Catalyst Deactivation 1987, Proceedings of the 4th International Symposium

646 Pages·1987·33.291 MB·iii-v, 1-661\646

by  B. Delmon and G.F. Froment (Eds.)

Studiesin SurfaceScienceand Catalysis
AdvisoryEditors:B.DelmonandJ.T. Yates
Vol. 34
CATALYST DEACTIVATION
1987
Proceedingsofthe 4th International Symposium, Antwerp,
September 29-0ctober 1, 1987
Editors
B. Delmon
Universite Catholique deLouvain, Louvain-Ia-Neuve, Belgium
and
G.F. Froment
RijksuniversiteitGent, Gent, Belgium
ELSEVIER Amsterdam- Oxford- NewYork - Tokyo 1987
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Catalyst deactivation 1987.
(Studies in surface science and catalysis ; vol. 34)
"Symposiumon Catalyst Deactivation"--Pref.
Bibliography: p.
Includes index.
1. Catalyst poisoning--Congresses. I. Delmon,
Bernard. II. Froment, Gilbert F. III. Symposium
on Catalyst Deactivation (4th : 1987 : Antwerp,
Belgium) IV. Series: Studies in surface science
and catalysis ; 34.
TP156.C35C379 1987 660.2'995'0289 87-19638
ISBN 0-444-42855-0
ISBN0-444-42855-0(Vol.34)
ISBN0-444-41801-6(Series)
©ElsevierSciencePublishersB.V., 1987
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PREFACE
This Symposium on Catalyst Deactivation came after those organized in differ-
ent styles successively in Berkeley (1978), Antwerp (1980) and Berkeley again
(1985).
For the present symposium, the emphasis was laid on three topics: the tech-
niques used in deactivation studies, the mechanisms of catalyst deactivation,
and modelling. With respect to the first, it became apparent that the study of
deactivation faces even more difficulties than the characterization of fresh
catalysts and the measurement of activity or selectivity. This is due to the
multiplicity of interacting processes occurring during deactivation. It was hoped
that these points would be clarified in the course of the symposium. Quite sub-
stantial progress has been made recently in the understanding of the mechanisms
of various processes, particularly coking. It was therefore desirable to accord
more time to these topics during the symposium. The third topic corresponds to a
problem which is very central to development studies and to the chemical engineer-
ing aspect of catalysis: it deals with the representativity of accelerated 5ests
and the modelling of the deactivation phenomena.
We are very grateful to the members of the Scientific Committee whose names
appear below, for their help in the difficult task of selecting, from the many
submitted contributions, the papers which are assembled in this volume.
Universite Catholique de Louvain BERNARD DELMON
Rijksuniversiteit Gent GILBERT F. FROMENT
Co-chairmen
SCIENTIFIC COMMITTEE MEMBERS
Professor M. BAERNS, Ruhr-Universitat Bochum, Germany
Professor E.G. DEROUANE, Facultes Universitaires N.D. de la Paix, Namur, Belgium
Dr C. GUEGUEN, Elf France, St Symphorien d'Ozon, France
Dr W.D. MROSS, BASF AG, Ludwigshafen, Germany
Dr N. PARKINS, British Gas Corporation, London, Great Britain
Dr. J. ROSTRUP-NIELSEN, Haldor Tops¢e A/S, Lyngby, Denmark
Professor J.B. UYTTERHOEVEN, Katholieke Universiteit Leuven, Belgium
Professor R.A. VAN SANTEN, Koninklijke/Shell Laboratorium, Amsterdam, The Nether-
lands
B.Delmonand G.F.Froment(Editors),CatalystDeactivation1987
©1987ElsevierScience PublishersB.V.,Amsterdam- PrintedinTheNetherlands
COKING OF REFORMING CATALYSTS
J. BARBIER
Universite de Poitiers, U.A. CNRS 350
40, Avenue du Recteur Pineau
86022 POITIERS - FRANCE
ABSTRACT
The purpose of this review is to link experimental worKing conditions of
reforming catalysts to the quantity, the chemical nature, the location and the
toxicity of deposited coke. Adiscusslon of the effect of the metallic and aci-
dic functions brings out that coking is a baianced reactlon between production
and destruction of COKe precursors, nucleation, growth and gasification of more
ordered carbon deposits.
INTRODUCTION
Metal based catalysts are used to promote a variety of reactions involving
carbon-bearing feedstocks. and coke is deposited on most catalysts. The coking
of catalysts containing noble metals has long been of interest, primarily as a
result of the industrlal importance of reforming process. Most studies nave
oeen carried out using a Pt/A1203 reforming catalys~. In sucn catalys~s, lt lS
well estaolished that total deactivation, as a result of coking, takes many
~housands of hours of operation. As a matter OT fact the selectivity of reTor-
wlng catalysts for coking reaction is low, since only one atom of carbon out of
200.000 activated by the catalyst is transformed into a non desorbaole oeoosit
of coke under the operating conditions used (1).
fhe thermodynamics of the reforming reactions are such tha~ it is deslrable
co work at high temperatures and low pressures (1). but tnese are the conoi-
Clons tnat favor coke formation. In recent years it nas been found possible ~o
use multimetallic catalysts supportea on alumina to promate reformlng. Conblna-
tlon of Pt-Re, Pt-Ir, Pt-Sn and Pt-Ge have been reported and are now used wide-
ly in industry (3)(4). The essential contribution of such catalysts nas been
greater stability in time. It never~heless can be seen tnat the staoilizing ef-
fect of additives has not yet received a simple or general explanation. The ad-
ditives used. however. can at least be arouped lnto two types (5):
i) additives such as rhenium ana iridium which really dlminish coke deposi~
rate
ii) additives such as germanium and ~in, for which the coking rate seems at
least equivalent, if no higher, than that observed wlth platinum alone.
2
It stands to reason that producing less coke under the reaction conditions
is not enough to counteract the effects of coking. Indeed the location of this
coke and its nature play an important role on its inhibiting effects in various
catalytic reactions.
The purpose of this paper is to study the modification of the quantity, the
location, the nature and the toxicity of coke induced by a change of the expe-
rimental conditions (temperature, pressure), or by a change of the nature of
the catalyst (support and metallic phase (dispersion, alloying)). Finally the
effect of these different parameters on the cOking reaction will be discussed
in relation to a described coke build up mechanism.
I) Location, composition and structure of carbon deposlts on bifunctional cata-
lysts :
I-i) Location
Coke may be removed by gasification with oxygen. The temperature programmed
combustion (TPC) of coked catalysts shows two oxidations, one at around 300°C
and the other at around 450°C. These two peaks are particularly well resolved
when platinum is supported on non acidic and non microporous alumina. Neverthe-
iess if such is the case, a low oxygen pressure durlng the TPC analysis, a'ilows
a good resolution of the two peaks (figure 1).
B
·C
100 200 300 400 500
Figure 1 iemperature programmea oxidation of coked Pt/A1203
catalyst
(A) C02 production \B) 02 consumption.
3
Astudy of these peaks shifting with an increasing temperature shows that
the activation energy of coke combustion is equal to 10 kcal for the first peak
and 15 kcal for the second one.
Many materials are known to catalyse the gasification of coke, and these
include several metals. So it has been found that the low temperature combus-
tion is due to the presence of coke on the metallic phase (6) (7) (8). As a
proof, when Pt/Si02 catalyst and pure alumina are coked as a mixture and analy-
sed apart by TPC experiments, coke deposited on Pt can be oxidized at 260°C
when coke deposited on alumina is oxidized at 550°C (figure 2). Such a diffe-
rence can be explained by assuming either that platinum catalyses the oxidation
of carbon or that coke deposited on the metal 1S different than coke deposited
on the alum1na.
15
B/....'
10 I \
I \
I \
I \
I \
I \
5 I \
/ \
/ \
'" ,
/ \ ,
o4.r.........~~rTTT~r'T'TT~;mrm.,....."'T"'TTT"T"T""TfTrrri"rrrr1
100 200 300 400 500 T"C
figure 2 Temperature programmed oxidation of coke
(A) deposited on Pt
(8) deposited on A1203.
The oxidation of coronene deposited on a Pt/A1203 catalyst proves the cata-
lytic part played by the metal (figure 3).
4
15
10
5
O+'-,.,..,..,.,..,..,.,..,..M'1'1=rnTTTTTTTTTTTTTTTTTT1"TT"rrrl'TT1rrnnT'l~T"-rr1
100 200 300 400 500 600 Toe
Figure 3 Temperature programmed oxidation of coronene deposited on
Pt/A1203·
In conclusion, distinction between coke deposited on the metallic surface
and on the support can be carried out by temperature programmed combustion. The
amount of coke deposited on the metallic phase of a bifunctional catalyst can
be determined by combustion at moderate temperature (300°C).
1-2) Composition
Measuring the oxygen consumed and the amount of carbon dioxide produced du-
rlng temperature-programmed combustion indicates that the coke deposited on
Pt/A1203 corresponds to the formula CHx' Table 1 shows that coke accumulated on
the metal is less dehydrogenated than coke deposited on the support.
catalyst coking conditions H/C
Platinum black cyclopentane + 10% of 1.05
cyclopentene (4000Cl
Chlorinated A1203 cyclopentane + 10% of 0,5
cyclopentene (400°C)
Table 1 Comparison of the H/C ratio of coke deposited on pure Pt and pure
A1203 catalysts.
5
When the coking time increases the hydrogen content of coke decreases (ta-
ble 2). Comparison of H/C values obtained on coke deposits with those of crude
oils (H/C =1.1-1.6) and of coals (H/C =0.6-0.8) shows that coke is between
po1yaromatics and coals.
Time of %C H/C
coking/min
15 0.61 0.70
60 0.84 0.65
135 1.53 0.63
300 2.53 0.55
600 4.64 0.39
Table 2 : Evolution of the H/C ratio with the coking time
1-3) Structure
Extraction of coke with various organic solvents, after dissolving the
inorganic matrix, allows a chemical analysis of the extractable compounds. It
has been shown that whatever the experimental conditions and the coking agent
may be, extractable coke consists essentially of polyaromatic compounds with a
possible branched chain like methyl, sometimes ethyl and uncommonly C3 or more
than C3 groups.
The unextractab1e coke can be analysed by X-ray diffraction. Table 3 shows
that ~uch deposits are composed of pseudo-graphitic phases with crystallogra-
phic characteristics very close to that of pure graphite.
Characteristics of coke graphite
aggregates
d(A) 3.45 3.35
Lc (A)
Thickness of 17
aggregates
La (A)
diameter of 100
aggregates
N(mean number
of sheets) 0
Table 3 characteristics of unextractab1e coke
6
II Influence of the nature of the catalyst on the amount of coke deposited
Abifunctional catalyst is defined by the nature of the metallic phase and
by that of the support.
11-1) Influence of the nature of the metallic phase:
The amounts of coke deposited on the metal can be determined by combustion
at moderate temperature (300°C).
Figure 4 shows the evolution of the variation of the number of carbon atoms
deposited on the metal per accessible atom of platinum, as a function of the
time on-stream, during the reaction of cyclopentane at 400°C on various cata-
lysts of variable dispersions. The curves obtained show that in the very first
moments, when the catalyst is brought into contact with hydrocarbon, the metal
attains a coke coverage that will remain constant, whereas the support conti-
nues to be charged with coke.
C
..
Pt
.
•
6
0 n 0
0 c;:
0
~ i
4 ~ '•;I * :~
•
% 0
0= 0.1% If= 1.0% e= 1.91% 0= 3.6% b. = 4.69%
2
...=6.59%.. =8.7%
o 2 3
Figure 4 : Effect of the time on stream on the coke coverage of the metallic
phase for different Pt/A1Z03 catalysts of different metal dispersion.
On the other hand the number of carbon atoms deposited per accessible metal
atom varies with the dispersion of the metal. Small platinum particles show a
greater resistance to deactivation by coke than do large particles (9). In the
7
same way, Lankhorst et al. (10) showed that the Pt/Si02 catalysts with a low
metallic dispersion are more sensitive to auto-deactivation than are well dis-
persed catalyst. This result is in agreement with that of Somorjai and co-
workers (11)(12) who proved that coke can settle more easily on planes than on
the corners and the edges of metallic crystallites.
The most remarkable progress that has been made in improving the staDility
in time of reforming catalysts is due to the use, with platinum, of different
additives like Re, Ir or S. Figure 5 shows the evolution of the amounts of co-
ke deposited on Re or Ir or S modified Pt/A1203 catalysts as a function of
time-on-stream during coking by cyclopentane at 400oC. The curves obtained show
that sulfur will promote the accumulation of coke whereas iridium and rhenium
have the reverse effect.
%C
4
o 3
Figure 5 Effect of the modification of Pt by different additives
on the coke formation
Pt/A1 (e), Pt(S)/A1 (0).
203 203
Pt-Re/A1203 (*). Pt-Re(S)/A1203 (~).
Pt-Ir/A1 (0), Pt-Ir(S)/A1 (A).
203 203