Perlite: an inexpensive natural support for heterogenization of HBF₄

Abstract

Nano-perlite-fluoroboric acid (n-PeFBA) has been synthesized by immobilization of HBF₄ on perlite to produce an efficient green heterogeneous reusable solid acid catalyst. The catalyst was characterized by (FE-SEM, EDX, BET and BJH). This catalyst was employed to prepare biologically important 2,4,5-trisubstituted and 1,2,4,5-tetrasubstituted imidazoles with high yields and selectivities. Advantages of solid acid include: low cost, facile handling, simple preparation, high stability, reusability and low toxicity. The catalyst could be recovered and reused for several runs without deterioration in catalytic activity.

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1. Introduction

Catalysis forms the foundation the chemical industry today. A catalytic process is utilized in the production of 90% of existing chemicals.¹ Clean and environmentally benign chemical processes have become a primary goal in synthetic organic chemistry. Homogeneous acids are widely used for organic transformations.^2–4 Traditional homogeneous catalytic systems suffer from drawbacks including: difficult removal of the catalyst from reaction products, difficulty with recycling and the generation of large amounts of waste. These difficulties can contribute to high manufacturing costs.^5,6 One approach to overcome the aforementioned problems is the replacement of environmentally hazardous Bronsted and Lewis acid catalysts with efficient and stable reusable solid acid catalysts that have been associated with high reactivity, operational simplicity, non-corrosive nature and the potential of the catalyst to be recyclable.^7–17

Perlite is a cheap mineral mixed oxide mixture which contains approximately 70–75% of SiO₂ and 12–18% of Al₂O₃ (ref. 18) that can be used as a platform for the preparation of heterogeneous catalysts. Advantages of perlite include: ease of handling, low cost, low toxicity, high mechanical and thermal stability and non-corrosiveness.^19–22

Imidazole derivatives are an important class of heterocycles with wide ranging biological and therapeutic activities such as anti-inflammatory,²³ anti-bacterial,²⁴ glucagon receptor antagonists,²⁵ modulators of Pgp-mediated multidrug resistance,²⁶ ligands of the Src SH2 protein,²⁷ antitumor agents,²⁸ and CB1 cannabinoid receptor antagonists,²⁹ as well as fungicides and herbicides,³⁰ plant growth regulators and therapeutic agents.³¹ Thus, the syntheses of imidazole derivatives is of great interest and numerous methods continue to be reported. The synthesis of 2,4,5-trisubstituted imidazoles can be accomplished via multicomponent cyclocondensation of a 1,2-diketone, α-hydroxy/acetoxy/silyloxyketone or 1,2-ketomonoxime, an aldehyde and ammonium acetatein refluxing acetic acid,³² in the presence of a homogeneous strong protic acid,³³ Lewis acids,^34–36 glycerol,³⁷ ionic liquids^38,39 and under microwave/ultrasonic/classical heating in the presence or absence of catalyst.^40–42 Similarly, the preparation of 1,2,4,5-trisubstituted imidazoles has also been accomplished via cyclocondensation of 1,2-diketone, α-hydroxy/acetoxy/silyloxyketone or 1,2-ketomonoxime, aldehyde, amine and ammonium acetate using microwaves,⁴³ ionic liquids,⁴⁴ BF₃·SiO₂,⁴⁵ L-proline,⁴⁶ HClO₄–SiO₂,⁴⁷ heteropolyacid,⁴⁸ sodium benzenesulfinate⁴⁹ and molecular iodine.⁵⁰ Some of these methodologies suffer from one or more drawbacks such as: harsh conditions, long reaction times, unsatisfactory yields, difficult work-up procedures, expensive reagents, difficulty in recovery and reusability of the catalysts.

Recently, we prepared the heterogeneous reagent ‘perlite sulfonic acid’ and investigated its application as an efficient catalyst in several organic reactions.⁵¹ Our studies illustrated that this catalyst had excellent reactivity in multicomponent one-pot synthesis of 3,4-dihydropyrimidin-2(1H)-ones, 2,4,5-trisubstituted imidazoles, coumarins and bis(indolyl)methanes. Moreover, the catalyst was successfully recovered and reused several times without significant decrease in activity.

Given the excellent properties of perlite and the significant of heterogeneous solid acid catalysis, we have continued our studies in this area and now report a novel nano-perlite-fluoroboric acid (n-PeFBA). This catalysis is suitable for the one-pot synthesis of 2,4,5-trisubstituted and 1,2,4,5-tetrasubstituted imidazoles. The reactivity, reusability, and stability of the catalyst during reactions are reported.

2. Results and discussion

As a part of our ongoing investigation in developing new and efficient heterogeneous solid acid catalysts,^8,51 we demonstrate here the synthesis of nano-perlite-fluoroboric acid (n-PeFBA) and discuss its performance as solid acid catalyst. The n-PeFBA catalyst was prepared by the reaction of perlite and fluoroboric acid at room temperature. The inorganic solid acid catalyst was characterized via EDX, FE-SEM and BET and BJH analysis.

2.1. Characterization of n-PeFBA

2.1.1. XRD energy-dispersive X-ray spectroscopy (EDX) analysis. Energy-dispersive X-ray spectroscopy (EDX) analysis of n-PeFBA indicated the presence of Si, Al, Fe, K, Na, Mg, O as the components of perlite structure, also the small peaks for fluorine and boron clearly indicate that HBF₄ is incorporated within the channels of perlite (Fig. 1).

Fig. 1 EDX spectra of n-PeFBA.

2.1.2. SEM analysis of the catalyst. Fig. 2 shows the field emission scanning electron microscopy (FE-SEM) images of perlite and modified perlite nanostructures. Fig. 2a and b show that the pure perlite has a layer structure with smoothed surface. Fig. 2c and d show the morphology of n-PeFBA upon functionalization. It is clear that the morphology of the n-PeFBA is a layer like structure but it is not smooth. The size and morphology distribution of the particles located on the layer are homogeneous. The layer thickness size is 50–70 nm and the particles diameter sizes are about 50–70 nm. It shows that the modification process has been performed successfully.

Fig. 2 FE-SEM images of perlite (a and b) and n-PeFBA (c and d).

2.1.3. BET and BJH texture analysis of the catalyst. The synthesized powders were characterized for their surface area, average pore size and average pore volume. Prior to N₂-physical adsorption measurement, the samples were degassed at 150 °C for 120 min in the nitrogen atmosphere. So, the specific surface area (SBET) of the obtained materials was determined with adsorption–desorption isotherms of N₂ at 77 K. The surface area, pore volume and average pore diameter of the synthesized materials are summarized in Table 1. From Table 1, it can be seen that the average surface area and pore volumes are about 0.21 m² g⁻¹ and 0.0003 cm³ g⁻¹ for pure perlite and 0.43 m² g⁻¹ and 0.0043 cm³ g⁻¹ for modified perlite, respectively. And also, for perlite and n-PeFBA, the average nanoparticles sizes were measured as 29 μm and 14 μm, respectively. Also, Table 2 shows the textural properties of the as-prepared materials. The data summarized in Table 2 shows that the specific surface area of pores of n-PeFBA is larger than that of pure perlite and the pore width and pore volume of n-PeFBA is larger than that of perlite. So, the investigated results of BET and BJH measurements suggest that the surface area of modified perlite is larger than that of pure perlite. It also shows that with modifying perlite, the average particle sizes are smaller than those of pure perlite that is in a good agreement with FESEM images.

Table 1 Pore structure parameters of perlite and n-PeFBA derived from the N₂ adsorption–desorption isotherms

Sample	BET surface area (m^2 g^−1)	Pore size (Å)	Pore volume (cm^3 g^−1)	Average particles size (μm)
Perlite	0.21	64	0.0003	29
n-PeFBA	0.42	408	0.0043	14

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Table 2 BJH data (textural properties) for the perlite and n-PeFBA

Property	Perlite	n-PeFBA
BJH adsorption cumulative surface area of pores between 17 and 3000 Å width	0.09 m^2 g^−1	0.41 m^2 g^−1
BJH adsorption cumulative volume of pores between 17 and 3000 Å width	0.0007 cm^3 g^−1	0.0052 cm^3 g^−1
BJH adsorption average pore width (4V/A)	64 nm	41 nm

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2.2. Application of n-PeFBA as heterogeneous catalyst in the synthesis of multisubstituted imidazoles

We decided to explore the use of this new nanocatalyst for the synthesis of various multisubstituted imidazoles under solvent-free condition. The n-PeFBA catalyst was employed in the preparation of 2,4,5-trisubstituted imidazoles from aromatic aldehydes, benzil and NH₄OAc as well as 1,2,4,5-tetrasubstituted imidazoles from aromatic aldehydes, benzil, NH₄OAc and aniline (Scheme 1).

Scheme 1 Synthesis of 2,4,5-trisubstituted and 1,2,4,5-tetrasubstituted using of n-PeFBA.

2.2.1. Optimization of the reaction parameters. The reaction of benzaldehyde (1 mmol), benzil (1 mmol), and ammonium acetate (2 mmol) was carried out in the presence of various amount of the n-PeFBA (Table 3, entries 1–4). The best yield was obtained with 0.01 mmol of catalyst (Table 3, entry 2). The usage of higher amounts of catalyst did not have any significant effect on the product yield. Also, when the reaction was attempted without the addition of catalyst, no desired product was obtained (Table 3, entry 1). The effect of temperature plays an important role in the catalytic synthesis of imidazoles. It was examined in the temperature range from 60 °C to 120 °C in the absence of solvent, using n-PeFBA as catalyst. At 60 °C the reaction has not high yield and with further increase in temperature to 100 °C the yield of product increased. By increasing the temperature to 120 °C, the yield of product decreased which may be due to the formation of some side product (Table 3, entries 2, 5–7). So 100 °C was chosen as the optimum temperature for performing the reaction. We also examined the effect of three solvents: H₂O, MeOH and CH₃CN (Table 3, entries 8–10). We found H₂O and MeOH to be suitable reaction media (Table 3, entries 8 and 10), whereas in CH₃CN did not afford any desired product (Table 3, entry 9). The reaction proceeded under solvent-free condition to generate the corresponding product in excellent yields in short reaction time in comparison with solvent conditions (Table 3, entry 2). So, to avoid the use of volatile solvent and to reduce environmental pollution, all the reactions were performed under solvent-free conditions. Our preferred reaction conditions were determined to be solvent-free, 100 °C, for 35 (Table 3, entry 2).

Table 3 Optimization of synthesis of imidazoles^a

Entry	Solvent	Condition	Amount of catalyst (mmol)	Time (min)	Yield (%)
a Reaction condition: benzaldehyde (1 mmol), benzil (1 mmol), NH4OAc (2 mmol).
1	Solvent-free	100 °C	—	35	Trace
2	Solvent-free	100 °C	0.01	35	98
3	Solvent-free	100 °C	0.02	35	84
4	Solvent-free	100 °C	0.025	35	82
5	Solvent-free	60 °C	0.01	35	70
6	Solvent-free	80 °C	0.01	35	79
7	Solvent-free	120 °C	0.01	35	90
8	H2O	Reflux	0.01	35	70
9	CH3CN	Reflux	0.01	35	79
10	MeOH	Reflux	0.01	35	90

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This reaction as applied to several substituted aromatic aldehydes to prepare the corresponding 2,4,5-trisubstituted imidazoles in good yields (Table 4). Aldehydes bearing either electron-withdrawing or electron-donating groups perform equally well in the reaction. The nature of the substituents on the aromatic aldehyde has not significant effect on yield of reaction. The reaction was also applied to the synthesis of 1,2,4,5-tetrasubstituted imidazoles from benzil, aniline, ammonium acetate and various aromatic aldehydes (Table 5).

Table 4 n-PeFBA-catalyzed one-pot synthesis of 2,4,5-trisubstituted imidazoles^a

Entry	Aldehydes	Time (min)	Yield (%)	Mp (°C)
				Found	Reported (ref.)
a Reaction condition aromatic aldehyde (1 mmol), benzil (1 mmol), NH4OAc (2 mmol), n-PeFBA (0.01 mmol), solvent-free, 100 °C.
1		35	98	267–268	267–269 (ref. 37)
2		60	88	211–213	204–205 (ref. 37)
3		45	89	208–210	210 (ref. 52)
4		40	87	196–198	195–197 (ref. 53)
5		48	97	264–266	264–267 (ref. 54)
6		41	93	253–255	252–255 (ref. 55)
7		35	96	220–222	220–222 (ref. 37)
8		34	94	233–235	232–234 (ref. 56)
9		65	75	257–258	256–258 (ref. 57)
10		120	70	320–322	319–321 (ref. 37)
11		30	92	220–222	222–224 (ref. 57)
12		48	89	236–238	228–231 (ref. 57)
13		80	94	264–266	268–269 (ref. 37)

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Table 5 n-PeFBA-catalyzed one-pot synthesis of 1,2,4,5-tetrasubstituted imidazoles^a

Entry	Aldehydes	Time (min)	Yield (%)	Mp (°C)
				Found	Reported
a Reaction condition aromatic aldehyde (1 mmol), benzil (1 mmol), NH4OAc (1 mmol), aniline (1 mmol), (0.01 mmol), solvent-free, 100 °C.
1		80	92	218–219	218–219 (ref. 37)
2		120	91	185–187	164–166 (ref. 58)
3		90	89	283–285	282–284 (ref. 56)
4		120	90	180–183	180–182 (ref. 56)
5		120	97	184–186	182–184 (ref. 56)
6		180	78	220–222	184–186 (ref. 56)
7		110	93	178–180	178–180 (ref. 59)

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We compared n-PeFBA with results reported in the literature using other catalysts for the synthesis of 2,4,5-trisubstituted imidazoles (Table 6) and 1,2,4,5-tetrasubstituted imidazoles (Table 7). This comparison revealed advantages of this nanocatalyst over most of the others in term of reaction conditions, time and product yield.

Table 6 Comparison of the efficiency of various catalysts with n-PeFBA in the synthesis of 2,4,5-trisubstituted imidazoles

Entry	Catalyst	Condition	Time (min)	Yield (%)	Ref.
a Sulfated zirconia.
1	Yb(OPf)3	C10F18/80 °C	360	80	60
2	Nano-crystalline SZ^a	Reflux in EtOH	45	87	56
3	NiCl2·6H2O/Al2O3	Reflux in EtOH	90	89	35
4	Zeolite	Reflux in EtOH	60	80	56
5	Montmorilonite K10	Reflux in EtOH	90	75	56
6	n-PeFBA	Solvent free/100 °C	35	98	Present work

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Table 7 Comparison of the efficiency of various catalysts with n-PeFBA in the synthesis of 1,2,4,5-tetrasubstituted imidazoles

Entry	Catalyst	Condition	Time (min)	Yield (%)	Ref.
1	BF3/SiO2	Solvent free/140 °C	120	92	61
2	NaHSO4/SiO2	Solvent free/140 °C	120	92	62
3	SbCl5/SiO2	Solvent free/140 °C	120	90	63
4	MgCl2	Solvent free/80 °C	45	60	45
5	n-PeFBA	Solvent free/100 °C	80	92	Present work

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2.3. Plausible mechanism

The suggested mechanism for the formation of trisubstituted imidazole using n-PeFBA is depicted in Scheme 2. The reaction can be initiated by protonation by n-perlite-HBF₄ at carbonyl oxygen to initiate a condensation to diamine intermediate 1. Intermediate 1, subsequently condenses with benzil to form intermediate 2, which in turn rearranges to the trisubstituted imidazole. Similarly, the plausible mechanism for the synthesis of tetrasubstituted imidazoles begins the formation of intermediate 3 by the reaction of an aldehyde, phenyl amine and ammonium acetate in the presence of catalyst. Intermediate 3 condenses with benzil to form intermediate 4, and then tautomerizes to the tetrasubstituted imidazole (Scheme 3).

Scheme 2 Proposed mechanism for the synthesis of trisubstituted imidazole.

Scheme 3 Proposed mechanism for the synthesis of tetrasubstituted imidazole.

2.4. Reusability of the catalyst

The activity of recycled catalyst was also examined under the optimized conditions. Upon completion, the heterogeneous catalyst was separated via centrifuge, washed with acetone/ethanol, dried in oven and the recycled catalyst was reused in the next reaction. The recycled catalyst could be reused 5 runs without distinct deterioration in catalytic activity (Fig. 3).

Fig. 3 Recyclability of n-PeFBA.

3. Experimental section

3.1. General remarks

All of the chemicals used in this study were purchased from Merck and Aldrich chemical companies and used without further purification. Perlite obtained from Iran, Semnan sources. All yields refer to the isolated products after purification. Products were characterized by their physical constants and comparison with authentic samples. The purity of products was accompanied by thin layer chromatography (TLC) on silica-gel polygram SILG-UV 254 plates using n-hexane/ethyl acetate mixture as mobile phase. Melting points were determined on Electro thermal 9100 without further corrections. The EDX characterization of the catalyst was performed using a Mira 3-XMU scanning electron microscope equipped with an energy dispersive X-ray spectrometer operating. Filed emission scanning electron microscope (FESEM) images were obtained on a SEM-Philips XL30. BET surface areas were acquired on a Beckman Coulter SA3100 surface area analyzer. The NMR spectra were measured in pure deuterated chloroform and dimethyl sulfoxide with a Bruker Avance 300 MHz instruments (¹H NMR 300 MHz) with tetramethylsilane (TMS) as the internal reference.

3.2. Purification of neat perlite powder

10 g of perlite powder was washed by stirring overnight in 200 mL methanol and ethanol to remove organic components, then were washed by distilled water, methanol and ethanol three times and following that were heated in aqueous solution of NaOH 5 N for 20 minutes. Precipitated perlite were filtered and rinsed with excess water until NaOH removed and then dried at 100 °C.⁵¹

3.3. Preparation of fluoroboric acid adsorbed on perlite (n-PeFBA)

The n-PeFBA was prepared following the originally reported procedure for the preparation of HBF₄–SiO₂.⁶⁴ A magnetically stirred suspension of perlite (26.7 g) in Et₂O (75 mL) was treated with HBF₄ (3.3 g, 15 mmol, 8.25 mL of a 40% aq. solution of HBF₄) and the mixture was stirred magnetically for 3 h at room temperature. The mixture was concentrated and the residue dried under vacuum at 100 °C for 72 h to afford n-PeFBA (0.5 mmol of HBF₄ g⁻¹) as a free-flowing powder.

3.4. n-PeFBA, solid acid catalyst for multi component one-pot synthesis of imidazoles

3.4.1. General procedure for the synthesis of 2,4,5-trisubstituted imidazoles. The stirred mixture of aldehyde (1 mmol), benzil (1 mmol) and NH₄OAc (2 mmol) was heated at 100 °C (bath temperature) in the presence of n-PeFBA (0.01 mmol) under neat conditions for the appropriate time until the reaction was complete. The reaction was monitored by thin layer chromatography (TLC) [7 : 3 hexane : ethyl acetate].

After the completion of the reaction, hot ethanol (5 mL) was added and the obtained solution was hotly filtered. The filtrate was then sufficiently cooled to precipitate crude products. The retaining catalyst was washed with EtOH (2 × 4 mL). The combined filtrates were concentrated under rotary vacuum evaporation and the crude product recrystallized from ethanol to afford pure imidazole derivatives. The products were confirmed by comparisons with authentic samples, IR, ¹H NMR spectra and melting points.

3.4.2. General procedure for the synthesis of 1,2,4,5-tetrasubstituted imidazoles. The mixture of aldehyde (1 mmol), benzil (1 mmol), NH₄OAc (1 mmol) and aniline (1 mmol) was heated at 100 °C (bath temperature) in the presence of n-PeFBA (0.01 mmol) under solvent free conditions for the appropriate time. After the completion of the reaction which was monitored by TLC, the reaction mixture was diluted with hot ethanol (5 mL) and the obtained solution was hotly filtered. The filtrate was then cooled to precipitate crude products. The retaining catalyst was washed with EtOH (2 × 4 mL). The combined filtrates were concentrated under rotary vacuum evaporation and the crude product recrystallized from ethanol to afford pure imidazole derivatives. The remaining reactions were performed following this general procedure and the physical data (mp, IR, NMR) of all known compounds were identical with those reported in the literature.

3.5. Selected data

3.5.1. Compound (entry 3, Table 4).
2-(2-Methoxy-phenyl)-4,5-diphenyl-1H-imidazole. Mp 208–210 °C, IR (KBr, cm⁻¹). 3400, 1650, 1600, 1475, 1180 ¹H NMR (DMSO-d₆) δ. 3.88 (s, 3H), 6.86–8.02 (m, 14H), 11.85 (s, 1H).

3.5.2. Compound (entry 11, Table 4).
2-(3,4-Dimethoxy-phenyl)-4,5-diphenyl-1H-imidazole. Mp 220–222 °C, IR (KBr, cm⁻¹). 3400, 3100, 2874, 1650, 1600, 1475, 1255, 1100, 900, 750, 690. ¹H NMR (DMSO-d₆) δ. 3.13 (s, 3H), 3.81 (s, 3H), 7.01–7.63 (m, 13H), 12.47 (s, 1H).

3.5.3. Compound (entry 13, Table 4).
2-Ethoxy-4-(4,5-diphenyl-1H-imidazol-2-yl)-phenol. Mp 264–266 °C, IR (KBr, cm⁻¹). 3224, 1589, 1473, 1180 ¹H NMR (DMSO-d₆) δ. 1.39 (t, 3H), 4.11 (q, 2H), 6.87 (d, 1H), 7.22–7.55 (m, 11H), 7.63 (d, 1H), 9.23 (s, 1H), 12.43 (s, 1H).

3.5.4. Compound (entry 4, Table 5).
1,4,5-Triphenyl-2-(4-methoxyphenyl)-1H-imidazole. Mp 180–183 °C, IR (KBr, cm⁻¹). 3035, 1596, 1396, 1087 ¹H NMR (CDCl₃) δ. 3.81 (s, 3H), 7.50 (d, 2H), 7–7.21 (m, 17H).

3.5.5. Compound (entry 5, Table 5).
1,4,5-Triphenyl-2-p-tolyl-1H-imidazole. Mp 184–186 °C, IR (KBr, cm⁻¹). 3042, 2944, 1667, 1591, 1092 ¹H NMR (CDCl₃) δ. 2.30 (s, 3H), 7.62 (d, 2H), 7.31–7.02 (m, 17H), 6.98 (d, J = 8.8 Hz, 2H).

4. Conclusion

In the present work, the novel nano solid acid catalyst was developed and exhibited highly catalytic activity for the one-pot synthesis of 2,4,5-trisubstituted imidazoles and 1,2,4,5-tetrasubstituted imidazoles under solvent-free conditions. Inexpensive, heterogeneous and non-toxicity nature, low-cost, easy preparation, high surface area, large pore size are significant properties of this catalyst. Moreover, the catalyst was proved to be a recyclable, green, and highly effective nano solid acid catalyst.

Acknowledgements

The authors are grateful to the Semnan University research council for the partial support of this work. My acknowledgements are also expressed for the comprehensive editing of this article by Dr Jakob Magolan from (Chemistry Department, Idaho University, Moscow, Idaho, United States).

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