I
V. C. F. HOLM, G. C. BAILEY, and ALFRED CLARK Research Division, Phillips Petroleum Co., Bartlesville,
Okla.
Nickel Oxide Polymerization Catalysts EfFects of Preparation Methods on Properties ,The activity of nickel oxide-silicaalumina catalysts for polymerizing ethylene depends on how they are made. Coprecipitated types may show activity 50% greater than impregnated types, apparently because of better nickel dispersion, although concentration i s the same. In both types, activity generally increases as nickel concentration decreases, but the optimum range i s 3 to 5%.
PREPARATION
and polymerization properties of catalysts containing nickel oxide on silica-alumina have been described ( 3 ) . Data obtained for this article on impregnated and coprecipitated catalysts showed differences in physical and catalytic properties. In the composition range producing maximum polymerization activity, the coprecipitated catalysts were more active; on the basis of x-ray diffraction studies and magnetic measurements, it was concluded that these catalysts had a higher degree of dispersion of nickel oxide than the impregnated catalysts. Materials and Methods
Catalyst Preparation. T h e impregnated catalysts were made by treating a commercial synthetic cracking catalyst with an appropriate nickel nitrate solution. T h e silica-alumina cracking catalyst (Houdry Process Corp.) contained about 90% silica, had a surface area of about 315 square meters per gram: and was obtained in the form of pills. These were broken and sized and a 16-30 mesh fraction was used for impregnation. For catalysts having about 3 7 , nickel, 50 ml. of the 16-30 mesh support was treated with 7.5 ml. of a n 0.8M nickel nitrate solution. After about 15 minutes, the support was drained and then dried in a n evaporating dish on a hot plate with constant stirring. After drying in a n oven a t 110' C.? the catalyst was heated gradually to 500' C. in a stream of dried air and then held at this temperature for 5 hours, in the course of which nickel nitrate decomposed to nickel oxide. Preparation of the coprecipitated catalysts may be presented by considering the procedure used in making a nickel oxidesilica-alumina catalyst containing about
250
5y0 nickel. Two solutions were prepared: one containing 180 grams of Nbrand sodium silicate (28.6y0 silicon dioxide, 8.427, sodium monoxide) in 1200 ml. of water and the other containing 42 grams of aluminum nitrate nonahydrate, 14.85 grams of nickel nitrate hexahydrate, and 6.3 ml. of concentrated nitric acid in 1000 ml. of water. These solutions were mixed during vigorous stirring, after which a solution of 5 grams of sodium hydroxide in 100 nil. of water was added, resulting in a p H of 8. A gel formed in about 30 seconds. This was allowed to stand for about an hour, then drained on a Biichner funnel and the excess liquid finally removed with suction. The gel was broken u p and placed in evaporating dishes under heat lamps and allowed to dry in this manner overnight, after which it was dried for several hours in an oven at 110' C. T h e dried catalyst was washed twice by decantation with water, then treated with six or eight successive portions of 57, ammonium chloride solution at 70' C. for removal of sodium by base exchange. The product was then washed eight 01' ten times with water, drained, and dried a t 110' C. The heat treatment consisted of heating slowly to 5.50' C. in a stream of dry air and holding this temperature for 16 hours. This produced catalysts with adequate surface areas: and sodium contents lower than 0.027,. Catalysts with other nickel contents were prepared to contain silica and alumina in the same ratio as in the example cited : 9 to 1. Polymerization Test. Ethylene polymerization tests were made using a 16mm. outside diameter glass reactor equipped with a n axial thermocouple well that permitted temperature measurements on the catalyst bed, which was supporred on a perforated glass platform in the hot zone of the reactor. T h e reactor was mounted in a vertical tube furnace fitted with a heavy metal sleeve for temperature equalization. Dried ethylene was admitted a t the top from a calibrated flowmeter and the reacted mixture was passed through a trap cooled by a dry ice bath below the reactor, T h e unreacted ethylene from the trap was passed through a second calibrated flowmeter. Thus, with a given flow rate of ethylene, readings of the second flowmeter made possible a satisfactory calculation of the instan-
INDUSTRIAL AND ENGINEERINGCHEMISTRY
taneous values for conversion of ethyIene to dimers, trimers, and higher polvmers. Prior to a run, the reactor was charged with 2 ml. of the catalyst and with a slow stream of dry nitrogen flowing, the furnace was heated to 400' C., held at this temperature for about an hour, and cooled to room temperature overnight. Then with the refrigerated receiver in place, the nitrogen was cut off and the flow of Phillips Petroleum Co. research grade ethylene was started a t a gaseous hourly space rate of 2300. The furnace was turned on a t a voltage to attain 300' C. in 2 hours. Flowmeter and temperature readings were made at 5minute intervals. Tests made in this manner were reproducible and although exact levels of conversion in duplicate runs might vary by a few per cent, the characteristic shapes of the conversiontemperature curves for a given catalyst were not altered. Kickel-promoted catalysts generally gave maximum conversion a t the highest temperature--300' C. When higher olefins were polymerized, the liquid product could be weighed and the yields were in satisfactory agrcement with values expected from the conversion-time relationships. One advantage of this method is that i t yields information on polymerization activity at atmospheric pressure over a usefuI range of temperatures. Hydrogen Reduction Test. I n order to determine whether unusual values of magnetic susceptibility of nickel oxide represented changes in the degree of dispersion of nickel or changes in the valence state, it was necessary to evaluate the latter by precise reductions of the nickel oxide catalysts with hydrogen. For reducible unsupported oxides, the combined oxygen can be determined easily by passing hydrogen over the oxide a t a suitable elevated temperature and collecting and weighing the amount of water formed. However, this procedure is not applicable when the metal oxide is present an a support that has appreciable surface area, because of gradual evolution of adsorbed water when the catalyst is heated. This difliculty may be avoided by conducting the reduction in a closed system and measuring consumption of hydrogen, as was done originally by Hill and Selwood (2). In this work, hydrogen was circulated over the heated catalyst by means of an
P R E P A R I N G CATALYSTS IN T H E L A B O R A T O R Y all-glass magnetically actuated pump (7) and the water produced was absorbed in a tube of anhydrous magnesium perchlorate in series with the catalyst tube. Pressure changes, indicating the course of the reaction, could be observed during reduction by means of a manometer of barometric height attached to the circulating system. When the reaction was complete, the system was cooled to room temperature and the volume of hydrogen, computed from the pressure reading, was compared with the hydrogen charged initially. Reduction temperatures for the catalyst samples were 500' or 550" C., depending on the ease of reduction, and run durations ranged from 6 to 24 hours. In preparing for a run, the apparatus was evacuated, charged with hydrogen, reevacuated, and recharged with hydrogen to approximately atmospheric pressure. This was done to minimize the amount of adsorbed osygen on the catalyst and walls of the apparatus. I n each test, a blank correction obtained by a similar reduction run with a silica-alumina catalyst in the reactor, was applied. Operation of the reduction apparatus was checked on samples of chemically pure cupric oxide and nickel oxide for which valence values of 2.00 and 1.99 were obtained. Magnetic Measurements. Evaluation of magnetic susceptibility was done a t room temperature using the Gouy method. Tests were made over a range of field strengths in order to detect any evidence of ferromagnetic impurities which would invalidate the results. The electromagnet used in this work was capable of producing a magnetic field of about 8 kilogauss with a gap of 1 3 / 8 inches. Sample tubes were of borosilicate glass, 6 mrri. in inside diameter and 140 mm. long; with careful packing, satisfactory measurements could be
1STS
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1SO^^-^;-I I
Figure 1. Effect of nickel concentration on magnetic per gram nickel susceptibility for heattreated Ni0-Si02-AI2O3 catalysts
p
i
~
$
E
j
\-copREclplTATED
IMPREGNATED A
4
30 - ____
~~-
~
40 60 WEIGHT PER CENT OF NICKEL
made either with 16-30 mesh granules or with powdered catalysts. W e i g h i n g s were made with a Chainomatic analytical balance. Corrections for the diamagnetic effects of the oxide supports were made in computing the data. Values for magnetic susceptibility were expressed in two ways: mass susceptibility or susceptibility per gram of sample (corrected for the support) and susceptibility per gram of nickel. T h e latter, obtained from the first by dividing by the nickel concentration, expressed in parts per hundred, furnishes a means for comparing the nickel susceptibility in different materials. Catalysts with nickel atoms in close proximity either because of clumping of nickel oxide or because of a high concentration of nickel are expected to show a low value of susceptibility per gram of nickel because of interaction.
Results a n d Discussion Polymerization Activity. The results of tests on impregnated and coprecipi-
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I
IUO
tated catalysts are presented in Table I. The maximum polymerization activity per unit volume in each series was developed with nickel contents of from 3 to 5%, and the maximum for the coprecipitated catalysts was about 50% greater than for those prepared by impregnation. For each series, the activityperunitweight of nickel increased continuously as the concentration of nickel decreased. Although differences in surface area may account for some differences in polymerization activity, any method of correction would not change the conclusions. Hydrogen Reduction Studies. Hydrogenreduction tests showed that in both impregnated and coprecipitated nickel oxide-silica-alumina catalysts, nickel exhibited a normal valence of 2. Thus, these materials are different from the nickel oxide-alumina catalysts studied by Selwood (4, for which he found the valence of nickel higher than 2. H e attributed this to a valence induction effect resulting from the influence of trivalent aluminum in the alumina lattice. ~~
Table I.
Catalyst NiO-SiOpAlzO,
Properties of Catalysts Prepared b y Coprecipitation and b y a Standard Impregnation Procedure
Preparation Impregnated
Approx. Surface Ni Content, Area, % ' Sq. M./G.
X-Ray Evidence, NiO No Yes Yes Yes
Magnetic Susceptibility Per G . per G. Catalysta Ni
x
10-6
50% PolymerizaReduction tion of by Hz* c ~ H ~ax. , Polymerizaby Hz Time, Temp., Conversion, tion/O.l G. Reduction hr. C. % Ni
Nickel Valence
..
..
1.99 1.95 2.07
0.8
440
0.8
400
1.94
..
2.0
500
2.04
..
..
1.3
.. .. .. ..
500
.. .. *. ..
NiO HT, air, 550' C. 77.8
