Surface engineering of a chromium metal-organic framework with bifunctional ionic liquids for selective CO2 adsorption: Synergistic effect between multiple active sites

Accepted Manuscript Surface engineering of a chromium metal-organic framework with bifunctional ionic liquids for selective CO2 adsorption: Synergistic effect between multiple active sites Chong Chen, Nengjie Feng, Qirui Guo, Zhong Li, Xue Li, Jing Ding, Lei Wang, Hui Wan, Guofeng Guan PII: DOI: Reference:

S0021-9797(18)30271-6 https://doi.org/10.1016/j.jcis.2018.03.029 YJCIS 23379

To appear in:

Journal of Colloid and Interface Science

Received Date: Revised Date: Accepted Date:

31 December 2017 4 March 2018 11 March 2018

Please cite this article as: C. Chen, N. Feng, Q. Guo, Z. Li, X. Li, J. Ding, L. Wang, H. Wan, G. Guan, Surface engineering of a chromium metal-organic framework with bifunctional ionic liquids for selective CO2 adsorption: Synergistic effect between multiple active sites, Journal of Colloid and Interface Science (2018), doi: https://doi.org/ 10.1016/j.jcis.2018.03.029

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Surface engineering of a chromium metal-organic framework with bifunctional ionic liquids for selective CO2 adsorption: Synergistic effect between multiple active sites Chong Chen, Nengjie Feng, Qirui Guo, Zhong Li, Xue Li, Jing Ding, Lei Wang, Hui Wan *, Guofeng Guan * State Key Laboratory of Materials-Oriented Chemical Engineering, College of Chemical Engineering, Jiangsu National Synergetic Innovation Center for Advanced Materials, Nanjing Tech University, Nanjing 210009, P. R. China

*Corresponding author Telephone: +86-25-83587198 E-mail: [email protected] (H. Wan) E-mail: [email protected] (G. Guan)

ABSTRACT Targeting CO2 capture application, a new strategy for building multiple adsorption sites in metal-organic framework MIL-101(Cr) was constructed through the incorporation of diethylenetriamine-based ionic liquid (DETA-Ac) via a post-synthetic modification approach. The DETA-Ac, with multi-amine-tethered cation and acetate anion, could not only provide additional binding sites, but also enhance the affinity of framework surfaces toward CO2. Simultaneously, the high surface area and large cage size of MIL-101(Cr) ensured the better dispersion of IL, thus exposing more active sites for CO2 adsorption. In addition, enough free space was still retained after functionalization, which facilitated CO2 transport and allowed the Cr(III) sites deep within the pores to be accessed. The multiple adsorption sites originating from IL and MOF were found to synergistically affect the CO2 capture performance of the composite. The adsorption capacity and selectivity of DETA-Ac@MIL-101(Cr) for CO2 were significantly improved. The higher isosteric heats of adsorption (Qst) evidenced the stronger interaction between the composite and CO2 molecules. Moreover, a possible two-step mechanism was proposed to reveal the manner in which CO2 bound to the IL-incorporated frameworks. Despite the relatively high initial Q st value, the DETA-Ac@MIL-101(Cr) could be easily regenerated with almost no drop in CO2 uptake during six cycles.

Keywords: Metal-organic framework; MIL-101(Cr); Bifunctional ionic liquids; Selective CO2 adsorption; Multiple active sites; Synergistic effect

1. Introduction The massive combustion of fossil fuels is causing a swift increase in atmospheric carbon dioxide (CO2) concentration year by year, which has become the culprit of greenhouse effect and has further led to severe global climate change [1]. In this regard, substantial efforts are being made to mitigate the emission of CO2. However, the efficient capture of CO2 from the flue gas (CO2/N2) is still an enormous challenge as the CO2 level contained is relatively low (10-15%) [2,3]. Various technologies have been implemented to alleviate this problem [4-6], wherein the chemical absorption of CO2 using aqueous amine solutions (e.g. monoethanolamine, MEA) is regarded as the most practical technology for the selective CO2 separation on an industrial scale [7,8]. Nevertheless, these amine-based absorbents often have some intrinsic drawbacks, such as solvent loss and degradation, equipment corrosion, high energy costs during regeneration step, and unfriendly to the environment [9,10]. This motivates the scientists to explore alternative absorbents for CO2 removal. Ionic liquids (ILs) are well recognized as novel green solvents, which have been applied extensively in different areas [11-14]. Their distinct properties, including negligible vapor pressure, good thermal/chemical stability, and tailorable structure make them potential replacements for organic amines in CO2 capture. Many early researchers have investigated and proved that the CO2 gas is highly soluble in ILs [15-17]. However, the absorption capacities of conventional ILs are rather low particularly at low CO2 partial pressures due to the nature of physical dissolution. An orthogonal pathway of improving the absorption performance is to introduce

additional functional groups that can afford chemisorption of CO2. Davis et al. [18] designed a new task-specific IL tethering a primary amine moiety to the imidazolium cation for CO2 capture. The chemical absorption process followed a 1:2 mechanism where per mole of CO2 reacted with two moles of IL to form the carbamate. The theoretical maximum of the amine-appended IL was 0.5 mol CO2/mol IL, and this value was comparable to that of MEA. Afterward, a large number of other functionalized ILs were synthesized [19-21], which greatly contributed to the progress in CO2 chemisorption. Based on the designability of ILs, further enhancement in CO2 uptake could be realized by incorporating more active sites into the anions. Lv and co-workers [22] reported the preparation of a dual amine-functionalized IL [APmim][Gly]. Owing to the reactivities of both amine and amino acid groups, the CO2 capacity reached 1.23 mol CO2/mol IL, almost twice of the single amine-containing ILs. It is well-documented that the anion species, such as azolate, phenolate, acetate, and sulfonate anions, also have obvious implications for capturing CO2 [23-26]. All these corroborated the huge potential of multiple-site based ILs as efficient absorbents. However, still several hurdles need to be overcome before the large-scale application. The synthesis of such functionalized ILs commonly requires complicated reaction steps, and the purification of products is difficult [27]. Moreover, the high viscosity of ILs will limit the mass transfer and diffusion, which is not conducive to the absorption of CO2 [28]. One feasible scenario that could improve the uptake efficiency would be the immobilization of ILs onto the surfaces of porous solids. The properties of supports are reported to affect the adsorption performances

of the supported ILs through host-guest interactions [29], therefore, seeking for suitable host materials is the main concern of this research field. Over the past decades, metal-organic frameworks (MOFs) with the features of large surface areas, accessible free pore volumes, abundant adsorption sites, and variable structures, have been in the limelight as multifunctional materials for various applications [30-34]. A large number of publications have reported the exceptional CO2 adsorption capacities of MOFs [35-39], which are more advantageous over traditional solid adsorbents like zeolite and activated carbon. However, there still remain concerns about their relatively low selectivities toward CO2 and the weak interactions between the frameworks and the adsorbate in the case of low-concentration CO2 removal from the gas mixtures. To solve these problems, different CO2-philic moieties have been introduced through the post-synthetic modification (PSM) of the frameworks [40-42]. More recently, the ILs are also employed for functionalization, where the MOFs act as the host materials to reduce the viscosity of ILs and facilitate CO2 transport [43,44]. Xue et al. [45] investigated the dispersion behaviors of [BMIM][SCN] in five different MOFs and compared the CO2/N2 separation performances of the MOF-supported ILs by the molecular simulation method. They found that introducing IL into the pores could lead to enhancements in capture performances, which was more evident with better IL dispersion. Ma and co-workers [46] synthesized a mixed-matrix membrane based on [C3NH2bim][Tf2N] and NH2-MIL-101(Cr) for CO2/N2 separation. Apart from the reduction in adsorption capacity, the amine-containing IL as the selective CO2

transport carrier could increase the CO2 permeability and further improve the CO2/N2 selectivity. Simultaneously, the MOF could control the dispersion of IL, and thus provided more effective adsorption sites. Although such IL-MOF composites exhibit considerable promise in CO2 capture, the related reports are still lacking. Much more attentions should be directed towards the theoretical and experimental studies to pave the way for practical application. In addition, despite the high selectivities of the composites for CO2, the adsorption capacities are often lower in comparison with pristine MOFs [46-48]. Thus, the oriented design and synthesis of novel IL-MOF composites combining both high CO2 adsorption capacities and selectivities are intensively desired. Herein, we present a way to construct a highly efficient adsorbent for CO2 capture through tailoring the active adsorption sites of the IL-MOF composites (Scheme 1). The designabilities of both ILs and MOFs can provide numerous possibilities for tuning the separation performances of materials. It is well known that more sites are generally accompanied by higher CO2 adsorption capacities, therefore, a diethylenetriamine-based IL (DETA-Cl) containing multiple amine groups is selected as the guest material. This IL can be synthesized by a simple one-step method using low-priced starting materials, which is more accessible and cost-competitive than traditional functionalized ILs. Moreover, the anions can be further tailored to afford more reactive sites. In this study, the basic acetate anions ([Ac]-) are introduced given their high CO2 solubility as well as the strong interactions with CO2 [49-53]. To fabricate the IL-MOF composite, the chromium(III) terephthalate (MIL-101(Cr))

featuring high physicochemical stability and large mesoporous cages is chosen as a representative model of host material. The coordinatively unsaturated Cr(III) centers in activated MIL-101(Cr) can be utilized to facilitate the grafting of DETA-based IL. In this way, better dispersion of IL inside the pores can be achieved, thus exposing more active sites and providing a selective pathway for CO2. Although part of the Cr(III) sites are occupied, additional binding sites from the IL are available, which can bring a higher density of the active adsorption sites to the material. In addition, the cage sizes of MIL-101(Cr) are large enough for CO2 movement even after the incorporation of IL. The synergistic effect between DETA-based IL and MIL-101(Cr) can also be expected owing to the accessibilities of both functional groups and open metal sites, which may result in further improvement in CO2 capacity. Following this strategy, the DETA-Ac@MIL-101(Cr) composite was assembled and applied for the selective adsorption of CO2/N2 (0.15/0.85). The isosteric heat of adsorption (Qst) of CO2 on the composite was estimated and the possible adsorption mechanism was also proposed.

2. Experimental section 2.1. Materials All chemicals used in synthesis, including chromium (III) nitrate nonahydrate (Cr(NO3)3·9H2O), terephthalic acid (H2BDC), N,N-dimethylformamide (DMF), diethylenetriamine (DETA), concentrated hydrochloric acid (HCl), sodium acetate anhydrous, hydrofluoric acid (HF, 48%), and anhydrous ethanol are of analytical

purity and commercially available. Unless otherwise mentioned, these reagents were used as purchased from Aladdin Industrial Corporation. 2.2. Synthesis of MIL-101(Cr) MIL-101(Cr) was synthesized via a simple hydrothermal method with reference to a reported procedure [54]. Generally, Cr(NO3)3·9H2O (4.0 g), H2BDC (1.66 g), and HF (0.4 mL) were added in 48 mL of deionized water. Then, the mixture was stirred for 30 min and transferred into a Teflon-lined autoclave (100 mL). The reaction container was kept at 220 oC in an oven for 8 h. After cooling down, the green powder was collected by centrifugation and washed with DMF for three times. The solid was then treated with hot ethanol at 80 oC overnight to remove the unreacted terephthalic acid. This process was repeated three times, and the MIL-101(Cr) product was finally obtained by filtration and dried at 80 oC for 12 h under vacuum. 2.3. Synthesis of diethylenetriamine-based ionic liquid The diethylenetriamine-based ionic liquid DETA-Cl was prepared through the one-step acid-base neutralization of DETA with HCl. In a typical process, equimolar HCl was added dropwise into DETA in ice-water bath under vigorous stirring. Then, the mixture was allowed to react at 25 oC for another 4 h with continuous agitation. After removing the water by evaporation, the light-yellow transparent liquid was dried under vacuum at 80 oC for 24 h to obtain the target ionic liquid DETA-Cl. 2.4. Synthesis of DETA-Ac@MIL-101(Cr) The post-synthetic modification (PSM) approach was employed for the incorporation of diethylenetriamine-based ionic liquid in MIL-101(Cr). First, an

excess of the DETA-Cl (0.5 g) was dissolved in 30 mL of ethanol. After that, 1.0 g of activated MIL-101(Cr) powder was well dispersed into the solution by ultrasonic treatment. The suspension was stirred at 30 oC for 24 h, and then the solid was separated by centrifugation. The unattached ionic liquid was eluted with ethanol, and the product 1 (DETA-Cl@MIL-101(Cr)) was achieved after vacuum drying. Second, the DETA-Ac@MIL-101(Cr) composite (product 2) was fabricated through anion exchange. In this step, an excess of sodium acetate anhydrous was mixed with 1.0 g of the product 1 and stirred in 30 mL of ethanol at room temperature for 24 h. Afterward, the resulting dark-green solid was filtered, washed three times with ethanol aqueous solution, and dried in a vacuum at 50 oC for 12 h. The DETA-Ac-Imp-MIL-101(Cr) sample was prepared by direct impregnating MIL-101(Cr) with an ethanol solution of DETA-Ac, and the addition amount of IL was determined based on the thermogravimetric analysis. 2.5. Adsorbent characterization Powder X-ray diffraction (XRD) data of the adsorbents were collected using a SmartLab diffractometer (Rigaku, Japan) equipped with Cu Kα radiation (λ = 1.5418 Å). The scanning angle was in the range of 2-10o (2θ) and the scanning rate was 0.02o per second. The Brunauer-Emmett-Teller (BET) surface areas of samples were calculated from the N2 adsorption-desorption isotherms measured by a Micromeritics ASAP 2020 apparatus at 77 K. Prior to measurement, the samples were evacuated at 100 oC for 8 h under vaccum. The morphologies of the as-prepared crystals were observed on a Hitachi S-4800 field-emission scanning electron microscope (SEM)

instrument. Fourier transform infrared (FT-IR) spectra from 4000 to 400 cm-1 were performed on a Thermo Nicolet 6700 spectrometer using the KBr/sample pellets. X-ray photoelectron spectroscopy (XPS) was adopted to investigate the surface chemical compositions and bonding structures of samples. The measurements were conducted by using a PHI-5000 spectrometer fitted with Al Kα radiation source (hν = 1486.6 eV). Thermogravimetry (TG) analyses were carried out up to 800 oC at a heating rate of 10 oC/min using a STA 409PC thermogravimetric analyzer under N2 atmosphere. Elemental analyses (C, H, and N) of samples were measured using the Elementar Vario EL cube elemental analyzer. 2.6. CO2 and N2 adsorption measurements The CO2 and N2 adsorption isotherms in the pressure range 0-1.0 bar were obtained using a BELSORP max instrument (MicrotracBEL, Japan) via the volumetric technique. Approximately 0.1 g of each sample was added into the analysis tube and evacuated at 100 oC for 8 h. Then, the adsorption measurement was then conducted at different temperatures (25, 35, and 45 oC) under water bath. The regenerability of the adsorbent was investigated by repeating the CO2 adsorption process for six times. After each test, the adsorbent was degassed under dynamic vacuum at 100 oC for 1 h.

3. Results and discussion 3.1. Characterization The

XRD

patterns

of

MIL-101(Cr),

DETA-Cl@MIL-101(Cr),

and

DETA-Ac@MIL-101(Cr) are shown in Fig. 1. The characteristic diffraction peaks at 2θ = 2.80o, 3.28o, 3.96o, 5.14o, 5.82o, 8.44o, and 9.04o could be clearly observed from Fig. 1(a) as the evidence for the formation of MIL-101(Cr) structure [55]. For the samples containing DETA-based ionic liquids, the positions of main peaks remained almost unchanged, indicating that the grafting of ionic liquids did not affect the crystal structure of MIL-101(Cr). However, the intensities of peaks at 3-7o decreased, especially that of the peak at 3.28o corresponding to the (222) plane. This was related to the changes in electron density within the pores of MIL-101(Cr) [56], implying that the ionic liquids were incorporated and partially occupied the mesocages. The XRD pattern

of

DETA-Cl@MIL-101(Cr)

closely

resembled

that

of

DETA-Ac@MIL-101(Cr), whereas the intensity of the (222) peak further declined. It was possibly arising from the anion exchange reaction of DETA-Cl with sodium acetate inside the MIL-101(Cr) pores. The N2 adsorption-desorption measurements of MIL-101(Cr) before and after loading ionic liquids were conducted to obtain their textural properties, as given in Fig. 2 and Table 1. It could be seen in Fig. 2(A) that the isotherms of all samples exhibited mixed type I/IV behavior on the basis of IUPAC nomenclature, which was indicative of microporous-mesoporous materials. The BET surface area and total pore volume of the original MIL-101(Cr) were 2082 m2/g and 1.25 cm3/g, respectively. In contrast, there was a sharp decrease in these values after the introduction of DETA-Cl, suggesting the partial occupation of the MIL-101(Cr) pores by the ionic liquid. After anion exchange, the surface area and pore volume slightly decreased from 1344 m2/g

and 0.81 cm3/g for DETA-Cl@MIL-101(Cr) to 1259 m2/g and 0.76 cm3/g for DETA-Ac@MIL-101(Cr). It was ascribed to the further pore filling of MIL-101(Cr) by acetate anions. As depicted in Fig. 2(B), the pore diameter distribution curves of the three samples revealed two distinct peaks. For MIL-101(Cr), the pore diameters centered at ~2.2 nm and ~1.6 nm, which were comparable to the results reported in literature [57]. However, the pore diameter distributions shifted toward smaller pores as the DETA-based ionic liquids were incorporated. The curves of the ionic liquid-containing samples exhibited peaks at ~1.9 nm and ~1.3 nm, confirming that the ionic liquids were successfully tethered to the inner pores of MIL-101(Cr) framework. Nevertheless, the free pore space preserved after modification was sufficient for the efficient transfer of CO2 molecules. These findings coincided well with those obtained from XRD analysis. The morphologies of MIL-101(Cr) crystals as well as the ionic liquid-containing samples were investigated by SEM, and the images are displayed in Fig. 3. The pure MIL-101(Cr) synthesized in the absence of HF had a regular octahedral morphology and an average particle size of 250-400 nm. As expected, no obvious differences in crystal shape and size were found after the grafting of DETA-based ionic liquids (Fig. 3(b1, b2) and (c1, c2)), which indicated that the structure of MIL-101(Cr) was well preserved during the post-synthetic modifications. To verify the incorporation of DETA-based ionic liquids, the FT-IR spectra of MIL-101(Cr), DETA-Cl@MIL-101(Cr), DETA-Ac@MIL-101(Cr), and DETA-Cl are shown for comparison in Fig. 4. For MIL-101(Cr), the broad peak centered at 3425

cm-1 could be ascribed to the O-H stretching vibration of adsorbed water molecules. The C-O-C asymmetric and symmetric stretching vibrations of dicarboxylate were observed at 1612 cm-1 and 1393 cm-1, while the C=C stretching vibration corresponding to the aromatic rings appeared at 1516 cm-1. The bands at 1165, 1016, 881, and 750 cm-1 were attributed to the C-H stretching vibrations in the benzene. Besides, a weak peak around 1705 cm-1 could also be observed as a consequence of the unreacted terephthalic acid inside the pores [58]. In the case of DETA-Cl, the bands located at 3379 cm-1 and 3298 cm-1 could be assigned to the asymmetric and symmetric stretching vibrations of primary amines (NH2). Two other bands resulting from the C-H stretching vibrations appeared at 2932 cm-1 and 2864 cm-1, which were related to the alkyl chain (CH2). The peaks at 1568, 1113, and 816 cm-1 corresponded to the N-H bending, C-N stretching, and N-H out-of-plane deformation modes, respectively. As seen in Fig. 4(b) and (c), the characteristic absorption peaks of MIL-101(Cr) were still remained after modification, while a few peaks related to the DETA-based ionic liquids were overlapped. For DETA-Cl@MIL-101(Cr), the peaks due to the aliphatic CH2 stretching in the 2800-3000 cm-1 region shifted slightly to higher wavenumber (2968 and 2880 cm-1) in comparison with DETA-Cl, which might be ascribed to the coordination between -NH2 and the unsaturated metal sites Cr3+ in MIL-101(Cr) [59]. In addition, the band at 1705 cm-1 completely disappeared on account of the neutralization of -COOH by amine groups. All these implied that the ionic liquid DETA-Cl was successfully grafted onto MIL-101(Cr). Further introduction of acetate anions, as displayed in Fig. 4(c), did not lead to visible

difference as the absorption bands of C=O (1591 cm-1) and C-O (1393 cm-1) were overlapped with those of BTC linkers. The interactions between DETA-Cl and MIL-101(Cr) were further confirmed by the XPS analysis (Fig. 5). The Cr 2p XPS spectrum of MIL-101(Cr) displayed two peaks at 577.5 and 587.3 eV, which were assigned to the Cr 2p1/2 and Cr 2p3/2 energy levels, respectively [60]. After grafting of amine-containing ionic liquid, slight shifts of Cr 2p peaks to lower binding energies (576.8 and 586.5 eV) were observed, suggesting that the electron density of Cr(III) species was increased [61]. This change was mainly caused by the covalent attachment of DETA-Cl with the MIL-101(Cr) framework through Cr-NH2 interactions. Moreover, the presence of N 1s and Cl 2p peaks at about 401.1 and 200.4 eV in the survey spectrum of DETA-Cl@MIL-101(Cr) was also indicative of the incorporation of ionic liquid. The

TG

curves

of

MIL-101(Cr),

DETA-Cl@MIL-101(Cr),

and

DETA-Ac@MIL-101(Cr) are depicted in Fig. 6. For all the as-synthesized samples, the absorbed water molecules were removed from the pores before 100 oC. In the second stage, the weight loss of pure MIL-101(Cr) between 100 and 250 oC was mainly due to the departure of coordinated water molecules, while the weight losses of the other two samples from 100 to 330 oC were caused by the decompositions of the corresponding DETA-based ionic liquids. The collapse of MIL-101(Cr) framework was detected up to 550 oC in all cases, indicating that the introduction of ionic liquids had no effect on the thermodynamic stability of material. According to the TG results, the weight percentages of DETA-Cl and DETA-Ac in MIL-101(Cr)

were 23.7% and 27.1%, respectively. To further confirm the loading amounts, the elemental analyses were also conducted and the results are given in Table 2. The contents of DETA-based ionic liquids were calculated to be 1.68 mmol/g (23.4%) and 1.65 mmol/g (26.9%) on the basis of N contents, matching well with those of TG. 3.2. CO2 and N2 adsorption isotherms Fig. 7 presents the CO2 and N2 adsorption isotherms of MIL-101(Cr), DETA-Cl@MIL-101(Cr), and DETA-Ac@MIL-101(Cr) at 25 oC and 1.0 bar. As expected, the IL-incorporated samples exhibited remarkable enhancements in CO2 uptake compared to the pristine MIL-101(Cr), which could be attributed to the higher density of active adsorption sites in the composites. The amount of CO2 adsorbed on DETA-Cl@MIL-101(Cr) was 2.35 mmol/g, representing 92.6% improvement relative to MIL-101(Cr) (1.22 mmol/g). Taking into account its lower surface area and pore volume, as listed in Table S1, it was easy to infer that the multi-amine-tethered IL had a dominant role in capturing CO2. When the acetate anions with high CO2 solubility were introduced, a further increase in CO2 adsorption capacity to 2.46 mmol/g was observed (increased by 101.6%). This revealed that the active sites of the composites could be tailored by adjusting the functional groups of ILs, which in turn affected the affinities of the framework surfaces for CO2 molecules. Actually, the partial pressure of CO2 in a flue gas was relatively low (~0.15 bar) [2]. Thus, a more important indicator for evaluating the performance of solid adsorbents was the CO2 uptake at a pressure near 0.15 bar. It was visible that the as-synthesized composites could adsorb ~3.4 times as much CO2 as MIL-101(Cr) at 25 oC and 0.15 bar, showing great

potential for practical application. On the other side, the adsorption amounts of N2 on the IL-containing samples were slightly lower than that on IL-free MOF as a result of the reduced accessible surface areas and pore volumes after modification (Fig. S1). This implied that the integration of DETA-based ILs with MIL-101(Cr) was conductive to the selective adsorption of CO2 over N2. It was conjectured that the distribution of IL in MOF also had a significant impact on the final performance of CO2 adsorption. To verify this assumption, the DETA-Ac-Imp-MIL-101(Cr) was synthesized by a direct impregnation method, and the characterization results including XRD, N2 adsorption-desorption, SEM, FT-IR, TG, and elemental analysis are also supplied for comparison. As shown in Fig. 1 and Fig. 3, there were no remarkable changes in the XRD pattern and SEM images of DETA-Ac-Imp-MIL-101(Cr). Unlike DETA-Ac@MIL-101(Cr), the IL-impregnated sample possessed lower BET surface area and pore volume, and the pore diameters were roughly consistent with those of MIL-101(Cr) (Fig. 2), implying that the DETA-Ac was mainly accumulated on the outer surfaces of the framework rather than grafted inside the pores. This was further confirmed by the FT-IR studies as no significant shifts in the adsorption peaks of the alkyl chain (CH 2) was observed. According to the results of TG and elemental analyses (Fig. S2 and Table 2), it could be calculated that the loading amounts of ILs in DETA-Ac@MIL-101(Cr) and DETA-Ac-Imp-MIL-101(Cr) were basically the same. However, the performance curves of the two samples varied considerably. It could be clearly found in Fig. 8 that the CO2 adsorption capacity of the impregnated sample was higher than that of

DETA-Ac@MIL-101(Cr) at low pressure, whereas the value was gradually overtaken at high pressure. The reason was that the IL moiety aggregated and formed a dense layer outside the surfaces of MIL-101(Cr), which resulted in a larger amount of functional groups on the outer surfaces that could be preferentially contacted at low CO2 loadings. With the pressure increasing, the dense IL layer restricted the transfer of CO2 to access the unsaturated Cr(III) sites deep within the pores, ultimately leading to the slightly inferior in CO2 adsorption performance (1.98 mmol/g). In sharp contrast, the better dispersion of IL could promote the exposure of more active sites and provide the accessibility to bulk metal sites, thereby improving the CO2 uptake of the IL-grafted sample. Moreover, the synergistic effect between DETA-Ac and MIL-101(Cr) was also responsible for the enhanced adsorption performance of the composite towards CO2. 3.3. CO2/N2 adsorption selectivity As mentioned before, the grafting of DETA-based ILs onto MIL-101(Cr) would dramatically enhance the CO2 uptake while restrain the N2 adsorption, therefore, these composites could serve as promising candidates for the post-combustion CO2 capture. In this work, the CO2/N2 adsorption selectivities at 25 oC were roughly estimated using the ideal adsorbed solution theory (IAST) based on the fitted isotherms of pure components (Supporting Information, Section 1) [62,63]. Given the rough composition of the flue gas, a gas mixture containing 15% CO 2 and 85% N2 was chosen to evaluate the separation performance. Thus, the selectivity (S) at a total pressure of 1.0 bar could be calculated from the ratio of the amount of CO2 adsorbed

on sample at 0.15 bar to the N2 uptake at 0.85 bar, and this value should be further normalized according to the given pressures (Equation (1)). (1) Fig. 9 depicts the IAST-predicted selectivities for CO2/N2 (0.15/0.85) mixture at 25 oC as a function of the bulk pressure. It was found that the adsorption selectivities of both samples decreased with increasing pressure. More importantly, the IL-incorporated sample always retained a higher selectivity for CO2 compared to the original MIL-101(Cr), especially in the relatively low-pressure region. At 0.10 bar, the CO2/N2 selectivity of DETA-Ac@MIL-101(Cr) was up to 297, whereas that of MIL-101(Cr) was only 6. Even the pressure reached 1.0 bar, the increase in adsorption selectivity after IL grafting was still apparent, which was almost 39 times as high as that of the unmodified sample. The significantly enhanced separation performance of the composite was mainly attributed to its higher affinity for CO2. Specifically, the DETA-based IL polarized the pore surfaces of MIL-101(Cr), thus affording stronger adsorption force toward the CO2 molecules with high polarizability and quadrupole moment. Besides, the highly dispersed IL inside the pores could facilitate the transport of CO2 while crowding out the non-polar N2, which imposed a positive effect on achieving the selective CO2 adsorption. 3.4. Isosteric heat of adsorption To visually access the interactions between the framework surfaces and gas molecules, the isosteric heats of CO2 adsorption (Qst) on MIL-101(Cr) and DETA-Ac@MIL-101(Cr) were estimated by the Clausius-Clapeyron equation

(Supporting Information, Section 2) after fitting the CO2 adsorption isotherms at different temperatures [64,65]. Fig. 10 illustrates the dependence of Qst on the adsorbed amounts of CO2 over the samples. The isosteric heats of adsorption for CO2 on DETA-Ac@MIL-101(Cr) were generally higher than those on bare MIL-101(Cr), indicating a stronger affinity of the IL-modified frameworks toward CO2. It was noticed that the composite displayed an initial Qst of 46 kJ/mol at low surface coverage, which was approximately 25 kJ/mol higher than that calculated for MIL-101(Cr) (21 kJ/mol). This high Qst value suggested a chemisorptive-type interaction between the pore surfaces and CO2 molecules [66]. When the DETA-based IL was grafted, partial unsaturated Cr(III) sites of MIL-101(Cr) were replaced by amine functional groups, thus leading to the distinct difference in interaction with CO2. Besides, the adsorption enthalpy of DETA-Ac@MIL-101(Cr) dropped to 19 kJ/mol at high CO2 loading due to the heterogeneity of the adsorption sites. In this respect, the CO2 molecules were more inclined to adsorb on the highly active sites introduced by IL at low pressure. As the adsorbed amount of CO2 increased, the additional binding sites from IL were gradually occupied and reached saturation. Afterward, the CO2 diffused deeply into the pore spaces of the composite under relatively high pressure, which was further captured by the reserved metal sites. All these evidenced that the introduction of multiple-site containing IL could endow the composite with enhanced adsorption enthalpy and improved affinity for CO2. 3.5. A possible mechanism of CO2 adsorption In order to

better understand the adsorption behavior of CO 2 on

DETA-Ac@MIL-101(Cr), a possible two-step mechanism was proposed to explain the manner in which CO2 bound to the modified frameworks (Fig. 11). On the initial stage, the CO2 molecules were mainly captured by the multiple amine groups tethered to IL (Step 1). It has been reported that the amine end groups grafted on the unsaturated Cr(III) sites could also take part in the binding of CO2 [67,68]. Therefore, it was inferred that the two unprotonated amines reacted with CO2 to form the carbamates through a 2:1 (amine:CO2) mechanism. During this process, the nucleophilic attack of primary amine was firstly occurred on the C atom of CO2 molecule to form the N-C bond, and then the secondary amine could act as a Lewis base to accept the H proton. It was referred in literature that rearrangement of the carbamate might also occurred to achieve a minimum energy structure, where the metal-nitrogen bond broke and the CO2 molecule inserted to form a metal-oxygen bond [67]. In the second phase, the CO2 adsorption took place on the sites of basic acetate anions ([Ac]-) (Step 2). Previous studies have proved the high CO2 solubility as well as the strong CO2-[Ac]- interaction in acetate-based ILs [51-53]. Shi et al. [69] proposed that the physisorption and chemisorption of CO2 corresponded to two different types of binding configurations between CO2 molecule and [Ac]- anion. This theory was mainly focused on the non-imidazolium type ILs, which was also applicable for this situation. Given the relatively low Q st for CO2 at high surface coverage (19 kJ/mol), we could speculate that the CO2 was physically adsorbed by [Ac] - anions. In this case, the CO2 molecule simultaneously interacted with both O atoms of [Ac] - to produce a 1:1 ([Ac]-:CO2) stoichiometry. Aside from the active

adsorption sites provided by DETA-Ac ionic liquid, the open metal sites of MIL-101(Cr) were also accessible and took effect during the adsorption process. Thus, the synergistic effect between the multiple adsorption sites might account for the remarkable improvements in CO2 retention. 3.6. Multiple cycles of CO2 adsorption-desorption Generally speaking, a porous adsorbent with high isosteric heat of adsorption usually owns high adsorption capacity and selectivity for CO2, however, the difficulty in

regeneration

is

also

accompanied.

To

examine

the

recyclability

of

DETA-Ac@MIL-101(Cr), the CO2 adsorption cycling measurements were conducted several times. In each cycle, the sample was saturated with CO2 under the condition of 25 oC and 1.0 bar, followed by desorption using a combined vacuum and temperature swing approach (100 oC, 1 h). As described in Fig. 12, the composite displayed a good regeneration performance with no significant loss in CO2 adsorption capacity during six cycles. In addition, almost no accumulation of irreversible bound CO2 was observed on the recycled sample, indicating that the effective desorption was achieved and the CO2 adsorption process was reversible. Bearing in mind its high thermal stability (based on the analysis of TG), the DETA-Ac@MIL-101(Cr) composite could remain intact during regeneration process, which was advantageous for realistic CO 2 capture.

4. Conclusions In this work, the multiple-site containing IL DETA-Ac was post-synthetically

grafted onto the unsaturated Cr(III) centers in MIL-101(Cr) for CO2 capture. At 25 oC and 1.0 bar, the CO2 uptake of DETA-Ac@MIL-101(Cr) was up to 2.46 mmol/g, whereas that of the original MIL-101(Cr) was only 1.22 mmol/g. This remarkable enhancement in adsorption capacity was mainly ascribed to the higher density of the active adsorption sites resulting from the amine functional groups and acetate anions introduced by IL. In addition, the good dispersion of DETA-Ac within MIL-101(Cr) gave rise to more available binding sites from IL and the accessibility of open metal sites deep inside the crystals, which further contributed to the synergistic effect during adsorption process. The IAST combined with DSLF model were also employed to predict the adsorption selectivities for CO2/N2 (0.15/0.85) mixture. At 1.0 bar, the CO2/N2 selectivity of DETA-Ac@MIL-101(Cr) was calculated to be 181, which was almost 39 times as high as that of MIL-101(Cr). The reason was that the IL polarized the frameworks surfaces and thus led to a higher affinity toward CO2 molecules. The adsorption behavior of CO2 in the composite suggested that there existed a possible two-step mechanism for CO2 capture. This assumption was further supported by the higher initial and average Qst values for DETA-Ac@MIL-101(Cr). Besides, no apparent change was observed in CO2 adsorption capacity during six cyclic tests, showing considerable stability and regenerability of the composite. This study revealed the enormous potential of the IL-MOF composites as efficient and reversible adsorbents for CO2 capture. Moreover, by tailoring the active adsorption sites, the gas affinities of such composites could also be tuned to satisfy the escalating separation requirements of various gas mixtures.

Acknowledgments This work was supported by the National Natural Science Foundation of China (21476110, 21706131), the Natural Science Foundation of Jiangsu Province of China (BK20151531), the Key Project for University Natural Science Foundation of Jiangsu Province (14KJA530001), the Prospective Joint Research Program of Jiangsu Province (BY2015005-09), and the Natural Science Fund for Colleges and Universities in Jiangsu Province (17KJB530004).

Appendix A. Supplementary material Supplementary data associated with this article can be found, in the online version, at http://dx.doi.org/10.1016/j.jcis.2017.xxxxx.

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Table captions Table 1. Pore structure parameters of MIL-101(Cr), DETA-Cl@MIL-101(Cr), DETA-Ac@MIL-101(Cr), and DETA-Ac-Imp-MIL-101(Cr). Table

2.

Elemental

analyses

of

MIL-101(Cr),

DETA-Cl@MIL-101(Cr),

DETA-Ac@MIL-101(Cr), and DETA-Ac-Imp-MIL-101(Cr).

Table 1. Pore structure parameters of MIL-101(Cr), DETA-Cl@MIL-101(Cr), DETA-Ac@MIL-101(Cr), and DETA-Ac-Imp-MIL-101(Cr). Sample

BET surface area

Total pore volume

Micropore volume

(m2/g)

(cm3/g)

(cm3/g)

MIL-101(Cr)

2082

1.25

1.04

DETA-Cl@MIL-101(Cr)

1344

0.81

0.64

DETA-Ac@MIL-101(Cr)

1259

0.76

0.61

DETA-Ac-Imp-MIL-101(Cr)

1101

0.67

0.52

Table

2.

Elemental

analyses

of

MIL-101(Cr),

DETA-Cl@MIL-101(Cr),

DETA-Ac@MIL-101(Cr), and DETA-Ac-Imp-MIL-101(Cr). Sample

Element C (%)

H (%)

N (%)

MIL-101(Cr)

29.54

2.46

–

DETA-Cl@MIL-101(Cr)

31.54

4.58

7.04

DETA-Ac@MIL-101(Cr)

32.03

4.31

6.93

DETA-Ac-Imp-MIL-101(Cr)

31.98

4.30

6.97

Scheme and Figure captions Scheme 1. Schematic illustration for the synthesis of DETA-Ac@MIL-101(Cr) adsorbent. Fig. 1. Powder XRD patterns of (a) MIL-101(Cr), (b) DETA-Cl@MIL-101(Cr), (c) DETA-Ac@MIL-101(Cr), and (d) DETA-Ac-Imp-MIL-101(Cr). Fig. 2. N2 adsorption-desorption isotherms (A) and pore diameter distributions (B) of (a) MIL-101(Cr), (b) DETA-Cl@MIL-101(Cr), (c) DETA-Ac@MIL-101(Cr), and (d) DETA-Ac-Imp-MIL-101(Cr). Fig. 3. SEM images of (a1, a2) MIL-101(Cr), (b1, b2) DETA-Cl@MIL-101(Cr), (c1, c2) DETA-Ac@MIL-101(Cr), and (d1, d2) DETA-Ac-Imp-MIL-101(Cr). Fig. 4. FT-IR spectra (A) and partial enlargement (B) of (a) MIL-101(Cr), (b) DETA-Cl@MIL-101(Cr),

(c)

DETA-Ac@MIL-101(Cr),

(d)

DETA-Ac-Imp-MIL-101(Cr), and (e) DETA-Cl. Fig. 5. Survey XPS spectra (A) and Cr 2p XPS spectra (B) of (a) MIL-101(Cr), and (b) DETA-Cl@MIL-101(Cr). Fig. 6. TG curves of (a) MIL-101(Cr), (b) DETA-Cl@MIL-101(Cr), and (c) DETA-Ac@MIL-101(Cr). Fig. 7. CO2 and N2 adsorption isotherms of MIL-101(Cr), DETA-Cl@MIL-101(Cr), and DETA-Ac@MIL-101(Cr) at 25 oC. Fig.

8.

CO2

adsorption

isotherms

of

DETA-Ac@MIL-101(Cr),

and

DETA-Ac-Imp-MIL-101(Cr). Fig. 9. The selectivity of CO2 over N2 on MIL-101(Cr) and DETA-Ac@MIL-101(Cr)

at 25 oC. Fig.

10.

Isosteric

heat

of

CO2

adsorption

on

MIL-101(Cr)

and

DETA-Ac@MIL-101(Cr). Fig. 11. A possible mechanism of CO2 adsorption in DETA-Ac@MIL-101(Cr). Fig.

12.

Six

consecutive

cycles

DETA-Ac@MIL-101(Cr) at 25 oC.

of

CO2

adsorption-desorption

on

the

Scheme 1. Schematic illustration for the synthesis of DETA-Ac@MIL-101(Cr) adsorbent.

Intensity (a.u.)

(d)

(c)

(b)

(a) 2

4

6

8

10

2 Theta (degree)

Fig. 1. Powder XRD patterns of (a) MIL-101(Cr), (b) DETA-Cl@MIL-101(Cr), (c) DETA-Ac@MIL-101(Cr), and (d) DETA-Ac-Imp-MIL-101(Cr).

0.7 800 (A)

(B)

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

-1

Pore volume (cm g nm )

600

0.5

3

-1

3

Volume adsorbed (cm /g)

0.6

400

200

0 0.0

(a) (b) (c) (d) 0.2

0.4

0.6

Relative pressure (P/Po)

0.8

0.4

0.3

0.2

0.1

1.0

0.0 1.0

1.5

2.0

2.5

3.0

3.5

4.0

Pore diameter (nm)

Fig. 2. N2 adsorption-desorption isotherms (A) and pore diameter distributions (B) of (a) MIL-101(Cr), (b) DETA-Cl@MIL-101(Cr), (c) DETA-Ac@MIL-101(Cr), and (d) DETA-Ac-Imp-MIL-101(Cr).

Fig. 3. SEM images of (a1, a2) MIL-101(Cr), (b1, b2) DETA-Cl@MIL-101(Cr), (c1, c2) DETA-Ac@MIL-101(Cr), and (d1, d2) DETA-Ac-Imp-MIL-101(Cr).

(A)

(B)

(e)

(e) 1113

816

3298

Transmittance (a.u.)

Transmittance (a.u.)

2864

1568

3379

(d)

2932

(c) 1591

(b)

(d)

(c)

1016 881

(b) (a) 3425

1705 1612

1165 1516 750 1393

4000 3500 3000 2500 2000 1500 1000 -1

2968

(a) 500 3050

3000

Wavenumber (cm )

2950

2880

2900

-1

2850

2800

Wavenumber (cm )

Fig. 4. FT-IR spectra (A) and partial enlargement (B) of (a) MIL-101(Cr), (b) DETA-Cl@MIL-101(Cr),

(c)

DETA-Ac@MIL-101(Cr),

DETA-Ac-Imp-MIL-101(Cr), and (e) DETA-Cl.

(d)

(B)

(A)

Cr 2p 576.8 586.5

O 1s

Intensity (a.u.)

Intensity (a.u.)

Cr 2p C 1s N 1s

(b) Cl 2p O 1s

(b)

577.5 587.3

Cr 2p

(a)

C 1s

(a) 0

200

400

600

800

Binding Energy (eV)

1000

570

575

580

585

590

595

600

Binding Energy (eV)

Fig. 5. Survey XPS spectra (A) and Cr 2p XPS spectra (B) of (a) MIL-101(Cr), and (b) DETA-Cl@MIL-101(Cr).

100

~1.6%

~3.9%

90 ~27.1%

~23.7%

Weight loss (%)

80 ~41.0% 70

60 ~31.1%

~32.8%

50

(a) (b) (c)

40

30 100

200

300

400

o

500

600

700

Temperature ( C)

Fig. 6. TG curves of (a) MIL-101(Cr), (b) DETA-Cl@MIL-101(Cr), and (c) DETA-Ac@MIL-101(Cr).

2.5 MIL-101(Cr)-CO2

Quantity adsorbed (mmol/g)

DETA-Cl@MIL-101(Cr)-CO2 DETA-Ac@MIL-101(Cr)-CO2

2.0

MIL-101(Cr)-N2 DETA-Cl@MIL-101(Cr)-N2 DETA-Ac@MIL-101(Cr)-N 2

1.5

1.0

15 kPa

0.5

0.0 0

20

40

60

80

100

Bulk pressure (kPa)

Fig. 7. CO2 and N2 adsorption isotherms of MIL-101(Cr), DETA-Cl@MIL-101(Cr), and DETA-Ac@MIL-101(Cr) at 25 oC.

Fig.

8.

CO2

adsorption

DETA-Ac-Imp-MIL-101(Cr).

isotherms

of

DETA-Ac@MIL-101(Cr),

and

400

MIL-101(Cr)

Selectivity of CO 2 vs N2

DETA-Ac@MIL-101(Cr)

300

200

100

0 0

20

40

60

80

100

Bulk pressure (kPa)

Fig. 9. The selectivity of CO2 over N2 on MIL-101(Cr), and DETA-Ac@MIL-101(Cr) at 25 oC.

60 MIL-101(Cr) DETA-Ac@MIL-101(Cr)

-Qst (kJ/mol)

50

40

30 20

10

0 0.1

0.2

0.3

0.4

0.5

0.6

0.7

0.8

0.9

1.0

CO2 adsorbed (mmol/g)

Fig.

10.

Isosteric

heats

DETA-Ac@MIL-101(Cr).

of

CO2

adsorption

on

MIL-101(Cr),

and

Fig. 11. A possible mechanism of CO2 adsorption in DETA-Ac@MIL-101(Cr).

Quantity adsorbed (mmol/g)

2.5

2.0

1.5 1

2

3

4

5

6

1.0

0.5

0.0

Cycles

Fig.

12.

Six

consecutive

cycles

DETA-Ac@MIL-101(Cr) at 25 oC.

of

CO2

adsorption-desorption

on

the

Graphical Abstract