Aluminum phosphate-modified red clay as a stable nanocomposite catalyst for the selective conversion of methanol to dimethyl ether

Abstract

Abstract Aluminum phosphate-modified Egyptian red clay (AlPO 4 /ERC) nanocomposites were developed as low-cost solid acid catalysts for methanol dehydration to dimethyl ether (DME). Structural and physicochemical analyses (XRD, FTIR, N₂ adsorption–desorption, TEM, SEM, and acid-probe reactions) show that AlPO 4 incorporation preserves the aluminosilicate framework while tuning surface acidity, porosity, and active-site accessibility. XRF and XRD confirm that ERC consists mainly of quartz, kaolinite, and iron oxides, whereas AlPO 4 introduces additional phosphate phases that interact with the clay matrix. Catalytic testing reveals a strong dependence on AlPO 4 loadings, with 5 wt.% AlPO 4 /ERC exhibiting optimal performance and achieving 90% methanol conversion with 100% DME selectivity at 200 °C. This enhancement correlates with an optimal balance of weak and intermediate acid sites (~ 2.5 mmol g −1 ) and a high specific surface area (52 m 2  g −1 ), which maximizes the number of accessible active centers. Higher loadings reduce activity because of pore blockage and decreased site accessibility. Mechanistically, methanol dehydration proceeds via acid-catalyzed surface methoxy intermediates, with activity governed by acid-site distribution rather than iron species. The catalyst shows excellent stability over 15 days without deactivation, confirming its strong structural robustness. Overall, controlled AlPO 4 incorporation optimizes the acidity-porosity synergy, enabling efficient and stable DME production.