High resolution Chromatographic characterization of depolymerized coals of different rank: aliphatic and aromatic hydrocarbons

High resolution chromatographic characterization of depolymerized coals of different rank: aliphatic and aromatic hydrocarbons Richard Ji-Zhou Joseph

E. Carlson, Scott Critchfield, William P. Vorkink, Dong, Ronald J. Pugmire, Milton L. Lee, Yuan Zhang*, Shabtai* and Keith D. BartIe?

Department of Chemistry, Brigham Young University, Provo, UT 84602, USA *Department of Fuels Engineering, University of Utah, Salt Lake City, UT 84112, tDepartment of Physical Chemistry, University of Leeds, Leeds, UK (Received 76 November 7990; revised 19 February 7991)

USA

A selective, low temperature depolymerization procedure has been applied to four coals of different rank to obtain products that are representative of the original structural units (monoclusters) of the coal macromolecular component, and that are amenable to chromatographic analysis. The monocluster products of this depolymerization procedure retain most of their original aromatichydroaromatic structures. whereas functional groups serving as intercluster linkages, e.g. alkylene and etheric groups, undergo predictable modification. A comparison of liquid 13C n.m.r. spectra of the products and solid state 13C n.m.r. spectra of the original coals showed only minor changes in the aromaticities between the solid and depolymerized products, but some loss of carbonyl carbons was detected in all of the coals. Both the tetrahydrofuran pre-extract and the depolymerized products of the four coals were separated into chemical classes by adsorption chromatography. Two of these fractions were found to consist predominantly of aliphatic hydrocarbons (parafftns and naphthenes) and alkylsubstituted bi-, tri- and (some) tetracyclic arenes. The fractions were analysed using gas chromatography-mass spectrometry. Structural identifications were based on a combination of chromatographic retention and mass spectral fragmentation data. For the lower rank coals, the compositions of the pre-extracts were quite different from the corresponding depolymerized coal products, and contained various molecular biological markers. The compositions of the pre-extracts became closer to those of the depolymerized coal products as rank increased. (Keywords: chromatography;

characterization;

depolymerized

Elucidation of the macromolecular structure of coal and its geochemical origin continues to attract wide interest. Knowledge concerning the coal molecular network is vital to understanding and enhancing the physical and chemical processes involved in liquefaction, gasification, combustion and refining of coal, as well as providing the data base for future products that eventually will be derived from coal. Coal is believed to behave as an irregular polymeric material containing an easily extractable mobile phase component within an insoluble skeleton. It has not been clear whether the relatively minor mobile phase is just the soluble portion of a continuum of increasingly larger coal molecules, or whether it is distinctly different in composition and origin. The mobile phase component is indicated to consist mainly of normal and branched alkanes, alkenes, arenes, naphthenes, fatty acids, esters, isoprenoids and terpenoid-like compounds, along with similar compounds of biological origin’-12. The predominant insoluble skeleton has been indicated to consist of polycyclic aromatic-hydroaromatic clusters connected by linkages such as alkylene, etheric, thioetheric, esteric and other groups.

0016-2361’92.010019~11 c 1992 Butterworth-Heinemann

Ltd.

coals; n.m.r.)

In an effort to understand coal chemistry, direct analysis of solid coals has been investigated, but due to its complex, heterogeneous nature, limited specific information concerning its macromolecular structure and/or building units can be obtained. Analysis of room and/or low temperature solvent extracts has provided information concerning portions of the mobile phase, leaving the predominant insoluble skeleton relatively unexplored. Extractions carried out at elevated temperatures can result in higher yields, however thermal cracking at these temperatures destroys the molecular integrity of the products. To study coal structure effectively, the macromolecular network must be carefully broken down under mild reaction conditions into representative coal units amenable to chromatographic analysis. Some of the chemical methods employed in the past to depolymerize coal include reductive alkylation, 0-alkylation, nonreductive alkylation, acid-catalysed depolymerization, oxidation and hydrogenation. In general, these methods are either destructive and/or non-specific, resulting in random bond cleavage, or they are too mild to depolymerize significant portions of the coal for structural studies. Reductive alkylation treatments produce radical anions which transfer electrons to the aromatic coal

FUEL,

1992,

Vol

71,

January

19

Characterization

of depolymerized

coals:

R. E. Cartson

et al

matrix, resulting in the formation of aromatic radical anions or dianions. The aromatic radical anions are then alkylated using alkyl halides13. Reductive alkylation has proved to be effective in dissolving large portions of the coal, but the resulting molecular fragments are too large for chromatographic analysis. 0-Alkylation involves the selective alkylation of acid hydroxyl groups in coal, such as phenolic and carboxylic hydroxyls14. This reaction is performed under very mild conditions (room temperature, 100 kPa Nz) and only 0-H bonds are affected. Yields of dissolved material in 0-alkylation are too low (-25 wt%) for significant structural studies”. Non-reductive alkylation takes advantage of the slightly acidic nature of coal. Coal anions are formed by placing the sample in a basic medium such as NH,. Alkylated coals are produced as the coal anions are reacted with alkyl halides. These treatments have been shown to predominantly cleave carbonheteroatom (S, alkylations also 0 and N) linkages ‘CL’’ . Non-reductive fail to effectively depolymerize the coal network, since such treatments cleave few, if any, covalent bonds. Significant depolymerization was found to occur when hvB coals were treated with a phenollBF, or phenol-p-toluenesulphonic acid systemls~ly. This was reflected in a major increase in coal solubility. The depolymerization of the coal was attributed to the cleavage of aliphaticcaromatic linkages and exchange of aromatic cluster units with phenol. The overall transalkylation reaction has been indicated to be of limited value as a tool for structural analysis of coals. Oxidation treatments have been carried out under drastic conditions (high temperatures and harsh reagents) when compared to other methods. In general, oxidation destroys to a large extent the aromatic units in the coal structure and is not selective when breaking the coal network20.21. Hydrogenation reactions have been shown to be more successful in solubilizing larger portions of the coal network than room temperature solvent extractions’2.‘“. However, hydrogenation treatments have been carried out at a variety of temperatures, often in the 300&400 C range, which may lead to free radical rearrangements. crosslinking and thermal devolatilization of the sample. As a result of current studies on coal depolymerization, a novel liquefaction procedure has been developed by Shabtai and co-workers24-2y. In general, it consists of subjecting the coal to exhaustive depolymerization by selective low-temperature catalytic reactions, prior to hydroprocessing. The depolymerization stage of the procedure consists of the following two sequential steps: (1) dispersion of catalytic amounts (3315 wt%) of ferric chloride (FeCl,) into the coal matrix, followed by mild hydrotreatment Table 1

Elemental

compositions

of intact

(HT) at temperatures ~290°C; and (2) hydrolysis (or alcoholysis) of the mildly hydrotreated product using a methanolic solution of KOH or other base, again at temperatures < 290°C. Depolymerized coals obtained by the above sequential treatment consist of mixtures of low molecular weight products, composed primarily of monocluster compounds24-29. It was of considerable interest in this study to analyse in detail the compositions of selected, mildly depolymerized coals to establish the chemical structures of the fundamental building units in the coal framework. Further, such structural analysis was considered to be of major value in clarifying the chemistry and mechanism of low-temperature, non-destructive coal depolymerization. It was hoped that this depolymerization method could produce molecular fragments which are representative of the original coal structure and are amenable to chromatographic analysis. EXPERIMENTAL Sunzple pr6jpuration Coul samples. High vitrinite, premium coal samples from Beulah Zap (North Dakota), Illinois no. 6 (Illinois), Blind Canyon (Utah), and Pocahontas no. 3 (W. Virginia) were supplied by the Argonne National Laboratory. These samples (< 149 pm particle size) were provided in brown glass ampoules sealed under N,, and were immediately used after opening. Elemental analyses of these samples are given in Tub/e I. Pre-estraction. Approximately 10 g of each sample were dried at 60°C in a vacuum oven for 6 h, then weighed and extracted with tetrahydrofuran (THF) in a Soxhlet apparatus for 48 h. The solvent was removed using a rotary evaporator, and the pre-extracts were further dried in a vacuum oven for 24 h at 40°C. Depol?‘nzeri=ution. In each experiment, _ 10 g of coal sample were pre-extracted with THF in a Soxhlet apparatus using a single thickness, Whatman cellulose extraction thimble. The extraction was carried out for 48 h or until the recycle extraction solvent became clear. The resulting, pre-extracted coal was dried under vacuum, and placed together with the desired amount of FeCl, (3315 wt%) in a 100 ml container. Acetone was added to the container in sufficient amount to bring the liquid level to _ 1 cm above the coal, and the resulting coallFeCl,-acetone mixture was agitated in an ultrafor 2 h. The excess acetone was sonic bath at -40°C then distilled off under a stream of nitrogen and the remaining FeCl,-impregnated coal was dried in a vacuum oven at 50°C overnight. The impregnated coal sample was quickly transferred

coals”

Coal (wt%, dmmf basis) State

Seam

Rank

c

H

0

S

Ash

ND

Lignite

12.9

4.8

20.3

0.8

9.7

Illinois no. 6

IL

High vol. bit.

17.7

5.0

13.5

4.8

15.5

Blind Canyon

UT

High vol. bit.

80.7

5.8

I I.6

0.6

4.7

2.5

0.7

4.8

Be&h

Zap

Pochahontas “From

20

no. 3

WV

Ref. 43

FUEL,

1992,

Vol 71, January

Low vol. bit.

91 .o

4.4

Characterization

to a 300 ml magnedrive autoclave and subjected to mild d29O’C; initial H, pressure, HT (temperature, 10.34 MPa; reaction time, 3 h). The hydrotreated product was then removed from the reactor, backextracted with acetone in a Soxhlet apparatus to remove the FeCl, catalyst, and the solvent-free extract was washed with water to remove metal halide, leaving a small amount of acetone-soluble product. The acetoneinsoluble partially depolymerized coal was then extracted with THF in a Soxhlet apparatus, and the small amount of THF-soluble product was dried and weighed. The total product from the HT step [partially amount solid coal plus a small depolymerized (- IO wt%) of solubles] was subjected to base-catalysed depolymerization (BCD) in a 300 ml stirred autoclave, using a 10% KOH solution in methanol as depolymerization agent. A mixture of this solution and the total product from the HT step (usually in a ratio of IO:1 by weight) was introduced into the autoclave, and the latter was purged of air and pressurized with N, to a pressure of 6.9 MPa. The autoclave was quickly heated to 29O’C, and the mixture was stirred at this temperature for 1 h. The autoclave was then quickly cooled down to room temperature and depressurized. The product was removed from the autoclave, transferred to a beaker and acidified with 2 N HCl to a pH of 2-3. The water-insoluble material was separated by vacuum filtration, washed with distilled water, and finally dried in a vacuum oven overnight. The product was then extracted with THF in a Soxhlet for 48-72 h, and the yield of THF-soluble product obtained in the BCD step was determined. The total THF-solubles were then fractionated into cyclohexane-solubles (oils) and some incompletely depolymerized products (benzeneand THF-solubles). The incompletely depolymerized products could be converted to a significant extent (15-40 wt%) into cyclohexane-solubles by recycling through the HT-BCD procedure.

Both the pre-extracts and the HT-BCD products were fractionated into four chemical classes on a neutral alumina column using the method described by Later et ~1.~‘. The four chemical classes were primarily composed of aliphatic hydrocarbons, neutral polycyclic aromatic compounds, and two fractions of increasing polarity (Polar I and Polar 2). Approximately 0.1-0.3 g of sample was dissolved in a few millilitres of chloroform and adsorbed onto 3 g of neutral alumina, after which the solvent was eliminated by stirring under a stream of N,. The alumina containing the adsorbed sample was then introduced at the top of an 11 mm i.d. column packed with neutral alumina. The fractions were sequentially eluted using the following solvents: 20 ml hexane, 50 ml benzene, 70 ml chloroform (containing 0.75% ethanol) and 50 ml THF/ethanol(9: 1 v/v) solution, respectively. GLIBchromatograplzp (g.c.) and gas chromatograph~~~mass spectromrtrj) (gx-m.s.) A gas chromatograph equipped with a flame ionization detector was used to separate and detect the compounds in each fraction. A 13 m x 200 pm i.d. SB-methyl-100 (0.25 pm film thickness) fused silica column and helium carrier gas maintained at a linear velocity of 40 cm s-l were used during the analysis of each fraction. For each

of depolymerized

coals:

R. E. Carlson

et al.

of the four fractions, the initial column temperature was 4O’C for 2 min, followed by temperature programming to 3OO“C at a rate of 3°C min- ‘. Compound identification was achieved using mass and retention data from a g.c.-mass selective detector operated at 70 eV; in this case, the chromatographic column was a 15 m x 200 pm i.d. polymethylsiloxane (0.25 pm film thickness) fused silica open tubular column.

The high resolution 13C and ‘H n.m.r. studies of the HT-BCD extracts were performed using a high resolution n.m.r. spectrometer operated at 500 MHz. The sample analysis time was adjusted to yield approximately 30000 transients. The nuclear Overhauser effect was suppressed by gated decoupling with a total recycle time of 2.3 s.

RESULTS

AND

DISCUSSION

Using the above described HT-BCD procedure, the cyclohexane-soluble product yields [reported as wt% of coal (dmmf)] obtained from a single depolymerization cycle were 54.2,58.3,55.6 and 22.0% for the pre-extracted Beulah Zap, Illinois no. 6, Blind Canyon and Pocahontas no. 3 samples, respectively. Pre-extraction with THF removed most of the easily extractable material within the coal network, leaving the porous system of the coal more susceptible to catalyst impregnation. During impregnation, the FeCl, becomes uniformly dispersed in the coal particles as recently demonstrated by electron microscopy31. The partial depolymerization of the coal during the HT step involves preferential hydrogenolytic cleavage of alkylene (e.g. ethylene), benzyl etheric. cycloalkyl etheric and some activated thioetheric linkages24p29. The BCD step completes the depolymerization by base-catalysed hydrolysis (or alcoholysis) of diary1 etheric, aryl cycloalkyl etheric, diary1 thioetheric and other bridging groups. Depolymerized coal samples obtained by the above sequential treatment consist of mixtures of low molecular weight products, composed primarily of monocluster compounds”4p2”. Under optimum processing conditions, which change to some extent with coal rank, the total yields of THF soluble, depolymerized products ranged from 85 to 93 wt%. Such depolymerized coals usually contain 22-60 wt% of cyclohexane-soluble material (oils), pIus small amounts of partially depolymerized products. The yields obtained from the chemical class fractionation of the coal pre-extracts and of the depolymerized products are reported in Table 2. The relative distributions in the fractionations of both the pre-extracts and the HT-BCD products show essentially the same pattern, which may indicate the non-destructive nature of the HT-BCD process. The largest fraction from the Beulah Zap and Pocahontas no. 3 products (both the HT-BCD and the pre-extract) eluted from the alumina column in the chloroform eluant (Polar 1 fraction). However, in the Illinois no. 6 and Blind Canyon coals, the final or more polar fraction (Polar 2) contained the greatest amount of product. This observation may provide further evidence to support the elimination of oxygen-containing functional groups as coalification proceeds, such as the removal of phenolic oxygen from lignin as suggested by Hayatsu et ~1.“~.

FUEL,

1992,

Vol 71, January

21

Characterization Table 2

Alumina

of depolymerized column

fractionation

coals:

R. E. Carlson

yields of pm-extracts

et al

and HT-BCD

products Coal (wt”%, dmmf basis)

Beulah Zap Fraction

Pre-extract”

Aliphatic Neutral

PAH

HTmBCDh

Illinois no. 6 Pm-extract”

HT-BCDh

Blind Canyon Pre-extract”

HT-BCDb

Pocahontas Pm-extract“

no. 3 HT-BCDh

0.19

2.4

0.x9

2.3

3.80

6.4

0.05

1.o

0.56

1.2

2.10

9.2

3.97

12.4

0.10

7.4

1

I .04

23.2

3.16

2.1

3.57

2.6

0.49

10.2

Polar 2

0.41

21.4

10.65

44.7

7.56

34.2

0.16

3.4

Coal (total wt%)

2.2

54.2

16.X

58.3

55.6

0.8

Polar

18.9

22.0

” THF soluble h Cyclohexane soluble

The chromatographic results of the aliphatic fractions obtained from the pre-extracts and HT-BCD products of the four coals studied are shown in Figures 1 and 2. respectively. Of particular significance in the Beulah Zap samples is the absence of the biological marker compounds in the HT-BCD product, as compared to the pre-extract. The pre-extract (Figure IA) is primarily composed of cyclic and branched alkanes, diterpanoids. tetracyclic terpanes, pentacyclic triterpanes, and their derivatives, whereas the HT-BCD product (Figure ?A) consists predominantly of more stable compounds, e.g. n-alkanes, alkylarenes and heterocyclics. The low yields obtained for the solvent pre-extracts were previously believed to account for much of the aliphatic component of the coal. However, following HT-BCD treatment, the aliphatic portion of the product represents 4.5 wt% of the total product or 3.2 wt”! of the Beulah Zap coal. These results give further indication that alkyl chains and alkylene bridges appear mostly as substituents in the macromolecular network. The chromatographic results of both the pre-extracts and the HT-BCD products show trends in the molecular structure as rank changes. For example, the analysis of the aliphatic fraction of the Illinois no. 6 pre-extract (Figure IB) shows fewer biological marker compounds than the lignite (Figure 1A). The aliphatic fractions of the Blind Canyon and Pocahontas no. 3 pre-extracts (Figures IC and D) reveal a similar trend; the products show a decreasing amount of biological marker material as the rank of the coal increases; in fact the Pocahontas no. 3 pre-extract contains primarily n-alkanes with shorter chain lengths than the Beulah Zap, Illinois no. 6 and Blind Canyon pre-extracts. This trend is well established and may result from thermal reactions during coalification’ 1.33. On the other hand, the aliphatic fractions of the four HT-BCD products show a normal hydrocarbon distribution from -C,, to C,,, with smaller amounts of branched alkanes and/or alkenes (Figure 2). As discussed above, the pre-extract yield from the low volatile bituminous (Pocahontas no. 3) coal was low and no biological marker compounds were found.The HT-BCD aliphatic fraction of the Pocahontas no. 3 coal contains many of the same alkanes present in the corresponding pre-extract but in greater amounts and spread over a wider molecular weight range (Figure 20). The Pocahontas no. 3 HT--BCD product also shows a slight odd over even preference for n-C*, to n-C,, (Figure 20). Likewise, the aliphatic fractions of the Illinois no. 6 and Blind Canyon HT-BCD products exhibit an odd over even carbon

22

FUEL, 1992, Vol 71, January

number preference for these same compounds. A similar odd over even hydrocarbon preference has been observed in soils”, living plants’ ’ and other coal products7,’ 1,32 and is also present in the pre-extraction products. On the other hand, the distribution of n-alkanes in the HT-BCD Beulah Zap product appears to be smooth, having no preference of odd over even carbon number hydrocarbons. These long chain aliphatic compounds may be the reduction products of straight chain fatty alcohols, acids and their esters in the coal structure such as reported by and Given7. Similar Snape et a1.34 and Mudamburi aliphatic products have been observed by other researchers3~s-7~10~1’. Table 2 points out that as rank increases, the amount of polar components (i.e. Polar 1 and Polar 2 fractions) decreases. Thus as expected, fewer fatty acids are present in the low volatile bituminous samples. In fact, Snape et ~1.~~ reported the absence of (i.e. ~0.1 wt%) fatty acids in a bituminous coal. Therefore, it is possible that the majority of the n-alkanes reported in the Pocahontas no. 3 coal are present in a mobile phase which is difficult to extract or, more likely as substituents or linking groups within the macromolecular network, which are released during the HT-BCD process. Nelson’ has shown the importance of such aliphatic structures in coal pyrolysis, and alkylene (longer CH,) groups have been considered3” to be major aliphatic components of the coal and are part of the macromolecular network. In an effort to explain the origin of the long chain aliphatic compounds identified in coal, Calkins and entities in plant Spackman 36 looked for polymethylene materials ranging from living tissues to peat and coal. They concluded that the aliphatic components of coal are the result of polymethylene groups present in the original plant material. Additional geochemical sources of these long chain aliphatic compounds may be from reactions between lipid-type materials (e.g. esters, fatty acids and long chain alcohols) and the remaining lignin fragments as coalification proceeds4. During diagenesis, cellulose can be easily degraded to CO, and water, while some humic acids, like lignin, being more resistive to degradation, can survive with only small changes in the chemical structure”.“932*33*37, or recombined to form other polymers37-40. Therefore, low rank coals may show a larger content of biological marker compounds because the non-vascular plant material experiences geochemical changes faster than the lignin. Furthermore, as rank increases, reactions between lignin

Characterization

of depolymerized

hopanes

coals:

8

R. E. Carlson

et al.

moretanes

I phytane

30

0

I

40

I 100

I

o

I 150

tempPC)

90

60

time(m~n:

I

I 2Oo

I

I

I

250

300

Figure 1 Gas chromatograms of the aliphatic fractions (THF pre-extracts) of: A, Beulah-Zap; B. Illinois no. 6; C, Blind Canyon; D, Pocahontas period. The peaks no. 3 coals. Chromatographic conditions: temperature programmed from 40 to 300 C at 3-C min-’ after a 2 min isothermal marked with an ‘0’ indicate normal alkanes

and lipids may result in the formation of additional polymethylene linkages which remain attached to the aromatic moieties derived from the lignin4.6. The chromatograms of the pre-extracts and the HT-BCD products of the polycyclic aromatic hydrocarbon (PAH) fractions are shown in Figures 3 and 4,

respectively. Table 3 lists the proposed compounds which correspond to the peak numbers in Figures 3 and 4. As seen in Figure 3A, the neutral aromatic fraction of the lignite pre-extract contains primarily partially aromatized pentacyclic triterpanoid-like compounds, along with smaller amounts of alkyl naphthalenes, biphenyls

FUEL,

1992,

Vol 71, January

23

Characterization

of depolymerized

coals:

R. E. Carlson

et al.

BHT

phylane

I

phytanc prlstm?

0

I

40

Figure 2 Gas chromatograms no. 3 coals. Chromatographic

30

tI rn, e ( In. n i

I 100

of the aliphatic fractions conditions are given in

I

I 150

temicC)

FUEL, 1992, Vol 71, January

90

I

I

I

200

250

300

I

(HT-BCD products) of: A, Beulah-Zap; B, Illinois no. 6; C, Blind Canyon; I. The peaks marked with an ‘0’ indicate normal alkanes

D, Pocahontas

Figure

and other aromatic compounds. Figure 3 indicates that as rank increases, the aromatic hydrocarbon fractions of the pre-extracts show a decrease in tetra- and pentacyclic triterpanoid-like compounds, and an increase in mono-, bi- and tricyclic arenes. These products could be the result of coalification in which thermal decomposition and transformation reactions of terpenoid-like compounds from waxes, resins, algae, lipids, etc., led to structures

24

60

I

which were eventually included in the coal framework as condensed aromatic entities. As with the aliphatic fractions, the aromatic fractions of the pre-extracts show changes in the thermal history and maturation of the coal. Analysis of the corresponding HT-BCD fractions resulted in the detection of alkylnaphthalenes and biphenyl compounds in addition to alkylated anisoles, fluorenes, benzofluorenes, phenanthrenes and similar

Characterization

of depolymerized

coals:

R. E. Carlson

et al.

c I I

30 I 40

Figure 3 Gas chromatograms no. 3 coals. Chromatographic

I 100

90

60

tlme(mlni

I

II

I I 150

teW°Ci

I 200

I

I

300

250

of the PAH fractions (THF pre-extracts) of: A, Beulah-Zap; B, Illinois conditions are given in Figure I. Peak numbers correspond to compounds

no. 6; C, Blind Canyon; listed in Table 3

FUEL,

1992,

D, Pocahontas

Vol 71, January

25

Characterization

of depolymerized

0 1

I

I

100

Figure 4 Gas chromatograms Chromatographic conditions

FUEL,

1992,

R. E. Carlson

30

40

26

coals:

et al.

liP>ejmln,

I

150

I t
60

90

I I 250

I

I

300

of the PAH (HT-BCD products) of: A, Beulah-Zap; 8, Illinois no. 6; C, Blind Canyon; are given in Figure I. Peak numbers correspond to compounds listed in Tuhfr 3

Vol 71, January

D, Pocahontas

no. 3 coals

Characterization

two- and three-ring aromatic compounds (Figure 4). As rank increases, the distribution of the HT-BCD products remains quite similar. For example, each sample contains essentially the same classes of compounds, such as the Table 3

Compounds

Identified

in the PAH

fractions Compounds

identtfied”

CI-Bcnrenes C,-Benzenes ‘-Mcthvlnaphthalene I-Methvlnaphthalene C,-Ankles Biphcnyl C,-Btphcnjls C,-Naphthalencs C,-Naphthalcnes CL-Biphenyls C,-Fluorenes C,-Fluorenes C,-Btphenyla C,-Fluorcncs C,-Phenanthrcncs Phenylnaphthalene C,-Fluorcncs Bcnzofluorene C,-Benzolluorenes Cz-Bcnrofluorenea C,-Phcnylnaphthalenes Benzopyrcnc “Identification based on relative g.c.-m.s. fragmentation patterns

chromatographic

retention

and

of depolymerized

coals:

R. E. Carlson

et al

C, to C, alkylnaphthalenes, alkylfluorenes and alkylphenanthrenes. However, as rank increases, a noticeable shift to higher boiling products is apparent. Comparing the Beulah Zap HT-BCD product to the Pocahontas no. 3 HT-BCD product indicates that as maturation proceeds, the number and size of condensed aromatic rings increases and the product distribution favours the more thermally mature compounds. Also of interest is the absence of methoxy and phenolic compounds in the Pocahontas no. 3 product. This is not surprising since the amount of oxygen-containing compounds decreases as maturation proceeds; however, it is interesting that such a pattern is found in the HT-BCD products, and clearly indicates the mild nature of this depolymerization as coal rank increases, the method. Furthermore, molecular profiles of the pre-extracts begin to resemble the molecular profiles of the HT-BCD products. Figure 5 shows an example of the chromatographic results of the Polar 1 and Polar 2 fractions obtained from the alumina fractionation of the Illinois no. 6 coal. These fractions are more polar than the polycyclic aromatic hydrocarbon fractions and are important in the complete characterization of depolymerized coals. Chloroform and THF-ethanol(9: 1 v/v) were used to elute these fractions, which are believed to consist mostly of phenols, accompanied by nitrogen-containing polycyclic aromatic compounds (N-PAC) and higher molecular weight PAC. Previous work has indicated that the major components in the depolymerized coals are alkyl-substituted mono-

A

tlme(mln)

I

40

Figure 5 Gas chromatograms are given in Fiyuw I

60

I

90

I

I

I

I

100

150

tompW)

I 200

of: A, Polar 1; B, Polar 2 fractions of the HT-BCD

I

I

250

300

products of an Illinois no. 6 coal. Chromatographic

FUEL, 1992,

Vol 71,

conditions

January

27

Characterization N.m.r. coals

aromaticity

Table 4

intact

of depolymerized values

coals:

R. E. Carlson

of depolymerized

products

and

a

Intact

coal

Seam

HT-BCD

Beulah Zap Illinois no. 6 Blind Canyon Pocahontas no. 3 -.

0.53 0.55 (0.63) 0.41 (0.46) 0.78 (0.86)

produ:

0.54 0.72 0.64 0.86

“The recycle time employed in acquiring the data resulted in a 10-I 5’/0 underestimation of the carbon aromaticity. The corrected values are given in parentheses

and bicyclic phenols4’. Current work is underway to fully characterize these predominant fractions. To evaluate the structural integrity of the products obtained from the HT-BCD process, ‘H and ’ 3C n.m.r. analyses were performed on the depolymerized products for comparison with solid state n.m.r. measurements on the original coals. The aromaticities (i.e. the percentage of total carbon found in aromatic rings), cluster sizes, substitution patterns on aromatic rings; distributions of various methyl groups, percentages of carbon found in alcohols, ethers, carboxylic acids, etc., and other structural elements have been determined for the intact standard coals and the HT-BCD products of these coals. Table 4 lists the aromaticities determined for these samples. The data indicate that for the low rank coal (Beulah Zap), the depolymerized HT-BCD product has an aromaticity which is essentially the same as the intact coal. The mid rank coals (Blind Canyon and Illinois no. 6) show the greatest decrease in aromaticity after depolymerization, and the higher rank Pocahontas no. 3 HT-BCD product shows good agreement with the measured solid state aromaticity. Each of the liquid spectra indicate the presence of minor components which were present at different concentrations in the original coals. Carbonyl groups were observed only in the lignite and Blind Canyon solid coals (0.13 and 0.01 mole fraction total carbon, respectively)42. Only traces of carbonyl groups were noted in the HT-BCD products from the four coals. This indicates a significant loss of carbonyl functionality during the depolymerization of the Beulah Zap, possibly via reduction and/or decarboxylation during the BCD step. Phenols and phenolic ethers are prominent in the Beulah Zap, Blind Canyon and Illinois no. 6 products but less apparent in the Pocahontas no. 3 HT-BCD product spectrum. Again, this is consistent with the mole fractions of total carbon that represent phenols or phenolic ethers (6 = 150-165 ppm) calculated from solid state 13C n.m.r. data for the intact coals (0.10, 0.07,0.06 and 0.02 for Beulah Zap, Blind Canyon, Illinois no. 6 and Pocahontas no. 3, respectively)42. Alkenes are present in the HT-BCD product spectra for Beulah Zap, Blind Canyon and Illinois no. 6 but are much less apparent in the Pocahontas no. 3 HT-BCD product spectrum. Alcohols (6 = 55590 ppm) are clearly present in the depolymerized lignite spectrum, with a decreasing amount in the Blind Canyon and Illinois no. 6 spectra and none in the Pocahontas no. 3 HT-BCD product. These data are also consistent with the solid state n.m.r. data on the corresponding intact ~oals~~. The purpose of the n.m.r. analysis was to assess the integrity of the HT-BCD process. The Beulah Zap

28

FUEL,

1992,

Vol 71, January

et al HT-BCD product appears to best retain its aromatic integrity from the intact coal (Table 4). Loss of the carbonyl functionality is predicted under the experimental conditions employed. The poor precision of the data above 165 ppm makes quantitation in this region unreliable. The other HT-BCD product functionalities appear to be similar to the intact coal functionalities in the lignite. The primary differences between the 13C n.m.r. spectra of the Blind Canyon, Illinois no. 6 and Pocahontas no. 3 HT-BCD products and corresponding intact coal spectra were the apparent losses of aromatic carbon. The experimental conditions employed in acquiring the data produced a 10-l 5% underestimation of the aromaticity (,f!J values. Corrected values for this partial saturation effect on the non-protonated carbon are given in Table 4. The Blind Canyon and Illinois no. 6 HT-BCD extract-intact coal differences in fi were most significant (loss of 28 and 1 l%, respectively). The Pocahontas no. 3 coal showed essentially no loss in f’i when the correction for partial saturation was made. The losses in aromaticity for the two mid rank coals during depolymerization was beyond experimental error. More rigorous n.m.r. analyses on repeat depolymerizations will be necessary to determine the precision of the aromaticity comparisons. The losses in aromaticity were probably due to hydrogen transfer reactions from the methanolKOH system during the BCD step, affecting some of the aromatic ring structures. The other structural features for the Blind Canyon, Illinois no. 6 and Pocahontas no. 3 HT-BCD products (e.g. alkenes, alcohols, aromatic ethers, etc.) were reasonably consistent with the intact coal spectra. The determination of the structural features of coals is a complex task. The approach taken was to look at the molecules of each coal and separately determine the presence and abundance of these molecules. The coals reported in this study involve a distribution of maceral types. The analysis and interpretation has not considered this complication at this point. The next series of coals that are to be studied includes a vitrinite enriched preparation of coal, which will initiate an attempt to determine maceral specific differences in organic structure.

CONCLUSIONS The HT-BCD depolymerization procedure produces a coal-derived product with minimal molecular structural changes. no evidence of significant polymeric material, and the majority of molecules in the mass range for high resolution chromatographic analysis. The molecular profiles of the pre-extracts (mobile phase) and the HT-BCD products (depolymerized building units of the macromolecular structure) are distinctly different in low rank coals. The differences between these two sets of products decreases as coal rank increases. One possible explanation for the obvious difference between the compounds of the soluble and insoluble portions in such coals is that they originate from different plant precursors. The chromatographic analysis of the four depolymerized coals and their pre-extracts showed some trends that follow thermal maturity. Pentacyclic triterpanoidlike and other biological marker compounds were most prominent in the Beulah Zap pre-extracts (both aliphatic

Characterization

and neutral PAH fractions) and decreased as rank increased. In addition, the HT-BCD products of all coals (neutral PAH fractions) resembled the low volatile bituminous Pocahontas no. 3 solvent pre-extract. The PAH fractions of the pre-extracts and HT-BCD products also showed a gradual increase in average ring size as rank increased. This is consistent with n.m.r. data and previous chromatographic data. Finally, chromatography of the two predominant polar fractions of the HT-BCD products showed no indication of polymeric material; chromatograms displayed discrete, separated compounds, which are currently under structural investigation. ACKNOWLEDGEMENTS Financial support for the analytical part of this work was provided by the Gas Research Institute, Contract no. 5084-260-l 129, and by the Brigham Young University/University of Utah Advanced Combustion Engineering Research Center. Funds for this Center are provided by the National Science Foundation (Cooperative Agreement no. CDR 8522618) the State of Utah, and 28 industrial and government participants. Financial support for the coal depolymerization studies was provided by the US Department of Energy through the Consortium for Fossil Fuel Liquefaction Science (DE-FC22-89PC89851). REFERENCES I 2 3 4 5 6 7 8 9 IO

Vahrman, M. C/rem. Brifain 1972, 8, I6 Given, P. H.. Marzce, A., Barton, W. A. et al. Fuel 1986,65, I55 Stock. L. M. and Wang, S-H. Energy & Furls 1990, 4, 335 Dong, J.-Z. and Ouchi, K. Fuel 1989, 69, 1354 Nelson, P. F. Fuel 1987, 66, 1264 Dong. J.-Z., Katoh, T., Itoh, H. et al. Fur/ 1987, 66, 1336 Mudamburi, 2. and Given. P. H. Org. Geochem. 1985, 8, 221 Hatcher, P. G.. Breger, I. A., Szeverenyi, N. et al. Org. Geochem. 1982, 4, 9 Hatcher, P. G., Breger. I. A. and Earl, W. L. Org. Geachem. 1981,3,49 Bartle. K. D., Martin, T. G. and Williams, D. F. Fuel 1975. 54, LLO

11 12

Brooks, J. D. and Smith, J. W. Geochim. Cosniochint. At/u 1967, 31, 2389; and references therein Chang, H-C. K., Nishioka, M.. Bartle. K. D. et (I/. Fuel 1988, 67, 45

I3 I4 I5 16 I7 I8 I9 20

21 22 23 24 25 26 27 28 29 30 31 32 33 34 35 36 37 38

39 40

41 42 43

of depolymerized

coals:

R. E. Carlson

et al.

Sternberg, H. W., Delle Donne, C. L., Pantages, P. e/ ul. Fuel 1971, 50, 432 Liotta, R. Fuel 1979, 56, 9 Shaw, P. M., Brassell, S. C., Assinder, D. J. et al. Fuel 1988, 67, 557 Larsen, J. W. and Kuemmerle, E. W. Fuel 1976, 55, 162 Wachowska, H.. Ignasiak, T., Strausz, 0. P. e/ a/. Fuel 1986, 65, 1081 Heredy, L. A. and Neuworth, M.B. Fuel 1962, 41, 221 Ouchi: K., Imuta, K. and Yamashita, Y. Furl 1965, 44, 29 Havatsu. R., Winans, R. E.. Scott. R. G. e/ a/. in ‘Oreanic Chemistry of Coal, ACS Symposium Series 71’ (Ed. f W. Larsen). American Chemical Society, Washington DC, 1978, p. I08 Mayo, F. R., Pavellka, L. A., Hirschon, A. S. et al. Furl 1988, 67, 612 Redlich, P. J., Carr, R. M., Jackson, W. R. et a/. Fuel 1990, 69, 764 Singleton. K. E., Cooks, R. G., Wood, K. V. er a/. Fuel 1987, 66, 74 Shabtai, J. S. and Zhang, Y. ‘Proc. 1989 Int. Coal Conf. Coal Sci.‘, Vol. II, NEDO. Tokyo. 1989, pp. 8077810 Shabtai, J. S. and Saito, I. US Pat. 4 728418, 1988 White, R. MS Thesis University of Utah, 1989 Skulthai, T. PhD Di.wer~a/ion University of Utah, 1987 Shabtai, J. S. and Skulthai, T. ‘Proc. 1987 Int. Conf. Coal Sci.‘. Elsevier, Amsterdam, 1987, pp. 761-764 Shabtai. J. S., Skulthai, T. and Saito. I. Am. Chem. Sot. Dir. Fuel Chew. Prepr. 1986, 31, 15 Later, D. L., Lee, M. L., Bartle, K. D. et al. And. Chem. 1981, 53, 1612 Wang, H. P., Lo, R., Huai, H. er a/., Fuel in press Hayatsu, R., McBeth, R. L., Neill, P. H. e/ rr/. Energy & Fuds 1990, 4, 456 van Krevelen, D. W. ‘Coal’, Elsevier, Amsterdam. 1961 Snape, C. E., Stokes, B. J. and Bartle, K. D. Fuel 1981,60,903 Derbyshire, F., Marzec, A.. Schulten, H.-R. CI a/. Fuel 1989.68. 1091. Calkins, W. H. and Spackman, W. In/. J. Coal Geol. 1986,6, I Sigleo, A. C. Geochim. Cosmochim. Acta 1978, 42, 1397 Flaig, W., Beutelspacher, H. and Rietz, E. in Soil Components’ (Ed. J. E. Gieseking), Vol. I, Springer-Verlag. New York, 1975, pp. ll2ll Martin, F. Fuel 1975, 54, 236 Hayatsu, R., Winans, R. E., McBeth, R. L. e/ al. in ‘Coal Structure’ (Eds M. L. Gorbaty and K. Ouchi), American Chemical Society, Washington DC, 1981, p. 133 Meuzelaar, H. L. C., Huai, H. and Shabtai, J. personal communication Solum, M. S.. Pugmire, R. J. and Grant, D. M. Energy & FW/S 1989, 3, 187 Vorres, K. S. ‘Users Handbook for the Argonne Premium Coal Sample Program’, Argonne National Laboratory, Argonne, 1989

FUEL,

1992,

Vol 71, January

29