Abstract
To clarify the dealumination behavior of zeolite during steaming, HZSM-5 zeolites with various degrees of dealumination were prepared by steaming and thermal treatment. The dealuminated zeolites were characterized by using FT-IR and water vapor adsorption. It was found that the dealumination rate is strongly dependent on the water vapor pressure. It was also found that the dealumination of HZSM-5 zeolite by steaming consists of two steps at least; the first step is a partial hydrolysis of four A1-O bonds of the framework aluminum, and the second step is a complete release of the alumin um from the zeolite framework.
Adsorption of n itrogen and water vapor was also studied to evaluate the state of nonframework aluminum in the zeolitic pore. The micropore volumes W0(N2) of the dealuminated zeolites calculated by the D-R plot of the adsorption isotherms of nitrogen hardly depended upon the degree of dealumination, indicating that the dealuminated zeolites maintain their framework structure. However, the micropore volumes W0(N2) from the adsorption isotherms of water vapor depended on the dealumination condition. In the case of the zeolites dealuminated by mild steaming or thermal treatment, the micropore volumes W0(N2) were consistent with those of untreated zeolites with the same concentration of framework aluminums. In the case of zeolites dealuminated by severe steaming, however, the micropore volumes W0(N2) were smaller than those expected from the concentration of framework aluminums. This suggests that the nonframework aluminum species released completely from the zeolite framework suppress strongly the adsorption of water molecules into the zeolitic pores.
