aspects of the chemistry
of hydrogen donor solvent
coal liquefaction
j.j.de vlieger i
TR diss
1626
aspects of the chemistry
of hydrogen donor solvent
coal liquefaction
aspects of the chemistry
of hydrogen donor solvent
coal liquefaction
Proefschrift ter verkrijging van de graad van doctor aan de Technische Universiteit Delft, op gezag van de
Rector Magnificus, Prof.dr. J . M . Dirken, in het
openbaar te verdedigen ten overstaan van een commissie door het College van Dekanen daartoe aangewezen, op dinsdag 19 april 1988 te 16.00 uur, door
Jan Jacobus de Vlieger
geboren te Middelburg scheikundig ingenieur
TR diss
1626
Prof.dr.ir. H. van Bekkum en
Prof.dr.ir. A.P.G. Kieboom
Typing: Mrs. M.A.A. van der Kooij-van Leeuwen Drawings: Mr. W . J . Jongeleen
from the Project Office for Energy Research of the Netherlands Energy Research Foundation ECN, within the framework of the Dutch National Coal Research Programme.
1. Introduction
2. Coal liquefaction
Introduction 11 Mechanism of coal liquefaction: classic view 11
Mechanistic studies using coal-related model compounds 12
Experimental mechanistic work 16 Use of deuterated solvent 16
Isotope effects 18 The role of (gas phase) hydrogen 18
Hydrogen shuttling 20 Process solvents 22 Literature 23
3. Behaviour of tetralin in coal liquefaction, examined in long run batch autoclave experiments
Introduction 27 Experimental 29 Materials 29 Apparatus 30 Procedure 30 Results 31 Decomposition of tetralin 31 Formation of 1-methylindan and indan 33
Formation of /7-butylbenzene and other hydrocracking products 33 Hydrogen transfer from tetralin to coal and naphthalene
formation 35 Discussion 38
Catalytic effect of mineral matter and autoclave wall 38 Formation of 1-methylindan, indan and «-butylbenzene 38 Tetralin/naphthalene interconversion and hydrogen transfer 40
Hydrogen transfer and coal conversion 41
Tetralin/coal ratio 41
Conclusions 42 Literature 42
from tetralin
Introduction 45 Experimental 45 Results and discussion 47
Literature 50
Effect of tetralin/coal ratio and temperature on liquefaction behaviour of a bituminous coal using different batch autoclave systems Introduction Experimental Materials Apparatus Procedure G.p.c. 13 1 C and H n.m.r. spectroscopy Results Coal conversion
Hydrogen transfer and coal conversion G.p.c.
N.m.r. spectroscopy Tetralin dimer formation Discussion
53
54
54
54
54
55
56
56
56
56
59
60
60
62
Effect of mixing, degree of filling and tetralin/coal ratioon coal conversion Hydrogen transfer Product distribution Aromaticity of products Mass balance Dimers Conclusions Acknowledgement Literature
62
63
64
64
64
(,r,65
6566
oligomeric compounds
Introduction 67 Experimental 68
Metal-ammonia reduction of naphthalene at -33 °C 68 Reaction between 1,4-dihydronaphthalene and metal amide 69
General procedures 69 Results and discussion 69
Reduction of naphthalene 69 Reduction of 1,4-dihydronaphthalene with alkali metal amide
at -33 °C 74 Acknowledgement 75 Literature 75
Enhanced extractability in coal liquefaction by alkali aetal-liquid aanonia pretreatment
Introduction 77 Experimental 78
Materials and apparatus 78 Reduction with Li or Na in liquid ammonia 79
Liquefaction procedure 80 13 1 C and H n.m.r. spectroscopy 80 G.p.c. 81 Curie-point pyrolysis g.c.-m.s. 81 Results 81 Coal liquefaction 81 Chemical analysis 83 Extractability 83 13 1 C and H n . m . r . s p e c t r o s c o p y 84 G . p . c . 87 Curie-point pyrolysis g.c.-m.s. 88
The tetrahydrofuran-hexane soluble fraction 90
Discussion 90 Literature 91
SuBBary 95
Samenvatting 97
CHAPTER 1
INTRODUCTION
Coal's present day main use is as a source of energy by direct combustion. Historically pyrolysis of coal has been a major route to basic chemicals like acetic acid, methanol, aromatics and heteroaromatics. From the beginning of this century coal has also been converted into liquid fuels and the production of a petroleum substitute expanded greatly during World War II. In 1944 twelve German coal hydrogenation plants and nine Fischer-Tropsch plants were in operation with a total capacity of heavy coal liquids of 5 x 10 tons per year and 7 x 10 tons per year, respectively. Because of the discovery of additional large resources of oil and natural gas in the late 1940's the domination of chemical engineering and technological organic chemistry by the carbonization of coal shifted towards continuous catalytic processing of these resources. At present only 30X of the world's total fuel use is derived from coal, as shown in Table 1.
However, the reserves of coal are much larger than those of oil and are more evenly distributed around the world than oil or natural gas. The oil crisis in 1973, accompanied by the high rise of oil prices and political uncertainties in world supplies, raised renewed interest, based more on strategic than on economic reasons, in the liquefaction of coal.
Processing techniques for the production of liquid fuels from coal can be
divided into three main categories: pyrolysis, indirect liquefaction and direct liquefaction.
During pyrolysis at temperatures above 400 °C coal is converted in an inert or in a hydrogen atmosphere to gases, liquids and char.
During indirect liquefaction coal is fragmented into CO, COg, H„ and CH. (gasification) which are subsequently converted into liquid fuels and chemicals by:
(i) the Fischer-Tropsch process, the catalytic conversion of synthesis gas into hydrocarbons. Two modifications of this process are in use on a commercial scale in South Africa (Sasol plants) and produce gaseous, liquid, solid (waxy) hydrocarbons, various oxygen containing chemicals,
ammonia and sulphur. The ratio of gasoline/diesel output can be changed amongst others by variation in temperature and ratio of CO/H„ in the synthesis gas. The basic reaction in the formation of hydrocarbons is:
CO + 2 H„ &H2) + H20 AH = -39.4 kcal/mol
(ii) the methanol-to-gasoline (MTG) process; first methanol is produced by the catalytic conversion of synthesis gas:
CO + 2 H0 CH3OH AH = -22 kcal/mol
Subsequently methanol is transformed into gasoline over zeolite ZSM-5 as a catalyst. The MTG process, developed by Mobil, has been successfully commercialised in New Zealand since 1986. Here natural gas is the starting product.
Table 1. Worldwide fuel reserves and consumption
Crude oil Natural gas Coal
Reserves Consump- Reserves Consump- Reserves Consump
tion tion tion
Area
North America Central and South
America Western Europe Eastern Europe and USSR Africa Middle East Far East and
Oceania Total xl01 8J 504 207 143 401 353 2262 234 4104 xl01 8J/a 40.8 8.2 27.6 25.0 3.7 5.0 23.0 133.3 xl01 8J 397 120 171 1533 206 838 201 3466 xl01 8J/a 22.9 1.5 8.3 20.3 0.8 1.2 3.0 58.0 xl01 8J 7176 — 2381 7494 1513 4332 22896 xl01 8J/a 18.6 — 12.9 25.2 2.9 25.4 85.0 a 1984 data. b 1982 data.
In direct coal liquefaction hydrogen is added, either in molecular form or in association with a hydrogen donor solvent which can be derived from the coal itself, to coal. During the process the coal is extractively disintegrated in the solvent. Solvent extraction and hydrogenation of the coal may be carried out in a single reactor or in two stages. In most operations catalysts are used to effect hydrogenation, molecular weight reduction and removal of heteroatoms and functional groups. These reactions may be carried out in the presence of conventional supported or disposable hydrocracking catalysts. Some of the catalytic functions can also be
2 supplied by minerals present in coal .
The research described in this thesis is related to the reactions that take place in direct liquefaction processes.
Some chemical and physical properties of coal in the various rank classes,
according to the ASTM system, are given in Table 2. By way of comparison the chemical constitution of a petroleum crude is also included. The rank of a coal is its degree of metamorphism, the extent to which plant debris over millions of years, by exposure to elevated pressures and temperatures, has been deoxygenated and aromatiaed along the pathway through lignite and bituminous coals to anthracite.
Apart from the practical ranking, the heterogeneous nature of coals can be better defined by their microscopically detectable and distinctive organic entities called macerals. The three principal classes of macerals
4
are vitrinite, inertinite and liptinite . The first two are derived from lignin and cellulose constituting plant cell wall structures. The biochemical precursors of liptinite exhibit a lipid-like chemical nature which for example can be found in the outer cell walls of various plant organs. Only the vitrinite (the most abundant maceral) and the liptinite of lignite and bituminous rank coals are readily amendable to liquefaction. As shown in Table 2 the hydrogen/carbon ratio in coal is much lower than that in crude oil. Conversion of coal into liquid or gaseous fuels will require hydrogenation or withdrawal of carbon. Heteroatoms like sulphur, nitrogen and oxygen, of which the amounts are usually higher in coal than in crude oil, will have to be removed not only for environmental reasons but also because of upgrading processes applied to coal liquids. In the order oxygen < sulphur < nitrogen the heteroatoms are present in ring structures (like sulphur in thiophenes and nitrogen in pyridines) from where their elimination is relatively difficult. Another major problem in the use of coal as energy source is the great environmental consequences of the ash
heavy bottoms produced by conversion processes.
Table 2. Composition of coals and crude oil
Lignite Subbi- High vol. Bitu- Anthracite Crude oil tuminous bitum. minous
C (*) H (*) 0 (X) N (X) S (X) H/C atom ratio 0 as OH (X) Aromatic C/ total C 65-72 4 . 5 20-30 1-2 1-2 0 . 8 10-15 72-76 5 15-20 1-2 1-2 0 . 8 10-12 76-87 5.5 4-13 1-2 1-6 0.8 3-9 89-90 4 3-4 1-2 1-5 0 . 5 - 0 . 6 0 . 2 93 2.5 2 1 1 0 . 3 - 0 . 4 0 83-87 11-14 0-1.5 0 . 1 - 0 . 5 0 . 2 - 3 . 0 1.7 0.50 0.65 0.75 0.80-0.90 0.90-0.95 Volatile matter (X) 40-50 35-50 31-45 10-31 Calorific value (kJ/kg) 16 23 28-34 35-37 210 35
Coal analyses on dry and ash-free basis.
Four major variations of direct liquefaction technology will be discussed
briefly: the Exxon Donor Solvent (EDS) process, the H-coal process, the Kohleöl process and the Two-stage processing.
The EDS process, see Figure 1, liquefies coal with recycle solvent obtained by catalytic hydrogenation of the middle boiling range (205-455 °C) of the liquid product. Recently, it has been found that recycling a fraction of the distillate residue to the reactors significantly increased the level of conversion to lighter products. The EDS process was in operation in a 200
The H-coal process, see Figure 2, slurries the coal with a mixture of distillate and nondistillable liquids and solids (vacuum bottoms) into an ebulliated-bed catalytic reactor. Catalyst (cobalt-molybdenum) can be withdrawn and replaced to maintain high catalyst activity while the reaction products, unconverted coal and ash are withdrawn from above the catalyst bed and separated into gaseous and liquid streams. The H-coal process was operated, while it was scaled up from 3 t/d to a 200 t/d pilot plant, for
two years in Catllesburg, Kentucky, and was closed down in August 1982.
EDS ctulyat
1 _
t o l v t n l h y d r o g e n ! ' o n r * C ] * l # M l « f i tf
C M I / O H ï l g r r y l f l f l n/droo*" , l i g u e f a c t l o n product M M i t Ion bo I to»-. b o t t o n M M M M r o o ^ n u t ' f l c a t i o n •no purl l c « t l a n Miroul tnaliquid eroducti 'u»t M I «ii/ipent
i r
C 4 t « l y t t
Figure 1. General flow diagram of EDS p r o c e s s .
l O l w n t 1 l u r r y ' n g M-CM c a u l r
t
" i«J»»f*tt lor product w p a r a t i o n l i q u i d p •nd oducti I MCVMi - • ■ - ■ .,
1
M i t n •nd p u n U*< M l lydragrn a t i o n i c i t t o n1
a i h / i p e n ' c a t a l / t tThe Kohleöl process, see Figure 3, has its origin in the I.G. Farben process (also known by the name of its inventor, Bergius) which operated during World War II in Germany. Currently a 200 t/d pilot plant is operating in Bottrop, West-Germany. The major difference between the Kohleöl and the I.G. Farben process is the lower hydrogen pressure (30 instead of 70 MPa) of the Kohleöl process which is facilitated by the change in recycle pattern in favour of lighter slurrying oil. The process uses a disposable catalyst and the products are separated by distillation into light oil (< 200 °C), middle oil (200-325 °C), heavy oil (> 325 °C) and vacuum residue. The recycle solvent is a mixture of heavy and middle distillate oil.
In all of the liquefaction processes the high molecular weight distillation residues are designed to be gasified which results in a production of excess of fuel gas and no yield of heavy oil.
coll u u l / s t r » e j f t ! f
1
C 0 I Ï / O W Hqu«*4Cttori N p l ■ t'OO totUat o . / ? * * | t n f $na 0 * '«ssm
t l q - l d p'0Cu:üi r
C 4 t i l / i :Figure 3. General flow diagram of Kohleöl process.
A comparison of the reaction conditions for three processes of single-stage direct liquefaction is shown in Table 3.
In the Two-stage coal liquefaction, see Figure 4, dissolution of coal and upgrading of coal products are optimised in two distinct processes. Upgrading reactions are performed with the aid of hydrocracking catalysts at temperatures lower than dissolution temperatures and therefore retrograde reactions which accompany thermal upgrading can be diminished. Other potential advantages are more efficient overall hydrogen usage, better control of overall product selectivity and increasing catalyst life.
One of the Two-stage pilot plant processes which has been in operation since 1976 is the 6 t/d Chevron coal liquefaction process in Richmond,
Q
California . This process uses a first stage temperature of 450 °C and a coal residence time of 10 minutes and a second stage temperature between 340 and 400 °C and a coal residence time of 60 min.
reeytlt coa /oil ïlur'/i»fl
I
product lepa-itlo*1..«If
^ «rt-o^>low Soil inn products
J
Mild* Md hydro^n " H I » I (qu<di r!
T
nyCogen «ssffittllon and purification|
1
•id/toent eatalyitFigure 4. General flow diagram of Two-Stage processing.
Despite large-scale direct coal liquefaction during World War II and recent pilot-plant studies the interactions and effects of different process variables are still not well understood. This is due to the lack of knowledge about the nature of the coal itself and the coal liquefaction mechanisms.
This thesis describes the results of mechanistic studies on the basis of laboratory coal liquefaction experiments with several coal samples, with coal pretreatment and with solvents like the hydrogen donor tetralin and the non-donor dodecane. Chapter 2 presents a literature review of the different liquefaction mechanisms proposed and studied so far. In Chapter 3 the behaviour of tetralin as a model hydrogen-donor solvent is investigated during non-catalytic liquefaction of bituminous coals at mild temperatures (400-450 °C) 10 Chapter 4 elaborates on the formation and entrapment of
oligomeric products derived from tetralin during coal liquefaction Chapter 5 describes the effect of several reaction parameters and the use of
12 different batch autoclaves upon conversion and product distribution Chapter 6 presents a study on the formation of oligomeric compounds during
13
ammonia reduction of naphthalene at -33 °C . The effect of metal-ammonia pretreatment of coal on both the conversion and the nature of the
14 liquefaction products is given in Chapter 7 .
Table 3. Comparison of Major Liquefaction Processes 6-8
Feed coal Reactor conditions Solvent/coal/bottoms (by weight) Pressure, MPa Temperature, °C Residence time, min Catalyst EDSa US Subbitum. 1.2/1/0.4 13.5 450 60
Ni-Mo for solvent hydrogenation H-Coal US Subbitum. 2.1/1/0 18.7 454 45 Co-Mo Kohleol German Bitum. 0.75/1/0 30 475 GO red mud (25-40* Fe203)
Products, X on dry coal
c
r°4
Distillate oil
Vacuum bottoms (incl. ash) Water CO, C 02, HC1, NHg, t^S H_ consumption, wt X of dry coal 11.0 41.5 37.5 9.0 5.4 4.5 14.8 44.0 33.5 8.5 4.2 17.3 45.1 29.9 7.4 5.1 5.0 4.8 Bottoms recycle. Syncrude mode.
Literature
1. Derbyshire, F.J. and Gray, D., Ullmann's Encyclopedia of Industrial Chemistry, Fifth edition, Volume A7, VCH Verlagsgesellschaft, Weinheim, BRD, 1986, p. 197.
2. Whitehurst, D.D., Mitchell, T.D. and Farcasiu, M., Coal Liquefaction, Academic Press, London, 1980, 163.
3. Given, P.H. Coal Science, Vol. 13, Eds. Gorbaty, M.L., Larsen, J.W. and Wender, I., Academic Press, London, 1984, 63.
4. Neavel, R.C., Phil. Trans. R. Soc. Lond. A300, 1981, 141.
5. Wade, D.T., Ansell, L.L. and Epperly, W.R., Chemtech., 1982, 242. 6. Hemminy, D.F., Holmes, J.H. and Teper, M., EAS Report E3/82/4, 1983. 7. Smith, G.B. and Callcott, T.G., Solvent Refined Coal, NERDDC Report,
1980.
8. Fu, Y.C. and Shah, Y.T., Reaction Engineering in Direct Coal Liquefaction, Addison-Westley, Reading (Massachusetts), 1981, 18.
9. Rosenthal, J.W., Dahlberg, A.J., Kuehler, C.W., Cash, D.R. and Freedman, W. , Fuel 1982, 61, 1045.
10. Vlieger, J.J. de, Kieboom, A.P.G. and Bekkum, H. van, Fuel 1984, 63, 334.
11. Vlieger, J.J. de, Leeuw, J.W. de, Kieboom, A.P.G. and Bekkum, H. van, Reel. Trav. Chim. Pays-Bas 1984, 103, 203.
12. Majchrowicz, B.B., Franco, D., fir-Ian, J. and Vlieger, J.J. de, Proc. Coal Science Conf. Maastricht 1987, 207.
13. Vlieger, J.J. de, Kieboom, A.P.G. and Bekkum, H. van, J. Org. Chem. 1986, 51, 1389.
CHAPTER 2
COAL LIQUEFACTION
Introduction
The production of coal liquids requires hydrogen addition to the coal during liquefaction. The actual amount of hydrogen consumed in the process is related to the average molecular weight (mw) of the coal liquid produced. A first approximation could be presented as:
7 Hg 2 H2 13 H2
Coal »• 2 High mw —£ ■ 4 Intermediate mw — » ■ 8 Low raw
mw a 1000 mw » 500 mw a 250
Possible hydrogen sources are: hydrogen gas and hydrogen-rich solvent, but also coal, coal products and residue. The latter four contribute their hydrogen through the following chemical transformations: dehydrogenation of hydroaromatics, aromatic substitution and char formation. A high rate of hydrogen addition occurs during the initial stage of the coal conversion process in order to cap therraally induced radicals and to eliminate reactive 3ulfur and oxygen species. The conversion of high mw products to inter mediate mw moieties requires only as much hydrogen as to break one bond per high mw species. As the conversion proceeds the rate of hydrogen consumption decreases and the major part of the hydrogen is used in heteroatom removal and in the formation of low molecular weight carbon gases and solvent range products.
Mechanisn of coal liquefaction: classic view
The overall macromolecular structure of a bituminous coal can be considered as condensed and highly substituted cyclic carbon structures that are mutually linked together by alkyl and ether bridges:
The classic view of coal liquefaction at 400-450 °C considers free radical 2
formation by homolytic bond scission , eq. (1):
Coal - Coal ... 2 Coal • -1
(1)
The arrows in the hypothetical coal structure indicate those bonds that will be cleaved preferentially during coal liquefaction. Subsequently, the coal radicals are thought to react with a hydrogen donor solvent denoted as RH or with hydrogen gas:
Coal. + RH -■ Coal H + R k- 2 (2) 3 Coal. + H9 -■ Coal H + H • £ k- 3 (3)
This explains that coal conversion is dependent upon the amount of hydrogen 3-5
transferred to the coal, as found experimentally
The formation of free radicals in non-catalytic coal liquefaction can further be rationalised by the fact that conversion only takes place at temperatures high enough for thermal bond breaking to occur. Although it was
2
originally stated that only coal is involved in the first rate determining step, rapid chain pyrolysis processes are also initiated by coal-solvent, coal-gas and solvent-solvent (gas) interactions.
Mechanistic studies using coal-related aodel compounds
When coal is looked upon as a homogeneous, non-ionic system, three modes of bond breaking may be distinguished at coal liquefaction temperatures: bond
horaolysis, free radical p-bond scission and concerted molecular decomposi tion. In addition, alternative pathways might be of importance.
Generally, at temperatures of 375-450 °C bonds with strengths greater than 65 kcal/mol will not undergo appreciable homolysis whereas bonds weaker than 50 kcal/mol will homolyse rapidly. Some dates of bond strengths, for comparison: the Ar-Ar bond in diphenyl is 114 kcal/mol, the C-Ar bond in diphenylmethane is 84 kcal/mol, the O-Ar bond in diphenyl ether is 73 kcal/mol, the C-C bond in dibenzyl is 61 kcal/mol and the C-0 bond in benzyl phenyl ether is 52 kcal/mol. Larger aromatic systems attached to the C-C or C-0 bond as present in coal will result in extra radical stabilisation and consequently, in bond energy lowering [e.g. the C-C bond in 1,2-dinaphthyl-ethane is 52 kcal/mol).
Calculated half-life times for bond homolysis of some model compounds K 7
at 400 °C ' , are summarised in Table 1.
Table 1. Calculated half-life times for bond homolysis at 400 °C
Model compound t'/s Model compound t'/z Model compound t'/ï
Some experimental results of the degree of decomposition of several structures at 400 °C in tetralin for 1 h are given in Table 2 ~ .
Table 2. Degree of decomposition of model compounds heated with excess tetralin at 400 °C for 1 h
Model compound Degree of Model compound Degree of
decomposition decomposition
a
e
"'-o
Q~.-~~.-Q
Q-CH.-CH , - Q
Q-CH
1-CH
1-CH,-0
56-/_
Q-CH
1-CM
l-CM
1-CH
1-Q
l 0 %0>
C H , _ C H
't5 «
Q_
CHi
_
CHl
_
CH]
|^yCH1-CHl-CM,-CH, 6-/_j ^ T
c H ,
i n
cc -o
IJ-^y-O-CHj-CH l ^ - C H j - O Ha°iD
Cg
10 V. 2C'i, 70-1. '•'• 30').Obviously, certain C-C bonds with relatively high energies, like in 1,3-diphenylpropane (see Table 1 ) , are not cleaved through simple bond homolysis, but by a chain reaction sequence including 0-bond scission, eq. ( 4 - 8 )1 1: R « + PhCH„CH2CH2Ph — » - R - H 4 PhCHCH2CH„Ph (4) PhCHCH2CH2Ph •PhCH = CH2 + PhCH2 PhCH2 + PhCH2CH2CH2Ph -PhCHCHgCHgPh 4 phCH-(5) (6)
As to the origin of the initiating radicals in eq. 4 the question remains whether these originate from tetralin C-H homolysis (see below) or from traces of oxygen present. The enhanced rated of decomposition of 2- and
4-hydroxydiphenylmethane compared to diphenylmethane is proposed to be due to keto-enol tautomerism
Since many of the linkages between aromatic moieties still have relatively long life times it is apparent that the rapid decomposition of coal cannot be explained by the classical view of homolysis of weak bonds only (eq. (1)). Nowadays, the rapid processes in the early stages of coal liquefaction (and equally rapid dehydrogenation of the donor solvent) are believed to be the result of chain processes initiated by rapid molecular disproportionation:
CO-CO- CO-CO
The initiating reaction of the molecular disproportionation has been shown 12 to be a stepwise, nonconcerted, nonstereospecific radical process . From thermochemical kinetics it is predicted that log k_ = (10 ± 1) -(151 + 8)/0, 0 = 0.0191 T kJ/mol1 3.
In coal liquefaction with hydrogen donor solvents like tetralin dispropor tionation might lead to hydrogenation of aromatic rings or dehydrogenation of alicyclic rings within the coal structure.
-2
When 10 mol % of dihydronaphthalene is present in nearly pure tetralin (assuming [tetralin] = 5 M) the concentration of 1-tetralyl radicals at 400 °C has been calculated to be 10 M . This means that apart from direct bond homolysis there is a major source of free radicals by molecular disproportionation. The possibility of solvent participation in the bond
14 breaking of linkages to aromatic structures needs further investigation :
xcr-co-xxf--co-yxF- «
The activation energy of cleavage via such a radical hydrogen transfer mechanism is predicted to be significantly lower than that via free
15
H-atoms . Related to this is the efficacy of various solvents in their ability of cleaving strong bonds by facilitating H-transfer.
Furthermore, concerted molecular decomposition has been proposed as a major mechanism of bond homolysis in coal liquefaction . However, experimental rates of product formation are much higher than can be explained by this
19 type of molecular decomposition
The postulation that aromatic reduction by hydroaromatic donors takes place
20 21 through concerted pathways has been met by strong objections because the
model compounds used (1,2- and 1,4-dihydronaphthalene) are thermally unstable at the reaction temperature (400 °C).
Experimental mechanistic work
Use of deuterated solvent
When deuterium is used as a tracer in coal liquefaction, either as completely or as partially deuterated tetralin as donor solvent, exchange with coal and coal-derived materials is rapid and extensive. Both aromatic and aliphatic carbon-hydrogen bonds prove to be reactive, especially the
22-24
benzylic carbon-hydrogen bonds ' . Exchange in the aromatic fragments of coal was much faster than exchange in the aromatic ring of tetralin due to the fact that aromatic rings in coal are often larger, condensed structures which are further activated by OH groups.
The following experimental results showed the preferential participation of the 1-tetralyl radical in the liquefaction reaction:
- coal was found to exchange selectively and reversibly deuterium atoms from the benzylic position of tetralin-d.„ while the recovered naphthalene
1 25 contained 7 times more H at the a than at the (3 position
- when coal was reacted with tetralin-d.„ in a D„ atmosphere at 400 and 425 °C the recovered tetralin contained up to 20% H distributed over the H , H„ and H positions as 65, 25 and 10%, respectively
oc p ar
The principal pathway for the scrambling of deuterium from the 1- to the 2-position of tetralin-1,l-d_ or tetralin-1,1,4,4-d. in experiments with coal at 427 °C is believed to involve the direct formation of 2-tetralyl
nn o p
radicals ' , eq. (9-10). This was underlined by the higher deuterium levels in the 2-position when tetralin-1,1,4,4-d. is used instead of tetralin-1,l-d„.
D O D O
D D
1 —
D (H) D D D(H)
(10)
Also an isotope effect was found in abstraction of hydrogen in preference to deuterium by the 2-tetralyl radical.
The estimated bond energies in tetralin are 82-85 kcal/mol for the benzylic C-H, 95-97 kcal/mol for the secondary C-H and 110 kcal/mol for the aromatic C-H, thus hydrogen abstraction is much faster from the benzylic position
25
than that from the other positions . However, the occurrence of both 1- and 2-tetralyl radicals are further supported by our observation that small amounts of various bitetralyl species are formed during liquefaction (see Chapter 4 ) .
The exchange of deuterium in the aromatic position of coal cannot be understood so easily because the aromatic C-H bond energy is relatively high (~ 25 kcal/mol stronger than benzylic C-H bonds). Possible mechanisms for the creation of aromatic deuterated structures are predicted in Scheme
6 13 22 29
I-IV ' ' ' . Adduction, like in Schemes I and II, will normally be more favourable when the aromatic system is larger. During a coal liquefaction experiment at 425 °C tetralin incorporation was found to be 4X
D T T 30 I I I I I
'AM
xö-xo -;có --xó
IV Schemes I-IVIsotope effects
When a subbitumi nous coal w a s reacted with tetralin-d.?, tetralin-1,1,4,4-d. and tetralin-2,2,3,3,-d. H/D kinetic isotope effects in the formation of
1 fi 17 naphthalene with relative rates k /lc_ of 3.7, 2.0 and 2.0 were found ' This w a s interpreted b y the authors as evidence for hydrogen transfer through a concerted pathway in the transition state of the hydrogen transfer
21
reaction (eq. ( 2 ) ) . The concerted mechanism was rejected by others since deuterated tetralin did not donate two hydrogens simultaneously. In addition
the conversion rate of tetralin in these experiments will always show an isotope effect since the rate constant k_ will depend on the strength o f the hydrogen-tetralin or deuterium-tetralin bond. Experimentally k „ / kn a 1.5 has
13
been found in the reaction of tetralin-1,1,4,4-d., tetralin-1,l-d„ and mixtures of tetralin-1,1,4,4-d. and tetralin-d_ with a bituminous coal, a subbitimunous coal or a lignite. When deuterated tetralin is used in the reaction with model compounds no effect upon the extent of conversion of
31
tetralin to naphthalene is observed . In the same experiments no deuterium was detected in aromatic or 0-positions of recovered tetralin which again underlines the high activity of coal with tetralin.
When coal is reacted with tetralin-d.„ instead of tetralin-d~ as hydrogen donor solvent at 4 0 0 , 425 and 4 5 0 °C lower conversions were
26 2 9 30
observed ' * . This lowered conversion is not consistent with the classic view of eq. (1) being rate limiting, since the nature of the hydrogen donor source should make no difference.
The role of (gas phase) hydrogen
The use of hydrogen gas at fairly high partial pressures is commonly thought to hydrogenate the solvent during the liquefaction towards hydroaromatic molecules. Normally in coal solvents the hydroaromatic concentrations are
32
below that allowed b y thermodynamics solvent rehydrogenation is slow compared to solvent hydrogen donation.
When a bituminous coal w a s reacted in the presence o f 7 MPa H„ and synthetic recycle solvents of varying tetralin contents the sources of the hydrogen transferred are shown in Table 3. In each experiment 2.5 g of hydrogen w a s consumed per 100 g coal.
Table 3 shows that in noncatalysed liquefaction hydrogen from the gas phase will only be used significantly when insufficient hydrogen donor is available in the solvent.
Table 3. Hydrogen donated by different sources to a bituminous coal
X Tetralin in the solvent
40 8.5 4 0
* of total H donated by: Hydroaromatics in solvent Hydrogen gas Coal
BO
020
32
31
37
1!)
43
38
0
57
43
Conditions: Reaction time 90 min, 7 MPa H„, hydrogen consumption 2.5 g/100 g coal.
33 34
Recently, it was shown ' that free hydrogen atoms, believed to be formed through dissociation of hydrogen gas via free radical abstraction also initiate hydrocracking reactions that further reduce the molecular mass of the coal structure. When dibenzyl was heated at 450 °C with an excess of tetralin the conversion was 47% and toluene was the only major product, whereas when the experiment was carried out under 11 MPa hydrogen pressure the conversion increased to 58% and significant amounts of benzene and ethylbenzene were produced. This means also that hydrogen can be responsible for undesirable reactions like dealkylation of alkylaromatic compounds and ring opening of hydroaromatic molecules. The dealkylation reaction, in which the metal wall of the reactor may play a role, is thought to occur by the following mechanism:
R a H H
R' d l )
Free hydrogen atoms are also formed under relatively mild conditions, when hydroaromatics are present:
C O * - C O ^ C O — 0 0 -
<12>
With a relatively high concentration of dihydronaphthalene (1 molar) naphthalene formation at 400 °C took place for only 60% through a direct bimolecular step, eq. (13):
14 while 40* was formed through the mechanism depicted in eq. (12)
At the moment it is not clear what fraction of light gases evolved in coal thermolysis (H~0, CO, C0„ and CH.) are formed by H-atom displacement (eq. (11)). For example, H atoms are expected to displace OH and SH (attack of the aromatic ring) far more slowly than to abstract H atoms from Ar-O-H and Ar-S-H . Anyway, compounds like benzylalcohol and a- and g-naphthol are not stable under liquefaction conditions and readily decompose to H„0 and
7 z
toluene and H„0 and naphthalene, respectively .
Also ethers like ir/VcM,-o-CH1-CH! , / ^ - O - C H , - C H , and (^ -O-CH,
Q
form water upon heating in tetralin . Perhaps ionic mechanisms play a major role here.
When coal is reacted with tetralin in the presence of deuterium gas the reaction gas formed contained CH., CH„D, CH-D-, CHD„ and CD. ' . Since aromatic ring hydrogenation is insignificant at 400 and 425 °C with no added
37 38
catalysts ' the aliphatic structures in the coal must produce the methane. Whether the methane is directly derived from the coal or formed from the liquid products, the presence of different isotopes of methane illustrates the extreme mobility of hydrogen. The mobility of hydrogen was also illustrated by the exchange found from the gas phase hydrogen with the hydrogen in the solvent either by direct reaction or indirect transfer through the coal
Hydrogen shuttling
Bituminous coals become plastic and mobile at liquefaction temperatures and
39
can therefore be efficient sources of hydrogen during liquefaction . The donation of hydrogen from hydroaromatic structures in coal can be assisted40
by certain highly condensed aromatic molecules in solvents . Such molecules are no net donors of hydrogen but can rapidly equilibrate with hydroaromatics in the coal and thus "shuttle" hydrogen from one region of the coal to another. The effect is very noticeable in short contact time liquefaction experiments (1-5 rain) when the overall hydrogen demand is still
29 relatively small but the rate of demand is high
When a solvent with limited hydrogen donor capacity is used the amount of coal becoming soluble is proportional to the concentration of polycondensed
41
coal was reacted at 340 °C with phenanthrene and 8* of the weight of 42
extracted coal was chemically bound phenanthrene .
However, the quantity of labile hydrogen available within the coal will be limited and other mechanisms might be responsible for the enhancement of coal conversion in the presence of polycondensed aromatics.
Pyrene-coal reaction products show the presence of hydro- and alkylated 43
pyrenes , even in inert atmosphere, although their concentrations were very small. The formation of dihydropyrene in hydrogen atmosphere is related to catalysis by coal mineral matter (especially pyrite). The majority of the hydrogen consumed by the coal was found to be derived from molecular hydrogen rather than from the coal itself. Probably the enhancement in conversion observed by using polycondensed aromatics is based on the fact that they function as intermediates between the hydrogen gas and the coal structure.
Furthermore the extensiveness of the alkylation processes was interesting: as many as one of each 60 carbon atoms in the coal may be transferred to pyrene. The alkylation reaction will be thus a source of free hydrogen atoms:
Pyrene + «CH» —*-Pyrene-CH„ + H» (14)
With process solvents it was observed that a blend of distillate hydrogen donors and polycondensed aromatics interact synergetically in enhancing coal
rsic 45
44
conversion . This was explained using model compounds tetralin and pyrene
-4H
Tetralin :l£L-»-Naphthalene (15)
Tetralin + 2 Pyrene ~* Naphthalene + 2 Dihydropyrene (16)
Pyrene + H„ ,, "* Dihydropyrene (17)
Dihydropyrene *-Pyrene (18)
Because of the combined effect of reactions (16) and (17) more dihydropyrene is present in the system tetralin-pyrene-coal-hydrogen than in the system pyrene-coal-hydrogen (dihydropyrene is 40 times as active as tetralin). At 427 °C this synergistic effect showed its maximum in coal conversion with a solvent containing 30 wt % tetralin and 70 wt % pyrene.
The relatively high ratio of dehydrogenation/hydrogenation rates for the pyrene/dihydropyrene equilibrium probably helps to limit overreaction (formation of tetrahydropyrene etc.).
Apart from polycondensed aromatic molecules also phenolic compounds play an active role in the redistribution of hydrogen from one part of the coal to
32
another . Phenol was only found to be active during coal liquefaction at 46
relatively high temperatures (460-480 °C) . The result was a soluble material enriched in hydrogen and a residue which is hydrogen poor. At these
temperatures 5 wt % of the products were derived from phenol.
Process solvents
If a coal conversion process is to be used commercially, the solvent must be derived from the coal itself. Normally such process-derived solvents are complex mixtures of compounds. The selectivity towards desirable or regressive reactions is largely determined by the nature and concentration of solvent components. A survey of most coal liquefaction process derived recycle solvents learns that they consist of 10% hydrocarbons, 20% mono- and polyphenols and 10% heterocyclic compounds. On the other hand the process by which free radicals are formed will essentially depend on the temperature and the coal structure.
In general, a good liquefaction recycle solvent must contain partially hydrogenated polycyclic aromatic compounds which are good hydrogen donors and capable of regeneration under coal conversion conditions. However, the hydrogen donor content of a solvent alone is not sufficient to completely define its effectiveness as a coal liquefaction solvent. This was
illustrated in experiments using different solvents but with the same level of donatable hydrogen (as determined by n.ra.r. or catalytic hydrogenation)
u u. , , ,47,48 whereby a wide variation in coal conversion was observed
A liquefaction recycle solvent must also be a good physical solvent for liquefaction products and have the ability to shuttle hydrogen. Therefore, it must contain polyaromatics, phenols and heterocyclic polyaromatics.
49
Polyaromatics were not only shown to be good physical solvents but also to be good hydrogen acceptors whereby the resultant free radicals act as
3 40
hydrogen donors ' . It was reported that hydroaromatic compounds containing phenol structures, such as 1,2,3,4-tetrahydro-5-hydroxynaphthalene and o-cyclohexylphenol were the best solvents for coal liquefaction (although
not stable at the reaction temperatures ) and that the addition of small quantities of cresol to tetralin resulted in a remarkable increase in coal conversion In the latter case these positive effects were reported by others mainly to occur during work-up procedures rather than during the conversion reactions.
However, phenols do greatly increase coal solubility and when present in sufficient concentrations even short-time contact products {e.g. preheater
conversions) are dissolved in the process solvent. In comparison, the EDS process solvent (see Chapter 1), because of the nature of the process, contained low concentrations of phenolic compounds and as a result, the coal products did not dissolve in the solvent until they were converted to high quality products like distillates.
Literature
1. Stephens, H.P., Int. Conf. on Coal Science Pittsburgh, 1983, 105. 2. Curran, G.P., Struck, R.T. and Gorin, E., Ind. Eng. Chem. Process. Des.
Dev. 1967, 6, 166.
3. Neavel, R.C., Fuel 1976, 55, 237.
4. Cronauer, D.C., McNeil, R.I., Young, D.C. and Ruberto, R.G., Fuel 1982, 61, 610.
5. De Vlieger, J.J., Kieboom, A.P.G. and Bekkum, H. van, Fuel 1984, 63, 334; Chapter 3, this thesis.
6. Stein, S.E., Am. Chem. Soc. Symp. Ser. 1981, 169, 122.
7. Shah, Y.T. and Cronauer, D.C, Catal. Rev.-Sci. Eng. 1979, 20, 209. 8. Cronauer, D.C, Jewell, D.M., Shah, Y.T. and Mock, R.J., Ind. Eng. Chem.
Fundam. 1979, 18, 153.
9. McMillen, D.F., Ogier, W.C and Ross, D.S., J. Org. Chem. 1981, 46, 3322.
10. Mallinson, R . C , Chao, K.C and Greenhorn, R.A., Prep. ACS Div. Fuel Chem. 1980, 25, 120.
11. Gilbert, K.E. and Gajewski, J.J., J. Org. Chem. 1982, 47, 4399. 12. Heesing, A. and Muellers, W., Chem. Ber. 1980, 113, 9.
13. Franz, J.A. and Camaioni, D.M., Fuel 1984, 63, 990.
14. McMillen, D.F., Malhotra, R., Chang, S.-J., Ogier, W . C , Nigenda, S.E., and Fleming, R.H., Fuel 1987, 66, 1611.
15. McMillen, D.F. and Malhotra, R., Int. Conf. on Coal Science Maastricht, 1987, 193.
16. Brower, K.R., J. Org. Chem. 1982, 47, 1889.
17. Brower, K.R. and Pajak, J., J. Org. Chem. 1984, 49, 3970. 18. Virk, P.S., Fuel 1979, 58, 149.
19. Stein, S.E., Fuel 1980, 59, 900.
20. Garry, M.J. and Virk, P.S., Fuel 1980, 25, 132. 21. King, H.H. and Stock, L.M., Fuel 1981, 60, 748.
22. Heredy, L.A., Skowronski, R.P., Rattro, J.J. and Goldberg, I.B., Am. Chem. Soc. Dir. Fuel. Chem. Progr. 1981, 26, 114.
23. Franz, J.A., Fuel 1979, 58, 405.
24. Schweighardt, F.K., Bockrath, B.C., Friedel, R.A. and Retcofsky, H.L., Anal. Chem. 1976, 48, 1254.
25. King, H.H. and Stock, L.M., Fuel 1982, 61, 257.
26. Skowronski, R.P., Rattro, J.J., Goldberg, I.B. and Heredy, L.A., Fuel 1984, 63, 440.
27. Franz, J.A. and Camaioni, D.M. , Int. Conf. on Coal Science Du'sseldorf, 1981, 327.
28. Franz, J.A. and Camaioni, D.M., Fuel 1980, 59, 803.
29. Cronauer, D.C, McNeil, R.I., Young, D.C. and Ruberto, R.G. , Fuel 1982, 61, 610.
30. Heredy, L.A., Skowronski, R.P., Rattro, J.J. and Goldberg, I.B., Prep. Am. Chem. Soc. Div. Fuel Chem. 1981, 26, 84.
31. King, H.H., and Stock, L.M., Fuel 1984, 63, 810.
32. Whitehurst, D.D., Mitchell, T.D. and Farcasiu, M., Coal Liquefaction, Academic Press, London, 1980.
33. Vernon, L.W., Fuel 1980, 59, 102.
34. Allen, D.T. and Gavalas, G.R., Int. J. Chem. Kin. 1983, 15, 219.
35. Wilson, M.A., Vassallo, A.M., Collin, P.J. and Batts, B.D., Fuel Process. Technol. 1984, 8, 213.
36. Wilson, M.A. Collin, P.J., Barron, P.F. and Vassallo, A.M., Fuel Process. Technol. 1982, 5, 281.
37. Wilson, M.A., Pugmuire, R.J., Vassallo, A.M., Grant, D.M., Collin, P.J. and Zilm, K.W., Ind. Eng. Chem. Prod. Rev. Dev. 1982, 21, 477.
38. Guin, J.A., Tarrer, A.R., Prather, J.W., Johnson, D.R. and Lee, J.M., Ind. Eng. Chem. Process. Des. Dev. 1978, 17, 118.
39. Marsh, H. and Neavel, R.C., Fuel 1980, 59, 511.
40. Whitehurst, D.D., Farcasiu, M. Mitchell, T.D. andDickert, J.J., Jr., EPRI Report AF-480, Project RP-410, 1977.
41. Derbyshire, F.J. and Whitehurst, D.D., High Temperatures-High Pressures 1981, 13, 177.
42. Heredy, L.A. and Fugassi, P., Adv. Chem. Ser. 1966, 55, 448. 43. Derbyshire, F.J. and Whitehurst, D.D., Fuel 1981, 60, 655.
44. Derbyshire, F.J., Odoefer, G.A., Varghese, P. and Whitehurst, D.D., Fuel 1982, 61, 899.
45. Derbyshire, F.J., Varghese, P. and Whitehurst, D.D., Fuel 1982, 61, 859. 46. Larsen, J.W., Sams, T.L. and Rodgers, B.R., Fuel 1981, 60, 335.
47. Curtis, C.W., Guin, J.A., Jeng, J. and Tarrer, A.R., Fuel 1981, 60, 677. 48. Curtis, C.W., Guin, J.A., Hale, M.A. and Smith, N.L., Fuel 1985, 64,
461.
49. Davies, G.O., Derbyshire, F.J. and Price, R., J. Inst. Fuel 1977, 121. 50. Orchin, M. and Storch, H.H., Ind. Eng. Chem. 1948, 40, 3225.
CHAPTEH 3
BBHAVIOUR OF TETRALIN IN COAL LIQUEFACTION, EXAMINED IN LONG RUN BATCH AUTOCLAVB EXPERIMENTS
Introduction
1-7
The classical hydrogen donor tetralin is commonly used for the study of coal liquefaction reactions because tetralin is considered as one of the most convenient hydroaromatic solvents with sufficient hydrogen donor ability. Tetralin as such, however, is not an optimal solvent for coal liquefaction; for instance, small quantities of phenolic compounds added to
8 9 tetralin or the use of pyrene/tetralin mixtures can increase the coal
conversion substantially. Also the shuttle mechanism between coal and tetralin and other thermal or coal catalysed isomerisation and degradation reactions prohibit the use of tetralin as a durable recycle solvent for coal liquefaction.
Three stages can be distinguished in the liquefaction reaction: (i) solubilisation of the coal at short reaction times whereby the presence of either good hydrogen donors or hydrogen shuttlers is necessary for high
2 11 12
conversion ' ' ; (ii) defunctionalisation of the coal and hydrogen transfer; (iii) rehydrogenation of the solvent. When hydroaromatic concentrations become low in the liquefaction solvent hydrogen gas or coal itself can become a significant source of hydrogen
It is well known that the extent of coal conversion is related to the hydrogen consumed in the process. As a measure of hydrogen consumption by the coal, the degree of tetralin dehydrogenation towards naphthalene has
1 2 4 5 7 1 7 often been used ' ' ' ' . Some of these studies used hydrogen gas ' , others
2 4 5
did not ' ' ; sometimes the dehydrogenation reaction was studied under 7
hydrogen pressure at short reaction times . However a complete and unequivocal analysis of the rol of gaseous hydrogen in relation to the
tetralin-naphthalene interconversion during coal liquefaction at longer reaction times is not available.
To obtain maximum hydrogenation of a medium-volatile bituminous coal in 13 excess tetralin, the presence of hydrogen ga3 was considered necessary .
However, the dehydrogenation of tetralin in the liquefaction of lignite was 14 reported to be independent of the gaseous atmosphere in the system . The rate limiting step in the liquefaction of coal at high temperatures (ï 400 °C) and hydrogen pressure was reported to be the transfer of gaseous hydrogen to the donor solvent, which is catalysed by coal minerals
12 15-17
(particularly pyrite) ' In addition to iron tin, cobalt, and molybdenum compounds (and combinations thereof) were reported to be active hydrogenation catalysts of the liquefaction solvent ' '
Isomerisation and thermal-degradation reactions of tetralin result in a lower H-donor activity of the solvent apart from the fact that the cracking reactions consume hydrogen, and destroy the possibility of regenerating tetralin by rehydrogenation. A brief review of studies of these reactions is given below.
Already in 1967 it was reported that tetralin during liquefaction trans formed to "C4-benzenes and indans". A closer look at the behaviour of neat tetralin with micro-autoclave experiments at 300-500 °C without hydrogen
22 23
pressure ' showed that tetralin conversion did not exceed 1% after 18 h reaction at 400 °C but became substantial at higher temperatures (43* after 6 h at 450 °C). The main products observed were naphthalene and
1-methyl-indan, the latter thought to be formed by reverse 1-2 aryl migration of the 2-tetralyl radical . Rate constants were determined in a micro-autoclave system at 400 and 450 °C at 10.2 MPa nitrogen from which activation energies of 116.8 kJ/mol (27.9 kcal/mol) and 196.3 kJ/mol (46.9 kcal/mol) were obtained for the tetralin to naphthalene and tetralin to 1-methylindan
con-25
version, respectively . Thermal cracking reactions of tetralin were studied 96
in a flow reactor at 460-540 °C and 1.0-16.0 MPa hydrogen . /r-Butylbenzene, styrene, ethene, 1-methylindan, and o-propyltoluene were identified as primary (hydro)cracking products.
3 The behaviour of tetralin in the presence of coal has been studied at 378-460 °C under low hydrogen pressure (0.98 MPa); at 400 °C and 10.2 MPa H.
7
in a batch recycle system with small residence times (minutes) , and at 450 °C with an initial hydrogen pressure (at room temperature) of 3.45
27
MPa . In the latter study it was found from the extent of the reaction that coal radicals have about the same selectivity in hydrogen abstraction as benzyl radicals in a tetralin/dibenzyl system. The dibenzyl in the tetralin/dibenzyl system is considered to act as a hydrogen acceptor partly as would be the case with coal because coal can also act as a hydrogen
2 28
subsequent cracking to indan was also studied in a tetralin/dibenzyl system 29
under a N„ atmosphere
In addition to isomerisation and (hydro)cracking reactions adduction is another solvent consuming reaction. Adduction of tetralin to coal was found to take place to a small extent (2-5 wt *) during the liquefaction reac tions ' ' . At higher temperatures (450 °C) radical addition of solvent to
31 dissolved coal can be followed by thermolysis of the newly formed bond These literature data show that there are no detailed studies concerning the behaviour of tetralin as such or in the presence of coal under liquefaction conditions and long reaction times. Therefore, the option was to investigate systematically dehydrogenation, isomerisation and hydrocracking reactions of tetralin. In particular, the influence of different coal samples and of hydrogen gas on the tetralin conversion were studied in long-run batch autoclave experiments.
Experimental
Materials
In all the experiments tetralin with a purity of 99* was used. Impurities were 0.3% naphthalene, 0.4S> cis-decalin and 0.3% trans-decalin. Tetrahydro-furan and hexane were reagent grade. Coal samples, obtained from Penn State University (PSOC-852) and Hoogovens, the Netherlands (Coal Mountain and Newdell coal), were ground (< 200 pm) and not dried before use. The analyses of the samples used are given in Table 1.
Table 1. Chemical analysis (wt X as received) of coals
c
H
N
S
H,0
Ash
V.M. PSOC-852 67.94.8
1.4
0.4
7.0
5.2
28.4 Coal Mountain 79.85.6
2.6
0.9
0.7
5.6
33.9 Newdell 72.15.2
2.1
0.7
2.5
9.5
35.7Apparatus
3 The batch reactor system is depicted in Figure 1. The reactor is a 0.5 dm stainless steel autoclave with a magnetically driven stirrer and with a maximum pressure of 30 MPa and a maximum temperature of 500 °C.
lr=CXJ-Figure 1. Batch reactor system, 1. Hydrogen or nitrogen delivery system; 2, Feed reservoir with piston; 3. Sample tube.
Procedure
The autoclave was usually charged with 125 g tetralin and flushed with H„ or N„. Heating to the required temperature (400 or 450 °C) with a 2.5 kW oven took about 45 min. Liquefaction reactions were started by injection of a cold coal-tetralin slurry (25 g/25 g) from a container equipped with a piston into the autoclave. This caused the temperature to drop a 10 °C but
the reaction temperature was regained within 3 min. Stirrer speed was kept at 800 rev min during all of the experiments. Aliquots of the reaction mixture were taken through the sample tube. After each sampling some pressure was let off and the pressure was restored by pressurising gas through the sample tube. In this way any coal and tetralin were forced back from the sampling tube into the bulk. The samples (2 ml) were diluted with hexane (15 m l ) , filtered and the filtrate analysed by a Varian Model 3700 gas chromatograph with a capillary CP Sil 5 column (25 m x 0.23 mm) using temperature programming, 60-130 °C at 5 °C/min , and flame ionisation detection. A correction was made for initial tetralin impurities. All
percentages mentioned are given as mol percentages. Identification of the different products was performed by g.c.-m.s. In the experiments which were performed to study different conversions of the same coal sample, it was found that the above mentioned procedure gave no representative samples of the solid products present in the reaction mixture. Hence, to measure accurately the coal conversion as a function of time, separate coal liquefaction experiments were carried out with different reaction times and the reactions were quenched by rapid air cooling. After cooling to room temperature the total content of the autoclave was removed with tetrahydro-furan, filtered and the insoluble part extracted with tetrahydrofuran in a Soxhlett apparatus for 24 h. The insoluble residue was dried and weighed. The conversion was defined by
weight coal (d.a.f.) - weight residue (d.a.f.)
conversion = x 100X weight coal (d.a.f.)
After removal of the tetrahydrofuran from the filtrate by distillation, 2 ml of the remaining solution were diluted with 15 ml hexane, filtered and analysed in the same way as mentioned above.
Results
Decomposition of tetralin
Table 2 gives detailed information of the autoclave experiments. The overall tetralin conversion at various conditions is shown in Figure 2. At 400 °C no difference in rate and product composition of tetralin was observed when H„ or N„ pressure was applied. To find out whether the wall of the stainless steel reaction vessel acted catalytically at 400 °C, tetralin was reacted under 10 MPa H_ pressure in an open pyrex glass tube located in the autoclave, which was partly filled with tetralin. Table 3 shows that the stainless steel does have some accelerating influence on the decomposition of tetralin to various products. In the presence of a small amount of liquefaction residue the conversion of tetralin increased from 10 to 30* after 24 h reaction time at 400 °C. The rate of decomposition of tetralin at 400 °C and 450 °C also increased in the presence of coal. Hardly any difference was noticed in the reactions of tetralin at 400 °C when Newdell
coal or PSOC-852 was used. At 450 °C the differences between the blank and the liquefaction experiment became very small after 24 h.
Table 2. Experimental conditions of tetralin decomposition
Symbol in figures
A
0
X
V
ü
0
AI
•
T«
Experiment number A BC
I)E
F
G
II 13
K Tetralinm
150
150
150
150
150
150
150
180
150
150
150
Coal sample—
—
Residue PSOC-852 PSOC-852 — PSOC-852 PSOC-852 Newdell Newdell Coal Mountain Coal(«)
—
—
3.3
25
25
—
25
L5
25
25
25
Temp.(°c)
400 400 400 400 400 450 450 400 400 400 400 Gash
N?
h
«9
•V
*?
H?H
7H
? N?
H
2 Pressure MPa 10 10 1C 16.5 10LG
10 10 10 10 10Residue (THF insoluble, ash content of 53%) from New Dell coal after liquefaction in tetralin at 400 °C for 24 h.
Table 3. Effect of the stainless steel autoclave on the decomposition of tetralin at 400 °C and 10 MPa Hg.
Reaction Tetralin Naphthalene 1-methyl- /r-Butyl- Other time indan benzene
(h) (X) (X) (*) (X) (X) Tetralin (pyrex) Tetralin (autoclave) Tetralin (pyrex) Tetralin (autoclave)
2
2
2222
96.7 96.5 89.2 85.80.8
0.9
1.3
2.7
0.7
0.9
7.1
8.1
0.4
0.3
1.6
1.7
1.4
1.4
0.8
1.7
molC/.) 100 80 60 40 20 O O 500 1000 1500 t(mln)
Figure 2. Tetralin conversion versus reaction tine. For conditions and symbols, see Table 2.
Formation of 1 -methyl indan and indan
The rate of formation of 1-methylindan at 400 °C (see Figure 3) was found to be independent of the gas atmosphere applied, whereas coal or a liquefaction residue accelerated the 1-methylindan formation. At 450 °C the 1-methylindan concentration reached a maximum after 6 h in both the blank and the liquefaction experiment (see Figure 4 ) . At the same time the rate of indan formation was found to increase whereas at 400 °C indan was formed in detectable amounts only after long reaction times (20 h ) .
Formation of n-butylbenzene and other hydrocracking products
The formation of w-butylbenzene from tetralin in seven different experiments is shown in Figure 5. When applying a H„ atmosphere substantially more «-butylbenzene was formed than in the presence of N_. The influence of hydrogen is further shown in liquefaction experiments J, I, and D (T, • and V in Figure 5 ) . The n-butylbenzene concentration goes through a maximum at 450 °C, obviously by consecutive hydrocracking reactions yielding smaller products.
rnolC/.) 16
-O 500 1000 1500 -t [mm)
Figure 3. 1-Methylindan formation from tetralin versus reaction time. For conditions and symbols, see Table 2.
moll*/.)
Figure 4 . 1-Methylindan and indan formation from t e t r a l i n versus r e a c t i o n t i m e . , 1-Methylindan; , indan; for c o n d i t i o n s and symbols, s e e Table 2.
moi (•/.)
-t (min)
F i g u r e 5 . 77-Butylbenzene f o r m a t i o n from t e t r a l in versus r e a c t i o n t i m e . F o r c o n d i t i o n s a n d s y m b o l s , s e e T a b l e 2 .
T h e c o m p l e t e c o m p o s i t i o n o f f o u r d i f f e r e n t t e t r a l i n - d e r i v e d r e a c t i o n products after 24 h is given in Table 4. At 450 °C the concentrations of toluene and ethylbenzene became considerable as can be seen from this Table. In the early stages of all the experiments the amount of cis- and trans-decalin formed was the same but gradually the concentration of cis-trans-decalin decreased and the concentration of rra^js-decalin increased. Applying a higher hydrogen pressure (16.5 MPa) increased the concentration of trans-decalin (experiment D, 1.5 wt % after 24 h ) . As was established by g.c.-m.s. analysis, dihydronaphthalene was present in only small amounts. The small amounts of alkyltetralins, alkylnaphthalenes and alkylindans will be formed from adduction and subsequent hydrocracking reactions. Coal does not seem to catalyse this type of reaction.
Hydrogen transfer from tetralin to coal and naphthalene formation
The formation of naphthalene from tetralin in seven different experiments is shown in Figure 6. A higher hV, pressure led to a lower naphthalene formation as would be expected. At a higher temperature (450 °C) a considerable increase in the naphthalene formation was observed from neat tetralin (17.5X after 24 h ) . Not all of the hydrogen produced by dehydrogenation of tetralin was transferred to the coal as shown by the presence of 10* H„ in the gas phase of experiment J at 1800 min. No difference in naphthalene formation was observed when converting Newdell coal or PSOC-852. At 450 °C the
naphthalene content decreased after 800 min, probably due to the hydrogen transfer to the coal reaching its maximum and tetralin/naphthalene inter-conversion becoming important.
Table 4. The concentration (wt X) of tetralin-derived products after 24 h reaction times in four different autoclave experiments
E x p e r i m e n t n o . T e t r a l i n N a p h t h a l e n e A l k y l n a p h t h a l e n e s + a l k y l t e t r a l i n s A l k y l i n d a n s c i s - D e c a l i n 1 - M e t h y l i n d a n Unknown r r a ^ - s - D e c a l i n / 7 - B u t y l b e n z e n e I n d a n s e c - B u t y l b e n z e n e l - E t h y l - 2 - m e t h y l b e n z e n e fl-Propylbenzene o - X y l e n e I s o p r o p y l b e n z e n e E t h y l b e n z e n e T o l u e n e A 8 8 . 8 1.7 0 . 3 0 . 1 0 . 2 5 . 9 0 . 1 0 . 3 2 . 1 < 0 . 1 < 0 . 1
—
—
—
—
0 . 4 0 . 1 F 1 7 . 7 1 7 . 6 1.6 1.1 0 . 1 1 7 . 6 0 . 7 0 . 7 2 . 2 5 . 9 0 . 4 0 . 3 0 . 8 0 . 7 0 . 2 1 8 . 5 1 3 . 9 5 6 . 3 2 0 . 8 0 . 4 0 . 2 0 . 2 1 3 . 9 0 . 2 0 . 4 4 . 3 0 . 5 0 . 3—
0 . 2 0 . 2—
1.0 1.1 G 1 1 . 1 3 4 . 7 1.1 1.0—
1 4 . 5 0 . 5 0 . 6 1.4 6 . 6 0 . 3 0 . 3 0 . 8 0 . 7 0 . 2 1 3 . 8 1 2 . 4Benzene not included, since g.c. detection was not possible. Experimental conditions, see Table 2.
Figure 7 shows the hydrogen transfer, calculated from the dehydrogenation of tetralin on the approximation that all the hydrogen is transferred to the coal, as g H donated per 100 g coal. A higher tetralin/coal ratio (experiment H) resulted in a substantial increase of the hydrogen transfer, which ultimately limited to 7.6 g H/100 g coal (42 wt X tetralin left).
500 1000 t (min)
1500
Figure 6. Naphthalene formation from tetralin versus reaction time. For
conditions and symbols, see Table 2.
1CO0 1500 2 0 0 0 " 4500 i 1 ( m m )
Figure 7. Tetralin dehydrogenation in g H per 100 g coal (d.a.f.) versus
reaction time. For conditions and symbols, see Table 2.
Separate liquefaction experiments (400 °C, 10 MPa H„) with Coal Mountain were interrupted at different reaction times to establish in an accurate way
both the hydrogen-transfer and the coal-conversion behaviour. The experi mental relation between the hydrogen tiansfer and coal conversion reflects a progressive hydrogen consumption (Figure 8 ) .
Htrons C >3 2 1 0 40 50 60 70 80 90 conversion (•/•)
Figure 8. Relation between hydrogen transferred from tetralin in g H per 100 g coal (d.a.f.) and conversion of Coal Mountain. For conditions and symbols, see Table 2.
Discussion
Catalytic effect of mineral matter and autoclave wall
Mineral matter plays a role in coal liquefaction by catalysing hydrogena-1 hydrogena-18 tion/dehydrogenation reactions. In the literature it is stated ' that mineral-matter enriched material (liquefaction residues) has no influence on tetralin isomerisation and dehydrogenation. However, the present investiga tion of the behaviour of "natural catalysts" by reaction of tetralin at 400 °C in the presence of a small amount of tetrahydrofuran-insoluble coal liquefaction residue, showed that the coal residue not only increased the dehydrogenation of tetralin but also enhanced isomerisation and hydro-cracking reactions (see Figures 3, 5 and 6 ) . This is not surprising as transition metal ions are known to initiate radical reactions. The observed catalytic effect of the stainless steel wall of the autoclave (Table 3) is considered to be of similar origin.
Formation of 1-methylindan, indan and n-butylbenzene
The formation of the major organic products from tetralin is summarised in Scheme I.
naphthalene tetralin 1-methylindan
/
n-butylbenzene
i
Indan
Scheme I. Tetralin conversion: a, dehydrogenation; b, isomerisation; c, (hydro)cracking.
Neat tetralin, without additives, is slowly thermally converted into 1-methylindan as the main product, irrespective of the use of H_ or N_ gas pressure (Figure 3 ) . The first order rate constant found at 400 °C (4.9 x 10 rain ) is slightly lower than literature values ' (5.7 x 10 min and 6 x 10 min ) . This might be due to the presence of small amounts of catalytically active species in the latter studies. In tetralin/coal or -4 tetralin/residue experiments at 400 °C the rate is increased (1.7 x 10 min ) which will be due to free-radical formation initiated by metal ions. This rate constant, however, still is a factor 10 lower than that found for
29 —3 —1
tetralin/dibenzyl (90/10 wt Si) mixtures (1.5 x 10 min ) where a higher free-radical concentration is to be expected.
-3 -1 At 450 °C the same rate of formation of 1-methylindan (1.8 x 10 min ) was observed in the blank and the tetralin/coal reactions (0 and k in Figure 4 ) , although in the latter tetralin conversion at relative short reaction times is more than twice as fast (see Figure 2 ) . This indicates that at this temperature mineral matter also has a catalytic function upon the rate of 2-tetralyl radical formation.
The 1-methylindan formed is subsequently converted into indan. This consecutive reaction is relatively 3low at 450 °C (Figure 4) which is in line with the reported "stability" of 1-methylindan in coal liquefaction at 450 °C after short reaction times (60 min) .
The rate of formation of the hydrocracking product w-butylbenzene (Figure 5) depends primarily on the concentration of hydrogen in the reaction mixture (cf. ref. ' ) . The n-butylbenzene formation in experiment J under N„ pressure (?) consumes the hydrogen which is liberated in the dehydrogenation of tetralin. The results in Figures 3 and 5 show that n-butylbenzene is directly formed from tetralin which is in contradiction to the statement
1 Q
be noted that under liquefaction conditions a-ring opening of tetralin due to attack of hydrogen atoms is strongly preferred to thermal p-ring opening since no o-propyltoluene was observed (Table 4 ) .
Tetralin/naphthalene interconversion and hydrogen transfer
The thermodynamic equilibrium
v
naphthalene + 2 L ._ "- tetralin
might b e c o m e relevant under liquefaction c o n d i t i o n s . Calculation o f the equilibrium constant K is however difficult because of the large uncertainty
in the estimation of the hydrogen concentration in the solution at high 34
temperature and pressure (cf. ref. ) . The best fit of K versus temperature 1 ft
has been given by the following equations:
(moles tetralin)/(moles n a p h t h a l e n e ) K = P -4 2 (P„ + 3.30 x 10 * PH T H2 H2 (P„ in atmosphere) H2 and log K = -13.3689 + 7158.47/T (T in Kelvin)
At 400 and 450 °C under 10 MPa H„ pressure the equilibrium tetralin/-naphthalene ratio is calculated to be 18.5 and 3.4, respectively. Starting a
liquefaction experiment - as formulated in this study - at 400 °C under N„ pressure and assuming an ultimate hydrogen transfer from tetralin to coal of 4 g/100 g coal one calculates an equilibrium tetralin/naphthalene ratio of a 2.0. The formation of free hydrogen under such conditions was observed in liquefaction experiments in micro-autoclaves without pressurising the system . Due to the sampling methods used, in experiment J (T), part of the free hydrogen is vented off and the tetralin/naphthalene ratio after 1750 min is 0.59 which is still higher than the equilibrium value 0.19 (partial hydrogen pressure at that point was 1 MPa). This explains the continuing naphthalene formation in Figure 6. Obviously the tetralin/naphthalene inter-conversion plays a role in the maximum found in the naphthalene formation at
