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 productio...
Objectives of
the Design of Solid Catalysts
 Most smooth penetration into market for
• Improved catalysts to be employed ...
Development of Solid Catalysts
 Mechanical strength
 Most important for technical applications
 Size of catalyst bodies...
Development of Solid Catalysts
 Transport of material and thermal energy to
and from the active sites
• Solid catalysts u...
Design of Solid Catalysts
 Catalytic activity and selectivity
 Structure and chemical composition of active surface
 Ex...
Supported Solid Catalysts
Usual separation of functions
 Support
 Size and mechanical strength of catalyst
bodies
 Por...
Supported Solid Catalysts
 However, sometimes catalytic function of support
also involved : Bi-functional catalysts
• Aci...
Unsupported Catalysts
 Reasons to employ unsupported catalysts :
 Active species providing sufficiently large and
thermo...
Unsupported Catalysts
 Examples :
• Pt or Pd gauze for the oxidation of ammonia to
nitrogen oxide in the production of ni...
Unsupported Catalysts
 Examples :
• Ethylbenzene dehydrogenation catalyst (iron
oxide promoted with potassium oxide)
 Po...
Catalyst Preparation :
Science or Art
 First example :
Preparation of VPO catalyst for oxidation of n-
butane to Maleic a...
Catalyst Preparation :
Art or Science
 Procedure (2) Katsumoto et al.
 Reduction of 15 g V2O5 at 120oC in 60 ml
1:1 (v/v...
Catalyst Preparation :
Art or Science
 Reduction of vanadium to mixture of vanadium(IV)
and vanadium(V) by either inorgan...
Solid Catalysts
 Usually supported catalysts in view of better control of
properties of catalysts
 Surface area and load...
Supported Catalysts
Supported catalyst
ThermostableUnsupported catalyst rapid sintering
Reduction
Supported MetalSupported...
Components of Solid Catalysts
 Support
 Shape and size of catalyst bodies
 Porous structure
 Mechanical strength
 Sur...
Types of Supported Catalysts
 Catalysts containing base metals, base metal oxides or
base metal sulfides
 Active surface...
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
 Applicati...
Preparation of Catalysts by
Selective Removal
 Resulting in
 powder, e.g., Raney nickel
 powder to be processed to bodi...
Catalyst Preparation: Art or
Science
 Second example :
Ammonia synthesis catalyst
 Trial and error
Iron ore
 About 97% ...
Catalyst Preparation: Science
or Art
 Selective removal of oxygen
 Minimum amount of Al2O3 to transport water rapidly
ou...
Effect of Structure of Active Surface
 Different surface structures
Different activity
per unit surface area
 Effect of ...
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
• Usuall...
Support for Precious Metals
 Precious metals used in liquid-phase processes :
 Activated carbon attractive support
• Car...
Activated Carbon Supports
 Since activated carbon is produced from peat or wood,
it is a natural product and therefore di...
Commercial
Pre-Shaped Support Bodies
 Main deficiency of alumina and silica supports : not
compatible with alkaline promo...
Preparation Procedures of
Supported Catalysts
 Application of active precursor onto
separately produced support
• Applica...
Preparation of Supported
Precious Metal Catalysts
 Most obvious procedure
Pore-volume impregnation and drying of pre-shap...
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 u...
Distribution of
Active Precious Metal Particles
Uniform distribution
of precious metal particles
Usually desired when
tran...
Preparation of Supported
Precious Metal Catalysts
 Alternative industrial procedure to arrive at egg-shell
distribution o...
Preparation of Supported
Precious Metal Catalysts
 Establishment of an egg-shell distribution of precious
metal particles...
Preparation of Supported
Precious Metal Catalysts
 Soaking of alumina support in solution of precious
metal or recirculat...
Preparation of Supported
Precious Metal Catalysts
 Uniform distribution of precious metal(s) can be
achieved more readily...
Preparation of Supported
Precious Metal Catalysts
 Control of location of active component within
support bodies
 Compet...
Preparation of Supported
Precious Metal Catalysts
 Adsorption of precious metal on activated carbon
• Freshly produced ac...
Adsorption of Precious
Metals on Activated Carbon
 Limited adsorption of positively charged complexes of
precious metals,...
Preparation of Supported
Precious Metal Catalysts
 With pre-shaped silica supports preferably impregnation
with positivel...
Impregnation of Pre-Shaped
Support Bodies and Drying
 Impregnating with active precursor solution not (strongly)
adsorbin...
Production of Supported
Catalysts by Impregnation and
Drying
 Impregnation with solutions of species not
strongly adsorbi...
Production of Supported
Catalysts by Impregnation and
Drying
 Impregnation and drying of pre-shaped support bodies
 Evac...
Production of Supported
Catalysts by Impregnation and
Drying
 Impregnation of porous support bodies filled with air
may l...
Impregnation and Drying of
Pre-Shaped Support Bodies
 Often Active Precursor selectively Deposited on
External Edge of Su...
Support Bodies having Wide
Pores
 Incipient wetness impregnation and drying
 Deposition of active precursor at external ...
Shape of Drop dried on Glass
Drop of solution of copper(II) nitrate dried on microscope glass slide
Note preferential buil...
Shape of Drop dried on Glass
Drop of solution of copper(II) citrate dried on microscope glass slide
Note absence of large ...
Preparation of Supported Catalysts
by Impregnation and Drying
 Also with supports having fairly narrow pores
deposition o...
Preparation of Supported Catalysts
by Impregnation and Drying
 Upon suitable impregnation pores of support are
uniformly ...
Preparation of Supported Catalysts
by Impregnation and Drying
 Hydrostatic pressure of liquid within porous system of
sup...
Stages during Evaporation of
Solvent
Initial stage
Second stage
Formation of
menisci at the
liquid-gas interface
Radii of ...
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...
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
e...
Rate of Drying of Porous Bodies
Impregnated with Water
GBHE A2ST a-alumina ring extrudates 0.27m2/g, 0.40 ml/g, void fract...
Rate of Drying of Porous Bodies
Impregnated with Water
GBHE A2ST silica spheres 70 m2/g, 0.85 ml/g, void fraction 0.66
GBH...
Impregnation and Drying of
Support Bodies
 Apparently transport of liquid either through filled pores or
as a liquid film...
Impregnation and Drying of
Support Bodies
 Impregnation with different dissolved iron(III)
species
 Evaluation of the di...
Impregnation of Silica with
Different Iron Precursors
Support : Silica extrudates 2.1 mm diameter
Prepared from Ox 50 Degu...
Impregnation of Silica with
Different Iron Precursors
 Simple, well soluble salts of iron(III) lead to deposition of rela...
Impregnation and Drying of
Support Bodies
 Employing a liquid the viscosity of which rises when
the solvent is evaporatin...
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 th...
Deposition of Copper Oxide
on Silicon Wafers
Deposition of clusters of copper(II) oxide particles from a solution of
Cu(NO...
Deposition of Copper Oxide
on Silicon Wafers
Silica layer on silicon wafers hydrophobic, treatment with ammonia and H2O2
r...
Deposition of Iron Oxide on
Silica Extrudates
 Confirming effect of interaction with surface of support
by experiments wi...
Impregnation and Drying of
Support Bodies
 Interaction of the species to be deposited with the
surface of the support cer...
Elution Experiments with Different
Iron Species adsorbed on Silicagel
 Solutions of different iron compounds brought on s...
Elution Experiments with Different
Iron Species adsorbed on Silica gel
 Apparently interaction of dissolved species behav...
Deposition of Iron Oxide on Silicon
Wafers by Spincoating
Before investigation in the AFM the wafers have been calcined
(N...
Deposition of Iron Oxide on
Silicon Wafers by Spin coating
 Apparently at room temperature interaction of support
(silica...
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 proce...
Conclusions about
Impregnation and Drying
 To achieve higher loadings adsorption of active
precursors on the surface of t...
Conclusions about
Impregnation and Drying
 Impregnation with solutions the viscosity of which
increases during evaporatio...
Design and Production of Heterogeneous Catalysts
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Design and Production of Heterogeneous Catalysts

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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 ......

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Design and Production of Heterogeneous Catalysts

  1. 1. Design and Production of Heterogeneous Catalysts Gerard B. Hawkins Managing Director
  2. 2. 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
  3. 3. 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
  4. 4. 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
  5. 5. 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
  6. 6. 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
  7. 7. 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
  8. 8. 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
  9. 9. 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
  10. 10. 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
  11. 11. 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
  12. 12. 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
  13. 13. 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
  14. 14. 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
  15. 15. 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
  16. 16. Supported Catalysts Supported catalyst ThermostableUnsupported catalyst rapid sintering Reduction Supported MetalSupported Metal Oxide
  17. 17. 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
  18. 18. 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.%
  19. 19. Production of Finely Divided Material Condensation of molecularly dispersed species Selective removal of some component
  20. 20. 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
  21. 21. 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
  22. 22. 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
  23. 23. 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
  24. 24. 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
  25. 25. Oxygen on Fe(100)
  26. 26. Penetration of Foreign Atoms into Surface Extended crystallographic plane Small crystallographic plane
  27. 27. 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
  28. 28. 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
  29. 29. 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
  30. 30. 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
  31. 31. 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
  32. 32. 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
  33. 33. Pore Volume Impregnation Incipient Wetness Impregnation Also known as “dry impregnation”
  34. 34. 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
  35. 35. 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
  36. 36. 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
  37. 37. 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
  38. 38. 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
  39. 39. 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 PtCl6 2- generating protons by exchange with water and dissociation of water, which proceeds slowly • PtCl6 2- + H2O = PtCl5(H2O)- + Cl- PtCl5(H2O)- + H2O = PtCl5(OH)- + H3O+
  40. 40. 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
  41. 41. 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
  42. 42. 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
  43. 43. 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
  44. 44. 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
  45. 45. 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
  46. 46. 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
  47. 47. 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
  48. 48. 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
  49. 49. Support Bodies having Wide Pores  Incipient wetness impregnation and drying  Deposition of active precursor at external edge  Result : Eggshell catalyst
  50. 50. 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
  51. 51. Shape of Drop dried on Glass Drop of solution of copper(II) citrate dried on microscope glass slide Note absence of large crystallites
  52. 52. 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)
  53. 53. 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
  54. 54. 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
  55. 55. 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
  56. 56. Haines Jump Filling of smaller pockets upon emptying of larger pockets
  57. 57. 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
  58. 58. Formation of Bubbles within Impregnated Support Bodies
  59. 59. 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
  60. 60. 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
  61. 61. 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
  62. 62. 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
  63. 63. 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
  64. 64. 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
  65. 65. 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
  66. 66. 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
  67. 67. Course of Viscosity of Liquid during Drying
  68. 68. 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)
  69. 69. 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
  70. 70. 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
  71. 71. 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)
  72. 72. 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 ?
  73. 73. 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
  74. 74. 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
  75. 75. 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
  76. 76. 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
  77. 77. Impregnation of a-Alumina with Different K3Fe(CN)6 Precursors No HEC 1 wt.% HEC 2 wt.% HEC HEC = hydroxy ethyl cellulose
  78. 78. Extrudates impregnated with Different Iron Precursors
  79. 79. 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
  80. 80. 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
  81. 81. 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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