POROUS MATERIALS Ordered macro-microporous metal- organic framework … · POROUS MATERIALS Ordered macro-microporous metal-organic framework single crystals Kui Shen, 1Lei Zhang,

POROUS MATERIALS

Ordered macro-microporous metal-organic framework single crystalsKui Shen,1 Lei Zhang,1 Xiaodong Chen,1 Lingmei Liu,2 Daliang Zhang,3 Yu Han,2

Junying Chen,1 Jilan Long,1 Rafael Luque,4 Yingwei Li,1* Banglin Chen5*

We constructed highly oriented and ordered macropores within metal-organic framework(MOF) single crystals, opening up the area of three-dimensional–ordered macro-microporous materials (that is, materials containing both macro- and micropores) insingle-crystalline form. Our methodology relies on the strong shaping effects of apolystyrene nanosphere monolith template and a double-solvent–induced heterogeneousnucleation approach.This process synergistically enabled the in situ growth of MOFs withinordered voids, rendering a single crystal with oriented and ordered macro-microporousstructure. The improved mass diffusion properties of such hierarchical frameworks,together with their robust single-crystalline nature, endow them with superior catalyticactivity and recyclability for bulky-molecule reactions, as compared with conventional,polycrystalline hollow, and disordered macroporous ZIF-8.

Porous materials have attracted considera-ble scientific attention because of theirwide industrial applications ranging fromgas separation to catalysis (1). Topologicalfeatures—particularly pore sizes at each di-

mension and the uniformity of these pores—inporous materials are key factors that determinetheir preferred function in a particular appli-cation (2, 3). The construction of hierarchicalporous structures that maintain their overallcrystalline order is desirable because the highstructural regularity may, in turn, afford mark-edly improved properties. A few advantageousexamples include substantially improved con-ductivities and electron mobilities for mesoporousTiO2 single crystals (4) as compared with nano-crystalline TiO2 and much stronger frameworkacidities and stabilities for mesoporous crystal-line zeolites with respect to amorphous molec-ular sieves (5, 6).The synthesis of highly ordered meso- or

macroporous crystalline materials at a widerange of length scales with different structuralcoherencies remains an important goal (7, 8),particularly with respect to enlarging the poresof metal-organic frameworks (MOFs). Most re-ported MOFs exhibit porosities restricted to themicroporous regime, which limits their appli-cations to diffusion-limited processes (9, 10).Synthetic strategies to generate controllableporosities within MOFs mainly include tem-

plating, etching, and template-free and defect-inducedmethods (11–14).However, theseapproachescannot generate orderedmacro-microporousMOFsingle crystals. Alternatively, extendable ligandsand three-dimensional (3D)–ordered templateshave been used to design periodic meso- andmacropores (15–19). The mesopores built fromlong ligands often suffer from rapid collapseafter guest removal, whereas template-duplicatedmacropores are generally derived from the as-sembly of intergrown MOF polycrystals. Untilnow, MOFs featuring both macropore ordering(pores with diameter > 50 nm) and single crys-tallinity have not been achieved. On the basis ofthese premises, the development of reliable meth-ods to design ordered meso- or macroporousMOF single crystals with robust framework struc-tures is imperative.We report a single-crystalline MOF with 3D

ordering of macro-micropores, and we used theZIF-8 structure as a proof of concept. This struc-ture’s 3D framework is built up from connectingZnII ions through 2-methylimidazole linkers, show-ing a periodicallymicroporous topology featuringlarge cavities (11.6 Å) and small apertures (3.4 Å)(20, 21). First, monodisperse polystyrene spheres(PSs) were assembled into highly ordered 3D PSmonoliths (Fig. 1A and figs. S1 and S2). ZIF-8precursors—that is, 2-methylimidazole and Zn(NO3)2—were then filled into the PS monolithinterstices to form “precursor@PS” monoliths,which were subsequently soaked in CH3OH/NH3·H2Omixed solutions. We chose the mixedsolutions as the solvent, with the idea thatNH3·H2O can induce rapid crystallization of theprecursors, whereas CH3OH can effectively sta-bilize such precursors and adjust the balancebetween nucleation versus growth of ZIF-8. As aresult, the precursors smoothly turned into ori-ented and3D-orderedZIF-8 single crystals (fig. S3)(22). Obviously, the proposed double-solvent–assisted method could overcome the energeticbarrier to homogeneous nucleation, which is

usually unavoidable in conventional nanocast-ing processes (23, 24). This new type of macro–microporousMOFwas denoted as SOM-ZIF-8(x, y)(SOMstands for single-crystal orderedmacropore),in which x and y represented the feedingmolarratios of 2-methylimidazole/Zn(NO3)2 and thevolume fraction of NH3·H2O in the solvent usedfor synthesis, respectively.Low-resolution scanning electron microscopy

(SEM) images (Fig. 1B and fig. S4) revealed thatSOM-ZIF-8(3, 0.5) displayed a tetrakaidecahedronmorphology, although some crystals looked dif-ferent as various directions were viewed. Theoriented arrangement of 3D-orderedmacroporescould be identified in the representative SEMimages of individual crystals taken from fourdifferent directions (Fig. 1C). On inspection, weidentified that the ordered arrangement of mac-ropores on the surface of six square planes andeight triangle planes inherited {100} and {111}planes of the colloidal PS template, respectively.The crystal growth of ZIF-8 is known to start

with the formation of cubes composed of 6 (100)facets, which can subsequently grow into trun-cated rhombic dodecahedra composed of 12 (110)and 6 (100) facets (25, 26). In our study, ZIF-8growing on the external surface of a PS templatealso displayed a truncated rhombic dodecahe-dra morphology with 6 (100) and 12 (110) well-developed facets (Fig. 1D and fig. S5). Given thatthe difference between the two crystal morphol-ogies was the existence of the PS template, weassumed that the PS template was the control-ling agent that allowed for the preferential for-mation of tetrakaidecahedron SOM-ZIF-8 withoriented macropore arrangement.To validate this assumption, we conducted a

controlled experiment in which the precursor@PSmonolith was first broken down to very smallparticles before soaking with CH3OH/NH3·H2Ofor further crystallization (under otherwise iden-tical synthetic conditions). As expected, almostall of the obtained crystals also exhibited an ob-vious truncated rhombic dodecahedra morphol-ogy (fig. S6). A closer inspection revealed that the(100) facets of many poorly developed crystalsalso inherited {100} planes of the colloidal PStemplate. A selected crystal (Fig. 1E) provideddirect insights into the effects of ordered PSs astemplate on the morphological features of SOM-ZIF-8(3, 0.5). In one part, the 3D PS templateguided the crystal growth into a macroporoustetrakaidecahedron with its (100) and (111) facetsmatching well with the {100} and {111} planes ofmacropore arrangements, respectively. However,the crystal had grown into conventionalmacropore-free truncated rhombic dodecahedra in the re-maining part, owing to the absence of PSs. Thisoriented growthmodel was further confirmed bya broken crystal composed of an internal oriented3D-ordered macroporous system and an externalcompact faceted surface, consistent with conven-tionalZIF-8morphology (Fig. 1F). Thus, the shapingand templating effects of the 3D-ordered PS tem-plate could directly guide themorphological evo-lution of SOM-ZIF-8 into a tetrakaidecahedronmorphology and match its crystal face to the

RESEARCH

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1School of Chemistry and Chemical Engineering, South ChinaUniversity of Technology, Guangzhou 510640, China.2Advanced Membranes and Porous Materials Center,Physical Sciences and Engineering Division, King AbdullahUniversity of Science and Technology (KAUST), Thuwal23955-6900, Saudi Arabia. 3Imaging and CharacterizationCore Lab, KAUST, Thuwal 23955-6900, Saudi Arabia.4Departamento de Química Orgánica, Universidad deCórdoba, Edificio Marie Curie, Carretera Nacional IV-A, Km396, E14014, Córdoba, Spain. 5Department of Chemistry,University of Texas at San Antonio, One UTSA Circle, SanAntonio, TX 78249, USA.*Corresponding author. Email: [email protected] (Y.L.);[email protected] (B.C.)

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oriented 3D-ordered macroporous arrangement(Fig. 1G).X-ray diffraction (XRD) patterns of SOM-ZIF-

8(3, 0.5) are compared with conventional (denotedC-ZIF-8, fig. S7) and simulated ZIF-8 in Fig. 2A.SOM-ZIF-8(3, 0.5) exhibited only XRD reflectionsascribed to ZIF-8 (20, 21), confirming the forma-tion of phase-pure ZIF-8 with good crystallinity.Nitrogen-adsorption experiments were furtherperformed to investigate the porosity in themate-rial (Fig. 2B). Both SOM-ZIF-8(3, 0.5) and C-ZIF-8showed similar type I isotherms with a high ni-trogen adsorption capacity at very low relativepressures, indicative of their microporous struc-tures (20, 21), as also confirmed by the correspond-ing micropore size distribution curves (Fig. 2B,inset). The Brunauer-Emmett-Teller surface areaandmicropore volume of SOM-ZIF-8(3, 0.5) were1540 m2/g and 0.59 cm3/g, respectively—valueshigher than those of C-ZIF-8 (1397 m2/g and

0.55 cm3/g). The formation of a 3D-ordered mac-roporous system did not influence its inherentmicroporous structure. The presence of macro-pores in SOM-ZIF-8(3, 0.5) was fully confirmedby means of mercury intrusion porosimetry. Asshown in fig. S8, SOM-ZIF-8(3, 0.5) displays aregular macroporous distribution with an aver-age size of ~80 nm, which could be assignedto the throat size of “ink-bottle” pores betweenneighboring macropores.SOM-ZIF-8(3, 0.5) was subsequently charac-

terized in detail by transmission electronmicros-copy (TEM) to further confirm its macroporousstructure and composition. A representative high-angle annular dark-field–scanning transmissionelectron microscopy (HAADF-STEM) image re-vealed that closely packed ordered PSs wereimprinted into entire SOM-ZIF-8(3, 0.5) crystals(Fig. 2C). The average diameter of the intercon-nected macropores was ~270 nm, similar to that

of PS latex, because the ordered macroporousstructure was a negative replica of the PS tem-plate. The highly ordered macropore arrange-ment in SOM-ZIF-8(3, 0.5) was clearly evidencedfrom low-magnification TEM images of differentdirections (fig. S9). Selected-area electron diffrac-tion (SAED) patterns taken from an entire crystal(Fig. 2D, inset) revealed the single-crystalline na-ture of SOM-ZIF-8(3, 0.5). High-resolution TEM(HRTEM) images at different magnificationsalong the <011> zone axis showed uniform latticefringes with consistent orientations over theentire image region, without domain boundariesor interfaces observed (Fig. 2, D to F). Fouriertransform (FT) patterns derived from HRTEMfurther confirmed the single crystalline nature ofthis material (Fig. 2E, inset). The elemental map-ping images (fig. S10) pointed to a homogeneousdistribution of elements (including C, N, and Zn)in whole SOM-ZIF-8(3, 0.5) crystals.

Shen et al., Science 359, 206–210 (2018) 12 January 2018 2 of 5

Fig. 1. In situ nanocasting synthesis of SOM-ZIF-8 and its structure con-firmation. (A) Schematic diagram of our strategy to design SOM-ZIF-8.THF,tetrahydrofuran. (B) Representative SEM image of SOM-ZIF-8(3,0.5). (C) SEMimages of individual crystals taken from four different directions. (D) SEMimage of the ZIF-8 grown on the external surface of the PS template.

(E and F) SEM images of two selected semifinished SOM-ZIF-8(3, 0.5)crystals, confirming their oriented growth within ordered voids. Yellow areas,{100} planes ofmacropore arrangements; red areas, {111} planes ofmacroporearrangements. (G) Illustration of the shaping and templating effects of the3D-ordered PS template on the morphologic evolution of SOM-ZIF-8.

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The prerequisite to achieve uniform SOM-ZIF-8 materials was the use of methanol/ammoniawater mixtures as suitable solvents. Methanol islargely employed in the preparation of uniformZIF-8 materials but generally does not providehigh synthetic yields (fig. S11). Pure ammoniawater could effectively increase the yield of ZIF-8, but with too high of a crystallization rate toprepare uniform ZIF-8 crystals (fig. S11). Wesuccessfully realized the controlled crystalliza-tion of SOM-ZIF-8(3, 0.5)within template-confinedspaces by combining the advantages of the twotypes of solvents, as stated above. In the absenceof NH3·H2O, all ZIF-8 crystals nucleated andgrew outside of the PS template, producing tinysolid ZIF-8 nanocrystals (Fig. 3A and figs. S12 toS14). Comparably, a homogeneous crystalliza-tion of the precursors occurred when using pureNH3·H2O as solvent, affording a monolithic SOM-ZIF-8 (figs. S12, S13, S15, and S16). Uniform SOM-ZIF-8 with single-crystalline nature could beefficiently obtained within a wide volume frac-tion of NH3·H2O in the solvent (Fig. 3, B and C,and figs. S17 and S18). The particle size grew from~1.3 to ~4.3 mm when the volume fraction ofNH3·H2O was increased from 0.33 to 0.67 (fig.

S19), which demonstrates that crystal sizes canbe flexibly tuned by simply changing the solvent’sNH3·H2O content. Additionally, variations in thefeedingmolar ratio of 2-methylimidazole/Zn(NO3)2from 2 to 4 indicated that uniform SOM-ZIF-8with similar sizes could be prepared in a ratherwide composition range (figs. S20 and S21). Lowertemperatures appeared to favor the tetrakaideca-hedronmorphology; syntheses at a slightly highertemperature of 55°C produced near-sphere SOM-ZIF-8(3, 0.5) with larger particles (3.7 mm) (fig.S22). More importantly, a series of SOM-ZIF-8withmacropore sizes ranging from~190 to~470nmcould be prepared by carefully controlling the di-ameter of PS templates, indicating the generalapplicability of the proposed strategy (Fig. 3, D toK, figs. S23 to S27, and table S1).The catalytic properties of SOM-ZIF-8 were

investigated and compared to those of bulk C-ZIF-8, polycrystal hollow ZIF-8 (denoted as PH-ZIF-8, figs. S28 and S29), andmacroporous ZIF-8synthesized by disordered PSs as the template (de-noted as M-ZIF-8, fig. S30) in the Knoevenagelreaction between benzaldehydes and malono-nitriles as a model (27). As expected, SOM-ZIF-8exhibited substantially improved activities with

respect to C-ZIF-8, as the conversion of benzal-dehyde could be completed after 2 hours for theoptimal SOM-ZIF-8(3, 0.33) versus 8 hours for C-ZIF-8 (Fig. 4A). SOM-ZIF-8(3, 0.33) also out-performed various state-of-the-art heterogeneouscatalysts, including amino-functionalized mo-lecular sieves and UiO-66-NH2, under identicalreaction conditions (figs. S31 to S37). Despitefeaturing similar macropore sizes, SOM-ZIF-8exhibited much improved catalytic activities ascompared with PH-ZIF-8 and M-ZIF-8. The in-terconnected macropores in SOM-ZIF-8 can beentered from the external crystalline surface,facilitating the accessibility to reactive sites. Incontrast, macropores in PH-ZIF-8 and M-ZIF-8were completely or partly occluded in the mi-croporous matrix and thus were much less ac-cessible to bulky reactants. To further supportthese hypotheses, the diffusion process of greenfluorescent protein (GFP) from the outside to theinterior of individual SOM-ZIF-8, M-ZIF-8, andC-ZIF-8 crystals was subsequently monitoredin situ (fig. S38). Compared with M-ZIF-8 andC-ZIF-8, SOM-ZIF-8 clearly allowed a much fasterGFP diffusion throughout the entire crystal,confirming the enhanced diffusion efficiency of

Shen et al., Science 359, 206–210 (2018) 12 January 2018 3 of 5

Fig. 2. Structure characterization of SOM-ZIF-8. (A) XRD patterns and(B) nitrogen adsorption-desorption isotherms of various samples. Theinset of (B) shows the corresponding micropore size distribution from theHorvath-Kawazoe model, where SOM-ZIF-8(3, 0.5) has been shifted upby 0.3 cm3 g−1 for clarity. a.u., arbitrary units. (C) HAADF-STEM image of

an individual SOM-ZIF-8(3, 0.5) crystal. (D to F) TEM images of SOM-ZIF-8at different magnifications taken along the <011> zone axis. The insetof (D) shows the corresponding SAED patterns, and the inset of (E) showsthe indexed FT patterns. (E) and (F) are magnified views of the areasoutlined in (D) and (E), respectively.

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3D-ordered macropores in SOM-ZIF-8 for bulky-molecule–involved applications. Furthermore,the reaction time for complete conversion ofbenzaldehyde was consistently 2 to 3 hours whenthe average particle size was decreased from4.3 mm for SOM-ZIF-8(3, 0.67) to 1.3 mm for SOM-ZIF-8(3, 0.33).Additionally, SOM-ZIF-8 exhibited superior

structural stability and improved recyclabilityas compared with PH-ZIF-8 composed of inter-grown polycrystals. SOM-ZIF-8(3, 0.33) could bereused at least seven times (Fig. 4B), with benz-aldehyde conversion only decreasing from 94.6to 87.0%. Comparably, a ≥5% decrease in catalyticactivity was observed in every subsequent run forPH-ZIF-8. The TEM image of used PH-ZIF-8 in-dicates that the recycling operation may cause asevere collapse in its hollow structure, resultingin utrasmall ZIF-8 nanocrystals that are lost inthe separation step (fig. S39). In sharp contrast,SOM-ZIF-8 experiencedmuch less structural dam-age, with its 3D-orderedmacro-microporous struc-ture being well preserved after seven reactioncycles. The durability of SOM-ZIF-8 can be attri-buted to its robust single-crystalline framework

with much lower defect density in comparisonwith PH-ZIF-8 (fig. S40).The reaction of various substituted benzalde-

hydes with malononitrile was also performedwith SOM-ZIF-8(3, 0.33) and C-ZIF-8 (table S2).

For all investigated substrates, SOM-ZIF-8(3, 0.33)always showed much higher catalytic activitiesthanC-ZIF-8 under identical reaction conditions.The activity enhancement was even notable forbenzaldehydeswith bulky groups (table S2, entries

Shen et al., Science 359, 206–210 (2018) 12 January 2018 4 of 5

Fig. 3. Controlled growth of SOM-ZIF-8 by regulating solvents and PSsizes. (A to C) SEM images of various samples prepared with differentvolume fractions of NH3·H2O: (A) 0, (B) 0.33, and (C) 0.67. (D to K)HAADF-STEM images of various SOM-ZIF-8 crystals with macropore sizes

ranging from 190 to 470 nm, as determined by carefully controlling thediameter of the PS templates: (D and H) 190 nm, (E and I) 340 nm, (F and J)400 nm, and (G and K) 470 nm. The images in (H) to (K) are magnifiedviews of the areas outlined by the squares in (D) to (G).

Fig. 4. Comparing the catalytic performance of SOM-ZIF-8 with C-ZIF-8, PH-ZIF-8, andM-ZIF-8. (A) Benzaldehyde conversions over various samples as a function of reaction time.(B) Recyclability tests of SOM-ZIF-8(3, 0.33) and PH-ZIF-8.

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11 to 16). In viewof the improvedcatalytic efficienciesin the Knoevenagel reaction, SOM-ZIF-8 could beexpected to show additionally enhanced catalyticactivities in various bulky-molecule reactions,which cannot be promoted or achieved usingpurely microporous C-ZIF-8.

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ACKNOWLEDGMENTS

We thank the National Natural Science Foundation of China (grants21606087, 21436005, 51621001, 21576095, and 21606088), theFundamental Research Funds for the Central Universities (grants

2017PY004 and 2017MS069), the Guangdong Natural ScienceFoundation (grants 2014A030310445, 2016A050502004, and2017A030312005), and the Welch Foundation (grant AX-1730)for financial support. K.S. thanks D. Tong for help with theschematic design and Y. Zhao for GFP preparation. K.S., Y.L., andB.C. conceived the idea and designed the experiments. K.S.synthesized the materials and carried out most of the structuralcharacterization. Y.H., L.L., and D.Z. performed the HRTEM imagesand analyzed the results. K.S., L.Z., X.C., J.C., and J.L. performedthe catalytic tests and some structural characterization. K.S.,Y.L., B.C., and R.L. cowrote the paper. All authors discussed andcommented on the manuscript. All data are reported in themain text and supplementary materials. K.S. and Y.L. are inventorson a patent/patent application (201710819873.9) held/submittedby South China University of Technology that covers the methodfor preparing ordered macroporous MOF single crystals.

SUPPLEMENTARY MATERIALS

www.sciencemag.org/content/359/6372/206/suppl/DC1Materials and MethodsFigs. S1 to S40Tables S1 and S2MS and NMR SpectraReferences (28–32)

10 July 2017; accepted 4 December 201710.1126/science.aao3403

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Ordered macro-microporous metal-organic framework single crystals

Li and Banglin ChenKui Shen, Lei Zhang, Xiaodong Chen, Lingmei Liu, Daliang Zhang, Yu Han, Junying Chen, Jilan Long, Rafael Luque, Yingwei

DOI: 10.1126/science.aao3403 (6372), 206-210.359Science 

, this issue p. 206Sciencebenzaldehydes and malononitriles and better catalyst recyclability.porous polystyrene template. These materials showed higher reaction rates for the Knoevenagel reaction betweenmicrocrystals of the ZIF-8 metal-organic framework, in which zinc ions are bridged by 2-methylimidazole linkers, inside a

grew ordered arrays ofet al.they are incorporated into a mesoporous structure with much larger pores. Shen The diffusion limitations on gas storage and catalytic reaction of microporous materials can often be overcome if

Mesoporous metal-organic frameworks

ARTICLE TOOLS http://science.sciencemag.org/content/359/6372/206

MATERIALSSUPPLEMENTARY http://science.sciencemag.org/content/suppl/2018/01/16/359.6372.206.DC1

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