Longmuir 1991, 7, 1695-1701
1695
Infrared and Gravimetric Study of an Aerosil and a Precipitated Silica Using Chemical and H/D Exchange Probes B.A. Morrow' and A. J. McFarlan Department of Chemistry, University of Ottawa, Ottawa, Ontario, Canada K I N 6N5 Received December 21,1990. I n Final Form: March 28, 1991 The accessibility of the hydroxyl groups on silica to chemisorption and H/D-exchan e probe molecules of different steric sizeshas been studied by using infraredspectroscopyand vacuum microhnce techniques. The results have been used to compare and characterize a nonporous aerosil and a nonporous precipitated silica of similar surface areas. In the fully hydroxylated "as received" state, the number of silanol groups oneither silica that react with various hydrogen sequesteringagents (HS) decreases as the size of the agent increases [ZnMez, BCb, Tic&,AlMes, and MeaSiNHSiMes (HMDS)in increasing size]. The number that undergo H/D exchange also decreases as the size of the probe molecule increases [D20, ND3, and deuterated methanol, isopropyl alcohol, and tert-butyl alcohol]. The total number of silanol groups on the precipitated silica is 6.8/nm2 of which 5.6 can exchange with the smallest H/D-exchange molecule (D20 or NDs), and on aerosil it is 3.1/nm2 of which 2.5 will exchange. The total number per nm2 that react with a given HS agent is about the same on either silica, and this is substantially smaller than the number that will undergo exchange. The difference between the extent of chemisorption versus exchange has been attributed to the steric blocking effect of the immobile chemisorbed product. HMDS can only react monofunctionally with surface SiOH,and the number of trimethylsilyl (TMS)groups that can be attached toeither silica is about the same, 1.4-1.5/nm2. Both hydroxylated silicascontain about 1.1isolated silanols/ nm2, all of which react with HMDS. However,the number of TMS species that end up attached to silanols that were originally H-bonded is 0.3-0.4/nm2.
Introduction The density of surface silanol groups [SiOH] on amorphous silicas can vary considerably accordingto the method of preparation.'-' On completely polymerized and fully hydrated silicas of whatever origin, there are about 4.55.0 hydroxyls/nm*, this being more or less independent of the surface area. However, this number can approach 12-14 OH/nm2 on uncalcined-gel or precipiated silicas where there are a large number of internal silanols in micropores and/or where many surface silicon atoms are bonded to more than one OH g r ~ u p . ~On J the other hand, on "as received" (i.e., nonrehydrated) pyrogenic or aerosil type silicas, which are produced by the high temperature hydrolysis of Sic&,the hydroxyl density is usually lower,617 being in the range 2.5-3.5 OH/nm2. The silanol groups on silica can strongly interact via hydrogen bonding with proton donor or acceptor molecules, and this property largely determines its interfacial chemistry. However, a normally hydrophilic hydroxylated surface can be rendered relatively hydrophobic by replacing the SiOH groups by other functionalities. For example, the SiOH groups can react with a variety of socalled hydrogen sequestering (HS) agents MX, via reaction 1 to yield a surface with very differing adsorption SiOH + MX,
-
SiOMX,,
+ HX
(1)
characteristics.8 When the HS agent contains a trialkyl(1) Zhuravlev, L. T.Langmuir, 1987,3, 316. (2)Tlmabe,K.;Mieono,M.;Ono,Y.; Hattori,H.Stud.Surf.Sci.Catal. 1989.51.92. (3) Khelev,A.V.; Lygin,V.I. Infrared Spectra of Surface Compounds; Wiley: New York, 1975. (4) Iler, R. K . The Chemistry of Silica; Wiley: New York, 1979. (5) Burneau, A.; BarrBs, 0.; Gallm, J. P.; Lavalley, J. C. Langmuir,
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-1990.6.1.W. -, -,- - -.
61.
(6) Mathim, J.; Wannemacher, G.J . Colloid Interface Sci. 1988,125, (7) Morrow, B. A.; McFarlnn, A.J. J. Non-Cryst. Solids 1990,120,61. (8) Morrow, B. A. Stud. Surf. Sci. Catal. 1990,57A,A161.
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silyl group, SiOH is converted into SiOSiRs, and a relatively inert hydrophobic material can be produced that might be more suitable for chromatographic or other purposes.9 Whatever the HS agent used, the extent to which the silanols react depends on steric factors such as the silanol density, or the size of the HS agent used, and this will ultimately influence the number of silanols that react. In this work, we have examined how steric factors influence the extent of chemical modification and H/D exchange of surface silanols on two silicas of differing origin. Chemical modification has been achieved, using HS agents of different size (ZnMe2, BC4, TiCL AlMe3, and MesSiNHSiMes (HMDS)), and exchange was carried out, using D20, NDs, and deuterated methanol, isopropyl alcohol, and tert-butyl alcohol. The results are used to develop a quasi-quantitative model of the silanol structure on these silicas.
Experimental Section This work was carried out by using (1)a pyrogenic or aerosil type silica, Cab-0-Si1HS5 and (2) a high purity (Na 60 ppm, Al < 100 ppm, Fe 20 ppm, Ti C 20 ppm) nonporous precipitated silica from Rh6ne-Poulenc, Paris, France. For IR transmission studies, 50 mg of powder was compacted at lo7 Pa into a selfsupporting disk 25 mm in diameter. One hundred milligram disks were used in the microbalance studies. The aerosil and precipitated silicas will be designated by the terms A-x and P-x respectively, where x is the temperature of activation under vacuum for 1 h in degrees C (generally 150 or 450 "C). The BET (Nn)surface area of the aerosil was 325 f 5 m*/gfor either A-150 or A-450; that of P-450 was 285 f 5 and of P-150 was 270 f 5 m2/g. As will be discussed later, the number of H-bonded silanols on precipitated silica that are eliminated a~ water upon heating from 150 to 450 O C is about 3 times greater than the number that are eliminated from aerosil. The increase in the BET surface area upon going from P-150 to P-450 is (9) Chmielowiec,J.; Morrow, B. A. J. Colloid Interface Sci. 1988,M,
319.
0 1991 American Chemical Society
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Morrow and McFarlan
1696 Langmuir, Vol. 7, No.8, 1991
to hydrogen-bonded ~ilanols.l-~9~a For aerosil, a shoulder near 3720 cm-l can just be discerned, and the 372013520 cm-l pair of bands have been assigned, respectively, to the H-bonded and free silanol groups of a pair or chain of silanols as follows:
Aerosil Silica
H.
H
P/ ', / 0 0
3520
1.
SI
Precipitated Silica
UI
3800
C'
3500 cm-'
3200
Figure 1. Infrared spectra A-150 (A) and A-450 (B)in the SiOH stretching region after vacuum activation for 1h at 150 or 450 O C , respectively. Curve C is the difference spectrum B -A. Curves A', B' and C' are the corresponding spectra for the precipitated silica. probably due to the removal of this H-bonded network of silanols, thus permitting increased adsorption of Nz. Therefore, we believe that an area of 285 m*/g is more representative of the real area of the precipitated silica for either temperature of activation, and this value has been used to calculate the silanol density data discussed later for P-150 and P-450. The 300-mL quartz IR celllo was connected via an Apiezon N greased ball joint to a pyrex vacuum line (300-mL volume) capable of attaining a base pressure of about lo-' Torr. All reactants were transferred as gases from the main manifold to the reaction cells,and pressures were measured with a capacitance manometer. Chemical reactions were carried out by using a large excess of reactant compared to the number of SiOH groups (see later for this parameter), and the reactions went to completion after a few minutes for AlMea, BCls, and Tic4 at 22 O C , 7 after about 20 min at 22 "C for ZnMeP, and after about 1h at 150 "C for HMDS. (A "complete reaction" was deemed to have resulted when there was insignificant further change over an additional period of about 20 min.) The exchange reactions were all carried out at 22 "C and the procedure will be described later. Fourier transform infrared (FTIR) spectra were recorded by using a Bomem DA3-02 instrument (MCTdetector) or a Bomem Michelaon MBl00 instrument (DTGS detector) at a resolution of 2 or 4 cm-l. Vacuum microbalance experiments were carried out by using a Sartorius model 4433 instrument having a sensitivity of 0.1 rg.
Results Untreated Silica. The surface silanol groups on silica exhibit characteristic OH stretching vibrations in the spectral region from 3800 to 3200 cm-l. The infrared spectrum of aerosil after vacuum activation at 150 "Cfor 1h (A-150) is shown in Figure 1as curve A, and that of the precipitated silica (P-150) as curve A'. Both spectra show a sharp peak near 3747-3737 cm-1 characteristic of isolated noninteractingsurfacesilanolsand a broad feature with amaximum near 3520 cm-l, which has been attributed (IO) Morrow, B. A.; hamurthy, P. J . Phys. Chem. 1973, 77,3062.
