Polyolefin catalyst and method for its preparation

A new catalyst and a method of polymerizing olefins in which the catalyst is prepared by forming a mixture by dispersing on a finely divided, difficult to reduce, inorganic support of the class consisting of silica, alumina, thoria, zirconia, titania, magnesia and mixtures or composites thereof, an organic chromium compound pyrolytically decomposable in the substantial absence of oxygen to deposit a catalytically active residue along with a carbon residue as a contaminant on the support, then activating the mixture by subjecting the mixture to non-oxidative pyrolysis to and at a temperature within the range of about 600-2000.degree. F., thereby depositing on the support the chromium-bearing residue and carbon residue as a by-product from the pyrolytic decomposition of the organic chromium compound, and subjecting the activated catalyst after this activating to heat and an oxidizing gas such as air, oxygen, carbon dioxide, nitrous oxide, and the like, to burn off a substantial amount of the carbon residue and to modify and improve the characteristics of the activated catalyst.

CROSS-REFERENCE TO RELATED APPLICATION 
This invention in one embodiment is related to the assignee's Hwang et al 
prior application Ser. No. 478,879, filed June 13, 1974, now U.S. Pat. No. 
3,953,413 issued Apr. 27, 1976. 
BACKGROUND OF THE INVENTION 
The new and improved catalysts of this invention are prepared by depositing 
on a finely divided and difficult to reduce inorganic oxide selected from 
silica, alumina, thoria, zirconia, titania, magnesia and/or mixtures 
thereof an organic chromium compound and then activating the resulting 
mixture in a non-oxidizing inert or reducing atmosphere at elevated 
temperatures up to about 2000.degree. F. followed by subjecting the 
activated catalyst to an oxidizing gas to burn off the carbon residue and 
to modify or improve the characteristics of the activated catalyst. This 
invention is also effective in improving the performance of such catalysts 
wherein the support is first chemically modified with metallic elements 
including zirconium, titanium and others. It has been found experimentally 
that a black color formed on the catalyst during activation is due to 
carbon deposits which are presumed to result from the decomposition of the 
organic chromium compound during the non-oxidative pyrolysis step. The 
carbon deposits are substantially removed by burning in an oxidizing gas 
and the resulting catalysts give much improved performance in 1-olefin 
polymerization. Notably and significantly improved are catalyst activity, 
solid polymer color of polymers produced with the new catalyst, and 
various other desirable polymer physical properties such as melt index, 
shear response, and melt elasticity. 
SUMMARY OF THE INVENTION 
The organic chromium compounds used to prepare the catalysts which are the 
subject of this invention can be any of those that provide an olefin 
polymerization catalyst when mixed with a support as defined herein and 
subjected to non-oxidative pyrolysis. One of the types of organic chromium 
compounds that fall within this description are the chromium chelates of 
the above Hwang et al Pat. No. 3,953,413. The chelates are derived from 
one or more beta-dicarbonyl compounds that may be either acyclic or 
cyclic, the chelates being essentially of the formula of the class 
consisting of 
##STR1## 
wherein R is individually selected from hydrogen, alkyl, alkenyl, aryl, 
cycloalkyl and cycloalkenyl radicals and combinations of these radicals 
with each R containing 0-20 carbon atoms and a corresponding 
valence-satisfying number of hydrogen atoms, R' is selected from alkylene, 
alkenylene, arylene, cycloalkylene and cycloalkenylene radicals and 
combinations of these bivalent radicals with R' containing 1-20 carbon 
atoms and a corresponding valence-satisfying number of hydrogen atoms, m 
is a whole number of 1 to 3, n is a whole number of 0 to 2 and m plus n is 
2 or 3 and X is an inorganic or organic negative group (relative to 
chromium) such as halide, alkyl, alkoxy, and the like. Typical compounds 
are chromium acetylacetonate, chromium benzoylacetonate, chromium 
5,5-dimethyl-1,3-cyclohexanedionate, chromium 2-acetylcyclohexanonate, and 
the like. 
A second group of organic chromium compounds are the .pi.-bonded 
organochromium compounds of the structure 
EQU (L).sub.x -- Cr -- (L').sub.y 
disclosed, for example, in U.S. Pat. Nos. 3,806,500 and 3,844,975 wherein L 
and L' are the same or different organic ligands which are adapted to 
being pi-bonded to the chromium atom, and x and y are each integers of 0 
to 3, inclusive, and x plus y equals 2 to 6, inclusive. Typical compounds 
of this group are bis(cyclopentadienyl) chromium (II), 
bis(benzene)chromium (O), cyclopentadienyl chromium tricarbonyl hydride, 
etc. 
A third group of organic chromium compounds are tetravalent organochromium 
compounds of the structure Y.sub.4 Cr disclosed, for example, in U.S. Pat. 
No. 3,875,132 wherein Y is individually selected from alkyl, alkenyl, 
cycloalkyl, cycloalkenyl, cycloalkyl-substituted alkyl, or 
aryl-substituted alkyl radicals containing 1 to about 14 carbon atoms and 
the tetravalent chromium atom is directly linked to one of the carbon 
atoms in each alkyl group. Typical compounds of this group are 
tetrakis(neopentyl)chromium(IV), tetrakis(tertiary-butyl)chromium (IV), 
etc. 
Another type of organic chromium compound which may be used in this 
invention is the reaction product of ammonium chromate and pinacol as 
disclosed in Hoff et al U.S. Pat. No. 3,986,983 issued Oct. 19, 1976, and 
also assigned to the assignee hereof. 
Still another group of chromium compounds which may be used in the present 
invention include several types of chromate esters. A simple type is 
organic chromate of the formula 
##STR2## 
wherein R is individually selected from hydrogen or a hydrocarbyl radical 
containing about 1-14 carbon atoms, preferably about 3-10 carbon atoms, 
including alkyl, aryl, arylalkyl, cycloalkyl, alkenyl and cycloalkenyl 
groups. Typical compounds are bis(triphenylmethyl) chromate, 
bis(tributylmethyl)chromate, etc. 
A second group of chromate ester is organosilyl chromate, such as described 
in Granchelli et al U.S. Pat. No. 2,863,891, and has the general formula 
##STR3## 
wherein R is individually selected from hydrogen and a wide range of 
hydrocarbyl groups similar to those just described immediately above. A 
typical compound is bis(triphenylsilyl)chromate. 
A third type of chromate ester which may be used in this invention is 
chromyl bis(trihydrocarbyltitanate), such as disclosed in U. S. Pat. No. 
3,752,795, and has the general formula 
##STR4## 
wherein R is individually selected from a wide range of hydrocarbyl 
radicals described immediately above. A typical compound is chromyl 
bis(tributyltitanate). 
Still another type of chromate ester is chromyl 
bis(dihydrocarbylphosphate), such as disclosed in U.S. Pat. No. 3,474,080, 
and has the general formula 
##STR5## 
wherein R is again individually selected from a wide variety of 
hydrocarbyl groups described immediately above. A typical compound is 
chromyl bis(diphenylphosphate). 
In accordance with this invention, the burning off of the carbon residue 
by-product from the activated catalysts gives improved catalytic 
performance when used for olefinic polymerization or copolymerization. 
In accordance with this invention, the new catalysts are prepared and 
activated in the following manner: 
1. The support or base 
The finely divided and difficult to reduce inorganic support is preferably 
silica, alumina, zirconia, thoria, magnesia, titania, or mixtures or 
composites thereof. These supports can have a pore volume in excess of 0.5 
cc/g and a surface area ranging from a few m.sup.2 /g to over 700 m.sup.2 
/g, but preferably above 150 m.sup.2 /g. A finely divided non-porous 
support with a high surface area such as "Cab-O-Sil" may also be used. 
It is sometimes advantageous to pretreat the support before addition of the 
organic chromium compound. Such pretreatment typically consists of 
adjusting the moisture content of the support by drying at elevated 
temperature or chemically modifying the support with compounds containing 
metallic elements such as zirconium, titanium, boron, vanadium, tin, 
molybdenum, magnesium, hafnium, or the like. Chemical modification can 
include adding compounds such as ammonium hexafluorosilicate which can 
react with the support or with the organic chromium compound during 
calcining and activation. Chemical modification using metal alkyls which 
react with the support can also be used. 
In calcining or adjusting the moisture content of the support, temperatures 
of from 300.degree.-2000.degree. F. are normally used for a time 
sufficient to drive off substantially all loosely held volatile material. 
The calcining or drying steps can be carried out by any process known in 
the art such as in a furnace or in a heated fluizided bed using dry gases 
such as nitrogen, air, carbon monoxide or other suitable reactive or inert 
gases as the fluidizing medium. 
2. Impregnating the support 
The organic chromium compound can be deposited on the support prior to 
thermal activation in a number of ways well known in the art. These 
include dry mixing the support and the organic compound, dissolving the 
chromium compound and mixing the solution and the support, and vaporizing 
the compound and contacting the vapor with the support. In the case of 
solution impregnation, it is often convenient to remove excess solvent by 
drying before proceeding with thermal activation. 
3. Thermal activation of the catalyst 
Up to this point, in most cases, the catalysts so prepared have little or 
no activity. To improve their performance, a process commonly known as 
activation or thermal aging is employed. In essence, this process requires 
subjecting the catalysts to elevated temperatures in the presence of an 
inert or reducing (nonoxidative) atmosphere. As demonstrated in Examples 
18-24, even a minute contamination of oxygen during the activation 
generally has a detrimental effect on catalyst activity. Understandably, 
such an adverse effect is greatly magnified when the chromium level is 
reduced to about 0.15% from a more typical 1% by weight. 
The activation step is usually carried out using a prescribed heating cycle 
which includes heating the catalysts up to a specific temperature, usually 
in the range of 600.degree.-2000.degree. F. (preferably 
800.degree.-2000.degree. F.), holding the catalyst at this temperature for 
a prescribed length of time, usually 30 minutes to 12 hours, followed by 
cooling. The cycle can include hold periods at temperatures below the 
maximum to permit diffusion of moisture or solvents from the catalyst 
pores, or to permit reactions such as decomposition of the organic 
chromium compound to take place. 
4. Treatment of the activated catalyst 
The new and improved catalyst of this invention is obtained by subjecting 
the activated catalyst described in the preceding section to a 
post-treatment with dry air, oxygen, carbon dioxide, nitrous oxide and 
other oxidizing gases for a short period of time, preferably in a fluid 
bed, at elevated temperatures up to but normally below the highest 
temperature at which the catalyst was previously held during the 
non-oxidative activation. In general, this treatment results not only in a 
partial or complete burning off of the carbon residue which eventually 
leads to the improved polymer color of a polymer prepared with the 
catalyst but also in a substantial modification of the catalyst which 
reveals itself in the improved activity and significantly different and/or 
improved polymer properties. 
As the presence of even a minute quantity of oxygen during the 
non-oxidative activation was known to be detrimental to the genesis of 
catalyst activity, or in other words oxygen is a catalyst poison, it was 
generally expected that any attempt to remove carbon residue by the air 
treatment or other oxidative methods would necessarily involve severe 
sacrifice of catalyst activity, possibly to the extent of completely 
deactivating the catalyst. It was therefore unexpected that catalyst 
activity was greatly improved instead and that the physical properties of 
the resulting polymer were also significantly modified or improved. 
As a matter of practical considerations, this post-activation treatment is 
normally carried out by using dry air or diluted air but other less 
obvious and weaker oxidizing gases such as carbon dioxide and nitrous 
oxide may also be used, as illustrated in Examples 25-28 with excellent 
results of improving the polymer color. In general, a mixture of the 
above-mentioned oxidizing gases may be used also. Furthermore, it is 
permissible, or sometimes useful from an operating point of view, to 
dilute the oxidizing gas or mixture of oxidizing gases with an inert gas, 
or a mixture of inert gases, such as nitrogen, helium, argon, neon, etc., 
in said treatment of the activated catalyst. The temperature and duration 
of said treatment are, as a rule, to be adjusted in each case so as to 
achieve the desired effects depending on the catalyst composition, type of 
oxidizing gas and other catalyst preparation variables. 
In the case of air treatment, the preferred temperature is 
900.degree.-1700.degree. F. In this range of from about 
900.degree.-1700.degree. F. a drastic improvement in polymer color and 
other resin properties such as lower ash, higher melt index, broader MWD, 
etc. is obtained. If a Hunter meter is used to measure the color of the 
polymer produced with catalysts which are the subject of this invention, 
it is found that air treated and untreated catalysts give typical values 
as follows: 
______________________________________ 
"B Value" 
"L Value" 
______________________________________ 
Polymer from untreated catalyst 
0.5-2.0 68-80 
Polymer from air treated catalyst 
0.1-1.0 86-91 
______________________________________ 
Where a higher "B" value indicates a more intense yellow color in the 
polymer, a high "L" value indicates better whiteness. 
Polymer properties do not depend on the length of time the catalyst is air 
treated, provided the time is long enough to eliminate the carbon residue 
evidenced by the characteristic black color of the untreated catalysts. It 
has been demonstrated experimentally that the air treatment can be as 
short as 20 seconds. 
5. Polymerization 
The new and improved catalysts prepared according to this invention may be 
used to polymerize 1-olefins in liquid phase or vapor phase processes. 
These processes may be either batch or continuous. The mode of charging 
catalyst, olefin, and solvent if required, to the reactor system may 
follow any conventional practice applicable to batch or continuous 
operation. Normally, agitation is provided in the reactor as well as a 
means to remove the heat of polymerization and a means to control the 
reactor temperature. In liquid phase processes, olefin polymer is normally 
recovered by flashing off solvent without any intervening steps for 
removal of the catalyst. The activity of the catalysts described in this 
invention is normally greater than 3000 lbs. of polymer/pound of catalyst 
so that catalyst removal for practical purposes is unnecessary. Reactor 
conditions are dependent on the type of olefin as well as the desired 
polymer properties. In the case of ethylene, reactor pressures may range 
from 50 to 1000 psig, temperatures from 150.degree. F. to 500.degree. F. 
and solids levels from 5-60% by weight. 
As a result of this invention, it is now possible to achieve the following 
improvements with catalysts derived from supported organic chromium 
compounds pyrolytically activated in a non-oxidative atmosphere: 
a. Improved solid polymer color when used for 1-olefin polymerization. 
b. Higher melt index especially when used for ethylene polymerization. 
c. Higher catalyst activity. 
d. Production of ethylene polymers having broader molecular weight 
distribution as indicated by higher melt flow shear response. 
e. Production of ethylene polymers having higher melt elasticity.

DESCRIPTION OF THE PREFERRED EMBODIMENTS 
The following examples illustrate the invention: 
EXAMPLE 1 
A silica base having a surface area of approximately 350 m.sup.2 /g and a 
pore volume of approximately 1.7 cc/gm was used as the catalyst support 
for this example. 
This type of material is available commercially from such sources as the 
Davison Chemical Company, and their designation for this type of material 
is 952 MS-ID silica gel. The catalyst of this example was prepared by 
thoroughly mixing this silica base with an aqueous solution of zirconium 
tetrachloride. A sufficient amount of zirconium tetrachloride was used to 
give 1% zirconium on the base. The base so impregnated was dried in an 
oven at 400.degree. F. until it was free flowing, at which point it was 
transferred to saggers and calcined in a muffle furnace at 1200.degree. F. 
for 4 hours. Upon cooling, this dried silica was then dry mixed with a 
sufficient amount of chromium acetylacetonate to give a chromium 
concentration of 1% by weight on the total dry catalyst. The dried 
catalyst was then transferred to an activator. 
The activator consisted of a 4 inches I.D. by about 48 inches long tube 
made of "Inconel" metal. The tube was provided with electric heaters 
around the outside of the tube. The heaters were capable of heating the 
tube plus its contents to temperatures of up to 2000.degree. F. The bottom 
of the tube was fitted with a distributor plate designed to give uniform 
distribution of the gas entering the bottom of the tube and flowing up 
through the tube. A bed of regenerated molecular sieves was used to dry 
the nitrogen to a total moisture content of less than 2 ppm (vol.) before 
it entered the tube. Before entering the desiccant bed, the nitrogen was 
passed through a deoxygenating bed containing a reduced copper oxide 
catalyst. In this bed, the oxygen level was reduced to less than 5 ppm 
(vol.). A flow measuring device to regulate the flow rate of gas through 
the activator tube was provided. A controller for the heating elements 
capable of raising the temperature of the fluidizing tube to elevated 
temperatues according to a predetermined cycle was also provided. 
In this tube, the catalyst was fluidized with nitrogen and heated to a 
temperature of 350.degree. F. and held for 3 hours; the temperature was 
then raised to 550.degree. F. and held for 3 hours; and the temperature 
was then raised to 1700.degree. F. and held for 6 hours. The heat up rate 
between hold temperatures was about 150.degree. F./hr. All the while the 
nitrogen flow was held constant to provide fluidization of the catalyst 
within the heated tube. The catalyst was then cooled to approximately 
ambient temperature while still fluidized and was then dumped from the 
tube into a predried flask which had been carefully purged to eliminate 
all traces of oxygen and moisture from the interior of the flask. This 
flask was then sealed, and the flask was stored in a container having a 
dry nitrogen environment until the catalyst was to be used in the 
polymerization process. The activated catalyst of this example was black 
in color. At a suitable time, the catalyst was charged to a continuous 
polymerization reactor and used to polymerize ethylene at a temperature of 
about 227.degree. F. in the presence of dry isobutane and with an ethylene 
concentration of about 5% by weight in the reactor. 
The reactor used for the polymerization tests consisted of a vessel 
provided with a jacket and a means for good agitation within the vessel. 
The volume of the vessel was about 90 gallons. Water was circulated 
through the jacket of the reactor to remove the heat liberated during the 
polymerization reaction. Means were provided to regulate the coolant 
temperature and the coolant flow so as to control the temperature of the 
reactor. Means were provided to feed a slurry of catalyst to the reactor 
at a controlled rate. Means were also provided to feed ethylene to the 
reactor at a controlled rate. Means were also provided for introducing a 
second monomer or comonomer to the reactor as well as modifying agents to 
control the molecular weight of the polymer formed in the reactor although 
these were not used in this example. Means were provided to feed a diluent 
separately to the reactor at a controlled rate. Means were provided to 
discharge a mixture of the polymer formed in the reactor, unreacted 
monomer and/or comonomer, and diluent from the reactor. The polymer 
mixture discharged from the reactor flowed to a heated flash vessel where 
the diluent and unreacted ethylene were removed as a vapor and the polymer 
was recovered with only traces of hydrocarbon. The recovered polymer was 
purged batchwise with inert gas to remove the traces of hydrocarbon and 
analysed for melt index, density and ash. These factors are determined by 
standard tests well known in the industry. The test used for determining 
melt index is ASTM D-1238, and the method for measuring the density is 
ASTM D-1505. Ash was determined by a pyrolysis method. In all cases, the 
polymer yield figures are calculated from the ash values. 
This example illustrated the performance of a typical nontreated chromium 
acetylacetonate catalyst. The polymer yield of this catalyst amounts to 
2,730 pounds of polymer collected per pound of catalyst fed to the 
reactor. The Hunter color evaluation indicated a whiteness value of 76.7. 
Other pertinent data are summarized in Table I. 
EXAMPLES 2-5 
The catalysts used in these examples were prepared in the same manner as in 
Example 1 except that air was introduced into the catalyst bed during the 
cool down period after catalyst activation, at various temperatures as 
specified in Table I (1100.degree.-1550.degree. F.), for a period of 
fifteen minutes. After the air treatment the normal nitrogen flow was 
restored in the activator and the catalyst was allowed to cool down to 
room temperature. 
These catalysts were then tested in the continuous polymerization reactor 
of Example 1. The results are summarized in Table I. 
These examples clearly demonstrated the beneficial effects obtained by this 
invention. An improved MI/synthesis temperature relationship along with 
higher Rd (broader molecular weight distribution), improved catalyst 
activity (lower ash), and significantly higher Hunter whiteness (L) are 
realized. 
EXAMPLES 6 and 7 
These examples illustrate the invention with a catalyst system involving 
the reaction product of ammonium chromate and pinacol. 
Untreated catalyst: 
Davison 952 base was impregnated with an aqueous solution of ammonium 
chromate and pinacol having a molar ratio of pinacol/ammonium chromate of 
4. A sufficient amount of ammonium chromate was used to give a 
concentration of 0.8% chromium by weight on the base. The impregnation was 
done in a round bottomed flask under constant nitrogen purge. The flask 
containing the impregnated base and still under nitrogen purge was then 
heated with a heat gun to remove the excessive moisture. 
The dried catalyst was carefully transferred under nitrogen atmosphere to 
the activator tube of Example 1. A mixture of nitrogen and carbon monoxide 
(7 vol.% carbon monoxide and 93 vol.% nitrogen) was used to fluidize the 
catalyst and the tube was heated to 1300.degree. F. at appromixately 
200.degree. F./hour heat up rate and held at 1300.degree. F. for five 
hours. After cooling down to 450.degree. F. pure nitrogen was used to 
purge the tube. After four hours of nitrogen purge the catalyst was then 
transferred to a well purged catalyst flask. 
Treated catalyst: 
Basically the air treated catalyst was prepared in a very similar fashion 
as the one described above except for the fluidizing gas after the 
1300.degree. F. hold period. In this case, air was introduced for 15 
minutes at the start of the cool down period after the non-oxidative 
activation. At the end of fifteen minutes, pure nitrogen replaced the air 
and the tube continued to cool. When it reached 750.degree. F. carbon 
monoxide replaced the nitrogen for fifteen minutes. After an additional 
nitrogen purge for four hours, the catalyst was removed and stored in a 
flask. 
When tested in the continuous reactor of Example 1, these two catalysts 
showed the advantages of this invention. The air treated catalyst 
demonstrated significantly improved catalyst activity and polymer color as 
summarized in Table I. 
EXAMPLES 8 and 9 
These examples illustrate the applicability of this invention to silica 
supported catalyst involving .pi.-bonded chromium compounds such as 
described in U.S. Pat. 3,806,500, an example of which is 
dicyclopentadienyl chromium (Chromocene). 
In Example 8, in accordance with the general procedure as disclosed in U.S. 
Pat. No. 3,806,500, the base catalyst activated but not treated was 
prepared as follows: 
1. 50 grams of Davison 952 MS-ID silica gel was dehydrated in a 38 mm O.D. 
27 inches long "Vycor" tube surrounded with a tubular electric heater and 
under a nitrogen fluidizing atmosphere. A fritted disc was provided in the 
midsection of the tube for the purpose of fluidizing the catalyst. The 
dehydration temperature was 1100.degree. F. and lasted for two hours. 
After cooling to ambient temperature, the dry silica gel was transferred 
to a well purged flask. 
2. 2 grams of Chromocene dissolved in 120 cc toluene was added to 40 grams 
of the dried silica base of step 1. The excess solvent was evaporated at 
ambient temperature by nitrogen sweep. 
3. About 15 grams of this catalyst was then charged to the "Vycor" tube and 
the catalyst was activated under 400 cc/min. of nitrogen flow by a heating 
cycle as follows: (a) hold at 250.degree. F. for one hour; (b) hold at 
350.degree. F. for one hour; (c) hold at 550.degree. F. for one hour; (d) 
hold at 1600.degree. F. for 2 hours via 200.degree. F./15 minutes heat up 
rate; and (e) cool down to ambient temperature. 
4. The activated catalyst was transferred into a closed flask equipped with 
a hose and clamp at both openings without exposing to air. 
In Example 9, in accordance with this invention, a new and improved 
Chromocene catalyst was prepared in an almost identical manner except for 
the additional air treatment on the cool down portion of the activation 
cycle. As stated in (3) above, the catalyst was allowed to cool down after 
being held at 1600.degree. F. for two hours. When it reached 1200.degree. 
F., the heater was turned on and the tube temperature was maintained at 
1200.degree. F. while the catalyst was treated with air for five minutes. 
The catalyst was then purged with nitrogen and allowed to cool to ambient 
temperature. 
The ethylene polymerization activity of these two catalysts was tested in a 
bench scale reactor using isobutane as the reaction medium. The reactor, 
essentially an autoclave 5 inches I.D. and about 12 inches deep, was 
equipped with an agitator rotating at 560 rpm, a flush bottom valve, and 
three ports for charging catalyst, isobutane and ethylene, respectively. 
The reactor temperature was controlled by a jacket containing methanol 
which was kept boiling by an electrical heater encircling the jacket. The 
control mechanism involved the automatic adjustment of jacket pressures in 
response to either cooling or heating requirements. 
The reactor was first thoroughly purged with ethylene at temperatures 
around 200.degree. F. followed by the transfer of a nominal 0.16 gram 
catalyst from the catalyst flask under nitrogen into the reactor via a 
transfer tube without exposing it to air. After the catalyst charge port 
was closed, 2900 ml of isobutane (dried and deoxygenated) was charged into 
the reactor, trapped ethylene was vented, and the reactor was allowed to 
warm up to 225.degree. F. The reactor was then pressurized with ethylene 
which was regulated at 550 psig and which was permitted to flow into the 
reactor whenever the reactor pressure dropped below 550 psig. An 
instantaneous flow rate of ethylene was monitored by rotameters of various 
capacity. The duration of the test run was 60 minutes. 
At the end of this test run, ethylene flow was cut off, the flush bottom 
valve was opened, and the reactor content was dumped into a recovery pot, 
approximately 5 inches I.D. and 10 inches deep, where isobutane was 
allowed to flash off through a 200 mesh screen into the vent. Polymer 
particles left in the pot were recovered and weighed. 
As shown in Table I, there was a substantial increase in yield 
(gm/polymer/gm catalyst/hr) as well as polymer whiteness (L value) with 
the treated catalyst. 
EXAMPLES 10 and 11 
These examples further illustrate the benefits realized under this 
invention with another catalytic system involving one type of chromate 
esters, namely silyl chromates, an example of which is 
bis(triphenylsilyl)chromate. 
The control sample under this set of examples was prepared by dissolving 
2.5 grams of bis(triphenylsilyl)chromate in sixty cubic centimeters of 
toluene and impregnating with this solution, twenty grams of Davison 952 
MS-ID silica base predried at 1300.degree. F. After being thoroughly 
purged and dried, a portion of this free-flowing, chromate impregnated 
silica was charged to a Vycor tube and thermally treated according to the 
method of Example 8 (without air treatment). 
The new and improved catalyst under this invention was prepared in a 
similar manner except for the additional air treatment during the cool 
down period after the non-oxidative activation. Air treatment procedure 
and temperatures identical to those given in Example 9 was used. 
Polymerization tests with the control catalyst and the catalyst of this 
invention were conducted in a bench scale reactor as described in Examples 
8 and 9. The resultant data summarized in Table I demonstrate again the 
significant improvement in polymer color and catalytic activity with the 
improved catalyst of this invention. 
TABLE I 
__________________________________________________________________________ 
Summary of Examples 1 through 11 
Example 
Catalyst Act. Air Reaction Yield Ash Color 
No. Composition 
Temp. Treatment 
Temp. .degree. F. 
MI Rd/Sw.sup.(1) 
#/#Cat. 
% (L)* 
__________________________________________________________________________ 
1 1% Cr/1% Zr 
1700.degree. F. 
None 227.0 0.58 
5.7/5.0 
2730 .038 
76.7 
as ZrCl.sub.4 
2 " 1700.degree. F. 
1100.degree. F. 
227.0 1.80 
7.8/4.3 
8340 .012 
90.0 
3 " 1700.degree. F. 
1200.degree. F. 
220.0 0.90 
7.5/4.6 
7700 .013 
89.7 
4 " 1700.degree. F. 
1300 20 F. 
226.5 1.90 
7.6/4.6 
5260 .019 
90.1 
5 " 1700.degree. F. 
1550.degree. F. 
225.0 0.70 
7.5/4.5 
5260 .019 
88.2 
6 0.8% Cr as 1300.degree. F. 
None 228.0 0.06 .180 
79.2 
Pinacol/NH.sub.4 Cro.sub.4 
7 " 1300.degree. F. 
1300.degree. F. 
227.5 0.16 .013 
89.4 
8 1.45% Cr as 
1600.degree. F. 
None 225.0 0.24 
NA 300** 37.9 
Chromocene 
9 " 1600.degree. F. 
1200.degree. F. 
225.0 0.42 
NA 1175** 86.6 
10 1% Cr as bis 
1600.degree. F. 
None 225.0 214** 68.5 
(triphenylsilyl) 
chromate 
11 " 1600.degree. F. 
1200.degree. F. 
225.0 623** 92.5 
__________________________________________________________________________ 
*Hunter color whiteness value. Test method from Hunter Laboratory Assoc. 
**Reactivity in gms. polymer per gms. catalyst per hour. 
.sup.(1) Shear sensitivity index (Prediction of High Density Polyethylene 
Processing Behavior from Rheological Measurements by M. Shida and L. 
Cancio) 
EXAMPLES 12 and 13 
These examples further illustrate the practical benefits of this invention 
with a silica-supported catalyst involving another type of chromate ester 
which is represented by bis(triphenylmethyl)chromate. 
The base catalyst used in these examples was prepared by impregnating 20 
grams of Davison 952 MS-ID silica, predried at 1300.degree. F. for 5 
hours, with 60 ml of toluene solution containing 2.4 grams of 
bis(triphenylmethyl)chromate, followed by evaporating off solvent. The 
chromate was prepared essentially by a method described in U.S. Pat. No. 
3,493,554 which comprises refluxing a mixture of 2.3 grams chromium 
trioxide, 6.0 grams triphenylcarbinol and 90 ml dichloromethane for 1 hour 
in a flask, filtering off an excess chromium trioxide and further 
purifying the resultant chromate ester by precipitation. 
The base catalyst was then activated in nitrogen in one case according to 
the method of Example 8 and in the other case activated and further 
treated with air according to the method of Example 9. Both catalysts were 
tested for ethylene polymerization in a bench scale reactor in accordance 
with the procedure described in Examples 8 and 9. 
For a charge of 0.1845g of the untreated catalyst and run time of 1 hour, 
we recovered 22 grams of polymer, corresponding to the reactivity of 119g 
polymer/g catalyst/hr. For the treated catalyst, we charged 0.0762g 
catalyst, recovered 19 grams polymer in 1 hour and obtained the improved 
reactivity of 249 g/g catalyst/hr. Again, the polymer color was very much 
improved by said treatment. 
EXAMPLE 14 and 15 
These examples are intended to illustrate the applicability of the present 
invention to a silica-supported catalyst involving still another type of 
chromate ester, namely chromyl bis(trihydrocarbyltitanate), an example of 
which is chromyl bis(tributyltitanate). 
The base catalyst used in these examples is prepared by impregnating 
Davison 952 MS-ID silica, predried at 1300.degree. F. for 4 hours in the 
muffle furnace, with carbon tetrachloride solution containing chromyl 
bis(tributyltitanate), which was prepared essentially by a method 
disclosed in U.S. Pat. No. 3,752,795, which comprises refluxing a mixture 
of 10 grams chromium trioxide, 18 ml tetrabutyltitanate and 250 ml carbon 
tetrachloride for 24 hours in an inert atmosphere, cooling the green 
reaction mixture, filtering off the unreacted chromium oxide, and 
recovering the remaining solution containing said chromate ester. The 
solution is then concentrated to give the chromium content of about 0.4g 
chromium per 100 ml so that the base catalyst may be prepared conveniently 
by impregnation to contain about 1% chromium by weight on the dry basis. 
The impregnated base catalyst is then activated by the method of Example 8 
in one case and is activated and further treated with air in another case 
by the method of Example 9. Both catalysts are subsequently tested for 
ethylene polymerization in a bench scale reactor according to the 
procedure described in Examples 8 and 9. The polymer yields are 432 and 
207 g/g catalyst/hr for the treated and untreated catalyst, respectively. 
The polymer color is again noticeably improved by said treatment of the 
activated catalyst. 
EXAMPLES 16 AND 17 
These examples are intended to further demonstrate the broad applicability 
of the present invention to various types of organic chromium compounds. A 
catalyst used in these examples involves another distinctive class of 
organochromium compounds featuring tetravalent chromium and typified by 
tetrakis (neopentyl) chromium(IV). 
Tetrakis(neopentyl)chromium is prepared essentially by a method disclosed 
in U.S. Pat. No. 3,875,132. The base catalyst is prepared by dispersing 
tetrakis(neopentyl)chromium in heptane solution onto Davison 952 MS-ID 
silica, predried at 1300.degree. F. for 5 hours in a fluid bed, in such a 
ratio as to give 1% chromium by weight on the dry basis in the impregnated 
catalyst. The base catalyst thus prepared is activated, in one case, by 
the method of Example 8 and, in the other case, by the method of Example 9 
which includes further treatment of the activated catalyst at 1200.degree. 
F. for a period of 15 minutes with dry air. 
The catalysts thus prepared are individually tested in a bench scale 
reactor according to the procedure described in Examples 8 and 9. In both 
cases, the duration of test run is 60 minutes. The reactivities indicated 
are 574 and 342g polymer/g catalyst/hour for the treated and untreated 
cases, respectively. The polymer color turns out to be gray for the 
untreated catalyst but looks quite white to the naked eye in the treated 
case. 
EXAMPLES 18-24 
These examples are intended to illustrate the detrimental effect of oxygen 
contamination during activation on catalyst reactivity. The zirconium 
modified 952 base used for these examples was prepared in the same fashion 
as the one described in Example 1. The zirconated base was then 
impregnated with chromium acetylacetonate dissolved in toluene. After 
drying in an inert atmosphere until it is free flowing, the Cr(AcAc).sub.3 
impregnated base was charged to the Vycor tube of Examples 8 and 9 and 
thermally treated as follows: 
1. 400 cc/minute dry nitrogen doped with a predetermined amount of oxygen 
was used to fluidize the catalyst. 
2. The heating cycle used was (a) hold at 250.degree. F. for one hour, (b) 
hold at 350.degree. F. for one hour, (c) hold at 550.degree. F. for one 
hour, (d) hold at 1700.degree. F. for 2 hours via 200.degree. F./15 
minutes heat up rate, (e) cool down to ambient temperature. 
3. The activated catalyst was transferred into a closed flask equipped with 
a hose and clamp at both openings without exposing to air. 
Polymerization tests with ethylene were carried out in the bench scale 
reactor of Examples 8 and 9. As shown in Table II below a loss in catalyst 
reactivity was noted when oxygen impurity was present during activation 
and the loss increases as oxygen impurities increase. 
TABLE II 
______________________________________ 
Effect of Trace Amounts of O.sub.2 in N.sub.2 
Activation Temperature = 1700.degree. F. 
Synthesis Temperature = 225.degree. F. 
Example Catalyst PPM O.sub.2 
Reactivity 
No. Composition in N.sub.2 
g/g/hr 
______________________________________ 
18 0.15% Cr/0.5% Zr 
0 400 
19 0.15% Cr/0.5% Zr 
160 0 
20 0.3% Cr/1% Zr 0 500 
21 0.3% Cr/1% Zr 57 125 
22 0.75% Cr/1% Zr 0 1200 
23 0.75% Cr/1% Zr 57 325 
24 0.75% Cr/1% Zr 160 100 
______________________________________ 
EXAMPLES 25-28 
These illustrative examples are to demonstrate the beneficial effects of 
this invention using oxidizing gases other than air. The polymer color is 
distinctively improved as indicated by the following examples: 
Carbon Dioxide 
Catalysts of these examples were prepared in a very similar fashion as 
those in Examples 2-5 except that carbon dioxide was used instead of air. 
Carbon dioxide treatment time was one hour. Catalysts so treated were 
evaluated in the continuous loop reactor of Example 1. The following 
results were obtained: 
______________________________________ 
Carbon 
Dioxide 
Example 
Catalyst Treatment Yield Color 
No. Composition Temperature 
#/#Cat. 
(L) 
______________________________________ 
25 1% Cr/1% Zr 1600.degree. F. 
2630 88.2 
as ZrCl.sub.4 
26 1% Cr/1% Zr 1300.degree. F. 
4160 82.6 
as ZrCl.sub.4 
27 1% Cr/1% Zr 1000.degree. F. 
2560 80.4 
as ZrCl.sub.4 
______________________________________ 
NITROUS OXIDE 
The catalyst of this example was prepared similarly as in Examples 18-24 
except that oxygen-free nitrogen gas was used to fluidize the catalyst 
during the activation and a nitrous oxide treatment lasting 60 minutes was 
conducted during cool down after the activation. The catalyst of this 
example was tested in the bench scale reactor of Examples 8 and 9. The 
following results were obtained: 
______________________________________ 
Nitrous 
Oxide 
Example 
Catalyst Treatment Yield Color 
No. Composition Temperature 
g/g Cat. 
(L) 
______________________________________ 
28 1% Cr/1% Zr 1200.degree. F. 
1747 90.0 
as ZrCl.sub.4 
______________________________________ 
Having described our invention as related to the embodiments disclosed 
herein, it is our intention that the invention be not limited by any of 
the details of description, unless otherwise specified, but rather be 
construed broadly within its spirit and scope as set out in the appended 
claims.