M. A. Massucci

Preparation and ion-exchange properties of a new phase of the crystalline titanium phosphate, Ti(HPO4)2·2H2O

The preparation of a new inorganic ion-exchanger with the formula Ti(HPO4)2·2H2O and its ion-exchange properties towards sodium and strontium ions, are reported and discussed. This exchanger is shown to be very stable to hydrolysis and to have high exchange capacity in strongly acid medium. Comparison with the corresponding zirconium phosphate dihydrated phase (γ-ZP), suggests that Ti(HPO4)2·2H2O possesses a different lattice structure from that reported for the monohydrated exchanger.

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.

Titanium and zirconium acid phosphate dihydrates: thermal behaviour and phase changes of their hydrogen, sodium and strontium forms

Abstract The thermal behaviour of crystalline titanium or zirconium bismonohydrogen orthophosphate) dihydrate, Ti(HPO4)2·2H2O (γ-titanium phosphate) and Zr(HPO4)2·2H2O (γ-zirconium phosphate) and their sodium- and strontium-exchanged forms have been investigated. TG and DTA curves are given. The X-ray diffraction patterns of the various phases obtained during the thermal treatment (taken with a high-temperature camera) are also reported. Some interesting similarities are found in the thermal behaviour of the hydrogen forms of γ-titanium and γ-zirconium phosphate. Furthermore, the discontinuous decrease in the first d-value in their respective X-ray diffraction patterns during the dehydration processes, suggests that these two exchangers possess a layered structure, as do the corresponding α-compounds (Ti(HPO4)2·H2O and Zr(HPO4)2·H2O). Comparisons are made between α- and γ-compounds, in hydrogen, sodium and strontium form. Although they have an identical chemical composition, the two series of compounds have different structural arrangements which persist over a wide termperature range. Only at high temperatures (800-900°C) do the γ-compounds give the same phases as obtained from the α-forms.

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.

X-ray photoelectron spectroscopic evidence of interlayer complex formation between Co(II) and N-heterocycles in α-Zr(hpo4)2 · H2O

Abstract Evidence from XPS Spectroscopy for coordination of the Co(II) in zirconium phosphate-phen composite is described. Analysis of the shape of the Co2 p photoelectron peak indicates that high-spin N-coordinated Co(II) is present mixed with O-coordinated Co(II). Comparison of the N1 s peak with those in parent α-Zr(HPO 4 ) 2 (phen) 0.5 · 2H 2 O confirms that Co(II) is indeed N-coordinated, but that some phenanthroline molecules anchored to the host are also present.

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.

Intercalation compounds of α-zirconium hydrogen phosphate with Rh3+ ions and Rh3+-diamine complexes. Part II. Their behaviour towards CO, CO2 and H2 and their use in the CO catalytic oxidation

The reactivity of Rh3+ ions and Rh3+-diamine α-Zr(HPO4)2·H2O complexes intercalated in α-zirconium hydrogen phosphate towards small molecules (CO, O2, H2) was studied. The compounds only containing Rh3+ ions, of composition ZrHxRhy(PO4)2·4H2O (x = 2 – 3y; 0 < y ≤ 0.66) react with CO at atmospheric pressure and temperatures ranging from 80 to 100°C, and undergo selective reduction of Rh3+ to Rh1+. The resulting materials containing Rh1+ are reoxidized to Rh3+ by molecular dioxygen under the same pressure and temperature conditions. The simultaneous action of a COO2 mixture determines the catalytic oxidation of the CO to CO2 and the system acts as a stable catalyst of this reaction. At higher temperatures, the reduction of Rh3+ is no longer selective and in these conditions Rh0 is formed, which escapes from the support and causes its deactivation. Similar behaviour is found in systems containing Rh3+-diamine complexes, which react with CO at temperatures higher than 120°C and undergo an irreversible reduction of Rh3+ to Rh0. The reaction with H2 (70 < T < 100°C) also causes a non selective reduction of the Rh3+ to Rh1+ and Rh0. The progress over time of the catalytic activity of some compounds with different contents of Rh3+ in converting CO to CO2 has shown not only that these materials maintain a constant catalytic activity, indicating the stability of the systems to the loss of metal during working cycles, but also that Rh3+ supported in these matrixes is more active and selective in this type of reaction than Rh3+ in solution.

Hydrothermal treatment of Zr, Ti, Sn and Ge hydrogenphosphates

The Zr, Ti, Sn and Ge hydrogenphosphates, generally prepared in a crystalline form by the refluxing method, have been submitted to hydrothermal treatment at 180° and 300°C in order to observe if the preparation time can be shortened maintaining their chemical composition and their α-structure. Simultaneous TG and DTA together with XRD revealed to be very suitable techniques for the characterization of the obtained products.Zr phosphate is the most stable and gives in all conditions the α-monohydrated hydrogenphosphate phase. Similar behaviour for the Ti phosphate at 180°C, while at 300°C the compound transforms into γ-Ti(HPO4)2·2H2O. Sn phosphate, even on maintaining its α-structure, gives rise to more and more dehydrated phases on the increasing of the temperature of the HT and the concentration of the used phosphoric acid.Ge hydrogenphosphate resulted the least stable under hydrothermal conditions, since even at 180°C it gives rise to GeOHPO4. Because of the easy hydrolyzability of Ge, the hydrothermal method is not a way to prepare the crystalline α-germanium hydrogenphosphate.ZusammenfassungDie Hydrogenphosphate von Zr, Ti, Sn und Ge wurden mittels der Refluxmethode in kristalliner Form hergestellt und bei 180° und 300°C einer hydrothermischen Behandlung unterzogen, um festzustellen, ob die Herstellungszeit bei gleichbleibender chemischer Zusammensetzung und ihrer α-Struktur verkürzt werden kann. Simultane TG und DTA zusammen mit Röntgendiffraction scheinen eine sehr gute Technik darzustellen, um die erhaltenen Produkte zu charakterisieren.Zr-Phosphate ist am stabilsten und ergibt unter allen Bedingungen eine α-monohydrierte Hydrogenphosphate-Phase. Ein ähnliches Verhalten zeigt das Titanphosphat bei 180°C, während die Verbindung sich bei 300°C in γ-Ti(HPO4)2·2H2O umwandelt. Sn-Phosphat gibt — gerade um seine α-Sruktur zu behalten — durch die steigende Temperatur und steigende Konzentration an Phosphorsäure Anlaß für mehr und mehr dehydratierte Phasen.Ge-Hydrogenphosphat erwies sich unter hydrothermischen Bedingungen als am instabilsten bereids bei 180°C liefert as GeOHPO4. Wegen der leichten Hydrolisierbarkeit von Ge ist die hydrothermische Methode kein Weg, um kristallines α-Germaniumhydrogenphosphat herzustellen.