Synthetic Factors Affecting the Scalable Production of Zeolitic

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Synthetic Factors Affecting the Scalable Production of Zeolitic-Imidazolate-Frameworks Rongfang Wu, Ting Fan, Junying Chen, and Yingwei Li ACS Sustainable Chem. Eng., Just Accepted Manuscript • DOI: 10.1021/ acssuschemeng.8b05436 • Publication Date (Web): 24 Jan 2019 Downloaded from http://pubs.acs.org on January 27, 2019

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ACS Sustainable Chemistry & Engineering

Synthetic Factors Affecting the Scalable Production of Zeolitic-ImidazolateFrameworks Rongfang Wu,† Ting Fan,‡ Junying Chen,‡ and Yingwei Li*‡ †Department of Environmental Monitoring, Guangdong Polytechnic of Environmental Protection Engineering, 98 Guidan West Road, Foshan 528216, People’s republic of China. ‡ State Key Laboratory of pulp and Paper Engineering, School of Chemistry and Chemical Engineering, South China University of Technology, 381 Wushan Road, Guangzhou 510640, People’s republic of China. AUTHOR INFORMATION Corresponding Author *E-mail: [email protected]

Abstract

Zeolite imidazolate frameworks (ZIFs), as an important part of metal-organic frameworks (MOFs), have received great attentions in many application fields, such as gas adsorption, separation and catalysis. It was found that the performances of these applications largely depend on ZIFs’ properties, such as particle size distribution, pore size, specific surface, which are essentially controlled by different synthetic methods. Among all the disclosed ZIF-type structures, researches on ZIF-8 is in the ascendant since it owns high chemical and thermal stability and flexible structure. Therefore, ZIF-8 was chosen as an example in this review to illustrate how synthetic factors affect the final properties of ZIF materials. We summarize the evolution process of ZIF-8 which is divided into three stages: supersaturation, nucleation and particle. Emphasis is placed on the discussion of the influences of various factors on the formation of ZIF-8. The factors are classified into several types such as various salt sources, concentration of reactants, solvents, temperature and so on. The challenges, prospects and outlook of ZIF-8 in the future are presented in the last. This perspective

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aims to provide necessary information for industrial synthesis of MOF and other porous materials in the future. KEYWORDS: ZIF-8; crystallization processes; morphology; crystal size; reaction rate

Introduction As a new type of porous materials, Metal organic frameworks (MOFs) has received extensive attention in the past few decades. 1 Typically, MOFs are composed of metal ions and organic ligands linked by coordination bonds. Due to the diversity of metal elements, organic ligands and coordination manners, the structures and topologies of MOFs can be rationally designed and tailored for specific functions.2 The characters of the high specific surface area, large pore volume and easily adjustable pore size of MOFs have pave the way for developing advanced MOFs materials in various applications including gas storage,3-7 catalysis,8-11 carbon capture,12,13 separation,14-23 sensing,24-27 proton conduction,28-31 and drug delivery.32-36 ZIFs as a ‘‘star’’ subclass37 of MOFs, consist of four-coordinated transition metals ions, i.e., Zn2+, Co2+, Fe2+, and Cu2+, and imidazolate linkers. The single crystal X-ray structures of ZIFs38 are shown in Fig. 1 in a stick and tailing mode. Table 1 shows the parameters of ZIF-1 to ZIF-12 in details. Based on the structures of ZIFs, it was found that they are very similar to aluminosilicate zeolites. Zn2+ or Co2+ ions of ZIFs are alike Si of zeolites; while imidazolate linkers work as the oxygen in zeolites. Therefore, the combined characters of MOFs and zeolites make ZIFs not only hold great promise in many fields including catalysis, separation and storage of gases, but also have the superior thermal and chemical stabilities than other MOFs. For example, ZIFs displayed better H2 adsorption properties at room temperature than some other MOFs, such as MIL-53 (MIII(OH)·[O2CC6H4CO2]·H2O, M = Fe, Al, materials of institute Lavoisier framework-53), HKUST-1 (Cu3(C9H3O6), Hong Kong University of Science and Technology-1).41 ZIF-8 can also separate paraffin isomers more efficiently than zeolite 5A. 42 Even more, due to its higher adsorption capacity, ZIF-8 could be recognized as an ideal substitute of zeolite 5A in the application of separating monobranches alkanes from isomer mixtures.43 Moreover, ZIF-95 and ZIF-100 can separate CO2 from CO, CH4, O2 and N2 efficiently. Their performances are far higher as compared with the industrial porous activated carbon materials.44


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Fig. 1 The single crystal x-ray structures of ZIFs. In each row, the net is shown as a stick diagram (left) and as a tailing (center). (Right) The largest cage in each ZIF is shown with ZnN4 tetrahedra in blue, and, for ZIF-5, InN6 octahedra in red. H atoms are omitted for clarity. Adapted with permission from ref 38. Copyright 2006 National Academy of Sciences. Recently, ZIFs are reported to be used for molecular sensing and used as the intracellular drug media. For example, as potential drug carriers, the three ZIFs, i.e., ZIF-7, ZIF-8 and ZIF-9, showed high capacities for the adsorption of 5-fluorouracil and hydroxyurea.45 Nanoscaled ZIF-90, which is formed by coordinating Zn2+ ions with the organic imidazolate, imidazole-2-carboxyaldehyde, can be used to study the movements of adenosine triphosphate (ATP) levels in living cells via targeting the subcellular mitochondria and recording the dynamics of ATP in mitochondria 46 ZIF-11 exhibited better extraction efficiency than ZIF-747 in solidphase extraction (SPE) analysis of polycyclic aromatic hydrocarbons (PAHs) in the samples from environmental water mainly due to its relatively larger cages. It revealed that the spatial structure effect on ZIFs’ SPE extraction ability can promote the application of ZIFs in chromatographic analysis in the future.



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Table 1. Composition, structure, and net parameters of ZIFs.38 ZIF-n

Composition

Neta

Zeoliteb

dc/Å

ZIF-1

Zn(IM)2

crb

BCT

6.94

ZIF-2

Zn(IM)2

crb

BCT

6.00

ZIF-3

Zn(IM)2

dft

DFT

8.02

ZIF-4

Zn(IM)2

cag

--

2.04

ZIF-5

In2Zn3(IM)12

gar

--

3.03

ZIF-6

Zn(IM)2

gls

GIS

8.80

ZIF-7

Zn (PhIM)2

sod

SOD

4.31

ZIF-8

Zn(MeIM)2

sod

SOD

11.60

ZIF-9

Co(PhIM)2

sod

SOD

4.31

ZIF-10

Zn(IM)2

mer

MER

12.12

ZIF-11

Zn(PhIM)2

rho

RHO

14.64

ZIF-12

Co(PhIM)2

rho

RHO

14.64

a

For definitions of three-letter abbreviations, see Reticular Chemistry Structure Resource (http://okeeffews1.la.asu.edu/RCSR/home.html) b The structure of zeolites which are defined by International Zeolite Association. c d is the diameter of the largest sphere that will fit into the framework

ZIFs have a high-grade crystalline structure area and pore system. This is useful to be employed as confinement voids for the growth of nanoparticles (NPs) and as supports for catalysts. For instance, Qin et


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al.48 studied the photocatalytic CO2 reduction by the cooperation of ZIF-67 and Ru-dye under mild reaction conditions. The results showed that ZIF-67 co-catalyst presented superior promotional effect for CO2 photoreduction than other typical MOFs (Cu-MOF, Fe-MIL-101-NH2) and ZIF-67 has superior stability. Zhao and coworkers investigated the selective hydrogenation of cinnamaldehyde with Pd loaded ZIF-8 catalysts (Pd/ZIF-8), which exhibited outstanding catalytic activity and selectivity in more than 5 runs. 49 Recently, using ZIF(s) as template or precursor to prepare metal/metal oxide/carbon materials with fine structures is another hot trend for the applications in heterogeneous catalysis,49-50 electrochemistry,50 and so on. For instance, an electrospun ZIF-7/carbon nanofiber (CNF)-derived nanocomposite was prepared and applied as a freestanding electrode in supercapacitors.51 It showed great capacitance retention with a high capacitance of 202 F·g-1 at 1 A·g-1 and energy density of 42 W·h·kg-1 when the nanocomposite was prepared from the ZIF-7/CNF precursor with an optimized polyacrylonitrile (PAN) content under a chosen temperature of 950 oC for carbonization. The good electrochemical performance was due to the formation of pores in the CNF at 950 oC, under which Zn from ZIF-7 could be completely removed and the proper C/N ratio maintained. The ZIF/CNF-derived nanocomposite, which is prepared by using a commercially scalable approach, was found to resemble a textile measuring 30×10 cm2 and can be used as freestanding electrodes of supercapacitor without a substrate or current collector. Verpoort and co-workers have a more detailed review in the field of preparing advanced material from ZIFs precursors.52 It is worth noting that the preparation of ZIFs with uniform particle sizes has strong influence on the spatial distribution of the derived metal/metal-oxide/carbon materials. A large scale of ZIFs are demanded due to a myriad of potential applications. One of the parameters to measure the yield of ZIFs is high space-time-yield (YST), which represents a measurement of the yield of ZIFs obtained from per stere of reactant mixture per day. To achieve high YST, it is essential to fully understand the whole synthetic procedure. To the best of our knowledge, there are a few reviews on the



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synthetic procedures of ZIFs,53-57 however, the synthetic parameters for the scalable production of ZIFs are so far only partially discussed. The most typical ZIFs is ZIF-8, which has been investigated more intensively than other ZIFs. ZIF-8 is a soft porous crystal (SPC). It is a sodalite-type framework with Zn2+ linked by 2-methylimidazole (2-MIM) 58

Among all the ZIF materials, ZIF-8 owns an ultra-high thermal stability with a decomposition temperature

up to 550 oC under an inert atomsphere,38,59 which can keep its structure in boiling water for 7 days, even in the aqueous solution of NaOH at the concentrations of 0.1 and 8M at 100 oC for 24 hours.60 Because of these outstanding properties, ZIF-8 have been applied in industrial applications. The potential applications of ZIF8 are illustrated in Fig. 2. For example, Pan and co-workers prepared ZIF-8 membranes in aqueous solutions, exhibiting excellent performance in separating C2/C3 hydrocarbon mixtures, i.e., H2/C3H8, H2/C3H6, C2H6/C3H8, C2H4/C3H6 and C2H4/C3H8, due to size exclusion.61 The permeabilities of H2 in the H2/C3H8 mixtures, C2H6 in the C2H6/C3H8 mixture and C2H4 in the C2H4/C3H8 mixture are calculated approximately to be 3600, 600 and 1200 GPU, respectively; while the factors of separation are ∼80, ∼10 and ∼167. It means that the separation performance of the as-prepared ZIF-8 membranes reach the commercial criteria: the permeability higher than 100 GPU, separation factors higher than 40. Thus, this ZIF-8 membrane will be commercially attractive in natural gas processing, petroleum refining and/or other chemical processes. In addition, it is reported that when ZIF-8 was exposed to seawater solutions, it could be used as desalination membranes.62 The results showed that ZIF-8 materials are highly stable and can maintain its crystal structure after exposure to fresh or sea water and can provide substantial permeation, which can be applied in the seawater desalination to produce more fresh water for arid area. Besides the unique characters of chemically and thermally more stable of ZIF-8 than those of other ZIF materials, one other feature for ZIF-8 is the easily controllable morphology through the variation of synthetic parameters, which will be discussed and summarized thoroughly thereafter. Therefore, ZIF-8 could be used as the representative to investigate multiple


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methodologies toward the design of various porous coordination polymers, and thus provide the fundamental aspects on material science with a large number of future uses for industrial purposes. Despite that the importance and the demand of ZIF-8 are increasing, production of ZIF-8 in mass is still in the stage of research and development.

Fig. 2 The potential applications of ZIF-8.63-68 (i) ZIF-8 as chromatographic serparation. Adapted with permission from ref 63. Copyright 2016 Royal Society of Chemistry. (ii) ZIF-8 as drug delivery agents. Adapted with permission from ref 64. Copyright 2012 Royal Society of Chemistry. (iii) ZIF-8 as chemical sensors. Adapted with permission from ref 65. Copyright 2010 American Chemical Society. (iv) ZIF-8 as separation membrane. Adapted with permission from ref 66. Copyright 2019 Elsevier. (v) ZIFs in a carbon– neutral energy cycle. Adapted with permission from ref 67. Copyright 2016 Nature. (vi) ZIF-8 as catalysts.



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Adapted with permission from ref 68. Copyright 2018 Wiley-VCH. There are many synthetic methods to prepare ZIF-8. One of the most common synthetic methods of ZIF-8 is hydrothermal method, which is shown in Fig. 3. First, mixing zinc ions and imidazole with a certain molar ratio in a known solvent, which is usually methanol, water or dimethylformamide. Then, the mixture is kept under agitation at a certain temperature for several hours and after that it is left to stand within several hours, and then ZIF-8 crystals are separated by centrifugation from the milky emulsion, washed with methanol and dried in air for further use. Considering the potential application of ZIF-8 materials, a various methods of preparing ZIF-8 have been developed, including traditional methods such as solvothermal,69 hydrothermal, mechanical,69-71 sonochemical,72,73 microwave-assisted69 methods, and some other industrial methods such as solvo-jet,74-75 T-mixer,6 and electro-chemical techniques.76

Fig. 3 The Schematic diagram of the most common synthesis process of ZIF-8 in laboratory. Though 10 years have passed since the first ZIF-8 is synthesized by Park and co-workers,38 as far as we know there have been only few systematic reviews focusing on the synthetic factors, which would affect the outcome of ZIF-8 production in general. Therefore, this review will systematically give a deeper understanding on the influences during the preparation of thermally stable ZIF-8 with high yield, which will provide scientists, who are working in the relevant areas, with a full picture on the synthesis of ZIF-8. This review is divided into three sections: (1) to summarize possible crystallization processes; (2) to discuss the


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factors on the formation of ZIF-8, including metal salts, solvents, temperature, reaction time and synthetic routes, and (3) to offer concluding remarks on the future development. Crystallization processes Among ZIFs, ZIF-8 has the sodalite (SOD) structure with a large pore size of 11.6 Å and small pore apertures of 3.4 Å.77 The crystal structure of ZIF-8 is shown in Fig. 4. The size of ZIF-8, usually determined by synthetic methods, is from ~ 20 nm to ~1.8 μm,58, 77, 78 resulting in the varied surface areas in the range of 900~1600 m2·g-1.79, 80 The catalytic performance of ZIF-8 particle is largely determined by the particle sizes. Normally, the material with smaller size can get larger external surface, which makes the ZIF-8 nanocrystals show higher catalytic activity. For example, when the size of ZIF-8 particles are small, e.g., 100-200 μm, it can be used as a heterogeneous catalyst in the Knoevenagel reaction between benzaldehyde and malononitrile in toluene at room temperature.81 Thus, understanding the fundamental insights on the formation mechanism of ZIF-8 is important to synthesize ZIF-8 with controlled size and structure.

Fig. 4 Crystal structure of ZIF-8: Zn (polyhedral), N (sphere), and C (line). Adapted with permission from ref 40. Copyright 2008 Science. Generally, there are three stages in the synthetic process of ZIF-8 particles: (1) supersaturation (the concentration of reactants is far more than the solution can hold); (2) nucleation (the number of ZIF-8 nuclei



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becomes more); (3) particle growth (the synthesized ZIF-8 nanoparticles become particle and the size is growing). LaMer diagram82 shows these three stages, which is illustrated in Fig.5. As shown in Fig.5A, before t1, the process is Stage I and concentration of reactants is increasing, until supersaturation state appears when the concentration of reactant increases to be Cmin*, Between t1 and t2, the formation of ZIF-8 crystals starts, and this process is Stage II: nucleation; After t2, the process is Stage III and ZIF-8 particles grow into larger size. This process can also be applied in the formation of ZIF-67.83 The formation mechanism of ZIF-8 could be slightly different due to various synthetic environments.84 For example, the formation of ZIF-8 crystals is a little different in methanol at room temperature. The process is shown in Fig.5B. Stage I disappears, which may be thought to be finished instantaneously without showing. It directly comes into Stage II: the process of nucleation. It is divided into two small processes. The first small process is ZIF-8 and a metastable phase, in which the metastable phase is coexisting with ZIF-8 size enlarged from ~50 to 230 ± 40 nm. The second small process is disappearance of the metastable phase and transformation of ZIF-8 from nanoparticles into homogenous pure crystalline ZIF-8 particles. Lastly, the growth process is stage III. In this process, the small crystals tend to growth to form larger ones according to the Ostwald87 ripening mechanism, which aims to achieve a lower energy state.


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Fig. 5 (A) Scheme of particle synthesis based on LaMer diagram. Cmin* and Cs represent critical supersaturation and the solubility of a reaction product, respectively. Adapted with permission from ref 82. Copyright 2013 Elsevier. (B) Proposed formation pathway of ZIF-8 in methanol as a function of synthetic time. Adapted with permission from ref 84. Copyright 2010 American Chemical Society. (C) Proposed formation pathway of the growth of ZIF-8 crystals in water. The purple spheres and green rods represent zinc ion and free Hmim, respectively. Adapted with permission from ref 85. Copyright 2015 Royal Society of Chemistry. (D) Proposed mechanism for mechanochemical dry conversion of ZnO to ZIF-8. Adapted with permission from ref 86. Copyright 2013 Royal Society of Chemistry. Similarly, the formation processes of ZIF-8 in an aqueous solution at room temperature was also proposed,88 as shown in Fig.5C. The process contains Stage I, II and III, and Stage III is divided into four small processes. In stage III, the first process is the formation of 2D layered structure units in larger sizes from



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ZIF-8 nuclei. Then, the 2D layer units randomly attached to form 2D flake products, which is named as the layer-by-layer growth. After that, the stacked layers break down to smaller crystals, which is called transformation. Finally, the small crystals grow larger, which is similar to the growth process in methanol. All the formation process can be proved by the SEM images as shown in Fig. 6, and the reason of different Stage III is attributed to the effect of excess Hmim, which can be served for the deprotonation of Zn(Hmim)n2+ during the formation of ZIF-8. More details about this will be discussed later.

Fig. 6 Shape evolution of synthesized ZIF-8 obtained from Zn(OAc)2 as a function of reaction time in water. Reprinted with permission from ref 85. Copyright 2015 Royal Society of Chemistry. Not only does the solvent influence the formation process, but also the synthetic methods do. ZIF-8 nanoparticles can be synthesized solely from ZnO without any other additives via a mechanochemical approach.86 The mechanism is shown in Fig.5D. Because ZnO is in bulky phase, which is largely different from reactants in the two cases mentioned above, the formation of ZIF-8 is different too. It has two formation pathways. In the first pathway, the bulky ZnO first breaks down to nanoparticles (about ~373 nm) via grinding. The left formation processes are similar to the general ones, nucleation and particle growth. In the second pathway, ZIF-8 directly forms from the bulky ZnO, which then transforms to be ZIF-8 polycrystal. During


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the milling, the mechanochemical energy was supplied in the synthetic procedure for reducing the particle size, defect-forming, dislocating in crystal lattices and local heating and melting.87 What’s more, detailed information on the morphology evolution of ZIF-8 was also investigated in different experimental conditions.88 But the conclusions were similar, as shown in Fig.7. Initial morphology of ZIF-8 is cubic, which cannot be detected exactly because its short lifetime, but analogous morphology such as cubes with round edges could be detected at early stages.89 The ZIF-8 with a cubic morphology could be changed to rhombic dodecahedron with time. Each stage is proved by many experiments.58, 77, 80, 83, 89-94 It is worthwhile to point out that different morphology can bring different catalytic effects.

Fig. 7 Illustration of the crystal morphology evolution with time: cube (a), cube with truncated edges (b), rhombic dodecahedron with truncated corners (truncated rhombic dodecahedron) (c and d) and rhombic dodecahedron (e). Reprinted with permission from ref 88. Copyright 2012 Royal Society of Chemistry. Factors on the formation of ZIF-8 The factors on the formation of ZIF-8 are classified into five types: (i) reactants (including concentration and molar ratio); (ii) solvents; (ii) reaction temperature (including drying temperature); (iv) reaction time (including aging time) and (v) synthetic routes. Among them, zinc salt variations and molar ratio of reactants (in terms of concentration of reactants) are the essential factors on the synthesis of ZIF-8. Effects of reactants Zinc salt variations



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According to the Pearson’s acid-base theory, the strength of interaction between the metal ions and counter anions in the metal salt can influence the rate of nucleation. It is also true in nucleation process of ZIF-8. Table 2 summarized the influences of Zn2+ salts on the properties of the afforded ZIF-8 crystals. When Zn(OAc)2 was used as the metal source, the morphology of truncated rhombic dodecahedron could be obtained in general. However, there is one exception that ZIF-8 with a multilayered 2D structure would be generated when the molar ratio of Hmim/Zn2+ was 10. In the case of using ZnSO4 with a Hmim/Zn2+ molar ratio of 70, the prepared particles exhibited bumpy surfaces with some truncated edges. It is interesting that typical rhombic dodecahedral ZIF-8 could also be synthesized from ZnSO4, which is similar to that from zinc acetate, at the Hmin/Zn2+ molar ratio of 20 or 35 in water. The rates of nucleation of ZIF-8 crystals prepared from different Zn2+ salts follows an order of nitrate > > chloride > acetate.80, 95 It implies that the high nucleation rate can not only result in more nuclei forming but also make a “reaction zone” , where the metal ions and ligands are highly concentrated to generate many smaller crystals. Meanwhile, the crystallinity of corresponding ZIF-8 was presented in the order of Zn(NO3)2·6H2O > Zn(OAc)2·6H2O > ZnSO4·7H2O > ZnCl2.96 With different zinc sources, the synthetic process of ZIF-8 was slightly different, due to different pHs caused by the anionic ion. Similarly, during the synthetic procedure of ZIF-67, the cobalt variations have also impacted the size and morphology and the nucleation process of ZIF-67.87


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Table 2. The influence of different Zn2+ salts on the physical properties of ZIF-8. Mean Particle

Pore

BET surface/

No.

Zn2+

Zn/Hmim molar ratio

morphology

size/nm

volumes/cm3/g

cm2/g

Ref.

1

Zn(acac)2

Zn/Hmim/MeOH =1/8/559

-

45/85

-

-

80

2

Zn(ClO4)2


-

224 /312

-

-

80

Zn/Hmim/MeOH 3

Zn(NO3)2

=1/2.35/1140

SOD

50

1.3

1530

97

4

Zn(NO3)2 ·4H2O

Zn/Hmim/DMF =1:0.9:18ml

SOD

-

0.663

1947

38

5

Zn(NO3)2·6H2O

Zn/ Hmim/ NH3·H2O=1/2/32

SOD

700

0.51

958.4

79

Zn/Hmim/NH3/ H2O = 6

Zn(NO3)2·6H2O

1/2/32/157

SOD

700

0.5

1079

98

7

Zn(NO3)2·6H2O

Zn/Hmim/H2O = 1/70/1238

SOD

85

0.31

1079/1173

99

8

Zn(NO3)2·6H2O

Zn/Hmim/MeOH =1:8:559

SOD

141 ± 48/192

0.66

1700 ± 30

80

8:31.5:1.8:1732

-

∼100–200

-

-

95

Zn/Hmin/HCOONa/MeOH= b

Zn(NO3)2 9

10

Zn(OAc)2

Zn/Hmim = 1/70

SOD

746

0.57

1126

85

11

Zn(OAc)2


SOD

500/480

0.55

1477 ± 36

80

SOD

2000

1.2

1960

97

Zn/Hmim/MeOH 12

Zn(OAc)2a

=1/3.35/1036



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Zn/Hmim/MeOH 13

Zn(OAc)2

=1/3.35/1036

SOD

300

0.9

1570

97

14

Zn(OAC)2·2H2O

Zn/Hmim/H2O= 1/29/1039

SOD

5000

0.58

1151.2

79

SOD

1000-1300

0.49

1060

98

8:31.5:1.8:1732

-

∼1000

-

-

95

Zn/Hmim/NH3/H2O = 15

Zn(OAC)2·2H2O

1/2/32/157

Zn/Hmin/HCOONa/MeOH= Zn(OAc)2b 16

17

ZnBr2

Zn/Hmim = 1/70

SOD

26.4

-

-

85

18

ZnBr2


SOD

1050/1160

0.63

1713 ± 45

80

19

ZnCl2

Zn/Hmim/H2O= 1/8/1120

Leaf-shaped

400

0.04

12.7

79

Zn/Hmim/NH3/H2O = 20

ZnCl2

1/2/32/157

-

600-1300

0.5

1092

32

21

ZnCl2


SOD

300/388

-

-

98

8:31.5:1.8:1732

-

∼700

-

-

95

Zn/Hmin/HCOONa/MeOH= b

ZnCl2 22

23

ZnI2


SOD

500/589

-

-

80

24

ZnSO4

Zn/Hmim = 1/70

SOD

231

-

-

85

25

ZnSO4

Zn/Hmim = 1/35

SOD

1640

-

-

85

26

ZnSO4


-

211 ± 60/247

-

1713 ± 45

80


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27


[ZnCO3]2·[Zn(OH)2]3

Zn/Hmin/MeOH=1:23:1000

-

∼80-90

a The mixing of reactant is in different ways. b The synthesis method is microwave synthesis.


-

-

100


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Concentration of reactants Considering the chemical equilibrium, increasing the concentration of reactants can accelerate most chemical reactions, and the formation process of ZIF-8 is no exception. In addition, the yield, morphology, sizes of ZIF-8 particles are strongly affected by the concentration of reactants. The reason why these phenomena occurred was illustrated below. The formation of ZIF-8 (reaction i in Scheme 1) includes two steps, deprotonation of Hmim (reaction ii in Scheme 1) and formation of Zn(mim)2 (reaction iii in Scheme 1). Reaction i shows higher concentration of Zn2+ or Hmim can speed up reaction rate. However, people often use excess Hmim instead of Zn2+ because Zn2+ has many side reactions, such as hydrolysis of Zn2+ (reactions iv to vii). Though the ideal Zn/Hmim molar ratio was found to be 1/2, the one used in experiments is much smaller, which is usually between 1:20 and 1:7058, 85, 92, 101, 102 in an aqueous solution.75, 102 The excess Hmim can promote the deprotonation of 2methylimidazole, which is beneficial for the formation of ZIF-8 crystals. On the basis of the reaction equilibrium of forming ZIF-8, reaction ii can be promoted by increasing the concentration of Hmim to produce more mim-, which is essential for the following reaction iii to produce ZIF-8. Moreover, excessive Hmim can also promote the procedures of supersaturation and nucleation, resulting in ZIF-8 particles with smaller sizes. 82

Scheme 1. Basic reactions of ZIF-8 production under an aqueous condition.


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Effects of solvents Solvent can modify the evolution of reactions, changing the rates of crystallization and the sizes of afforded nanocrystals. Based on the thermodynamic and kinetic effects of solvent on the formation of MOFs, the use of solvents can be classified into four categories: (i) as ligands; (ii) as guest molecules; (iii) as both ligands and guest molecules, (iv) as a structure directing agent (SDA).103 In the procedures of synthesizing ZIF-8 particles, solvents are mostly used as structure directing agents.38, 102, 104 The most common solvents used are water, ammonia, methanol, dimethylformamide (DMF). According to their chemical structure, solvents can be classified into inorganic solvents and organic solvents, which are discussed below. Effects of inorganic solvents The mostly used inorganic solvents in the preparation of ZIF-8 crystals are water and ammonia. Water is always recognized as to an ideal and economical solvent for synthesizing ZIF-8 particles.78, 85, 98-103, 105 As shown in scheme 1, hydrolysis exerts an effect on the preparing ZIF-8. When increasing the concentration of water in the reaction media, the hydrolysis of zinc ions may happen and as a result the yield of by-products increased.101 On the other hand, because increasing the usage of water increased pH of reaction solution,96 the rate of deprotonation of 2-methylimidazole became slow. The reaction rate between mim- and Zn2+ could be slowed down by decreasing the concentration of mim-. Therefore, the crystallization of ZIF-8 would be postponed. Since the pH value is another vital parameter in the synthesis of ZIF-8, adding appropriate base like ammonia hydroxide to modulate the pH of reaction solutions is a simple way to accelerate the reaction rate. Particularly, increasing solution basicity can shift the equilibrium of reaction i in scheme 1; while in reaction ii, the organic ligands are deprotonated and release mim- to react with Zn2+ (reaction iii). Thereby, it can make the crystal grow in all directions, and well-inter-grown crystals with larger sizes can be produced.98 In



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experiment, it was found that when the usage of ammonia hydroxide was changed from 8:1 to 16:1 based on molar ratio of NH4+/Zn2+, the ZIF-8 structure changed from a sod-like structure to a regular cubic structure. When the NH4+/Zn2+ molar ratio is 32:1, the cubic morphology of the ZIF-8 structure keeps the same but with larger crystal size. In addition, ZIF-8 particles still keep its cubic structure although the molar ratio of NH3/Zn is as high as 45:1. Similar base solvents like sodium hydroxide or potassium hydroxide can also be used in the synthesis of ZIF-8 crystals.71, 106 Cho et al.15 used NaOH (aq) to adjust pH to improve the yield of ZIF-8 through a sonochemical method. The product yield was immediately improved from 22% to 93% via changing the pH by using a 10 M NaOH solution although the molar ratio of Zn: Hmim: DMF was kept constant. Demir et al. used sodium hydroxide to adjust the pH of the recycled mother liquid for the synthesis of ZIF-8. As a result, the total yield of ZIF-8 could be improved from 38% to 80%, and the microstructural characteristics of ZIF-8 were almost unchanged.107 Effects of organic solvents The most common organic solvents used in synthesis of ZIF-8 are methanol, DMF, triethanolamine (TEA) and diethylformamide (DEF). ZIF-8 was firstly synthesized in a DMF solution.38 Due to high cost and environmental unfriendliness of DMF, methanol and TEA71 were chosen as substitute solvents. Eugenia et al.102 have systematically investigated the solvents effect on the synthesis of ZIF-8 nanocrystals at room temperature. They employed a series of aliphatic alcohols, e.g, CH3OH, C2H5OH, C3H7OH and C4H9OH, DMF and acetone. Because of hydrogen bond donation, organic solvents can modify the formation of ZIF-8, such as the rate of crystallization, the size of crystals and so on. ZIF-8 possessed pill-like crystals with round edges if it was synthesized in the solvents of aliphatic alcohols or acetone. However, ZIF-8 cannot be obtained if changing the solvent to water while keeping other reaction conditions the same. These experimental results proved that solvent could be served as a SDA, which could alter the formation of ZIF-8 phase. The existence


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of hydrogen bond may increase the polarization of pyrimidine hydrogen, promote the deprotonation of Hmim, and further promote its coordination with zinc ion, thus facilitate the reaction. Besides, some other unusual organic solvents are used in the synthesis of ZIF-8. For example, an excess of various simple auxiliary monodentate ligands, which were functionalized by different chemical groups, such as carboxylate, N-heterocycle and alkylamine, were successfully applied to control the crystal size of ZIF-8. During the synthesis processes, these functionalized ligands played important roles, such as the competitive ligands and bases in the equilibria of coordination and deprotonation, respectively.89 Another example, Pebax 1657, which is a PEO based poly(ether-block-amide), has been used as a SDA in the synthesis of ZIF-8.108 It is found that by increasing of the content of Pebax 1657, the porosity and surface area of the synthesized ZIF-8 could be increased correspondingly. That’s because hydrophobic effects due to Pebax 1657. Furthermore, Liu et al. reported a new route for preparing ZIF-8 in a eutectic solvent of choline chloride-urea. 109

Wang et al. have improved this method by using cooling-induced crystallization in a deep eutectic solvent

(DES).110 ZIF-8 particles were formed and kept soluble in this eutectic solvent, then water with methanol109 or cooling step110 was introduced to make ZIF-8 crystals precipitate for separation. It may be the strong dissolving capacity of this eutectic solvent that make the ZIF-8 crystals homogeneously disperse. The precipitation of ZIF-8 synthesized in DES is different from the classical crystallization mechanism utilized by zeolites and just follows a fast cooling crystallization process, as illustrated in Scheme 2. It is also interesting to note that ZIF-8 and ZIF-67 can be prepared in the microemulsions solvent.111 By using microemulsions, ZIF-8 particles can be prepared with uniform small particle size distribution and higher surface areas and thermal stability.



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Scheme 2. Illustration of ZIF-8 crystals precipitating from DES. Reprinted with permission from ref 110. Copyright 2014 Elsevier. Effects of combination of inorganic/organic solvents Considering the effects of inorganic and organic solvents on the formation of ZIF-8, combination of these two kinds of solvents can enhance their synergistic effects, which can initiate the formation of ZIF-8 while reducing the use of Hmim ligand. For example, based on the fact that PEO-PPO-PEO (a triblock copolymer, poly(ethylene oxide)–poly(propylene oxide)–poly(ethylene oxide) can electrostatically attracts metal ions, Gross et al. prepared ZIF-8 by using stoichiometric amounts of metal salt and ligand in an aqueous solvent of ammonia in the presence of PEO-PPO-PEO.112 2-Methylimidazole cold be deprotonated by ammonium hydroxide, which is highly required for coordinating metal ions. For another example, Shen et al. has synthesized 3D ordered macro-microporous ZIF-8 single crystal for the first time by using a double-solvent– induced heterogeneous nucleation approach.91 CH3OH/NH3·H2O mixed solutions were chosen as the solvent. A rapid crystallization of the precursors could be induced by NH3·H2O, whereas methanol can act as structure directing agent. Effects of reaction temperature Comparing with the above-mentioned influences, another the key parameter is the reaction temperature for the formation of

ZIF-8 based on the following reasons: firstly, the reaction temperature affects the


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solubility of Hmim; secondly, it may play a crucial role in coordination mode of Hmim; finally, it directly affects the reaction energy in reaction thermodynamics and kinetics. Consequently, the reaction temperature can be used as a structure direct factor in the synthesis of ZIF-8. Specifically, Tsai and co-workers have systematically studied the effect of temperature on the particle size distribution of nano-ZIF-8. 78 In their experiment, the average ZIF-8 nanoparticle size decreased to 26 nm from 78 nm with increasing the reaction temperature from -15 to 60 oC. At the same time, the external surface area increased from 220 to 336 m2·g-1. As a result, an improved catalytic activity could be achieved in comparison with ZIF-8 micro-particles. The reason why the size decreased as the temperature increased is that the relationship between nucleation and temperature. At higher temperatures, faster nucleation happens which results in a high concentration of nanoparticles, whereas nucleation rate slows down at lower temperature and leads to the less amount of nucleating particles, which would grow up to form larger particles further in the presence of a bunch of starting materials. The temperature-dependent size control is achieved not alone via traditional methods, but also via a jetmixing reactor. The nanoparticle size of ZIF-8 decreased gradually when increasing the temperature from 90 oC

to 150 oC. However, if the sample was synthesized at 90 oC, the particles exhibit low uniformity of size

distribution, which might be ascribed that the temperature of 90 oC is too low for affording highly uniform ZIF-8 nanoparticles. Assuming that the determination of the size of crystal formed by chemical reactions is the size of nuclei, which is formed due to the limitation of solubility, the nanoparticle size of ZIF-8 can be deduced from the reaction temperature by the following Kelvin equation113: L=

4𝑉𝑚𝜎 𝑣𝑅𝑇𝑙𝑛𝛼

(1)

where L is the size of crystals afforded by the chemical reaction, Vm is the molar volume of the crystal, σ is the average solid-liquid interfacial tension, ν is the number of ions per molecule of solute (for molecular crystals, ν = 1), R is Avogadro number which is normally taken as 6.02  1023, T is the absolute temperature



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(K), and α is the ratio of concentration of the supersaturated and saturated solution. According to the equation, the crystal size L is inversely proportional to T. Therefore, α would increase with increasing the reaction temperature, leading to the reduced nanoparticle size of ZIF-8 subsequently. Effects of reaction time Generally, it is well accepted that the product yield is increased with time, which is also suitable for the production of ZIF-8 nanoparticles. In a sonochemical route, the ZIF-8 crystal synthesized in 1 h was reported to have fine textural properties albeit with low yield; while for the ZIF-8 crystal synthesized during a 3 h period, the product yield was high yield, however, its textual properties was inferior. Balancing the yield and textual properties, if ZIF-8 was synthesized from a 2 h reaction, it could be afforded in a moderate yield of 85% with good textural properties.71 In a fluidic system, the nanoparticle size of the ZIF-8 depended on the flow rate of reactants. As the flow rate was reduced, particles with uniform size and good crystallinity could be obtained in the process. This could be due to the residence period of the reactants in the tubular reactor in the jet stream region was insufficient to react with unreacted ZIF-8 precursors.73 Effect of different synthetic routes In 2006, Yaghi and coworkers reported a solvothermal method for the preparation of ZIF-8 in DMF at 140 oC for the first time.38 From then, various methods and techniques of synthesizing ZIF-8 were developed, including microwave, sono/mechanochemical, dry-gel conversion, solvent-free oxide/hydroxide-based, microfluidic, and electrochemical methods (Table 2). These processes are briefly summarized in Figure 8. Solvothermal/Hydrothermal method is the most commonly used way to produce ZIF-8 materials, which is simple and easy to handle. The disadvantage of this method is hard to scale up and the yield is low. On the basis of this method, microwave heating technique was performed to prompt the rapid synthesis of nano ZIF-8 under solvo/hydrothermal conditions via increasing nucleation rate and controlling the size of ZIF-8 particles.


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On the other hand, in the sonochemical synthesis, the high-energy ultrasound could be introduced to the mixture containing reactants, which would led to a rapid and homogeneous nucleation, resulting in a shortened time for crystallization and decreased particles. Mechanochemical synthesis introduced the mechano energy into solid reactants mixture dominated by reactions on the surface. In the synthesis of ZIF-8 by a dry-gel method (or water-steam assisted synthesis), steamed water was introduced into the eutectic mixture upon heating the reactants mixture to assist the formation process of ZIF-8 nanocrystals. Table 3 showed the textural properties of ZIF-8, which is prepared by using different synthetic conditions. The experimental results by Lee and co-workers,114 i.e., from entry 1 to entry 8, showed that although the samples prepared via the dry-gel and sonochemical routes display the BET surface areas within a range of 1250 - 1700 m2·g-1, the particle sizes were much smaller than that prepared by other methods. Due to a variety of controllable parameters in mixing manner, ZIF-8 crystals could be synthesised with slightly different textual property using the same synthetic routes. Just like entry 7, entry 9 and entry 11, these synthetic routes are all in a microfludic system, which has obvious merits in facilitating the mixing of reactants and heat and mass transfer process in comparison with the conventional method. The difference is the way reactants mixed. Entry 7 and entry 12 used T-mixer reactor, and entry 10 used a jet-mixing reactor. Different mixing reactor can achieve different degrees of mass transfer between zinc ions and Hmim, which influcenced the rate of ZIF-8 formation and resulted in different sizes of ZIF-8.


ACS Sustainable Chemistry & Engineering 1 2 3 4 5 6 7 8 9 10 11 12 13 14 15 16 17 18 19 20 21 22 23 24 25 26 27 28 29 30 31 32 33 34 35 36 37 38 39 40 41 42 43 44 45 46

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Table 3. Synthetic conditions of ZIF-8 and its textural properties No.

Synthesis method

Zn2+

MeIMa

(mmol)

(mmol)

Synthetic condition Solvent

Conditions

Particle size

SBET b

Vporec

Sext d

Yield e

μm

(m2/g)

(cm3/g)

(m2/g) (%)

Ref.

1

Solvothermal (DMF f)

2

2

DMF

140 oC, 24 h

150 ~ 200

1370

0.51

6.7

60

114

2

Solvothermal (MeOH g)

2

2

MeOH

25 oC, 24 h

3~5

1549

0.59

32.9

43

114

3

Microwave

2

2

DMF

120 oC, 3h, 80 W

5 ~ 10

1250

0.53

22.1

62

114

4

Sonochemical

2

2

DMF (and TEAi)

1 h, 300 W

0.3 ~ 0.5

1249

0.71

53.7

62

114

5

Mechanochemical

0.79

1/56

No solvent (add NH4NO3)

45 min, 25 Hz

3 ~ 15

1256

0.64

31.8

82

114

6

DGC h

0.5

5

H2O

120 oC, 24h

0.3 ~0.5

1306

0.52

54.0

84

114

7

Microfluidic

2

2

DMF

150 oC, 0.5 mL·h-1

5 ~ 15

1435

0.42

18.4

58

114

8

Commercial product

-

-

-

-

0.5 ~ 200

1580

0.64

17.8

-

114

9

Salt assisted mechanosynthesis

1j

2

Zinc acetate

100rpm,50YTZ balls

-

1720

0.63

-

~100

69

10

Synthesis in a jet-mixing reactork

2

1

MeOH

Vr/Vj=1.4/3.2,0 h

0.074±0.015

1534

0.635

-

87l

75

11

Sonochemical

10

10

TEA and NaOH

300 W, 20KHz

0.7

1253

0.61

367

93

71


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12

Microfluidic (T - mixer)

2.25

157.5

H2O

10min,100oC

0.3 ~ 0.9

1730±14.7

0.56

-

-

115

13

Steam- assisted

0.5

5

H2O

at 120oC for 24 h

-

1470

0.69

-

17

116

a

2-methylimidazole.

b

Specific surface area calculated by the BET method.

c

Total pore volume.

d

External surface calculated by the t-plot method.

e

Based on the limiting reactant of mIM.

f

Dimethylformamide.

g

Methanol.

h

Dry-gel conversion.

i Trimethylamine. j

the zinc source is using nanosized ZnO with acetate, and the total molar of zinc is composed of nanosized ZnO and acetate.

k

the usage of zinc and MeIM is their molar ratio, Vr is the flow velocity through the line, Vj is the flow velocity through the jet.

l

Based on the limiting reactant Zn.



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Fig. 8 The synthesis method of ZIF-8. (a) solvothermal method, (b) microwave method, (c) sonochemical method, (d) mechanochemical method, (e) dry-gel conversion method, (f) microfludic method, (g) electrochemical method. Reprinted with permission from ref 117. Copyright 2014 Springer.


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Conclusion and outlook Tremendous progress has been made over the last 15 years in understanding the fundamental insights of the crystallization processes for synthesizing ZIF-8. No matter which method is used, the crystal growth of ZIF-8 particles can be generally divided into three stages: supersaturation, nucleation and crystal growth. Solvents and synthetic methods will slightly influence the mechanism of the formation process. For example, the state of supersaturation in the formation of ZIF-8 is not monitored in methanol while it is observed in water or via a mechanochemical method. The formation of ZIF-8 involves five reactions as shown in Scheme 1. Based on reactions ii, iv and v, the formation closely depends on the pH values of reaction mixture. With different anionic ions, the relative crystallinity of the as-prepared ZIF-8 is in the order of: Zn(NO3)2·6H2O > Zn(OAc)2·6H2O > ZnSO4·7H2O > ZnCl2, which largely depends on the different pH values resulted from anionic ions. Increasement of concentration of both reactants can accelerate the reaction rate. Excessive Hmim is used instead of zinc ions to promote the deprotonation of Hmim, which is the key steps for the formation of ZIF-8. Solvents are usually thought as structure-directing agents in the ZIF-8 formation processes, which could accelerate the crystallization rate and adjust the particle size of ZIF-8. Inorganic solvent such as ammonia can speed up the reaction rate via modulating the pH of reaction mixture, which is widely applied in size control of ZIF-8. Organic solvents can modify the evolution processes of the reaction, changing the rate of crystallization and sizes of nanocrystals. It mostly depends on the polarity or the hydrogen bond donation ability of organic solvents, which facilitates deprotonation of Hmim. The control of reaction temperature on the formation of ZIF-8 via different synthetic routes achieves the same influences. Higher reaction temperature not only enhances the solubility of Hmim, but also speeds up



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the reaction rate by producing smaller particles. What is more, different synthetic routes use different mixing reactors, which directly influence the degree of mixing between zinc ions and mim-. It can affect the mass transfer and result in higher reaction rate and smaller sizes of ZIF-8. The investigation on the synthesis of ZIF-8 mainly focuses on how to regulate the formation rate and the morphology such as particle size and BET surface. This review gives comprehensive summary on the factors on the process of synthesizing ZIF-8 and gives good understanding of the mechanism of crystal growth. Although ZIF-8 and its derivatives have shown significant potentials in gas adsorption, catalysis, separation and many other fields, it is a long way to industrialize the production of ZIF-8 and its derivatives. It is worth pointing out that few attentions are paid on the optimization of reaction parameters such as yield and reaction conversion rate to realize the industrial process, which may be the future research hotspots in this field. On the other hand, sustainable and green production of ZIFs for large-scale applications is critical from a viewpoint of environment protection, such as the use of environmentall-benign solvents (ideally without using any solvents) and mild synthesis conditions. Especially, for scaled-up production of ZIFs out of the laboratory, it should be greatly emphasized to realize the preparation of ZIFs under low or even normal pressures inside conventional reactors in further investigation.

ACKNOWLEDGMENTS This work was supported by the National Natural Science Foundation of China (21825802, 21436005, 21576095), the Fundamental Research Funds for the Central Universities (2017PY004), the Science and Technology Program of Guangzhou (201804020009), the State Key Laboratory of Pulp and Paper Engineering (2017ZD04, 2018TS03), and the Natural Science Foundation of Guangdong Province (2016A050502004, 2017A030312005).


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TOC

The synthetic factors affecting the scalable and sustainable production of zeolitic-imidazolate-frameworks are discussed, including the varied reactants, solvents, temperature, time, and synthetic routes.



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Biographies

Dr. Rongfang Wu is currently a lecture at Guangdong Polytechnic of Environmental Protection Engineering. She received her B.S. degree in Chemical Technology from Beijing Institute of Technology, China in 2008 and her Ph.D. degree in Chemical Engineering from the Institute of Processing Engineering, Chinese Academy of Science in 2013. She worked as a Visiting Scholar at School of Chemistry and Chemical Engineering in South China University of Technology from 2017 to 2018. Her research interests focus on developing synthetic pathways for the large-scale production of metal–organic frameworks under sustainable environmental-friendly conditions.

Dr. Ting Fan received B.Sc. on chemistry at Wuhan University. In 2014, she obtained Ph.D. for research under the supervision of Prof. Zhenyang Lin at Hong Kong University of Science and Technology. She was a postdoc in the lab of Dr. Mårten Ahlquist at KTH Royal Institute of Technology for two years. Now she is an associate professor at South China University of Technology. Her current research interests are mechanistic studies on reactions catalyzed by MOF-derived materials.


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Dr. Junying Chen received her B.S. degree from Dalian University of Technology in 2007. She joined Prof. Wonwoo Nam’s group in Ewha Womans University and received her Ph.D. in inorganic chemistry in 2014. Then she moved to South China University of Technology as a postdoctoral fellow, after a short postdoctoral program with Prof. Nam. She is currently an associate professor at the School of Chemistry and Chemical Engineering, South China University of Technology. Her research interests include the development of efficient catalysts for photocatalysis and biomimetic oxidation reactions.

Dr. Yingwei Li received his BS degree in 1998 and his PhD in 2003, both from the Department of Chemistry of Tsinghua University, and he then conducted postdoctoral work at the University of Calgary and the University of Michigan (Ann Arbor) from 2003 to 2007. He is currently a full professor at the School of Chemistry and Chemical Engineering, South China University of Technology. His research focuses on the design and synthesis of new metal–organic framework materials for heterogeneous catalysis, and gas adsorption and separation.


Synthetic Factors Affecting the Scalable Production of Zeolitic

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