Characterization of Covalent Linkages in Organically Functionalized MCM-41 Mesoporous Materials by Solid-State NMR and Theoretical Calculations

Abstract

The covalent linkages formed during functionalization of MCM-41 mesoporous molecular sieves with five chloroalkylsilanes ((EtO)3Si(CH2Cl), (MeO)3Si(CH2CH2CH2Cl), Cl3Si(CH2CH2CH3), Cl2Si(CH3)(CH2Cl) and Cl2Si(CH3)2) have been investigated using high-resolution solid-state NMR spectroscopy and DFT calculations. Structural information was obtained from 1H-13C and 1H-29Si heteronuclear (HETCOR) NMR spectra, in which high resolution in the 1H dimension was obtained by using fast MAS. The 1H-13C HETCOR results provided the assignments of 1H and 13C resonances associated with the surface functional groups. Sensitivity-enhanced 1H-29Si HETCOR spectra, acquired using Carr-Purcell-Meiboom-Gill refocusing during data acquisition, revealed the identity of 29Si sites (Qn, Tn, and Dn) and the location of functional groups relative to these sites. Optimal geometries of local environments representing the Qn, Tn and Dn resonances were calculated using molecular mechanics and ab initio methods. Subsequently, DFT calculations of 29Si, 13C, and 1H chemical shifts were performed using Gaussian 03 at the B3LYP/6-311++G(2d,2p) level. The theoretical calculations are in excellent accord with the experimental chemical shifts. This work illustrates that state-of-the-art spectroscopic and theoretical tools can be used jointly to refine the complex structures of inorganic-organic hybrid materials.

Application of Advanced Solid‐State NMR Techniques to the Characterization of Nanomaterials: A Focus on Interfaces and Structure

“…Nevertheless, it is trickier to prove that a homogeneous functionalization occurred at the pore surface. The group of Pruski demonstrated functionalization of organic compounds at silica surface mainly via 1 H-13 C and 1 H- 29 Si HETCOR experiments [161,163,165,166]. In all cases, a big effort was put into the development of robust and adapted pulse sequences, which could provide unambiguous (e.g., use of LG homonuclear decoupling to prevent 1 H spin diffusion) and high-quality data (e.g., sensitivity enhancement via very fast MAS or Carr-Purcell-Meiboom-Gill (CPMG) pulse sequences).…”

29Si NMR in solid state with CPMG acquisition under MAS

“…Most of the CPMG-based experiments are conducted using three samples of MCM-41-type mesoporous silica nanoparticle (MSN) materials whose synthesis and characterization have been described in our earlier reports: (i) a non-functionalized sample, referred to as MSN [54]; (ii) a sample functionalized via co-condensation with covalently bound allyl groups (ACH 2 ACH@CH 2 ), referred to as AL-MSN [54]; and (iii) a sample functionalized via grafting with chloromethyltriethoxysilane ((EtO) 3 Si(CH 2 Cl)), referred to as CMTES-MSN [50]. Based on the deconvolution of 29 Si DPMAS spectra, we estimated that 11 ± 2% and 15 ± 2% of all silicon atoms in AL-MSN and CMTES-MSN, respectively, are bound to carbon (functionalized) [50,54].…”

“…Based on the deconvolution of 29 Si DPMAS spectra, we estimated that 11 ± 2% and 15 ± 2% of all silicon atoms in AL-MSN and CMTES-MSN, respectively, are bound to carbon (functionalized) [50,54]. All silicas were studied in the absence of templating molecules, which were removed by acid extraction (MSN and AL-MSN) or calcination (CMTES-MSN).…”

“…3c, low trace). This scheme can be easily extended to reconstruct 2D spectra, such as 29 Si{ 1 H} HETCOR [49,50].…”

“…We recently demonstrated that highly resolved 29 Si{ 1 H} HET-COR NMR spectra of silica-based catalytic systems can be obtained with excellent sensitivity using multiple pulse CPMG refocusing and appropriate data processing [49,50]. These experiments were made possible by the use of fast MAS, at rates of 25-40 kHz, which provided adequate 1 H-1 H decoupling.…”