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Shape Control of Zn(II) Metal−Organic Frameworks by Modulation Synthesis and Their Morphology-Dependent Catalytic Performance Mohammad Yaser Masoomi, Saeideh Beheshti, and Ali Morsali* Department of Chemistry, Faculty of Sciences, Tarbiat Modares University, Tehran, Islamic Republic of Iran S Supporting Information *

ABSTRACT: Micro- and nanorods and plates of three porous Zn(II)-based metal−organic frameworks, [Zn2(oba)2(4-bpdb)]n·2(DMF) (TMU-4), [Zn(oba)(4-bpdh)0.5]n·1.5(DMF) (TMU-5), and [Zn(oba)(4-bpmb)0.5]n 1.5(DMF) (TMU-6) were synthesized by the coordination modulation method. The effects of concentration of modulator, temperature, and time of reaction on size and morphology have been investigated. Also, catalytic performance of TMU-5 nanosized metal−organic framework in Knoevenagel condensation reaction was evaluated.

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three MOFs, [Zn2(oba)2(4-bpdb)]n·2(DMF) (TMU-4), [Zn(oba)(4-bpdh)0.5]n·1.5(DMF) (TMU-5), and [Zn(oba)(4bpmb)0.5]n 1.5(DMF) (TMU-6) (H2oba = 4,4-oxybisbenzoic acid, 4-bpdb = 1,4-bis(4-pyridyl)-2,3-diaza-1,3-butadiene, 4-bpdh = 2,5-bis(4-pyridyl)-3,4-diaza-2,4-hexadiene, and 4-bpmb = N1,N4-bis((pyridin-4-yl)methylene)benzene-1,4diamine, Figure S1). Using acetic acid and pyridine led to competition between these capping reagents and oba and N-donor ligands, respectively, for coordinating the Zn(II) ions. This competition can direct crystal growth along the particular direction. Furthermore, the catalytic activity of TMU-5 with different morphologies in the Knoevenagel condensation reaction of malononitrile with benzaldehyde has been investigated. In fact, using MOFs as heterogeneous catalysts is one of the earliest and most prolific areas of MOF research.22−26

INTRODUCTION Metal−organic frameworks (MOFs), resulting from selfassembly of inorganic secondary building units (SBUs) and organic linkers, are highly attractive because of their potential applications as functional materials in structure-dependent applications, such as gas storage and separation, ion exchange, sensing, catalysis, molecular recognition, and drug delivery.1−3 Nano- and microscale MOFs (nMOFs or mMOFs) often exhibit some novel, interesting, and/or enhanced size-dependent physical and chemical properties that cannot be observed in their bulk analogues.4−7 They can provide combination features of nanomaterials in addition to properties of MOFs.8,9 Moreover, for the nanosized porous materials, the diffusion length is decreased, which is very important in catalysis and sorption behavior. Despite the fact that the shape of MOF nanoparticles is not necessarily an extension of their molecular geometry and a very challenging task,10 but still by choice of appropriate geometries and structures, selection of some preparation methods that can dictate specific morphology, particular shape and size, and also by controlling the growth conditions, MOFs with different morphologies can be successfully obtained.11−13 Modulating synthesis is a method in which finely controlled formation of MOFs can be obtained by adding capping reagents (modulators) with the same chemical functionality as the linkers. This results in a competition between organic linkers and modulators for coordinating the metal ions during the growth process.5,14−21 In that sense, MOFs can be formed on the desired scale ranging from the nano regime to the lower end of the micro regime. Here, we report a simple strategy, the coordination modulation method, which can be easily scalable using acetic acid and pyridine as capping reagents to achieve efficient control over the nucleation process and, thus, control over the size of © XXXX American Chemical Society

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EXPERIMENTAL SECTION

Materials and Physical Techniques. Zinc(II) acetate dehydrate, acetic acid, pyridine, and 4,4′-oxybis(benzoic acid) (H2oba) were purchased from Aldrich and Merck Company and used as received. The ligands 1,4-bis(4-pyridyl)-2,3-diaza-1,3-butadiene (4-bpdb), 2,5-bis(4pyridyl)-3,4-diaza-2,4-hexadiene (4-bpdh), and N1,N4-bis((pyridin-4yl)methylene)benzene-1,4-diamine (4-bpmb) were synthesized according to previously reported methods.27 Apparatus. Melting points were measured on an Electrothermal 9100 apparatus. IR spectra were recorded using Thermo Nicolet IR 100 FT-IR. X-ray powder diffraction (XRD) measurements were performed using a Philips X’pert diffractometer with monochromated Cu Kα radiation. The samples were characterized with a field emission scanning electron microscope (FE-SEM) Philips XL30, TESCAN MIRA (Czech), Received: March 4, 2015 Revised: April 6, 2015

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DOI: 10.1021/acs.cgd.5b00304 Cryst. Growth Des. XXXX, XXX, XXX−XXX

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Figure 2. Field-emission scanning electron microscopy (SEM) images of TMU-5 samples obtained with various combinations of modulator/ temperature/time; r represents the ratio between pyridine and 4-bpdh: (a) r = 50, T = 100 °C, 24 h (inset is high-magnification image, scale 2 μm); (b) r = 30, T = 100 °C, 24 h; (c) r = 10, T = 100 °C, 24 h (inset is high-magnification image, scale 1 μm); (d) r = 10, T = RT, 24 h; (e) r = 5, T = RT, 2 h, (inset is high-magnification image, scale 1 μm) ; and (f) r = 2, T = RT, 2 h (inset is high-magnification image, scale 1 μm). The concentration of 4-bpdh (c = 0.0125 M) is the same in all experiments.

Figure 1. Field-emission scanning electron microscopy (SEM) images of TMU-5 samples obtained with various combinations of modulator/ temperature/time; r represents the ratio between acetic acid and oba: (a) r = 15, T = 100 °C, 24 h; (b) r = 10, T = 100 °C, 24 h; (c) r = 5, T = 100 °C, 24 h; (d) r = 2, T = 100 °C, 24 h (inset is TEM image, scale 1500 nm); (e) r = 2, T = RT, 2 h (inset is high-magnification image, scale 500 nm); and (f) r = 0.5, T = RT, 2 h. The concentration of oba (c = 0.025 M) is the same in all experiments. and ZEISS SIGMA VP (Germany) with gold coating. GC runs were performed on an Echrom GC A90 gas chromatograph. Synthesis of Bulk TMU-4, TMU-5, and TMU-6. In a typical experiment TMU-4, TMU-5, and TMU-6 were obtained by mixing oba (0.5 mmol), N-donor ligands (4-bpdb, 4-bpdh, and 4-bpmb) (0.25 mmol), and Zn(OAc)2·4H2O (0.5 mmol) in 20 mL DMF in a round-bottom flask at 100 °C for 24 h. The resulted powders were isolated by centrifugation, washed with DMF 3 times, and dried in air for characterization. Synthesis of TMU-4, TMU-5, and TMU-6 Nanostructures. The same process was applied to synthesizing TMU-4, TMU-5, and TMU-6 nanorods and nanoplates, by adding different concentrations of acetic acid and pyridine as capping reagents (modulator) at 100 °C and room temperature (RT) for 24 h and 2 h. For TMU-4, r factors (where r is defined as the ratio of concentration of modulator to ligand) for acetic acid were 10 and 5, and r factors for pyridine were 20, 10, and 2. For TMU-5, r factors for acetic acid were 15, 10, 5, 2, 2, and 0.5, and r factors for prydine were 50, 30, 10, 10, 5, and 2. For TMU-6, r factors for acetic acid were 20, 10, 5, 1, and 0.5, and r factors for prydine were 30, 15, 5, and 1.

Activation Method. All samples were heated in an oven at 140 °C for 24 h under vacuum. The structure remains intact upon removal of guest DMF molecules. Knoevenagel Condensation. To a mixture of malononitrile (1.1 mmol) and benzaldehyde (1 mmol) was added 2 mol % of activated catalyst. The resulting mixture was stirred at room temperature for a desired time. The course of the reaction was followed by GC analysis. Reaction mixture diluted with 5 mL of CH2Cl2. On completion of the reaction, the catalyst was recovered by centrifugation and the supernatant liquid was collected and evaporated to dryness. Pure product was obtained by recrystallization from EtOH. The catalyst was washed repeatedly with EtOAc, dried, and reused when required.

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RESULTS AND DISCUSSION Micro and nanostructures of three zinc(II) based MOFs, [Zn 2 (oba) 2 (4-bpdb)] n ·2(DMF) (TMU-4), [Zn(oba)(4bpdh)0.5]n·1.5(DMF) (TMU-5), and [Zn(oba)(4-bpmb)0.5]n 1.5(DMF) (TMU-6), were synthesized by simply mixing a B

DOI: 10.1021/acs.cgd.5b00304 Cryst. Growth Des. XXXX, XXX, XXX−XXX

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Figure 3. (a) Powder X-ray diffraction patterns of TMU-5. For rodshaped, conditions are r = 10, T = 100 °C, 24 h, and for plate-shaped, conditions are r = 10, T = RT, 24 h. (b) Representation of the (200) and (002) planes in TMU-5.

constant concentration of initial reagents (Zn(OAc)2·2H2O, oba, 4-bpdb, 4-bpdh, and 4-bpmb, Figure S1) using different concentrations of acetic acid and pyridine as capping reagents. Monocarboxylic acid, acetic acid, was chosen because it has the same carboxylate functionality as the oba ligand and is expected to efficiently prevent the coordination interactions between zinc and oba ligand. Also, the nucleation rate can be controlled by acetic acid through controlling of deprotonation of oba during the synthesis and therefore moderate size and aspect ratio of MOFs. On the other hand, an amine containing a nitrogen atom with a lone pair, pyridine,15 was chosen to create competition with the N-donor pillar ligands for coordinating to Zn(II) centers. Moreover, zinc(II) acetate was chosen because upon mixing with oba and N-donor ligands at room temperature it readily forms a yellow precipitate of these three MOFs, while with zinc(II) nitrate, high temperatures and longer reaction times are needed before the appearance of precipitate is observed. This feature suggests a faster reaction with zinc(II) acetate, which is more suitable for control through the coordination modulation. Also two reaction temperatures, 100 °C and RT in 24 and 2 h of time, have been studied. Among these three MOFs, TMU-5 is the best candidate for modulation synthesis15,20 because of its binuclear paddlewheel Zn(II) units constructed from nonlinear dicarboxylic acid, oba, and linear N-donor pillar ligand, 4-bpdh (Figure S2).28 Various “r” ratios (where r is defined as the ratio of concentration of modulator to ligand) have been used to produce rod- and platelike morphologies. It has been found that when the ratio between acetic acid and oba was reduced from r = 15 to r = 2 at 100 °C in constant concentration of oba (c = 0.025 M), microrods with reduced size have been obtained (Figures 1a−d and S3). This is attributed to the fact that a higher concentration of monocarboxylate in the solution could lead to a decrease in the nucleation rate and the number of nucleation centers, leading to

Figure 4. FE-SEM images of TMU-4 samples obtained with various combinations of modulator/temperature/time; r represents the ratio between acetic acid and oba: (a) r = 10, T = 100 °C, 24 h; (b) r = 5, T = 100 °C, 24 h; (c) r = 10, T = RT, 24 h; (d) TEM image of sample c; (e) r = 5, T = RT, 2 h; and (f) TEM image of sample e. The concentration of oba (c = 0.025 M) is the same in all experiments.

larger particle sizes.14,29 Moreover, in r = 2, upon decreasing temperature from 100 °C to RT in the shorter time of reaction (2 h) nanorods of TMU-5 have been obtained (Figure 1e) in which the aspect ratio of length to width is reduced. This is due to a higher nucleation rate in lower temperature that is followed by a slow phase of particle growth.30−32 On the other hand, plate-like morphologies have been acquired using pyridine as a modulator (Figures 2a−f and S4). Similar to acetic acid, sizes of plates have been reduced upon decreasing r from 50 to 10; meanwhile, lower temperature leads to preparation of nanoplates (Figure 2e). The X-ray diffraction (XRD) patterns of rod and plate are similar to the simulated pattern of TMU-5 indicating the unchanged structures (Figure 3a). It is also noted that for the microrods the relative intensities of (200) and (002) reflections are, respectively, larger and smaller than that in the simulated pattern. This is attributed to crystal growth along the [100] direction and prevention of crystal growth along the [001] direction in the presence of acetic acid (Figure 3a,b). On the contrary, for nanoplates the reflection intensity becomes reversed because pyridine is used (Figure 3). Unlike TMU-5 with paddlewheel SBU, TMU-4 is based on a binuclear Zn2 unit, both of which have tetrahedral geometry (Figure S2).28 In this case we examined the efficiency of acetic acid to direct morphology to rods, by influence of the nucleation C

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process, but despite our expectations, plate-like morphology was obtained under examination of various concentrations of acetic acid. There is no relation between r factor and morphology or size of particles (Figure 4). Because of the tendency in TMU-4 to choose a plate-like morphology even without using a modulator (Figure 5a,b) we examined three concentrations of pyridine as

Figure 5. (a) FE-SEM image and (b) TEM image of bulk TMU-4. (c−e) FE-SEM images of TMU-4 samples obtained with various combinations of modulator/temperature/time; r represents the ratio between pyridine and 4-bpdb: (c) r = 20, T = 100 °C, 24 h; (d) r = 10, T = RT, 2 h; (e) r = 2, T = RT, 2 h. The concentration of 4-bpdb (c = 0.0125 M) is the same in all experiments.

Figure 6. FE-SEM images of TMU-6 samples obtained with various combinations of modulator/temperature/time; r represents the ratio between acetic acid and oba: (a) r = 20, T = RT, 24 h; (b) r = 10, T = RT, 24 h; (c) r = 5, T = RT, 24 h; (d) r = 1, T = RT, 24 h; (e) r = 0.5, T = RT, 24 h; and (f) magnified image of e. The concentration of oba (c = 0.025 M) is the same in all experiments.

capping agent to tune plate morphology (Figure 5c−e). This reveals that TMU-4 has a desire to selectively choose plate-like morphology, and it is possibly related to the twisted structure of TMU-4 (Figure S2). TMU-6 structure is also similar to TMU-4 and it is expected that the modulation synthesis will hardly influence the morphology (Figure S2).33 Meanwhile, in the case of TMU-6, acetic acid and pyridine can direct the morphology to rod and plate, respectively (Figures 6 and 7), but not well enough as in TMU-5. Similar to previous observations, the efficacy of capping reagents increased upon decreasing the r factor and led to more uniform morphology with reduced sizes of rods or plates. The X-ray diffraction (XRD) patterns of TMU-4 and TMU-6 upon using acetic acid or pyridine are consistent with their simulated patterns confirming that in all cases the structure remained unchanged (Figures S5 and S6). In our previous study, these three MOFs were used as heterogeneous catalysts in the Knoevenagel condensation reaction.33 Among them, TMU-5 with narrow and interconnected pores in three dimensions showed the highest catalytic activity because of increasing basicity and better interaction between azine groups (as Lewis base) in catalyst and substrate. More basicity was attributed to lone pair−lone pair electron repulsion and the alpha effect.34−36 Consequently, different morphologies of TMU-5 were chosen and their catalytic performance in solventfree Knoevenagel condensation reaction of benzaldehyde with malononitrile was investigated (Scheme 1). Condensation is carried out under atmospheric pressure at room temperature. According to our results presented here, nanorod morphology of TMU-5 with 96% yield after 4 h has much higher activity toward the condensation of benzaldehyde with malononitrile as compared to other morphologies of TMU-5 (Figure 8). Catalytic activity recorded for nanoplate morphology of TMU-5 is to some extent lower than nanorod morphology (92% yield after 4 h) but still higher than its microrod morphology (82% yield after 4 h).

Figure 7. FE-SEM images of TMU-6 samples obtained with various combinations of modulator/temperature/time; r represents the ratio between pyridine and 4-bpmb: (a) r = 30, T = RT, 24 h; (b) r = 15, T = RT, 24 h; (c) r = 5, T = RT, 24 h (inset is high-magnification image, scale 200 nm); and (d) r = 1, T = RT, 24 h. The concentration of 4-bpmb (c = 0.0125 M) is the same in all experiments.

Bulk sample of TMU-5 is also tested as a reference to shed light on the effect of the MOF structure on catalytic activity. Its catalytic activity is the least among the others (75% yield after 4 h). This slightly different catalytic performance can be attributed to more accessible pore apertures in the nanorod morphology and more available azine groups rather than nanoplates or microrod morphology. Blank reaction is carried out under identical D

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Scheme 1. Knoevenagel Condensation Reaction Catalysed by Different Morphologies of TMU-5

conditions to clarify catalytic activity of TMU-5. No conversion is achieved in the absence of catalyst. The catalyst MOFs could be reused for at least 4 runs without any loss in activity, so their PXRD patterns measured after reactions show the same profile as the simulated pattern of TMU-5, indicating the unchanged coordination framework, although the crystallinity seems to be weakened due to broadening of diffraction peaks (Figures S7 and S8).

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CONCLUSIONS Modulation synthesis was used to produce micro- and nanorods and plates of three porous Zn(II)-based metal−organic frameworks, [Zn2(oba)2(4-bpdb)]n·2(DMF) (TMU-4), [Zn(oba)(4bpdh)0.5]n·1.5(DMF) (TMU-5), and [Zn(oba)(4-bpmb)0.5]n 1.5(DMF) (TMU-6). Based on the structure of these MOFs, different capping reagents can direct the morphology to rod or plate shape. Regarding the variations in concentration of modulator (or r factor) and temperature, it is found that upon decreasing these parameters uniform morphologies with reduced size were obtained. Furthermore, nanorods and nanoplates of TMU-5 show better catalytic activity in solid-state Knoevenagel condensation reaction compared with other morphologies of TMU-5. Also, this study clearly unveils the connection of framework structure with its performance. ASSOCIATED CONTENT

* Supporting Information S

More FE-SEM images, XRPD, and FT-IR spectra. This material is available free of charge via the Internet at http://pubs.acs.org.

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Figure 8. Yield (%) vs time (h) for Knoevenagel condensation reaction of benzaldehyde with malononitrile in the presence of bulk, nanorod, nanoplate, and microrod morphologies of TMU-5.

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AUTHOR INFORMATION

Corresponding Author

*E-mail: [email protected]. Tel: (+98) 21-82883449. Notes

The authors declare no competing financial interest.

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ACKNOWLEDGMENTS Support of this investigation by Tarbiat Modares University is gratefully acknowledged. E

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DOI: 10.1021/acs.cgd.5b00304 Cryst. Growth Des. XXXX, XXX, XXX−XXX