With time and patience the mulberry leaf becomes a silk gown. Chinese Proverb
Lecture 5
Catalyst Materials, Properties and Preparation
I. Catalyst Materials
A. Typical Heterogeneous Catalysts
B. Three Main Components
C. Molecular Sieves / Zeolites
II. Catalyst Properties
A. Catalyst Engineering
B. Physical, Chemical, and Catalytic Properties
III. Catalyst Preparation and Forming
A. General Approach: adsorption, impregnation, precipitation
B. Impregnation
Outline
INTRODUCTION
Commercial catalysts are complex, sophisticated materials based on decades of catalytic art and science.
Design and preparation of catalysts previously an art; now
becoming a science.
Catalyst research and development highly interdisciplin-ary—involves chemistry, chemical engineering, material science and physics.
Properties are classified as (1) dynamic, (2) chemical and
(3) physical.
Catalysts are composed of (1) active material, (2) promoter, and (3) support.
macro-poresmeso-pores
Pt crystallites
high SA alumina
(10 nm)
(100-200 nm)
(1-5 nm)
Heterogeneous CatalystPt supported on alumina
(a)
A. Active phase - metal that provides active sites where thechemical reaction takes place
B. Support or Carrier - high surface area oxide whichdisperses and stabilizes the active phase
(adds efficiency, physical strength, sometimes selectivity)
C. Promoter(s) - additive which improves catalyst properties, e.g. activity, selectivity, catalyst life
Components of a Typical Heterogeneous Catalyst
Active Catalytic Phases and Reactions They Typically Catalyze
Active Phase Elements/Compounds Reactions Catalyzed
metals Fe, Co, Ni, Cu, Ru, Pt,Pd, Ir, Rh, Au
hydrogenation, steam reforming, HCreforming, dehydrogenation, ammoniasynthesis, Fischer-Tropsch synthesis
oxides oxides of V, Mn, Fe,Cu, Mo, W, Al, Si,Sn, Pb, B
complete and partial oxidation ofhydrocarbons and CO, acid-catalyzedreactions (e.g. cracking, isomerization,alkylation), methanol synthesis
sulfides sulfides of Co, Mo,W, Ni
hydrotreating (hydrodesulfurization,hydrodenitrogenation, hydrodemetallation),hydrogenation
carbides carbides of Fe, Mo, W hydrogenation, FT synthesis
Support/Catalyst BET area (m2/g) Pore Vol. Pore Diam. (nm)
Activated Carbon 500-1500 0.6-0.8 0.6-2
Zeolites (Molecular Sieves) 500-1000 0.5-0.8 0.4-1.8
Silica Gels 200-600 0.40 3-20
Activated Clays 150-225 0.4-0.52 20
Activated Al2O3 100-300 0.4-0.5 6-40
Kieselguhr ("Celite 296") 4.2 1.14 2,200
Typical Physical Properties of Common Carrier (Supports)
.
3% La2O3/Al2O3
2% La2O3/Al2O3
pure Al2O3
700 800 900 1000 1100
200
180
160
140
120
100
80
60
40
Temperature C
Spec
ific
sur
face
m2
g-1
1. The crown jewels of catalysis
2. Catalyst and adsorbents designed at the molecular scale
3. Can be an effective carrier or support
4. Contain acid sites of strong acidity for catalytic reactions, e.g.
catalytic cracking and isomerization
Structure General formula Mv (AlO2)x (SiO2)y . z H2O
AlO2 and SiO2 species are the primary units that share oxygen ions to form tetrahedral AlO4 and SiO4 building blocks for the zeolite unit cell.
Formation of three common zeolites from primary SiO4 and AlO4 tetrahedral units
Si4+, Al3+ etc.
O2-
(a) Primary Units
(b) Secondary Units
(c) Tertiary Units or Building Polyhedra
(d) Zeolite Structures
4R 6R
D4R D6R
Type A Sodalite Faujasite(Type X, Y)
Compositions and Limiting Pore Diameters for Common Zeolites
Type Composition, per unit cell Aperature
Na AlO2 SiO2 H2O size (Å)
A 12 12 12 27 4.2
Faujasite X 86 86 106 264 8.0
Y 56 56 136 264 8.0
Erionite 4.5 9 27 27 4.4
Mordenite 8 8 40 24 6.6
Pentasil ZSM-5 9 9 87 16 5.5
Silicalite 0 0 96 16 5.5
CH3 OH +(b)
+
(a)
Molecular Sieves: Shape Selectivity
(a) reactant selectivity for cracking of a straight-chain versus branched C7.
(b) product selectivity for p-xylene over o- and m- forms
Major Commercial Catalytic Processes Using Zeolites [Vaughan, 1988].
Process Zeolite Products $/tona
Catalytic cracking faujasite gasoline, fuel oil 1.5-3000
Hydrocracking faujasite kerosene, jet fuelbenzene, toluene,xylene
12,000 Pta
Hydroisomerisation mordenite i-hexane, heptane(octane enhancer)
12,000 Pta
iso/n-paraffinseparation
Ca-A pure n-paraffins 5,000
Dewaxing ZSM-5,mordenite
low pour point lubes 60,000 Pta
Olefin drying K-A polyolefin feed 4,000
Benzene alkylation ZSM-5 styrene 60,000
Xylene isomerization ZSM-5 paraxylene 60,000a
CATALYST
DESIGN
Mechanical Properties strength attrition
Catalytic Properties activity/selectivty stability
Chemical/Physical Properties
surface area, porosityacidity, composition,density
Catalyst Design
Catalyst design is an optimized combination of interdependentmechanical, chemical/physical, and catalytic properties
.
Cru
sh S
tren
gth,
kg/
cm2
Porosity,
200
100
00.5 1.0
normalrange
0
b.
Frac
tion
al C
onve
rsio
n
Pellet Density, g/cm3
Porosity,
Ideal
Practical
0.5 1.000
0.5
1.01.0 0.5 02.0 1.5
a.
Porosity versus Strength
Physical and Mechanical Properties: Definitions and Specifications
Property Definition/Specification
densitybulk density,
particle density,
solid density,
mass per unit vol. of bulk catalyst, i.e., bed, packed densitymass per unit vol. of pellet; also called apparent densitymass per unit vol. of solid; also called skeletal or true density
pore volume, Vpore
macropore volumemesopore volumemicropore volume
volume of macropores per unit mass (dpore > 50 nm)volume of mesopores per unit mass (dpore of 3-50 nm)volume of micropores per unit mass (dpore < 3 nm)
pore size and size distr.macroporesmesoporesmicropores
average size (diam.) and distribution for dpore > 50 nmaverage size (diam.) and distribution for dpore of 3-50 nmaverage size (diam.) and distribution for dpore < 3 nm
surface area, Sint
meso and macropore SAmicropore surface area
SA of pores with diameters of 3-5,000 nmSA of pores with diameters of less than 3 nm
catalyst particle size diameter and/or length of pellets/extrudates or hole size (pitch)of monoliths
crushing strengthparticlebulk
force necessary to crush particle in axial or radial directiondisplacement or percentage of fines versus hydraulic pressure
attrition percentage loss per time due to tumbling
30% solid volume
30% pore volume
40% void volume
Distribution of solid, pore and void volumes in a typical catalyst particle
Property Definition/Specification acidity Bronsted acidity Lewis acidity
ability of a material to donate protons ability of a material to capture electrons
chemical composition bulk surface
chemical make-up by element
oxidation state bulk surface
chemical state or valence state
chemical structure bulk surface
geometric arrangement of atoms, arrangement and properties of electrons, and bonding characteristics of atoms
chemisorptive site density
surface concentration of chemisorption sites per mass of catalyst determined by chemisorption; may be used to determine active site density (concentration)
Chemical Properties:Definitions and Specifications
S iAc
O
Al
O
S i
OH H+
S i
O
Al
O
S i
Lewis Acid Sitee- acceptor
BronstedAcid State(proton donor)
+
Catalyst Acidity
Qualitative Ranking of Aqueous and Solid Acids(Listed in decreasing order of acid strength)..
H3O+
HClO4, HNO3
H2SO4
H2SO3
H3PO4
HNO2
HCO2H
H2CO3
H2S
NH4+, H3BO4
HCO3-
HPO42-
HS-
H2O
Solid Acids
AlCl3
Zeolites
SiO2-Al2O3
HF treated Al2O3
SiO2-MgO
- Al2O3
- Al2O3
Nb2O3
TiO2
ZrO2
- Al2O3
MgAl2O4
SiO2
UO2
CaO
MgO
Aqueous Acids
Dec
reas
ing
acid
ity
Dynamic (Catalytic) Properties of Catalysts: Definitions and Specifications.
Property Definition/Specification
intrinsic specific activityturnover frequencyspecific rate (SA basis)
specific reaction rate based on surface area or number of sitesmeasured in the absence of heat/mass transport and deactivationdisguises at specified T, Preactants, and conversion; TOF is themolecules converted or produced per catalytic site per second.
catalytic activityrate based on SArate based on mass, vol.T for required conv.T for given prod. quality
reaction rate or equivalent measured at specified T, Preact., conv.rate based on catalytic surface area, intrinsic or non intrinsicrate based on catalyst mass or volume, intrins. or non intrinsictemp. for required conv. of react., usually non intrinsictemp. for specified product quality, usually non intrinsic
selectivityrate-basedprod. distribution-based
amount or relative rate of prodn. of specified product rel. to othersrate of prodn. of spec. prod. divided by rate for another product or percentage of specified product in total product mixture
stabilitydeactivation rateresistancetolerance
measure of activity decline at specified conditionsrate of activity loss, da/dt where a is normalized activityinverse of deact. rate, e.g., half-life or time to reach spec. act.residual activity after complete poisoning or fouling
Catalyst Preparation and Forming: General SchemePreparation Of The Support
Precip colloidal sol
Aging to form gel
Washing to purify
Separation of a xerogel
Drying
Calcination
Pellets
Deposition Of Active Component
Washing
Filtering
Drying
Calcination
Precipitation, Adsorption, or Impregnation
Formulation
Pelletization
Extrusion
Sphere prilling
Activation
Reduction
Sulfiding
Pelletformulation
mayfollow
deposition.
Monoliths
Depositionmay be doneprior tomonolithextrusion.
Calcination
Drying
Wash-coating
HT calcination
Extrusion
Prepare ceramic paste
(a) Schematic of steps in the preparation of supported catalysts byimpregnation to incipient wetness.
dehydratedpellets pore-filling
solution
pore-saturatedpellets
drying
impregnated pellets
liquid-filledpores
enlarged pelletprecursorcrystallite
in pore