Examinando por Autor "Martos, Carmen"

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A new method for the determination of wax precipitation from non-diluted crude oils by fractional precipitation
(ELSEVIER, 2008) Coto, Baudilio; Martos, Carmen; Peña, José L.; Espada, Juan J.; Robustillo, María D.
Wax precipitation is one of the most important flow assurance problems. Unfortunately, experimental data are very scarce to confirm existing models for prediction of such precipitation problem. This work reports a new experimental method. Multistage fractional wax precipitation was carried out by decreasing the temperature of crude oil. No solvent dilution was used, so the effect of solvent on temperature wax precipitation was avoided. The reproducibility of the method was tested with comparable results. The precipitation curve and the wax appearance temperature were obtained for two crude oils with different chemical nature using the reported method.
Catalytical advantages of Hf-MOFs in benzaldehyde acetalization
(Elsevier, 2024-05-15) García-Rojas, Elena; Tapiador, Jesús; Leo, Pedro; Palomino, Carlos; Martos, Carmen; Orcajo, Gisela
The potential use of acetals as bioadditives based on sustainable feedstocks in the automotive industry is of great interest. If such feedstock is a residual stream, the process would represent a dual environmental challenge, such as the valorization of glycerol obtained as a by-product of biodiesel to produce acetals. There are just scarce works about this synthetic route due to the challenge in finding efficient catalytic converters for glycerol valorization in a single process. Herein four different Metal-Organic Frameworks (MOFs) are evaluated as heterogeneous catalysts for the acetalization reaction of benzaldehyde with methanol, specifically, two structures, MOF-808 and UiO-66, containing two structural metal ions, zirconium and hafnium. The aim of this work is to investigate the influence of the MOF structure, the metallic active sites and the reaction conditions on the catalytic performance of this acetal production-type reaction. Furthermore, the recyclability of the catalyst is evaluated, and a potential reaction mechanism is suggested. As relevant results, Hf-MOFs showed higher benzaldehyde conversion than Zr-based ones, and significant improvements in reaction conditions were achieved, such as a significant reduction in catalyst loading of 99.6 % using UiO-66-Hf, and an optimization of reagent ratios reducing methanol consumption by 50 % for a 92 % of benzaldehyde conversion. The Brønsted character of UiO-66-Hf seems to enhance the reaction rate more than the metal center accessibility and larger pore volumes offered by MOF-808
Copper MOFs performance in the cycloaddition reaction of CO2 and epoxides
(Elsevier, 2023) Tapiador, Jesús; García-Rojas, Elena; Leo, Pedro; Martos, Carmen; Calleja, Guillermo; Orcajo, Gisela
The research in advanced materials for effective CO2 capture and conversion is an imperative duty for the next years. In this sense MOF materials are being intensively tested for this purpose. Herein, seven different copperbased MOF materials with different functional organic groups in their linkers have been assessed to comprehend some structural features that influence the CO2 transformation via cycloaddition reaction with epoxides: CuURJC-1 (tetrazole group), Cu-URJC-8 (primary amine group), Cu-MOF-74 (hydroxyl group), JUC-62 (azo group), PCN-16 (triple bond group), HNUST-1 (amide group) and HKUST-1. When epichlorohydrin was used as a substrate, Cu-URJC-8 produced the best epoxide conversion (90%) and selectivity to cyclic carbonate (>99%), despite showing a modest surface area. This result can be explained by the higher basicity of primary amines than other basic functional organic groups of the MOFs. However, when styrene oxide was used as substrate, CuURJC-1 led to the best reaction results with a conversion and selectivity toward cyclic carbonate of 60 and 86%, respectively. This result was achieved because this material presents the synergic effect of having in its flexible structure 12 Å channels, that favors the styrene oxide diffusion, and a great number of basic nitrogen atoms, which can increase the reactivity of carbon dioxide. In summary, the presence of nitrogen atoms in the organic linker increases the CO2 conversion by promoting the reactivity of this molecule.
Influence of divalent metal ions on CO2 valorization at room temperature by isostructural MOF-74 materials
(Elsevier, 2023) Tapiador, J.; García-Rojas, E.; López-Patón, P.; Calleja, Guillermo; Orcajo, Gisela; Martos, Carmen; Leo, Pedro
At present, the concentration of carbon dioxide (CO2) has increased to unprecedented levels. The valorization of CO2 into high value-added chemicals can be a very interesting alternative to stabilize the concentration of this atmospheric pollutant while the current energy transition takes place. The well-known M-MOF-74 family (M= Mg, Co, Ni, Cu and Zn) have been evaluated as heterogeneous catalysts to carry out several cycloaddition reactions at room temperature with several epoxides (epichlorohydrin; 1,2-epoxyhexane and styrene oxide) as substrate, to evaluate the influence of the metal in this reaction. Zn-MOF-74 showed the highest catalytic activity, always with conversion and selectivity values above 93% regardless of the epoxides studied and the carbonates obtained, due to the higher acid character of the Zn2+ ion. Therefore, it is confirmed that the influence of the metal is key in this reaction, despite the fact that Cu-MOF-74 with an adsorption capacity of 5 mmol/g of CO2 (45 ◦C, 5 bar) converts 26% of styrene oxide under the conditions evaluated compared to ZnMOF-74, which is the material with the lowest adsorption capacity (3 mmol/g of CO2) and shows a conversion of 93%.
Novel and Versatile Cobalt Azobenzene-Based Metal-Organic Framework as Hydrogen Adsorbent
(Wiley, 2019-01-18) Montes-Andres, Helena; Leo, Pedro; Orcajo, Gisela; Rodríguez-Dieguez, Antonio; Choquesillo-Lazarte, Duane; Martos, Carmen; Botas, Juan Ángel; Martínez, Fernando; Calleja, Guillermo
A novel URJC-3 material based on cobalt and 5,5′-(diazene-1,2-diyl)diisophthalate ligand, containing Lewis acid and basic sites, has been synthesized under solvothermal conditions. Compound URJC-3, with polyhedral morphology, crystallizes in the tetragonal and P43212 space group, exhibiting a three-dimensional structure with small channels along a and b axes. This material was fully characterized, and its hydrogen adsorption properties were estimated for a wide range of temperatures (77–298 K) and pressures (1–170 bar). The hydrogen storage capacity of URJC-3 is quite high in relation to its moderate surface area, which is probably due to the confinement effect of hydrogen molecules inside its reduced pores of 6 Å, which is close the ionic radii of hydrogen molecules. The storage capacity of this material is not only higher than that of active carbon and purified single-walled carbon nanotubes, but also surpasses the gravimetric hydrogen uptake of most MOF materials.
Synthesis of Sn-silicalite from hydrothermal synthesis of SiO2-SnO2 xerogels
(Microporous Mesoporous Mater. 119 (2009) 176-185, 2009) van Grieken, Rafael; Martos, Carmen; Sánchez-Sánchez, Manuel; Serrano, David P.; Melero, Juan A.; Iglesias, José; Cubero, Alvar G.
For the first time, the synthesis of Sn-zeolites starting from SiO2-SnO2 xerogels is described. The influence of several synthesis parameters in the physical-chemical properties of the xerogels and the zeolites subsequently generated has been investigated. Two different tin sources were assayed in the synthesis of xerogels (anhydrous SnCl4 and SnCl4¿5H2O) as well as two gelation agents (NH3 and TPAOH). Though both bases led SiO2-SnO2 xerogels mainly showing tetrahedrally coordinated tin centres, regardless the tin source, only TPAOH derived materials were easily transformed into MFI-type zeolites showing tin incorporation. On the other hand, NH3 gelified xerogels were difficult to crystallize, being this fact attributed to the retention of NH3 species in the xerogels because of the tin Lewis acidity, avoiding its inclusion in the zeolite crystalline framework. Studies on the crystallization time and temperature allowed to optimize the transformation of TPAOH gelified SiO2-SnO2 xerogels into tin-containing MFI zeolites showing isomorphically incorporated tin centres. The so-obtained materials were compared with conventionally synthesized SnS-1 zeolite showing better quality in terms of tin incorporation.
Synthesis, Structural Features, and Hydrogen Adsorption Properties of Three New Flexible Sulfur-Containing Metal–Organic Frameworks
(American Chemical Society, 2020-08-31) Montes-Andres, Helena; Leo, Pedro; Orcajo, Gisela; Rodríguez-Dieguez, Antonio; Choquesillo-Lazarte, Duane; Martos, Carmen; Botas, Juan Ángel; Calleja, Guillermo
Three novel flexible sulfur-containing MOF materials named Co-URJC-5, Cu-URJC-6 and Zn-URJC-7, based on the 5,5′-thiodiisophthalic acid linker have been synthesized through solvothermal methods and characterized by different physicochemical techniques. Hydrogen adsorption analysis at room temperature reveals that these compounds display a gate-opening type adsorption mechanism at low pressures, attributed to the flexible nature of the H4TBTC ligand. This behavior is even more noticeable for Cu-URJC-6, since the layer arrangement by π–π stacking interactions between the aromatic layers could contribute to the flexibility of the structure. These results can be considered as a representative example to elucidate how MOF structures are built using flexible ligands and more significantly as a promising route for designing materials with selective gas sorption properties.
URJC-1: Stable and Efficient Catalyst for O-Arylation Cross-Coupling
(MDPI, 2024-06-27) García-Rojas, Elena; Leo, Pedro; Tapiador, Jesus; Martos, Carmen; Orcajo, Gisela
The design of metal–organic frameworks (MOFs) allows the definition of properties for their final application in small-scale heterogeneous catalysis. Incorporating various catalytic centers within a single structure can produce a synergistic effect, which is particularly intriguing for crosscoupling reactions. The URJC-1 material exhibits catalytic duality: the metal centers act as Lewis acid centers, while the nitrogen atoms of the organic ligand must behave as basic centers. The impact of reaction temperature, catalyst concentration, and basic agent concentration was evaluated. Several copper-based catalysts, including homogeneous and heterogeneous MOF catalysts with and without the presence of nitrogen atoms in the organic ligand, were assessed for their catalytic effect under optimal conditions. Among the catalysts tested, URJC-1 exhibited the highest catalytic activity, achieving complete conversion of 4-nitrobenzaldehyde with only 3% mol copper concentration in one hour. Furthermore, URJC-1 maintained its crystalline structure even after five reaction cycles, demonstrating remarkable stability in the reaction medium. The study also examined the impact of various substituents of the substrate alcohol on the reaction using URJC-1. The results showed that the reaction had high activity when activating substituents were present and for most cyclic alcohols rather than linear ones.