Homogeneous Catalysis HMC Dr. K.R.Krishnamurthy National Centre for Catalysis Research Indian Institute of Technology,Madras Chennai

 Polymerization  Ethylene & Propylene polymerization  Ziegler –Natta catalysts  Metallocene catalysts

Chemical Industry-The Fact Sheet products 10 Million direct employees 50 Million indirect employees Wide range of products/processes / feed-stocks Enabling better quality of life Annual growth rate 2.4 % Global enterprise valued at $2.2 Trillion …… and growing

Chemical Industry- Products pattern Polymers constitute 20 % of Mega Chemical Industry

Polymers -Types POLYMERS BIO-POLYMERS Proteins, Nucleic acid NATURAL POLYMERS Rubber, Starch, Cellulose SYNTHETIC POLYMERS Polyethylene Nylon,PVC Elastomers & Plastics Thermosets & Thermoplastics Single Polymer chains as seen by AFM

Polymerization-Types Mode of formation Addition Polymerization - Polyethylene, Polypropylene Grafting, cross-linking Condensation Polymerization - PET, Nylon 66 Mechanism of formation Free radical - LDPE, PVC Co-ordination - PP,PBR Ionic - Cationic, Anionic- Poly acrylonitrile

Addition Polymers-Types

ABS- Acrylonitrile-Butadiene-Styrene co-polymer PAN-Strong fibre character PBR- Rubber-Elasticity-shock absorber PS - Tough & hard The co-polymer has very high strength & toughness Block Random Graft

Polymers- Finger prints Catalyst & Process controlled Reflected in RegioselectivityMelting point Cis/Trans IsomerismCrystallization temp. StereoselectivityGlass Transition temp. Mol. Wt distributuion Modulus PolydispersityCrystallinity Viscosity Morpohology Hardness Stiffness Transparency Catalysts & process dictate the property of polymer

Polymers-The journey Monomer Type of polymerization Type of process/reactor Reaction conditions Type of catalyst Polymer resin Processing End Product Type of Processing, Processing aids, Machinery Co-monomer(s) Co-catalysts Donors etc.

History of PE & PP (1933) (1955) Propylene polymerization on similar catalysts by Natta (1956) - 3 generations of catalysts Silica supported chromia catalyst for ethylene polymerization Banks & Hogan- Phillips (1958)- Low pressure/Temp process Metallocene catalysts- Kaminsky (1989) Post metallocene catalysts Ziegler & Natta awarded Nobel prize in 1963 Global consumption- PP - 45 Mill.MT; Value- $65 Billn. (2007) PE - 65 Mill.MT (2008)

monomer initiation Ethylene (C2H4) forms polyethylene (PE) in the presence of free radical R (catalyst or initiator) propagation Free radical ploymerization- Ethylene

Ploy ethylene – Types Vs Properties PE MWDensity Tensile strength Branching Mill. g/cc MPa HDPE > Low LDPE Med & Short LLDPE Short Co-monomers for LLDPE → 1-Butene/1-Hexene/1-Octene PE by free radical route Extensive branching Long and short branches Lower crystallinity 30-60% Density Vary P, T during synthesis UHMWPE – MW- 3-6 Million

Surface structure of Chromium based PE catalysts 1.CrO 3 / Silica- Phillips 2.Chromocene/Silica- Union Carbide Choice of silica (~300 m 2 /g), Cr loading (1%), promoters & pretreatment (calcination, pre-reduction) of the catalysts are crucial % of PE is produced by Phillips process

Z-N- Polymerization of ethylene

Catalytic cycle for polymerization of ethylene - Cossee-Arlman mechanism - Direct insertion of olefin across M-alkyl bond

Processes for PE production High Pressure Autoclave Tubular Low Pressure Slurry phase Gas phase Solution phase

Employs free radical catalyst for polymerization Energy intensive process Product with easy processability PE Technologies High Pressure Processes Tubular Autoclave Tubular- More of Long chain branching (LCB) & less Short chain branching (SCB) Autoclave- More SCB & less LCB

Polyolefin (PE/PP) Process Technologies Slurry PhaseGas PhaseSolution Phase (PE Only) CSTR Heavy Diluent Light Diluent FBD Stirred bed Low Pressure PE Processes

Block diagram of a typical slurry polymerization process

Typical process flow diagram for gas phase polymerization

Processes for Polyethylene

Conventional Ziegler-Natta Catalyst Catalyst componentsTiCl 4 & AlEt 3 –co-catalyst Organo aluminium compound reduces TiCl 4 to generate TiCl 3 Active phase TiCl 3 Different crystalline forms- α, β, γ, & δ β Chain structure; α, γ, & δ have layer structure Layer structure ensures vacant co-ordination sites Activity & Isotacticity differs α – Hexagonal Close packed – hcp of Cl ions γ - Cubic close packed –ccp of Cl ions Ti ions occupy Octahedral holes of Cl - matrix α, γ & δ yield high isotacticity while β- gives low isotacticity DEAC is a better co-catalyst than TEAC since over reduction of TiCl 4 beyond Ti 3+ (to Ti 2+ ) is avoided

Ball-milling of the catalysts decreases the particle size and hence increases Surface area by forming smaller TiCl 3 crystallites. Such treatment increases polymerization activity Use of electron donors increases the stereoselectivity Isotacticity Index DEAC % TEAC % Type of co-catalyst influences Isotacticity Index EADC is converted to DEAC

Stereochemistry of PP- a) Three different orientation of methyl groups in PP backbone b) Stereo chemical relationship between two adjacent methyl groups

Z-N Polymerization-Stereochemistry

PP- Streochemistry Vs Properties PP Elastic HardnessMP Mech.props Modulus-Gpa Mpa° C Isotactic Stiff/Brittle Syndiotactic Robust, transparent Atactic < 0

Polymerization of propylene- Reaction scheme

Polymerization of propylene- Steps 1.Replacement of one Cl by alkyl group of Al alkyl 2.Bonding of Propylene to a vacant site 3.Insertion of propylene into Metal-alkyl bond- Initiation 4.Creation of vacant site for propylene adsorption 5.Repetition of steps 2,3 & 4 leading to chain growth/propagation 6.Catalyst configuration decides the configuration of added propylene 7.Termination of polymer chain with hydrogen- Termination

Streochemistry of active site Ti Cl CH 3 P Cl 4 Bridging Cl 1Terminal Cl replaced by alkyl 1 Vacant site for propylene adsorption C C CH 3 H H  124.3° 1.336Å 1.501Å H

Polypropylene- Stereoregulation Methyl group of the incoming propylene prefers a trans position vis-à-vis the polymer chain-p – Right ; cis orientation as shown on left is not favoured

PP Stereochemistry- Effect of metal & ligand

M P P 3 M M 3 1 P 2 M P 1,2 Insertion 2,1 Insertion 3,1 Insertion Possible insertion modes for Propylene across Metal- Alkyl bond- Different orientations of methyl group

Z-N catalysts- Generations catalyst First TiCl 3 and AlEt 2 Cl Second TiCl 3 + AlEt 2 Cl + Mono/Di ethers, Mono/Di esters –Effect of TiCl 3 Crystallite size, ball-milling & increase in surface area Third TiCl 4 supported on MgCl 2 + Al-Alkyl + Phthalate esters -3 rd component MgCl 2 has layered structure; Ionic radii of Mg 2+ & Ti & 068 nm Structural compatibility MgCl 2 & TiCl 4 Polymer yield > 30 Kg/g ; Isotacticity index % Fourth Morphology controlled catalysts Spherical polymer product-No extrusion Removal of Atactic PP and catalyst residue Catalyst residue removed by washing with alcohols & water-Deashing High activity catalyst → Elimination of catalyst removal deashing & extrusion

Removal of catalyst residue and atactic PP are the two critical steps

PP- First Generation Process Large plant size High capital and operating cost Large number of equipments Large inventory of solvent Energy intensive Solvent purification APP removal Catalyst deashing Polymerization Degassing Centrifugation and Catalyst deactivation Drying PP Solvent recovery Propylene recovery Extrusion Deashing Total process steps: 8

PP-Second Generation Process Hexane Slurry Plant size small Reduced capital and operating cost as compared to first generation process No catalyst deashing Large inventory of solvent Energy intensive due to solvent purification step Still involved removal of APP Total process steps: 7 Polymerization Degassing Centrifugation and Catalyst deactivation Drying PP Solvent recovery Propylene recovery Extrusion

PP-Second Generation Process Liquid Pool (Bulk Loop) Spheripol Plant size further reduced Capital and operating cost reduced considerably as compared to Hx slurry process Very simple to operate Energy efficient Removal of APP not required Polymerization Degassing And Steaming PP Propylene recovery Extrusion Total process steps: 5

Plant size reduced Capital cost high (10-15%) but operating cost reduced considerably Very simple to operate Energy intensive – Extrusion step required Removal of APP not required Polymerization Degassing & deactivation Polypropylene Propylene recovery Extrusion PP-Third Generation Process Gas Phase Total process steps: 5

PP-Third Generation Process Liquid Pool (Spheripol + Adipol) Plant size reduced Capital cost and operating cost reduced considerably vs. Hx slurry Very simple to operate Energy intensive–Extrusion not required Removal of APP not required Polymerization Degassing and deactivation Polypropylene Spheribeads Propylene recovery No Extrusion Total process steps: 4

Vessel AVessel B Diameter = 4.0m Length = 20.0 m Diameter = 4.0m Length = 20.0 m Material of Construction = Carbon Steel Design Pressure = 1 atm Design Pressure = 400 atm Installed Cost = $228,700 Installed Cost = $1,840,000