Marta Santiago

Metal-substituted hexaaluminates for high-temperature N2O abatement

Metal-substituted hexaaluminates are highly active, selective, stable, and inexpensive catalytic materials for high-temperature N2O abatement in the chemical industry, such as in nitric acid and caprolactam plants, and in combustion processes.

In situ studies during thermal activation of dawsonite-type compounds to oxide catalysts

The thermal decomposition of metal-substituted ammonium aluminium carbonate hydroxide (AACH) materials with dawsonite-like structure has been studied by using in situ XRD, FT-IR, TGA, and TPD-MS in the temperature range 298–1573 K. The in situ approach enables accurate monitoring of the decomposition process and the nature and stability of the resulting oxide phases. This information is essential to optimize the thermal activation of these versatile materials for subsequent catalytic applications. Pure AACH and the corresponding substituted systems with La and/or transition metals (Fe or Mn) were synthesized by co-precipitation using the carbonate route. In situ XRD indicated the destruction of the AACH phase at 473 K, independent of the composition of the material, leading to an amorphous alumina phase. This transition temperature is in excellent agreement with FT-IR, TGA, and TPD-MS data. The later techniques substantiate the one-step removal of H2O, NH3, and CO2 within a narrow temperature range. In pure AACH, amorphous alumina is transformed into α-Al2O3 at 1373 K, while La-containing samples display the hexaaluminate structure above 1423 K. Incorporation of transition metals into the Al and La–Al systems promotes the formation of α-Al2O3 and hexaaluminate phases at significantly lower temperatures.

In-line dispersion–precipitation method for the synthesis of metal-substituted dawsonites. Genesis of oxide materials with superior properties

Thermal activation of metal-substituted dawsonite-type compounds synthesized by a novel in-line dispersion–precipitation (ILDP) method leads to oxide materials with superior properties of component dispersion, porosity, thermal stability, and improved catalytic performance.

Perturbing the properties of layered double hydroxides by continuous coprecipitation with short residence time

A previous work (S. Abello and J. Perez-Ramirez, Adv. Mater., 2006, 18, 2436) revealed an unanticipated variation in the textural properties of Mg–Al hydrotalcite, prepared by continuous coprecipitation with short residence time, τ = 1 s which, at that time, was not fully understood. Herein, we report the generalisation of such variation in physical properties to layered double hydroxides (LDHs) of different composition (Ni–Al, Mg–Al, and Mg–Fe hydrotalcite-like compounds). In particular stable colloidal suspensions and, on drying, impervious LDH particles have been prepared using the in-line dispersion precipitation (ILDP) method with τ = 1 s. This is thought to be a consequence of variation in the mechanism of inter-crystallite interactions with decreasing crystallite size. The resulting materials are characterised using multiple techniques and are compared to analogous materials attained at longer residence times (τ = 12 s). We show that despite the apparent compositional similarity and structural isomorphicity of the precipitates, their textural and morphological properties and their thermal stability differ strongly. Thermal activation of the LDHs, however, resulted in the development of comparable textural properties in the corresponding oxides, independent of the residence time.

Assembly of a hierarchical zeolite-silica composite by spray drying

We report a one-step spray drying synthesis of a multimodal composite based on a polymer-templated silica matrix enclosing a mesoporous ZSM-5 zeolite. The first level of porosity, comprising the silica matrix with 13–25 nm mesopores, is generated by evaporation-induced assembly of the silica nanoparticles together with the non-ionic surfactant using spray drying. The mesoporous zeolite contributes with micropores (0.56 mm) and intracrystalline mesopores (∼7 nm) introduced by controlled alkaline leaching. The nanostructured composite, featuring agglomerated zeolite microgranules uniformly covered by the silica, displayed tuneable properties of the matrix mesoporosity and particle size by changing both the silica source and the spray drying conditions. The structural and acidic properties of the zeolite phase were not altered by the assembly method. The technique was successfully applied to other zeolite structures (USY, mordenite and ferrierite) with no impact in the characteristics of the matrix. This fast, versatile, reproducible, economical and scalable technique represents a powerful strategy for creating hierarchically structured materials with adjustable micro-, meso- and macroporous properties and enhanced agglomeration.