A MAS NMR investigation of aluminosilicate, silicophosphate, and aluminosilicophosphate gels and the evolution of crysta
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I. INTRODUCTION
Alumina, silica, and phosphorus pentoxide are important compounds in glass chemistry.1 They form glasses both in binary and ternary compositions. With the advent of novel preparative techniques such as the sol-gel method,23 it has been possible to prepare a variety of glasses with unusual compositions which can be crystallized into useful ceramics. Since these oxides react chemically among themselves, it is of some importance to investigate the evolution of product structures during gel-glass-crystal transformations. For example, mullite is one of the technologically important crystalline phases formed in aluminosilicates. A structurally interesting aspect that is worthy of note is a recent report on the presence of octahedrally coordinated silicon in silicophosphates.4 We have been for some time interested in understanding the evolution of glassy and crystalline structures from xerogels through the elimination of water. High resolution (HR) magic-anglespinning (MAS) NMR spectroscopy employing 27A1, 29 Si, and 31P nuclei is eminently suited for the study of systems such as aluminosilicates, silicophosphates, and aluminosilicophosphates in the solid state.5"12 Chemical shifts of these nuclei have been documented in a variety of materials of known structure and chemistry, as shown in Table I.13"21 The sensitivity of chemical shifts a)
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J. Mater. Res., Vol. 6, No. 3, Mar 1991 Downloaded: 14 Mar 2015
of silicon to the second neighbor environment makes Si a particularly appropriate probe in such studies.19 In this paper, we report the results of extensive 27 A1, 29Si, and 31P MAS NMR studies of multicomponent gels prepared from alumina, silica, and P2O5. We have investigated the spectra of a variety of compositions subjected to different heat treatments and have obtained new insight into the nature of the transformations occurring in these systems. We have found it instructive to understand the results in terms of the basicities of the oxide reactants. 29
II. EXPERIMENTAL PROCEDURES
Silica gel was prepared by first hydrolyzing tetra ethoxy silane (TEOS) (MERCK, purity >98%) with water into a sol in ethanol as a solvent. The sol was stirred thoroughly for a few hours and kept in an oven at 333 K and allowed to gel. Gelation occurred within a day and the gel was transparent. A12O3 gel was similarly prepared by hydrolyzing aluminum isopropoxide (Fluka, assay >98% Al) with water using nitric acid as a catalyst. Aluminosilicate gels of different compositions were prepared as follows: first, appropriate quantities of ethanol and TEOS were mixed thoroughly and appropriate quantities of aluminum nitrate (Qualigens, India, AR grade) and water were added to it and stirred again for a few hours. The homogeneous sol thus formed was © 1991 Materials Research Society IP address: 129.49.23.145
S. Prabakar, K. J. Rao, and C. N. R. Rao: A MAS NMR investigation
TABLE I. Chemical shift ranges for different structural units reported in the liter
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