A waste to wealth approach through utilization of nano-ceramic tile waste as an accessible and inexpensive solid support to produce a heterogeneous solid acid nanocatalyst: to kill three birds with one stone

Abstract

Regarding the subject of waste products/materials, recycling or reusing has had a pivotal role on account of environmental and economic reasons. As a consequence, the essential task, nowadays, can be invention of new and practical pathways to reuse or even recycle them. In this framework, until now, recycling of some waste products such as tile wastes has not been taken into consideration; so, we were prompted by the possibility of recycling tile wastes in the catalytic direction. To this end, we use them as cost-effective, available, and nontoxic support materials for the heterogenization of sulfuric acid, in order to prepare the novel nano-ceramic tile waste supported sulfonic acid catalyst (nano-ceramic tile waste-SO₃H or n-CTW-SA). This solid acid catalyst was well characterized through FT-IR, XRD, FE-SEM, EDX, TEM, TGA, BET, BJH, pH analysis and Hammett acidity function. The as-prepared nano-ceramic tile waste supported sulfonic acid catalyst proved to be an active heterogeneous catalyst in multicomponent reactions (MRCs) for the rapid and efficient one-pot synthesis of 2,4,5-trisubstituted and 1,2,4,5-tetrasubstituted imidazoles in high yields and selectivity. In comparison with some other homogeneous and heterogeneous catalysts, nano-ceramic tile waste supported sulfonic acid displayed a greater activity. Moreover, being highly stable, inexpensive, accessible, retrievable, reusable, and having low toxicity are some other beneficial points of this catalyst.

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1. Introduction

Waste products as resources, have opened an avenue to catalysis and, therefore, to waste and energy minimization. The catalytic opportunities offered by waste materials, in a further step, can contribute to plenty of beneficial factors connected to economy and environment. In this regard, as reported in the literature, waste materials can be put to practical use either by their application directly as catalysts, pre-catalysts, and catalyst precursors or by modifying them to produce active catalysts.¹ As a consequence, the catalytic opportunities provided by waste products can be broadened from research laboratories to the chemical industry. Till now, numbers of examples have been reported in which large scale waste materials/products from industrial process (as by-products) or biological resources are considered and their catalytic applications are taken into consideration such as red mud,² aluminum dross,³ fly ash,⁴ raw blast furnace slag,⁵ chicken egg shell,⁶ shrimp shell,⁷ rice husk and rice husk ash.⁸ However, there is still a great demand that research will be pointed toward more efficient, eco-friendly and economic solutions so as to put other recyclable waste products in the direction of recycling. One example of these waste products is ceramic tile wastes whose chemical compositions, in general, are included large amounts of clay, carbonate, quartz, and minor amount of talc, which make the ceramic tiles rich in the contents of SiO₂ (32–67%) and Al₂O₃ (8–31%). However, a source of TiO₂, Fe₂O₃, and CaO is existed in slight amounts in the body of the ceramic tile clay.^9–12 The Fe₂O₃ + TiO₂ as well as CaO must be maintained at low amount to avoid undesired colour in the ceramic product.¹³ Thanks to their body raw materials, ceramic tiles exhibit peculiar characteristics such as high thermal stability, high porosity, strong reactivity and sorption, low moisture expansion, low cost, low toxicity, large surface areas, and ease of handling.¹¹ Therefore, the above-mentioned advantages have laid the foundations of their applications in civil construction architecture for covering floors or external/internal wall in the building and for decorative or aesthetic purposes.¹⁰

While homogeneous catalysts possess the advantages of easy accessibility of reactants to the catalytic sites in reaction media, high activity, and selectivity, they most of the time render difficulties related to their separation from products and reaction solvents; therefore, vast quantities of homogeneous catalysts will be consumed in each reaction run.^14–16 To address this thorny issue, a step toward enhancement the separation and recycling of homogeneous catalysts has been to apply thermal or chemical recovery, membrane processes, and multiple phase transfers.¹⁵ In this framework, soluble supporting materials for the immobilizing catalysts have also attracted an immense focus of attention because not only they can help the reaction conditions in liquid-phase to maintain, but they also offer an opportunity to solve problems of insoluble heterogeneous material.^17–24 Nevertheless, due to generating huge amounts of waste and, thus, being uneconomic, this approach is impractical in chemical and pharmaceutical industry.²⁵ To tackle this problem, micro-sized-heterogeneous catalysts are introduced to the industry.^26,27 However, in comparison with homogenous systems, they suffer from lower activities caused by steric or diffusion factors.²⁶ Additionally, in such a heterogeneous system, reactants encounter the difficulties in terms of accessing to the catalytic sites since a high proportion of these catalysts are deep within the supporting matrix.²⁶ Nano-scale supporting materials or catalysts have set a milestone effect on the improvement of the activity and selectivity of catalysts through their increased ratio of surface areas to volumes. Consequently, the support materials can be evenly dispersed in solution, generating a homogenous emulsion.^26,28 Moreover, in the system where nanocatalysts are presented the problems regarding the porosity and transport of reactants and/or products to and from the catalytic sites can be approached.

Based on our knowledge, there were no reports on recycling ceramic tile wastes; we were, thus, inspired by the feasibility of recycling waste ceramic tiles through their utilization as support materials for the heterogenization of sulfuric acid catalysts. As mentioned before, quartz and clay are found in ceramic tiles. Clay structures composed of sheets of tetrahedral silicon dioxide (SiO₂) and octahedral aluminium oxide (Al₂O₃) linked through bridging oxygen atoms. Some of the oxygens on the surface of the ‘sheets’ and within the clay structure are in the form of OH groups.²⁹ Besides, clay minerals exhibit interesting catalytic properties that have already been examined extensively.³⁰ Therefore, we thought that ceramic tile waste could offer golden opportunities to use them as supports for immobilizing of homogeneous catalysts (such as sulfuric acid), thereby drawing organic transformations into the realm of green and sustainable chemistry. Following up our research group's attempts in the context of investigation and application of solid acids in chemical transformations,^31–36 herein, we have designed a novel, cost-effective, low toxic, recyclable, and reusable nano-ceramic tile wastes supported sulfonic acid, by the aid of chlorosulfonic acid as sulfonating agent. It is worthy to mention that supporting sulfonic acid through sulfonating with chlorosulfonic acid, provided by the procedure first reported by Zolfigol,³⁷ have garnered much attention for the sake of being a convenient, fast and efficient method for heterogenization of sulfuric acid.^35,38–44 Subsequently, we studied the catalyst's performance in the one-pot synthesis of 2,4,5-trisubstituted and 1,2,4,5-tetrasubstituted imidazoles subsequent to the deeply investigation of the reactivity, reusability, and stability of the as-obtained catalyst.

Our main targets are as follow:

(i) Recycling ceramic tile wastes in an effort to minimize cost and land pollution as well.

(ii) Utilizing them as porous, low cost, readily available, low-toxic, and thermally stable support materials to produce the active, novel nano-ceramic tile wastes supported sulfonic acid catalyst.

(iii) Applying the designed catalyst in the multicomponent (MRCs) reaction conditions of 2,4,5-trisubstituted and 1,2,4,5-tetrasubstituted imidazoles synthesis because of their applications in biological, therapeutic and agricultural sectors.^45–53

2. Experimental section

2.1. Instrumentation, analysis, and starting materials

All chemicals were purchased from Merck and Aldrich companies and used without any further purification. Ceramic tiles waste was prepared from Semnan Tile (Iran). According to manufacturing formula of the Semnan Tile factory, ceramic tile is produced from clay, sand, feldspar, quartz, and water. Ceramic tile powder was prepared by two-cup planetary ball mill of Iran. 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 checked by thin layer chromatography (TLC) on glass plates coated with silica gel 60 F254 using n-hexane/ethyl acetate mixture as mobile phase. Melting points were determined in open capillaries using an Electrothermal 9100 without further corrections. Fourier transform infrared spectroscopy (FTIR) was recorded on a Shimadzu 8400s spectrometer using KBr pressed powder discs. The NMR spectra were measured with a Bruker Avance 300 spectrometer (¹H NMR 300 MHz and ¹³C NMR 75 MHz) in pure deuterated chloroform with tetramethylsilane (TMS) as the internal standard. Field emission scanning electron microscope (FE-SEM) images were acquired using Zeiss field emission scanning electron microscope (Sigma, Germany) instrument operating at 15 kV, equipped with an Oxford X-ray detector (EDX; Oxford Instruments, Oxford, UK). TEM images were also obtained through transmission electron microscope (TEM; Philips–CM300–150 kV). The average particle size distribution was determined by using Image software. Thermogravimetric analyses (TGA) were carried out on a Du Pont 2000 thermal analysis apparatus at a heating rate of 5 °C min⁻¹ under air atmosphere. BET surface area was measured using a BELSORP-mini apparatus (BEL Japan, Inc.). X-ray diffraction (XRD) was detected by Philips using Cu-Kα radiation of wavelength 1.54 Å. Presented UV-vis spectra were obtained as carbon tetrachloride solutions (10⁻⁴ M) on a Shimadzu UV-1650PC spectrophotometer.

2.2. Preparation of neat ceramic tile powder

Initially, ceramic tile wastes were crushed with ball mill to get a soft and tiny powder. Then, 20 g of the as-obtained powder was carefully washed with 200 mL methanol and ethanol, three times, to take all organic compounds away from the powder. After washing with excess amount of distilled water, the precipitated ceramic tile wastes powder was sent through a filter and, then was dried at 120 °C.

2.3. Preparation of n-CTW-SA

To a suction flask equipped with a constant-pressure dropping funnel, containing chlorosulfonic acid (0.3856 g, 0.0033 mol), and a gas inlet tube for conducting HCl gas into water as an adsorbing solution, 1.0 g of ceramic tile waste powder in dry CH₂Cl₂ (20 mL) was added. Then, neat chlorosulfonic acid added dropwise over a period of 15 min at room temperature. During stirring the mixture, HCl gas immediately evolved from the reaction vessel. After addition, the mixture continued stirring for 1 h at room temperature while the residual HCl was seized by suction.³⁷ Subsequently, the solid powder was washed with water (10 mL) and dried at 80 °C. Finally, the as-obtained solid acid catalyst n-ceramic tile waste supported sulfonic acid in 1.44 g is ready to use in organic transformation.

2.4. Application of n-CTW-SA catalyst for multicomponent one-pot synthesis of heterocyclic compounds

2.4.1. General procedure for the synthesis of 2,4,5-trisubstituted imidazoles. To a test tube equipped with a stir bar, a mixture of aromatic aldehyde (1.0 mmol), benzil (1.0 mmol), ammonium acetate (2 mmol) and n-CTW-SA (0.006 g, 20 mol%) as catalyst was added. The mixture was reacted under solvent free condition at 100 °C (in an oil bath) for the appropriate time until the reaction was completed. The progress of the reaction was followed by thin layer chromatography (TLC) [7 : 3 hexane : ethyl acetate]. After completion of the reaction (checked by TLC), hot ethanol (5 mL) was added and centrifuged. Then the resulted solution was filtered in order to separation of the catalyst from the mixture. The filtrate was 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.

2.4.2. General procedure for the synthesis of 1,2,4,5-tetrasubstituted. A mixture of benzil (1 mmol), ammonium acetate (1 mmol), aldehyde (1 mmol), primary aromatic amine (1 mmol) in the presence of n-CTW-SA (0.003 g, 10 mol%) as catalyst was stirred with a glass bar at 120 °C under solvent free condition for the appropriate time. After the reaction was completed, monitored by TLC, the reaction mixture was dissolved in ethanol (5 mL) and the catalyst was separated by filtration after being centrifuged. The filtrate was concentrated on a rotary evaporator under reduced pressure and the solid crude product obtained was washed with water and recrystallized from ethanol to afford pure imidazole derivatives. The remaining reactions were performed following this general procedure and the physical data (mp, IR, and NMR) of all known compounds were identical with those reported in the literature.

3. Results and discussion

In the present study, our prime target is to demonstrate the synthesis of n-ceramic tile waste-sulfonic acid and elucidate its performance as solid acid catalyst. Aiming at recycling ceramic tile wastes, we use these wealthy waste materials as resources for catalytic applications in the form of surface decorated with sulfonic groups. Through this study, we found that ceramic tile wastes can provide a cheap, available, recoverable, and reusable platform to heterogenize sulfuric acid. The preparation of this catalyst is showed in the schematic (Scheme 1). In the procedure, ceramic tile waste powder nicely reacted with chlorosulfonic acid during which HCl gas evolved from the reaction vessel very soon. Therefore, any further work-up is needed which can serve as an easy and clean procedure. Other beneficial traits of the present work, that can be mentioned, are simple catalyst preparation by using inexpensive and easily accessible waste materials that helped creating the heterogeneous, stable solid acid catalyst. The “inorganic solid acid catalysts” were fully characterized by X-ray diffraction (XRD), thermogravimetric analysis (TGA), Fourier transform infrared spectroscopy (FT-IR), energy dispersive X-ray spectroscopy (EDX), scanning electron microscopy (SEM), transmission electron microscopy (TEM), BET, BJH and Hammett acidity function method. Additionally, in order to find the relationship between the effect of different size of catalysts on the reaction efficiency, a set of two samples of CTW with two mesh sizes of 250 and 500 μm were functionalized with SO₃H function and tested in the reaction condition. The loading of SO₃H per g was calculated by pH analysis, and it is found to be 2.88, 1, 0.5 mmol g⁻¹ of n-CTW-SA, CTW-SA₂ and CTW-SA₁, respectively. Related data was summarized in the Table 1. Subsequent to the initial characterization of the catalyst, its catalytic activity was examined in the multicomponent reaction for the one-pot synthesis of some heterocyclic compounds such as of 2,4,5-trisubstituted and 1,2,4,5-tetrasubstituted imidazoles as showed in the brief route in Schemes 2 and 3.

Scheme 1 Preparation of nano-ceramic tile waste supported sulfonic acid.

Table 1 Acid capacity measurement of catalysts with different particle sizes

Entry	Sample	Sieve mesh size	Equivalent particle diameter (μm)	Catalyst	Acid capacity^a (mmol H^+ per g)
a Determined by pH analysis.b CTW (500 μm mesh size) functionalized with SO3H groups.c CTW (250 μm mesh size) functionalized with SO3H groups.
1	n-CTW	<35	0.02–0.03	n-CTW-SA	2.88
2	CTW1	35	500	CTW-SA1^b	0.5
3	CTW2	60	250	CTW-SA2^c	1

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Scheme 2 Synthesis of 2,4,5-trisubstituted imidazoles in the optimum condition.

Scheme 3 Synthesis of 1,2,4,5-tetrasubstituted imidazoles in the optimum condition.

3.1. Characterization of n-CTW-SA

3.1.1. pH analysis of catalyst. The acid capacities of n-CTW-SA were determined by acid–base potentiometric titration in which the aqueous suspension of the weighed amount of the catalyst was titrated with standard NaOH solution. Before titration, 100 mg n-CTW-SA was dispersed in 20 mL H₂O by ultrasonic bath for 60 min. By addition of standard NaOH solution (0.08 N), the amount of the acid was neutralized, till the equivalence point of titration that required 3.5 mL of NaOH. The optimum concentration of H⁺ sites was ascertained to be 2.8 mmol gram⁻¹ of catalyst (values calculated by the weight of n-CTW-SA) at 25 °C. Through consecutive experiments to measure the concentrations of the residual H⁺ on the recovered catalyst, it was found that the loss of H⁺ was very small or marginal, which was a sign that the SO₃H groups were tightly anchored to ceramic tile waste base, probably through a covalent linkage.

3.1.2. Surface acidity of the catalyst. The acid strength of n-CTW-SA was determined through the Hammett indicator function (H₀).⁵⁴ It can be calculated using the following equation:

H₀ = pK(I)aq + log([I]s/[IH⁺])

In the equation, ‘I’ is the indicator base (mainly substituted nitroanilines) and [I]s and [IH⁺]s are the molar concentrations of the un-protonated and protonated forms of the indicator, respectively. The pK(I)aq values, which are known (for instance the pK(I)aq value of 4-nitroaniline is 0.99), can be obtained from many references. According to the Lambert–Beer law, the value of [I]s/[IH⁺]s can be determined and calculated using the UV-visible spectrum. In our experiment, 4-nitroaniline and CCl₄ were selected as the basic indicator and the aprotic solvent, respectively. The maximal absorbance of the un-protonated form of 4-nitroaniline was observed at 330 nm in CCl₄. When n-CTW-SA catalyst was added to the indicator solution, the absorbance of the un-protonated form of the indicator weakened, indicating that the indicator was partially in the form of [IH⁺] (Fig. 1). The obtained results are listed in Table 2, showing the acidity strength of n-CTW-SA (Table 2). According to the results of the Hammett acidity function (H₀), it is confirmed that the new catalyst was synthesized with a good density of acid groups (–SO₃H groups) on its surface (Table 2).

Fig. 1 Absorption spectra of (a) 4-nitroaniline (indicator), (b) nano-ceramic tile waste-SO₃H (catalyst) in CCl₄.

Table 2 Hammett acidity function (H₀) data for n-CTW-SA^a

Entry	Catalyst	Amax	[I]s (%)	[IH^+]s (%)	H0
a Condition for UV-visible spectrum measurement: solvent, CCl4; indicator, 4-nitroaniline (pK(I)aq = 0.99), 1.44 × 10^−4 mol L^−1; catalyst, n-CTW-SA (20 mg), 25 °C.
1	—	2.74	100	0	—
2	n-CTW-SA	1.49	54.27	45.73	1.32

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3.1.3. FT-IR spectral analysis of the catalyst. The infrared spectra of n-CTW and n-CTW-SA are presented in Fig. 2. In the case of n-CTW, the peaks at ca. 3419 and 1632 cm⁻¹ correspond to the –OH stretching vibration of the adsorbed water and the bands at 1074 and 796, and 777 cm⁻¹ can be collectively attributed to Si–O stretching vibrations.⁵⁵ The peaks appeared at 464 and 532 cm⁻¹ are assigned to the Si–O–Si and Si–O–Al stretching vibration.⁵⁶ The spectrum of sulfonated n-CTW displays almost the same pattern as that of pristine n-CTW, but the n-CTW-SA spectrum shows a relatively broad band around 2700 to 3600 cm⁻¹ that is due to OH stretching absorption of the SO₃H group.⁵⁷ The broad band around 1100 cm⁻¹ is assigned to stretching modes of Si–O and S–O bands which are overlapped together.

Fig. 2 The FT-IR spectra of n-CTW and n-CTW-SA.

3.1.4. Thermogravimetric analysis. Thermogravimetric analysis (TGA) of n-CTW-SA in comparison with n-CTW is displayed in Fig. 3. The TGA curve of n-CTW (Fig. 3a) showed a weight loss (5 wt%) below 100 °C which corresponds to the loss of the physically adsorbed water. For the n-CTW-SA, the thermogravimetric curve seems to indicate three-stage decomposition. The first weight loss found below 130 °C (12 wt%) to be due to the loss of physically adsorbed solvent or trapped water from the catalyst. The mass loss occurred between 120 °C and 310 °C (about 15 wt%), is related to the slow mass loss of SO₃H groups and the last loss of weight (about 20 wt%) started from 310 °C to 740 °C, was attributed to the sudden mass loss of covalently bounded SO₃H groups.^57,58 Additionally, it is understood from the TGA measurement that n-CTW-SA has a great thermal stability (until 220 °C) and can be safely applied in organic reactions at temperatures up to 220 °C.

Fig. 3 The TGA curve of n-CTW and n-CTW-SA.

3.1.5. Field emission scanning electron microscopy. The samples of n-CTW, n-CTW-SA, CTW-SA₁ and CTW-SA₂ were analyzed by field emission scanning electron microscopy (FE-SEM), as represented in Fig. 4a–e. From picture a, it was found that the material is composed of macro porous structure. Besides, it is clear that there are some particles on the surface of the material. The particles sizes are about 30 nm to 500 nm. However, with modifying the material with SO₃H function, as shown in Fig. 4b and c, the morphology was changed to the multilayer structures in nano to micrometer size range. The thickness size of the layers is about 30 nm to 250 nm. Besides, there are some spherical particles on the surface of the material. Their diameter sizes are about 20–50 nm. As can be seen from Fig. 4d and e, the CTW-SA₁ and CTW-SA₂ samples have the multilayer structures morphology. The material used to produce CTW-SA₁ and CTW-SA₂ catalysts had mesh sizes of 500 and 250 μm. However, functionalization with chlorosulfonic acid caused the sizes of CTW-SA₁ and CTW-SA₂ catalysts to be smaller than the bear ones.

Fig. 4 The FE-SEM images of n-CTW (a), n-CTW-SA (b and c), CTW-SA₂ and CTW-SA₁ (d and e).

3.1.6. Transmission electron microscopy (TEM). Fig. 5a–d shows the TEM images as well as the particle size distribution profile. As could be seen from the figures, the materials are consisted of nearly homogeneous narrow size distributed spherical particles. It also indicates that the maximum particle size distributions were in the range of 20–30 nm.

Fig. 5 The TEM images of n-CTW-SA (a–c), particle size distribution profile (d).

3.1.7. Energy-dispersive X-ray spectroscopy (EDX). The elemental composition of the n-CTW from EDX analysis was exhibited in Fig. 6a that represents the presence of O, Si, Al, Ca, Fe, and Ti elements. The EDX spectrum of n-CTW-SA was also showed in Fig. 6b which showed the appearance of the sulfur peak and clearly confirms the presence of –SO₃H groups from chlorosulfonic acid (Fig. 6b). However, the acid treatment could cause the elimination of mineral impurities and exchangeable metal cations from the clay material in the tile body.⁵⁶ Therefore, Ca, Ti and Fe are partially washed out from the n-CTW structure through modification with chlorosulfonic acid as can be seen from the EDX spectrum of n-CTW-SA.

Fig. 6 The EDX spectra of n-CTW (a), and n-CTW-SA (b).

3.1.8. XRD (X-ray diffraction) spectral analysis. Fig. 7a and b show the XRD patterns of the n-ceramic tile waste and n-CTW-SA, respectively. The phases identification of the XRD spectral was carried out according to previous reports.^59,60 It is clear that with modifying the tile material, the crystalline count for the material was reduced. It may be due to the addition of a non-crystalline phase (SO₃H) to the tile material. Besides, there is another confirmation for the presence of the non-crystalline phase in the material. It is the broadening of the XRD pattern showed in Fig. 7b in the 2θ range of about 20–30° compared to the XRD pattern showed in Fig. 6a.

Fig. 7 The XRD patterns of n-CTW (a), and n-CTW-SA (b).

3.1.9. BET and BJH texture analysis of the catalyst. The n-CTW-SA, CTW-SA₁ and CTW-SA₂ catalysts and the n-CTW were characterized for their surface area, pore volume and pore size by using the Brunauer–Emmett–Teller (BET) and Barrett–Joyner–Halenda (BJH) equations. BET surface area was evaluated using a BELSORP-mini apparatus (BEL Japan, Inc.). Prior to N₂-physical adsorption measurement, the samples were degassed at 150 °C for 120 min in the nitrogen atmosphere. The specific surface area (S_BET) of the as-prepared catalyst was determined with adsorption–desorption isotherms of N₂ at 77 K. The obtained results related to the textural properties of the sample (in terms of BET surface area, total pore volumes, BJH pore size) are listed in Table 3. The investigated results in the table suggested that the n-CTW-SA has larger surface area and total pore volume than that of n-CTW. However, the BJH pore size diameter of n-CTW-SA is smaller than that of n-CTW. Moreover, the BET surface area of CTW-SA₂ is larger than that of CTW-SA₁.

Table 3 BET and BJH data showing the textural properties of n-CTW, n-CTW-SA, and CTW-SA

Entry	Samples	SBET^a [m^2 g^−1]	Vp^b [cm^3 g^−1]	Pore size^c [nm]
a BET surface area.b Total pore volume.c BJH pore size diameter.
1	n-CTW	1.16	0.009	6.96
2	n-CTW-SA	3.93	0.015	1.21
3	CTW-SA2	0.94	0.006	18.94
4	CTW-SA1	0.03	0.005	25.55

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3.2. Application of n-ceramic tile waste-supported sulfonic acid as heterogeneous solid acid catalyst in the synthesis of multisubstituted imidazoles

When the initial characterization of the n-ceramic tile waste supported sulfonic acid catalyst was performed, we were inspired by the investigation of its catalytic performance, in multicomponent reactions, not only for the preparation of 2,4,5-trisubstituted imidazoles from aromatic aldehydes, benzil and NH₄OAc but also for the production of 1,2,4,5-tetrasubstituted imidazoles from aromatic aldehydes and amines, benzil, and NH₄OAc in solvent free conditions (Schemes 3 and 5).

3.2.1. Optimization of the reaction parameters for one-pot synthesis of 2,4,5-trisubstituted imidazoles. At the onset of this study, we examined the above-mentioned reaction of benzaldehyde, benzil, NH₄OAc for the synthesis of 2,4,5-trisubstituted imidazole at 100 °C in the absence of the catalyst under solvent-free condition, that low yielding reaction in the long reaction time was observed (Table 4, entry 1). In the next step, with the view to optimizing the reaction conditions, we investigated the reaction in the presence of different amounts of n-CTW-SA as catalysts (ranging from 5–20 mol%) under solvent-free condition at 100 °C. It was observed that the optimal amount of catalyst was 20 mol% with the product yielding of 99% in 10 min (Table 4, entries 2–5). Additionally, these results were compared with the reactions performed in the presence of unfunctionalized ceramic tile waste powder (without the SO₃H groups appended), confirming the fact that the n-CTW-SA catalyst is a more suitable option. The greater catalytic activity of n-CTW-SA was most likely attributed to the catalyst surface decoration with SO₃H groups, which could offer efficient acidic sites. When optimized the catalyst loading, we decided to probe the model reaction by adding 20 mol% of the n-CTW-SA as catalyst under solvent free condition in the different temperatures (70, 80, 90, and 100 °C), because temperature is a key factor for the synthesis of imidazole compounds (Table 4, entries 5–9). According to the obtained data, within 10 min, the most efficient reaction temperature was 100 °C at which 99% product yield was provided (Table 4, entry 5). The further study was the effect of solvent on the model reaction. In order that, different classical solvents such as EtOH, MeOH, CH₃CN, CH₂Cl₂, and H₂O were tested in the reaction condition. As obvious in the Table 4 (entries 10–14), using these solvents gave products in lower yields and longer reaction times. These results proved that solvent-free reaction condition is the best condition in which no volatile solvents are used, therefore this condition kept pace with the green chemistry protocols. As a result, the reaction parameters which we optimized were: 100 °C, 20 mol% of the n-CTW-SA catalyst, and solvent-free conditions (Table 4, entry 5). In an attempt to investigate how catalyst particle size can affect the reaction outcome, CTW-SA₁ and CTW-SA₂ were put in the optimized reaction condition (i.e. 100 °C, 20 mol% of the catalyst, and solvent-free conditions). Comparison of the results showed that the reaction operated in the presence of n-CTW-SA rendered higher yields of product in 10 minutes (Table 5). It was suggested that the higher BET surface area and acid capacity, smaller particle sizes and total pore sizes were the effective parameters which caused n-CTW-SA to give the satisfactory results. Moreover, with the presence of higher weighed amount of CTW-SA₁ and CTW-SA₂, the pathways for attacking substrates to each other are presumably obstructed, thereby lowering the reaction efficiencies. Besides, the larger particle sizes of these catalysts may hinder reactants to react with each other in the reaction vessel (Table 5, entries 2 and 3). In the final step, the scope of the reaction was further investigated under the established optimal reaction conditions. As shown in Table 6, a wide range of aromatic aldehydes including electron-rich (deactivated) and electron-deficient (activated) were subjected to the cyclization with benzil and ammonium acetate which prepared the corresponding 2,4,5-trisubstituted imidazoles in good yields (Table 6). It should be noticed that 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 (Table 6).

Table 4 Optimization of the reaction condition for the synthesis of 2,4,5-trisubstituted imidazoles^a

Entry	Solvent	Condition	Catalyst [mmol]	Time [h [min]]	Yield [%]
a Reaction condition: benzil (1 mmol), benzaldehyde (1 mmol), and NH4OAc (2 mmol).
1	Solvent-free	100 °C	—	2	70
2	Solvent-free	100 °C	0.05	[10]	92
3	Solvent-free	100 °C	0.1	[10]	94
4	Solvent-free	100 °C	0.15	[10]	96
5	Solvent-free	100 °C	0.2	[10]	99
6	Solvent-free	90 °C	0.2	[10]	90
7	Solvent-free	80 °C	0.2	[10]	83
8	Solvent-free	70 °C	0.2	[10]	76
9	Solvent-free	60 °C	0.2	1 [20]	72
10	H2O	Reflux	0.2	[10]	70
11	CH3CN	Reflux	0.2	[10]	75
12	CH2Cl2	Reflux	0.2	[10]	67
13	EtOH	Reflux	0.2	[10]	85
14	MeOH	Reflux	0.2	[10]	80

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Table 5 Catalysts particle size effect on the synthesis of 2,4,5-trisubstituted imidazoles reaction condition^a

Entry	Catalyst	Catalyst amount^b [g]	Yield [%]
a Reaction condition: benzil (1 mmol), benzaldehyde (1 mmol), and NH4OAc (2 mmol), solvent-free, 100 °C.b Catalyst amount is equal to 0.2 mmol H^+.
1	n-CTW-SA	0.06	99
2	CTW-SA1	0.4	73
3	CTW-SA2	0.2	80

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Table 6 n-CTW-SA-catalyzed one-pot synthesis of 2,4,5-trisubstituted imidazoles^a

Entry	Aldehyde	T [min]	Yield^b [%]	Mp [°C]	Lit. mp [°C] (ref.)
a Reaction condition: benzil (1 mmol), aromatic aldehyde (1 mmol), NH4OAc (2 mmol) and n-CTW-SA (0.2 mmol), solvent-free, 100 °C.b Yields refer to isolated products.
1	Benzaldehyde	10	99	271–273	272–273 (62)
2	4-Chlorobenzaldehyde	20	95	261–262	262–263 (62)
3	2-Chlorobenzaldehyde	15	88	200–201	199–201 (63)
4	2,4-Dichlorobenzaldehyde	10	95	175–177	174–175 (64)
5	4-Fluorobenzaldehyde	20	96	261–262	260–262 (65)
6	3-Bromobenzaldehyde	45	90	306–309	>300 (66)
7	3-Nitrobenzaldehyde	45	83	265–266	265–267 (62)
8	4-Nitrobenzaldehyde	35	87	237–239	239–242 (67)
9	2-Nitrobenzaldehyde	45	80	230–232	230–233 (68)
10	2-Methoxybenzaldehyde	25	89	209–211	210 (69)
11	2-Hydroxybenzaldehyde	35	90	210–212	209–211 (70)
12	3-Hydroxybenzaldehyde	20	92	258	258 (68)
13	4-Hydroxybenzaldehyde	15	94	256–257	256–257 (62)
14	4-Methylbenzaldehyde	20	94	227–228	226–227 (62)
15	3-Ethoxy-4-hydroxybenzaldehyde	50	94	265–267	268–269 (66)
16	3,4-Dimethoxybenzaldehyde	20	93	221–223	222–224 (71)
17	4-Isopropylbenzaldehyde	35	94	252–255	252–255 (72)
18	4-N,N-Dimethylbenzaldehyde	40	82	256–257	255–256 (73)

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3.2.2. Optimization of the reaction parameters for one-pot synthesis of 1,2,4,5-tetrasubstituted imidazoles. For the optimization of the reaction condition for the one-pot synthesis of 1,2,4,5-tetrasubstituted compound, initially, a mixture of benzil, ammonium acetate, benzaldehyde, and aniline was reacted without adding the catalyst at 120 °C under solvent free condition. However, the desired product, obtained in this reaction, was in low yields (Table 7, entry 1). Disappointed with the obtained results about the model reaction performed in the presence of bare ceramic tile waste support, we examined the above-mentioned reaction in the presence of different amounts of the n-CTW-SA as catalysts (ranging from 5–20 mol%) under solvent-free condition at 120 °C and 10 mol% was proved to be superior in reaction efficiency and time (Table 7, entries 2–5). In further step, reaction temperature was optimized in view of the fact that temperature can affect the synthesis of highly substituted imidazoles. In order that, the model reaction was conducted under solvent-free conditions at 90 °C, 100 °C, 110 °C, and 120 °C (Table 7, entries 3, 6–9). According to the Table 7, by increasing the reaction temperature up to 120 °C, the efficiency of the reaction was improved; therefore, the most effective temperature was 120 °C (Table 7, entry 3). Eventually, the model reaction was conducted under solvent conditions, in order to investigate the effect of different classical solvents such as EtOH, MeOH, CH₃CN, CH₂Cl₂ and H₂O (Table 7, entries 10–14). The results showed that the reaction performed in the presence of these solvents rendered desired products in good yields in 60 min, but solvent-free reaction condition offered the product in significantly higher yields and lower reaction times. As a consequent, solvent-free reaction condition was chosen as the key factor to design a sustainable protocol. Further substrate scope investigations, to prepare 1,2,4,5-tetrasubstituted imidazoles, were all carried out under solvent-free conditions using 10 mol% n-CTW-SA as catalysts, at 120 °C (Table 7, entry 3). According to the obtained data about the reactions in which the several substituted aromatic aldehydes and amines were participated under optimal reaction conditions, aldehydes with either electron-withdrawing or electron-donating substituents perform equally well in the reactions (Table 8). The nature of the substituents on the aromatic aldehyde has not significant effect on the reaction efficiencies.

Table 7 Optimization of the reaction condition for the synthesis of 1,2,4,5-tetrasubstituted imidazoles^a

Entry	Solvent	Condition	Catalyst [mmol]	Time [h [min]]	Yield [%]
a Reaction condition: benzil (1 mmol), benzaldehyde (1 mmol), and NH4OAc (1 mmol), aniline (1 mmol).
1	Solvent-free	120 °C	—	4	Trace
2	Solvent-free	120 °C	0.05	1 [30]	85
3	Solvent-free	120 °C	0.1	[40]	94
4	Solvent-free	120 °C	0.15	1 [20]	90
5	Solvent-free	120 °C	0.2	1 [40]	87
6	Solvent-free	110 °C	0.1	1 [20]	90
7	Solvent-free	100 °C	0.1	2	85
8	Solvent-free	90 °C	0.1	2 [35]	80
9	Solvent-free	60 °C	0.1	3 [15]	75
10	H2O	Reflux	0.1	[60]	60
11	CH3CN	Reflux	0.1	[60]	67
12	CH2Cl2	Reflux	0.1	[60]	50
13	EtOH	Reflux	0.1	[60]	79
14	MeOH	Reflux	0.1	[60]	70

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Table 8 n-CTW-SA-catalyzed one-pot synthesis of 1,2,4,5-tetrasubstituted imidazoles^a

Entry	Aldehyde	Amine	T [h [min]]	Yield^b [%]	Mp [°C]	Lit. mp [°C] (ref.)
a Reaction condition: benzil (1 mmol), aromatic aldehyde (1 mmol), NH4OAc (1 mmol), aromatic amine (1 mmol) and n-CTW-SA (0.1 mmol), solvent-free, 120 °C.b Yields refer to isolated products.
1	Benzaldehyde	Aniline	[40]	94	218–219	218 (74)
2	4-Chlorobenzaldehyde	Aniline	1	91	161–163	160–163 (75)
3	4-Nitrobenzaldehyde	Aniline	1 [20]	87	185–187	184–186 (76)
4	4-Methoxybenzaldehyde	Aniline	1 [35]	93	182–184	180–182 (76)
5	4-Hydroxybenzaldehyde	Aniline	[45]	82	281–282	280–281 (77)
6	4-Methylbenzaldehyde	Aniline	1 [10]	97	191–193	189 (74)
7	4-Methylbenzaldehyde	4-Chloroaniline	2	87	165–167	166–168 (76)
8	4-Chlorobenzaldehyde	4-Chloroaniline	2 [10]	80	188–190	187–189 (78)
9	4-Nitrobenzaldehyde	4-Methylaniline	1	90	220–222	219–220 (79)
10	3-Nitrobenzaldehyde	4-Methylaniline	1 [15]	85	148–150	149–151 (79)
11	4-Methylbenzaldehyde	4-Methylaniline	1 [35]	95	187–190	187–190 (76)
12	4-Hydroxybenzaldehyde	4-Methylaniline	1 [45]	92	277–290	>275 (78)

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As compared n-CTW-SA performance with other catalysts reported in the literature for the one-pot synthesis of 2,4,5-trisubstituted imidazoles (Table 9) and 1,2,4,5-tetrasubstituted imidazoles (Table 10), it was revealed that n-CTW-SA is superior to most of the others regarding reaction conditions, time and product yield. Therefore, it can be seen that n-CTW-SA is a very useful and efficient catalyst in the synthesis of multisubstituted imidazoles.

Table 9 Comparison of the efficiency of n-CTW-SA with different catalysts in the synthesis of 2,4,5-trisubstituted imidazoles

Entry	Catalyst	Condition	Time [h [min]]	Yield [%]	Ref.
a Nano silica phosphoric acid.b Sulfated zirconia.
1	Nano-SPA^a	Solvent-free/140 °C	3	90	69
2	Nano-crystalline SZ^b	Reflux in EtOH	[45]	87	80
3	KH2PO4	Reflux in EtOH	[40]	93	81
4	Yb(OPf)3	C10F18/80 °C	6	80	82
5	NiCl2·6H2O/Al2O3	Reflux in EtOH	1 [30]	89	83
6	Zeolite	Reflux in EtOH	1	80	80
7	Montmorillonite K10	Reflux in EtOH	1 [30]	75	80
8	InCl3·3H2O	MeOH/r.t	12	73	84
9	Zr(acac)4	Reflux in EtOH	2 [30]	90	64
10	n-CTW-SA	Solvent-free/100 °C	[10]	99	Present work

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Table 10 Comparison of the efficiency of n-CTW-SA with different catalysts in the synthesis of 1,2,4,5-tetrasubstituted imidazoles

Entry	Catalyst	Condition	Time [h [min]]	Yield [%]	Ref.
a Wells–Dawson heteropolyacid supported on silica.
1	Bf3/SiO2	Solvent-free/140 °C	2	92	85
2	NaHSO4/SiO2	Solvent-free/140 °C	2	92	86
3	SbCl5/SiO2	Solvent-free/140 °C	2	90	77
4	MgCl2	Solvent-free/80 °C	[45]	60	85
5	AlCl3	Solvent-free/140 °C	2	53	87
6	WD^a/SiO2	Solvent-free/140 °C	2	85	76
7	H3PMo12O40	Reflux in EtOH	[15]	88	87
8	n-CTW-SA	Solvent-free/120 °C	[40]	94	Present work

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3.2.3. Plausible mechanism for the synthesis of 2,4,5-trisubstituted and 1,2,4,5-tetrasubstituted imidazoles catalyzed by n-CTW-SA. The probable mechanism for the formation of trisubstituted imidazole using n-CTW-SA is represented in Scheme 4. The reaction can be started through increasing the electrophilicity of the carbonyl group of the aldehyde by the help of n-CTW-SA and then condensing to form diamine intermediate 1 afterwards, intermediate 1 condenses with benzil to form intermediate 2, which in turn rearranges to the trisubstituted imidazole. Similarly, the possible mechanism for the synthesis of tetrasubstituted imidazoles initiates with 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 5).

Scheme 4 Proposed mechanism for the synthesis of 2,4,5-trisubstituted imidazole.

Scheme 5 Proposed mechanism for the synthesis of 1,2,4,5-tetrasubstituted imidazole.

3.3. Reusability of the catalyst

From the view point of green chemistry, catalysts with good recovery and reusability are highly preferable. Therefore, in this work, we studied the level of reusability of the n-CTW-SA in the model reaction for the synthesis of multisubstituted imidazoles under the optimum conditions.

After each run, the catalyst is simply recovered from the reaction mixture by filtration. The recovered catalyst was first washed with ethanol and water, and then dried at 110 °C. The recovered catalyst was added to reaction mixture of fresh reactants under same conditions for seven runs with no remarkable drop in yield and its catalytic activity (Fig. 8). For determination of the percent leaching of the acid,⁶¹ the model reaction, for the synthesis of tetra substituted imidazole, was performed in the presence of n-CTW-SA for 20 min and at that point the catalyst was removed by filtration. The residual solution was then allowed to react, but no significant progress was observed after 2 h. Therefore, this experiment signified that SO₃H moiety was tightly anchored to the support, which is a further testimony to the heterogeneous nature of the catalytic system.

Fig. 8 Recyclability of n-CTW-SA.

4. Conclusion

n-CTW-SA was synthesized, not only to develop the current important areas of heterogenization of sulfonic acid, but also to utilize waste products. The n-CTW-SA heterogeneous solid acid catalyst displayed great activities in the multi component reactions for the simple, efficient, and rapid one-pot synthesis of heterocyclic compounds such as 2,4,5-trisubstituted as well as 1,2,4,5-tetrasubstituted imidazoles. The proposed reaction conditions which are used the recyclable catalyst, can be considered as a green process due to be kept pace with the green chemistry principles such as high yields, low-cost, short reaction times, low-toxicity, solvent-free conditions and so on. Additionally, being heterogeneous, non-toxic, low-cost, easily accessible, simple preparation, recoverable and reusable are significant beneficial properties of this catalyst.

Acknowledgements

We gratefully acknowledge the Faculty of Chemistry of Semnan University for supporting this work.

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Footnote

† Electronic supplementary information (ESI) available. See DOI: 10.1039/c6ra11923a

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