Aluminofosfatos e magnesoaluminofosfatos mesoporosos obtidos sobre arranjos supramoleculares

AUTOR(ES)
DATA DE PUBLICAÇÃO

2000

RESUMO

Mesoporous aluminophosphates and magnesium-aluminophosphates have been synthesized with the use of cetyltrimethylammonium bromide (CTAB) as the structure-directing agent in the presence of tetramethylammonium hydroxide (TMAOH). The influence of the concentration of the structure-directing agent, duration of synthesis, temperature of the hydrothermal treatment, order of addition of reagents, presence of mineralizing agent and influence of Mg/AI molar ratios was analyzed. A hexagonal mesostructure has been evidenced by the presence of two very well defined diffractions within the range of 2q, between 2° and 5°. The same mesostructure was observed in the aluminophosphates and magnesium-aluminophosphates that have been prepared with a concentration of the structure-directing agent of 0.5 minimum, regardless of the concentration of magnesium added to the gel, within the range of 0.005 to 0.030 that has been studied. A method to remove the organic phase which would not lead to the collapse of the structure, was also studied. Among the usual processes described in the literature, the extraction with water followed by the addition of ethanol, the calcination and the Soxhlet extraction in acid solutions resulted in the collapse of the mesostructure, probably because of the incomplete polymerization of the inorganic matrix. As an alternative to those procedures, the Soxhlet extraction has been conducted in the alkaline medium, with the use of a solution of ethanol/n-butylamine that has proved to be efficient for the removal of CTAB. It was observed the partial disorganization of the structure to non-parallel tubes which remains as such after calcination. The products have been characterized by elemental analyses (ICP-AES), powder X-ray diffraction (XRD), small angle X-ray scattering (SAXS), infrared spectroscopy (IR), thermogravimetry (TG) and derivative thermogravimetry analyses (DTG), magic-angle spinning nuclear magnetic resonance spectroscopy (MAS-NMR) of P and AI, transmission electron microscopy (TEM), scanning electron microscopy (SEM) surface area and pore-size measurements by N2 adsorption. The confirmation of the presence of Brönsted acidic sites generated in the structure by the incorporation of magnesium in the framework has been made by thermal analyses (DTG) on NH4 exchanged samples. It has been observed that the increase of the concentration of magnesium in the material determines an increase in the total amount of Brönsted acidic sites, what can be confirmed by the increase of the ammonia evolution in approximately 600K from ammonium exchanged samples. FTIR spectrum of adsorbed pyridine on a dehydrated magnesium-aluminophosphate sample showed only a band in 1447 cm suggesting that Lewis acidity is prevalent in this type of material.

ASSUNTO(S)

extração (quimica) fosfatos porosidade

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