Control of Physical Structure of Silica-Alumina Catalyst J
IC. D. ASHLEY
AND
W. B. INNES
Stamford Research Laboratory, American Cyanamid Co., Stamford, Conn.
I
that the cracking activity is associated with acid groups on the surface of the gel and that the average ultimate particle size of agreed to manufacture 15 tons per day of silica-alumina gel of fresh catalyst is about 50 A. (6, I?', 18). a particle size suitable for the newly developed fluid cracking Although there has been little direct evidence presented to process. The high octane gasoline and Ca hydrocarbons produced show that silica-alumina gel has a cross-linked chain or brush by catalytic cracking were then badly needed for war purposes, heap structure, it is otherwise difficult to account for the low and a catalyst manufacturing process had to be worked out in a solids content, of the hydrogel. Tamele ( I O ) , however, has very short time. pointed out that the ultimate particles which are derived from the This presented the problem of developing a process suitable for sol precursor could be spheroidal if these particles tended to the handling of over 160 tons per day of gelatinous silica. The bridge t o form chains. The present study has been concerned old method, based on the concept that the silica gel had to be with obtaining a better understanding of the structure of silicapartly dried with low temperature air and then washed free of alumina gel, the factors controlling it, and their effect on catalyst sodium before applying the aluminum, appeared impractical for performance. such a large scale operation. Probably the most common method of manufacture of fluid It was soon established that washing the hydrogel with the silica-alumina catalyst (5, 7 ) and the one used in this work inlarge volume of water required to free it of sodium would require volves as a first step the formation of silica gel by mixing acid a reslurrying operation because of cracking of the filter cake. with silicate and addition of an aluminum salt such as the sulfate The most economical way of carrying out such an operation is to followed by addition of ammonia. According t o Tamele (20) wash on a series of vacuum drum-type rotary filters with repulpthe hydrolyzing aluminum salt reacts with the surface silicic acid ing of the cake between each stage of filtration. Attempts to do to form a complex in which the aluminum is tetrahedrally cothis soon demonstrated that the gel slurry prepared by mixing ordinated. Washing to remove soluble salts and drying then acid and sodium silicate with moderate agitation had too broad a yields the final catalyst. range of particle size. The semicolloidal material quickly blinded It would appear that the structure of silica-alumina hydrogel a filter cloth so that the gel could not be washed on a rotary filter. is essentially the same as that of the silica gel preceding it since The problem was first solved by one of the authors ( 5 )by use of only a surface reaction appears to be involved in its formation. a screen-shrouded impeller-agitator during the mixing of acid The silica hydrogel is the aggregation product of the sol preceding and silicate, followed by addition of a flocculating agent (4)-i.e., it. Therefore, it seemed that studies on the sol would throw glue or aluminum hydroxide. This gave a gel slurry of optimum light on both the nature of the ultimate particles of silica and particle size for rotary filtration and washing. The flocculating silica-alumina gel. effect of aluminum hydroxide made it seem that addition of an aluminum salt and ammonia before filtering, washing, or drying SILICA SOL AND GEL the hydrogel would solve the washing problem and greatly simPhysical Properties. There is a meat deal of literature on colplify operations. Therefore, this procedure was tried while varyloidal silica and silica gel, ing temperature, concentrasome of which has been sumtion, and silica gel aging time marized by Ephraim (8). prior to alumina addition, alMore recently the subject though it was generally behas been reviewed by Tamele lieved a t that time that this ($0). However, there does would give an unstable catanot appear to be a good lyst. The results of this inunderstanding of their strucvestigation showed how a ture. stable catalyst was prepared The most direct approach by such a method and why in studying the sol is by use earlier attempts where there of the electron microscope. was insufficient aging of the However, the ultimate parsilica gel had yielded unstable ticle size of the colloidal catalysts. silica made by mixing acid Since 1942 the use of with silicate solution is of ' 'flu i d silica-alumina gel the order of 50 A., which apcracking catalysts as well as proaches the limits of resotheir manufacture has aslution of the electron microsumed great economic imscope (20 to 40 A.). It is portance. Along with this, a possible, though, to prepare great deal of research has colloidal silica and silica gels been carried out on silicawith a larger particle size by alumina gels. This work (10, Gelatinous Silica-Alumina Slurry Is Dewatered and removing the electrolyte and 18, 13, 19, 20, $ 1 ) appears to Washed with Acidified Wash Water on Rotary Vacuum subjecting the sol to heat have fairly well established Filters to Eliminate Sodium Compounds
N 1942 the American Cyanamid Go., a t government request,
2857
INDUSTRIAL AND ENGINEERING CHEMISTRY
2858
treatment. Salt removal may be accomplished by dialysis, passage through ion exchange resins, or by forming and washing the gel and then redispersing it by heating in a closed vessel. Electron micrographs of sols prepared in the latter manner and the xerogel prepared from one of the sols by evaporation are shown in Figures 1 and 2. The sol in Figure 1A was prepared from a 4yo silica hy-
A
B
Figure 1. Electron 5licrographs of Colloidal Silica 1. B.
Made by heat treatment of washed silica gel (15 hr. at 100' C.) After further treatment (16 hr. at 250' C.)
drogel made by adding silicate to acid t o give a pH of 4.5 The EO^ was obtained by heating the gel in a closed vessel a t 100" C. During this process the p H increased from 4.5 t o 9.0. Disperhion occurred in about 3 hours, although the heat treatment was continued for 15 hours. As shown in Figure l A , the colloidal particles (about 150 A. in diameter) can be resolved though they are somewhat aggregated. Figure 1B shows the same material after additional heat treatment in a closed vessel (16 hours at 250" C.). Here particles appear to be still spheroidal hut to have grom-n to a diameter of about 650 A. Other evidence of the spheroidal nature of the colloidal silica particles is that "surface area diameters" calculated from the area of the dried sol on the assumption of spheroidal paiticle shape are in good agreement with those from micrographs (Table I).
Vol, 44, No. 12
the silica as obtained by measuring the skeletal density of the dried colloid (dried a t 100' C.) in water. This density figure as well as refractive index nieasurcments on the sol and x-ray diffraction measurements on the dried colloid lend support to the idea that the principal spacings of the ultimate particles are similar t o cristobalite but that, as suggested by TS'arrcn ( 8 3 ) for silica gel, no orderly array of regular repeating structure is PI esent. If no orderly array exists, it is reasonable t o expect that particle grox t h would take place equally Tell in any direction, thus leading to spheroidal particles. Some of the u-ray diffraction data on both silica arid silicaalumina gel are shown in Figure 3. These were obtained with a North American Phillips recording Geiger counter x-ray spectrometer using nickel filtered copper radiation. The main maxima a t sin (6'/A) = 0.13 corresponds to the principal spacing of cristobalite reported by Warren ( 2 3 ) for silica gel; but the secondary maxima a t sin (O/L) = 0.06, also obtained with other low area silica gels, does not appear to have been observed beforr. This may be because it is masked by small angle scattering \\-hen high area silica gel is used. From a formal point of view it suggests the existence of a secondary spacing about twice the primary spacing. Direct evidence that ordinary silica gel and silica-a1uinm:t catalysts have a spheroid aggrcgate structure is shown in Figure 4. These electron micrographs show colloidal silica immediatcaly prior t o gel formation and n ~ilica-aluminaaerogel derived from such a sol. The sol specimen ~ n prepared s by immediate ciilu-
A OF ELECTRON ~IICROSCOPE AND SL-RE. ICE TABLE I. COXPARISON
-4RE.4 PARTICLE DIAMETERS
Specific Area, Sq. X / G . 81 191 500 185
Sample Low area silica gel Medium area silica gel High area silica gel Steamed Si02-Ql208 catalyst 0
Diameter =
Av. Particle Diain., 1. ElectronmicroSurface areaQ graphs'~ 322 350 136 52 141
150
70
180
6 specific area X skeletal density
b Based on measurements of representative-appearing particles.
Viscosity data, as sholi-n in Table 11, on sols of this type indicate that they have viscosities close to that predicted by Einstein's equation for spherical particles:
7 = po =
C,
=
tion of a sample of sol taken a feiy seconds prior t o gelation, evaporation of a drop of the sample on a nitrocellulose base, and washing in situ t o leach out the sodium sulfate. The colloid~l silica was prepared by addition of sulfuric acid to sodium silicate to a p H of 7.0 a t a 5.0% silica rontent a t 28" C. The silicaalumina aerogel was prepared by the addition of alum and ammonia to a silica hydrogel which was made by adding silkate to acid to a p H of 4.5 and a silica content of 5y0a t 28" C., folloxved by washing. The usual aerogel technique of displacting the water with ethyl alcohol and displacing the alcohol with diethyl ether mas then followed. The ether gel was then dried at 100° C. The aerogel, which had a specific area of 775 q u a r e mrters per gram and a pore volume of 2.5 cc. per gram, was prepared rather than the xerogel to obtain a more open structure in ortlc~
viscosity of the colloidal solution viscosity of the medium a t the same temperature, Trater volume fraction occupied by the solute, silica
If the particles had any other shape such as acicular, if they were aggregated, highly hydrated, or porous, a higher viscosity would be expected. The data obtained on several preparations are given in Table 11. The higher solids materials were concentrated by evaporation. The volume fraction was calculated assuming a density of 2.3 for
B
Figure 2. Colloidal Silica (.1) and Xerogel Derilerl from It ( B )
OF COI,I,OII~.~L SILICADISPER~IOXS TABLE 11. VISCOSITY
Grama Sample No.
Q
pH
SiOr/100
311.
Rleasured by pipet drainage t i m e
C ? . Vol. c7C Sios
,,P, Viscosit9 Relative
to Kater
Ilr
->:.-
Cv
December 1952
INDUSTRIAL AND ENGINEERING CHEMISTRY
better t o resolve the ultimate particles. Typical spraydried silica-alumina c a t a l y s t (pore volume = 0.75 cc. per gram, specific area = 600 Cried Silia-Alumina Gel W c i t r oreo. 5\Om!/g. square meters per gram) before and after steaming is shown in Figure 5. B e c a u s e of t h e dense structure of the fresh catalyst, i t is only possible t o resolve the ultiDried Silica Gel Specific area*800nP/g mate particles a t t h e edges of the large aggregates. The particles appear much larger in the steamed catalyst, indicating considerable IC A from Brapa Equation growth has oc2 0 I IO 5 14 ,3 I curred. T h i i s cor0 ' 1 I f 1 roborated by the SIN e/x surface area calFigure 3. X-Ray Diffraction culated diameter Patterns which is in agreement with what appears t o be the new ultimate particles, as seen from Table I. Chemical Properties. The composition of silica gel includes silicic acid groups on the surface of the colloidal particles as well as silica. These groups react with the hydrolyzing alumina when a base is added to an aluminum-salt-silica gel system according to the views of Tamele ($0). There are several approaches t o the measurement of such groups, including titration and water loss on heating. It should also be possible t o calculate the number of silicic acid groupe from the surface area by use of the equation:
%\
i
2859
silicic acid group is equal t o t h e molecular volume of silica t o the "3 power. Titration offers some difficulty because silicic acid is a very weak solid acid. However, it is believed t h a t the alkali requirement in going from p H = 5 to 10.5-less t h a t required for water under fixed conditions of silica, salt content, and temperaturedoes give a proportional measure of this (see Figure 6). Silicic acid is apparently too weak a n acid to have appreciable catalytic effect for cracking, judging from the poor activity of pure silica gel (do). T o study the water content after heat treatment, two silica xerogels of different surface areas were utilized. The procedure in making the medium area gel (185 square meters per gram, silica) was similar t o that described for the sol shown in Figure 1A. However, the sol was dried a t 110" C., acid washed, water washed, and redried at 110' C. to give a salt-free xerogel. The high area silica gel was prepared a t a pH of 5.0, a silica content of 5%, and a temperature of 30" C. The gel, after 30 minutes aging, was washed with dilute acid and water t o render it salt free. D a t a on water content as determined by weight losses after heat treatment for 16 hours a t t h e indicated temperatures are given in Figures 7 and 8. The samples were heated successively a t increasingly higher temperatures up t o 1000' C. Earlier experience indicated t h a t losses above this temperature were negligible. It appears that a t 110" C. the water content corresponds approxi-
.
BIeq./gram = loi I\T-1/3 V 7 , , - 2 /S3 = 0.1355 or Meq./100 square meters = 1.35 (2) A; = Avogadro's number 60 V,, = molar volume of silica = 2.3 S = specific area in square meters per gram (ignited basis) hIeq./gram = milliequivalents per gram (ignited basis) This equation assumes t h a t every silicon atom a t the surface is attached t o one OH group t o give silicic acid so t h a t the area per
A
Figure 5. '4.
R.
B
Spray-Dried Silica-Alumina Catalyst
Fresh commercial catalyst After steaming (120 hr., 560' C., 15 Ib./sq. in. gage)
mately t o the theoretical value for surface silicic acid. At higher temperatures up to 590" C. the water is gradually lost without a corresponding loss in surface. To test the reversibility of water uptake, part of the high area silica gel which had been heated t o 590" C. was rehydrated by soaking in water for I hour. T h e process was then repeated as shown in Figures 7 and 8. It appears t h a t nearly all the irreversibility can be attributed t o surface area loss. Table I11 compares the silicic acid groups as determined by the different methods and gives t h e corresponding weight per cent alumina if a n equivalent quantity of alumina were added. Discontinuities in skeletal density and base exchange capacity with increasing alumina content a t about 30% alumina reported by Milliken, Mills, and Oblad ( I $ ) may be due t o exceeding the equivalent amount of silicic acid. SILICA-ALUiMINA CATALYST
A
Figure 4. A. B.
B
Spheroid Aggregate Structures
Colloidal silica prior t o gel formation Silica-alumina aerogel from similar colloidal silica
Preparation. I n order t o arrive a t the optimum conditions for catalyst manufacture, a study was made of the effect of temperature, silica content, p H , and salt content during silica gel formation and aging on final catalyst properties. The effect of time and temperature during and after alumina addition prior t o washing was also investigated but was found t o be of little importance. This might be explained on the basis t h a t the added
2860
INDUSTRIAL AND ENGINEERING CHEMISTRY
JTt
1
-xMEASURE OF ACID GROUPSx\
'f
g 2Wcc. F SILICA ( bGEL ESLURRY D
N
I
Z
20000.BOILED WATER E D
SURFACE AREA A N D POREVOLUME. Surface area and pore volunie measurements were made using nitrogen adsorption methods described previously (11). Area results are reported on a n ignited ( l O O O o C.) basis and 15.4 sq. A./nitrogen molecule cross section. PARTICLE SIZE DISTRIBUTIOS. This was determined by a combination of screening and "hydrometer" sedimentation ( 3 ) . ATTRITIONRESISTAKCE. The attrition resistance was measured using a n apparatus vhich will be described in detail in a later publication. It was similar t o t h a t described by Forsythe and HertTyig (9) in that the attrition was brought about by a high velocity air stream entering a catalyst bed through a inch diameter hole. However, three holes were used with the same total air flow (15 cubic feet per hour). The fines formation was measured by the amount passing overhead out of a 5-inch diameter upper tube. With the indicated air flow and the density of fresh silica-alumina catalyst, material under about 16 niicrons in diameter Tvill pass overhead according to Stokes' law. A size fraction (140 to 200 mesh) was used for these tests. Fiftygram samples were employed. CATALYST ACTIYITY BY U.O.P. RIETHOD. The standard conditions under which these activity tests vc-ere made are as follows:
e
1412
108
6
4
2
0
2
4
6
8
IO
CC. 1.2 N NaOH CC. 1.2 N HC1
Figure 6 . Typical Titration Curve for Washed Silica Hydrogel
alumina by reacting with the silicic acid, blocks further condensation of the silicic acid and thereby acts to stabilize the hydrogel.
1. Sulfuric acid (6 N ) was added over a 30-minute period to sodium silicate (Philadelphia Quartz Co. N brand) solution of a concentration to give the indicated silica content. 2. The mixer consisted of a 16-gallon kettle with a n 8-mesh screen-shrouded turbine agitator. 3. The silica gel slurry was aged as indicated with slow agitation prior to the addition of a 33% alum solution equivalent to a 10% alumina content in the finished catalyst. The p H of the mixture was raised to 4.6 with 14% ammonia solution, added slowly over a 30-minute period. 4. Filtration and washing (with deionized water) were carried out on 15-inch filter crocks to a sodium content less than 0.005% on a dry basis. 5. Drying and heat treatment of the washed gel were carried out i n a n indirect gas-fired American Gas furnace (8 X 41 inches). About 8500 grams of wet cake was fed into the kiln in a 20-minute period in the form of 11/4-inch pieces. T h e temperature of the tube was controlled so as to raise the temperature to 700' C. in about 60 minutes. 6. The calcined catalyst was stage ground with a Quaker City mill to give the following particle size distribution: Mesh size +40 40-100 Per cent of sample 0.0 40-50
100-200 20-30
200-300 10-14
-300 10-20
Test Methods. BULK DEXSITY. Direct measurement of porosity was made only in some cases. Instead, the apparent bulk density was determined by a method developed by the Universal Oil Products Co. This is believed to reflect accurately the porosity of ground catalyst when the particle size is controlled and the amount of fines is not large enough for electrostatic effects t o be important (see Figure 9). The theoretical relation between porosity and apparent bulk density is: A.B.D.
=
1
Vol. 44, No. 12
(3)
,4.B.D. = apparent bulk density, grams per cc. d, = skeletal density = 2.3 grams per cc. V , = pore volume, cc. per gram f = a packing factor which is greater than 1 and is determined by the interparticle void space which is a function of particle size distribution and shape under conditions of test. -4value of 2 which corresponds to void volume being equal to particle volume was found to give good agreement with ground silica-alumina gel. The apparent bulk density is measured by slowly pouring the catalyst powder minute) through a funnel (150-mm. analytical funnel, cut off t o give 1-inch stem length) into a 25-ml. graduated cylinder (cut off a t the 25-ml. mark), and then immediately scraping off the excess with a straightedge, taking care not to shake or t a p the graduate. The filled graduate is then weighed to an accuracy of 1 0 . 0 5 gram.
Catalyst volume, ml. Catalyst temp., C. Space velocity, vol./vol./hour Process period, hours Charging stock
25 500 4 2 Mid-continent gas oil
O
The weight of gas plus gasoline (liquid distilled over, up to 204" C. during an Engler distillation) divided by the recovery gives the per cent conversion. This is translated into relative volume activity by comparing the space velocity a t which this conversion is realized x i t h a standard catalyst. Weight activity was obtained from the volume activity by use of the following relation: Keight activity
=
A.B.D. of standard = 0.50 . X volume activity A.B.D. of sample
THERMAL STABILITY. This was determined by measuring the U.O.P. activity after a 6-hour thermal treatment in a temperature controlled muffle a t 900" C. T o maintain a constant enough temperature in the catalyst (zk 3' C.) for reasonably reproducible results it was found necessary carefully t o position the sample trays 1inch from the muffle floor in a central portion of the muffle, t o employ enough "fixed heat" almost t o maintain the temperature, and to use about 5 inches of insulation around the muffle as well as magnesia brick in the front.
TABLE111. SILICIC ACID GROUPSOK SILICA GELS.4ND EQrIVBLEST AkLUJIINA
Specific Area of Silica Gel, Sq. M./G. SiOn, after Drying a t 1100 C. 800 185 81
Meq. H/G. SiOa Calcd. from Titration, specific pH 5 to 10.5 area 10.7 2.2 2.5
1.1
0.6
0.25
Water Con- Equiv. Wt., tent after 16 % Aluminaa Hours, on SlOz110" C. A1203 Basis 8.2 36 2.7 1;
...
Assuming the reaction of silicic acid with alumina is such t h a t one aluminum group reacts with each silicic acid group a n d using the surface area values for meq. hydrogen per gram silica. Possibly the surface reactions can be written as given below in the absence of ions which might exchangewiththe OH-orthe". a
1
-SiOH
I
+ 4 l + + + + 30H- +
T h a t exchange may take place is evidenced by the fact that. it is necessary t o wash silica-alumina hydrogel a t low pH to remove sodium completely b y a washing process, while i t is necessary to wash a t a pH > 5 to reduce the sulfate content below about 2% (dry basis).
December 1952
2861
INDUSTRIAL AND ENGINEERING CHEMISTRY
STEAMSTABILITY.Two types of steam treatment were utilized. I n one method the catalyst was charged to a tube which was inserted in a temperature controlled lead bath. Steam a t 15 pounds per square inch gage was then continuously bled through the catalyst bed. The temperature of the catalyst during steaming as measured by a central thermowell was maintained a t 565" i2" C . by this method. I n the other method, the catalyst was charged to catalyst tubes in the temperature controlled bronze block used for U.O.P. activity measurement and steamed for 7 hours a t 600" f 4" C. a t 15 pounds per square inch absolute. The U.O.P. activity was then determined.
1
5
$
6
7
8
1 0
9
I
P O R E VOLUME, CC/G.
Figure 9. Apparent Bulk Density of Ground Catalyst of Varying Porosity
INTERPRETATION OF POROSITY MEASUREMENTS
The variation of porosity of the catalyst with aging time and temperature of the silica gel is shown in Figure 10. The effect of temperature of gelation and silica content on porosity is shown in Figure 11. Figure 12 shows the influence of pH. These data may be compared with those of Plank (14,15) for coprecipitated silica-alumina gel. The effect of aging is also covered in a patent (6). The results indicate that porosity increases with :
1. Increasing time and temperature of aging of the silica gel 2 . Increasing silica content during silica gel formation in the range 3 to 7% silica 3. Higher pH of aging in the range of pH 5.5 to 7.5 All these changes favor reaction between silicic acid and/or silicate ions on adjacent particles of the hydrogel so as t o give a more rigid shrinkage-resistant structure. Decreased shrinkage during drying would lead to the higher observed porosities. During silica gel aging it was observed that the p H increases as much as several p H units This was also observed by Plank ( 1 4 ) and is evidence that reaction between silicic acid groups is occurring since the reaction can be written:
1 1
-SiO-
I I + HOSiI +-Si-OSi-
I
I
+ OH-
I
The time required for gel formation in the p H range 2 t o 7.5 is shorter the higher the p H but increases a t p H values greater I
I
I
I
I
I
1
,
I
I
10
15
I
SILICA G E L AGING T I M E , HOURS
Figure 10.
Increase of Porosity with Temperature and Aging Time S.Op/. Si02 content of silica mix Sihca gel aging pH 6.5
than 7.5. This might be explained on the basis that both silicate ion and silicic acid are necessary for the above reaction t o occur. At very high pH and e large degree of ionization, electrostatic repulsive forces would be expected t o slow down the reaction. Porosity was also found to be influenced by other factors. When silicate and acid were added simultaneously so as to give a fixed pH close t o 7 without removing gel as it formed, a final product of extremely high final porosity was obtained (Table IV).
;kL 1 HIGH AREA GEL
TABLE IV. EFFECTOF MISCELLANEOUS VARIABLES ON POROSITY"
I ST DEHVDR4TIDN
2ND DEHVDRATIDN
MEDIUM AREA GE
0
5
100
200
300
400
500
600
ASSUMED
700
800
900
1000
T E M P E R A T U R E , "C.
Figure 7.
Water Content of Silica Gels
0
Method Pore Volume, Cc./G. Acid t o silicate addition 0.60 Simultaneous addition 1.15 Kiln drying 0.60 Spray drying 0.75 4.3% NazS04 added during gelation 0.52 2.5% NazS04 added during gelation 0.42 Other conditions were the same for samples compared.
18
$-E
16
~6
14 I2
08
*c p
IO
4H
4
k=
2
X-MEDIUM &REA SILICA CEL
T E M P E R A T U R E , 'C.
Figure 8. Water Content of Silica Gels after Heat Treatment, Calculated on Equal Surface Area Basis
The hydrogel was quite rigid and felt rather grainy, and the filter cake had a high solids content. These observations can be explained on the basis of penetration of silicate and acid into the previously formed silica hydrogel. Subsequent reaction would serve t o reinforce the original structure so as to make it more resistant to shrinkage. Very rapid dehydration of the silica-alumina hydrogen as is realized in a spray drying operation ( 2 )was found t o give a somewhat greater porosity than obtained by kiln drying (Table IV). Possibly this can be attributed to lack of time during dehydration for the ultimate particles to become rearranged into so compact a structure.
2862
INDUSTRIAL AND ENGINEERING CHEMISTRY
I
I
10 eo 3 . 5 40 50 T E M P E R A T U R E O F G E L A T I O N A N D AGIKG, OC.
Figure 11. Increase of Porosity w-ith Temperature and Silica Content of Silica Mix Silica gel aging time 2 h r . ; pH 6.5
Electrolyte concentration during gel formation also appears t o affect the porosity (Table IV). Additional sodium sulfate was added other than t h a t derived from the silicate and sulfuric acid to obtain a weight concentration of 4.3% sodium sulfate which is equal t o that obtained with 6% silica rather than the actual 3.5% silica. The2.5% sodium sulfate catalyst was obd tained without addition .50 of sodium sulfate 17-ith the same silica concentration (3.5%). Aging 2 80 time of theand silicatemperature gels were 51
: : ; :
2
* 2
hours at 27" C. The 30 -" effect of electrolyte can " A G I K G pH. possibly be attributed to the salt lowering the reFigure 12. Increase of p u 1 s i v e electrostatic Porosily with Gel Aging pH forces(zeta potentia1)be2.5% s i o z of silica nlix Silica gel aging time 2 hr.; tempertween silicate groups and ature 27' C. thus catalyzing the reaction indicated above. This would lead t o increased cementing of the gel structure on increasing electrolyte concentration, n-ith resultant greater shrinkage resistance and xerogel porosity. Catalyst Activity and Stability. Examination of initial U.O.P. weight activity data indicated only minor variation with change in silica hydrogel solids content, aging time, or temperature ( S o t o 30' C.). (There is evidence t h a t large increases in p H (3 4 7) and temperature of gelation result in decreased initial surface area.) However, the catalyst thermal stability was markedly affected, and i t appeared t h a t the variation could be fairly well expressed as a function of porosity regardless of preparative variables in the ranges investigated, as shown in Figure 13. The thermal stability appears t o increase markedly with increasing porosity a t least up t o a pore volume of 0.7 cubic centimeters per gram. This parallels the findings of Van Nordstrom, Kreger, and Ries ( 2 2 ) on silica gel. Steam stability was not determined on these samples. However, for control purposes, U.O.P. activity and bulk density were measured on a routine basis on silica-alumina (10 to 13% alumina) production samples. Pore volumes m-ere determined only in a few cases. The averages of these data (70 points) on kiln-dried, ground catalyst expressed in terms of pore volume are plotted in Figure 14. As there was appreciable variation in particle size distribution, the bulk density can only be considered as a fair indicator of porosity, As might be expected, the plot showed considerable scatter. The average deviation from the curve is 11/2
Vol. 44, No. 12
5.5 activity units. This compares with a n average deviation in check determinations of about 4 activity unitfi. The average deviation from the mean in check A.B.D. determinations on samples containing appreciable -200 mesh material is about 0.015 gram per cubic centimeter, corresponding to about 0.025 cubic centimeters per gram. The large scatter is probably greater than can be attributed t o particle size variations or to experimental error and thus may imply t h a t the steam stability of this type catalyst cannot be uniquely expressed as a function of porosity. Deviations from the avwage stability-porosity relationship would be expected t o shed light on other factors influencing catalyst stability. Other steam stabilitj- data in terms of specific surface and porosity after steaming (about 0.1 cubic centimeter per gram less than initial pore volume) are given in Figure 15. D a t a presented by Polack, Segura, and Walden ( 1 6 ) also appear to indicate t h a t increasing porosity in the range 0.4 to 0.8 cc. per gram increases the steam stability. Increased stability with increasing porosity might be attributed to the increasing average distance between ultimate particles making it more difficult for mergers or other transport processes to occur. Typical data on the effect of steam and thermal treatment and use on silica-alumina catalyst are given in Table V. These are in substantial agreement with similar data reported by Van Nordstrom, Kreger, and Ries (22).
TABLE I-, TYPICAL S U R F A C E A R E 4 8 BEFORE 9x1) AFTER THERJIAI. STEAM TREATMENT AFD USE Specific Area, 3q. hI., G.
Specific P o w Volriine, Cc./G.
600
0.73
185
0.60
190 100
0.28
Fresh
Steamed (350 hr., 565'' C . , 15 1b.isq. in. gage)
Thermal treatment (6 hr., 900' C.) Equilibrium, used
0.30
From the data of Table V and the electron mirrographs (Figure 5), it seems probable that thermal treatment brings about a normal sintering such as would occur on heating packed metal spheres t o near the melting point of the metal, whereas steaming brings about a growth in ultimate particle size so t h a t an approximate spheroid aggregate structure is maintained. The growth of colloidal silica on heating (Figure 1) may be cited as another example of such a process. The effect of use appears to be intermediate and varies considerably with different cracking units. The pore volume or volume/area ratio probably gives an indication of the relative importance of steam and thermal deactivation in the unit in question. Attrition Resistance. T o see if porosity affected attrition resistance, samples of kiln-dried ground catalyst samples of varying
6 4
.5 .5
.e
7
P O R E VOLUME, CC./G.
Figure 13.
Increase of Thermal Stability w-ith Porosity
December 1952
INDUSTRIAL AND ENGINEERING CHEMISTRY
porosity (140 to 200 mesh) were tested for attrition resistance. The data are given in Figure 16. The results, as might be anticipated, indicate decreased attrition resistance with increasing porosity. On the other hand, it would be expected that the erosive action of the catalyst on equipment would decrease with increasing porosity, and qualitative indications are in this direction. Ease of Regeneration. This laboratory has not made a study of ease of regeneration because burning rates are generally satisfactory on fluid silica-alumina catalyst. However, it is reasonable t o expect t h a t large pores would favor increased ease of regeneration since the interdiffusion of oxides of carbon and oxygen is required. Agafonov and Kaliko ( 1 ) reported this t o be true. Webb ($4) concluded t h a t the regeneration difficulties experienced with fluid silica-magnesia catalyst were to a considerable degree due t o its fine pore structure. Selectivity. There is reason t o believe that large hydrocarbon molecules would have difficulty in penetrating very small pores and t h a t consequently selective cracking of lower hydrocarbons would occur with low pore diameter catalysts leading to greater gas yields. Such observations have been reported by Agafonov and Kaliko (1), Bench scale $3 4 testing by this s2 laboratory on 5 6 7 PORE VOLUME, CC./G. fresh and steamed cataFigure 14. Increase of Steam Stability with Porosity lyst with a test unit and test Weight activity after steaming 7 hr., 600’ C., 15 lb./sq. in. gage conditions similar to those described by Birkhimer, Macuga, and Leum (6)-except that the catalyst was tested in powder instead of pellet formfailed t o show appreciable differences in gas or coke yields for a given conversion level in the porosity range 0.58 t o 0.95 cc. per gram. However, it is reasonable to expect that differences would show up on fresh catalyst a t porosities lower than 0.5 cc. per gram.
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Figure 16. Attrition Resistance of Fresh Commercial Ground Silica-Alumina Catalyst of Varying Porosity Size fraction 140 to 200 m e s h ACKNOWLEDGMENT
Many members of the Stamford Research Laboratories contributed t o this project. The electron microscopy was done by F. G. Rowe, Martin Botty, and E. G. Davis. The x-ray diffraction studies were made by L. A. Siege]. Credit for catalyst testing and reparation is largely due G. Davis, J. Thomas, E. Paul, and R. aolmstead. Others assisting in this investigation include R. Wetherbee and.C. Nichols. The authors are grateful to the American Cyanamid Co. for permission t o publish this work. LITERATURE CITED
(1) Agafonov, A. V., and Kaliko, M. A., Zhur. Obshchel. Khim., 19, 39 (1949); or Chem. Abstracts, 43, 6336 (1949).
Ashley, K. D., U.S. Patent 2,555,282 (May 1951); Brit. Patent 644,322 (Oct. 1950). (3) Ashley, K. D., and Innes, W. B., Proc. Am. Petroleum Inst., 27,
(2)
I11 (1947). (4)
Ashley, K. D., and Jaeger, A. O., U. S. Patent 2,411,820 (Nov. 1946).
(5) Ashley, K. D., and Jaeger, A. O., Ibid., 2,478,519 (Aug. 1949). (6) Birkhimer, E. R., Macuga, S. J., and Leum, L. N., Proc. Am. Petroleum Inst., 27, I11 (1947). (7) Chem. Eng., 58, No. 11,218 (1951). (8) Ephraim, F., “Inorganic Chemistry,” ed. by P. C. Thorne and E. R. Roberts, 4th ed., New York, Nordeman Publishing Co., (1943). (9) Forsythe, W., Jr., and Hertwig, W. R., IND.ENG.CHEM.,41, 1200 (1949).
(10) Hansford, R. C., Ibid., 39,849 (1947). (11) Innes, W. B., Anal. Chem., 23, 759 (1951). (12) Milliken, T. H., Jr., Mills, G. A,, and Oblad, A. G., TTans. Faraday SOC.,1950,279. (13) Mills, G. A., Boedeker, E. R., and Oblad, A. G., J . Am. Chem.
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Figure 15. Steam Stability Determined by Surface Area IMeasurernent 0 Before steaming
0 After steaming
Optimum Porosity. Consideration of all factors seems to indicate t h a t the optimum porosity for this type of catalyst ranges from 0.6 to 0.9 cubic centimeters per gram, depending on the efficiency of dust collection, etc. Lower porosities give relatively poor stability and higher porosities mag lead t o excessive attrition losses.
Soc., 72,1554 (1950).
Plank, C. J., J . ColEoid Sci., 2, 413 (1947). Plank, C. J., and Drake, L. C., Ibid., 2, 299 (1947). Polack, J. A., Segura, M. A,, and Walden, G . H., paper presented a t the 120th Meeting of the AMERICAN CHEMICAL SOCIETY, New York, N. Y., Sept. 1951. (17) Ritter, H. L., and Erioh, L. C., Anal. Chem., 20, 665 (1948). (18) Shull, C. G., Elkin, P. B., and Roess, L. C., J . Am. Chem. SOC.,
(14) (15) (16)
70,1410 (1948).“
Tamele, M. W., Chemical Architecture,” p. 175, New York, Interscience Publishers, 1948. (20) Tamele, M. W., Trans. Faraday Xoc., 1950, 270. ENQ.CHEM.,41, 2564 (1949). (21) Thomas, C. L., IND. (22) Van Nordstrom, R. A., Kreger, W. E., and Ries, H. E., J . Phys. (19)
& CoZloidChem.,55,621 (1951).
Warren, B. E., J . Applied Phys., 8 , 645 (1937). Webb, G. M., paper presented at the annual meeting of the Am. Inst. Chem. Engrs., Kansas City, Kans., May 1951. R E C E I V ~for D review March 12, 1952. ACCEPTED August 15, 1952. Presented before the Division of Colloid Chemistry, Symposium on Colloid Chemical Properties of Catalysts, atc the 120th Meeting of the AMERICAN (23) (24)
SOCIETY, New York, N. Y . CHEMICAL
