Study on Desorption Process of n-Heptane and Methyl Cyclohexane

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Study on Desorption Process of n-Heptane and Methyl Cyclohexane Using UiO-66 with Hierarchical Pores Sijia Chen, Lin Zhang, Zhao Zhang, Gang Qian, Zongjian Liu, Qun Cui, and Haiyan Wang ACS Appl. Mater. Interfaces, Just Accepted Manuscript • DOI: 10.1021/acsami.8b04931 • Publication Date (Web): 06 Jun 2018 Downloaded from http://pubs.acs.org on June 6, 2018

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ACS Applied Materials & Interfaces

Study on Desorption Process of n-Heptane and Methyl Cyclohexane Using UiO-66 with Hierarchical Pores Sijia Chen,† Lin Zhang,† Zhao Zhang, Gang Qian, Zongjian Liu, Qun Cui* and Haiyan Wang* College of Chemical Engineering, Nanjing Tech University, Nanjing 210009, China. KEYWORDS: UiO-66, hierarchical pores, n-heptane, methyl cyclohexane, reverse shape selectivity, desorption activation energy

ABSTRACT: UiO-66 (UiO for University of Oslo), is a zirconium-based MOF with reverse shape selectivity, gives an alternative way to produce high purity n-heptane used for the manufacture of high-purity pharmaceuticals. Couple of studies have shown that UiO-66 gives a high selectivity on the separation of n-/iso-alkanes. However, the microporous structure of UiO-66 causes poor mass transport during the desorption process. In this work, hierarchical-pore UiO-66 (H-UiO-66) was synthesized and utilized as an adsorbent of n-heptane (nHEP) and methyl cyclohexane (MCH) for systematically studying the desorption process of n/iso-alkanes. A suite of physical methods, including XRD patterns verified the UiO-66 structures and HRTEM showed the existence of hierarchical pores. N2 adsorption-desorption isotherms further confirmed the size distribution of hierarchical pores in H-UiO-66. Of particular note, the MCH/nHEP selectivity of H-UiO-66 is similar with UiO-66 in the same adsorption conditions, 1 ACS Paragon Plus Environment

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the desorption process of nHEP/MCH from H-UiO-66 is dramatically enhanced, viz, the desorption rates for nHEP/MCH from H-UiO-66 is enhanced by 30%/23% as comparing to UiO-66

at

most.

Moreover,

desorption

activation

energy

(Ed)

derived

from

temperature-programmed desorption (TPD) experiments indicate that the Ed for nHEP/MCH is lower on H-UiO-66, i.e., the Ed of MCH on H-UiO-66 is ~37% lower than that on UiO-66 at most, leading to a milder condition for the desorption process. The introduction of hierarchical structures will be applicable for the optimization of desorption process during separation on porous materials.

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INTRODUCTION Recently more and more studies are focus on high-purity n-heptane solvents (mass fraction higher than 99%), which are an integral part of high-purity pharmaceuticals

1-2

. Adsorption

separation based on 5A zeolite is the most widely studied method among the production of high-purity n-heptane

3-4

. However, the challenging desorption conditions (high temperature

and/or high vacuum) cause high energy consumption

2, 5

, and the higher purity of n-heptane

cause the lower recovery of n-heptane based on this separation process via 5A zeolite. In order to solve this problem, the researchers focus their studies on developing new adsorbent and new separation process. Metal-organic frameworks (MOFs), a class of hybrid crystalline materials, with possibilities in designing both geometrical shape and chemical properties of the internal surface are promising candidates in this separation process. In addition, MOFs have been applied in various applications including adsorption 6, catalysis sensing

13-14

7-10

, gas storage

11

, gas separation

12

,

, to name a few. The adsorption separation mechanism of alkanes on majority of

MOFs is shape selectivity, in which n-alkanes were absorbed preferentially than iso-alkanes

15

.

So the separation process based on shape selectivity leads to high purity iso-alkane production. Inverse/reverse shape selectivity was firstly proposed by Santilli et al 16, where iso-alkanes were absorbed selectively over n-alkanes in zeolites with 7-7.4 Å pores, which was explained by the attractive forces between zeolite wall and adsorbed hydrocarbons. The separation process based on reverse shape selectivity proposes a route which can directly produce high purity n-heptane. UiO-66 as the first MOF that follows reverse shape selectivity mechanism

17-18

has shown

potential in n-/iso-alkanes separation. In our previous work 4, UiO-66 has been demonstrated to 3 ACS Paragon Plus Environment


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be an effective adsorbent for high purity n-heptane production from rectified No.120 solvent oil (32.5% n-heptane) and chemical pure n-heptane (95.8% n-heptane). However, UiO-66 with microporous structure needs a relatively high desorption temperature which is a bit lower than separation process via 5A zeolite. As we all know, by systematic tuning the pore size of MOFs, the interactions with guest molecules can be accordingly adjusted

19-20

, thereby their desorption

process. To enhance mass transportation, hierarchical structure has already been introduced to mesoporous carbon materials

21

also facilitate mass transport n/iso-alkanes (C1-C6)

24

and zeolites 22. Similarly, MOFs with hierarchical structure can 23

and thus be applied in chromatographic separation of

and catalysis

25-26

. Recently, researchers found that hierarchical-pore

MOFs (H-MOFs) with both micropores and mesopores/macropores could be simplely synthesized by template-free 27 or template-assisted 28-29 strategy with tunable structures as well as properties. Herein, H-UiO-66 was synthesized via in situ self-assembly template strategy

28

. The

structures of H-UiO-66 and typical UiO-66 were confirmed by XRD, N2 adsorption-desorption isotherms and HRTEM. Eventually, the separation and desorption performances of UiO-66 and H-UiO-66 were evaluated and compared, and n-heptane/methyl cyclohexane (nHEP/MCH) showed varied desorption behaviors based on different adsorbents and desorption conditions. The n/iso-alkanes desorption behavior on UiO-66/H-UiO-66 was systematic studied, and the introduction of hierarchical structures is referential for the optimization of industrial adsorbents, thus leading to a reduced energy consumption. EXPERIMENTAL SECTION 4 ACS Paragon Plus Environment

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Materials. Zirconium tetrachloride (ZrCl4, 98%), zinc nitrate hexahydrate (Zn(NO3)2·6H2O, 99%), 1,4-benzenedicarboxylic acid (H2BDC, 99%) and methyl cyclohexane (MCH, 99%) were purchased from Aladdin Reagent Co. Ltd., Shanghai, China. Benzoic acid (BA, 99.5%), hydrochloric acid (HCl, 36%), acetone (CH3COCH3, 99.5%) and concentrated sulfuric acid (H2SO4, 98%) were purchased from Lingfeng Chemical Reagent Co. Ltd., Shanghai, China. N, N′-dimethylformamide (DMF, 99.5%) was purchased from Kelong Chemical Co., Ltd., Chengdu, China. Ethanol (C2H5OH, 99.7%) was purchased from Yasheng Chemical Reagent Co. Ltd., Wuxi, China. Hydrogen peroxide (H2O2, 30%), n-heptane (nHEP, 97%) and sodium nitrate (NaNO3, 98.5%) were supplied by Sinopharm Chemical Reagent Co., Ltd., Shanghai, China. Sodium hydroxide (NaOH, 96%) was supplied by Xilong Chemical Co. Ltd., Shantou, China. All reagents were used as received without further purification. Synthesis of UiO-66 and H-UiO-66. UiO-66 was synthesized according to reported literature 30

, ZrCl4 (0.96 g, 4 mmol) and H2BDC (1.328 g, 8 mmol) were dissolved in 160 mL DMF at

room temperature in a Teflon reaction still (200 mL) and sonicated for 20 min. Then mixture was sealed and placed in a preheated oven at 393 K for 24 h, after which it was allowed to cool down to room temperature in air. Generated solid was centrifuged, washed DMF (twice) and then ethanol (twice) and dried at 333 K for 12 h. Dried sample was further activated at 573 K in a furnace under air atmosphere for 3 h. H-UiO-66 was synthesized based on previous report

28

, in which an in situ self-assembly

template strategy was used. ZrCl4 (0.96 g, 4 mmol), Zn(NO3)2·6H2O (1.785 g, 6 mmol), H2BDC (1.328 g, 8 mmol) and BA (14.64 g, 120 mmol) were added into 160 mL DMF at room 5 ACS Paragon Plus Environment


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temperature in a Teflon reaction still (200 mL) and sonicated for 20 min. Mixture was then sealed as well and placed in a preheated oven at 393 K for 24 h. After cooling to room temperature in air the solid was centrifuged and washed with DMF (twice). 10 mL HCl solution (pH=1) was added to the solid and stirred for 10 min, followed by the washing of DMF (twice) and acetone (once). Product was collected by centrifugation and then dried in the oven at 333 K for 12 h. Finally, dried sample was activated at 573 K for 3 h in a furnace under air atmosphere. Characterizations. The crystallinity of powder was examined by X-ray diffraction (XRD, D8 ADVANCE-TXS, Bruker AXS, Germany) with a monochromatized Cu-Kα radiation (λ=1.5418 Å), operation voltage and current maintained at 40 kV and 100 mA, respectively. Diffraction patterns were monitored between 5 º and 50 º 2θ at a scanning speed of 10 º/min. Specific surface of samples was evaluated using the nitrogen isotherms measurement at 77 K after degassing of samples at 473 K for 12 h (ASAP2020, Micromeritics, USA). Surface morphology and structure of samples were characterized by high-resolution transmission electron microscopy (HRTEM, JEM-2100, JEOL, Japan). Energy dispersive spectra (EDS) was carried out by scanning electron microscope (SEM, S4800, Hitachi, Japan). The concentrations of the elements were determined by inductively coupled plasma-optical emission spectroscopy (ICP-OES, Optima 7000 DV, PerkinElmer, USA). Potentiometric acid-base titration was carried out on the basis of reported literature

31

,

samples were ground and activated at 423 K for 3 h prior to titration. 50 mg activated sample was dispersed in 50 mL aq. NaNO3 solution (0.01 M) solution and equilibrated for 18 h. Each titration solution was charged with a magnetic stir bar, acidity of the solution was adjusted using 6 ACS Paragon Plus Environment

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aq. HCl solution (0.1 M) to a pH of 3, and aq. NaOH solution (0.1 M) was used to titrate at an injection rate of 0.015 mL/min. Titration curves were collected for different samples. The curves were fit with Lorentzian functions. Temperature-programmed desorption of helium (He-TPD, AutoChem II 2920, Micromeritics, USA) experiments were performed in the temperature range between 313 and 773 K. 25 mg sample was placed in the sample chamber and purged with He (99.999%) at a flow rate of 50mL/min at room temperature for 30 min. He-TPD was performed upon varied heating rate (in the range of 5-13 K/min) once the baseline of TCD signal became steady. Vapor Phase Adsorption/Desorption Experiments. Prior to adsorption/desorption experiment, UiO-66 powder was firstly compacted into disks in an infrared press upon a pressure of 1.5 MPa. Then the disks were crushed into fragments, and sieved into 0.250-0.425 mm pellets. Sample (~0.8 g) was stored in adsorption column with a length of 300 mm and inner diameter of 3.4 mm

32

. In adsorption section, nHEP/MCH was firstly injected into fixed bed vaporizing

chamber by a microliter syringe, and vaporized mixture flowed through fixed bed using high purity N2 as carrier gas. In desorption section, the valves on both sides of fixed bed were closed upon the completion of the adsorption. Flow rate of high pure N2 was adjusted to 20 mL/min and the valves were switched on simultaneously, thus the desorption begun. The outlet-gas was analyzed by a gas chromatograph (GC, SP 6890, Lunan, China) with a FID detector and SE30 capillary column. RESULTS AND DISCUSSION X-ray Diffraction. XRD patterns were used to determine bulk crystallinity of 7 ACS Paragon Plus Environment


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UiO-66/H-UiO-66 (Fig. 1). It is clear that XRD pattern of H-UiO-66 resembles that of UiO-66, and both correspond well to simulated theoretical XRD patterns. Broaden peaks of H-UiO-66 are consistent with the decrease of crystallite size, based on Scherrer equation 33. N2 Adsorption-Desorption Isotherms and HRTEM Measurements. To gain further insight into the pore structure of UiO-66/H-UiO-66, N2 adsorption-desorption isotherms measurements were conducted. As shown in Fig. 2a, N2 adsorption-desorption isotherms of UiO-66 are of type I, which corresponding to microporous materials. While H-UiO-66 shows type IV isotherms with hysteresis loops, indicating the existence of typical mesopores structures. The sharp N2 uptake near P/P0~1 should be ascribed to the presence of slit pores 27. Table 1 summarized the resulting BET surface areas and pore volumes of UiO-66/H-UiO-66. The decrease of surface area for H-UiO-66 (~42% decrease compared to UiO-66) should result from the loss of micropores (~51% decrease compared to UiO-66, based on Horvath-Kawazoe method). Changes in pore size evidently would be of particular importance to understand the adsorption/desorption behavior of UiO-66/H-UiO-66

34

. Hence, to obtain the most accurate pore size analysis possible, nonlocal

density functional theory (NLDFT) was applied, which provides pore size distribution in the entire range of micro- and mesopores from adsorption branch of isotherms. UiO-66 only has micropores and the corresponding pore sizes are 6, 9 and 15 Å. In contrast, H-UiO-66 is consisted of micropores and mesopores, and the corresponding pore sizes are 6, 14 and 18 Å for micropores, and 20-100 Å for mesopores. It should be mentioned that the pore sizes of mesopores are mainly 27, 47 and 81 Å, which agreed well with Huang et al. calculated from DFT theory). 8 ACS Paragon Plus Environment

28

(~100 Å

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As can be seen from Fig. 2c, UiO-66 particles show octahedral morphology with a size of ~80 nm. HRTEM measurement performed on H-UiO-66 (Fig. 2d) shows that mesopores are distributed evenly throughout H-UiO-66, which is in accordance with the NLDFT analysis of H-UiO-66 aforementioned. Furthermore, we also checked the effect of defect and residue of un-washed Zn element. Potentiometric acid-base titration method was utilized to quantitatively analyze the defects

35

.

The numbers of defects per SBU were calculated from the titration curves of UiO-66 and H-UiO-66 (Fig. S2), which is approximately 1.0 and 1.5, respectively. In terms of defects, the difference between UiO-66 and H-UiO-66 is quite finite. ICP and SEM-EDS (Fig. S3) were performed to further prove the Zn-base template can be removed by acid treatment. ICP for the H-UiO-66 gives contents of Zr 29.60% and Zn 0.15%, illustrating that the content of Zn is negligible, which may come from Zn ions anchoring (through coordinating with carboxylate, etc.). Dynamic Adsorption Separation Performance. The nHEP/MCH equimolar binary breakthrough experiments were measured under the hydrocarbon pressure of 10 kPa at 473 K to detect the difference of selectivity of MCH/nHEP between UiO-66 and H-UiO-66. Experimental conditions were shown in Table 2. As is shown in Fig. S4, due to the loss of micropores in H-UiO-66, the breakthrough adsorbed capacity of nHEP and MCH from H-UiO-66 decreased by 34% and 28% compared to that of UiO-66. Meanwhile, MCH/nHEP selectivity of H-UiO-66 is similar with UiO-66 (2.50 and 2.31, respectively), illustrating that the adsorption separation performance of H-UiO-66 will not be impacted by the introduction of hierarchical pores. 9 ACS Paragon Plus Environment


Dynamic Desorption Performance. Generally, main difference between UiO-66 and H-UiO-66 is hierarchical pores, which reduce adsorbed capacity but remain MCH/nHEP selectivity. For desorption performance evaluation, the adsorption/desorption of single component (nHEP or MCH) were measured under the hydrocarbon pressure of 10 kPa with varied temperatures (453-513 K). To probe the effect of varying pore structures on desorption process on UiO-66/H-UiO-66, desorption experiments were conducted at the same temperature and pressure with a N2 flow of 20 mL/min right after adsorption process. The concentration changes of nHEP or MCH as a function of time in the outlet-gas calculated via equation (2) (seen in Supporting Information) was presented in Fig. 3. As for the concentration of MCH from H-UiO-66, it drops sharply after reaching the peak value, implying a faster desorption behavior. The faster desorption behavior of nHEP or MCH from H-UiO-66 compare with UiO-66 originates from the difference in aperture and pore size. Desorption rate of nHEP or MCH at different desorption time from UiO-66 and H-UiO-66 was further studied (Fig. 4). Generally, desorption of nHEP or MCH from H-UiO-66 is faster than that from UiO-66 under the same conditions. Note that the desorption behavior of nHEP from UiO-66 is faster than that of MCH under the same conditions, which is attributed to the reverse shape selectivity of UiO-66. In the meantime, the specific desorption capacity and desorption rate at 10 min are calculated using equation (3) and (4), and the results were presented in Table 3. The desorption rates of nHEP or MCH from H-UiO-66 is 90.3% or 50.2%, respectively, at the desorption time of 10 min 10 ACS Paragon Plus Environment

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at 513 K, making the former nearly 2 times larger than the latter. An obvious increase in desorption rate can be seen when the temperature rises from 453 K to 513 K (11% or 18% for nHEP or MCH from UiO-66 and 45% or 14% from H-UiO-66, respectively), indicating that the desorption of nHEP from H-UiO-66 can dramatically benefit from the rise of desorption temperature. Specifically, the desorption rate of nHEP from H-UiO-66 (66.2% at 473 K) is higher than that from UiO-66 (59.9% at 513 K), and so does that of MCH (36.4% at 453 K from H-UiO-66 compare to 33.3% at 513K from UiO-66). These results suggest that the introduction of mesopores may lead to a reduction of desorption temperature ~40 K. Temperature Program Desorption. Given that nHEP or MCH dynamic desorption performance between UiO-66 and H-UiO-66 was quite different, comparisons of desorption activation energy (Ed) calculated from experimentally measured TPD profile (Fig. 5) are often insightful

36

. nHEP or MCH desorbed gradually from UiO-66/H-UiO-66 with the increase of

temperature under different heating rates. Both nHEP and MCH have three peaks, in which the first peak temperature is around 345.3-370.8 K, corresponding to the desorption of nHEP or MCH from the surface of UiO-66/H-UiO-66. UiO-66 consists of an array of tetrahedral and octahedral cavities which are 8 and 11 Å in diameter, respectively 17. Adsorbate molecules can access cavities through triangle windows within the 5-7 Å range. The kinetic diameter of C5-C8 cycloalkanes (~ 6 Å) are nearly identical with UiO-66 tetrahedral cavities, which make cycloalkanes particularly challenging to desorb from adsorbents. Therefore, we speculate that the second peak temperature (399.6-438.2 K) and the third peak temperature (508.0-525.7 K) correspond to the desorption process of alkanes in octahedral and tetrahedral cavity, respectively. 11 ACS Paragon Plus Environment


The area of first peak is much larger than that of the second or third (Table S1), indicating that the adsorption of nHEP mainly occurred on the surface rather than in the cavities of UiO-66/H-UiO-66. Meanwhile, for MCH, peak 2 shows the largest area, implying that the adsorption behavior occurred preferentially in the cavities rather than on the surface of UiO-66/H-UiO-66 owing to the strong adsorption behavior between MCH and the cavities, thus leading to a higher desorption temperature 37. Note that the desorption temperature of H-UiO-66 for both nHEP and MCH are lower than that of UiO-66 upon every heating rate, revealing that the introduction of mesopores contribute to faster mass transport thus reduce the desorption temperature. To ascertain the difference of desorption behaviors of nHEP and MCH on UiO-66/H-UiO-66 visually, desorption activation energy (Ed) was closely approximated by means of equation (6) from the slope of a plot of ln(βH/RTp2) vs. 1/Tp. And the calculated Ed values were listed in Table 4. Ed for MCH of UiO-66/H-UiO-66 are lower than that of nHEP, which should be ascribed to stronger interaction between MCH and UiO-66/H-UiO-66. To our delight, apart from smaller peak 3 area percentage (Table S1), Ed1 (Ed of peak 1) for MCH of H-UiO-66 (103.80 kJ/mol) is ~37% lower than that of UiO-66 (65.06 kJ/mol). Unlike minimum difference between Ed1 and Ed2 of UiO-66, Ed2 is 1.5 times larger than Ed1 for H-UiO-66, implying that nHEP or MCH adsorbed on the surface of adsorbent was easily desorbed due to hierarchical pores. In all cases, Ed of three desorption peaks of H-UiO-66 are lower than that of UiO-66, further confirming that the introduction of mesopores is facilitates to the desorption performance of nHEP or MCH. CONCLUSION 12 ACS Paragon Plus Environment

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In summary, hierarchical pore structure of H-UiO-66 was confirmed by XRD, HRTEM and N2 adsorption/desorption isotherms. The MCH/nHEP selectivity of UiO-66 and H-UiO-66 is almost the same (2.31 and 2.50, respectively). TPD experiments indicate that the Ed for nHEP/MCH on H-UiO-66 is 6%~36% and 10%~37% lower than that on UiO-66, respectively. However, H-UiO-66 exhibit both the lowest Ed and highest desorption rate under similar conditions. The desorption temperatures of nHEP or MCH is reduced by ~40 K. As a proof of concept, the direct participation of a hierarchical network can significantly enhanced mass transfer which is of practical relevance to improve the efficiency of desorption processes. To the extent that best practices can be widely adopted to accelerate the exploration and application of MOFs on significant industrial problems.

ASSOCIATED CONTENT Supporting Information The Supporting Information is available free of charge at the ACS Publications website or from the author. Methods and equations of adsorption and desorption experiments, potentiometric acid-base titration curves of benzoic acid and UiO-66/H-UiO-66, EDS element images of H-UiO-66, peaks area percentage integrated from TPD, breakthrough curves of nHEP/MCH on UiO-66 and H-UiO-66 under 473 K, breakthrough curves of nHEP or MCH on UiO-66 and H-UiO-66 under 453~513 K.



AUTHOR INFORMATION Corresponding Author * E-mail: [email protected] (Q. Cui); [email protected] (H. Wang) Author Contributions † S.C. and L.Z. contributed equally to this work. ORCID Sijia Chen: 0000-0002-3129-0851 Lin Zhang: 0000-0001-5853-3107 Zhao Zhang: 0000-0002-9555-2179 Gang Qian: 0000-0002-1681-3554 Zongjian Liu: 0000-0002-4611-5423 Haiyan Wang: 0000-0001-7742-9583 Notes The authors declare no competing financial interest. ACKNOWLEDGMENT Financial support from the Project of the Natural Science Foundation of China (51476074) is gratefully acknowledged. We also acknowledge the comments of the anonymous reviewers that help us to improve the content of the paper. REFERENCES (1) Li, K.; Kennedy, E. M.; Chen, S. Adsorption of n-Butane and n-Heptane on 5A Zeolite. Sep. Purif. Technol. 1998, 33, 1571-1584. 14 ACS Paragon Plus Environment

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Page 21 of 26

FIGURES

Relative Intensity (a.u.)

H-UiO-66

UiO-66

Simulated

10

20

30

40

50

2 Theta (degree)

Figure 1. XRD patterns of simulated UiO-66, synthesized UiO-66 and H-UiO-66. (a) 1000 H-UiO-66-ADS H-UiO-66-DES

dV/dlog(W) (cm /g.Å)

800

(b) 0.3 UiO-66-ADS UiO-66-DES

3

N2 Uptake at 77K (cm /g STP)

600 400

200 0 0.0

H-UiO-66 0.2

3

1 2 3 4 5 6 7 8 9 10 11 12 13 14 15 16 17 18 19 20 21 22 23 24 25 26 27 28 29 30 31 32 33 34 35 36 37 38 39 40 41 42 43 44 45 46 47 48 49 50 51 52 53 54 55 56 57 58 59 60


0.1

UiO-66

0.0 0.2

0.4

0.6

0.8

1.0

Relative Pressure (P/P0)

1

10

100

Pore Diameter (Å)

Figure 2. (a) N2 adsorption-desorption isotherms of UiO-66 and H-UiO-66 at 77 K; (b) NLDFT pore size distribution of UiO-66 and H-UiO-66. HRTEM images of (c) UiO-66 and (d) H-UiO-66. 21 ACS Paragon Plus Environment


UiO-66

7 6 5 4 3 2 1 0 0

10

5 4 3 2 1 0 0

c (mol%)

H-UiO-66

UiO-66

453 K nHEP 20

30

473 K nHEP 10

5 4 3 2 1 0 0

20

30

493 K nHEP 10

5 4 3 2 1 0 0

20

30

513 K nHEP 10

20

30

5 4 3 2 1 0 0

H-UiO-66

453 K MCH 10

5 4 3 2 1 0 0

20

30

473 K MCH 10

5 4 3 2 1 0 0

20

30

493 K MCH 10

5 4 3 2 1 0 0

20

30

513 K MCH 10

20

30

Desorption time (min)

Figure 3. The variation of concentration of nHEP (left) and MCH (right) vs. time in the outlet-gas at varied temperature. 453 K 473 K 493 K 513 K 513 K

80 60

(b) 100

453 K 473 K 493 K

Desorption rate (%)

(a) 100

Desorption rate (%)

1 2 3 4 5 6 7 8 9 10 11 12 13 14 15 16 17 18 19 20 21 22 23 24 25 26 27 28 29 30 31 32 33 34 35 36 37 38 39 40 41 42 43 44 45 46 47 48 49 50 51 52 53 54 55 56 57 58 59 60

Page 22 of 26

40 20 0

0

1

2

3

4

5

6

7

8

9

10

453 K 473 K 493 K 513 K

80

453 K 473 K 493 K 513 K

60 40 20 0

0

1

2

3

4

5

6

7

8

9



Figure 4. The desorption rate of (a) nHEP and (b) MCH vs. time at varied temperature on UiO-66 (solid) and H-UiO-66 (hollow).


10

Page 23 of 26

(b)

370.8

TCD Signal (a.u.)

βH=7 K/min βH=5 K/min

363.7 359.8

438.2 432.3 428.1

360

400

440

480

520

560

408.1

412.5

358.3 362.3

320

360

518.7 515.9 509.7 512.3 400

(d)

355.2 352.3

437.3 433.4 421.8

426.1 320

360

400

440

TCD Signal (a.u.)


520

560

β H=10 K/min

406.9

β H=7 K/min

356.2 402.5

β H=5 K/min

350.6 399.6

518.0 515.2 511.5 508.0

345.3

523.5 521.2 513.1

508.0 480 520

β H=13 K/min

358.9

βH=10 K/min

480

410.8

βH=13 K/min

362.6

440

Temperature (K)

(c)


364.8

Temperature (K) 367.0

βH=10 K/min

415.1

368.1

525.7 520.9 514.8 509.9

423.5

βH=13 K/min

βH=10 K/min

366.6

320

418.8

βH=13 K/min

TCD Signal (a.u.)

(a)

TCD Signal (a.u.)

1 2 3 4 5 6 7 8 9 10 11 12 13 14 15 16 17 18 19 20 21 22 23 24 25 26 27 28 29 30 31 32 33 34 35 36 37 38 39 40 41 42 43 44 45 46 47 48 49 50 51 52 53 54 55 56 57 58 59 60


320

560

360

400

440

480

520

560

Temperature (K)

Temperature (K)

Figure 5. TPD spectra for nHEP desorption (a) UiO-66, (c) H-UiO-66 and MCH desorption for (b) UiO-66, (d) H-UiO-66 with various heating rate.



Page 24 of 26

TABLES Table 1. BET surface area and pore structure parameters of UiO-66 and H-UiO-66. Adsorbent SBET(m2/g) Vt(cm3/g) VBJH(cm3/g) P/P0 Range C R2 UiO-66 1264 0.91 0.0081-0.0492 1603 0.9999 H-UiO-66 731 1.47 1.41 0.0085-0.0491 428 0.9999 Table 2. The experimental conditions and parameters of dynamic adsorption separation experiments. Adsorbent Adsorbate nHEP MCH nHEP MCH

UiO-66 H-UiO-66

Alkane pressure (kPa) 5.21 5.05 5.09 4.45

Breakthrough adsorbed capacity (g/100g) 2.76 6.06 1.81 4.34

Adsorption selectivity 2.31 2.50

Table 3. The experimental conditions and parameters of dynamic adsorption/desorption experiments.

AdsorbentAdsorbate

UiO-66 nHEP H-UiO-66

UiO-66 MCH H-UiO-66

Temp. (K)

Alkane pressure (kPa)

453 473 493 513 453 473 493 513 453 473 493 513 453 473 493 513

10.04 9.94 10.24 10.19 9.88 10.11 10.21 10.18 10.11 9.86 10.52 9.93 9.58 9.52 9.51 9.64

Breakthrough Desorption Desorption Nitrogen capacity at rate at 10 adsorbed flow rate a capacity min (%) 10 min (mL/min) (g/100g) (g/100g) 8.84 19.9 5.74 49.2 6.72 20.0 5.23 53.0 4.91 19.9 4.74 64.3 3.26 20.1 4.15 59.9 4.77 20.1 2.63 45.9 2.82 20.1 3.09 66.2 2.56 20.1 3.49 84.0 1.25 19.9 2.25 90.3 13.16 19.8 2.41 15.5 11.97 20.1 2.39 17.2 10.26 19.9 3.49 27.4 9.27 20.1 3.64 33.3 6.34 19.8 2.46 36.4 6.03 19.9 2.70 40.1 4.83 20.0 2.89 48.7 4.55 20.1 2.72 50.2


Page 25 of 26 1 2 3 4 5 6 7 8 9 10 11 12 13 14 15 16 17 18 19 20 21 22 23 24 25 26 27 28 29 30 31 32 33 34 35 36 37 38 39 40 41 42 43 44 45 46 47 48 49 50 51 52 53 54 55 56 57 58 59 60


a

Breakthrough adsorbed capacity was calculated via equation (1) (seen in Supporting

Information). Table 4. The desorption activation energy (Ed) of nHEP and MCH on UiO-66 and H-UiO-66. Adsorbate Adsorbent nHEP MCH

UiO-66 H-UiO-66 UiO-66 H-UiO-66

Peak 1

R2

Peak 2

R2

Peak 3

R2

93.02 ± 2.20 59.52 ± 1.69 103.80 ± 1.49 65.06 ± 0.65

0.9764 0.9716 0.9856 0.9900

94.96 ± 2.67 86.86 ± 0.86 123.49 ± 1.99 107.87 ± 1.29

0.9719 0.9901 0.9839 0.9880

125.61 ± 0.40 117.95 ± 2.64 223.15 ± 1.12 200.41 ± 0.00

0.9968 0.9776 0.9950 1.0000



Table of Content


Page 26 of 26

Study on Desorption Process of n-Heptane and Methyl Cyclohexane

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