Solid State NMR Spectroscopy Studies of the Nature of Structure Direction of OSDAs in Pure-Silica Zeolites ZSM‑5 and Beta
journal contributionposted on 12.06.2017, 12:50 by Xiaolong Liu, Qing Luo
2D 1H DQ-SQ NMR spectra of zeolites TPA-OH-ZSM-5 and TEA-OH-ZSM-5 clearly demonstrate the specific spatial correlations between the SiO–··HOSi hydrogen bonds within the framework and the alkyl chains of TPA+ and TEA+. For zeolite TPA-OH-ZSM-5,the 2D 1H DQ-SQ NMR spectrum shows that the SiO–··HOSi hydrogen bonds within the framework are spatially close to the methyl groups of TPA+ cations. For zeolite TEA-OH-ZSM-5, the 2D 1H DQ-SQ NMR spectrum shows that the SiO–··HOSi hydrogen bonds within the framework are spatially close to both the methyl and methylene groups of TEA+ cations. These observations imply that the position and distribution of the negative charge centers such as F anions, SiO–··HOSi hydrogen bonds and T3+ atoms in the MFI framework are influenced by TPA+ or TEA+ cations for the strong electrostatic interactions. By analyzing the variable contact time 1H–13C CP/MAS NMR experimental results, the 13C signal with δ = 10.6 ppm can be assigned to the methyl groups of TPA+ cations located in zigzag channels and the 13C signal with δ = 11.6 ppm can be assigned to the methyl groups of TPA+ cations located in straight channels. Both 2D 1H DQ-SQ NMR spectrum and 1H–13C CP/MAS NMR spectra of TEA-OH-Beta show that the SiO–··HOSi hydrogen bonds within the framework are spatially further from the alkyl chains of TEA+ in Beta than those in ZSM-5, which indicates that van der Waals interactions play the dominant roles during the crystallization process of zeolite Beta. According to our NMR observations, it can be inferred that the nature of structure direction of OSDAs roots in the complex relationship between van der Waals interactions and electrostatic interactions in the inorganic–organic composites formed in the induction period.