Thermal hydrogenolysis of dibenzo-p-dioxin and dibenzofuran

Article abstract of DOI:10.1002/1099-0690(200208)2002:16<2792::AID-EJOC2792>3.0.CO;2-H

The behaviour of dibenzo-p-dioxin (DD) and dibenzofuran (DF) was studied in flow reactors in an excess of hydrogen, at atmospheric or elevated pressure (up to 35 bar), in the temperature range 890-1262 K. The experiments at atmospheric pressure were performed with DF or DD as such, while at increased pressures the compounds were introduced as an admixture with benzene. DD gave CO, but also DF as an important product. The rates clearly depended on the hydrogen concentration, and were about an order of magnitude higher than that of the hydrodechlorination of chlorobenzene. The reaction apparently started with the fission of a C-O bond, induced by H atom attack. DF reacted much more slowly, to give CO and hydrocarbons, especially naphthalene and benzene. Its rate was insensitive to the concentration of H₂, and the degradation has been interpreted as thermolysis, through C-O bond homolysis, isomerisation and fragmentation, primarily to naphthalene, and C₂O as the intermediate to CO. The apparent resistance of DF to hydrogenolysis can be understood from its relatively favourable thermodynamic stability. The consequences for the behaviour of polychlorinated DDs and DFs under similar conditions - relevant for the possible application of thermal hydrogenolysis as a waste management technology - are also discussed briefly. Wiley-VCH Verlag GmbH, 69451 Weinheim, Germany, 2002.

Full text of DOI:10.1002/1099-0690(200208)2002:16<2792::AID-EJOC2792>3.0.CO;2-H

FULL PAPER
Thermal Hydrogenolysis of Dibenzo-p-dioxin and Dibenzofuran
[a]
[b]
[a]
Mariusz K. Cieplik, Onno J. Epema, and Robert Louw*
Keywords: Dibenzo-p-dioxin / Oxygen heterocycles / Hydrogenation / Kinetics / Reaction mechanisms
The behaviour of dibenzo-p-dioxin (DD) and dibenzofuran
DF) was studied in flow reactors in an excess of hydrogen,
and benzene. Its rate was insensitive to the concentration of
, and the degradation has been interpreted as thermolysis,
through C−O bond homolysis, isomerisation and fragmenta-
tion, primarily to naphthalene, and C O as the intermediate
to CO. The apparent resistance of DF to hydrogenolysis can
be understood from its relatively favourable thermodynamic
stability. The consequences for the behaviour of polychlorin-
(
H
2
at atmospheric or elevated pressure (up to 35 bar), in the
temperature range 890−1262 K. The experiments at atmo-
spheric pressure were performed with DF or DD as such,
while at increased pressures the compounds were introduced
as an admixture with benzene. DD gave CO, but also DF
2
as an important product. The rates clearly depended on the ated DDs and DFs under similar conditions — relevant for
hydrogen concentration, and were about an order of magni- the possible application of thermal hydrogenolysis as a waste
tude higher than that of the hydrodechlorination of chlorob- management technology — are also discussed briefly.
enzene. The reaction apparently started with the fission of a
C−O bond, induced by H atom attack. DF reacted much more
slowly, to give CO and hydrocarbons, especially naphthalene
( Wiley-VCH Verlag GmbH, 69451 Weinheim, Germany,
2002)
Introduction
breakdown of aromatic rings is slow. Nevertheless, even
quite complex PAHs are smoothly degraded in the vapour
phase, ultimately to methane, which offers potential for
this ‘‘dry’’ gasification as a waste management technology.
Halogens can be tolerated even at high concentrations, as
these — and other — substituents are displaced by H at a
Benzene, under a pressure of 20Ϫ30 bar of hydrogen, re-
acts smoothly at around 1200 K (927 °C), in seconds, to
afford high yields of methane.[ This process appears to
proceed through (reversible) H atom addition and isomeris-
ation, hydrogenation to C H species and subsequent split-
1]
much faster rate than that at which arene rings are de-
[1,3,4]
graded.
Under these reductive conditions, organic
6
8
chlorine derivatives exclusively become HCl — unlike in the
case of combustion, which also gives free chlorine. Further-
more, ‘‘dioxins’’ (polychlorinated dibenzodioxins and
·
·
[1]
ting into CH and C H (e.g. cyclopentadienyl) radicals.
3
5
5
Naphthalene reacts about 10 times more rapidly; this is
understood — also on a thermokinetic basis — in terms of
an overall easier ring-opening hydrogenolysis.[ The fact
that anthracene reacts only marginally more rapidly than
naphthalene is explained by the extra energy barrier re-
quired for splitting of its central ring, leaving degradation
-furans, PCDD/Fs) cannot arise, and, if present, will be
2]
detoxified, at least by their hydro-dechlorination to the par-
ent compounds DD and DF.
DD, and certainly DF, can be looked upon as heterocyc-
lic PAH analogues, and so it is of basic interest, both from
a scientific viewpoint and to obtain a better insight into the
fate of PCDD/Fs when present in waste streams, to deter-
mine rates and pathways of conversion of DD and DF and
to compare their behaviour with that of, for example, ben-
zene or naphthalene.
[
2]
of a side ring as the only effective reaction pathway.
Activated carbon, ‘‘the ultimate PAH’’ (PAH ϭ poly-
cyclic aromatic hydrocarbon), is considerably more resist-
ant; its lifetime for gasification to methane is over an hour
at 1250 K (970 °C). This highly condensed structure is less
open to reaction with H /H , and therefore the effective
·
2
Results
[
[
a]
b]
Center for Chemistry and the Environment, Leiden Institute of
Chemistry, Gorlaeus Laboratories, Leiden University, P.O. Box
9
502, 2300 RA Leiden, The Netherlands
Dibenzo-p-dioxin (DD)
Fax: (internat.) ϩ31-71/527-4492
E-mail: R.Louw@chem.LeidenUniv.NL
The conversion of DD was first investigated at ca. 1 bar,
in set-up A (see Exp. Sect.) between 960 and 1153 K, with
residence times τ in the quartz flow reactor of 2.2Ϫ2.6 s.
Institute for Inland Water Management and Waste Water
Treatment (RIZA), Ministry of Transport, Public Works and
Water Management,
Ϫ4
P.O. Box 17, 8200 AA Lelystad, The Netherlands
The DD level in the bath gas (H ϩ N ) was around 10
2
2
2792
 WILEY-VCH Verlag GmbH, 69451 Weinheim, Germany, 2002 1434Ϫ193X/02/0816Ϫ2792 $ 20.00ϩ.50/0 Eur. J. Org. Chem. 2002, 2792Ϫ2799

Thermal Hydrogenolysis of Dibenzo-p-dioxin and Dibenzofuran
FULL PAPER
(
mixing ratio). Three representative examples (IϪIII) are was again an important product, with selectivities of as
[
8]
outlined in Table 1. Remarkably, next to CO and ethene as much as 60Ϫ70% at T ϭ 1030Ϫ1050 K. The rate-enhan-
rather prominent degradation products, dibenzofuran (DF) cing effect of formaldehyde was also verified for DD alone,
appears as an important product as well — with selectivities as illustrated by examples IVϪVI (Table 1) involving ca. 0.6
of around 50% — together with smaller proportions of ben- mol-% CH O relative to H . Roughly speaking, the same
2
2
zene, naphthalene and hydroxy-DF.
rates as in series IϪIII were obtained, but at temperatures
about 100 degrees lower (see also Figure 1).
Table 1. Hydrogenolysis of dibenzo-p-dioxin at atmospheric pres-
sure (n.d. ϭ not detected, detection limit ca. 0.03 µmol/h)
Dibenzofuran (DF)
Exp. No.[a]
The reactivity of DF was determined analogously. With
I
II
III
IV
V
VI
ca. 0.1% DF in H and τ ഠ 4.0Ϫ4.2 s, degrees of conversion
2
Temperature [K]
Res. time [s]
Intake [µmol/h]
DD
Trioxane
Output [µmol/h]
Benzene
Naphthalene
DF
DD
1053
2.4
1083 1133 953 983 1033 ranged from Ͻ10 to 64% between 1133 and 1213 K, the
2.3 2.2 4.6 4.4 4.2
50% level of reaction being reached at ca. 1195 K. Clearly
DF was reacting much more slowly than DD, which would
have already been nearly completely converted at the onset
temperature for DF (see Figure 1). Naphthalene and CO
were rather prominent products, together with benzene, in-
dene, and lighter hydrocarbons (Table 2). The selectivity to
CO was essentially 100%, and the carbon was also quantit-
atively accounted for (100 Ϯ 2%).
13.0
Ϫ
6.54 9.72 7.53 6.22 6.87
241 207 239
Ϫ
Ϫ
0.39
0.13
1.05
7.07
0.62 1.08 n.d. n.d. n.d.
0.29 0.88 n.d. n.d. 0.10
1.67 4.42 0.36 0.49 1.27
3.30 0.33 4.25 2.62 1.63
2
4
-Hydroxydiphenyl ether ഠ 0.03 n.d. n.d. n.d. n.d. n.d.
-Hydroxydibenzofuran 0.98
1.21 0.49 0.62 0.72 1.01
3.01 7.23 481 514 687
2.32 2.58 98.2 42.5 19.6
0.13 0.33 n.d. n.d. n.d.
A separate series in which the H concentration was va-
2
CO
1.90
2.13
0.10
45
ried by admixting with nitrogen had no perceptible effect
on the rates.
C
C
2
H
H
4
2
6
[
b]
When plotted as a first-order reaction, the rate constant
DD conversion [%]
49
97
44
58
77
Ϫ1
k for the disappearance of DF obeyed logk/s
ഠ
[
a]
[8]
Pressure: 1.4 bar; total inflow 146 mmol/h; H
2
/N
2
ϭ3.5 (exp. 13.3Ϫ76.7 kcal/mol/2.3RT. This was suggestive of a
IϪIII); 2.4 (exp. IVϪVI). [ Other data on DD conversion:
b]
‘‘spontaneous’’ pyrolytic conversion rather than a reaction
1
9
032 K ഠ 20%; 993 K Ͻ 5% (in absence of trioxane); 1073 K ഠ
0%, 1113 K Ͼ 97% (with trioxane).
induced by hydrogen, and in this light we thought it worth-
while to consider the hydrogenolysis of DF at elevated H2
pressures, also comparing its reactivity with those of aro-
Next, competitive runs of DD with chlorobenzene (PhCl) matic hydrocarbons.[1,2]
were conducted, with the aim of determining relative rates.
This was done by admixing DF with several PAHs dis-
In the temperature range 950Ϫ1090 K, with τ ϭ 4.2Ϫ4.6 s, solved in an excess of benzene (Table 3). With residence
Ϫ3
a PhCl inflow level of ca. 10 mol/mol of (H ϩ N ) and times of 21 Ϯ 2 seconds, pressures of ca. 35 bar and a molar
2
2
DD/PhCl ratios of around 0.04, the rates of disappearance H /benzene inflow ratio of 33, runs were conducted in set-
2
of DD were similar, and about a factor of 10 higher than up B (section 5.2). Data for experiments at temperatures
those of PhCl. The degree of conversion of PhCl was only between 1157 and 1262 K are shown in Table 3. At the
about 20% at the highest temperature. This implies a rate highest T, conversion of both PAH and DF was essentially
that is a factor of 10 lower than that observed previously, complete — with a recovery of benzene of ca. 7% — again
when much higher arene/H intake ratios (e.g., 1:5) were with a quantitative yield of CO from DF.
2
[
5]
used. The reaction of PhCl involves removal of Cl by a
hydrogen atom. The ‘‘spontaneous’’ splitting of H is very plotted in Figure 1. As a measure of the levels of conversion
Outputs of CO as a percentage of the inflow of DF are
[
5]
2
slow, which means that radical-molecule reactions must be of the hydrocarbons, the output of methane (plus the small
sufficiently abundant to establish (near) equilibrium con- amounts of C hydrocarbons) as a percentage of the total
2
·
[6]
Ϫ3
centrations of H. The very low levels (10 or less) of carbon in the feed is also depicted.
aromatics applied in the experiments discussed here may
therefore have been insufficient for this.
It can be seen that the degree of conversion of DF at
1170 K is ca. 30%, seemingly the same value as for with ca.
In order to get around that problem, the effect of addi- 1 bar of H . While it should be noted that the residence
2
tion of sym-trioxane was studied. This cyclic trimer of times were ca. five times longer — and that data from
formaldehyde decomposes above 400 °C into formaldehyde, greatly different set-ups were being compared — there was
a known source of H atoms,[ without making the chem- no sign of a higher rate with increased H concentration.
7]
2
istry of interest too complex. Repetition of the DD/PhCl The 35 bar result can also be compared with that previously
competition experiment with added trioxane (ca. 2 mol per reported for naphthalene (N) — 30% conversion at 1133 K
[
2]
mol of PhCl) indeed produced considerably higher rates, (860 °C) at approximately the same pressure and reaction
for both compounds. From a series of reactions conducted time — meaning that DF reacts considerably more slowly
[8]
between 890 and 1090 K (see ref. for details) with τ ഠ 6 s, than N under the given conditions.
it was inferred that PhCl reached the 50% conversion level
After we had concluded our work, Winkler et al. pre-
at ca. 1050 K, while DD had already done so at 970 K. DF sented rate and product data on the pyrolysis of DF in a
Eur. J. Org. Chem. 2002, 2792Ϫ2799
2793

M. K. Cieplik, O. J. Epema, R. Louw
FULL PAPER
Figure 1. Overview of DD/DF conversions; x: DD conversion (with trioxane, 1.4 bar, 4.2Ϫ4.6 s); ᭺: DD conversion (1.4 bar 2.2Ϫ2.4 s);
[
9]
᭿
᭜
DF conversion (1.1 bar, 4.0Ϫ4.2 s), based on DF recovery; ᮀ DF conversion (35 bar, 20 s), based on CO output; ∆ DF conversion;
CH output (1.4 bar, 4 s), ؉ CH output (35 bar, 20 s)
4
4
Table 2. Hydrogenolysis of DF
Temperature[a] 1133 1143 1153 1163 1173 1183 1193 1203 1213
Table 3. High-pressure hydrogenolysis of dibenzofuran/hydrocar-
bon mixtures
[K]
Temperature[a]
K]
1157
1198
1214
1262
[
Intake [µmol/h]
DF
Output [µmol/h]
CO
14.5 14.5 14.2 14.1 14.0 13.8 13.7 13.6 13.6
Intake [mmol/h]
Benzene
Indene
Naphthalene
Biphenyl
Phenanthrene
DF
14.8
0.25
1.11
0.16
0.12
0.29
16.2
0.26
1.10
0.16
0.12
0.28
14.9
0.24
1.03
0.15
0.11
0.27
15.0
0.24
1.07
0.16
0.12
0.29
0.60 1.65 2.04 3.12 3.87 5.44 7.07 7.42 8.94
0.79 1.91 2.45 4.42 5.92 10.4 15.6 17.8 23.5
0.29 0.59 0.66 0.72 0.66 0.64 0.78 0.70 0.74
0.21 0.81 1.05 1.64 2.04 3.06 4.14 4.09 4.60
0.17 0.15 0.28 0.51 0.56 0.90 1.12 1.05 1.20
CH
4
C
2
C
2
C
2
H
2
H
4
H
6
Cyclopentadiene 0.19 0.27 0.34 0.38 0.38 0.36 0.35 0.32 0.27
Output [mmol/h]
CO
Benzene
Toluene
Indene
Naphthalene
Biphenyl
Biphenylene
DF
0.73 0.81 1.06 1.53 2.07 2.57 3.02 3.72 2.70
n.d. 0.03 0.03 0.03 0.04 0.04 0.07 0.07 0.07
0.27 0.31 0.44 0.60 0.82 0.97 1.05 1.18 1.24
0.51 0.62 0.78 0.98 1.42 1.76 2.04 2.38 2.77
0.05 0.05 0.05 0.04 0.04 0.03 0.04 0.03 0.03
0.03 0.03 0.03 0.03 0.03 0.03 n.d. n.d. n.d.
13.0 12.5 11.8 10.9 9.7 8.5 7.1 6.2 5.0
0.07
48.92
1.44
7.05
0.01
0.13
71.83
0.55
5.80
0.19
78.37
0.40
3.25
0.28
CH
4
86.26
0.101
0.91
2 6
C H
Benzene
Toluene
Indene
Naphthalene
Biphenyl
Ͻ0.01 Ͻ0.01 Ͻ0.01
Ͻ0.01 Ͻ0.01 Ͻ0.01 n.d.
0.05
0.04
Ͻ0.01 0.01
24
0.04
0.03
0.05
0.05
Ͻ0.01 n.d.
70
71.2
Ͻ0.01
Ͻ0.01
DF conversion (8.5) ഠ13 17 23 30 38.5 48 54.5 63.5
[%]
Phenanthrene [
b]
DF conversion [%]
46.5
61.2
96.5
77.8
[
a]
At 1.1 bar; carrier gas: H
2
only, 60 mmol/h, residence time τ ഠ
CH
4
[% of total carbon input] 44.7
4
.2 s (1133 K) to 4.0 s (1213 K).
[
a]
/benzene molar intake ratio ca. 33. [b]
2 2
, τ ഠ 21 s, H
3
5 bar of H
Based on CO output.
large excess of argon (1 bar, 1173 K) with residence times
[
9]
of ca. 20 s. The pyrolysate contained 74% of DF, and N
was the most prominent product. Rate data were not re-
ported explicitly, but for complete materials balance the de-
Keeping in mind that the studies were performed in dif-
gree of conversion should have been 26%; for less complete ferent laboratories and with different set-ups, it is still strik-
recoveries the conversion would have been accordingly ing that the rate of decomposition of DF under 1 bar of
higher. The data point for the ‘‘minimum’’ value has been argon apparently differs little from that under 35 bar of hy-
included in Figure 1.
drogen.
2794
Eur. J. Org. Chem. 2002, 2792Ϫ2799

Thermal Hydrogenolysis of Dibenzo-p-dioxin and Dibenzofuran
FULL PAPER
Discussion
minor importance, as IV was only found at trace levels at
‘‘lower’’ temperatures. (At higher T, compound IV would
Dibenzo-p-dioxin
also be rapidly hydrogenolysed, either via radical I or not).
As the rate of thermal hydrogenolysis clearly depends on
Splitting of I into o-quinone and phenyl [reaction (f)] to
the concentration of (atomic) hydrogen, the first step in the explain the formation of benzene, CO and C
conversion of DD would appear to be the reaction between is attractive, but the activation energy would probably be
hydrocarbons
2
·
[13]
H and a CϪO bond. Upon ipso-addition to C and sub- too high to compete successfully with steps bϪd.
In-
sequent breaking of the relatively weak CϪO bond, the stead, radical I could provide a route to these degradation
substituted phenoxyl radical I would be formed [Scheme 1, products — as well as naphthalene — after rearrangement
reaction (a)].
[pathway (g)], as further illustrated in Scheme 2.
By analogy with the parent phenoxyl radical,[14] I would
be a candidate to split off CO directly to give radical V
[reaction (h)] but for thermochemical-kinetic reasons this
would not be likely to be important. Rather, a rearrange-
ment culminating in loss of CO, by, for example, pathway
(i), producing radical VI, seems straightforward. Both V
and VI could be degraded to benzene etc., but further re-
arrangement could result in loss of another CO molecule,
as in pathway (j), with radical VII as a precursor of naph-
thalene.
Naphthalene may also be formed by condensation of two
[
15]
cyclopentadienyl radicals,
should these act as interme-
diates. Substituted analogues such as V can react accord-
ingly, with loss of (phenoxy) substituents either during the
process or after the naphthalene structure has been estab-
lished.
Scheme 1
·
The key intermediate I could isomerize to species II by
The first step in the conversion of DD with H [reaction
intramolecular H abstraction [step (b)]; it could also arise (a), Scheme 1] should be irreversible, because conversion of
·
by direct attack of H on oxygen in DD. The conversion I I to DF and III via radical II would be easier, in view of
Ǟ II is endothermic (by ca. 27 kcal/mol[ ), and therefore the lower energy barriers.
10]
[10]
That DD reacts considerably
reversible and very probably equilibrated. Radical II is the more rapidly than chlorobenzene is primarily due to the
·
logical precursor of both DF — by loss of OH [reaction statistical factor of 4; the per site reactivity may also differ
(
c)] — and of its 4-hydroxy derivative III, formed by ring- somewhat, although in general, rates for — straightfor-
·
closure with loss of H [reaction (d)]. Intermolecular hydro- ward — displacements from monosubstituted benzenes
genation of I and/or II [reaction (e)] would appear to be of PhZ ϩ H Ǟ PhH vary little with Z.
·
[3]
Scheme 2
Eur. J. Org. Chem. 2002, 2792Ϫ2799
2795

M. K. Cieplik, O. J. Epema, R. Louw
to 68.5[18] kcal/mol) differ widely.
FULL PAPER
[
11]
Dibenzofuran
and of C O (from 58
2
If values of 11 and 58 kcal/mol, respectively, were taken,
As reported above, the reaction rates of dibenzofuran do
the overall (standard) heat of reaction of DF to C O and
2
not appear to depend on the hydrogen concentration/pres-
0
sure. Moreover, Winkler et al.[9] have recently found that
naphthalene (∆
f
H ϭ 36 kcal/mol) would be 74 kcal/mol,
in fair agreement with the observed Arrhenius activation
energy of 77 kcal/mol.
pyrolysis of DF gave not only similar rates, but also similar
main products: carbon monoxide and naphthalene. All this
strongly points to a unimolecular reaction. Scheme 3 pre-
sents an interpretation, based on the overall stoichiometry
Interestingly, benzofuran derivatives could not be de-
tected. This could mean two things: either that, if formed,
they were rapidly converted, or that no such species were
formed. This would be consonant with the fact that hydro-
genolysis of DF is insignificant compared with its direct
thermolysis. To understand why DF — which at first sight
is a regular (di)substituted benzene — resists hydrogenolytic
splitting, consider Schemes 4 and 5.
DF Ǟ naphthalene ϩ C O.
2
·
Scheme 4 depicts the opening of the central ring by H to
give radicals XI and XII. No doubt the forward reaction
(
L) should occur easily, by analogy with the splitting of bi-
[
20]
phenyl,
but there is no other fate than to revert to DF
·
and H , a step that is even mildly exothermal. Similarly, the
formation of XII [reaction (m)] should be easy. Although
reversal of XII to DF and H should — on the basis of
·
current thermochemistry — be endothermic by some 30
kcal/mol, it still appears to be fast at the high temperature
involved, compared with possible ‘‘exits’’ through bimo-
Scheme 3
Upon CϪO bond cleavage, DF could isomerise to species
VIII (5H-5,9-methanobenzo[7]annulen-10-one[
16]
)
and,
through radical IX, ultimately to compound X (5H-
benzo[7]annulen-5-ylidenemethanone[ ). This could split
16]
[
17]
off C O, by analogy with cheletropic decarbonylation.
2
Dicarbon monoxide is known to decompose even at rela-
tively mild temperatures,[ yielding atomic carbon and CO.
The thermochemical-kinetic details of this mechanism
are not discussed here, one reason being that the published
18]
[
19]
Scheme 4
heats of formation both of DF (from 11 to 20 kcal/mol
)
Scheme 5
796
2
Eur. J. Org. Chem. 2002, 2792Ϫ2799

Thermal Hydrogenolysis of Dibenzo-p-dioxin and Dibenzofuran
FULL PAPER
lecular reactions, such as formation of o-hydroxybiphenyl Conclusions
XIII and its hydrogenolysis.
Scheme 5 shows the addition of H to positions 1 and 4
·
Dibenzo-p-dioxin was relatively easily converted under a
in DF, resulting in radicals XIV and XV (additions to the hydrogen atmosphere at T around 1000 K, about 10 times
other two unsubstituted positions 2 and 3 are not depicted, more rapidly than the reaction of chlorobenzene to give
because the behaviour of the resulting radicals would be benzene and HCl. Interestingly, together with CO, dibenzo-
analogous to that of XIV and XV).
If XIV and XV were to react analogously with benzene,
(
ther hydrogenolysis (with loss of one carbon as CH3
would give rise to cyclopentadiene derivatives (P). These are
furan (DF) was an important product. Both processes were
sensitive to the concentration of H atoms, which appeared
[
1]
reversible) isomerisation to XVI/XVII would occur. Fur- to react by addition to (Cl- or O-) substituted carbons as a
·
[1]
)
first step.
DF was much more resistant to degradation. Moreover,
analogues of dihydropentalene, known to be highly the rates did not seem to depend on the hydrogen (atom)
[
21]
strained.
XVI and XVII, so altogether even their formation is heavily genolysis — from 1Ϫ35 bar of H
This ‘‘handicap’’ is already present in species concentration, as shown by comparison between hydro-
— and a recent report on
2
disfavoured compared with the base case benzene.
thermolysis in argon. Naphthalene (N) was an important
product throughout, and a thermolysis mechanism invol-
ving CϪO fission and isomerization has been put forward.
While the overall rates for the conversion of DF and N
were quite comparable, hydrogenolytic ring-opening in DF
appeared to be slow. An explanation for this has also been
presented.
Relative Rates
In our earlier papers,[1,2] the relative rates for the conver-
sions of benzene (B), naphthalene (N), and chlorobenzene,
The results are of interest for possible application of ther-
mal hydrogenolysis in the management of (hazardous)
waste. On the basis of the found or calculated relative rates,
the expected fate of polychlorinated DDs is that conversion
into chlorinated DFs would be a more important pathway
than hydrodechlorination; DD and analogues — if present
in the starting mixture — would disappear relatively rapidly
anyway. The polychlorinated DFs should be smoothly
dechlorinated, just like chlorobenzenes etc. (and hence, de-
toxified) to leave DF — together with other (aromatic)
compounds, especially hydrocarbons — as a rather stable
compound. With, for example, Ͼ99% ‘‘mineralisation’’ of
organic chlorine to HCl, this could be a very acceptable
result. There is little need to destroy DF for health or safety
reasons (or N; some carcinogenic PAHs may have been
formed), but, if desired, temperatures above 1300 K should
be chosen, so as to gasify thoroughly even benzene and de-
modelled for 1250 K and 25 bar H , were shown to be B/
N/PhCl ϭ 0.1:1:35.
2
To put DD on this scale, the result (faster than PhCl by
a factor of 10 at ca. 1000 K) has to be translated to 1250 K,
and a value of ca. 8 would be expected.[ Obviously, DF
reacted very much more slowly than DD: Figure 1 shows
that the T50% values for DD and DF are some 230 °C apart.
On this basis, DF has been calculated to react ca. 4 ϫ 103
times more slowly than DD (ca. 14 times more slowly than
N) under the model conditions used.[ All this implies the
following order (Table 4, relative rates compared with
N ϭ 1).
22]
24]
Table 4. Relative rates at 1250 K, 25 bar H
2
Compound (in order of DF ഠ B < N << PhCl < DD
increasing rates)
[
1,2]
rivatives, if not soot and carbon, to methane;
questions
about exact mechanisms, molecular or free-radical, of con-
versions of DD/DFs then become quite pointless.
Relative rate
ഠ 0.07 0.1
1
35
ഠ3·102
Experimental Section
On going from 25 to 1 bar of hydrogen (with sufficiently
high concentrations of organics to ensure equilibration be-
Atmospheric Pressure Set-Up (A): Atmospheric pressure experi-
·
tween H and H), the rates for PhCl and DD — and pre-
2
ments were performed in the set-up shown schematically in Fig-
sumably also for B and N — went down (by a factor of 5),
with little if any change in their relative rates. While DF is
insensitive to H2 concentration, its relative rate can ap-
[8]
ure 2. The effective volume of the reactor is ca. 6 mL. Hydrogen
and nitrogen were introduced by means of electronic mass flow
controllers (Brooks 5850TR). Methane was used as a reference gas.
proach that of N. In any case, the fact remains that hydro- Liquid and solid reagents were evaporated/melted in an impinger
dechlorination (of PhCl) and hydrogenolytic conversion of vessel and carried into the reactor system by a calibrated nitrogen
DD are much faster than the degradation (thermolysis) of flow.
DF. Polychlorinated DFs will therefore undergo repeated
hydrodechlorination (ultimately to DF) rather than being
degraded. Likewise, polychlorinated DDs will be subject to
hydrodechlorination, but, by analogy with DD itself,
High Pressure Set-Up (B): The high pressure set-up and the experi-
mental procedures applied in this study have been described else-
where.[
1]
formation of chlorinated DFs as intermediate products Sampling and Analyses. Atmospheric Pressure Experiments:
would be expected to be more important.
Samples of the effluent gases were taken by means of six-port sam-
Eur. J. Org. Chem. 2002, 2792Ϫ2799
2797

M. K. Cieplik, O. J. Epema, R. Louw
FULL PAPER
Figure 2. Experimental setup A: 1. mass flow controllers; 2. evaporation vessels; 3. valve; 4. bypass; 5. oven; 6. reactor; 7. sample valve;
. sample loops; 9. split/splitless capillary injection port; 10. splitless packed injection port; 11. capillary column (mol. sieves 5 A); 12.
capillary solumn (CP-SIL 5); 13. packed column (Carbosphere); 14. methanizer; 15. FID; 16. TCD
8
pling valves (Valco C6WT-HC) mounted in a HP 5890 series II
Gas Chromatograph, equipped with three columns: a packed one
Products, 99.995%), methane (Air Products, 99.995%), nitrogen
(Air Products 99.995%).
2 1 3
connected to a FID methaniser (CO, CO and C /C hydrocarbons
analyses); a capillary Chrompack CP-Sil5 (50 m, 0.32 mm ID) col-
˚
5
umn connected to a FID (C ϪC18 hydrocarbons) and a 5 A mol.
sieves column connected to a TCD.
[
1]
W. B. van Scheppingen, M. K. Cieplik, R. Louw, Eur. J. Org.
Chem. 2001, 2101Ϫ2106.
[2]
M. K. Cieplik, R. Louw, W. B. van Scheppingen, Eur. J. Org.
Chem. 2001, 4517Ϫ4524.
3]
Unknown products were identified by off-line GC/MS analysis on
a HP 5970/HP 5890 instrument.
[
J.A. Manion, R. Louw, J. Phys. Chem. 1990, 94, 4127Ϫ4134.
R. Louw, P. Mulder, J. Environ. Sci. Health 1990, 25, 555Ϫ569.
J.A. Manion, J.H.M. Dijks, P. Mulder, R. Louw, Recl. Trav.
Chim. Pays-Bas 1988, 107, 434Ϫ439.
S.W. Benson, R. Shaw, J. Chem. Phys. 1967, 47, 4052Ϫ4055.
W. Hogg, D. M. McKinnon, A. F. Trotman-Dickenson, G. J.
O. Verbeke, J. Chem. Soc. 1961, 1403Ϫ1404.
O. J. Epema, On Gas-Phase Thermal Conversions of Dibenzofu-
ran and Dibenzo-p-Dioxin, thesis, Leiden University, Leiden,
The Netherlands, 1996.
[4]
[5]
High-Pressure Experiments: For each run, two liquid (organic) frac-
tions were collected: one in the pressurised trap of the reactor sys-
tem and one in a cold trap. Samples were analysed by GC/FID
[6]
[7]
[8]
(HP5890 quantification) and GC/MS (HP5890/HP5972 scan mode,
MM range 30Ϫ350; with a standard NIST NBS75 spectrum library
Ϫ qualitative analysis). Standard method: init. temp. 50 °C; init.
time 5 min; rise 10 °C/min; final temp. 280 °C, final time 10 min;
column: Chrompack CP-SIL 5 CB, 50 m ϫ 0.32 mm ϫ 0.4 µm;
[9]
J. K. Winkler, W. Karow, P. Rademacher, J. Analyt. Appl. Pyro-
lysis 2001, 57, 133Ϫ144.
carrier gas: H
kPa. Monobromobenzene was applied as a co-injection standard
ca. 0.1 g per sample).
2
(GC/FID), He (GC/MS); column head pressure 90
[10]
Basically, it is the difference between a benzene CϪH bond (ഠ
11]
1
10 kcal/mol) and the OϪH bond of phenol (ഠ87).[ The o-
(
phenoxy substituent in II probably lowers the latter value by
several kcal/mol.[ With heats of formation of Ϫ1 and ϩ26
kcal/mol for I and II, respectively, the heat of the reaction I o
II is ca. 27 kcal/mol.
12]
During each run, eight to twelve 0.5 mL gaseous samples were
taken from the sampling point at the end of the system. Analyses
of C
matograph equipped with a packed column (Alltech, Carbosphere,
6Ϫ100 MESH), a FID detector and a methaniser. Peak areas were
compared to those of a standard mixture of CO, CO , methane,
1 2
/C compounds were carried out on a Packard 428 gas chro-
[11]
Data from the online NIST Standard Thermodynamic Database
at: http://webbook.NIST.gov
8
[12]
M. Jonsson, J. Lind, T.E. Eriksen, G. Merenyi, J. Chem. Soc.,
Perkin Trans. 2 1993, 1567Ϫ1568.
2
[13]
0
0
acetylene, ethylene, and ethane, each 1% (v/v) in nitrogen.
f 6 5 f
With ∆ H (C H ) ϭ ϩ77 kcal/mol; ∆ H (o-quinone) ϭ Ϫ25
[
11]
kcal/mol, the product level of channel (f) (ഠ 52 kcal/mol) is
To take the collecting time into account, results are expressed in
time-dependent units (mmol/h).
·
·
well above those for DF ϩ OH, or (III) ϩ H (each around 29
kcal/mol) and also ca. 15 kcal/mol higher than the starting level
·
of DD (Ϫ15) and H (ϩ52 kcal/mol).
Chemicals: The following chemicals were utilised: benzene [Merck
p.a., 99% (distilled)], dibenzo-p-dioxin (synthesised, method de-
scribed by Dittus et al. in ref.[ ), dibenzofuran (Fluka, Ͼ99%),
monobromobenzene (Baker, Ͼ 99.5%), naphthalene (Janssen Chi-
mica, Ͼ 99%), n-pentane (Baker Analysed, Ͼ 99%), hydrogen (Air
[14]
D. L. Baulch, C. J. Cobos, R. A. Cox, C. Esser, P. Frank, T.
H. Just, J. A. Kerr, M. J. Pilling, J. Troe, R. W. Walker, J. War-
natz, J. Phys. Chem. Ref. Data 1992, 21, 411Ϫ429.
E. Gey, B. Ondruschka, G. Zimmermann, J. Prakt. Chem.
1987, 329, 511Ϫ519.
25]
[15]
2798
Eur. J. Org. Chem. 2002, 2792Ϫ2799

Thermal Hydrogenolysis of Dibenzo-p-dioxin and Dibenzofuran
FULL PAPER
[
[
16]
17]
[23]
Names obtained from ACD/IUPAC Name Free v.5.0, using the
ACD/I-Lab service.
R. B. Woodward, R. Hoffmann, in The Conservation of Orbital
Symmetry, Verlag Chemie GmbH Ϫ Academic Press Inc.,
Weinheim, 1970.
H. B. Palmer, W. D. Cross, Carbon 1966, 3, 475Ϫ481.
R. Notario, M. V. Roux, O. Casta n˜ o, Phys. Chem. Chem. Phys.
R. Louw, H. Dijks, P. Mulder, Chem. Ind. 1983, 759Ϫ760.
24]
[
2
The relative rate for DF (at 1250 K in 25 bar of H ) can be
[24a]
approached in two ways:
(1) Compare with N: 30% conver-
sion at 1133 K, 4 s,[ vs. same conversion for DF, but at
1173 K, 21 s; this fivefold difference in experimental unimolec-
ular rate constant, accepting the temperature coefficient of DF
2]
[
[
18]
19]
(76.7 kcal/mol), gives a relative rate N/DF of ca. 18 at 1233 K,
and extrapolated to 1250 K, of ca. 14 (or 1:0.07). [
24b]
(2) Cover
2
001, 3, 3717Ϫ3721.
[
[
[
20]
21]
22]
J. A. Manion, P. Mulder, R. Louw, Envir. Sci. Tech. 1985, 19,
80Ϫ282.
the gap between DF and DD, of ca. 230 °C (Figure 1). Same
rates at 1173 and 943 K, respectively, imply a difference in
(free) activation energy of ca. 15 kcal/mol. With this value the
2
S. Kato, R. Gareyev, C. H. DePuy, V. M. Bierbaum, J. Am.
Chem. Soc. 1998, 120, 5033Ϫ5042.
To allow for an ‘‘exact’’ factor of 10 in the rate at ca. 1000 K,
3
relative rate constant for DD/DF at 1250 K would be ca. 4·10 ,
again giving 0.07 in Table 4 for DF.
Ϫ1
[25]
the overall parameters for PhCl, logA/s ϭ 10.9, E
kcal/mol,
a
ϭ 57.3
G. Dittus, W. Lurken, B. Zeeh, in Methoden der Organische
Chemie 6, part 4, 4th edition, Theime Verlag, Stuttgart, 1966.
Received February 5, 2002
[
22]
were modified for DD to logA ϭ 11.5 and 55.5,
respectively. For 1250 K the relative rate then becomes ca. 8.
[O02073]
Eur. J. Org. Chem. 2002, 2792Ϫ2799
2799