A highly catalytically active Hf(IV) metal-organic framework for Knoevenagel condensation

Microporous and Mesoporous Materials 284 (2019) 459–467

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A highly catalytically active Hf(IV) metal-organic framework for Knoevenagel condensation

T

Aniruddha Dasa, Nagaraj Anbub, Amarajothi Dhakshinamoorthyb,∗∗, Shyam Biswasa,∗ a b

Department of Chemistry, Indian Institute of Technology Guwahati, Guwahati, 781039, Assam, India School of Chemistry, Madurai Kamaraj University, Madurai, 625021, Tamil Nadu, India

ARTICLE INFO

ABSTRACT

Keywords: Metal-organic framework Hafnium Hydrazine functionalization Catalysis Knoevenagel condensation

We report a highly crystalline, Hf(IV) metal-organic framework (MOF) containing 2-hydrazinyl-1,4-benzenedicarboxylic acid (H2BDC-N2H3) as the ligand. The resulting Hf-UiO-66-N2H3 MOF (1) exhibited efficient and recyclable catalytic activity in its activated form (1′) for Knoevenagel condensation of benzaldehyde with malononitrile. Moreover, in traditional solvothermal synthesis, compound 1 was produced with a very high yield. Compound 1′ showed very high N2 adsorption capacity, excellent chemical and thermal stability. Material 1′ could catalyze Knoevenagel condensation of benzaldehyde with malononitrile, giving quantitative yield under the optimum reaction conditions. Moreover, several control experiments indicated that Hf-based MOFs exhibit superior activity than Zr-based MOFs under identical conditions. Furthermore, the stability of 1′ was surveyed by conducting reusability and leaching experiments. These results indicate that 1′ is active up to five cycles and catalytic reaction is heterogeneous in nature. Furthermore, the scope of 1′ was also studied with a series of aldehydes differing from their substituents and molecular sizes, and high yields were observed in many cases. Interestingly, the order of reactivity of the aldehydes with malononitrile in the presence of 1′ under identical conditions was: benzaldehyde > 1-naphthaldehyde > 9-anthracenealdehyde. These observations suggest that the catalysis is size-selective.

1. Introduction Knoevenagel condensation reaction is a versatile and well-known C-C coupling reaction in organic transformation [1–4]. In this reaction, nucleophilic addition takes place to a carbonyl compound by an active methylene compound, which is accompanied by dehydration. It often results in an α,β-conjugated enone. Many organic condensation reactions such as the Henry and Claisen-Schmidt reactions also occur in the similar fashion as the Knoevenagel reaction involving different types of nucleophiles [5–8]. In case of these type of reactions, researchers faced many problems such as poor recyclability, low loading of catalyst and contamination of catalyst, when homogenous catalysts were used [9,10]. In order to overcome these issues, significant research efforts are currently being devoted to develop novel heterogeneous catalysts which are highly stable, recyclable and easy to handle. Some solid catalysts used in the Knoevenagel condensation reaction are ZIF-8, ZIF9, carbon nanotubes, mesoporous titanosilicate, Cu3TATAT, etc. [11–14]. In some cases, the preparation of the catalysts are tedious, catalysts have low loading capacity and the catalysis suffers from

∗

several other problems [15]. Tremendous research efforts are being dedicated now-a-days involving metal-organic frameworks (MOFs) [16–18]. Among all the rapidly growing organic-inorganic materials, MOFs are very unique materials, since they combine both inorganic and organic chemistry. The prime hallmarks of MOFs include excellent crystallinity, huge surface area and tunable pore aperture [18–20]. They are highly promising materials in chemical and material science, not only for their versatile structural topology but also for their thermal and hydrolytic stability [21], tunable functionality in different applications including gas storage and purification [22], molecular sensing [23], photoluminescence [24], molecular based magnetism [25], drug delivery [26], biomedicine [27] and photo-catalysis [28]. MOFs have been also used as heterogeneous catalysts in many organic reactions [29–32]. The catalytically active sites in MOFs involve metal nodes, nanoparticles incorporated inside the pores or functional groups attached with the organic ligand [18,33]. Several MOFs such as Al-CAU-1-NH2 [34], Zn-MOF-NH2 [35], Cr-MIL-101-NH-RNH2 [36] have been reported as catalysts in Knoevenagel condensation reaction. On the other hand, Zr(IV) and Hf(IV)

Corresponding author. Corresponding author. E-mail addresses: [email protected] (A. Dhakshinamoorthy), [email protected] (S. Biswas).

∗∗

https://doi.org/10.1016/j.micromeso.2019.04.057 Received 14 March 2019; Received in revised form 11 April 2019; Accepted 24 April 2019 Available online 25 April 2019 1387-1811/ © 2019 Elsevier Inc. All rights reserved.

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containing UiO-n (UiO: University of Oslo) MOFs are very effective for Knoevenagel condensation reaction due to the presence of Mn+‒O2(M = Zr/Hf) Lewis acid-base pair inside the framework [37]. The Ru/ Zr-UiO-66 [37] and Zr-UiO-66-NH-RNH2 [36] have been previously employed successfully as heterogeneous catalysts in Knoevenagel condensation reaction due to the availability of Mn+‒O2- (M = Zr) Lewis acid-base pair. Research attention in the area of Zr- and Hf-based MOFs is growing expeditiously because of their robust structural frameworks, high thermal and chemical stability, low toxicity and stability towards the inclusion of various functional groups in the side chains of the organic linkers [38,39]. Since the discovery of Zr-based UiO-n series of MOFs in 2008 [38], many Zr-based MOFs have been reported [40] until today. On the other side, only few reports on the synthesis and application of Hf-based MOFs have appeared in the literature till date [39,41–46]. In addition, reports on the utilization of Hf-based MOFs as heterogeneous catalysts are very rare in the literature [47,48]. Moreover, Hf(IV)-based MOFs have shown higher stability and more oxophilic character compared to their Zr(IV) counterparts. It has been reported in the literature that Hf(IV)-based MOFs possess lower pKa than Zr(IV)-based MOFs [41]. These exceptional features of Hf MOFs encouraged us to develop new water-stable, hydrazine-functionalized Hf MOFs and their successful utilization as heterogeneous catalysts. It is worthy to mention that developing highly water and thermally stable hydrazine functionalized Hf-based MOFs may also find wide interests in luminescence sensing applications [49]. This study involves the preparation of Hf-UiO-66 and Hf-UiO-66N2H3 MOFs and comparison of their catalytic activity with their analogous Zr-based MOFs under identical reaction conditions in Knoevenagel condensation as a model reaction. It describes modulated solvothermal synthesis of the hydrazine-functionalized Hf-UiO-66 MOF namely Hf-UiO-66-N2H3 (1). The MOF compound was produced in extremely high yield. Moreover, it displayed noticeable surface area and good thermal stability. The compound also showed high chemical stability towards water and acid. In addition, the activated form of 1 (called 1′) exhibited high efficiency and recyclability towards Knoevenagel condensation of benzaldehyde with malononitrile (Scheme 1). The catalysis was found to be size-selective as well as heterogeneous in nature.

Table 1 Knoevenagel reaction between malononitrile and benzaldehyde catalyzed by various MOFs.a Entry

Catalyst (mg)

Time (h)

Yield (%)

1

Hf-UiO-66 (20 mg)

2

Hf-UiO-66-NH2 (20 mg)

3

Zr-UiO-66 (20 mg)

4

Zr-UiO-66-N2H3 (20 mg)

5

Hf-UiO-66-N2H3 (activated at 150 °C) (20 mg)

6

HfCl4 (15.6 mg)

7

H2BDC-N2H3 (9.5 mg)

0.5 1 2 4 0.5 1 2 4 0.5 1 2 4 0.5 1 2 4 0.5 1 2 4 0.5 1 2 4 0.5 1 2 4

76 92 93 93 72 76 96 97 23 46 77 83 51 69 83 94 77 87 95 99 0 0 0.7 0.9 4 6 14 31

a Conditions for reaction: malononitrile (1 mmol), benzaldehyde (1 mmol), ethanol (0.3 mL), room temperature, 4 h.

66 [45], Hf-UiO-66-NH2 [45], Zr-UiO-66 [50] and Zr-UiO-66-N2H3 [51]. 2.2. Activation of compound 1 After stirring compound 1 (100 mg) in methanol (30 mL) at ambient temperature over 24 h, its methanol-exchanged form was obtained. After that the drying of the compound was performed at 80 °C in an oven for 6 h. The activated form of 1 (called 1′) was obtained after heating this powder sample at 120 °C in high vacuum for one day.

2. Experimental section 2.1. Synthesis of [Hf6O4(OH)4(BDC-N2H3)6]∙6H2O∙7DMF (Hf-UiO-66N2H3, compound 1)

2.3. Procedure for Knoevenagel condensation Aldehyde (1 mmol), malononitrile (1 mmol), solvent (0.3 mL) and catalyst 1′ (20 mg) were added to a 10 mL Schlenk tube. Then, this mixture was homogeneously mixed and allowed to stand at ambient conditions. The various reaction time periods are shown in Table 1. Gas chromatography was used to regulate the progress of the reaction. The aliquots were sampled at various reaction time intervals. Then, the final reaction mixture was examined by gas chromatography for its purity and selectivity. The internal standard method was applied to estimate the product yield. The characterization of the obtained products was accomplished by GC-MS technique. For the recyclability experiments, the identical procedure was followed except the fact that filtration of the catalyst was performed after completion of each cycle of reaction. Then, it was washed three times with dichloromethane, followed by drying for 1 h at 80 °C.

Reaction of a mixture of HfCl4 (66 mg, 0.204 mmol), H2BDC-N2H3 linker (40 mg, 0.204 mmol) and formic acid (769 μL, 6.12 mmol) in N,N-dimethylformamide (DMF; 3 mL) resulted in a light yellow precipitate when the reaction mixture was allowed to heat inside a sealed tube using a pre-heated aluminum heating block for one day at 120 °C. After filtering the precipitate, it was washed with acetone several times. The drying of the powder sample was accomplished at 60 °C in an oven for 4 h. Yield: 80 mg (0.03 mmol, 97%) considering HfCl4. IR (KBr, cm−1): 3435 (br), 1681 (m), 1650 (w), 1621 (w), 1583 (vs), 1503 (m), 1432 (m), 1393 (s), 1305 (m), 1268 (w), 1159 (w), 1100 (w), 1017 (m), 973(m), 769 (s), 748 (w), 676 (vs), 552 (w), 476 (s). Literature procedures were followed for the preparation of Hf-UiO-

3. Results and discussion 3.1. Synthesis and procedure for activation Scheme 1. Knoevenagel condensation of benzaldehyde with malononitrile catalyzed by 1′ in ethanol at room temperature.

All possible combinations of reactions were carried out with HfCl4 and HfOCl2∙8H2O in three solvents (DMF, dimethylacetamide, and 460

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Table 2 Knoevenagel condensation reaction between aldehydes and malononitrile in the presence of 1′ as a heterogeneous solid catalyst.a Entry

Aldehyde

Product

Yield (%)b

1

98

2

98

3

98

4

99

5

96

6

99

7

99

8

65

9

99

10

76

11

93

12

67

13

53

14

92

15

98

16

56

Fig. 1. (a) Theoretical XRPD pattern of Hf-UiO-66 and observed XRPD patterns of (b) 1 and (c) 1′.

1583 cm−1, respectively [53,54]. Thus, the existence of ligand molecules in the framework of the compound was corroborated. 3.3. XRPD analysis The XRPD patterns of 1 and 1′ are shown in Fig. 1. From Fig. 1, it is obvious that the peak positions of both 1 and 1′ appear at the same positions (i.e. 2-theta values) as the simulated Hf-UiO-66 compound. These results disclose that the framework topology of 1 is identical with the un-functionalized Hf-UiO-66 material [45]. It can be also inferred that the structural framework of 1 remains intact even after the solventexchange (in methanol) and activation (at 120 °C) processes. 3.4. Structure description The XRPD experiment (Fig. 1) performed with as-synthesized 1 indicates that the Hf(IV)-based UiO-66 MOF containing hydrazine functionality has the same structural framework as the parent Hf-UiO-66 MOF [45]. The structure of Zr-UiO-66 MOF has been formerly presented in detail in the literature [55]. The Zr-UiO-66 framework consists of [Zr6O4(OH)4]12+ cluster as the building blocks (Fig. 2a) which interconnect with each other through the BDC linkers [55]. The coordination geometry of every Zr(IV) atom is square anti-prismatic [55]. Each hexanuclear cluster is connected with twelve BDC ligands. In the structure of Hf-UiO-66-N2H3 (Fig. 2), the incorporated ligand is BDCN2H3 instead of BDC ligand as in the structure of Zr-UiO-66. The cubic, three-dimensional framework has octahedral (free diameter ∼11 Å) as well as tetrahedral cages (free diameter ∼8 Å). Eight tetrahedral cages surround each octahedral cage and narrow triangular windows (free diameter ∼6 Å) connect them with each other.

a Reaction conditions: aldehyde (1 mmol), malononitrile (1 mmol), ethanol (0.3 mL), 1′ (20 mg), room temperature, 4 h. b Yields were determined by GC.

diethylformamide) using various modulators. The reaction temperatures varied from 100 to 150 °C. The modulators included trifluoroacetic acid, acetic acid, benzoic acid and formic acid) [52]. The formation of compound 1 having high crystallinity was achieved after putting a reaction mixture containing HfCl4, H2BDC-N2H3 ligand, formic acid and DMF in a sealed tube, followed by a heating for one day at 120 °C. In the activation step, 100 mg of as-synthesized 1 was stirred in 30 mL of methanol for a time period of 24 h at room temperature. Afterwards, the compound was filtered off and the powder solid was allowed to heat at 120 °C under a dynamic vacuum for 24 h.

3.5. Thermal stability The thermogravimetric (TG) experiment disclosed that the material is stable up to 400 °C in argon atmosphere. Therefore, 1 and 1′ possess high thermal stability, which is similar as the existing, UiO-66 type MOFs [45]. Compound 1 displayed three distinct weight loss steps in its TG profile (Fig. S5, Supporting Information). The first weight loss of 3.9 wt % occurs from 50 to 150 °C owing to the elimination of six water (guest)

3.2. Infrared spectroscopy The IR spectra of 1 and 1′ are exhibited in Fig. S4 (Supporting Information). As displayed in Fig. S4 (Supporting Information), the strong symmetric and asymmetric stretching vibrations for the coordinated BDC-N2H3 molecules are noticed at approximately 1432 and 461

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Fig. 2. (a) Structures of the [Hf6O4(OH)4]12+ SBUs and (b) 3D cubic framework of 1′. Colour codes: Hf, sky blue polyhedra; C, grey; O, red; N, blue. The figures were drawn by using the structural model obtained by employing Materials Studio (version 5.0, Accelrys Inc., San Diego, 2009) program. (For interpretation of the references to colour in this figure legend, the reader is referred to the Web version of this article.)

3.7. Surface area analysis

molecules per formula unit (calculated: 3.6 wt%). The second weight loss of 17.0 wt% is observed from 150 to 400 °C due to the elimination of seven DMF (guest) molecules per formula unit (calculated: 17.1 wt %). The third weight loss begins to take place at 400 °C. In this step, the coordinated organic ligand molecules start to escape from the framework of 1 causing its structural decomposition. Compound 1′ displayed the first weight loss of 4.9 wt% in the range of 50–150 °C in its TG trace due to the elimination of water from the porous cages, which was adsorbed from the moisture during the storage of the activated sample under air atmosphere. The sharp weight loss step occurring at 400 °C signifies the decomposition temperature of the MOF compound.

The accessible surface area and micropore volume of the thermally activated Hf-UiO-66-N2H3 MOF compound were deduced from the N2 sorption experiment. The N2 sorption measurement (Fig. S11, Supporting Information) points out that 1′ is a microporous material since it shows characteristic type-I adsorption isotherm. The accessible surface area and pore volume of 1′ were calculated by the BET method and they correspond to 1356 m2/g and 0.75 cm3/g (p/p0 = 0.5), respectively. Thus, 1′ possesses high BET surface area and pore volume, which are higher or analogous with the functionalized UiO-66 MOF materials reported in the literature till date [50,58,59]. The BET surface areas of typical UiO-66 MOFs fall in the range of 850–1000 m2/g but 1′ has a BET surface area of 1356 m2/g in spite of having larger ligand and metal ion in its framework. The availability of high BET surface area is probably can be explained by the ‘coordination defects’ within the framework of 1′ [60,61]. For studying the influence of activation temperature on the sorption property, we have also measured the BET surface area of 1 activated at 150 °C (Fig. S12, Supporting Information). The obtained BET surface area was 1346 m2/g. The result suggests that the increase in the activation temperature had negligible effect on the sorption property of 1.

3.6. Chemical stability We have studied the chemical stability of 1′ in aqueous, acidic and basic medium. Compound 1′ was stirred in water, glacial acetic acid, 1(M) HCl and 0.1(M) NaOH solutions at room temperature for a time period of 4 h. Subsequently, the samples were collected by vacuum filtration and these powder materials were dried at 100 °C for 6 h. The stability of the dry compounds was investigated by XRPD measurement. The XRPD (Fig. S6, Supporting Information) of the recovered solid samples after stirring in water, acetic acid and 1(M) HCl showed similar peak positions and intensities as the as-synthesized sample. In order to check the possibility of leaching of any Hf(IV) ion or ligand during the chemical stability test, UV–Vis spectra of the supernatants were recorded. Furthermore, EDX spectra were collected with the solid residues obtained after evaporating the supernatants. The EDX spectra (Figs. S7–S9, Supporting Information) revealed the existence of small amounts of Hf atoms in all the solid residues. Moreover, the existence of minute amounts of ligands in all the supernatants was corroborated by the UV–Vis spectra (Fig. S10, Supporting Information). The amounts of leached Hf(IV) atoms and ligands were relatively higher in 1(M) HCl and acetic acid, as compared to water. Thus, slow decomposition of the framework of 1′ occurred in all the tested liquids. Unfortunately, the structural framework of the material was completely decomposed in presence of 0.1(M) NaOH solution. These results point out that 1′ possesses similar chemical stability as the formerly reported UiO-66 MOF materials [56,57].

3.8. Catalytic studies Knoevenagel condensation is a well-known organic transformation in which α,β-conjugated enone is generated by the reaction of an active methylene compound with a carbonyl compound. Many research groups are focusing to develop heterogeneous solid catalysts for this organic transformation to obtain the above mentioned products mainly due to the fact that these intermediates are frequently used in perfumes, polymers, fine chemicals, cosmetics, drugs and pharmaceuticals. Furthermore, one of the commonly employed test reactions to examine the catalytic activity of MOFs bearing free amino groups in its crystal structure is Knoevenagel condensation reaction between malononitrile and benzaldehyde. This reaction was reported with a series of MOF catalysts consisting of free amino groups in the organic ligand or diamino groups grafted with the metal center. The various MOF catalysts reported for the Knoevenagel condensation include UiO-67–Tz–NH2 [62], NH2-MIL-101(Fe) [63], Fe-MIL-101-NH2 [64], Cr-MIL-101-NHRNH2 and UiO-66-NH-RNH2 [65], JUC-199 [66], Cd and Zn-pillared MOF [67], MIL-101-NH2-SO3H [68], TMU-25 [69], TMU-22 [70], TMU462

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Fig. 5. Time-conversion plots for Knoevenagel condensation between benzaldehyde and malononitrile: (a) with 1′, (b) hot filtration experiment (catalyst filtered after 15 min) and (c) control experiment without catalyst. Conditions for reaction: malononitrile (1 mmol), benzaldehyde (1 mmol), 1′ (20 mg), ethanol (0.3 mL), room temperature, 4 h.

Fig. 3. Effect of solvents for the Knoevenagel condensation between malononitrile and benzaldehyde utilizing 1′ as solid catalyst. Conditions for reaction: malononitrile (1 mmol), benzaldehyde (1 mmol), 1′ (20 mg), solvent (0.3 mL), room temperature, 4 h.

performed at 5, 10, 15 and 20 mg of catalyst loading. The kinetic timeconversion profiles (Fig. 4) show that the initial reaction rates as well as the final product yield were higher with 20 mg catalyst than the other three loadings. Therefore, the use of 20 mg catalyst was considered as the optimal loading for the further experiments. A blank control experiment afforded 24% conversion of benzaldehyde after 4 h in ethanol at room temperature. In contrast, 100% conversion of benzaldehyde was accomplished using 1′ in ethanol at room temperature after 4 h. Fig. 5 displays time-conversion plot comparing the reaction rate in the presence and absence of solid MOF catalyst. The outcomes firmly confirm that this reaction is promoted only in the presence of solid catalyst. Furthermore, a leaching experiment was also conducted under identical conditions to make sure that the reaction is promoted only by the solid catalyst and not due to the active sites leached into the solution. In this aspect, the reaction between benzaldehyde and malononitrile was initiated in the presence of 1′ under identical conditions. Later, the solid catalyst was removed by filtration from the reaction mixture after 15 min and the resulting solution was allowed to continue up to 4 h. The results of the leaching experiment are provided in (Fig. S13, Supporting Information) with more clarity by excluding the contribution from blank run. It becomes clear from Fig. 5 and (Fig. S13, Supporting Information) that the reaction rate was significantly reduced in the absence of catalyst upon filtration. These data indicate that the reaction is exclusively catalyzed by the presence of 1′ and no active site is leached from the solid catalyst into the solution. However, a slight increase in the conversion of benzaldehyde after removal of the catalyst may be because of the contribution from the blank reaction as shown in Fig. 5 and (Fig. S13, Supporting Information). Furthermore, to support this hypothesis, the aqueous extract of the reaction mixture in the absence of catalyst after 4 h was investigated by ICP-OES, which showed the absence of Hf(IV) ions. This observation corroborates the stability of the framework during the catalysis experiments. For realizing the type of active sites accelerating Knoevenagel condensation with 1′, several control experiments were performed. The observed results are shown in Fig. 6 and Table 1. The comparison of the reaction rate of 1′ with Hf-UiO-66, HfCl4 and H2BDC-N2H3 ligand clearly indicates that Hf-UiO-66 exhibits higher initial reaction rate than 1′ due to the lack of diffusion restrictions while 1′ shows higher

Fig. 4. Effect of catalyst loading for the Knoevenagel condensation between benzaldehyde and malononitrile: 5 mg (black line), 10 mg (red line), 15 mg (blue line) and 20 mg (pink line). Conditions for reaction: malononitrile (1 mmol), benzaldehyde (1 mmol), catalyst 1′, ethanol (0.3 mL), room temperature. (For interpretation of the references to colour in this figure legend, the reader is referred to the Web version of this article.)

5 [71], CAU-1-NH2 [34], Cd(II)-MOFs [72], adenine-based zinc(II) MOF [73], NH2-Tb-MOF [74], Pd NP-loaded NMOF [75], Yb-BDC-NH2 [76], ZIF-8 and ZIF-67 [77], MOF-74 [78], cation-exchanged anionic MOFs [79] and IRMOF-3 [80]. We have examined the catalytic behavior of 1′ in Knoevenagel condensation between malononitrile and benzaldehyde at room temperature using different organic solvents like acetonitrile (ACN), dichloromethane (DCM), benzene, toluene and ethanol. The observed catalytic results are shown in Fig. 3. These results suggest that the reaction is efficiently promoted in ethanol compared to other solvents and hence further experiments for the optimization of catalyst loading were performed in ethanol. The Knoevenagel condensation of malononitrile with benzaldehyde using 1′ in ethanol at room temperature was 463

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Fig. 8. Reusability study for the Knoevenagel condensation reaction of malononitrile with benzaldehyde using 1′.

Fig. 6. Time conversion plots for Knoevenagel condensation between malononitrile and benzaldehyde: (a) Hf-UiO-66-N2H3, (b) Hf-UiO-66, (c) H2BDCN2H3 and (d) HfCl4.

Introduction section, the use of Hf-based MOFs has received considerable interest among researchers not only to develop new MOFs but also to achieve better activity than the parent MOFs. In this aspect, the catalytic activity of 1′ is compared with Zr-UiO-66 and Zr-UiO-66-N2H3 under similar experimental conditions (Fig. S14, Supporting Information). These data clearly suggest that 1′ shows higher initial reaction rate and final yield than the analogous Zr-based MOFs. These experimental observations are in good agreement with the lower pKa values of Hf-MOFs than Zr-MOFs [41]. These results undoubtedly highlight the importance of developing Hf-based MOFs than Zr-MOFs. Furthermore, this enhanced activity of 1′ and Hf-UiO-66 compared to analogous ZrUiO-66 and Zr-UiO-66-N2H3 catalysts provides convincing message for the material scientists to develop series of Hf-based MOFs for catalytic applications. A heterogeneous catalyst must be recovered and recycled in the

Fig. 7. Time conversion plots for Knoevenagel reaction between malononitrile and benzaldehyde: (■) 1′, (●) Hf-UiO-66-NH2 and (▲) Hf-UiO-66.

activity at final reaction time. These data imply that the active sites mostly originate from the metal nodes/structural defects than the ligand [81–83]. Furthermore, Fig. 7 shows the reaction rates of 1′, HfUiO-66-NH2 and Hf-UiO-66 under identical reaction conditions. These data also prove that Hf-UiO-66 exhibits higher initial reaction rate than the other two catalysts indicating that the activity arises due to the metal nodes (i.e. uncoordinated Hf sites or structural defects) than the ligand. Furthermore, the slightly lower activity of Hf-UiO-66-NH2 and 1′ catalysts may be attributed to diffusion limitation of reactants compared to Hf-UiO-66. Thus, the activity of 1′ was higher than HfUiO-66-NH2 and Hf-UiO-66 catalysts after 4 h under identical conditions indicating the effect of functionalized linker in promoting this reaction. It is also worth noting here that both Hf-UiO-66-NH2 and 1′ show similar reactivity (Table 1), which further confirms the major role played by metal nodes/structural defects than ligand. It is relevant to point out here that Knoevenagel condensation is catalyzed by acidic and basic sites. Hence, in the present catalytic system, the activity is dominated by acid sites than basic sites. As discussed in the

Fig. 9. XRPD patterns of 1′ (a) before catalysis, (b) recovered after one cycle and (c) recovered after five cycles. 464

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subsequent reaction cycles without losing its activity. This experiment also provides sufficient knowledge on the catalyst stability under the reaction conditions. Hence, the stability of 1′ was assessed by recycling the catalyst in the subsequent runs. It can be obviously noticed from Fig. 8 that the catalyst was used for five cycles without any loss in its performance. These data suggest that the solid MOF catalyst is stable in the present catalysis reaction conditions and the product yield is retained in all the five cycles. Moreover, the stability of this catalyst during the reusability experiments was ascertained by comparing the XRPD pattern of the fresh solid with the recovered and five times used catalysts. The observed XRPD patterns are presented in Fig. 9. These data clearly substantiate that the crystalline nature of the solid is retained for five cycles and no structural damage is noticed. Furthermore, the FE-SEM images (Figs. S15–S16, Supporting Information) confirmed that 1′ preserved its crystallinity as well as morphology even after five catalytic cycles. All these experimental and analytical data reveal that the solid MOF catalyst enjoys higher stability under the optimized reaction conditions. One of the ideal methodologies to assess the catalytic activity of a heterogeneous catalyst with the formerly reported catalysts involves calculation of turnover number or turnover frequency, provided that the active sites are unambiguously identified. We have compared the activity of 1′ with earlier reports and the data are displayed in Table S1 (Supporting Information). In most of the cases, the activity is reported in terms of conversion or yield at a given time due to uncertainty in identifying the active sites. In any way, the yield achieved with 1′ as catalyst at room temperature is preferable to those catalysts operating at 60–130 °C or requiring longer reaction times (12–24 h). The high yield along with catalyst stability during many cycles demonstrates the improvement of 1′ compared to earlier reports [64,84,85]. The preliminary results obtained for 1′ prompted us to investigate the scope of this catalyst with a large range of aldehyde substrates containing electron withdrawing and electron donating substituents. The reaction between benzaldehyde and malononitrile afforded the expected condensation product in 98% yield at room temperature after 4 h (entry 1, Table 2). On the other hand, p-tolualdehyde and p-anisaldehyde reacted with malonononitrile using 1′ giving the condensation product in 98% yield under identical conditions (entries 2–3, Table 2). Benzaldehyde with electron withdrawing substituents like 4fluoro, 4-chloro, 4-bromo and 4-nitrobenzaldehydes also condensed with malononitrile with 1′ as a solid catalyst to provide the respective products in quantitative yields (entries 4–7, Table 2). Furthermore, the reaction between salicylaldehyde and malononitrile employing 1′ gave 65% yield of the product. Heterocyclic aldehyde like 2-furfural conveniently condensed with malononitrile to its respective product in 99% yield under identical conditions. In addition, phenylacetaldehyde reacted with malononitrile in the presence of 1′ to obtain 76% yield (entry 10, Table 2). The reaction between 1-naphthaldehyde and 9anthracenealdehyde with malononitrile afforded 93% and 67% yields, respectively under identical conditions (entries 11–12, Table 2). This difference in the yields may be due to the larger dimensions of the later substrate as compared to the former one. Considering the reactivity of benzaldehyde, 1-naphthaldehyde and 9-anthracenealdehyde, it can be seen undoubtedly that the order of reactivity decreases (benzaldehyde >1-naphthaldehyde> 9-anthracenealdehyde) as the molecular size of the aldehyde increases. These experimental data further indicate that the catalytic reaction occurs within the voids of 1′. Furthermore, unsaturated aldehyde like cinnamaldehyde reacted with malononitrile to afford 53% of the condensed product under similar conditions (entry 13, Table 2). Substrates like 4-t-butylbenzaldehyde and 3,4,5-trimethoxybenzaldehyde exhibited 92 and 98% of the respective Knoevenagel condensation product under the optimized conditions (entries 14–15, Table 2). Finally, 4-hydroxy-3-ethoxybenzaldehyde reacted with malononitrile to give 56% yield of the expected product after 4 h at room temperature. This moderate yield may be due to the larger dimension of the substrate and it may encounter diffusion limitation to

reach the active sites. In any case, the data presented in Table 2 and Figs. S17–S32 (Supporting Information) clearly point out that the solid catalyst 1′ can be conveniently used to perform Knoevenagel reaction in short reaction time at ambient temperature. The salient features of this catalyst include exhibition of high activity in the Knoevenagel reaction, working under milder reaction conditions like room temperature, short reaction time, possessing wide substrate scope and high stability as evidenced by reusability as well as leaching experiments. 4. Conclusions A new Hf based MOF material called Hf-UiO-66-N2H3 (1) having H2BDC-N2H3 ligand in its framework has been synthesized successfully and characterized comprehensively. The catalytic activity of the activated compound (1′) was examined in the Knoevenagel reaction between malononitrile and benzaldehyde and quantitative yield was observed at room temperature after 4 h. In addition, it was unambiguously proved that Hf-based MOFs exhibit better activity than analogous Zr MOFs in same catalysis conditions. Furthermore, the substrate scope of 1′ was also screened with a series of aldehydes containing substituents with various electronic properties and having different molecular dimensions. For the tested aldehyde substrates, moderate to high yields were obtained. Size selectivity was also observed for benzaldehyde, 1naphthaldehyde and 9-anthracenealdehyde under identical reaction conditions, which points out that the reaction takes place within the voids of the framework solids. The reusability experiments revealed that the solid MOF catalyst is stable for five cycles and no evidence was observed for the leaching of metal ions. Hence, the catalytic reaction is heterogeneous in nature. Further work is in progress to elucidate the activity due to the functionalized linker in 1′. Acknowledgements S.B. acknowledges financial support from SERB, New Delhi (grant no. EEQ/2016/000012). A.D.M. thanks SERB, New Delhi (EMR/2016/ 006500) for financial assistance. A.D.M. thanks the University Grants Commission, New Delhi, for the award of an Assistant Professorship under its Faculty Recharge Programme. Appendix A. Supplementary data Supplementary data to this article can be found online at https:// doi.org/10.1016/j.micromeso.2019.04.057. References [1] R.C.M.A. Sobrinho, P.M.D. Oliveira, C.R.M. D'Oca, D. Russowsky, M.G.M. D'Oca, Solvent-free Knoevenagel reaction catalysed by reusable pyrrolidinium base protic ionic liquids (PyrrILs): synthesis of long-chain alkylidenes, RSC Adv. 7 (2017) 3214–3221. [2] F. Martínez, G. Orcajo, D. Briones, P. Leo, G. Calleja, Catalytic advantages of NH2modified MIL-53(Al) materials for Knoevenagel condensation reaction, Microporous Mesoporous Mater. 246 (2017) 43–50. [3] L.F. Tietze, Domino reactions in organic synthesis, Chem. Rev. 96 (1996) 115–136. [4] S. Hasegawa, S. Horike, R. Matsuda, S. Furukawa, K. Mochizuki, Y. Kinoshita, S. Kitagawa, Three-dimensional porous coordination polymer functionalized with amide groups based on tridentate ligand: selective sorption and catalysis, J. Am. Chem. Soc. 29 (2007) 2607–2614. [5] H. Mei, X. Xiao, X. Zhao, B. Fang, X. Liu, L. Lin, X. Feng, Catalytic asymmetric Henry reaction of nitroalkanes and aldehydes catalyzed by a chiral N,N′-Dioxide/Cu(I) complex, J. Org. Chem. 80 (2015) 2272–2280. [6] M. Phukan, K.J. Borah, R. Borah, Henry reaction in environmentally benign methods using imidazole as catalyst, Green Chem. Lett. Rev. 2 (2009) 249–253. [7] H. Qian, D. Liu, Synthesis of chalcones via claisen-schmidt reaction catalyzed by sulfonic acid-functional ionic liquids, Ind. Eng. Chem. Res. 50 (2011) 1146–1149. [8] V.G. Sadvilkar, S.D. Samant, V.G. Gaikar, Claisen-schmidt reaction in a hydro tropic medium, J. Appl. Chem. Biotechnol. 62 (1995) 405–410. [9] X.-C. Yi, M.-X. Huang, Y. Qi, E.-Q. Gao, Synthesis, structure, luminescence and catalytic properties of cadmium(II) coordination polymers with 9H-carbazole-2,7dicarboxylic acid, Dalton Trans. 43 (2014) 3691–3697. [10] J. Chen, R. Liu, H. Gao, L. Chen, D. Ye, Amine-functionalized metal-organic

465

Microporous and Mesoporous Materials 284 (2019) 459–467

A. Das, et al.

[11] [12] [13] [14] [15] [16] [17]

[18] [19] [20] [21] [22] [23] [24] [25] [26] [27] [28] [29] [30] [31] [32] [33] [34] [35] [36]

[37] [38]

frameworks for the transesterification of triglycerides, J. Mater. Chem. 2 (2014) 7205–7213. Z. Miao, Y. Luan, C. Qi, D. Ramella, The synthesis of a bifunctional copper metal organic framework and its application in the aerobic oxidation/Knoevenagel condensation sequential reaction, Dalton Trans. 45 (2016) 13917–13924. J. Liang, G. Zhang, J. Yang, W. Sun, M. Shi, TiO2 hierarchical nanostructures: hydrothermal fabrication and application in dyesensitized solar cells, AIP Adv. 5 (2015) 017141–017153. A. Villa, J.-P. Tessonnier, O. Majoulet, D.S. Su, R. Schlögl, Transesterification of triglycerides using NitrogenFunctionalized carbon nanotubes, ChemSusChem 3 (2010) 241–245. L.T.L. Nguyen, K.K.A. Le, H.X. Truong, N.T.S. Phan, Metal–organic frameworks for catalysis: the Knoevenagel reaction using zeolite imidazolate framework ZIF-9 as an efficient heterogeneous catalyst, Catal. Sci. Technol. 2 (2012) 521–528. B. Helms, S.J. Guillaudeu, Y. Xie, M. McMurdo, C.J. Hawker, J.M.J. Fréchet, OnePot reaction cascades using star polymers with core-confined catalysts, Angew. Chem. Int. Ed. 44 (2005) 6384–6387. J. Liu, L. Chen, H. Cui, J. Zhang, L. Zhang, C.-Y. Su, Applications of metal–organic frameworks in heterogeneous supramolecular catalysis, Chem. Soc. Rev. 43 (2014) 6011–6061. S.R. Batten, B.F. Hoskins, R. Robson, Two interpenetrating 3D networks which generate spacious sealed-off compartments enclosing of the order of 20 solvent molecules in the structures of Zn(CN)(NO3)(tpt)2/3.solv(tpt = 2,4,6-tri(4-pyridyl)1,3,5-triazine, solv = ∼3/4∼C2H2Cl4.3/4CH3OH or -3/∼CHCl3.1/3CH3OH, J. Am. Chem. Soc. 117 (1995) 5385–5386. A.H. Chughtai, N. Ahmad, H.A. Younus, A. Laypkov, F. Verpoort, Metal–organic frameworks: versatile heterogeneous catalysts for efficient catalytic organic transformations, Chem. Soc. Rev. 44 (2015) 6804–6849. K. Vikrant, V. Kumar, K.-H. Kim, D. Kukkar, Metal–organic frameworks (MOFs): potential and challenges for capture and abatement of ammonia, J. Mater. Chem. 5 (2017) 22877–22896. H. Furukawa, K.E. Cordova, M. O'Keeffe, O.M. Yaghi, The chemistry and applications of metal-organic frameworks, Science 30 (2013) 1230444–1230457. A.V. Desai, B. Joarder, A. Roy, P. Samanta, R. Babarao, S.K. Ghosh, Multifunctional behavior of sulfonate-based hydrolytically stable microporous metal-organic frameworks, ACS Appl. Mater. Interfaces 10 (2018) 39049–39055. B. Li, H.-M. Wen, W. Zhou, B. Chen, Porous Metal−Organic frameworks for gas storage and separation: what, how, and why? J. Phys. Chem. Lett. 5 (2014) 3468–3479. A. Das, S. Banesh, V. Trivedi, S. Biswas, Extraordinary sensitivity for H2S and Fe(III) sensing in aqueous medium by Al-MIL-53-N3 metal–organic framework:in vitro and in vivo applications of H2S sensing, Dalton Trans. 47 (2018) 2690–2700. I.T. Weber, A.J.G.D. Melo, M.A.D.M. Lucena, M.O. Rodrigues, S.A. Junior, High photoluminescent metal organic frameworks as optical markers for the identification of gunshot residues, Anal. Chem. 83 (2011) 4720–4723. X. Zhao, S. Liu, Z. Tang, H. Niu, Y. Cai, W. Meng, F. Wu, J.P. Giesy, Synthesis of magnetic metalorganic framework (MOF) for efficient removal of organic dyes from water, Sci. Rep. 5 (2015) 11849–11858. M.-X. Wu, Y.-W. Yang, Metal–organic framework (MOF)-Based drug/cargo delivery and cancer therapy, Adv. Mater. 29 (2017) 1606134–1606153. P. Horcajada, R. Gref, T. Baati, P.K. Allan, G. Maurin, P. Couvreur, G. Férey, R.E. Morris, C. Serre, Metal-organic frameworks in biomedicine, Chem. Rev. 112 (2012) 1232–1268. Y. Li, H. Xu, S. Ouyang, J. Ye, Metal–organic frameworks for photocatalysis, Phys. Chem. Chem. Phys. 18 (2016) 7563–7572. H.-C. Kim, S. Huh, S.-J. Kim, Y. Kim, Selective carbon dioxide sorption and heterogeneous catalysis by a new 3D Zn-MOF with nitrogen-rich 1D channels, Sci. Rep. 7 (2017) 17584–17595. S. Goswami, H.S. Jena, S. Konar, Study of heterogeneous catalysis by iron-squarate based 3D metal organic framework for the transformation of tetrazines to oxadiazole derivatives, Inorg. Chem. 53 (2014) 7071–7073. A. Dhakshinamoorthy, A.M. Asiri, H. Garcia, Tuneable nature of metal organic frameworks as heterogeneous solid catalysts for alcohol oxidation, Chem. Commun. 53 (2017) 10851–10869. G.-J. Chen, H.-C. Ma, W.-L. Xin, X.-B. Li, F.-Z. Jin, J.-S. Wang, M.-Y. Liu, Y.-B. Dong, Dual heterogeneous catalyst Pd−Au@Mn(II)-MOF for one-pot tandem synthesis of imines from alcohols and amines, Inorg. Chem. 56 (2017) 654–660. L. Zhu, X.-Q. Liu, H.-L. Jiang, L.-B. Sun, Metal−Organic frameworks for heterogeneous basic catalysis, Chem. Rev. 117 (2017) 8129–8176. A. Dhakshinamoorthy, N. Heidenreich, D. Lenzen, N. Stoc, Knoevenagel condensation reaction catalysed by Al-MOFs with CAU-1 and CAU-10-type structures, CrystEngComm 19 (2017) 4187–4193. A. Taher, D.-J. Lee, B.-K. Lee, I.-M. Lee, Amine-functionalized metal-organic frameworks: an efficient and recyclable heterogeneous catalyst for the Knoevenagel condensation reaction, Synlett 27 (2017) 1433–1437. Y. Luan, Y. Qi, H. Gao, R.S. Andriamitantsoa, N. Zheng, G. Wang, A general postsynthetic modification approach of amino-tagged metal–organic frameworks to access efficient catalysts for the Knoevenagel condensation reaction, J. Mater. Chem. 3 (2015) 17320–17331. Q. Yang, H.-Y. Zhang, L. Wang, Y. Zhang, J. Zhao, Ru/UiO-66 catalyst for the reduction of nitroarenes and tandem reaction of alcohol oxidation/knoevenagel condensation, ACS Omega 3 (2018) 4199–4212. J.H. Cavka, S. Jakobsen, U. Olsbye, N. Guillou, C. Lamberti, S. Bordiga, K.P. Lillerud, A new zirconium inorganic building brick forming metal organic frameworks with exceptional stability, J. Am. Chem. Soc. 130 (2008) 13850–13851.

[39] V. Bon, I. Senkovska, I.A. Baburin, S. Kaskel, Zr- and Hf-based Metal−Organic frameworks: tracking down the polymorphism, Cryst. Growth Des. 13 (2013) 1231–1237. [40] Y. Bai, Y. Dou, L.-H. Xie, W. Rutledge, J.-R. Li, H.-C. Zhou, Zr-based metal–organic frameworks: design, synthesis, structure, and applications, Chem. Soc. Rev. 45 (2016) 2327–2367. [41] S. Rojas-Buzo, P. García-García, A. Corma, Hf-based metal–organic frameworks as acid–base catalysts for the transformation of biomass-derived furanic compounds into chemicals, Green Chem. 20 (2018) 3081–3091. [42] Z. Hu, E.M. Mahdi, Y. Peng, Y. Qian, B. Zhang, N. Yan, D. Yuan, J.-C. Tan, D. Zhao, Kinetically controlled synthesis of two-dimensional Zr/Hf metal–organic framework nanosheets via a modulated hydrothermal approach, J. Mater. Chem. 5 (2017) 8954–8963. [43] S. Waitschat, H. Reinsch, M. Arpacioglua, N. Stock, Direct Water-Based Synthesis and Characterization of New Zr/Hf-MOFs with Dodecanuclear Clusters as IBUs, CrystEngComm 20 (2018) 5108–5111. [44] M. SK, S. Nandi, R.K. Singh, V. Trivedi, S. Biswas, Selective sensing of peroxynitrite by Hf-based UiO-66-B(OH)2 metal–organic framework: applicability to cell imaging, Inorg. Chem. 57 (2018) 10128–10136. [45] Z. Hu, A. Nalaparaju, Y. Peng, J. Jiang, D. Zhao, Modulated hydrothermal synthesis of uio-66(Hf)-Type Metal−Organic frameworks for optimal carbon dioxide separation, Inorg. Chem. 55 (2016) 1134–1141. [46] M.A. Nasalevich, C.H. Hendon, J.G. Santaclara, K. Svane, B.V.D. Linden, S.L. Veber, M.V. Fedin, A.J. Houtepen, M.A.V.D. Veen, F. Kapteijn, A. Walsh, J. Gascon, Electronic origins of photocatalytic activity in d0 metal organic frameworks, Sci. Rep. 6 (2016) 23676–23684. [47] L.H.T. Nguyen, T.T. Nguyen, H.L. Nguyen, T.L.H. Doan, P.H. Tran, A new superacid hafnium-based metal-organic framework as a highly active heterogeneous catalyst for the synthesis of benzoxazoles under solvent-free condition, Catal. Sci. Technol. 7 (2017) 4346–4350. [48] M. Rimoldi, A.J. Howarth, M.R. DeStefano, L. Lin, S. Goswami, P. Li, J.T. Hupp, O.K. Farha, Catalytic zirconium/hafnium-based Metal−Organic frameworks, ACS Catal. 7 (2017) 997–1014. [49] W.P. Lustig, S. Mukherjee, N.D. Rudd, A.V. Desai, J. Li, S.K. Ghosh, Metal–organic frameworks: functional luminescent and photonic materials for sensing applications, Chem. Soc. Rev. 46 (2017) 3242–3285. [50] S. Nandi, S. Banesh, V. Trivedi, S. Biswas, A dinitro-functionalized metal–organic framework featuring visual and fluorogenic sensing of H2S in living cells, human blood plasma and environmental samples, Analyst 143 (2018) 1482–1491. [51] A. Das, S. Das, V. Trivedi, S. Biswas, A dual functional MOF-based fluorescent sensor for intracellular phosphate and extracellular 4-nitrobenzaldehyde, Dalton Trans. 48 (2019) 1332–1343. [52] N. Stock, S. Biswas, Synthesis of metal-organic frameworks (MOFs): routes to various MOF topologies, morphologies, and composites, Chem. Rev. 112 (2012) 933–969. [53] A. Das, S. Biswas, A multi-responsive carbazole-functionalized Zr(IV)-based metalorganic framework for selective sensing of Fe(III), cyanide and p-nitrophenol, Sens. Actuators, B 250 (2017) 121–131. [54] R. Dalapati, B. Sakthivel, M.K. Ghosalya, A. Dhakshinamoorthy, S. Biswas, A cerium-based metal–organic framework having inherent oxidase-like activity applicable for colorimetric sensing of biothiols and aerobic oxidation of thiols, CrystEngComm 19 (2017) 5915–5925. [55] L. Valenzano, B. Civalleri, S. Chavan, S. Bordiga, M.H. Nilsen, S. Jakobsen, K.P. Lillerud, C. Lamberti, Disclosing the complex structure of uio-66 metal organic framework: a synergic combination of experiment and theory, Chem. Mater. 23 (2011) 1700–1718. [56] M. Kandiah, M.H. Nilsen, S. Usseglio, S. Jakobsen, U. Olsbye, M. Tilset, C. Larabi, E.A. Quadrelli, F. Bonino, K.P. Lillerud, Synthesis and stability of tagged UiO-66 ZrMOFs, Chem. Mater. 22 (2010) 6632–6640. [57] K.E. DeKrafft, W.S. Boyle, L.M. Burk, O.Z. Zhou, W. Lin, Zr- and Hf-based nanoscale metal–organic frameworks as contrast agents for computed tomography, J. Mater. Chem. 22 (2012) 18139–18144. [58] M.A. Nasalevich, C.H. Hendon, J.G. Santaclara, K. Svane, B.V.D. Linden, S.L. Veber, M.V. Fedin, A.J. Houtepen, M.A.V.D. Veen, F. Kapteijn, A. Walsh, J. Gascon, Electronic origins of photocatalytic activity in d0 metal organic frameworks, Sci. Rep 6 (2016) 23676–23684. [59] S. Waitschat, D. Fröhlich, H. Reinsch, H. Terraschke, K.A. Lomachenko, C. Lamberti, H. Kummer, T. Helling, M. Baumgartner, S. Henninger, N. Stock, Synthesis of MUiO-66 (M = Zr, Ce or Hf ) employing 2,5-pyridinedicarboxylic acid as a linker: defect chemistry, framework hydrophilisation and sorption properties, Dalton Trans. 47 (2018) 1062–1070. [60] H. Wu, Y.S. Chua, V. Krungleviciute, M. Tyagi, P. Chen, T. Yildirim, W. Zhou, Unusual and highly tunable missing-linker defects in zirconium metal–organic framework uio-66 and their important effects on gas adsorption, J. Am. Chem. Soc. 135 (2013) 10525–10532. [61] P. Ghosh, Y.J. Colóna, R.Q. Snurr, Water adsorption in UiO-66: the importance of defects, Chem. Commun. 50 (2014) 11329–11331. [62] X.-C. Yi, F.-G. Xi, Y. Qi, E.-Q. Gao, Synthesis and click modification of an azidofunctionalized Zr(IV) metal–organic framework and a catalytic study, RSC Adv. 5 (2015) 893–900. [63] D. Wang, Z. Li, Bi-functional NH2-MIL-101(Fe) for one-pot tandem photo-oxidation/Knoevenagel condensation between aromatic alcohols and active methylene compounds, Catal. Sci. Technol. 5 (2015) 1623–1628. [64] M. Hartmann, M. Fischer, Amino-functionalized basic catalysts with MIL-101 structure, Microporous Mesoporous Mater. 164 (2012) 38–43. [65] Y. Luan, Y. Qi, H. Gao, R.S. Andriamitantsoa, N. Zheng, G. Wang, A general post-

466

Microporous and Mesoporous Materials 284 (2019) 459–467

A. Das, et al.

[66]

[67]

[68] [69] [70] [71] [72]

[73] [74] [75]

synthetic modification approach of amino-tagged metal–organic frameworks to access efficient catalysts for the Knoevenagel condensation reaction, J. Mater. Chem. 3 (2015) 17320–17331. H. He, F. Sun, B. Aguila, J.A. Perman, S. Ma, G. Zhu, A bifunctional metal–organic framework featuring the combination of open metal sites and Lewis basic sites for selective gas adsorption and heterogeneous cascade catalysis, J. Mater. Chem. 4 (2016) 15240–15246. H. Ghasempour, A.A. Tehrani, A. Morsali, J. Wang, P.C. Junk, Two pillared metal–organic frameworks comprising a long pillar ligand used as fluorescent sensors for nitrobenzene and heterogeneous catalysts for the Knoevenagel condensation reaction, CrystEngComm 18 (2016) 2463–2468. H. Liu, F.-G. Xi, W. Sun, N.-N. Yang, E.-Q. Gao, Amino- and sulfo-bifunctionalized metal–organic frameworks: one-pot tandem catalysis and the catalytic sites, Inorg. Chem. 55 (2016) 5753–5755. H. Ghasempour, A. Morsali, Ultrasound-assisted synthesized and catalytic studies of two nano-structured metal–organic frameworks with long N-donor ligand as a pillar, Polyhedron 151 (2018) 58–65. M. Ghorbanloo, V. Safarifard, A. Morsali, Heterogeneous catalysis with a coordination modulation synthesized MOF: morphology-dependent catalytic activity, New J. Chem. 41 (2017) 3957–3965. M.Y. Masoomi, S. Beheshti, A. Morsali, Mechanosynthesis of new azine-functionalized Zn(II) metal–organic frameworks for improved catalytic performance, J. Mater. Chem. 2 (2014) 16863–16866. B. Ugale, S.S. Dhankhar, C.M. Nagaraja, Construction of 3D homochiral metal–organic frameworks (MOFs) of Cd(II): selective CO2 adsorption and catalytic properties for the Knoevenagel and Henry reaction, Inorg. Chem. Front. 4 (2017) 348–359. S. Zhang, H. He, F. Sun, N. Zhao, G. Zhu, A novel adenine-based zinc(II) metalorganic framework featuring the Lewis basic sites for heterogeneous catalysis, Inorg. Chem. Commun. 79 (2017) 55–59. J. Huang, C. Li, L. Tao, H. Zhu, G. Hu, Synthesis, characterization and heterogeneous base catalysis of amino functionalized lanthanide metal-organic frameworks, J. Mol. Struct. 1146 (2017) 853–860. W.-L. Jiang, Q.-J. Fu, B.-J. Yao, L.-G. Ding, C.-X. Liu, Y.-B. Dong, Smart pH-

[76] [77] [78] [79] [80] [81]

[82] [83]

[84] [85]

467

responsive polymer-tethered and Pd NP-loaded NMOF as the pickering interfacial catalyst for one-pot cascade biphasic reaction, ACS Appl. Mater. Interfaces 9 (2017) 36438–36446. Y. Zhang, Y. Wang, L. Liu, N. Wei, M.-L. Gao, D. Zhao, Z.-B. Han, Robust bifunctional lanthanide cluster based metal–organic frameworks (MOFs) for tandem deacetalization–knoevenagel reaction, Inorg. Chem. 57 (2018) 2193–2198. S.F. Amarante, M.A. Freire, D.T.S.L. Mendes, L.S. Freitas, A.L.D. Ramos, Evaluation of basic sites of ZIFs metal organic frameworks in the Knoevenagel condensation reaction, Appl. Catal. Gen. 548 (2017) 47–51. P. Valvekens, M. Vandichel, M. Waroquier, V.V. Speybroeck, D.D. Vos, Metal-dioxidoterephthalate MOFs of the MOF-74 type: microporous basic catalysts with well-defined active sites, J. Catal. 317 (2014) 1–10. S. Beheshti, A. Morsali, Post-synthetic cation exchange in anionic metal–organic frameworks; a novel strategy for increasing the catalytic activity in solvent-free condensation reactions, RSC Adv. 4 (2014) 41825–41830. J. Gascon, U. Aktay, M.D. Hernandez-Alonso, G.P.M.v. Klink, F. Kapteijn, Aminobased metal-organic frameworks as stable, highly active basic catalysts, J. Catal. 261 (2009) 75–87. M. Vandichel, J. Hajek, F. Vermoortele, M. Waroquier, D.E.D. Vos, V.V. Speybroeck, Active site engineering in UiO-66 type metal–organic frameworks by intentional creation of defects: a theoretical rationalization, CrystEngComm 17 (2015) 395–406. S. Ling, B. Slater, Dynamic acidity in defective UiO-66, Chem. Sci. 7 (2016) 4706–4712. O.V. Gutov, M.G. Hevia, E.C. Escudero-Adán, A. Shafir, Metal–organic framework (MOF) defects under control: insights into the missing linker sites and their implication in the reactivity of zirconium-based frameworks, Inorg. Chem. 54 (2015) 8396–8400. M. Almáši, V. Zeleňák, M. Opanasenko, J. Čejka, A novel nickel metal–organic framework with fluorite-like structure: gas adsorption properties and catalytic activity in Knoevenagel condensation, Dalton Trans. 43 (2014) 3730–3738. M. Opanasenko, A. Dhakshinamoorthy, M. Shamzhy, P. Nachtigall, M. Horáček, H. Garcia, J. Čejka, Comparison of the catalytic activity of MOFs and zeolites in Knoevenagel condensation, Catal. Sci. Technol. 3 (2013) 500–507.