layered pyrophosphate phase) they transform into sites of medium-high Strength.

Tetrakis(selenodiazole)porphyrazines 1: Tetrakis(selenodiazole)porphyrazine and its Mg(II) and Cu(II) Derivatives. Evidence for their Conversion to Tetrakis(pyrazino)porphyrazines through Octaaminoporphyrazines

The new phthalocyanine-like macrocycle tetrakis(selenodiazole)porphyrazine, TSeDPzH2, and its Mg(II) and Cu(II) complexes have been prepared and their general, spectroscopic (IR, UV-vis), and magnetic properties investigated. It has been observed that the peripheral selenodiazole rings of the TSeDPz skeleton can be opened by the action of H2S, with release of the Se atoms and formation of a new macrocycle, i.e. octaaminoporphyrazine, which is easily converted into tetrakis(pyrazino)porphyrazine derivatives.

TiO2 supported vanadyl phosphate as catalyst for oxidative dehydrogenation of ethane to ethylene

Abstract Bulk and TiO 2 supported VOPO 4 has been investigated for the oxidative dehydrogenation of ethane. XRD, SEM, TG analyses and BET surface area measurements indicated that vanadyl phosphate is highly dispersed on the support up to mono-layer coverage. A fraction of vanadium is present as V(IV) in the calcined samples as evaluated by EPR and TPR techniques. Both reducibility and acidity of vanadium phosphate is strongly enhanced by deposition on TiO 2 with respect to the bulk phase, as shown by TPR and NH 3 TPD technique, respectively. The supported catalysts are active and selective in the oxidative dehydrogenation of ethane to ethylene in the temperature range 450–550°C, the mono-layer catalyst giving the best performances. Ethylene selectivity decreases with the contact time but increases with the temperature. The former effect indicates that ethylene is further oxidized to CO x at high contact times. The effect of the temperature was attributed to the formation of V(IV), favoured at increasing temperature. This hypothesis was supported by TPR experiments carried out after catalytic tests at 550°C that indicated a significant increase of the fraction of V(IV) after the reaction.

TPD study of NH3 adsorbed by different phases of zirconium phosphate

Abstract Thermodesorption of NH 3 has been used to measure the acidity of α-zirconium hydrogen phosphate and different phases obtained from this material by thermal treatments. It was found that samples treated at temperatures lower than 300 °C, consisting of hydrated or anhydrous α-phases, adsorbed an amount of ammonia corresponding almost to neutralization of all acidic -POH groups and formation of a well-characterized diammonium phase. Samples treated at t ≥ 400 °C, in which partial or total condensation of interlayer -POH groups occurred, showed a strongly reduced capacity for adsorption of NH 3 , because of the formation of P-O-P bridges between layers which hindered diffusion of NH 3 . After pretreatment of Zr hydrogen phosphate at 600 °C, the TPD spectrum of ammonia adsorbed at room temperature showed only the signal of NH 3 adsorbed by surface -POH sites, indicating that its interaction with internal sites was now precluded. From the shape of the TPD curves from these samples information on the strength of surface acidic sites was deduced.

A fourier-transform infrared and catalytic study of the evolution of the surface acidity of zirconium phosphate following heat treatment

The surface acidity of zirconium phosphate at different stages of dehydration and heat treatments has been studied by Fourier-transform infrared spectroscopy of adsorbed pyridine, acetonitrile and acetone and by catalytic activity in the isomerization of but-1-ene. Bronsted-acidic surface POH and P(OH)2 groups are identified [v(OH)= 3670–3660 and 3600 cm–1, respectively] whose strength increases slightly on bulk dehydration. They are thought to be responsible for the activity in but-1-ene isomerization, which also increases during condensation to pyrophosphate. Lewis-acidic sites of medium-high strength have also been found, and responsible for the formation of chemisorbed forms of pyridine (v8a= 1610 cm–1), acetonitrile [v(CN) Fermi resonance doublet at 2322 and 2295 cm–1] and acetone [v(CO) 1684 cm–1]. Surface ZrOH groups are also detected on the layered ZrP2O7 surface. The results illustrate the role of exposed planes, both parallel and perpendicular to the layered structure.

Tg/dta, Xrd and NH3-TPD Characterization of Layered VOPO4·2H2O and its Fe3+-Substituted Compound

Iron(III)-substituted vanadyl phosphate, [Fe(H2O)]0.20VO0.80PO4·2.25H2O (FeVOP), has been prepared and characterized by XRD and TG/DTA analyses. The new compound is isomorphous with layered tetragonal VOPO4·2H2O (VOP), but it possesses a lower interlayer distance. Information on the reactivity and surface acidity of both VOP and FeVOP has been obtained by NH3-TPD experiments. The hydrated materials adsorb high amounts of NH3 (up to 2 mmol g-1). Different ammonia-containing phases are formed, characterized by lower interlayer distances in comparison with the NH3-free parent compounds. NH3 is intercalated between the layers without displacement of water. The materials dehydrated by heat treatment at 450°C retain the layered structure but adsorb NH3 only on the external surface. A wide variety of acid sites, from weak to strong, was observed. A mechanism is proposed for the NH3- acid sites interaction. SEM micrographs of VOP and FeVOP are shown.

Thermal, structural and acidic characterization of some vanadyl phosphate materials modified with trivalent metal cations

A set of new materials with general formula [M(H2O)]X(VO)1−XPO4·2H2O (M3+=Al, Cr, Ga, Mn), isomorphous with layered tetragonal VOPO4·2H2O and having potential catalytic properties, have been characterized by TG and DTA, X-ray diffraction and surface acid strength. During heating the compounds transform in the monohydrated and anhydrous phases, all maintaining a layered structure, with a proper interlayer spacing. Catalytic tests performed with 1-butene show that theM3+-vanadyl phosphates greatly improve the conversion of the olefine with respect to pure vanadyl phosphate.

Studies on Water and Ammonia Programmed Thermodesorption of Mixed M(III)-vanadyl Phosphates

Hydrated M(III)-vanadyl phosphates (M (III)=Mn, Fe, Ga, Al) have been prepared and studied for water and ammonia adsorption properties by TG/DTA, NH3 TPD, FTIR and XRD techniques. The compounds have the same tetragonal layered structure of VOPO4 ⋅2H2 O, but shorter interlayer distances. Ammonia adsorption leads to intercalation of large amounts (0.19–0.39 mol/mol) of base between the layers of the materials, without displacement of water. The ammoniated phases obtained from these compounds have interlayer distances shorter than that of the corresponding precursors. In this connection an interaction mechanism NH3 -host is proposed. Treated at 450°C the materials adsorb ammonia only on the external surface because of the large decrease of the interlayer distance that prevents NH3 from entering the interlayer space. All M(III)-vanadyl phosphates present a wide distribution of strength of ammonia adsorbing sites.

Preparation and characterisation of histidine– and iron–histidine–α-zirconium phosphate intercalation compounds. Catalytic behaviour of the iron derivatives in oxidation reactions with H2O2

The amino acid histidine (His) easily diffuses into layered α-zirconium phosphate if the layers are first pre-swelled via preparation of a metastable ethanol intercalate α-Zr(HPO4)2·2EtOH. Different materials are obtained by varying the contact time and the His solution concentration. A new compound of composition α-Zr(HPO4)2His1.2·5H2O has been selected for its higher His content and remarkable interlayer spacing of 18.4 A. The FT-IR spectra of this compound show that His interacts with the P–OH groups of the host through the NH2 primary group of the amino acid. Zr(HPO4)2His1.2·5H2O takes up Fe(II) and Fe(III) ions from the respective iron sulfate solutions by ion-exchange. This process, accompanied by a partial release of His to the contact solutions, gives rise to intercalates in which the metal ion∶His molar ratios are close to 1∶1. The reflectance spectra account for the presence, in the interlayer region, of a Fen+∶His = 1∶1 coordinated species. The iron–histidine intercalation compounds have been tested as catalysts in the oxidation of an organic substrate, Indigo Carmine, by H2O2 and the results compared with those obtained with the H2O2 solution alone, with Fentons and Ruffs reagents (a H2O2 aqueous solution containing respectively iron(II) or iron(III) ions as catalysts) or with H2O2 solutions containing the Fen+–His (1∶1) complex species. The solid catalysts, recovered after reaction, have been re-used for further runs.

Layered zirconium-tin phosphates : I. Chemical and physical characterization

Abstract Crystalline zirconium-tin phosphates of formula Zr x Sn 1− x (HPO 4 ) 2 · H20 (0 ≤ x ≤1) were synthesized and characterized in terms of their physical and chemical properties. From X-ray and thermal analysis it was found that solid solutions are formed for every composition. The tin content in mixed phases has a marked influence on thermal behaviour, speeding up the kinetics of transformation to layered pyrophosphates (L-Py). Mixed phosphates possess surface areas which are markedly higher than pure phosphates. The method of ammonia thermodesorption (TPD) was employed for the acidity measurements. This method has allowed the evaluation of the concentration and strength of surface acid sites present on the external surface of L-Py or mixed hydrogen-L-Py phases obtained after a variety of thermal treatments. The acid strength of L-Py phases increases with increasing tin content.