A. La Ginestra

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.

On the existence of pyrophosphates of tetravalent metals having a layered structure

Abstract After an accurate investigation of the thermal behaviour (TG and DTA curves) of several samples of α-Zr(HPO 4 ) 2 ·H 2 O and α-Ti(HPO 4 ) 2 ·H 2 O, prepared both by the refluxing and HF procedures, γ-Zr(HPO 4 ) 2 ·2 H 2 O and γ-Ti(HPO 4 ) 2 ·2 H 2 O, new pyrophosphate phases of tetravalent metals have been isolated. The structure of these compounds, deduced from X-ray diffraction data, density measurements and considerations of bond length and angles, arises from the packing of layers of α and γ type, identical to those present in the starting materials and held together by POP bridges. These layered M(IV)P 2 O 7 are obtained after the condensation process of the HPO 4 groups present in α and γ-M(IV)(HPO 4 ) 2 and are stable until the transition to the respective cubic pyrophosphates takes place. The range of stability of these phases changes on going from the α to the γ phase and in part depends on the degree of crystallinity and/or the crystal size of the starting layered exchanger. The chemical implications of α- and γ-L-M(IV)P 2 O 7 are examined and discussed.

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.

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.

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.

Nickel complexes with sulphur-containing ligands as initiators of the polymerization of phenylacetylene

Abstract The polymerization reaction of phenylacetylene in the presence of Ni( ii ) and Ni( iii ) catalysts, i.e. [Ni(dtb) 2 ] (dtb = dithiobenzene), [Ni( N , N -ddtc) 2 ] ( N , N -ddtc = N , N -diethyldithiocarbamate), [Ni(oatp) 2 ] (oatp = o -aminothiophenolate), [Ni(mtb) 2 ] (mtb = monothiobenzoate) and (Bu 4 N)[Ni(tdt) 2 ] (tdt = toluene-3,4-dithiolate), gave polyphenylacetylene (PPA) in high yields and under mild reaction conditions. The complex [Ni(mtb) 2 ] reacts at −40°C, leading to highly stereoregular PPA. Infra-red and nuclear magnetic resonance spectra of PPA and conductivities (upon doping with I 2 and FeCl 3 ) are reported and discussed in comparison with literature data.

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.

Ag-zirconium phosphate system: Characterization of the phases obtained at different temperatures

Abstract The products obtained by ion exchange of zirconium phosphate loaded with Ag + (from 13% to 96% of conversion) have been characterized by thermal and X-ray methods. The materials maintain a layered structure until around 550–600°C, with a d 002 of about 7,6–7,8 A. At low Ag conversion solid solutions can be obtained. For all samples, above 600°C the layered structure disappears and the phase AgZr 2 (PO 4 ) 3 is produced. Other phases, ZrP 2 O 7 , Ag 4 P 2 O 7 or Ag 3 PO 4 (depending on the initial composition) are formed together with AgZr 2 (PO 4 ) 3 . The conditions of formation and possible transformation of some of these phases are discussed.

Determination of the surface acidity of layered metal phosphates by NH3 temperature-programmed desorption.

Abstract The acid properties of layered Me phosphates (MeTi, Ge, Zr, Sn) were determined by NH3 temperature-programmed desorption. Careful establishment of the preheating conditions suppressed interference from NH3 intercalation and allowed the surface acidity to be measured. A scale of acidity strength in line with the catalytic behaviour of these phosphates in acid-catalysed reactions was obtained.