Aluminosilicate

About: Aluminosilicate is a research topic. Over the lifetime, 6004 publications have been published within this topic receiving 133486 citations. The topic is also known as: aluminium silicate & aluminium silicates.

The geopolymerisation of alumino-silicate minerals

Abstract: Geopolymers are similar to zeolites in chemical composition, but they reveal an amorphous microstructure. They form by the co-polymerisation of individual alumino and silicate species, which originate from the dissolution of silicon and aluminium containing source materials at a high pH in the presence of soluble alkali metal silicates. It has been shown before that geopolymerisation can transform a wide range of waste alumino-silicate materials into building and mining materials with excellent chemical and physical properties, such as fire and acid resistance. The geopolymerisation of 15 natural Al–Si minerals has been investigated in this paper with the aim to determine the effect of mineral properties on the compressive strength of the synthesised geopolymer. All these Al–Si minerals are to some degree soluble in concentrated alkaline solution, with in general a higher extent of dissolution in NaOH than in KOH medium. Statistical analysis revealed that framework silicates show a higher extent of dissolution in alkaline solution than the chain, sheet and ring structures. In general, minerals with a higher extent of dissolution demonstrate better compressive strength after geopolymerisation. The use of KOH instead of NaOH favours the geopolymerisation in the case of all 15 minerals. Stilbite, when conditioned in KOH solution, gives the geopolymer with the highest compressive strength (i.e., 18 MPa). It is proposed that the mechanism of mineral dissolution as well as the mechanism of geopolymerisation can be explained by ion-pair theory. This study shows that a wide range of natural Al–Si minerals could serve as potential source materials for the synthesis of geopolymers.

Amphiphilic organosilane-directed synthesis of crystalline zeolite with tunable mesoporosity

Abstract: Zeolites are a family of crystalline aluminosilicate materials widely used as shape-selective catalysts, ion exchange materials, and adsorbents for organic compounds. In the present work, zeolites were synthesized by adding a rationally designed amphiphilic organosilane surfactant to conventional alkaline zeolite synthesis mixtures. The zeolite products were characterized by a complementary combination of X-ray diffraction (XRD), nitrogen sorption, scanning electron microscopy (SEM) and transmission electron microscopy (TEM). The analyses show that the present method is suitable as a direct synthesis route to highly mesoporous zeolites. The mesopore diameters could be uniformly tailored, similar to ordered mesoporous silica with amorphous frameworks. The mesoporous zeolite exhibited a narrow, small-angle XRD peak, which is characteristic of the short-range correlation between mesopores, similar to disordered wormhole-like mesoporous materials. The XRD patterns and electron micrographs of the samples taken during crystallization clearly showed the evolution of the mesoporous structure concomitantly to the crystallization of zeolite frameworks. The synthesis of the crystalline aluminosilicate materials with tunable mesoporosity and strong acidity has potentially important technological implications for catalytic reactions of large molecules, whereas conventional mesoporous materials lack hydrothermal stability and acidity.

Synthesis and characterisation of materials based on inorganic polymers of alumina and silica: sodium polysialate polymers

Abstract: Inorganic polymers based on alumina and silica polysialate units were synthesised from dehydroxylated aluminosilicate clay (metakaolinite) condensed with sodium silicate in a highly alkaline environment. Reaction of the aluminosilicate with alkali polysilicates yields polymeric Si–O–Al three-dimensional structures with charge-balancing positive ions such as hydrated Na+ in the framework cavities. A statistical study of the effect on the polymerisation process of the molar ratio of the component oxides and the water content of the mixture showed the latter to be a critical parameter. The polymerisation mechanism and structures of the products were investigated using NMR, XRD and FTIR spectroscopy. 29Si liquid-state NMR shows that some compositions do not cure properly because of incomplete reaction of the sodium silicate with the metakaolinite. FTIR confirms that during drying of the incompletely cured samples, Na migrates to the surface where it undergoes atmospheric carbonation. The cured polymers were found to be essentially X-ray amorphous, with bulk densities of 1.3–1.9. During polymerisation the coordination of Al in the metakaolinite reactant (IV, V and VI) changes almost completely to IV in all the polymer compositions. The environment of the Na is unchanged irrespective of the polymer composition. The solid-state 29Si NMR spectra indicate a range of Si–O–Al environments. Typical mechanical properties of the best polymers were: Mohs hardness >7, Vickers hardness about 54, and compressive strength (after drying for 1 h at 65°C) 48.1 MPa.

Studies on mesoporous materials: I. Synthesis and characterization of MCM-41

Abstract: The mesoporous molecular sieves MCM-41 are synthesized and characterized by X-ray powder diffraction, nitrogen adsorption/desorption, cyclohexane and water adsorption, transmission electron micrographs (TEM), thermogravimetric analysis (TGA), ammonia temperature-programmed desorption (TPD), FTIR, FT-Raman, 29Si, 27Al and 13C magic angle spinning (MAS) NMR spectroscopy. Raman spectra from these materials exhibit a common band at ∼610 cm−1 assignable to cyclic trisiloxanes (3-membered rings), indicating in combination with TEM, IR and NMR results that the inorganic portion of MCM-41 resembles amorphous silicas or aluminosilicates rather than crystalline molecular sieves in terms of the local structure and bonding. Pure-silica MCM-41 can be heated to at least 850°C in dry air or 800°C in air with 8 Torr water vapor before structural collapse begins. Using sodium aluminate as the aluminum reagent, aluminosilicate MCM-41 can be prepared with a Si/Al ratio as low as 29 without observing the presence of octahedral aluminum. Such is not the case when using Catapal alumina. Adsorption of cyclohexane and water reveals that pure-silica and aluminosilicate (Si/Al = 39) MCM-41 are both hydrophobic. Based on ammonia TPD results, aluminosilicate MCM-41 has acidity similar to that of amorphous aluminosilicates.