Papers by Maria Luisa Cervantes
Reducing the cost and the processing time of carbon gels
The carbon gels obtained by polymerization of resorcinol (R) and formaldehyde (F) are very attrac... more 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 book shows the strategies applied at different stages of the global process, synthesis, gelation-curing, and drying, which have been developed in order to reduce the cost and the processing time of carbon gels. The use of monomers cheaper than resorcinol, 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 techniqu...
100Cias Uned, 2008
Enseñanza 100cias@uned n º 1 ( n u e va é poca) 2008 Facultad de Ciencias 100cias@uned TALLER Y L... more Enseñanza 100cias@uned n º 1 ( n u e va é poca) 2008 Facultad de Ciencias 100cias@uned TALLER Y LABORATORIO eXpeRIMenToS caSeRoS con aGua oXIGenaDa "Catálisis homogénea, heterogénea y enzimática", "el genio en la lámpara" y "pasta de dientes para elefante"
Origen y control de los contaminantes
Journal of Molecular Catalysis, 1993
Anionic alkylation of imidazole with alkyl halides to produce intermediates in the synthesis of p... more Anionic alkylation of imidazole with alkyl halides to produce intermediates in the synthesis of pharmaceutically important antiviral products, such as famciclovir and acyclovir, have been carried out using alkaline carbons as base catalysts. Previously to the alkylation of imidazole, the condensation of benzaldehyde with malonic esters has been used as test reaction of basicity on a series of lithium, sodium, potassium and cesium exchanged Norit RX 1 Extra activated carbon. The importance of active sites and the measurement of surface basicity have been emphasized. Under reaction conditions, it has been found that most of the basic sites in alkaline carbons have 10.7d~K~g13.3, but there is a considerable number of sites able to abstract protons of 13.3<pK,< 16.5. Norit RX 1 Extra which was not subject to alkaline exchange retains most of its basic sites capable of abstracting protons of 9 d pK, < 10.7. Taking into account that the pK,, of the NH group of imidazole is 14.5, the alkaline carbons present the required basic strength to catalyze the alkylation of this N-heterocycle with alkyl halides.
100cias@ uned, 2008
El curso académico 2010/2011 comenzará a impartirse en la Facultad de Ciencias el título de Gradu... more El curso académico 2010/2011 comenzará a impartirse en la Facultad de Ciencias el título de Graduado/a en Química. Este título tiene un interés considerable para los estudiantes, tanto desde el punto de vista científico y académico como profesional. Desde el primer punto de vista, cabe indicar que la Química, con su permanente capacidad de innovación, ha tenido siempre un enorme impacto sobre el progreso, desarrollando productos y tecnologías que inciden en todos los campos de actividad de los seres humanos, convirtiéndose en uno de los pilares de la capacidad competitiva de un país. Así, la Ciencia Química juega un papel muy importante en la protección de la salud y el medio ambiente, en la mejora de las condiciones higiénicas y sanitarias, en la obtención cualitativa y cuantitativa de alimentos para toda la humanidad, y en la fabricación de nuevos y más baratos materiales que permiten mejorar la calidad de nuestras vidas. Por otra parte, desde el punto de vista profesional, el interés del título está más que justificado. Según el Consejo General de Colegios Oficiales de Químicos de España, los titulados en Química o en Ciencias Químicas pueden desempeñar su actividad profesional en empresas del sector químico, farmacéutico, sanitario, óptico y cosmético; empresas de materias plásticas, vidrio, papel, colorantes, fertilizantes, fibras sintéticas; en el sector industrial el petróleo, energía, metalurgia, gas, aguas, cementos, materiales avanzados, energías alternativas, reciclaje, gestión de residuos, medio ambiente y sector agroalimentario, así como en hospitales, la Administración Pública y organismos internacionales.
Physico-chemical properties of low-rank coals
Powder Technology, 2004
Five low-rank coals, including leonardites and lignites, have been physico-chemically characteris... more Five low-rank coals, including leonardites and lignites, have been physico-chemically characterised. The effects of the carbonisation and demineralisation of some coals on mass and physical properties have also been studied. The characterisation was carried out by the chemical analyses of the coals and also by Fourier transform infrared (FT-IR) spectroscopy, gas adsorption (N2, −196 °C; CO2, 0 °C), mercury porosimetry,
A study of hydrogenated carbon fibers by scanning electron microscopy and confocal laser scanning microscopy
Microscopy Research and Technique, 2009
The hydrogen absorption process is studied in carbonaceous fibers produced from a mixture of meth... more 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 &amp;amp;amp;amp;amp;amp;amp;amp;amp;amp;amp;amp;amp;amp;amp;amp;quot;as-grown&amp;amp;amp;amp;amp;amp;amp;amp;amp;amp;amp;amp;amp;amp;amp;amp;quot; 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.
Journal of Chemical Technology & Biotechnology, 2004
α,β‐Unsaturated nitriles have been synthesized by Knoevenagel condensation of a carbonylic compou... more α,β‐Unsaturated nitriles have been synthesized by Knoevenagel condensation of a carbonylic compound with malononitrile, assisted by sonochemical irradiation. Two alkaline‐promoted clays (Li+‐ and Cs+‐exchanged saponites) have been employed as catalysts. The influence of the carbonylic compound (benzaldehyde or cyclohexanone) and the use of a solvent on the catalytic activity have been studied. Remarkable increase in the conversion values has been found when the reaction is activated by ultrasound, as compared with the thermal activation. In this green, solvent‐free procedure, α,β‐unsaturated nitriles have been produced in very high yields (97%) when the Cs+‐saponite is used as catalyst. Copyright © 2004 Society of Chemical Industry
Interaction of triton X‐100 on silica: A relationship between surface characteristics and adsorption isotherms
Journal of Chemical Technology & Biotechnology, 1995
Adsorption of Triton X‐100 on various silica substrates has been investigated. A number of solids... more Adsorption of Triton X‐100 on various silica substrates has been investigated. A number of solids, including a natural quartz, this quartz washed with HCl acid and subsequently heated at 1273 K; two aerosils and one Kieselgel silicas were studied. These solids exhibit surface areas in the range of 5 to 430 m2 g−1. All the Triton adsorption isotherms display an S‐shape at the adsorption temperatures studied (298 and 308 K). It has been found that the pretreatments of natural quartz (by water washing, impurities removed by acid and/or high temperature calcination) affect considerably the amounts of TX‐100 adsorbed. Measurements of surface composition have been made by X‐ray photoelectron spectroscopy (XPS) with particular emphasis on the presence of impurities and on the number of OH groups at the surface of the samples. The nature of the surface hydroxyl has also been studied by infrared spectroscopy. Furthermore, the specific number of hydroxyl groups on the surface of the silica sa...
Festival ciencia para la juventud en la Uned
100cias@ uned, 2009
... En la mayoría de los casos ha existido una gran acogida por parte de los centros implicados, ... more ... En la mayoría de los casos ha existido una gran acogida por parte de los centros implicados, siendo muy elevado el número de participantes ... En cuanto a la divulgación y publicidad del proyec-to, hay que indicar que la Televisión Educativa de la UNED grabó un programa1 el ...
Catalysis Today, 2006
Vanadia-titania catalysts prepared by different sol-gel procedures were studied as heterogeneous ... more Vanadia-titania catalysts prepared by different sol-gel procedures were studied as heterogeneous catalysts for the liquid phase oxidation of limonene. The catalysts were characterized by XRD, XPS, ICP and nitrogen adsorption. According to the XRD results the catalyst samples can be divided in two different groups: anatase samples and anatase + rutile samples. XRD signals of vanadia are not found in the diffractograms. The main reaction products are polymers. Limonene oxide, limonene glycol, carveol and carvone are obtained in small amounts. A number of autoxidation products, alcohols, aldehydes and ketones, are also obtained. The effects of titania composition on the reaction orientation are discussed.
Bulletin of Materials Science, 2003
Coal basically consists of two parts-a crystalline, inorganic part, and an amorphous, organic par... more Coal basically consists of two parts-a crystalline, inorganic part, and an amorphous, organic part. Based on this, we intended to study the changes that occurred on the composition and on the chemical structure of coals after carbonization at 1000 or 900°°C and demineralization treatments with hydrochloric and hydrofluoric acids. For this, four coals of different categories (or levels) were chosen: semianthracite (A-O) and high volatile bituminous coal (B-O), which are high level coals, and lignite (Li-O) and leonardite (Le-O), these being low level coals. The coals were first analysed in terms of their proximate and elemental compositions and then carbonized and demineralized. Also, the starting coals and the prepared samples were examined by infrared spectroscopy. In addition, a study of the optimization of the application of this technique for only A-O was carried out. For A-O and B-O, the spectra recorded intense absorption bands that are ascribable to vibration modes in mineral components as quartz and aluminosilicates, such as kaolinite. For Li-O and Le-O, the spectra displayed some other bands as well, also quite intense, which have been assigned to bond vibrations in functional groups and structures of their organic part. The carbonization of the coals resulted in significant changes in their inorganic part as the content of quartz increased and the content of aluminosilicates decreased. In addition, the thermal decomposition of mineral carbonates occurred. The carbonization greatly affects the organic part of the coals, especially in Li-O and Le-O, as most functional groups and structures are not thermally stable under heating conditions. With regard to demineralization, HF is a more effective agent than HCl, achieving products with higher organic content. The mass losses are higher in Li-O and Le-O than in A-O and B-O. So, the infrared spectroscopy allows the analysis of both inorganic and organic parts of the coals and of their carbonization and demineralization products. These processes facilitate subsequent analysis of the inorganic and organic parts of coals by infrared spectroscopy. In the application of this technique, both the coal : KBr ratio and the thickness for the disks should be controlled, owing to the influence on the infrared absorption.
Revista CENIC Ciencias …, 2002
Composition influence over delivery ability of hydroxyapatite cements RESUMEN. Se prepararon ceme... more Composition influence over delivery ability of hydroxyapatite cements RESUMEN. Se prepararon cementos de hidroxiapatita divididos en dos partes, una sólida compuesta por una mezcla de hidroxiapatita, yeso y dos polímeros, alginato de sodio y poli-N-vinil-2-pirrolidona, y una parte líquida conformada en este caso por una mezcla de sulfato y fosfato de potasio en solución acuosa a una concentración específica. Todas las formulaciones fueron dopadas con cefazolina y ciprofloxacina y se estudió la cinética de liberación de los fármacos en regulador fosfato. Los resultados se ajustan al modelo de Higuchi regulado por la difusión en dos etapas. Se determinó la influencia de los contenidos de yeso y de inhibidor sobre la cantidad total de antibiótico liberada que osciló, en dependencia del tipo de fármaco y de la formulación entre un 10 y un 50 % a la semana de iniciado el estudio, alterando de manera inversa la cantidad de yeso y de forma directa la cantidad de inhibidor, sobre la cantidad total de fármaco liberada, en ambos casos, por la competencia de las reacciones de fraguado entre el fosfato de potasio y el yeso y la del alginato de sodio con el segundo. Se discute y demuestra la no existencia de diferencias entre la liberación cuando se habla de ciprofloxacina y lo contrario, si es el caso de la cefazolina. Estos resultados permiten valorar la posibilidad real de la obtención de un cemento de hidroxiapatita de producción nacional, similar a los ofertados hoy día en el mercado internacional y dotar al sistema de salud cubano de una formulación que puede ser un potencial rubro exportable. ABSTRACT. Hydroxyapatite cements divided in two parts, a solid part composed by a mixture of hydroxyapatite, gypsum and two polymers, sodium alginate and poly-N-vinyl-2-pyrrolidone and a liquid part formed by a mixture of salts, potassium sulphate and potassium phosphate in water solution, were prepared. All the formulations were doped with cefazolin and ciprofloxacine. The kinetics of drug delivery in buffer phosphate was studied. The results were adjusted to the Higuchi model regulated by the diffusion in two steps, conditioned by the drug exposed at the sample surface in first place, and a second step oriented by the diffusion through the matrix until to achieve the surface and from these one to the solution. One week after initiate the study, the influence of the gypsum and inhibitors contents, on the liberated antibiotic quantity was determined, oscillating between 10-50 %, depends on drug type and the formulation. The inverse effect of gypsum quantity, and direct effect of the inhibitor quantity, on the liberated drug, occurs in both cases, for the reactions competition that have between the gypsum with potassium phosphate and sodium alginate. In other side, the authors discussed and probed it that no exist differences between drug delivery when studies the accelerator/inhibitor relationship in case of ciprofloxacine and the contrary if it is talking about cefazolin. These results allow to value the real possibility to obtain a hydroxyapatite cement of national production, similar to the offered today in the international market and to endow to Cuban Health System of a formulation that can be a potential exportable item.
Applied Surface Science, 2006
N-Propargyl imidazole has been synthesized by Knoevenagel condensation of benzaldehyde with propa... more N-Propargyl imidazole has been synthesized by Knoevenagel condensation of benzaldehyde with propargyl bromide, assisted by microwave irradiation. Two alkaline-promoted clays (Li + -and Cs + -exchanged saponites) have been used as catalysts. The influence of several factors, such as irradiation power, irradiation time and alkaline promoter has been studied. The catalysts were characterized by XRD and chemical analysis. The basicity enhancement is directly connected to the presence of alkaline metal promoters in the saponite structure. In addition, a significant increase in the conversion values has been found when the reaction is activated by microwave irradiation, as compared with thermal activation. The yield to the N-propargyl imidazole shows a maximum for the Cs + -saponite at 750 W in only 5 min of microwave irradiation. This green and solvent-free procedure can be extended to the preparation of other N-substituted heterocycles, which could serve as precursors in the primary route to pharmaceutical compounds of interest.
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Papers by Maria Luisa Cervantes