| Zeolites are widely used as catalysts, especially as acidic catalysts in petrochemical industry, for example, zeolite Y for fluid catalytic cracking (FCC) and hydrocracking processes; ZSM-5 for xylene isomerization, hydrodewaxing, alkylation and aromatization processes. The hydrothermal stability of zeolites used in the above processes is extremely important because the regeneration in the presence of steam at the temperature higher than 600℃is usually needed. In these conditions, dealumination of the zeolite is favored, resulting in a loss in Al-OH-Si groups responsible by the Brφnsted acid. So preventing or decreasing this dealumination is a topic of continuous interest in the field of acidic zeolite catalysts, especially in catalytic cracking applications.In order to improve the stability of zeolite, P or rare earth metal were used to modify zeolite. However, zeolites modified by phosphorus or rare earth reulted in the total elimination of strong B acid sites accompanied by the formation of new B type acid sites with reduced acid strength. It is reported that the dealumination of zeolites in steam at high temperature was caused by protonic acid (H+) sites in zeolites. On the base of the oxidation-reduction properties of silver, we modified ZSM-5 zeolites by silver to improve their hydrothermal stability. Silver was incorporated into the cationic positions of zeolites by ion-exchange, it was expected that silver cations can suppress the dealumination of zeolites under steaming. We studied the hydrothermal stability, the change of acidity and n-hexane cracking conversion of Ag modified ZSM-5. The main experimental results and conclusions are summarized as follows:AgHZSM-5 was prepared by ion-excange of silver with HZSM-5. The adsorption isotherm of nitrogen, NH3-TPD, Py (pyridine)-IR and n-hexane cracking conversion of samples steamed in different condition were measured. The n-hexane cracking conversion, adsorption peak of NH3 and Brφnsted acid sites in Py-IR spectra of AgHZSM-5 samples steamed at 600℃were higher than those of HZSM-5 sample steamed at the same condition; the remained amount of acid sites after the steam treatment increased with the silver content of AgHZSM-5 catalyst. These results indicated that silver species in the cationic positions of zeolites (in the condition of oxidizing atmosphere) can effectively improve the hydrothermal stability of zeolites. The n-hexane cracking conversion and acidity of AgHZSM-5 samples steamed for 4 h in nitrogen or 100% steam were lower than those of the samples steamed in air. The n-hexane cracking conversion and acidity of AgHZSM-5 samples steamed for 8 h in nitrogen or 100% steam decreased much, which was close to the sample HZSM-5 steamed for 4 h. The n-hexane cracking conversion and acidity of AgHZSM-5 samples steamed for 8 h in air were the same with AgHZSM-5 steamed for 4 h. The n-hexane cracking conversion, adsorption peak of NH3 and Brφnsted acid sites in Py-IR spectra of the reduced AgHZSM-5 samples steamed in nitrogen were almost the same with those of HZSM-5 sample steamed for 4 h. It is concluded that the reduced silver species could not stabilze the framework structure. The above results indicated that Ag species can improve the hydrothermal stability of zeolite only under steaming in air. Silver cations can be completely reduced to silver particles or silver clusters by hydrogen or hydrocarbons, thus protonic cations located in the cationic positons of samples instead of Ag+. Unlike the modification of phosphorus, silver modification did not change the strength of the acid sites.AgHZSM-5 catalyst still have protonic cations leading to dealumination of zeolite, so AgNaZSM-5 catalyst prepared by ion-excange of silver with NaZSM-5 was also investigated. The properties of AgNaZSM-5 samples steamed in different conditions and extended the steam time were studied. It is concluded that AgNaZSM-5 steamed in air had the good hydrothermal stability. After 16 h of the steam treatment of AgNaZSM-5 catalyst, there was no dealumination found. In non-oxidative atmosphere, the reaction H2O+2Ag+(cationic position)=2Ag+2H+ (cationic position)+1/2O2, happened leading to the dealumination of zeolite.After Ag+ in zeolite was reduced, Brφnsted acid site were shown in Py-IR spectra. There was no peak correlated to Brφnsted acid sites at high temperature (400℃), but the Ag+ peak at higher temperature peak (530℃) was found when helium was used as the carrier gas of NH3-TPD. We speculated that in non-reducing gas, the reduced Ag species reentered the cationic position becoming to Ag+ cations. After we carried out NH3-TPD measurements using hydrogen as the carrier gas, it is known that Ag+ in zeolite can be easily reduced by hydrogen, reduced Ag species and H+ cations can stablely exist in hydrogen atmosphere. |
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