Versatile and Efficient Mechanochemical Synthesis of Crystalline

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Versatile and Efficient Mechanochemical Synthesis of Crystalline Guest#ZIF Complexes (ZIF = Zeolitic Imidazolate Framework) by in situ Host–Guest Nanoconfinement Peng Gao, Xiaomeng Shi, Xiaoyan Xu, and Wei Wei Cryst. Growth Des., Just Accepted Manuscript • DOI: 10.1021/acs.cgd.8b00528 • Publication Date (Web): 05 Sep 2018 Downloaded from http://pubs.acs.org on September 6, 2018

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Crystal Growth & Design

Versatile and Efficient Mechanochemical Synthesis of Crystalline Guest⊂ZIF Complexes (ZIF = Zeolitic Imidazolate Framework) by in situ Host– Guest Nanoconfinement Peng Gao, Xiaomeng Shi, Xiaoyan Xu, Wei Wei* Department of Chemistry, Capital Normal University, Beijing, 100048, P. R. China.

Abstract

In this work, the one-pot mechanochemical synthesis has been demonstrated to be a versatile and efficient method to prepare hybrid guest⊂ZIF materials with high crystallinity, and up to 18 functional guest molecules with different sizes, shapes and properties have been encapsulated into interior cavities of ZIFs with high guest loading. These guest molecules can be accommodated within the different cavities of sod- or rho-ZIFs, depending on the sizes of guest. Because of the relatively small opening of ZIFs, the guest molecules can be immobilized by physical imprisonment and cannot be released without destroying the host matrix. More importantly, the obtained guest⊂ZIF materials have been endowed with various interesting properties originated from the encaged guest molecules, which significantly extends the

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functionality of MOFs. For instance, PEG-decorated nanoparticles of a sod-ZIF (i.e. ZIF-8) encapsulating gadolinium complex exhibit interesting property of magnetic resonance imaging (MRI) and a rho-ZIF (i.e. MAF-6) with metalloporphyrin embedded can be used as an effective heterogeneous catalyst for epoxidation of styrene.

Introduction Growth of metal-organic frameworks (MOFs) has been explosive in the past two decades, and chemists have widely explored the applications of these porous materials in various areas such as gas storage and separation,1–5 catalysis,6–8 sensing9 and drug delivery.10–12 However, rational design of new MOFs for new/improved functions to satisfy the practical application is still highly desirable. Recently, a forefront approach to construct made-to-order MOFs for particular application is hybridization. It has been demonstrated that mixing different components from atomic, molecular and nanoscale to mesoscale allows for the exquisite control of properties of MOFs, even leading to synergistic effect. As proposed by Kitagawa et al,13 hybrid MOFs have been divided into three broad classes: isomorphous MOFs of Class I prepared by mixing analogous metal ions and/or ligands, core−shell structures of Class II with distinct phases, and MOF with accommodated guest (guest⊂MOF) of Class III. Different from Class I and II, hybrid MOFs of Class III confine the functional guests within the well-defined nanospace at the molecular level, which could avoid the aggregation and unpredictable interaction of guest molecules and may lead to the uniform and even enhanced functions. Therefore, it is particularly interesting for synthesis of ideal guest⊂MOF materials with quantitative guest loading and perfect structural regularity. However, the most frequently used methods to prepare guest⊂MOF, that is, one-pot solution synthesis and infiltration route14,15 usually afford non-ideal hybrid MOFs


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with the partial occupation and uneven dispersion of guest molecules. Moreover, some other intrinsic disadvantages of existing methods such as excessive reagents in bulky organic solvents, low loading efficiency and expensive equipment also have hindered the development of hybrid MOF of Class III. As an excellent alternative to traditional solution-based and solvothermal ones, mechanochemical techniques,16–21 usually in the form of milling or grinding, possess advantages of clean, and energy-efficient synthesis with no need for bulk solvents, excess reagent or agressive conditions. Friščić and co-workers22–27 recently developed some improved approaches including liquid-assisted grinding (LAG) and ion- and liquid-assisted grinding (ILAG) to rapidly and mildly prepare a number of microporous MOFs in gram-scale. Among these MOFs, zeolitic imidazolate frameworks (ZIFs) are very attractive host matrices for preparing guest⊂MOF materials, since they usually possess large interior cages and relatively small windows, which is beneficial for accommodating and immobilizing bulky guest molecules.28–30 Very recently, Wang31 and Xu32 have reported that the polyoxometalates can be accommodated into the cavities of the in situ formed sod-ZIF (cavity diameter ~13 Å, windows diameter 3.4 Å) or rho-ZIF (cavity diameter ~22 Å, windows diameter ~7.6 Å) in a mechanochemical process, and the obtained complexes can be used for small molecule adsorption/sensing and heterogeneous catalysis. Because of the hydrophobic nature of nanocavities in ZIFs, it is reasonable to expect that bulky hydrophobic guest molecules are more favorable to be encapsulated via in situ host– guest nanoconfinement in the mechanochemical synthesis, which may endow the hybrid guest⊂ZIF complexes with brand-new properties.


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In this work, the one-pot mechanochemical synthesis has been used as a versatile and efficient method to prepare hybrid guest⊂ZIF materials with high crystallinity, and up to 18 functional guest molecules with different sizes, shapes and properties have been encapsulated into interior cavities of ZIFs with high guest loading. Because of the relatively small opening of ZIFs, the guest molecules can be immobilized by physical imprisonment and cannot be released without destroying the host matrix. More importantly, the obtained guest⊂ZIF materials have been endowed with various interesting properties originated from the encaged guest molecules, such as magnetic resonance imaging (MRI) and heterogeneous catalysis for styrene oxidation, which significantly extends the functionality of MOFs. Results and Discussion Syntheses Scheme 1. The schematic illustration for the one-pot mechanochemical synthesis of guest⊂ ⊂ZIF complexes.


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The synthesis of the hybrid host-guest guest⊂ZIF complexes by one-pot mechanochemical method is illustrated in Scheme 1. The hybrid sod-ZIF complexes were synthesized by an improved LAG method. A solid mixture of zinc oxide, 2-methylimidazole (2.5-3 equivalents) and targeted guest molecules (0.1 equivalents) and small amounts of liquid (methanol, DMF or DEF) along with twelve 10 mm and ten 7mm diameter zirconia balls were put into a 70 mL zirconia milling pot. After the mixture was ball-milled for 30 min twice with a 5 min break, the powdery product was obtained. Pristine ZIF-8 was also synthesized in a similar way for comparison. Different from the sod-ZIFs, small amounts of salt (NH4)2SO4 must be added in order to prepare the hybrid rho-ZIF complexes with a ILAG method, and the formation conditions of rho-ZIF are more subtle owing to the metastable nature of rho-ZIF in thermodynamics.33 For instance, an overlong milling time, an inappropriate ratio of reactants and excess mounts of salts will lead to the appearance of impurity with other topologies. After a series of comparative experiments, the optimized conditions of 1.0 mmol ZnO, 3.0-3.5 mmol 2ethylimidazole, 10 mg (NH4)2SO4, 200 µL DEF, 0.05 mmol corresponding guest molecules, twenty 10 mm zirconia balls in a 70 mL zirconia milling pot were employed, and reaction time is 2×30 minutes with a 5 min break. It should be noted that excess imidazole ligands are necessary in the synthesis process of sod- and rho-ZIFs to avoid leaving unreacted ZnO. To determine loading ratios of various guest, all the obtained powdery product were firstly washed at least three times with ethanol and distilled water, and then washed by Soxhlet extraction for 24 hours with chloroform to thoroughly remove excessive guest molecules and impurities on the solid surface. The guest contents in these solvent washings were found to be much less than what was added in the original synthesis (Figure S14), indicating that the reduced guest may be included into ZIFs. After drying in vacuum at 120 oC for 12 h, the obtained ZIF


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samples were collected for further characterization. Systematic investigations show that up to 18 bulky guest molecules with different sizes, shapes and properties can be encapsulated within interior cavities of ZIFs, which is preliminarily evidenced by the consistent color of the hybrid ZIF complexes and guest molecules. The results of 1H NMR (Figure S2-S11), inductively Coupled Plasma-Optical Emission Spectrometer (ICP-OES), inductively Couple Plasma mass spectrometer (ICP-MS) and elemental analysis of C/H (Table S1) demonstrated that the majority of guest⊂ZIF complexes have very high guest loading, and some guests are almost quantitatively encapsulated (Table 1). Obviously, the behaviors of guest/host confinement largely depend on the relative diameters of guest and host cavity. For example, the cavities of sod-ZIF (~12 Å)28 can accommodate guest 5, but not the substituted molecules 7–10 with larger size, which can only be encapsulated within the cavities of rho-ZIF (~22 Å).29,30 Interestingly, the relatively small guest 5 cannot be immobilized in rho-ZIF, although the windows of rho-ZIFs (~7.6 Å) seem smaller than guest 5, and similar phenomena were observed in the previous work.34–36 For 14⊂rho-ZIF, it maintained the intrinsic color of metalloporphyrin after Soxhlet extraction to thoroughly remove the unencapsulated guest on the solid surface. In sharp contrast, when sod-ZIF was synthesized under the similar conditions by using 2-methylimidazole instead of 2-ethylimidazole, guest molecules of 14 can be thoroughly removed by washing the powders several times, only leaving the white pristine ZIF-8 solids. This result provides another evidence to confirm that guest molecules indeed occupy the interior cages of rho-ZIF at the molecular level. More subtle encapsulation behaviors were observed for guest 8/10 and 11/12. The results show that each rhoZIF cavity can accommodate two molecules of guest 8 or 11, but only one molecule of guest 10 or 12, despite the fact that there are only a slight enlargement of 10/12 compared with 8/11.


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Table 1. Various guest⊂ ⊂ZIF complexes and their corresponding loading ratios

ZIF type

Number per cage

Loading ratios (%)

1

sod

1

99 (3)a

2

sod

1

85 (4)b

3

sod

1

62 (3)

sod

1

76 (2)

5

sod

1

99 (4)d

6

sod

1

82 (3)d

2

98 (3)d

2

99 (4)d

2

100 (4)d

guest

4

7

8 9

Structures

C60

rho

rho

rho

b

c


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10

rho

1

99 (3)d

11

rho

2

97 (4)d

12

rho

1

100 (2)d

13

rho

1

84 (3)d

14

rho

1

84 (1)

b

15

rho

1

81 (2)

b

16

rho

1

69 (4)

b

17

rho

1

97 (3)

b


8

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rho

18

a

Determined by ICP-OES;

b

Determined by ICP-OES;

1

c

86 (3)

b

Determined by elemental analysis of C and H;

d

Determined by 1H NMR.

Structural Characterization

Figure 1. a) PXRD patterns of sod-ZIF, 1⊂sod-ZIF, 4⊂sod-ZIF, 5⊂sod-ZIF, 6⊂sod-ZIF and ZnO; b) PXRD patterns of rho-ZIF, 7⊂ rho-ZIF, 8⊂ rho-ZIF, 11⊂ rho-ZIF, 14⊂ rho-ZIF, 18⊂ rho-ZIF and ZnO.


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Powder X-ray diffraction (PXRD) patterns show that all the guest⊂sod-ZIF and guest⊂rhoZIF were in consistence with those of simulated ones from the single-crystal XRD data (Figure 1 and Figure S1). High crystallinity of these complexes is indicated by the sharp diffraction peaks, which are close to the samples through solution synthesis.28 No diffraction peaks of guest molecules were observed in these patterns, indicating that the aggregated guest crystals were not formed in the mechanochemical synthesis. Therefore, the original ordered structure of ZIFs remained intact after encapsulation of guest molecules. As typical tests of chemical stability, the dried samples 1⊂sod-ZIF and 14⊂rho-ZIF were immersed in various solvents such as water, methanol, ethanol, acetonitrile and chloroform for 12 h, UV-vis spectra suggested that there were no absorption of the colored guest leaching out from the host matrix and the complexes kept stable crystallinity in these solvents (Figure S13). Considering the impurity and metastability of pristine rho-ZIF by ILAG method,22,33 guest⊂rho-ZIF materials have much higher chemical stability, which is probably due to strong host-guest interaction from the complementary shape. This phenomenon also indicates the occurrence of guest inclusion for porous ZIF materials.


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Figure 2. a) N2 adsorption and desorption isotherms of sod-ZIF, 1⊂sod-ZIF and 4⊂sod-ZIF, b) N2 adsorption and desorption isotherms of rho-ZIF and 8⊂ rho-ZIF and 14⊂ rho-ZIF.

The N2 adsorption isotherms of the hybrid guest⊂ZIF complexes were measured at 77 K to evaluate the effect of guest encapsulation into the interior cages of ZIFs (Figure 2). The BET surface areas of the pristine sod-ZIF and rho-ZIF are 1533 and 1050 m2/g, while those of 1⊂sodZIF and 14⊂rho-ZIF were reduced to 913 and 640 m2/g, respectively. Other guest⊂ZIF materials


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also display similar behaviors. These results not only indicate that the host-guest hybridization did not damage the integrity of the framework, but also further confirm that the cages in rho-ZIF are indeed occupied by these bulky guest molecules.

Magnetic Resonance Imaging of 1⊂ ⊂sod-ZIF Magnetic resonance imaging (MRI) has become a powerful tool in the clinical diagnosis of disease and imaging in vivo owing to the noninvasive character and high spatial resolution in soft issue.37,38 Due to the highly paramagnetic nature, GdIII ions are often chosen as the metal connectors to construct MOF contrast agents for MRI.39–41 Herein we attempt to investigate the MRI property of the captured GdIII centers within cavities of ZIF. Figure S17 shows representative TEM images of the as-synthesized 1⊂sod-ZIF nanoparticles prepared by the mechanochemical method, whose sizes are distributed roughly from 70 to 200 nm. For evaluating the performance of the nanoscale 1⊂sod-ZIF for MRI, the as-prepared nanoparticles were redissolved in an aqueous solution containing PEG. The Gd concentration of the solution was determined by ICP-MS, and then solutions with different Gd concentrations diluted by water were measured to determine T1 relaxation times. The longitudinal (r1) value was determined to be 9.4007 mM−1s−1 from the slope of the plot of 1/T1 versus the Gd3+ concentration (Figure 3). This r1 value was much higher than the commonly used commercial contrast agent such as GdDTPA (r1=3.63 mM−1s−1),42 which can be explained that the porous nature of the ZIF-8 facilitated the contact of water molecules with gadolinium ions. In addtion, T1-weighted images with same concentration gradient of Gd solution was investigated. Therefore, these results suggest that 1⊂sod-ZIF is a potential T1-weighted MRI contrast agent.


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Figure 3. r1 relaxivity curve of 1⊂sod-ZIF. Inset shows corresponding T1-weighted MRI images of nanoscale 1⊂sod-ZIF.

Catalytic Epoxidation of Styrene by 14⊂ ⊂rho-ZIF Metalloporphyrin complexes has been a hot topic in the field of catalytic oxidation over the past two decades.43–46 Nevertheless, owing to the formation of bridged µ-oxide dimers which hindered access to catalytic sites and oxidative self-degradation,47 the majority of homogeneous metalloporphyrins catalysts have a limited lifetime activity. Therefore, rationally designed platforms to isolate and protect the catalytic site of metalloporphyrin have attracted intense interest from chemists. As epoxidation reaction of styrene by Mn-porphyrin has been well studied,48,49 we chose this reaction to evaluate the catalytic activity of 14⊂rho-ZIF encapsulating tetraphenylporphine manganese(III) chloride. 14⊂rho-ZIF can efficiently catalyze the epoxidation process in the presence of styrene as substrate, iodosobenzene (PhIO) as oxidant, acetonitrile as solvent, in sharp constrast to the blank and pristine rho-ZIF (Table 2). To obtain


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the optimal reaction conditions, several reaction variables such as temperature, solvent, reaction time and the amount of catalyst and oxidant were investigated (Table 2 and Table S2). As the temperature varied, the conversion is maximized at 40 oC. The effect of the solvent on the reaction was also investigated and acetonitrile shows a higher catalytic activity compared with other solvents. Moreover, the conversion of styrene reached ~85% just within 15min, and the reaction was nearly completed within 30min, which shows a high catalytic reactivity of 14⊂rhoZIF for styrene epoxidation. The amount of catalyst and oxidant was also examined for the reaction. About 7 mg 14⊂rho-ZIF can efficiently catalyze 0.5 mmol styrene epoxidation, and when the ratio of PhIO/styrene is enhanced from 1 to 1.5, the conversion rate increased from 92% to 98%. The TON of this catalyst is calculated to be ~502. Moreover, the recovered catalyst can be reused in epoxidation reactions for at least three times with only a slight loss of activity. Table 2. Effect of reaction conditions in epoxidation of styrene with PhIOa

Temperature Reaction time (°C) (min)

Conversionb (%)

Entry

Catalyst

PhIO/Styrene

1

−

1

40

30

16

2

rho-ZIF

1

40

30

18

3

14⊂rho-ZIF

1

25

30

65

4

14⊂rho-ZIF

1

40

30

92

5

14⊂rho-ZIF

1

50

30

89

6

14⊂rho-ZIF

1

60

30

82

7

14⊂rho-ZIF

1

40

5

41

8

14⊂rho-ZIF

1

40

15

85

9

14⊂rho-ZIF

1

40

60

93

10

14⊂rho-ZIF

1.25

40

30

95

11

14⊂rho-ZIF

1.5

40

30

98

12

14⊂rho-ZIF

1.5

40

30

93c


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a

Reaction condition: 0.5 mmol styrene, PhIO, 7 mg catalyst and 2 mL CH3CN. b Conversions were determined

by gas chromatography. c Catalytic activity for the third run.

Conclusion In summary, the one-pot mechanochemical synthesis has been demonstrated to be a versatile and efficient method to prepare hybrid guest⊂ZIF materials with high crystallinity, and up to 18 functional guest molecules with different sizes, shapes and properties have been encapsulated within interior cavities of ZIFs. These guest molecules are immobilized by physical imprisonment and cannot be released without destroying the host matrix. Moreover, the obtained guest⊂ZIF materials have been endowed with various interesting properties such as the effective MRI and heterogeneous catalytic activity, originated from the encaged guest molecules, which significantly extends the functionality of MOFs. Experimental Section Materials and instruments. All chemicals used in this work were of reagent grade from commercial sources and used without further purification. Gadolinium(III) acetylacetonate hydrate were purchased from Strem Chemical. Mn(III) meso-Tetratolylporphyrin were purchased from Frontier Scientific. The one-pot mechanochemical synthesis was performed in a QM-3C ball mill (Nanjing University Instrument Factory, China). Powder X-ray diffraction (PXRD) patterns were characterized by a Purkinje General XD-3 X-ray Diffractometer equipped with a Cu-Kα radiation (36 kV, 20 mA). The samples were digested in DMSO-d6 by the addition of several drops of deuterated hydrochloric acid, and 1H NMR data were collected using Bruker AVANCE III 600 spectrometers. C, H and N elemental contents were analysed on a Vario EL cube elemental analyzer and Zn, Mn, Fe, Co, Pd elements were determined by an Agilent 730


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Inductively Coupled Plasma-Optical Emission Spectrometer (ICP-OES). Gd3+ concentration was measured by an Agilent 7700 Inductively Couple Plasma mass spectrometer (ICP-MS). N2 sorbtion studies were performed on a 3H-2000PSI Specific surface analyzer. GC analysis were performed on a Shimadzu GC-2014C with a FID detector equipped with an HP-5 ms capillary column inflowing N2 with a heating rate of 5 °C min−1. Thermogravimetric analysis (TGA) was performed using a Shimadzu TGA-60 equipped with an alumina pan and heated at a rate of 5 °C/min under nitrogen atmosphere. Guest⊂ ⊂sod-ZIF. Synthetic reactions were carried in a 70 mL zirconia milling pot with twelve 10 mm and ten 7 mm diameter zirconia balls. A solid mixture of 1 mmol zinc oxide, 2.5 mmol 2methylimidazole and 0.1 mmol targeted guest molecules and 200 µL of solvent were put into the pot, then the mixture was milled for 30 min twice with a 5 min break at 50 Hz, The obtained solids were first washed with substantial deionized water and alcohol and then washed by Soxhlet extraction for 24 hours. After drying in vacuum at 120 oC for 12 h, the samples were collected for further characterization. The added organic solvents are methanol for guest 1-3, DMF for guest 4 and DEF for guest 5-6. To avoid the appearance of unreacted ZnO, 3 equivalents of 2-methylimidazole are needed for 4⊂sod-ZIF.

Guest⊂ ⊂rho-ZIF. Synthetic reactions were carried in the same pot with twenty 10 mm zirconia balls. A solid mixture of 1.0 mmol ZnO, 3.0 mmol 2-ethylimidazole, 10 mg (NH4)2SO4 and 0.05 mmol corresponding guest molecules and 200 µL DEF were put into the pot, then the mixture was milled for 30 min twice with a 5 min break at 45 Hz. The following process was same as that of sod-ZIF. To avoid the appearance of unreacted ZnO, 3.5 mmol 2-ethylimidazole are needed for 12-18⊂rho-ZIF.


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1⊂ ⊂sod-ZIF-PEG. 10mg 1⊂sod-ZIF and 200mg PEG were added in to 1.5mL CH3OH , then the reaction mixture was vigorously stirred for 2h. After the reaction was completed, the product was washed with water and then dried for MRI characterization. T1 Measurement: The longitudinal relaxation times (T1) of the 1⊂sod-ZIF were measured with a MicroMR20-025H-I scanner at 0.5T. 1⊂sod-ZIF solutions were prepared with Gd3+ concentrations of 0.045, 0.090, 0.180, 0.362, 0.723, 1.446 and 2.893 mM separately in methanol and then were placed in 1.5 mL tubes. The T1 values were measured with a standard inversionrecovery sequence. (1/T1)obs = (1/T1)d + r1[M] (1) (1/T1)obs and (1/T1)d are observed water proton relaxation rates in the presence and absence of the paramagnetic species, and [M] is the concentration of the paramagnetic species. T1-Weighted MRI Images. The T1-weighted MR images were recorded with a Siemens Medical Systems, The instrumental parameters were 0.5 T, repetition time (TR) =350 ms, echo time (TE) = 14 ms, FOV read = 100 mm, base resolution = 256, slice thickness = 3.5 mm. Catalytic epoxidation of styrenes. In a typical experiment, styrene (0.5 mmol), iodosobenzene, 7mg 14⊂rho-ZIF and 2 mL acetonitrile were added to a Schlenk tube. Then reaction mixture was vigorously stirred in a Wattecs parallel reactor. After the reaction was completed, the products were analyzed by gas chromatography. ASSOCIATED CONTENT Supporting Information


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The Supporting Information is available free of charge on the ACS Publications website.

X-ray powder diffraction patterns, TGA curves, TEM images, 1H NMR patterns and the data of catalyst performances. (PDF).

AUTHOR INFORMATION Corresponding Author *E-mail: [email protected]. (w.w.) ORCID Wei Wei: 0000-0002-8123-1374 Author Contributions All authors have given approval to the final version of the manuscript. Notes The authors declare no competing financial interest. ACKNOWLEDGMENT This study was supported by the National Natural Science Foundation of China (Project No. 11474204 and 21271025). REFERENCES (1) Li, J.-R.; Sculley, J.; Zhou, H.-C. Metal–Organic Frameworks for Separations. Chem. Rev. 2012, 112, 869–932.


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For Table of Contents Use Only

Versatile

and

Efficient

Mechanochemical

Synthesis

of

Crystalline

Guest⊂ZIF Complexes (ZIF = Zeolitic Imidazolate Framework) by in situ Host–Guest Nanoconfinement

Peng Gao, Xiaomeng Shi, Xiaoyan Xu, Wei Wei*

The one-pot mechanochemical synthesis has been demonstrated to be a versatile and efficient method to prepare hybrid guest⊂ZIF materials with high crystallinity and guest loading, and up to 18 functional guest molecules with different sizes, shapes and properties have been encapsulated into interior cavities of ZIFs with sod- or rho-topology.


26

Versatile and Efficient Mechanochemical Synthesis of Crystalline

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