Shapeable Fibrous Aerogels of MetalâOrganic-Frameworks

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Shapeable Fibrous Aerogels of Metal-Organic-Frameworks Templated with Nanocellulose for Rapid and Large-Capacity Adsorption Luting Zhu, Lu Zong, Xiaochen Wu, Mingjie Li, Haisong Wang, Jun You, and Chaoxu Li ACS Nano, Just Accepted Manuscript • DOI: 10.1021/acsnano.8b00566 • Publication Date (Web): 09 May 2018 Downloaded from http://pubs.acs.org on May 10, 2018

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Shapeable Fibrous Aerogels of Metal-Organic-Frameworks Templated with Nanocellulose for Rapid and Large-Capacity Adsorption Luting Zhu, †,1,2 Lu Zong, †,2 Xiaochen Wu,2 Mingjie Li,2 Haisong Wang,1,3,* Jun You,2,* and Chaoxu Li2,*

1

Key Laboratory of Pulp &Paper Science and Technology, Qilu University of Technology, Jinan,

Shandong 250353, China 2

CAS Key Lab of Biobased Materials, Qingdao Institute of Bioenergy and Bioprocess Technology,

Chinese Academy of Sciences, Songling Road 189, Qingdao 266101, P. R. China. 3

Liaoning Key Laboratory of Pulp and Papermaking Engineering, Dalian Polytechnic University,

Dalian, Liaoning, 116034, China. †

These authors contributed equally to this work.

*Corresponding

author:

Chaoxu

Li

(Email:

[email protected]),

[email protected]), Haisong Wang (Email: [email protected])

1 ACS Paragon Plus Environment

Jun

You

(Email:

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ABSTRACT Conventional metal-organic framework (MOF) powders have periodic micro/mesoporous crystalline architectures tuned by their three-dimensional coordination of metal nodes and organic linkers. In order to add practical macroscopic shapeability and extrinsic hierarchical porosity, fibrous MOF aerogels were produced by synthesizing MOF crystals on the template of TEMPO-cellulose nanofibrils. Cellulose nanofibrils not only offered extrinsic porosities and mechanic flexibility for the resultant MOF aerogels, but also shifted the balance of nucleation and growth for synthesizing smaller MOF crystals, and further decreased their aggregation possibilities. Thanks to the excellent shapeability, hierarchical porosity up to 99% and low density below 0.1 g/cm3, these MOF aerogels could make the most of their pores and accessible surface areas for higher adsorption capacity and rapid adsorption kinetics of different molecules, in sharp contrast to conventional MOF powders. Thus this scalable and low-cost production pathway is able to convert MOF powders into a shapeable and flexible form and hereby extend their applications in more broad fields, e.g. adapting for conventional filtration setup.

KEYWORDS: nanocellulose, metal-organic-framework, aerogel, fibrous template, rapid adsorbing


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Metal-organic frameworks (MOFs) refer to crystalline porous coordination polymers with specific pore apertures (e.g. 1~10 nm) tunable by three-dimensional (3D) coordination networks of metal nodes and organic linkers.1 Besides high crystallinity and thermal stability, their specially-designed periodic micro/mesoporous architectures lead to large specific surface areas, high porosities and low densities, being appealing to trap or transport small and large molecules for diverse applications in gas storage,2 separation,3 catalysis,4 water purification,5 sensing6 and energy storage.7 However, due to the crystalline nature of MOFs, they were mostly synthesized in the form either of bulk powders, or of colloidal crystals (e.g. with the size of 101~103 nm). Their crystal shapes were controlled by the crystallization balance of nucleation and growth.8 These stubborn MOF powders and crystals, on one hand, depressed processability and usability for their practicable and broad applications as membranes, filters, columns etc..9 On the other hand, their intrinsic micro-/meso-porosities also limited accessible surface areas and diffusion kinetics, particularly for applications dealing with large molecules and nanomaterials. Therefore, it remains challenging to combine easy-handling shapes, hierarchical porosities and rapid diffusion kinetics into MOFs for industrial and commercial applications. Distinct pathways have been attempted to introduce extrinsic meso-/macro-porosities into MOF layers, composites, gels and aerogels for shapeability and hierarchical porosities. MOF crystals were initially tried to deposit as porous layers on functional substrates, natural and synthetic fibers, large inorganic particles and nanosheets, by means of layer-by-layer,10 spray,11 and in-situ growth.12 However, in most cases, the MOF crystal sizes and contents were greatly limited by the surface wettability, roughness and charged properties. And surface modification was required to increase the compatibility between MOFs and the matrix.13 MOF crystals were also embedded into 3 ACS Paragon Plus Environment

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hollow chitin fibers,14 electrospun nanofiber mats15 and polymer films (e.g. chitosan,16 alginate,17 polyamide18 and polyimide19) to prepare composite membranes, particularly for separation and catalysis. In analogue, poor particle-polymer matrix interaction and particle aggregation might lead to low MOF contents (e.g. ＜60 %) and hereby low specific surface areas. Metal-organic gels and aerogels were also produced by growing and aggregating MOF particles into interconnected 3D networks through solvothermal heteronuclear method20 and high-internal-phase emulsion.21 But their experimental conditions had to be tuned precisely to achieve gelation over crystallization and precipitation,22 which thus decreased their tunability in pore apertures, overall porosities and densities. Due to weak inter-particle interactions and intrinsic rigidity of MOFs, these gels and aerogels also lacked sufficient flexibility and robustness for large deformation highly desired when molding into various shapes. Irregular aggregation of MOF particles also decreased their extrinsic surface areas valid for applications. Mechanical flexibility and shapeability of aerogels rely strongly on their porosity and cross-linking, as well as intrinsic toughness, dimensionality and aspect ratio of their building blocks. For example, many nanofibrils of natural macromolecules, such as denatured lysozyme,23,24 silk fibroin,25 and cellulose,26 have the diameter of 3-10 nm and aspect ratio up to 103, whose super toughness and mutual interactions (e.g. physical interactions and entanglements) offered the opportunity of producing fibrous 3D matrix with sustainability, flexibility, shapeability and hierarchical porosity. In order to introduce the merits of nanofibrous aerogels, we endeavored to in-situ synthesize MOF crystals around TEMPO-oxidized CNFs. Fibrous nanocellulose had abundant polar groups (e.g. hydroxyl and carboxyl) and large specific surface area, offering copious reaction sites and strong binding interactions for nucleation, growth and adhesion of MOF crystals. 4 ACS Paragon Plus Environment

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During the MOF synthesis, nanocellulose, on one hand, was expected to favor crystal nucleation over growth, and thus decrease the size of MOF crystals. On the other hand, they also decreased the aggregation possibilities of MOF particles, and thus benefitted to retain optimal performance of MOF particles, in sharp contrast to simply blending MOF crystals with cellulose nanocrystals.5 Cellulose nanofibrils also had the large aspect ratio and inter-fibril interactions, enabling sufficient cross-linkers (e.g. H-bonding and physical entanglements) in the resultant fibrous MOF aerogels, being advantageous to withstand large-strain deformation. Thus these fibrous MOF aerogels not only maintained high micro/mesoporosities of MOF crystals, but also combined hierarchical porosities (up to 99%), flexibility, shapeability, low density (e.g. below 0.1 g/cm3) of cellulose aerogels. This combination offers a low-cost, sustainable and scalable platform for superior adsorption capacity and kinetics to MOF powders, as well as variable shapes highly desired in different supporters and devices. These aerogels were also in sharp contrast to MOF loaded porous CNF membranes produced by filtration,27 which may lack flexibility in control of micro/mesoporosities and shapeability, and thus could be invalid for wide applications other than gas separation.

RESULTS AND DISCUSSION CNFs are one of the most abundant organic nanomaterials in nature and have the global production of tens of billions tons per year. Their merit lies not only in nanoscale dimension, low cost, sustainability and lightweight, but also in high solvent-resistance, strength, aspect ratio and specific area. When being exfoliated from softwood pulp by a TEMPO-oxidization technique,26 CNFs could achieve a surface carboxylate content up to 1.38 mmol/g and an aspect ratio up to 103 5 ACS Paragon Plus Environment

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(Figure 1a, 1b & S1). They not only offered the possibility of gelation by chelating with multivalent metal ions (Figure S2a), but also were capable of serving as functional template to in-situ synthesize and capture a large amount of MOF crystals. Fibrous MOF aerogels scaffolded by CNFs were produced through the sequential processes of ionic gelation, template synthesis of MOF and freeze-drying. When exposing to multivalent metal ions (e.g. Zn2+, Cu2+ and Co2+ in Figure 1C), carboxylic CNFs would interact ionically with metal ions. These ionic interactions served as ionic cross-linkers and favoured a “phase transition” from colloidal CNFs to a homogeneous fibrous hydrogel (CNFs-M2+). In the presence of appropriate ligand precursors, the binding metal ions would participate to form MOF crystals around the CNFs (Figure 1d), e.g. zeolitic imidazolate framework-8 (ZIF-8), Hong Kong University of Science and Technology-1 (HKUST-1) and zeolitic imidazolate framework-67 (ZIF-67). In these homogenous MOF hydrogels, CNFs seemed to be physically cross-linked by H-bonding and physical entanglement of CNFs as well as strong interactions between MOF crystals and CNFs. And MOF crystals were nucleated and grew gradually on fibrous CNF networks. The fibrous MOF networks were also stable enough to survive from freeze-drying and formed bulk MOF aerogels (Figure 1e). The resultant MOF aerogels showed fibrous and hierarchically porous microstructures, being ultralight, flexible and mechanically robust enough to be handled without breaking their structural integrity. To be noted, direct mixing of MOF crystals and colloidal CNFs failed to form homogeneous hydrogels and aerogels due to the strong aggregation tendency of MOF particles and CNFs (see Figure S3a & S3b). When using ZIF-8 as the MOF example, Zn2+ initially interacted ionically with carboxylic 6 ACS Paragon Plus Environment

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CNFs. Then they coordinated with 2-methylimidazole and nucleated for the growth of ZIF-8 crystals on CNFs-Zn2+ (Figure 2a & S4). The loading of ZIF-8 crystals within the hybrid aerogels can be easily tuned by controlling the concentration of zinc ions and organic ligands (Table S1). The mass fraction of ZIF-8 crystals in the hybrid aerogels is directly analyzed by weighting method. The calculated content of ZIF-8 could be tuned within a broad range of 11-81 wt%. Because of high specific surface areas of CNFs, MOF nucleation would compete with the growth of MOF crystals. And as-synthesized ZIF-8 crystals (with the size of 18~65 nm) were smaller than those (with the size of ~75 nm) synthesized without the presence of CNFs-Zn2+ (Figure 2b-2e & S3b). Under the same concentration of CNFs-Zn2+ gels, their sizes were progressively promoted by the higher concentrations of Zn2+ and organic ligands (Figure 2b-2e & S5). For example, under the lower MOF content of 33 wt% (Figure S5), small ZIF-8 crystals with ~18 nm in diameter anchored discretely on CNFs. And with the ZIF-8 content of 81% (Figure 2d), relatively larger ZIF-8 crystals with ~65 nm in diameter assembled continuously on CNFs. In addition to ligand concentration, higher crystallization temperature also slightly promoted the size of ZIF-8 crystals (Figure S6). ZIF-8 crystals adhered to CNFs via H-bonding and ionic interactions. These strong interactions not only ensured minimal MOF loss from the aerogels during practical applications, but also ensured ultra-low densities of MOF aerogels comparable to those of CNF-Zn2+ aerogels (Figure 2e & S2c), e.g. 9.1 mg/cm3 for 33 wt% MOF content and 65.1 mg/cm3 for 81 wt% MOF content (Figure 2f). These values were also superior to other porous materials with the corresponding MOF contents.5,14,17,27-30 Powder X-ray diffraction (XRD) was used to confirm the crystal structure of ZIF-8 in the 7 ACS Paragon Plus Environment

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aerogels. As shown in Figure 3a & S7a, CNFs-Zn2+ aerogels showed only two broad diffraction peak around 2θ of 15.7 and 22.3°, which are typical of cellulose I crystalline structure.31 In the MOF aerogels, strong diffractions at 2θ of 7.2, 10.3, 12.6, 16.4 and 17.9° showed up, corresponding to the (110), (200), (211), (310) and (222) diffraction peaks of ZIF-8 crystals.32 The ZIF-8 powders and the aerogels showed similar XRD patterns, indicating the formation of pure phase of ZIF-8 in the aerogels (Figure S7b). Moreover, the diffraction intensities correlated directly to the MOF contents evaluated by thermogravimetry analysis in Figure S7c. The FT-IR and XPS spectra of CNFs-Zn2+, ZIF-8 aerogels and ZIF-8 powders were also collected to evaluate the interactions between CNF and ZIF-8 crystals. As shown in Figure 3b, after hybridization with ZIF-8, the broad adsorption at 3416 cm-1, which was ascribed to the stretching vibrations of –OH groups of CNFs, broadened and underwent a low-frequency shift to 3342 cm-1. The sharp peak at 1140 cm-1, which was identified as C-N bonds in ZIF-8, showed a high-frequency shift to 1147 cm-1. Meanwhile, the XPS peaks at 532.4 eV and 398.7 eV, which were attributed to the typical signals of oxygen in hydroxyl groups and nitrogen in imidazole groups, represented opposite shift directions in ZIF-8 aerogels (Figure 3c & 3d). These characteristics indicated the existence of strong hydrogen bonding between CNFs and ZIF-8 crystals, which promoted the tight immobilization of ZIF-8 nanocrystals on CNF surfaces. These aerogels have a series of mechanic advantages such as flexibility and robustness (Figure 3e). With comparison to conventional ionically-crosslinking CNF aerogels33 (Figure S2), the MOF aerogels had the reducing ionic cross-linkers of -COO- M2+ -OOC-, which enhanced their deformability under compression. For example, the ZIF-8 aerogels showed a strain of ＞80% 8 ACS Paragon Plus Environment

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without disruption, in contrast to ＜60 % of CNFs-Zn2+ aerogel under the same compression stress up to 6 MPa. This super deformability offered the possibility of moulding MOF into shapes desired for diverse applications, e.g. cylinder, plate and other complex 3D shapes (Figure 3f). An impressive advantage of MOFs is their high specific area, which is favourable for a series of practical applications such as catalysis, adsorption and separation.34 Figure 4a & S8a gives the nitrogen adsorption isotherms of pristine ZIF-8 powders and ZIF-8 aerogels. As shown in Figure S8a, ZIF-8 powders exhibited typical type-I adsorption-desorption isotherms, revealing its microporous nature. When MOF crystals were synthesized on the template of CNFs, the obtained ZIF-8 aerogels displayed quite different adsorption-desorption isotherms. The N2 uptake at high relative pressures was dramatically increased and an obvious hysteresis loop was observed, suggesting the introduction of abundant mesoporous. Such mesoporous with a size of 7-40 nm was attributed to the fibrous porosity of ZIF-8 aerogels, which was beneficial to the exposure of more external surface of MOF crystals. In addition to this, the utilization of CNFs as template barely affected the microporous nature of ZIF-8 crystals, ZIF-8 powders and ZIF-8 aerogels showed similar microporous distributions within the sub 2 nm region (Figure S8b & S8c). Moreover, the specific surface area of ZIF-8 aerogels within 273.4 ~ 890.9 m2/g were well proportional to the ZIF-8 content, suggesting that fibrous aggregations of MOF crystals did not depress their surface areas valid for adsorption and transportation.34 Among several organic dyes (Figure S9), Rhodamine B was utilized as a model tracer to monitor the adsorption kinetics of ZIF-8 aerogels in Figure 4b. After adding the aerogels, the pink dye solution gradually faded into colorlessness, and its UV–vis absorption maximum at 554 nm 9 ACS Paragon Plus Environment

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reduced significantly. Moreover, the ZIF-8 aerogels showed more efficient adsorption than conventional CNFs-Zn2+ aerogels imaged in Figure S2 and ZIF-8 powders synthesized without CNFs and imaged in Figure S3. The prominent adsorption performance of ZIF-8 aerogels might be resulted from the hierarchical fibrous porosity and smaller MOF crystals in this MOF aerogel, which could supply more external surface and accelerate mass transfer for rapid adsorption. In order to directly evaluate the utilization efficiency of ZIF-8 crystals in the hybrid aerogels, adsorption kinetics and corresponding parameters (based on mass of MOFs) for various ZIF-8 aerogels were illustrated in Figure 4c & 4d. Their time-dependence curves of UV-Vis adsorption at 554 nm fitted well with the pseudosecond-order kinetic model (Figure S10b), which implied that the adsorption performance was determined mainly by intra-particle diffusion.35 With the ZIF-8 loading within 0~33 wt% (see Figure S5), the hybrid aerogels had a positive dependence of adsorption efficiency on the ZIF-8 content (Figure S10a). Considering large Rhodamine B molecules (15.9×11.8 ×5.6 Å3 in size) might not enter ZIF-8 nanopores (3.4 Å in window size), this phenomenon could be attributed to stronger surface adsorption ability of ZIF-8 crystals than that of CNFs. The aerogels with ZIF-8 content of 33 wt% possessed the highest adsorption rate (k2, 0.036 g mg-1 h-1) and equilibrium adsorption capacity (qe, 83.3 mg/g), being significantly higher than ZIF-8 powders (~ 75 nm, 0.02 g mg-1 h-1 and 16.81 mg/g, respectively) and conventional CNFs-Zn2+ aerogels. The ZIF-8 content of > 33 wt% would result in much larger crystal sizes and close packing of ZIF-8 crystals in the hybrid aerogels, which thus decreased the external surfaces of ZIF-8 crystals valid for adsorption (Figure S10b). In order to determine the maximum adsorption capacity of 33 wt% ZIF-8 aerogels for 10 ACS Paragon Plus Environment

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Rhodamine B, the adsorption isotherm (based on the total mass of hybrid aerogel) was performed in Figure S10c. The equilibrium adsorption data fitted well to a Langmuir model with a maximum adsorption capacity up to 81 mg/g for the aerogel with 33 wt% ZIF-8 content. To quantify the affinity of this aerogel adsorbent for Rhodamine B, its distribution coefficient (Kd) was thus calculated according to the equation of Kd=qe/ce to be 8800 mL/g at equilibrium concentrations (ce) of 2.5 mg/L.36 Such high adsorption capacity and distribution coefficient surpassed most of previous absorbent materials (e.g. MOFs of Fe3O4/MIL-100(Fe), MIL-125(Ti) and JLU-Liu 39; Natural polymers of seed hull, rhizopus oryzae, bagasse etc.; Inorganic materials of Australian zeolite, sodium montmorillonite, kaolinite etc.; synthetic resins) (Figure 4e).37-39 Such excellent adsorption performance was also valid for other organic dyes such as Methyl Violet and Methylene Blue (Figure S11a). In addition, MOF aerogels also exhibited good performance for the selective adsorption of methyl orange from methyl violet on the basis of different adsorption abilities (Figure S11b). The combination of deformability and rapid adsorption kinetics offered the facileness of employing ZIF-8 to many specific circumstances which were inappropriate for MOF powders. As illustrated in Figure 4f, the ZIF-8 aerogel could deform and fit in a filtration setup for continuous filtering adsorption. Under a fast filtration flux (e.g. 2.31-4.72×103 L•m-2•h-1) for three cycles, the dye concentration characterized by the UV-Vis absorption peak at 554 nm was reduced rapidly to 2 % of the initial concentration. In sharp contrast, the utilization of ZIF-8 powders led to much lower adsorption efficiency and serious leakage through the filtration setup (Figure S12). To be noted, this production pathway of fibrous MOF aerogels was not limited only to ZIF-8. 11 ACS Paragon Plus Environment

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Other MOF aerogels (e.g. HKUST-1 and ZIF-67) could also be produced by using different metal clusters and organic linkers. Both SEM and XRD measurements (Figure S13 & S14) confirmed the successful formation of MOF aerogels with lightness, mechanical robustness and free-standing.

CONCLUSIONS In brief, shapeable and fibrous MOF aerogels were produced successfully by synthesizing MOF crystals on the template of TEMPO-cellulose nanofibrils. The nanofibrous template not only offered extrinsic porosities and mechanical flexibility for the resultant MOF aerogels, but also shifted the balance of nucleation and growth for synthesizing smaller MOF crystals, and further decreased their aggregation possibilities. With shapeability, hierarchical porosity up to 99% and low density below 0.1 g/cm3, these MOF aerogels could make the most of their pores and accessible surface areas for higher adsorption capacity and rapid adsorption kinetics of different molecules. Thus this scalable and low-cost production pathway is able to convert MOF powders into a shapeable and flexible form and hereby extend their application in more broad fields.

EXPERIMENTAL SECTION Materials Needle bleached kraft pulp was supplied by Hangzhou Wohua Filter Paper Co. Ltd. 2,2,6,6-Tetramethylpiperidyl-1-Oxyl (TEMPO) was purchased from Sigma-Aldrich. Sodium hypochlorite solution (6-14%), copper nitrate trihydrate, cobalt nitrate hexahydrate, zinc nitrate hexahydrate, 2-methylimidazole and trimesic acid were purchased from Aladdin Industrial Corporation. Other reagents (HCl, NaOH, NaBr, Rhodamine B, Methyl Blue, Methylene Blue,


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Methyl Orange, Methyl Violet etc.) were purchased from Sinopharm Chemical Reagent Co. Ltd. Deionized water (resistivity: 18.2 MΩ/cm) were used to prepare aqueous solutions. Fabrication of CNFs CNFs were exfoliated from softwood pulp according to the typical TEMPO oxidation protocol.26 Dry kraft pulp (5 g) was subjected to oxidation in a mixed solution (500 mL) of TEMPO (0.08 g), NaClO (80 mmol) and NaBr (0.5 g) under alkaline conditions (pH = 10.5) for 7 h with vigorous stirring. After washing with distilled water for >3 times, the resultant floccules were homogenized with 3 passes at 50 MPa and pH ~7.0 in a pressure micro-fluidizer (MRT model CR5). The viscous product (0.18 wt%) was then centrifuged at 9000 rpm to remove un-exfoliated aggregates and stored at 4 °C for characterization. Ionic gelation of CNFs with M2+ The thixotropic CNF suspensions (3 mL) were first stirred vigorously and degassed centrifugally. After dropwise adding 2 mL of M2+ solution (1.68 M) without stirring, the suspensions underwent the gelation process for 24 h. The resultant hydrogels were washed with distilled water, following by exchanging the solvent to t-BuOH and freeze-drying. The obtained metal-chelated aerogels were denoted as CNFs-M2+ aerogels, which was used as control sample in the whole tests. Fabrication of MOF aerogel scaffolded by CNFs CNFs-Zn2+ gels were solvent-exchanged in methanol (or ethanol) and incubated in the solutions of metal salts in methanol (or ethanol) for 24h. The gels were then immerged in the corresponding solutions of ligand precursors in alcohol for MOF growth under specific experimental conditions (Table S1), e.g. ZIF-8 with 16.8~336 mM zinc nitrate hexahydrate and 0.13~2.69 M 2-methylimidazole at 25 ℃ for 12 h, ZIF-67 with 34.4 mM cobalt nitrate hexahydrate and 0.13 M 13 ACS Paragon Plus Environment

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2-methylimidazole, methanol at 85 ℃ for 12 h, HKUST-1 with 0.12 M copper nitrate trihydrate and 0.14 M trimesic acid at 25 ℃ for 12 h (ethanol). The aerogels were obtained after washing with ethanol for 3 times, solvent-exchanging in t-BuOH and freeze-drying (-58 oC, < 50 Pa). In order to avoid the shrinkage of the hydrogels during the solvent-exchanging process, the alcogels were immerged in the mixed solutions of methanol (alcohol) and t-BuOH with increased t-BuOH content step by step. Subsequently, these gels placed in a plastic tube were directly frozen by immersing the tube in liquid nitrogen before freeze-drying. MOF powders were prepared by directly mixing of 336 mM zinc nitrate hexahydrate and 2.69 M 2-methylimidazole in methanol at 25 oC for 12 h. The mass fraction of ZIF-8 crystals in the hybrid aerogels is directly analyzed by weighting method. If setting the weight of pristine CNF-Zn2+ aerogel and corresponding MOF aerogels as x and y, the ZIF-8 content (ϕZIF-8) was calculated according to the equation ϕZIF-8=(y-x)/y. The calculated content of ZIF-8 could be tuned within a broad range of 11 wt%-81 wt%. Adsorption evaluation of MOF aerogels Five organic dyes were used to evaluate adsorption performance of MOF aerogels in aqueous solutions at room temperature. The aerogels (1.2 mg) were incubated in 5 mL of aqueous solutions of dyes (10 mg/L) under the monitor of UV-vis spectroscopy. Characterization Transmission electron microscopy (TEM) measurements were carried out with a Hitachi TEM (H-7650) instrument operating at a voltage of 100 kV. Scanning electron microscopy (SEM) measurements were conducted on a Hitachi S-4800 instrument operated at 2 kV for gold-sputtered samples. X-ray diffraction patterns (XRD) were recorded on a Bruker D8 ADVANCE X-ray diffractometer with a Cu Kα radiation (λ=1.5418A). Powder was leveled on sample holders and 14 ACS Paragon Plus Environment

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scanned with a 2θ angle from 5 to 50° with a step speed of 5 °/min. FTIR measurements were conducted on a Nicolet 6700 Fourier transform infrared spectrometer. X-ray photoelectron spectra (XPS) were obtained with an ESCALAB 250Xi (Thermo Scientific) X-ray photoelectron spectrometer, using monochromatic Al Kα (1486.6 eV) radiation as the excitation source. Binding energy was charge corrected to 284.6 eV for C 1s. Thermal gravimetric analysis (TGA) was conducted on a thermogravimetric analyzer (Ulvac TGD 9600) at a ramp rate of 10 °C/min from room temperature to 900 °C under air purge at a flow rate of 10 mL/min. Nitrogen physisorption measurements were performed at 77 K by a Quantachrome autosorb iQ surface analyzer (Quantachrome, USA). Brunauer Emmett Teller (BET) analysis was performed for relative vapor pressures of 0.01–0.3. The samples were degassed at 90 °C in vacuum to remove all the adsorbed species. Compression tests were performed on a universal tensile-compressive machine (CMT 6503, MTS systems China Co. Ltd).

ASSOCIATED CONTENT Supporting Information The Supporting Information is available free of charge on the ACS Publications website at DOI: Photographs, SEM images, TGA curves, Zeta potential, XRD patterns, adsorption isotherms, and tables summarizing the MOF aerogels properties and adsorptive model parameters (PDF).

ACKNOWLEDGEMENTS


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The National Natural Science Foundation of China (No. 21474125, 51603224, 31370582 and 31370584), Chinese “1000 Youth Talent Program”, Shandong “Taishan Youth Scholar Program”, Natural Science Foundation of Shandong Province (JQ201609), Key Laboratory of Pulp and Paper Science & Technology of Ministry of Education of China (KF201404) and Shandong Collaborative Innovation Centre for Marine Biomass Fiber Materials and Textiles are kindly acknowledged for financial support.

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(11) Yao, M. S.; Lv, X. J.; Fu, Z. H.; Li, W. H.; Deng, W. H.; Wu, G. D.; Xu, G. Layer-by-Layer Assembled Conductive MOF Nanofilms for Room Temperature Chemiresistive Sensing. Angew. Chem. Int. Ed. 2017, 56, 16510-16514. (12) Lemaire, P. C.; Zhao, J.; Williams, P. S.; Walls, H. J.; Shepherd, S. D.; Losego, M. D.; Peterson, G. W.; Parsons, G. N. Copper Benzenetricarboxylate Metal-Organic Framework Nucleation Mechanisms on Metal Oxide Powders and Thin Films Formed by Atomic Layer Deposition. ACS Appl. Mater. Interfaces 2016, 8, 9514-9522. (13) Kitao, T.; Zhang, Y.; Kitagawa, S.; Wang, B.; Uemura, T. Hybridization of MOFs and Polymers. Chem. Soc. Rev. 2017, 46, 3108-3133. (14) Wisser, D.; Wisser, F. M.; Raschke, S.; Klein, N.; Leistner, M.; Grothe, J.; Brunner, E.; Kaskel, S. Biological Chitin-MOF Composites with Hierarchical Pore Systems for Air-Filtration Applications. Angew. Chem. Int. Ed. 2015, 54, 12588-12591. (15) Wu, Y.; Li, F.; Liu, H.; Zhu, W.; Teng, M.; Jiang, Y.; Li, W.; Xu, D.; He, D.; Hannam, P. Electrospun Fibrous Mats as Skeletons to Produce Free-Standing MOF Membranes. J. Mater. Chem. 2012, 22, 16971-16978. (16) Neufeld, M. J.; Lutzke, A.; Tapia, J. B.; Reynolds, M. M. Metal-Organic Framework/Chitosan Hybrid Materials Promote Nitric Oxide Release from S-Nitrosoglutathione in Aqueous Solution. ACS Appl. Mater. Interfaces 2017, 9, 5139-5148. (17) Zhu, H.; Zhang, Q.; Zhu, S. Alginate hydrogel: A Shapeable and Versatile Platform for in situ Preparation of Metal-Organic Framework-Polymer Composites. ACS Appl. Mater. Interfaces 2016, 8, 17395-17401. (18) Sorribas, S.; Gorgojo, P.; Téllez, C.; Coronas, J.; Livingston, A. G. High Flux Thin Film Nanocomposite Membranes based on Metal-Organic Frameworks for Organic Solvent Nanofiltration. J. Am. Chem. Soc. 2013, 135, 15201-15208. (19) Rodenas, T.; van Dalen, M.; García-Pérez, E.; Serra-Crespo, P.; Zornoza, B.; Kapteijn, F.; Gascon, J. Visualizing MOF Mixed Matrix Membranes at the Nanoscale: Towards Structure-Performance Relationships in CO2/CH4 Separation over NH2-MIL-53 (Al)@ PI. Adv. Funct. Mater. 2014, 24, 249-256. (20) Mahmood, A.; Xia, W.; Mahmood, N.; Wang, Q.; Zou, R. Hierarchical Heteroaggregation of Binary Metal-Organic Gels with Tunable Porosity and Mixed Valence Metal Sites for Removal of Dyes in Water. Sci. Rep. 2015, 5, 10556. (21) Zhang, B.; Zhang, J.; Liu, C.; Peng, L.; Sang, X.; Han, B.; Ma, X.; Luo, T.; Tan, X.; Yang, G. High-Internal-Phase Emulsions Stabilized by Metal-Organic Frameworks and Derivation of Ultralight Metal-Organic Aerogels. Sci. Rep. 2016, 6, 21401. (22) Li, L.; Xiang, S.; Cao, S.; Zhang, J.; Ouyang, G.; Chen, L.; Su, C.-Y. A Synthetic Route to Ultralight Hierarchically Micro/Mesoporous Al (III)-Carboxylate Metal-Organic Aerogels. Nat. Commun. 2013, 4, 1774. (23) Li, C.; Born, A. K.; Schweizer, T.; Zenobiwong, M.; Cerruti, M.; Mezzenga, R. Amyloid-Hydroxyapatite Bone Biomimetic Composites. Adv. Mater. 2014, 26, 3207-3212. (24) Li, C.; Adamcik, J.; Mezzenga, R. Biodegradable Nanocomposites of Amyloid Fibrils and Graphene with Shape-Memory and Enzyme-Sensing Properties. Nat. Nanotechnol. 2012, 7, 421.


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(25) Lv, L.; Han, X.; Zong, L.; Li, M.; You, J.; Wu, X.; Li, C. Biomimetic Hybridization of Kevlar into Silk Fibroin: Nanofibrous Strategy for Improved Mechanic Properties of Flexible Composites and Filtration Membranes. ACS Nano 2017, 11, 8178-8184. (26) Saito, T.; Kimura, S.; Nishiyama, Y.; Isogai, A. Cellulose Nanofibers Prepared by TEMPO-Mediated Oxidation of Native Cellulose. Biomacromolecules 2007, 8, 2485-2491. (27) Matsumoto, M.; Kitaoka, T. Ultraselective Gas Separation by Nanoporous Metal-Organic Frameworks Embedded in Gas-Barrier Nanocellulose Films. Adv. Mater. 2016, 28, 1765-1769. (28) Küsgens, P.; Siegle, S.; Kaskel, S. Crystal Growth of the Metal-Organic Framework Cu3(BTC)2 on the Surface of Pulp Fibers. Adv. Eng. Mater. 2009, 11, 93-95. (29) Zhao, J.; Lee, D. T.; Yaga, R. W.; Hall, M. G.; Barton, H. F.; Woodward, I. R.; Oldham, C. J.; Walls, H. J.; Peterson, G. W.; Parsons, G. N. Ultra-Fast Degradation of Chemical Warfare Agents using MOF-Nanofiber Kebabs. Angew. Chem. Int. Ed. 2016, 55, 13224-13228. (30) Lange, L. E.; Obendorf, S. K. Functionalization of Cotton Fiber by Partial Etherification and Self-Assembly of Polyoxometalate Encapsulated in Cu3(BTC)2 Metal-Organic Framework. ACS Appl. Mater. Interfaces 2015, 7, 3974-3980. (31) You, J.; Cao, J.; Zhao, Y.; Zhang, L.; Zhou, J.; Chen, Y. Improved Mechanical Properties and Sustained Release Behavior of Cationic Cellulose Nanocrystals Reinforeced Cationic Cellulose Injectable Hydrogels. Biomacromolecules 2016, 17, 2839-2848. (32) Park, K. S.; Ni, Z.; Côté, A. P.; Choi, J. Y.; Huang, R.; UribeRomo, F. J.; Chae, H. K.; O'Keeffe, M.; Yaghi, O. M. Exceptional Chemical and Thermal Stability of Zeolitic Imidazolate Frameworks. Proc. Natl. Acad. Sci. 2006, 103, 10186-10191. (33) Dong, H.; Snyder, J. F.; Williams, K. S.; Andzelm, J. W. Cation-Induced Hydrogels of Cellulose Nanofibrils with Tunable Moduli. Biomacromolecules 2013, 14, 3338-3345. (34) Lu, A. X.; Mcentee, M.; Browe, M. A.; Hall, M. G.; Decoste, J. B.; Peterson, G. W. MOFabric: Electrospun Nanofiber Mats from PVDF/UiO-66-NH2 for Chemical Protection and Decontamination. ACS Appl. Mater. Interfaces 2017, 9, 13632-13636. (35) Plazinski, W.; Dziuba, J.; Rudzinski, W. Modeling of Sorption Kinetics: the Pseudo-Second Order Equation and the Sorbate Intraparticle Diffusivity. Adsorption 2013, 19, 1055-1064. (36) Aguila, B.; Sun, Q.; Perman, J. A.; Earl, L. D.; Abney, C. W.; Elzein, R.; Schlaf, R.; Ma, S. Efficient Mercury Capture Using Functionalized Porous Organic Polymer. Adv. Mater. 2017, 29, 100665. (37) Eftekhari, S.; Habibi-Yangjeh, A.; Sohrabnezhad, S. Application of AlMCM-41 for Competitive Adsorption of Methylene Blue and Rhodamine B: Thermodynamic and Kinetic Studies. J. Hazard. Mater. 2010, 178, 349-355. (38) Yao, S.; Xu, T.; Zhao, N.; Zhang, L.; Huo, Q.; Liu, Y. An Anionic Metal-Organic Framework with Ternary Building Units for Rapid and Selective Adsorption of Dyes. Dalton. Trans. 2017, 46, 3332-3337. (39) Selvam, P. P.; Preethi, S.; Basakaralingam, P.; Thinakaran, N.; Sivasamy, A.; Sivanesan, S. Removal of Rhodamine B from Aqueous Solution by Adsorption onto Sodium Montmorillonite. J. Hazard. Mater. 2008, 155, 39-44.


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Figure 1. Pathway followed to fabricate fibrous MOF aerogels with the template of CNFs. (a) Schematic illustration of nanocellulose in wood. (b) TEMPO-exfoliation to produce carboxylic CNFs. (c) Ionic gelation of CNFs with ionic interactions between metal ions and carboxylic CNFs. (d) Template synthesis of MOF crystals around CNFs-M2+. (e) Freeze-drying for fibrous MOF aerogels.


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Figure 2. Morphological characterization of fibrous ZIF-8 aerogels. (a) Schematic illustration of hydrogen binding between CNFs and ZIF-8 crystals. (b-d) SEM images of ZIF-8 aerogels with different ZIF-8 contents: 51 wt% (b), 75 wt% (c) and 81 wt% (d). (e) Dependence of aerogel density and ZIF-8 crystals size on ZIF-8 contents. (f) Specific density and ZIF-8 contents of MOF aerogels in comparison to other porous materials with MOF loading (detailed in Table S2).


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Figure 3. Physical characterization of fibrous ZIF-8 aerogels. (a) XRD patterns. (b) FT-IR spectra. (c) O 1s XPS analysis. (d) N 1s XPS analysis. (e) Compression performance. (f) Shapeability evaluation for cutting, molding and patterning. Different ZIF-8 contents were indicated.


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Figure 4. Adsorption characterization of fibrous ZIF-8 aerogels for dyes. (a) N2 adsorption-desorption analysis. (b) UV-vis spectra of aqueous solutions of Rhodamine B after exposing to ZIF-8 aerogels. Initial concentration of Rhodamine B: 10 mg/L; Exposing time: 8 h; Weight ratio of solution and aerogel: 0.042. The inset gives the corresponding photographs (from left to right) of the aqueous solutions after exposition. (c) Adsorption curve normalized to ZIF-8 mass. (d) Dependence of rate constant and equilibrium adsorptive capacities on ZIF-8 contents (based on mass of MOFs). (e) Maximum uptake amount and kd value in comparison to other porous absorbents (based on mass of total aerogels, detailed in Table S3). (f) Molding ZIF-8 aerogels for conventional filtration setup for adsorption.


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TOC

Shapeable Fibrous Aerogels of Metal-Organic-Frameworks Templated with Nanocellulose for Rapid and Large-Capacity Adsorption Luting Zhu, †,1,2 Lu Zong, †,2 Xiaochen Wu,2 Mingjie Li,2 Haisong Wang,1,3,* Jun You,2,* and Chaoxu Li2,*


Shapeable Fibrous Aerogels of MetalâOrganic-Frameworks

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