FULLERENES FROM KEROGEN BY LASER-ABLATION FOURIER-TRANSFORM ION-CYCLOTRON RESONANCE MASS-SPECTROMETRY

Raw oil shale, kerogen (demineralized shale) and carbonaceous residues from kerogen pyrolysis in the range 350-700-degrees-C (at 50-degrees- C intervals) were studied by laser ablation Fourier transform ion cycl otron resonance mass spectrometry using the fundamental frequency of N d:YAG laser (1064 nm). Normally, pyrolysis of the raw materials produc es oil and the resulting residues have decreased hydrogen to carbon ra tios and exhibit relative increases in aromatic carbons. Raw shale and kerogen give positive-ion spectra with mainly protonated species of m /z 100-400. Laser ablation positive-ion mass spectra of the pyrolysis products of the kerogen show the presence of C60, C70 and other fuller ene ions with a distribution of higher mass fullerene ions up to m/z 4 000. Using high laser powers (100-3000 MW cm-2), the residue from pyro lysis at 350-degrees-C initially did not produce any fullerene ions (a part from traces of C60 and C70), but after continued ablation a cavit y was formed in the target and a wide distribution of fullerene ions w as obtained with subsequent laser pulses. Residues obtained from the p yrolysis of kerogen at 400-500-degrees-C produced fullerene ions at bo th low (4-200 kW cm-2) and high laser powers. The 550-degrees-C pyroly sis residue gave only small amounts of C60 and C70 positive ions at lo w laser power whereas residues from the pyrolysis of kerogen above 550 -degrees-C did not give fullerene ions over a wide range of laser powe rs. It is proposed from the above results that the changes in the arom atic nature of the kerogen residues with increasing pyrolysis temperat ure are directly related to the ease of fullerene formation. This is p ossibly due to the formation of large polycyclic aromatic systems at p yrolysis temperatures above 400-degrees-C, formed in the residues. It should be noted that the shale samples (raw or pyrolysed) did not gene rate fullerene ions under any of the conditions employed in these expe riments.