Effects of low-temperature gel aging on the synthesis of zeolite Y at different alkalinities
Introduction
The size of zeolite crystals is a significant parameter that may affect the performance in several applications related to catalysis, diffusion, adsorption, ion exchange, etc. Although the filtration and recovery of small zeolite crystals may be a technological challenge, small crystals are desired for obtaining higher catalytic effectiveness, lower coke formation, faster diffusion and higher external surface area as well as for easier cation exchange and template extraction [1]. Thin zeolite films to be utilized in potential applications, such as those related to the membrane separation [2] or sensing of molecules [3] can also be prepared by using the synthesis conditions that allow the formation of small crystals. The utilization of larger crystals, on the other hand, may be favorable for other purposes. The determination of crystalochemical parameters, the generation of intrinsic crystal diffusion and adsorption data and obtaining higher selectivity in shape-selective catalysis requires the employment of larger crystals [4]. In some applications, especially those related to adsorption and catalysis, the utilization of an optimum size of zeolite crystals may be required for achieving the desired performance [5], [6].
The crystal size of zeolites is determined by the relative rates of the two competing phenomena occurring during synthesis, namely, nucleation and crystal growth. The higher the nucleation rate, the smaller the zeolite crystals obtained. Several parameters take part in the establishment of the relative rates of nucleation and crystal growth. Varying these parameters may lead to changes in the sizes of zeolite crystals. For example, the rates of both nucleation and growth are known to decrease as the temperature is lowered [7], [8]. Since the growth rate seems to vary more strongly than the nucleation rate with temperature, as the temperature is lowered, the decrease occurring in the nucleation rate remains less significant, resulting in the preparation of smaller zeolite crystals. Using seeding techniques, modifying the alkalinity, dilution, ionic strength and stirring conditions of the synthesis medium and aging the reaction mixture may also affect the relative rates of nucleation and crystal growth [1]. Aging the synthesis mixture prior to crystallization is a common method used to tailor the size of zeolite crystals. Aging, especially at room temperature, may favor the process of nucleation significantly. It is generally hypothesized that the nucleation rate is significant at room temperature while the growth rate is negligible and nuclei lie dormant until the temperature is raised. Thus, aging at relatively low temperatures may allow for an increase in the number of nuclei or nuclei precursors, which in turn allows the preparation of smaller zeolite crystals.
The effect of aging on synthesis has been investigated and described for some zeolites, especially for zeolite A [9], [10]. Aging is generally considered to play a significant role in the synthesis of zeolite Y, especially to suppress phases other than faujasite. Some studies have already been carried out to determine how aging influences the synthesis of zeolite Y and smaller zeolite Y crystals have been obtained by using different methods in which aging is a noteworthy tool [11], [12], [13], [14]. However, the studies performed up to now, some of which employ two-step synthesis methods, seem to have excluded some significant points. For example, the effects of aging at temperatures lower than room temperature and the relationship of aging with the composition of the reaction mixture have not been investigated in the synthesis of zeolite Y. Additionally, the variations occurring in the crystal size with gel aging have been monitored only by taking into consideration the average diameter of the crystals, as observed by SEM, while possible changes in the crystal size distribution have been overlooked. In this study, the effects of aging on the synthesis of zeolite Y, prepared by using single-step conventional synthesis, were investigated. The variations of certain parameters, such as crystal size, size span of particles, crystallinity and Si/Al ratio of the zeolite, with aging time were monitored at two different temperatures and compositions. In the former case, the purpose was to determine the effects of aging in a relatively cold environment (4°C). In the latter case, the alkalinity of the reaction mixture was varied in order to observe the effects of low-temperature aging at a composition of higher alkalinity. Chemical analyses, X-ray diffraction (XRD) and particle size measurements were carried out to obtain the results.
Section snippets
Experimental procedure
Zeolite Y was synthesized from two different gel compositions, in which only the amount of Na2O was varied. The mole oxide compositions of the two gels were 4.16 Na2O:1 Al2O3:10 SiO2:205 H2O and 5.3 Na2O:1 Al2O3:10 SiO2:205 H2O, respectively. The former composition was reported in the literature to yield pure zeolite Y [15]. The aluminosilicate gels were prepared by using precipitated silica (EgeKimya), sodium aluminate, sodium hydroxide and distilled water. Sodium aluminate (13.97% Al2O3,
Results and discussion
The effects of aging on the synthesis of zeolite Y were investigated. Two distinct gel compositions were utilized in the experiments. As mentioned before, the compositions were 4.16 Na2O:1 Al2O3:10 SiO2:205 H2O and 5.3 Na2O:1 Al2O3:10 SiO2:205 H2O. From now on in this study, these compositions will be referred to as C1 (lower alkalinity) and C2 (higher alkalinity), respectively. The aging of the aluminosilicate gel was carried out at two different temperatures (4°C and 25°C) for different
Conclusions
Smaller zeolite Y crystals could be prepared by the aging of aluminosilicate gels having two different compositions. The lower aging temperature of 4°C favored the formation of smaller zeolite Y crystals, especially when the less alkaline gel composition was used. The favorable effect of the low-temperature aging on the crystal size indicates that significant synthesis activity, which probably results in a structural arrangement in the gel during aging, prevails even at temperatures as low as
Acknowledgements
This work was carried out as part of a project sponsored by NATO's Scientific Affairs Division in the framework of the Science for Peace Programme.
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