Superstructure of a MetalâOrganic Framework Derived from

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Superstructure of a Metal-Organic Framework Derived from Microdroplet Flow Reaction: An Intermediate State of Crystallization by Particle Attachment Ying Wang, Liangjun Li, Huimin Liang, Yanlong Xing, Liting Yan, Pengcheng Dai, Xin Gu, Guoming Zhao, and Xuebo Zhao ACS Nano, Just Accepted Manuscript • Publication Date (Web): 07 Mar 2019 Downloaded from http://pubs.acs.org on March 7, 2019

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Superstructure of a Metal-Organic Framework Derived from Microdroplet Flow Reaction: An Intermediate State of Crystallization by Particle Attachment Ying Wang,1,3 Liangjun Li,*2 Huimin Liang,1 Yanlong Xing,4 Liting Yan,1 Pengcheng Dai,2 Xin Gu,2 Guoming Zhao,5 and Xuebo Zhao, * 1, 2 1

State Key Laboratory of Heavy Oil Processing, College of Chemical Engineering, China

University of Petroleum (East China), Qingdao, 266580, China 2

Institute of New Energy, China University of Petroleum (East China), Qingdao, 266580, China

3

Key Laboratory of Macromolecular Science of Shaanxi Province, Key Laboratory of Applied

Surface and Colloid Chemistry, Ministry of Education, School of Chemistry & Chemical Engineering, Shaanxi Normal University, Xi’an, 710119, China 4 Leibniz

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Institute for Analytical Sciences, Berlin 12489, Germany

College of Chemical and Environmental Engineering, Shandong University of Science and

Technology, Qingdao 266590, China

ACS Paragon Plus Environment

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ABSTRACT: Understanding the crystallization pathway is of fundamental importance in controlling structures and functionalities for metal-organic frameworks (MOFs), but only few studies have been reported on the mechanism of crystallization for MOFs to date. Here, by using a microdroplet flow (MF) reaction technique, we successfully revealed the different status of HKUST-1 during its crystal growth process. The morphologies and structures of crystals at different stages were recorded and characterized by scanning electron microscopy, transmission electron microscopy and small-angle X-ray diffraction. Experimental observations clearly demonstrate a process of crystallization by particle attachment (CPA) for crystal growth of HKUST-1 under MF conditions. The superstructure of HKUST-1, which is assembled from oriented attachment of nano-sized particles of HKUST-1, is observed at early stage of crystal growth. This type of superstructure gradually transforms to true single-crystals through a ripening effect upon increasing residence time, accompanied by increase in dimensions of crystals. Thus, the superstructure is the intermediate state during crystallization and acts as the bridge between disordered reactants and highly ordered single-crystals. Based on these findings, the crystal growth of HKUST-1 in MF reaction can be elucidated as a process involving three steps: the generation of nano-sized primary particles, the following assembly of the primary particles into a superstructure and the ripening of superstructure into a crystal. Furthermore, the superstructure of HKUST-1 shows superior performance for CO2 and CH4 adsorptions. The CPA mechanism in the crystallization of HKUST-1 demonstrated in this work is in clear contrast to the monomer-bymonomer addition mechanism in classic models of crystal growth. This mechanism could have important reference meaning for understanding the crystal growth mechanism of other type of MOFs or other special morphologies.


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KEYWORDS: metal-organic frameworks, microdroplet flow reaction, superstructure, crystal growth mechanism, crystallization by particle attachment, methane storage Metal-organic frameworks (MOFs) represent an emerging type of organic-inorganic hybrid crystalline materials consisting of metal clusters and organic linkers.1-3 In the past two decades, scientists have witnessed the great progress in the development of MOFs.4-6 These achievements enable MOFs to be promising candidates for a variety of applications including gas storage/separation,7-9 catalysis,10-13 energy storage,14 sensing and optical apparatus,15-17 etc. In stark contrast with the fast development in aforementioned research fields, the crystallization process that plays a central role in the synthesis of MOFs has rarely been reported.18, 19 As a result, very little is known about the crystallization process in MOFs’ synthesis.20, 21 As a basic scientific issue, to understand the detailed mechanism of crystallization for MOFs is a crucial aspect that permits the control of pore structure, morphology, functionality and forms of defects in MOFs’ crystals.22,

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Currently, the interpretation of crystallization process of MOFs mainly relies on

classical nucleation and growth model, in which the monomer-by-monomer addition mechanism was thought to take effect in the course of crystal growth. Some researchers also proposed the metathesis model to explain the possible driving force giving rise to single-crystals of MOFs in hydrothermal/solvothermal conditions. They believed that the ligand addition/ dissolving or exchanging mechanisms dominated the crystal growth process.24-27 However, whether or not the classical or metathesis mechanisms can be applied to the crystallization of MOFs is still doubtful. In fact, it has been found that a certain amount of phenomena associated with crystallization of solid-state materials cannot be explained satisfactorily by classical theory of crystal growth.28, 29 Compared to the relatively deep understanding in crystal growth for some inorganic materials including nano-sized metals, metal oxides and minerals,30, 31 the knowledge about crystallization


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process of MOFs is limited. The slow progress of researches in crystallization process of MOFs compared to other materials could be attributed to following two aspects. Firstly, the crystallization of MOFs is a reaction-crystallization process in which chemical reaction among reactants and crystal growth from solution is both involved. The interplay between these two steps leads to a great complexity in analyzing this crystallization course.32, 33 In addition, the crystallization of MOFs is largely influenced by many physico-chemical factors including architectures of resulting MOFs, chemical properties of components and reaction environments.23,

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Therefore, it is

questionable that the intractable problems of crystallization for MOFs can be resolved within the framework of classical models of crystal growth. Furthermore, the absence of an effective research approach makes it rather difficult in exploring the detailed crystallization mechanism. Although the in-situ characterization technique,35, 36 especially in-situ electron microscopy, is considered as a powerful mean to investigate the crystallization process,37-39 it remains a great challenge to investigate the crystallization process for MOFs by using this technique, due to the susceptibility of MOFs’ framework to electron-beam and their weak electro-conductivity.40-42 In order to overcome this problem, other techniques, i.e. extended X-ray absorption fine structure (EXAFS) spectroscopy,43 off-line analysis,44 in-situ spectroscopic techniques,45 fancy laboratory equipments46 and theoretical simulations,47 have been employed to explore the mechanism of crystallization for MOFs in recent years. These studies have made some progress in explaining the formation of MOFs crystals. However, it is still challenging to get a full landscape of crystallization process and understand the detailed mechanism of the reaction-crystallization process of MOFs. Herein, we utilized a microdroplet flow (MF) reaction system, combined with ex-situ scanning electron microscopy (SEM), transmission electron microscopy (TEM) and small-angle XRD


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(SAXD) to investigate the crystallization process of MOFs. By using HKUST-1 as the representative MOF, the status of crystallization at different stages of crystal growth were captured and characterized. These activities enable us to track the crystal growth process along the time axis. Based on these results, a process of crystallization by particle attachment (CPA), a nonclassical model of crystallization which postulates that crystals can grow via oriented attachment or addition of particles,48, 49 has been found to dominate the crystal growth process for HKUST-1 under MF conditions. A superstructure of HKUST-1, the most direct evidence of CPA process, is observed at the early stage of crystal growth for HKUST-1. As a kinetically stabilized meso-crystal assembled from the oriented attachment of nanoparticles,50-52 the superstructure can be viewed as an intermediate state of CPA process, bridging disordered monomers and highly ordered singlecrystals of MOFs. Based on the CPA model, the crystallization of HKUST-1 can be interpreted as a process comprising of several stages. Other than HKUST-1, the crystallization process of other two MOFs with different structures and topologies: MOF-5 and NOTT-100, were also examined by using a MF technique. It has been revealed that the crystal growth process of MOF-5 and NOTT-100 could also be interpreted using a CPA model. These results validate the viability of this mechanism to crystal growth process of MOFs.

Results/Discussion The MF reaction is a variant of microfluidic reaction that is an emerging synthetic technique applicable to a wide range of compounds,53-58 such as nano-materials, biochemical compounds and MOFs, et al.59-63 Compared to common batch-scale reactors, MF reaction shows its advantage in finely controlling of time of reaction process, rendering it to be a potential technique in investigating the crystallization process of MOFs. The MF system employed in this work consists


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of three injection pumps, one heating zone, one cooling zone and one product-collecting device (see schematic diagram in Figure 1a and actual apparatus in Figure 1b). In a typical synthesis procedure, the Cu(NO3)2∙3H2O (0.72 g) was dissolved in the mixed solution of deionized water (H2O, 10 mL), ethanol (EtOH, 2.5 mL) and N,N-dimethylformamide (DMF, 2.5 mL), while the 1,3,5-benzenetricarboxylic acid (H3BTC, 0.42 g) was dissolved in the mixed solution of Ethanol (EtOH, 2.5 mL) and N,N-dimethylformamide (DMF, 12.5 mL). These two homogeneous solutions were sucked into syringe B and C, respectively. Then these solutions were pumped continuously into a micro-mixer equipped with a magnetic stirring to form a homogeneous solution. Afterwards, silicon oil (carrier phase) from syringe A and the solution of reactants coming from micro-mixer were injected co-currently into a T-junction, where the solution of reactants was sheared into uniform microdroplets by shear forces between two fluid flows immiscible to each other. These microdroplets passed through the heating zone (T= 90 ︒ C), giving rise to micro blue crystals suspended in colorless microdroplets (as shown in Figure 1c). Although this reaction temperature is higher than the boiling point of EtOH, the combining effect of low concentration of EtOH, positive operating pressure and encapsulation of microdroplets by silicon oil ensures the liquid phase reaction in these droplets without the vaporization of solvents. After flowing through cooling zone and the sample-collecting device, the blue-colored powder was obtained (termed as MFHKUST-1-t, where t represents residence time in minutes, see in Figure 1c and Table S1). The residence time was controlled by adjusting the length of heating coil. In addition, the HKUST-1 derived from conventional solvothermal synthesis was prepared for comparison (named as STHKUST-1). The powder X-ray diffraction (PXRD) patterns of samples prepared at different residence times were in good agreement with the simulated pattern of HKUST-1 (Figure 3h), suggesting the same crystallographic structures and high purities of these samples.


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Figure 1. (a) Schematic diagram of the home-built microdroplets flow-reaction system; (b) The picture of the microdroplets flow-reaction system; (c) The appearance of microdroplets passing through the reaction coil and the obtained blued products.


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Figure 2. (a) SEM images of the front view of MF-HKUST-1-8. Blue arrows show the angles of 60o between two stripes; (b) SEM of the broadside view for MF-HKUST-1-8; (c) TEM of these nano-sized particles in the MF-HKUST-1-8; (d) TEM of the layer-by-layer structure for MFHKUST-1-8. In the beginning, we prepared the HKUST-1 at a residence time of 8 minutes, at which point a pure phase of HKUST-1 with a high degree of crystallinity was produced (MF-HKUST-1-8). SEM images reveal a phase of octahedral crystals with clear edges and corners for MF-HKUST-1-8 (see Figure 2a and Figure S1). A considerable amount of small particles ranging from ~30 nm to ~3


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μm in size co-exist with these larger crystals. Close observation at the front view of crystals reveals a series of orderly aligned, staggered and parallel arranged stripes, with an exact angle of 60 ° between two staggered stripes (Figure 2a and Figure S2). Some pores with a mean size of 60 nm can be observed on the surface and the interior of these crystals (see Figure 2a and Figure S2 and Figure S3 in supporting information). These pores belong to the macro-pores according to the IUPAC scheme. The magnified SEM image discloses a considerable amount of nano-sized particles tiled on the crystal surface (as shown in Figure S4 in supporting information). The orderly and close packing of the nano-sized particles composes the crystal surface, resembling wall of house formed by orderly stacking of bricks. A cross-section view showing truncated corner of crystals demonstrates that the interior of crystal possesses the same structure as that on the surface (Figure S5). Thus, the arrangements of particles are homogeneous on the surface and in the middle of crystals. Viewing from the lateral side of the crystal, a series of periodically arranged layers were observed (Figure 2b). SEM image shows that the bulk crystal is formed by the layer-by-layer stacking of these lamellar structures. This layered structure is similar with 2D materials,64-66 however, it is constructed by 3D MOFs’ framework. These layers are parallel to each other, with an inter-layer distance of about 12 nm which is much larger than the unit cell dimension for HKUST-1 (2.63 nm). On the edges of these layers, clear boundaries between layers and many nano-sized particles that are dislocated from layers are observed (as shown in Figure S6 in supporting information). TEM image discloses a large number of nano-sized particles in the bulk crystals of MFHKUST-1-8, confirming that this crystal is constructed by the assembly of several nano-sized particles (see TEM image in Figure 2c). The median size of these particles is 11.4 nm, which is comparable with those dislocated particles seen from side-view of SEM image. Thus, it can be


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supposed that these nano-sized particles are building units of these layers in MF-HKUST-1-8. Furthermore, the side-view of TEM image of MF-HKUST-1-8 shows a layered structure, in good agreement with that observed from the SEM (see TEM image in Figure 2d). Other than thick layers, a minor phase of lamellar products with a high degree of crystallinity can be observed on the outer surface of thick layer structure. At the later stage of crystallization process, they can transform to the final product. Therefore, this type of morphologically different product could be immature (by-) products, temporarily stabilized phase or intermediate product during the chemical reaction or crystal growth process of HKUST-1. On the basis of these observations, we can speculate that the structure units of the layers are these nano-sized particles, which attached together in an oriented way to form the layers, and the orderly stacking of these layers gives rise to the bulk crystals. Notably, several parallel stripe-like profiles on coarse surfaces and boundaries between layers can be observed on crystals of MF-HKUST-1-8. This morphology shows a clear contrast to the smooth surface of ST-HKUST-1 derived from conventional solvothermal synthesis, despite that they possess the same crystallographic structure. This type of periodically aligned layers can be identified as a MOF superstructure,30, 67 in which the nano-sized particles acting as the primary particles. The stripes and boundary between layers observed in SEM images are originated from dislocations and small misalignments during attachments of these particles, and they can be identified as visualized features of superstructure. To capture the states of crystallization at different stages of crystal growth, the morphology of HKUST-1 formed at different residence time (the time passing through the reaction zone at a constant velocity, Figure S7) were characterized by ex-situ SEM. After conducting a series of experiments with different residence time, we found that the residence time of 4 minute is a critical point at which product of HKUST-1 can be produced. PXRD pattern shows that the sample


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prepared in 4 minutes’ of residence time (MF-HKUST-1-4) has a same crystallographic structure as HKUST-1, despite that it has a lower degree of crystallinity (see PXRD in Figure 3h). SEM image of ST-HKUST-1-4 shows a phase of nano-sized spherical particles which aggregate with each other into clusters (as shown in Figure 3a). When the residence time is increased to 6 minute, the obtained sample shows a higher degree of crystallinity (MF-HKUST-1-6). SEM image of MFHKUST-1-6 exhibits a phase of octahedral crystals and co-existence of nano-sized sphere particles. It can be seen from the SEM images that these octahedral crystals are actually formed by the assembly of small particles (as shown in Figure 3b, inset). As can be seen from the comparison of SEM images, the nano-sized particles in MF-HKUST-1-4 are indeed the building units which assembled into the octahedral shaped crystals in MF-HKUST-1-6. The dislocations and gaps between these particles lead to a rough surface of these octahedral crystals for MFHKUST-1-6. Compared to MF-HKUST-1-6, MF-HKUST-1-8 shows a much larger sizes and more regular shapes (see SEM image in Figure 3c). Notably, at a residence time of 8 minutes, the obtained MOF exhibits the most apparent characteristics of superstructures. Clear stripes and boundaries can be observed from the front and side view of the SEM images (see SEM images in Figures 3c and Figure S8). With the increasing residence time, those small nano-crystals gradually disappear, and crystal sizes become larger (see SEM images in Figure 3d and 3e). Meanwhile, upon the elongated residence time, the characteristics of the superstructure become more and more obscure, and finally disappear at residence time of 80 minutes (see magnified images in Figure 3f). The flat and smooth surface for MF-HKUST-1-80 is similar to single-crystal of HKUST-1 (as shown in Figure 3g). These observations demonstrate that the HKUST-1 gradually transformed from superstructure to single-crystals with the increasing residence time. As shown in statistical analysis of crystal sizes


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derived from SEM images (Figures 3i), median size of crystals increases with the increasing residence time. The median crystal size of MF-HKUST-1-4 is 23.5 nm, gradually increasing to 0.13, 2.40, 6.22, 7.21 and 9.77 μm for MF-HKUST-1-6, 8, 16, 32 and 80, respectively (Figures 3a-3f inset). From the plot of particle size versus time, it can be found that the particle size increases exponentially with time at the early stage of crystal growth (less than 8 minutes). In contrast, at the later stage of crystal growth, the particle size increase in a nearly linearly trend with the increasing residence time (when the residence time is longer than 8 minutes). The different variation trend of crystal sizes may reflect the different stages of crystal growth. At the early stage of crystal growth, the oriented attachment of nano-sized particles into superstructures plays a major role, while in the later stage, the ripening of superstructures dominates the crystal growth process.


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Figure 3. SEM micrographs of MF-HKUST-1-t with residence time of (a) 4 min; (b) 6 min; (c) 8 min; (d) 16 min; (e) 32 min and (f) 80 min; (g) SEM image of ST-HKUST-1; (h) PXRD patterns of MF-HKUST-1-t prepared at different residence times; (i) The size distribution of HKUST-1 crystals obtained from different residence time under the MF conditions; (j) The SAXD pattern for MF-HKUST-1-t and ST-HKUST-1. (Blue squares represent the magnified images and red bar graphs in figures a-g represent the size distribution of crystals) The evolution process of superstructure of HKUST-1 can be validated by small-angle X-ray diffraction (SAXD), an effective mean to study meso-scale ordered structures. As shown in Figure 3j, a sharp peak at the small angle of 2θ = 0.71± 0.05° is observed on the SAXD pattern of MFHKUST-1-8. This peak clearly indicates the periodically arranged meso-scale structure,68 which is corresponding to orderly packed layers observed in this superstructures (Figure 2). This type of meso-scale superstructure shows an obvious contrast with ST-HKUST-1 which exhibits no peaks on SAXD in small angles (Figure 3j and Figure S9). With the increasing residence time, the peak intensity in SAXD patterns gradually decreases. This phenomenon indicates that the superstructure gradually vanished and converted into normal monolith crystals, which is accordance with that observed from SEM studies. By indexing the cell parameters of superstructures on the basis of SAXD, it can be found that these superstructures exhibit a hexagonal crystal system, with ultralarge cell parameters that are comparable with the dimension of primary particles (Table S2). The cell parameters of superstructures gradually become larger with the elongated residence time. This phenomenon can be rationalized by the enlarged nanoparticles that assembly into superstructure with the elongated residence time, consistent well with the phenomenon that observed in SEM images.


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The combination of SEM, TEM images and SAXD patterns gives a whole picture of crystal growth for HKUST-1 under MF conditions. At the different points of timeline of crystallization process for HKUST-1, at least three kinds of condensed states of MOFs appear. According to their characteristics and sequence of appearance, these condensed materials can be classified as nanosized primary particles of MOF, superstructure of MOF and micro-size true crystals, respectively. The superstructure is a meso-scale assembly formed by the periodically alignment of nano-sized primary particles of MOFs, and it can be recognized as the imprint left by attachment of particles. The appearance of this type of superstructure accompanied by their gradual disappearance upon increasing time are signatures of CPA process, a non-classical pathway of crystallization that has been observed in crystal growth in some multi-ion complexes,69 oligomers70 and nanocrystal.71-73 The oriented attachment of nano-sized primary particles is the main cause of superstructure of HKUST-1. The appearance of small aggregates at short residence time combined with the following disappearance of these aggregates and the increment of crystal sizes indicate an Ostwald ripening process, which is also an essential step in crystallization process dominated by CPA mechanism.30 Based upon above phenomenon, it can be concluded that the crystallization of HKUST-1 under MF condition undergoes a CPA pathway. According to the characteristics of CPA process,74 it can be supposed that the crystallization of HKUST-1 under MF conditions comprise of the following three stages: a) the formation of nanosized primary particles; b) the oriented attachment of these primary particles into superstructure; c) the ripening of superstructure into single crystals (as illustrated in Figure 4). In detail, at the beginning of crystal growth, the coordination reaction between the H3BTC ligand and Cu2+ ions happen, and numerous nano-sized primary particles of HKUST-1 are generated, giving rise to a colloid solution composed of nano-sized primary particles. These primary particles are usually too


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small to be visible by naked eyes and they are hard be collected by filtration. Then, these primary particles aggregated into a superstructure through a CPA process.67,

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During the course of

aggregation, some rules should be followed. From the SEM image of MF-HKUST-16, it can be found that some nano-sized crystals aggregate with each other with the same orientation and at a certain position (see Figure S10). The intersection lines between these two crystals are parallel to edges of crystals (see inset picture in Figure S10). This phenomenon may reflect some underlying rules of crystal packing: the nano-sized crystals are likely to assembly with other crystals with the same lattice plane to minimize the strain between these crystals as well as entropy of the bulk crystal.76 Guided by this lattice-matching rule, the nano-sized primary particles are prone to assembly with each other at a certain angle and position to match their lattice plane, resulting in the orderly packing of these nano-crystals in superstructure.49 At the early stage of crystal growth process, it can be found that the superstructure of HKUST-1 is composed by the order packing nano-sized particles with irregular shapes. Although these nano-sized primary particles of HUKST-1 do not have regular shapes, they are indeed nano-sized MOF crystals, as revealed by PXRD pattern. In different directions, these nano-sized crystals could have different lattice plane on the surface, exhibiting anisotropic properties in different directions. According to the latticematching rule of CPA process, these nano-sized particles can be aligned and attached together when their lattice plane are matched.30, 77 Thus, these nano-sized crystals prefer to attach with each other with a certain orientation during the CPA process, giving rise to regular polyhedrons rather than disordered aggregations. The anisotropic properties on different lattice plane also results in the packing style of nano-sized particles as well as distinct appearance of crystal surface in different directions. Therefore, the phenomenon observed in this work is agreeable with the general


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principle of CPA process, and it could be another evidence of CPA mechanism for the crystal growth process of HKUST-1. Although the oriented attachment of nano-particles is a main reason that leads to a superstructure, it is not the only factor in crystallization process dominated by CPA mechanism. Indeed, the CPA process is a dynamic process in which attachment and Ostwald ripening of nanoparticles occur simultaneously.78 At the early stage of crystallization of HKUST-1, the time is too short for Ostwald ripening to obliterate the packing defects such as protrusions, pinholes, dislocations and misalignments, as revealed by SEM images (Figure 2b and Figure S2-S6). However, with the elongated residence time, these packing defects gradually being eliminated through the rearrangement or recrystallization of primary particles under the effects of Ostwald ripening. The Ostwald ripening is driven by the effect of Gibbs-Thomson relation in which the particle solubility increases as the radius decreases.79 Caused by this ripening effect, the dislocations or defects on superstructure are gradually repaired at the later stage of crystal growth. Via Ostwald ripening, those small aggregates and dislocated particles are dissolved into primary particles, and in the meantime those vacant sites on large crystals are re-attached by primary particles.79-81 During this process, these small aggregates exchange materials with the large crystals, leading to the more and more smooth surface for large crystals with increasing residence times. The continuous attachment of primary particles on the surface or edges of crystals also leads to increment in crystal dimensions. Under the combined effects of CPA and Ostwald ripening, the superstructure gradually transforms to true single-crystals that resemble the crystals derived from solvothermal synthesis. Thus, the superstructure can be regarded as the intermediate state of crystallization process, bridging the disordered monomers and highly ordered single-crystals of HKUST-1.


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To examine the function of MF conditions in the preparation of HKUST-1, conventional solvothermal synthesis at short reaction times were performed. In clear contrast to fast production of HKUST-1 under MF conditions, no products of HKUST-1 were obtained in the time range of 4~80 min under solvothermal synthesis. This observation highlights the advantages of MF in MOF synthesis, which offers a dynamic process, in contrast to the static reaction environment in conventional solvothermal synthesis. In addition, the movement of droplets caused by the convective flow lead to the high transfer rate of heat and mass,82-85 enabling high reaction speed between ligands and metal ions and fast generation of primary crystalline particles. Guided by the lattice matching rule,86, 87 these nano-sized primary particles align in an ordered way and form the regular shape of the resulting superstructure of HKUST-1. However, the movement of these nanosized primary crystalline particles also increases the possibility of dislocation and misalignments for these primary particles in the CPA process. As a result, the remnant imprints of CPA of primary particles are remained, providing the visualized evidence of CPA at the early stage of crystallization process. To examine the generality of CPA process, we also examined crystal growth process for other MOFs by using a MF technique. Two MOFs: MOF-5 and NOTT-100, were prepared with different residence times under MF conditions, and the morphologies at the different stage of crystal growth for these two MOFs were captured. As revealed by PXRD patterns, the MOF-5 synthesized at residence time of 32, 80 and 160 minutes under MF conditions exhibit a high degree of crystallinity (termed as MF-MOF-5-t, t=32, 80 and 160) (as shown in Figure S11a). Some additional peaks in PXRD patterns at short residence time (32 min) could be ascribed to the immature (by) products or intermediate products of MOF-5. Furthermore, the defects of linkers or metal-nodes in MOF's framework can lead to different lattice strain as well as lattice parameters, which can also give rise


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to different PXRD patterns.88-90 In addition, these samples exhibit distinct morphologies, as revealed by SEM images (as shown in Figure S11b-d). Notably, the MF-MOF-32 shows a type of special “crystals” which has a regular shape of crystals but rough surfaces (as shown in Figure S11b). If magnified, it can be found that these special “crystals” are indeed formed by the assembly of a large number of small particles with a similar shape and size (see Figure S11b, inset). This type of structure can be viewed as the superstructure of MOF-5 which is composed by uniform stacking of nano-sized building units. At a prolonged residence time (e.g. 80 min), the obtained sample (MF-MOF-5-80) exhibit a phase of cubic shaped crystals, except of several joints and gaps on crystal surfaces (see Figure S11c). When the residence time is increased to 160 min, the obtained sample exhibit perfect crystals which are featured with regular cubic shapes and smooth surfaces (see Figure S11d). Other than MOF-5, we also monitored the growth process of NOTT-100, a Cu-based MOF with an NbO topology. PXRD patterns also reveal a crystalline phase of NOTT-100, even if it was prepared at a short residence time (16 min) (see Figure S12a). SEM images reveal different morphologies for these samples prepared at different residence time. At a residence time of 16 min, small and round shaped particles with the similar sizes were observed. These small particles aggregated together to form larger clusters. Interestingly, some cubic shaped particles formed by the aggregation of these small particles can be observed (see Figure S12b). These cubic shaped aggregations could be viewed as the embryo of superstructure for NOTT-100. At a residence of 32 min, the superstructure of NOTT-100 which is featured with regular cubic shape but roughness surface is produced (see Figure S12c). Closer inspection reveals boundaries of aggregation for these particles, similar with that observed on superstructures of HKUST-1 and MOF-5 (see Figure


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S12c, inset). When the residence is increased to 80 min, single crystals of NOTT-100 with perfect regular shapes and smooth surface is produced (see Figure S12d). Above experiments provide a clear picture of crystal growth process for HKUST-1, MOF-5 and NOTT-100: at the early stage of growth of crystal growth, superstructure for MOFs is produced; with the increasing residence time, the superstructure transforms to single-crystals with regular shapes and smooth surface under the effects of ripening. The appearance of superstructure and the following transformation of superstructure with increasing residence time suggest that all of three MOFs may undergo a crystal growth pathway dominated by CPA mechanism. From these results, it could be postulated that the CPA mechanism may be the general mechanism which is applicable to other MOFs with different structures or compositions. Owing to their intriguing structures and properties, MOF superstructures have recently aroused particular attentions, and some special preparation methods have been developed. For example, using templates,91, 92 self-templated,93 spontaneous higher-order assembly,94 spray-drying,95 selfassembly of polyhedral colloidal MOF particles67 and et al., have been proven to be effective in constructing MOFs superstructures. Despite of large difference between these methods, similar coarsen and patterned surface were observed in these MOF superstructures. The orientedattachment or CPA of nano-sized MOF particles can give rise to several external morphologies, such as spherical superstructures, hollow superstructures, polyhedral superstructures, accordionlike superstructures, 2-dimensional superstructures and et al.96-98 The results of these studies suggest that the CPA mechanism for MOF crystallization may be applicable to many synthetic methods. The micro-droplet synthesis proposed in this work facilitate us to monitor the different status of crystallization for MOFs during the crystal growth process and validate the CPA


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mechanism. Therefore, we could infer that the CPA mechanism is not limited to microdroplet reaction, in contrary, it is a general phenomenon present in various reaction system for MOFs.

Figure 4. Schematic diagram of the pathway of CPA process for HKUST-1 under MF conditions. The rapid formation of superstructure and relatively slower process of ripening of superstructure indicate the interplay between thermodynamics and kinetics during the course of crystallization. The high transfer rate of heat and mass in MF reaction system can overcome the energy barrier of coordination reaction, thus a great number of primary particles can be obtained in a short time under the large chemical potential due to the high concentration of reactants combined with low solubility of HKUST-1. Owing to their high surface energy,49, 99 these nano-particles can aggregate quickly via CPA, giving rise to the micro-meter superstructure. In contrast with the above rapid process, the ripening of these superstructure is a kinetically controlled process, during which the attachment and dislocation of primary particles happens simultaneously.100,

101

Thus, the

superstructure is a kinetically stabilized state of crystallization. From the thermodynamically view, it is a metastable phase which represents a minimum on the free-energy landscape, and it will be


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replaced by a more stable phase of the single-crystal of HKUST-1 when given enough residence time. To explore pore structure of HKUST-1 at different stages of crystallization, N2 adsorptions at 77 K were performed on the activated MF-HKUST-1-t samples. As shown in Figure 5 and Figure S13, all the samples exhibit the Type-I isotherms. The steep increase in the low pressure range and the plateau in the high pressure range suggest an overall microporous nature for these MOFs. Notably, a shoulder on the isotherm of MF-HKUST-1-8 is observed, which is a characteristic of sequential filling induced by meso-scale structures (see magnified isotherm in Figure 5a, insert).102 The Brunauer-Emmett-Teller (BET) surface areas of MF-HKUST-1-8 and ST-HKUST-1 are calculated as 1794 and 1870 m2∙g-1, with pore volumes of 0.769 and 0.762 cm3∙g-1, respectively. Compared with ST-HKUST-1, MF-HKUST-1-t series show similar surface areas and pore volumes (see pore structure parameters in Table S3). Pore size distribution (PSD) derived from the Nonlocal density function theory (NLDFT) reveals a small portion of meso-pores centered at 3.0 ~ 6.5 nm for MF-HKUST-1-t (Figures 5b, S13 and Table S3), suggesting a small portion of meso-pores. Owing to the special configuration of superstructure, a void space could be expected when nano-sized particles are assembled orderly into a superstructure. To figure out whether these meso-pores were originated from these voids, a simplified model was proposed on the basis of structural features of superstructure of MF-HKUST-1-8 (as shown in Figure S13). Based on geometric relations, a dimension of 4.6 nm of the void space can be determined (see details in Figure S14). This value is consistent to that derived from N2 adsorption analysis, suggesting that these meso-pores could be originated from the void space between the orderly packing nano-scale grains in the superstructures. Upon the elongated residence time, the resulting MF-HKUST-1-t exhibit gradually decreased meso-pore volumes. MF-HKUST-1-80 shows a microporous structure


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which is almost identical with ST-HKUST-1. The variation trend of pore structure with residence time suggests that the meso-scale voids are gradually filled by the micro-porous crystals. Upon a longer residence time, the ripening of superstructures results in the total microporous MOF which resembles ST-HKUST-1, as proved by the results of SEM images and SAXD patterns. Thermal gravimetric analysis (TGA) demonstrates that MF-HKUST-1-t are stable up to 309 oC in air (see TGA plots in Figure S15). The differences in weight loss at plateaus on TGA curves between MF-HKUST-1 and ST-HKUST-1 could be attributed to their different amount of void space which results in different loading of guest molecules involved in the pores of these MOFs. The decomposition temperature of MF-HKUST-1 is only slightly lower than ST-HKUST-1 (~320 oC), suggesting

that the formation of the superstructure of HKUST-1 does not significantly weaken

its stability. The comparable thermal stability also indicates that the thermal stability of MOF is determined by its intrinsic structure of MOF rather than the way of assembly. The infrared (IR) spectra for MF-HKUST-1-t series MOFs exhibit similar patterns as ST-HKUST-1, indicating the same functional groups in these superstructures (see IR spectrum in Figure S16). To further examine their compositions, X-ray photoelectron spectroscopy (XPS) was carried out on the activated samples of these MOFs. Their identical XPS survey spectra, combined with similar high resolution spectra for C 1s, O 1s and Cu 2p, suggest that these superstructures have identical composition and share the same coordination spheres as ST-HKUST-1 (see XPS spectra in Figure S17). It is different from our previous study on MF synthesis of hierarchical porous UiO-66 in which mesopores are caused by the metal cluster defects.90 Above results demonstrate that the chemical composition, functional groups and coordination spheres remain unchanged during the course of crystal growth. These phenomena also support the CPA pathway for crystallization process.


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Figure 5. (a) N2 sorption isotherms at 77 K for MF-HKUST-1-8 and ST-HKUST-1. The insets show the featuring of adsorption sorption hysteresis of MF-HKUST-1-8. (b) Pore size distribution of MF-HKUST-1-8 and ST-HKUST-1 calculated using Nonlocal density function theory (NLDFT). (c) The CO2 adsorption isotherms on MF-HKUST-1-8 and ST-HKUST-1 at 298 K and 0~ 20 bar. (d) The CH4 adsorption isotherms on MF-HKUST-1-8 and ST-HKUST-1 at 298 K and 0~100 bar. The intriguing superstructure of HKUST-1 encourages us to explore its functions on gas adsorption, including CO2 capture and CH4 storage. As shown in Figure 5c, the CO2 isotherms were measured for MF-HKUST-1-8 and ST-HKUST-1 at room temperature and in the pressure range of 0~20 bar. When the pressure increases to 20 bar, MF-HKUST-1-8 shows an absolute uptake of 13.3 mmol∙g-1 of CO2, which is approximately 17 % higher than ST-HKUST-1 (11.4 mmol∙g-1). Figure 5d shows CH4 sorption isotherms that were measured at room temperatures and


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in the pressure range of 0~100 bar. At 298 K and 100 bar, the absolute adsorption capacity of CH4 in MF-HKUST-1-8 is high up to 14.8 mmol∙g-1, which is 6.5 % higher than ST-HKUST-1 (13.9 mmol∙g-1). The corresponding volumetric CH4 uptake capacity is 292.1 v(STP)/v, which is among the highest values of CH4 storage capacity for known porous adsorbents.103-105 The isosteric adsorption enthalpy (Qst) of CO2 and CH4 on MF-HKUST-1-8 are 26.6 kJ∙mol-1 and 14.1 kJ∙mol1

at a loading of 1 mmol·g-1, respectively, which are similar to that on ST-HKUST-1 (Figures S18-

19). Thus, the higher uptake capacities of CO2 and CH4 could be attributed to the functions of superstructure in MF-HKUST-1-t series MOF. The exposed external surface of periodically arranged nano-sized particles in the superstructures can serve as additional active adsorption sites. Given the fact that the CH4 storage capacity of pristine HKUST-1 is among the top ranking for MOFs, the further improvement in CH4 storage capacity is rather challenging. The considerable improvements in CO2 capture and CH4 storage capacity highlights the functions of superstructures in gas adsorption applications. Conclusions In summary, the crystallization process of HKUST-1 was studied in detail by tracking different stages of crystallization along the time axis of crystal growth by using a MF technique. A MOF superstructure which is assembled from oriented attachment of nano-sized particles of HKUST-1 is observed at the early stage of crystallization. TEM validate the primary particles that assembled into the superstructure. Upon increasing residence time, the stripes and lamellar structures which can be viewed as the remnant imprints of particle attachment, gradually disappears under the ripening effect. In addition, SAXD patterns demonstrate that the superstructure of HKUST-1 gradually vanishes with increasing residence times and transfers to single-crystals, given enough time of crystallization. Based on combined results of SEM, TEM images and SAXD patterns for


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crystals obtained at different stages, a process of crystallization by particle attachment (CPA), a non-classical model of crystallization, can be proposed for crystal growth of HKUST-1 under MF conditions. The pathway of crystal growth of HKUST-1 under MF conditions can thus be interpreted as a process with three-stages: the generation of nano-sized primary particles, the oriented attachment of primary particles into superstructures and the ripening of superstructure into single-crystals. The superstructure of HKUST-1 is an intermediate state of crystallization of CPA process that acts as the bridge linking disordered monomers and highly ordered singlecrystals. Pore structural characterization validates the transforming process of superstructure into single-crystals. Furthermore, this type of superstructure exhibits considerable improvements in CO2 capture and CH4 storage capacity, compared to single-crystals of HKUST-1 derived from conventional solvothermal synthesis. The observations in this work are very helpful to understand the mechanism of crystallization for other types of MOFs and may shed a light in the engineering of crystal morphology and functionality for MOFs in the future. Methods/Experimental Materials. Cu(NO3)2∙3H2O and Zn(NO3)2∙6H2O were purchased from Sinopharm Chemical Reagent Co. Ltd., Trimesic Acid (H3BTC), Terephthalic acid (H2BDC) and Biphenyl-3,3′,5,5 ′ -tetracarboxylate (H4bptc) were purchased from Aladdin Reagent Co. Ltd., China., Ethanol (EtOH) and N, N-Dimethylformamide (DMF), 1,4-dioxane and HCl (37%) was purchased from Sinopharm Chemical Reagent Co. Ltd., China. Silicone oil (PMX-200, viscosity is 500 cs) was purchased from Dow Corning, USA. All of these reagents were used without further purification. High-pure (99.999%) nitrogen and carbon dioxide were purchased from Qingdao Tianyuan Gas Co., Ltd., China. High-pure (99.999%) methane was provided by Jiangsu Hongren Gas Co., Ltd., China.


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Synthesis of MF-HKUST-1-t under the microdroplet flow-reaction system. In the homebuilt microdroplet flow-reaction system, the Cu(NO3)2∙3H2O (0.72 g) was added into the mixed solution of deionized water (H2O, 10 mL), ethanol (EtOH, 2.5 mL) and N, N-dimethylformamide (DMF, 2.5 mL); the 1,3,5-benzenetricarboxylic acid (H3BTC, 0.42 g) was added into the mixed solution of Ethanol (EtOH, 2.5 mL) and N, N-dimethylformamide (DMF, 12.5 mL). The two mixed solutions were stirred for 30 min until complete dissolution of the metallic salt and the organic ligand, respectively. Then these solutions were pumped continuously into a static micromixer equipped with a magnetic stirring to form a homogeneous solution.60,

61

At a constant

velocity ratio (υsilicone oil : υreactant = 9:4), the solution of reactants are dispersed into uniform microdroplets by shear forces between the two co-current fluid flows which are immiscible to each other. These micro-droplets pass along the channel (2 mm i.d. and 3 mm o.d.) and enter into the reaction coil that is immersed in an oil heating bath (T = 90︒C). The products are activated by Soxhlet extraction using petroleum ether and acetone as the eluent and is dried at 60 °C for 12 hours. Synthesis of ST-HKUST-1. ST-HKUST-1 was prepared via a conventional solvothermal synthesis. Typically, Cu(NO3)2∙3H2O (0.72 g) and 1,3,5-benzenetricarboxylic acid (H3BTC, 0.42 g) were added into the mixed solutions of H2O (10 mL), ethanol (EtOH, 5 mL), and N,N`dimethylformamide (DMF, 15 mL). The solution was stired for 30 mintues at amibent conditions and was transfered into a Telfon-lined reactor. Then the reactor was heated at 90 ˚C for 24 hours. After cooling to room temperature, filtrated and washed with organic solvent, a blue powder was collected. The obtained sample was activated by Soxhlet extraction using acetone as the eluent and was dried at 60 °C for 12 hours.The thus obtained mixture was stirred and placed in a vessel at 90 ˚C for 24 hours. After cooling to room temperature, filtrated and washed with organic solvent, a


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blue powder was collected. The products are activated by Soxhlet extraction using acetone as the eluent and is dried at 60 °C for 12 hours. Synthesis of MF-MOF-5-t under the microdroplet flow-reaction system. In the same homebuilt microdroplet flow-reaction system, the Zn(NO3)2∙6H2O (1.19 g) and the terephthalic acid (H2BDC, 0.22 g) were desolved N,N-dimethylformamide (DMF, 15 mL), respectively. Then the Zn(NO3)2/DMF and H2BDC/DMF solutions were pumped continuously into the microdroplet system, and the series of MF-MOF-5-t were obtained at the temperature of 130 oC by using the same method as MF-HKUST-1-t. Synthesis of ST-MOF-5. ST-MOF-5 was prepared via a conventional solvothermal synthesis.60 Zn(NO3)2∙6H2O (1.19 g) and the terephthalic acid (H2BDC, 0.22 g) were added into the N,N`dimethylformamide (DMF, 30 mL). Then, the solution was transferred into a Teflon-lined reactor and heated at 130 ˚C for 24 hours. Synthesis of MF-NOTT-100-t under the microdroplet flow-reaction system. In the same home-built microdroplet flow-reaction system, the Cu(NO3)2∙3H2O (0.28 g) was dissolved into the mixed solution of H2O (7.5 mL), 1,4-dioxane (7.5 mL) and 2 drops of aqueous HCl (37%); the biphenyl-3,3′,5,5′-tetracarboxylate (H4bptc, 0.1 g) were dissolved into the mixed solution of N,N-dimethylformamide (DMF, 15 mL) and 2 drops of aqueous HCl (37%). Then these solutions were pumped continuously into the microdroplet system respectively and the series of MF-MOFNOTT-100-t were obtained at the temperature of 90 oC by using the same method of MF-HKUST1-t. Synthesis of ST-NOTT-100. ST-NOTT-100 was prepared via a conventional solvothermal synthesis.106 Cu(NO3)2∙3H2O (0.28 g) and the biphenyl-3,3′,5,5′-tetracarboxylate (H4bptc, 0.1


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g) were dissolved in the mixed solution of H2O (7.5 mL), 1,4-dioxane (7.5 mL), N,Ndimethylformamide (DMF, 15 mL) and 4 drops of aqueous HCl (37%). Then, the solution was transferred into a Teflon-lined reactor and heated at 90 ˚C for 24 hours. Characterization. Powder X-ray Diffraction (XRD) analysis of the MF/ST-HKUST-1-t, MF/ST-MOF-5-t and MF/ST-NOTT-100-t series were used to confirm the crystallinity as well as the phase purity of the simulated pattern of HKUST-1, MOF-5 and NOTT-100. Powder X-ray diffraction (XRD) data were performed on a XRD-7000 diffractometer (Shimadzu, Japan) with Cu Kα radiation from 5 to 70 o (λ = 1.5406 Å). Small-angle X-ray Diffraction (0.5~10°) was performed on D8 ADVANCE and DAVINCI DESIGN. Thermal gravimetric analysis (TGA) experiments were conducted on a ZRT-A thermogravimetric analyzer (Gaoke instrument Co. Ltd., China.), with a heating rate of 10 °C/min. Fourier transform infrared (FT-IR) measurements were performed on a FTIR-850 spectrometer (Guangdong instrument Co. Ltd., China.). And the spectra were collected from 4000 to 500 cm-1. X-ray photoelectron spectroscopy (XPS) spectra is conducted on an EscaLab 250Xi photoelectron spectrometer. The shift of the binding energy due to the surface electrostatic charging is corrected using the C 1s as an internal standard at 284.6 eV. The scanning electron microscopy (SEM) images were acquired using a Merlin microscope (Carl Zeiss, Germany). The TEM images were obtained using a JEM2100F transmission electron microscope (JEOL Co. Ltd., Japan). Nitrogen sorption isotherms were measured at 77 K using an Autosorb volumetric gas sorption analyzer (Quantachrome, USA). All of samples were degassed in vacuum for 12 h at 100 °C to fully remove guest molecules prior to analysis. The pore size distribution is obtained from Non-local density function theory (NLDFT) based on N2 isotherms. High pressure CO2 and CH4 adsorption isotherms were measured by using a XEMIS magnetic suspension balance sorption analyser (Hiden, UK) equipped with a circulating water bath. Before


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sorption measurements, samples were activated at 100 °C for 8 h under ultrahigh vacuum (10-6 mbar). The pressure range of CO2 and CH4 are increasing up to 20 bar and 100 bar, respectively.

ASSOCIATED CONTENT Supporting Information. Supplementary Information (SI) available: SEM, TEM images, TGA plots, IR, XPS, adsorption isotherms, pore structure parameters, calculation of mesopores and adsorption enthalpies. This material is available free of charge via the internet at http://pubs.acs.org. AUTHOR INFORMATION Corresponding Author *Xuebo Zhao Email: [email protected] Liangjun Li Email: [email protected] ACKNOWLEDGMENTS This work was supported by grants from the Natural Science Foundation of China (Grant No. 21473254 and 21401215), the Special Project Fund of “Taishan Scholars” of Shandong Province (ts201511017) and the Fundamental Research Funds for the Central Universities (18CX02068A).

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