Structural, textural and toluene adsorption properties of NH4HF2 and alkali modified USY zeolite

The non-framework aluminum is inevitably formed in the process of preparing the ultra-stable Y zeolite (USY), which severely blocks the mesopores of USY zeolite. It is necessary to treat the USY with the appropriate acid and/or alkali. In this paper, the USY zeolite is firstly dealuminated with NH4HF2 solution and then desiliconized with NaOH or NH4OH solution. The effect and mechanism of NH4HF2 and different alkali treatments on the structural and textural property of USY zeolite are elaborately demonstrated. It is found that the non-framework Al remaining in the mesopores of USY is significantly removed by the NH4HF2 solution. The desiliconization reaction is more likely to start near the mesoporous wall because the Al concentration in here is low and the repulsive force against OH− is weak. The OH− ions from NaOH solution not only etch the framework Si near the inner wall of the mesopores, but also remove the Si on the surface of NH4HF2-etched USY zeolite. However, the concentration of OH− ions is too low when NH4OH is used as the desiliconization agent, the OH− ions just react with silicon around the mesopores. Therefore, the NaOH/NH4HF2-etched USY contains both some micropores and macropores, and the NH4OH/NH4HF2-etched USY only possesses more mesopores with pore sizes of ～10 nm. In addition, NaOH/NH4HF2-etched USY exhibits excellent toluene adsorption performance at low pressure stage, while NH4OH/NH4HF2-etched USY has the highest adsorption capacity on toluene at high pressure stage. [Display omitted] •The non-framework Al remaining in the mesopores of USY is significantly removed by NH4HF2 solution.•The desiliconization reaction is more likely to start near the mesopores of NH4HF2-etched USY.•NaOH/NH4HF2-etched USY contains a large number of micropores and macropores.•NH4OH/NH4HF2-etched USY possesses more mesopores with pore sizes of ～10 nm.•NH4OH/NH4HF2-etched USY has the highest toluene adsorption capacity.