Ascención Montoya

Comparison between sol-gel, coprecipitation and wet mixing synthesis of ZnAl2O4

ZnAl2O4 was prepared by hydrolyzing a mixture of aluminum alkoxide with zinc nitrate dissolved in hexylene glycol and calcining at 800°C. The results are compared with those obtained by wet mixing and coprecipitation. The sol-gel method produces solids whose surface areas and pore volumes are 100% larger and with a more homogeneous pore size distribution.

Fly ash from a Mexican mineral coal. II. Source of W zeolite and its effectiveness in arsenic (V) adsorption

Coal-fired plants in Coahuila (Mexico) produce highly reactive fly ash (MFA), which is used in a one-step process as a raw material in producing zeolite. We explored two routes in the synthesis of zeolite: (a) direct MFA zeolitization, which resulted in the formation of W zeolite with KOH and analcime with NaOH and (b) a MFA fusion route, which resulted in the formation of zeolite W or chabazite with KOH and zeolite X or P with NaOH. No residual crystalline phases were present. When LiOH was employed, ABW zeolite with quartz and mullite were obtained. For both zeolitization routes, the nature of the alkali (KOH, NaOH, LiOH), the alkali/MFA ratio (0.23-1.46), and the crystallization temperature and time (90-175 degrees C; 8-24 h) were evaluated. Additionally, the effect of temperature and time on MFA fusion was studied. W zeolite was obtained by both zeolitization methods. The direct route is preferred because it is a straightforward method using soft reaction conditions that results in a high yield of low cost zeolites with large crystal agglomerates. It was demonstrated that aluminum modified W zeolite has the ability to remove 99% of the arsenic (V) from an aqueous solution of Na(2)HAsO(4).7H(2)O originally containing 740 ppb.

Oxidative dehydrogenation of 1-butene over ZnAl ferrites

Abstract The aluminum introduction into the zinc ferrite ZnFe 2− x Al x O 4 in the range 0.0 ≤ x ≤ 1.0 was studied. These ferrites were calcined at two temperatures, 550°C and 750°C. X-ray diffraction (XRD) patterns showed that they have a spinel structure with Al 3+ replacing Fe 3+ in octahedral sites. The crystallite size decreased as the aluminum introduction increased. At high calcination temperature, sintering and segregation processes occur. Linear correlations were found between XRD and Mossbauer parameters. Mossbauer spectra showed that the symmetry of the electron distribution decreased around the nucleus of Fe 3+ , as the aluminum content increased. Such asymmetrical distribution of the electron density may be associated to the spatial accommodation of aluminum in the neighboring octahedral sites. Oxygen atoms turn out to be more basic due to the charge transfer from Fe 3+ to O 2− in the FeO bond. Hence the acid-base type dissociation of the CH bond for the n -butenes activation is favored. The highest yield of butadiene was obtained for the samples treated at 750°C whereas for the samples calcined at 550°C, double bond isomerization of 1-butene occurs in competition with the oxidative dehydrogenation.

Preparation of Ni/ZrO2–SO42− catalysts by incipient wetness method: effect of nickel on the isomerization of n-butane

Abstract A series of Ni-promoted sulfated zirconia catalysts with different nickel concentration (from 1 to 9.6 wt.%) were prepared by incipient wetness method. Ni and SO 4 2− promoters were co-impregnated to a parent zirconium hydroxide by a solution of Ni(NO 3 ) 2 ·6H 2 O in H 2 SO 4 . After calcination at 948 K, the solids isomerized n -butane at 338 K. Up to 4.5 wt.% Ni content, nickel increases activity of ZrO 2 –SO 4 2− , afterwards, the catalytic activity decreases. The temperature-programmed desorption of ammonia (TPD-NH 3 ) and IR of pyridine adsorbed results show that enhanced activity cannot be completely explained in function of a higher acid strength. The increase of the isomerizating activity is better explained in terms of a bimolecular mechanism, as proposed by Guisnet et al. [M.R. Guisnet, Acc. Chem. Res. 23 (1990) 392], involving olefins as intermediates. In this mechanism, Ni causes an enhancement in the surface concentration of olefins. In spite of the relatively high Ni concentration, X-ray diffraction results showed no evidence of any NiO phase due to this oxide is well-dispersed on the surface of ZrO 2 –SO 4 2− in form of small particles. The inhibition of isomerizating properties of the catalysts when hydrogen was present in the reactor feed confirmed this bimolecular mechanism. Interestingly, unpromoted ZrO 2 –SO 4 2− exhibited also the usual induction period observed on nickel-promoted sulfated zirconia catalysts. Then, this bimolecular mechanism for the n -butane isomerization could also apply in the unpromoted zirconia sulfate catalyst.

Synthesis and characterization of MTiO3(M = Mg, Ca, Sr, Ba) sol-gel

Multiple oxide titanates have been prepared via the sol-gel method by the reaction of an alcoholic solution of titanium n-butoxide, Ti(OC4H9)4, with nitrate solutions, M(NO3)2(M = Mg, Ca, Sr, Ba), at room temperature. The amorphous gels so obtained under acidic conditions were calcined at temperatures between 700 and 900 °C to obtain high-purity, crystalline MTiO3 powders. The samples were characterized by powder X-ray diffraction (XRD), scanning (SEM) and energy-dispersive electron microscopy, thermogravimetric (TG) and differential thermal analysis (DTA) and BET surface area measurements. The stability and large surface area obtained for the Sr and Ba compounds make them suitable for use as catalyst supports.

Oxidative dehydrogenation of 1 -butene to butadiene on α-Fe2O3/ZnAl2O4 and ZnFexAl2-xO4 catalysts

The oxidative dehydrogenation of 1-butene to butadiene was studied over a series of catalysts of iron impregnated on ZnAl2O4 used as a support and also Fe coprecipitated with zinc and aluminum in order to obtain ZnFexAl2-xO4 type catalysts. Results were compared with bulk α-Fe2O3, ZnAl2O4 and ZnFe2O4. X-ray diffraction (XRD) and Mössbauer spectroscopy suggest that in the impregnated catalysts, Fe ions are in strong interaction with the support. These samples have higher butadiene selectivity than coprecipitated ZnFexAl2-xO4 catalysts, in which iron is incorporated into the ZnAl2O4 spinel network. Results suggest also that iron content has a greater effect on butadiene selectivity than the zinc aluminate-iron oxide interaction.

Role of bentonite clays on cell growth.

Bentonites, naturally occurring clays, are produced industrially because of their adsorbent capacity but little is known about their effects on human health. This manuscript reports on the effect of bentonites on cell growth behaviour. Bentonites collected from India (Bent-India), Hungary (Bent-Hungary), Argentina (Bent-Argentina), and Indonesia (Bent-Indonesia) were studied. All four bentonites were screened in-vitro against two human cancer cell lines [U251 (central nervous system, glioblastoma) and SKLU-1 (lung adenocarcinoma)] supplied by the National Cancer Institute (USA). Bentonites induced growth inhibition in the presence of U251 cells, and growth increment in the presence of SKLU-1 cells, showing that interactions between bentonite and cell surfaces were highly specific. The proliferation response for U251 cells was explained because clay surfaces controlled the levels of metabolic growth components, thereby inhibiting the development of high-grade gliomas, particularly primary glioblastomas. On the other hand, the proliferation response for SKLU-1 was explained by an exacerbated growth favoured by swelling, and concomitant accumulation of solutes, and their hydration and transformation via clay-surface mediated reactions.

Lipid peroxidation and cytotoxicity induced by respirable volcanic ash.

This paper reports that the main component of respirable volcanic ash, allophane, induces lipid peroxidation (LP), the oxidative degradation of lipids in cell membranes, and cytotoxicity in murin monocyle/macrophage cells. Naturally-occurring allophane collected from New Zealand, Japan, and Ecuador was studied. The quantification of LP was conducted using the Thiobarbituric Acid Reactive Substances (TBARS) assay. The cytotoxic effect was determined by the 3-[4,5-dimethylthiazol-2-yl]-2,5-diphenyltetrazolium bromide colorimetric assay. Electron-Paramagnetic Resonance (EPR) determinations of naturally-occurring allophane confirmed the incorporation in the structure and clustering of structural Fe(3+), and nucleation and growth of small-sized Fe (oxyhydr)oxide or gibbsite. LP induced by allophane varied with time, and solid concentration and composition, reaching 6.7 ± 0.2 nmol TBARS mg prot(-1). LP was surface controlled but not restricted by structural or surface-bound Fe(3+), because redox processes induced by soluble components other than perferryl iron. The reactivity of Fe(3+) soluble species stemming from surface-bound Fe(3+) or small-sized Fe(3+) refractory minerals in allophane surpassed that of structural Fe(3+) located in tetrahedral or octahedral sites of phyllosilicates or bulk iron oxides. Desferrioxamine B mesylate salt (DFOB) or ethylenediaminetetraacetic acid (EDTA) inhibited LP. EDTA acted as a more effective inhibitor, explained by multiple electron transfer pathways. Registered cell-viability values were as low as 68.5 ± 6.7%.

Mössbauer study of supported Pt—Sn

Pt—Sn supported on alumina was characterized before and after treatment with hydrogen by Mössbauer spectroscopy and X-ray diffraction. For the calcined sample tin is present as SnO2 and platinum as metal. After reduction with hydrogen, Sn(IV), Sn(II), Sn(0), Pt, Pt3Sn, PtSn alloys are formed. SnO interacts strongly with the support.

Influence of the synthesis parameters on textural and structural properties of MCM-41 mesoporous materials

In the search of catalysts with outstanding properties for treatment of heavy oil fractions, MCM-41 type silicoaluminates are of interest because their textural properties. MCM-41 mesoporous materials were synthesized under different hydrothermal conditions using organic templates, i.e. CTMA-Br, DTMA-Br. Reaction temperature (120 to 195 C) and reaction time (24 to 120 hr) varied and their influence on the structure properties of MCM-41 was sought i.e. d = 36 to 41.6 {angstrom} for distinct micelles diameters. Also, insertion of polyaromatics into the surfactant micelles was determinant in the final pore diameter, i.e. d = 35.4 to 48.6 {angstrom}.