A fast, economic, and green synthesis of benzimidazole derivatives using iron oxide modified sepiolite (IMS) as a catalyst has been reported. IMS showed excellent catalytic properties and the reactions completed within 20–30 min to give products in high yield. The adsorption mechanism of formic acid on IMS was studied by infrared (IR) spectroscopy at temperature range 120–400 °C. Thermal desorption of pyridine was followed by IR and thermal analysis techniques to estimate the acidity of IMS. Lewis acid-bound pyridine bands at 1,618–1,631 and 1,443–1,445 cm−1 were observed even after IMS sample were heated above 400 °C.