Efficient and Clean Catalytic Hydrogenolysis of Aromatic Ketones by Silica Supported Schiff Base Modify Chitosan-Palladium Catalyst ()
1. Introduction
The reduction of aldehydes or ketones, especially selectively reduce C=O group to methylene (CH2) is an important organic reaction, which is used to the direct conversion of aromatic ketones to synthesize linear alkylbenzenes [1]. The linear alkylbenzenes are frequently used as intermediates in chemical industries. The classical reported procedures for the reduction of C=O group to CH2 are the Clemmensen and Wolff-Kishner reducetions [2,3]. However, not only Clemens reaction, but also Wolff-Kishner reaction is not environment-friendly, these reactions are either carried out in the presence of Zn-Hg/concentrated HCl or employed lot of hydrazine as reactant. Subsequently, the catalytic hydrogenation of C=O group to CH2 has been reported, Cu-Cr, Fe or Ni had proved less active for the conversion of C=O into CH2 at high temperatures (473 - 573 K) [4-6]. Avnir [7] once reported that the catalytic hydrogenolysis of aromatic ketones by a sol-gel entrapped combined Pd- [Rh(cod)Cl]2 catalyst, however, the selectivity of hydrogenation reaction towards the alkylbenzenes is low and the aromatic rings were fully hydrogenated. The result that reported by Prof. De Vos [8] suggests that the mixed choline-betainium ionic liquids has promotional effect on the hydrogenolysis of aromatic ketones catalytic by Pd catalyst, the conversion of aromatic ketones and selectivity to alkylbenzenes were all improved.
Chitosan (CS) is a natural biopolymer, which can be easily obtained from chitin that is widely dispersed in living organisms [9]. More recently, with the development of environmentally friendly industries, CS-supported metals catalysts have attracted a lot of attention because CS shows these advantages of nontoxicity and desirable physical and mechanical properties [10-13]. In previous papers [14-18], a silica-supported CS-palladium complex abbreviated as SiO-CS-Pd has been found to catalyze the hydrogenation of nitrotoluene, nitrobenzene, chlorophenol, 2-octene and 3-octene, et al. Some ketones can reduce to chiral alcohols catalyzed by silica supported CS-Pd complex via asymmetric hydrogenation [19,20]. However, the successful application of CS stabilized palladium catalysts in hydrogenolysis reactions of C=O to CH2 groups has not yet been reported.
In the presence of active amino group, CS also exhibits the possibility of chemical modifications, including the preparation of Schiff bases by reaction with aldehydes and ketones [21-23]. Recently, we has prepared a silica supported Schiff base modified CS-Pd catalyst and found that it exhibited good catalytic activity in the hydrogenaolysis of 1-tetralone. In this paper, we validated this finding and revealed the application scope of the catalyst. It can be found that this immobilized combined catalyst promotes the total hydrogenolysis of some aromatic ketones to give corresponding aromatic hydrocarbons under mild conditions.
2. Experimental
2.1. Materials
Chitosan (CS) finely purified to a de-acetyl degree of 90.0% was purchased from Sinopharm Chemical Reagent Co, Ltd. China. Salicylaldehyde and 1-tetralone was purchased from Aladdin Reagent Co, Ltd. China and salicylaldehyde was distilled before used. Other reagents were of analytical grade and were used as received.
2.2. Preparation of Catalyst
Silica supported chitosan (SiO2-CS) was first prepared, 4.0 g CS was added into 250 ml 1.5% CH3COOH and stirred until CS was all dissolved at room temperature, and then 8.0 g SiO2 was added. After continued stirred for 2 h, the PH of solution was modified by 1 mol/L NaOH to 13, the solid was separated by filtration, washed with water (until the PH = 8), ethanol and acetone, respectively, and then dried at 333 K under vacuum for 10 h to give white solid, the nitrogen content was determined to be 2.52 wt% by elemental analysis.
Then the silica supported chitosan Schiff base (SiO2-CS-Schiff base) was prepared refer to the procedure in the literatures 24 - 25. 10 g SiO2-CS and excess salicyal (15 ml) and acetic acid (12 ml) were added to methanol (120 ml), and then the mixture was refluxed for 10 h. After the resultant mixture was cooled, the solid was separated by filtration, washed with methanol and then dried at 333 K under vacuum for 12 h to give bright yellow solid.
Finally, the silica supported Schiff base modify chitosan-palladium (SiO2-CS-Schiff base-Pd) was prepared according to the method of reported in literatures [18,19, 23]. The SiO2-CS Schiff-base and PdCl2 were weighed in 10:1 mass ratio and were added in ethanol solvent. After the mixture was stirred at 303 K for 72 h, the solid product was filtered and washed with ethanol to the filtrate became transparent and colorless, and dried at 323 K under vacuum to obtained brown catalyst particles, which was then used to catalyze the hydrogenolysis of 1-tetralone with hydrogen. The metal contents of catalyst determined by ICP are 5.4%.
2.3. Characterization of the Catalyst
The X-ray diffraction analysis was carried out using a X-ray diffractometer (Beijing Purkinje General Instrument Co. Ltd) with Ni filtered Cu Kα radiation (λ = 1.542 Å ) and a scanning range 2θ of 5˚ - 80˚. The FT-IR spectra were measured on a FT6700 spectrophotometer in the range 400 - 4000 cm−1. X-Ray Photoelectron Spectroscopy (XPS) measurements were performed with a VG Scientific ESCALAB 250 instrument with Mg Kα radiation (1253.6 eV). The TG analyses were performed on a STA449 thermogravimetric analyzer (NETZSCH, Germany). The morphology and elemental composition of sample was analyzed by a JSM6380LV scanning electron microscopy equipped with energy dispersive X-ray (SEM-EDX) elemental analysis system. The content of Pd before and after reaction was identified by an OPTIMA2000DV Inductive Coupled Plasma Emission Spectrometer (ICP).
2.4. Hydrogenolysis of 1-Tetralone
The hydrogenolysis experiment was performed in a 50 mL tube with a side neck equipped with a magnetic stirrer and an automatic temperature controller. In a typical hydrogenolysis experiment, to a mixture of 1-tetralone substrates (0.5 ml) and catalyst (0.2 g) was added ethanol (15 ml). The reaction vessel was flushed (three times) with pure hydrogen and then retained the 40 ml/min current velocity of hydrogen. The reaction mixture was stirred magnetically at a rate of 150 rpm at 313 K for 5 h. After the reaction, the hydrogenolysis products were identified and quantified by 1H NMR (CDCl3, 400 MHz) and an HP 6890/5973 GC/MS instrument (FID, column: HP-5MS). The by-products of the reaction were 1-tetralin alcohol.
3. Results and discussion
3.1. Characterization Results
The XRD pattern of prepared catalyst is shown in Figure 1. From the pattern, it can be seen that SiO2-CS-Schiffbase-Pd only exhibits two broad peaks at about 20˚, which ascribes the amorphous silica and the crystalline macromolecule of CS, respectively. The diffraction peaks of palladium (0) (2θ = 38˚, 46˚, et al., JCPDS: 05-618) or other palladium-contained compounds are not observed.
The FT-IR spectrums of the prepared SiO2-CS-Schiffbase and SiO2-CS-Schiff-base-Pd catalyst had reported in previous works [25]. The results exhibited that a band due to the C=N stretching vibration appeared at 1630 cm−1, which indicated that the formation of Schiff base. And the characters bands of silica were also retained, which means that the structure of silica was no change in the preparation.
The TG data of the SiO2-CS-Schiff-base-Pd catalyst is depicted in Figure 2. From this result, the destruction
Figure 1. XRD pattern of silica supported CS-Schiff-basePd catalyst.
Figure 2. The TG data of silica supported CS-Schiff-basePd catalyst.
temperature of catalyst is 490 K, which suggests that the catalyst of SiO2-CS-Schiff-base-Pd have a good thermal stability between room temperature and 490 K. The weight loss of the catalysts before the destruction is attributed to desorption of water.
The surface morphology of the catalyst is shown in Figure 3. From the SEM image it is clear that the catalyst has irregular block structure. And the existence of palladium in the catalyst was confirmed by SEM-EDX analysis (Figure 3), chloride also was detected for SiO2- CS-Schiff-base-Pd catalyst. The elemental composition of catalyst analyzed by with SEM-EDX is listed in Table 1, and it can be seen that the molar ratio of chlorine: palladium is about 2:1. The appearance of the SiO2-CSSchiff-base-Pd catalyst and SiO2-CS-Schiff-base is different in color. The former was brown and the latter was bright yellow.
Figure 4 shows the XPS analysis of SiO2-CS-Schiffbase-Pd catalyst. The XPS analysis is a technique essentially limited to the very first external layers of the material (on a thickness limited to 2 - 3 times the wavelength