Tuning the moisture and thermal stability of metal-organic frameworks through incorporation of pendant hydrophobic groups

Article abstract of DOI:10.1021/cg4009224

An isostructural series of NbO-type porous metal-organic frameworks (MOFs) with different dialkoxy-substituents of formula Cu₂(TPTC-OR) (TPTC-OR = 2′,5′-di{alkyl}oxy-[1,1′:4′,1″-terphenyl]-3, 3″,5,5″-tetracarboxylate, R = Me, Et, ⁿPr, ⁿHex) has been synthesized and characterized. The moisture stability of the materials has been evaluated, and a new superhydrophobic porous MOF has been identified. The relationship between pendant side chain length and thermal stability has been analyzed by in situ synchrotron powder X-ray diffraction, showing decreased thermal stability as the side chain length is increased, contradictory to thermogravimetric decomposition studies. Additionally, the four materials exhibit moderate Brunauer-Emmett-Teller (BET) and Langmuir surface areas (1127-1396 m² g^-1 and 1414-1658 m² g ^-1) and H₂ capacity up to 1.9 wt % at 77 K and 1 bar.

Full text of DOI:10.1021/cg4009224

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Article
Tuning the Moisture and Thermal Stability of Metal-Organic
Frameworks through Incorporation of Pendant Hydrophobic Groups
Trevor Makal, Xuan Wang, and Hong-Cai (Joe) Zhou
Cryst. Growth Des., Just Accepted Manuscript • DOI: 10.1021/cg4009224 • Publication Date (Web): 13 Sep 2013
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Crystal Growth & Design
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Tuning the Moisture and Thermal Stability of Metal-Organic
Frameworks through Incorporation of Pendant Hydrophobic
Groups
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Trevor A. Makal†, Xuan Wang, and Hong-Cai Zhou*
Department of Chemistry, Texas A&M University, College Station, Texas 77843, United States
† Current address: Department of Natural Sciences, The University of Virginia’s College at Wise, Wise, Virginia 24293,
United States
KEYWORDS Metal-organic framework, MOF, isoreticular chemistry, hydrophobic material
ABSTRACT: An isostructural series of NbO-type porous metal-organic frameworks (MOFs) with different dialkoxy-substituents
n
n
of formula Cu2(TPTC-OR) (TPTC-OR = 2',5'-di{alkyl}oxy-[1,1':4',1''-terphenyl]-3,3'',5,5''-tetracarboxylate, R = Me, Et, Pr, Hex)
has been synthesized and characterized. The moisture stability of the materials has been evaluated and a new superhydrophobic
porous MOF has been identified. The relationship between pendant side chain length and thermal stability has been analyzed by in
situ synchrotron powder X-ray diffraction, showing decreased thermal stability as the side chain length is increased, contradictory
to thermogravimetric decomposition studies. Additionally, the four materials exhibit moderate Brunauer-Emmett-Teller (BET) and
Langmuir surface areas (1127 – 1396 m2 g-1 and 1414 – 1658 m2g-1), and H2 capacity up to 1.9 wt% at 77 K and 1 bar.
particular, NbO-type MOFs are well known as highly porous
and robust frameworks with exceptional gas sorption proper-
ties, and can predictably be assembled from solvothermal re-
INTRODUCTION
Metal-organic frameworks (MOFs)1-5 are porous, crystalline
materials that have been steadily growing in popularity due to
their exciting chemistries and potential applications in gas
storage,6-10 separations,11-14 catalysis,15-18 magnetism,19 lumi-
nescence,20-22 and drug delivery and storage,23 to name a few.
The applicability of MOFs stems from their chemical tunabil-
ity, high surface areas and porosities, and impressive thermal
stability. MOFs for the storage of alternative fuels (hydrogen
and natural gas) along with carbon capture from flue gas from
coal-fired power plants have been at the forefront of the de-
velopment of future materials for environmental conservation
and remediation.24-25 However, since MOFs are constructed
from coordination bonds between metal ions or clusters and
organic linkers (typically carboxylate or pyridyl moieties), the
frameworks are often susceptible to decomposition resulting
from ligand displacement by atmospheric water vapor.26 This
is a significant concern in clean energy technologies, as water
is certainly present in flue gas, and is often an impurity in nat-
ural gas supplies. To this end, a number of researchers have
investigated methods for enhancing the water stability of
MOFs, including using high oxidation state metals (such as
Fe3+, Al3+, and Zr4+) which form strong coordination bonds
with linkers,27-31 incorporation of methyl groups near coordi-
nation sites,32-34 and post-synthetic grafting of hydrophobic
groups onto linkers,35 among others.36-40
actions of copper(II) salts and rectangular planar tetracarbox-
ylic acids.41-49 A series of NbO-type MOFs (NOTT-10x) has
been previously reported by Schröder and coworkers studying
the role of pore size, ligand functionalization, and exposed
metal sites in isoreticular copper(II) tetracarboxylate MOFs.50
This previous work highlighted the predictability and feasibil-
ity of designing and synthesizing NbO-type MOFs and their
applicability in future gas sorption applications.
From an inexpensive starting material, hydroquinone, four
rectangular planar terphenyl tetracarboxylate organic linker
precursors (H4TPTC-OR, Scheme 1) were designed and syn-
thesized by grafting different dialkoxy substitutions on the
central phenyl ring in order to modify the hydrophobic proper-
ties. By incorporating these hydrophobic groups prior to MOF
formation we can ensure full inclusion of functionalized link-
ers without any detrimental effects on the structure of the
framework. We report here a series of four NbO-type Cu(II)-
based MOFs with different alkoxy substitutions that modify
the moisture and thermal stability of the isostructural frame-
works, as well as H2 capacity. Through in situ synchrotron
powder X-ray diffraction (PXRD) measurements, we thor-
oughly investigated the stability of the crystalline materials.
The hydrogen storage capacity is reported and the role of pen-
dant alkoxy groups on the linkers toward hydrogen uptake
and, most importantly, thermal and moisture stability evaluat-
ed.
With the ready availability and inexpensiveness of cop-
per(II) salts, along with the predictable nature of formation of
dicopper(II) paddlewheel secondary building units (SBUs), we
have turned our attention toward investigating methods for
enhancing the water stability of MOFs formed from combina-
tion of dicopper paddlewheel SBUs and carboxylate linkers. In
EXPERIMENTAL SECTION
General Information. All initial reagents were purchased
from commercial sources and used as received without further
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Scheme 1. Synthesis and structures of ligand precursors H4TPTC-OR
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purification. Nuclear magnetic resonance (NMR) 1H data were
collected on a 300 MHz Mercury 300 spectrometer. Thermo-
gravimetric analyses (TGA) were performed under N2 flow on
a Shimadzu TGA-50 thermogravimetric analyzer with a heat-
ing rate of 3 ˚C min-1.
Synthesis of 2,5-diethoxy-1,4-di-iodo-benzene (B, R=Et) B,
R=Et, was synthesized by modification of a previously report-
ed procedure.52 To 20 mL methanol at 0˚C were added drop-
wise 1.26 mL (3.91 g, 24.1 mmol) iodine monochloride, fol-
lowed by 1,4-diethoxybenzene (A, R=Et, 1.0 g, 6.0 mmol).
The reaction mixture was stirred and heated to reflux for 4
hours, the white precipitate collected by vacuum filtration,
washed with methanol, and dried in air to yield 0.681 g. An-
other 0.123 g was recovered upon cooling the filtrate in refrig-
erator (32.0 % combined). 1H NMR (300 MHz CDCl3) 7.19 (s,
2H), 4.00 (q, 4H, J = 7.0 Hz) 1.44 (t, 6H, J = 7.0 Hz).
Synthesis of 1,4-di-n-propoxybenzene (A, R=nPr) A, R=nPr,
was synthesized by modification of a previously reported pro-
cedure.51 Hydroquinone (2.10 g, 19.1 mmol) and excess potas-
sium carbonate were purged under vacuum and back-filled
with N2 three times in a dry 100 mL Schlenk flask. N,N-
dimethylformamide (DMF, 30 mL) was added via canula and
1-bromopropane (4.1 mL, 45.2 mmol) was added via syringe.
The reaction mixture was heated at 60 ˚C under N2 atmosphere
overnight. After cooling to room temperature, water was add-
ed and the light brown crystals collected by vacuum filtration.
Recrystallization from methanol yielded colorless crystals
Synthesis of 1,4-di-iodo-2,5-di-n-propoxybenzene (B,
R=nPr) B, R=nPr, was synthesized similarly to B, R=Et, ex-
cept that 1,4-di-n-propoxybenzene (A, R=nPr, 0.937 g, 4.82
mmol) was used instead of A, R=Et, in 15 mL methanol with
1
1.0 mL (19.1 mmol) ICl. Yield: 57.7% H NMR (300 MHz
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(26.5 %). H NMR (300 MHz CDCl3): δ 6.80 (d, 4H, J = 2.2
CDCl3) 7.11 (s, 2H), 3.83 (t, 4H, J = 6.4 Hz), 1.75 (m, 4H),
1.00 (t, 6H, J = 7.4 Hz).
Hz), 3.85 (td, 4H, J = 6.6 Hz, 2.2 Hz), 1.78 (m, 4H), 1.01 (td,
6H, J = 7.4 Hz, 2.2 Hz).
Synthesis of 2,5-di-n-hexyloxy-1,4-di-iodo-benzene (B,
R=nHex) B, R=nHex, was synthesized similarly to B, R=Et,
except that 1,4-di-n-hexyloxybenzene (A, R=nHex, 2.03 g,
7.29 mmol) was used instead of A, R=Et, in 20 mL methanol
Synthesis of 1,4-di-n-hexyloxybenzene (A, R=nHex) A,
R=nHex, was synthesized similarly to A, R=nPr, except that 1-
bromohexane (6.3 mL, 45.2 mmol) in place of 1-
bromopropane. Recrystallization of the light brown crystals
from ethanol yielded a flaky off-white precipitate (40.1 %). 1H
NMR (300 MHz CDCl3): δ 6.80 (s, 4H), 3.88 (t, 4H, J = 6.6
Hz), 1.73 (m, 4H), 1.50-1.26 (m, 12H), 0.88 (t, 6H, J = 7.0
Hz).
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with 1.65 mL (31.5 mmol) ICl. Yield: 82.2% H NMR (300
MHz CDCl3) 7.17 (s, 2H), 3.92 (t, 4H, J = 6.4 Hz), 1.80 (qn,
4H, J = 6.3 Hz), 1.58-1.43 (m, 4H), 1.41-1.29 (m, 8H), 0.91 (t,
6H, J = 7.0 Hz).
Synthesis of diethyl 2',5'-dimethoxy-[1,1':4',1''-terphenyl]-
3,3'',5,5''-tetracarboxylate (C, R=Me) 1,4-Dibromo-2,5-
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Crystal Growth & Design
dimethoxybenzene (0.51 g, 1.72 mmol), diethyl 5-(4,4,5,5- filled with N2 three times. 1,2-Dimethoxyethane (DME, 100
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tetramethyl-1,3,2-dioxaborolan-2-yl)-1,3-benzene-
mL) and distilled water (10 mL) were mixed and purged with
N2 for 30 minutes, then transferred under N2 to the reaction
flask via canula. The reaction solution was heated at reflux for
5 days. Organic solvent was removed in vacuo and water add-
ed. The aqueous phase was extracted with dichloromethane
three times and organic extracts combined. The organic solu-
tion was washed with water, dried over MgSO4, filtered, and
reduced in vacuo. The white solid was collected after recrys-
dicarboxylate53 (2.61 g, 7.5 mmol), NaHCO3 (1.23 g), CsF (3.0
g), and Pd(PPh3)4 (0.15 g) were mixed in a 250 mL Schlenk
flask, then purged and refilled with N2 three times. 1,2-
Dimethoxyethane (DME, 100 mL) and distilled water (10
mL) were mixed and purged with N2 for 30 minutes, then
transferred under N2 to the reaction flask via canula. The reac-
tion solution was heated at reflux for 7 days. Organic solvent
was removed in vacuo and water added. The aqueous phase
was extracted with dichloromethane three times and organic
extracts combined. The organic solution was washed with
water, dried over MgSO4, filtered, and reduced in vacuo. The
off-white solid was collected after washing with hot acetone
and drying in air (43.8 %). 1H NMR (300 MHz CDCl3): δ 8.67
(t, 2H, J = 1.6 Hz), 8.43 (d, 4H, J = 1.6 Hz), 7.00 (s, 2H), 4.43
(q, 8H, J = 7.1 Hz), 3.82 (s, 6H), 1.43 (t, 12H, J = 7.1 Hz).
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tallization from acetone and drying in air (44.0 %). H NMR
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(300 MHz CDCl3): δ 8.66 (t, 2H, J = 1.6 Hz), 8.47 (d, 4H, J =
1.6 Hz), 7.02 (s, 2H), 4.42 (q, 8H, J = 7.1 Hz), 3.95 (t, 4H, J =
6.4 Hz), 1.68 (qn, 4H, J = 6.3 Hz), 1.43 (t, 12H, J = 7.1), 1.39-
1.19 (m, 12H) 0.83 (t, 6H, J = 6.8 Hz).
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Synthesis
of
2',5'-dimethoxy-[1,1':4',1''-terphenyl]-
3,3'',5,5''-tetracarboxylic acid (H4TPTC-OMe) To a suspen-
sion of C, R=Me, (0.435 g, 0.753 mmol) in 50 mL
THF/methanol (1/1 by volume) was added KOH (3 g in 25 mL
water). The reaction mixture was heated at reflux until no
more solid was present. The organic solvent was removed in
vacuo and the peach colored solution acidified with 20% HCl
(in water). The white precipitate was collected by vacuum
filtration, washed with water, and dried in air (quant. yield).
1H NMR (300 MHz DMSO-d6): δ 13.35 (br, 4H), 8.46 (t, 2H,
J = 1.6 Hz), 8.32 (d, 4H, J = 1.6 Hz), 7.18 (s, 2H), 3.81 (s,
6H).
Synthesis of diethyl 2',5'-diethoxy-[1,1':4',1''-terphenyl]-
3,3'',5,5''-tetracarboxylate (C, R=Et) B, R=Et, (0.804 g, 1.63
mmol), diethyl 5-(4,4,5,5-tetramethyl-1,3,2-dioxaborolan-2-
yl)-1,3-benzene-dicarboxylate53 (2.26 g, 6.52 mmol), NaHCO3
(1.00 g), CsF (2.5 g), and Pd(PPh3)4 (0.1 g) were mixed in a
250 mL Schlenk flask, then purged and refilled with N2 three
times. 1,2-Dimethoxyethane (DME, 100 mL) and distilled
water (10 mL) were mixed and purged with N2 for 30 minutes,
then transferred under N2 to the reaction flask via canula. The
reaction solution was heated at reflux for 5 days. Organic sol-
vent was removed in vacuo and water added. The aqueous
phase was extracted with dichloromethane three times and
organic extracts combined. The organic solution was washed
with water, dried over MgSO4, filtered, and reduced in vacuo.
The off-white solid was collected after recrystallization from
acetone and drying in air (42.5 %). 1H NMR (300 MHz
CDCl3): δ 8.60 (t, 2H, J = 1.6 Hz), 8.43 (d, 2H, J = 1.6 Hz),
6.97 (s, 2H), 4.36 (q, 8H, J = 7.1 Hz), 3.98 (q, 4H, J = 6.9 Hz),
1.36 (t, 12H, J = 7.1 Hz), 1.27 (t, 6H, J = 7.0 Hz).
Synthesis of 2',5'-diethoxy-[1,1':4',1''-terphenyl]-3,3'',5,5''-
tetracarboxylic acid (H4TPTC-OEt) To a suspension of C,
R=Et, (0.401 g, 0.690 mmol) in 50 mL THF/methanol (1/1 by
volume) was added KOH (3 g in 25 mL water). The reaction
mixture was heated at reflux overnight. The organic solvent
was removed in vacuo and the peach colored solution acidified
with 20% HCl (in water). The white precipitate was collected
by vacuum filtration, washed with water, and dried in air (60.0
1
%). H NMR (300 MHz DMSO-d6): δ 13.30 (br, 4H), 8.45 (t,
2H, J = 1.6 Hz), 8.39 (d, 4H, J = 1.6 Hz), 7.20 (s, 2H), 4.09 (q,
4H, J = 7.1 Hz), 1.24 (t, 6H, J = 7.0 Hz).
Synthesis
of
diethyl
2',5'-di-n-propoxy-[1,1':4',1''-
terphenyl]-3,3'',5,5''-tetracarboxylate (C, R=nPr) B, R=nPr,
(1.24 g, 2.78 mmol), diethyl 5-(4,4,5,5-tetramethyl-1,3,2-
dioxaborolan-2-yl)-1,3-benzene-dicarboxylate53 (2.60 g, 7.5
mmol), NaHCO3 (1.2 g), CsF (3.0 g), and Pd(PPh3)4 (0.15 g)
were mixed in a 250 mL Schlenk flask, then purged and re-
filled with N2 three times. 1,2-Dimethoxyethane (DME, 100
mL) and distilled water (10 mL) were mixed and purged with
N2 for 30 minutes, then transferred under N2 to the reaction
flask via canula. The reaction solution was heated at reflux for
5 days. Organic solvent was removed in vacuo and water add-
ed. The aqueous phase was extracted with dichloromethane
three times and organic extracts combined. The organic solu-
tion was washed with water, dried over MgSO4, filtered, and
reduced in vacuo. The white solid was collected after washing
Synthesis
of
2',5'-di-n-propoxy-[1,1':4',1''-terphenyl]-
3,3'',5,5''-tetracarboxylic acid (H4TPTC-OnPr) To a suspen-
sion of C, R=nPr, (1.07 g, 1.69 mmol) in 50 mL THF/methanol
(1/1 by volume) was added KOH (3 g in 25 mL water). The
reaction mixture was heated at reflux overnight. The organic
solvent was removed in vacuo and the peach colored solution
acidified with 20% HCl (in water). The white precipitate was
collected by vacuum filtration, washed with water, and dried
1
in air (quant. yield). H NMR (300 MHz DMSO-d6): δ 13.26
(br, 4H), 8.45 (t, 2H, J = 1.6 Hz), 8.39 (d, 4H, J = 1.6 Hz),
7.19 (s, 2H), 4.00 (t, 4H, J = 6.2 Hz), 1.64 (m, 4H, J = 6.7 Hz),
0.90 (t, 6H, J = 7.4 Hz).
Synthesis
of
2',5'-di-n-hexyloxy-[1,1':4',1''-terphenyl]-
3,3'',5,5''-tetracarboxylic acid (H4TPTC-OnHex) To a suspen-
sion of C, R=nHex, (0.896 g, 1.25 mmol) in 50 mL
THF/methanol (1/1 by volume) was added KOH (3 g in 25 mL
water). The reaction mixture was heated at reflux overnight.
The organic solvent was removed in vacuo and the peach col-
ored solution acidified with 20% HCl (in water). The white
precipitate was collected by vacuum filtration, washed with
1
with acetone and drying in air (60.7 %). H NMR (300 MHz
CDCl3): δ 8.60 (t, 2H, J = 1.6 Hz), 8.42 (d, 4H, J = 1.6 Hz),
6.96 (s, 2H), 4.36 (q, 8H, J = 7.1 Hz), 3.87 (t, 4H, J = 6.3 Hz),
1.65 (m, 4H), 1.36 (t, 12H, J = 7.1 Hz), 0.90 (t, 6H, J = 7.4
Hz).
Synthesis
of
diethyl
2',5'-di-n-hexyloxy-[1,1':4',1''-
terphenyl]-3,3'',5,5''-tetracarboxylate (C, R=nHex) B, R=nHex,
(1.50 g, 2.82 mmol), diethyl 5-(4,4,5,5-tetramethyl-1,3,2-
dioxaborolan-2-yl)-1,3-benzene-dicarboxylate53 (2.60 g, 7.5
mmol), NaHCO3 (1.2 g), CsF (3.0 g), and Pd(PPh3)4 (0.15 g)
were mixed in a 250 mL Schlenk flask, then purged and re-
1
water, and dried in air (quant. yield). H NMR (300 MHz
DMSO-d6): δ 13.30 (br, 4H), 8.45 (t, 2H, J = 1.6 Hz), 8.35 (d,
4H, J = 1.6 Hz), 7.17 (s, 2H), 4.00 (t, 4H), 1.57 (m, 4H), 1.45-
1.05 (m, 12H), 0.76 (t, 6H).
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Figure 1. PXRD patterns for Cu2TPTC-OR MOFs as-synthesized (as), activated (ac), after being exposed to atmosphere after activation for
1 h, and after being exposed to atmosphere (50% r.h.) after activation for 16 days, compared to the PXRD pattern for NOTT-106 (simulat-
ed from single-crystal data).
Synthesis of Cu2(TPTC-OR)·xS (S=unidentified guest spe-
cies). Cu2(TPTC-OMe)·xS To a 10 dram glass vial was added
0.050 g (0.107 mmol) H4TPTC-OMe and 0.150 g (0.645
mmol) Cu(NO3)2 ∙ 2.5 H2O dissolved in 15 mL N,N-
dimethylacetamide (DMA) with 40 drops HBF4 (48% w/w
aqueous solution) and 40 drops water. The reaction was kept
at 85 °C in an oven for 4 days. The resultant blue-teal crystal-
line powder of Cu2(TPTC-OMe)·xS was then decanted,
washed with fresh DMA, and collected. Yield: 45.3% (activat-
ed sample) based on H4TPTC-OMe.
Cu2(TPTC-OnPr)·xS To a 10 dram glass vial was added
0.050 g (0.096 mmol) H4TPTC-OnPr and 0.150 g (0.645
mmol) Cu(NO3)2 ∙ 2.5 H2O dissolved in 15 mL N,N-
dimethylacetamide (DMA) with 20 drops HBF4 (48% w/w
aqueous solution). The reaction was kept at 85 °C in an oven
for 4 days. The resultant blue-teal crystalline powder of
Cu2(TPTC-OnPr)·xS was then decanted, washed with fresh
DMA, and collected. Yield: 66.5% (activated sample) based
on H4TPTC-OnPr.
Cu2(TPTC-OnHex)·xS To a 10 dram glass vial was added
0.050 g (0.082 mmol) H4TPTC-OnHex and 0.150 g (0.645
mmol) Cu(NO3)2 ∙ 2.5 H2O dissolved in 15 mL N,N-
dimethylacetamide (DMA) with 20 drops HBF4 (48% w/w
aqueous solution). The reaction was kept at 85 °C in an oven
for 4 days. The resultant blue-teal crystalline powder of
Cu2(TPTC-OnHex)·xS was then decanted, washed with fresh
DMA, and collected. Yield: 63.2% (activated sample) based
on H4TPTC-OnHex.
Cu2(TPTC-OEt)·xS To a 10 dram glass vial was added 0.050
g (0.101 mmol) H4TPTC-OEt and 0.150 g (0.645 mmol)
Cu(NO3)2
∙
2.5 H2O dissolved
in 15 mL N,N-
dimethylacetamide (DMA) with 20 drops HBF4 (48% w/w
aqueous solution). The reaction was kept at 85 °C in an oven
for 4 days. The resultant blue-teal crystalline powder of
Cu2(TPTC-OEt)·xS was then decanted, washed with fresh
DMA, and collected. Yield: 39.4% (activated sample) based
on H4TPTC-OEt.
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Figure 2. Views of (a) the node structures in NOTT-101; (b) simplified 4-connected node; (c) NbO type framework assembled by two 4-
connected nodes; (d and e) structures of the two types of cages. Reprinted with permission from Ref. 50. Copyright 2009 American Chem-
ical Society.
X-ray Crystallography. Powder X-ray diffraction (PXRD)
patterns were collected on a Bruker D8-Focus Bragg-Brentano
X-ray Powder diffractometer equipped with a Cu sealed tube
(λ = 1.54178 Å) and graphite monochromator at a scan rate of
1 s deg-1, solid state detector, and a routine power of 1600 W
(40 kV, 40 mA). The samples were dispersed on Si single
crystal zero diffraction plate for analysis. Simulation of the
PXRD pattern was carried out by the single-crystal data and
diffraction-crystal module of the Mercury program available
the procedure repeated once more with dry methanol, and
three subsequent times with dry dichloromethane. After the
removal of dichloromethane by decanting, the samples were
activated by drying under a dynamic vacuum at room tempera-
ture and at 80 °C overnight to produce activated samples. Be-
fore the measurement, the samples were further activated us-
ing the “degas” function of the surface area analyzer for 5 h at
80 °C. Other activation temperatures were tested, with the
reported methods providing the best sorption properties. UHP
grade (99.999%) N2 and H2 were used for all measurements.
free-of-charge
via
the
internet
at
http://www.ccdc.cam.ac.uk/products/mercury/.
RESULTS AND DISCUSSION
In situ synchrotron PXRD measurements were conducted on
Beamline 1-BM-C at the Advanced Photon Source, Argonne
National Laboratory. Samples were heated from 25 to 400 ˚C
at a rate of 3 ˚C/min. For each diffraction pattern, the sample
was exposed to synchrotron X-ray radiation (λ = 0.60505 Å)
20 times over 6 sec, rocking the sample +/- 5˚ to reduce arti-
facts from non-monodisperse particle sizes.
Synthesis and Structural Identification of Cu2TPTC-
OR·xS. Under solvothermal conditions, reactions between
Cu(NO3)2·2.5 H2O and H4TPTC-OR in DMA afforded teal-
blue microcrystalline powders of Cu2TPTC-OR·xS. No crys-
tals were large enough for structural characterization by sin-
gle-crystal X-ray diffraction, so a previously synthesized
Cu(II)-based MOF with similarly substituted terphenyl tetra-
carboxylate ligand backbone, NOTT-106, was used as a struc-
tural analogue.50 Using the powder pattern of NOTT-106
Low-Pressure Gas Adsorption Measurements. Gas sorption
isotherm measurements were performed on an ASAP 2020
Surface Area and Pore Size Analyzers. As-synthesized sam-
ples of Cu2TPTC-OR were immersed in dry methanol for 24 h
and the extract decanted. Fresh dry methanol was subsequent-
ly added and the crystals remained in the solvent for an addi-
tional 24 h. Each sample was collected by decanting and
Figure 3. Dry (a, c, e) and wet (b, d, f) powder samples of
Cu2TPTC-OEt, -OnPr, and –OnHex, respectively. After additions
of a single drop of water the –OEt and –OnPr frameworks readily
absorb the water drop, while the –OnHex framework acts as a
superhydrophobic surface.
Figure 4. Thermogravimetric analysis of Cu2TPTC-OR MOFs.
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Figure 5. Top view of combined patterns from in situ synchrotron PXRD thermal decomposition experiments (heating from 25 – 350 ˚C at
a rate of 3 ˚C min-1; a, c, e), and single diffraction patterns taken at 100 ˚C (b, d, f) for Cu2(TPTC-OEt), –OnPr, and –OnHex, respectively.
simulated from single-crystal data, the PXRD data collected
for Cu2TPTC-OMe, -OEt, -OnPr, and –OnHex were compared
and determined to be isostructural with NOTT-106 (Figure 1).
Furthermore, a new method developed in our lab was used to
generate a difference envelope density for highly porous
MOFs from PXRD data. In conjunction with overlaying the
predicted structure (NOTT-106), the core structures of the
MOFs were confirmed and positions of pendant alkoxy chains
identified.54
r.h.) after full removal of all guest species from the pores (1 h
exposure and 16 days exposure, Figures 1 and S1-S4).
It is apparent from the PXRD data that the –OMe substitut-
ed framework is the least stable of the studied systems and, in
fact, begins to decompose during the desolvation process,
when guest molecules are removed from the pores, as evident
from the increasing intensity of the reflection at ~36.4˚ 2θ
(Figure 1). This reflection is also observed in the as-
synthesized pattern, and is attributed to decomposition of the
framework (formation of CuO). The other three structures,
however, are observed to retain crystallinity upon solvent re-
moval, activation, and a minimum of 1 hour exposure to at-
mospheric conditions (50% r.h.) after activation. As is com-
mon when comparing experimental powder patterns with a
simulated structure, reflection intensities of experimental pat-
terns do not match perfectly with the simulated pattern, but all
experimental patterns (under similar conditions) match quite
well with one another. The deviation of as-synthesized forms
from the simulated pattern is attributed to the fact that the sin-
gle crystal data includes no contributions from solvent mole-
cules which reside in the pores of the structure. While devia-
tions apparent in the activated structure arise from slight dis-
tortions in the framework to minimize the energy after the
pores are evacuated. The most notable trend observed is seen
in the PXRD patterns after 16 days exposure to atmospheric
conditions (50% r.h.). The pattern for the –OEt substituted
framework shows significant broadening of diffraction peaks
and decrease in signal to noise ratio, indicative of significant
distortions in the framework structure and the initial stages of
decomposition. Whereas the –OnHex substitution leads to sig-
nificant enhancement in the stability of the system to atmos-
pheric conditions, identified from the minimal broadening of
reflection peaks. The –OnPr substituted framework falls in
Therefore, the structures of Cu2TPTC-OR may be described
as being constructed from 4-connected square planar Cu2-
paddlewheel SBUs and 4-connected rectangular planar TPTC-
OR4- organic SBUs. The Cu2-paddlewheels are sustained by
four carboxylates from four separate TPTC-OR4- linkers bridg-
ing between two Cu(II) ions. When two types of planar 4-
connected nodes are combined, it often results in the formation
of NbO type networks of 64·82 topology.50 The NbO network
may be described as being composed of two types of cages or
channels, as previously reported (Figure 2).49-50
To confirm no cleavage of the side chains occurred during
solvothermal reactions, a sample of each MOF was collected,
1
digested in HCl, washed with water, air dried, and H NMR
spectra confirmed to match with synthesized acid precursors
of each ligand.
Moisture and Thermal Stability Properties. The role of
pendant alkoxy groups in affecting the moisture stability of the
four Cu2TPTC-OR MOFs was evaluated through PXRD stud-
ies and observation of interaction of water drops with the func-
tionalized frameworks. PXRD patterns for each sample were
measured after each stage of solvent exchange, guest removal
(activation), and exposure to air (laboratory conditions, 50%
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Page 7 of 18
Crystal Growth & Design
between the –OEt and –OnHex substituted frameworks, as
expected, when comparing peak broadening, but, interestingly,
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shows a clear reflection attributed to the formation of CuO, a
feature not observed in what should be the less stable –OEt
substituted framework.
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The observation of excellent moisture stability led us to
study the nature of water droplets on the surfaces of the three
stable MOFs. When a single drop of water was dropped on to
the surface of a ~10 mg sample of powdered MOF, both the –
OEt and –OnPr substituted frameworks were observed to ab-
sorb the water droplet (Figure 3a-d). However, the long chain
–OnHex substitution created an exceptionally hydrophobic
material and resulted in the surface of the water drop being
coated in the powder and retaining its shape before rolling off
the surface of the powder (Figure S5). Another drop was then
added to the same sample with similar result, but remaining in
a divot in the powder formed when the drop landed on the
surface of the sample (Figure 3f).
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Figure 6. Nitrogen sorption isotherms at 77 K. Filled symbols =
adsorption; empty symbols = desorption.
The thermal stability of the MOFs was evaluated by both
TGA and in situ synchrotron PXRD decomposition studies.
Based upon TGA studies, the MOFs exhibit a three step de-
composition process. The first step occurs from 30 to ~100˚C,
and is attributed to the removal of uncoordinated (“free”)
guest molecules. The second step occurs around 250 – 300 ˚C,
and is attributed to coordinated guest species, with full de-
composition beginning at 300, 325, 340, and 355 ˚C for –
OMe, –OEt, –OnPr, and –OnHex substituted frameworks, re-
spectively (Figure 4). Surprisingly, the in situ study showed
contradictory thermal stability results, indicating that rather
than increasing the stability of the framework, longer alkoxy
chains resulted in decreased crystallinity at lower temperatures
(Figure 5). The –OEt, –OnPr, and –OnHex substituted frame-
works underwent amorphization at 315, 298, and 292 ˚C, re-
spectively. The explanation for such reversal in thermal stabil-
ity may be explained by considering the length and flexibility
of pendant groups allows for more disruptive movements in
the crystal structure with the input of thermal energy. This
increased thermal motion places further strain on the crystal-
line framework, leading to collapse. These results show that
TGA studies may not provide the entire picture when investi-
gating thermal stability of porous materials. This is an inherent
problem when utilizing TGA data, as TGA is based solely
upon combustion of the sample and cannot take into consider-
ation the crystallinity or porosity of the system. However, in
situ PXRD study allows for the direct observation of the crys-
tallinity of the structure and confirmation of the porous struc-
ture.
respectively. This trend was somewhat unexpected, as it would
seem that incorporating longer alkoxy chains on the ligands
should decrease the uptake and apparent surface area, since
they would serve to occupy more of the free void space within
the porous structure. This trend is observed between the –OnPr
and –OnHex frameworks, but not from the –OEt to the –OnPr
(because of the impurity observed in the activated PXRD pat-
tern for the –OMe substituted MOF, it will not be further con-
sidered when evaluating trends). However, we can see that the
difference between surface areas for the –OEt and –OnPr
frameworks differ only slightly, as compared to the difference
between the surface areas of the –OnPr and –OnHex frame-
works. This unexpected juxtaposition in trends is proposed to
arise from the pendant –OnPr groups effectively reducing the
pore sizes so as to enhance the interactions between the host
framework and guest gas molecules. Since gas adsorption in
MOFs occurs through weak van der Waal’s interactions, gas
storage in the system may be enhanced by tuning pore sizes to
very near the kinetic diameter of target guest species to in-
crease the potential overlap between host and guest.
The N2 sorption measurements were repeated for the –OEt,
–OnPr, and –OnHex samples after being exposed to atmospher-
ic conditions (50% r.h.) for 16 days without and with reactiva-
tion of the sample prior to measurement (thermal activation,
80 °C for 5 hours under dynamic vacuum), Figure S6. Signifi-
cant decrease in the amount of gas adsorbed was observed for
each of the three samples, with the –OnPr sample exhibiting no
N2 sorption and the –OEt sample exhibiting the greatest sorp-
tion (reduced from 372 to 119 cm3g-1). This follows in line
with the observations from PXRD data (i.e. decomposition of
the –OnPr sample). The –OnHex sample showed N2 sorption
reduced from 292 to 59 cm3g-1. While the N2 isotherm for the
–OnHex sample still exhibited a Type I isotherm with no hys-
teresis, the –OEt isotherm showed significant hysteresis,
which may be attributed to collapse of the framework. The
observation of Type I isotherm with diminished uptake for the
–OnHex sample, paired with observations from PXRD, indi-
cate that the MOF is structurally stable to moisture under 50%
r.h. conditions but retains a significant amount of water upon
thermal activation under reduced pressure.
Surface Area and Hydrogen Storage Properties. The
stability and significant uptake typical of NbO type MOFs led
us to investigate the porosity and hydrogen uptake properties.
The freshly prepared samples of Cu2TPTC-OR were first sol-
vent exchanged with methanol, then dichloromethane, result-
ing in a color change from teal in DMA, to green in methanol,
to blue in dichloromethane, and finally deep purple upon acti-
vation, typical of dicopper(II)-paddlewheel frameworks. The
N2 sorption isotherms, Figure 6, reveal that all four MOFs
exhibit typical Type I sorption behavior, a characteristic of
microporous materials,55 and congruent with the expected
crystal structure. By applying the BET model, the apparent
surface areas are estimated to be 1127, 1293, 1396, and 1083
m2 g-1 (1414, 1612, 1658, and 1269 m2g-1 Langmuir) for the –
OMe, –OEt, –OnPr, and –OnHex substituted MOFs,
The hydrogen storage properties of the four MOFs were
evaluated at both 77 K and 87 K (Figures 7a and S7). All
MOFs adsorb a fairly significant amount of H2 at 77 K and
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Crystal Growth & Design
Page 8 of 18
bility is increased upon extending the hydrophobic pendant
chains, resulting in a superhydrophobic material when –OnHex
is the substituent on the central phenyl ring. However, the
thermal stability of the systems are decreased with increasing
chain length, as identified through in situ synchrotron powder
X-ray diffraction measurements, in which the crystallinity of
the frameworks were monitored as a function of temperature.
This result shows that the standard use of thermogravimetric
analysis, which measures combustion temperatures, may not
always provide an accurate description of the thermal stability
of a MOF. The H2 sorption properties of the materials were
evaluated and found to scale according to accessible surface
area, showing no other significant impact from changing the
length of pendant groups, and heats of adsorption match well
with other previously reported Cu-based MOFs with uncoor-
dinated metal centers. This work is a successful demonstration
of the simplicity and feasibility of modifying organic linkers
to produce water stable, superhydrophobic materials for future
gas sorption applications. Future work will focus on tuning the
pore sizes and geometries to enhance targeted storage of gas
molecules while maintaining hydrophobicity so as to preferen-
tially exclude water.
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ASSOCIATED CONTENT
Supporting Information. H2 uptake at 87 K, normalized H2 up-
take at 87 K, full PXRD analyses, and additional photograph from
water drop analyses. This material is available free of charge via
the Internet at http://pubs.acs.org.
AUTHOR INFORMATION
Corresponding Author
* E-mail: zhou@chem.tamu.edu Tel: 1-979-845-4034
Fax: 1-979-845-1595
Figure 7. Excess hydrogen uptake at 77 K (a) with isosteric heats
of adsorption (inset), and normalized hydrogen uptake at 77 K (b):
(H2 uptake, wt%)/(BET surface area, m2 g-1) v. pressure.
ACKNOWLEDGMENT
This work was supported as part of the Center for Gas Sepa-
rations Relevant to Clean Energy Technologies, an Energy Fron-
tier Research Center funded by the U.S. Department of Energy
(DOE), Office of Science, Office of Basic Energy Sciences under
Award Number DE-SC0001015 and the Welch Foundation (A-
1725). A detailed description is given in the Supporting Infor-
mation. The in situ diffraction study was conducted by Andrey A.
Yakovenko and Dr. Mario Wriedt on Beamline 1-BM-C at the
Advanced Photon Source at Argonne National Laboratory.
810 Torr: 1.57 wt%, –OMe; 1.79 wt%, –OEt; 1.96 wt%, –
OnPr; and 1.48 wt%, –OnHex. As a way to identify the role of
the pendant alkoxy groups on the H2 sorption properties, the
isotherms were normalized by dividing the H2 uptake (in wt%)
by the surface area of the sorbent (in m2g-1) (Figures 7b and
S8). As has been shown in previous reports, the H2 sorption
properties of MOFs are highly dependent upon accessible sur-
face area and pore volume,56-59 and much less dependent upon
functional groups. To further analyze the H2 uptake properties,
the isosteric heats of adsorption, Qst, were analyzed using the
Clausius-Clapeyron equation.60 At zero-coverage, the four
MOFs exhibit Qst values (Figure 7a inset) similar to that of
other Cu(II)-based MOFs, ranging from 6.22 – 6.72 kJ mol-1,
attributable to interactions of H2 with open Cu sites.61-64 This is
further highlighted by the decreasing Qst as loading increases
and the exposed Cu sites are saturated, resulting in steady Qst
of approximately 5.3 – 5.6 kJ mol-1. The lower Qst observed
for the –OnHex framework at zero-loading may be explained
by the same reason it exhibits higher moisture stability; that is,
the pendant hexyloxy groups serve to decrease access to the
open metal sites.
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Tuning the Moisture and Thermal Stability of Metal-Organic Frameworks through Incorporation of Pendant Hy-
drophobic Groups
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Trevor A. Makal, Xuan Wang, and Hong-Cai Zhou
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A series of NbO type metal-organic frameworks are synthesized with various pendant alkoxy groups,
exhibiting remarkable moisture and thermal stability relationships dependent on the pendant group.
While longer pendant alkoxy group enhances moisture stability, thermal stability is observed to de-
crease; contrary to results from commonly used TGA data.
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Synthesis and structures of ligand precursors H4TPTC-OR
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PXRD patterns for Cu2TPTC-OR MOFs as-synthesized (as), activated (ac), after being exposed to
atmosphere after activation for 1 h, and after being exposed to atmosphere after activation for 16 days,
compared to the PXRD pattern for NOTT-106 (simulated from single-crystal data).
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Views of (a) the node structures in NOTT-101; (b) simplified 4-connected node; (c) NbO type framework
assembled by two 4-connected nodes; (d and e) structures of the two types of cages. Reprinted with
permission from Ref. 50. Copyright 2009 American Chemical Society.
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Dry (a, c, e) and wet (b, d, f) powder samples of Cu2TPTC-OEt, -OnPr, and –OnHex, respectively. After
additions of a single drop of water the –OEt and –OnPr frameworks readily absorb the water drop, while the
–OnHex framework acts as a superhydrophobic surface.
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Thermogravimetric analysis of Cu2TPTC-OR MOFs.
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Top view of combined patterns from in situ synchrotron PXRD thermal decomposition experiments (heating
from 25 – 350 ˚C at a rate of 3 ˚C min-1; a, c, e), and single diffraction patterns taken at 100 ˚C (b, d, f)
for Cu2(TPTC-OEt), –OnPr, and –OnHex, respectively.
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Nitrogen sorption isotherms at 77 K. Filled symbols = adsorption; empty symbols = desorption.
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