Synthesis of Desilicated Chabazite for Non-Oxidative Conversion of Methane (NOCM)

Non-oxidative conversion of methane (NOCM) into value-added longer-chain hydrocarbons facilitates the transport of natural gas resources from remote drilling locations to centralized chemical plants and refineries. The NOCM reaction is hypothesized to be catalyzed by edge-defect sites on carbon-based supports at high (>1100 K) temperatures to overcome thermodynamic constraints. Desilicated chabazite (CHA) zeolite is one material proposed to form large densities of these edge defects through carbon deposition. The procedure to synthesize desilicated CHA involves preferentially removing silicon from the crystalline framework through an alkaline (NaOH) treatment while preserving the structure-directing agents (SDAs) occluded within microporous voids during crystallization to mitigate pore collapse. This selectively creates mesopores at crystallite exteriors to increase the accessible surface area for carbon formation during NOCM. The crystallinity and pore volume of the resultant materials were studied via X-ray diffraction and gas adsorption characterization. Broad applicability of desilicated CHA as a catalyst necessitates maintaining the number of Brønsted acid sites throughout the synthesis process, which is quantified through ammonia titration and temperature-programmed desorption (TPD). The structural stability of desilicated CHA depended strongly on the Si/Al ratio of the parent material and the temperature used for SDA removal. TPD results revealed an increase in the amount of ammonia desorbed at lower temperatures in mesoporous materials, possibly indicating the creation of weaker-binding, under-coordinated acid sites around regions of mesopore formation. These results collectively indicate a complex relationship between synthesis conditions and material stability, with application to NOCM necessitating stability analysis at conditions best suited for the reaction.