Optimized separation of C2H2 from binary mixtures of C2H2/CO2 and C2H2/C2H4 by coordinative copper(I) metal nodes modified from HKUST-1

초록

Acetylene (C 2 H 2 ), an industrially important gas, is produced by steam cracking of hydrocarbons and inevitably contains traces of ethylene (C 2 H 4 ) and carbon dioxide (CO 2 ) as impurities. Metal-organic frameworks (MOFs) and further post-modified MOFs are versatile and promising adsorbents for energy-efficient C 2 H 2 /C 2 H 4 and C 2 H 2 /CO 2 separation by selective adsorption of C 2 H 2 compared to energy-consuming conventional methods. Herein, Cu(I) ion generated structures from post-synthetic modification of HKUST-1 are chosen as C 2 H 2 selective sites over both C 2 H 4 and CO 2 gases. Protective Cu(I) ions were generated by post-coordinative reduction of Cu(II) nodes of a HKUST-1 using hydroquinone as a reducing agent to create robust C 2 H 2 adsorptive sites. In the metal node reduced to Cu(I) by hydroquinone (HKUST-1-HQ), C 2 H 2 selectivity was enhanced not only for C 2 H 2 /CO 2 but also C 2 H 2 /C 2 H 4 by the presence of the Cu(I) site, by virtue of the copper(I)-alkynyl chemistry. In a breakthrough test, separation of C 2 H 2 and C 2 H 4 did not occur at the coordinatively unsaturated site (HKUST-1), whereas separation capability appeared after monovalent copper was introduced. Enhanced C 2 H 2 /CO 2 separation was also observed for HKUST-1-HQ. The adsorption behavior of C 2 H 2 (triple bond), C 2 H 4 (double bond), and CO 2 (opposite quadruple moment) at the copper(II) open metal site and copper(I) site originating from the same structure respectively is studied to understand the C 2 H 2 gas sorption. In HKUST-1-HQ, the study of four types of possible Cu structures (Nodes II, III, IV, and V) and adsorbates revealed that Node IV is a preferential site over the other three sites, and the CUS site (bare HKUST-1). DFT computational simulation successfully demonstrated that the Node IV type Cu(I) is a plausible adsorption site for C 2 H 2 . Overall, HKUST-1-HQ exhibits greater moisture stability, improved selectivity, and repeatability as C 2 H 2 adsorbent.

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