Zeolitic imidazolate frameworks for kinetic separation of propane and propene

Article abstract of DOI:10.1021/ja9039983

(Figure Presented) Propane/propene separation by cryogenic distillation is one of the most energy and cost intensive industrial processes. Adsorptive separation is a more energy-efficient alternative. Three isostructural zinc imidazolate zeolitic framewor

Full text of DOI:10.1021/ja9039983

Published on Web 07/10/2009
Zeolitic Imidazolate Frameworks for Kinetic Separation of Propane and
Propene
Kunhao Li,† David H. Olson,† Jonathan Seidel,† Thomas J. Emge,† Hongwei Gong,‡ Heping Zeng,‡
and Jing Li*,†,‡
Department of Chemistry and Chemical Biology, Rutger, The State UniVersity of New Jersey, 610 Taylor Road,
Piscataway, New Jersey 08852, and College of Chemistry, South China UniVersity of Technology,
Guangzhou, 510006, P. R. China
Received May 21, 2009; E-mail: jingli@rutgers.edu
Industrial olefin/paraffin separations heavily rely upon energy-
intensive distillation-based technologies, which represent a class
of the most important and also the most costly processes in the
chemical industry.1 Adsorptive separation is widely considered as
a more energy and cost-efficient alternative. The structures and
properties of the adsorbent materials often dictate the separation
mechanisms that apply: molecular sieving or steric size exclusion,
equilibrium or kinetics based separation.2 While most adsorbents
(exclusively zeolites or amorphous adsorbents) studied for olefin/
paraffin separations achieve such separations by their preferential
equilibrated uptake of one component versus the other or by size
exclusion, there are a limited number of cases where the separations
are accomplished by differences in the diffusion rates of the
adsorbates (olefin and paraffin) into and out of the adsorbents.2,3
Microporous metal organic frameworks (MMOFs), as a new type
of crystalline adsorbent materials, have shown great potential in
hydrocarbon separations.4 Very recently, the possibility of equi-
librium-based olefin/paraffin separations by MOFs was investi-
gated.5 Herein, we report the first examples of MMOFs that are
capable of kinetic separation of propane and propene (propylene),
which is one of the most difficult chemical separations due to their
very close relative volatilities and molecular sizes.
the main structural directing factor in the formation of zeolite-like
frameworks.6 In this case, both 1 and 2 have an expanded soda-
lite (sod)7 framework structure (Figure 1a). The square faces of
the truncated octahedral cages are essentially blocked by the
chlorine or bromine atoms (Figure 1b); therefore, the cages are
interconnected along 3-fold axes through the small openings
delimited by H atoms on imidazolate ligands (Figure 1c).
Figure 1. (a) Expanded sodalite (sod) framework formed by connecting
the tetrahedral ZnII centers; (b) space-filling model of one sod cage; (c)
view of the pore opening along one 3-fold axis; (d) labeling scheme of the
structural features determining the pore opening size. C: gray; H: white;
N: blue; Zn: light blue; Cl or Br (or methyl in 3): light green. All disordered
solvent molecules were removed for clarity. Yellow spheres indicate the
cavity inside the cages (∼12.5 Å in diameter).
The MMOFs we studied belong to a recently emerged group of
materials named Zeolitic Imidazolate Frameworks (ZIFs).5 Their
structures are constructed upon metals of tetrahedral coordination
geometry (e.g, ZnII and CoII) and imidazole ligands (e.g., 2-chlor-
oimidazole (2-cim) for 1 and 2-bromoimidazole (2-bim) for 2
herein), which closely resemble the structures of zeolites. In addition
to their excellent thermal and chemical stability, they also lack
Lewis and Brønsted acid sites that may catalyze polymerization of
olefins inside the pores. For zeolites, only those with high silica
content can be used for separation of olefin/paraffin whereas their
synthesis can be difficult.
Compound 1 and 2 are isostructural to the recently reported ZIF-8
structure (3, with 2-methylimidazole as the ligand, Supporting
Information).6 Considering the comparable sizes of chloro, bromo,
and methyl groups, the similarity in their overall structure is
expected. However, the slight differences in the sizes of the three
substituents (and possibly in combination with their different
electronic inductive effects) lead to small but significant perturba-
tions to the Zn-N bond distances (d), Zn-Im-Zn angles (θ) and
the dihedral angles between the imidazole ring and the hexagonal
faces enclosed by 6 ZnII centers (Φ), and Zn-Zn distances (L),
which result in different effective pore opening (aperture) sizes (A,
Figure 1d and Table 1). It is shown in the following study that the
seemingly small differences in their effective pore opening sizes
Reacting Zn(NO3)2 · 6H2O with 2-cim and 2-bim in methanol
under solvothermal conditions resulted in good quality polyhedral
crystals of [Zn(2-cim)2] · 2.1(CH3OH) (1) and [Zn(2-bim)2] ·
0.16(H2O)·0.16(C2H5OH) (2), respectively (see Supporting Infor-
mation). Single crystal X-ray diffraction revealed that 1 and 2 are
j
iso-structural to each other, both crystallizing in the cubic I43m
space group with almost identical unit cell constants (Supporting
Information). ZnII centers are invariably tetrahedrally coordinated
to four N atoms from four imidazolate ligands. Each imidazole
coordinates to two ZnII with Zn-Im-Zn angles (θ, Figure 1d) close
to 145°, which are coincident with the typical Si-O-Si angles
found in many zeolites. This has been reasonably attributed to be
o
)
(<0.2 Å) are actually critical to their propane/propylene (C3 /C3
)
separation capabilities.
Under equilibrium conditions, propane and propene adsorption
measurements on 3 revealed essentially identical adsorption capaci-
ties for both, 155 and 160 mg/g at 30 °C and 600 Torr. In addition,
their isosteric heats of adsorption at zero loading are similar, 34
and 30 kJ/mol, respectively (Supporting Information). While
† The State University of New Jersey.
‡ South China University of Technology.
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10.1021/ja9039983 CCC: $40.75  2009 American Chemical Society

C O M M U N I C A T I O N S
Table 1. Structural Factors Determining the Effective Size of the
Pore Openings, A (Excluding van der Waals Radius of H)
on the remarkable differences in their diffusion rates through the
pore systems. The effective size of the pore opening is believed to
be the controlling factor determining the separation capability. In-
depth studies to assess the full separation capability of these
materials, e.g., the breakthrough performance, process efficiency,
and cost simulations, etc., are currently underway.
θ (deg)
d (Å)
Φ (deg)
L (Å)
A (Å)
1
2
3
142.62
144.35
144.77
1.983
1.995
1.984
5.328
7.032
10.764
6.004
6.032
6.015
3.37
3.54
3.26
Acknowledgment. The authors gratefully acknowledge the
financial support from DoE (Grant No. DE-FG02-08ER46491). J.L.
is a Cheung Kong Scholar associated with South China University
of Technology.
thermodynamic separation seems impractical, the rates of adsorption
are markedly different (Figure 2). At 30 °C, the ratio of their
o
diffusion rate coefficients, D(C3 )/D(C3)), is 125, suggesting that
3 has great potential for the kinetic separation of these two very
similar molecules. Adsorption rate measurements for 1 have also
yielded the relative adsorption rates of propene and propane. At
30 °C the ratio of their diffusion rates is 60, approximately one-
half that of 3 (Figure 2).
Supporting Information Available: Syntheses of 2-cim; crystal
growths and selected crystallographic data of 1, 2, and 3; adsorption
measurement procedures and additional figures. These materials are
available free of charge via the Internet at http://pubs.acs.org.
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Figure 2. Propene and propane uptake by 3 (left) and 1 (right) as a function
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In summary, the single-component diffusion rate study reveals
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