Article pubs.acs.org/crystal
Local and Extended-Order Evolution of Synthetic Talc during Hydrothermal Synthesis: Extended X‑ray Absorption Fine Structure, X‑ray Diffraction, and Fourier Transform Infrared Spectroscopy Studies Angela Dumas,*,†,‡ Martín Mizrahi,† François Martin,‡ and Felix G. Requejo† †
Instituto de Investigaciones Fisicoquímicas Teóricas y Aplicadas, INIFTA (CONICET-UNLP), diag. 113 y calle 64, 1900 La Plata, Argentina ‡ ERT 1074 “Géomatériaux”, GET (Géosciences Environnement Toulouse), UMR 5563 (UPS-CNRS-IRD-CNES), 14 Avenue Edouard Belin, 31400 Toulouse, France ABSTRACT: Synthetic talc is a new filler of industrial interest due to its submicron size, its chemical and mineral purity, and its hydrophilic character. To develop this filler on a preindustrial scale, this work aimed to understand the mechanisms of transformation from the amorphous talc precursor to crystalline synthetic talc. X-ray diffraction, Fourier transform infrared spectroscopy, and extended X-ray absorption fine structure at Ni−K edge techniques were used to study a Ni-talc series composed of a talc precursor sample and talc samples synthesized at 100, 200, and 300 °C for 1 or 6 h. As soon as the Ni-talc precursor precipitated, a tetrahedral−octahedral− tetrahedral-type structure appeared that was characterized by 2−3 Ni-octahedra distanced 3.07 Å from each other and by 3−4 Si tetrahedra distributed on the top and bottom of the octahedral “sheet” and distanced 3.29 Å from Ni. Simultaneously following the synthesis temperature, the octahedral sheet grew, and the tetrahedral sheet expanded; the distances Ni−Ni and Ni−Si also gradually shortened. The intracrystalline distribution of the octahedral sheet was also studied. At 300 °C, a random distribution was obtained. Cluster distribution was not observed at low temperature, which we hypothesize is a function of crystallite size.
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INTRODUCTION Synthetic talc is the submicron-scale counterpart of natural talc (Mg3Si4O10(OH)2) (Figure 1). It appeared 10 years ago as a technical solution for the performance of new lubricant composite materials.1 In collaboration with industrial partners, the synthetic processes were recently revised and optimized by the ERT 1074 Géomatériaux (GET, Toulouse, France) to obtain a simpler, faster, and more efficient talc synthesis that complies with industrial requirements.2,3 The preparation of synthetic talc requires two steps: (i) preparation of a talc precursor at room temperature with the proper Mg/Si talc ratio and (ii) hydrothermal treatment at 300 °C under autogenous pressure (85 bar) for a few hours. Currently, synthetic talc appears in various industrial sectors as a competitive and original filler.4,5 The industrial demand for this new material will lead to a significant technology transfer in the near future. However, the crystallogenesis mechanisms should be considered before scaling up the process. Although synthetic talc is a well-documented product,6−9 very little information about its precursor exists. Lèbre8 described the talc precursor as an amorphous and hydrated product. More recently, Dietemann10 studied the influence of precipitation parameters on the talc precursor’s properties (reactant addition mode and order, reactant molalities and ultrasound). These authors © XXXX American Chemical Society
confirmed its amorphous character and revealed a highly agglomerated and porous (micropores and mesopores) product.11 Moreover, understanding synthesis talc growth mechanism is also highly applicable to understanding the crystallization of talc in natural environments ranging from low temperature (sedimentary) to high temperature (metamorphic/hydrothermal) environments. In this paper, we report diffraction and spectroscopic results to understand the transformation mechanisms from talc precursor to crystalline nano-talc. While X-ray diffraction (XRD) data inform the crystalline structure at a length scale of approximately 50 Å, Fourier transform infrared spectroscopy (FTIR) and X-ray absorption spectroscopy (XAS) provide details about the local structure. Extended X-ray absorption fine structure (EXAFS) was used to probe the immediate environment of the octahedral cation: the distances and coordination number (CN) of the octahedral absorbing atom was determined. EXAFS spectroscopy is particularly interesting because this technique is chemically selective and well suited for Received: July 28, 2015 Revised: September 28, 2015
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DOI: 10.1021/acs.cgd.5b01076 Cryst. Growth Des. XXXX, XXX, XXX−XXX
Crystal Growth & Design
Article
(100% Ni substitution). In the light of the results from the first part, the second part addresses the intracrystalline distributions of Ni and Mg as a function of both the synthetic hydrothermal temperature and the talc precursor precipitation method (50% Ni substitution).
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EXPERIMENTAL SECTION
Sample Overview. Fourteen samples were prepared to monitor the transformation of the talc precursor in synthetic talc. Three series of Ni-talcs were prepared that differed by the percentage of Mg substitution (100% or 50%) or by the method used to obtain the talc precursors (2P or M; see sample preparation description). Each series was composed of the talc precursor sample (PT) obtained at room temperature and talc samples (T) synthesized at 100, 200, and 300 °C for 1 or 6 h. Table 1 summarizes the description of the samples and their nomenclatures. All experiments on the synthesis temperature were realized using the same talc precursor. Starting Materials. The starting materials for the experiment were as follows: nickel acetate tetrahydrate (Ni(CH3COO)2·4H2O), magnesium acetate tetrahydrate (Mg(CH3COO)2·4H2O), sodium metasilicate pentahydrate (Na2SiO3·5H2O), sodium acetate trihydrate (NaCH3COO·3H2O), and acetic acid. All reagents were purchased from Aldrich and used without further purification. A 1 N solution of acetic acid was prepared using deionized water. Sample Preparation. The talc precursor substituted entirely by Ni was prepared at room temperature by precipitation of a sodium metasilicate solution and a nickel acetate solution in the presence of sodium acetate, according to the following equation:
Figure 1. Structure of talc (2:1 phyllosilicate); yellow ball: silica; light blue ball: magnesium; dark blue ball: hydroxide; red ball: oxygen.
poorly crystalline and noncrystalline materials.12 FTIR spectroscopy is a sensitive method used to probe the local structural environment of the OH groups in clay minerals13,14 because the vibrations of the OH groups are sensitively affected by the variation in the mineral structure.15,16 Moreover, the OH vibrations are more sensitive in the NIR region than in the middle infrared region (MIR).16 The aim of this paper was to study the structural evolution of synthetic talc during its synthesis with the aim to understand the mechanisms of transformation from the amorphous talc precursor to the crystalline synthetic talc. More precisely, in this work, we studied the influence on the final product of (i) the temperature of hydrothermal treatments, (ii) the synthesis duration, and (iii) the mixture of starting materials and its impact on the cationic distribution. To study the structural evolution of the octahedral sheet, synthetic Ni-talcs were used because Ni atoms have more suitable energy absorption for the use of EXAFS measurements than Mg atoms (8333 and 1305 eV for Ni and Mg, respectively). The talc synthetic procedure allows partial or complete substitution of the magnesium cation by nickel in the octahedral sheet (ionic radius of 0.72 and 0.69 Å for nickel and magnesium, respectively).17 This paper is organized in two parts that differ by the percentage of magnesium substitution by nickel. The first part determines the structural evolution of talc entities (form and size) from the talc precursor to crystalline synthetic Ni-talc
4[Na 2SiO3] + 3[Ni(CH3COO)2 ] + 2CH3COOH + mH 2O + yCH3 COONa → Ni3Si4O11, n′H 2O + (8 + y)CH3COONa + (m − n′ + 1)H 2O More details about the talc precursor preparation are available in the examples of the synthetic talc patent.2 The talc precursor was centrifuged and rinsed to remove sodium salt before being freezedried. The Ni-talc precursor then underwent three different hydrothermal treatments at 100 °C (
