Synthesis of Na-X zeolite from Longkou oil shale ash by alkaline fusion hydrothermal method

•High-crystallinity Na-X zeolite was synthesized from oil shale ash by the alkaline fusion hydrothermal method.•The optimal NaOH fusion temperature was 600 °C at ash to NaOH ratio 1:1.2 g·g−1 and solid-liquid ratio 1:4 g·mL−1.•The required corresponding crystallization temperature was 80 °C and crystallization time was 24 h.•Almost all quartz of oil shale ash was converted into silicate and aluminosilicate at fusion temperature above 600 °C.