Browsing by Author "Borges, C."
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- 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.
- Structural state and redox behavior of framework Co(II) in CoIST-2: a novel cobalt-substituted aluminophosphate with AEN topologyPublication . Borges, C.; Ribeiro, M. F.; Henriques, C.; Lourenço, J. P.; Murphy, D. M.; Louati, A.; Gabelica, Z.A highly crystalline cobalt-substituted aluminophosphate with an AEN topology, labeled CoIST-2, has been synthesized and characterized by powder XRD, TG/DSC, diffuse reflectance UV-vis spectroscopy, FTIR, EPR spectroscopy, and cyclic voltammetry. Both water and methylamine were shown to play a complementary role as dual template and space-filling agents during the hydrothermal synthesis of this material. The spectroscopic (EPR, UV-vis, and FTIR) and electrochemical characterization of the as-synthesized and calcined CoIST-2 material revealed the presence of two structurally different Co(II) species incorporated in its crystalline framework. The redox behavior and acidity was found to be different for these two Co(II) species. While one of these cobalt species corresponded to Bro¨nsted Co(OH)P acid sites, the other seemed to be associated with the presence of Lewis acid sites (Co0P type species). UV-vis spectra, typical of divalent cobalt in tetrahedral coordination, along with FTIR, demonstrated that only a fraction of the Co(II) ions undergo a reversible oxidative process after calcination in air and that some extraframework CoxOy type species are also generated through oxidation. The presence of divalent Co(II) in a slightly distorted tetrahedral coordination, both in the as-synthesized and calcined phase, was also detected with use of EPR data. The electrochemical oxidation of the two different framework Co(II) species and their further reduction could be evaluated semiquantitatively by cyclic voltammetry. The redox ability of CoIST-2 was also compared with that of two other cobaltsubstituted aluminophosphates involving different pore structures, namely CoAPO-40 and CoAPO-37, by using FTIR of adsorbed NO. It was found that the most readily oxidized ions were present in the more dense AEN topology, followed by the more open AFR and FAU structures, thereby emphasizing how the redox properties of transition metal ions in porous aluminophosphates are intimately linked to both the structure type/density of the host lattice and framework positions of the heteroelement.
- Structure analysis of the novel microporous aluminophosphate IST-1 using synchrotron powder diffraction data and HETCOR MAS NMRPublication . Jordá, J. L.; McCusker, L. B.; Baerlocher, C.; Morais, C. M.; Rocha, J.; Fernandez, C.; Borges, C.; Lourenço, J. P.; Ribeiro, M. F.; Gabelica, Z.A combination of advanced powder diffraction and NMR techniques have allowed the structure of the novel microporous aluminophosphate IST-1 (|(CH3NH2)4(CH3NHþ 3 )4(OH )4|½Al12P12O48 ) to be elucidated. The framework structure was determined in the non-centrosymmetric space group Pca21 (a ¼ 9:61523ð1Þ AA, b ¼ 8:67024ð1Þ AA, c ¼ 16:21957ð2Þ AA) from high-resolution synchrotron powder diffraction data using the program FOCUS. Extra framework species were then located on difference electron density maps. A hydroxyl group was found to bridge between two of the framework Al atoms, and one methylamine species, presumably protonated, could be located in the channels where it H-bonds to three framework oxygens. The most unusual feature of the structure is the second methylamine molecule, which bonds directly to a framework Al atom. The structure is entirely consistent with 31P and 27Al MAS NMR studies, which showed there to be three P (all 4-coordinate) and three Al (one 4-, one 5- and one 6-coordinate) sites, and with 13C MAS NMR, which showed there to be two different types of methylamine species in equal amounts. Assignment of the 31P, 27Al and 13C MAS NMR signals could be deduced from the crystallographic data,31P-27Al HETCOR spectra and ab initio calculations.
- Synthesis and characterization of new CoAPSO-40 and ZnAPSO-40 molecular sieves. Influence of the composition on the thermal and hydrothermal stability of AlPO4-40 -based materialsPublication . Lourenço, J. P.; Ribeiro, M. F.; Borges, C.; Rocha, J.; Onida, B.; Garrone, E.; Gabelica, Z.Highly crystalline CoAPSO-40 and ZnAPSO-40 have been synthesized and characterized by powder XRD, TG/ DSC, SEM, EDX, 27Al, 31P and 29Si solid-state NMR, di use re¯ectance UV±VIS spectroscopy, FT IR and catalytic tests using the m-xylene isomerization and n-heptane cracking as model reactions. The simultaneous incorporation of silicon and a divalent metal in the framework of the AFR structure results in the generation of Brùnsted acid sites similar to those found in SAPO-40 and MeAPO-40. The catalytic activity of the samples for acid catalyzed reactions is higher than that of SAPO-40 with a homogeneous framework silicon distribution. The framework incorporation of cobalt or zinc results in a decrease of the thermal and hydrothermal stability of the metal bearing AFR framework, when compared with AlPO4-40 and SAPO-40.
- Two new aluminophosphates, IST-1 and IST-2: first examples of a dual templating role of water and methylamine in generating microporous structuresPublication . Fernandes, A.; Ribeiro, M. F.; Borges, C.; Lourenço, J. P.; Rocha, J.; Gabelica, Z.This study is aimed at exploring the ability of very small sized N-bearing molecules to generate and stabilize microporous aluminophosphates. Two new AlPO4-n materials, called IST-1 and IST-2, have been obtained in aqueous media using, as main template, methylamine (MA), directly added, or generated in situ from methylformamide (MF) degradation. While IST-1 topology proved to be novel, IST-2 appears structurally related to AlPO4-53(A). The obtained materials were characterized by powder XRD, TG/DSC, SEM and solid-state NMR. Tetraalkylammonium (TEA) cations were used as potential co-templates but only MA and water were found incorporated in the pore volumes of both structures, which argues for their true templating role. In IST-1, 13C solid-state NMR studies showed that half of MA species, presumably protonated, is H-bonded to framework oxygens while the other half surprisingly bonds directly to framework Al atoms. 13C NMR showed that only protonated MA occurs in IST-2 channels. TEA+ cations definitely do not play any specific template role. They indirectly favor the crystallization of IST-1 or IST-2 devoid from other crystalline or amorphous side phases, by interacting with part of the Al and P in solution and forming soluble [AlPO4(OH)]–[TEA,HMA] complexes, substantially modifying the compositions of gels precursors to each phase during nucleation and/or growth steps. While both IST-1 and IST-2 crystallize from gels of similar initial compositions, it was demonstrated that the new MA/T ratio (T = Al or P) obtained after in situ complexation was the key parameter that specifically governs the crystallization of each phase.