Ren Bingnan, 1 Yang Gengliang,1,2 Wang Lijuan,1 Ma Zhengyue, 1
(1 College of Pharmacy, Hebei University, Baoding 071002, China; 2 Key Laboratory of Pharmaceutical Quality Control, Hebei Province. Baoding 071002, China)
Abstract Monolithic silica was used as an effective heteropoly acid H3PW12O40 (PW) support for the synthesis of bis (indoly) methanes. The present method offers short reaction times, mild condition and a recyclable catalyst with a very easy process.
Heteropoly acids (HPAs), promising solid acids, have been extensively studied as acid catalysts for many reactions and found industrial applications in several processes.  As a strong acid, HPAs generally exhibit higher catalytic activities than conventional catalysts, such as mineral acids, ion-exchange resins, zeolites, SiO2/Al2O3, H3PO4/SiO2, etc.  Furthermore, HPAs catalysis lacks side reactions, such as sulfonation, chlorination, etc., that frequently occur with mineral acids. The Keggin-type H3PW12O40 (PW), in particular, was the target catalyst among the Keggin series in many earlier reports due to its stronger acidity. [3, 4] Supported H3PW12O40 catalysts have excellent activity and can be easily recovered and reused from reaction mixtures. Acidic or neutral substances, such as SiO2, MCM-41, SBA-15, active carbon, acidic ion-exchange resin, etc., are all suitable supports.5 They show fantastic properties such as easy of functionality, high surface areas and narrow pore size distributions.
TMOS was supplied by Hangzhou Guibao Chemical Co. Ltd, 95%. PEG was supplied by Sigma-Aldrich Co., average mol wt 10,000. H3PW12O40 was supplied by Shanghai Mayao Co., 98%.All reagents used were of AR grade.
Monolithic silica with various sizes of skeletons (1每2 mm) and through-pores (2每8 mm) were prepared with a sol-gel reaction accompanied with phase separation. Some subsequent post-gelation treatments were done to enhance the ripening of finely textured wet gel matrix. The prepared monolithic was characterized by SEM and TEM. Monolith supported PW were prepared by impregnation method. The preparation conditions of monolithic silica were similar to those reported earlier. [6-8] Typical conditions are as follows: a mixture of TMOS (14 mL), PEG 10 000 (2.0 g) and urea (2.4 g) in 0.01mol/L acetic acid (30 mL)was stirred at 0 ⊥ for 30 min to form a homogenous solution. The resultant solution was poured into a PTFE tube (10 mm℅5 mm I.D.) and to gel at 40 ⊥ for 24 h. The wet rod was transferred to an autoclave and immersed into appropriate amount of external solution. The autoclave was heated up to 120 ⊥ and kept 12 h to form mesopores in the silica gel. After that, the silica rod was washed with N, N-dimethylformamide and then dried at 60每100 ⊥ for 12 h. Subsequently, the monolithic silica gel was heated at 700 ⊥ for 2 h to decompose organic materials and stabilize the hydrophilic surface. The obtained silica monolith was refluxed in 6M HCl aqueous solution for 24 h and finally dried at 110 ⊥ for 4 h.
Supported catalyst was prepared by impregnation of H3PW12O40 (PW) on biporose silica monolith by adopting the reported procedure.  Silica monolith was used as the support. 5 g of silica monolith was impregnated into a saturated aqueous H4PW12O40 solution (10 cm3) at 0.5 torr/0.5 h at room temperature. The wet sample was then dried in an oven at 100 ⊥ overnight. The resulting supported monolith were calcined at 300 ⊥ for 5 h and denoted as the monolithic silica supported H3PW12O40. This procedure allowed a maximum PW loading of 46.5 wt %.
2.2 Synthesis of Bis(indoly) methanes
The mixture of indole (2 mmol), an aldehyde, water (4 mL), monolithic silica supported PW (0.017 g) was stirred at room temperature until the indole disappeared (1每4 h). When the reaction completed, the silica monolith was taken out of the mixture. The reaction mixture was filtrated and washed with water. The crude product was characterized by melting points and purified by recrystallization or column chromatography (EtOAc每petroleum ether).
Scheme 1 Reaction of indole with aldehyde in the presence of a heteropoly acid.
The reactions were carried out in the simplest manner, only by stirring the mixture of indole, an aldehyde and catalytic amount of commercially available supported PW at room temperature in a 50 mL flask under nitrogen protection.
- RESULTS AND DISCUSSION
A typical SEM image of the monolith showed in the Figure 1A. Uniform macropores can be seen. The size of the macropores is about 2 米m, which can be controlled by adjusting the parameters of the sol concentration, reaction temperature and time.10 The TEM image shows the distribution of micropore. It should be noted that it is a rough estimate because of the difficulty in size determination.
Figure 1. (A) Scanning electron microscopy (SEM) image and (B) transmission electron microscopy (TEM) image of the silica monolith
Figure 2 Synthesis of 3, 3′-bis (indolyl) phenylmethane using recycled monolith supported H3PW12O40
The catalyst stabilities were studied by recycling the silica monolith from reactions. After each catalytic running the supported catalyst were separated, and attempts were made to regenerate the catalysts by washing with water, acetone and acetic ether, and drying at 40 ⊥ for 5 h, and then the catalysts were calcined at 300 ⊥ for 1 h. The catalysts with the max PW loadings exhibited relatively stability, however, the catalytic activity decreased slightly after 5 cycles (Figure 2).
Table 1 Synthesis of bis (indoly) methanes in water catalysed by monolithic silica supported PW.
|Enter||Aldehhydes||Time(h)||Yield (%)||Mp. ⊥|
The monolith supported PW was used as the catalyst in the electrophilic substitutions of indole with a variety of aldehydes in water to afford bis (indolyl) methanes with excellent yields (Table 1). First, the synthesis could not be achieved in the presence of the monolithic silica without PW. It was observed that both unsupported PW and monolith supported PW obtained short reaction times for the coupling of an aldehyde and indole. It should be emphasized that monolith supported PW could be easily recycled and regenerated for the next running. That procedure represented a clean, practical, simple, and eco-friendly method for synthesis of bis (indoly) methanes.
The study demonstrates that the porous monolithic silica can be used as an efficient catalyst support in an eco-friendly route for indole leading to formation of bis (indoly) methanes. The monolith supported PW is an efficient, reusable catalyst with an easy process.
This work was supported by National Natural Science Foundation of China (Grant No. 20675084 and 20677066).