Chemistry of organically bound sulphur forms during the mild oxidation of coal

Short

Chemistry oxidation

of organically of coal

bound

sulphur

forms

during

Communications

the mild

Martin L. Gorbaty, Graham N. George and Simon R. Kelemen Corporate Research Laboratories, Exxon Research and Engineering Company, Annandale, NJ 08801, USA (Received 7 February 1990; revised 30 April 7990)

X-ray absorption near edge structure (XANES) spectroscopy and X-ray photoelectron spectroscopy (XPS) were used to study the chemistry of organically bound sulphur species in coal during mild coal oxidation. Spectra of preserved and oxidized samples were obtained and compared. Both techniques indicate that sulphide sulphur forms are converted to oxidized forms while the thiophenic forms remain largely untouched under these experimental conditions. (Keywords:

oxidation;

sulphur;

chemistry)

X-ray absorption near edge structure (XANES) spectroscopy and X-ray photoelectron spectroscopy (XPS) are two techniques which have been applied recently for the direct speciation and approximate quantification of organically bound forms of sulphide and thiophenic sulphur in non-volatile liquid and solid carbonaceous materials 1-6. XANES spectroscopic data are obtained on the entire sample, while almost the entire XPS signal originates from the first 50 A of surface. Consequently, the data from both approaches are complementary and use of both sets allows not only characterization of the sulphur forms as they exist in resources of interest, but also in reaction products, thus providing insights into the chemical reactivity of organic sulphur forms under different processing conditions. This paper describes use of these approaches to determine the fate of organically bound sulphur in coal during mild oxidation.

I

Rasa Coal

172.0

168.0

164.0

160.0

Binding Energy (eV) I

EXPERIMENTAL

Oxidized

A sample of Rasa coal was used in this study. It was chosen because it has an unusually high amount of organically bound sulphur (12 wt%), and an unusually low level of pyritic sulphur7*8. In addition, it has been determined previously by XPS and XANES (neglecting slight distortions of the XANES data due to thickness effects] that this coal sample contains 30 mol% sulphide and 70 mol% thiophenic sulphur3-6. The coal, sized to pass a 100 mesh screen, was spread in a thin layer in a large crystallizing dish and exposed to air (ambient relative humidity x60%) in an oven kept at 125 f 0.2”C. Small samples were withdrawn at various times, placed in a nitrogen filled dry box, purged in nitrogen, stored in nitrogen filled bottles, and examined by XANES and XPS. Detailed experimental procedures for XANES and XPS have been described 00162361/90/08106543 0 1990 Butterworth-Heinemann

Ltd.

5 Days 125% Thiophenic -Like

I 160.0

164.0

168.0

I 172.0

Binding Energy (eV) Figure

1

XPS spectra

of Rasa coal: a, as-received;

b, oxidized

FUEL, 1990, Vol 69, August

1065

Short Communications Third derivatives of previously1*3v4. XANES spectra allow resolution and approximate quantification of sulphide and thiophenic sulphur forms in samples which mixtures of these contain species1,2,5,6. Peaks at about 2469.8 eV are indicative of sulphides, and those at about 2470.4 eV of thiophenes. XPS spectra were curve-resolved using peak values fixed at 163.3 eV for sulphides and 164.1 eV for thiophenes3s4. In addition, individual contributions of different oxidized sulphur forms were determined from XPS spectra using idealized single component sulphur line shapes with peaks at 166.0 eV, 168.2 eV and 169.0 eV, of model compounds characteristic containing sulphoxides, sulphonesg*” and sulphonic acidsg. RESULTS

AND DISCUSSION

XPS analysis The sulphur to carbon (S/C) atomic ratio of the as-received coal determined by XPS (0.062) was in good agreement with that determined by elemental analysis (0.060), indicating that surface and bulk sulphur concentrations were quite similar. Upon oxidation at 125°C for 5 days, there was a substantial increase in the oxygen to carbon (O/C) atomic ratio as determined by XPS. There was much less of an increase in the O/C atomic ratio in samples held for 5-30 days. This behaviour is similar to that previously reported I1 for Illinois no. 6 coal, in which a slowing of the accumulation of surface oxygen was observed after the first few days of oxidation at 125°C. The XPS data for oxidation of Rasa coal are shown in Table 1. A representative XPS sulphur 2p spectrum of as-received Rasa coal is shown in Figure la. Figure 2 shows an example of the calculated summation spectrum for unoxidized forms and the actual spectrum of fresh Rasa coal. The summation peak generated in all cases is within the scatter of the experimental data. The contribution due to oxidized forms is not included in Figure 2, and the lack of overlap in this region illustrates the representative noise in the data. The very low level of oxidized forms in the

K

._.i, 160

of air oxidized

Rasa coal at 125°C Mole per cent

Atomic ratio

(x 100) Sample

O/C

SIC

Sulphidic (163.3 eV)

Initial 5 days 15 days 30 days

5.7 20.4 20.8 23.1

6.2 6.8 6.9 6.3

26 I 7 5

Thiophenic (164.1 eV)

Sulphoxide (166.0 eV)

70 69 68 65

1

1

4 4 5

8 8 9

fresh coal sample is also evident from this figure. The values found in Table 1 for sulphonic and sulphone groups in the fresh sample represent an upper limit, as some signal intensity in this energy region may originate from a n to Z* shake-up signal”. Air oxidation for 5 days at 125°C changes the XPS sulphur 2p spectrum of Rasa coal as shown in Figure lb. The presence of oxidized sulphur forms is apparent at high binding energies and the sum of all oxidized forms comprises up to 24% of the total sulphur on the surface of Rasa coal after 5 days at 125°C. The individual contributions of the different oxidized sulphur forms were determined by curve resolution of the spectra, and the results are also shown in Table 1. The concentration of organic sulphur forms on the surface remains about the same following oxidation in air at 125°C for up to 30 days, as shown by the S/C atomic ratios (Table I ). However, upon oxidation, the distributions change significantly: the amount of thiopheniclike components remains nearly constant, but the sulphidic components drop dramatically and are replaced by oxidized forms. The changes were most dramatic for the first 5 days, again indicating a slowing of the oxidation rate at longer exposure times. XANES analysis Figure 3 shows three XANES spectra for as-received and oxidized samples of Rasa coal. The intensities of these spectra are normalized to the height of the edge jump, obtained by extrapolating the EXAFS spline function to the edge, and are thus directly comparable. The dashed line corresponds to as-received and (as shown by XPS) virtually unoxidized Rasa

Sulphone (168.2 eV)

Sulphonic acid (169.0 eV) 2 12 13 15

coal. The dotted line in the figure corresponds to a sample oxidized for 5 days, and the solid line to a sample oxidized for 30 days. The initial sulphur absorption K-edge appears at about 2470 eV, and a second less sharp feature appears at about 2478 eV. The latter has been attributed” largely to shape resonance absorptions, but the possibility that this peak could arise from oxidized sulphur species was not ruled out. From Figure 3, it is clear that the intensity of the spectral feature centred at about 2478 eV increases dramatically following air exposure at 125°C for 5 days. Just as found by XPS, the changes seen by XANES are less substantial on further exposure for 30 days. The 2478 eV feature is undoubtedly due to contributions from oxidized sulphur species (in addition to shape resonances). The third derivatives of the XANES spectra are displayed in Figure 4. The feature at 2469.8 eV in the as-received sample (sulphide) decreases in intensity with increasing oxidation time, and disappears in the sample held for 30 days. The peak at 2470.4 eV also decreases, but is still quite prominent. A small peak at 2469.1 eV appears in the third derivative of the sample oxidized for 30 days. This is probably due to either elemental sulphur or disulphide. (The presence of elemental sulphur could go undetected by XPS analysis because its binding energy is quite close to that of thiophenic sulphur3s4). Taken together, these data suggest that sulphide sulphur is preferentially attacked to produce oxidized forms such as elemental sulphur, sulphoxides, sulphones and sulphonic acids on mild while thiophenic sulphur oxidation, remains largely untouched. These results are in complete agreement with the XPS

AcuaI XPS S”l‘W /,2P) spectrum forFresh Aa*a Caal

k, 164 168 Eliming Energy (ev,

Figure 2 Comparison of the calculated summation peak for sulphidic and thiophenic forms with the actual XPS spectrum of fresh

Rasa coal

1066

XPS analysis

I CalCYlated S”mm~*io” Pea* For s”l,i*ic and ThlOphenlC speaes

I =? 2 .i ” & ; = z : P

Table 1

FUEL, 1990, Vol 69, August

Figure 3 XANES spectra oxidized Rasa coal

of as-received

and Figure 4

Third derivatives

of XANES spectra

Short Communications data, and show that the thiophenic-like sulphur components in Rasa coal are much less reactive toward oxidation than sulphide forms. Quantifications of oxidized sulphur forms are not possible by XANES at this point, because calibrations for a system containing oxidized and unoxidized sulphur forms have not yet been established. The feature in the XANES spectrum at about 2478 eV is relatively broad, so its intensity is considerably diminished in the third derivative plot. Previous XPS studies on coal oxidation in air showed that there is a fairly close correspondence between the degree of oxidation at the external coal surface and the bulk of the sample’ ‘.l ‘,i4. While the total amounts of unoxidized and oxidized sulphur forms were quantified in these studies, problems associated with interference from inorganic species’ ‘,I3 and poor signal to noise ratios’ ‘,i3,i4 precluded detailed analysis of organic sulphur forms. In this work, both XPS and XANES analyses show the disappearance of sulphide sulphur, indicating that substantial bulk oxidation has taken place along with surface oxidation. An oxidation approach to distinguish between different forms of organically bound sulphur by XPS has been reportedg. Performic acid in methanol was used as the reagent on a vitrinite sample from a high volatile bituminous coal. The complete disappearance of the sulphur 2p 164 eV peak and the appearance of a peak at 167.9 eV was taken to mean that sulphones were the major product of the oxidation and that they arose from sulphides and thiophenes. Tailing at higher binding energies suggested that low levels of sulphonic acids were also formed. Since thiophenes survive mild air oxidation, the performic acid oxidation procedure is judged to be more severe than air oxidation, and it is not possible to distinguish between sulphides and thiophenes by that procedure. The XPS data in Table 1 lead to some interesting speculations concerning the forms of organically bound sulphur in Rasa coal. About half of the oxidized sulphur product appears in the form of sulphonic acids. From studies on pure compounds, it is known that sulphonic acids are oxidation products of either

mercaptans or disulphides, while sulphides produce either sulphoxides or sulphones15116. Ignoring the possibility that some new and unprecedented oxidation chemistry is taking place within the coal, the pure compound and coal oxidation results could mean that a significant fraction of what XPS and XANES techniques identify as ‘sulphide sulphur’ in unoxidized Rasa coal is actually mercaptans and/or disulphides. In support of the speculation that mercaptans and/or disulphides may be present in the as-received coal, it is noted that mercaptans (R-SH) and sulphides (R-S-R) are not distinguishable by XANES’, or sulphides and disulphides by XPS3*4. The detection limits of both methods are such that less than about 10% of either mercaptan or disulphide forms would probably go undetected as such, and be reported as sulphides. For example, 7% mercaptan sulphur and 7% disulphide sulphur would be counted as 14% ‘sulphide’, but would appear after oxidation as 14% sulphonic acids. However, chemical tests carried out by Ignasiak et al.’ ruled out the presence of mercaptans in their sample of Rasa coal. This speculation will be tested by derivatization by base catalysed Salkylation using i3C labelled methyl iodide” followed by solid state i3C n.m.r. analysis.

trations than pristine samples. especially true for XANES.

is

ACKNOWLEDGEMENTS X-ray absorption spectra were recorded at the Stanford Synchrotron Radiation Laboratory, which is funded by the Department of Energy, under contract DE-AC03-82ER-13000, Office of Basic Energy Sciences, Division of Chemical Sciences and the National Institutes of Health, Biotechnology Resource Program, Division of Research Resources. The authors wish to thank P. J. Kwiatek for technical assistance in obtaining the XPS spectra and C. White (PETC) for providing a sample of Rasa coal. REFERENCES 1

2

3 4 5

CONCLUSIONS XPS and XANES spectroscopies have shown that organically bound sulphide sulphur in coal is preferentially converted to oxidized forms compared with thiophenic sulphur under mild oxidation and that the oxidation conditions, process proceeds in a similar way at the surface and in the bulk. The broad feature in the XANES spectra at about 2478 eV may be attributable largely to oxidized sulphur forms. Further work is necessary for quantification of the effect by XANES, but in principle it should be possible to extract kinetic data from XANES spectra to complement data from XPS. This work demonstrates the importance of careful coal acquisition, preparation and storage techniques to prevent air oxidation of samples used for determining the forms of organically bound sulphur, since highly oxidized samples will show lower sulphide and higher thiophene concen-

This

10

11 12 13

14 15

16

17

FUEL,

George, G. N. and Gorbaty, M. L. J. Am. Chem. Sot. 1989, 111, 3182 George, G. N., Gorbaty, M. L. and Kelemen, S. R. in ‘Geochemistry of Sulfur in Fossil Fuels’ (Eds. W. L. Orr and C. M. White), American Chemical Society, Washington, DC, USA Kelemen, S. R., George, G. N. and Gorbaty, M. L. Am. Chem. Sot. Div. Fuel Chem. Prepr. 1989, 34, 129 Kelemen, S. R., George, G. N. and Gorbaty, M. L. Fuel 1990,69,939 Gorbaty, M. L., George, G. N. and Kelemen, S. R. Am. Chem. Sot. Div. Fuel Chem. Prepr. 1989, 34, 738 Gorbaty, M. L., George, G. N. and Keleman, S. R. Fuel 1990, 69, 945 Given, P. H. Prog. Energy Cornbust. Sci. 1984, 10, 149 Ignasiak, B. S., Fryer, J. F. and Jadernik, P. Fuel 1978,57, 578 Jones, R. R., McCourt, C. B. and Swift, P. ‘Proc. Int. Conf. Coal Sci.’ Dusseldorf, FRG, 1981, p. 657 Gardella Jr., J. A., Ferguson, S. A. and Chin, R. L. Applied Spectroscopy 1986, 40, 224 Kelemen, S. R. and Freund, H. Energy & Fuels 1989, 3, 498 Huffman, G. P., Huggins, F. E., Mitra, S. et al. Enerav & Fuels 1989. 3. 200 Wu, M. M.: Robbins, G. A.,‘Winschell, R. A. and Burke, F. P. Energy & Fuels 1989,2, 150 KeIemen, S. R. and Freund, H. Energy & Fuels 1990, 4, 165 Suter, C. M. in ‘Organic Chemistry of Sulfur’, John Wiley and Sons, London, UK, 1944 Reid, E. E. in ‘Chemistry of Divalent Sulfur’, Chemical Publishing Co., New York, USA, 1960 Liotta, R. Fuel 1979, 58, 724

1990,

Vol 69, August

1067