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 4
Catalysts preparation – continuation
- Shaping
- Single site catalysts
styskalik@chemi.muni.cz
styskalik.sci.muni.cz
Heterogeneous catalysis
Lecture 4
Catalyst synthesis – continuation
- Shaping
- Single site catalysts
Shaping
• Why is shaping important?
– Accessibility
(micro+meso+macro porosity)
– Mechanical resistance
(application of binders)
– Reducing pressure drop
(p1 > p2)
– Hydrothermal stability, coking, poisoning,…
p2
p1
→ Industrial point of view
Shaping
• Industrial example:
Fluid catalytic cracking
Shaping
• Fluid catalytic cracking
„agglomerated catalyst“
Shaping
• Industrial example:
Fluid catalytic cracking
micrograin
Elementary particles
Shaping
• Macroscopic shaping
– Extrusion
– Pelletization
– Molding/casting
grain
Shaping
• Shaping as a „mechanochemical synthesis“?
Single site catalysts
• What is it?
Active site in cytochrome P450
Enzymatic catalysis
Grubbs catalyst
Homogeneous catalysis
Single site catalysts
• What is it?
Single site catalysts
• What is it?
Single site catalysts
• Any continuous material (metal, oxide), where
catalysis relies mainly on structure deffects
• Instead
– One (or more) atoms
– Spatially isolated
– Same energy of interaction with reactant
– Structurally well characterized (similar to homo)
Single site catalysts
• What is it?
– Individual isolated ion/atom/molecular
complex/cluster
• Grafted on porous support
• Created within the material by building block approach
– „Ship in bottle“ structures– e.g. molecular
complex/enzyme trapped within a zeolitic cage
– Crystalline, open-structure, microporous solids
(e.g. zeolites) with active sites uniformly
distributed throughout the bulk
Single site catalysts
• Why?
• Why?
– Number of active sites
– Turn-over frequency, TOF
– Characterization of active site
– Quantum-chemical calculations
↓
– Understanding mechanism
– Understanding kinetics
↓
– Bridging gap between homo-and heterogeneous cata
Single site catalysts
Single site catalysts
• Bridging gap between homogeneous and heterogeneous
catalysis – example:
– Phillips catalyst = „CrO3“ dispersed on silica
– Ethylene polymerization (50 % of global production of linear HDPE)
– No co-catalyst (compare with Ziegler-Natta + MAO)
– What is the actual active site???
Single site catalysts
• CrO3 dispersed on silica
• Ethylene exposure induces „mild reduction“
• Only 10 % of Cr sites are active
• CrII or CrIII??? Big discussion…
Single site catalysts
CrII CrIII
Single site catalysts
CrIII is active!
Single site catalysts
• What is it?
– Individual isolated ion/atom/molecular
complex/cluster
• Grafted on porous support
• Created within the material by building block approach
– „Ship in bottle“ structures– e.g. molecular
complex/enzyme trapped within a zeolitic cage
– Crystalline, open-structure, microporous solids
(e.g. zeolites) with active sites uniformly
distributed throughout the bulk
Single site catalyts
• Grafting on silica
– Detailed knowledge of surface (number of OH
groups per nm2)
– Rigorous water- and oxygen-free environment
Single site catalysts
Single site catalysts
• Metal precursors (almost all d + f elements)
– Organometallic compounds
– Metal amides and silylamides
– Metal alkoxides and siloxides
– Metal halogenides
Single site catalysts
Single site catalysts
• In many cases silica needed all around the
active site (no unreacted chlorines etc.)
+ SiO2
2. ΔT
− butene
1.
− butanol
Single site catalysts
• Grafting on silica – other types of bonding
Single site catalysts
• Grafting on silica – other types of bonding
Single site catalysts
• Grafting on carbon – π-π stacking
Single site catalysts
• Grafting on carbon – π-π stacking
– Boomerang effect
Single site catalysts
• What is it?
– Individual isolated ion/atom/molecular
complex/cluster
• Grafted on porous support
• Created within the material by building block approach
– „Ship in bottle“ structures– e.g. molecular
complex/enzyme trapped within a zeolitic cage
– Crystalline, open-structure, microporous solids (e.g.
zeolites) with active sites uniformly distributed
throughout the bulk
Single site catalysts
+ n MClx
Si8O20(SnMe3)8
= SnMe3 MClx = PCl3, SiCl4, BBr3, AlCl3, GaCl3, TiCl4,
ZrCl4, VCl4, VOCl3, SnCl4, WCl6,
MoO2Cl2
-ClSnMe3 M
O
O
O
O
M
M
M
M
1. N.N. Ghosh, J.C. Clark, G.T. Eldridge, C.E. Barnes, Chem. Comm., (2004) 856-857.
2. M.-Y. Lee, J. Jiao, R. Mayes, E. Hagaman, C.E. Barnes, Catalysis Today, 160 (2011) 153-164.
Single site catalysts
Si8O12(OSnMe3)8
O
Al
O
O
Si
S iC l4
Me3SnCl
RO
Al
Cl
Cl
Cl
R
Si
O
Si
Me3SnCl
Al
O
O
O
O
R R
limiting
amount
R R
Single site catalysts
• Targeting connectivity
Al
O
O
O
L
3C-Al←L
L = py (1), THF (2)
Al
O
O
O
O
X+
[X][4C-Al]
X = NBu4
+ (3)
Single site catalysts
• Targeting geometry
– Tetrahedral
– Square planar
Ti
O
O
O
O
4C-Ti
Single site catalysts
• What is it?
– Individual isolated ion/atom/molecular
complex/cluster
• Grafted on porous support
• Created within the material by building block approach
– „Ship in bottle“ structures– e.g. molecular
complex/enzyme trapped within a zeolitic cage
– Crystalline, open-structure, microporous solids (e.g.
zeolites) with active sites uniformly distributed
throughout the bulk
CHARACTERIZATION?
Single site catalysts
• What is it?
– Individual isolated ion/atom/molecular
complex/cluster
• Grafted on porous support
• Created within the material by building block approach
– „Ship in bottle“ structures– e.g. molecular
complex/enzyme trapped within a zeolitic cage
– Crystalline, open-structure, microporous solids
(e.g. zeolites) with active sites uniformly
distributed throughout the bulk
Single site catalysts
Cobalt phthalocyanine within zeolite Y
Single site catalysts
• Single site very well preserved (no
dimerization,…)
• Diffusion of molecules small enough in the
micropore system of zeolite – both reactants
and products – enhancing specificity and
selectivity
Single site catalysts
• Synthesis?
– Cation exchange (2Na+ for Co2+)
– Complexation with ligands within the zeolite pores
„Co2+-exchanged zeolite-Y was heated with excess amount
of 1,2-dicyanobenzene (DCB) under vacuum. Excessive DCB
was then extracted (Soxhlet).
The dicyanobenzene molecule is small enough (∼6.5 Å) to
diffuse through the 7.4 Å windows of the supercage of the
zeolite and condense around the metal ion to form cobalt
phthalocyanine.“
Single site catalysts
• Enzyme agglomerates inside a hollow silica sphere
Chem. Sci., 2020,11, 954-961
Single site catalysts
• What is it?
– Individual isolated ion/atom/molecular
complex/cluster
• Grafted on porous support
• Created within the material by building block approach
– „Ship in bottle“ structures– e.g. molecular
complex/enzyme trapped within a zeolitic cage
– Crystalline, open-structure, microporous solids
(e.g. zeolites) with active sites uniformly
distributed throughout the bulk
Single site catalysts
• Zeolites
– ≡Si−O(H)−Al≡
• MAlPOs
– ≡MII−O(H)−P≡
– M = Mg, Co, Zn, Mn…
Single site catalysts
• Zeolites and MAlPOs
– Microporous
– Crystalline
– Ideal to study by many analytical techniques
Single site catalysts
• Zeolites and MAlPOs
– Isolated sites???
• Si/Al ratio ≥ 200; perfect crystallinity
– Equivalent sites???
– Amorphous grains/surface species
Single site catalysts
• Zeolites and MAlPOs
– Isolated sites???
– Equivalent sites???
• Different positions in cage
– Amorphous grains/surface
species
Single site catalysts
• Zeolites and MAlPOs
– Isolated sites???
– Equivalent sites???
– Amorphous grains/surface species
• e.g. extraframework aluminum species
– From not fully embedded (connectivity = 3) surface Al atoms
to amorphous alumina (nano)particles
– Tetra, penta, hexacoordinated
– Easy to overlook (99 % of the material is crystalline), but
important to notice
– Their role in catalysis???
Single site catalysts
• Similar but different (Markus Antoinetti et al.)
• Doped graphene sheets
• Isolated sites located by HR-TEM
Single site catalysts
• Doped graphene sheets
– Single metal atoms within carbon layer
• Single atom = „complex like“
• Conductive carbon layer, electrons shared = „metal like“
• Unique
• Single-site Au highly active in Haber-Bosch ammonia
synthesis, mild conditions
Single site catalysts
• Doped graphene sheets
– Single Au atoms within N-doped graphene
References