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Hydrogenation of brown coal
Hydrogenation IV Model
compound
of brown studies
coal
and mechanistic
considerations Peter J. Cassidy,
W. Roy Jackson
and Frank
P. Larkins
Department of Chemistry, Monash University, Clayton, Victoria, Australia 3168 (Received 8 March 1983; revised 16 May 1983)
A series of lignin-related phenyl, benzyl and alkyl ethers have been reacted with hydrogen in the presence of iron (II) acetate and tin metal as additives. Iron reduced the rate of reaction of the phenyl ethers and increased the rate of reaction of the benzyl and alkyl ethers compared to the reactionswithout additive. Tin decreased the rate of reaction of the phenyl and benzyl ethers and had no effect on the reaction of the alkyl ether. Neither iron nor tin changed the products from the reactions of the ethers compared to the reactions without additive. The results are discussed in terms of the use of iron and tin compounds as additives in coal liquefaction reactions. (Keywords:
coal; brown
coal; model compounds;
hydrogenation)
Iron and tin additives have been shown to catalyse the conversion of coal to liquids in both batch autoclave’- 3 and continuous reactor systems4. It has been shown in autoclave reactions using a 1:l by weight coal/tetralin slurry that they lead to an increase in the yield of asphaltenes (CH,Cl,-soluble, X4 petrol insoluble) whereas in the continuous reactor studies using 1:3 by weight coal/tetralin the metals resulted in the formation of additional amounts of oil (X4 petrol soluble). Neither additive had any effect on the chemical nature (‘H n.m.r., acidic oxygen, non acidic oxygen, C/H ratio) of the asphaltene and oil products compared with those obtained from the reaction of untreated coalj. Although the nature of the products from the treated and untreated reactions is similar, the mode of operation of the iron and tin appears to be different. For example, the reaction of tin-treated coal has a very marked hydrogen pressure dependence3 which manifests itself as increased yields of asphaltene. Iron does not show such a marked hydrogen pressure dependence. The synergistic effect of adding a small amount of tin to an iron-treated coal reaction2s3 also supports the view that iron and tin are operating by different mechanisms. Hatswell’ showed that both iron and tin are particularly active during the first minutes of reaction which would suggest that they are possibly reacting with the weaker bonds in the coal structure. Victorian brown coal is a low rank coal with up to 25 wt% organically bound oxygen of which approximately one third is in the form of ether linkage&‘. The ether linkages are up to 28 kJ mol- ’ weaker than the equivalent carbon-carbon linkages (e.g. PhCH,aPh 221 kJ mall ’ cj PhCH,-CH,Ph 249 kJ mol-‘) and thus provide a potential region for bond cleavage within the brown coal structure which may be of central importance in the overall liquefaction reaction sequence. Infra-red and artificial coalilication7 studies suggest that Victorian brown coal may consist largely of demethylated, dehydrated lignin to which -CO,H groups have been introduced during coalification. However, it is 0016-2361/83/12140408~3.00 @ 1983 Butterworth & Co. (Publishers)
1404
FUEL,
1983,
Ltd.
Vol 62, December
possible that significant changes to the parent lignin structure may have occurred’. Lignin is formed from substituted phenylpropene units such as coniferyl alcohol : (HO
0
CH = CH-
CH,OH)
0 Me0
The ethers formed from the condensation of the phenyl propene units can be broadly classified into three classes depending on the type of radical formed after cleavage of the ether linkage. The classes are phenoxy ethers (PhOR), benzyl ethers (PhCH,-OR) and alkyl ethers @a-R’). The model compounds studied (Table I) were selected on the basis of the above classification and are representative of the ether environments found in lignin. To eliminate other effects the models were restricted to single-ring unsubstituted compounds. It is noted that there are chemical differences between coal-derived products and the models. For example, the average aromatic ring size of brown coal-derived asphaltenes is approximately two9’i0 and the distribution of phenolic substituents in the coal is uneven with more than one-OH group per aromatic ring. However, McMillen et al. ’ 1 have examined the effects of increasing the aromatic ring size and the addition of phenolic-OH substituents to coal-related model-ether linkages and concluded that both effects contribute to Tab/e 7 The set of coal-related
Phenoxy
model compounds
studied Abbreviation
ethers
Diphenyl ether Benzyl phenyl ether 2 Phenylethyl phenyl ether 3 Phenylpropyl phenyl ether
(PhOPh) (PhCH,OPh) (PhCH,CH,OPh) (PhCH,CH,CH,OPh)
DPE BPE PEPE PPPE
(PhCH,OCH,Ph)
DBE
(PhCH,CH,OCH,CH,Ph)
DPEE
Benzyl ethers Dibenzyl
ether
Aliphatic -____.-
ethers
Di-2-phenylethyl
ether
Hydrogenation
weakening of the linkage bond. Therefore, if single-ring, unsubstituted model compounds react in the presence or absence of the metal then it is highly probable that the equivalent coal molecules will be likewise affected. The corollary is that if there is no reaction of the models then no firm conclusions can be drawn as to the metal’s effect on the equivalent coal molecule. The present study investigates the effect of iron and tin on the reactions of the model compounds over a range of reaction temperatures (22&425”(Z) at constant initial hydrogen pressure (6 MPa) and reaction time (1 h). EXPERIMENTAL Dibenzyl ether (DBE) and diphenyl ether (DPE) were purchased from Aldrich and shown to be > 99% pure by ‘H n.m.r. and g.1.c. analysis. The phenyl alkyl ethers (PhO(CH,),Ph) were prepared by the Williamson synthesis12. Sodium metal (0.2 mol) was reacted slowly with absolute alcohol (100 ml). Phenol (0.2 mol) in absolute alcohol (20 ml) was added to the sodium ethoxide solution followed by the appropriate nphenylalkyl bromide (0.28 mol) and the solution refluxed gently for 3 h. The product was successively washed with water, 2 aliquots of dilute NaOH, water, dilute H2S04 and water. The impure ether was dried over anhydrous MgSO,, extracted with ether and vacuum distilled until the purity was >98% by g.1.c. and ‘H n.m.r. analysis. Di-2-phenylethyl ether was preparedI by refluxing 2phenylethyl alcohol (PhCH,CH,OH) with sodium hydrogen sulphate at 16&17o”C for 24 h. The ether was purified by vacuum distillation and the purity checked by ‘H n.m.r. and g.1.c. Approximately l-2 g of the model compound was placed in a 70 ml reactor similar to that used for the coal hydrogenation studies3. The iron was added as ferrous acetate while tin was added as powdered metal. The additive concentration was 300 mmol kg- ’ of compound. The reactor was sealed, evacuated and then 6 MPa initial pressure of hydrogen was added. The reaction temperature (220-46O”C) was reached within 10 min and held for 1 h after which the reactor was quenched in an ice bath. The liquid product was analysed by a Varian 3700 gasliquid chromatograph using a 1.83 m x 3.175 mm (6’ x $)r) stainless steel column packed with Carbowax 20M on Chromosorb W. Detection was by means of a flameTable 2 Effect of reaction temperature Reaction temperature
on the untreated,
Phenol
Iron-treated
Tin-treated
RESULTS The product distribution from the untreated and ironand tin-treated reactions of the phenoxy ethers are shown in Tables 2-4. Diphenyl ether (DPE) was unaffected by reaction temperature and by the addition of iron or tin up to 460°C. It can be concluded that a single-ring di-aromatic ether would be essentially unaffected during the coal liquefaction reaction. The products from the untreated reactions of benzyl phenyl ether (BPE), 2-phenylethyl phenyl ether (PEPE) and 3-phenylpropyl phenyl ether (PPPE) were generally phenol and the corresponding alkyl benzene which is in agreement with other work on this class of compounds14. Above 350°C cracking of the alkyl benzene occurred yielding alkyl benzenes of reduced chain length and benzene. Dibenzyl was formed in small amounts in the reactions of BPE and DPPE while l-phenylprop1-ene (PhCH = CHCH,) and 1-phenylprop-2-ene (PhCH,CH =CH,) were also detected from the untreated and treated reactions of PPPE. Polymerization reactions occurred with all three compounds leading to the formation of small amounts of brown, gummy material on the lining of the autoclave. In most cases the percentage of gummy material was ~20 wt% of the starting material and in all cases the percentage of non-volatile material decreased with increasing reaction temperature. BPE was most prone to form polymeric material and an untreated reaction at 220°C formed reactions of benzyl phenyl ether (BPE)a Toluene
Benzene
Dibenzyl
(mot %)
(“C) Untreated
ionization detector and cyclohexanone was used as an internal standard. The standard was added to the reactor to prevent loss of material and the sample to be analysed was taken directly from the reactor. Acetone was used as a solvent, where necessary, to ensure all of the material was washed into the reactor. The reproducibility of the data presented below was determined to be + 2% on the basis of having repeated a number of reactions over the range of compounds studied. It is noted that the reactions of the model compounds were particularly sensitive to the condition of the reactor wall and to fluctuations in the reaction temperature profile. For example, the effects of adding the metals were reproduced in a different autoclave but the conversion profiles were displaced by 2040°C.
and iron- and tin-treated
Conversionb
of brown coal. IV: P. J. Cassidy et al.
_
220 260 300 425
61 100 100 100
25 65 63 82
12 28 35 52
trace trace 2 3
240 260 300 425
32 63 100 100
8 32 51 65
6 23 36 60
_
220 260 300 425
31 68 100 100
trace 36 61 77
trace 18 28 45
-
_
_ 15
2 2 2
_ _ 21
_ 7
trace 5 4 5
a Reaction time = 1 h; initial H, pressure = 6 MPa /J Conversion = [l - ((ni - nf)/“i)] x 100. ni = initial no. of moles of reactant; nf = final number of moles Of reactant
FUEL,
1983,
Vol 62, December
1405
Hydrogenation Table 3
of brown
coal. IV: P. J. Cassidy et al.
Effect of reaction temperature
on uncatalysed
Reaction temperature
and iron- and tin-catalysed
Conversionb
Phenol
reactions of 2phenylethyl Ethylbenzene
phenyl ether (PEPE)a Tol uene
Benzene
(mol %)
(“C)
Untreated
Iron-treated
Tin-treated
300 325 350
19 43 82
9 25 60
7
_
27 50
_
_ _
5
trace
8
365
84
56
50
300 325
3 13
4 8
2 13
-
365 400
40 96
29 57
31 78
19
300 325
12 33
8 22
6 15
365 385
77
43
45
8
90
49
62
14
_ _
7
=l IO _ _
_ 3
trace 3
a Initial Hz pressure = 6 MPa; reaction time = 1 h b Refer to comment b, Tab/e 2 for the definition of conversion
Tab/e 5 Effect of iron and tin catalysts on the conversion of the model ethers. Temperature (“C) of 50% conversion after 1 h of reactiona
BPE PEPE PPPE DBE DPEE a Initial hydrogen
10 t
I
I
300
,
I
320
*,
340 360 360 Reaction temperature (‘C)
Figure 7 Conversion versus reaction temperature 0, Untreated; n . iron-treated; A, tin-treated
43 wt% but this decreased
I
I
400
420
for PPPE
to 20wt% at 425°C. The structure of these polymeric materials is the subject of further investigation. Addition of iron or tin to the reactions of the phenylalkyl ethers had no effect on the range of products formed compared to the untreated reactions. There was no evidence that the metals promoted the hydrogenation of the aromatic rings. The significant feature was that both iron and tin slowed the rate of disappearance of the parent compound compared to the untreated reaction. Figure 1 shows a plot of total conversion versus reaction temperature for PPPE which is representative of the effects of iron
1406
FUEL,
1983,
Vol
62, December
Untreated
Iron
Tin
208 326 354 312 392
255 373 376
242 340 384
277
318
358
392
pressure = 6 MPa
and tin on the phenoxy ethers. The reaction temperatures (T’,) at which 50% conversion occurred were determined from interpolation of the results given in Tables Z-4. The findings are detailed in Table 5. Both iron and tin suppressed the decomposition reaction resulting in an increase in T,, compared to the untreated reaction for these phenoxy ethers. Iron was the more effective inhibitor as it increased T,, by up to 47°C compared to tin which only increased T,, by a maximum of 34°C. Dibenzyl ether (DBE) in the absence of additives gave toluene, benzaldehyde, benzyl alcohol,. benzene and dibenzyl in broad agreement with previous work15,16. The effects of reaction temperature on the product distributions from the untreated and iron- and tin-treated reactions of DBE are summarized in Table 6. As with the phenoxy ethers the addition of iron and tin had no effect on the range of products observed compared to the untreated reaction. However, in contrast to the phenoxy ethers, iron and tin had opposite effects on the conversion. Iron decreased T,, by 35°C while tin increased T,, by 6°C (Table 5 annd Figure 2). It should be noted that tin significantly increased the benzyl alcohol:benzaldehyde ratio compared to the untreated and iron-treated reactions (Table 7). The untreated reaction of di-2-phenylethyl ether (DPEE) yielded ethyl benzene, toluene, benzene and a small amount of 1-phenylethanol (Table 8). Addition of iron decreased T&, by 34°C (Table 5) and increased the yield of l-phenylethanol compared to the untreated reaction. The presence of tin had no observable effect on the reaction of the ether compared to the untreated reaction.
45 56
88 94 100
350 365
375 400 425
350 365 375 390 400
365 385 400 425
1
8 16 18 11
4
8 7 18 24 31
14 32 34 42
trace
31 38 27
42 53 51
11 16 24 47 56
14 15
22 28
trace
9
18
Phenol
3 9 6 16 29
7 5 9 7 3
4 12 15 32
1
_ trace 2 3 26
5 16 20 53
_ 3 2 7 9
4 5 4 1
-
4 10 11 24 28
trace 5 19
7 18 36
_
_ 2
3 11
2 2 4 5 5
-
Benzene (mot %)
--
2 9 4
1 5
trace 3 9 10 7
WI 5 4 8 7
_
_
Dibenzyl
ether (PPEEja
_
Toluene
phenyl
trace 2 7 8 16 21
-
Ethyl Benzene
reactions of 3-phenylpropyl
5
1 -Phenyl prop-l ,2ene
and iron- and tin-treated
Propyl Benzene
on untreated
a Reaction time = 1 h; initial H, pressure = 6 MPa b Refer to comment 6, Tab/e 2 for the definition of conversion c ND = not determined
8
27 69 77 99
310
Tin-treated
10
20 32 46 85 96
310
Iron-treated
23
Conversionb
310
Untreated
T (“C)
Tab/e 4 Effect of reaction temperature
1 _ 1 IO 22
_ 2 9
8 31
5 16
NDC
NDC
22
_
_
CH4
-
CO
2
3
_ -
_ _
4 4
-
.~
h’-‘a
9
3
2
C2H4
-.-
23
5 11
-v-
---
C2H6
_
Hydrogenation Table 6
of brown coal. IV: P. J. Cassidy et al.
Effect of reaction temperature
on the untreated,.iron-
and tin-treated
Conversionb
reactions of dibenzyl Benzaldehyde
Tol uene
ether (DBE)a
Benzvl alcohol
Benzene
Dibenzyl
Temperature (“C)
(mol %)
Untreated
300 310 330 425
19 44 97 100
19 42 115 126
11 19 15 -
8 15 17 _
_ _ _ 58
2 3 10 3
Iron-treated
269 280 300
13 64 96
10 49 56
10 12 26
3 17 23
_ _ 1
2 11 20
Tin-treated
300 310 320 330
12 30 56 74
11 20 64 98
4 3 6 4
6 12 28 29
_ _ -
2 1 5 8 -___
.~_
a Reaction time = 1 h; initial H, pressure = 6 MPa b Refer to comment b, Tab/e 2 for the definition of conversion
100
mechanism similar to that account for the products phenyl ether. The initiation reaction (aliphatic) bond (Reaction
/‘.
go/
/
80 z 0 k a E
t 70
is homolysis of the 0-CH, Scheme (RS): Equation 1) to
Reaction
scheme: the free radical mechanism
R-O-R’
LRb.
R6
R’
+ R-O-R’
-1 R +
60 t
invoked by other workersi to from the reaction of benzyl-
H-H
R-O_{’ -
1
-
ROH
+ R-0-i’
_
RH
l li
20 I
Rb + RCH= CHs
(I)
PhCHO
(ii)
O-R’
l
PhkH,
0 + ri’
-
2 b - propagat Ion
(iir)
PhOH 20
t
.’
L I, ,I--_-
rn
250
260
270
280
Reactron
, , ,II, , Ai
290
300
temperature
310
320
330
for DBE.
Tab/e 7 Comparison of the benzyl alcohol: benzaldehyde ratio from the untreated, iron- and tin-treated reactions of dibenzyl ether Benzyl alcohol: Untreated
Iron-treated 0.30 1.42
-
300 310 320 330
0.73 0.79 _ 1.13
0.88 _ _ -
1.50 4.00 4.67 7.25
DISCUSSION considerations
The product distributions from the reactions of the phenylalkyl ethers can be accounted for by a free-radical
FUEL,
1983,
Vol
-
ROR’
-
R’R’
+
li
-
ROH
-
R’H
-
RCH,CH,
+li
2RCHsCHs (i) (ii) (iii)
reaction reaction reaction
3a I - termrnation I + RCHs =CHs
3b
J 3c
1
of PEPE and DPEE of DEE of the phenyl alkyl ethers
Intramolecular
reactions n
of DBE” CH,PH
DBE
62, December
Ph-CHs-CH2 W _gLO
Tin-treated
_ -
1408
4’ R’
and DPEE _
PhCHO+
PhCHs
Benzaldehyde
260 280
General mechanistic
Rb -1 R
+ +
(“C)
Figure 2 Conversion versus reaction temperature 0, Untreated; n , iron-treated; A. tin-treated
Reaction temperature (“C)
Rb -1 R
ph_;H $ DPEE
z
-
PhCH,
+
HCHO
+ PhCH=CHs
z >3OO”C I 17 CO+ H,
Model
R _
R’ _
PhOPh
Ph
Ph
PhOCH,Ph
Ph
PhCHs
PhOCH,CH,Ph
Ph
PhCH2CH2
PhOCH&H,CHaPh
Ph
PhCHZCH2CH2
PhCH,
PhCHa
PhCHsOCHaPh PhCHsCHsOCHzCHzPh Reaction Scheme (RS)
PhCHaCHz PhCHaCHz Free-radical mechanism
Hydrogenation Table 8 Effect of reaction temperature
Reaction temperature
on the untreated
and iron- and tin-treated
Conversion b
Ethyl benzene
of brown
coal. IV: P. J. Cassidy et al.
reactions of di-2-phenylethyl
Tol uene
ether (DPEEja
Benzene
1 -Phenyl ethanol
(mol %I
(“C)
_ Untreated
Iron-treated
Tin-treated
330
1
1
360 385 405 425 460
12 34 81 99 100
12 25 76 103 41
_
_
5 9 19 39 22
_ 2 14 42 123
2 12
trace _
ml 5 9 2 5 _
300
_
trace
330 360 385
25 55 69
17
8
405
93
425 460
100 100
10 45 73 109 102 46
_
31 36 29
23 43 106
20 6 trace
385 425
34 100
22 98
10 47
_ 38
8 trace
trace 12 38
22
a Reaction time = 1 h, initial H, pressure = 6 MPa b Refer to comment b, Table 2 for the definition of conversion
yield two free radicals which propagate the reaction by abstracting H. from a parent molecule of H, gas (RS: Equation 2a). The radical formed by hydrogen atom abstraction from PEPE can rearrange to yield styrene and phenoxy radicals which further propagate the reaction. The radicals formed by hydrogen atom abstraction from BPE and PPPE are not capable of propagating the reaction in this way. However, the reaction can propagate by reaction of the initially formed radical with hydrogen gas yielding a hydrogen atom which attacks the aromatic ring of the parent molecule to ultimately yield phenol and the corresponding phenylalkyl radical (RS: Equation 2b(iii)). The free radical reactions may be terminated by combination of two radicals (RS: Equation 3a), stabilization of a radical by reaction with fi (RS: Equation 3b) or by disproportionation (RS: Equation 3~). The reactions of DBE and DPEE cannot be wholly accounted for by the above free-radical scheme. For example, the initiation reaction and, hence, TsO, are related to the energy of the weakest bond in the molecule. DBE has a CH,-O bond energy of 286? 10 kJ mol- ’ which is x66 kJ mole1 greater than both PEPE and PPPE. However, 7”, for DBE is significantly lower than 7”0 of PEPE and PPPE. Likewise DPEE, with an ether bond energy similar to the unreactive diphenyl ether had a much lower T,, than would be expected from a pure freeradical reaction. Cronauer et
iron
The effects of iron on the reactions of model compounds can be summarized as follows: (1) no hydrogenation of the aromatic rings in any of the compounds; (2) diphenyl ether - no effect at 460°C; (3) dibenzyl and aliphatic ethers - (a) increased the rate of reaction with DBE and DPEE and (b) increased the yield of PhCH,CH,OH from DPEE compared to the untreated reaction; (4) phenyl-alkyl ethers - PhO(CH,),Ph (n= 1,2, 3Ha) reduced the rate of reaction and (b) no significant change in the product distributions compared to the untreated reactions. Presumably adsorption of the ethers onto the iron surface occurs in all cases. Iron catalyses cleavage of the C-0 bonds in dibenzyl and aliphatic ethers leading to an increase in concentration of the propagating radicals and thus an increase in reaction rate. In the case of DPEE the increase in radical formation leads to increased yields of PhCH,CH,OH relative to ‘the products of thermal rearrangement which dominate the reaction without added metals. The retardation of reactions of phenyl ethers by added iron could be due to the formation of a relatively strong bond between the iron and the phenoxy radicals generated at the surface. Iron has been shown to form stable alkoxy and carboxylate ‘s*19intermediates upon reaction with alcohols and acids and it is not surprising that a similar effect is observed with phenoxy radicals. Furthermore, phenoxy radicals generated away from the iron surface must have been transported to the iron for the rate of reaction to be reduced below that of the untreated reaction. The effect was restricted to the phenoxy radical and not observed for a species such as PhCH,O because the resonance-stabilization energy associated with PhO would result in it having a longer half-life thus allowing it time to be transported to the catalyst surface without picking up a stabilizing hydrogen. The fact that no reduction in the rate of reaction was observed when Fe was added to dibenzyl ether which can form the stable benzyl radical (PhcH,), shows that the affinity of iron for oxygen is important to the radical stabilization process.
FUEL, 1983, Vol 62, December
1409
Hydrogenation
of brown coal. IV: P. J. Cassidy et al.
Effects of tin
The effects of tin on the reactions of the model compounds can be summarized as follows: (1) no hydrogenation of the aromatic rings in any of the compounds; (2) no effect with DPE or DPEE; (3) decrease in the rate of reaction of- (a) the benzyl ether - DBE and (b) the phenyl-alkyl ethers - BPE, PEPE, PPPE; (4) significant increase in the benzyl alcohol:benzaldehyde ratio from the reaction of DBE. These results can also be understood as a perturbation on the free-radical route. In contrast to iron, tin decreased the rate of reaction (increased T,,) of both the benzyl and phenyl ethers. It is noted that T,, for DBE was increased by only 6°C compared to an average of 26°C for the phenyl ethers. However the slope of the conversion versus reaction temperature curve (Figure 2) was high and at 33O”C, for example, the conversion of the tin-treated reaction was 23% lower than the untreated case (Table 6). Unlike iron, tin had no effect on the reaction of the alkyl ether. As tin did not increase the rate of reaction of any of the ethers it is believed that tin did not dissociatively adsorb the parent molecules but reacted, forming a relatively strong bond, with stable phenoxy and benzyl radicals thermally generated away from the metal surface. The ability of tin to stabilize free radicals is not surprising in view of its use to increase the thermal stability of polymers 20-22. Polymers generally un-zip by a free-radical mechanism and the tin presumably captures any free radicals that are thermally generated and thus prevents the initial radical attack from occurring. The increased benzyl alcohol :benzaldehyde ratio from the tin-treated reaction of DBE suggests that tin was also capable of utilizing hydrogen from the gas to form benzyl alcohol rather than benzaldehyde which has been shown to form in a hydrogen-deficient environment16. Tin (m.p. = 232°C) is molten at the reaction temperature used and dissolves significant amounts of hydrogen23. The hydrogen is dissociatively dissolved as atomic hydrogen24 so there is a potential supply of atomic hydrogen at the catalyst surface that can stabilize a free radical. Furthermore, tin readily forms hydrides at lower temperatures which will react by both polar and free-radical mechanisms with a wide range of organic compounds25. In the light of this evidence it is not surprising that tin showed an interaction at high temperatures with free radicals and hydrogen. Activity of iron and tin in the liquefaction of Mctorian brown coal
These studies have shown that iron can catalyse the degradation of aliphatic ether linkages while tin does not accelerate the breakdown of any of the compounds studied. In addition it was shown that the lignin-related aliphatic and benzylic ethers decomposed thermally at temperatures < 400°C. It has been recognized that tin is a more efficient promoter of coal liquefaction reactions than iron and for these reasons it is uncertain whether the capability of iron to catalyse carbon-oxygen bond cleavage in lignin-related ethers is of primary importance in coal liquefaction reactions. In agreement with other work26 it was found that the thermal reaction of hydrogen with diphenyl ether was very slow. No catalysis of the reaction with iron and tin compounds was observed. However, the effect of iron and tin on the reactions of diary1 ethers with hydrogen, where
1410
FUEL, 1983, Vol 62, December
the aryl group contains oxygen substituents or is a polycyclic aromatic system, are not known. Petrakis and Grandy” showed, from in-situ e.s.r. studies of the coal liquefaction reaction of Powhatan No.5 coal, that the concentration of free radicals increased rapidly with reaction time and the concentration of radicals using naphthalene was more than double that when tetralin was used as the vehicle. This evidence corroborates the interpretation of Neave128 that the initial chemical reactions generally involve bond homolysis to yield free radicals which attract hydrogen atoms from the tetralin in preference to undergoing polymerization reactions. However, it is noted that other high energy intermediates have been postulated to account for the reaction of coa129,30. It is believed that the ability ofthe iron and tin additives to capture radicals as exhibited in these model compound studies, is crucial in the initial stages of coal hydrogenation where radical propagation reactions and, hence, repolymerization reactions are retarded. Heating rate studies’ have shown that slow heating to reaction temperature resulted in high conversions to methylene chloride-soluble liquids at temperatures as low as 375°C. Coal has a range of chemical environments with varying bond strengths of the linkages and slow heating results in a gradual homolysis of the various bonds resulting in lower concentration of radicals at any time31*32. This allows the radical-trapping ability ofthe iron and tin to be more efficiently utilized, Neave12* demonstrated that in the first few minutes of a bituminous coal reaction at 400°C there was little conversion of tetralin to naphthalene. He concluded that during this time the tetralin was primarily solvating the coal and little chemical reaction was occurring. In contrast this model compound study demonstrates that significant chemical reaction of lignin-related ether linkages must be occurring in the first minutes of the reaction of brown coal at >4OO”C. It has been shown3 that neither iron nor tin has any effect on the conversion of asphaltene to oil and it is concluded that the major effect of iron and tin is most probably restricted to stabilizing radicals formed during the important first minutes of reaction5. Use of a rapid heat-up autoclave for a coal liquefaction study3 resulted in lower conversions when compared to results obtained using a larger, slower-heating reactor’, even in the presence of a catalyst. This result could be associated with an increase in the relative rate of repolymerization reactions or a relative decrease in catalyst activation reactions, e.g. reduction of a metal oxide to a metal. Increasing the reaction temperature results in degradation of the polymer as the threshold temperature for the stronger bonds is reached. The iron and tin can then commence to stabilize the radicals generated from decomposition of the polymer. Iron and tin also have complementary roles as to which radicals they will stabilize as shown in the model compound experiments. Iron appeared to be the most effective when phenoxy radicals were involved while tin interacted with phenoxy and benzylic radicals. It is also possible that iron catalysed the cleavage of stable aliphatic ethers although infra-red evidence7 would suggest that the concentration of these ethers is negligible in the brown coal. The synergism between the iron and tin observed in the coal liquefaction studies2s3 cannot be fully accounted for
Hydrogenation
by this study. It is possible that the synergism is due to cooperative interaction of the greater stabilization ability of iron with the ability of tin to transfer hydrogen atoms to adsorbed free radicals. The similarity in the chemical nature of the products from the untreated and iron- and tin-treated coal reactions observed in an earlier study3 may be explained by the lack of any catalysis of carboncarbon33 bond cleavage, carbon-oxygen bond cleavage and hydrogenation of aromatic rings. CONCLUSION The results of this study have shown that ethers closely related to structural features of lignin-derived brown coal will decompose below 400°C. A primary role of the iron and tin is to reduce the rate of retrograde polymerization reactions. This type of activity explains why there are no chemical differences in the products from the reactions of untreated and iron- and tin-treated coals. The activity does not conform to the conventional definition of a catalyst where reaction rates are increased and the equilibrium position is unchanged. ACKNOWLEDGEMENT The authors thank the Victorian Brown Coal Council for continued financial support for this research. One of us (PJC) gratefully acknowledges the award of a VBCC scholarship. The views expressed in this paper are entirely the responsibility of the authors. REFERENCES 1 2 3
Hatswell, Rash, D. Marshall, Rash, D. Cassidy, Rash, D.
M. R., Jackson, W. R., Larkins, F. P., Marshall, M., and Rogers, D. E. Fuel 1980,59, 442 M., Jackson, W. R., Larkins, F. P., Hatswell, M. R. and Fuel 1982, 61, 121 P. J., Hertan, P. A., Jackson, W. R., Larkins, F. P. and Fuel 1982,61,939
4 5 6 7 8 9 10 11
12 13 14 15 16 17 18 19 20 21 22 23 24 25 26 27 28 29 30 31 32 33
of brown coal. IV: P. J. Cassidy et al.
White, R. T., Thewlis, P. and Agnew, J. B. Abstracts and papers 7th Australian Workshop on Coal Hydrogenation, 1982, 114 Hatswell, M. R. Ph.D. 7hesis, Monash University (Chemistry) 1982 McPhail, I. and Murray, J. B. SECT/ Miscellaneous Report MRIS,1969 Durie, R. A., Lynch, B. M. and Stemhell, S. Aust. J. Chem. 1960, 13, 156; Sternhell, S. Aust. J. Chem. 1964, 17, 1236 Yoshida, T., Nakata, Y., Yoshida, R., Veda, S., Kanda, N. and Maekawa, Y. Fuel 1982,61, 824 Cassidv. P. J. Ph.D. 7hesis. Monash Universitv lchemistrv) 1982 ’ ~ -’ Charlesworth, J. M. Fuel’1980, 59, 865 McMillen, D. F., Ogier, W. C. and Ross, D. S. in ‘Proceedings: International Conference on Coal Science, Dusseldorf 1981, Verlag Gliickauf GmbH, Essen, 1981, p. 104 Vogel, A. I. in ‘A Textbook of Practical Organic Chemistry’, Longmans, London, 1967, p. 671 Senderens, J. B. Compt. Rend. 1929, 188, 1074 Schlosberg, R. H., Davis, W. H. and Ashe,T. R. Fuel 1981,60,201 Schlosberg, R. H., Ashe,T. R., Pancirou, R. J. and Donaldson, M. Fuel 1981, 60, 155 Cronauer, D. C., Jewell, D. M., Shah, Y. J. and Modi, R. J. Ind. Eng. Chem. Fundam. 1919, 18, 153 Walker, J. F. in ‘Formaldehyde’, Am Chem Sot Monograph Series, Reinhold, New York, 1964 Benzig, J. B. and Madix, R. J. J. Catalysis 1980, 65, 36 Benzig, J. B. and Madix, R. J. J. Catalysis 1980,65, 49 Smirnov, L. N. and Kharitonov, V. M. Volokna Sin Polim. 1970, 236 Sieron, J. K. Rubber Chem. Echnol. 1966, 39(4), 1141 Thoma, W. and Oertel, H. Patent. Chem. Abs. 1963,59, 14171C Mantell, C. I. in ‘Tin - Its mining, production, technology and applications’, Hafner Publishing Co., New York, 1970 Fast, J. D. in ‘Gases in Metals’, MacMillan Press, London, 1976 Kuivila, H. G. Accounts Chem. Res. 1968, 1, 229 Carson, D. W. and Ignasiak, B. Fuel 1980,59, 760 Petrakis, L., Grandy, D. W. and Jones, G. L. Fuel 1982,61, 21 Neavel, R. C. Coal Science 1982, 1, 1 Virk, P. S. Fuel 1979, 58, 149 Brewer, K. R. J. Org. Chem. 1982,47, 1889 Hill, G. R., Hariri, H., Reed, R. I. and Anderson, L. L. Am. Chem. Sot. Adv. in Chem. Ser. 1966,55, 427 Curran, G. P.,Struck, R. T. and Gorin, E. Ind. Eng. Chem. Process Des. Deu. 1967, 6, 166 Thewlis, P., Monash University, Dept. of Chemistry, personal communication
FUEL, 1983, Vol 62, December
1411
compound
of brown studies
coal
and mechanistic
considerations Peter J. Cassidy,
W. Roy Jackson
and Frank
P. Larkins
Department of Chemistry, Monash University, Clayton, Victoria, Australia 3168 (Received 8 March 1983; revised 16 May 1983)
A series of lignin-related phenyl, benzyl and alkyl ethers have been reacted with hydrogen in the presence of iron (II) acetate and tin metal as additives. Iron reduced the rate of reaction of the phenyl ethers and increased the rate of reaction of the benzyl and alkyl ethers compared to the reactionswithout additive. Tin decreased the rate of reaction of the phenyl and benzyl ethers and had no effect on the reaction of the alkyl ether. Neither iron nor tin changed the products from the reactions of the ethers compared to the reactions without additive. The results are discussed in terms of the use of iron and tin compounds as additives in coal liquefaction reactions. (Keywords:
coal; brown
coal; model compounds;
hydrogenation)
Iron and tin additives have been shown to catalyse the conversion of coal to liquids in both batch autoclave’- 3 and continuous reactor systems4. It has been shown in autoclave reactions using a 1:l by weight coal/tetralin slurry that they lead to an increase in the yield of asphaltenes (CH,Cl,-soluble, X4 petrol insoluble) whereas in the continuous reactor studies using 1:3 by weight coal/tetralin the metals resulted in the formation of additional amounts of oil (X4 petrol soluble). Neither additive had any effect on the chemical nature (‘H n.m.r., acidic oxygen, non acidic oxygen, C/H ratio) of the asphaltene and oil products compared with those obtained from the reaction of untreated coalj. Although the nature of the products from the treated and untreated reactions is similar, the mode of operation of the iron and tin appears to be different. For example, the reaction of tin-treated coal has a very marked hydrogen pressure dependence3 which manifests itself as increased yields of asphaltene. Iron does not show such a marked hydrogen pressure dependence. The synergistic effect of adding a small amount of tin to an iron-treated coal reaction2s3 also supports the view that iron and tin are operating by different mechanisms. Hatswell’ showed that both iron and tin are particularly active during the first minutes of reaction which would suggest that they are possibly reacting with the weaker bonds in the coal structure. Victorian brown coal is a low rank coal with up to 25 wt% organically bound oxygen of which approximately one third is in the form of ether linkage&‘. The ether linkages are up to 28 kJ mol- ’ weaker than the equivalent carbon-carbon linkages (e.g. PhCH,aPh 221 kJ mall ’ cj PhCH,-CH,Ph 249 kJ mol-‘) and thus provide a potential region for bond cleavage within the brown coal structure which may be of central importance in the overall liquefaction reaction sequence. Infra-red and artificial coalilication7 studies suggest that Victorian brown coal may consist largely of demethylated, dehydrated lignin to which -CO,H groups have been introduced during coalification. However, it is 0016-2361/83/12140408~3.00 @ 1983 Butterworth & Co. (Publishers)
1404
FUEL,
1983,
Ltd.
Vol 62, December
possible that significant changes to the parent lignin structure may have occurred’. Lignin is formed from substituted phenylpropene units such as coniferyl alcohol : (HO
0
CH = CH-
CH,OH)
0 Me0
The ethers formed from the condensation of the phenyl propene units can be broadly classified into three classes depending on the type of radical formed after cleavage of the ether linkage. The classes are phenoxy ethers (PhOR), benzyl ethers (PhCH,-OR) and alkyl ethers @a-R’). The model compounds studied (Table I) were selected on the basis of the above classification and are representative of the ether environments found in lignin. To eliminate other effects the models were restricted to single-ring unsubstituted compounds. It is noted that there are chemical differences between coal-derived products and the models. For example, the average aromatic ring size of brown coal-derived asphaltenes is approximately two9’i0 and the distribution of phenolic substituents in the coal is uneven with more than one-OH group per aromatic ring. However, McMillen et al. ’ 1 have examined the effects of increasing the aromatic ring size and the addition of phenolic-OH substituents to coal-related model-ether linkages and concluded that both effects contribute to Tab/e 7 The set of coal-related
Phenoxy
model compounds
studied Abbreviation
ethers
Diphenyl ether Benzyl phenyl ether 2 Phenylethyl phenyl ether 3 Phenylpropyl phenyl ether
(PhOPh) (PhCH,OPh) (PhCH,CH,OPh) (PhCH,CH,CH,OPh)
DPE BPE PEPE PPPE
(PhCH,OCH,Ph)
DBE
(PhCH,CH,OCH,CH,Ph)
DPEE
Benzyl ethers Dibenzyl
ether
Aliphatic -____.-
ethers
Di-2-phenylethyl
ether
Hydrogenation
weakening of the linkage bond. Therefore, if single-ring, unsubstituted model compounds react in the presence or absence of the metal then it is highly probable that the equivalent coal molecules will be likewise affected. The corollary is that if there is no reaction of the models then no firm conclusions can be drawn as to the metal’s effect on the equivalent coal molecule. The present study investigates the effect of iron and tin on the reactions of the model compounds over a range of reaction temperatures (22&425”(Z) at constant initial hydrogen pressure (6 MPa) and reaction time (1 h). EXPERIMENTAL Dibenzyl ether (DBE) and diphenyl ether (DPE) were purchased from Aldrich and shown to be > 99% pure by ‘H n.m.r. and g.1.c. analysis. The phenyl alkyl ethers (PhO(CH,),Ph) were prepared by the Williamson synthesis12. Sodium metal (0.2 mol) was reacted slowly with absolute alcohol (100 ml). Phenol (0.2 mol) in absolute alcohol (20 ml) was added to the sodium ethoxide solution followed by the appropriate nphenylalkyl bromide (0.28 mol) and the solution refluxed gently for 3 h. The product was successively washed with water, 2 aliquots of dilute NaOH, water, dilute H2S04 and water. The impure ether was dried over anhydrous MgSO,, extracted with ether and vacuum distilled until the purity was >98% by g.1.c. and ‘H n.m.r. analysis. Di-2-phenylethyl ether was preparedI by refluxing 2phenylethyl alcohol (PhCH,CH,OH) with sodium hydrogen sulphate at 16&17o”C for 24 h. The ether was purified by vacuum distillation and the purity checked by ‘H n.m.r. and g.1.c. Approximately l-2 g of the model compound was placed in a 70 ml reactor similar to that used for the coal hydrogenation studies3. The iron was added as ferrous acetate while tin was added as powdered metal. The additive concentration was 300 mmol kg- ’ of compound. The reactor was sealed, evacuated and then 6 MPa initial pressure of hydrogen was added. The reaction temperature (220-46O”C) was reached within 10 min and held for 1 h after which the reactor was quenched in an ice bath. The liquid product was analysed by a Varian 3700 gasliquid chromatograph using a 1.83 m x 3.175 mm (6’ x $)r) stainless steel column packed with Carbowax 20M on Chromosorb W. Detection was by means of a flameTable 2 Effect of reaction temperature Reaction temperature
on the untreated,
Phenol
Iron-treated
Tin-treated
RESULTS The product distribution from the untreated and ironand tin-treated reactions of the phenoxy ethers are shown in Tables 2-4. Diphenyl ether (DPE) was unaffected by reaction temperature and by the addition of iron or tin up to 460°C. It can be concluded that a single-ring di-aromatic ether would be essentially unaffected during the coal liquefaction reaction. The products from the untreated reactions of benzyl phenyl ether (BPE), 2-phenylethyl phenyl ether (PEPE) and 3-phenylpropyl phenyl ether (PPPE) were generally phenol and the corresponding alkyl benzene which is in agreement with other work on this class of compounds14. Above 350°C cracking of the alkyl benzene occurred yielding alkyl benzenes of reduced chain length and benzene. Dibenzyl was formed in small amounts in the reactions of BPE and DPPE while l-phenylprop1-ene (PhCH = CHCH,) and 1-phenylprop-2-ene (PhCH,CH =CH,) were also detected from the untreated and treated reactions of PPPE. Polymerization reactions occurred with all three compounds leading to the formation of small amounts of brown, gummy material on the lining of the autoclave. In most cases the percentage of gummy material was ~20 wt% of the starting material and in all cases the percentage of non-volatile material decreased with increasing reaction temperature. BPE was most prone to form polymeric material and an untreated reaction at 220°C formed reactions of benzyl phenyl ether (BPE)a Toluene
Benzene
Dibenzyl
(mot %)
(“C) Untreated
ionization detector and cyclohexanone was used as an internal standard. The standard was added to the reactor to prevent loss of material and the sample to be analysed was taken directly from the reactor. Acetone was used as a solvent, where necessary, to ensure all of the material was washed into the reactor. The reproducibility of the data presented below was determined to be + 2% on the basis of having repeated a number of reactions over the range of compounds studied. It is noted that the reactions of the model compounds were particularly sensitive to the condition of the reactor wall and to fluctuations in the reaction temperature profile. For example, the effects of adding the metals were reproduced in a different autoclave but the conversion profiles were displaced by 2040°C.
and iron- and tin-treated
Conversionb
of brown coal. IV: P. J. Cassidy et al.
_
220 260 300 425
61 100 100 100
25 65 63 82
12 28 35 52
trace trace 2 3
240 260 300 425
32 63 100 100
8 32 51 65
6 23 36 60
_
220 260 300 425
31 68 100 100
trace 36 61 77
trace 18 28 45
-
_
_ 15
2 2 2
_ _ 21
_ 7
trace 5 4 5
a Reaction time = 1 h; initial H, pressure = 6 MPa /J Conversion = [l - ((ni - nf)/“i)] x 100. ni = initial no. of moles of reactant; nf = final number of moles Of reactant
FUEL,
1983,
Vol 62, December
1405
Hydrogenation Table 3
of brown
coal. IV: P. J. Cassidy et al.
Effect of reaction temperature
on uncatalysed
Reaction temperature
and iron- and tin-catalysed
Conversionb
Phenol
reactions of 2phenylethyl Ethylbenzene
phenyl ether (PEPE)a Tol uene
Benzene
(mol %)
(“C)
Untreated
Iron-treated
Tin-treated
300 325 350
19 43 82
9 25 60
7
_
27 50
_
_ _
5
trace
8
365
84
56
50
300 325
3 13
4 8
2 13
-
365 400
40 96
29 57
31 78
19
300 325
12 33
8 22
6 15
365 385
77
43
45
8
90
49
62
14
_ _
7
=l IO _ _
_ 3
trace 3
a Initial Hz pressure = 6 MPa; reaction time = 1 h b Refer to comment b, Tab/e 2 for the definition of conversion
Tab/e 5 Effect of iron and tin catalysts on the conversion of the model ethers. Temperature (“C) of 50% conversion after 1 h of reactiona
BPE PEPE PPPE DBE DPEE a Initial hydrogen
10 t
I
I
300
,
I
320
*,
340 360 360 Reaction temperature (‘C)
Figure 7 Conversion versus reaction temperature 0, Untreated; n . iron-treated; A, tin-treated
43 wt% but this decreased
I
I
400
420
for PPPE
to 20wt% at 425°C. The structure of these polymeric materials is the subject of further investigation. Addition of iron or tin to the reactions of the phenylalkyl ethers had no effect on the range of products formed compared to the untreated reactions. There was no evidence that the metals promoted the hydrogenation of the aromatic rings. The significant feature was that both iron and tin slowed the rate of disappearance of the parent compound compared to the untreated reaction. Figure 1 shows a plot of total conversion versus reaction temperature for PPPE which is representative of the effects of iron
1406
FUEL,
1983,
Vol
62, December
Untreated
Iron
Tin
208 326 354 312 392
255 373 376
242 340 384
277
318
358
392
pressure = 6 MPa
and tin on the phenoxy ethers. The reaction temperatures (T’,) at which 50% conversion occurred were determined from interpolation of the results given in Tables Z-4. The findings are detailed in Table 5. Both iron and tin suppressed the decomposition reaction resulting in an increase in T,, compared to the untreated reaction for these phenoxy ethers. Iron was the more effective inhibitor as it increased T,, by up to 47°C compared to tin which only increased T,, by a maximum of 34°C. Dibenzyl ether (DBE) in the absence of additives gave toluene, benzaldehyde, benzyl alcohol,. benzene and dibenzyl in broad agreement with previous work15,16. The effects of reaction temperature on the product distributions from the untreated and iron- and tin-treated reactions of DBE are summarized in Table 6. As with the phenoxy ethers the addition of iron and tin had no effect on the range of products observed compared to the untreated reaction. However, in contrast to the phenoxy ethers, iron and tin had opposite effects on the conversion. Iron decreased T,, by 35°C while tin increased T,, by 6°C (Table 5 annd Figure 2). It should be noted that tin significantly increased the benzyl alcohol:benzaldehyde ratio compared to the untreated and iron-treated reactions (Table 7). The untreated reaction of di-2-phenylethyl ether (DPEE) yielded ethyl benzene, toluene, benzene and a small amount of 1-phenylethanol (Table 8). Addition of iron decreased T&, by 34°C (Table 5) and increased the yield of l-phenylethanol compared to the untreated reaction. The presence of tin had no observable effect on the reaction of the ether compared to the untreated reaction.
45 56
88 94 100
350 365
375 400 425
350 365 375 390 400
365 385 400 425
1
8 16 18 11
4
8 7 18 24 31
14 32 34 42
trace
31 38 27
42 53 51
11 16 24 47 56
14 15
22 28
trace
9
18
Phenol
3 9 6 16 29
7 5 9 7 3
4 12 15 32
1
_ trace 2 3 26
5 16 20 53
_ 3 2 7 9
4 5 4 1
-
4 10 11 24 28
trace 5 19
7 18 36
_
_ 2
3 11
2 2 4 5 5
-
Benzene (mot %)
--
2 9 4
1 5
trace 3 9 10 7
WI 5 4 8 7
_
_
Dibenzyl
ether (PPEEja
_
Toluene
phenyl
trace 2 7 8 16 21
-
Ethyl Benzene
reactions of 3-phenylpropyl
5
1 -Phenyl prop-l ,2ene
and iron- and tin-treated
Propyl Benzene
on untreated
a Reaction time = 1 h; initial H, pressure = 6 MPa b Refer to comment 6, Tab/e 2 for the definition of conversion c ND = not determined
8
27 69 77 99
310
Tin-treated
10
20 32 46 85 96
310
Iron-treated
23
Conversionb
310
Untreated
T (“C)
Tab/e 4 Effect of reaction temperature
1 _ 1 IO 22
_ 2 9
8 31
5 16
NDC
NDC
22
_
_
CH4
-
CO
2
3
_ -
_ _
4 4
-
.~
h’-‘a
9
3
2
C2H4
-.-
23
5 11
-v-
---
C2H6
_
Hydrogenation Table 6
of brown coal. IV: P. J. Cassidy et al.
Effect of reaction temperature
on the untreated,.iron-
and tin-treated
Conversionb
reactions of dibenzyl Benzaldehyde
Tol uene
ether (DBE)a
Benzvl alcohol
Benzene
Dibenzyl
Temperature (“C)
(mol %)
Untreated
300 310 330 425
19 44 97 100
19 42 115 126
11 19 15 -
8 15 17 _
_ _ _ 58
2 3 10 3
Iron-treated
269 280 300
13 64 96
10 49 56
10 12 26
3 17 23
_ _ 1
2 11 20
Tin-treated
300 310 320 330
12 30 56 74
11 20 64 98
4 3 6 4
6 12 28 29
_ _ -
2 1 5 8 -___
.~_
a Reaction time = 1 h; initial H, pressure = 6 MPa b Refer to comment b, Tab/e 2 for the definition of conversion
100
mechanism similar to that account for the products phenyl ether. The initiation reaction (aliphatic) bond (Reaction
/‘.
go/
/
80 z 0 k a E
t 70
is homolysis of the 0-CH, Scheme (RS): Equation 1) to
Reaction
scheme: the free radical mechanism
R-O-R’
LRb.
R6
R’
+ R-O-R’
-1 R +
60 t
invoked by other workersi to from the reaction of benzyl-
H-H
R-O_{’ -
1
-
ROH
+ R-0-i’
_
RH
l li
20 I
Rb + RCH= CHs
(I)
PhCHO
(ii)
O-R’
l
PhkH,
0 + ri’
-
2 b - propagat Ion
(iir)
PhOH 20
t
.’
L I, ,I--_-
rn
250
260
270
280
Reactron
, , ,II, , Ai
290
300
temperature
310
320
330
for DBE.
Tab/e 7 Comparison of the benzyl alcohol: benzaldehyde ratio from the untreated, iron- and tin-treated reactions of dibenzyl ether Benzyl alcohol: Untreated
Iron-treated 0.30 1.42
-
300 310 320 330
0.73 0.79 _ 1.13
0.88 _ _ -
1.50 4.00 4.67 7.25
DISCUSSION considerations
The product distributions from the reactions of the phenylalkyl ethers can be accounted for by a free-radical
FUEL,
1983,
Vol
-
ROR’
-
R’R’
+
li
-
ROH
-
R’H
-
RCH,CH,
+li
2RCHsCHs (i) (ii) (iii)
reaction reaction reaction
3a I - termrnation I + RCHs =CHs
3b
J 3c
1
of PEPE and DPEE of DEE of the phenyl alkyl ethers
Intramolecular
reactions n
of DBE” CH,PH
DBE
62, December
Ph-CHs-CH2 W _gLO
Tin-treated
_ -
1408
4’ R’
and DPEE _
PhCHO+
PhCHs
Benzaldehyde
260 280
General mechanistic
Rb -1 R
+ +
(“C)
Figure 2 Conversion versus reaction temperature 0, Untreated; n , iron-treated; A. tin-treated
Reaction temperature (“C)
Rb -1 R
ph_;H $ DPEE
z
-
PhCH,
+
HCHO
+ PhCH=CHs
z >3OO”C I 17 CO+ H,
Model
R _
R’ _
PhOPh
Ph
Ph
PhOCH,Ph
Ph
PhCHs
PhOCH,CH,Ph
Ph
PhCH2CH2
PhOCH&H,CHaPh
Ph
PhCHZCH2CH2
PhCH,
PhCHa
PhCHsOCHaPh PhCHsCHsOCHzCHzPh Reaction Scheme (RS)
PhCHaCHz PhCHaCHz Free-radical mechanism
Hydrogenation Table 8 Effect of reaction temperature
Reaction temperature
on the untreated
and iron- and tin-treated
Conversion b
Ethyl benzene
of brown
coal. IV: P. J. Cassidy et al.
reactions of di-2-phenylethyl
Tol uene
ether (DPEEja
Benzene
1 -Phenyl ethanol
(mol %I
(“C)
_ Untreated
Iron-treated
Tin-treated
330
1
1
360 385 405 425 460
12 34 81 99 100
12 25 76 103 41
_
_
5 9 19 39 22
_ 2 14 42 123
2 12
trace _
ml 5 9 2 5 _
300
_
trace
330 360 385
25 55 69
17
8
405
93
425 460
100 100
10 45 73 109 102 46
_
31 36 29
23 43 106
20 6 trace
385 425
34 100
22 98
10 47
_ 38
8 trace
trace 12 38
22
a Reaction time = 1 h, initial H, pressure = 6 MPa b Refer to comment b, Table 2 for the definition of conversion
yield two free radicals which propagate the reaction by abstracting H. from a parent molecule of H, gas (RS: Equation 2a). The radical formed by hydrogen atom abstraction from PEPE can rearrange to yield styrene and phenoxy radicals which further propagate the reaction. The radicals formed by hydrogen atom abstraction from BPE and PPPE are not capable of propagating the reaction in this way. However, the reaction can propagate by reaction of the initially formed radical with hydrogen gas yielding a hydrogen atom which attacks the aromatic ring of the parent molecule to ultimately yield phenol and the corresponding phenylalkyl radical (RS: Equation 2b(iii)). The free radical reactions may be terminated by combination of two radicals (RS: Equation 3a), stabilization of a radical by reaction with fi (RS: Equation 3b) or by disproportionation (RS: Equation 3~). The reactions of DBE and DPEE cannot be wholly accounted for by the above free-radical scheme. For example, the initiation reaction and, hence, TsO, are related to the energy of the weakest bond in the molecule. DBE has a CH,-O bond energy of 286? 10 kJ mol- ’ which is x66 kJ mole1 greater than both PEPE and PPPE. However, 7”, for DBE is significantly lower than 7”0 of PEPE and PPPE. Likewise DPEE, with an ether bond energy similar to the unreactive diphenyl ether had a much lower T,, than would be expected from a pure freeradical reaction. Cronauer et
iron
The effects of iron on the reactions of model compounds can be summarized as follows: (1) no hydrogenation of the aromatic rings in any of the compounds; (2) diphenyl ether - no effect at 460°C; (3) dibenzyl and aliphatic ethers - (a) increased the rate of reaction with DBE and DPEE and (b) increased the yield of PhCH,CH,OH from DPEE compared to the untreated reaction; (4) phenyl-alkyl ethers - PhO(CH,),Ph (n= 1,2, 3Ha) reduced the rate of reaction and (b) no significant change in the product distributions compared to the untreated reactions. Presumably adsorption of the ethers onto the iron surface occurs in all cases. Iron catalyses cleavage of the C-0 bonds in dibenzyl and aliphatic ethers leading to an increase in concentration of the propagating radicals and thus an increase in reaction rate. In the case of DPEE the increase in radical formation leads to increased yields of PhCH,CH,OH relative to ‘the products of thermal rearrangement which dominate the reaction without added metals. The retardation of reactions of phenyl ethers by added iron could be due to the formation of a relatively strong bond between the iron and the phenoxy radicals generated at the surface. Iron has been shown to form stable alkoxy and carboxylate ‘s*19intermediates upon reaction with alcohols and acids and it is not surprising that a similar effect is observed with phenoxy radicals. Furthermore, phenoxy radicals generated away from the iron surface must have been transported to the iron for the rate of reaction to be reduced below that of the untreated reaction. The effect was restricted to the phenoxy radical and not observed for a species such as PhCH,O because the resonance-stabilization energy associated with PhO would result in it having a longer half-life thus allowing it time to be transported to the catalyst surface without picking up a stabilizing hydrogen. The fact that no reduction in the rate of reaction was observed when Fe was added to dibenzyl ether which can form the stable benzyl radical (PhcH,), shows that the affinity of iron for oxygen is important to the radical stabilization process.
FUEL, 1983, Vol 62, December
1409
Hydrogenation
of brown coal. IV: P. J. Cassidy et al.
Effects of tin
The effects of tin on the reactions of the model compounds can be summarized as follows: (1) no hydrogenation of the aromatic rings in any of the compounds; (2) no effect with DPE or DPEE; (3) decrease in the rate of reaction of- (a) the benzyl ether - DBE and (b) the phenyl-alkyl ethers - BPE, PEPE, PPPE; (4) significant increase in the benzyl alcohol:benzaldehyde ratio from the reaction of DBE. These results can also be understood as a perturbation on the free-radical route. In contrast to iron, tin decreased the rate of reaction (increased T,,) of both the benzyl and phenyl ethers. It is noted that T,, for DBE was increased by only 6°C compared to an average of 26°C for the phenyl ethers. However the slope of the conversion versus reaction temperature curve (Figure 2) was high and at 33O”C, for example, the conversion of the tin-treated reaction was 23% lower than the untreated case (Table 6). Unlike iron, tin had no effect on the reaction of the alkyl ether. As tin did not increase the rate of reaction of any of the ethers it is believed that tin did not dissociatively adsorb the parent molecules but reacted, forming a relatively strong bond, with stable phenoxy and benzyl radicals thermally generated away from the metal surface. The ability of tin to stabilize free radicals is not surprising in view of its use to increase the thermal stability of polymers 20-22. Polymers generally un-zip by a free-radical mechanism and the tin presumably captures any free radicals that are thermally generated and thus prevents the initial radical attack from occurring. The increased benzyl alcohol :benzaldehyde ratio from the tin-treated reaction of DBE suggests that tin was also capable of utilizing hydrogen from the gas to form benzyl alcohol rather than benzaldehyde which has been shown to form in a hydrogen-deficient environment16. Tin (m.p. = 232°C) is molten at the reaction temperature used and dissolves significant amounts of hydrogen23. The hydrogen is dissociatively dissolved as atomic hydrogen24 so there is a potential supply of atomic hydrogen at the catalyst surface that can stabilize a free radical. Furthermore, tin readily forms hydrides at lower temperatures which will react by both polar and free-radical mechanisms with a wide range of organic compounds25. In the light of this evidence it is not surprising that tin showed an interaction at high temperatures with free radicals and hydrogen. Activity of iron and tin in the liquefaction of Mctorian brown coal
These studies have shown that iron can catalyse the degradation of aliphatic ether linkages while tin does not accelerate the breakdown of any of the compounds studied. In addition it was shown that the lignin-related aliphatic and benzylic ethers decomposed thermally at temperatures < 400°C. It has been recognized that tin is a more efficient promoter of coal liquefaction reactions than iron and for these reasons it is uncertain whether the capability of iron to catalyse carbon-oxygen bond cleavage in lignin-related ethers is of primary importance in coal liquefaction reactions. In agreement with other work26 it was found that the thermal reaction of hydrogen with diphenyl ether was very slow. No catalysis of the reaction with iron and tin compounds was observed. However, the effect of iron and tin on the reactions of diary1 ethers with hydrogen, where
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FUEL, 1983, Vol 62, December
the aryl group contains oxygen substituents or is a polycyclic aromatic system, are not known. Petrakis and Grandy” showed, from in-situ e.s.r. studies of the coal liquefaction reaction of Powhatan No.5 coal, that the concentration of free radicals increased rapidly with reaction time and the concentration of radicals using naphthalene was more than double that when tetralin was used as the vehicle. This evidence corroborates the interpretation of Neave128 that the initial chemical reactions generally involve bond homolysis to yield free radicals which attract hydrogen atoms from the tetralin in preference to undergoing polymerization reactions. However, it is noted that other high energy intermediates have been postulated to account for the reaction of coa129,30. It is believed that the ability ofthe iron and tin additives to capture radicals as exhibited in these model compound studies, is crucial in the initial stages of coal hydrogenation where radical propagation reactions and, hence, repolymerization reactions are retarded. Heating rate studies’ have shown that slow heating to reaction temperature resulted in high conversions to methylene chloride-soluble liquids at temperatures as low as 375°C. Coal has a range of chemical environments with varying bond strengths of the linkages and slow heating results in a gradual homolysis of the various bonds resulting in lower concentration of radicals at any time31*32. This allows the radical-trapping ability ofthe iron and tin to be more efficiently utilized, Neave12* demonstrated that in the first few minutes of a bituminous coal reaction at 400°C there was little conversion of tetralin to naphthalene. He concluded that during this time the tetralin was primarily solvating the coal and little chemical reaction was occurring. In contrast this model compound study demonstrates that significant chemical reaction of lignin-related ether linkages must be occurring in the first minutes of the reaction of brown coal at >4OO”C. It has been shown3 that neither iron nor tin has any effect on the conversion of asphaltene to oil and it is concluded that the major effect of iron and tin is most probably restricted to stabilizing radicals formed during the important first minutes of reaction5. Use of a rapid heat-up autoclave for a coal liquefaction study3 resulted in lower conversions when compared to results obtained using a larger, slower-heating reactor’, even in the presence of a catalyst. This result could be associated with an increase in the relative rate of repolymerization reactions or a relative decrease in catalyst activation reactions, e.g. reduction of a metal oxide to a metal. Increasing the reaction temperature results in degradation of the polymer as the threshold temperature for the stronger bonds is reached. The iron and tin can then commence to stabilize the radicals generated from decomposition of the polymer. Iron and tin also have complementary roles as to which radicals they will stabilize as shown in the model compound experiments. Iron appeared to be the most effective when phenoxy radicals were involved while tin interacted with phenoxy and benzylic radicals. It is also possible that iron catalysed the cleavage of stable aliphatic ethers although infra-red evidence7 would suggest that the concentration of these ethers is negligible in the brown coal. The synergism between the iron and tin observed in the coal liquefaction studies2s3 cannot be fully accounted for
Hydrogenation
by this study. It is possible that the synergism is due to cooperative interaction of the greater stabilization ability of iron with the ability of tin to transfer hydrogen atoms to adsorbed free radicals. The similarity in the chemical nature of the products from the untreated and iron- and tin-treated coal reactions observed in an earlier study3 may be explained by the lack of any catalysis of carboncarbon33 bond cleavage, carbon-oxygen bond cleavage and hydrogenation of aromatic rings. CONCLUSION The results of this study have shown that ethers closely related to structural features of lignin-derived brown coal will decompose below 400°C. A primary role of the iron and tin is to reduce the rate of retrograde polymerization reactions. This type of activity explains why there are no chemical differences in the products from the reactions of untreated and iron- and tin-treated coals. The activity does not conform to the conventional definition of a catalyst where reaction rates are increased and the equilibrium position is unchanged. ACKNOWLEDGEMENT The authors thank the Victorian Brown Coal Council for continued financial support for this research. One of us (PJC) gratefully acknowledges the award of a VBCC scholarship. The views expressed in this paper are entirely the responsibility of the authors. REFERENCES 1 2 3
Hatswell, Rash, D. Marshall, Rash, D. Cassidy, Rash, D.
M. R., Jackson, W. R., Larkins, F. P., Marshall, M., and Rogers, D. E. Fuel 1980,59, 442 M., Jackson, W. R., Larkins, F. P., Hatswell, M. R. and Fuel 1982, 61, 121 P. J., Hertan, P. A., Jackson, W. R., Larkins, F. P. and Fuel 1982,61,939
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White, R. T., Thewlis, P. and Agnew, J. B. Abstracts and papers 7th Australian Workshop on Coal Hydrogenation, 1982, 114 Hatswell, M. R. Ph.D. 7hesis, Monash University (Chemistry) 1982 McPhail, I. and Murray, J. B. SECT/ Miscellaneous Report MRIS,1969 Durie, R. A., Lynch, B. M. and Stemhell, S. Aust. J. Chem. 1960, 13, 156; Sternhell, S. Aust. J. Chem. 1964, 17, 1236 Yoshida, T., Nakata, Y., Yoshida, R., Veda, S., Kanda, N. and Maekawa, Y. Fuel 1982,61, 824 Cassidv. P. J. Ph.D. 7hesis. Monash Universitv lchemistrv) 1982 ’ ~ -’ Charlesworth, J. M. Fuel’1980, 59, 865 McMillen, D. F., Ogier, W. C. and Ross, D. S. in ‘Proceedings: International Conference on Coal Science, Dusseldorf 1981, Verlag Gliickauf GmbH, Essen, 1981, p. 104 Vogel, A. I. in ‘A Textbook of Practical Organic Chemistry’, Longmans, London, 1967, p. 671 Senderens, J. B. Compt. Rend. 1929, 188, 1074 Schlosberg, R. H., Davis, W. H. and Ashe,T. R. Fuel 1981,60,201 Schlosberg, R. H., Ashe,T. R., Pancirou, R. J. and Donaldson, M. Fuel 1981, 60, 155 Cronauer, D. C., Jewell, D. M., Shah, Y. J. and Modi, R. J. Ind. Eng. Chem. Fundam. 1919, 18, 153 Walker, J. F. in ‘Formaldehyde’, Am Chem Sot Monograph Series, Reinhold, New York, 1964 Benzig, J. B. and Madix, R. J. J. Catalysis 1980, 65, 36 Benzig, J. B. and Madix, R. J. J. Catalysis 1980,65, 49 Smirnov, L. N. and Kharitonov, V. M. Volokna Sin Polim. 1970, 236 Sieron, J. K. Rubber Chem. Echnol. 1966, 39(4), 1141 Thoma, W. and Oertel, H. Patent. Chem. Abs. 1963,59, 14171C Mantell, C. I. in ‘Tin - Its mining, production, technology and applications’, Hafner Publishing Co., New York, 1970 Fast, J. D. in ‘Gases in Metals’, MacMillan Press, London, 1976 Kuivila, H. G. Accounts Chem. Res. 1968, 1, 229 Carson, D. W. and Ignasiak, B. Fuel 1980,59, 760 Petrakis, L., Grandy, D. W. and Jones, G. L. Fuel 1982,61, 21 Neavel, R. C. Coal Science 1982, 1, 1 Virk, P. S. Fuel 1979, 58, 149 Brewer, K. R. J. Org. Chem. 1982,47, 1889 Hill, G. R., Hariri, H., Reed, R. I. and Anderson, L. L. Am. Chem. Sot. Adv. in Chem. Ser. 1966,55, 427 Curran, G. P.,Struck, R. T. and Gorin, E. Ind. Eng. Chem. Process Des. Deu. 1967, 6, 166 Thewlis, P., Monash University, Dept. of Chemistry, personal communication
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