SlLlCA=ALUMlNAGELS Specific Surface and Particle Size Distribution THREE SERIES of silica-alumina gels of various concentrations, made by (A) precipitation i n sequence, ( 8 ) mixing wet gels, and (C) impregnation of partially dried silica gel, were examined by nitrogen adsorption, small angle x-ray scattering, and x-ray diffraction. Specific surfaces were smaller for A and larger for B than those expected for mixtures of t h e dried silica and alumina gels. M a x i m a In curves for specific surface os. alumina content for B and C a t about 670by weight correlated with m i n i m a in average particle size. No alumina was detected by x-ray diffraction for gels containing as much a t 26% by weight alumina.
P. 6.ELKIN, C . 0 . SHULL, A N D L. C. ROESS T H E T E X A S COMPANY, BEACON, N. Y .
I
N T H E routine examination of commercial silica-alumina catalysts by x-ray diffraction, it was observed that no diffraction features characteristic of the alumina were obtained for catalysts containing appreciable amounts of alumina. When dried; pure silica gel gives a broad band indicating an amorphous structure, while dried alumina gel is crystalline. The nature of the crystalline alumina pattern depends on the temperature of drying. The present investigation was planned in order to study the effect of the method of preparing silica-alumina gels on the physical structure of the gels. Three methods, representative of commercial preparations, were chosen: (A) precipitation in sequence, in which alumina is precipitated from an aluminum chloride solution in the presence of a dilute slurry of silica gel; (E) mixing of the two wet gels in a dilute slurry; and (C) impregnation of silica gel (dried at 120' C.) with aluminum nitrate and subsequent thermal decomposition to form alumina. Several samples of varying alumina content were made for each of these series. To obtain additional information, the gel preparations were examined by low-temperature nitrogen adsorption and small-angle x-ray scattering in addition to the usual diffraction technique. From adsorption measurements the specific surface may be calculated, and from small-angle x-ray scattering the particle size distribution may be obtained. These examinations afforded a comparison of the specific surfaces calculated from particle size distribution data and from adsorption measurements.
another container, leaving behind about 3 liters containing the coarser particles of gel. The concentration of dry silica in the second container was found to be 2.4 grams per 100 cc. of slurry. This silica gel slurry vas used in the preparation of six silicaalumina gels of series .4. A portion was dried at 120' C. for x-ray diffraction, small-angle x-ray scattering, and adsorption measurement. SILICAGEL11. A solution of 2000 grams of the N brand sodium silicate in 10 liters of water was stirred for 30 minutes and acidified by the slow addition of 625 cc. of dilute hydrochloric acid, making the system just acid t o phenolphthalein. After stirring for a half hour, 325 cc. more of the acid were addedi leaving the system just basic to methyl red. After agitation for a half hour, the addition of 50 cc. of ammonium hydroxide loft the system just acid to litmus. The gel was filtered and washed six times by slurrying in 5 liters of water containing 5 cc. of concentrated hydrochloric acid. After washing three times by slurrying with 5 liters of water, the gel was sucked dry on the filter. This wet gel was found to contain 92.9% water. Portions were used for the preparation of one silica-alumina sample in series A and all of the samples of series B. Another portion was dried at 120" C. and screened to 4-10 mesh for the preparation of silica-alumina samples of series C. The dried gel w w found t o contain 37.2% water. SERIES A. Varying amounts of Baker's C.P. A1Cl3.6H10 IA solution were added to portions of silica gel I. Alumina was precipitated by the addition of dilute ammonium hydroxide until the system was just basic to litmus. The gel mixture was filtered and washed twice by slurrying for 30 minutes in 1 liter of water containing 1 cc. of concentrated ammonium hydroxide and once by slurrying in 1 liter of water. The washed gel was slurried in sufficient water to make a total of 1400 cc., filtered, and evaporated to dryness at 120' C. The quantities of materials used, together with the alumina contents found by analysis of the final dried gels, follow:
PREPARATION O F GELS
SILICAGELI. A solution of 2000 grams of Philadelphia Quartz Company's N brand sodium silicate in 10 liters of water was precipitated by slow addition of dilute hydrochloric acid with good mixing until the system was just faintly pink to phenolphthalein. After stirring for 20 minutes, the solution was made slightly acid to Congo red by the addition of more hydrochloric acid. The mixture was stirred for 30 minutes, and dilute ammonium hydroxide was added, leaving the system just acid t o litmus. After stirring for another half hour, the gel was filtered and washed five times by slurrying for one hour each time in 8 liters of water containing 8 cc. of concentrated hydrochloric acid. The gel was then washed by slurrying for one hour in 8 liters of water containing 80 grams of AlC13.6HsO. The gel was finally washed three times by slurrying in 8 liters of water for 30-minute periods. The gel was diluted to 8 liters, agitated for 8 hours, and allowed to stand for 2 days; then the gel was again stirred for an hour and allowed to settle for a half hour. The slurry was siphoned to
Wet Silica Gel I, C c . 1340 1280 I160 1040 680 440
AICls.6H20. Grams 7.1 14.2 28.4 42.6 85 113
HrO, cc. 160 1720 3840 3960 4320 4560
AltOa by Analyais, Wt. % ' 5.2 10.0 19.8 26.5 51.2 69.5
One silica-alumina sample was made by this method from silica gel 11. To 678 grams of silica gel I1 in 500 cc. water, a solution of 18.4 g:ams of Baker's C.P. A1(NOs)r.9Hf0in 50 cc. of , water was added. After stirring for a half hour, the alumina waa
327
328
Vol. 37, No. 4
INDUSTRIAL AND ENGINEERING CHEMISTRY
precipitated by dropwise addition of dilute ammonium hydroxide until the system was basic to litmus. The gel mixture was filtered and washed five times by slurrying in 500 cc. of water containing 1 cc. of dilute ammonium hydroxide. After final washing in 500 cc. of water, the gel was filtered and dried, Chemical analysis of this sample was not made. The theoretical alumina concentration was calculated to be 5% by weight. SERIESB. An alumina gel was prepared by dissolving 1000 grams of Baker's C.P. AlC1~6Wz0in 10 liters of mater and adding 1450 cc. of dilute ammonium hydroxide t o make the system basic to litmus. The gel was washed five times by slurring with 5 liters of water containing 10 cc. of dilute ammonium hydroxide each time. A final wash with 5 liters of water peptized the gel and made filtration very slow. The water content of the gel was found to be 92.2%. Varying amounts of this wet gel were mixed with portions of wet silica gel I1 and stirred in 1 liter of water for an hour. The gels were then filtered, dried at 120' C. for 6 hours, and calcined for 6 hours a t 540". The quantities of materials and the percentages of alumina found by analysis of the final prepara&ionsare listed below: Wet Alumina Gel, Grams
16 32
64 96 192 250
Wet Silica Gel 11, Grams 332
314 280 246 140 70
0
.41z?z b y Bnalysis, Wt. % 5.2 12.0 17.5 77.5
a4
0.6
0.8
1.0
5 00
31.0 59.5
02
600
E
a
2 400 OI L
SERIESC. Portions of silica gel 11, which was dried at 120" C. (containing 37.2% water) and screened to 4-10 mesh, were added to varying amounts of a solution of Baker's C.P. A1(Pi03'18.9H,0. One liter of this solution contained 367 grams of the salt which was equivalent to 0.05 gram of AlLh per cc. Four samples were prepared using amounts as listed below with the final chemical analysis. The samples were dried a t 120' C. for 6 hours and calcined at ,540" for 6 hours.
2 0)
Q
e
300
& 0
0 0)
.Q
200
L
0 u)
T)
A I ( N O ~ )CC. ~, 5 25 50 100
Silica Gel 11, Grams
A1108 by Analysis, Wt. oil.
39
5.2 7.9 13.4 25.5
38 36
32
a
E -a 0
> 0
ANALYSES
SPECIFIC SURFACE MEASUREMENTS. Low-temperature nitrogen adsorption isotherms were obtained by measuring the amount of nitrogen gas adsorbed a t various pressures from a few millimeters of mercury up to the saturation pressure. The apparatus was of the volumetric type, similar to the one described by Emmett ( 2 ) . The adsorbent was baked out under vacuum for 2 hours a t 200" C. and, when cooled, immersed in a liquid nitro-
100
0
1
600 -
0.2
0.4
0.6
1
Figure 1. Nitrogen Adsorption Isotherms of Series A Gels, Made by Precipitation in Sequence on Silica Gel I 1, silica gel I; 2, 10% alumjna; 3, 26.5% alumina; 4, 69.5% alumina; 5, 5% a l u m l n a o n silica gel II; 6, silica gel I I
Figure 2. Nitrogen Adsorption Isotherms of Series B Gels, Made by Mixing Wet Silica Gel I I w i t h Alumina Gel 1 silica gel I I ; 2, 5.2% alumina; 3, 12% alumina; 4, l i . 5 % a l u m i n a , 5, 31% alumina; 6, 59.5% alumina; 7, 7715% alumina; 8, alumina gel
Figure 3. Nitrogen Adsorption isotherms of Series C Gels, Made by Impregnation of Bartially Dried Silica Gel II 1 , silica gel 11. 2, 5.2% a l u m i n a . 3, 7.9% alGmina; 4, 13.4% a l u m i n a ; 5, 25.51% alumina
Relative
Pressure, p i c
0.8
IO
INDUSTRIAL AND ENGINEERING CHEMISTRY
April, 1945
gen bath. The dead air space around the adsorbent was measured by the pressure produced by admitting a known amount of helium into the adsorption system. After the helium was pumped out, known volumes of nitrogen were added; when equilibrium was attained, the pressure of the adsorption system was recorded. The temperature of the liquid nitrogen bath, from which the saturatioa pressure may be calculated, was not measured; instead, the saturation pressure was measured directly a t the end of each run by condensing nitrogen in the adsorption tube and reading the pressure. The adsorption isotherms were interpreted according to the method of Brunauer, Emmett, and Teller (I). The experimental values for p / V ( p , - p ) were plotted as a function of p / p o , where V is the amount of gas adsorbed per gram a t pressure p , and p , is the saturation pressure. Straight lines were drawn through these points; from the values of the slope and intercept, the amount of nitrogen, Vm,necessary to cover the surface of the adsorbent with a monomolecular layer was calculated. The specific surface, expressed as square meters per gram, was obtained by multiplying the V , value, expressed as cc. of nitrogen (N.T.P.) per gram of adsorbent, by 4.38 square meters per cc, This constant is the value for the calculated area that the molecules of 1 cc. of nitrogen will cover if liquid density and close packing of spherical molecules in a monomolecular layer are assumed. X-RAY DIFFRACTION.Photograms were obtained by the Debye-Scherrer powder method using filtered copper radiation. Compound identification was performed by comparison of the photograms with accepted or published diffraction patterns. SMALL-ANGLE X-RAY SCATTERING. From a study of the intensity of x-ray scattering at small angles (from 0.1' to 2.2'), it is possible to obtain the particle size distribution in the scattering specimen. This technique has been described by Guinier (3) and Hosemann (4, 5 ) . Crystal monochromated CuKa radiation was used in an evacuated scattering camera for this study. X-ray intensities were recorded photographically and determined with a microphotometer. Additional details regarding the interpretation of the scattered x-ray intensities will be published elsewhere. ADSORPTION ISOTHERMS
The nitrogen adsorption isotherms for the gel preparations dried at 540' C. from series A, B, and C are shown in Figures 1, 2, and 3, respectively. Figure 1 shows that, for the samples prepared with silica gel I by precipitation in sequence, the amount
600
329
of nitrogen adsorbed at lower pressures decreases (curves 2, 3, and 4) as the concentration of alumina is increased. On the other hand, the amount adsorbed at the higher pressures does not vary uniformly with the .alumina concentration. The adsorption isotherm of silica gel I1 (curve 6) has a different shape from that of silica gel I. The former is a typical S-shaped isotherm, whereas the latter is almost a Langmuir type of curve. The amount adsorbed near saturation pressure ( p / p o = 1.0) is much greater for silica.ge1 11. Table 1. Results of Nitrogen Adsorption a n d X - R a y Diffraction Examination of Three Series of Silica-Alumina Preparations Specific Surface, X-Ray Diffraction ALO3 Wt. /o 0
5.2 10.0 19.8 26.5 61.2 69 >5
6 0 5.2 12.0 17.5 31.0 59.5 77.5 100
Sq. M./G. Identification Series A, Pptn. in Sequence on Silica Gel I ................ 760 ... Amorphous SiOP 523 Amorphous SiOP ... Amorphous Si025 448 Amorphous SiOwb Amorphous Si02 a-AhOa.Hy0 h--A1zOr3HnOa 285 Amorphous St& a-.41~0a*HzO,h.-AbOr3HzOa Series A , Pptn. in Sequence on Silica Gel I1 373 ................ Series B , Mixing Wet Gels (Silica Gel 11) 414 ................ 541 ................ 454 ................ 388 Amorphous Si02 377 Amorphous SiOzb 345 Amorphous SiOz, 7-AlaOr Amorphous SiOz, 7-AlzOo ?ZS
...
................
173
Series C, Impregnation of Silica Gel I1 0 414 ................ 452 ................ 6.2 397 ................ 7.9 370 ................ 13.4 272 Amorphous Si02 25.6 Samples dried at 120' C.: all others weie dried at 540'. b Faint pattern of crystalline alumina was obtained by electron diffraction. Q
Figure 2 gives adsorption isotherms for the series B samples prepared by mixing wet gels. I n contrast with the behavior shown in Figure 1, the amount of adsorption a t lower pressures does not decrease uniformly as the alumina concentration is increased but first increases (curves 2 and 3) to values higher than that for the pure silica gel (curve 1) and then decreases (curves 4 through 8). This initial increase shown by the samples containing 5.2 and 12.0% alumina was so unexpected that the results were checked by examination of another series prepared by the same method but using different silica and alumina gels. Exactly the same type of behavior was found. The data are not included here. Figure 2 also shows that the amount adsorbed a t higher pressures for the samples is in about the same order as t h e amount adsorbed a t lower pressures. The curves for series C, made b impregnation of silica gel I1 with alumina formed by thermal ecomposition of aluminum nitrate, are shown in Figure 3. The variation of amount adsorbed with alumina concentration is similar to that observed for series B. The sample containing 5.2% alumina adsorbs more a t lower pressures than the pure silica gel. To determine whether the uniform variation of amount adsorbed for series A was due to any unusual properties of the base silica gel I, a single sample containing about 5% alumina was
d
a
\ N
400
$
0 r L
s f-
v)
.-c u ::200 0
v)
Figure 4.
o
20
40
Weight
Per
60 Cent
Alz03
ao
100
Variation of Specific Surface w i t h Alumina Concentration
Gels A I and A l l prepared by precipitation in sequence on sillca gels I and II; B, by mlxlng silica and a l u m i n a gels (wet); C, by impregnation of silica gel (partially dried) by a l u m i n a ; D and E represent d r y mechanlcal mixtures of a l u m i n a gel w i t h silica gels I and II, respectively
I N D U S T R I A L A N D E N G I N E E R I N G CHEMISTRY Figure 5.
Vol. 37, No. 4
Small-Angle X-Ray Scattering for Series A Preparations
1, silica gel I ; 2, 5.2% alumina; 3, 19.8% alumina
Figure 6.
Small-Angle X-Ray Scattering for Series B Prepapations
1, sllloa gel II; 2, 5.2% alumina; 3,,17.5% a l u q i n a
0.4
0.6
I
2
4
6
IO
15
served for samples containing as much as 26.5, 31.0, and 25.5% alumina for series A, B, and C, respectively. For comparison, a mechanical mixture of 5% dried alumina gel and 95% dried silica gel was prepared and examined by x-ray diffraction. A good pattern of crystalline C Y - ~ ~ ~ & H was Z Oobtained. Therefore, the failure of the diffraction method to detect alumina in these silica-alumina gels shows that less than 5% of crystalline alumina ia present. These x-ray results were confirmed by electron diffraction examination, which might be expected to detect surface concentrat'ions more readily. For gels having higher alumina contents, patterns of 01A~zO~~H (boehmite) ZO and a-Al.103.3HaO (hydrargillite) are obtained for samples dried a t 120" and patterns of y-312O3 for samples dried a t 540" C. The diffraction lines of the aluminas are broad, indicating small cryst'als. SMALL-ANGLE X-RAY SCATTERING
0.4 0.6 SCATTERING
I 2 4 ANGLE SQUARED ( 1 0 ' ~
6
IO
sa. RADIANS)
IS
prepared by the precipitation in sequence of alumina on silica gel 11. Curve 5 of Figure 1 is the isotherm for this sample; it lies below curve 6 for the base silica gel 11,just as do the curves of series A. Hence it is concluded that the uniform variation is not specific to the base gel. SPECIFIC SURFACE AND X - R A Y D I F F R A C T I O N
Specific surface values, calculated from the above adsorption data, are listed in Table I and plotted on Figure 4 as a function of alumina concentration. Dashed line D represents the expected variation of specific surface for mechanical mixtures of dried silica gel I and dried alumina gel, and curve E represents that expected for dried silica gel 11 and dried alumina gel. It is evident that the specific surfaces of the samples prepared by precipitation in sequence are less than would be expected for a mechanical mixture of the dried gels. The curve for series B (mixture of wet gels) lies above the expected curve for a mechanical mixture and shows a maximum a t 5% alumina. A similar peak is observed for series C, made by the impregnation procedure. No attempt was made to measure the specific surface of the pure alumina so formed. The results of x-ray diffraction examination are also listed in Table I. ;".io diffraction pattern attributable to alumina is ob-
Three members of series A and three of series B were examined by the 'small-angle x-ray scatbering technique. The scattering data for these two groups of preparations are given in Figures 5 and 6. Here the scattered intensity is plotted against the scatt,ering angle squared, both on a logarithmic scale. Each of t,he curves on these figures is a composite of several experimental curves. Since it is impossible to measure reliably x-ray intensity changes larger than a factor of 5 on any one film, several exposures \=,we taken of each material with different exposure times, and the data Fere combined into one curve. From these data it is possible t o determine the particle size distribution in the specimen under examination if certain assumptions as to scattering 4 complete discussion of theory and particle geometry are made. ' these assumptions will be published elsewhere. I n the abscnce of more precise information, the gel particles have been assumed to possess a symmetrical shape and to be defined in size by a particle diameter. Figures 7 and 8 give the particle size distributions for the members of series A and series B, respectively. The average particle diameter (defined a8 that diameter at which as much material existe in smaller sizes as in larger sizes) is shown on each of these curves. Figure 7 shows that, the average particle size increases regularly with increasing alumina content for members of series A. On the other hand, for series B members, the average particle size for the 5.2% alumina sample is definitely smaller than for either the origina1,silica gel or the 17.5% alumina sample. This shifting of the particle size distribution curves for the various gel preparations is in agreement with the specific surface findings given above. A greater state of subdivision with a smaller particle size should result in an increased specific surface. The difference in particle size distributions for the two original silica gels is striking. Silica geJ I has an average particle size of about 32 A. as compared to 58 A. for silica gel 11. This large difference in state of subdivision is to be correlated with the marked difference in specific surface values obtained experimentally. It is interesting to calculate the specific surfaces which should exist for the particle size distribut,ions given in Figures 7 and 3. For simplicit,y, spherical particles arc assumed. The results of these calculations are shown in Table 11; the calculat'ed specific surfaces show the same trends iyith increasing alumina content as do the experimental values. The surface anomaly with series B
.
x
April, 1945
INDUSTRIAL AND ENGINEERING CHEMISTRY
gels is thus confirmed; in addition, the surface variations are shown to be caused by changes in the particle size distribution existing in the gel microstructure. The calculated values in Table I1 are all higher than the experimental values for the specific surface. Perhaps the simplest explanation for this is the shielding of particle surface from adsorbed gas a t regions of particle-particle contact.
331
I
5
cn cn
s4 w
z 3
a -1 W
a 2
DISCUSSION OF RESULTS
.
I The results show definitely that silicaalumina catalysts prepared by the three methods are not simple mixtures of silica 0 20 40 60 80 100 particles and alumina particles such as PARTICLE DIAMETER (8.) PARTICLE DIAMETER (s.) would be expected from mixing fine powders of the two materials. Since there is no difFigure 7. Particle Size Distributions Figure 8. Particle Size Distributions fraction evidence of chemical compound for Series B Preparations for Series A Preparations formation between the silica and alumina, I , slllca gel 11; 2, 6.2% alumlna; 3, 17.5% 1, alllea gel I ; 2, 6.2% alurnlna; 3, 19.8% it seems reasonable to assume that the alumlna alurnlna observed phenomena are due mainly to physical effects. The possibility of a caused by a small number of contact regions for any one particle. solid solution of alumina in the silica gel is doubtful, particuThere is no reason for expecting the magnitude of this shielding larly in the series prepared by the impregnation of pareffect to remain constant from material to material since the partially dried silica gel, It may be postulated that the particle ticle packing need not remain constant. sizes of the wet gels and the partially dried silica gels are much smaller than the sizes observed for the dried gels. The addition SUMMARY of the alumina gel in the wet state may, therefore, prevent the Silica-alumina gels precipitated in sequence were found to 1 . formation of larger particles upon drying. Further investigation have lower specific surfaces than would be expected for mechaniof particle size measurement of wet and partially dried gels i s incal mixtures of the dry gels. dicated. 2. Mixing of the wet gels gave specific surfaces greater than those expected for a mechanical mixture of the dry gels. Table I I . Comparison of Specific Surfaces Measured by 3. Specific surfaces of gels made by impregnation of partially Nitrogen Adsorption and Calculated f r o m Particle Size dried silica were greater at small alumina concentrations and Distribution lower at large alumina concentrations than those expected for a Ay. -Sp. Surface, Sq. M./G.mechanical mixture of the dried gels. AlaOa, Particle Nz adSize disWt. % ' Diam.,H. sorption tribution Ratio 4. Crystalline alumina was not detected by x-ray or electron Series A, Pptn. in Sequence on Silica Gel I diffraction for gels with as much as 26% by weight of alumina, 829 1080 1.30 0. 31.6 but was found in a sample containing 501, alumina made by mix675 1020 1.61 6.2. 34.6 700 1.21 43.5 580b 19.8" ing the dried gels. Series B, Mixing of Wet Gels (Silica Gel 11) 5. Particle size distributions for the gels were determined by 414 560 1.35 58.3 0 small-angle x-ray scattering technique. These showed maxima 1.14 54.2 571 650 5.2 I .37 65.7 388 530 17.5 in particle size a t from 30 to 65 A. Samples dried for 4 hours a t 120' C. instead of 540'. 6. A maximum was observed in the specific surface us. conb Value obtained by interpolation on a graph of specific surface us. composition. centration curve a t about 5% alumina for the two series prepared by mixing the wet gels and impregnating the partially dried silica gel. The average particle sizes of these gels, as determined The agreement between the results found by nitrogen adsorpby small-angle x-ray scattering, had minima corresponding to the tion and by small-angle x-ray shattering appears satisfactory, and specific surface maxima. promotes confidence in the usefulness of this relatively new x-ray 7. Specific surfaces calculated from particle size distribution technique. In all cases of specific surface variation, either in one were found to change uniformly with experimental values but preparation series or with members of both series, the changes are were always higher than the experimental values. This last efreflected in the particle size distribution curves in a manner to fect is ascribed to a shielding of particle surface from adsorbed account for the observed surface phenomena. It must be adnitrogen a t particle contact regions. mitted that the assumption of a spherical'shape for the gel parACKNOWLEDGMENT ticles is not true in detail. Certainly the particles are irregular in The authors wish to express their appreciation to M. M. Stewsize and shape, but when attention is centered on a large number art and C. E. Moser for the preparation of the silica-alumina gels of particles (as is done experimentally), it is believed that the and to F. F. Coleman for carrying out the electron diffraction exsize distributions obtained on the picture of spheres are repreamination. sentative of the true gel particles. The differences between calculated and observed specific surLITERATURE CITED faces, shown in Table 11, are believed to be indicative of the par(1) Brunauer, S., Emmett, P. H., and Teller, E., J. Am. Chem. Soc., ticle-particle packing. A simple calculation shows that nitrogen 60,309 (1938). (2) Emmett, P. H., in "Advances in Colloid Science", Vol. I, p. 3, would be excluded from a small percentage of the particle surface New York, Interscience Publishers, 1942. a t a particle-particle contact point. With irregularly shaped (3) Guinier, A,, thesis, Univ. of Paris, 1939. particles, this shielding effect will besmore pronounced, and hence (4) Hosemann, R., 2.Physik, 113,751 (1939). it is possible to explain the specific surface differences as being (6) laid., 114, 133 (1939). Q
