Direct pyrolysis of rock asphalt from Buton Indonesia: characterization of liquid product and the role of calcite

Buton rock asphalt (RA) majority composed of a mixture of calcite and petroleum-like material that is abundantly available in Buton, Indonesia. Direct pyrolysis of RA reveals the catalytic role of calcite in efficiently transforming the fused aromatic rings of asphaltene in RA into diesel-range hydrocarbons. Pyrolysis at 400 °C for 180 min produced a 74% liquid product yield, significantly higher than that obtained from pyrolysis of decalcified RA (22.9%), or decalcified RA using commercial CaCO₃ (34.2%). The resulting liquid hydrocarbon contains 70.30% diesel fraction, 25.72% heavy oil, and 3.98% gasoline, and exhibits a higher cetane number (69) than commercial diesel fuel. These findings highlight the importance of strong calcite/asphaltene interactions for efficient heat transfer during hydro-dearomatization of aromatic rings. Furthermore, the high thermal stability and strong basicity of calcite catalyze the cracking process into diesel-range hydrocarbons. This study offers a roadmap for the efficient conversion of low-quality Buton rock asphalt into high-quality diesel fuel through direct pyrolysis at 400 °C.