Isomorphic silicon/aluminum substitution on layered ilerite – Structural study and calorimetry of copper interaction

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Abstract

Layered ilerite (Na-il) and ilerite with structural aluminum incorporated (Na–[Al]il) through isomorphic substitution of silicon atom in the silicic network were synthesized by hydrothermal procedures. X-ray diffraction (XRD) and scanning electron microscopy confirmed the structure and morphology. The incorporated metal was detected and quantified by nuclear magnetic resonance in the solid state and X-ray fluorescence. Both classes of materials sorbed copper and these interactions were calorimetrically followed through isothermal titrations. The thermodynamic data for Na-il and Na–[Al]il gave ΔG −4.2 ± 0.4 and −4.7 ± 0.5 kJ mol−1, ΔH 1.54 ± 0.02 and 2.9 ± 0.01 kJ mol−1 and ΔS 19 ± 1 and 25 ± 1 J K−1 mol−1, respectively. These values are relatively close and the differences are explained by the higher acidity for the Na–[Al]il surface, which leads to stronger interaction between the copper species and the solid material. The copper-ilerite structures were characterized by XRD, the environment surrounding copper has been studied by X-ray absorption spectroscopy and EXAFS was used to determine copper coordination, metal–metal and copper-oxygen distances. For both materials octahedral coordination was determined, whose axial and equatorial distances for Cu–O gave 196 and 234 pm for Na-il, respectively. For the Na–[Al]il sample two equatorial distances, 189 and 196 ppm and an axial distance of 247 pm were found. These results indicate that hydrated Cu2+ species interact with silicate layers by electrostatic forces. The XAS results are in agreement with the calorimetric results and with acid–base principles, as well described for these systems.

Highlights

► Layered ilerite containing structural aluminum were successfully synthesized. ► Thermodynamic data of copper-surface interaction were calorimetrically determined. ► Atomic distances of copper species were calculated by XAFS.

Introduction

Ilerite, also known as octosilicate or RUB-18, is a well-organized layered hydrated silicate that naturally occurs or can be synthesized through hydrothermal procedures [1]. This structurally self-arranged compound is one of the hydrous layered silicate members containing sodium that also include makatite, kanemite, magadiite and kenyaite [2]. The interlayer cavity space includes hydrated cations, usually sodium or hydrogen, to maintain charge neutrality. These internal ions can be exchanged by charged inorganic or organic species. In a similar way as happens with zeotype materials, the lamellar structure can be slightly modified by the isomorphic substitution of silicon by other atoms, such as iron, aluminum or titanium [3]. The isomorphic chemical exchange in the original structure causes some surface property modifications in agreement with the new atom incorporated and also its degree of concentration.

The synthesized ilerite in sodium form, (Na-ilerite), has precise and normal structure and a determined chemical composition [4]. Thus, its unit cell has the formula Na8Si32O64(OH)8·32H2O, with an interlayer distance and a layer thickness of 1.10 and 0.73 nm, respectively, indexed as (0 0 1) diffraction planes, with the crystal structure details elucidated also by X-ray diffraction patterns and solid-state NMR spectroscopy [1], [5]. The secondary structural building units of Na-ilerite silicate layer consists of four- and five-membered rings. This kind of [54] cage, common for many zeolites, was observed for the first time in a layered silicate. The remaining silicon atoms are bonded to hydroxyl groups or have a negative charge that is compensated by octahedral coordinated sodium cations. Weak and strong hydrogen bonds among the structural water and siloxane or silanol groups are expected [6].

Structural aluminum inserted throughout the silicate network increases the layer acidity due to the misbalance in charge. Aluminosilicate materials have a wide range of potential applications, mainly as catalysts and sorbents in many reactions and separation uses [7]. For example, layered silicate materials with aluminum have already been synthesized for magadiite and ilerite structures [3], [8]. For magadiite the metal inserted into the silicic network was successfully investigated by 27Al NMR spectra. On the other hand, gallium containing samples were also synthesized, with the majority of this element remaining as oxide in the interlayer space [3]. It is worth to mention that ilerite samples containing structural aluminum or tin inserted were not obtained, but magadiite was obtained instead, with small amounts of the metal in the tetrahedral sites and oxides on the surface or in the interlayer spaces [8].

Among the metal contaminants, copper is one of the most investigated in sorption processes, due to its toxicity in Wilson disease and damage to the gastro-intestinal tract, followed by its abundance as contaminant in many environments and also its many chemical applications [9]. In this context, zeolites and other silica materials containing exchanged copper cations have activity in nitrous oxide catalytic decompositions [10], catalytic conversion of methane to methanol [11] and wet hydrogen peroxide catalytic oxidation, among other applications [12].

Investigations involving interactive copper processes with many sorbents can be performed by some techniques, but the thermodynamic data can be obtained from isothermal calorimetric titration by a direct measurement [13], [14]. These data are closely correlated with solid–liquid interface interactions and the effect of isomorphic substitutions and other modifications can be clearly shown. In addition, the use of X-ray absorption spectroscopy for samples after sorption processes also aids the elucidation of this kind of process and mainly the metal species characterization on the final material [15], [16].

Taking into account investigations associated with ilerite, not only involving clay science, but also due to structural similarities with the zeolite field, are relevant and can give outstanding contributions to better understanding of some processes. Thus, the purpose of the present study is to synthesize and characterize layered ilerite samples containing aluminum, followed by the investigation of copper sorption processes at the solid/liquid equilibrium, to elucidate the exchange effect of this cation in this well-organized solid derivative of silicic acid.

Section snippets

Chemicals

Silica gel, aluminum isopropoxide and copper nitrate were purchased from Aldrich and sodium hydroxide from Synth. All reagents were used as supplied.

Synthesis

Sodium ilerite silicate was synthesized as previously described [17]. Aluminum containing samples were made by a similar method, but aluminum alkoxide was added before hydrothermal treatment. The gel composition in molar ratio was established as 0.025Al2O3:SiO2:0.5NaOH:7.0H2O and reactions were done inside a Teflon-lined autoclave at 378 K from 3 to

Ilerite synthesis

X-ray diffraction patterns of ilerite samples are shown in Fig. 1. Pure silica samples start their crystallization after 3 days of hydrothermal treatment, as given by very low signals (curve a). After 6 days the ilerite structure is already obtained (curve b), presenting all characteristic diffraction signals [4], however, part of it remains in amorphous phase. Finally, the pure octosilicate structure is obtained after 9 days of reaction (curve c).

Aluminum insertion into the original structure

Conclusions

The synthesized precursor layered silicate ilerite in sodium form had the element aluminum successfully incorporated in the silicic network, to give a final compound with high crystallinity. Different crystallization pathways were detected for the synthesis with or without aluminum, going from amorphous to ilerite phase or transformed to another phase, which is very similar to magadiite. These differences reflect on morphology and, consequently, on copper sorption capacity. The isomorphic

Acknowledgements

The authors are indebted to CAPES, CNPq and FAPESP for fellowships and financial support, and also LME-LNLS and MAX-Lab for TEM and XAS measurements and technical support. Also Dr. Alviclér Magalhães is acknowledged for NMR measurements.

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