The Catalyst Review June 2020 - 11

SPECIAL FEATURE
and appropriate electron density are expected to obtain PP with high isotacticity index. It is reported that this bulky group could
protect the silane against removal from the catalyst surface when contacting with aluminum alkyl activator. By far, cyclohexyl(methyl)
dimethoxysilane (donor C) and dicyclopentyldimethoxysilane (donor D) have been most commonly used. In respect to the
polymerization performance, donor C shows high hydrogen sensitivity while donor D allows particularly high stereospecificity and
broader MWD. However, it is often difficult for a single external donor to yield desirable control over multiple properties of the final
polymer. To overcome this problem, mixed external donors or separate addition in different reactors could be considered to tune
polymer properties (Wang, 2019).
Now, we would add some further notes on the polymerization
activators (organometals) and catalyst support technology
advances. Large amount of aluminium alkyls are used in ZN
and chromium oxide coordination catalysts for producing HDMD-LLDPE, PP, EPR/EPDM and BR. These co-catalysts are used
to activate the transition metal catalyst as both a reducing
agent and an alkylating agent. As said, they also function to
scavenge water and oxygen that usually poison the catalyst.
See in Table 4 a list of some commercial Al-Alkyls co-catalysts.
Triethylaluminium (TEAL) is the most used Al-Alkyls for ZN and
chromium oxide catalysis. The multistep process to produce TEAL
uses aluminum metal, hydrogen gas, and ethylene, summarized
as follows: 2Al + 3H2 + 6C2H4 → Al2Et6.

Table 4: Commercial Al-Alkyls Activators.

Source: Author

Because of this efficient synthesis, TEAL is one of the most available organoaluminium compounds. Methylaluminoxane (MAO)
is the most widely used co-catalyst for metallocene catalysts, since it is able to activate the largest number of metallocenes. It is
obtained by the controlled hydrolysis of Al(CH3)3 and consists of a mixture of several different compounds, included residual AlMe3
and possibly AlO3 units in dynamic equilibrium. The main drawback of MAO is the high cost due to the expensive AlMe3 parent
compound. To solve the above problems, MAO alternatives have been investigated. An improvement toward alternative systems has
been the use of boron compounds in combination with metallocene dialkyls. As these systems are not able to scavenge impurities,
and a large part of the activated catalyst has to be sacrificed for that purpose, much better results have been achieved by adding
small amounts of AlR3, such as TIBAL and TEAL, to the reaction system. Although these boron activators can be used in small
amounts, close to equimolar with the metallocene (1-1.2:1), they are still quite expensive. In the past decade many research efforts
were made with the ultimate aim of improving the cost in use (productivity) of silica supported single-site (metallocene) catalyst
systems for olefin polymerization. ActivCat of Grace is a MAO-based commercially available catalyst activator technology, which
is claimed to provide improved performance, lower cost-in-use, in both mPE & mPP platforms without compromising important
polymer resin-related properties. This technology includes the use of novel haloaluminoxane proprietary compositions, originally
developed by Albemarle (Sangokoya Patent, 2007) but now owned and commercialized by Grace. The halogen can be fluorine,
chlorine, and/or bromine; the fluoroaluminoxanes seem to provide the greatest efficacy in increasing activity. Such compositions
typically have considerable stability under inert, anhydrous conditions, while maintaining their solubility in hydrocarbon solvents,
especially aromatic hydrocarbon solvents (TCGR, 2011).
Supports are an important component of the catalyst system because make it possible to control particle size distribution and
morphology of the catalyst. Proper control of catalyst PSD is desirable for minimizing large (and small) particles. Because on-site
industrial polymer transfer is achieved primarily by pneumatic conveyance, large particles can lead to flow problems, such as plugged
transfer lines. Fine particulates may cause instead clogged filters and heighten the possibility of dust explosions. Morphology control
increases bulk density of the finished resin and improves fluidization dynamics in gas phase processes. Many inorganic compounds
were tested as supports, but magnesium salts and silica provide the most serviceable catalysts. Magnesium compounds such as
MgO, Mg(OH)2, HOMgCl, CLMgOR and Mg(OR)2, have been used, but anhydrous MgCl2 is the most widely employed in commercial
Ziegler-Natta catalysts. Titanium active centers are chemisorbed on the surface of the magnesium compound also due to the
similarity of crystal structures between MgCl2 and TiCl3. Silica is sometimes called a carrier since the catalyst may be simply deposited
onto the support, not so for supported chromium catalysts, where the catalyst is strongly bonded to the support. Even in cases of
simple deposition of catalyst, however, the PSD and morphology of the silica dictates the PSD and shape of the polymer particle.
Special polymerization cases use also clay as special support to obtain PP nanocomposites that have applications in food packaging,
structural, electrical/electronic, and medical fields. Bi-supported Ziegler-Natta catalysts (MgCl2/clay/internal donor/TiCl4 based)
containing varying amounts of organoclay are used to synthesize spherical particles of polypropylene/clay nanocomposites (PCN).
The organoclay is introduced into the catalyst support formulation and PCN are obtained using an in situ polymerization technique in
presence of triethylaluminium as cocatalyst and silane as external electron donor (Cardoso, 2018).
The Catalyst Review

June 2020

The Catalyst Review June 2020

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