Catalysis Chemical marriage-brokers

Catalyst

Nobel Prize in Chemistry 2005"for the development of the metathesis method in organic

synthesis"

Yves Chauvin

Robert H. Grubbs

Richard R. Schrock

Prof. Grubbs main interests in organometallic chemistry and synthetic chemistry are catalysis, notably Grubbs’ Catalyst for olefin metathesis and ring-opening metathesis polymerization with cyclic olefins such as norbornene. He also contributed to the development of so-called “living polymerization".

Grubbs’ CatalystGrubbs' Catalyst is a transition metal carbene complex named

after the chemist by whom it was first synthesized, Robert H. Grubbs. There are two generations of the catalyst, as shown on the

below. In contrast to other olefin metathesis catalysts, Grubbs'Catalysts tolerate other functional groups in the alkene and are compatible with a wide range of solvents. For these reasons,

Grubbs' Catalysts are extraordinarily versatile.

Grubbs' Catalyst 1st GenerationGrubbs' Catalyst 2nd Generation

Ru

P(Cy)3

P(Cy)3

Ph

Cl

Cl

Ru

P(Cy)3PhCl

Cl

NN

Me

Me

Me

Me

Me

Me

Olefin Metathesis

Olefin metathesis was first used in petroleum reformation for the synthesis of higher olefins in the Shell jigher olefin process (SHOP) under high pressure and high temperatures.

Olefin metathesis

R1 R2

R4R3

R5 R6

R8R7

+

R5

R6R2

R1

R7

R8R4

R3

+

Olefin metathesisR1

R2

M

R3

M MR1

R2 R3

R1

R2 R3

+

Ring-opening metathesis polymerization

The reaction uses strained cyclic olefins to produce stereoregularand monodisperse polymers and co-polymers.

Polymerization of cyclic olefine

M

R

M

R

M Rn

Catalysis: Chemical marriage-brokers• Many synthetic materials found in our homes as well as

medicines and agricultural chemicals are made using catalysts• Plastics, antifreeze etc..• Speed up impossibly slow reactions• Most molecular processes in living organisms are mediated by

biological catalysts called enzymes• Chemists search for more efficient catalysts to create energy-

saving, more environmentally-friendly processes for chemical industry

• Fermenting fruit and grains to make wine and beer – the natural sugars are converted to ethanol by the mediation of enzymes released by yeasts

• Fuel, plastics, antifreeze, and exhaust gases• Synthetic materials and nature• A substance that participates in a chemical reaction and

increases its rate without a net change in the amount of the catalyst in the system

• A reaction at lower temperatures costs less money to run

Rise of the use of catalysts in industry• The exploitation of catalysis on a major industrial

scale began late in the 19th century with the production of sulfuric acid over a platinum catalyst

• Selectivity ( in other words to produce the desired product with the minimum amount of waste products)

How does a catalyst work?• There is still considerable debate over

the detailed mechanisms of nearly all catalysed processes

• Energy barrier (activation energy) –all catalysts reduce the size of this barrier, usually by providing an alternative pathway

• The formation of a transitionaryintermediate compound of the reacting molecules and the catalyst

• Regeneration of pure catalyst after reaction

Haber-Bosch process

Nobel Prize in Chemistry 1918

Haber-Bosch process - an industrial process for producing ammonia from nitrogen and hydrogen by combining them under high pressure in the present of an iron catalyst

The use of fertilizers in the world increased as the population increased. In the late 1890’s, scientists began to wonder if the sources of nitrogen being destroyed would cause the making of chemical fertilizer to cease. Plants could not convert atmospheric nitrogen to soluble nitrogen compounds directly, therefore, a solution to this problem was needed.

The early 1900’s produced three methods in search of this must needed solution. The electric arc process and cyanamid process worked originally, but never made a lasting impact in the United States. The third method created was the Haber-Bosch Process. This process has changed the way nitrogen fertilizers are produced and has increased the availability and use of fertilzers. Over the years, the fertilizer produced by this process.

N2 (g) + 3H2(g) 2NH3

The ammonia is oxidised to nitrogen monoxide gas.

This is cooled. At ordinary temperatures and in the presence of excess air, it is oxidised further to nitrogen dioxide.

The nitrogen dioxide (still in the presence of excess air) is absorbed in water where it reacts to give a concentrated solution of nitric acid.

Haber-Bosch process

4NH3(g) + 5O2(g) 4NO(g) + 6H2O(g)Pt/Rh

2NO(g) + O2(g) 2NO2(g)

2H2O(l) + 4NO2(g) 4HNO3(aq)

What makes a good catalyst• These materials are often metals or metal oxides

particularly those of the transition elements• Transition elements have complex, highly versatile

electronic structures which participate in chemical reactions to form variable numbers of differing types of bonds with many kinds of atoms and molecules

• A solid surface – heterogeneous catalysis• Metal complexes – homogeneous catalysis• Acids, enzymes• Easy bond formation and dissociation• The active sites must have the appropriate geometry

to give access to the reactants – the configuration around the active site often determines the selectivity

• Activity vs separation

Phase: Separation

http://www.chemguide.co.uk/physical/catalysis/introduction.html

Heterogeneous Homogeneous

Heterogeneous catalysis• 85 % of all catalysed chemical processes• Crude oil - a complex mixture of hydrocarbons which

includes long-chain alkanes, branched alkanes, alkenes and aromatic hydrocarbons

• To make crude oil suitable for use – reforming ( linear alkanes are converted into branched or cyclic versions) and cracking ( breaking up the heavier into lighter)

• Linear alkanes cause the phenomenon of knocking• Reforming – platinum particles and aluminum oxide

Heterogeneous catalysis: hydrogenationNi

CH2=CH2 + H2 CH3CH3

1

2

3

4

5

http://www.chemguide.co.uk/physical/catalysis/introduction.html

Zeolites- good catalysts• Zein- to boil, lithos – stone• Huge absorbing surface area anything up to 500

square metres per gram• The channels are formed by stacking rings made up

from a lattice of tetrahedral units of silica (SiO4)• 30 natually occuring zeolites – SiO4 or AlO4• 150 synthetic zeolites – ZnO4, GeO4 and PO4• In the 1940s as absorbents and filters rather than

catalysts• Control acidity; Bronsted acid sites vs Lewis acid

site• Control the pore size

Zeolites- good catalysts

http://www.galleries.com/minerals/silicate/zeolites.htm

Zeolites: CrackingZeolite catalyst

C15H32 2C2H4 + C3H6 + C8H18ethene propene octane

Zeolites: Isomerization

Zeolites: Reforming

+ 4H2

hexane benzene

+ 4H2

heptane methylbenzene

Tailored catalysts• Sophisticated zeolite catalysts with channels of

precise dimensions – templating• Computer program called ZEBEDDE which can

predict suitable templates to produce particular zeolite structures

• Pore sizes ranging from 1 to 10 nanometres• Graft other molecules – take advantage of

homogeneous ones

Fuel cell Energy for Future

A fuel cell in the boot of a Vauxhall Zaphira.

Traditional fuel cell. Hydroxide

ions combine with hydrogen to

release electrons at the negative

terminal. Oxygen combines with the

electrons to produce the

hydroxide ions.

Fuel cells are attracting a great deal of interest in the search for alternative energy supplies to fossil fuels, since they are very efficient and virtually pollution free. In a fuel cell a fuel gas such as hydrogen is combined with oxygen to produce electricity. This conversion happens at electrodes that are coated with catalysts. Unlike an ordinary battery which has a fixed life span, the fuel cell will continue to function for as long as the gases are supplied to it.

Fuels for the future

• Fischer-Tropsch catalysis – coal is first broken down into a mixture of hydrogen and carbon monoxide-so-called synthesis gas, or syn-gas - using a process called coal gasification in the presence of metal catalysts such as cobalt, iron, nickel and ruthenium.

• Uneconomical – a better understanding of the reaction mechanism would help to develop more efficient catalysts.

C + H2O CO + H2

H2 + CO

H C C H C C C C CO O H O H O H O H3C OH

HH HH H

+ H2

H2O

+ H2

Two suggested mechanism for the Fischer-Tropsch reaction

(B)

(A)

O C CH2 CH2 CH3 H2CCH3

+ H2H2O + H2CO

Methanol – a fuel for the future?• Feedstock for the chemical industry, burnt as a fuel

or a source of hydrogen for the fuel cells• Copper/zinc oxide catalyst in relatively low T and P

– 200 ~ 300 C, 50 ~ 100 atm• Mechanism is still in debate

CO/CO2/H2 CH3OHCO2

CO2

C

H

O O

Copper

O

CH3

HO CH3

1/2 H2 + H21/2 H2?

Known intermediates in the synthesis of methanol from carbonDioxide and hydrogen over a copper catalyst. Debate is now centred on how the carbon dioxide is activated at the copper surface, since experiments suggest that carbon dioxide on its does not react with copper fast enough to support the synthesis of methanol.

Seeing atoms and molecules at surface• SPM (Scanning probe microscopy), • STM (Scanning tunnelling microscopy)• AFM (Atomic force microscopy)• To study mechanism on metal surface

Kern, K. et. al. Nature Materials, 2004, 3, 229.

2D Coordination Network on Cu(100)

Versatile Transition Metal Complexes• Core metal atoms can bind to a wide variety of

molecules or ions• Spectator ligands are key importance because they

can be carefully chosen to tune the reactivity of the metal so as to modify its catalytic properties – they can be simple ions, small neutral molecules, or larger complicated structures

• Phosphines : modify the hydrocarbons to tune the properties – monodentate, bidentate

• Substrates will bind to active site whose geometry is influenced by the size and shape of the ligands

• Detaching one or more ligands – this can often be promoted thermally or photochemically

• Altering the metal at the heart of the complex –electronic structure change

• Easy to study for mechanism – discrete molecules

Homogeneous catalysis

• Intermediates can be studied individually• Rate determining step – accelerate this step• Organometallic chemistry• Sandwich structure of ferrocene in 1954 – geoffrey

Wilkinson and Ernst Otto Fischer – the manufacture of polymers

• Wilkinson’s catalyst – hydrogenation of a C-C double bond

• Fine chemicals• Transition metal complexes• Mild conditions• Greater selectivity• Easy to study

The catalytic cycle: Hydrogenation

http://pslc.ws/mactest/mcene.htm

Sandwich Complexes

The catalytic cycle: Polymerization

A case history: a homogeneous route to acetic acid

Nickel, H2OMethane (CH4) Synthesis gas (CO + H2)

Copper/zincMetanol(CH3OH)

Cobalt, rhodium or iridiumAcetic acid (CH3COOH)

Heterogeneou catalysts – step 1 and 2Homogeneou catalysts – step 3

Enantioselective Alkylations

Eric N. Jacobsen et al J. Am. Chem. Soc., 127 (1), 62 -63, 2005.

Preliminary Screen of Tributyltin Enolate Alkylation

Supramolecular allosteric catalytic signal amplifier

Chad A. mirkin et al J. Am. Chem. Soc., 127 (6), 1644 -1645, 2005.

Photo: Taken under UV lamp (365 nm); Reaction time = 6 h. Graph: Fluorescence vs time plot (λex = 368 nm, λem = 415 nm).

Chad A. mirkin et al J. Am. Chem. Soc., 127 (6), 1644 -1645, 2005.

Supramolecular allosteric catalytic signal amplifier

Supramolecular allosteric catalytic signal amplifier. A slow background reaction occurs in the absence ofanalyte (Cl- or CO). Analyte binding opens the cavity and allows substrate molecules to enter where they undergo a fast intramolecular reaction generating acetic acid which protonates a pH-sensitive fluorescent probe.

Chiral Porous Hybrid Solids for Practical Heterogeneous Asymmetric Hydrogenation of Aromatic Ketones

Wenbin Lin et al J. Am. Chem. Soc., 125 (38), 11490 -11491, 2003.

Interaction of Nitric Oxide with Tetrathiolato Iron(II) Complexes: Relevance to the Reaction Pathways of Iron Nitrosyls in Sulfur-Rich Biological

Coordination Environments

Stephen J. Lippard et al J. Am. Chem. Soc., . Am. Chem. Soc., 128 (11), 3528 -3529, 2006.

Nature’s catalysts - enzymes• Recognize• Lock and key• Too selective and so active in mild conditions• Mimic enzyme to create the synthetic catalyst

• The hydrolysis of esters and amides• Chiral selectivity – three dimensional space• Hydrolase, reductase (associate with cofactor)• Ketones to alchols

Acid Catalysis in Organic Chemistry

H3PO4CH2=CH (g) + H2O (g) CH3CH2OH (g)

esterification reaction

hydrolysis of esters

conc H2SO4CH3COOH + CH3CH2OH CH3COOCH2CH3 + H2O

dilute acidCH3COOCH2CH3 + H2O CH3COOH + CH3CH2OH

Acid Catalysis in Organic Chemistry

Future challenges• More active and selective catalysts• The selective conversion of hydrocarbons into more

valuable chemicals• New process for undiscovered reactions – propene to

propene oxide• Natural gas to valuable chemicals (methanol)

Mirror image catalysis• Asymmetric catalysis, where a chiral product is made in

preference to its mirror-image partner• Vital importance for pharmaceuticals and other

biologically-related products• Homogeneous to heterogeneous – for the chemical

industry (recovery and recycle: more economical process)• Immobilize onto a substrate (external and internal)• Organic molecules as a template – control the

configurations of the intermediates• Selectivity, energy-efficient, and environmental-

friendly process• Design the next generation of catalysts

Examples for Immobilization of Homogeneous Catalysts onto Silica

Reek, N. H. et al. JACS. 2004, 126, 14557.

Alper, H. et al. JACS. 2001, 123, 10521.

Van Leeuwen P. W. N. M. et al. Angew. Chem. Int. Ed.. 1999, 38, 3231.

• The immobilization of homogeneous catalysts into solid materials has a great attention, because it can overcome the impractical aspects of homogeneous catalysts.

–Low density of catalytic active sites remains as a challenge part.

Immobilization of Palladium(II) Species onto Silica

Noncovalent Anchoring

Sol-gel Immobilization

Application of Micro- and Nanoparticles

Homochiral Metal-Organic Porous Materials

Immobilization of a Metalloligandsinto a Porous Coordination

Polymer

Using enantiopure building blocksThe chiral pores and functionalities, which may be exploited in chiral-selectivity(enantioselective separations)

Using metalloligands as building blocks

The highly selective molecular recognition (regioselectivity or shape/size selectivity) and heterogeneous catalysis

These materials have shown the great success in magnetics, molecular recognition, catalysis, gas-storageand guest-host interaction.Most metal-organic networks are constructed from organic spacers based on pyridine or carboxylate groups and metal ions or metal clusters.

Hot issues in Coordination Networks

Metal-Organic Coordination Networks (MOCN)

Preparation of Micro- and Nanoparticles Made from Coordination Polymers M-BMSB-M

Addition of initiation solvent (diethyl ether or pentane) into the reaction mixture containing 1and 2 in pyridine results in particle formation and precipitation.

The resulting particles are stable in several organic solvents, water, and in the dried state.

Addition of excess pyridine dissolves the particles and redisperses the ligand and metal ion building blocks.

M: Regulate of photophysical and catalytic properties

M’: Regulate of morphology and structure of particles

Initiation solvent and addition methods: Regulate particle size

Two routes in a typical synthesis: Fast addition and slow diffusion of initiation solvent into precursor solution

Moonhyun Oh, et. al., Nature, 2005, 438, 651.

Microparticles Zn-BMSB-Zn

5 µµµµm

OM and FM images

5 µµµµm

SEM ImagesAverage diameter = 1.60 ± 0.47 µµµµm, (s.d., n = 100)

EDX SpectrumEmission Spectra of Zn-BMSB-Zn Particle and Zn-BMSB Building Block

Moonhyun Oh, et. al., Nature, 2005, 438, 651.

Examples for Immobilization of Homogeneous Catalysts onto Silica

Reek, N. H. et al. JACS. 2004, 126, 14557.

Alper, H. et al. JACS. 2001, 123, 10521.

Van Leeuwen P. W. N. M. et al. Angew. Chem. Int. Ed.. 1999, 38, 3231.

• The immobilization of homogeneous catalysts into solid materials has a great attention, because it can overcome the impractical aspects of homogeneous catalysts.

–Low density of catalytic active sites remains as a challenge part.

Immobilization of Palladium(II) Species onto Silica

Noncovalent Anchoring

Sol-gel Immobilization