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

Abstract

Acetylene (C2H2), an industrially important gas, is produced by steam cracking of hydrocarbons and inevitably contains traces of ethylene (C2H4) and carbon dioxide (CO2) as impurities. Metal-organic frameworks (MOFs) and further post-modified MOFs are versatile and promising adsorbents for energy-efficient C2H2/C2H4 and C2H2/CO2 separation by selective adsorption of C2H2 compared to energy-consuming conventional methods. Herein, Cu(I) ion generated structures from post-synthetic modification of HKUST-1 are chosen as C2H2 selective sites over both C2H4 and CO2 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 C2H2 adsorptive sites. In the metal node reduced to Cu(I) by hydroquinone (HKUST-1-HQ), C2H2 selectivity was enhanced not only for C2H2/CO2 but also C2H2/C2H4 by the presence of the Cu(I) site, by virtue of the copper(I)-alkynyl chemistry. In a breakthrough test, separation of C2H2 and C2H4 did not occur at the coordinatively unsaturated site (HKUST-1), whereas separation capability appeared after monovalent copper was introduced. Enhanced C2H2/CO2 separation was also observed for HKUST-1-HQ. The adsorption behavior of C2H2 (triple bond), C2H4 (double bond), and CO2 (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 C2H2 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 C2H2. Overall, HKUST-1-HQ exhibits greater moisture stability, improved selectivity, and repeatability as C2H2 adsorbent.