A new approach on the coordination of al in non-crystalline gels and glasses of the system Al2O3-SiO2

Structural Role of Phosphate in Metaluminous Sodium Aluminosilicate Glasses As Studied by Solid State NMR Spectroscopy

In this contribution we present a detailed study of the effect of the addition of small to intermediate amounts of P₂O₅ (up to 7.5 mol %) on the network organization of metaluminous sodium aluminosilicate glasses employing a range of advanced solid state NMR methodologies. The combined results from MAS, MQMAS (multiple quantum MAS), or MAT (magic angle turning) NMR spectroscopy and a variety of dipolar based NMR experiments-²⁷Al{³¹P}-, ²⁷Al{²⁹Si}-, ²⁹Si{³¹P}-, and ³¹P{²⁹Si}-REDOR (rotational echo double resonance) NMR spectroscopy as well as ³¹P{²⁷Al}- and ²⁹Si{²⁷Al}-REAPDOR (rotational echo adiabatic passage double resonance) NMR-allow for a detailed analysis of the network organization adopted by these glasses. Phosphate is found as Q_P², Q_P³, and Q_P⁴ (with the superscript denoting the number of bridging oxygens), the Q_P⁴ units can be safely identified with the help of ³¹P MAT NMR experiments. Al exclusively adopts a 4-fold coordination. The withdrawal of a fraction of the sodium cations from AlO₄ units that is needed for charge compensation of the Q_P² units necessitates an alternative charge compensation scheme for these AlO₄ units via formation of Q_P⁴ units or oxygen triclusters. The dipolar NMR experiments suggest a strong preference of P for Al with an average value of ca. 2.4 P-O-Al connections per phosphate tetrahedron. P is thus mainly integrated into the network via P-O-Al bonding, the formation of Si-O-P bonding plays only a minor role.

Structural and dynamic properties of aluminosilicate melts: a molecular dynamics study

In the present work, the structural and dynamic properties of aluminosilicates (Al₂O₃) _x -(SiO₂)_1-x (AS) as a function of the Al₂O₃ concentration x are studied by means of molecular dynamics simulations. Firstly, the parametrization of the Born-Mayer-Huggins type potential developed recently for the more general CaO-Al₂O₃-SiO₂ ternary system is assessed. Comparison of local structural properties, such as the x-ray structure factor, partial pair-correlation functions, distributions of coordination numbers and bond angles, as well as the dynamics through the viscosity and self-diffusion coefficients to experimental data and other molecular dynamics simulations found in the literature, shows that this potential is transferable to AS melts for all compositions and is more reliable than other empirical potentials used so far. The evolution of viscosity with temperature in stable liquid and undercooled regions is studied in the whole composition range and results show a progressive increase of the fragility with increasing Al₂O₃ content correlated to that of local structural entities like the triply bonded oxygen (TBO), AlO₅ and AlO₆.

Structure and diffusion in amorphous aluminum silicate: A molecular dynamics computer simulation

The amorphous aluminum silicate (Al2O3)2(SiO2) [AS2] is investigated by means of large scale molecular dynamics computer simulations. We consider fully equilibrated melts in the temperature range 6100 K> or =T> or =2300 K as well as glass configurations that were obtained from cooling runs from T=2300 to 300 K with a cooling rate of about 10(12) K/s. Already at temperatures as high as 4000 K, most of the Al and Si atoms are fourfold coordinated by oxygen atoms. Thus, the structure of AS2 is that of a disordered tetrahedral network. The packing of AlO4 tetrahedra is very different from that of SiO4 tetrahedra in that Al is involved with a relatively high probability in small-membered rings and in triclusters in which an O atom is surrounded by four cations. We find as typical configurations two-membered rings with two Al atoms in which the shared O atoms form a tricluster. On larger length scales, the system shows a microphase separation in which the Al-rich network structure percolates through the SiO2 network. The latter structure gives rise to a prepeak in the static structure factor at a wave number q=0.5 A(-1). A comparison of experimental x-ray data with the results from the simulation shows good agreement for the structure function. The diffusion dynamics in AS2 is found to be much faster than in SiO2. We show that the self-diffusion constants for O and Al are very similar and that they are by a factor of 2-3 larger than the one for Si.

Surface Acidity and Hydrophilicity of Coprecipitated Al(2)O(3)-SiO(2) Xerogels Prepared from Aluminium Nitrate Nonahydrate and Tetraethylorthosilicate

Amorphous aluminosilicate xerogels with various chemical compositions were prepared by coprecipitation, and their surface acidity and hydrophilicity were investigated by NH(3) gas temperature programed desorption (TPD), water vapor adsorption-desorption isotherms, and (27)Al magic angle spinning nuclear magnetic resonance (MAS NMR). The xerogels were synthesized by adding conc. NH(4)OH to an ethanol solution of calculated amounts of aluminium nitrate nonahydrate and tetraethylorthosilicate, and calcined at 300 degrees C for 4 h. All the NH(3) TPD spectra of the xerogels showed similar asymmetric peak profiles at around 200 degrees C tailing to the higher temperature side. The amount of acidity evaluated from the peak area of the TPD spectra showed a maximum at around 10 mol% Al(2)O(3) composition. The change as a function of composition showed a good correlation with the total amount of four and five coordinated Al atoms in the xerogels deduced from the (27)Al MAS NMR spectra. The water vapor adsorption isotherms of the xerogels were all of type IV irrespective of the composition. The maximum amounts of water vapor adsorbed by these xerogels were about 600-700 ml(STP)/g and were relatively high compared with those for various other adsorbents reported so far. Since the thickness of the adsorbed water vapor layer of the xerogels in the low relative pressure region increased with increasing Al(2)O(3) content, the surface of the xerogels is considered to become more hydrophilic with increasing Al(2)O(3) content of the xerogels. Copyright 1999 Academic Press.