Rapid mechanochemical synthesis of metal-organic frameworks using exogenous organic base

Article abstract of DOI:10.1039/d0dt01240h

Metal-organic frameworks (MOFs) bearing coordinatively unsaturated metal centers, exemplified by the MOF-74 family of frameworks, are promising for applications ranging from gas separations and storage to Lewis acid catalysis. However, the scalable synthe

Full text of DOI:10.1039/d0dt01240h

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ARTICLE
Rapid Mechanochemical Synthesis of Metal‒Organic Frameworks
using Exogenous Organic Base
Zihao Wang,a Zongzhe Li, a Marcus Ng, a Phillip J. Milner*,a
Received 00th January 20xx,
Accepted 00th January 20xx
Metal–organic frameworks (MOFs) bearing coordinatively unsaturated metal centers, exemplified by the MOF-74 family of
frameworks, are promising for applications ranging from gas separations and storage to Lewis acid catalysis. However, the
scalable synthesis of MOF-74 analogues remains a significant challenge. Recently, mechanochemistry has emerged as a
sustainable strategy for the preparation of MOFs in the solid state with minimal solvent waste. Mechanochemical methods
typically rely on metal salts bearing basic anions to deprotonate the conjugate acid of the organic linker and a small amount
of organic solvent or water to facilitate liquid assisted grinding. Here, we demonstrate that the liquid exogenous organic
base Hünig’s base (N,N-diisopropylethylamine) can fulfill both roles, enabling the mechanochemical synthesis of M2(dobdc)
analogues (M = Mg, Mn, Co, Ni, Cu, Zn; dobdc4‒ = 2,5-dioxidobenzene-1,4-dicarboxylate) using metal nitrate salts in only 5
minutes at room temperature. Importantly, we demonstrate that this straightforward method can be generalized to prepare
the isomeric framework Mg2(m-dobdc) (m-dobdc4‒ = 2,4-dioxidobenzene-1,5-dicarboxylate) and the expanded framework
Mg2(dobpdc) (dobpdc4‒ = 4,4′-dioxidobiphenyl-3,3′-dicarboxylate) under solvent-free conditions for the first time. The MOFs
prepared using this method possess high crystallinities and surface areas, with the Mg2(m-dobdc) prepared herein
representing the first reported permanently porous variant of this framework. This new sustainable mechanochemical
synthesis of MOF-74 analogues should enable their preparation on a large scale for industrial applications.
DOI: 10.1039/x0xx00000x
As with many MOFs, MOF-74 analogues are typically prepared
under ultra-dilute solvothermal conditions using amide solvents such
as N,N-dimethylformamide (DMF) (Figure 2a).28,29 The role of the
Introduction
Metal–organic frameworks (MOFs) are a class of porous, crystalline
solids constructed from polytopic organic “linkers” and inorganic
“secondary building units” (SBUs) with myriad potential applications
ranging from chemical separations to catalysis.1,2 Of these, MOFs
bearing coordinatively unsaturated or open metal centers are
particularly intriguing due to their ability to strongly polarize and
bind guest molecules.3 In addition, MOFs with open metal centers
are promising heterogeneous Lewis acid catalysts.4,5 Among open
metal site MOFs, the MOF-74 or CPO-27 family stands out in terms
of structural tunability and density of open metal sites (Figure 1).6
amide solvent is to decompose at high temperatures (>100 °C) to
produce amines (e.g. N,N-dimethylamine) that deprotonate the
conjugate acid of the linker prior to MOF self-assembly. While these
solvothermal syntheses produce highly crystalline frameworks,12
they employ solvents that are undesirable on industrial scale29 and
result in significant generation of organic waste. In addition, a lack of
control over the strength of the base and the rate of its addition
limits the inherent synthetic tunability of these methods. This is
important because parameters such as pH and temperature can have
a significant effect on framework crystallinity and polymorph
formation, as has been observed previously for MOF-74
analogues.30–34 Recently, it has been shown that the base can be
decoupled from the reaction solvent by instead employing metal
salts bearing basic counteranions (e.g. acetate) under solvothermal
conditions (not shown).35–40 More generally, it is possible to
completely separate the base from the MOF precursors or solvent by
utilizing exogenous organic41 or inorganic42,43 bases to prepare MOF-
74 frameworks (Figure 2b). Although these methods allow more
synthetic control over framework formation, they still lead to
significant solvent waste. Therefore, while it is possible to avoid the
use of amide solvents during the preparation of select MOF-74
analogues, the generation of significant waste limits the
sustainability of these methods. In addition, isomeric and expanded
MOF-74 analogues are still widely prepared using DMF-based
solvothermal approaches (Figure 1).
Following the initial report of the synthesis of Zn2(dobdc) (dobdc4‒
2,5-dioxidobenzene-1,4-dicarboxylate) in several
2005,6
=
isostructural metal analogues, including Mg,7 Mn,8 Co,9 Ni,10 and
Cu11,12 congeners, as well as isomeric13 and expanded14–21 variants,
have been reported (Figure 1). Due to their hexagonal one-
dimensional channels lined with a high density of five-coordinate M2+
centers, activated MOF-74 variants hold numerous records for gas
storage capacities at low pressures, including for CO2 and H2.22,12 In
addition, MOF-74 analogues demonstrate promising catalytic activity
for Lewis acid-mediated23,24 and C–H oxidation reactions.17,25 Given
the promise of MOF-74 analogues for industrial applications,
sustainable methods for their preparation on scale are desirable.26,27
a. Department of Chemistry and Chemical Biology, Cornell University, Ithaca, NY
14853, United States
Electronic Supplementary Information (ESI) available: [details of any supplementary
information available should be included here]. See DOI: 10.1039/x0xx00000x
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Figure 1. Structures of the open metal site metal‒organic frameworks M2(dobdc) (M = Mg, Mn, Co, Ni, Cu, Zn; dobdc4‒ = 2,5-dioxidobenzene-1,4-dicarboxylate), Mg2(m-dobdc) (m-
dobdc4‒ = 2,4-dioxidobenzene-1,5-dicarboxylate), and Mg2(dobpdc) (dobpdc4‒ = 4,4′-dioxidobiphenyl-3,3′-dicarboxylate). The cross-pore diameters in these MOFs are 15.4 Å, 14.9
Å, and 22.8 Å, respectively. Gray, white, red, and green spheres represent carbon, hydrogen, oxygen, and magnesium, respectively.
Figure 2. Overview of methods for the synthesis of MOF-74 analogues, with the source of base highlighted in red. a) Traditional solvothermal synthesis using N,N-dimethylformamide
(DMF); b) solvothermal synthesis using exogenous organic or inorganic base; c) mechanochemical synthesis using basic metal precursors, and d) mechanochemical synthesis using
exogenous organic base (this work, highlighted in blue).
Recently, mechanochemical syntheses of MOFs in the solid state preparation of high-quality MOF-74 frameworks in a scalable and
have emerged as promising alternatives to solvothermal methods sustainable manner. In addition, we show that this strategy can be
because they avoid the generation of significant solvent waste.44–47 readily translated to prepare isomeric and expanded MOF-74
In particular, liquid assisted grinding (LAG), in which precursors solids analogues in the solid state for the first time.
are mechanically ground together in the presence of a small amount
of solvent, has been shown to enable MOF synthesis on large
scale.47,48 Select MOF-74 frameworks can be prepared by LAG using
basic metal precursors, albeit with variable crystallinities and surface
areas (Figure 2c).49,50 Building upon this precedent, we hypothesized
that we could mechanochemically synthesize MOF-74 frameworks
Results and Discussion
We began exploring the mechanochemical synthesis of MOF-74
analogues with exogenous base by simply grinding an excess of
Mg(NO3)2·6H2O, the most commonly used metal precursor to
prepare Mg-based MOF-74 analogues,7,13,14,19 and H4dobdc together
for 5 min at room temperature using a mortar and pestle (see
Supporting Information or SI Section 2 for details). Unsurprisingly
due to the lack of a sufficiently strong base, Mg2(dobdc) did not form
under these conditions. However, the addition of 4.4 equiv. of
Hünig’s base to the solid mixture led to the clean formation of
using
a liquid exogenous base, such as Hünig’s base (N,N-
diisopropylethylamine), to completely decouple the base from the
metal precursor (Figure 2d).49,50 In these reactions, Hünig’s base
would play a dual role as both the base to enable MOF formation as
well as the liquid to assist in mechanical grinding. Herein, we
demonstrate that LAG with Hünig’s base does in fact allow for the
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desolvated at 180 °C under high vacuum to remove residual
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DOI: 10.1039/D0DT01240H
methanol. Importantly, the measured N2 adsorption isotherms of
these materials at 77 K are comparable to reference material
prepared under solvothermal conditions (Figure 4).52 In particular,
the Langmuir surface areas of Mg2(dobdc) prepared on 2 mmol scale
(1852 ± 53 m²/g, blue circles) and 8 mmol scale (1992 ± 49 m²/g,
green triangles) using a ball mill are similar to MOF prepared under
solvothermal conditions (1914 m²/g, purple squares).52 These results
are summarized in Table 1 and confirm that high-quality Mg2(dobdc)
can be prepared by mechanochemical synthesis using exogenous
organic base.
Figure 3. Powder X-ray diffraction (PXRD) patterns (λ = 1.5406 Å) of Mg2(dobdc) prepared
under mechanochemical conditions with a mortar and pestle (red) or ball mill (blue,
green) and Mg2(dobdc) prepared under standard solvothermal conditions (purple).52 The
simulated pattern based on the previously reported single-crystal X-ray diffraction
structure of the isostructural framework Zn2(dobdc) is included for reference (gray).53
nanocrystalline Mg2(dobdc), as confirmed by powder X-ray
diffraction (Figure 3, red curve; see SI Section 3 for details). Hünig’s
base was chosen because it possesses similar basicity (pKb = 3.2) to
dimethylamine (pKb = 3.3) and triethylamine (pKb = 3.2), which have
previously been used to prepare MOF-74 analogues,6,41 but is
significantly less coordinating and volatile. This result confirms that
the combination of organic linker, metal salt, and amine base is
sufficient to prepare Mg2(dobdc) in the solid state.
Figure 4. Comparison of the 77 K N2 adsorption (filled) and desorption (open) isotherms
of Mg2(dobdc) prepared under mechanochemical conditions (blue circles, green
triangles) and standard solvothermal conditions (purple squares).52
We next evaluated the generality of this mechanochemical
strategy towards the synthesis of other M2(dobdc) congeners (see SI
Sections 4–8 and Section 12 for details). Gratifyingly, the
corresponding M2(dobdc) (M = Mn, Co, Ni, Cu, Zn) frameworks could
be prepared on ~0.5 g scale following the same procedure outlined
above, as confirmed by powder X-ray diffraction (Figure 5) and
Infrared spectroscopy (SI Sections 4–8). Qualitative analysis of these
powder X-ray diffraction patterns using the Scherrer equation (K =
0.89, λ = 1.5406 Å) suggests that the average crystalline domain size
of all prepared MOFs are in the nanocrystalline regime38,41 and
smaller than MOF prepared under solvothermal conditions (Table
1).52 Soaking the as-synthesized M2(dobdc) frameworks in methanol
followed by desolvation under vacuum at 180 °C allowed us to
evaluate their 77 K N2 surface areas (Table 1; see SI Sections 4–8 for
details). Similar to the Mg congener, Co2(dobdc) and Zn2(dobdc)
produced under mechanochemical conditions possess Langmuir
surface areas close to those reported for frameworks prepared
under solvothermal conditions (Table 1).12 Although the Langmuir
surface area of Ni2(dobdc) is slightly lower than expected,
nanocrystalline samples of MOF-74 variants have previously been
shown to have low surface areas due to a higher prevalence of
defects.38 Similarly, the unexpectedly low Langmuir surface area of
Cu2(dobdc) may arise due to the presence of defects or amorphous
contaminants, in line with the known difficulty of preparing high
surface area samples of this framework.12 Unfortunately, the
porosity of Mn2(dobdc) prepared herein was quite low, likely due its
oxidative degradation in air upon filtration. Nonetheless, these
results confirm that of M2(dobdc) variants generally can be prepared
using this mechanochemical strategy.
In order to standardize our mechanochemical synthesis, we
employed a ball mill to prepare Mg2(dobdc) with more consistent
grinding (see SI Section 2 for details).51 Using tungsten carbide
vessels and balls (ϕ = 2.0 mm) and a milling rate of 600 rpm, we
successfully prepared Mg2(dobdc) on 2 mmol scale (~0.5 g) at room
temperature in only 5 min, as confirmed by powder X-ray diffraction
and IR spectroscopy (Figure 3, blue curve). Notably, the crystallinity
of Mg2(dobdc) synthesized using a ball mill was superior to that
prepared using a mortar and pestle. We were able to further scale
up this simple method to prepare Mg2(dobdc) on 8 mmol (~2.0 g)
scale with similar results (Figure 3, green curve). It should be noted
that grinding times longer than 5 min led to a significant reduction in
crystallinity, indicating that reaction time likely has a significant
impact on the observed crystallite size. As a result, a standard
reaction time of 5 min was used for subsequent experiments.
To evaluate whether mechanochemical synthesis produces MOF
of comparable porosity to material prepared under solvothermal
conditions,12,52 we soaked the large-scale samples of Mg2(dobdc) in
methanol at 65 °C, changing the solvent every day for three days.
Digestion of the samples using DCl (35 wt% in D2O) in DMSO-d6 and
analysis by 1H NMR confirmed that soaking in methanol is sufficient
to remove residual Hünig’s base and ammonium salts from the
framework pores (SI Figure S27). Next, these samples were
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Table 1. Summary of average crystalline domain sizes and Langmuir surface areas for
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DOI: 10.1039/D0DT01240H
MOF-74 analogues.
Average
Crystalline
Domain Size
(nm)a
Langmuir
Surface
Area
Solvothermal
Langmuir
Framework
Surface Area
(m2/g)b
(m2/g)b
Mg2(dobdc)
Mn2(dobdc)
Co2(dobdc)
Ni2(dobdc)
21 (51)c
28
1992 ± 49
385 ± 14d
1334 ± 51
1281 ± 48
1115 ± 6
1914
179712
23
143812
13
157412
Cu2(dobdc)
Zn2(dobdc)
Mg2(m-dobdc)
Mg2(dobpdc)
39
151512
21 (15)e
22
1204 ± 16
1793 ± 50
3137 ± 71
127712
Nonporous54
378014
10
aDetermined by the Scherrer equation using the first PXRD reflection. bDetermined
from N2 adsorption isotherm at 77 K. cFrom solvothermal synthesis.52 dFramework
oxidized in air. ePrepared using 2,6-lutidine as base in place of Hünig’s base.
Figure 6. Powder X-ray diffraction patterns (λ = 1.5406 Å) of Mg2(m-dobdc) (blue) and
Mg2(dobpdc) (red) prepared under mechanochemical conditions. The simulated
patterns based on the previously reported single-crystal X-ray diffraction structures of
the isostructural frameworks Co2(m-dobdc)54 and Zn2(dobpdc)56 are included for
reference.
SI Sections 9–10 for details). In both cases, the successful formation
of these frameworks was confirmed by powder X-ray diffraction
(Figure 6) and Infrared spectroscopy (SI Sections 9–10). Our
preliminary results suggest that Co2(dobpdc) and Zn2(dobpdc) can
also be prepared under these conditions (SI Figures S28–29). Using
the Scherrer equation, we confirmed that the average crystalline
domain size of Mg2(m-dobdc) is similar to that of Mg2(dobdc), which
is understandable given their structural similarity (Table 1). In
contrast, smaller crystallites of Mg2(dobpdc) were obtained.
Consistent with our findings for Ni2(dobdc), the 77 K N2 Langmuir
surface area of Mg2(dobpdc) prepared herein is lower than that for
material prepared under solvothermal conditions (Table 1, SI Figure
S26).14 Importantly, the Mg2(m-dobdc) synthesized herein was found
to be highly porous (1793 ± 50 m2/g), with a surface area similar to
that of the isomeric framework Mg2(dobdc) (Table 1, SI Figure S23).
This is in contrast the material prepared under solvothermal
conditions, which was previously reported to be non-porous due to
the difficulty of removing residual DMF from the pores.21,54 To the
best of our knowledge, this is the first report of the permanent
porosity of Mg2(m-dobdc), further expanding the limited pool of Mg-
based frameworks with coordinatively unsaturated metal centers.55
Overall, LAG with Hünig’s base appears to be a general protocol for
the synthesis of MOF-74 analogues.
Figure 5. Powder X-ray diffraction patterns (λ = 1.5406 Å) of M2(dobdc) (M = Mn, Co, Ni,
Cu, Zn) prepared under mechanochemical conditions. The simulated pattern based on
the previously reported single-crystal X-ray diffraction structure of the framework
Zn2(dobdc) is included for reference.53
While the mechanochemical synthesis of selected M2(dobdc)
variants has been previously described using basic metal salts,49,50
the solid-state synthesis of related M2(m-dobdc) (m-dobdc4‒ = 2,4-
dioxidobenzene-1,5-dicarboxylate) and M2(dobpdc) (dobpdc4‒ = 4,4′-
dioxidobiphenyl-3,3′-dicarboxylate) frameworks (Figure 1) has not,
to the best of our knowledge, been reported to date. The sustainable
synthesis of these frameworks is desirable because they are
promising for H2 storage22 and CO2 capture,21 respectively.
Therefore, we further extended our methodology by preparing
Mg2(m-dobdc) and Mg2(dobpdc) in a ball mill using Hünig’s base (see
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prove beneficial for the scalable synthesis of both known and
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new porous MOF-74 variants. Current efDfoOrIt:s10in.10o3u9/rDl0aDbTo0r1a2t4o0rHy
are focused on exploring the mechanism of this novel LAG
strategy and extending it to the synthesis of other industrially
relevant MOFs.
Conflicts of interest
The authors have no conflicts of interest to declare.
Acknowledgements
The authors acknowledge Cornell University for financial
support of this work. Z.W. thanks Nankai University and Z.L.
thanks Beihang University for additional financial support. The
authors thank Prof. Francis DiSalvo (Cornell University) for the
use of the ball mill in his laboratory. The powder X-ray
diffraction pattern of Co2(dobpdc) was collected using the mail-
in service at beamline 11-BM of the Advanced Photon Source.
Use of the Advanced Photon Source at Argonne National
Laboratory was supported by the U. S. Department of Energy,
Office of Science, Office of Basic Energy Sciences, under
Contract No. DE-AC02-06CH11357. 1H NMR data were collected
on a Bruker INOVA 500 MHZ spectrometer that was purchased
with support from the National Science Foundation (CHE-
1531632). We thank Dr. Arjun Halder (Cornell University) for
assistance with the preparation of this manuscript.
Figure 7. Powder X-ray diffraction patterns (λ = 1.5406 Å) of Zn2(dobdc) prepared under
mechanochemical conditions with different organic bases. The simulated pattern based
on the previously reported single-crystal X-ray diffraction structure of Zn2(dobdc) is
included for reference.53
To gain preliminary mechanistic insight into the role of organic
base in MOF formation, we explored mechanochemical synthesis
using different nitrogen bases (Figure 7; see SI section 8 for details).
For these studies, Zn2(dobdc) was chosen because its mechanism of
formation under mechanochemical conditions has previously been
shown to proceed by step-wise deprotonation of the carboxylates of
the linker followed by the less acidic phenols.50 As with Mg2(dobdc),
no MOF formation was observed in the absence of base.
Interestingly, when the weak organic base pyridine (pKb = 8.8) was
employed for LAG, Zn2(dobdc) was not observed. This is likely
because pyridine is insufficiently basic to deprotonate the phenols of
the organic linker, although we cannot rule out that the coordinating
nature of pyridine also plays a role in inhibiting MOF formation.
Consistently, when the stronger and more sterically-hindered
organic base 2,6-lutidine (pKb = 7.3) was used instead, we were able
to successfully prepare Zn2(dobdc). However, the use of even more
basic Hünig’s base (pKb = 3.2) led to more crystalline material. This is
likely because Hünig’s base is sufficiently basic to deprotonate both
the carboxylic acid (pKa1 ≈ 3.0) and phenol (pKa ≈ 13.8) of the linker
to some degree. These results confirm tha2t base of sufficient
strength to deprotonate both the phenol and carboxylic acid of the
linker are required to form MOF, suggesting that the strength of the
base is a potential handle for controlling MOF formation. Further
studies are underway in our laboratory to explore the role of organic
base in MOF formation.
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synthesis of MOF-74 variants using Hünig’s base as both the
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Dalton Transactions
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DOI: 10.1039/D0DT01240H
We describe the mechanochemical, solvent-free synthesis of metal–organic frameworks using
liquid organic base for the first time.