design and production of heterogeneous catalysts
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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
Unsupported Catalysts
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
Unsupported Catalysts
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”
Preparation of Supported Precious Metal Catalysts
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
Preparation of Supported Precious Metal Catalysts
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
Preparation of Supported Precious Metal Catalysts
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
Preparation of Supported Precious Metal Catalysts
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
Preparation of Supported Precious Metal Catalysts
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+
Preparation of Supported Precious Metal Catalysts
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
Preparation of Supported Precious Metal Catalysts
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
Preparation of Supported Precious Metal Catalysts
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
Production of Supported Catalysts by Impregnation and
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
Production of Supported Catalysts by Impregnation and
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)
Preparation of Supported Catalysts by Impregnation and Drying
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
Preparation of Supported Catalysts by Impregnation and Drying
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 and Drying of Support Bodies
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
Impregnation and Drying of Support Bodies
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
Impregnation and Drying of Support Bodies
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)
Impregnation and Drying of Support Bodies
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
Conclusions about Impregnation and Drying
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
Conclusions about Impregnation and Drying
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
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