Ionic liquid immobilized on modified magnetic FSM-16: an efficient and magnetically recoverable nanocatalyst

In the present article, a nanocomposite was prepared by immobilizing ionic liquid on the magnetic mesoporous FSM-16 with a core–shell structure (Fe3O4@FSM-16-SO3/IL). Subsequently, the structural properties of the synthesized nanocatalyst were characterized and analyzed by various techniques such as XRD, FT-IR, TEM, FE-SEM, BET, VSM, TGA, and EDS. Fe3O4@FSM-16-SO3/IL was used as a recoverable and efficient nanocatalyst for the synthesis of polyhydroquinoline derivatives. The magnetic nanocatalyst showed remarkable stability and reusability and was reused six consecutive times without considerable loss of its activity.


Introduction
In recent years, various types of mesoporous materials with high surface area, high pore volume, and large pore size have been synthesized using different surfactant templating procedures to expand their applications in the eld of heterogeneous catalysts. 1,20][11] Some important studies reported on the FSM-16 based nanocatalysts are FSM-16@imine-thiophen/Pd, 12 Fe 3 O 4 @FSM-16-SO 3 H, 13 FSM-16-Met, 14 FSM-16-SO 3 H, 15 and FSM-16/AEPC-SO 3 H. 16 Although porous nanomaterials have been widely used as supports for the preparation of heterogeneous catalysts, one of the major problems of them is the difficulty of separation from the reaction mixture by ltration and centrifugation.8][19] The combination of the properties of magnetic nanoparticles and mesoporous materials in a single material is particularly attractive from the point of view of catalysis because of possibility of combining the various functional groups with the advantages of the magnetic properties of magnetic nanoparticles.][22] New and innovative research in the eld of green chemistry extensively discusses waste and damage reduction, innate atom economy, energy conservation, ease of use and the avoidance of perilous chemicals.Another important aspect of green chemistry is the use of ionic liquids (ILs) as new, green, and environmentally friendly solvents compared to toxic solvents for synthetic chemistry.4][25][26] The zwitterionic salts (ZIs) are organic salts that are composed of covalently bonded cations and anions.Zwitterions do not dissociate into ions, thus avoiding unwanted partitioning of ions can be avoided.The intramolecular covalent bonding of the cationic and anionic sites of zwitterions makes them stronger hydrogen bond donors or acceptors than their ionic liquid counterparts. 19,27,28Another interesting feature of ZIs is their ability to act as precursors for Brønsted acidic ionic liquids, which are widely used as solvents and catalysts in various organic reactions.However, the problems of recyclability and high viscosity have limited the use of ionic liquids use in catalytic reactions.A combination of interesting properties of the ionic liquid with those of the support material will develop novel performances when synergistic effects occur.0][31][32][33] To date, many ionic liquids supported on magnetic mesoporous materials have been prepared and used in many chemical reactions.][45] Nitrogen-containing heterocycles oen play an important role as the scaffolds for pharmacological compounds.7][48] Therefore, considering the medicinal and biological signicance, the synthesis of this class of heterocyclic compounds has attracted considerable attention from synthetic chemists.][51] In continuation of our studies program for the synthesis of novel heterogeneous nanocatalysts and also considering the importance of mesoporous materials in the catalysis synthesis process, 12,[52][53][54][55][56][57][58] herein, a recyclable magnetic catalyst based on FSM-16 (Fe 3 O 4 @FSM-16-SO 3 /IL) has been synthesized and characterized.Furthermore, Fe 3 O 4 @FSM-16-SO 3 /IL was used as an effective catalyst in the synthesis of polyhydroquinoline derivatives via the one-pot reaction of various aromatic aldehydes, dimedone, ethyl acetoacetate, and ammonium acetate.

Synthesis of Fe 3 O 4 nanoparticles
Initially, Fe 3 O 4 NPs were prepared by adding FeCl 3 $6H 2 O (2.7 g, 10 mmol) and FeCl 2 $4H 2 O (1 g, 5 mmol) to deionized water (50 mL), followed by a dropwise addition of sodium hydroxide solution (5 mL, 10 M) with stirring at 80 °C under an N 2 atmosphere for 1 h.The magnetic precipitate was then collected with a magnet and washed through deionized water and ethanol.The precipitate obtained was dried in an oven at 70 °C for 2 h. 54

Synthesis of Fe 3 O 4 @FSM-16
Sodium hydroxide (3 g) was dissolved in deionized water (30  mL).Tetraethyl ortho silicate (TEOS) (16.6 mL) was added dropwise to the NaOH solution over 1 h and the resulting mixture was stirred for 24 h at 80 °C.The mixture was centrifuged and washed with deionized water, and dried at 80 °C for 12 h.The resulting product was then calcined in an oven at 650 °C for 5 h, and nally, the product of kanemite (d-Na 2 Si 2 O 5 ) was synthesized.Kanemite (5 g) was added to deionized water (50 mL) and stirred at 30 °C for 3 h.The resulting suspension was then ltered to obtain the wet kanemite dough.Next, Fe 3 O 4 nanoparticles (0.106 g, 0.457 mmol) were dispersed in deionized water (50 mL) by ultrasonic waves and cetyltrimethylammonium bromide (CTAB) was added to this solution by slowly raising the temperature to 70 °C.Then, the kanemite paste was added and then stirred at 70 °C for 3 h.At this stage, the pH value of the suspension was 11.5-12.5.Aer 3 h, the pH of the medium was adjusted to 8.5 with HCl (2 M), and the mixture was stirred at 70 °C for another 3 h.Aer cooling the mixture, the resulting solid was separated by centrifugation and washed with deionized water.The magnetite mesoporous silicate (Fe 3 O 4 @FSM-16) was dried in an oven at 80 °C for 2 h and then calcined in an oven at 650 °C for 5 h to burn the surfactant and synthesis of the nal magnetite mesoporous silicate, Fe 3 O 4 @FSM-16. 134.Synthesis of Fe 3 O 4 @FSM-16-SO 3 H Fe 3 O 4 @FSM-16 (0.5 g) was then sonicated for 15 min in dry CH 2 Cl 2 (5 mL) in a 10 mL round bottom ask.Then cholorosulfunic acid (ClSO 3 H) (0.15 mL) was added dropwise to the reaction mixture for 15 min at room temperature.The reaction mixture was stirred for 2 h.Finally, the brown solid was

Paper
RSC Advances separated with an external magnet and was dried in an oven at 100 °C for 2 h to obtain Fe 3 O 4 @FSM-16-SO 3 H. 13 2.5.Preparation of zwitterionic salts [(CH 2 ) 4 SO 3 TEA] The zwitterionic solid was synthesized by a one-step reaction between triethylamine (1 mmol) and 1,4-butane sultone (1 mmol) under solvent-free conditions and stirring at room temperature for 24 h.The resulting solid salt was washed several times with diethyl ether and dried in a vacuum at 60 °C.was stirred under reux conditions.The progress of the reaction was monitored by TLC (n-hexane/ethyl acetate, 2 : 1).At the end of the reaction, the Fe 3 O 4 @FSM-16-SO 3 /IL nanocatalyst was separated by an external magnet.Finally, the pure products were puried by recrystallization from MeOH.

Recovery
Fe 3 O 4 @FSM-16-SO 3 /IL (0.004 g) was added to the mixture of aldehyde (1 mmol), dimedone (1 mmol), ethyl acetoacetate (1 mmol) and ammonium acetate (1.2 mmol) in (5 mL) ethanol and stirred under reux conditions.Aer completion of the reaction, hot methanol (10 mL) was added to the reaction mixture; the catalyst was separated by an external magnet, washed three times with MeOH, and dried.The isolated catalyst was used directly for the subsequent runs under similar conditions.
Scheme 1 Functionalization of FSM-16 and electrostatic immobilization of IL.

Synthesis and characterization of the catalyst
In this work, for the preparation of Fe 3 O 4 @FSM-16-SO 3 /IL, the acidic ionic liquid immobilization on Fe 3 O 4 @FSM-16 nanocomposite was performed in several steps as shown in Scheme 1. Firstly, Fe 3 O 4 nanoparticles were prepared and then coated with CTAB and kanemite (d-Na 2 Si 2 O 5 ).The resulting product was calcined to produce Fe 3 O 4 @FSM-16.Subsequently, Fe 3 -O 4 @FSM-16-SO 3 H was prepared by the reaction of between Fe 3 O 4 @FSM-16 with chlorosulfunic acid.Then, the reaction of triethylamine with 1,4-butane sultone produced the ionic liquid (IL).Finally, the Fe 3 O 4 @FSM-16-SO 3 /IL nanocatalyst was prepared by the electrostatic stabilization the IL on Fe 3 O 4 @-FSM-16-SO 3 H.The synthesized nanocatalyst was characterized by XRD, FT-IR, TGA, FE-SEM, EDS, TEM, BET, and VSM analyses.
In addition, the low angle X-ray diffraction (LXRD) analysis of the Fe 3 O 4 @FSM-16-SO 3 /IL catalyst in Fig. 3 shows a high intensity peak at 2q = 0.91°corresponding to (1 0 0), which is characteristic of hexagonally ordered mesoporous materials.
The surface morphology, particle size and structural features of Fe 3 O 4 , FSM-16, 12 and Fe 3 O 4 @FSM-16-SO 3 /IL nanocatalyst were identied by using eld effect scanning electron microscopy (FE-SEM) (Fig. 4).The FE-SEM images show that the synthesized nanocatalyst has an almost regular spherical morphology and uniformity and is less than 100 nm in particle size.Furthermore, it is obvious from Fig. 4 that the average  As shown in Fig. 5, energy-dispersive X-ray (EDX) analysis was used to determine the elements of nanoporous Fe 3 O 4 @-FSM-16-SO 3 /IL.The results conrm the presence of C, O, N, S, Si, and Fe in the synthesized nanocatalyst, and it could be inferred that the target catalyst was successfully synthesized.
The TEM images of Fe 3 O 4 @FSM-16-SO 3 /IL catalyst are shown in Fig. 6.As seen in the images, the black cores of Fe 3 O 4 NPs are surrounded by a grey shell of FSM-16-SO 3 /IL.
The magnetic properties of the Fe 3 O 4 and Fe 3 O 4 @FSM-16-SO 3 /IL were investigated using the vibrating sample magnetometer (VSM) technique at room temperature (Fig. 7).Based on the VSM curves, the magnetization was measured to be Fe 3 O 4 53.03 emu g −1 , and for Fe 3 O 4 @FSM-16-SO 3 /IL 15.5 emu per g nanocatalyst.It is essential to notice that the decrease in the magnetization value Fe 3 O 4 @FSM-16-SO 3 /IL nanocatalyst compared to Fe 3 O 4 may be due to the immobilization of the shell of FSM-16-SO 3 H and then IL around the magnetic Fe 3 O 4 cores.The results showed that even with the reduction of the magnetization; the catalyst can be successfully separated from the reaction mixture by an external magnet.
To investigate the thermal stability of the Fe 3 O 4 @FSM-16-SO 3 /IL nanocatalyst, the thermo gravimetric analysis (TGA) was performed in the range of 25-900 °C (Fig. 8).Based on the TGA spectrum, the rst weight loss of about 4.6% at a temperature below 220 °C was related to the removal of water, physically and chemically absorbed solvents, and surface hydroxyl groups on the Fe 3 O 4 @FSM-16-SO 3 /IL surface.The second and largest weight loss in the range of 220-480 °C is about 28%, related to the thermal decomposition of the sulfuric acid group, amine   group, and organic groups on the surface of the Fe 3 O 4 @FSM-16 nanocomposite.The third and last weight loss of about 8% between 480 and 900 °C is related to the thermal complete decomposition of Fe 3 O 4 @FSM-16 and the ionic liquid (IL) stabilized on its surface, conrming the thermal stability of the prepared Fe 3 O 4 @FSM-16-SO 3 /IL nanocatalyst.
In addition, based on the physicochemical and structural parameters of Fe 3 O 4 @FSM-16-SO 3 /IL nanocatalyst, the nitrogen adsorption-desorption isotherms were measured (Fig. 9).The type of N 2 adsorption-desorption isotherm of prepared nanocatalyst, according to the IUPAC classication, is a type III isotherm, which indicating of the typical mesoporous structure.Furthermore, according to the Brunauer-Emmett-Teller (BET) analysis, the surface area, total pore volume, and mean pore diameter of the Fe 3 O 4 @FSM-16-SO 3 /IL catalyst 180 m 2 g −1 , 0.02 cm 3 g −1 , and 5.83 nm were obtained respectively.

Synthesis of polyhydroquinoline derivatives
Aer the successful characterization of the Fe 3 O 4 @FSM-16-SO 3 / IL nanocatalyst, its catalytic application was investigated in the synthesis of polyhydroquinoline derivatives with the multicomponent reaction of aromatic aldehyde, dimedone, ethyl acetoacetate, and ammonium acetate (Scheme 2).
Initially, to obtain the best reaction conditions, a one-pot reaction between benzaldehyde, dimedone, ethyl acetoacetate, and ammonium acetate as a model reaction was investigated.For this purpose, the effect of catalyst loading, temperature, and solvent was studied.Primarily, the catalytic efficiency of the Fe 3 O 4 @FSM-16-SO 3 /IL nanocatalyst was investigated.The catalyst loading study showed that the use of 0.004 g of Fe 3 -O 4 @FSM-16-SO 3 /IL gave the highest conversion.Then, to obtain the optimum solvent, the model reaction was carried out in Scheme 3 The suggested mechanism for the synthesis of polyhydroquinoline derivatives 5 using Fe 3 O 4 @FSM-16-SO 3 /IL.different polar and non-polar solvents including toluene, CH 3 CN, MeOH, EtOH, H 2 O, DMSO, and also under solvent-free conditions.It was found that the best result was obtained in reuxed ethanol.The reaction temperature was also affected and the best result was observed at reux conditions in ethanol.Finally, according to the mentioned results, the use of 0.004 g of Fe 3 O 4 @FSM-16-SO 3 /IL, and EtOH solvent at reux conditions was selected as the optimum conditions (Table 1).Subsequently, different aryl aldehydes containing electron-donating and electron-accepting groups were used to prepare polyhydroquinoline derivatives.As shown in Table 2, the products were successfully obtained in good to excellent yields.As shown in Scheme 3, a mechanism for the synthesis of polyhydroquinoline derivatives (5a-i) via the Hantzsch reaction in the presence of Fe 3 O 4 @FSM-16-SO 3 /IL nanocatalyst is proposed.Firstly, dimedone and aldehyde were activated by the base site and the acidic sites of the nanocatalyst, respectively.Then, the Knoevenagel condensation was performed by the nucleophilic addition of active methylene of dimedone to the activated carbonyl group of aldehyde to produce intermediate I.
On the other, intermediate II (enamine) was formed through the condensation of ammonia with catalyst-activated ethyl acetoacetate.Then, intermediate I performed a Michael addition with enamine to give intermediate III.Finally, the nal product was synthesized by intramolecular cyclization of III followed by H 2 O elimination.

Efficiency of the catalyst
In order to investigate the heterogeneous or homogeneous nature of the Fe 3 O 4 @FSM-16-SO 3 /IL nanocatalyst, a leaching test was performed under optimum reaction conditions in the model reaction.To this end, the catalyst was separated by an external magnet aer almost 50% of the reaction had taken place.Next, the residue of the reaction mixture under optimum conditions was stirred, but no signicant progress in the reaction was observed.The result of this test conrms that the catalyst acts heterogeneously.To evaluate the recyclability and reusability of the Fe 3 O 4 @FSM-16-SO 3 /IL magnetic solid acid nanocatalyst, the reaction of benzaldehyde, dimedone, ethyl acetoacetate, ammonium acetate under optimum reaction conditions was carried.Aer completion of the reaction process, the catalyst was separated by the external magnetic eld and was washed with ethanol, dried, and reused six times without any signicant decrease in its catalytic activity (Fig. 10).
In the following, the FT-IR spectrum of the recovered Fe 3 -O 4 @FSM-16-SO 3 /IL nanocatalyst is shown in Fig. 11.This analysis conrms the high strength and stability of the catalyst structure aer recycling.Also, the XRD diffraction pattern of the recovered catalyst was analyzed.As shown, the relative intensity and position of all peaks, and the structural stability were conrmed (Fig. 12).
Next, the comparison of the performance of the Fe 3 O 4 @FSM-16-SO 3 /IL catalyst was investigated with other reported catalysts for the synthesis of polyhydroquinoline derivatives was investigated.Based on the results in Table 3, the Fe 3 O 4 @FSM-16-SO 3 / IL nanocatalyst is comparable with other catalysts in terms of recycling times and reusability, short reaction time, amount of the catalyst, reaction temperature, type of solvent, and product yield.

Conclusions
This study presents a new ionic liquid based on magnetic FSM-16 nanocomposite and its catalytic application in the synthesis of polyhydroquinoline derivatives.The FE-SEM and TEM images of the Fe 3 O 4 @FSM-16-SO 3 /IL catalyst showed a spherical morphology with a highly ordered structure.Furthermore, the XRD, TGA, EDX, and FT-IR analyses conrmed the high stability, and also the immobilization of the ionic liquid on the magnetic mesoporous framework.Also, the VSM analysis showed well the magnetic properties of this nanocomposite.Some of the remarkable advantages of this catalyst are short reaction time, excellent product yield, and chemical stability.More importantly, the catalyst can be recovered using an external magnet and reused 6 times without signicant loss of catalytic activity.

Table 1
Optimization of the reaction conditions for the synthesis of 5a a b Isolated yields.

Table 3
Comparison results of Fe 3 O 4 @FSM-16-SO 3 /IL with other catalysts for the synthesis of polyhydroquinoline derivatives a Isolated yields.b This work.