Analyzing microporosity with vapor thermogravimetry and gas pycnometry

The complementary use of thermogravimetry and pycnometry is demonstrated to expand the toolbox for experimental micropore analysis <1 nm. Thermogravimetry is employed to assess the uptake of water, methanol, ethanol, 1-propanol and cyclohexane vapors in microporous structures at room temperature and derive quantitative micropore volumes and minimum pore entrance sizes together with qualitative information on surface chemistries. Pycnometry is employed to measure the uptake and adsorption of helium, argon and nitrogen gas in microporous structures at room temperature and derive semi-quantitative surface-to-volume ratios, surface areas and micropore cavity sizes and qualitative information on the surface chemistries. The method is validated and calibrated by applying it to a series of zeolites with known micropore structures. The results are compared with data from conventional N₂ adsorption at −196 °C and CO₂ adsorption at 0 °C. Main advantages of the demonstrated method are that diffusion limitations due to cryogenic temperatures are eliminated, adsorption is studied with non-polar gases, micropore cavity sizes are probed separate from micropore entrances and data can be interpreted in a straightforward fashion without requiring theoretical models on molecular behavior. Micropores <1 nm can thus be analyzed with increased accuracy as compared to conventional adsorption isotherm analysis.