6.6 Optimization of the Procedure for Synthesis
of the Metallocene Molecule
The initial synthetic procedure developed for the synthesis and isolation of the pure
metallocene dichloride samples included steps that required subambient reaction
temperatures and chlorinated solvents such as methylene chloride [19, 20]. The
recipe, though very convenient at smaller scales, was not very practical for the
preparation of large quantities of, e.g., structures 1 or 6. A new synthetic procedure
that did not require the application of sub-zero temperatures and also eliminated the
methylene chloride as solvent by replacing it with pentane resolved this issue [187,
188]. In the new procedure for the preparation of the metallocene molecule, the
exact stoichiometric amount of the ligand’s di-anion and ZrCl 4 are suspended in
pentane and are reacted together. The procedure is effective in such a way that it
does not necessitate any purification steps if the starting reagents are reacted
together in stoichiometrically exact molar ratios. In this new “pentane procedure”
the yield is practically quantitative.
6.7 Proper Choice of the Silica and MAO
For the industrial preparation of supported metallocene catalysts, selection of the
inorganic support, generally a type of silica, is very important. There are many
varieties of silica available in the market and each has different specifications with
respect to the particle surface area, pore size (diameter, volume), bulk density, and
mechanical properties [189–194]. The silica pore diameter and volume must be
chosen bearing in mind that these pores will be partially filled with MAO.
Once the silica with desired properties is selected, the physically absorbed and
chemically bonded (OH groups) excess water molecules have to be removed to
reduce the number of OH groups and adjust their concentration to the selected
MAO type and concentration in order to minimize excess usage the of MAO. The
amount of fines formation during the polymerization, the bulk density of the
polymer particles, and even the morphology of the final polymer particles depend,
aside from the initial silica type, largely on the silica heat treatment regime and
proper MAO treatment [189–194]. The heat treatment of the silica is also essential
in ensuring optimum catalyst activity and, as mentioned, reducing the amount of
MAO needed. It helps to reduce the amount of catalyst required and consequently
the overall production cost. In contrast to silica, there are only a few known sources
of MAO available commercially, with more or less similar compositions but
different tri-methyl aluminum contents. No matter which of these MAO types is
chosen, care must be taken to ensure the freshness and constancy of the product in
each delivered batch to maintain the consistency of the resulting catalyst properties
and of polymer production in the plant.
Syndiotactic Polypropylene: Discovery, Development, and Industrialization. . .
97
of the Metallocene Molecule
The initial synthetic procedure developed for the synthesis and isolation of the pure
metallocene dichloride samples included steps that required subambient reaction
temperatures and chlorinated solvents such as methylene chloride [19, 20]. The
recipe, though very convenient at smaller scales, was not very practical for the
preparation of large quantities of, e.g., structures 1 or 6. A new synthetic procedure
that did not require the application of sub-zero temperatures and also eliminated the
methylene chloride as solvent by replacing it with pentane resolved this issue [187,
188]. In the new procedure for the preparation of the metallocene molecule, the
exact stoichiometric amount of the ligand’s di-anion and ZrCl 4 are suspended in
pentane and are reacted together. The procedure is effective in such a way that it
does not necessitate any purification steps if the starting reagents are reacted
together in stoichiometrically exact molar ratios. In this new “pentane procedure”
the yield is practically quantitative.
6.7 Proper Choice of the Silica and MAO
For the industrial preparation of supported metallocene catalysts, selection of the
inorganic support, generally a type of silica, is very important. There are many
varieties of silica available in the market and each has different specifications with
respect to the particle surface area, pore size (diameter, volume), bulk density, and
mechanical properties [189–194]. The silica pore diameter and volume must be
chosen bearing in mind that these pores will be partially filled with MAO.
Once the silica with desired properties is selected, the physically absorbed and
chemically bonded (OH groups) excess water molecules have to be removed to
reduce the number of OH groups and adjust their concentration to the selected
MAO type and concentration in order to minimize excess usage the of MAO. The
amount of fines formation during the polymerization, the bulk density of the
polymer particles, and even the morphology of the final polymer particles depend,
aside from the initial silica type, largely on the silica heat treatment regime and
proper MAO treatment [189–194]. The heat treatment of the silica is also essential
in ensuring optimum catalyst activity and, as mentioned, reducing the amount of
MAO needed. It helps to reduce the amount of catalyst required and consequently
the overall production cost. In contrast to silica, there are only a few known sources
of MAO available commercially, with more or less similar compositions but
different tri-methyl aluminum contents. No matter which of these MAO types is
chosen, care must be taken to ensure the freshness and constancy of the product in
each delivered batch to maintain the consistency of the resulting catalyst properties
and of polymer production in the plant.
Syndiotactic Polypropylene: Discovery, Development, and Industrialization. . .
97