Advanced Analysis of Clay Microporosity Using High-Resolution N₂-Ar Adsorption Isotherms Coupled with the Derivative...

Area of Science:

• Materials Science
• Surface Chemistry
• Clay Science

Background:

• Understanding clay textural properties is crucial for applications in adsorption, catalysis, and separation.
• The influence of cation exchange on clay porosity, particularly microporosity, requires detailed characterization.
• Distinguishing between intra-particle and external surface adsorption sites is key to interpreting clay behavior.

Purpose of the Study:

• To characterize the textural properties of sodium and tetramethylammonium (TMA+) exchanged clays.
• To quantify microporous and mesoporous volumes and specific surface areas.
• To investigate the contributions of different adsorption sites (intra-particle, basal, lateral) to overall clay porosity.

Main Methods:

• Physisorption of nitrogen (N2) and argon (Ar) at cryogenic temperatures.
• Application of Brunauer-Emmett-Teller (BET) and t-plot models for surface area and pore volume determination.
• Utilizing the Derivative Isotherm Summation (DIS) method, with modifications, to analyze high-resolution Ar isotherms and quantify adsorption site energies and surface fractions.

Main Results:

• TMA+ cation exchange significantly increased the microporosity of clays compared to their sodic forms.
• High-energy adsorption sites at low pressures were attributed to intra-particle microporosity from stacking defects or open inter-layer spaces.
• The modified DIS method improved the analysis of adsorption site contributions and experimental data fitting.

Conclusions:

• Interlayer expansion induced by TMA+ exchange is the primary cause for increased microporosity in clays.
• The study successfully differentiated adsorption contributions from intra-particle sites versus basal and lateral surfaces.
• Advanced physisorption analysis provides valuable insights into the complex porosity of modified clay materials.