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JOSEPH D. DANFORTH AND DEANF. MARTIN
Vol. 60
THE EFFECT OF ALKALI METAL IONS ON THE ACTIVITY OF CRACKING CATALYSTS BY JOSEPH D. DANFORTH AND DEANF. MARTIN Department of Chemistry, Grinnell College, Cfn'nnell,Zowa Received September id, 1866
The conversion of cetane in a standard activity test haa been reported aa a function of the concentration of four alkali metal ions for a silica-ma nesia catalyst, an alumina-boria catalyst, and for several silica-alumina catalysts before and after the partial destruction o f active sites by calcination. The loss in conversion per milliequivalent of each alkali waa considered to be a fundamental characteristic of each catalyst. The constancy of this value before and after site destructive calcination waa consistent with a mechanism of site destruction in which large sections of active sites become buried by condensation with an adjacent surface, while the spacing between those sites available for reaction remained relatively unchanged.
Because of the amorphous nature of cracking catalysts, the application of physical methods to the determination of structure has been relatively unsuccessful. An indirect approach to the structure of catalysts has been attempted by studying the reaction of the catalyst with various substances, usually poisons, in an effort to relate cracking activity to chemical changes brought about at the active sites. It was previously shown that the conversion of cetane was inversely proportional to the milliequivalents of an alkali ion over a major portion of the range in which cetane conversion changed appreciably with added alkalisa The larger alkali ions were more effective poisons than the smaller ones, and the loss in conversion per milliequivalent of alkali ion appeared to be a function of the radius of the covering ion. On the assumption that the loss in conversion per milliequivalent of alkali ion was an indirect measure of the distance between the active. sites, the losses in conversion per milliequivalent of lithium, sodium, potassium and cesium hydroxide have been determined for cracking catalysts of different chemical constitution. A comparison of the losses in conversion per milliequivalent of alkali for two silica-alumina catalysts before and after the partial destruction of active sites by calcination indicated no definite change in catalyst spacings, and a mechanism of site disappearance on calcination is implied.
Development Company as calcined granules containing 11.8% boria on grade A Alorco alumina. Du Pont Cetane was used in all tests. Apparatus and Procedure.-The calcined catalyst sample! were impregnated as previously described,a dried at 110 and calcined 2 hr. a t 500" before charging t o the reactor. Appreciable quantities of boria were found to dissolve in the aqueous impregnation layer above the alumina-boria catalyst, and this introduced an uncertainty in the aluminaboria data. The method for determining conversions previously describedSwas modified to charge 25 ml. of cetane at 485" and liquid hourly space velocity, 4, to 12.5 ml. of catalyst. The wei h t of non-volatile product remaining after stabilization of tie liquid to 180"was determined to the nearest mg. and expressed aa per cent. of cetane recovered. The conversion of cetane was the difference between this value and 100. The tables summarizing the conversion data are not given because they do not add appreciably to the information which is presented in the figures.
Recision and Accuracy of Data.-An inspection of the points of Figs. 1-6 shows a definite spread in the data, and lines of somewhat different slopes could be used to represent the data in several cases. The possible error increases as the size of the ion increases because of the very steep slopes obtained on the addition of cesium ion. As is sometimes encountered in catalytic work, an occasional point will fall completely off of a line by an amount far greater than the precision of the standard test would permit. On the other hand, there is a repeating pattern of points which enables lines of approximately reproducible slopes to be drawn to show the change in conversion as a function of the milliequivalents of alkali ion. Because of the posMaterials.-A Shell Development Company experimental sible variation in slopes, ranges of values obtained catalyst was received as 8 to 14 mesh granules containing by the manner in which the line was drawn have ap roximately 2570 alumina on silica. Site destructive cahnation was accomplished by heating for 8. hr. at 550". been given in Table I. In some cases this range is The catalyst gave 547? conversion of cetane, initially, and quite small, while in others the range is large. 47% conversion after site destructive calcination. A 25% alumina on silica catalyst was prepared by the h drolysis of alcoholic solutions of ethyl orthosilicate and aLminum isopropoxide as described by Thomas.P The catalyst was dried, calcined several hours a t 500" and pressed to 6-16 mesh granules. The granules were calcined 15 hr. at 500°, and site destructive calcination was accomplished by an additional 6 hr. at 650' (cetane conversion, 33%). A silica-magnesia catalyst from the American Cyanamid Company represented a composite of laboratory samples containing approximately 31% MgO. The catalyst as received wai calcined for 15 hr. a t 500' (cetane conversion, 32%). The alumina-boria catalyst waa obtained from the Shell
(1) G. A. Mille, E. R. Boedeker and A. G. Oblad, J . A m . Chsm. Soc., 72, 1554 (1850). ( 2 ) Charles L. Thomas, I n d . Eng. Chem., 41, 2564 (1949). (a) Joseph D. Danforth, T H IJOURNAL, ~ 68, 1030 (1954).
Results The conversion of cetane in a standard activity test has been plotted as a function of the milliequivalents of lithium, sodium, potassium and cesium hydroxides in Figs. 1-6 for two high activity silica-alumina composites, before and after site destructive calcination, and for a silica-magnesia catalyst and an alumina-boria catalyst. Over that portion of each curve in which added alkali caused a significant change, the conversion appears to be inversely proportional to the milliequivalents of alkali ion per gram of catalyst. At higher concentrations of alkali (not shown) the conversion remains relatively constant. For very small amounts of alkali there is, in some cases, a
THEACT~VITY OF CRACKING CATALYSTS
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0.4 0.6 0.8 1.0 1.2 Meq. MeOH/g. catalyst. Fig. 4.-Shell high alumina catal st after site destructive calcination: 0, LiOH; 0 , NaO€f; A, KOH; A, CsOH.
0.2
d
'Ec 20 0.2 0.4 0.6 0.8 1.0 Meq. MeOH/g. catalyst. Fig. 2.-25% A1208on SiOz after site destructive calcination: 0, LiOH; 0 , NaOH; A, KOH; A, CsOH.
50
I-
8
V
0.2 0.4 0.6 Mea. MeOHla. Fig. 5.-Silica-magnesia catalyst: . _. catalvst. 0, LiOH; 0 , NaOH; A, KOH; A, CsOH.
1 0.4 0.6 0.8 1.0 1.2 Meq. MeOH/g. catalyst. Fig. 3.-Shell high alumina catal st before site destructive calcination: 0, LiOH; 0 , NaO&; A, KOH; A, CsOH. 0.2
rather sharp decrease in conversion, which is emphasized in the figures by the observation that the straight line portion of every curve does not necessarily extrapolate to the determined conversion for zero alkali content. The curves of Figs. 1-6 have been drawn to emphasize the straight line portions, rather than to depict accurately the conversion of cetane as a function of added alkali over the entire range of alkali content. The slopes of the straight line portion of each curve have been divided by the weight of the catalyst sample to give a value having the units-loss in conversion per milliequiv-
alent of alkali. This value appears to be characteristic for each catalyst as long as comparisons are made with a constant volume of catalyst a t the same conditions of operation. These values have been recorded for each alkali ion and each catalyst in Table I. A study of Table I indicates that only insignificant variation in the loss in conversion per milliequivalent of alkali is observed on two silica-alumina catalysts of different origin before and after site destructive calcination. The Shell catalyst decreased in conversion from 54 to 47% and the prepared silica-alumina catalyst decreased from 41 to 33% conversion. The fact that the loss in conversion per milliequivalent of any one of the four alkali ions did not change on either catalyst as a result of calcination, has been interpreted to mean that the distance between the active sites does not change, as sites disappear on calcination. This assumes, of course, that the increase of poisoning effect with increase of ion size is a function of the size of the ion, and not an implicit function of some other variable directly related to the size of the ion. A mechanism for the disappearance of active sites, which is consistent with the observation that
JOSEPH D. DANFORTH AND DEANF. MARTIN
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Vol. 60
TABLE I Loss IN CONVERSION PER MEQ.ALKALIION 485"; L.H.S.V., 4; duration, hr., 0.5: 12.5 ml. cata.: 25 ml. cetane. Metal ion
Radius of ion,
A.
Li
Na
K
CS
0.60
0.95
1.33
1.69
3.2-3.6 3.5-3.6
6.0-6.2 5.7-6.0
8.5-11.2 9.2-9.6
14.0-15.5 15 .0-15.2
2.8-3.3 2.8-3.3 5.3-5.7 6.2-7.0
6 1-6.8 6.0-6.3 9.5-11.0 9.4-9.8
8.1-10.3 9.7-11.7 16.0-16.5 12 5-13.8
15.1-17.0 16.4-17.6 20-22 16.4-20.4
Catalyst
Shell high alumina catalyst Before site dest. calc. After site dest. calc. Prepared 25% A1203 on SiOI Before site dest. calc. After site dest. calc. Silica-magnesia Alumina-boria
the spacings between the active sites remain constant during moderate site destructive calcination, can be represented by the disappearance of large sections of active sites by condensation with an adjacent catalyst micelle. Thus, the chains of active sites which have been represented as >-OH4 enter into condensation with hydroxyls on an adjacent surface and thereby become buried and unavail-
40
.E
c
u" 20
0.2 0.4 0.6 Meq. MeOH/g. catalyst. Fig. 6.-Alumina-boria catalyst: 0,LiOH; @, NaOH; A , KOH; A, CsOH.
able to hydrocarbon molecules. By this mechanism the active sites which do not enter into condensation remain relatively unchanged. This mechanism of active site djsappearance is consistent with the observation that only a portion of the acid hydrogens present in precipitated gels of alumina and silica and presumed to be the source of active sites, remains to be titrated after the composite has been calcined in the range of 500". Presum(4) Joseph D. Danforth, THISJOURNAL, SO, 564 (1955).
ably a high proportion of the active sites is buried in the catalyst by these condensation processes. When the losses in conversion per milliequivalent of each alkali ion from Table I are plotted as a function of the radius, or as a function of the radius squared for the silica-alumina catalysts, the alumina-boria catalyst and the silica-magnesia catalyst, a dependence of the loss in conversion per milliequivalent of alkali ion on the size of the ion is observed. The precision of the data does not permit a definite choice to be made between the radius and the square of the radius as the correlating variable for these catalysts. It is definitely shown that the silica-magnesia and alumina-boria catalysts are more sensitive to alkali ions than those silica-alumina catalysts. This comparison is particularly emphasized for the two smaller ions, lithium and sodium, where the precision of the values is greater. Thus the loss in conversion per milliequivalent of lithium ion is 2.8 to 3.6 on the silica-alumina composites, about 5.5 for the silica-magnesia, and 6.2 to 7.0 for the aluminaboria catalyst. It would be convenient for purposes of correlation to assign specific drops in conversion per milliequivalent to each different chemical composite. However, data from other sources show that certain high density, low activity silica-alumina catalysts have losses in conversion per milliequivalent that correspond to those observed for the silicamagnesia and alumina-boria catalysts. The variation for the losses in conversion per milliequivalent was from 3 to 8 with the lithium hydroxide, and from 14 to 22 for cesium hydroxide with the ranges for sodium and potassium falling a t intermediate values. The reasons why different catalysts may have diff erent sensitivities to poisoning by alkali ions cannot be given with certainty. Summary.-The conversion of cetane as a function of the amounts of certain alkali ions has been determined for several silica-alumina catalysts, an alumina-boria catalyst and a silica-magnesia catalyst. A mechanism for the disappearance of active sites on calcination which is consistent with the data has been suggested. Acknowledgment.-The assistance of Mr. Wayne Ohline and the financial assistance of the Office of Naval Research and the Research Corporation are greatly appreciated.
