J. de D. Lopez-Gonzalez

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.

Activated carbons from almond shells. I: Preparation and characterization by nitrogen adsorption

Abstract Several series of activated carbons have been prepared from almond shells by mean of carbonization in a flow of nitrogen followed by activation in a flow of carbon dioxide. The carbonized material is essentially microporous with pore dimensions close to those of the nitrogen molecule as deduced from the comparison of nitrogen adsorption isotherms at 77 and 90 K. Activation with carbon dioxide leads to the appearance of micropores and to a considerable increase in surface area. The effects of preparation conditions on the adsorptive capacity of the carbons are also discussed.

Preparation and characterization of active carbons from olive stones

Abstract Olive stones have been carbonized under a flow of nitogen in the temperature range from 700 to 900°C and activated in a CO2 flow in the range from 675 to 875°C. ZnCl2 was used in some of the activation processes. The adsoptive characteristics of the carbonized and activated samples have been determined by adsorption of nitrogen (77 and 90 K), carbon dioxide (195 and 273 K), n-butane (273 K) and methylene blue (aqueous solution at 298 K). Meso and macroporosity have been followed by mercury porosimetry. The resulting activated carbons have very large surface areas as well as a highly developed microporosity. The most adequate experimental conditions for the preparation of active carbons, highly microporous but with a well developed meso and macroporosity, are discussed. All active carbons prepared have a very low ash content and complete absence of sulphur, both very attractive characteristics.

The controlled reaction of active carbons with air at 350°C—I: Reactivity and changes in surface area

Abstract Two activated carbons prepared from almond shells and olive stones were reacted with air at 350°C to different percentages burn-off. The reactivity was studied in the temperature range 350–500°C where the reaction is relatively slow. The activated carbon from almond shells is more resistant to the reaction with air and the activation energy of that reaction is 101 kJ mol −1 . The adsorption of N 2 at 77 K has been used to characterize the adsorptive properties and surface area of all the obtained products, which have high surface areas (around or above 1000 m 2 g −1 ). The gas adsorption results, together with mercury porosimetry have allowed a study of the variation of surface area and porosity as a function of the burn-off. In any case, the exposure of the active carbons to air at 350°C for several days does not considerably affect their adsorptive properties even for a weight loss up to 50%.

Activated carbons from almond shells—II: Characterization of the pore structure

Abstract Several series of activated carbons have been prepared from almond shells by carbonization in nitrogen followed by activation in a flow of carbon dioxide. The adsorption of CO2 at 195 and 273 K and n-C4H10 at 273 K confirms that the carbonized materials are essentially microporous with dimensions or constrictions in the range 0.3–0.5 nm. Upon activation with carbon dioxide there is a considerable increase in the aperture of micropores and an increase in the apparent surface area. The effect of preparation conditions on the adsorptive capacity of the carbons are also discussed.

The two-stage air-CO2 activation in the preparation of activated carbons. II: Characterization by adsorption from solution

Olive stones and almond shells have been used as raw materials to prepare activated carbons following three different experimental methods: (a) carbonization in N2 followed by activation in CO2, (b) direct activation in CO2 and (c) treatment in dry air at 573 K followed by activation in CO2. The carbons have been characterized by the adsorption of paranitrophenol, methylene blue, orange II, crystal violet and victoria blue, all in aqueous solution. Methods (a) and (b) yield carbons with very similar adsorptive capacities and carbons prepared by method (c) have larger adsorptive capacities for similar overall yields. On the other hand, carbons from almond shells (more microporous) would be more suitable for adsorption of small solutes and carbons from olive stones (more macroporous and consequently, with larger rate of adsorption) for larger dimension molecules.

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.

Kinetics of the formation of graphite oxide

Abstract The retention of ethylene glycol and X-ray-induced photo-electron spectroscopy (XPS) have been used to follow the oxidation process of two different types of graphite, a natural based and an artificial one. From both methods it is apparent that the oxidation process is almost complete by about 24–32 hr. The specific velocity of the oxidation process has been calculated from both methods, giving values in reasonable agreement; consequently it appears that these methods are consistent when the oxidation kinetics are deduced. In all cases the amount of oxygen and the specific velocity in graphite oxide from the artificial graphite is higher than that in graphite oxide from the natural based one.

Carbonization of Olive Wood: Evolution of Surface Area and Porosity with Treatment Temperature

A study has been made of the influence of pyrolysis temperature on the surface area and porosity of carbonized samples prepared by heating olive tree wood in flowing N2 (100 cm3, min−1) at different temperatures (473, 573, 673, 773, 873, 973, 1073, 1173 and 1273 K using a heating rate of 5 K per min). At temperatures above 723 K carbon is produced. The adsorptive properties of the samples have been determined by N2 adsorption of 77 K. Meso and macroporosity have been followed by mercury porosimetry. Surface area was found to increase up to 873 K, and thereafter to remain practically constant to 1173 K. The average pore diameter was not influenced.