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)

TEM of platinum supported on alumina

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

Examples of typical Heterogeneous Catalyst

Automotive Catalytic Converter

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

Zeolites, the Crown Jewels of Catalysis

Para-xylene in Zeolite Theta-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

Mordenite

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

Pore Size Breakdown

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

Increasing Rate Of Drying