Aluminosilicates with varying alumina–silica ratios: synthesis via a hybrid sol–gel route and structural characterisation

Abstract

Aluminosilicates with varying Al2O3:SiO2 molar ratios (3:1, 3:2, 3:3 and 3:4) have been synthesized using a hybrid sol-gel route using boehmite sol as the precursor for alumina and tetraethyl orthosilicate (TEOS) as the precursor for silica. The synthesis of boehmite sol from aluminium nitrate, and its use as the alumina precursor, is cost effective compared to alkoxide precursors. Structural aspects, including bonding and coordination, are studied in detail for samples calcined in the temperature range 400-1400 °C using both NMR and FTIR spectroscopy: the results are correlated with phase formation data (spinel and high temperature phases) obtained from XRD and thermal analysis. FTIR results show a broadening of peaks at 800 °C indicating a disordered distribution of octahedral sites caused by crosslinking between AlO6 octahedral and SiO4 tetrahedral units prior to the formation of mullite. (27)Al MAS NMR spectra are consistent with a progressive decrease in the number of AlO6 polyhedra with increasing temperature corresponding to Al in these units being forced to adopt a tetrahedral coordination due to the increasing presence of similarly coordinated Si species. XRD results confirm the formation of pure mullite at 1250 °C for a 3Al2O3:2SiO2 system. At 1400 °C, phase pure mullite is observed for all compositions except 3Al2O3:SiO2 where α-Al2O3 is the major phase with traces of mullite. The synthesis of aluminosilicates through a hybrid sol-gel route and the detailed insight into structural features gained from spectroscopic and diffraction techniques contributes further to the development of these materials in applications ranging from nanocatalysts to high-temperature ceramics.

Tunable white light emission by variation of composition and defects of electrospun Al2O3–SiO2 nanofibers

“…4 , and the corresponding assignments are listed in Table 1 . The IR spectra of our samples are similar to those reported for natural and synthetic silica/alumina composites [ 29 – 37 ]. In the wavenumber range of 400–1300 cm −1 , nine, obvious, characteristic, IR peaks from the composite samples are observed and are located at 475, 530, 573, 618, 730, 790, 848, 878, and 1120 cm −1 .…”

“…The IR peak at ≈475 cm −1 is due to the vibrations of O–Si–O bending modes (SiO 4 ), whose intensity increases with an increase in the Si content. Typically, the stretching modes of an AlO 6 moiety are expected in the region 500–680 cm −1 , whilst comparable modes for AlO 4 appear in the region of 680–880 cm −1 [ 29 – 30 ]. Thus, the characteristic, broad adsorption peaks at 530, 575, and 618 cm −1 are assigned to Al–O stretching modes (AlO 6 ), and the peaks at 730, 848, and 878 cm −1 are due to the vibrations of Al–O stretching modes (AlO 4 ) [ 31 – 32 ].…”

Characteristics of aluminosilicates prepared from rice husk silica and aluminum metal

“…When the sintering temperature was further increased to 1400 °C (Figure 4c) a substantial change in crystallinity of the sample was observed, as characterized by sharp increase of mullite phase and diminish of quartz as well as alumina phase, suggesting that both quart and alumina have completely transformed into mullite. In previous studies 30,31,32 , it has been reported that at temperature around 1400 °C, quartz was converted into liquid silica which penetrated alumina grain aggregates, and the two reacted to produce mullite. This is in good agreement with this study where mullite was the major phase present at 1400 °C.…”

Thermal behaviour of mixed alumina–silica gels obtained from alkoxides: Phase formation and morphology of powders

“…Hence as the practical use of aluminosilicates increases so does the number of research papers dealing with the phase equilibria that occur in the SiO 2 -Al 2 O 3 binary system (in which mullite is the intermediate phase) and with the mechanism and kinetics of crystalline mullite formation [1][2][3]. However, the phase equilibrium diagrams found in the literature differ depending on the source.…”