Synthesis and characterization of metals loaded fibrous mordenite zeolite for carbon monoxide methanation

Abstract

Carbon monoxide (CO) methanation is one of the most viable and sustainable ways for methane (CH4) production to replace fossil fuels (coal, petroleum, and natural gas) and alleviate the adverse environmental impacts of carbon-intensive industries. Thermodynamically, CO methanation is a feasible reaction which can proceed at low temperatures. However, to meet the requirements of reaction kinetics of CO methanation, a suitable and highly active catalyst is mandatory for high CH4 yield. In this study, fibrous zeolites were successfully synthesized through the microemulsion method using commercial zeolites, namely mordenite (MOR), ZSM-5, and beta zeolite (BEA) as seed. All the zeolite samples were characterized by different techniques, including X-ray diffraction (XRD), nitrogen physisorption, field emission scanning electron microscopy (FESEM), transmission electron microscopy (TEM), fourier-transform infrared spectroscopy (FTIR), pyrrole adsorbed FTIR, hydrogen temperature-programmed reduction (H2-TPR) and electron spin resonance (ESR) spectroscopy. The synthesized fibrous mordenite (FSMOR), fibrous ZSM-5 (FSZSM-5) and fibrous beta zeolite (FSBEA) were compared with MOR, ZSM-5 and BEA zeolites to study their activity towards CO methanation. The CH4 yield was found in the order of FSMOR (50%) > FSZSM-5 (44%) > FSBEA (41%) > MOR (37%) > BEA (25%) > ZSM-5 (21%) at 450oC. The catalytic activity of the synthesized zeolites was strongly correlated to the existence of mesoporosity, inter- and intra-particle pores, intrinsic basic sites, and oxygen vacancies. The fibrous mordenite (FSMOR) displayed superior catalytic activity among all zeolites as a result of the high basicity and oxygen vacancies. To further enhance the catalytic activity, transition metals including Fe, Co, Ni, Ru, Pd, and Ag were loaded on FSMOR by the wet impregnation method. It was found that the transition metals loading significantly improved the catalytic activity towards CO methanation. The Ru-FSMOR unveiled a superior CH4 yield of 78% at 400oC compared to the other catalysts, in the order of Ru-FSMOR > Ni-FSMOR > Co-FSMOR > Ag-FSMOR. The catalytic performance of the Ru-FSMOR was boosted because of the high reducibility of well-dispersed Ru nanoparticles (Ru-NPs) and the synergistic effect between the Ru-NPs and oxygen vacancies in the FSMOR support. The FSMOR and Ru-FSMOR revealed high stability and suppressed the coke formation caused by the undesired side reactions during CO methanation. Moreover, in the proposed reaction mechanism of CO methanation, it was discovered that the FSMOR and Ru-FSMOR followed an associative reaction pathway via linearly adsorbed CO* as an essential intermediate, dissociated into adsorbed C* to form methane by hydrogenation. For FSMOR, the oxygen vacancies conducted the activation of CO and H2 into C* and H* during methane formation. Whereas for Ru-FSMOR, the active Ru phase conducted the activation of H2 and CO molecules followed by migration onto the FSMOR surface to form adsorbed CO* and adsorbed H*. The adsorbed CO* appeared in two forms, namely linear and bridged forms. Based on the above observations, this work provides fundamental insights into the robust catalytic system relating to CO methanation using zeolite-based catalysts with unique fibrous morphology, which can potentially be applied to produce substitute natural gas on a commercial scale.