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The influence of sulfur on activity and selectivity of reforming catalysts in the conversion of individual hydrocarbons
Applied Catalysis,31(1987)
99-111 Elsevier Science Publishers B.V., Amsterdam - Printed in The Netherlands
THE INFLUENCE CONVERSION
M. WILDE, Central
OF SULFUR
ON ACTIVITY
OF INDIVIDUAL
T. STOLZ,
Institute
Department
(Received
21 August
OF REFORMING
IN THE
CATALYSTS
HYDROCARBONS
R. FELDHAUS
of Organic
of Basic
AND SELECTIVITY
99
Organic
and K. ANDERS
Chemistry
of the Academy
Materials,
1986, accepted
Permoserstr.
9 December
of Sciences
15, DDR-7050
of the GDR, Leipzig,
G.D.R.
1986)
The conversion of n-hexane, n-heptane and methylcyclopentane was performed at atmosoheric oressure with unsulfided and oresulfided catalvsts (Pt/Al2D3, Pt-Re/ A1203, Pt-Re-CrjAl203, Pt-Ir/Al203). In each'case presulfidation effects higher conversion, lower formation of methane, lower dealkylation activity (n-heptane test) and enhanced yield of aromatics. The improved activity stability is due to the retarded self-poisoning of the metal surface by coke. The higher selectivity of presulfided catalysts for nondestructive reactions is due to the more effective dilution of metal ensembles by sulfur than by coke. The aromatization of n-heptane occurs by direct 1,6-ring closure; only small amounts of coke are deposited. On the other hand, in the aromatization of methylcyclopentane, the more stable dehydrogenation activity of the presulfided catalysts brings about a higher concentration of methylcyclopentadienes and hence an increase in coke formation.
INTRODUCTION Sulfur treated often
is well
known as a poison
for reforming
feeds are used in the reformer
include
a sulfidation
of the run. Such unwanted containing compared
reforming
step to avoid hyperactivities
catalysts.
to platinum
Studies
In the low temperature poisoning,
sulfur
the extent
of poisoning
of sulfur
range
operates
[3]. In the range of typical
flicting
results
are contradictory observed
under
OX&9834/87/$03.50
follows
[4,8-121.
the following
reaction
for Re or Ir
activity
of these metals
reforming
depends
hydrogenolysis
[7]. In an extreme
temperatures
around
non-destructive
but the findings
in activity
0 1987 Elsevier Science Publishers B.V.
over
773 K, conreactions
concerning
due to sulfidation
conditions:
on
is far
is only dehydrogenated
catalyzed
sulfidation,
are well documented.
[3-71. Selectivity
paths;
n-hexane
For metal
A decline
at the beginning
to a low level of self-
[6] or aromatization
platinum
hydro-
procedures
pronounced
catalysts
as a poison
content)
have been found.
hydrogenolysis
hydrogenolysis
(6 700 K), equivalent
than dehydrogenation
For this reason hand start-up
[1,2].
in the various
in selectivity
excessive
on reforming
exclusively
case (e.g., 643 K, high sulfur
an increase
catalysts.
On the other
are especially
The higher
is well documented
on the influence
more suppressed
units.
activity
has been
100 Hydrocarbon
Temperature
Pressure
/K
/MPa
HJHC
Molar
ratio
Ref.
n-heptane
763
1.5
19.4
methylcyclopentane
748
1.38
5
Cl01
n-hexane
680-740
0.1
8
c91
c71
methylcyclopentane, n-hexane n-hexane
(Pt/A1203)
In the following stability)
cases,
773
0.1
773
0.1
however,
after a relatively
sulfided
pulse system
[41
11
catalysts
Cl11
showed
higher
activity
(i.e.,
short time-on-stream:
methylcyclopentane, n-hexane n-hexane
(Pt-Re/A1203)
methylcyclohexane
Improved
activity
apparently
decreasing
H2/HC
version
resulted
[I21
11
Cl11
773 K
0.1 MPa
H2/HC
9
(H2/N2
1)
for sulfided
with
ratio
reforming
conditions
of sulfur
carbons
which
can be easily
(i)
Does sulfidation
of reforming
poisoning
in higher
(ii)
How does sulfidation
result
after
pressure
catalysts
and
in the con-
of strong
self-
on the coking
sulfidation,
[12]), unchanged
ones
known
the dehydrocyclization
of sulfur
sulfur
(n-hexane
the spectrum
rate
addition
[12]) or
of coke. To give a general
on coking we extended
is to answer
It is well
under conditions
activity
[8]) deposition
ment on the influence
The aim of this work
of sulfided
C61
to sulfur-free
decreasing
also favour
on the influence
(methyl-cyclopentane
(methyl-cyclohexane
to n-heptane,
temperature,
feed hydrocarbons
Reports
compared
self-poisoning.
into properties
In cases of improved
in enhanced
catalysts
of strong
increasing
[133. These
seems warranted.
are conflicting.
decreased
4
An investigation
of typical
poisoning
H2/HC H2/HC
under conditions
accelerates
of paraffins.
0.5 MPa 0.1 MPa
behaviour
occurs
that coking
778 K 773 K
state-
of feed hydro-
dehydrocyclized.
the questions: catalysts
under conditions
of strong
self-
conversions?
influence
coke formation?
EXPERIMENTAL Catalyst
samples
Catalysts diameters H2PtC16,
were prepared
of about HRe04,
both calcination
by simultaneous
2 mm, SBET 240 m2 g-l)
(Cr03)x,
H21rC16
and reduction
impregnation
of y-Al203
(using an excess
and HCl. After
were performed
in
overnight situ.
water drying
Before
(globules
technique) ez
situ
with by
at 393 K
chemisorption
101 TABLE
1
Chemical
composition
and CO chemisorption
Catalyst
Pt
Pt/wt%
capacity
of the catalysts.
Pt-Ir
Pt-Re
0.55
0.60
0.60
0.38
0.38
0.55
Re/wt%
Pt-Re-Cr
0.03
Ir/wt%
0.20
Cr203/wt% Cl/wt%
1.10
1.05
CO/urn01 gilt
measurements
were taken the catalysts
chemisorption (Pt, Pt-Ir)
capacity
in Table
were
was determined
or hydrogen
are presented
(Pt-Re,
1.10
25.0
20.8
1.10
28.1
calcined
25.0
and reduced
in a pulse apparatus
Pt-Re-Cr)
as the carrier
at 773 K. The CO
at 298 K with
argon
gas [143. Catalyst
data
1.
Test series Up to 12 consecutive
tests with
the same feed
charge
hydrocarbon
(40 cm3, diluted
flow reactor
with each catalyst
was calcined
for 3 h at 773 K in a stream
were
performed
with quartz).
of dry air before
starting
in a
The catalyst the program,
which was as follows: heating
to 623 K in H2 (20 1 h-') in 1 h
maintain
at 623 K for 1 h (H2, 20 1 h-')
begin feed addition perform
catalytic
at 623 K (0.5 g/g h; H2/HC
= 5.5) and heat to 773 K in 0.5 h
test at 773 K for 1 h (analysis
by means
of capillary
gas
chromatography) stop feed addition
and cool to 323 K in H2 (20 1 h-')
purge with N2 and heat to 773 K in 2 h (air/N2 maintain
= 1)
at 773 K for 2 h in air (20 1 h-l), determine
CO2 absorbed
in NaOH
solution cool in flowing The catalyst
air
was sulfided
of dimethyldisulfide period
before
starting
the feed
tests at 773 K the atomic platinum
amounts
(Pt+Re)s
value of rhenium
Before
in the 3rd, 8th and 10th test by continuous
(corresponding
to 0.2 wt% S in the catalyst)
injection.
containing
from the 5th test was burnt-off
catalysts
the conditions chemisorbed
to Sirr /PtS ? 0.5 in accordance
the 6th test of each series
(corresponding
Under
ratio of irreversibly
with
exceeds
the catalysts
to the catalyst).
of the catalytic
sulfur
to surface
[6]; however,
the Sirr/
0.5 [15].
were
and Ccl4 was injected
to 0.4 wt% Cl addition
addition
over a 5 min
reactivated;
the coke
into the air stream
MCP
n-C6
96.8 99.6 96.8 99.8 83.2 96.1
98.3
99.3
98.0
99.9
91.0
99.2
92.3
99.2
83.3
93.4
n-C7
98.4
10
8
3
Feed
99.8
10'
Pt 8' 8
99.1 99.8
3
99.6 99.9
99.2 99.9 98.0 99.5 84.0 98.3
93.2 99.5 69.9 97.8
8
3
97.7
8'
Pt-Re 3'
99.8
(3, 8, 10).
3'
Pt-Ir
(3', 8', 10') and with sulfidation
8'
(%) without
3'
Conversions
TABLE 2
94.3
98.8
99.0
65.1
92.0
97.7
79.5
99.8
91.1
97.6
99.4
3
3'
99.8
10
10'
97.1
79.3
99.1
97.4
99.7
99.3
8
8'
Pt-Re-Cr
91.8
79.5
98.4
96.4
10
10'
103
reactivation
sulfidation
sulfidchon
sulf idation
I I
I
Pt
---
20.
Pt-Re
-.-- Pt-Re-Cr 1
2
3
4
5
6
test FIGURE
1
Benzene
formation
7
8
9
10
12 -
11
J
sequence
from n-hexane
as a function
of test sequence.
RESULTS Figure
1 presents the benzene yields in n-hexane conversion
the test sequence
for the catalysts
3,8,10)
as well as reactivation
between
the benzene
with
increasing
yields
number
To investigate
Pt, Pt-Re and Pt-Re-Cr.
(test 6) enhances
of sulfided
of sulfidations
the influence
an average
value
of strong
in each case following
sulfidation.
Comparison following yields
2 shows
sulfidation.
catalysts.
in n-heptane
detectable
Figure
compared level.
in Figure number
typically
catalysts.
behaviour
we compared
experiments,
decreased,
in Table
thus 2 show
is reached
sulfidation.
in aromatic
selectivity
2 verify
the levelled
in the case of rhenium
of sulfided
without
product;
decreased
containing
conversion
following
(tests
The difference
of tests
selectivity
experiments
greatly
a higher
aromatics
catalysts
presulfidation
after sulfidation
of
about
is clear 100 components
the number
and only the proportions
of
of toluene
increased.
3 illustrates
the formation
and subsequent
of aromatics
increasing
in the reaction
hydrocarbons
and hydrogen
on catalyst
2 shows an increase
The improved In
preceding
yield
The results
conversion.
be detected
samples
in the case of rhenium
self-poisoning,
2 and Figure
(cf. x and xi) with
containing
could
the enhanced
of Table
Sulfidation
formation.
of tests 2 and 4 (Table 2). Results
that, under conditions
Figure
and non-sulfided
of presulfidation
tests 3,8 and IO with the respective 3' denotes
benzene
as a function
the remarkable
of higher
to the other
aromatics.
catalysts,
drop
in both toluene
Pt-Ir shows superior and this,in
demethylation
dealkylation
the sulfided
state,
and
activity
is at a lower
104
feed :n-C,
Pt
s
Pt-lr
Pt-Re
Pt-Re-Cr
60
feed:n-Cs
E
IJJ 40 0 ; 0 :
20
3
8
10
3
Pt
8
10
3
Pt-Re
8
10
Pt-Re-Cr
80 feed:MCP 60
3
FIGURE
2
Aromatics
8
Pt
10
3
q
yields.
8
10
3
Pt-Re
without
a
10
Pt-Re-Cr
sulfur
(tests
3', 8', IO');
w
with
sulfur.
The influence ethane
ratios
of sulfur
(n-hexane
(Figure
3), the decreasing
imposes
the effect.
Figure coking
5 summarizes
is decreased
methylcyclopentane situation
on hydrogenolysis
tests,
Figure
selectivity
the amounts
by sulfidation conversion
is observed
4). with
may be demonstrated
increasing
of deposited
carbon.
(with exception
coking
when n-hexane
by the methane/
As in the case of demethylation
is increased
number
of tests super-
In the n-heptane
tests
of the Pt-Ir catalyst); by sulfidation.
in
An intermediate
is the feed hydrocarbon.
DISCUSSION Activity
and selectivity
The important aromatization
result
activity
of these
experiments
and selectivity.
is the considerable
An increase
in inherent
improvement activity
in
caused
105
0,
!ij0.15' 0 < Q)
z
O.lO’
z
," 0.05.
L-
i
3
810
3810
38
Pt-Re
Pt-lr
Pt
38
Pt-Re-Cr
feed:
3
810
3
Pt
FIGURE
3
version.
Dealkylation 0
without
3
8
and formation sulfur
810
Pt-Re
Pt-lr
of higher
3
8
Pt-Re-Cr
aromatics
n
(tests 3', 8', IO');
in the n-heptane with
1feed
1I 8
10
Pt
FIGURE 3'9
4
Methane/ethane
8’ and 10');
l
Pt-Re
ratios with
in the n-hexane
sulfur.
n-C7
i
con-
sulfur.
: n-C6
8
Pt-Re-Cr
tests.
0
without
sulfur
(tests
106
I
1.2 :
ifeed:n-C,
r
0.8 Od
r
Pt
Pt-lr
Pt-Re
Pt-Re-Cr
Ii 1.2 a. -m ;;; 0.8 u ;
feed:n-C6
0.4
:
3
8
10
3
8
10
-PtIRe.-
Pt
Pt-R&d;
II-II
eed:MCP
3
8
10
3
Pt
FIGURE
5
IO');
Carbon m
contents
cannot
It can be assumed function
chemisorption
sulfided
0
sulfur
withou
(tests
is known as a catalyst
with non-sulfided
of bifunctional
of sulfur
catalysts
is very small
The total coke content
Interpretation
component,
3', 8' and
[3]. Under
[15]. Coking
such as coking
Smaller
in the coked
almost
reaction
state.
entirely
conditions
of the metal
at the beginning
be used to characterize
of coked metal
et al. [11,18,19]
in sulfided
The dilution
[11,19].
cannot
catalysts
is restricted
poison
to H2S
in reforming of the
the deactivation
as will be shown later.
of the activity
of Sachtler ensembles
Re-S species. structure
8
is far more rapid than that of the support
run [16,17].
platinum
3
Pt-Re-Cr
since sulfur
that the influence
on the support
state of the metal
hypothesis
of the catalysts.
be proposed,
13-71. We have to compare
catalysts
10
with sulfur.
by sulfidation
the metal
8
Pt-Re
Pt-Re
also occurs ensembles
via C, fragments
which
catalysts
surfaces
states
by dilution
of larger with
inert
Pt-S, but in a less rigid
mean that structure-sensitive
are suppressed
on the
that the number
is reduced
in the system
is based
[lQ,ZO].
reactions,
From the results
it follows
107
I
50
60
80
70 %
FIGURE
6
3+4 MCP=/CH=
l
without
A
after
n
with
sulfur
ratios
reactivation sulfur
for Pt;
over sulfided
and dehydrocyclization dehydrogenation The classical comprises hexene
0
0
n
@
exhibit
dealkylation
high activity.
dehydrogenation
mechanism
must
catalysts
by sulfur,
result
(Figure
metal
of n-heptane
catalyzed
is in
cyclization activity
accordance
occurs
deposits.
selectivity
is needed.
sulfidation It follows
conversion
(Figure
6).
of methylcyclopentane
isomerization
ratio,
[Zl].
to cyclo-
If the acidity
as is shown
containing
by the sulfided
of the metal
that the atomic
surface
established which
is much more (carbide-like)
of
of a
by 5. Davis
catalysts.
because
et
This
the dehydro-
site [23]. To sum up, the improved
discussion
of platinum
platinum
hypothesis,
for non-destructive
It is well
- hydrogenolysis
non-acid
samples)
as a consequence
as was demonstrated
atom as active
A more detailed
properties
(over sulfided
can be understood
with the ensemble
samples
protection
carbonaceous
on the catalytic
MCP=/CH=
C6 ring closure
at one metal
of presulfided
by the effective
to be true for the
of hydrogenation-dehydrogenation
of the dehydrocyclization
al. [22] for the case of sulfur assumption
are
dehydrogenation
6).
and especially
predominant
MCP=,
to benzene
the improvement
in an increased
The high selectivity n-hexane
(test 3).
paths,
is shown
of the aromatization
to methylcyclopentene
1,2,4,5,7).
(test 6),
and ring rupture
reaction
This
CH= at acid sites and dehydrogenation
activity
for Pt-Re-Cr
but the desired
(tests
for Pt-Re-Cr
by the data from methylcyclopentane
bifunctional
is not influenced
A
m
hydrogenolysis,
in the MCP tests.
for Pt-Re-Cr
for Pt-Re;
for Pt-Re;
catalysts,
activity
of conversion
for Pt-Re;
for Pt;
that the rates of multiple retarded
as a function
for Pt;
1 IO
90
conversion
reactions
against
deactivation
to understand
that coking
suppressed
by
their superior
exerts
is qualitatively
carbon
can be explained
an influence
similar
to
than dehydrogenation
formed
in the early
[24-271.
stages
of
108
the run acts on the platinum samples
system
of the diluents
Pt-C contains
platinum
the coked platinum smaller
carbon
773 K sulfur
cannot
between
platinum
sulfides
is a highly
whereas
sulfur
relatively
pretation
between
Pt-S bond
of sulfur
results
of
of on platinull
dilution
on
Pt-Sn whose
(super-lattices).
(on platinum) diluents.
The
influence
and sulfur.
linearly
the
of sulfur
[32,33]
cannot
and the inter-
assertions
Probably
about
the electronic
than the geometric
bonded
CO varies
[35]. For this reason
which,
to aggregate,
Considering
lead to contradictory
platinum
vibrations
adsorbed
Pt-C on the one hand and Pt-S and
the electronic
hypothesis
of at
The chemical
by the existence
of sulfur
of the work function
[22,34]
Clearly,
with the system
of carbon
of CO is suppressed,
is based on the ensemble
on
model
gives a very efficient
and efficient
between
in its stretching
interpretation,
feature
is in any case of less importance
bridge-bonding
(if at all)
the systems
[31,32],
relation
carbon
areas.
by inter-metallic compounds
In fact, measurements
of catalytic
documented
it is analogous
hand is the tendency
the donor-acceptor influence
is well
[30], which
respect
the
there are islands
solid phase as does carbon.
or tin act as stable
strong
be ruled out.
on the surface
and a typical
is characterized
difference
1283. Carbide-like
are not known. A realistic
distributed
structure
in the
equilibrium
by S. Davis et al. [29] is that besides
and sulfur
In this
the non-sulfided
difference
In thermodynamic
sites as well as graphite-covered
(PtS, PtS2),
Pt-Sn on the other
Nevertheless,
is a decisive
and graphite
carbides
form a separate
symmetrical
there
and sulfur.
developed
widely
platinum
scale.
phase diagram essential
surface
platinum
affinity
the atomic
platinum
residues
free contiguous
carbon
only platinum
is metastable;
as a diluent.
because
are far less selective,
efficiency
while
surface
in our opinion,
one;
only slightly
our discussion
provides
a satisfactory
to a first approximation.
Coke formation The most
striking
is the suppression conformity tests.
with
Because
result
concerning
of coke formation
the influence
[12], an acceleration hydrogenolysis
of coking
activity
the interpretation
formation
on the basis of hydrogenolysis
the reaction (formation formation process.
[12,36]
that over sulfided
path of coking with
of alkylcyclopentanes restricts
Analogous
of methylcyclohexane
coking, results
toluene
becomes
the great degree
or olefins). toluene
were found
coking
selective.
of coke deposition
of sulfur
should
on total coke
occurs
selectively,
be via side reactions
The relatively
of strong
by sulfidation,
of the catalysts.
high rate of toluene
does not contribute
by Coughlin
but, in
the methylcyclopentane
C6 ring closure
the feed n-heptane
under conditions
highly
behaviour
catalysts
on coking
conversion
is lowered
of the influence
because
tion of Pt and Pt-Re catalysts
during
in both cases
we reject
If we accept
of presulfidation
in the case of n-heptane
much
to this
et al. [8] in the dehydrogenatios'
self-poisoning
is suppressed,
because
viz, after the conversion
We do not have a satisfactory on the sulfided
explanation
Pt-Ir catalyst.
sulfidato for
109
n El
_r *.0i
1.3
I+2 MCP” FIGURE
7
Carbon
MCP (tests tests. , 2
content
as a function
4 ST)without
Pt-Re-Cr'(ieit'6).
sulfur
/
A
after
with sulfur
3+4 MCP’
1
of the 1+2 MCP==/3+4
for Pt;
n
1.4 -
0
MCP= ratios @
for Pt-Re;
reactivation
a
for Pt;
for Pt;
0
in the
for Pt-Re-Cr
for Pt-Re;
A
q
for Pt-Re;
for
for Pt-Re-
Cr (test 3).
In the case of methylcyclopentane precursors form coke), causes
conversion
of the methylcyclopentadienes are formed
an enhanced
sites. Also, is heavily
on the main
concentration
the MCP==/MCP=
reaction
equilibration
/3+4 MCP= ratios
sulfided Figure
samples
(i.e.,
Barbier
low hydrogen
activity
desired
importance
pressure,
reaction
direct
C6 ring closure
in the case of the
dehydrogenation
the MCP==/MCP=
is related
during
of n-hexane
important.
depending prevails.
n-hexane
Indeed,
Therefore,
on whether
dehydrogenation
Hence
leads to the the advantages
the special
and the acid function.
conversion
is not clear.
than with
n-heptane,
in the 90 to 95% conversion coking
enhanced
at
from a practical
sulfidation
(of the support).
is less favoured
conversion
to the actual
seems essential
of both the metal
function).
ratio and the extent
that for the cyclopentane
improvement,
1.4 to 0.7 wt% MCP have been found. by sulfidation
are observed
between
coking
surface of course
(cf. the variation
in the MCP conversion
on coking
routes also become
tarded
are reached
a matter
conversion
of an exact adjustment
C6 ring closure
Since the metal
in n-heptane
by accelerated
activity
7).
ratios
coke formation
to
"drain off" via the acid
it is by no means
This relationship
in that whereas
The role of sulfur direct
correlation
all round selectivity
are accompanied
in Figure
et al. [36] verified
of the catalysts.
point of view
cannot
equilibria
MCP==/MCP=
tendency
path. A high dehydrogenation
in the case of the protected
7 shows the direct
of coking. very
the highest
have a strong
is favoured.
pressure
that the hydrogenation-dehydrogenation
As expected,
(which
of MCP= which
coked under atmospheric
in the I+2 MCP
the methylcyclopentenes,
MCP==
can be accelerated
MCP==
formation
The so other range or re-
or increased
110 On the influence Figures
of repeated
last tests of the series maintained
which
of aromatics
yields
metal
decreasing
yields
an activity
sulfur
at 8 and 8' caused
pressure,
that the metal
coverage
is surpassed).
reduction
about under
its content Moreover, with
surface
During
reaction
an
adsorbed
The increasing
with
portion
Probably,
From this
irreversibly
chemisorbed
(the optimal
sulfur
the Pt catalyst
[4,6,381
under conditions
high
H2S partial
reduction.
sulfate
of sulfate
et al. C371 by means
in the sense of an activity
pressures
of sulfidic
in test 10 (with
reducibility
of Pt-Re
greater
in a relatively
for a considerable
out by Engels
sensitivity
catalysts
that in the
amount
leads to higher
of slow sulfate
is not sufficient
was pointed
We presume
enhancement
the three presulfidations
sulfur
is operative
of higher H2S partial
(cf.
conversion,
and Pt-Re-Cr
This results
is not only covered
conditions.
be mentioned.
the Pt-Re
activity
test 3, because
0.15 wt% which
the enhanced
Pt should
for x',
MCP conversion
leads to a correspondingly
sulfur which
with a reversibly
in the catalyst
this phenomenon
in the pre-
reactivation
test 9 a considerable
but a restricted
with
but additionally
which
of sulfate.
owing to the additional
sulfur,
accumulates
during
is
This levelling
yields
In the n-hexane
after coke burn-off. with
by reduction
in comparison
it follows
of sulfur,
beginning
and selectivity,
presulfidation)
detectable
(increasing
by Cl addition
on that trend.
on the support
experiments
has been formed
activity
in the
is observed.
the amount
accumulation
"sulfur-free"
conversion
with the Pt and Pt-Ir catalysts
about double
sulfate
by presulfidation.
in the case of the bifunctional
is superimposed
situation
catalysts:
a high level of aromatization
increased
n-heptane
for x) whereas
maximum
In comparison adsorb
sulfidation
in the tests x and x' is clearly
catalyzed
2, Pt-Re)
intermediate
without
is not appreciably
dominantly
Figure
sulfidations
1 and 2 show a striking feature of the Re containing
in comparison
of strong decay
with of TPR.
self-poisoning
only
in the case
(test 10).
REFERENCES 1 2 3 4 5 6 7 8 9 10 11 12 13
Sinfelt, Adv. Catal., 23 (1973) 91. S. Engels, P. Mahlow, W. Pfeiffer, 0. Sager and M. Wilde, Chem. Techn., 36 (1984) 426. E. Baumgarten and H. HBffkes, Z. Phys. Chem.(N.F.), 121 (1980) 107. P.G. Menon and J. Prasad, Proc. 6th Int. Congr. Catal., G.C. Bond, P.B. Wells and F.C. Tompkins, (Eds), The Chemical Society, London, 1977, 11-1061. P.G. Menon, G.B. Marin and G.F.Froment, Ind. Eng. Chem., Prod. Res. Dev., 21 (1982) 52. C.R. Apesteguia and J. Barbier, J. Catal., 78 (1982) 352. P.A. van Trimpont, G.B. Marin and G.F.Froment, Appl. Catal., 17 (1985) 161. R.W. Coughlin, A. Hasan and K. Kawakami, J. Catal., 88 (1984) 150; 163. 7. Schay, K. Matusek and L. Guczi, Appl. Catal., 10 (1984) 173. G.B. McVicker, P.J. Collins and J.J. Ziemiak, J. Catal., 74 (1982) 156. V.K. Shum, J.B. Butt and W.M.H. Sachtler, J. Catal., 96 (1985) 371. J.M. Parera, J.N. Beltramini, E.J. Churin, P.E. Aloe and N.S. Figoli, Proc. 9th Ibero-Amer. Symp. Catal., Lisbon 1984, 259. N.S. Figoli, J.N. Beltramini, E.E. Martinelli, M.R. Sad and J.M. Parera, Appl. Catal., 5 (1983) 19.
J.H.
111 W. GrUnert, unpublished results. T. Stolz, unpublished results. Parera, N.S Figoli, E.M. Traffano, J.N. Beltramini and E.E. Martinelli, 16 Appl. Catal., 5 (1983) 33. 17 J. Barbier, G. Corro, Y. Zhang, J.P. Bournonville and J.P. Franck, Appl. Catal., 13 (1985) 245; 16 (1985) 169. 18 P. Biloen, J.N. Helle, H. Verbeek, F.M. Dautzenberg and W.M.H. Sachtler, J. Catal., 63 (1980) 112. 19 W.M.H. Sachtler, J. Mol. Catal., 25 (1984) 1. 20 A. Sarkany, H. Lieske, T. Szilagyi and L. Toth, Proc. 8th Int. Congr. Catal., Berlin (West) 1984, Verlag Chemie, Frankfurt/Main, 1984, 11-613. D.M. Selman and A. Voorhies, Jr., Prepr. Div. Petr. Chem., 18 (1973) 171. ;: B.H. Davis, G.A. Westfall, J. Watkins and J. Pezzanite, Jr., J. Catal., 42 (1976) 247. 23 J.K.A. Clarke, Chem. Rev., 75 (1975) 291; J.K.A. Clarke and J.J. Rooney, Advan. Catal., 26 (1976) 125. B. Coq and F. Figueras, J. Catal., 85 (1984) 197. Z. Paal, M. Dobrovolszky, J. VUlter and G. Lietz, Appl. Catal., 14 (1985) 33. 26 R.P. de Pauw and G.F.Froment, Chem. Eng. Sci., 30 (1975) 789. 27 Z. Paal, M. Dobrovolszky and P. Tetenyi, J. Catal., 46 (1977) 65. 28 M. Hansen, Der Aufbau der Zweistofflegierungen, Julius Springer, Berlin, 1936, p.371. 29 S.M. Davis, F. Zaera and G.A. Somorjai, J. Catal., 77 (1982) 439. 30 J. Dudar, Catal. Rev.-Sci. Eng., 22 (1980) 171. J.G. McCarty, K.M. Sancier and H. Wise, J. Catal., 82 (1983) 92. :: T.E. Fischer and S.R. Kelemen, Surf. Sci., 69 (1977) 1. C.-M. Pradier and Y. Berthier, Bull. Sot. Chim. France, (1985) 317. :;? J. Cosyns, J.-P. Franck and J. Marin Gil, Compt. Rend., 287 (1978) C-85. 35 A. Melchor, E. Garbowski, M.V. Mathieu and M. Primet, React. Kinet. Catal. Lett., 29 (1985) 379. 36 J. Barbier, L. Elassal, N.S. Gnep, M. Guisnet, W. Molina, Y.R. Zhang, J.P. Bournonville and J.P. Franck, Bull. Sot. Chim. France, (1984) I-245. 37 S. Engels, H. Lausch and L. Peitzsch, unpublished results. 38 M.A. Pacheco and E.E.Petersen, J. Catal., 88 (1984) 400.
1: J.M.
99-111 Elsevier Science Publishers B.V., Amsterdam - Printed in The Netherlands
THE INFLUENCE CONVERSION
M. WILDE, Central
OF SULFUR
ON ACTIVITY
OF INDIVIDUAL
T. STOLZ,
Institute
Department
(Received
21 August
OF REFORMING
IN THE
CATALYSTS
HYDROCARBONS
R. FELDHAUS
of Organic
of Basic
AND SELECTIVITY
99
Organic
and K. ANDERS
Chemistry
of the Academy
Materials,
1986, accepted
Permoserstr.
9 December
of Sciences
15, DDR-7050
of the GDR, Leipzig,
G.D.R.
1986)
The conversion of n-hexane, n-heptane and methylcyclopentane was performed at atmosoheric oressure with unsulfided and oresulfided catalvsts (Pt/Al2D3, Pt-Re/ A1203, Pt-Re-CrjAl203, Pt-Ir/Al203). In each'case presulfidation effects higher conversion, lower formation of methane, lower dealkylation activity (n-heptane test) and enhanced yield of aromatics. The improved activity stability is due to the retarded self-poisoning of the metal surface by coke. The higher selectivity of presulfided catalysts for nondestructive reactions is due to the more effective dilution of metal ensembles by sulfur than by coke. The aromatization of n-heptane occurs by direct 1,6-ring closure; only small amounts of coke are deposited. On the other hand, in the aromatization of methylcyclopentane, the more stable dehydrogenation activity of the presulfided catalysts brings about a higher concentration of methylcyclopentadienes and hence an increase in coke formation.
INTRODUCTION Sulfur treated often
is well
known as a poison
for reforming
feeds are used in the reformer
include
a sulfidation
of the run. Such unwanted containing compared
reforming
step to avoid hyperactivities
catalysts.
to platinum
Studies
In the low temperature poisoning,
sulfur
the extent
of poisoning
of sulfur
range
operates
[3]. In the range of typical
flicting
results
are contradictory observed
under
OX&9834/87/$03.50
follows
[4,8-121.
the following
reaction
for Re or Ir
activity
of these metals
reforming
depends
hydrogenolysis
[7]. In an extreme
temperatures
around
non-destructive
but the findings
in activity
0 1987 Elsevier Science Publishers B.V.
over
773 K, conreactions
concerning
due to sulfidation
conditions:
on
is far
is only dehydrogenated
catalyzed
sulfidation,
are well documented.
[3-71. Selectivity
paths;
n-hexane
For metal
A decline
at the beginning
to a low level of self-
[6] or aromatization
platinum
hydro-
procedures
pronounced
catalysts
as a poison
content)
have been found.
hydrogenolysis
hydrogenolysis
(6 700 K), equivalent
than dehydrogenation
For this reason hand start-up
[1,2].
in the various
in selectivity
excessive
on reforming
exclusively
case (e.g., 643 K, high sulfur
an increase
catalysts.
On the other
are especially
The higher
is well documented
on the influence
more suppressed
units.
activity
has been
100 Hydrocarbon
Temperature
Pressure
/K
/MPa
HJHC
Molar
ratio
Ref.
n-heptane
763
1.5
19.4
methylcyclopentane
748
1.38
5
Cl01
n-hexane
680-740
0.1
8
c91
c71
methylcyclopentane, n-hexane n-hexane
(Pt/A1203)
In the following stability)
cases,
773
0.1
773
0.1
however,
after a relatively
sulfided
pulse system
[41
11
catalysts
Cl11
showed
higher
activity
(i.e.,
short time-on-stream:
methylcyclopentane, n-hexane n-hexane
(Pt-Re/A1203)
methylcyclohexane
Improved
activity
apparently
decreasing
H2/HC
version
resulted
[I21
11
Cl11
773 K
0.1 MPa
H2/HC
9
(H2/N2
1)
for sulfided
with
ratio
reforming
conditions
of sulfur
carbons
which
can be easily
(i)
Does sulfidation
of reforming
poisoning
in higher
(ii)
How does sulfidation
result
after
pressure
catalysts
and
in the con-
of strong
self-
on the coking
sulfidation,
[12]), unchanged
ones
known
the dehydrocyclization
of sulfur
sulfur
(n-hexane
the spectrum
rate
addition
[12]) or
of coke. To give a general
on coking we extended
is to answer
It is well
under conditions
activity
[8]) deposition
ment on the influence
The aim of this work
of sulfided
C61
to sulfur-free
decreasing
also favour
on the influence
(methyl-cyclopentane
(methyl-cyclohexane
to n-heptane,
temperature,
feed hydrocarbons
Reports
compared
self-poisoning.
into properties
In cases of improved
in enhanced
catalysts
of strong
increasing
[133. These
seems warranted.
are conflicting.
decreased
4
An investigation
of typical
poisoning
H2/HC H2/HC
under conditions
accelerates
of paraffins.
0.5 MPa 0.1 MPa
behaviour
occurs
that coking
778 K 773 K
state-
of feed hydro-
dehydrocyclized.
the questions: catalysts
under conditions
of strong
self-
conversions?
influence
coke formation?
EXPERIMENTAL Catalyst
samples
Catalysts diameters H2PtC16,
were prepared
of about HRe04,
both calcination
by simultaneous
2 mm, SBET 240 m2 g-l)
(Cr03)x,
H21rC16
and reduction
impregnation
of y-Al203
(using an excess
and HCl. After
were performed
in
overnight situ.
water drying
Before
(globules
technique) ez
situ
with by
at 393 K
chemisorption
101 TABLE
1
Chemical
composition
and CO chemisorption
Catalyst
Pt
Pt/wt%
capacity
of the catalysts.
Pt-Ir
Pt-Re
0.55
0.60
0.60
0.38
0.38
0.55
Re/wt%
Pt-Re-Cr
0.03
Ir/wt%
0.20
Cr203/wt% Cl/wt%
1.10
1.05
CO/urn01 gilt
measurements
were taken the catalysts
chemisorption (Pt, Pt-Ir)
capacity
in Table
were
was determined
or hydrogen
are presented
(Pt-Re,
1.10
25.0
20.8
1.10
28.1
calcined
25.0
and reduced
in a pulse apparatus
Pt-Re-Cr)
as the carrier
at 773 K. The CO
at 298 K with
argon
gas [143. Catalyst
data
1.
Test series Up to 12 consecutive
tests with
the same feed
charge
hydrocarbon
(40 cm3, diluted
flow reactor
with each catalyst
was calcined
for 3 h at 773 K in a stream
were
performed
with quartz).
of dry air before
starting
in a
The catalyst the program,
which was as follows: heating
to 623 K in H2 (20 1 h-') in 1 h
maintain
at 623 K for 1 h (H2, 20 1 h-')
begin feed addition perform
catalytic
at 623 K (0.5 g/g h; H2/HC
= 5.5) and heat to 773 K in 0.5 h
test at 773 K for 1 h (analysis
by means
of capillary
gas
chromatography) stop feed addition
and cool to 323 K in H2 (20 1 h-')
purge with N2 and heat to 773 K in 2 h (air/N2 maintain
= 1)
at 773 K for 2 h in air (20 1 h-l), determine
CO2 absorbed
in NaOH
solution cool in flowing The catalyst
air
was sulfided
of dimethyldisulfide period
before
starting
the feed
tests at 773 K the atomic platinum
amounts
(Pt+Re)s
value of rhenium
Before
in the 3rd, 8th and 10th test by continuous
(corresponding
to 0.2 wt% S in the catalyst)
injection.
containing
from the 5th test was burnt-off
catalysts
the conditions chemisorbed
to Sirr /PtS ? 0.5 in accordance
the 6th test of each series
(corresponding
Under
ratio of irreversibly
with
exceeds
the catalysts
to the catalyst).
of the catalytic
sulfur
to surface
[6]; however,
the Sirr/
0.5 [15].
were
and Ccl4 was injected
to 0.4 wt% Cl addition
addition
over a 5 min
reactivated;
the coke
into the air stream
MCP
n-C6
96.8 99.6 96.8 99.8 83.2 96.1
98.3
99.3
98.0
99.9
91.0
99.2
92.3
99.2
83.3
93.4
n-C7
98.4
10
8
3
Feed
99.8
10'
Pt 8' 8
99.1 99.8
3
99.6 99.9
99.2 99.9 98.0 99.5 84.0 98.3
93.2 99.5 69.9 97.8
8
3
97.7
8'
Pt-Re 3'
99.8
(3, 8, 10).
3'
Pt-Ir
(3', 8', 10') and with sulfidation
8'
(%) without
3'
Conversions
TABLE 2
94.3
98.8
99.0
65.1
92.0
97.7
79.5
99.8
91.1
97.6
99.4
3
3'
99.8
10
10'
97.1
79.3
99.1
97.4
99.7
99.3
8
8'
Pt-Re-Cr
91.8
79.5
98.4
96.4
10
10'
103
reactivation
sulfidation
sulfidchon
sulf idation
I I
I
Pt
---
20.
Pt-Re
-.-- Pt-Re-Cr 1
2
3
4
5
6
test FIGURE
1
Benzene
formation
7
8
9
10
12 -
11
J
sequence
from n-hexane
as a function
of test sequence.
RESULTS Figure
1 presents the benzene yields in n-hexane conversion
the test sequence
for the catalysts
3,8,10)
as well as reactivation
between
the benzene
with
increasing
yields
number
To investigate
Pt, Pt-Re and Pt-Re-Cr.
(test 6) enhances
of sulfided
of sulfidations
the influence
an average
value
of strong
in each case following
sulfidation.
Comparison following yields
2 shows
sulfidation.
catalysts.
in n-heptane
detectable
Figure
compared level.
in Figure number
typically
catalysts.
behaviour
we compared
experiments,
decreased,
in Table
thus 2 show
is reached
sulfidation.
in aromatic
selectivity
2 verify
the levelled
in the case of rhenium
of sulfided
without
product;
decreased
containing
conversion
following
(tests
The difference
of tests
selectivity
experiments
greatly
a higher
aromatics
catalysts
presulfidation
after sulfidation
of
about
is clear 100 components
the number
and only the proportions
of
of toluene
increased.
3 illustrates
the formation
and subsequent
of aromatics
increasing
in the reaction
hydrocarbons
and hydrogen
on catalyst
2 shows an increase
The improved In
preceding
yield
The results
conversion.
be detected
samples
in the case of rhenium
self-poisoning,
2 and Figure
(cf. x and xi) with
containing
could
the enhanced
of Table
Sulfidation
formation.
of tests 2 and 4 (Table 2). Results
that, under conditions
Figure
and non-sulfided
of presulfidation
tests 3,8 and IO with the respective 3' denotes
benzene
as a function
the remarkable
of higher
to the other
aromatics.
catalysts,
drop
in both toluene
Pt-Ir shows superior and this,in
demethylation
dealkylation
the sulfided
state,
and
activity
is at a lower
104
feed :n-C,
Pt
s
Pt-lr
Pt-Re
Pt-Re-Cr
60
feed:n-Cs
E
IJJ 40 0 ; 0 :
20
3
8
10
3
Pt
8
10
3
Pt-Re
8
10
Pt-Re-Cr
80 feed:MCP 60
3
FIGURE
2
Aromatics
8
Pt
10
3
q
yields.
8
10
3
Pt-Re
without
a
10
Pt-Re-Cr
sulfur
(tests
3', 8', IO');
w
with
sulfur.
The influence ethane
ratios
of sulfur
(n-hexane
(Figure
3), the decreasing
imposes
the effect.
Figure coking
5 summarizes
is decreased
methylcyclopentane situation
on hydrogenolysis
tests,
Figure
selectivity
the amounts
by sulfidation conversion
is observed
4). with
may be demonstrated
increasing
of deposited
carbon.
(with exception
coking
when n-hexane
by the methane/
As in the case of demethylation
is increased
number
of tests super-
In the n-heptane
tests
of the Pt-Ir catalyst); by sulfidation.
in
An intermediate
is the feed hydrocarbon.
DISCUSSION Activity
and selectivity
The important aromatization
result
activity
of these
experiments
and selectivity.
is the considerable
An increase
in inherent
improvement activity
in
caused
105
0,
!ij0.15' 0 < Q)
z
O.lO’
z
," 0.05.
L-
i
3
810
3810
38
Pt-Re
Pt-lr
Pt
38
Pt-Re-Cr
feed:
3
810
3
Pt
FIGURE
3
version.
Dealkylation 0
without
3
8
and formation sulfur
810
Pt-Re
Pt-lr
of higher
3
8
Pt-Re-Cr
aromatics
n
(tests 3', 8', IO');
in the n-heptane with
1feed
1I 8
10
Pt
FIGURE 3'9
4
Methane/ethane
8’ and 10');
l
Pt-Re
ratios with
in the n-hexane
sulfur.
n-C7
i
con-
sulfur.
: n-C6
8
Pt-Re-Cr
tests.
0
without
sulfur
(tests
106
I
1.2 :
ifeed:n-C,
r
0.8 Od
r
Pt
Pt-lr
Pt-Re
Pt-Re-Cr
Ii 1.2 a. -m ;;; 0.8 u ;
feed:n-C6
0.4
:
3
8
10
3
8
10
-PtIRe.-
Pt
Pt-R&d;
II-II
eed:MCP
3
8
10
3
Pt
FIGURE
5
IO');
Carbon m
contents
cannot
It can be assumed function
chemisorption
sulfided
0
sulfur
withou
(tests
is known as a catalyst
with non-sulfided
of bifunctional
of sulfur
catalysts
is very small
The total coke content
Interpretation
component,
3', 8' and
[3]. Under
[15]. Coking
such as coking
Smaller
in the coked
almost
reaction
state.
entirely
conditions
of the metal
at the beginning
be used to characterize
of coked metal
et al. [11,18,19]
in sulfided
The dilution
[11,19].
cannot
catalysts
is restricted
poison
to H2S
in reforming of the
the deactivation
as will be shown later.
of the activity
of Sachtler ensembles
Re-S species. structure
8
is far more rapid than that of the support
run [16,17].
platinum
3
Pt-Re-Cr
since sulfur
that the influence
on the support
state of the metal
hypothesis
of the catalysts.
be proposed,
13-71. We have to compare
catalysts
10
with sulfur.
by sulfidation
the metal
8
Pt-Re
Pt-Re
also occurs ensembles
via C, fragments
which
catalysts
surfaces
states
by dilution
of larger with
inert
Pt-S, but in a less rigid
mean that structure-sensitive
are suppressed
on the
that the number
is reduced
in the system
is based
[lQ,ZO].
reactions,
From the results
it follows
107
I
50
60
80
70 %
FIGURE
6
3+4 MCP=/CH=
l
without
A
after
n
with
sulfur
ratios
reactivation sulfur
for Pt;
over sulfided
and dehydrocyclization dehydrogenation The classical comprises hexene
0
0
n
@
exhibit
dealkylation
high activity.
dehydrogenation
mechanism
must
catalysts
by sulfur,
result
(Figure
metal
of n-heptane
catalyzed
is in
cyclization activity
accordance
occurs
deposits.
selectivity
is needed.
sulfidation It follows
conversion
(Figure
6).
of methylcyclopentane
isomerization
ratio,
[Zl].
to cyclo-
If the acidity
as is shown
containing
by the sulfided
of the metal
that the atomic
surface
established which
is much more (carbide-like)
of
of a
by 5. Davis
catalysts.
because
et
This
the dehydro-
site [23]. To sum up, the improved
discussion
of platinum
platinum
hypothesis,
for non-destructive
It is well
- hydrogenolysis
non-acid
samples)
as a consequence
as was demonstrated
atom as active
A more detailed
properties
(over sulfided
can be understood
with the ensemble
samples
protection
carbonaceous
on the catalytic
MCP=/CH=
C6 ring closure
at one metal
of presulfided
by the effective
to be true for the
of hydrogenation-dehydrogenation
of the dehydrocyclization
al. [22] for the case of sulfur assumption
are
dehydrogenation
6).
and especially
predominant
MCP=,
to benzene
the improvement
in an increased
The high selectivity n-hexane
(test 3).
paths,
is shown
of the aromatization
to methylcyclopentene
1,2,4,5,7).
(test 6),
and ring rupture
reaction
This
CH= at acid sites and dehydrogenation
activity
for Pt-Re-Cr
but the desired
(tests
for Pt-Re-Cr
by the data from methylcyclopentane
bifunctional
is not influenced
A
m
hydrogenolysis,
in the MCP tests.
for Pt-Re-Cr
for Pt-Re;
for Pt-Re;
catalysts,
activity
of conversion
for Pt-Re;
for Pt;
that the rates of multiple retarded
as a function
for Pt;
1 IO
90
conversion
reactions
against
deactivation
to understand
that coking
suppressed
by
their superior
exerts
is qualitatively
carbon
can be explained
an influence
similar
to
than dehydrogenation
formed
in the early
[24-271.
stages
of
108
the run acts on the platinum samples
system
of the diluents
Pt-C contains
platinum
the coked platinum smaller
carbon
773 K sulfur
cannot
between
platinum
sulfides
is a highly
whereas
sulfur
relatively
pretation
between
Pt-S bond
of sulfur
results
of
of on platinull
dilution
on
Pt-Sn whose
(super-lattices).
(on platinum) diluents.
The
influence
and sulfur.
linearly
the
of sulfur
[32,33]
cannot
and the inter-
assertions
Probably
about
the electronic
than the geometric
bonded
CO varies
[35]. For this reason
which,
to aggregate,
Considering
lead to contradictory
platinum
vibrations
adsorbed
Pt-C on the one hand and Pt-S and
the electronic
hypothesis
of at
The chemical
by the existence
of sulfur
of the work function
[22,34]
Clearly,
with the system
of carbon
of CO is suppressed,
is based on the ensemble
on
model
gives a very efficient
and efficient
between
in its stretching
interpretation,
feature
is in any case of less importance
bridge-bonding
(if at all)
the systems
[31,32],
relation
carbon
areas.
by inter-metallic compounds
In fact, measurements
of catalytic
documented
it is analogous
hand is the tendency
the donor-acceptor influence
is well
[30], which
respect
the
there are islands
solid phase as does carbon.
or tin act as stable
strong
be ruled out.
on the surface
and a typical
is characterized
difference
1283. Carbide-like
are not known. A realistic
distributed
structure
in the
equilibrium
by S. Davis et al. [29] is that besides
and sulfur
In this
the non-sulfided
difference
In thermodynamic
sites as well as graphite-covered
(PtS, PtS2),
Pt-Sn on the other
Nevertheless,
is a decisive
and graphite
carbides
form a separate
symmetrical
there
and sulfur.
developed
widely
platinum
scale.
phase diagram essential
surface
platinum
affinity
the atomic
platinum
residues
free contiguous
carbon
only platinum
is metastable;
as a diluent.
because
are far less selective,
efficiency
while
surface
in our opinion,
one;
only slightly
our discussion
provides
a satisfactory
to a first approximation.
Coke formation The most
striking
is the suppression conformity tests.
with
Because
result
concerning
of coke formation
the influence
[12], an acceleration hydrogenolysis
of coking
activity
the interpretation
formation
on the basis of hydrogenolysis
the reaction (formation formation process.
[12,36]
that over sulfided
path of coking with
of alkylcyclopentanes restricts
Analogous
of methylcyclohexane
coking, results
toluene
becomes
the great degree
or olefins). toluene
were found
coking
selective.
of coke deposition
of sulfur
should
on total coke
occurs
selectively,
be via side reactions
The relatively
of strong
by sulfidation,
of the catalysts.
high rate of toluene
does not contribute
by Coughlin
but, in
the methylcyclopentane
C6 ring closure
the feed n-heptane
under conditions
highly
behaviour
catalysts
on coking
conversion
is lowered
of the influence
because
tion of Pt and Pt-Re catalysts
during
in both cases
we reject
If we accept
of presulfidation
in the case of n-heptane
much
to this
et al. [8] in the dehydrogenatios'
self-poisoning
is suppressed,
because
viz, after the conversion
We do not have a satisfactory on the sulfided
explanation
Pt-Ir catalyst.
sulfidato for
109
n El
_r *.0i
1.3
I+2 MCP” FIGURE
7
Carbon
MCP (tests tests. , 2
content
as a function
4 ST)without
Pt-Re-Cr'(ieit'6).
sulfur
/
A
after
with sulfur
3+4 MCP’
1
of the 1+2 MCP==/3+4
for Pt;
n
1.4 -
0
MCP= ratios @
for Pt-Re;
reactivation
a
for Pt;
for Pt;
0
in the
for Pt-Re-Cr
for Pt-Re;
A
q
for Pt-Re;
for
for Pt-Re-
Cr (test 3).
In the case of methylcyclopentane precursors form coke), causes
conversion
of the methylcyclopentadienes are formed
an enhanced
sites. Also, is heavily
on the main
concentration
the MCP==/MCP=
reaction
equilibration
/3+4 MCP= ratios
sulfided Figure
samples
(i.e.,
Barbier
low hydrogen
activity
desired
importance
pressure,
reaction
direct
C6 ring closure
in the case of the
dehydrogenation
the MCP==/MCP=
is related
during
of n-hexane
important.
depending prevails.
n-hexane
Indeed,
Therefore,
on whether
dehydrogenation
Hence
leads to the the advantages
the special
and the acid function.
conversion
is not clear.
than with
n-heptane,
in the 90 to 95% conversion coking
enhanced
at
from a practical
sulfidation
(of the support).
is less favoured
conversion
to the actual
seems essential
of both the metal
function).
ratio and the extent
that for the cyclopentane
improvement,
1.4 to 0.7 wt% MCP have been found. by sulfidation
are observed
between
coking
surface of course
(cf. the variation
in the MCP conversion
on coking
routes also become
tarded
are reached
a matter
conversion
of an exact adjustment
C6 ring closure
Since the metal
in n-heptane
by accelerated
activity
7).
ratios
coke formation
to
"drain off" via the acid
it is by no means
This relationship
in that whereas
The role of sulfur direct
correlation
all round selectivity
are accompanied
in Figure
et al. [36] verified
of the catalysts.
point of view
cannot
equilibria
MCP==/MCP=
tendency
path. A high dehydrogenation
in the case of the protected
7 shows the direct
of coking. very
the highest
have a strong
is favoured.
pressure
that the hydrogenation-dehydrogenation
As expected,
(which
of MCP= which
coked under atmospheric
in the I+2 MCP
the methylcyclopentenes,
MCP==
can be accelerated
MCP==
formation
The so other range or re-
or increased
110 On the influence Figures
of repeated
last tests of the series maintained
which
of aromatics
yields
metal
decreasing
yields
an activity
sulfur
at 8 and 8' caused
pressure,
that the metal
coverage
is surpassed).
reduction
about under
its content Moreover, with
surface
During
reaction
an
adsorbed
The increasing
with
portion
Probably,
From this
irreversibly
chemisorbed
(the optimal
sulfur
the Pt catalyst
[4,6,381
under conditions
high
H2S partial
reduction.
sulfate
of sulfate
et al. C371 by means
in the sense of an activity
pressures
of sulfidic
in test 10 (with
reducibility
of Pt-Re
greater
in a relatively
for a considerable
out by Engels
sensitivity
catalysts
that in the
amount
leads to higher
of slow sulfate
is not sufficient
was pointed
We presume
enhancement
the three presulfidations
sulfur
is operative
of higher H2S partial
(cf.
conversion,
and Pt-Re-Cr
This results
is not only covered
conditions.
be mentioned.
the Pt-Re
activity
test 3, because
0.15 wt% which
the enhanced
Pt should
for x',
MCP conversion
leads to a correspondingly
sulfur which
with a reversibly
in the catalyst
this phenomenon
in the pre-
reactivation
test 9 a considerable
but a restricted
with
but additionally
which
of sulfate.
owing to the additional
sulfur,
accumulates
during
is
This levelling
yields
In the n-hexane
after coke burn-off. with
by reduction
in comparison
it follows
of sulfur,
beginning
and selectivity,
presulfidation)
detectable
(increasing
by Cl addition
on that trend.
on the support
experiments
has been formed
activity
in the
is observed.
the amount
accumulation
"sulfur-free"
conversion
with the Pt and Pt-Ir catalysts
about double
sulfate
by presulfidation.
in the case of the bifunctional
is superimposed
situation
catalysts:
a high level of aromatization
increased
n-heptane
for x) whereas
maximum
In comparison adsorb
sulfidation
in the tests x and x' is clearly
catalyzed
2, Pt-Re)
intermediate
without
is not appreciably
dominantly
Figure
sulfidations
1 and 2 show a striking feature of the Re containing
in comparison
of strong decay
with of TPR.
self-poisoning
only
in the case
(test 10).
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1: J.M.