Boosting kinetic separation of ethylene and ethane on microporous materials via crystal size control

Abstract

The adsorptive separation of C2H4 and C2H6, as an alternative to distillation units consuming high energy, is a promising yet challenging research. The great similarity in the molecular size of C2H4 and C2H6 brings challenges to the regulation of adsorbents to realize efficient dynamic separation. Herein, we reported the enhancement of the kinetic separation of C2H4/C2H6 by controlling the crystal size of ZnAtzPO4 (Atz = 3-amino-1,2,4-triazole) to amplify the diffusion difference of C2H4 and C2H6. Through adjusting the synthesis temperature, reactant concentration, and ligands/metal ions molar ratio, ZnAtzPO4 crystals with different sizes were obtained. Both single-component kinetic adsorption tests and binary-component dynamic breakthrough experiments confirmed the enhancement of the dynamic separation of C2H4/C2H6 with the increase in the crystal size of ZnAtzPO4. The separation selectivity of C2H4/C2H6 increased from 1.3 to 98.5 with the increase in the crystal size of ZnAtzPO4. This work demonstrated the role of morphology and size control of adsorbent crystals in the improvement of the C2H4/C2H6 kinetic separation performance.

1. Introduction

Ethylene (C2H4) is a key feedstock to many chemicals and polymers [1]. During the production of ethylene from the steam-cracking reaction of carbon-based feedstocks, ethane (C2H6) is one of the main impurities. C2H6 must be removed from C2H4 to produce high-pure C2H4 for the polymer production process. The present industrial method to enrich C2H4 from C2H4/C2H6 mixtures mostly relied on cryogenic distillation, requiring a large energy consumption, ca. 0.3% of global energy [2]. Under the global urgent obligation of carbon neutralization, all kinds of energy-saving transformations of chemical plants will be a long-term significant reform, including the introduction of adsorption separation technology [3–8]. And for the adsorptive separation of C2H4/C2H6 mixtures, the exploration of efficient adsorbents is of great significance.

However, the intrinsically small and similar molecular sizes [9,10] of C2H4 (0.328 nm × 0.418 nm × 0.484 nm) and C2H6 (0.381 nm × 0.408 nm × 0.482 nm) make it challenging to explore efficient adsorbents with high C2H4/C2H6 selectivity. Many researchers have explored various types of materials to selectively adsorb C2H4 from C2H4/C2H6 mixture [10–36]. So far, the design strategy of adsorbents with efficient separation performance can be mainly classified into two categories. For the first class of adsorbent design, transition-metal ions and unsaturated metal sites were introduced into porous materials, such as Fe-MOF-74 [12] and PAF-1-SO3Ag [22], performing efficient separation by thermodynamic effect. The IAST (ideal adsorbed solution theory) separation selectivity of C2H4/C2H6 on Fe-MOF-74 and PAF-1-SO3Ag were as high as 13–18 and 27, respectively. However, the preferentially interact of PAF-1-SO3Ag with the p-electrons of C2H4 molecules resulted in relatively high adsorption energy (106 kJ·mol−1) [17]. The second class of design strategy was controlling the pore size and pore structure of adsorbents to realize kinetic separation based on the diffusion rate. In an ideal scenario, approximately selective molecular sieving of C2H4 and C2H6 based on large diffusion rates was illustrated by UTSA-280 (pore size 0.3–0.4 nm) [19] and Mg-gallate (pore size 0.356 nm) [26]. Other adsorbents such as ITQ-55 [16], Cu(OPTz) [22] showed potential separation performance by kinetic effect. The selectivity for kinetic separation of C2H4/C2H6 was up to 50 on ITQ-55 with heart-shaped cages and framework flexibility. Recently, our group [17] reported an ultra-microporous metal–organic framework [Zn3(Atz)3(PO4)]·(ZnAtzPO4, Atz = 3-amino-1,2,4-triazole) for efficient separation of C2H4 and C2H6 by exploiting equilibrium-kinetic synergetic effect. The ZnAtzPO4 adsorbent showed high C2H4/C2H6 selectivity and low adsorption energy.