J. Phys. Chem. 1991, 95, 9391-9396 This means, taking into account the lattice parameter, that
on a (100) face about 0.18 of the anionic sites should be vacant. This percentage is probably too high to be due exclusively to preexisting vacancies (i); the role of the second class of vacancies must thus be considered. A confirmation of this idea comes from the hyperfine structure observed for a fraction of the paramagnetic centers on Li/MgO and due to one or two ’Li ions in the first coordination sphere of the trapped electron. Preliminary computer simulation results indicate that the fraction of sites exhibiting hyperfine coupling with one or two Li nuclei is about 25%. The whole process occurring at the surface can be, in conclusion, described as the ionization of a metal that releases one or two electrons. Some electrons are stabilized in surface vacancies deriving from the thermal treatment and the remaining in new anion vacancies formed by a (partial) reconstruction of the surface or, better, of the cationic sublattice of the surface.
9391
A schematic view of the process is shown in Figure 6. The surface reconstruction begins, very likely, at the morphological defects of the surface such as kinks, steps, or terraces, because in these points the electrostatic potential (or, in other words, the coordination number attained by the added ion) is higher than on a flat ( 1 00) face. Moreover, the number of additional ions required to complete a 5-coordinated anion vacancy at the mentioned morphological defects is limited to one or two, whereas the reconstruction of the same site on a flat (100) face needs four additional ions in vicinal positions and is therefore less probable. A picture of two possible “reconstructed” sites, involving one and two additional ions, is shown in Figure 7. The hyperfine structure shown by some F,+ centers on Li/MgO, and involving the presence of one or two Li nuclei in the site, once again confirms the above description of the surface. Registry No. MgO, 1309-48-4;Li, 7439-93-2;Mg, 7439-95-4.
Structure and Dynamics of Amino Functional Silanes Adsorbed on Silica Surfaces Hye-Jung Kang and Frank D. Blum* Department of Chemistry and Materials Research Center, University of Missouri-Rolla, Rolla, Missouri 65401 (Received: October 22, 1990)
Deuterated (aminopropy1)triethoxysilane(DAPES) and deuterated (aminobutyl)triethoxysilane (DABES) have been adsorbed on high-surface-area silica (CabOSil), dried, and probed with 2H (wide-line)and %i (CP/MAS) solid-state NMR techniques. When adsorbed, the DABES reacted to form siloxane linkages with the surface or other DABES molecules. The amount of DABES adsorbed on silica was dependent on the concentration of the silane coupling agent solution. The molecular motion of the adsorbed silane coupling agents on silica have been probed as a function of the amount and type of coupling agent. The slow-motional theory of Freed was used to simulate the resulting ZHspectra. The spectra were simulated by a superposition of a rigid and a mobile species. The C-D bonds of the mobile species undergo anisotropic diffusion (d, = 70-500 kHz and dxy = 10-100 kHz) plus faster two-site jumps about a diffusion axis tilted at 54.7’ (rate = 1-2 MHz). When treated with a solution resulting in approximately monolayer coverage, the molecular motion of the coupling agent was the slowest. From treating solutions resulting in coverages above and below monolayer coverage, the spectra of the coupling agents on the surface were indicative of more mobile species. At the higher concentrations, the increase mobility a was similar to that of a polysilsesquioxane layer which was indirectly bonded to the surface was observed. Surfaces treated to be at submonolayer coverages were also less restricted. The 2HNMR spectra for DAPS and DABS on the silica surface also showed that the difference in the alkyl chain length of silane coupling agents did not significantly affect the molecular motions of silane when they were adsorbed on silica surfaces for these short (C, and C,) chains.
Introduction Silane coupling agents are widely used to improve the bonding between silica and polymeric resins by altering the properties of the interfacial layer. In addition to the bulk properties of the component species, the properties of the interfacial region are very important in determining the properties of the composite materials. For example, in glass fiber/polyester composites, the treatment of glass with silane coupling agents has been shown to affect the mechanical properties of the composite as a function of both the amount and type of coupling agent used.’ Since the interfacial layers are very thin, it is necessary to probe their behavior on a molecular level. Microscopic interfacial properties can conveniently be classified as probing either structure or dynamics. These two are clearly interrelated, but different experimental techniques are usually sensitive to different kinds of properties. A structural property might be the conformation of a molecule on a solid surface or the mechanism by which it was attached to the surface. A dynamic or motional property might be the rate and manner in which the molecule moves while attached to the surface. In order to relate the microscopic interfacial properties to the mechanical properties in composite systems, a good deal more needs to be known about interfacial species. To whom correspondence should be addressed.
A significant amount of work, based various techniques, has been done to elucidate the structure of aminosilanes in solution and on substrates. Chiang et al.* have used FTIR spectroscopy to study the structure of (y-aminopropy1)triethoxysilane (APS) in aqueous solution and on glass surfaces. They reported that the hydrolyzed amino functional silane exists in two structural forms, internal cyclic ring and non-ring-extended structures. It was found that the hydrolyzed aminosilane adsorbed onto high-surface-area silica approximately as a monolayer and that the amino group is hydrogen bonded to the surface. Johansonn et aL3 used radiochemical tracers and found that APS formed uniform films on E-glass surfaces with covalent bonding a t the interface. Schrader4 has proposed a combination of physisorbed silane and two types of chemisorbed species on silica surfaces. The physisorbed silane coupling agents were not reacted with the E-glass surface and were removed with solvent washing. The two chemisorbed silanes were distinguishable by their ability to be removed with boiling water. The nonremovable species corresponded to ( 1 ) Eckstein. Y . J . Adhes. Sci. Technol. 1989, 3, 337. (2) Chiang, C. H.; Ishida, H.; Koenig, J. L. J . Colloid Interface Sci. 1980, 74, 396. (3) Johansonn, 0. K.; Stark, F. 0.;Vogel, K. M.; Fleischmann, R. M. J . Compos. Mater. 1967, I , 278. (4) Schrader, M . E. J . Adhes. 1970, 2, 202.
0022-3654/91/2095-9391%02.50/00 1991 American Chemical Society
9392 The Journal of Physical Chemistry, Vol. 95, No. 23, 1991
a monolayer or less. Culler et aL5 reported structural differences between physisorbed and chemisorbed APS on silicon powder based on FTlR diffuse reflectance spectra (DRIFT). Waddell et aL6 have shown that trifunctional alkylsilanes have higher stability than di- and monofunctional ones. Solid-state N M R techniques a t natural abundance with cross-polarization and magic-angle spinning (CP/MAS) of 13C,’-12 or both1G23have also been very useful for studying reacted chemical species on surfaces. Together with high-power decoupling, one can obtain high-resolution spectra and readily observe the structural changes of adsorbed organosilanes to surfaces. Many of these studies have focused on the structure of APSmodified silicas. In addition, the use of isotopes such as 2H2k28 or I3Cz9can be used to obtain information on specific portions of the molecule. 29Sispectra have been shown to be very useful for surface species on silica because the CP process is most effective for those silicon atoms near protons.I3 These are the ones near the surface. In addition, the long relaxation times of the bulk of the 29Sinuclei also reduce their intensity.I6 From the 29Si CP/ MAS spectra, three different silane chemical environments corresponding to the silanes having one, two, or three siloxane bonds at the silane-silica interface were observed.18 The formation of siloxanes was promoted by the presence of water and higher curing temperatures. From the I3C CP/MAS spectra, it was observed that the amino groups in samples prepared in dry toluene were hydrogen bonded or protonated by acidic silanols at the silica surface.22 In addition to structure, knowledge of the molecular motions in coupling agents may provide a better understanding of their behavior and relationship to the physical properties of composites. Only limited quantitative work has been done on the dynamics of these species, especially as it relates to composite materials. Sindorf and MacielIo have observed differences in the mobility of aliphatic silanes which increase in mobility with chain length. These differences have been confirmed by 2H s t ~ d i e s ,but ~ ~in, ~ ~ (5) Culler, S.R.; Ishida, H.; Koenig, J. L. J . Colloid Inrerface Sci. 1985, 106, 334. (6) Waddell, T.G.;Leyden, D. E.; deBello, M. T. J. Am. Chem. Soc. 1981, 103, 5303. (7) Leyden, D.E.; Kendall, D. S.;Waddell, T. G. Anal. Chim. Acta 1981, 126, 207. ( 8 ) Chiang, C.-H.; Liu, N.-I.;Kwnig, J. L. J. Colloid Inreflace Sci. 1982, 86. 26. (9) Hays, G. R.; Clague, A. D. H.; Huis, R.; van der Velden, G.Appl. Surj. Sei. 1982,IO, 247. (IO) Sindorf, D. W.; Maciel, G. E. J . Am. Chem. Soc. 1983,105,1848. (11) ZaDer. A. M.: Koenia. J. L. Polvm. ComDos. 1985.6 , 156. (1 2) Z a k r , A. M.; KoenigYJ. L. Ado. Colloid titerface Sci. 1985,22,113. (13) Maciel, G.E.; Sindorf, D. W. J . Am. Chem. Soc. 1980, 102,7606. (14) Maciel, G.E.; Sindorf, D. W.; Bartuska, V. J . Chromatogr. 1981, 205,438. (15) Sindorf, D. W.; Maciel, G. E. J . Phys. Chem. 1982,86, 5208. (16) Fyfe, C. A.; Gobbi, G. C.; Kennedy, G. J. J . Phys. Chem. 1985,89, 277. (17) Hoh, K.-P.; Ishida, H.; Koenig, J. L. Polym. Compos. 1990,I / ,121. (18) Sindorf, D. W.; Maciel, G.E. J . Am. Chem. Soc. 1983,105. 1487. (19) Bayer, E.; Albert, K.; Reiners, J.; Nieder, M.; Muller, D. J . Chromaiogr. 1983,264,197. (20) Sudholter, E. J.; Huis, R.; Hays, G. R.; Alma, N. C. J . Colloid Interface Sci. 1985, 103, 554. (21) deHaan, J. W.; Van Den Bogaert, H. M.;Ponjee, J. J.; Van De Ven, L. J. J . Colloid Interface Sci. 1986. 110. 591. (22) Caravajal, G.‘S.; Leyden, D. E.; Quinting, G.R.; Maciel, G. E. Anal. Chem. 1988,60, 1776. (23) Van Kan. J. M.; Ponjee, J. J.; DeHaan, J. W.; van de Ven, L. J. J . Colloid Inrerface Sei. 1988, 126, 604. (24) Keluskv. E. C.; Fyfe. C. A. J . Am. Chem. Soc. 1986. 108, 1746. (25) Gangoda, M.; Gilpin, R. K.; Figueirinhas, J. J . Phys. Chem. 1989, 93, 481 5. (26) Kang, H.-J.; Meesiri, W.; Blum, F. D. Mater. Sei. Eng. 1990,A126, 265. (27) Blum, F. D.; Funchess, R. B.; Meesiri, W. In Solid State NMR of
Kang and Blum a qualitative way. Hoh et a1.I’ have used 29Sicross-polarization times to study the relative mobilities of certain silane species under different conditions; however, lack of precise distance information makes quantitation of their data in terms of molecular motion difficult. In contrast, solid-state N M R methods have been used in a more quantitative way for studying the molecular motions in solid polymers. The line shapes of solid-state *H NMR spectra for selectively deuterated polymers have been shown to be a powerful tool for a number of systems.3w34 In the present paper, 29Si CP/MAS has been used to verify bonding of deuterated coupling agents to silica, and 2H wide-line N M R was then used to yield information on the nature of the structure and dynamics of the selectively deuterated amino functional silanes adsorbed on silica surfaces as a function of composition. In order to obtain more detailed information on the motional rate and the types of motion, the deuterium line shapes have been simulated using the slow-motional theory of Freed?”’ Experimental Section Sample heparation. Deuterated (aminopropyl)triethoxysilane (DAPES) and deuterated (aminobuty1)triethoxysilane (DABES) were prepared by deuteration of (cyanoethy1)triethoxysilane(CES) and (cyanopropy1)triethoxysilane (CPS), respectively.26 The synthesis scheme was
(CH,CH20),Si(CH2),CN
-Ni. D2
(CH3CH20),Si(CH2),CD2ND2
H10
(X),Si(CH2),CD2NH2
where X represents a variety of possible species including -OCH2CH,, -OH, or -OS-. When the coupling agents are hydrolyzed and adsorbed on the surface, they are referred to as DAPS and DABS to signify the loss of the ethoxy group. Both CES and CPS were purchased from Petrarch Systems Inc. (Bristol, PA). High-surface-area fumed silica, Cab-0-Si1 (S-17), having a normal surface area of 400 m2/g from the Cabot Corp. (Tuscola, IL), was used for the surface. Prior to use, it was dried under vacuum at 120 OC for 2 days to remove moisture. This treatment did not affect the surface area or adsorption behavior of the coupling agent.38 For the deposition of the coupling agents onto the silica surface, silane-treating solutions of different concentrations were prepared. About 0.1 5 g of silica was placed in a test tube and treated with an 8-mL aliquot of freshly prepared coupling agent solution in water. The solution was left in contact with the silica for about 3 h with occasional agitation using a vortex mixer. After that, the sample was centrifuged at about 3000 rpm for 1 h, washed twice with distilled water, and then dried in a vacuum oven at about 90 OC at 10 mmHg for 12 h. NMR Spectroscopy. The N M R experiments were performed on a Varian VXR-200 NMR spectrometer at room temperature. The solid-state 29SiN M R spectra were obtained at frequency of 39.7 MHz with cross-polarization and magic-angle spinning (CP/MAS) using a Varian PLE probe with zirconia stator and vespel rotor. Magic-angle spinning at 2.7 kHz, cross-polarization with a contact time of 2.5 ms, recycle delay time of 3 s, 90° pulse width of 10 ps, and high-power proton decoupling of 44.6 kHz to remove proton dipolar broadening were used. Approximately
~
Polymers; Mathias, L., Ed.; Plenum Press: New York, in press. (28) Blum, F. D.; Meesiri, W.; Kang, H.-J.; Gambogi, J. E. J . Adhes. Sei. Technol. lqgl, 5. 479. (29) HulJgen. T. P.; Angad Gaur, H.; Weeding, T. L.; Jenneskens, L. W.; Schurrs. H. E.; Huysmans, W. G.;Veeman, W. S.Macromolecules 1990.23, 3063.
(30) Jelinski, L. W.; Dumais, J. J.; Engle, A. K. Macromolecules 1983. 16, 492. (31) Bovey, F. A.; Jelinski, J. J . Phys. Chem. 1985,89, 511. (32) Spe.iss, H. W. Adu. Polym. Sei. 1985, 66, 23. (33) Smith, P. 9.; Bubeck, R. A.; Bales, S.E. Macromolecules 1988.21, 2058. (34) Hirschinger, J.; Miura, H.; Gardner, K. H.; English, A. D. Macromolecules 1990,23, 215. (35) Schneider, D. J.; Freed, J. H. In Spin Lubelling Theory and A p plications; Berliner, L.,Ed.; Academic Press: New York, 1976; Vol. I. (36) Schneider, D. J.; Freed, J. H. In Spin Labelling: Theory and A p plications; Berliner, L., Ed.; Academic Press: New York, 1989; Vol. 8. (37) Jagannathan, S.;Blum,F. D.; Polnaszek, C. F. J . Chem. In/. Comput. Sci. 1987,27, 167. (38) Meesiri, W. Personal communication.
Amino Functional Silanes Adsorbed on Silica Surfaces 6000 and 10000 scans were used for pure and treated silica, respectively. The chemical shifts were given relative to external tetramethylsilane. The spectrometer was equipped with a wide-line probe operated at a deuterium frequency of 30.7 MHz. All 2HN M R spectra were collected using a variant of the solid-echo pulse sequence39 [ 1 80°-(alternate pulse)4elay-90°x-rl-900,,-~2-echo] with the 1 80° prepulse and subtraction used on each alternate scan. This entire sequence was also phase cycled and has the effect of reducing coherent noise and acoustic ringing while coherently adding the nuclear signals. For all of the 2H NMR spectra, a delay time, 71, of 30 ps, repetition time of about 2 s, and 90° pulse width of 2.0 ps were used. Typically 2000-10000 echos were required per spectrum, depending on the sample because of the dilution effect of the material providing the surface.40 No attempt was made to correct the spectra for finite pulse widths. Simulation of the Deuterium Spectra. Deuterium line shapes were simulated using the slow-motional theory developed by Freed.35-37 With this method the Hamiltonian matrix is constructed, made tridiagonal by the Lanczos algolithm, and then used to calculate the spectrum. The simulated spectra were compared to the experimental ones to obtain quantitative information about the molecular motion of the silica-bound silane. The calculations were performed on a Silicon Graphics Personal Iris 4D/20 and required about 3-5 min per simulation. Each simulation requires the specification of the tilt angle between the C-D bond vector and the external field, the quadrupole coupling constant, two rotational diffusion rates (d, and d,,), and the jump rate. In addition, the amounts of a rigid (% R, Pake powder pattern) and mobile (% M) components were varied. The "best fits" were made by eye. In order to estimate the uncertainty of the quantities measured, each of the "best fit" parameters was varied by a factor beyond which a good fit was no longer obtainablee2* Thus, we estimate that the reorientation rates are known to within a factor of 2 and the percentages of rigid and mobile components to within *lo% absolute.
Results 2%i NMR Spectra. The degree of condensation of the coupling agent with the silica surface is known to increase with an increase in curing temperature;22 so care is needed in sample comparisons. In this study, all of the samples used were prepared under similar hydrolysis and curing conditions so that a meaningful comparison could be made. The CP/MAS 29SiN M R spectra of bulk and treated silica (Cab-O-Sil) are shown in Figure 1 along with assignments from the l i t e r a t ~ r e . ' ~The - ~ ~treated silica was mixed with an aqueous solution containing 2 and 10 wt % deuterated (aminobuty1)triethoxysilane (DABES). The 29SiN M R spectrum of the pure silica, shown in Figure IA, has two major resonances at -1 10.6 and -102.0 ppm, denoted as 4 4 and 43,respectively. These resonances are assigned to %i nuclei having four Si-0-Si linkages (44) and %i nuclei having three S i U S i linkages and one - O H (43)as shown be lo^:'^-^^ -(Si-O)$i -(Si-O)3SiOH
44
43
-(Si-O),SiCH2CH2CH2CH2NH c3 The majority of the spectral intensity appears in the 43 region, although quantitation of these resonance intensities has not been attempted. The 29Sispectrum of Cab-O-Si1 suffers from a poorer signal-to-noise ratio than silica gel2' due to its different and lower hydroxyl content. When the silica was treated with 2% DABES solution, the 29Si spectrum shown in Figure 1 B exhibited a new broad resonance (C3)a t about -67.4 ppm. This resonance is mainly due to the 2%i nuclei from the (aminobuty1)silane in the interfacial region (39) Davis, J. H.;Jeffrey, K. R.;Bloom, M.;Valic, M.1. Chem. Phys. Lert. 1976, 42. 390. (40) Blum, F. D.Colloids Surj. 1990, 45, 361.
The Journal of Physical Chemistry, Vol. 95, No. 23, 1991 9393 a Q 4
Ib" 1 1 1 1
3
1 1 1 1
- 0
1 1 1 1
-;o
/ I l l
1111
-iom
Figure 1. %i CP/MAS NMR spectra: (A) bulk silica (CabO-Sil), (B) silica treated from a 2% DABES solution, (C) silica treated from a 10% DABS solution. Q3,Q4, and C3 represent the -(SiO)3SiOH, -(SiO),Si, and -(SiO),SiR species, respectively. The C3 resonance may also include contributions from other -SIR species (see text).
which have three Si-0-Si linkages.'8-22 Figure 1B also shows a change in the relative intensities of surface sites, 44 and 4 3 , compared with the spectrum for pure silica. The 4 4 peak is the most intense for silica treated with DABES solution while the 43 peak is the most intense for pure silica. This difference is because the reaction of coupling agents with the surface silanols on the silica surface eliminates many of the 43 species. When the silanols on the silica surface react with DABES, the 43 sites are converted to 44 sites. This is consistent with other silica gel/coupling agent s t u d i e ~ . 'In ~ the spectrum of silica treated with a 10% DABES solution (Figure lC), the intensity for the C3 site in the interfacial layer is increased relative to the 4 4 species. This change is due to an increase in the amount of polysilsesquioxane in the interfacial layer. It indicates, qualitatively, that the amount of the coupling agent in the interfacial layer is increased with the concentration of treating solution. It is known that the amount of coupling agent adsorbed on a substrate is dependent upon the solution concentration!I The spectrum shows only a relatively small contribution from 4 3 sites which suggests that relatively few silanols remain unreacted when treated at high concentration. 2H NMR Spectra. In order to probe the molecular motion of silanes adsorbed on silica as a function of the amount and type of silane coupling agent (DAPES and DABES), solid-state 2H NMR spectra were collected. Shown in Figure 2 are the spectra of silica treated with DABES as a function of treating solution concentration. The concentrations of the treating solution were increased from 0.5 to 10 wt %. Figure 2 shows that the resonances become broader with an increase in the concentration of the DABES solution and then narrower with further increases. The broadest spectrum is that from the 2% solution. The spectrum from the 2% solution also shows the appearance of two components, assigned to a rigid (Pake pattern with splitting of about 115 kHz) and mobile (narrower center resonance) component. From solution concentrations above 2%,the spectra show narrower resonances for the mobile component, which narrows with increased concentration of the treating solution. The spectra for the silica treated with 1% and 10%solution were obtained as a function of 71 in the range 71= 30-100 ps. Although there is a small change between the spectra with 7, = 30 and 50 ps, the spectra did not show any significant change in the line shape. (41) Ishida,
H.;Koenig, J. L. J . Colloid
Interface Sci. 1978, 64, 555.
9394 The Journal of Physical Chemistry, Vol. 95, No. 23, 1991
Kang and Blum
TABLE I: Half-Widths for Ex~erirmatalS~ectraand Panmeters for Simulated SDectrr for 3H NMR Soectra of DABS and DAW on Silica experiment simulation concn of treating s o h % AvI2. kHz %R %M jump rate k, MHz d - kHz d... kHz ~
DABS 74 78 78 62 42
0 0 20 15 5
0.5 1 .o 2.2 3.3
74 77 78 73
10.0
60
0 0 25 15 5
0.5 1 .o 2.0 3.5
10.0
100 100 80 85 95
IO IO IO IO
1.5 1 .o 1.o
1.6 2.0
70 70 70
70 500
100
DAPS 100
I .5
IO
1.5
10 10 10
2.0
IO
1 .o 1 .o
IO0 75 85 95
Exp.rlmontel
SlmulMw
.1bo
d
160
Frequency (KHr)
260
-260
-1bo
d
do
do
-2b0
Frequrncy (KHr)
Figure 2. Experimental and simulated solid-state 2H NMR spectra for silica treated with DABES solutions as a function of the composition of the treating solution: (A) 0.5%, (B) 1 .O%, (C) 2.0%, (D) 3.576,and (E) 10.0% DABES in water.
Simulated deuterium line shapes have been calculated to obtain information on the motional mechanism and the rate, plus the relative amounts of the rigid and the mobile components. Motional models based on only rotational diffusion or two-site jump motions alone did not produce spectra that simulated the experimental ones. Two-site jumps, superimposed on rotational diffusion, were required for proper simulation of the mobile component. The best fits of the experimental spectra have been simulated by assuming jumps between two equivalent sites about a diffusion axis tilted at 54.7'. For all of the calculated spectra, a quadrupolar coupling constant of 167 kHz was used. The motion of the bond vector was characterized by a rotational diffusion tensor, which was assumed to be symmetric about the z axis so that it is specified by two values, d,, and dxy. The parallel ( t i z z ) and perpendicular (dxy)rotational diffusion rates and the jump rates (k)have been varied to obtain the best fits. The parameters used for the simulations are shown in Table I for both systems, along with the experimental line width at half-height ( A u I I Z ) .The simulated spectra are shown in Figure 2 for comparison with the observed ones. There is a good agreement between the simulated and experimental spectra. In order to probe the effect of the type of coupling agent on the molecular motion, solid-state fH N M R spectra for D A B on silica were collected and shown as a function of the amount of coupling agent in solution along with the simulated spectra in Figure 3. The spectra of DAPS on silica show almost the same line shapes as those for DABES from solution in the concentration range from 0.5% to 2%. This similarity suggests that the difference
Slmuirtwl
A
C
.200
70 70 70 70 70
.1bo
d
do
Frequency ( K H r )
I
220 -200
I
I
I
.loo
0
100
2d0
Frrquency (KHz)
Figure 3. Experimental and simulated solid-state ' H N M R spectra for silica treated with D A B solutions as a function of composition of the treating solution: (A) 0.5%, (B) 1.0%, (C) 2.0%, (D) 3.5%, and (E) 10.0%DAPES in water.
in the carbon chain length does not significantly affect the m e lecular motion of the silane when adsorbed on the silica surface. This similarity is not observed for longer alkyl chain silanes.'0*2s The spectra for DAPS on silica, as those for DABS, show narrower spectra for samples from the 0.5% and 1% solutions with the broadest line shape from the 2% solution. This also suggests that the monolayer coverage for DAPS on the silica surface might be obtained from a concentration of 2% under our conditions. For samples from solutions above this concentration (3.5% and lo%), the spectra for the DAPS samples are the broader than those for DABS at same concentrations. This broader resonance of DAPS at high concentrations, compared with the case of DABS, indicates either less mobility or more anisotropic motion of DAPS molecules from the polysilsesquioxane layer not in intimate contact with the surface. These results are consistent with the notion that the molecular motion of D A B is more restricted than that of DABS when it is polymerized to the bulk h o m ~ p o l y m e r . ~ ~ ~ ~ ~ ~ ~ ~ In order to probe the reproducibility of these experimental results, a second set of DAPS/silica samples were prepared as a function of treating-solution concentration and their spectra taken. These spectra showed almost identical line shapes as the spectra shown in Figure 3, although there were small differences (3-5 kHz) in the line widths between them. In general, small (42) Kang, H.-J.; Blum, F. D. Unpublished results. (43) Kang, H.-J. Ph.D. Thesis, University of Missouri-Rolla, Rolla, MO, 1990.
Amino Functional Silanes Adsorbed on Silica Surfaces differences in preparation can yield small, but measurable, differences in the final material.
Discussion The 29SiN M R spectra provide structural information on the adsorbed hydrolyzed a m i n o s i l a n e ~ . I ~In- ~the ~ spectra of silica treated with DABES solutions, the broad single C 3 resonance arising from 29Sinuclei having three Si-0-Si linkages and an aminobutyl group suggests three. possible structures of silane adsorbed on silica: (i) a silane having a tridentate linkage to the silica surface (although this is unlikely due to the molecular geometry), (ii) a silane having two Si-0-Si linkages with the surface (this may also be unlikely for Cab-O-Si12’) and one Si+Si linkage with a DABS molecule; and (iii) a silane having one S i 4 4 3 linkage with the surface and two Si-0-Si linkages with other DABS molecules. Previous work21 suggests (iii) may be most likely due to fewer (but more reactive) silanols on CabO-Si1 as compared to silica gel. In addition, the breadth of the C3 resonance cannot preclude the presence of C2 species which is due to 29Sinuclei with two Si-0-Si linkages, one -OH,and an aminobutyl group. Neither spectrum of silica treated with DABES shows evidence of any intramolecular five-membered rings, with the amine group pentacoordinated to a silicon atom as proposed by Plueddemann.” The %i chemical shift of this species would be shown in the region between -120 and -1 80 p ~ m Photoacoustic . ~ ~ infrared results also suggest an orientation for (aminopropy1)silane perpendicular to the surface.46 Amino functional silane coupling agents exhibit unique behavior, unlike other neutral silanes. Most neutral silanes in aqueous solution exist predominantly as a monomer below 1 wt %. In contrast, aminosilanes are generally oligomers even at I wt %.47 The instability the of aminosilanetriol a t concentrations less than 1% is considered to be due to the self-catalyzed condensation of the silanetriol by the amino group. Ishida et al.48 have studied (aminopropy1)silane (APS) in aqueous solution at concentrations ranging between 2 and 80 wt % by Fourier transform infrared spectroscopy (FTIR). They reported that APS is oligomeric ,in this concentration range by observing the band around 1115 cm-I, which is due to the S i U S i antisymmetric stretching mode. In the concentration range studied, the presence of both monomeric and oligomeric species is expected with the latter more likely at high concentrations. The %i solid-state spectra also suggest that more surface silanols are reacted at higher treating solution concentrations. The calculated spectra for both coupling agents, DABS and DAPS on silica, are consistent with adsorbed silane molecules undergoing anisotropic rotation plus two-site jump motions. The tilt angle of 5 4 . 7 O (half of the jump angle) used is similar to the 103’ jumps found in poly(buty1ene tere~hthalate).”~~ The jump rates and percent mobile component decrease with increasing concentration of treating solution until the concentration for “monolayer coverage” (ca. 2%) is reached. For the spectra of silica treated with 3.5% and 10% DAPES or DABES, the calculated spectra suggest a decrease in the relative amount of rigid component and a faster motional rate of mobile component. The solid-state 2H NMR spectra can be understood on the basis of surface coverage. Adsorption isotherms for (aminopropy1)silane on Cab-0-Si1 have been reported.28 With the known amounts of coupling agent and surface used, the adsorption isotherm can be used to predict the amount of surface coverage. The narrower resonances for the 0.5%and I%, compared to 2%, can be explained if many silanols on the silica surface have not reacted with DABES. For the amounts of materials used here, treated surfaces from 0.5% and 1% solutions correspond to submonolayer coverage (44) Plueddemann, E. P. Chemistry of Silane Coupling Agents; Plenum Press: New York, 1982. (45) Marsmann, H. N M R Basic Princ. Prog. 1981, 17, 65. (46) Urban, M. W.; Koenig, J. L. Appl. Spectrosc. 1986, 40, 513. (47) Ishida, H.; Naviroj, S.;Tripathy, S.K.; Fitzgerald, J. J.; Koenig, J. L. J . Polym. Sci.. Poly. Phys. Ed. 1982, 20, 701. (48) Ishida, H.; Chaing, C. H.; Koenig, J. L. Polymer 1982, 23. 251.
The Journal of Physical Chemistry, Vol. 95, NO. 23, 1991 9395 and that from the 2% solution corresponds to approximately monolayer ~ o v e r a g e . ~ *Submonolayer *~~ coverage would result in moie space between the adsorbed molecules and consequently less restricted molecular motion of the coupling agents. The most rigid spectrum is that for treated silica from a 2% solution. This is consistent with the silica surface being highly covered with coupling agent. This crowded surface results in the most restricted motion of silane on silica as evidenced by (i) the spectrum with the highest amount of rigid component and (ii) the slowest mobile component for both DABS and DAPS. The adsorption isotherm of completely hydrolyzed APES from a 1011 acetone/water mixture showed a rapid increase until about OS%, then a gradual increase, and finally a leveling off, corresponding to ‘monolayer coverage”, at a concentration corresponding to about 2-3%. We estimate that there are roughly four coupling agents adsorbed per 100 A* surface a t monolayer coverage or one per surface hydroxyl.21*2**mAt higher adsorbed amounts, the excess coupling agent will form a polysilsesquioxane layer which is bonded indirectly to the silica surface. The molecular motion of aminosilane in this layer is faster or more isotropic in comparison with the motion of aminosilane in this layer is faster or more isotropic in comparison with the motion of silane adsorbed directly onto the silica surface. Therefore, it is appropriate for the resonance in the center of the spectra for silica treated with aminosilane to have a contribution from two mobile components in addition to a rigid component, although the line shapes do not show separate moieties for each. For simplicity, we denote these as mobile and highly mobile species. The silane molecules adsorbed directly on the silica surface correspond to the mobile species. The silane molecules in the second polysilsesquioxane layer are indirectly adsorbed onto silica and are consequently of relatively mobile. For silica treated with higher concentrations (3.5% and 10%) of DAPS, the broader resonances in the line shapes than those for DABS can be explained on the basis of the molecular motion of DAPS and DABS in condensed polymers. (We assume multilayer formation, although the adsorption isotherms for these higher concentrations have not been measured by us.) The spectrum of DAPS in the condensed polymer showed a larger fraction of rigid component and a lower mobility of mobile component than those from the DABS condensed polymer!2 These differences are due to the difference in the carbon chain lengths. The slower or less isotropic motion of DAPS is probably due to the shorter propyl chain portion which has less internal flexibility than that of the longer butyl chain in DABS. (We remind the reader that this effect is not evident on the surface because of chain packing.) Therefore, for silica treated with highest concentrations of treating solutions, the broader resonance in the spectrum for DAPS than those for DABS suggests that the structures of the polysilsesquioxane layers of DAPS and DABS are similar to the bulk condensed polymers. The simulations suggest that the rotational motion of the DABS in the mobile component from the 10% solution is approximately an order of magnitude faster than DAPS under the same conditions, although we note that the uncertainties in these quantities are about a factor of 2. On the basis of these experiments, we suggest an explanation for the spectra observed in terms of dynamics. At the low concentrations, corresponding to the submonolayer or monolayer coverage, the silane molecules may be adsorbed on the silica surface through the formation of siloxane bonds. Consequently, the silanes are constrained by neighboring silanes. Silane molecules adsorbed on surface would roughly be confined to a cylinder because of the neighboring constraints and the surface bonding. At the higher surface coverages, the polysilsesquioxane layer forms a structure which may be similar to that of their bulk polymers. This proposed mechanism is consistent with our earlier results26*28 in which the 2H solid-echo NMR spectrum for silica treated with 2% DAPES solution showed a more motionally restricted spectrum (49) Meesiri, W. Ph.D. Thesis, University of Missouri-Rolla, Rolla, MO, 1988. (50) Cab-0-Si1 Properties and Functions, Cabot Corp., Tuscola, IL, 1983.
J. Phys. Chem. 1991, 95, 9396-9402
9396
than that for the DAPES condensed polymer. Conclusions
The 29si CP/MAS N M R spectra of the Cab-0-Si1 silica and treated Cab-0-Si1 silica with coupling agent solutions provide information on the nature of the structure of the silane coupling agents adsorbed on the silica surface similar, but not identical to, that of silica The 29Si N M R spectra of treated silica showed that the silane molecules were chemically adsorbed to the surface through the formation of a siloxane bond with the silanol on the surface. The amount of coupling agent adsorbed on the silica surface was increased with an increased concentration of treating solution. The solid-state 2H N M R spectra as a function of the amount of silane coupling agent provide information on the molecular motion of silanes adsorbed on silica and the adsorption behavior. The simulated deuterium line shapes suggest possible motional mechanisms, rates, and the relative amounts of rigid and mobile components. The spectra are consistent with rigid and mobile fractions with the mobile fractions undergoing anisotropic rotation plus two-site jumps about a diffusion axis tilted at 54.7O. The calculated spectrum for the sample from the 2% treating solution, which may correspond to monolayer coverage, shows the highest
relative amount of rigid component and the slow reorientation rate, indicative of the most crowded surface environment. 29SiNMR data and 2H N M R spectra for DAPS and DABS are consistent with the notion that silane molecules are adsorbed and reacted directly onto the silica surface until monolayer coverage and then form a polysilsesquioxane layer which is intermolecularly condensed above the monolayer and indirectly bonded to the surface. From solution concentrations above this for monolayer coverage, the molecular motion has a contribution from a highly mobile component resulting from the silane molecules of the polysilsesquioxane layer. The *HN M R spectra for both DAPS and DABS on a silica surface suggest that the molecular motion of silane directly adsorbed on silica is not affected by the difference in carbon chain length of amino functional silane coupling agent. It appears to be affected by the amount of coupling agent for these fairly short chain lengths. This is in contrast with that observed for longer chains.
Acknowledgment. The authors thank the Office of Naval Research for their financial support. They also thank Drs. Schneider and Freed for supplying the original program and R. B. Funchess for adapting it for deuterium line shape calculations.
Promoter Action of Alkali Nitrate in Raney Ruthenium Catalyst for Activation of Dinitrogen Tokihisa Hikita, Yasushi Kadowaki, and Ken-ichi Aika* Research Laboratory of Resources Utilization, Tokyo Institute of Technology, 4259 Nagatsuta, Midori-ku. Yokohama 227, Japan (Received: October 23, 1990)
Alkali nitrate promoted Raney Ru catalysts were prepared by decomposition of alkali nitrates (CsNO,, RbNO,, KNO,, and NaNO,) with hydrogen over Raney Ru. These catalysts were as active as Raney Ru promoted with metallic potassium at 573 K in N2 activation (ammonia synthesis and especially isotopic equilibration reaction (IER) of N2). The promotional behavior of alkali nitrates on Raney Ru was different from that on the supported Ru catalysts. The alkali was estimated to work as a metallic on Raney Ru, whereas it was estimated to be hydroxide on supported Ru. The more reduced form on Raoey Ru-CsNO, was considered to give a higher turnover frequency of IER of N2 than that over alumina-supported Ru-CsNO,. Since the rate of IER of N2 is a rate of tracer atom moving from N2 to adsorbed N under the condition of adsorption equilibrium, it should be slower than the rate of ammonia synthesis whose adsorption step is rate-determining in a dynamic condition. To the contrary, the rates of ammonia synthesis were slower than IER rates of N2 over Raney Ru-CsNO,, suggesting hydrogen inhibition in the N2 activation process. Indeed, the IER of N2 over Raney Ru-CsNO, was proved to be retarded by the presence of hydrogen. A kinetic analysis disclosed that N(a) and H(a) compete with each other on the Ru surface where H(a) adsorption is stronger than N(a) adsorption at 473-523 K. The heats of adsorption of N 2 and H2 were estimated from the kinetics.
Introduction
Ruthenium has been known to be quite active in ammonia synthesis when it is promoted with electron donors or basic oxides.' Two different forms of Ru catalysts can be prepared: supported Ru2+ and Raney R u . ~For ~ ~supported catalysts, a metallic alkali promoter resulted in higher activity than did an alkali nitrate promoter such as C S N O , . ~However, ~ water-sensitive metallic (1) Oulki, A,; Aika, K. In Catalysis-Science and Technologv; Anderson, J. R., Boudart, M., Eds.; Springer-Verlag: Berlin, 1981; Vol. 1, Chapter 3, p 81. (2) Aika, K.; Hori, H.;Ozaki, A. J . Catal. 1972, 27, 424. (3) Aika, K.; Shimazaki, K.; Hattori, Y.; Ohya, A.; Oshima, S.; Shirota, K.; Ozaki, A. J. Catal. 1985, 92, 296. (4) Aika, K.; Ohya, A.; Ozaki, A.: Inoue, Y.; Yasumori, 1. J . Catal. 1985, 92, 305. ( 5 ) Urabe, K.; Yoshioka, T.; Ozaki, A. J . Coral. 1978, 54, 52. (6) Ogata, Y.; Aika, K.; Onishi, T. J. Catal. 1988, 112, 496.
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alkali is not easily handled, whereas the alkali nitrate can be safely treated for preparing the catalysts. During the activation process, alkali nitrate decomposition is promoted by the presence of Ru in hydrogen, which results in an active promoter form., The promoter action of alkali nitrates has been studied comprehensively on supported Ru c a t a l y ~ t s . ~The . ~ alkali promoter is considered to interact not only with the Ru surface directly but also with the support. Thus, the direct interaction should be studied in detail on the catalyst without the support. However, a support-free Ru powder-alkali nitrate catalyst does not provide enough activity for detailed study because it has a low surface area (1 m2/g). The Raney Ru-alkali nitrate system was thus chosen as a model system, although the surface was covered partly by residual Ala6 The surface area was about 50 m2/g in the last study' and 40 m2/g (7) Aika, K.; Ogata, Y.; Takeishi, K.; Urabe,K.; Onishi, T. J. Coral. 1988. 114. 200.
0 199 1 American Chemical Society
