DICP Course - Dalian, 2012
Preparation of solid catalysts
Part 7
Supported by the Chinese Academy of Sciences
Charles Kappenstein, Professor Emeritus, University of Poitiers, France
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Outline
Introduction and general aspects
Interfacial chemistry – Electrostatic adsorption
Impregnation, drying, calcination and/or reduction
Sol-gel chemistry processing
Deposition – Precipitation and Coprecipitation
Bimetallic catalysts
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Outline
Shaping of solid catalysts – Monolith-based catalysts
Zeolite-based catalysts
Ordered mesoporous materials
Case studies:
- Noble metal catalysts
- Methanol catalysts
- Hydrotreating catalysts
- ……
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Outline
Shaping of solid catalysts – Monolith-based catalysts
1) Introduction
2) Fixed bed catalysts
3) Cellular ceramics
- nature of the support
- different shapes
- geometric parameters
- catalyst manufacture
- applications
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Introduction
Industrial catalysts are shaped bodies with different appearances
The shape depends on the type of catalytic bed in the reactor
- fixed bed: applicability limited by pressure drop
- mowing bed
- fluidized bed (40 to 500 µm)
Size and shape are decisive for avoidance
for internal and external mass transport
limitations and for heat transfer
Characteristic length V/A maximum
V = pellet volume
A = external surface area
Catalyst efficiency η close to 1
minimum fluid velocity to avoid external
transport resistance
Exemple of catalyst shapes
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Introduction
Industrial catalysts are shaped bodies with different appearances
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Introduction
Industrial catalyst carriers
Shapes of fixed-bed carriers: foams with 20 and 45 ppi
400-cpsi honeycomb, and spheres with diameters of 3.3 and 1.5 mm.
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Outline
Shaping of solid catalysts – Monolith-based catalysts
1) Introduction
2) Fixed bed catalysts
3) Cellular ceramics
- nature of the support
- different shapes
- geometric parameters
- catalyst manufacture
- applications
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Fixed bed catalysts: pellets
Diameter not be smaller than 1-2 mm
to avoid high pressure drop
The shape affect the bed porosity ε
ε = bed void volume
Ex.: packing of uniform spheres
voidage between 0.35 and 0.4
Use of particles with different diameters
bed porosity decreases
Use of Raschig rings
voidages 0.5 to 0.8
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Fixed bed catalysts: pellets
How to make pellets?
- pelleting
Dry catalyst powder is compressed
In a die by applying forces 50 to 80 kN
Use of die
- granulation
Size enlargement by wet-growth agglomeration
(like a rolling snow ball)
Pan granulation spherical beads 2 to 20 mm
Examples of catalyst particles
- extrusion
A paste is passed through a profiled die
possibility to control the shape
making of monoliths
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Outline
Shaping of solid catalysts – Monolith-based catalysts
1) Introduction
2) Fixed bed catalysts
3) Cellular ceramics
- nature of the support
- different shapes
- geometric parameters
- catalyst manufacture
- applications
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Cellular ceramics
Drawbacks of pellet-based catalysts
- High pressure drop
- Formation of fines due to mechanical attrition
- Pressure oscillations
- Scaling effects from lab-scale to full-scale
- Use of specific alumina able to resist thermal shocks (strategic stocks)
Cellular ceramics represent an attractive alternative for catalyst supports
The fabrication of cellular ceramics is a mature technology.
Several companies are able to procure the needed cellular ceramic supports
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Cellular ceramics
Tubes
Honeycomb Monoliths
Foams
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Cellular ceramics
Advantages of cellular ceramics (mainly honeycomb monoliths)
- Low pressure drop
- Good mechanical properties and thermal stability, Resistance to thermal shock
- Low thermal expansion coefficient
- Thin layer of catalyst: shorter diffusion path length
- Compatibility with washcoat and catalysts
- Large heritage from cleaning of exhaust gas
Drawbacks of monoliths
Laminar flow: large residence time distribution unfavorable for high conversion levels
Poor radial heat conductivity
Low diffusivity for liquid phase
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Outline
Shaping of solid catalysts – Monolith-based catalysts
1) Introduction
2) Fixed bed catalysts
3) Cellular ceramics
- nature of the support
- different shapes
- geometric parameters
- catalyst manufacture
- applications
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Cellular ceramics: nature of the support
Physical properties of some ceramic support materials
Support material
Melting point
/°C
Thermal
conduct.
Thermal shock
resistancea
/gm
Compressive
strength
/psi
modulus of
ruptureb /psi
thermal
expansion
/10-6.K-1
Mullite 3Al2O3⋅2SiO2
1700
high
1.1
108
160
—
α-Al2O3
1870
high
1.3
301
418
7.5
Zirconia-alumina
2100
low
0.9
181
150
8.0
Stabilized ZrO2
2470
low
0.4
146
256
7.9
Silicon carbide SiC
1720
v. high
0.1
155
240
5.5
Cordierite
2MgO⋅2Al2O3⋅5SiO2
1470
—
0.0
215
192
2.0
Lithium aluminum
silicate
1367
—
0.0
150
190
1.2
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Cellular ceramics: nature of the support
Metallic honeycomb monolith
- Segments of metallic honeycombs with 400–1200 cpsi.
- S-shaped design of a metallic honeycomb used as a turbocharger.
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Outline
Shaping of solid catalysts – Monolith-based catalysts
1) Introduction
2) Fixed bed catalysts
3) Cellular ceramics
- nature of the support
- different shapes
- geometric parameters
- catalyst manufacture
- applications
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Cellular ceramics: different shapes
1) Small honeycomb monoliths: square channels
10 mm
2) Small honeycomb monoliths: triangular
channels
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Cordierite
Cordierite
400 cpsi
600 cpsi
SiC
SiC
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Cellular ceramics: different shapes
3) Foams: interconnected pores
block of foam (alumina)
cylindrical-shaped foam (mullite).
4) Large honeycomb monoliths
Monolith before
preparation
100 or 400 cpsi
Cordierite or mullite
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100 mm
50 mm
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Outline
Shaping of solid catalysts – Monolith-based catalysts
1) Introduction
2) Fixed bed catalysts
3) Cellular ceramics
- nature of the support
- different shapes
- geometric parameters
- catalyst manufacture
- applications
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Cellular ceramics: geometric parameters
Honeycomb monolithic supports: three parameters
channel shape: square, triangular, hexagonal
channel size
wall thickness
A case study: square channels
Enlargement
400 cpsi (channel per square inch)
Channel size
dch /mm
Open fraction area
(OFA)
Wall thickness
Geometric surface
area (GSA) /mm-1
lch /mm
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Cellular ceramics: geometric parameters
Honeycomb monolith supports with square channels
dch
n
GSA
OFA
: channel size
: Channel density (cpsi)
: Geometric surface area (mm-1)
: Open fraction area
Ich
Dh
MIF
TIF
Ideal support high GSA, OFA, MIF, TIF,
: wall thickness
: Hydraulic diameter (mm)
: Mechanical integrity factor
: Thermal integrity factor
low Dh
Mathematical formulas
1
n= 2
dch
GSA = 4n(dch − ech )
OFA = n(d ch − ech ) 2
OFA
Dh = 4 × (
)
GSA
d
TIF = ch
ech
ech2
MIF =
d ch (d ch − ech )
Channel density
dch
/mm
Ich
/mm
n
/mm-2
n
/cpsi
GSA
/mm-1
OFA
Dh
/mm
MIF
TIF
400 cpsi
1.30
0.35
0.592
382
2.25
0.534
0.95
0.099
3.71
100 cpsi
2.40
0.40
0.174
112
1.39
0.690
2.00
0.033
6.00
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Cellular ceramics: geometric parameters
Surface and volume of foams: ppi = pores per inch
Determination of the open volume from density measurements.
Determination of the internal surface area models.
Simple model: interconnected hollow spheres, constant diameter, half the surface is open
Ex.: Alumina
porosity
20 ppi
30 ppi
Volume
/cm3
weight
/g
foam density
/g cm-3 (CTI)
apparent
density /g
cm-3
foam volume
/cm3 g-1
free volume
/cm3 g-1
GSA
30 ppi
12.6
11.516
0.914 (0.83)
2.59
1.094
0.708
3.24
20 ppi
12.6
7.315
0.581 (0.67)
2.59
1.721
1.335
2.33
400 cpsi
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/mm2 mm-3
2.25
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Cellular ceramics: surface picture
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Outline
Shaping of solid catalysts – Monolith-based catalysts
1) Introduction
2) Fixed bed catalysts
3) Cellular ceramics
- nature of the support
- different shapes
- geometric parameters
- catalyst manufacture
- applications
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Cellular ceramics: catalyst manufacture
Four steps
1) Preliminary treatment of monoliths (< 0.5 m2 g(monolith)-1)
- nitric acid, ultra-pure water, calcination
- remove impurities from the pores and create macro-porosity
2) Preparation of colloidal suspension of boehmite AlO(OH)
- control temperature, mixing rate, viscosity and aging
- compare different procedures
A: pseudo-boehmite + diluted HNO3 + urea
B: AlCl3 + Al + urea
255 m2 g-1
100 m2 g-1
3) Washcoating of supports by dipping into the boehmite sol, drying and calcination
homogeneous deposition of washcoat layer (~22 m2 g(monolith)-1)
Up to three successive washcoating steps to get a given washcoat wt.-%
4) Impregnation with active phase precursor followed by drying and thermal
treatments under oxidative or reductive atmosphere.
to get 10 to 40 wt.-% based on the washcoat layer.
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Cellular ceramics: catalyst manufacture
Examples of catalysts
10 mm
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Cellular ceramics: catalyst manufacture
Comparison of procedures A and B for washcoating step
Sol A more homogeneous and smooth layer
Sol B
Sol A
1000
Intensity /counts
1400
800
Intensity /counts
1200
Pt
1000
GC013 (+300)
600
Pt
400
800
Pt
Pt
600
GS045 (+300)
400
200
GC023
200
GS056
0
30
35
40
45
50
2 theta
0
30
35
40
45
50
2 theta
XRD profile of Pt-based catalyst on cordierite (left) and SiC (right) monoliths.
Average Pt size about 13 nm. The other peaks origin from the support.
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Outline
Shaping of solid catalysts – Monolith-based catalysts
1) Introduction
2) Fixed bed catalysts
3) Cellular ceramics
- nature of the support
- different shapes
- geometric parameters
- catalyst manufacture
- applications
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Cellular ceramics: applications
- car exhaust catalysts
- nitrate abatement
- reduction of NOx
Three-way catalyst
2 NO + 2 CO N2 + CO2
2 CO + O2 2 CO2
Hydrocarbons + O2 CO2 + H2O
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Cellular ceramics: applications
- Propulsion: exemple for ignition of hydrogen at low temperature
Parameters:
(i) nature of the ceramic: mullite or cordierite
(ii) channel density: 100 or 400 cpsi
(iii) nature of the active phase: Ir, Pt, Pd or Rh
(iv) wash-coating procedure
(v) content of active phase:15 to 40 wt.-% of the washcoat mass
Typical values of iridium catalysts prepared on different supports
support
Cordierite,
400 cpsi
Cordierite,
100 cpsi
Mullite, 400
cpsi
Initial mass /g
134.30
106.07
154.34
Volume /cm3
196
196
196
Wash-coat
mass /g
18.35
9.37
13.10
Iridium mass /g
4.43
2.57
4.25
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Ir catalyst after tests
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End of part 7
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