Polymerization Catalysts (Part - 2) - Chemical Engineering PDF Download

Fragmentation of polymerization catalysts

Many industrial polymerization reactions are carried out with supported catalysts. Typically porous silica, MgCl₂ or certain polymers are used as supports. For supported catalysts on initiation of polymerization, the active sites on the catalyst surface are rapidly fouled due to encapsulation by the polymer product. However, the catalyst may undergo fragmentation due to accumulation of polymers within the catalyst particles. This fragmentation results in exposure of new active catalytic sites and maintains the catalytic activity. The fragmentation process ensures access of the monomers to the active catalyst sites. The fragmentation of catalyst particles are typically observed for olefin polymerization reactions such as polyethylene and polypropylene productions with Ziegler–Natta catalysts. Fragmentation of catalyst particles results in higher polymer yield. Since recovery of the catalyst particles from polymer product is difficult and expensive, fragmentation of catalyst makes the catalyst particles small enough so that final product quality is not affected. In the final product, the size of the catalysts particles are in the range of ~ 100 nm which are embedded in large polymer particles of 200 -1000 µm diameter.

Fragmentation and polymer growth models

1. Core – shell model

According to this model, catalyst particles do not break up in the beginning of the polymerization process. Initially, polymerization occurs on the surface of the particle which acts as a core. Then, the polymer grows in the form of a shell around the core. After formation of accumulated polymer shell, the monomer has to diffuse through the polymer layer to reach the catalyst surface, where it reacts. The model is more applicable for catalysts with low porosity for which monomer diffusion is limited

Fig.  4. Core – shell model for polymer growth

2. Multigrain model

For highly porous catalyst monomer diffusion is less limited and monomer can penetrate into the pores of the catalyst more easily. Consequently polymer can grow throughout the particle and result in immediate fragmentation of the catalyst particles (Fig. 5).

Fig 5. Growth of polymer within the pores of porous catalysts

After initial breaking of catalyst particles into small fragments (microparticles), polymerization reaction occurs on surface of microparticles according to core–shell model. These microparticles together form porous macroparticles. This is the most accepted model for particle growth in olefin polymerization. Scheme of polyethylene morphology development during gas phase polymerization is shown in Fig. 6.

Fig. 6. Scheme of development of polymer with catalysts fragmentation

Several researchers have studied fragmentation of Ziegler–Natta catalyst for olefin polymerization. The fragmentation behavior of the emulsion-based Ziegler–Natta catalyst for propylene polymerizations was observed to be faster and more uniform than that of the MgCl₂ -supported and silica-supported catalysts of similar chemical composition [1-2].

Liquid phase polymerization

Radical polymerizations can be carried out both by homogeneous and heterogeneous process depending on whether the initial reaction mixture is homogeneous or heterogeneous. Bulk polymerization and solution polymerization are homogeneous processes while suspension and emulsion polymerization are heterogeneous processes. By heterogeneous polymerization thermal and viscosity problems can be controlled more efficiently.

1.  Bulk polymerization

Bulk polymerization of pure liquid monomer is the simplest process and carried out by using initiator in the absence of diluent or solvent. For this process reaction rate is high due to high monomer concentration and result in high yield per volume of reactor. Another advantage is that the relatively pure product is produced. However, control of the bulk polymerization, exothermic in nature, is difficult. The viscosity of the reaction system increases rapidly even at relatively low conversion. The heat removal is difficult due to high viscosity and low thermal conductivity of the polymer melt. Consequently local hot spots may occur resulting in degradation and discoloration of the polymer product. Bulk polymerization requires careful temperature control and strong elaborate stirring equipment. Though, bulk polymerization is commercially less used, polymerization of ethylene, styrene and methyl methcrylate are carried out by this method. The heat dissipation and viscosity problem are reduced by carrying out polymerization at low conversion. Bulk polymerization can be carried out in conventional stirred tank reactor, long tubular reactor with high surface to volume ratio and screw extruder reactors.

2.  Solution polymerization

Solution polymerization of monomers is carried out with dissolved monomers and initiators in solvent. Typical solvents include aromatic and aliphatic hydrocarbons, esters, ethers, alcohol or water. The solvent acts as diluent and aids in transfer of the heat of polymerization. In presence of solvent the stirring becomes easier since the viscosity of the reaction mixture is decreased. Consequently controlling of process temperature is much easier in solution polymerization compared to bulk polymerization. However, in presence of solvent purity of the product is reduced particularly if there is a difficulty in removal of solvent. Vinyl acetate, acrylonitrile and ester of acrylic acid are polymerized in solution.

3.  Suspension polymerization

Suspension polymerization is carried out by suspending relatively large droplets (10-1000µm) of insoluble monomers along with catalyst in water. The water to monomer weight ratio varies from 1:1 to 4:1 in most polymerization. The monomer droplets are prevented from coalescing by agitation and presence of stabilizers. The suspension stabilizers are typically used in less than 0.1 wt% of the aqueous phase. Two types of stabilizer are used:

1. Water soluble polymers such as poly vinyl alcohol, sodium poly styrene sulfonate, hydroxypropyl cellulose etc. 
    
2. Water insoluble inorganic compounds such as talc, barium sulfate, kaolin, calcium phosphate etc.

Styrene, acrylic and methacrylic esters, vinyl chloride, vinyl acetate and tetrafluoro ethylene are polymerized by suspension method.

4.  Emulsion polymerization

Emulsion polymerization involves finely divided droplets of insoluble monomers suspended in water. Hydrophobic monomer droplets, of diameter in the range of 0.5 -10 µm, are dispersed in water which also serves as heat transfer medium. In emulsion polymerization water soluble initiators such as persurphates are used. The difference between emulsion polymerization and suspension polymerization lies in the type and size of the particles in which polymerization occurs and kind of initiator employed. Many industrial polymers are produced by emulsion polymerization such as polybutadiene and PVC.

 Gas phase polymerization
Large scale production of polyethylene and polypropylene from gaseous monomer is carried out using heterogeneous catalyst. Powdered catalysts are mixed with gaseous monomers at the reactor entrance. Reactors are fluidized bed or stirred reactors. The major advantage of this process is that monomers can be easily separated from polymers. Catalyst residues are not separated from polymers.

Coordination polymerization

The polymerization catalyzed by transition metal complex such as Zieglar-Natta catalysts or metallocene catalysts is also known as coordination polymerization. The Ziegler –Natta catalysts system may be heterogeneous (some titanium based system) or soluble (most vanadium containing species). The best known are derived from TiCl₄ or TiCl₃ and aluminium trialkyl. These catalysts are highly stereospecific and can orient the monomer in specific direction before addition to the chain. The Ziegler-Natta and metallocene initiators are considered as coordination initiators that perform stereoselectivity by co-ordination. The olefin polymerization is carried out in presence of Ziegler–Natta catalyst (TiCl₄ supported on MgCl₂).

Mechanism and rate
Radical chain polymerization involves initiation, propagation, termination, chain transfer and inhabitation. For free radical polymerization the mechanism of formation of polymer using peroxide catalysts can be represented as follows:

Assuming that (a) overall rate of reaction is determined by rate of propagation and (b) rate of initiation of free radical is equal to rate of their termination, the overall rate equation can be derived as:

The coordination polymerization on Ziegler–Natta catalyst is assumed to be initiated by adsorption of monomer at an electron deficient surface vacant site on octahedral structure of titanium metal alkyl complex. A transition complex is formed by opening of the double bond. The complex is then rearranged by insertion of the monomer into the growing chain. When the insertion occurs at the original chain growing site with respect to metal ion and original vacant site is retained then the growth corresponds to isotactic growth. However if the chain growth site and vacant site interchange, then the chain growth corresponds to syndiotactic growth. The mechanism is shown in Fig. 7.

Fig. 7. Polymerization of ethylene with Ziegler-Natta catalysts

The Ziegler Natta catalysts are mostly heterogeneous in nature and adsorption processes are most likely to occur during polymerization reactions. Various kinetic schemes have been proposed assuming that polymerization centers are formed by the adsorption of metal alkyl species on the surface of a crystalline transition metal halide and then chain propagation occurs between the adsorbed metal alkyl and monomers. Langmuir Hinshelwood rate law for adsorption and reaction on solid is frequently adopted for this kind of reaction scheme. The rate expression for the heterogeneous Ziegler–Natta catalyzed polymerization process can be derived by using following model.

Assuming the rate of initiation and termination to be equal and that the overall rate is summation of rate of propagation and transfer, the overall rate can be derived as:

Industrial processes

Most polymerization processes are carried out in the liquid phase in batch reactor or CSTR and only few are continuous. For continuous process plug flow or fluidized bed with low residence time is used. Long residence time should be avoided in batch /CSTR as it is associated with many disadvantages such as catalysts decay and accumulation, polymer degradation, production of non-uniform polymer etc.

1.  Polyethylene production (PE)

Different grades of polyethylene such as low density polyethylene (LDPE), high density polyethylene (HDPE) or linear low density polyethylene (LLDPE) are produced commercially.  

Low density polyethylene (LDPE) and high density polyethylene (HDPE)

The LDPE or high pressure polyethylene is produced by radical polymerization. The HDPE or low pressure polyethylene is synthesized by co-ordination polymerization. Except LDPE, all other polymers of olefins are produced by co-ordination catalysts. LDPE obtained by radical polymerization differs structurally from HDPE produced by traditional Ziegler Natta co-ordination catalyst. The LDPE is more highly branched (both short and long branch) than HDPE and is therefore lower in crystallinity and density. The crystallinity of LDPE lies in the range of 40-60% and while that of HDPE in 70-90% . The density of LDPE and HDPE lie in the range 0.91 -0.93 g/cm³and 0.94-0.96 g/cm³ respectively. Compared to LDPE, HDPE has increased tensile strength, stiffness, chemical resistance and upper used temperature. Most HDPE have number average molecular weights in the range of 50000 -250,000 and have wide range of applications such as bottles, housewares, toys, pails, film for grocery bags and food packing, pipe, tubing, cables etc.

Linear low density polyethylene (LLDPE)

Co-ordination copolymerization of ethylene in presence of small amount of α-olefins such as 1-butene, 1-hexene or 1-octene results in polyethylene that have structure, properties and applications equivalent to the branched LDPE produced by radical polymerization. This polyethylene is known as linear low density polyethylene (LLDPE) and has controlled amount of ethyl, n-butyl and n-hexyl branches respectively.

The polyethylene can be produced by following methods :

1. LDPE is produced by free radical high pressure bulk polymerization process 
    
2. HDPE and LLDPE are produced by slurry-suspension process at moderately low pressure. The process is carried out over supported catalysts such as supported Cr or Cr organometallic or coordination catalysts such as metallocene or Ziegler –Natta catalysts. The reaction conditions are 80- 150°C and 20-35 atm pressure. Supported catalyst is typically suspended in alkene or cyclohexane solvent which also serve as the heat transfer medium. The Ziegler–Natta(TiCl₄ /Al - alkyl/MgCl₂) catalyst is more active than Cr based catalysts. 
    
3. HDPE and LLDPE can also be produced by gas phase fluidized bed polymerization over supported CrCO₃ or Ti based catalysts.

2.  Polypropylene production  

Isotactic isomers of polypropylene are most useful. It is stronger and harder than polyethylene and frequently used in block copolymer production. Various catalysts are used for this process.

1. Using Zeigler – Natta catalyst : The process is carried out at 70°C and ~13 atm using slurry reactors. Catalyst are prepared by reducing TiCl 4 with Al (C₂H₅)₃ in a cold hydrocarbon liquid to produce stereo-unselective form of TiCl₃. On heating to 100-200°C, TiCl₃ form convert to the stereo-selective form. Isotactic yield of propylene is about 92 %. The final active catalysts contain TiCl 3 and AlCl 3 . 
    
2. Using MgCl₂ supported TiCl 3 catalyst : The process is carried out at 70°C and 13-20 atm pressure. It gives around ~ 95 % isotactic polypropylene yield. The catalyst is prepared by first milling MgCl₂ with ethy bezanoate extensively to produce a highly active disordered state. Then it is treated with TiCl₄ at 100°C . 
    
3. Metallocene catalysts are m ore active with higher stereo selectivity. 100 % yield of isotactic or syndiotactic is possible.

Deactivation: CO, O₂ and S compounds act as poison for the catalysts. Reactants are passed through molecular sieve adsorbent column before treating with catalysts.