Umberto Costantino

New Synthetic Routes to Hydrotalcite-Like Compounds − Characterisation and Properties of the Obtained Materials

Hydrotalcite-like anionic clays of general formula [M(II)1–xM(III)x(OH)2]x+[CO32–x/2]x– · m H2O with M(III) = Al and M(II) = Mg, Ni, Zn have been prepared by precipitation of the hydroxycarbonates from the “homogeneous” solution after the thermally induced hydrolysis of urea. The effect of the temperature of precipitation, of the total metal cations concentration, of the molar fraction M(III)/M(III) + M(II) and of the molar fraction urea/M(II) + M(III) in solution on the composition and on the crystallinity degree of the samples has been investigated. The optimum conditions are reported to obtain micro-crystalline powders with a narrow distribution of particle size in a short time with a simple procedure. The compounds obtained have been characterised for chemical composition, thermal behaviour, particle-size distribution and BET-surface area. In addition, the crystal structure of Mg0.67Al0.33(OH)2 (CO3)0.165 · 0.4 H2O has been refined by X-ray diffraction powder methods. The carbonate form has been converted into the chloride form by letting gaseous HCl flow over the hydrotalcite-like compounds, heated at 150 °C. The exchange of Cl– anions with some alkoxide anions in the presence of the respective alkanols has been investigated. The exchange reaction was driven by the segregation of NaCl crystals poorly soluble in alkanols and led to the co-intercalation of the alkoxide ions together with the alkanol with the formation of a bi-layer of extended alkyl chain in the interlayer region of the Mg-Al hydrotalcite. The intercalation compound, washed with water, produces a hydrotalcite with Cl– and OH– as balancing anions.

Crystalline Zr(R-PO3)2 and Zr(R-OPO3)2 compounds (R = organic radical): A new class of materials having layered structure of the zirconium phosphate type

Abstract By slowly decomposing zirconium fluoro-complexes in the presence of hydroxymethane phosphonic acid, benzene phosphonic acid or triethylphosphate, micro-crystalline compounds with formulae Zr(HOCH 2 PO 3 ) 2 ·H 2 O, Zr(C 6 H 5 PO 3 ) 2 and Zr(C 2 H 5 OPO 3 ) 2 respectively are formed. Crystals of suitable size for X-ray crystal structure determination were not obtained but several experimental observations such as the shape of the crystals, the linear relation between the first d -value of the X-ray patterns and the formula-weight/density ratio, the discontinous change of the first d -value during the dehydration of Zr(HOCH 2 PO 3 ) 2 ·H 2 O as well as intercalation of organic molecules provides strong evidence that these materials possess a layered structure of the zirconium phosphate type. The possibility to obtain other Zr(R-PO 3 ) 2 or Zr(R-OPO 3 ) 2 compounds and inorganic-organic ion-exchangers as well as their possible applications are finally discussed.

Inorganic ion-exchange pellicles obtained by delamination of α-zirconium phosphate crystals

Abstract During systematic research into the intercalation of alkylamines in layered α-[Zr(PO4)2]H2 · H2O, some favorable cases have been found in which the intercalate, in the presence of water, exfoliates giving rise to a colloidal dispersion of platelike particles of zirconium phosphate partially or totally neutralized by alkylammonium ions. This exfoliation occurs dramatically during the intercalation of propylamine. From this dispersion, very similar to that obtained with smectite clays, a suspension of very thin lamellae of highly hydrated α-[Zr(PO4)2]H2 is formed after a deintercalation process. The thin lamellae may be easily recomposed in the form of films, membranes, or flexible pellicles by filtering the suspension on a porous plastic filter or by spraying the suspension on a suitable support and leaving it to dry. The present paper reports the conditions for the formation of the colloidal dispersion as well as the preparation of the pellicles, films, and membranes. Finally, some properties of the colloidal dispersion and the membranes are reported. Pellicular zirconium phosphate shows promise for applications in the field of ion-exchange, heterogeneous catalysis, chromatographic supports, and protonic conductors.

Recent progress in the intercalation chemistry of layered α-zirconium phosphate and its derivatives, and future perspectives for their use in catalysis

Abstract After a brief discussion of the present status of knowledge of the mechanism of intercalation processes in α-zirconium phosphate, recent developments particularly in the synthesis of new layered materials with α-structures are examined. Derivatives of α-zirconium phosphate with one or two pendant R groups as well as the intercalation of organic molecules in such layered compounds are reported. Finally, some pillared zirconium phosphates, perspectives for obtaining new pillared compounds with tailor-made porosity and their potential applications as molecular sieves, as shape-selective ion-exchangers and as materials for shape-selective catalysis are discussed.

On the mechanism of diffusion and ionic transport in crystalline insoluble acid salts of tetravalent metals—I Electrical conductance of zirconium bis (monohydrogen ortho-phosphate) monohydrate with a layered structure☆

Abstract The specific conductance of zirconium phosphate decreases considerably with increase in the degree of crystallinity; however the energy of activation for the conduction apparently does not depend on the degree of crystallinity and is surprisingly low (11–13 KJ/mole). In order to explain these results, different samples of Zr(HPO 4 ) 2 ·H 2 O of the same degree of crystallinity but having different specific surface area were obtained by sedimentation and the amount of surface counter-ions per cm 3 of microcrystals and the specific conductance measured. It was found that a large fraction of the total current was transported by the surface counter-ions, their mobility being ⩾10 4 times that of the internal ones. Thus, the low activation energy for ionic conduction in the crystalline material is due essentially to the transport of surface counter-ions. These results demonstrate that the counter-ions present at the surface of microcrystals of zirconium phosphate make an important contribution to the total conduction, to the activation energy, and to the electrochemical properties of membranes consisting of microcrystals of zirconium phosphate.

Protonic conductivity of layered zirconium phosphonates containing −SO3H groups. I. Preparation and characterization of a mixed zirconium phosphonate of composition Zr(O3PR)0.73(O3PR′)1.27·nH2O, with R=−C6H4−SO3H and R′ = −CH2−OH

Abstract A zirconium phosphonate of formula: Zr(O3PC6H4SO3H)0.73(O3PCH2OH)1.27·nH2O was prepared and characterized by X-ray powder diffraction, thermogravimetric analysis, EMF and ac conductivity measurements. The compound seems to have a layer structure of α-type and the number of water molecules in the interlayer region strongly depends on relative humidity so that hydrated phases with different interlayer distances are formed. EMF measurements showed the absence of electronic conduction, Both the conductivity and the parameters of the Arrhenius equation are strongly influenced by the water content. At 30°C and 90% relative humidity (n = 7.5) a conductivity of 1.6×10−2 S cm−1, with an activation energy of 4.5 kcal mol , was found. A correlation between changes in activation energy and interlayer distance was observed indicating the possible presence of a non-negligible component of bulk transport.

Heterogeneous Catalysis in Trimethylsilylation of Alcohols and Phenols by Zirconium Sulfophenyl Phosphonate

Abstract Layered zirconium sulfophenyl phosphonate was found to be an efficient heterogeneous catalyst for the trimethylsilylation of alcohols and phenols.

Recent progress in the synthesis and application of organically modified hydrotalcites

Abstract This review is focused on the preparation and potential applications of hydrotalcite like compounds organically modified by ion-exchange procedure and the data reviewed have been supplemented with unpublished results. It is divided in two Parts. Part I deals with intercalation of biologically active species such as amino-acids, anti-inflammatory and antibiotic drugs, UV-absorbers to produce nano-hybrids with versatile application as biomolecule reservoir and in the pharmaceutical and personal care fields. Part II deals with the intercalation of several anions with either hydrophobic or hydrophilic properties in order to make the inorganic sheets compatible with different polymers. Moreover, if the guest is an active molecular anion such as antimicrobial, antioxidant, antibiotic or anti-inflammatory, the polymer can acquire the peculiar properties of the guest opening novel interesting application fields.

Crystalline insoluble acid salts of tetravalent metals—XX Forward and reverse Cs+/H+ and Rb+/H+ ion exchange on crystalline zirconium phosphate

Abstract Forward and reverse Cs+/H+ and Rb+/H+ ion exchange have been investigated by the batch-procedure in 3rder to obtain both the titration and uptake curves, as well as X-ray diffractograms and water contents of the samples at the various Cs+ or Rb+-loadings. The interlayer distance of the exchanger increases discontinuously with increasing Cs+-loading and three different phases, ZrHCs(PO4)2.2H2O, ZrH0·5Cs1·5(PO4)2.3H2O, and Zr(CsPO4)2.6H2O with interlayer distances of 11·3, 11·7 and 14·2 A, respectively, have been found in the forward process. For the analogous Rb+-ion exchange process the phases were ZrH0·5Rb1·52H2O (10·6 A) and Zr(RbPO4)2.H2O (9·2 A). Reverse ion exchange processes both for Cs+ and Rb+ ions showed a considerable irreversibility especially in the range of 0–50 per cent Cs+ or Rb+-loadings. The replacement of both Cs+ and Rb+ ions by H+ ions occurred together with the formation of solid solutions and therefore the large ion exchange hysteresis loops were ascribed to the formation of different phases in the forward and reverse processes. The X-ray diffraction patterns of the various Rb+ and Cs+-phases dried under different conditions are reported. Finally, a general discussion of the Cs+ or Rb+-ion exchange behavior on zirconium phosphate at various degree of crystallinity is made. It is concluded that the low, apparent selectivity of crystalline zirconium phosphate towards large monovalent ions is due to steric hindrance and therefore enlarged phases of this exchanger, such as Zr(HPO4)2.8H2O (10·4 A), are expected to have a high selectivity towards Cs+ or even larger cations.

Preparation, characterization and proton conductivity of titanium phosphate sulfophenylphosphonate

Abstract Titanium phosphate sulfophenylphosphonate, Ti(HPO 4 ) 1.00 (O 3 PC 6 H 4 SO 3 H) 0.85 (OH) 0.30 · n H 2 O was synthesised and characterized by X-ray diffraction, solid state NMR, impedance measurements, TG and DSC analysis. The MAS-NMR spectrum of 31 P nucleus presents two resonances characteristic of monohydrogen phosphate and sulfophenylphosphonate groups coordinated to three Ti(IV) atoms through three non-protonated oxygen atoms. Water content, protonic conductivity ( σ ) and activation energy for conduction ( E a ) were determined in the range 20 to −20°C for materials equilibrated at relative humidity (r.h.) in the range 5–90%. By increasing the number of water molecules per sulfonic group from 2 to 14, σ rises by 3 orders of magnitude up to about 0.1 S cm −1 , while E a decreases from 12.6 to 4.3 kcal/mol. Measurements were also carried out at 100°C as a function of relative humidity in the range 30–100%; σ values higher than 0.1 S cm −1 were found at r.h. ≥65%. The titanium phosphonate was used as a filler of Nafion 1100 hybrid membranes. At 100°C and 80% r.h., the conductivity of the hybrid membranes containing from 5% to 20% phosphonate loading is very similar to that of a Nafion 1100 membrane prepared in-house by casting technique, but it drops by an order of magnitude when the loading is increased to 30%.