Fraction Of Tetrahedral Sites Research Articles

Pyridine adsorption on NiSn/MgO–Al2O3: An FTIR spectroscopic study of surface acidity

The acid–base properties of MgO–Al2O3 supports and NiSn/MgO–Al2O3 catalysts were evaluated by IR spectroscopy using pyridine as a probe molecule. The results indicate that only Lewis acid sites were detected on the surface of the supports as well as on the catalysts. Nevertheless, Brønsted acid sites were not detected. In the support without MgO three kinds of coordinatively unsaturated acid sites were detected: Al3+ cations occupying octahedral, tetrahedral and tetrahedral with cationic vacancy in the neighbourhood. The last sites appear as the strongest. Moreover, they are able to activate the pyridine molecules leading to the formation of an intermediate α-pyridone complex. When MgO or NiO were added to the alumina, the number and strength of the Lewis acid sites decreased and significant changes were observed in the tetrahedral sites with adjoining cation vacancies. The incorporation of the Mg2+ cations into the alumina's structure takes place on the vacant tetrahedral positions, forming spinel MgAl2O4. As a result, the fraction of tetrahedral sites with adjoining cationic vacancy diminished and the intermediate α-pyridone complex in the support with the highest MgO loading was hardly detected. The addition of Ni2+ cations leads to the filling of the free octahedral positions, resulting in the formation of a NiAl2O4 spinel structure and the thermal stability of the α-pyridone species decreases. In the catalysts, the progressive reduction of the number and strength of the Lewis acid sites is due to a competitive formation of the two types of MgAl2O4 and NiAl2O4 spinels. In the catalyst NiSn/30MgO–Al2O3 no cationic vacancies were detected and the surface reaction with α-pyridone formation did not occur.

Raman and11B Nuclear Magnetic Resonance Spectroscopic Studies of Alkaline‐Earth Lanthanoborate Glasses

Glasses from the RO.La2O3.B2O3(R = Mg, Ca, and Ba) systems have been examined. Glass formation is centered along the metaborate tie line, from La(BO2)3to R(BO2)2. Glasses generally have transition temperatures >60°C and expansion coefficients between 60 × 10‐7/°C and 100 × 10‐7/°C. Raman and solid‐state nuclear magnetic resonance spectroscopies reveal changes in the metaborate network that depend on both the [R]:[La] ratio and the type of alkaline‐earth ion. The fraction of tetrahedral sites is generally reduced in alkaline‐earth‐rich glasses, with magnesium glasses possessing the lowest concentration of B[4]. Raman spectra indicate that, with increasing [R]:[La] ratio, the preferred metaborate anion changes from a double‐chain structure associated with crystalline La(BO2)3to the singlechain and ring metaborate anions found in crystalline R(BO2)2phases. In addition, disproportionation of the metaborate anions leads to the formation of a variety of other species, including pyroborates with terminal oxygens and more‐polymerized species, such as diborates, with tetrahedral borons. Such structural changes are related to the ease of glass formation and some of the glass properties.