Separation of diamondoids in crude oils using molecular sieving techniques to allow compound-specific isotope analysis

Correlation of condensates with other condensates or oils has been a long standing problem due to the significantly depleted abundance of biomarkers in condensates. On the contrary, diamondoids are thermally stable and are present in relatively high abundance in highly mature oils and condensates. Hence, stable carbon isotope measurements of individual diamondoids using compound specific isotope analysis (CSIA) can potentially assist in solving this correlation problem (He, M., Moldowan, J.M., Nemchenko-Rovenskaya, A., Peters, K.E., 2012. Oil families and their inferred source rocks in the Barents Sea and northern Timan-Pechora Basin, Russia. American Association of Petroleum Geologists Bulletin 96, 1121-1146). However, CSIA analysis requires well resolved diamondoid concentrates, a daunting task due to the low concentration of diamondoids relative to non-diamondoids in oils and even condensates generated from carbonates and coals. The diamondoids co-elute substantially with the abundant iso- and cycloalkanes in gas chromatography (GC) chromatograms of oils. Here, isolation of adamantane and its methylated derivatives was achieved by way of concurrent desorption and adsorption processes in a column with two segments. The top segment contained 60 Å pore silica gel powder with a pre-adsorbed hydrocarbon fraction containing the diamondoid mixture. The bottom segment was filled with a microporous adsorbent - a form of a commercial zeolite, β CP814 E∗, heated at 550 °C for 13 h under open air conditions to give a uniform pore size of ca. 9.5 Å. With the larger micropore size (9.5 Å) than the molecular dimensions of adamantane (7.4 Å), molecular sieving to exclude diamondoids through the microporous adsorbent segment is induced by an entropy barrier arising from the mismatch between the bulky shape of diamondoid molecules and the elliptical shape of pore entrance in this adsorbent. It was also found that the separation was sensitive to operating temperature. The impact of pore size and shape of the microporous adsorbent on the quality of the diamondoid separation is addressed, as well as diamondoid abundance in the original hydrocarbon fraction.