Chemistry Reference
In-Depth Information
Following the heating, the oxide is reduced with H
2
or NaBH
4
. Sometimes, powdered Al
2
O
3
or
MoO
3
-Al
2
O
3
is added prior to reduction.
For an Al
2
O
3
-SiO
2
support, a 3% metal nitrate solution is used. The saturated bed is heated to
500
C to decompose the nitrate. This is followed by a reduction. Many modifications of the above
procedure developed since.
Preparations based on molybdenum oxide are called Standard Oil Co. catalysts, while those based
on chromium oxide are known as the Phillips Petroleum Co. catalysts. Following is a description of a
preparation of a molybdenum based catalyst. Alumina is saturated with a solution of ammonium
molybdate and then subjected to heating in air at 500-600
C. The oxide that forms is reduced with
hydrogen at 430-480
C. Reducing agents like CO, SO, or hydrocarbons are also used. Hydrogen,
however, is preferred at pressures of approximately 75 psi. The catalyst may contain between 5 and
25% of the molybdenum compound dispersed on the surface [
309
].
A typical chromium based catalyst is prepared similarly. It is usually supported by a 9:1 of SiO
2
:
Al
2
O
3
[
347
] carrier. Either Cr(NO
3
)
3
9H
2
OorCrO
3
solutions in nitric acid are used to impregnate
the support. The nitrates are decomposed in air at 400-1,000
C[
310
]. Optimum chromium content in
these catalysts is 2-3%.
The mechanism of polymerization was initially believed to be as follows [
212
]. Cr-O-Si bonds are
present on the catalyst surface. Contact with ethylene results in an oxidation-reduction reaction and
formation of an ethylene-chromium complex. The initiations are accompanied by gains of hydride
ions and formations of terminal methyl groups. Polymer growth was pictured as follows:
Cr
Cr
Cr
catalyst surface
The termination reactions were believed to result from hydride-ion transfers [
262
]. Later, however,
polyethylene, formed with these catalysts, was found to contain approximately equal numbers of
saturated and unsaturated chain ends. This contradicts the above concept. It may also mean that the
initiation involves
p
-complexes that disproportionate to yield coordination of the metal to -CH
ΒΌ
CH
2
andto-CH
2
-CH
3
groups [
262
].
Polymerization of propylene with these catalysts yields polymers with very little crystallinity.
Higher
a
-olefins yield completely amorphous polymers. Formation of partially crystalline polymers
was reported for vinylcyclohexane, allylcyclohexane, and 4-phenyl-1-butene. Styrene does not
polymerize at all. Isoprene forms a
trans
-1,4-polymer [
262
].
4.5.8
Isomerization Polymerizations with Coordination Catalysts
Polymerizations of many internal olefins, like 2-butene, 2-pentene, 3-heptene, 4-methyl-2-pentene, 4-
phenyl-2-butene and others, with Ziegler-Natta catalysts are accompanied by monomer
rearrangements. The isomerizations take place before insertions into the chains [
311
-
318
]:
R
CH
2
R
n
n
CH
2
R
The double bonds migrate from the internal to the
a
-positions:
