Natural gas purification using a porous coordination polymer with water and chemical stability

Article abstract of DOI:10.1021/ic5030058

Porous coordination polymers (PCPs), constructed by bridging the metals or clusters and organic linkers, can provide a functional pore environment for gas storage and separation. But the rational design for identifying PCPs with high efficiency and low energy cost remains a challenge. Here, we demonstrate a new PCP, [(Cu₄Cl)(BTBA)₈·(CH₃)₂NH₂)·(H₂O)₁₂]·xGuest (PCP-33?guest), which shows high potential for purification of natural gas, separation of C₂H₂/CO₂ mixtures, and selective removal of C₂H₂ from C₂H₂/C₂H₄ mixtures at ambient temperature. The lower binding energy of the framework toward these light hydrocarbons indicates the reduced net costs for material regeneration, and meanwhile, the good water and chemical stability of it, in particular at pH = 2 and 60 °C, shows high potential usage under some harsh conditions. In addition, the adsorption process and effective site for separation was unravelled by in situ infrared spectroscopy studies.

Full text of DOI:10.1021/ic5030058

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Article
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Natural Gas Purification Using a Porous Coordination Polymer with
Water and Chemical Stability
Jingui Duan,*,† Wanqin Jin,† and Rajamani Krishna‡
†State Key Laboratory of Materials-Oriented Chemical Engineering, College of Chemistry and Chemical engineering, Nanjing Tech
University, Nanjing 210009, China
‡Van ‘t Hoff Institute for Molecular Sciences, University of Amsterdam, Science Park 904, 1098 XH Amsterdam, The Netherlands
S
* Supporting Information
ABSTRACT: Porous coordination polymers (PCPs), con-
structed by bridging the metals or clusters and organic linkers,
can provide a functional pore environment for gas storage and
separation. But the rational design for identifying PCPs with
high efficiency and low energy cost remains a challenge. Here,
we demonstrate a new PCP, [(Cu4Cl)(BTBA)8·(CH3)2NH2)·
(H2O)12]·xGuest (PCP-33⊃guest), which shows high poten-
tial for purification of natural gas, separation of C2H2/CO2
mixtures, and selective removal of C2H2 from C2H2/C2H4
mixtures at ambient temperature. The lower binding energy of
the framework toward these light hydrocarbons indicates the
reduced net costs for material regeneration, and meanwhile,
the good water and chemical stability of it, in particular at pH = 2 and 60 °C, shows high potential usage under some harsh
conditions. In addition, the adsorption process and effective site for separation was unravelled by in situ infrared spectroscopy
studies.
organic frameworks (MOFs),4,10−21 demonstrate significant
promise for such task, since their easy tailorability could lead to
INTRODUCTION
■
Due to the energy crisis and environmental concerns, the
higher surface and functional pore environments for different
efficient storage and separation of C1 and C2 hydrocarbons
recognition abilities of each component.22−26 Thus, with great
(CH4, C2H2, C2H4, and C2H6) have attracted a great deal of
versatility of their structures and pore surface nature, PCP
research interest. This is because CH4 with high heat of
materials have been considered as the most promising
combustion (55.7 kJ/g) is poised to become one of the most
candidates for such separations.
important energy sources in the future.1 Acetylene and ethylene
Generally, the optimal candidate of PCP materials for gas
are widely used as feedstocks in industrial reactions of
separation should satisfy at least four important factors:27−30
polymerization, oxidation, alkylation, hydration, oligomeriza-
tion, and hydroformylation.2,3 To produce ethylene, acetylene
(1) high separation selectivity; (2) high separation capacity; (3)
good water and chemical stability; (4) low binding energy.
as the byproduct from the steam cracking of ethane shows
deleterious effect to the polyethylene reaction, and also it is
Although recent studies showed that some of the strategies,
such as immobilizing the strong recognition sites for higher gas
well-known to be a highly reactive molecule: it cannot be
selectivity,31−33 improving the pore volume for higher gas
compressed above 0.2 MPa or it explodes without oxygen, even
at room temperature. Furthermore, the separation of C2H2/
capacity,34 and enhancing the coordination strength for stable
framework,28,35−37 worked very well in each aspect, the
CO2 mixtures is important in industry for production of pure
disadvantages of these strategies are also very obvious. For
C2H2, which is required for a variety of applications in the
example, the net energy cost for the regeneration process will
petrochemical and electronic industries. The C2H2/CO2
separation is particularly challenging in view of the similarity
be increased in frameworks with high binding energies (50−90
of the molecular dimensions and boiling points.4 Thus, the
kJ/mol), but much lower binding energy will result in lower
selectivity.33 In many cases, the modified frameworks are very
challenges we now face are becoming more practical in nature,
sensitive to water and offer significantly reduced pore volume.
that is, how to design and synthesize the safe and effective
materials for economic separation.5−7
There is a need to optimize the required characteristics and
integrate all of these into one PCP framework. Our objective is
In order to fully utilize these light hydrocarbons, the
the construction of porous coordination polymers for a variety
separations are generally done by cryogenic distillation, which
entails high danger and large energy costs. Recently, in contrast
with carbon tube8 and zeolites,9 a new class of porous materials,
Received: December 16, 2014
porous coordination polymers (PCPs) also called metal−
© XXXX American Chemical Society
A
DOI: 10.1021/ic5030058
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Figure 1. Structure of PCP-33: (a) molecular structure of well designed C2v symmetric ligand (H3BTBA); (b) the square-planar Cu4Cl cluster
surrounded by six tetrazolate and two carboxylate groups; (c, d) the view of two kinds of cages in PCP-33; (e) the packing view of the 3,8-connected
three-dimensional framework derived from the structure of PCP-33.
of applications.27−29,38 Recently, we reported two water and
the size of the cross point of the 3-D channel reached 20 Å. In
addition, the anionic charge of the framework was balanced by
chemical stable frameworks for selective capture of CO2. The
((CH3)2NH2)+, derived from the hydrolysis of DMF in the presence
high coordination number of the La3+ atom and well-packed
of HCl, which shows a little difference from the counterions
organic walls provide the increased bond energy and hydro-
phobic environment for a water-resistant framework. However,
([Mn(CH3OH)6]2+) in Mn-BTT. The total accessible volume of the
fully desolvated PCP-33 is ca. 47.4%, calculated using the PLATON
the heats of adsorption are too low to achieve higher
performance for gas separation. In order to further optimize
the function of the candidate material, herein, we demonstrate a
PCP based on a new C2v symmetric ligand (3,5-bis(2H-tetrazol-
5-yl)-benzoic acid, H3BTBA) with heterocoordination groups
(Figure 1a). The generated PCP, with suitable pore size
distributions and exposed open metal sites, exhibits promising
characteristics of high separation capacity and high selectivity of
light hydrocarbons at room temperature. In addition, the lower
binding energy of the framework toward these light hydro-
carbons indicates the reduced net costs for material
regeneration, and meanwhile, the good water and chemical
stability of it shows high potential for usage under some harsh
conditions. Furthermore, the adsorption process and adsorp-
tion site for separation was well revealed by using an in situ
infrared spectroscopy study. Thus, PCP-33 is one of the
promising candidates that possess good performance character-
istics natural gas purification.
program.40 The powder X-ray diffraction (PXRD) of the as-
synthesized and degassed form of PCP-33 was collected (Figure S6,
Supporting Information). With very good reliability factors (Rp
=
0.03306 and Rwp = 0.04469), Le Bail analysis of as-synthesized form
shows that the refined parameters (a = 17.5229 Å) are very close to
the data from the single crystal (a = 17.5355(12) Å), reflecting good
phase purity and also well-defined structure (Figure S7, Supporting
Information). In addition, the peak positions of the degassed form are
consistent with its as-synthesized form. Therefore, the framework of
PCP-33 is stable even after removing the guest molecules.
Thermogravimetric analysis (TG) of PCP-33 shows that PCP-33 is
thermally stable up to 200 °C under N2 atmosphere.
Gas Adsorption. The permanent porosity of desolvated PCP-33
was established by N2 sorption experiments at 77 K, which exhibits a
complete reversible type-I isotherm (Figure 2). The estimated
apparent Brunauer−Emmett−Teller surface area is ∼1248 m2·g−1
(Langmuir surface ∼1419.3 m2·g−1). The calculated pore size
distributions according to N2 isotherm (NLDFT/GCMC method)
are in the range 9−22 Å, which matches well the parameters of the
EXPERIMENTAL SECTION
■
General procedures for the experiments, ligand synthesis, and
simulations can be found in the Supporting Information.
Synthesis and Structure of PCP-33. For synthesis of [(Cu4Cl)-
(BTBA)8·(CH3)2NH2)·(H2O)12]·xGuest, (PCP-33·xGuest), copper-
(II) chloride (10 mg), H3BTBA (6 mg), and HCl (30 μL) were mixed
with 2 mL of DMF/H2O (5:1) in a 4 mL glass container, and the
container was tightly capped with a Teflon vial, and the mixture was
heated at 65 °C for 2 days. After cooling to room temperature, the
resulting green polyhedral crystals were harvested with high yield (65%
based on ligand) and washed by DMF.
Solvothermal reaction of CuCl2·2H2O with H3BTBA in DMF/H2O
containing HCl afforded blue crystals of PCP-33·xGuest. Single crystal
X-ray diffraction shows that the chloride-centered square-planar
[Cu4Cl] units are linked by eight ligands to form a three-dimensional
framework as shown in Figure 1. The cubic sodalite-type framework of
PCP-33 is similar as that of Mn-BTT, even though their ligands have
different symmetry.39 The size of the open pore is around 11 Å, while
Figure 2. N2 and Ar adsorption isotherms of PCP-33 at 77 and 87 K.
Inset, calculated pore size distributions. The calculated pore volume
reached 0.50 cm3·g−1.
B
DOI: 10.1021/ic5030058
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crystal structure. The total pore volume calculated from the maximum
amount of N2 adsorbed is 0.50 cm3·g−1. The C1 and C2 hydrocarbons
and CO2 adsorption isotherms of PCP-33 at 273 and 298 K have been
collected and fitted (Figure 3). The Langmuir−Freundlich parameters
potential for separation. The predicted CO2/CH4 selectivity, as well as
the C2H2/CO2 selectivity, is around 6−10. In addition, we note that
the equimolar C2H2/C2H4, C2H2/C2H6, and C2H6/C2H4 selectivities
are all close to unity, showing the separation of individual components
of C2 hydrocarbons is difficult. Thus, the IAST calculations in Figure 4
demonstrate high potential for natural gas purification, as well as the
possibility for selective removal of CO2 from C2 hydrocarbons and
separation of C2H2/CO2 mixtures at room temperature.
Isosteric Heat of Adsorption. To understand such high
separation ability better, the adsorption enthalpies were calculated.
The binding energies of C2H2, C2H4, C2H6, and CO2 in PCP-33 are
reflected in the isosteric heat of adsorption, Qst. These values were
determined using the pure component isotherm fits. Figure 5 presents
Figure 3. Comparison of absolute component loadings for CH4, C2
hydrocarbons, and CO2 at 298 K in PCP-33 with the isotherm fits.
for each pure component isotherms in PCP-33 are provided in Table
S1, Supporting Information. The adsorption hierarchies of these five
gases are very distinct, indicating good separability. The total uptake of
C2H2, C2H6, C2H4, CO2, and CH4 in PCP-33 reached 121.8, 102.4,
86.8, 58.6, and 6.9 cm3·g−1, respectively, at 1 bar and 298 K. In
addition, all of the isotherms are completely reversible, and no
hysteresis is observed. Thus, these results motivated us to examine the
potential application of PCP-33 for C1 and C2 hydrocarbon and CO2
separation, given the fact that these five molecules have comparable
molecular sizes (Table S2, Supporting Information).
Figure 5. Isosteric heats of adsorption for C2H2, C2H4, C2H6, and CO2
in PCP-33. The determination of the Qst is based on the Clausius−
Clapeyron equation.
data on the loading dependence of Qst for C2H2, C2H4, C2H6, and CO2
in PCP-33. We note that the binding energies are in the narrow range
of 22−27 kJ mol−1. It is particularly note worthy that the Qst for C2H2
in PCP-33 is about half the value reported for other MOFs with open
metal sites such FeMOF-74, MgMOF-74, CoMOF-74, and CuBTC,42
implying the significantly lower energy consumption during
regeneration of adsorbed C2 hydrocarbons in fixed bed absorbers of
PCP-33. This is because the large pore size (9−22 Å) and organic
counterions of PCP-33 somewhat reduced the heat of adsorption.43
Transient Breakthroughs. We also performed transient break-
through simulations in a fixed bed adsorber to investigate the
separation potential of PCP-33. Such simulations reflect the combined
influences of adsorption selectivity and uptake capacity. The
breakthroughs of an equimolar component mixture including CH4,
C2H2, C2H4, C2H6, and CO2, using the methodology described in
earlier works,44,45 were explored at 298 K. The relative concentrations
of outflowing gas are shown in Figure 6. The simulation results for
transient breakthrough are presented in terms of a dimensionless time τ,
defined by dividing the actual time, t, by the characteristic time (Lε/u).
The breakthrough hierarchy is dictated by the adsorption strengths;
the weaker the adsorption, the earlier the breakthrough.
Adsorption Selectivity. The ideal adsorbed solution theory
(IAST) of Myers and Prausnitz41 was employed to predict
multicomponent adsorption behaviors from the experimental pure-
gas isotherms. Figure 4 presents IAST calculations of the component
Natural gas usually contains CO2 and C2 hydrocarbons that require
removal by selective adsorption. Figure 6a presents simulation results
for equimolar five-component CH4/C2H2/C2H4/C2H6/CO2 mixtures.
The partial pressures of these five gases were set as 20 kPa. It is clear
that pure CH4 can be recovered because it is the least strongly
adsorbed component and elutes first. In addition, the result shown in
Figure 6b demonstrates that after recovery of CH4, the remaining
C2H2/C2H4/C2H6/CO2 mixture can be separated to yield two
fractions: CO2 and C2 hydrocarbons. Thus, we further simulate the
separation behavior of C2H2/CO2, since it is particularly challenging in
view of their similarity molecular dimensions and also boiling point.3
In view of the high selectivity for adsorption of C2H2, it is possible to
recover pure CO2 during the adsorption phase in a fixed bed adsorber
(Figure 6c). In addition, the significant time interval between the
Figure 4. IAST calculations of CH4, C2H2, C2H4, C2H6, and CO2
adsorption selectivities in PCP-33 at 298 K.
loadings for CH4, C2H2, C2H4, C2H6, and CO2 for adsorption of a five-
component equimolar mixture in PCP-33 at 298 K. The IAST
calculations show the following loading hierarchies at 100 kPa. On the
basis of the component loadings, we calculate the selectivities of
separation of the six constituent binary pairs: C2H2/CH4, C2H2/C2H4,
C2H2/C2H6, C2H2/CO2, C2H6/C2H4, and CO2/CH4. The C2H2/CH4
selectivity is the highest and falls in the range of 40−65, indicating the
C
DOI: 10.1021/ic5030058
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Figure 6. Transient breakthrough simulations for separation of equimolar 5-, 4-, 2-, and 2-component CH4/C2H2/C2H4/C2H6/CO2 mixtures using
PCP-33 at 298 K, with partial pressures of 20, 25, 50, and 50 kPa for each component, respectively.
breakthroughs of C2H2 and CH4 in PCP-33 indicates excellent
separation at 298 K (Figure 6d).
We now consider C2H2/C2H4 separations. In steam cracking of
C2H6 to produce C2H4, one of the byproducts is C2H2. C2H2 has a
deleterious effect on end-products, such as polyethylene. Therefore,
recovery or removal of C2H2 from C2H4 streams is essential. Typically,
C2H2 forms about 1% of the C2H4 streams, and the impurity level of
40 ppm of C2H2 needs to be met for C2H4 feed to a polymerization
reactor. The selective removal of C2H2 is conventionally carried out by
absorption in dimethylformamide; this process is energy-intensive.
Selective adsorption with PCP-33 could be an energy-efficient
alternative. The transient breakthrough simulations for C2H2/C2H4
(1/99) mixtures are shown in Figure 7. We note that for τ < 170, the
outlet gas contains less than 40 ppm of C2H2. The adsorption cycle
needs to be terminated at τ = 170 and the regeneration process needs
to be initiated. Due to the significantly low binding energy of C2H2 in
Figure 7. Transient breakthrough of C2H2/C2H4 mixture containing
1% C2H2 mixture in an adsorber bed packed with PCP-33. The partial
pressures of C2H2 and C2H4 in the inlet feed gas mixture are,
respectively, p1 = 1 kPa, p2 = 99 kPa.
PCP-33, the regeneration costs can be expected to be lower than that
of FeMOF-74, MgMOF-74, CoMOF-74, and CuBTC, which are also
suitable for this separation task.42
In order to further confirm such high separation capability of PCP-
33, pulse chromatographic simulations for varied gas mixtures were
carried out. Figure S12, Supporting Information, demonstrates the
good potential of PCP-33 for the purification of natural gas. The
remaining C2H2/C2H4/C2H6/CO2 mixture can also be separated to
yield two fractions: CO2 and C2 hydrocarbons. In addition, the
separation potential of CO2/C2H2 in PCP-33 is clearly indicated. The
result of pulse chromatographic simulations match well with that of
the breakthrough curves presented in Figure 6.
In Situ Infrared Spectroscopy. Although the average adsorption
heat of the isolated frameworks can be estimated by the
calculation,46,47 exploring the interactions and adsorption behaviors
of the guest on each adsorption site becomes more important, since
the precise understanding of the information between host and guest
will guide the next material design. Indeed, it is better to load C2H2
inside the framework for the IR measurements, but because of its
D
DOI: 10.1021/ic5030058
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