(AI 3+) centers of Pt/AI203 and Pt-Sn/AI203 catalysts, ... and in the isomerization rate of heptane, respectively. ... various catalysts against its formation. Coke is ...
React. Kinet. Catal. Lett., Vol. 41, No. 2, 277-282 (1990)
DEACTIVATION
OF R E F O R M I N G METALLIC
N.M.
Ostrovskii,
E.M.
Department
COKE F O R M A T I O N
AND A C I D I C
CENTERS
Chalganov,
Yu.K.
Kolomytsev Omsk
CATALYSTS.
and O.B.
of I n s t i t u t e
Demanov,
ON
Yu.N.
Bogomolova of Catalysis,
Omsk,
USSR Received July 28, 1989 Accepted October 12, 1989
To c h e c k (AI 3+)
coke
formation
centers
of Pt/AI203
the a p p l i c a t i o n detecting bonds
Pt~
(2070 cm -I)
of active
Pt ~ and A 1 3 +
that
in the d e h y d r o g e n a t i o n
and
in the
~ns
KOHTpOnS
AI203
UeHTpOB
HcHonbaOBaHHe
(2070
CM -I) H A I 3 + - C O qHc~a
SKTHBHbIX
CKOpOCTH
H3OMepHBa~HH
The m a i n is coke
of coke
support
(2190
to c o r r e l a t e
of c y c l o h e x a n e respectively.
(Pt ~
Pt/AI203
H KHCH Pt-Sn/
N~-cneKTpOCKOnHH CBSae~
CM-I).
~eHTpOB
~eFH~pHpOBaHH~
reason
catalysts
(2190 cm -I)
a~cop-
Pt~
HOKaSaHO,
qTO CHH-
Pt ~ K o p p e ~ H p y e T
C yMeHb-
I~HK~OFeKCaHa , a A13+
FeHTaHa.
formation.
the nature various
rate
MeTannHqecKHX
CO c ~ e T e K T H p O B a H H e M
~eHHe
is shown
KaTanMsaTOpOB
CO for
in the number
rate of heptane,
3aKOKCOBaHHS
O6OCHOBSHO
6HpOBaHHOFO
meHHeM
centers
catalysts,
of a d s o r b e d
Decrease
with
(AI 3+)
and acidic
and A I 3 + - C O
substantiated.
isomerization
(Pt ~
and P t - S n / A I 2 0 3
of IR s p e c t r o s c o p y
has been
nOTHbIX
both
on m e t a l l i c
for the d e a c t i v a t i o n The a v a i l a b l e formed,
against
(AI203)
its its
of r e f o r m i n g
literature
localization formation.
and m e t a l s
has m a n y and
Coke
(Pt, Pt-Re,
catalysts data about
stability
is d e p o s i t e d
Pt-Sn).
of on
Nevertheless,
Akad~miai Kiad6, Budapest
OSTROVSKII e t a ] .
: REFORMING CATALYSTS
the l i t e r a t u r e direct
blocking
acidic
(AI 3+)
contains
almost
no q u a n t i t a t i v e
of m a i n
active
sites
and s p e c i f i c
to Pt 2+ and w h i c h Besides,
exist
the m a i n
those
involved
in reforming)
In the p r e s e n t using
state
(Pt~ is close
s y s t e m [i]. solved,
namely
of w h a t r e a c t i o n
and to w h a t e x t e n t
investigated
of a d s o r b e d
dehydrogenation
whose
the
so
(among
is d e a c t i v a t e d
center?
study we have
IR s p e c t r o s c o p y
hexane
has not been
to the q u e s t i o n
of a p a r t i c u l a r
about
that are m e t a l l i c
centers,
in the P t ( C I ) / A I 2 0 3
problem
far we have no a n s w e r
by b l o c k i n g
(P~)
data
this p r o b l e m
CO and also k i n e t i c s
and h e p t a n e
of cyclo
reforming.
EXPERIMENTAL TO r e c o r d
IR spectra,
dried
samples
in v a c u u m
directly
vessel
were
drogen
at 773 K for 1 h and then a g a i n
773 K for 1 h. S p e c t r a were recorded
in the r e a c t i o n
at 393 K for 1 h, r e d u c e d
of CO a d s o r b e d
on a S p e c o r d - 7 5
treated
by hy-
in v a c u u m
at
on Pt ~ and A 1 3 + c e n t e r s -I cm and T=293 K
IR at 1 8 0 0 - 2 3 0 0
and 77 K, r e s p e c t i v e l y . The a c t i v i t y hexane
of f r e s h l y
hydrogenation
prepared
was d e t e r m i n e d
at 0.i M P a and 553-613
K with
and c o k e d
samples
in a g r a d i e n t l e s s
simultaneous
studies
in cyclo reactor
of k i n e t i c s
like in Ref.[2]. Heptane reactor
dehydrocyclization
same c o n d i t i o n s Catalyst 0.36%
7-A1203
surface
with
were
were:
278
in a flow isothermal
covered
Centers.
in coked,
m2/g
w i t h the coke
in 02 a d s o r p t i o n
and V ~ = 0 . 6
Measurements
supported
present
Under
certain
catalysts
[5]. Only CO c h e m i s o r p t i o n
C3-
Sn s u p p o r t e d
on
cm3/g.
of the a c c e s s i b l e by t r a d i t i o n a l
difficulties.
and H 2 t r a n s f e r
the
formed.
C1 - 0.5% Pt, C2 - 0.4% Pt,
02 and H 2 and the H 2 - O 2 t i t r a t i o n results
studied
K like in Ref.[3].
Sn and C4 - 0.3% Pt + 0.22%
SBET=I80-230
Active
tion m e t h o d s tion
catalysts
samples
Pt + 0.27%
Metallic
was
at 1.0 MPa and 808-823
metal
chemisorp-
Due to coke p a r t i c i p
[4], the a d s o r p t i o n
do not p r o v i d e
of
reliable
[5] can be c o n s i d e r e d
as
OSTROVSKI et al,: REFORMN I GCATALYSTS reliable m e t h o d
to measure
Dehydrogenation
coke-free
of naphthene
on metal centers of catalysts formation
platinum.
is known to take place only
for reforming.
Hence d u r i n g o c o k e
a c o r r e l a t i o n m u s t be o b s e r v e d between
of the intensity
the decrease
(measured at the band maximum)
in the IR
spectra of CO adsorbed on Pt ~ (2070 cm -I [6])
and the catalyst
activity
is c o n f i r m e d
in c y c l o h e x a n e
both q u a l i t a t i v e l y
dehydrogenation.
and q u a n t i t a t i v e l y
sults given in Fig.
This
by the e x p e r i m e n t a i
ci
3.0
b)
o~
e - C = 0.85%
Q)
1.0
re-
la.
o
Jo
~-C = 0.13%
o
2.0 C~
0.6 0
'- tO
0.2 0
I
i
0 C
Fig.
r
0
T
i
8
2.0
1.0
(wt.~
I. D e a c t i v a t i o n
I
t
of Pt ~ centers.
a) Decrease of free Pt ~ (~) and of relative constant
for c y c l o h e x a n e
dehydrogenation
growing coke c o n c e n t r a t i o n b) Reaction kinetics r - reaction
I
I
12 16 PEH (kPa)
rate,
rate
(aD) with
(C).
on coked catalysts
at 553 K.
PCH - partial pressure
of
cyclohexane
This permits centers
to suggest that coke only blocks
and does not change
experiments
them qualitatively.
active
Special
on coked Pt, Pt-Re and Pt-Sn catalysts
kinetic
confirm
279
OSTROVSKIi et a l . : REFORMING CATALYSTS
this suggestion. equation
A type of the kinetic
curve
(Fig.
ib),
its
[2]
r=k PCH #/ (l+b PB ) z
y=l - PBzP~2/KpPCH ,
and also values
of the activation
energy
heat of benzene
Q are independent
of the coke concentration,
which evidences
qualitative
freshly prepared values were: AI203
identity
kJ/mol
increase
the nonuniformity
centers
For Pt/AI203
and Q=147•
does its deactivation
thus indicating
of active
and coked catalysts.
E=80•
E and chemisorption
kJ/mol.
on
the measured
Only on Pt-Sn/
E from 109•
to 134•
of their active
centers
kJ/no] for
dehydrogenation. Aprotic
Acidic
also measured
Centers
bond
coked catalysts
causes
intensity
proportional centration
is possible
reactions
kinetic
on AAC.
AAC,
to
of the
(y), which
is
on the coke con-
catalytic
properties
since none of them takes
participation
of the supported
of
place
metal [9].
step of some of these
In accordance
with Refs.
are likely to be isomerization
cyclohexane
dehydrogenation,
period of catalyst Hence
coke
operation (Co
(Fig.
they,
[i0-i13,
and dehydrocyc-
data should be carried
however,
in the flowing
2a))
out only
of spectral
need a
raw materi-
partly blocks
in this case a comparison
ing y and rate constants
280
of this method
2a.
is to determine
only when the limiting
al. The accumulated and AAC.
CO [8] with monitor-
of paraffins.
Unlike certain
in Fig.
of AAC A13+ are
The dependence
of coke-free
reactions,
proceeds
these reactions lization
no difficulties.
is repesented
only on AAC without
Extension
for the band at 2190 cm -I
to the portion
AAC in reforming
of adsorbed
(2190 cm-l).
Much more difficult
This
Concentrations
using spectoscopy
ing the AI3+-CO
relative
(AAC).
both Pt ~ and
for C > Co, normaliz-
of isomerization
and aromatization
OSTROVSKII et al.: REFORMINGCATALYSTS 1.0 ~ a )
o -C2
1.0
9 -C3 9 - C4
0.8
A
C3
b)
0.8
,z. D
0.6
0.6
0./.
0.4
I I I I
0
Fig.
I
1
I
I
3 C(wt?/o) 2
4
5
0
[
2
I
f
5
3 C(wt,%)
2. AAC-AI 3+ deactivation. a) Dependence
of protion
concentration b) Variations
of relative
in Fig.
in heptane
(aA) on
at Co:
aA=kA/k ~
activities.
of these parameters 2b for catalyst
C3.
range for a I variations
well with a decrease isomerization
values
a i = K i / k ~,
Here a I and a A are relative
the confidence
rate constants
8 =u
(k I and k A) by the c o r r e s p o n d i n g
The dependence
(y) on coke
(a I) and aromatization
Pt-Sn/AI203.
6=WY O,
of free AAC
(C).
isomerization
are given
I
1
in the Portion
in its reforming
on coke concentration It is easily
seen that
correlates
sufficiently
of free AAC.
Thus heptane
can be treated
as a test reac-
tion for AAC deactivation. Dehydrocyclization
rate
than that of isomerization,
(aA) decreases although
than the rate of dehydrogenation, If one suggests
a bifunctional
[12],
taking
its steps
taking
mechanism
place on metal
1.5-2 times
faster
to a much lesser degree place only on Pt ~ of d e h y d r o c y c l i z a t i o n
and support
have compar-
281
OSTROVSKII et al .: REFORMING CATALYSTS
able rates. On the other hand, tivity of catalysts
as has been shown in Ref.
in heptane
aromatization
to the number of their Pt Y centers. suggested
compared to Pt ~ and a test reaction
adequate
In this case P ~
to possess higher stability
Pt Y centers can be heptane conclusions,
come d i f f i c u l t i e s
for the d e a c t i v a t i o n
it is n e c e s s a r y
in m e a s u r i n g
can be
against coke formation
aromatization.
tion steps of d e h y d r o c y c l i z a t i o n
[i], the ac-
is p r o p o r t i o n a l
of
To obtain more
to e s t a b l i s h what reac-
take place on P ~
and to over-
the number of Pt Y in coked cata-
lysts.
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282