Examinando por Autor "Carrero, Alicia"

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Chromium oxide/metallocene binary catalysts for bimodal polyethylene: hydrogen effects.
(Elsevier, 2012-12) Paredes, Beatriz; Grieken, Rafael van; Carrero, Alicia; Moreno, Jovita; Moral, Alberto
Bimodal resins came up to meet application requirements: low molecular weight for good processability and high molecular weight for mechanical properties. For obtaining this bimodality there are several strategies: physical melt mixing of the two components produced separately, a single catalyst in two different serial reactors and a single reactor technology employing a tailor made catalyst and/or switching conditions. This last method has many advantages such as lower investment costs, less process complexity and intimate mixing of high and low molecular weight components (improved product quality). By means of this single reactor technology, bimodal polyethylene was synthesized using a mesostructured catalyst based on Al-SBA-15 where two active centers, chromium and metallocene, were incorporated. Ethylene polymerizations were carried out over binary catalysts (hybrid and mixed Crmetallocene) and the polyethylenes obtained were compared with those obtained with individual catalysts in order to determine the contribution of each active centre. As well, the effect on polymer properties of the partial pressure of hydrogen in the reactor was evaluated. Results indicate that the hybrid catalyst (metallocene supported over Cr-Al- SBA-15) and physical mixture (Cr-Al-SBA-15 mixed with met-Al-SBA-15) lead to bimodal polyethylenes which combine high molecular weight, crystallinity and melting point with good processability (high melt index).
Evaluation of bimodal polyethylene from chromium oxide/metallocene hybrid catalysts for high resistance applications
(Wiley, 2020-08) Paredes, Beatriz; Moreno, Jovita; Carrero, Alicia; van Grieken, Rafael
An interesting alternative to the industrial two-stage cascade process for the production of bimodal polyethylene, used for high resistance applications such as pressure pipes, has been developed. The key point is a binary catalytic system with chromium and metallocene sites incorporated together on AlSBA-15 mesostructured material. This hybrid catalyst is able to produce bimodal polyethylene in a single reactor. In the present work, it is shown that, in the presence of hydrogen and comonomer (1-butene or 1-hexene) in the reaction medium, the obtained polyethylenes exhibit appropriate mechanical properties for pipes manufacture, such as resistance to rapid crack propagation (RCP) and low crack growth (SCG), reaching standards for PE100 and even for PE100RC grades.
Hydrogen Production Technologies: From Fossil Fuels toward Renewable Sources. A Mini Review
(ACS, 2021) Megía, Pedro J.; Vizcaíno, Arturo J.; Calles, José A.; Carrero, Alicia
The global economic growth, the increase in the population, and advances in technology lead to an increment in the global primary energy demand. Considering that most of this energy is currently supplied by fossil fuels, a considerable amount of greenhouse gases are emitted, contributing to climate change, which is the reason why the next European Union binding agreement is focused on reducing carbon emissions using hydrogen. This study reviews different technologies for hydrogen production using renewable and non-renewable resources. Furthermore, a comparative analysis is performed on renewablebased technologies to evaluate which technologies are economically and energetically more promising. The results show how biomass-based technologies allow for a similar hydrogen yield compared to those obtained with water-based technologies but with higher energy efficiencies and lower operational costs. More specifically, biomass gasification and steam reforming obtained a proper balance between the studied parameters, with gasification being the technique that allows for higher hydrogen yields, while steam reforming is more energy-efficient. Nevertheless, the application of hydrogen as the energy vector of the future requires both the use of renewable feedstocks with a sustainable energy source. This combination would potentially produce green hydrogen while reducing carbon dioxide emissions, limiting global climate change, and, thus, achieving the so-called hydrogen economy
Hydrogen Production through Oxidative Steam Reforming of Acetic Acid over Ni Catalysts Supported on Ceria-Based Materials
(MDPI, 2022-11-27) Megía, Pedro J.; Morales, Anabel; Vizcaíno, Arturo J.; Calles, José A.; Carrero, Alicia
Oxidative steam reforming allows higher energy efficiency and lowers coke deposition compared to traditional steam reforming. In this work, CeO2-based supports have been prepared with Ni as the active phase, and they were tested in the oxidative steam reforming of acetic acid. The influence of the O2/AcOH molar ratio (0–0.3) has been evaluated over Ni/CeO2. The results stated that by increasing oxygen content in the feeding mixture, acetic acid conversion increases too, with a decrease in coke deposition and hydrogen yield. To have a proper balance between the acetic acid conversion and the hydrogen yield, an O2/AcOH molar ratio of 0.075 was selected to study the catalytic performance of Ni catalysts over different supports: commercial CeO2, a novel mesostructured CeO2, and CeO2-SBA-15. Due to higher Ni dispersion over the support, the mesostructured catalysts allowed higher acetic acid conversion and hydrogen yield compared to the nonporous Ni/CeO2. The best catalytic performance and the lowest coke formation (120.6 mgcoke·gcat -1·h-1) were obtained with the mesostructured Ni/CeO2. This sample reached almost complete conversion (>97%) at 500 ºC, maintaining the hydrogen yield over 51.5% after 5 h TOS, being close to the predicted value by the thermodynamic equilibrium that is due to the synergistic coordination between Ni and CeO2 particles.
Oxidative steam reforming of acetic acid on Ni catalysts: Influence of the La promotion on mesostructured supports
(Elsevier, 2023) Megía, Pedro J.; Morales, Anabel; Vizcaíno, Arturo J.; Calles, José A.; Carrero, Alicia
In this work, the support effect and the La2O3 promotion of Ni-based catalysts on the oxidative steam reforming of acetic acid as bio-oil model compound has been studied. Ni/ SBA-15 showed the worst catalytic performance with an acetic acid conversion dropping below 30% at 500 C ascribed to active phase oxidation. In contrast, Ni supported over mesoporous CeO2 (CeO2-m) reached better catalytic performance and lower coke formation due to the higher Ni dispersion and oxygen mobility of the support. On the other hand, La2O3 promotion to SBA-15 and CeO2-m led to even higher Ni dispersion and prevented sintering during the reforming reaction. This effect resulted in an improvement in the catalytic performance for both promoted samples. As a consequence of low Ni crystallite size and high oxygen mobility, Ni/La2O3eCeO2-m reached almost complete conversion (~96%), the highest hydrogen yield (~53%) maintained for 5 h with the lowest coke formation...