Separation of the major components of landfill and natural gas by four adsorbent materials was investigated, and a high selectivity was obtained in some materials. The adsorbents, prepared from a natural clay, were two porous clay heterostructures (PCH) obtained by use of tetraethoxysilane (TEOS) and phenyltriethoxysilane (PhOS), and two other PCH prepared by use of 3-aminopropyltriethoxysilane (APTES) and TEOS. A pillared clay, with aluminum oxide pillars, was used also. CH₄, C₂H₆, and CO₂ high-pressure adsorption isotherms (measured up to 1000 kPa) were used to estimate the selectivity and phase diagrams for separation of binary mixtures of these gases. The significant differences found between the materials were related to the surface chemistry. The material modified with APTES presents the highest CO₂ affinity, suggesting a strong interaction between this gas and the amine group. The new materials presented interesting properties regarding CO₂/CH₄ separation. The porosity and apparent specific surface areas of the samples were characterized by low-temperature nitrogen adsorption. The samples were also characterized by diffuse reflectance infrared Fourier transform (DRIFT) spectroscopy and ¹³C, ²⁹Si, and ²⁷Al magic-angle spinning nuclear magnetic resonance (MAS NMR). These techniques confirmed the presence of organic groups in the organic/inorganic hybrid samples and provided insights into the Al coordination.