Pillar-layer Zn-triazolate-dicarboxylate frameworks with a customized pore structure for efficient ethylene purification from ethylene/ethane/acetylene ternary mixtures

Abstract

The selective adsorption of C₂H₆ and C₂H₂ over C₂H₄ from C₂H₂/C₂H₄/C₂H₆ ternary mixtures for one-step C₂H₄ purification represents a crucial yet challenging task in industry. The pore structure of the adsorbents must be finely tailored to meet the demanding requirements for the separation considering the very similar physicochemical properties of the three gases. Herein, we report a Zn-triazolate-dicarboxylate framework, HIAM-210, featuring a novel topology which possesses one-dimensional channels decorated with adjacent uncoordinated carboxylate-O atoms. The suitable pore size and customized pore environment enable the compound to selectively capture C₂H₆ and C₂H₂ with high C₂H₂/C₂H₄ and C₂H₆/C₂H₄ selectivities of both 2.0. Breakthrough experiments show that polymer-grade C₂H₄ can be directly harvested from C₂H₂/C₂H₄/C₂H₆ (34/33/33 and 1/90/9) ternary mixtures. The underlying mechanism of the preferential adsorption was uncovered by grand canonical Monte Carlo simulations and DFT calculations.

Fig. 1. Structure of HIAM-210. (a) The building blocks; (b) the 2D Zn–TZ–NDC layer structure; (c) the pillaring mode by NDC; (d) the structure viewed along the 1D channels; (e) a close-up view of the array of the four different cages in the channel; (f) the distribution of uncoordinated carboxylate–O atoms in the four cages.

Fig. 2. Gas adsorption on HIAM-210. (a) N₂ adsorption–desorption isotherms at 77 K and pore size distribution (inset); (b) C₂H₂/C₂H₄/C₂H₆ adsorption–desorption isotherms at 298 K; (c) IAST selectivity of equimolar C₂H₂/C₂H₄ and C₂H₆/C₂H₄ mixtures at 298 K; (d) comparison of the separation performance of representative MOFs (C₂H₂/C₂H₄: 50/50, v/v; and C₂H₆/C₂H₄: 50/50, v/v).

Fig. 3. The preferential adsorption sites for (a) C₂H₂, (b) C₂H₆, (c) C₂H₄ in Cage C and (d) preferential adsorption site for C₂H₂ in Cage B, and the C–H⋯π and C–H⋯O interactions are represented by blue and red dashed lines respectively.

Fig. 4. Dynamic breakthrough tests on HIAM-210. The breakthrough curves for binary mixtures with the composition of (a) C₂H₂/C₂H₄ (50/50, v/v) and (b) C₂H₆/C₂H₄ (50/50, v/v), the breakthrough curves for ternary mixtures with the composition of (c) C₂H₂/C₂H₄/C₂H₆ (34/33/33, v/v/v) and (d) C₂H₂/C₂H₄/C₂H₆ (1/90/9, v/v/v), all collected at 298 K; (c) comparison of the breakthrough curves under humid and dry environments; (d) four consecutive breakthrough cycles at 298 K.