FSM-16-SO₃H nanoparticles as a novel heterogeneous catalyst: preparation, characterization, and catalytic application in the synthesis of polyhydroquinolines

The catalyst characterization

FSM-16 and FSM-16-SO₃H were characterized using Fourier transform infrared spectroscopy (FT-IR), scanning electron microscope (SEM), Brunauer–Emmett–Teller (BET), and X-ray diffraction (XRD) techniques. The SEM micrographs of FSM-16 and FSM-16-SO₃H are shown in Figure 1A and B, respectively. SEM micrographs show spherical nanoparticles with sizes of approximately 100 nm. Both materials have similar texture and there is no significant difference.

Figure 1:

SEM images of (A) FSM-16 and (B) FSM-16-SO₃H.

The FT-IR spectra of FSM-16 and FSM-16-SO₃H are shown in Figure 2. The spectrum of FSM-16 (Figure 2A) shows characteristic peaks at 1216, 1082, and 804 cm⁻¹ due to symmetric and asymmetric stretching vibrations of Si-O-Si and also the peak of 966 cm⁻¹ due to Si-OH groups. The peak at 463 cm⁻¹ is assigned to the bending vibration of Si-O-Si. For FSM-16-SO₃H (Figure 2B), the FT-IR bands of the O=S=O asymmetric and symmetric stretching modes lie in 1120–1230 and 1010–1080 cm⁻¹, respectively, and that of the S-O stretching mode lies in 600 cm⁻¹. The FT-IR spectrum of FSM-16-SO₃H shows the overlap of asymmetric and symmetric stretching bands of SO₂ with Si-O-Si stretching bands in the region of 1000–1250 cm⁻¹, which results in an increase of intensity of this peak.

Figure 2:

FT-IR spectra of (A) FSM-16 and (B) FSM-16-SO₃H.

The low angle XRD patterns of FSM-16 and FSM-16-SO₃H are shown in Figure 3. The characteristic peaks of FSM-16 (Figure 3A) appeared at 2θ=2.38°, 4.11°, and 4.73° in accordance with the literature (Araki et al., 2003). As shown in Figure 3B, after surface modification with SO₃H groups, the intensity of the main peak (2θ=2.38°) decreased and shifted to the higher angle (2θ=2.76°). Any increase in the width of the main peak of the FSM-16-SO₃H is due to a decrease in the long-range order of the hexagonal mesostructure of FSM-16 after the incorporation of -SO₃H groups into the channels of FSM-16. The shift in the main peak of FSM-16-SO₃H to a higher angle is due to a decrease in the pore diameter because of the insertion of sulfonic acid groups on the inner surface of mesopores.

Figure 3:

Low angle XRD patterns of (A) FSM-16 and (B) FSM-16-SO₃H.

The distribution of both Brønsted and Lewis acid sites of FSM-16-SO₃H was detected using FT-IR spectroscopy by means of pyridine absorption. Figure 4 shows the pyridine adsorbed spectra of FSM-16-SO₃H heated at various temperatures. The spectrum of pyridine adsorbed FSM-16 shows only peaks of pyridine-bonded Lewis acid sites at 1446 and 1598 cm⁻¹ (Figure 4B). The spectrum of pyridine-adsorbed FSM-16-SO₃H before heat treatment (Figure 4C) shows the contribution of the pyridine adducts in the region of 1400–1650 cm⁻¹. The peak at 1487 cm⁻¹ is attributed to the combination of pyridine bonded to Lewis and Brønsted acid sites. The peak appearing at 1530 cm⁻¹ is assigned to Brønsted acid sites (pyridinum ion). The peak at 1603 cm⁻¹ is attributed to the pyridine-bonded Lewis acid sites of the catalyst. The peak at 1633 cm⁻¹ in the spectrum of the catalyst before treatment with pyridine (Figure 4A) is due to the presence of water during the preparation of the pellet sample. This sharp peak overlapped with another weak peak from the Brønsted acid site at 1642 cm⁻¹. However, these results confirm the distribution of both Brønsted and Lewis acid sites on the surface of the catalyst. As shown in Figure 4B–E, with increasing temperature, the characteristic peak of Lewis acid sites at 1603 cm⁻¹ and the peak of Brønsted acid sites at 1530 cm⁻¹ still remained. These results confirm that -SO₃H functionalization has strengthened both Brønsted and Lewis acid sites on the catalyst.

Figure 4:

FT-IR spectra of (A) FSM-16-SO₃H, (B) pyridine adsorbed FSM-16, and (C) FSM-16-SO₃H at ambient temperature; pyridine-adsorbed FSM-16-SO₃H heated at (D) 100°C and (E) 200°C.

The strength and number of acid sites of the catalysts were determined by potentiometric titration of samples with 0.02 N solution of n-butylamine in acetonitrile. According to this method, the initial electrode potential (E_i) indicates the strength of the acid sites, and the meq of the consumed base indicates the number of acid sites (Pizzio et al., 2003). As shown in Figure 5, the very low initial potential of FSM-16 shows that the acid strength of these samples is very low. By placing the -SO₃H groups on the surface of FSM-16, FSM-16-SO₃H displays higher strength than that of FSM-16. Because of the higher meq of the used base per gram of the FSM-16-SO₃H, this sample also has a higher number of acidic sites than FSM-16.

Figure 5:

Potentiometric titration of (A) FSM-16 and (B) FSM-16-SO₃H.

To investigate the number of -SO₃H acidic sites supported on the FSM-16, the elemental composition of the FSM-16-SO₃H was determined using energy dispersive X-ray (EDX) analysis, and the results are shown in Figure 6 and Table 1. Based on these results, the amount of sulfur in the FSM-16-SO₃H was 7.08 wt.%, and thus the number of -SO₃H on the catalyst was 0.87 mmol per gram of FSM-16-SO₃H.

Figure 6:

Energy-dispersive X-ray spectrum of FSM-16-SO₃H.

Figure 7 shows the N₂ adsorption–desorption isotherms of the FSM-16 and FSM-16-SO₃H catalysts. In the isotherms of FSM-16, a mesoporous inflection was observed at the medium p/p° partial pressure region (p/p°=0.1–0.3) due to the capillary condensation of N₂ in the mesopores. A sharper hysteresis was observed at higher p/p° (p/p°>0.8) for both catalysts. The hysteresis in this region is because of the condensation of N₂ within the voids formed by nanoparticles. In the isotherms of FSM-16-SO₃H, the curvature of the mesoporous structure in the area of P/P°=0.2–0.4 decreased because the hexagonal cavities were occupied by -SO₃H groups. The textural properties of FSM-16-SO₃H were also studied by N₂ adsorption–desorption isotherms. According to the obtained results, the BET surface area for FSM-16-SO₃H was measured to be 634 m²/g and the average pore diameter using the Barret–Joyner–Halenda desorption method was calculated to be 5.97 nm.

Figure 7:

N₂ adsorption–desorption isotherms of (A) FSM-16 and (B) FSM-16-SO₃H.

Catalytic activity of FSM-16-SO₃H

After the catalyst characterization was done successfully, the catalytic activity of FSM-16-SO₃H nanoparticles was investigated in the synthesis of polyhydroquinolines. Initially, the reaction of benzaldehyde, dimedone, ethyl cyanoacetate, and ammonium acetate was selected as the model reaction and the results are summarized in Table 2.

The effect of solvent and temperature was investigated by performing the model reaction in the presence of 40 mg of catalyst in various solvents and temperatures (Table 2, entries 1–6). Among them, ethanol was found to be the best solvent at reflux condition in terms of the reaction time and yield of product (Table 2, entry 6). The lower yield and longer reaction time was achieved by performing the model reaction in the presence of ethanol as the solvent at r.t. condition (Table 2, entry 5).

To optimize the required catalyst amount, the model reaction was also performed in the presence of various amounts of catalyst, and according to the obtained results (Table 2, entries 6 and 9–11), 40 mg of the catalyst was chosen as the best amount. Clearly, the FSM-16 support strongly affected the efficiency of the heterogeneous catalyst. To show this, the model reaction in the presence of 40 mg FSM-16 was performed under the same reaction conditions and lower yield of the product was obtained after 90 min (Table 2, entry 8).

The scope and generality of this methodology has been illustrated with respect to the reaction of different types of benzaldehydes that have electron-withdrawing and electron-donating groups with dimedone, ammonium acetate, and ethyl cyanoacetate and the results are summarized in Table 3.

Table 3:

Synthesis of polyhydroquinolines in the presence of FSM-16-SO₃H.

Entry	Aldehyde (1)	Polyhydroquinoline (5)	Time (min)	Yielda (%)	mp (°C)
					Found	Lit. [ref]
a			30	93	157–160	150–155 (Kumar et al., 2008)
b			20	88	155–157	150–153 (Abdollahi-Alibeik and Rezaeipoor-Anari, 2016)
c			20	94	190–195	190–195 (Abdollahi-Alibeik and Rezaeipoor-Anari, 2016)
d			40	90	139–141	135–137 (Kumar et al., 2008)
e			15	88	135–136	140–143 (Abdollahi-Alibeik and Rezaeipoor-Anari, 2016)
f			10	89	183–184	–
g			60	80	172–175	179–181 (Abdollahi-Alibeik and Rezaeipoor-Anari, 2016)
h			5	94	182–183	183–185 (Abdollahi-Alibeik and Rezaeipoor-Anari, 2016)
i			55	83	128–130	122–125 (Kumar et al., 2008)
j			25	93	155–158	–
k			15	91	222–226	–

1. ^aIsolated yield.

The reaction of various types of aryl aldehydes with both electron-donating and electron-withdrawing substituents were carried out under optimal conditions and the corresponding polyhydroquinolines were obtained in high to excellent yields (Table 3, entries 2–11). As exemplified in Table 3, this protocol is rather general for a wide variety of electron-rich as well as electron-deficient aromatic aldehydes. Although a regular trend was not observed based on the activity of aldehyde derivatives, the results in Table 3 show that aldehydes with electron-withdrawing groups such as -NO₂, Cl, Br, and -F have maximum rates of reaction.

To investigate the reusability of FSM-16-SO₃H, it was separated from the reaction mixture and washed with ethanol. The catalyst was dried in an oven at 120°C for 2 h. The recycled catalyst was reused in the model reaction. The catalyst was found to be reusable for at least three cycles without considerable loss of activity (Table 4).

Table 5 shows a comparison between the activity of the FSM-16-SO₃H and the other reported catalysts for the polyhydroquinoline synthesis through reaction of benzaldehyde, dimedone, ethyl cyanoacetate, and ammonium acetate. Results show that FSM-16-SO₃H is comparable with other catalytic systems in terms of reaction time and/or yield.

Materials and methods

All chemicals were commercial grade and were used as-received. The reactions were monitored using thin layer chromatography (TLC). The yields of products refer to isolated compounds. Melting points were obtained with a Buchi B-540 apparatus and are uncorrected. ¹H NMR and ¹³C NMR spectra of polyhydroquinolines were recorded in DMSO-d₆ on a Bruker DRX-500 AVANCE spectrometer (500 MHz for ¹H and 125.72 MHz for ¹³C). Infrared spectra of the reaction products and catalysts were recorded on a Bruker FT-IR Equinax-55 in KBr disks. The XRD patterns were recorded on a Bruker D8 ADVANCE X-ray diffractometer using Ni-filtered CuKa radiation. The morphology was studied using a KYKY-EM3200 SEM. The BET surface area was measured using a micromeritics model ASAP2020 from the nitrogen adsorption–desorption isotherms at 77 K. All samples were degassed at 120°C under flowing nitrogen for 2 h. The specific surface area (S_BET) was calculated from the adsorption data using the BET equation, and the pore volume (V_pore) was estimated from the volume of adsorbed N₂ at relative pressure (p/p°) of 0.99. The pore size distribution was calculated using the Barret–Joyner–Halenda method.

Preparation of FSM-16 nanoparticles

A typical procedure for the preparation of kanemite was as follows: to a solution of NaOH (3 g) dissolved in deionized water (30 mL), tetraethylortosilicate (16.6 mL) was added dropwise and then the mixture was stirred for another 12 h at room temperature. The solution was transferred into an oven and heated at 355 K for 4 h. The resultant product was calcined at 923 K for 5 h to obtain δ-Na₂Si₂O₅ (kanemite). The kanemite was deliquescent and immediately used for further treatment. Kanemite (5 g) was dispersed in deionized water (50 mL) and then stirred for 3 h at 300 K. Then, the suspension was filtered out to obtain wet kanemite paste. All of the kanemite paste was dispersed in an aqueous solution (40 mL) of cetyltrimethylammonium bromide (0.125 mol L⁻¹) and then stirred at 343 K for 3 h. The pH value of the suspension was 11.5–12.5 at this stage. Afterward, the pH value was adjusted carefully to 8.5 by adding 2 mol L⁻¹ of hydrochloric acid with stirring. The suspension was continuously stirred at 343 K for 3 h while keeping the pH value at 8–9. After cooling to r.t., the solids were separated using a centrifuge and washed with distilled water (20 mL) and dried in an oven at 393 K for 2 h to yield mesoporous silicate, FSM-16, while retaining the template. The product was calcined at 823 K to burn off the surfactant to obtain the final FSM-16.

Preparation of FSM-16-SO₃H nanoparticles

FSM-16 (0.278 g) was added to dry CH₂Cl₂ (3 mL) in a 5 mL round-bottomed flask equipped with a gas outlet tube and a dropping funnel containing a solution of chlorosulfonic acid (0.6 mL) in dry CH₂Cl₂ (4.5 mL). The chlorosulfonic acid solution was added dropwise to the obtained suspension over a period of 30 min at room temperature. After completion of the reaction, the sediment was separated using a centrifuge and then washed with deionized water (2×3 mL). The obtained solid was dried in an oven at 120°C for 2 h to obtain FSM-16-SO₃H.

General procedure for the synthesis of polyhydroquinoline derivatives in the presence of FSM-16-SO₃H

A mixture of dimedone (1 mmol), aldehyde (1 mmol), ethyl cyanoacetate (1.1 mmol), ammonium acetate (1.2 mmol), and a catalytic amount of FSM-16-SO₃H (40 mg) was stirred in ethanol (1.5 mL) under reflux conditions. After completion of the reaction (monitored by TLC, eluent; n-hexane: ethyl acetate, 1:1), the catalyst was separated from the reaction mixture using a centrifuge and washed with ethanol (2×3 mL). After evaporation of solvent, the crude products were crystallized from ethanol to give pure polyhydroquinolines.

Physical and spectroscopic data for selected compounds