The crystal morphology of zeolite A. The effects of the source of the reagents
Introduction
Zeolite crystal morphology results from the gel composition, the nucleation process, the chemistry at the crystal surface and the kinetics of the crystal growth. The rates of the reaction correlate directly with the basic units involved in the growth of the crystal [1]. Industrial synthesis is guided by using economical hydrothermal conditions. It has developed to allow the construction of defined frameworks with specific geometry, adjustable acid strengths and selectable electric fields [2].
Zeolite A, discovered by Milton in 1956 3, 4, is of great industrial importance as an adsorbent and detergent builder. Large crystals have been grown in sodium systems to study their catalytic, sorptive and ion-exchange properties [5]. Work with many known zeolite structure types has been carried out to study the influential factors affecting general synthesis. This includes molar composition of the starting gels, synthesis time and temperature, the cation source, ageing procedures, stirring, seeding and the order of the mixing of the gels 6, 7, 8, 9, 10, 11, 12, 13. It is generally accepted that the nucleation process is kinetically controlled and the chemical and physical nature of the reactants before crystallisation is a kinetic variable [14]. The primary influence of the molar ratios of reagents is also well documented [15], as are the effects of organic substances used in synthesis procedures [16]. The source of the reagents including mixed alkali–organic base systems (i.e. NaOH, KOH and TMAOH) have been studied [17]. Single and binary cation systems using cations from both inorganic and organic sources [18]have also been used.
The effects of using different sources of aluminium and/or silicon have been studied more recently in zeolites NaX [19], Omega [20], Beta [21], ZSM-5 [22]and Mordenite [23]. However, recent work appears to have neglected the study of effects of the starting materials used in the synthesis of zeolite A, in particular the source of the aluminium. This research investigates the use of inorganic and organic silicon and aluminium compounds, and their effects on morphology and crystallinity, in the synthesis of zeolite A.
Section snippets
Experimental
The molar composition of the starting reaction gel is given in Table 1. Chemical analysis of the starting materials was determined by X-ray fluorescence (XRF) using an ARL 8410 sequential X-ray spectrometer. Samples of ∼1 g were sandwiched between 4 μm of Prolene (X-ray transparent film) prior to analysis. The reagents used were aluminium isopropoxide as aluminium isopropylate (98+% Aldrich Chimie) analysed as containing 0.9% Ca; aluminium metal as fine powder, B.D.H., with 0.88% Fe; deionised
Mixing procedure
A preliminary investigation was undertaken. All sources of reagents were used to establish an order of addition and a mixing procedure which produced the most consistently crystalline product. The mixing of each system differed slightly due to the nature of the source reagent. The procedures are described on the basis of the aluminium source, the silica source and the method of combination.
Experimental methods—the systems investigated
The stoichiometry as calculated produced 200 g of zeolite gel using mixing procedure B. The pH was measured after the 30 min ageing period, and the characteristic behaviour of the solutions in each system was observed. The gel was divided between six PTFE bottles which were then sealed. No intentional seeding of the gels was employed.
The gels were placed in an oven, controlled at the synthesis temperature, and sampled over a 14 day period. After removal from the oven the samples were quenched and
The hydroxide concentration
Sufficient sodium cations were combined with the reagent sources in systems 4 and 5, using sodium silicate and sodium aluminate, to omit the NaOH (Table 1). In systems 6 and 7, using sodium aluminate, the stoichiometric volume of NaOH was reduced by 50% as the cations were available in the aluminium source. This adjustment also reduced the [OH]− by the equivalent mole fraction. These four syntheses were then repeated to include the stoichiometric quantity of [OH]−. Table 2 lists the
Results
System one, using tetraethyl-orthosilicate and aluminium isopropoxide, the aluminium source took several minutes to dissolve. The addition of the silica source was exothermic with a distinct odour of alcohol. Stirring resulted in an opaque smooth gelatinous liquid. Over the 14 day sampling period the pH value reduced by 1.8. This system reached a maximum of 62% crystallinity (Table 4), and matched XRD reflections with reference to JCPDS pattern 38–241. SEM (Table 5) showed the sample to consist
Discussion
From the summary of crystal morphologies (Table 5) it is clear that the source of the aluminium reagent had a predictable effect. Using organic alumina the cubes always had deep chamfered edges (systems 1, 3 and 5). In contrast, the systems using sodium aluminate always produced cubes with sharp edges (systems 4, 6 and 7). This was consistent in systems with a low hydroxide concentration (systems 4, 5, 6, and 7), and in systems which produced poorly crystalline material (systems 1, 4a, 6 and
Conclusions
The source of the reagents used in the synthesis of zeolite A affects the gel rheology, the kinetics of the gel chemistry and the activity at the crystal surface, producing predictable crystal morphologies. Systems using aluminium isopropoxide always produced cubic morphology with deep chamfered edges. Systems using sodium aluminate always produced sharp edged cubic morphology. The system using teraethyl-orthosilicate with elemental aluminium produced an irregular hexagonal morphology.
These
Acknowledgements
The authors thank Mr Jon Allen for XRF analysis, Mr Brian Bucknall for XRD analysis, Mr David Crane for electron microscopy and Dr C.V.A. Duke for FTIR analysis.
References (29)
J. Cryst. Growth
(1971)J. Colloidal Interface Sci.
(1968)- et al.
Zeolites
(1993) - et al.
Zeolites
(1987) - et al.
Zeolites
(1988) - et al.
Zeolites
(1982) - F. Fajula, NATO ASI Series B (1989)...
- R.M. Barrer, Hydrothermal Chemistry of Zeolites, Academic Press,...
- et al.
J. Am. Chem. Soc.
(1956) - et al.
J. Am. Chem. Soc.
(1956)
J. Phys. Chem.
Adv. Chem. Ser.
Adv. Chem. Ser.
Am. Chem. Soc. Symp. Ser.
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