Pascual Olivera-Pastor

Surfactant‐Assisted Synthesis of a Mesoporous Form of Zirconium Phosphate with Acidic Properties

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Catalytic behavior of chromia and chromium-doped alumina pillared clay materials for the vapor phase deep oxidation of chlorinated hydrocarbons

Abstract The catalytic behavior of a chromium-doped alumina pillared clay (ACrPA) and a series of chromia-pillared clay materials (XSCr) for the deep oxidation of methylene chloride has been studied. Both types of catalysts showed a high activity (conversion > 80%) at T > 350°C. The ACrPA sample showed a virtually constant catalytic activity (conversion > 99%) in the 300–400°C temperature range while the activity of the XSCr samples markedly increased with the temperature reaching an almost total conversion at 400°C. XPS (X-ray photoelectron spectroscopy) and temperature programmed reduction (TPR) studies demonstrated that ACrPA is a Cr(VI) Cr(III) mixed valence catalyst, whereas XSCr samples contain only chromium(III) ion. The high activity shown by ACrPA at low temperatures was attributed to the presence of highly dispersed Cr(VI) species, which exhibited an unusually high stability. The metal oxidation state of the two chromium based systems seemed to be important in determining the catalytic activity. The increase of the activity with the Cr content in XSCr samples suggested that the acid sites in these catalysts were located on the chromia pillars.

“Breathing” in Adsorbate-Responsive Metal Tetraphosphonate Hybrid Materials

The structures of various layered calcium tetraphosphonates (CaH6DTMP; H8DTMP=hexamethylenediamine tetrakis(methylenephosphonic acid)), have been determined. Starting from CaH6DTMP.2H2O, thermal treatment and subsequent exposure to NH3 and/or H2O vapors led to four new compounds that showed high storage capacity of guest species between the layers (up to ten H2O/NH3 molecules) and a maximum volume increase of 55 %. The basic building block for these phosphonates consists of an eight-membered ring chelating Ca2+ through two phoshonate groups, and the organic ligand is located within the layers, which are held together by hydrogen bonds. The structural analysis revealed that the uptake/removal of guest species (H2O and NH3) induces significant changes in the framework not only by changing the interlayer distances but also through important conformational changes of the organic ligand. An anisotropic breathing motion could be quantified by the changes of the unit-cell dimensions and ligand arrangements in four crystalline derivatives. Complete characterization revealed the existence of interconversion reactions between the different phases upon gas uptake and release. The observed behavior represents, to the best of our knowledge, the first example of a breathing-like mechanism in metal phosphonates that possess a 2D topology.

Porous chromia-pillared α-zirconium phosphate materials prepared via colloid methods

The reaction of Cr(CH3CO2)3[Cr(OAc)3] with colloidal n-propylammonium α-zirconium phosphate and subsequent calcination of the products have been investigated. Depending on [Cr(OAc)3] : [initial phosphate] ratios and Cr3+ concentrations, a series of polyhydroxy acetato-Cr3+ intercalated precursor materials can be obtained, in which topotactic interface reactions have occurred to give materials with interlayer distances (d002) ranging from 13.0 to 39.0 A. These precursors show higher layer expansions than the analogous pillared clays (PILCS; d001= 16.8–27.6 A). A model invoking ordered in situ polymerisation of the Cr(OAc)3 on the phosphate surfaces is put forward.Calcination of these precursors under N2(400 °C) leads to a series of chromia-pillared materials in which the interlayers do not collapse to a single (much lower) value, as found previously for most PILCS, but instead provide a wide range of interlayer distances (10–27 A). These correspond to free heights of 3.5–20.5 A, the widest ranging and highest yet found for such materials. These nanoscale oxide-pillared materials have N2 surface areas (B.E.T., 77 K) of 250–330 m2 g–1, with pore radii (cylindrical pore method) ranging from 8.5 to 13.8 A, and very narrow pore-size distributions. Calcination conditions are crucial for obtaining porous solids. If calcination is carried out in air at 400 °C, although pillared powders and films are again obtained, surface areas are only ca. 40 m2 g–1(B.E.T., N2, 77 K).Furthermore, higher calcination temperatures (500 °C, under N2) give rise to X-ray amorphous materials, again having high surface areas and narrow pore-size distributions. All the materials can be processed in thin-film form without loss of textural characteristics.

Porous cross-linked materials formed by oligomeric aluminium hydroxides and α-tin phosphate

The intercalation of the tridecameric polyhydroxyaluminium Keggin-type cation, formally [AlO4Al12(OH)24(OH2)12]7+, into α-Sn(HPO4)2·H2O via the colloidal tetramethylammonium-exchanged intermediate α-Sn[NMe4]0.9–1.0H1.1–1.0(PO4)2·4H2O and the alumina-pillared materials obtained after calcination are described. Two different intercalated precursor materials are obtained, depending on whether the Keggin ion inserted derives from the commercial product (‘Chlorhydrol’) or from the polyhydroxyaluminium cation generated in situ. Calcination leads to materials differing in free heights and in alumina contents. Their surface areas (B.E.T., N2, 77 K) are quite high: 190 m2 g–1[chlor-SnP (400 °C)] and 228 m2 g–1[Al13-SnP (400 °C)]. Pore-size calculations show them to be mainly mesoporous, but with some micropore contribution (> 50% of pores in width range 15–40 A). The higher microporosity of the former with respect to the latter is ascribed to lateral-order differences between the alumina pillars.High cation-exchange capacities (for Co2+, Ni2+ and Cu2+) confirm that both solids are porous and have more accessible interlayer sites than does the parent material. Optical spectra of the transition-metal ion-exchanged materials indicate that the sites available in the two solids differ, and that both differ from those present in the starting α-tin phosphate. Site geometries are suggested.

Structural Mapping and Framework Interconversions in 1D, 2D, and 3D Divalent Metal R,S-Hydroxyphosphonoacetate Hybrids

Reactions of divalent cations (Mg(2+), Co(2+), Ni(2+), and Zn(2+)) with R,S-hydroxyphosphonoacetic acid (HPAA) in aqueous solutions (pH values ranging 1.0-4.0) yielded a range of crystalline hydrated M-HPAA hybrids. One-dimensional (1D) chain compounds were formed at room temperature whereas reactions conducted under hydrothermal conditions resulted in two-dimensional (2D) layered frameworks or, in some cases, three-dimensional (3D) networks incorporating various alkaline cations. 1D phases with compositions [M{HO(3)PCH(OH)CO(2)}(H(2)O)(2)].2H(2)O (M = Mg, Co, and Zn) were isolated. These compounds were dehydrated in liquid water to yield the corresponding [M{HO(3)PCH(OH)CO(2)}(H(2)O)(2)] compounds lacking the lattice water between the 1D chains. [M{HO(3)PCH(OH)CO(2)}(H(2)O)(2)] (M = Mg, Ni, Co, Zn) compounds were formed by crystallization at room temperature (at higher pH values) or also by partial dehydration of 1D compounds with higher hydration degrees. Complete dehydration of these 1D solids at 240-270 degrees C led to 3D phases, [M{HO3PCH(OH)CO(2)}]. The 2D layered compound [Mg{HO(3)PCH(OH)CO(2)}(H(2)O)(2)] was obtained under hydrothermal conditions. For both synthesis methods, addition of alkali metal hydroxides to adjust the pH usually led to mixed phase materials, whereas direct reactions between the metal oxides and the hydroxyphosphonoacetic acid gave single phase materials. On the other hand, adjusting the pH with acetate salts and increasing the ratio M(2+)/HPAA and/or the A(+)/M(2+) ratio (A = Na, K) resulted in 3D networks, where the alkali cations were incorporated within the frameworks for charge compensation. The crystal structures of eight new M(II)-HPAA hybrids are reported herein and the thermal behavior related to dehydration/rehydration of some compounds are studied in detail.

Porous chromia-pillared α-tin phosphate materials

Abstract The reactions of Cr(OAc)3 (OAc− = CH3OCO−) solutions with colloidal suspensions of tetramethylammonium α-tin phosphate ( α- Sn[Me 4 N] 0.9 – 1.1 H 1.1−0.9 (PO 4 ) 2 · n H 2 O = “Me 4 NSnP”) have been investigated over a wide range of [Cr(OAc)3]:[Me4NSnP] ratios, Cr(OAc)3 concentrations, and heating conditions. On aging + reflux, Me4NSnP takes up oligomeric Cr3+ species to give crystalline, highly expanded materials having interlayer distances between 24 and 33 A. Conversely, Cr(OAc)3 solutions treated in the same way, but separately, and then added to colloidal Me4NSnP gave composites with little expansion (interlayer distance, d002 = 15 A), collapsing to nonporous oxide-phosphates (d002 = 10 A) on calcination. Separately polymerized CrCl 3 NaOH or [Cr3O(OAc)6(OH2)3]+ (standard precursors in clay-pillaring) also gave poorly defined delaminated materials. Chemical, TGA/DTA, and visible/near UV spectroscopic evidence shows the intercalates are polyhydroxyacetato-Cr3+ species, and some may be formulated as [Cr3(OH)6(OAc)]2+, [Cr4(OH)7(OAc)3+, and [Cr5(OH)7(OAc)3]5+. They are formed in situ on the α-tin phosphate surfaces; possible orientations within the layers (taking into account the presence of zeolite-type water) is discussed. Calcination under N2 at 400°C gives chromium oxide-pillared materials with d002 in the ranges 12.5–14.0 A (from precursors prepared at higher Cr3+ concentrations) or 15–20 A (from precursors at lower Cr3+ concentrations). The surface areas (BET, N2, 77K): 264–386 m2 g−1, compare well with analogues in clay chemistry (150–400 m2 g−1). Pore-sized distributions (cylindrical pores model) are narrow, >70% of pores having widths

Divalent metal vinylphosphonate layered materials: compositional variability, structural peculiarities, dehydration behavior, and photoluminescent properties

A family of M-VP (M = Ni, Co, Cd, Mn, Zn, Fe, Cu, Pb; VP = vinylphosphonate) and M-PVP (M = Co, Cd; PVP = phenylvinylphosphonate) materials have been synthesized by hydrothermal methods and characterized by FT-IR, elemental analysis, and thermogravimetric analysis (TGA). Their structures were determined either by single crystal X-ray crystallography or from laboratory X-ray powder diffraction data. The crystal structure of some M-VP and M-PVP materials is two-dimensional (2D) layered, with the organic groups (vinyl or phenylvinyl) protruding into the interlamellar space. However, the Pb-VP and Cu-VP materials show dramatically different structural features. The porous, three-dimensional (3D) structure of Pb-VP contains the Pb center in a pentagonal pyramid. A Cu-VP variant of the common 2D layered structure shows a very peculiar structure. The structure of the material is 2D with the layers based upon three crystallographically distinct Cu atoms; an octahedrally coordinated Cu(2+) atom, a square planar Cu(2+) atom and a Cu(+) atom. The latter has an unusual co-ordination environment as it is 3-coordinated to two oxygen atoms with the third bond across the double bond of the vinyl group. Metal-coordinated water loss was studied by TGA and thermodiffractometry. The rehydration of the anhydrous phases to give the initial phase takes place rapidly for Cd-PVP but it takes several days for Co-PVP. The M-VP materials exhibit variable dehydration-rehydration behavior, with most of them losing crystallinity during the process.

Layered and pillared metal carboxyethylphosphonate hybrid compounds

A series of carboxyethylphosphonate hybrid materials has been prepared: Mn(II)(O3PCH2CH2COOH) *H2O (1), Mn(III)(OH)(O3PCH2CH2COOH)*H2O (2), Al3(III)(OH)3(O3PCH2CH2CO2)2 *3H2O (3) and Cr2(III)(OH)3(O3PCH2CH2CO2) *3H2O (4). Compounds 1 and 2 were synthesized from Mn(III)(CH3COO)3 *2H2O under hydrothermal, or refluxing treatments, respectively. The crystal structures of the manganese-bearing solids have been solved ab initio from laboratory X-ray powder diffraction data and refined by the Rietveld method. 1 crystallises in a orthorhombic cell and 2 in monoclinic symmetry. Both solids have inorganic 2D layered structures with the acid carboxylic groups pointing towards the interlayer space, and the layers linked only through hydrogen bonds. The inorganic layers of these compounds are formed by manganese atoms in distorted octahedral environments linked together by the phosphonate groups. The crystal structure of 3 has been solved ab initio from synchrotron X-ray powder diffraction data. This solid shows a pillared structure with the phosphonate and carboxylate groups cross-linking the inorganic layers. These layers contain chains of aluminium octahedra running parallel to each other. 4 is amorphous and the IR-UV-VIS spectra suggest a framework with Cr(III) cations in octahedral environments. Thermal, spectroscopic and magnetic data for manganese and chromium compounds as well as the structural details of these solids are discussed.

PILLARED CLAYS PREPARED FROM THE REACTION OF CHROMIUM ACETATE WITH MONTMORILLONITE

Refluxing chromium (III) acetate with a Na+-montmorillonite suspension gives rise to the intercalation of linear Cr(III) polyhydroxo-acetate oligomers. Thermally stable chromia pillared mont-morillonite materials are obtained upon calcination under ammonia up to 625°C, and basal expansions up to 6 Å are maintained. The porous materials retain high surface areas (366–464 m2 g−1), a micropore volume of 0.1 cm3 g−1 and narrow pore size distributions centered between 7.5 and 12 Å. The most thermally stable materials in air were those prepared under ammonia at 625°C, containing NH4+ as the exchangeable ion.