J.L. Sotelo

Anomalous crystallization mechanism in the synthesis of nanocrystalline ZSM-5

Abstract Nanocrystalline ZSM-5 (crystal size in the range 10–100 nm) has been synthesized in only 24 h by hydrothermal crystallization of clear supersaturated homogeneous synthesis mixtures. The influence of the aluminum source, aging time, pH, water content and presence of alkaline cations on the properties of the final nanocrystalline zeolite have been studied. Samples with different degrees of crystallinity have been prepared and characterized by X-ray fluorescence, X-ray diffraction, solid state MAS-NMR, transmission electron microscopy and nitrogen adsorption (BET) in order to investigate the crystallization mechanism. The process involves the formation of an initial amorphous solid prior to the zeolite crystallization, which is gradually transformed into nanocrystalline ZSM-5 through solid–solid transformations. Additionally, the conventional formation of ZSM-5 from crystalline nuclei generated in the remaining liquid solution is observed after 48 h.

Henry's law constant for the ozone-water system

Abstract Ozone absorption in water in the presence of several salts has been studied in an agitated tank. The influence of the operating variables: ionic strength, temperature, pH, gas flow rate, ozone partial pressure and agitation speed, on the dissolved ozone concentration has been investigated. From an ozone balance in the reactor, the liquid phase volumetric mass transfer coefficient and the equilibrium ozone concentration were calculated. Finally, equations for the Henrys law constant as a function of temperature, pH and ionic strength for every salt have been deduced. These equations reproduce the experimental results with deviations less than ±15%. The equations allow us to know the ozone solubility for a given operating condition.

Adsorption of pharmaceutical compounds and an endocrine disruptor from aqueous solutions by carbon materials

Adsorption has been used to study the removal of atenolol, caffeine, diclofenac and isoproturon, pharmaceutical compounds as emerging contaminants and an endocrine disruptor from ultrapure water and a municipal wastewater treatment plant effluent with three carbonaceous materials: activated carbon, multiwalled carbon nanotubes and carbon nanofibers. The adsorption capacities were studied in the temperature range of 25–65°C and pH range from 3 to 9. Several model isotherms were used to model the adsorption equilibrium data. Also, the competitive adsorption was evaluated.

Surface modified amorphous titanosilicate catalysts for liquid phase epoxidation

Abstract Titanium-containing mesoporous silica has been prepared by silylation of mixed oxides obtained via sol–gel method. These samples displayed enhanced activity in the epoxidation of styrene with Bu t OOH in comparison to normal xerogels or other titanium-containing catalytic systems. The influence of the extension of the silylation reaction, as well as the use of different silylating agents, have been investigated. Samples were characterised by FTIR, XRF, 29 Si-MAS NMR, diffuse reflectance UV–Vis and N 2 adsorption–desorption isotherms, X-ray photoelectron spectroscopy (XPS) and FTIR of adsorbed deuteroacetonitrile. The improvement in the catalytic activity achieved has been related to the removal of OH groups by means of calcination or silylation treatments.

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.

Toluene disproportionation over ZSM-5 zeolite: Effects of crystal size, silicon-to-aluminum ratio, activation method and pelletization

Abstract The catalytic activity and selectivity of ZSM-5 zeolite in toluene disproportionation were studied as a function of crystal size, silicon-to-aluminum ratio, acidity, activation method and pelletization of zeolite. Although para -xylene formation was slightly favoured by increasing the crystal size, high para -selectivity was not achieved. At the lowest silicon-to-aluminum ratio, catalytic activity was enhanced with a drop on the selectivity to disproportionation, suggesting the presence of “super acid sites” related with extra lattice aluminum species. From IR spectroscopy measurements with adsorption/desorption of pyridine it was observed that during the activation step, a high H + concentration in the cation exchange increases the strength of Brensted sites, increasing the number of acid sites which are active in toluene disproportionation. Subsequent calcination completes zeolite activation, increasing the number of Bronsted acid sites active in that reaction. In the pelletization there is a cation exchange between Na + from the montmorillonite and H + from the zeolite when both are mixed and suspended in water. Nevertheless, sodium montmorillonite proved to be a suitable binder of ZSM-5 provided that the catalysts are reactivated after pelletization.

Carbon dioxide/methane separation by adsorption on sepiolite

In this work, the use of sepiolite for the removal of carbon dioxide from a carbon dioxide/methane mixture by a pressure swing adsorption (PSA) process has been researched. Adsorption equilibrium and kinetics have been measured in a fixed-bed, and the adsorption equilibrium parameters of carbon dioxide and methane on sepiolite have been obtained. A model based on the LDF approximation has been employed to simulate the fixed-bed kinetics, using the Langmuir equation to describe the adsorption equilibrium isotherm. The functioning of a PSA cycle for separating carbon dioxide/methane mixtures using sepiolite as adsorbent has also been studied. The experimental results were compared with the ones predicted by the model adapted to a PSA system. Methane with purity higher than 97% can be obtained from feeds containing carbon dioxide with concentrations ranging from 34% to 56% with the proposed PSA cycle. These results suggest that sepiolite is an adsorbent with good properties for its employment in a PSA cycle for carbon dioxide removal from landfill gases.

Deactivation of toluene alkylation with methanol over magnesium-modified ZSM-5 Shape selectivity changes induced by coke formation

Abstract Coke deposition during toluene alkylation with methanol over a Mg-modified ZSM-5 catalyst leads to drastic changes in the zeolite shape selectivity with the time-on-stream. Besides a decrease in toluene and methanol conversions, a maximum in para selectivity is observed due to coke deposition. Nevertheless, the deactivated catalysts are less para-selective than the fresh ZSM-5 when they are compared at the same toluene conversion. Para selectivity of the primary product decreases due to coking below the value of 100% corresponding to the uncoked catalyst. At low times-on-stream, coke deposition takes place mainly by pore filling. At higher times-on-stream, the proportion of polyaromatic species in the coke deposits is increased, which leads to a partial blockage of the zeolite pore structure, lowering the length of the diffusional pathway. Therefore, the intracrystalline diffusional control on the internal xylene isomerization is less pronounced in the coked catalysts, which causes the lower para selectivity of the primary product observed in these samples.

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.