Aqueous-System-Enabled Spray-Drying Technique for the Synthesis

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Article http://pubs.acs.org/journal/acsodf

Aqueous-System-Enabled Spray-Drying Technique for the Synthesis of Hollow Polycrystalline ZIF‑8 MOF Particles Shunsuke Tanaka*,†,‡ and Ryo Miyashita† †

Department of Chemical, Energy and Environmental Engineering, Faculty of Environmental and Urban Engineering, and Organization for Research and Development of Innovative Science and Technology (ORDIST), Kansai University, 3-3-35 Yamate-cho, Suita-shi, Osaka 564-8680, Japan ‡

S Supporting Information *

ABSTRACT: Zeolitic imidazolate framework-8 shares the same topology with sodalite zeolite but consists of Zn nodes bridged by imidazolate linkers to form a neutral openframework structure. ZIF-8 has been recognized as a unique molecular sieving material with a flexible framework enabling interesting “gate-opening” functionality. Controlling the crystal size and shape is crucial for regulating the structural flexibilities and mass transport properties. The present study demonstrates that an aqueous-system-enabled spray-drying process enables the shape engineering of ZIF-8 with a hollow polycrystalline structure. It is notable that our synthesis route produces an amorphous zinc complex compound, which possesses a continuous random network partially with crystalline fillers, after spray drying followed by an amorphous-to-crystal transition via activation treatment using polar organic solvents. The size of primary ZIF-8 crystals consisting of secondary polycrystals depends on the kind of the organic solvent. The macro-/microscopic structures of hollow polycrystalline ZIF-8 significantly structurally enhanced the adsorption capacity and uptake rate. The largescale, rapid production and enhanced adsorption performances make this continuous method a very promising candidate for industrial applications and shaping of MOF.

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INTRODUCTION Metal−organic frameworks (MOFs)1−4 offer many interesting opportunities in adsorption technology, with unprecedented capacities and chemical and structural tunabilities. Among the many MOFs, zeolitic imidazolate framework-8 (ZIF-8, Zn(2methylimidazole)2) is undoubtedly the most extensively studied because of its facile synthesis coupled with its exceptional chemical and thermal stabilities.5−7 ZIF-8 is crystallized into the sodalite topology, forming large cages (a diameter of 11.6 Å) interconnected via narrow six-ring windows (3.4 Å). Such a framework structure gives ZIF-8 particularly interesting “gateopening” functionality. Numerous experimental and computational studies have revealed that the pore apertures swing open by the reorientation of imidazolate linkers and expand when probed with guest molecules.8−15 More recently, the influences of the crystal size and shape on the adsorption-induced structural transition and framework flexibility have been recognized.16−18 To take advantage of this unique adsorption property for adsorption, control in the framework flexibility is of crucial importance. Besides the nanostructure control, the control of the crystal size and shape is a key breakthrough toward the development of practical applications. A great deal of effort has been made to develop a method of synthesizing MOFs, including solvothermal, microwave-assisted,19 sonochemical,20,21 mechanochemical,22,23 and electrochemical24 techniques. Recently, spray-drying technique has been demonstrated as a continuous and scalable process.25 © 2017 American Chemical Society

However, most of these methods share a conventional method that MOFs are crystallized in organic solution using N,Ndimethylformamide (DMF) and methanol. On the other hand, aqueous synthesis has opened up environmental friendly and efficient ways to synthesize MOFs.26−28 Compared with the crystallization in organic solutions, the aqueous synthesis of ZIF-8 has particular advantages in terms of operation, economy, and high-yield production. However, the aqueous synthesis requires excessive 2-methylimidazole for the synthesis of ZIF-8 than the stoichiometric proportion of Zn(2methylimidazole)2. In the aqueous synthesis of ZIF-8,28 free Zn2+ ions are easily complexed with OH to form solute species of type Zn(OH)i2−i with i = 1−4 and then byproducts such as zinc hydroxide are yielded. For the crystallization of phase-pure ZIF-8 in water, it is necessary to increase the population of free deprotonated 2-methylimidazole by increasing its concentration. Herein, we describe an aqueous-system-enabled spray-drying method for the synthesis of ZIF-8. It is notable that our synthesis route produces an amorphous zinc complex compound, which possesses a continuous random network with crystalline fillers, after spray drying followed by an amorphous-to-crystal transition via activation treatment using Received: September 7, 2017 Accepted: September 26, 2017 Published: October 5, 2017 6437

DOI: 10.1021/acsomega.7b01325 ACS Omega 2017, 2, 6437−6445

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polar organic solvents. Our spray-drying method is a rapid, continuous, and organic solvent minimization technique, enabling not only large-scale production but also the formation of unique hollow polycrystalline particles with several micrometers in diameter. This method is advantageous for avoiding the use of large excess of 2-methylimidazole and allowing the synthesis of ZIF-8 in stoichiometric proportion. Furthermore, it is found that the polycrystalline micron-sized particles consisting of primary ZIF-8 nanocrystals clearly show the advantages of bulky rhodamine B adsorption capacity and uptake rate and hexane uptake rate over ZIF-8 monocrystals.

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RESULTS AND DISCUSSION The coordinated reaction occurred immediately after the precursor solution was stirred for 1 min before feeding to the spray dryer. Therefore, the obtained white suspension was continuously fed and spray-dried, and the white powdery SDasmade was obtained. On the other hand, nonSD-asmade was collected from the precursor solution by centrifugation. Powder X-ray diffraction (PXRD) patterns of initial efforts to characterize SD-asmade are shown in Figure 1. To the best

Figure 2. FESEM (left, scale bar: 1 μm) and TEM (right, scale bar: 500 nm) images of the nonSD-asmade and SD-asmade.

asmade have a dense solid structure. Both of them hardly adsorbed nitrogen, as shown in Figure S1. Combining PXRD and TEM results, these results reveal that the SD-asmade and nonSD-asmade possess a dense framework. When particles are prepared from spray drying of a precursor containing colloidal particles, the colloidal particles are included in the spray-dried particles. With the cross-shaped nonSDasmade contained inside, the surface of the spherical spraydried particles should become irregular because of the dispersion of the nonSD-asmade and heterogeneous shrinkage of the droplet. However, in our case, the fact that the SDasmade has a smooth spherical shape may be caused by the shape/structure evolution of the nonSD-asmade including amorphization and crystal downsizing during the spray-drying process. Figure 3 shows the Fourier transform infrared (FTIR) spectra of the SD-asmade and nonSD-asmade particles with a comparison to monocrystalline ZIF-8. The FTIR spectra confirmed that the bands of the SD-asmade were same as those of the nonSD-asmade, with the exception of a broad band at around 3200 cm−1 observed in the spectra of the SD-asmade. The broad band is assigned to the physically adsorbed water and Zn−OH groups. The narrow band observed at 3650 cm−1 is assigned to the free Zn−OH groups. The presence of these bands indicates that both SD-asmade and nonSD-asmade are in the hydrated form. In addition, the bands of the SD-asmade and the nonSD-asmade at 500−550 and 900 cm−1 are attributed to the presence of acetic acid. On the other hand, the convoluted bands of ZIF-8 at 420, 700−750, 1000−1300, 1350−1500, 1580, and 2929/3135 cm−1 are ascribed to Zn−N, out-of-plane bending and in-plane bending of the imidazole ring, entire ring stretching, CN, and aliphatic/aromatic C−H bonds of the imidazole ring, respectively. Although the spectral intensity loss of Zn−N coordination bond is realized for the SD-asmade and nonSD-asmade compared to ZIF-8, both of them possess a Zn−(2-methylimidazole) coordination framework. The thermogravimetric analysis (TGA) and TGA−mass spectrometry (MS) curves of the SD-asmade and nonSDasmade are shown in Figure 4. TGA performed on the nonSD-

Figure 1. PXRD patterns of nonSD-asmade, SD-asmade, and SD-x.

of our knowledge, such a crystal structure of nonSD-asmade has been unreported. The reflection peaks cannot be correlated with those of basic zinc hydroxides. Although the crystal structure of the nonSD-asmade remains unknown, the structure was found to have high density (on the order of decreasing of the d spacing: 7.3, 5.1, 4.8, 3.6 Å, and so on). The PXRD pattern of SD-asmade shows some reflection peaks attributed to the nonSD-asmade. On the other hand, the decrease in the nonSD-asmade peak intensity and broad band at around 2θ = 15°, visible for the SD-asmade, indicate the conversion from the crystalline to the amorphous form and/or crystal downsizing in the process of spray drying. Figure 2 shows the field emission scanning electron microscopy (FESEM) and transmission electron microscopy (TEM) images of the SD-asmade and nonSD-asmade particles. It is interesting to note that the shape of the particles differs greatly between the SD-asmade and the nonSD-asmade. The nonSD-asmade particles are cross-shaped crystals. On the other hand, the SD-asmade particles show a smooth spherical shape with an average diameter of 3.9 μm. The diameter of spraydried particles is directly related to the spray droplet size. The TEM images suggested that both SD-asmade and nonSD6438


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Figure 3. FTIR spectra of nonSD-asmade, SD-asmade, and ZIF-8.

total weight loss was 54% for the nonSD-asmade, indicating that 2-methylimidazole is insufficient for the formation of openframework structure with sodalite topology. On the other hand, the SD-asmade showed a 75% total weight loss because it contains 2-methylimidazole coordinated with Zn, free 2methylimidazole, water, and acetic acid in the structure. Therefore, the SD-asmade is hydrophilic and can be easily dispersed in polar solvents. In a general spray-drying process, the solvent evaporates, forming a layer of higher concentration of the precursor at the outer boundary of the particle.29 During this process, hollow superstructures can be formed by a nonlinear change in precursor concentration at the droplet and subsequent formation of an impermeable shell and generation of gas at the core.30,31 However, in our case, the SD-asmade was dense spherical particles with a smooth surface. The spray-drying process starts with atomization injection of the precursor solution into a formation of microdroplets including the nonSD-asmade particles. Thus, the droplets contact a gas stream heated at 150 °C, causing the solvent to begin evaporating. The rapid solvent evaporation enhances the reaction between residual Zn2+ ions and 2-methylimidazole or water. Zinc acetate reacts with 2-methylimidazole and/or water, releasing acetic acid as a byproduct. As the evaporating droplet shrinks, its receding droplet surface leads to increasing particle density of the nonSD-asmade in the droplet. On the other hand, the increasing concentration of the byproduct acetic acid in droplets induces the dissolution/amorphization of the nonSD-asmade particles and rearrangement of Zn−(2-methylimidazole) and Zn−(water) coordination bonds, resulting in the formation of amorphous dense spherical particles with nanosized nonSD-asmade fillers. Consequently, the droplet including the nonSD-asmade particles changes its shape into dense spheres with a smooth surface. Surprisingly, such an amorphous zinc complex compound was converted into ZIF-8 by contact with polar organic solvents. Figures 1 and S2 show the PXRD patterns of SD-x. After contacting the SD-asmade with a polar organic solvent, new reflection peaks appeared, which correspond to a family of lattice planes of sodalite ZIF-8. The changes in the PXRD patterns revealed the conversion from the amorphous to the sodalite crystalline form in the organic solvent. In addition, the reflection peaks of SD-methanol are broader than those of SDbutanol (Figure 1), suggesting that the crystallite size varies

Figure 4. TGA curves (top) of nonSD-asmade, SD-asmade, zinc acetate, and 2-methylimidazole. TGA−MS curve (bottom) of SDasmade.

asmade revealed structural stability up to 400 °C, which is comparable to ZIF-8. The weight loss in the temperature range 400−500 °C can be mainly attributed to the decomposition of 2-methylimidazole strongly coordinated with Zn. On the other hand, for the SD-asmade, two weight loss peaks centered at around 200 and 400 °C were observed. The weight loss at around 200 °C accompanied the evolution of species with m/z of 59.6 (probably for acetic acid) and 80.6 (probably for 2methylimidazole). These results indicate that the SD-asmade possesses two types of 2-methylimidazole in the structure: (1) coordination state with Zn and (2) relatively “free” state. Pure ZIF-8, Zn(2-methylimidazole)2, is decomposed and oxidized to pure ZnO, resulting in 65% of the starting weight loss.23,28 The 6439


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depending on the kind of the organic solvent, as discussed in more detail later. On the other hand, apolar solvents, such as toluene, benzene, and hexane, cause little or no change in structural transition. In SD-water, the structure was converted from amorphous to the same crystal similar to the nonSDasmade (Figure S3). 2-Methylimidazole is highly symmetric and exists in two equivalent tautomeric forms because the proton can be located on either of the two nitrogen atoms. The formation of ZIF-8 involves the coordination of Zn2+ ion with the electron-deficient pyridinic nitrogen of 2-methylimidazole. This coordination polarizes the proton of the pyridinic group, then allowing another Zn2+ ion to coordinate with the other nitrogen of 2-methylimidazole. In polar organic solvents capable of generating hydrogen bonding, a destabilization of the pyridinic proton could induce, facilitating the evolution of coordination bond relocation. The interaction between the amorphous zinc complex compound and the polar solvent provokes the structural rearrangement and amorphous-tocrystal phase transition. The SD-asmade particles hardly adsorbed nitrogen, indicating a nonporous structure. On the other hand, the type-I adsorption isotherm behavior for all of the products, except for SD-hexane and SD-water, revealed their microporous nature (Figure S4). Table 1 summarizes the results of the structure

Figure 5. FESEM (left, scale bar: 1 μm) and TEM (right, scale bar: 500 nm) images of SD-methanol and SD-butanol.

hand, the primary crystal sizes varied according to the kind of the organic solvent (Figure 6). In particular, in the case of alcohols, the primary crystal size did increase with increasing carbon chain length. The proposed structural transition steps are as follows: (i) penetration and diffusion of the solvent molecules into the SDasmade particles, (ii) interaction between solvent molecules and water (and acetate) coordinated with Zn, leading to coordination bond breaking and dissolution of water and acetate in the organic solvent, (iii) interaction between solvent molecules and 2-methylimidazole molecules, leading to reforming new coordination bonds and rearrangement of Zn−(2methylimidazole), and (iv) nucleation and crystal growth, yielding polycrystalline ZIF-8. It is considered that highly diffusive solvent molecules for the hydrophilic SD-asmade particles induce nucleation at a high density. The higher the polarity and hydrogen bond donating ability of organic solvents, the higher the nucleation density and the smaller the primary crystal size. This crystallization process contains two competitive processes: (1) nucleation and crystal growth and (2) the dissolution of unnecessary components, water and acetate, for ZIF-8 formation at the interface between the solvent and the SD-asmade particle. Such a mass transfer from the core to the outer surface and preferential nucleation and crystal growth near the surface of particles associated with the dissolution process may result in the formation of a hollow superstructure. Comparing our findings with the literature, structural transition in zeolitic imidazolate frameworks from ZIF-L to ZIF-8 can be discussed. ZIF-L is a two-dimensional layered structure that is composed of the same building blocks as ZIF8.32−34 The layers are stacked onto each other via hydrogen bonds between free 2-methylimidazole molecules. The close similarity between ZIF-L and ZIF-8 offered new insights concerning the crystallization mechanisms of zeolitic imidazolate frameworks. Low et al. demonstrated that the crystal transition of ZIF-L to ZIF-8 occurs in ethanol.34 Their results show that ZIF-L undergoes structural transition to form ZIF-8 upon heating at 60 °C and it takes at least 72 h for complete structural transition. On the other hand, in our case, the SD-

Table 1. Structural Characteristics of the Products SD-methanol SD-ethanol SD-propanol SD-butanol SD-pentanol SD-hexanol SD-acetone SD-DMF SD-benzene SD-toluene SD-hexane SD-water 12ZIF-8 6ZIF-8

SBETa (m2/g)

Vmicrob (cm3/g)

Vtotalc (cm3/g)

1330 1360 1350 1300 1280 1390 1330 1440 260 260 20 20 1560 1570

0.46 0.47 0.45 0.44 0.45 0.52 0.47 0.49 0.11 0.11 0.01 0.01 0.61 0.61

1.58 0.69 0.69 0.47 0.55 0.60 0.59 0.60 0.13 0.11 0.09 0.14 0.94 1.16

a BET surface area. bMicropore volume calculated by the αs-plot method. cTotal pore volume calculated as the amount of nitrogen adsorbed at a relative pressure of 0.99.

analysis. Combined with PXRD results, these results strongly suggest that the obtained ZIF-8 possesses an open-framework structure with sodalite topology. It was found that intracrystalline microporosity was developed in our spray-drying-mediated process. The Brunauer−Emmett−Teller (BET) surface areas and the micropore volumes amounted to ∼1440 m2/g and ∼0.52 cm3/g, respectively. The experimentally determined surface areas reported for ZIF-8 range from ca. 530 to 1700 m2/g, whereas the majority of the values are around 1200 m2/ g.5,26 Therefore, the porous structure of the ZIF-8 prepared in this study is comparable to that of conventional ZIF-8. Of particular interest is that the SD-x particles converted using alcohols have a hollow polycrystalline shape, as shown in Figures 5 and S5. The macroscopic particle size was preserved during amorphous-to-crystal phase transition. The secondary particle size of hollow polycrystalline ZIF-8 particles is comparable to that of the SD-asmade particles. On the other 6440


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Figure 6. Secondary particle size distributions (left) of SD-methanol, 12ZIF-8, and 6ZIF-8. Average primary crystal size (right) of SD-x. The insets show the FESEM images of 6ZIF-8 and 12ZIF-8.

Figure 7. Uptake curves (left) and plots of the pseudo-second-order kinetic model (right) of rhodamine B on 12ZIF-8 and SD-methanol.

adsorption dynamics of ZIF-8. Further studies are under way to control the grain size of polycrystalline particles over a wide range from nanometer to micrometer size for the development of adsorption performance. The above properties would make hollow polycrystalline ZIF-8 an attractive candidate for applications in adsorption. Herein, we discuss the adsorption performances of the dye pollutant on the developed hollow polycrystalline ZIF-8, SDmethanol, to demonstrate its potential as a liquid-phase adsorbent for pollutant removal in practical environmental remediation. We take the bulky basic dye rhodamine B (4.3 × 9.8 × 15 Å3)37 as an example to demonstrate the enhanced adsorption performance of the macro-/microscopic structures of hollow polycrystalline ZIF-8. Note that the apertures of the six-membered ring of ZIF-8 are 3.4 Å. It is apparent that the rhodamine B molecules hardly enter the pore cages through the narrow apertures. Therefore, adsorption should occur mainly on the external surfaces of ZIF-8.

asmade undergoes structural transition to form ZIF-8 at room temperature within 10 minutes. It is believed that such an ease of structural transition is allowed by high coordination substitution and rearrangement abilities due to the amorphous structure of the SD-asmade. ZIF-8 has been recognized as a unique molecular sieving material with a flexible framework. It is interesting to note that ZIF-8 can adsorb molecules with a kinetic diameter larger than its window size, such as propane, butanol, 1,2,4-trimethylbenzene (4.0 × 7.6 × 9.1 Å3), and so on.8−10,35,36 This unexpected adsorption behavior has been speculated to be due to the flexible apertures that swing open by the reorientation of imidazolate linkers enforced by guest adsorption. Our previous results revealed that such flexible functionality and transport property of ZIF-8 are strongly affected by the crystal size,17 suggesting a high potential for the control of adsorption performance with crystal size. An interesting approach is to utilize the crystal size dependency of flexible functionality and 6441


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All adsorption measurements were performed at 25 °C. Rhodamine B was initially dissolved in distilled water to obtain solutions with different initial concentrations. According to the Beer−Lambert law, the maximal absorption wavelength for the detection of rhodamine B is 554 nm. A measure of 10 mg of SD-methanol was added to each solution (10 mL) with agitation and maintained for enough time to reach adsorption equilibrium. For comparison, rhombic dodecahedral ZIF-8, 12ZIF-8 (0.90 μm) (Figure S5), was also tested as an adsorbent. On the other hand, the average secondary particle size of SD-methanol is 3.9 μm. Let us begin by discussing the adsorption kinetics. Figure 7 shows the uptake curves of rhodamine B by the ZIF-8 samples. Furthermore, it was found that the kinetic adsorption performance of rhodamine B on the ZIF-8 samples can be well-described by a pseudo-second-order kinetic model,38,39 where qt and qeq are the transient and equilibrium adsorption amounts, respectively, and k is the pseudo-second-order rate constant. The constant k can thus be obtained from the intercept of the plot t/qt against t. Figure 7 shows a good linear relationship between t/qt and t, indicating that the kinetic adsorption behavior is well-correlated with the pseudo-secondorder kinetic model.

dq t dt

Figure 8. Adsorption isotherms of rhodamine B on 12ZIF-8 and SDmethanol. The insets show the structures of rhodamine B and part of six-ring aperture.

ZIF-8, 6ZIF-8 (0.16 μm) (Figure S5). Our approach to highlight the effect of particle shape on intrinsic uptake kinetics is based on the selection of a fast diffusing and apolar adsorbate, n-hexane (a kinetic diameter of 4.3 Å). We then measured the adsorption equilibrium data of isomeric 2,3-dimethylbutane, 2,3-DMB, coupled with n-hexane, on SD-methanol at 25 °C. As a result of the rotational freedom of the linkers that constitute the ZIF-8 apertures, n-hexane is capable of being adsorbed in the pores of ZIF-8 in spite of its kinetic diameter (4.3 Å) being larger than the aperture size of ZIF-8 (3.4 Å). Figure 9 shows that SD-methanol adsorbs n-hexane selectively. The adsorption capacity of n-hexane on the hollow polycrystalline ZIF-8 is comparable to that on conventional ZIF-8.40 Along with the elucidation of equilibrium adsorption properties, the understanding of mass-transfer mechanisms is essential toward the development of separation technologies.

= k(qeq − qt)2

1 1 t = + t 2 qt qeq kqeq

The initial adsorption rate, H, can be estimated as follows H = kqeq 2

The initial adsorption rate of rhodamine B on SD-methanol was determined to be 17.2 μmol/(g·min), which is much higher than that [2.70 μmol/(g·min)] of rhodamine B on 12ZIF-8 at the same initial concentration of 100 μM. The higher adsorption rate of SD-methanol demonstrated it as a better adsorbent for bulky dye pollutant removal than 12ZIF-8, which was likely due to the limited contact time in the water treatment processes, achieving more efficient elimination by hollow polycrystalline ZIF-8. The isothermal adsorption curves revealed that the macro-/ microscopic structures of hollow polycrystalline ZIF-8 significantly structurally enhanced the adsorption capacity of the dye pollutant (Figure 8). Surprisingly, the saturated adsorption amount of rhodamine B on SD-methanol (285 μmol/g) is about 20 times higher than that on 12ZIF-8 (14.7 μmol/g), indicating relatively favorable affinity to rhodamine B for the polycrystalline grain boundary structure compared with a monocrystalline surface structure. Note that the only adsorption isotherm for 12ZIF-8 can be successfully correlated with the Langmuir model. The maximum adsorption capacity, q0, and Langmuir constant, KL, were determined to be q0 = 14.7 μmol/g and KL = 10.9 L/μmol, respectively. The high correlation is due to the adsorption behavior used only on the external surface of 12ZIF-8 monocrystals. On the other hand, it is suggested that the intercrystalline interfaces of hollow polycrystals involve the multiple adsorption processes instead of monomolecular adsorption. Furthermore, we investigated the vapor-phase adsorption performances of the developed hollow polycrystalline ZIF-8, SD-methanol, as compared with 12ZIF-8 and cubic hexahedral

Figure 9. Adsorption isotherms of n-hexane and 2,3-DMB on SDmethanol (upper). Uptake curves of n-hexane by 6ZIF-8, 12ZIF-8, and SD-methanol at P/P0 of 0.01 (lower). 6442


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recorded on a JEM-2010 (JEOL) at an acceleration voltage of 200 kV. Nitrogen adsorption/desorption isotherms were measured at 77 K using a BELSORP-max (MicrotracBel, Japan). The samples were degassed at 200 °C under vacuum. TGA was carried out with a DTG-60H (Shimadzu) under air flow at a heating rate of 5 °C/min. The combination of TGA with MS was carried out with a TG8120 (Rigaku)/GCMSQP2010SE (Shimadzu) system to analyze the gases evolved during TGA analysis under helium flow. Liquid-phase batch adsorption experiments were carried out by agitating 10 mg of sample in 10 mL of dye solution with different initial concentrations. After adsorption, the dye concentration was determined using a UV−visible spectrophotometer UV-2450 (Shimadzu). Vapor-phase adsorption experiments were performed on an environment-controlled QCM, BELQCM (MicrotracBel, Japan). During the measurements, the total pressure was constant at atmospheric pressure and the adsorbate partial pressure was controlled by independent mass flow controllers with regulating the saturator temperature. The Au-coated QCM substrates were dipped in the ZIF-8 colloidal solution. The quantity of ZIF-8 sample was controlled by the particle concentration of the colloidal solution. After achieving the desired sample loading of several tens to hundred nanograms, the sample was dried at 200 °C under helium flow. Quantitative mass change of the sample mounted on the QCM substrate was obtained from the frequency change according to the Sauerbrey equation.41

The accurate characterization of mass transfer is, however, a very challenging task because it may be severely affected by extracrystalline resistances, including thermal effects and bed diffusion. In this study, a quartz crystal microbalance (QCM) technique was employed for nanogram-order resolution. We then confirmed that the thermal effect and bed diffusion are negligible. In addition, it was also proven that no significant mass-transfer resistances might result from an external fluid film. Surprisingly, despite the largest particle size (3.9 μm), the kinetic adsorption performance of n-hexane on SD-methanol is better than that on 12ZIF-8 (0.90 μm) and almost same as that on nanosized 6ZIF-8 (0.16 μm) (Figure 9). Besides the nanostructure control, the shaping of the adsorbent is a key breakthrough toward the development of practical applications.

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CONCLUSIONS We describe a highly versatile and effective strategy to produce ZIF-8 with a hollow polycrystalline structure. This method also enables large-scale, rapid, and continuous production. The polycrystalline structure, the crystallite size, and the grain boundary phase may be important factors for the application in adsorption and separation. The products developed in our spray-drying-mediated process possess the combined architecture of the secondary agglomerates formed by polycrystalline ZIF-8 with a controllable primary crystal size. Further, controlling the macro-/microscopic structures of hollow polycrystalline ZIF-8 opens new opportunities for numerous applications, including adsorption, drug delivery, catalysis, contrast agents, and sensing devices.

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ASSOCIATED CONTENT

S Supporting Information *

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The Supporting Information is available free of charge on the ACS Publications website at DOI: 10.1021/acsomega.7b01325. Adsorption isotherms of nitrogen in nonSD-asmade and SD-asmade, additional PXRD patterns of SD-x, PXRD pattern and FESEM image of SD-water, adsorption isotherms of nitrogen in SD-x, additional FESEM and TEM images of SD-x, and additional adsorption isotherms and pore size distributions (PDF)

EXPERIMENTAL METHODS A precursor solution of 0.025 M zinc acetate and 0.05 M 2methylimidazole in 300 mL of distilled water was stirred and spray-dried using a two-fluid nozzle at a feed rate of 5 mL/mim, a spray air pressure of 70 kPa, and an inlet temperature of 150 °C, using an SD-1000 spray dryer (Tokyo Rikakikai). The spray-dried sample referred to as SD-asmade was then dispersed in a pure organic solvent at room temperature and then collected by centrifugation. The immersion time in the pure organic solvent was 20 min including centrifugation (4000 rpm, 10 min). The products were then dried at 40 °C under vacuum overnight. The resulting samples are designated as SDx, where x denotes the organic solvent. For reference, the product collected from the precursor solution by centrifugation is designated as nonSD-asmade. Conventional ZIF-8 samples were prepared according to the literature17 for the comparison of adsorption test with SD-x. Briefly, rhombic dodecahedral ZIF-8 (0.90 μm), referred to as 12ZIF-8, was prepared in water at a molar ratio of 1.0 zinc acetate/80 2-methylimidazole. Cubic hexahedral ZIF-8 (0.16 μm), referred to as 6ZIF-8, was prepared in water at a molar ratio of 1.0 zinc nitrate/60 2-methylimidazole using cetyltrimethylammonium chloride as a surface-specific capping agent. The products were collected by repeated centrifugation, washed three times with fresh methanol, and then dried at 40 °C under vacuum overnight. PXRD patterns were recorded on a MiniFlex 600 (Rigaku) by using Cu Kα radiation with λ = 1.5418 Å; the copper anode was operated at 30 kV and 15 mA. FESEM images were recorded on an S-4800 (Hitachi High-Tech). The diameters of more than 200 particles in the FESEM images were measured to determine the average particle size. TEM images were

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AUTHOR INFORMATION

Corresponding Author

*E-mail: [email protected]. ORCID

Shunsuke Tanaka: 0000-0001-5157-3317 Author Contributions

The manuscript was written through the contributions of all authors. All authors have given approval to the final version of the manuscript. Notes

The authors declare no competing financial interest.

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ACKNOWLEDGMENTS This work was financially supported by Yazaki Memorial Foundation for Science and Technology and Yashima Environment Technology Foundation.

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REFERENCES

(1) Kondo, M.; Yoshitomi, T.; Matsuzaka, H.; Kitagawa, S.; Seki, K. Three-Dimensional Framework with Channeling Cavities for Small Molecules: {[M2(4,4′-bpy)3(NO3)4]·xH2O}n (M = Co, Ni, Zn). Angew. Chem., Int. Ed. 1997, 36, 1725−1727.

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