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The catalytic hydrogenation of benzene over supported metal catalysts
73
Applied Catalysis, 29 (1987) 73-90 Elsevier Science Publishers B.V., Amsterdam
THE CATALYTIC I. GAS-PHASE
HYDROGENATION
of Chemical
2628 BL Delft, *Author
OF BENZENE
HYDROGENATION
M.C. SCHOENMAKER-STOLK, Department
OF BENZENE
OVER SUPPORTED
METAL
CATALYSTS
OVER RUTHENIUM-ON-SILICA
J.W. VERWIJS,
J.A. DON and J.J.F.
Delft
Technology,
University
SCHOLTEN*
of Technology,
Julianalaan
136,
The Netherlands.
to whom all correspondence
Dept. of Catalysis,
(Received
Printed in The Netherlands
2 June
Central
should
be addressed;
Laboratories,
DSM, Geleen,
1986, accepted
26 August
also affiliated
with the
The Netherlands.
1986)
ABSTRACl A study is reported of the gas-phase hydrogenation of benzene over a rutheniumon-silica catalyst in the temperature range from 300 to 415 K and at a total pressure of 130 kPa. The texture of the catalyst is studied by TEM, physisorption of nitrogen, chemisorption of hydrogen and by mercury penetration. Surface analysis is performed by XPS/AES. The adsorption enthalpies of the reaction intermediates 1,4-cyclohexadiene and cyclohexene and of cyclohexane on ruthenium are determined. On the basis of the kinetics, a reaction mechanism is proposed in which the rate-determining step shifts from the first hydrogen addition to benzene at low temperatures to the hydrogen addition to cyclohexene at higher temperatures. An approximate diagram is constructed representing the change of the free enthalpy as a function of the extension of the reaction.
INTRODUCTION It is well of catalysts temperature
known that the gas-phase is impossible,
of hydrogen,
reported
by Sabatier
over many metals
plots
superior
provided conditions occurring Steen
However,
the catalytic
in 1901 [I], proceeds
of the second
activities
of cyclohexene
hydrogenation,
under relatively
of several
paper of this series,
the behaviour
activity
Don and Scholten and pressures,
in the absence
below the dissociation
metals
mild conditions are summarised
in the form of Tanaka-
[3].
Less is known about reports
isomers.
[Z]. The catalytic
in the discussion Tamaru
of benzene
and the same is true for the hydrogenation
and of the two cyclohexadiene first
hydrogenation
at least at temperatures
of ruthenium
of ruthenium-on-silica,
[5], working
with nonsupported
found very high hydrogenation
the surface
is free of impurities
the catalyst in periods
and Scholten
0166-9834/87/$03.50
stability
of about
is very
as compared ruthenium
activities like oxygen
limited
12 h in the presence
[6] used highly
purified
as a catalyst.
temperatures
as compared
with platinum,
and chlorine.
Under these
of water
01987 Elsevier Science Publishers B.V.
[4]
with other metals.
at ambient
however,
ruthenium
Kubicka
total deactivation in the feed.
powders,
Van der
as evidenced
by
74 XPS analysis.
In this case the lifetime
As to the kinetics
of the reaction
of the catalyst
was 6 hrs at the utmost.
all investigators
agree
below about 400 K and at atmospheric
pressure
near zero and in hydrogen
0.5 and 1.0 [7-g]. Such kinetics
with practically
catalyst
the catalyst Finally, change
we report on the kinetics
in the temperature
will
be discussed
we will
reflect
of the standard
and products
order
in benzene
is
are found
all other metals.
In this article silica
in between
the reaction
that at temperatures
range
of the reaction
of
extensively.
on the reaction
free enthalpy
as a function
over a 0.4% Ru-on-
from 300 to 400 K. Also the texture
mechanism
on the basis of the gradual
of the reactants,
of the extension
reaction
intermediates
of the reaction.
EXPERIMENTAL Materials Benzene, Merck,
"for die Spectroscopic",
FRG. 1,4-Cyclohexadiene
Belgium.
Drying
and further
by adding
an excess
tillation
in an ultra-pure
shelter
Sulphur
S(BET)
of "Drina",
being a strong
97%) was supplied of these
a 9:l Pb/Na alloy
Pyrex glass apparatus
poison
was extended
in catalytic
by slurrying
with
= 6 m2 g-l, under dry, oxygen-free
Hydrogen,
from Hoek-Loos,
in the section spherical,
describing
by Aldrich
reactants
from Merck,
from
Chemie,
were performed
followed
under oxygen-free
from Shell,
particles
with minimum
by dis-
nitrogen
as a
by Drijfhout,
with
purified support
as indicated was silica,
to the manufacturer
is 53 nm. This material
S(BET)
was crushed
size 1.7 mm. Ruthenium
= to
trichloride
Amsterdam.
and adsorption
distribution
at pressures
the benzene
Ru powder,
for 30 min.
The catalyst
According
pore diameter
hydrogenation, of reduced
was further
size 0.85 mm and maximum
characterization
The pore volume tration
nitrogen
The Netherlands,
type CLA 33569C.
(spec pure) was supplied
Catalyst
benzene -3
4 g dm
the flow apparatus.
130 m* g-' and the average
measurements
of the support
up to 200 MPa, applying
was determined
a Porosimeter,
by mercury
pene-
type 2000, from Carlo
Italy.
Volumetric
adsorption
measurements
out in a "micro-BET-apparatus" Instruments
Inc.,
The BET surface taking
(purity
desulphurization
were obtained
gas.
pretreatment
Erba,
as well as cyclohexene,
Burlington,
chemisorption platinum
Mass.,
surface
with
a Baratron
USA) equipped
area was calculated
for the cross-sectional
The free-metal
of N2, H2 and cyclohexene
provided
area of a nitrogen
at 295 K. On the analogy
[lo], the transition
with single
from the N2 adsorption
area was calculated
molecule
point between
strong
recipe and weak
carried
gauge
sided
(MKS
sensors.
isotherm
at 78 K,
0.162 nm2.
from the extent
of Anderson's
were
pressure
of strong
H2
for the case of chemisorption
was
75 arbitrarily
taken at 133.23
to one H atom per surface Adsorption apparatus",
Pa. The number
which
to 78 K. Dosing
of cyclohexene
for this special
connected
were
occasion
to the apparatus
of cyclohexene
the Dewar cooling
was performed
by opening
for a short time. For every
vessel
from the Clausius-Clapeyron
equation.
at the operating with
standard
conditions
values
via a valveless
Electron Philips
were pretreated
EM 420T apparatus.
we decided
on a "spec pure" London.
from the hydrogen
catalyst
were
and embedded After
E, Reichert)
with
type Puratron, is analogous
a 70 nm thick a diamond
being unknown
a TPD measurement
taken with a
in a mixture
curing
cutter.
in the literature, of this compound
from Johnson
and
to that described
[I21 for the case of non-dissociative
on silica
flow equipment
in the pressure
gas by leading
adsorption
purified
sieves,
type 3A
trollers,
type F-201,
and 1,4-cyclohexadiene,
from
by with
from oxygen
benzene
pressure
through
(4).
by leading
which
was adjusted
gas and the temperature
reactor
it over finely
regulated Benzene, finely
by mass
thermostatted
of the liquid.
dispersed
over flow con-
and also cyclohexene
divided
in a stainless-steel
by regulating
was removed
(BASF FRG, type R-
(2). Both gases were dried
which was stored
with a heating jacket
catalyst
by leading
Instruments
was evaporated
the liquid,
1, the numbers
a fixed-bed
range from 0.1 to 0.5 MPa. Oxygen
(3). The gas flows were Inacom
see Figure with
it over a Pd/A1203
(BASF FRG, type R-3-111)
molecular
through
of the catalyst,
a continuous
O-20 (I), and helium was
carrier
sample,
of measurement
the performance
was used, operating
provided
to the
equipment
(1) to (11) inclusive,
The partial
connected
at
readsorption.
For determining
helium
(Ultracut
by performing
ruthenium
The method
and Scholten
Hydrogenation
copper
was mortared
of 1,4-cyclohexadiene
its value
adsorbed
simultaneous
and passivated
The catalyst
an ultra microtome
Matthey, Konvalinka
or prereduced
chamber
and 30% butylmethyacrylate.
of adsorption
to estimate
type LHS-10
eV) was applied
lines were compared
ir, VQCUD
in a preparation
of the reduced
of 70% methylmethacrylate
The enthalpy
(FRG),
1253.6
point
of adsorption
UHV lock.
micrographs
slice was cut with
(energy
of 13 kV and 20 mA. Spectral
[II]. Samples
670 K in a 90% Ar + 10"; H2 mixture XPS/AES
source
down
and removing
measuring
enthalpy
out in a Leybold-Heraeus
A Mg Ka excitation
apparatus.
cooled
the stopcock
isothermal
was calculated
was carried
with a side tube
with cyclohexene,
The isosteric
XPS/AES
to correspond
out in a "micro-BET-
had been provided
and filled
time of 20 min was taken.
analysis
atoms was assumed
was assumed
carried
an equilibrium
Surface
surface
stoichiometry
Ru atom.
measurements
with stopcock
of ruthenium
per m*, and the surface
to be 1.63 x 10"
water
hydrogen vessel
or
(5)
was circulated.
the space velocity
of the
76
FIGURE
1
numbers
Continuous
flow fixed-bed
hydrogenation
passing
the reactor
the evaporator
(6). The reactor
(5) the gas mixture
was a stainless-steel
of 10 mm and a length of 40 mm. The catalyst two stainless-steel put around thyristor
grids.
the reactor. controller
The product Samples
were
They were
The temperature
gas mixture
led through
ionisation
detector,
The product cell
flowed
in the reactor
in between by an oven
through
heated
tubes
Catalyst
preparation
solution
needed
From
a heated
to arrive
The catalyst was slowly Inductive
a ruthenium
Inspection homogeneously
content
The peak
LCI 11/03).
back pressure-regulator
the oxygen
mesh.
by a flame
electrometer. (Digital
support
material,
reduction
replaced
which
(9), a
[13], and through
a
to the atmosphere.
method
point".
Next,
by measuring the sample
in hydrogen
temperature
Plasma
spectrometry
the metal
into a vessel stirred.
The
the amount was dried
of
in air
(60 cm3 (STP) min-')
was reached
down to room temperature
by air, by which
C141. A 0.06 M
slowly
was regularly
beforehand
was performed
was cooled
wetness
in 1 M HCl was dropped
at the "caking
Coupled
content
5704A
3 mm)
W-HP, 80-100
were analysed
the gas was vented
of liquid was determined
673 K for 3 h. The final
hydrogen
(inner diameter
on Chromosorb
Packard
by the incipient
trichloride
at 400 K for 17 h. Reduction
-1 .
steel
(7).
(8).
and characterization
the pre-dried
volume
to the gas chromatograph
by a microprocessor
(11). Finally,
was prepared
of ruthenium
containing maximum
of a Eurotherm
Carle valve
of stainless
to a Hewlett
and integrated
gas was led through
The catalyst
by means
actuated
was kept at 335 K. The samples
condenser
to
up to 770 K could be realised
by a pneumatically
a 4 m long column
coupled
tubes
tube with an inside diameter
was kept constant
(IO), in order to determine
water-cooled
was led via heated
was fixed
with 30% 1,2,3-tris(2-cyanoethoxy)propane
areas were recorded
min
of
0.5 K.
taken automatically
temperature
water
Temperatures
to within
The column
Hersch
For explanation
see text.
After
filled
equipment.
under hydrogen surface
(Jobin Yvon,
at
at a rate of 0.5 K and finally
was passivated.
type JY 38 apparatus)
of 0.4 wt% was found.
of the electron distributed
micrographs
showed
over the support.
the ruthenium
Broken
catalyst
particles
particles
to be
showed
the
77
number of partlcles
1% )
particlediameter(nm)
FIGURE
2
Histogram
giving
the Ru particle
diameter
distribution,
from EM micro-
graphs.
catalyst
not to have a mantle
tribution. cation
A sample of about
factor
this figure
of 420,000
dEM = dvs = cn ii'i
From hydrogen
the ruthenium
in which
at. Assuming particles
of the total metal
the metal
volume
fixed
from:
in Figure
the mean particle
to hydrogen
diameter
follows
this percentage
the main
the ruthenium,
as it was not detected
grams
do not show directly
considering 151.
material.
area. Hence
of the ruthenium analysis
impurity,
the carbon
The absence
the high amounts
-1
and from:
that SH
the fraction
of chlorine
area
the spectro-
on the metal
on the surface
detected
is
stems from
However,
is located
of this element
surface
of the catalyst
probably
on the bare support.
whether
g
may be of the order of 45%
XPS/AES
trace
2
It turns out that dH is
surface
support
to hydrogen.
3. Carbon,
or on the support
indi-
area, SH, of 100 m
accessible
per gram of ruthenium.
area, and/or
represented
samples
surface
to be totally
to the silica
and not accessible
powder
2. From
low, dEM gives only a rough
55'; of the total metal
is poisoned
particles
in Figure
of 2.7 nm was calculated
this value with the dEM value of 2.7 nm, it follows
area strongly
remarkable,
is very
a free ruthenium
to be spheres,
(2) is about
recorded
presented
dis-
size.
chemisorption
4.9 nm. Comparing
of metal
counted
particle
1" is the metal
in equation
diameter
particle
with a magnifi-
(I)
of particles
of the mean
Ru was arrived
particles
the size histogram
mean particle
metal
in a photograph
d3 / cn .d2 ii i
As the number cation
300 metal
yielded
a volume-surface
but a homogeneous
character,
on ruthenium
is
78
3.0 r
CPSxlO
k.energyeV FIGURE
3
XPS/AES
spectrogram
The BET surface
area of the catalyst
the support
area provided
detrimental
action
From mercury
during
penetration
2
g
This
catalyst
a pore volume
with a sharp maximum
with the value of 26 nm mentioned from mercury
was 115 m
by the manufacturer.
on the support
penetration
of r) resulted,
of 0.4 wtX Ru-on-silica. -1
, in fair agreement with
points
to the absence
of any
preparation.
distribution
at a pore radius
curve
(dV/dr as a function
of 25 nm, in good accordance
by the manufacturer. The total pore volume 3 -1 3 -1 (manufacturer: 0.85 cm g ). g
was 0.72 cm
RESULTS Adsorption
measurements
For the construction of the catalytic
reaction
of the reactants, are discussed
of a plot of the change
in the first
The mode of adsorption mented
[15-171,
from benzene
the intermediates
of free enthalpy
to cyclohexane,
and the products
in the course
the enthalpies are needed.
of adsorption
These
values
part of this section. of benzene
and its strength
on several
is influenced
transition
metals
by the degree
is well docu-
of electron
filling
of the d-orbitals.
For the case of ruthenium
we select,
the enthalpy
given by van Meerten
1181 for the case of nickel-on-silica,
the number
value
of d-electrons
15 and 16 respectively. The TPD profile excess
of argon
and a larger
molecules the weakly
in ruthenium
being very close
We then arrive
at an approximate
of 1,4-cyclohexadiene,
as a shelter
gas, shows
one at 533 K. Applying
with simultaneous
readsorption
is of the order
two peaks,
to that in nickel,
viz. -1 value of -59 kJ mol .
according
to ref. [12] in an
viz. a small peak at 343 K
the TPD equation
enthalpy
part. The enthalpy
of -198
measured
as a first approximation,
for first order desorption
1121, and assuming
to be zero, an adsorption adsorbed
C.S.
the entropy of the adsorbed -1 of -122 kJ mol was calculated for
of adsorption
kJ mol -I. The reason
of the strongly
for the existence
held part
of the strongly
held
79
294 K
2volume adsorbed
(moleculesCgH10 x108.m-* Ru
FIGURE
4
Reversible
I
part of the adsorption
isotherm
of cyclohexene
on ruthenium
powder.
1,4-cyclohexadiene pound occurs, Figure prepared
but this requires
4 presents
Don c.s., their
that some polymerization
B in Table
on ruthenium
30% of the amount
of cyclohexene
totally
and 337 K are plotted.
on ruthenium
at 1470 K as described
1 [19]. Part of the cyclohexene
into benzene adsorbed.
in the isotherms;
of this com-
research.
isotherms
of RuO2 after calcining
sample
is not included
further
the adsorption
by reduction
disproportionate about
we suggest
is unknown;
and hydrogen.
appeared
This corresponds
This type of destructive
only the reversible
to to
adsorption
parts adsorbed
at 294 K
the following -1 ; at adsorption enthalpies are calculated: at 8 = 0.05, -aHads = 26.5 kJ mol -1 . Hence an 6 = 0.1, -aHads = 37.2 kJ mol-I; at e = 0.25, -aHads = 44.2 kJ mol indication
From the Clausius-Clapeyron
powder,
by J.A.
is found of an increasing
coverage.
This might
molecules
with increasing
In the literature of cyclohexane
no values
the enthalpy
of adsorption by Walker,
Kinetics
are to be found for the enthalpy
For the time being we adopt
of cyclohexane
on activated
for the case of low coverage
of the hydrogenation
of benzene,
as a catalyst
(instability)
cyclohexyl-
caused
kinetic
increasing of cyclohexene
of adsorption
as a first
approximation -1 , of 34 kJ mol
carbon
~201, in view of the metallic
lytic activity
as a function
1.4-cyclohexadiene
measurements
by the gradual
and 1,4-dicyclohexylcyclohexane
deposition
conditions
and cyclohexene
are hampered
by a decreasing
of by-products
[6]. In Figure
of time is plotted
303 and 400 K under the experimental figure.
with
interaction
of carbon.
With ruthenium activity
of adsorption lateral
coverage.
on ruthenium.
as tabulated character
enthalpy
be due to an increasing
equation
5 the change
for experiments indicated
like of cata-
at respectively
in the legend
of the
80
TON 24 (moleculesC6H6 x103.sitem'. 16 s-11 8
0
0
720
1440
2160
reaction time lmin) FIGURE
5
The change
as a function pressure
of time at: total pressure
2 kPa and space velocity
in the product
TABLE
of the rate of benzene
conversion
over 0.4 wt.% Ru-on-silica
110 kPa, hydrogen
pressure
40 kPa, benzene
5 x lo3 cm3 (STP) m -2 Ru h-l. The oxygen
content
gas stream was 4 ppm.
1
Reaction
orders
pressure
130 kPa and space velocity
is reached
in the hydrogenation
in each case by adding
Temperature
Order
of benzene
over
0.4 wt% Ru-on-silica.
2 x IO4 cm3 (STP). m -' Rh h-l. Total
Total pressure
helium. Order
in benzene
in hydrogen
/K 303 400
aWithin
o.oa
l.Zb
0.2a
Z.Ob
the range of benzene
pressures
pressure of 79 kPa. b. Within the range of hydrogen
from 1.9 - 7.9 kPa and at a hydrogen
pressures
from
13 - 89 kPa and at a benzene
pressure
of 5 kPa.
The low stability of Don and Scholten
of the catalyst
[5] and of van der Steen and Scholten
at 400 K a very stable lower coverages temperature,
much
faster
behaviour
of benzene
as a result
is appreciably
at 303 K is in accordance
lower.
is observed.
and of cyclohexene
of which
Moreover,
we think,
during
reaction
products,
the results
[6]. Interestingly,
This,
the rate of alkylation
the alkylation
with
is related
to the
at this higher
of benzene if present,
by cyclohexene will desorb
at 400 K than at 303 K.
The reaction
orders
are determined
from the curves
and 6b. At 303 K the rate of hydrogenation is 1.2. Experimental
conditions
appeared
are indicated
represented
in Figures
to be zero order
in Table
6a
in hydrogen
1. Surprisingly,
at 400 K
81
(a)
(b)
In TON
Ln TON
(moleculesC6H6.
(molecules
site-'.s-'1
sith'.s-'1 303 K +Z@==J -4.0
I0
1.5
1.o
0.5
-71 25
25
2.0
3.0
In PC6H6 (kPa1
FIGURE
6
genation
Benzene
deviating
the order
pressure
in hydrogen, the results
the measurements
pressure
dependence
which (Table
were
dependence increased
was found,
1) we took into account
performed
at differential
103; b) every
necessary,
corrections
were made for deactivation.
Apparent
activation
under the conditions
energies indicated
rate constant,
temperature
is assumed
catalyst
In the figure ture levels
were
hydro-
with respect
the following
calculated
increase
to
(as was found by van Meerten
and, when
carried
7. In calculating
of the reaction
orders
a)
the benzene
in triplicate
from measurements
of Figure
factors:
conditions,
point was measured
in the legend
a linear
especially
reactor
being below
nickel
(b) of the benzene
from 1.2 (303 K) to 2.0 (400 K). In cal-
conversion
reaction
4.5
rate.
a strongly
culating
(a) and hydrogen
40
3.5
ln PH2 (itPa)
with
C.S. for reactions
out the
increasing over a
[9]). the apparent
are indicated
activation
between
energies
in kJ mol-'
at various
tempera-
brackets.
Temperature IK) -8
416
385
357
333
313
2.4
2.6
2.8
3.0
3.2
I
I
I
I
I
-9In k (moleculesCgH6. site-ls-',divided -10 by [PC6H61aX[p~~d-? -11
-
-12 -
1000/T (K-l) FIGURE
7
Arrhenius
Ru-on-silica.
plot of the rate of hydrogenation
of benzene
over 0.4 wt%
82 TABLE
2
Product
gas composition
Ru-on-silica.
Total
in the hydrogenation
pressure
h-'. The total pressure
of 1,4-cyclohexadiene
9 x IO4 cm3 (STP) mm2 Ru
130 kPa and space velocity
is reached
in each case by adding Partial
Temperature
over 0.4 wt%
helium.
pressures
ofa:
H2 Pressure/kPa
Pressure/kPa
/K
C6H6 /kPa
'gH1O /kPa
/kPa
62.5
2.03
305
0.06
0.04
1.93
358
0.66
0.27
1.10
62.5
2.03
62.5
2.03
392
0.85
0.17
1.01
13.2
2.08
400
1.15
0.56
0.37
52.6
2.08
400
0.95
0.10
1.03
86.8
2.08
400
0.84
0.03
1.21
78.3
0.89
400
0.33
0.01
0.55
78.3
3.25
400
1.25
0.01
1.99
78.3
5.47
400
2.09
0.01
3.37
aThe partial verted
pressures
are a measure
of the amounts
of C6H8 con-
into C6H6, C6H,0 and C6H,2 per unit time.
The curious
change
ture, and especially were
in the off-gas
also observed
of the apparent the negative
by Kubicka
activation
values
energy
as a function
of this quantity
of tempera-
found above
[4] for ruthenium-on-silica
catalysts.
355 K, We return
to this in the discussion. In the study of reaction kinetics
of the presumed
of 1,4-cyclohexadiene be ruled out that intermediate, stable
surface
complex
high even at room temperature in measuring
conversion
Besides
as a hydrogenation
excess
of hydrogen,
argument
found with
number
the formation
by-product,
a relatively
separately.
Though
in the mechanism as this isomer
pressure.
it cannot
as an forms
a very
2) was extremely
Therefore be higher
we did not than the total
turns out to be 0.6 C6H8 molecules of cyclohexane,
and, notwithstanding of benzene
and dehydrogenation
the main
(see Table
but it should
large amount
in the final discussion
1,4-cyclohexadiene
the
the hydrogenations
[21].
and 130 kPa total
The fact that both hydrogenation strong
were studied
the rate of reaction,
served
to investigate
Therefore,
is also involved
of 1,4-cyclohexadiene
per unit time. The latter
per site per second.
helpful
steps.
our study to the 1,4-isomer,
The rate of hydrogenation
succeed
it is often
reaction
and of cyclohexene
1,3-cyclohexadiene
we limited
bidentate
mechanisms
elementary
the presence was formed
occurred
of the mechanism. product
cyclohexene
observed
will
Contrary
was obof an
as well. serve as a to the results
in the hydrogenation
of
83 TABLE
3
Gas composition
in the reactor
0.4 wt% Ru-on-silica, m -' Ru h-l. Total
Total
pressure
exit,
pressure
in the hydrogenation
is reached
in each case by adding Partial
Temperature
'gH1O Pressure/kPa
H2 Pressure/kPa
of cyclohexene
130 kPa and space velocity
helium.
pressures
/K
C6H6 /kPa
'gH12 /kPa 6.19
62.5
6.25
305
0.01
62.5
6.25
358
0.05
5.07
62.5
6.25
400
0.14
4.75
13.1
7.18
305
0.01
1.42
39.5
7.18
305
0.01
4.46
65.8
7.18
305
0.01
6.81
77.6
1.97
305
0.01
1.95
77.6
3.42
305
0.01
3.34
77.6
5.67
305
0.01
5.49
aThe partial and C6H,2
pressures
was cyclohexane
It is important to be always
and only small amounts
than that of benzene
in Table
3. Again
sure that we measured to present
cyclohexene
(compare
pressure
and lines,
is to be expected
coverage.
A first order
This problem
reaction
Therefore,
be dealt with
whereas conclusions
in mind
it is not
but from Table
in both the hydrogen
for the order
in cyclohexene). with
3 it
and the in
Such kinetic
a low cyclohexene
is less easy to understand,
pressure
leads to a high hydrogen
in the discussion. and cyclohexene
and their
ratios
are
of temperature.
the distance
temperature.
equilibrium,
this
of the reaction,
for hydrogen
hydrogen
rates for benzene
as a function
As said before,
orders
was found The results
high, so that we are not
Keeping
as long as we are dealing
relationship
will
rates were
were detected,
conditions.
lines 4, 5 and 6 in the Table
that the high partial
In Table 4 reaction presented
into C6H6
hydrogenation
comparable
to be first order
7, 8 and 9 for the order
behaviour
considering
under
the reaction
here the precise is likely
coverage.
converted
of benzene
far from equilibrium.
is seen the reaction
hydrogen
of C6H,0
to note here that the rate of cyclohexene
higher
are summarized
possible
of the amount
ofa:
per unit time.
cyclohexene
fully
are a measure
over
6 x IO4 cm3 (STP)
from equilibrium
In the case of benzene
hydrogenation
in the case of cyclohexene from Table
that the rates
of reaction
An explanation
of this phenomenon
is strongly
nature
when the temperature
and its consequences
by the
we are far from
this situation
4 are of a qualitative
run antiparallel
influenced
is given
is never only.
realised.
It is found
is increased. in the discussion.
84 TABLE
4
Rate of cyclohexene
and benzene
range from 305 to 410 K. Total (cyclohexene)
pressure
hydrogenation pressure
and their
ratio
130 kPa, hydrogen
6 kPa, helium
pressure
305
340
in the temperature
pressure
62 kPa, benzene
62 kPa and space velocity
6 x lo4
cm3 (STP) me2 Ru h-l. Temperature/K
322
350
360
400
380
415
TON C6H6a
0.02
0.03
0.06
0.07
0.07
0.07
0.06
TON C6H,oa
2.35
2.26
2.17
2.17
2.18
2.11
2.07
TON C6H,o/TON
C6H6
118
aTON = number
of benzene
68
33
(cyclohexene)
28
30
molecules
30
reacted
0.05
34
per site per second.
DISCUSSION For the heterogeneous the following
c&j (g)
H2
reaction
(g)
H2
observation
2 C6H8(ads )+2H(ads)
mechanism
is based
hydrogenation;
this has never
the cyclohexadienes
C6H10(g)
H2 (9)
11
= C6H10
cyclohexene
been observed
are too strongly
over group VIII metals,
adopted
among other
on,
that the intermediate
of benzene
is generally
(g)
11
(ads) + PH(ads)
The stepwise
hydrogenation
mechanism
11
11 C6H6
catalytic
stepwise
[22,23]:
C6H,2 (g)
11
(ads)+PH(ads)
things,
11 2 C6H,2
the often
can desorb
before
quoted
further
for the cyclohexadienes.
adsorbed
to desorb
before
(ads
[4 6,231 catalytic
Apparently
further
hydro-
genation. Derbentsev,
Paal and Tetenyi
mechanism
of benzene
included.
They started
and unlabeled product
hydrogenation
was smaller
Starting formed,
is so strongly
adsorbed
more C6H,o
Table
than C6H,2
radioactivity
Derbentsev
is about
VIII metals, ruthenium 14 of C labeled benzene
fraction.
mechanism.
C.S. and by Tetenyi
is
and Table
is extremely
both C6H6 and a C6H,o/C6H,2
2)
lowered.
mixture
are
times as high as the hydrogenation
from C6H8. Therefore,
in the product
This was
conclusion
section
2, line 4, shows that under certain is formed
in the C6Hlo
of the adsorbed
Their
(see Results
that the C6H6 coverage
a hundred
the reaction
mixtures
full hydrogenation
the stepwise
from this high C6H8 coverage, at a rate which
group
that the radioactivity
in the light of our observation
rate of benzene.
between
and found
of a direct
C6H6 ring in one step, besides
that C6H8
equimolecular
and Paal [25] studied
than in the C6H,2 product
by the introduction
questionable
over several
from nearly
1,3-cyclohexadiene
fraction
explained
[24] and Tetenyi
fractions
reaction
we think the difference
of C6H10 and C6H,2
C.S. can be explained
conditions
without
as observed
by
the introduction
of
85
AG' ikl ml-‘1
extensm
FIGURE
8
An approximate
enthalpy
diagram
in the hydrogenation
reaction,
Lower
levels:
extremely
high activation
energies.
Temperature
(ads) and C6H,,
around
full hydrogenation benzene
We now first
should
discuss
types of adsorbed measurable
article
to values
transition
absolute
chosen.
value
hydrogen
of binding
metals
&HadS,
value
atoms,
the chances
forming
adsorption accepted
around
is involved
on transition
and adsorption
means
[26]. The answer reaction
is indicative
will be higher
depends
for ~~~~~~
corresponding
(weakly
bound
hydrogen)
between
on the
migration desorption.
a low of hydroAlso
as the -aHads/RT
of the free enthalpy
8), an enthalpy
to weakly
adsorbed
-1
to the question
in this respect;
according
of a diagram
(see Figure
being
the absolute
as low as 36 kJ mol
that the rate of surface
in the construction
room temperature
metal
enthalpies
In general
range from values
in a catalytic
of hydrogen
has to be chosen
value
that
a C 6H ,* transition
that there are at least five
seems to be marginal
of this quantity
is lower. Hence,
the reaction
lines:
(ads), C6Hg
-1 , and in this respect the difference
The value of -nH ads/RT
reactivity
Dashed
of C6H7
gen is high, and the same is the case for the rate of hydrogen the catalytic
mixture
gas-phase
but low activation
levels
of hydrogen
[26] it appears
enthalpies,
type of hydrogen
temperature
over ruthenium.
unknown
of the TPD technique.
as high as 170 kJ mol
the various
levels:
in this diagram.
their populations
by means
value of the adsorption
of the
be low.
hydrogen,
especially
lines:
Upper
step of the C6H6 ring. Moreover,
the modes
From a survey
free
of the extension
point.
300 K. The free enthalpy
reacts with 6 adsorbed
in one step,
reaction
For the hydrogen/benzene
hydrogenation
Dotted
(ads) are not included
a direct
surfaces.
as a function
mechanism.
catalytic
energies.
adsorbed
which
of benzene
of the standard
aG ' is taken zero as a reference
in the gas-phase,
state
of the change
for the case of the stepwise
hydrogenation.
reactmn (%I
of the
of
of hydrogen
hydrogen.
is about 40 kJ mol-l
A generally [26].
86 Around hydrogen
300 K we observed pressure.
bound types of hydrogen coverage
to be nearly
notwithstanding
is often
case of benzene
called
which weakly
order.
values
differential
the adsorption
values
enthalpies
starting
= -120 kJ mol-' K-'
[261 Cl81 1281
cyclohexene
.
cyclohexane
:
1,s z-285 kJ mol-’ ads 2s = -100 kJ mol-' ads ASads = -100 kJ mol-'
stances
is higher Figure
due to their
reaction C6H8 (step
(ads) level.
It follows
can proceed,
(ads) level,
can be explained
(2)) was found
(1)). Hence the population the very fast reaction continues,
to the right
are rough of these subof these molecules
is about 41 kJ mol -' higher of activation
to that value.
the thermodynamically
the unfavourable
unfavourable
high
The rate of C6H8 hydrogenation
is continuously
that the production thermodynamic
Furthermore,
by the relatively
for
The fact that the
than the rate of C6H6 hydrogenation
of the C6H8 (ads) level
(1) to the right.
is increased
mobility
that the free enthalpy
step (2). This means
notwithstanding
of r-Go for step
the surface
entropies
of adsorption.
as follows.
to be much faster
and cyclohexane
the gas-phase
be at least equal
notwithstanding
the following
K-'
that the C6H8 (ads) level
(1) to the right should
in the section
K-'
to that of benzene,
lower enthalpies
8 demonstrates
than the C6H6 step
equal
K-'
for cyclohexene
based on the fact that, whereas
are nearly
from the
are introduced:
x5ads = -140 kJ mol-' K-'
chosen
the free enthalpies
given
AS
entropies
of the
for the non catalytic
from AH values,
:
estimates
TPD
the reaction
by subtracting
:
The differential
in which
from this,
hydrogen
1,4-cyclohexadiene:
a rate, around
as a function
values
benzene
ads
this type
held monolayer"
observed
diagram
are calculated
of IGO values,
entropy
bound causing
of the hydrogen
steps are plotted free enthalpy
over ruthenium
free enthalpy
that,
of Aben C.S. [27] for the
authors
free enthalpy
reaction
The virtual
In the calculation
In the literature
in the population
are taken from Janz [28]. Starting reaction
is present,
of the strongly
These
the
Hence we conclude of the strongly
in
hydrogen.
a reaction
of the various
of the surface
approximate
8 in ref. C261) shows
the results
over platinum.
adsorbed
of the reaction.
gas-phase
for the weakly
adsorption
in excess with
is first order
8 we present
free enthalpies
to be first
"hydrogen
hydrogenation
peak representing
Results.
is first order
isotherms
pressure.
hydrogen
is in accordance
room temperature,
hydrogenation
Figure
in hydrogen
a weak
unity,
hydrogenation
[26]. Our conclusion
extension
rate which
adsorption
(see for instance
first order
equals
the rate of benzene
In Figure
hydrogenation
of hydrogen
the fact that the sum of the coverages
types of hydrogen
of hydrogen
a benzene
Inspection
by
of C6H8 (ads)
situation
the rate of reaction
high initial
depleted
(step
pressures
of an increase of step
(1)
of C6H6 and
and their respective populations on the surface. The kinetic situation H2' described above has recently been theoretically treated by Boudart 1291. In his
87 terminology
the stimulation
the depletion
of step
(1) by pressure
(ads) level by step
of the C6H8
is called
(2) "pumping
“Pumping
down"
UP”,
and
of the C6H8
(ads
concentration. The high rates we observed determining reaction
around
around
be explained We
believe
atoms
this temperature
level
in the holes
to
Step
in the pressure
surface
of benzene
C6H6 coverage
layer over the small
in the ruthenium
being rate
(1)
that the total
equals
chemisorbed
(for instance
can unity.
hydrogen
at the so-called
[26]). that during
are formed is about
level.
is zero order
C6H6 to form a chemisorbed
Our observation mixture
point
the observation
correct,
from the fact that at room temperature
bound
C8 sites
for step (2) and (3)
300 K. If this is
C6H8 hydrogenation
can be explained
both C6H6 and a C6H18/C6H,2
from the fact that
(see Figure 8) the C6H8 -1 above the C6H10
kJ mol-' above the C6H6 level and 38 kJ mol
41
The extremely
high rates of these reactions,
indicate
that the activation
the C6H6
(ads) level,
energy
and between
barriers
the C6H8
even at room temperature,
between
the C6H8
(ads) level and
(ads) level and the C6H10
(ads) level,
are very low. At 300 K, where
it is likely that the addition
(I), is rate determining, apparent
activation
in the same range 30 kJ mol".
temperature. energy
of 3.8 kJ mol
of temperatures
However
of the logarithm
we calculated
energy
Kubicka
from the Arrhenius
-1
. By contrast,
the much
plotted
of the reaction
higher
work
in quite
330 K the apparent
occurs
(van Meerten
energy
is accompanied
especially,
(1) to
good accordance
right change AS
activation
energy
is observed,
by a dramatic
with
chemisorbed,
time
is very
as
we are dealing reactions,
are given
of
rate instead
of the reciprocal
Kubicka's
other
a negative group
value.
value.
VIII metals
in the sign of the apparent
change
in the reaction
orders
so that its coverage
in turn means going
however,
strongly
is supported
In Kubicka's it also activation
of benzene
and,
increased.
is strongly
that the rate of step
lowered
Our
view is illustrated
step
(1)
the situation and
Step
by, respectively:
step
lowered.
(1) to the
by the antiparallel 4.
(step 3)) has become
that this step is preceded
(2). The equilibrium
is
at higher
(3) is appreciably
from 300 K to 400 K, the rate of step
the last hydrogenation with
step from
by the fact that cyclohexene
of TON C6H6 and TON IZ~H,~ as can be seen from Table soon
energy
is the fact that at temperatures reaches
and with
C.S. ES]). The change
(3). This view
This
same
of the reaction
as a function
7) an
[4] reports
activation
this to be due to a shift of the rate determining
step
temperatures.
At the
(Figure
of hydrogen.
We speculate
very weakly
plot
step
If we adopt the same procedure we arrive at an apparent activation -1 ,
of 38 kJ mol
the same phenomenon
step
to benzene,
Kubicka
apparent
the logarithm
rate constant
In our work the most striking observation around
of hydrogen
constants
rate determining,
by two equilibrium of step
(1) and
(2)
88 @C6H8
K, = eC6H6
(a)
(a)
x
(3)
@H (a)*
and @C&l,0
(a)
K2 = @C6H8(a)
x
In both equations
@H(a)*
that in the overall finally
become
Meerten
C.S.
an order
rate equation
coverage result
and hence one might
to the benzene
coverage
increase
have to explain
in the temperature
that the nH values
positive).
It follows
the rate-determining
in the order of benzene the occurrence
range above
it appears
(3), cannot
full
will finally
pressure. negative
activation
7). From our calculations
(1) and (2) are very small
(1) and (2), as equilibria
explain
at still be found.
from 0.0 to 0.2.
temperatures
330 K (see Figure
steps
will
at an order
from the original
of an apparent
of the equilibria
that reaction step
deviating increasing
changes
by them, observed
also,
in hydrogen
will
at bY van
arrived
that on ruthenium
in benzene
slowly
that further
investigated
It follows
in hydrogen
were arrived
We, however,
above 400 K, this high order
at 303 K. We expect
in a further
energy
temperatures
expect
pressure.
of the order
and conclusion
from 303 to 400 K the order
We finally
value
of 3 in the case of nickel.
temperatures,
Proceeding
to the hydrogen
the maximum
[gl, who, at the highest
in hydrogen
This points
is proportional
3. The same explanation
of 2 in hydrogen higher
(4)
@ H(a) '
the negative
apparent
(and
preceding activation
energy. A better
explanation
the coverages this will
seems to be the following.
of C6H6 (ads) and especially
have a retarding
in the form of an apparent the coverage
effect
the AH values
of the equilibria:
C6H6(gas) =
C6H6
C6H,0
on the reaction
negative
of dissociatively
C6H,0
activation
adsorbed
With
rate, which
energy,
hydrogen.
increasing
(ads) decrease
temperature
drastically,
manifests
itself
and the same is true for
Or, to put it in another
way,
(5)
(ads)
(gas) 2 C6H,g
and
(ads)
(6)
-AHa2
and
H2 (gas)
=
as equilibria activation
2H (ads)
preceding
energy
(7)
-AHa3 the rate-determining
in the following
way:
step, will
change
the apparent
89
= "H'step E app.
(3) - tHal
-3cHas
- CHa2
(8)
with
[AH,,
+
bH,2
+
3rHa31
>
rH2step
(9)
(3)
CONCLUSION The mechanism plicated.
and presented
reaction
nuclei,
and kinetic
by a more detailed
is highly
by the present
com-
authors
it is seen that we are dealing
in which
we are even dealing
that for a full mechanistic especially
steps
metals
VIII
adopted
If the elementary
steps.
split up into reaction
the cyclic
over group
mechanism,
at the start of the discussion,
least with nine elementary further
hydrogenation
of benzene
From the stepwise
steps
only one hydrogen
with twelve analysis
steps.
more
study of the kinetics
at
(I), (2) and (3) are atom
Therefore
research
is added
to
it is obvious
is needed,
of the elementary
steps as
such.
In the second article of this series we will report on the kinetics of the hydrogenation
of benzene
over copper-on-silica.
ACKNOWLEDGEMENTS We thank Mr. A.P. Pijpers Central
Laboratories,
measurements
and Mr. J. Cremers
DSM, Geleen,
and for their
skilful
help with
Thanks
are also due to Mr. A. Pijpers,
study,
and to Mr. P. van Oeffelt
measurements. of Technology,
Mr. J. Teunisse assisted
The investigations Chemical
Research
the Advancement
(Department
The Netherlands)
the interpretation
from the same department,
for carrying
in the study of the catalyst
were supported
(partly)
of Pure Research
Chemistry,
out the XPS/AES of the spectra. for the TEM
out the 1,4-cyclohexadiene
and Mr. N. van Westen,
(SON) with financial
of Physical
for carrying
TPD
both from Delft University texture.
by The Netherlands
aid from The Netherlands
Foundation Organization
for for
(ZWD).
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J.A. Konvalinka, J.J.F. Scholten and J.C. Rasser, J. Catal., 48 (1977) 374. McMillan, London, 1973, p.199. A. Verdin, "Gas Analysis Instrumentation", Y.W. Chen, H.I. Wang and J.G. Goodwin, Jr., J. Catal., 83 (1983) 415. P.W. Selwood, 3. Am. Chem. Sot., 79 (1957) 3346. D.M. Haaland, Surf. Sci., 102 (1981) 405. J.S. Lehwald, H. Ibach and J.E. Demuth, Surf. Sci., 78 (1978) 577. R.Z.C. v. Meerten and J.W.E. Coenen, J. Catal., 46 (1977) 13. J.A. Don, A.P. Pijpers and J.J.F. Scholten, J. Catal., 80 (1983) 296. P.L. Walker, Jr., "Chemistry and Physics of Carbon", M. Dekker, Inc., New York, 1970, ~01.6, p.90. A.J. Pearson, "Metallo-organic Chemistry", John Wiley and Sons, Chichester, 1985, p.255. S. Siegel, "Advances in Catalysis", (P.P. Eley, H. Pines and P.B. Weisz, Eds.), Academic Press, New York, 1966, p.123. F. Hartog and P. Zwietering, J. Catal., 2 (1963) 79. V.I. Derbentsev, Z. Paal and P. Tetenyi, Z. Phys. Chem. Neue Folge, 80 (1972) 51. P. Tetenyi and Z. Paal, Z. Phys. Chem. Neue Folge, 80 (1972) 63. J.J.F. Scholten, A.P. Pijpers and A.M.L. Hustings, Catal. Rev. -Sci. -Eng., 27 (1985) 151. P.C. Aben, H. v.d. Eyk and J.M. Oelderik, "Proceedings of the 5th International Congress on Catalysis", Miami Beach, Florida, (J.W. Hightower, Ed.), North Holland Publishing Company, Amsterdam, 1972, vol. 1, p. 717. G.J. Janz, J. Chem. Phys., 22 (1954) 751. M. Boudart, J. Phys. Chem., 87 (1983) 2786.
Applied Catalysis, 29 (1987) 73-90 Elsevier Science Publishers B.V., Amsterdam
THE CATALYTIC I. GAS-PHASE
HYDROGENATION
of Chemical
2628 BL Delft, *Author
OF BENZENE
HYDROGENATION
M.C. SCHOENMAKER-STOLK, Department
OF BENZENE
OVER SUPPORTED
METAL
CATALYSTS
OVER RUTHENIUM-ON-SILICA
J.W. VERWIJS,
J.A. DON and J.J.F.
Delft
Technology,
University
SCHOLTEN*
of Technology,
Julianalaan
136,
The Netherlands.
to whom all correspondence
Dept. of Catalysis,
(Received
Printed in The Netherlands
2 June
Central
should
be addressed;
Laboratories,
DSM, Geleen,
1986, accepted
26 August
also affiliated
with the
The Netherlands.
1986)
ABSTRACl A study is reported of the gas-phase hydrogenation of benzene over a rutheniumon-silica catalyst in the temperature range from 300 to 415 K and at a total pressure of 130 kPa. The texture of the catalyst is studied by TEM, physisorption of nitrogen, chemisorption of hydrogen and by mercury penetration. Surface analysis is performed by XPS/AES. The adsorption enthalpies of the reaction intermediates 1,4-cyclohexadiene and cyclohexene and of cyclohexane on ruthenium are determined. On the basis of the kinetics, a reaction mechanism is proposed in which the rate-determining step shifts from the first hydrogen addition to benzene at low temperatures to the hydrogen addition to cyclohexene at higher temperatures. An approximate diagram is constructed representing the change of the free enthalpy as a function of the extension of the reaction.
INTRODUCTION It is well of catalysts temperature
known that the gas-phase is impossible,
of hydrogen,
reported
by Sabatier
over many metals
plots
superior
provided conditions occurring Steen
However,
the catalytic
in 1901 [I], proceeds
of the second
activities
of cyclohexene
hydrogenation,
under relatively
of several
paper of this series,
the behaviour
activity
Don and Scholten and pressures,
in the absence
below the dissociation
metals
mild conditions are summarised
in the form of Tanaka-
[3].
Less is known about reports
isomers.
[Z]. The catalytic
in the discussion Tamaru
of benzene
and the same is true for the hydrogenation
and of the two cyclohexadiene first
hydrogenation
at least at temperatures
of ruthenium
of ruthenium-on-silica,
[5], working
with nonsupported
found very high hydrogenation
the surface
is free of impurities
the catalyst in periods
and Scholten
0166-9834/87/$03.50
stability
of about
is very
as compared ruthenium
activities like oxygen
limited
12 h in the presence
[6] used highly
purified
as a catalyst.
temperatures
as compared
with platinum,
and chlorine.
Under these
of water
01987 Elsevier Science Publishers B.V.
[4]
with other metals.
at ambient
however,
ruthenium
Kubicka
total deactivation in the feed.
powders,
Van der
as evidenced
by
74 XPS analysis.
In this case the lifetime
As to the kinetics
of the reaction
of the catalyst
was 6 hrs at the utmost.
all investigators
agree
below about 400 K and at atmospheric
pressure
near zero and in hydrogen
0.5 and 1.0 [7-g]. Such kinetics
with practically
catalyst
the catalyst Finally, change
we report on the kinetics
in the temperature
will
be discussed
we will
reflect
of the standard
and products
order
in benzene
is
are found
all other metals.
In this article silica
in between
the reaction
that at temperatures
range
of the reaction
of
extensively.
on the reaction
free enthalpy
as a function
over a 0.4% Ru-on-
from 300 to 400 K. Also the texture
mechanism
on the basis of the gradual
of the reactants,
of the extension
reaction
intermediates
of the reaction.
EXPERIMENTAL Materials Benzene, Merck,
"for die Spectroscopic",
FRG. 1,4-Cyclohexadiene
Belgium.
Drying
and further
by adding
an excess
tillation
in an ultra-pure
shelter
Sulphur
S(BET)
of "Drina",
being a strong
97%) was supplied of these
a 9:l Pb/Na alloy
Pyrex glass apparatus
poison
was extended
in catalytic
by slurrying
with
= 6 m2 g-l, under dry, oxygen-free
Hydrogen,
from Hoek-Loos,
in the section spherical,
describing
by Aldrich
reactants
from Merck,
from
Chemie,
were performed
followed
under oxygen-free
from Shell,
particles
with minimum
by dis-
nitrogen
as a
by Drijfhout,
with
purified support
as indicated was silica,
to the manufacturer
is 53 nm. This material
S(BET)
was crushed
size 1.7 mm. Ruthenium
= to
trichloride
Amsterdam.
and adsorption
distribution
at pressures
the benzene
Ru powder,
for 30 min.
The catalyst
According
pore diameter
hydrogenation, of reduced
was further
size 0.85 mm and maximum
characterization
The pore volume tration
nitrogen
The Netherlands,
type CLA 33569C.
(spec pure) was supplied
Catalyst
benzene -3
4 g dm
the flow apparatus.
130 m* g-' and the average
measurements
of the support
up to 200 MPa, applying
was determined
a Porosimeter,
by mercury
pene-
type 2000, from Carlo
Italy.
Volumetric
adsorption
measurements
out in a "micro-BET-apparatus" Instruments
Inc.,
The BET surface taking
(purity
desulphurization
were obtained
gas.
pretreatment
Erba,
as well as cyclohexene,
Burlington,
chemisorption platinum
Mass.,
surface
with
a Baratron
USA) equipped
area was calculated
for the cross-sectional
The free-metal
of N2, H2 and cyclohexene
provided
area of a nitrogen
at 295 K. On the analogy
[lo], the transition
with single
from the N2 adsorption
area was calculated
molecule
point between
strong
recipe and weak
carried
gauge
sided
(MKS
sensors.
isotherm
at 78 K,
0.162 nm2.
from the extent
of Anderson's
were
pressure
of strong
H2
for the case of chemisorption
was
75 arbitrarily
taken at 133.23
to one H atom per surface Adsorption apparatus",
Pa. The number
which
to 78 K. Dosing
of cyclohexene
for this special
connected
were
occasion
to the apparatus
of cyclohexene
the Dewar cooling
was performed
by opening
for a short time. For every
vessel
from the Clausius-Clapeyron
equation.
at the operating with
standard
conditions
values
via a valveless
Electron Philips
were pretreated
EM 420T apparatus.
we decided
on a "spec pure" London.
from the hydrogen
catalyst
were
and embedded After
E, Reichert)
with
type Puratron, is analogous
a 70 nm thick a diamond
being unknown
a TPD measurement
taken with a
in a mixture
curing
cutter.
in the literature, of this compound
from Johnson
and
to that described
[I21 for the case of non-dissociative
on silica
flow equipment
in the pressure
gas by leading
adsorption
purified
sieves,
type 3A
trollers,
type F-201,
and 1,4-cyclohexadiene,
from
by with
from oxygen
benzene
pressure
through
(4).
by leading
which
was adjusted
gas and the temperature
reactor
it over finely
regulated Benzene, finely
by mass
thermostatted
of the liquid.
dispersed
over flow con-
and also cyclohexene
divided
in a stainless-steel
by regulating
was removed
(BASF FRG, type R-
(2). Both gases were dried
which was stored
with a heating jacket
catalyst
by leading
Instruments
was evaporated
the liquid,
1, the numbers
a fixed-bed
range from 0.1 to 0.5 MPa. Oxygen
(3). The gas flows were Inacom
see Figure with
it over a Pd/A1203
(BASF FRG, type R-3-111)
molecular
through
of the catalyst,
a continuous
O-20 (I), and helium was
carrier
sample,
of measurement
the performance
was used, operating
provided
to the
equipment
(1) to (11) inclusive,
The partial
connected
at
readsorption.
For determining
helium
(Ultracut
by performing
ruthenium
The method
and Scholten
Hydrogenation
copper
was mortared
of 1,4-cyclohexadiene
its value
adsorbed
simultaneous
and passivated
The catalyst
an ultra microtome
Matthey, Konvalinka
or prereduced
chamber
and 30% butylmethyacrylate.
of adsorption
to estimate
type LHS-10
eV) was applied
lines were compared
ir, VQCUD
in a preparation
of the reduced
of 70% methylmethacrylate
The enthalpy
(FRG),
1253.6
point
of adsorption
UHV lock.
micrographs
slice was cut with
(energy
of 13 kV and 20 mA. Spectral
[II]. Samples
670 K in a 90% Ar + 10"; H2 mixture XPS/AES
source
down
and removing
measuring
enthalpy
out in a Leybold-Heraeus
A Mg Ka excitation
apparatus.
cooled
the stopcock
isothermal
was calculated
was carried
with a side tube
with cyclohexene,
The isosteric
XPS/AES
to correspond
out in a "micro-BET-
had been provided
and filled
time of 20 min was taken.
analysis
atoms was assumed
was assumed
carried
an equilibrium
Surface
surface
stoichiometry
Ru atom.
measurements
with stopcock
of ruthenium
per m*, and the surface
to be 1.63 x 10"
water
hydrogen vessel
or
(5)
was circulated.
the space velocity
of the
76
FIGURE
1
numbers
Continuous
flow fixed-bed
hydrogenation
passing
the reactor
the evaporator
(6). The reactor
(5) the gas mixture
was a stainless-steel
of 10 mm and a length of 40 mm. The catalyst two stainless-steel put around thyristor
grids.
the reactor. controller
The product Samples
were
They were
The temperature
gas mixture
led through
ionisation
detector,
The product cell
flowed
in the reactor
in between by an oven
through
heated
tubes
Catalyst
preparation
solution
needed
From
a heated
to arrive
The catalyst was slowly Inductive
a ruthenium
Inspection homogeneously
content
The peak
LCI 11/03).
back pressure-regulator
the oxygen
mesh.
by a flame
electrometer. (Digital
support
material,
reduction
replaced
which
(9), a
[13], and through
a
to the atmosphere.
method
point".
Next,
by measuring the sample
in hydrogen
temperature
Plasma
spectrometry
the metal
into a vessel stirred.
The
the amount was dried
of
in air
(60 cm3 (STP) min-')
was reached
down to room temperature
by air, by which
C141. A 0.06 M
slowly
was regularly
beforehand
was performed
was cooled
wetness
in 1 M HCl was dropped
at the "caking
Coupled
content
5704A
3 mm)
W-HP, 80-100
were analysed
the gas was vented
of liquid was determined
673 K for 3 h. The final
hydrogen
(inner diameter
on Chromosorb
Packard
by the incipient
trichloride
at 400 K for 17 h. Reduction
-1 .
steel
(7).
(8).
and characterization
the pre-dried
volume
to the gas chromatograph
by a microprocessor
(11). Finally,
was prepared
of ruthenium
containing maximum
of a Eurotherm
Carle valve
of stainless
to a Hewlett
and integrated
gas was led through
The catalyst
by means
actuated
was kept at 335 K. The samples
condenser
to
up to 770 K could be realised
by a pneumatically
a 4 m long column
coupled
tubes
tube with an inside diameter
was kept constant
(IO), in order to determine
water-cooled
was led via heated
was fixed
with 30% 1,2,3-tris(2-cyanoethoxy)propane
areas were recorded
min
of
0.5 K.
taken automatically
temperature
water
Temperatures
to within
The column
Hersch
For explanation
see text.
After
filled
equipment.
under hydrogen surface
(Jobin Yvon,
at
at a rate of 0.5 K and finally
was passivated.
type JY 38 apparatus)
of 0.4 wt% was found.
of the electron distributed
micrographs
showed
over the support.
the ruthenium
Broken
catalyst
particles
particles
to be
showed
the
77
number of partlcles
1% )
particlediameter(nm)
FIGURE
2
Histogram
giving
the Ru particle
diameter
distribution,
from EM micro-
graphs.
catalyst
not to have a mantle
tribution. cation
A sample of about
factor
this figure
of 420,000
dEM = dvs = cn ii'i
From hydrogen
the ruthenium
in which
at. Assuming particles
of the total metal
the metal
volume
fixed
from:
in Figure
the mean particle
to hydrogen
diameter
follows
this percentage
the main
the ruthenium,
as it was not detected
grams
do not show directly
considering 151.
material.
area. Hence
of the ruthenium analysis
impurity,
the carbon
The absence
the high amounts
-1
and from:
that SH
the fraction
of chlorine
area
the spectro-
on the metal
on the surface
detected
is
stems from
However,
is located
of this element
surface
of the catalyst
probably
on the bare support.
whether
g
may be of the order of 45%
XPS/AES
trace
2
It turns out that dH is
surface
support
to hydrogen.
3. Carbon,
or on the support
indi-
area, SH, of 100 m
accessible
per gram of ruthenium.
area, and/or
represented
samples
surface
to be totally
to the silica
and not accessible
powder
2. From
low, dEM gives only a rough
55'; of the total metal
is poisoned
particles
in Figure
of 2.7 nm was calculated
this value with the dEM value of 2.7 nm, it follows
area strongly
remarkable,
is very
a free ruthenium
to be spheres,
(2) is about
recorded
presented
dis-
size.
chemisorption
4.9 nm. Comparing
of metal
counted
particle
1" is the metal
in equation
diameter
particle
with a magnifi-
(I)
of particles
of the mean
Ru was arrived
particles
the size histogram
mean particle
metal
in a photograph
d3 / cn .d2 ii i
As the number cation
300 metal
yielded
a volume-surface
but a homogeneous
character,
on ruthenium
is
78
3.0 r
CPSxlO
k.energyeV FIGURE
3
XPS/AES
spectrogram
The BET surface
area of the catalyst
the support
area provided
detrimental
action
From mercury
during
penetration
2
g
This
catalyst
a pore volume
with a sharp maximum
with the value of 26 nm mentioned from mercury
was 115 m
by the manufacturer.
on the support
penetration
of r) resulted,
of 0.4 wtX Ru-on-silica. -1
, in fair agreement with
points
to the absence
of any
preparation.
distribution
at a pore radius
curve
(dV/dr as a function
of 25 nm, in good accordance
by the manufacturer. The total pore volume 3 -1 3 -1 (manufacturer: 0.85 cm g ). g
was 0.72 cm
RESULTS Adsorption
measurements
For the construction of the catalytic
reaction
of the reactants, are discussed
of a plot of the change
in the first
The mode of adsorption mented
[15-171,
from benzene
the intermediates
of free enthalpy
to cyclohexane,
and the products
in the course
the enthalpies are needed.
of adsorption
These
values
part of this section. of benzene
and its strength
on several
is influenced
transition
metals
by the degree
is well docu-
of electron
filling
of the d-orbitals.
For the case of ruthenium
we select,
the enthalpy
given by van Meerten
1181 for the case of nickel-on-silica,
the number
value
of d-electrons
15 and 16 respectively. The TPD profile excess
of argon
and a larger
molecules the weakly
in ruthenium
being very close
We then arrive
at an approximate
of 1,4-cyclohexadiene,
as a shelter
gas, shows
one at 533 K. Applying
with simultaneous
readsorption
is of the order
two peaks,
to that in nickel,
viz. -1 value of -59 kJ mol .
according
to ref. [12] in an
viz. a small peak at 343 K
the TPD equation
enthalpy
part. The enthalpy
of -198
measured
as a first approximation,
for first order desorption
1121, and assuming
to be zero, an adsorption adsorbed
C.S.
the entropy of the adsorbed -1 of -122 kJ mol was calculated for
of adsorption
kJ mol -I. The reason
of the strongly
for the existence
held part
of the strongly
held
79
294 K
2volume adsorbed
(moleculesCgH10 x108.m-* Ru
FIGURE
4
Reversible
I
part of the adsorption
isotherm
of cyclohexene
on ruthenium
powder.
1,4-cyclohexadiene pound occurs, Figure prepared
but this requires
4 presents
Don c.s., their
that some polymerization
B in Table
on ruthenium
30% of the amount
of cyclohexene
totally
and 337 K are plotted.
on ruthenium
at 1470 K as described
1 [19]. Part of the cyclohexene
into benzene adsorbed.
in the isotherms;
of this com-
research.
isotherms
of RuO2 after calcining
sample
is not included
further
the adsorption
by reduction
disproportionate about
we suggest
is unknown;
and hydrogen.
appeared
This corresponds
This type of destructive
only the reversible
to to
adsorption
parts adsorbed
at 294 K
the following -1 ; at adsorption enthalpies are calculated: at 8 = 0.05, -aHads = 26.5 kJ mol -1 . Hence an 6 = 0.1, -aHads = 37.2 kJ mol-I; at e = 0.25, -aHads = 44.2 kJ mol indication
From the Clausius-Clapeyron
powder,
by J.A.
is found of an increasing
coverage.
This might
molecules
with increasing
In the literature of cyclohexane
no values
the enthalpy
of adsorption by Walker,
Kinetics
are to be found for the enthalpy
For the time being we adopt
of cyclohexane
on activated
for the case of low coverage
of the hydrogenation
of benzene,
as a catalyst
(instability)
cyclohexyl-
caused
kinetic
increasing of cyclohexene
of adsorption
as a first
approximation -1 , of 34 kJ mol
carbon
~201, in view of the metallic
lytic activity
as a function
1.4-cyclohexadiene
measurements
by the gradual
and 1,4-dicyclohexylcyclohexane
deposition
conditions
and cyclohexene
are hampered
by a decreasing
of by-products
[6]. In Figure
of time is plotted
303 and 400 K under the experimental figure.
with
interaction
of carbon.
With ruthenium activity
of adsorption lateral
coverage.
on ruthenium.
as tabulated character
enthalpy
be due to an increasing
equation
5 the change
for experiments indicated
like of cata-
at respectively
in the legend
of the
80
TON 24 (moleculesC6H6 x103.sitem'. 16 s-11 8
0
0
720
1440
2160
reaction time lmin) FIGURE
5
The change
as a function pressure
of time at: total pressure
2 kPa and space velocity
in the product
TABLE
of the rate of benzene
conversion
over 0.4 wt.% Ru-on-silica
110 kPa, hydrogen
pressure
40 kPa, benzene
5 x lo3 cm3 (STP) m -2 Ru h-l. The oxygen
content
gas stream was 4 ppm.
1
Reaction
orders
pressure
130 kPa and space velocity
is reached
in the hydrogenation
in each case by adding
Temperature
Order
of benzene
over
0.4 wt% Ru-on-silica.
2 x IO4 cm3 (STP). m -' Rh h-l. Total
Total pressure
helium. Order
in benzene
in hydrogen
/K 303 400
aWithin
o.oa
l.Zb
0.2a
Z.Ob
the range of benzene
pressures
pressure of 79 kPa. b. Within the range of hydrogen
from 1.9 - 7.9 kPa and at a hydrogen
pressures
from
13 - 89 kPa and at a benzene
pressure
of 5 kPa.
The low stability of Don and Scholten
of the catalyst
[5] and of van der Steen and Scholten
at 400 K a very stable lower coverages temperature,
much
faster
behaviour
of benzene
as a result
is appreciably
at 303 K is in accordance
lower.
is observed.
and of cyclohexene
of which
Moreover,
we think,
during
reaction
products,
the results
[6]. Interestingly,
This,
the rate of alkylation
the alkylation
with
is related
to the
at this higher
of benzene if present,
by cyclohexene will desorb
at 400 K than at 303 K.
The reaction
orders
are determined
from the curves
and 6b. At 303 K the rate of hydrogenation is 1.2. Experimental
conditions
appeared
are indicated
represented
in Figures
to be zero order
in Table
6a
in hydrogen
1. Surprisingly,
at 400 K
81
(a)
(b)
In TON
Ln TON
(moleculesC6H6.
(molecules
site-'.s-'1
sith'.s-'1 303 K +Z@==J -4.0
I0
1.5
1.o
0.5
-71 25
25
2.0
3.0
In PC6H6 (kPa1
FIGURE
6
genation
Benzene
deviating
the order
pressure
in hydrogen, the results
the measurements
pressure
dependence
which (Table
were
dependence increased
was found,
1) we took into account
performed
at differential
103; b) every
necessary,
corrections
were made for deactivation.
Apparent
activation
under the conditions
energies indicated
rate constant,
temperature
is assumed
catalyst
In the figure ture levels
were
hydro-
with respect
the following
calculated
increase
to
(as was found by van Meerten
and, when
carried
7. In calculating
of the reaction
orders
a)
the benzene
in triplicate
from measurements
of Figure
factors:
conditions,
point was measured
in the legend
a linear
especially
reactor
being below
nickel
(b) of the benzene
from 1.2 (303 K) to 2.0 (400 K). In cal-
conversion
reaction
4.5
rate.
a strongly
culating
(a) and hydrogen
40
3.5
ln PH2 (itPa)
with
C.S. for reactions
out the
increasing over a
[9]). the apparent
are indicated
activation
between
energies
in kJ mol-'
at various
tempera-
brackets.
Temperature IK) -8
416
385
357
333
313
2.4
2.6
2.8
3.0
3.2
I
I
I
I
I
-9In k (moleculesCgH6. site-ls-',divided -10 by [PC6H61aX[p~~d-? -11
-
-12 -
1000/T (K-l) FIGURE
7
Arrhenius
Ru-on-silica.
plot of the rate of hydrogenation
of benzene
over 0.4 wt%
82 TABLE
2
Product
gas composition
Ru-on-silica.
Total
in the hydrogenation
pressure
h-'. The total pressure
of 1,4-cyclohexadiene
9 x IO4 cm3 (STP) mm2 Ru
130 kPa and space velocity
is reached
in each case by adding Partial
Temperature
over 0.4 wt%
helium.
pressures
ofa:
H2 Pressure/kPa
Pressure/kPa
/K
C6H6 /kPa
'gH1O /kPa
/kPa
62.5
2.03
305
0.06
0.04
1.93
358
0.66
0.27
1.10
62.5
2.03
62.5
2.03
392
0.85
0.17
1.01
13.2
2.08
400
1.15
0.56
0.37
52.6
2.08
400
0.95
0.10
1.03
86.8
2.08
400
0.84
0.03
1.21
78.3
0.89
400
0.33
0.01
0.55
78.3
3.25
400
1.25
0.01
1.99
78.3
5.47
400
2.09
0.01
3.37
aThe partial verted
pressures
are a measure
of the amounts
of C6H8 con-
into C6H6, C6H,0 and C6H,2 per unit time.
The curious
change
ture, and especially were
in the off-gas
also observed
of the apparent the negative
by Kubicka
activation
values
energy
as a function
of this quantity
of tempera-
found above
[4] for ruthenium-on-silica
catalysts.
355 K, We return
to this in the discussion. In the study of reaction kinetics
of the presumed
of 1,4-cyclohexadiene be ruled out that intermediate, stable
surface
complex
high even at room temperature in measuring
conversion
Besides
as a hydrogenation
excess
of hydrogen,
argument
found with
number
the formation
by-product,
a relatively
separately.
Though
in the mechanism as this isomer
pressure.
it cannot
as an forms
a very
2) was extremely
Therefore be higher
we did not than the total
turns out to be 0.6 C6H8 molecules of cyclohexane,
and, notwithstanding of benzene
and dehydrogenation
the main
(see Table
but it should
large amount
in the final discussion
1,4-cyclohexadiene
the
the hydrogenations
[21].
and 130 kPa total
The fact that both hydrogenation strong
were studied
the rate of reaction,
served
to investigate
Therefore,
is also involved
of 1,4-cyclohexadiene
per unit time. The latter
per site per second.
helpful
steps.
our study to the 1,4-isomer,
The rate of hydrogenation
succeed
it is often
reaction
and of cyclohexene
1,3-cyclohexadiene
we limited
bidentate
mechanisms
elementary
the presence was formed
occurred
of the mechanism. product
cyclohexene
observed
will
Contrary
was obof an
as well. serve as a to the results
in the hydrogenation
of
83 TABLE
3
Gas composition
in the reactor
0.4 wt% Ru-on-silica, m -' Ru h-l. Total
Total
pressure
exit,
pressure
in the hydrogenation
is reached
in each case by adding Partial
Temperature
'gH1O Pressure/kPa
H2 Pressure/kPa
of cyclohexene
130 kPa and space velocity
helium.
pressures
/K
C6H6 /kPa
'gH12 /kPa 6.19
62.5
6.25
305
0.01
62.5
6.25
358
0.05
5.07
62.5
6.25
400
0.14
4.75
13.1
7.18
305
0.01
1.42
39.5
7.18
305
0.01
4.46
65.8
7.18
305
0.01
6.81
77.6
1.97
305
0.01
1.95
77.6
3.42
305
0.01
3.34
77.6
5.67
305
0.01
5.49
aThe partial and C6H,2
pressures
was cyclohexane
It is important to be always
and only small amounts
than that of benzene
in Table
3. Again
sure that we measured to present
cyclohexene
(compare
pressure
and lines,
is to be expected
coverage.
A first order
This problem
reaction
Therefore,
be dealt with
whereas conclusions
in mind
it is not
but from Table
in both the hydrogen
for the order
in cyclohexene). with
3 it
and the in
Such kinetic
a low cyclohexene
is less easy to understand,
pressure
leads to a high hydrogen
in the discussion. and cyclohexene
and their
ratios
are
of temperature.
the distance
temperature.
equilibrium,
this
of the reaction,
for hydrogen
hydrogen
rates for benzene
as a function
As said before,
orders
was found The results
high, so that we are not
Keeping
as long as we are dealing
relationship
will
rates were
were detected,
conditions.
lines 4, 5 and 6 in the Table
that the high partial
In Table 4 reaction presented
into C6H6
hydrogenation
comparable
to be first order
7, 8 and 9 for the order
behaviour
considering
under
the reaction
here the precise is likely
coverage.
converted
of benzene
far from equilibrium.
is seen the reaction
hydrogen
of C6H,0
to note here that the rate of cyclohexene
higher
are summarized
possible
of the amount
ofa:
per unit time.
cyclohexene
fully
are a measure
over
6 x IO4 cm3 (STP)
from equilibrium
In the case of benzene
hydrogenation
in the case of cyclohexene from Table
that the rates
of reaction
An explanation
of this phenomenon
is strongly
nature
when the temperature
and its consequences
by the
we are far from
this situation
4 are of a qualitative
run antiparallel
influenced
is given
is never only.
realised.
It is found
is increased. in the discussion.
84 TABLE
4
Rate of cyclohexene
and benzene
range from 305 to 410 K. Total (cyclohexene)
pressure
hydrogenation pressure
and their
ratio
130 kPa, hydrogen
6 kPa, helium
pressure
305
340
in the temperature
pressure
62 kPa, benzene
62 kPa and space velocity
6 x lo4
cm3 (STP) me2 Ru h-l. Temperature/K
322
350
360
400
380
415
TON C6H6a
0.02
0.03
0.06
0.07
0.07
0.07
0.06
TON C6H,oa
2.35
2.26
2.17
2.17
2.18
2.11
2.07
TON C6H,o/TON
C6H6
118
aTON = number
of benzene
68
33
(cyclohexene)
28
30
molecules
30
reacted
0.05
34
per site per second.
DISCUSSION For the heterogeneous the following
c&j (g)
H2
reaction
(g)
H2
observation
2 C6H8(ads )+2H(ads)
mechanism
is based
hydrogenation;
this has never
the cyclohexadienes
C6H10(g)
H2 (9)
11
= C6H10
cyclohexene
been observed
are too strongly
over group VIII metals,
adopted
among other
on,
that the intermediate
of benzene
is generally
(g)
11
(ads) + PH(ads)
The stepwise
hydrogenation
mechanism
11
11 C6H6
catalytic
stepwise
[22,23]:
C6H,2 (g)
11
(ads)+PH(ads)
things,
11 2 C6H,2
the often
can desorb
before
quoted
further
for the cyclohexadienes.
adsorbed
to desorb
before
(ads
[4 6,231 catalytic
Apparently
further
hydro-
genation. Derbentsev,
Paal and Tetenyi
mechanism
of benzene
included.
They started
and unlabeled product
hydrogenation
was smaller
Starting formed,
is so strongly
adsorbed
more C6H,o
Table
than C6H,2
radioactivity
Derbentsev
is about
VIII metals, ruthenium 14 of C labeled benzene
fraction.
mechanism.
C.S. and by Tetenyi
is
and Table
is extremely
both C6H6 and a C6H,o/C6H,2
2)
lowered.
mixture
are
times as high as the hydrogenation
from C6H8. Therefore,
in the product
This was
conclusion
section
2, line 4, shows that under certain is formed
in the C6Hlo
of the adsorbed
Their
(see Results
that the C6H6 coverage
a hundred
the reaction
mixtures
full hydrogenation
the stepwise
from this high C6H8 coverage, at a rate which
group
that the radioactivity
in the light of our observation
rate of benzene.
between
and found
of a direct
C6H6 ring in one step, besides
that C6H8
equimolecular
and Paal [25] studied
than in the C6H,2 product
by the introduction
questionable
over several
from nearly
1,3-cyclohexadiene
fraction
explained
[24] and Tetenyi
fractions
reaction
we think the difference
of C6H10 and C6H,2
C.S. can be explained
conditions
without
as observed
by
the introduction
of
85
AG' ikl ml-‘1
extensm
FIGURE
8
An approximate
enthalpy
diagram
in the hydrogenation
reaction,
Lower
levels:
extremely
high activation
energies.
Temperature
(ads) and C6H,,
around
full hydrogenation benzene
We now first
should
discuss
types of adsorbed measurable
article
to values
transition
absolute
chosen.
value
hydrogen
of binding
metals
&HadS,
value
atoms,
the chances
forming
adsorption accepted
around
is involved
on transition
and adsorption
means
[26]. The answer reaction
is indicative
will be higher
depends
for ~~~~~~
corresponding
(weakly
bound
hydrogen)
between
on the
migration desorption.
a low of hydroAlso
as the -aHads/RT
of the free enthalpy
8), an enthalpy
to weakly
adsorbed
-1
to the question
in this respect;
according
of a diagram
(see Figure
being
the absolute
as low as 36 kJ mol
that the rate of surface
in the construction
room temperature
metal
enthalpies
In general
range from values
in a catalytic
of hydrogen
has to be chosen
value
that
a C 6H ,* transition
that there are at least five
seems to be marginal
of this quantity
is lower. Hence,
the reaction
lines:
(ads), C6Hg
-1 , and in this respect the difference
The value of -nH ads/RT
reactivity
Dashed
of C6H7
gen is high, and the same is the case for the rate of hydrogen the catalytic
mixture
gas-phase
but low activation
levels
of hydrogen
[26] it appears
enthalpies,
type of hydrogen
temperature
over ruthenium.
unknown
of the TPD technique.
as high as 170 kJ mol
the various
levels:
in this diagram.
their populations
by means
value of the adsorption
of the
be low.
hydrogen,
especially
lines:
Upper
step of the C6H6 ring. Moreover,
the modes
From a survey
free
of the extension
point.
300 K. The free enthalpy
reacts with 6 adsorbed
in one step,
reaction
For the hydrogen/benzene
hydrogenation
Dotted
(ads) are not included
a direct
surfaces.
as a function
mechanism.
catalytic
energies.
adsorbed
which
of benzene
of the standard
aG ' is taken zero as a reference
in the gas-phase,
state
of the change
for the case of the stepwise
hydrogenation.
reactmn (%I
of the
of
of hydrogen
hydrogen.
is about 40 kJ mol-l
A generally [26].
86 Around hydrogen
300 K we observed pressure.
bound types of hydrogen coverage
to be nearly
notwithstanding
is often
case of benzene
called
which weakly
order.
values
differential
the adsorption
values
enthalpies
starting
= -120 kJ mol-' K-'
[261 Cl81 1281
cyclohexene
.
cyclohexane
:
1,s z-285 kJ mol-’ ads 2s = -100 kJ mol-' ads ASads = -100 kJ mol-'
stances
is higher Figure
due to their
reaction C6H8 (step
(ads) level.
It follows
can proceed,
(ads) level,
can be explained
(2)) was found
(1)). Hence the population the very fast reaction continues,
to the right
are rough of these subof these molecules
is about 41 kJ mol -' higher of activation
to that value.
the thermodynamically
the unfavourable
unfavourable
high
The rate of C6H8 hydrogenation
is continuously
that the production thermodynamic
Furthermore,
by the relatively
for
The fact that the
than the rate of C6H6 hydrogenation
of the C6H8 (ads) level
(1) to the right.
is increased
mobility
that the free enthalpy
step (2). This means
notwithstanding
of r-Go for step
the surface
entropies
of adsorption.
as follows.
to be much faster
and cyclohexane
the gas-phase
be at least equal
notwithstanding
the following
K-'
that the C6H8 (ads) level
(1) to the right should
in the section
K-'
to that of benzene,
lower enthalpies
8 demonstrates
than the C6H6 step
equal
K-'
for cyclohexene
based on the fact that, whereas
are nearly
from the
are introduced:
x5ads = -140 kJ mol-' K-'
chosen
the free enthalpies
given
AS
entropies
of the
for the non catalytic
from AH values,
:
estimates
TPD
the reaction
by subtracting
:
The differential
in which
from this,
hydrogen
1,4-cyclohexadiene:
a rate, around
as a function
values
benzene
ads
this type
held monolayer"
observed
diagram
are calculated
of IGO values,
entropy
bound causing
of the hydrogen
steps are plotted free enthalpy
over ruthenium
free enthalpy
that,
of Aben C.S. [27] for the
authors
free enthalpy
reaction
The virtual
In the calculation
In the literature
in the population
are taken from Janz [28]. Starting reaction
is present,
of the strongly
These
the
Hence we conclude of the strongly
in
hydrogen.
a reaction
of the various
of the surface
approximate
8 in ref. C261) shows
the results
over platinum.
adsorbed
of the reaction.
gas-phase
for the weakly
adsorption
in excess with
is first order
8 we present
free enthalpies
to be first
"hydrogen
hydrogenation
peak representing
Results.
is first order
isotherms
pressure.
hydrogen
is in accordance
room temperature,
hydrogenation
Figure
in hydrogen
a weak
unity,
hydrogenation
[26]. Our conclusion
extension
rate which
adsorption
(see for instance
first order
equals
the rate of benzene
In Figure
hydrogenation
of hydrogen
the fact that the sum of the coverages
types of hydrogen
of hydrogen
a benzene
Inspection
by
of C6H8 (ads)
situation
the rate of reaction
high initial
depleted
(step
pressures
of an increase of step
(1)
of C6H6 and
and their respective populations on the surface. The kinetic situation H2' described above has recently been theoretically treated by Boudart 1291. In his
87 terminology
the stimulation
the depletion
of step
(1) by pressure
(ads) level by step
of the C6H8
is called
(2) "pumping
“Pumping
down"
UP”,
and
of the C6H8
(ads
concentration. The high rates we observed determining reaction
around
around
be explained We
believe
atoms
this temperature
level
in the holes
to
Step
in the pressure
surface
of benzene
C6H6 coverage
layer over the small
in the ruthenium
being rate
(1)
that the total
equals
chemisorbed
(for instance
can unity.
hydrogen
at the so-called
[26]). that during
are formed is about
level.
is zero order
C6H6 to form a chemisorbed
Our observation mixture
point
the observation
correct,
from the fact that at room temperature
bound
C8 sites
for step (2) and (3)
300 K. If this is
C6H8 hydrogenation
can be explained
both C6H6 and a C6H18/C6H,2
from the fact that
(see Figure 8) the C6H8 -1 above the C6H10
kJ mol-' above the C6H6 level and 38 kJ mol
41
The extremely
high rates of these reactions,
indicate
that the activation
the C6H6
(ads) level,
energy
and between
barriers
the C6H8
even at room temperature,
between
the C6H8
(ads) level and
(ads) level and the C6H10
(ads) level,
are very low. At 300 K, where
it is likely that the addition
(I), is rate determining, apparent
activation
in the same range 30 kJ mol".
temperature. energy
of 3.8 kJ mol
of temperatures
However
of the logarithm
we calculated
energy
Kubicka
from the Arrhenius
-1
. By contrast,
the much
plotted
of the reaction
higher
work
in quite
330 K the apparent
occurs
(van Meerten
energy
is accompanied
especially,
(1) to
good accordance
right change AS
activation
energy
is observed,
by a dramatic
with
chemisorbed,
time
is very
as
we are dealing reactions,
are given
of
rate instead
of the reciprocal
Kubicka's
other
a negative group
value.
value.
VIII metals
in the sign of the apparent
change
in the reaction
orders
so that its coverage
in turn means going
however,
strongly
is supported
In Kubicka's it also activation
of benzene
and,
increased.
is strongly
that the rate of step
lowered
Our
view is illustrated
step
(1)
the situation and
Step
by, respectively:
step
lowered.
(1) to the
by the antiparallel 4.
(step 3)) has become
that this step is preceded
(2). The equilibrium
is
at higher
(3) is appreciably
from 300 K to 400 K, the rate of step
the last hydrogenation with
step from
by the fact that cyclohexene
of TON C6H6 and TON IZ~H,~ as can be seen from Table soon
energy
is the fact that at temperatures reaches
and with
C.S. ES]). The change
(3). This view
This
same
of the reaction
as a function
7) an
[4] reports
activation
this to be due to a shift of the rate determining
step
temperatures.
At the
(Figure
of hydrogen.
We speculate
very weakly
plot
step
If we adopt the same procedure we arrive at an apparent activation -1 ,
of 38 kJ mol
the same phenomenon
step
to benzene,
Kubicka
apparent
the logarithm
rate constant
In our work the most striking observation around
of hydrogen
constants
rate determining,
by two equilibrium of step
(1) and
(2)
88 @C6H8
K, = eC6H6
(a)
(a)
x
(3)
@H (a)*
and @C&l,0
(a)
K2 = @C6H8(a)
x
In both equations
@H(a)*
that in the overall finally
become
Meerten
C.S.
an order
rate equation
coverage result
and hence one might
to the benzene
coverage
increase
have to explain
in the temperature
that the nH values
positive).
It follows
the rate-determining
in the order of benzene the occurrence
range above
it appears
(3), cannot
full
will finally
pressure. negative
activation
7). From our calculations
(1) and (2) are very small
(1) and (2), as equilibria
explain
at still be found.
from 0.0 to 0.2.
temperatures
330 K (see Figure
steps
will
at an order
from the original
of an apparent
of the equilibria
that reaction step
deviating increasing
changes
by them, observed
also,
in hydrogen
will
at bY van
arrived
that on ruthenium
in benzene
slowly
that further
investigated
It follows
in hydrogen
were arrived
We, however,
above 400 K, this high order
at 303 K. We expect
in a further
energy
temperatures
expect
pressure.
of the order
and conclusion
from 303 to 400 K the order
We finally
value
of 3 in the case of nickel.
temperatures,
Proceeding
to the hydrogen
the maximum
[gl, who, at the highest
in hydrogen
This points
is proportional
3. The same explanation
of 2 in hydrogen higher
(4)
@ H(a) '
the negative
apparent
(and
preceding activation
energy. A better
explanation
the coverages this will
seems to be the following.
of C6H6 (ads) and especially
have a retarding
in the form of an apparent the coverage
effect
the AH values
of the equilibria:
C6H6(gas) =
C6H6
C6H,0
on the reaction
negative
of dissociatively
C6H,0
activation
adsorbed
With
rate, which
energy,
hydrogen.
increasing
(ads) decrease
temperature
drastically,
manifests
itself
and the same is true for
Or, to put it in another
way,
(5)
(ads)
(gas) 2 C6H,g
and
(ads)
(6)
-AHa2
and
H2 (gas)
=
as equilibria activation
2H (ads)
preceding
energy
(7)
-AHa3 the rate-determining
in the following
way:
step, will
change
the apparent
89
= "H'step E app.
(3) - tHal
-3cHas
- CHa2
(8)
with
[AH,,
+
bH,2
+
3rHa31
>
rH2step
(9)
(3)
CONCLUSION The mechanism plicated.
and presented
reaction
nuclei,
and kinetic
by a more detailed
is highly
by the present
com-
authors
it is seen that we are dealing
in which
we are even dealing
that for a full mechanistic especially
steps
metals
VIII
adopted
If the elementary
steps.
split up into reaction
the cyclic
over group
mechanism,
at the start of the discussion,
least with nine elementary further
hydrogenation
of benzene
From the stepwise
steps
only one hydrogen
with twelve analysis
steps.
more
study of the kinetics
at
(I), (2) and (3) are atom
Therefore
research
is added
to
it is obvious
is needed,
of the elementary
steps as
such.
In the second article of this series we will report on the kinetics of the hydrogenation
of benzene
over copper-on-silica.
ACKNOWLEDGEMENTS We thank Mr. A.P. Pijpers Central
Laboratories,
measurements
and Mr. J. Cremers
DSM, Geleen,
and for their
skilful
help with
Thanks
are also due to Mr. A. Pijpers,
study,
and to Mr. P. van Oeffelt
measurements. of Technology,
Mr. J. Teunisse assisted
The investigations Chemical
Research
the Advancement
(Department
The Netherlands)
the interpretation
from the same department,
for carrying
in the study of the catalyst
were supported
(partly)
of Pure Research
Chemistry,
out the XPS/AES of the spectra. for the TEM
out the 1,4-cyclohexadiene
and Mr. N. van Westen,
(SON) with financial
of Physical
for carrying
TPD
both from Delft University texture.
by The Netherlands
aid from The Netherlands
Foundation Organization
for for
(ZWD).
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