Yinji Wan,Dekai Kong,Feng Xiong,Tianjie Qiu,Song Gao,Qiuning Zhang,Yefan Miao,Mulin Qin,Shengqiang Wu,Yonggang Wang,Ruiqin Zhong,,Ruqiang Zou,

1 State Key Laboratory of Heavy Oil Processing, China University of Petroleum (Beijing), Beijing 102249, China

2 Beijing Key Laboratory for Theory and Technology of Advanced Battery Materials, School of Materials Science and Engineering, Peking University, Beijing 100871, China

Keywords:Core-shell Mg-MOF-74@ZIF-8 CO2 capture Hydrophobic effect

ABSTRACT Developing metal-organic framework (MOF) materials with the moisture-resistant feature is highly desirable for CO2 capture from highly humid flue gas.In this work,a new core-shell MOF@MOF composite using Mg-MOF-74 with high CO2 capture capacity as a functional core and hydrophobic zeolitic imidazolate framework-8 (ZIF-8) as a protective shell is fabricated by the epitaxial growth method.Experimental results show that the CO2 adsorption performance of the core-shell structured Mg-MOF-74@ZIF-8 composites from water-containing flue gas is enhanced along with their improved hydrophobicity.The dynamic breakthrough results show that the Mg-MOF-74@ZIF-8 with three assembled layers(Mg-MOF-74@ZIF-8-3) can capture 3.56 mmol·g-1 CO2 from wet CO2/N2 (VCO2: VN2=15:85) mixtures,which outperforms Mg-MOF-74 (0.37 mmol·g-1) and most of the reported physisorbents.

1.Introduction

The Paris Agreement has set up a target to limit global temperature increaseto 1.5-2 °C above pre-industrial levels,and many countries are committed to reach global carbon peaking as soon as possible to achieve a climate neutral by mid-century [1,2].In this process,carbon capture and storage (CCS) will play an irreplaceable role unless a green renewable energy is applied to the electricity and industry sectors.Over the past two decades,great efforts have been made to develop a variety of porous physical adsorbents with efficient CO2adsorption performance,easy regeneration,and strong tolerance to impurities [3-6].

Among various types of solid porous materials,metal-organic frameworks (MOFs) that combine well-defined adsorptive site,tunable pore structure,high selectivity,large working capacity,and low energy penalty for regeneration are promising adsorbents for trapping CO2[7-13].However,there is a strongly competitive adsorption behavior between CO2and water vapor in MOF adsorbents in a water-containing flue gas,leading to the significantly decrease of CO2capture capacity [14].For instance,Mg-MOF-74 has drawn great attentions owing to its exceptionally high CO2adsorption capacity (～8 mmol·g-1) and stability in dry air [15-18].Nevertheless,the CO2adsorption ability of Mg-MOF-74 is significantly vulnerable in the presence of moisture,and only about 16% of its initial CO2capacity can be maintained [18].That is because the coordinatively unsaturated metal sites (CUSs) of Mg-MOF-74 can strongly adsorb water due to their large dipole moment.If the influence of water vapor can be eliminated,Mg-MOF-74 will be a popular adsorbent for high-humidity flue gas carbon capture.

Great efforts have demonstrated that the combination of MOFs with functional materials to form MOF composites can overcome the drawbacks of single MOF while maintaining the original advantages of MOFs [19].Various functional materials like metal nanoparticles,metal oxides,quantum dots,polymers,etc.have been coupled with MOFs to form versatile MOF composites for high-efficiency energy storage and conversion[20],heterogeneous catalysis [21-23],and gas adsorption and separation applications[24].Recently,Jinet al.[25] enhanced the CO2adsorption behaviors of Mg-MOF-74 from wet gases by introducing the polymer hydrophobic polydimethylsiloxane.Compared with the abovementioned incorporated functional components,MOFs with diverse structures,tunable chemical composition,flexible designability,and multifunction are anticipated to be promising complementary functional materials to create MOF/MOF composites.Among multicomponent MOF composites,the core-shell structured composite can integrate properties of different MOFs into one system and respectively endow specific functions of both core and shell to meet the requirements of bi/multi-functionalities for practical application [26-29].For instance,Gonget al.[26] developed a core-shell MOF@MOF composite (i.e.UiO-66@UiO-67-BPY) and it possessed a 98% catalytic activity conversion rate for the Knoevenagel condensation,higher than 87% of shell and 25%of core,respectively,which was ascribed to the rich channel of the core UiO-66 and the existence of Lewis basic sites in the UiO-67-BPY shell.In another work,Kimet al.[30] synthesized a coreshell MOF catalyst with the core acting as catalytically active sites and inactive shell playing the sieving effect for reactants for aerobic oxidation of alcohols,which can improve selectivityviathe enhanced size discrimination of reactants.Renet al.[31]fabricated a core-shell MIL-101@UiO-66 nanocrystal by introducing microporous UiO-66 into mesoporous MIL-101 for H2storage.The asprepared MIL-101@UiO-66 composites presented a greatly improved H2storage capacity,thermos-stability,and moisture tolerance in comparison to individual MOF.In addition,MOF@MOF composites and their derivatives have been widely used in the field of photo/electrocatalysis,and their catalytic performance and durability are superior to single MOF due to their synergistic effects and emerging integrated properties[32,33].So far,the literature on MOF@MOF adsorbents with hydrophobic structures and prominent CO2capture performance for real industrial applications is scarce.

Zeolitic imidazolate frameworks (ZIF) that belong to a subclass of hydrophobic porous MOFs are stable in water,which is attributed to the presence of anionic nitrogen-containing ligands within the framework of MOF[34,35].Among numerous ZIFs,ZIF-8 owns good hydrophobicity.Its pore openings (i.e.apertures) are too small (0.34 nm) to permit water clusters and the lack of polar groups further helps exclude liquid water [36].In consequence,ZIF-8 is a potential material to be used under humid conditions.Keeping all those considerations in mind,herein,a novel core-shell structured MOF@MOF (i.e.,Mg-MOF-74@ZIF-8) composite was constructed by a pre-seeding process and a two-step temperature-controlled crystallization strategy (Fig.1),where Mg-MOF-74 acts as the functional core to perform the outstanding CO2adsorption performance,while ZIF-8 as the water-proofing shell layer to hinder the water enters into the core.The influence of different coated layers on morphologies,structures,hydrophobicity,and CO2adsorption performance of Mg-MOF-74@ZIF-8 composites are fully investigated by scanning electron microscope(SEM),high-angle annular dark-field scanning transmission electron microscopy (HAADF-STEM),energy dispersive spectrometer(EDS),X-ray diffraction (XRD),Fourier transform infrared spectroscope (FT-IR),contact angle (CA),etc.Moreover,the dynamic CO2adsorption performance and cyclic stability in the simulated highly humid flue gas are evaluated by a homemade fixed bed.

Fig.1.Schematic illustration of the synthesis approach of Mg-MOF-74@ZIF-8.

2.Experimental

2.1.Fabrication of Mg-MOF-74@ZIF-8 composites

All the reagents were purchased (details in Supplementary Material) and used without further purification.Mg-MOF-74 was synthesized by the previously reported method [15,17] (details in Supplementary Material).For the synthesis of Mg-MOF-74@ZIF-8 composites,firstly,0.372 g of Zn(NO3)2·6H2O was dissolved in 25 ml of anhydrous methanol,denoted as solution A;0.308 g of 2-methylimidazole was dissolved in 75 ml of anhydrous methanol,denoted as solution B.After that,0.3 g of as-prepared Mg-MOF-74 was placed into a 10 ml sample bottle,then add 1 ml of solution A and 3 ml of solution B were in sequence.The mixed solution was sonicated and fully dispersed.Subsequently,the abovementioned solution was dried in an oven at 100 °C,and ZIF-8 was initially seeded and crystallized on the surface of Mg-MOF-74 material.The dried solid powder material was put into a 200 ml round-bottomed flask,then the remaining 24 ml of solution A and 74 ml of solution B were added in sequence and well mixed by sonication.The mixed solution was subjected to two-step temperature-controlled crystallization.The mixed solution was slowly stirred at -15 °C for 2 h and then stirred at 25 °C for 2 h.After that,the resulting products were washed three times with anhydrous methanol to remove remnant solvents.Finally,the solid powder material was collected and vacuum-dried at 100 °C for 12 h.The above processes were performed three times,and the product obtained for the first time was named Mg-MOF-74@ZIF-8-1.Correspondidingly,the products obtained the second and third times were named Mg-MOF-74@ ZIF-8-2 and Mg-MOF-74@ZIF-8-3,respectively.

2.2.Characterization

The SEM and EDS tests were carried out on Regulus 8200(Hitachi,Japan).Double aberration-corrected high-angle annular darkfield scanning transmission electron microscopy (HAADF-STEM)measurements were conducted on a modified FEI Tiatan microscope(TEAM0.5,FEI,USA)operated at 300 kV with a HAADF detector.The STEM probe semi-angle is 30 mrad.N2adsorptiondesorption isothermal curve was collected by a Autosorb-iQ(Quantachrome,USA)at-196°C,and the Brunauer-Emmet-Teller(BET) method was used to calculate the specific surface area of samples.Pore size distribution was calculated by the non-local density functional theory (NLDFT) model,and N2at -196 °C on carbon(slit pore,NLDFT equilibrium model)was used as the calculation model to obtain the pore size distribution.XRD patterns were obtained on a SmartLab(Rigaku,Japan)using Cu Kα radiation(45 kV and 200 mA).The content of H,C,and N of all the samples were measured by elemental analysis (Vario EL CUBE,Elementar,Germany).FT-IR was recorded using Tensor 27 FT-IR (Bruker,Germany).The water contact angle (CA) was performed on a water contact angle analyzer SL-200B (Solon Tech.,China).X-ray photoelectron spectroscopy (XPS) analysis was performed under the Krayos AXIS Ultra DLD electronic energy spectrometer (Shimadzu,Japan).The binding energies of each element were corrected by standard carbon spectrum (C 1s=284.6 eV).

2.3.Gas adsorption measurements

All gases (N2,CO2) used in this study were ultra-high purity(99.999%).The static adsorption experiments of N2,CO2,and H2O for all the samples were performed on a Quantachrome Autosorb-IQ at 25 °C using a circulating ethylene glycol/H2O(1:1) bath.The dynamic CO2/N2adsorption performance under highly humid conditions was evaluated by a self-built fixed bed,and a schematic diagram of the corresponding experimental setup can be found in our previous work[37].The humidity of the tested environment can be provided by connecting a high-pressure reaction kettle with a saturated sodium bicarbonate aqueous solution in front of the adsorption bed.The reaction gas with a certain humidity can be obtained by passing through the reaction kettle and the humidity value can be detected by the humidity recorder.The simulated flue gas is composed of 85% (vol) N2,15% (vol) CO2with 50% relative humidity.Prior to the measurement,the tested adsorbent was degassed under 150°C for 8 h to remove impurities of pore in vacuum condition.

3.Results and Discussion

The morphology and structure of the as-prepared Mg-MOF-74@ZIF-8 composite were observed by SEM and HAADF-STEM.As shown in Fig.S1(a)and(b),SEM images present a flower-like shape composed of agglomerated needle-type crystals,which was in line with the previous report[17].ZIF-8 displays a cubic structure with an average particle size of 100 nm.In Fig.2(a)and Fig.S2(a),(b),it can be seen that ZIF-8 nanoparticles grew onto the surface of Mg-MOF-74 in its original cubic morphology named Mg-MOF-74@ZIF-8-1,Correspondidingly,the surface of Mg-MOF-74 gradually turns into rough because of the nucleation and growth of ZIF-8 at the interface.HAADF-STEM-EDS elemental mappings and corresponding line scans reveal that Mg elements belonging to Mg-MOF-74 are well concentrated in the core of the composite.Moreover,Zn and N elements of the ZIF-8 layer can be identified at the edge of composites from Fig.2(d),(e).HAADF-STEM images show the contrast in the central and outer region,further indicating that the Mg-MOF-74@ZIF-8-1 adopts a core-shell structure with an average shell thickness of 28 nm (Fig.2(b),(c)) [38].From Fig.2(a),(d)and Fig.S2(b),the surface of Mg-MOF-74 is not entirely covered by ZIF-8 nanoparticles,which can be settled by the repeating growth of the ZIF-8 layer on the surface of Mg-MOF-74.After the second assembled cycle (Mg-MOF-74@ZIF-8-2),the core Mg-MOF-74 is almost coated by ZIF-8 nanocrystals (Fig.S3(a),(b)).With ZIF-8 nanoparticles assembled onto Mg-MOF-74 up to three cycles,a slight crack on the surface of Mg-MOF-74@ZIF-8-3 composites can be observed from Fig.S4(a),(b) due to significantly thickening and densification of ZIF-8 shell.

Fig.2.Morphological and structural characteristics of Mg-MOF-74@ZIF-8-1 composite:(a)SEM image(inset:the corresponding model);(b)HAADF-STEM image;(c)HAADFSTEM image recorded from(b);(d)HAADF-STEM image recorded from(b)and HAADF-STEM-EDS elemental mappings(color online);(e)corresponding line-scans obtained from (d) along the arrow.

The elemental mapping results show that O,Mg,N,and Zn elements are homogeneously dispersed in Mg-MOF-74@ZIF-8-1(Fig.S5(a)-(e)),suggesting that ZIF-8 is uniformly coated on Mg-MOF-74.Besides,the N amounts of composites obtained by elemental analysis gradually increase with the increase of the crystallization times of the ZIF-8 (Table S1).These results effectively confirmed that ZIF-8 nanocrystals were successfully grown on the surface of Mg-MOF-74.Furthermore,the porosities of all the samples are investigated by the N2adsorption-desorption isotherms at-196°C.It can be seen from Fig.S6 that all the samples show I-type isotherm adsorption curves relating to microporous characteristics.The BET area is in the increasing sequence of 1416.11,1423.37,1436.91,1473.77,1568.51 m2·g-1for Mg-MOF-74,Mg-MOF-74@ZIF-8-1,Mg-MOF-74@ZIF-8-2,Mg-MOF-74@ZIF-8-3,and ZIF-8,respectively.In the pore size distribution curves (Fig.S7(a),(b)),Mg-MOF-74@ZIF-8 composites mainly show micropores similar to the pore size of Mg-MOF-74,indicating the coating of ZIF-8 does not block the pores of Mg-MOF-74.Except for microspores,there are a few mesopores that may be attributed to slight cracks (see SEM results) in the Mg-MOF-74@ZIF-8-2 and Mg-MOF-74@ZIF-8-3 samples,which were caused by the accumulation of ZIF-8 nanoparticles on the surface of Mg-MOF-74.

The evolution of crystal structure during the formation of Mg-MOF-74@ZIF-8 composites was characterized by XRD and FT-IR.As shown in Fig.3,all peaks of as-synthesized Mg-MOF-74 and ZIF-8 are in line with those of the simulated Mg-MOF-74 and ZIF-8 XRD pattern,indicating the successful synthesis of Mg-MOF-74 and ZIF-8,respectively.In comparison with the reference samples of Mg-MOF-74 and ZIF-8,the XRD patterns of Mg-MOF-74@ZIF-8 composites show the existence of the characteristic diffraction peaks of both ZIF-8 and Mg-MOF-74.And the intensity of characteristic peaks of composites is getting stronger with ZIF-8 shell layers increasing.Similar changes were observed in FT-IR spectra (Fig.S8).Compared Mg-MOF-74@ZIF-8 with pristine Mg-MOF-74,it can be found that the peaks at 995,1145,1508 cm-1appear in the FT-IR spectrum of Mg-MOF-74@ZIF-8-1 composite,which attributes to the characteristic peaks of ZIF-8 [39].The wavenumber of 995 and 1145 cm-1appear caused by the stretching vibration of C—N,while 1508 cm-1belongs to C-N stretches.Other peaks are related to C=C and C—H stretches.With the ZIF-8 assembly three cycles,the intensities of these peaks in Mg-MOF-74@ZIF-8 composites were enhanced.These results further confirmed that the functional groups of ZIF-8 have been introduced onto the surface of Mg-MOF-74.In XPS spectra of Mg-MOF-74@ZIF-8 (Fig.S9),the characteristic peaks of Zn can be clearly seen,which is absent in Mg-MOF-74,proving the successful introduction of ZIF-8.The intensity of Zn 2p peaks in Mg-MOF-74@ZIF-8 composites was enhanced with ZIF-8 assembly three cycles.Moreover,compared with unmodified Mg-MOF-74,the binding energy of Mg 1s in Mg-MOF-74@ZIF-8 shifts to higher binding energy,which suggests that Mg-MOF-74 might have a stronger electronic interaction with ZIF-8 and enhance the Mg-MOF-74 and ZIF-8 interaction through the physically electronic interaction,thus enhancing the stability of Mg-MOF-74@ZIF-8 materials.

Fig.3.XRD patterns of simulated Mg-MOF-74,Mg-MOF-74,simulated ZIF-8,ZIF-8,Mg-MOF-74@ZIF-8-1,Mg-MOF-74@ZIF-8-2,Mg-MOF-74@ZIF-8-3 composites.

The hydrophilic and hydrophobic properties of new material can be predicted through water CA.On account of this,the surface hydrophobicity of all the powder samples was investigated through CA measurement.The water contact images and angles are shown in Fig.4.Prior to the measurement,each sample powder was pressed into a tablet and dried.In Fig.4(a),when the surface of the Mg-MOF-74 plate encountered a water droplet,it was immediately adsorbed,and CA was extremely small which displays that Mg-MOF-74 is extraordinarily hydrophilic.However,as shown in Fig.4(e),ZIF-8 presents good hydrophilicity which is in accordance with previous work [40].For as-synthesized core-shell structured Mg-MOF-74@ZIF-8 composites (Fig.4(b)-(d)),as expected,they exhibit moderate hydrophobic characters between Mg-MOF-74 and ZIF-8 due to the shielding protection of the ZIF-8 shell layer,which demonstrates highly improved hydrophilicity than unmodified Mg-MOF-74.The water contact angle gradually enlarged with the increase of assembled cycles of the ZIF-8 layer,which means Mg-MOF-74@ZIF-8-3 possesses the best hydrophobicity among the three composites.

Fig.4.Water contact angles on(a)Mg-MOF-74,(b)Mg-MOF-74@ZIF-8-1,(c)Mg-MOF-74@ZIF-8-2,(d)Mg-MOF-74@ZIF-8-3,(e)ZIF-8.(f)Water stability test digital photo of Mg-MOF-74 and Mg-MOF-74@ZIF-8-3 after 24 h in water vapor at 110 °C.

In addition,water stability tests for pure Mg-MOF-74 and coreshell structured Mg-MOF-74@ZIF-8-3 were conducted on 110 °C water vapor for 24 h,respectively.It can be observed from Fig.4(f) that Mg-MOF-74 material was completely immersed in condensed water after being treated with steam at 110 °C for 24 h which further demonstrates Mg-MOF-74 shows a good water affinity,leading to the pristine framework decomposition of Mg-MOF-74 confirmed by BET (Fig.S10).On the contrary,for the Mg-MOF-74@ZIF-8-3(Fig.4(e)),it still remains dry under same conditions.Besides,the Mg-MOF-74@ZIF-8-3 composites after the water stability test keep the original structure confirmed by BET(Fig.S11),XRD (Fig.S12),and EDS mapping (Fig.S13) results,which means the hydrophobic capacity of Mg-MOF-74@ZIF-8-3 composites is promising in practical CCS applications.

The H2O adsorption isotherms for Mg-MOF-74,ZIF-8,and three kinds of Mg-MOF-74@ZIF-8 composites are tested at 25 °C.As shown in Fig.5,H2O adsorption capacity gradually rises for most samples with the relative pressure increasing.The H2O adsorption capacity over Mg-MOF-74 is about 4.3 times as much as CO2.Nevertheless,ZIF-8 shows an almost ignorable H2O uptake in theP/P0＜0.90 region,which reveals an outstanding hydrophobic character.For Mg-MOF-74@ZIF-8 composites with different numbers of layers,Mg-MOF-74@ZIF-8 materials present a middle level for H2O capture capacity in comparison with pristine Mg-MOF-74 and ZIF-8.Additionally,the uptake capacity of H2O decreases with the ZIF-8 shell layer increasing from monolayer to multilayer.The H2O adsorption quantity for all samples at 25 °C,0.1 MPa shows a descending tendency: Mg-MOF-74 (31.85 mmol·g-1) ＞Mg-MOF-74@ZIF-8-1 (28.63 mmol·g-1) ＞ Mg-MOF-74@ZIF-8-2 (25.80 mmol·g-1) ＞ Mg-MOF-74@ZIF-8-3 (19.01 mmol·g-1) ＞ ZIF-8(13.42 mmol·g-1).These results are in line with observations from the water CA and water stability,further confirming the outstanding hydrophobic ability of ZIF-8 and Mg-MOF-74@ZIF-8 composites.

Fig.5.H2O adsorption isotherms of Mg-MOF-74,ZIF-8,Mg-MOF-74@ZIF-8-1,Mg-MOF-74@ZIF-8-2,and Mg-MOF-74@ZIF-8-3 samples at 25 °C.

The adsorption capacity of CO2and CO2/N2separation factor under static conditions are crucial parameters to assess the feasibility of adsorbent.The pure CO2and N2adsorption and desorption isotherms for all the samples are obtained at 25 °C,respectiverly.Besides,on the basis of the adsorption isotherm,the adsorption separation factor of CO2/N2was calculated and obtained by the IAST model,and calculated details can be found in our previous works [8,10].As shown in Fig.6(a),all the adsorbents show a low N2capture capacity (＜1.20 mmol·g-1).While the CO2adsorption capacity for the five captors display a wide fluctuation,their adsorption capacity at 0.1 MPa is in the sequence of Mg-MOF-74(7.46 mmol·g-1) ＞Mg-MOF-74@ZIF-8-1 (6.62 mmol·g-1) ＞Mg-M OF-74@ZIF-8-2 (5.09 mmol·g-1) ＞ Mg-MOF-74@ZIF-8-3(4.46 mmol·g-1) ＞ ZIF-8 (0.76 mmol·g-1).From Fig.6(b),the adsorption separation factor of CO2/N2for almost most of adsorbents has a high value between 150 and 230 at 0.1 MPa except ZIF-8 (15),indicating the outstanding separation performance of as-prepared Mg-MOF-74@ZIF-8 composites.It can be clearly seen that CO2uptakes of the core-shell Mg-MOF-74@ZIF-8 composites continuously reduce with the increase of ZIF-8 layers.Compared with Mg-MOF-74,ZIF-8 shows very poor CO2uptakes because of the absence of open metal sites [40].Therefore,multilayer ZIF-8-coated Mg-MOF-74 results in the decline of CO2capture capacity.However,too many ZIF-8 layers modified-Mg-MOF-74@ZIF-8 composite has better hydrophobic performance which can protect Mg-MOF-74 from water damage.It should be pointed out that CO2adsorption capacity and CO2/N2selectivity (232) of Mg-MOF-74@ZIF-8-3 still outperforms most of adsorbents at 25°C,0.1 MPa.

Fig.6.(a) CO2 and N2 adsorption and desorption isotherms and (b) CO2/N2 selectivity of samples at 25 °C.

Considering that core-shell Mg-MOF-74@ZIF-8-3 adsorbent exhibits a large working CO2capacity,outstanding hydrophobic character,high CO2selectivity,and low energy penalty for regeneration (Fig.S14).The dynamic CO2adsorption performance of Mg-MOF-74@ZIF-8-3 composite is investigated at 25 °C and 0.1 MPa under simulated industrial flue gas (VCO2:VN2=15:85) with 48.3%relative humidity(RH).As shown in Fig.7(a),Mg-MOF-74 exhibits poor CO2/N2separation performance.CO2from wet CO2/N2breaks through the fixed bed at 52 s and the CO2adsorption capacity merely reaches 0.37 mmol·g-1,which is consistent with the previous report [18].While,for the core-shell Mg-MOF-74@ZIF-8-3 adsorbent,CO2breaks through the column at 479 s from wet CO2/N2and the CO2uptake is 3.56 mmol·g-1,where the CO2uptake is higher than most of the reported physical adsorbents including carbons [41-43],zeolites [40,44-46] and MOFs [11,18,47-51](Fig.7(d)).Our findings lead us to conclude that the CO2adsorption performance of the Mg-MOF-74@ZIF-8 composite is not fully influenced by water,indicating ZIF-8 shell layer can effectively hamper the penetration of H2O into the Mg-MOF-74 core,while the CO2molecules still could freely diffuse into the inside Mg-MOF-74 core,maintaining a good affinity to CO2(Fig.7(c)).In addition,the cyclic stability experiments show that the CO2adsorption capacity of Mg-MOF-74@ZIF-8-3 in high humid flue gas can maintain about 50% of its initial CO2capacity (1.76 mmol·g-1) after five cycles(Fig.S15).Compared with unmodified Mg-MOF-74,ZIF-8-coated Mg-MOF-74 endows Mg-MOF-74@ZIF-8-3 composite with outstanding adsorption capacity and relatively good recyclability in capturing CO2from wet gas mixtures.

Fig.7.Dynamic breakthrough curves of CO2/N2 over (a) Mg-MOF-74 and (b) Mg-MOF-74@ZIF-8-3.(c) Schematic diagram of the protection mechanism of core-shell structured Mg-MOF-74@ZIF-8.(d) The comparison of CO2 capture capacity of this work and reported physiadsorbents under different RH.

4.Conclusions

In summary,by combining a pre-seeding process and a twostep temperature controlling crystallization,we successfully synthesized a new type of MOF@MOF core-shell (namely Mg-MOF-74@ZIF-8) composite.In this composite,Mg-MOF-74 serveds as the core while ZIF-8 with hydrophobicity actes as the shell which significantly enhances the surface hydrophobic property,and thus preventes the penetration of H2O into Mg-MOF-74 core.According to H2O adsorption isotherms,CA,and water stability test results,the Mg-MOF-74@ZIF-8 with three assembled layers demonstrates a superior hydrophobic ability among these composites.Moreover,the dynamic adsorption capacity of CO2over Mg-MOF-74@ZIF-8-3 adsorbents in highly RH is greatly improved compare with pure Mg-MOF-74,and the CO2capture capacity is maintained as high as 1.76 mmol·g-1after five cycles,which is better than many other adsorbents.Our findings pave a new way for designing functional MOFs for high-humidity flue gas CO2capture.

Data Availability

Data will be made available on request.

Declaration of Competing Interest

The authors declare that they have no known competing financial interests or personal relationships that could have appeared to influence the work reported in this paper.

Acknowledgements

This work was supported by the National Natural Science Foundation of China (51772329,51972340,and 51825201).

Supplementary Material

Supplementary data to this article can be found online at https://doi.org/10.1016/j.cjche.2023.03.002.