1Hyperpolarized 129Xe NMR investigation of multifunctional organic/inorganic hybrid mesoporous silica materials
Type: Object |
Advantage: None |
Novelty: New |
ConceptID: Obj1
1
Hyperpolarized 129Xe NMR investigation of multifunctional organic/inorganic hybrid mesoporous silica materials
Type: Object |
Advantage: None |
Novelty: New |
ConceptID: Obj1
2
An extensive study has been made on a series of multifunctional mesoporous silica materials, prepared by introducing two different organoalkoxysilanes, namely 3-[2-(2-aminoethylamino)ethylamino]propyltrimethoxysilane (AEPTMS) and 3-cyanopropyltriethoxysilane (CPTES) during the base-catalyzed condensation of tetraethoxysilane (TEOS), using the variable-temperature (VT) hyperpolarized (HP) 129Xe NMR technique.
Type: Object |
Advantage: None |
Novelty: New |
ConceptID: Obj2
3
VT HP-129Xe NMR chemical shift measurements of adsorbed xenon revealed that surface properties as well as functionality of these AEP/CP-functionalized microparticles (MP) could be controlled by varying the AEPTMS/CPTES ratio in the starting solution during synthesis.
Type: Result |
Advantage: None |
Novelty: None |
ConceptID: Res1
4
Additional chemical shift contribution due to Xe-moiety interactions was observed for monofunctional AEP-MP and CP-MP as well as for bifunctional AEP/CP-MP samples.
Type: Result |
Advantage: None |
Novelty: None |
ConceptID: Res2
5
In particular, unlike CP-MP that has a shorter organic backbone on the silica surface, the amino groups in the AEP chain tends to interact with the silanol groups on the silica surface causing backbone bending and hence formation of secondary pores in AEP-MP, as indicated by additional shoulder peak at lower field in the room-temperature 129Xe NMR spectrum.
Type: Conclusion |
Advantage: None |
Novelty: None |
ConceptID: Con1
6
The exchange processes of xenon in different adsorption regions were also verified by 2D EXSY HP-129Xe NMR spectroscopy.
Type: Result |
Advantage: None |
Novelty: None |
ConceptID: Res3
7
It is also found that subsequent removal of functional moieties by calcination treatment tends to result in a more severe surface roughness on the pore walls in bifunctional samples compared to monofunctional ones.
Type: Conclusion |
Advantage: None |
Novelty: None |
ConceptID: Con2
8
The effect of hydrophobicity/hydrophilicity of the organoalkoxysilanes on the formation, pore structure and surface property of these functionalized mesoporous silica materials are also discussed.
Type: Object |
Advantage: None |
Novelty: New |
ConceptID: Obj3
Introduction
Type: Background |
Advantage: None |
Novelty: None |
ConceptID: Bac1
10
To realize these applications, it is essential to obtain capabilities in custom-tailoring the surface properties and in controlling the particle and pore morphologies of these materials.
Type: Background |
Advantage: None |
Novelty: None |
ConceptID: Bac1
11
Recently, we have reported an approach that allows a synergistic control over the pore structure, particle morphology, and surface functionalization by introducing different molar ratios of organoalkoxysilane precursors that could provide different non-covalent interactions with the cationic surfactant template, cetyltrimethylammonium bromide (CTAB), in a base-catalyzed condensation of silicate.5
Type: Method |
Advantage: None |
Novelty: Old |
ConceptID: Met1
12
For example, a series of bifunctional mesoporous silica materials were prepared by introducing different molar ratios of two organoalkoxysilanes, namely 3-[2-(2-aminoethylamino)ethylamino]propyltrimethoxysilane (AEPTMS) and 3-cyanopropyltriethoxysilane (CPTES) to a base-catalyzed condensation of tetraethoxysilane (TEOS) in the presence of CTAB.
Type: Method |
Advantage: None |
Novelty: Old |
ConceptID: Met1
13
These bifunctional mesoporous organic–inorganic hybrid materials exhibited fine-tuned chemical accessibility in terms of Cu2+ adsorption capability.5a
Type: Method |
Advantage: Yes |
Novelty: Old |
ConceptID: Met1
14
However, an unexpected nonlinear correlation between the Cu2+ adsorption capability with the amount of AEP groups in such AEP/CP bifunctional silicas were evident, indicating that the CP functionality, which is relatively more hydrophobic in nature, should also play an active role in the observed decrease in chemical accessibility per AEP group.5a
Type: Background |
Advantage: None |
Novelty: None |
ConceptID: Bac2
15
Thus, a more detailed understanding of the conformation of organic moieties and related guest-host interactions in the confined space demands further investigation.
Type: Motivation |
Advantage: None |
Novelty: None |
ConceptID: Mot1
Type: Method |
Advantage: None |
Novelty: Old |
ConceptID: Met2
17
Unlike conventional 129Xe NMR in which the nuclear spin polarization is governed by Boltzmann equilibrium, laser-polarized 129Xe NMR facilitates a metastable polarization transfer from the alkali-metal electronic spin to the 129Xe nucleus by the spin exchange optical pumping method,11 thus capable of enhancing signal sensitivity by 103–105 folds even at dilute Xe loading.12
Type: Method |
Advantage: Yes |
Novelty: Old |
ConceptID: Met2
18
Thus, the hyperpolarized (HP) 129Xe NMR technique is particularly useful for systems with low surface areas and/or long spin–lattice relaxation times (T1), and has found widespread applications, for examples, in surfaces, nanocrystals or thin films,13 porous materials,12,14 magnetic resonance imaging and medical imaging,15 and sensors,16etc.
Type: Method |
Advantage: None |
Novelty: Old |
ConceptID: Met2
19
In the case of conventional 129Xe NMR, the observed 129Xe chemical shift of Xe adsorbed on porous substrates can be expressed as:7δ = δ0 + δs + σXe–XeρXe where δ0 = 0 ppm is the reference chemical shift, δs represents the characteristic chemical shift due to Xe–wall interactions, corresponding to the shift at zero Xe loading and hence should reflect not only the chemical composition but also the geometry of Xe environment of the surface adsorption site.
Type: Background |
Advantage: None |
Novelty: None |
ConceptID: Bac3
20
The last term, σXe−XeρXe, is the contribution due to Xe–Xe interactions, which normally increases linearly with Xe loading, ρXe.
Type: Background |
Advantage: None |
Novelty: None |
ConceptID: Bac3
21
Whereas for HP-129Xe NMR, since the experiments are normally carried out at dilute Xe loading, the chemical shift contribution arising from Xe–Xe interactions can be greatly depressed.
Type: Background |
Advantage: None |
Novelty: None |
ConceptID: Bac3
Type: Background |
Advantage: None |
Novelty: None |
ConceptID: Bac3
23
The objective of this work is to investigate the structural and surface properties of the aforementioned organic–inorganic hybrid mesoporous materials using variable-temperature (VT) HP-129Xe NMR and two-dimensional exchange spectroscopy (2D EXSY) NMR techniques.20
Type: Goal |
Advantage: None |
Novelty: None |
ConceptID: Goa1
24
A series of bifunctional mesoporous silicas synthesized by introducing different molar ratios of AEPTMS and CPTES precursors to a base-catalyzed co-condensation of silicate have been examined along with AEP- and CP-monofunctional silicas.
Type: Object |
Advantage: None |
Novelty: New |
ConceptID: Obj4
25
Various information regarding to the conformation, hydrophilicity/hydrophobicity, and functionalities of the AEP/CP groups and their correlations with the structural porosity, surface roughness and related Xe-moiety interactions, which can be inferred from the temperature dependence of 129Xe characteristic chemical shift (δs), has been investigated and compared with the 13C CP-MAS NMR results reported earlier5.
Type: Object |
Advantage: None |
Novelty: New |
ConceptID: Obj4
Experimental section
Materials
26
Various mono- and bifunctional mesoporous silicas were prepared following the procedures described in an earlier report.5
Type: Method |
Advantage: None |
Novelty: New |
ConceptID: Met3
27
Accordingly, a series of micrometre-sized particles (MP) of organic–inorganic hybrid materials with varying molar concentrations of organoalkoxysilane precursors ranging from 100% AEPTMS to 100% CPTES, with the total amount of AEPTMS + CPTES relative to TESO fixed at 12.8 mol% were obtained.
Type: Experiment |
Advantage: None |
Novelty: None |
ConceptID: Exp1
28
Herein, we denote the monofunctionalized microparticles as AEP-MP or CP-MP and the bifunctionalized materials as AEP/CP(x/y)-MP, where x/y denotes the relative ratio of AEPTMS/CPTES in the initial reaction mixture.
Type: Experiment |
Advantage: None |
Novelty: None |
ConceptID: Exp1
29
All samples were template extracted either by acid extraction or by calcination treatments.
Type: Experiment |
Advantage: None |
Novelty: None |
ConceptID: Exp1
30
Further experiments by 13C CP-MAS NMR confirmed that no extra organic groups other than the designated functional groups are present in the AEP/CP(x/y)-MP samples, indicating a complete removal of template by the aforementioned treatments.5a
Type: Result |
Advantage: None |
Novelty: None |
ConceptID: Res4
31
Separate experiments by field-emission scanning electron microscopy (FE-SEM) revealed that, in contrast to the pure MCM-41 mesoporous silica synthesized under the same conditions, the AEP/CP-MP samples exhibit spherical shapes, whereas AEP-MP and CP-MP resulted in sphere and rod shapes and sizes, i.e., from spheres with an average diameter of 3 μm to rods with an average size of 1.0 × 0.2 μm.5b
Type: Result |
Advantage: None |
Novelty: None |
ConceptID: Res5
32
The average diameters of AEP/CP-MP spheres decreased as the relative ratio (x/y) changed from 5/5 to 1/9.
Type: Observation |
Advantage: None |
Novelty: None |
ConceptID: Obs1
33
Calcination treatments of the samples were carried out at 823 K under flowing air for 4 h, they are denoted as AEP-, CP- and AEP/CP(x/y)-MPC, respectively.
Type: Experiment |
Advantage: None |
Novelty: None |
ConceptID: Exp2
HP-129Xe NMR studies
34
All samples were compressed by pressure of 100 MPa for 10 min, crashed into small pellets, and then dehydrated under vacuum at 423 K for 12 h prior to the HP 129Xe NMR experiments.
Type: Experiment |
Advantage: None |
Novelty: None |
ConceptID: Exp3
35
The above post-synthesis treatment procedure effectively eliminate the effect arising from fast exchange between Xe adsorbed in the intra- and inter-particle voids while avoid jeopardizing the textural structures of the samples.21
Type: Experiment |
Advantage: None |
Novelty: None |
ConceptID: Exp3
36
Detailed descriptions of the continuous-flow optical pumping apparatus for fabricating the HP-129Xe gas can be found elsewhere.19
Type: Experiment |
Advantage: None |
Novelty: None |
ConceptID: Exp3
37
Briefly, the spin-exchange optical pumping process was facilitated by introducing a gas mixture of 97% He, 1% N2, and 2% Xe (natural abundance) into the pumping cell (containing 1 g Rb metal and placed under a magnetic field of ca. 200 G) maintained at 4 atm and 423 K, while subjecting to irradiation of left-circularly polarized light (wavelength 794.8 nm) generated by a diode laser (Coherent; FAP-30).
Type: Experiment |
Advantage: None |
Novelty: None |
ConceptID: Exp3
38
The resulted gas mixture containing HP-129Xe (polarization ca. 2.4%) was introduced into a home-designed NMR tube via 1/16 inch Teflon tubing and continuously flew through the sample at ca. 100 scc min−1 at 1 atm, the gas mixture was eventually vent to the atmosphere.
Type: Experiment |
Advantage: None |
Novelty: None |
ConceptID: Exp3
39
As such, this corresponds to an equilibrium Xe partial pressure of ca. 15.2 Torr.
Type: Experiment |
Advantage: None |
Novelty: None |
ConceptID: Exp3
40
It has been found that the presence of He and N2 buffer gases has no significant effect on the resultant HP-129Xe NMR spectrum.12
Type: Background |
Advantage: None |
Novelty: None |
ConceptID: Bac4
41
Separate experimental tests using the batch setup confirmed that the achievable signal enhancement factor was ca. 3500.
Type: Result |
Advantage: None |
Novelty: None |
ConceptID: Res6
42
All HP-129Xe NMR spectra were acquired on a Bruker Avance 300 NMR spectrometer at Larmor frequency of 83.012 MHz using a single-pulse sequence with a π/2 pulse (15 μs) and a recycle delay of 5 s.
Type: Experiment |
Advantage: None |
Novelty: None |
ConceptID: Exp4
43
Typically, 64–256 free induction decay (FID) signals were accumulated for each spectrum.
Type: Experiment |
Advantage: None |
Novelty: None |
ConceptID: Exp4
44
VT experiments were carried out in the temperature range 160–298 K. The actual sample temperature was calibrated using the 207Pb NMR signal of solid Pb(NO3)2.22
Type: Experiment |
Advantage: None |
Novelty: None |
ConceptID: Exp4
45
The 129Xe NMR chemical shift was referenced to extremely diluted xenon gas (as 0 ppm).
Type: Experiment |
Advantage: None |
Novelty: None |
ConceptID: Exp4
46
To verify the origins of the observed 129Xe resonances, additional experiments were also carried out by 2D EXSY 129Xe NMR technique20 at room temperature (298 K) using a 90°–t1–90°–tm–90°–t2 pulse sequence.
Type: Experiment |
Advantage: None |
Novelty: None |
ConceptID: Exp5
47
The 2D NMR spectrum was acquired with a spectral width of 12.5 kHz and 512 and 2048 points were acquired for the t1 and t2 dimensions, respectively, while changing the mixing time (tm) from 1 to 25 ms.
Type: Experiment |
Advantage: None |
Novelty: None |
ConceptID: Exp5
Results and discussion
VT HP-129Xe NMR
48
Figs. 1 and 2 display the VT HP-129Xe NMR spectra of various functionalized materials before and after the calcination, respectively.
Type: Observation |
Advantage: None |
Novelty: None |
ConceptID: Obs2
49
In general, most spectra exhibit two main resonances, the sharp peak at ca. 0 ppm can be unambiguously assigned to free gaseous Xe whereas the peak with higher chemical shift (or lower field) is attributed to Xe adsorbed in the mesopores.
Type: Result |
Advantage: None |
Novelty: None |
ConceptID: Res7
50
The observed chemical shift should be a weighted average of the fast exchange between Xe residing in different adsorption regions in the mesopores (see below).
Type: Model |
Advantage: None |
Novelty: None |
ConceptID: Mod1
51
An expected increase in 129Xe chemical shift of the adsorbed Xe with decreasing temperature was observed for each sample, as depicted in Fig. 3.
Type: Observation |
Advantage: None |
Novelty: None |
ConceptID: Obs3
52
In view of the very low Xe loading (15.2 Torr) in the continuous-flow HP-129Xe NMR experiments, it is anticipated that only a dilute adsorption layer with a very weak contribution of Xe–Xe interactions to the observed chemical shift.
Type: Model |
Advantage: None |
Novelty: None |
ConceptID: Mod2
53
As the temperature decreases, the adsorbed Xe should spend a longer time residing on the pore surface and thus causing the 129Xe NMR resonance to shift downfield.
Type: Model |
Advantage: None |
Novelty: None |
ConceptID: Mod2
54
For T < 200 K, a sharp increase in chemical shift is evident, which may be attributed to the condensation of xenon.
Type: Result |
Advantage: None |
Novelty: None |
ConceptID: Res8
55
Only below such low temperature does the chemical shift contribution due to Xe–Xe interactions become significant.
Type: Result |
Advantage: None |
Novelty: None |
ConceptID: Res9
56
Eventually, the 129Xe chemical shift is expected to approach that of bulk liquid Xe (ca. 255 ppm) and solid Xe (ca. 305 ppm).
Type: Conclusion |
Advantage: None |
Novelty: None |
ConceptID: Con3
57
Unlike calcined samples whose 129Xe NMR spectra of the adsorbed Xe all exhibited a symmetrical narrow peak throughout the temperature examined (Fig. 2), broader and somewhat asymmetric lines were observed for most of the functionalized AEP/CP-MP samples.
Type: Observation |
Advantage: None |
Novelty: None |
ConceptID: Obs4
58
For example, two overlapped peaks at 87 and 98 ppm were observed for AEP-MP (Fig. 1a) at room temperature (298 K).
Type: Observation |
Advantage: None |
Novelty: None |
ConceptID: Obs5
59
These two peaks gradually merged to form a broad line at lower temperature (T < 260 K).
Type: Observation |
Advantage: None |
Novelty: None |
ConceptID: Obs6
60
Similar phenomenon was also observed for a dodecylsilane-grafted MCM-41 mesoporous silica at high surface coverage (1.31 nm−2).19
Type: Observation |
Advantage: None |
Novelty: None |
ConceptID: Obs7
61
In this context, the AEP groups existing on the mesoporous silica surfaces might form an ‘organic phase’ in which the diffusional motion of Xe would be hindered provided that the surface coverage of the organic groups was high enough.
Type: Conclusion |
Advantage: None |
Novelty: None |
ConceptID: Con4
62
The effect of such ‘organic phase’ would be two-fold.
Type: Conclusion |
Advantage: None |
Novelty: None |
ConceptID: Con4
63
First, it might block Xe from accessing to the bare silica surface.
Type: Conclusion |
Advantage: None |
Novelty: None |
ConceptID: Con4
64
Second, it might interact with Xe to result in an additional contribution to the 129Xe chemical shift.
Type: Conclusion |
Advantage: None |
Novelty: None |
ConceptID: Con4
65
Thus, we attribute a priori the shoulder peak at 98 ppm to Xe residing in the surface ‘organic phase’ and the main peak at ca. 87 ppm to ‘mobile’ Xe in mesopores.
Type: Result |
Advantage: None |
Novelty: None |
ConceptID: Res10
66
Upon decreasing temperature, the latter Xe tends to spend longer time on the pore surfaces grafted with AEP groups, hence, resulting the resonance to move toward down-filed (higher chemical shift) and eventually the two lines merge to reveal a singlet at low temperature.
Type: Result |
Advantage: None |
Novelty: None |
ConceptID: Res11
67
Similarly, the increase in 129Xe chemical shift (Fig. 3) and slight linewidth broadening observed for bifunctional samples (Fig. 1) compared to the calcined samples (Fig. 2) may also be attributed to the presence of organic groups on the surface of mesoporous silicas, which tends to slow down the exchange of Xe between different local environments.
Type: Conclusion |
Advantage: None |
Novelty: None |
ConceptID: Con5
68
The interconnectivity between different Xe adsorption regions in the mesopores of AEP-MP was further examined by 2D EXSY NMR spectroscopy (Fig. 4a–c).
Type: Object |
Advantage: None |
Novelty: New |
ConceptID: Obj5
69
Accordingly, the exchange between regions with different chemical shifts was revealed by the cross-peaks between the signals of the exchanging sites.
Type: Observation |
Advantage: None |
Novelty: None |
ConceptID: Obs8
70
Fig. 4a–c showed the 2D spectra of an AEP-MP sample using an exchange time (tm) of 1, 5 and 25 ms, respectively.
Type: Observation |
Advantage: None |
Novelty: None |
ConceptID: Obs9
71
The observed three diagonal peaks at ca. 0, 87, and 98 ppm in the vertical (F2) axis were assigned (vide supra) to gaseous Xe, ‘mobile’ Xe adsorbed in the mesopores, and Xe residing in the ‘organic phase’, respectively.
Type: Result |
Advantage: None |
Novelty: None |
ConceptID: Res12
72
In addition, the off-diagonal peaks intensified with increasing exchange times set in the experimental pulse sequence.
Type: Observation |
Advantage: None |
Novelty: None |
ConceptID: Obs10
73
For example, traces of cross peaks appeared at tm = 1 ms (Fig. 4a) indicating that the exchange between the ‘mobile’ (87 ppm) and gaseous (0 ppm) Xe occurred even on a time scale less than a fraction of 1 ms.
Type: Result |
Advantage: None |
Novelty: None |
ConceptID: Res13
74
As tm is increased to 5 ms, wherein such exchange became more pronounced, the exchange between ‘mobile’ Xe adsorbed in the mesopores and Xe in the ‘organic phase’ (98 ppm) began to take place (Fig. 4b).
Type: Result |
Advantage: None |
Novelty: None |
ConceptID: Res14
75
Eventually, the exchange between the ‘organic phase’ and the gaseous Xe’s was also evident with tm = 25 ms (Fig. 4c), indicating that the exchange between Xe species in different adsorption regions and the gas phase appeared to be completed.
Type: Result |
Advantage: None |
Novelty: None |
ConceptID: Res15
76
Consequently, it can be concluded that the exchange rates between different environments follow the following trend: kgm > kmo > kgo, where kmo denotes the exchange rate between ‘mobile’ Xe in the mesopores and Xe residing in the ‘organic phase’ and kgm and kgo denote exchange rates between gaseous-‘mobile’ Xe and gaseous-‘organic phase’ Xe, respectively, as illustrated in Fig. 5.
Type: Conclusion |
Advantage: None |
Novelty: None |
ConceptID: Con6
77
It is envisaged that the slower rate observed for kgo may manifest itself via the other two exchange processes.
Type: Conclusion |
Advantage: None |
Novelty: None |
ConceptID: Con7
78
Accordingly, the dynamic phenomena obtained from the 129Xe 2D EXSY experiments provide strong supports to the aforementioned peak assignments for the room-temperature 129Xe NMR spectrum observed for AEP–MP.
Type: Conclusion |
Advantage: None |
Novelty: None |
ConceptID: Con8
79
In the VT HP-129Xe NMR experiments, the concentration of Xe in the gas mixture flowing through the sample placed in the detection (NMR coil) region was extremely low (ca. 2%, equivalent to a Xe partial pressure of 15.2 Torr) and thus could be fitted adequately to the dilute adsorption (Henry’s Law) regime.
Type: Model |
Advantage: None |
Novelty: None |
ConceptID: Mod3
80
Therefore, the temperature dependence of the observed 129Xe chemical shift, particularly for T > 200 K, could be approximated by the following equation derived based on the fast-exchange model:23where δs is the characteristic chemical shift, representing the shift arising from Xe–surface interactions, V and S represent the free volume and specific surface area of the mesoporous materials, respectively, R is the universal gas constant, K0 is the pre-exponent of Henry’s constant, and ΔHa is the adsorption enthalpy.
Type: Model |
Advantage: None |
Novelty: None |
ConceptID: Mod3
81
Values of V and S can be derived from N2 adsorption/desorption data measured at 77 K and the other parameters δs, K0 and ΔHa can be determined by least-square fittings of the observed VT HP-129Xe NMR chemical shift results to eqn. (2), as summarized in Table 1.
Type: Model |
Advantage: None |
Novelty: None |
ConceptID: Mod3
82
Since the chemical shift contribution arising from Xe–Xe interactions becomes more significant at low temperature, only those experimental results obtained at T > 200 K were used during data fittings.19,21,24
Type: Method |
Advantage: None |
Novelty: New |
ConceptID: Met4
83
All observed ΔHa values fall in the range of 9–17 kJ mol−1, as expected for physical adsorption of Xe.
Type: Result |
Advantage: None |
Novelty: None |
ConceptID: Res16
Analysis of characteristic chemical shifts (δs)
84
Since the characteristic chemical shift, δs, reflects interactions between Xe and the surface of the substrate.
Type: Background |
Advantage: None |
Novelty: None |
ConceptID: Bac5
85
In this context, for the functionalized porous materials, the values of δs should arise from Xe interacting with the bare surface of the mesoporous silica as well as that with the soft organic moieties.
Type: Hypothesis |
Advantage: None |
Novelty: None |
ConceptID: Hyp1
86
Therefore, it would be helpful to compare the δs contribution due to Xe-moieties interactions observed for various bifunctional mesoporous MCM-41 samples investigated herein.
Type: Goal |
Advantage: None |
Novelty: None |
ConceptID: Goa2
87
Since the δs values observed for CP-MPC and AEP-MPC samples after removal of organic ligands by calcination were 104.9 and 106.0 ppm, respectively (Table 1), which are nearly identical (within experimental error; ca. ±1 ppm) to that of 105.4 ppm found previously for a bulk siliceous MCM-41 material (pore diameter ca. 2.54 nm)19,21 synthesized using the conventional procedure.25
Type: Result |
Advantage: None |
Novelty: None |
ConceptID: Res17
88
Foreseeably, the surface properties of these materials, as detected by the adsorbed Xe, should be practically the same.
Type: Hypothesis |
Advantage: None |
Novelty: None |
ConceptID: Hyp2
89
On the other hand, an averaged δs an increase of ca. 7–9 ppm, was observed (Fig. 6) for the calcined samples originally prepared with various AEPTMS/CPTES (or x/y) ratios compared to the calcined AEP-MPC or CP-MPC samples, indicating a more severe surface roughness26 in the bifunctional than monofunctional mesoporous silica materials after calcinations.
Type: Result |
Advantage: None |
Novelty: None |
ConceptID: Res18
90
It is also evident that the δs values observed for bifunctionalized AEP/CP-MP materials, which seem to scatter in the 118–122 ppm range, are significantly higher than the calcined samples (Fig. 6 and Table 1).
Type: Observation |
Advantage: None |
Novelty: None |
ConceptID: Obs11
91
Xenon–organic-moiety interactions should be responsible for the observed differences.
Type: Result |
Advantage: None |
Novelty: None |
ConceptID: Res19
92
Accordingly, characteristic chemical shift contribution due to Xe–moiety interactions, δs,moiety, can be derived by the simple relation:δs,moiety = δs,fun − δs,calwhere δs,fun and δs,cal represent the δs values obtained from the functionalized AEP/CP-MP and calcined AEP/CP-MPC samples, respectively.
Type: Model |
Advantage: None |
Novelty: None |
ConceptID: Mod4
93
However, since Xe–moiety interactions and hence δs,moiety should be strongly related to the arrangements, functionality, chain length and coverage (ρ) of the organic moieties, it is preferable to infer the Xe–moiety interactions in terms of deshielding medium contribution (−σmoiety), which can be derived by:19Accordingly, the values of −σmoiety derived from eqn. (4) are also depicted in Table 1.
Type: Result |
Advantage: None |
Novelty: None |
ConceptID: Res20
94
It is intriguing that the −σmoiety values obtained from the monofunctional and bifunctional materials fall into two distinct regions, the former observed for CP-MP and AEP-MP samples are higher than 17.7 ppm nm2 whereas the latter from series of AEP/CP-MP samples are lower than 11.2 ppm nm2 (Table 1).
Type: Result |
Advantage: None |
Novelty: None |
ConceptID: Res21
95
This could be attributed to the more inhomogeneous distribution of organic moieties in the bifunctional materials compared to the monofunctional ones, as could also be inferred from the discrepancies in the observed values of δs,cal and consequently the values of δs,moiety listed in Table 1.
Type: Conclusion |
Advantage: None |
Novelty: None |
ConceptID: Con9
96
Unfortunately, further justification of the variation of −σmoiety with AEPTMS/CPTES ratios could not be made directly based on the results obtained from the present study.
Type: Conclusion |
Advantage: None |
Novelty: None |
ConceptID: Con9
97
Alternatively, complementary information may be inferred from a similar system.
Type: Background |
Advantage: None |
Novelty: None |
ConceptID: Bac6
98
In a previous study,19 we employed the similar VT HP-129Xe NMR technique to probe the surface properties of mesoporous MCM-41 silica grafted with various alkylsilanes by post-synthesis silylation treatment.
Type: Background |
Advantage: None |
Novelty: None |
ConceptID: Bac6
99
It was found that the characteristic chemical shifts responsible for Xe-surface interactions exhibited strong correlations with both the surface coverage and carbon chain length (nC) of the grafted alkylsilanes.
Type: Background |
Advantage: None |
Novelty: None |
ConceptID: Bac6
100
Consequently, the deshielding medium contribution due to the individual alkyl ligand could be deduced based on the group contribution analysis by the following empirical relationship:19–σmoiety = 2.96nC + 2.35
Type: Background |
Advantage: None |
Novelty: None |
ConceptID: Bac6
101
Regardless of the differences in the chemical nature of organic ligands on the pore surface of the mesoporous silica between the two sample systems, if one take nC = 5 for CPTES, an estimated −σmoiety value of ca. 17.2 ppm nm2 is derived for CP-MP using eqn. (5), comparable to the value 17.7 ppm nm2 obtained from this study.
Type: Result |
Advantage: None |
Novelty: None |
ConceptID: Res22
102
This is, of course, based on an oversimplified assumption that similar Xe-moieties were adsorbed in the two different mesoporous materials, namely alkylsilane-grafted MCM-41 and CP-MP that have CH3 and CN as end groups, respectively, i.e., the intrinsic chemical shift of the Xe–C and Xe–N moieties are practically the same because of the same atomic number of the organic backbones.
Type: Conclusion |
Advantage: None |
Novelty: None |
ConceptID: Con10
103
By the same token, if one takes nC = 10 for AEPTMS, an estimated −σmoiety value of ca. 32.0 ppm nm2 for the AEP moiety in AEP-MP could be obtained.
Type: Result |
Advantage: None |
Novelty: None |
ConceptID: Res23
104
However, a significantly smaller value of 23.6 ppm nm2 was observed, which could be fitted to a value of nC = 7 by using eqn. (5).
Type: Result |
Advantage: None |
Novelty: None |
ConceptID: Res23
105
Furthermore, it is noteworthy that the presence of two broad, overlapping 129Xe resonances in the spectra of AEP-MP around room temperature (Fig. 1a) clearly suggested the existence of a strong Xe-AEP interaction considering its medium coverage (0.56 nm−2) and backbone length.
Type: Result |
Advantage: None |
Novelty: None |
ConceptID: Res24
106
In this context, the observation mentioned above could be explained by a “prone” or “bent” conformation of the AEP moiety on the mesopore surfaces as depicted in Fig. 7.
Type: Conclusion |
Advantage: None |
Novelty: None |
ConceptID: Con11
107
As we reported in a previous study,5a the basic amino groups of the AEP functionality indeed could strongly interacted (hydrogen-bonded) with the silanol groups on the surface of the mesoporous silica.
Type: Conclusion |
Advantage: None |
Novelty: None |
ConceptID: Con12
108
Consequently, the AEP group will be less flexible and hence result in a decrease in interactions between Xe and each constitutional atom due to such geometric constraints.
Type: Conclusion |
Advantage: None |
Novelty: None |
ConceptID: Con12
109
Conversely, such a bent backbone arrangement was likely to increase the atomic density of the organic phase causing Xe to be ‘entangled’ in the organic phase and thus gave rise to a shoulder peak at 98 ppm.
Type: Conclusion |
Advantage: None |
Novelty: None |
ConceptID: Con13
110
In contrast, the CP and the alkyl moieties existing in the CP-MP and the aforementioned alkyl-grafted MCM-41 samples do not have any basic amino groups that can interact with the surface silanols.
Type: Conclusion |
Advantage: None |
Novelty: None |
ConceptID: Con14
111
In fact, conformations with flexible organic chains experienced by the adsorbed Xe were attributed to the obtained −σmoiety.
Type: Conclusion |
Advantage: None |
Novelty: None |
ConceptID: Con14
112
Although this hypothesis is based on a simplified model, it is in good agreement with our previous findings using 13C CP-MAS NMR, which revealed that the CP backbone in CP-MP is more mobile than that of AEP in AEP-MP.5a
Type: Conclusion |
Advantage: None |
Novelty: None |
ConceptID: Con15
113
Furthermore, should the bending of AEP backbones on the silica surface indeed occur, it will give rise to a shorter dendritic surface together with the co-existing shorter and hydrophobic CP moieties causing an effective reduction in chelating effect of the AEP group to Cu2+.
Type: Conclusion |
Advantage: None |
Novelty: None |
ConceptID: Con15
114
Consequently, lower Cu2+ adsorption capacities were found for bifunctional AEP/CP-MP samples than AEP-MP5.
Type: Background |
Advantage: None |
Novelty: None |
ConceptID: Bac7
Formation mechanism and surface properties
115
Assuming that the calcination treatment was sufficient in removing all organic moieties without jeopardizing the structure of the mesoporous silica, the aforementioned surface roughness observed for the calcined AEP/CP-MP samples (Fig. 6) should be strongly related to the synthesis conditions.
Type: Hypothesis |
Advantage: None |
Novelty: None |
ConceptID: Hyp3
116
These functionalized mesoporous silicas were prepared via a co-condensation reaction of TEOS with organoalkoxysilanes in a diluted aqueous solution of cetyltrimethylammonium bromide (CTAB).
Type: Method |
Advantage: None |
Novelty: New |
ConceptID: Met5
117
Based on the conventional surfactant-templated mechanism25 proposed for the formation of mesoporous MCM-41 materials under similar conditions, the oligomeric silicate polyanions acting as multidentate ligands should interact with cationic surfactant head groups to form surfactant/silica bilayers of lamellar phase.
Type: Conclusion |
Advantage: None |
Novelty: None |
ConceptID: Con16
118
Subsequent condensation of these silicate clusters would lead to a reduction of negative charges at the interface.
Type: Conclusion |
Advantage: None |
Novelty: None |
ConceptID: Con16
119
As a result, the different charge-density matching between the surfactant and the silicate provoked the formation of a hexagonal surfactant–silicate composite which yielded the mesoporous structure after removal of surfactant template.
Type: Conclusion |
Advantage: None |
Novelty: None |
ConceptID: Con16
120
However, for the multifunctional materials studied herein, the presence of additional organic groups (AEP and/or CP) that are covalently bonded to the silicate surface would foreseeably alter the interfacial property of the surfactant-silicate assemblies.
Type: Conclusion |
Advantage: None |
Novelty: None |
ConceptID: Con17
121
More specifically, the presence of the hydrophobic CP groups are likely to penetrate into the surfactant phase during formation of the surfactant–silicate interface in a way analogous to MCM-41 formation thus leading to a similar silicate ordering and hence a more uniform (smoother) silica surface, as evidenced by the similar δs values observed for CP-MPC (104.9 ppm) and Si-MCM-41 (105.4 ppm).
Type: Conclusion |
Advantage: None |
Novelty: None |
ConceptID: Con17
122
On the other hand, the more hydrophilic AEP moieties tend to occupy the space between the surfactant and the silicate forming a surfactant/AEP-silicate interface.
Type: Conclusion |
Advantage: None |
Novelty: None |
ConceptID: Con17
123
Although only a slightly higher δs value was observed for AEP-MPC (106 ppm) compared to CP-MPC and Si-MCM-41, indicating a slightly rougher silica surface.
Type: Result |
Advantage: None |
Novelty: None |
ConceptID: Res25
124
It is noteworthy that the interactions between the amino groups on the AEP chains and the neighboring silanol groups indeed play an important role in curling the surfactant micelles in the surfactant/silicate composites as mentioned in our previous reports.5
Type: Background |
Advantage: None |
Novelty: None |
ConceptID: Bac8
125
Thus, unlike MCM-41, the resultant template-free AEP-MP material possesses a disordered, wormhole-like porous structure.5
Type: Background |
Advantage: None |
Novelty: None |
ConceptID: Bac8
126
Owing to the presence of AEP moieties, the bifunctional AEP/CP(x/y)-MP samples also possessed the wormhole-like mesoporous structures to various extents dictated by the designated x/y ratios.5
Type: Background |
Advantage: None |
Novelty: None |
ConceptID: Bac8
127
It was anticipated that a random distribution of AEP and CP moieties would lead to a non-uniform surfactant/silicate interfaces.
Type: Conclusion |
Advantage: None |
Novelty: None |
ConceptID: Con18
128
For example, the more hydrophobic CP moieties would tend to closely associate themselves with the surfactant micelles, whereas the hydrophilic and spatially demanding AEP-bonded silicas would like to be slightly further away from the micellar interface in order to accommodate a less favorable charge-density matching.
Type: Conclusion |
Advantage: None |
Novelty: None |
ConceptID: Con18
129
Such an effect could perhaps provide an explanation to the more sever surface roughness (i.e., higher δs value; see Table 1) observed in the bifunctional AEP/CP-MPC materials in comparison with those of monofunctional AEP-MPC or CP-MPC samples.
Type: Conclusion |
Advantage: None |
Novelty: None |
ConceptID: Con18
Conclusions
130
We have performed extensive VT HP-129Xe NMR chemical shift measurements of Xe adsorbed in a series of novel multifunctional organic/inorganic hybrid mesoporous silica materials.
Type: Object |
Advantage: None |
Novelty: New |
ConceptID: Obj6
131
Unlike conventional 129Xe NMR method, HP-129Xe renders highly sensitive detection of Xe local environments even at very dilute loading and thus capable of measuring 129Xe chemical shift primary associated with interactions between Xe and surface of the substrate.
Type: Object |
Advantage: Yes |
Novelty: New |
ConceptID: Obj6
132
As demonstrated in the present work, the use of VT HP-129Xe NMR technique allows us to probe the surface properties, pore structure, and local arrangement of the functional groups covalently anchored on the surfaces of the multifunctional AEP/CP-MP materials and hence should provoke further understanding and future developments of novel nanostructured materials.
Type: Conclusion |
Advantage: None |
Novelty: None |
ConceptID: Con19