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

Abstract

The interlayer expanded zeolite COE-4 was synthesized using silylation agents such as dichlorodimethylsilane (DCDMS) as layer linker 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 29Si MAS NMR results show that after calcination the layer spacing of COE-4 is enlarged by 2.6 Å 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) 129Xe NMR results indicate that after interlayer expansion, Xe atoms can be adsorbed in the 10-MR channels of COE-4, and the 10-MR channels are homogeneously distributed like ZSM-5. For COE-4, Xe has a stronger interaction with the channel walls containing dual silanol groups. 1H MAS NMR combined with theoretical calculations make 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-d5, 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 1H NMR signal of dual silanol groups shows a weak broad peak at 5.7 ppm after pyridine adsorption.