Distribution of Polycyclic Aromatic Sulfur Heterocycles in Three Saudi Arabian Crude Oils as Determined by Fourier Transform Ion Cyclotron Resonance Mass Spectrometry Saroj K. Panda, ²,‡ Wolfgang Schrader, § Adnan al-Hajji, | and Jan T. Andersson* ,² Institute of Inorganic and Analytical Chemistry and NRW International Graduate School of Chemistry, UniVersity of Mu ¨nster, Corrensstrasse 30, 48149 Mu ¨nster, Germany, Max-Planck-Institut fu ¨r Kohlenforschung, Mu ¨lheim a.d. Ruhr, Germany, and Chromatography Unit, R & D Center, Saudi Aramco, Saudi Arabia ReceiVed October 12, 2006. ReVised Manuscript ReceiVed December 22, 2006 The distribution pattern of polycyclic sulfur aromatics was established in three Saudi Arabian crude oils using a combination of liquid chromatography on a bonded Pd(II) phase and Fourier transform ion cyclotron resonance (FT-ICR) mass spectrometry. The highest double bond equivalent (DBE) measured was 17 for the Arabian heavy crude oil and 14 for both Arabian medium and Arabian light crudes. The high DBE number indicates the presence of a large variety of parent ring systems; knowledge of them is important in the characterization of crudes with respect to their processing properties. A higher sulfur concentration was correlated with more alkyl carbons in the aromatic compounds and with a higher DBE. A representation of the mass spectral data in a pseudogram, a form that resembles a chromatogram, is introduced to facilitate the quick visual inspection and comparison of the mass spectral data. Introduction Due to the increasingly rare availability of low-sulfur crude oils, high-sulfur crudes are gaining ground for the production of low-sulfur fuels. Refineries can achieve this only through efficient desulfurization processes that lower the sulfur content to below the legal limits regulating transportation and heating fuels in many countries. Although many processes have been suggested and tested for desulfurization purposes, in practice, hydrodesulfurization (HDS), involving elevated temperatures and hydrogen pressures in the presence of, typically, Ni/Mo and/or Co/Mo catalysts, is the generally used process. Although it is quite effective for low-boiling and middle distillates such as gas oils, it has not yet been shown to operate as efficiently for high-boiling fractions like vacuum gas oils (VGO) or vacuum residues. 1 The reasons for this are at present not fully understood, but one hypothesis is that molecular characteristics influence the ease of removal of the sulfur. The polycyclic aromatic sulfur heterocycles (PASHs) are known to be a major chemical class of sulfur-containing compounds in crudes, and as the sulfur concentration usually goes up with the boiling range of the distillation fractions, it can be expected that the PASHs are predominant species in fractions of higher boiling points. Among the molecular characteristics that may have an influence on the ease of desulfurization are, in analogy to the known reactivity of more volatile compounds, the parent aromatic structures and the respective positions of alkyl groups. Using standard compounds, it has been established that the resistance to hydrodesulfurization increases in the following order: thiophenes < benzothiophenes < benzonaphthothiophenes < tetrahy- drobenzonaphthothiophenes < dibenzothiophenes. 2 A more recent investigation on VGO extended the observations to include a finer gradation in the difficulty of hydrodesulfurization as follows: nonaromatic sulfides < thiophenes ∼ ben- zothiophenes , catacondensed (“extended”) five-ring thiophenes ∼ six-ring thiophenes < benzonaphthothiophenes ∼ pericon- densed (“compact”) five-ring thiophenes < C 0 /C 1 -diben- zothiophenes < C 2+ -dibenzothiophenes < (pericondensed) phenanthrothiophenes. 3 It has also been reported that it is evident that the alkylation pattern has an influence on the resistance to HDS. Thus among the dibenzothiophenes of middle distillates, substituents in the 4 and/or 6 positions confer a high degree of stability to the compounds under HDS conditions and these accumulate in the desulfurized product. 4-6 Recently, it was also noted that a methyl group in the 1 position of dibenzothiophene can have a similar effect. 7 Structural studies of the PASHs present in a crude oil * Corresponding author. E-mail: anderss@uni-muenster.de. ² Institute of Inorganic and Analytical Chemistry, University of Mu ¨nster. ‡ NRW International Graduate School of Chemistry, University of Mu ¨nster. § Max-Planck-Institut fu ¨r Kohlenforschung. | Saudi Aramco. (1) Mochida, I.; Choi, K. An overview of hydrodesulfurization and hydrodenitrogenation. J. Jpn. Petrol. Inst. 2004, 47, 145-163. (2) Nag, N. K.; Sapre, A. V.; Broderick, D. H.; Gates, B. C. Hydrodes- ulfurization of polycyclic aromatics catalyzed by sulfided CoO-MoO3/γ- Al2O3: The relative reactivities. J. Catal. 1979, 57, 509-512. (3) Choudhary, T. V.; Malandra, J.; Green, J.; Parrott, S.; Johnson, B. Towards clean fuels: Molecular-level sulfur reactivity in heavy oils. Angew. Chem., Int. Ed. 2006, 45, 3299-3303. (4) Ma, X. L.; Sakanishi, K. Y.; Mochida, I. Hydrodesulfurization reactivities of various sulfur-compounds in diesel fuel. Ind. Eng. Chem. Res. 1994, 33, 218-222. (5) Ma, X. L.; Sakanishi, K.; Mochida, I. Hydrodesulfurization reactivities of various sulfur compounds in vacuum gas oil. Ind. Eng. Chem. Res. 1996, 35, 2487-2494. (6) Schade, T.; Roberz, B.; Andersson, J. T. Polycyclic aromatic sulfur heterocycles in desulfurized diesel fuels and their separation on a novel palladium(II)-complex stationary phase. Polycyclic Aromat. Compd. 2002, 22, 311-320. 1071 Energy & Fuels 2007, 21, 1071-1077 10.1021/ef060511s CCC: $37.00 © 2007 American Chemical Society Published on Web 02/06/2007