34
T. Taniike et al. / Journal of Catalysis 311 (2014) 33–40
(3) The said hierarchical agglomeration leads to the formation
ization results of the Mg particles are shown in Table 1. The size
of Mg particles becomes smaller in the order of A ? C ? B ? D.
Ethanol (purity >99.5%) was dried over 3A molecular sieve with
N2 bubbling. Heptane (purity >99.5%), toluene (purity >99.5%) and
di-n-butylphthalate (DBP) (purity >98%) were dried over 4A molec-
ular sieve with N2 bubbling. Cyclohexylmethyldimethoxysilane
(CMDMS) was purified by distillation under reduced pressure. Eth-
ylene of research grade donated by Sumitomo Chemical Co., Ltd.
was used as delivered.
of a range of porosity from micro to macropores, whose dis-
tributions and shapes are sensitively affected by employed
preparation methods and conditions [20]. In Ziegler–Natta
olefin polymerization, polymer initially formed in accessible
pores build up mechanical stresses inside catalyst particles
to trigger particle fragmentation (called the pore-breakage
process), in which fresh catalyst surfaces which are origi-
nally hidden in inaccessible pores are continuously exposed.
These processes enable industrial catalysts to retain stable
polymerization activity over hours. In this way, inner struc-
tures of catalyst macroparticles significantly affect the frag-
mentation process [21,22] and the kinetic behavior during
polymerization [12].
2.2. Mg(OEt)2 synthesis
Mg(OEt)2 was synthesized based on a patent [29] with several
modifications. 0.25 g of MgCl2 (as an initiator) and 31.7 mL of
dehydrated ethanol were introduced into a 500 mL jacket-type
separable flask equipped with a mechanical stirrer rotating at
180 rpm under N2 atmosphere. After the dissolution of MgCl2 at
75 °C, 2.5 g of Mg and 31.7 mL of ethanol were introduced. 2.5 g
of Mg and 31.7 mL of ethanol were again added 10 min after the
reaction was initiated by MgCl2. Thereafter, 2.5 g of Mg and
31.7 mL of ethanol were added repeatedly 4 times every 10 min,
followed by aging at 75 °C for 2 h. Finally, the temperature was de-
creased to 40 °C, and the product was washed with ethanol. In this
study, four kinds of Mg(OEt)2 particles (MGE A–D) were synthe-
sized from Mg A–D under the same conditions.
Roughly speaking, the first structural feature at the atomic scale
is mainly related to the primary structure and polydispersity of
produced polymer through the performance of active sites
[14,23], while the latter two at larger scales from nm to lm mainly
affect the kinetic profile of a catalyst and the resultant polymer
particle morphology through fragmentation and replication phe-
nomena during polymerization [11,24,25]. However, from a quan-
titative viewpoint, all these issues are believed to more or less
exert influence on each said performance.
Though several research studies have been undertaken with the
aim to understand relationships between catalyst structures and
performances in Ziegler–Natta olefin polymerization, quantitative
structure–performance relationships have not yet been reached.
One of the main drawbacks in the previous studies can be attrib-
uted to the absence of multilateral characterization: They deter-
mined only one or a few structural parameter(s) of catalyst
samples such as the crystalline disorder of MgCl2 [26], surface area
[27], total pore volume [12,28], and average pore size [12], without
considering other parameters which also affect a targeted perfor-
mance. Characterization and parameterization of the structures
of Ziegler–Natta catalysts are actually not trivial in terms of the
complexity and heterogeneity in chemical and physical structures
as well as of their extreme sensitivity to moisture. Nevertheless,
reliable quantitative structure–performance relationships are not
to be acquired without parameterizing catalyst structures as pre-
cisely as possible with various characterization methods.
Based on these backgrounds, we have set as our primary objec-
tive to first establish and apply multilateral characterization for
structures of Ziegler–Natta catalysts. Four catalysts were prepared
based on the chemical conversion of Mg(OEt)2 precursor, which is
one of the most commonly employed preparation routes in
industry due to superior activity and copolymerization ability of
resultant catalysts. They were subjected to a variety of character-
ization methods in order to achieve structural parameterization
over multi-length scales such as electron microscopy, N2 adsorp-
tion/desorption, Hg intrusion, UV/vis spectroscopy, and gas
chromatography. We also examined impacts of the determined
structural parameters on the ethylene/1-hexene copolymerization
ability of the catalysts.
2.3. Catalyst preparation
The preparation of Ziegler–Natta catalysts from Mg(OEt)2 was
conducted again based on a patent [30] with several modifications.
10 g of Mg(OEt)2 and 140 mL of toluene were charged in a 300 mL
3-neck flask equipped with
a mechanical stirrer rotating at
180 rpm under N2 atmosphere. 20 mL of TiCl4 was added dropwise,
while the temperature of the suspension was kept within 0–5 °C.
Thereafter, the temperature was first elevated to 90 °C to add
3.0 mL of DBP, and then, it was brought to 110 °C. The reaction
slurry was continuously stirred at 110 °C for 2 h. Subsequently,
the reaction product was washed with toluene twice at 90 °C and
further treated with 20 mL TiCl4 at 90 °C for 2 h. After that, the
product was washed with n-heptane 7 times to get the final cata-
lyst. Four kinds of Ziegler–Natta catalysts (Cat A–D) were obtained
from MGE A–D under the same conditions.
2.4. Polymerization
Ethylene/1-hexene copolymerization was performed in a 1 L
autoclave equipped with
a mechanical stirrer rotating at
350 rpm. 407 mL of n-heptane was introduced into the reactor.
TEA ([Al] = 10 mmol/L), CMDMS (Al/E ꢀ D = 10) and 93 ml of
1-hexene (corresponding to 0.75 mol) were introduced into the
reactor, and the solution was saturated with 0.5 MPa of ethylene
at 50 °C. A catalyst ([Ti] = 0.005 mmol/L) was fed into the reactor
by a bomb injection technique to initiate the polymerization. The
polymerization was conducted for 30 min with a continuous sup-
ply of ethylene gas at 0.5 MPa. The polymer was recovered by
pouring the reaction slurry into mixture of acetone and methanol
kept at 0 °C and subsequent filtration.
2. Experimental
2.5. Characterization
2.1. Materials
2.5.1. Scanning electron microscopy
Anhydrous MgCl2, triethylaluminium (TEA), and four kinds of
poreless Mg particles (termed Mg A–D) were donated from Toho
Titanium Co., Ltd., Tosoh Finechem Corporation and Yuki Gousei
Kogyo Co., Ltd., respectively. The morphologies of Mg A,C are
flake-like, while those of Mg B,D are spherical (Fig. 1). Character-
Particle morphological characteristics of Mg(OEt)2 and catalyst
particles were studied with scanning electron microscopy (SEM,
Hitachi S-4100) operated at an accelerating voltage of 20 kV. Be-
fore the measurements, particles were subjected to Pt sputtering
for 100 s. To quantify observed particle morphology, SEM images