Raquel Trujillano

New synthesis strategies for effective functionalization of kaolinite and saponite with silylating agents

Functionalization of Brazilian São Simão kaolinite and Spanish Yunclillos saponite with the alkoxysilanes 3-aminopropyltriethoxysilane and 3-mercaptopropyltrimethoxysilane is reported. The resulting hybrids were characterized by X-ray diffraction, thermal analysis, infrared absorption spectroscopy, and scanning electron microscopy, which demonstrated the effectiveness of the interlamellar grafting process. The X-ray diffractograms revealed incorporation of the alkoxide molecules into the interlayer space of the clays. The displacement of the stretching bands of interlayer hydroxyls in the infrared spectra of the modified kaolinites and the increased intensity of the Mg-OH vibrations in the spectra of the modified saponites confirmed the functionalization of the clays. The thermal behavior of the organoclays confirmed the stability of the hybrids, which was dependent on the clay used for preparation of the materials.

Cobalt–iron hydroxycarbonates and their evolution to mixed oxides with spinel structure

Cobalt(ii)–iron(iii) mixed hydroxides with hydrotalcite-like structure have been synthesized. Depending on the experimental conditions during synthesis, CoII becomes partially oxidized to CoIII . The solids have been characterized by X-ray diffraction, FTIR spectroscopy, temperature-programmed reduction and nitrogen adsorption at low temperature for specific surface area assessment. Reduction of FeIII takes place at lower temperatures than that of bulk Fe3O4 . Thermal treatment in air leads to oxidation of CoII to CoIII , in the same temperature range where collapsing of the layered structure by decarbonation is observed. The nature of the spinel phases formed upon calcination at high temperature depends both on the calcination temperature and the calcination time.

Acetylene hydrogenation on Ni–Al–Cr oxide catalysts: the role of added Zn

Abstract Ni-containing catalysts for selective acetylene hydrogenation to ethylene have been prepared by controlled calcination of hydrotalcite-like precursors. In addition to Ni and Al, Cr and Zn were added to improve the catalytic performance. The hydrotalcite-like precursors and the calcined catalysts have been characterized by powder X-ray diffraction, FT-IR and Vis–UV/Diffuse Reflectance spectroscopies, temperature-programmed reduction, and specific surface area assessment by nitrogen adsorption at 77 K. Despite addition of Zn hinders coke formation, the activity to gaseous products decreases as the Ni content is increased. An increase in coke concentration increases activity and selectivity to ethylene, specially in those samples with not too high Ni contents. The highest selectivity to ethylene is achieved for Zn/Ni≈4 (molar ratio).

Porphyrin-kaolinite as efficient catalyst for oxidation reactions.

The preparation, characterization, and application in oxidation reactions of new biomimetic catalysts are reported. Brazilian Sao Simao kaolinite clay has been functionalized with [meso-tetrakis(pentafluorophenyl)porphinato]iron(III), Fe(TPFPP). To obtain the functionalized clay, the natural clay was purified by dispersion-sedimentation, expanded by insertion of dimethyl sulfoxide (DMSO), and functionalized with amino groups by substitution of DMSO with ethanolamine. These previous steps allowed clay functionalization with Fe(TPFPP), leading to a layered material with a basal spacing of 10.73 A. Clay functionalization with the porphyrin was confirmed by formation of the secondary amine, as demonstrated by FTIR bands at 3500-3700 cm(-1). UV-vis spectroscopy revealed a red shift in the Soret band of the iron porphyrin in the functionalized material as compared to the parent iron porphyrin catalyst in solution, indicating Fe(III)P --> Fe(II)P reduction. The catalytic performance of the functionalized clay was evaluated in the epoxidation of cyclooctene, with complete selectivity for the epoxide (100% epoxide yield), and ketonization of cyclohexane, cyclohexanone being the major product. The novel catalyst was also evaluated in the Baeyer-Villiger (BV) oxidation of cyclohexanone, with 85% conversion of cyclohexanone in epsilon-caprolactone, with total selectivity to epsilon-caprolactone.

Use of hydrotalcites as catalytic precursors of multimetallic mixed oxides. Application in the hydrogenation of acetylene

Abstract The effect of the Ni/Zn molar ratio on the activity, selectivity, and coke formation of NiO·ZnO·Al2O3 catalysts (modified with Fe3+ or Cr3+) during acetylene hydrogenation has been studied. Coke formation is decreased significantly in the presence of ZnO, and a similar effect is also found when the catalysts are doped with Cr3+ instead of with Fe3+. An optimum Ni/Zn ratio for activity, selectivity and coke formation performance has been found. The existence of this maximum implies the necessity of adding ZnO to the support in order to modulate the catalytic properties of Ni. Furthermore, if the Ni concentration is increased, the conversion, selectivity, and yield to ethylene not only fails to increase, but actually decreases, while coke formation simultaneously increases. The existence of the above-mentioned optimum is the consequence of a minimum concentration of the hydrogenolytic (naked) metallic sites, the majority being hydrogenating metallic sites covered by a monolayer of ethylidines. A kinetic model of coke growth is proposed assuming the existence of two types of coke associated with the hydrogenolitic and hydrogenating sites respectively.

A FTIR spectroscopic study of surface acidity and basicity of mixed Mg, Al-oxides obtained by thermal decomposition of hydrotalcite

Abstract A hydrotalcite with a Mg:Al ratio of 2.5:1 has been synthesized and characterized. The solids obtained upon calcination of this material at 770 and 1023 K have been also characterized, and have been found to be a mixture of MgAl 2 O 4 and MgO. Surface acidity and basicity of the solid calcined at 1023 K have been assessed by FTIR monitoring of pyridine and formic acid adsorption. It has been found that the surface is not acid, although a residual acidity, larger than that in pure MgO, exists. Adsorption of formic acid takes place dissociatively on surface, isolated hydroxyl groups, contrary to previous results with MgO, where dissociative adsorption took place through interaction with exposed oxide ions.

New highly luminescent hybrid materials: terbium pyridine-picolinate covalently grafted on kaolinite.

Luminescent hybrid materials derived from kaolinite appear as promising materials for optical applications due to their specific properties. The spectroscopic behavior of terbium picolinate complexes covalently grafted on kaolinite and the influence of the secondary ligand and thermal treatment on luminescence are reported. The resulting materials were characterized by thermal analysis, element analysis, X-ray diffraction, infrared absorption spectroscopy, and photoluminescence. The thermogravimetric curves indicated an enhancement in the thermal stability up to 300 °C for the lanthanide complexes covalently grafted on kaolinite, with respect to the isolated complexes. The increase in the basal spacing observed by X-ray diffraction confirmed the insertion of the organic ligands into the basal space of kaolinite, involving the formation of a bond between Al-OH and the carboxylate groups, as evidenced by infrared spectroscopy. The luminescent hybrid material exhibited a stronger characteristic emission of Tb(3+) compared to the isolated complex. The excitation spectra displayed a broad band at 277 nm, assigned to a ligand-to-metal charge transfer, while the emission spectra presented bands related to the electronic transitions characteristic of the Tb(3+) ion from the excited state (5)D(4) to the states (7)F(J) (J=5, 4, and 3), with the 4→5 transition having high intensity with green emission.

Hybrid materials prepared by interlayer functionalization of kaolinite with pyridine-carboxylic acids

This paper presents the results of the functionalization of Brazilian São Simão kaolinite with pyridine-2-carboxylic and pyridine-2,6-dicarboxylic acids. The functionalization involved refluxing of the pyridine-carboxylic acid in the presence of kaolinite previously intercalated with dimethyl sulfoxide; both acids effectively displaced dimethyl sulfoxide from the clay interlayer. The resulting materials were characterized by X-ray diffraction, thermal analysis, infrared absorption spectroscopy, and C and N elemental analysis. The X-ray diffractograms revealed the incorporation of the acid molecules into the interlayer space of kaolinite. The thermogravimetric curves of the kaolinite samples functionalized with the pyridine-carboxylic acids indicated that the materials were thermally stable up to 300 degrees C. The displacements of the bands due to interlayer hydroxyls in the infrared absorption spectra also confirmed the functionalization of the kaolinite with the pyridine-carboxylic acids.

Thermal behaviour of Zn–Cr layered double hydroxides with hydrotalcite-like structures containing carbonate or decavanadate

Hydrotalcite-like compounds containing ZnII and CrIII in the brucite-like layers have been prepared, containing carbonate or decavanadate in the interlayer, with the formulae [Zn0.67Cr0.33(OH)2](CO3)0.165· 1.34H2O and [Zn0.67Cr0.33(OH)2](CO3)0.165· 1.34H2O, respectively. The samples have been characterized by XRD, XAS, UV-VIS, FTIR and Raman spectrocopies, while the specific textuies of the samples were assessed by nitrogen adsorption, and reducibilities were studied by temperature-programmed reduction. A similar characterization study has been carried out on samples obtained after calcination of the parent samples in air at increasing temperatures. Thermal decomposition of the ZnCr hydrotalcite containing carbonate leads to oxidation of CrIII to CrO42– species at intermediate calcination temperatures, ZnII cations then retaining their original octahedral coordination; at higher calcination temperatures ZnO and ZnCr2O4 are formed. However, the presence of interlayer decavanadate species inhibits the CrIII→ CrO42– oxidation during thermal decomposition of the vanadate-containing hydrotalcite, owing to preferential formation of vanadates. In this case the final product is a mixture of Zn pyrovanadate, Zn2V2O7, and spinel ZnCr2O4, together with a phase consisting of a mixture of partially depolymerized decavanadate to β-VO3– chains, together with monomeric VO43– species.

Surface and textural properties of hydrotalcite-like materials and their decomposition products

Abstract Layered double hydroxides (LDHs) with the hydrotalcite-like (HT) structure, and the formula [Mg 1-x Al x (OH) 2 ] (CO 3 ) x/2 .nH 2 O, have been synthetised following the constant pH method. Samples have been submitted to hydrothermal treatment (samples I) at 400 K or left under room conditions (samples 0) for 7 days. X-ray diffraction, thermal analysis, FT-IR spectroscopy and transmission electron microscopy results confirm the HT-like structure. Surface texture assessment by nitrogen adsorption at 77 K indicates type II nitrogen-adsorption isotherms with a narrow hystheresis cycle, due to pores open on both ends, but is not recorded in the isotherms of the solids obtained upon calcination of the samples at increasing temperatures. This treatment increases the specific surface area up to ca. 85 m 2 /g when the samples are calcined at 1100 K, where X-ray diffraction data indicate formation of MgAl 2 O 4 and MgO. Such a change in the specific surface area (specially upon calcining at moderate temperatures) is even marked in the hydrothermally-treated sample.