Development of zeolitic imidazolate framework-67

Colloids and Surfaces A 552 (2018) 16–23

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Colloids and Surfaces A journal homepage: www.elsevier.com/locate/colsurfa

Development of zeolitic imidazolate framework-67 functionalized Co-Al LDH for CO2 adsorption

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Yingli Yanga, Xinlong Yana, , Xiaoyan Hua, Rui Fenga, Min Zhoua, Wenlong Cuib a Key Laboratory of Coal Processing and Efficient Utilization (Ministry of Education), School of Chemical Engineering & Technology, China University of Mining and Technology, XuZhou, 221116, PR China b Jiangsu Key Laboratory of Advanced Catalytic Materials and Technology, Advanced Catalysis and Green Manufacturing Collaborative Innovation Center, Changzhou University, Changzhou, 213164, PR China

G R A P H I C A L A B S T R A C T

A R T I C LE I N FO

A B S T R A C T

Keywords: ZIF-67 LDH composite CO2 adsorption

The effective capture of CO2 has become increasingly important for environment protection. In this work, LDH@ ZIF-67 composite was synthesized through in-situ growth of ZIF-67 on the surface of Co-Al LDH for CO2 adsorption. The as-synthesized LDH@ZIF-67 composite was characterized by different techniques and the results confirmed that the successful formation of ZIF-67 on the surface of Co-Al LDH. The CO2 adsorption performance of the adsorbents was measured by thermogravimetric analysis and volumetric method, respectively. After introducing ZIF-67 onto the Co-Al LDH’s surface, the CO2 adsorption capacity of LDH@ZIF-67 could be as high as 22.16 mg/g at 30 °C, which was 88% of the capacity of pure ZIF-67 and much higher than that of Co-Al LDH. More importantly, the material showed rapid adsorption kinetics with ultrahigh selectivity for CO2 over N2. The kinetic data were analyzed by pseudo-first order model, pseudo-second order model, Avrami model and the intraparticle diffusion model, respectively. It was found that the behaviors of CO2 adsorption could be best described by Avrami’s kinetic model. Moreover, the LDH@ZIF-67 composite could be regenerated and the efficiency retained more than 94% up to 5 cycles of the regeneration.

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Corresponding author. E-mail address: [email protected] (X. Yan).

https://doi.org/10.1016/j.colsurfa.2018.05.014 Received 19 March 2018; Received in revised form 3 May 2018; Accepted 6 May 2018 Available online 06 May 2018 0927-7757/ © 2018 Elsevier B.V. All rights reserved.


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1. Introduction

2. Experimental

With the rapid development of industry, large amount of carbon dioxide was released from fossil fuel combustion, such as coal, oil and natural gas [1,2]. Consequently, the concentration of atmospheric carbon dioxide increased year by year, which is considered to be responsible for global warming and climate change resulting from the greenhouse effect [3,4]. Hence, it is of great importance to explore effective methods to reduce the emissions of carbon dioxide. So far, a well-known method for reducing global CO2 emission is Carbon Capture and Utilization (CCU) [5]. The first step is the CO2 capture. Among different methods developed, adsorption is considered as the most attractive method owing to its low energy cost, simple operation and high efficiency [6,7]. Different porous solid materials have been extensively studied as adsorbent for the capture of carbon dioxide, such as porous metal oxide [8], zeolite [9], activated carbons [10], carbon nanotubes [11] and Metal-Organic Frameworks (MOFs), [12,13] etc. Unfortunately, most of the above adsorbents suffered from problems of high regeneration energy, low capacity, and so on. Thus, developing efficient adsorbents for the removal of CO2 is still urgently needed. Zeolitic imidazolate frameworks (ZIFs), a sub-class of MOFs with highly porosity and adjustable structure, consist of inorganic metal ions bridged by imidazolate ligand [14]. It has been widely studied for their potential application for CO2 capture in recent years [15,16]. However, disadvantages such as high cost and being difficult to be separated caused by its ultrafine particle size, have deterred the commercial uses of ZIFs. To overcome these weaknesses, remarkable progress on the synthesis of zeolitic imidazolate frameworks (ZIFs) based composite has been achieved in recent years. Various ZIFs composites such as ZIFs/ silica oxide [17], ZIFs/graphene oxide [18] and ZIFs/polymer [19] et al have been reported. Anindita [20] et al found that the CO2 uptake of ZIF-8@Amino Clay was doubled compared with that of ZIF-8 nanoparticles at 298 K. Yeo [21] et al reported that CO2/N2 and CO2/CH4 selectivity of ZIF-8/Al2O3 was up to 3.4 and 4.0, respectively. Wang [22] et al. synthesized ZIF-8 membrane with silicon nitride hollow fibers and found it could facilitate the H2/CO2 separation. However, the above mentioned composites mostly presented physical mixed phase with part of ZIFs embedded in the composites. Layered double hydroxides (LDH) are a catagory of two-dimensional anionic clays. The structure of LDH based on brucite (Mg(OH)2) –like layers consist of metal cations and charge-balancing anions in the hydrated interlayer regions [23–26]. Due to presence of numerous metal cations on the surface, LDH could also be used as support for in situ nucleation and directed growth of ZIFs. Liu [27] et al. fabricated LDH@ZIF-8 composite by in-situ growth of ZIF-8 on the surface of ZnAl LDH, and found that CO2 adsorption capacity of the composite was larger than that of Zn-Al LDH and ZIF-8. ZIF-67 is a modification of ZIF8 framework through Zn substitution with Co, which was found catalytically efficient towards chemical fixation of CO2 with high selectivity, good reusability and economic-environmental friendly in recent years [28]. Considering these features, in situ assemble of ZIF-67 with LDH material is expected to show a good CO2 adsorption performance. Therefore, in this work, LDH@ZIF-67 composite was synthesized by in-situ growth of ZIF-67 on the surface of Co-Al LDH. The characteristics of the as-synthesized samples were analyzed by XRD, SEM, XPS, TG, CHN elemental analysis, and N2 adsorption-desorption technics et al. The CO2 adsorption performance of the samples was measured by using thermogravimetric analysis and volumetric method, respectively. The kinetics and thermodynamics were studied in detail. Furthermore, the regeneration of the adsorbent was also investigated.

2.1. Materials All chemicals used in this study were of analytical grade without additional purification: Co(NO3)2·6H2O (Sinoreagent), Al(NO3)3·9H2O (Sinoreagent), (NH2)2CO (Sinoreagent), methanol (Sinoreagent) and 2methylimidazole (J&K Scientific Company). Highly pure Ar, N2 and CO2 (99.99%) were used in the experiments. 2.2. Preparation of ZIF-67, Co-Al LDH and LDH@ZIF-67 composite ZIF-67 was synthesized according to the reported method in literature [29]. Typically, Co(NO3)2·6H2O (0.291 g) and 2-methylimidazole (0.66 g) were dissolved in 30 mL of methanol and the mixture was stirred for 24 h at room temperature. After that, the solid sample was obtained by centrifuge, wash with methanol for several times and dry at 60 °C overnight. Co-Al LDH was synthesized through a hydrothermal method. Typically, 200 mL of solution was prepared by dissolving Co (NO3)2·6H2O (2.32 g), Al(NO3)2 9H2O (1.5 g) and (NH2)2CO (20 g) in DI water. Then the mixture was kept at 100 °C for 7 h in a water bath. Finally, the pink precipitate was formed and collected by centrifugation, washing with DI water and drying overnight. The preparation of LDH@ZIF-67 composite was similar to a previously described method [30]. Briefly, 50 mL of Co(NO3)2·6H2O methanol solution (0.1 M) was added dropwise into a 60 mL of methanol solution containing 0.75 g Co-Al LDH. Subsequently, 50 mL of 2-methylimidazole dissolved in methanol (0.8 M) was quickly poured into the above mixed solution under magnetic stirring at 20 °C. After 20 min of stirring, the color of the mixture changed from pink to purple. The purple mixture was centrifuged and washed by methanol repeatedly, and dried at 60 °C for 24 h to yield the final composite material. 2.3. Characterization Powder X-ray diffraction was performed to characterize the physical phase of the samples using a Bruker D8 ADVANCE diffractometer with Cu Kα radiation (40 kV, 30 mA). The morphology was studied by Scanning Electron Microscope (SEM) (FEI Quanta 400 FEG), combined with energy dispersive X-ray spectroscopy (EDX) for the element mapping investigations. The elemental CHN analysis was carried out on a EURO EA-3000 elemental analyzer. X-ray photoelectron spectrum (XPS) measurements were performed on a Thermo Scientific Escalab 250Xi instrument equipped with Al Kα radiation. Binding energies for the high-resolution spectra were calibrated by setting C1s at 284.8 eV. Thermogravimetric (TG) analysis was carried out on a thermogravimetric analyzer (Netzsch STA 449 F5). The samples were heated from room temperature to 800 °C with a temperature ramp of 10 °C min −1 under Ar gas with flow rate of 100 cm3 min−1. N2 adsorption-desorption isotherms were measured by a gas adsorption analyzer (IQ2 Quantachrome porosimeter). The samples were outgassed at 100 °C for 12 h before analysis. The BET and t-plot method was used to analyze the specific curface area and micropore volume (Vmicro) of the samples, respectively. 2.4. CO2 adsorption measurements Dynamic adsorption measurements were conducted using thermogravimetric analysis on a thermogravimetric setup (Netzsch STA 449 F5) according to the previously published work with some changes [31]. About 10 mg of adsorbent was taken into the TGA measuring crucible. The sample was pretreated by heating to 100 °C and maintained for 1 h under pure Ar at a flow rate of 30 cm3 min−1 to remove water molecules from the adsorbent before the measurement. Then, the temperature was reduced to the sorption temperature (30 °C) and 17


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Fig. 3. The TGA curves of ZIF-67, Co-Al LDH and LDH@ZIF-67 samples. Fig. 1. XRD patterns of ZIF-67, Co-Al LDH and LDH@ZIF-67.

formation of ZIF-67 in the composite. The morphology of the as-prepared materials was characterized by SEM (Fig. 2). It could be seen that the Co-Al LDH particle consisted of several thin hexagonal platelets (Fig. 2a). Obviously, the surface of LDH@ZIF-67 composite (Fig. 2c) presented rhombic dodecahedron shape in accordance with that of the ZIF-67 crystals (Fig. 2b). In addition, ZIF-67 was homogeneously distributed on the surface of Co-Al LDH as proved by elemental mapping (Fig. 2e–h). Furthermore, it was concluded that some special interaction occurred between Co-Al LDH and 2-methylimidazole, since the solution color changed from pink to purple with mixing with 2-methylimidazole solution [30]. These results revealed the effective formation of ZIF-67 onto the surface of Co-Al LDH. The element analysis of LDH@ZIF-67 composite gave 27.87%, 3.91% and 16.17% for carbon, hydrogen and nitrogen element, respectively. Consequently, the content of ZIF-67 in the composite was estimated to be 64.4 wt.% based on the nitrogen content in the composite. The TGA curves of ZIF-67, Co-Al LDH and LDH@ZIF-67 are shown in Fig. 3. The weight loss of all samples could be attributed to desorption of moisture with temperature lower than 100 °C. Similar tendency was observed during the temperature range of 100–330 °C for CoAl LDH and LDH@ZIF-67 composite, with weight loss taking place at 100–200 °C and 200–330 °C, corresponding to loss of water in the interlayers of Co-Al LDH and decomposition of NO3− and OH− in the Co-

maintained for 1 h under pure Ar. The sorption was carried out at 30 °C for 20 min by switching the gas from pure Ar to pure CO2. Afterwards, desorption was performed at 100 °C for 1h under pure Ar gas. Several adsorption-desorption cycles was further conducted over the adsorbents to test their reusability. The CO2 adsorption capacity was calculated based on the weight change during the sorption processes. The CO2 adsorption isotherms at different temperatures and pressures were also measured by volumetric method using IQ2 Quantachrome porosimeter. Before adsorption, the samples were outgassed at 150 °C for 3.5 h to remove the guest molecules.

3. Results and discussion 3.1. Characterization of the samples Crystallinity of the samples was measured by XRD, as shown in Fig. 1, the patterns of Co-Al LDH exhibited five peaks at 11.6°, 23.4°, 34.5°, 39.1° and 46.5°, corresponding to (003), (006), (012), (015) and (018) planes of Co-Al layered double hydroxides, respectively [32,33]. After in-situ growth of ZIF-67 on LDH’s surface, the LDH@ZIF-67 composite displayed some new peaks at 7.4°, 7.8°, 10.4° and 14.8°, which was in good accordance with the ZIF-67. No impurity peaks were observed, indicating high purity of the products and successful

Fig. 2. SEM images of (a) Co-Al LDH; (b) ZIF-67; (c) LDH@ZIF-67 and (d–h) elemental mapping of LDH@ZIF-67. 18


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Fig. 5. N2 adsorption-desorption isotherms (a) and NLDFT pore size distributions (b) of ZIF-67, Co-Al LDH and LDH@ZIF-67 samples.

the abundant N element from imidazole ligand [39]. The textural properties of the samples were investigated by N2 adsorption-desorption measurements (Fig. 5). A mesoporous structure was concluded to be incorporated in the Co-Al LDH, since the isotherm was identified as type IV behavior with rapid uptake at high relative pressure. After in-situ growth of ZIF-67 on the surface of Co-Al LDH, the composite exhibited a type I isotherm, similar with that of ZIF-67, indicating the existence of micropores. Moreover, the BET surface area and total pore volume of the LDH@ZIF-67 were 1196.49 m2/ g and 0.55 cm3/g, notably higher than Co-Al LDH (31.74 m2/g and 0.22 cm3/ g) but smaller than pure ZIF-67 (2019.49 m2/ g and 0.96 cm3/g). The pore size distribution of the samples is shown in Fig. 5 (b). There is a broad distribution of pore sizes observed for Co-Al LDH sample. With the growth of ZIF-67 onto LDH, the pore size of the composite centered at 1–3 nm. The smaller pore is convinced from the inheritance of microporous ZIF-67.

Fig. 4. XPS spectra of Co-Al LDH and LDH@ZIF-67 samples.

Al LDH respectively [34,35]. Somewhat differently, an extra weight loss was observed for LDH@ZIF-67 composite at temperature higher than 500 °C. It should be resulted from the decomposition of the ligand in ZIF-67 [36]. This result also suggested the existence of ZIF-67 in the composite, which was consistent with the XRD and SEM results. The surface chemical information of the composite was analyzed by XPS (Fig. 4). All the binding energies were calibrated using the C 1s peak at 284.8 eV. The full XPS spectrum (Fig. 4a) of Co-Al LDH and LDH@ZIF-67 displayed signals of Co, Al, N, O and C element. The existence of Co2+ in Co-Al LDH and LDH@ZIF-67 composite was evidenced by two main peaks located at 781.2 eV and 797.4 eV (Fig. 4b), which was assigned to Co2p3/2 and Co2p1/2 [37], respectively. After wrapped by ZIF-67, the lower intensity of signals should be ascribed to the fact that cobalt was surrounded by imidazole ligand in the composite. Furthermore, there was a slight shift of the Co2p binding energy of LDH@ZIF-67, demonstrating the change of chemical environment of cobalt [38]. A peak at 398.9 eV could be observed in N1s region (Fig. 4c), with much stronger intensity for the composite, resulting from

3.2. CO2 adsorption performance Fig. 6a shows CO2 adsorption capacity of the three samples (Co-Al LDH, ZIF-67 and LDH@ZIF-67) at 30 °C under atmospheric pressure measured by thermogravimetric analysis. Both ZIF-67 and LDH@ZIF-67 sample exhibited fast adsorption kinetics, and approximately 4 min were required to achieve adsorption equilibrium. A low adsorption capacity of 8.65 mg/g was obtained for the Co-Al LDH. After in-situ growth of ZIF-67 over the surface, the uptake of LDH@ZIF-67 composite reached 22.16 mg/g, which was only 12% less than that of pure ZIF-67 (25.12 mg/g). Considering its lower cost and easy to separate because of large particle size, LDH@ZIF-67 still has a great potential for CO2 capture. The obtained volumetric adsorption capacity of different 19


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Fig. 7. Experimental adsorption capacity of ZIF-67, Co-Al LDH and LDH@ZIF67 along with corresponding fit to kinetic models.

85 kPa, respectively. P1 and P2 represent the equilibrium partial pressure of CO2 (15 kPa) and N2 (85 kPa) in the bulk gas phase, respectively. The result showed that the CO2/N2 adsorption selectivity of LDH@ZIF-67 could be as high as 17.1. This high selectivity may derives from increased electrostatic interactions arising from the appreciable charge overlap between the ZIFs framework and one of the CO2 oxygens as compared to the weaker framework-N2 interactions [41,42]. Furthermore, the comparison of CO2 adsorption capacity on various adsorbents was listed in Table 1. It can be seen that LDH@ZIF-67 were comparable with those ZIFs or LDH based adsorbent obtained by others. 3.2.1. Kinetics study To further investigate the kinetics of CO2 adsorption onto the above adsorbents, four different models including pseudo-first order model, pseudo-second order model, Avrami model and the intraparticle diffusion model were employed to fit the experimental data. Pseudo-first order model and pseudo-second order model are the most commonly used adsorption rate models; the former assumes that the adsorption rate is in proportional relationship with the number of free adsorption sites and the later proposes that the adsorption capacity is in proportion to the amount of active sites over the sorbent. Avrami model is used to simulate phase transition and crystal growth of materials initially and it has been applied to describe the CO2 adsorption onto amine-modified materials in recent years [45,46]. The intraparticle diffusion model is based on the assumption that the uptake of CO2 is in linear relation with the square root of time if the adsorption process is governed by intraparticle diffusion. The intraparticle diffusion model usually consist of three steps, i.e., the external diffusion adsorption or boundary layer diffusion of the gas phase, the gradual adsorption stage where the intraparticle diffusion of gas molecules occurs, and finally the adsorption equilibrium stage [47]. The pseudo-first order model equation:

Fig. 6. Adsorption curves of thermogravimetric analysis (a) and volumetric method (b) for different samples at 30 °C.

Table 1 Comparison of CO2 adsorption capacity onto different adsorbents at ∼1 atm. Adsorbent

Adsorption capacity mg/g (Adsorption Temperature)

References

ZIF-8/PS GO/MWCNT-LDH ZIF-8 ZIF-L ZIF-67 LDH@ZIF-67

∼45 (273 K) ∼20.2 (573 K) ∼35 (298 K) ∼41 (298 K) 28.7 (303 K) 22.2 (303 K)

[19] [25] [43] [44] This work This work

adsorbents was shown in Fig. 6b. With the pressure increased, the CO2 uptake of all the samples gradually increased. The adsorption capacity was basically close to that measured by thermogravimetric analysis with the pressure approaching atmospheric pressure. The CO2 adsorption capacity of LDH@ZIF-67 composite (22.19 mg/g) was a little lower than the pure ZIF-67 (28.72 mg/g) at 30 °C, but it was greatly higher than that of Co-Al LDH (5.45 mg/g). Moreover, the N2 adsorption capacity of the adsorbent at 30 °C was also measured to investigate the selectivity for CO2/N2 separation. The results showed that a tiny amount of N2 was adsorbed by LDH@ZIF-67 (Fig. 6b). Since exhaust gas from post-combustion fossil-fuel power station commonly consists of approximately 85% N2 and 15% CO2 at constant pressure. Thus, the CO2/N2 adsorption selectivity of LDH@ZIF-67 was estimated according to the following equation [40]: q1

S=

P1

qt = qe (1−e−kf t )

(2)

The pseudo-second order model equation:

qt =

qe2 ks t 1 + qe ks t

(3)

Avrami model equation: n

qt = qe [1−e (−ka t ) ]

(4)

The intraparticle diffusion model equation: q2 P2

qt = ki t 0.5 + C

(1)

(5) −1

−1

−1

Where kf (min ), ks (g (mg min) ), ka (min ) and ki (mg (g min0.5)−1) are rate constants. qt and qe represent the adsorption

where q1 and q2 are the adsorbed amount of CO2 at 15 kPa and N2 at 20


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Table 2 Kinetic parameters for CO2 adsorption on ZIF-67, Co-Al LDH and LDH@ZIF-67 at 30 °C. Sample

Co-Al LDH ZIF-67 LDH@ZIF-67 Sample

Co-Al LDH ZIF-67 LDH@ZIF-67

qe

exp

(mg g−1)

8.65 25.12 22.16 qe exp (mg g−1)

8.65 25.12 22.16

Pseudo-first-order kinetic model

Pseudo-second-order kinetic model

kf (min−1)

qe (mg g−1)

R2

ks (g (mg min)−1)

qe (mg g−1)

R2

0.2601 0.4782 0.4231

9.4473 25.7338 23.0527

0.9744 0.9625 0.9758

0.01852 0.01894 0.01698

12.482 30.321 27.683

0.9555 0.9182 0.9443

Avrami model ka (min−1)

qe (mg g−1)

n

R2

0.2919 0.4876 0.4266

8.7825 25.0325 22.3415

1.5450 1.7299 1.5208

0.9994 0.9985 0.9976

Fig. 8. Curves of the intraparticle diffusion model for CO2 adsorption over CoAl LDH, ZIF-67 and LDH@ZIF-67 at 30 °C.

capacity at a given point time and equilibrium time, respectively. n is the order of kinetic equation and C is related to the boundary layer thickness. Experimental adsorption capacity of Co-Al LDH, ZIF-67 and LDH@ ZIF-67 and kinetic parameters are shown in Fig. 7 and Table 2. It can be clearly seen that neither pseudo-first-order nor pseudo-second-order kinetic model fitted well with the adsorption data. An overestimation of the adsorption uptake at the initial phase, but underestimation of the adsorption approaching equilibrium and at the equilibrium stage was observed for the results analyzed by pseudo-first-order and pseudosecond-order models. The result in Table 2 demonstrated that the Avrami model with higher coefficients value could match well with the adsorption of CO2 on Co-Al LDH, ZIF-67 and LDH@ZIF-67, suggesting that both physisorption and chemisorption was included into the adsorption process [48]. The data clearly showed an increase in adsorption rate of LDH@ ZIF-67 compared to Co-Al LDH, owing to the presence of ZIF-67 in the composite. Furthermore, the experimental qe,exp value was in well agreement with the theoretical qe value calculated from the Avrami model, indicating that the Avrami model was more suitable for describing the CO2 adsorption behavior over LDH@ZIF-67. The kinetic data were further studied by simulating with the intraparticle diffusion model, for separating different diffusion stages of the adsorption process. As shown in Fig. 8, the plots are not linear, indicating that intraparticle diffusion is not the rate-controlling step [49]. The curves all go through three stages, which are film diffusion, gradual adsorption and equilibrium stage, respectively. During the first stage, CO2 diffusion through the gas phase to the surface of adsorbent.

Fig. 9. (a) Adsorption isotherms at 0 °C, 10 °C and 30 °C and (b) Isosteric heat of CO2 adsorption on LDH@ZIF-67 sample.

The second portion was related to the gradual adsorption stage, where the intraparticle diffusion of CO2 occurs. In the third step, the adsorption gradually reached the equilibrium, where the CO2 molecules bound with the surface active sites. Yet there would be only one ratelimiting step, which could be inferred from the slope of the linear portions. It can be seen that the minimum value was found for the slope of the first stage, indicating that the diffusion of CO2 from bulk phase to the surface of the adsorbent was the rate-limiting step [50].

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ZIF-67 decreased slightly with increasing cycle numbers. The value decreased from 22.88 mg/g to 21.56 mg/g, with only 5.77% loss of initial capacity after five adsorption/desorption cycles, moreover, XRD pattern (Fig. 10b) remained almost unchanged compared to those of the pristine LDH@ZIF-67, demonstrating that LDH@ZIF-67 had good stability during adsorption-desorption cycles. 4. Conclusions In this work, LDH@ZIF-67 composite was successfully prepared and its adsorption performance for CO2 was studied. At 30 °C, the CO2 uptake of LDH@ZIF-67 was 22.16 mg/g, much higher than Co-Al LDH and nearly 88% of the capacity of pure ZIF-67. Moreover, the composite showed rapid adsorption kinetics with ultrahigh selectivity for CO2 over N2. The kinetic data could be well described by the Avrami model, suggesting that physisorption and chemisorption coexisted in the adsorption process. The results simulated by intrapariticle diffusion model revealed that there were three stages in the adsorption process, and film diffusion stage was recognized as the rate-controlling step. The isosteric heat of adsorption remained approximately 28.39 kJ/mol at high CO2 coverage. In addition, the LDH@ZIF-67 composite exhibited good regenerability with more than 94% of its original capacity retained after 5 cycles of adsorption/desorption. Acknowledgments This work was supported by National Natural Science Foundation of China (No. 21606252 and 21506247), Natural Science Foundation of Jiangsu Province (No. BK20140260), National key R&D program of China (No. 2016YFE0102500) and the Priority Academic Program Development of Jiangsu Higher Education Institutions. References [1] E. Kintisch, The greening of synfuels, Science 320 (2008) 306–308. [2] R.J. Andres, T.A. Boden, F.M. Bréon, P. Ciais, S. Davis, D. Erickson, J.S. Gregg, A. Jacobson, G. Marland, J. Miller, T. Oda, J.G.J. Olivier, M.R. Raupach, P. Rayner, K. Treanton, A synthesis of carbon dioxide emissions from fossil-fuel combustion, Biogeosciences 9 (2012) 1845–1871. [3] T.R. Andersona, E. Hawkins, P.D. Jones, CO2, the greenhouse effect and global warming: from the pioneering work of Arrhenius and callendar to today’s earth system models, Endeavour 40 (2016) 178–187. [4] J.W. Akitt, Some observations on the greenhouse effect at the earth’s surface, Spectrochim. Acta A 188 (2018) 127–134. [5] J. Patricioa, A. Angelis-Dimakisb, A. Castillo-Castillob, Y. Kalmykovaa, L. Rosado, Region prioritization for the development of carbon capture and utilization technologies, J. CO2 Util. 17 (2017) 50–59. [6] S. Ahmed, A. Ramli, S. Yusup, S. Ahmed, A. Ramli, S. Yusup, Development of polyethylenimine-functionalized mesoporous Si-MCM-41 for CO2 adsorption, Fuel Process. Technol. 167 (2017) 622–630. [7] J. Kim, L.C. Lin, J.A. Swisher, Predicting large CO2 adsorption in aluminosilicate zeolites for postcombustion carbon dioxide capture, J. Am. Chem. Soc. 134 (2012) 18940–18943. [8] R. Bargougui, N. Bouazizi, J.-F. Hochepied, F. Le Derf, J. Vieillard, S. Ammar, Microwave-assisted polyol synthesis of mesoporous Ta doped mixed TiO2/SnO2: application for CO2 capture, J. Alloys Compd. 728 (2017) 391–399. [9] M.R. Hudson, W.L. Queen, J.A. Mason, D.W. Fickel, R.F. Lobo, C.M. Brown, Unconventional, highly selective CO2 adsorption in zeolite SSZ-13, J. Am. Chem. Soc. 134 (2012) 1970–1973. [10] R.V. Siriwardane, M. Shen, E.P. Fisher, J.A. Poston, Adsorption of CO2 on molecular sieves and activated carbon, Energy Fuels 15 (2001) 279–284. [11] D.J. Babu, M. Bruns, J.J. Schneider, Unprecedented CO2 uptake in vertically aligned carbon nanotubes, Carbon 125 (2017) 327–335. [12] X. Yan, S. Komarneni, Z. Zhang, Z. Yan, Extremely enhanced CO2 uptake by HKUST1 metal-organic framework through chemical treatment, Micropor. Mesopor. Mater. 183 (2014) 69–73. [13] K.S. Walton, A.R. Millward, D. Dubbeldam, H. Frost, J.J. Low, O.M. Yaghi, R.Q. Snurr, Understanding inflections and steps in carbon dioxide adsorption isotherms in metal-organic frameworks, J. Am. Chem. Soc. 130 (2008) 406–407. [14] K.S. Park, Z. Ni, A.P. Côté, J.Y. Choi, R. Huang, F.J. Uribe-Romo, H.K. Chae, M. O’Keeffe, O.M. Yaghi, Exceptional chemical and thermal stability of zeolitic imidazolate frameworks, Proc. Natl. Acad. Sci. U. S. A. 103 (2006) 10186–10191. [15] Y. Guana, J. Shi, M. Xia, J. Zhang, Z. Pang, A. Marchetti, X. Wang, J. Cai, X. Kong, Monodispersed ZIF-8 particles with enhanced performance for CO2 adsorption and heterogeneous catalysis, Appl. Surf. Sci. 423 (2017) 349–353.

Fig. 10. (a) Reusability of LDH@ZIF-67 for CO2 adsorption and (b) XRD pattern of LDH@ZIF-67 after 5 cycles of regeneration.

3.2.2. Adsorption isotherms The equilibrium adsorption isotherms of LDH@ZIF-67 at 0 °C, 10 °C and 30 °C were investigated by low pressure static CO2 adsorption measurements (Fig. 9a). The CO2 uptake dropped from 75.29 mg/g to 22.19 mg/g with the increase of temperature from 0 °C to 30 °C as expected, indicating the exothermic characteristic of the adsorption. The isosteric heat was obtained by calculations of the isotherms at constant coverage according to the Clausius-Clapeyron equation expressed in Eq. (6) [51]. The value of ΔHads can be calculated from the slope of the linear plots of 1/T vs ln(p).

ΔHads = R (

p T1 T2 )ln 2 T2−T1 p1

(6)

As shown in Fig. 9b, with the coverage increased, the value of the isosteric heat gradually decreased from 30.91 kJ/mol to 28.39 kJ/mol, indicating that the adsorption sites on the adsorbent were heterogeneous. The change of the curve can be explained by the fact that the interaction between two adsorbate molecules inside pores gradually became weaker than the adsorbent-adsorbate interaction as the coverage of CO2 increased. As a result, the isosteric heat of adsorption decreased firstly and then reached a constant value at high CO2 coverage [52]. 3.2.3. Recyclability To evaluate the regeneration performance of LDH@ZIF-67, desorption process was carried out at 100 °C for 1 h under pure Ar gas by using TGA and five cycles were measured in this work. The results displayed in Fig. 10a showed that the CO2 adsorption capacity of LDH@ 22


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