AmericanChemicalSociety.com

The principal investigator wrote a proposal to National Synchrotron Light Source (NSLS) at Brookhaven National Laboratory (BNL) for use of an x-ray beamline that would span the K-edge energy of the sulfur atom. The proposal was submitted on January 31, 2010. During June, 2010, two students from Rose-Hulman Institute of Technology (RHIT), Chad Wine, and Sean Gorsky, accompanied the principal investigator (PI), Sudipa Mitra-Kirtley, to the NSLS, and spent two and a half days gathering x-ray absorption data on the fossil-fuel samples. These samples were provided by Andrew Pomerantz, of Schlumberger-Doll Research, Boston, MA; the set consisted of several coal-derived oil and asphaltene samples, and three humic substances. 

The particular beamline used at NSLS was X-19A, which spans an energy range of 2.1-17 KeV, perfect for sulfur K-edge studies (K-edge energy is around 2472 eV). This beamline is equipped with torroidal and spherical mirrors to collimate and focus the beam. The monochromator at X19A slews through the energy increments, and is made of Si[111] crystals in a double-crystal configuration. The wavelengths of the incident radiation are comparable to the bond lengths of crystals, and dispersion is achieved by Bragg reflection.  A 10 micron thick Beryllium window separated the sample hutch from the beamline. The sample chamber was purged with helium gas to reduce x-ray absorption. A Passivated Implanted Planar Silicon (PIPS) detector, made by Canberra, was used for the studies. This detector was used in the fluorescence mode, and bulk observations were made. A Pentium PC with Windows operating system software was used to change beamline parameters, and to collect the XANES data. The samples were mounted on thin flexible, self-adhesive parafilm. The oils and the humic substances were made into thin films and then mounted on the film. In the case of asphaltenes and some oils, the samples were first dissolved in toluene, and then mounted on the film. 

After collection of the data, the RHIT students and the PI brought back the data to RHIT campus, and analyzed the data using different approaches. All three used the following procedure:

1.     Each model spectrum was fitted to a number of resonances and a single arc-tangent step.

2.     Each fossil fuel sample spectrum was fitted with several resonances occurring at the same energy positions as the major resonances of the model spectra. In the process, the width and the energy position of the resonances were held to be the same as the major resonances of the model spectra. The arc-tangent step was held fixed to one set of values (all position, width, and height variables) for all reduced forms of sulfur models, and to a different position (but same height and width) for the sulfone and sulfate model. In some of the fossil-fuel samples, where there was considerable oxidized (particularly sulfone and sulfate) forms of sulfur, two arc-tangent steps were used, one representing the reduced forms, and the other oxidized forms.

3.     Percentages of the different sulfur forms were extracted using the values for the areas under the resonances, obtained from the fitting procedure.

4.     A new spectrum was generated by summing the different percentages of the model spectra (and an arc tangent step) for each unknown fossil fuel sample. This spectrum was then overlaid on the actual fossil fuel spectrum to observe discrepancies.  Excel spreadsheet program was used to do this part of the analysis.

Chad Wine used the software WinXAS for the fitting procedure, steps 1-3. He tried different profiles, including Lorentzian, Gaussian, and Pseudo-Voigt, as the software provided. Sean Gorsky used a software package called Athena, which has been developed at the University of Chicago, and is available on the internet, for the same steps. The difficulty with this program was that only a limited number of resonances can be fitted at one time.  Therefore, multiple fits were done on the same sample, using different portions of the spectra. The PI oversaw the entire analysis procedure of Chad and Sean. She also used WinXAS to fit all the data, both the sulfur model set and the fossil fuel set, and compared the sulfur model values to the ones performed earlier. Numerous combinations of resonances were tried, and she used both Chad's and Sean's data to regenerate the fits in WinXAS. Many older sulfur model and fossil-fuel files were tried to obtain reproducibility.

Pyrite, organic sulfide, thiophene, sulfoxide, sufone, and sulfate forms of sulfur were investigated in all the fossil fuel samples. All the fossil fuel samples showed thiophene, and/or sulfide.  For Tanito Harum coal sample, the oil shows pyrite, and the sulfoxide content is less than in the asphaltene. In the Wyoming coal fractions, the oil shows more sulfoxide than the asphaltene. Both the samples also show significant sulfone and sulfate structures. The sulfoxide component in the Adora coal oil and asphaltene fractions are comparable, and although both the samples show both sulfone and sulfate components, the oil is richer in sulfate. No large differences between the thiophene to sulfide ratios are found between the oil and the asphaltene fractions. In the fulvic acid reference sample, humic acid standard, and the Suwanee River sample, large quantities of sulfone, sulfate, and pyrite are noted.

The next step of the project will be to investigate the coal fractions (extracted oil and asphaltene) as a function of coal rank. This work will be the first of its kind in investigating coal oils and asphaltenes, and comparing the results to petroleum products.  Another branch of this project will aim towards different kerogen samples, subjected to various degrees of thermal maturation treatments, and also to different oxidation treatments.