Hierarchical MetalâOrganic Framework Hybrids - American Chemical

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Hierarchical Metal−Organic Framework Hybrids: PerturbationAssisted Nanofusion Synthesis Yanfeng Yue,*,†,⊥ Pasquale F. Fulvio,‡ and Sheng Dai*,†,§ †

Chemical Sciences Division, Oak Ridge National Laboratory, Oak Ridge, Tennessee 37831, United States Department of Chemistry, University of Puerto Rico, Rio Piedras Campus, San Juan, Puerto Rico 00931, United States § Department of Chemistry, University of Tennessee, Knoxville, Tennessee 37996, United States ⊥ Department of Biology, Geology, and Physical Science, Sul Ross State University, Alpine, Texas 79832, United States ‡

CONSPECTUS: Metal−organic frameworks (MOFs) represent a new family of microporous materials; however, microporous−mesoporous hierarchical MOF materials have been less investigated because of the lack of simple, reliable methods to introduce mesopores to the crystalline microporous particles. State-of-the-art MOF hierarchical materials have been prepared by ligand extension methods or by using a template, resulting in intrinsic mesopores of longer ligands or replicated pores from template agents, respectively. However, mesoporous MOF materials obtained through ligand extension often collapse in the absence of guest molecules, which dramatically reduces the size of the pore aperture. Although the templatedirected strategy allows for the preparation of hierarchical materials with larger mesopores, the latter requires a template removal step, which may result in the collapse of the implemented mesopores. Recently, a general template-free synthesis of hierarchical microporous crystalline frameworks, such as MOFs and Prussian blue analogues (PBAs), has been reported. This new method is based on the kinetically controlled precipitation (perturbation), with simultaneous condensation and redissolution of polymorphic nanocrystallites in the mother liquor. This method further eliminates the use of extended organic ligands and the micropores do not collapse upon removal of trapped guest solvent molecules, thus yielding hierarchical MOF materials with intriguing porosity in the gram scale. The hierarchical MOF materials prepared in this way exhibited exceptional properties when tested for the adsorption of large organic dyes over their corresponding microporous frameworks, due to the enhanced pore accessibility and electrolyte diffusion within the mesopores. As for PBAs, the pore size distribution of these materials can be tailored by changing the metals substituting Fe cations in the PB lattice. For these, the textural mesopores increased from approximately 10 nm for Cu analogue (mesoCuHCF), to 16 nm in Co substituted compound (mesoCoHCF), and to as large as 30 nm for the Ni derivative (mesoNiHCF). While bulk PB and analogues have a higher capacitance than hierarchical analogues for Na-batteries, the increased accessibility to the microporous channels of PBAs allow for faster intercalated ion exchange and diffusion than in bulk PBA crystals. Thus, hierarchical PBAs are promising candidates for electrodes in future electrochemical energy storage devices with faster charge−discharge rates than batteries, namely pseudocapacitors. Finally, this new synthetic method opens the possibility to prepare hierarchical materials having bimodal distribution of mesopores, and to tailor the structural properties of MOFs for different applications, including contrasting agents for MRI, and drug delivery.

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INTRODUCTION Hierarchical porous materials are of great interest for separations, catalysis, and energy storage applications. Pores are defined according to the IUPAC as micropores (widths smaller than 2 nm), mesopores (widths between 2 and 50 nm), and as macropores (widths larger than 50 nm).1 While micropores contribute to the bulk of the surface area of a material, mesopores and macropores provide the required accessibility to gases and larger molecules to quickly diffuse through large crystal particles and finally reach the storage or reactive sites within micropores.2 Zeolites are among the first examples of widely investigated microporous materials with periodic crystal lattices. © 2015 American Chemical Society

Following the reports on the hydrothermal synthesis of zeolites, the discovery of surfactant templated ordered mesoporous siliceous materials (OMMs) in the 1990s, that is, MCM-41,3 SBA-15,4 KIT-6,5 and FDU-1,6 renewed the interest for the preparation of hierarchical porous structures. The use of surfactants allows for the preparation of materials with periodic packing of mesopores with controlled geometry and size. However, the walls of these materials are essentially amorphous. Received: July 28, 2015 Published: December 4, 2015 3044

DOI: 10.1021/acs.accounts.5b00349 Acc. Chem. Res. 2015, 48, 3044−3052

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Accounts of Chemical Research

multiple solvent extractions of templates are preferred over high temperature calcinations. Thus, the aforementioned methods are energy intensive, time-consuming, and the increased preparation costs hinders the upscale production of hierarchical MOFs. In the absence of a template, the first question in mesopore construction is how to generate SBUs that do not assemble into bulk MOF crystals. Recent investigations demonstrated that by transferring the organic linker to the metal ion precursor solution, under vigorous stirring could yield mesoporous aggregates of MOF nanosized units.22 The strong stirring kinetically limits the nucleation of large MOF crystals by maximizing the surface between the two mixing phases and permitting multiple coordination between organic linkers and metal cations. This induces formation of fewer SBUs and limits the size of MOF nanoparticles. It also prevents the slow nucleation of large MOF crystals with preferred facet orientation. Thus, the kinetic control over the formation and dissolution of SBUs, or system perturbation, is key to inducing the formation and aggregation of small polymorphic microporous crystals. Over time, randomly precipitated SBUs bind the larger polymorphic MOF crystals (nanofusion) (Scheme 1).22

To date, only a few examples of alkyl ammonium surfactant or silane modified surfactant template crystalline microporousmesoporous zeolites are known.7,8 The enhanced catalytic activities reported for these hierarchical zeolites over their microporous analogues have increased the scientific and technological interest over other types of microporous frameworks, namely, metal organic frameworks (MOFs).9−12 While some methods for the preparation of mesoporous MOFs exist, simpler methodologies are necessary for their upscale preparation and commercial exploration of the novel properties introduced by hierarchical pore structures. Like zeolites, MOFs are crystalline inorganic−organic hybrid materials with the atomic periodicity extending to the pores and channels. In zeolites, however, the periodic walls form from the condensation of silicate precursors with an ammine as structure directing agent. MOFs form via the self-assembly of secondary building units (SBUs) containing metal ions and oligotopic organic linkers. The self-assembly is strongly dependent on the coordination preference of the SBUs and the length and rigidity of the linkers, and in the host−guest chemistry of the host MOF units with the solvent molecules.13 Due to the large number of metal ions, oxidation states, and of organic ligands capable of forming MOFs, these have also been widely investigated for gas storage, separations, catalysis, photochemistry, and sensing devices.12 Despite their versatility and rich chemistry, hierarchical porous MOFs have been less investigated.

Scheme 1. Schematic Illustration of “Perturbation-Assisted Nanofusion” Mechanism for the Formation of Hierarchical MOFsa

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SYNTHESIS OF HIERARCHICAL METAL−ORGANIC FRAMEWORK HYBRIDS To date, MOFs displaying mesoporous cavities have been made by the ligand extension method. By extending the ligands, the final cavities could reach widths of nearly 4 nm.14 Large porous crystals were isolated, but drawbacks include restrictive synthesis conditions and limited diameter of mesopores, and crystal phase collapse upon removal of the guest solvent molecules. Alternatively, more methods have been reported for the preparation of mesoporous particles having microporous MOF. The latter include surfactant templating,15 microwaveassisted synthesis,16 solvent evaporation diffusion method,17 and ionic liquid/supercritical CO2 emulsion route,18 or CO2expanded liquids as switchable solvents.19 In particular, MOFs obtained from induced or cooperative supramolecular assembly with ionic surfactant templates required chelating agents or functionalized precursor/template additives. These functional groups bind the metal ions to the templates, thus avoiding phase separation of MOFs during assembly. These chelating agents are incorporated into the final MOF structure, and their removal negatively impacts the structural integrity. When combining the ionic surfactants with ionic liquids, chelating agents are not required.20 Nonetheless only platelet particles with small pores were obtained. Hence the surfactant-ionic liquid mixtures produced lamellar mesophases that coordinated the growth of platelet MOF particles. Another strategy was developed for MOF nanocomposites with silica and alumina, using anionic triblock copolymer surfactants.21 Despite the desirable large mesopores and good thermal stability of nanocomposites, the same strategy could not be extended to pure hierarchical MOFs. Besides compatibility issues with MOF, a drawback of using surfactant templates and other additives is the added step to remove these compounds. Because the decomposition temperature of most surfactants exceeds the thermal stability range of MOFs,

a

Reprinted with permission from ref 22. Copyright 2013 American Chemical Society.

The second question is how to balance the speed of crystallization and condensation. Crystallization of the nanosized MOF particles requires, in conjunction with other supramolecular interactions such as hydrogen bonding and π−π stacking, a dissociation−recoordination force for forming the “interunit” bridges on which the condensation depends. Accordingly, it is important to choose a proper solvent to control the rate of crystallization and crystal growth. When the rate of particle condensation is faster than that of the individual MOF particle crystallization, the products obtained are nanoparticles embedded in an amorphous substrate. Even faster rates of particle condensation over precipitation of crystallites can induce the formation of an essentially amorphous material. On the contrary, when the condensation of polycrystallites and amorphous phases is significantly slower than the speed of crystallization, bulk microporous crystals are obtained.23 Finally, the precipitated MOF materials have 3045


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Accounts of Chemical Research textural mesopores due to the small size of the aggregated crystallites. The latter partially redissolves when left stirring in the mother liquor for extended periods of time. This induces a thinning of the crystal pore walls, resulting in the enlargement of the textural mesopores. Due to the metastable nature of the mesopores, when exposed to prolonged solvothermal conditions, the most thermodynamically stable microporous crystalline phase forms. Consequently, the textural mesopores collapse due to the excessive growth of the microporous crystals.22 Following this perturbation-assisted nanofusion (PNF) method, hierarchically structured porous Zn-MOF-74 [Zn3(DHBDC)2·(guest)n, DHBDC = 2,5-dihydroxy-1,4-benzenedicarboxylic acid] were prepared in dimethylformamide (DMF) solvent.22 Several gram batches of these hierarchical materials are obtained within 1 h. The transmission electron microscopy (TEM) and high-resolution TEM (HRTEM, Figure 1) reveal the continuous networks of MOF with

Figure 2. N2 at −196 °C isotherms (a) and corresponding PSDs calculated by density functional theory method assuming slitlike pore geometry (b) of the microporous Zn-MOF-74 and Zn-MOF-74/t samples. For clarity, the PSDs were vertically offset in increments of 0.2 cm3/g nm. The units in (b) have been corrected from its originally published version. Reprinted with permission from ref 22. Copyright 2013 American Chemical Society.

microscopy (SEM, Figure 3) confirmed the presence of large textural mesopores. Pore volume analysis from the adsorption

Figure 3. SEM images for bimodal Zn-MOF-74 series synthesized with different reaction times (a) 0.25 h, (b) 1 h, (c) 24 h, and (d) 240 h. Reprinted with permission from ref 22. Copyright 2013 American Chemical Society. Figure 1. SEM (a), TEM (b), and HRTEM (c,d) images of one representative hierarchical Zn-MOF-74 material. For the HRTEM images (c,d), besides the textural pores, lattice fringes are highlighted in yellow and are seen for several particles. Amorphous and less periodic particles are also present. Reprinted with permission from ref 22. Copyright 2013 American Chemical Society.

isotherms reveals the large fractions of micropores and of mesopores. It is also clear from these isotherms that for longer reaction times, the mesopore volumes increase at the expense of diminishing microporosity. In detail, the standard Zn-MOF74 shows a type I isotherm with high amount of gas uptake at low relative pressure, characteristic of microporous materials. In the hierarchical MOFs, the isotherms are type IV, most with hysteresis loops characteristic of large constricted mesopores.1b The shifts in the capillary condensation steps toward higher relative pressures result from the enlargement of the textural mesopores with increasing syntheses times. Concomitantly, the diminishing amount of gas uptake at low relative pressures offers clear evidence of the decreasing amounts of microporous particles, or lack of accessibility to part of the micropores in the frameworks. These reflected on the calculated PSDs, with distribution maxima shifting to pores exceeding 15 nm, with a decrease in the pore fraction below 2 nm. As previously

disordered textural mesopores.22 Lattice fringes matching those of crystalline MOF are observed for some of the aggregated particulates, whereas particles having disordered structure are also present. The spacing of such fringes varied from 0.7 to 2.0 nm. The powder X-ray diffraction (XRD) patterns (not shown) had broader diffraction peaks than a standard MOF-74 sample. This broadening was attributed to the small size of the MOF-74 crystallites and to the presence of amorphous material in the pore walls. Both calculated pore size distributions (PSDs) from nitrogen adsorption isotherms (Figure 2) and scanning electron 3046


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Accounts of Chemical Research detailed, the nanofusion mechanism includes the redissolution and reagglomeration of the nanocrystalline polymorphic MOF particles with the binding amorphous material, in agreement with the HRTEM images. Hence, the hierarchical structure of pores is the result of this concerted redissolution−precipitation for the rearrangement of the nanocrystallites into mesoporous frameworks. The polarity of the solvent proved crucial for controlling both crystallites size and mesopore widths during PNF synthesis. For instance, polar aprotic solvents control crystal growth and nanofusion via coordination forces between solvent molecules and metal salts. For example, hierarchical Zn-MOF74 samples prepared with dimethylacetamide (DMA), and Nmethylpyrollidone (NMP), had smaller mesopores than the same MOF prepared in DMF.22 Furthermore, various other MOFs were prepared following the PNF synthesis,23b including IRMOF-3 [Zn4O(NH2BDC)3· (guest)n] (NH2BDC = 2-amino-1,4-benzenedicarboxylate),24 Cu-BDC [Cu(BDC)·(guest)n] (BDC = 1,4-benzenedicarboxylate),25 and Cu-BTEC [Cu2(BTEC)·(guest)n] (BTEC = 1,2,4,5-benzenetetracarboxylate)26 (Figure 4). In all cases, the pore walls of these hierarchical MOFs also partially retain the crystal structure of their corresponding single crystalline materials. In some of these hierarchical MOF phases, the coordination ability of the ligands and the existence of functional groups are also important for forming mesopores. For example, the hydroxyl groups in DHBDC ligands form strong H-bonds, thus facilitating the formation of SBUs, their interaction, and consequently accelerating the crystal growth speed. When

attempting to use 1,4-benzenedicarboxylic acid (H2BDC) to prepare hierarchical MOF-5, [Zn4O(BDC)3·guests],28 under same reaction conditions, only large single crystals were obtained. Possibly because of the additional strong H-bonding interactions in the ligand, the crystal growth is thermodynamically favored over the precipitation of polycrystalline nanoparticles and amorphous coordination polymers. Hence, the speed of crystallization surpasses the speed of condensation and redissolution. The anions of the selected salt precursors also balance the speeds of crystallite formation and crystal growth through the coordination, nucleation, and thereafter nanoparticle aggregation. During preparation of hierarchical Zn-MOF-74, preformed units of a derivative of the metal-acetate bidentate bridging [M(II) (CH3COO)4] were dissolved in DMF. Upon the addition of H2DHBDC, nanosized MOF-74 crystals precipitated through a ligand exchange process, that is, the deprotonated DHBDC replaced the coordinated acetate ions. This ligand exchange process is not expected to take place with ZnCl2, ZnSO4, and Zn(NO3)2. The basic acetate ions are required for the fast proton abstraction off the organic linkers. Consequently, the acetate anion accelerates the precipitation of MOF nanosized crystals.9a Hence, the construction of hierarchical MOF materials relies on the synergistic effects of the ligands, metal salts, solvents and the strong mixing.

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DYE UPTAKE BY HIERARCHICAL METAL−ORGANIC FRAMEWORK HYBRIDS Important applications of porous materials include water purification27 and sensing.28 To date, the separation and sensing properties of MOFs have been greatly impaired by the low organic pollutants and dyes uptake in the small MOF micropores. In contrast to microporous MOFs, hierarchical MOFs adsorb large dye molecules, and have high adsorption capacities due to the large mesopores. The mesopores further facilitate the diffusion of dye molecules, and their interaction with the framework walls. For instance, hierarchical Zn-MOF-74/18 was soaked in a concentrated methanol solution of Brilliant Blue R250 (BBR-250) having a molecular dimension of ∼1.8 nm × 2.2 nm,22 which is larger than the micropores of Zn-MOF-74. After separating the saturated solid adsorbents, the UV analysis showed the hierarchical Zn-MOF-74/18 adsorbed 17.9 wt %, or 543 mg/g, in contrast to only 2.7% amount adsorbed by the microporous Zn-MOF-74. The remarkable difference of the dye uptake ability between the hierarchical and microporous MOF materials indicated the additional mesopores provide access to large molecules in Zn-MOF-74/18, whereas the same dye adsorbed only on the small external surfaces of bulk ZnMOF-74. Different MOF phases and organic dyes of different sizes are presented in Table 1.9,10d,11b,12c Compared to those, the hierarchical Zn-MOF-74 exhibits significantly higher dye uptake. In order to use MOFs as sensors, MOF-dye composites are prepared by the innate entrapment (Scheme 2).23b In this method, a selected dye compound is mixed to the synthesis precursors of the MOF framework. Since molecules are trapped within the bulk of MOF walls, a dye cannot be leached out or exchanged. For enhanced sensing, higher quantities of trapped dye molecules are desired. The amount of dye, however, is restricted to the quantities that do not interfere with the crystallization of the MOF. Also, the volume of the immobilized dye is restricted by the narrow cavities present between

Figure 4. SEM (a, c, and e) and TEM (b, d, and f) images for IRMOF3, Cu-BDC, and Cu-BTEC, respectively, with crystal structure of related MOF as insets (guests molecules omitted for clarity). Reproduced from ref 23b with permission from The Royal Society of Chemistry. 3047


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Accounts of Chemical Research Table 1. Representative Adsorption Results for Various Microporous MOF Sorbents and Dyes MOFs

dye adsorption

uptake (mg/g)

ref

MIL-101(Cr)a MIL-100(Fe)c MOF-177e MOF-235g MIL-100(Fe)

xylenol orangeb orange IId astrazon orange Rf methylene blueh malachite greeni

97.17 43.98 400 187 205j

9a 10d 9b 11b 12c

a MIL-101(Cr) = chromium-benzenedicarboxylate. bInitial concentration of xylenol orange is 2000 ppm. cFe(BTC), BTC = 1,3,5benzenetricarboxylate. dInitial concentration of Orange II is 10 mg/g. e MOF-177= Zn4O(1,3,5-benzenetribenzoate)2. fSaturated solution of dye in CH2Cl2. gMOF-235 = [Fe3O(terephthalate)3(guest)3][FeCl4]. h Initial concentration of dye is 1000 ppm. iinitial concentration of dye is 100 mg L−1). jAt 25 °C.

Figure 5. Emission spectra of RB solution in ethanol and RB@ IRMOF-3 excited at 355 nm. Reproduced from ref 23b with permission from The Royal Society of Chemistry.

Scheme 2. Illustration of the PNF Synthesis of Hierarchical MOF with Entrapped Large Dye Moleculesa

a

I, initial state with ligands and metal ions dispersed in the solution; II, single crystals with ordered structure obtained without any perturbation; III, hierarchical superstructure fabricated under strong stirring. Reproduced from ref 23b with permission from The Royal Society of Chemistry.

Figure 6. Fluorescence spectra of RB@IRMOF-3 (a) and the emission peak-height ratios between ligand and dye moieties in RB@IRMOF-3 (b). Reproduced from ref 23b with permission from The Royal Society of Chemistry.

particles forming bulk microporous crystals. Excess quantities of dyes adsorbed on the external crystal surfaces are washed away in the synthesis. To overcome these issues, the PNF synthesis was successfully combined with the innate entrapment. The smaller size of the particles in the aggregates entrapped larger quantities of dyes. The large mesopores in the final composites also favored the immobilization of large molecules and their accessibility to the active dye-MOF surfaces. This was demonstrated for the hierarchical Rhodamine B (RB) and IRMOF-3 composite. Rhodamine B, a brightly colored synthetic pigment, is often used as a water tracer fluorescent dye to determine the rate and direction of flow, due to its strong absorption and very high fluorescence quantum yield.29 The florescence measurements showed the RB@IRMOF-3 composite displayed the characteristic emissions of both RB dye and IRMOF-3 after excitation at 355 nm in the solid state (Figure 5).23b The red-shift in the emission of RB in IRMOF-3 results from the increased environment polarity in IRMOF-3, indicating the RB dye molecules were encapsulated into IRMOF-3. When the RB@IRMOF-3 composite was further exposed to volatile organic compounds (VOCs) (Figure 6a),

the luminescence peak of the RB@IRMOF-3 treated with orthodichlorobenzene at 450 and 606 nm exhibited a red-shift with an intensity ratio between ligand and dye moieties of 6.35. For samples exposed to all other VOCs including chlorobenzene, benzene, toluene, p-xylene, only the luminescence peak at 606 nm exhibited a red-shift, with intensity ratios between ligand and dye moieties of 4.14, 4.45, 4.54, and 4.63 for chlorobenzene, toluene, p-xylene, and benzene, respectively (Figure 6b).23b After exposure to methanol, the RB@IRMOF-3 composite exhibited red-shifts in the luminescence peaks at 450 nm; however, an RB@IRMOF-3 sample exposed to ethanediol exhibited a blueshift for the peak at 450 nm. Exposing the RB@ IRMOF-3 to methanol and ethanediol, the emission intensity of ligand increased while the emission intensity of rhodamine moieties decreased, resulting in significantly decreased intensity ratios between ligand and dye moieties of 0.67 and 0.53, respectively. The differences of the fluorescent property of the RB@IRMOF-3 composite with different solvent molecules is possibly due to the energy transfer efficiency between the host matrix and the guest molecules being affected by the guest molecule within the MOF channels. These results suggest that 3048


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Accounts of Chemical Research RB@IRMOF-3 composites prepared by the PNF synthesis method can be used as a novel ratiometric luminescent sensor for detection of VOCs.

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PERTURBATION-ASSISTED NANOFUSION SYNTHESIS OF HIERARCHICAL PRUSSIAN BLUE ANALOGUES Besides the various hierarchical MOFs, the PNF synthesis was successfully extended to Prussian Blue (PB) and PB analogues (PBAs). PB is a mixed-valent iron cyanide complex with a repeating unit of sodium or potassium ferrous ferricyanide (A[FeIIIFeII(CN)6], A+ = Na+ or K+).30 It has a cubic facecentered structure (Fm3̅m). The FeII and FeIII ions sit on alternate corners of corner-shared octahedra bridged by small conjugated cyanide (CN) anions, forming a large ionic channel along the ⟨100⟩ direction.31 PB has a structural arrangement that allows for partial substitution of Fe by other transition metal ions in different oxidation states, such as CoII, CoIII, NiII, and CuII, forming PBAs. Despite the flexibility to cation substitution in the crystal lattice, PB and PBAs easily grow large crystals. Control over particle size and the ability to form mesoporous frameworks were previously limited to PBsupported carbonaceous or siliceous nanocomposites.32 When following the PNF synthesis (Scheme 3), mesoporous PB analogues were obtained (mesoMHCF, where HCF = Figure 7. N2−196 °C isotherms of the mesoMHCF series (a) and corresponding BJH PSD curves (b). Reproduced with permission from ref 32. Copyright 2014 Wiley-VCH.

Scheme 3. Schematic Illustration of the Formation of Hierarchical Prussian Blue Analogues through PerturbationAssisted Nanofusion Methoda

(mesoCuHCF) to 16 nm (mesoCoHCF) and 30 nm (mesoNiHCF). The specific surface areas of these materials were between 100 and 300 m2/g, which is much higher than that for bulk PB materials obtained from solvothermal method. While the adsorption and structural properties of PBAs are tailored by this simple metal substitution during PNF synthesis, the exact mechanism to explain changes remains unclear. Despite the inaccessibility of PB micropores to gaseous species, the alkaline cations located in the micro channels are reversibly exchanged. This occurs without symmetry changes to the original crystal lattice, and it is driven by lattice volume changes during reduction/oxidation of the FeIII/FeII pair.34 This property has made PBAs attractive for the development of inexpensive cathodes for Li-ion and Na-ion batteries as well as for electrochemical pseudocapacitors.33a When tested for rechargeable battery cathodes, the mesoporous PB analogues displayed lower energy storage capacity compared to a bulk crystalline phase. The latter is a consequence of large nonporous crystals having a higher volume of microchannels per unit mass. This leads to higher amounts of intercalated ions compared to mesoporous materials with thinner pore walls. The charge/discharge cycles in batteries are slow, thus facilitating the ionic exchange and diffusion through large crystals. Nonetheless, for the pseudocapacitive materials, the electric double layer, charge transfer-ion diffusion reactions occur at much faster rates than those in Li-ion or Na-ion batteries.35 In fact, the reactions occurring in electrochemical capacitors or pseudocapacitors are directly proportional to the accessible surface areas of the electrode materials.36 Consequently, the gravimetric capacitance and the energy and power densities of pseudocapacitors can be improved by designing high surface area electrodes.

a

Reproduced with permission from ref 32. Copyright 2014 WileyVCH.

hexacyanoferrate, M = NiII, CoII, and CuII).33 As with the MOF materials, the key to the fabrication of mesoMHCF lies in the formation of small nanocrystals, and the subsequent assembly of these building units into interconnected porous frameworks. In first stage, the reaction between the transition metal ions and Fe(CN)64− enables nanocrystals to form, aggregate, and fuse together. Textural mesopores form between the particles. In a second stage, and possibly concurrently, the aggregates grow into larger particles, thus enlarging the textural mesopores. Finally, for prolonged reaction times, large single-grained particles grow, with consequent collapse of the mesopores. The mesoporosity of these materials were confirmed by N2 adsorption isotherms and corresponding PSDs (Figure 7).33b All three samples showed type IV isotherms, with hysteresis loops typical of textural mesopores. The shifts in the capillary condensation steps toward higher relative pressures from the Cu, Co, and Ni demonstrated that mesopores are enlarged by the cation substitution. The calculated PSDs, and the broadening of these distributions agree well with the isotherms. The maxima in the PSDs fall within ranges from 10 nm 3049


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preparation of textural mesopores with nanosized crystalline domains, without surfactants or other organic additives, and in very short times. This single stage method allows for the tailoring of the framework crystallinity and adsorption parameters, namely, specific surface areas, pore volumes, and pore size distributions. Future challenges include the synthesis of materials having uniform mesopores without surfactant templates. In this direction, the combination of the PNF with the ligandextension synthesis14 has the potential to lead to bimodal distribution of mesopores: one corresponding to small and uniform mesoporous cages within crystalline MOF particles, and another corresponding to large textural mesopores. Compared to microporous−mesporous materials, a bimodal distribution of mesopores may significantly extend the applicability of hierarchical MOFs for the adsorption of larger and more complex molecules, of interest for water purification, and for sensors design. The preparation of more three-dimensional complex MOFs, zeolitic imidazolate frameworks (ZIFs), ion-intercalated frameworks, metal chalcogenides, and electronic conductive carbon composites is also largely desirable. The latter combined with surface modification of these nanomaterials with biocompatible groups, biodegradable composites, and strong magnetic properties could lead to novel materials for drug delivery, and MRI contrasting agents.39 Hence, this novel synthesis opens the way to the preparation of a wide range of hierarchical porous materials with novel properties introduced by the combination of micropores, mesopores, and macropores for applications in separations, sensing, medicine, catalysis, and energy storage.

Indeed, for the bulk PBA, no redox electrochemical peaks were detected within the 0.1−1.2 V potential windows studied (Figure 8a). Whereas mesoporous PBAs exhibited strong redox

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AUTHOR INFORMATION

Corresponding Authors

*E-mail: [email protected]. *E-mail: [email protected].

Figure 8. CVs of mesoMHCF series and macroNiHCF in 1.0 M Na2SO4 with a scan rate of 5 mVs−1 (a) and specific capacitance derived from CVs at different scan rates (b). Copyright 2015 WileyVCH. Reproduced with permission from ref 33b.

Notes

The authors declare no competing financial interest. Biographies

peaks in the cyclic voltammetry experiments. Their high capacitance approached 300 F/g in case of mesoCoHCF material.33b The rate performances of all materials were significantly poor due to the low electronic conductivity of the PBAs (Figure 8b). Other factors affecting the rate performances include the relatively high activation energy barrier for the redox process. Preliminary results, however, are very promising. Especially that the PNF method has the potential to be used in large-scale preparation of unique mesoporous PBAs. Improvements to the electrochemical properties could be achieved by testing different metal substitutions, and ratios in the PB lattice. This could lower the activation energy required for the FeIII/FeII redox pair and ionic diffusion in the microchannels.37 Also, the preparation of PB-carbon composites could significantly improve the electronic conductivities, and consequently the rate performances of the mesoporous PBAs.38

Yanfeng Yue received his Ph.D. in inorganic chemistry from Peking University (China) in 2008. After his postdoctoral training at University of Liverpool, University of Texas at San Antonio, and Oak Ridge National Laboratory (ORNL), he serves as a visiting assistant professor in Sul Ross State University. His research focuses on functional nanoporous materials. Pasquale F. Fulvio received his B.Sc. in Chemistry in 2003 from Univ. Federal do Espirito Santo (Brazil), and his Ph.D. in Chemistry from Kent State University in 2009. He was a postdoctoral researcher at ORNL until 2013. He joined the Chemistry Dept. of Univ. of Puerto Rico, Rio Piedras Campus in 2014 as an assistant professor. Sheng Dai obtained his B.S. degree (1984) and M.S. degree (1986) in Chemistry at Zhejiang University (China) and his Ph.D. (1990) in Chemistry at University of Tennessee, Knoxville. He is currently a Corporate Fellow and Group Leader in Chemical Sciences Division at ORNL and professor of chemistry at the University of Tennessee.

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CONCLUSION AND OUTLOOK A new and simple PNF synthesis for hierarchical microporousmesoporous materials was discussed with recent examples. The novel frameworks include different MOF phases, as well as PB and its analogues. The PNF method features the high-yield

ACKNOWLEDGMENTS This research was sponsored by the Division of Chemical Sciences, Geosciences, and Biosciences, Office of Basic Energy Sciences, US Department of Energy, under Contract DE-AC053050


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00OR22725 with Oak Ridge National Laboratory, which is managed and operated by UT-Battelle, LLC.

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