Preparation of Nano-ZIF-8 in Methanol with High Yield

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ABSTRACT. This article systematically presented analysis results of factors affecting the preparation of nano-. ZIF-8 in methanol for high yield. Samples were ...

Article Preparation of Nano-ZIF-8 in Methanol with High Yield† Don N. Ta,1* Hong K. D. Nguyen,1 Bai X. Trinh,1 Quynh T. N. Le,2 Hung N. Ta3 and Ha T. Nguyen4

1

School of Chemical Engineering, Hanoi University of Science and Technology, 1 Dai Co Viet,

Hanoi, 10000, Vietnam 2 3 4 *

PhamVanDong University, 968 Quang Trung, QuangNgai, Vietnam University of Tasmania, Churchill Ave, Hobart TAS 7005, Australia Industrial University of Ho Chi Minh City, Thanh Hoa Campus, Thanh Hoa, Vietnam Corresponding Author E-mail: [email protected]



This article has been accepted for publication and undergone full peer review but has not been through the copyediting, typesetting, pagination and proofreading process, which may lead to differences between this version and the Version of Record. Please cite this article as doi: [10.1002/cjce.23155]

Received 12 July 2017; Revised 19 October 2017; Accepted 20 October 2017 The Canadian Journal of Chemical Engineering This article is protected by copyright. All rights reserved DOI 10.1002/cjce.23155

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ABSTRACT This article systematically presented analysis results of factors affecting the preparation of nanoZIF-8 in methanol for high yield. Samples were characterized by X-ray diffraction (XRD), scanning electron microscopy (SEM), transmission electron microscopy (TEM), Fourier transform infrared spectroscopy (FTIR), Brunauer-Emmett-Teller (BET) adsorption, and thermogravimetry-differential thermal analysis (TG/DTA). Synthesized nano-ZIF-8 had advantages over commercial ZIF-8 (Basolite® Z1200 from SigmaAldrich) such as a higher surface area, consisting of not only micropores like Basolite® Z1200 but also subordinate mesopores, formed by an assembly of nano-ZIF-8 crystals, which was 30 nm. Specifically, for the first time, nano-ZIF-8 was prepared in methanol with the yield of 61.2 %. This article is protected by copyright. All rights reserved KEYWORDS: zeolitic imidazolate framework-8, nanocrystals, synthesis, characterization

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INTRODUCTION ZIFs are material family owning organic-metallic frameworks (MOFs). They are a new material family that has a crystalline structure possessing unique properties of both zeolite and MOFs, with uniform micropores and a high surface area.[1–4] ZIFs have zeolite-like structural connections in which divalent metallic cations bond with an imidazolate anion in tetrahedra.[5] Thanks to their chemical and hydrothermal stability as well as high porosity,[4,6] ZIFs have become intriguing in recent years,[2] particularly for application in gas storage and separation,[7– 17]

catalysis,[18–26] chemical sensors,[27–30] and drug delivery.[31]

ZIF-8 is one of the most frequently studied ZIFs, because it possesses a microporous structure with 11.6 Å in width linking through small 3.4 Å windows.[32–34] Crystallization of ZIF-8 is similar to that of zeolite, which undergoes stages of formation of saturated liquid, nucleation, and growth to form complete and stable ZIF-8 crystals.[35–38] Following this process, in the first and last stages of crystallization, liquid in gel transforms from a stable form to a metastable one and finally an unstable form. Owning a weak bonding energy, unstable forms tend to assemble to a structural unit inside the crystal. In the nucleation stage, there is separation of a heterogeneous part from the saturated solution. This is considered to be the determining stage of properties, as well as the structure of the finished product. Next, molecules in the solution continue to precipitate on existed nucleation sites to form crystals. The crystals grow in a specific direction depending on different natural gel precursors. Synthesis of ZIF-8 with particle size and crystalline morphology control[5,38–40] is opening application opportunities in catalysis,[5,38,41–43] gas adsorption and separation,[8,11,44–55] as well as layered material synthesis.[5,56] There have been ZIF-8 synthesis methods, like electrospinning, ultrasound,[38,58-61] the selftemplate strategy, microwave,[61,64–66] mechanochemical,[61,67] dry-gel conversion,[61,68] or solvothermal, that were commonly studied.[2,5,29,22,32,35,37–40,42–44,47–49,51,53,54,61,69–91] In the thermal solvent method, zinc salt and 2-methylimidazole (Hmim) were dissolved in different mole ratios in different solvents and were carried out in different conditions of temperature and time. Zinc salt could be Zn(NO3)2.6H2O,[5–6,22,37,39,40,42,47–49,51,61,70,71,74–79,82–85,88– 97]

Zn(NO3)2.4H2O,[2,38,72,73] Zn(CH3COO)2.2H2O,[55,64] ZnCl2,[46,53,69,80,86,97] or Zn(OH)2.[32,44]

The zinc salt to Hmim molar ratio (Zn:Hmim) is often taken excessively (1:4–1:6),[5,38,4749,51,54,71,75,77,78,80,83–85,88,89,91,92,97]

and in some studies, the Hmim seemed to be extremely high

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(1:56–1:70).[37,70,74]. In several cases, Zinc is residual,[2,22,42,46,61,73,76,81,82,86] but there were few studies actually using the proper reaction ratio.[40,41,72,79,89,91,92] Common solvents used for ZIF-8 preparation were dimethylformamide (DMF),[2,6,22,35,42,61,72,73,76,81,82,84, 91] methanol,[5,38,46–49,51,54,61,71,72,75,78,79,85,88–90,92,97] methanol and NH4OH,[44] methanol and HCOONa,[69,80,86,95,97] water,[5,55,64,74,77,83,93,96] water and surfactant,[5,51,70,73] or a series of solvents such as ethanol, n-propanol, 2-propanol, n-butanol, 2butanol, n-octanol, and acetone.[75] Generally, ZIF-8 synthesized in a less toxic solvent usually employed a largely excess amount of Hmim. Because it is a dilute reacting solution, a comparatively low yield was gained. The surfactant worked as a control agent of crystallite size (from 100–4000 nm) and morphology (from hexagon to rhombic dodecahedron).[37,70] Reactions were often carried out above 100 oC under self-generated pressure or under 100 oC with stirring. Reaction time was mostly in the range of 4–6 h or from 1–3 d.[2,22,40,42,46,61,71,72,74,76,79,81,82,85,86,88,95,98] Notably, nano ZIF-8 crystals synthesized by thermal solvent (≤ 100 nm)[5,47,54,71-73,75,77,83,85] had surface areas of 962–1816 m2/g and 1173–1720 m2/g according to BET and Langmuir, respectively, which were lower than micro ZIF-8 crystals and they always underwent strict conditions of equipment, reaction mode, and also post-crystalline product handling. Published in 2016, the highest BET surface area ZIF-8, prepared from Zn(NO3)2.6H2O, excess Hmim, and methanol at room temperature over 8 h, which accounted for 1971 m2/g,[99] was sized in micrometres. Most synthesized ZIF-8 was microporous because its adsorption-desorption isotherms belonged to type I without a hysterysis loop. There were a few publications on mesoporous ZIF-8. Besides, the former nano-ZIF-8 exhibited a low thermal stability of 250–350 oC when being calcined in atmosphere.[72,75,77,96,98,99] In addition, they were prepared in a small amount,[37–39] with low yield.[2,38,47,75] Although it has been 10 years since the first ZIF-8, published by Yaghi et al.,[2] there has been no systematic research about factors affecting ZIF-8 synthesis in general and nano-ZIF-8 in particular. Most studies have been concerned with discrete factors like zinc source,[39,98,100] solvent source,[5,75] solvent content,[5,89] Hmim content,[40,91,92] preparation time,[40,74,89] preparation temperature,[5,74,98] and product drying temperature.[47,96]

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Therefore, this study systematically indicates results of factors affecting the preparation of nanoZIF-8 for high yield and thermal stability in normal conditions. The formed Nano-ZIF-8 possessed a high surface area and high thermal stability, and its pore distribution contained both micro and mesopores. EXPERIMENTAL Materials In this work, Zn(NO3)2.6H2O (ZnN, 98 %), Zn(CH3COO)2.2H2O (ZnA, 98 %), ZnCl2 (ZnC, 98 %), and 2-methylimidazole (Hmin, 99 %) from Sigma-Aldrich, and solvents from Merck, methanol (MeOH, 99.8 %), ethanol (EtOH, 99.5 %), n-propanol (n-Pro, 99.5 %), i-propanol (iPro, 99.5 %), were used as received without further purification. Double distilled water was used where necessary. Commercial ZIF-8 as Basolite® Z1200 from Sigma-Aldrich was used for comparing with the best synthesized sample. Research on Factors Affecting Synthesis of ZIF-8 Zinc precursor was first dissolved into solvent (solution A), and Hmim into solvent (solution B); next, solution A was poured into B. The mixtures were then stirred or non-stirred at 20–150 oC and 6–30 h for the crystallization of ZIF-8. After the reaction, ZIF-8 powders were collected by repeated centrifugation (at 5000 rpm for 20 min) and washed by methanol three times, followed by vacuum drying at 70–180 oC overnight. The aluminum and solvent sources, molar composition of components (such as ZnA, MeOH, and Hmim), as well as the conditions of reaction temperature, time, drying time, stirring, or non-stirring on each sample were illustrated in Table 1. To examine nano-ZIF-8’s durability, a sample was calcined from 400–700oC in air with heating rate of 5 oC/min. To investigate repetition of the procedure, a sample was prepared with the zinc salt mass increased by 40 times in initial reaction mixtures. Characterization XRD analyses were carried out at room temperature in θ–2θ reflection mode using a D8 Advance-Bruker diffractometer. SEM and TEM images were obtained on an S-4800 and JEM 1010, respectively. Specific surface area determinations (BET) and pore size distributions were measured on a Micromeritics Gemini VII 2390. FTIR was performed using a Nicolet impact

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FTIR 410 Spectrometer. DTA/TGA was performed with a NETZSCH STA 409 PC/PG. Crystallinity was calculated according to Nguyen et al.,[101] crystallite size was determined from the Scherrer equation,[102] and ZIF-8 yield was based on the equation reported by Eugenia et al.[75]

RESULTS AND DISCUSSION Effect of Different Zn Sources Figure 1 revealed the XRD results of samples derived from distinct zinc salts. The XRD pattern of Z24-ZnN (Figure 1b) prepared from Zn(NO3)2.6H2O showed a flat baseline without unusual phases, with only one appropriate to the crystalline structure of ZIF-8. Meanwhile, XRD patterns of Z24-ZnC (Figure 1a) and Z24-ZnA (Figure 1c) possessed abnormal phases (*) beside the main one, attributed to the dense structure of Zn, which is similar to reports.[40,68] Thus, different salts had considerable impact on the formation of ZIF-8. Sample Z24-ZnN crystallized best, with no Zn phase included because it was prepared from Zn(NO3)2.6H2O, which was much more soluble in water than ZnCl2 or Zn(CH3COO)2.2H2O, solubility in water at 20 oC is 184.3 compared to 4.3 and 0.31 mg/l, respectively.[103,104] The XRD pattern also showed that Z24-ZnN owned the largest specific peak width, proving that the sample had the smallest particle size, and this is consistent with the literature.[39] Therefore, among the employed salts, Zn(NO3)2.6H2O was the best one for ZIF-8 synthesis.

Effect of Organic Solvent Organic solvent plays a significant role in the synthesis of ZIF-8. In this study, various solvents were employed in order to prepare ZIF-8 in the same condition including H2O, MeOH, EtOH, nPro, and i-Pro. XRD patterns, TEM, and SEM images were displayed in Figures 2, 3. and 4. Results of the characterizations were summarized in Table 2, and the product yields were calculated according to the equation[75] Y = M1:M2, %, where Y is the yield of ZIF-8 (%), M1 is the mass of Zn in synthesized ZIF-8 (g), and M2 is the mass of Zn employed in Zn(NO3)2.6H2O (g).

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As could be observed in Figure2, samples crystallizing in alcohol solvents all indicated high crystallinity. Z24-Wat (Figure 2a) crystallizing in water did not form ZIF-8. The appearance of abnormal peaks in Z24-Wat, which was not owing to Zn(OH)2, ZnO, or Zn, might be attributed to a zinc hydrate complex accompanying unknown phases, as in discussed by Chen et al., Q. Shi et al., and K. Kida et al.[40,68,105]

However, differences in crystallinity and particle size were illustrated in Table 2. Materials formed in the solvents MeOH and EtOH possessed the highest crystallinity and the smallest particle size. Nevertheless, all 4 samples prepared in the alcohol solvent had crystals in the nanometre scale, similar to results reported by Eugenia et al.[75] Comparing the crystallinity and particle size according to XRD, TEM, and SEM, all the samples had a reliable linearity. Among them, sample Z24-Met prepared in the MeOH solvent always demonstrated the best results. The SEM and TEM images displayed explicit and uniform crystals (Figure 3). The solvent dissolved zinc salt and Hmim, and dissociated H+ from Hmim to form Mim-. The Zn2+ cations combined with Mim- to form a polymer, and then ZIF-8. The four investigated alcohol solvents were all protic solvents (can dissolve anions) with polarities of MeOH > EtOH > n-Pro > i-Pro (1.70, 1.69, 1.68, and 1.66 D, respectively).[106] Clearly, the higher the polarity of the solvent, the easier H+ dissociated from Hmim, resulting in a higher ZIF-8 yield. This statement was consistent to the actual yields obtained, as shown in Table 2, which is arranged in the order of polarity.

In terms of the yield based on Zn, the sample prepared in the MeOH solvent was also the highest one at 52.4 %, and it was higher than former reports (36.6,[75] 50,[84] and 45–50 %[91]). These results lead to a conclusion: the MeOH solvent was the most appropriate one among the employed solvents and with the experimental conditions.

According to the literature,[107] the solvent could cause thermodynamic and kinetic effects in the formation of MOFs and could work as a ligand, reaction medium, both a ligand and reaction This article is protected by copyright. All rights reserved

 

medium, or a structure-directing agent (SDA). These effects could be changed by adding different aromatic compounds to the reacting mixtures.[108] Park et al.[2] suggested that structure was formed not only by using a suitable linker, but also by the support of SDA as an amide-like solvent. Tian et al.[109] also claimed that several organic solvents could work as an SDA or as a primary SDA in the synthesis process of imidazolates material, which had similar structure to zeolites. The effect of primary SDA occurred when the solvent acted as a nucleating agent and encapsulated the framework by different valence interactions.[110] Apart from that, the effect of an SDA occurred while contributing to the growth of MOF’s crystal. Based on the aforementioned discussions and results, ZIF-8 could be formed in distinct alcohol solvents with differences in crystallinity, particle size, and product yield. Therefore, the solvent also participated in the synthesis process of ZIF-8 as an SDA, similar to reports in the literature.[75] Effect of Zn(NO3)2.6H2O Content

As Figure 4 described, different salt contents possessed different XRD patterns. According to that, the result of sample Z24-Z0.5 (Figure 4a), which had a Zn:Hmim molar ratio of0.5:4, with excess Hmim (4 times excess by the molecular composition of ZIF-8), had almost no ZIF-8 phase, but only formed the dense structure of Zn (*).[40,68] The XRD patterns of sample Z24-Z1.5 (Figure 4c), with a Zn:Hmim ratio of 1.5:4 and the least amount of excess Hmim, showed a strong phase of ZIF-8, but there were still weak peaks of Zn (*). Figure 4b, which was the XRD pattern of sample Z24-Z1.0 with Zn:Hmim of 1:4, displayed a peak series of ZIF-8 only. It was evident that the molar ratio of Zn(NO3)2.6H2O to Hmim in mmol equalled 1:4 and was appropriate for synthesizing ZIF-8 with high crystallinity without abnormal phases.

Effect of Methanol Solvent Content MeOH was used with various content to prepare ZIF-8. XRD patterns, TEM, SEM images, and structural characterizations were summarized in Figures 5 and6, and Table 3.

As can be observed in Figure 5, while sample Z24-Met10 with a Zn:Hmim:MeOH ratio of 1:4:20 (Figure 5a), and sample Z24-Met20 with a ratio of 1:4:30 (Figure 5b) had only a single

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crystalline phase of ZIF-8, sample Z24-Met30 with a corresponding ratio of 1:4:30 (Figure 5c) possessed weak peaks of the Zn crystal (*) beside strong ones of ZIF-8. Accordingly, the low solvent content (Figure 5a) produced pure ZIF-8 with a large particle size (narrow peak width) similar to the results reported by Zhao et al. and Polyzoidis et al.,[5,89] while high solvent content (Figure 5c) resulted in incomplete crystallization. Proper solvent content based on the reacting composition of Zn:Hmim:MeOH was 1:4:20 (mmol:mmol:ml).

Both of the SEM and TEM images (Figure 6) proved that the sample prepared with a ratio of 1:4:20 indicated explicit and uniform crystals. The ZIF-8 morphology formed characteristically hexagonal. Statistics in Table 3 demonstrated the aforementioned discussions. ZIF-8 yield based on Zn was 50.8 % for a sample corresponding to a ratio of 1:4:20, which was the highest one in the experimental conditions.

Effect of Hmim Content Hmim was one of three reacting components and was the agent directly linking with Zn to form ZIF-8. As a result, Hmim content in the reacting solution was one of the deciding factors in the formation of ZIF-8.

In this study, the effect of Hmim content on the prepared sample was apparent, as shown by the XRD measurement. In Figure 7, the XRD patterns of Z24-Hmim4 with a Zn:Hmim ratio of 1:4 (Figure 7b), demonstrated the best result with a flat baseline and no abnormal phase except the ZIF-8, while thr Z24-Hmim2 sample (Figure 7a) showed only ZnO peaks and an undulating baseline. It can be concluded that despite the Zn:Hmim ratio of 1:2, which was the precise ZIF-8 formation ratio (formula: Zn(Mim)2), it did not produce ZIF-8. Sample Z24-Hmim6 (Figure 7c), with Zn:Hmim ratio of 1:6, presented a ZIF-8 peak at low intensity; other abnormal peaks are unidentified as discussed in the Effect of Organic Solvent section, along with the amorphous background. Generally, in the experimental conditions, Hmim content in solution had a substantial impact on the formation of ZIF-8. Accordingly, ZIF-8 was only efficiently produced if Zn:Hmim mmol ratio was 1:4. This was an essential result when most of the reports on single crystalline ZIF-8

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synthesis had a high (1:4–1:16) or even extremely high (1:56 to 1:70) Zn:Hmim ratio, as mentioned in the Introduction.

Effect of Stirring in the Crystallization Stage In this study, the effect of stirring was investigated. Figures 8 and 9 described the XRD patterns, and TEM and SEM images of samples prepared in the same condition with either stirring or nonstirring. Accordingly, both samples were well crystallized (Figure 8) with a crystallinity of 100 % (Table 4). However, the stirred sample had bigger peak width than the non-stirred one and the crystallite sizes from Scherrer’s equation were 31 and 32 nm respectively.

In Figure 9, the crystals were apparently uniform. Their corresponding particle sizes prepared by stirring and non-stirring are 38 and 50 nm (TEM), 35 and 58 nm (SEM), as seen in Table 4. Among the three methods determining the medium size of ZIF-8 crystals, the non- stirred sample Z6-Not illustrated close values, proving that this sample has a uniform particle size. Thus, stirring resulted in narrow particle distribution in contrast to wide distribution obtained by nonstirring. This tendency could be attributed to secondary nucleation caused by the stirring chaos, as mentioned by Janosch et al. and Mullin et al.[71,112] Product yield based on acquired Zn from the corresponding non-stirring and stirring cases were 58.2 and 54.7 %. In general, ZIF-8 synthesis without stirring presented higher yield, and smaller and more uniform particle size. Effect of Crystallization Time Crystallization time was another important factor affecting the formation of ZIF-8. XRD patterns, and TEM and SEM images of non-stirred samples crystallizing from 6–30 h were demonstrated in Figures 10 and 11. Table 5 summarized statistics obtained from the aforementioned methods and calculated product yield based on Zn. As could be seen in Figure 10, samples were crystallized constantly with only the crystalline phase of ZIF-8, and there was no significant difference between the XRD patterns of sample Z6Not and Z12-Not. When the crystallization time increased to 18, 24, and 30 h, characteristic peak intensity had a downward trend accompanied by the narrowing of peak width and the baseline tended to be higher. Hence, the crystallinity determined by Scherrer’s equation decreased by This article is protected by copyright. All rights reserved

 

prolonging the crystallization time and it fell sharply after 30 h of crystallization (Table 5). In contrast, the crystallite size calculated by XRD tended to grow. As could be seen in the TEM and SEM images (Figure 11), crystals were formed evenly and were bigger in size as the crystallization time increased, as reported by Polyzoidis et al.[89] Product yield based on Zn decreased from 58.2 % for a sample crystallizing in 6 h, to 42.7 % for a sample crystallizing in 30 h. Among the 5 samples crystallizing in different times, sample Z6Not crystallizing in 6 h was the best one with 100 % crystallinity, as well as the smallest size and the highest yield. When the Zn:Hmim mmolar ratio was 1:4, with a crystallization time of 6 h in 50 oC with nonstirring, the sample possessed a very high yield compared to the result reported by Yao et al.,[74] which had a corresponding mmolar ratio of 1:70, 24 h of crystallization, 50 oC with stirring.

Effect of Crystallization Temperature Figures 12 and 13 revealed the XRD patterns and TEM and SEM images of samples prepared in a crystallization temperature varying from 20–150 oC. In Figure 16, the XRD patterns proved that all of the samples have good crystallinity, a single phase, and a large characteristic peak. The crystallinity was 100 % for crystallizing at 20 and 50 oC, and 98 % for that at 90, 120, and 150 o

C. The crystallite sizes from Scherrer’s equation were small at around 32 nm.

In Figure 13, the TEM and SEM images of the 5 corresponding samples presented uniform crystals in the nanometre scale. The statistics summarized in Table 6 indicated that crystallite size was minimal when crystallizing at 50 oC. Nevertheless, crystallinity and product yield were maximal at this temperature. Comparing the XRD, TEM, and SEM, crystallite sizes obtained from sample Z6-Cry50 were very close to each other, demonstrating that the sample had the most uniform crystals. In the investigated temperature range, the sample crystallizing at 50 oC was the optimal one, consistent with the report by Yao et al.,[74] and was also better at forming uniform nanocrystals instead of micro ones. Results from this study also lead to an outcome: low crystallization temperatures tended to result in smaller crystals, similar to the report of Wang et al.[98] on microcrystals and Zhao et al.[5] on nanocrystals. It could be explained that at a low temperature, nucleation occurred faster, which increased the concentration of nanoparticles, leading to smaller crystals. Therefore, the

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nucleation rate or crystallite size depended on temperature, as reported recently by Chih-Wei et al.[38] Product yield in MeOH for sample Z6-Cry50 was 61.2 % based on Zn, and can be considered the highest ever, far exceeding recent reports which had high yields of 36.6 %,[75] 40 %,[47] and 35– 46 %[38] when also prepared in MeOH. Effect of Product Drying Temperature Drying temperature after synthesis was the factor affecting morphology and shape of MOFs. Its purpose was to vapourize water and solvent adsorbed on the surface and in pores, but this process must be carried out in order to leave the structure intact. The research of Song et al.[47] suggested that vacuum drying ZIF-8 at 60 and 230 oC in 18 h could lead to different results: the BET surface of the sample dried at 60 oC was 1645 m2/g, but that of the sample dried at 230 oC was only 1353 m2/g, but the structure might partly collapse from heat impact. Another report by Yin et al.[96] investigated air drying ZIF-8 at 65 oC in 12 h and 100 oC in 3 h. XRD patterns illustrated that the former had a lower characteristic peak intensity than the latter. It was evident that the drying temperature has a significant effect on the material structure.

In this study, the investigated vacuum drying temperatures were 70, 120, 150, and 180 oC. The corresponding XRD, TEM, and SEM results are displayed in Figures 14 and 15. Table 7 lists results from the characterization method and yield based on Zn. As could be observed in Figure 14, the XRD intensity remained constant with a drying temperature ≤ 120 oC, but when the drying temperature was ≥ 150 oC, characteristic peak intensity reduced, alongside a higher baseline, which was linear with the crystallinity calculated and summarized in Table 7. Notably, the characteristic full width at half the maximum of sample Z24-Dry120 (Figure 14b) was the largest, which means that it had the smallest crystallite size. TEM and SEM images (Figure 15) revealed that drying at a temperature ≥ 150oC, crystals tended to clump together rather than be discrete, as they were when drying at a temperature ≤ 120 oC. In particular, the average crystallite size, as measured by XRD, TEM, and SEM, all hit the bottom for sample Z24-Dry120. Its approximately similar size proved that crystals in this sample are very uniform.

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Z24-Dry120’s ZIF-8 yield also was the highest. Therefore, the proper drying temperature was 120 oC in vacuum.

Characterization of Nano-ZIF-8 Prepared in Optimal Conditions The XRD patterns of synthesized nano-ZIF-8 (Nano-ZIF-8) and commercial ZIF-8 (Basolite® Z1200) taken in the same conditions are displayed in Figure 16. It can be seen that they shared similar diffraction peak shapes in common with neither abnormal nor amorphous phases. Generally, their structures were the same and pure.

However, it was apparent from the height and width of 3 peaks at planes (011), (002), and (112) that the Nano-ZIF-8 peaks were shorter and not as sharp as that of Basolite® Z1200. It could be attributed to the much smaller crystallite size of Nano-ZIF-8 compared to Basolite® Z1200. Crystallite size was calculated by Scherrer’s equation:[102] D = Kλ/βcosθ where D is the particle diameter, K is the shape parameter (K = 0,94[38]), λ is the wavelength of the X-ray (λCu = 1542 Å), β is the full width at half maximum, and θ is Bragg’s diffraction angle. According to that, the FHWM of 3 peaks, representing for diffraction planes (011), (002), and (112) determined from XRD pattern, were employed to calculate the average ZIF-8 crystallite size. The two values for Nano-ZIF-8 and Basolite® Z1200 were 30 and 4760 nm, respectively (Table 8). Similarly, their relative crystallinities were also measured from XRD by equation[101] C = S1/S where C is the relative crystallinity of ZIF-8, S1 is the aggregate peak area of ZIF-8, and S is the aggregate peak area of all peaks in the sample. The relative crystallinity is 100 % once again, confirming that both the Nano-ZIF-8 and Basolite® Z1200 were pure. The size and morphology of the crystals were also examined by the TEM and SEM methods (Figure 17). These methods revealed the sharp difference between the nanometre scale of NanoZIF-8 and the micrometre scale of Basolite® Z1200. The corresponding results were 32 and This article is protected by copyright. All rights reserved

 

4920 nm (TEM), and 32 and 5105 nm (SEM). They were all bigger than that calculated from Scherrer’s equation (30 and 4760 nm, in Table 8). All of the methods possessed small errors, but proved good analytical conditions and confirmed that the crystals were formed evenly. Their crystal morphology was all rhombic dodecahedra. This was a common type for ZIF-8, as reported by Pan et al.[113] However, nanocrystals[5,47,73,96] did not have as high a resolution as microcrystals.[37,39,51,93] Figure 18 revealed the FTIR spectra of Nano-ZIF-8 and Basolite® Z1200 in the same measuring conditions. The shape of the spectra were absolutely similar, confirming the corresponding chemical bonds as reported in the literature.[2,35,37,39,75,114] Figure 18 showed that there was no wave number at 1850 cm–1 in the spectra of the 2 investigated samples, confirming the formation of imidazolate.[75] The band at 1585 cm–1 corresponds to the C = N double bond.[39,73,93] The bands found in the range of 1350–1500 cm–1 are assigned to a cyclic bond in ZIF-8.[37,39] The replacement of aliphatic C-H at 1117 cm–1[115] by 1145 cm–1 was ascribed to the conversion of imidazole into imidazolate.[75,115] Bands appearing at ~752 cm–1 were characteristic for vibrations outsides the cyclic bond (framework),[37,75] and bands showing up at 421 cm–1 correspond to presence of the Zn-N bond.[37,39,75,93,114] The emergence of a band at 421 cm–1 is evidence that Zn2+ cations bonded with a nitrogen atom in methylimidazole to form imidazolate.[37,39,75,93,114] In Figure 19, the N2 adsorption and desorption isotherm of Nano-ZIF-8 and Basolite® Z1200 demonstrated both similarity and difference. On the one hand, both of samples adsorbed a high nitrogen amount at a very low p/po proving that both of their surface areas were considerably high. On the other hand, while Basolite® Z1200 showed nearly no hysteresis loop characterized by microporous material with a type I isotherm, Nano-ZIF-8 presented one in p/po = 0.82–0.98, with a type IV isotherm and H1 hysteresis loop.[116] Its appearance was caused by the condensation of N2 molecules inside mesopores. Its shape demonstrated that the pores were cylindrical. In terms of the Basolite® Z1200, the adsorption as well as the desorption branch were identical in p/po = 0.97–0.99, which meant there was no condensation of nitrogen in the macropores. BJH pore size distribution (small figure in Figure 19) confirmed the aforementioned discussion. Nano-ZIF-8 contained mesopores concentrating at ~3.8 nm, while Basolite® Z1200 was comprised of macropores converging at ~78.2 nm. The BET and Langmuir surfaces were 1570

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and 1877 m2/g (for Nano-ZIF-8) and were 1215 and 1509 m2/g (for Basolite® Z1200), respectively. In addition, the microporous surface area (Smicro) and external surface area (Sexter) were 1453 and 117 m2/g (for Nano-ZIF-8) and 1148 and 67 m2/g (for Basolite® Z1200), respectively. Cumulative pore volumes measured from t-pot were 0.709 cm3/g (for Nano-ZIF-8) and 0.543 cm3/g (for Basolite® Z1200), as shown in Table 8.

Generally, Nano-ZIF-8’s surface area was 29.2 % larger than Basolite® Z120’s. The former’s cumulative pore volume was also 30.5 % higher than that of the latter’s. It is notable that the external surface was 74.6 % larger since crystallite size sharply decreases (30 nm comparing to 4760 nm in Scherrer’s equation). The small crystallite size and high porosity were considered to be significant in developing the adsorption and catalysis benefits, and particularly in accelerating the dynamic of mass transfer.[71] The synthesized material contained mesopores so it could broaden applications in adsorption and catalysis to bigger molecules, which previous iterations of ZIF-8 could not handle. So far, there have been few reports meticulously analyzing both the isotherm and pore size distribution of ZIF-8. Published papers on Micro ZiF-8 with the type I isotherm are relatively common.[37,39,92,93,98,99,117-119] However, there have been fewer studies of Nano ZIF-8 with the type I isotherm,[5,73,77,92,103] and almost none on Nano ZIF-8 with the type IV isotherm, and the H2[71,117] or H1[100] hysteresis loop. It could be concluded that most of the synthesized ZIF-8 up to now were microporous, even though the nano ones were not mesoporous due to no hysteresis loop.[5,73,77,92,103] The appearance of the hysteresis loop only occurred in Nano-ZIF-8 that had a very small size (18 nm,[71] 20 nm,[117] 47 nm,[100] and 32 nm (TEM) in this report). The surface areas in this report were 1570 m2/g (BET) and 1877 m2g (Langmuir), which were lower than recent reports (1617 - 1971 m2/g [48,71,98,99,118]), but still higher than reports updated from 2010 until the present (1047 - 1503 m2/g [5,37,39,47,73,77,92,93,100,117,119]). A DTA/TGA analysis of Nano-ZIF-8 and Basolite® Z1200 calcined in atmosphere with similar conditions was presented in Figure 20. In the TGA curve of Nano-ZIF-8, there were two mass losses:

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From room temperature to 565 oC, the mass decreased by 8.2 %. This was the

(i)

evaporation of adsorbed water and solvent.[5,73,87] (ii) From 565–800 oC, the mass decreased by 59.3 %. There were two effects peaking at 625 and 658 oC. This was associated with the decomposition of ZIF-8 to form ZnO.[37,39,96,99] The total mass loss accounted for 67.5 %. Similarly, there were two mass losses on the TGA curve of Basolite® Z1200: from room temperature to 350 oC, the mass reduced by 7.3 % and by 59.6 % from 350–550 oC. The total mass loss was 66.9 %. Because ZIF-8 is a MOF tjat has the same structure as zeolite, its thermal stability is higher than other MOFs. However, the aforementioned statistics demonstrated that this sample was highly thermal stable and it was also the first ZIF-8 to reach s stability of 565 oC calcining in atmosphere, which was much higher than micro-ZIF-8 (Basolite® Z1200) with a stability of 350 o

C in similar conditions, or than other reports so far.[5,37,39,71,82,96,98,99]

According to results on the thermal stability of ZIF-8, it could be analyzed based on two aspects: (1)

The sample calcined in atmosphere was less stable than that in N2. Reports[96]

showed that with the same ZIF-8 sample, it decomposed at 350 oC in air, but reached 500 o

C in N2.

(2)

The smaller the crystallite sizes were, the less stable they were. According to the

research,[5] ZIF-8, with a particle size of 18 nm, showed stability until 250 oC, while bigger ones (65 nm) attained 300 oC in the same condition. In order to investigate thermal stability, Nano-ZIF-8 was calcined in different temperatures: 450, 500, 550, 575, 600, 650, and 700 oC. Their corresponding XRD patterns are displayed in Figure 21. One of the main advantages of ZIF-8 was its ordered structure like zeolite, but its higher surface area and porosity than zeolite; however, its common disadvantage was its much lower thermal stability. Therefore, this study on ZIF-8 synthesis and post-synthesis treatment to form a highly thermal stable material was essential for widening the application of ZIF-8 in catalysis.

It was apparent from Figure 21 that at calcination temperatures ≤ 500 oC (Figure 21b–c), there was no change compared to that before calcination (Figure 21a). Decomposition of ZIF-8 started from 550 oC, and the structure was mostly degraded to ZnO at 650 oC. The structure of as This article is protected by copyright. All rights reserved

 

synthesized ZIF-8 in this study was completely destroyed from 700 oC, which was consistent with the DTA/TGA result in Figure 20. Synthesizing a huge amount of MOFs in general and ZIF-8 in particular has always been difficult. Most former reports carried it out with a small amount of Zn salts, mainly from 1.7–6 mmol,[37–39,48,71,77,92,98–100,118] particularly from 12–16.8 mmol.[42,47,51,73] To investigate the repetition of the Nano-ZIF-8 synthesis procedure, a sample with 20 times mass bigger (50 mmol Zn(NO3)2.6H2O) was prepared in similar conditions. The XRD pattern of Nano-ZIF-8 in the small amount and the large amount was almost identical. Therefore, the procedure was reliable. To examine the efficiency of ZIF-8 synthesis, in addition to the crystallinity, surface area, structural stability, we needed to consider pore size distribution, product yield, and sample mass for each synthesis process with reliable repetition. This report analyzed several notable points from the former reports: (i)

Samples owning the highest BET surface area so far of 1971 m2/g[98] and 1950

m2/g[48] were prepared in MeOH, but both of them were microporous and had low thermal stable (250 oC in atmosphere). (ii)

ZIF-8 only contained mesopores when the crystallite size was in the nanometre

scale,[71,100,117] but their thermal stability was still low (300–350 oC in atmosphere). (iii) The ZIF-8 yield based on Zn was 35–46 % in MeOH[38,47,75] and 45–50 % in the DMF solvent.[84,107] The highest published yield was 76–79 % for preparation in DES,[98] and 73 % in NH4OH.[39] However, the thermal stability was not improved (250–300oC in atmosphere). (iv) A sample amount for each preparation of ZIF-8 possessing a BET surface area ≥ 1600 m2/g[38,48,71,92,98,99,118] was 1.7–6 mmol Zn salt (approximately 0.5–1.5g), which was still low. To sum up, there was no ZIF-8 synthesis report meeting all the demands of crystallinity, surface area, thermal stability, and containing mesopores besides the characteristic micropores of traditional ZIF-8. Therefore, results from this study confirmed that as synthesized, ZIF-8 has three main advantages:

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(1)

The highest thermal stability (565 oC in atmosphere);

(2)

The highest yield in MeOH solvent (61.2 %) and a larger amount of synthesized

sample compared to the average level by 10–20 times (15 g for each process); (3)

Possessing mesopores with the type IV isotherm, H1 hysteresis loop, and the

highest BET surface area (1570 m2/g).

CONCLUSION Factors affecting the formation of ZIF-8 have been systematically investigated. According to that, all examined factors had their effects on the formation of ZIF-8. The study also indicated the emergence of ZIF-8 in an alcohol solvent in which the solvent worked as a structuredirecting agent. Simultaneously, the higher the polarization of the solvent, the more likely it was to produce ZIF-8 with high efficiency. New efficient Nano-ZIF-8 synthesis methods from Zn(NO3)2.6H2O and Hmim in MeOH were proposed. The reaction was carried out at 50 oC, non-stirring, in 6 h, with a Zn:Hmim:MeOH ratio of 1:4:20 (mmol:mmol:ml). This procedure showed a reliable repetition, with a huge synthesized amount. The fabricated product had a relative crystallinity of 100 %, BET surface area of 1570 m2/g, H1 type mesopores concentrating at 3.8 nm, and yield accounting for 61.2 %.

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Table 1. Composition and synthesis conditions of nano-ZIF-8 No

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

Code

Z24-ZnC Z24-ZnN Z24-ZnA Z24-Wat Z24-Met Z24-Eta Z24-nPro Z24-iPro Z24-Zn0.5 Z24-Zn1.0 Z24-Zn1.5 Z24-Met10 Z24-Met20 Z24-Met30 Z24-Hmim2 Z24-Hmim4 Z24-Hmim6 Z24-Dry70 Z24-Dry120 Z24-Dry150 Z24-Dry180 Z6-Stir Z6-Not Z6-Not Z12-Not Z18-Not Z24-Not Z30-Not Z6-Cry20 Z6-Cry50 Z6-Cry80 Z6-Cry120 Z6-Cry150

Zn

Solvent

Salt content

MeOH solvent

Hmim

source

source

(Zn:Hmim,

content

content

mmol:mmo

(Zn:Hmim:MeOH,

(Zn:Hmim,

l)

mmol:mmol:ml)

mmol:mmol)

1:4 1:4 1:4 1:4 1:4 1:4 1:4 1:4 0.5:4 1:4 1.5:4 1:4 1:4 1:4 1:4 1:4 1:4 1:4 1:4 1:4 1:4 1:4 1:4 1:4 1:4 1:4 1:4 1:4 1:4 1:4 1:4 1:4 1:4

1:4:20 1:4:20 1:4:20 1:4:20 1:4:20 1:4:20 1:4:20 1:4:20 1:4:20 1:4:20 1:4:20 1:4:10 1:4:20 1:4:30 1:4:20 1:4:20 1:4:20 1:4:20 1:4:20 1:4:20 1:4:20 1:4:20 1:4:20 1:4:20 1:4:20 1:4:20 1:4:20 1:4:20 1:4:20 1:4:20 1:4:20 1:4:20 1:4:20

1:4 1:4 1:4 1:4 1:4 1:4 1:4 1:4 1:4 1:4 1:4 1:4 1:4 1:4 1:2 1:4 1:6 1:4 1:4 1:4 1:4 1:4 1:4 1:4 1:4 1:4 1:4 1:4 1:4 1:4 1:4 1:4 1:4

ZnC ZnN ZnA ZnN ZnN ZnN ZnN ZnN ZnN ZnN ZnN ZnN ZnN ZnN ZnN ZnN ZnN ZnN ZnN ZnN ZnN ZnN ZnN ZnN ZnN ZnN ZnN ZnN ZnN ZnN ZnN ZnN ZnN

MeOH MeOH MeOH H2O MeOH EtOH n-Pro i-Pro MeOH MeOH MeOH MeOH MeOH MeOH MeOH MeOH MeOH MeOH MeOH MeOH MeOH MeOH MeOH MeOH MeOH MeOH MeOH MeOH MeOH MeOH MeOH MeOH MeOH

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Stirring

Crystallization

Crystallization

Drying

time, h

temperature

toC

oC

yes yes yes yes yes yes yes yes yes yes yes yes yes yes yes yes yes no no no no yes no no no no no no no no no no no

24 24 24 24 24 24 24 24 24 24 24 24 24 24 24 24 24 6 6 6 6 6 6 6 12 18 24 30 6 6 6 6 6

20 20 20 20 20 20 20 20 20 20 20 20 20 20 20 20 20 50 50 50 50 20 20 20 20 20 20 20 20 50 80 120 150

120 120 120 120 120 120 120 120 120 120 120 120 120 120 120 120 120 70 120 150 180 120 120 120 120 120 120 120 120 120 120 120 120

 

Table 2. Structural properties of the obtained ZIF-8 nanocrystals No.

Sample

Solvent

1 2 3 4 5

Z24-Wat Z24-Met Z24-Eta Z24-nPro Z24-iPro

H2O MeOH EtOH n-Pro i-Pro

Relative Crystallite Aggregate Aggregate crystallinity size from size from size from from XRD Scherrer’s TEM SEM (%) equation (nm) (nm) (nm) 0 100 32 58 59 82 38 68 67 80 40 76 77 80 50 88 90

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Product yield based on Zn (%) 52.4 51.6 49.1 48.5

 

Table 3. Structural properties of the obtained ZIF-8 nanocrystals No.

1 2 3

Sample

Solvent content, Relative Crystallite Zn:Hmim:MeOH crystallinity size from (mmol:mmol:ml) from XRD Scherrer’s (%) equation (nm) Z24-Met10 1:4:10 100 62 Z24-Met20 1:4:20 100 32 Z24-Met30 1:4:30 82 47

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Aggregate Aggregate Product size from size from yield TEM (nm) SEM (nm) based on Zn (%) 95 100 50.5 58 59 50.8 55 and 92 40 and 38.0 115

 

Table 4. Structural properties of the obtained ZIF-8 nanocrystals

No. Sample

1 2

Z6-Stir Z6-Not

Stirring

Yes No

Relative crystallinity from XRD (%) 100 100

Crystallite size from Scherrer’s equation (nm) 32 31

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Aggregate size from TEM (nm)

Aggregate size from SEM (nm)

Yield based on Zn (%)

50 38

58 35

54.7 58.2

 

Table 5. Structural properties of the obtained ZIF-8 nanocrystals

No.

Sample

Crystallization time (h)

1 2 3 4 5

Z6-Not Z12-Not Z18-Not Z24-Not Z30-Not

6 12 18 24 30

Relative crystallinity from XRD (%) 100 100 98 97 85

Crystallite size from Scherrer’s equation (nm) 31 31 32 34 48

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Aggregate Aggregate Product size from size from yield TEM (nm) SEM (nm) based on Zn (%) 38 35 58.2 58 52 54.4 58 54 52.3 58 67 50.8 100 102 42.7

 

Table 6. Structural properties of the obtained ZIF-8 nanocrystals No.

1 2 3 4 5

Sample

Crystallization Relative Crystallite size temperature crystallinity from (oC) from XRD Scherrer’s (%) equation (nm) Z6-Cry20 20 100 31 Z6-Cry50 50 100 30 Z6-Cry80 80 98 32 Z6-Cry120 120 98 32 Z6-Cry150 150 98 35

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Aggregate size from TEM (nm) 38 32 45 50 52

Aggregate Product size from yield SEM (nm) based on Zn (%) 35 58.2 32 61.2 43 55.7 48 55.1 52 54.4

 

Table 7. Structural properties of the obtained ZIF-8 nanocrystals No.

Sample

Drying temperature (oC)

Relative crystallinity from XRD (%)

Crystallite size from Scherrer’s equation (nm)

1 2 3 4

Z24-Dry70 Z24-Dry120 Z24-Dry150 Z24-Dry180

70 120 150 180

100 100 98 95

32 30 34 45

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Aggregate Aggregate Product size from size from yield TEM (nm) SEM (nm) based on Zn (%) 42 40 51.1 32 32 61.2 51 48 51.8 73 70 43.2

 

Table 8. Structural characterizations of the obtained ZIF-8 nanocrystals and Basolite® Z1200 Sample

SBET

SLangmuir Smicro

(m2/g) (m2/g)

No.

Sexter

Vpore

Pore size

Thermal Relative

Crystallite size (nm)

(m2/g) (m2/g) (cm3/g) distribution stability crystallinity XRD TEM (nm)

(oC)

SEM

from XRD (%)

1

Nano-

1570

1877

1453

117

0.709

ZIF-8 2

0.34; 1.16; 565

100

30

32

32

3.80

Basolite® 1215

1509

Z1200

(1300–

1148

67

0.543

0.34; 1.16; 350 78.20

1800)*

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100

4760 4920 (4900)*

5105

 

Figure 1: XRD patterns of samples prepared from different Zn salts: a) ZnCl2, b) Zn(NO3)2.6H2O, and c) Zn(CH3COO)2.2H2O. Figure 2. XRD patterns of samples prepared in different solvents: a) H2O, b) MeOH, c) EtOH, d) n-Pro, and e) i-Pro. Figure 3. TEM and SEM images of samples prepared in different solvents: a) MeOH, b) EtOH, c) n-Pro, d) i-Pro, 1) MeOH, 2) EtOH, 3) n-Pro, and 4) i-Pro. Figure 4. XRD patterns of samples with different salt contents: a) Zn:Hmim = 0.5:4, b) 1:4, and c) 1.5:4. Figure 5. XRD patterns of samples prepared in different solvent contents: Zn:Hmim:MeOH a) 1:4:10, b) 1:4:20, and c) 1:4:30. Figure 6. TEM and SEM images of samples prepared in different solvent contents: Zn:Hmim:MeOH a)1:4:10, b) 1:4:20, c) 1:4:30, 1) 1:4:10, 2) 1:4:20, and 3) 1:4:30. Figure 7. XRD patterns of samples prepared in different Hmim contents: Zn:Hmim a) 1:2, b) 1:4, and c) 1:6. Figure 8. XRD patterns of prepared samples: a) stirring, and b) non-stirring. Figure 9. TEM and SEM images of prepared samples: a) stirring, b) non-stirring, c) stirring, and d) non-stirring. Figure 10. XRD patterns of prepared samples in different crystallization times: a) 6, b) 12, c) 18, d) 24, and e) 30 h. Figure 11. TEM images of prepared samples in different crystallization times: a) 6, b) 12, c) 18, d) 24, e) 30, 1) 6, 2) 12, 3) 18, 4) 24, and 5) 30 h . Figure 12. XRD patterns of prepared samples at different crystallization temperatures: a) 20, b) 50, c) 80, d) 120, and e) 150 oC. Figure 13. SEM images of prepared samples at different crystallization temperatures: a) 20 , b) 50, c) 80, d) 120, e) 150, 1) 20, 2) 50, 3) 80, 4) 120, and 5) 150oC. Figure 14. XRD patterns of prepared samples at different drying temperatures: a) 70, b) 120, c) 150, and d) 180 oC. Figure 15. TEM and SEM images of prepared samples at different drying temperatures: a) 70, b) 120, c) 150, d) 180, 1) 70, 2) 120, 3) 150, and 4) 180 oC. Figure 16. XRD patterns of a) Nano-ZIF-8, and b) Basolite® Z1200.

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Figure 17. TEM and SEM images of a) Nano-ZIF-8, b) Basolite® Z1200, 1) Nano-ZIF-8, and 2) Basolite® Z1200. Figure 18. FTIR spectra of a) Nano-ZIF-8, and b) Basolite® Z1200. Figure 19. Nitrogen adsorption/desorption isotherms and pore size distribution of a) NanoZIF-8, and b) Basolite® Z1200. Figure 20. DTA/TGA curves of a) Nano-ZIF-8, and b) Basolite® Z1200. Figure 21. XRD patterns of a) Nano-ZIF-8, and b) Nano-ZIF-8 calcined in different temperatures: b) 450, c) 500, d) 550, e) 575, f) 600, g) 650, and h) 700 oC.

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Figure 1

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Figure 2

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Figure 3

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Figure 4

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Figure 6

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Figure 7

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Figure 8

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Figure 9

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Figure 10

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Figure 11

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Figure 12

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Figure 14

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Figure 15

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Figure 16

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Figure 17

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Figure 18

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Figure 19

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Figure 20

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Figure 21

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