Selective Oxidation Of Benzyl Alcohol On Aluminum-modified Fe₃O₄ Magnetic Catalysts

Benzyl alcohol-H₂O₂-Fe₃O₄-H2O liquid-phase catalytic reaction is a potential green process for producing chlorine-free benzaldehyde?BzH?,due to the simple,environmentally friendly and chlorine-free characteristics of this process.However,it remains a great challenge to improve the catalytic activity of conventional Fe₃O₄ catalysts to activate H₂O₂ for practical applications.The surface modification of Fe₃O₄ is a simple and feasible way to improve its catalytic activity.Aluminum?Al?is the most abundant metal element in the Earth's crust,and the aluminum compounds have been used as catalysts in many organic reactions.However,Al-modified Fe₃O₄ catalyst used for the title reaction has been rarely reported.Therefore,the effect of Al-containing additives on the catalytic performance of Fe₃O₄ was studied for selective oxidation of benzyl alcohol to BzH.In this thesis,Fe₃O₄ main catalyst and Al-containing additives were screened by characterization techniques and catalytic performances.Fe₃O₄ particles were modified by different methods,and then the catalytic performances of the modified Fe₃O₄ catalysts were compared.It showed that there were synergistic effects between iron?Fe?and Al.The main results and conclusions are as follows:1.The catalytic performances of Fe₃O₄ synthesized in different reaction media with various iron sources were investigated and characterized by XRD,SEM,BET,iron content analysis,etc.The results showed that Fe₃O₄ synthesized by different methods had different crystallinity,particle size and specific surface area,which led to the difference of their catalytic performances.By comparison,the Fe₃O₄ particles that were synthesized from soluble Fe³⁺sources in ethylene glycol have better crystallinity,higher yield and stronger stability,easy for industrial production.Therefore,these Fe₃O₄ particles were selected as the main catalyst.2.The selective oxidation of benzyl alcohol with the mixture of different metal oxides and Fe₃O₄ was investigated.The results showed that the mixture ofg-Al₂O₃ and Fe₃O₄ as catalyst has better catalytic activity?BzH yield:18.0%?.Very low yield of BzH?5.6%or0.7%?was obtained by using pure Fe₃O₄ org-Al₂O₃ as catalyst.However,a small amount of Al?The mass ratio of Fe₃O₄ tog-Al₂O₃ is 50:1?can significantly enhance the catalytic activity of Fe₃O₄?BzH yield:16.3%?,indicating that Al is a good promoter in the catalytic system.Althoughg-Al₂O₃ made by different methods had different properties?e.g.,crystallinity,morphology and particle size?,they all showed similar performances as co-catalysts?Bz H yield:14.5%-18.0%?on the physical mixture of Fe₃O₄ and variousg-Al₂O₃.3.The catalytic properties of Al-modified Fe₃O₄ magnetic catalysts by different modification methods?such as ultrasound-assisted two-step method and physical mixing method?were obviously different?the yield of BzH was 25.2%and 18%,respectively?,which may be related to the different contact degree between Al and Fe₃O₄.The catalytic activity of the modified Fe₃O₄ particles are 3-5 times that of the blank Fe₃O₄,indicative of the synergistic effect between Al and Fe.4.Al-modified Fe₃O₄ magnetic catalysts were prepared by in-situ solvothermal synthesis in ethylene glycol and characterized by XRD,SEM,XPS,BET and VSM techniques.The results showed that Al is uniformly deposited on the surface of Fe₃O₄microspheres.During the investigation of the effects of Al sources and content,aluminum isopropoxide?AIP?was found to be the best aluminum source with the optimized Al content of 1.42 wt%to make Al-modified Fe₃O₄ with the highest catalytic activity?BzH yield:43.7%on Fe₃O₄-AIP-1.42%?.Fe₃O₄-AIP-1.42%was selected in the following investigation.The reaction conditions?such as catalyst loading,H₂O₂ dosage,reaction temperature and time?were optimized.Fe₃O₄-AIP-1.42%catalyst was magnetically recoverable and reusable at least five times without loss of catalytic activity?BzH yield:36.0%-43.0%?after being rejuvenated with NaBH₄ reduction.The synergistic effect between Fe and Al sites contributes to the much better catalytic activity of Fe₃O₄-AIP-1.42%as compared to pure Fe₃O₄,g-Al₂O₃ or their physical mixtures as catalysts.The catalytic decompositions of H₂O₂ on Fe₃O₄ or Fe₃O₄-AIP-1.42%were studied.It was found that the decomposition rate of H₂O₂ was enhanced and the apparent activation energy was decreased from 52.3 kJ/mol to 41.1 kJ/mol after modification by AIP.Theoretical calculations confirmed that the Al promoter introduced new Al active sites besides the primary Fe sites on the surface of Fe₃O₄ microspheres,facilitated the formation of Al–peroxo?Al–OOH?intermediates,and promoted the oxidation of benzyl alcohol.