Raman Evidence of Multiple Adsorption Sites and Structural Transformation in ZIF-4
Priyanka Jain, Gayatri Kumari, Meha Bhogra, Premakumar Yanda, Boby Joseph, Umesh V. Waghmare, Chandrabhas Narayana
Abstract
The zeolitic imidazolate framework, ZIF-4, exhibits soft porosity and is known to show pore volume changes with temperatures, pressures, and guest adsorption. However, the mechanism and adsorption behavior of ZIF-4 are not completely understood. In this work, we report an open to narrow pore transition in ZIF-4 around T ∼ 253 K upon lowering the temperature under vacuum (10 –6 Torr) conditions, facilitated by C–H···π interactions. In the gaseous environment of N 2 and CO 2 around the framework, characteristic Raman peaks of adsorbed gases were observed under ambient conditions of 293 K and 1 atm. A guest-induced transition at ∼153 K resulting in the opening of new adsorption sites was inferred from the Raman spectral changes in the C–H stretching modes and low-frequency modes (<200 cm –1 ). In contrast to a single vibrational mode generally reported for entrapped N 2, we show three Raman modes of adsorbed N 2 in ZIF-4. The adsorption is facilitated by dispersive and quadrupolar interactions. From our temperature-dependent Raman results and theoretical analysis based on the density functional tight-binding approach, we conclude that the C–Hs are the preferred adsorption sites on ZIF-4 in the following order: C4–H, C5–H > C2–H > center of the Im ring (interacting with C–H centers) > center of the cavity. We also show that with an increasing concentration of N 2 adsorbed at low temperatures, the ZIF-4 structure undergoes shear distortion of the window formed by 4-imidazole rings and consequent volumetric expansion. Our results have immediate implications in the field of porous materials and could be vital in identifying subtle structural transformations that may favor or hinder guest adsorption.