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Timestamp: 2019-04-20 12:22:09+00:00

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The design of novel heterogeneous catalysts with multiple adjacent functionalities is of high interest to heterogeneous catalysis. Herein, we report a method to obtain a majority of bifunctional acid–base pairs on SBA15. Aniline reacts with SBA15 by opening siloxane bridges leading to N-phenylsilanamine–silanol pairs. In contrast with ammonia treated surfaces, the material is stable under air/moisture. Advanced solid state MAS NMR (2D 1H–1H double-quantum, 1H–13C HETCOR) experiments and dynamic nuclear polarization enhanced 29Si and 15N spectra demonstrate both the close proximity between the two moieties and the formation of a covalent Si–N surface bond and confirm the design of vicinal acid–base pairs. This approach was successfully applied to the design of a series of aniline derivatives of bifunctional SBA15. A correlation between the substituent effects on the aromatic ring (Hammett parameters) with the kinetics of a model Knoevenagel reaction is observed.
In the soft templating method, the inorganic materials provide the surface area and the porosity. The organic active site, linked to the surface via an alkyl spacer, can be randomly distributed or organized.12–21 However, the resulting materials are composed of complex mixtures of surface species statistically and randomly spread on the surface which are consequently difficult to characterize. Accordingly, the structure of their active site is generally not known at the molecular level.
In the SOMC methodology,7 the generation of well-defined surface species is achieved by understanding the reaction of organometallic complexes with the inorganic materials which act as a rigid ligand. This approach presents the advantage of establishing a structure–activity relationship, and provides molecular-level insight for the design and prediction of new catalysts for new reactions.22,23 Indeed, SOMC has been successful in designing “multifunctional” single site catalysts that are able to perform alkane metathesis via a multistep mechanism. However, the surface–complex bond is usually a σ-bonded oxygen ligand, e.g., siloxy [( Si–O–)MLn], with M = metal and L = ligands, in the primary coordination sphere and the requirement for oxygen limits the development of SOMC methods. It would then be highly desirable to tune the coordination sphere of the metal center by designing surface ligands in close proximity to the surface to preserve the rigidity of the ensemble “surface ligand/complex”. By tuning the electronic and/or steric properties of the surface ligand, new catalysts and new reactions will be discovered.
Scheme 1 Synthesis of paired N-phenylsilanamine/silanols 1via the chemisorption of dry aniline on SBA151100 in toluene at 80 °C for 20 h.
Fig. 1 FT-IR spectra of SBA151100 (black) and 1 (red).
Fig. 2 (A) 1H MAS NMR spectrum of 1. (B) DQ rotor-synchronized 2D 1H MAS NMR spectrum of 1 (see ESI† for details).
The two intense signals at 6.6 ppm correspond to the protons of the aromatic ring in ortho (Ho) and para (Hp) positions. The proton in the meta position (Hm) appears at 7 ppm (Fig. 2A). To confirm these proton assignments, the synthesis of a model molecular silsesquioxane bearing a N-phenylsilanamine group was carried out (see ESI, Fig. S1†). The 1H liquid-state NMR spectrum of the resulting silsesquioxane is in agreement with the 1H MAS spectrum of 1 and confirms the formation of a σ bond between the nitrogen of aniline and the SBA15 surface.
Further information about the presence of vicinal functionalities are obtained from the 2D 1H–1H double quantum (DQ) NMR spectrum (Fig. 2). The 2D DQ spectrum of 1 shows a weak correlation between the proton resonance of SiOH at 1.9 ppm and the proton resonance of SiNHPh at 3.4 ppm [5.3 ppm in F1: δH(OH) + δH(NH) = 1.9 + 3.4] (Scheme 2.1), indicating that the two protons are in close proximity, typically <5 Å.
Scheme 2 Schematic showing the observed proximities of the paired N-phenylsilanamine/silanol groups in 1 from the 2D 1H–1H DQ NMR spectrum.
Moreover, the paired organization is reinforced by the presence of a correlation between the proton resonance of SiOH and the aromatic proton resonances of the neighboring SiNHPh at 6.6 ppm [8.5 ppm in F1: δH(OH) + δH(Ho) = 1.9 + 6.6] (Scheme 2.2). The correlation most likely arises from ortho position Ho. Note that Hm does not correlate with the silanol indicating that the aromatic ring is oriented in such a way that Ho is closest to the silanol. An additional correlation is observed between the proton resonance of SiNHPh at 3.4 ppm and the proton of the aromatic ring in ortho position Ho at 6.6 ppm [10 ppm in F1: δH(OH) + δH(Ho) = 3.4 + 6.6] (Scheme 2.3). Finally, strong correlation peaks are observed between all the protons of the aromatic ring at 13.6 ppm (Scheme 2.4).
Interestingly, the fact that no correlations between two silanols [ SiOH] [ SiOH] (1.9 × 2 = 3.8 ppm in F1) as well as between two [ SiNHPh] [ SiNHPh] (2 × 3.4 = 6.8 ppm in F1) are observed clearly demonstrates that the majority of sites are isolated “acid–base” pairs [ SiNHPh]: [ SiOH].
We conclude that dry aniline reacts with strained siloxane bridges to generate vicinal N-phenylsilanamine and silanol groups. These results are consistent with those obtained with FT-IR spectroscopy.
In the 13C CP-MAS NMR spectrum (Fig. S4A†), the signals of the aromatic ring appear clearly at 118, 120, 129 and 143 ppm corresponding to the CoH, CpH, CmH and to the quaternary carbon linked to the nitrogen surface Si–NH–CIV, respectively. These assignments are in accordance with those of the silsesquioxane model (see ESI, Fig. S2†). The 1H–13C HETCOR spectrum demonstrates a strong correlation between these four carbon resonances and the proton resonances at 6.6 and 7 ppm attributed to the proton of aromatic group (see ESI, Fig. S4B†). These results confirm that the integrity of the organic fragment is maintained under the reaction conditions.
To identify the formation of a covalent bond between the silica surface and the organic fragment, SiNHPh, 29Si and 15N solid state NMR spectra are required, but are not practical using conventional methods due to their low sensitivity at natural isotopic abundance.
Fig. 3 400 MHz DNP SENS spectra of 1 (20 mg) impregnated with a 16 mM solution of TEKPOL53 in 1,1,2,2-tetrachloroethane at 8 kHz MAS frequency with a sample temperature of 100 K. (a) 29Si DNP enhanced CP/MAS with a CP contact time of 5 ms, a 3 s polarization delay and 1024 scans. Exponential line broadening of 60 Hz was applied prior to Fourier transformation. (b) 15N DNP enhanced CP/MAS with 5 ms CP contact time, a 3 polarization delay and 16 000 scans. Exponential line broadening of 150 Hz was applied prior to Fourier transformation. In both (a) and (b) for comparison, spectra are shown with both for both μwave on and off.
The natural abundance 15N DNP SENS spectrum shows a single peak at 66 ppm (Fig. 3b) and it is in agreement with the 15N liquid-state NMR spectrum of the model molecular silsesquioxane, for which the 15N chemical shift appears at 62 ppm (Fig. S3, ESI†).
Textural characterization is used to evaluate the preservation of the mesoporous materials, here by nitrogen sorption porosimetry, small angle X-ray diffraction (XRD), and Transmission Electronic Microscopy (TEM). The small angle X-ray diffraction patterns of 1 (Fig. S6, ESI†) exhibit three clear peaks (d100, d110 and d200) in the 2θ range of 0.7–4°. They confirm the presence of a well-ordered hexagonal mesophase with a d100 spacing of 86.28 Å (see ESI, Table S1†). The structure of the mesoporous materials is thus maintained throughout the chemisorption of dry aniline.
Analyses of the nitrogen adsorption/desorption isotherms yielded BET surface areas of 1 of approximately 512 m2 g−1 (versus 679 m2 g−1 for SBA1100) and pore volumes of 0.65 cm3 g−1 (versus 0.9 cm3 g−1 for SBA1100). Also, 1 showed type IV isotherms (Fig. S7, ESI†), with clear H1-type hysteresis loops associated with capillary condensation in the mesopores and with regular pore sizes of 50 Å. The textural parameters of sample 1 are summarized in Table S1† and are characteristic of mesoporous materials. The surface coverage α of the organic moieties based on the carbon content was calculated as described by Jaroniec et al.54 Using eqn (S1) (see ESI†), a carbon content of 1.42 wt% was determined for 1, which translates into a surface coverage of 0.3 μmol m−2. The low surface coverage supports the results of N2 sorption experiments and indicates the functionalization of the SBA151100.
Further evidence for a well-ordered hexagonal mesostructure is provided by the TEM images (Fig. 4), which are representative of mesoporous SBA15. After the high thermal treatment (1100 °C, 10−5 mbar) and the dissociative chemisorption of aniline (80 °C, toluene, 20 h), the mesoporous structure is still regular over the whole particle of 1.
Fig. 4 Transmission electron micrographs of 1 at different tilt angles: in the direction perpendicular to the pore axis (B), and in the direction of the mesopores axis (B).
Scheme 3 Knoevenagel condensation between benzaldehyde and diethylmalonate. (i) The reaction was performed in sealed flask in which each reactant (2 mmol) and the catalyst (20 mg) were added to dry ethanol (5 mL) and the reaction mixture was refluxed at 80 °C for 24 h.
For comparison purposes, a series of bifunctional mesoporous materials with different electronic properties were successfully synthesized through the same approach (Scheme 4). All the materials were characterized by FT-IR and 1H-MAS solid state NMR spectroscopy (Fig. S8 and S9†).
Scheme 4 Synthesis of acid–base paired catalysts via the chemisorption of dry aniline derivatives on SBA151100 in toluene at 80 °C for 20 h.
All the FT-IR spectra of catalysts 2–5 display the characteristic vibration bands of ν(OH) (3745 cm−1), ν(NH) and δ(NH), 3435 and 1500 cm−1 respectively. Vibrational bands of the aromatic group are still present at 3089–3023 cm−1 [ν(CH)], at 1606 and 1500 cm−1 [δ(C C)] (overlapping a NH band). The shoulder characteristic of the electronic interactions of the π system of the aromatic group with the newly formed adjacent silanol (π–OH interactions) in the range of 3690–3585 cm−1 is again observed for all catalysts (weak in the case of catalyst 5).
All the 1H NMR spectra feature the characteristic signal of SiOH and SiNH at around 2 ppm and 3.5–3.9 ppm, respectively. As expected, the protons in ortho and meta position to electron donating (OMe) and electron withdrawing (Cl, NO2) substituents show distinct upfield and downfield shifts (Fig. S9†).
Their catalytic performance were tested (Table 1, entry 1–5) and all the samples showed good catalytic ability. Entry 1 showed higher activity than entry 2. The nitro group is a strongly electron-withdrawing group (EWG) and thus, catalyst (2) is a weaker base than catalyst (1). A chloro group in the para position is a slightly EWG, so catalyst (3) exhibits better activity than (2) and is slightly less active than (1). Introducing an electron-donating group (EDG) as a p-methoxy group in the catalyst (4) enhances the catalytic performance in the Knoevenagel reaction. Among all these catalysts, (4) exhibits the best performance whereas (5) exhibits the lowest due to the base weakening effect.
a Determined by elemental analysis. b Determined by GC analysis after 24 h. c Turnover number (TON) = number of moles of product per number of moles of active amine site.
In parallel, the stability of (7) and (1) towards ethanol was monitored by FT-IR spectroscopy. After 5 min in contact with ethanol, the FT-IR spectrum (Fig. S10, ESI†) of (7) shows complete disappearance of the characteristic bands of the SiNH2 group [νs(NH2) = 3535, νs(NH2) = 3445 and δ(NH2) = 1550 cm−1]. However the FT-IR spectrum of (1) shows the characteristic bands of Si–NHPh, [ν(NH) = 3435 cm−1] even after 1 h in contact with dry ethanol. In addition, during the catalytic test with catalyst 1, no leaching of aniline was detected by both GC-FID and GC-MD (Fig. S11, ESI†).
The opening siloxane bridges approach was successfully established to create an atomic organization of well-defined bi-functional acid–base pairs on mesoporous SBA15. This approach is based on an analogy between organic epoxides and strained siloxanes ( Si–O–Si ) of mesoporous SBA151100. The generation of well-defined adjacent N-phenylsilanamine–silanol pairs was unambiguously determined through FTIR, 2D solid state NMR, XRD, nitrogen sorption and TEM. This way to design bi-functionalized mesoporous surface offers new opportunities to modify the electronic and steric properties of mesoporous silica useful for heterogeneous catalysis.
This work received support from the King Abdullah University of Science and Technology (KAUST) Office of Sponsored Research (OSR) under Award No. CRG_R2_13_BASS_KAUST_1. We thank Dr Olivier Ouari and Paul Tordo for providing the TeKPol radical.
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