Crystal Transformation in Zeolitic-Imidazolate Framework - Crystal

Article pubs.acs.org/crystal

Crystal Transformation in Zeolitic-Imidazolate Framework Ze-Xian Low,† Jianfeng Yao,*,† Qi Liu,† Ming He,† Zhouyou Wang,† Akkihebbal K. Suresh,‡ Jayesh Bellare,‡ and Huanting Wang*,† †

Department of Chemical Engineering, Monash University, Clayton VIC 3800, Australia Department of Chemical Engineering, Indian Institute of Technology Bombay, Bombay, Maharashtra 400076, India

‡

S Supporting Information *

ABSTRACT: The phase transformation of a zinc-2-methylimidazole-based zeolitic-imidazolate framework (ZIF), from a recently discovered ZIF-L to ZIF-8, was reported. ZIF-L is made up of the same building blocks as ZIF-8, having two-dimensional crystal lattices stacked layer-by-layer. Results indicated that the phase transformation occurs in the solid phase via the geometric contraction model (R2), a kinetic model new to ZIF. The phase transformation was monitored by means of ex situ powder X-ray diffraction, nitrogen sorption, Fourier transform infrared spectroscopy, selected-area electron diffraction, scanning electron microscopy, and in situ nuclear magnetic resonance spectroscopy. This work also demonstrates the first topotactic phase transformation in porous ZIFs, from a 2D layered structure to a 3D structure, and provides a new insight into metal−organic framework crystallization mechanisms.

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INTRODUCTION Metal−organic frameworks (MOFs) are formed by linking inorganic and organic units by strong bonds.1 MOFs and zeolites feature certain similarities, and therefore, the mechanisms for zeolite formation have been adopted to describe the crystallization process of MOFs. Zeolites are usually synthesized in a solution2,3 or by exposure of a suitable solid precursor mixture to a solvent vapor at elevated temperature,4,5 and thus, their formation mechanisms mainly involve a solution-mediated reaction−crystallization process and solid-state transformation. In particular, direct conversion of amorphous components into crystalline products is better represented in solid-state transformation, where internal, bondswitching rearrangement from amorphous to crystalline material takes place.3 MOFs, often produced in a one-pot synthesis,6 typically follow a solution-mediated reaction crystallization process.7−10 However, in some rare cases, the solid-state synthesis of MOFs is also reported.11−13 Crystal−crystal phase transformation is not far different from the amorphous-to-crystalline transformation process except that the precursor is also crystalline.3 Similarly, the phase transformation may occur via a solution-mediated process involving dissolution−recrystallization14−16 and pure solid-state transformation.17,18 As one of the most common zeolitic-imidazolate frameworks (ZIFs, a subclass of MOFs), ZIF-8 is made up of a Zn metal ion linked by 2-methylimidazole (MeIm) as an organic linker and possesses a sodalite (SOD) topology with a pore cavity of 11.6 Å accessible through the theoretical pore aperture of 3.4 Å.19 It has gained considerable interest in chemistry and materials © XXXX American Chemical Society

science due to its permanent porosity and high thermal and chemical stability.20 For instance, bulk ZIF-8 particles21−24 and supported ZIF-8 membranes25−28 have shown excellent performance in gas separation, liquid separation, sensing, and heterogeneous catalysis. Several synthesis methods have been reported to produce ZIF-8 in a large quantity, adopting simpler and greener approaches.29−32 The crystallization processes of ZIF-8 have been investigated by various techniques, including in situ time-resolved static light scattering,33 ex situ X-ray diffraction (XRD) and selected-area electron diffraction (SAED),34 in situ small-angle and wideangle X-ray scattering (SAXS/WAXS),35 and in situ atomic force microscopy (AFM).36 Venna et al. identified various structural evolution stages of ZIF-8 that followed Avrami’s regime.34 Cravillon et al., studied the homogeneous nucleation and early growth events of ZIF-8 and identified the formation and gradual disappearance of the clusters, suggesting the involvement of clusters in the particle nucleation process.35 In Moh et al.’s work,36 they provided strong evidence that crystal growth of ZIF-8 involves direct addition of simple monomeric 2-methylimidazole ion (MeIm−) and solvated Zn2+ and not larger clusters or secondary building units such as [Zn(MeIm)4]2−. They also demonstrated that MeIm− ions must interact with additional species, presumably the solvent molecules, which induce a degree of ordering of the molecules.36 All of these studies have collectively led to a Received: October 9, 2014 Revised: November 7, 2014

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dx.doi.org/10.1021/cg501502r | Cryst. Growth Des. XXXX, XXX, XXX−XXX

Crystal Growth & Design

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Figure 1. Views of the crystal structure (ball-and-stick model and wire-frame model) of (A) ZIF-L and (B) ZIF-8. For the ball-and-stick model, the polyhedron is built on a ZnN4 unit. For the wire-frame model, Zn−MeIm−Zn bonds are replaced with sticks and hydrogen atoms are omitted for clarity. Note: Quadrilaterals that are in the same plane are labeled as I. Quadrilaterals that are in the same plane, but different from plane I, are labeled as II.

the area of zeolite syntheses.49−54 This bottom-up concept produces some resultant new structures with different topologies (e.g., MCM-22 (MWW),55 FER,51,56,57 SOD,52,58,59 NU-6(2) (NSI),60 EU-20b (CAS),61 ZSM-5 (MFI),62 RUB-24 (RWR),63 RUB-37 (CDO),64 and RUB-41 (RRO)65) from their layered precursors (e.g., MCM-22P,55 PREFER,51 NU6(1),60 EU-19,61 RUB-15,52,58 18,63 −36,64 −38,64 −48,64 and −3965), which may not be obtained by direct synthesis, including new topologies such as MWW, RRO, RWR, CDO, and NSI out of close to 200 different structural types of zeolites.66 To the best of our knowledge, the conversion from ZIF-L to ZIF-8 is the first example of a topotactic transition of porous ZIF, from a 2D layered structure to a 3D structure. A few notable ZIF materials that also undergo structural transformations are ZIF-7,67−70 ZIF-8,71−73 and CoNIm (RHO).15 ZIF-7 lp-phase68 (or ZIF-7-I;67 lp = large pore) undergoes structural transformation to form ZIF-7 np-phase (or ZIF-7-II; np = narrow pore) upon heating, which can be regarded as a distorted ZIF-7. ZIF-8, on the other hand, also undergoes structural transformation after undergoing highpressure treatment, forming ZIF-8-II71 (or ZIF-8HP,73 a highpressure phase).71,73 Similar effects are also observed in ZIF-7 at higher pressure.69,70 These phenomena are now known as a breathing effect74−78 (reversible swelling with solvent or gas adsorption) or a gate-opening effect69,74,76,79 (reversible transition from a “closed” to “open” phase above a threshold pressure), although other terms are also used to describe such flexibility.75 CoNIm (RHO), on the other hand, does not show such an effect but undergoes phase transformation (not topotactic) to form ZIF-65 (SOD), a less porous ZIF upon prolonged heating in N,N-diethylformamide.70

good understanding of the solution-mediated crystallization of ZIF-8, but they are limited to the use of methanol as the solvent. In other ZIF-8 synthesis systems, a much higher ratio of Zn to MeIm (1:70)32 or modulating ligand to deprotonate MeIm is required.33,37−39 It is well-accepted that the synthesis conditions (i.e., the metal−linker ratio, type of solvent, and the presence of modulating ligand) have important effects on the successful synthesis of ZIF materials. For instance, a new ZIF (named ZIF-L) was recently synthesized in a Zn2+ and 2methylimidazole aqueous solution,40 in which the recipe was slightly modified from a ZIF-8 synthesis recipe by replacing the solvent with water.31 This demonstrates the complexity of the ZIF crystallization process, and thus more work is still needed to better understand the growth mechanisms of ZIF in different systems. ZIF-L is a two-dimensional layered ZIF structure that is made up of the same building blocks as ZIF-8. The layers are stacked onto each other along the c axis via the hydrogen bonds between MeIm molecules. The ZIF-L layer is also part of the SOD topology found in ZIF-8. Figure 1 shows the crystal structure of these two ZIFs. Unlike ZIF-8, the two neighboring two-dimensional sod layers in ZIF-L are not bridged by MeIm, but by hydrogen bonds; they are parallel to each other and 3.97 Å apart.40 Compared to ZIF-8, the ZIF-L layers are 6.64 Å “out of phase” along the b direction (Figure 1); there is no displacement along the a direction in ZIF-L (see the Supporting Information, Figures S1 and S2, for a more detailed comparison of ZIF-8 and ZIF-L in terms of structure and morphology). The close similarity between ZIF-L and ZIF-8 provides a unique opportunity to investigate the crystallization mechanisms of ZIFs. In the present paper, we report our findings on the topotactic phase transition from ZIF-L to ZIF-8. Topotactic transition, a structural change of crystalline solid where the final lattice is related to the initial lattice by one of more crystallographyically equivalent and orientational relationships,41 was also observed in supramolecular networks18,42,43 and a coordination polymer (or MOF).44−48 Also, the topotactic phase transition from a two-dimensional layered structure to a three-dimensional framework material is classic to

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EXPERIMENTAL SECTION

Materials. Zinc nitrate hexahydrate (Zn(NO3)2·6H2O; 99.9%), 2methylimidazole (MeIm; 99.0%), and 1-methyl-2-pyrrolidinone (NMP) were purchased from Sigma-Aldrich, Australia. Methanol, ethanol, N,N-dimethylformamide (DMF), isopropanol, and acetone were purchased from Merck Millipore, Australia. The water used for the experiments was purified with a water purification system (Milli-Q

B



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integral water purification system, Merck Millipore Australia) with a resistivity of 18.2 MΩ/cm. Synthesis of ZIF-L. ZIF-L was synthesized as reported.40 Typically, 0.59 g of zinc nitrate hexahydrate (Zn(NO3)2·6H2O) and 1.30 g of 2methylimidazole (MeIm) were dissolved in 40 mL of deionized water, respectively, and then the aqueous solution of zinc nitrate was added into the aqueous solution of MeIm under stirring. The mixture was stirred at room temperature for 4 h. The product was collected by repeated centrifugation (8000 rpm, 20 min) and washed with fresh methanol for three times, and then dried in an oven at 70 °C overnight. Conversion of ZIF-L in Organic Solvents at 60 °C. ZIF-L solution was prepared by dispersing 0.08 g of ZIF-L in 8 mL of ethanol. The prepared solution was placed in the oven at 60 °C for 4 h. The sample was collected by evaporation of solvent in a ventilated fume cupboard at room temperature (RT). The experiments were also conducted at other heating times (8, 12, 16, 24, 36, 48, 72, and 168 h). To determine the effect of other solvents as well as the stability of the products, fresh samples of ZIF-L were also dispersed in other solvents, including methanol, N,N-dimethylformamide (DMF), isopropanol, 1methyl-2-pyrrolidinone (NMP), and acetone, respectively. The samples were placed in an oven at 60 °C for 168 h for phase transformation and were recovered by evaporation of solvent. Conversion of ZIF-L under Vapor-Assisted Heating at 80 °C. The vapor-assisted heating method was carried out similar to the method reported by Shi et al.30 A 0.02 g portion of ZIF-L sample was placed in a 5 mL beaker that was elevated 2 cm from the bottom surface of the autoclave (Figure S3, Supporting Information). A few drops of ethanol were added to the bottom of the autoclaves. The transformation was then carried out at 80 °C for 24 h. After cooling the autoclave to room temperature, the solid products were separated by evaporation of solvent in a ventilated fume cupboard. To determine the stability of ZIF-8 after phase transformation, the same vaporassisted heating experiments were conducted for 168 h in solvents including ethanol, methanol, DMF, isopropanol, NMP, and acetone. Characterization Techniques. Field emission scanning electron microscopy (FESEM) images were taken using a Nova NanoSEM 450 (FEI, USA). Selected-area electron diffraction (SAED) were taken using a Tecnai G2 T20 TWIN TEM (FEI, USA). Low-dose imaging conditions were employed to reduce radiation damage to the ZIF sample. Powder X-ray diffraction (PXRD) patterns were recorded in the 2θ range of 5−40° at room temperature using a Miniflex 600 diffractometer (Rigaku, Japan) in transmission geometry using Cu Kα radiation (15 mA and 40 kV) at a scan rate of 2°/min and a step size of 0.02°. Quantitative XRD pattern analysis was done using a FULLPAT (full pattern quantitative analysis) program to compare the peaks with the simulated peaks based on individual ZIF-8 and ZIF-L samples.80 The program uses least-squares minimization to optimize the fit of the standard references to the observed patterns.80 Fourier transform infrared spectra (FT-IR) were recorded by means of an FTIR spectrophotometer (PerkinElmer Spectrum 100, USA) using the KBr wafer technique. The N2 sorption test was carried out on a physisorption analyzer (Micromeritics ASAP 2020, USA) at liquid nitrogen temperature, and the samples were degassed at 100 °C for 12 h before the measurements. Solid-state NMR experiments were collected on Bruker Avance 300 spectrometer (7.05 T magnet) with a 4 mm 13C multinuclear solid-state probe at room temperature. The ZIF-L sample was first degassed at 100 °C to remove any remaining solvent. The sample was then packed into a 4 mm zirconia rotor with a Kel-F cap, and spectra were recorded using cross-polarization/magicangle spinning (CP/MAS) techniques. For the data acquisition, 13C MAS CP at 100.6 MHz, spin rate of 10000 Hz, SW of 50000 Hz (662 ppm); acquisition time of 20 ms, 1992 data points, a 4 s delay, and a 2 ms contact time were used. The spectra were referenced to a glycine external reference. Solution spectra were recorded on a Bruker Avance III 400 instrument (9.4 T magnet) with a 5 mm 1H broadband inverse probe with Z-gradients. A 21 mg portion of ZIF-L was added to 0.7 mL of CD3OD, shaken, and then loaded into the spectrometer and spun at 20 Hz at 60 °C. After temperature equilibration, initial shimming and gain setup, experiments were recorded (24 scans, 11

ppm sweep width) every 30 min for 3 days. The spectra were phased, baseline-corrected, and integrated with the residue methanol peak being assigned a value of 100 in each spectrum. The spectra were processed using the Topspin 1.3 (for Avance 300) or Topspin 2.1 program (Avance III 400).

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RESULTS AND DISCUSSION Phase Transformation in Organic Solvents. We have observed that ZIF-L can transform to ZIF-8 in various organic solvents, such as dimethylformamide, N-methylpyrrolidone, acetone, isopropanol, ethanol, and methanol, at elevated temperatures in our experiments (see the Supporting Information, Figure S4, for PXRD patterns). Ethanol was chosen as an example to investigate the phase transformation of ZIF-L in detail. The samples collected from ZIF-L solvated in ethanol between 4 and 168 h at 60 °C were examined via PXRD. Figure 2 shows the evolution of the PXRD patterns of

Figure 2. PXRD patterns of the samples collected after heating in ethanol at 60 °C for different reaction times.

the solid sample with time. The intensities of the characteristic peaks at 2θ of 7.3°, 12.7°, and 18.0° corresponding to the (011), (112), and (222) planes of ZIF-8 increased with increasing time. Between 4 and 48 h, a mixture of ZIF-L and ZIF-8 was obtained. During the transformation, no peaks from any other phase were observed, which may suggest a direct phase transformation from ZIF-L to ZIF-8. Complete phase transformation was observed after 72 h. The FULLPAT (full pattern quantitative analysis) program was used to analyze the composition of the product by comparing the area under the peaks of the experimental PXRD patterns to those of the simulated PXRD patterns of ZIF-L and ZIF-8 mixed powders with varying compositions.80 Detailed information about FULLPAT PXRD quantitative analysis and all simulated zinc−MeIm−ZIF mixtures can be found in the Supporting Information (Figure S5). The experimental result is in good agreement with the simulated result, which confirms the inexistence of an intermediate phase during the phase transformation. The final product exhibited the typical pure ZIF-8 phase. The nitrogen sorption technique was used to analyze the samples collected during the transformation (Table 1). Since ZIF-8 has a much greater surface area than ZIF-L, the nitrogen sorption capacity of the sample increases with the transformation time (Figure 3). The increase in the surface area may be due to the rearrangement of the layered crystal structures to produce more accessible pores and internal surface area. Figure 4 shows the conversion of ZIF with time based on PXRD and Langmuir surface area. The Langmuir model was used to C



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Table 1. Structure Characteristics of the Products at Various Stages conversion time/h

SBETa/m2 g−1

SLb/m2 g−1

Vtotalc/cm3 g−1

Vt‑plotd/cm3 g−1

xBETe/%

xXRDf/%

ZIF-L 12 24 36 48 72 ZIF-8 (DMF)g ZIF-8 (H2O)h ZIF-8 (MeOH)i

18 167 586 683 929 1333 1630 ∼1500 962

25 219 774 899 1225 1757 1810 ∼1800 1765

0.02 0.09 0.29 0.33 0.46 0.66 0.64 0.64 0.36

0.00 0.07 0.27 0.32 0.43 0.61 -

0.00 11.29 43.20 50.53 69.29 100.00 -

0.00 12.20 34.90 43.00 79.30 96.50 -

a

BET surface area. bLangmuir surface area. cSingle-point adsorption total pore volume calculated as amount of N2 adsorbed at P/P0 of 0.97. dt-plot micropore volume. eConversion based on Langmuir surface area result. fConversion based on PXRD result. gZIF-8 synthesized in DMF.19 hZIF-8 synthesized in water.29 iZIF-8 synthesized in methanol.31

Figure 3. Nitrogen sorption isotherm curves of the samples collected after heating in ethanol at 60 °C for (a) 0 h (ZIF-L), (b) 12 h, (c) 24 h, (d) 36 h, (e) 48 h, and (f) 72 h.

Figure 5. SEM images showing the morphology change in the transformation of ZIF-L to ZIF-8 in ethanol at 60 °C for (a) 4 h, (b) 8 h, (c) 16 h, (d) 24 h, (e) 72 h, and (f) 168 h (scale bar: 5 μm).

calculate the rate of transformation (see the Supporting Information for method of calculation) as it is more suitable for evaluating the surface area of ZIF-8.19 The rate of transformation of ZIF-L in both the techniques was comparable. The ZIF-8 obtained after 72 h reaction time showed a Langmuir surface area of 1756.8 m2 g−1 and a singlepoint pore volume of 0.663 cm3 g−1, which are similar to ZIF-8 synthesized in DMF and in water (Table 1).19,29 These values indicated that the ZIF-8 transformed from ZIF-L contains minimal residual species and is of high quality. In addition, SEM images (Figure 5) show the morphological change of the ZIF-L nanoflakes through phase transformation. The breakup of nanoflakes to particles should be due to the

stress cumulating from the phase transformation. No regrowth of ZIF-8 particles was observed, which is evidence of direct solid-phase transformation of ZIF-L to ZIF-8. Selected-area electron diffraction (SAED) patterns (Figure 6) were obtained for pure ZIF-L, ZIF@36h (partially converted ZIF-L), and ZIF@72h (ZIF-8). The d-spacing values of the planes (011), (002), (112), (222), and (114) are in good agreement with those obtained from the XRD pattern and reported values of the d-spacing for crystalline ZIF-8 particles.27 Similarly, the d-spacing values for ZIF-L (020), (023), (314), (243), and (443) are also in good agreement with the values obtained from the XRD pattern. However, the d-spacings of ZIF@36h (6.7, 5.3, and 4.0 Å) can be assigned to both the

Figure 4. Conversion of ZIF-L to ZIF-8 with time in ethanol at 60 °C, determined by (a) PXRD quantitative analysis and (b) Langmuir surface area. D



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Figure 7. FT-IR spectra of samples after heating in ethanol at 60 °C for (a) 0 h (ZIF-L), (b) 4 h, (c) 8 h, (d) 12 h, (e) 24 h, (f) 36 h, (g) 72 h, and (h) ZIF-8.

cm−1 are attributed to the aliphatic and aromatic C−H stretching of the imidazole, respectively.81 In the FT-IR spectrum of ZIF-L, the adsorption band at 2426 and 1771 cm−1 can be assigned as N−H···N hydrogen bonds.82 The strong and broad adsorption band between 2250 and 3700 cm−1 can be ascribed to the N−H···N hydrogen bonds bridging the ZIF-L layers and the N−H group in MeIm.83 It should also be noted that several absorbance bands (675, 693, 1771, and 2426 cm−1) in ZIF-L coincide with that of MeIm, signifying a larger number of N−H groups.84 With increasing reaction time, the intensity of the few more pronounced peaks at 675, 1771, and 2426 cm−1 gradually reduced, revealing the disappearance of hydrogen bonds between the ZIF-L layers. The results also confirmed the rapid phase transformation without formation of an intermediate phase. The composition of ZIF-L was previously determined by elemental analysis as Zn(mim)2·(MeIm)1/2·(H2O)3/2.40 Compared to ZIF-8 (Zn(mim)2), ZIF-L contains an additional MeIm per 2 Zn centers (mim = C4H5N2). Thus, the phase transformation from ZIF-L to ZIF-8 must proceed with the release of MeIm into the solution. 13C cross-polarization/ magic-angle spinning solid-state NMR (13C CP MAS SSNMR) was used to identify various forms of MeIm molecules coordinated to the Zn centers in ZIF-8 and ZIF-L. Figure 8

Figure 6. SAED ring patterns of (a−c) ZIF-L, (d, e) ZIF@36h, and (f) ZIF@72h.

values obtained from the XRD of ZIF-L and ZIF-8 as they have very similar d-spacings below 8.5 Å (Table 2). Nevertheless, SAED results confirm that the particles after phase transformation are ZIF-8. Table 2. Selected-Area Electron Diffraction (SAED) Patterns: Correspondence of d-Spacing to the Lattice Planes d-spacing (Å)

ZIF-8 (hkl)

ZIF-L (hkl)

figure

4.0 5.3 6.7 4.9 6.9 8.5 12.0 3.0 3.5 3.9 5.2 8.7

(114) (013) (112) (222) (112) (002) (011)

(314) (023) (221)

6d,e,f 6d,e 6d,e 6f 6f 6f 6f 6b 6c 6a 6a 6a

(443) (243) (314) (023) (020)

The change of chemical structure in the phase transformation was also confirmed by FT-IR spectroscopy (Figure 7). Comparison between FT-IR spectra of ZIF-L and ZIF-8 (Figure S6, Supporting Information) shows a close similarity between the two as both the ZIFs were made up of the same metal centers and organic linkers. Most of the absorption bands are associated with the vibrations of the imidazole units except for Zn−N stretching, which is observed at 421 cm−1. In the fingerprint region, the bands in the spectral region of 600−800 cm−1 are associated with out-of-plane bending of the imidazolate ring, whereas those between 900 and 1350 cm−1 arise from the in-plane bending. The convoluted bands at 1350−1500 cm−1 are ascribed to the entire ring stretching. In the functional group region, the peak at 1584 cm−1 is assigned as the CN stretch mode, while the bands at 2927 and 3135

Figure 8. 13C CP MAS SSNMR spectra of (a) ZIF-8 and (b) ZIF-L. The numbers on the top of the peaks correspond to the numbers on the molecular formula. E



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Figure 9. (a) Time-resolved in situ 1H NMR spectra of ZIF-L heated in deuterated methanol at 60 °C. (b) Plot of extent of crystallization (α) curves against time (t) obtained by integration of the peak at 2.4 ppm in the in situ NMR data and by assigning a value of 100 to the residue methanol peak (3.3 ppm). Inset: analysis by the method of Sharp and Hancock with the line that is the result of linear regression analysis. (c) Plot of selected function g(α) curves against time (t) based on various models.

shows the 13C CP MAS SSNMR spectra of ZIF-8 and ZIF-L. The signal at about 150 ppm can be assigned to the carbon atom between the two nitrogen atoms in MeIm. The other signals at 10 and 120 ppm can be assigned to CH3− and CH− groups of MeIm, respectively.85 Since both ZIF-8 and ZIF-L share the same organic linker (MeIm), resonances at the same chemical shift regions are expected. Nevertheless, in ZIF-L, the bidentate MeIm exists in three forms. One form of the MeIm in common with ZIF-8 is the one bridging two Zn centers. The additional two forms of MeIm found only in ZIF-L are (1) free MeIm located between the ZIF-L layer structures, which helps to stabilize the layers by hydrogen bonds, and (2) single-sidecoordination-bonded MeIm located between each ZIF-L layer.40 Thus, the phase transformation must occur with the release of additional forms of MeIm in ZIF-L into the solvent as ZIF-8, formed with each bidentate MeIm linker bridging two Zn centers, which contains fewer MeIm compared to ZIF-L. The release of MeIm during the phase transformation process could provide indirect, but vital, evidence to the phase transformation mechanism. The phase transformation of ZIF-L in ethanol at 60 °C was monitored by in situ 1H NMR spectroscopy over 3 days. Figure 9a shows the NMR spectra of the sample during the phase transformation. The two resonance peaks at 2.4 and 7 ppm, whose intensity ratio is 3:2,86 correspond to CH3− and CH− of MeIm, respectively. The other two signals (3.3 ppm to 4.4 ppm) can be ascribed to the impurities in the deuterated solvent (residue methanol and H2O, respectively). The amount of MeIm released into the solvent during the phase transformation from ZIF-L to ZIF-8 increases with time, as indicated by the fact that the intensity of the two signals at 2.4 and 7 ppm increased with time. This provides strong evidence that the transformation does not proceed via a dissolution−recrystallization route, as there was no consumption of the built-up MeIm during the progression of transformation. In Khan and Jhung’s work,14 during the phase transition of chromium-benzenedicarboxylates (from MIL-101 to MIL-53), the concentration of TPA increased to a maximum value before decreasing. The conversion of MIL101 into MIL-53 occurred through the recovery of terephthalic acid/chromium species from MIL-101, followed by successive crystallization.14 Figure 9b shows the normalized transformation (α-time) curves by integration of the peak at 2.4 ppm and assigning a value of 100 to the residue methanol peak (3.3 ppm). The induction period was absent from the α-time plot, and the

transformation was deceleratory throughout. This is different from the synthesis of ZIF-8, which follows Arvami’s classical model.34 The shape of the α-time plot also provides a graphical representation of the deceleratory model, which can be based on either diffusion mechanisms (D1-D4), geometrical contraction models (R1-R3), or order of reaction (F0-F3). To determine the exact model, analysis of the crystallization curve was performed by the method of Sharp and Hancock87 (Figure 9b, inset). This powerful method allows the groups of functions to be distinguished readily. An α value from 0.15 to 0.50 was used to exclude the influence by uncertainty (initial conditions, particle size distribution, and other geometrical factors), but the same conclusion was also found using the whole α range. A linear plot with a slope of 0.89 was obtained, indicating either (i) a geometrical contraction model (probably one described by R2(α) or R3(α)) or (ii) a reaction based on order with respect to α (F0/R1(α) or F1(α)). To find out the most appropriate model within the two groups, g(α) vs time over the whole range of α was plotted (Figure 9c). The g(α)-time plots indicate that the transformation from ZIF-L to ZIF-8 followed the contracting area model (R2), with a coefficient of determination of 0.988 (adjusted R-squared). The geometrical contraction model88 (also known as the phase-boundary-controlled reaction89 or advancing interface reaction90) is generally expressed as 1 − (1 − α)1/n = kt, where n is the number of dimensions in which the interface advances. n = 3 for contracting volume, n = 2 for contracting area, and when there is linear advance of the interface in a single direction, n = 1, which is also known as zero-order kinetics. The geometrical contraction model involves consideration of different rates of interface progress in different crystallographic directions and of variations in crystallite shapes and dimensions.90 For a contracting area (disc, cylinder, or rectangle) equation, where n = 2, the reaction occurs only at the edges of a disc or platelike particle, inward toward the center. This equation has been used to describe the dehydrations of certain substances containing layer-type lattices.91−93 The R2 model was supported by microscopy observation (Figure 5), where the two-dimensional leaflike morphology was transformed sequentially to an elongated hexagon and irregular thin hexagonal prism as the transformation advanced from the edges of the crystal layer inward, but not in directions perpendicular to the layer. To the best of our knowledge, it is the first time to find that the phase F



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transformation of a zeolitic-imidazolate framework (ZIF) follows a geometrical contraction model. Organic Solvent Vapor-Assisted Phase Transformation. Recently, solid phase synthesis MOFs or coordination polymers, particularly ZIFs, have gained tremendous attention as an environmental-friendly and energy-efficient synthesis approach, such as in mechanochemistry12,94−96 and vaporassisted solid phase reaction.30 The similarity between ZIF-8 and ZIF-L topology inspires us to investigate the feasibility of the solid phase transformation from ZIF-L to ZIF-8, which may also provide useful information on the phase transformation process. Phase transformation of ZIF-L by heating in air is unlikely, and therefore, we adopt a vapor-assisted method reported elsewhere,30 where an organic solvent was used as the vapor phase, and ZIF-L nanoflakes were used as the solid phase (see the Supporting Information for the detailed experimental procedure). Remarkably, the complete phase transformation was obtained after exposure to various organic solvent vapors at 80 °C for 24 h. To examine the stability of the ZIF-8 produced via this method, the experiment was prolonged to 168 h. The PXRD patterns of the products shown in Figure 10 indicate

Figure 11. SEM images showing the morphology change in phase transformation of ZIF-L to ZIF-8 under exposure to ethanol vapor at 80 °C for (a) 2 h, (b) 4 h, (c) 6 h, (d) 8 h, (e) 16 h, and (f) 24 h (scale bar: 5 μm).

Mechanisms of Phase Transformation in Organic Solvent. On the basis of the experimental results above, it is clear that the phase transformation of ZIF-L nanoflakes to ZIF8 particles does not involve a dissolution−recystallization process, and it occurs in a solid phase. In other words, the process can be classified as a so-called topotactic phase transformation, considering the similarity of the two ZIF framework structures. Two phase transformation mechanisms are postulated below. Scheme 1 presents two possible phase Scheme 1. Proposed Phase Transformation Pathwaysa

Figure 10. XRD patterns of ZIF-L under vapor-assisted heating at 80 °C for 168 h in (a) DMF, (b) NMP, (c) acetone, (d) isopropanol, (e) ethanol, and (f) methanol.

that ZIF-8 was successfully produced and remained stable in various organic solvent vapors after 168 h (see the Supporting Information, Figure S7, for corresponding SEM images). The SEM images of the samples undergoing phase transformation in ethanol vapor at 80 °C were taken at various time intervals. As shown in Figure 11, during the phase transformation process, ZIF-L flakes break down to smaller crystals. This is similar to that observed in the phase transformation in an organic solvent, again confirming that no dissolution−recrystallization occurs (see the Supporting Information, Figure S8, for PXRD patterns of the sample collected after organic solvent vapor-assisted phase transformation). Attempt to transform ZIF-8 back to ZIF-L has not been successful, and therefore, the transformation appears to be irreversible. Since ZIF-L contains an additional MeIm per 2 Zn centers compared to ZIF-8, it is likely that the transformation is irreversible. Also, ZIF-8 with the SOD topology is also more stable than the layered structure of ZIF-L based on previous studies,40 in which the decomposition of ZIF-L occurs at 300 °C compared to 580 °C of ZIF-8.19,97 On the basis of the above results, we conclude that ZIF-L is kinetically favored, whereas ZIF-8, the more stable phase, is thermodynamically favored.

a

Zn−MeIm−Zn bonds have been represented by sticks, and H atoms have been omitted for clarity. A: Sliding of semi-SOD adjacent crystal layers along the [100] direction. B: Sliding of adjacent semi-SOD crystal layers along the [010] direction. I: breakage of H-bonds and displacement of the adjacent layers away from each other. II: Breakage of Zn−MeIm bonds and formation of new Zn−MeIm−Zn bonds.

transformation pathways. Scheme 1A shows the sliding of the two adjacent crystal layers of ZIF-L on the (001) plane in the [100] direction with an overall displacement of 12.37 Å. Scheme 1B shows the sliding of the two adjacent crystal layers in the [010] direction and an overall displacement of 6.95 Å. The detailed calculation of overall displacement can be found in the Supporting Information (Figure S9). The proposed transformation steps are as follows: (i) Interactions between the solvent molecules and the interlayer MeIm molecules lead G



Article

assistance of Xi-Ya Fang at the Monash Centre for Electron Microscopy. The authors also acknowledge the assistance of Peter Nichols for his help in NMR spectroscopy. Z.-X.L. is grateful for the Ph.D. top-up scholarship from the National Centre of Excellence for Desalination Australia, which is funded by the Australian Government through the Water for the Future initiative. J.Y. thanks Monash University for the Monash Fellowship. H.W. thanks the ARC for a Future Fellowship (FT100100192).

to breaking and reforming new hydrogen bonds with the MeIm molecules of alternating layers to allow displacement of the layers away from each other. (ii) Breakage of Zn−MeIm bonds in ZnN4 units exposed between the layers, freeing MeIm molecules. (iii) Rotation of MeIm molecules and formation of new Zn−MeIm−Zn bonds, yielding ZIF-8. The change in dimensionality of MOFs can be completed by the removal of structural organic linkers.98 However, the removal of structural linkers normally reduces the dimensionality of the MOF, which increases its flexibility at the loss of its level of connectivity.98,99 On the contrary, in ZIF-L to ZIF-8 phase transformation, removal of MeIm linkers actually results in an increase in the dimensionality of the ZIFs material, i.e., from 2D to 3D. The cavity size of the framework also increases from 9.4 Å × 7.0 Å × 5.3 Å (or ∼349 Å3 assuming a rectangular cavity) to ∼817 Å3 (based on the diameter of the largest sphere that will fit into ZIF-8 = 11.6 Å).19,40 This work demonstrates that the removal of structural linkers does not always lead to reduced dimensionality. ZIF-L layers were supported mainly by hydrogen bonds (not shown in the figure) and the intertwinement of the single-sidebonded imidazole molecules (presented in Scheme 1 (BI)). This intertwinement may restrict the movement of the layers along the a axis. Compared to pathway B, pathway A also requires a much larger overall interlayer displacement (12.37 Å against 6.95 Å). Even though the possibility of having both pathways occurring simultaneously cannot be ruled out completely, the topotactic phase transformation was more likely to follow pathway B with a lower overall interlayer displacement.

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ABBREVIATIONS MOF, metal−organic framework; ZIF, zeolitic-imidazolate framework; MeIm, 2-methylimidazole

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CONCLUSIONS We have shown that the phase transformation from ZIF-L to ZIF-8 occurs in various organic solvents at elevated temperatures, or under exposure to various organic solvent vapors. This is the first topotactic transformation observed in two functional, porous ZIFs, which occurred via a geometric contraction model (R2). Our study provides a better understanding of the structural evolution of zinc-2-methylimidazole-based ZIF materials, which have an important implication for understanding complex MOF crystallization. The phase transformation process may also provide an interesting strategy for the synthesis and manipulation of the structure and properties of ZIFs and MOFs.

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ASSOCIATED CONTENT

S Supporting Information *

Crystal structure, SEM images, PXRD pattern, and FT-IR spectra of the samples; calculation for rate of conversion; and overall displacement of ZIF-L layer. This material is available free of charge via the Internet at http://pubs.acs.org.

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REFERENCES

AUTHOR INFORMATION

Corresponding Authors

*E-mail: [email protected] (J.Y.). *E-mail: [email protected] (H.W.). Notes

The authors declare no competing financial interest.

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ACKNOWLEDGMENTS This work was, in part, supported by the Australia-India S & T Fund. The authors acknowledge the use of the facilities and the H



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Crystal Transformation in Zeolitic-Imidazolate Framework - Crystal

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