M.L. Rojas-Cervantes

Study of oxygen-containing groups in a series of graphite oxides: Physical and chemical characterization

Some graphite oxides with different degrees of oxidation have been prepared by using a modified Staudenmaier method. The oxidation process has been studied by using different techniques: elementary analysis, gas adsorption, X-ray diffraction, FT-IR, XPS and 13C-NMR. The influence of the oxidation degree on the appearance and evolution of new chemical groups in the samples has been analyzed. The oxidation process produces a development of porosity, especially microporosity. FT-IR, XPS and 13C-NMR are complementary techniques, which confirm the existence of hydroxyl, carbonyl, ether, epoxy, and peroxy groups in the more oxidized samples. These surface groups may play an important role in the acid-base bifunctional catalysis.

Characterization of basic sites of alkaline carbons by Knoevenagel condensation

The characterization of the carbon surface basicity by condensation of benzaldehyde with ethyl cyanoacetate, ethyl acetoacetate, diethyl malonate and ethyl bromoacetate as test reactions were studied with a series of lithium, sodium, potassium and cesium exchanged activated carbon (Norit RX 1 Extra). The importance of active sites and the measurement of the surface area in determining the reactivity of the carbons is emphasized. The activity of the carbons increases when the ionic radius of the alkali cation increases; the activity increases in the order Li < Na < K < Cs. Under reaction conditions, it was found that most of the basic sites in alkaline carbons have 10.7 ≤pK a ≤ 13.3 and there are a few sites in the range of 13.3 ≤ pKa ≤ 16.5. The treated activated carbon that had not been subjected to exchange by alkali metal cations exhibited mostly basic sites able to abstract protons with 9 ≤ pKa ≤ 10.7.

Ultrasound enhanced reactions involving activated carbons as catalysts: synthesis of α,β-unsaturated nitriles

Abstract α,β-Unsaturated nitriles have been synthesized by sonochemical activated reactions of carbonylic compounds with malononitrile using two basic carbons (Na+– and Cs+–Norit) as catalysts. The catalysts were characterized by thermal analysis, X-ray photoelectron spectroscopy and nitrogen adsorption isotherms. Under the experimental conditions, unsaturated nitriles can be prepared with a high activity and selectivity. The role of alkaline promoters (Na+ and Cs+) in the carbon has been studied. It is evidenced that the greater the basicity and the amount of catalyst, the higher is the conversion. For comparison, the condensation reaction has also been carried out in a batch reaction system and the influence of the temperature in both (batch and ultrasonic) systems has been studied.

Preparation of xTiO2·(1 − x)Al2O3 catalytic supports by the sol-gel method: physical and structural characterisation

A series of xTiO2·(1 − x)Al2O3 mixed oxides has been prepared by the sol-gel method with variable amounts of TiO2, from pure alumina to pure titania. Textural, bulk and surface characterisation of the samples has been carried out by nitrogen physisorption (SBET, porosity), thermal analysis (TGA, DTA), X-ray diffraction (XRD), zero point charge (ZPC) and surface acidity. The textural results show that at low titania contents, higher surface areas than those of pure alumina are obtained, and in the titania-rich samples, higher surface areas than for pure titania are stabilised. The titanium content also affects the crystallization process. Furthermore, the strength and distribution of the acid sites may be varied by changing the composition of the mixed oxide.

Sonocatalysis and Basic Clays. Michael Addition Between Imidazole and Ethyl Acrylate

Under ultrasound activation, imidazole was condensed with ethyl acrylate using two basic clays (Li+ and Cs+ montmorillonites). The clay catalysts were characterized by chemical analyses, nitrogen adsorption isotherms, and X-ray diffraction. Under sonochemical conditions N-substituted imidazole can be obtained with a high activity and selectivity. The conversion increases with the basicity of the clays and the ultrasonic exposition time. The yield presents a maximum for 0.1 g of Cs+ montmorillonite after 60 min of sonication.

Some strategies to lower the production cost of carbon gels

The carbon gels obtained by polymerization of resorcinol (R) and formaldehyde (F) are very attractive materials because a great variety of porous structures to be fitted in specific applications can be tailored by modifying the synthesis conditions. However, the application of RF carbon gels at industrial scale is principally limited by their high cost and by the long duration of the synthesis process. This article shows the strategies applied at different stages of the global process, synthesis, gelation-curing, and drying, which have been developed in order to lower the cost and the processing time of carbon gels. The use of monomers cheaper than R, as phenol or cresols, or of natural resources can lead to the formation of carbon gels with similar structure to that of RF carbon gels by selecting the appropriate synthesis conditions. The stages of gelation and curing can be shortened by using an inorganic acid as polymerization catalyst or by applying the ultrasound technique during the synthesis. Finally some drying techniques, as microwave or convective air drying, cheaper and/or faster than drying by supercritical CO2 are revised.

Influence of the synthesis methods on the physical and structural characteristics of 0.6TiO2·0.4Al2O3 catalytic supports

A series of 0.6TiO2·0.4Al2O3 catalytic supports has been prepared by both non-hydrolytic route and hydrolytic sol-gel processes. Some synthesis variables, such as the use of ethylene glycol as complexing agent, the generation in situ of the hydrolysis water, or the separated hydrolysis of the respective alkoxides, have been studied for the samples prepared by the hydrolytic sol-gel method. The physical and structural characterisation of the samples has been carried out by different techniques. The textural results show that the surface area and the pore size distribution are influenced by the synthesis method, which also affects the thermal behaviour and the crystallization process of the samples. The distribution of acid sites is also different, depending on the synthesis conditions.

Preparation of V/ZrO2 catalysts by the sol-gel method: Physical and structural characterization

Some zirconia-supported vanadium oxides have been prepared by sol-gel method and characterized by different techniques. The vanadium content exerts an effect on the area and pore-size of precursors and samples calcined. The presence of vanadium also affects the decomposition of precursors into ZrO2 and to its phase transformation, as well as the formation of fibres, as shown by SEM-EDX. XPS analyses show the presence of two species of vanadium at the surface, V5− and V4+, the percentage of each one depending on the load of vanadium in the catalyst.

A study of hydrogenated carbon fibers by scanning electron microscopy and confocal laser scanning microscopy

The hydrogen absorption process is studied in carbonaceous fibers produced from a mixture of methane and hydrogen. The absorption of the hydrogen was examined in two types of fibers, in “as‐grown” state and after a process of desorption during an annealing to 1.473 K under vacuum. Later to its production process, the fibers withstand an oxidation in air to 973 K. The fibers were examined by means of scanning electron microscopy (SEM) and confocal microscopy by reflection. Differences in the behavior during the oxidation were observed between the fibers in as‐grown state and those subjected to a further annealing. It could be verified that the fibers were really constituted by two different phases. In one of the phases, the storage of the hydrogen absorbed took place, whereas in the other phase there was no alteration. The process of annealing prior to the absorption of the hydrogen has an appreciable effect on the desorption rate of the hydrogen. Microsc. Res. Tech., 2009.