Introduction
Both acetylene
(C2H2)
and ethylene (C2H4) are widely used as
basic chemical materials in the petrochemical industry. The production
of C2H4 is from the fractional
distillation of petroleum, which usually coexists with 1%
C2H2.1-3 In addition,
carbon dioxide (CO2) appears in the production of
C2H2 by the thermal cracking
process.4,5 Thus, selectively separation of
C2H2 from
C2H4/C2H2or CO2/C2H2 mixtures is
important to meet the requirement of polymer-grade
C2H4 and high-purity
C2H2 in petrochemical
industrials.6,7 However, considering the small
difference of molecular dimension of
C2H2(3.3
Å×3.3 Å×5.7 Å), C2H4 (3.3
Å×4.2 Å×4.8 Å) and
CO2 (3.2 Å×3.3 Å×5.4 Å), as well as their similar
physical properties, it is a very challenging to separate
C2H2 from
C2H4 or
CO2.8,9 Although the cryogenic
distillation for C2H2 separation is a
very mature operation, it requires huge capital and energy input
primarily due to the requirement of low temperature
operation.10-12 It is necessary to develop a more
efficient separation process such as porous materials based adsorptive
separation with relatively low energy consumption and favorable
regeneration.13,14
Porous materials, such as zeolites, metal-organic frameworks (MOFs) and
covalent-organic frameworks (COFs), with porous structures, huge
specific surface area, and designable frameworks have received extensive
attention in recent years,15-17 as they present great
potential to revolutionize some industrial applications, especially in
separation, purification, and the storage of
gases.18-20An
ideal adsorbent for C2H2 separation is
expected to be chemically stable, be able to capture trace
C2H2 from
C2H4 or other feed gas mixtures, and be
able to regenerate easily. In recent years, a number of porous materials
have been reported for
C2H2/C2H4separation through a synergistic approach of pore tuning and
functionalization.21,22 In 2016, Xing’s
group14 reported the anion-pillared hybrid porous
materials SIFSIX-2-Cu-i, with the specific binding sites and suitable
pore space to effectively overcome the trade-off effect for
C2H2 separation. In 2017, the first case
of an ideal molecular sieve for
C2H2/C2H4was found called SIFSIX-14-Cu-i (UTSA-200)23 , which
with the ultrafine tuning of its pore size (3.4 Å), can not only
effectively block C2H4 but also adsorb
high amounts of C2H2, thus setting up
the benchmarks for both C2H2 adsorption
and
C2H2/C2H4separation. However, UTSA-200 can also take up a large amount of
CO2 or C3H4 at the same
conditions,24,25 which significantly restricts its
separation performance for
C2H2/CO2 and other
multicomponent gases mixtures.
Traditional research on porous materials for gas separation mainly
focused on those porous structures in the one-dimensional channel
(Figure 1a) or cage type pore (Figure 1b). During our exploration of
porous materials for C2H2 separation, we
realized that one unique class of porous MOFs, which have
interlayer pore cavities (Figure
1c)26,27 in their layered structures, had been
overlooked. Compared to the traditional pore types, this kind of pore
cavity has a narrow pore space, may exhibit multiple host-guest
interactions with gas molecules, and thus could be utilized for
selective separation of some specific gas component.
Herein, we report an ultramicroporous MOF
(Zn2(bpy)(btec))28,29 that
incorporates two-dimensional interlayer cavities, which enabled the full
entrance of C2H2 and effectively blocked
the C2H4 and CO2, thus
exhibiting the benchmark
C2H2/C2H4and C2H2/CO2 uptake
ratios. According to the breakthrough experiments,
C2H2 can be directly removed from
C2H2/C2H4(1/99, v/v) or
C2H2/CO2(50/50, v/v) mixtures and high-purity
C2H4 (>99.9999 %),
CO2 (>99.999 %), and
C2H2 (>98%) can be
obtained in the single separation process. More importantly,
Zn2(bpy)(btec) can be straightforwardly synthesized at
the kilogram scale under room temperature in an aqueous solution. Its
good chemical stability, water stability, thermal stability, and cyclic
stability are well satisfied requirements of industrial application.