Browsing by Author "Fernandes, A."
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- Al-containing MCM-41 type materials prepared by different synthesis methods: hydrothermal stability and catalytic propertiesPublication . Lourenço, João P.; Fernandes, A.; Henriques, C.; Ribeiro, M. F.Al-containing MCM-41 type materials were prepared by three different synthesis methods (aluminum grafting on the calcined MCM-41, pH adjustment during the crystallization period and crystallization in the presence of zeolite seeds). The samples were characterized by powder XRD, nitrogen adsorption, 27Al MAS NMR and FTIR with pyridine as probe molecule. All the samples exhibit a high hydrothermal stability at high temperature just with a minor structural degradation. N2 adsorption data obtained for the sample prepared from a gel containing zeolite seeds suggest a higher structural resistance to the hydrothermal treatment in comparison with the samples prepared by other methods. A different type of pyridine adsorbed, probably interacting simultaneously with a Brönsted and a Lewis acid site, is observed upon pyridine adsorption. This type of adsorption is stronger for the sample prepared in the presence of zeolite seeds and is not observed on Al-grafted MCM-41. 2 The catalytic activity towards the isomerization of α-pinene depends mainly on the number of Brönsted acid sites. The data obtained in this study strongly indicate that for this type of materials, the Lewis acid sites do not influence the selectivity for the isomerization products.
- Determination of the botanical origin of honey by sensor fusion of impedance e-tongue and optical spectroscopyPublication . Ulloa, P. A.; Guerra, Rui Manuel Farinha das Neves; Cavaco, A. M.; Costa, Ana M. Rosa da; Figueira, A.C.; Fernandes, A.The aim of this study was to discriminate four commercial brands of Portuguese honeys according to their botanical origin by sensor fusion of impedance electronic tongue (e-tongue) and optical spectroscopy (UV–Vis–NIR) assisted by Principal Component Analysis (PCA) and Cluster Analysis (CA). We have also introduced a new technique for variable selection through one-dimensional clustering which proved very useful for data fusion. The results were referenced against standard sample identification by classical melissopalynology analysis. Individual analysis of each technique showed that the e-tongue clearly outperformed the optical techniques. The electronic and optical spectra were fitted to analytical models and the model coefficients were used as new variables for PCA and CA. This approach has improved honey classification by the e-tongue but not by the optical methods. Data from the three techniques was then considered simultaneously. Simple concatenation of all matrices did not improve the classification results. Multi-way PCA (MPCA) proved to be a good option for data fusion yielding 100% classification success. Finally, a variable selection method based on one-dimensional clustering was used to define two new approaches to sensor fusion, and both yielded sample clusters even better defined than using MPCA. In this work we demonstrate for the first time the feasibility of sensor fusion of electronic and optical spectroscopy data and propose a new variable selection method that improved significantly the classification of the samples through multivariate statistical analysis.
- Ethylene polymerisation with zirconocene supported in Al-modified MCM-41: catalytic behaviour and polymer propertiesPublication . Campos, João M.; Ribeiro, M. R.; Lourenço, J. P.; Fernandes, A.Heterogeneous olefin polymerisation catalysts were prepared by combining zirconocene with a series of purely siliceous and Al-containingMCM- 41 supports with different Si/Al ratios, using simplified and improved versions of the well-known direct impregnation and “MAO pre-treatment” methods. The catalysts were tested in ethylene polymerisation. Analysis of polymerisation activities and kinetic profiles led to a rationalisation on the effect of support Si/Al ratios and methods used for catalyst preparation over the catalytic behaviour for ethylene polymerisations. Polymers were characterised by scanning electron microscopy and differential scanning calorimetry. A mechanism is proposed to associate the formation or absence of fibrous morphology with polymerisation activities.
- Ethylene removal by Ag-based ZSM-5 adsorbents for the preservation of climacteric fruitsPublication . Ferreira, R.; Lopes, H.; Lourenço, J. P.; Silva, J. M.; João, I. M.; Ribeiro, M. F.; Fernandes, A.Ethylene removal is crucial for fruits and vegetables preservation because even a very low concentration (<0.1ppm) can induce ripening during storage and transportation. Ag+ exchanged ZSM-5 zeolites were investigated as adsorbents for the efficient removal of ethylene. Two ZSM-5 materials (Si/Al ratios of 15 and 40), with different amounts of Ag (up to 6 % by weight) and two compensating cations, Na+ and H+, were used for this purpose. The adsorbents were characterized by X-ray Diffraction, diffuse reflectance spectroscopy and H2- TPR experiments. Their performance in ethylene adsorption was determined by the means of breakthrough curves experiments, mimicking the atmospheric conditions of industrial fruits cold storage chambers, particularly the high relative humidity levels. Results show that adsorbents are highly efficient in removing ethylene (max. 500 µmol.g-1 in the absence of water). Monovalent Ag+ species have been identified as the main contributors to the excellent performance of the different adsorbents, as they can easily interact with ethylene through strong π interaction. Moreover, statistical analysis (ANOVA) results confirmed that, in the presence of water, Ag-based ZSM-5 materials with a higher Si/Al ratio and Na+ as the charge balancing cation, i.e., those with a higher hydrophobic character, are the best adsorbents for ethylene removal.
- Ethylene removal with metals-based zeolites for climacteric fruits preservationPublication . Ferreira, R.; Lopes, H.; Fernandes, A.; Lourenço, João P.; Silva, J. M.; João, I. M.; Ribeiro, FilipaNowadays, there is a high importance of consuming fresh fruits and vegetables (F&V) in a well-balanced diet. One main issue concerning fruit preservation is ethylene molecule, a natural plant hormone that is responsible for fruits ripening and senescence. Thus, its removal from transportation and cold storage chambers is essential for industrial companies to prevent deterioration or spoilage of F&V and extend their post-harvest life [1].
- Functionalization of mesoporous MCM-41 (Nano)particles: preparation methodologies, role on catalytic features, and dispersion within polyethylene nanocompositesPublication . Bento, A.; Lourenço, J. P.; Fernandes, A.; Cerrada, María L.; Rosário Ribeiro, M.MCM-41 micro- and nanoparticles are prepared and functionalized with silane coupling agents. These pristine and silane-decorated mesoporous MCM-41s are further used in the synthesis of MCM-41/ethylene nanocomposites by in situ polymerization, utilizing either supported or non-supported catalytic systems. The catalytic behavior of those systems is discussed, taking into consideration the effects of the particle size, surface characteristics, and functionalization. The results have shown that the initial surface state of MCM-41 is of paramount importance to achieve high catalytic activity if a polymerization catalyst is to be supported. Functionalization changes the surface chemistry, thus, a positive or negative effect may be observed, depending on the previous features of the surface. The particle size of MCM-41 and its functionalization affect its final dispersion within the polymeric matrix, this distribution is evaluated by SEM, TEM, and microhardness measurements.
- Gas permeability properties of decorated MCM-41/polyethylene hybrids prepared by in-situ polymerizationPublication . Bento, A.; Lourenço, J. P.; Fernandes, A.; Ribeiro, M. R.; Arranz-Andrés, J.; Lorenzo, V.; Cerrada, María L.Mixed matrix membranes, MMM, based on polyethylene,PE, and silica mesoporous material, MCM-41, have been prepared using these hybrid materials obtained by in situ polymerization in order to achieve higher permeability, selectivity or both magnitudes relative to the existing polymeric PE membranes. Different methodologies are used to enhance interfacial adhesion between components, PE matrix and MCM-41. These basically consist of promoting the modification of MCM-41 surface with undecenoic acid, UA/triisobutylaluminum, TIBA, system before the polymerization and of incorporating UA in the polymer chains by copolymerization. The influence of UA on the degradation behavior is evaluated for all of the hybrids and the transport properties, in terms of permeability and diffusivity coefficients as well as permselectivity at different gases and temperatures are checked in some of the resulting materials.
- Gas-phase dehydration of glycerol over thermally-stable SAPO-40 catalystPublication . Lourenço, J. P.; Fernandes, A.; Bértolo, R. A.; Ribeiro, M. F.SAPO-40 was used as catalyst for the gas-phase dehydration of glycerol towards acrolein. At 350 ºC the catalyst attained full conversion of glycerol with a negligible deactivation in the first 48 h, a glycerol conversion above 50 % after 120 h on stream and a nearly constant selectivity to acrolein above 70%. This catalyst proved to be highly resistant under the experimental conditions used and can be regenerated without loss of activity or significant structural damage. The comparison of SAPO-40 with SAPO-34 and SAPO-11 illustrates the importance of the porous structure and emphasizes the good catalytic performance of this material.
- HDPE/mesoporous silica nanocomposites by in-situ polymerisation: structural details and mechanical responsePublication . Bento, A.; Lourenço, J. P.; Fernandes, A.; Pérez, Ernesto; Cerrada, María L.; Ribeiro, M. R.Mesoporous silicas exhibit stable three-dimensional structures, made of ordered channels with well-defined geometries and dimensions at nanometer scale, able to host intercalation reactions.
- Mesoporous Ga-MCM-41: a very efficient support for the heterogenisation of metallocene catalystsPublication . Campos, João M.; Lourenço, J. P.; Fernandes, A.; Ribeiro, M. R.Mesoporous Ga-containing silicates prepared by direct synthesis, Ga-MCM-41, are used in combination with metallocene Cp2ZrCl2 to provide very effective supported catalysts for ethylene polymerisation. A pre-treatment with methylalumoxane is not required in order to achieve high polymerisation activity levels with these catalysts, whose stability is improved when compared to the case where a purely siliceous support was used for the immobilisation of the metallocene.