Introduction
1,3-Butadiene
(C4H6) is one of the basic and important
raw
material for petrochemical industry,
which not only can be used in the production of
synthetic rubber, but also in the
synthesis of synthetic resins and other organic chemical products, such
as tetramethylene sulfone,
tetrahydrofuran.1-4It is mainly obtained from the by-product
of vapour cracking of
naphtha.5 However, this process is usually accompanied
by the presence of other C4 components, such as
n-butene
(n-C4H8), iso-butene
(iso-C4H8), n-butane
(n-C4H10) and iso-butane
(iso-C4H10), which
will cause structure transformation
of polybutadiene in the preparation of synthetic rubber and affect
product quality and even reduce polymerization activity by interrupting
the polymerization reaction.6 Hence, the separation of
C4H6 from the other C4hydrocarbons is very imperative. In addition, due to
the similar boiling point, molecular
size and physical properties of C4 hydrocarbons,
especially for diolefin and mono-olefins,7 the
separation of C4H6 from other
C4 hydrocarbons is still remain a particular challenge.
At present, the separation of C4H6 from
other C4 hydrocarbons is mainly carried out by precise
controlled extraction distillation in industry.8However, the extraction distillation usually needs high operation
temperature (323 K to 393 K) with more than 110 trays of high
towers,9 and consume large amounts of organic
solvents. In addition, the polymerization of high reactive
C4H6 is inevitable at above high
temperature distillation process.10 Therefore, the
extraction distillation process is energy intensive and environmental
unfriendly process, and it is very important to seek for an efficient
and low-cost technology to separate C4H6from C4 hydrocarbons.
Among the existing separation technologies,
adsorption separation featuring
simple operating conditions, energy saving, and high adsorption accuracy
has been proved to be a potential separation technology with broad
prospects for gas separation.11-13As
an emerging advanced adsorbent, metal-organic frameworks (MOFs) are
attracting more and more attention by virtue of their multifarious
structural topologies, precise structural determination and tunability
of pore surface functionalities.14-16 In recent years,
MOFs have been widely applied to various separation applications,
including CO2 capture,17propane-propylene
separation,18,19 hydrogen storage20and so on.21-25 From the point of view of the
adsorption mechanism, the efficient separation of gas mixtures by MOFs
adsorbents usually depends on the
thermodynamic separation based on
interaction force,26,27kinetic separation determined by
adsorption rate,28-30molecular sieving
separation31,32 and
gate-opening flexible
separation.33,34 Owing to the
similarities of molecular sizes,
shapes and properties between C4H6 and
other C4 hydrocarbons,35 the potential
separation of C4H6 from other
C4 hydrocarbons by MOFs adsorbents is still a challenge.
The information about physical and chemical properties of
C4 hydrocarbons are illustrated in Table S1 .
Nowadays, most reported MOF adsorbents used for
C4H6separation from other C4 hydrocarbons mainly based on
the reinforcing interactions between
C4H6 and MOF over other
C4 hydrocarbons, however the other C4hydrocarbons could be simultaneously adsorbed to a certain
extent,36-38 especially for the separation of
mono-olefins and diolefin. The co-adsorption separations will cause the
low adsorption selectivity and low separation
efficiency.39 As well known,
molecular sieving is an ideal
efficient separation model and has been well achieved on some
olefin/alkane separations.40 However, since the very
similar molecular size of
C4H6 with other C4hydrocarbons, the adsorbent that can separate
C4H6 with other C4hydrocarbons by complete molecular sieving effect have not been reported
up to now. Ingeniously, considering the difference that
C4H6 has two C=C double bonds, whereas
the other C4 hydrocarbons either have one or none C=C
double bond, gate-opening flexible MOF hopefully can be deemed as the
prospective candidate for the separation of
C4H6 to achieve the molecular sieving
effect. As a very rare example, Kitagawa et al.41reported a flexible SD-65 MOF that can selectively capture
C4H6 and exclude other
C4 hydrocarbons. However, the gate-opening pressure
for
C4H6 is up to 0.6 bar at 298 K. The
high
gate-opening pressure is disadvantageous for the actual purification and
separation of C4H6 and the operating
range will be relatively narrow, especially, it is difficult to achieve
efficient separation at practical
low partial pressure of
C4H6.42 Considering
this problem, the ideal MOF with gate-opening effect
on
C4H6/C4 hydrocarbons
separation should not only can sensitively capture
C4H6 at very low pressure but also can
exclude other C4 hydrocarbons even at 1 bar, to realize
efficient separation of C4H6 over a wide
ranges of operation pressures, especially at low partial pressure of
C4H6.
Herein, we adopt a guest induced flexible
Mn-bpdc
MOF with neat one-dimensional channels for the separation of
C4H6 from other four major
C4 hydrocarbons, including two mono-olefins and two
paraffins. The intrinsic flexibility of Mn-bpdc MOF is confirmed by
thermal responded variable temperature X-ray diffraction (VT-XRD) and
guest-depended structure transformations, while its gate-opening effects
on olefins and paraffins are explored by the adsorption isotherms of
C2H4,
C3H6,
C2H6 and
C3H8. Surprisingly, the single-component
adsorption isotherms of C4 hydrocarbons with Mn-bpdc
demonstrates that Mn-bpdc can specifically and sensitively capture
C4H6 with very low gate-opening pressure
and exclude for other C4 components even to 1 bar,
including n-C4H8,iso-C4H8,
n-C4H10 and
iso-C4H10. At the same time, the uptake
selectivities of
C4H6/n-C4H8and
C4H6/iso-C4H8in Mn-bpdc exceed the overall reported adsorbents. The four column
breakthrough experiments of gas mixtures of
C4H6/n-C4H8,
C4H6/iso-C4H8,
C4H6/n-C4H10and
C4H6/iso-C4H10furtherly credibly verify that the efficient dynamic separation effect
on Mn-bpdc to separate C4H6 from other
C4 components can be achieved. In addition, the Mn-bpdc
MOF possesses outstanding water stability, well regeneration and cyclic
utilization performance, which comprehensively affirm the great
potential of Mn-bpdc adsorbent for the challenging separation of
C4H6 from other C4hydrocarbons.