Structural investigation of interlayer-expanded zeolite by hyperpolarized Xe-129 and H-1 NMR spectroscopy

摘要

The interlayer expanded zeolite COE-4 was synthesized using silylation agents such as dichlorodimethylsilane (DCDMS) as layer linlcer from layered silicate RUB-36 precursor. Solid-state NMR techniques combined with theoretical calculations have proven to be a useful characterization method to investigate the structure and porosity of the interlayer expanded zeolites. XRD and Si-29 MAS NMR results show that after calcination the layer spacing of COE-4 is enlarged by 2.6 angstrom compared with that of RUB-37, indicating that pore windows between the layers are enlarged from 8-membered ring (8-MR) pores to 10-MR pores due to the formation of Si(OH)(2) pillar between adjacent layers. Hyperpolarized (HP) Xe-129 NMR results indicate that after interlayer expansion, Xe atoms can be adsorbed in the 10-MR channels of COE-4, and the 10-IVIR channels are homogeneously distributed like ZSM-5. For COE-4, Xe has a stronger interaction with the channel walls containing dual silanol groups. H-1 MAS NMR combined with theoretical calculations malce a clear assignment of dual silanol groups at 3.3 ppm, and the concentration of dual silanols is 2.2 mmol/g, which is 1-2 orders of magnitude larger than the concentration of the other two types of existing isolated and hydrogen-bonded silanol groups. After introduction of pyridine-d(5), one Si-OH from the dual silanol groups has a weak hydrogen bond interaction with pyridine molecule, and the adsorbed pyridine molecule may experience chemical exchange with the other Si-OH of the dual silanol group and therefore H-1 NMR signal of dual silanol groups shows a weak broad peak at 5.7 ppm after pyridine adsorption.