Organic vapor sorption in a high surface area dysprosium(III)-phosphine oxide coordination material

Article abstract of DOI:10.1021/ic301415p

PCM-16 is a phosphine coordination material comprised of Dy(III) and triphenylphosphine oxide, which displays the highest reported CO₂ BET surface area for a Ln(III) coordination polymer of 1511 m² g ^-1. PCM-16 also adsorbs 2.7 wt % H₂ and 65.1 wt % O ₂ at 77 K and 0.97 bar. The adsorption-desorption behavior of a series of organic vapors has been studied in PCM-16 to probe the nature of certain host-guest interactions in the pores. Aromatic and polar guest species showed high uptakes and marked adsorption/desorption hysteresis, while aliphatic vapors were less easily adsorbed. The surface area of PCM-16 could be increased significantly (to 1814 m² g^-1) via exchange of Me ₂NH₂⁺ cations in the pores with smaller NH ₄⁺ groups.

Full text of DOI:10.1021/ic301415p

Article
pubs.acs.org/IC
Organic Vapor Sorption in a High Surface Area Dysprosium(III)−
Phosphine Oxide Coordination Material
Ilich A. Ibarra,† Ji Woong Yoon,‡ Jong-San Chang,‡ Su Kyoung Lee,‡ Vincent M. Lynch,†
and Simon M. Humphrey*,†
†Department of Chemistry and Biochemistry, The University of Texas at Austin, 1 University Station A5300, Austin, Texas
78712-0165, United States
‡Catalysis Center for Molecular Engineering, Korea Research Institute of Chemical Technology, P.O. Box 107, Yusung, Daejeon,
305-600, Korea
S
* Supporting Information
ABSTRACT: PCM-16 is a phosphine coordination material comprised of
Dy(III) and triphenylphosphine oxide, which displays the highest reported
CO2 BET surface area for a Ln(III) coordination polymer of 1511 m2 g−1.
PCM-16 also adsorbs 2.7 wt % H2 and 65.1 wt % O2 at 77 K and 0.97 bar.
The adsorption−desorption behavior of a series of organic vapors has been
studied in PCM-16 to probe the nature of certain host−guest interactions in
the pores. Aromatic and polar guest species showed high uptakes and marked
adsorption/desorption hysteresis, while aliphatic vapors were less easily
adsorbed. The surface area of PCM-16 could be increased significantly (to
+
1814 m2 g−1) via exchange of Me2NH2 cations in the pores with smaller
+
NH4 groups.
carboxylate bonds, so the phosphine oxide behaves as an
asymmetric, 4-connected ligand that is tetrahedral at P. Porous
coordination polymers constructed from lanthanide metals
remain uncommon in comparison to the plethora of examples
of d-block materials. This is perhaps due to the comparatively
unpredictable variation in coordination numbers and geo-
metries exhibited by f-block metals.10 Thus far, literature
examples of lanthanide coordination polymers have been
focused mostly on their magnetic11 and photoluminescence
properties.12 Somewhat surprisingly, the gas and vapor
adsorption properties of homometallic f-block coordination
polymers have received less attention.13−25 Here, we report the
low-temperature synthesis and solid-state properties of a
Dy(III)-based PCM and present an in-depth study of its
small molecule adsorption properties: these include high H2
and O2 sorption capacity and hysteretic adsorption/desorption
of a number of organic vapors.
INTRODUCTION
■
Porous coordination materials that incorporate esoteric organic
substituents are currently of great interest because they may
induce particular host−guest selectivity.1 For example, inclusion
of polar or charged organic groups in a coordination polymer
may improve the adsorption enthalpy and/or spatial orientation
of certain polarizable guest adsorbates such as CO2.2
Phosphine ligands are ideally suited for construction of such
materials because established P(III)/P(V) chemistry provides
easy access to a variety of polar or ionic derivatives by reaction
at the nucleophilic phosphine-P. Furthermore, phosphine-based
coordination polymer building blocks can be modified both
pre- or postsynthetically to provide unique routes to porous
materials that are decorated with organic moieties that have not
previously been studied in the realm of coordination polymer
chemistry.3,4 Phosphines with three acid-functionalized aryl
rings are examples of uncommon 3-connected organic building
blocks that exhibit trigonal geometry and thus favor
construction of 3-dimensional coordination networks, as
opposed to lower- dimensional (layered) solids. This has
been demonstrated by recent work, where tris(p-carboxylato)
triphenylphosphine,5,6 its corresponding oxide,7 and also a
methylphosphonium derivate5,8 have provided diverse examples
of so-called phosphine coordination materials (PCMs).
EXPERIMENTAL SECTION
■
Materials and Methods. 1,4-Dibromobenzene and PCl3
(Aldrich), Dy(NO3)3·xH2O (Alfa Aesar), and HCl and H2O2 (Fisher
Scientific) were used as received. Tetrahydrofuran, N,N-dimethylfor-
mamide, diethyl ether, chloroform, and dichloromethane (Fisher
Scientific) were purified by degassing followed by column distillation
on an Innovative Technologies Inc. PureSolv system and stored on
molecular sieves under dry N2 prior to use. PCM-16 was synthesized
A recent extension of this research has focused on
preparation of lanthanide-based PCMs using tris(p-
carboxylato)triphenylphosphine oxide;9 the polar PO moiety
favors direct coordination to Ln(III) cations in addition to Ln−
Received: July 1, 2012
Published: November 1, 2012
© 2012 American Chemical Society
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Figure 1. (A) Two asymmetric units of PCM-16 detailing the overall ligand connectivity and picturing a single three-quarter paddlewheel motif. (B)
View of the open porous extended structure of PCM-16.
by heating mixtures in 40 capped scintillation vials using graphite
thermal baths, with the vials submerged below the internal solvent
level. Infrared spectra were collected on crystalline analyte using a
Nicolet Avatar 330 FT-IR spectrometer fitted with attenuated total
reflectance apparatus. Thermogravimetric analysis (TGA) was
performed under a He atmosphere at a scan rate of 2.0 °C min−1 in
the range 25−800 °C using a TA Instruments Q50 analyzer. NMR
analyses, 1H and 31P, were recorded in house using a 300 MHz Oxford
Instruments Cryomagnetic Systems spectrometer. Elemental micro-
analyses were performed by QTI Intertek, NJ.
(solid/cm−1): 2929 w, 1699 m br, 1652 m, 1565 w, 1395 m, 1262 m
1
br, 1161 m, 1103 s, 1017 m, 962 s, 933 s, 894 br s, 704 m. H NMR
(dmso; 300 MHz): δ = 7.92 (d, 6H), 8.15 (d, 6H) ppm. 31P NMR
(dmso; 300 MHz): δ = 26.0 ppm.
S y n t h e s i s
o f
P C M - 1 6
[ M e N H ] -
[Dy2(tctpo)2(O2CH)]·3dmf·3H2O. tctpoH3 (20 mg, 48 μmol) 2and
Dy(NO3)3·xH2O (65 mg, 186 μmol) were mixed in dmf (3.0 cm3), thf
(4.0 cm3), H2O (1.0 cm3), and HCl (36.5%, 1 drop); the resulting pH
of the mixture was 2.0. The slurry was stirred until complete
dissolution occurred. The solution was then heated in a scintillation
vial at 85 °C in a graphite thermal bath for 4 days to yield colorless
rods of the target material, which were isolated by decanting away any
residual amorphous solid with the mother liquor (yield 23.0 mg, 64%).
The same synthetic procedure was repeated in which HCl was
substituted for formic acid (HCO2H) to give pH = 2.0, which similarly
afforded 23.7 mg, 66%. Anal. Calcd for C54H60Dy2N4O22P2: C, 43.12;
H, 4.02; N, 3.73. Found: C, 43.33; H, 4.04; N, 3.74. νmax (solid/cm−1):
3662 w, 3587 m, 2896 s, 2784 m, 2482 m, 2325 m, 1729 br m, 1465 w,
1377 s, 1276 m, 1156 s, 1079 s, 857 w, 756 m, 719 s.
2
Synthesis of Trilithium Salt of Tris(p-carboxylato)-
triphenylphosphine ({P(C6H4-p-CO2Li)3}; tctpLi3). This ligand
was prepared using the reported method,5 which is a modified version
of the original procedure reported by Amengual et al.26 that directly
provides the trilithium salt. The salt was dried under vacuum to afford
a pale yellow solid that was stored under N2 (yield 68% based on the
1
tris(p-bromo)triphenylphosphine intermediate). H NMR (D2O; 300
MHz): δ = 7.38 (t, 6H); 7.70 ppm (dd, 6H). 31P NMR (D2O; 162
MHz): δ = −6.66 ppm.
Synthesis of Tris(p-carboxylic)triphenylphosphine Oxide
({P(O)(C6H4-p-CO2H)3}; tctpoH3). TctpLi3 (100 mg, 2.4 mmol)
was dissolved into H2O (10 cm3) in a round-bottomed glass reactor
tube fitted with a magnetic stir bar and heavy-duty Teflon-sealed screw
cap. H2O2 (5 cm3, 30%) was added, and the mixture was vigorously
stirred for 24 h. The resulting mixture was then cooled in an ice bath
and acidified with ice cold HCl solution (1.0 M) to yield a white
precipitate of tctpoH3 that was isolated by vacuum filtration, washed
with ether, and dried under vacuum (yield 614 mg, 63%). Anal. Calcd
for C21H15O7P·2H2O: C, 56.5; H, 4.29. Found: C, 56.8; H, 3.94. νmax
Preparation of NH4 + Exchanged PCM-16 [NH4 ]-
+
[Dy2(tctpo)2(O2CH)]·2dmf·3H2O. A NH4 -exchanged sample in
+
+
which Me2NH2 was replaced by NH4 was prepared by immersing
crystals of as-synthesized PCM-16 in a saturated solution of NH4Cl
predissolved in a mixture of dmf (3.0 cm3), thf (4.0 cm3), and H2O
(3.0 cm3). Crystals were soaked for 4 days, during which time the
NH4Cl solution was refreshed three times per day. After decanting
away the NH4Cl solution, the cation-exchanged crystals of PCM-16-
+
NH4 were rinsed and soaked in a mixture of dmf (3.0 cm3), thf (4.0
cm3), and H2O (3.0 cm3) for 1 day, repeatedly, to remove residual free
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NH4Cl. Anal. Calcd for C49H49Dy2N3O21P2: C, 41.95; H, 3.52; N,
3.00. Found: C, 41.87; H, 3.58; N, 3.02. νmax (solid/cm−1): 3696 w,
3575 m, 2897 s, 2784 m, 2465 m, 2316 m, 1721 br m, 1471 w, 1379 s,
1256 m, 1165 s, 868 w, 761 m, 714 s.
provided by a formate anion (O1F−C1F−O1FA) that bridges
two adjacent Dy(III) ions. The presence of formate in the
structure is explained by acid-catalyzed hydrolysis of N,N-
dimethylformamide solvent. PCM-16 could be synthesized in
high yield (64−66%) either using a small amount of HCl to
drive the hydrolysis reaction in situ or by direct addition of
formic acid to the reaction mixture. The resulting Dy dimers in
PCM-16 are essentially three-quarter paddlewheel motifs with a
single formate bridge perpendicular to the carboxylate bridges.
The formate ligand necessitates charge balance by a single
cation per formula unit. An obvious cation was not located in
the single-crystal structure. It is most likely that the cation is a
delocalized Me2NH2+ resident on solvent within the channel or
on the framework itself.7a PCM-16 has infinite 3-dimensional
connectivity with quadrilateral pore windows in all three
crystallographic planes (Figure 1B and Supporting Informa-
tion), the largest of which have accessible corner-to-corner
openings of 11.8 Å. As is expected for network solids
constructed using high-coordinate metals and unusual 3-
dimensional ligands, PCM-16 has a convoluted net topology
in which the metal and phosphine oxide both act as 5-c nodes,
in addition to the 2-c formate bridge (Supporting Information
Figure S2). Microwave-assisted synthesis and X-ray crystal
structures of a related family of materials of formula
[Me2NH2][Ln2(tctpo)2(O2CH)]·4dmf·6H2O (Ln = Sm−Lu)
with a network topology that is isostructural to PCM-16 were
reported recently;9b however, no solid-state analysis was
performed on the Dy-based material.
Thermogravimetric analysis (TGA) of a crystalline sample of
PCM-16 showed a continual mass loss of 21.1 wt % in the
range 25−200 °C, which, supported by microanalytical data,
corresponds closely to the calculated value for 3 dmf and 3
H2O molecules per formula unit (20.0 wt %) (Supporting
Information Figure S3). The fully desolvated material remained
stable up to 440 °C. In situ variable-temperature powder X-ray
diffraction (PXRD) of PCM-16 under vacuum (1 × 10−10 bar)
confirmed retention of framework crystallinity at 150 °C.
Above 200 °C and under vacuum, a phase transition was
observed to a new crystalline phase that was stable up to 350
°C (Supporting Information Figure S4). Although it was not
possible to ascertain the absolute structure of this new phase via
single-crystal X-ray diffraction, the original low-temperature
PXRD pattern corresponding to PCM-16 was easily recovered
by exposure of the material to humid air (see Supporting
Information Figure S5). On the basis of this observation, all
subsequent textural measurements of PCM-16 were performed
on crystalline samples that were activated below the phase
transition temperature.
Single-Crystal X-ray Diffraction. Crystals were mounted on thin
fiber loops using perfluoropolyether oil, which was frozen in situ by a
nitrogen gas Cryostream flow. Data for PCM-16 was collected on a
Rigaku AFC-12 diffractometer fitted with a Saturn 724+ CCD detector
using monochromated Mo Kα radiation (λ = 0.71075 Å). Cell
refinement and data reduction were performed using the Crystal-
Clear27 utility. Absorption corrections were made based on multiple ω
scans using the SADABS28 program. Structures were solved using
direct methods and refined on F2 using the program SIR-9729 and then
refined using SHELXTL-9730 software. All non-hydrogen atoms were
refined anisotropically for all atoms in the framework of PCM-16;
uncoordinated dmf and H2O solvent molecules were refined with
isotropic displacement parameters. dmf molecules were refined with
geometric restraints in order to stabilize the refinement process, and
free variables were initially applied to determine the site occupancies of
all solvent atoms, which were then set to 1.0 or 0.5 in the final
refinement cycle. The Squeeze31 utility in PLATON32 was applied to
the solution postrefinement in order to remove residual peaks due to
remaining disordered solvent. This resulted in only small improve-
ments to the final statistics (see CIF). All hydrogen atoms were fixed
based on idealized coordinates and refined with Uiso values set to 1.5
times that of the carrier atom. Solvent H2O H atoms were not directly
located in the peak difference map; compensatory alterations were
made to finalize the structural formula in the CIF to account for
solvent H atoms that were not directly located in the electron peak
difference map. See Supporting Information for full data in CIF
format.
Crystal Data for PCM-16. C21.5H12.5DyO8P; MW = 592.29,
monoclinic, space group P2/c, a = 14.537(3) Å, b = 10.492(2) Å, c =
23.403(5) Å, β = 107.32(3)°, V = 3407.8(12) Å3, Z = 4, ρ = 1.154 g
cm−3, μ(Mo Kα) = 2.268 mm−1, R1 = 0.057, 49 072 measured
reflections (5990 independent reflections, I > 2σ(I)), wR2 = 0.146 (all
data), Rint = 0.124, GoF = 1.03; CCDC 883819.
Powder X-ray Diffraction. Phase purity and thermal stability of
PCM-16 were confirmed by analysis of powdered crystalline samples
that were sealed inside borosilcate capillary tubes under inert
atmosphere conditions and spun in situ to prevent preferential
orientation of the crystallites. Diffractograms were recorded on a
Rigaku Spider diffractometer equipped with an image plate detector,
operating in Debye−Scherrer geometry using Cu Kα radiation (1.5412
Å). The 2-dimensional image plate data was converted to give a 1-
dimensional pattern using Rigaku Corp. 2DP software.33 Reflection
data in the range 5.0−35.0° 2θ was extracted from the entire data set.
The PXRD patterns were then compared directly to their
corresponding simulated patterns that were generated using the
SimPowPatt facility in Platon32 using hkl reflection data obtained from
the single-crystal experiment.
Gas Adsorption Isotherms. CO2, N2, O2, Ar, and H2 isotherms
were recorded on an Autosorb-1 system (Quantachrome) at The
University of Texas at Austin under ultrahigh vacuum in a clean system
with a diaphragm and turbo pumping system. Ultrahigh purity
(≥99.9995%) CO2, N2, O2, Ar, and H2 were purchased from Praxair.
The surface area was calculated using the BET method based on
adsorption data in the partial pressure (p/p0) range 0.05−0.30 for CO2
and N2.
Primary assessment of the surface area of an activated sample
of PCM-16 (150 °C, 1 × 10−10 bar, 12 h) using N2 and CO2
probe gases gave BET surface areas of 620 and 1511 m2 g−1,
respectively (Figure 2); the estimated CO2 micropore volume
for PCM-16 was 0.83 cm3 g−1. The disparity in the observed
BET surface areas suggested that PCM-16 has a significantly
greater affinity for CO2 adsorption, as has been previously
reported for other porous f-block coordination polymers.13 The
measured surface area of PCM-16 surpasses the recently
reported Tb(III)-based material (PCM-15) which showed the
highest CO2 surface area (1187 m2 g−1) reported to date for an
f-block porous coordination polymer.9a The lower N2 BET
surface area is similar to other important recent examples of
Ln(III)-based porous coordination polymers, such as Tb(btc)
(678 m2 g−1),20 the PCN-17 series (Er, 606 m2 g−1; Dy, 738 m2
RESULTS AND DISCUSSION
■
The 3-dimensional porous coordination polymer [Me2NH2]-
[Dy2(tctpo)2(O2CH)]·3dmf·3H2O (PCM-16) is comprised of
a single phosphine oxide trianion, [P(O)(C6H4-4-CO2)3]3−
(tctpo), that is multiply coordinated by a single Dy(III) center
(Figure 1A). Each Dy(III) has a distorted square antiprismatic
coordination sphere that consists of carboxylate donors
(chelating and bridging) and a single PO−Dy bond (Figure
1A). These account for seven interactions; the eighth is
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prepared and activated at 150 °C and 1 × 10−10 bar for 12 h.
Interestingly, the CO2 gas adsorption experiment at 196 K gave
a BET surface area of 1814 m2 g−1 (Supporting Information
Figure S15), which is 17% higher than the as-synthesized PCM-
16. Schroder et al. previously showed that replacement of
̈
Me2NH2 by smaller Li+ cations enhanced the porosity of a
+
MOF-type material by ca. 25%.35 Thus, it seems very likely in
this instance that the observed increase of surface area was
+
indeed due to replacement of larger Me2NH2 groups by
+
smaller NH4 . Successful exchange of cations inside PCM-16
using the method employed herein was confirmed by elemental
and TGA analyses (Supporting Information Figure S16).
Selective vapor sorption at ambient temperature by highly
porous coordination polymers is an interesting and largely
unexplored potential application.25 Since PCM-16 appeared to
be an ideal candidate, the uptake of benzene, toluene, ethanol
(Figure 3), n-hexane, and cyclohexane (Figure 4) were all
Figure 2. Observed type-I adsorption/desorption isotherms for PCM-
16.
g−1; Yb, 820 m2 g−1),22,23 or Tb(btb)(OH2) (730−930 m2
g−1).17 Due to the apparent preference of PCM-16 to adsorb
CO2 over N2, the isosteric heat of adsorption of CO2 was
derived from adsorption studies conducted at 278 and 298 K.
Fitting of both isotherms to a virial-type equation34 gave an
estimated value of 35.7 kJ mol−1 (Supporting Information
Figures S8 and S14). Kaneko et al. previously reported a heat of
adsorption for CO2 of 30.10 kJ mol−1 for Er2(pda)3.15
Following confirmation of the inherent porosity of PCM-16,
adsorption experiments using alternative probe analytes were
carried out on fresh, desolvated samples. H2 sorption studies at
77 K revealed rapid uptake kinetics and reversibility without
hysteresis (Figure 2; purple triangles). Total H2 uptake was
2.66 wt % at 77 K and p/p0 = 1.0 (295.61 cm3 g−1, 26.60 mg
g−1). This uptake is also significantly higher than the previously
highest reported H2 uptake for a Ln(III)-based coordination
polymer (1.96 wt %).9a The isosteric heat of adsorption of H2
in PCM-16 was estimated as 8.1 kJ mol−1 using data at 77 and
87 K (see Supporting Information Figure S7 and Figure S13).
Hong et al. previously reported a value of 7.8 kJ mol−1 for
Gd(tctpo).13 O2 adsorption at 77 K showed a total uptake of
456 cm3 g−1 (65.1 wt % at 0.97 bar; Figure 2, red triangles).
Total Ar uptake was measured as 254 cm3 g−1 (Figure 2, blue
circles).
Figure 3. Observed adsorption−desorption isotherms for benzene,
toluene, and ethanol vapors in PCM-16 at 298 K. Solid symbols
represent adsorption, and open symbols show desorption.
studied. These analytes were specifically chosen to allow for a
comparative analysis of preferential host−guest interactions
within the pores. First, the relative hydrophilicity/hydro-
phobicity of PCM-16 was probed by vapor sorption of apolar
aromatics versus ethanol (Figure 3). Benzene, toluene, and
A single cation is required in PCM-16 per formula unit to
achieve overall charge balance. The identity of this cation was
initially unclear to us, particularly in the desolvated (activated)
form of PCM-16, in which vapor sorption analyses were
+
performed. It is reasonable to assume that Me2NH2 could be
formed due to hydrolysis of dmf that also gave the formate
anion which was incorporated into the framework of PCM-16.
1
In previous work, ex situ H NMR studies of the framework
postdigestion in DCl revealed a peak that was attributed to
9b
+
Me2NH2 . This study did not unequivocally determine the
location of H+ species in the activated material, while it seemed
plausible that H+ could move to several alternative locations in
the framework upon heating under vacuum. To test this
hypothesis from an alternative route, we decided to attempt
cation exchange of the as-synthesized PCM-16.35
A sample of PCM-16 was subjected to cation exchange using
a saturated solution of NH4Cl in solvent of synthesis (see
Experimental Section), and the surface area of the resulting
material was studied by CO2 sorption analysis in addition to
other characterization studies. Thus, NH4−PCM-16 was
Figure 4. Adsorption−desorption isotherms for n-hexane and
cyclohexane in PCM-16 at 298 K; solid symbols represent adsorption,
and open symbols show desorption. (Inset) Corresponding time-
dependence plot.
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ethanol each underwent rapid uptake at low pressure, leading to
saturation below p/p0 = 0.2, indicative of favorable host−guest
interactions in all cases. The internal pore surfaces of PCM-16
are comprised primarily of aromatic rings, so it is not difficult to
understand why the aromatic vapors would be easily adsorbed
via π−π interactions. Polar species such as ethanol most likely
form hydrogen-bonded interactions with the charged moieties
(Dy−carboxylates in this instance). The highest vapor uptake
in PCM-16 was observed for benzene (34.8 wt % or 10.5
molecules per unit cell; Supporting Information Figure S17).
Toluene showed a lower total uptake, as expected due to the
larger adsorbate size (25.2 wt %, kinetic diameter = 5.9 Å; 6.5
molecules per unit cell; Figure 3, orange triangles). While there
are no examples of analogous vapor adsorption studies in other
Ln-based porous coordination polymers, the present data can
be contrasted against similar measurements on d-metal-based
polymers, such as Ni(bpb) (bpb = 1,4-bis(4-tetrazolyl)-
benzene) which exhibited a benzene uptake of 5.8 mmol g−1,
corresponding to 45.3 wt % under comparable conditions.36
The higher wt % uptake observed in the latter case is to be
expected, since the density of the Ni-based material is
significantly lower than the Dy-based PCM-16.
Ethanol adsorption in PCM-16 exhibited an uptake of 28.8
wt % (15 molecules per unit cell). Strong hysteresis was
observed for all three probe molecules, with marked stepped
profiles in the desorption phase (Figure 3, open symbols). The
largest pore openings in PCM-16 are significantly larger than
the kinetic diameters of the adsorbates (toluene, 5.9 Å;
benzene, 5.8 Å; ethanol, 4.5 Å).25 Therefore, it does not seem
appropriate to invoke ‘kinetic trap’ arguments that have been
suggested by others.37 Instead, the observed hysteresis is likely
due to moderately strong host−guest interactions, which result
in physical ‘sticking’ of the guests within the pores.
were only weakly bound inside PCM-16 and showed no
marked desorption hysteresis. However, chemically similar
adsorbates that have significantly different kinetic diameters
(e.g., n-hexane and cyclohexane) could still be discriminated
from one another, based on significantly different adsorption
rates through the pore windows in PCM-16.
ASSOCIATED CONTENT
* Supporting Information
■
S
Additional structural pictures, thermogravimetric analysis,
PXRD patterns, derivation of the isosteric heats of adsorption
for H2 and CO2, and solvent vapor adsorptions for PCM-16.
This material is available free of charge via the Internet at
http://pubs.acs.org.
AUTHOR INFORMATION
Corresponding Author
*E-mail: smh@cm.utexas.edu.
■
Notes
The authors declare no competing financial interest.
ACKNOWLEDGMENTS
■
The authors thank the Welch Foundation (F-1738) and the
Institutional Research Program (ISTK, KK-1201-F0) for
financial support and the National Science Foundation
(Grant No. 0741973) for X-ray equipment.
REFERENCES
■
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In an additional study to support this hypothesis, the kinetic
sorption of the apolar aliphatic species cyclohexane and n-
hexane was studied (Figure 4). The former showed a 19.0 wt %
uptake (5.3 molecules per unit cell; Supporting Information
Figure S18), while 16.3 wt % (4.5 molecules per unit cell) of n-
hexane was adsorbed at 298 K in PCM-16. A similar alkane
vapor adsorption study was made on the Cr-based MIL-53, in
which Trens and co-workers reported adsorption of 5.3
molecules of n-hexane per unit cell at 1 bar and 313 K.38 In
comparison to the former adsorbates, the aliphatics should be
less likely to engage in substantial host−guest bonding
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Most notably, however, the lack of any pronounced desorption
hysteresis for the alkanes is consistent with the lack of strong
host−guest interactions. Kinetic analysis of the aliphatic C6
adsorbates showed that although the total uptakes were similar,
the sorption rate of n-hexane was much faster than for
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CONCLUSION
■
The high surface area Dy(III)−phosphine oxide coordination
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