Surprising separation selectivity of ethylene from ethane over pure siliceous zeolites with framework flexibility

Abstract

Recovery and purify of light olefins from the off-gas of the catalytic crackers have been focused for more than 50 years, driven largely by the escalating demand for feedstock for production of polyethylene and polypropylene, which requires the separation of C 2 H 4 /C 2 H 6 and C 3 H 6 /C 3 H 8 mixtures in which the olefins and paraffins are present at roughly equal concentrations [1,2]. Distillation operated at low temperatures (cryogenic distillation) or high pressure is one of the most useful methods for the separation and purify of these light olefins. However, these processes are energetically inefficient and therefore costly since the differences in volatility between the olefins and corresponding paraffins are so small, which require the separation column operated at high reflux, with more than 100 plates [2]. As a consequence, more than 0.3% of the global energy was consumed in the separation of ethylene and propylene from the corresponding paraffins. For the sake of energy-saving and cost-reducing, developing the efficient adsorption systems has been regarded as one of alternative technologies. However, no economically viable adsorptive separation process has yet emerged up to now, due to the relatively strong interaction between the olefins and selective adsorbents, which results in the complex regeneration process. As a result, new kinetically selective adsorbents for light olefins separation have been paid much attention in recent years.Porous materials are promising candidates for efficient separation of gas mixtures by distinguishing the differences in molecular sizes, shapes, and polarities. Recently, custom-designed porous materials, in termed of metalorganic frameworks (MOFs) and/or porous coordination polymers (PCPs) with open lattices formed from inorganic centers (nodes) and organic linking groups have been paid much attention due to their inherent diversity, which afford precise control over pore chemistry and pore size and consequently excellent performance in gas separation or capture such as ethylene [3] and 1,3-butadiene [4]. However, the low thermal stability still limits their wide applications in the separation of light olefins from corresponding paraffins since thermal treatments are always required for regeneration of the adsorbents once the pores are blocked.Zeolites, especially pure siliceous zeolites with suitable microporous structure would be ideal adsorbents for the separation of light olefins with paraffins because of their high thermal stability for regeneration and neutral framework for olefins non-oligomerization. Previous literatures have confirmed that pure siliceous 8-membered ring (MR) zeolites, e.g., CHA or deca-dodecasil 3R (DD3R) have shown superior performance on the separation of propylene and propane [1]. But the suitable zeolites for separation of ethylene and ethane are still a great challenge.Excitingly, a joint team lead by Corma, Corcoran, Rey, and Ravikovitch [5] recently reported new pure siliceous zeolite (ITQ-55), which could kinetically separate ethy...