Catalysis by Zeolites

Transcription

EUCHEME 11/08/2012
CRERG - IBB
Carlos Henriques
Department of Chemical Engineering
Instituto Superior Técnico – IST, Lisbon
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1. Introduction to Zeolites
2. Composition, Porous Structure and Active
Sites
3. Shape-Selectivity with Zeolites
4. Zeolites in Industrial Catalysis
a. Catalytic cracking
b. Hydrocracking
c. Metanol to gasoline (MTG)
5. Zeolites and Green Chemistry
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Zeolites
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Swedish mineralogist A.F. Crönstedt
• upon rapidly heating, the mineral Stilbite
produces large amounts of steam
from adsorbed water
hydrated calcium aluminium silicate
(natural zeolite)
ZEOLITE → (zeo – boiling; lithos – stone)
A.F. Crönstedt, Akad. Handl. Stockholm,18 (1756) 120
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Zeolites
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¾ Crystalline alumino-silicates, with regular open
tridimensional nanosized porous framework
ZSM-5 (MFI)
MORDENITE (MOR)
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What is so special about zeolites?
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9 Zeolites have pores with nanosized
dimensions (0.3 – 0.8 nm) Î Shape Selectivity
9 As crystalline materials, zeolites present a
narrow range of pore sizes Î gives better
selectivity than non-crystalline materials
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What is so special about zeolites?
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¾ Ion-exchange properties
Î Acidity
ÎTransition metals
Catalytic
Active sites
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Kinetic diameter (nm)
Pore Size
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Kinetic Diameters of molecules, when
compared with Zeolite Pore Sizes
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0.8
Zeolite-X, Y
0.6
ZSM-5
0.4
Zeolite-4A
0.2
0
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Zeolite Framework (1)
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Î Zeolite framework is composed of SiO4 and
AlO4 tetrahedral units (Al, Si Î T- atoms),
sharing oxygen between every two consecutive
units
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Arrangement of Primary Building Units
oxygen tetrahedral
solid tetrahedral
negative charge
—
TO4
O
- Al or Si
c - oxygen
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How zeolites are built
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Î Cations (Na+, NH4, H+, transition metals)
located inside the channels or cavities of
zeolites, to balance negative charges in the
framework:
Exchange Positions
Zeolite sodium form
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FAU structure
(Y zeolite)
Exchange sites
1.3 nm
Al positions
Framework and
EFAL
Supercage ~ 1.7 nm
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Y zeolite
(FAU structure)
1.3 nm
0.77 nm
0.71 nm
ZSM-5
(MFI structure)
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Zeolites Morphology
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SEM photos
W.J. Mortier, Leuven
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Why Zeolites as Catalysis ?
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acidity (both Brönsted – protons – and Lewis –
electrons acceptors acid sites)
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ion-exchange capacity
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shape selectivity (separations, catalysis)
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confinnement effects (small cages act like nanoreactors)
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allows the stabilization of particular species of metal
active sites
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acceptable stability of both framework and active sites
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Key Structural Features in Zeolites
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9 Zeolites can be synthesized with a wide range of
pores sizes and shapes
9 Composition (Si/Al ratio) can be modified during
synthesis or by post-synthesis treatments
(dealumination; desilication)
9Pure silica zeolites (e.g., silicalite, with MFI
structure) tend to be hydrophobic
9 High
alumina
content
zeolites
have
significant amount of charge balancing extraframework cations (in exchange positions) and
have a very high affinity to polar molecules:
hydrophilic
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Composition of Zeolites
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¾ Zeolites are crystalline microporous
alumino-silicates, constituted by a tridimensional arrangement of TO4 tetrahedra,
linked by oxygen atoms, forming different
construction units and large frameworks,
where identical blocks constitute unit cells :
+
M nx/n
(AlO 2− ) x (SiO 2 ) y
n – M cation charge
x+y – number of tetrahedra per unit cell
y/x – atomic Si/Al ratio
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Porous Structure (1)
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¾ Porous Structure: most zeolites have
pores in a 4-10 Å (0.4-1 nm) range
¾ They can act as sieves, allowing to
separate molecules by their size – Molecular
Sieves
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¾ Most zeolites are classified accordingly to the
number of oxygen atoms in the opening (“ring”) of
larger pores:
• small-pore zeolites, with 8-membered oxygen
rings and a “free” diameter of 3 - 4.5 Å
• medium-pore zeolites, with 10-member
oxygen rings and a “free” diameter of 4.5 - 6 Å
• large-pore zeolites, with 12-member oxygen
rings and a “free” diameter of 6 - 8 Å
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¾ Porous Structures: 3 letters code, accordingly to
IZA definition: FAU, MFI, MOR, BEA, …
hexagonal
supercages prysms
sodalite
cages
Porous Structure of
some zeolites:
a) FAU
b) MOR
c) MFI
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Active Sites in Zeolites (1)
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¾ Active sites in zeolites
9 Each zeolite can be obtained with a large range
of compositions, both during synthesis or by
post-synthesis modifications
9 Furthermore, different compounds can be
introduced (or even synthesized) inside zeolite
pore system
9 Consequently, zeolite can behave like acid
catalysts, basic catalysts, redox catalysts, metal
catalysts, bi (multi)-functional catalysts
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91. Acid Catalysts: cracking, isomerization
9 Several hydrocarbon reactions are catalysed by
Brönsted acid sites (proton donators)
9 Lewis acid sites (electron acceptors – ex: extraframework Al species) often seem not participate
directly in the reactions mechanism, but can
increase Brönsted acidity strength
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¾ in acidic catalysis, zeolites activity depend on the
number of protonic sites and on their intrinsic
activity;
¾ active sites located in very small micropores,
where reactants cannot access, are inactive;
¾ even for accessible sites, different issues related
to reaction intermediate species can determine
catalysts activity;
¾ important parameters are also acid strength and
acid density (namely for bimolecular reactions)
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9Acid Catalysts
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sodium form
ion-exchange
ammonium form
thermal treatement
Brönsted Acid Sites
Brönsted acidity
dehydroxylation (- H2O)
Lewis Acid Sites
Basic sites
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9Acid Catalysts
¾ Protonic acid sites comes essentially from
bridged hydroxyl groups in the framework………..
Al-(OH)-Si
(no Al-(OH)-Al exist: Lowenstein rule)
¾ The maximum concentration of protonic acid
sites is equal to the concentration of Al atoms in
the framework of the zeolite
¾ BUT the real concentration is usually lower: noncrystalline fraction, dehydroxylation or even
dealumination, during thermal treatments at high
temperature (T> ~450ºC) decrease Al species in
framework.
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9 Acid Catalysts
Acid sites strength in zeolites is much higher than in
amorphous silica alumina:
A (zeolite)
B (amorph. Si/Al)
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9 Acid Catalysts
9 in zeolite structures (A), Al-O and Si-O bonds are
almost equivalent and the strong interaction Al-O
result in a weaker bond O-H, increasing the
strength of the proton
9 on the contrary in amorphous silica-alumina (B),
the acid site is represented by a silanol with a
weak acid-base interaction between OH group and
the Al atom
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9Acid Catalysts
9 there is a relationship between the protonic acid
strength and the angle of TOT bonds (T = Si or Al).
The higher the angle, the strongest the acid sites:
H-MOR (143º-180º) > H-MFI (133º-177º) > H-FAU 138º-147º
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9Acid Catalysts
¾ protonic acid sites strength depend on their
proximity: is maxima when sites can be considered
isolated (Barthomeuf et al.), i.e., when sequences …AlO-Si-O-Al… does not exist (but mainly ...Si-O-Al-O-Si-OSi…)
¾ relationship between acidic activity and Al
concentration has been shown for different reactions
and it is only observed for pure protonic zeolites,
without any extra-framework aluminium species, which
are obtained by dealumination
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2. Metal Catalysts
9 redox and hydrogenation sites
¾ Metal catalysis in zeolites is carried out by
metallic species introduced in zeolite pores or
framework during synthesis or post-synthesis
modifications
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9 Metal Catalysts
Metal Species can be located:
¾ in the zeolite framework, like Silicalite-1 (TS-1), a
pure titanium MFI structure molecular sieve;
¾ in extra-framework locations;
¾ in exchange positions (transition metals able to
change their valence);
¾ impregnated in the pore system (as when metal
oxides are introduced).
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9 Bifunctional Catalysts
¾ different catalytic processes run in the
simultaneous presence of different type of active
sites (catalytic functions) – multifunctional catalysis
¾ bifunctional catalysts – e.g. ,acidic and
hydrogenating functions, are mainly used in refining
processes: light alkanes isomerization,
hydrocracking, catalytic dewaxing, aromatization of
light alkanes, isomerization of C8 aromatic fraction…
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¾Bifunctional Catalysts
9Hydrogenating/dehydrogenating catalytic functions
are usually fulfilled by:
9 dispersed transition (Ni) or noble metals (Pt,
Pd,…) in exchange positions in the zeolite or in
another support (Pt/Al2O3)
9 metal (Ni, Co, Mo) sulphides, when sulphur is
present in the feed
9 metal oxides (Ga2O3/MFI) as in the aromatization
of C6-C7 cuts
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How did nanoporosity influences catalysis ?
Two ways, mainly:
9 1. Shape selectivity
P.B. Weisz et al., J. Phys. Chem. 64 (1960) 382
9 2. Confinement effects
E. G. Derouane, J. M. André, A. A. Lucas, J. Catal 110 (1988) 58
Zeolite cages, channels and channels
intersections really act like nanoreactors
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Shape Selectivity (1)
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¾ Shape Selectivity is a particular type of
selectivity that is originate in the fact that active
sites, in zeolites, are included in a microporous
framework with dimensions similar to those of
molecules of reactants and products
¾ This micropore framework is constituted by
cages, channels and channels intersections, that
can really be considered as nanoreactors (or
molecular reactors)
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¾ It means that their shape and size, the shape and
size of the inter-connecting rings will determine the
selectivity of catalyzed reactions
¾ This micropore system will also influence the
activity and stability of zeolite catalysts
¾ Furthermore, this particular type of catalyst
structure (framework) allows the stabilization of
particular, well defined and relatively homogeneous
types of active sites (i.e., metal active sites)
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¾ Consequently, it becomes a real possibility, with
zeolite catalysts
i. to tune active sites properties
ii. to choose the appropriate zeolite structure in
order to obtain, for a given reaction (or set of
reactions) an “ideal” catalyst on base of a
scientific approach
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Molecular Sieving
Reactant Selectivity
Branched molecules
do not access
channels
nC6/iC6 separation
+ nC6
transformation
Product Selectivity
Bulker products are
formed insid pores
but their exit is
hampered by slower
diffusion
toluene
dismutation w/
increasing
p-Xy
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I - Molecular sieving of reactants
Reactant type shape selectivity: competitive cracking of noctane and 2,2,4- trymethylpentane, the last being too bulky to
enter the pores of the zeolite and is hindered from reaching
the active sites inside pores.
- n-octane, on the contrary, is readily converted
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1.5 nm
¾ First Shape Selectivity to be found is in the base of
Selectoforming Process (Mobil) Erionite Zeolite
¾ Selective dehydration of 1-butanol over a 5A zeolite (0.5nm
pores), in a mixture with 2-butanol or iso-butanol
¾ Selective hydrogenation of n-butene in a mixture with isobutene over a Pt-5A catalyst
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1. Shape Selectivity (6)
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i) Molecular Sieving of Reactants (cont)
¾ This Molecular Sieving phenomena is based in the
difference of the rate of diffusion of the
considered molecules (that can be expressed by
the ration between their Diffusion Coefficients
DA/DB): Shape Selectivity usually occurs when this
ratio tends to infinite – one of the molecules
doesn’t enter in the pores, so it cannot diffuse!!!
¾ Nevertheless, lower ratios can also configure a
Shape Selectivity behaviour, depending on the
relative rates of diffusion and reactivity
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ii) Molecular Sieving of Products
¾ Applied by Mobil
to p-xylene synthesis
over ZSM-5 zeolite
processes:
benzene
pxylene
Product Selectivity:
Toluene dismutation
Bulkier products can
be formed inside pores
but their exit is
hampered by slower
diffusion
o-xylene
m-xylene
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9 the formation of bulkier products become limited
by their desorption
9 diffusion coefficient of p-xylene is several orders
greater than o- or m-xylenes in ZSM-5 catalyst
9 furthermore, o- and m- isomers, trapped inside
zeolite structure, are converted in p-xylene
9 for other reactions, when inter-conversion is not
rapid, entrapped species concentration can increase
and deactivate the catalysts
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Toluene
Ethylene
¾ Product shape-selectivity: acid catalyzed alkylation of Toluene
with Ethylene (aromatic C8 formation)
¾ Both reactants are small enough to enter the zeolite pores, but
from the potential products (o-, m- and p-ethyltoluene), only the
slim p-ethyltoluene is small enough to leave the pore system 44
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Molecular Sieving Selectivity depends on
• the relative reaction and diffusion rates
• the diffusion coefficients
molecules and pores sizes)
ratio
(size
of
• length of diffusion (zeolite crystallites size)
There are the possibility of deactivate external
(non-shape selective) active sites, by coke
deposition, selective poisoning (bulkier bases), …
Î Increasing global Shape Selectivity
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iii) Transition-State Selectivity
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Reaction intermediates and/or
transition states are sterically
limited by available space near
active sites
Dimethy-benzene dismutation over HMOR catalysts
diphenyl - methane intermediates
¾ Reactants and products can easilly diffuse in catalyst
structure, BUT intermediairy species formation, in the vicinity
of active sites (cages, channels, channels intersections) are
sterically limited: no 1,3,5 trimethylbenzene is formed!!!
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In an appropriate size zeolite no room exists for the formation
of bimolecular reaction intermediates:
Î no toluene or trimethyl benzenes are observed !!!
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Transition State Shape Selectivity: m-Xylene can
undergo acid-catalyzed isomerization into p-xylene
(and o-xylene, omitted from the figure) and
transalkylation into toluene and one of the
trimethylbenzene isomers.
ÎTransalkylation is a bimolecular reaction and bulkier
intermediate species are formed, when compared with
monomolecular isomerization
ÎShape Selective catalysts hampered their formation
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9 Contrary to Molecular Sieve effect, Transition-State
Selectivity does not depends on crystallites sizes, on
relative rates of reaction and diffusion, on Diffusion
Coefficients ratio,
9 Only depends of the porous structure of the zeolite
and the size of transition-state species.
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¾ This transition-state effect can co-exist with
molecular sieve effects (namely products);
¾ Transition-State Selectivity mainly concerns all
transformations that occur by inter-molecular
(bimolecular) reactions that originate intermediary
species bulkier than those arising from
monomolecular reactions, for similar reactants.
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¾ This can explain the key role of the porous
structure of zeolite catalysts on reactions
mechanism, in the case that both type of
interactions (intra- or inter-molecular) are possible.
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Transition-State Selectivity also plays a key role in
what heavy adsorbed products (coke) formation is
concerned:
¾ Coke formation occurs via bimolecular steps as
condensation reactions, that are very sensitive to
steric constraints
¾ Coke rate formation is strongly dependent of
the size of zeolite structure: smaller cavities do
not favours coke formation inside pores
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Confinement Effect (1)
Confinement Effect Î Is due to the strong interaction
between zeolite frameworks and molecules:
zeolites act as solid solvents
¾ One of the most important consequences of this solid
solvent effect, is that the concentration of reactants is
much higher inside the structure than it is outside
¾ This concentration of reactants inside the pores
presents a positive effect on reactions rates
¾ This effect evidences the role of interaction forces
between molecules and zeolite framework .
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¾ The very important increase of reactants
concentrations inside zeolite pores is one of the
explanations for the high activity of zeolite catalysts,
when compared with other structures
¾Other important issue related to Confinement
Effect is the fact that bimolecular reactions play a
major role in zeolite catalysts when compared with
other structures: zeolites allowed to put in evidence,
for the first time, the bimolecular character of
different reactions.
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¾ Kinetic models must take into account, besides
classic
chemisorption
steps
on
reaction
mechanisms, physisorption steps on micropores
(Langmuir model)
¾ Assuming a simple first order reaction A → B, the
correct expression for the reaction rate is not
r = k.[A] but is better described by LangmuirHinshellwood kinetics:
r = k. θA (θA fraction of sites occupied by A)
Î r = k.K A .[A]/(1 + K A .[A] + K B .[B])
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Shape Selectivity (Conclusions)
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¾ Shape Selectivity phenomena clearly highlight the
important role of the size and shape of pore, cages
and channels that constitute the zeolite framework
¾ Zeolite nanopore structure (succession of cages
act as nanoreactors) makes zeolite unique tools for
the development of selectivity in heterogeneous
catalysis
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Shape Selectivity (Conclusions)
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¾ In all processes that use zeolites as catalysts,
their activity, selectivity and stability depends not
only on the type of active sites, but also on their
location inside the zeolite structure.
¾ Shape Selectivity properties of zeolites
constitute one of the main reasons for the
applications of these solids in catalysis
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Zeolites in Industrial Catalysis (1)
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¾ Zeolites are widely used both in refining and
petrochemical industries
¾ We will look for three different processes where
zeolites are used
9 Catalytic Cracking (acid catalysis)
9 Methanol to Gasoline (MTG/Mobil)
(acid catalysis with shape selectivity)
9 Hydrocracking of paraffins
(bifunctional catalysis)
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Simplified flowsheet diagram of refinery operation
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Catalytic Cracking (1)
9 Catalytic Cracking - Fluidized Catalytic Cracking/FCC
intends to transform heavy products in a refinery process
(mainly from vacuum distillation) into lighter products:
gasoline, C3-C7 olefins
9 FCC catalyst is mainly a FAU zeolite in acidic (protonic)
form: USHY (Ultra-Stable HY)
9 Ultra-Stability arises from controlled dealumination
(during preparation) leading to a more resistant
framework (high T and H2O presence)
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a. Catalytic Cracking
¾ FCC processes are quite flexible, as they can
transform different fractions (paraffinic,
naphthenic or aromatics).
¾ One of the key points on these process is the
formation of significant amounts of heavy
unsaturated polyaromatic surface products (coke,
1- 1.5% of the feed) Î Catalysts Deactivation Î
ÎRegeneration
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Catalytic Cracking
9 Coke rapidly deactivates the catalyst that needs
to be continuously regenerated by coke burning
9 Heat released during regeneration is recovered
for the endothermic cracking process
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Catalytic Cracking – FCC Process
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490-540 ºC
coke ≈ 1-1.5%
Catalyst
stripper
680-760 ºC
560-600 ºC
650-750 ºC; coke < 0.05%
FEED
Pre heating
150-300 ºC
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Catalytic Cracking - Main Reactions
Compounds
Reaction
Products
Paraffins
cracking
paraffins + olefins
cracking
olefins
cyclization
naphthenes
isomerization
iso-olefins –(hydrogen transfer)Æ iso-paraffins
hydrogen transfer
paraffins
Olefins*
cyclization, condensation
coke
dehydrogenation
Naphthenes
Aromatics
cracking
olefins
dehydrogenation
cyclo-olefins –(hydrogen transfer)Æ aromatics
isomerization
naphthenes w/different rings
cracking of side chain
unsubstituted aromatics + olefins
transalkylation
alkylaromatics
dehydrogenation,
condensation
polyaromatics –(dehydrogenation condensation)Æ coke
* - mainly from cracking but also as charge impurities
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Catalytic Cracking- The Mechanism (1)
9The mechanism of catalytic cracking proceeds
via the formation of charged organic species –
carbocations – as intermediate species
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9 1st step corresponds to the formation of
carbocations (R+) - Initiation:
i) hydride abstraction on a Brönsted site
ii) hydride abstraction on a Lewis site
iii) paraffin protonation on a Brönsted site
(formation of penta-coordinated species)
iv) olefins (as impurities of the feed or arising
from thermal cracking) protonation
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1. Initiation
(Hydride abstraction)
R1 - CH2 - CH2 - R2 + H+ Z- →
(paraffin)
(carbocation)
R1 - CH2 – C+H
- R2 + Z- + H2
(Brönsted site)
(Hydride abstraction)
(carbocation)
R1 - CH2 - CH2 - R2 + L+ → R1 - CH2 - C+H - R2 + HL
(paraffin)
(Lewis site)
(Protonation)
R1 - CH = CH - R2 + H+ Z- →
(olefin/feed)
(carbocation)
R1 - CH2 – C+H
- R2 + Z -
(Brönsted site)
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92nd step corresponds to the cracking of C-C
bonds via β-scission of formed carbocation
(cracking takes place in the C-C bond in β position,
considering the positive charge)
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Chain Propagation (hydrogen transfer reactions)
R1- CH2- C+H-R2 + R3- CH2- CH2-R4 → R1- CH2- CH2-R2 + R3- CH2- C+H-R4
(carbocation)
(paraffin)
(paraffin)
(carbocation)
Cracking (β scission)
R3 - CH2 – C+H - R4 →
(carbocation)
R3+
(carbocation)
+
CH2 = CH - R4
(olefin)
FCC - important
source of olefins
Chain Termination
Occurs when the surface carbocations are desorbed (as olefins) and
the Brönsted sites of the catalyst are regenerated:
R3+ → R3= + H+
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FCC catalysts - The HY zeolite (1)
Sodalite cage
TO4
tetrahedra
T = Si, Al
Final Structure:
Sodalite cages
linked by hexagonal
prisms, in a
tetrahedral
arrangement,
defining supercages
(13Å)
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9 The industrial cracking catalyst: 5-40% HY
zeolite in a ceramic matrix (alumina, silica,
amorphous silica-alumina)
9 This matrix intents to protect zeolite crystallites:
(i) from abrasion, (ii) retaining metal species (V, Ni
as asphaltenes) and (iii) N - containing molecules
(acidity poisons) from HC feed
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¾ The fresh zeolite continuously fed to the raiser is
rapidly deactivated;
¾ It spends ~80% of time inside regenerator (coke
burning (680 ºC < T < 750 ºC), in the presence of steam;
¾ Under this conditions, zeolite suffers dealumination
(extraction of aluminium atoms from the framework).
¾ Such a phenomena, if in a large extent, can destroy
the zeolite structure – collapse
72
EUCHEME 11/08/2012
To avoid this:
9 a previous controlled and limited dealumination
treatment (high temperature steaming followed by
acid leaching of extra-framework aluminium atoms,
EFAL) is performed during preparation Î Ultra Stable
Y zeolite
9 elimination of EFAL atoms is important, as their
presence increases protonic acidity strength and coke
and gases formation, become too important.
9 this controlled dealumination also results in the
formation of some mesoporosity, what facilitates the
diffusion of bulkier molecules
73
EUCHEME 11/08/2012
9 Dealumination also results on a decrease of
acid sites, although an increase of acid strength
of remain sites is normally observed
9 As acid sites become more isolated, (less sites
density), so bimolecular reactions like hydrogen
transfer are reduced: FCC products become more
olefinic and less aromatic.
9 Also condensation reactions (coke formation)
are reduced
74
EUCHEME 11/08/2012
FCC - CONCLUSIONS
9 Very important process to obtain transportation
liquid fuels from crude;
9 Y zeolite in acidic form are normally used in a
dealuminated-stabilized form
9 Continuous regeneration of catalyst is required to
maintain its activity (coke formation)
9 Heath balance is reached in industrial processes:
heath released during regeneration is enough to
balance endothermal reactions
75
EUCHEME 11/08/2012
FCC - CONCLUSIONS
9 Hydrogen transfer reactions (bimolecular
reactions) increase paraffins and aromatics yield
9 This effect is more important in zeolites, due to:
i) the concentration of reactants increases
(confinement effect)
ii) the density of acid sites is higher in zeolites
9 Aromatic rings are not transformed in catalytic
cracking
9 Release of pollutants (NOx, SOx, COx) is a major
concern
76
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Simplified flowsheet diagram of refinery operation
77
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Hydrocracking - Bifunctional Catalysts (1)
¾ Transformation of heavy feedstocks in the
presence of a high pressure of hydrogen (~200 bar)
¾ Hydrocracking presents a higher flexibility in
what concerns both
¾ the charges to transform: from heavy gasoline
to cuts from vacuum distillation, in a refinery
¾ the obtained products: gasoline but mainly
middle distillates (jet, diesel, fuel, lube oil)
accordingly to the market
78
EUCHEME 11/08/2012
¾ An important issue concerns catalyst selectivity
and stability: high H2 pressure increases the
hydrogenation of unsaturated compounds Î
9 no unsaturated compounds are obtained as
products
9 (mono)- aromatic rings are transformed under
such conditions, via hydrogenation
9 coke precursors are also hydrogenated –
stability catalysts ©, Î fixed bed reactors are
used (~2 years lifetime)
79
EUCHEME 11/08/2012
9 Hydrocracking catalysts are Bifunctional: they
possess two different catalytic functions
9 (i) an acid function – cracking and isomerization
9 (ii) a metallic function – hydrogenation /
dehydrogenation
• Acid function – zeolite in acidic form (HY)
• Metallic function – noble metals (Pt, Pd), Ni, or
metal sulfides (Co, Ni, Mo)
80
IMPORTANT REMARK
EUCHEME 11/08/2012
CATALYTIC FUNCTION
9 A Catalytic Function is a set of a given
type of active sites that catalyze a given
reaction (catalytic cycle)
81
EUCHEME 11/08/2012
Hydrocracking Mechanism (1) (ex: n-C7)
Products
Feed
+ H2 - H 2
+ H2
+ H2
- H2
M
M
M
- H2
H+ Z- + H2
M
H+ Z-
H+ ZH+ Z-
H+ ZH+ Z-
H+ ZH+ Z-
X
Bifunctional transformation of n-heptane (n-C7):
(m-C6) methylhexanes; (dm-C5) dimethylpentanes; (C+)
carbocation; (O) olefins
82
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Hydrocracking Mechanism (2)
9 n-paraffins are dehydrogenated into olefins on metal
sites
9 olefins are transformed into carbocations on acid
sites
9 formed carbocations can undergo:
9 isomerisation (skeletal)
9 cracking by β-scission of isomerised
compounds (“cracking is consecutive to
isomerization”)
9 iso- and n-olefins are hydrogenated into isoand n-paraffins in metal sites
83
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Hydrocracking Mechanism (3)
Balance between the two catalytic functions:
¾ Strong hydrogenating function will result in
diminishing coke formation and, consequently, an
increase of catalysts stability
¾ Ratio [AA/AH] can be adjusted to optimize activity
and selectivity:
9 an optimum in the balance between both
functions results in the maximum activity
9 the selectivity can be tuned to favour lighter or
middle distillate products (more or less cracking)
84
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Hydrocracking - Effect of hydrogenation/acidity ratio (1)
¾ the performance of a bifunctional catalyst
depends on the balance of both catalytic functions
9 when AH (hydrogenation) is weak, the rate
limiting step can be (i) the initial dehydrogenation
on n-paraffins or (ii) the hydrogenation of isoolefins. Consequently, catalyst activity increases
with AH
9 when AA (acidity) is weak, the rate limiting step
is the isomerization of n-olefins, so catalyst
activity increases with AA
85
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Hydrocracking - Effect of hydrogenation/acidity ratio (2)
n-decane
transformation on
Pt/HFAU
¾ Catalyst activity initially
increases with nPt/nA.
¾After a given value (≈ 0.03) it
remains constant: this means
that there is enough metal
sites in order to fed all acid
sites into intermediary olefins,
maintaining the maximum
activity.
¾ From this point on, catalyst
activity depends on acidity
A
nPt/nA
86
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Hydrocracking Reactions
¾ in the presence of a hydrocracking catalyst, further hydrogenation
of aromatics take place, followed by naphthene ring opening
(hydrodecyclization)
¾ hydrogenation reactions lead to total (or partial) saturation of
87
olefinic and aromatic hydrocarbons
EUCHEME 11/08/2012
Hydrocracking Process
High stability catalysts → fix-bed reactors
Simplified
flow diagram
of a singlestage, dual
catalysts
hydrocracking
process
88
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Conversion Processes: FCC vs. Hydrocracking
89
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Methanol to Gasoline - MTG
Integrated processes NG J Syngas J MeOH J Olefins (UOP)
P Gasoline (Mobil)
Reaction pathway:
-H2O
MeOH → dimethyl-ether → light olefins → HC (alkanes, aromatics)
ZSM-5 : light olefins C2, C3
MTG on H/MFI
catalysts
370ºC, 1bar
Products
distribution as a
function of contact
time
90
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Methanol to Gasoline – Mechanism (1)
Methanol
DME
1st step Î formation of DME on acidic sites of zeolite
91
EUCHEME 11/08/2012
2nd step Î
formation of a
C=C bond.
Carbocationbased
mechanism
for the
formation of
olefins
W.W. Kaeding, S.A. Butter, J. Catalysis 61 (1980) 155
92
EUCHEME 11/08/2012
3rd reaction step corresponds to the formation of alkanes and
aromatics, from light alkenes by
(O) oligomerization-cracking reactions, isomerization
reactions,
(C) cyclisation reactions
(HT) hydrogen transfer reactions, with the participation of
carbocations, as intermediate species
93
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Methanol to Gasoline – MTG Process
Simplified flowsheet of MTG fixed-bed reactor process/Mobil
94
Zeolites in Green Chemistry (1)
EUCHEME 11/08/2012
“Catalysis is the key to waste minimization”
(R.A. Sheldon, J. Chem, Tech. Biotechnol., 68 (1997) 381-388)
95
¾
EUCHEME 11/08/2012
New Processes – Phenol
(Production of Bis-Phenol A; Phenolic resins;
Caprolactam; Aniline)
¾
Direct hydroxylation of Benzene to Phenol with N2O:
Fe/ZSM-5 (Monsanto)
Eliminates cumene as intermediate
z Enables use of N O as oxidant
2
z Prevents by-production of propanone
96
EUCHEME 11/08/2012
Traditional Process
Phosphoric Acid
AlCl3
Zeolite Advantages:
• catalyst can be removed,
regenerated and returned for reuse
• no hazardous waste or acidic
emission results from the use of
zeolite catalysts
• they work with lower quality
feedstock, yet produce a higher
quality product
• they produce a higher
proportion of cumene, and little
propylbenzene (by-products)
reducing energy used in
purification
97
Zeolites in Green Chemistry
EUCHEME 11/08/2012
New Processes - Cumene
(manufacturing of phenol and its co-product acetone)
CH3-CH-CH3
+ CH2=CHCH3
Benzene + Propene
Catalyst
Cumene
98
EUCHEME 11/08/2012
Catalyst: (acid sites)
z
solid phosphoric acid (SPA) (UOP)
z
aluminum chloride (AlCl3) (Monsanto – Kellog)
z
Zeolites: (Mobil-Raytheon - MCM-22; UOP - Beta;
Dow-Kellogg - Mordenite ; EniChem - Beta Zeolite)
CH3 CH3
Benzene
H/Zeolite
H+
CH2=CH2-CH3 ← → CH3-CH+-CH3 ←
Propene
CH
→
Cumene
99
EUCHEME 11/08/2012
Zeolites Advantages
¾
AlCl3-based process has the highest total capital
investment (additional equipment required for
catalyst disposal and the use of more expensive
materials of construction)
¾
Differences in capital costs between the zeolitebased process (using refinery-grade propylene)
and the modern SPA (Solid Phosphoric Acid) based processes: < 5%
100
EUCHEME 11/08/2012
Zeolites Advantages
¾ Because of its high benzene-to-cumene selectivity,
the zeolite-based process has the lowest production
costs
¾ In addition, the zeolite-based process offers higher
product purity and uses a regenerable catalyst,
eliminating the waste disposal problems associated
with the SPA and AlCl3 catalysts.
101
EUCHEME 11/08/2012
New Processes
Caprolactam and Adipic Acid
(manufacturing of NYLON – polyamide fibers)
O
NHO
Beckmann
rearrangement
O
C
NH
benzene
cyclohexanone
cyclohexanone
oxime
caprolactam
102
EUCHEME 11/08/2012
Benzene
Benzene
+ 2 H2
+ 3 H2
Routes of
Synthesis of
Caprolactam
and Adipic Acid
Cyclohexane
Cyclohexene
+ O2
H-ZSM 5
Ciclohexylperoxide
+ H2O
Cyclohexanol/-one
Cyclohexanone
- (NH4)2SO4
Cyclohexanone oxime
Beckmann rearrangment →
+ H2SO4
- (NH4)2SO4
Caprolactam
Asahi
Process
Cyclohexanol
Adipic
Acid
+ O2
+ NH2OH + H2SO4
Ru cat./
Delft Univ.
+ O2
Cyclohexanone
+ H2O2 + NH3
TS-1
Enichem
Cyclohexanone oxime
Zeolites
Caprolactam
W.F. Holderich et al., Catal. Today 37 (1997) 353-366
103
EUCHEME 11/08/2012
Synthesis of Caprolactam over Zeolite Catalysts
OXIME FORMATION
1st step – ammonia reacts inside TS-1,
forming the hydroxylamine
hydroxylamine
ketone
oxime
2nd step – hydroxylamine reacts with
ketone, giving rise to the formation of the
oxime
G. Bellusi et al., Cattech 4 (2000) 4-16
104
EUCHEME 11/08/2012
Zeolites Advantages
¾ Synthesis of Caprolactam over Zeolite Catalysts
¾ Beckmann rearrangement in gas-phase catalized by MFISilicalite
Time on stream
(hr)
1.25
46.25
CHO conv. (%)
99.9
96.7
CPL
selectivity(%)
96.4
96.4
Europ. Patent 544530 (1992) Sumitomo
105
EUCHEME 11/08/2012
Zeolites Advantages
¾
1 mol H2 is saved in the first step
¾
one reaction step less
¾
¾
¾
avoid the formation of the toxic hydroxyl amine,
NH2OH
avoid using hazardous and corrosive sulfuric acid
in the oxime formation step, as well as in the
Beckmann rearrangement
avoid the formation of ammonium sulfate (NH4)2SO4
(up to 4 Ton/Ton caprolactame)
106
Zeolites as Catalysts
EUCHEME 11/08/2012
Conclusions
¾
Zeolite structures can be considered as very active
nanoreactors, allowing to get insight reaction
mechanisms (molecular level)
¾
Zeolite can be shape-selective catalysts
¾
Increasing performances due to Confinement effects
¾
Possible determination of real active sites for each
step of heterogeneous process
107
Zeolites as Catalysts
EUCHEME 11/08/2012
Conclusions
¾ Is possible to achieve the stabilization of well
defined metal species
¾ Different catalytic functions can be finely tuned
¾ Zeolites are well adapted to refining and
petrochemical processes, but also to environmental
friendly processes
108
EUCHEME 11/08/2012
Thank You
for
Your Attention
Carlos Henriques
109

Catalysis by Zeolites

Transcription

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