Geochemistry of Sulfur in Fossil Fuels - American Chemical Society

Chapter 18 Characterization of Organic Sulfur Compounds in Coals and Coal Macerals 1

2

1

Stephen R. Palmer , Edwin J . Hippo , Michael A. Kruge , and John C. Crelling 1

1

Department of Geology, Southern Illinois University, Carbondale, IL 62901 Department of Mechanical Engineering and Energy Processes, Southern Illinois University, Carbondale, IL 62901

Downloaded by COLUMBIA UNIV on February 25, 2013 | http://pubs.acs.org Publication Date: June 29, 1990 | doi: 10.1021/bk-1990-0429.ch018

2

Peroxyacetic acid oxidation has been used to investigate the type and distribution of organic sulfur species in samples of vitrinite, sporinite and inertinite, separated from the Herrin No.6 and an Indiana No.5 coal seam. It was established that organic sulfur species were selectively preserved during oxidation and their analysis lead to some of the first sulfur-33 NMR spectra obtained for coal. The effects of maceral separation processes on model compounds were also studied. Results from our studies support the following conclusions: 1). Different macerals have different distributions and types of organic sulfur species. 2). Organic sulfur compounds in coal occur at the ends of macromolecular structures. 3). Maceral separation techniques do not affect organic sulfur species in coal. 4). Maceral separation is essential for the chemical characterization of coal. 5). GC-MS and sulfur-33 NMR data agree. The combustion of s u l f u r - c o n t a i n i n g coals leads to the environmentally unacceptable problems associated with acid r a i n ( l ) . Although current coal cleaning technologies can remove most of the inorganic sulfur from coal (2), no technology i s presently i n use f o r the e f f e c t i v e removal of organic sulfur. The f a i l u r e of organic d e s u l f u r i z a t i o n processes i s due i n part to a lack of information regarding the types of organic sulfur present i n coal. The optimum approach t h e r e f o r e , would seem t o be the initial characterization of the organic sulfur forms i n coal followed by the design of appropriate d e s u l f u r i z a t i o n technologies. There are methods f o r the d i r e c t determination of organic sulfur i n coal (3.4), but d e t a i l s regarding individual molecular structures are much harder to obtain. Much of the research designed to obtain t h i s information has concentrated on the analysis of coal extracts (5.6) and pyrolysis products (7.8). Unfortunately the sulfur species from pyrolysis processes may be highly modified and usually account for only a very small percentage of the t o t a l sulfur i n the coal. 0097-6156/90/0429-0296S06.00/0 © 1990 AmericanChemicalSociety

In Geochemistry of Sulfur in Fossil Fuels; Orr, W., et al.; ACS Symposium Series; American Chemical Society: Washington, DC, 1990.


18.

PALMER ET A L

Organic Sulfur Compounds in Cook and Macérais

297

Extrapolating the data from these analyses to characterize the whole coal can be very misleading. Other approaches have been used to characterize the organic sulfur i n coal. Programmed temperature reduction (PTR) (9-14) as used by Attar i s one such method as i s programmed temperature oxidation (PTO) (15-17). Both methods r e l y on differences i n r e a c t i v i t y of the d i f f e r e n t sulfur species. However, both procedures involve high temperatures and under such conditions transformation between sulfur species i s possible. This reaction i s highly probable i f pyrite or elemental sulfur are present i n the sample (18-20). Further techniques for the determination of molecular structure of organic sulfur species i n coal include Curie point pyrolysis (21) and X-ray absorption fine structure (XAFS) spectroscopy (22). In addition there are techniques which provide an overall view of the r e l a t i o n s h i p s between s u l f u r , metals and c o a l p e t r o l o g y . These include optical microscopy/electron probe microanalysis (10), X-ray p h o t o e l e c t r o n spectroscopy (XPS) (23.) and scanning e l e c t r o n microscopy-energy dispersive X-ray analysis (SEM-EDAX) (10). Although the above mentioned methods provide some i n f o r m a t i o n r e g a r d i n g organic sulfur species i n coal, and some advances have been made, a r o u t i n e , low-temperature, unambiguous method f o r organic s u l f u r characterization i n coal i s yet to be found. In this study we have investigated selective oxidation as a potential organic sulfur characterization approach. In p a r t i c u l a r we have used the peroxyacetic acid oxidation procedure. Although t h i s selective oxidant has received some attention i n the study of l i g n i n (24) and humic acid structures (25), i t s a p p l i c a t i o n to the study of coal has been l i m i t e d to only a few instances (26.27) with very l i t t l e information about organic sulfur species being reported. Peroxyacetic acid oxidation is similar to the peroxytrifluoroacetic acid (Deno) oxidation (2£). These peroxide systems are reported to s e l e c t i v e l y oxidize the aromatic portions of molecules while leaving a l i p h a t i c portions intact (29). Peroxyacetic acid w i l l oxidize aromatic units to phenolic units v i a hydroxylation. These phenolic moieties w i l l oxidize rapidly to ortho and para quinones, the l a t t e r of which are unstable are undergo ring f i s s i o n to form diene carboxylic acids (30). The selective oxidation of aromatic portions of molecules was demonstrated using model compounds such as toluene, ethylbenzene, npropylbenzene and iso-propylbenzene (28.). The major o x i d a t i o n products from these compounds were acetic, propionic, butyric and isobutyric acids respectively. In each case the carboxylic acid group marks the position that the aromatic unit used to occupy. Although the selective oxidation of coal has been extensively studied (31-33). surprisingly l i t t l e has been reported about sulfur species i n the oxidation products. Even less i s known about the d i s t r i b u t i o n of organic sulfur species between d i f f e r e n t coal macérais despite the fact that this information i s important f o r the development of any future desulfurization technology. In view of these shortcomings we have combined the need to c h a r a c t e r i z e organic forms of s u l f u r with the recent progress obtained i n the separation of coal into i t s single maceral fractions (34) . This affords an opportunity to compare the sulfur Chemistries of individual macérais with that of their parent coals.


298

G E O C h e m I S T R Y OF SULFUR IN FOSSIL FUELS

Experimental


Sample Preparation. Herrin No. 6 ( I l l i n o i s No.6) and Indiana No. 5 ( I l l i n o i s No.5) coals obtained from the I l l i n o i s Geological SurveySample Bank were used i n this study. The whole coals were s p l i t into four fractions each of which was placed i n a sealed, 5-gallon drum. The fourth f r a c t i o n was ground to minus 200 mesh and then introduced into a nitrogen gas powered (100 psi) Sturtevant f l u i d energy m i l l . In t h i s device the coal p a r t i c l e size i s reduced to the micron l e v e l by impaction between c o a l p a r t i c l e s themselves and w i t h the impaction chamber walls. Proximate and elemental data f o r these micronized coals are reported i n Table I.

Table I. Proximate and Elemental Data

Herrin No.6

Indiana No. 5

Moisture V o l a t i l e Matter Fixed Carbon H-T Ash

14..7 40..5 49..5 10,.5

10.,4 39,.6 51.,4 9,.0

Carbon Hydrogen Nitrogen Oxygen

69..20 5,.12 1..28 9,.49

71.,73 4,.85 1.,71 8,.93

Sulfate Sulfur P y r i t i c Sulfur Organic Sulfur

0.,05 1,.27 3..00

0.,01 1..85 1.,91

Total Sulfur Total Chlorine

4..32 0.,12

3,.76 0.,02

A l l percentages moisture.

are reported on a moisture

free

basis except f o r

Aliquots of the micronized coal were treated with HC1 and HF to remove carbonate and s i l i c a t e mineral matter, and aliquots of the micronized, acid treated coal were floated i n a 1.67 s p e c i f i c gravity solution of CsCl to eliminate pyrite. Some of the floated samples were then separated by density gradient centrifugation (DGC) into sporinite, v i t r i n i t e and i n e r t i n i t e maceral fractions (34)· Soxhlet extractions were performed on each of the coal and maceral samples. The micronized, acid treated and floated samples were extracted successively with hexane, toluene and f i n a l l y THF. The maceral fractions were extracted with THF only. A l l extracts and extraction residues were isolated, weighed and the d i s t r i b u t i o n of sulfur between them established. In addition, the extracts were examined using FTIR, proton and carbon-13 NMR spectroscopy, GLCFID/FPD and GC-MS.


18. PALMER ET AL.

Organic Sulfur Compounds in Coals and Macérais


Determination of the Effects of Separation Processes on Organic Sulfur Forms i n Model Compounds. Three substituted dibenzothiophenes were s u b j e c t e d to the c o a l p r e p a r a t i o n and maceral s e p a r a t i o n processes. The model compounds used are shown below.

1.

X - CH SH

2.

X = SCH

3.

X = SC H CH

2

6

3

5

Each compound was soaked f o r 1 hr i n 38% (wt/v) HCl followed by soaking i n 55% (wt/v) HF f o r 1 hr. Each was then rinsed exhaustively with water and dried under vacuum at room temperature. In a separate experiment each of the model compounds were exposed to a solution of cesium c h l o r i d e . A f t e r s u f f i c i e n t exposure each compound was recovered and washed with copious quantities of d i s t i l l e d water. The percentage recovery, the melting point and spectroscopic data such as FTIR , proton and carbon-13 NMR were recorded. Mild Oxidation of Coal and Macérais. Two grams of extracted coal was placed i n a 3-necked 250mL r.b.flask. F i f t y mL of g l a c i a l acetic acid was then added followed by 20 mL of 30% (wt/v) hydrogen peroxide. The f l a s k was then heated under reflux and a f t e r 1 hr a further 40 mL of hydrogen peroxide was added dropwise so as to maintain the reaction. The reaction was allowed to reflux for a t o t a l of 24 h. Maceral oxidations were performed on a 0.5g scale. Other oxidations were performed using 5g of coal with, i n some cases, only 40 mL of hydrogen peroxide. These experiments were conducted to generate samples f o r t o t a l sulfur analysis and to study the p a r t i a l oxidation, d i s s o l u t i o n and d e s u l f u r i z a t i o n of coal. Aliquots of the soluble oxidation products were retained for analysis including t o t a l s u l f u r , FTIR and NMR analysis. Another aliquot was methylated using the diazomethane method p r i o r to GC-MS and GLC-FID/FPD analysis. Instrumentation. Proton, carbon-13 and s u l f u r - 3 3 n u c l e i were observed at 300, 75 and 23 MHz respectively, using a Varian VXR 300 MHz NMR spectrometer. Proton spectra were recorded using a pulse width of 12us and a pulse delay of 20s. T y p i c a l l y 10 transients were obtained. Carbon-13 spectra were obtained using a pulse width of 4.3us and a pulse delay of 5s. The number of transients recorded varied from 500 - 10,000 depending on sample concentration. Sulfur-33 spectra were obtained using a pulse width of 65us and a pulse delay of 5ms. T y p i c a l l y between 400,000 and 500,000 t r a n s i e n t s were c o l l e c t e d . Extracts were dissolved i n either deuterated chloroform or deuterated THF and Chemical s h i f t s measured against internal TMS. Oxidation products were dissolved i n acetone/deuterium oxide (9:1) and Chemical s h i f t s measured against internal TMS f o r proton and carbon-13 s p e c t r a and e x t e r n a l ammonium s u l f a t e f o r s u l f u r - 3 3 spectra. GC-MS analysis was performed on a Hewlett Packard 5970B MSD f i t t e d with a 30m 0V-1 column. Two temperature programs were used. F i r s t l y the oven temperature was ramped from 100 - 300°C at


299

300

GEOCHEMISTRY OF SULFUR IN FOSSIL FUELS

3°C/min and secondly a ramp from 100 - 300°C at 20°C/min was used. Injector and detector temperatures were maintained at 270°C and 300°C respectively. GLC-FID/FPD analysis was performed on a Varian 3400 gas chromatograph using the same chromatographic conditions as used i n the GC-MS runs. FTIR spectroscopy was performed on a N i c o l e t instrument using the t h i n f i l m on sodium chloride plates technique for l i q u i d s and the potassium bromide p e l l e t technique for s o l i d s .


Results and Discussion Treatment of Model Compounds. As anticipated, each of the sulfurcontaining model compounds were recovered quantitatively a f t e r t h e i r exposure to the m i c r o n i z a t i o n , a c i d treatment and f l o a t a t i o n processes used i n coal and maceral preparation. In a l l cases the proton and carbon-13 NMR spectra, the FTIR spectra and the melting points of the recovered materials matched those of the starting materials, indicating that these sulfur species remain unchanged during the processing. Solvent Extraction. The y i e l d s of extracts and extraction residues obtained by successive hexane, toluene and THF extraction of the floated coal samples, plus the percent of the t o t a l of the organic sulfur that each extract contained are shown i n Table II.

Table I I . Extraction yields and sulfur d i s t r i b u t i o n s

Herrin Extract Type

Y i e l d Wt%

No.6 %Tot.0.S.

Indiana

No.5

Y i e l d Wt%

%Tot.0.S.

Hexane

0..5

2..0

0..6

6..6

Toluene

2.,5

2..9

1.,2

4.,6

THF

7,.1

4,.2

14..9

14..2

89..9

90..9

83..3

74..6

Extract Residue

It i s clear from Table II that most of the organic sulfur remains i n the insoluble coal matrix and cannot be extracted. The Indiana No. 5 coal has a higher quantity of extractable organic sulfur than the Herrin No.6. Both the e x t r a c t a b i l i t y and the organic sulfur d i s t r i b u t i o n varies between the two coals. It i s interesting that the hexane extracts from both coals and the toluene extract of the Indiana No 5. coal, contain a disproportionately high percentage of the t o t a l organic sulfur. Thus, there i s organic sulfur enrichment i n these extracts and a consequential depletion of the organic sulfur i n the extraction residues. This would appear to be good c r i t e r i a f o r a


18. PALMER ET AL.

Organic Sulfur Compounds in Coals and Macerah


desulfurization process but the low e x t r a c t a b i l i t i e s obtained these solvents prevents this from being a viable process.

with

Characterization of Sulfur Compounds i n Extracts. Each of the extracts and extraction residues were examined by FTIR spectroscopy. The hexane and toluene extracts gave spectra dominated by a l i p h a t i c f e a t u r e s while the THF e x t r a c t s had more aromatic and p o l a r functional group c h a r a c t e r i s t i c s . The extraction residues gave FTIR spectra dominated by aromatic and p o l a r functional group c h a r a c t e r i s t i c s with minor a l i p h a t i c features. A l l spectra contained peaks that could have arisen from organosulfur groups but the regions occupied by these peaks overlap with those occupied by other nonsulfur functional groups. Hence assignment of sulfur structures was not possible. This demonstrates the f u t i l i t y of t r y i n g to i d e n t i f y a minor constituent of a complex substance such as coal using nonselective techniques. The same type of information was obtained from proton and carbon-13 NMR. Once again there are absorptions that may be due to carbon and hydrogen bonded to sulfur but contributions from other non-sulfur containing structures are highly l i k e l y and hence a firm assignment cannot be made. It i s obvious from the FTIR and NMR analyses of these extracts that i n order to p o s i t i v e l y i d e n t i f y organosulfur structures we need an a n a l y t i c a l technique that i s s u l f u r s e l e c t i v e . That i s , a technique that responds to sulfur uniquely. One such technique, applicable to the problem i n hand, i s GLC-FID/FPD where the flame photometric detector i s set i n the sulfur selective mode. Aliquots of hexane, toluene and THF extracts were mixed together proportionately and the r e s u l t i n g combined extract separated by opencolumn l i q u i d chromatography to give a saturate, an aromatic, a polar 1 and a polar 2 f r a c t i o n . The saturate fractions from both coals d i d not contain any sulfur compounds detectable i n our GC-FID/FPD system. However, the aromatic fractions from both coals contained a complex series of sulfur compounds. Subsequent GC-MS analysis of these f r a c t i o n s u s i n g the s e l e c t e d i o n mode (SIM) i d e n t i f i e d these compounds as a series of benzothiophenes having one, two or three methyl group substituents and a series of dibenzothiophenes with one through four methyl group substituents. Dibenzothiophene i t s e l f was also detected but benzothiophene was not. Despite the inherent p e c u l i a r i t i e s of the FPD d e t e c t o r ( n o n - l i n e a r a l i t y , compound dépendance and quenching), comparison of the FPD traces with the composite SIM traces afforded a very good c o r r e l a t i o n , i l l u s t r a t i n g the fact that the composite SIM method d i d not miss major sulfur compounds. GC analysis of underivatized polar fractions d i d not reveal any v o l a t i l e s u l f u r compounds. However, once these f r a c t i o n s were methylated with diazomethane, a number of sulfur compounds were detected. (Presumably, the diazomethane methylated either carboxylic acid, phenolic, thiophenolic, sulfonic acid or even alcohol or t h i o l groups and thereby increased t h e i r parent molecules v o l a t i l i t y ) . These additional s u l f u r compounds are currently under investigation i n our laboratories and the results of these studies w i l l be reported l a t e r . Comparison of the extracts taken from the i s o l a t e d macérais show them a l l to be very similar, not only to each other , but to the


301

302

GEOCHEMISTRY OF SULFUR IN FOSSIL FUELS

extracts taken from the unfractionated coal. This indicates that the composition of the extract does not vary with maceral type and suggests the extract i s probably free to migrate throughout the coal matrix and disperse evenly among the various coal components.


Oxidation of Coal and Coal Macérais. Extraction residues were oxidized as outlined i n the experimental section. Two oxidation procedures were adopted:- i ) . excess oxidant oxidation and i i ) . oxidant starved oxidation ( p a r t i a l oxidation). i ) Excess oxidant oxidation:- Under the conditions employed the extraction residues were oxidized to soluble products leaving very l i t t l e r e s i d u a l matter. T y p i c a l l y the percentage of the c o a l dissolved was around 90-95% although some figures as high as 99% were recorded. The exceptions to this were the sporinite and i n e r t i n i t e macérais, both of which were more resistant to this oxidative d i s s o l u t i o n , with only 50-60% d i s s o l v i n g . This no doubt r e f l e c t s differences i n the properties and chemical structures of these macérais. I t i s also important to note that the y i e l d s of oxidation products were higher f o r floated coal samples vs the micronized and the acid treated samples. This suggests that minerals influence the oxidation of the organic portion of the coal, perhaps catalyzing the production of carbon dioxide. In the larger scale reaction (5g sample size) i t was established that the soluble oxidation products had considerably enhanced sulfur contents when compared with the unoxidized samples. This i s explained by the oxidation of carbon i n the coals to carbon dioxide and the subsequent concentration of the organic sulfur that remained. I t was also established that v i r t u a l l y a l l of the organic sulfur i n the extracted and unoxidized samples could be accounted f o r by the organic sulfur found i n the soluble oxidation products, i n d i c a t i n g that very l i t t l e was l o s t during the reaction and work-up procedures. It was a concern to us that some of the organic sulfur may have been oxidized to s u l f a t e . To check f o r this we tested the soluble oxidation products f o r sulfate using barium chloride solution. Both the micronized and the acid treated coal samples tested positive f o r sulfate. This i s not surprising since each contains pyrite that would be oxidized to sulfate during the reaction. However, the floated coal samples and the macérais (which have very l i t t l e residual pyrite

Geochemistry of Sulfur in Fossil Fuels - American Chemical Society

Recommend Documents