Growth Pattern Control and Nanoarchitecture Engineering of Metal-Organic Framework Single Crystals by Confined Space Synthesis.

The nanoarchitecture engineering of metal-organic frameworks (MOFs) is a fascinating but intellectually challenging concept that opens up avenues for both tailoring the properties of MOFs and expanding their applications. Herein, we report the confined growth of ZIF-8 single crystals in a three-dimensionally ordered (3DO) macroporous polystyrene replica and reveal that their growth patterns, morphologies, and nanoarchitectures can be highly engineered using the concentration of the precursor. Impressively, the favorable in situ confined growth enables the successful fabrication of 3DO sphere-assembled ZIF-8 single crystals or 3DO single-crystalline ZIF-8 sphere arrays when a low- or high-concentration precursor solution, respectively, is used as the feedstock. Furthermore, our strategy can be extended to the preparation of other 3DO MOF single crystals, including ZIF-67 and HKUST-1, with similar controllable hierarchical nanoarchitectures. With the successful preparation of a series of diameter-tunable ZIF-8 single-crystalline spheres, we further unravel their interesting size-performance relationship in the Knoevenagle reaction between benzaldehyde and malononitrile, wherein the smallest spheres show the fastest first-order reaction kinetics. This study not only develops a general strategy for engineering the nanoarchitectures of MOF single crystals but also provides fundamental knowledge of the mechanism for the growth of hierarchical single crystals under confined spaces.

Introduction

Engineering the nanoarchitecture and porosity of crystalline materials is of paramount importance in many applications because it provides an effective strategy for tailoring mechanical, optical, electronic, and catalytic properties of the materials for a particular application.[1−3] Accordingly, various approaches have been developed for this purpose over the last few decades, among which confined-space synthesis has proved to be a powerful strategy for preparing three-dimensionally ordered (3DO) sphere arrays of various nanomaterials with controllable sizes, porosities, spatial arrangements, and thus desired functionalities.[4,5] For example, several groups reported that the confined growth of zeolite crystals in 3DO macro- or mesoporous carbon templates can produce corresponding 3DO zeolite sphere arrays, which can further be employed as seeds for the epitaxial growth of zeolite films due to their high uniformity and good redispersibility.[6−8] In addition, the assembly of uniform nanoscale crystal units into hierarchical single crystals in a periodic arrangement via confined-growth synthesis has also attracted great research attention because the good structural regularity and highly ordered imprinted macro- or mesoporosity of the resultant complex architectures can, in turn, afford remarkably improved properties and new applications.[9,10] Despite the great success of confined-space synthesis in the fabrication of a range of 3DO nanostructures, the applications of this strategy are mainly limited to inorganic materials (such as zeolites, carbons, metal oxides, and metals)[11−16] and its expansion to functional materials that require mild synthesis environments remains a synthetic bottleneck.

As a rapidly growing class of organic–inorganic hybrid materials constructed from the assembly of metal centers with organic linkers, metal–organic frameworks (MOFs) have aroused enormous research interest because of their unique structural properties and consequent wide potential applications, from gas storage and separation to catalysis.[17−26] The conventional synthesis of MOFs generally involves in the reaction between metal ions and organic ligands in a bulk solution, where the limited spatial control over their nucleation and crystallization makes it difficult to precisely fabricate the designed MOF architectures.[27,28] In contrast, the confined growth of MOF crystals in a predesigned template is expected to provide more precise control over the MOF’s nanoarchitecture features, including morphology, particle size, porosity, lattice orientation, and surface regularity, which may open a new avenue to diversify their functions and applications.[29−33] For instance, ZIF-8 nanorods, nanotubes, and nanowires with controllable nanoarchitectures can be successfully prepared by controlling the confined growth of ZIF-8 crystals in nonoporous polymer membranes, and the materials show good potential for membrane-based gas–liquid separation applications.[34,35] In this respect, our group also proposed an in situ nanocasting strategy to fabricate a new class of MOF structures featuring ordered macro- and micropores that coexist within discrete ZIF-8 single crystals. The combination of such ordered micro- and macropores with the robust single-crystalline nature endows the resultant frameworks with improved mass diffusion, superior catalytic activity, and good recyclability for reactions involving bulky molecules.[36] Although MOF synthesis by confinement has been proven effective for the preparation of various templated structures,[37−39] this strategy exhibits ambiguous success in the control of the growth pattern of templated MOFs due to the poor understanding of the mechanism for the growth of single crystals in confined spaces. In particular, to the best of our knowledge, the confined growth of MOF crystals to construct either 3DO MOF single-crystalline sphere arrays or sphere-assembled MOF single crystals is rare despite their great technological potentials for the manufacture of oriented MOF films and the realization of new diffusion-limited catalysis methods. The main synthetic obstacles result from the difficulty in exploring competent templates and the lack of a powerful method to regulate the crystallization properties of MOFs in confined spaces.

Accordingly, herein we propose a general bottom-up strategy for the simultaneous fabrication of 3DO sphere-assembled ZIF-8 single crystals (denoted as 3DOSA-ZIF-8) and 3DO single-crystalline ZIF-8 sphere arrays (denoted as 3DOSC-ZIF-8) by carrying out the controllable confined growth of ZIF-8 crystals in a removable 3DO macroporous polystyrene (denoted as 3DOM-PS) template, which was prepared by replicating a silica opal via a low-temperature assisted vacuum method. We demonstrate that the concentration of the precursor can be used as an efficient tool to control the growth pattern and engineer the nanoarchitectures of hierarchical ZIF-8 single crystals in 3DOM-PS. Confined growth using a low precursor concentration favors the formation of 3DOSA-ZIF-8, while that using a high precursor concentration benefits the crystallization of 3DOSC-ZIF-8. Furthermore, this strategy can also be extended to the preparation of hierarchical ZIF-67 and HKUST-1 single crystals with similar controllable nanoarchitectures and is expected to provide a versatile methodology for enriching the family of 3DO nanoarchitecture materials. In addition, the detailed sampling analysis provides fundamental knowledge of the mechanism for the growth of MOF crystals in confined spaces. Based on the successful preparation of isolated ZIF-8 spheres with tunable diameters by sonication, we further investigate the size-dependent activity of ZIF-8 for Knoevenagle condensation between benzaldehyde and malononitrile as a probe reaction.

Results and Discussion

Synthesis and Characterization of 3DOSA-ZIF-8 and 3DOSC-ZIF-8

The synthetic procedures for the preparation of 3DOSA-ZIF-8 and 3DOSC-ZIF-8 with two quite different morphologies are schematically illustrated in Figure . Briefly, an opal consisting of ordered closely packed colloidal silica spheres was employed as an initial template to fabricate the 3DOM-PS template though the chemical polymerization of the styrene monomer in its interstices, followed by the removal of silica spheres via a dilute HF solution. After that, the obtained inverse opal template was filled with a methanol solution with a low or a high concentration of the 2-MeIM solution (holding the feeding molar ratio of 2-MeIM/Zn(NO3)2 to be 3) to afford the corresponding low- or high-c-precursor@PS composite (Figure S1). Subsequently the above low- or high-c-precursor@PS composite was independently immersed in an ammonia–methanol mixed solution to induce the confined growth of ZIF-8 crystals. Finally, the PS template can be easily removed by selective dissolution using a dimethylformamide solvent, leaving behind 3DOSA-ZIF-8 or 3DOSC-ZIF-8 produced from the low- or high-c-precursor@PS composite, respectively (see details in the Supporting Information).

Figure 1

Schematic illustration of the synthetic procedures for the synthesis of 3DOSA-ZIF-8 and 3DOSC-ZIF-8 using the concentration of the precursor as the key controller.

The scanning electron microscopy (SEM) images in Figure S2 clearly reveal that 3DOM-PS templated from silica opal possesses a highly uniform macroporous structure wherein the diameter of the pores is around 200 nm, smaller than that of the initial silica spheres (around 235 nm). Due to its ordered face-centered cubic (FCC) structure with highly interconnected spaces, tailorable cavities, and easily removal feature, the obtained 3DOM-PS should be particularly attractive for the confined synthesis of MOFs with relatively fragile structures. The low-magnification SEM image (Figure a) shows that discrete 3DOSA-ZIF-8 particles have well-developed polyhedron morphologies with a uniform size of ∼1.9 μm (Figure S3). Each well-defined particle is assembled by a number of spherical crystals about 200 nm in diameter, and all these ZIF-8 spheres are interconnected (Figure S4). The void spaces within the neighboring spheres endow each 3DOSA-ZIF-8 particle with an advantageous open structure. The 3DO arrangement of highly uniform ZIF-8 spheres can be further identified by the representative SEM images of individual 3DOSA-ZIF-8 particles, which were taken from the [100], [110], and [111] directions, and their corresponding schematic models (Figures b–d, respectively). The close inspection of a selected semifinished crystal by SEM (Figure e) reveals that 3DOSA-ZIF-8 displays an obviously truncated rhombic dodecahedral morphology with 12 {110} facets and 6 {100} facets, which is strongly in accordance with conventional ZIF-8 (denoted as C-ZIF-8) prepared by a previously reported method[40] (Figures S5, S6, and 2f). These results directly confirm the single-crystalline nature of 3DOSA-ZIF-8. Representative transmission electron microscopy (TEM) images (Figure g and S7) of the selected individual 3DOSA-ZIF-8 particles further confirm their 3D open structure constructed by uniform ZIF-8 spheres in an ordered FCC arrangement, which agrees well with the observation from the aforementioned SEM images. Therefore, 3DOSA-ZIF-8 has not only intrinsic micropores from the molecular framework of the ZIF-8 phase but also the desired interconnected macropores between neighboring spheres and thus presents as a unique multilevel or hierarchical porous single-crystalline material. The corresponding selected area electron diffraction (SAED) pattern (Figure h) displays an ordered diffraction spot array from which we can infer that the crystal axis is along the [111] direction, reconfirming the single-crystalline nature of 3DOSA-ZIF-8.[41] The high-angle annular dark-field scanning transmission electron microscopy (HAADF-STEM) image and the corresponding elemental mapping image of 3DOSA-ZIF-8 (Figure i and j, respectively) suggest the uniform distributions of Zn, N, and C elements over the entire architecture.

Figure 2

Characterization of 3DOSA-ZIF-8 and 3DOSC-ZIF-8. (a) Low-resolution SEM image of 3DOSA-ZIF-8. (b–d) SEM images of 3DOSA-ZIF-8 crystals from three unique directions and their corresponding schematic models. SEM images and corresponding schematic models of (f) a poorly templated 3DOSA-ZIF-8 crystal and (e) an analogous C-ZIF-8 crystal. (g) TEM image, (h) SAED pattern, (i) HAADF-STEM image, and (j) EDX elemental mappings of 3DOSA-ZIF-8. (k–n) SEM images of 3DOSC-ZIF-8 at different resolutions (the insets of panels l and m are the FT patterns for the white square zones). (o–r) STEM and lattice-resolution TEM images of 3DOSC-ZIF-8. (s) TEM image, (t) SAED pattern, (u) HAADF-STEM image, and (v) EDX elemental mappings of 3DOSC-ZIF-8.

In sharp contrast, when the high-c-precursor@PS composite is used for the confined growth, 3DOSC-ZIF-8 with a monolith structure can be obtained. As shown in Figure k–m, as-prepared 3DOSC-ZIF-8 has a long-range highly ordered sphere array structure, suggesting that the printed product perfectly inherits the densest lattice structure of the starting silica opal. The three-dimensional ordering throughout the monolithic superstructure can also be visualized well by the representative cross-section and edge SEM images in Figures S8 and S9. Additionally, the SEM images of the {100} and {111} planes of the ZIF-8 sphere array further confirm the FCC superlattice structure of 3DOSC-ZIF-8, which inherited the corresponding {100} and {111} planes from the silica opal and the 3DOM-PS template (Figure S10). The same 3D highly ordered feature can be found in the high-magnification SEM and HAADF-STEM images of 3DOSC-ZIF-8, wherein no cracked or poorly grown ZIF-8 spheres can be observed over the entire image region (Figures n and o and S11). We also noticed that the unique FCC arrangement of sphere arrays in 3DOSC-ZIF-8 is similar to the inherent topological structure of the ZIF-8 crystal (Figure S12). The diameter of the uniform ZIF-8 spheres was measured to be approximately 200 nm (Figure S13), which is identical to the average macropore size of 3DOM-PS, suggesting that the dried precursor completely fills the macropores in the PS inverse opal and that the ZIF-8 spheres do not contract during the removal of the PS template. One nearly perfect spherical crystal selected from 3DOSC-ZIF-8 appears as a single crystal, as evidenced by the lattice-resolution TEM image (Figure S14) where consistent and continuous lattice fringes without any interfaces or domain boundaries can be clearly observed throughout the entire particle. The lattice fringes and fast Fourier transform (FFT) patterns in two different HRTEM images shot for one ZIF-8 sphere share a common crystallographic orientation, which is in good agreement with the {220} planes of ZIF-8 structure having a lattice spacing of ∼11 Å[41,42] (Figures p–r and S15). Meanwhile, the clear SAED pattern taken from a representative ZIF-8 sphere along the [111] zone axis direction further confirms the single-crystalline nature of the spheres in 3DOSC-ZIF-8 (Figure s and t), which is very consistent with the above lattice-resolution TEM results. Note that a particle of the 3DOSC-ZIF-8 monolith may be a few millimeters in its size; thus, the entire structure is not a single crystal. In addition, the energy-dispersive X-ray (EDX) spectroscopy elemental mapping images demonstrate the homogeneous distributions of Zn, N, and O elements over the entire spherical crystal (Figure u and v). Furthermore, given the high morphological regularity of the spherical elements, we can easily obtain dispersive single-crystalline ZIF-8 spheres (denoted as SC-ZIF-8) with good regularity from the disassembly of 3DOSC-ZIF-8 via a simple sonication method (Figure S16).

Structural Analyses and Demonstrations of the High Tunability of 3DOSA-ZIF-8 and 3DOSC-ZIF-8

The crystalline structures and phase purity of 3DOSA-ZIF-8 and 3DOSC-ZIF-8 were subsequently investigated by X-ray powder diffraction (XRD). As shown in Figure a1, all the diffraction peaks of 3DOSA-ZIF-8 and 3DOSC-ZIF-8 match well with those of C-ZIF-8 and simulated ZIF-8, confirming that the as-synthesized samples have pure ZIF-8 phase structures with high crystallinity (Figures S17 and S18).[43] In addition, a nitrogen adsorption measurement was performed to determine the pore structures of these samples, and the results are shown in Figure a2. Obviously, 3DOSA-ZIF-8, 3DOSC-ZIF-8, and C-ZIF-8 all display a type I sorption isotherm with high N2 uptakes at very low pressures (P/P0 < 0.05), implying the presence of abundant micropores; these micropores were also evidenced by the corresponding pore distribution curves.[36] Accordingly, the Brunauer–Emmett–Teller (BET) surface areas and micropore volumes, respectively, were calculated to be 1510 m2/g and 0.72 m3/g for 3DOSA-ZIF-8 and 1580 m2/g and 0.73 m3/g for 3DOSC-ZIF-8, larger than those of 1400 m2/g and 0.66 m3/g of C-ZIF-8. These results demonstrate that the confined growth of ZIF-8 crystals does not affect the inherent microporous structures of 3DOSA-ZIF-8 and 3DOSC-ZIF-8. Interestingly, the long-range and three-dimensional highly ordered structure of 3DOSC-ZIF-8 also makes it a dynamic photonic material, which can be revealed by a visible-light microscope. When saturated with methanol, 3DOSC-ZIF-8 shows a beautiful bright green color through Bragg diffraction with visible light,[44] which directly suggests its desired photonic crystal property and thus good potential of this method for fabricating new MOF-based photonic materials (Figure a3). However, neither C-ZIF-8 (white) nor 3DOSA-ZIF-8 (yellow) displays a similar photonic crystal property due to its unordered or short-range ordered structure (Figures a4 and a5 and S19). Another interesting optical phenomenon is that the color of the methanol-saturated 3DOSC-ZIF-8 changed from bright green to dark red when the light was switched from a reflected pattern to a transmitted pattern, whereas no color change was observed for C-ZIF-8 under the same conditions (Figure S20). These results can in turn prove the excellent long-range-order feature of 3DOSC-ZIF-8 and its high structural regularity.

Figure 3

(a1) XRD patterns and (a2) N2 sorption isotherms (the inset shows the corresponding micropore distributions) of various samples. Optical images of (a3) 3DOSC-ZIF-8, (a4) 3DOSA-ZIF-8, and (a5) C-ZIF-8 after saturation with methanol. SEM images of 3DOSA-ZIF-8(z) at the following resolutions: (b1 and b2) 330, (c1 and c2) 425, (d1 and d2) 500, and (e1 and e2) 610 nm. (f1–i1 and f2–i2) SEM images and (f3–i3) HAADF-STEM plus EDX mapping images of 3DOSC-ZIF-8(z) at the following magnifications: (f1–f3) 330, (g1–g3) 425, (h1–h3) 500, and (i1–i3) 610 nm.

Furthermore, to manifest the good tunability of this concentration-assisted confined-growth strategy, we also successfully synthesized a series of 3DOSA-ZIF-8(z) and 3DOSC-ZIF-8(z) (z represents the average diameters of the ZIF-8 spheres) (Table S1) with sphere diameters ranging from 330 to 610 nm. As shown in Figure b–e, all 3DOSA-ZIF-8(z) samples display similar well-defined truncated rhombic dodecahedral morphologies with narrow particle size distributions and high crystallinities (Figures S21–S24), while all 3DOSC-ZIF-8(z) samples are composed of uniform pure-phase ZIF-8 spheres, which are assembled into similar long-range and highly ordered array structures (Figures f–i and S25–S32). The average diameters of their spherical elements are about 330, 425, 500, and 610 nm, which match well with the macropore sizes of the corresponding 3DOM-PS templates. However, the average diameters of the corresponding silica sphere templates are about 370, 460, 530, and 635 nm, suggesting that the macropores of 3DOM-PS would discriminatively shrink during the removal of silica spheres (Figures S33–S37 and Table S2). Thus, Figure S38 correlates the diameters of 3DOSA-ZIF-8(z) and 3DOSC-ZIF-8(z) with those of the employed SiO2 sphere templates, providing guidance for the synthesis of desired 3DOSA-ZIF-8 and 3DOSC-ZIF-8 with a specific size and more.

Formation Mechanism of 3DOSA-ZIF-8 and 3DOSC-ZIF-8

Given that the growth pattern control of MOFs in confined-space synthesis can be accomplished just by adjusting the concentration of the precursor, we further monitored the structural evolution of the products harvested at different stages using three representative concentrations of the precursor, namely 0.075, 0.135, and 0.675 g/mL, on a 2-MeIM basis (holding the molar ratio of 2-MeIM/Zn(NO3)2 at 3; see theSupporting Information for details). As shown in Figure a1, when the lowest 0.075 g/mL precursor solution was used for the synthesis, only some small discrete macroporous regions in 3DOM-PS were occupied by the dried precursor gel due to the limited precursor supply, whereas the occupied regions became larger when the concentration of the precursor was increased to 0.135 g/mL (Figure b1). Correspondingly, in the subsequent crystallization step, these two types of discrete dried precursor gels can be transformed in situ into monodispersed 3DOSA-ZIF-8 with average particle sizes of about 1 and 2 μm, respectively (Figure a2, a3, b2, and b3). In contrast, when the concentration of the precursor is further increased to 0.675 g/mL, there are enough precursor molecules in solution to ensure that all macropores of 3DOM-PS are occupied by the dried precursor gel after the removal of the solvent. The dried precursor gel can then grow into 3DOSC-ZIF-8 after experiencing a double-solvent-induced crystallization process (Figures c1–c3). These results suggest that the different concentrations of the precursor can lead to the different distribution states of the dried precursor gel in 3DOM-PS, which finally determine the growth pattern and nanoarchitecture of the resultant ZIF-8 single crystals in confined spaces. To further confirm this from another perspective, we carried out a control experiment in which a freeze-drying method was employed to control the distribution state of the dried precursor gel (the concentration of precursor used was 0.135 g/mL). We designed this experiment by holding onto the idea that the removal of solvents (mainly water) by cryogenic sublimation can minimize the movement of the precursor solution in 3DOM-PS and thus keep the precursor in its place during the freeze-drying process (Figure S39).[45,46] As expected, SEM images reveal that the freeze-dried precursor gel (denoted as F-precursor) is uniformly distributed in the entire 3DOM-PS framework without serious aggregation (Figure d1 and d4). Subsequently, the uniformly distributed F-precursor can be converted in situ into a three-dimensionally ordered ZIF-8 nanoarchitecture (denoted as F-3DO-ZIF-8) with many more macropores than 3DOSA-ZIF-8 (Figures d2, d3, and d5). A close examination of the high-magnification SEM image (Figure d6) reveals that F-3DO-ZIF-8 is composed of small irregular ZIF-8 particles, which connect to each other to form a highly porous 3D structure. All these results provide solid evidence proving the favorable in situ crystallization behavior of ZIF-8 from the dried precursor gel, which enables us to realize good control over the growth pattern, morphology, and particle size of ZIF-8 single crystals in confined-space synthesis by simply adjusting the concentration of the precursor. Accordingly, we further describe the formation mechanisms of 3DOSA-ZIF-8, 3DOSC-ZIF-8, and F-3DO-ZIF-8, which for the sake of clarity are shown are schematic illustrations in Figures e and f.

Figure 4

Formation mechanism studies of 3DOSA-ZIF-8 and 3DOSC-ZIF-8. SEM images of (a1–c1) precursor@PS, (a2–c2) ZIF-8@PS, and (a3–c3) templated ZIF-8, which were obtained using (a1–a3) 0.075, (b1–b3) 0.135, and (c1–c3) 0.675 g/mL precursor solutions. (d1–d6) SEM images showing the morphological evolution of F-3DO-ZIF-8 in its synthesis process for (d1 and d4) F-precursor@PS, (d2 and d5) F-3DO-ZIF-8@PS, and (d3 and d6) F-3DO-ZIF-8. Schematic illustrations of the formation mechanisms of (e) 3DOSA-ZIF-8 and 3DOSC-ZIF-8 and (f) F-3DO-ZIF-8.

Extension of Our Strategy to Other MOFs

More importantly, the good generality of our strategy can be verified through the successful preparation of other MOFs currently used most often, such as ZIF-67 and HKUST-1. When low-concentration solutions of the ZIF-67 and HKUST-1 precursors are used as feedstocks, 3DO sphere-assembled ZIF-67 and HKUST-1 single crystals (denoted as 3DOSA-ZIF-67 and 3DOSA-HKUST-1, respectively) can be prepared successfully. The single crystals of ZIF-67 and HKUST-1 show obvious dodecahedral and octahedral crystal morphologies, respectively, coincident with those of their corresponding conventional crystals (Figures a1–a4 and b1–b4 and S40–S47).[47,48] The single-crystalline natures of 3DOSA-ZIF-67 and 3DOSA-HKUST-1 were revealed by the corresponding SAED patterns taken from the individual crystals, both of which display clear diffraction spot matrices (Figures a5 and b5). In addition, the EDX elemental mappings manifest the uniform distributions of Co, N and C in 3DOSA-ZIF-67 (Figures a6 and a7), and C, Cu, and O in 3DOSA-HKUST-1 (Figures b6 and b7). As expected, when high-concentration solutions of the precursors are used as feedstocks, 3DO single-crystalline ZIF-67 and HKUST-1 sphere arrays can be fabricated successfully (denoted as 3DOSC-ZIF-67 and 3DOSC-HKUST-1, respectively), both of which are constructed by highly uniform MOF spheres assembled in a long-range-ordered arrangement as convincingly confirmed by their corresponding SEM, TEM, STEM, and EDS elemental mapping measurements (Figures c1–c5 and d1–d5 and S48–S54). These results highlight the good capability of our strategy to precisely control the growth patterns and engineer the nanoarchitectures of various MOFs by confined space synthesis.

Figure 5

Extension of this strategy to ZIF-67 and HKUST-1. Low-resolution SEM images and structures of (a1 and a2) 3DOSA-ZIF-67 and (b1 and b2) 3DOSA-HKUST-1. Comparison of the SEM images and schematic models of (a3) individual 3DOSA-ZIF-67 and (b3) 3DOSA-HKUST-1 particles with (a4 and b4, respectively) their conventional counterparts. TEM, SAED, and HAADF-STEM, and EDX elemental mapping images of (a5–a7) 3DOSA-ZIF-67 and (b5–b7) 3DOSA-HKUST-1. Low-resolution SEM and STEM images, respectively, of (c1 and c2) 3DOSC-ZIF-67 and (d1 and d2) 3DOSC-HKUST-1. TEM, SEM, and EDX elemental mapping images, respectively, of individual spheres from (c3–c5) 3DOSC-ZIF-67 and (d3–d5) 3DOSC-HKUST-1.

Exploration of the Structure–Activity Relationships of Various Catalysts

In recent years, great attention has been paid to the utilization of MOF-based materials as heterogeneous catalysts due to their attractive structures and catalytic performances in many important reactions.[49,50]

Most previous advances were focused on enhancing the catalytic activities of MOFs, yet the exploration of their size-dependent activity was rarely achieved due to the lack of an efficient strategy to precisely control the size of these materials.[51,52] Herein, the good size control of 3DO ZIF-8 single crystals with high uniformity enables us to unravel their size–performance relationship using the Knoevenagle condensation between benzaldehyde and malononitrile as a probe reaction, which is a widely employed method for carbon–carbon bond formation.[53] As shown in Figure a, both 3DOSA-ZIF-8(200) and 3DOSC-ZIF-8(200) exhibit significantly enhanced activities as compared with that of microporous C-ZIF-8, suggesting that the introduction of 3DO macropores into the both samples favorably boosts their catalytic performances. Interestingly, despite featuring similar 3D sphere-assembled structures, 3DOSA-ZIF-8(200) showed a much better catalytic activity than 3DOSC-ZIF-8(200), since the complete conversion of benzaldehyde to benzylidene malononitrile could be achieved within 4 h for 3DOSA-ZIF-8(200) versus 6 h for 3DOSC-ZIF-8(200) under identical reaction conditions. This is because the micrometer-sized open void structure of 3DOSA-ZIF-8 can endow it with a fast mass transfer capability and a short diffusion length to facilitate reactant diffusion and surface adsorption during the catalytic process. In contrast, the oversized superstructure of 3DOSC-ZIF-8 would inevitably lead to prolonged diffusion distance and thus sluggish reaction kinetics relative to its 3DOSA-ZIF-8 counterpart. As expected, the catalytic activity of 3DOSC-ZIF-8(200) was markedly improved by its sonication-assisted disassembly to SC-ZIF-8(200), which showed the highest catalytic activity (only 3 h were needed to realize the complete conversion of benzaldehyde) among all the studied catalysts due to having the smallest diffusion resistance and the largest external surface area. Subsequently, we investigated the detailed size–performance relationship of ZIF-8 by testing and comparing the catalytic activities of SC-ZIF-8(z) with diameters ranging from 200 to 610 nm for this Knoevenagel condensation reaction. It can be clearly seen in Figure b that the time to achieve the complete conversion of benzaldehyde gradually increased as the diameter of SC-ZIF-8(z) increased from 200 to 610 nm. Furthermore, a good linear relationship between ln(1 – X) (X represents the conversion of benzaldehyde) and the reaction time was observed for all the SC-ZIF-8(z) catalysts (Figure c), were the smallest SC-ZIF-8(200) showed the fastest first-order reaction kinetics.[54] Note that the molecular dynamics size of benzaldehyde is about 0.6 nm, which is much larger than the six-membered-ring pore windows of ZIF-8 (0.34 nm). Thus, as all accessible catalytic sites for the reaction are located on the external surface of ZIF-8, the high catalytic activity of SC-ZIF-8(200) can be reasonably ascribed to its large external surface with the abundant active sites (Figure S55).

Figure 6

(a and b) The conversion of benzaldehyde as a function of the reaction time using various catalysts. (c) The relationship between ln(1 – X) (X is the conversion of benzaldehyde) and the reaction time over various sizes of SC-ZIF-8. (d) Recyclability tests of SC-ZIF-8 and 3DOSA-ZIF-8 over eight cycles. (e) Structural characteristics of the various catalysts.

The most attractive advantages of heterogeneous catalysts are their easy separation from the reaction solution and thus good reusability as compared with those of their homogeneous counterparts.[52] Therefore, recycling experiments were performed on 3DOSA-ZIF-8(200) and SC-ZIF-8(200) for the same Knoevenagel condensation reaction. As shown in Figure d, 3DOSA-ZIF-8 could be reused at least eight times without a remarkable decrease in catalytic activity after each run. Comparably, there was an appreciable loss of catalytic efficiency for SC-ZIF-8(200) after eight reaction cycles under the same investigated conditions, decreasing from 72.2 to 62.1%. The SEM image of used SC-ZIF-8(200) (Figure S56) shows some broken spheres and small fragments that may be lost in the subsequent separation process, leading to the slightly inferior reusability of SC-ZIF-8(200). In contrast, the 3DO sphere-assembling structure of 3DOSA-ZIF-8 was preserved well after eight reaction runs (Figure S57). Therefore, 3DOSA-ZIF-8 can exhibit exceptional recyclability as compared to SC-ZIF-8(200) by virtue of the highly protective effect of its robust single-crystalline framework and well-ordered arrangement with high regularity. Accordingly, we further summarize and compare the structural characteristics of C-ZIF-8, 3DOSA-ZIF-8, 3DOSC-ZIF-8, and SC-ZIF-8 using the schematic illustration in Figure e, which demonstrates that both the introduction of additional macroporsity and the reduction of particle size can be employed as good tactics to achieve efficient catalysis for MOFs.

Conclusions

In conclusion, we propose a novel bottom-up strategy based on the controllable confined growth of ZIF-8 crystals in a 3DO macroporous PS replica for the simultaneous synthesis of 3DOSC-ZIF-8 and 3DOSA-ZIF-8. We prove that the concentration of the precursor can be used to control the growth pattern and engineer the nanoarchitectures of 3DO ZIF-8 singe crystals in their confined growth process. This versatile strategy can be extended to ZIF-67 and HKUST-1 with similar good control over their growth patterns and nanoarchitectures, indicating its excellent generality for the preparation of 3DO MOF single crystals of various topologies. Furthermore, the detailed studies of the morphological evolution processes uncover that the nanoarchitecture transformation from 3DO single-crystal sphere arrays to 3DO sphere-assembled single crystals is strongly dependent on the distribution states of the dried precursor gel in 3DOM-PS after the evaporation of the solvent. In addition, the successful fabrication of isolated ZIF-8 single-crystalline spheres with predictable sizes allows us to determine their size–performance relationship using the Knoevenagle condensation between benzaldehyde and malononitrile as a probe reaction. This study points out a general and effective methodology for controlling the growth patterns and engineering the nanoarchitectures of hierarchical MOFs single crystals, providing a new toolbox for enriching the family of hierarchical single-crystalline architectures with high structural regularity.

Safety statement: Caution should be taken when using HF during the etching of SiO2 because HF is extremely toxic and corrosive.