The Types of Catalysis

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Constance Simon
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1 The Types of Catalysis Heterogeneous Catalysis: Homogeneous Catalysis: Enzyme Catalysis: catalyst and reactants in different phase most 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 catalyst and reactants in same phase typical use: fine chemicals, pharmaceuticals + good contact, often highly active & selective - separation problem, often T-sensitive, expensive enzymes = proteins which catalyze biochemical reactions typical use: YOU! (plus some biochemical applications) + most active and selective catalysts known L extremely complex, highly sensitive

2 Organometallic Catalysis: Example Hydroformylation of propene to butanal (butyraldehyde) ( oxo-synthesis ) Largest homog. catalyzed process: > 15 bn lb alcohols/aldehydes per year (inventor: Otto Roelen, Ruhrchemie, 1938) Starring: the catalyst: Rh-organometallic complex the reactants: propylene, CO, H 2 the product: butyraldehyde the catalytic reaction cycle L27-2

3 L27-3 Hom. Catalysis & Polymerization Ziegler-Natta Catalysts for Polyethylene/polypropylene formation (Nobel Price 1963) Catalyst: mixture of titanium tetrachloride and aluminum alkyl..and so on

4 L27-4 Hom. Catalysis & Polymerization

5 L27-5 Stereoregular Polymers Isotactic Polypropylene All methyl groups have Syndiotactic Polypropylene Methyl groups have Atactic Polypropylene Methyl groups have

6 L27-6 The New Guy on the Block Metallocene Catalysts Ferrocene A metallocene is A cyclopentadienyl anion is made from a little molecule called cyclopentadiene: macrog/mcene.htm

The Catalyst Compounds discovered in the

7 The Catalyst Compounds discovered in the 1950s, but catalytic activity for polymerization only discovered by W. Kaminski in mid-1980s. Rapid development since then! An indenyl ligand Carbon bridge Derivative of bis-chlorozirconocene Metallocene Catalyst co-catalyst catalyst Metallocene-related patents L27-7

8 Polymerization Mechanism - I L27-8 Cl atoms are replaced by CH 3 groups from MAO Incoming of a propylene monomer (the indenyl ligands are cut-off in the following graphs to save space!) Source: Fall off of one of the CH 3 groups Temporary bond between C=C and Zr

other propylene-c Insertion of the incoming monomer to

9 Polymerization Mechanism - II Electrons shifting Bond formation between Zr and one propylene-c, methyl-c and the other propylene-c Insertion of the incoming monomer to form a chain L27-9 Source:

10 L27-10 Polymerization Mechanism - III Incoming of another propylene monomer Temporary bond between C=C and Zr Source:

the other propylene-c Insertion of the incoming monomer

11 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 L

12 Iso- vs Syndiotactic PP As 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. How can one obtain syndiotactic PP? Use a different metallocene! -> metallocenes denotes a large and highly flexible group of catalysts! Isopropyl (η 5 -cyclopentadienyl-η 3 -fluorenyl) zirconium chloride L27-12 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).

13 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 & atatic blocks? L27-13

14 L27-14 Mixtures of Isotatic & Atactic PP Dual-state, single-site metalocene Different isomers produce different blocks rotation of ligands governs properties of polymer

15 Ziegler Natta vs Metallocenes + Pros + Versatility of metallocenes for greater range Unrivaled control over polymer structure: e.g. lower melting points, better optical characteristics, better heat stability Extremely active: for example, 100 g of Hoechst zirconocene catalyst - Cons - High concentration of Al needed for the cocatalyst = Metal to Al ratio in Ziegler-Natta catalysts: Metal to Al ratio in metallocene catalysts: Cost of Metallocenes is several thousand dollars per pound which is L27-15

solvents Difficult to characterize Only surface atoms used Poisons easily Low

16 Hetero- vs Homogeneous Catalysis Pt(111) Heterogeneous Cat s Simple to prepare Very stable No solvent restrictions Easy to separate from products and solvents Difficult to characterize Only surface atoms used Poisons easily Low selectivity Homogeneous Catalysts L27-16 Empirical matching of catalyst and substrate

17 Homogeneous Catalysis: Problems Separation 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 L27-17

18 Hydroformulation: Catalyst Recovery L27-18 (back to our initial example: Hydroformulation) 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 NaHCO 3 solution CoH(CO) 4 UCC chemical decomposition Co(OH) 2 (J Chem Ed. (1984), 61, 961) Shell distillation distillation residue Note that typically only the metal is recovered while the ligand 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!

19 Hom. Cat.: More Issues 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!) How about Homogeneous Catalysis and Transport Limitations?? How about Homogeneous Catalysis and Reactor Design Equations?? L27-19

20 A Few Words About Fine Chemicals Undifferentiated 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 chemicals, like methanol, ammonia, ethylene Differentiated 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 and performance Examples: aspirin, ibuprofen, vitamins, agrochemicals, shampoo, toothpaste, chocolate, i.e.: pharmaceuticals, healthcare products, agrochemicals, food stuff, specialty chemicals L

21 Product Life Cycle L (example from UK, patent expires after 17 yrs!) Typical traces for good, medium and bad performance 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, then further 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/or use of flexible (multipurpose) facilities Economic situation, risk associated with the product Lead time required to expand capacity Company culture and politics

22 Product Life Cycle L (example from UK, patent expires after 17 yrs!) Patents Patentsgive give protection protection only only for for a a limited limited amount amount of of time time (20 (20 years years in in the the US) US) Many Many fine fine chemicals chemicals products products also also require require some some form form of of registration registration (e.g. (e.g. FDA) FDA) to to demonstrate demonstrate safety, safety, efficacy, efficacy, and and environmental environmental acceptability, acceptability, e.g.: e.g.: drugs, drugs, herbicides, herbicides, pesticides, pesticides, food food additives additives Obtaining Obtaining product product registration registration can can take take many many years, years, which which reduces reduces the the time time window window for for exploitation exploitation of of a a product product free free from from competition competition Fast Fast lead lead times times between between decision decision to to invest invest and and production production are are desirable desirable Profit Profit margins margins need need to to be be large, large, particularly particularly for for drugs drugs

Reactors for Fine Chemicals Batch reactors are almost exclusively used for the following reasons: Batch reactors (stirred vessels typically of stainless steel or lined with glass) are easily modified

23 Reactors for Fine Chemicals Batch reactors are almost exclusively used for the following reasons: Batch reactors (stirred vessels typically of stainless steel or lined with glass) are easily modified for use with new products and/or to increase production. -> > 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). L Bad product batches are easily identified and product quality problems quickly contained (food, drugs!!).

Abstract Process Economics Program Report 153C SINGLE-SITE CATALYSTS FOR PROPYLENE-BASED POLYMERS (June 2002)

Abstract Process Economics Program Report 153C SINGLE-SITE CATALYSTS FOR PROPYLENE-BASED POLYMERS (June 2002) Single-site catalysts (SSC) are considered one of the most significant innovations in the polymer