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Development of supported manganese oxides catalysts for partial oxidation: Preparation and hydrogenation properties
13
Applied Catalysis,28 (1986) 13-33 Elsevier Science Publishers B.V.. Amsterdam - Printed in The Netherlands
DEVELOPMENT
OF SUPPORTED
PREPARATION
AND HYDROGENATION
Miguel
Alvin
A. BALTANASa,
Center
for
University
Catalytic
3000 Santa 'Mobil
(Received
6. STILES and
- Newark,
FOR PARTIAL
and James
R. KATZERb
Tecnnology,
OXIDATION:
Department
Tecnologico
para
Corporation,
1985, accepted
of
Chemical
Engineering,
I!.S.A.
To whom correspondence
and Development
5 December
CATALYSTS
De. 19716,
de Desarrollo
Fe, Argentina.
Research
OXIDES
PROPERTIES
Science
of Delaware
- Instltuto
aINTEC
MANGANESE
la Industria should
Guemes
345n
be addressed.
Paulsboro,
28 August
Quimica,
N.J., U.S.A.
1986)
ABSTRACT Supported manganese oxide catalysts were prepared by impreonating manqanese salts Mn(OHj2 in situ usinq ammonium onto TiOp, CeOp, A1203 and MgA1204 and precipitating hydroxide. X-ray diffraction and ion scattering spectroscopy showed that the preparation procedure left well-dispersed overlayers on the support surfaces without formation of separate crystalline Mn oxide phases. The catalytic activity for ethylene hydrogenation varied by over three orders of The changes are a function of the support and are magnitude on all the catalysts. apparently related to the reducibility of the surfaces, the hydrolysis of Mn-support bonds and/or structural changes in the MnOx overlayers.
INTRODUCTION The understanding
of catalysts
lyst
structure
and
ture
of Oxides
always
and cations of
a
bination
of
of different
methanol
oxidation
supports,
varied
widely,
model
where
under
of the
Additional
capable
and because
oxidation
oxidation
for oxide
we
and
controlled of measuring
have
and
different
reactions
consist
requires
a com-
[1,2]. reactions
manganese
oxides
of the
-such
on dif-
supports
conditions,
individual
strucgroups,
catalyst
properties
reproducible
of cata-
hydroxyl
hydrogenation
prepared
chemical
knowledge
The surface
anions,
viz. it is polyfunctional
catalysts
physical
implies
reaction.
of oxygen
selective
formaldehyde-
the
oxidation
catalyzed
were
and we have
catalyst
properties
[3].
paper
focuses
MnOx at submonolayer ties
a
of sites, novel
carefully
reactions
or functions This
to
vacancies,
steps,
types
of the
of a combination
anion
to develop
ferent
used
with
elementary
In an effort as
consists
associated
series
for selective
of the mechanisms
on the coverage
resultant
reaction
0166-9834/86/$03.50
preparation on different
catalysts
studies
using
to test
and
characterization
supports
ethylene
highly
dispersed
and on the hydrogenation
hydrogenation
the deep oxidation
0 1986 Elsevier Science Publishers B.V.
of
as a model
and partial
oxidation
properreaction. activi-
14 ties of these ture
of
which
catalysts
the
are
surface
catalyst
as spectroscopic
surfaces
reactants
under
as well
or
and
products
reducing
and
the
of
structure
oxidation
oxidizing
studies and
to characterize
bonding
reactions,
conditions
are
of
the struc-
simple
molecules,
adsorbed
on the
subject
of-other
the
catalyst papers
C4,81.
EXPERIMENTAL
Materials All obtain
and Catalyst
supports materials
deleterious
Preparation
were
synthesized
of high
promoter
type) were prepared
effects
decomposition
preparation
of aqueous
preparation
through
of
a 35 mesh
incipient
wetness
of manganese the
Trial
incipient Since
with
supports
(Tyler)
sieve.
range;
diluted
(NH4)OH,
nitrates
prepared
K with
criteria
experiments
for
Titania
(anatase)
(A type)
MgA1204
outlined
Alp03
synthesized
obtained
Further
above
material
a dilute
(R
by
by neutra-
details
were
was then
on the
solution
the
were made to determine
of Mn(N03)2.
appropriate
the exact
first
are
uptake
by the
An amount
which
amount
liquid
screened
impregnated
of MnO, on the Support
estimating
and
to any
CeO2 was prepared was
was
(NH4)$03.
or salts
to minimize
[S].
The -35 mesh
at 298
seeking
isopropoxides.
and
as
to give an overlayer
compounds
purity,
Al203
with
are given elsewhere
five
added
impurities.
bicarbonate.
of their
technique
was
monolayer
below.
the
by
of highly
cerium
Al(N03)3
solution
of the supports
Aliquots
caused
of
of the metal
area and very high
by hydrolysis
by thermal
lizing the acidic
by hydrolysis
surface
was below discussed
which
caused
wetness. manganese
nitrate
surface
of the grains
the Mn
ions
were
upon
could
migrate
out
drying
giving
a high
precipitated
in situ,
of the
in the
pores
and
concentration pores
of the
precipitate
on the
of Mn at the surface, particles
using
NH40H,
before drying. After
the
supports
mixed with glass
were
was admitted
cations
the drying.
geneties
hours.
vacuum-NH3
method
in the
Mn-impregnated The dry
system.
the drying
The process
This drying
to a rotary
to the
of multiple
unagglomerated,
with
the Mn solution,
beads to avoid the formation
which was then attached NH3
impregnated
Mn
moist
admission
flask
distribution.
were
uniform
was
Vacuum
cycles.
When
was put in a warm
took about
solid
of lumps and placed
drier.
The precipitation-drying
4 hours
did not work with
powders
This gave apparently
vacuum
then
powders bath
were
and then moist involved
became
appli-
loose
and
(333 K) to accelerate
for each catalyst.
under
calcined
the
solids
into a round flask
was applied procedure
water
Ti02 because
Therefore, dried
the moist
it always
for the vacuum
at 473
inhomo-
of MnO,/Ti02,
at 353 K in a vacuum
for 2 hours
Mn distribution.
led to visual
preparation
K under
oven
the
for 48
flowing
air.
15 Apparatus
and Procedures
X-ray diffraction. to
for
80'
intensity with The
[9]
powdered
Ion
and
built
The finely
on
voltage
of
beam
1.5
surface
KeV
The was was
after
o.o4 ,I.75 =
Z
=
Corporation)
slides.
their
peak
diffractometer using
Cu powder.
To determine
equation
to
the
was applied
support
to the most
of
an
special
into wafers,
a CMA voltage
improve
positioned
the
was
signals
sur-
mounted
on a
being
introduced
of producing
designed a stable
at 6 = 90' and a channel-
1.5
Ti-Mn
interest their
kas a laboratory
capable
ion gun,
gun
the
values
relative
;
height
KeV.
A
Ne ion
resolution
for
beam
with
a
MnOx/Ti02.
The
were at room temperature,
and
were achieved.
oxides
was
determined
from derivative
to oxygen,
energy
as suggested
by
computing
spectra
the
atomic
and empirical
by Wheeler
scat-
[lo]:
Co
intensities
minimizes
(1)
of the metal
and oxygen, and oxygen,
respectively; respectively;
M (oxygen is assigned
an arbitrary
of one). surface
roughness
effects
that
are always
present
in pressed
[ll].
Ethylene
hydrogenation.
tial flow microreactor the
presseo
10m5 tort-. All measurements
of
of
[HICM
cross-section
Powders
were
samples
by ISS to determine
number;
atomic
method
catalyst
The instrument
r
gaseous
detector. 02
increase
at 473 K for 2 hours before
c”,c; = atomic concentration of the metal M g = relative cross-section of the metal
and
in the range 2e = 10'
was calibrated
analyzed
ions,
ion
steady-state
sections
IM,I~ = peak
gas
He
composition
,M =
were
powders
composed
used
using peak-to-peak cross
Three
(Iss).
ground
of noble
in the chamber
ratios,
of
[niano
Scherrer
of the spectrometer.
analyzer.
data were taken
This
recorded
(-50 urn) to
microscope the
sizes,
and then evacuated
ISS spectrometer
energy
tering
were
mesh
Electric
and MnOx/A1203,3)
chamber
mono-energetic
The
325
The diffractometer
mounted
spectroscopy
ISS holder,
the main
vacuum
were
MnOx/CeO2
composition.
tron
General
radiation.
samples
scattering
standard into
XDS-5
patterns
to
lines.
(MnOx/Ti02, face
an
CuKa
ground
over-layer crystal
MnOx
intense
samples
using
Ni filtered
and/or
X-ray diffraction
powdered
Matheson).
U.H.P.
studies
in the range 473-673
components
A schematic were
The hydrogenation
was
of the grade
done
with
experimental
(Matheson);
Gases were purified
by passing
were
carried
K and at 1 atmosphere a HP set-up
the
5750
was
oxygen
pressure.
chromatograph
is shown
ethylene
through
gas
out in a differen-
in Figure research
traps
Analysis
using
a TC
1. The H2, He purity
(CuO activated
(99.98% for 4
16
-
I
BY Pass
L-
Molecular sieve Hz0 trap CuO oxygen trop Rotameter w/ NRS volve Detail of the Pyrex reactor Fine metering valve CAJON@ water injection FIGURE 1 hours
Experimental
at 573
Water was
injected
stainless
steel Cajon
Catalyst (conversion
was
using
The experimental
perature
catalytic on
reoxidation
the
rC2H6 Fp
activity
of
catalyst;
measuring
(1) the effect
effect
reaction
except
where
= molar
5A) cartridges. using a standard
conditions
catalyst
charge
cat./h. reduction of the
catalyst;
(3)
standard
temperature
reduction the
and water
on the reaction under
reactor
Typical
of the effect
of reoxidation
rate
tem-
effect
of
addition
on
rate. These effects conditions
after
indicated.
were determined
Fgxi/W
= ethane
(2) the
of temperature
rates at low conversion
flow
in hydrogen.
He-preactivated
(4) the
the
to the reactor
was lo-13 m3 STP/kg
catalyst;
fresh,
sieve
septum.
differential
of 5% ethylene
determined:
had been achieved
The reaction rC2H6 =
program
(molecular
prior
under
space velocity
of fresh
by
hydrogenation.
feed just
determined
and (5) the effect
determined
steady-state
into the
activity
on used
used catalyst; were
for ethylene
a stream
was 500 mg; the volumetric
on the
port
vacuum tee with a silicone
activity <5%)
5
Hz) and water traps
flowing
on-line
7”
T Vent 01 Flow meters
apparatus
K under
-4
from the following
relation: (2)
formation
rate, kmol/kg
flow rate of reactant
cat./h;
i, kmol/h;
17 TABLE
1
Pretreatments
for atmospheric
Pretreatment
Specimen
Description
Reduction
5
w/hydrogen
II ,I
C
01 ,I
Activation
2 A
w/pure
Reduction ,I
B C
I,
D
Reoxidation
w/oxygen w/pure
In the out
first
at the
conducted increased carrying
from
the
the desired
673 K
2h
473 K
2h
573 K
2h
helium
helium
573 K
2h
(500 ue)
573 K 473 K
2h
i;
to
sequences
catalysts at which
were
were
prereduced,
He
(6~10~~
m3
pure
H2
during
the
for 8 hours
applied
the'catalyst
in
out the reaction
obtaining
2h
kg.
hydrogen zero
4h
573 K
w/hydrogen
same temperature using
473 K
2h
pretreatment
sequence,
2h
2h
of reactant
Two experimental
573 K
473 K
of water
Reduction
weight,
2h
473 K
w/pure
Addition
= catalyst
2h
w/oxygen
Activation
W
573 K 673 K
w/hydrogen
Reoxidation
= conversion
4h
II
Reduction E
473 K
helium
w/hydrogen ,,
Activation
xi
hydrogenation
Code
1 A
Specimen
ethylene
which
half
catalyst reaction
reduced.
The
assured
rate data, the catalyst
and the
was
STP/h). last
to each
carried were
concentration
was
of pretreatment.
achieving
was reduced
was
1).
The reductions
hydrogen hour
(Table
steady-state
After
and after
at the next temperature
in
the sequence.
In the second 573
K
before
completion 1% oxygen
sequence
undergoing
in helium. were
Following
the study
again
reoxidized
During
activated
catalysts
the
of Step C (Table
catalysts
ua
the
same
activated
were
again
of ethylene
in pure
studies
He before
hydrogenation In this
in flowing
procedure
reoxidized
the last half hour a stream
and activated.
the reaction
first
reduction-reaction
l), the catalysts
pulse of water at 573 K before After
were
as
at
After of
was used. The
at 473 K with
at this temperature,
case the catalyst
before.
first with a stream
of pure oxygen
reduction
helium
hydrogen.
the catalyst
was contacted
was
with a 500
reduction.
at 673 K were completed,
the temperature
dependency
of
properties
of impregnation:
298.9
0.91
240.7
2.61
473 K/2 hr
298K; vacuum drying
which was grey-brownish
where 353 K employed. (W. Germany).
Color was dark brown in all cases except MnOx/Ce02
Ebach
temperature
c
Laboratorien,
K in all cases except MnOx/Ti02
wetness;
250.8
From Analitische
at 298-343
Method of preparation:
107.3
0.29
3.63
4.15
1.61
473 K/2 hr
473 K/2 hr
473 K/2 hr
Yes
Yes
MnOx/A1203,B
MnOx/A1203,A
Yes
MnOx/MgA1204
Yes
MnOx/Ce02
prepared
b
a
incipient
1.13
;;l;o;;3afK;;rb,c 2
86.5
5.22
% Mn (A.A.)~
B.E.T. (N2) S.A. (m2lgram)
473 K/2 hr
No
MnOx/Ti02
of the catalysts
Calcination
NH3 treatment
Catalyst
Physical
TABLE 2
FIGURE
2
calcined
X-ray diffraction
patterns.
at 473 K; C) MnOx/Ti02,
A) Ti02
calcined
, calcined at 673 K; B) MnOJTi02,
at 673 K.
49” 368
C
FIGURE
3
calcined
X-ray diffraction
patterns:
at 473 K; C) MnOx/Ce02,
A) Ce02,
calcined
calcined
at 673 K.
at 673 K; B) MnOx/Ce02,
20 TABLE
3
X-Ray diffraction
patterns
of supports
and catalysts
Crystallite Size, i 106
GO
Remarksa
Small amount
of orthorhombic
Same pattern
as Ti02,
More crystallized
Ti02 or Ti0224U
no Mn-related
anatase;
peaks
indication
of some amount
of MnTi04
(haussmanite)
82.8 107
Some Ce(C03)2 Some Ce(C03)2 Manganese
causes
Bayerite
(AI(OH)3)
y-A1203 24
Some y-AlO broad
peaks,
Weak residue Bohmite-
45
No Mn-related
MS-Al-O,
synthetic
II
of y-AlO(
one weak
aluminium
phase related
composition spinels
oxide"
to y-Al203
is a monoclinic
to the spine1
is unknown.
of molecular
50.6
Also peaks from Mg5A1204
73.7
Both systems
is a I:1 ratio
No indication
of Mn-related
tetragonal
peak assigned
peaks
"magnesium
intermediate chemical
hydroxyde-
II
50
- U = centered
peaks
no Mn peaks
oxide
II
42
39.1
broad
peaks
- Bohmite-peaks
BUhmite-aluminium
45
and Bdhmite
no manganese
peaks
y-A1203
48,6
band broadening;
It is observed
ratios
. 15 H20 peaks
structure.
Mg0:A1203
metastable Its
only for larger
than 1:2.5
21
Sample
Treatment
Ti02
Calcined
673 K, 14 h
MnOx/Ti02
Calcined
473 K, 2 h
Patternb
II
CeO,
0,
II
II
673 K, 14 h
CeO,
Calcined
573 K, 14 h
CeO:
MnOx/Ce02
Calcined
473 K, 2 h
CeO;
I,
Calcined
673 K, 2 h
Ce02
Al,O,, L
J
typeA
Dry
Amorphous
(D<20i) (D<2Oi)
II
Calcined
598 K, 14 h
Amorphous
11
Calcined
773 K, 14 h
Y-A1203
Calcined
473 K, 2 h
Amorphous
MnOx/A1203,
A
A1203, type E
y-AlO
Dry
II
Calcined
598 K, 14 h
y-AlO
,I
Calcined
773 K, 25 h
n-Al 203
Calcined
473 K, 2 h
y-AlO
Calcined
673 K, 2 h
n-A1203
MnOx/A1203,
B
II
Calcined
,I
II
MnOx/MgA1204
a Arabic
Amorphous
Dry
MN1204
numeraks
b Ref. 35.
Ti0212U
,I
Amorphous
Dry Calcined
Anatase,
673 K, 14 h
Mg-Al-O,
amorphous
Calcined
773 K, 14 h
II
Calcined
923 K, 16 h
MS-Al-0
Calcined
1073 K, 16 h
Mg-Al-0
Calcined
1273 K, 14 h
MS-Al-O,
MgA1204
Calcined
473 K, 2 h
MS-Al-O,
amorphous
after formulae
stand for the number
II
of atoms
per unit cell
FIGURE
X-ray
4
MnOx/AI203,B,
reaction
diffraction
calcined
rate was determined.
at each temperature mation
After
was increased
temperature.
achieved
A)
First,
Al$l3,
These
independent
and again
experiments
of temperature
showed history
B,
calcined
steady-state the
and that
59B
stepwise
and the desired
at the lowest
that
at
K;
B)
at 673 K.
was decreased
was established
data was obtained stepwise
type
calcined
the temperature
until a steady-state
was achieved.
temperature each
patterns:
at 473 K; C) MnDx/A1203,B
desired
and held
rate infor-
temperature,
the
rate data were obtained same
there
steady-state was
rate
no measurable
at was
aging
observed.
RESULTS
AND DISCUSSION
Preparation A summary
and Structural
2. The surface
areas
same
as
of
MnO,
was
mostly
those
accounted
anatase
impregnation area
of
the
calcination
Characterizations
of the physical
properties
(B.E.T.)
the
for.
(Figure
measured
supports The 2).
X-ray
patterns
(Tables
2 and
3).
prepared after
is given
in Table
Mn addition
were the
addition
when
the
indicated
that
the calcined
changes
were
TfD2 had the largest The MgA1204
weight
observed
crystallites spine1
was
of the
in the
Ti02
and lowest formed
added
titania
was
after
surface
only
after
at 923 K.
The type A Al203 was amorphous nation
for the catalysts to Mn
No significant
and recalcination. supports
prior
of the catalysts
at 598 K, but reverted
bohmite;
to bayerite
the type B Al203 was bohmite during
the Mn impregnation
after
(Figure
calci3). The
23 TABLE
4
Uniform
slab thickness
% Mn before
firing
of hypothetical
at 673 K
Mn(W2
overlayers
of Mn
TiO2
Ce02
A1203,A
A1203,B
MgA1204
5.153
3.23
3.66
4.75
4.16
2.55
1.60
1.81
2.35
2.06
1.65
1.03
1.17
1.52
1.33
2.94
1.49
0.60
0.97
0.82
1.90
0.96
0.39
0.63
0.53
(m3/g of cat.108)a Mn02
Mn(OW2 (m3/m2 of cat.lOl'
=A)
Mn02
aFrom
average
sp. gr. [36]
(i-1)
-b
E 2.U Zdb 0 L. t.s5 2.55 .I T
2
CHAINS
HCP
(2-2)
c
336
FIGURE
5
Single
oxihydroxide,
chain,
double
and dihydroxide
chain,
polymorphs
and
layer
[37].
structure
of the manganese
oxide,
24 I
I
I
I
I
Ti
I
I
MnOx/TiOz
1
I
I
I
I
I
I
I
I
I
I
I
I
I
I
I
I
I
I
I
I
I
’
0 I)
Mn
Al
I 30
I
I
I
40
50
MnOx/Al203,a
60
I
I
70
80
I
90
I
100
170
E/EoxlOOO
FIGURE
6
ISS spectra
of selected
beam at 1.5 KeV. Spectra after
5 scans.
correspond
catalysts
evacuated
to steady
state
at 473 K. Conditions: values of intensity
Het
ratioes
25 TABLE
5
Atomic
ratios
from ion scattering
spectroscopya
CATALYST
M
O/M
O/Mn
M/Mn
MnOx/Ti02
Ti
3.49
7.33
2.10b
MnOx/Ce02
Ce
1.57
2.75
1.74
MnOx/A1203,B
Al
2.27
6.87
3.3
acalculated
revertion by
its
light
remained The
estimated The
involved at 673
yellow
to
infinite
which
assumed flat
catalysts support
a
added
slab
of the
(Figure
5). the
slab
the
model oxide.
of segregation
dispersion
Figure
6
summarizes manganese
shows values
the
that
the
formation
as it should
some
ratios
the
in Figure
be dehydrated
Pattern
water
(Figure
of
surface
6.
only
4), but
hydration
The average of Figure
was
still
specific
that
far
too
Further
Results
is
simple,
or agglomeration indicated
it
If the surface
or MnO2. the
semi-
overlayers area of the
unit area (m2/g) of each
are
within
summarized oxide the
indicates
phases
in Table
compound
4.
is uni-
submonolayer
no need
XPS [8] studies
to bulk
was
of Mn(flH)2 and
5, and the hypothetical
it
ESR [7] and
over
gravity
if the manganese
surface,
supports
of Mn(OH)2
deposited
by the total
thickness.
indicate
of the
of an overlayer
thickness
were divided
range
for multiple
were unable
to find
of MnO,. This very high
by X-ray diffraction
and ISS as shown by
ISS
They
2-6.
derivative
of the
calculated
surface
could
a Ce02
per gram of catalyst.
support
is
cover
uniform
of
intensity atomic were
the ratio,
ratios low,
and A1203 the oxygen-to-support
value,
evident
could
slab
of Mn was also
3 and 5 and Figures
steady-state
Ti02
on
of manganese
surface Tables
gave
support.
volumes
In all cases the calculations
any evidence
Mn
of
a hypothetical
distributed
bayerite
ceria
from the structures
was used, these to give
Although
was
as m3 of overlayer
formly
layers
it
the hypothetical
surface
expressed
the
that
Mn02 was calculated were
described
[12].
by assuming
model
and the
rehydration,
K. The calcined color,
ln the solid extent
conditions
1.5 KeV.
probably
recalcination
from
1, for the experimental
using equation
Ne+ ion beam,
bFrom
be for surfaces
intensity
usually
using
Equation
indicating cation with
ratio
data.
reached
(1). The
excellent is higher
OH groups,
after
correspond
5 scans. support
dispersion
of
to
Table
5
metal-toMn.
For
than the stoichiometric
even after
evacuation
at 473 K
and beam sputtering. On the MnO,/Ce02
the
oxygen-cerium
ratio was
below two which
is abnormally
low.
for ethylene
473 573 673 473 573 673 473 573
1A
1B
1C
2A
28
2c
2D
2E
4.42d
0.257
1.87
16.0d
163.b
3.51
0.72
1.89
MnOx/Ti02
1.69
6.31
2.57
1.58
7.41d
2.30
1.07
6.00d
MnOx/Ce02
pressure
4.73
o.688c
1.55
2.77d
1.91
1.26
1.78
0.383
MnOx/MgA1204
RATE (kmol ethane/kg
at atmospheric
REACTION
(5%) hydrogenation
16.8
16.6
2.41
3.37
7.70
1.92
3.29
9.58
MnOx/A1203,A
catalyst. h *104) a
10.6d
10.6
2.7Zd
5.54
2.00d
1.97
4.23
3.07
MnOx/A1203,B
z Values at pseudo steady state - as of eight hours onstream. Non differential conditions. i Reaction at 573 K. Transient value. * Pretreatments for protocol 1: 1A) H2 reduction, 4 h @ 473 K; 1B) H2 reduction, 2 h @ 573 K; 1C) H2 reduction, 2 h @ 673 K. Pretreatments for protocol 2: 2A) He preactivation, 2 h @ 573 K, then H2 reduction, 4 h @ 473 K; 2B) H2 reduction, 2 h @ 573 K; 2C) H2 2 H @ 673 K; 2D) 02 2 h @ 473 K, then He preactivation, reduction, 2 h @ 573 K, then H2 reduction, reoxidation, 2 h @ 473 K; 2E) 02 reoxidation, 2 h @ 473 K, then He activation, 2 h @ 573 K, then addition of 0.5ml H20 @ 573 K, then H2 reduction, 2 h @ 473 K.
(K)
Reaction Temperature
activities
Pretreatment
Catalytic
TABLE 6
S?
21 Whether
this
conditions not clear. of
was
a consequence
[12] or was
pure
Therefore, Ce02
samples
showed
stil i
suggestion Additional
XPS
either
surface
experiments
of
anion
by the heavy
0
to
confirmed
that
consistent
surface low
ratio
samples
with
reduction
0-Ce
U.H.V.
[IO] was
The control
ratios,
or the
under
of Ce atoms
studied.
Ce
vacancies
vacancies
matrix
of pure Ce02 were lower
anion
[8]
appearance
caused
control
powder
of
of the
an artifact
to
was
the
Ce203.
not
an
ISS
artifact. From all the results concluded each
that
of
the
supports
separate
crystalline
Ethylene
Hydrogenation
K there
673
K this
MnOx/Ce02 most
was
were
to
the
effect
almost
The
was
of these
higher
MnO,/TiO2
was
high
(close
further
in
lo-fold the
with
increase
first
because
were
no obvious
decreased
again
patterns.
for
the
activation
for ethylene
as compared This
in helium
after
hydrogenation
since
dilution
at
MnO,/Ti02.
The high
condition
was the
increased.
in activity
narrow.
at
but at
by H2 reduction
activity
true
e.g.,
of the high
with further activity than
was made
K. These a
very
with
[14].
reaction
rate
conversion
of ethylene,
was there
set of
activity After
and
reoxidation.
the sequence that
to a value reoxidation
the
the actito
reoxidizing,
catalytic
activity
of reoxidations
was made
reoxidation reoxidation
that
did
An attempt
by successively
by observing
that
and H2 reduction,
For the MnO,/A1203,
MnO,/Ti02
showed
activity,
catalyst.
doubled
levels.
work
experiments
low
reactivation
it was before
K, followed
at 473 K. In this
(nonactivated)
In the final
especially [13,14],
catalysts
the catalysts
at 673 K MnOx/Ti02
differences
differences
at 473
in catalytic
vity of the MnOx/Ti02
was
these
previous
changes
reducing
573 and 673
a catalyst
was
was even higher
for C2H4 hydrogenation at 473,
in
no other pro-
rise.
these and
There
to a differential
of the catalysts,
reoxidation
explore
of
or as
was investigated
among
hydrogenation
catalysts
However,
not bring them back to their
preactivating,
3-fold.
chromia
correspond
little temperature
vity after
oxides
For the nonactivated
in activity
remarkable
this
superior
to 90%).
The reoxidation
manganese
of catalysts
catalysts
observation
reductions
not
two
showed
catalysts;
temperature did
it is
on the surfaces
took place cleanly;
at 473 K, whereas
at 673 K their
be a common
by H2 produces
for very
to
to only
catalyst
catalysts
nonactivated
For
on the Series
difference
reduced
activity
He-activated
reduction
multilayer
of pretreatment.
a 25-fold
difference
seems
as
and ISS data,
dispersed
detected.
the effect
573 K, but upon H2 reduction The
present
of C2H4 to C2H6
was the lnost active
active.
not
diffraction
very well
range 473 K to 673 K. The reaction
6 summarizes
473
were
the X-ray
were
phases.
or side reactions
Table
including
overlayers
and
The hydrogenation the temperature ducts
obtained
the manganese
was
at 673
at 473 K resulted at
573
identical
K caused with
that
K, the catalytic
a of
acti-
over that of the 573 K reoxidation.
experiments
of the second
sequence
(Table 1) after
reoxida
7
8
1.6
1
\@I \
I
the reactor
the reactor
, 2.0
4 2.2
temperature).
on the rate of C2Hq hydrogenation
temperature),
B
?
5
100 1.4
2
4
5
6
7
8
101 9
using MnOJCeO2
using MnOx/Ti02
'3 d
s
I
0 r( . -
t
on the rate of C2Hq hydrogenation
1.8 . 10~ l/T
Effect of temperature
0 increasing
FIGURE 8
Cl\
5
Effect of temperature
0 increasing
FIGURE
1.4
100
5 -
2
MnOf/TiO
l/T
(0 decreasing
2.0
the reactor
the reactor
1.8 IO3 . (0 decreasing
1.6
temperature;
temperature;
2.2
29 TABLE
7
Activation
energy
at 673 K for ethylene
Activation Catalyst
hydrogenation
Energy
Correlation
a
E(kcal/gmol)
Coefficient
MnOx/Ti02
11.2 + 0.9
0.979
MnOx/Ce02
8.6 t 2.0
0.964
MnOx/MgA1204
5.4 -+ 0.4
0.977
MnO,/Al203,A
6.5 + b
MnOx/Al203,B
5.7 + 0.8
0.976
MnOx-Ce02
4.6 t 1.2
0.970
MnO,-MgA1204
5.6 t 1.8
0.762
aFor a confidence bOnly
interval
two temperatures
then
studies
reduced
revealed
673 K.
at 573 K, water
at 473
K and ethylene
the following
. For MnOx/A1203,
treatment
(500 us?) was added at 573 K. The catalysts hydrogenation
a sharp
The activity
increase
then slowly
after 4 hours on stream
. ,For MnOx/Ceo2a sharp decay in activity . For MnOx/Ti02 several
and MnO,/MgA1204,
hours,
cantly
higher
The temperature exemplified reaction
by
in
Table
are
behavior
possibly
perature
Bulk Khodakow
case
7.
apparent
With
the
(Figure
8).
(Table
These
catalytic
levels
occurred
of the non-
6).
activity
activity
rise occurred
remained
rate for catalysts
activation
exception showed
in the
activity
to the
over
at a signifi-
value.
from a linear
MnO,/CeO2
due to a change
is lowered
the
in catalytic
decayed
transient
of the reaction
The
satisfactory.
of
only
the
the
correlation
are
coef-
in its rate temperature
rate-determining
catalyst
hydrogenation
of the rate data)
MnOx/A1203,A change
reduced at 673 K iS
for
regression
a clear
reaction
It is the
energies
to show
step as the temthis
type
of non-
behavior. manganese
oxides
[I53 has reported
to 673
K) while
K.
this
In
7.
was evaluated.
was observed.
a sharp
steady-state
573 K (as calculated
ficients
typical
in this
dependence
Figure
above
summarized
but
(>8-fold)
activity
behavior:
A or B types,
with time on stream. hydrated
with a = 0.05.
around
tion and He preactivation were
___
MnO
study
are
poor
that Mn02
has an activity he concludes
that
catalysts
for
iS completely of 4~10~~ lower
the
kmol/kg
metal
hydrogenation
inactive
for this
h (5x10-10
oxides
are
of
olifins.
reaction
kmol/m2/h)
qenerally
more
(473 K at 473 active
than
the
y-Al203 some
ones
exhibiting
shows
small
at fJ3
X [&xID-~
activity
not active Our
for ethylene
results
become
behavior
"critical
the the
MnOx/CeO2
profile
that
vior
might
The
cally recover
does
their step
the different
supports
inactive:
and anatase rI?,IR>;
shows
Ce$
is
influence
Our reaction
temperatures
are
dissociation
of hydrogen
can
which
the
in ethylene
lack
data
of the
defines on
the
series,
[ZO], but
the main function adsorption
of
of the
the
olefin
with
a temperature-activity
step
(Figure
8). This beha-
mobile
OH- at higher
in rate-determining of Mn
hydrogenation
cations
with
below.
not bring
about
catalytic
regain,
is reoxidized
markedly
oxide species.
reoxidation
or
significant
activity.
even
reason for this may be associated catalyst
are also
for
exception
by
initial
to
supports kmn)}mjh)
the H2 that
we
of a change
be caused
reoxidation
reoxidation
to activate
as discussed
temperatures,
TX,
although
is the only
is typical
also
The
at 623 K [16],
ur I.IxID-~~
step on pure metals
[21-251,
itself.
that
of the manganese temperature",
the ability
catalysts
states. activity
kmn)}kgjh
demonstrate
rate-determining
it is probably oxide
oxidation
hydrogenation
hydrogenation.
patently
the catalytic above
higher
a very
exceed,
changes
initial
with the reducible
and activated
with
hwer,
MnOx/TiO2, its
and the catalysts
rewires
catalytic
nature
a 573 K
activity.
of the support.
He, a new hydrogen
typi-
pretreatment
The
Once the may cause
the reaction
l/2
H2 + CP2 + OH- + H-
to occur sence
extensively.
of both
did not
properties The
the
and
same
anion
dissociation
sites.
behavior
unsdturdteb
with
as MnO,Ti02
helium
at
573
might
would
also
be enough
reducible
its easier
pre-
oxide,
reoxidation
for MnOx/CeO2.
chemical
bti_ytmte-s %ve
in the
occur
another
be due to
K removes
as W&-I ax
j-ites.,
of H2 may
The fact that CeO2,
[26]. Thus a 473 K reoxidation
pretreatment
cb?n;itiVIS?I_Y support
A heterolytic
catIon
show
(3)
water
and
leaves
coor-
man_mnrieJe h_y%mX*lb?s mb ni-e
: OH
HO ‘Mn/
(4)
HO' HO
'OH OH
+
'x'
This effect tic activity
is remarkable reported
aL1.26 ta%a>ysts H2 activated The effect
&Lay
in Table \n
as shown by the enhancement
on MnOx/Ti02
6. At higher
artivib
to
a
reaction
IeqE{
tkat
temperatures is
of the catalythe He-preactivtQ
tkat
of
samples. of ~20 on the reaction
performance
is also revealing.
On ZnO and
t&
31
Hydrated
Dehydrofed
Heierolytic
Reductive
H2 adsorption
0
02- (fIrsi H+ H-
.
0
FIGURE
9
Changes
row )
I)
Mn"+
@
M”(“-l)+
x
in a MnO, overlayer
H2 adsorption
C2H4
on a support
basal plane,
under the influence
of H20, H2 and C7H4
y-Al203
a small
amount
of H2D
deactivation
cI6,27]
metal
the presence
oxides
dinatively
of
unsaturated
bonds,
ethylene
a monolayer)
hydrogenation
(CUS)
and
is enough
activity.
to
partially
1281, or to even hydrolyze
be due
reduction
to a decrease
in the activity
in CUS
performance
after the helium
H2 at 473°K
that
occupy
followed
hours
MnO, to
supported
"normal"
on
due
enhancement
after preactivation
anatase
or hydrolysis The
supported
MnOx/Ce+, perature,
C2H4 with
of the
bonds
different
hydrogenation lower
has also
with
been
are
activities observed
this
catalyst
OH- groups
effects
for
MnO,/Ti02
activity the
permanent,
perhaps,
pretreatments complex.
either
with
The
by Denisova
the
metal
oxide
improve
as H20 and
after
is far
reveals
of
several
catalytic
less
than
a decrease of manganese
supported
of
the
et al. on Cr2D3/Al&
the
in CUS ions
MnOx.
on the catalytic
behavior of
its with
preactivation.
decays
oxidation
values
might
reduction
enhancement but
hydrogen
singular
intermediate
as in
MUx clusters.
did not
of tne helium
initial
is
of coor-
catalyst,
that may desorb
-or both- for the alumina
at
the
after the H20 treatment
pure He. This again,
and,
transition
less reducible
high
pretreatment
catalyst
of the Al-0-Mn
effects
for
$0
supported
the total amount
the carrier/transition
that
surface
the
a complete
is then understandable:
In contrast
performance
on the
the
fact
any beneficial
alumina
values.
to
The
can produce
CUS so as to minimize
For
[30].
to cause
reoxidize
of MnOx/Ce+
pretreatment
On
shown to affect
as in the case of CrO,/.SlD2 [29], stabilizing
The observed
vity
of
of H20 has been
sites
the case of MoOx/A12D3
(-2%
acti-
MnO,/Tifl2 and reduction
[31].
tem-
They also
32 reported that, decreases Most
if starting
certainly
endothermic,
the
but
reaction
These
with
types
of
OH- groups
473
can be adsorbed
though
might
be reproduced.
above
temperatures
catalysts,
using
a simple
oxides,
K is not
on the
sup-
thus decreasing
the
-notably
occur
of
groups
(Figure
either
form
of manganese
chains
would of
MnDx/Ti02
and
at 673 K and this would
model
A1203
not produce Mn
atoms
for the catalysts.
of Mn over the preferenor Cd+,
with
mononuclear
[32,33]
ClOSe
species,
resembling
its
but own
5).
-pyrolusite
cations
structural
a deposition
such as anatase,
most certainly
structure
9. The figure
which
at higher
the rate steadily
cannot
H2 activation
of water
the
mobile
and
activity
activity.
overlayer
the first type of regular in Figure
solids
that was used involved
This method
isomorphic
dehydration,
different
some
is lowered,
catalytic
be deeper
of OH- covered
oxide and hydroxide Upon
On
method
plane
packed anions. an
the
can be summarized
The impregnation basal
will
for a higher
ideas
of
the evolution
reduction
a reoxidation
be responsible
rather
reduction
performance.
Mn02/Ce02-
low-temperdture
involves
ports [29]. The
tial
from 673 K the temperature
and the original
or
coordinated
ramsdellite with
oxygen
chainsanions.
will
has been used to model the MnO, OVerlaYer,
shows the surface
under one set of conditions
lead
to
For simplicity, as shown
among the many
that may be possible. This simple
model
gen and further
least two adjacent (c) and reductive
[Mnn+]2(02-)2
offers
reactions,
a description following
Mn cations,
of the heterolytic
the Burwell
model
each one with an anion
dissociation
[34], with
vacancy
of hydro-
involvement
of at
both after unreductive
(d) conditions:
+ H2 + C2H4 +
+ [Mnn+C2H4][Mn n+H-]02+ [Mnnt]2(02-)2
The formation
OH- +
(5)
+ C2H6
of a hydrolyzed
support
manganese
oxide
overlayer
or the reduction
of
the metal + [Mn (n-2)+]DH-
[Mn"+H-](O*-)
follows
directly
from the model.
ACKNOWLEDGMENTS This
work
was
Industry-University the Center
R&D
by
Cooperative
for Catalytic
ISS experiments Central
supported
were
Department,
Science
a
National
Research
Science
Program
Foundation
and by the
grant
Industrial
from Sponsors
the of
and Technology.
performed Wilmington,
by
Dr.
D.G.
Delaware.
Swartzfager G.C.A.
Schuit
of the
DuPont
contributed
Company, generously
33 through
many helpful discussions.
REFERENCES 1 2 3
a 9 10 11 12 13 14 15 16 17 :: 20 21 22 23 24 25 26
33 34 35
B.C. Gates, J.R. Katzer and G.C.A. Schuit, "Chemistry of Catalytic Processes", pp. 325-390, McGraw-Hill, 1979. G.C.A. Schuit, Int. J. Quantum Chem. 12, Suppl. 2 (1977) 43. PhD Dissertation, Univ. of Delaware, Dissertation Abstracts M.A. Baltanas, International 43(7) (1983) 2272-B. M.A. Baltanas, A.B. Stiles and J.R. Katzer, Applied Catal., in press. Ibid., Applied Catal., in press. (a) Ibid., J. Catal. 88 (1984) 362; (b) Ibid., J. Catal. 88 (19a4) 516. M.A. Baltanas, S. OeCanio, J.R. Katzer and C. Oybowski, Acta Chimica Hungarica, lla(4) (1985) -285. M.A. Baltanas, A.B. Stiles and J.R. Katzer, Acta Chimica Hungarica, accepted for publication. E.F. Kaelbe, "Handbook of X-rays", Chapter 17, McGraw Hill, New York,.l966. M.A. Wheeler, Anal. Chem. 47(l) (1975) 146. G.C. Nelson, J. Colloid Interface Sci. 55(2) (1976) 289. F.A. Cotton and G. Wilkinson, "Advanced Inorganic Chemistry", 3rd. ed., Wiley, New York, 1972. H. Schaefer and E. Buchler, Z. Natuurforsch 22a (1967) 2117. G.C.A. Schuit, private communication. Yu S. Khodakov, V.V. Zasypkin, N.I. Galan-Sanches and Kh.M. Minachev., Bull. of the Acad. of Sci. of USSR 26(7) (1977) 1550. S.G. Hindin and S.W. Welle, Adv. in Catal. 10 (195B) 70. D.L. Harrison, D. Nicholls and H. Steiner, J. Catal. 7 (1967) 350. R. Rukwei Kyi, PhD Dissertation, Columbia University, 1966. Kh. M. Minachev, Yu S. Khodakov, and V.S. Nakhshunov, J. Catal. 49 (1977) 207. J. Horiuti and K. Miyahara, 4th Int. Congr. Catal., Moscov (196B), Preprint 43. D.A. Dowden, N. Mackenzie and B.M.W. Trapnell, Proc. Roy. Sot., A237 (1956) 245. O.V. Krilov, "Catalysis by Nonmetals", Academic Press, New York, 1970. K. Tanaka and T. Okuhara, J. Catal. 65 (1980) 1. (a) R.P. Eischens, W.A. Pliskin and M.J.D. Low, J. Catal. 1 (1962) 180; (b) R.J. Kokes, Catal. Rev. 6 (1972) 1. P.P.M.M. Wittgen, C. Groeneveld, J.H.G.C. Jansenns, M.L.J.A. Wetzels and G.C.A. Schuit, J. Catal. 59 (1979) 168. (a) M. Breysse, M. Guenin, B. Claudel, H. Laetrille and J. Vernon, J. Catal. 28 (1973) 54; (b) Ibid. 27 (1972) 275. J.F. Woodman and H.S. Taylor, J. Amer. Chem. Sot. 62 (1940) 1393. J. Engelhardt, J. Catal. 62 (1980) 243. G.C.A. Schuit and N.H. deBoer, Rec. Trav. Chim. 70 (1951) 1076. T. Yamaguchi, N. Ikeda, H. Hattori, K. Tanabe, J. Catal. 67 (1981) 324. T.A. Denisova, K.P. Lavrovkii and A.L. Rozental, Neftekhimiya 8 (1968) 326. P.W. Selwood, T.E. Moore, M. Ellis and K. Wethington, J. Amer. Chem. Sot. 71 (1949) 693. R.B. Spooner and P.W. Selwood, J. Amer. Chem. Sot. 71 (1949) 2184. R.L. Burwell Jr. and C.J. Loner, Proc. 3rd. Int. Congr. Catal., hsterdam, 804 (1964). "Powder Diffraction File, Inorganic Compounds", J.C.P.D.S. International Center for Diffraction Data (1977). "Lange"s Handbook of Chemistry", 12th. ed., McGraw Hill, New York, 1979. W.F. Nye, S.B. Levin and H.H. Kedesky, Power Sources Conf. Proc. 13 (1959) 125.
Applied Catalysis,28 (1986) 13-33 Elsevier Science Publishers B.V.. Amsterdam - Printed in The Netherlands
DEVELOPMENT
OF SUPPORTED
PREPARATION
AND HYDROGENATION
Miguel
Alvin
A. BALTANASa,
Center
for
University
Catalytic
3000 Santa 'Mobil
(Received
6. STILES and
- Newark,
FOR PARTIAL
and James
R. KATZERb
Tecnnology,
OXIDATION:
Department
Tecnologico
para
Corporation,
1985, accepted
of
Chemical
Engineering,
I!.S.A.
To whom correspondence
and Development
5 December
CATALYSTS
De. 19716,
de Desarrollo
Fe, Argentina.
Research
OXIDES
PROPERTIES
Science
of Delaware
- Instltuto
aINTEC
MANGANESE
la Industria should
Guemes
345n
be addressed.
Paulsboro,
28 August
Quimica,
N.J., U.S.A.
1986)
ABSTRACT Supported manganese oxide catalysts were prepared by impreonating manqanese salts Mn(OHj2 in situ usinq ammonium onto TiOp, CeOp, A1203 and MgA1204 and precipitating hydroxide. X-ray diffraction and ion scattering spectroscopy showed that the preparation procedure left well-dispersed overlayers on the support surfaces without formation of separate crystalline Mn oxide phases. The catalytic activity for ethylene hydrogenation varied by over three orders of The changes are a function of the support and are magnitude on all the catalysts. apparently related to the reducibility of the surfaces, the hydrolysis of Mn-support bonds and/or structural changes in the MnOx overlayers.
INTRODUCTION The understanding
of catalysts
lyst
structure
and
ture
of Oxides
always
and cations of
a
bination
of
of different
methanol
oxidation
supports,
varied
widely,
model
where
under
of the
Additional
capable
and because
oxidation
oxidation
for oxide
we
and
controlled of measuring
have
and
different
reactions
consist
requires
a com-
[1,2]. reactions
manganese
oxides
of the
-such
on dif-
supports
conditions,
individual
strucgroups,
catalyst
properties
reproducible
of cata-
hydroxyl
hydrogenation
prepared
chemical
knowledge
The surface
anions,
viz. it is polyfunctional
catalysts
physical
implies
reaction.
of oxygen
selective
formaldehyde-
the
oxidation
catalyzed
were
and we have
catalyst
properties
[3].
paper
focuses
MnOx at submonolayer ties
a
of sites, novel
carefully
reactions
or functions This
to
vacancies,
steps,
types
of the
of a combination
anion
to develop
ferent
used
with
elementary
In an effort as
consists
associated
series
for selective
of the mechanisms
on the coverage
resultant
reaction
0166-9834/86/$03.50
preparation on different
catalysts
studies
using
to test
and
characterization
supports
ethylene
highly
dispersed
and on the hydrogenation
hydrogenation
the deep oxidation
0 1986 Elsevier Science Publishers B.V.
of
as a model
and partial
oxidation
properreaction. activi-
14 ties of these ture
of
which
catalysts
the
are
surface
catalyst
as spectroscopic
surfaces
reactants
under
as well
or
and
products
reducing
and
the
of
structure
oxidation
oxidizing
studies and
to characterize
bonding
reactions,
conditions
are
of
the struc-
simple
molecules,
adsorbed
on the
subject
of-other
the
catalyst papers
C4,81.
EXPERIMENTAL
Materials All obtain
and Catalyst
supports materials
deleterious
Preparation
were
synthesized
of high
promoter
type) were prepared
effects
decomposition
preparation
of aqueous
preparation
through
of
a 35 mesh
incipient
wetness
of manganese the
Trial
incipient Since
with
supports
(Tyler)
sieve.
range;
diluted
(NH4)OH,
nitrates
prepared
K with
criteria
experiments
for
Titania
(anatase)
(A type)
MgA1204
outlined
Alp03
synthesized
obtained
Further
above
material
a dilute
(R
by
by neutra-
details
were
was then
on the
solution
the
were made to determine
of Mn(N03)2.
appropriate
the exact
first
are
uptake
by the
An amount
which
amount
liquid
screened
impregnated
of MnO, on the Support
estimating
and
to any
CeO2 was prepared was
was
(NH4)$03.
or salts
to minimize
[S].
The -35 mesh
at 298
seeking
isopropoxides.
and
as
to give an overlayer
compounds
purity,
Al203
with
are given elsewhere
five
added
impurities.
bicarbonate.
of their
technique
was
monolayer
below.
the
by
of highly
cerium
Al(N03)3
solution
of the supports
Aliquots
caused
of
of the metal
area and very high
by hydrolysis
by thermal
lizing the acidic
by hydrolysis
surface
was below discussed
which
caused
wetness. manganese
nitrate
surface
of the grains
the Mn
ions
were
upon
could
migrate
out
drying
giving
a high
precipitated
in situ,
of the
in the
pores
and
concentration pores
of the
precipitate
on the
of Mn at the surface, particles
using
NH40H,
before drying. After
the
supports
mixed with glass
were
was admitted
cations
the drying.
geneties
hours.
vacuum-NH3
method
in the
Mn-impregnated The dry
system.
the drying
The process
This drying
to a rotary
to the
of multiple
unagglomerated,
with
the Mn solution,
beads to avoid the formation
which was then attached NH3
impregnated
Mn
moist
admission
flask
distribution.
were
uniform
was
Vacuum
cycles.
When
was put in a warm
took about
solid
of lumps and placed
drier.
The precipitation-drying
4 hours
did not work with
powders
This gave apparently
vacuum
then
powders bath
were
and then moist involved
became
appli-
loose
and
(333 K) to accelerate
for each catalyst.
under
calcined
the
solids
into a round flask
was applied procedure
water
Ti02 because
Therefore, dried
the moist
it always
for the vacuum
at 473
inhomo-
of MnO,/Ti02,
at 353 K in a vacuum
for 2 hours
Mn distribution.
led to visual
preparation
K under
oven
the
for 48
flowing
air.
15 Apparatus
and Procedures
X-ray diffraction. to
for
80'
intensity with The
[9]
powdered
Ion
and
built
The finely
on
voltage
of
beam
1.5
surface
KeV
The was was
after
o.o4 ,I.75 =
Z
=
Corporation)
slides.
their
peak
diffractometer using
Cu powder.
To determine
equation
to
the
was applied
support
to the most
of
an
special
into wafers,
a CMA voltage
improve
positioned
the
was
signals
sur-
mounted
on a
being
introduced
of producing
designed a stable
at 6 = 90' and a channel-
1.5
Ti-Mn
interest their
kas a laboratory
capable
ion gun,
gun
the
values
relative
;
height
KeV.
A
Ne ion
resolution
for
beam
with
a
MnOx/Ti02.
The
were at room temperature,
and
were achieved.
oxides
was
determined
from derivative
to oxygen,
energy
as suggested
by
computing
spectra
the
atomic
and empirical
by Wheeler
scat-
[lo]:
Co
intensities
minimizes
(1)
of the metal
and oxygen, and oxygen,
respectively; respectively;
M (oxygen is assigned
an arbitrary
of one). surface
roughness
effects
that
are always
present
in pressed
[ll].
Ethylene
hydrogenation.
tial flow microreactor the
presseo
10m5 tort-. All measurements
of
of
[HICM
cross-section
Powders
were
samples
by ISS to determine
number;
atomic
method
catalyst
The instrument
r
gaseous
detector. 02
increase
at 473 K for 2 hours before
c”,c; = atomic concentration of the metal M g = relative cross-section of the metal
and
in the range 2e = 10'
was calibrated
analyzed
ions,
ion
steady-state
sections
IM,I~ = peak
gas
He
composition
,M =
were
powders
composed
used
using peak-to-peak cross
Three
(Iss).
ground
of noble
in the chamber
ratios,
of
[niano
Scherrer
of the spectrometer.
analyzer.
data were taken
This
recorded
(-50 urn) to
microscope the
sizes,
and then evacuated
ISS spectrometer
energy
tering
were
mesh
Electric
and MnOx/A1203,3)
chamber
mono-energetic
The
325
The diffractometer
mounted
spectroscopy
ISS holder,
the main
vacuum
were
MnOx/CeO2
composition.
tron
General
radiation.
samples
scattering
standard into
XDS-5
patterns
to
lines.
(MnOx/Ti02, face
an
CuKa
ground
over-layer crystal
MnOx
intense
samples
using
Ni filtered
and/or
X-ray diffraction
powdered
Matheson).
U.H.P.
studies
in the range 473-673
components
A schematic were
The hydrogenation
was
of the grade
done
with
experimental
(Matheson);
Gases were purified
by passing
were
carried
K and at 1 atmosphere a HP set-up
the
5750
was
oxygen
pressure.
chromatograph
is shown
ethylene
through
gas
out in a differen-
in Figure research
traps
Analysis
using
a TC
1. The H2, He purity
(CuO activated
(99.98% for 4
16
-
I
BY Pass
L-
Molecular sieve Hz0 trap CuO oxygen trop Rotameter w/ NRS volve Detail of the Pyrex reactor Fine metering valve CAJON@ water injection FIGURE 1 hours
Experimental
at 573
Water was
injected
stainless
steel Cajon
Catalyst (conversion
was
using
The experimental
perature
catalytic on
reoxidation
the
rC2H6 Fp
activity
of
catalyst;
measuring
(1) the effect
effect
reaction
except
where
= molar
5A) cartridges. using a standard
conditions
catalyst
charge
cat./h. reduction of the
catalyst;
(3)
standard
temperature
reduction the
and water
on the reaction under
reactor
Typical
of the effect
of reoxidation
rate
tem-
effect
of
addition
on
rate. These effects conditions
after
indicated.
were determined
Fgxi/W
= ethane
(2) the
of temperature
rates at low conversion
flow
in hydrogen.
He-preactivated
(4) the
the
to the reactor
was lo-13 m3 STP/kg
catalyst;
fresh,
sieve
septum.
differential
of 5% ethylene
determined:
had been achieved
The reaction rC2H6 =
program
(molecular
prior
under
space velocity
of fresh
by
hydrogenation.
feed just
determined
and (5) the effect
determined
steady-state
into the
activity
on used
used catalyst; were
for ethylene
a stream
was 500 mg; the volumetric
on the
port
vacuum tee with a silicone
activity <5%)
5
Hz) and water traps
flowing
on-line
7”
T Vent 01 Flow meters
apparatus
K under
-4
from the following
relation: (2)
formation
rate, kmol/kg
flow rate of reactant
cat./h;
i, kmol/h;
17 TABLE
1
Pretreatments
for atmospheric
Pretreatment
Specimen
Description
Reduction
5
w/hydrogen
II ,I
C
01 ,I
Activation
2 A
w/pure
Reduction ,I
B C
I,
D
Reoxidation
w/oxygen w/pure
In the out
first
at the
conducted increased carrying
from
the
the desired
673 K
2h
473 K
2h
573 K
2h
helium
helium
573 K
2h
(500 ue)
573 K 473 K
2h
i;
to
sequences
catalysts at which
were
were
prereduced,
He
(6~10~~
m3
pure
H2
during
the
for 8 hours
applied
the'catalyst
in
out the reaction
obtaining
2h
kg.
hydrogen zero
4h
573 K
w/hydrogen
same temperature using
473 K
2h
pretreatment
sequence,
2h
2h
of reactant
Two experimental
573 K
473 K
of water
Reduction
weight,
2h
473 K
w/pure
Addition
= catalyst
2h
w/oxygen
Activation
W
573 K 673 K
w/hydrogen
Reoxidation
= conversion
4h
II
Reduction E
473 K
helium
w/hydrogen ,,
Activation
xi
hydrogenation
Code
1 A
Specimen
ethylene
which
half
catalyst reaction
reduced.
The
assured
rate data, the catalyst
and the
was
STP/h). last
to each
carried were
concentration
was
of pretreatment.
achieving
was reduced
was
1).
The reductions
hydrogen hour
(Table
steady-state
After
and after
at the next temperature
in
the sequence.
In the second 573
K
before
completion 1% oxygen
sequence
undergoing
in helium. were
Following
the study
again
reoxidized
During
activated
catalysts
the
of Step C (Table
catalysts
ua
the
same
activated
were
again
of ethylene
in pure
studies
He before
hydrogenation In this
in flowing
procedure
reoxidized
the last half hour a stream
and activated.
the reaction
first
reduction-reaction
l), the catalysts
pulse of water at 573 K before After
were
as
at
After of
was used. The
at 473 K with
at this temperature,
case the catalyst
before.
first with a stream
of pure oxygen
reduction
helium
hydrogen.
the catalyst
was contacted
was
with a 500
reduction.
at 673 K were completed,
the temperature
dependency
of
properties
of impregnation:
298.9
0.91
240.7
2.61
473 K/2 hr
298K; vacuum drying
which was grey-brownish
where 353 K employed. (W. Germany).
Color was dark brown in all cases except MnOx/Ce02
Ebach
temperature
c
Laboratorien,
K in all cases except MnOx/Ti02
wetness;
250.8
From Analitische
at 298-343
Method of preparation:
107.3
0.29
3.63
4.15
1.61
473 K/2 hr
473 K/2 hr
473 K/2 hr
Yes
Yes
MnOx/A1203,B
MnOx/A1203,A
Yes
MnOx/MgA1204
Yes
MnOx/Ce02
prepared
b
a
incipient
1.13
;;l;o;;3afK;;rb,c 2
86.5
5.22
% Mn (A.A.)~
B.E.T. (N2) S.A. (m2lgram)
473 K/2 hr
No
MnOx/Ti02
of the catalysts
Calcination
NH3 treatment
Catalyst
Physical
TABLE 2
FIGURE
2
calcined
X-ray diffraction
patterns.
at 473 K; C) MnOx/Ti02,
A) Ti02
calcined
, calcined at 673 K; B) MnOJTi02,
at 673 K.
49” 368
C
FIGURE
3
calcined
X-ray diffraction
patterns:
at 473 K; C) MnOx/Ce02,
A) Ce02,
calcined
calcined
at 673 K.
at 673 K; B) MnOx/Ce02,
20 TABLE
3
X-Ray diffraction
patterns
of supports
and catalysts
Crystallite Size, i 106
GO
Remarksa
Small amount
of orthorhombic
Same pattern
as Ti02,
More crystallized
Ti02 or Ti0224U
no Mn-related
anatase;
peaks
indication
of some amount
of MnTi04
(haussmanite)
82.8 107
Some Ce(C03)2 Some Ce(C03)2 Manganese
causes
Bayerite
(AI(OH)3)
y-A1203 24
Some y-AlO broad
peaks,
Weak residue Bohmite-
45
No Mn-related
MS-Al-O,
synthetic
II
of y-AlO(
one weak
aluminium
phase related
composition spinels
oxide"
to y-Al203
is a monoclinic
to the spine1
is unknown.
of molecular
50.6
Also peaks from Mg5A1204
73.7
Both systems
is a I:1 ratio
No indication
of Mn-related
tetragonal
peak assigned
peaks
"magnesium
intermediate chemical
hydroxyde-
II
50
- U = centered
peaks
no Mn peaks
oxide
II
42
39.1
broad
peaks
- Bohmite-peaks
BUhmite-aluminium
45
and Bdhmite
no manganese
peaks
y-A1203
48,6
band broadening;
It is observed
ratios
. 15 H20 peaks
structure.
Mg0:A1203
metastable Its
only for larger
than 1:2.5
21
Sample
Treatment
Ti02
Calcined
673 K, 14 h
MnOx/Ti02
Calcined
473 K, 2 h
Patternb
II
CeO,
0,
II
II
673 K, 14 h
CeO,
Calcined
573 K, 14 h
CeO:
MnOx/Ce02
Calcined
473 K, 2 h
CeO;
I,
Calcined
673 K, 2 h
Ce02
Al,O,, L
J
typeA
Dry
Amorphous
(D<20i) (D<2Oi)
II
Calcined
598 K, 14 h
Amorphous
11
Calcined
773 K, 14 h
Y-A1203
Calcined
473 K, 2 h
Amorphous
MnOx/A1203,
A
A1203, type E
y-AlO
Dry
II
Calcined
598 K, 14 h
y-AlO
,I
Calcined
773 K, 25 h
n-Al 203
Calcined
473 K, 2 h
y-AlO
Calcined
673 K, 2 h
n-A1203
MnOx/A1203,
B
II
Calcined
,I
II
MnOx/MgA1204
a Arabic
Amorphous
Dry
MN1204
numeraks
b Ref. 35.
Ti0212U
,I
Amorphous
Dry Calcined
Anatase,
673 K, 14 h
Mg-Al-O,
amorphous
Calcined
773 K, 14 h
II
Calcined
923 K, 16 h
MS-Al-0
Calcined
1073 K, 16 h
Mg-Al-0
Calcined
1273 K, 14 h
MS-Al-O,
MgA1204
Calcined
473 K, 2 h
MS-Al-O,
amorphous
after formulae
stand for the number
II
of atoms
per unit cell
FIGURE
X-ray
4
MnOx/AI203,B,
reaction
diffraction
calcined
rate was determined.
at each temperature mation
After
was increased
temperature.
achieved
A)
First,
Al$l3,
These
independent
and again
experiments
of temperature
showed history
B,
calcined
steady-state the
and that
59B
stepwise
and the desired
at the lowest
that
at
K;
B)
at 673 K.
was decreased
was established
data was obtained stepwise
type
calcined
the temperature
until a steady-state
was achieved.
temperature each
patterns:
at 473 K; C) MnDx/A1203,B
desired
and held
rate infor-
temperature,
the
rate data were obtained same
there
steady-state was
rate
no measurable
at was
aging
observed.
RESULTS
AND DISCUSSION
Preparation A summary
and Structural
2. The surface
areas
same
as
of
MnO,
was
mostly
those
accounted
anatase
impregnation area
of
the
calcination
Characterizations
of the physical
properties
(B.E.T.)
the
for.
(Figure
measured
supports The 2).
X-ray
patterns
(Tables
2 and
3).
prepared after
is given
in Table
Mn addition
were the
addition
when
the
indicated
that
the calcined
changes
were
TfD2 had the largest The MgA1204
weight
observed
crystallites spine1
was
of the
in the
Ti02
and lowest formed
added
titania
was
after
surface
only
after
at 923 K.
The type A Al203 was amorphous nation
for the catalysts to Mn
No significant
and recalcination. supports
prior
of the catalysts
at 598 K, but reverted
bohmite;
to bayerite
the type B Al203 was bohmite during
the Mn impregnation
after
(Figure
calci3). The
23 TABLE
4
Uniform
slab thickness
% Mn before
firing
of hypothetical
at 673 K
Mn(W2
overlayers
of Mn
TiO2
Ce02
A1203,A
A1203,B
MgA1204
5.153
3.23
3.66
4.75
4.16
2.55
1.60
1.81
2.35
2.06
1.65
1.03
1.17
1.52
1.33
2.94
1.49
0.60
0.97
0.82
1.90
0.96
0.39
0.63
0.53
(m3/g of cat.108)a Mn02
Mn(OW2 (m3/m2 of cat.lOl'
=A)
Mn02
aFrom
average
sp. gr. [36]
(i-1)
-b
E 2.U Zdb 0 L. t.s5 2.55 .I T
2
CHAINS
HCP
(2-2)
c
336
FIGURE
5
Single
oxihydroxide,
chain,
double
and dihydroxide
chain,
polymorphs
and
layer
[37].
structure
of the manganese
oxide,
24 I
I
I
I
I
Ti
I
I
MnOx/TiOz
1
I
I
I
I
I
I
I
I
I
I
I
I
I
I
I
I
I
I
I
I
I
’
0 I)
Mn
Al
I 30
I
I
I
40
50
MnOx/Al203,a
60
I
I
70
80
I
90
I
100
170
E/EoxlOOO
FIGURE
6
ISS spectra
of selected
beam at 1.5 KeV. Spectra after
5 scans.
correspond
catalysts
evacuated
to steady
state
at 473 K. Conditions: values of intensity
Het
ratioes
25 TABLE
5
Atomic
ratios
from ion scattering
spectroscopya
CATALYST
M
O/M
O/Mn
M/Mn
MnOx/Ti02
Ti
3.49
7.33
2.10b
MnOx/Ce02
Ce
1.57
2.75
1.74
MnOx/A1203,B
Al
2.27
6.87
3.3
acalculated
revertion by
its
light
remained The
estimated The
involved at 673
yellow
to
infinite
which
assumed flat
catalysts support
a
added
slab
of the
(Figure
5). the
slab
the
model oxide.
of segregation
dispersion
Figure
6
summarizes manganese
shows values
the
that
the
formation
as it should
some
ratios
the
in Figure
be dehydrated
Pattern
water
(Figure
of
surface
6.
only
4), but
hydration
The average of Figure
was
still
specific
that
far
too
Further
Results
is
simple,
or agglomeration indicated
it
If the surface
or MnO2. the
semi-
overlayers area of the
unit area (m2/g) of each
are
within
summarized oxide the
indicates
phases
in Table
compound
4.
is uni-
submonolayer
no need
XPS [8] studies
to bulk
was
of Mn(flH)2 and
5, and the hypothetical
it
ESR [7] and
over
gravity
if the manganese
surface,
supports
of Mn(OH)2
deposited
by the total
thickness.
indicate
of the
of an overlayer
thickness
were divided
range
for multiple
were unable
to find
of MnO,. This very high
by X-ray diffraction
and ISS as shown by
ISS
They
2-6.
derivative
of the
calculated
surface
could
a Ce02
per gram of catalyst.
support
is
cover
uniform
of
intensity atomic were
the ratio,
ratios low,
and A1203 the oxygen-to-support
value,
evident
could
slab
of Mn was also
3 and 5 and Figures
steady-state
Ti02
on
of manganese
surface Tables
gave
support.
volumes
In all cases the calculations
any evidence
Mn
of
a hypothetical
distributed
bayerite
ceria
from the structures
was used, these to give
Although
was
as m3 of overlayer
formly
layers
it
the hypothetical
surface
expressed
the
that
Mn02 was calculated were
described
[12].
by assuming
model
and the
rehydration,
K. The calcined color,
ln the solid extent
conditions
1.5 KeV.
probably
recalcination
from
1, for the experimental
using equation
Ne+ ion beam,
bFrom
be for surfaces
intensity
usually
using
Equation
indicating cation with
ratio
data.
reached
(1). The
excellent is higher
OH groups,
after
correspond
5 scans. support
dispersion
of
to
Table
5
metal-toMn.
For
than the stoichiometric
even after
evacuation
at 473 K
and beam sputtering. On the MnO,/Ce02
the
oxygen-cerium
ratio was
below two which
is abnormally
low.
for ethylene
473 573 673 473 573 673 473 573
1A
1B
1C
2A
28
2c
2D
2E
4.42d
0.257
1.87
16.0d
163.b
3.51
0.72
1.89
MnOx/Ti02
1.69
6.31
2.57
1.58
7.41d
2.30
1.07
6.00d
MnOx/Ce02
pressure
4.73
o.688c
1.55
2.77d
1.91
1.26
1.78
0.383
MnOx/MgA1204
RATE (kmol ethane/kg
at atmospheric
REACTION
(5%) hydrogenation
16.8
16.6
2.41
3.37
7.70
1.92
3.29
9.58
MnOx/A1203,A
catalyst. h *104) a
10.6d
10.6
2.7Zd
5.54
2.00d
1.97
4.23
3.07
MnOx/A1203,B
z Values at pseudo steady state - as of eight hours onstream. Non differential conditions. i Reaction at 573 K. Transient value. * Pretreatments for protocol 1: 1A) H2 reduction, 4 h @ 473 K; 1B) H2 reduction, 2 h @ 573 K; 1C) H2 reduction, 2 h @ 673 K. Pretreatments for protocol 2: 2A) He preactivation, 2 h @ 573 K, then H2 reduction, 4 h @ 473 K; 2B) H2 reduction, 2 h @ 573 K; 2C) H2 2 H @ 673 K; 2D) 02 2 h @ 473 K, then He preactivation, reduction, 2 h @ 573 K, then H2 reduction, reoxidation, 2 h @ 473 K; 2E) 02 reoxidation, 2 h @ 473 K, then He activation, 2 h @ 573 K, then addition of 0.5ml H20 @ 573 K, then H2 reduction, 2 h @ 473 K.
(K)
Reaction Temperature
activities
Pretreatment
Catalytic
TABLE 6
S?
21 Whether
this
conditions not clear. of
was
a consequence
[12] or was
pure
Therefore, Ce02
samples
showed
stil i
suggestion Additional
XPS
either
surface
experiments
of
anion
by the heavy
0
to
confirmed
that
consistent
surface low
ratio
samples
with
reduction
0-Ce
U.H.V.
[IO] was
The control
ratios,
or the
under
of Ce atoms
studied.
Ce
vacancies
vacancies
matrix
of pure Ce02 were lower
anion
[8]
appearance
caused
control
powder
of
of the
an artifact
to
was
the
Ce203.
not
an
ISS
artifact. From all the results concluded each
that
of
the
supports
separate
crystalline
Ethylene
Hydrogenation
K there
673
K this
MnOx/Ce02 most
was
were
to
the
effect
almost
The
was
of these
higher
MnO,/TiO2
was
high
(close
further
in
lo-fold the
with
increase
first
because
were
no obvious
decreased
again
patterns.
for
the
activation
for ethylene
as compared This
in helium
after
hydrogenation
since
dilution
at
MnO,/Ti02.
The high
condition
was the
increased.
in activity
narrow.
at
but at
by H2 reduction
activity
true
e.g.,
of the high
with further activity than
was made
K. These a
very
with
[14].
reaction
rate
conversion
of ethylene,
was there
set of
activity After
and
reoxidation.
the sequence that
to a value reoxidation
the
the actito
reoxidizing,
catalytic
activity
of reoxidations
was made
reoxidation reoxidation
that
did
An attempt
by successively
by observing
that
and H2 reduction,
For the MnO,/A1203,
MnO,/Ti02
showed
activity,
catalyst.
doubled
levels.
work
experiments
low
reactivation
it was before
K, followed
at 473 K. In this
(nonactivated)
In the final
especially [13,14],
catalysts
the catalysts
at 673 K MnOx/Ti02
differences
differences
at 473
in catalytic
vity of the MnOx/Ti02
was
these
previous
changes
reducing
573 and 673
a catalyst
was
was even higher
for C2H4 hydrogenation at 473,
in
no other pro-
rise.
these and
There
to a differential
of the catalysts,
reoxidation
explore
of
or as
was investigated
among
hydrogenation
catalysts
However,
not bring them back to their
preactivating,
3-fold.
chromia
correspond
little temperature
vity after
oxides
For the nonactivated
in activity
remarkable
this
superior
to 90%).
The reoxidation
manganese
of catalysts
catalysts
observation
reductions
not
two
showed
catalysts;
temperature did
it is
on the surfaces
took place cleanly;
at 473 K, whereas
at 673 K their
be a common
by H2 produces
for very
to
to only
catalyst
catalysts
nonactivated
For
on the Series
difference
reduced
activity
He-activated
reduction
multilayer
of pretreatment.
a 25-fold
difference
seems
as
and ISS data,
dispersed
detected.
the effect
573 K, but upon H2 reduction The
present
of C2H4 to C2H6
was the lnost active
active.
not
diffraction
very well
range 473 K to 673 K. The reaction
6 summarizes
473
were
the X-ray
were
phases.
or side reactions
Table
including
overlayers
and
The hydrogenation the temperature ducts
obtained
the manganese
was
at 673
at 473 K resulted at
573
identical
K caused with
that
K, the catalytic
a of
acti-
over that of the 573 K reoxidation.
experiments
of the second
sequence
(Table 1) after
reoxida
7
8
1.6
1
\@I \
I
the reactor
the reactor
, 2.0
4 2.2
temperature).
on the rate of C2Hq hydrogenation
temperature),
B
?
5
100 1.4
2
4
5
6
7
8
101 9
using MnOJCeO2
using MnOx/Ti02
'3 d
s
I
0 r( . -
t
on the rate of C2Hq hydrogenation
1.8 . 10~ l/T
Effect of temperature
0 increasing
FIGURE 8
Cl\
5
Effect of temperature
0 increasing
FIGURE
1.4
100
5 -
2
MnOf/TiO
l/T
(0 decreasing
2.0
the reactor
the reactor
1.8 IO3 . (0 decreasing
1.6
temperature;
temperature;
2.2
29 TABLE
7
Activation
energy
at 673 K for ethylene
Activation Catalyst
hydrogenation
Energy
Correlation
a
E(kcal/gmol)
Coefficient
MnOx/Ti02
11.2 + 0.9
0.979
MnOx/Ce02
8.6 t 2.0
0.964
MnOx/MgA1204
5.4 -+ 0.4
0.977
MnO,/Al203,A
6.5 + b
MnOx/Al203,B
5.7 + 0.8
0.976
MnOx-Ce02
4.6 t 1.2
0.970
MnO,-MgA1204
5.6 t 1.8
0.762
aFor a confidence bOnly
interval
two temperatures
then
studies
reduced
revealed
673 K.
at 573 K, water
at 473
K and ethylene
the following
. For MnOx/A1203,
treatment
(500 us?) was added at 573 K. The catalysts hydrogenation
a sharp
The activity
increase
then slowly
after 4 hours on stream
. ,For MnOx/Ceo2a sharp decay in activity . For MnOx/Ti02 several
and MnO,/MgA1204,
hours,
cantly
higher
The temperature exemplified reaction
by
in
Table
are
behavior
possibly
perature
Bulk Khodakow
case
7.
apparent
With
the
(Figure
8).
(Table
These
catalytic
levels
occurred
of the non-
6).
activity
activity
rise occurred
remained
rate for catalysts
activation
exception showed
in the
activity
to the
over
at a signifi-
value.
from a linear
MnO,/CeO2
due to a change
is lowered
the
in catalytic
decayed
transient
of the reaction
The
satisfactory.
of
only
the
the
correlation
are
coef-
in its rate temperature
rate-determining
catalyst
hydrogenation
of the rate data)
MnOx/A1203,A change
reduced at 673 K iS
for
regression
a clear
reaction
It is the
energies
to show
step as the temthis
type
of non-
behavior. manganese
oxides
[I53 has reported
to 673
K) while
K.
this
In
7.
was evaluated.
was observed.
a sharp
steady-state
573 K (as calculated
ficients
typical
in this
dependence
Figure
above
summarized
but
(>8-fold)
activity
behavior:
A or B types,
with time on stream. hydrated
with a = 0.05.
around
tion and He preactivation were
___
MnO
study
are
poor
that Mn02
has an activity he concludes
that
catalysts
for
iS completely of 4~10~~ lower
the
kmol/kg
metal
hydrogenation
inactive
for this
h (5x10-10
oxides
are
of
olifins.
reaction
kmol/m2/h)
qenerally
more
(473 K at 473 active
than
the
y-Al203 some
ones
exhibiting
shows
small
at fJ3
X [&xID-~
activity
not active Our
for ethylene
results
become
behavior
"critical
the the
MnOx/CeO2
profile
that
vior
might
The
cally recover
does
their step
the different
supports
inactive:
and anatase rI?,IR>;
shows
Ce$
is
influence
Our reaction
temperatures
are
dissociation
of hydrogen
can
which
the
in ethylene
lack
data
of the
defines on
the
series,
[ZO], but
the main function adsorption
of
of the
the
olefin
with
a temperature-activity
step
(Figure
8). This beha-
mobile
OH- at higher
in rate-determining of Mn
hydrogenation
cations
with
below.
not bring
about
catalytic
regain,
is reoxidized
markedly
oxide species.
reoxidation
or
significant
activity.
even
reason for this may be associated catalyst
are also
for
exception
by
initial
to
supports kmn)}mjh)
the H2 that
we
of a change
be caused
reoxidation
reoxidation
to activate
as discussed
temperatures,
TX,
although
is the only
is typical
also
The
at 623 K [16],
ur I.IxID-~~
step on pure metals
[21-251,
itself.
that
of the manganese temperature",
the ability
catalysts
states. activity
kmn)}kgjh
demonstrate
rate-determining
it is probably oxide
oxidation
hydrogenation
hydrogenation.
patently
the catalytic above
higher
a very
exceed,
changes
initial
with the reducible
and activated
with
hwer,
MnOx/TiO2, its
and the catalysts
rewires
catalytic
nature
a 573 K
activity.
of the support.
He, a new hydrogen
typi-
pretreatment
The
Once the may cause
the reaction
l/2
H2 + CP2 + OH- + H-
to occur sence
extensively.
of both
did not
properties The
the
and
same
anion
dissociation
sites.
behavior
unsdturdteb
with
as MnO,Ti02
helium
at
573
might
would
also
be enough
reducible
its easier
pre-
oxide,
reoxidation
for MnOx/CeO2.
chemical
bti_ytmte-s %ve
in the
occur
another
be due to
K removes
as W&-I ax
j-ites.,
of H2 may
The fact that CeO2,
[26]. Thus a 473 K reoxidation
pretreatment
cb?n;itiVIS?I_Y support
A heterolytic
catIon
show
(3)
water
and
leaves
coor-
man_mnrieJe h_y%mX*lb?s mb ni-e
: OH
HO ‘Mn/
(4)
HO' HO
'OH OH
+
'x'
This effect tic activity
is remarkable reported
aL1.26 ta%a>ysts H2 activated The effect
&Lay
in Table \n
as shown by the enhancement
on MnOx/Ti02
6. At higher
artivib
to
a
reaction
IeqE{
tkat
temperatures is
of the catalythe He-preactivtQ
tkat
of
samples. of ~20 on the reaction
performance
is also revealing.
On ZnO and
t&
31
Hydrated
Dehydrofed
Heierolytic
Reductive
H2 adsorption
0
02- (fIrsi H+ H-
.
0
FIGURE
9
Changes
row )
I)
Mn"+
@
M”(“-l)+
x
in a MnO, overlayer
H2 adsorption
C2H4
on a support
basal plane,
under the influence
of H20, H2 and C7H4
y-Al203
a small
amount
of H2D
deactivation
cI6,27]
metal
the presence
oxides
dinatively
of
unsaturated
bonds,
ethylene
a monolayer)
hydrogenation
(CUS)
and
is enough
activity.
to
partially
1281, or to even hydrolyze
be due
reduction
to a decrease
in the activity
in CUS
performance
after the helium
H2 at 473°K
that
occupy
followed
hours
MnO, to
supported
"normal"
on
due
enhancement
after preactivation
anatase
or hydrolysis The
supported
MnOx/Ce+, perature,
C2H4 with
of the
bonds
different
hydrogenation lower
has also
with
been
are
activities observed
this
catalyst
OH- groups
effects
for
MnO,/Ti02
activity the
permanent,
perhaps,
pretreatments complex.
either
with
The
by Denisova
the
metal
oxide
improve
as H20 and
after
is far
reveals
of
several
catalytic
less
than
a decrease of manganese
supported
of
the
et al. on Cr2D3/Al&
the
in CUS ions
MnOx.
on the catalytic
behavior of
its with
preactivation.
decays
oxidation
values
might
reduction
enhancement but
hydrogen
singular
intermediate
as in
MUx clusters.
did not
of tne helium
initial
is
of coor-
catalyst,
that may desorb
-or both- for the alumina
at
the
after the H20 treatment
pure He. This again,
and,
transition
less reducible
high
pretreatment
catalyst
of the Al-0-Mn
effects
for
$0
supported
the total amount
the carrier/transition
that
surface
the
a complete
is then understandable:
In contrast
performance
on the
the
fact
any beneficial
alumina
values.
to
The
can produce
CUS so as to minimize
For
[30].
to cause
reoxidize
of MnOx/Ce+
pretreatment
On
shown to affect
as in the case of CrO,/.SlD2 [29], stabilizing
The observed
vity
of
of H20 has been
sites
the case of MoOx/A12D3
(-2%
acti-
MnO,/Tifl2 and reduction
[31].
tem-
They also
32 reported that, decreases Most
if starting
certainly
endothermic,
the
but
reaction
These
with
types
of
OH- groups
473
can be adsorbed
though
might
be reproduced.
above
temperatures
catalysts,
using
a simple
oxides,
K is not
on the
sup-
thus decreasing
the
-notably
occur
of
groups
(Figure
either
form
of manganese
chains
would of
MnDx/Ti02
and
at 673 K and this would
model
A1203
not produce Mn
atoms
for the catalysts.
of Mn over the preferenor Cd+,
with
mononuclear
[32,33]
ClOSe
species,
resembling
its
but own
5).
-pyrolusite
cations
structural
a deposition
such as anatase,
most certainly
structure
9. The figure
which
at higher
the rate steadily
cannot
H2 activation
of water
the
mobile
and
activity
activity.
overlayer
the first type of regular in Figure
solids
that was used involved
This method
isomorphic
dehydration,
different
some
is lowered,
catalytic
be deeper
of OH- covered
oxide and hydroxide Upon
On
method
plane
packed anions. an
the
can be summarized
The impregnation basal
will
for a higher
ideas
of
the evolution
reduction
a reoxidation
be responsible
rather
reduction
performance.
Mn02/Ce02-
low-temperdture
involves
ports [29]. The
tial
from 673 K the temperature
and the original
or
coordinated
ramsdellite with
oxygen
chainsanions.
will
has been used to model the MnO, OVerlaYer,
shows the surface
under one set of conditions
lead
to
For simplicity, as shown
among the many
that may be possible. This simple
model
gen and further
least two adjacent (c) and reductive
[Mnn+]2(02-)2
offers
reactions,
a description following
Mn cations,
of the heterolytic
the Burwell
model
each one with an anion
dissociation
[34], with
vacancy
of hydro-
involvement
of at
both after unreductive
(d) conditions:
+ H2 + C2H4 +
+ [Mnn+C2H4][Mn n+H-]02+ [Mnnt]2(02-)2
The formation
OH- +
(5)
+ C2H6
of a hydrolyzed
support
manganese
oxide
overlayer
or the reduction
of
the metal + [Mn (n-2)+]DH-
[Mn"+H-](O*-)
follows
directly
from the model.
ACKNOWLEDGMENTS This
work
was
Industry-University the Center
R&D
by
Cooperative
for Catalytic
ISS experiments Central
supported
were
Department,
Science
a
National
Research
Science
Program
Foundation
and by the
grant
Industrial
from Sponsors
the of
and Technology.
performed Wilmington,
by
Dr.
D.G.
Delaware.
Swartzfager G.C.A.
Schuit
of the
DuPont
contributed
Company, generously
33 through
many helpful discussions.
REFERENCES 1 2 3
a 9 10 11 12 13 14 15 16 17 :: 20 21 22 23 24 25 26
33 34 35
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