KE-40 9910 Preparation

7/29/2019 KE-40 9910 Preparation

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Preparation of solid catalysts

Dr. Sanna Airaksinen

Aalto University

School of Science and Technology



Outline• What is a catalyst

• Preparation methods for supports and bulk

catalysts

• Preparation methods for supported catalysts

• More information from e.g.

Handbook in Heterogeneous Catalysis ,Ed. Ertl, Knözinger, Schüth, Weitkamp,

2nd edition, vol 2.



What is a catalyst?• Catalysts are substances that increase the

reaction rate by which a chemical systemapproaches equilibrium without changing or

being used up themselves in the process

Figure: Puurunen (2002)



What is a catalyst?

A catalyst

1. Increases the chemical reaction rate

(kinetic effect)

2. Does not influence the thermodynamicequilibrium of a chemical reaction

3. Does not change in a chemical reaction



Ammonia synthesis



Classes of catalysis

• Heterogeneous catalysis

• Homogeneous catalysis• Photocatalysis

• Enzymatic catalysis



Solid catalysts

• Bulk catalysts

• Supported catalysts

– Support

– Active component

– Promoter(s)

Different catalysts from Süd Chemie



Properties of catalyst supports

4.50.90380Aluminium

silicate

<20.711040Activated carbon

5.40.67220Alumina, Al2O3

2.20.72450Silica, SiO2

Mean pore

diameter, nm

Pore

volume,cm3 /g

Surface

area,m2 /g

Support



Preparation of supports

and bulk catalysts

• Precipitation and co-precipitation – Alumina

• Sol-gel synthesis

– Silica

• Zeolite synthesis

• Preparation of skeletal metal catalysts

• Other methods also available



Supersaturated metalsalt solution

Precipitation physically (e.g. evaporation)or chemically (adding precipitating agent,base)

Precipitate

[Aging, modification], filtering

Filter cake

Dry precursor

Shaping

Preformed

precursor

Calcination

Catalyst/support

Active phase

[Aging, modification],spraydrying

Calcination Shaping

Catalyst/support

Precipitation

andcoprecipitation

e.g. metal hydroxide



Precipitation and coprecipitation• Precipitation

– Very pure materials can be obtained – Product needs to be separated from the solution

• Coprecipitation

– Simultanous precipitation of more than one component

– Homogeneous distribution of catalyst components

– Difficult to follow product quality during precipitation

• Examples

– Al2O3 (mostly γ ): support, catalyst (e.g. isomerization) – SiO2: support

– Sn-Sb-oxides: catalyst in selective oxidation (e.g. isobutene tomethacrolein)



Preparation of alumina by

precipitation• Reactants in salt solutions

– Aluminium sulphate and ammonium hydroxide

– Precipitation and crystallization of Al(OH)3 (pH, T, time)

• pH 7..12

• Elevated T, high pH → bayerite (Al(OH)3)• Lower pH → pseudoboehmite → boehmite (AlOOH)

• Hydrolysis of aluminum alcoholates

– Al(OR)3 + (2+x )H2O→

3ROH + AlOOH· x H2O – High-purity pseudoboehmite obtained

• Drying

• Calcination



Calcination of alumina

SA=200-300 m2 /g

SA<5 m

2

/g



https://www.llnl.gov/str/May05/Satcher.html

Sol-gel synthesis

• Precursor in dissolved state→ pH adjustment

• Conversion (activation) of dissolved precursor to the reactive state

• Polycondensation of activated molecular precursors into nanoclusters

(micelles)→ colloidal solution, the sol

• Gelation

• Aging→

hydrogel• Washing

• Drying→

xerogel (evaporation) or

aerogel (autoclave)

• Stabilization



Preparation of silica• Can be prepared by

– Sol-gel process (silica gel)

– Precipitation (precipitated silica)

– Flame hydrolysis (fumed silica)

• Sol-gel process and precipitation

– Starting material sodium silicate (waterglass),

SiO2:Na2O=1.6-3.9

– Preparation methods quite similar

– Precipitation at neutral or alkaline solution

– Sol-gel process at acidic solution→ real gel is formed

(if gel formation is prevented e.g. by stirring a precipitate is

formed)



Preparation of silica gelSodium silicate solution

pH adjustment with H 2 SO 4

Silicic acid

→ polycondensation products

→ colloidal silica particles (sol)

Hydrogel

Agglomeration of colloidal silica

Drying (evaporation)Xerogel

Washing

Formulation (e.g. pelletization or extrudation)

Silica particles

Colloidal silicawww.grace.com



Zeolites

• Crystalline aluminiumsilicate – M2/ n O · Al2O3 · x SiO2 · y H2O

Zéo boilingLíthos stone

• Molecular sieve

• Porous material,

pore size = molecule size, 0.3-2.0 nm

Linde Type A (LTA)

Faujacite (FAU)



Hydrothermal zeolite synthesis

Si source

Al source

Base

Templating agent

Reaction mixture

Low T (<60°C)

Synthesis gelSilicate and aluminate monomers and oligomers in solution in equilibrium with condensed silicate and aluminate units in the gel phase

Zeolite crystallization

T increased to crystallization T (~100..200°C)

Separation, washing, drying and calcination

Zeolite

After induction period

- Si source: colloidal silica, waterglass,

pyrogenic silica, silicon alkoxides- Al source: gibbsite, pseudoboehmite,aluminate salts, aluminum alkoxides, metal

powder- Base (pH 9..12): alkali metal hydroxide,

quaternary ammonium hydroxide etc.- Template/structure-directing agent (SDA):cationic or neutral organic molecules

Starting materials and synthesis conditions (T, p, pH) depend on the wanted zeolite (Si/Al ratio)



ZEOLITE PLANT (1/2)

Sulphate Aluminate

SilicateAluminium

Sodium

Sodium

SeedsML-Gel

Sulphate

Beltfilter

Effluent

Aluminium

Water

BeltfilterNa-Y Zeolite

RE-Y Zeolite

(NH4)2SO4

RECl3

/

Water

Beltfilter

Effluent

NH4-Y /



Bag Filter System

Calciner

US-Y Zeolite

CREY /

RE-Y Zeolite

NH4-Y /

Hot Air

Dryer

ZEOLITE PLANT (2/2)



Skeletal metal catalysts• Raney metals: active porous metals

– Ni (Fe, Co, Cu)

• Metal forms an alloy with Al

→ Al is leached with NaOH

→ porous metal with high surface area

• Used in selective hydrogenation

reactions in liquid phaseslurry reactors

Dry Raney nickelPicture:http://commons.wikimedia.org/wiki/File:Dry_Raney_nickel.jpg



Supported catalysts

• Support

• Active component

• Promoter(s)



Examples of active components of

catalysts

Al2O3, SiO2,zeolites

SiO2-Al2O3

Isomerization,cracking,dehydration

Insulating oxides

NiO, ZnO, CuO

Cr2O3, MoS2

Selectivehydrogenation,hydrogenolysis

Semiconductiveoxides andsulfides

Fe, Ni, Pt

Pd, Cu, Ag

hydrogenation

hydrogenolysis

Metals

CatalystReactionActive

component



Examples of promoters

Enhances C–S and C–N hydrogenolysis

Improves dispersion of MoO3

Ni, Co

P, B

MoO3 /Al2O3

Decreases hydrogenolysis and sinteringRePt/Al2O3

Increases acidity and thermal stability

Enhances hydrogenation

Re ion

Pt

Zeolites

Enhances oxidation of COPtSiO2-Al2O3

Increases thermal stabilityPrevents cokingIncreases acidityPrevents sintering of active sites

SiO2, ZrO, PK2OHClMgO

Al2O3

FunctionPromoterCatalyst



• Selection of support• Pretreatment of support

• Selection of method to add the active material

on the support• Selection of precursor material to produce the

active component

• Drying

• Calcination

• Activation

Preparation of supported catalysts



Preparation of supported catalysts

1. If reactor price determines

⇒ maximize active surface area per

volume unit of the catalyst

2. If catalyst price determines(e.g. noble metal catalysts)

⇒ maximize active surface area per

weight unit of the active phase



Methods to deposit

the active component

• Deposition-precipitation• Impregnation

• Adsorption

• Ion exchange

• CVD / ALD

(chemical vapour deposition / atomic layer deposition)



Deposition-precipitation (1)• Metal hydroxide/carbonate is precipitated on the support surface

from metal salt solution when pH is increased

• Support in powder form

• Rapid mixing essential: nucleation on the surface of the support, notin the bulk of solution

• Homogeneous distribution of active component, small particles

• Generally used to obtain high loadings with good dispersion

• Also: low loadings of very small particles of e.g. noble metals

Metal salt solutionoxalatenitrate

sulfatechloride

+

Supportpowderparticles

Base

NaOHKOHNH4OHNa2CO3

etc.

Metal

hydroxide orcarbonate on

support

Calcination(activation)(shaping)

Catalyst



Deposition-precipitation (2)• Using urea as the OH- source instead of base

→ better control over uniform precipitation

Metal salt solution with ureamixed with support powder

T increased to 70-90°C

Urea hydrolyzes to form OH-

Metal hydroxide precipitateson the support

Precursor-support interaction →

nucleation at support surface at lower concentration than in the bulk

Filtering, washing, drying, etc.



Impregnation

Solution with activephase precursor

Calcinedand driedsupport

Aging, e.g. overnight

Drying, calcination,activation (e.g. hydrogenation

with metal catalysts)

• Incipient wetness/

dry/pore volumeimpregnation

– Precursor in salt solution,

amount of solution equal to

or slightly lower than the

pore volume of the support

• Wet impregnation

– Amount of solution higher

than the pore volume

Impregnation (1)



Impregnation (2)• Pre-shaped support bodies and powdered supports can

both be used• Waste water is not produced

• Active phase precursors are not lost

• Several impregnation steps might be needed to obtainhigh loadings (precursor solubility)

• Drying important→ can result in uneven distribution if

not done properly



Adsorption• Support material in metal salt solution adsorbs salt ions

– Support dehydrated and soaked in the appropriate solution for asuitable time

– Cations and anions in the solution react with support surface

groups depending on surface properties

SOH+ + C+↔ SOC+ + H+

S(OH)- + A-↔ SA- + (OH)-

• Good method for depositing small amounts

– Higher loadings usually require multiple adsorption steps

• Results in uniform deposition



Ion exchange• Similar to adsorption

• Lower valency ions exchange with ions having

higher charge, e.g.

SNa+

+ Ni2+↔

SNi2+

+ Na+

• Useful in removing harmful agents and adding

promoters

• Zeolites



Catalyst preparation by

Atomic Layer Deposition (ALD)

• Materials growth technique allowing coating ofcomplex shapes with a high quality materiallayer (thin films)

• Surface controlled process• Surface saturating gas–solid reaction

(chemisorption)





• In catalyst preparation

– Solid surface is the surface of the catalyst support

– Gaseous reactant is a precursor for catalytically

active sites

• Highly dispersed and active supported metal

and oxide catalysts prepared

• Tailored support/catalyst modification possible

• No waste water formed during the preparation



P u u r u n e n , D o c t o r a l T h e s i s ,

H U T 2 0 0 2





• Preparation of modified supports

– AlN/Al2O3 (Puurunen, PhD Thesis 2002)

– ZrO2 /Al2O3 (Kytökivi et al. Langmuir 13 (1997)

2717–2725.)

• Preparation of catalysts

– CrOx /Al2O3 (Hakuli, PhD Thesis 1999) – Pt and Ru on carbon fibres (Plomp et al. Appl.

Catal., A 351 (2008) 9-15.)



Preparation of

chromia/alumina by ALD• Chromia/alumina used in dehydrogenation of propane

and isobutane• Chromia ALD precursor Cr(acac)3

Ligand removal,

calcination Cr6+ (mono- and/orpolychromates)

Cr3+ (isolated Cr3+ ,

amorphous Cr2O3,cryst. Cr2O3)

P i f



Preparation of

chromia/alumina by ALD



Catalyst calcination and activation• Calcination

– Salt is converted to oxide – Surface stabilized

• Activation

– Metals: hydrogenation

• Deposited oxide converted to the metal

– Sulfides: sulfidation• CoMo and NiMo used in HDS



Catalyst preparation process



Catalyst particles



Literature• James Richardson, Principles of Catalyst

Development , Plenum Press, New York, 1989.• Handbook of Heterogenous Catalysis , Ed. Ertl,

Knözinger, Schüth, Weitkamp, 2nd edition, vol 2.

• Catalysis: An Integrated Approach , Ed. van

Santen, van Leeuwen, Moulijn, Averill, 2nd

edition





Properties of supports• Support surface groups important

– Interact with the active-phase precursors – Hydroxyl groups (acidic, neutral, basic)

– Coordinatively unsaturated metal sites (Lewis acids)

• E.g. alumina



Properties of supports• Surface OH groups are in equilibrium with the solution

depending on solution pH → surface may be charged – M-OH + H+

↔ [M-OH2]+, or M-OH+H2O↔ [M-OH2]

+ + OH-

– M-OH + OH-↔M-O- + H2O

• pzc = point of zero charge = pH at which the surfacecharge is zero

• Effect of solution pH

– pH>pzc: support surface negatively charged→ will attract cations

– pH<pzc: support surface positively charged

→ will attract anions



Preparation of MoO3

/Al2

O3

by impregnation

• Disproportionation (metathesis) catalyst

• Impregnation of γ -Al2O3 with

– an aqueous solution of ammonium heptamolybdate (AHM) or

– ammoniacal solution of ammonium dimolybdate (ADM)

• With AHM

– (1) HM ions react with most basic OH groups:

Mo7O24(6-) +8 Al-OH -> 4 (Al)2MoO4 + 3 MoO4

(2-) + 4 H2O

– (2) physisorption of AHM on c.u.s. Al sites

• With ADM

– Solution basic, MoO4

(2-)

molecules present: no interaction with basic aluminaOH groups initially (pH>pcz)

– With drying: ammonia released, pH inside the pores drops, HM ions formed:

7 MoO4(2-) + 4H2O↔ Mo7O24

(6-) + 8 OH-

– The HM ions formed react with surface groups (mainly reaction (2))

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KE-40 9910 Preparation