1030
JOSEPH D. DANFORTH
cient datal2 would seem to confirm such postulation for some surfactants. Acknowledgment.-Acknowledgment is made for the cooperation and advisory assistance of fellow
Vol. 58
workers of the Paint and Chemical Laboratory, Aberdeen Proving Ground, Md., Drs. C. F. Pickett, Chief, and Myer Rosenfeld; also to Rebecca Flickinger for help in obtaining many of these data.
THE EFFECT OF ALKALI AND ALKALINE EARTH METAL IONS ON THE ACTIVITY OF CRACKING CATALYSTS BY JOSEPHD. DANFORTH Depaytmen( of Chemistry, Grinnell College, Grinnell, Iowa Received April 19, 1964
The poisoning effect of the alkali metal ions and barium ion on three cracking catalysts has been determined. When a small fraction of the catalyst surfaces is covered b lithium ion, a residual activity is attained which does not show an appreciable decrease on the addition of more lithium !&-oxide. The amount of lithium hydroxide required to reach this base conversion, appears to be a measure of the acid sites which are active for the cracking of cetane. The poisoning effect of barium ion is identical with the poisoning effect of potassum ion on a molar basis. The dependence of the poisoning effect of the alkali ions on the radius indicates that the catalyst may be formed by the deposition of polymeric chains of hydrated alumina on the silica micelle.
In a previous paper' the effect of lithium hydroxide on the activity of several cracking catalysts was reported. It was assumed that the amount of lithium hydroxide required to eliminate the activity of the catalyst for cetane cracking represented a measure of the active acid sites. It was also indicated that potassium hydroxide was a more effective poison than lithium hydroxide, and the greater poisoning effect was attributed to the larger ionic radius of potassium. Since the effect of ion size would appear to give some indication of the arrangements of active sites on the catalyst surface, an extensive investigation of the effect of ion size and ion charge on the activity of cracking catalysts was initiated. Bitepazh2 has studied the effect of the alkali metal ions on cracking catalysts. In his work freshly formed silica-alumina gels containing alkali metal ions were exchanged with increasing quantities of hydrogen ion, and the activity of the calcined catalyst determined for .cracking kerosene and reforming a naphtha fraction. A large increase in catalyst activity was observed for the first amount of acid added. The large alkali metal ions were observed to have a greater poisoning effect than the smaller ions. Millsa has reported the effect of potassium ion and quinoline as poisons for calcined and uncalcined silica-alumina cracking catalysts. The present paper reports the data obtained when three calcined cracking catalysts were impregnated with dilute solution of the hydroxides of lithium, sodium, potassium, cesium and barium, and their activities for the cracking of cetane determined. Several interesting deductions can be drawn from the data relative to some of the structural characteristics of the catalyst. Materials.-The U.O.P. Type B catalyst contained 86.2.% silica, 9.4% zirconia and 4.3% alumina. It was keceived as '/g" pills which had been calcined a t their source. It had picked up 3.5 wt. % of material which was volatile on calcination for two hours at 500"; 25-ml. samples of P. Stright and J. D. Dsnforth, T H IJOVRNAL, ~ 57, 448 (1953). Yw. A. Bitepaah, J . Gen. Chem. (U.S.S.R.1, 17, I99 (1947). (3) G.A. Milh, E. R.Bdedeker and A. Q..Oblad, J . Am. G'h'hlm. doc., k9, 1554 (1960). (1) (2)
this catalyst represented a dry weight of 14.73 g. The reported surface area (B.E.T. method) waA 346 m.2/g. The Socony-Vacuum synthetic bead catalyst contained 10% by weight A1203 on silica. Although calcined a t its source, this catalyst contained 1.88% volatile at 500-550'; 16.0 g. of catalyst represented 25 ml. and calculations of milliequivalents per gram were based on a dry weight of 15.70 g. The surface area was 420 m.2/g. A second large batch of Socony-Vacuum synthetic bead catalyst was presumably identical in composition with the earlier batch and was so represented. A large portion of this batch was calcined a t 500' for 3 hours before impregnation. The weight of the 25-ml. sample thus dried was 17.3 g. indicating a higher density than the earlier batch. Subsequent testing showed this batch of catalyst to be markedly inferior to the early batch giving 17 .O% conversion of cetane versus 25% conversion obtained in the earlier sample. This catalyst has been designated as low activity synthetic bead catalyst. The various alkalies used were designated as reagent grade. Cesium hydroxide was prepared by the solution of double distilled cesium metal in distilled water under an atmos here of nitrogen. Du cetane was the charging stock in all tests.
gent
Apparatus and Procedure Impregnation of Catalysts.-The indicated weight of 25 ml. of each catalyst was weighed on an analytical balance to the nearest ill or bead. The catalysts contained several per cent. vo!$tile matter due to the adsorption of water on standing. All calculations were made on the basis of the dry catalyst which was arbitrarily taken after heating a sample in a muffle furnace for 3 hours a t 500'. The catalyst was covered with distilled water, the amount of alkali required to give the desired milliequivalents per g. wa8 added, and the solution made up to 200 ml. by the additiqn of distilled water. When the supernatant liquid was acid to phenolphthalein, it was considered that equilibrium had been attained and the liquid was decanted. The catalyst was dried overnight a t 110" and calcined for 2 hours at 500'. A t high concentrations of caustic the solutions did not become acid to phenolphthalein. In these cases the catalysts were allowed to stand for a minimum time of 6 days with occasional shaking, the liquid decanted and the unadsorbed caustic determined by titration with standard acid. Catalyst activities were determined in an apparatus similar to that previously described' using 25-ml. samples of catalyst and charging 100 ml. of cetane for a period of 1hour a t atmospheric pressure and 500'. Weight percentages of gas, gasoline (to 200') and bottoms (over 200') were determined by conventional procedures. Conversions were recorded as the weight percentage of gas lus asoline on a loss free basis. Coke on the catalyst was found to represent lesa than 0.2% conversion at the highest
Nov., 1954
EFFECT OF
ALKALIO N
THE ACTIVITY O F CRACKING
CATALYSTS
1031
was obtained with glass chips. On each of the three catalysts the poisoning effectper milliequivalent of alkali ion becomes greater as the size of the ion increases. In a rather arbitrary manner, it was decided that the points of intersection of the h'orizontal portions of the curves and the portions in which the decrease in conversion depended directly on the amount of alkali, represented the milliequivalents of the particular ion necessary to cover the active acid sites. The curves have been drawn to emphasize this value, and these points of intersection have been termed the milliequivalents of alkali per g. of catalyst required to reach the base conversion. An additional useful value has been obtained from the slopes of the curves in the range in which the catalyst activity decreases 0 0.1 0.2 0.3 0.4 0.5 0.6 0.7 0.8 0.9 directly with added alkali. By Meq. metal per g. catalyst. Fig. 1.-High activity synthetic bead catalyst-conversion (gas plus gasoline) vs. dividing the slope of the curves meq. of metal per gram of catalyst: @, LiOH; 0, NaOH; X, KOH; A, CsOH. by the weight of 25 cc. of catalyst, a value representing the loss in conversion per milliequivalent of alkali can be obtained. The milliequivalents of alkali per g. of catalyst required to reach the base conversion, the loss in conversion per milliequivalent of alkali and the ionic radius of the impregnating ions have been summarized in Table I. In Fig, 4 the milliequivalents of alkali required to reach $he base conversion have been plotted as a function of the radii of the impregnating ions. On each of three catalysts the values for sodium, potassium and cesium 0 0.1 0.2 0.3 0.4 lie on a straight line and shorn Meq. metal per g. catalyst. that the milliequivalents of alFig. 2.-UOP type B catalyst-conversion (gas plus gasoline) us. meq. of metal per liali per g. of catalyst required gram of catalyst: @, LiOH; 0, NaOH; X , KOH; A, CSOH. to reach the base conversion is a direct function of the ionic radius. Results The conversion of cetane has been plotted as a 20 function of the milliequivalents of the adsorbed metal ion in Figs. 1 , 2 and 3 for the Socony-Vacuum synthetic bead, the U.O.P. Type B, and the low activity synthetic bead catalysts, respectively. The plot of the activity against alkali content may show .an initially high drop in conversion for very small 1 l o amounts of alkali ion, followed by a straight line 2 portion in which the decrease in conversion is pro- u portional to the amount of the ion. At higher concentrations of the alkali ion, the dependence of conversion on alkali concentration becomes less and the approximately horizontal portions of the 0 0.1 0.2 0.3 curves have been considered as the base activity of Meq. metal per g. Catalyst. t'he catalyst after covering the active acid sites. Fig. %-Low activity synthetic bead catalyst-conversion This base activity appears to lie in the range of 4 to (gas plus gasoline) us. milliequivalents of metal per gram of 8% conversion, while a thermal conversion of 1.7% catalyst: @, LiOH; 0 , NaOH; x, KOH; A , CaOH.
conversion reported, and was not determined or accounted for in the remaining runs. The tables summarizing the data have been omitted because of their voluminous size and because they do not add appreciably to the information which is presented in the figures.
JOSEPH D. DANFORTH
1032
Vol. 58
The loss in conversion per milliequivalent of alTABLE I SUMMARY OF THE Loss IN CONVERSIONS PER MEQ. ALKALI kali is plotted as a function of the ionic radii in Fig. A N D MEQ. ALXALI/G.REQUIREDTO ELIMINATE CETANE 5. Straight lines are obtained on three catalysts and this implies certain interesting conclusions CRACKINQ Meq. alkali/g.
to reaoh base conv.
Loss in conv. per meq. alkali
High Activity Synthetic Bead 0.46 1.59 .31 3.90 .21 5.15 .13 7.60
LiOH NaOH
KOH CsOH
Radius of
alkali icm,
A.
concerning the geometry of the cracking catalyst.
0.6 0.95
1.33 1.69
U.O.P. Type B 0.31 3.8
LiOH NaOH KOH CaOH
.21 -15 .I1
6.2 8.15 11.2
Low Activity Synthetic Bead 0.25 2.9 .165 4.7 .125 6.5 -10 8.4
LiOH NaOH
KOH CsOH
The smaller lithium ion does not fall on this st,raight line, but extrapolated values t o zero ionic radius for two of the three catalysts give values which correspond quite closely to the milliequivalents of lithium ion per g. of catalyst required t o reach the base conversion. These observations bb
0.40
d. 0.30 4-
d
' 8 0.20
a
Y
2 0.10
la' 0.6 0.8 1.0 1.2 1.4 1.6 1.8 Ionic radius, A. Fig. 4.-Meq. Me per g. catalyst to reach break point vs. ionic radius, b.: 0 , UOP type B; 0 , high activity synthetic bead; x, low activity synthetic bead. 0
0.2
0.4
have been interpreted to mean.that the very small lithium ion is capable of poisoning only a single active center, and that the milliequivalents of lithium hydroxide required to reach the base conversion represents a close approximation to the milliequivalents of acid sites active in cetane cracking. Since soluble hydroxides of ions smaller than lithium do not exist, it appears to be difficult t o prove conclusively that a lithium ion covers one, and only one, active site. It is not exactly clear why different base conversions should be obtained with the different alkali metal ions, but there is a general tendency for the base conversion t o increase with a decrease in size of the metal ion. Since the base conversions for sodium, potassium and cesium ions are essentially the same, and the only marked increase in base conversion occurs with the lithium ion, this could be interpreted to mean that the lithium ion may coordinate in the structure and may contribute acidity and some activity in its own right.
1.0 1.2 1.4 1.6 1.8 Ionic radius, A. Fig. 5.-Loss in conversion per meq. Me+ us. ionic radius, A.: 0 , UOP type B; 0 , high activity synthetic bead; X, low activity synthetic bead. 0.4
0.6
0.8
-
The fact that the ion size has such a significant effect on the poisoning of a catalyst implies that adjacent active sites must be so close together that a single large alkali ion can cover or make inaccesible more than one active site. If the poisoning is a function of the radius, rather than the square of the radius, as shown in Fig. 5, it would seem t o imply that the active sites are in a linear chain rather than in clusters. The chain could be cyclic provided the radius of the circle were large relative t o the distance between the active sites. That this proposed structure is not inconsistent with the inorganic chemistry involved in catalyst preparation is supported by a considerable amount of literature on isopolybases. Jander4 and Souchay6 have shown beyond any doubt that the neutralization of a soluble aluminum salt gives polymeric ions before the precipitation of aluminum hydroxide occurs. The degree of polymerization and the exact structure of the polymer may be questionable but there is no doubt that polymeric hydrated alumina exists in solution before precipitation. On this basis it is believed that the laying down of a chain of alumina on the surface of a silica micelle results in a chain of active acid sites. The data do not indicate whether the alumina is four or six coordinated, or whether the acid sites are Brfinsted or Lewis acids. The zirconia in the Type B catalyst would be expected to be deposited in a manner similar to that described for alumina. Although three different catalysts have been used in determining the indicated correlations, it is significant that the activities, the percentage compositions and the degree of calcination are very similar on the three catalysts. It is planned to extend this study to high alumina catalysts a t varying degrees of calcination as well as to other catalytic composites. (4) G. Jander and K. E. Jahr, KolIoid Be&. 48,295 (1936). ( 5 ) P. Souchay, Bull. Soc. Chim. Francs, 914 (1947).
Nov., 1954
EFFECT ON ALKALI ON THE ACTIVITY OF CRACKING CATALYSTS
1033
Since considerable emphasis has been placed on the fact that poisoning of the cracking catalyst is a function of the size of the poisoning ion, it seemed worthwhile to investigate the poisoning effect of the barium ion. The barium ion was chosen because 20 it was available as a soluble hydroxide, had the same 8 ionic radius as a potassium ion and had a dipositive charge. The data of Figs. 6 and 7 compare the relB ative poisoning effects of potassium hydroxide and 10 barium hydroxide on the Type B catalyst and the high activity synthetic bead catalyst. On the Type B catalyst it is clearly and quantitatively demonstrated that the poisoning effect of a mole of barium ion is exactly the same as that of a mole of 0 0 0.1 0.2 0.3 potassium ion. This relationship does not hold for the synthetic bead catalyst as shown in Fig. 7. Millimoles metal ion per g. catalyst. The discrepancy could not be due to sulfates on the Fig. 6.-Conversion us. moles metal ion on UOP type B catalyst, since the catalyst had been calcined above catalyst: 0 , KOH; 0 , Ba(OH)2. the decomposition point for sulfates before impregnation, and no sulfates could be found after exhaustive extraction of the original catalyst with boiling water. The discrepancy observed in 20 Fig. 7, and, indeed, the occasional large variation for cer- ‘I8 tain points of Figs. 1, 2 and 3 ti have been rationalized on the basis of the following considera- V tions. 10 When metal hydroxides are added to a catalyst, the hydroxide may be adsorbed randomly on the surface of the catalyst, or it may exchange the metal ion 0 for hydrogen ion a t the active 0 0.1 0.2 0.3 0.4 acid sites. Dilute solutions of Millimoles metal ion per g. catalyst. hydroxides favor exchange Fig. 7.-Conversion us. moles metal ion on high activity synthetic bead catalyst: 0, KOH; 0, Ba(OH)*. rather than adsomtion. Alkali metal ions are muih less strongly adsorbed than divalent metal ions such as barium.6 consistent with the recent suggestion that the cataFor reasons which are not clear the adsorption of lyst can be treated as a liquid.’ Summary.-Evidence is presented that the millibarium ion represented a significant amount on the synthetic bead catalyst and only exchange was sig- equivalents of lithium hydroxide required to poison nificant on the Type B catalyst. The amounts of a catalyst for cetane cracking represent a measure adsorbed alkali metal ion are considered to be neg- of the active acid sites of the catalyst. A barium ligible when compared to exchanged metal ion when ion is equal in its poisoning effect to a potassium ion impregnations were made from very dilute solutions of the same size. Large alkali metal ions are more of the metal ion, and when equilibrium conditions effective poisons per ion than the small alkali metal corresponded to substantially complete removal of ions, and the poisoning effect is a function of the radius of the ion. the metal ion from the solution. The data indicate that a cracking catalyst can If the mechanical adsorption of barium ion can be accepted as an explanation for the discrepancy of be represented as containing chains of active sites, Fig. 7, the exact agreement observed between the which represent only a small portion of the total poisoning effect of equal molar amounts of barium catalyst surface. Such chains could be formed by ion and potassium ion shown in Fig. 6 can be taken the condensation of polymeric alumina chains on t o prove that the poisoning of cracking catalysts by the surface of a silica micelle. metal ions of this type is almost entirely a function Acknowledgment.-The assistance of Beverly of the size of the ion. It would appear that this Carlson and Dean F. Martin and the financial observed dependence upon ion size would be in- assistance of the Office of Naval Research are greatly appreciated. (6) Q. E.Boyd, J. Schubert and A. W. Adamson, J . Am. Chem. Soe., .d
P
69, 2818 (1947).
(7) A. 0. Oblad, 9.0. Rindin and 0. A. Mills, ibid., TS,4088 (1953).
