View PDF VersionPrevious ArticleNext Article
 
DOI: 10.1039/C6CE02664H (Paper) CrystEngComm, 2017, 19, 4187-4193

Knoevenagel condensation reaction catalysed by Al-MOFs with CAU-1 and CAU-10-type structures†

Amarajothi Dhakshinamoorthy *ab, Niclas Heidenreich b, Dirk Lenzen b and Norbert Stock *b
aSchool of Chemistry, Madurai Kamaraj University, Madurai-625 021, Tamil Nadu, India. E-mail: admguru@gmail.com; Tel: +91 99764 73669
bInstitut fur Anorganische Chemie, Christian-Albrechts-Universität, Max-Eyth Strasse 2, 24118 Kiel, Germany. E-mail: stock@ac.uni-kiel.de; Fax: +49 4318801775; Tel: +49 4318801675

Received 2nd January 2017 , Accepted 25th January 2017

First published on 2nd February 2017

Abstract

The metal–organic frameworks (MOFs) CAU-1-NH2 ([Al4(OH)2(OCH3)4(p-BDC-NH2)3], (p-BDC-NH2)2− = 2-aminoterephthalate) and CAU-10-NH2 ([Al(OH)(m-BDC-NH2)], (m-BDC-NH2)2− = 5-aminoisophthalate) which possess two different pore sizes were studied for their catalytic activity as heterogeneous solid base catalysts in the Knoevenagel condensation reaction between benzaldehyde and malononitrile under mild reaction conditions (40 °C, 7 h, ethanol). For comparison, isoreticular MOFs containing a smaller amount of –NH2 groups (CAU-1-NH2/H) or no –NH2 groups (CAU-10-H) were synthesized. A two-step synthesis route including the synthesis of CAU-1-NH2 and its use as crystal seeds was developed to obtain the mixed-linker CAU-1-NH2/H compound. Only for CAU-1-NH2, up to 100% selectivity towards the desired Knoevenagel condensation product is observed. Hence, the catalytic activity of CAU-1-NH2 was tested between various benzaldehydes containing different substituents with malononitrile. By employing benzaldehyde and malononitrile as the starting materials, it is found that the mixed-linker MOF CAU-1-NH2/H catalyses the formation of the acetal (benzaldehyde diethyl acetal), while both CAU-10 materials lead to mixtures of the two products. Furthermore, the catalyst stability was also examined through reusability and leaching experiments and it is observed that the catalyst can be reused with no significant drop in its activity.

Introduction

In the field of catalysis, porous crystalline materials are attractive candidates due to their high specific surface areas and their well-ordered pore dimensions. In the category of porous crystalline materials, metal–organic frameworks (MOFs) that consist of inorganic building units (IBUs) bridged by organic linkers are particularly suited for catalysis.1 MOFs present, in addition to very large specific surface areas, the option to tailor their pore surface properties towards the requirements of certain catalytic processes. This has been accomplished by direct synthesis using functionalized organic linker molecules or via post-synthetic modification.2 Both methods preserve the parent framework structure and leave the porous structure accessible for the reactants of a chemical reaction.

As MOFs contain organic moieties that decompose at higher temperatures, these compounds are primarily applicable to catalytic processes at lower temperatures. The thermal stability of a MOF strongly depends on its composition and the structure of the IBU and some Al-based MOFs have been shown to be both thermally and chemically stable.3 For example, the compound [Al(OH)(m-BDC)] also known as CAU-10-H (m-H2BDC: 1,3-benzenedicarboxylic acid) can be used in heat transformation applications and is stable towards water sorption/desorption for at least 10[thin space (1/6-em)]000 cycles without loss of crystallinity and water adsorption capacity.4 This compound can also be directly synthesized with various functional groups on the linker molecule and the sulfonated derivative (CAU-10-SO3H) has been successfully used as a catalyst in the gas phase dehydration of ethanol.5 The structure of CAU-10-H is composed of helical chains consisting of cis-corner-sharing AlO6 polyhedra which are connected to four adjacent chains via m-BDC2− ions. Accordingly, CAU-10-H exhibits a framework with square-shaped one-dimensional modulated pores with a diagonal opening of ∼3.6 Å taking van der Waals radii into account (Fig. 1). CAU-1-NH2 ([Al4(OH)2(OCH3)4(p-BDC-NH2)3]·xH2O) is an Al-MOF with a three-dimensional pore system and two different pore sizes. The structure consists of eight-membered rings composed of corner- and edge-sharing AlO6 polyhedra. Each IBU is bridged via 2-aminoterephthalate (p-BDC-NH22−) ions to 12 other IBUs. The resulting structure possesses tetrahedral and octahedral cavities with diameters of ∼5 and ∼10 Å, respectively (Fig. 1).6


image file: c6ce02664h-f1.tif
Fig. 1 Section of the crystal structures of CAU-1 (left) and CAU-10 (right, view along the channel axis ([001])). H-atoms have been omitted for clarity.

The Knoevenagel condensation is one of the fundamental reactions in organic chemistry for the formation of carbon–carbon bonds.7 The condensation reaction occurs between carbonyl compounds and activated methylene compounds. This carbon–carbon bond coupling reaction is widely used for the production of important drug intermediates. Furthermore, this reaction is employed as a classical test reaction to examine the activity of newly synthesised Brønsted-base catalysts. Due to their significance from an industrial and synthetic point of view, a large number of catalytic systems have been reported for Knoevenagel condensation reactions. For example, the reaction of benzaldehyde with malononitrile has been studied using [Cd(4-btapa)2(NO3)2]·6H2O·2DMF (4-btapa: 1,3,5-benzenetricarboxylic acid tris[N-(4-pyridyl)amide]),8 [Gd2(tnbd)3(DMF)4]·4DMF·3H2O (tnbd2−: 2,6,2′,6′-tetranitro-biphenyl-4,4′-dicarboxylate),9 ZIF-8,10 ZIF-9,11 Cu3(BTC)2 or Fe(BTC) (BTC3−: 1,3,5-benzenetricarboxylate)12 as solid Lewis-acid catalysts. In contrast, the same reaction was also catalyzed by basic Brønsted sites available in MOFs like DETA-Cr-MIL-101 (ref. 13) (DETA: diethylenetriamine), Fe-MIL-101-NH2,14 Zn2(tpt)2(p-BDC-NH2)I2 (ref. 15) (tpt: tris(4-pyridyl)triazine, p-H2BDC-NH2: 2-aminoterephthalic acid) and others.16

Al-MOFs have also been tested in catalysis. Thus, Gascon et al. have reported the basic properties of IRMOF-3 and Al-MIL-53-NH2 in the Knoevenagel condensation of benzaldehyde with ethyl cyanoacetate wherein the latter catalyst is less active than the former one.17 Furthermore, the catalytic activities of Fe-MIL-101-NH2, Al-MIL-101-NH2 and CAU-1-NH2 were tested in the Knoevenagel condensation of benzaldehyde with malononitrile and ethyl cyanoacetate, respectively.14 Al-MIL-101-NH2 has been reported as a bifunctional Lewis-acid/Brønsted-base heterogeneous catalyst in a tandem Meinwald rearrangement/Knoevenagel condensation reaction.18 Recently, a post-synthetic modification strategy was employed for the introduction of a primary alkyl amino group in Al-MIL-53 and its activity was examined in Knoevenagel condensation.19

Herein, we report a highly active and selective recyclable catalyst, namely CAU-1-NH2, for the Knoevenagel condensation reaction between various benzaldehyde derivatives and malononitrile which is catalyzed by primary amine groups. The activities were compared with those of the CAU-10-H, CAU-10-NH2 and CAU-1-NH2/H heterogeneous catalysts under mild reaction conditions. The catalyst stability was also ascertained by performing leaching and reusability experiments.

Experimental

Materials and methods

The chemicals used for the synthesis of CAU-1-NH2, CAU-1-NH2/H, CAU-10 and CAU-10-NH2 are commercially available and were employed without further purification. The powder X-ray diffraction (PXRD) measurements were carried out on a STOE Stadi-P powder diffractometer equipped with a MYTHEN1K detector system (Cu Kα1 radiation). MIR spectra were recorded on an ATI Matheson Genesis spectrometer equipped with an ATR unit in the spectral range of 400–4000 cm−1. Sorption experiments were carried out using a Belsorp-max instrument (BEL JAPAN INC.). Before the sorption experiments, all samples were activated at 150 °C under vacuum (102 kPa) overnight. These conditions were chosen to prevent the exchange of methoxy by hydroxyl groups in the case of CAU-1-type samples.20 Gas chromatography (6890N Network, Agilent Technologies, FID detector) was used for determining the conversion and selectivity. The product was confirmed by GC-MS measurements on a Hewlett Packard 5971A GC-MS instrument. CAU-1-NH2/H was prepared using an Anton Paar Synthos 3000 multimode microwave oven. The EDX spectroscopic measurements were carried out on a Philips ESEM-XL 30 while 1H-NMR spectra were collected using a Bruker DRX 500.

Synthetic procedures

Synthesis of CAU-1-NH2. CAU-1-NH2 ([Al4(OH)2(OCH3)4(p-BDC-NH2)3]·xH2O) was synthesized with a slightly altered synthesis route based on the method reported previously.20 Briefly, CAU-1-NH2 was synthesised using MW-assisted heating (Biotage Initiator). A mixture of AlCl3·6H2O (232 mg, 0.961 mmol) and p-H2BDC-NH2 (58 mg, 0.312 mmol) was suspended in methanol (3.175 mL) in a 5 mL glass vial. After sealing, the reaction mixture was heated under stirring (300 r s−1) at 145 °C for 6 min. The reaction mixture was rapidly cooled to room temperature and a yellow microcrystalline dispersion was obtained. After centrifugation, the product was redispersed in water (25 mL) to remove Cl− ions. This procedure was repeated (three times) until no residual Cl− ions could be detected by EDX spectroscopy. The final product was dried in air to yield CAU-1-NH2. To evaluate the methylation of the amino groups, a small amount of the MOF was dissolved in a solution of NaOD/D2O (5%) and analysed via1H-NMR spectroscopy. The degree of methylation was determined to be ∼4%. Elemental analysis of the washed phase: observed, C: 41.22, H: 3.65, N: 5.46; calculated (based on [Al4(OH)2(OCH3)4(p-BDC-NH2)3]), C: 41.84, H: 3.64, N: 5.22%.
Synthesis of CAU-1-NH2/H. The synthesis of CAU-1-H containing only unfunctionalized terephthalate ions has not yet been reported. Hence, a two-step procedure was developed for the formation of the mixed-linker MOF CAU-1-NH2/H containing predominantly p-BDC2− ions. In a typical synthesis, 4 mg of previously synthesized CAU-1-NH2 was used as crystallization seeds and added to 8.2 mg of terephthalic acid (p-H2BDC). Subsequently, 1235 μl of a 0.12 mol l−1 solution of AlCl3·6H2O in methanol was added and the mixture was heated to 120 °C under stirring using MW-assisted heating. After a reaction time of 60 min, the product was filtered and washed twice with DMF to remove any remaining unreacted linker. As in the case of CAU-1-NH2, part of the product was dissolved in NaOD/D2O (5%) and analysed by 1H-NMR spectroscopy. The ratio of p-BDC2− to p-BDC-NH22− in the product was evaluated by integrating and comparing the signals corresponding to each linker in the 1H-NMR spectrum (S4 in the ESI†). The amount of linkers in the product was quantified to be ∼76% p-BDC2− and ∼24% p-BDC-NH22−.
Synthesis of CAU-10-H and CAU-10-NH2. The unfunctionalized and the amino-functionalized forms of CAU-10 were obtained according to the published literature procedure.5a

Experimental procedure for Knoevenagel condensation

A 25 mL round-bottomed flask was charged with the catalyst (25 mg) and ethanol (3 mL). To this slurry, benzaldehyde (0.5 mmol) and malononitrile (0.5 mmol) were added. The reaction mixture was stirred for the required time. The reaction was monitored periodically by analyzing the sample by GC until completion of the reaction. The percentage conversion and selectivity were determined by GC analysis. The product was confirmed by GC-MS analysis. Prior to the experiments, the respective sample was activated at 120 °C for 3 h under reduced pressure.

Leaching experiment

The Knoevenagel condensation reaction was started as mentioned in the experimental procedure. Then, the catalyst was removed by filtration using a 0.45 μm syringe filter after 15 min and the resulting reaction was continued for 7 h. Afterwards, the reaction mixture was dissolved with water and dichloromethane. The aqueous layer was submitted for AAS analysis to detect the Al3+ which leached into the reaction mixture.

Results and discussion

Four different Al-MOFs, CAU-1-NH2, CAU-1-NH2/H, CAU-10-H and CAU-10-NH2, were synthesized and used in the Knoevenagel condensation reaction at a low reaction temperature of 40 °C.

While CAU-1-NH2, CAU-10-H and CAU-10-NH2 were obtained according to literature procedures, the synthesis of the mixed-linker compound CAU-1-NH2/H was accomplished using a two-step approach using CAU-10-NH2 as crystal seeds. Thus, highly crystalline CAU-1-NH2/H was obtained with ∼76% BDC2− ions as linkers (Fig. S1†). IR spectroscopy unequivocally proves the presence of –NH2 groups in CAU-1-NH2 and CAU-1-NH2/H (Fig. S2†). As expected, CAU-1-NH2 exhibits a smaller specific surface area and micropore volume compared to CAU-1-NH2/H (1300 m2 g−1 and 0.54 cm3 g−1 (ref. 20) vs. 1400 m2 g−1 and 0.58 cm3 g−1) which is due to the replacement of –NH2 groups by H-atoms. The N2-sorption isotherms are presented in Fig. S3 and Table S2.†

The other two MOFs, CAU-10-H and CAU-10-NH2, were also obtained as microcrystalline samples (Fig. S5†). The presence of amino groups and other characteristic stretching frequencies corresponding to CAU-10-H and CAU-10-NH2 is visible in the IR spectra (Fig. S6 and S7†). The results of the N2 sorption measurements (Fig. S8†) for CAU-10-H are in agreement with those reported previously (a specific surface area of 656 m2 g−1 and a micropore volume of 0.25 cm3 g−1).5a On the other hand, the BET surface area of CAU-10-NH2 was measured to be 159 m2 g−1 with a micropore volume of 0.08 cm3 g−1 (Fig. S6†). The hysteresis in the N2 sorption curve is due to the small pore size in CAU-10-NH2 and low measurement temperature (77 K).

The Knoevenagel condensation (Scheme 1) reaction is one of the well-known C–C bond-forming reactions in organic chemistry which occurs between aldehydes and activated methylene compounds containing two electron-withdrawing groups.21,22 This condensation reaction is often catalysed by bases or acids and it requires high reaction temperatures or microwave irradiation.23–26 Although many catalytic systems have been reported,15,27 it is very challenging to develop a catalyst which can promote this reaction without the formation of by-products due to consecutive self-condensation and oligomerization reactions of the primary reaction product.


image file: c6ce02664h-s1.tif
Scheme 1 Knoevenagel condensation reaction between benzaldehyde (1) and malononitrile in ethanol as the solvent to obtain benzylidene malononitrile (2) and the undesired product benzaldehyde diethyl acetal (3).

In addition to these by-products, often a reaction between the solvent and the aldehyde is observed (Scheme 1).

To evaluate the catalytic activity of the four Al-MOFs, at first, the Knoevenagel condensation reaction was performed by taking benzaldehyde (1) and malononitrile as model substrates in ethanol as the solvent. The reaction afforded the desired benzylidene malononitrile (2) and benzaldehyde diethyl acetal (3) through Brønsted-base and Lewis-acid catalysis, respectively, under the present experimental conditions. This observation is well supported by earlier precedents in the literature in which the Knoevenagel condensation can be performed with either acidic or basic MOFs.1c The time conversion plots for the reaction between benzaldehyde (1) and malononitrile in the presence of these catalysts are shown in Fig. 2. The selectivities to the reaction products are shown in Table 1. The reaction using CAU-1-NH2 as the catalyst resulted in 98% conversion with complete selectivity towards 2 after 7 h, while CAU-1-NH2/H showed 84% conversion of 1 with 95% product selectivity to 3. Although it would be interesting to study the activity of pure CAU-1-H, such studies cannot be carried out since we have not been able to synthesize this compound. Recently, the activities of Fe-MIL-101-NH2 and Al-MIL-101-NH2 were compared with that of CAU-1-NH2 in the Knoevenagel condensation between benzaldehyde and malononitrile at 80 °C in toluene. The former catalyst exhibited around 90% yield after 3 h, while CAU-1-NH2 showed low activity (25% yield) under identical reaction conditions.14 This was attributed to the narrow pore dimensions of CAU-1-NH2 compared to those of the MIL-101 catalysts. In any case, the present data clearly show that CAU-1-NH2 can also promote the Knoevenagel condensation if the substrate fits within the pore limits. In contrast, CAU-10-H and CAU-10-NH2 afforded complete conversion of 1 but a mixture of 2 and 3 is obtained with very similar product selectivities in both catalytic systems. This may be explained from the fact that the Brønsted-base-catalysed reaction is only dominant when the amino groups of the functionalized linker molecules are readily accessible as in CAU-1-NH2. In contrast, the mixed-linker MOF CAU-1-NH2/H which contains ca. 26% functionalized linker molecules possesses sufficient Lewis acidity, thus giving 3 as the main product. Hence, the nature of the active sites and the pore properties in the respective catalyst determine the product selectivity as well as the conversion.


image file: c6ce02664h-f2.tif
Fig. 2 Time conversion plots for the reaction between 1 and malononitrile in the presence of CAU-1-NH2 (black square) and CAU-1-NH2/H (blue triangle) as catalysts and the time conversion plot for the first (black square) and second (red circle) reuse using CAU-1-NH2. Conversion was determined by GC and referred as the disappearance of 1 and the selectivity data is given in Table 1.
Table 1 Knoevenagel condensation reaction catalyzed by the different title compoundsa
S. no Catalyst Conversion of 1b (%) Product selectivityb (%)
2 3
a Reaction conditions: benzaldehyde (0.05 mL), malononitrile (0.03 mL), catalyst (25 mg), EtOH (3 mL), 40 °C, 7 h. b Determined by GC. Conversion was determined by using nitrobenzene as the internal standard. c Second reuse.
1 CAU-1-NH2 98 100 —
2 CAU-1-NH2c 98 100 —
3 CAU-1-NH2/H 84 — 95
4 CAU-10-H 99 56 43
5 CAU-10-NH2 99 54 45

Furthermore, Table 2 provides the comparison of the catalytic data using CAU-1-NH2 with those for catalysts reported in the literature in terms of reaction temperature, time, conversion and selectivity, and various pieces of evidence supported to ascertain the stability of the catalysts under the experimental conditions.

Table 2 Comparison of the activity of CAU-1-NH2 with those of the catalysts reported for the Knoevenagel condensation reaction between benzaldehyde (1) and malononitrile
Catalyst T (°C) Time Conv. (%) Selec. (%) Stability Ref.
a Catalyst stability is ascertained by leaching and XRD studies.
[Cd(4-btapa)2(NO3)2]·6H2O·2DMF RT 12 h 98 100 XRD 8
[Gd2(tnbd)3(DMF)4]·4DMF·3H2O RT 20 min 96 100 3 reuses, XRD 9
ZIF-8 RT 1 h 97 100 5 reuses, leaching 10
ZIF-9 RT 2 h 99 100 5 reuses, XRD, IR 11
Cu3(BTC)2 60 6 h 88 100 XRD, leaching 12
DETA-Cr-MIL-101 RT 1 h 97 100 3 reuses, XRD 13
Fe-MIL-101-NH2, Al-MIL-101-NH2a 80 3 h 90 100 Leaching, XRD 14
Zn2(tpt)2(p-BDC-NH2)I2 60 2 h 99 100 XRD, TGA 15
Zn-MOF-NH2 80 4.5 h 98 100 5 reuses, XRD 16
CAU-1-NH2 40 1 h 94 100 2 reuses, XRD, IR, leaching This work

The stability of the catalysts was studied using PXRD, atomic absorption spectroscopy and hot filtration tests (Fig. S9†).

PXRD and IR spectroscopy of the fresh and recovered catalysts show no significant changes (Fig. S10–S12†). Also, atomic absorption spectroscopy analysis reveals that no detectable amount of Al3+ is present in the reaction mixture after 7 h. These experiments clearly indicate the framework stability of CAU-1-NH2 during the course of the reaction.

Due to its good catalytic performance and high selectivity, CAU-1-NH2 was used in a consecutive reaction (Fig. 2). No degradation of the activity or selectivity was observed and the activity was retained with a similar initial reaction rate.

The preliminary results for the catalytic activity of CAU-1-NH2 obtained by the reaction between benzaldehyde (1) and malononitrile prompted us to expand the scope of this catalyst with substituted benzaldehydes having electron-donating or -withdrawing substituents and aldehydes with larger substrates like phenylacetaldehyde and 1-naphthaldehyde. In addition, the reaction products were isolated and washed before the yield was determined. The observed results are given in Table 3.

Table 3 Knoevenagel condensation reaction between substituted benzaldehydes and malononitrile catalyzed by CAU-1-NH2a
S. no Substrate Product Yieldb [%]
a Reaction conditions: substrate (1 mmol), malononitrile (1 mmol), CAU-1-NH2 (25 mg), EtOH (3 mL), 40 °C, 7 h. b Isolated yield.
1 image file: c6ce02664h-u1.tif image file: c6ce02664h-u2.tif 91
2 image file: c6ce02664h-u3.tif image file: c6ce02664h-u4.tif 92
3 image file: c6ce02664h-u5.tif image file: c6ce02664h-u6.tif 90
4 image file: c6ce02664h-u7.tif image file: c6ce02664h-u8.tif 69
5 image file: c6ce02664h-u9.tif image file: c6ce02664h-u10.tif 82
6 image file: c6ce02664h-u11.tif image file: c6ce02664h-u12.tif 77
7 image file: c6ce02664h-u13.tif image file: c6ce02664h-u14.tif 80
8 image file: c6ce02664h-u15.tif image file: c6ce02664h-u16.tif 92
9 image file: c6ce02664h-u17.tif image file: c6ce02664h-u18.tif 70
10 image file: c6ce02664h-u19.tif image file: c6ce02664h-u20.tif 85
11 image file: c6ce02664h-u21.tif image file: c6ce02664h-u22.tif 60

The reaction between benzaldehyde (1) and malononitrile resulted in a yield of 91% (entry 1, Table 3) of the corresponding product after 7 h in the presence of CAU-1-NH2 and also the use of other aldehydes (substrates) resulted predominantly in the formation of the Knoevenagel condensation product (60–92% yield). In comparison to homogeneous catalysis where the presence of a substituent on benzaldehyde strongly influences the reactivity, with electron-withdrawing groups leading to higher reactivities, in heterogeneous catalysis of microporous materials, the pore size also plays an important role in determining the reaction pathway and selectivity, in addition to the electronic effects of substrates. In the present case, the use of electron-withdrawing groups afforded high yields except for 4-bromobenzaldehyde (entries 2–5, Table 3). On the other hand, benzaldehyde with electron-donating groups also showed a comparable yield only with 4-methoxybenzaldehyde, but in the other two cases, lower yields were obtained (entries 6–8, Table 3). Apart from the electronic nature of the substrates, the molecular size and the position of the carbonyl groups can also determine the yield (entries 9–11, Table 3).

Our results demonstrate that the product yield in Knoevenagel condensation can be influenced by not only the electronic factors on the substrate but also the molecular size of the substrate and the position of the aldehyde group under the present experimental conditions. Thus, a size–selectivity screen has been studied to reveal the relationship between the size of the aldehyde substrate and the catalytic activity of CAU-1-NH2. Benzaldehyde has a size of 5 × 5.6 Å, and can freely penetrate into the pores of CAU-1-NH2 having pore sizes of 5 to 10 Å. Similarly, malononitrile has a molecular size of 6.9 × 4.5 Å, and again can diffuse into the pores of CAU-1-NH2. On the other hand, the use of a slightly larger aldehyde substrate, such as 1-naphthaldehyde (5.6 × 7.5 Å), is again within the pore limit of CAU-1-NH2. Hence, the comparison of the pore nature of CAU-1-NH2 with that of the reactants employed in the present work clearly suggests that the reaction takes place mostly inside the pores without having any diffusion limitations. However, moderate yields were observed when 4-bromobenzaldehyde and cinnamaldehyde were used as substrates, thus suggesting the diffusion limitation experienced in these substrates.19

Conclusions

The catalytic activity of CAU-1-NH2 was tested in Knoevenagel condensation reactions and its activity was compared with those of analogous catalysts like CAU-10-H, CAU-10-NH2 and CAU-1-NH2/H. The observed data explain that CAU-1-NH2 exhibits high activity and selectivity towards the Knoevenagel condensation product while CAU-10-H and CAU-10-NH2 result in a mixture of products derived through acid and base catalysis namely acetal and Knoevenagel condensation products. The available catalytic data clearly establish that the product selectivity mainly depends on the nature of the MOFs and the reaction occurs within the pores of CAU-1-NH2 as evidenced from the comparison of the molecular dimensions of the reactants with the pore sizes of CAU-1-NH2. In addition, CAU-1-NH2 was reused and no significant change in its activity is observed. PXRD measurements of the fresh and reused catalyst show no structural changes and a leaching experiment indicates the absence of Al3+, thus proving the catalyst stability under the present experimental conditions. Future work will be aimed at synthesizing CAU-1-H and also methoxy-free CAU-1-NH2 in order to fully evaluate the influence of the methoxy and amino groups of the catalysts on Knoevenagel condensation reactions.

Acknowledgements

ADM thanks the University Grants Commission, New Delhi for the award of Assistant Professorship under its Faculty Recharge Program and also thanks the Department of Science and Technology, India for the financial support through the Fast Track project (SB/FT/CS-166/2013). ADM thanks DFG for the financial support to the three month research stay at the University of Kiel, Germany under the INSA-DFG bilateral exchange program.

Notes and references

  1. (a) D. Farrusseng, S. Aguado and C. Pinel, Angew. Chem., Int. Ed., 2009, 48, 7502 CrossRef CAS PubMed ; (b) A. Corma, H. Garcıa and F. X. Llabres i Xamena, Chem. Rev., 2010, 110, 4606 CrossRef CAS PubMed ; (c) A. Dhakshinamoorthy, M. Opanasenko, J. Čejka and H. Garcia, Adv. Synth. Catal., 2013, 355, 247 CAS .
  2. K. K. Tanabea and S. M. Cohen, Chem. Soc. Rev., 2011, 40, 498 RSC .
  3. (a) N. Stock, Metal-Organic Frameworks: Aluminium-Based Frameworks, in Encyclopedia of Inorganic and Bioinorganic Chemistry, John Wiley & Sons, Ltd, 2011 Search PubMed ; (b) T. Loiseau, C. Volkringer, M. Haouas, F. Taulelle and G. Férey, C. R. Chim., 2015, 18, 1350 CrossRef CAS ; (c) L.-H. Schilling, H. Reinsch and N. Stock, Synthesis, Structure, and Selected Properties of Aluminum-, Gallium-, and Indium-Based Metal–Organic Frameworks, in The Chemistry of Metal–Organic Frameworks, Wiley-VCH Verlag GmbH & Co. KGaA, 2016, pp. 105–135 Search PubMed .
  4. D. Fröhlich, E. Pantatosaki, P. D. Kolokathis, K. Markey, H. Reinsch, M. Baumgartner, M. A. van der Veen, D. E. De Vos, N. Stock, G. K. Papadopoulos, S. K. Henninger and C. Janiak, J. Mater. Chem. A, 2016, 4, 11859 Search PubMed .
  5. (a) H. Reinsch, M. A. van der Veen, B. Gil, B. Marszalek, T. Verbiest, D. de Vos and N. Stock, Chem. Mater., 2013, 25, 17 CrossRef CAS ; (b) H. Reinsch, S. Waitschat and N. Stock, Dalton Trans., 2013, 42, 4840 RSC ; (c) H. Reinsch, F. M. Hinterholzinger, P. Jäker, F. Hesse, B. Reimer, T. Bein, M. Položij, D. Nachtigallová, P. Nachtigall and N. Stock, J. Phys. Chem. C, 2015, 119, 26401 CrossRef CAS ; (d) A. Weiss, N. Reimer, N. Stock, M. Tiemann and T. Wagner, Phys. Chem. Chem. Phys., 2015, 17, 21634 RSC ; (e) A. Weiss, N. Reimer, N. Stock, M. Tiemann and T. Wagner, Microporous Mesoporous Mater., 2016, 220, 39 CrossRef CAS ; (f) N. Reimer, B. Bueken, S. Leubner, C. Seidler, M. Wark, D. De Vos and N. Stock, Chem. – Eur. J., 2015, 35, 12517 CrossRef PubMed .
  6. T. Ahnfeldt, N. Guillou, D. Gunzelmann, I. Margiolaki, T. Loiseau, G. Férey, J. Senker and N. Stock, Angew. Chem., 2009, 121, 5265 CrossRef .
  7. (a) E. Knoevenagel, Ber. Dtsch. Chem. Ges., 1898, 31, 2585 CrossRef CAS ; (b) G. Jones, Org. React., 1967, 15, 204 CAS .
  8. S. Hasegawa, S. Horike, R. Matsuda, S. Furukawa, K. Mochizuki, Y. Kinoshita and S. Kitagawa, J. Am. Chem. Soc., 2007, 129, 2607 CrossRef CAS PubMed .
  9. R. K. Das, A. Aijaz, M. K. Sharma, P. Lama and P. K. Bharadwaj, Chem. – Eur. J., 2012, 18, 6866 CrossRef CAS PubMed .
  10. U. P. N. Tran, K. K. A. Le and N. T. S. Phan, ACS Catal., 2011, 1, 120 CrossRef CAS .
  11. L. T. L. Nguyen, K. K. A. Le, H. X. Truong and N. T. S. Phan, Catal. Sci. Technol., 2012, 2, 521 CAS .
  12. M. Opanasenko, A. Dhakshinamoorthy, M. Shamzhy, P. Nachtigall, M. Horcek, H. Garcia and J. Cejka, Catal. Sci. Technol., 2013, 3, 500 CAS .
  13. S.-N. Kim, S.-T. Yang, J. Kim, J.-E. Park and W.-S. Ahn, CrystEngComm, 2012, 14, 4142 RSC .
  14. M. Hartmann and M. Fischer, Microporous Mesoporous Mater., 2012, 164, 38 CrossRef CAS .
  15. Y. Tan, Z. Fu and J. Zhang, Inorg. Chem. Commun., 2011, 14, 1966 CrossRef CAS .
  16. A. Taher, D.-J. Lee, B.-K. Lee and I.-M. Lee, Synlett, 2016, 27, 1433 CrossRef CAS .
  17. J. Gascon, U. Aktay, M. D. Hernandez-Alonso, G. P. M. van Klink and F. Kapteijn, J. Catal., 2009, 261, 75 CrossRef CAS .
  18. R. Srirambalaji, S. Hong, R. Natarajan, M. Yoon, R. Hota, Y. Kim, Y. Ho Ko and K. Kim, Chem. Commun., 2012, 48, 11650 RSC .
  19. Y. Luan, Y. Qi, H. Gao, R. S. Andriamitantsoa, N. Zheng and G. Wang, J. Mater. Chem. A, 2015, 3, 17320 CAS .
  20. T. Ahnfeldt, D. Gunzelmann, J. Wack, J. Senker and N. Stock, CrystEngComm, 2012, 14, 4126 RSC .
  21. F. Freeman, Chem. Rev., 1981, 80, 329 CrossRef .
  22. L. F. Tietze, Chem. Rev., 1996, 96, 115 CrossRef CAS PubMed .
  23. S. A.-E. Ayoubi, F. Texier-Boullet and J. Hamelin, Synthesis, 1994, 258 CrossRef .
  24. D. Prajapati, K. C. Lekhok, J. S. Sandhu and A. C. Ghosh, J. Chem. Soc., Perkin Trans. 1, 1996, 1, 959 RSC .
  25. S. Saravanamurugan, M. Palanichamy, M. Hartmann and V. Murugesan, Appl. Catal., A, 2006, 298, 8 CrossRef CAS .
  26. J. S. Yadav, B. V. Subba Reddy, A. K. Basak, B. Visali, A. V. Narsaiah and K. Nagaiah, Eur. J. Org. Chem., 2004, 546 CrossRef CAS .
  27. Y. K. Hwang, D.-Y. Hong, J.-S. Chang, S. H. Jhung, Y.-K. Seo, J. Kim, A. Vimont, M. Daturi, C. Serre and G. Ferey, Angew. Chem., Int. Ed., 2008, 47, 4144 CrossRef CAS PubMed .

Footnote

† Electronic supplementary information (ESI) available. See DOI: 10.1039/c6ce02664h
This journal is © The Royal Society of Chemistry 2017
Click here to see how this site uses Cookies. View our privacy policy here.