Percorrer por autor "Gabelica, Z."
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- An elegant way to increase acidity in SAPOs: use of methylamine as co-template during synthesisPublication . Fernandes, Auguste; Ribeiro, F. R.; Lourenço, J. P.; Gabelica, Z.SAPO-34 and SAPO-11 with an increased number of acid sites were synthesized by adding methylamine (MA) as co-template to tetraethylammonium hydroxide (TEAOH) or dipropylamine (DPA) respectively used as true templates.
- 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.
- 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.
- 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.
- Síntese e caracterização de silicoaluminofosfatos do tipo SAPO-40Publication . Lourenço, J. P.; Ribeiro, M. F.; Ribeiro, F. R.; Rocha, J.; Gabelica, Z.Os aluminofosfatos e silicoaluminofosfatos constituem duas das novas famílias de sólidos microporosos com estrutura tipo zeolítico. Estes materiais que apresentam elevada estabilidade térmica e hidrotérmica,podem possuir propriedades ácidas importantes, que associadas à sua estrutura de poros, lhes conferem novas e interessantes potencialidades de utilização no domínio da catálise, adsorção e permuta iónica.
- Solid-state NMR and powder XRD studies of the structure of SAPO-40 upon hydration-dehydration cyclesPublication . Lourenço, J. P.; Ribeiro, M. F.; Ribeiro, F. R.; Rocha, J.; Gabelica, Z.It is well known that after the removal of the template many porous aluminophosphates and related materials are very sensitive to water.' Depending on the type of structure, reversible or irreversible phase transitions, loss of crystallinity and changes in the coordination of some framework A1 upon rehydration are observed. For example, solid-state NMR shows that the rehydration of SAPO-5 leads to the formation of octahedral Al. Subsequent dehydration restores the initial tetrahedral coordination of Al. Template-free SAPO-37 becomes totally amorphous to X-rays after exposure to water and stays so after subsequent thermal treatment^.,,^ In contrast, Barthomeuf and co-workers have shown recently, that, on hydration, template-free SAPO-34, an analogue of chabasite, shows the opening of some Si-0-A1 bonds, the effect being reversible upon dehydrati~n.T~h e hydrated distorted structure was found to be stable for months with no further modifications and the ordered material could be regenerated by removal of water. Here we wish to report that the structure of template-free SAPO-40 undergoes a similar reversible modification.
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