Acta Physico-Chimica Sinica ›› 2020, Vol. 36 ›› Issue (4): 1905035.doi: 10.3866/PKU.WHXB201905035

Special Issue: Solid-State Nuclear Magnetic Resonance

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Hydrogen-Bond Induced Crystallization of Silicalite-1 Zeolite as Revealed by Solid-State NMR Spectroscopy

Xiaolong Liu1,2,Qiang Wang1,Chao Wang1,Jun Xu1,*(),Feng Deng1,*()   

  1. 1 State Key Laboratory of Magnetic Resonance and Atomic and Molecular Physics, National Centre for Magnetic Resonance in Wuhan, Wuhan Institute of Physics and Mathematics, Innovation Academy for Precision Measurement Science and Technology, Chinese Academy of Sciences, Wuhan 430071, P. R. China
    2 School of Materials, Sun Yat-Sen University, Guangzhou 510275, P. R. China
  • Received:2019-05-08 Accepted:2019-06-14 Published:2020-03-12
  • Contact: Jun Xu,Feng Deng E-mail:xujun@wipm.ac.cn;dengf@wipm.ac.cn
  • Supported by:
    the National Natural Science Foundation of China(21473246);the National Natural Science Foundation of China(21622311);the National Natural Science Foundation of China(21573278);the National Natural Science Foundation of China(21673282);Key Program for Frontier Science of the Chinese Academy of Sciences(QYZDB-SSW-SLH027);Hubei Provincial Natural Science Foundation, China(2017CFA032)

Abstract:

The flexible chemical composition of the frameworks with tunable pore size and geometry of molecular dimensions makes zeolites widely used in chemical and petrochemical industry fields. The understanding of crystallization mechanism is important for a rational design of new zeolite with target structure and property, which however is still a big challenge in the field of material science. In this work, the specific spatial correlations/interactions between the SiO-···HO―Si hydrogen bonds within the charged framework of silicalite-1 (MFI topology) zeolite and the alkyl chains of tetrapropylammonium ion (TPA+) organic structure direction agents (OSDAs) were studied by one-dimensional (1D) and two-dimensional (2D) solid state-NMR spectroscopy in combination with other techniques, with the aim to shed light into the crystallization mechanism of silicalite-1. The "solvent-free" route was used to study the crystallization process. Silicalite-1 crystals were also prepared following the hydrothermal synthesis route. The structural properties of as-synthesized TPA-silicalite-1 samples during the crystallization were characterized by XRD and scanning electron microscopy (SEM) images, which showed the evolution of long-range periodic structure and cyrtal growth. The 1H-29Si CP/MAS NMR experiments showed that the reorganization of the silica or silicates occurred in the crystallization process. The lH-13C CP/MAS NMR experiments performed on the samples synthesized with different time indicated that the TPA+ ions in the amorphous samples experienced a constrained environment, forming the inorganic-organic composites. The splitting of the methyl carbon signal from TPA+ ions was observed in the 13C NMR spectra, which is the direct reflection of the interactions between the methyl groups and the silicate framework in the straight and zig-zag channels of silicalite-1. Two types of SiO-···H―OSi hydrogen bonds (SiO-···H―OSi hydrogen bond in-cage and SiO-···H―OSi hydrogen bond between lamellae) have been identified by 2D 1H double quantum (DQ)-single quantum (SQ) MAS NMR and 2H MAS NMR during the crystallization of silicalite-1. The SiO-···H―OSi hydrogen bonds between lamellae are formed and gradually transformed into the in-cage ones during the crystallization process. Their functions have been revealed in the formation of silicalite-1: the SiO-···H―OSi hydrogen bond in-cage provides the stereoscopic counterbalance for the positive charges from TPA+ ions and this stereoscopic electrostatic interaction is the key factor to transform inorganic-organic composites with the MFI structure property, even though the long-range periodic MFI structures have not been established yet; the SiO-···H―OSi hydrogen bond between lamellae acts as a connector to assemble the silicate species together to generate the zeolite framework. 2H MAS NMR spectra show that the SiOH nests exist in the zeolite framework even though the long-range periodic structures have been fully established.

Key words: Zeolite, Crystallization mechanism, Solid-state NMR, Structure direction agent, Hydrogen bond

MSC2000: 

  • O641