Communications to the Editor Bull. Korean Chem. Soc. 1999, Vol. 20, No. 11 1261
New NiSO
4/ZrO
2Catalyst for Ethylene Dimerization
JongRackSohn* andWonCheonPark
Department of Industrial Chemistry, EngineeringCollege, KyungpookNational University, Taegu702-701, Korea Received August 31, 1999
Nickeloxideonsilicaorsilica-aluminaisaneffective cat
alystforethylene dimerization andn-butene isomerization at room temperature.1~6 It has been suggested that the active sitefor ethlyenedimerizationis formedby an interaction of a low valent nickel ion with an acidsite.7 In the previous papersfrom our laboratory, ithasbeenshown that NiO-TiO2
and NiO-ZrO2 modified with sulfate or tungstate ion are very active forethylene dimerization.8"11 High catalyticactivi ties in the reaction were attributed to the enhanced acidic properties of the modified catalysts, which originated from the inductive effectofS=O orW=O bonds of the complex formedbythe interactionofoxides with sulfate ortungstate ion. As an extension of the study onthe ethylene dimeriza tion, wehaveprepared a new catalystof NiSO4/ZrO2 which is active forethylenedimerizationevenat room temperature.
The catalysts were prepared asfollows. Theprecipitate of Zr(OH)4 was obtainedby adding aqueous ammonia slowly into an aqueous solution ofzirconium oxychloride at room temperature with stirring until the pH of mother liquid reached about 8. The precipitatethus obtained waswashed thoroughly with distilled water until chloride ion was not detectedwithAgNO3 solution, and was driedat 100 oC. The precipitatepowdered below 100 meshwas impregnatedwith aqueous solution of NiSO4 -6H2O followed by evaporating water, drying, andcalcining inairat 500oC for 1.5h. It was used ascatalystafterevacuationatdifferent temperatures for 1 h. This series ofcatalysts are denoted by the weight per
centage of NiSO4. For example, 15-NiSO4/ZrO2 means the catalysthaving 15 wt%of NiSO4.
The catalytic activity forethylene dimerization was deter mined at 20oC by a conventional static systembymonitor ing pressure decrease froman initial pressure of 300 Torr.
Freshcatalystsample of 0.2 g was used forevery run andthe catalytic activity was calculated as the amount of ethylene consumed intheinitial 5 min. Reactionproductswere ana
lyzed by gas chromatography witha VZ-7 column atroom temperature.
The catalyticactivities of NiSO4/ZrO2 for the reaction of ethylene dimerization were examined and the results are shownas a functionof NiSO4 content in Figure 1,wherethe catalystswereevacuatedat 500oC for 1 hbeforereaction. It is confirmed that the catalytic activity gives a maximum at 15 wt% ofNiSO4. Thisis due to theincrease of specific sur face area and the subsequent increase ofactive sites. Cata lytic activity is closely correlated to the acidity of catalysts.
Namely, the more the acidity, the higherwas the catalytic activity. As listed in Table 1,the BET surface areaand acid
ity attained a maximum when the NiSO4 contentinthe cata
lystis 15wt%. The acidity of NiSO4/ZrO2 was determined
bytheamount of NH3 irreversibly adsorbed at230 oC.6,11 It is known that acidity is required forthe ethylene dimeriza tion.5,6 Although the ZrO2 without NiSO4 was inactive as catalystforethylenedimerization,the NiSOq/ZrO? exhibited high catalyticactivityevenatroomtemperature.
The effect of evacuation temperature on the catalytic activity of 15-NiSO4/ZrO2 was also examined, where the catalysts were evacuated for 1 h. As shown inFigure 1,the maximumactivityisobtained with thecatalystevacuatedat 500oC. To examinethe effectof evacuation temperatureon surface area,BET surfacearea of 15-NiSO4/ZrO2 atvarious evacuation temperature (300-700 oC) was measured. How
ever, no particular changes of surface area were observed, giving about 136-141 m2/gregardless ofevacuation temper
ature. Therefore,itseemslikely that thevariationof catalytic activityisnotduetothechangeofsurfaceareabutdueto the easy formation of active site upon calcination temperature.
Allthe NiSO4/ZrO2 catalystsdimerizedethylene selectively to n-butenes.The compositionofn-butenes analyzedbygas chromatography was suchthat 1-butene was found to bethe dominant species at the initial reaction time as compared with cis-buteneortrans-butene. However, it was shownthat the amount of 1-butene decreases, whilethe amount of 2-
L= Eg -으 으- 匕E
>
으
>>
a5AE o
Evacuation temperature, °C
NiSO4 content, wt%
Figure 1. Variations of catalytic activity for ethylene dimerization with NiSO4 content and evacuation temperature: (
O
) NiSO4/ZrO2having different NiSO4 contents and evacuated at 500 oC; (
•
) 15-NiSO4/ZrO2 evacuated at different temperatures.
1262 Bull. Korean Chem. Soc. 1999, Vol. 20, No. 11 Communications to the Editor Table 1. Specific Surface Area and Acidity of NiSO4/ZrO2
Catalysts Calcined at 500 oC for 1.5 h
Weight percentage of NiSO4 Surface area (m2/g) Acidity (#mole/g)
0 64 71
5 109 161
10 136 237
15 141 262
20 123 239
30 93 237
40 60 232
100 30 79
Table 2. Acid Strength of Samples Hammett indicator pKa value of
indicator ZrO2 5-NiSO4/ZrO2
Benzeneazodiphenylamine +1.5 + +
Dicinnamalacetone -3.0 - +
Benzalacetophenone -5.6 - +
Antraquinone -8.2 - +
Nitrobenzene -12.4 - +
2,4-Dinitrofluorobenzene -14.5 - +
buteneincreases with the reactiontime. Therefore,it seems likely that the initiallyproduced 1-butene is alsoisomerized to2-buteneduringthe reaction.
The acid strength of the sampleswasexamined bya color change method, using Hammett indicator12 in sulfurylchlo ride. Since itwas verydifficult to observe thecolor of indi catorsadsorbedon thecatalyst of high nickelsulfate content, the low percentageofnickel sulfate (5 wt%)was used in this experiment. The results are listedinTable 2. In thistable, + means that the color of base form was changed to that of conjugate acid form. Theacid strength of ZrO2 wasfound to be Ho < +1.5,while NiSO4/ZrO2 wasestimatedto havea Ho V -14.5, indicating the formation of new strongeracid site thanthatof ZrO2. An acid strongerthan Ho= -11.93, which correspondsto an acid strength of 100% H2SO4,is known as a superacid.13 Consequently, NiSOq/ZrO? wouldbe a solid superacid. Such superacid properties are attributed to the inductive effect of S=O ofsulfateionsupportedon ZrO2.14
ZrO2 alonewithout NiSO4, whose acid strength wasfound tobe Ho V +1.5, wastotally inactive for the dimerization reaction at room temperature. Theseresultsindicate that eth ylene dimerization requires acid sites stronger than Ho = +1.5. The catalyst calcinedat 900 oC, which hasno sulfate
iondueto thecompletedecomposition,wasalsoinactive for dimerization. Therefore, it is believed that the active site responsible for dimerization consistsof a low-valentnickel and anacid, as observed in theNiO-containingcatalysts.7-10, 15 In fact, it is known thatZrO2 modified withsulfateion is an acid.14 The term low-valent nickel originated from the fact thatthe NiO-SiO2 catalystwas drasticallypoisoned by carbon monoxide, since a low-valent nickel is prone to chemisorb carbon monoxide.7 In this work, all catalysts added with NiSO4 were poisoned by 1 〃mol g-1 ofcarbon monoxide for dimerization. It seems that the formation of low-valentnickelis caused by evacuation at hightempera
ture.
The existence of both Bronsted and Lewis acid siteswas shown by the IR spectra of ammonia adsorbed on NiSO4/ ZrO2 catalyst. X-ray powder diffraction showed that for the calcination temperatureof 500 oCNiSOq/ZrO? having 10-30 wt%of NiSO4 wasamorphous, but 5-NiSO4/ZrO2 exhibited onlytetragonalphaseof ZrO2.
Acknowledgment. This work was supported by the Korea Science and Engineering Foundation through the Research Center for Catalytic Technology at Pohang Uni versityof Science and Technology.
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