Riccardo Vivani

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.

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like compounds organically modified by ion-exchange procedure and the data reviewed have been supplemented with unpublished results.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.

Proton conductivity of mesoporous zirconium phosphate pyrophosphate

The conductivity of a mesoporous zirconium phosphate pyrophosphate of composition Zr(P 2 O 7 ) 0.81 (O 3 POH) 0.38 and surface area of 215 m 2 /g has been determined as a function of temperature at controlled relative humidity. Ac and dc conductivity measurements carried out on pellets with platinum electrodes, in nitrogen or hydrogen flow, showed that the charge carriers are protons. At 20°C and relative humidity decreasing in the range 90-20%, the conductivity changes from 1.3 × 10 -3 S cm - 1 to 4 × 10 -7 S cm - 1 . The activation energy for conduction in the range 20/ - 20°C increases from 4.6 to 8.7 kcal/mol with decreasing relative humidity. At 100°C and 90% relative humidity the pyrophosphate groups undergo a slow rehydration and the conductivity rises up to 2.6 × 10 -3 S cm -1 as a consequence of the increase of the number of -POH groups.

Proton conductivity of zirconium carboxy n-alkyl phosphonates with an α-layered structure

Abstract Alpha-layered zirconium carboxy n -alkyl phosphonates, Zr(O 3 P(CH 2 ) n COOH) 2 with n =1−5 have been prepared and characterized by X-ray diffraction, TG-DTA, DSC and ac-conductivity measurements. The interlayer distance of the phosphonates increases almost linearly with increasing n from 11.1 to 18.7 A. Compounds with n =2, 4, 5 undergo a reversible phase transition at 165, 95 and 215°C, associated with a weak endothermic effect and with a discontinous increase of the interlayer distance of 1.0, 1.3 and 0.8 A respectively. Structural considerations indicated a different arrangement of the terminal-COOH groups in the interlayer region as a consequence of the odd or even number of methylene groups in the alkyl chain. ac-conductivity measurements, performed on pressed pellets by the impedance method, in fact showed that compounds with n =2 and 4 have a conductivity much lower than that of compounds with n =1, 3, 5. The conductivity of these latter is 2×10 -9 , 1.6×10 -8 and 1.4×10 -7 S cm -1 at 200°C, with activation energies of 77, 104 and 132 kJ mol -1 respectively. Finally the high frequency limit capacitance of the pellets made up of compounds with n =2 and 4 was found to change significantly at the phase transition temperature indicating a change in dielectric properties of these compounds.

On the Intercalation of the Iodine−Iodide Couple on Layered Double Hydroxides with Different Particle Sizes

Molecular iodine was intercalated from nonaqueous solution into microsized ZnAl-layered double hydroxide (LDH) in the iodide form, generating the I(3)(-)/I(-) redox couple into the interlayer region. Chloroform, ethanol, acetonitrile, or diethyl ether were used as solvents to dissolve the molecular iodine. The intercalation compounds were characterized by thermogravimetric analysis, X-ray powder diffraction, UV-vis spectroscopy, and scanning and transmission electron microscopy. The stability of iodine-solvent adducts and the iodine concentration affected the LDH iodine loading, and samples with I(2)/I(-) molar ratio ranging from 0.14 to 0.82 were prepared. Nanosized, well dispersible LDH, synthesized by the urea method in water-ethylene glycol media, were also prepared and successfully functionalized with the I(3)(-)/I(-) redox couple applying the conditions optimized for the micrometric systems.

Advances in the Chemistry of Nanosized Zirconium Phosphates: A New Mild and Quick Route to the Synthesis of Nanocrystals

Simple addition of zirconyl propionate to phosphoric acid in alcoholic media surprisingly led to the formation, in few minutes, of transparent gels containing solvent intercalated zirconium phosphate (ZrP) nanoparticles with hexagonal shape and a planar size of about 40 nm. With the help of elemental analysis, inductively coupled plasma-optical emission spectrometry (ICP-OES), and (31)P magic angle spinning (MAS) NMR, the nanoparticle composition was formulated as Zr(R)(w)(HPO(4))(x)(H(2)PO(4))(y), in which R can be an hydroxyl or a propionate group. The stoichiometric coefficients for propanol intercalated ZrP are x = 1.43, y = 0.83, and w = 0.32. Solvent elimination at 60 °C gave rise to an increase in the x value and a decrease in the y and w values. X-ray powder diffraction analysis and transmission electron microscopy (TEM) observations showed a concomitant increase in the particle size: planar size and thickness ranged from 90 to 200 nm and from 20 to 85 nm, respectively, depending on the nature of the solvent. A possible mechanism explaining the change in the x, y, and w values, the growth of nanoparticles, and the role of the solvent is proposed. Finally, the possibility of using these gels to disperse the ZrP nanoparticles within the polymer matrix of Nafion117 is shown.

Preparation of Mesoporous Zirconium Phosphate-Pyrophosphate with a Large Amount of Thermally Stable Acid Groups on the Pore Surface

Mesoporous zirconium phosphate pyrophosphate of composition Zr(P2O7)0.81(O3POH)0.38 was obtained by calcination at 600°C of a mesoporous zirconium phosphite diphosphonate, Zr[(O3P-C6H4-PO3)0.54 (O3P-C6H4-PO3H2)0.13(O3PH)0.79]. The surface area of this completely inorganic material (215 m2/g) was lower than that of its inorgano-organic precursor (350 m2/g); however, both materials exhibited the same pore distribution, with an average pore diameter of ≈4 nm, and similar values of the mesopore volume (0.21 and 0.28 cm3/g, respectively). These results showed that the thermal treatment at 600°C leaves nearly unaltered the mesoporous structure of the precursor. 31P solid state NMR and TEM investigations were also carried out and discussed. The material obtained represents a considerable advancement in the preparation of zirconium phosphates with high surface area. Due to the good thermal stability of its surface acid groups (700–800°C), it is of interest for potential uses as an acid catalyst at high temperature.

Shaping Solid-State Supramolecular Cavities: Chemically Induced Accordionlike Movement ofγ-Zirconium Phosphate Containing Polyethylenoxide Pillars

The hydrophilic oxygen atoms of polyethylenoxide chains inserted as pillars in gamma-zirconium phosphate form hydrogen bonds with the acid groups of the host. As a result the pillars are almost perpendicular to the gamma layers. Upon changing the pH level of the supernatant solution the hydrogen bonds are broken and the pillars become almost perpendicular to the layers (shown schematically). Thus there is a reversible enlargement-shortening of the interlayer space.

Crystal engineering on layered zirconium phosphonates. Crystal structure (from X-ray powder data) and non-covalent interactions on the layered zirconium compound of 4-[bis(phosphonomethyl)amino]butanoic acid

A new layered zirconium diphosphonate fluoride, ZrHF(O3PCH2)2NHC3H6CO2, has been prepared by the reaction of zirconyl chloride with 4-[bis(phosphonomethyl)amino]butanoic acid in the presence of HF. Its structure has been determined “ab initio” by X-ray powder data. It crystallizes in the monoclinic space group P21/c (No. 14), with a = 12.9640(3) A, b = 8.9900(4) A, c = 10.7924(4) A, β = 101.854(4)°, and Z = 4. Both of the phosphonic groups of each diphosphonate building block are bonded to zirconium atoms on the same side of the layers. Only one organic residue is associated with two phosphonate tetrahedra. The packing of layers creates an interdigitated arrangement of organic groups in the interlayer region. Two strong non-covalent interactions are present in the structure. One of them involves neighbouring P–OH and amino groups, while the other interaction engages terminal carboxylic groups and fluorine atoms belonging to adjacent layers. Thermal treatment at 240 °C causes the loss of one mole of HF per mole of zirconium, with the formation of a stable compound in which carboxylate groups probably coordinate to the zirconium atoms belonging to adjacent layers. Preliminary experiments of intercalation with ammonia and short alkylamines are also reported.

A Layered Mixed Zirconium Phosphate/Phosphonate with Exposed Carboxylic and Phosphonic Groups: X-ray Powder Structure and Proton Conductivity Properties

A novel mixed zirconium phosphate/phosphonate based on glyphosine, of formula Zr2(PO4)H5(L)2·H2O [L = (O3PCH2)2NCH2COO], was synthesized in mild conditions. The compound has a layered structure that was solved ab initio from laboratory PXRD data. It crystallizes in the monoclinic C2/c space group with the following cell parameters: a = 29.925(3), b = 8.4225(5), c = 9.0985(4) Å, and β = 98.474(6)°. Phosphate groups are placed inside the sheets and connect the zirconium atoms in a tetradentate fashion, while uncoordinated carboxylate and P-OH phosphonate groups are exposed on the layer surface. Due to the presence of these acidic groups, the compound showed remarkable proton conductivity properties, which were studied in a wide range of temperature and relative humidity (RH). The conductivity is strongly dependent on RH and reaches 1 × 10(-3) S cm(-1) at 140 °C and 95% RH. At this RH, the activation energy of conduction is 0.15 eV in the temperature range 80-140 °C. The similarities of this structure with related structures already reported in the literature were also discussed.