Preparation And Catalytic Performances Of Aluminum-modified Iron Oxides For Selective Oxidation Of Benzyl Alcohol With H₂O₂

Benzaldehyde has wide applications in medicine,perfume and other fields.Chlorination of toluene followed by hydrolysis is the main method for industrial production of benzaldehyde,but trace chlorinated impurities in the product limit its applications in medicine and perfume.Liquid phase oxidation of benzyl alcohol（Bz OH）with H₂O₂ in H₂O is a potential green and clean route to prepare chlorine-free benzaldehyde,which has the advantages of easy-to-operate under mild conditions.Aluminum（Al）and iron（Fe）elements are abundant in the Earth’s crust.Iron with variable valence states has the capability to activate H₂O₂,which can be further enhanced via the synergistic effect between Al and Fe.However,there are very few reports on the preparation of chlorine-free benzaldehyde by liquid phase oxidation of Bz OH with H₂O₂ in H₂O over Al-modified iron oxides.In this thesis,Al-modified iron oxides with different morphologies（including Fe₃O₄ microspheres,Fe₃O₄nanorods and Fe₂O₃ nanosheets）were prepared from cheap and easily available raw materials,and then applied to catalyze selective oxidation of Bz OH with H₂O₂ in H₂O（target reaction）to make chlorine-free benzaldehyde.The main research results are as follows:1.Al-modified Fe₃O₄ microspheres were in-situ prepared by solvothermal synthesis using inorganic aluminum salts as raw materials.The effects of type,particle size,amount and adding sequence of inorganic aluminum sources on the catalytic performance of the Al-modified Fe₃O₄ microspheres were investigated.Modification by inorganic aluminum significantly improved the catalytic activity of the blank Fe₃O₄microspheres(Bz OH conversion(C_{Bz OH}):47.3%vs.5.7%;benzaldehyde yield(Y_{Bz H}):41.2%vs.5.6%).The catalytic activity and reproducibility of Fe₃O₄-AS-1.0microspheres prepared under optimized conditions,that is,using ground Al₂（SO₄）₃·18H₂O particles as aluminum source with Fe/Al ratio of 7.4 and adding raw materials in the order of ferric salt-aluminum salt-alkali source,were significantly improved in comparison with the initial Al-modified Fe₃O₄ microspheres(C_{Bz OH}:45.5%±0.9%vs.34.7%±5.8%;benzaldehyde selectivity(S_{Bz H}):85.5%±0.6%vs.87.8%±5.3%).However,the activity of the recovered Fe₃O₄-AS-1.0 microspheres decreased sharply after the reaction.EDS analysis and Na BH₄regeneration experiment suggested that the deactivation of the Fe₃O₄-AS-1.0 microspheres was closely related to Al leaching and Fe²⁺oxidation during the Bz OH oxidation with H₂O₂.Therefore,the deactivated Fe₃O₄-AS-1.0 microspheres were rejuvenated by simultaneous alumination and reduction in ethylene glycol.The regenerated Fe₃O₄-AS-1.0 microspheres could be recycled for at least five times without significant loss of catalytic performance(C_{Bz OH}:44.2%-50.3%and S_{Bz H}:80.0%-85.9%).2.Magnetic Fe₃O₄ nanorods were synthesized in two steps.Rice-shapedβ-Fe OOH nanoparticles were firstly prepared by hydrothermal method and then converted into Fe₃O₄ nanorods by solvothermal method.The average length and diameter of Fe₃O₄nanorods measured from TEM images were respectively 565.8±6.2 nm and 83.5±9.7nm.The concentration of ferric salts had a great influence on the yield ofβ-Fe OOH synthesized by hydrothermal method.The highest yield（about 43.9%）of rice-shapedβ-Fe OOH with good crystallinity was obtained by hydrothermal treatment of 40.0 g/L of Fe Cl₃·6H₂O aqueous solution at 110 ^oC for 24 h.Based on the synthesis method of blank Fe₃O₄ nanorods（30 g/L ofβ-Fe OOH dispersion in alkaline ethylene glycol reacted at 198 ^oC for 14 h）,Al-modified Fe₃O₄nanorods were produced with Al₂（SO₄）₃·18H₂O as inorganic aluminum source.The Al-modified Fe₃O₄nanorods prepared with Fe/Al ratio of 300（denoted Al-Fe₃O₄-0.01）had the best catalytic performance(C_{Bz OH}:52.0%and S_{Bz H}:87.5%),which was much higher than the activity of blank Fe₃O₄ nanorods(C_{Bz OH}:32.5%and S_{Bz H}:89.4%).The particle length and diameter of Al-Fe₃O₄-0.01 nanorods were 499.3±6.8 nm and 79.5±5.6 nm,respectively.The similar morphology and particle size of Al-Fe₃O₄-0.01 and blank Fe₃O₄ nanorods indicated that the growth process and microstructure of Fe₃O₄ particles were not significantly changed by introducing trace aluminum.Quenching experiment by tert-butanol implied that·OH was the main reactive oxygen species in the Bz OH oxidation with H₂O₂ over Al-Fe₃O₄-0.01.Under optimal reaction conditions,that is,20mmol of Bz OH,40 mmol of H₂O₂ and 50 mg of Al-Fe₃O₄-0.01 were reacted at 100 ^oC for 1.5 h,52.0%of C_{Bz OH} and 87.5%of S_{Bz H} were achieved.After simultaneous alumination and reduction in ethylene glycol,the deactivated Al-Fe₃O₄-0.01 could be regenerated and recycled for at least 5 times with good catalytic performances(C_{Bz OH}:52.0%-63.8%and S_{Bz H}:76.1%-87.5%).3.Al-modifiedα-Fe₂O₃ nanosheets were prepared by direct calcination method.The influence of calcination temperature and Al amount on the catalytic performance of Al-modifiedα-Fe₂O₃ nanosheets were investigated.The optimal calcination temperature and Al/Fe ratio were found to be 500°C and 0.5（the corresponding sample denoted as 0.5Al-α-Fe₂O₃-500m）.XRD,SEM and XPS results indicated that 0.5Al-α-Fe₂O₃-500m is a mixture of Fe Al₂O₄ andα-Fe₂O₃ with low crystallinity.The nanosheet structure of 0.5Al-α-Fe₂O₃-500m with～78 nm in thickness was observed by SEM.Compared with single-componentα-Fe₂O₃ and Al₂O₃ flakes,0.5Al-α-Fe₂O₃-500m nanosheets had the highest catalytic activity for the target reaction(C_{Bz OH} 33.8%,1.8%and 3.1%).More importantly,0.5Al-α-Fe₂O₃-500m nanosheets were stable after the Bz OH oxidation and could be directly reused（i.e.,no requirement of regeneration）.After five cycles,no significant loss of activity occurred(C_{Bz OH}:44.5%-49.1%;S_{Bz H}:86.4%-88.9%).By taking 0.5Al-α-Fe₂O₃-500m nanosheets as catalyst,45.0%of C_{Bz OH}and 87.0%of S_{Bz H} were obtained under the optimal reaction conditions,that is,20 mmol of Bz OH,40 mmol of H₂O₂ and 50 mg of 0.5Al-α-Fe₂O₃-500m were reacted at 80°C for 1.5 h.