Study On The Preparation Of Fe/ZSM-5 Zeolite Catalysts And The Reaction Process For The One Step Oxidation Of Benzene To Phenol Using N₂O As An Oxidant

Fe/ZSM-5 zeolite catalyst applied on N₂O oxidizing benzene to phenol has obtained good initial catalytic performance and phenol selectivity,however,the fast deactivation casuing by carbon deposition severely restricts the industrialization process of this application.This article focuses on the research over the controlled synthesis of ZSM-5 zeolite with small particle size,and mesoporous ZSM-5 zeolite,and modification of ZSM-5 zeolite,and the catalytic mechanism of the oxidation of benzene to phenol with N₂O,which lays the foundation of Fe/ZSM-5 zeolites for the further development and industrial application.Ultrasound-assisted synthesis of nano-sized ZSM-5 zeolite was studied.The effects of the molar ratio of the synthesis system,ultrasonic duration,ultrasonic power and crystallization conditions on the synthesis of ZSM-5 zeolites were systematically investigated.The results illustrated that ultrasound promoted the polymerization of silicon species and nucleation,and inhibited the agglomeration of silicon species in the sol-gel system,which affected the morphology and exposure of（001）crystal face,and particle size positively.The average particle size of ZSM-5 zeolite pre-treated with 4.5h ultrasonic duration and an ultrasonic power of 240 W,exhibiting a c-axis oriented octagonal shape,reduced from 960 nm to 320 nm;the amount of tetrapropylammonium hydroxide influenced the exposure ratio of（101）crystal face aggressively;when ZSM-5 was crystallized at low temperature,it tended to grow into spherical-shaped ZSM-5zeolite.Therefore,it can be concluded that ultrasound induces the formation of ZSM-5zeolite with specific morphology.Based on the commercial nano-ZSM-5 zeolite with a Si/Al molar ratio of 12.5,the effects of high temperature calcination,controlled Fe ion exchange,and water vapor pre-treatment on physicochemical properties were systematically studied.The results showed that with the increase of calcination temperature（773-1073 K）on the H-ZSM-5 zeolite,its Br?nsted acid sites（BAS）significantly reduced,whereas Lewis acid sites（LAS）gradually increased,resulting in the decrease of total BAS/LAS（B/L）and strong B/L ratios.The LAS of the zeolite catalyst prepared by controlled Fe ion modification increased sharply,while BAS decreased aggressively.H-ZSM-5-ST and Fe/ZSM-5-ST zeolite catalysts pre-treated with water vapor generated more mesopores and both their mean pore size was 2.7 nm;the total BAS and LAS of two zeolite catalysts decreased sharply,and strong BAS and LAS were completely eliminated,nevertheless,the B/L value of previous catalyst was 2.00,which was much higher than 0.67 of the latter one.Besides,part of Fe₂O₃ nanoparticles aggregated on the Fe/ZSM-5-ST zeolite catalyst was converted into four-coordinate and eight-coordinate isolated Fe ions.It can be summarized that all three pre-treatment routes are proper ways to regulate the acid distribution of zeolite catalyst,and the water vapor pre-treatment facilitates the formation of mesopores and adjusts the distribution of Fe species.A series of mesoporous Fe/ZSM-5 zeolites with a Si/Al molar ratio of 50 utilizing CTAB as the mesoporous template was synthesized.A systematic examination of the effects over the molar ratio of CTAB to aluminium iso-propoxide（AIP）,tailoring temperature（20-60°C）,and ultrasonic power（300 and 600 W）on the synthesis of Fe/ZSM-5 zeolites was studied.The results showed that when pre-treated without ultrasound,and CTAB/AIP=1.4,and 40°C and 60°C,both mesoporous Fe/ZSM-5zeolites achieved the biggest mean pore size of 4.2 nm;when CTAB/AIP=0.7 and pre-treated without ultrasound,with the temperature increasing from 20°C to 60°C,the mean mesoporous pore size of Fe/ZSM-5 zeolites increased from 2.95 nm to 3.81 nm.When CTAB/AIP=0.7 and ultrasonic power was 300 W,with the temperature rose to40°C or 60°C,the pore size distribution of the mesoporous region split into two clear peaks;with 600 W ultrasonic power,the pore size distribution in the mesoporous region depicted mild double peaks.Thus,it can be drawn that proper temperature for CTAB micelles,ultrasonic power and CTAB amount adjust the pore texture of Fe/ZSM-5zeolite effectively.Based on the Fe/ZSM-5 zeolite catalysts prepared by zeolites above,the relationship between the acidic-pore structure characteristics and catalytic activity over one-step oxidation of benzene to phenol（BTOP）with N₂O was systematically investigated.The results showed that the Fe/ZSM-5-ST pre-treated with water vapor attained an initial phenol yield of 78.8%and an effective catalyst life of 90 min;Fe³⁺ions contributed more to N₂O conversion,while water vapor pre-treatment improved phenol selectivity significantly;there was a negative correlation between LAS and phenol initial selectivity（during time on stream of 20 min）.Fe/MZ-0.7C-60-300 zeolite catalyst（Si/Al=50）,which was prepared at 60℃,CTAB/AIP=0.7 and 300 W ultrasound power,performed an initial phenol yield of 71.1%at time on stream of 30min and effective catalyst life of 390 min,which prolongs the catalyst span predominantly.Above all,a comprehensive consideration of both acid properties and micro-mesoporous properties is the key to a good performance of catalyst.Catalytic reaction-N₂O regeneration cycles on the oxidation of benzene to phenol with N₂O over the hierarchical Fe/ZSM-5-Hi2.8 catalysts in a continuous fixed-bed reactor were perfomred 10 cycles in total.A 100%N₂O conversion,93.3%phenol selectivity,and high phenol formation rate of 16.49±0.06 mmol_phenol g^-1_catalyst h^-1 at time on steam of 5 min,and a good phenol productivity of 147.06 mmol_phenol g^-1_catalystduring the effective catalyst life-time of 1800 min were obtained on the fresh hierarchical Fe/ZSM-5-Hi2.8 catalyst.Upon the reaction-regeneration cycle,N₂O conversion was fully recovered within time on stream of 3 h,indicating that reaction time within one reaction should not be too long.The phenol selectivity descended gradually.The main reason was related to the carbonous products deposited on the catalyst surface(the average coke accumulation speed was 0.28 mg_c g^-1_catalyst min^-1within time on stream of 3 h),resulting in the coverage of the active Fe-O-Al sites,and their agglomeration on catalyst surface upon each cycle,and a further decreased amount,and finally contributing to the deactivation of catalysts.The phenol productivity was decreased ca.2.2±0.8%after each cycle,which accumulated a total phenol productivity of ca.0.44 ton_phenol kg^-1_catalyst estimated for 300 cycles.Hence,the modification of catalysts cooperated with the proper catalytic reactor is the key point to faciliate this industrial implimentation of the oxidation of benzene to phenol.