The catalytic effects of minerals on the formation of diamondoids from kerogen macromolecules

Abstract To investigate the catalytic effects of minerals on the formation of diamondoids, a series of anhydrous and hydrous pyrolysis experiments were conducted at 340 °C for 72 h on six kerogens of four types (I, II, II-S, and III) in the presence and absence of different minerals (montmorillonite K10, acidic aluminosilicate, kaolinite, illite, CaCO 3 , CaSO 4 , and S 0 ) and their mixtures in different ratios. Regardless of the accessibility of water, direct decomposition of kerogen upon thermal stress produces various quantities of diamondoids depending on the type of kerogen. Montmorillonite K10 and acidic aluminosilicate greatly promote the formation of diamondoids at 340 °C during kerogen pyrolysis because they are strong Lewis acids. Rearrangements of polycyclic carbonium ion intermediates at Lewis super-acid sites probably are responsible for the observed relatively large quantities of diamondoids produced at elevated temperatures in the presence of montmorillonite K10 or acidic aluminosilicate. In contrast, CaCO 3 appears to inhibit the formation of diamondoids. Kaolinite is a less active catalyst because the yield of diamondoids is only slightly elevated, while no catalytic effect is observed from illite. The presence of elemental sulfur appears to “poison” the catalytic activity of montmorillonite K10, but it is counteracted to some extent by the presence of CaCO 3 . The yield of diamondoids was slightly improved in the anhydrite catalytic reaction with kerogen in the presence of water. The addition of elemental sulfur to the kerogen of any type may initiate the C–C bond cleavage and is not favorable for diamondoid formation. The yield of diamondoids is very sensitive to the amount of montmorillonite K10 mixed with the kerogen up to a montmorillonite K10/kerogen ratio of ca. 15:1. At higher montmorillonite K10/kerogen ratios (⩾15:1), the accessible Lewis sites at montmorillonite K10 might be completely saturated with the organic precursors of diamondoids from thermal degradation of kerogen.