Phase-Transition and Phase-Selective Synthesis of Porous Chromium

Feb 1, 2010 - Pill Won Seo , Nazmul Abedin Khan , Zubair Hasan , and Sung Hwa Jhung. ACS Applied Materials & Interfaces 2016 .... Highly selective ads...
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DOI: 10.1021/cg901562d

Phase-Transition and Phase-Selective Synthesis of Porous Chromium-Benzenedicarboxylates

2010, Vol. 10 1860–1865

Nazmul Abedin Khan and Sung Hwa Jhung* Department of Chemistry, Kyungpook National University, Daegu 702-701, Korea Received December 11, 2009; Revised Manuscript Received January 15, 2010

ABSTRACT: A study of the crystallization of two topical porous chromium- benzenedicarboxylates, denoted as MIL-53 and MIL-101, has been reported. Both conventional electric (CE) and microwave (MW) heating have been explored in order to understand phase-selectivity and phase-transition between these solids. One chromium benzenedicarboxylate, the kinetically favorable MIL-101 (lower density phase), is the phase obtained at the early stage of the reaction, while the thermodynamically favorable MIL-53 phase (higher density phase) is obtained at the expense of MIL-101 at longer synthesis time. Phase-transition from MIL-101 to MIL-53 does not occur by direct conversion. Instead, at the longer synthesis time the MIL-101 is degraded, and subsequently, MIL-53 is observed via the reorganization of the decomposed species. Moreover, it is suggested that MW irradiation provides a phase-selective synthesis of MIL-101 due to rapid synthesis, preventing the conversion into the thermodynamically favorable phase (MIL-53). Therefore, the MW synthesis may lead to a new way to find new metal-organic frameworks (MOFs) especially those that are hard to synthesize due to interconversion into a more stable phase or dense phase.

Introduction The number of materials exhibiting permanent nanoporosity has rapidly expanded in recent years, due in large part to the discovery of hybrid inorganic-organic materials including metal-organic frameworks (MOFs) and coordination polymers, etc.1 The porous MOFs attract considerable attraction due to an easily tunable crystalline hybrid network with a high and regular porosity. The major applications currently being considered for these compounds involve gas storage/ adsorption,2 catalysis,3 separations,4 drug delivery,5 carriers for nanomaterials,6 luminescence,7 and magnetism.8 Among the numerous MOFs reported so far, two of the most topical solids are the porous chromium-benzenedicarboxylates (Cr-BDCs), such as MIL-539 and MIL-10110 (MIL stands for Material of Institut Lavoisier), which are largely studied for their potential applications. MIL-53 with a chemical formula of Cr(OH)[C6H4(CO2)2] 3 nH2O has an orthorhombic structure and pore volume of 0.6 mL/g.9 MIL-101, Cr3O(F/OH)(H2O)2[C6H4(CO2)2], has a cubic structure and huge pore volume of 1.9 mL/g.10 The pore sizes of MIL-53 and MIL-101 are around 0.85 and 2.9-3.4 nm, respectively.9,10 MIL-53 is very interesting due to the breathing effect11 and has been widely studied for adsorption12 and drug delivery.5a,13 MIL-101 is a very important material due to mesoporous structure and huge porosity, and is widely studied for adsorption,14 catalysis,15 and drug delivery.5a,13 So far, most of the research on MOFs has been focused mainly on the synthesis of new structures and structure determination even though facile, reproducible, and effective syntheses are very important for viable applications. Only a few studies to estimate the effect of reaction conditions on the syntheses are known16 even though studies on the effect of process parameters on the MOF synthesis are important. *To whom correspondence should be addressed. Fax: (þ)82-53-950-6330; e-mail: [email protected]. pubs.acs.org/crystal

Published on Web 02/01/2010

Syntheses of porous materials with microwave (MW) have advantages such as rapid syntheses,17 narrow particle size distribution,18 easy morphology control,19 facile evaluation of process parameters,20 and phase-selective syntheses,21,22 etc. In particular, an aluminophosphate (AlPO) molecular sieve having a less stable pore structure (large pore size or low framework density) is preferentially obtained with MW due to decreased synthesis time. For example, VPI-5 molecular sieve (having 18-membered rings; 18 MR) and AFI (having 12 MR) molecular sieves are selectively synthesized rather than AFI (12 MR) or AEL (10 MR) and CHA (8 MR) structures, respectively, in reduced time under MW heating.22 Recently not only conventional electric (CE) heating but also several synthesis methods such as MW23 and ultrasonic (US)24 syntheses have been applied for MOFs syntheses. MW syntheses of MOFs have the advantage of fast crystallization23 and decreased size,23c etc. However, to the best of our knowledge, phase-selective synthesis by MW and phaseconversion with reaction time have not been reported in the case of MOFs, so far. In this work, the phase-selective synthesis of relatively unstable Cr-BDC by MW heating is reported. Additionally, the conversion of the less stable phase into the more stable one, with the progress of reaction, is be discussed. The phaseselectivity and phase-conversion is explained with thermodynamic stability and concentrations of products and reactants such as terephthalic acid (TPA or H2BDC, one of the reagents for Cr-BDCs), respectively. Experimental Section Cr-BDCs were synthesized from chromium chloride hexahydrate (CrCl3 3 6H2O, Aldrich), TPA (99%, Junsei), and deionized water similar to the reported methods9,10 under autogenous pressure at 210 °C unless otherwise specified. The reactant composition was CrCl3 3 6H2O/TPA/(250-550) H2O. The gel of 20 g was loaded in a 100 mL Teflon autoclave, which was sealed and placed in a microwave oven (Mars-5, CEM, maximum power of 1200 W). The autoclave in a microwave oven was heated for 2 min to the reaction r 2010 American Chemical Society

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Figure 2. Yields of Cr-BDCs depending on the reaction time and methods: (a) CE heating; (b) MW heating. The reactant composition is CrCl3 3 6H2O/TPA/400 H2O.

Figure 1. XRD patterns of as-synthesized Cr-BDCs depending on the reaction time and methods: (a) CE heating; (b) MW heating. The reactant composition is CrCl3 3 6H2O/TPA/400 H2O. Panel (c) shows the calculated XRD patterns of MIL-53 and MIL-101. temperature of 210 °C and kept for a predetermined time. The microwave power was 400 W throughout the whole synthesis steps including heating-up stage. Detailed procedures for MW syntheses were described elsewhere.25 For conventional electric crystallization, the gel of 20 g was loaded in a Teflon-lined autoclave and put in a preheated electric oven (temperature was the same as that of the microwave oven) for a fixed time. During the microwave and conventional reactions, the reactant mixtures were kept without agitation. After the reaction, the autoclave was cooled to room temperature and solid products were recovered by centrifugation and purification by treatment with

N,N-dimethylformide following the reported method.26 The purified Cr-BDCs were dried overnight at 100 °C and, after cooling to room temperature, stored over saturated ammonium chloride water solution. The yield of the solid product was calculated by comparing the amount of recovered solid with the expected weight based on chromium. The crystal phase of the samples was verified using an X-ray diffractometer (MO3X-HF, model no. 1031, CuKR radiation). The morphology was examined using a field emission scanning electron microscope (Hitachi, S-4300). The nitrogen adsorption isotherms and surface area were measured at -196 °C with a surface area and porosity analyzer (Micromeritics, Tristar II 3020) after evacuation at 150 °C for 12 h. The surface area and micropore volume were obtained using the BET equation and t-plot, respectively. The content of uncoordinated TPA was analyzed using the X-ray diffraction (XRD) peak intensities (at 2 theta; 17.5°). After synthesis, the solid products including TPA were recovered by centrifugation and dried at 100 °C for 6 h because TPA is nearly insoluble and sublimes at 402 °C. The solid products were ground well into a homogeneous fine powder. The comparative XRD intensities of pure TPA and free TPA (that is remained in the products) were used to calculate the content of TPA in the products.

Results Figure 1 shows the XRD patterns of Cr-BDCs depending on the reaction time and heating methods. Compared with previous XRD patterns,9,10 it can be observed that the MIL-53 and MIL-101 are mainly obtained with the CE and MW heating, respectively, even though the content of MIL-53

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Figure 3. Nitrogen adsorption isotherms of MIL-53 obtained in 3 days by CE heating and MIL-101 obtained by MW heating for 1 h. The reactant composition is CrCl3 3 6H2O/TPA/400 H2O for both cases.

Figure 5. XRD patterns of as-synthesized Cr-BDCs depending on the reaction time and methods: (a) CE heating; (b) MW heating. The reactant composition is CrCl3 3 6H2O/TPA/550 H2O.

Figure 4. SEM images of (top) MIL-53 obtained in 3 days by CE heating and (bottom) MIL-101 obtained by MW heating for 1 h. The reactant composition is CrCl3 3 6H2O/TPA/400 H2O for both cases.

increases with increasing reaction time. MIL-53 is the main product after a long reaction time of 3 or 4 days under CE heating; however, pure MIL-101 is obtained by MW synthesis for 1 h from the same reaction mixture. The absence of other Cr-BDC such as MIL-88B27 cannot be ruled out, however, due to the low signal/noise ratio of the XRD patterns. The content of MIL-53 increases steadily with the progress of reaction even under MW heating, as evidenced by the presence

of the peak at 2-theta close to 15°, even though the concentration is relatively low. The yields of MIL-53 and MIL-101, calculated from XRD intensity and product weight, are displayed in Figure 2 for both CE and MW heating. The yield of MIL-53 increases steadily with the decrease of the MIL-101 yield (with increasing time) in both CE and MW syntheses. XRD-pure MIL-53 and MIL-101 were analyzed to check the physicochemical properties of the Cr-BDCs. As shown in Figure 3, the nitrogen adsorption isotherms of MIL-53 and MIL-101 are very similar to those reported earlier.9,10 The adsorption isotherm of MIL-53 is typical type I, confirming the microporous structure of MIL-53. The BET surface area, micropore volume, and total pore volume of MIL-53, calculated from adsorption isotherm, are 1438 m2/g, 0. 53 mL/g, and 0.55 mL/g, respectively, which are similar to the previous results.9 The adsorption isotherm of MIL-101 is similar to type IV because of the mesoporous pore structure of MIL-101,10 and the BET surface area and total pore volume are 3303 m2/g and 1.69 mL/g, respectively, which are similar to the previous results.10 Scanning electron microscopy (SEM) images (Figure 4) show that MIL-53, obtained with CE heating, is composed of relatively homogeneous big particles (2-3 μm), and MW synthesis leads to the very small MIL-101 particles in accordance with the previous work23c to synthesize nanoparticles under MW irradiation especially for a short reaction time. The Cr-BDCs have been synthesized in a more diluted condition to obtain phase-pure MIL-101 in wide reaction time because MIL-101 is contaminated easily with MIL-53 in

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Figure 6. Yields of Cr-BDCs depending on the reaction time and methods: (a) CE heating; (b) MW heating. The reactant composition is CrCl3 3 6H2O/TPA/550 H2O.

the previous condition (H2O/Cr = 400, Figures 1 and 2). It has been found that a diluted reagent is helpful to get MIL-101 rather than MIL-53.28 As shown in Figures 5 and 6, the pure MIL-101 can be obtained by MW synthesis for 1-4 h. On the contrary, CE heating is helpful to produce MIL-53 especially after 2 or 5 days. So, the phase-selective syntheses with MW (for MIL-101) or CE (for MIL-53) are confirmed again. Moreover, the increase of MIL-53 concentration with reaction time is also observed with CE synthesis (especially from 1 to 2 days, Figure 5a). The change of yields of MIL-53 and MIL-101 (Figure 6a) with reaction time under CE confirms the replacement of MIL-101 with MIL-53 with increasing reaction time. In order to understand the phase-conversion between MIL-53 and MIL-101, we analyzed the concentration of uncoordinated TPA (or H2BDC), one of the precursors for Cr-BDC, including the yields of MIL-53 and MIL-101 with reaction time for both CE and MW syntheses (Figure 7). Two different synthesis conditions where both the Cr-BDCs in the same batch survive for long times were selected, that is, diluted condition for CE heating and concentrated conditions for MW heating. For CE synthesis, the yield of MIL-101 and the concentration of TPA are serially maximized at different times while the yield of MIL-53 steadily increases. The concentration of TPA slightly decreases at the initial stage of the synthesis but reaches a maximum value on the way that the yield of MIL-101 goes down to a minimum value. For MW

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Figure 7. Changes of yields of MIL-53, MIL-101, and TPA depending on the reaction time: (a) CE heating and the reactant composition is CrCl3 3 6H2O/TPA/400 H2O; (b) MW heating and the reactant composition is CrCl3 3 6H2O/TPA/250 H2O.

synthesis, the same trend as in CE synthesis is observed although the formation of MIL-53 requires an induction period up to 2 h. Discussion The influence of several parameters such as reaction time and heating method (CE and MW) in the synthesis of Cr-BDCs has been investigated. The MIL-101 is preferentially obtained over MIL-53 with MW synthesis; on the contrary, MIL-53 is the main product with CE synthesis. Moreover, the yield of MIL-53 gradually increases with increasing time, while the yield of MIL-101 decreases after reaching a maximum value. The change of yields of Cr-BDC with time suggests that the thermodynamic stability of MIL-53 is higher than that of MIL-101 in agreement with the pore sizes or crystal densities of the two Cr-BDCs.9,10 In other words, MIL-101 converts into MIL-53 with the progress of the reaction because the stability of MIL-101 is less than that of MIL-53. Therefore, the phase-selective synthesis of MIL-101 with MW technique can be explained with the less stable and kinetically favored MIL-101 and rapid synthesis with the MW technique. Interestingly, MW irradiation leads to a phaseselective synthesis of more stable hybrid inorganic-organic material (tetragonal nickel glutarate),29 which is quite different from this study to produce a less stable structure. Therefore, phase-selective synthesis under MW needs further research. MW syntheses of porous materials are very interesting, especially for the syntheses of AlPOs, to lead to a new phase21

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that is not obtained by CE heating due to rapid synthesis by MW. Relatively unstable material having a large pore (or low framework density) such as VPI-5 is selectively obtained in a short reaction time by MW heating.22 A relatively unstable material may convert into the more stable phase with increasing time by CE heating due to a long syntheses time.21,22 The phase-selective synthesis of MIL-101 in this study is similar to the selective synthesis of AlPO molecular sieves such as VPI-5.21,22 Therefore, MIL-101 and MIL-53 can be regarded as a kinetically and thermodynamically favorable Cr-BDC, respectively. This phase-selective synthesis of the less stable structure with MW may lead to a new method to synthesize a phase which has not been observed by a conventional synthesis (from the identical reaction mixture). Therefore, this MW synthesis, because of decreased synthesis time, may be be utilized as a new way to produce a new MOF that is especially hard to synthesize due to interconversion into the more stable phases or dense phases. To date, the conversion of a phase into a new phase (for example, VFI into AFI; AFI into CHA etc.) has not been clearly explained due to the difficulty of analysis under the reaction conditions (high temperature and high pressure) and inadequate intermediates or reactants for analysis.21,22 However, in this work, the phase-selectivity and phase-conversion of Cr-BDCs are studied with the analysis of the contents of TPA and products in the reaction system. As TPA is the organic linker of MIL-53 and MIL-101, the higher concentration of free TPA indicates the lower conversion into Cr-BDCs or the decomposition of formed Cr-BDCs. As presumed, the TPA concentration decreases with time at the early stage of the reaction (1 day and 1-2 h under CE and MW heating, respectively), resulting in the formation of MIL-101 (Figure 7). After then, the TPA concentration increases with time and reaches maximum values at about 2 days and 2.5 h under CE and MW heating, respectively. The increase of TPA content correlates with the fast disappearance of MIL-101 and the steady formation of MIL-53 (Figure 7). The variation of TPA concentration can be rationalized by the fact that MIL-101 degrades at a specific time into TPA and chromium species; the obtained TPA and chromium species simultaneously reassembles to form MIL-53. In other words, the direct interconversion of MIL-101 into MIL-53 is not possible; instead, the conversion of MIL-101 into MIL-53 occurs through the recovery of TPA/chromium species from MIL-101 and successive crystallization. However, the analysis in this study is semiquantitative, and part of TPA is entrained in Cr-BDCs and wasted in the mother liquor; therefore, more quantitative work will be necessary to confirm the mechanism of the phase transition. Conclusions The syntheses of porous Cr-BDCs with MIL-101 and MIL53 structures have been compared using microwave and conventional electric heating methods. The influence of several parameters in the syntheses of Cr-BDCs indicated that MIL-101 phases are the kinetically favorable phases which are formed at the early stage of the reaction, while MIL-53 phases are thermodynamically favorable and easily obtained at longer synthesis time. On the basis of the phasetransition behavior between MIL-101 and MIL-53, the conversion of MIL-101 into MIL-53 is assumed to be indirectly achieved as follows: the degradation of MIL-101 to release TPA and chromium ion species to the synthesis mixtures and

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reorganization of these species to crystallize the corresponding MIL-53 phase. The selective formation of MIL-101 by MW irradiation is mainly ascribed to fast kinetics of nucleation and crystallization for kinetically favorable phases with the MW method. This paves the way for the selective synthesis of highly porous MOFs that are hard to get because of facile conversion into a more stable phase with a long reaction time. Acknowledgment. This work was supported by the National Research Foundation of Korea (NRF) grant funded by the Korea government (MEST) (R01-2008-0055718, 20090083696). The authors thank Dr. Jong-San Chang, Dr. Young Kyu Hwang (Korea Research Institute of Chemical Technology), Dr. C. Serre and Dr. P. Horchada (Universite de Versailles) for their helpful discussions and comments.

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