Multinuclear NMR Spectroscopy Studies of Silica Surfaces - American

14 Multinuclear NMR Spectroscopy

Downloaded by CORNELL UNIV on October 9, 2016 | http://pubs.acs.org Publication Date: May 5, 1994 | doi: 10.1021/ba-1994-0234.ch014

Studies of Silica Surfaces Gary E . M a c i e l , Charles E. B r o n n i m a n n , Robert C . Zeigler, I-Ssuer Chuang, D a v i d R. Kinney, and E l l e n A . Keiter 1

Department of Chemistry, Colorado State University, F o r t Collins, CO 80523

H --> Si cross polarization with magic-angle spinning is a valuable approach for observing local silicon environments on the silica surface. The H combined rotation and multiple-pulse spectroscopy (CRAMPS) approach distinguishes clustered and isolated surface silanols. Correlations of the variations in Si and H peak intensities with silane loading level in silica gels derivatized with (CH ) SiCl suggest an incorrect model of the silica surface that is inconsistent with models derived from more detailed considerations. Other nuclides (e.g., H, C, and N, along with Si) provide valuable approaches for the characterization of local structure and motion in derivatized (silylated) silicas. 1

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HIGH-RESOLUTION NMR SPECTROSCOPY TECHNIQUES FOR SOLIDS developed during the past 15 years (1-4) have been applied to the study of surfaces (5). N M R spectroscopy, with its typical focus on short-range (local) order (chemical structure), provides a powerful complement to diffraction techniques, which require long-range order (i.e., a high degree of crystallinity). O f course, N M R spectroscopy has notoriously poor intrinsic sensitivity properties, so it has typically been limited to systems that have relatively large numbers of the relevant nuclei (~10 ) at the surface. Because most surface systems of technological interest (e.g., in catalysis and in separations) have high surface areas and are not highly regular at 19

J

O n leave from Eastern Illinois University.

0065-2393/94/0234-0269$08.00/0 © 1994 American Chemical Society

Bergna; The Colloid Chemistry of Silica Advances in Chemistry; American Chemical Society: Washington, DC, 1994.


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the surface, N M R techniques are often well suited to their study. Silica surfaces often fall into this category. The most generally useful line-narrowing technique in modern solidstate N M R spectroscopy is magic-angle spinning (MAS), in which the sample is mechanically rotated rapidly (thousands of revolutions per second) about an axis that makes an angle of 54.7° relative to the direction of the static magnetic field (6-9). Sufficiently rapid M A S brings about the coherent averaging of inhomogeneous line-broadening effects, such as the chemical-shift anisotropy (CSA) and inhomogeneous magnetic dipole-dipole interactions. This effect is a coherent, mechanical analog of the incoherent motional averaging accomplished by Brownian motion in liquids—the reason that high-resolution N M R spectroscopy enjoyed popularity for liquid samples for 25 years before it did for solids.

Underivatized Silica Surface Figure 1 (bottom) shows the S i (spin 1/2, 4.70% natural abundance) M A S N M R spectrum of the silica gel system (10). This spectrum shows a small peak at - — 9 0 ppm due to =Si(OH)2 groups at the surface, a large peak at - — 1 0 0 ppm due to = S i - O H groups at the surface, and a large peak at 109 ppm due to silicon atoms in the surface region with no directly attached hydroxyl groups. The lack of line broadening due to 8 ΐ - Ή dipolar interactions in this spectrum is due to the effects of both M A S and high-power Ή decoupling. The fact that the large peak at 109 ppm doesn't completely dominate the spectrum is due in part to the very high surface area of the silica gel sample (about 260 m /g), but primarily results from the fact that a highly effective surface-selective technique has been employed—cross polarization (CP) (11-13). In the cross-polarization approach, spin polarization from a more abundant spin set that has a larger nuclear magnetic moment (in this case, Ή ) is transferred via a double-resonance method to a less-abundant spin set that has a smaller nuclear magnetic moment (in this case, Si). The mechanism for the C P process in this double-resonance experiment involves a static component of the magnetic dipole-dipole interaction (in this case, a * H M S i dipolar interaction) and hence falls off rapidly with increasing distance. Therefore, because nearly all of the protons in this system are present at the surface as covalently attached O H groups or as physisorbed H 2 O , there is a dramatic selectivity of the S i spin polariza tion for the surface environment. O n the basis of this approach, first used as a surface-selective strategy by Sindorf and Maciel (13-19), numerous studies of the effects of dehydration and rehydration or derivatization of the silica surface were carried out (13-29). Figure 1 (middle and top) shows examples of this approach. 29

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-Τ—ι—ι—ι—ι—j—ι—ι—ι—ι—j—ι—ι—ι—ι—ι—ι—>—ι—>—Γ"* 50

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Figure 1. Si CP-MAS spectra of hydrated Fisher S-157 silica gel (bottom), silica gel heated under vacuum at 200 °C (middle), and silica gel derivatized by (CH ) SiCl (top) (10). m

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The other magnetic nuclei present in significant numbers at the typical silica surface are 0 (0.037% naturally abundant) and Ή (99.99% naturally abundant). Oldfield and co-workers (30, 31) obtained Ή —• 0 C P spectra of silica sμrfaces i n experiments that normally require isotopic enrichment because the small nuclear magnetic moment (about 14% of that of Ή ) and quadrupolar broadening exacerbate both sensitivity and 1 7

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resolution problems. 0 N M R spectroscopy is likely to play a significant role i n future N M R studies of the silica surface. Ή N M R studies of silica surfaces have been numerous. The possibility of high local concentrations of protons (e.g., clustering of O H groups) can render the Ή - Ή dipolar interactions homogeneous and thus limit the success of moderate-speed M A S i n averaging the Ή - Ή dipolar line broadening. Nevertheless, apparently useful Ή M A S spectra on silica-type samples have been reported (32, 33), although some may be "distorted" in the manner indicated here (34). The possibility of line broadening or spectral distortions due to Ή - Ή dipolar interactions can be largely eliminated by the use of multiple-pulse homonuclear dipolar line-narrow ing techniques (35) i n conjunction with M A S . This combination of techniques, combined rotation and multiple-pulse spectroscopy (CRAMPS) (36), is capable of providing line widths of 0.2-0.3 ppm i n favorable cases, i f there is not extensive line broadening due to (1) molecular motion, (2) unaveraged dipolar interactions with quadrupolar nuclei, or (3) large inhomogeneous effects from chemical inhomogeneities or magnetic susceptibility effects (37, 38). Figure 2 shows Ή C R A M P S spectra (39) of silica gels. Parts A , A ' , and A " of Figure 2 show that the spectrum obtained on the untreated sample can be computer-simulated as the sum of contributions from relatively sharp, symmetrical peaks centered at 1.7 and 3.5 ppm and an asymmetrical peak or band extending from about 8 to about 1 ppm. The spectrum of the silica gel after a 25 °C vacuum dehydration (Figure 2B) allows the peak centered at 3.5 ppm i n Figure 2 A to be easily identified as physisorbed water. A CRAMPS-based Ή technique that is extremely useful in the study of surfaces and greatly aids i n identifying the origins of the other two peaks in the spectra shown in Figure 2 is a dipolar-dephasing experiment (39). In this experiment, a "dephasing period*' is inserted between the initial pulse that first generates transverse Ή magnetization and the C R A M P S detec tion period. During the dipolar-dephasing period, no multiple-pulse dipolar line narrowing occurs, so the Ή magnetization evolves under the influence of whatever Ή - Ή dipolar interactions may be present i n the system. F o r those protons that are isolated from other protons (e.g., i n isolated silanols), the magnetization is little affected by the dephasing period. However, the magnetization due to those protons that feel strong dipolar interactions with other protons (e.g., i n clustered, H-bonded silanols or relatively stationary water clusters) should lose phase coherence and hence experience a substantial attenuation during a dephasing period of 5 0 - 1 0 0 ^s. O n this basis, the sharp peak at 1.7 ppm i n the spectra of Figure 2 was identified as "isolated silanols", and the broad asymmetrical band was identified as "clustered silanols". It is interesting, and not


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Figure 2. 187-MHz H CRAMPS spectra of Fisher S-679 silica gel (A) untreated, (B) evacuated at 25 °C, (C) evacuated at 200 °C, and (D) evacuated at 500 °C. Plot A" is the deconvolution of spectrum A; plot A ' is a computer simulation based on A". (Reproduced from reference 39. Copyright 1988 American Chemical Society.) 2


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surprising, that the isolated silanols are retained after sample evacuation at 500 °C (Figure 2D), whereas the " c l u s t e r e d " silanols are eliminated. The silylation of a silica surface not only gives rise to a new, silane peak in the S i spectrum (~15 ppm in Figure 1, top), but also causes an alteration of the intensities in the =Si(OH)2, ^=SiOH, and =Si= peaks. These spectral changes permit the use of silylation for studying relative reactivities, as has been demonstrated by Sindorf and Maciel (16, 18, 19). The silylation process also changes the Ή C R A M P S spectrum (40): new peak(s) due to the surface-attached silane moieties are introduced, and intensities in the silanol peaks are altered. Thus, the silylation process can be used to correlate Ή and S i N M R views of the silica surface. F o r this purpose, silylation by trimethylchlorosilane (TMCS) was used. 29


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For TMCS-derivatized silicas with any but the lowest surface loading levels, the Ή C R A M P S spectra are overwhelmed by the dominating (CH3)3Si-0- peak; hence, (CEfoJaSiCl that was largely deuterated was employed to markedly attenuate this signal. Experiments of this nature were carried out as a function of the extent of silylation. Figure 3 shows the Ή C R A M P S and S i C P - M A S spectra obtained on a series of T M C S derivatized silica gels with various ( C H b ^ S i - O - loading levels. As the loading level increases, the relative intensities within the Ή and S i spectra are altered, the most conspicuous change being the increasing intensity of the sharp (CH3)3Si-0- peaks at —0.5 and —15 ppm i n the Ή C R A M P S and S i C P - M A S spectra, respectively. To compare changes in the various peak intensities as a function of ( C H 3 ) s S i - 0 - loading level, deconvolutions of the Ή and S i spectra were carried out, and suitable corrections for spin dynamics were applied. O n this basis, comparisons of the Ή and S i intensities of the individual contributing peaks can be made. These comparisons can be visualized from the plots given in Figure 4. 29

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Comparisons of the trends in the upper and lower portions of Figure 4 reveal intensity patterns with the following apparent correlations: the sharp Ή resonance of "isolated" surface O H groups appears to correlate with the S i resonance of =Si(OH)2 groups, and the broad Ή peak of hydrogen-bonded (clustered) surface O H groups appears to correlate with the S i resonance of = S i - O H . This pattern suggests a surface structure in which there are large hydrogen-bonded clusters of = S i O H g P and isolated islands of =Si(OH)2 groups on the surface. However, this model is not entirely consistent with the C R A M P S determined Ή dipolar-dephasing behavior (39), the main basis for distinguishing between the clustered and isolated protons in Ή C R A M P S spectra. This model is also inconsistent with related Ή spin-diffusion behavior, as reflected in some preliminary Ή - 8 ί dipolar-dephasing experiments, Si-detected Ή - Ή spin-diffusion results, and S i C P - M A S spectra detected in the absence of Ή decoupling. Such experiments, 29

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Figure 3. H CRAMPS (left) and Si CP-MAS spectra (right) of silica gel derivatized by (CHs) SiCl [perdeuterated, except for 1% (C H3)sSiCl]. Loading levels (percent silylation) are shown in the center. l

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which are reported i n detail separately (41), indicate that the protons responsible for the broad Ή C R A M P S peak identified with H-bonded silanols are the protons responsible for cross polarization of =Si(OH)2 silicons and some ^ S i O H silicons. The protons identified as isolated silanols (non-H-bonded) i n the Ή C R A M P S spectrum are the protons responsible for cross polarization of the remaining = S i O H silicons in the S i C P - M A S spectrum. These correlations, which are more compatible 29


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Figure 4. Signal intensities versus silane loading level (percent silylation) in derivatized silica gels shown in Figure 3: A, Si CP-MAS intensities of =Si(OH) , =SiOH, and =Si=; and B, m CRAMPS intensities of clustered (Hhonded) and isolated SiOH groups. 29

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with the relevant infrared work (42-44), lead to a surface structure that we hypothesize to be composed of Η-bonded =Si(OH)2 and = S i O H groups along with other = S i O H groups that exist in somewhat more isolated, nonH-bonding sites on the surface. Ή - Ή dipolar interactions and spindiffusion interactions within the Η-bonded silanols are much stronger— because of much smaller Ή - Ή distances—than within the non-H-bonded = S i O H surface moieties. Future avenues that should be explored to clarify these points include Ή multiple-quantum techniques (for estimating H bonded cluster sizes) and Ή - 8 ί two-dimensional (2-D) chemical-shift correlation spectroscopy (for verifying or correcting tentative correla tions). 2 9


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Derivatized Silica Surfaces In the preceding discussion, reference to derivatized silica surfaces focused entirely on the information that the silylation process can shed, via N M R spectroscopy, on the silica surface itself. W i t h (CH3)3SiCl as the derivatization agent, the gross nature of the (CH3)3Si-0- attachment to the surface, or its behavior on the surface, is largely predictable via induction, without the need for N M R data. F o r some systems, especially when multifunctional silylation agents (X SiR4-n, where η > 1) are employed, details of the attachment to the surface and the behavior of the attached moiety on the surface may be much less predictable. A n example is provided by silica systems that have been derivatized by the (coupling) agent (CH CH20)3SiCH2CH CH2NH2 (APTS). Figure 5 shows Si C P - M A S spectra of two series of APTS-modified silica samples (24). The variety of spectral patterns of the = S i - C H C H 2 C H N H 2 silicon resonance in the 45-70-ppm region of the S i C P - M A S spectra reveals the range of APTS attachments to the surface and displays the power of S i N M R techniques for elucidation of such details.


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In contrast, C C P - M A S spectra analogous to those in Figure 5 are generally less informative. However, the ^-carbon (relative to the NH2 group) is sensitive to the details of hydrogen bonding or protonation (Bronsted acid-base complex formation) of the NH2 group at the surface (Figure 6). O f course, the N (0.37% naturally abundant) chemical shift of the NH2 group itself would be expected to be more sensitive to H bonding and protonation than C chemical shifts. Figure 7 displays the natural abundance N C P - M A S spectra of APTS-derivatized silica gels. The observed spectral differences presumably reflect the relative simplicity of the protonated form and the relatively complex distribution of hydrogenbonded species present in the other two samples. These preliminary results indicate that the N N M R spectroscopy approach is worth pursuing and may warrant the effort that will be required to synthesize the corresponding N-labeled samples. 1 3

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Another type of derivatized silica surface that has received consider able attention is Cis-derivatized silica gel; that is, silica gel that has been silylated with reagents such as Cl(CH3)2Si(CH )i7CH (45-51). This system was studied via C N M R (47) and H N M R spectroscopy (45) as a function of surface loading of the Cis chains and of certain added liquids. The ability of cross polarization to select static components of a surface derivative relative to more mobile components—which would be emphasized by non-CP (single-pulse) techniques that rely on direct C spin-lattice relaxation—can be seen in Figure 8, which provides a comparison between the single-pulse and cross-polarization C M A S N M R spectrosco py results on four Cis-silica samples. Dramatic differences in line shapes and intensities, which can be interpreted at least qualitatively in terms of 2

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Figure 5. Si CP-MAS spectra of APTS-modified silica gels. The spectra on the left and right correspond to silica gels dried under vacuum at 200 °Cand room temperature, respectively, prior to reaction in dry toluene. The post-reaction treatment (curing) temperature is shown on the left (RT, room temperature). Structural assignments are given at the bottom. (Reproduced from reference 24. Copyright 1988 American Chemical Society.) m


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Figure 7. N CP-MAS spectra (natural abundance) of APTS-modified silica gels (top) after treatment with H2SO4, (middle) untreated, and (bottom) after treatment with 0.1 M NaOH. 15


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local motion within the Cis chain, can be noted readily i n these compari sons. More detailed information on motion within the C i s chains is obtained by wide line H N M R spectroscopy (a technique that does not have high resolution) on Cie-silica samples i n which deuterium has been selectively substituted for protons. In this approach, the details of which are reported elsewhere (45), the line-narrowing effects of motion on the broad, quadrupole-based H N M R line width of a mechanically static sample is modeled theoretically for specific trial motions to elucidate the detailed nature of the motion. 2

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Experimental Details S i and C N M R measurements were made at 39.7 and 50.3 M H z , respectively, on a drastically modified Nicolet NT-200 spectrometer; a home-built C P - M A S probe with a sealed-tube M A S system based on the Gay design was used (52). N N M R measurements were made on the same spectrometer at 20.3 M H z with a 2.5-cm large-volume M A S system described previously (53). Ή C R A M P S spectra were obtained at 187 M H z on a rebuilt NT-200 spectrometer described previously (37, 38); a probe with a M A S system based on the Gay design was used. 29

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Acknowledgments The authors gratefully acknowledge partial support of this research by the National Science Foundation (Grant C H E - 9 0 2 1 0 0 3 ) and use of the Colorado State University Regional N M R Center (funded by N S F Grant CHE-8616437).

References


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