Influence of framework substitution of Al3+ by Fe3+ on the sorption characteristics of β zeolite

Abstract

The sorption characteristics of Al-β(SiO₂ : Al₂O₃= 39) and Fe-β(SiO₂ : Fe₂O₃= 39–46) zeolites in both sodium and protonic forms have been compared. Sorption capacities at P/P₀= 0.8 and 298 K for water, hexane, cyclohexane and butylamine (BA) yielded estimates of the void volume of the β zeolites. Sorption kinetics with these probe molecules also confirmed the highly crystalline nature of the samples. BA sorption isotherms in both the zeolites in the temperature range 333–483 K have also been measured to characterize the acidic centres in the zeolites. Analysis of the BA sorption data in terms of different sorption models yielded useful information on the nature of sorbent surface and sorption centres. Sorption selectivities were also compared by evaluation of the chemical potential of the sorption at isotherm temperatures. Isosteric heats (Q_st) of BA sorption were evaluated by the application of a thermodynamic approach to the sorption energetics. Over the entire coverage Q_st values varied within 40–60 kJ mol^–1 for H/AI-β and within 18–40 kJ mol^–1 for H/Fe-β zeolites.