Introduction
Natural gas (NG), as a promising clean energy, has been widely used with an increasing trend in recent years.[1] Natural gas consists primarily of methane, including location-dependent ratio of 12 ~ 39% of other heavier light hydrocarbons, such as ethane, propane, n-butane, etc.[2,3] Among these light hydrocarbons, C2H6 and C3H8 are of great value in the petrochemical industry. C2H6 is the most important raw material for the ethylene production which serves as the main component of the polyethylene, polyvinyl chloride and other polymer. C3H6 is also the basic feedstock for the propylene and polypropylene production.[3,4] Direct combustion of NG for heat supply without recovery of C2H6 and C2H8 would cause an enormous waste of these ethane (C2) and propane (C3) resources. Therefore, in order to fully utilize these hydrocarbons, it is essential to recover the ethane (C2) and propane (C3) from the natural gas for the high-purity alkene production. Although the cryogenic distillation, which is based on the small differences in boiling point of each component, may be used for the separation of C1/C2/C3, it is an energy-intensive separation technology.[3,5] Adsorptive separation is considered as one of the most promising techniques because of its low energy consumption and high separation efficiency.[1,6,7]
In adsorption technology, the adsorbent with excellent separation properties is the core.[7] Therefore, considerable efforts have been made to develop porous materials with high adsorption capacity and selectivity for separation of C1/C2/C3. Traditional porous materials, such as zeolites, carbonaceous materials, are employed to separate the C1/C2/C3 mixtures but most of them exhibited either low capacity or low selectivity for these hydrocarbons.[8,9] In recent years, metal-organic frameworks (MOFs), comprised of metal ions/clusters and organic linkers, has emerged and the diversity of the organic/inorganic linkers and exquisite control over pore aperture size promise MOFs great potential in the areas of gas storage,[10]catalysis,[11,12]sensing,[13,14] gas separations.[16-27]
Since the concentrations of ethane and propane are about 5% and 10% in natural gas respectively,[2] a MOF material with high gas capacity under low pressure area (5 kPa ~ 10 kPa) is highly demanded to address the issue of separation of C1/C2/C3. As a benchmark material for hydrocarbon separations, MOF-74(Co) was reported to adsorb 3 mmol/g propane at 5 kPa and 3.1 mmol/g ethane at 10 kPa, while it was humid unstable.[28,29] In addition, the Gly@HKUST-1 exhibited 4.22 mmol/g propane at 5 kPa and 1.19 mmol/g ethane at 10 kPa.[26] However, most of other MOFs reported for separation of C1/C2/C3 exhibited low adsorption capacity at low pressure region.[26-27, 36-38] In order to improve the C2/C3 low-pressure adsorption ability, a microporous MOF material with pore size slightly larger than molecule sizes of propane and ethane is required, which may lead to the enhancement of the interaction between framework and propane/ethane molecule. A pillared-layer MOF, Ni(TMBDC)(DABCO)0.5[30], was found to show strong affinity towards C2H6 at low pressure area. In the structure of this material, the 2D-layer is connected by nickel paddle-wheels and 2,3,5,6-tetramethylterephthalic acid (TMBDC) and bridged by 1,4-Diazabicyclo[2.2.2]octane (DABCO) to produce a 3D network with the topology of pcu . The pore size of Ni(TMBDC)(DABCO)0.5 is 0.59 nm, which is slightly larger than molecular sizes of propane (0.50 nm) and ethane (0.44 nm). Therefore, Ni(TMBDC)(DABCO)0.5 would be a promising MOF material for the separation of C1/C2/C3.
Herein, we reported the synthesis of Ni(TMBDC)(DABCO)0.5and its performance of separating light hydrocarbons for the recovery of C3H8 and C2H6 from natural gas. The stability of the material was estimated by TG analysis and PXRD characterization. The CH4, C2H6 and C3H8 adsorption isotherms on Ni(TMBDC)(DABCO)0.5 were measured and the separation performance of C1/C2/C3 ternary mixture was further evaluated by breakthrough experiment. The selectivity of C2H6/CH4 and C3H8/CH4 were predicted by ideal adsorbed solution theory (IAST) model. In addition, molecule simulation was applied to investigate the adsorption mechanism of these light hydrocarbons in the pores of Ni(TMBDC)(DABCO)0.5.