Research paperAl–Mn-silicate nanobubbles phase as an intermediate in zeolite formation
Graphical abstract
Introduction
Clay minerals have always been of great interest due to their attractive properties especially their high specific surface area and natural abundance. For these reasons, clays have been widely used to catalyze different reactions (Adams and McCabe, 2006, Patel et al., 2008, Pawar et al., 2009). Much work has been carried out to synthesize smectites, chlorites, or kaolinites in hydrothermal conditions (Jaber et al., 2013). The major advantage of synthetic materials over natural is that it can be produced in high purity. Synthetic clay minerals may also offer improved specific properties (textural and structural properties) as compared to natural ones (Di et al., 2010); in addition, they provide the possibility to fine-tune the chemical composition of the clay network, introducing elements that are not present in great amounts in most natural clays. Thus, in the last few years, transition metal (Cu, Co, Zn and Mn)-rich clay minerals have attracted much interest to establish new specific catalytic applications (Di et al., 2010, Sivaiah et al., 2011). In addition, synthetic clays may offer many advantages: controlled composition, high active sites dispersion and better reproducibility of the properties. Recently, some researchers have been trying to improve the physicochemical properties of kaolinite by incorporating transition metals into its framework. Very few data exist on the synthesis of Mn-containing kaolinite. Komarneni's group has reported that it is possible to prepare Mn-kaolinite under hydrothermal conditions using as starting materials silicic acid, aluminum nitrate and Mn carbonate mixed with aqueous solutions of NaOH or KOH (Choi et al., 2009, Seliem et al., 2010). The synthesis was performed at 100–200°C for 24–96 h. The best result was found using the composition 3.83K2O:0.08Al2O3:0.30MnO:0.43SiO2:1380H2O. Based on XANES spectroscopy, these authors have reported that manganese is octahedrally coordinated and its oxidation state is (+ III). Thus, they deduced that both Al and Mn lie in the octahedral sites. When attempting to reproduce these published syntheses using NaOH in smaller batches, the main reaction product was a zeolite of the analcime type, which is interesting in its own respect: the synthesis of analcime was reported in previous papers (Bejar et al., 2014, Bejar et al., 2015). When kaolinite is synthesized from amorphous aluminosilicate in alkaline solutions at temperatures between 175 and 230°C, unlike the Na+ ions, the K+ ions prevented the formation of zeolite (De Kimpe, 1976, De Kimpe and Rivard, 1981). The same phenomenon seems to take place even when manganese is present in the reactive media.
In this study, different experimental parameters (reaction temperature, reaction time and reactants concentrations) were modified with the aim to pinpoint the effect of each parameter on the formation of the final zeolite product and of the intermediate phases. A sample free of analcime was characterized by different techniques to identify intermediates and to elucidate the formation mechanisms.
Section snippets
Hydrothermal synthesis
The synthesis procedure is based on the protocol proposed by Choi and al. (Choi et al., 2009). The starting materials were a mixture in aqueous solution of NaOH (Acros, purity: pure pellets), silicic acid (Sigma Aldrich, purity: 99.9%), manganese (II) carbonate (Sigma Aldrich, purity: 99.9%) and aluminum nitrate nonahydrate (Acros, purity: 99.9%). All chemicals were mixed in distilled water (final volume of dispersion: 15 mL) in Teflon-lined stainless steel Parr reactors (25 mL capacity). The
Effect of synthesis parameters
First, varying alkali concentrations were used in series A (Fig. 1). For the most alkaline solutions, a zeolite material of the analcime type is formed as reported in previous work (Azizi and Ehsani Tilami, 2013, Bejar et al., 2014). On the other hand, two reflections at 2 theta = 12.44° (d = 7.1 Å) and 2 theta = 24.39° (d = 3.2 Å) are observed that do not behave like the analcime peaks: they reach a maximum at an intermediate alkalinity (Fig. 1-c) and decrease for the highest alkalinity (Fig. 1-d). The
Conclusion
The aim of the present work was to understand the nature of the intermediate silicate phase formed in hydrothermal syntheses whose final product is Mn-analcime. The formation of a lamellar, kaolinite-like material in moderate conditions (175°C for 48 h) is shown; however, in contrast to previously published studies, it only constitutes a minority species and the majority phase is an X-ray amorphous Al–Mn silicate with a particular morphology consisting in hollow spheres with a 8–10 nm diameter
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2019, Advances in Colloid and Interface Science