Gabriel Ovejero

Adsorption of anionic and cationic dyes on activated carbon from aqueous solutions: equilibrium and kinetics.

Activated carbon was utilized as adsorbent to remove anionic dye, Orange II (OII), and cationic dye, Methylene blue (MB), from aqueous solutions by adsorption. Batch experiments were conducted to study the effects of temperature (30-65 degrees C), initial concentration of adsorbate (300-500 mg L(-1)) and pH (3.0-9.0) on dyes adsorption. Equilibrium adsorption isotherms and kinetics were investigated. The equilibrium experimental data were analyzed by the Langmuir, Freundlich, Toth and Redlich-Peterson models. The kinetic data obtained with different carbon mass were analyzed using a pseudo-first order, pseudo-second order, intraparticle diffusion, Bangham and Chien-Clayton equations. The best results were achieved with the Langmuir isotherm equilibrium model and with the pseudo-second order kinetic model. The activated carbon was found to be very effective as adsorbent for MB and OII from aqueous solutions.

Preparation of TS-1 by wetness impregnation of amorphous SiO2—TiO2 solids: influence of the synthesis variables

Abstract TS-1 has been synthesized by wetness impregnation of amorphous SiO 2 TiO 2 solids with TPAOH solutions and subsequent crystallization under autogenous pressure, the influence of the different variables involved being studied. The SiO 2 TiO 2 cogel used as raw material has been prepared following a two step (acid-base) sol-gel process which leads to the formation of Si O Ti bonds previously to the zeolite crystallization. The titanium content of TS-1 can be controlled through the variation of the raw SiO 2 TiO 2 composition, although a limit of the Ti/(Ti + Si) × 100 molar ratio around 2.3% in titanium effectively incorporated to the zeolite framework has been observed, even when starting from cogels with low Si/Ti ratios. The size and morphology of the crystals depend on the synthesis variables, small crystallites with 0.2 μm of diameter being obtained at high TPAOH concentration. The kinetics of TS-1 crystallization by this method is very fast in spite of taking place in static conditions. Thus, pure TS-1 can be achieved at 170°C in just 6 h of synthesis. All TS-1 samples were tested as catalysts for n-hexane oxyfunctionalization, high activities and selectivities being obtained with most of them.

Evidence of solid-solid transformations during the TS-1 crystallization from amorphous wetness impregnated SiO2TiO2 xerogels

Abstract The crystallization mechanism of TS-1 has been investigated when this Ti-containing material is obtained from amorphous SiO2TiO2 solids impregnated with TPAOH solutions followed by thermal treatment under autogenous pressure. Samples with different degrees of crystallinity have been prepared and characterized by elemental analysis, X-ray diffraction (XRD), infrared (IR) and diffuse reflectance ultraviolet visible (DR UV-Vis) spectroscopies, scanning and transmission electron microscopy (SEM and TEM), n-hexane adsorption and catalytic tests of n-hexane oxyfunctionalization. In contrast to the classical mechanism proposed to describe the zeolite synthesis through a mediated solution process, in the system studied here the TS-1 crystallization is mainly governed by solid-solid transformations, and a true crystal growth step is not observed. The major role of the solution is to provide the TPA+ species necessary to complete the solid crystallization and to favour the migration of the secondary particles. During the first stages of the synthesis, a small amount of the raw material is dissolved yielding soluble Ti-rich species ( Si Ti = 5 ), which are no longer involved in the crystallization. Simultaneously, the starting polymeric SiO2TiO2 cogel is converted into a particulate amorphous material formed by a tight packing of primary particles with sizes around 50 nm. In a subsequent step, the latter are aggregated leading to secondary particles that become independent of the cogel as they reach a critical diameter of 0.5–0.7 μm. Once individualized, the secondary particles undergo a zeolitization process, being gradually transformed into TS-1 crystals of approximately the same size. In addition, the coordination of Ti sites become tetrahedral as the crystallization progresses due to the higher connectivity between Ti and Si atoms existing in the TS-1 network compared to the raw amorphous xerogel.

Synthesis of TS-1 by wetness impregnation of amorphous SiO2TiO2 solids prepared by the sol-gel method

Abstract TS-1 has been synthesized following a method based on the wetness impregnation with a template solution of amorphous SiO2TiO2 solids followed by crystallization under autogenous pressure. The preparation of the raw SiO2TiO2 solids has been carried out by different two-step sol-gel processes in order to achieve the formation of SiOTi bonds prior to the zeolite synthesis. The properties of the TS-1 samples obtained strongly depend on the method used to prepare the starting amorphous cogel. All TS-1 samples showed high catalytic activity for n-hexane oxyfunctionalization with H2O2. X-Ray diffraction (XRD), infrared (IR) and catalytic measurements evidence the Ti incorporation into the zeolite framework. Diffuse reflectance ultraviolet-visible (DR UV-VIS) spectroscopy shows that the Ti atoms are occupying tetrahedral positions in the TS-1 lattice, whereas bulk anatase and extraframework Ti species are not detected. Thermogravimetric analysis (TGA) and n-hexane adsorption measurements confirm the high crystallinity of the material obtained by this procedure. The size of the crystallites ranges between 0.4 and 5 μm depending on the method used to prepare the raw SiO2TiO2 cogel. When compared to the conventional procedures of TS-1 preparation by hydrothermal crystallization of a liquid gel, the method studied here is simpler, requires a lower reaction volume and proceeds with shorter synthesis time. In addition, TS-1 prepared by wetness impregnation of SiO2TiO2 cogels exhibits properties similar to those of the material obtained following the recipe of the original TS-1 synthesis patent.

Adsorption of lindane from water onto GAC: effect of carbon loading on kinetic behavior

Abstract Batch kinetic experiments of adsorption of lindane in dilute aqueous solution onto F-400 granular activated carbon (GAC) with different carbon weight and initial concentration were analyzed. Concentration decay of lindane was measured by means of SPME (solid phase micro extraction). A high level of energetic and structural heterogeneity of the F-400 activated carbon was observed by means of TG analysis. An adsorption model including film-, macropore-, and micropore-diffusion based on the branched pore model was fitted to experimental data. The effect of carbon loading on model parameters can be summarized as follows: (i) the effective macropore diffusivity, including pore and surface diffusivity, and the micropore rate coefficient have little dependence on carbon loading, and (ii) the fraction of adsorptive capacity in macropores depends strongly on carbon loading, which is attributed to the difference in adsorptive affinity in macro- and micropores.

Synthesis and characterisation of iron-containing SBA-15 mesoporous silica

Mesoporous silica SBA-15 molecular sieve has been synthesised and modified with Fe via a post-synthesis procedure by reacting as-made SBA-15 with different iron precursors in dry ethanol and followed by calcination. The influence of variables such as the nature of the starting silica support and the iron source (FeCl 3 6H 2 O and Fe (O-CH 2 CH 3 ) 3 in absolute anhydrous conditions) on the grafting of metallic species into SBA-15 materials has been studied. Likewise, direct synthesis by co-condensation of Fe chloride with silica species templated with Pluronic 123 has been also checked. Different strategies of synthesis lead to significant changes in the bonding and environment of iron species within the silica materials. These differences have been monitored by means of spectroscopic techniques such as: PAS-FTIR, DRUV-VIS, 29 Si NMR and EPR.

Adsorption of dyes on carbon nanomaterials from aqueous solutions.

The removal of methylene blue (MB), a cationic dye and orange II (OII), an anionic dye, from aqueous solution by using carbon nanomaterials as multiwalled carbon nanotubes (MWNTs) and carbon nanofibers (CNF) as adsorbents was studied in batch experiments. The effect of pH, temperature and surface modification of adsorbent on the removal of MB and OII was also investigated. The removals of OII and MB by adsorption on MWNT were maximum at pH 3.0 and pH 7.0, respectively. However, in the case CNF was employed as adsorbent, the optimum values of pH were 9.0 and 5.0 for OII and MB, respectively. Langmuir and Freundlich isotherms are applied to fit the adsorption data of both dyes. Equilibrium data were well described by the typical Langmuir adsorption isotherm. Overall, the study demonstrated that MWNTs and CNFs can effectively remove cationic and anionic dyes as MB and OII from aqueous solutions under these experimental conditions.

TS-2 synthesis from wetness-impregnated SiO2-TiO2 xerogels

TS-2 has been synthesized from SiO 2 -TiO 2 xerogels by wetness impregnation with TBAOH solutions and subsequent autoclaving at 170°C. The preparation of the raw materials through sol-gel methods leads to a stabilization of the Ti atoms by dispersion in isolated positions of the amorphous SiO 2 network, which prevents the formation of undesired TiO 2 phases during both gel and zeolite synthesis. Additional advantages of this method compared to the conventional TS-2 preparations are the use of lower amounts of TBAOH and smaller reactor volumes as well as the higher zeolite yields that are obtained. Characterization of the samples obtained by i.r., DR u.v.-vis, and n-hexane oxyfunctionalization measurements confirms that the Ti species are located in tetrahedral positions of the MEL framework, whereas extraframework Ti is not detected, t.g., n-hexane and N 2 adsorption, and TEM measurements reveal the presence of a relatively high external surface and a secondary mesoporosity in these samples. This mesoporosity is originated by the assembly of small crystallites (0.03 μm width) to yield zeolite macroparticles (1 μm). TS-2 prepared by wetness impregnation exhibits catalytic properties for the oxyfunctionalization of n -hexane with H 2 O 2 similar to those of the conventional TS-2. The TS-2 crystallization under these conditions takes place through a nonconventional mechanism governed mainly by solid-solid transformations, leading to a reordering of the amorphous starting solid into the TS-2 crystalline structure.

Synthesis of titanium silicalite-1 from an SiO2–TiO2 cogel using a wetness impregnation method

An alternative to the conventional methods of TS-1 synthesis is based on the wetness impregnation of an SiO2–TiO2 cogel, which leads to more effective Ti incorporation in a simpler way.

Crystallization of TS-1 and TS-2 zeolites with contribution of solid–solid transformations

The synthesis of TS-1 and TS-2 zeolites from SiO2–TiO2 xerogels wetness impregnated with solutions of the structure-directing agent proceeds mainly through solid–solid transformations which allow the raw amorphous network to be reordered into crystalline frameworks.