P. Patrono

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.

Conversion of 1-butene over WO3-TiO2 Catalysts

Abstract The conversion of pure 1-butene over WO3-TiO2 Catalysts has been studied in a flow microreactor at 693 K and at 723 K. For comparison, experiments have also been carried out on the bare TiO2 support. Significant activity in the skeletal isomerization has been found, together with double bond migration, cis-trans isomerization, hydrogenation and cracking. By increasing time-on-stream the conversion of 1-butene is increasingly limited due to the formation of carbon residues on the Catalyst surfaces. Working with τ = 6.44 h at 723 K (9.33% WO3 by weight), the selectivity to products different from n-butenes decreases from nearly 75% after 15 min time-on-stream to less than 6% after 3 h. Initial selectivity to isobutene also decreases from nearly 14% to less than 2% after 3 h. A Fourier transform infrared study suggests that Catalyst deactivation is due to the oligomerization of butenes on the surface Bronsted acid sites. Carbon residues formed are largely constituted of aromatized butene polymers anchored to the surface through car☐ylate and alcoholate groups. Burning of the carbon residue occurs at 623–723 K and restores Catalytic activity. The activity and the deactivation of the bare commercial TiO2 support is attributed to the weaker Bronsted acidity arising from silica impurities observed on the surface.

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.

Layered zirconium-tin phosphates. II, Catalytic properties in the oxydehydrogenation of ethylbenzene to styrene

Abstract Zirconium-tin mixed phosphates have been studied as catalysts for the oxydehydrogenation of ethyl-benzene to styrene at T =400–500°C and atmospheric pressure. It has been found that these mixed compounds are much more active than pure zirconium and tin phosphates. Ethylbenzene conversion of ca. 50% with selectivity to styrene up to 90% has been obtained, the by-products being mainly CO x with lower amounts of benzaldehyde. The effect of operating parameters, such as contact time, reaction temperature and feeding ratio, has been the subject of a preliminary investigation. A reaction scheme is proposed in order to interpret these results. The influence of surface acidity on the reaction through the formation of a catalytically active coke is discussed.

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.

Comparative study of catalytic behaviour of bulk-like and highly dispersed supported vanadyl orthophosphate catalysts in the oxidative dehydrogenation of ethane

The catalytic behaviour in the oxidative dehydrogenation of ethane of TiO2- and SiO2-supported catalysts containing bulk-like VOPO4 particles has been compared to that of TiO2-supported highly dispersed VOPO4. The catalysts have been characterised by XRD analysis and BET measurements. Redox properties have been studied by TPR experiments. A correlation of catalytic activity and C2H4 selectivity with redox properties has been proposed.

Acidic properties of Ge, Ti, Zr and Sn phosphates and arsenates

A comparison of the acidic properties of isostructural phases of Ge, Ti, Zr and Sn bis-(monohydrogenphosphates) or arsenates and the derived layered- or cubic-pyrophosphates is reported. This study has been performed by investigating their thermal behaviour, structural changes, FT-TR spectra of the compounds before and after adsorption of probe molecules, NH3 TPD and by means of acid-catalysed reactions, such as isopropanol dehydration and 1-butene isomerisation.