Article pubs.acs.org/cm
In Situ NMR, Ex Situ XRD and SEM Study of the Hydrothermal Crystallization of Nanoporous Aluminum Trimesates MIL-96, MIL100, and MIL-110 Mohamed Haouas,*,† Christophe Volkringer,‡,§ Thierry Loiseau,‡,§ Gérard Férey,‡ and Francis Taulelle† †
Institut Lavoisier de Versailles (UMR CNRS 8180), Tectospin Group and ‡Institut Lavoisier de Versailles (UMR CNRS 8180), Porous Solids Group, Université de Versailles Saint Quentin en Yvelines, 45, avenue des Etats-Unis, 78035 Versailles, France S Supporting Information *
ABSTRACT: In an attempt to understand the relationship between the chemistry occurring in solution and the topology of final crystalline structures, this paper investigates through ex situ and in situ 27Al and 1H NMR methods, the formation mechanisms of three porous aluminum trimesates MIL-96, MIL-100, and MIL-110, the hydrothermal synthesis conditions of which are very similar, despite distinct crystallographic structures. The nature of the starting organic reactanttrimesic acid (H3btc) or trimethyltrimesate ester (Me3btc)is of paramount importance for the targeted product. Both the solution and solid part of the reactive mixture were studied along the synthesis process. In situ speciation in solution of both inorganic and organic parts during hydrothermal crystallization were monitored using 27Al and 1H NMR spectroscopy. The nature and morphology of the various intermediate solid phases, at different reaction times, were monitored by means of ex situ powder X-ray diffraction (XRD) and scanning electron microscopy (SEM), in relation with the solution speciation. This multimodal approach allowed assignment of the 27Al NMR signals. Besides the dominant signal of the aluminum monomers (0−1 ppm), two additional small resonances at 4 and 7 ppm were assigned to two different types of Al dimersa corner-shared μ2-O and an edge-shared μ2-Ostabilized by two and one trimesate ligands, respectively. These units can be found in MIL-96 and MIL-100 structure for the first type of dimer and in MIL-110 structures for the second type of dimer. Depending on the nature of the solid product, the structure of the soluble species is proposed, and a scheme of the reaction pathways occurring in solution is proposed. The final structure is strongly dependent on the nature of the species formed in solution. KEYWORDS: nuclear magnetic resonance, in situ NMR, time-resolved, MOF materials, benzenetricarboxylates
1. INTRODUCTION Nanoporous metal−organic frameworks (MOFs) represent a topical class of solids, mostly obtained by hydrothermal synthesis.1−3 Their structural properties and chemical reactivities are related to applications in different fields, such as gas storage,4−8 separation, 9 catalysis, 10 and pharmaceutical uses.11−15 Beyond the usual syntheses based on a trial-anderror strategy,16,17 the need for detailed knowledge at the atomic/molecular level in order to understand their formation, therefore is of paramount importance to elaborate rational syntheses of novel framework structures. Crystallization mechanisms of MOFs, up to now, have been hardly studied and the role of building units of the crystal from solution is still not described at an atomic level.18−21 Despite the very few experimental studies on speciation in solution during MOFs synthesis, compared to zeolites and other related materials, some valuable information has been obtained by means of various techniques, including mass spectroscopy,22,23 extended X-ray absorption fine structure (EXAFS),24 X-ray scattering25 or diffraction,26,27 and atomic force microscopy (AFM).28 Preformed units have been © 2012 American Chemical Society
identified in solution during the synthesis of MIL-88 that could be implicated directly in the crystal growth mechanism.24 However, a recent AFM work of Shoaee and co-workers on HKUST-1 does not support such a conclusion and rather suggests simply the monomer and the isolated ligand as the most likely growth units.28 In situ NMR spectroscopic characterization of the synthesis medium under real growth conditions should provide further insights on the reactive species. Most of the MOF solids are often prepared with transition metals, but compounds with trivalent elements of block III columns are of particular interest, for their potential as inexpensive and useful materials. In this context, the elaboration of a series of aluminum-based MOF compounds has been thus investigated29−45 and the 27Al nucleus is the candidate of choice as a solid-state NMR probe.46−55 In 27Al NMR, when oxygen is the first coordination shell, there is a well-established relationship between the Received: February 8, 2012 Revised: June 12, 2012 Published: June 13, 2012 2462
dx.doi.org/10.1021/cm300439e | Chem. Mater. 2012, 24, 2462−2471
Chemistry of Materials
Article
further purification. Al(NO3)3·9H2O (98%), trimethyl 1,3,5-benzenetricarboxylate (Me3btc) (98%) and HNO3 (Rectapur) were purchased from Carlo Erba Regenti, Aldrich, and Prolabo, respectively. The solids were synthesized from the reaction of appropriate amounts of Al(NO3)3 and Me3btc in water. When needed, the pH was adjusted using an aqueous solution of HNO3 (1 and 4 M). The mixtures were heated at 180 °C for various reaction times for ex situ study and continuously during 19−24 h for in situ NMR measurements. The increase in temperature from room temperature was achieved within 1 h. The time origin of the kinetics was considered to be the exact moment the system reaches the synthesis temperature (180 °C). After each reaction time for ex situ study as well as at the end of the in situ study, the powdered samples were collected by filtration, washed with purified water, and dried at room temperature. In all cases, intermediate and final products were analyzed by scanning electron microscopy (SEM) and powder XRD. 2.2. NMR Experiments. All NMR experiments were conducted in liquid state at a Larmor frequency of 130.32 MHz for 27Al, and 500.13 MHz for 1H NMR using a Bruker Model Avance 500 spectrometer. The standard reference for 27Al chemical shifts was aluminum nitrate aqueous solution where aluminum is present as Al(H2O)63+. For 1H NMR, the scale of the chemical shifts was calibrated relative to the standard reference, tetramethylsilane. Single-pulse NMR experiments were used. The 27Al spectra were recorded with a recycle delay of 0.1 s, a pulse length of 5.7 μs (corresponding to a flip angle of π/12), and an acquisition time of 0.3 s. In typical kinetics experiment, 2048 FIDs were accumulated and averaged within ca. 15 min measurement time, and then were recorded every 10 min at 180 °C. After each 27Al NMR experiment, a quick record (within ca. 4 s) of an 1H NMR spectrum was carried out.
chemical shifts and the coordination number of the aluminum observed, from hexacoordination to tetracoordination environments.56 The use of in situ 27Al NMR under hydrothermal conditions has been successfully carried out on AlPO57 and SAPO58 materials, providing unique results that could not be obtained ex situ under ambient conditions. This was made possible thanks to the specific homemade NMR tubes for high temperature and pressure (up to 220 °C and 20 bar) acting as hydrothermal reactors.59 The porous aluminum trimesate solids MIL-96,48 MIL-100,49 and MIL-110,47,50 crystallize from identical precursor reactants and only the pH of the starting mixture and/or the reaction time fixes the final product. Furthermore, two of the solids, MIL-96 and MIL-100, exhibit a common building unit in their respective structure, while MIL-110 is constructed from different building units. For these reasons, they could serve as model compounds for studying the synthesis mechanism of MOF aluminum carboxylates. Therefore, the outline of this paper is as follows: after a short experimental section (see section 2), a detailed description of experimental results is presented, both for ex situ SEM and powder XRD measurements (see section 3.1) and for timeresolved ex situ and in situ 27Al and 1H NMR experiments (see section 3.2). In the discussion (section 4), an interpretation of results is provided, leading to the elaboration of a mechanistic scheme from early chemical speciation to condensed species in solutions and the building of units specific to each phase.
3. RESULTS 3.1. Ex Situ Structural Study of the Formation of Aluminum Trimesates MIL-96, MIL-100, and MIL-110. Powder XRD and SEM enabled following the synthesis steps of the three compounds to be followed by analyzing all intermediate solid products at different reaction times. SEM and XRD are complementary techniques. XRD detects crystallized materials, while SEM provides data about the morphology and size of crystallites as well as amorphous particles. The crystal structure of each aluminum trimesate had been solved, yielding distinct XRD pattern signatures for MIL49 96 (P63/mmc),48 for MIL-100 (Fd3m ̅ ), and for MIL-110 50 (P6̅2c). The crystal morphologies of each compound are also easily distinguished by SEM. Single-crystal MIL-96 has a flat hexagonal shape,48 MIL-100 has an octahedral shape,49 and MIL-110 has a hexagonal needlelike shape.50 3.1.1. MIL-96. Figure 1 shows a stack plot of the powder XRD patterns of intermediate solids obtained during MIL-96 synthesis. At t = 0, just after reaching the synthesis temperature during 1 h of heating, the only crystalline solids were the starting reactant Me3btc and a hydrated form of H3btc (2θ = 27°). These results indicate the slow hydrolysis process of the carboxylate ester and its quick recrystallization under its hydrated acid form.61 This is clearly an indication of the low solubility of the organic acid in such a solution. After 1 h at 180 °C, all the reactants were hydrolyzed and the major crystallized materials were the hydrated form of H3btc with a trace of MIL100 (2θ = 4°). Between 2 and 8.5 h of reaction, XRD patterns showed a mixture of MIL-100 and MIL-96 peaks with a maximum intensity for MIL-100 at 8 h. An SEM image of the 2h solid product (Figure 2a) showed that the main crystallites are small octahedra (ca. 0.1−0.3 μm), corresponding to MIL100. However, the SEM image of solid collected after 8 h of reaction (Figure 2b) showed that the solid is constituted mainly of 3−5 μm ill-defined hexagonal crystals of MIL-96. After 16 h
2. EXPERIMENTAL SECTION 2.1. General Procedures. All syntheses of both ex situ and in situ studies were performed in the specific NMR devices, which include a Teflon-lined Vespel tube especially developed for hydrothermal NMR experiments57,60 (see the Supporting Information). The use of the same reactor allows direct comparison between the two independent studies. In the ex situ study, syntheses were conducted using an oven under static conditions, while in situ study syntheses were realized inside the NMR magnet via contact with a heated air flux using a NMR Eurotherm temperature unit. The synthesis recipes of MIL-96, MIL100, and MIL-110 originally described for Teflon-lined Parr autoclaves reactions under autogenous pressure at 210 °C47−50 have been slightly modified. Experimental conditions for the three syntheses have been optimized, with respect to the NMR tube reactor, as well as to a lower temperature fixed at 180 °C. Lowering the temperature ensures slower kinetics reactions more suitable for a mechanistic study. The major changes of synthesis parameters, compared to synthesis with Parr
Table 1. Starting Molar Composition of Synthesis Precursor of MIL-96, MIL-100, and MIL-110 Molar Composition phase
Al(NO3)3
Me3btc
H2O
HNO3
MIL-96 MIL-100 MIL-110
1.2 1.5 1.5
1.0 1.0 1.0
500 400 250
0.0 2.0 4.5
autoclave reactor, involve the reactant molar ratio (see Table 1). These differences may be due to the difference in thermal conductivity between the thin Vespel tube and the thick steel autoclave. Indeed, the rate of increase/decrease of the heating temperatureand, thus, the thermal equilibriumis much faster with the Vespel tube than the Parr autoclave. For instance, at the end of a synthesis at 180 °C, the material decays spontaneously to room temperature after ca. 5 h with a Parr autoclave, but within only 10 min with the NMR device. All reactants were commercially available and used as such, without 2463
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Figure 1. Powder XRD patterns of solid product intermediates during MIL-96 synthesis after different reaction times : (a) 0 h, (b) 1 h, (c) 2 h, (d) 4 h, (e) 7 h, (f) 8 h, (g) 8.5 h, and (h) 16 h at 180 °C. The 1 h of heating from room temperature to 180 °C is not taken into account. Selected characteristic peaks of final and intermediate products are shown: Me3btc (orange square), H3btc (aqua green tilted square), MIL-100 (pink triangle), and MIL-96 (purple solid circle).
Figure 3. Powder XRD patterns of solid product intermediates during MIL-100 synthesis after different reaction times : (a) 0 h, (b) 0.5 h, (c) 1 h, (d) 2 h, (e) 3 h, (f) 6 h, (g) 7 h, (h) 8 h, (i) 9 h, (j) 16 h, and (k) 19 h at 180 °C. The 1 h of heating from room temperature to 180 °C is not taken into account. Selected characteristic peaks of final and intermediate products are shown: Me3btc (orange square), H3btc (aqua green tilted square), MIL-100 (pink triangle), and MIL-96 (purple solid circle).
of heating, the unique final crystalline product was MIL-96, according to XRD, indicating the end of the reaction. This is consistent with the SEM image (Figure 2c) showing larger MIL-96 crystals (5−10 μm) with well-defined hexagonal faces. 3.1.2. MIL-100. Just after reaching the synthesis temperature (t = 0 at 180 °C), the only crystalline solid present (Figure 3) was the starting reactant Me3btc. After 0.5 h of heating at 180 °C, the ester Me3btc was transformed to the crystalline hydrated acid form H3btc, and 0.5 h later, it disappeared with a simultaneous appearance of the first peaks of MIL-100. During a period of 1−6 h, those later grew continuously, and MIL-100 appeared as the unique crystalline product. The SEM images during this period (1−5 h) depicted in Figure 4 show the evolution of morphology of MIL-100 crystallites during their growth. After 1 h of synthesis, the crystallites were an aggregation of spherical particles. The 2-h product was composed of small crystals (
