Received March 22, 2013; revised May 4, 2013; accepted May 21, 2013

ABSTRACT

Catalysts based on pillared clays were synthesized, by preadsorption of amines between the clay layers prior to pillaring with aluminium precursora, from Moroccan Ghassoulite clay. A comparison of catalytic activity, in the synthesis of benzothiazole and benzimidazole derivatives, of these catalysts is discussed. It is shown that Al-pillared (x CEC) catalyst can be quantitatively recovered and be reused effectively for many times.

Keywords:

Cation Exchange Capacity (x CEC); Benzimidazole; Benzothiazole; Catalyst; Pillared

1. Introduction

Clay minerals are attractive materials for catalysis and adsorption due to low toxicity, high availability and economics. Various methods have been proposed for modifying their properties to obtain materials for specific applications [1]. Treating cationic layered minerals with acid change surface acidity influence textural properties and chemical composition [2]. Intercalation of cationic layered minerals with metal oxide pillars (e.g. Al₂O₃, TiO₂, ZrO₂) is another important method used for modi- fication of these clays [3]. These materials, which are called pillared interlayered clays (PILCs), are character- ized by a relatively high surface area (150 - 350 m ² /g), developed microporous structure, ion-exchange proper- ties and surface acidity. However, in the last two decades, the interest in pillared clays as catalysts has been in- creased due to their textural and catalytic properties in different reactions [4,5]. Aluminium is found to be among the most common cations used to build the pillars of the structure, yielding pillared clay materials with thermal and mechanical stability [6,7].

The Al₁₃-PILC material used in the present study was prepared with a purified Moroccan clay mineral from Moulouya known as a Ghassoulite or Rhassoulite [8].

The benzimidazole and benzothiazole skeletons may be found in numerous pharmaceutical agents with a di- verse spectrum of biological properties [9-12]. Al- though a wide range of methods are available for synthe- sizing benzimidazoles [12-21], and benzothiazoles [22-26], a real need exists for new and simple procedures that support many kinds of structural diversity and vari- ous substitution patterns in the target library. Recently, some microwave-assisted methods were reported for the synthesis of 2-substituted benzothiazoles [27]. Strong oxidant [28] or, more interestingly, catalytic aerobic oxidation involving oxygen as terminal oxidant [29,30] has received considerable attention in the building block construction of benzimidazoles [31-34].

In continuation of our attempts to explore the catalytic activity of some new inorganic supports like Animal Bone Meal (ABM) and Al-pillared clay for useful or- ganic transformations [34-38], we herein report a study of the influence of number and size of the pillars in the interlayer region, which is in turn influenced by the cation exchange capacity (x CEC) of the original clay mineral, in the synthesis of benzimidazole and benzothi- azole derivatives (Scheme 1).

The modified clay support has paved the way for bet- ter dispersion of the Alumina which results in higher catalytic activity.

2. Experimental

2.1. Aluminum Oxide-Pillared Clay Synthesized with Preadsorption of Diethylamine

The starting material is the Na-exchanged ghassoulite. It is added to the vigorously stirred solution of an organic amine compound (diethylamine) dissolved in water. The amounts of amine were 1/4, 1/2 and 3/4 of the cation exchange capacity (CEC) of the clay. The pH of this mixture was adjusted to 8 and the mixture was stirred over night at room temperature until the exchange was complete. Adding the pillaring solution, previously pre- pared, performs a further exchange reaction, drop wise to the vigorously stirred solution of the partially intercalated Na-Gh clay. This mixture is stirred at room temperature over a night. After centrifugation, the solid phase was washed until no chloride was present in the wash water. The solid obtained was then dried at 100˚C and calcined at a heating rate of 2˚C/min to a designated temperature T = 450˚C and kept at this temperature for 4 h. During this calcination process the aluminum precursors are con- verted into rigid aluminum oxide pillars, and the ammo- nium ions are removed. The resulting materials are de- nominated x CEC and the x values are 1/4, 1/2 and 3/4 as shown in Figure 1 and analyzed and interpreted as in previous studies (Figure 1) [8].

2.2. Characterization

The clay’s cation exchange capacity (CEC) was meas- ured using the method generated by Glaeser [39]. Four samples of 0.5 g of the protonated form of the clay sam- ples are placed in contact with four different quantities of NaOH ( 0.1 M ) solutions. The suspensions are agitated and then centrifuged. The excess of OH⁻ in the centri- fuged solutions are titrated by HCl ( 0.1 M ).

By extrapolating the plot of n(OH)titrated = f(n(OH) added) to the intercept with the x-axis, we determined the

Scheme 1. Synthesis of 2-arylbenzimidazoles and benzothi- azoles catalyzed by Al-pillared (x CEC).

CEC value. The experiment revealed that the negative charge was reduced from 79 for the raw mineral to 49 for the Na-Gh, expressed as milliequivalents of the anionic sites per 100 g of the clay. X-ray diffraction (XRD) patterns were recorded at a scanning rate of 1˚C/min (2θ) with a Siemens D-5000 Diffractometer (Cu Kα radiation, λ = 1.5405 A˚).

The X-ray diffraction data was used to determine the basal spacing of the pillared products. Samples for X-ray diffraction (XRD) measurements were recorded after heating for 2 h in vacuum (0.1 Pa) at 100˚C. Nitrogen adsorption isotherms were carried out at a liquid-nitrogen temperature with an ASAP 2000 M Adsorption Analyzer from Micrometrics on sample pre-calcined at 250˚C, the surface area was obtained from the BET treatment of the isotherms in the range of relative pressures 0.05 - 0.25, while the BJH [40] method was used to determine the pore surface areas, the pore volumes and the micropore volumes (Figures 1 and 2, Tables 1 and 2) [8].

2.3. General Procedure for the Synthesis of Bis-Enzimidazoles or Benzothiazoles

The catalyst Al-PILC (x CEC) (100 mg) was added to a mixture of aldehyde 2 (1.0 mmol) and o-phenylenedia- mine 1a or o-aminothiophenol 1b (1.1 mmol). The mix- ture was stirred at room temperature in methanol. Pro-

gress of the reaction was monitored by TLC (n-hexane/ EtOAc 2:1). After complete conversion, the reaction mixture was filtered, the catalyst was washed, and the solvent was removed under reduced pressure. The crude material was purified by column chromatography or recrystallization to afford the pure product 3 or 4.

3. Results and Discussion

Al-pillared clays were prepared by a reaction of Na- ghassoulite (Na-Gh) with base-hydrolyzed solutions of Al³⁺ salts and a subsequent thermal conversion of the intercalated polycations. These pillared clays have rela- tively low micropore volumes [37,38]. By preadsorption of amines between the clay layers prior to pillaring with aluminium precursor, it was possible to increase the mi croporosity of the obtained material. This improvement can be explained by the decrease of the Al-pillars density

^aPores between 1.7 and 300 nm diameter.

between the clay sheets. This explanation was proved by the decrease of the aluminium amounts in the treated samples determined by chemical analyses. The amount of amine was successively 1/4, 1/2 and 3/4 of the cation exchange capacity of the clay mineral.

As a result of preadsorption of amines, the pillar density decreases because a part of the interlayer space is occu- pied by the amine and during the calcination; the pillaring precursors are converted into rigid aluminum oxide pillars and the organic compounds are removed reflecting an apparent improvement in the orientation of the clay layers. The surface area of this product increase to reach 624 m²∙g⁻¹ and the micropore volume becomes three times higher than that prepared without preadsorption of diethylamine.

Recently we have reported that Al-PILC used in methanol at room temperature or microwave can promote the synthesis of benzimidazole with good yields [37,38]. In this work we report the effect of a family of Al-pil- lared Moroccan clay, differing only in cation exchange capacity (CEC), in the synthesis of benzimidazole or benzothiazoles derivatives.

The reaction was first optimized using benzaldehyde 2a (1.1 equiv) and o-phenylenediamine 1a or o-aminothio- phenol 1b in the presence of Al-pillared (x CEC), an in- expensive and readily available catalyst (Table 3). Without catalyst in Methanol, reaction of benzaldehyde 2a with o-phenylenediamine 1a proceeded smoothly (en- try 1). With o-aminothiophenol 1b (entry 6) no conden- sation was observed regardless of the reaction time. Ab- sence of air [33] prevents the required oxidative step ben- zimidazole and benzothiazole were not detected (entries 5, 10).

In air and in the presence of Al-pillared (x CEC), reac- tions between 2a and o-phenylenediamine or o-ami- nothiophenol furnished the desired cycloadducts 3a and 4a in good (entries 2, 3, 7, 8) to excellent yields (entries 4, 9). The synthesis mechanism of benzimidazole and benzothiazole formation was mostly based on an addi- tion-elimination process (Scheme 2).

High yields are obtained in a reasonably short time in the presence of the catalyst Al-pillared (1/4 CEC) than Al-pillared (1/2 CEC, 3/4 CEC) (Figure 3, Table 4).

To explain this phenomenon, we must clarify the in- fluence of preadsorption of amine between the clay lay- ers prior to pillaring with aluminum precursor.

^aYields in pure isolated products. ^bNR: no reaction.

The preadsorption of amine increase the porosity of pillared clay (Table 1). This improvement can be explanation was proved by the decrease of the aluminum amounts in the treated sample (Table 2) from 1/4 CEC to 3/4 CEC.

The reduction of yields from catalyst 1/2 CEC to 1/4 CEC (Tables 3 and 4) can be explained by complete satu-

Scheme 2. Proposition of mechanism synthetic of benzimidazoles and benzimidazoles via Al-pillared (x CEC).

^aYields in pure products isolated by distillation under vacuum identified by ¹H-NMR and IR spectroscopy.

ration of interlayer volume with amine molecules which hinders the exchange with the Al₁₃ oligomers. Since these oligomers will be responsible for the formation of pillars, which produce the permanent and final porosity of materials after calcination. The saturation of the inter- layer volume with amine will have negative effect poros- ity (Vp) (Table 1). The Al-pillared (1/4 CEC) sample pre- pared in this study has narrow enough pores to adsorb the molecules and to produce a good catalysis efficiency.

In the presence of pillared clay (1/4 CEC), various sol- vents were tested. Thus, after 20 min the yields of prod- uct 3a obtained were 95%, 87%, 63% and 11% in the presence of methanol, ethanol, chloroform and dichloro- methane respectively. In the cases of acetone, dimethyl- formamide, tetrahydrofuran, dioxane and hexane, no product was observed under the reaction conditions; only the starting material was isolated.

In the absence of solvent, only a 10% yield of 3a was obtained. This behavior indicates that some solvent is needed to facilitate contact between the reagents and the active site.

The Al-pillared (1/4 CEC) was quantitatively recovered by simple filtration and regenerated by calcination at 200˚C over 2 h and was reused in future reactions. Inves- tigations were performed on benzaldehyde 2a and o- phenylenediamine 1a as model substrates (Table 5).

Whatever the assay, completion of the reaction was always achieved in 25 min but a decrease in yield was observed at the fourth round. The catalyst was fully re- covered after the first reaction, recovery remained stable till the fifth reaction but the amount of promoter gradually decreased after each assay.

4. Conclusions

To summarize, we have reported a study of the influence of number and size of the pillars in the interlayer region, which is in turn influenced by the cation exchange ca- pacity (x CEC) of the original clay mineral, in the synthesis of benzimidazole and benzothiazole derivatives. High yields are obtained in a reasonably short time in the presence of the catalyst Al-pillared (1/4 CEC) than Al-

pillared (1/2 CEC, 3/4 CEC).

The Al-pillared (1/4 CEC) sample in this study was prepared with pores narrow enough to adsorb the mole- cules and to have an enhanced catalysis efficiency com- pared to other Al-pillared samples. The modified clay support has paved the way for better dispersion of the Alumina which results in higher catalytic activity.

References