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
„malaxed-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?
Výsledek obrázku pro cytochrome p450 active site Výsledek obrázku pro grubbs catalyst
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
i
C
l
4
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
•EXAFS + XANES


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
https://pubs.rsc.org/image/article/2020/sc/c9sc04615a/c9sc04615a-s2_hi-res.gif
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