Browsing by Author "Onida, B."
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- 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.
- Spectroscopic characterization of hydroxyl groups in SAPO-40. Part I: Study of the template-free samples and their interaction with ammoniaPublication . Onida, B.; Gabelica, Z.; Lourenço, J. P.; Garrone, E.FTIR, MAS 1H-NMR, and molecular graphics data are reported concerning the hydroxyl species in a series of SAPO-40 samples with variable Si contents, as well as FTIR data concerning the interaction of ammonia with the different acid sites. Up to seven types of Si(OH)Al Brønsted sites are present, mainly differing by the O-H stretching frequency. Three high-frequency bands (3637-3624 cm-1) correspond to hydroxyls located in the 12-member channels and two low-frequency bands (3559 and 3528 cm-1) to OH species in eight-member rings and side pockets of the AFR structure. Two bands at exceptionally low frequency (3485 and 3445 cm-1 ) are due to OH species interacting with silica islands. All low-frequency bands behave with temperature as H-bonded species. All types of hydroxyls form ammonium ions, the thermal decomposition of which shows some difference in acidity among the OH species. IR evidence indicates that NH4 + species interact strongly with the basic oxygen atoms of the lattice. No Lewis sites were evidenced by ammonia adsorption.
- Spectroscopic characterization of the hydroxyl groups in SAPO-40. Part II: interaction with CO and N2Publication . Onida, B.; Gabelica, Z.; Lourenço, J. P.; Ribeiro, M. F.; Garrone, E.SAPO-40 exhibits five main OH species, describable as high-frequency ones (HF, at 3737, 3730, and 3724 cm-1) and low-frequency ones (LF1 and LF2, at 3559 and 3528 cm-1, respectively), the former probably sitting in 12-member-ring channels and the latter either in side pockets or eight-member-ring channels of the AFR structure. The low-temperature interaction of such OH species with CO shows that HF species have strictly similar acidity (comparable to that of HY zeolites), in contrast with previous results concerning H-transfer to NH3. The ease of interaction and the spectral features are in accord with the location in large cavities. The vibration of H-bonded hydroxyl has a noticeable structure with subbands. The interaction of CO with LF1 and LF2 species occurs with difficulty and yields nonlinear complexes because of the steric hindrance brought about by small cavities. Adsorption of N2 follows similar patterns.
- 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.