Hierarchically Porous MetalâOrganic Frameworks: Green Synthesis

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Materials and Interfaces

Hierarchically porous metal-organic frameworks: green synthesis and high space-time yield Feier Li, chongxiong Duan, Hang Zhang, Xin Yan, Jinqing Li, and Hongxia Xi Ind. Eng. Chem. Res., Just Accepted Manuscript • DOI: 10.1021/acs.iecr.8b00470 • Publication Date (Web): 18 Jun 2018 Downloaded from http://pubs.acs.org on June 29, 2018

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Industrial & Engineering Chemistry Research

Hierarchically porous metal-organic frameworks: green synthesis and high space-time yield Feier Li, † Chongxiong Duan, † Hang Zhang, † Xin Yan, † Jinqing Li, † Hongxia Xi†, ‡ * †

School of Chemistry and Chemical Engineering, South China University of Technology,

Guangzhou 510640, P. R. China. ‡

Guangdong Provincial Key Laboratory of Atmospheric Environment and Pollution Control,

South China University of Technology, Guangzhou Higher Education Mega Centre, Guangzhou 510006, P. R. China

ABSTRACT Two major goals of hierarchically porous metal-organic frameworks (HP-MOFs) research are the reduction of energy consumption and the enhancement of production rate. Here, we develop a simple and versatile method for the rapid synthesis of three HP-MOFs (Cu-BTC, Cu-BDC, and ZIF-8) through introducing zinc oxide (ZnO) and template simultaneously. The synthesis condition was facile and the synthesis time could be shorten to 30 min. Moreover, the porosity and morphology of HP-MOFs could be adjustable with variable amounts and types of template. A synergistic effect of the hydroxy double salt (HDS) and template is crucial for the synthesis, whereas no meso- and macropores were formed in the individual employing of either the HDS or the template. The rapid synthesis mechanism was further elucidated by the mesoscopic dynamics simulation. Furthermore, the synthetic approach of HP-MOFs is readily scalable with a space–time yield (STY) of up to 1322 kg·m−3·d−1, which allows the large-scale production. The prepared HP-MOFs exhibited improved catalytic activity as compared with

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microporous MOFs for the Henry reaction involving large molecules. This synthetic route with low energy consumption and high production rate may open a door for the large-scale production and a wide range of application of HP-MOFs.

Introduction Porous materials, such as mesoporous silica,1 hierarchical zeolite,2 activated carbon,3, 4 and metal-organic frameworks (MOFs),5-8 have been widely applied in the fields of gas storage,8-10 adsorption/separation,11-13 supercapacitors,14 and catalysis,15, 16 due to their possession of high specific surface area, superior adsorption capacity, and excellent catalytic activity.6,

17-19

However, challenges remain in the desirable fabrication of porous materials. For example, conventional porous materials are relatively difficult in precisely designing and modifying their structure and function, especially at the level of molecular or atomic.20,

21

By contrast,

hierarchically porous MOFs (HP-MOFs), constructed through the self-assembly of organic ligands and metal ions (or metal clusters), can easily meet these requirements,5, 22 thus, HPMOFs have recently received increased attention.23-25 To date, six synthetic strategies of HP-MOFs with desirable functionalization have been developed including ligand-extension method,18,

26

mixed-ligand method,27 post-modification

method,28 crystal defects method,24 composite method,29 and template method.25,

30, 31

These

methods could meet with various demands appearing in practical applications.32-35 However, with respect to the large-scale production and commercial application of HP-MOFs, several key factors are indispensable to be considered:36 (i) A facile synthesis route that avoid the harsh synthetic conditions such as high temperature and pressure, will reduce operation cost whilst minimizing environmental pollution. (ii) A high space-time yield (STY, kg of HP-MOF


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produced per m3 of reaction mixture per day) will increase the production rates and be available to scale up. (iii) A versatile method is crucial to accommodate various HP-MOFs. With respect to the first challenge, in previous studies, three HP-MOFs (hierarchical Cu-BTC, ZIF-8, and ZIF90) have been fabricated at room temperature by template method.37 However, the synthesis time still exceeds 20 h, resulting in a low production rates. For example, the STYs of HP-MOFs synthesized with a long time (> 20 h) are commonly less than 300 kg·m−3·d−1.38 For the increase of STYs, Zhao et al.39 reported a facile synthetic route of MOFs with a record STY (3.6 × 104 kg·m−3·d−1) by employing (Zn, Cu) hydroxy double salt (HDS), which exhibits greatly promising potential toward the large-scale production of MOFs. But the obtained MOF products only possess micropores rather than hierarchical pores. In addition, there are only a few of reports concerns about resolving the challenges of both the synthesis condition and STY simultaneously. For example, Huo et al.40 reported the room-temperature synthesis of hierarchically porous HKUST-1 by tuning the copper source and reaction time, with a maximum STY of up to 2035 kg·m−3·d−1, demonstrating the viability in scale-up production of HP-MOFs. However, challenges still remain in popularizing the synthesis of other HP-MOFs. Therefore, a versatile and facile method to synthesize various HP-MOFs with high STY is highly desired. In this work, a simple, green and versatile synthetic route for three HP-MOFs (Cu-BTC, CuBDC, and ZIF-8) has been developed, through the simultaneous introduction of ZnO and template (e.g., 1-bromodecane). The synthetic procedure can be accomplished in 30 min at room temperature and pressure and the morphology and porosity of the HP-MOFs can be controlled by tuning the amount and type of template. During the synthesis, ZnO was employed to accelerate framework construction through the HDS layer formed by a fast reaction with Cu(NO3)2, while the template micelles guided the formation of meso- and macropores through a synergistic effect


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with the HDS. The formation of template micelles has been elucidated elaborately by mesoscopic dynamics simulation. In addition, our rapid synthetic approach for HP-MOFs is readily scalable with an STY as high as 1322 kg·m−3·d−1, which exhibits enormous potentiality in the large-scale production. The prepared HP-MOFs exhibited improved catalytic activity as compared with microporous MOFs for the Henry reaction involving large molecules. Experimental Section Rapid room-temperature synthesis of hierarchically porous Cu-BTC Zinc oxide (ZnO, 3.6 mmol) powder was dispersed in the mixture of 8 mL deionized water and 16 mL of dimethylformamide (DMF) under ultrasonic treatment for 15 min. Then, Cu(NO3)2·3H2O (7.2 mmol) was dissolved in 8 mL deionized water and added to the stirring ZnO nanoslurries for 5 min (denoted as solution A). Next, template (1-bromodecane, 7.2 mmol) and linker trimesic acid (4 mmol) were dissolved in 16 mL ethanol (denoted as solution B). Under fast magnetic stirring, solution A was added into solution B. After stirring for 30 min at room temperature and pressure (RTP), the suspension crystal was collected by suction filtration and then extracted with ethanol for 4 times in 2 days. The obtained product was vacuum dried under 393 K, denoted as Cu-BTC_A1 (molar ratio T/Cu2+ = 1.0, where T represents template). Similarly, HP-MOF products synthesized with various T/Cu2+ molar ratios are denoted as CuBTC_A2 (T/Cu2+ = 2.0) and Cu-BTC_A3 (T/Cu2+ = 4.0), respectively. Moreover, the HP-MOFs synthesized with different kinds of template (T/Cu2+ = 2.0) are denoted as Cu-BTC_B1 (1bromotetradecane), Cu-BTC_C1 (1-bromohexane), Cu-BTC_D1 (tetradecanonitrile) and CuBTC_E1 (hexylamine), respectively. Control sample (conventional Cu-BTC) was prepared through solvothermal route at 393 K,7 denoted as C-Cu-BTC.


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Rapid room-temperature synthesis of hierarchically porous ZIF-8 ZnO (5 mmol) powder and zinc acetate ((C4H6O4)Zn·2H2O, 5 mmol) were dispersed in the mixture of 10 mL of deionized water and 5 mL of DMF, and stirred for 24 h. 3 mL mixture was taken out, denoted as solution A. 2-Methylimidazole (6 mmol) and template 1-bromodecane (2 mmol) were dissolved in 9 mL of DMF (denoted as solution B). Under magnetic stirring, solution A and Solution B were mixed and stirred for 30 min at RTP. The suspension was collected by suction filtration and then extracted with ethanol for 4 times in 2 days. The obtained product was vacuum dried under 423 K, denoted as HP-ZIF-8. Rapid room-temperature synthesis of hierarchically porous Cu-BDC Similarly, zinc oxide powder (3.6 mmol) and Cu(NO3)2·3H2O (7.2 mmol) were dispersed in 20 mL DMF (denoted as solution A) under ultrasonic treatment for 15 min. Template 1bromodecane (14.4 mmol) and linker terephthalic acid (7.2 mmol) were dissolved in 20 mL DMF (denoted as solution B). Under magnetic stirring, solution A was added into solution B, and the mixture was stirred for 30 min at RTP. The suspension crystal was collected by suction filtration and then extracted with ethanol for 4 times in 2 days. The obtained product was vacuum dried under 393 K, labeled as HP-Cu-BDC. Catalyse the Henry reaction In a Schlenk tube with a magnetic bar under a N2 atmosphere, 2.4 mL of nitromethane (CH3NO2), 0.2 mmol of 4-nitrobenzaldehyde and 0.02mmol of catalyst were added. The mixture was heated at 343 K for 24 h. Then the reaction was quenched, the solvent was removed by rotary evaporator, and the product was purified by flash column chromatography on silica gel


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(EtOAc/petroleum ether=5:1) as eluent to yield the product. The identity of the obtained product was performed by the 1H-NMR spectroscopy. Results and discussion Figure 1 presents the powder wide-angle X-ray diffraction (XRD) patterns and Fourier transform infrared (FTIR) spectroscopy of Cu-BTC_An (n = 1, 2, 3) and C-Cu-BTC. As shown in Figure 1a, the high-intensity characteristic diffraction peaks of all Cu-BTC_An are in good agreement with the C-Cu-BTC and the simulated results, indicating the obtained products are crystalline Cu-BTC.25 No additional peaks are observed, confirming the absence of crystalline impurities.41 However, the reflection peak intensities of Cu-BTC_An are inferior to C-Cu-BTC, most likely owing to the effect of the template on the MOFs crystallization.37 From Figure 1b, the absorbance in the 1600 cm-1 is related to the anti-symmetric stretching vibration, and the doublet peaks at 1450 cm-1 and 1370 cm-1 are associated with the symmetrical stretching vibration of carboxylate group. The stretching vibration of C-O-Cu is responsible for the adsorption peaks at 1020 cm-1 and 940 cm-1. The presence of doublet peaks near 750 cm-1 can be ascribed to the fact that the groups on the benzene ring are replaced by Cu ions. These results indicates that the infrared characteristic absorption peaks of Cu-BTC_An are consistent with the result of XRD diffraction.


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Figure1. (a) Powder wide-angle X-ray diffraction patterns and (b) FTIR patterns of various samples. Figure 2 exhibits the N2 adsorption–desorption isotherms and pore size distributions (PSDs) of Cu-BTC_An (n=1, 2, 3) and C-Cu-BTC samples. As shown in Figure 2a, the N2 adsorption– desorption isotherms of C-Cu-BTC showed a type I curve (provided by IUPAC),42 which is a typical characteristic of microporous materials.30 By contrast, the N2 adsorption–desorption isotherms of Cu-BTC_An exhibited a combination of type I and type IV model. At the low relative pressure of P/P0 < 0.4, the isotherms have a good correlation with Freundlich isotherm model43 with high adsorption capacity due to the abundant microporous structure. While at the high relative pressure of P/P0 > 0.4, the adsorption capacity increased again after saturation, ascribed to the capillary condensation of guest molecules in the larger channels.44 However, these phenomena were absent in the C-Cu-BTC. These analyses confirm the coexistence of hierarchical pores in the Cu-BTC_An samples. From Figure 2b, other than the micropores with a diameter of 0.9 nm, all Cu-BTC_An MOFs contain a broad pore size distributions from 30nm to 100nm, which were absent in C-Cu-BTC MOF. The porosity properties of Cu-BTC_An and CCu-BTC MOFs are listed in Table S2. With the increase of molar ration of T/Cu2+, the BET


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surface area (SBET) and total pore volume (Vt) decreased, respectively. Moreover, the Smeso/Smicro ratio of mesopore surface area (Smeso) to micropore surface area (Smicro) increased from 0.06 to 0.18, while the mesopore volume (Vmeso) increased from 0.09 cm3·g−1 (Cu-BTC_A1) to a maximum value of 0.15 cm3·g−1 (Cu-BTC_A2), and then decreased to 0.08 cm3 g−1 (CuBTC_A3). It is clear that the porosities of Cu-BTC_An could be controlled by changing the amount of template, which allows us to tailor HP-MOFs with desire porosities easily, such as SBET and Vmeso.

Figure 2. (a) The N2 adsorption–desorption isotherms and (b) pore size distributions (PSDs) of various samples. The morphology of Cu-BTC_An samples could be clearly observed in the SEM and TEM images. As shown in Figure 3, the morphology of Cu-BTC_A1 maintains a basic octahedral structure, similar to the C-Cu-BTC (Figure 3a). However, apparent macroscopic trenches can be observed on the surface of Cu-BTC_A1 crystal, resembling the etched samples reported by ElHankari et al45. With the increase of the amount of template (1-bromodecane), the morphologies of Cu-BTC_A2 and Cu-BTC_A3 present a defective polyhedron structure, as shown in Figure 3b-c. More visual and definitive interpretations of the local porous structural of Cu-BTC_An


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materials were observed by using TEM. As shown in Figure 3d-f, the TEM images of CuBTC_An clearly show the defect and hollow structure of the as-synthesized samples, but the different morphology can be observed in the Cu-BTC_An (n =1, 2, 3) samples. These analyses indicate that the morphologies of HP-MOFs depend on the amount of template.

Figure 3. The SEM and TEM images of various samples: (a) and (d) Cu-BTC _A1, (b) and (e) Cu-BTC _A2, and (c) and (f) Cu-BTC _A3. The weight loss curves of samples were obtained by using thermogravimetric analysis (TGA). As shown in Figure S3a, low-boiling molecules such as moisture were evaporated first, corresponding to the initial stage of the thermal gravimetric curves. The continuous weight loss between 100 ℃ and 220 ℃ could be ascribed to the loss of the molecules trapped in the channels. The framework of Cu-BTC crystal started to decompose after 300 ℃ with a sharp mass loss in the curve. The as-synthesized Cu-BTC_An samples show clearly identical weight change tendency compared to the C-Cu-BTC, indicating the Cu-BTC_An samples have good thermal stability. The purity of samples were discussed by energy dispersive spectrometer. As shown in


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Table S3, all the ratio of Cu/Br are far below the ratio of added T/Cu2+, indicating the removal of the template. In order to assess the versatility of our synthesis strategy, other four template molecules with similar structure were carried out under the same synthesis conditions, and the resulting products are labelled as Cu-BTC_X1, in which X (X = B, C, D, E; B: 1-bromotetradecane, C; 1bromohexane, D: tetradecanonitrile, E：hexylamine; molar ratio of T/Cu2+ = 2.0) represents different template (Table S4). The XRD patterns and FTIR spectroscopy of the obtained CuBTC_X1 products match well with that of C-Cu-BTC (Figure S4), respectively, confirming that these products synthesized from different template are crystalline Cu-BTC. The SEM and TEM images of Cu-BTC_X1 revealed a clear void structures, as shown in Figure S5. The coexistence of micro-, meso- and macropores in Cu-BTC_X1 were further verified by the N2 adsorption– desorption isotherms and the PSDs (Figure S6). The as-synthesized samples also exhibited good thermal stability (Figure S3b). Nevertheless, samples Cu-BTC_X1 (X = A-E) show a minor discrepancy in weight change curves indicating that the thermal stability of the HP-MOFs was affected by the type of template. In addition, we note that among Cu-BTC_X1 (X = A-E) samples show different adsorption capacities and hysteresis loop shapes, implying the difference in porosity.46 As shown in Table S5, the type of template significantly affected on hierarchical structure fabrication. The maximal ratio of Vmeso/Vmicro belongs to sample Cu-BTC_E1 synthesized with template C6H15N while larger SBET could be observed in those sample synthesized with template C14H29Br and C14H27N. These results indicate that the porous framework of the HP-MOFs is highly adjustable, depending on the type of template. In order to investigate the rapid room temperature synthesis mechanism at room temperature and pressure, other three control experiments were carried out (Supporting Information): (i) The


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addition of ZnO individually in the Cu-BTC synthesis system, and the obtained sample is labeled as Cu-BTC_A1-1X; (ii) The addition of 1-bromodecane individually in the synthesis system, while no precipitate was obtained; and (iii) Conventional solvothermal method synthesis of CuBTC with 1-bromodecane as template at 12 h for 120 °C, and the obtained product is denoted Cu-BTC_A1-1Y. The XRD patterns of Cu-BTC_A1-1X and Cu-BTC_A1-1Y indicate that the obtained products are Cu-BTC crystalline (Figure S7). However, the N2 adsorption–desorption isotherm, PSD, SEM and TEM data indicate the absence of meso-/macropores in Cu-BTC_A11X and Cu-BTC_A1-1Y (Figures S8–11). These analyses demonstrated that the simultaneous introduction of a hydroxy double salt (HDS) and surfactants had a synergistic effect, whereas no meso-/macropores were formed in the presence of either the HDS or the surfactants individually. During the synthetic process, the template played a critical role on tuning porosity and morphology of HP-MOFs.47, 48 Hence, the study on the phase behavior of template is beneficial to understand the MOFs synthesis mechanism.49 Thus, the micelle formation process of 1bromodecane was elucidate by mesoscopic dynamics (MesoDyn) simulation50-52. 1-bromodecane and the trimesic acid in ethanol solution were selected as the objects, treated as a Gaussian chain (Figure S1). The process of mesoscopic phase and order parameter changed with simulation time are shown in Figure 4. At the initial stage, the solution transform from a homogeneous phase system (Figure 4a1) to a continuous phase system rapidly, which can be attributed to that the template with a hydrophobic alkyl chain and a hydrophilicity group self-assemble into spherical micelle (Figure 4a2). A sharp increase of phase order parameter could be observed in Figure 4b (stage І), correspondingly. After a steady shearing action was introduced, the order parameter exhibited a rapid decrease and then increased immediately (stage ІІ), corresponding to the transformation of spherical micelles into columnar micelles (Figure 4a3). Finally, an equilibrium


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system was formed (Figure 4a4) while the order parameter remained stable (stage ІІІ). These results revealed the formation of micelles, which is valuable for investigating the synthesis mechanism. Furthermore, the geometry optimization on template 1-bromodecane were carried out and molecular electrostatic potential (MEP) maps of template 1-bromodecane were calculated by DFT-B3LYP method.53 As shown in Figure 4c, the negative potential region (red), related to electrophilic reactivity,

54

is mainly located on the hydrophobic group, indicating that

this moiety is the preferable site for co-assembly with MOF precursors.53, 55 The MEP maps of other template used in this work are displayed in Figure S12.

Figure 4. (a) The snapshots of micelle at (a1) 0ms, (a2) 0.1ms, (a3) 0.3ms and (a4) 1.0ms, where the different beads represent: red for -CH2- segment, green for -CH2-Br segment, blue for benzene ring, orange for carboxyl group, cyan for ethanol ; (b) Time evolution of the order parameter P during the mesoscopic phase formation process, which can be divided into stage І, ІІ and ІІ, respectively ; (c) The molecular electrostatic potential (MEP) maps of 1-bromodecane;


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Upon the basis of the above conclusions and previous literatures,56 a possible mechanism for rapidly synthesizing HP-MOFs at room temperature and pressure is illustrated in Scheme 1. Prior to the MOFs synthesis, the template molecules self-assemble into micelles when the concentration of template is greater than the critical micelle concentration (CMC).57 Then, the (Zn, Cu) hydroxy double salt ((Zn, Cu)(OH)NO3) has been prepared by dispersing the ZnO in a solution of Cu(NO3)2, which is comprised of the cationic sheets (Cu2+ and Zn2+) connected by NO3- anions.58 After that, the BTC3- solution, containing the template micelles, has been exposed to (Zn, Cu)-HDS, the MOFs precursor are formed when the NO3- anions are replaced by the BTC3- due to the anion exchange capacity of HDS. In addition, the anion exchange process enhances the electrostatic interaction between MOFs precursor and micelle, thus the MOFs precursor self-assemble under the direction of the template and crystalline on the surface of surfactant micelle. The meso-/macropores were formed after the removal of the template, hence the pore structure of the obtained MOFs depends on the size of template micelles. Besides, the amount and size of micelles are influenced greatly by the template concentration. For example, insufficient micellar are formed under the condition of lower template concentration, less extend pores will be created. In contrast, excessive micellar will occupy the react volume and thus hinder the MOFs crystallizing. Thus the porosity of the HP-MOFs is adjustable by the amount of the template used. Furthermore, the different porosities of Cu-BTC _X1 (X = A-E) might be mainly attributed to the amount of micelle (Table S5), owing to the CMC of the template depends on the structural formula. Under the same ratio of T/Cu2+, the extended long carbon chain will lead to the increase of the CMC thus the insufficient micellar, resulting in the increase of SBET and decrease of Vmeso/Vmicro (Cu-BTC _B1 and Cu-BTC _D1).


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Scheme 1. A possible mechanisms for the rapid room-temperature synthesis of HP-MOFs Synthetic conditions and production rates are important factors in determining the possibility of materials for the large-scale production and application 36, 59 Comparing with previous reports that templating synthesis of HP-MOFs with a long synthesis times at high temperature and pressure, our synthesis strategy allows the HP-MOFs to be synthesized within 30 min at room temperature and pressure. Besides, the STY is a key parameter for the scale-up production.60 The STYs of the H-Cu-BTC and industrial Cu-BTC materials synthesized in this work are summarized in Table 1. Czaja et al. 61reported the minimum STY of industrial MOF synthesis as 20 kg·m−3·d−1 (Basolite F300, Fe-EMOF, Table S6), which could be recognized a critical value for industrial process viability of HP-MOF materials. But the STY of the as-synthesized HP-CuBTC (Cu-BTC_A2) was as high as 1322 kg·m-3·d-1, two orders of magnitude higher than the critical value as well as surpassing that of Basolite C300 (Cu-BTC-MOF, 225 kg·m−3·d−1),


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indicating that the facile route for scaling-up the synthesis of HP-MOFs to an industrial level is feasible. Table 1. STYs of industrial MOFs and the HP-MOF sample. Time

SBET [m2·g−1]

STY [kg·m−3·d−1]a

H-Cu-BTC

30

673

1322

Basolite C300

－

1680

225

Sample

b a

Space-time yield, STY=m MOFs/Vsolution•τ×1.44×106 kg·m−3·d−1, where m MOFs represents the dried mass (g) of the obtained HP-Cu-BTC product, Vsolution represents the total volume (cm3) for the mixture, and ߬ represents the reaction time (min) ;b Commercial Cu-BTC sample, data from Ref61. The hierarchically porous structure can be beneficial for catalytic reactions involving large

molecules, in which the diffusion rate and mass transfer resistance of reactant or product molecules into/toward the internal active sites are important.32 Here, the catalytic activities of the HP-MOF materials were investigated using the Henry reaction of 4-nitrobenzaldehyde with nitromethane as a probe reaction that involves bulky reactant and product molecules (Figure S13). As shown in Table 2, H-Cu-BTC (26.3% yield, entry 2) exhibited a higher activity for this reaction than that of C-Cu-BTC (16.5% yield, entry 1) (Table 2). Taking into account that the surface area of Cu-BTC_A2 (672.89 m2·g−1) is much smaller than that of C-Cu-BTC (1399 m2·g−1, Table S2), the enhanced catalytic activity of Cu-BTC_A2 can be attributed to the increased amount of meso-/macropores in Cu-BTC_A2. Similar conclusion has been drawn by Huang et al.62 and Xue et al.,63 in which the catalytic sites are more accessible for large substrates and products diffusing through meso-/macropore channels. Therefore, the introduction of extended pores into conventional MOFs will be favorable for the enhanced catalytic efficiency of MOFs in a reaction involving large molecules.


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Table 2. Henry reaction of 4-nitrobenzaldehyde with nitromethane catalyzed by different MOF materials. Entry

Catalysts

Yield (%)a

1

C-Cu-BTC

16.5

2

Cu-BTC_A2

26.3

a

The yield calculated by the mass ratio of actual products to theoretical products obtained from stoichiometry, as Yield=m p-actual/m p-calculated×100% In addition, hierarchically porous Cu-BDC (HP-Cu-BDC) and ZIF-8 (HP-ZIF-8) were successfully fabricated in 30 min at room temperature and pressure. The XRD patterns of the obtained HP-Cu-BDC product were consistent with the simulated one (Figure S15), confirming that the resultant product is crystalline Cu-BDC. The N2 adsorption-desorption isotherms of HPCu-BDC exhibited a combination of type I and type IV model with a hysteresis loop (type H3), confirming the coexistence of hierarchical porous structure in HP-Cu-BDC (Figure S16a), which was further proved by the PSDs in Figure S16b. Similarly, HP-ZIF-8 displayed a hierarchical structure, as proved by the XDR, N2 adsorption-desorption and PSD analyses (see Figure S17-18 in Supporting Information for detailed data). Moreover, the as-synthesized samples also exhibited good thermal stability (Figure S19). Conclusions In summary, the present work demonstrates a simple and versatile method for the rapid synthesis of various HP-MOFs (Cu-BTC, Cu-BDC, and ZIF-8). The synthetic procedure can be accomplished at room temperature and pressure through employing ZnO and template simultaneously. Various organic molecule can be applied as template to synthesize various HPMOFs by this method. The pore structure and crystal morphology can be easily tuned by


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adjusting the amount and type of template. In the synthetic process, the formed HDS accelerates the rate of anion exchange while the template micelle directs the formation of extend pores. However, no meso-/macropores were formed in the individual presence of either the HDS or the templates. The formation of template micelle was further verified with mesoscopic dynamics simulation. In addition, our rapid synthetic approach is readily scalable with a space–time yield (STY) of up to 1322 kg·m−3·d−1, which will allow ton-scale production of this material. The resulting HP-MOF exhibited enhanced catalytic activity for a Henry reaction (yield, 26.3%) compared to that of a conventional microporous MOF (yield, 16.5%). The increased catalytic activity can be attributed to the fast diffusion rate and reduced resistance of mass transfer through the meso-/macropores channels in HP-MOFs. Overall, the rapid room-temperature synthesis strategy developed in this work should be very promising for the large-scale industrial synthesis of various HP-MOFs with excellent performance for a wide range of industrial applications, e.g. gas adsorption, separation, sensors, catalysis, etc. ASSOCIATED CONTENT Supporting Information. The following files are available free of charge. The synthesis process of samples C-Cu-BTC, Cu-BTC_A1-1X, Cu-BTC_A1-1Y, the simulation details and more detailed results for the as-synthesized samples as Figure S1to S19, Table S1 to S6 (PDF) AUTHOR INFORMATION Corresponding Author *E-mail: [email protected]


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Notes The authors declare no competing financial interest. ACKNOWLEDGMENT We gratefully acknowledge the financial support from the National Natural Science Foundation of China (No. 21576094), SRFDP (No.20130172110012), Guangdong Natural Science Foundation (2017A030313052). ABBREVIATIONS HP-MOFs, hierarchically porous metal-organic frameworks; RTP, room temperature and pressure; HDS, hydroxy double salt; STY, space–time yield; CMC, critical micelle concentration; PSDs, pore size distributions. REFERENCES (1) Wu, S. H.; Mou, C. Y.; Lin, H. P. Synthesis of mesoporous silica nanoparticles. Chem. Soc. Rev. 2013, 42, (9), 3862-75. (2) Serrano, D. P.; Escola, J. M.; Pizarro, P. Synthesis strategies in the search for hierarchical zeolites. Chem. Soc. Rev. 2013, 42, (9), 4004-4035. (3) Sá, J.; Goguet, A.; Taylor, S. F. R.; Tiruvalam, R.; Kiely, C. J.; Nachtegaal, M.; Hutchings, G. J.; Hardacre, C. Influence of Methyl Halide Treatment on Gold Nanoparticles Supported on Activated Carbon. Angew. Chem. 2011, 50, (38), 8912-6. (4) Tang, Y. B.; Liu, Q.; Chen, F. Y., Preparation and characterization of activated carbon from waste ramulus mori. Chem. Eng. J. 2012, 203, (5), 19-24.


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Abstract Graphics


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Prior to the MOFs synthesis, the template molecules self-assemble into micelles when the concentration of template is greater than the critical micelle concentration (CMC).57 Then, the (Zn, Cu) hydroxy double salt ((Zn, Cu)(OH)NO3) has been prepared by dispersing the ZnO in a solution of Cu(NO3)2, which is comprised of the cationic sheets (Cu2+ and Zn2+) connected by NO3- anions.58 After that, the BTC3- solution, containing the template micelles, has been exposed to (Zn, Cu)-HDS, the MOFs precursor are formed when the NO3- anions are replaced by the BTC3- due to the anion exchange capacity of HDS. In addition, the anion exchange process enhances the electrostatic interaction between MOFs precursor and micelle, thus the MOFs precursor self-assemble under the direction of the template and crystalline on the surface of surfactant micelle. The meso-/macropores were formed after the removal of the template, 188x97mm (300 x 300 DPI)


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