Characterization of lignite low-severity depolymerization products

Fuel Processing Technology, 11 (1985) 87--98 Elsevier Science Publishers B.V., Amsterdam --Printed in The Netherlands

87

C H A R A C T E R I Z A T I O N OF LIGNITE LOW-SEVERITY DEPOLYMERIZATION PRODUCTS

ANA MARIA MASTRAL and VICENTE L. CEBOLLA Instituto de Carboqu~mica, Plaza Parafso 1, 50004-Zaragoza (Spain) (Received November 16th, 1984;accepted April 4th, 1985)

ABSTRACT Oils and asphaltenes derived from direct extraction and several mild depolymerization processes have been studied. The aspbaltenes have been fractionated by column adsorption chromatography (with deactivated silica gel) and benzene, THF and MeOH were used in sequence as eluothropic series. Clear chemical separation between one aromatic and two polar fractions has been obtained, giving high percentages recovery. The fractions have been characterized by VPO, FT-IR, ~H-NMR and elemental analysis. Several structural parameters of oils have been calculated. These oils can be assimilated to equivalent average hydrocarbons having between 10 and 20 carbon atoms and an aromatic carbon percentage oscillating between 47 and 66%. In general, the degree of substitution in aromatic rings is low and the presence of phenolic groups is limited. The majority of the carbons are aromatic and these rings show low degrees of condensation.

INTRODUCTION T h e liquids derived f r o m a n y coal t r a n s f o r m a t i o n process are c o m p l e x m i x t u r e s , w h i c h , with respect to analysis, m u s t be f r a c t i o n a t e d a c c o r d i n g to their solubility p r o p e r t i e s [ 1 ]. N o r m a l l y , low m o l e c u l a r weight h y d r o c a r b o n s are used in a first fractionat i o n and hence, t h e separated g r o u p s o f c o m p o u n d s are referred to as paraffin-insolubles (asphaltenes) and paraffin-solubles (mal.tenes or oils). O f course, these are o p e r a t i o n a l d e f i n i t i o n s [2] w h i c h o n l y provide general inf o r m a t i o n o n t h e m i x t u r e to be a n a l y z e d . This m e t h o d o f f r a c t i o n a t i o n is n o t c o m p l e t e l y r e p r o d u c i b l e because the solubility o f these kinds o f c o m p o u n d s s o m e t i m e s d e p e n d s o n the presence or absence o f o t h e r c o m p o u n d s w h i c h can act as co-solvents [ 3 ] . T h e r e f o r e , s u b s e q u e n t f r a c t i o n a t i o n s normally carried o u t b y m e a n s o f d i f f e r e n t c h r o m a t o g r a p h i c t e c h n i q u e s are necessary. A review o f the a p p l i c a t i o n o f these t e c h n i q u e s to coal-derived liquids, and s p e c t r o s c o p i c c h a r a c t e r i z a t i o n o f the o b t a i n e d fractions has been given b y Pourier and G e o r g e [ 4 ] . These t e c h n i q u e s have e x p e r i e n c e d a spectacular advance in t h e last few years due to the a d v e n t o f F T I R [ 5 , 6 ] , 1HNMR [7] and especially solid-state '3C-NMR [8] (cross polarization, magic angle spinning) s p e c t r o s c o p i c t e c h n i q u e s . 0378-3820/85/$03.30

O 1985 Elsevier Science Publishers B.V.

88 Due to the mild conditions used, coal depolymerization catalyzed by Lewis acids does not modify the original molecules to large degree as the main original structure remains unchanged, and so its derived products represent acceptable portions of coal molecules. Analyses and characterizations of asphaltenes and oils recovered from depolymerization processes [10,11] are presented in this paper, using: anisole, m-methylanisole and 1,3-dimethoxybenzene as solvents; -- BF3, A1C13, ZnC12, SbC13 as catalysts; 1/1, 4/1 as ponderal ratios catalyst/lignite. Column adsorption chromatography was applied to asphaltenes derived from these reactions with a chosen elution sequence of solvents. Schwager and Yen's eluothropic series [12] were also used. The fractions were studied with the help of spectroscopic techniques, MW determination and elemental analysis. Several structural parameters of the oils were determined by 1H-NMR spectroscopy according to Bartle et al.'s method [7]. Although assuming a value x for (Hal/Cal), these authors state that the determination by ~H-NMR agrees with the information obtained by ~3C-NMR and IR. At present, methods joining ~3C- and 1H-NMR [13--15] on the one hand, and 1H-NMR and IR [16,17] on the other hand, have been developed, which allow for the calculation of x, based, in the case of ~H-NMR/IR, on Haley's equations [18]. Nevertheless, Shu-An Qian et al. [16] suggest that, even with the advantages manifested by these methods, the estimation of parameters by IH-NMR is valid and especially suitable for structures of low aromaticity with long straight-chain alkyl substituents [16]. -

-

-

-

EXPERIMENTAL The coal used was a Spanish lignite from Utrillas (UL). Proximate and ultimate analyses were reported in a previous paper [10]. Depolymerization processes and fractionation were carried out under the conditions mentioned in the same paper [10]. IR spectra of KBr pellets were recorded using Nicolet MX-10 and PerkinElmer Model 457 spectrometers. 1H-nuclear magnetic resonance spectra were made using a Varian FT-80 spectrophotometer. Elemental analyses were carried out in an automatic analyzer Leco CHN-600, and molecular weight determinations by VPO in a Hitachi--Perkin-Elmer apparatus, using toluene as a solvent. THF was also tested as a solvent, but it was found impossible to stabilize the apparatus when using it. Adsorption chromatography was performed in a column (25 × 500 mm) of 10% deactivated [19] silica gel (Kieselgel 60, Merck, 70--230 mesh). The ratio sample weight/gel weight used was 1/20. Benzene, tetrahydrofuran and methanol were used as solvents. Samples were introduced into the column after previous adsorption on CaCO3. The columns were packed with benzene as packing liquid.

89 UL m e t h y l a t i o n was carried out in N2 atmosphere, adding to 40.00 g UL 29 g K2CO3, 20 ml Me2SO4 and an excess acetone. The mixture was heated to th e boiling point of acetone and stirred for 12 hours. After the 12 hours, the same amounts o f the reagents were added and the operation was repeated. Subsequently, the liquid was distilled off by rot ary evaporation. The solid was washed with water acidified with HC1 and t hen washed with water only in order t o eliminate the carbonates present. Methylated UL was vacuum filtered and dried at 105°C. RESULTS AND DISCUSSION One of the i m p o r t a n t problems in the processes of coal transformation results f r o m its solid nature and limited solubility, which makes it difficult to handle. Catalytic d e p o l y m e r i z a t i o n [10,19] gives a solubility increase of coal by means o f the selective breaking of labile bonds in its molecules influenced by a catalyst (protic or Lewis acid) and an aromatic solvent. The breaking of coal molecules and the incorporation of the aromatic solvent give compounds of low molecular weight which are more soluble than the original lignite. Mechanism and general scheme were shown in a previous paper [ 10].

L Asphaltenes Asphaltenes derived f r om : -- the described process of depolymerization, d e p o l y m e r i z a t i o n with phenol--BF3 (cat/UL=l/1) (this experiment gives the best yield), -- direct extraction of the lignite with solvents, have been fractionated by column adsorption chromatography. Several combinations of solvents were tested before choosing the eluothropic series used (benzene, T H F , MeOH). First of all, Schwager and Yen's sequence of solvents [12] (benzene, Et20, T H F ) was applied to the asphaltenes derived from t ol uene- - et hanol direct extraction giving a low recovery (60%). Likewise, the fractions eluted with Et20 and T H F showed the same IR spectra which were similar to those of the original asphaltenes. A n o t h e r sequence which was used was based on Farcasiu's m e t h o d [ 3 ] . Benzene, Et20, CHC13, EtOH, MeOH, T H F and pyridine (Py) were used. This series also yielded a low percentage recovery and the separations between the fraction were o f poor quality. Finally the selected sequence was: benzene, T H F and MeOH. High percentages recovery were observed (~100%) except in the case of the preasphaltenes, because t h e y are exclusively constituted of very polar comp o u n d s and can interact irreversibly with the silica gel (see Table 1). In these preasphaltenes, the polar fraction eluted with T H F is the largest portion. Th e nature of the fractions is described in the following sections. -

-

90 TABLE 1 E l u t i o n a n d r e c o v e r y p e r c e n t a g e s o f some c o l u m n c h r o m a t o g r a p h i c f r a c t i o n s

P r e a s p h a l t e n e s f r o m anisole--BF 3 A s p h a l t e n e s derived f r o m : a n i s o l e - ZnC12 m - m e t h y l a n i s o l e -- ZnC12 1 , 3 - d i m e t h o x y b e n z e n e -- ZnC12 p h e n o l -- B F 3 anisole -- B F 3 A s p h a l t e n e s f r o m direct e x t r a c t i o n w i t h : THF

"

Recovery %

% Fraction eluted with

Sample

\

~

Benzene

THF

MeOH

2.98

66.73

30.29

64

9.63 34.59 23.91 22.75 40.12

85.93 63.20 74.03 74.57 53.89

4.44 2.21 2.06 2.68 5.99

96 100 100 95 92

14.91

78.07

7.02

95

~

°

d

' 3500 3000

2000

' 1600

12O0

'

8'00

Fig. 1. IR spectra o f t h e f r a c t i o n s e l u t e d w i t h b e n z e n e (a) d e p o l y m e r i z a t i o n w i t h p h e n o l - - B F 3 ( e x t r a c t i o n w i t h z a t i o n w i t h p h e n o l - - B F 3 ( e x t r a c t i o n w i t h T H F ) ; (c) first derived f r o m d e p o l y m e r i z a t i o n w i t h anisole--BF3; (d) extraction with toluene--ethanol (1:1).

'

' Ocm" /~O

f r o m a s p h a l t e n e s derived f r o m : t o l u e n e - E t O H ) ; (b) d e p o l y m e r i fraction (saturated); asphaltenes s e c o n d f r a c t i o n ; a n d (e) direct

91 Ia. Fraction eluted with benzene This f r a c t i o n is m a i n l y c o m p o s e d o f c o m p o u n d s o f a r o m a t i c n a t u r e , according to the IR and IH-NMR spectra. A l t h o u g h bands d u e to e t h y l i c esters ( 1 7 3 0 cm -1 IR; 4.15,c; 1,2,t 1H-NMR) are observed in samples exclusively t r e a t e d with e t h a n o l . In t h e case o f the asphaltenes e x t r a c t e d with T H F , these signals are n o t observed. These bands are also observed in the a r o m a t i c fractions o f p h e n o l - - B F 3 asphaltenes e x t r a c t e d e i t h e r with tol u e n e - e t h a n o l or with T H F {Fig. 1). Several f r a c t i o n s have b e e n o b t a i n e d b y eluting with b e n z e n e in some e x p e r i m e n t s . C o m p l e t e l y saturated c o m p o u n d s , which are similar to oils, a p p e a r in the first f r a c t i o n eluted (see Fig. 1). This is n o r m a l , because, in n - p e n t a n e f r a c t i o n a t i o n , these kinds o f substances can appear s o m e t i m e s in the oils, and s o m e t i m e s in the asphaltenes d e p e n d i n g on t h e i r relative solubilities with respect to o t h e r c o m p o u n d s .

b

a

I

10

I

I

I

i

I

I

I

I

i

9

8

7

6

5

Z.,

3

2"

1

P~

Fig. 2. NMR spectra of the fractions eluted with benzene from asphaltenes derived from: (a) depolymerization with phenol--BF3; (b) depolymerization with anisole--BF3; and (c) depolymerization with anisole--ZnC12.

92

In the aromatic fraction of the asphaltenes extracted with toluene-ethanol (Fig. 1), the presence of - N H : IR bands and the absence of - O H bands can be observed. In depolymerized asphaltenes, - O H bands appear, probably due to demethylation of solvent incorporated in the depolymerization [10], so, in the case of phenol--BF3, - O H absorption is broad and strong. 1H-NMR spectroscopy confirms the information obtained by IR. In Fig. 2 the incorporation of the depolymerization solvent can be detected (anisole, 3.5,s; phenol, an increase of aromaticity; m-methylanisole, 3.5,s; and 2.2,s). The structural similarity between the aromatic fractions of the asphaltenes and the corresponding oils is remarkable. Data obtained from 1H-NMR and molecular weight determinations of these fractions are shown in Table 2. TABLE 2 M o l e c u l a r w e i g h t a n d p e r c e n t a g e o f d i f f e r e n t t y p e s o f H w i t h r e s p e c t to t o t a l H o f s o m e f r a c t i o n s e l u t e d w i t h b e n z e n e (~H-NMR) Sample Asphaltenes extracted with: Toluene -- ethanol (1/1) THF A s p h a l t e n e s derived f r o m : phenol -- BF 3 anisole - - B F 3 anisole - - ZnC12 S o l u b l e p a r t in t o l u e n e f r o m asph. derived f r o m : anisole - - BF 3

MW

Hat, %a

He, %b

Ha , %c

Ho ' %d

301 430

25 29

19 -

12 28

44 43

344 292 275

39 40 32

10 31 20

10 -

41 29 48

250

39

32

-

28

1600

1200

a A r o m a t i c H. b E t h e r i c a n d e s t h e r i c H. c H o f a - g r o u p s to a r o m a t i c rings. d O t h e r t y p e s o f H.

3500

3000

2000

800

cm

Fig. 3. I R s p e c t r u m o f t h e f r a c t i o n e l u t e d w i t h T H F f r o m a s p h a l t e n e s derived f r o m phenol--BF 3 depolymerization.

93 Ib. Fraction eluted with THF This is a mixture of polar compounds and constitutes the major fraction. It shows IR absorptions at 3500 cm -~ and between 1000--1200 cm -1 that can be attributed to - O H groups. A minor concentration of ester- and carbonyl-groups is found, compared to those in the aromatic fractions (Fig. 3). In some cases, IR spectra of these fractions are very similar to those of the original asphaltenes, due to the complexity o f the mixture. Elemental analysis shows that the percentage C varies between 51 and 75 (though the majority of the samples contain approximately 70%), percentage H oscillates between 4.75 and 7.75 and percentage N between 0.35 and 1.25. These fractions contain the major portion of nitrogenated compounds. Ic. Fraction eluted with MeOH This is the most polar fraction and the smallest one with respect to the total amount. The - O H absorption is greater than that of the other fractions and greater sharpness in IR bands is observed, which could indicate the presence of relatively few compounds. Gas chromatography of this fraction of original asphaltenes extracted with toluene--ethanol conforms this hypothesis. H. Oils Structural characteristics of different oils have been deduced from the 1HNMR data, starting from: - - o i l s obtained by UL and methylated UL direct extraction with several solvents; -- oils derived from depolymerization processes. These parameters are not completely accurate due to the supposed value of x (Hal/Cal) and to difficult integration of the signs, but they serve to charac+~erize the average molecule and, therefore, its chemical behaviour. Battle et al.'s method, applied in this paper, shows greater variety of H types than the old Brown--Ladner m e t h o d [20] and overcomes some of its limitations. The calculated parameters and the corresponding formulae, together with the types of H and the chemical shifts are shown in Table 3. As a,c,d coefficients 2.3, 2.3, 3 have been selected, respectively, and H'/b has been eliminated as negligeable from this study because, in practice, signs do not appear between 1.6 and 2.0 ppm. In the formulae for calculation of the parameters of equivalent hydrocarbon in which the addition relating to non-phenolic oxygen appears, the oxygen due to ester double bonds (~C=O) has been discounted, because typical signs of ethylic ester (4.15,c; 1.25,t coupled; 1735 cm -1) appear in the oils in large concentrations. The number of oxygens due to esters has been calculated from integration of the 4.15 ppm signal. Therefore, the

94 TABLE 3 a. H-types and chemical shifts considered in 1H-NMR T y p e of H H'ar H'OH H' F H'est H'et H'a H'~

Chemical shift (ppm) Aromatic Phenolic Ar--CH2--Ar R--CO~--CH~--CH 3 R--OCH3 CH3, CH: and CH a to an aromatic ring CH3, CH~ and CH t~ or further from an aromatic ring CH~ ~ or further from an aromatic ring

H' 7

6.0- 9.0 5.0--6.0 3.3--4.5 a 4.15,c 3.7 ,s 1.9-- 3.3 1.0--1.9 0.5--1.0

a H,est and H'et are not considered. b. Parameters and corresponding formulae Parameter

Symbol

Formula

Total carbon Total hydrogen Alkyl and naphthenic carbon N u m b e r of alkyl and naphthenic groups A r o m a t i c and ring-joining carbon Ring-joining m e t h y l e n e Ring-joining carbon A r o m a t i c and ring-joning hydrogen N u m b e r of aromatic rings and rings containing ring-joining m e t h y l e n e groups. % A r o m a t i c carbon Degree of alkyl substitution Degrees of alkyl and phenolic substitution Measure of the degree of condensatio n Alternative measure o f the degree of condensation

C' E //1E C'al

C'+n-pO'+IV +S' H'+2(n-pO'+N'+S') ITa/a + IT~tic + H'7/d

AG

H'a/a

C'R

RJM C'i H' R

(a=2.3, c=2.3, d=3) C'E-C'al

H'F/2 + n-pO'+N'+S' C'R-H'ar-H'oH-A G - R J M C'R-C'i+RJM

Ra+ RJM (C'i+2)/2 Car [ l O0( C'-C'al-tT F/ 2-IT et/ 3-H'est) ] / C' AG/(AG+H'ax+H'oH) ~

(AG+H'oH)/(AG+H'ar+I-ToH)

cd

(/TR--6)/(C'R- 6 )

cd~

C'j/ C'R-6

number of oxygens which are neither phenolic nor due to ester double bonds, and which cannot be substituted by hypothetic - C H 2 - to calculate the equivalent hydrocarbon, will be: H'OH + H'est/2. Taking into account the ester and methylic ether (3.7 ppm, s) signs of the depolymerization solvents, the formula of C aromatic percentage is 1 0 0 [ C ' - C ' al - ( / / ' F / 2 ) - (H' e t / 3 ) - H e s t ]

Car =

C'

95

0

o

0

~

0

0

0

0

0

ddo

odddd

doo

oooS

0

0

0

0

0

0

0

0

0

0

0

d d S S o g d o

0

0

0

0

0

0

0

~

0

0

0

0

0

0

0

0

o S d o o d s S

~2

.o

b

0 "0

o

0

~

r.

b

0 ~ 0 ~

dod

I O 0

I

I

I

I

I

I

O I I I I

d

od

~

II

I

I

I

l

l

l

l

l

0

e~

o

0

~

0

~

<

[...

0

~

~

~

~

~

~

'

96 The values o f the parameters are shown in Table 4. The average molecules show a low number of C and H atoms. However, in the reaction with anisole--SbC13 (1/1), the oils recovered contained 26 C and 35H. The number of aliphatic carbons and alkylic groups is not high except in the case of the methylated and toluene-extracted lignite oils, which show a high concentration in groups to aromatic rings and a low C'j value. High RJM values, sometimes superior to those of C'al , a r e observed. This is due to the high concentration of ester and ether forms in the oils. The average equivalent hydrocarbon associated with the oils extracted with toluene--ethanol has C19H26 as an empirical formula. Aproximately 21% of these carbons are aliphatic and aproximately 7% of these are due to aromatic groups. These oils have three --CH2-- groups and four rings; between the aromatics groups and the rings there are ring-joining methylene groups. Thirty two percent of the C atoms are internal aromatic carbons or aromatic carbons joined to ring-joining methylene groups. The number of hydrogens joined to aromatic rings and ring-joining methylene groups (when alkyl and naphthenic groups are replaced by hydrogen) is 45% with respect to the total a m o u n t of H. The average equivalent hydrocarbon associated with the oils of the depolymerization process carried out with BF3--DMR (1/1) (which gives the best yield) has CllH~6 as an empirical formula. Therefore, it has a lower molecular size than that of the original oil. The a-group contribution to alkylic groups of the average molecule is greater in this case (~27%), although the aliphatic C percentage is less. The average of the two --CH2-- bridges and the average of three carbons between internal aromatics, and aromatics joined to ring-joining methylene groups have been calculated. The average number of rings is 2. Two considerations must be taken into account concerning aromaticity: firstly, the aromaticity increases in oils derived from direct extraction of previously methylated lignite, compared with those which are derived from nonmethylated oils. Secondly, the aromaticity increases with the process yield for each solvent separately, although with some exceptions. The low degree of substitution (alkylic and phenolic) is also noticeable. The majority of phenolic compounds appears in the asphaltenic fraction. The discrepancy observed in Car and a with respect to the values previously calculated [11] can be attributed to the different manner of interpreting and integrating the H signals because B r o w n - L a d n e r ' s method only considers three different kinds of H. The numbers cdl and cd2 represent two different ways of interpreting the degree of condensation [7]. While cd~ diminishes with the increase of the degree of condensation of aromatic structure, cd2 increases. The oils derived from BF3--DMR, ZnC12--mMA and SbC13--anisole are among the more condensed structures, and the ones extracted are less condensed from methylated UL.

97 CONCLUSIONS T h e asphaltenes f r o m e x t r a c t i o n and d e p o l y m e r i z a t i o n processes have been f r a c t i o n a t e d into o n e a r o m a t i c and t w o p o l a r s u b f r a c t i o n s b y c o l u m n a d s o r p t i o n c h r o m a t o g r a p h y with high p e r c e n t a g e s recovery. T h e n a t u r e o f t h e a r o m a t i c fractions f r o m these asphaltenes is very similar to the n a t u r e o f a r o m a t i c fractions f r o m their c o r r e s p o n d i n g oils. S o l v e n t i n c o r p o r a t i o n can be c o r r o b o r a t e d in the spectra o f the a r o m a t i c f r a c t i o n r e c o v e r e d after lignite d e p o l y m e r i z a t i o n . The polar s u b f r a c t i o n s eluted with T H F are t h e largest for all the processes and show the highest p e r c e n t a g e s o f n i t r o g e n c o m p o u n d s . T h e polar s u b f r a c t i o n s eluted with MeOH are practically insignificant. A low s u b s t i t u t i o n degree and also a low c o n d e n s a t i o n degree can be ded u c e d f r o m the s t r u c t u r a l p a r a m e t e r s calculated for the oils r e c o v e r e d f r o m Utrillas lignite b y e x t r a c t i o n and d e p o l y m e r i z a t i o n processes. Their equivalent h y d r o c a r b o n associated with the d e p o l y m e r i z a t i o n oils being smaller t h a n their equivalent e x t r a c t i o n oils. ACKNOWLEDGEMENTS T h e a u t h o r s wish to t h a n k C A I C Y T for the financial s u p p o r t , Project No. 1 7 0 6 - 8 2 C 0 2 - 0 1 , and o n e o f us (VLC) gratefully a c k n o w l e d g e s the help and s u p p o r t o f M ° de I n d u s t r i a y Energia.

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98 11 Mastral, A.M., Cebolla, V.L. and Gavil~in, J.M., 1985. Alkyl aryl ethers in lignite solubilization. 2. Analysis of oils. Fuel, 64: 316. 12 Schwager, I. and Yen, T.F.~ 1979. Chromatographic separation and characterization of coal-derived asphaltenes. Fuel, 58 : 219. 13 Snape, C.E. and Ladner, W.R., 1984. The chemical nature of asphaltenes from some coal liquefaction processes Fuel Processing Technology, 8: 155. 14 Dickinson, E.M., 1980. Structural comparison of petroleum fractions using proton and 13C-NMR spectroscopy. Fuel, 59: 290. 15 Seshadri K.S., Albaugh, E.W. and Bacha, J.D., 1982. Characterization of needle coke feedstocks by NMR. Fuel, 61 : 336. 16 Q i a n S.A., Li, C.F. and Zhang, P.Z., 1984. Study of structural parameters on some petroleum aromatic fractions by ~H-NMR/IR and 13C/~H-NMR spectroscopy. Fuel, 63: 268. 17 Haley, G.A., 1972. Unit sheet weights of asphalt fractions determined by structural analysis Anal. Chem., 44: 580. 18 Membrado, L., Mastral, A.M. and Gavil~in, J.M., 1985. Fraccionamiento de bitumen mediante cromatograffa de adsorci6n. Afinidad, 4 2 : 1 6 5 . 19 Mastral, A.M., Membrado, L. and Rubio, B., 1984. Despolimerizaci6n qufmica de lignitos a baja temperatura en fenol (II). Estudio de los maltenos. An. Qufm., 80{B): 161. 20 Brown, J. and Ladner, W.R., 1960. A study of the hydrogen distribution in coal-like materials by high-resolution nuclear magnetic resonance spectroscopy. II. A comparison with infrared measurement and the conversion to carbon structure. Fuel, 39: 87.