Pericyclic Pathways in the Mechanism of Coal Liquefaction - American

19 Pericyclic Pathways in the Mechanism of Coal Liquefaction P. S. V I R K , D. H. BASS, C . P. E P P I G , and D . J . E K P E N Y O N G

Downloaded by UNIV OF ARIZONA on April 26, 2017 | http://pubs.acs.org Publication Date: October 14, 1980 | doi: 10.1021/bk-1980-0139.ch019

Department of Chemical Engineering, Massachusetts Institute of Technology, Cambridge, MA 02139

The object of this paper is to draw attention to the possible importance of concerted molecular reactions, of the type termed pericyclic by Woodward and Hoffman (1), in the mechanism of coal liquefaction. In outline of what follows we will begin by brief reference to previous work on coal liquefaction. The present approach will then be motivated from considerations of coal structure and hydrogen-donor activity. A theoretical section follows in the form of a pericyclic hypothesis for the coal liquefaction mechanism, with focus on the hydrogen transfer step. Experiments suggested by the theory are then discussed, with presentation of preliminary results for hydrogen transfer among model substrates as well as for the liquefaction of an Illinois No. 6 coal to hexane-, benzene-, and pyridine-solubles by selected hydrogen donors. Previous literature on coal liquefaction is voluminous (2). Molecular hydrogen at elevated pressures is effective (3) but more recent work has employed hydrogen donor 'solvents', such as Tetralin (4), which are capable of liquefying coal at relatively milder conditions. Donor effectiveness is strongly dependent upon chemical structure. Thus, among hydrocarbons, several hydroaromatic compounds related to Tetralin are known to be effective (5·, J5, 7_9 8) , while the corresponding fully aromatic ®§) or fully hydrogenated £ 0 compounds are relatively ineffective Ç7, 9). Further, among alcohols, isopropanol H0-( and o-cyclohexyl phenol HO-fi) are effective donors (7) whereas tbutanol HO-^ is not rÇ (10) . These observations have been theoretically attributed (e.g. 2^, 9) to a free-radical mechanism according to which, during liquefaction, certain weak bonds break within the coal substrate, forming radical fragments which abstract hydrogen atoms from the donor, thereby becoming stable compounds of lower molecular weight than the original coal. According to this free-radical mechanism, therefore, donor 0-8412-0587-6/80/47-139-329$05.00/0 © 1980 American Chemical Society Whitehurst; Coal Liquefaction Fundamentals ACS Symposium Series; American Chemical Society: Washington, DC, 1980.


330

COAL LIQUEFACTION FUNDAMENTALS

e f f e c t i v e n e s s i s r e l a t e d to the a v a i l a b i l i t y of a b s t r a c t a b l e hy drogen atoms, and t h i s n o t i o n seems to have won general accep tance i n the l i t e r a t u r e because i t r a t i o n a l i z e s the e f f e c t i v e n e s s of T e t r a l i n , which possesses b e n z y l i c hydrogen atoms, r e l a t i v e to naphthalene and D e c a l i n , which do not. However, the foregoing mechanism i s l e s s than s a t i s f a c t o r y because indane @ φ » which has j u s t as many b e n z y l i c hydrogens as T e t r a l i n , i s r e l a t i v e l y i n e f f e c t i v e (7) as a donor. A l s o , the a c t i v i t y of the a l c o h o l donors, l i k e i s o p r o p a n o l , would have to i n v o l v e a b s t r a c t i o n of hydrogen atoms bonded e i t h e r to oxygen or to an s p ^ - h y b r i d i z e d carbon; t h i s seems u n l i k e l y because both of these bonds are r e l a t i v e l y strong compared to any that the hydrogen might form with a c o a l fragment r a d i c a l . The present approach to the c o a l l i q u e f a c t i o n mechanism evolved from contemporary knowledge of c o a l s t r u c t u r e (e.g. 11, 12, 13), which emphasizes the existence of hetero-atom-containing s t r u c t u r e s comprising 2- to 4-ring fused aromatic n u c l e i j o i n e d by short methylene b r i d g e s . From t h i s i t i s apparent that sigma bonds between s p ^ - h y b r i d i z e d atoms i n c o a l are seldom more than one bond removed from e i t h e r a p i - e l e c t r o n system or a hetero-atom containing substituent. Such a molecular topology i s f a v o r a b l e f o r p e r i c y c l i c r e a c t i o n s , which are most prone to occur on s k e l e tons with proximal Π- and σ-bonds a c t i v a t e d by s u b s t i t u e n t s . We t h e r e f o r e hypothesize that the o v e r a l l i n t e r a c t i o n between the c o a l s u b s t r a t e and hydrogen donor, which e v e n t u a l l y leads to l i q u e f a c t i o n , i n v o l v e s a sequence of concerted, p e r i c y c l i c steps, which w i l l be i n d i c a t e d i n the next s e c t i o n . The novelty of t h i s approach i s twofold; f i r s t , the mechanistic concept i s e s s e n t i a l l y d i f f e r e n t from any that has h i t h e r t o been proposed i n c o a l - r e l a t e d l i t e r a t u r e and second, i t lends i t s e l f to q u a n t i t a t i v e t e s t s and p r e d i c t i o n s s i n c e the p e r i c y c l i c r e a c t i o n s envisioned must obey the Woodward-Hoffman r u l e s (1) f o r the conservation of o r b i t a l symmetry. I t should a l s o be pointed out that i f the pre sent approach proves v a l i d , then i t w i l l have the engineering s i g n i f i c a n c e that the l a r g e volume of r e c e n t l y developed p e r i c y c l i c r e a c t i o n theory (14, 15, 16) could be a p p l i e d to the prac t i c a l problem of d e f i n i n g and improving a c t u a l c o a l l i q u e f a c t i o n processes. Theory In d e l i n e a t i n g a c o a l l i q u e f a c t i o n mechanism we d i s t i n g u i s h three b a s i c steps, namely rearrangements, hydrogen-transfer, and fragmentation, a l l of which are hypothesized to occur v i a t h e r mally-allowed p e r i c y c l i c r e a c t i o n s . Some t y p i c a l r e a c t i o n s ap p r o p r i a t e f o r each step are i n d i c a t e d below u s i n g WoodwardHoffman (1) terminology: Rearrangements: Sigmatropic s h i f t s , e l e c t r o c y c l i c r e a c t i o n s Hydrogen-Transfer : Group-transfers Fragmentation: Group-transfers, retro-ene, c y c l o - r e v e r s i o n s .

Whitehurst; Coal Liquefaction Fundamentals ACS Symposium Series; American Chemical Society: Washington, DC, 1980.

19.

viRK ET AL.

Pericyclic

331

Pathways

An i l l u s t r a t i o n of how the o v e r a l l p e r i c y c l i c mechanism might apply to the decomposition of 1,2 diphenylethane, a model s u b s t r a t e , i n the presence of A ^ d i h y d r o n a p h t h a l e n e , a model hy drogen-donor, has r e c e n t l y been given (17). In the present work, a t t e n t i o n i s focussed on the hydrogen-transfer s t e p . Hypothesis. The t r a n s f e r o f hydrogen from donors to the c o a l s u b s t r a t e during l i q u e f a c t i o n occurs by concerted p e r i c y c l i c r e a c t i o n s of the type termed 'group t r a n s f e r s ' by Woodward and Hoffman ( 1 ) .


Consequences. (A)

Allowedness

The Woodward-Hoffman r u l e s (1) s t a t e t h a t : "A ground s t a t e p e r i c y c l i c change i s symmetry allowed when the t o t a l number of (4q + 2) s u p r a f a c i a l and (4r) a n t a r a f a c i a l components i s odd". To i l l u s t r a t e how t h i s a p p l i e s i n the present circumstances we consider a p a s s i b l e group t r a n s f e r r e a c t i o n between A ^ d i h y d r o naphthalene, (gQ) , a hydrogen donor, and p h e n a n t h r e n e , j 6 n f r > s u b s t r a t e (hydrogen acceptor) which models a p o l y n u c l e a r aromatic moiety commonly found i n c o a l . In the o v e r a l l group t r a n s f e r r e action: f^^l ._ 2

a

; oo +

substrate 2e(n)

donor 6e(aIIa)

hydrogenated substrate

dehydrogenated donor

2

hydrogen i s t r a n s f e r r e d from A - d i a l i n to the phenanthrene, pro ducing 9,10-dihydrophenanthrene and naphthalene; t h i s r e a c t i o n i s s l i g h t l y exothermic, with ΔΗ£ ^ -8 k c a l as w r i t t e n . The e l e c t r o n i c components i n v o l v e d are 2e(TI) on the s u b s t r a t e and 6ε(σΤΤσ) on the donor and from the Woodward-Hoffman r u l e s i t can be seen that the r e a c t i o n w i l l be thermally forbidden i n e i t h e r the suprasupra stereochemistry (which i s s t e r i c a l l y most f a v o r a b l e ) or the antara-antara stereochemistry ( s t e r i c a l l y the most unfavorable) but w i l l be thermally allowed i n e i t h e r the antara-supra or supra-antara modes, both of the l a t t e r being p o s s i b l e , but s t e r i c a l l y d i f f i c u l t , s t e r e o c h e m i s t r i e s . A r e a c t i o n p r o f i l e f o r (Rl) i s sketched i n F i g u r e 1, showing energy versus r e a c t i o n c o o r d i n ate as w e l l as t r a n s i t i o n s t a t e stereochemistry f o r both the f o r bidden supra-supra and the allowed antara-supra modes. We note next that changing the donor from A - d i a l i n to A - d i a l i n (Ç^j) , changes the donor e l e c t r o n i c a l l y from a 6e(aIIa) component 2

to a 4e(ao) component, of c o n t r a r y o r b i t a l symmetry. overall reaction:

1

Thus f o r the




332


19.

viRK ET AL.

Vericyclic

(R2) Group T r a n s f e r : (gj^f 2β(Π)


333

Pathways

+

jg£)

-

(gQf

+

4β(σσ)

the supra-supra (and antara-antara) modes w i l l be thermally allowed while the antara-supra (and supra-antara) modes are f o r bidden. The p r a c t i c a l upshot o f t h i s i s that the c h e m i c a l l y very s i m i l a r d i a l i n isomers should e x h i b i t s t r i k i n g l y d i f f e r e n t r e a c t i v i t i e s i n concerted group t r a n s f e r s with a given s u b s t r a t e , on account of t h e i r opposite o r b i t a l symmetries. Note that no r e a sonable f r e e - r a d i c a l mechanism could p r e d i c t profound d i f f e r e n c e s i n the donor c a p a b i l i t i e s o f these d i a l i n isomers. O r b i t a l sym metry arguments can a l s o be extended to d i f f e r e n c e s i n the sub s t r a t e s . Thus anthracene Ι ^ @ φ i s a 4ë(IUI) component, o f symmetry opposite to that of phenanthrene fctâp which i s a 2e(TI) component; t h e r e f o r e , with a given donor, such as A - d i a l i n j group t r a n s f e r s that are symmetry-forbidden with phenanthrene w i l l be symmetry-allowed with anthracene i n the same sterochemistry. A c c o r d i n g l y , the r e a c t i o n : H H 2

(R3) Group T r a n s f e r 4e(nn)

6ε(σΠσ)

i s a 4e(nn) + 6e(aIIa) group t r a n s f e r , which i s thermally-allowed i n the supra-supra stereochemistry whereas the analogous r e a c t i o n (Rl) with phenanthrene was f o r b i d d e n . G e n e r a l i z a t i o n of the preceding suggests that there e x i s t two b a s i c c l a s s e s o f donors (and a c c e p t o r s ) , of opposite o r b i t a l symmetries, which w i l l r e s p e c t i v e l y engage i n group t r a n s f e r r e a c t i o n s e i t h e r as (4n+2) e l e c t r o n components or as (4n) e l e c t r o n components. Donors with (4n+2)e w i l l , i n general, t r a n s f e r hydrogen to (4m)e acceptors, the most f a v o r a b l e supra-supra s t e r o chemistry being presumed i n each case. Among the (4n+2)e c l a s s of hydrogen donors, the f i r s t (n=o) member i s molecular hydrogen and the second (n=l) member i s the but-2-ene moiety, w h i l e among the (4n)e c l a s s o f donors, the f i r s t (n=l) member i s the ethane moiety, the second (n=2) the hexa-2,4-diene moiety. Among hydrogen acceptors, the (4m+2)e c l a s s has the ethylene and hexa-1,3,5t r i e n e m o i e t i e s as i t s f i r s t two members, w h i l e the (4m)e c l a s s of acceptors possesses the buta-1,3-diene and octa-1,3,5,7-tetraene moieties as i t s f i r s t two members. Each of the foregoing s e r i e s can be continued s t r a i g h t f o r w a r d l y f o r n>2. I n t e r e s t i n g l y , the nature of allowed donor-acceptor i n t e r a c t i o n s suggests that donor c l a s s w i l l be conserved i n any hydrogen t r a n s f e r sequence. Thus a (4n+2)e donor, say, w i l l t r a n s f e r hydrogen to a (4m)e acceptor, and the l a t t e r , upon a c c e p t i n g the hydrogen, w i l l e v i d e n t l y become a (4m+2)e donor, of the same c l a s s


334

COAL

LIQUEFACTION

FUNDAMENTALS

as the o r i g i n a l donor. The donor and acceptor c l a s s e s i l l u s t r a t e d w i t h hydrocarbons can be d i r e c t l y extended to i n c l u d e hetero-atoms. For example, the a l c o h o l moiety ^ Q _ / H would be a 4e donor, o f the same o r b i t a l symmetry as the ethane moiety ^ S i m i l a r l y the carbonyl moiety 0= would be a 2e acceptor, analogous t o an ethylene moiety = i n terms o f o r b i t a l symmetry. Thus hydrogen t r a n s f e r r e a c t i o n s of molecules with hetero-atoms should have the same allowedness as r e a c t i o n s o f t h e i r i s o - e l e c t r o n i c hydrocarbon analogues. N


(B)

Reactivity

The a c t u a l r a t e s o f thermally-allowed p e r i c y c l i c r e a c t i o n s vary v a s t l y , and f r o n t i e r - o r b i t a l theory (14, 15, 16) has proven to be the primary b a s i s f o r q u a n t i t a t i v e understanding and c o r r e l a t i o n o f the f a c t o r s r e s p o n s i b l e . I t i s t h e r e f o r e o f i n t e r e s t to f i n d the dominant f r o n t i e r o r b i t a l i n t e r a c t i o n s f o r the group t r a n s f e r r e a c t i o n s hypothesized to occur. The HOMO (highest occupied molecular o r b i t a l ) and LUMO (low est unoccupied MO) l e v e l s f o r hydrogen donors used i n c o a l l i q u e f a c t i o n are not y e t w e l l known, but the p r i n c i p l e s i n v o l v e d can be i l l u s t r a t e d w i t h the group t r a n s f e r r e a c t i o n between molecular hydrogen, a (4n+2)e donor w i t h n=0, and naphthalene, a (4m)e acceptor w i t h m=l: „ (R4) Group T r a n s f e r :

An approximate f r o n t i e r o r b i t a l (F0) i n t e r a c t i o n diagram f o r t h i s system i s presented i n F i g u r e 2. T h i s shows that the dominant F0 i n t e r a c t i o n , i . e . , that w i t h the lowest energy gap, i s between H0M0(H2)-LUM0 ( @ ί φ ) ; d e t a i l s of the i n t e r a c t i o n are given t o wards the bottom of the f i g u r e showing the r e s p e c t i v e phases and coefficients involved. The preceding i n d i c a t i o n s regarding the dominant F0 i n t e r a c t i o n i n hydrogen-transfer r e a c t i o n s suggests that they would be f a c i l i t a t e d by a r e d u c t i o n i n the HOMO (hydrogen donor)-LUMO (hy drogen acceptor) energy s e p a r a t i o n . Thus donor e f f e c t i v e n e s s should be enhanced by i n c r e a s i n g the donor HOMO energy, e.g., by e l e c t r o n - r e l e a s i n g or conjugative s u b s t i t u t i o n , whereas acceptor e f f e c t i v e n e s s should be enhanced by lowering the acceptor LUMO l e v e l , e.g. by electron-withdrawing o r conjugative s u b s t i t u t i o n , or by complexation with Lewis a c i d s . As a p r a c t i c a l example r e l e v a n t to our experiments, we should expect A - d i a l i n (gYj) to be a more e f f e c t i v e (4n+2)e type o f hydrogen donor than T e t r a l i n ( g A because v i n y l i c s u b s t i t u t i o n i n the former should raise the^r l e v e l o f the hydrogen-containing HOMO. F o r the same reason, Δ - d i a l i n ( ^ ( ^ should a l s o be a more e f f e c t i v e (4n)e type o f donor than Tetralin " ° t e that the Δ - and 2

1

N

1


viRK E T A L .

F encyclic Pathways

335


19.

Figure 2. Frontier orbital energy levels for the reaction of hydrogen and naph lene



336 2

A - d i a l i n s have the opposite o r b i t a l symmetries ; t h e i r r e a c t i v i t i e s cannot t h e r e f o r e be meaningfully compared, inasmuch as t h e i r r e a c t i o n s with a common acceptor cannot both be allowed w i t h the same stereochemistry. Experiments


(A)

Model Compounds

From the t h e o r e t i c a l d i s c u s s i o n i t f o l l o w s that the present hypothesis f o r the hydrogen t r a n s f e r mechanism can be t e s t e d by a study of r e a c t i o n s between hydrogen donors and acceptors of oppos i t e o r b i t a l symmetries. Our experimental g r i d i s shown i n Table 1.1. The c o a l s u b s t r a t e was modelled by anthracene, a (4m) e acceptor, and by phenanthrene, a (4m+2)e acceptor; both of these aromatic C14 moieties e x i s t i n c o a l and are found i n c o a l derived l i q u i d s . The hydrogen donor s o l v e n t was modelled by a number of c y c l i c CIO compounds d e r i v e d from naphthalene by hydrogénation. Among these, the Δ - and A ^ d i a l i n isomers were of p r i n c i p a l i n t e r e s t , being r e s p e c t i v e l y hydrogen-donors of the (4n+2)e and (4n)e c l a s s e s ; the T e t r a l i n and D e c a l i n served as c o n t r o l s o l v e n t s , the former being very commonly used i n c o a l l i q u e f a c t i o n experiments. For the 2 x 2 matrix of p o s s i b l e hydrogent r a n s f e r r e a c t i o n s between the model C14 s u b s t r a t e s and CIO d i a l i n s o l v e n t s , the Woodward-Hoffman r u l e s p r e d i c t that r e a c t i o n i n the f a v o r a b l e supra-supra stereochemistry should be t h e r m a l l y allowed f o r ( A ^ d i a l i n + phenanthrene) and ( A - d i a l i n + anthra cene) and t h e r m a l l y - f o r b i d d e n f o r ( A ^ d i a l i n + anthracene) and ( A - d i a l i n + phenanthrene). These p r e d i c t i o n s are shown i n Table 1.1 w i t h / denoting the allowed and χ the forbidden r e a c t i o n s . The experiments were conducted batchwise i n small s t a i n l e s s s t e e l pipe-bombs immersed i n a m o l t e n - s a l t bath that was main tained at a d e s i r e d , constant temperature. Pipe-bomb heat-up and quench times, on the order of 1 min each, were n e g l i g i b l e com pared with r e a c t i o n times, which were on the order of 1 h r . The reagents used were obtained commercially; a l l were of p u r i t y > 98% except f o r the A - d i a l i n which had a composition of ( @ ô ) , ® O @ 0 > © Q ^ = » 9, 20, 64) mol%. The p r o p o r t i o n s of Subs t r a t e to s o l v e n t were maintained constant, w i t h the C14 subs t r a t e s as l i m i t i n g r e a c t a n t s i n a l l cases. The extent of r e a c t i o n was measured by proton-NMR spectroscopy on samples of the whole r e a c t i o n batch, as w e l l as of each of the CIO and C14 f r a c t i o n s separated by vacuum d i s t i l l a t i o n and l i q u i d chromatography. Experimental r e s u l t s are shown i n Table 1.2, which quotes the observed percentage conversion of each of the model C14 subs t r a t e s to t h e i r di-hydro d e r i v a t i v e s by each of the model CIO s o l v e n t s . Consider f i r s t the column f o r the anthracene s u b s t r a t e , showing i t s conversion to 9,10 dihydroanthracene a f t e r 2 hr at 300 C i n v a r i o u s s o l v e n t s . The conversion by @Q) (58%), i s an order of magnitude greater than that by (5%) , i n s t r i k i n g 2

2

2

2

7


viRK ET AL.

19.

Table 1.1

Model Compound Experimental G r i d

Coal Model C14

Anthracene

Phenanthrene

Supra-Supra Allowedness

Solvent CIO Downloaded by UNIV OF ARIZONA on April 26, 2017 | http://pubs.acs.org Publication Date: October 14, 1980 | doi: 10.1021/bk-1980-0139.ch019

337

F encyclic Pathways

Decalin

©o

Tetralin

@3

χ

ÉO

/

A^Dialin 2

A -Dialin

Table 1.2

Model Compound Experimental Results

Reaction Conditions Coal Model C14

Temp = 300 C Time = 2 Hr

Temp = 400 C Time = 2 Hr

Anthracene

Phenanthrene

(QlPJ % conversion to 9,10-dihydro-- d e r i v a t i v e

Solvent C10 Decalin Tetralin Δ ^Dialin 2

A -Dialin

Note:

00 @0 m

"SO

See text f o r reagent

0

0

3

2

5

16

58

10

purities.



338

accord with the t h e o r e t i c a l p r e d i c t i o n s according to which the r e a c t i o n with was allowed while that with was f o r b i d d e n . Note too that conversions w i t h the c o n t r o l s o l v e n t s (3%) and (no r e a c t i o n ) were l e s s than those w i t h the t e s t s o l vents, v e r i f y i n g that the l a t t e r were indeed the more a c t i v e . Reactions with phenanthrene s u b s t r a t e r e q u i r e d r a t h e r more severe c o n d i t i o n s , namely 2 hr at 400 C, than anthracene. While the lower r e a c t i v i t y o f phenanthrene r e l a t i v e to anthracene i s gener a l l y w e l l known, i n the present context i t can d i r e c t l y be a t t r i buted to the phenanthrene possessing the higher energy LUMO. The conversions observed, @3 (16%) > (§Xj) (10%), are i n accord with t h e o r e t i c a l p r e d i c t i o n s , and a p p r e c i a b l y exceed the conversions obtained with the c o n t r o l s o l v e n t s (§0 ( ^) 00 ( reac t i o n ) . I t i s i n t e r e s t i n g that the observed s e l e c t i v i t y of hydro gen-transfer from the Δ - and A - d i a l i n s to phenanthrene, r e s p e c t i v e l y (0.6/1), i s not as great as that to anthracene, r e s p e c t i v e l y (12/1). P o s s i b l e reasons f o r t h i s are f i r s t that where as anthracene i s e s s e n t i a l l y always c o n s t r a i n e d to i n t e r a c t with supra-stereochemistry at i t s 9, 10 p o s i t i o n s , the phenanthrene s t r u c t u r e admits a p o s s i b l e a n t a r a - i n t e r a c t i o n across i t s 9, 10 p o s i t i o n s and t h i s l a t t e r might have permitted a thermally-allowed (antara-supra) hydrogen-transfer from A - d i a l i n . Second, the A - d i a l i n used contained some Δ ^ d i a l i n impurity, which could not c o n t r i b u t e to anthracene conversion (forbidden) whereas i t might have c o n t r i b u t e d to the phenanthrene conversion (allowed); t h i r d , the d i a l i n s have a tendency to d i s p r o p o r t i o n a t e , to naphthalene and T e t r a l i n , at elevated temperatures and t h i s might have i n fluenced the r e s u l t s f o r phenanthrene, which were at the h i g h e r temperature.


2

1

a

n

d

n o

2

2

2

1

2

In summary, the Δ - and A - d i a l i n isomers have been shown to be a p p r e c i a b l y more a c t i v e than e t r a l i n (and d e c a l i n ) i n t r a n s f e r r i n g hydrogen to anthracene and phenanthrene. The observed s e l e c t i v i t y of t h i s hydrogen t r a n s f e r i s i n accord with the Woodward-Hoffman r u l e s f o r group t r a n s f e r r e a c t i o n s , anthracene conversions being i n the r a t i o (@Q) / @0 ) 1 /1 1 while phenanthrene conversions are i n the r a t i o (@3/@(5 ) 0.6/1 < 1. The q u a n t i t a t i v e d i f f e r e n c e s i n the s e l e c t i v i t i e s observed with anthracene and phenanthrene are being f u r t h e r explored. =

2

=

(B)

Coal

Conversion

The d i a l i n donor s o l v e n t s were a l s o used d i r e c t l y i n c o a l l i q u e f a c t i o n s t u d i e s . Inasmuch as d e t a i l s of c o a l s t r u c t u r e are unknown, the present theory can only be t e s t e d i n a q u a l i t a t i v e way, as f o l l o w s . F i r s t , i f the l i q u e f a c t i o n of c o a l occurs under k i n e t i c c o n t r o l w i t h hydrogen-transfer from the donor s o l v e n t i n volved i n the rate-determining step, then we should expect the d i a l i n donors to be more e f f e c t i v e than the c o n t r o l s o l v e n t T.etr a l i n (and a l s o D e c a l i n ) . This i s suggested by the theory because the d i a l i n s possess higher energy HOMOs than T e t r a l i n and



19.

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339

Pathways

according to the f r o n t i e r - o r b i t a l a n a l y s i s given p r e v i o u s l y , the hydrogen-transfer r e a c t i o n s of the d i a l i n s should t h e r e f o r e be k i n e t i c a l l y favored r e l a t i v e to those of T e t r a l i n . Second, ac cording to the present theory, donor symmetry i s preserved during hydrogen t r a n s f e r , i . e . , a donor of a given c l a s s i s capable only of i n t e r a c t i o n with acceptors of the complementary c l a s s . Now s i n c e the c o a l s u b s t r a t e l i k e l y contains hydrogen acceptors of both (4m)e and (4m+2)e c l a s s e s , a mixture of s o l v e n t s c o n t a i n i n g donors of both (4n+2)e and (4n)e c l a s s e s should be more e f f e c t i v e i n hydrogen t r a n s f e r than a s i n g l e s o l v e n t of e i t h e r c l a s s which could i n t e r a c t with only one of the two p o s s i b l e c l a s s e s i n c o a l . Thus, i n p r i n c i p l e , f o r each c o a l there should e x i s t an O p t i m a l ' s o l v e n t which contains hydrogen donors of opposite symmetries i n p r o p o r t i o n s that are matched to the p r o p o r t i o n s of the complemen t a r y hydrogen acceptors i n the c o a l . For these reasons a mixture of Δ - and A - d i a l i n s might be a b e t t e r donor s o l v e n t than e i t h e r the Δ - or the A - d i a l i n alone, and there may e x i s t an optimal mixture composition that i s c h a r a c t e r i s t i c of a given c o a l . L i q u e f a c t i o n experiments were performed on a sample (18) of I l l i n o i s No. 6 c o a l , from Sesser, I l l i n o i s . Proximate and e l e mental analyses of t h i s high v o l a t i l e A bituminous c o a l are given i n Table 2. The c o a l , of p a r t i c l e s i z e 600-1200 microns (32x16 mesh), was d r i e d at 110 C i n a n i t r o g e n blanketed oven p r i o r to l i q u e f a c t i o n . A s o l v e n t to c o a l weight r a t i o of 2.0 was used i n a l l experiments, which were conducted i n tubing bomb r e a c t o r s that were immersed i n a constant-temperature bath f o r the de s i r e d time w h i l e being rocked to a g i t a t e the r e a c t o r contents. At the end of each experiment, the r e a c t o r s were quenched and t h e i r contents analyzed to a s c e r t a i n the amounts of each of hexane-, benzene-, and p y r i d i n e - s o l u b l e s (plus gas) produced from the o r i g i n a l c o a l . The procedure used f o r a l l analyses i s d e s c r i b e d f o r the p y r i d i n e - s o l u b l e s (plus gas). F i r s t the r e a c t o r contents were e x t r a c t e d with p y r i d i n e and the r e s i d u e d r i e d on a preweighed a s h l e s s f i l t e r paper to provide the g r a v i m e t r i c conver s i o n to p y r i d i n e - s o l u b l e s , d e f i n e d as 100(1-(daf residue/daf coal)). Second, the r e s i d u e was ashed i n a furnace f o r 3 hours at 800 C, y i e l d i n g the ash-balance conversion to p y r i d i n e s o l u b l e s , d e f i n e d as 100(1-(a/A))/(1-(a/100)) where A and a were r e s p e c t i v e l y the weight percentages of ash i n the r e s i d u e and i n the o r i g i n a l c o a l . The conversions obtained from each of the two methods normally agreed to w i t h i n ±2 weight percent and were averaged to provide f i n a l v a l u e s . Results showing the e f f e c t i v e n e s s of the Δ - and A - d i a l i n s i n c o a l l i q u e f a c t i o n r e l a t i v e to c o n t r o l s o l v e n t s , naphthalene, D e c a l i n , and T e t r a l i n , are presented i n Tables 3.1 and 3.2. In both these t a b l e s , each row provides the conversion of the c o a l sample to each of hexane-, benzene-, and p y r i d i n e - s o l u b l e s (plus gases) by the i n d i c a t e d s o l v e n t . Table 3.1 contains data de r i v e d at a temperature of 400 C and a r e a c t i o n time of 0.5 h r . Among the c o n t r o l s o l v e n t s , i t can be seen that the naphthalene 1

2

1

2

1

2


340

COAL

Table 2.


Origin: Rank:

LIQUEFACTION FUNDAMENTALS

Coal Sample C h a r a c t e r i z a t i o n

I l l i n o i s No. 6 from Sesser, Bituminous, High V o l a t i l e A

Illinois

Proximate A n a l y s i s : (wt% dry b a s i s )

VM 37.3

FC 56.7

Ash 6.0

Elemental A n a l y s i s : (wt% daf)

H 5.4

C 82.0

Ν 1.6

Table 3. 1.

Total 100.0 0 10.2

S 0.8

Coal L i q u e f a c t i o n Results

Temperature = 400 C : Reaction Time = 0.5 hr

Solvent

Coal Conversions to HexaneBenzenePyridineSolubles (plus g a s ) , wt%

Naphthalene

f^ÏQ)

Decalin

00

8.0

1 0

·

8

22. 5±2.6

29.7 32.5

Tetralin

20.9

43.9±1.4

70.2

A^Dialin

24.0

39.6±1.9

71.3

28.6

44.9±1.5

81.4

2

A -Dialin

2.

Temperature = 300 C : Reaction Time = 0.5 hr

Decalin

Tetralin 2

A -Dialin

CO @0 @0

ο 0

0 2.4±0.6

0

9.0±1.5

3.7 19.3



19.

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341

and d e c a l i n give s i m i l a r r e s u l t s and are both much l e s s e f f e c t i v e than t e t r a l i n , the y i e l d s of each of hexane-, benzene-, and p y r i d i n e - s o l u b l e s obtained with the former being roughly h a l f of those obtained with the l a t t e r . The g r e a t e r e f f e c t i v e n e s s of T e t r a l i n as a donor-solvent r e l a t i v e to naphthalene and d e c a l i n i s i n agreement w i t h previous s t u d i e s (2, 7_, 9 ) . Further, the absolute conversion to benzene-solubles (plus gases) obtained w i t h t e t r a l i n i n the present work, namely 44 weight percent, compares f a v orably w i t h the values of 36 and 47 weight percent reported by Neavel (9) f o r comparable HVA and HVB bituminous c o a l s at s i m i l a r r e a c t i o n c o n d i t i o n s . The accord between the c o n t r o l s o l v e n t l i q u e f a c t i o n data shown i n Table 3.1 and the l i t e r a t u r e permits us to p l a c e some confidence i n the present experimental proce dures. Turning now to the d i a l i n donors, f o r which c o a l l i q u e f a c t i o n data have not h i t h e r t o been r e p o r t e d , i t can be seen from Table 3.1 t h a t , r e l a t i v e to T e t r a l i n , the Δ -dialin y i e l d e d ap p r e c i a b l y more hexane-solubles, somewhat l e s s benzene-solubles, and approximately the same p y r i d i n e - s o l u b l e s . A l s o r e l a t i v e to t e t r a l i n , the A - d i a l i n y i e l d e d a p p r e c i a b l y more hexane-solu b l e s , approximately the same benzene-solubles, and a p p r e c i a b l y more p y r i d i n e - s o l u b l e s . I t i s apparent that the d i a l i n s , especc i a l l y the A - d i a l i n , were more e f f e c t i v e donor s o l v e n t s than t e t r a l i n i n l i q u e f a c t i o n of the present c o a l sample. While no p r e c i s e chemical i n t e r p r e t a t i o n can be attached to the q u a n t i t i e s used to measure l i q u e f a c t i o n , the p y r i d i n e - s o l u b l e s roughly r e present the extent to which the c o a l s u b s t r a t e i s converted, whereas the hexane-solubles are a measure of the f i n a l , o i l , pro duct (the benzene-solubles represent an i n t e r m e d i a t e ) . Accord i n g l y , from Table 3.1, the A - d i a l i n i n c r e a s e d the c o a l conver s i o n by 16 percent and product o i l formation by 37 percent r e l a t i v e to T e t r a l i n . A few l i q u e f a c t i o n experiments were a l s o con ducted at a temperature of 300 C and a r e a c t i o n time of 0.5 h r , with r e s u l t s reported i n Table 3.2. Of the c o n t r o l s o l v e n t s . D e c a l i n y i e l d e d n e i t h e r hexane- nor benzene-solubles, w h i l e T e t r a l i n y i e l d e d no hexane-solbules but d i d y i e l d the i n d i c a t e d small amounts of benzene- and p y r i d i n e - s o l u b l e s . The A - d i a l i n y i e l d e d no hexane-solubles but provided a p p r e c i a b l e amounts of benzene- and p y r i d i n e - s o l u b l e s . The conversions seen i n Table 3.2 are a l l much lower than the corresponding conversions i n Table 3.1, undoubtedly a consequence of the lower r e a c t i o n tem perature, 300 C versus 400 C, r e a c t i o n times being equal. Fin a l l y , Table 3.2 shows that the A - d i a l i n s o l v e n t produced 3 times the benzene-solubles and 5 times the p y r i d i n e - s o l u b l e s produced by T e t r a l i n , a s t r i k i n g re-inforcement of the i n d i c a t i o n from Table 3.1 that the d i a l i n s were the more e f f e c t i v e donors. A second s e r i e s of l i q u e f a c t i o n experiments were conducted to t e s t the t h e o r e t i c a l suggestion that a mixture of Δ - plus A - d i a l i n isomers might be a more e f f e c t i v e s o l v e n t than e i t h e r one of the d i a l i n s alone. P r e l i m i n a r y r e s u l t s at 400 C and 0.5 hr r e a c t i o n time, are shown i n Table 4 which quotes the r a t i o of 1

2

2

2

2

2

1

2


342

COAL LIQUEFACTION FUNDAMENTALS Table 4.

Coal L i q u e f a c t i o n by Mixed D i a l i n Solvent

Temperature = 400 C : Reaction Time = 0.5 1

Solvent:

1:1 mixture of Δ -

2

and

A -dialins

Note:

Pyridine-

BenzeneSolubles

Hexane-

Conversion Ratio r

hr

1.11

1.01

1.08

r = p /0.5(pi+P2) where p , P2 are r e s p e c t i v e l y the wt% conversions obtained with mixed solvent,Δ - d i a l i n and A -dialin. m

m


2

the conversion of each of hexane-, benzene-, and p y r i d i n e - s o l ubles obtained with a s o l v e n t mixture c o n t a i n i n g equal amounts of Δ - and A - d i a l i n s r e l a t i v e to the average of the corresponding conversions obtained with each of the Δ ^ d i a l i n and A - d i a l i n s o l v e n t s s e p a r a t e l y . ( G e n e r a l l y , i f p ^ was the conversion to say, p y r i d i n e - s o l u b l e s obtained with a s o l v e n t mixture c o n t a i n i n g a f r a c t i o n χ of s o l v e n t 1, w h i l e p-^ and p2 were the conversions r e s p e c t i v e l y obtained with the pure s o l v e n t s 1 and 2 s e p a r a t e l y , then the departure of the r a t i o r = p / ( x p ^ + (l-x)p2) from u n i t y w i l l e v i d e n t l y measure the a d d i t i o n a l e f f e c t i v e n e s s of the solvent mixture r e l a t i v e to the separate pure s o l v e n t s . ) In Table 4 i t can be seen that the c o a l conversion to hexane-, ben zene-, and p y r i d i n e - s o l u b l e s with the d i a l i n mixture was respec t i v e l y 8, 1, and 11 percent greater than the average f o r the se parate s o l v e n t s . Further work i s being undertaken using purer A - d i a l i n s o l v e n t to seek the g e n e r a l i t y of t h i s r e s u l t and to d i s c e r n an optimum s o l v e n t mixture. 1

2

2

x

m x

2

Conclusions At t y p i c a l c o a l l i q u e f a c t i o n c o n d i t i o n s , namely temperatures from 300 to 400 C and r e a c t i o n times on the order of 1 h r , hydro gen t r a n s f e r from model C10 donors, the Δ - and A - d i a l i n s , to model C14 acceptors, anthracene and phenanthrene, occurs i n the sense allowed by the Woodward-Hoffman r u l e s f o r supra-supra group t r a n s f e r r e a c t i o n s . Thus, i n the conversion of the C14 sub s t r a t e s to t h e i r 9, 10 dihydro d e r i v a t i v e s the d i a l i n s e x h i b i t e d a s t r i k i n g r e v e r s a l of donor a c t i v i t y , the Δ ^ d i a l i n causing about twice as much conversion of phenanthrene but only one-tenth as much conversion of anthracene as d i d A - d i a l i n . The d i a l i n s were a l s o found to be more e f f e c t i v e donor s o l vents than T e t r a l i n i n the l i q u e f a c t i o n of an I l l i n o i s No. 6 HVA bituminous c o a l . For example, at 400 C and 0.5 hr r e a c t i o n time, A - d i a l i n y i e l d e d 16% more p y r i d i n e - s o l u b l e s and 37% more hex1

2

2

2



19.

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343

ane-solubles than Tetralin; at 300 C and 0.5 hr reaction time, the A 2 -dialin yielded 5 times the pyridine-solubles and 3 times the benzene-solubles yielded by Tetralin. Finally, a mixture containing equal parts of Δ 1 - and Δ 2 dialin was found to be a more effective donor solvent than either of the Δ 1 - or A 2 -dialins separately. At 400 C and 0.5 hr reac tion time, the mixture of donors yielded 11% more pyridine-sol ubles and 8% more hexane-solubles than the average for the sepa rate donors. The preceding experiments offer preliminary support to our notion that pericyclic pathways might be intimately involved in the mechanism of coal liquefaction. More specifically, the re sults indicate that pericyclic group transfer reactions consti tute a plausible pathway for the transfer of hydrogen from donor solvents to coal during liquefaction. Abstract We hypothesize that the mechanism of coal liquefaction might involve three general steps, namely rearrangement, hydrogen transfer, and fragmentation, each proceeding via concerted peri cyclic reactions. This mechanism is subject to decisive tests because each step must conform to the Woodward-Hoffman rules. Thus, in the hydrogen transfer reaction, there are predicted to be two distinct classes of hydrogen donors (and acceptors), of opposite orbital symmetries, and reaction should be facile be tween complementary donor-acceptor classes, either a 4n donor + 4n+2 acceptor or v.v. Within a given donor (or acceptor) class, relative reactivity is governed by frontier orbital inter actions between the HOMO of the donor and the LUMO of the accep tor. These predictions were tested by experiments. We used Δ1- and Δ2-dialins as hydrogenation solvents and anthracene and phenanthrene as model coal moieties and acquired kinetic data for their respective thermal reactions at temperatures of 200400 C. By the Woodward-Hoffman rules, the thermal reaction between anthracene and Δ1-dialin is forbidden whereas that with Δ2-dialin is allowed. The experimental data, e.g. at 300 C and 2 hr holding time, showed anthracene conversions of 6% with Δ1dialin and 58% with Δ2-dialin (Tetralin yielded negligible an thracene conversion under these conditions). Experiments were also conducted with selected donor solvents in batch liquefac tion of coal. The dialin donors, which possess HOMOs of higher energy than Tetralin, were rather more effective; e.g. after 30 min at 400 C, an Illinois No. 6 coal yielded (hexane-solubles, pyridine-solubles) wt% of (21,70) with Tetralin and (29,81) with Δ2-dialin. The agreement between orbital symmetry predictions and the experimental data offers preliminary support of our hypo thesis.


344


Acknowledgment This work was supported by the US DOE through the Energy Laboratory at Massachusetts Institute of Technology. Literature Cited


1. Woodward, R.B., and Hoffman, R. "The Conservation of Orbital Symmetry"; Verlag Chemie GmbH: Weinheim, 1970. Pericyclic reaction terminology defined in this text is used in the present paper. 2. Whitehurst, D.D. "Asphaltenes in Processes Coals"; Annual Report EPRI-AF-480 (1977) and references therein. 3.

Bergius, F.

German Patents 301,231 and 299,783 (1913).

4.

Pott, A. and Broche, H. Glückauf, 1933, 69, 903.

5. Weller, S., Pelipetz, M.G., and Freidman, S. Chem., 1951, 43, 1572.

Ind. & Eng.

6. Carlson, C.S., Langer, A.W., Stewart, J., and R.M. H i l l . Ind. & Eng. Chem., 1958, 50 (7), 1067. 7. Curran, G.P., Struck, R.T., and Gorin, E. Proc. Des. & Dev., 1967, 6 (2), 166.

Ind. & Eng. Chem.

8. Ruberto, R.G., Cronaner, D.C., Jewell, D.M., and Seshadri, K.S. Fuel, 1977, 56, 25. 9.

Neavel, R.C., Fuel, 1976, 55, 161.

10. Ross, D.S., and Blessing, J.E. prints, 1977, 22 (2), 208.

ACS Div. of Fuel Chem. Pre-

11. Given, P.M., Fuel, 1960, 39, 147 and 1961, 40, 427. 12. Wiser, W.H., in Wolk, R.M. ACS Div. of Fuel Chem. Preprints, 1975, 20 (2), 116. 13. Wender, I., ACS Div. of Fuel Chem. Preprints, 1975, 20 (4), 16. 14. Fujimoto, Μ., and Fukui, D. "Intermolecular Interactions and Chemical Reactivity and Reaction Paths", in "Chemical Reactivity and Reaction Paths"; G. Klopman, Ed ; Wiley-Interscience: New York, 19 74; pp. 23-54.


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15. Klopman, G. "The Generalized Perturbation Theory of Chemical Reactivity and Its Applications", in "Chemical Reactivity and Reaction Paths", G. Klopman, Ed.; Wiley-Interscience: New York, 19 74, pp. 55-166 16. Houk, K.N. "Application of Frontier Molecular Orbital Theory to Pericyclic Reactions", in "Pericyclic Reactions", A.P. Marchand & R.E. Lehr, Eds.; Academic Press: New York, 1977, pp. 182-272.


17.

Virk, P.S.

Fuel, 1979, 58, 149.

18. Our coal sample was obtained from Inland Steel Company Research Laboratories, Chicago, Illinois through the courtesy of Professor J.B. Howard of the Department of Chemical Engineering Massachusetts Institute of Technology, Cambridge, Massachusetts. RECEIVED May 12,

1980.


Pericyclic Pathways in the Mechanism of Coal Liquefaction - American

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