Ultrahigh Surface Area Zirconium MOFs and Insights into the Applicability of the BET Theory

Article abstract of DOI:10.1021/ja512973b

An isoreticular series of metal-organic frameworks (MOFs) with the ftw topology based on zirconium oxoclusters and tetracarboxylate linkers with a planar core (NU-1101 through NU-1104) has been synthesized employing a linker expansion approach. In this se

Full text of DOI:10.1021/ja512973b

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Article
Ultrahigh Surface Area Zirconium MOFs and
Insights into the Applicability of the BET Theory
Timothy C. Wang, Wojciech Bury, Diego A Gomez-Gualdron, Nicolaas A. Vermeulen,
Joseph E Mondloch, Pravas Deria, Kainan Zhang, Peyman Z. Moghadam, Amy A.
Sarjeant, Randall Q. Snurr, J. Fraser Stoddart, Joseph T. Hupp, and Omar K. Farha
J. Am. Chem. Soc., Just Accepted Manuscript • DOI: 10.1021/ja512973b • Publication Date (Web): 27 Feb 2015
Downloaded from http://pubs.acs.org on March 2, 2015
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Ultrahigh Surface Area Zirconium MOFs and Insights into the
Applicability of the BET Theory
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Timothy C. Wang1‡, Wojciech Bury1,3‡, Diego A. Gómez-Gualdrón2, Nicolaas A. Vermeulen1, Jo-
seph E. Mondloch1, Pravas Deria1, Kainan Zhang1, Peyman Z. Moghadam2, Amy A. Sarjeant, Ran-
dall Q. Snurr2*, J. Fraser Stoddart1*, Joseph T. Hupp1* and Omar K. Farha1,4*
1Department of Chemistry, Northwestern University, Evanston, Illinois 60208, United States
2Department of Chemical and Biological Engineering, Northwestern University, Evanston, Illinois 60208, United
States
3Department of Chemistry, Warsaw University of Technology, Noakowskiego 3, 00-664 Warsaw, Poland
4Department of Chemistry, Faculty of Science, King Abdulaziz University, Jeddah, Saudi Arabia
Supporting Information Placeholder
ABSTRACT: An isoreticular series of metal-organic frameworks (MOFs) with the ftw topology based on zirconi-
um oxoclusters and tetracarboxylate linkers with a planar core (NU-1101 through NU-1104) has been synthe-
sized employing a linker expansion approach. In this series, NU-1103 has a pore volume of 2.91 cc g-1 and a geo-
metrically calculated surface area of 5,646 m2 g-1, which is the highest value reported to date for a zirconium-
based MOF, and among the largest that have been reported for any porous material. Successful activation of the
MOFs was proven based on the agreement of pore volumes and BET areas obtained from simulated and exper-
imental isotherms. Critical for practical applications, NU-1103 combines for the first time ultrahigh surface ar-
ea and water stability, where this material retained complete structural integrity after soaking in water. Pressure
range selection for the BET calculations on these materials was guided by the four so-called “consistency crite-
ria”. The experimental BET area of NU-1103 was 6,550 m2/g. Insights obtained from molecular simulation sug-
gest that, as a consequence of pore-filling contamination, the BET method overestimates the monolayer load-
ing of NU-1103 by ~16%.
area MOFs, including MOF-2105, NU-1004, NU-1096, NU-
1. INTRODUCTION
1106, DUT-327, and DUT-4911, have been synthesized using
Metal-organic frameworks (MOFs) are a class of crystal-
this strategy and have registered specific surface areas as
line materials formed by coordination bonds between
high as 7,140 m2 g-1 (NU-110), as determined by applying
metal-based nodes and organic linkers.1-3 By judicious
choice of building blocks, MOFs with many desirable
BET theory to measured nitrogen isotherms. While iso-
reticular expansion may appear deceptively simple, sever-
physical properties have been synthesized. Some of the
al potential pitfalls still need to be circumvented en route
most desirable characteristics of MOFs are their perma-
to ultrahigh surface area materials.12,13 For instance, linker
nent porosity and ultrahigh (Brunauer–Emmett–Teller
(BET) ≥ 6000 m2 g-1) specific surface areas. The realization
expansion engenders a tendency for MOFs to collapse
upon solvent removal. Gentle supercritical CO2 activation,
of ultrahigh surface areas within MOFs has, in turn,
a technique developed at Northwestern University, has
pushed the limit of practical sorption based gas storage
been shown useful, however, in addressing this issue.14-17
technologies, e.g., molecular hydrogen and natural gas.4-7
Although long linkers may also lead to the formation of
Indeed, Toyota will start selling hydrogen fueled vehicles
interpenetrated structures, synthesizing MOFs in topo-
in California in early 2015.8 Forward looking, Mercedes
logical nets that are not prone to interpenetration can
Benz is pursuing the use of MOFs in hydrogen storage
alleviate this tendency. In fact, this rationale was used in
systems for next generation fuel cell vehicles.9
the synthesis of NU-1106 and DUT-327, which are based,
The synthesis of ultrahigh surface area MOFs has been
advanced largely by utilizing the concept of "isoreticular
expansion"—the expansion of organic linker length in a
given MOF topology.10 Some of the highest known surface
respectively, on the rht and umt topologies that inher-
ently forbid interpenetration.
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One key limitation in exploiting ultrahigh surface area
to moisture.18 Herein, we utilize the isoreticular approach
to synthesize series of water stable zirconium-
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in practical applications of MOFs has been their lack of
water (or moisture) stability. For example, it is often ad-
vantageous for materials to be stable towards oxygen and
moisture during material processing (e.g., pellet for-
mation). To date, ultrahigh surface area MOFs have been
based exclusively on Zn4O or Cu2 metal-based clusters
with linkers containing carboxylate groups, which unfor-
tunately lose crystallinity and surface area when exposed
a
oxo/hydroxo cluster-based MOFs, NU-1101 through NU-
1104 based on the ftw topology. One of these MOFs, NU-
1103, which has a geometric surface area of 5,646 m2 g-1
(and BET area of 6,550 m2 g-1), exhibits the highest geo-
metric surface area (and BET area) reported for any zirco-
nium oxocluster containing MOF.
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Figure 1. Structural features of the ftw topology illustrated in NU-1103 with large pores (indicated with blue spheres) located at
the center of the cubic unit cell, and small pores (indicated as purple spheres) located at the edges of the cubic unit cell.
in an oven at 120 ˚C for 3 days. After three days, dark
purple crystals were formed on the walls and bottom of
the vials and the mother liquor was removed.
2. EXPERIMENTAL SECTION
All air- or water sensitive reactions were carried out
under a dry nitrogen atmosphere using Schlenk tech-
niques. Chemicals were purchased from Oakwood
Products, Inc. (West Columbia, SC), Strem chemicals.
(Newburyport, MA) and Aldrich Chemicals Co. (Mil-
waukee, WI) and used without further purification. All
gases used for adsorption and desorption measurements
were ultra high purity grade 5 and were obtained from
Airgas Specialty Gases (Chicago, IL).
Synthesis of NU-1103
In an ambient condition, Py-PTP (25 mg, 0.023
mmol), benzoic acid (1 g, 8.92 mmol) were added into a
6-dram vial. 20 mL of DMF is then added and the vial
was sonicated to fully dissolve the reagents. Then,
ZrOCl2ꢀ8H2O (10 mg, 0.03 mmol) was quickly weighed
into the vial containing the reagents, and the vial was
sonicated again until no solid can be found in the vial.
The solution prepared was divided into two portions,
which is achieved by transferring half of the solution
into an empty 6-dram vial by pipet. The 2 vials prepared
were heated in a heating block on a hotplate, and the
temperature of the heating block is set at 120˚C by using
thermocouple. After 24 hours, vials were taken out from
the heating block, and the mother solution was removed
by centrifuging (10 mins, 7000 rpm).
Synthesis of NU-1101
ZrOCl2ꢀ8H2O (96 mg, 0.30 mmol) and benzoic acid
(2.7 g, 22 mmol) were mixed in DMF (8 mL in a 6-dram
vial) and sonicated until the solid dissolved. The clear
solution was incubated in an oven at 80 ˚C for 1h. After
cooling down to room temperature, Py-XP (66 mg, 0.06
mmol) was added to this solution and the mixture was
sonicated for 10 min. The colorless solution was heated
in an oven at 120⁰C for 48 h. After cooling down to room
temperature, colorless cubic-shaped crystals were isolat-
ed by filtration and washed with fresh DMF.
Synthesis of NU-1104
Synthesis of NU-1102
ZrOCl2ꢀ8H2O (16 mg, 0.05 mmol) and benzoic acid
(600 mg, 4.9 mmol) of were mixed in 8 mL of DMF (in a
6-dram vial) and sonicated until the solid dissolved. The
clear solution was incubated in an oven at 80⁰C for 1h.
After cooling down to room temperature, Por-PTP
(12 mg, 0.01 mmol) was added to this solution and the
resulting mixture was sonicated for 10 min. The dark
purple solution was heated in an oven at 120⁰C for 48 h.
After cooling down to room temperature, dark purple
In an O2 and H2O free drybox, ZrCl4 (37.5 mg , 0.16
mmol) was weighed into a 4-dram vial. The vial was
removed from the glovebox and Por-PP (59 mg, 0.054
mmol) along with 4 mL DEF was added into the vial.
The solution was sonicated until green homogeneous
slurry was obtained. Next, benzoic acid (675 mg, 5.5
mmol) was added into the slurry and sonicated until
dissolved. Finally, the 4-dram vial was sealed and placed
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with the ftw topology and Py-XP, Por-PP, Py-PTP,
cubic-shaped crystals were separated from amorphous
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phase by centrifuging in DMF. Approximately 25 mg of
solvated crystalline material was placed in a 4-dram vial
with 3 mL of DMF. Then a solution of 30 mg of
NiCl2ꢀH2O in DMF (1 mL) was added and the vial was
swirled gently and placed in an oven at 90⁰C for 2 days
to metallate the porphyrin core.
Por-PTP as the linkers. This combination results in an
isoreticular series of MOFs which we have termed NU-
1101–NU-1104 (Figure 2).
It is noteworthy that inconsistencies in the applica-
tion of the consistency criteria,37 for comparison of ma-
terials based on their reported Brunauer–Emmett–Teller
(BET) areas, may be problematic for these and other
materials with ultrahigh surface areas.7 In order to cir-
cumvent this issue and benchmark target materials
properly, we have calculated surface areas geometrically
using a rolling-probe method, which is independent of
the nitrogen isotherm and the BET theory.38 The calcu-
lated geometric surface areas for NU-1101 through NU-
1104 are shown in Table 1, along with the calculated ge-
ometric surface areas for other ultrahigh surface area
MOFs. Somewhat pleasingly, NU-1103 exhibits a calcu-
lated geometric surface area of 5,646 m2 g-1, a value
which is only 10% lower than the current record holder
(NU-110).
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3. RESULTS AND DISCUSSION
Pre-synthesis design In an attempt to address the
issue of moisture and water stability we decided to pur-
sue MOFs based on zirconium-oxoclusters and carbox-
ylate-based linkers. Water stable MOFs based on other
building units have been reported in the literature19,20
,
but they are not suitable to reach ultrahigh surface are-
as.21 Zirconium-oxocluster MOFs22-24 have been shown to
exhibit general stability towards water and acid, as well
as a wide range of organic solvents.24-33 The remarkable
stability of zirconium-based MOFs is driven by the
strong bonding between zirconium ions and carboxylate
oxygens present in the linkers. On account of the high
molecular weight of the Zr-oxoclusters (vs Cu2 or Zn4O
based nodes), it might seem counterintuitive to pursue
ultrahigh surface areas materials with Zr-based MOFs. It
is notable that the highest surface area (as calculated by
BET theory) reported28,34 to-date for Zr-oxocluster-based
MOFs is ~4,500 m2 g-1.
Table 1. Summary of Calculated Geometric Surface
Areas for Ultrahigh Surface Area MOFs
MOF
Geometric surface
area (m2 g-1)
Reference
NU-110
NU-109
NU-100
MOF-210
NU-1103
DUT-32
NU-1104
DUT-49
NU-1102
MOF-177
NU-1101
NU-1100
MOF-5
6229
6175
5822
5770
5646
5297
5290
5176
4712
4673
4422
3892
3434
(6)
(6)
(4)
(5)
Recently, however, through calculation of geometric
surface areas, Gomez-Gualdron et al.35 demonstrated
that ultrahigh surface areas are achievable for Zr-based
MOFs with reasonable expansion of tetratopic linkers
with (nearly) square planar symmetry in the csq, scu, or
ftw topologies, wherein the ftw topology (Figure 1)
shows the highest potential surface area. Initial calcula-
tions (see SI for additional details) also indicated that
the ftw topology minimizes the propensity for catena-
tion. Zirconium-based MOFs with this topology are
based on a cubic unit cell with zirconium oxoclusters at
the vertices connected by planar tetratopic linkers
across the faces. The zirconium nodes have the same
structure as the well-known UiO-66.22 Although the ftw
topology has been depicted28,36 as a collection of cubic
cages, Figure 1 shows that this topology features two
types of pores — (i) large pores (indicated with blue
spheres) located at the center of the cubic unit cell, and
(ii) small pores (indicated as purple spheres) located at
the edges of the cubic unit cell.
this work
(7)
this work
(10)
this work
(5)
this work
(23)
(34)
Our selection of tetratopic linkers was based on the in
silico evaluation of geometric surface areas of MOFs
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Figure 2. Tetratopic organic linkers (Py-XP, Por-PP, Py-PTP, Por-PTP) used and the crystal structures of NU-1101 to NU-1104.
The increasing dimensions of the organic linkers result in the reticular expansion of the MOF structure, with unit cell parame-
ters of the cubic cells being 24.3, 25.3, 27.9 and 28.7Å, respectively. Two-thirds of the small cavities (purple spheres) were ne-
glected for clarity when showing the unit cells of NU-110x series. The calculated pore diameter for NU-110x series: NU-110x (dl,
ds)= NU-1101 (17.2, 9.5), NU-1102 (20.5, 11.1), NU-1103 (22.7, 12.7), NU-1104 (24.2, 13.5). dl=large pore diameter, ds=small pore diam-
eter. All units are in Å
Synthesis and experimental characterization.
In light of the computational results, the candidate
tetratopic organic linkers, Py-XP, Por-PP, Py-PTP, Por-
PTP, were synthesized by Sonogashira or Suzuki cou-
NU-1104
pling, followed by saponification in basic aqueous solu-
tion. (See SI for details.)
Solvothermal reactions between zirconium salts and
each linker in DMF at elevated temperature all afforded
cubic crystals, and the crystal structures were deter-
mined by single crystal X-ray diffraction, (see SI) and as
anticipated, each structure shows cubic symmetry and
adopts the ftw topology, with no interpenetration of the
structure. All MOFs in the NU-110x series showed excel-
lent agreement between the simulated (Figure 3) and
measured powder X-ray diffraction (PXRD) patterns,
demonstrating the bulk purity of the materials.
NU-1104 (Sim.)
NU-1103
NU-1103 (Sim.)
The thermal stability of NU-1101 through NU-1104 was
assessed by thermogravimetric analysis (TGA). The re-
sults revealed that all the materials are stable at elevated
temperature, having decomposition temperatures
around 470 °C (See SI section S8.). The TGA of the solv-
ated samples all show significant weight loss due to
guest solvent molecules in the cavity prior to 200 °C,
consistent with the highly porous nature of these mate-
rials.
NU-1102
NU-1102 (Sim.)
NU-1101
NU-1101 (Sim.)
Figure 3. Powder X-ray diffraction of the as-synthesized
MOFs. The agreement between the simulated and meas-
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ured diffraction patterns indicates the phase purity of the
treatment. This full retention of porosity demonstrates
that the material is water-stable under the conditions
tested.
bulk material.39
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An important consideration when working with ultra-
high surface area MOFs is the removal of solvent mole-
cules from the pores of the crystals while leaving the
framework pores intact and accessible.16 On moving
from DMF to acetone40, a lower boiling point solvent,
followed by heating under dynamic vacuum, NU-1101
and NU-1102 could both be activated, as evidenced by
the PXRD patterns (Figure S7.1 and S7.2.) as well as by
the N2 adsorption isotherm (Figure. 4a). When a similar
procedure was used with NU-1103 and NU-1104, howev-
er, their crystallinity was lost and the resulting materials
exhibited little porosity. In order to activate NU-1103
and NU-1104 fully, supercritical CO2 drying14 was uti-
lized so as to avoid capillary forces collapsing the MOFs.
Under these conditions, both NU-1103 and NU-1104 re-
tained their crystallinity (Figure S7.3 and S7.4.) and po-
rosity (Figure 4a) upon activation.
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Nitrogen isotherms were measured (Figure. 4a) at 77
K for NU-1101 through NU-1104 to assess their porosities
and surface areas (vide infra). The pore volumes extract-
ed from the nitrogen saturation loadings measured for
all samples agree very well with those extracted from
nitrogen isotherms simulated by Grand Canonical Mon-
te Carlo (GCMC) simulations (see SI section 9). Indeed,
the agreement between measured and simulated nitro-
gen isotherms indicates the successful nature of the ac-
tivation and suggests that essentially all of the calculat-
ed geometric surface area should be accessible.
Figure 4. a) Nitrogen isotherms for NU-1101 to NU-1104.
(solid lines: simulated isotherm; symbols: experimental
isotherm. black: NU-1101, blue: NU-1102, green: NU-1103,
red: NU-1104.) b) PXRD patterns (inset) and nitrogen iso-
therms remained identical before and after water treat-
ment. (blue: reactivated sample after soaking in water, red:
NU-1103 right after activation step.)
The porosity of the NU-110x series rises as the linker
length increases, and the observed pore volumes are
1.72, 2.00, 2.91, and 2.79 cc g-1 for NU-1101, NU-1102, NU-
1103 and NU-1104, respectively. The pore size distribu-
tion, calculated from nitrogen isotherms with the BJH
desorption model, also agree well with the simulated
data. Each MOF showed two types of pores, which were
anticipated from the crystal structures, i.e., a larger pore
centered at (½,½,½), and one smaller centered at
(½,0,0). Similar to the pore volume, the pore sizes also
become larger with increasing linker length. The size of
the larger pore was found to increase from 17.2 to 24.2 Å
throughout the series, while the size of the smaller pore
is 9.5 Å to 13.5 Å. These pore sizes range from mi-
croporous (<20Å) to mesoporous (≥20Å), and are con-
sistent with the observed steps in the N2 isotherms of
NU-1103 and 1104.
Calculation of the BET area and discussion of the
applicability of BET theory. Since the PXRD and sorp-
tion measurements suggested excellent agreement be-
tween the simulated and synthesized structures, it is
reasonable to assume that the members of the NU-110x
series possess the calculated geometric surface areas
listed in Table 1, with NU-1103 registering the highest
one among these zirconium-based MOFs. To obtain
experimental estimates for N2-accessible surface areas,
the BET analysis of nitrogen isotherms is commonly
employed. Although several assumptions made in BET
theory may not apply rigorously to MOFs, the theory has
still been a valuable predictor of relative gas-storage
performance for members of a related series MOFs.41
In order to demonstrate the water stability of the ul-
trahigh surface area Zr-oxocluster based MOFs, we se-
lected the material with the largest geometric surface
area in the series, namely NU-1103, and soaked it in wa-
ter at room temperature for 18 hours. Given the propen-
sity towards capillary force-driven collapse when acti-
vated from water 24, the solvent was then changed to
ethanol. Subsequent supercritical CO2 drying resulted in
the activated MOF. Remarkably, the nitrogen isotherm
and PXRD pattern measured for this water-treated sam-
ple were identical (Figure 4b) to those measured before
The BET equation
ꢀ/ꢀꢁ
1
ꢈ ꢄ 1
ꢆ
ꢉ
ꢃꢀ/ꢀꢁꢅ
ꢂꢃ1 ꢄ ꢀ/ꢀꢁꢅ ꢂꢇꢈ ꢂꢇꢈ
is derived42 from the assumptions of BET theory and has
been commonly used to get an experimental value of the
surface area of a material by plotting the left-side of the
equation as a function of p/po. From such a plot, a linear
region is identified. From the fitted slope (ꢍꢊꢋꢌꢊ) and in-
ꢎ
ꢌ
tercept (ꢍ ꢅ, the value of the monolayer capacity nm
ꢊ
ꢎ
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and the BET constant C can be extracted. A critical step, stable zirconium-based MOFs, as demonstrated by the
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therefore, in the determination of the BET area is to
select the “correct” linear pressure range, as the value of
the BET area obtained can be greatly affected by this
choice. Indeed, there may appear more than one linear
range to select from, and thus application of consistency
criteria, as suggested by Rouquerol et al.,43 is recom-
mended to guide the selection of the appropriate pres-
sure range.
isoreticular expansion in the NU-110x series.
Given the importance of the BET method for experi-
mental characterization of MOFs, and taking advantage
of the fact that an isoreticular series of structures was
synthesized and completely activated, we decided to
analyze the molecular configurations from the simulat-
ed nitrogen isotherms to gain insights into deviations
from the BET theory in structures with different pore
sizes and the discrepancy between BET areas and geo-
metric surface areas of some members of the NU-110x
series. Using a simple automated algorithm, for each
structure and selected pressure, we counted the total
number of nitrogen molecules (blue line in Figure 5) in
the cubic unit cell and identified those in contact with
the pore walls with a tolerance of 1.0 Å (pink line in Fig-
ure 5), which represent the formation of the monolayer.
9
Briefly, the consistency criteria state that: 1) Only a
range where ꢂꢃ1 ꢄ ꢀ/ꢀꢁꢅ increases monotonically with
ꢀ/ꢀꢁ should be selected. 2) The value of the BET con-
stant C, resulting from the linear fit, should be positive.
3) The monolayer loading nm should correspond to a
pressure in the selected range. 4) The calculated value
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ꢌ
for monolayer formation (√ꢊꢏꢌ), should be equal to the
pressure determined in criterion 3. We used these crite-
ria to select the pressure ranges in the measured and
simulated nitrogen isotherms of the NU-110x series44 to
be able to consistently compare experimental and simu-
lated BET area values. Detailed BET calculations and
discussion of the consistency criteria are reported in the
SI.
We discovered that, while the application of the first
and second consistency criteria was straightforward, the
application of the third and fourth criteria was often
fraught with problems. For all structures, a tolerance of
around 10% was needed to satisfy the third consistency
criterion, which requires the pressure corresponding to
monolayer formation to be within the chosen pressure
range. For instance, for NU-1103, the selected pressure
range for calculating the BET area was 0.036 < ꢀ/ꢀꢁ <
0.164, although the ꢀ/ꢀꢁvalue corresponding to mono-
layer capacity ꢂꢇ was 0.169, which is 3.0% above the
upper pressure limit. Indeed, the use of reasonable tol-
erances may be needed to account for small deviations
relating to some of the BET assumptions. In fact, a simi-
lar tolerance has been proposed by Rouquerol et al.43 for
Figure 5. Simulated N2 isotherm (blue) and the number of
molecules in monolayer contact (pink). The vertical blue
line is the pressure corresponding to the monolayer for-
mation as determined by BET calculations, and the shaded
blue area indicates the amount of adsorbed N2 that is not
forming the monolayer in the simulation but is counted
toward the monolayer in the BET analysis.
Notice that at low pressures, all of the nitrogen is ad-
sorbed on the walls, thus contributing to the monolayer
formation. As the pressure is increased beyond a struc-
ture-dependent value, pore filling starts to occur while
the “monolayer” is still being formed. The vertical line
indicates the pressure predicted to correspond to the
monolayer formation from the BET calculation using the
the fourth criterion, where a 20% tolerance for the rela-
ꢌ
tion that ꢃꢀ/ꢀꢁꢅꢇ ꢆ √
is suggested.
ꢊꢏꢌ
Similar deviations from the consistency criteria were
observed when applying BET calculations to simulated
isotherms, with similar tolerances needed to satisfy the
criteria. Therefore, to calculate an apparent BET area44
for each of these materials, we chose the pressure range
that satisfied the largest number of criteria, and mini-
mized the error for any criterion not satisfied (see Table
S9.1). From the measured (simulated) isotherms we ob-
tained BET areas of 4340 (4350), 4830 (4730), 6550 (6860)
and 6230 (6262) m2 g-1. Notice that the application of the
consistency criteria as guidelines to select the pressure
range for BET calculation allowed a consistent compari-
son between measured and simulated values. On these
grounds, the agreement of the BET areas obtained from
simulated versus measured isotherms (in addition to
pore volumes and PXRDs) indicates the successful acti-
vation of the materials and further confirms that it is
possible to achieve ultrahigh surface areas in water-
simulated isotherms.
Intriguingly, this pressure,
ꢃꢀ/ꢀꢁꢅꢇ, seems to be a good indication of when the
monolayer is essentially complete (i.e. the number of
molecules contacting the pore walls reaches saturation),
as indicated by the pink curves reaching a plateau after
this specific pressure. The degree of pore filling "con-
tamination" of the BET estimates of monolayer loading
at ꢃꢀ/ꢀꢁꢅꢇ is largest for NU-1103 and NU-1104 — mate-
rials whose porosities place them in the transition range
between microporosity and mesoporosity. We hypothe-
size that this pore filling contamination contributes to
the consistency criteria not being rigorously fulfilled,
and it evidently contributes to overestimation of the
monolayer loading. This phenomenon seems to general-
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Journal of the American Chemical Society
ly occur in other structures with pores in the transition
range between microporosity and mesoporosity, and it
will be thoroughly investigated for a comprehensive set
of structures in upcoming work.
AUTHOR INFORMATION
1
2
3
4
Corresponding Author
o-farha@northwestern.edu;
stoddart@northwestern.edu; snurr@northwestern.edu
j-hupp@northwestern;
3. CONCLUSIONS
5
We found that by employing hexa-zirconium
oxo/hydroxo nodes and tetra-carboxylated linkers, wa-
ter-stable MOFs featuring the non-catenating ftw topol-
ogy, and characterized by ultrahigh surface areas, can be
synthesized and fully activated. To our knowledge, all
previous experimental forays into the realm of ultrahigh
surface area MOF chemistry, including our own, have
been plagued, from an applications perspective, by sen-
sitivity of the activated versions of the materials to water
vapor (i.e., degradation by water).
6
7
8
9
Present Addresses
Author Contributions
‡These authors contributed equally.
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60
Funding Sources
Notes
ACKNOWLEDGMENT
Each of the MOFs examined here features a pair of
pore types, which is signaled in N2 adsorption isotherms
by a sharp and sizable step at a p/p0 value between 0.1
and 0.2. From GCMC simulations, the physical signifi-
cance of the unusual isotherm shape includes in some
instances somewhat significant pore filling before
monolayer coverage of the surfaces of both pores is
achieved, which leads the BET theory to overestimate
the monolayer coverage. In turn, this finding suggests
that the Rouquerol consistency criteria for data-range
selection for accurate application of the BET analysis
may be impossible to satisfy. Indeed, we find that to
varying degrees this is the case. A practical consequence,
that will be explored in quantitative detail elsewhere, is
that BET areas may exceed true N2-accessible surface
areas for these complex materials by up to several hun-
dred m2 g-1 (i.e. up to ~15%).45 Nevertheless, the analysis
returns reliable relative surface areas for the series of
compounds examined. Notice that if experimental BET
areas are to be used to assess the activation of a MOF,
comparison to BET areas from simulated N2 isotherms
are more reliable than comparison to geometrically cal-
culated areas.
With 5646 m2g-1, NU-1103 possesses the highest geo-
metric surface area (and a BET area of 6,550 m2 g-1) yet
reported for a water-stable MOF. This record-breaking
property renders NU-1103 attractive for practical appli-
cations such as hydrogen storage, as it simultaneously
addresses the well-known need for high capacity and
the much less well appreciated, but equally important
issue of stability in the presence of contaminants. The
adsorption behavior of this material towards various
gases is under investigation, with predicted hydrogen
uptake from simulation being comparable to those of
record-holding materials.
O.K.F., R.Q.S. and J.T.H.: This work (modeling, MOF as-
sembly and experimental characterization) was supported
as part of the Inorganometallic Catalyst Design Center, an
Energy Frontier Research Center, funded by the U.S. De-
partment of Energy, Office of Science, Basic Energy Scienc-
es, under Award DESC0012702. J.F.S: This research is part
(Project 34-944) of the Joint Center of Excellence in Inte-
grated Nano-Systems (JCIN) at King Abdulaziz City for
Science and Technology (KACST) and Northwestern Uni-
versity (NU). The authors would like to thank both KACST
and NU for their continued support of this research.
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