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Big World in the Small Space: Constructing Hollow Zeolite Microspheres Using a Sustainable Template Meng Pan, Jiajun Zheng, Yujian Liu, Qinglan Kong, Huiping Tian, and Ruifeng Li Cryst. Growth Des., Just Accepted Manuscript • DOI: 10.1021/acs.cgd.7b00850 • Publication Date (Web): 15 Aug 2017 Downloaded from http://pubs.acs.org on August 17, 2017

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Crystal Growth & Design

Big World in the Small Space: Constructing Hollow Zeolite Microspheres Using a Sustainable Template Meng Pan,† Jiajun Zheng,*, † Yujian Liu,‡ Qinglan Kong,† Huiping Tian,‡ and Ruifeng Li*, † †

Research Centre of Energy Chemical & Catalytic Technology, Taiyuan University of

Technology, Taiyuan 030024, China ‡

Sinopec Research Institute of Petroleum Processing, Beijing, 100083, China

KEYWORDS: Zeolite; Hollow spheres; CMC; ZSM-5

ABSTRACT: Hollow zeolite microspheres have attracted considerable interest due to their unique properties and great potential applications. In this work, we report a simple and costeffective approach for constructing hollow zeolite microspheres based on a biomass-derived template, i.e., carboxymethylcellulose sodium (CMC). As an example, a hollow ZSM-5 microsphere with a hollow core smaller than 1 µm in diameter and a complete crystal shell is synthesized. This approach overcomes most of the limitations associated with the existing methods, such as complex operations and costly spherical templates. By studying the growth process in detail, a possible formation mechanism is proposed. The crystallization of the hollow zeolite microsphere is through a “surface to core” process, in which the interaction between the CMC and the zeolite gel and the decomposition of the polymer network play the critical roles.

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1. INTRODUCTION

Hollow microspheres have a broad application prospect in the fields of electronics, biotechnology and catalysis due to their unique features such as the selective permeability of the shell and the specific micro-environment within the hollow core.1-6 Among various hollow materials, hollow zeolite spheres have been demonstrated to be a biocompatible, nontoxic and thermal/hydrothermal stable material.5-7 In particular, the access to the interior of the hollow zeolite spheres is controlled by their regular micropores.7-9 Therefore, this material has attracted great interests of the researchers who devoted their efforts to the studies on controlled release capsules, bioreactors, confined-space catalysts, etc.4-11 To date, a series of innovative methods have been developed to fabricate hollow zeolite spheres. The most common way is to grow zeolite crystals on a spherical template via the layer-by-layer (LbL) technique and subsequently remove the template by calcining or dissolving.4-7 Nevertheless, the complicated and labor intensive LbL coating process as well as the costly spherical template such as polystyrene sphere4, carbon sphere5 and mesoporous silica sphere6,7 limits the practical application of this approach. In addition, hollow zeolite spheres can be fabricated by synthesizing the zeolite crystals in the first step and selectively dissolving the crystal cores with a base in the second step.9,10 However, this method also involves a multi-step process. Besides, the permeability of the prepared hollow spheres may be seriously affected by the debris resulting from the dissolution of the crystals. Recently, a relatively simple approach to preparing hollow zeolite spheres is developed in an emulsion system in which the zeolite crystals grow along the interface.11 This method usually yields large hollow spheres with diameters of more than 10 micrometer. Up to now, there are few


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reports focused on utilizing a simple and cost-effective approach to synthesize hollow zeolite spheres on micrometer or sub-micrometer scale. Carboxymethylcellulose sodium (CMC) is a biopolymer derived from natural cellulose which is the most abundant renewable raw material in the world. CMC has been widely used in various fields ranging from food and medicine industry to petrochemical technology due to its availability, relatively low cost and favorable environmental profile.12 In addition, using CMC to prepare high value-added materials has always been the focus of many researchers because it can provide an efficient way to utilize the agricultural waste cellulose sources.13,14 Taking into consideration that the long chain of CMC contains abundant hydroxy and carboxylic groups,15 we deduce it can interact with the zeolite gel and further affect the crystal growth. Here, CMC is employed to prepare hollow zeolite microspheres for the first time. By adding CMC into the synthesis system of a traditional ZSM-5, hollow ZSM-5 microspheres with special structures were obtained. This approach is proved to be a desirable strategy for constructing hollow zeolite microspheres in a simple and costeffective manner.

2. EXPERIMENTAL SECTION

2.1 Sample Preparation. Hollow ZSM-5 microspheres were prepared by a simple one-pot hydrothermal method, in which CMC was added into the synthesis gel of a traditional ZSM-5. In a typical synthesis, 0.50 g of sodium hydroxide (98 wt. %, Aladdin) and 0.43 g of sodium aluminate (41 wt. % of Al2O3, 35 wt. % of Na2O, Sinopharm Chemical Reagent Co., Ltd.) were successively dissolved in 89 mL of distilled water. Then 0.5−2.0 g of carboxymethylcellulose sodium (CMC, 98 wt. %, Sinopharm Chemical Reagent Co., Ltd.) was added slowly under


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vigorous stirring. After the complete dissolution of the CMC in the solution, 19 mL of silica sol (40 wt. %, Qingdao Haiyang Chemical Co., Ltd.) was added dropwise. Finally, 4 mL of ethylenediamine (EDA, 99 wt. %, Aladdin) was added to the mixture. The mixture was stirred vigorously for another 4 h to form a gel. Then the gel was transferred into autoclave and heated at 180 ℃ for 72 h. The as-synthesized products were recovered by filtrating, washing and drying and denoted as HZ-m (“m” stands for the additive amount of CMC, for example, HZ-1 represents the product synthesized by adding 1.0 g of CMC into the above system). For comparison, the traditional ZSM-5 was also prepared under the similar conditions except without using CMC and named as TZ. The molar composition is as following: 5 Na2O: 90 SiO2: 1 Al2O3: 34.7 EDA: 3471 H2O. Moreover, a commercial ZSM-5 (provied by Sinopec Research Institute of Petroleum Processing) was also used as a reference and marked as CZ. In order to remove the concealed organic templates in the cages and the pore channels, all the samples were calcined in air at 800 ℃ for 6 h. 2.2 Glucose Detection. The glucose generated from the decomposition of CMC was detected by the Benedict’s method.16 Typically, 10 mL of the liquid under test was mixed with 2 mL of Benedict’s solution (Shanghai Haling Biotechnology Co., Ltd.) firstly. The mixture was then heated at 80 ℃ for 5 min. After cooling, the color of the mixture was recorded. 2.3 Characterization. X-ray powder diffraction patterns (XRD) were recorded using a Shimadzu XRD-6000 X-ray diffractometer, Ni-filtered Cu Kα radiation, 40 kV and 30 mA. Framework infrared spectra (FT-IR) were obtained on a Shimazdu FTIR-8400 spectrometer in KBr pellets. Crystal sizes and morphology of the as-synthesized samples were investigated on a Hitachi S4800 scanning electron microscopy (SEM) coupled with energy dispersive X-ray spectroscopy (EDX and EDS). Crystal internal structure was studied on a JEM-200CX


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transmission electron microscopy (TEM) coupled with selected area electron diffraction (SAED). Macropore size distribution was studied with an AutoPore IV 9500 intrusive mercury apparatus. N2 adsorption-desorption measurement was performed in a NOVA 1200e gas sorption analyzer. Thermogravimetric analysis (TG) was performed in a Netzch SAT449 F3 instrument.

3. RESULTS AND DISCUSSION

All the samples reported here except for HZ-2 are pure MFI zeolites with high crystallinity as confirmed by XRD patterns (Figure 1). It is known that CMC is usually used as a thickener.12 Perhaps the excessive CMC made the sol too thick to form a uniform system, so an impurity of quartz phase was finally generated in the sample HZ-2 (marked by arrow in Figure 1). Although the other four samples are all MFI phases, they display the different morphologies. Commercial ZSM-5 is made up of the typical coffin-shaped crystals,17 as shown in Figure 2A. The product TZ prepared without using CMC has a similar morphology to the commercial ZSM-5 despite the different crystal sizes (see Figure 2B and Figure S1 in Supporting Information). However, the products HZ-0.5 and HZ-1 with the addition of CMC are composed of spherical particles with a diameter of about 1.5 µm, as shown in Figure 2C−D and Figure S2 in Supporting Information. Moreover, the enlarged image in the inset of Figure 2D shows that the shell of the microsphere consist of the nanosized coffin-shape crystals. Combining with the results of XRD, these microspheres should be MFI phases. When adding a small amount of CMC, there were a few microspheres with the incomplete shells can be observed in the SEM image of the product HZ0.5 (Figure S2A, Supporting Information). Interestingly, the hollow interiors of these microspheres are very obvious (see the inset in Figure S2A). After increasing CMC moderately, almost all the microspheres in the product HZ-1 had the complete shells, as shown in Figure S2B


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in Supporting Information. In addition, a small number of twin microspheres also existed in the product HZ-1. When adding CMC in excess, besides the quartz crystals, some accumulational microspheres can still be detected in the SEM image of the product HZ-2 (Figure S3, Supporting Information). These observations indicate that the formation of the microsphere was a result of the addition of CMC in the zeolite gel.

HZ-2

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HZ-1

HZ-0.5

TZ

CZ 5

10

15

20

25

30

35

2 Theta (degree)

Figure 1. XRD patterns of the samples.

A

B

C

D

Figure 2. SEM images of the samples. (A) Commercial ZSM-5; (B) traditional ZSM-5; (C) HZ-0.5; (D) HZ1.


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In order to reveal the inner structure of the microsphere, the sample HZ-1 was crashed by 20 MPa and then observed by SEM. The result (Figure 3A) indicates that the microsphere has a hollow interior and a dense shell layer constituted by the intergrown MFI crystals. The diameter of the hollow core is about 800 nm and the thickness of the shell is about 300 nm. Figure 3A also suggests the hollow microspheres have a high mechanical strength because most of them still keep the complete shells after being crushed by 20 MPa. TEM image (Figure 3B) further proves that the MFI microspheres have the hollow interiors and the complete shells. The SAED image (the inset in Figure 3B) exhibits the polycrystal-like diffraction pattern, which confirms that the shell of the hollow microsphere is composed by the polycrystalline MFI zeolites. The hollow structure can also be observed in the SEM-EDX image (Figure 3C). The mercury intrusion porosimetry was also conducted on the sample HZ-1 after being crushed by 20 MPa and ground to detect the hollow structure. Two obvious peaks can be observed in the macropore size distribution curve (Figure 4). The pore structures with the sizes range from 300 nm to 1 µm should be associated with the hollow interior of the microsphere. This result is similar to the observation of SEM and TEM. The spherical hollow rather than columnar pore led to the present of a broad pore size distribution in this range. In addition, the pore structures greater than 1 µm in diameter can be attributed to the spaces between the microsphere particles. The chemical compositon of a given zeolite is an important characteristic, which defines its properties.18 EDS analysis (Figure 3D) shows the Si/Al ratio of the microsphere HZ-1 is 43.7 which is close to the 40.4 of the traditional ZSM-5. This makes clear that the hollow microsphere is ZSM-5 zeolite phase. Based on the above-mentioned results, we can draw a solid conclusion that the hollow ZSM-5 microsphere was successfully prepared by employing CMC as template. To the best of our knowledge, this is the first report on the synthesis of a hollow zeolite microsphere by using


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such a sustainable template. Micro-mesoporous structure of the hollow microsphere is detected by N2 adsorption-desorption measurement and the results are shown in Figure 5 and Table 1. The porosity of the shell of the microsphere HZ-1 is proved by its similar micropore area and micropore volume to those of TZ and CZ. Furthermore, the negligible mesopore volume of HZ-1 further reflects the integrality of the shell. The results suggest that the interiors of the hollow microspheres can only be accessed though the micropores of ZSM-5 zeolite. This unique feature increases the value of the synthesized material in practical applications, such as drug controlled release and catalytic microreactor.7-9

A B

C D

Figure 3. Characterization of the sample HZ-1. (A) SEM image of the sample after being crushed by 20 MPa; (B) TEM image; (C) SEM-EDX image; (D) EDS analysis.


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Log Differential Intrusion (mL/g)

1.0

0.8

0.6

0.4

0.2

0.0 100

1000

10000

Pore Diameter (nm)

Figure 4. Macropore size distribution curve of HZ-1 after being crushed by 20 MPa and ground.

160

HZ-1 TZ CZ Volume Adsorbed (a.u.)

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140

+30 120

+20

100

80 0.0

0.2

0.4

0.6

Relative Pressure (P/P0)

0.8

1.0

Figure 5. N2 adsorption-desorption isotherms of the samples. Table 1. Physico-chemical properties of the samples.

Samples HZ-1 TZ CZ a d

Si/Al ratioa 43.7 40.4 16.4

SBETb (m2/g) 381 377 400

Vmicc (cm3/g) 0.13 0.14 0.14

Smicc (m2/g) 324 342 343

Sextc (m2/g) 57 35 57

Vmesd (cm3/g) 0.03 0.02 0.03

Results of the EDS analysis. b Value determined by the BET method. c Value determined by the t-plot method. Value determined by BJH adsorption.

In order to explore the formation mechanism of the hollow zeolite microspheres, FT-IR and TG characterizations were conducted on the samples firstly. The results are shown in Figure 6 and Figure 7. Figure 6 indicates that the characteristic vibration bands of ZSM-5 zeolite and CMC are both present in the FT-IR spectrum of the hollow microsphere HZ-1 before calcination.


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For instance, the band around 547 cm-1 is attributed to the five-membered ring vibration of ZSM5 zeolite and the band at about 1605 cm-1 is due to the asymmetric stretching of carboxylate groups of CMC.19,20 However, there are also some differences can be observed. In the spectrum of CMC, there is only one band at about 1356 cm-1 which is attributed to symmetrical deformation of C–OH group.12,15 But there are two bands around this location in the spectrum of HZ-1 before calcination. The similar phenomenon also occurs around 787 cm-1 and 1398 cm-1. These additional bands exist likewise in the spectrum of the initial gel of HZ-1 and disappear after calcination. The above differences suggest that some new chemical bonds should be formed when adding CMC into the synthetic system of ZSM-5 zeolite. It has been reported by Madusanka et al. that the strong interaction between CMC and montmorillonite (MMT) made the band around 1350 cm-1 in the FT-IR spectrum become broaden.21 This interaction was essentially the chemical reaction between the –COOH or C–OH groups from CMC and the Si–OH groups from MMT.22,23 Kim et al. also reported that the Si–OH groups can react with the oxygenated functional groups (such as –COOH and C–OH) to form Si–O–C=O and Si–O–C linkages respectively.24 Thus, during the formation of the hollow zeolite microspheres, CMC should have interacted with the zeolite gel to form Si–O–C=O or Si–O–C linkage. TG curves (Figure 6B) show the total mass loss of HZ-1 is greater than it of TZ, which further indicates CMC or its hydrolysis products should be contained in HZ-1 before calcination. On the other hand, TG curve of CMC (Figure 7A) indicates the mass loss occurs in three major steps and DTG curve also shows three sharp peaks. The maximum peak observed at 275 ℃ is associated with the chain cleavage of CMC.19 However, the shape of TG curve of HZ-1 (Figure 7B) is different and the staged mass drops can’t be observed. Moreover, the sharp peaks below 400 ℃, especially the maximum peak at 275 ℃, disappear in the DTG curve of HZ-1. Only a peak above 600 ℃ is


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obvious, which is associated with the further oxidation of the residues after the chain cleavage of CMC.19 The above results indicate that the long chain of CMC should break down during the crystallization of the hollow zeolite microspheres. In order to further prove that CMC decomposed during crystallization, the glucose in the synthesis system was detected by Benedict’s reagent. The results are shown in Figure 8. The solution containing the supernatant of the sample TZ after crystallization or the initial gel of HZ-1 was blue (Figure 8A or 8B), indicating the absence of the reducing sugar. However, the solution containing the supernatant of the sample HZ-1 after crystallization turned dark red (Figure 8C). The change of the color means that the glucose may be generated in the synthesis system of HZ-1 during crystallization. The reason why there was no red precipitate formed is probably the existence of the organic amine in the synthesis system which can react with the metal cation to form a stable complex and thus interfere with the detection. Then an experiment was conducted to avoid the interference. According to the formula of HZ-1, 1.0 g of CMC and 0.69 g of sodium hydroxide were dissolved in 108 mL of distilled water to form a mixture which was then transferred into an autoclave and heated at 180 ℃ for 72 h. Before the hydrothermal treatment, the result of the glucose detection shows a pale blue solution (Figure 8D). After the mixture being hydrothermally treated at 180 ℃ for 72 h, the glucose can be obviously detected in the system (i.e., red precipitate can be observed in Figure 8E). The above results demonstrate that CMC decomposed during crystallization which produces glucose in the synthesis system. This consequence is in agreement with the literature which reported that the CMC chains can hydrolyze to different oligomers or glucose when it was hydrothermally treated at high temperature.14


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787 787

1356

(a) (b) (c) (d)

Intensity (a.u.)

Intensity (a.u.)

1605

1356 (a) 1398

(b)

547

(c)

(e)

(d) 400

800

1200

-1

Wavenumber (cm )

1600

2000

750

800

1400 -1

Wavenumber (cm )

Figure 6. FT-IR spectra of the samples. (a) CMC; (b) the initial gel of HZ-1; (c) HZ-1 before calcination, (d) HZ-1 after calcination; (e) TZ before calcination.

100

A

0

100

B

0.0

105

60

Mass (%)

-4 610

-0.1

DTG (% / min)

Mass (%)

-2 80

DTG (% / min)

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96

-0.2

-6 -0.3

92 275

200

400

o

Temperature ( C)

600

-8 800

200

400

o

Temperature ( C)

600

800

Figure 7. (A) TG-DTG curves of CMC. (B) TG-DTG curves of TZ (dotted line) and HZ-1 (solid line) before calcinations.

Figure 8. The results of the glucose detection. (A) The supernatant of the synthesis system of TZ after crystallization; (B) the initial gel of HZ-1; (C) the supernatant of the synthesis system of HZ-1 after crystallization; (D) the mixture of distilled water, sodium hydroxide and CMC before hydrothermal treatment; (E) the mixture after hydrothermal treatment.


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To gain a further insight into the formation process of the hollow zeolite microspheres, the products collected at different crystallization times according to the formula of the sample HZ-1 were studied by XRD and SEM. The results are presented in Figure 9 and Figure 10. XRD patterns (Figure 9) show that only a small diffraction peak attributed to CMC phase can be detected in the initial gel (marked by arrow with reference to Figure S4 in Supporting Information). By increasing the crystallization time to 20 h, a partial crystalline zeolite phase was obtained, which developed gradually into a fully crystalline MFI zeolite within the time range of 72 h. It’s worth noting that the characteristic peak of CMC disappeared after 20 h of crystallization. Combining the results of FT-IR and TG, this phenomenon can be explained by the destruction of the long-range order of CMC under the ongoing hydrothermal synthesis with high temperature (180 ℃). SEM images (Figure 10) show that the spherical particles or the twins exist not only in the initial gel but also in the intermediate products of HZ-1 despite having different sizes and surface morphologies. In the initial gel, the microspheres were made up of tiny amorphous debris. An extension of the crystallization time to 20 h made the microsphere transform into a larger one composed by the wormlike nano-particles. Moreover, many voids can be observed in the interior of the microspheres. As the crystallization time was increased to 48 h, the wormlike nano-particles disappeared and the intergrown coffin-shape crystals formed the shell of the microspheres. After 72 h, the hollow zeolite microspheres with fully crystalline shells were obtained. However, these microspheres can’t be detected in the SEM images of the products collected after different crystallization times without using CMC (see Figure S5 in Supporting Information). This reflects that the addition of CMC influenced the process of crystal growth and further led to the formation of the hollow zeolite microspheres.


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Intensity (a.u.)


72 h

48 h 20 h 0h 5

10

15

20

25

30

35

2 Theta (degree)

Figure 9. XRD patterns of HZ-1 collected after different crystallization times.

A

B

C

D

E

F

Figure 10. SEM images of HZ-1 collected after different crystallization times. (A) and (B) 0 h; (C) and (D) 20 h; (E) and (F) 48 h.


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Figure 11. Schematic representation of the formation process of hollow zeolite microspheres.

Based on the aforementioned studies, a possible growth mechanism is proposed and illustrated in Figure 11. The formation of the hollow zeolite microspheres follows a “surface to core” crystallization process.6,7,20 Initially, CMC is dissolved in the zeolite gel and further interacts with the soluble precursors (Figure 11A). The abundant hydroxy and carboxylic groups of CMC can react with the Si–OH groups of the zeolite gel to form Si–O–C=O or Si–O–C linkage.21-24 This strong interaction makes the zeolite precursors been entrapped in the CMC polymer network to form the amorphous microspheres or the twins in the aging process (Figure 11B). At the subsequent crystallization stage, the surface of the amorphous microspheres should possess higher nucleation rate than the interior because the precursors on the surface can sufficiently absorb nutrients from the external gel (Figure 11C).20 This finally facilitates the formation of the MFI seeds on the external surface of the microspheres (Figure 11D). On the other hand, the continual high temperature (180 ℃) causes CMC to decompose, which results in the damage of


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the polymer network. Thus, the internal zeolite precursors can easily move out to participate in crystallization and then the internal voids are generated (Figure 11D). The following ongoing crystallization ultimately leads to the formation of the highly crystalline ZSM-5 shells and the hollow interiors (Figure 11E). During this stage, the steric hindrance provoked by the concurrently growing crystals in the shell should facilitate the construction of the wellintergrown layers.4,25 On the other hand, the results of Figure 2 and Figure 3 show the size of the crystals in the shell is not uniform. Thus the interspaces between the large crystals can be filled by the small crystals (marked by blue circles in Figure 2D, Figure 10F and Figure S6), which makes the closed polycrystalline shells formed. After removing the organics by calcination, hollow zeolite microspheres with the complete microporous shells are obtained (Figure 11F).

4. CONCLUSIONS

This work reports a hollow zeolite microsphere fabricated using a cost-effective and sustainable template, namely, CMC. The resultant product has a hollow core with a dimension below 1 µm and a complete ZSM-5 zeolite shell which ensures that the interior can only be accessed though the regular micropores. This material also exhibited a high mechanical strength. During the crystallization of the hollow zeolite microsphere, the interaction between CMC and the zeolite gel induced the crystal growth to undergo a “surface to core” process and the hydrolysis of the CMC chains facilitated the formation of the final hollow structure. The simple and efficient method as well as the special crystallization path described in this article can provide value information for constructing other hollow microspheres and may open a door for their practical application in industry.

ASSOCIATED CONTENT


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Supporting Information The Supporting Information is available free of charge on the ACS Publications website at DOI: Figures containing the XRD pattern and the SEM images of the different samples. AUTHOR INFORMATION Corresponding Author *Address: Research Centre of Energy Chemical & Catalytic Technology, Taiyuan University of Technology, 79# West Yingze Street, Taiyuan 030024, China. Email: [email protected] (Jiajun Zheng); [email protected] (Ruifeng Li). Notes The authors declare no competing financial interest. ACKNOWLEDGMENT This work is supported by the Joint Funds of the National Natural Science Foundation of ChinaChina Petroleum and Chemical Corporation (the State Key Program Grant No. U1463209); the National Natural Science Foundation of China (Grant Nos. 21371129; 21376157) and Sinopec (No.116050). REFERENCES (1) Hu, J.; Chen, M.; Fang, X.; Wu, L. Fabrication and application of inorganic hollow spheres. Chem. Soc. Rev. 2011, 40, 5472–5491. (2) Zhou, J.; Sun, Z.; Chen, M.; Wang, J.; Qiao, W.; Long, D.; Ling, L. Macroscopic and mechanically robust hollow carbon spheres with superior oil adsorption and light-to-heat evaporation properties. Adv. Funct. Mater. 2016, 26, 5368–5375. (3) Shen, L.; Yu, L.; Yu, X. Y.; Zhang, X.; Lou, X. W. Self-templated formation of uniform NiCo2O4 hollow spheres with complex interior structures for lithium-ion batteries and supercapacitors. Angew. Chem. Int. Ed. 2015, 54, 1868–1872. (4) Valtchev, V. Silicalite-1 hollow spheres and bodies with a regular system of macrocavities. Chem. Mater. 2002, 14, 4371–4377.


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For Table of Contents Use Only Manuscript Title Author List

Big World in the Small Space: Constructing Hollow Zeolite Microspheres Using a Sustainable Template Meng Pan, Jiajun Zheng, Yujian Liu, Qinglan Kong, Huiping Tian, and Ruifeng Li TOC Graphic

Hollow zeolite microsphere which could be used as drug controlled release agent or catalytic nanoreactor is constructed based on a biomass-derived sustainable template.


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Big World in the Small Space: Constructing Hollow ... - ACS Publications

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