Design and Production of Heterogeneous Catalysts

Design and Production of

Heterogeneous Catalysts

Gerard B. Hawkins Managing Director

Objectives of the Design of Solid Catalysts

Development of Improved catalysts to be employed within

existing production units for existing reactors Improved catalysts for existing reactors using

new procedures calling for new equipment Integration of catalyst and reactor

Heat transfer and mass transport Integration of catalytic reaction and separation of

reactants or reaction products Catalytic distillation as an example :

performing a catalytic reaction within a distillation column

Objectives of the Design of Solid Catalysts

Most smooth penetration into market for • Improved catalysts to be employed within existing

production units for existing reactors • Relatively small rise in conversion and/or selectivity

leads to large increase in profit at low costs of investment

Other possibilities are calling for higher costs of investment and are thus more difficult to get accepted • Smaller units more easy access to the market; new

concepts to be introduced first in small-scale units

Development of Solid Catalysts

Mechanical strength Most important for technical applications

Size of catalyst bodies Determining pressure drop

Active surface area Sufficiently large active surface area per unit volume of

catalyst Active surface area stable at temperatures of

pretreatment and catalytic reactions Desired structure and chemical composition

Development of Solid Catalysts

Transport of material and thermal energy to and from the active sites • Solid catalysts usually highly porous and thus

thermally isolating materials • Length of pores more important than diameter of

pores (Thiele’s modulus) Tri-lobs, quadri-lobs, rings

Liquid-phase catalysts completely different constraints than gas-phase catalysts • Development of gas-phase catalysts much more

advanced

Design of Solid Catalysts

Catalytic activity and selectivity Structure and chemical composition of active surface Extent of active surface area

Transport properties To catalyst bodies

Size and shape of catalyst bodies; pressure drop Within catalyst bodies

Size of catalyst bodies Pores size distribution

Supported Solid Catalysts

Usual separation of functions Support

Size and mechanical strength of catalyst bodies

Porous structure Active component

Structure and chemical composition of catalytically active surface

Supported Solid Catalysts

However, sometimes catalytic function of support also involved : Bi-functional catalysts • Acid function of support with precious metal

function in catalytic reforming catalysts Support promoting dissociation of carbon monoxide

in some Fischer-Tropsch catalysts • Different selectivity of titania-supported cobalt

catalysts Reduced titanium ions at the periphery of the supported metal particles take up oxygen of carbon monoxide and thus promote dissociation

Unsupported Catalysts

Reasons to employ unsupported catalysts : Active species providing sufficiently large and

thermo stable surface area as well as suitable pore structure Catalytically active species capable of providing

sufficiently strong porous bodies

Reaction of suitable supports with required promoters, which prevents promoters to be effective


Examples : • Pt or Pd gauze for the oxidation of ammonia to

nitrogen oxide in the production of nitric acid and Pt gauze in Andrussow’s process for the production of HCN from methane and ammonia

• Silica-alumina cracking catalyst • Raney metal catalysts • High-temperature carbon monoxide shift

conversion catalyst : iron oxide-chromium oxide


Examples : • Ethylbenzene dehydrogenation catalyst (iron

oxide promoted with potassium oxide) Potassium oxide promoter reacting with the

usual alumina and silica support • V-P-O catalyst for the selective oxidation of n-

butane to Maleic anhydride Alumina support reacts with phosphoric acid

and disturbs vanadium/phosphorous ratio; vanadium difficult to apply to silica support

Catalyst Preparation : Science or Art

First example : Preparation of VPO catalyst for oxidation of n-butane to Maleic anhydride

Procedure (1) Centi et al. Reduction of 6.7 g of V2O5 for 16 hours in

80 ml of HCl at 100oC Addition of 9.3 g 85% H3PO4 and refluxing the

solution for 1 hour Evaporate to dryness Dry resulting green viscous mass in nitrogen

flow for 10 hours at 125oC

Catalyst Preparation : Art or Science Procedure (2) Katsumoto et al.

Reduction of 15 g V2O5 at 120oC in 60 ml 1:1 (v/v) i-butanol/cyclohexanol mixture

Cooling to room temperature and addition of 21 g of o-H3PO4 mixed with 30 ml butanol

Refluxing for 6 hours leads to blue-green suspension

Filtering of suspension and drying in nitrogen flow for 12 hours at 125oC

Catalyst Preparation : Art or Science

Reduction of vanadium to mixture of vanadium(IV) and vanadium(V) by either inorganic (HCl) or organic reducing agent

Catalysts produced in either way call for being at least for 24 h on stream to exhibit a reasonable selectivity and activity

Solid Catalysts Usually supported catalysts in view of better control of

properties of catalysts Surface area and loading of the support with the active

component as well as the distribution of the active component over the surface of the support determining • Extent of catalytically active surface area per unit

volume • Thermo stability together with the interaction of the

active component with the surface of the support

Supported Catalysts

Supported catalyst Thermostable Unsupported catalyst rapid sintering

Reduction

Supported Metal Supported Metal Oxide

Components of Solid Catalysts

Support Shape and size of catalyst bodies Porous structure Mechanical strength Surface area

Active component Size and number (loading) of supported active moieties Distribution over surface of support Interaction with support

Types of Supported Catalysts

Catalysts containing base metals, base metal oxides or base metal sulfides Active surface area per unit volume decisive

Limiting size of reactor Usually high loadings of support with active

component(s) Loadings of 20 to 50 wt.% usual

Catalysts containing precious metals Active surface area per unit weight of precious

metal decisive Low loadings of active component(s)

Less than 1 wt.% usual, sometimes up to about 5 wt.%

Production of Finely Divided Material

Condensation of molecularly dispersed species

Selective removal of some component

Preparation Procedures of Supported Catalysts

Application of active precursor to separately produced support Application of active precursor into pre-shaped

support bodies Application on powdered support and subsequent

shaping Selective removal of one or more constituents from

essentially non-porous precursor of support and active component Examples Raney metals; ammonia synthesis

catalyst; methanol and low-temperature carbon monoxide shift catalyst based on copper/zinc oxide

Preparation of Catalysts by Selective Removal Resulting in

powder, e.g., Raney nickel powder to be processed to bodies, e.g.,

methanol synthesis catalyst porous solid bodies, e.g., ammonia synthesis

catalyst

Non-porous Precursof

Powdered catalyst

Porous catalyst body

Shaped catalyst body

Catalyst Preparation: Art or Science

Second example : Ammonia synthesis catalyst

Trial and error Iron ore

About 97% Fe3O4 2% Al2O3 1% K2O Double-promoted iron catalyst Alumina structural promoter Potassium required to maintain activity

at higher pressures

Catalyst Preparation: Science or Art

Selective removal of oxygen Minimum amount of Al2O3 to transport water rapidly

out of porous catalyst bodies Al2O3 effectively prevents sintering Role of potassium still debated

Presumably potassium oxide promoting desorption of ammonia, which is required at elevated pressures

Effect of Structure of Active Surface

Different surface structures

Different activity per unit surface area

Effect of size of active particles Large active particles mainly atomically flat

surfaces

Small active particles penetration of atoms

into surface layer

Oxygen on Fe(100)

Penetration of Foreign Atoms into Surface

Extended crystallographic plane

Small crystallographic plane

Generally Employed Supports

By far the most preferred commercial support : Alumina due to elevated bulk density • Usually g-alumina, surface area from about 300 to 100

m2/g; most preferable needles from boehmite (AlOOH), less preferable from gibbsite or bayerite (Al(OH)3)

• a-alumina is support with relatively inert surface and surface area of usually less than 1 m2/g and exceptionally 10 m2/g

When alumina cannot be employed, silica is the second best

Support for Precious Metals

Precious metals used in liquid-phase processes :

Activated carbon attractive support • Carbon is not attacked by acids and alkaline liquids • Carbon bodies of about 50 mm therefore often used

suspended in liquids • To reclaim the precious metal a carbon support can

be removed by simple combustion

Activated Carbon Supports

Since activated carbon is produced from peat or wood, it is a natural product and therefore difficult to reproduce accurately

Mechanical strength of carbon supports is often problematic

(Carbon supports cannot be calcined in air) Apparent surface area of activated carbon about 1200

m2/g • Activated carbon contains many micropores • Besides very small particles also stacking of

graphite layers present

Commercial Pre-Shaped Support Bodies

Main deficiency of alumina and silica supports : not compatible with alkaline promoter species

Silica is volatile with steam at high pressure and/or high temperatures

Other supports, such as, zirconia or titania more difficult to process to mechanically strong bodies of an elevated surface area; supports are much more expensive • Zirconia and titania compatible with alkaline materials • Alternative supports much more expensive

Important producer of alternative supports : HAISO

Preparation Procedures of Supported Catalysts

Application of active precursor onto separately produced support • Application of active precursor into pre-

shaped support bodies • Application on powdered support and

subsequent shaping Employing commercial pre-shaped support

bodies most obvious • Wide range of different shapes and sizes of

alumina and silica available

Preparation of Supported Precious Metal Catalysts

Most obvious procedure

Pore-volume impregnation and drying of pre-shaped support bodies Actually adsorption of active precursor on surface of support

Alumina impregnation with acid, negatively charged precursors Silica impregnation with positively charged ammonia complexes With alumina neutralization of acid components of

impregnating liquid often important No risk of loss of precious metal with, e.g., waste water Selection of size, shape, pore structure, and mechanical strength of

support bodies viable from large range of commercial support bodies

Pore Volume Impregnation

Incipient Wetness Impregnation

Also known as “dry impregnation”


Precious metal precursor often adsorbing on surface support

At the usual low loadings of precious metals adsorption brings about inhomogeneous distribution of the precious metal over the support bodies

Chromatographic effect

Distribution of Active Precious Metal Particles

Uniform distribution of precious metal particles

Usually desired when transport limitations are

not expected

Egg-shell distribution of precious metal particles

Resulting from adsorption from the impregnating liquid

Only desired with transport limitations


Alternative industrial procedure to arrive at egg-shell distribution of precious metal particles : Spraying of solution of dissolved precious metal precursor onto agitated volume of support bodies

To limit penetration of solution of active precursor into pores of support, support is often pre-heated

Egg-shell distribution of active precious metal particles on support bodies often desired with catalysts intended for liquid-phase processes


Establishment of an egg-shell distribution of precious metal particles on alumina support bodies

Fill pore volume of pre-shaped support bodies completely with water

Pass acid solution of precious metal along water-filled support bodies • Neutralization of acid solution at external surface of

alumina support and consequent deposition of palladium compound

• Slow transport of dissolved precious metal species through water present within pore system of support


Soaking of alumina support in solution of precious metal or recirculation of solution of precious metal for long periods of time leads to uniform distribution and high loading


Uniform distribution of precious metal(s) can be achieved more readily by employing less strongly adsorbing species • With alumina supports change H2PtCl6 for K2PtCl6

Generation of adsorbing Al+ sites by reaction of proton with surface OH- groups of alumina Reportedly PtCl62- generating protons by exchange with water and dissociation of water, which proceeds slowly

• PtCl62- + H2O = PtCl5(H2O)- + Cl- PtCl5(H2O)- + H2O = PtCl5(OH)- + H3O+


Control of location of active component within support bodies Competitive adsorption with dibasic organic acids,

e.g., oxalic acid, tartaric acid, citric acid or aromatic acids with hydroxyl group besides carboxyl group, as, e.g., salicylic acid

Homogeneously applied Egg-shell Egg-white


Adsorption of precious metal on activated carbon • Freshly produced activated carbon hydrophobic • Storage without exposure to atmospheric air

maintains hydrophobicity • Usual activated carbon hydrophilic due to surface

oxidation leading to carboxylic acid groups • Oxidation, e.g., by hydrogen peroxide, nitric acid

(cautious for explosions) or ozone can increase number of carboxylic acid sites and thus raises hydrophilicity

Adsorption of Precious Metals on Activated Carbon

Limited adsorption of positively charged complexes of precious metals, such as, ammonia complexes

More extensive loading from strongly acid solutions of precious metals • Reason not completely clear

Adsorption on positively charged carboxyl acids due to uptake of additional proton

More likely good wetting of carbon by acidic solution and deposition by evaporation of liquid as species badly crystallizing from acid film on carbon surface


With pre-shaped silica supports preferably impregnation with positively charged precious metal complexes • Ammonia complexes attractive • Organic nitrogen complexes may lead to reduction of

precious metals at slightly elevated temperatures At pH levels above about 2 silica increasingly negatively

charged due to dissociation of surface hydroxyl groups; only at pH levels above about 6 sufficient reactivity of silica

At more elevated pH levels dissolution of finely divided silica to be considered

Impregnation of Pre-Shaped Support Bodies and Drying

Impregnating with active precursor solution not (strongly) adsorbing on surface of support

Most rapid procedure to arrive at industrial catalysts Selection of appropriate support from wide range of

commercial supports Pore volume and solubility of active precursor determine

maximum loading Difficult to achieve uniformly distributed active

component(s) Often concentration of active component at external

edge of support bodies

Production of Supported Catalysts by Impregnation and

Drying Impregnation with solutions of species not

strongly adsorbing on surface of support

Higher loadings can be achieved than with adsorbing species

Difficult to achieve : Uniform distribution within support body


Drying Impregnation and drying of pre-shaped support bodies Evacuation of pre-shaped support bodies

• Laboratory-scale catalyst preparation : employ vapor • Addition of volume of impregnating solution equal to

pore-volume to evacuated support Dry (under vacuum) at room temperature and

subsequently at increasingly higher temperatures up to about 120 to 150oC

Subsequent calcination at about 350 to 500oC


Drying Impregnation of porous support bodies filled with air

may lead to fracture of bodies when the amount of liquid is larger than the pore volume • Experiments on sol-gel silica spheres produced by

GBHE upon immersion in water With pore volume impregnation some volume

elements may not be penetrated by impregnating liquid • Volume elements not containing active components

Impregnation and Drying of Pre-Shaped Support Bodies

Often Active Precursor selectively Deposited on External Edge of Support Bodies With Supports of Wide Pores to be Expected

With all Hydrophilic Supports Migration of Liquid

to External Edge

Support Bodies having Wide Pores Incipient wetness impregnation and drying Deposition of active precursor at external edge Result : Eggshell catalyst

Shape of Drop dried on Glass

Drop of solution of copper(II) nitrate dried on microscope glass slide Note preferential build up of crystallites at the rim of the drop

Shape of Drop dried on Glass

Drop of solution of copper(II) citrate dried on microscope glass slide Note absence of large crystallites

Preparation of Supported Catalysts by Impregnation and Drying

Also with supports having fairly narrow pores deposition of active precursor at external edge of bodies

Evaporation of liquid at external edge and transport of liquid to external edge

Achieve uniform distribution throughout support bodies by using solutions of badly crystallizing active precursors the viscosity of which raises when the solvent is removed by volatilization Citric acid complexes EDTA complexes Addition of, e.g., sugar (prevent explosive

decomposition)


Upon suitable impregnation pores of support are uniformly filled with solution of active precursor, provided no substantial adsorption on the surface of the support proceeds

Accumulation of active species at external edge of support bodies is established during drying • During the main part of the drying process the

evaporation of solvent takes place exclusively at the external edge of the support bodies

• Transport of water vapor within porous structure proceeds too slowly to lead to significant gradients in partial pressure


Hydrostatic pressure of liquid within porous system of support bodies determined by capillary forces ΔP = 2γ/r • ΔP pressure difference between air pressure outside

pore system and hydrostatic pressure within system having pores of radius r at the external edge

• g surface energy of the liquid/gas boundary; water 72.88 dyne/cm 20oC 71.40 dyne/cm 30oC

• Since r the radius of curvature of the meniscus is negative, the hydrostatic pressure in the liquid is lower than the air pressure

Stages during Evaporation of Solvent

Initial stage

Second stage Formation of

menisci at the liquid-gas interface

Radii of menisci between different

particles equal

Four stages during the evaporation of the solvent of an impregnated support body

Third stage Haines jump by emptying of volume V1

Fourth stage Transport through

adsorbed liquid film to external surface within

funicular region

Haines Jump

Filling of smaller pockets upon emptying

of larger pockets

Evaporation of Solvent from Impregnated Support Bodies

Transport of liquid also as a liquid film over the surface of the support much more rapid than transport of vapor through emptied pores of support • Evident from the fact that the rate of evaporation is

constant during a large fraction of the drying process

• In the last stage of the drying process the liquid film has disappeared from a significant fraction of the volume of the support body, this is stage is known as the pendular state

Formation of Bubbles within Impregnated Support Bodies

Formation of Bubbles within Liquid present in Impregnated Support Bodies

Formation of bubbles of vapor of solvent during evaporation of solvent

from impregnated support bodies

Bubbles can only arise within pores of a diameter larger than the diameter of the

necks between the elementary particles at the external edge

Rate of Drying of Porous Bodies Impregnated with Water

GBHE A2ST a-alumina ring extrudates 0.27m2/g, 0.40 ml/g, void fraction 0.62

GBHE A2ST silica spheres 70 m2/g, 0.85 ml/g, void fraction 0.66

GBHE A2ST

Rate of Drying of Porous Bodies Impregnated with Water

GBHE A2ST silica spheres 70 m2/g, 0.85 ml/g, void fraction 0.66

GBHE A2ST a-alumina cyl. Tablets 1.0 m2/g, 0.20 ml/g, void fraction 0.45

GBHE A2ST

Impregnation and Drying of Support Bodies

Apparently transport of liquid either through filled pores or as a liquid film on the surface of the support to the external edge of the support bodies

With the liquid the dissolved species is migrating, which leads to deposition of the active precursor at the external edge of the support body

Important parameters : viscosity of the liquid, especially as a function of the concentration, and the interaction of the liquid with the surface of the support


Impregnation with different dissolved iron(III) species

Evaluation of the distribution obtained after drying and calcination by X-ray diffraction • Finally divided material evident from absence of

sharp X-ray diffraction profiles

Impregnation of Silica with Different Iron Precursors

Support : Silica extrudates 2.1 mm diameter Prepared from Ox 50 Degussa Surface area 44 m2/g; pore volume 0.8 ml/g void fraction 0.65

Fe EDTA pH 8.5 Fe EDTA pH 10.1 Fe gluconate NH4 Fe citrate

Fe(NH4)2(SO4)2.6H2O Fe2(SO4)3.5H2O) FeCl3.6H2O Fe(NO3)3.9H20

Impregnation of Silica with Different Iron Precursors

Simple, well soluble salts of iron(III) lead to deposition of relatively large crystallites, mainly at the external edge of the support bodies

Complexes of iron with organic ligands or the presence of dissolved organic species containing hydroxyl groups or other hydrophilic groups substantially improves the distribution of the iron species over the support

Interesting is the effect of the pH value of the impregnating liquid with the iron(III) EDTA complexes


Employing a liquid the viscosity of which rises when the solvent is evaporating suppresses motion of the liquid as an adsorbed film to the external edge of the support bodies

Interesting to establish the viscosity of the impregnating liquid as a function of the concentration taking into account that evaporation of the solvent leads to an increase in concentration

Course of Viscosity of Liquid during Drying


Effect of interaction of the species of the active precursor deposited from the solution with the surface of the support • An effect of interaction with the support indicated by

the effect of the pH of the impregnating solution with the iron(III) EDTA complexes

Experiments with silicon wafers covered by a thin silica layer upon exposure to atmospheric air • Application of thin layer of solution by spin coating • Evaluation of the deposition by AFM (Atomic Force

Microscopy)

Deposition of Copper Oxide on Silicon Wafers

Deposition of clusters of copper(II) oxide particles from a solution of Cu(NO3)2 in cyclohexane on “natural” oxide layer present on silicon

wafers

When some small particles have been deposited, the

remaining solution is taken up within

the pores in between the particles due to capillary

forces

Note high magnification

Deposition of Copper Oxide on Silicon Wafers

Silica layer on silicon wafers hydrophobic, treatment with ammonia and H2O2 required to produce a hydrophilic silica layer.

On the hydrophilic silica surface, much more

interaction with the solution, which leads to

deposition of well distributed small copper

oxide particles

Deposition of Iron Oxide on Silica Extrudates

Confirming effect of interaction with surface of support by experiments with silica extrudates • Impregnation with a solution of Fe(NO3)3 and

subsequent drying

Calcined at 750oC Fresh Treated at 100oC with NH3/H2O2/H2O)


Interaction of the species to be deposited with the surface of the support certainly important as evident from the effect of pretreatment of silica surfaces

Effect of organic species only rise in viscosity or is organic species also enhancing the interaction with the surface of the support ?

Elution Experiments with Different Iron Species adsorbed on Silicagel

Solutions of different iron compounds brought on silica gel column

Subsequently eluted with water of the same pH as the initial iron solution

Elution volumes reflecting interaction with silica surface Precursor salt Elution volume (ml) (NH4)2Fe(SO4)2 6.2 FeCl3 6.3 Fe(NO3)3 7.7 NH4 Fe citrate 5.6

Elution Experiments with Different Iron Species adsorbed on Silica gel

Apparently interaction of dissolved species behaving completely differently during drying after impregnation not significantly different

Spin-coating experiments with silicon wafers pretreated with ammonia and hydrogen peroxide

Deposition of Iron Oxide on Silicon Wafers by Spincoating

Before investigation in the AFM the wafers have been calcined

(NH4)2Fe(II)(SO4)2 FeCl3

Fe(NO3)2 NH4 Fe citrate

Wafer surfaces pre-treated with NH4/H2)2/H20

Deposition of Iron Oxide on Silicon Wafers by Spin coating

Apparently at room temperature interaction of support (silica) surface with dissolved species containing organic molecules not substantially different

At more elevated temperatures interaction much more stronger leading to tenaciously adhering film to surface of the support

Due to elevated viscosity growth of crystal nuclei impeded within the film layer

Other organic molecules containing hydroxy groups, such as, sugar or HEC exhibit same effect

Impregnation of a-Alumina with Different K3Fe(CN)6 Precursors

No HEC 1 wt.% HEC 2 wt.% HEC

HEC = hydroxy ethyl cellulose

Extrudates impregnated with Different Iron Precursors

Conclusions about Impregnation and Drying

Impregnation and drying of pre-shaped support bodies excellent and rapid procedure to produce supported catalysts • No waste water; no loss of active species • Scale up can be performed readily

At low loadings of active species adsorption on the surface of the support can allow one to control the range within the support body where the active precursor will be deposited


To achieve higher loadings adsorption of active precursors on the surface of the support can not effectively be employed

Bad distributions of the active precursors within the support bodies is due to migration of liquid elements during evaporation of the solvent at the external edge of the support bodies


Impregnation with solutions the viscosity of which increases during evaporation of the liquid is very effective in establishing a uniform distribution of the active species throughout the support bodies

The agent raising the viscosity generally also increases the interaction with the surface of the support, but as required only at elevated temperatures when the solvent has largely evaporated

Design and Production of Heterogeneous Catalysts

Technology

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