Browsing by Author "Ribeiro, M. F."
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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.
- Characterization of stability and porosity of SAPO-40 using m-xylene as model reactionPublication . Lourenço, J. P.; Ribeiro, M. F.; Ribeiro, F. R.; Rocha, J.; Gabelica, Z.The thermal and hydrothermal stability of SAPO-40 has been evaluated using the isomerization of m-xylene as model reaction. The characterization of the porosity was also studied and compared to other well known structures, namely Y zeolite, mordenite and ZSM-5. The structural changes that occur under thermal treatments influence the activity and selectivity of SAPO-40, while hydrothermal treatments do not significantly change those properties. The results confirm the high stability of SAPO-40 under thermal and hydrothermal treatments. Catalytic tests and adsorption of cyclohexane indicate that regular large side pockets or lobes exist along the main channels of SAPO-40, which create a large void volume responsible for the high rate of adsorption, high amount of adsorbed cyclohexane and the low shape selectivity in m-xylene isomerization.
- Disproportionation of ethylbenzene over SAPO-40Publication . Lourenço, J. P.; Ribeiro, M. F.; Ramôa Ribeiro, F.; Rocha, J.; Gabelica, Z.SAPO-40 exhibits strong acid sites, which catalyse the reaction of disproportionation of ethylbenzene. The distribution of diethylbenzene isomers suggests a high void volume in the structure of this molecular sieve.
- Efficient co-templating roles of amines and amides admixed with alkylammonium salts for the stabilisation of new AlPO4-n topologiesPublication . Borges, C.; Ribeiro, M. F.; Henriques, C.; Duarte, M.; Lourenço, J. P.; Gabelica, Z.Two different aluminophosphate structures were synthesised in aqueous media using as main template methylamine (MA), directly added, or generated in situ from methylformamide (MF). Both involve novel AlPO4-n topologies that undergo structural transformations upon template removal, yielding above 350°C microporous thermostable materials with interesting properties. Tetraalkylammonium (TEA) cations were used optionally as co-templates along with MA. Only non protonated MA was found incorporated into the pore volume of both structures, in relatively strong interaction with the framework oxygens. TEA appeared to stabilise one of the phases at defined stages of nucleation and/or growth processes, without playing any specific structure-directing role.
- 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.
- Evidence of a solvent screen effect affecting the redox properties of Co(II) ions in CoAPO-37, CoAPO-40 and CoIST-2 (AEN), by cyclic voltammetryPublication . Gabelica, Z.; Louati, A.; Borges, C.; Ribeiro, M. F.; Lourenço, J. P.; Murphy, D. M.Cyclic voltammetric experiments of framework Co(II) ions in three different CoAPO structures have provided evidence for a screening effect created by solvent or template molecules which results in a facile oxidation (and a difficult reduction) of the Co(II) ions incorporated into the most open (the less strained) structures. An apparent contradiction of these findings with respect to theoretical expectations is discussed.
- 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.
- Generation of acid sites by incorporation of cobalt in the AFR structurePublication . Lourenço, J. P.; Ribeiro, M. F.; Ribeiro, F. R.; Rocha, J.; Gabelica, Z.; Onida, B.; Garrone, E.Pure CoAPO4-40 and CoAPSO-40 samples have been synthesized. The CoII <=> CoIII framework transformation, and hence the number of potential acid sites has been evaluated by spectroscopic techniques and using m-xylene isomerization as model reaction.
- Incorporation of cobalt in the AFR structure and characterization of the resulting acid sitesPublication . Lourenço, J. P.; Ribeiro, M. F.; Ribeiro, F. R.; Rocha, J.; Gabelica, Z.; Onida, B.; Garrone, E.The AFR structure with the silicoaluminophosphate composition (SAPO-40) was known since 1984. However the synthesis of AIPO4-40 and MeAPO4-40 (Me:Co, Zn) was only reported in 1994, hence offering new potential applications of this structure.
- Methylamine as true template and TEAOH as purifying agent: unexpected roles of current organic additives in the hydrothermal synthesis of microporous aluminophosphatesPublication . Fernandes, A.; Ribeiro, M. F.; Barroso, S.; Lourenço, J. P.; Gabelica, Z.Methylamine (MA), TEA+ and water were shown to play a concerted role during the synthesis of two new aluminophosphates IST-1 and IST-2. Both structures start to nucleate after the dramatic change of the gel composition due to preliminary interactions between TEA+ cations.
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