Juan Carlos Medina - Academia.edu (original) (raw)
Uploads
Papers by Juan Carlos Medina
Revista Española de Cardiología, 2004
Analytical Chemistry, 2001
Experimental peak capacities for capillary gas chromatography (GC), capillary liquid chromatograp... more Experimental peak capacities for capillary gas chromatography (GC), capillary liquid chromatography (CLC), and capillary electrochromatography (CEC) were compared. To obtain a meaningful comparison, the following constraints were applied. First, the same sample (mixture of alkylbenzenes) was used as a test mixture for all three techniques; second, the same packing material and column diameter were used in CLC and CEC; and third, isothermal conditions were used in GC, while isocratic conditions were used both in CLC and in CEC. Comparison of peak capacities for the same total column efficiency (∼36 000 plates) showed that the peak capacity of GC is greater than those of the liquid-phase separation techniques. Comparison of CEC and CLC for constant retention factor was also carried out. For this condition, the results depend on the particle size used; for 3-µm porous particles, CEC had a peak capacity larger than CLC due to higher efficiency from the flow profile generated by electroosmotic flow. However, when 1.5-µm nonporous particles were used, the peak capacities were approximately the same for both techniques. The effect of linear velocity on peak capacity was also studied for all three techniques. Practical conditions aimed at increasing peak capacities of liquid-phase separation techniques are discussed.
Journal of Molecular Catalysis A-chemical, 1997
The synthesis and characterization of an iron cyclam-type complex within the cavities of a Y-zeol... more The synthesis and characterization of an iron cyclam-type complex within the cavities of a Y-zeolite is described. By comparing the performance of the heterogeneous system with the free complex in the oxidation of cyclohexene using iodosylbenzene as oxygen donor, a ...
Journal of Microcolumn Separations, 2000
Abstract In this study, fast chiral separations were demonstrated using near-critical carbon diox... more Abstract In this study, fast chiral separations were demonstrated using near-critical carbon dioxide as mobile phase. Most separations were carried out in less than 1 min at 30 C using 1215 cm capillary columns packed with 5 μm porous (300 Å) silica particles deactivated ...
Journal of Microcolumn Separations, 2000
Abstract In this study, fast chiral separations were demonstrated using near-critical carbon diox... more Abstract In this study, fast chiral separations were demonstrated using near-critical carbon dioxide as mobile phase. Most separations were carried out in less than 1 min at 30 C using 1215 cm capillary columns packed with 5 μm porous (300 Å) silica particles deactivated ...
Energy & Fuels, 2000
Kinetic data from the literature were used to predict formation rates and product yields of oil a... more Kinetic data from the literature were used to predict formation rates and product yields of oil and gas at typical low-temperature conditions of coal maturation. These data indicate that gas formation rates from hydrocarbon thermolysis are several orders of magnitude too low to have generated known coal-seam natural gas reserves, assuming bulk first-order kinetics defined by a single activation energy and preexponential factor. By assuming distributed activation energies, thermal cracking of liquid hydrocarbons and coal kerogen to methane can occur at sufficiently high rates to produce commercial quantities over long periods of geologic time. Acid-mineralcatalyzed cracking, transition-metal-catalyzed hydrogenolysis of liquid hydrocarbons, and transition-metal-catalyzed CO 2 hydrogenation form gas at very high rates at geologic temperatures. Rates of gas production in these reactions are orders of magnitude higher than those predicted from thermolysis; moreover, the gaseous products for metal-catalyzed hydrogenolysis of hydrocarbon liquids and for CO 2 hydrogenation are nearly the same as those of typical natural coalbed gases, while gases from thermal and catalytic cracking differ from most coalbed gases. The available data are most consistent with a model involving thermal and catalytic cracking of kerogen to oil followed by iron-and nickel-metal-catalyzed hydrogenolysis of oil to natural gas. In CO 2 -containing coal gases, natural gas may also be formed by iron-catalyzed CO 2 hydrogenation.
Geochimica Et Cosmochimica Acta, 2000
Hydrocarbon hydrogenolysis and CO 2 hydrogenation in the presence of Fe/SiO 2 and Ni/SiO 2 cataly... more Hydrocarbon hydrogenolysis and CO 2 hydrogenation in the presence of Fe/SiO 2 and Ni/SiO 2 catalysts were evaluated as potential mechanisms contributing to natural gas formation in coalbeds. The hydrocarbons used as reactants in hydrogenolysis included butane, octane, 1-octene, and 1-dodecene. The reactions carried out in a laboratory batch reactor produced gas that contained methane concentrations greater than 90%, which resembles the composition of natural gas. Reaction temperatures were selected to resemble natural coalbed conditions. Evidence is presented to show that iron and nickel minerals, which can be present in coals at levels of 2000 and 10 ppm, respectively, can become active under geologic conditions. The oxides (Fe 2O 3 and NiO) used as precursors of the active catalysts (Fe and Ni metals) were reduced at 200°C under a hydrogen atmosphere. Mössbauer spectroscopy showed that ca. 6% of the iron oxide was converted to the metal; in the case of nickel, oxygen titration showed that the extent of reduction to the metal was ca. 29%. The resultant fractions of the active metals in coals are adequate to catalyze generation of appreciable amounts of methane over geologic time.
Geochimica Et Cosmochimica Acta, 2000
Hydrocarbon hydrogenolysis and CO 2 hydrogenation in the presence of Fe/SiO 2 and Ni/SiO 2 cataly... more Hydrocarbon hydrogenolysis and CO 2 hydrogenation in the presence of Fe/SiO 2 and Ni/SiO 2 catalysts were evaluated as potential mechanisms contributing to natural gas formation in coalbeds. The hydrocarbons used as reactants in hydrogenolysis included butane, octane, 1-octene, and 1-dodecene. The reactions carried out in a laboratory batch reactor produced gas that contained methane concentrations greater than 90%, which resembles the composition of natural gas. Reaction temperatures were selected to resemble natural coalbed conditions. Evidence is presented to show that iron and nickel minerals, which can be present in coals at levels of 2000 and 10 ppm, respectively, can become active under geologic conditions. The oxides (Fe 2 O 3 and NiO) used as precursors of the active catalysts (Fe and Ni metals) were reduced at 200°C under a hydrogen atmosphere. Mössbauer spectroscopy showed that ca. 6% of the iron oxide was converted to the metal; in the case of nickel, oxygen titration showed that the extent of reduction to the metal was ca. 29%. The resultant fractions of the active metals in coals are adequate to catalyze generation of appreciable amounts of methane over geologic time.
Revista Española de Cardiología, 2004
Analytical Chemistry, 2001
Experimental peak capacities for capillary gas chromatography (GC), capillary liquid chromatograp... more Experimental peak capacities for capillary gas chromatography (GC), capillary liquid chromatography (CLC), and capillary electrochromatography (CEC) were compared. To obtain a meaningful comparison, the following constraints were applied. First, the same sample (mixture of alkylbenzenes) was used as a test mixture for all three techniques; second, the same packing material and column diameter were used in CLC and CEC; and third, isothermal conditions were used in GC, while isocratic conditions were used both in CLC and in CEC. Comparison of peak capacities for the same total column efficiency (∼36 000 plates) showed that the peak capacity of GC is greater than those of the liquid-phase separation techniques. Comparison of CEC and CLC for constant retention factor was also carried out. For this condition, the results depend on the particle size used; for 3-µm porous particles, CEC had a peak capacity larger than CLC due to higher efficiency from the flow profile generated by electroosmotic flow. However, when 1.5-µm nonporous particles were used, the peak capacities were approximately the same for both techniques. The effect of linear velocity on peak capacity was also studied for all three techniques. Practical conditions aimed at increasing peak capacities of liquid-phase separation techniques are discussed.
Journal of Molecular Catalysis A-chemical, 1997
The synthesis and characterization of an iron cyclam-type complex within the cavities of a Y-zeol... more The synthesis and characterization of an iron cyclam-type complex within the cavities of a Y-zeolite is described. By comparing the performance of the heterogeneous system with the free complex in the oxidation of cyclohexene using iodosylbenzene as oxygen donor, a ...
Journal of Microcolumn Separations, 2000
Abstract In this study, fast chiral separations were demonstrated using near-critical carbon diox... more Abstract In this study, fast chiral separations were demonstrated using near-critical carbon dioxide as mobile phase. Most separations were carried out in less than 1 min at 30 C using 1215 cm capillary columns packed with 5 μm porous (300 Å) silica particles deactivated ...
Journal of Microcolumn Separations, 2000
Abstract In this study, fast chiral separations were demonstrated using near-critical carbon diox... more Abstract In this study, fast chiral separations were demonstrated using near-critical carbon dioxide as mobile phase. Most separations were carried out in less than 1 min at 30 C using 1215 cm capillary columns packed with 5 μm porous (300 Å) silica particles deactivated ...
Energy & Fuels, 2000
Kinetic data from the literature were used to predict formation rates and product yields of oil a... more Kinetic data from the literature were used to predict formation rates and product yields of oil and gas at typical low-temperature conditions of coal maturation. These data indicate that gas formation rates from hydrocarbon thermolysis are several orders of magnitude too low to have generated known coal-seam natural gas reserves, assuming bulk first-order kinetics defined by a single activation energy and preexponential factor. By assuming distributed activation energies, thermal cracking of liquid hydrocarbons and coal kerogen to methane can occur at sufficiently high rates to produce commercial quantities over long periods of geologic time. Acid-mineralcatalyzed cracking, transition-metal-catalyzed hydrogenolysis of liquid hydrocarbons, and transition-metal-catalyzed CO 2 hydrogenation form gas at very high rates at geologic temperatures. Rates of gas production in these reactions are orders of magnitude higher than those predicted from thermolysis; moreover, the gaseous products for metal-catalyzed hydrogenolysis of hydrocarbon liquids and for CO 2 hydrogenation are nearly the same as those of typical natural coalbed gases, while gases from thermal and catalytic cracking differ from most coalbed gases. The available data are most consistent with a model involving thermal and catalytic cracking of kerogen to oil followed by iron-and nickel-metal-catalyzed hydrogenolysis of oil to natural gas. In CO 2 -containing coal gases, natural gas may also be formed by iron-catalyzed CO 2 hydrogenation.
Geochimica Et Cosmochimica Acta, 2000
Hydrocarbon hydrogenolysis and CO 2 hydrogenation in the presence of Fe/SiO 2 and Ni/SiO 2 cataly... more Hydrocarbon hydrogenolysis and CO 2 hydrogenation in the presence of Fe/SiO 2 and Ni/SiO 2 catalysts were evaluated as potential mechanisms contributing to natural gas formation in coalbeds. The hydrocarbons used as reactants in hydrogenolysis included butane, octane, 1-octene, and 1-dodecene. The reactions carried out in a laboratory batch reactor produced gas that contained methane concentrations greater than 90%, which resembles the composition of natural gas. Reaction temperatures were selected to resemble natural coalbed conditions. Evidence is presented to show that iron and nickel minerals, which can be present in coals at levels of 2000 and 10 ppm, respectively, can become active under geologic conditions. The oxides (Fe 2O 3 and NiO) used as precursors of the active catalysts (Fe and Ni metals) were reduced at 200°C under a hydrogen atmosphere. Mössbauer spectroscopy showed that ca. 6% of the iron oxide was converted to the metal; in the case of nickel, oxygen titration showed that the extent of reduction to the metal was ca. 29%. The resultant fractions of the active metals in coals are adequate to catalyze generation of appreciable amounts of methane over geologic time.
Geochimica Et Cosmochimica Acta, 2000
Hydrocarbon hydrogenolysis and CO 2 hydrogenation in the presence of Fe/SiO 2 and Ni/SiO 2 cataly... more Hydrocarbon hydrogenolysis and CO 2 hydrogenation in the presence of Fe/SiO 2 and Ni/SiO 2 catalysts were evaluated as potential mechanisms contributing to natural gas formation in coalbeds. The hydrocarbons used as reactants in hydrogenolysis included butane, octane, 1-octene, and 1-dodecene. The reactions carried out in a laboratory batch reactor produced gas that contained methane concentrations greater than 90%, which resembles the composition of natural gas. Reaction temperatures were selected to resemble natural coalbed conditions. Evidence is presented to show that iron and nickel minerals, which can be present in coals at levels of 2000 and 10 ppm, respectively, can become active under geologic conditions. The oxides (Fe 2 O 3 and NiO) used as precursors of the active catalysts (Fe and Ni metals) were reduced at 200°C under a hydrogen atmosphere. Mössbauer spectroscopy showed that ca. 6% of the iron oxide was converted to the metal; in the case of nickel, oxygen titration showed that the extent of reduction to the metal was ca. 29%. The resultant fractions of the active metals in coals are adequate to catalyze generation of appreciable amounts of methane over geologic time.