CATALYTIC PROPERTIES OF FLUORINEPROMOTED ALUMINA VERNON C. F. HOLM AND ALFRED CLARK Research Division, Phillips Petroleum Co., Bartlesville, Okla.
The effect of fluorine additions to alumina was studied for catalysts with fluorine contents in the range 1.6 to 23%. Variation in catalytic activity was determined for propylene polymerization, o-xylene isomerization, and n-octane cracking. Addition of fluorine transforms alumina from a material with moderate dehydrogenation-dehydrocyclization activity to one which readily promotes these acid-catalyzed reactions. The most abrupt change occurs in the range between about 1.6 and 3% fluorine, and the maximum effecl appears to b e obtained with about 6%. Ammonia adsorption studies on some of the catalysts permitted calculation of isosteric heats of adsorption. These indicated that fluorine reduced the strength of the alumina acid sites. Weaker sites are believed to be desirable for good catalytic activiiy because they allow higher surface mobility and permit easier desorption of adsorbed molecules.
N THE COURSE
of a n investigation of acid-type catalysts. a
I detailed study of the silica-alumina system has shown ( 7 )
that the typical, acid-catalyzed, hydrocarbon conversion reactions-such as cracking, isomerization, hydrogen transfer, and olefin polymerization-occur most readily on relatively weak acid sites. This conclusion resulted from a study of these reactions on a series of silica-alumina gels ranging in composition from pure alumina to pure silica and a comparison of reaction data with the thermodynamics of ammonia adsorption on the catalyst surfaces. Alumina. Lvhich is a poor catalyst for these reactions, has strong sites as indicated by high values for heats of adsorption for ammonia, and low values for the differential entropy of the adsorbed molecules, suggesting low surface mobility. C n the other hand, active silica-alumina catalysts have a number of sites that are characterized by low heats of adsorption. and the entropy studies indicate that adsorbed molecules have considerable surface mobility. The present paper describes experiments i n a similar study of alumina catalysts which have been treated to contain varying amounts of fluorine. The drastic changes produced in alumina by fluorine treatment are well known and the acid properties developed are utilized in many commercial catalysts. The authors' results indicate that fluorine, like silica, modifies alumina by developing a spectrum of low-energy sites. The catalysts tested had fluorine contents ranging from 1.6 to 22.7Yc. Activities were determined for propylene polyI
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h
merization, o-xylene isomerization, and n-octane cracking. Ammonia adsorption studies v-ere made on three of the catalysts. Data are compared with those obtained on pure alumina. Experimental
Materials. The catalysts were prepared by impregnating 20- to 60-mesh granules of high purity alumina (Davison Chemical Corp.), with appropriate solutions of ammonium acid fluoride. After separation of excess liquid, the granules were dried a t 110' C., rapidly heated to 550' C., and held at this temperature for 16 hours in a stream of dried air. Catalysts were prepared containing 1.6, 3.3. 6.3, 9.6, and 22.7% fluorine by analysis. The respective surface areas were 357, 277, 183, 236. and 152 square meters per gram. The alumina used for impregnation was in the form of beta-alumina trihydrate in all cases except for the one with 6.37, fluorine, in which case the alumina had been calcined previously a t 540' C. The prop) lene used for polymerization tests was Phillips Research grade. The n-octane and o-xylene were higher than 99% purit). The ammonia was of the same purity (99.99y0) used in previous work ( 7 ) . Procedures. The catalytic tests were made with 10 ml. of catalyst in a 16-mm. 0.d. glass reactor, fitted with a coaxial thermocouple well for temperature measurements, and mounted within a metal shield in a vertical resistance furnace.
, ISO-ALKANES n-ALKANES (c4-c5)
AROMATICS n-Cg CRACKED
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Figure 1 . 38
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IO 12 14 PER CENT FLUORINE
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Activity tests on fluorine-treated aluminas
l & E C P R O D U C T RESEARCH A N D D E V E L O P M E N T
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Figure 2. Changes in mole ratios of selected products in n-octane cracking as a function of fluorine content
A 5-ml. bed of loiv-surface-area alpha alumina pills was used above the catalyst for preheat. Liquid product was collected in an ice-cooled condenser-receiver below the reactor. In the cracking tests, off-gas was sampled and measured with suitable bulbs and a gas meter. Liquid feed was metered from a weighed! graduated feed-buret. The propylene polymerization tests \vere made a t a gaseous hourly space rate of 250 while the temperature was increased a t a steady rate from 30' to 300' C. in 2 hours as described previously (2). Conversions \vere determined from readings of the feed and effluent flow meters. Catalysts Ivere compared on the basis of conversions a t 200' C. The o-xylene isomerization tests Lvere made at 350' C . with a liquid hourly space rate of two. The n-octane cracking tests \vere made at 550' C. and a liquid hourly space rate of one. Liquid products were analyzed by chromatographic techniques and gaseous products by mass spectrometry. The ammonia adsorption tests xvere made with a vacuummicrobalance as described previously ( 7 ) . Catalyst samples of 0.2 gram ivere heated for 1 hour a t 500' C . in diffusion-pump vacuum before dropping the temperature to 400' C . and admitting ammonia a t a selected pressure. N'eighings Xvere made at 50' intervals between 400' and 100' C. while corresponding pressure measurements were made with a triplerange McLeod gage. Ten or 12 samples of catalyst were used a t various pressures of ammonia up to 15 mm. of Hg to define each family of isotherms. Results and Discussion
The results of the activity tests for propylene polymerization, o-xylene isomerization, and n-octane cracking are shown in Figure 1. The high conversions produced by moderate fluorine additions caused some scatter in the data but the over-all effects of fluorine treatment are clearly illustrated. The smallest addition of fluorine. 1.67,. did increase the cracking activity and developed a slight amount of polymerization activity but did not produce activity for xylene isomerization. The addition of 3.3yc fluorine, however, caused marked increases in activity for all three reactions. At a slightly higher fluorine content. the catalytic activity reached a maximum. and further additions of fluorine caused little or no change. The nature of the changes produced by fluorine addition are perhaps best shown in Figure 2. based on product distri-
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butions in the n-octane cracking tests. The curve for the mole ratio. iso-alkanes to n-alkanes, starts a t a low value for alumina and rises steadily to a maximum a t around 6y0 fluorine: indicating a peak in isomerization activity. The curve for the mole ratio, hydrogen to n-octane cracked, starts a t a high value for alumina, drops in linear fashion with fluorine content and tends to flatten out a t about 6% fluorine. A similar behavior is shown by the curve for the mole ratio, aromatics to n-octane cracked, except that the drop is most abrupt in the range between 1.6 and 3.37c fluorine. Thus, fluorine additions transform alumina from a moderately active dehydrogenation-dehydrocyclization catalyst to materials that exhibit isomerization as \vel1 as polymerization activities. t)-pica1 of conventional cracking catalysts. Ammonia Adsorption. Adsorption studies \vex made on catalysts containing 1.6, 6.3, and 9.6% fluorine and isosteric heats of adsorption were calculated by the use of the ClausiusClapeyron equation. Computations were made at coverages differing by small increments between adjacent isotherms. and some values \vere obtained between several pairs of isotherms at the same coverage. These were averaged and the average values of AH us. coverage were plotted for each catalyst as sho\sn in Figure 3. The data for alumina are included for comparison. .4t the lower levels of coverage, the heats of adsorption are high for .4I2Osand for the catalysts \vith 1.6 and 6.3% fluorine. The heats decrease with increasing coverage, and, at the higher coverages, the heats of adsorption are lowest for the catalysts with highest fluorine content. Thus, additions of fluorine produce a progressive reduction in the strengths of a considerable portion of the acid sites on alumina. The development of a spectrum of low-strength sites is accompanied by an increase in the capacity for catalyzing the acid-type reactions of polymerization, isomerization, and cracking. I t is not possible to designate precisely the level of site strength that is desirable for optimum catalytic activity, but presumably this is obtained by a fluorine addition of about 6Yc. Addition of more fluorine produces some weaker sites that may not be effective for catalytic activity. Consequently. it is not reasonable to expect a linear and continuous correlation between site strength and activity over the entire range of fluorine contents studied, The advantage of suitable low-strength sites presumably is related to both a critical mobility of adsorbed species on the surface and a critical ease of desorption of reacted molecules. Similar conclusions concerning the effect of fluorine in increasing the mobi1it)- of adsorbed molecules can be obtained by considering changes in the differential entropy of the adsorbed layer. These entropy effects were also observed with coprecipitated catalysts although it is difficult to compare the two series in detail a t the present time, These problems \vi11 be investigated further. Ac knowledgrnent
The authors wish to express their appreciation to Phillips Petroleum Co. for permission to publish this Lvork. Literature Cited (1) Clark, A , , Holm, V. C. F.?Blackburn, D. M., J . Catalysis 1,244
(1962). 0
02 04 06 08 1.0 12 14 ADSORBED "3, MOLECULES PER CM2. x
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Figure 3. Differential heats of adsorption (kc.l'gr.am mole) as' functions of coverage for three fluorine-promoted aluminas and pure alumina
(2) Holm. V. C , F., Bailey. G. C.: Clark, A , ; Ind. Eng. Chem. 49, 250 (1957).
RECEIVED for review November 21, 1962 ACCEPTED December 31, 1962 Division of Petroleum Chemistry, 142nd Meeting. ACS? Atlantic City, N. J.. September 1962. VOL. 2
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