I
MYRON MALANCHUKI and EDWARD B. STUART Chemical Engineering Department, Engineering Research Division, University of Pittsburgh, Pittsburgh, Pa.
Effect of Heat Treatment on Silica Gel This report should aid users of silica gels to evaluate the effects of retorting on final physical properties, for better precision in catalysis or adsorption
SILICA
GEL, widely used in adsorption processes, may find application in selective adsorption. Its nature-activity and structure-is still obscure to the physical chemist. Many investigators have studied the silica gels but only recently has enough information been available to define a pattern. Extensive work by Kiselev and others (4) has helped to catalog the knowledge about activated silica. Silica gel is activated by removing water from the amorphous gel to produce a porous solid, T h e gel is heated to moderate temperatures, 150". to 400" C., as excessively high temperature, 800" to 1000' C., decreases the activity of the product. Silica gel is commonly prepared by treating a sodium silicate solution with a n acid or acid salt solution, breaking up the resulting mass of gelatinous material, washing, and then drying it. T h e usual product is a hard, glassy substance, which may contain well over 5% of water. I n controlling the preparation of silica gels to obtain particular characteristics of pore sizes and distribution, Kiselev and others studied extensively three types of gel of different structures:
A.
Fine-pored, glassy gel
B. Coarse, uniform-pored glassy gel V. Chalky gel of mixed porosity
They retorted these gels a t temperatures from 115' to 1000' C. for a much longer period (12 hours) than was used by Bartell and Almy and Milligan and Rachford. T h e plots of pore volume distribution were considerably different for the three types. T h e glassy types, A and B, were prepared by reaction of sodium silicate solution with sulfuric acid; type B gel received in addition a soaking treatment with a 0.02N ammonia solution. T h e chalky type, V, was prepared by reaction of the sodium silicate solution with copper sulfate solution. Plots of surface area, determined by the BET method (5) as a function of temperPresent address, Mellon Institute of Industrial Research, Pittsburgh 13, Pa.
ature of heating, showed a sharp change in slope a t 500" C. for the glassy gels; their areas dropped, off rapidly in the interval up to 900" C. The chalky gel, on the other hand, decreased smoothly in area in the same temperature range; this stability was attributed to the presence of the highly developed network of macropores protecting the gel structure from full sintering even a t 1000" C.
Experimental
Materials. A commercial grade of silica gel (from Davison Chemical Corp.) was of 28- to 200-mesh (U. S. standard) size and designated grade 912. The gel was a fine-pored material, specified as having an average pore diameter of 24 A. and a surface area of800 square meters per gram, glassy in appearance. I n preparation for selective adsorption experiments, the gel was activated by heating at 300' C. for 4 hours. Apparatus. T h e furnace used for retorting the silica gel samples was of a heavy box construction, electrically heated through a Powerstat control. Pyrometer readings within f 5 " C. of the set point were obtained as temperature control. T h e adsorption-desorption isotherms for the retorted gel samples were determined a t liquid nitrogen temperatures with the conventional gas adsorption equipment. Such an apparatus is illustrated and well described by Joyner (7). Procedure. From the stock supply of the grade 912 gel, a 75-gram portion was weighed out for each heat treatment, and placed in a shallow porcelain evaporating dish laid in a quartz tray.
Soon after the furnace was started heating, the tray was put in and the dish of gel positioned in the middle of the fur'nace with the thermocouple junction directly above it. T h e furnace was raised to the retorting temperature as rapidly as possible; then the power supply rheostat was turned back to give a much lower rate of heating. T h e time of retorting was measured from this point. Upon completion of heating, the sample was put in a desiccator to cool. I n the case of the sample retorted a t 926' C. (1700" F.), two portions were heated in the furnace: the 75-gram portion and a 1-gram portion spread out in the evaporating dish. Gel samples of 0.5 to 0.6 gram were used in the gas adsorption apparatus for the adsorption-desorption isotherms. Each sample was heated under a vacuum less than 1 micron for 2 hours at 200' to 210' C. before points for the isotherm's were determined. As each point was obtained, it was plotted and the isotherm curve developed; the desorption curve was followed closely to see that it fell in line with the adsorption part, as an indication that the points were valid and not out of line because of leaks or other experimental errors. At the conclusion of the desorption run, the sample bulb was pumped out under high vacuum for a t least 0.5 hour and the weight of sample was checked again. For the sample retorted at 926" C. (1700" F.), only three or four points of the adsorption curve were determined; these were used to calculate the surface area. Adsorption-desorption isotherms can be used to obtain a comprehensive pic-
Literature Background Ref.
Author
Subject
Patrick, Brazer, Rush Van Bemmelen Holmes Kistler Bqrtsll and Almy Krejci and Ott Milligan and Rachford
Adsorption capacity factors Gel porosity a result of purged capillary water Long drying period Controlled drying through organic agents in capillaries Temperature effect on gel structure and activity Crystalline aspects of silica gel Temperature effect on porosity
VOL. 50, NO. 8
AUGUST 1958
(10) (13) (6) (7) (8)
(8) (9)
1207
4 HR.
AT 300-C.
47 HR. AT 5 3 8 - C .
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0.02 46
,
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%. Figure 1. Nitrogen isotherms give a picture of the pore character of a silica gel
1 1
ture of the pore character of a gel. Isotherms were obtained for the original ge! and for portions heated from 538" to 982' C. T h e data are given in table page 1210 and plots, according to the technique described by Joyner (7). are shown in Figure 1 . Complete adsorption-desorption isotherms were determined for gel heated to each temperature except 926' C. For these only the lou pressure portion of the nitrogen adsorption isotherm was obtained. Distributions of pore volume and area with respect to pore radius Ivere computed by the method of Barrett, Joyner, and Halenda (2).
Pore Characteristics of Original Gel The silica gel (Grade 912) !vas characterized by pores having a radius of less than 15 A . Its nitrogen gas adsorption-desorption isotherm (Figure 1) is typical of a fine-pored glassy gel-a steep rise in the volume of gas adsorbed a t low relative pressures with a leveling off in the high pressure region and only a small desorption hysteresis indicative of little capillary condensation. The pore size distribution curve computed from the sorption isotherm data (Figure 2) emphasizes the relatively narrow range of pore sizes for the silica gel. T h e pores of less than 15 A. radius accounted for 70% of the total pore volume as measured by the area underneath the curve. Pores of less than 25 A.-those not contributing to the hysteresis that results from capillary condensation-account for nearly 95% of the total pore volume. Because of almost insignificant capillary condensation, surface effects predominate in this gel. 1 208
0.00 0
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- -
20 30 PORE RADIUS,r ( A )
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Figure 2. The original silica gel has a relatively narrow range of pore sizes
Change of Pore characteristics with Heat Treatment Pore Volume. T h e total pore volume of the gel decreased with increasing temperature throughout the temperature range studied. The plot of nitrogen gas adsorption by the gel, both at the low relative pressure of Pl'Po = 0.15 and a t saturation P;Po = 1 (Figure 3), showed nearly the same rate of drop in adsorption with increasing temperature of retorting. A comparison emphasizes the relatively high adsorption capacity at low relative pressures-in this case, 70% of the total pore volume as given by values at saruration. \Vhile the curve for total pore volume drops off smoothly with increasing temperature, the low pressure (PiPo = 0.15) curve is marked by a sharp drop after 875' C. A similar characterization \\,as obtained by Kiselev and others for a finepored gel: one of three types studied. Retorting times of 12 hours lvere used and methanol isotherms were developed, from which the pore characters of the gels were determined. The fine-pored gel showed an adsorptive capacity at P,/Po= 0.2 about 55% of the saturation volume, Ivhich may indicate a gel not so finely porous as the one used in this investigation. Riselev determined that a t 800' C. the silica gel was partly converted into quartz glass. The effect on pore size distribution was not discussed. Surface Area. Adsorption values from the l o ~ vpressure portion of the iso-
INDUSTRIAL AND ENGINEERING CHEMISTRY
therms were used to calculate the specific surface of the gels, by the method of Brunauer, Emmett, and Teller (5). The relation of specific surface to retorting temperature is plotted in Figure 4 for gels retorted 47 -to 49 hours -in 75gram portions; the point at 300' C. represents the original gel, dried at that temperature lor a t least 4 hours, while those at 926' C. represent two portions retorted lor 2 hours. Of these last two values, the higher is for a I-gram sample scooped out of the center of the 75-gram portion and the lower is for the 1-gram portion spread thinly in a porcelain dish during retorting. Surface areas underwent nearly the very same rate of decrease with increasing firing temperature as with the extent of gas adsorption-i.e., pore volume. These parallel rates of surface area and pore volume decrease reaffirm the predominance of fine pores wherein physical adsorption exists xvithout or with negligible capillary condensation which requires the relatively larger pores. T h e plot of surface area in relation to retorting temperature (Figure 4) reveals the same shape of curve, with the sharp drop after 875' C. as for the pore volume plot of Figure 3. T h e drop in surface area to a negligible value at 982' C. makes it appear that the gel has lost all its pores and only particle surfaces exist. Pore Size Distribution. The nitrogen gas sorption isotherms for the various temperatures of retorting indicdte that the pore size distribution re-
H E A T T R E A T M E N T O F S I L I C A GEL 800
-
600
-
4 Figure 3. Adsorption capacity of finepored glassy silica gel shows nearly thesame rate of drop with increased retorting temperature at low pressure and at saturation
#x! 8
c
5
400-
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Y (L
f
200.
200
400
600
800
1000
RETORTING TEMPERATURE, 'C.
Figure 4. Decrease of specific surface of silica g e l with increased retorting temperature parallels that of pore volume
RETORTING TEMPERATURE. *C.
mained about the same with respect to the point of maximum volume change; the initial steep slope of the isotherm did not change much with temperature. From a study of the original gel, it is known (Figure 2) that 70y0 of the total pore volume was contained in pores of less than 15 A. radius. These fine pores have little latitude to get smaller. If they became larger, the change would probably be defined by a n increase in total pore volume. This was not so in the case of this gel; rather, the total pore volume decreased. The fact that the extent of gas adsorption a t the low relative pressure of P/Po = 0.15 continued to be about 70y0 of the adsorption a t saturation when the retorting temperature was increased indicates that the same pore size distribution persisted through the range of temperatures. At 982' C., the material had crystallized and was substantially nonporous. If calculated from the experimental values of pore volume and surface area on the assumption of Barrett, Joyner, and Halenda ( 2 ) that the pores of the adsorbent are cylindrical : Av. r p = 2 V / A
values of 12.26, 12.05, 11.41, and 11.14 A. are obtained for the average pore radius of the gels retorted at 300', 538', 760°, and 860" C., respectively. Thus, the pore size distribution plot of Figure 2 is typical for the retorted gels, and only the height of the peak (AV/Ar) changes with temperature of retorting. Seybert ( 7 7) suggested that heat treat-
ment would reduce the pore size and surface area of a glassy silica gel (Davison grade 7C). Because the original gel was finely porods, its average pore size could not be reduced significantly at a temperature below that where the rate of crystallization became appreciable. There was indication of a slight reduction in pore size, however. Milligan and Rachford, on the other hand, found no appreciable change in pore size distribution with increasing temperature, which could be the case if the time of heating were short. Retorting Time. I n many cases a heating period like 2 hours may not give uniform treatment of the particles throughout the layer of gel. Temperature equilibrium conditions may not be reached when samples of 20 grams and more are retorted. I n much of the work on the effect of heat treatment of silica gel on its pore structure, Patrick, Frazer, and Rush (70), Bartell and Almy (3) and Milligan and Rachford (Y), used a relatively short heating period, usually no more than 2 hours. That temperature equilibrium may not be reached within 2 hours is evident from the two different gels resulting from heat treatment at 926' C. (1700' F.) and the sirme heating time. The 1-gram portion spread out would reach furnace temperature very quickly. The center of the 75gram portion did not reach that temperature, certainly not for the same 2-hour period of heating. Not only does the depth of silica gel insulate the inner grains of gel from the furnace tempera-
ture, but the pore structure of each grain helps build up the insulating shield. Kiselev and others attributed the thermostability of their Type V gel to the existence of a highly developed system of macropores that protected the structure of that gel from the sintering action at high temperatures. The two portions of gel fired at 926' C. (1700' F.) possessed different surface areas after heat treatment. As anticipated, the sample of gel scooped out of the center of the 75gram portion showed less of a change in specific surface than the I-gram portion fully exposed to the furnace heat. Their difference in gas adsorption capacity is even more striking (see data). I t might be supposed that change in pore volume or surface area of a gel with the temperature of retorting would have time of retorting as a parameter. T h e evidence about the small and large eamples retorted at 1700' F. shows that time is a factor as inherent in the rate of heat transfer through the mass of gel. Consequently, porosity of the gel, bulk density, and extent of surface exposed to heat or depth of gel must also be considered. T o say that a gel's pore volume would be some specific quantity after retorting at a particular temperature, one must allow sufficient time for the entire mass of gel to reach an equilibrium temperature which would be maintained throughout the gel. Only thus could each particle be exposed to the same intensive force such that a gel of uniform characteristics would be obtained. Thus, because of the high insulation properties of some silica gels, adequate time of heating must be provided for the quantity of gel being retorted when the effect of heat treatment on the pore structure is studied. VOL. 50, NO, 8
AUGUST 1958
1209
Nitrogen Adsorption-Desorption Isotherm Values a t N2
Relative
Adsorbed Calcd.,
- 195” C. for Glassy Silica Gel
cc./g.
Relative Pressure,
(STPI
P/Po
0.5850-Gram Sample, Dried 4 Hours at
Desorption step
300‘ C .
0.8960 0.7548 0.6156 0.5010 0.4387 0.3296 0.2407 0.1891 0.1045 0,0678 0.0331 0.0197
Adsorption step 0.0051 0.1225 0.2623 0.3286 0.4275 0.5460 0.6853 0.8176 0.9221 0.976
57.6 105.3 125.9 134.0 144.8 154.2 157.9 159.8 160.7 161.2
98.5 180.0 215.3 229.0 247.5 263.7 270.0 273.1 274.8 275.6
155.7 154.0 143.9 134.0 122.0 106.6 100.1 88.9
266.2 263.3 245.8 228.8 208.3 182.2 171.0 151.8
Desorption step 0.5794 0.4986 0.4136 0.3238 0.2300 0.1315 0.0959 0.0492
0.6291-Gram Sample, Retorted 47 Hours at 538‘ C
64.2 65.6 102.3 110.5 118.1 132.1 140.9 147.3 151.5 154.0 156.2 157.0 157.7
102.1 104.25 162.7 175.7 187.8 209.9 224.0 234.1 240.8 244.7 248.3 249.6 230.8
0.0813 0.1348 0.1806 0.2199 0.2526 0,3240 0.4410 0.6446 0.8387 0.9888
154.7 153.2 151.6 143.9 134.6 125.9 120.1 108.5 101.0 95.1 90.5 84.1 79.5
246.0 243.6 241.0 228.8 214.0 200.0 190.9 172.4 160.5 151.2 143.9 133.6 126.3
0.5205-Gram Sample, Retorted 49 Hours at 780’ C. Adsorption step 0.0480 0.0848 0.1902 0.2450 0.3246 0.4505 0.5952 0.7134 0.9167 0.9860
56.6 63.3 76.1 81.1 87.7 95.5 101.0 101.8 103.2 103.5
108.9 121.9 146.3 156.1 168.7 183.6 194.4 195.7 198.5 199.1
0.9307 0.7725 0.6290 0.5010 0.4327 0.3700 0.3118 0.2286 0.1752 0.1132 0.0548 0.0355 0.0245
(STP)
103.2 102.6 101.8 100.6 97.7 88.7 80.4 75.5 66.3 60.8 53.2 48.5
198.5 197.3 195.8 193.4 188.0 170.5 154.6 145.2 127.5 117.0 102.3 93.2
56.5 61.8 65.4 68.2 70.4 74.7 80.0 83.9 85.1 85.8
106.3 116.3 123.1 128.3 132.5 140.5 150.5 157.8 160.1 161.4
85.6 85.0 84.4 83.4 81.4 77.3 74.1 68.9 65.1 60.0 53.5 50.3 48.1
161.1 159.9 158.8 156.9 153.2 145.4 139.4 129.6 122.5 112.9 100.7 94.6 90.5
ax
0.5771-Gram Sample, Retorted 48 Hours at 982’ C. Adsorption step 0.0757 0.1147 0.1660 0.2365 0.4060 0.6465 0.9712
2.70 3.11 3.47 3.90 4.69 5.61 6.60
4.68 5.39 6.01 6.76 8.13 9.72 11.44
5.21 4.71 4.42 3.96 3.20 2.93 2.79 2.07
9.02 8.17 7.66 6.87
Desorption step 0.4647 0.3874 0.3156 0.1977 0.1157 0.0737 0.0527 0,0050
5.55 5.07 4.84 3.59
0.5411-Gram Sample”, Retorted 2 Hours at 926’ C. Adsorption step 0.01377 0.1802 0.2447
23.18 25.98 30.57
42.86 48.04 56.52
0.5506-Gram Sampleb, Retorted 2 Hours at 926’ C. Adsorption step 0.0540 0.0902 0.1202 0.1898
Taken from 1-gram portion spread thinly in furnace. portion piled together in furnace.
1 2 10
cc./g.
(STP)
_
Desorption step
Desorption step 0.5873 0.5004 0.4593 0.3984 0.3083 0.2353 0.1935 0.1207 0.0827 0.0595 0.0441 0.0270 0.0188
Calcd.,
Cc.
0.5315-Gram Sample, Retorted 48 Hours 860‘ C. Adsorption step
Adsorption step 0.00643 0.00717 0.0932 0.1372 0.1848 0.2922 0.3752 0.4525 0.5335 0.6215 0.7860 0.8770 0.9830
Adsorbed ~ Nq _ _ _
INDUSTRIAL AND ENGINEERING CHEMISTRY
33.24 36.74 39.17 43.86
60.40 66.74 71.15 79.66
’Scooped from center of 75-gram bulk
Conclusions Modification of the pore characteristics of silica gel by retorting has been investigated with the aim of suiting the gel to a process of selective adsorption. The control over production of a gel with characteristics to suit specifications seems possible. Temperature of retorting represents one method of achieving some control. For a fine-pored, glassy silica gel, increasing temperature decreases total pore volume and surface area but changes pore size distribution only slightly. This is in agreement Lvith the results of Milligan and Rachford ( 9 ) . The fine-pored gel loses its porosity at abour: 1000° C. Retorting periods of much more than 2 hours are necessary to attain equilibrium temperature conditions for silica gel samples of 20 grams or more. The periods vary with the grade of gel treated. Past investigators limited the retorting time to 2 hours or less, excepi that Kiselev and others used 12 hours. The pore structure of the gel may effect a strong insulating barrier against the heating treatment, in which case a considerable time of retorting is required to attain the desired temperature uniformly distributed through the silica gel. Only if such a temperature condition is attained will there be produced a material of uniform pore characteristics from one particle to the next.
References Barr, W.E., Anhorn, V. J., “Scientific and Industrial Glass Blowing and Laboratory Techniques,” p. 257, Instruments Publishing. Co., Pittsburgh, 1949. Barrett, E. P., Joyner, L. G., Halenda, P. P., J . Am. Chem. Sac. 73, 373 (1951). Bartell, F. E., Almy, E. G., J . Phys. Chem. 36, 475 (1932). Boreskov, G. K., Borisova, M. S., Dzisko, V. A , Kiselev, .‘i. V., Likacheva, 0. A , , Morokhovets, T. N., Doklady Akad. ?\‘auk. S.S.S.R. 62, 649-52 (1948). Brunauer, S., Emmett, P. H., Teller, E., J . Am. Chem. Sac. 60, 309 (1938). Holmes, H. N., Anderson, J. A., IND. ENG.CHEM.17, 280 (1925). Kistler, S. S., Nature 127, 741 (1931). Krejci, L. E., Ott, E., J . Phys. Chem. 35, 2061 (1931). Milligan, W. O., Rachford, H.H., Jr., J . Phys. Colloid Chem. 51, 333 (1947). Patrick, W. A,, Frazer, J. C. W., Rush, R. I., J . Phys. Chem. 31, 1511 (1927). Seybert, E. K., Davison Chemical Gorp., private correspondence. Thomson, W., Phil. Mag. (4) 42, 448 (1871). Van Bemmelen, J. M., “Die Xbsorption Gesammelte Abhandlungen uber Kolloide und Absorption,” p. 210, Theodor Steinkopff, Dresden, 1910. RECEIVED for review March 27, 1957 ACCEPTEDFebruary 24, 1958
