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.
The Argon at 87 K Advantage
Figure 1 shows the difference in pore filling pressure ranges for Ar (87 K) and N2 (77 K) adsorption in a faujasite type zeolite. In addition to being more accurate and faster than N2 adsorption (Ar experiments may take as little as half the time of N2 experiments). Ar is also capable of resolving small differences in pore filling pressure in zeolites of different pore size and geometry (Figure 2). Figure 2 shows zeolites of different pore size, some only as different as 0.1 nm, and the pressure for each pore filling step is at a distinct relative pressure. IUPAC recommendations clearly state the benefits of Ar (87 K) adsorption for the characterization of zeolites.
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