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Heterogeneous Catalysis:Heterogeneous Catalysis: catalyst and reactants in different phasecatalyst and reactants in different phase

most common example: solid catalyst, fluid reactantsmost common example: solid catalyst, fluid reactants

by far the largest segment of catalysis

typical use: petrochemical industry

+ separation product/catalyst easy

- often contact problems, not as selective

Enzyme Catalysis:Enzyme Catalysis: enzymes = proteins which catalyze biochemical reactionsenzymes = proteins which catalyze biochemical reactions

typical use: YOU! (plus some biochemical applications)

+ most active and selective catalysts known

- extremely complex, highly sensitive

The ‘Types’ of Catalysis

Homogeneous Catalysis:Homogeneous Catalysis: catalyst and reactants in same phasecatalyst and reactants in same phase

typical use: fine chemicals, pharmaceuticals

+ good contact, often highly active & selective

- separation problem, often T-sensitive, expensive

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Organometallic Catalysis: ExampleHydroformylationHydroformylation of of propenepropeneto to butanalbutanal ((butyraldehydebutyraldehyde))

the catalytic reaction cycle

(“oxo-synthesis”)

Largest homog. catalyzed process: > 15 bn lb alcohols/aldehydes per year

(inventor: Otto Roelen, Ruhrchemie, 1938)

Starring:the catalyst: Rh-organometallic complex

Starring:the catalystthe catalyst: : RhRh--organometallicorganometallic complexcomplexthe reactantsthe reactants: : propylene, CO, Hpropylene, CO, H22

the productthe product: : butyraldehydebutyraldehyde

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Hom. Catalysis & PolymerizationZiegler-Natta Catalysts for Polyethylene/polypropylene formation (Nobel Price 1963)

..and so on…

Catalyst: mixture of titanium tetrachlorideand aluminum alkyl

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Hom. Catalysis & Polymerization

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Stereoregular Polymers

Atactic Polypropylene

Syndiotactic Polypropylene

Isotactic Polypropylene

http://www.chee.iit.edu/~m1/pictures.htm

All methyl groups have

Methyl groups have

Methyl groups have

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The New Guy on the Block…

A metallocene is

FerroceneMetallocene Catalysts

A cyclopentadienyl anion is made from a little molecule called cyclopentadiene:

http://www.psrc.usm.edu/macrog/mcene.htm

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The Catalyst

Derivative of bis-chlorozirconocene

An indenyl ligand

Carbon bridge

Metallocene Catalyst

http://www.psrc.usm.edu/macrog/mcene.htm

Compounds discovered in the 1950s, but catalytic activity for polymerization onlydiscovered by W. Kaminski in mid-1980s. Rapid development since then!

Metallocene-related patentsMetalloceneMetallocene--related patentsrelated patents

coco--catalystcatalyst

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Polymerization Mechanism - I

Cl atoms are

replaced by CH3 groups from MAO

Fall off of one of the CH3

groups

Incoming of a

propylene monomer Temporary bond

between C=C and ZrSource: http://www.psrc.usm.edu/macrog/mcene.htm

(the indenyl ligands are‘cut-off’ in the followinggraphs to save space!)

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Polymerization Mechanism - IIElectrons shifting

Insertion of the incoming monomer to form a chain

Bond formation between Zr and one propylene-C, methyl-C and the other

propylene-C

Source: http://www.psrc.usm.edu/macrog/mcene.htm

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Polymerization Mechanism - III

Source: http://www.psrc.usm.edu/macrog/mcene.htm

Incoming of another propylene monomer

Temporary bond between C=C and Zr

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Polymerization Mechanism - IV

Electrons shifting

Bond formation between Zr and one

propylene-C, methyl-C and the other propylene-C

Insertion of the incoming monomer to the chain

http://www.psrc.usm.edu/macrog/mcene.htm

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Iso- vs Syndiotactic PPAs you could see above, the direction of approach switches with each monomer added.

However, notice that this yields an isotactic polymer: the methyl groups are always on the same side of the polymer chain.

Isopropyl (η5-cyclopentadienyl-η3-fluorenyl) zirconium chloride

How can one obtain syndiotactic PP?Use a different metallocene!-> metallocenes denotes a large and

highly flexible group of catalysts!

Syndiotactic polypropylene could not be produced as a pure polymer before the discovery of metallocenes. The material is significantly different from isotactic polypropylene in its physical properties - much softer, but also much tougher and much clearer (uses in medical applications).

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Mixtures of Isotatic & Atactic PP

• Isotatic polypropylene:– Rigid but brittle

– Has good optical properties (clear)

• Atatic polypropylene:– Hard to crystallize

– Poor optical properties

– Flexible & tough

• A single polymer chain having both isotatic and atactic segments will combine the advantages of both!– Tunable properties

– Value-added designer polymers

Can a single catalyst produce alternating isotatic & ataticblocks?

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Mixtures of Isotatic & Atactic PP

Dual-state, single-site metalocene

Different isomers produce different blocks—rotation of ligands governs properties of polymer

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Ziegler Natta vs Metallocenes

VersatilityVersatility of metallocenes for greater range

Unrivaled controlUnrivaled control over polymer structure:

e.g. lower melting points, better optical characteristics, better heat stability

Extremely activeExtremely active: for example, 100 g of Hoechst zirconocenecatalyst

+ Pros ++ Pros ++ Pros +High concentration of AlHigh concentration of Al needed for

the cocatalyst =

Metal to Al ratio in Ziegler-Natta catalysts:

Metal to Al ratio in metallocenecatalysts:

Cost of Cost of MetallocenesMetallocenes is several thousand dollars per pound which is

- Cons --- Cons Cons --

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Hetero- vs Homogeneous Catalysis

Heterogeneous Cat’sHeterogeneous Cat’s Homogeneous CatalystsHomogeneous Catalysts

Simple to prepareSimple to prepare

Very stableVery stable

No solvent restrictionsNo solvent restrictions

Easy to separate from Easy to separate from products and solventsproducts and solvents

Difficult to characterize

Only surface atoms used

Poisons easily

Low selectivity

Empirical matching of catalyst and substrate

Pt(111)Pt(111)

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Homogeneous Catalysis: Problems

SeparationSeparation of Catalyst from Product Phase::

• Distillation often not possible due to thermal

• Extraction or thermal/chemical decomposition is

• Separation often dominates the flowsheet

• Recent development: immobilization of ‘homogeneous’ catalysts on solid

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Hydroformulation: Catalyst Recovery

Process Catalyst Separation Method Returned as

Ruhrchemie thermal decomposition Co solids

BASF chemical decomposition Co acetate

Mistubishi chemical decomposition oil-soluble Co soaps

Kuhlmann extraction under pressure (10 atm) with dilute NaHCO3 solution

CoH(CO)4

UCC chemical decomposition Co(OH)2

Shell distillation distillation residue

(J Chem Ed. (1984), 61, 961)(back to our initial example: (back to our initial example: HydroformulationHydroformulation))

Note that typically only the metal is recovered while the Note that typically only the metal is recovered while the ligandligand is lost.is lost. The catalyst is hence destroyed, and needs to be re-synthesized. While the valuable material component is thus recovered, much of the cost of homogeneous catalysts is in the complex multi-step synthesis!

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Hom. Cat.: More Issues

How about Homogeneous Catalysis and Transport Limitations??

How about Homogeneous Catalysis and Reactor Design Equations??

Corrosion is another typical problem in homogeneous catalysis, since many solutions used in homogeneous catalysis are highly corrosive. (Corrosion-resistant materials are usually readily available – but expensive!)

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A Few Words About Fine ChemicalsUndifferentiated ProductsUndifferentiated Products

• Sold simply on the basis of their chemical composition and purity

• Likely to be a number of manufacturers producing the product

• Competition essentially on the basis of price

• Examples: bulk chemicalsbulk chemicals, like methanol, ammonia, ethylene

Differentiated ProductsDifferentiated Products

• Products sold for what they do, rather than chemical composition (“effect chemicals”)

• Chemical formula might not sufficiently characterize the product(especially true for formulated products)

• Typically only a few specific applications

• Other manufacturers may make similar products, but with different composition andperformance

• Examples: aspirin, ibuprofen, vitamins, agrochemicals, shampoo, toothpaste, chocolate,i.e.: pharmaceuticals, healthcare products, pharmaceuticals, healthcare products, agrochemicals, food stuff, specialty chemicalsagrochemicals, food stuff, specialty chemicals

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• Typical traces for good, medium and badperformance of a product(indistinguishable in the early stages!)

• Therefore: tendency to make low initial capital investment to produce small quantity. If the product sells well, thenfurther investments to increase capacity.

• Consequence: Rapid growth in demand requires quickly bringing new capacity on stream!

• Choice of initial capacity will depend on

– Availability of capital

– Availability of existing plants, and/oruse of flexible (multipurpose) facilities

– Economic situation, risk associatedwith the product

– Lead time required to expand capacity

– Company culture and politics

Product Life Cycle

(example from UK, patent expires after 17 yrs!)

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• Patents give protection only for a limited amount of time (20 years in the US)

• Many fine chemicals products also require some form of registration (e.g. FDA) to demonstrate safety, efficacy, and environmental acceptability, e.g.: drugs, herbicides, pesticides, food additives

• Obtaining product registration can take many years, which reduces the time window for exploitation of a product free from competition

– Fast lead times between decision to invest and production are desirable

– Profit margins need to be large,particularly for drugs

• PatentsPatents give protection only for a limited amount of time (20 years in the US)

• Many fine chemicals products also require some form of registration (e.g. FDA) to demonstrate safety, efficacy, and environmental acceptability, e.g.: drugs, herbicides, pesticides, food additives

• Obtaining product registration can take many years, which reduces the time window for exploitation of a product free from competition

–– Fast lead times between decision to Fast lead times between decision to invest and production are desirableinvest and production are desirable

–– Profit margins need to be large,Profit margins need to be large,particularly for drugsparticularly for drugs

Product Life Cycle

(example from UK, patent expires after 17 yrs!)

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Reactors for Fine ChemicalsBatch reactorsBatch reactors are almost exclusively used for the following reasons:

• Batch reactors (stirred vessels typically of stainless steel or lined withglass) are easily modified for use with new products and/or to increaseproduction. --> Flexibility!> Flexibility!

• Technology is easy and fast to scale up from laboratory experiments (large beakers). Pilot plants may be eliminated from the design process, reducing lead times and cost.

• BR is much more robust to inaccurate knowledge! A batch reaction can be left longer if it is incomplete, additional reagents can be added, temperatures can be changed. This flexibility means that less detailed information is required (again reducing lead times) and operators can modify processes without significant equipment modifications (reduced cost and times).

• Bad product batches are easily identified and product quality problems quickly contained (food, drugs!!).