Efficient Mechanochemical Synthesis of MOF-5 for Linear Alkanes

Jun 21, 2017 - An efficient mechanochemical method was proposed to synthesize MOF-5 with high BET area within minutes. The effects of parameters such ...
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Efficient Mechanochemical Synthesis of MOF‑5 for Linear Alkanes Adsorption Daofei Lv,† Yongwei Chen,† Yujie Li,† Renfeng Shi,† Houxiao Wu,† Xuejiao Sun,*,‡ Jing Xiao,† Hongxia Xi,† Qibin Xia,*,† and Zhong Li† †

School of Chemistry and Chemical Engineering, South China University of Technology, Guangzhou 510640, PR China School of Chemical Engineering and Materials Science, Quanzhou Normal University, Quanzhou 362000, PR China

‡

S Supporting Information *

ABSTRACT: An efficient mechanochemical method was proposed to synthesize MOF-5 with high BET area within minutes. The effects of parameters such as solvents activation, the metal/ ligand ratio, grinding speed and time were carefully studied and the optimized MOF-5-B was used to investigate its adsorption properties of linear alkanes (C1−nC7). The results showed that solvents activation played an important role in the formation of MOF-5. Besides, the Zn2+/BDC ratio had a great impact on the formation of crystalline MOF-5, and the appropriate Zn2+/BDC ratio was 3:1 for mechanochemical synthesis of MOF-5. Grinding for 60 min could lead to a better crystallinity and the highest surface area of MOF-5-B. The resulting MOF-5-B possessed BET area of 3465.9 m2·g−1. More importantly, MOF-5-B showed a preferential adsorption for long alkanes over short alkanes at low pressures. The saturated adsorption capacities of nC4−nC7 decreased with the increase of hydrocarbon chain length. The isosteric heats of C1−nC7 increased with the increase of the alkyl chain length. Furthermore, the adsorption capacities of the alkanes (C3−nC7) on MOF-5-B were much higher than those of conventional activated carbons and zeolites.

1. INTRODUCTION Hydrocarbon mixtures are fractionated into different fractions on large scales for the preparation of eldings and chemical feedstocks.1 Among various types of hydrocarbons, linear alkanes (C1−nC7) have been widely used as eldings or feedstocks for commodity chemicals. To fully utilize these linear alkanes, it is essential to have high quality and purity of such basic chemicals. Therefore, the efficient separation of linear alkanes is of great significance in the petrochemical industries.2 Currently, adsorption is a cost-effective and highly efficient energy technique for alkanes separation.3,4 An adsorbent is the key of adsorption technology. Thus, extensive efforts have been made to develop new adsorbents with superior linear alkanes separation. Metal−organic frameworks (MOFs), as novel nanoporous functional materials, have attracted much attention for their potential application of linear alkanes separation due to their high specific area and pore volume, tunable pore structures and flexible surface functionality.5−8 For instance, MOF-5, also known as IRMOF-1, is an attractive candidate for the adsorption and separation of linear alkanes due to its high surface area and large pore volume. Krishna et al.9 simulated the adsorption and diffusion of linear alkanes C1−nC6 on MOF-5 and found that the adsorption capacities of C1−nC6 increased with increasing chain length. Sandler et al.10 also simulated the adsorption of C1−nC5 and reported that long alkanes were preferentially adsorbed at low pressures, while the reverse was © XXXX American Chemical Society

found at high pressures. However, there is still not any experimental study of alkane adsorption on MOF-5 to be reported. So far, most MOFs were generally synthesized by hydrothermal or solvothermal method, which not only took long reaction time but also used a large amount of organic solvent in the synthesis process, as shown in Table S1. Recently, mechanochemical method have attracted enormous interest worldwide and have been widely used to synthesize some MOFs, such as HKUST-1,11−13 MOF-14,14 ZIF-8,15 and UIO66.16 Mechanochemical synthesis could not only efficiently synthesize MOFs within short reaction time, but also need small amounts of solvents or even none solvent. Prochowicz et al.17 synthesized MOF-5 using liquid-assisted mechanochemical synthesis and found that the product presented relative low BET area and existed plenty of amidate analogue byproduct. Up to now, an efficient mechanochemical synthesis to prepare MOF-5 with high BET area has not been reported yet. Herein, we successfully provided a mechanochemical method to efficiently synthesize MOF-5. The effects of parameters, such as solvent activation, the metal/ligand ratio, grinding speed and time on the structure, and porosity of MOF-5, were carefully studied. Then, the MOF-5 synthesized under optimized Received: January 17, 2017 Accepted: June 7, 2017

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alkanes (nC5−nC7) adsorption. The vapor generation system included a stainless steel chamber with a hard seal, manual cutoff valve to be attached in place of the Psat tube, and a heating mantle to control the temperature of the chamber at an operator-specified temperature between ambient and 316 K. The constant adsorption temperature was achieved by putting sample cell into circulating water bath. In each case, 60 mg of sample degassed at 423 K for 10 h before each run.

condition was further characterized and its adsorption performance of linear alkanes (C1−nC7) was investigated. Besides, the isosteric heats of linear alkanes adsorption on MOF-5 were also calculated.

2. EXPERIMENTAL SECTION 2.1. Materials. Zinc acetate dihydrate [Zn(OAc)2·2H2O, 99.0%, A.R.] was purchased from Shanghai Xinbao Chemical Co., Ltd. (China). Terephthalic acid (H2BDC, >99.0%, A.R.) was purchased from Alfa Chemicals. N,N-dimethylformamide (DMF, 99.8%, A.R.) and chloroform (CHCl3, 99.0%, A.R.) were purchased from Guangdong Guanghua Sci-Tech Co. Ltd. (China). Methane (C1, 99.99%), ethane (C2, 99.99%), propane (C3, 99.95%), and n-butane (nC4, 99.99%) were purchased Guangzhou Zhuo Zheng Gas Co. Ltd. (China). n-Pentane (nC5, 99%, A.R.), n-hexane (nC6, 99%, A.R.), and n-heptane (nC7, 99%, A.R.) were obtained from J&K Scientific Ltd. (China). 2.2. Synthesis of MOF-5. The MOF-5 samples were prepared in a QM-3C high speed vibrating ball mill (Nanning NanDa Instrument Plant) via an efficient room-temperature mechanochemical synthesis. First, Zn(OAc)2·2H2O (1.97 g, 9.00 mmol) and H2BDC (0.50 g, 3.00 mmol) were added in an 80 mL stainless steel jar with four 10 mm diameter stainless steel balls. Second, the mixtures were ground for the required time and grinding speed. Then, the products were activated with solvents, to be exact, the products were soaked in DMF for 36 h and then in CHCl3 for another 36 h (refreshing the solvents every 12 h). Finally, the samples were dried in a vacuum at 423 K for 8 h. In addition, a series of metal/ligand molar ratio (1:1, 3:1, 5:1), grinding speed (900, 1000, 1100 r· min−1) and grinding time (30, 60, 90 min) were used to study their roles in the formation of MOF-5. The sample was characterized by elemental analysis (found (calcd): C, 36.40 (37.44); H, 1.96 (1.57); N, 0.25 (0)). 2.3. Characterization. Powder X-ray diffraction (PXRD) patterns were obtained on a Bruker D8 Advance X-ray diffractometer, with Cu Kα radiation at 40 kV and 40 mA and a scanning step size of 0.02° in the 2θ range of 5−50°. Scanning electron microscope (SEM) was conducted on a Hitachi S-4800 instrument after gold deposition. Transmission electron microscopy (TEM) was performed on a JEM2100(HR) instrument operating at an accelerating voltage of 200 kV. Thermogravimetric analyses (TGA) were performed on a Netzsch TG 209 F3 Tarsus instrument from room temperature to 873 K in nitrogen atmosphere at a heating rate of 10 K/min. Nitrogen isotherms were measured at 77 K on a Micromeritics ASAP 2020 instrument. The pore structure and specific surface area of the samples were estimated using BET equation and the pore size distribution was calculated using density functional theory (DFT) method. 2.4. Adsorption Experiments. The isotherms of linear alkanes were measured at 288, 298, and 308 K on 3 Flex Surface Characterization Analyzer (Micromeritics, American). Data points on isotherms of linear alkanes were obtained when the change rates of adsorbed values lower than 0.01% for 10 s. Ultrahigh purity grade methane (C1, 99.99%), ethane (C2, 99.99%), propane (C3, 99.95%), and n-butane (nC4, 99.99%) were used for short chain alkanes (C1-nC4) adsorption. nC5nC7 were dehydrated by adding some silica gels before the adsorption experiments. And dissolved gases in nC5−nC7 were removed via degassing under vacuum for 5 h. nC5−nC7 vapors were supplied by the vapor generation system for long chain

3. RESULTS AND DISCUSSION 3.1. Preparation of MOF-5. 3.1.1. Solvent Requirements for MOF-5 Production. We measured the PXRD patterns of MOF-5-B without being activated and with solvent activation, as shown in Figure 1. Figure 1 shows that the sample without

Figure 1. PXRD patterns of samples without being activated and with solvent activation.

being activated has not formed MOF-5 and exhibited a new diffraction peaks differing from starting materials (Figure S1) at 11.3°, which indicated the formation of a new intermediate phase during grinding (Figure S2).1 But after being activated with DMF, we could observe the appearance of MOF-5 characteristic reflection at 6.8°, 9.7°, 13.7° and 15.4°, which were in agreement with those of MOF-5 reported elsewhere.18 The formation of MOF-5 after being activated by DMF is because DMF can enhance the mobility of intermediate phase and remove byproducts, and DMF solvent can help intermediate phase transform to MOF-5 crystal.19 The transformation can also be observed from SEM pictures (Figure S3a−d). Furthermore, PXRD peak at 6.8° of the activated sample with DMF and CHCl3 was sharper than that of the activated sample with DMF. Meanwhile, the sample activated by DMF and CHCl3 has a lower intensity ratio of the PXRD peak at 9.7° to the peak at 6.8° than that activated by DMF. Compared with the activated sample with DMF, there were fewer zinc species trapped in the cavities of MOF-5 sample for the activated sample with DMF and CHCl3.20 3.1.2. Effect of Metal/Ligand Ratio. We synthesized MOF-5 samples with different Zn2+/BDC ratios (1:1, 3:1, and 5:1), which were correspondingly labeled as MOF-5-A, MOF-5-B, and MOF-5-C, as shown in Table 1. Figure 2 shows the PXRD patterns of MOF-5-A, MOF-5-B, and MOF-5-C samples. All the samples exhibited the characteristic peaks at 6.8°, 9.7°, 13.7°, and 15.4°, which were in agreement with those reported in the literature,20 indicating B

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Table 1. BET Area of MOF-5 Samples Prepared Mechanochemically Under Various Conditions sample

metal/ligand ratio

grinding speed (r·min−1)

grinding time (min)

BET area (m2·g−1)

MOF-5-A MOF-5-B MOF-5-C

1:1 3:1 5:1

1100 1100 1100

60 60 60

924.5 3465.9 2993.1

many small particles around the MOF-5 crystals in Figure 3b− d. The small particles were not impurities and just the crystals of MOF-5 with small size. Because the samples synthesized via mechanochemical tended to have crystal defects in crystal lattices caused by the grinding process.13 Besides, the PXRD patterns (Figure 4), SEM (Figure S3), and TEM (Figure S5)

Figure 2. PXRD patterns of the as-synthesized MOF-5 samples with different metal/ligand ratios.

that MOF-5 products were successfully synthesized by mechanochemical method. However, a new peak was seen at 10.3°, suggesting that MOF-5-A was probably impure crystal phase. It was ascribed to existing intermediate product [Zn(BDC)(H2O)].20,21 Because Zn2+/BDC ratio of MOF-5-A (1:1) was lower than 4:3 of MOF-5 [Zn4O(BDC)3], resulting in the BDC group binding to the Zn2+ metal center in the monodentate ligand and forming the lower dimensional crystals such as [Zn(BDC)(H2O)].20,21 It was also noticed that MOF5-A exhibited the distinct weight loss step in the range of 473− 550 K (Figure S4), suggesting that the BDC group in MOF-5-A might partly bind to the Zn2+ metal center in the monodentate ligand or multidentate ligands instead of coordinating in a bidentate fashion.22 Moreover, there was obvious morphology difference for MOF-5-A-C samples (Figure 3). MOF-5-B exhibited a well-defined cubic structure (Figure 3b and c) and

Figure 4. PXRD patterns of MOF-5 samples synthesized under different grinding speed and time.

were also verified the purity of samples. But MOF-5-A (Figure 3a) and MOF-5-C (Figure 3d) showed less defined crystalline features compared to MOF-5-B. As shown in Table 1, the BET areas of the samples followed the order: MOF-5-B > MOF-5-C > MOF-5-A (Figure S6a−c). BET area of MOF-5-A was only 924.5 m2·g−1, much lower than that of MOF-5-B (3465.9 m2·g−1). It suggested that the Zn2+/ BDC ratio had a great impact on the formation of crystalline MOF-5, in agreement with the PXRD and TG results (Figure S4). MOF-5-C (Zn2+/BDC = 5:1) could have more zinc species trapped in the cavities than MOF-5-B (Zn2+/BDC = 3:1), which leaded to BET area of MOF-5-C lower than that of MOF-5-B.20 Moreover, these results indicated that the appropriate Zn2+/ BDC ratio was 3:1 for mechanochemical synthesis of MOF-5, which was consistent with that for hydrothermal method.23 3.1.3. Role of Grinding. Grinding can provide mass transfer and kinetic energy, which will have a significant impact on the final grinding product.24 Hence, we further studied the effects of grinding speed and time on the MOF-5 formation. Preparation conditions of MOF-5 samples under different grinding speed and time are listed in Table 2. Figure 4 shows PXRD patterns of these samples synthesized at different grinding speed and time. The major characteristic peaks of these samples were nearly the same as those of the MOF-5 reported elsewhere.25 But these samples presented different surface topographies (Figure 3b and c and Figure 5a− d). MOF-5-D and MOF-5-E exhibited smaller particle sizes and less defined crystalline features compared to that of MOF-5-B. This was because the lower grinding speed ( nC5 > nC4 > C3 > C2 > C1. The trend was related to the physical properties of alkanes and their adsorption interactions between adsorbents and host MOF-5-B frameworks. Table 3 lists the physical properties of selected alkanes.30,31 It showed that the molecular length and cross-sectional area of

could not offer enough energy to facilitate the formation of MOF-5. MOF-5-F synthesized with 30 min grinding presented an irregular structure. After 60 min grinding, MOF-5-B exhibited a well-defined cubic structure. However, when grinding time was extended to 90 min, MOF-5-G showed smaller particle size and less defined crystalline features. The reason was that when the grinding time was further prolonged, the kinetic energy could lead to the formation of defects and dislocations in crystal lattices, even amorphization and crystalline phase transition.24 In addition, Table 2 indicated that the MOF-5-B sample had the largest BET area of 3465.9 m2·g−1 (Figure S6b and d−f). Therefore, we acquired the optimized conditions of mechanochemical synthesis of MOF-5 (MOF-5-B): grinding the reactant mixture with a molar composition of 3:1 [Zn(OAc)2· 2H2O:H2BDC] for 60 min at 1100 r·min−1. Nitrogen isotherms were measured at 77 K as shown in Figure S7. N2 isotherms of MOF-5-B exhibited the typical typeI profile, which was the characteristic of microporous structure. MOF-5-B-G showed a dominant pore size around 12.5 Å, as shown in Figure S8, which was close to the theoretical value of about 13 Å.26 MOF-5-A (Figure S8) showed a different pore size distribution because of the existence of impure phase. In addition, the MOF-5-B sample had large Langmuir and BET area (3934.9 and 3465.9 m2·g−1), as well as large total pore volume of 1.40 cm3·g−1. BET area of MOF-5-B was very close to the theoretical value (3656 m2·g−1) and much higher than those of MOF-5 samples via solvothermal and microwave synthesis,27−30 as shown in Table S1. The MOF-5 sample synthesized via solvothermal synthesis and activated as the same conditions (including the activation temperature and

Table 3. Physical Properties31,32 and Saturated Adsorption Capacities of Selected Alkanes on MOF-5-B adsorbates

molecular length (Å)

molecular cross-sectional area (Å2)

Qa (mmol·g−1)

C1 C2 C3 nC4 nC5 nC6 nC7

3.829 3.809 6.606 7.855 9.101 10.344 11.589

16.4 22.7 32.0 39.7 45.0 51.0 57.3

11.5 10.3 9.0 8.0

alkane increased as the alkyl chain length of alkane increased. The results indicated that the molecular sizes of alkanes followed the order: nC7 > nC6 > nC5 > nC4 > C3 > C2 > C1. At the region of low pressure, monoadsorption was dominant, the interactions between alkanes and the surface of MOF-5-B was the most important factor. The larger the adsorbate molecular D

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Figure 7. Adsorption capacities at saturation of nC4−nC7 on MOF-5-B as functions of their molecular length and cross-sectional area. L is molecular length, and S is molecular cross-sectional area.

similar conditions. Therefore, the MOF-5-B is a promising adsorbent for separation of linear alkanes. 3.3. Isosteric Heat of Linear Alkanes Adsorption. The isosteric heat of adsorption can be calculated from experimental isotherms at various temperatures (Figures S12−S18) by the Clausius−Clapeyron equation (eq S2).34 The isosteric heats of linear alkanes (C1−nC7) adsorption on MOF-5-B are shown in Figure 8. The isosteric heats of C1−nC7 increased with the

size was, the stronger the adsorbate−adsorbent interaction for an adsorbent with fixed pore size and volume was. Consequently, alkanes with larger molecule size could have higher adsorption capacity, and thus the adsorption capacity of MOF-5-B followed the order at the region of low pressure: nC7 > nC6 > nC5 > nC4 > C3 > C2 > C1. The long alkane was found to approach saturation at a lower pressure than that of a short alkane, which is consistent with the simulated results.9 The saturated adsorption capacities (Qa) of nC4−nC7 decreased with the increase of hydrocarbon chain length. This was because the saturated adsorption capacities were dominated by the limited pore volume of adsorbent. The larger the alkane molecular size was, the less the amounts of alkanes adsorbed within the pores of MOF-5-B were. As a result, the saturated adsorption capacities of nC4-nC7 on MOF5-B followed the order: nC4 > nC5 > nC6 > nC7, which were 11.5, 10.3, 9.0, and 8.0 mmol·g−1, as shown in Table 3. Figure 7 shows the plots of Qa of nC4−nC7 versus their molecular length and cross-sectional area. It indicated that the saturated adsorption capacities of nC4-nC7 declined linearly with the increase of their molecular length and cross-sectional area. For further comparison, Table S2 lists alkanes adsorption capacities of MOF-5 and some other adsorbents. Adsorption isotherms of linear alkanes (C1−nC6) on MOF-5-B were like the simulated data,9 and the adsorption capacities of C1−nC6 on the MOF-5-B were slightly lower than simulated results because of its lower BET area. The shapes of C1−nC6 isotherms were very close to those of the reported data (Figure S10). C1−nC4 isotherms of the solvothermal synthesized MOF-5 were like those of MOF-5-B (Table S2 and Figure S11).33 Adsorption capacities of methane and ethane on MOF-5-B were close to solvothermal synthesized MOF-5. But the adsorption capacities of C3 and nC4 on MOF-5-B were higher than solvothermal synthesized MOF-5 because of its higher BET area. For the short alkanes, the adsorption capacities of C1 and C2 on the MOF-5-B were close to the conventional absorbents and comparatively lower than the MOFs. However, the adsorption capacities of C3 and nC4 were about 11.1 and 15.3 mmol·g−1, which were much higher than that on the conventional absorbents and MOFs. It was also noticed that adsorption capacities of the long alkanes (nC5-nC7) on MOF-5B were comparatively higher than MOFs and about 1.2−8.2 times higher than those of activated carbon and zeolites at

Figure 8. Isosteric heats of linear alkanes adsorption on MOF-5-B.

increase of the alkyl chain length, which followed the order: nC7 > nC6 > nC5 > nC4 > C3 > C2 > C1. It indicated that the bigger the molecular size, the stronger the interaction force between the alkane molecular and MOF-5-B. In addition, the isosteric heats of adsorption for C1-nC7 varied only slightly as the loading increased, indicating that MOF-5-B had the energetically homogeneous surface toward C1−nC7.

4. CONCLUSIONS In summary, in this Article, we provided an efficient mechanochemical route for the preparation of MOF-5 with an appreciable BET area (3465.9 m2·g−1). Solvents activation played an important role in the formation of MOF-5. The optimized conditions of mechanochemical synthesis of MOF-5 were as following: grinding the reactant mixture with a molar composition of 3:1 [Zn(OAc)2·2H2O/H2BDC] for 60 min at 1100 r·min−1. The adsorption capacities of C1−nC7 on MOF-5B increased with the increase of hydrocarbon chain length at the region of low pressure. The isosteric heats of C1−nC7 E

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increased with the increase of the alkyl chain length. The adsorption capacities of the short alkanes C3 and nC4 on MOF5 were higher than those of the conventional adsorbents and MOFs. The adsorption capacities of the long alkanes nC5−nC7 on MOF-5-B were comparatively higher than MOFs and about 1.2−8.2 times higher than those of conventional activated carbons and the zeolites. The excellent adsorption properties of linear alkanes make MOF-5 become a promising candidate for the hydrocarbons adsorption and separation.

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ASSOCIATED CONTENT

S Supporting Information *

The Supporting Information is available free of charge on the ACS Publications website at DOI: 10.1021/acs.jced.7b00049. PXRD of starting materials, nitrogen adsorption/ desorption isotherms, and pore size distribution of MOF-5-B without being activated at 77 K, SEM images of MOF-5-B and the samples without being activated, TG curves for MOF-5-A-G, TEM of MOF-5-B, plot of the linear regions on the N2 isotherms of MOF-5 samples for the BET equation, nitrogen adsorption/ desorption isotherms of MOF-5-B, pore size distribution of MOF-5-A-G, summary of the results from the literatures for MOF-5 prepared by various methods, plot of the linear regions on the N2 isotherms of MOF-5 samples synthesized by solvothermal method for the BET equation, adsorption isotherms of linear alkanes (C1-nC7) on MOF-5-B at 298 K, C2 and C3 isotherms of MOF-5 synthesized by solvothermal method at 298 K, alkane adsorption capacities of MOF-5 and some other adsorbents, isosteric heats of alkanes adsorption and C1nC7 isotherms of MOF-5 at 288, 298, and 308 K (PDF)

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AUTHOR INFORMATION

Corresponding Authors

*E-mail: [email protected]. *E-mail: [email protected]. ORCID

Qibin Xia: 0000-0002-8563-6715 Funding

This work was supported by the National Natural Science Foundation of China (Nos. 21576092, 21276092, and 21606144). Notes

The authors declare no competing financial interest.

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REFERENCES

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DOI: 10.1021/acs.jced.7b00049 J. Chem. Eng. Data XXXX, XXX, XXX−XXX