Downloaded by NORTH CAROLINA STATE UNIV on April 16, 2013 | http://pubs.acs.org Publication Date: July 31, 1989 | doi: 10.1021/bk-1989-0398.ch004
Chapter 4
Structure and Properties of Aluminosilicate Solutions and Gels 1
Gillian Harvey and Lesley S. Dent Glasser Department of Chemistry, University of Aberdeen, Aberdeen AB9 2UE, Scotland A range of aluminosilicate solutions were investigated. The gelation behaviour, the species in solution (as observed by NMR) and the zeolite crystallization products are described. The effect of concentration and type of alkali metal cation present in solution gives information about the formation of aluminosilicate complexes and how they interact, under the influence of the cation, to form an aluminosilicate gel, the precursor to zeolite crystallization.
A typical zeolite synthesis involves mixing together silicate and aluminate solutions or sols to form an aluminosilicate gel, usually instantaneously, which is then treated hydrothermally to give the crystalline product. The composition and structure of the aluminosilicate gel are of considerable interest and characterization of the aluminosilicate species present would give insight into the crystallization process. It is important to know what species are present at the beginning of the reaction. Silicate and aluminate solutions have been well studied so that one can be reasonably sure what species are present in a given solution of known concentration and pH. Aluminate solutions have been shown to contain only one type of ion at high pH; the tetrahedral AI(OH)4" ion (1). It is only when the pH drops towards neutral that other, polymeric ions appear which ultimately give way to ΑΙ(Η2θ)β in acid conditions. The tetrahedral aluminate ion is the important species for normal zeolite synthesis. Silicate solutions, again at high pH, contain a range of small silicate polymers, formed from corner-sharing tetrahedral S1O4 units. These polymers depolymerise quickly in response to increased pH or dilution. Rings and cages are the preferred form of the silicate species, while chains larger than trimer are rare (2.3). Various techniques have been used to 3+
1
Current address: Institute for Crystallography and Petrography, ΕΤΗ, CH-8092, Switzerland 0097-6156/89/0398-ΟΟ49$Ο6.00/0 ο 1989 American Chemical Society In Zeolite Synthesis; Occelli, M., et al.; ACS Symposium Series; American Chemical Society: Washington, DC, 1989.
ZEOLITE SYNTHESIS
50
investigate the silicate species in solution; the most successful being trimethylsilylation (4) and S i NMR. S i NMR using enriched silica and selected Si-Si decoupling has made it possible to identify and quantify many of the small species present (5-7). It was also found that the range of species present is largely independent of the type of alkali metal cation (Na+ - Cs+) present (4). A crystalline zeolite can also be studied by a variety of methods including crystallography and NMR but the intermediate phase, the gel, has proved very resistant to any type of study. The aim of this work, therefore, was to delay the formation of the gel long enough to investigate the precursor solution because solutions are generally easier to study than gels. Aluminosilicate solutions, containing sodium or alkylammonium cations, have been studied previously. Guth et al (8-10) studied changes as aluminate and silicate solutions were mixed using pH, conductivity and Laser Raman measurements. They showed that aluminosilicate complexes were formed. Other experiments studied the crystallization of zeolites directly from solution (11-13). In solutions from which zeolites A and X crystallized, it was possible to observe aluminosilicate complexes by AI NMR. It was also stated that crystal nuclei, equivalent to one or two unit cells, were present in the solution. Bell et al (14) observed aluminosilicate species by both AI and S i NMR in solutions of monomeric silicate solutions doped with aluminate. AI NMR spectra showed a broadened and shifted AI(OH) " peak, and two additional peaks, attributed to aluminium attached to one and two silicate species. The S i NMR spectra show the Q° peak (monomeric silicate) considerably broadened and shifted. Such complexes were also observed in tetramethylammoniun aluminosilicate solutions, again by AI NMR (15-17). These solutions are much less prone to gelling and so can be prepared much more concentrated than other aluminosilicate solutions. Eventually double-four-ring aluminosilicates crystallize from solution. The AI NMR spectra of these solutions showed four distinct peaks which were interpreted as aluminium attached to zero, one, two and three silicate groups. In this work aluminosilicate solutions containing Na , K+, Rb or C s cations were studied by a range of different methods: the length of time the solution takes to gel, the light scattering behaviour as it gels, and the species present in solution as determined by AI and S i NMR. The factors that influence the gelation of the solutions were found to be; composition, cation present and the silicate species present at the beginning of the reaction, i.e. the sequence of mixing. All solutions or gels were then treated hydrothermally to investigate the relationship between the gel properties and the type of zeolite produced.
Downloaded by NORTH CAROLINA STATE UNIV on April 16, 2013 | http://pubs.acs.org Publication Date: July 31, 1989 | doi: 10.1021/bk-1989-0398.ch004
2 9
2 9
2 7
27
2 9
27
4
2 9
27
27
+
27
+
+
2 9
EXPERIMENTAL
Solutions; preparation and composition Silicate solutions of ratio 1 : 0.6 S i 0 : alkali (MOH) were prepared from fumed silica and AnalaR alkali hydroxides. Aluminate solutions were 2
In Zeolite Synthesis; Occelli, M., et al.; ACS Symposium Series; American Chemical Society: Washington, DC, 1989.
Aluminosilicate Solutions and Geh
Downloaded by NORTH CAROLINA STATE UNIV on April 16, 2013 | http://pubs.acs.org Publication Date: July 31, 1989 | doi: 10.1021/bk-1989-0398.ch004
4. HARVEY AND CLASSER
51
prepared by direct reaction of aluminium wire and alkali hydroxide. There were two methods of mixing the solutions together: 1) All excess water and alkali were added to the aluminate solution, which was then mixed with the silicate solution. This ensured that the range of silicate species was identical at the start of every reaction. 2) All or some of the excess alkali ( 22, 44, 66 and 88% of total alkali) was added to the silicate solution, which was allowed to equilibriate and then mixed with the aluminate solution. Thus the silicate species were progressively depolymerised in response to the extra alkali at the beginning of the reaction. Forty different compositions of solution were studied (Figure 1 ) with special emphasis on nine representative solutions (shown in the boxes of Figure 1). The compositions of these nine solutions are given in Table I. Solutions containing sodium, potassium, rubidium or caesium ions were studied. Al: S i
1:1
1:2
1:3
1:4
2e
3e
4e
2d
3d
4d
3c
4c
2b
3b
4b
2a
3a
4a
5le
Id
1:5
1:6
1.7
5e
6e
7e
5d
6d
7d
5c
6c
7c
5b
6b
7b
5a
6a
7a
L ι—ι
3
Ο lc
·-·
