1364 (14) (15) (16) (17) (18) (19) (20) (21) (22)
(23) (24)
BI. 0. BAKER,
S . D . CHESSUTT,
.4SD THOMSS P. WIER, JR.
LoNaswomH, L. G.: J. Am. Chem. Soc. 66, 449 (1944). LONGSWORTH, L. G.: J . Am. Chem. SO?. 67, 1109 (1945). LONGSKORTH, L. G.: J. Phys. Colloid Chem. 51, 171 (1947). LONGSWORTH, L. G., A N D X~ACISSES, D. -4.: Cheni. Revs. 11, 171 (1932). LOSGSWORTH, L. G., AXD hf.4CINXES, D. A . : J. Am. Chem. sac. 62, 705 (1940). ROTHEN, A.: J. Gen. Physiol. 24, 203 (1940-41). SCATCHARD, G., ONCLEY, J . L., W I m I A m , J. W., ANI) BROWK, A.: J. Am. Chem. SOC. 66, 19SO (1944). SCYNER, J. B., A N D €lam,D. B.: J . Am. Chem. Sac. 51, 1255 (1929). SVEDBERG, T., ASD PEDERSEN, K. 0.: T h e Ultracentrifuge. Cniversity Press, Oxford (1940). SVENSSOX, H.: Arkiv. Kemi Mineral. Geol. 22A, No. 10 (1946). THEORELI,, H.: Riochem. ,J. 233, 293 (19373.
A CONTRIBUTIOX TO THE KSOWLEDGE O F SODIUM C: O S TAXII N AT ION O X C R h C KI SG CATALYSTS MAURICE 0. RAKER, S. D. CHCSSUTT,
AXD
THOSIAY P. WIER, JR.
Research Laboratories. Hoirston Re$nwy, Shell 011Co7tipnnil. Incorporated, Houston, Texas Rereiiwl A p i i l P, 1.948
Small amounts of sodium present in c ,>mmercialsilica-alumina catalysts used for the cracking of petroleum hydrocarbons have been considered t o have an adversc effect on the cracking activity of the catalyst. I n attempting t o obtain a quantitative relationship for this effect, it has been customary t o determine the total sodium content of the catalpqt irreqpectiw of whether 1he sodium might be distributed over the internal surface of the catalyst pores or buried in thc solid phase. Since catalyst activity is a wrface phenomenon, it seems reasonable that only that portion of the sodium which is on the surface should appreciably affect the activity. Surface concentrations of sodium h a w been measured for several silicaalumina catalyst samples in order t o determine \That proportion of the total sodium cwntent was on the surface and t o learn the changes which may take plat? in the surface concentnition during US of the catalyst for the cracking of hydrocarbons. Further, since carbon is deposited on the catalyst during the cracking process, thereby possibly covering some of the sodium, a brief study of surface sodium with respect to carbon deposition has been made. S o attempt is mil&: within this paper t o esamine the relationship hetween (*atalpticactivity and tlic presence of sodium in the catalyst. EXPERIMENTa'.L
Experimentally, the study was quite simple. The amounts of carbon and extractable sodium were determined on samples of spent catalyst, regenerated
SODIUM COKTAMINATION ON CHACKISG CATALYSTS
1365
catalyst, and catalyst from which all of the carbon had been removed by careful combustion. Surface areas and total sodium contents were also measured, giving the following pattern: Spent catalyst (a) Per cent extractable sodium (b) Per cent carbon Partially regenerated catalyst (from unit) (a) Per cent extractable sodium (b) Per cent carbon (c) Surface area Completely regenerated catalyst (from laboratory furnace) (a) Per cent extractable sodium (b) Per cent total sodium This was repeated for samples taken at intervals during deactivation by use in a fluid-bed type pilot plant. The samples and analytical methods are more fully described below. S o data on run conditions have been included, since no attempt t o relate these to catalyst changes is made herein. It is sufficient to say that the catalysts were subjected to operating ronditions in ranges ordinarily encountered in commercial application. CATALYST S S M P L E S
T w o forms of silica-alumina (3-A) cracking catalysts for fluid-bed operation were studied. One of these was the commercial ground product of -American Cyanamid and Chemical Corporation (referred to hereafter as “low sodium” catalyst), in which the only sodium present was that remaining after washing of the hydrogel with distilled water. The other was from an experimental batch of microspheroidal catalyst prepared by spray drying of a hydrogel furnished by the same company, which is of interest in this study not because of its microspheroidal character but because in this particular batch the greater part of thr sodium present n-as introduced by the use of tap water in reslurrying the hydrogel for spray drying. This material is referred to hereafter as “high sodium” catalyst. Samples of each were taken before use and from both the reactor and thr regenerator at intervals during fluid-bed pilot plant runs made to study rates of activity decline. An important point which must be emphasized is the fact that no fresh catalyst was added during the runs; thus catalyst samples are relatively homogeneous, rather than mixtures of new and old catalyst. .LK.%LYTIC.&L METHODS
The amount of sodium which could be extracted from the catalyst by strong acid in the presence of the existing carbon was quantitatively determined. This vas accomplished, without removing the carbon, by shaking 5 g. of catalyst with 50 ml. of concentrated hydrochloric acid for 1 hr., centrifuging, removing exactly 25 ml. of solution for sodium analysis, adding 25 ml. of fresh acid, and repeating the extraction process. Even low concentrations of hydrogen ions are knou-n t o have a strong displacing effect for surface cations on silica gel (3), zeolites (1).
1366
11. 0 . BAKER, S . D. CHESXUTT, -4” THOMAS P. WIER, JR.
clays (2), and similar materials. Thus this extraction technique allowed the calculation of a value closely approximating the desired quantity, and in the case of the carbon-free samples the leachable sodium is considered to be equal t o the entire amount of sodium on the surface of the catalyst. The probable leaching of a portion of the surface alumina has been assumed not to affect the results. The total sodium nhich the catalyst contained both on the surface and in the mass was determined by dissolving the catalyst sample in hydrofluoric acid and sulfuric acid. Sodium in the extracts and solutions was determined gravimetrically as sodium uranyl zinc acetate hexahydrate, using blank determinations with careful duplication of technique to eliminate errors involved in determining the small quantities of sodium encountered. For catalysts containing of the order of 0.01 per cent by weight of sodium oxide, analyses Tvere reproducible to f 4 per cent of the mean, while for catalysts caontaining of the order of 0.1 per cent by weight of sodium oxide, the values were reproducible to f l per cent of the mean. I n order t o give a standard basis for interpretation of the sodium contents, all sample weights have been corrected to the basis of no carbon and only that moisture retained on heating a t 600°C. for 1 hr. The use of the term “per cent by weight of sodium oxide” to express the sodium concentration is merely to conform t o practice and should not be construed as indicating the form in which the sodium is actually present. Surface areas were determined by low-temperature nitrogen adsorption, the regenerated catalyst being used for reasons not pertinent to this study. The effect of carbon on surface area measurements (other than the effect on the m i g h t of sample) has not been taken into account, since this is usually quite small. High-temperature combustion analyses accurate to f O . O 1 per cent by weight on an absolute basis vere used to determine the carbon contents of spent and regenerated cataly-st. I n producing the completely regenerated catalyst, the carbon was carefully and completely burned off in dry air at 600°C. This technique was known (4)not to affect the gel structure or extractability of sodium from silica gel, so that it is reasonable to assume no deleterious effect in this case. RESULTs -\XD DIzCUdSIOS
The results of the determinationb described in the previous section are arranged in tables 1 and 2, the samples being listed in the order of increasing extent of pilot-plant use. Upon examining these tables it is seen immediately that the total sodium caontent of the “high sodium” catalyst is approximately six times that of the ”lou- sodium” catalyst, 90 that the use of tap n-ater in reslurrying the hydrogel introduces an appreciable amount of sodium into the catalyst. The percentage of the total sodium in each case nhich is on the catalyst su:face is approximately 90 per cent for thc “high sodium” catalyst and 65 per cent for the “ l o ~ rsodium” catalyst. These figures are h s e d on the percentage of the total sodium nhich is extractablc from the fresh sample. Thus, in both cases, the greater pari of the sodium is located on the available interior surface rather than buried \rithiii thc solid phase of the catalyst.
1367
SODIUM CONTAMINATION ON CRACKIKG CATALYSTS
The use of these catalysts over an extended period in the pilot plant did not result in any appreciable change in the total sodium content; notably, there naa no increase from sodium 11-hich is sometimes present in hydrocarbon feed stockh. During thi.: period of use, however, there was a definite decline in both the surface area and the proportion of the sodium which is on the surface. Comparing the decline of these quantities graphically, using the original values as 100 per cent :is is done in figures 1 and 2, it is seen that for both ratalysts the surface area dec.rease9 somen-hat more rapidly than does the amount of sodium on the surfacc TABLE I ...
l’r7biilatior~ u j results* for "lair sodium” c a t a l p s t ........ _______
._ ........
TOI.\L
EPUPLE D T S 1 6 S k T I O X .~
~
Sara
Sa20
.~____..
per. cent b y useight
~
SURF.\CE: .\RE 1 ~~
.~
~
ChRBOS ~
~
p e r cent b y weigh1
p e r cent b y u e i g h l
m.2igvam
0.0160
,354
A-Spent A-Regenerated .i-Carhnn-free
0.0140 0.0137 0.0159
435
1.82 1.08 0
H- Spen t B-Regenerated R-Carbon-free
0.0114 0.0145 0.0154
-100
1 .oo 0.25 0
C-Spent C-Regenerated
365
C-Carhoti-free
0 0074 0 0109 0 0131
0 85 0 24 0
D-Spe ti t . . . . . . . . . . . D-Regenerated.. . . . . D-Carbon-free . . . . . . . . . .
0.0080 0.0115 0.0138
341
0.83 0.33 0
E-Spent . . . . . . . . . . . . . E-Regenerated . . . . . . . E-Carbon-free . . . . . . . .
0.0105 0.0118 0.0132
327
0.58 0.26 0
Fresh
. . . . . . . . . . . .
0.026
~
’
0 ’
* I n calculating each of these results, sample weights were coirected for volatile matter, using the loss i n weight on heating in air at 600°C. for 1 hr., t o give a standard basis for comparison This leads to a slight increase in the concentration of sodium per square meter of surface area as the catalyst is used, a factor which must be taken into account if there is to be any quantitative study of sodium as a contaminant in cracking reactions. Other surface chemical changes, such as a change in the surfart. alumina concentration, etc., may also take place. The extent to which the presence of a carbon deposit interferes with the extraction of surface sodium may be learned by esamining the data presented in figurw 3 and 4 for the extractable sodium on samples of spent, regenerated, and carbonfree catalyst. The effect is most pronounced for the t lo^ sodium” samples, for
1368
M. 0. BAKER, S . D. CHESNCTT, -4ND THOMAS P. WIER, JR.
TABLE 2 Tabulation of results” f o r “high sodium” catalyst
’
I
A-Spent A-Regener ated A-Carbon-free
383
1.48 0.59 0
364
1.35 0.46 0
0 0984 0 1001 0 1132
349
1.99 1.03 0
0 0944 0 0971 0 1108
334
0.03 0.35 0
E-Spent E-Regenerated E-Carbon-free
0 OS56 0 0942 0 1078
323
1.09 0.42 0
F-Spent . . . . , . . . , . . , . , , ...., . F-Regenerated F-Carbon-free. . . . . . . . , . ,
0.0912 0.0967 0.1059
278
1.70 0.96 0
B-Spent . . . . . . , . , , , , B-Regenerated . . , . , . B-Carbon-free., , . ,
C-Spent C-Regenerated C-Carbon-free
I
0 1029 0 1068 0.1146
’
O.OS89
’ l
. . I
D-Spent D-Regenel ated D-Carbon-free
0.147
0.0896 0.1123
* I n calculating each of these results, sample weights were corrected for volatilematter, using the loss in weight on heating in air a t E O O T . for 1 hr., t o give a standard basis for comparison.
FIG. 1. “Low sodium” catalyst
1369
SODIUM COKTAMIKATION ON C R k C K I K G CATALYSTS
which linear relationships were observed. It is interesting to calculate the percentage of the surface which is covered by a given amount of carbon (assuming
3
FIG.2. “High sodium” catalyst 0016
0014
0012
2
c
3
0010
o m
0,
3
0006
0004
0002
om0 (
1
’
02
I
i 04
06
08
Yow
IO 12 C A R EON
14
I 16
18
3
FIG.3. Decrease in extractability of sodium by carbon deposition: “low sodium” catalyst.
a close-packed monolayer of carbon atoms) and compare this figure with the percentage of surface sodium which is rendered non-extractable by the same
1370
31. 0 . B-IKER,
S. Y. CHESKUTT, A S D THOMAS P. WIER, J R .
amount of carbon. The following equation is used t o calculate the surface covering power of the carbon: Per cent of surface covered by assumed carbon-atom monolayer
where Ji‘ is the molecular weight (12.01 for carbon), is Avogadro’s number, C, is the concentration of carbon in grnms of carbon per 100 g. of catalyst, S, is the specific surface area in square meters per gram of catalyst, and d is the density
3 %w
CARBON
FIG.4. Decrease in extractability of sodium by carbon deposition : “high sodium’! catalyst.
(2.% g./ml. for graphite). Insertion of the proper values in this equation reduces it to the form:
Per cent of surface covered
=
3
2.35 X 10
L c A%
Tlie given percentage of carbon at which the comparison is made is taken here t u be 1.00 per cent, the results of the calculation being given in table 3. There appears t o be sufficient difference in the behacior of the “low sodium” and “high sodium” catalysts with respect t o carbon deposition t o warrant their separate discussion. For the ‘‘low sodium’’ catalyst, the deposition of 1 per cent by weight of carbon renders a much larger percentage of the sodium non-extractable than could be accounted for by simple uniform covering of the catalyst surface with a monolayer. .iwiming that the sodium atoms are rather uniformly
1371
SODIC11 COSTA31IiiSTION ON C R I C K I S G CSTALYSTS
distributed over the catalyst surface, the above behavior suggests that either ( I ) carbon has been selectively deposited on sodium sites, or ( 2 ) carbon is able to block off much more surface (and the sodium present on that surface) than if it were in the form of a monolayer of close-packed carbon atoms. This may be done by (a) closing of pore mouths with a small deposit or ( b ) non-penetration by the aqueous solution due to non-n-etting of the surface, though there is not an actual sealing off of the pore or even complete covering of the surface by carbon. Assuming the sodium to be distributed non-uniformly leads t o a third possible explanation: namely, (3) surface sodium may be concentrated on the surface of the smaller (or larger) pores of the catalyst and during the cracking reaction carbon may be selectively deposited in these pores merely because of their size. The selective deposition of carbon on sodium sites would involve a rather large TABLE 3 Comparison of the surface area covered b y carbon nconolaUer and the sodium rendered non-extractable b y the same amount of ca, bon AT
1.OO PER CEST BY WEIGHT CllRBON
.................
“Lou: sodium” catalyst: Sample A , , . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . Sample B . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . Sample C . ,. . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . Sample D . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . Sample E. . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . “High sodium” catalyst: Sample A . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . Sample B . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . Sample C . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . Sample D . . .. . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . Sample E . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . Sample F . .. . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . .
p e r cent
per cent
5.4 5.9 6.4 6.9 7.2
8 26 54 50 35
8 0 2 8 6
12 20 11 12 20 8
2 6 7 6
6.1 6.5 6.7 7.0 7.3 5.5
I
4
5
number of carbon atoms being deposited in the vicinity of each sodium atom. Considering the data for sample B, which exhibits an intermediate slope in figure 3, it n-ould be necessary t o assume approximately 640 carbon atoms deposited in the vicinity of each sodium atom. There is probably sufficient space between sodium atoms for this t o occur (there being an average of only one sodium atom for each 10,000 -I.? of surface). In this case sodium must exert its influence directly on the deposition of carbon a t the great distances involved for most of the carbon atoms. Holyever, it is still possible that sodium atoms may merely initiate carbon deposition, these carbon atoms in turn causing further deposition. Considering the second explanation, the closing of pore entrances by a small deposit of carbon does not seem reasonable, in vien- of the extremely pronounced
1372
11. 0. BAKER, S. D. CHESNUTT, AXD THOJMS P. TTIER, JR.
effect which 1 per cent of carbon would thus be expected to produce in the cracking reaction, an effect of the necessary magnitude never being observed. Furthermore, non-wetting is not a satisfactory explanation, because the extractability of the sodium on the “high sodium” catalyst was not greatly decreased by carbon contents as high as 2.0 per cent by weight. A positive evaluation of the explanations given above cannot be made at the present time. Because the covering of sodium by carbon may be very extensive in some practical cases, any study of the effect of sodium on cracking activity must take into account the amount of carbon deposit present during reaction. Inspection of the data for the “high sodium” catalyst in figure 4 and table 3 leads t o the conclusion that the sodium introduced by adding t a p water to the hydrogel must have assumed a different position on the catalyst surface or a different mode of attachment t o the surface so that carbon is not selectively deposited on the sodium t o any marked degree. At low carbon concentrations there does appear t o be selectivity, which might be explained by the postulation of two kinds of sodium: ( I ) that originally present in the hydrogel and ( 2 ) that introduced from the tap n-ater. SUhIMARY
Sodium contaminant on commercial silica-alumina cracking catalyst is considered t o be effective only when present on the surface, the surface sodium concentration being measured by extraction n i t h strong acid -In increase in the concentration of sodium per square meter of surface is shown to occur, without any increase in total sodium, as the structure changes t o one of lower area during use. The effect of carbon deposition on the extractability of the surface sodium is determined and interpreted in terms of the surface distribution and mode of attachment of the sodium t o the surface. Both of these effects must be taken into account in any quantitative study of sodium as a contaminant. REFERENCICS (1) ACSTERWEIL. (3,. J . So?. Cheni. Ind. 53, 185-9T (1934). (2) J E N N Y , H m s : J. Phys. Chem. 36, 217-58 (1932); WILLIAJIS,RICE: J. Agr. Sci. 22, 538-14 (1932) ; arid others. (3) TAMELE, M. W ,(Shell Development Company) : Unpublished data. (4) TAMELE, 11,IT,, . ~ N D\VIER, T. P., JR. (Shell Development Company): Unpublished data.
