Influence of Acid–Base Synthesis Conditions on Metal-Oxide Catalysts for Sulfur Compound Removal from Natural Gas

Ethyl mercaptan is one of the most persistent sulfur-containing impurities in natural gas and requires efficient adsorbent/catalyst systems for removal under mild conditions. In this study, two types of halloysite-supported metal oxide catalysts, namely Mn- and Cu-based catalysts, were prepared via reflux-wet impregnation using three alkaline precipitation agents (NaOH; Ca(OH)2; Ba(OH)2), with and without the use of acetic acid added during the synthesis process. The objective was to evaluate how acidic and alkaline synthesis environments influence catalyst structure, surface chemistry, active metal distribution, and ethyl mercaptan adsorption performance. Characterization techniques used to examine the catalysts included Brunauer–Emmett–Teller surface area analysis (BET), X-ray diffraction (XRD), Fourier-transform infrared spectroscopy (FTIR), scanning electron microscopy (SEM), energy dispersive spectroscopy (EDS), and X-ray photoelectron spectroscopy (XPS). The characterization results showed that halloysite (support) possessed a mesoporous structure with a surface area of 51.2 m2/g, while metal incorporation modified the pore structure, surface composition, crystallinity, and morphology of the support. Acid addition during synthesis produced base-dependent effects: it improved the textural properties of Mn-Ca(OH)2 and Cu-NaOH catalysts but generally reduced active metal surface concentration, modified hydroxyl environments, and increased structural disorder in several systems. XRD and XPS studies suggested that Cu catalysts were comprised primarily of well-defined CuO/Cu2O surface domains, while Mn catalysts were mostly amorphous or poorly crystallized MnOx species. Adsorption testing using 200 ppm ethyl mercaptan in methane demonstrated that several synthesized catalysts significantly outperformed the commercial catalyst, which showed a breakthrough time of approximately 1200 min. The best Mn catalyst was Mn-Ba(OH)2 synthesized without acid, reaching approximately 3600 min. The Cu catalysts showed superior performance overall, with Cu-Ca(OH)2 synthesized without acid achieving the highest breakthrough time of approximately 8550 min, corresponding to more than a six-fold improvement over the commercial catalyst. These findings demonstrate that catalyst performance is governed not by surface area alone, but by the combined effects of active metal chemistry, precipitation environment, surface composition, crystallinity, metal–support interactions, and accessibility of adsorption-active sites.