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...
Author: Julia Garrett
7 downloads 0 Views 791KB Size
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 ?

CH2

21%

CH3 20% Ethane

22% Ethene

Methane 22% 14% Butenes Propane p 1%

+

H2

+ T: 773K, p: 3 kPa n-butane, catalyst: H-ZSM5, 5% conv.

7

Rate [mol /g s mbar] [[*109]

Conversion of light alkanes over H H--MFI 200

120 kJ/mol

100

Bond CH C-H C-C

130 kJ/mol

50 Cracking

20

BE (kJ/mol) 413 347

140 kJ/mol

10 155 kJ/mol

5

Dehydrogenation

2 1

1

2 3 4 5 6 Number of carbon atoms

7

• 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 Skeletal Isomerization Cracking Alkylation/Oligomerization

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.

18