Studies on partial hydrogenation of 1,5,9-cyclo-dodecatriene towards cyclo-dodecene

Article abstract of DOI:10.1016/j.apcata.2011.09.003

The selective hydrogenation of 1,5,9-cis,trans,trans-cyclo-dodecatriene (1,5,9-ctt-CDT) towards cyclo-dodecene (CDE) depends strongly on the pressure of hydrogen, respectively the hydrogenation rate. High yields of CDE (>90%) can only be reached at extremely low hydrogen pressure. In order to elucidate this exceptional reaction performance the course of reaction has been studied for a wide range of hydrogen pressure, 0.01>pH2>2.5MPa, taking into consideration data of other research groups. The CDT hydrogenations were discontinuously carried out in liquid phase on Pd/Al₂O₃ at T = 353 K. The resulting hypothesis of this study is that the very low reaction rate at low pH2 is necessary in order to realize a dense surface coverage of 1,5,9-CDT and 1,5-cyclo-dodecadiene (CDD) isomers where these molecules show adsorption on Pd via two double bonds so that readsorption of formed CDE and subsequent hydrogenation to cyclo-dodecane (CDA) is hardly possible. On the whole this new hypothesis on the reaction course of CDT hydrogenation gives a sound and fully consistent view on this rather complicated reaction.

Full text of DOI:10.1016/j.apcata.2011.09.003

Applied Catalysis A: General 409–410 (2011) 21–27
Contents lists available at SciVerse ScienceDirect
Applied Catalysis A: General
journal homepage: www.elsevier.com/locate/apcata
Studies on partial hydrogenation of 1,5,9-cyclo-dodecatriene towards
cyclo-dodecene
J. Gaubea,∗, W. Davida, R. Sanchayana, N. Wuchtera, H.-F. Kleinb
a Ernst Berl-Institut für Technische und Makromolekulare Chemie, Technische Universität Darmstadt, Petersenstr. 20, D-64287 Darmstadt, Germany
b Eduard Zintl-Institut für Anorganische und Physikalische Chemie, Technische Universität Darmstadt, Petersenstr. 18, D-64287 Darmstadt, Germany
a r t i c l e i n f o
a b s t r a c t
Article history:
Received 18 July 2011
Received in revised form 2 September 2011
Accepted 3 September 2011
Available online 10 September 2011
The selective hydrogenation of 1,5,9-cis,trans,trans-cyclo-dodecatriene (1,5,9-ctt-CDT) towards cyclo-
dodecene (CDE) depends strongly on the pressure of hydrogen, respectively the hydrogenation rate.
High yields of CDE (>90%) can only be reached at extremely low hydrogen pressure. In order to eluci-
date this exceptional reaction performance the course of reaction has been studied for a wide range of
hydrogen pressure, 0.01 > pH > 2.5 MPa, taking into consideration data of other research groups. The
2
CDT hydrogenations were discontinuously carried out in liquid phase on Pd/Al2O3 at T = 353 K.
Dedicated to Herbert Vogel on the occasion
of his 60th birthday.
The resulting hypothesis of this study is that the very low reaction rate at low pH is necessary in
2
order to realize a dense surface coverage of 1,5,9-CDT and 1,5-cyclo-dodecadiene (CDD) isomers where
these molecules show adsorption on Pd via two double bonds so that readsorption of formed CDE and
subsequent hydrogenation to cyclo-dodecane (CDA) is hardly possible.
On the whole this new hypothesis on the reaction course of CDT hydrogenation gives a sound and fully
consistent view on this rather complicated reaction.
Keywords:
Hydrogenation
1,5,9-Cyclo-dodecatriene
Cyclo-dodecene
Kinetics
© 2011 Elsevier B.V. All rights reserved.
1. Introduction
hydrogenation of CDT with Pd supported on alumina at rather low
hydrogen pressures. Such high yields could also be obtained in a
Starting material of the synthesis of laurolactam the monomer
for the production of Nylon-12 is 1,5,9-ctt-cyclo-dodecatriene
which is obtained by catalytic cyclo-trimerisation of butadiene.
The usual synthetic route proceeds via oxidation of cyclo-dodecane
(CDA) to cyclo-dodecanone which is unsatisfactory with respect to
selectivity.
The route via the selective hydrogenation of CDT towards CDE
is a promising alternative, Fig. 1. CDE could be oxidised with H2O2
to the epoxide which can be catalytically transformed into cyclo-
dodecanone as proposed by Wilke [1]. CDE can also be oxidised
with N2O to cyclo-dodecanone as recently patented by BASF [2].
Furthermore CDE can serve as starting material for the production
of 1,10-decane dicarboxylic acid.
In a group of studies CDT hydrogenations have been carried out
under low hydrogen pressures achieving rather high CDE yields.
McAlister reported a CDE yield of 90% obtained in a liquid phase
batch hydrogenation procedure applying a powdered catalyst of 5%
Pd on charcoal at stepwise reduced hydrogen pressure between 0.2
and 0.025 MPa [3]. Wießmeier and Hönicke [4] and Herwig et al. [5]
could also reach CDE yields of about 90% by continuous gas phase
liquid phase batch procedure with Pd supported on alumina at
stepwise reduced hydrogen pressure of 0.15 down to 0.007 MPa
by Gaube et al. [6].
Another group of studies have been carried out under elevated
pH with the result of a markedly lower CDE yield.
2For the liquid phase hydrogenation of CDT using a Pd/Al2O3
catalyst in a slurry reactor Stüber et al. [7] and Julcour et al. [8]
observed an increasing CDE yield with decreasing hydrogen pres-
sure. Benaissa et al. [9] extended these studies by detailed isomer
analysis at hydrogen pressures in the range of 0.15–1.2 MPa. At
1.5 MPa a CDE yield of 74% was attained.
In a more detailed study of Zieverink [10] the dependences
on hydrogen pressure are given for all analysed isomers, c,c,t-,
c,t,t-, t,t,t-CDT; c,c-, c,t-, t,t-CDD; c-,t-CDE, for pH between 0.33
and 2.5 MPa. The results show also an increasing2CDE yield with
decreasing pH . At pH = 0.33 MPa a CDE yield of 68% was attained.
2
These data a2re reasonably represented by a model based on
the reaction scheme, Fig. 2, taken from preceding works [11,12].
The increasing CDE yield with decreasing pH has been formally
2
described by an additional direct hydrogenation of CDD isomers
towards CDE which was characterised by a second order in hydro-
gen.
Despite the knowledge of increasing CDE yield with decreas-
∗
ing pH the experiments of Benaissa et al. and of Zieverink were
Corresponding author. Fax: +49 6151 16 4788.
E-mail address: gaube@hrzpub.tu-darmstadt.de (J. Gaube).
2
not extended to lower pH Therefore, the aim of the present work
2
0926-860X/$ – see front matter © 2011 Elsevier B.V. All rights reserved.
doi:10.1016/j.apcata.2011.09.003

22
J. Gaube et al. / Applied Catalysis A: General 409–410 (2011) 21–27
Fig. 1. Synthesis of laurolactam and of HOOC–(CH2)10–COOH.
is to study and interpret the influence of hydrogen pressure on the
interdependence of isomerisation and hydrogenation and in partic-
ular on the CDE yield covering the range 0.007 < pH < 2.5 MPa
by experiments of the present study and for pH ≥ 20.33 MPa by
2
experiments of Zieverink.
Fig. 3. Experimental unit of liquid phase hydrogenation. P, circulation pump; S,
sieve; GC, gas chromatograph.
2. Experimental
mol/l at 353 K. This liquid flows through a sieve in order to avoid
entering of gas bubbles into the catalyst layer of egg-shell type Pd
catalyst. The flow rate is kept high so that only a very small part
of hydrogen is consumed during one passage through the catalyst
layer, thus the concentration of reactants is nearly constant along
the catalyst layer. The experiment is started by quick dosing of CDT
using a syringe.
The experiments of the present study were carried out in a
batchwise operated recycle reactor, Fig. 3.
The circulated liquid mixture of reactants (0.196 mol/l in n-
decane) is saturated with hydrogen enhanced by a stirrer (about
500 rpm) to intensify the contact of gas and liquid. In equilibrium
the concentration of hydrogen in n-decane is cH = 0.045 pH2 /MPa
2
The hydrogenation of 1,5,9-ctt-CDT has been studied at different
hydrogen pressures and for stepwise reduced hydrogen pressures.
For elucidation of the reaction course also the hydrogenation of the
intermediate product 1,5,9-ttt-CDT was carried out.
For all experiments of the present study a catalyst of egg-shell
type was used. The properties of this catalyst are listed in Table 1.
For all experiments carried out with this catalyst an influence of
mass transfer on the course of reaction could be excluded [6].
The discontinuous hydrogenation of CDT and CDD needs sev-
eral hours while the time required for a 90%-approach of steady
state concentration within the active layer of the catalyst is about
20 s. Therefore, the concentration profiles in the active layer of the
catalyst pellets are in quasi steady state because of the very slow
change of concentrations in the surrounding liquid.
In order to extend the pressure range of hydrogenation experi-
ments for theoretical discussion we used the experimental work of
Table 1
Structural properties of the catalyst.
Diameter of pellets [mm]
Thickness of active layer [␮m]
BET surface [m2/g]
2–3
20
108
67
Porosity %
Pore volume > 7.6 nm [cm3/g]
Pore volume < 7.6 nm [cm3/g]
Maximum of pore distr. [nm]
Pd content wt%
0.52
0.06
6–7
0.1
Pd surface [m2/g]
0.05–0.1
Degree of dispersion %
Pd particle size [nm]
18
2–3
Fig. 2. Reaction scheme of 1,5,9-CDT hydrogenation.

J. Gaube et al. / Applied Catalysis A: General 409–410 (2011) 21–27
23
Fig. 4. Experiment 1. Product composition of 1,5,9-ctt-CDT hydrogenation. 3.75 g
Fig. 6. Experiment 3. Product composition of 1,5,9-ctt-CDT hydrogenation. 4.8 g cat-
alyst, nCDT = 0.0274 mol, cCDT = 0.196 mol/l, T = 353 K, pH = 0.05 MPa (xdb = 0–0.35),
pH = 0.025 MPa (xdb = 0.35–0.65), pH = 0.007 MPa (xdb2 = 0.65–).
catalyst, pH = 0.15 MPa, nCDT = 0.0274 mol, cCDT = 0.196 mol/l, T = 353 K.
2
2
2
Zieverink [10]. Together with our study this dependence on pH can
2
now be discussed for the wider range between 0.007 and 2.5 MPa.
Zieverink employed a stirred batch autoclave filled with the
slurry of powdered Pd/alumina in CDT diluted with n-decane at
a concentration of 0.35 mol/l. Just as for our experiments it could
be proved that mass transfer does not affect the conversion so
that all results used in our study represent true kinetics. Product
analysis has been carried out by gas chromatography using a sil-
ica capillary: L = 30 m, dint. = 0.25 mm coated with 0.25 ␮m DB-wax.
It should be noted that the identification of compounds in the gas
chromatogram of our study is in agreement with that of Zieverink
[10].
In order to study the interdependence of isomerisation and
hydrogenation batchwise liquid phase and continuously operated
gas phase experiments with c-2-butene as model reaction were
carried out. The experimental unit for the latter is described by
Wuchter [13].
For all experiments of the present study the hydrogenation is first
order with respect to hydrogen [6].
The fractions are plotted versus the degree of double bond
hydrogenation x(db) which is defined as Xdb = x0 − xdb/x0 with
db
db
xdb the fraction of double bonds.
Xdb = 1 indicates complete hydrogenation of CDT.
Much higher CDE yields are obtained with further decreasing
pH2 . In order to avoid too long reaction times in our experiments
the hydrogen pressure is reduced stepwise. Figs. 5 and 6 show the
product composition of experiment 2 (maximum CDE yield 0.8),
and experiment 3 (maximum CDE yield 0.9).
A further increase of CDE yield up to 0.93 was attained in the
presence of CO as explained by Gaube et al. [6].
Fig. 7 shows an experiment of Zieverink under a hydrogen pres-
sure of 0.8 MPa.
The experiment of Zieverink was carried out at T = 433 K, 80 K
higher than for our experiments. However, following his study the
influence of temperature is small so that a comparison with our
data is justified.
In all these experiments cct-CDT and cc-CDD show fractions
less than 0.03. The formation of the intermediate ttt-CDT strongly
3. Results
Figs. 4–6 show the results of ctt-CDT conversions. The hydro-
genation rate of experiment 1 (maximum CDE yield 0.68) is
3.78 mol (H2)/h g(Pd), constant up to 55% double bond conversion.
Fig. 5. Experiment 2. Product composition of 1,5,9-ctt-CDT hydrogenation.
4.76 g catalyst, nCDT = 0.0274 mol, cCDT = 0.196 mol/l, T = 353 K, pH = 0.15 MPa
Fig. 7. 1,5,9-ctt-CDT hydrogenation by Zieverink [10]. cCDT = 0.35 mol/l, pH
0.8 MPa, T = 433 K, x(db) is the degree of double bond hydrogenation.
=
2
2
(xdb = 0–0.28), pH = 0.03 MPa (xdb = 0.28–).
2

24
J. Gaube et al. / Applied Catalysis A: General 409–410 (2011) 21–27
Table 2
Maximum fraction of ttt-CDT.
Fraction ttt-CDT
pH2 /MPa
Zieverink
”
”
”
Experiment 1
Experiment 2
0.023
0.038
0.045
0.059
0.107
0.127
2.5
1.6
0.8
0.33
0.15
0.05
depends on pH as shown by the maximum fraction listed in Table 2
2
including some further results of Zieverink.
For a better understanding of the reaction course the hydro-
genation of ttt-CDT is presented in Fig. 8.
In the study of Zieverink [10], and in the present study positional
isomers beside 1,5,9-CDT and 1,5-CDD show very small fractions
and disappear already at moderate degree of conversion. There-
fore, in the reaction scheme, Fig. 2, only cis/trans isomerisations are
considered.
Fig. 9. Gas phase isomerisation and hydrogenation of c-2-butene towards t-
2-butene and 1-butene (≈10%),
pc-2-butene = 2000 Pa, T = 333 K, mcat = 65 mg (see
Table 1).
4. Discussion
Table 3
Liquid phase isomerisation and hydrogenation of c-2-butene. T = 333 K, 5.6 g c-2-
butene in 65 g pentane, mcat. = 100 mg.
4.1. Interdependence of isomerisation and hydrogenation
In order to give a better insight into the interrelation between
hydrogenation and isomerisation experiments with c-2-butene as
a model were carried out in both continuous gas phase and discon-
tinuous liquid phase operation. Fig. 9 shows the reaction rates of
hydrogenation and isomerisation of c-2-butene towards t-2-butene
and to a minor extent of about 10% to 1-butene.
pH2 /MPa
r (isom.)/r (hydr.)
0.4
0.1
0.05
1.3
5.8
9.8
For the isomerisation this diagram reveals with respect to
hydrogen a change from first order at very low hydrogen pressure
up to zero order and finally to negative order at high hydrogen
pressure. The hydrogenation towards butane is of first order with
respect to hydrogen. This dependence of hydrogenation and iso-
merisation on hydrogen pressure could be well represented by a
kinetic model based on the reaction scheme, Fig. 10, already pro-
posed by Horiuti and Polanyi [14] and still widely accepted. This
model is based on the assumption of a dissociative adsorption of
hydrogen
ꢀH denotes a formal measure of hydrogen availability. For mod-
elling ꢀH is formally expressed by:
KH · pH
2
ꢀH
=
1 + KH · pH
2
where KH is a constant. [C4H9*] symbolizes a formal surface cover-
age of this intermediate and can be eliminated by the steady state
condition r1 = r2 + r3.
The curves in Fig. 9 were generated with the parameters:
k3
k1/mol kg−1h−1 Pa−1 = 0.0265,
= 0.257
= 0.0054 and KH/Pa−1
+H
∗
r1 = k1 · P1-C 8 ꢀH, c-C4H8−→ C4H9
k2
H
4
−H
r2 = k2[C4H9∗](1 − ꢀH), C4H9∗−→ t-C4H8
+H
r3 = k3[C4H9∗] · ꢀH, C4H9∗−→ C4H10
At very low pH the rate determining step is assumed as the
2
formation of the alkyl intermediate while with increasing pH the
abstraction of hydrogen is retarded by the decreasing numb2er of
hydrogen accepting sites.
For reaction conditions which are close to those of CDT hydro-
genation Table 3 gives the ratios of isomerisation/hydrogenation for
the conversion of c-2-butene in liquid phase at different hydrogen
pressures. As for the gas phase reaction also the liquid phase proce-
dure shows a strong increase of the isomerisation/hydrogenation
ratio with decreasing hydrogen pressure.
Table 4 gives the ratio for the isomerisation/hydrogenation
of ctt-CDT estimated from the diagrams listed in the table. The
Table 4
Liquid phase isomerisation and hydrogenation of ctt-CDT.
Experiment
pH2 /MPa
r (isom.)/r (hydr.)
Zieverink [10], Fig. 7
Exp. 1, Fig. 4
Exp. 2, Fig. 5
0.8
0.98
1.92
1.88
3.93
0.15
0.15
0.05
Fig. 8. Hydrogenation of 1,5,9-ttt-CDT. 4.8 g catalyst, pH = 0.1 MPa,
2
Exp. 3, Fig. 6
nCDT = 0.0274 mol, cCDT = 0.196 mol/l, T = 393 K.

J. Gaube et al. / Applied Catalysis A: General 409–410 (2011) 21–27
25
Fig. 10. Reaction scheme of hydrogenation/isomerisation (Horiuti and Polanyi).
dependence of this ratio on pH is less strong than for c-2-butene
which may be due to the lower2mobility within the CDT ring.
Up to now the early reaction scheme proposed by Polanyi and
Horiuti [14] is well accepted as a hypothesis. Starting with a ␲-
coordinated alkene a semihydrogenation step leads to an alkyl
which is bound to the Pd surface. The next steps are either the
abstraction of hydrogen from ␤ position of the alkyl forming again
a ␲-coordinated alkene or the irreversible hydrogenation of the
alkyl to alkane.
The abstracted hydrogen atom moves to a free site on Pd. If
this abstraction happens after rotation in the alkyl group cis/trans
isomerisation occurs.
The actions of hydrogen depend on the specific bonding of H
atoms in different positions, on the surface, in the subsurface, and
dissolved in the Pd lattice, Fig. 11 [15,16].
In the modelling by Benaissa et al. [9] the isomerisation of ctt-
CDT towards cct-CDT and its hydrogenation to ct-CDD appears as
main reaction route while the direct hydrogenation of ctt-CDT is
excluded. In general these authors assumed that cis double bonds
are preferred in hydrogenation because of much stronger adsorp-
tion compared with trans double bonds.
The reason of this discrepancy between the interpretation of
Benaissa et al. and that of Zieverink as well as ours could be clari-
fied by a critical review of experimental data. In contrast to the data
of Zieverink and of the present study the experiments of Benaissa
et al. show for the formation of cct-CDT a very strong increase
and after a short time a strong decrease which causes this wrong
interpretation.
␲-Coordinated alkene is prone to nucleophilic attack by surface
hydride which generates a surface alkyl. Subsequent electrophilic
attack at the ␣-C atom requires a positively charged H from the
subsurface region.
Many attempts have been made to identify intermediates in the
hydrogenation of ethene as a model case by various spectroscopic
methods as compiled in a review by Rupprechter [17]. However at
room temperature no adsorbed species and intermediates could be
identified by SFG spectroscopy for hydrogenations on Pd (1,1,1) [18]
and model catalysts of Pd crystallites on Al2O3 support [19,20]. Nev-
ertheless ␲-coordinated ethene is regarded as the reactive species.
4.2. Course of CDT hydrogenation
The diagrams of Zieverink, Fig. 7, and of the present study,
Figs. 4 and 5, show a linear increase of ct-CDD formation and an
s-shaped one of tt-CDD formation indicating the direct formation
of ct-CDD by hydrogenation of ctt-CDT and the formation of tt-CDD
by a consecutive reaction via ttt-CDT. The hydrogenation experi-
ment of ttt-CDT, Fig. 8, is the direct proof of the formation of tt-CDD
by hydrogenation of ttt-CDT. ct-CDD is formed by isomerisation
of tt-CDD which must be assumed as a second route to ct-CDD in
the conversion of ctt-CDT. The isomerisation of ttt-CDT to ctt-CDT
is marginal due to the equilibrium in the group of 1,5,9-CDTs as
already discussed by Zieverink [10].
Fig. 11. Properties of hydrogen for different positions [15,16].

26
J. Gaube et al. / Applied Catalysis A: General 409–410 (2011) 21–27
Fig. 12. Dependence of the degree of double bond hydrogenation and of CDA fraction on reaction time. (a) Experiment of Zieverink for pH = 0.8 MPa [10], (b) Experiment
2
1, pH = 0.15 MPa, (c) Experiment 2, pH = 0.03 MPa (xdb = 0.28–), (d) Experiment 3, pH = 0.007 MPa (xdb = 0.65–).
2
2
2
Zieverink has found that for pH ≥ 0.33 MPa the rate of hydro-
genation follows first order in the 2concentration of double bonds.
The plot xdb, degree of double bond hydrogenation, as func-
tion of time, Fig. 12a, shows the corresponding decrease of the
slope with increasing time. This kinetic analysis has lead to the
assumption that the surface of the catalyst is not densely covered
by CDT/CDD [10]. Consequently, in this case CDE gets a chance
of adsorption followed by consecutive hydrogenation towards
CDA.
The results of Zieverink and of the present study show that the
activity of hydrogenation of a double bond in trans conformation
is similar or even preferred to that in a cis conformation of CDT in
contradiction to the assumption of Benaissa et al. [9].
4.3. Dependence of selectivities on hydrogen pressure
The competition between isomerisation and hydrogenation is
best demonstrated by the ratio of tt-CDD/ct-CDD formation. As
With decreasing pH the first order in the concentration of
with increasing pH the ratio of hydrogenation and isomerisation
double bonds changes 2towards zero order. For pH = 0.15 MPa,
2
increases experiments at high pH as given in Fig. 7 show a ratio ct-
2
CDD/tt-CDD >1 and at low pH as2given in Figs. 4–6 show a ratio < 1
Fig. 12b, strict zero order is observed in the range up to xdb < 0.55.
2
Above this range the course of reaction approaches first order. For
because the formation of ct-CDD depends mainly on the hydro-
genation of ctt-CDT while the first step of tt-CDD formation is the
isomerisation of ctt-CDT to ttt-CDT. Table 2 shows that the maxi-
pH = 0.03 in the range xdb > 0.3 the zero order is extended over the
2
whole range, Fig. 12c. This is also demonstrated for experiment 3,
Fig. 12d, at further reduced pH2 . As demonstrated in these diagrams,
Fig. 12a–d, the increase of the CDA fraction correlates with the devi-
ation from zero order in the fraction of double bonds towards first
order.
mum fraction of ttt-CDT increases with decreasing pH due to the
2
increasing ratio isomerisation/hydrogenation of ctt-CDT to ttt-CDT
and to ct-CDD and due to decreasing hydrogenation of ttt-CDT to
tt-CDD.
This leads to the assumption that the adsorption of CDE is
prevented by densely adsorbed CDT and CDD isomers. The pref-
erence for CDT/CDD adsorption with respect to CDE adsorption
is explained by the further assumption that CDT and CDD always
utilise two double bonds for contact with the surface of the cata-
lyst and thus are more strongly adsorbed than CDE following the
early concept of Bond and co-workers as discussed for many other
systems [21].
Therefore, we considered the adsorption of tt-, ct- and cc-1,5-
CDD molecules on a Pd surface using standard molecular models
as exemplified for ct-1,5-CDD in the graphical abstract. All iso-
mers show a suitable conformation for contact with two double
bonds.
4.4. The effect of pH on the CDE yield at very low pH
2
2
The CDE yield obtained by Zieverink for pH = 0.8 MPa is 0.63.
2
Reduction of pH leads to considerably increased CDE yields as
2
shown for experiments 1–3, Figs. 4–6. The maximum CDE yields
are:
0.68 at pH = 0.15 MPa, 0.80 at pH = 0.15 MPa (xdb
2
2
= 0–0.28), pH =0.03 MPa (xdb = 0.28–), 0.90 at pH
2
2
= 0.05 MPa (xdb = 0–0.35), pH = 0.025 MPa (xdb
2
= 0.35–0.65), pH = 0.007 MPa (xdb = 0.65–)
2

J. Gaube et al. / Applied Catalysis A: General 409–410 (2011) 21–27
27
Of course this mechanistic consideration lacks strict physical
support. However, it is still very difficult to calculate the dynamic
adsorption process of CDT or CDD molecules from a liquid on a Pd
surface.
surface is not completely covered with CDT and CDD isomers so
that CDE can get access to the surface and can be hydrogenated
towards CDA thus lowering the CDE yield. At low pH respec-
tively low hydrogenation rate we have a regime of dense2coverage
of the surface by CDT/CDD so that readsorption and subsequent
hydrogenation of formed CDE is to a large extent prevented. The
preference of CDT/CDD adsorption over CDE is explained by adsorp-
tion of CDT/CDD via two double bonds. With this interpretation of
the dependence of CDE yield on the hydrogen pressure a consistent
hypothesis is presented.
We assume that at very low pH the surface is densely cov-
ered first with CDT and then with 2CDD isomers down to a low
concentration of CDD. However, at high hydrogen pressure and
consequently high hydrogenation rate this dense adsorption layer
is not replenished fast enough and CDE gains an increasing chance
of adsorption and hydrogenation towards CDA.The contact via two
double bonds suggests that isomerisation and hydrogenation can
simultaneously occur at both adsorbed double bonds. However, at
the moment of hydrogenation of the double bond the very high
reaction enthalpy can be assumed as concentrated on the molecule
just formed. Dissipation of this energy may occur by collision with
neighbouring molecules and also via the chemisorption bonds to
the Pd lattice. Thus, vibrations perpendicular to the surface are
strongly activated so that desorption of this molecule occurs [22].
This may be the reason that only one of the double bonds can be
hydrogenated within the residence time of adsorption. Therefore,
we expect and in all experiments find a strictly consecutive hydro-
genation CDT → CDD → CDE → CDA. Therefore, the assumption of
an additional direct hydrogenation of CDD to CDA by Zieverink
is excluded. On the other hand isomerisations can occur at both
adsorbed double bonds. This may be the reason that we find CDE
formed by hydrogenation of CDD in the equilibrated c/t ratio of
about ½ even in a range of reaction where readsorption of CDE can
be excluded because its hydrogenation is negligible.
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be found which describes this complicated interdependence of
CDT/CDD adsorption and readsorption of CDE as a function of pH
.
2
However, for such attempt too many assumptions would be neces-
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The comparison of experiments over a wide range of hydrogen
pressure respectively reaction rate shows an increasing CDE yield
with decreasing pH . At very low pH CDE yields >0.9 are reached.
2
The interpretation 2of the present study is that at high pH the Pd
2