Synthesis Of Hierarchical ZSM-5 And Its Application In Benzene Alkylation With Methanol

Hierarchical ZSM-5 zeolite has advantages of both zeolites and mesoporous materials, and possesses a good prospect of industrial applications. Hierarchical ZSM-5 was directing synthesized by solid-phase hydrothermal crystallization method with organosilane as additive. The influences of gel composition and crystallization conditions on synthesis were investigated. Hierarchical ZSM-5 was also prepared by treating ZSM-5(SiO2/Al2O3=70) with sodium hydroxide solution. The effects of the concentration of sodium hydroxide solution, treating temperature and time on the formation of mesoporous structure were studied. The samples were characterized by XRD, SEM, FT-IR, NH3-TPD, N2 adsorption-desorption and TG-DTA. The results showed that with the gel composition of SiO2/Al2O3=360, TPAOH/SiO2=0.2, C2H5OH/SiO2=20, HTS/SiO2=5%, hierarchical ZSM-5 was prepared by hydrothermal crystallization method with the highest relative crystallinity at 180℃for 72h. At this condition, the crystal size was 500nm, mesoporous volume 0.34cm3/g and mesopore diameter 10nm. The optimal treatment condition (zeolite treated with 0.2mol/1 NaOH solution at 338K for 30min) led to a drastic increase in mesopore surface areas from 59m2/g for the untreated ZSM-5 to 180m2/g for the alkaline-treated ZSM-5, and the mesoporous size was about 40nm.Compared to the conventional ZSM-5, the hierarchical ZSM-5 displayed high benzene conversion, selectivity and yields for the toluene and xylene in the benzene alkylation with methanol. The benzene conversion over the zeolites synthesized by solid-phase hydrothermal method and alkaline treatment method increased by 8% and 6%, respectively; the selectivity and yields for the toluene and xylene increased by 3% and 7% over the zeolites prepared by the two methods, respectively. The selectivity of xylene over the zeolite fabricated by the first method had been a 6 percent increase.