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Article

Halloysite Nanoclay with High Content of Sulfonic Acid-Based Ionic Liquid: A Novel Catalyst for the Synthesis of Tetrahydrobenzo[b]pyrans

by
Samahe Sadjadi
1,*,
Fatemeh Koohestani
1,
Neda Abedian-Dehaghani
2 and
Majid M. Heravi
2,*
1
Gas Conversion Department, Faculty of Petrochemicals, Iran Polymer and Petrochemical Institute, Tehran P.O. Box 14975112, Iran
2
Department of Chemistry, School of Physic and Chemistry, Alzahra University, Vanak, Tehran P.O. Box 1993891176, Iran
*
Authors to whom correspondence should be addressed.
Catalysts 2021, 11(10), 1172; https://doi.org/10.3390/catal11101172
Submission received: 22 August 2021 / Revised: 27 September 2021 / Accepted: 27 September 2021 / Published: 28 September 2021
(This article belongs to the Special Issue Advances in Supported Catalytic Nanomaterials)
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Abstract

:
One of the main drawbacks of supported ionic liquids is their low loading and consequently, low activity of the resultant catalysts. To furnish a solution to this issue, a novel heterocyclic ligand with multi imine sites was introduced on the surface of amino-functionalized halloysite support via successive reactions with 2,4,6-trichloro-1,3,5-triazine and 2-aminopyrimidine. Subsequently, the imine sites were transformed to sulfonic acid-based ionic liquids via reaction with 1,4-butanesultone. Using this strategy, high loading of ionic liquid was loaded on halloysite nanoclay. The supported ionic liquid was then characterized with XRD, SEM, TEM, EDS, FTIR, BET, TGA and elemental mapping analysis and utilized as a metal-free Brønsted acid catalyst for promoting one-pot reaction of aldehydes, dimedone and malononitrile to furnish tetrahydrobenzo[b]pyrans. The catalytic tests confirmed high performance of the catalyst. Moreover, the catalyst was stable upon recycling.

1. Introduction

Considering the importance of environmental concerns, development of green methodologies for the synthesis of chemicals has gained growing attention. In this regard, use of natural materials for the preparation of catalysts is a hot research topic [1,2,3,4,5]. On the other hand, metal-free catalysts have been introduced as environmentally benign catalysts. One class of metal-free catalysts that has broad applications is ionic liquids (ILs). These compounds are defined as organic salts [6,7,8,9,10] that consist of organic cations [11,12,13,14]. As these compounds benefit from outstanding features [15,16], such as low-toxicity and high electric conductivity, they can be applied both as catalysts and solvents. More importantly, the features of ILs can be readily tuned by altering their cations and anions [12,17,18,19]. In recent years, development of state-of-the art ILs, such as bio-based ILs and chiral ILs expanded the utilities of this class of compounds [13,20]. One of the advances in the field of ILs is heterogenation of ILs through their stabilization on supporting materials. Among various potential supports, clays have received increasing interest. One of these clays is halloysite nanoclay (Hal) [21,22,23] that is dioctahedral 1:1 clay mineral with tubular morphology [24,25,26,27,28,29,30,31]. The outstanding performance of this clay [32,33] as a support has induced many studies and to date [34,35], various catalytic species have been stabilized on Hal. Noteworthy, not only Hal exterior surface can be decorated with catalytic species, but also Hal lumen can be targeted for embedding the catalytic species. Other features of Hal that render it a potential candidate for the catalysis is its electrically charged surfaces [36,37,38]. In fact, the interior and exterior surfaces of Hal are oppositely charged. Furthermore, both Hal surfaces can be readily functionalized through conventional chemical reactions [39,40,41]. Introduction of the functional groups on Hal improves its features as a supporting material and leads to the preparation of heterogeneous catalysts with improved activity and recyclability [42,43].
To prepare supported ILs, two strategies can be utilized. In one strategy, IL with proper functional group can be prepared separately and, then immobilized on a support, while in another method, IL can be grown on the supporting material step-wisely [11,12,13,14]. In both methods, the loading of IL on the supporting materials is mostly low. This can be due to the nature of the support or unsuitable supporting procedure. Hence, development of supported ILs with high loading of ILs is of great importance [44].
Tetrahydrobenzo[b]pyran derivatives are heterocycles with biological and pharmaceutical properties [45,46,47]. Conventionally, these heterocycles can be synthesized through catalytic Knoevenagel condensation [48,49,50]. Considering the importance of this class of heterocycles, to date myriad catalysts have been devised for promoting their synthesis [51,52,53,54]. Despite considerable advances, some methodologies suffer from some drawbacks, such as usage of toxic solvents, sever condition, moderate yields and formation of waste.
In the continuation of our studies on the catalysts, we have studied metal-free catalysts that have been achieved through immobilization of ILs on naturally occurring materials [55,56]. In this context, we wish to introduce a new Brønsted acid catalyst that has been prepared through covalent conjugation of high loading of sulfonic acid-based IL on Hal nanoclay, Figure 1. The catalyst, Hal-Py-IL, has been characterized and its catalytic activity and recyclability for the synthesis of 4H-benzo[b]pyrans were investigated.

2. Results and Discussion

2.1. Catalyst Characterization

The as-prepared Hal-Py-IL was then characterized. The first feature of the catalyst that has been evaluated was its morphology. In Figure 2, two SEM images of Hal-Py-IL are depicted. As illustrated, the nano tubes of Hal can be discerned in the SEM images. This fact inferred that tubes of Hal did not collapse upon introduction of Py-IL. Moreover, on Hal tubes, small aggregates can be observed that can be ascribed to Py-IL.
In Figure 3, the TEM image of Hal-Py-IL is illustrated. As shown, Hal tubes are detected in the TEM image of Hal-Py-IL, implying the stability of Hal structure upon functionalization. Furthermore, in some parts, the thickening of Hal tubes can be observed. This is related to the growth of Py-IL on Hal.
To further affirm the conjugation of Py-IL moiety on Hal clay, EDS analysis was conducted. The result, depicted in Figure 4, implied that C, N, O, Al, Si and S atoms are present in the framework of Hal-Py-IL. Although EDS analysis is not sufficient for affirming the structure of Hal-Py-IL, observation of S atom that can be indicative of (−SO3−), as well as C and N atoms established the presence of Py-IL. Moreover, the absence of Cl atom can indicate successful substitution reaction between TCT and 2-aminopyrimidine. Furthermore, to shed light into the dispersion of Py-IL, elemental mapping analysis of Hal-Py-IL was conducted. As shown in Figure 4, S, C and N atoms are dispersed uniformly, implying that Py-IL was homogeneously formed on Hal.
Hall-Py-IL was also characterized via XRD analysis, Figure 5. In this context, XRD patterns of both Hal-Py-IL and Hal were obtained and compared. The comparison of the two X-ray diffraction patterns, Figure 5, inferred that all of the bands of Hal are present in the XRD pattern of Hal-Py-IL. However, their intensities significantly decreased. According to the literature, this issue can be indicative of incorporation of Py-IL [57].
In Figure 6, the comparison of the recorded FTIR spectra of Hal, Hal-TCT, Hal-TCT-Py, Hal-Py-IL is presented. According to the literature [42], the absorbance bands of Hal can be listed as the bands at 1054 cm−1 (Si-O), 3696 cm−1 and 3623 cm−1 (inner –OH), and 536 cm−1 (Al-O-Si). These characteristic bands can be observed in the FTIR spectra of Hal-TCT, Hal-TCT-Py and Hal-Py-IL, confirming the fact that Hal structure was stable in the course of introduction of the organic species. The comparison of the FTIR spectra of Hal and three other samples established that in the Hal-TCT, Hal-TCT-Py and Hal-Py-IL spectra, a new band at 1660–1687 cm−1 is observable. This absorbance band is ascribed to the –C=N functionality of TCT and 2-aminopyrimidine. Furthermore, the absorbance band at 2931 cm−1 in the FTIR spectrum of Hal-Py-IL is due to –CH2 and proves conjugation of 1,4-butanesultone.
To validate conjugation of the organic moiety, TG analysis of Hal, Hal-TCT-Py and Hal-Py-IL was conducted and the obtained TG curves were compared, Figure 7. As shown, in Figure 7, in Hal thermogram, two weight loss steps can be discerned that are assigned to the loss of H2O and dehydroxylation of Hal (at 480 °C) [58,59]. In the case of Hal-TCT-Py, apart from the aforementioned weight losses, a new loss occurred at 340 °C that is assigned to the degradation of TCT-Py. According to the TGA, the loading of TCT-Py was calculated as 18 wt. %. Comparison of the thermograms of the catalyst with that of Hal-TCT-Py implied that the content of the loaded 1,4-butanesultone was 3 wt. %.
BET analysis of Hal-Py-IL and Hal was conducted and the surface properties of the two samples have been compared. As illustrated in Figure 8, the N2 adsorption-desorption isotherms of both Hal-Py-IL and Hal are of type II. Regarding the specific surface area, it was found that by growth of Py-IL on Hal, the specific surface area decreased from 51 to 5.2 m2·g−1. This significant decrease of this value can establish coverage of Hal with Py-IL.

2.2. Activity

In the next section, the activity of the as-prepared Hal-Py-IL was evaluated. In this regard, the performance of the catalyst for the preparation of 4H-benzo[b] pyrans through one-pot reaction of aldehyde, malononitrile and dimedone was appraised, Figure 9.

2.2.1. Optimization of Reaction Condition

First, a model 4H-benzo[b] pyran, derived from reaction of 4-nitrobenzaldehyde, malononitrile and dimedone, was selected to optimize the effective reaction parameters and find the best reaction condition. In this respect, the influence of the reaction solvent was first examined by conducting the reaction in EtOH, H2O and H2O:EtOH (1:2). Among the studied solvents, EtOH was recognized as the most efficient solvent, Table 1. Next, the model synthesis was conducted in the presence of different dosage of Hal-Py-IL (20–40 mg). It was affirmed that this is an influential factor. In fact, increasing of the catalyst content from 20 to 30 mg led to the increase of the yield of the reaction. However, further increment to 40 mg was not effective. Hence, 30 mg of Hal-Py-IL was selected as the optimum catalyst dosage. Finally, the reaction temperature was optimized by varying the temperature (ambient temperature to 60 °C). The results, Table 1, indicated that increasing the temperature to 50 °C resulted in accelerating the reaction rate and increase of the yield of the reaction. Notably, elevating the reaction temperature to 60 °C had no significant effect. Considering these results, the optimum condition for the reaction was found to be use of 30 mg of Hal-Py-IL in EtOH at 50 °C.

2.2.2. Comparison of the Activity of Hal-Py-IL with Control Catalysts

To disclose the beneficial effect of conjugation of the catalyst components, the catalytic activity of Hal-Py-IL for promoting the model reaction under the optimum condition was compared with some control catalysts. First, the catalytic activity of Hal for the model reaction was examined. As tabulated in Table 2, Hal exhibited low catalytic activity and furnish the desired product in 25% yield after 3 h. Similarly, the catalytic activity of Hal-N (obtained from reaction of Hal and APTES), Hal-TCT (prepared from reaction of Hal-N with TCT) and Hal-Py (prepared from reaction of Hal-TCT with 2-aminopyrimidine) was also examined for the model reaction. As listed in Table 2, functionalization of Hal with APTES, TCT and 2-aminopyrimidine did not affect the catalytic activity of the resultant catalyst.
Next, the catalytic activity of Py (prepared from reaction of TCT and 2-aminopyrimidine) was appraised. This control catalyst showed low catalytic activity.
Py-IL was also prepared through successive reaction of TCT with 2-aminopyrimidine and 1,4-butanesultone and its catalytic activity was examined for the model reaction. It was found that the catalytic activity of Py-IL was slightly lower than that of Hal-Py-IL. More precisely, Py-IL promoted the reaction to give the desired product in 87% yield. In fact, the catalytic activity of Py-IL is due to the instinct activity of IL. Higher activity of Hal-Py-IL can be ascribed to the synergistic effect between Hal and Py-IL.
Finally, the catalytic activity of sulfonic acid (SA), prepared from reaction of 2-aminopyrimidine with 1,4-butanesultone was examined for the model reaction. As shown in Table 2, the activity of SA was lower than that of Hal-Py-IL and furnished the product in 80% after 65 min.
These results confirmed that conjugation of Hal and Py-IL is beneficiary for the catalytic activity.

2.2.3. Generality

Next, the substrate scope of the present methodology was appraised. In this context, various aldehydes were applied for the synthesis of 4H-benzo[b] pyrans. The results, listed in Table 3, approved that Hal-Py-IL can efficiently promote the reaction of various substrates to furnish the corresponding products in high yields. It is worth noting that the reaction yield for the aldehydes with electron-withdrawing groups was slightly higher than that of the substrates with electron-donating groups. Moreover, it was found that heterocyclic aldehydes underwent the reaction to give the products in high yields.

2.2.4. Reaction Mechanism

In Figure 10, the reaction mechanism for the synthesis of 4H-benzo[b] pyrans under Hal-Py-IL catalysis is depicted. As shown, Hal-Py-IL can activate the aldehyde and malononitrile and promotes their Knoevenagel condensation to furnish compound 1. In the next step, Michael addition of compound 1 and enolate form of dimedone forms intermediate 2. Next, nucleophilically attack of enolate oxygen to nitrile group leads to the formation of a cyclic intermediate, compound 3, which tolerates tautomeric proton shift to furnish 4H-benzo[b] pyran.

2.2.5. Comparative Study

To investigate whether the efficiency of Hal-Py-IL for the preparation of 4H-benzo[b] pyrans is comparable with other reported catalysts, a comparative study has been conducted, Table 4. As tabulated, various catalysts, ranging from magnetic catalysts to porous polymer have been reported for catalyzing this organic synthesis. Comparison of the catalytic performance of the summarized catalysts indicated that the catalytic activity of Hal-Py-IL is comparable with the previous catalysts.

2.3. Catalyst Recyclability

The recyclability of Hal-Py-IL was appraised under the optimized condition for the multi-component condensation reaction of 4-nitrobenzaldehyde, malononitrile and dimedone. In this regard, after completion of the first run of the model reaction, Hal-Py-IL was filtered off, washed with hot EtOH, dried and used for the next run of the same condensation reaction. The recovered Hal-Py-IL was sequentially reused and the yields of the reactions were calculated for eight reaction runs, Figure 11. As illustrated in Figure 11, reuse of Hal-Py-IL for the second run did not cause any decrement of the activity of the catalyst. However, using Hal-Py-IL for the third time led to slight loss of the activity. Further reuse also induced slight decrease of the catalytic activity and after eighth run the yield of the reaction was 80%.
The reused Hal-Py-IL after eight runs was analyzed via FTIR spectroscopy and the recorded FTIR spectrum was compared with spectrum of fresh Hal-Py-IL. As depicted in Figure 12, FTIR spectrum of the reused Hal-Py-IL is identical to that of the fresh one and contains all of the absorbance bands of the fresh catalyst, implying that reusing of Hal-Py-IL did not induce structural collapse and Hal-Py-IL was stable in the course of recycling.

3. Materials and Methods

The used reagents and solvents for the preparation of the catalyst included Hal, 3-(aminpropyl)-triethoxysilane (APTES), 2-aminopyrimidine, 1,4-butanesultone, potassium carbonate (K2CO3), 2,4,6-trichloro-1,3,5-triazine (TCT), tetrahydrofuran (THF), toluene, all was purchased from Sigma-Aldrich. The chemicals that have been applied for the evaluation of the catalytic performance of the catalyst included dimedone, malononitrile, ethanol (EtOH), aldehydes. All was received from Sigma-Aldrich (Germany, Taufkirchen).
In order to confirm the formation of the designed catalyst, several analyses, including, thermo gravimetric analysis (TGA), X-ray diffraction (XRD), scanning electron microscope (SEM), Fourier transform infrared (FTIR), energy dispersive spectroscopy (EDS) and elemental mapping analyses were conducted. The technical data of the applied apparatuses are as follow: BRUKER TENSOR 35 (Germany, Berlin) was used for performing FTIR spectroscopies using KBr pellet. Using METTLER TOLEDO thermogravimetric analysis instrument (model Leicester, Leicester, UK) with heating rate of 10 °C/min, the TG analyses were carried out under N2 atmosphere. Siemens, D5000 instrument (Karlsruhe, Germany) with graphite monochromatic Cu-Kα was applied for conducting XRD analysis. SEM /EDS and elemental mapping analyses were performed on MIRA 3 TESCAN-XMU (Kohoutovice, Czech Republic). Transmission electron microscopy (TEM) was conducted by using Philips CM30300Kv (Beaverton, OR, USA) field emission transmission electron microscope. The specific surface area of the catalyst and Hal were evaluated by Brunauer–Emmet–Teller (BET) via Belsorp Mini II (BEL Japan, Inc., Osaka, Japan). Pre-heating was conducted at 100 °C for 3 h.

3.1. Catalyst Fabrication

3.1.1. Hal Functionalization with APTES: Hal-N

To functionalize Hal with APTES, suspension of Hal (4 g) in toluene (80 mL) was ultrasounded (power of 230 W for 10 min) to furnish a homogenized suspension. Afterwards, APTES (4.5 mL) was added to the as-prepared Hal suspension and the obtained mixture was refluxed overnight at 100 °C under Ar atmosphere. At the end of the reaction, the solid was separated, washed with dry toluene and dried in oven at 80 °C for 1 day.

3.1.2. Preparation of Hal-TCT

In order to prepare Hal-TCT, Hal-N (3 g) and K2CO3 (1 mmol) as a base were dispersed in THF (40 mL) and subjected to ultrasonic waves (power of 230 W) for 10 min. Then, the homogenized mixture was stirred in ice bath for 30 min. Afterwards, a solution of TCT (8 mmol in 10 mL THF) was added and the mixture was kept under stirring at 0 °C for 1 day. At the end, the precipitate was collected via conventional filtration, rinsed with THF and dried at 60 °C.

3.1.3. Preparation of Hal-Py

Suspension of Hal-TCT (3 g) in THF (40 mL) was well dispersed by using ultrasonic irradiation (power of 230 W for 10 min). Then, a solution of 2-aminopyrimidine (16 mmol in 10 mL THF) was injected to the Hal-TCT suspension. Subsequently, the obtained mixture was refluxed overnight at 70 °C under inert gas atmosphere. Afterwards, the solid was filtered, rinsed with THF repeatedly and dried at 60 °C.

3.1.4. Introduction of Ionic Liquid: Synthesis of Hal-Py-IL

In the final step, Hal-Py (1.5 g) was dispersed in dry toluene (35 mL) under ultrasonic irradiation (power of 230 W for 10 min). Subsequently, a solution of 1,4-butanesultone (20 mmol) in toluene (15 mL) was injected into the mentioned suspension in a dropwise manner. The mixture was then refluxed at 110 °C for 24 h. At the end of the reaction, the precipitate was filtered off, washed with toluene and dried at 70 °C in oven for 12 h, Figure 1.

3.2. General Procedure for the Synthesis of 4H-Benzo[b]pyran Derivatives

To synthesize 4H-benzo[b]pyran derivatives, dimedone (1 mmol), malononitrile (1 mmol), aryl aldehyde (1 mmol) were dissolved in EtOH (10 mL). Then, Hal-Py-IL (30 mg) as a catalyst was added to the reaction vessel and the mixture was stirred at 50 °C. At the end of the reaction, which was affirmed by TLC, the reaction mixture was allowed to cool to the room temperature and then Hal-Py-IL was readily filtered via filtration paper. The solvent in reaction vessel was evaporated and the resulting residue was purified with recrystallization from EtOH or column chromatography. The recovered Hal-Py-IL on the other hand, was washed with EtOH several time and dried in oven at 70 °C overnight. The synthesis of the products was confirmed via FTIR, NMR and Mass spectroscopies (Figures S1–S22).

4. Conclusions

A novel sulfonic acid-based IL was grown on Hal surface through successive reactions of Hal-N with TCT, 2-aminopyrimidine and 1,4-butanesultone. This catalyst that benefits from high loading of IL was then characterized and utilized for catalyzing one-pot three component reaction of aldehydes, dimedone and malononitrile to furnish tetrahydrobenzo[b]pyrans. The results affirmed that the catalyst was highly active and could promote reaction of various substrates to furnish the corresponding tetrahydrobenzo[b]pyrans in high yields. Moreover, Hal-Py-IL was highly recyclable and could be recovered and reused for eight consecutive reaction runs with slight loss of the catalytic activity.

Supplementary Materials

The following are available online at https://www.mdpi.com/article/10.3390/catal11101172/s1, Figure S1: Figure S1. 1HNMR spectrum of 2-amino-4-(4-methoxyphenyl)-7,7-dimethyl-5-oxo-5,6,7,8-tetrahydro-4H-chromene-3-carbonitrile, Figure S2. Mass spectrum of 2-amino-4-(4-methoxyphenyl)-7,7-dimethyl-5-oxo-5,6,7,8-tetrahydro-4H-chromene-3-carbonitrile, Figure S3. FTIR spectrum of 2-amino-4-(4-methoxyphenyl)-7,7-dimethyl-5-oxo-5,6,7,8-tetrahydro-4H-chromene-3-carbonitrile, Figure S4. 1HNMR spectrum of 2-amino-4-(4-bromophenyl)-7,7-dimethyl-5-oxo-5,6,7,8-tetrahydro-4H-chromene-3-carbonitrile, Figure S5. 13CNMR spectrum of 2-amino-4-(4-bromophenyl)-7,7-dimethyl-5-oxo-5,6,7,8-tetrahydro-4H-chromene-3-carbonitrile, Figure S6. FTIR spectrum of 2-amino-4-(4-bromophenyl)-7,7-dimethyl-5-oxo-5,6,7,8-tetrahydro-4H-chromene-3-carbonitrile, Figure S7. 13CNMR spectrum of 2-amino-7,7-dimethyl-4-(3-nitrophenyl)-5-oxo-5,6,7,8-tetrahydro-4H-chromene-3-carbonitrile, Figure S8. Mass spectrum of 2-amino-7,7-dimethyl-4-(3-nitrophenyl)-5-oxo-5,6,7,8-tetrahydro-4H-chromene-3-carbonitrile, Figure S9. FTIR spectrum of 2-amino-7,7-dimethyl-4-(3-nitrophenyl)-5-oxo-5,6,7,8-tetrahydro-4H-chromene-3-carbonitrile, Figure S10. 13CNMR spectrum of 2-amino-4-(4-hydroxyphenyl)-7,7-dimethyl-5-oxo-5,6,7,8-tetrahydro-4H-chromene-3-carbonitrile, Figure S11. 1HNMR spectrum of 2-amino-4-(4-chlorophenyl)-7,7-dimethyl-5-oxo-5,6,7,8-tetrahydro-4H-chromene-3-carbonitrile, Figure S12. FTIR spectrum of 2-amino-4-(4-chlorophenyl)-7,7-dimethyl-5-oxo-5,6,7,8-tetrahydro-4H-chromene-3-carbonitrile, Figure S13. Mass spectrum of 2-amino-4-(4-chlorophenyl)-7,7-dimethyl-5-oxo-5,6,7,8-tetrahydro-4H-chromene-3-carbonitrile, Figure S14. 13CNMR spectrum of 2-amino-4-(4-chlorophenyl)-7,7-dimethyl-5-oxo-5,6,7,8-tetrahydro-4H-chromene-3-carbonitrile, Figure S15. FTIR spectrum of 2-amino-7,7-dimethyl-5-oxo-4-phenyl-5,6,7,8-tetrahydro-4H-chromene-3-carbonitrile, Figure S16. 13CNMR spectrum of 2-amino-7,7-dimethyl-5-oxo-4-phenyl-5,6,7,8-tetrahydro-4H-chromene-3-carbonitrile, Figure S17. 1HNMR spectrum of 2-amino-7,7-dimethyl-5-oxo-4-phenyl-5,6,7,8-tetrahydro-4H-chromene-3-carbonitrile, Figure S18. FTIR spectrum of 2-amino-7,7-dimethyl-4-(4-nitrophenyl)-5-oxo-5,6,7,8-tetrahydro-4H-chromene-3-carbonitrile, Figure S19. 1HNMR spectrum of 2-amino-7,7-dimethyl-4-(4-nitrophenyl)-5-oxo-5,6,7,8-tetrahydro-4H-chromene-3-carbonitrile, Figure S20. Mass spectrum of 2-amino-7,7-dimethyl-4-(4-nitrophenyl)-5-oxo-5,6,7,8-tetrahydro-4H-chromene-3-carbonitrile, Figure S21. 1HNMR spectrum of 2-amino-4-(2-chlorophenyl)-7,7-dimethyl-5-oxo-5,6,7,8-tetrahydro-4H-chromene-3-carbonitrile, Figure S22. 1HNMR spectrum of 2-amino-4-(2,6-dichlorophenyl)-7,7-dimethyl-5-oxo-5,6,7,8-tetrahydro-4H-chromene-3-carbonitrile.

Author Contributions

Conceptualization, S.S. and M.M.H.; methodology, F.K. and N.A.-D.; validation, F.K. and S.S.; formal analysis, F.K. and N.A.-D.; investigation; resources, S.S. and M.M.H.; data curation, F.K. and N.A.-D.; writing—original draft preparation, F.K. and S.S.; writing—review and editing S.S.; visualization, F.K. and N.A.-D.; supervision, S.S. and M.M.H.; project administration, S.S. and M.M.H.; funding acquisition, S.S. and M.M.H. All authors have read and agreed to the published version of the manuscript.

Funding

This research received no external funding.

Data Availability Statement

The data presented in this study are available in article.

Acknowledgments

The authors appreciate the partial support of Iran Polymer and Petrochemical Institute and Alzahra University.

Conflicts of Interest

The authors declare no conflict of interest.

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Catalysts 11 01172 g001 550
Figure 1. Procedure for the preparation of Hal-Py-IL.
Figure 1. Procedure for the preparation of Hal-Py-IL.
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Catalysts 11 01172 g002 550
Figure 2. SEM images of Hal-Py-IL.
Figure 2. SEM images of Hal-Py-IL.
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Catalysts 11 01172 g003 550
Figure 3. TEM image of Hal-Py-IL.
Figure 3. TEM image of Hal-Py-IL.
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Catalysts 11 01172 g004 550
Figure 4. (A) EDS and (B) elemental mapping analyses of Hal-Py-IL.
Figure 4. (A) EDS and (B) elemental mapping analyses of Hal-Py-IL.
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Catalysts 11 01172 g005 550
Figure 5. XRD patterns of Hal and Hal-Py-IL.
Figure 5. XRD patterns of Hal and Hal-Py-IL.
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Catalysts 11 01172 g006 550
Figure 6. FTIR spectra of Hal, Hal-TCT, Hal-TCT-Py, Hal-Py-IL.
Figure 6. FTIR spectra of Hal, Hal-TCT, Hal-TCT-Py, Hal-Py-IL.
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Catalysts 11 01172 g007 550
Figure 7. TG thermograms of Hal, Hal-TCT-Py, Hal-Py-IL.
Figure 7. TG thermograms of Hal, Hal-TCT-Py, Hal-Py-IL.
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Catalysts 11 01172 g008 550
Figure 8. N2 adsorption-desorption isotherms of Hal (right) and Hal-Py-IL (left).
Figure 8. N2 adsorption-desorption isotherms of Hal (right) and Hal-Py-IL (left).
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Catalysts 11 01172 g009 550
Figure 9. Preparation of 4H-benzo[b] pyrans.
Figure 9. Preparation of 4H-benzo[b] pyrans.
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Catalysts 11 01172 g010 550
Figure 10. Plausible mechanism of preparation of 4H-benzo[b] pyrans under Hal-Py-IL catalysis.
Figure 10. Plausible mechanism of preparation of 4H-benzo[b] pyrans under Hal-Py-IL catalysis.
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Catalysts 11 01172 g011 550
Figure 11. Reusability of Hal-Py-IL for the preparation of 4H-benzo[b] pyrans for the reaction of 4-nitrobenzaldehyde, malononitrile, dimedone at 50 °C in EtOH.
Figure 11. Reusability of Hal-Py-IL for the preparation of 4H-benzo[b] pyrans for the reaction of 4-nitrobenzaldehyde, malononitrile, dimedone at 50 °C in EtOH.
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Catalysts 11 01172 g012 550
Figure 12. The FTIR spectra of fresh and reused Hal-Py-IL after eight runs.
Figure 12. The FTIR spectra of fresh and reused Hal-Py-IL after eight runs.
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Table
Table 1. Optimization of the reaction variables for the model reaction.
Table 1. Optimization of the reaction variables for the model reaction.
EntryHal/Py-IL (mg)SolventTemp. (°C)Time (h)Yield (%)
120H2Or.t.250
220H2O:EtOH (1:2)r.t.250
320EtOHr.t.260
420EtOH40275
520EtOH501.580
620EtOH601.580
730EtOH501.5100
840EtOH501.5100
Table
Table 2. Comparison of the activity of Hal-Py-IL with some control catalysts.
Table 2. Comparison of the activity of Hal-Py-IL with some control catalysts.
Entry Catalyst Time (h:min)Yield (%)
1Hal325
2Hal-N325
3Hal-TCT325
4Hal-Py325
5Py315
6Py-IL287
7Hal-Py-IL1:5100
8SA2:580
Table
Table 3. Synthesis of 4H-benzo[b] pyrans a.
Table 3. Synthesis of 4H-benzo[b] pyrans a.
EntryAldehydeTime
(h:min)		Conversion (%)Yield (%)
1Benzaldehyde29090
24-Nitrobenzaldehyde1:5100100
33-Nitrobenzaldehyde1:59595
42-Nitrobenzaldehyde1:5100100
54-Chlorobenzaldehyde1:59595
62-Chlorobenzaldehyde1:59595
72, 6-Di-chlorobenzaldehyde1:59790
84-Methoxybenzaldehyde2:59588
94-Hydroxybenzaldehyde29690
104-Bromobenzaldehyde1:59595
114-Cyanobenzaldehyde1:59690
124-(Dimethylamino)benzaldehyde2:58574
132-Naphthalenecarboxaldehyde2:58061
14Furfural29081
153-Pyridinecarboxaldehyde28975
16Thiophene-2-carbaldehyde2:58872
a Reaction condition: aldehyde (1 mmol), malononitrile (1 mmol), dimedone (1 mmol), catalyst (30 mg), at 50 °C in EtOH.
Table
Table 4. Comparison of the activity of Hal-Py-IL with other catalysts for the synthesis of the model product.
Table 4. Comparison of the activity of Hal-Py-IL with other catalysts for the synthesis of the model product.
EntryCatalystCatalyst AmountSolventConditionTime (min)Yield (%)Ref.
1NH4H2PO4/Al2O30.03 g-Solvent-free3092[60]
2Ce1Mg0.6Zr0.4O20.2 gEtOHReflux4090[61]
3mPMF a0.02 g-Ball milling, r.t.6092[62]
4Fe3O4@SiO2-guanidine-poly acrylic acid0.05 gH2O70 °C2098[63]
5Aminopropylated silica gel10 mol%H2O70 °C4590[64]
6MnFe2O4@SiO2NH–NH2–PTA b0.04 g-80 °C2595[65]
7Nano ZnO10 mol%EtOH: H2O (1:1)r.t.18091[66]
8CuO-CNS c15 w%H2Or.t.2090[67]
9SB-DABCO d0.06 gEtOHr.t.2090[68]
10Hal-Py-IL0.03 gEtOH50 °C90100This Work
a: Nitrogen-rich porous organic polymer. b: MnFe2O4@SiO2@NHPhNH2–phosphotungstic acid. c: CuO on cellulose nanocrystals. d: Silica bonded n-propyl-4-aza-1-azoniabicyclo[2.2.2]octane chloride.
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Sadjadi, S.; Koohestani, F.; Abedian-Dehaghani, N.; Heravi, M.M. Halloysite Nanoclay with High Content of Sulfonic Acid-Based Ionic Liquid: A Novel Catalyst for the Synthesis of Tetrahydrobenzo[b]pyrans. Catalysts 2021, 11, 1172. https://doi.org/10.3390/catal11101172

AMA Style

Sadjadi S, Koohestani F, Abedian-Dehaghani N, Heravi MM. Halloysite Nanoclay with High Content of Sulfonic Acid-Based Ionic Liquid: A Novel Catalyst for the Synthesis of Tetrahydrobenzo[b]pyrans. Catalysts. 2021; 11(10):1172. https://doi.org/10.3390/catal11101172

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Sadjadi, Samahe, Fatemeh Koohestani, Neda Abedian-Dehaghani, and Majid M. Heravi. 2021. "Halloysite Nanoclay with High Content of Sulfonic Acid-Based Ionic Liquid: A Novel Catalyst for the Synthesis of Tetrahydrobenzo[b]pyrans" Catalysts 11, no. 10: 1172. https://doi.org/10.3390/catal11101172

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