S. Mendioroz

Preparation of Al-pillared montmorillonite from concentrated dispersions

Abstract Clays may be modified by introducing hydroxycations between the layers to obtain microporous solids with catalytic or adsorption properties. This study presents the differences occurring in samples when different Al/clay ratios are applied in pillaring a montmorillonite with aluminum. XRD, FTIR, 27 Al MAS NMR, nitrogen adsorption isotherms and ammonia adsorption were applied in order to study the structure and properties of pillared materials. Besides the aluminum-forming pillars, we found a fraction of aluminum retained on the clay surface, its extent depending on the Al/clay ratio. This fraction, mainly in tetrahedral coordination, is a result of the pillaring process and especially affecting the textural properties and, to a lesser degree, the acidity. A relation between pillar density and acidity was also found.

Influence of preparation method on thermal stability and acidity of Al-PILCs

In general, clays may be modified introducing hydroxications between the constituent layers to obtain microporous solids, known as pillared clays (PILCs), useful in adsorption and catalysis. In this work, XRD, FT-IR, N2 adsorption isotherms and ammonia adsorption techniques were applied to study the changes in structure and properties provided to a montmorillonite through pillaring when different preparation methods, Al/clay ratio and drying procedures are used in the synthesis. Above a certain Al/clay ratio, pillar density is addressed by the maximum exchange capacity of the clay at the pH of the pillaring solution. Under such a situation, micro- and meso-porosity remain almost unchanged whereas macroporosity is fully dependent on the concentration of the clay suspension and more on the drying conditions of the flocculated exchanged clay layers. Thermal analysis has shown as a useful tool to understand the changes in acidity with the temperature.

Synthesis and characterization of pillared montmorillonite catalysts

Abstract Pillared montmorillonite was synthesized by the direct cation exchange method, varying the hydroxyl/aluminium and aluminium/clay ratios in the preparation in order to find the optimum conditions, within the limits of this study, for obtaining stable products with a microporous structure and high specific surface area, capable of being used as cracking catalysts. The minimum aluminium/clay ratio required was 7.5 meq Al/g clay, independent of the hydroxyl/aluminium molar ratio. The specific surface area of the original sample (61 m2/g) was increased by pillaring up to 366 m2/g. The structure of the aluminium oxide pillared montmorillonite proved to be stable up to 500°C.

Structural and textural evolution of Al- and Mg-rich palygorskites, I. Under acid treatment

Abstract The structural and textural evolution of two Spanish palygorskites, one magnesium-rich and another aluminium-rich, activated with hydrochloric acid of different concentrations, has been studied. The maximum activation is obtained in both cases with a 2 N concentration. The activation is much higher in the magnesium-rich palygorskite, which undergoes a greater dissolution of its octahedral sheet as well as a more extensive alteration of its structure and a greater increased creation of microporosity.

Structural and textural evolution under thermal treatment of natural and acid-activated Al-rich and Mg-rich palygorskites

The structural and textural evolution under thermal treatment of two Spanish palygorskites, one magnesium-rich and the other aluminium-rich, and also of the same palygorskites previously activated with HCl, has been studied. Using IR and X-ray techniques, the structural modification that the samples undergo on heating, is shown. By means of N2 adsorption-desorption isotherms and Hg porosimetry the changes in the texture were followed until 1000°C. The texture of both natural palygorskites is maintained until 600°C, despite the structural changes which they undergo. On heating at 1000°C a great decrease of surface area is observed. In samples previously activated with HCl, a greater evolution of the texture is evident. On heating above 300°C there is a sharp decrease of the surface area, owing to the disappearance of the microporosity created by acid-leaching. This decrease of the surface area is greater in the magnesium-rich sample.

Catalytic activity of pillared clays in methanol conversion

Abstract A number of pillared clay based catalysts have been used in methanol conversion and the obtained results related to the physico-chemical characteristics of the samples. Zr-PILCs samples have proven to be quite effective in dehydration to dimethyl ether and hydrocarbons due to their acidity and porosity. In special, well-pillared samples are suitable to obtain hydrocarbons. Doping with Cu does not improve dehydration due to the loss in acidity caused by Cu addition, but it renders the materials useful for methanol dehydrogenation. Cu/pillared clays have shown promising catalysts for methanol dehydrogenation to methyl-formate, despite the existence of interactions with the acid–base properties of the support, which contribute to dimethyl ether formation and degradation of methyl-formate to CO 2 , CO and formaldehyde. X-ray diffraction, chemical analysis, N 2 and pyridine adsorption and XPS helped to understand the catalytic performance of the prepared catalysts.

Nickel supported on natural silicates: Activity and selectivity in sunflower seed oil hydrogenation

Abstract Supported nickel catalysts have been prepared by two different methods on several clays. The precursors were studied by temperature-programmed reduction and subsequently reduced in the temperature range 350–500 °C. Metal areas were determined by transmission electron microscopy and X-ray diffraction. The chemical state of the nickel prior to reduction was investigated by X-ray photoelectron spectroscopy. In the reaction of triglyceride hydrogenation of sunflower seed oil, catalysts prepared by precipitation show a larger metal area and activity than those prepared by impregnation. Activity was almost independent of the nature of the support, whereas differences in the selectivity were ascribed to differences in the morphology of the support.

Al-pillared montmorillonite-based Mo catalysts: effect of the impregnation conditions on their structure and hydrotreating activity

Abstract The effects of the pH and medium of the impregnation solution on the surface structure and activity for gas oil hydrodesulfurization (HDS) of molybdenum catalysts supported on an Al-pillared montmorillonite have been studied. Catalysts were characterized by using X-ray diffraction, nitrogen and ammonia sorption measurements, temperature-programmed reduction, Raman spectroscopy, and X-ray photoelectron spectroscopy. It was found that impregnation at basic pH leads to the incorporation of molybdenum mainly in form of monomeric and polymeric species, with a substantial part of Mo species in the interlayer space of the Al-pillared clay, probably in interaction with the alumina pillars. Whereas, formation of small particles of bulk MoO3 deposited on the external surface of the Al-pillared clay was favoured by impregnation at acid pH. The impregnation at pH 8.5 in a H2O2 solution produced abundant formation of incipient MoO3 crystallites relatively well dispersed, partially located inside the porous system. The catalysts prepared at basic pH values had slightly higher HDS activity than those prepared at acid pH values.

Mercury elimination from gaseous streams

Abstract The elimination of mercury vapours from gaseous streams has been studied using sulfurized silicates as adsorbents. A new method to sulfurize the materials based in hydrogen sulfide oxidation at low temperature is proposed. The sulfur produced in the reaction is deposited on the material, its dispersion depending on the process temperature and physico-chemical properties of the silicates used. The adsorbents were tested in static and dynamic systems for mercury depuration of gaseous streams. Commercial sulfurized active carbon was used as a reference. X-ray diffraction, atomic absorption spectroscopy, X-ray photoelectron spectroscopy, nitrogen adsorption-desorption and mercury intrusion porosimetry were used in raw materials and adsorbents characterization.

Kinetic study of the Claus reaction at low temperature using γ-alumina as catalyst

Abstract A kinetic study of the oxidation on H 2 S on γ-alumina by SO 2 following a modified Claus process at low temperature (373–473 K) was carried out. To the analytical data, at varying SO 2 and H 2 S concentrations, several Langmuirian type expressions for the reaction rate were applied. The models were submitted to statistical and physical criteria to establish the best fitting of the experimental results. The resulting kinetic expression applied to sulphurized surfaces shows the chemical reaction between adsorbed species—one of them H 2 S dissociatively adsorbed—as the controlling step of the process. The reaction was also effected under integral regime, the obtained results confirming the validity of the proposed kinetic expression.