Nitrogen Adsorption on Chemically Modified James B. Sorrel11 and Robert Rowan, Jr. New Mexico State University, Las Cruces, N. Ma Studies have been made of silica gels modified by chemical reaction of an alcohol or a chlorosilane with the surface hydroxyls of the gel. The over-all objective was correlation of the physical properties of such preparations with their behavior as adsorbents in gassolid and other types of chromatography. The present paper presents the results of the studies of physical characteristics which can be deduced from nitrogen adsorption isotherms. These include surface area, average heats of adsorption, and site energy distribution. In substantially ail cases, surface properties were changed by the chemical modification. The concentration of high-energy adsorption sites was generally reduced, and this resulted in a more uniform surface, superior for chromatography. The average adsorption energy was also reduced.
IT IS GENERALLY accepted that exposed hydroxyl groups are present on the surface of a silica gel, distributed somewhat irregularly ; the surface is thus physically and chemically nonuniform. The small radius and acidity of the hydrogen atoms on the surface hydroxyls results in pronounced specific interaction with certain molecules (1-6). There exists a possibility of making a more uniform silica gel surface by substitution of more inert groups for the surface hydroxyls. This substitution should reduce the adsorption of molecules containing either x electrons, OW, or =NH groups. Two substitution reactions of this type have been investigated in the literature. These reactions may be written as:
I
-SOH
I
+ CiSiR,
1 4
surface chlorosilane
I
-SOH
I surface
-SiOSiRa
!
+ HC1
(1)
modified surface
1 + ROH -+ -SiOR + HzO
I
(2)
alcohol modified surface
where R represents the organic groups of the alcohol or chlorosilane. Both reactions lead to the formation of stable compounds with Si-R or Si-OR bonds on the surface (7-14). Present address, Freeport Kaolin Co., Gordon, Ga. (1) A. V. Kiselev and V. I. Lygin, Cull. J . , 21, 561 (1959). (2) G. 1. Young,J. ColloidSci.: 13,67 (1958). (3) A. V. Kiselev,J. Phys. Chem. USSR,38, 1501(1964). (4) 0. M. Dzhigit, A. V. Kiselev, and G. G. Muttik, Coll. J., 23,461
(1961). (5) A. 6. Benus and A. V. Kiselev, J. Phys. Chem. U S S R , 40, 311 (1966). (6) A. V. Kiselev and A. V. Lygin, Surface Sci., 2,236 (1964). (7) A. V. Kiselev, A. Ya. Korolev, and R. S. Petrova, CON.J., 22, 669 (1960). (8) I. B. Slinyakova and I. E. Neimark, ibid., 24, 188 (1962). (9) R. K. Iler, “The Colloid Chemistry of Silica and Silicates,” Cornel1 University Press, New York, N. Y . , 1955, pp 170, 257. (10) W. Stober, G. Bauer, and K. Thomas, Justus Liebigs Ann. Chem., 604, 104 (1957). (11) C. C. Ballard, E. C . Broge, R. K. Iler, D. S. St. John, and J. R. McWhorter,J. Phys. Chem., 65,20 (1961). (12) W. Stober, KuIca(loidZ.,149,39(1956). (13) A. V. Kiselev, V. I. Lygin, and I. N. Solomonova, Coll. J., 26, 273 (1964). (14) A. V. Kiselev, in “The Structure and Properties of Porous Materials,” Butterworths, London, 1958,p 195. 1712
*
Both the chlorosilane (15-17) and the alcohol (9-11, 18-20) modifications reduce the adsorption of molecules such as water and benzene. Although a number of chemically modified silica gels have been prepared and some of their adsorptive properties determined, the effect of chemical modification on the distribution of adsorption energies has not been reported. Whalen (21) has reported that the energy distributions on silica gel reflect interactions with oxide and hydroxyl surface domains, In this work, the effect of type and degree of chemical modification on the distribution of adsorption energies is reported. EXPERIMENTAL
The starting material silica gels were Davison Chemical Company’s (W. R. Grace and Co., Baltimore, Md.) grades 12 and 62. Grade 12 has a pore volume of 0.43 cma/gramand a mesh size of 28-200 while grade 62 has a pore volume of 1.15 cm3/gram and a mesh size of 35-60. The BET surface areas of these gels, measured by the adsorption of nitrogen, were 795 and 275 m2/gram for grades 12 and 62, respectively. The starting material silica gels were treated with either an alcohol or a chlorosilane and the modified gel was dried and evacuated at 200 ‘C for 2 or more hours to remove any excess reagent and water. The amount of reaction was determined by carbon combustion analysis and is reported as micromoles reacted per square meter of available surface. The alcohols used were reagent or spectroscopic-grade and were used without further purification. The chlorosilanes were obtained from the General Electric Co. (Silicone Products Dept., Waterford, N. Y . )and were used as supplied. In general the preparations involving alcohols were made by covering a weighed amount of the silica gel with the liquid alcohol and heating the mixture to boiling for 1 or 2 hours. Some reactions were carried out at higher temperatures and pressures in a stainless steel bomb. Two different reaction conditions were used in the preparation of the gels modified with chlorosilanes. One preparation was made by evaporation of the chlorosilane vapor through a packed column of silica gel. In some cases, the silica gel column was heated with steam. The time of exposure to the chlorosilane vapors was either 1 or 2 hours. In the other preparation using chlorosilanes, a solution of the reagent in benzene was heated to boiling for 1 or 2 hours while in contact with the gel. Adsorption isotherms were obtained gravimetrically using a Cahn “Gram” electrobalance (Cahn Instrument Co., Paramount, Calif.) equipped with a vacuum bottle. The 0- to 10mg and 0- to 1-mg scales were used. The sensitivity on these scales was +lo-a and 110-4 mg, respectively. Sample weights were 80 to 150 mg. The pressure was measured on one of four gauges depending upon its magnitude. A McLeod gauge was used for the (15) I. Yu. Babkin, A. V. Kiselev, and A. Ya. Korolev, Chem. Abstr., 56,9511d(1962). (16) A. V, Kiselev, in “Gas Chromatography,” M. Van Swaay, Ed., Butterworths, London, 1962,p XXXIV. (17) A. V. Kiselev, Quart. ReG., 15,99 (1961). (18) W. K. Lowen and E. C . Broge, J . Phys. Chern., 65,16 (1961). (19) B. G. Aristov, I. Yu, Babkin, and A. V. Kiselev, Cull. J., 24, 550 (1962). (20) I. V. Dragaleva, A. V. Kiselev, A. Ya. Korolev, Yu. A. El’Tekov, ibid., p 128. (21) J. W. Whalen, J. Phys. Chem., 71, 1557 (1967).
ANALYTICAL CHEMISTRY, VOL. 42, NO. 14, DECEMBER 1970
3.00
c
-
r
% 2.00
I
amyl alcohol, I I 5 " C ) I- butanol, 107O C 1
2.00
a
u
8
N
n
BE,
-
;
3.00
N
alcohol, 1 2 7 6 )
\(n-arnyl
1.00
U
f
1.00
Lo a
I
0.50
- propanol, i18* C )
H. 0.50
(2-proponoh 79. C)
f-' Surface coverage us. reciprocal temperature
Figure 2. for grade 12 silica gels modified by various alcohols
(OK)
I
1
2.2
2.4
I
I
2.6
28
I
3.0
10-3
rl Figure 1. Surface coverage cs. reciprocal temperature (OK) for grade 62 silica gels modified by various alcohols range 10-6 to 8 X 10-2 mm of Hg. A thermocouple gauge (Consolidated Vacuum Corp., Rochester, N. U.)was used for the range to 2 mm. An absolute vacuum gauge (Roger Gilmont Instruments Inc., Great Neck, N. Y . ) was used for the range 0.1-10 mm. The latter is a duBrovin type gauge which expands the 0- to 10-mm range into 0.1-mm scale divisions. The range from 10-200 mrn of Hg was measured with a mercury manometer using a cathetometer to read the meniscus. The accuracy of the pressure reading was from 1-5 depending upon the pressure range and gauge.
The concentration of reactive OH groups on the surface before the reaction was 6 pmolesjm2. This i s based on the 12 pmoles/m* total maximum concentration estimated by Kiselev (14) and on the experimental data (22) indicating that about 50 of the OM groups are replaceable. The concentration of unreacted OH groups is 6 pmoles/m2 minus the suface coverage. The concentration of ester groups at equilibrium is the surface coverage. The equilibrium constants were estimated at two temperatures for the silica gels used. The equilibrium constants for grade 62 gel were 2.37 x 1W3 at 127 "C and 1.11 X at 162 "C. For grade 12 gel, the values were 1.18 X at 92 "C. Heats of reaction were at 79 'C and 1.67 x calculated from the equilibrium constants at two temperatures using the equation:
RESULTS AND DISCUSSION As the temperature of the preparation increased the surface coverage increased. Broge (11, 18) and Stober ( I O ) found the same relationship. In Figures 1 and 2, the logarithm of surface coverage has been plotted against reciprocal preparation temperature for a number of preparations. The linearity of these plots suggests that the degree of surface coverage must be substantially proportional to the equilibrium constant for the preparation reaction. It is interesting to follow this line of reasoning through to an estimate of the heat of reaction of the alcohol with the gel surface. Writing the reaction between the silica surface and the alcohol as:
the equilibrium constant may be written as K , = (-SiOR) (H20) Where (-SiOR) is the concentration of ester (SOH) (ROH)' groups on the surface and (SiOH) is the concentration of unreacted OH groups. By making certain assumptions about the concentrations, equilibrium constants were estimated. The assumptions were: The equilibrium concentration of water is just the amount of water produced in the reaction divided by the volume of alcohol used. The equilibrium concentration of the alcohol can be approximated by the molar concentration of the pure alcohol at the temperature of the reaction.
The heats of reaction vtere 13.9 and 7.8 kcal/mole on grades 62 and 12, respectively. These estimates are in agreement with the endothermic heat of 12.1 kcal/rnole for an alcohol on silica gel found by Broge (11) from kinetic measurements. It is evident from the estimated heats of reaction that there is a distinct difference between the behavior of the two silica surfaces. The difference in the AH values estimated from the equilibrium constants is about 6 kcal/mole. The explanation for this difference is related to the difference in pore sizes and chemical heterogeneity of the gels. The surface areas and heats of adsorption were calculated from the low temperature nitrogen adsorption isotherm data by means of the BET theory (23) for multilayer adsorption. The BET (Brunauer-Emmett-Teller) equation can be written c-1 P P 1 =__ .- where V , is the volume as V(P, - P ) v,c V,C Po equivalent to a monolayer coverage and Po is the vapor pressure of adsorbate. The value of C can be given by the equation C = exp (E] - ELL)/RTwhere El is the average heat of adsorption of the first layer and EL the heat of liquification of the adsorbate. A plot of P/V(P, - P> against PiPo should result in a straight line with a slope equal to (C - l)/CV, and an intercept of l/CV,. It is found that the linear BET
+
(22) V. Ya Davydov, L, T. Zhuralev; and A. V. Kiselev, J . Phys. Chein., USSR, 38,1108 (1964). (23) S.Brunauer, P. H. Emmett. and E. Teller, J. Amer. Chem. SOC., 60,309 (1938).
ANALYTICAL CHEMISTRY, VOL. 42, NO. 14, DECEMBER 1970
e
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Table I. Adsorption Isotherm Data for N, on Various Grade 12 Silica Gels Gel no.
...
. . ~
1 2
0.16 0.45 0.47
1,4-Dihydroxymethylcyclohexane 2-Propanol Trimethy lchlorosilane 2-Butanol 1-Propanol 1-Butanol 1-Decanol Trimethylchlorosilane Ethylene glycol Chloromethyldimethylchlorosilane Phenylethyl alcohol Trirnethylchlorosilane
3
4 5 6 I 8
9 ,
Surface coverage, (pmoles/m2)
Modifying agent
10 11
12 13
Surface area (m2/grarn)
BET C value
534
138 179 131 145 129 125 133 113
195
0.51
678 745 655 739
1.27
677
1.40 1.50
698
1.51 1.82
523
104
652
2.23
420
2.70 2.81
565
84 89 129 57
374
El
RTlnCfEL (kcal/mole) 2.08 2.11 2.06 2.08 2.06 2.05
=
2.06
2.04 2.04 2.00 2.00 2.06 1.94
Table 11. Adsorption Isotherm Data for N, on Various Grade 62 Silica Gels Gel no.
Modifying agent
1
.,.
2
2-Propanol 2-Butanol 2-Propanol 1-Propanol 2-Propanol 2-Propanol 1-Butanol Iso-Amyl alcohol n-Amyl alcohol Methylphenyldichlorosilane 1-Decanol
3 4 5
6 7 8 9 10 11 12
Surface coverage (prnolesjm2)
region was from PIP, values of 0.02 to 0.25, and the slopes and intercepts were taken from the region giving the best least squares fit. Site energy distributions were calculated from the adsorption isotherms using the method of Adamson and Ling (24). This method allows one to estimate the site energy distribution from the experimental adsorption isotherm and an arbitrarily chosen local isotherm function. The Langmuir equation for localized adsorption without lateral interaction was taken as the local isotherm function. Tables I and I1 contain some of the nitrogen adsorption data on the various silica gels. In all cases the modified gels had lower surface areas than the starting material. The actual area depends upon the size of the organic molecules and upon the density of the layer. This is illustrated by the three preparations made using trimethylchlorosilane, the data for which are shown in Table I. The surface coverages of these preparations were 0.47,1 S I , and 2.81 pmolesjm2, while the surface areas were respectively 655, 523, and 374 mZ/grarn. The effect of the size of the organic molecule can be seen by comparing several of the preparations which have the same surface coverages. Comparing preparations 9 and 10 of Table I, one can see that the surface area of the trimethylchlorosilane gel was 523 m*/gram, somewhat less than that of the ethylene glycol gel (652 m2/gram)although the surface coverage of the latter was greater (1.81 z~s.1.51 pmoles/m*). A distinct difference can be seen between the gels prepared from the narrow pore gel (Table I) and those prepared from the wide pore gel (Table 11). The modification of the wide pore (grade 62) gel did not greatly reduce its surface area while the gels prepared from the narrow-pore (grade 12) gel had much lower surface areas than the unmodified grade 12 gel. (24) A. W. Adamson and I. Ling, Advan. Chem. Ser., 33,62 (1962).
1714
0
...
Surface area (mZ/gram) 27 5
0.35
267
0.43 0.69
264 263 267
0.95
1.06 2.02 2.04 2.24 2.67 3.04 3.35
El
BET C value
108 103 90 95 83
252 23 6
84
247
62 36
252 253
256 225
52 32 34
17
+
RTIn C EL (kcal/mole) 2.03
=
2.03 2.01
2.02 1.99 1.99 1.92 1.95 1.87 1.85
1.86 1.76
This is taken to mean that few pores were blocked by the chemical modification of the wide-pore gel. Although the surface areas were decreased by a larger factor by modification of the narrow-pore gel than of the wide pore gel, the heats of adsorption were decreased by a smaller factor. It appears that a large percentage of the high energy adsorption sites were blocked or eliminated by the chemical reaction on the wide-pore gel, judging from the heats of adsorption, while the same degree of substitution on the narrowpore gel did not change the heats of adsorption as much. The fact that chemical modification of the wide-pore gel produced a more uniform adsorbent is also shown by the site energy distributions. The average heats of adsorption of nitrogen on the wide-pore gels decreased as the degree of modification (surface coverage) increased. This indicates that the high energy adsorption sites eliminated were the hydroxyl groups on the surface. Whalen (21) represented the hydroxyl groups as being high energy nitrogen adsorption sites. Figure 3 shows an adsorption isotherm for nitrogen on the grade 62 silica gel. Figure 4 shows the site energy distributions obtained for various treated grade 12 (narrow-pore) gels. The site energy distributions are plotted as dF/dQ, us. Q, where F is the fraction of sites having adsorbent-adsorbate interaction energies equal to or greater than Q. In general, chemical modification shifted the curves toward slightly lower energies and lowered the heats of adsorption. Figure 5 compares the distributions for the two untreated silica gels. The grade 62 (wide-pore) gel had a lower concentration of adsorption sites in the 2.5 to 3.0-kcal range and lower average heat of adsorption. This result is in agreement with the estimated heats of reaction for alcohols on these gels. The grade 12 (narrow-pore) gel had a higher energy surface
ANALYTICAL CHEMISTRY, VOL. 42, NO. 14, DECEMBER 1970
1.5
I .0 0
W
b W ,5 \ \
0 1.0
1.5
2.0
2.5
3.0
0 (kcal./mola)
Id
Io2
IO
lo4
IO6
Figure 5. Differential site energy distributions for nitrogen on unmodified silica gel;
$(atm.‘’)
-_
----
Figure 3. Adsorption isotherms for nitrogen on grade 62 silica gel +Sf Trial 1; surface area = 277 m2/gram,C value = 108 0000 Trial 2; surface area = 275 m2/gram,C value = 96
Grade 12 Grade62
+
7
1.0
I.5
0 1.0
1.5
2 .0
2.5
2.o
2.5
3.0
Q (kcol. /mole9
3.0
Figure 6 . Differential site energy distributions for nitrogen on grade 62 silica gels
O (kcol. / mole)
Figure 4. Differential site energy distribution for nitrogen on grade 12 silica gels - Unmodified - 1-Decanol . . . . Trimethylchlorosilane .-. Chloromethy ldimethy lchlorosilane
---
for physical adsorption and also a higher-energy surface for chemical reaction, as is reflected in its lower endothermic heat of reaction with alcohols, than the wide-pore gel. The high energy sites for physical adsorption and the chemical reaction sites are the surface hydroxyls. Figure 6 shows some of the distributions for the wide-pore gels. The average heat of adsorption for the 1-decanol gel was 1.76 kcal/mole, which is 0.27 kcal,/mole less than the heat of adsorption for the unmodified gel. From the distribution it can be seen that the sites having energy above 2.2 kca1:mole have been eliminated. The peak at about 1.7 in the distribution for the 1-decanol modified gel is very close to the value of 1.76 kcal/mole average heat of adsorption for nitrogen on this gel. Many of the higher-energy sites were eliminated by the replacement of high-energy hydroxyl groups with lowenergy SiOR groups. Similar results are seen for the widepore gel modified with methylphenyldichlorosilane and with
___-..... -._._ . , ._. * .
Unmodified
1-Decanol Methylphenyldichlorosilane iso-Amylalcohol n-Amylalcohol
the iso-amyl and n-amyl alcohol-modified gels. The isoamyl and n-amyl alcohol modified gels have similar surface areas and heats of adsorption. Apparently the slight difference in the structure of these two alcohols makes very little difference in the shape of the site energy distribution curve. The distribution of adsorption energies for nitrogen on the wide pore gels depends more upon the surface coverage than upon the type of modifying layer on the surface. In the case of the narrow-pore gels, many of the pores were blocked by the organic groups on the surface. The trend in the shifting of the distributions to lower energies and in the lowering of average heat of adsorption was more pronounced with chemical modification of the wide-pore gel, while the modified narrow-pore gels showed a larger decrease in surface area. RECEIVED for review December 29,1969. Accepted August 24, 1970. This work has been supported by Grants No. GP-4206 and GP-8361 from the National Science Foundation.
ANALYTICAL CHEMISTRY, VOL. 42, NO. 14, DECEMBER 1970
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