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support interface in toluene steam reforming over rhodium-alumina catalysts
Cotalyui.s, 5 (1983) 219-226 Eleevier Scientific Publishing Company, Amsterdam - Printed in The Netherlands
219
Applied
THE ROLE OF THE METAL/SUPPORT ALUMINA
INTERFACE
IN TOLUENE STEAM REFORMING OVER RHOOIUM-
CATALYSTS
D. DUPREZa,
J. LITTLEC and J. BOUSQUETd
A. MILOUDIapb,
aLaboratoire
de Catalyse
40 Avenue du Recteur Pineau, 86022 Poitiers
Organique,
Cedex, France. b
On leave from the University
'Department Pembroke dCentre
of Metallurgy Street,
and Materials
Cambridge
de Recherche
(Received 6 October
of Sciences,
Algiers.
Sciences,
University
of Cambridge,
CB2 342, U.K.
Elf Solaise,
1982, accepted
BP 22 , 69360 St. Symphorien
23 November
d'Ozon,
France.
1982)
ABSTRACT Toluene steam reforming has been studied using two series of well characterized Rh/Y-Al 03 catalysts. On series with variable loading, xm, and variable dispersion, D , of z he metallic phase, turnover frequencies appeared to be proportional to the t&m (Do2xm)i-n where n is the kinetic order with respect to toluene. These results confirm the validity of the previous model 123 in which the rate determining step of the reaction was found to be the surface migration of hydroxyl groups throuoh the support/metal interface. The same model accounts well for the results obtained with the second series, comprising catalysts with variable metal profile inside the pellets.
INTRODUCTION In a recent patent catalysts
are more active
with a homogeneous (i) to a higher effects
Cl], it has been shown that crown-impregnated
intrinsic
reaction
support
activity
followed
interface.
mechanism
from the support
properties
including
the same metal
site "a" was given by the following
(Rh) deposited
in which
of the catalysts the support the
the rate determining
to the metal
through
step
the metal/
that, for a series of catalysts
on the same support
(y-A1203),
equation:
xm)lmn
0166-9834/63/0000-00001$03.00
either
(ii) to diffusional
[23, we have shown that, on Rh catalysts,
This led to the conclusion
= c(* (Do2
catalysts
On the other hand, by studying
per metal
a = ciIo'+
catalysts,
of the structural
a bifunctional-like of OH groups
than standard
This result could be ascribed
of the patented
of the metal).
in steam dealkylation
was the migration
steam dealkylation
of the metal.
or (iii) to a modification
(e.g. higher dispersion effects
in toluene
distribution
rhodium-alumina
0 1983 Elsevier Scientific Publishing Company
the activity
220 where
I, is the perimeter
Do is the percentage
of the metal/support
dispersion
n is the kinetic order with respect series of Rh/v-A1203
The purpose
area,
in weight
and u and c(* are constants
that the specific
to the dispersion
activity
and to the metal
%,
for a
of a Rh/A1203
catalyst
loading by the relation
(1).
proofs of the validity
above and (ii) to show that the crown effect may be precisely
by this model. Thus, two series of Rh/y-A1203
and investigated, profile
to toluene
of the present work is (i) to provide additional
of the model developed explained
per unit of catalyst
xm is the metal loading
catalysts.
It is clear, consequently, can be related
interface
of the metal,
a series with variable
catalysts
have been prepared
loading and a series with a variable
metal
in the pellets.
EXPERIMENTAL The apparatus
for steam dealkylation
present work have been described in an isothermal
and the g.c. pulse system used in the
elsewhere
The sample weights
were adjusted
to obtain
co, cop, gaseous and condensed the initial activity
[2-41. The catalysts
argon
essively
impregnation. was pretreated contacted
The support
of the catalysts
have been described
in detail
(OT) and hydrogen
medium
solution
amount of metal.
GFS 300 y-A1203,
of rhodium
Exchange
The pH of the pretreatment
chloride
of rhodium was complete
to dryness;
calcined
and immediately
containing
the
in less than 15 h.
under flowing
air at 723 K.
was not complete
aliquot
at different
by wet
200 m2 g-', high purity)
hydrate
(5 and 10% Rh), exchange
portions
temperatures,
solution
is a determining
parameter
have been used: these were pure water,
(HCl 0.04 N) and a high acidity medium Two series of catalysts
medium
(HT) were succ-
after
of the same resulting
in
of the metal for the same loading.
Three types of solution
I) with metal
by ultrapure
were prepared
dried at 393 K and calcined
batch were subsequently dispersion
titration
The catalysts
for 4 h. It was then filtered
15 h. The solution was then evaporated
(series
in the p.c. pulse system by H2 chemisorption
(Rhone Poulenc
in the aqueous
For highly loaded catalysts
a variable
15-25%.
used for
titration
The solid was then filtered,
catalyst
.
by gas chromatography.
100*3%. The methods
(0.245 cm3 per pulse).
with an aqueous
appropriate
-1
for 3 h and then cooled to 333 K in argon. Hydrogen
(HC), oxygen
performed
of about
The samples were reduced in situ at 723K,purged
(N 60 Air liquide)
chemisorption
were analyzed
to within
were characterized
and H2-O2 titrations.
out
and water flow rates were 2 cm3 h
initial conversion
hydrocarbons
"2' Mass and carbon balances were performed determining
[2,33. The reaction was carried
flow reactor at 713 K; toluene
(HCl 0.25 N).
have been prepared:
loadings
ranging
of the preparation.
a low acidity medium
these were an initial series
from 0.03 to lo%, prepared
and a second series of three catalysts
the three different
modes of pretreatment.
profile of rhodium
inside the pellet, which
in low acidity
at 0.5 to 0.6% Rh prepared
Each of these modes
by using
induces a different
have been characterized
by microprobe
221 analysis.
Electron
microprobe
The catalyst approximately sectioning
analysis
pellets were
in the form of small cylinders
5 mm in length).
to avoid artefacts
Such small pellets
in the finally measured
the pellets were set into a cold mounting edge of the final mount. the direction
carryover profile
as the microtome
should merely
and thus ambiguous
leaving a cross section
edge appeared microprobe
absorption
results
This should ensure
Thus
that, if there is
the pellet,
then this
should not occur. The cuts were made at intervals
until half of the cylinders
ofthepellet
with maximum
for microprobe
to be fully
analysis,
the X-ray peak monitored
could become
had been cut away thus
area. When choosing
suitable
care was taken to ensure
intact under a medium
power optical
that the
microscope.
of approximately
The
40nA. Under these
was the Rh La,at 2.696 keV. At this low energy,
of the X-rays by the evaporated
conductive,
profile.
so that the cut was made in
knife slices through
was run at 20 kV with a beam current
conditions,
concentration
go down on areas of the pellet with the same concentration
of 20 urn and were continued
areas on the pellets
and
very careful
resin with the long axes flat to the
These were then microtomed
of the axes of the pellets.
any powder carryover
(1.2 mn diameter
necessitate
significant
carbon
layer applied
if the carbon
to make the surface
layer were locally
very thick
( a 5 urn carbon layer would reduce the intensity to 90%). To circumvent any problems
of measuring
or estimating
the C layer thickness,
both the rhodium
and the samples were coated at the same time in the carbon evaporator both suffered An
correction
Rh La, in the aluminum using the machine
did
matrix.
have
to
be applied
smooth. This surface
can lead to increases
roughness
of absorption
off angle to the X-ray detector
surface
the average
features
of approximately across
in the emitted
of
program
should only
and the surface of the catalyst
is not
X-ray path
have the same effect as increasing
and thus a program was run calculating
take off angles on the true X-ray intensity.
the catalysts,
ZAF computer
these calculations
surface
Such increases
by virtue of absorption
This was run on a standard
take off angle of 75". However
be valid for a smooth polished
of different
and thus
equal carbon absorption.
absorption
length.
standard
the take the effect
From the SEM images of
value of large pore size (2-3 urn) and the height of
(l-l.5 urn) led to the use of an effective twice that of a polished
surface.
the pellet profile were then incorporated
path length
increase
These final concentrations
into Figure
1.
RESULTS AND DISCUSSION Catalyst
with variable
The results
metal
concerning
is seen that the temperature dispersion
loading
(series
I)
this series of catalysts
of the catalysts
of calcination
are reported
has a significant
(see e.g. samples
in Table
1. It
effect on the final
6 and 7, 8 and 9): an elevated
222
2.0
1.0
8 _- __------__
-sm.
0
FIGURE
1
Profile of metal concentration
tempearture
increases
have reported the dispersion
the dispersion.
This is in agreement
that, for the highly loaded catalysts increased
the specific
ation varies by a factor of 18 between (sample 7). The model
with Yao et aZ. who
(5.5 wt % Rh in their study),
activity
at 400°C to about
the less active
leading to Equation
in toluene (sample
the series of catalysts
steam dealkyl-
1) and the most
(1) is tentatively
n = 0.1, 0.3 and 0.5. It is clear that this model accounts across
verified
and that the best correlation
for n = 0.3. One may argue that the kinetic order with respect
reported
in the literature
see in the following
omenon appears
increases
of the metal deposit section
to be more marked
inside the pellets.
that the concentration
the specific
activity
of the catalysts.
and it is noteworthy
As this phen-
Equation
higher value of n. Nevertheless,
of the correlation
We will,
of the metal at the
for the least loaded catalyst,
better with the data for a slightly not affect the validity
is to toluene
[6-83 is rather close to 0.1. This slight difference
might be due to non-homogeneity
pellet periphery
with
well for the variations
obtained
however,
A, B and C.
at 700°C.
For the series of catalysts,
of activity
Catalysts
from 30% when the sample was calcined
100% when it was calcined
active
inside the pellets.
(I) fits
this does
that the term
223 TABLE
1
Characteristics
and activities
Catalyst
xm
Tea
Do
number
/%Rh
/K
1%
of Rh/y-Al203
catalysts
(Series
I)
a/(Do2xm)1-n
ab n = 0.1
n = 0.3
n = 0.5
1
0.031
723
100
60
0.34
1.08
3.4
2
0.063
723
100
160
0.48
1.75
6.3
3
0.18
723
95
180
0.23
1.02
4.5
4
0.58
723
92
430
0.21
1.12
6.1
5
0.60
723
96
460
0.20
1.10
6.2
6
4.82
723
45
650
0.17
1.05
6.6
4.87
873
61
1070
0.16
1.12
7.9
8
7
10.1
400
13.7
200
0.22
1.02
4.6
9
10.3
723
32
700
0.17
1.07
6.8
atemperature of calcination prior to reduction of the catalyst. b specific activity in toluene conversion at 713 K per hour per metal respective
partial
a/ (Do2 xrn)lNn is Similar effects dealkylation catalysts
pressures
of toluene
practically
constant
catalysts
[lo]. These results
reactions
Catalyst with variable The characteristics The dispersion HC/OT/HT
ascribed
to a particle
variations
taking
(I) with n = 0.4 - 0.5 and even alkane
II) and their activities is deduced
It is worth
are reported
notinp that the specific
of the catalysts; Nevertheless,
in Table 2.
from the stoichiometries activity
this result could be simply this is not consistent
with the
1 as far as the first five samples are concerned. Therefore, the
of activity
are likely to be due to the interface
(I), i.e. to a combined
concentration. analysis,
(series
size effect.
by Equation
steam
on Rh/A1203
catalysts.
Do of the three samples
results of Table
Equation
profile
in toluene
steam reforming
seems to be valid for aromatic
of the samples
with the dispersion
have been reported
C91 and in heptane
on Rh/A1203
equal to l/2/4 [2,11].
increases
for all the series of catalysts.
are well correlated
[9] and n = 0 [IO]. Thus, the model steam reforming
and water are 0.145 and 0.855 atm.
of the metal concentration
on Rh/a-Cr203
site; the
The profile
is shown in Figure
into account
If the cylindrical metal concentration
of the dispersion obtained
1; Rh* corresponds
the surface pellet
effect
in the catalysts,
roughness
is divided
of the sample
as constant,
leading to
and of the local metal
by electron
to the corrected
into j elementary
xi* may be considered
effect
microprobe values of % Rh
(see Experimental volumes
section).
llVi in which the
one verifies
that:
224
" xm v
where xm is the global metal loading obtained of the pellet and the reduced
analysis,
V is the volume
ratio R/R,.
Hence, the corrected consistent
by chemical
clVi is the elementary volume of a hollow cylinder centered around metal concentration
xi * deduced
with the result of the chemical
from EMA measurements
is
analysis.
TABLE 2 Dispersion
and activity
measurements
of Rh/y-Al203
catalysts
with variable
metal
profile Catalysts
Pretreatment
Dispersion
a
HC/Rh
DT/Rh
HT/Rh
Do
/h-l
A
0.52% Rh
pure water
0.937
1.87
3.63
94
685
B
0.53% Rh
low acidity (0.04 N HCl)
0.874
1.76
3.46
88
445
C
0.56% Rh
high acidity (0.25 N HCl)
0.824
1.53
2.92
a2
300
A pellet may be considered
as a juxtaposition
in which the dispersionis D,andthemetal a homogeneous
particle
The summation
on the whole pellet
j
(x~*)~-~
1
a _
a*
2-2n
D
with a specific
weight activity
of elementary
catalyst
grains
% is xi*; each grain behaves given by Equation
(see Apendix)
as
(1).
leads to the relation:
Avi
i=l
(2)
0
'rn '
The dispersion
is assumed
of fact, Do is practically
to be constant independent
along a section of the pellet:
as a matter
of the metal profile and is rather favoured
by the crown-impregnation. The calculations are reported
made from the profile
xi* vs. R/R0 in the catalysts
in Table 3. The term 6, which appears
term of Equation
in this table,
(2): a = a*6. It is clear that the model represented
(1) and (2) accounts
well for the differences
of activity
A, B and C
is the second by Equations
of the catalysts
A, B
and C. Another limitations catalyst
explanation
for the higher activity
which may be expected
A, because
the equivalent
in this sample than in catalyst
of catalyst
to affect catalyst diameter
A might be diffusional
C to a greater
of the active particle
C. Therefore,
catalyst
effectiveness
extent than is much smaller factors
have
225 TABLE 3 Validity
of Equation
Catalyst
(2) for catalysts
with variable
metal profiles
6
Do/%
a/6
n = 0.1
n = 0.3
n = 0.1
n = 0.3
A
94
4620
694
0.148
0.98
B
88
3055
498
0.146
0.89
C
82
1805
338
0.166
0.89
been computed
for heat and mass transfer
Under our experimental
conditions,
better than 0.9. In the most unfavourable n = 0.902 at 20% conversion. The maximal
Moreover,
error due to diffusional
and cannot account
limitations.
the global effectiveness case (catalyst
n decreases
limitations
for the differences
between
factor
nG is always
C, first kinetic order),
very slowly with conversion.
may thus be estimated
the catalysts
at 10%
A, B and C.
CONCLUSION The kinetic model developed reasonable
representation
A1203 catalysts
of the variations
with variable
valid whenever
a bifunctional
one site ensemble
in [2] for toluenesteamdealkylation
reaction
to another
of activity
loading and with variable is controlled
(for instance,
versa) and could thus be applied
provides
a
for two series of Rh/yprofile.
The model
by surface migration
is from
from a metal to a support or vice-
to any reaction
system of this type.
APPENDIX
In an elementary the number of moles
volume hVi in which the surface area of the catalyst is aAi, reacted
per hour is
r = a Mi"Ai
where Mi is the metal area)
site density
in the elementary
volume,
and "a" the turnover
(1). For the series of Rh/y-A1203
a = a*(Do2
catalysts,
* l-n 'i
(x~*)~-~
The integration
on the whole
Do3-2n
z(x~*)~-~
frequency
we have:
Mi = ED~x~*. hence:
)
Ar = U*E Do3-2n
r = a*c
(number of metal sites per unit of catalyst
4Ai
AA~
pellet
leads to
given by Equation
226 The apparent
turnover
frequency
j a
=
r
=
MA
a*D
2-2n
C i=l
of the pellet will be:
(Xi*)2-nAAi
0 xnl
A
where M is the mean metal site density BET area is proportional by
and A the BET area of the pellet.
to the volume of the catalyst,
As the
AA and A may be replaced
AV and V, respectively.
REFERENCES 1 2 3 4 5 6 7 8 9 IO 11
M. Grand and D. Duprez (S.N.E.A.) Fr. Patent, 2,423,469. D. Duprez, P. Pereira, A. Miloudi and R. Maurel, J. Catal., 75 (1982) 151. D. Duprez, P. Pereira, M. Grand and R. Maurel, Bull. Sot. Chim. Fr., I (1980) 35. D. Duprez, R. Maurel, A. Miloudi and P. Pereira, Nouv. J. Chim., 6 (1982) 163. H.C. Yao, S. Japar and M. Shelef, J. Catal., 50 (1977) 407. P. Beltrame, I. Ferino, L. Forni and S. Torrazza, Chim. Ind. (Milano), 60 (19781 .._. _, 191. ._._ D.C. Grenoble, J. Catal., 51 (1978) 203. P. Pereira, Thesis, Poitiers (1979). K. Kochloefl, Proc. Sixth Intern. Congr. Catalysis, London, (1976), p 1122, The Chemical Society, London, 1977. E. Kikuchi, K. Ito, T. Ino and Y. Morita, J. Catal., 46 (1977) 382. T. Paryjczak, W.K. Jozwiak and J. Goralski, J. Chromatog., 166 (1978) 65 and 75.
219
Applied
THE ROLE OF THE METAL/SUPPORT ALUMINA
INTERFACE
IN TOLUENE STEAM REFORMING OVER RHOOIUM-
CATALYSTS
D. DUPREZa,
J. LITTLEC and J. BOUSQUETd
A. MILOUDIapb,
aLaboratoire
de Catalyse
40 Avenue du Recteur Pineau, 86022 Poitiers
Organique,
Cedex, France. b
On leave from the University
'Department Pembroke dCentre
of Metallurgy Street,
and Materials
Cambridge
de Recherche
(Received 6 October
of Sciences,
Algiers.
Sciences,
University
of Cambridge,
CB2 342, U.K.
Elf Solaise,
1982, accepted
BP 22 , 69360 St. Symphorien
23 November
d'Ozon,
France.
1982)
ABSTRACT Toluene steam reforming has been studied using two series of well characterized Rh/Y-Al 03 catalysts. On series with variable loading, xm, and variable dispersion, D , of z he metallic phase, turnover frequencies appeared to be proportional to the t&m (Do2xm)i-n where n is the kinetic order with respect to toluene. These results confirm the validity of the previous model 123 in which the rate determining step of the reaction was found to be the surface migration of hydroxyl groups throuoh the support/metal interface. The same model accounts well for the results obtained with the second series, comprising catalysts with variable metal profile inside the pellets.
INTRODUCTION In a recent patent catalysts
are more active
with a homogeneous (i) to a higher effects
Cl], it has been shown that crown-impregnated
intrinsic
reaction
support
activity
followed
interface.
mechanism
from the support
properties
including
the same metal
site "a" was given by the following
(Rh) deposited
in which
of the catalysts the support the
the rate determining
to the metal
through
step
the metal/
that, for a series of catalysts
on the same support
(y-A1203),
equation:
xm)lmn
0166-9834/63/0000-00001$03.00
either
(ii) to diffusional
[23, we have shown that, on Rh catalysts,
This led to the conclusion
= c(* (Do2
catalysts
On the other hand, by studying
per metal
a = ciIo'+
catalysts,
of the structural
a bifunctional-like of OH groups
than standard
This result could be ascribed
of the patented
of the metal).
in steam dealkylation
was the migration
steam dealkylation
of the metal.
or (iii) to a modification
(e.g. higher dispersion effects
in toluene
distribution
rhodium-alumina
0 1983 Elsevier Scientific Publishing Company
the activity
220 where
I, is the perimeter
Do is the percentage
of the metal/support
dispersion
n is the kinetic order with respect series of Rh/v-A1203
The purpose
area,
in weight
and u and c(* are constants
that the specific
to the dispersion
activity
and to the metal
%,
for a
of a Rh/A1203
catalyst
loading by the relation
(1).
proofs of the validity
above and (ii) to show that the crown effect may be precisely
by this model. Thus, two series of Rh/y-A1203
and investigated, profile
to toluene
of the present work is (i) to provide additional
of the model developed explained
per unit of catalyst
xm is the metal loading
catalysts.
It is clear, consequently, can be related
interface
of the metal,
a series with variable
catalysts
have been prepared
loading and a series with a variable
metal
in the pellets.
EXPERIMENTAL The apparatus
for steam dealkylation
present work have been described in an isothermal
and the g.c. pulse system used in the
elsewhere
The sample weights
were adjusted
to obtain
co, cop, gaseous and condensed the initial activity
[2-41. The catalysts
argon
essively
impregnation. was pretreated contacted
The support
of the catalysts
have been described
in detail
(OT) and hydrogen
medium
solution
amount of metal.
GFS 300 y-A1203,
of rhodium
Exchange
The pH of the pretreatment
chloride
of rhodium was complete
to dryness;
calcined
and immediately
containing
the
in less than 15 h.
under flowing
air at 723 K.
was not complete
aliquot
at different
by wet
200 m2 g-', high purity)
hydrate
(5 and 10% Rh), exchange
portions
temperatures,
solution
is a determining
parameter
have been used: these were pure water,
(HCl 0.04 N) and a high acidity medium Two series of catalysts
medium
(HT) were succ-
after
of the same resulting
in
of the metal for the same loading.
Three types of solution
I) with metal
by ultrapure
were prepared
dried at 393 K and calcined
batch were subsequently dispersion
titration
The catalysts
for 4 h. It was then filtered
15 h. The solution was then evaporated
(series
in the p.c. pulse system by H2 chemisorption
(Rhone Poulenc
in the aqueous
For highly loaded catalysts
a variable
15-25%.
used for
titration
The solid was then filtered,
catalyst
.
by gas chromatography.
100*3%. The methods
(0.245 cm3 per pulse).
with an aqueous
appropriate
-1
for 3 h and then cooled to 333 K in argon. Hydrogen
(HC), oxygen
performed
of about
The samples were reduced in situ at 723K,purged
(N 60 Air liquide)
chemisorption
were analyzed
to within
were characterized
and H2-O2 titrations.
out
and water flow rates were 2 cm3 h
initial conversion
hydrocarbons
"2' Mass and carbon balances were performed determining
[2,33. The reaction was carried
flow reactor at 713 K; toluene
(HCl 0.25 N).
have been prepared:
loadings
ranging
of the preparation.
a low acidity medium
these were an initial series
from 0.03 to lo%, prepared
and a second series of three catalysts
the three different
modes of pretreatment.
profile of rhodium
inside the pellet, which
in low acidity
at 0.5 to 0.6% Rh prepared
Each of these modes
by using
induces a different
have been characterized
by microprobe
221 analysis.
Electron
microprobe
The catalyst approximately sectioning
analysis
pellets were
in the form of small cylinders
5 mm in length).
to avoid artefacts
Such small pellets
in the finally measured
the pellets were set into a cold mounting edge of the final mount. the direction
carryover profile
as the microtome
should merely
and thus ambiguous
leaving a cross section
edge appeared microprobe
absorption
results
This should ensure
Thus
that, if there is
the pellet,
then this
should not occur. The cuts were made at intervals
until half of the cylinders
ofthepellet
with maximum
for microprobe
to be fully
analysis,
the X-ray peak monitored
could become
had been cut away thus
area. When choosing
suitable
care was taken to ensure
intact under a medium
power optical
that the
microscope.
of approximately
The
40nA. Under these
was the Rh La,at 2.696 keV. At this low energy,
of the X-rays by the evaporated
conductive,
profile.
so that the cut was made in
knife slices through
was run at 20 kV with a beam current
conditions,
concentration
go down on areas of the pellet with the same concentration
of 20 urn and were continued
areas on the pellets
and
very careful
resin with the long axes flat to the
These were then microtomed
of the axes of the pellets.
any powder carryover
(1.2 mn diameter
necessitate
significant
carbon
layer applied
if the carbon
to make the surface
layer were locally
very thick
( a 5 urn carbon layer would reduce the intensity to 90%). To circumvent any problems
of measuring
or estimating
the C layer thickness,
both the rhodium
and the samples were coated at the same time in the carbon evaporator both suffered An
correction
Rh La, in the aluminum using the machine
did
matrix.
have
to
be applied
smooth. This surface
can lead to increases
roughness
of absorption
off angle to the X-ray detector
surface
the average
features
of approximately across
in the emitted
of
program
should only
and the surface of the catalyst
is not
X-ray path
have the same effect as increasing
and thus a program was run calculating
take off angles on the true X-ray intensity.
the catalysts,
ZAF computer
these calculations
surface
Such increases
by virtue of absorption
This was run on a standard
take off angle of 75". However
be valid for a smooth polished
of different
and thus
equal carbon absorption.
absorption
length.
standard
the take the effect
From the SEM images of
value of large pore size (2-3 urn) and the height of
(l-l.5 urn) led to the use of an effective twice that of a polished
surface.
the pellet profile were then incorporated
path length
increase
These final concentrations
into Figure
1.
RESULTS AND DISCUSSION Catalyst
with variable
The results
metal
concerning
is seen that the temperature dispersion
loading
(series
I)
this series of catalysts
of the catalysts
of calcination
are reported
has a significant
(see e.g. samples
in Table
1. It
effect on the final
6 and 7, 8 and 9): an elevated
222
2.0
1.0
8 _- __------__
-sm.
0
FIGURE
1
Profile of metal concentration
tempearture
increases
have reported the dispersion
the dispersion.
This is in agreement
that, for the highly loaded catalysts increased
the specific
ation varies by a factor of 18 between (sample 7). The model
with Yao et aZ. who
(5.5 wt % Rh in their study),
activity
at 400°C to about
the less active
leading to Equation
in toluene (sample
the series of catalysts
steam dealkyl-
1) and the most
(1) is tentatively
n = 0.1, 0.3 and 0.5. It is clear that this model accounts across
verified
and that the best correlation
for n = 0.3. One may argue that the kinetic order with respect
reported
in the literature
see in the following
omenon appears
increases
of the metal deposit section
to be more marked
inside the pellets.
that the concentration
the specific
activity
of the catalysts.
and it is noteworthy
As this phen-
Equation
higher value of n. Nevertheless,
of the correlation
We will,
of the metal at the
for the least loaded catalyst,
better with the data for a slightly not affect the validity
is to toluene
[6-83 is rather close to 0.1. This slight difference
might be due to non-homogeneity
pellet periphery
with
well for the variations
obtained
however,
A, B and C.
at 700°C.
For the series of catalysts,
of activity
Catalysts
from 30% when the sample was calcined
100% when it was calcined
active
inside the pellets.
(I) fits
this does
that the term
223 TABLE
1
Characteristics
and activities
Catalyst
xm
Tea
Do
number
/%Rh
/K
1%
of Rh/y-Al203
catalysts
(Series
I)
a/(Do2xm)1-n
ab n = 0.1
n = 0.3
n = 0.5
1
0.031
723
100
60
0.34
1.08
3.4
2
0.063
723
100
160
0.48
1.75
6.3
3
0.18
723
95
180
0.23
1.02
4.5
4
0.58
723
92
430
0.21
1.12
6.1
5
0.60
723
96
460
0.20
1.10
6.2
6
4.82
723
45
650
0.17
1.05
6.6
4.87
873
61
1070
0.16
1.12
7.9
8
7
10.1
400
13.7
200
0.22
1.02
4.6
9
10.3
723
32
700
0.17
1.07
6.8
atemperature of calcination prior to reduction of the catalyst. b specific activity in toluene conversion at 713 K per hour per metal respective
partial
a/ (Do2 xrn)lNn is Similar effects dealkylation catalysts
pressures
of toluene
practically
constant
catalysts
[lo]. These results
reactions
Catalyst with variable The characteristics The dispersion HC/OT/HT
ascribed
to a particle
variations
taking
(I) with n = 0.4 - 0.5 and even alkane
II) and their activities is deduced
It is worth
are reported
notinp that the specific
of the catalysts; Nevertheless,
in Table 2.
from the stoichiometries activity
this result could be simply this is not consistent
with the
1 as far as the first five samples are concerned. Therefore, the
of activity
are likely to be due to the interface
(I), i.e. to a combined
concentration. analysis,
(series
size effect.
by Equation
steam
on Rh/A1203
catalysts.
Do of the three samples
results of Table
Equation
profile
in toluene
steam reforming
seems to be valid for aromatic
of the samples
with the dispersion
have been reported
C91 and in heptane
on Rh/A1203
equal to l/2/4 [2,11].
increases
for all the series of catalysts.
are well correlated
[9] and n = 0 [IO]. Thus, the model steam reforming
and water are 0.145 and 0.855 atm.
of the metal concentration
on Rh/a-Cr203
site; the
The profile
is shown in Figure
into account
If the cylindrical metal concentration
of the dispersion obtained
1; Rh* corresponds
the surface pellet
effect
in the catalysts,
roughness
is divided
of the sample
as constant,
leading to
and of the local metal
by electron
to the corrected
into j elementary
xi* may be considered
effect
microprobe values of % Rh
(see Experimental volumes
section).
llVi in which the
one verifies
that:
224
" xm v
where xm is the global metal loading obtained of the pellet and the reduced
analysis,
V is the volume
ratio R/R,.
Hence, the corrected consistent
by chemical
clVi is the elementary volume of a hollow cylinder centered around metal concentration
xi * deduced
with the result of the chemical
from EMA measurements
is
analysis.
TABLE 2 Dispersion
and activity
measurements
of Rh/y-Al203
catalysts
with variable
metal
profile Catalysts
Pretreatment
Dispersion
a
HC/Rh
DT/Rh
HT/Rh
Do
/h-l
A
0.52% Rh
pure water
0.937
1.87
3.63
94
685
B
0.53% Rh
low acidity (0.04 N HCl)
0.874
1.76
3.46
88
445
C
0.56% Rh
high acidity (0.25 N HCl)
0.824
1.53
2.92
a2
300
A pellet may be considered
as a juxtaposition
in which the dispersionis D,andthemetal a homogeneous
particle
The summation
on the whole pellet
j
(x~*)~-~
1
a _
a*
2-2n
D
with a specific
weight activity
of elementary
catalyst
grains
% is xi*; each grain behaves given by Equation
(see Apendix)
as
(1).
leads to the relation:
Avi
i=l
(2)
0
'rn '
The dispersion
is assumed
of fact, Do is practically
to be constant independent
along a section of the pellet:
as a matter
of the metal profile and is rather favoured
by the crown-impregnation. The calculations are reported
made from the profile
xi* vs. R/R0 in the catalysts
in Table 3. The term 6, which appears
term of Equation
in this table,
(2): a = a*6. It is clear that the model represented
(1) and (2) accounts
well for the differences
of activity
A, B and C
is the second by Equations
of the catalysts
A, B
and C. Another limitations catalyst
explanation
for the higher activity
which may be expected
A, because
the equivalent
in this sample than in catalyst
of catalyst
to affect catalyst diameter
A might be diffusional
C to a greater
of the active particle
C. Therefore,
catalyst
effectiveness
extent than is much smaller factors
have
225 TABLE 3 Validity
of Equation
Catalyst
(2) for catalysts
with variable
metal profiles
6
Do/%
a/6
n = 0.1
n = 0.3
n = 0.1
n = 0.3
A
94
4620
694
0.148
0.98
B
88
3055
498
0.146
0.89
C
82
1805
338
0.166
0.89
been computed
for heat and mass transfer
Under our experimental
conditions,
better than 0.9. In the most unfavourable n = 0.902 at 20% conversion. The maximal
Moreover,
error due to diffusional
and cannot account
limitations.
the global effectiveness case (catalyst
n decreases
limitations
for the differences
between
factor
nG is always
C, first kinetic order),
very slowly with conversion.
may thus be estimated
the catalysts
at 10%
A, B and C.
CONCLUSION The kinetic model developed reasonable
representation
A1203 catalysts
of the variations
with variable
valid whenever
a bifunctional
one site ensemble
in [2] for toluenesteamdealkylation
reaction
to another
of activity
loading and with variable is controlled
(for instance,
versa) and could thus be applied
provides
a
for two series of Rh/yprofile.
The model
by surface migration
is from
from a metal to a support or vice-
to any reaction
system of this type.
APPENDIX
In an elementary the number of moles
volume hVi in which the surface area of the catalyst is aAi, reacted
per hour is
r = a Mi"Ai
where Mi is the metal area)
site density
in the elementary
volume,
and "a" the turnover
(1). For the series of Rh/y-A1203
a = a*(Do2
catalysts,
* l-n 'i
(x~*)~-~
The integration
on the whole
Do3-2n
z(x~*)~-~
frequency
we have:
Mi = ED~x~*. hence:
)
Ar = U*E Do3-2n
r = a*c
(number of metal sites per unit of catalyst
4Ai
AA~
pellet
leads to
given by Equation
226 The apparent
turnover
frequency
j a
=
r
=
MA
a*D
2-2n
C i=l
of the pellet will be:
(Xi*)2-nAAi
0 xnl
A
where M is the mean metal site density BET area is proportional by
and A the BET area of the pellet.
to the volume of the catalyst,
As the
AA and A may be replaced
AV and V, respectively.
REFERENCES 1 2 3 4 5 6 7 8 9 IO 11
M. Grand and D. Duprez (S.N.E.A.) Fr. Patent, 2,423,469. D. Duprez, P. Pereira, A. Miloudi and R. Maurel, J. Catal., 75 (1982) 151. D. Duprez, P. Pereira, M. Grand and R. Maurel, Bull. Sot. Chim. Fr., I (1980) 35. D. Duprez, R. Maurel, A. Miloudi and P. Pereira, Nouv. J. Chim., 6 (1982) 163. H.C. Yao, S. Japar and M. Shelef, J. Catal., 50 (1977) 407. P. Beltrame, I. Ferino, L. Forni and S. Torrazza, Chim. Ind. (Milano), 60 (19781 .._. _, 191. ._._ D.C. Grenoble, J. Catal., 51 (1978) 203. P. Pereira, Thesis, Poitiers (1979). K. Kochloefl, Proc. Sixth Intern. Congr. Catalysis, London, (1976), p 1122, The Chemical Society, London, 1977. E. Kikuchi, K. Ito, T. Ino and Y. Morita, J. Catal., 46 (1977) 382. T. Paryjczak, W.K. Jozwiak and J. Goralski, J. Chromatog., 166 (1978) 65 and 75.