Improving the Structural Characterization of Zeolite with Argon Adsorption
IUPAC recommends characterization of zeolites to be performed using argon gas at 87 K. The advantages of argon over nitrogen are discussed. Examples of Ar isotherms for various zeolites are shown. Finally, the characterization of mesoporous zeolites (hierarchical materials) using Ar adsorption is discussed.
Physical adsorption experiments studying the structural aspects of microporous materials require measurements at substantially lower relative pressures than measuring mesoporous materials. The characterization of zeolites with nitrogen at 77 K is difficult because filling 0.5-1 nm pores occurs at relative pressures of 10-7 to 10-5, where the rate of diffusion and adsorption equilibration is extremely slow. Additionally, the N2 quadrupole can interact with a variety of surface functional groups and exposed ions within the zeolite. These factors influence the orientation of the adsorbed N2 molecule on the adsorbent surface, thus affecting subsequent calculations. It also strongly affects the micropore filling pressure, often shifting it to even lower relative pressures. Because of the specific interactions with the surface functional groups, the pore filling pressure is not clearly correlated with pore size and structure. Hence, it is advantageous to analyze zeolites consisting of such narrow oxidic micropores by using argon gas as the adsorptive at 87 K. Argon fills micropores of dimensions 0.5-1 nm at much higher relative pressures of 10-5 to 10-3 compared to N2, which leads to accelerated diffusion and equilibration. This results in a much shorter analysis time and higher accuracy in data reduction. Unlike N2, Ar does not specifically interact with zeolite oxidic sites or ions, resulting in a clear correlation between micropore filling pressure and pore size. If liquid argon is not readily available, the CryoSync accessory allows one to easily achieve 87 K while using liquid nitrogen.
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