27 Al and 29 Si NMR study of sol-gel derived aluminosilicates and sodium alumino (original) (raw)
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27Al and 29Si NMR study of sol-gel derived aluminosilicates and sodium aluminosilicates
Journal of Materials Science, 1988
Solid state 27AI and 29Si NMR was used to examine the structures of aluminosilicates and sodium aluminosilicates prepared by the sol-gel method from metal alkoxides. In contrast to the borosilicate system, where B-O-Si bonds are not formed until heat treatment above 1 50 ~ C, AI-O-Si formation appears complete upon gelation. Aluminium occupies tetrahedral [AIO4]-sites in the polymer network and octahedral [AI(H20)6] 3+ (or similar) sites in the intersticies for charge balance. When sodium is added as a counter ion the octahedral aluminium is converted to tetrahedral aluminium in the oxide network. In gels of high aluminium content prepared from (BuSO)2AI-O-Si(OEt)3, some aluminium in five coordinate environments is also observed. All gels remain amorphous on heating to 800 ~ C.
Physical chemistry of aluminosilicate gels
Microporous and Mesoporous Materials, 1999
The influence of the batch molar ratio YbN=[SiO2]bN/[Al2O3]bN (the subscript bN denotes different batches b (=1, 2, 3 and 4) of different systems N (=I, II, III and IV)) and alkalinity of systems on the distribution of Na, Al and Si between the solid (S) and the liquid (L) phases of sodium aluminosilicate hydrogels prepared at different batch concentrations [Al2O3]bN and different batch alkalinities defined by the molar ratios a=[Na2O/Al2O3]0 (Na2O/Al2O3 molar ratio in the starting aluminate solution) and b=[Na2O/SiO2]0 (Na2O/SiO2 molar ratio in the starting silicate solution) were investigated by measuring the concentrations of Na, Al and Si in both the solid (S) and liquid (L) phases by atomic absorption spectroscopy. Analysis of the experimental results have shown that (1) [Na2O]S=[Al2O3]S=0.0631−0.0638AN−(0.216AN−0.228)/([Na2O]bNYbN), (2) YS=5.925−0.335AN−(6.575−0.415AN)/YbN, and (3) lnYL[Al2O3]S=9.8−(32−2.3AN)/YbN, where YS=[SiO2]S/[Al2O3]S, YL=[SiO2]L/[Al2O3]L and AN=aN+bN. Based on these relationships, it is found that the equation (32−2.3A)YS+(0.415A−6.575)lnYL+20.2605A−0.7705A2=125.17 satisfactorily describes the equilibrium distribution of Na2O, Al2O3 and SiO2 between the solid and liquid phases of aluminosilicate hydrogels prepared in a wide range of batch concentrations [Na2O]bN, [Al2O3]bN [SiO2]bN and [H2O]bN as well as their molar ratios XbN=[Na2O]bN/[Al2O3]bN, YbN=[SiO2]bN/[Al2O3]bN and ZbN=[H2O]bN/[Al2O3]bN.
Chemistry of Materials, 2001
Sol-gel processes have been applied extensively to the preparation of single-and multicomponent oxide gels. 1 In bicomponent systems containing the metals M and M′, homogeneous condensation of MOM′ bridges occur only if the homocondensation reactions for M and M′ (i.e., formation of MOM and M′OM′ bridges) have comparable reaction rates. Therefore, a high degree of homogeneity of bicomponent gels is difficult to achieve by the hydrolytic sol-gel route. 2 The nonhydrolytic solgel approach, pioneered by Corriu et al., reduces the difference in reactivity and is much more efficient for the preparation of homogeneous bicomponent gels. 2 A number of mixed oxide gels with a high level of homogeneity have been recently prepared by using several routes; among them are etherolysis of a mixture of metal chlorides (eq 1), condensation of metal halides with alkoxides (eq 2), 3,4 and transformation of heterometallic alkoxides with pre-existing MOM′ linkages (eq 3). 5 However, in these routes the formation of a MOM′ joint network is still based on a series of complex condensation processes during the gelation stage. This restricts the choice of precursors and limits the preparation of new multicomponent materials.
27Al and 29Si MAS–NMR studies of structural changes in hybrid aluminosilicate gels
Journal of the European Ceramic Society, 2002
Aluminosilicate gels with stoichiometric and nonstoichiometric compositions were synthesized by means of colloidal sol-gel method and their mullitization behavior was studied by X-ray diffraction (XRD), 27 Al and 29 Si magic angle spinning nuclear magnetic resonance (MAS-NMR) experiments. Particular attention was given to the structural changes of matrix accompanying the formation of mullite. The various coordinated Al occupancies were clarified by simulating the 27 Al MAS-NMR spectra with Gaussian lines. The results demonstrate that the so-synthesized aluminosilicate gel is a hybrid gel containing a mixture of a singlephase gel and a diphasic gel. The mullitization of so-formed hybrid gel exhibits a consecutive one-step conversion process, but not a two-step process, much similar to that of a true diphasic mullite gel. The mullite formation from hybrid aluminosilicate gel mainly depends on the nature of dominant matrix part, but not on the nature of minor matrix part in gel. During the formation process of mullite, amorphous Si-rich phase appears as a transitional phase. The effects of gel composition and heating rate on the phase transformation behavior of hybrid aluminosilicate gel were also discussed here. #
Journal of Non-Crystalline Solids, 2005
Solid-state 29Si, 27Al, and 23Na magic angle spinning (MAS) NMR techniques in combination with X-ray powder diffraction (XRD) are used to characterize aluminosilicate gels as a function of composition, pH, and reaction times. These gels were prepared at 80°C using initial solutions with low Si/Al ratios, high alkaline and salt concentrations that are characteristic of nuclear tank wastes. XRD data show that cancrinite and sodalite are the main crystalline phases in the aluminosilicate gels produced. It is found that the pH and the salt content have significant effects on the nature of the aluminosilicate gels. Higher pH appears to increase the rate of crystallization, the degree of overall crystallinity and the percentage of cancrinite phases in aluminosilicate gels, whereas the high-salt concentration promotes the formation of cancrinite and sodalite and prohibits the formation of other zeolites. Complementary to XRD, NMR is extremely useful for providing the information on the structure of amorphous intermediate gels with no long-range order.
Phase segregation of non-stoichiometric aluminosilicate gels characterized by 27Al and 29Si MAS-NMR
Journal of Non-Crystalline Solids, 2002
Polymeric aluminosilicate gels with Al 2 O 3 /SiO 2 molar ratios of 1.3/2, 2.5/2, 3/2 and 4.7/2 were prepared by gelling a mixture of tetraethoxysilane and ethyl acetoacetate aluminium diisopropoxide. Mullite, without any other crystalline phase, directly crystallizes from the gel matrix at about 1000°C for all investigated samples. The Al 2 O 3 or SiO 2 crystalline phase can only be detected at relatively high temperatures accompanying the modification of mullite in the lattice structure. 27 Al and 29 Si MAS-NMR studies indicate that segregation of Al and Si atoms occurs in all samples below 900°C in the amorphous state, regardless of the gel composition. However, the species of segregated units are very different and strongly dependent on the composition of the starting gels. This segregation may not cause the exothermic effect in differential thermal analysis examination, but it appears to be responsible for the composition change behavior of crystalline mullite with different heat treatment.