(30g) Engineering Pore Size of Pillared-Layer Coordination Polymers Enables Highly Efficient Adsorption Separation of Acetylene from Ethylene

14.5pt">Engineering Pore Size 14.5pt">of Pillared-Layer
Coordination Polymers Enables Highly Efficient Adsorption Separation of
Acetylene from Ethylene position:relative;top:0pt">

Fang Zheng†, Lidong
Guo†, Bixuan Gao†, Liangying Li†, Zhiguo Zhang†, Qiwei Yang†, Yiwen Yang*†,
Baogen Su†, Qilong Ren†, Zongbi Bao*†

Key Laboratory of
Biomass Chemical Engineering of Ministry of Education, College of Chemical and
Biological Engineering, Zhejiang University, Hangzhou 310027, P. R. China

" times new roman>Abstract

Separation of acetylene and ethylene is an
industrially important but challenging separation task. Ethylene and acetylene
are both widely used as basic feedstocks to produce polymers and other
chemicals in the petrochemical industry. The origin of ethylene derives from
the fractional distillation of petroleum, tending to coexist with 1% acetylene.
On account of the high activity of acetylene, a p.p.m. level of acetylene can
poison Ziegler-Natta catalyst during ethylene polymerizations, and even
explosion once compressed surpass 0.2 MPa. However, the most commonly employed
method for the commercial separation of acetylene from the ethylene stream is a
cryogenic distillation, which requires both capital and energy input primarily
due to the extremely low temperature and high pressure. It is necessary to
develop such efficient separation approaches as pressure swing adsorption with
relatively low energy consumption and favourable regeneration.

Metal-organic
frameworks (MOFs), emerging as a new class of porous polymer materials, show
great potential in adsorptive removal of trace acetylene from ethylene. For the
microporous adsorbents, the pore size plays a vital role in determining the
overall separation performance of gas mixtures. By modifying and optimizing
ligands, fine-tuned aperture sizes and unique functionality of MOFs can be
devised and used for gas storage and separations.

In
this work, the pore apertures of coordination pillared-layer (CPL) were
systematically controlled by adjusting the length of pillared ligands. Rational
pore-size engineering can effectively hinder the larger C₂H₄ molecule
(empirical kinetic diameters of C₂H₂/C₂H₄
are 3.3 Å and 4.2 Å, respectively), thus affording even higher C₂H₂/C₂H₄
selectivity. We selected
different length of ligands synthesizing CPL-1(L= pyrazine), CPL-2(L= 4,
4¡ä-bipyridine), and CPL-5[L=1, 2-di-(4-pyridil)-ethylene]. Pore size was
adjusted to approach the molecular dimension of acetylene (C₂H₂)
(Figure 1). Among three complexes, CPL-2 exhibits the highest uptake of C₂H₂,
3.13mmol/g at 298 K, surpassing the uptake of 3.01 mmol/g and 2.07 mmol/g in
CPL-5 and CPL-1, respectively. Interestingly, CPL-1 has excellent performance
in nearly excluding C₂H₄ (0.31 mmol/g). The dynamic
breakthrough experiments
indicate that all adsorbents especially CPL-1 offer the potential to
effectively separate C₂H₂ from C₂H₄
and simultaneously produce both gases in high purity (Figure 2). The detailed
computational investigations were performed using first-principles DFT-D
(dispersion-corrected density functional theory) method and the specific
adsorption sites were identified (Figure 3). All adsorbents show marvelous
regeneration and thermal and hydrothermal stability.

" times new roman>Figure 1 12.0pt;font-family:" times new roman>. Representations of
the crystal structures of [Cu₂(pzdc)₂(L)] (Cu, green; N,
blue; C, grey; O, pink; H, white) font-family:" times new roman> (a) Schematic representation of the CPL
structure. Orthographic views down the channel axis of (b) CPL-1; (c) CPL-2;
(d) and CPL-5, showing different sizes of channels can be controlled by
changing different lengths of ligands. From the perspective of c axis
exhibiting the channel of (e) CPL-1 4 12.0pt;font-family:" times new roman>X6Ų£»(f) CPL-2 9X6 Ų;
(g) CPL-5 11X6 Ų.

Figure 2. Gas
sorption isotherms and column breakthrough experiments on the activated CPL-n.
(a) CPL-1; (b) CPL-2; (c) CPL-5; (d) C₂H₂/C₂H₄
adsorption selectivity at 298 K. (e) C₂H₂/ C₂H₄
mixed gas containing 1% C₂H₂; (f) C₂H₂/
C₂H₄ mixed gas containing 50% C₂H₂.
Acetylene and ethylene desorption and sorption curves with open and closed
symbols, respectively; C₂H₂ and C₂H₄
with round and triangle symbols, respectively. For the breakthrough
simulations, the following parameter values were used, t=ut/eL.

Figure
3. The
DFT-D calculated C₂H₂ and C₂H₄
adsorption binding sites in CPL-1 (a. C₂H₂ and d. C₂H₄),
CPL-2 (b. C₂H₂ and e. C₂H₄), CPL-5
(c. C₂H₂ and f. C₂H₄),
respectively. Color code: N, blue; C, gray; O, pink; Cu, green; H, white.

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