Groups on the Adsorption Performances
3.4 Physico-chemical Characterization
3.4.3 Interactions between Basic Groups and Surficial Silanols
A study focused on the interactions between the amino groups present in the organic chains and the silanol groups present on the surface, was performed. This point is crucial for understanding the reactivity of these samples towards carbon dioxide. In fact, recently in the literature it has been hypothesized that such interaction between silanols and amino groups can favour the CO2 adsorption. [8] In detail, this specific interaction was studied by evaluating the mobility of the organic chains, assuming that the interaction with silanol groups reduces the mobility of the organic chains.
This behaviour was studied initially by using single pulse excitation 1H MAS NMR spectroscopy (Figure 3.12, Frame A). This technique in fact allows to distinguish mobile from rigid components by analyzing the width of the peaks: narrow peaks correspond to high mobility, while broad signals correspond to reduced mobility. [26]
76
Figure 3.12. 1H MAS NMR (Frame A) and Hahn-echo NMR spectra (Frame B) of hybrid SBA-15 samples. [26]
In Figure 3.12 A, A-SBA-15 and P-SBA-15 sample show narrow resonances at around 0.7 ppm due to CH3 protons near silicon atoms and a sharp peak at around 1.2 ppm due to the remaining methylene protons. In addition, a peak at 2 ppm attributed to surface silanols is also visible. This suggest that in A-SBA-15 and P-SBA-15 samples organic chains are free to move. On the contrary, no sharp resonances are present for E-SBA-15 sample, suggesting that all the EAPTS chains are in a confined space, where they experience reduced mobility. This effect is probably due to the inter- and/or intra-molecular interactions. For example, intermolecular interactions can occur with the silica surface through bindings by silanols. Finally, a broad resonance peak at 5.7 ppm is visible in all the spectra. This peak is attribute to the interactions between NH2 groups and the physisorbed water remaining after the thermal treatment and H-bonded silanols. [26]
A
A-SBA-15 E-SBA-15 P-SBA-15
E-SBA-15
A-SBA-15 P-SBA-15
B
77 In order to better investigate the mobility of the organic chains in grafted samples, rotor synchronized Han-eco sequence (π/2-τ- π- τ) with long time delay (τ : 6700 μs) was also applied (Figure 3.15, Frame B). This sequence allows the selective detection of mobile species, while rigid component are not resolved from the baseline. From spectra in Figure 3.12 B, it can be observed that A-SBA-15 and P-SBA-15 samples show narrow peaks due to the methylene protons of the organic chains. In addition, a resonance at about 2 ppm, attributed to isolated silanols, is also visible. Instead, in E-SBA-15 sample no resonance can be observed, confirming that for this sample a reduced mobility of the organic chains is present due to inter- and/or intra-molecular interactions in/among the organic chains. [26]
The possibility that the grafted chains can interact each other can be estimated by comparing an approximation of the molecular length of the organic chains with the average area around each organo-silane. In Table 3.4, the free average space of each silane domain (calculated as the inverse of the organo-silane density) and the average intermolecular distance (calculated as the square root of this area) are reported; the chain lengths were obtained from ab initio geometry optimizations (see below).
Sample Silane Density
Table 3.4. Distances between chains of the organo-silane in grafted silica samples.
In order to better investigate the arrangement of organic chains in the space, theoretical calculations have been performed, by using Gaussian09 program at Density Functional Theory (DFT) level. (In detail, hybrid functional B3LYP [4,35]
using Dunning’s correlation consistent cc-PVDZ basis set on light atoms[36,37] and LANL2DZ effective core potentials and the basis set for silicon [38] were used. In
78 order to estimate the contribution from dispersion forces to energies and geometrical structures, an atom-atom pairwise algorithm proposed by Grimme[39] and implemented in Gaussian09 was also used. The silica surface was simulated by using two different clusters of amorphous silica reported in the literature [40]: Si39O112H68
(Cluster I) and Si52O152H92 (Cluster II), with a surface of approximately 2.5 and 4 nm2, respectively. The silanol concentration on the model surface was set to 5 SiOH/nm2, according to experimental data reported before. In order to simulate the surface after the functionalization, APTS, EAPTS and PAPTS molecules were introduced on the surface by elimination three water molecules per chain from SiOH groups, thus forming Si-O-Si bonds.[26] Initially, one molecule of each organo-silane was added to cluster I, and the optimized geometries are reported in Figure 3.13.
Figure 3.13. Optimized structures of APTS, EAPTS and PAPTS adducts on silica model cluster I.
[26]
From optimized structures reported in Figure 3.13 it can be noted that APTS chain is not bended towards the surface due to its relatively short chain, while EAPTS and PAPTS grafted chains can lie down on the silica surface, forming hydrogen bond
A-SBA-15 E-SBA-15
P-SBA-15
79 between silanols and amino groups. In order to deepen also intermolecular interactions, adducts with two molecules grafted on cluster II were optimized (Figure 3.14).
Figure 3.14. Optimized structures of two molecules of APTS, EAPTS and PAPTS adducts on silica model cluster II. [26]
By adding a second organic molecule, the behaviour of the organo-silane on the surface does not present many variations with respect to the presence of a single chain: APTS chain remains perpendicular to the surface, while EAPTS and PAPTS chains lie on the surface. However, in PAPTS grafted sample, not all the amino groups are involved in hydrogen bond with silanols, because there are not enough silanols in suitable positions. This may be the reason why PAPTS chains can move more easily, as observed through NMR spectroscopy analysis (even if this is just one of the possible conformations). [26] Since we are using a cluster (cluster II) with an area of about 4 nm2, and given that the estimated amount of silane introduced is about
A-SBA-15 E-SBA-15
P-SBA-15
80 1 chain/nm2 (see Table 4) we decided to add a third EAPTS and PAPTS chain to cluster II. We decided to study only these two systems because in this longer chains steric effects are expected to be more relevant. The optimized structure are given in Figure 3.15.
Figure 3.15. Optimized structures of three molecules of EAPTS and PAPTS adducts on silica model cluster II. [26]
After the introduction of the third organic molecule on cluster II, steric hindrance effects became more relevant. In particular, in both structures the third organic chain remains perpendicular and has no access to the silica surface.