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\begin{document}

\title{Efficient separation of 1,3-butadiene from C4 hydrocarbons by flexible
metal-organic framework with gate-opening effect}



\author[1]{Wang Lu}%
\author[1]{Hongliang Huang}%
\author[1]{Zhu Hejin}%
\author[1]{Chang  Yanjiao}%
\author[1]{Guo Xiangyu}%
\author[1]{Yang Fan}%
\author[1]{Chongli Zhong}%
\affil[1]{Tiangong University}%


\vspace{-1em}



  
  \date{June 17, 2024}


\begingroup
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\selectlanguage{english}
\begin{abstract}
Efficient and economical separation of 1,3-butadiene (C4H6) from C4
hydrocarbons is imperative yet challenging in industrial separation
processes. Herein, a guest-induced flexible Mn-bpdc MOF has been
employed to separate C4H6 from C4 hydrocarbons, including n-butene
(n-C4H8), iso-butene (iso-C4H8), n-butane (n-C4H10) and iso-butane
(iso-C4H10). Significantly, C4H6 can instantaneously induce gate-opening
of Mn-bpdc MOF at 0.13 bar and 298 K, thus significant amounts of C4H6
can be adsorbed, while other C4 hydrocarbons cannot induce the
gate-opening even at 1 bar. The uptake selectivities of Mn-bpdc MOF for
C4H6/n-C4H8 and C4H6/iso-C4H8 are up to 40.0 and 45.0 at 298 K and 1
bar, respectively, both surpassing all the reported adsorbents. In
addition, breakthrough experiments verified that C4H6/n-C4H8,
C4H6/iso-C4H8, C4H6/n-C4H10 and C4H6/iso-C4H10 mixture can be
efficiently separated. More importantly, Mn-bpdc possesses excellent
water stability and outstanding regeneration ability for C4H6
separation, making it a new benchmark for C4H6 purification.%
\end{abstract}%



\sloppy


\textbf{Efficient separation of 1,3-butadiene from C4 hydrocarbons by
flexible metal-organic framework with gate-opening effect}

Lu Wang\textsuperscript{1,3}, Hongliang Huang\textsuperscript{1,2}*,
Hejin Zhu\textsuperscript{1,3}, Yanjiao Chang\textsuperscript{1,2},
Xiangyu Guo\textsuperscript{1,2}, Fan Yang\textsuperscript{1,2}, Chongli
Zhong\textsuperscript{1,2}

1. State Key Laboratory of Separation Membranes and Membrane Processes,
Tiangong University, No. 399 Binshui West Road, Xiqing District, Tianjin
300387, P. R. China.

2. School of Chemical Engineering, Tiangong University, No. 399 Binshui
West Road, Xiqing District, Tianjin 300387, P. R. China.

3. School of Textile Science and Engineering, Tiangong University, No.
399 Binshui West Road, Xiqing District, Tianjin 300387, P. R. China.

*Corresponding authors:

Hongliang Huang Email:\emph{huanghongliang@tiangong.edu.cn}.

\textbf{Abstract:}Efficient and economical separation of 1,3-butadiene
(C\textsubscript{4}H\textsubscript{6}) from C\textsubscript{4}
hydrocarbons is imperative yet challenging in industrial separation
processes. Herein, a guest-induced flexible Mn-bpdc MOF has been
employed to separate C\textsubscript{4}H\textsubscript{6} from
C\textsubscript{4}hydrocarbons, including n-butene
(n-C\textsubscript{4}H\textsubscript{8})\textsubscript{,} iso-butene
(iso-C\textsubscript{4}H\textsubscript{8}), n-butane
(n-C\textsubscript{4}H\textsubscript{10}) and iso-butane
(iso-C\textsubscript{4}H\textsubscript{10}). Significantly,
C\textsubscript{4}H\textsubscript{6}can instantaneously induce
gate-opening of Mn-bpdc MOF at 0.13 bar and 298 K, thus significant
amounts of C\textsubscript{4}H\textsubscript{6} can be adsorbed, while
other C\textsubscript{4} hydrocarbons cannot induce the gate-opening
even at 1 bar. The uptake selectivities of Mn-bpdc MOF for
C\textsubscript{4}H\textsubscript{6}/n-C\textsubscript{4}H\textsubscript{8}and
C\textsubscript{4}H\textsubscript{6}/iso-C\textsubscript{4}H\textsubscript{8}are
up to 40.0 and 45.0 at 298 K and 1 bar, respectively, both surpassing
all the reported adsorbents. In addition, breakthrough experiments
verify that
C\textsubscript{4}H\textsubscript{6}/n-C\textsubscript{4}H\textsubscript{8},
C\textsubscript{4}H\textsubscript{6}/iso-C\textsubscript{4}H\textsubscript{8},
C\textsubscript{4}H\textsubscript{6}/n-C\textsubscript{4}H\textsubscript{10}and
C\textsubscript{4}H\textsubscript{6}/iso-C\textsubscript{4}H\textsubscript{10}mixture
can be efficiently separated. More importantly, Mn-bpdc possesses
excellent water stability and outstanding regeneration ability for
C\textsubscript{4}H\textsubscript{6} separation, making it a new
benchmark for C\textsubscript{4}H\textsubscript{6} purification.

\textbf{Keywords:} metal-organic frameworks, gate-opening,
1,3-butadiene, adsorption/gas, gas purification.

\textbf{Introduction}

1,3-Butadiene (C\textsubscript{4}H\textsubscript{6}) 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.\textsuperscript{1-4}It is
mainly obtained from the by-product of vapour cracking of
naphtha.\textsuperscript{5} However, this process is usually accompanied
by the presence of other C\textsubscript{4} components, such as n-butene
(n-C\textsubscript{4}H\textsubscript{8}), iso-butene
(iso-C\textsubscript{4}H\textsubscript{8}), n-butane
(n-C\textsubscript{4}H\textsubscript{10}) and iso-butane
(iso-C\textsubscript{4}H\textsubscript{10}), 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.\textsuperscript{6} Hence, the
separation of C\textsubscript{4}H\textsubscript{6} from the other
C\textsubscript{4}hydrocarbons is very imperative. In addition, due to
the similar boiling point, molecular size and physical properties of
C\textsubscript{4} hydrocarbons, especially for diolefin and
mono-olefins,\textsuperscript{7} the separation of
C\textsubscript{4}H\textsubscript{6} from other C\textsubscript{4}
hydrocarbons is still remain a particular challenge. At present, the
separation of C\textsubscript{4}H\textsubscript{6} from other
C\textsubscript{4} hydrocarbons is mainly carried out by precise
controlled extraction distillation in
industry.\textsuperscript{8}However, the extraction distillation usually
needs high operation temperature (323 K to 393 K) with more than 110
trays of high towers,\textsuperscript{9} and consume large amounts of
organic solvents. In addition, the polymerization of high reactive
C\textsubscript{4}H\textsubscript{6} is inevitable at above high
temperature distillation process.\textsuperscript{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
C\textsubscript{4}H\textsubscript{6}from C\textsubscript{4}
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.\textsuperscript{11-13}As 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.\textsuperscript{14-16} In recent years, MOFs have been
widely applied to various separation applications, including
CO\textsubscript{2} capture,\textsuperscript{17}propane-propylene
separation,\textsuperscript{18,19} hydrogen
storage\textsuperscript{20}and so on.\textsuperscript{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,\textsuperscript{26,27}kinetic
separation determined by adsorption
rate,\textsuperscript{28-30}molecular sieving
separation\textsuperscript{31,32} and gate-opening flexible
separation.\textsuperscript{33,34} Owing to the similarities of
molecular sizes, shapes and properties between
C\textsubscript{4}H\textsubscript{6} and other C\textsubscript{4}
hydrocarbons,\textsuperscript{35} the potential separation of
C\textsubscript{4}H\textsubscript{6} from other C\textsubscript{4}
hydrocarbons by MOFs adsorbents is still a challenge. The information
about physical and chemical properties of C\textsubscript{4}
hydrocarbons are illustrated in Table \textbf{S1} . Nowadays, most
reported MOF adsorbents used for
C\textsubscript{4}H\textsubscript{6}separation from other
C\textsubscript{4} hydrocarbons mainly based on the reinforcing
interactions between C\textsubscript{4}H\textsubscript{6} and MOF over
other C\textsubscript{4} hydrocarbons, however the other
C\textsubscript{4}hydrocarbons could be simultaneously adsorbed to a
certain extent,\textsuperscript{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.\textsuperscript{39} As well known, molecular sieving is an
ideal efficient separation model and has been well achieved on some
olefin/alkane separations.\textsuperscript{40} However, since the very
similar molecular size of C\textsubscript{4}H\textsubscript{6} with
other C\textsubscript{4}hydrocarbons, the adsorbent that can separate
C\textsubscript{4}H\textsubscript{6} with other
C\textsubscript{4}hydrocarbons by complete molecular sieving effect have
not been reported up to now. Ingeniously, considering the difference
that C\textsubscript{4}H\textsubscript{6} has two C=C double bonds,
whereas the other C\textsubscript{4} 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
C\textsubscript{4}H\textsubscript{6} to achieve the molecular sieving
effect. As a very rare example, Kitagawa et
al.\textsuperscript{41}reported a flexible SD-65 MOF that can
selectively capture C\textsubscript{4}H\textsubscript{6} and exclude
other C\textsubscript{4} hydrocarbons. However, the gate-opening
pressure for C\textsubscript{4}H\textsubscript{6} is up to 0.6 bar at
298 K. The high gate-opening pressure is disadvantageous for the actual
purification and separation of C\textsubscript{4}H\textsubscript{6} and
the operating range will be relatively narrow, especially, it is
difficult to achieve efficient separation at practical low partial
pressure of C\textsubscript{4}H\textsubscript{6}.\textsuperscript{42}
Considering this problem, the ideal MOF with gate-opening effect on
C\textsubscript{4}H\textsubscript{6}/C\textsubscript{4} hydrocarbons
separation should not only can sensitively capture
C\textsubscript{4}H\textsubscript{6} at very low pressure but also can
exclude other C\textsubscript{4} hydrocarbons even at 1 bar, to realize
efficient separation of C\textsubscript{4}H\textsubscript{6} over a wide
ranges of operation pressures, especially at low partial pressure of
C\textsubscript{4}H\textsubscript{6}.

Herein, we adopt a guest induced flexible Mn-bpdc MOF with neat
one-dimensional channels for the separation of
C\textsubscript{4}H\textsubscript{6} from other four major
C\textsubscript{4} 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
C\textsubscript{2}H\textsubscript{4},
C\textsubscript{3}H\textsubscript{6},
C\textsubscript{2}H\textsubscript{6} and
C\textsubscript{3}H\textsubscript{8}. Surprisingly, the single-component
adsorption isotherms of C\textsubscript{4} hydrocarbons with Mn-bpdc
demonstrates that Mn-bpdc can specifically and sensitively capture
C\textsubscript{4}H\textsubscript{6} with very low gate-opening pressure
and exclude for other C\textsubscript{4} components even to 1 bar,
including
n-C\textsubscript{4}H\textsubscript{8,}iso-C\textsubscript{4}H\textsubscript{8},
n-C\textsubscript{4}H\textsubscript{10} and
iso-C\textsubscript{4}H\textsubscript{10}. At the same time, the uptake
selectivities of
C\textsubscript{4}H\textsubscript{6}/n-C\textsubscript{4}H\textsubscript{8}and
C\textsubscript{4}H\textsubscript{6}/iso-C\textsubscript{4}H\textsubscript{8}in
Mn-bpdc exceed the overall reported adsorbents. The four column
breakthrough experiments of gas mixtures of
C\textsubscript{4}H\textsubscript{6}/n-C\textsubscript{4}H\textsubscript{8},
C\textsubscript{4}H\textsubscript{6}/iso-C\textsubscript{4}H\textsubscript{8},
C\textsubscript{4}H\textsubscript{6}/n-C\textsubscript{4}H\textsubscript{10}and
C\textsubscript{4}H\textsubscript{6}/iso-C\textsubscript{4}H\textsubscript{10}furtherly
credibly verify that the efficient dynamic separation effect on Mn-bpdc
to separate C\textsubscript{4}H\textsubscript{6} from other
C\textsubscript{4} 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
C\textsubscript{4}H\textsubscript{6} from other
C\textsubscript{4}hydrocarbons.

\textbf{Results and Discussion}\selectlanguage{english}
\begin{figure}[H]
\begin{center}
\includegraphics[width=0.70\columnwidth]{figures/image1/image1}

\end{center}
\end{figure}

\textbf{Figure 1. a)} 2, 2'-bipyridyl-4, 4'-dicarboxylic acid ligand in
Mn-bpdc MOF; \textbf{b)} Local coordination geometry of the central Mn
metal atoms; \textbf{c)} The 3D structure and connolly surface of
Mn-bpdc MOF front view of the structure along the c axis; \textbf{d)}The
one-dimensional channels of Mn-bpdc MOF. The inner and external surfaces
are lilac and grey, respectively. Color codes: Grey violet, red, dark
blue, and gray nodes represent manganese metal, oxygen, nitrogen and
carbon atoms, respectively.

The Mn-bpdc MOF was synthesized by hydrothermal method through the
reaction of commercially available 2,2'-bipyridine-4,4'-dicarboxylic
acid ligand (H\textsubscript{2}BPDC) and manganese
sulfate.\textsuperscript{43} The experimental PXRD pattern of the
as-synthesized Mn-based MOF is accorded with the simulated ones,
revealing the successful synthesis of Mn-bpdc MOF (\textbf{Figure S1} ).
The framework of Mn-bpdc consists of infinite chains of corner-sharing
Mn(II) centers connected with two nitrogen atoms from bipyridyl and four
oxygen atoms from carboxylic of BPDC ligands, possessing uniform one
dimensional channels (\textbf{Figure 1a-d} ). The scanning electron
microscopy (SEM) of Mn-bpdc MOF is showed in \textbf{Figure S2,} which
reveals that the Mn-bpdc MOF has rod-shaped laminated microstructures.\selectlanguage{english}
\begin{figure}[H]
\begin{center}
\includegraphics[width=0.70\columnwidth]{figures/image2/image2}

\end{center}
\end{figure}

\textbf{Figure 2. a)} VT\textbf{-} PXRD patterns of Mn-bpdc MOF at
different temperatures under atmospheric environment; \textbf{b)} PXRD
patterns of Mn-bpdc MOF after different treatments by immersing in
acetone, dichloromethane and water, respectively; \textbf{c)} Adsorption
isotherms of Mn-bpdc MOF for single
component\textbf{C\textsubscript{2}H\textsubscript{4}}
and\textbf{C\textsubscript{2}H\textsubscript{6}} at different
temperatures;\textbf{d)} Adsorption isotherms of Mn-bpdc MOF for single
component\textbf{C\textsubscript{3}H\textsubscript{6} and
C\textsubscript{3}H\textsubscript{8}} at different temperatures.

To investigate the flexible structure of Mn-bpdc, firstly, the VT-PXRD
measurements at different temperatures under atmospheric environment
were measured, as drawn in \textbf{Figure 2a} . At the temperatures of
303 K and 323 K, the PXRD patterns are consistent with the simulated
ones. When the temperature raises from 373 K to 473 K, the diffraction
peak at around 11.3\selectlanguage{ngerman}° slightly shifts to the high angle, while new
diffraction peak appears at 26.3° and the pristine diffraction peaks at
24.1°of Mn-bpdc at 303 K and 323 K completely disappears, which
illustrate temperature responded structural transformation of Mn-bpdc.
It is worth noting that when the temperature drops from 473 K to 303 K,
the PXRD patterns were restored to the original state at 303 K, which
indicates that the reversible structure transformation of Mn-bpdc upon
thermal stimulation. To explore the guest molecule induced structural
transformation of Mn-bpdc, we measured the PXRD of the Mn-bpdc MOF
samples by immersing in water, dichloromethane and acetone,
respectively. The PXRD pattern of Mn-bpdc under water environment is
consistent with that simulated one (\textbf{Figure 2b} ). However, the
PXRD patterns change obviously when the MOF immersed in acetone or
dichloromethane, and the diffraction peaks at 24.1° disappear while new
diffraction peaks appear at 26.3°, which indicates that guest molecules
can also induce the structure transformation of Mn-bpdc. In addition,
when the dichloromethane treated Mn-bpdc MOF was exchanged by water, the
consistent PXRD pattern of water exchanged Mn-bpdc MOF \textbf{(Figure
S4)} with simulated ones credibly verifies the structural reversibility
of Mn-bpdc MOF. In a word, it illustrates that both thermal stimulation
and guest molecule induction can trigger the structural transformation
of the Mn-bpdc MOF.

Taking into account the peculiarity of both temperature and guest
molecular can induce structural transformation, gas molecules with
different polarity might also be able to regulate the structural
transformation of the Mn-bpdc MOF at proper temperature. To study the
stimulus-response behavior of Mn-bpdc towards gas molecules, the
single-component gas adsorption isotherms of
C\textsubscript{2}H\textsubscript{4}and
C\textsubscript{3}H\textsubscript{6}were collected at 273 K, 288 K, 298
K and 323 K, respectively, as shown in \textbf{Figure 2c} . For the
C\textsubscript{2}H\textsubscript{4} adsorption at 323K, there is almost
no observed adsorption even up to 1 bar, indicating that
C\textsubscript{2}H\textsubscript{4} cannot open the door of Mn-bpdc at
323 K. However, when the temperature reduces, obvious adsorption begins
to be observed at 273 K, which shows typical S-typed curves and implies
structural transformation in the adsorption
process.\textsuperscript{44,45} In particular, the lower adsorption
temperatures, the lower gate-opening pressures can be achieved,
illustrating that low temperature is more conducive for
C\textsubscript{2}H\textsubscript{4} to open the door of Mn-bpdc.
Similar results also can be observed for
C\textsubscript{3}H\textsubscript{6} adsorption (\textbf{Figure 2d} ).
We can obviously find that it fails to open the door at both 298 K and
323 K for C\textsubscript{3}H\textsubscript{6} adsorption, but the
gate-opening effects are appeared at 273 K and 288 K, corresponding to
the gate-opening pressure points at 0.35 bar and 0.80 bar, respectively.
These results mean that the gate-opening pressure points of adsorption
can be well regulated by different temperatures and object gas molecule.
In addition, the single component gas sorption isotherms of Mn-bpdc MOF
for C\textsubscript{2}H\textsubscript{6} (\textbf{Figure 2c} ) and
C\textsubscript{3}H\textsubscript{8} (\textbf{Figure 2d} ) were measured
both at 273 K and 298 K. Obviously, no matter at 273 K or 298 K, they
have no ability to open the door of Mn-bpdc MOF, which suggests that the
occurrence of gate-opening can be induced by the C=C double bond of
olefins. Furtherly, compared the adsorption curves of
C\textsubscript{2}H\textsubscript{4} and
C\textsubscript{3}H\textsubscript{6} at 273 K, it should be noticed that
the gate-opening points of C\textsubscript{2}H\textsubscript{4} (0.2
bar) is lower than that of C\textsubscript{3}H\textsubscript{6} (0.35
bar). Similar changes of gate-opening pressure can also be observed for
C\textsubscript{2}H\textsubscript{4} and
C\textsubscript{3}H\textsubscript{6} adsorption at 298 K. It verifies
that C\textsubscript{2}H\textsubscript{4} has higher capability to
induce the gate-opening behavior of Mn-bpdc than that of
C\textsubscript{3}H\textsubscript{6}, and gate-opening pressure of
Mn-bpdc will increase when the number of carbon atoms increased in
mono-olefines.

Enlightened by the above experimental results, it implies that Mn-bpdc
depending on its structural response to different types olefins can
achieve the separation of C\textsubscript{4}H\textsubscript{6} from
other C\textsubscript{4} at room temperature. In detail, since
C\textsubscript{4}H\textsubscript{6} has two C=C double bonds, it could
be more easily to open the door of Mn-bpdc MOF with lower gate-opening
pressure. On the contrary, for other C\textsubscript{4} components, the
gate-opening ability will be weaker due to the increases of carbon atoms
numbers. Since C\textsubscript{3}H\textsubscript{6} cannot open the door
of Mn-bpdc even up to 1 bar at 298 K, we can infer that
n-C\textsubscript{4}H\textsubscript{8} and
iso-C\textsubscript{4}H\textsubscript{8} will fail to open the door at
298 K although they have one C=C double bond. On the other hand,
n-C\textsubscript{4}H\textsubscript{10} and
iso-C\textsubscript{4}H\textsubscript{10} also certainly cannot open the
gate of Mn-bpdc at 298 K owning to the absence of C=C double bond.
Hence, theoretically, the Mn-MOF can efficiently separate
C\textsubscript{4}H\textsubscript{6} from other
C\textsubscript{4}hydrocarbons by recognizing
C\textsubscript{4}H\textsubscript{6} and rejecting other
C\textsubscript{4} hydrocarbons to enter the framework of Mn-bpdc MOF at
room temperature.\selectlanguage{english}
\begin{figure}[H]
\begin{center}
\includegraphics[width=0.70\columnwidth]{figures/image3/image3}

\end{center}
\end{figure}

\textbf{Figure 3. a)} Adsorption isotherms of Mn-bpdc MOF for single
component C\textsubscript{4}H\textsubscript{6} at different
temperatures;\textbf{b)} Adsorption isotherms of Mn-bpdc MOF for single
component C\textsubscript{4}H\textsubscript{6},
n-C\textsubscript{4}H\textsubscript{8},
iso-C\textsubscript{4}H\textsubscript{8},
n-C\textsubscript{4}H\textsubscript{10} and
iso-C\textsubscript{4}H\textsubscript{10} at 273 K;
\textbf{c)}Adsorption isotherms of Mn-bpdc MOF for single component
C\textsubscript{4}H\textsubscript{6},
n-C\textsubscript{4}H\textsubscript{8},
iso-C\textsubscript{4}H\textsubscript{8},
n-C\textsubscript{4}H\textsubscript{10} and
iso-C\textsubscript{4}H\textsubscript{10} at 298 K,
respectively;\textbf{d)} The comparison of uptake selectivity of
C\textsubscript{4}H\textsubscript{6}/n-C\textsubscript{4}H\textsubscript{8}and
C\textsubscript{4}H\textsubscript{6}/iso-C\textsubscript{4}H\textsubscript{8}with
reported adsorbents at 298 K and 1 bar.\selectlanguage{english}
\begin{figure}[H]
\begin{center}
\includegraphics[width=0.70\columnwidth]{figures/image4/image4}

\end{center}
\end{figure}

\textbf{Figure 4. (a)} Adsorption isotherms of Mn-bpdc marked with four
adsorption positions for C\textsubscript{4}H\textsubscript{6} at 298
K;\textbf{(b)} \emph{In situ} PXRD experiments collected under different
gas pressures corresponding to the four adsorption positions as shown
in\textbf{(a)} , point A (0 bar), point B (0.11 bar), point C (0.17
bar), point D (1 bar).

To confirm our hypothesis, firstly, the adsorption isotherms of
C\textsubscript{4}H\textsubscript{6} at different temperatures were
collected. As shown in \textbf{Figure 3a} , the Mn-bpdc MOF exhibits
abrupt adsorption phenomenon for C\textsubscript{4}H\textsubscript{6} at
all temperatures, including 273 K, 288 K, 298 K, 308 K,318 K and 323 K.
As expected, the gate-opening pressure for
C\textsubscript{4}H\textsubscript{6}is found to be as low as 0.13 bar at
298 K by virtue of the presence of two C=C double bonds. In addition,
the C\textsubscript{4}H\textsubscript{6}uptake at 0.20 bar is about 30
cm\textsuperscript{3}g\textsuperscript{-1}, 93 \% of that at 1 bar
(32.12 cm\textsuperscript{3} g\textsuperscript{-1}), which both
demonstrates that C\textsubscript{4}H\textsubscript{6}with double C=C
bonds has strong ability to open the doors of Mn-bpdc
MOF.\textsuperscript{46} Besides, the gate-opening pressure is observed
to be increased with the increasing of adsorption temperature. In
detail, at 273 K, 288 K, 308 K,318 K and 323 K, the abrupt gate-opening
pressures are 0.02 bar, 0.06 bar, 0.26 bar, 0.50 bar, 0.92 bar
(\textbf{Figure S5} ), corresponding to the uptake of 33.57
cm\textsuperscript{3}g\textsuperscript{-1}, 32.81
cm\textsuperscript{3}g\textsuperscript{-1}, 31.40
cm\textsuperscript{3}g\textsuperscript{-1}, 30.51 cm\textsuperscript{3}
g\textsuperscript{-1}and 26.04 cm\textsuperscript{3}
g\textsuperscript{-1} at 1 bar, respectively. Consisting with the
observations of C\textsubscript{2}H\textsubscript{4} and
C\textsubscript{3}H\textsubscript{6} adsorptions, it can be found that
lower temperatures are more conducive to opening the door of Mn-bpdc for
C\textsubscript{4}H\textsubscript{6}, which dues to the gas molecules
slow down at low temperatures and are more likely to be bound by MOF,
thus providing adequate gate-opening capacity per unit
time.\textsuperscript{47,48} In addition, compared with the gate-opening
pressures of C\textsubscript{2}H\textsubscript{4} (nearly 1 bar) and
C\textsubscript{3}H\textsubscript{6} (cannot open the gate even at 1
bar) at 298 K, C\textsubscript{4}H\textsubscript{6} can open the
flexible door of Mn-bpdc MOF at very low pressure (0.13 bar)
(\textbf{Figure S5} ), which is consistent with the previous inferences.
It should be noticed that although the SD-65 MOF with gate-opening
effect has been reported for
C\textsubscript{4}H\textsubscript{6}separation from other
C\textsubscript{4} mono-olefines and paraffins, the gate-opening
pressure of SD-65 reaches up to 0.6 bar at 298 K.\textsuperscript{41}
Obviously, the very low gate-opening pressure of Mn-bpdc for
C\textsubscript{4}H\textsubscript{6}is a huge advantageous to achieve
efficient separation of C\textsubscript{4}H\textsubscript{6}from other
C\textsubscript{4} hydrocarbons at lower partial pressure of
C\textsubscript{4}H\textsubscript{6}.

Subsequently, the adsorption isotherms for
n-C\textsubscript{4}H\textsubscript{8},
iso-C\textsubscript{4}H\textsubscript{8},
n-C\textsubscript{4}H\textsubscript{10} and
iso-C\textsubscript{4}H\textsubscript{10} were measured at 273 K
(\textbf{Figure 3b} ) and 298 K (\textbf{Figure 3c} ), respectively. At
273 K and 1 bar, there are almost no uptakes for
n-C\textsubscript{4}H\textsubscript{8} (1.78
cm\textsuperscript{3}g\textsuperscript{-1}),
iso-C\textsubscript{4}H\textsubscript{8} (1.48 cm\textsuperscript{3}
g\textsuperscript{-1}), n-C\textsubscript{4}H\textsubscript{10} (0.61
cm\textsuperscript{3}g\textsuperscript{-1}) and
iso-C\textsubscript{4}H\textsubscript{10}(0.05 cm\textsuperscript{3}
g\textsuperscript{-1}), but high uptake for
C\textsubscript{4}H\textsubscript{6} (34.12
cm\textsuperscript{3}g\textsuperscript{-1}) is retained. Additionally,
the uptake of C\textsubscript{4}H\textsubscript{6} at the gate-opening
pressure of 0.02 bar is closed to the maximum adsorption capacity. The
distinct difference between adsorption isotherms of
C\textsubscript{4}H\textsubscript{6} and other
C\textsubscript{4}hydrocarbons means great theoretical selectivity could
be achieved and this MOF has huge potential for actual separation of
C\textsubscript{4}H\textsubscript{6} over other
C\textsubscript{4}hydrocarbons. Importantly, the very low uptakes of
n-C\textsubscript{4}H\textsubscript{8} (0.80
cm\textsuperscript{3}g\textsuperscript{-1}),
iso-C\textsubscript{4}H\textsubscript{8} (0.74 cm\textsuperscript{3}
g\textsuperscript{-1}), n-C\textsubscript{4}H\textsubscript{10} (0.29
cm\textsuperscript{3}g\textsuperscript{-1}) and
iso-C\textsubscript{4}H\textsubscript{10}(0.02
cm\textsuperscript{3}g\textsuperscript{-1}) can be also observed at 298
K and 1 bar. The extreme low uptakes illustrate that the Mn-bpdc MOF is
very negative to capture n-C\textsubscript{4}H\textsubscript{8},
iso-C\textsubscript{4}H\textsubscript{8},
n-C\textsubscript{4}H\textsubscript{10} and
iso-C\textsubscript{4}H\textsubscript{10}. On the contrary, by virtue of
much stronger polarity of C\textsubscript{4}H\textsubscript{6}, it can
actively push open the molecular door of Mn-bpdc MOF, which is
definitely verified by the very low gate-opening pressure (0.13 bar) and
close saturation adsorption capacity of
C\textsubscript{4}H\textsubscript{6} (32.2
cm\textsuperscript{3}g\textsuperscript{-1}) at the low pressure. As we
known, the adsorption selectivity is a critical factor to evaluate the
separation performance for adsorbent materials.\textsuperscript{49}
Notably, the calculated uptake selectivities of Mn-bpdc MOF for
C\textsubscript{4}H\textsubscript{6}/n-C\textsubscript{4}H\textsubscript{8}and
C\textsubscript{4}H\textsubscript{6}/iso-C\textsubscript{4}H\textsubscript{8}are
40.0 and 45.0 at 298 K and 1 bar, respectively, both of which are much
higher than that all previous porous materials. (Mg-gallate: 1.3,
15.1;\textsuperscript{50} Ni-gallate: 2.4,
15.4;\textsuperscript{50}GeFSIX-14-Cu-i: 4.7, 6.4;\textsuperscript{37}
NbFSIX-2-Cu-i: 1.2, 5.7;\textsuperscript{37} GeFSIX-2-Cu-i: 1.1,
2.9;\textsuperscript{37}ZJNU-30a: 1.0, 1.2;\textsuperscript{36} and
SD-65: 22.3, 25.4;\textsuperscript{41} \textbf{Figure 3d} and
\textbf{Table S2} ). Those results powerfully indicate the potential
utility of Mn-bpdc MOF for the separation of
C\textsubscript{4}H\textsubscript{6} from other C\textsubscript{4}
hydrocarbons under ambient conditions.

Furtherly, to explore the structure variations upon the adsorption of
C\textsubscript{4}H\textsubscript{6} and directly observe the
guest-induced structural transformations, we measured the \emph{in
situ}PXRD of Mn-bpdc under C\textsubscript{4}H\textsubscript{6}
environment with different pressures (0 bar, 0.11 bar, 0.17 bar and 1
bar) at 298 K (\textbf{Figure 4b} ). The four different pressures points
at \emph{in situ} PXRD data are corresponded to the detailed adsorption
points in the adsorption curves of C\textsubscript{4}H\textsubscript{6}
at 298 K (Figure 4a). As shown in \textbf{Figure 4b} , compared with the
PXRD pattern of Mn-bpdc under vacuum environment (point A), obvious new
diffraction peak at 25.4\selectlanguage{ngerman}° appears and slight peak at 24.4° begins to
appear when the pressure increases to 0.11 bar (point B), which means
small amount adsorption of C\textsubscript{4}H\textsubscript{6} before
isotherm obviously jumping can lead to the initial transformation of the
structure of Mn-bpdc MOF and proves the sensitivity of the structure for
C\textsubscript{4}H\textsubscript{6}. Subsequently, the diffraction peak
at 26.5° completely disappears and the peak at 24.4° gradually increases
when the pressure raises to 0.17 bar (point C), which corresponds to a
large amount of C\textsubscript{4}H\textsubscript{6} (28.4
cm\textsuperscript{3} g\textsuperscript{-1}) enter the pore structure
and furtherly facilitate the structural changes. Then, when the pressure
reaches 1 bar and the amount adsorption of
C\textsubscript{4}H\textsubscript{6} reaches the saturated state, the
diffraction peak at 25.4° disappears and the intensity of diffraction
peak at 24.4° reaches its maximum value (point D). These results
together reveal gradual transformation of the structure of Mn-bpdc in
the adsorption process of C\textsubscript{4}H\textsubscript{6}. In
addition, it is found that the strong peaks at around 11.6° under vacuum
transforms into several unidentifiable weaker diffraction peaks when the
pressures increased to 0.11 bar and 0.17 bar, then, furtherly splits
into two strong peaks at around 11° at 1 bar, which due to the formation
of metastable distorted structure of Mn-bpdc at the adsorption process
of C\textsubscript{4}H\textsubscript{6} and also verifies the huge
structure transformation of Mn-bpdc induced by
C\textsubscript{4}H\textsubscript{6}.\textsuperscript{41,51} Furtherly,
the PXRD pattern of Mn-bpdc after
C\textsubscript{4}H\textsubscript{6}desorption is highly consistent with
that of initial vacuum state, which means the well reversibility of
structure transformation for C\textsubscript{4}H\textsubscript{6}
adsorption (\textbf{Figure S6} ).

\par\null

\textbf{Figure 5.} Column breakthrough experiment for
C\textsubscript{4}H\textsubscript{6}/n-C\textsubscript{4}H\textsubscript{8}(1/1
(v/v)) \textbf{(a)} ;
C\textsubscript{4}H\textsubscript{6}/iso-C\textsubscript{4}H\textsubscript{8}(1/1
(v/v)) \textbf{(b)} ,
C\textsubscript{4}H\textsubscript{6}/n-C\textsubscript{4}H\textsubscript{10}(1/1
(v/v)) \textbf{(c)} ,
C\textsubscript{4}H\textsubscript{6}/iso-C\textsubscript{4}H\textsubscript{10}(1/1
(v/v)) \textbf{(d)} at 298 K, respectively.

To explore the actual separation performance, dynamic fixed-bed column
breakthrough experiments were conducted by using binary gas mixtures of
C\textsubscript{4}H\textsubscript{6}/n-C\textsubscript{4}H\textsubscript{8}(1:1
v/v),
C\textsubscript{4}H\textsubscript{6}/iso-C\textsubscript{4}H\textsubscript{8}(1:1
v/v),
C\textsubscript{4}H\textsubscript{6}/n-C\textsubscript{4}H\textsubscript{10}(1:1
v/v) and
C\textsubscript{4}H\textsubscript{6}/iso-C\textsubscript{4}H\textsubscript{10}(1:1
v/v) at 298 K, respectively. As shown in \textbf{Figure 5(a-d)} , the
n-C\textsubscript{4}H\textsubscript{8},
iso-C\textsubscript{4}H\textsubscript{8},
n-C\textsubscript{4}H\textsubscript{10} and
iso-C\textsubscript{4}H\textsubscript{10} all begin to break through the
bed gradually as soon as the feeding of target gas mixture, meanwhile,
C\textsubscript{4}H\textsubscript{6} keeps continuous adsorption with
breakthrough retention times of 167.1 s, 171.8 s, 175.7 s, 177.4 s,
respectively, which attributes to its super sensitive gate-opening
effect on C\textsubscript{4}H\textsubscript{6}and unrecognized responses
for n-C\textsubscript{4}H\textsubscript{8},
iso-C\textsubscript{4}H\textsubscript{8},
n-C\textsubscript{4}H\textsubscript{10} and
iso-C\textsubscript{4}H\textsubscript{10}. In consequence, the excellent
dynamic separation performance toward
C\textsubscript{4}H\textsubscript{6} for all the binary
C\textsubscript{4}H\textsubscript{6}/n-C\textsubscript{4}H\textsubscript{8},
C\textsubscript{4}H\textsubscript{6}/iso-C\textsubscript{4}H\textsubscript{8},
C\textsubscript{4}H\textsubscript{6}/n-C\textsubscript{4}H\textsubscript{10}and
C\textsubscript{4}H\textsubscript{6}/iso-C\textsubscript{4}H\textsubscript{10}gas
mixtures furtherly verifies that this MOF is a very promising soft MOF
adsorbent for C\textsubscript{4}H\textsubscript{6} purification.\selectlanguage{english}
\begin{figure}[H]
\begin{center}
\includegraphics[width=0.70\columnwidth]{figures/image6/image6}

\end{center}
\end{figure}

\textbf{Figure 6. a)} PXRD patterns of Mn-bpdc MOF after different
treatments by immersing in water (RT) for 60 days, boiled water (100 )
for 24 hours and air for 60 days, respectively; \textbf{b)} Adsorption
isotherms of single component C\textsubscript{4}H\textsubscript{6} at
298 K for Mn-bpdc samples after different treatments;\textbf{c)}
Adsorption isotherms of single component
C\textsubscript{4}H\textsubscript{6} for 10 cycles at 298 K. Adsorption
and desorption profiles are shown in closed and open symbols,
respectively; \textbf{d)} Cycling breakthrough tests for
C\textsubscript{4}H\textsubscript{6}/n-C\textsubscript{4}H\textsubscript{8}(1/1
(v/v)) separation with Mn-bpdc MOF at 298 K for five times. The column
was reactivated with a helium flow at 100  after each breakthrough
test.

The excellent stability and regeneration of absorbents are prerequisites
for the practical applications in the adsorptive separation of
C\textsubscript{4}H\textsubscript{6} from
C\textsubscript{4}hydrocarbons.\textsuperscript{52} In order to examine
the water stability of Mn-bpdc MOF, we carried out stability tests of
Mn-bpdc at water (RT) for 60 days, air for 60 days and boiled water (100
) for 24 h. As shown in \textbf{Figure 6a} , the PXRD patterns verifies
that the framework of Mn-bpdc MOF can well maintain even in those harsh
conditions. Furtherly, the C\textsubscript{4}H\textsubscript{6} sorption
isotherms of Mn-bpdc MOF after different stability tests are shown
in\textbf{Figure 6b} . The almost identical adsorption isotherms with
original simple demonstrate that it can still preserve its intact
structure without phase transition and framework collapse after
stability treatments.\textsuperscript{53}In addition, thermogravimetric
analysis (TGA) was performed on the Mn-bpdc MOF to survey its thermal
stability. As shown in \textbf{Figure S3} , Mn-bpdc MOF can undergo a
sustaining 4 \% loss of weight up to 360 \selectlanguage{ngerman}°C, which dues to the loss of
all free molecules. Then, the weight remains constant until the
framework begins to collapse at 480 °C. The excellent water and thermal
stability of Mn-bpdc MOF can be attributed to the strong coordinate
bonds with high coordination numbers formed between central hard acids
(Mn\textsuperscript{2+}) and connected four hard bases
(CH\textsubscript{3}COO\textsuperscript{-}) and two hard-medial bases
(C\textsubscript{5}H\textsubscript{5}N).\textsuperscript{54} The
regeneration ability of Mn-bpdc MOF was firstly evaluated by cycling
single component adsorption of C\textsubscript{4}H\textsubscript{6} at
298 K for ten times. As shown in \textbf{Figure 6c} , all the adsorption
curves at each cycle are almost consistent and the uptake capacities do
not have any loss. Notably, the sample only needs regenerate by vacuum
treatment at room temperature instead of degassing by high temperature
before each cycle. Furtherly, the PXRD pattern of Mn-bpdc MOF after ten
times cycles was measured (\textbf{Figure S7)} , which is consistent
with activated MOF. In addition, the recycling breakthrough test of
Mn-bpdc for
C\textsubscript{4}H\textsubscript{6}/n-C\textsubscript{4}H\textsubscript{8}separation
is shown in \textbf{Figure 6d} and the breakthrough times for
C\textsubscript{4}H\textsubscript{6} remain nearly unchanged after five
cycles. Those results comprehensively reveal that the integrality of
frameworks can be retained after repeated uses and the Mn-bpdc MOF
possesses outstanding recyclability for
C\textsubscript{4}H\textsubscript{6} adsorption and separation.
Importantly, it is a scarce report that confirms Mn-bpdc can be well
recycled many times by easily regenerating under vacuum with room
temperature and the cost of regeneration will be relatively low. Hence,
the excellent water-stability and cycling tests furtherly manifest that
it is a great valuable adsorbent for industrial
C\textsubscript{4}H\textsubscript{6} purification.

\textbf{Conclusions}

In this work, we have employed a flexible Mn-bpdc MOF featuring distinct
gate-opening and reversible switch behavior for
C\textsubscript{4}H\textsubscript{6} purification. The intrinsic
flexibility is confirmed by VT-XRD with gradient temperature control. In
addition, the different polarity guest-depended molecule can lead to
structure transformation with different gate-opening pressure points at
different temperatures, such as C\textsubscript{2}H\textsubscript{4} and
C\textsubscript{3}H\textsubscript{6}. Importantly, it exhibits prominent
gate-opening and abrupt adsorption for
C\textsubscript{4}H\textsubscript{6} at very low pressure (0.13 bar) but
no stimulus-response behavior for all
n-C\textsubscript{4}H\textsubscript{8,}iso-C\textsubscript{4}H\textsubscript{8},
n-C\textsubscript{4}H\textsubscript{10} and
iso-C\textsubscript{4}H\textsubscript{10} even at 1 bar and ambient
conditions. Notably, the uptake selectivities of Mn-bpdc MOF for both
C\textsubscript{4}H\textsubscript{6}/n-C\textsubscript{4}H\textsubscript{8}and
C\textsubscript{4}H\textsubscript{6}/iso-C\textsubscript{4}H\textsubscript{8}surpass
whole the reported adsorbents. Furtherly, four dynamic breakthrough
experiment results of
C\textsubscript{4}H\textsubscript{6}/n-C\textsubscript{4}H\textsubscript{8},
C\textsubscript{4}H\textsubscript{6}/iso-C\textsubscript{4}H\textsubscript{8},
C\textsubscript{4}H\textsubscript{6}/n-C\textsubscript{4}H\textsubscript{10}and
C\textsubscript{4}H\textsubscript{6}/iso-C\textsubscript{4}H\textsubscript{10}mixtures
verifies that this Mn-bpdc MOF can practically separate
C\textsubscript{4}H\textsubscript{6} from other
C\textsubscript{4}paraffins and mono-olefins. In addition, the
unexpected water stability, benign regeneration ability, signifies that
this MOF has great potentiality for industrial
C\textsubscript{4}H\textsubscript{6} purification and makes it a new
benchmark of flexible adsorbent for the separation of
C\textsubscript{4}H\textsubscript{6} over other
C\textsubscript{4}hydrocarbons.

\textbf{Acknowledgements}

This work was supported by the National Natural Science Foundation of
China (No. 22038010, 21878229, and 21978212), National Key Projects for
Fundamental Research and Development of China (No. 2016YFB0600901), and
the Science and Technology Plans of Tianjin (19PTSYJC00020 and
20ZYJDJC00110) and we would like to thank the Analytical \& Testing
Center of Tiangong University for \emph{in situ} XRD measurements.

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