HW3: Ethanol dehydration over 192 mg of Al-SiO2
catalyst, gas phase, fixed bed reactor, 240 °C
Feed: 4.4 mol% ethanol, the rest being flowing N2 (40 cm3 min−1 @STP)
Analysis of the products by GC-FID:
2.326 mol% ethylene, 0.332 mol% diethylether, water from
dehydration rxn, ethanol 1.553 mol%, the rest being N2 (constant)
Calculate conversion, selectivity to ethylene and diethylether + their
yields. Calculate carbon balance. Calculate WHSV.
C2H5OH → C2H4 + H2O
2 C2H5OH → C2H5OC2H5 + H2O
Heterogeneous catalysis (C9981)
Lecture 3
Catalysts preparation
styskalik@chemi.muni.cz
styskalik.sci.muni.cz
Types of catalysts
• Bulk catalysts
• Supported catalysts
• Agglomerated catalysts
Types of catalysts
• Bulk catalysts
• Supported catalysts
• Agglomerated catalysts
– Intermediate category
– Catalysts obtained by mixing active species with
support (powders+precursors)
– Ill defined catalysts – broad application in industry
Types of catalysts
• Bulk catalysts
• Supported catalysts
• Agglomerated catalysts
Preparation of bulk catalysts
• The whole body of the catalyst is made of one
active substance
• = It must be cheap, thermally, mechanically,
and chemically stable, porous,…!
– Silica-alumina (cracking)
– Raney nickel, Co molybdate (hydrogenation)
– Catalyst supports: silica, alumina, silica-alumina,
zeolites, TiO2, MgO, carbon
Preparation of bulk catalysts
• Ceramic route (heat and beat)
– e.g. Molybdates Bi2O3 + MoO3 → Bi2MoO6
– Used as selective oxidation catalysts (propylene to acrolein)
– Homogeneity, time, temperature
• (Co-)Precipitation
– E.g. NH4VO3 + Al(NO3)3 → V-Al oxynitride (used in propene
ammoxidation to acrylonitrile)
– change of pH leads to precipitation
• Sol-gel method
• Combustion synthesis
• Solvothermal method
– e.g. zeolites
Preparation of bulk catalysts
• Ceramic route, (co-)precipitation, sol-gel
method, combustion synthesis, and
solvothermal method:
#InorgMatChem
• Heat and beat quickly and effectively?
• Sol-gel continuously???
Preparation of bulk catalysts
Preparation of bulk catalysts
Preparation of bulk catalysts
◊ γ-AlOOH
● α-Al2O3
Δ α-AlOOH
Preparation of bulk catalysts
Preparation of bulk catalysts
Preparation of bulk catalysts
• Flame spray pyrolysis
Preparation of bulk catalysts
• Flame spray pyrolysis
Types of catalysts
• Bulk catalysts
• Supported catalysts
• Agglomerated catalysts
Preparation of supported catalysts
• Properties of catalyst supports
– Cheap
– Porous
– Thermally, mechanically, and chemically stable
– …
• Role of catalyst supports (non-innocent)
– Porosity/hydrophobicity vs. Activity/selectivity
– O, H, N,…bank (MvK mechanism)
– Electron donor/acceptor (Haber-Bosch)
– …
Preparation of supported catalysts
• Three types of interaction between support
and active phase
– Weak
– Electrostatic
– Covalent
Preparation of supported catalysts
• Weak interaction = Impregnation methods
– Wet impregnation
• Suspension of support (e.g. SiO2 in water)
• Solution of metal salt (e.g. Cu(NO3)2 in water)
• Mixed together (suspension)
• Cu2+ cations diffuse into the pores of SiO2 support (=we
need to wait for enough long time, Fick laws)
• Drying, calcination (= formation of Cu2O NPs on SiO2),
(reduction of Cu2O to Cu NPs)
• Disadvantages: Time, homogeneity.
Preparation of supported catalysts
• Weak interaction = Impregnation methods
– Dry impregnation (Incipient wetness impregnation)
• Dry support, pores full of gas (e.g. SiO2 in ambient air)
• Solution of metal salt, V = pore volume (e.g. Cu(NO3)2 in
water)
• Mixed together (paste)
• Capillary elevation (fast) brings Cu2+ solution directly
into the pores (concentration gradients might occur –
pore size!)
• Drying, calcination (= formation of Cu2O NPs on SiO2),
(reduction of Cu2O to Cu NPs)
• Solubility limits!
Preparation of supported catalysts
• Weak interaction = Impregnation methods
– Dry impregnation (Incipient wetness impregnation)
• Big issue: Gas trapped inside the pores
Pore collapse
Mechanical degradation
Attrition
…
Preparation of supported catalysts
• Weak interaction = Impregnation methods
– Dry impregnation (Incipient wetness impregnation)
• Big issue: Gas trapped inside the pores
Possible solutions:
- No gas (impregnation under vacuum)
- Lower surface tension (addition of a surfactant)
- Wait until gas dissappears and impregnation is completed
(either gas dissolves in solution or bubbles move out from
the pores)
Preparation of supported catalysts
• Weak interaction = Impregnation methods
– Dry impregnation (Incipient wetness impregnation)
• Big issue: Gas trapped inside the pores
Preparation of supported catalysts
• Three types of interaction between support
and active phase
– Weak
– Electrostatic
– Covalent
Preparation of supported catalysts
• Electrostatic interaction = Ion exchange
– Classic ion exchangers: clays, layered hydroxides,
zeolites
– Oxides with the help of pH
Preparation of supported catalysts
• Electrostatic interaction = Ion exchange
– Zeolites, how does it work?
– Cation compensating the negative charge?
– Na+ from the synthetic step
–
Preparation of supported catalysts
• Electrostatic interaction = Ion exchange
– Zeolites, how does it work?
– Na+ → H+ directly?
– Na+ → ??? → H+
–
Preparation of supported catalysts
• Electrostatic interaction = Ion exchange
– Zeolites, how does it work?
– Na+ → NH4
+ → H+
– Zeolite NaY
– Up to 73 % of Na+ can
be exchanged at R.T.
– 4 consecutive steps
or
continuous ion exch.
ΔT
Preparation of supported catalysts
• Electrostatic interaction = Ion exchange
– Enhancement…trap for leaving ion (shifting
equilibrium)!
– e.g. Exchange of Ni2+ for H+ in zeolite mordenite
– Ni(NO3)2 vs. Ni(OAc)2
Preparation of supported catalysts
• Electrostatic interaction = Ion exchange
– Trap
• OAc- + H+ → AcOH (rarely dissociated)
• OH- + H+ → H2O (rarely dissociated)
• SCN- + Ag+ → AgSCN (rarely dissociated)
Salt - trap Support
Preparation of supported catalysts
• Electrostatic interaction = Ion exchange
– Classic ion exchangers: clays, layered hydroxides,
zeolites
– Oxides with the help of pH
Preparation of supported catalysts
• Electrostatic interaction = Ion exchange
– Oxides with the help of pH
Preparation of supported catalysts
• Electrostatic interaction = Ion exchange
– Oxides with the help of pH
• Isoelectric point
Preparation of supported catalysts
• Electrostatic interaction = Ion exchange
– Oxides with the help of pH
• Exchange capacity: the farther you are from isoelectric
point, the higher the exchange capacity.
Preparation of supported catalysts
• Electrostatic interaction = Ion exchange
– Oxides with the help of pH
• Stability of support at different pH!
• Precursor stability at different pH!
0
10
20
30
40
50
60
70
80
90
100
0 2 4 6 8 10 12 14
pH
Distribution(%)
[Pd(H2O)4]2+
[Pd(OH)2(H2O)2]
[Pd(OH)(H2O)3]+
[Pd(OH)4]2-
[Pd(OH)3(H2O)]Cationic
species Neutral species Anionic species
0
10
20
30
40
50
60
70
80
90
100
0 2 4 6 8 10 12 14
pH
Distribution(%)
[Pd(H2O)4]2+
[Pd(OH)2(H2O)2]
[Pd(OH)(H2O)3]+
[Pd(OH)4]2-
[Pd(OH)3(H2O)]Cationic
species Neutral species Anionic species
Preparation of supported catalysts
• Electrostatic interaction = Ion exchange
– Oxides with the help of pH
• Competitive ion exchange
• Example: very high affinity between Al2O3 and Pt
• Result: heterogeneous distribution of Pt (only outer
surface)
Preparation of supported catalysts
• Electrostatic interaction = Ion exchange
– Oxides with the help of pH
• Competitive ion exchange
• NH4OH dissociates readily, NH4
+ competes with Pt for
sites (= inverse to trap)
• Result: homogeneous distribution of Pt through the
support
2–
Preparation of supported catalysts
• Electrostatic interaction = Ion exchange
– Oxides with the help of pH
• Competitive ion exchange
[NH4
+]
Preparation of supported catalysts
• Electrostatic interaction = Ion exchange
– Oxides with the help of pH
• Competitive ion exchange
Preparation of supported catalysts
• Electrostatic interaction = Ion exchange
– Oxides with the help of pH
• Competitive ion exchange
Preparation of supported catalysts
• Electrostatic interaction = Ion exchange
– Oxides with the help of pH
• Competitive ion exchange: Note
• We say homogeneous Pt distribution…
• But how homogeneous???
• Latest research from de Jong group (University Utrecht,
HR-TEM, perfect control of Pt deposition on atomic
scale) shows that uniform Pt homogeneity is not ideal
for hexane isomerization!
Preparation of supported catalysts
• Electrostatic interaction = Ion exchange
– Oxides with the help of pH
• Detailed knowledge of isoelectric point, precursor
behavior, control over pH:
Preparation of supported catalysts
• Electrostatic interaction = Ion exchange
– Oxides with the help of pH
• Detailed knowledge of isoelectric point, precursor
behavior, control over pH:
Preparation of supported catalysts
– NPs by wet (dry) impregnation vs. impregnation
with electrostatic interaction
calcination
Electrostatic
interaction
Uniform NPs
+ + + + +
− − − − −
Preparation of supported catalysts
• Electrostatic interaction = Ion exchange
– Oxides with the help of pH
Preparation of supported catalysts
• Three types of interaction between support
and active phase
– Weak
– Electrostatic
– Covalent
Preparation of supported catalysts
• Covalent bonds = grafting
– Formation of metal oxide on top of catalyst support
– Is calcination necessary? More in lecture on single
site catalysts…
surface grafting
+ calcination
OH OH OH
Oxide
Oxide 2
+ MClx = active metal
OH OH OH OH OH
Oxide
RO
M
OR
OR
R'
M = Al, Ti, Zr, ...n
MClx
or
Organic (O)
Inorganic (I)
surface grafting
+ calcination
OH OH OH
Oxide
Oxide 2
+ MClx = active metal
OH OH OH OH OH
Oxide
RO
M
OR
OR
R'
M = Al, Ti, Zr, ...n
MClx
or
Organic (O)
Inorganic (I)
Preparation of supported catalysts
• Covalent bonds = grafting
– Formation of metal NPs on top of catalyst support
Preparation of supported catalysts
• Covalent bonds = grafting
– NPs by impregnation vs. grafting
calcination
Covalent bonds
Uniform NPs
Preparation of supported catalysts
• Covalent bonds = grafting
– Formation of metal NPs on top of graphene oxide
Pd
OAcEt2HN
AcO NHEt2
Pd
OAcEt2HN
AcO NHEt2
Carbon
C
OO
Pd
OAcEt2HN
complex [Pd(OAc)2
(Et2
NH)2
]
Content of COOH groups
increased by oxidation
(HNO3, H2O2,O3,…)
Preparation of supported catalysts
• Covalent bonds = grafting
– Formation of metal NPs on top of graphene oxide
– Last step: Pd reduction
200 nm200 nm
Without
functionalization
by oxidation
(thus almost no
grafting)
With
functionalization
by oxidation
(thus grafted Pd
complexes)
200 nm200 nm
no visible
metal
aggregates!
Preparation of supported catalysts
• Covalent bonds = Deposition/precipitation
– Starts as a dry impregnation
– Metal salt is then selectively precipitated at the
catalyst surface
– Precipitation mainly by pH change –
hydrolysis/condensation