Small Angle X-ray Diffraction

Figure 5.4 depicts the XRD pattern obtained for the calcined MCM-41 material obtained from the pseudomorphic route and for surface modified MCM-41 samples. Calcined MCM-41 silica shows three Bragg peaks at low angles, a strong (100) as well as the (110) and (200) reflections of lower intensity, which characterize the highly ordered hexagonal pore structure in the sample. The surface-modified materials exhibit almost identical XRD pattern which clearly indicates that the basic MCM-41 pore structure remains unchanged after surface modification. In the case of the endcapped materials, the (110) and (200) reflections become weaker and are broadened. This observation points to a lower long-range order because of (unordered) additional scattering material inside the pores.

Figure 5.4: XRD patterns for MCM-41 materials before and after surface modification with trifunctional silanes (left), and monofunctional silanes (right).

5.3.5 ²⁹Si NMR Spectroscopy

29Si NMR spectroscopy was employed for the determination of the surface species, amount of alkyl chain attachment and degree of cross linking of the attached alkylsilanes.

29Si CP/MAS NMR spectra of the unmodified and modified MCM-41 samples are shown in Figure 5.5, and the ²⁹Si chemical shifts are reported in Table 5.3. The ²⁹Si resonances around -92, -102 and -110 ppm originate from the structural units of the MCM-41 support and reflect surface silanol groups, i.e., Q² and Q³ units, and interior, completely condensed Q⁴ units, respectively (Qⁿ = Si(OSi)n(OH)4-n, with n=1 to 4) [95,97]. After attachment of the butyl or octyl silanes, the intensities of Q² and Q³ units, bearing surface hydroxyl groups, are significantly reduced, while the intensity for the Q⁴ units increases.

This trend continues for the endcapped samples which exhibit an additional considerable growth for the Q⁴ signal intensity.

The Q³ signal intensity for the samples modified with monofunctional alkylsilanes is found to be higher than for those obtained from trifunctional alkylsilanes, which is consistent with the lower surface coverage. This can be related to the lower reactivity of the former silylation reagent due to the lower probability (only one reactive group available) to react with surface silanols. Further reasons might be the bulky methyl groups of the monofunctional alkylsilane, which provide a steric hindrance for the binding of other alkylsilane chains in close vicinity to a surface-bound alkylsilane, and the lack of cross-linking reactions due to the presence of a single reactive group.

For the endcapped materials, an additional peak at about 13 ppm [M = R3Si(OSi-)] is found arising from the trimethylsilyl groups of the endcapping reagent. For sample, M-C8ME (derived from the monofunctional alkylsilane) this peak also contains a small signal component from the attached monofunctional alkylsilane chains of the first surface modification step. Hence, for sample M-C4M this latter signal is clearly visible in Figure 5.5, while for sample M-C8M, due to the low surface coverage, the respective resonance is relatively weak.

Table 5.3: ²⁹Si chemical shifts for MCM-41 materials before and after surface modification.

29Si Chemical Shift (ppm) Material

code Q² Q³ Q⁴ T¹ T² T³ M

MCM-41 -93.0 -102.5 -111.8 - - - -

M-C4T -92.2 -102.9 -110.0 -49.8 -57.9 -68.2 -

M-C8T -92.4 -102.3 -110.0 -49.3 -56.9 -65.8 -

M-C8TE - -102.7 -110.4 -50.5 -57.5 -66.7 12.7

M-C4M -92.9 -102.4 -111.3 - - - 13.3

M-C8M -92.7 -102.2 -110.6 - - - 13.1

M-C8ME - -102.4 -109.7 - - - 12.6

The presence of T¹, T² andT³ peaks(at about -49 ppm, -57 ppm and -66 ppm) for the MCM-41 samples treated with trifunctional alkylsilanes clearly prove the attachment and cross-linking of these chains at the MCM-41 silica surface (Tⁿ=RSi(OSi) n(OH) 3-n, n=1, 2, 3) [54]. They refer to trifunctional groups without (T¹), with partial (T²) and complete cross-linking (T³). For sample M-C4T, the intensity of T¹ and T² peaks is higher as compared to sample M-C8T which is in agreement with the findings for the surface coverage.

Figure 5.5: ²⁹Si CP/MAS NMR spectra of MCM-41 materials before and after surface modification with trifunctional silanes (left), and monofunctional silanes (right).

All ²⁹Si NMR spectra discussed so far were recorded under ²⁹Si{ H}¹ cross polarization conditions to increase the signal-to-noise ratio. Cross-polarization experiments, however, do not give the correct signal intensities, and provide enhanced intensities for the resonances of those ²⁹Si nuclei with adjacent protons. Therefore, single pulse ²⁹Si NMR spectra, given in Figure 5.6, were recorded to determine the relative amount of silanols on the silica surface, and to compare the amounts before and after surface modification. The ²⁹Si chemical shifts of the various species and their relative intensities obtained by spectral deconvolution are reported in Table 5.4.

Table 5.4: Spectral parameters from deconvolution of ²⁹Si single pulse spectra (chemical shifts (δ), line widths (FWHM) and relative intensities (I).

Q² Q³ Q⁴

Material code δ

(ppm)

FWHM (Hz)

I (%) δ

(ppm)

FWHM (Hz)

I

(%) δ (ppm)

FWHM (Hz)

I (%)

T¹ δ (ppm)

T² δ (ppm)

M δ (ppm) MCM-41 -92.7 330 1 -102.0 986 44 -110.0 787 55 - - - M-C4T - - - -101.9 724 22 -111.2 778 78 -50.2 -57.9 - M-C8T - - - -101.7 680 24 -111.0 790 76 -50.6 -57.5 -

M-C8TE - - - -110.0 - 100 -51.1 -58.0 12.7

M-C4M - - - -102.0 673 28 -111.3 737 72 - - -

M-C8M - - - -102.2 760 32 -110.8 713 68 - - -

M-C8ME - - - -110.3 - 100 - - 12.4

The derived intensities for the Q³ and Q⁴ species, as given in Table 5.4, support the expected structural changes by surface modification with the alkylsilanes. Hence, the relative amount of the Q³ species decreases, while - due to the formation of new Si-O-Si units - the amount of Q⁴ species increases. In general, the derived signal intensities are in complete agreement with the surface coverage data, given in Table 5.2. That is, an increasing surface coverage is accompanied by an intensity reduction of the Q³ units along with an intensity rise of the Q⁴ group resonance. Single pulse ²⁹Si NMR spectra also show the presence of cross-linking of the chains. However, the intensity of the T³ peaks is very low.

Figure 5.6: ²⁹Si single pulse NMR spectra of MCM-41 materials before and after surface modification with trifunctional silanes (left) and monofunctional silanes (right).

5.3.6 ¹³C NMR Spectroscopy

13C NMR spectroscopy was used to study the organic components after attaching alkylsilanes to the MCM-41 silica surface. For alkylsilanes with longer alkyl chains (i.e., with 10 or more methylene segments), the ¹³C resonances of the inner methylene groups can be used to get qualitative information about the chain conformational order.

Figure 5.7 depicts experimental ¹³C CP/MAS NMR spectra for the present MCM-41 materials after surface modification. The signals can be assigned on the basis of solution NMR data for the respective alkylsilanes [148-149]. The ¹³C chemical shifts of the various species are reported in Table 5.5, and are in good agreement with these literature values.

As can be seen, the signal-to-noise ratio varies with the actual sample which again reflects the differences in surface coverage. For the MCM-41 samples, surface modified with trifunctional silanes, carbon C-1 resonates at about 13 ppm, while for those obtained from monofunctional silanes the respective ¹³C NMR signal occurs at about 17 ppm.

For the samples modified with trifunctional silanes, an additional peak around 50 ppm shows up, which can be assigned to methoxy groups bound to silicon. This signal could be explained either by non-reacted methoxy groups of the alkylsilanes. Another, more likely explanation is the presence of methoxy groups which were released during the hydrolysis of the trifunctional silanes, and which are subsequently bound to the surface silanol groups [146].

Figure 5.7: ¹³C CP/MAS NMR spectra of MCM-41 materials after surface modification with trifunctional silanes (left), and monofunctional silanes (right).

The trimethylsilyl groups after endcapping with HMDS resonate at about 0.4 ppm and 0.1 ppm for sample M-C8TE and M-C8ME, respectively. In the latter case, this signal also contains a spectral component from the methyl groups of the attached dimethyloctylsilane. For samples M-C8M and M-C4M (i.e., without endcapping), these methyl groups bound to the silicon atom of the alkylsilane resonate at about -3.0 ppm.

Table 5.5: ¹³C chemical shifts and assignment of surface modified MCM-41 materials.

Sample Carbon position ¹³C shift (ppm)

M-C4T OCH3 49.3

C-4 10.5

C-1 12.7

C-3 25.8

C-2 29.6

M-C8T OCH3 50.2

C-8 11.0

C-1 13.2

C-2, C-7 23.2

C-4, C-5 29.9

C-3, C-6 32.8

M-C8TE OCH3 50.3

Si(CH3)3 0.4

C-8 11.0

C-1 13.5

C-2, C-7 23.4

C-4, C-5 30.1

C-3, C-6 33.0

M-C4M Si(CH3)2 R -3.5

C-4 12.7

C-1 17.0

C-3 25.7

C-2 29.1

M-C8M Si(CH3)2 R -3.1

C-8 13.2

C-1 17.3

C-2, C-7 23.1

C-4, C-5 29.2

C-3, C-6 32.2

M-C8ME Si(CH3)2 R, Si(CH3)3 0.1

C-8 13.1

C-1 17.9

C-2, C-7 23.0

C-4, C-5 29.8

C-3, C-6 32.5

Im Dokument Synthesis, characterization, and applications of organic-inorganic hybrid mesoporous silica materials (Seite 88-96)