An silica supported chitosan-Schiff base Pd(II) catalyst was prepared in a simple way and characterized by XRD, FT-IR, SEM-EDS, XPS and TG, and the ability of this complex to catalyze hydrogenolysis of 1-tetralone into 1,2,3,4-tetrahydronaphthalene was also investigated in the presence of hydrogen. It has been revealed that the catalyst had high catalytic activity for hydrogenolysis of 1-tetralone at ambient temperature and normal pressure of hydrogen. Especially, the hydrogenolysis of 1-tetralone in ethanol solvent gave excellent results and the 100% conversion of 1-tetralone and the 100% selectivity for 1,2,3,4-tetrahydronaphthalene were obtained under optimized reaction conditions. The influences of reaction temperature, reaction time and solvent on the hydrogenolysis of 1-tetralone were also investigated. It has been also revealed that the catalyst was efficient and eco-friendly for the hydrogenolysis of carbonyl that connected with a benzene ring to give corresponding aromatic hydrocarbons.
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 [
Chitosan (CS) is a natural biopolymer, which can be easily obtained from chitin that is widely dispersed in living organisms [
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.
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.
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%.
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).
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.
The XRD pattern of prepared catalyst is shown in
The FT-IR spectrums of the prepared SiO2-CS-Schiffbase and SiO2-CS-Schiff-base-Pd catalyst had reported in previous works [
The TG data of the SiO2-CS-Schiff-base-Pd catalyst is depicted in
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