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.