Introduction Role of the catalyst. Nature of active sites Acid base catalysis Metal catalysis Redox catalysis
Outline • Introduction • Role of the catalyst • Principal reaction mechanisms • Nature of active sites • Acid – base catalysis • Metal catalysis • Red...
Outline • Introduction • Role of the catalyst • Principal reaction mechanisms • Nature of active sites • Acid – base catalysis • Metal catalysis • Redox catalysis
Definition of catalysis Catalysis is a process in which the rate of a reaction is enhanced by a relatively small amount of a different substance (catalyst) that does not undergo any permanent change itself. – 1835 Berzelius – 1900 Bodenstein, Ostwald, van't Hoff After one cycle the catalyst has to reach the same state as before the reaction. • Opposed to surface reaction or stoichiometric reaction with a coreactant. How catalysts act • Catalysts reduce the energy, which is necessary to proceed along the reaction pathway. • Catalysts offer new reaction pathways. • Catalysts concentrate the reactants at the surface.
1
Energy pathway of catalyzed reaction THERMAL REACTION
Potential Energy Et
CATALYTIC REACTION
Ec
ΔH react.
ΔHads Reactants Reactants adsorbed
CH3
H3C H
H
H3C
CH3
H3C
H
H
H
*
*
Products
ΔHdes.
*
Products adsorbed
*
H CH3
H3C H
H *
*
CH3
H
H3C H
CH3
Elementary steps in Heterogeneous Catalysis Internal diffusion
External diffusion
Adsorption
Surface reaction
A A
Internal transport of products
A
B
Desorption
B B
External transport of products
2
Types of elementary steps Langmuir Hinshelwood mechanism 2HC
CH3
– CH2
H
2HC
*
*
*
*
Eley - Redeal mechanism 2HC
– CH2
*
*
*
CH3
H2
*
2HC
H
*
*
Mars-van Krevelen mechanism
O2
≡
O C M–O–M–O
O M–
–M–O
M–O–M–O
Acid -base catalysis
3
Acid and bases - definitions Brønsted – Lowry acid-base theory Acid: Hydrogen containing species able to donate a proton Base: Species capable of accepting a proton AH
+
B
A- +
BH+
Lewis acid-base theory c d Spec Species es ab able e to o accep accept a an e electron ec o pa pair to o form o a Acid: dative or coordinative bond Base: Species possessing a non-bonding electron pair able to form a dative or coordinative bond A
+
B
Aδ- +
Bδ+
Acid solutions and solid acids • Aqueous solutions of acids contain only one type of acid: H 3 O+ • Only the extensive factor of acidity is measured. • On solid surfaces: - Every OH group is potentially a Brønsted acid site - Therefore, distribution of site strengths exist.
The term “acidity” acidity may describe • an extensive property, i.e., the density of acid sites on the catalyst surface (mol/gcat) better called acid site density. • an intensive property, i.e., the ability to protonate bases of different strengths. This property is better called acid strength.
4
Acidity and basicity of solids • The intrinsic acid-base strength of a solid depends upon its average electronegativity - High average electronegativity – strong acid - Low average electronegativity – weak acid • The average electronegativity according to Sanderson is the geometric mean of the electronegativities of the elements in a compound. • Many existing concepts fail to predict nature and concentration of acid sites.
(
Sint = S ⋅ S ⋅ S p P
q Q
)
1 r p+q+r R
Acid sites generated by defects and substitution
But, when substituting some Si4+ by Al3+ STRONG ACIDITY develops
OH groups in silica (SiO2) weakly acidic O
O Si
Si
H O
O
O Si
Si
H O
H O
O Si
Al
H O
O Si
Si
5
Acid and base sites in zeolites
H
Brønsted acid site Lewis acid site
Sorption and catalysis are governed by crystalline void space and substitution. Solid acids provide a low concentration of protons.
Acid – base catalysis Increasing g difficulty of product to desorb
Increa asing reactivity of sub bstrate
• Acid catalyzed alkane activation • Carbenium ion based catalysis y – Isomerization – Cracking – Oligomerization, alkylation • Alcohol reactions • Shape selectivity
6
Overall mechanism of protolytic cracking
H3C
H2 C
C H2
H2 C
CH3
H3C
H2 C
C H2
H2 C
CH3
[SiOHAl] [SiOHAl] [SiO-Al] +
CH2 H3C
CH
H3C
+
[SiO-Al]
Dehydrogenation EA = 200 – 220 kJ/mol Cracking g
CH2 H3C
Si-O-Al
[SiO-Al]
CH3
CH2
CH2 H3C
CH2
H2 H2 C H C C + CH3 H3C H H
CH2
H3C
CH2
Products in protolytic nn-butane conversion CH4 +
C2H6 +
1 H3 C
+
Propene
2 CH
Are products characteristic of protolytic cleavage ?
• The logarithm of the rate of cracking g and dehydrogenation y g increases linearly with the size of the alkane.
Tr=773K
Micropores contribute strongly to alkane sorption
Brønsted ø acid site
Heat of ads sorption [kJ/mol]
120 100 80 60 40 H-MFI MFI H-FAU FAU
20 0
2
4
6 Carbon number
8
Lewis acid site Protons contribute 7 resp. 12 kJ/mol 10 to the alkane bonding.
8
Reaction pathway and energy profile
ENERGY Y
Et
THERMAL REACTION
Ec
Reactants
CATALYTIC REACTION
ΔH react.
ΔHads. Reactants adsorbed
Products
ΔHdes.
rTOF = k react .Θ react .
Products adsorbed
rTOF = Ae
rTOF = k react . K ads. preact .
rTOF = Ae
E − A RT
e
−
EA RT
o ΔH ads . − RT
e
e
−
o ΔGads . RT
o ΔS ads . R
preact .
preact .
Rate [mol /g s mbar] [*109]
Conversion of light alkanes over H H--MFI 200
Energy [kJ/mol]
120 kJ/mol
100
250
130 kJ/mol 200
50 Cracking
20
150
140 kJ/mol
10
100
155 kJ/mol
5
50
Dehydrogenation
2 1
Apparent Ea
0
1
Tr=773K
2 3 4 5 6 Number of carbon atoms
7
Heat of adsorption C3
C4
C5
Carbon number
C6
• The true energy of activation is identical for all n-alkanes. • The activity depends upon the concentration of alkanes in the pores, their transition entropy and the concentration of Brønsted acid sites.
9
Acid strength and acid site concentration Polanyi relationship
E A = E A0 - γP ΔH R
Borges et al., Journal of Molecular Catalysis A: Chemical 229 (2005) 127
W.O. Haag et al. Stud Surf Sci. Catal. (1994).
Activation of hydrocarbons via generation of carbenium ions • A double bond in an olefin can be attacked by a proton (Brønsted acid site) forming a carbocation (carbenium ion) • A hydride can be abstracted from a paraffin by a Lewis site (carbenium ion) • A very strong acid site could protonate a paraffin forming a penta-coordinated carbocation (carbonium ion), which decomposes into hydrogen/alkane and a carbenium ion ion.
10
Proton addition to olefins
H O
O
O
O
O
Si
Si
H2 H C CH3 C H 3C O O Si
Si
Al
• Carbenium ions of small olefins exist onlyy as transition states. • In the ground state they form alkoxy groups. • Carbenium ions of sterically hindered olefins are stable.
H3C
O
O
O
Al
H2 H C + CH3 C O
Si
Si
O
Al
Hydride abstraction and dehydrogenation
H3C
C H2
H2 C
H3C
CH3
H C
C H
CH3
H-
O
O Si
H O
O Si
O
O Si
H+ O
H O Si
• Lewis acid sites abstract hydride, which leads to dehydrogenation and formation of an olefin. • The olefin is protonated at a Brønsted acid site.
11
Decomposition of carbonium ions H3C
C H2
Si
CH3
H O
O
O
H2 C
Si
O
O
O Si
Al
H2 H3C H C C + CH3 HH O OSi
Al
- H2
• Carbonium ions are transition states that decompose readily into alkoxy groups. • Carbenium ions of sterically hindered olefins are stable
H2 H C CH3 C H 3C O O
O Si
Si
O
Al
Relative stabilities of carbenium ions Tertiary carbenium ion is more stable than secondary and secondary more stable than primary
12
Acid - catalyzed reactions of carbenium ions Elementary steps • Hydride Shift • Methyl shift (via 3C 3C- ring) • ß-scission • Addition to carbenium ions • Hydrogen Transfer
Double-bond isomerization Doubleof 11-butene (hydride shift) • Proton addition to the double bond on the Brønsted acid site H 2C
•
H C
C H2
CH3
+
H+
Internal hydride shift H + C
H3C
C H2
CH3
H 3C
H+ C
C H2
CH3
sec-butyl cation
H3C
H2 C
+ CH 3
C H
• Proton elimination
H 3C
H2 C
+ CH
C H
3
H3C
H C
C H
CH3
+
H+
13
n-Butene isomerization on acid catalysts H3C
C H2
H + C
H2 2C 3 + C CH2 H3C H21
CH3
if bond 3 breaks
if bonds 1 or 2 break H3C
C H2
H + C
CH3 C + H3C H CH2
CH3 CH3 C + H3C H CH2
However, the formation of primary carbenium ion results in a high activation energy and much lower rates than for npentene isomerization.
H2 + C H C CH2 H3C H
18 kcal/mol
3 kcal/mol H3C
H2 C
+ CH
C H
13 kcal/mol
3
CH3
H3C
C
+
CH3
n-Pentene isomerization on acid catalysts H3C
H + C
C H2
H2 C
H2 C H+ H C C H CH3 H3C
CH3
CH3
H3C
C
+
C H2
CH3
H- shift
CH3
regardless of where the ring opens
C + CH3 H3C H C H
-H+ CH3
H3C
C
C H
CH3
14
Carbenium ion cracking H3 C
H3 C
H C
H C
C H
C H
H2 C
H2 C
C H2
C H2
CH3
H3 C
H C
CH2
+
H C
H3 C
CH3
H C
H3 C
C H
H2 C
[SiOHAl]
C H2
CH3
Si-O-Al
[SiO-Al]
H3C
CH2
CH2
+
CH2
CH2
CH2
H3 C
[SiOHAl] H3 C
[SiO-Al] +
CH2 H3C
CH
CH2 C H3C
H + C
C H2
H2 C
C H2
CH3
[SiO-Al]
CH2
• Secondary carbenium ions form at random. • Scission of bond ß to carbon with positive charge yielding an α-olefin and a primary carbenium ion • Primary carbenium ion undergoes rapid hydride shift to form more stable secondary ion
H2 C
C H
CH3
Ethene hardly formed in catalytic cracking Methane and ethane are less observed in catalytic cracking in contrast to thermal and protolytic cracking. H3 C
H + C
C H2
H2 C
Catalytic cracking
C H2
CH3 H2C CH2
β - scission H 3C
H3 C
H + C
Radical cracking
H C
CH2
CH3
ß - scission not possible
H2 C
+
H2 C
CH3
energetically not favored
15
Activation via hydride abstraction
H O
O
O
H 3C O
O
O
Si
Si
H3C
H2 C
C H2
H2 H C CH3 C O
Si
Si
Al
CH3 C H3C H CH3
O
Al
CH3
CH3 H3 C Si
CH3 O
O
O
O
C
Si
Al
Alkylation – the reverse reaction to cracking Olefin addition CnH2n
CnH2n + H+ → [CnH2n+1]+ Carbenium ion Ester
Alkene
Alkene
I iti ti Initiation [i-C4H9]+ H3C
H C
C H
CH3
CH3 H 2 H C CH3 C H3C H3C CH3
CH3
+
C H3C H CH3
tert - Butyl carbenium ion
[C2nH4n+1]+ Octyl carbenium ion
i-C2nH4n+2
i-C4H10
iso-Alkane
iso-Butane
Hydride transfer
C4 =
kC1
C4 +
C8 +
kA1 kB1
C8 =
kC2 kA2 kB2
C4
C12+
C16+
kA3 kB3
C8
C12=
kC3
Low temperatures !
C12
16
Design criteria for alkylation catalysts Sufficient space for the hydride transfer transition state
CH3
H3C CH3
H3C
O
HO
C H2
CH3 CH3
H3C
CH3
H3C
S O
CH3 C + H3C H CH3 C C CH3 H2 *
O
Appropriate strength of the C-O bond High concentration of acid sites
Elimination of alcohols on acidacid-base catalysts H3C CH3 HC C H2 HO
H3C CH3 HC C H2 HO
ACID
OH-
CH3 H3C H+ CH2 C
H3C H+ OH-
BASE
C C H H
Dehydration
H+
H3 C CH3 HC C H2 O
H 3C
CH3 H- H+
CH3 C C H2 O
Dehydrogenation
17
Types of shape selectivity induced by zeolites Reactant exclusion Dewaxing
Product diffusion control Para-directed aromatic reactions
Restricted transition state Prevention of transalkylation
Isomerization of xylenes
Ultimate selectivity m→ p-xylene
• Reaction is proportional to OH coverage, if all H+ are equally active. • Product selectivity determined by transition state and diffusion limitations.