Molecular weight distributions and size fractionations of H-coal liquids

Molecular weight distributions and size fractionations of H-coal liquids Mohammed Department (Received

Mahfooz

Khan*

of Chemistry, Al-Fateh 21 October 1981)

University,

PO Box

13203,

Tripoli,

Libya

This Paper deals with a comparative study on the use of gel permeation chromatography (g.p.c.) and vapour pressure osmometry (v.p.0.) to obtain molecular weight data for the hexanesoluble fractions of three H-coal liquids. The use of two types of column packing materials, polyvinylacetate and styrene divinylbenzene copolymer gels, is described. A successful, preparative use of the polyvinylacetate gel to fractionate the hexane-soluble fraction of H-coal liquid, atmospheric still overhead (ASO), has been established. Molecular weight data obtained by v.p.0. for the benzenesolublefraction and the pyridine soluble fraction of the three H-coal liquids are reported. Solvent extraction has been utilized also to find the amount of oil, asphaltenes and asphaltols in the three H-coal liquids. (Keywords: analysis)

molecular

weight;

H-coal

liquids;

particle

Several processes for the conversion of coal into coalderived liquids are currently being developed. Coals differ in the ease with which they can be solubilized and each of the processes has its own advantages and disadvantages and each produces a different product mixture. One of the more advanced liquefaction methods is the H-coal process developed by Hydrocarbon Research Inc., which is carried out in an ebulliated-bed reactor in the presence of hydrogen and a desulphurization and hydrogenation catalyst ‘q2. The aim of the investigation in this Paper was to determine the composition of various soluble fractions of coal-derived liquids obtained via the H-coal process. Previously3 _ ‘, coal-derived liquids have been characterized using chromatography and ‘H n.m.r., ’ 3C and mass spectrometry. Gel permeation chromatography (g.p.c.) has been used for petroleum and coal liquefaction products’ and has been shown, recently, to be one of the better methods for molecular weight characterization of coal-derived liquids”. G.p.c. fractionates the coal-derived liquids according to molecular size in solution. To relate the retention times of the fractions to their molecular weights, it is necessary to calibrate the g.p.c. column. This is achieved by (1) taking liquids of known molecular weight, having molecules of similar sizes to those in the coal-derived liquids and narrow molecular weight distributions, and fractionating them on the column to obtain the retention times; and (2) determining the molecular weights of the resultant fractions by vapour phase osmometry (v.p.0.). This Paper describes the determination of molecular weight data for the hexane-soluble fractions (HSF) of various samples of coal-derived liquids by combining g.p.c. and v.p.0. * The work described in this Paper was carried out at the Department of Chemistry, University of Louisville, Louisville, Kentucky 40208, USA

001~2361/82/060553~4$3.00 @ 1982 Butterworth & Co (Publishers)

Ltd.

size distribution;

instrumental

methods

of

EXPERIMENTAL Materials

The coal-derived liquids were obtained from the Institute for Mining and Minerals Research, University of Kentucky, Kentucky, USA. The liquefaction was performed by the H-coal process in the ‘syncrude’ mode with the reactor temperature at 454°C and a reactor partial pressure of hydrogen of 15.5 MPa. The samples were designated atmospheric still overhead (ASO), atmospheric still bottom (ASB), vacuum still overhead (VSO), and vacuum still bottom (VSB). The nominal boiling ranges of these samples are: ASO, = 200°C; ASB, 200-350°C; VSO, 35s520°C; VSB is a solid. Vapour pressure osmometry

Molecular weights (MW) were determined on a Wescan model 233 molecular weight apparatus at 50°C with toluene and pyridine as solvents. Three solutions of each coal-derived liquid were prepared; i.e. 2,3 and 6 mg/ml of solvent. For each solution, three readings of the change in bridge current were taken and averaged to obtain a value for AV (average bridge current). Calibration was accomplished using sucrose octa-acetate. The intercept of the A; versus C (concentration

of sample, g/100 ml)plot

multiplied by the known molecular weight of the sucrose octa-acetate (678.6) gave the calibration factor K. Molecular weight values obtained experimentally for known compounds were: naphthalene (MW, 128.2) experimental value, 129.8; triptycene (MW, 254.3) experimental value, 257.0; acenaphthalene (MW, 154.2) experimental value, 156.1. Gel permeation

chromatography

The chromatograph consisted of a M 6000 A pump, a U6K injector and model 440 absorbance detector

FUEL,

1982,

Vol 61, June

553

Molecular weight data of H-coal liquids: M. M. Khan Tab/e 1 Solvent fractions obtained from the four H-coal samples (1) Atmospheric

still overhead

97.63%

(a) Hexane-soluble fraction (b) Benzene-soluble fraction fc) Pyridine-soluble fraction (2) Atmospheric

1.4% 0.56%

still bottom 92.10% 3.44% 3.98%

Ia) Hexane-soluble fraction fb) Benzene-soluble fraction fc) Pvridine-soluble fraction (3) Vacuum

(4) Vacuum

Fractionation of the coal-derived liquids

A solution of the hexane-soluble fraction of AS0 was prepared in the eluting solvent at a concentration of 360 mg ml-‘. Injections of 5&60 ~1 were made on the Frectogel columns. The eluent from the U.V.detector was passed to an automatic fraction collector which took fractions every 30 s. Appropriate fractions were the methanolchloroform solvent was combined, removed by evaporation, and the remaining coal fractions subjected to v.p.0. The fractionation was carried out with the U.V.detector set at 313 nm.

still overhead

92.70%

(a) Hexanesoluble fraction fb) Benzene-soluble fraction fc) Pyridinesoluble fraction

4.60% 0.37%

still bottom 7.30% 49.60% 42.0%

(a) Hexane-soluble fraction fb) Benzene-soluble fraction fc) Pyridinesoluble fraction

distribution. 22965 theoretical plates per metre were obtained by injecting 2.0% acetone onto the column. The column was standardized by injecting fractions of coalderived liquids of known molecular weights into it. A plot of log M, versus V, (retention volume) was rectilinear. The values for M,, M, and M WD for the coal-derived liquids were obtained as described previously.

Solvents

operating at 254 and 313 nm, all obtained from Waters Associates, Milford, Massachusetts. Frectogel PVA 500. Polyvinylacetate 500 (37-74 pm) was obtained from Rainin Instrument Co. Inc., Brighton Mass. This material was slurry-packed at 5.5 MPa into two 610 mm x 7.8 mm stainless-steel columns and elution was carried out at a flow rate of 1 ml mini with chloroform-methanol (3:l). The number of theoretical plates obtained by injecting 2% acetone onto this column was 1148 m-i. These columns, connected in series, were standardized by injecting samples of coal-derived liquids of known molecular weight into them. A plot of log M, versus V,(retention volume) was found to have a concave shape. Injections of 2&30 ~1 of samples of coal-derived liquid, with concentrations of 14.00 g l- ‘, were made, and values for M,, M, and M WD for the coal-derived liquids were obtained from the Following equations:

M w

=i$,i”i i$l Hi

(1)

IT?Hi

Mn= ni=l

MWD=2

(2)

”

where: Hi, peak height of molecular species i; Mi, molecular weight obtained from the coal-derived liquid calibration plot based on V, of species i”; M,, weightaverage molecular weight; M,, number-average molecular weight; M WD, molecular weight distribution. Toyo Soda column, G 2000HlO. This type of column,

made up of cross-linked styrene+livinylbenzene copolymer, is a high-resolution g.p.c. gel which can have a large number of theoretical plates because it has a finely controlled small pore size and structure, and a narrow size

554

FUEL, 1982,

Vol 61, June

All the solvents described in this Paper were of analytical grade and were used without further purification. RESULTS AND DISCUSSION Generally, the quality of coal products has been classified in terms of solubility classes. Thus, the solubility of the four coal-derived liquids from the H-coal process in hexane, benzene and pyridine are given in Table 1. Some characterization of the components of the various solubility classes of coal-derived liquids has been described previously4*‘2*‘3. Fractionation of coal-derived liquids and calibration of Frectogel and Toyo Soda column

The size of samples used in previous investigations into varies widely. For analytical g.p.c. separation determinations the sample sizes are small, whereas for preparative size separation, larger quantities may be used. The use of g.p.c. in coal research has been limited mainly to analytical separations14,i5. To separate large amounts of coal-derived liquids rapidly it was necessary to establish a suitable column; consequently a column packed with polyvinylacetate 500 (PVC 500) Frectogel was investigated. The injection of the hexane-soluble fraction of AS0 onto the PVC 500 column resulted in a chromatogram which could be separated into 15 fractions. With the U.V.detector set at a wavelength of 254 nm, only samples of analytical size could be injected onto this column; for larger sample sizes (e.g. 20 mg per injection) it was more expedient to use a higher wavelength (i.e. 313 nm) to maintain a linear response by the detector by virtue of lower extinction coefficients. Figure 1 shows high-pressure liquid chromatography (h.p.1.c.) chromatograms for the hexane-soluble fractions of VSO, VSB, AS0 and ASB on the PVC 500 Frectogel coumn. Calibration of columns and correlation of gel permeation chromatography and vapour pressure osmometry

The method usually for the determination derived liquids may elemental composition

used to calibrate the g.p.c. columns of molecular weight data for coalbe inaccurate because (1) the of the coal-derived liquids is not

Molecular weight data of H-coal liquids: M. M. Khan

volumes, and the retention volumes of the fractions on Toyo Soda columns. On the basis of these calibration curves, the gel permeation chromatograms for HSF of ASO, ASB, VSO and VSB were analysed according to the standard technique”. Table 3a gives (1) the number-average molecular weight (M,); (2) the weight average molecular weight (M,); and (3) the molecular weight distribution (MWD)for the hexanesoluble fractions of the various coal-derived liquids, Table 4 gives the molecular weight data obtained by v.p.0. for the benzene-soluble fractions and pyridinesoluble fractions of ASB and VSB. The molecular weight data for these fractions were determined using pyridine as a solvent in which all the fractions were completely soluble. 4

8

12

16 20 24 28 Retentron volume

32 36 (ml)

40

44

48

CONCLUSION From Table 2, it is evident that the PVC 500 Frectogel column separated the hexane-soluble fractions of AS0 according to the size of the coal molecules, i.e. the molecules of highest-molecular-weight species eluted lirst lowest-molecular-weight species last. and the Furthermore, it is evident from Table 3 that M, values for the hexane-soluble fractions of ASO, ASB, VSO and VSB,

Figure 1 G.p.c. runs on Frectogel PVC 500 for hexane-soluble fractions of A, VSO; B, VSB; C, ASO; D, ASB; at 313 nm

1.8 14

1.0’ 0

’

’

4

8

’

12

’

16

’

20

Retentron figure

2

Calibration

’

24

’

28

’

32

’

36

’

40

’

44

volume (ml)

curve for hexane-soluble

fraction

of AS0

fully known; and (2) the composition and structure of the coal-derived liquids can vary between samples. Thus, in this Paper, in an attempt to minimize the problems, the Frectogel and Toyo Soda columns were calibrated using very narrow fractions of the HSF of ASO, after establishing the number-average molecular weight (M,) for these fractions by v.p.0. The results presented here show that the separation of coal extracts into hexanesoluble fractions yields products of sufficiently narrow molecular weight distribution to be useful in calibration. Figures 2 and 3 summarize the M, data for the hexanesoluble fractions of AS0 obtained by v.p.0. The calibration curves, obtained on PVC 500 Frectogel and Toyo Soda columns by plotting log M, versus V, were concave and straight, respectively. For the Frectogel calibration curve, the down curvature at the lower end is probably due to the poor fractionation efficiency of this in the very-lowparticular column combination molecular-weight range, which is normal behaviour for any g.p.c. column near the total permeation point. Table 2 shows the molecular weight data obtained by v.p.0. for the HSF of AS0 after fractionation on the Frectogel column. This table also shows the amount of each fraction collected on this column and their retention

0

I

I

4

8

I 12

I

I

I

I

16

20

24

28

Retention Toyo Figure 3 fraction of AS0

volume

Soda column calibration

1

I

32

(ml) curve for hexane-soluble

Table 2 Molecular weight data obtained by v.p.0. for the fractionated HSF (ASO) on the PVC 500 Frectogel column and the retention volumes of each fraction on PVC 500 Frectogel and Toyo Soda columns

Combined fractions

M, from V.P.O.

l-23 24-35 36-42 46-49 50-58 60-62 63-65

627 603 304 242 206 196 115

Log M,

2.7973 2.7603 2.4629 2.3838 2.3139 2.2923 2.0607

V, on Frectogel (ml)

Vr on Toyo Soda (ml)

14.8 15.0 19.6 24.0 27.0 30.6 32.0

12.0 12.2 14.0 14.4 15.6 16.0 16.4

FUEL, 1982, Vol 61, June

555

Molecular weight data of H-coal liquids: M. M. Khan Tab/e 3 A comparison of the molecular weight data for the hexanesoluble fractions of ASO. ASB, VSO, and VSB on polyvinylacetate PVC-500 and styrenedivinylbenzene copolymer gels a G.p.c. on polyvinylacetate

gel and V.P.O.

G.p.c. - polyvinylacetate v.p.0.

COal fraction

Mn

KV

MWD

Mn

HSF HSF HSF HSF

193.0 204.4 208.8 230.0

213.8 215.0 215.7 244.0

1 .lO 1.05 1.03

183 203 229

1.06

294

(ASO) (VSO) (ASB) (VSB)

b G.p.c. on styrenedivinylbenzene G.p.c. -

copolymer

gel and v.p.0.

styrene-divinylbenzene copolymer v.p.0.

Coal fraction

Mn

M,

MWD

Mn

HSF HSF HSF HSF

193.0 197.0 200.0 239.6

206 224 211 267

1.06 1.13 1.06 1.16

183 203 229 294

(ASO) (VSO) (ASB) (VSB)

obtained from the chromatograms on PVC 500 Frectogel and styrene-divinylbenzene (SDB) copolymer gel (Toyo Soda) columns produce similar results, indicating the reliability of using a polyvinylacetate gel and an SDB copolymer gel for size separation. However, the M, data obtained from g.p.c. chromatograms as described previously compared with the data obtained from v.p.o (Table 3a and b) for hexanesoluble fractions of ASO, ASB, VSO and VSB, show that these data are in two categories. One, in which the M, data obtained from g.p.c. are in full agreement with the data obtained from v.p.o. for the hexane-soluble fractions of AS0 and VSO, and a second category, which shows a significant difference between the M, values obtained from g.p.c. and v.p.0. for the hexane-soluble fractions of ASB and VSB. From this it can be inferred that hexanesoluble fractions of AS0 and VSO closely resemble one another in chemical composition, whereas ASB and VSB samples are quite different from AS0 and VSO. Generally, hexane-soluble fractions (oils) have little function, but current processes which convert coal into liquids can change the function as well as the structure. It is also evident that calibration of the g.p.c. column with a sample of coal-derived liquid has some limitations and cannot be used for determining the molecular weight of other coal samples by the standard method”, because the structure and function of coal-derived liquids play an important role in size separation.

556

FUEL, 1982, Vol 61, June

Table 4 Molecular-weight

data by v.p.0.

Coal fraction Benzene-soluble Benzene-soluble Pyridine-soluble Pyridine-soluble

Mn fraction fraction fraction fraction

(ASB) (VSB) (ASB) (VSB)

312 433 419 542

The molecular weight data for benzene-soluble fractions and pyridine-soluble fractions of ASB and VSB, obtained by v.p.o., agree with molecular weights reportedby earlier workers4*‘3*14 for benzene-soluble fractions (asphaltenes) and benzene-insoluble fractions (asphaltols) and were found to be in the range of 300-700 and 400-2000, respectively. ACKNOWLEDGEMENT The author wishes to thank Professor John L. Wong, Chemistry Department, University of Louisville, Louisville, Kentucky, for his support of this work and for his many valuable comments, and also Mrs A. Ikram for typing this manuscript. REFERENCES 1 2

3 4 5 6 I 8 9 10

11

12

13 14 15

Katz, D. L. ‘Evaluation of Coal Conversion Process to Provide Clean Fuels’, EPRI 206-0-O Final Report, Parts II, III, Feb. 1974 Kermode, R. I. ‘Project 2. Gasification and Liquefaction of Kentucky Coal’, Institute for Mining and Minerals Research Annual Report, Sept. 30, 1973 Sternberg, H. W., Raymond, R. and Schweighardt, F. K. Science 1975, 188, 49 Farcasiu, M. Fuel 1977, 56, 9 Wooton, D. L., Coleman, W. M., Taylor, L. T. and Dorn, H. C. Fuel 1978, 57, 17 Seshadri, K. S., Ruberto, R. G., Jewell, D. M. and Malone, H. P. Fuel 1978,57, 111 Dark, W. A., McFadden, W. H. and Bradford, D. L. J. Chromatogr. Sci. 1977, 15,454 Thomas, A. Erdol Kohle, Erdger, Petrochem. Brennst.-Chemie 1973,26(l), 27 Dark, W. A. J. Chromatogr. Sci. 1978, 16, 289 (and references therein) Yoshii, T. and Sato, Y. Fuel 1979, 58, 534 Ouano, A. C., Barrall, E. M. and Johnson, J. F. ‘Polymer Molecular Weights’, Part II (Ed. P. E. Slade, Jr.), Marcel Dekker, New York, 1975, Chap. 6 Whitehurst, D. D., Farcasiu, M. and Mitchell, T. 0. ‘The Nature and Origin of Asphaltenes in Processed Coals’, EPRI AF-252, Project 410-1, Annual Report, February 1976 Farcasiu, M., Mitchell, T. 0. and Whitehurst, D. D. Chem. Tech. 1977, 7, 680 Dark, W. A. Am. Lab. 1975, August,

50 Prather, J. W.,Tarrer, A. R.,Guin, J A., Johnson, D. R. and Neely, W. C. Am Chem. Sot., Div. Fuel Chem. Preprints 1976,21(5), 144