Langmuir 1992,8, 2753-2757
2753
Pore Size and Surface Texture Modification of Silica via Trialkylsilylation Duen-Wu Hua and Douglas M. Smith* UNMINSF Center for Micro-Engineered Ceramics, University of New Mexico, Albuquerque, New Mexico 87131 Received March 6,1992 Although surface modification of silica via silylation is often undertaken to change surface chemistry and, hence, adsorption characteristics,the use of silylation for modifying pore size, size distribution, and surface texture is rarely considered. In principle, by use of silylatingagents of the form R,SiCL-, where n is typically 3, pore size and texture may be varied by changing the R group, n, and/or surface coverage. When the silica is microporous, this approach should be particularly effective for changing the pore size. However,many questionsexist concerningthe effect of small-scalesurface roughness, the fractionalsurface coverage, and R group size on the effective pore size. Modification of various silicas (Vycor, CPG-75, two-step acidlbase catalyzed silica gel) was conducted by trialkylsilylation. The degree of silylation/ surfacecoverage was monitored by thermogravimetric analysis and elementalanalysis. Pore structure was studied by nitrogen adsorption and condensation and small angle X-ray scattering. The results show that surface area, surface texture, pore size distribution, and total pore volume are indeed changed in a controlled fashion due to silylation. Generally, the mean pore size is smaller,the pore size distribution is significantly narrower, and the pore surface is smoother after silylation. The magnitudes of those changes are shown as a function of R, coverage, and surface roughness.
Introduction Surface modification of porous silica has been widely studied and used in various fields such as chromatography supports and ads0rbants.l It is well-known that the active -OH groups on the surface of silica can react with alcohol or alkylchlorosilane, changing pore chemistry and rendering the surface hydrophobic. However, the use of silylation for modifying pore size and size distribution as well as the physical texture of the pore structure is seldom considered. Avnir2 has suggested the use of silylation as a surface structure probe. Nakamura and co-workers3 varied the degree of trimethylsilylation on a single silica gel. A slight decrease in pore size was noted with silylation but the principal thrust of that work was on the relationship between silylation and water adsorption and not pore size/ texture. Pfeifer and co-workers4analyzed the results from Nakamura and concluded that the silica gel was relatively , smooth with a surface fractal dimension (D)of ~ 2 . 1 but the number of data points was limited. Okubo and Inoue5 covered the surface of a single gel with different R groups and measured the reduction in surface area and pore volume using nitrogen adsorption. No particular attention was paid to the effect of silylation on pore morphology but subsequent analysis by Pfeifer and co-workers4indicated that the silica gel was nearly smooth ( D= 2.15). Recently, Schmidt and co-workers6used smallangle X-ray scattering (SAXS)to study silylation on gels containing a wide range of molecular weight R groups (R 5 18)and demonstrated that the interpretation of scattering curves can be complicated by the presence of a diffuse layer at the solidpore boundary. Changes in macroscopic pore geometry (i.e. pore size distribution) were not addressed in that work. (1) Iler, R.K. The Chemistry of Silicu; Wiley: New York, 1979;p 420. (2) Avnir, D. Personal communication, 1990. (3) Nakamura, Y.;Shinoda, M.; Danjo, K.; Iida, K.; Otauka, A. Adu. Powder Technol. 1990, 1 (l),39. (4) Pfeifer, P.; Johnston, G. P.; Deshpande, R.; Smith, D. M.; Hurd, A. J. Lungmuir 1991, 7,2833. (5) Okubo, T.; Inoue, H. MChE J. 1988,34, 1031. (6)Schmidt, P. W.; Avnir, D.; Levy, D.; Hober, A.; Steiner, M.; Roll, A. J. Chem. Phys. 1991,94, 1474.
0743-746319212408-2753$03.00/0
Unger and co-workers' have studied the effect of various derivatization schemes on the surface chemistry of silica. In contrast to silica, the use of various pore size/texture modification treatments using silane, disilane, borane, etc. is fairly common for zeolites.8 This study considers the effect of silylation on various silicas, such as phase-separated glasses (Vycor glass from Corning, control pore glass [CPGI from Electronucleonics), and a sol-gel derived silica gel obtained from the two-step acidtbase catalyzed reaction of tetraethyl orthosilicate. The effects of the size of the alkyl groups and pore surface roughness are also inqestigated.
Experimental Section Samples used in this study are Vycor, CPG-75, and a waterwashed two-step acidlbase catalyzed silica xerogel. This silica xerogel commonly denoted as BP has been used for several previous studies of pore Davis and co-worked* have shown that the morphology and surface chemistry (SiOR vs SiOH) of this gel could be significantlychanged dependingon the washing schemeemployedbefore drying. Washing in ethanol removes unreacted and partiallyreacted silicateswhich can cause phase separation if the wet gel is placed in water. b o , ethanol washing promotes increased surface esterification and network depolymerizationleadingto higher surfacearea and lower xerogel pore volumes. Washingin water promotes further condensation/ polymerizationreaction leading to a decrease in surface area and increased pore volume. For this work, the wet gel is washed 3 times with EtOH and 3 times with water, each wash taking about 24 h. Drying at 393 K for 2 days, the final xerogel is obtained, which will have a surface which is primarily Si-OH. Vycor and CPG-75were rehydroxylated in water at 363 K for 60hand dried at 393 K for 2 days and then stored in a desiccator. Silylationwas conducted by placing a certain quantity of dried silica (for CPG and Vycor = 0.1 g, for B2 gel = 0.5 g) into a (7) Unger, V. K.; Berg, K.; Nyanah, D.;Lothe, Th. Colloid Polym. Sci. 1974, 252, 317. ( 8 ) Vansart, E. F. Pore Size Engineering in Zeolites; J. Wiley and Sons: New York, 1990. (9) Brinker, C.J.; Keefer, K. D.; Schaefer, D. W.; Ashley, C. S.J. NonCryst. Solids 1982,48,47. (10) Glaves, C. L.; Brinker, C. J.; Smith, D. M.; Davis, P. J. Chem. Muter. 1989, 1, 34. (11) Davis.P. D. M. J. Non-Crwt.Solids 1992. . J.:. 3rinker.C. . J.:Smith. . 142, 189. (12) Davis, P. J.; Brinker, C. J.; Smith, D. M.; Aasink, R. A. J.NonCryst. Solids 1992,142, 197.
0 1992 American Chemical Society
Hua and Smith
2754 Langmuir, Vol. 8, No. 11, 1992 Table I. Nitrogen Adsorption Results for Trimethylsilylated CPG-76 SA PV (m2/g) (cmVg)
CPGMl CPGM2 CPGM3 CPGM4 CPGM5
E
U
O
f
140
0.3 1.51 1.48 1.86 2.18
0.568 0.515 0.600 0.502 0.500
152 134 130 129 129
N
%C
TMS
%
groupstg 0 1.52 X lP 1.47 X 1020 2.11 X l @ 2.64 X lP
coverage 0 50.5 49.3 55.7 78.6
I“1
m
u
so
040.3%
0 78.6%
0 55.7%
120
I
0.0
10
1
BO 100
Figure 2. Pore size distributions for TMCS silylated CPG.
0
a
70
10
Radlur (A)
0 50.3%
bL m
50
40
I
0.1
I
0.2
I
I
I
0.3
0.4
0.5
[TMCSI ( M I
Figure 1. Effect of TMCS silylation on surface area of CPG.
Table 11. Nitrogen Adsorption Rerults for TrimethylsilylatedVycor SA PV TMS % (m2/g) (cm3/g) % C groupstg coverage Low Concentration VycMlL VycM2L VycM3L VycM4L
VycM5L benzene solution with various trimethylchlorosilane (TMCS, Aldrich Co.) concentrations, normally below 5 mL of TMCS in 10 mL of benzene (or 2 out of 5 mL). After the mixture was shaken for 24 h at room temperature, TMCS/silica was filtered, washed with benzene and then acetone, dried at 413 K for 2 days, and stored in a desiccator until analysis? Triethylchlorosilane (TECS) and tri-n-propylchlorosilane (FPCS) were employed for the B2 gel under the same concentrations and procedures as used for TMCS for a comparison of the R group size effect. The degree of silylation was calculated from elementalanalysis and represented by carbon contentsin the trialkyleilylatedsilica. The blank carbon value wa8 subtracted from the silica sample treated only with benzene and acetone. Pore structure analysis was conducted using nitrogen adsorption/condensation at 77 K. Samples were outgassed at 403 K overnight before analysis. Surface area, total pore volume and pore size distribution data were calculated from the adsorption branch of the isotherm in the relative pressure (PI&) range of 0.05 I P/Po I 0.30 (surface area) and the desorption branch (porevolume/size distribution). Small angle X-ray scattering was performed at Oak Ridge National Laboratory with a pinhole camera using a 64 X 64 2D position sensitive detector. Cu K a line is used (A = 1.54 A) and the scattering wave vector ( q ) range covered is from 0.005 to 0.5 A-l. The samples were prepared by loosely packing a 0.5 mm path length cell in order to have sufficient transmission. The volume and weight of silica in each cell were carefully noted. Absolute intensity is calibrated by using a standard “Lupolen” sample from Oak Ridge and converting to scattered cross section per unit volume.
Results and Discussions Nitrogen adsorption data and TMS surface coverage for the CPG-75 are listed in Table I. Surface coverage (in terms of the fraction of the surface covered by trialkylsilyl group) is calculated from the carbon contents by elemental analysis and the estimated size of the trialkylsilyl groups (0.38 nm2 for TMS by Bondi13). The nitrogen surface area and total pore volume decrease with increasing TMCS concentration (higher surface coverage). The surface area of TMCS silylated CPG-75 is plotted in Figure 1. As can be clearly seen, the surface area decreases rapidly and (13)Bondi, A. J. Phys. Chem. 1964,68,441.
VycM6L VycM7L
VycMlH VycM2H VycM3H
VycM4H VycM5H VycM6H
179 172 166 153 148 149 131
0.251 0.240 0.239 0.219 0.212 0.212 0.190
0.3 0.76 1.14 1.30 1.54 1.69 1.72
0
0.77 X lP 1.4OX lP 1.67 X lP 2.07 X lP 2.32 X lP 2.38 X 1P
0 16.3 29.8 35.4 44.0 49.3 50.0
High Concentration 156 129 125 122 123 115
0.243 0.203 0.202 0.206 0.195 0.192
0.65 1.94 2.03 2.42 2.18 2.45
0 2.16 X 2.31 X 2.96 X 2.56 X 3.01 X
lP
0 62.7
1P 1P
72.3
lP lP
56.4
62.6
73.5
reaches a minimum. This effect is opposite to that expected for a smooth surface and clearly shows the presence of small scale surface roughness in the material. As has been previously noted,l the surface cannot be completely silylated as a result of TMCS steric effects and our maximum degree of coverage (78.6%) is similar to previous ~ o r k Pore . ~ size ~ ~distributions ~ ~ (Figure 2) also change significantly with silylation from plain CPG75 to a highly silylated one (78.6% ) resulting in a decrease of approximately 4 A in pore radius. The more subtle change in pore size between samples with various concentrations of TMCS are illustrated with subsequent samples. Two batches of Vycor samples were silylated a t low range (0-0.2 M) and high range (0-1.6 M) of TMCS concentrations and studied by nitrogen adsorption (Table II). Figure 3shows the surface area change with varying TMCS concentration. The circles are the lower TMCS concentrations and triangles are the higher concentration. The difference in the surface area of the two blank Vycor samples is probably the result of different degrees of hydroxylation since our BET surface area measurements are reproducible to better than 5%. The decreasing trend of the surface area is similar to that shown for CPG-75, and the significant changes in surface area with various low TMCS concentrations are clearly shown. The surface coverage is also shown in the figure. A similar value for the maximum coverage is obtained for Vycor as found for the CPG-75. Pore size distributions of Vycor show very small changes for dilute TMCS concentration. For the higher TMCS concentrations, the trend is similar to that of CPG-75. Since the pore size of Vycor (r = 28 A) and
Langmuir, Vol. 8, No.11, 1992 2166
Modification of Silica via Silylation
4
2oo
1 .o
0.5
1.5
2.0
[TMCSI (MI Figure 3. Effect of TMCS silylation on surface area of Vycor. The circle ie the lower concentration batch and inverse triangle is the higher concentration batch.
I,..
0
500
0 0
052%
a
osax
0 40%
ln 400 I
0
(m2/g) (cma/g) % C groups/g coverage Methyl (cross sectional area of TMS group ie estimated to be 0.38 nm2P 539 0.897 0.5 0 0 530 0.854 1.17 1.12 x lP 7.89 525 0.809 2.24 2.91 X lP 20.5 490 0.736 3.65 5.27 X lP 37.1 461 0.693 4.91 7.38X lP 52.0 447 0.684 5.21 7.88X lP 55.5 439 0.674 5.55 8.45 X 1P 59.5 446 0.667 5.43 8.24 X loa0 58.0 0.673 5.71 436 8.71 X lP 61.4 Ethyl (croessectional area of TES group iS estimated to be 0.48 nm2p 582 0.806 0.42 0 0 547 0.766 1.94 2.54X 1P 21.0 529 0.743 2.55 3.56X 1P 29.5 526 0.698 3.41 5.00X lP 41.4 494 0.678 4.06 6.09X lP 50.4 517 0.675 4.52 6.86X lP 56.8 502 0.644 4.54 6.89X lolo 57.1 494 0.634 6.31 9.85 X lP 81.6 n-Propyl (cross sectional area of P P S group is estimated to be 0.55 nm2)0 0.798 0.42 0 597 0 0.768 1.38 557 1.61 X 1010 14.8 554 0.749 1.66 2.07XlP 19.1 0.748 2.13 540 2.86 X lP 26.3 0.714 2.65 3.73 X loa0 34.4 534 0.709 3.43 5.03 X lP 528 46.4 536 0.685 4.46 6.76XlP 62.2 0.665 4.06 6.09X l@ 519 56.1
B2M1 B2M2 B2M3 B2M4 B2M5 B2M6 B2M7 B2M8 B2M9
.--0.0
4 E
Table 111. Nitrogen Adlorption Ebrultr for Variour Trialkylrilylated B2 Gels SA PV TMS %
0 61%
I
I
I
I
I
1
2
3
4
5
1
B2E1 B2E2 B2E3 B2E4 B2E5 B2E6 B2E7 B2E8 B2P1 B2P2 B2P3 B2P4 B2P5 B2P6 B2P7 B2P8
Eetimated from Bondi'smodel, see test.
I
6
[TMCSI ( M I Figure 4. Effect of TMCS silylation on surface area of B2 gel.
-
4
:
b
L
t
ft ,
L
U
4 m
A
Blank (Wl)
0
Mothy1 (M4)
0
Ethyl (E5)
o n-Propyl (PO)
n L
U
-
m
4
0
modlum rllylatod
2
SI 1
n
10
30
50
70
100
200
Radlur (A) Figum 6. Size effect of R group on pore size distributions of B2 gel. 10
30
50
70
100
Radlur (A) Figure 5. Pore size distributions for TMCS silylated B2 gel.
CPG (r = 55 A) is large, as corresponds to the size of R, porous samples with smaller pores should better illustrate the effects of silylation on pore size. Surface area data (Figure 4) of TMCS silylated B2 gels indicate a similar trend to Vycor and CPG. Pore size distributionsof TMCS silylatedB2gels are shown in Figure 5 and the effect is clear, higher TMCS concentration narrows down the size distribution and shifts the distribution to the smaller pore region. Silylation appears to selectively react in the larger pores which results in a significant narrowing of the pore size distribution. This
may be the result of steric hindering of the silylation reaction in smaller pores. For a given TMCS concentration, the equilibrium coverage in the larger pores is presumably greater than in the smaller pores. Even a t only 20% surface coverage, most large pores have been eliminated. TECS and 'PPCS silylated B2 gels both exhibit similar trends as the TMCS silylated ones; the detailed nitrogen adsorption data and calculated surface coverage are given in Table 111. The effect of different alkyl groups on the pore size distributionof B2 gelasilylatd with various silylating agents (similarnumber of silylating group on the surface) is shown in Figure 6. The difference between TMCS and the others is quite large and appears to exceed the expected size change effect based solely on
2756 Langmuir, Vol. 8, No. 11, 1992 -0*03 -0.04
7 TMCS - TPCS
4 "
-0.06
:
v
-
m
0
-0.07
I
\V \
'...%. \ '.._ \
E 0
100
e a e
10-1
a
-
4-
0
\
-0.01
--
-1.2
-1.0
log(PV,-
I
-0.11
-
>
10-2
-3.85
a
E
1o
- ~ -4.00
5
PV,)
Figure 7. log-log plot of surface area of TMS f i i on the B2 gels as a function of f i i volume (both normalized by the values of the unsilylated B2 gel).
R group size. However, the difference between TECS and P P C S is only barely noticeable. Small-scalepore surface roughness is another important issue for many practical applications such as adsorption and catalysis. In a previous paper, Pfeifer et al.4 showed that by using nitrogen adsorption data measured for a samplewith various thicknesses of a preadsorbedfilm such as water,14one could derive the surface fractal dimension. For a fractal surface, the measured film surface area, A, decreases with increasing adsorbed film volume, V, as A
-3.67
a
c U
0
-0.oe -0.10 I -1.4
hlgh (81.6%)
=
+r
I
io'
U
0
I
(50.4%)
c
\
-
0
mrdlum
n
\
-
.+I
blank (0%) 10'
V\ 'b....,,
n
10'
TECS
0.. ....
-0.05
Hua and Smith
cc v(2-D1/(3-D)
.-
10-4
0.001
I
0.01
I
, * . . . .,I 0.1
I
